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Soil moisture is an essential climate variable of the Earth system. Understanding its spatiotemporal dynamics is essential for predicting weather patterns and climate variability, monitoring and mitigating the effects and occurrence of droughts and floods, improving irrigation in agricultural areas, and sustainably managing water resources. Here we review in depth how soils can remember information on soil moisture anomalies over time, as embedded in the concept of soil moisture memory (SMM). We explain the mechanisms underlying SMM and explore its external and internal drivers; we also discuss the impacts of SMM on different land surface processes, focusing on soil‐plant‐atmosphere coupling. We explore the spatiotemporal variability, seasonality, locality, and depth‐dependence of SMM and provide insights into both improving its characterization in land surface models and using satellite observations to quantify it. Finally, we offer guidance for further research on SMM. Plain Language Summary: Our review paper takes an in‐depth look at soil moisture memory, which is how soil records its moisture history over time and space. Analogous to human psychology, which seeks to understand how a person's/society's memory influences his/her present and future behavior, understanding soil moisture memory encourages consideration of how such memory determines present state and might determine future behavior of soils exposed to environmental disturbances. Soil moisture memory can be affected by a variety of factors, both external (e.g., weather extremes) and internal (soil's unique properties). It affects everything from the air to the way our landscapes respond to disasters like droughts, wildfires, and floods. We also studied how this phenomenon affects the balance of water and energy in our environment, the health of our plants, and even how it communicates with the atmosphere. We show how it can change depending on where you are on the planet, the time of year, and how deep you dig into the soil. We offer scientists insights into how weather and land surface models can become more accurate by accounting for soil moisture memory. Its understanding not only helps us predict and manage our environment, but also provides opportunities for exciting scientific discoveries. Key Points: Atmospheric forcings, land use and management, and soil processes and mechanisms explain how and why soil moisture memory emerges in ecosystemsNonlocality of moisture memory, its spread across different regions, and its interaction with large‐scale climate phenomena are underexploredFurther advances in land surface models and closer integration of model simulations and observations are needed to better characterize moisture memory [ABSTRACT FROM AUTHOR]

The functions of soils are intimately linked to their three‐dimensional pore space and the associated biogeochemical interfaces, mirrored in the complex structure that developed during pedogenesis. Under stress overload, soil disintegrates into smaller compound structures, conventionally named aggregates. Microaggregates (<250 µm) are recognized as the most stable soil structural units. They are built of mineral, organic, and biotic materials, provide habitats for a vast diversity of microorganisms, and are closely involved in the cycling of matter and energy. However, exploring the architecture of soil microaggregates and their linkage to soil functions remains a challenging but demanding scientific endeavor. With the advent of complementary spectromicroscopic and tomographic techniques, we can now assess and visualize the size, composition, and porosity of microaggregates and the spatial arrangement of their interior building units. Their combinations with advanced experimental pedology, multi‐isotope labeling experiments, and computational approaches pave the way to investigate microaggregate turnover and stability, explore their role in element cycling, and unravel the intricate linkage between structure and function. However, spectromicroscopic techniques operate at different scales and resolutions, and have specific requirements for sample preparation and microaggregate isolation; hence, special attention must be paid to both the separation of microaggregates in a reproducible manner and the synopsis of the geography of information that originates from the diverse complementary instrumental techniques. The latter calls for further development of strategies for synlocation and synscaling beyond the present state of correlative analysis. Here, we present examples of recent scientific progress and review both options and challenges of the joint application of cutting‐edge techniques to achieve a sophisticated picture of the properties and functions of soil microaggregates. [ABSTRACT FROM AUTHOR]

Soils in hyper‐arid climates, such as the Chilean Atacama Desert, show indications of past and present forms of life despite extreme water limitations. We hypothesize that fog plays a key role in sustaining life. In particular, we assume that fog water is incorporated into soil nutrient cycles, with the inland limit of fog penetration corresponding to the threshold for biological cycling of soil phosphorus (P). We collected topsoil samples (0–10 cm) from each of 54 subsites, including sites in direct adjacency (<10 cm) and in 1 m distance to plants, along an aridity gradient across the Coastal Cordillera. Satellite‐based fog detection revealed that Pacific fog penetrates up to 10 km inland, while inland sites at 10–23 km from the coast rely solely on sporadic rainfall for water supply. To assess biological P cycling we performed sequential P fractionation and determined oxygen isotope of HCl‐extractable inorganic Pδ18OHCl−Pi$$ \mathrm{P}\ \left({\updelta}^{18}{\mathrm{O}}_{\mathrm{HCl}-{\mathrm{P}}_{\mathrm{i}}}\right) $$. Total P (Pt) concentration exponentially increased from 336 mg kg−1 to a maximum of 1021 mg kg−1 in inland areas ≥10 km. With increasing distance from the coast, soil δ18OHCl−Pi$$ {\updelta}^{18}{\mathrm{O}}_{\mathrm{HCl}-{\mathrm{P}}_{\mathrm{i}}} $$ values declined exponentially from 16.6‰ to a constant 9.9‰ for locations ≥10 km inland. Biological cycling of HCl‐Pi near the coast reached a maximum of 76%–100%, which could only be explained by the fact that fog water predominately drives biological P cycling. In inland regions, with minimal rainfall (<5 mm) as single water source, only 24 ± 14% of HCl‐Pi was biologically cycled. We conclude that biological P cycling in the hyper‐arid Atacama Desert is not exclusively but mainly mediated by fog, which thus controls apatite dissolution rates and related occurrence and spread of microbial life in this extreme environment. [ABSTRACT FROM AUTHOR]

Soil water status, which refers to the wetness or dryness of soils, is crucial for the productivity of agroecosystems, as it determines nutrient cycling and uptake physically via transport, biologically via the moisture‐dependent activity of soil flora, fauna, and plants, and chemically via specific hydrolyses and redox reactions. Here, we focus on the dynamics of nitrogen (N), phosphorus (P), and sulfur (S) and review how soil water is coupled to the cycling of these elements and related stoichiometric controls across different scales within agroecosystems. These scales span processes at the molecular level, where nutrients and water are consumed, to processes in the soil pore system, within a soil profile and across the landscape. We highlight that with increasing mobility of the nutrients in water, water‐based nutrient flux may alleviate or even exacerbate imbalances in nutrient supply within soils, for example, by transport of mobile nutrients towards previously depleted microsites (alleviating imbalances), or by selective loss of mobile nutrients from microsites (increasing imbalances). These imbalances can be modulated by biological activity, especially by fungal hyphae and roots, which contribute to nutrient redistribution within soils, and which are themselves dependent on specific, optimal water availability. At larger scales, such small‐scale effects converge with nutrient inputs from atmospheric (wet deposition) or nonlocal sources and with nutrient losses from the soil system towards aquifers. Hence, water acts as a major control in nutrient cycling across scales in agroecosystems and may either exacerbate or remove spatial disparities in the availability of the individual nutrients (N, P, S) required for biological activity. [ABSTRACT FROM AUTHOR]

Aims: Agricultural soils in Germany store 2.54 Pg of organic carbon (C). However, information about how and when this C entered the soils is limited. This study illustrates how depth profiles of organic matter can shed light on different entry paths of organic C. Methods: Machine learning was used to explain total organic C (TOC), C:N, particulate organic C (POC), δ13C and δ15N values down to 100 cm depth based on pedology, geology, climate and management-related variables from the German Agricultural Soil Inventory. We estimated TOC turnover rates based on the relationship between the proportion of maize (only C4 plant) in crop rotations and soil δ13C values. Results: In the upper 30 cm of cropland, fresh photosynthates added on average 0.2 to 0.8 Mg C ha− 1 year− 1. Organic fertiliser was another source of topsoil C, especially in grassland. Sandy sites in north-west Germany contained historic C from past heathland and peatland. One third of German agricultural land was found to be on colluvial and alluvial deposits, in which allochthonous C from upstream and upslope areas evidently increased the TOC content of subsoils. In and below hardpans, TOC content and C:N and POC:TOC ratios were low, indicating restricted root-derived C input. Conclusions: Our data indicate that ongoing management in German agricultural soils mainly affects topsoil C, while C storage in subsoils reveals significant legacies from allochthonous, buried or translocated C inputs. Specific attention should be focused on the sustainable loosening of hardpans that could result in a slow, but significant increase in subsoil C stocks. [ABSTRACT FROM AUTHOR]

Soil carbon transformation and sequestration have received significant interest in recent years due to a growing need for quantitating its role in mitigating climate change. Even though our understanding of the nature of soil organic matter has recently been substantially revised, fundamental uncertainty remains about the quantitative importance of microbial necromass as part of persistent organic matter. Addressing this uncertainty has been hampered by the absence of quantitative assessments whether microbial matter makes up the majority of the persistent carbon in soil. Direct quantitation of microbial necromass in soil is very challenging because of an overlapping molecular signature with nonmicrobial organic carbon. Here, we use a comprehensive analysis of existing biomarker amino sugar data published between 1996 and 2018, combined with novel appropriation using an ecological systems approach, elemental carbon–nitrogen stoichiometry, and biomarker scaling, to demonstrate a suit of strategies for quantitating the contribution of microbe‐derived carbon to the topsoil organic carbon reservoir in global temperate agricultural, grassland, and forest ecosystems. We show that microbial necromass can make up more than half of soil organic carbon. Hence, we suggest that next‐generation field management requires promoting microbial biomass formation and necromass preservation to maintain healthy soils, ecosystems, and climate. Our analyses have important implications for improving current climate and carbon models, and helping develop management practices and policies. [ABSTRACT FROM AUTHOR]

