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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]

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]

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]

Background and aims: Phosphorus (P) is an essential element for crop growth. However, while links of P turnover in soils to carbon (C) and nitrogen (N) availability have been described, it remains to be clarified how combinations of fertilizer C and N additions affect stocks and cycling of distinct P fractions at different soil depths. The objectives of our study were (1) to assess how soil total P stocks are affected by organic amendments and N fertilization, (2) to evaluate how different soil P fractions respond to N fertilization, and (3) to verify whether N fertilization increases soil biological P cycling. Methods: We collected soil samples from a long‐term field experiment established in 1984 in Rauischholzhausen, Germany. The soil is a Haplic Luvisol and received either no organic fertilizer (NOF), farmyard manure (FYM) or a combination of organic and mineral N fertilizer (OMF). Each treatment additionally received three levels of mineral N: 0 kg ha−1 y−1 (N0), 100 kg ha−1 y−1 (N100), and 200 kg ha−1 y−1 (N200). The organic fertilizers were applied by a manure spreader and the N fertilizer (calcium ammonium nitrate) was applied in spring as top dressing by a plot fertilizer machine. We estimated stocks of P in fractions isolated by sequential P fractionation, and assessed the oxygen isotopic composition of 1 M HCl‐extractable phosphate (δ18OP). Results: We found that increased organic matter (OM) addition and mineral N inputs caused significant decreases in the stocks of resin‐ and NaHCO3‐extractable P in the topsoil (0–30 cm). Mineral N fertilization alone resulted in significant increases in stocks of resin‐, NaHCO3‐, and NaOH‐extractable P in the upper subsoil (30–50 cm). These changes occurred for both inorganic and organic P. The subsoil δ18OP values were closer to expected equilibrium values in soil fertilized with mineral N, indicative of more intensive biological P cycling than in the treatments without mineral N inputs. Conclusions: These findings suggest that long‐term OM and mineral N fertilization promotes topsoil P losses from labile fractions by crop uptake with an enrichment of these P forms in the subsoil, and an overall increase in biological P cycling in both top‐ and subsoil horizons upon N fertilization. [ABSTRACT FROM AUTHOR]

Background: In search for more sustainable crop production, the subsoil has recently come into focus as considerable reservoir of nutrients and water. Aims: Dimensions of subsoil phosphorus (P) reserves are yet largely unknown but crucial for identifying regions suitable to include subsoil into sustainable management strategies. Methods: We analyzed stocks of total and plant‐available (calcium acetate lactate‐extractable) P in 96 representative soil profiles of German arable land down to 1 m depth. Results: We found that the German arable soils stored, on average, 8 t ha−1 of total P, of which nearly 500 kg ha−1 were readily plant‐available. Notably, one third of plant‐available P was located below the plow layer and one fifth even at depths below 0.5 m. The depth gradients of plant‐available P stocks were affected more by major reference soil group than by texture. Generally, Chernozem but also Anthrosol, Gleysol and Fluvisol exhibited the largest P stocks in German cropland. The contribution of plant‐available P to total P stocks was larger in sandy and extremely acidic (pH < 4.5) soils compared with more fine‐grained and slightly acidic to alkaline soils, possibly because fertilization compensated for overall lower total P stocks at these sites. Generally, the more P was stored in topsoils, the more P was stored also in subsoils. Conclusions: A hypothetical crop utilization of 10% from plant‐available P stocks and 0.1% from total P stocks from shallow subsoil could compensate for P fertilization by ca. 8 kg ha−1, but the rate of plant‐available P replenishment in subsoil likely remains the crucial factor for the role of subsoil P stocks in crop nutrition. Generally, the large P reserves found in subsoil could act as an 'insurance' system for crops. [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]

Understanding and quantification of phosphorus (P) fluxes are key requirements for predictions of future forest ecosystems changes as well as for transferring lessons learned from natural ecosystems to croplands and plantations. This review summarizes and evaluates the recent knowledge on mechanisms, magnitude, and relevance by which dissolved and colloidal inorganic and organic P forms can be translocated within or exported from forest ecosystems. Attention is paid to hydrological pathways of P losses at the soil profile and landscape scales, and the subsequent influence of P on aquatic ecosystems. New (unpublished) data from the German Priority Program 1685 ' Ecosystem Nutrition: Forest Strategies for limited Phosphorus Resources' were added to provide up-to-date flux-based information. Nitrogen (N) additions increase the release of water-transportable P forms. Most P found in percolates and pore waters belongs to the so-called dissolved organic P (DOP) fractions, rich in orthophosphate-monoesters and also containing some orthophosphate-diesters. Total solution P concentrations range from ca. 1 to 400 µg P L−1, with large variations among forest stands. Recent sophisticated analyses revealed that large portions of the DOP in forest stream water can comprise natural nanoparticles and fine colloids which under extreme conditions may account for 40-100% of the P losses. Their translocation within preferential flow passes may be rapid, mediated by storm events. The potential total P loss through leaching into subsoils and with streams was found to be less than 50 mg P m−2 a−1, suggesting effects on ecosystems at centennial to millennium scale. All current data are based on selected snapshots only. Quantitative measurements of P fluxes in temperate forest systems are nearly absent in the literature, probably due to main research focus on the C and N cycles. Therefore, we lack complete ecosystem-based assessments of dissolved and colloidal P fluxes within and from temperate forest systems. [ABSTRACT FROM AUTHOR]