Abstract In Bavarian alpine forests, long-term input of organic matter from conifer species has built up thick organic surface layers, thus immobilized substantial soil organic phosphorus upon calcareous parent materials. The composition of these phosphorus stocks, however, remained unrevealed. Therefore, we analyzed organic phosphorus species in three so called Tangelhumus (Histosol according to World Reference Base) profiles with different radiocarbon ages using alkaline extraction followed by one-dimension and two-dimension 31P nuclear magnetic spectroscopy in order to understand changes in organic phosphorus composition with soil organic matter genesis. A diverse phosphorus pool composed of monoesters, diesters, and phosphonates was detected. However, in these Tangelhumus soils only monoesters accumulated overtime and eventually dominated the organic phosphorus pool after 1558 years. Diesters and phosphonates did not show any significant accumulation pattern. The origin of monoesters varied with soil depth and soil age: monoesters in young surface soils were actually hydrolysis products of diesters released by plant and microbial cells during alkaline extraction; whereas in older soils monoesters originated largely from Ca-bound precipitates formed after long-term in situ decomposition of organic matter and stabilization by Ca2+. Myo -inositol hexakisphosphate was below detection limit in all soils. The presence of its scyllo and other isomers likely pointed to direct synthesis by microorganisms instead of microbial epimerization from myo -inositol hexakisphosphate. In general, the overall picture of phosphorus speciation manifested as the continuous degradation of organic matter and accumulation of monoesters. Therefore, the current data indicated no involvement of humification process but favored the soil continuum model. Highlights • Only monoesters showed dramatic accumulation after 1500 years of Tangelhumus formation. • Origin of monoesters varied with soil age/depth. • Ca2+ acted the stabilizer for SOP accumulation. • Myo -inositol hexakisphosphate was missing in all Tangelhumus soils. • Current data favored soil continuum model rather than humification. [ABSTRACT FROM AUTHOR]

Abstract The cycling of iron (Fe) is often closely linked with that of carbon, nitrogen, phosphorus and manganese. Therefore, alterations in the Fe cycle may be indicative of concurrent overall changes in the biogeochemistry of terrestrial and aquatic ecosystems. Biogeochemical processes taking part in the Fe cycle frequently fractionate stable Fe isotopes, leaving soil, plant and other compartments of the ecosystems with varied Fe isotopic signatures. In this work, we reviewed the Fe isotope fractionation processes that have been reported so far for terrestrial ecosystems. While parent materials vary in Fe isotope compositions, pedogenic processes can further fractionate Fe isotope signatures, resulting in soil profiles with δ56Fe values (relative to isotope standard IRMM-014) from −0.52 to +0.72‰. Different soil Fe pools, as a result of cycling processes, show an even broader range of δ56Fe values, with secondary Fe oxides being isotopically the lightest, and with Fe sequestered in silicate minerals being the heaviest, due to preferential release of light Fe isotopes during dissolution of minerals. Actually and potentially plant-available Fe in soil can be extracted by 0.5 M HCl, which includes pools of water-extractable and exchangeable Fe, organically bound or adsorbed Fe, and poorly crystalline Fe oxides, altogether showing a depletion of heavy Fe isotopes with δ56Fe values down to −1.08‰. Depending on the Fe speciation and concentration present in the growth medium, plants can adapt their uptake strategy for Fe. Plants of the strategy I type especially take up light iron isotopes, while strategy II plants fractionate less towards light isotopes. Aboveground tissues usually show lighter Fe isotope signatures than the roots, with flowers (δ56Fe: −2.15 to −0.23‰) being isotopically the lightest. In freshwater systems, the most distinct Fe isotope fractionation is usually found at the oxic-anoxic interface, where redox conditions change and thus Fe speciation controls the degree of Fe isotope fractionation. Similar to soils, the δ56Fe values of unfiltered water mainly reflect averaged Fe isotope compositions across fractions with different particle sizes. In filtered freshwater (<0.45 μm), isolated colloid-sized fractions can exhibit either positive or negative δ56Fe values, depending on the chosen size fraction and the origin of the (nano) particles, with δ56Fe values up to +2.79‰ for fractions smaller than 0.003 μm from an arctic stream or down to −1.73‰ for dissolved Fe (<0.02 μm) from a boreal forested catchment. Most freshwater studies showed that rivers with elevated contents of dissolved organic carbon (DOC) tend to be isotopically heavier than those with lower DOC contents, while some studies also showed that rivers with high DOC can display light Fe isotopic signatures owing to the input of groundwater- and/or soil water-derived Fe. Finally, anthropogenic impacts can contribute to Fe isotope fractionation in freshwaters and may widen the range of δ56Fe values in the environment, with the lowest records found down to −5.29‰. Overall, our compilation reveals that Fe pools in different terrestrial system compartments vary in stable Fe isotope compositions, although the current database is still small. In order to use stable Fe isotopes as proxies to reconstruct the biogeochemical processes, future works should not solely rely on bulk δ56Fe assessments, but also involve the assessment of different fractionation factors for all biogeochemical pathways, which includes isotopic analyses among various pools of the terrestrial Fe cycle. [ABSTRACT FROM AUTHOR]

Aims: Root exudates contain polymers that form crosslinks and can create a jelly like substance known as mucilage, which adheres to soil and thus promotes the formation of rhizosheaths, i.e. soil that remains attached to the roots after gentle shaking. We hypothesized that rhizosheath formation is optimal at an intermediate chia seed mucilage concentration and water content, but that its formation is limited at both a high concentration of chia seed mucilage and under dry conditions as well as at a low concentration of chia seed mucilage and under wet conditions. We used an artificial root soil system in which soil moisture and mucilage concentrations could be varied independently from one another with respect to their effect on rhizosheath formation. Methods: Jute cords were disposed in sandy loam soil and in quartz sand. In a subsequent study, they were also amended to different moisture contents with five different concentrations of mucilage (from 0 to 0.2 g dry mucilage g−1 water), before being isolated from chia and flaxseed mucilage after swelling of the respective seeds in distilled water for 15 min. Results: We found that in dry soil, rhizosheath formation peaked at an intermediate chia seed mucilage concentration. This behavior was supported by our conceptual model of mucilage spreading and rhizosheath formation, which relies on a radial diffusion equation and assumes that at low mucilage concentration, molecule numbers are insufficient to support polymer-like networks that stick soil particles together. In a very concentrated gel, however, mucilage is too sticky to diffuse far into the soil. Increasing soil moisture promotes rhizosheath formation both in a low and a high mucilage concentration range, although only up to an intermediate volumetric water content of 0.15cm3 cm–3. Conclusions: We conclude that both water and chia seed mucilage concentration are important drivers of rhizosheath formation. The effects are not additive but can combine to an optimum range, with a maximum formation of rhizosheaths observed in this study at 0.12 g mucilage g−1 rhizosphere water. [ABSTRACT FROM AUTHOR]

Abstract: All soils harbor microaggregates, i.e., compound soil structures smaller than 250 µm. These microaggregates are composed of diverse mineral, organic and biotic materials that are bound together during pedogenesis by various physical, chemical and biological processes. Consequently, microaggregates can withstand strong mechanical and physicochemical stresses and survive slaking in water, allowing them to persist in soils for several decades. Together with the physiochemical heterogeneity of their surfaces, the three‐dimensional structure of microaggregates provides a large variety of ecological niches that contribute to the vast biological diversity found in soils. As reported for larger aggregate units, microaggregates are composed of smaller building units that become more complex with increasing size. In this context, organo‐mineral associations can be considered structural units of soil aggregates and as nanoparticulate fractions of the microaggregates themselves. The mineral phases considered to be the most important as microaggregate forming materials are the clay minerals and Fe‐ and Al‐(hydr)oxides. Within microaggregates, minerals are bound together primarily by physicochemical and chemical interactions involving cementing and gluing agents. The former comprise, among others, carbonates and the short‐range ordered phases of Fe, Mn, and Al. The latter comprise organic materials of diverse origin and probably involve macromolecules and macromolecular mixtures. Work on microaggregate structure and development has largely focused on organic matter stability and turnover. However, little is known concerning the role microaggregates play in the fate of elements like Si, Fe, Al, P, and S. More recently, the role of microaggregates in the formation of microhabitats and the biogeography and diversity of microbial communities has been investigated. Little is known regarding how microaggregates and their properties change in time, which strongly limits our understanding of micro‐scale soil structure dynamics. Similarly, only limited information is available on the mechanical stability of microaggregates, while essentially nothing is known about the flow and transport of fluids and solutes within the micro‐ and nanoporous microaggregate systems. Any quantitative approaches being developed for the modeling of formation, structure and properties of microaggregates are, therefore, in their infancy. We respond to the growing awareness of the importance of microaggregates for the structure, properties and functions of soils by reviewing what is currently known about the formation, composition and turnover of microaggregates. We aim to provide a better understanding of their role in soil function, and to present the major unknowns in current microaggregate research. We propose a harmonized concept for aggregates in soils that explicitly considers the structure and build‐up of microaggregates and the role of organo‐mineral associations. We call for experiments, studies and modeling endeavors that will link information on aggregate forming materials with their functional properties across a range of scales in order to better understand microaggregate formation and turnover. Finally, we hope to inspire a novel cohort of soil scientists that they might focus their research on improving our understanding of the role of microaggregates within the system of aggregates and so help to develop a unified and quantitative concept of aggregation processes in soils. [ABSTRACT FROM AUTHOR]

At least 300 Mio t of plastic are produced annually, from which large parts end up in the environment, where it persists over decades, harms biota and enters the food chain. Yet, almost nothing is known about plastic pollution of soil; hence, the aims of this work are to review current knowledge on i) available methods for the quantification and identification of plastic in soil, ii) the quantity and possible input pathways of plastic into soil, (including first preliminary screening of plastic in compost), and iii) its fate in soil. Methods for plastic analyses in sediments can potentially be adjusted for application to soil; yet, the applicability of these methods for soil needs to be tested. Consequently, the current data base on soil pollution with plastic is still poor. Soils may receive plastic inputs via plastic mulching or the application of plastic containing soil amendments. In compost up to 2.38–1200 mg plastic kg − 1 have been found so far; the plastic concentration of sewage sludge varies between 1000 and 24,000 plastic items kg − 1 . Also irrigation with untreated and treated wastewater (1000–627,000 and 0–125,000 plastic items m − 3 , respectively) as well as flooding with lake water (0.82–4.42 plastic items m − 3 ) or river water (0–13,751 items km − 2 ) can provide major input pathways for plastic into soil. Additional sources comprise littering along roads and trails, illegal waste dumping, road runoff as well as atmospheric input. With these input pathways, plastic concentrations in soil might reach the per mill range of soil organic carbon. Most of plastic (especially > 1 μm) will presumably be retained in soil, where it persists for decades or longer. Accordingly, further research on the prevalence and fate of such synthetic polymers in soils is urgently warranted. [ABSTRACT FROM AUTHOR]

Liquid-state, one-dimension 31P nuclear magnetic resonance spectroscopy (NMR) has greatly advanced our understanding of the composition of organic phosphorus in the environment However, the correct assignment of signals is complicated by overlapping and shifting signals in different types of soils. We applied therefore for the first time diffusion-ordered spectroscopy (DOSY) to soil extracts, allowing us to separate phosphorus components in the second domain based on their translational diffusion coefficients. After successful application to a mixture of 14 model compounds, diffusion rates correlated closely with the molecular weight of the individual compound in aqueous solution (R² = 0.97). The method was then applied to NaOH/ EDTA extracts of a grassland soil, of which paramagnetic contaminations were removed with sodium sulfide following high-velocity centrifugation (21 500g, 45 min) at 4 °C. Diffusion rates in soil extracts were again closely related to molecular weight (R² = 0.98), varying from 163.9 to 923.8 Da. However, our DOSY application failed for a forest soil with low organic phosphorus content. Overall, DOSY did help to clearly identify specific NMR signals like myo- and scyllo-inositol hexakisphosphate. It thus provides a more confident signal assignment than ID 31P NMR, although currently the ubiquitous use of this novel methodology is still limited to soils with high organic phosphorus content. [ABSTRACT FROM AUTHOR]

In January 2011, fuel quality in inland water vessels was changed by EU regulation 2009/30/EC, aiming at improving air quality along waterways. We hypothesized that the implementation of this regulation both lowered the total deposition of polycyclic aromatic hydrocarbons (PAHs) and changed their composition in river valleys. We analyzed parent-, alkylated- and thio-PAHs in soil and vine leaves, at two waterways (Rhine and Moselle, Germany), as well as in one ship-free reference area (Ahr, Germany). Samples were taken annually (2010–2013) in transects perpendicular to the rivers. We did not find any relation of PAH concentration and composition on vine leaves to inland navigation, likely because atmospheric exchange processes distorted ship-specific accumulation patterns. We did find, however, an accumulation of ship-borne PAHs in topsoil near the waterways (1543 ± 788 and 581 ± 252 ng g − 1 at Moselle and Rhine, respectively), leading to larger PAH concentrations at the Moselle Valley than at the reference area (535 ± 404 ng g − 1 ) prior to EU fuel regulation. After fuel regulation, the PAH concentrations decreased in topsoils of the Moselle and Rhine Valley by 35 ± 9 and 62 ± 28%, respectively. These changes were accompanied by increasing proportions of dibenzothiophene (DBT) and low molecular weight PAHs. Both, changes in PAH concentrations and composition were traceable within 200 and 350 m distance to the river front of Moselle and Rhine, respectively, and likely favored by erosion of topsoil in vineyards. We conclude that the EU regulation was effective in improving soil and thus also air quality within only three years. The impact was greater and spatially more relevant at the Rhine, which may be attributed to the larger traffic volume of inland navigation. [ABSTRACT FROM AUTHOR]

Nitrite (NO ) as an important intermediate of the biological nitrogen cycle is particularly reactive in acidic soils and acts as a source of NO and NO (NO and NO). However, abiotic and biotic pathways of NO -driven NO and NO production in forest soil and the role of soil organic matter (SOM) in these processes are still unclear. In this study, NO was applied to both unsterile and sterilized soil samples as well as to different SOM fractions from a Norway spruce forest. Biotic and abiotic NO emission was measured with an infrared absorption analyzer and gas chromatography, while NO emission was quantified with a chemiluminescence analyzer. Isotopic signatures of NO (δN, δO, and N-NO site preference) were analyzed with an isotope ratio mass spectrometer. After NO addition, a large amount of NO was emitted immediately, while NO emission occurred 15-60 min later and was much lower compared to NO. Sterilization of soil decreased NO emission significantly, but not NO emission. The N site preference of NO ranged from 7.98 to 11.58‰ for abiotic and 4.69-7.42‰ for biotic sources. The fulvic acid fraction contributed the most to abiotic NO emission, while the fastest NO and NO emission occurred after NO application to the humin fraction, followed by the humic acid fraction. These results are important for the future understanding of NO and NO sources, as well as the use of isotopic signatures for source-partitioning NO emission from soil. Graphical Abstract: [ABSTRACT FROM AUTHOR]

• Chronological sequence of aggregate formation processes. • Organic gluing agents initiate and accelerate aggregation process. • Minerals stabilize aggregates in the longer term. • Root-mediated processes further stabilize macroaggregates against ultrasonic dispersion. Microaggregates (<250 µm) are key structural subunits of soils. However, their formation processes, rates, and transformation with time are poorly understood. We took advantage of multiple isotope labelling of potential organic gluing agents and inorganic building units to unravel their role in soil aggregation processes being initiated with and without plant growth. We added 13C-labelled extracellular polymeric substances (EPS), 15N-labelled bacteria, 57Fe-labelled goethite, and 29Si-labelled montmorillonite to fine soil <250 µm of an Ap horizon from a Stagnic Luvisol, which was planted with Festuca heteromalla or kept bare in a climate chamber. Samples were taken after 4, 12, and 30 weeks, and separated into free (f) and occluded (o) microaggregates of different sizes (<20 µm, 53–20 µm, 250–53 µm), and in stable macroaggregates (>250 µm) that resisted 60 J mL−1 ultrasonic dispersion. Afterwards, we assessed the C, N, Fe, and Si stable isotope composition in each size fraction. After four weeks we found a rapid build-up of stable macroaggregates comprising almost 50 % of soil mass in the treatment with plants and respective soil rooting, but only 5 % when plants were absent. The formation of these stable macroaggregates proceeded with time. Soil organic carbon (SOC) contents were elevated by 15 % in the large macroaggregates induced by plant growth. However, the recovery of EPS-derived 13C was below 20 % after 4 weeks, indicating rapid turnover in treatments both with and without plants. The remaining EPS-derived C was mainly found in macroaggregates when plants were present and in the occluded small microaggregates (<20 µm) when plants were absent. The excess of bacterial 15N closely followed the pattern of EPS-derived 13C (R2 = 0.72). In contrast to the organic gluing agents, the goethite-57Fe and montmorillonite-29Si were relatively equally distributed across all size fractions. Overall, microaggregates were formed within weeks. Roots enforced this process by stabilizing microaggregates within stable macroaggregates. As time proceeded the labelled organic components decomposed, while the labelled secondary oxides and clay minerals increasingly contributed to aggregate stabilization and turnover at the scale of months and beyond. Consequently, the well-known hierarchical organization of aggregation follows a clear chronological sequence of stabilization and turnover processes. [ABSTRACT FROM AUTHOR]

• Chronological sequence of aggregate formation processes. • Organic gluing agents initiate and accelerate aggregation process. • Minerals stabilize aggregates in the longer term. • Root-mediated processes further stabilize macroaggregates against ultrasonic dispersion. Microaggregates (<250 µm) are key structural subunits of soils. However, their formation processes, rates, and transformation with time are poorly understood. We took advantage of multiple isotope labelling of potential organic gluing agents and inorganic building units to unravel their role in soil aggregation processes being initiated with and without plant growth. We added 13C-labelled extracellular polymeric substances (EPS), 15N-labelled bacteria, 57Fe-labelled goethite, and 29Si-labelled montmorillonite to fine soil <250 µm of an Ap horizon from a Stagnic Luvisol, which was planted with Festuca heteromalla or kept bare in a climate chamber. Samples were taken after 4, 12, and 30 weeks, and separated into free (f) and occluded (o) microaggregates of different sizes (<20 µm, 53–20 µm, 250–53 µm), and in stable macroaggregates (>250 µm) that resisted 60 J mL−1 ultrasonic dispersion. Afterwards, we assessed the C, N, Fe, and Si stable isotope composition in each size fraction. After four weeks we found a rapid build-up of stable macroaggregates comprising almost 50 % of soil mass in the treatment with plants and respective soil rooting, but only 5 % when plants were absent. The formation of these stable macroaggregates proceeded with time. Soil organic carbon (SOC) contents were elevated by 15 % in the large macroaggregates induced by plant growth. However, the recovery of EPS-derived 13C was below 20 % after 4 weeks, indicating rapid turnover in treatments both with and without plants. The remaining EPS-derived C was mainly found in macroaggregates when plants were present and in the occluded small microaggregates (<20 µm) when plants were absent. The excess of bacterial 15N closely followed the pattern of EPS-derived 13C (R2 = 0.72). In contrast to the organic gluing agents, the goethite-57Fe and montmorillonite-29Si were relatively equally distributed across all size fractions. Overall, microaggregates were formed within weeks. Roots enforced this process by stabilizing microaggregates within stable macroaggregates. As time proceeded the labelled organic components decomposed, while the labelled secondary oxides and clay minerals increasingly contributed to aggregate stabilization and turnover at the scale of months and beyond. Consequently, the well-known hierarchical organization of aggregation follows a clear chronological sequence of stabilization and turnover processes. [ABSTRACT FROM AUTHOR]