Both biochar and compost may improve carbon sequestration and soil fertility; hence, it has been recommended to use a mixture of both for sustainable land management. Here, we evaluated the effects of biochar-compost substrates on soil properties and plant growth in short rotation coppice plantations (SRC). For this purpose, we planted the tree species poplar, willow, and alder in a no-till field experiment, each of them amended in triplicate with 0 (= control) or 30 Mg ha−1 compost or biochar-compost substrates containing 15% vol. (TPS15) and 30% vol. biochar (TPS30). For three years running, we analyzed plant growth as well as soil pH, potential cation exchange capacity (CEC), stocks of soil organic carbon (SOC), total N, and plant-available phosphate and potassium oxide.Biochar-compost substrates affected most soil properties only in the topsoil and for a limited period of time. The CEC and total stocks of SOC were consistently elevated relative to the control. After three years the C gain of up to 6.4 Mg SOC ha−1 in the TPS30 plots was lower than the added C amount. Especially in the case of TPS30 treatment, C input was characterized by the greatest losses after application, although the black carbon of the biochar was not degraded in soil. Additionally, tree growth and woody biomass yield did not respond at all to the treatments. Overall, there were few if any indications that biochar-compost substrates improve the performance of SRC under temperate soil and climate conditions. Therefore, the use of biochar for such systems is not recommended. [ABSTRACT FROM AUTHOR]

Past land-use changes, intensive cropping with large proportions of root crops, and preferred use of mineral fertilizer have been made responsible for proceeding losses of soil organic C (SOC) in the plough layer. We hypothesized that in intensive agriculturally managed regions changes in SOC stocks would be detectable within a decade. To test this hypothesis, we tracked the temporal development of the concentrations and stocks of SOC in 268 arable sites, sampled by horizon down to 60 cm in the Cologne-Bonn region, W Germany, in 2005 and in 2013. We then related these changes to soil management data and humus balances obtained from farmers' surveys. As we expected that changes in SOC concentrations might at least in part be minor, we fractionated soils from 38 representative sites according to particle size in order to obtain C pools of different stability. We found that SOC concentrations had increased significantly in the topsoil (from 9.4 g kg−1 in 2005 to 9.8 g kg−1 in 2013), but had decreased significantly in the subsoil (from 4.1 g kg−1 in 2005 to 3.5 g kg−­1 in 2013). Intriguingly, these changes were due to changes in mineral-bound SOC rather than to changes in sand-sized organic matter pools. As bulk density decreased, the overall SOC stocks in the upper 60 cm exhibited a SOC loss of nearly 0.6 t C (ha · y)−1 after correction by the equivalent soil mass method. This loss was most pronounced for sandy soils [−0.73 t SOC (ha · y)−1], and less pronounced for loamy soils [−0.64 t SOC (ha · y)−1]; silty soils revealed the smallest reduction in SOC [−0.3 t SOC (ha · y)−1]. Losses of SOC occurred even with the overall humus balances having increased positively from about 20 kg C (ha · y)−1 (2003-2005) to about 133 kg C (ha · y)−1 (2005-2013) due to an improved organic fertilization and intercropping. We conclude that current management may fail to raise overall SOC stocks. In our study area SOC stocks even continued to decline, despite humus conservation practice, likely because past land use conversions (before 2005) still affect SOC dynamics. [ABSTRACT FROM AUTHOR]

Phosphorus (P) is an indispensable element for all life on Earth and, during the past decade, concerns about the future of its global supply have stimulated much research on soil P and method development. This review provides an overview of advanced state-of-the-art methods currently used in soil P research. These involve bulk and spatially resolved spectroscopic and spectrometric P speciation methods (1 and 2D NMR, IR, Raman, Q-TOF MS/MS, high resolution-MS, NanoSIMS, XRF, XPS, (µ)XAS) as well as methods for assessing soil P reactions (sorption isotherms, quantum-chemical modeling, microbial biomass P, enzymes activity, DGT, 33P isotopic exchange, 18O isotope ratios). Required experimental set-ups and the potentials and limitations of individual methods present a guide for the selection of most suitable methods or combinations. [ABSTRACT FROM AUTHOR]

Precipitation and topsoil samples from a climate transect over the Scandinavian Mountains, Norway, were analyzed for bulk and compound-specific δ18O values. The natural abundance of 18O in the plant-derived hemicellulose biomarkers arabinose and xylose correlates positively with δ18O of bulk soil, but not with δ18O of precipitation. This suggests that other factors than δ18Oprec, such as evaporative 18O enrichment of leaf water, exert a strong influence on the natural abundance of 18O in soils. [ABSTRACT FROM AUTHOR]

A letter to the editor is presented in response to the comment by C. Brock on an article about the monitoring of organic carbon stocks from agricultural soils in Western Germany.