Soils are faced with man-made chemical stress factors, such as the input of organic or metal-containing pesticides, in combination with non-chemical stressors like soil compaction and natural disturbance like drought. Although multiple stress factors are typically co-occurring in soil ecosystems, research in soil sciences on this aspect is limited and focuses mostly on single structural or functional endpoints. A mechanistic understanding of the reaction of soils to multiple stressors is currently lacking. Based on a review of resilience theory, we introduce a new concept for research on the ability of polluted soil (xenobiotics or other chemical pollutants as one stressor) to resist further natural or anthropogenic stress and to retain its functions and structure. There is strong indication that pollution as a primary stressor will change the system reaction of soil, i.e., its resilience, stability and resistance. It can be expected that pollution affects the physiological adaption of organisms and the functional redundancy of the soil to further stress. We hypothesize that the recovery of organisms and chemical-physical properties after impact of a follow-up stressor is faster in polluted soil than in non-polluted soil, i.e., polluted soil has a higher dynamical stability (dynamical stability = 1 / recovery time), whereas resilience of the contaminated soil is lower compared to that of not or less contaminated soil. Thus, a polluted soil might be more prone to change into another system regime after occurrence of further stress. We highlight this issue by compiling the literature exemplarily for the effects of Cu contamination and compaction on soil functions and structure. We propose to intensify research on effects of combined stresses involving a multidisciplinary team of experts and provide suggestions for corresponding experiments. Our concept offers thus a framework for system level analysis of soils paving the way to enhance ecological theory. [ABSTRACT FROM AUTHOR]

Highlights: [•] Soil texture and soil organic carbon content do not affect 15N dynamic. [•] 15N release depends on fertilizer (chicken manure, mushroom compost, mineral N). [•] Mustard intercropping is a tool to enhance N efficiency. [Copyright &y& Elsevier]

Abstract: In arable farming systems, the term ‘subsoil’ refers to the soil beneath the tilled or formerly tilled soil horizon whereas the latter one is denoted as ‘topsoil’. To date, most agronomic and plant nutrition studies have widely neglected subsoil processes involved in nutrient acquisition by crop roots. Based on our current knowledge it can be assumed that subsoil properties such as comparatively high bulk density, low air permeability, and poverty of organic matter, nutrients and microbial biomass are obviously adverse for nutrient acquisition, and sometimes subsoils provide as little as less than 10% of annual nutrient uptake in fertilised arable fields. Nevertheless, there is also strong evidence indicating that subsoil can contribute to more than two-thirds of the plant nutrition of N, P and K, especially when the topsoil is dry or nutrient-depleted. Based on the existing literature, nutrient acquisition from arable subsoils may be conceptualised into three major process components: (I) mobilisation from the subsoil, (II) translocation to the shoot and long-term accumulation in the Ap horizon and (III) re-allocation to the subsoil. The quantitative estimation of nutrient acquisition from the subsoil requires the linking of field experiments with mathematical modelling approaches on different spatial scales including Process Based Models for the field scale and Functional–Structural Plant Models for the plant scale. Possibilities to modify subsoil properties by means of agronomic management are limited, but ‘subsoiling’ – i.e. deep mechanical loosening – as well as the promotion of biopore formation are two potential strategies for increasing access to subsoil resources for crop roots in arable soils. The quantitative role of biopores in the nutrient acquisition from the subsoil is still unclear, and more research is needed to determine the bioaccessibility of nutrients in subsoil horizons. [Copyright &y& Elsevier]

Abstract: Paddy soils make up the largest anthropogenic wetlands on earth. They may originate from any type of soil in pedological terms, but are highly modified by anthropogenic activities. The formation of these Anthrosols is induced by tilling the wet soil (puddling), and the flooding and drainage regime associated with the development of a plough pan and specific redoximorphic features. Redox potential oscillations due to paddy management control microbial community structure and function and thus short-term biogeochemical processes. After flooding, microbial reduction processes sequentially use NO3 −, Mn4+, Fe3+, SO4 2 − as electron acceptors, accompanied by the emission of the trace gases N2O, N2, H2S, CH4 and — due to reduction-induced increasing pH — NH3. This results in N losses and low N fertilizer use efficiency. However, transport of atmospheric O2 to the roots via the rice plant''s aerenchyma modifies conditions in the rhizosphere, leading to nitrification and methane oxidation, and precipitation of Mn and Fe oxides. High concentrations and fluxes of dissolved organic matter (DOM) in paddy soils from plant debris trigger microbial activity and thus the emission of greenhouse gases. Retention of DOM by soil minerals and its subsequent stabilisation against microbial decay depend on the redox state (e.g. DOM precipitation by Fe2+ under anaerobic conditions). Oscillation in redox conditions may enhance retention and stabilisation of DOM by Fe oxyhydroxides. Induced by the periodic short-term redox cycles, paddy management over long periods has strong effects on long-term biogeochemical processes. Frequent irrigation intensifies mineral weathering and leaching processes. High concentrations of DOM during flooding seasons enhance the changes and the release of structural iron in clay minerals, and support the formation of ferrihydrite. Repeated redox alternations lead to a translocation of iron in various directions, and particularly increase the crystallinity of iron oxides. This results also in higher total iron oxide contents in paddy compared to non-paddy soils. The large accumulation of soil organic matter (SOM) observed in some, but not all paddy soils, is considered to be due to high input of plant residues and charred material associated with retarded decomposition under anaerobic conditions. There is also evidence of SOM stabilisation via occlusion into aggregates and phytoliths as well as interactions with clay minerals and iron oxides. SOM accumulation in paddy subsoils can be explained by downward movement of DOM and its stabilisation by interaction with iron oxides. A specific feature of paddy soils is the coupling of organic matter turnover with mineral transformations and fluxes, which seem to be intensified by the alternating redox conditions with increasing age of paddy soil development. Bioavailability of soil organic N is strongly coupled to SOM cycling and is a crucial parameter determining crop yield. Anaerobic conditions inhibit N mineralization, with a high risk of gaseous N losses. In paddy soils the management-induced, microbially mediated redox processes control the dynamics of soil minerals and soil organic matter, which are strongly related to the microbial accessibility of C and N, but also of Fe. [Copyright &y& Elsevier]

Abstract: It is well known that termites enrich organic matter in their nests and alter its properties, but their influence on phosphorus forms is less clear. The aim of our study was to quantify and systemize phosphorus forms in different nest parts (outer wall, inner wall, and central part) of termites of varying feeding guilds and soils in seven dominant Brazilian ecosystems. Organic (Po) and inorganic P (Pi) were analyzed after sequential extraction of six labile and stable P fractions in samples from the Amazon region (Terra firme, and Várzea and Igapó floodplains), the Pantanal, the Atlantic rain forest (Mata Atlântica), the Cerrado and the Caatinga. In addition, selected alkaline extracts were characterized by means of liquid-state 31P NMR spectroscopy. The central part of termite nests contained between 275 and 1871mgkg−1 total P, dominated by Pi in nests of soil/wood interface-feeder termites, but with balanced Po and Pi contents in nests of wood-feeding termites. All Po fractions, and, in most nests, also labile Pi forms were enriched in termite mounds in comparison to soils. The P enrichment was most pronounced in nests of the Terra firme (enrichment factors for labile Pi forms and for Po fractions were 1.7–36.5 and 1.1–16.8, respectively), and the Igapó (labile Pi: 4.1–34.0; Po: 1.1–25.7) where the total P content of the soil was the lowest with 141 and 171mgkg−1. Nests of wood-feeding termites mainly accumulated highly labile resin-Pi (enrichment factor 10.5–36.5) and partially the stable HClconc-Pi fractions, apart from Po, which can be all attributed to the organic food source and food digestion. The soil/wood interface-feeder termites had more variations in their nests, but in general had higher portions of P bonded to minerals. Hence, we conclude that (i) termite activity results in a gross enrichment of labile P forms in the nests, (ii) the P composition in termite nests reflects the major feeding guild, and (iii) the degree of this P enrichment is partly driven by the P stock in the soil. [Copyright &y& Elsevier]

Tracing back the N use efficiency of long-term fertilizer trials is important for future management recommendations. Here we tested the changes in natural N-isotope composition as an indicator for N- management within a long-term fertilization lysimeter experiment in a low mountain range pasture ecosystem at Rengen (Eifel Mountains), Germany. Cattle slurry (δ15N = 8.9 ± 0.5‰) and mineral fertilizers (calcium ammonium nitrate; δ15N = −1.0 ± 0.2‰) were applied at a rate between 0 and 480 kg N ha−1 yr−1 throughout 20 years from 1985 onwards. In 2006, samples were taken from different grass species, coarse and fine particulate soil organic matter, bulk soil and leachates. Total soil N content hardly changed during fertilization experiment. As also N leaching has been small within the stagnant water regime, most N was lost through the gaseous phase beside plant uptake and cutting. Unlike N uptake by plants, the process of N volatilization resulted in strong discrimination against the 15N isotope. As a consequence, the δ15N values of top soil samples increased from 1.8 ± 0.4‰ to 6.0 ± 0.4‰ and that of the plants from −1.2 ± 1.3‰ to 4.8 ± 1.2‰ with increasing N fertilizer rate. Samples receiving organic fertilizer were most enriched in δ15N. The results suggest that parts of the fertilizer N signal was preserved in soils and even discovered in soil organic matter pools with slow N turnover. However, a 15N/14N isotope fractionation of up to 1.5‰ added to the δ15N values recovered in soils and plants, rendering the increase in δ15N value a powerful indicator to long-term inefficient N usage and past N management in the terrestrial environment. [ABSTRACT FROM AUTHOR]

Abstract: Termites concentrate nutrients in their mounds, which may reenter soil through leaching. We studied the effect of the grass-feeder and litter-forager termite Cornitermes silvestrii on soil solution chemistry. Free-draining lysimeters were installed below mounds of this primary termite, below mounds build by C. silvestrii but inhabited by secondary termites (mainly by Nasutitermes kemneri), and reference soils. The solution was regularly collected during a 1-year period and analyzed for dissolved organic carbon (DOC), phosphate, nitrate, ammonium and pH. The volume of drainage water was lower under the mounds than under the reference soil, and so was the flux of DOC and nutrients. Only nitrate concentrations were enhanced under secondarily inhabited termite mounds. Hence, an effect of the termite mounds on drainage water and soil nutrients through leaching is evident, but the latter only below older mounds with secondary termites. [Copyright &y& Elsevier]

The article reports on the results of a multiresidue determination of pesticides in acid-clay soils from Thailand. A description of the experimental set-up and measurement method is presented. The study showed that the recoveries of all pesticides from soil ranged from 68.6 to 104 percent. Also the loss of internal standards compensated for the losses of most pesticides during workup.

Abstract: There is evidence of black carbon (BC) contributing to stable humus in the soil environment but its quantity and fate are poorly examined. We used benzene polycarboxylic acids (BPCAs) as markers to assess the contents and distribution of BC in soils and soil fractions of the long-term field experiments in Halle (Haplic Phaeozem), Bad Lauchstädt (Haplic Chernozem), Rotthalmünster (Haplic Luvisol) and an additional site near Bad Lauchstädt (Haplic Phaeozem), Germany. Black carbon comprised 11.9–13.2% of organic C in the top soils (0–10 cm) of the black soils located around Halle and Bad Lauchstädt while it explained only 2.7% of organic carbon in the Haplic Luvisol of Rotthalmünster. The BPCA pattern at Bad Lauchstädt and Halle suggested that two thirds of BC were of fossil origin. In general, BC contents (in g kg−1 soil) decreased with increasing soil depth. The C-normalized BC concentrations, however, increased and reached 35±7% of organic carbon at a depth of 87–114 cm. With increasing depth BC was increasingly localized in the coarse silt and sand fractions and the heavy mineral fraction. This indicated that BC was connected to the minerals of this size and preserved there. Inorganic fertilization for >30 years did not affect BC contents. [Copyright &y& Elsevier]

Abstract: Soils contain significant amounts of black carbon (BC) from biomass and fossil fuel combustion. However, its origin, morphology, and chemistry have remained obscure. Here, we examined BC in particle-size and density fractions of the surface soil of a Haplic Chernozem using a scanning electron microscope (SEM) coupled to an energy-dispersive X-ray spectrometer (EDX) in order to investigate the morphological and chemical properties of BC as a function of its origin and fate in soils. The results showed that BC did not only occur as well-defined but also as SEM-amorphous particles. The BC particles exhibited different morphologies ranging from spherical to irregular shapes and from smooth to rough surfaces. Particles with similar morphologies were found in different soil fractions, indicating that BC from different sources is present in the soil, dominated by soot-BC from coal (and oil) combustion and char-BC from coal combustion and biomass burning. The identity of BC was ascertained by atomic O/C ratios ≤0.33. Within a BC particle, the O/C ratio increased from interior to exterior surfaces. The mean degree of oxidation increased significantly with an increase in the size of the particle fraction and an increase in the density of the fraction. The presence of inherently light BC in heavy mineral fractions as well as SEM-visible mineral associations with BC particles provided evidence that the partially oxidized BC chemically interacted with the mineral phase, presumably resulting in a protection of the enclosed BC against further decomposition in soil. [Copyright &y& Elsevier]

We know little about the mechanisms that cause rapid losses in the soil organic N pool during cropping. As the analysis of amino acid enantiomers can provide insight into both the fate of microbial N and the ageing of cells in the environment, we used this technique as a tool to examine how the pool of protein-bound N in subtropical Plinthosols responds to increasing duration of arable cropping. The samples comprised bulk soils (0–20 cm) and clay fractions from each of three agro-ecosystems in semiarid South Africa; the sites have been cropped for periods varying from 0 to 98 years. The amino acid enantiomers contributed 34% to the total N content. With increasing number of years a piece of land has been cropped, the amino acid concentrations declined bi-exponentially to about 30% of their initial level in the native grasslands. Changes of the remaining soil protein-N pool were indicated by alterations in the -content of individual amino acids. As the years of arable cropping increased, the proportions of -alanine and -glutamic acid increased relative to the respective -enantiomers. This was attributed to an accumulation of N in residues of bacterial cell walls. In contrast, the /-ratios of leucine and aspartic acid declined in the long-term cultivated plots, probably reflecting losses of old amino acid-N reserves at the most degraded arable land. [ABSTRACT FROM AUTHOR]

Understanding the global distribution of PAHs requires knowledge of their sources. The objective of our work was to test the hypothesis that termites and woody plants are sources of naphthalene (NAPH), phenanthrene (PHEN), and perylene (PERY) for soils of different tropical climates. We determined the concentrations of 20 PAHs in soil, wood, and different compartments of termite nests (central part, inner wall, outer wall) in the Amazon (Terra firme, Va´rzea, and Igapo´), Pantanal, Cerrado, Mata Atlaˆntica, and Caatinga regions. The sum of 20 PAH concentrations was low in all soils (4.8–347 μg kg−1, n=47) and much higher in many of the wood (47–3894, n=31) and termite nest samples (29–4208, n=121). In general, NAPH, PHEN, or PERY were most abundant. In the Amazon region, wood samples contained up to 3785 and termite nest samples up to 3645 μg kg−1 of NAPH. In all regions, most termite nests contained higher PERY concentrations (up to 1109 μg kg−1) than wood or soil, indicating that PERY was produced or accumulated in these nests. In many termite nests, the central part had larger NAPH and PERY concentrations than the wall, indicating that these compounds may have been produced within the nests. With few exceptions, NAPH dominated the PAH pattern in the Amazon and west Cerrado regions, PHEN in the Mata Atlaˆntica and Caatinga regions, and PERY in the Pantanal and central Cerrado regions. Our results suggest that there are large unknown sources of NAPH, PHEN, and PERY in the tropical environment. It is likely that part of these sources is biological. The release of NAPH and PHEN from these sources seems to depend on climatic factors. [Copyright &y& Elsevier]

Examines the organic matter preservation of the sediment in an acidic mining lake. Oxidation of carbon following iron and sulfate reduction; Neutralization capacity of the lake system; Distribution of organic matter within the sediment.

This study aimed at investigating the potential of x-ray photoelectron spectroscopy (XPS) for investigating soil organic matter at secondary soil particles. The XPS was applied to microaggregates of the A horizon of a Typic Haplustoll (<20-μm equivalent diameter, > 53μm maximum real diameter) and to the fine-earth fraction (<2 mm) of the Bs horizon of a Typic Haplorthod. Carbon and N, as well as Si (both samples), Ca (Haplustoll), and Al (Haplorthod) were detected. Removing the particle surface layer (<50 nm) by bombarding with Ar[sup +] resulted in a strong reduction of the signals of C and N, while those attributed to inorganic components increased relatively. Consequently, in both soils, organic matter was concentrated at the surface of soil aggregates. We conclude that Ar[sup +] sputtering followed by XPS analysis is a useful tool in identifying the accumulation of elements at the surfaces of soil particles. [ABSTRACT FROM AUTHOR]

High termite abundance and diversity in rainforests results in the creation of different termite nests, which are formed from excreted food waste. The objective of this study was to elucidate the properties of nest material of common rain forest termites by analysing residues of potential food sources. In the Amazonian rain forest near Manaus, Brazil, we sampled nests of six different termite genera as well as surrounding wood, microepiphytes and soil. After a density separation into <1.6, 1.6–1.8, 1.8–2.0, 2.0–2.4, and >2.4 g cm−3 fractions, we determined the contents of C, N, and lignin-derived phenols in the samples. As particle density increased, the element and lignin concentrations decreased in all samples, the remaining lignin showed evidence of increased side-chain oxidation (P<0.05). The termite nests contained 7.3–22 times more C, 14–220 more lignin-derived phenols, and 5.6–260 times more light fraction material than the surrounding surface soils (0–10 cm). The nests of the wood-feeding guild contained 1.2–15 times more lignin and 1.1–46 times more light material than those of the soil/wood-interface feeders. Among the different nests, the particle density increased in the order Nasutitermes sp.

Studies the persistent organic pollutants (PO) in native grassland soils along a climosequence in North America. Role of soil organic matter (SOM) and climate in controlling the fate of hazardous POP; Description of study sites and soils; Polycyclic aromatic hydrocarbon (PAH) and polychlorinated biphenyl (PCB) concentrations and patterns.

Rice cultivation requires high amounts of phosphorus (P). However, significant amounts of P fertilizer additions may be retained by iron (Fe) oxides and are thus unavailable for plants. At the same time, rice cultivation has a high demand for silicic acid (Si), reducing Si availability after short duration of rice cultivation. By studying a paddy chronosequence with rice cultivation up to 2000 years, we show that Si limitation, observed as early as a few decades of rice cultivation, is limiting P availability along the paddy soils chronosequence. Using near edge X-ray absorption fine structure spectroscopy (NEXAFS) in a scanning transmission (soft) X-ray microscope (STXM) we show release of available P was linked to a Si-induced change in speciation of Fe-phases in soil particles and competition of Si with P for binding sites. Hence, low Si availability is limiting P availability in paddy soils. We propose that proper management of Si availability is a promising tool to improve the P supply of paddy plants. [ABSTRACT FROM AUTHOR]

• Organic matter accumulation depends on pedogenic soil differentiation. • Pedogenic processes lead to soil-group-specific organic matter accrual in subsoil. • Reference Soil Groups have unique process combinations of organic matter accrual. Soil organic matter (SOM) accumulation is different in certain soil groups with differences in parent material, degree of weathering and mineral composition. These differences are modulated by climatic factors, but also by pedogenesis, in particular by the formation of reactive mineral surfaces, by soil aggregation, as well as by translocation processes such as eluviation and illuviation and different types of turbation. However, there is still a lack of conceptualization of how such processes and thus important Reference Soil Groups influence the composition and properties of OM. Here we summarize the basic processes of OM storage as they differ from soil group to soil group, in order to present a first overview of the processes of OM formation in the different terrestrial soils of the world. We distinguish between soils of different climatic zones, i.e. Cryosols in permafrost regions, soils of limited development (Cambisols), Podzols, Phaeozems, Chernozems, Kastanozems, and Luvisols in temperate climate zones, as well as Acrisols, Ferralsols, Plinthosols and Nitisols in the subtropics and tropics. We also include soils derived from a specific parent material (Andosols, Vertisols), as well as Anthrosols (paddy soils, Terra Preta, plaggen soils) as examples for human-made SOM accumulations. The compilation of the literature shows that research on OM is clearly focused on specific Reference Soil Groups in temperate climate zones and some man-made soils, while other soils such as Nitisols and Acrisols are clearly underrepresented. The contribution of the different soil groups to global organic carbon (OC) stocks varies, with large amounts of OC found for the first metre in Cryosols, Cambisols, and Podzols, due to the large land area they cover, followed by Acrisols and Ferralsols. In part, these differences can be attributed to differences in the formation of SOM, which we ascribe to three main mechanisms. We emphasize that in all major Reference Soil Groups, both the mechanism of sorptive conservation as well as the protection within the aggregates contribute to the storage of OM. However, the reactant partners and aggregate forming agents and therewith the intensity of these stabilisation processes vary among the Reference Soil Groups. As a result, there are differences in the SOM composition in the topsoil. Within the entire soil profile, however, pedogenic processes lead as third mechanism to soil-group-specific accrual of SOM in the subsoil, e.g. by means of illuviation, by cryo-/bio-, and peloturbation, as well as by management. We conclude that the specific pedogenic environment must be considered in the assessment of global SOM storage potentials and thus also in future global C models. [ABSTRACT FROM AUTHOR]

Lake sediments can provide useful archives to reconstruct past vegetation changes or fire history. To comprehend how vegetation and fire history have correlated during the last 130,000 years, we used two lake sediment records with known patterns of pollen and botanical macro remains and supplemented this data by analyses of lignin-derived phenols as markers for local vegetation inputs and by benzene polycarboxylic acids (BPCAs) as markers for total fire residue inputs (black carbon, BC). The two sediment archives originated from two maar lakes in the Eifel, which is part of the low mountain ranges in central Germany. A lignin-derived phenol index showed woody angiosperms and gymnosperms as fire fuel in the periods with the highest BC amounts. We recorded 3 g BC per kg sediment for phases covered by forest, while BC contents during colder and drier climates were much smaller (≤0.2 g BC per kg sediment), confirming the hypothesis that fires mostly occurred during humid periods in which forest were established. Both records pointed to forested phases with strong fire activity from 48,000–60,000 years before the year 2000 (b2k). Furthermore, the record reaching back to 130,000 years showed a strong fire activity at about 28,000–30,000 yr b2k, and was free of vegetation macro remains and fire markers from 60,000–118,000 yr b2k. All paleobotanical and chemical biomarker results documented the next oldest forested and fire interval, from 118,000 to 130,000 yr b2k, which includes the Eemian of the last interglacial. Thus, nearly all forested phases were accompanied by fire peaks. Overall, biomarker analyses confirmed that analyses of pollen and botanical macro remains. Fire events were highest during warm and humid forested phases. • Paleovegetation and -fire reconstruction via lignin and black carbon. • Black carbon marker peaked at periods when forests were most abundant. • The comparison of two different maar cores indicate local vegetation patches. [ABSTRACT FROM AUTHOR]

Background: Even with extensive root growth, plants may fail to access subsoil water and nutrients when root-restricting soil layers are present. Biopores, created from decaying roots or soil fauna, reduce penetration resistance and channel root growth into the deeper soil. Further positive effects on plants result from biopore traits, as the pore walls are enriched in nutrients, microbial abundance, and activity relative to bulk soil. However, negative effects on plant growth have also been observed due to root clumping in biopores, less root-soil contact than in the surrounding bulk soil and leaching of nutrients. Scope: We discuss methods for biopore research, properties of biopores and their impact plant performance based on a literature review and own data. We elucidate potential implications of altered root-soil contact for plant growth and the consequences of root growth in pores for the rhizosphere microbiome. Conclusions: Biopores play an important but ambiguous role in soils. The effects of biopores on plant growth depend on soil properties such as compaction and moisture in an as-yet-unresolved manner. However, pore properties and root-soil contact are key parameters affecting plant yield. Knowledge gaps exist on signaling pathways controlling root growth in pores and on mechanisms modifying rhizosphere properties inside biopores. The degree to which negative effects of biopores on plant growth are compensated in the bulk soil is also unclear. Answering these questions requires interdisciplinary research efforts and novel imaging methods to improve our dynamic understanding of root growth and rhizosphere processes within biopores and at the rhizosphere-biopore interface. [ABSTRACT FROM AUTHOR]

Antibiotic resistance, along with its dynamics in different environments, has attracted increasing attention because of the potential for resistance gene transfer into human pathogens. Therefore, several researchers have focused on combating the increasing prevalence of antibiotic resistance genes (ARGs) in diverse environments, using various carbon-based amendments to resolve issues regarding emerging contaminants. However, information on systematic knowledge regarding carbon-based material performance and mechanisms for alleviating ARGs remains lacking. To this end, we summarize carbon-based materials that are used as additives, amendments, adsorbents, and other functional materials in compost, soil, and water environments. The underlying mechanisms of alleviating ARG pollution using carbon-based materials are mainly related to 1) environmental factor improvement, 2) microbial community structure alteration, 3) chemical contaminant-caused co-selective pressure reduction, 4) mobile genetic element (mediating horizontal gene transfer processes) reduction, and 5) direct adsorption and/or damage to extracellular DNA. This review aimed to enrich our understanding of the functional roles of carbon-based materials and provide a basis for management strategy development to mitigate ARG pollution. [ABSTRACT FROM AUTHOR]

The hyper-arid soils of the Atacama frequently served as model systems for tracing life under extreme dry conditions such as on Mars. Plants rely on fog moisture (coastal zones) or on irregular rainfalls (mainly north-eastern part), which are controlled by the El Niño Southern Oscillation (ENSO) and the Pacific Decadal Oscillation (PDO) and vary with latitude and altitude. We therefore hypothesized that traces of life in the Atacama Desert are patchy, but nevertheless also follow latitudinal and altitudinal gradients. To test this hypothesis, we sampled surface soils (0-10 cm) along an altitude transect in the region of Quebrada Aroma spanning from the arid Precordillera of the Andes towards the hyper-arid core of the desert (1,300 m to 2,700 m a.s.l). We assessed the contents of organic carbon (OC), total nitrogen (N), extractable total S as well as of n-alkanes and n-fatty acids for tracing organic residues in these soils. We screened the element composition in order to derive information on potential soil formation processes along these gradients. In addition, we analyzed living microbiota using a combination of cultivation-dependent and independent approaches and we performed DNA extraction for amplification of the 16S rRNA gene fragments with subsequent analyses of PCR products by denaturing gradient gel electrophoresis. Additional information on bacterial and archaeal community composition was obtained from phospholipid fatty acids (PFLA) and glycerol dialkyl glycerol tetraethers (GDGTs). OC contents in the surface soils increased from the hyper-arid core in the desert to wetter sites at higher altitudes and presence of sparse vegetation. The abundances of aliphatic lipid biomarkers (short chain n-alkanes, n-fatty acids) increased along the altitudinal transect. Most n-alkanes and n-fatty acids at the drier, lower altitude (1,340 m) were short-chain homologues suggesting a microbial origin, whereas at higher elevation (2,721 m; marginally wetter) more long-chain compounds were present suggesting a larger contribution of plant-derived OC. Ongoing radiocarbon analysis will give further insights into OC sources (old versus young). Living cells were detectable at all sites with a consistent decrease in cell numbers as aridity increased. Lowest cell numbers below the detection limit of 5 CFUs g-1 soil were found at the driest sites. Intriguingly, we did not find pronounced changes in bacterial diversity using cell counting and simple denaturing gradient gel electrophoresis. The distributional patterns of the PLFAs suggest a significantly larger bacterial diversity at the highest, wettest altitude than at the lower, dryer sites. In addition, both archaeal isoprenoid GDGTs and bacterial branched GDGTs were detected in all samples also following the altitudinal trend with similar concentrations at sites located at 2,020 and 2,720 m altitude. These results suggest that several microorganisms are able to survive extreme dryness. The abundance of these organisms spans along an altitudinal gradient that follows the pattern of rare precipitation events, correlating also with the presence of sparse vegetation remains. After cell death, many of these biomolecules are preserved, now offering novel potentials also for tracing the evolution of past life forms from their cell wall remains. [ABSTRACT FROM AUTHOR]

Background: Microaggregates are suspected to protect soil organic carbon (SOC) from microbial decay, but its residence time is not well understood. Aims: We aimed at unraveling the relevance of microaggregates for C storage and testing the hypothesis that C in the interior of aggregates is older, compared to the exterior. Methods: We sampled soil under C3 vegetation and at a site where cropping shifted to C4 vegetation 36 years ago. We isolated free and macroaggregate‐occluded size fractions (250–53 µm) by wet sieving and ultrasound, manually isolated aggregates therefrom, and analyzed whether vegetation‐related differences in δ13C could be traced at the interior and exterior of microaggregate cross‐sections using elemental and laser ablation‐isotope ratio mass spectrometry. Results: Size fraction weights comprised <5% of microaggregates. Based on a source partitioning approach including C3‐ and C4‐derived C, we found mean residence times of SOC in occluded and free microaggregates of 62 and 105 years, respectively. Thus, C storage was longer than that in size fractions (35 years) and bulk soil (58 years). The small‐scale variability of δ13C within aggregate cross‐sections was considerable, both in C3 and C4 soil, yet without significant (p = 0.46) differences between interior and exterior locations. Conclusions: We conclude that microaggregates do not persist in an intact form in such a long‐term that systematic differences in δ13C patterns between exterior and interior parts can develop. [ABSTRACT FROM AUTHOR]

Background and aims: Paddy management results in frequent redox cycles of the soil and thus changes in the terrestrial iron (Fe) cycle. We intended to test that the increasing duration of paddy management and the increasing frequency of soil redox cycles leave their fingerprint on Fe isotope composition of paddy systems, which could subsequently be used to deduce the origin of rice plants as related to the extent of past soil paddy management. Methods: We sampled soil and rice plants of a paddy chronosequence in China with rice cultivation history up to 2000 years and determined the changes of soil Fe pools and Fe isotope composition of the soil and rice plants. Results: Prolonged paddy management reduced Fe concentrations in submerged topsoil leading to an enrichment of heavy Fe isotopes, with the δ56Fe values 0.12‰ heavier than the parent material after 2000 years. Particularly, Fe oxides were lost quickly, while exchangeable and organic-associated Fe continuously accumulated during paddy management and played an increasing role in the plant-available Fe pool in the topsoil. The Fe content in rice also increased with paddy age, while its Fe isotope composition did not reflect that of paddy soil but resembled that of the Fe plaques on the roots. Conclusion: Prolonged rice cropping altered the biological contribution in the terrestrial Fe cycle. However, while soil Fe pools that are closely linked with biological activities were affected rather quickly, the changes in the whole soil Fe system were detectable only after a millennium of paddy management. [ABSTRACT FROM AUTHOR]

Phosphorus (P) is a key limiting nutrient in tropical and subtropical soils but with unknown responses to climate change. We hypothesized that storage and distribution of P pools differ between reference soil groups, but P availability increases with an increase in precipitation in native sub‐Saharan Africa, an underrepresented region in the global P database. To test these hypotheses, we sampled topsoils (0–10 cm; Arenosols, Lixisols, Acrisols) of uncultivated Savannah woodlands along climate gradients stretching from Mozambique to Zambia. Mean annual precipitation (MAP) ranged from 365 to 1227 mm. We extracted P fractions using Hedley's sequential fractionation, yielding resin‐exchangeable P, P in 0.5 M NaHCO3, 0.1 M NaOH, 1 M HCl and aqua regia. Extracts were analysed for total P using inductively coupled plasma–optical emission spectroscopy and inorganic P using the colorimetric molybdenum blue method. We found that total P contents were highest in Acrisols (235 ± 76 mg kg−1), followed by Lixisols (214 ± 45 mg kg−1) and Arenosols (133 ± 74 mg kg−1). Beside P storage, also P distribution differed between soils. Mean annual temperature (MAT) did not reveal significant correlations with P fractions. However, with increasing MAP, concentrations of almost all P pools increased; available Pi (Resin‐P) increased significantly in Arenosols while potentially bioavailable P (NaHCO3‐Po) increased in all soils, with MAP explaining 43% of data variability in Arenosols, 74% in Lixisols and 85% in Acrisols. Hence, P availability increased with an increase in MAP to different degrees, that is, climatic effects on P dynamics were soil‐group‐specific. Therefore, different regions in the native sub‐Saharan woodlands are likely prone to different soil fertility responses when climate changes. [ABSTRACT FROM AUTHOR]

As emerging contaminants, microplastics and nanoplastics (MPs/NPs) are increasingly widespread in soil environment. The present paper reviews available analytical methods for detecting MPs/NPs in soil and plants, and examines the migration processes of MPs/NPs at multiple interfaces as well as the potential toxic effects of MPs/NPs. Without standard methods to trace MPs/NPs in different environmental compartments, indoor simulation experiments were mainly adopted in previous studies, differing greatly from MPs/NPs in actual environments. Therefore, we propose that new research should particularly 1) focus on realistic environmental concentrations and surrogates that mimic types, shapes, and sizes of MPs/NPs in natural environments; 2) develop novel methods to improve our understanding of multi-interface migration and transformation of MPs/NPs in the soil-plant system; and 3) advance towards an in-depth assessment of potential toxic effects of MPs/NPs on soil organisms to provide a scientific basis for the effective control of MPs/NPs pollution. [Display omitted] • Microplastics/nanoplastics (MPs/NPs) exhibit adverse effects on soil organisms. • The migration of MPs/NPs in soils at multiple interfaces is described in detail. • Stable isotope labelling can be applied to characterize the migration of MPs/NPs. • Standardized analytical methods are needed for MPs/NPs in soils and plants. [ABSTRACT FROM AUTHOR]

• Fan ages in the Atacama dated to ∼13.6 ka and ∼56.4 ka. • The younger fan with rough surfaces was dominated by nano and fine colloids. • The older fan showed smooth surface but heterogenous size distribution of colloids. • Soil depth distributions pointed to colloidal transport in the younger fan. • Plants enhanced accumulation of nanocolloids with elevated OC and Ca content. As located in one of the oldest and driest deserts on Earth, soils in the Atacama Desert are greatly affected by atmospheric dust deposited on soil surface and the related fate of water-dispersible colloids (WDCs, <300 nm). We hypothesize that formation and content of these WDCs change with topography and age of natural soils. To highlight the processes involved, we investigated a mid-sized and gently (∼5°) sloping alluvial fan system of multi-phase evolution at 1480 m a.s.l. in the Paposo region of the hyper-arid Atacama Desert, which is considered typical for this part of the Coastal Cordillera. Sampling was done along a topographic transect in 11 pits, and assessed the distribution and composition of WDCs by means of asymmetric flow field-flow fractionation (AF4). The younger fan section (optically stimulated luminescence (OSL)-age of ∼13.6 ka) exhibited a pronounced surface roughness and steep slopes. Here, WDCs from the top soils (0–1 cm) free of plants contained nearly 54 ± 7% of medium-sized colloids (MCs, 210–300 nm) with a dominance of Si and Al. The elevated concentrations of fine colloids (FC, 24–210 nm) and particularly nanocolloids (NCs, 0.6–24 nm) was shown in levelled surface soils near shrubs with predominance of organic carbon (OC) and Ca. With higher collodial OC and Ca content in soils near shrubs, more WDC-P was formed concomitantly through increased OC-Ca-P associations. Larger variations in total WDC content were detected in the surface soils of the older fan section, which was dated to ∼56.4 ka. Here, the peaking NC had almost disappeared and thus MC dominated, probably reflecting re-aggregation and wind erosion over longer periods of time across a relatively smooth land surface. The WDCs and WDC-P peaked at 5–10 cm depth in the older fan section, as here a solid mineral/salt layer was present, while in the younger fan section the WDCs were more likely to be translocated from 'permeable' surface into deeper layers, likely reflecting leaching with occasional heavy rainfall. Overall, forms and distribution of WDCs depended on both topographic position and sediment age, thus making colloids as unique tracers of soil development processes during myriad or more years. [ABSTRACT FROM AUTHOR]

Deep‐ploughing far beyond the common depth of 30 cm was used more than 50 years ago in Northern Germany with the aim to break root‐restricting layers and thereby improve access to subsoil water and nutrient resources. We hypothesized that effects of this earlier intervention on soil properties and yields prevailed after 50 years. Hence, we sampled two sandy soils and one silty soil (Cambisols and a Luvisol) of which half of the field had been deep‐ploughed 50 years ago (soils then re‐classified as Treposols). The adjacent other half was not deep‐ploughed and thus served as the control. At all the three sites, both deep‐ploughed and control parts were then conventionally managed over the last 50 years. We assessed yields during the dry year 2019 and additionally in 2020, and rooting intensity at the year of sampling (2019), as well as changes in soil structure, carbon and nutrient stocks in that year. We found that deep‐ploughing improved yields in the dry spell of 2019 at the sandy sites, which was supported by a more general pattern of higher NDVI indices in deep‐ploughed parts for the period from 2016 to 2021 across varying weather conditions. Subsoil stocks of soil organic carbon and total plant‐available phosphorus were enhanced by 21%–199% in the different sites. Root biomass in the subsoil was reduced due to deep‐ploughing at the silty site and was increased or unaffected at the sandy sites. Overall, the effects of deep‐ploughing were site‐specific, with reduced bulk density in the buried topsoil stripes in the subsoil of the sandy sites, but with elevated subsoil density in the silty site. Hence, even 50 years after deep‐ploughing, changes in soil properties are still detectable, although effect size differed among sites. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • Charcoals and fire condensates are differentiated by aromatic compound composition. • Charcoals have smaller B5CA/B6CA, IP/(IP + B[ghi]P) and Flua/(Flua + Py) ratios than fire condensates. • Dimethylated phenanthrene composition is a marker for the fuel origin of fires. Abstract Fire condensates from vegetation and household burnings occur ubiquitously in the environment. Until now, however, it was difficult to estimate their origin and any relation to pyrogenic carbon deposited as charcoal. Our aim here was: (i) to differentiate the chemical composition of fire condensates from charcoal particles, and (ii) to relate this to fuel origins from grass, softwood or hardwood. We analysed δ13C and δ15N isotope composition, lignin-derived phenols, benzene polycarboxylic acids (BPCA) and polycyclic aromatic hydrocarbons (PAH), in lab-produced charcoals and condensates at combustion temperatures of 300, 350, 400, 450, 500 and 600 °C. We found that the BPCA and PAH composition of condensates differed significantly from that of charcoals. Condensates exhibited larger portions of benzene penta- to hexacarboxylic acids (B5CA to B6CA), phenanthrene (p < 0.01) and four-ring PAH (fluoranthene, pyrene, chrysene and benz[ a ]anthracene, p < 0.01). PAH ratios of indeno[1,2,3- cd ]pyrene to benzo[ ghi ]perylene (IP/(IP + B[ ghi ]P) and fluoranthene to pyrene (Flua/(Flua + Py) were diagnostic for condensates, but independent from fuel type. Composition of the 1,2-, 1,7- and 2,6|3,5 dimethylphenanthrenes (DMP) was fuel specific, with the ratio of the (1,7 + 2,6|3,5)/(1,2 + 1,7 + 2,6|3,5) isomers separating hardwood (0.2–0.6), from grass (0.6–0.9), and softwood (<0.9), thus enabling the identification of both condensate and charcoal fuel sources. [ABSTRACT FROM AUTHOR]

The specific properties of nanoparticles can induce toxic or promoting effects on plant growth. We performed a meta-analysis of 173 studies to evaluate the uptake and accumulation of metallic nanoparticles (MNPs) by crops. Studies on the main cereal/vegetable crops (wheat, tomato, bean, maize, rice and cucumber) and Ag, Zn, Ce, Fe, Cu and Ti containing MNPs were mostly found; 63% of the studies used MNPs with size <30 nm, and root application accounted for 84% of all studies. Under root exposure, metal concentrations were reduced by up to 2 orders of magnitude from root through shoot to leaf. Moreover, the uptake preference of MNPs or dissolved metals was element-specific and varied among tissues. Plants accumulated generally higher concentrations of the three essential elements (Cu, Zn and Fe) than of the non-essential ones (Ag, Ce and Ti). Also, foliar application was more efficient than root exposure. Metal concentrations in shoots increased with decreasing particle size for non-essential elements, but showed a variable increase for essential elements as size decreased. Besides, the uptake of MNPs increased with more negative zeta potential (especially for Cu and Zn). Overall, the plant essential elements (Cu, Zn and Fe) were preferred taken up as MNPs, while for non-essential elements (Ag and Ce) uptake as dissolved metals was more common. We conclude that for plant nanofertilizers, foliar application of essential elements as small MNPs might be more efficient for crop uptake than the uptake of dissolved metals. [ABSTRACT FROM AUTHOR]

Background and Aims: Root restricting layers often hinder crops from accessing the large reservoir of bioavailable mineral nutrients situated in subsoil. This study aims to explore changes in the mean nutrient uptake depth of cereal crops when removing root restricting layers through subsoil management. Methods: Subsoil management was performed by deep loosening, cultivation of lucerne as deep-rooting pre-crop, and their combination with compost incorporation. Management effects were evaluated by means of shoot biomass and element concentrations in shoots and soil compartments. The mean nutrient uptake depth was fingerprinted by graphically matching the 87Sr/86Sr ratios in shoots with the 87Sr/86Sr ratios in the exchangeable fraction in soil. Nutrient uplift from depth to topsoil was inferred from element concentrations in the exchangeable fraction in soil. Results: Shoot biomass remained constant in management and control plots. The mean nutrient uptake depth changed with subsoil management in the order: deep loosening < control < deep loosening with compost incorporation. The latter coincided with a reallocation of compost-derived Na and hence resulted in increased levels of bioavailable Na below the depth of compost incorporation, which may have led to an improved water use efficiency of the crops. Thus, Na relocation triggered the deepening of the mean uptake depth of water and nutrients. Moreover, nutrient uplift from depth to topsoil was evident 21 months after subsoiling. Conclusion: Subsoil management by deep loosening with compost incorporation provides a sustainable use of soil resources because otherwise unused deep geogenic-derived nutrient reservoirs were additionally involved in crop nutrition. [ABSTRACT FROM AUTHOR]

Although the evaluation of soil organic carbon (SOC) stocks across different types of land use and major reference soil groups is essential for mitigating climate change, there remains, to date, limited comprehensive understanding of whole tropical soil profiles. Therefore, this study aimed to explain the amount of SOC stocks in different land-use systems and across various soil groups, as well as its spatial pattern in the topsoil (0–30 cm) and subsoil (30–100 cm) within the savannah zone of Burkina Faso. Roughly 70 soil profiles were considered along with additional auger sampling to account for spatial variation in both cropland (CR) and savannah (SA). The machine learning technique random forest regression (RFR) and multiple linear regression (MLR) were used for modeling the surface and subsurface SOC stocks. For model calibration, covariates including land use, topographic, texture, and climatic data were considered as surrogate for soil forming factors. The prediction maps produced by the calibrated models were validated by an independent dataset. The results indicated that about 53% of the SOC stock over 1 m depth was held in the upper 30 cm. Only a marginal difference was recorded between the topsoil SOC stock in SA (41.4 t C ha −1 ) and CR (39.1 t C ha −1 ) soils. For the subsoil, a significant difference (p < 0.05) was observed between the SOC stock of CR soils recording about 40.2 t C ha −1 and SA soils with 26.3 t C ha −1 . Among the reference soil groups, the Gleysols located at lower elevation positions revealed the highest SOC stocks over 0–30 cm (44 t C ha −1 ) and 100 cm depth (86.6 t C ha −1 ). The Stagnosols (45.2 t C ha −1 ) followed by the Gleysols (42.7 t C ha −1 ) recorded the highest SOC stocks over 30–100 cm. The variability of SOC stock in the topsoil was primarily related to site-specific elements, such as particle-size fraction and wetness index, while its distribution in the subsoil was mainly associated with the topographical orientation represented by the slope aspect. Compared to the MLR, RFR estimated mean top- and subsoil SOC stocks of the catchment fairly well, along with lower statistical error metrics, though extreme values were not covered. Nevertheless, the findings on SOC stocks reinforce the view that the semi-arid ecosystems of West Africa still offer a significant opportunity for carbon sequestration for both topsoil and subsoil, and these results represent a baseline for future modeling of SOC dynamics in the region. [ABSTRACT FROM AUTHOR]

Reseeding grasses into degraded cropland and using it as permanent secondary pasture may combat desertification and promote sequestration of soil organic carbon (SOC). Our objective was to assess the contribution of lignin and grass-derived carbon to the rates of SOC accrual upon such land-use change. We sampled secondary pastures of different age (1 to 31 years old), established on former degraded cropland at three agro-ecosystems in the semi-arid Highveld of South Africa; old degraded croplands and primary grassland used for cattle grazing served as controls. Topsoil samples (0–5 and 5–10 cm) were characterized by their natural 13 C and 15 N abundances and by their contents of lignin-derived phenols (VSC) after alkaline CuO oxidation. The results showed that the δ 13 C signature in secondary pasture soil was variable and not a sensitive indicator of land restoration in these sites. Soil δ 15 N values declined from 8 to 6‰ with increased duration of secondary pasture use and approached the values of the primary grassland, suggesting that N-use efficiency improved. The VSC contents increased in the surface soils and reached a new steady-state equilibrium after 38 years. Lignin sequestration was accelerated relative to total SOC in the 0–5 cm soil depth but delayed in the 5–10 cm soil depth, reflecting different amounts of depth-dependent inputs of plant debris into the pastures. Even if absolute VSC contents did not reach the respective concentrations in the primary grassland, the contribution of VSC to total SOC (g VSC kg − 1 C) was similar to that of the primary grassland. Decreasing ratios of syringyl to vanillyl structural units reflected that after 8–19 years all remaining lignin derived primarily from the new grassland vegetation. Lignin enhances SOC accumulation only in the very surface soil (0–5 cm) but not below this depth, rendering the restoration of subsoil C stocks difficult in these ecosystems. [ABSTRACT FROM AUTHOR]

Farmers in South Asia increasingly switch from continuous paddy-rice cropping to rotations including non-flooded crops, such as growing maize in the dry season. We hypothesized that the introduction of maize into a permanent paddy-rice cropping system boosts drainage and leaching losses of nitrogen (N) and dissolved organic carbon (DOC) in the initial years of maize establishment, due to the disturbance of the equilibrated soil conditions established under continuous paddy cropping. We tested this hypothesis in a 3.5-year field experiment using monolith lysimeters cropped with either (i) single paddy rice in the wet season and maize in the dry season (maize-paddy rice, M-MIX), or (ii) double paddy rice (R-WET) as control. Expandable and compressible pads minimized the formation of a gap at the interface between soil monolith and lysimeter casing during shrinking and swelling of the clay soil. In the first year of introducing maize, drainage (606 l m −2 yr −1 ) and leaching of total nitrogen (TN, 6.8 g N m −2 yr −1 ) and DOC (2.7 g m −2 yr −1 ) were significantly larger in M-MIX than in R-WET (water: 149 l m −2 yr −1 , TN: 0.1 g m −2 yr −1 , DOC: 0.7 g m −2 yr −1 ). However, the additional losses of water, nitrogen, and DOC caused by the introduction of maize disappeared in the following years. In the last two dry seasons of our study, drainage and leaching losses of TN, and DOC were even significantly smaller in M-MIX than in R-WET. In the dry seasons of the 2nd to 4th year after introducing maize (2013–2015), M-MIX saved on average 388 l m −2 of percolation water losses compared to R-WET and leaching losses of TN and DOC under maize were reduced on average by 0.6 g m −2 and 1.6 g m −2 , respectively. We conclude that leaching losses of water and nutrients are only transiently boosted during the first year after introducing maize in perennial rice cropping systems, so that maize cropping in the dry season could save water and reduce nutrient leaching in comparison to continuous paddy-rice cropping in the long run. Long-term field trials are necessary to validate the lysimeter results. [ABSTRACT FROM AUTHOR]