Search

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]

Nature conservation and restoration in terrestrial ecosystems is often focused on increasing the numbers of megafauna, expecting them to have positive impacts on ecological self‐regulation processes and biodiversity. In sub‐Saharan Africa, conservation efforts also aspire to protect and enhance biodiversity with particular focus on elephants. However, elephant browsing carries the risk of woody biomass losses. In this context, little is known about how increasing elephant numbers affects carbon stocks in soils, including the subsoils. We hypothesized that (1) increasing numbers of elephants reduce tree biomass, and thus the amount of C stored therein, resulting (2) in a loss of soil organic carbon (SOC). If true, a negative carbon footprint could limit the sustainability of elephant conservation from a global carbon perspective. To test these hypotheses, we selected plots of low, medium, and high elephant densities in two national parks and adjacent conservancies in the Namibian component of the Kavango Zambezi Transfrontier Area (KAZA), and quantified carbon storage in both woody vegetation and soils (1 m). Analyses were supplemented by the assessment of soil carbon isotopic composition. We found that increasing elephant densities resulted in a loss of tree carbon storage by 6.4 t ha−1. However, and in contrast to our second hypothesis, SOC stocks increased by 4.7 t ha−1 with increasing elephant densities. These higher SOC stocks were mainly found in the topsoil (0–30 cm) and were largely due to the formation of SOC from woody biomass. A second carbon input source into the soils was megaherbivore dung, which contributed with 0.02–0.323 t C ha−1 year−1 to ecosystem carbon storage in the low and high elephant density plots, respectively. Consequently, increasing elephant density does not necessarily lead to a negative C footprint, as soil carbon sequestration and transient C storage in dung almost compensate for losses in tree biomass. [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 is the most important terrestrial carbon (C) reservoir but is greatly impacted by land use change (LUC). Previous analyses of LUC impacts on soil C have focused on biophysical variables, leaving aside the influence of socioeconomics. The aim of our study was to determine global soil organic carbon (SOC) change patterns after LUC and to assess the impacts of both biophysical and socioeconomic factors that influence stocks of SOC after LUC simultaneously. This was done at a global scale using 817 sites from 99 peer-reviewed publications. We performed separate analyses for cases in which there were gains and losses of SOC. The best predictors of SOC stock changes were the type of LUC and predictors related to sampling depth, climate, biome, soil order, relief, geology, years since LUC, and primary productivity. However, also, socioeconomic variables such as indices of poverty, population growth, and levels of corruption were important. They explained 33% of the variability in SOC on their own and helped improve model accuracy from 42 to 53% when considered in combination with biophysical variables. SOC losses were highly correlated to the type of LUC and social variables, while SOC gains correlated most strongly with years since LUC and the biophysical variables. The analyses confirm that one of the biggest drivers of SOC loss is conversion to agroindustrial scale cropping, whereas with regard to the recuperation of SOC after LUC, the factor "time since conversion" emerged as the most important predictive variable, which must be better integrated in respective policy expectations. We conclude that policies should more than ever incentivize holistic approaches that prevent additional loss of native SOC, while at the same time promoting sustainable intensification of existing agricultural regions. Finally future investments on LUC to regain SOC should be aligned with efforts to alleviate poverty and corruption for their potential to achieve mutual gains in soil fertility and socio-economic parameters. [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]

• Deep soil was included in the assessment of different restoration approaches. • Soil organic carbon accrual in deep soil did not follow aboveground biomass growth. • Deep soil organic carbon stock in reforestation areas was lower than in adjacent cropland sites. Carbon (C) sequestration following land restoration is an efficient measure of climate change mitigation. However, most studies including the international "4 per 1000″ Initiative focus on topsoil C only. Here, we studied the response of deep soil C to different land restoration approaches. Samples were taken from rural sites in the Atlantic Forest Biome (Brazil), using a paired site design with triplicates of different 16-to-19-year-old restoration approaches of former arable land: reforestation, natural regeneration, and agroforestry sites. Adjacent arable land and a > 25-year-old secondary forest served as references. We determined aboveground and belowground C pools, including deep soil until 3 m depth. We did not find statistical differences among the three restoration methods for the C sequestration in the whole ecosystem. However, the C stock in the aboveground living biomass in the reforestation site was 85 ± 15 Mg ha−1, significantly exceeding the respective aboveground C in natural regeneration (40 ± 13 Mg ha−1) and agroforestry system (25 ± 5 Mg ha−1), respectively. This order, however, was reversed in soil, where reforestation caused a loss of soil organic carbon (SOC) by 27 Mg ha−1 below 40 cm depth, whereas SOC stocks in the agroforestry system and natural regeneration site increased by 41 and 25 Mg C/ha, respectively, compared to arable land. We thus conclude that deep soil carbon losses can negate aboveground carbon accrual after land restoration, which emphasizes the importance of including this pool in C sequestration studies. [ABSTRACT FROM AUTHOR]

Soils hold the largest pool of organic carbon (C) on Earth; yet, soil organic carbon (SOC) reservoirs are not well represented in climate change mitigation strategies because our database for ecosystems where human impacts are minimal is still fragmentary. Here, we provide a tool for generating a global baseline of SOC stocks. We used partial least square (PLS) regression and available geographic datasets that describe SOC, climate, organisms, relief, parent material and time. The accuracy of the model was determined by the root mean square deviation (RMSD) of predicted SOC against 100 independent measurements. The best predictors were related to primary productivity, climate, topography, biome classification, and soil type. The largest C stocks for the top 1 m were found in boreal forests (254 ± 14.3 t ha−1) and tundra (310 ± 15.3 t ha−1). Deserts had the lowest C stocks (53.2 ± 6.3 t ha−1) and statistically similar C stocks were found for temperate and Mediterranean forests (142 - 221 t ha−1), tropical and subtropical forests (94 - 143 t ha−1) and grasslands (99-104 t ha−1). Solar radiation, evapotranspiration, and annual mean temperature were negatively correlated with SOC, whereas soil water content was positively correlated with SOC. Our model explained 49% of SOC variability, with RMSD (0.68) representing approximately 14% of observed C stock variance, overestimating extremely low and underestimating extremely high stocks, respectively. Our baseline PLS predictions of SOC stocks can be used for estimating the maximum amount of C that may be sequestered in soils across biomes. [ABSTRACT FROM AUTHOR]

Historically, large areas of forest in Europe were managed as coppice woodland to produce wood-based fuel for the smelting industry. We hypothesized that this practice produced a legacy effect on current forest ecosystem properties. Specifically, we hypothesized that the historical form of coppicing may have produced a legacy of elevated stocks of soil organic carbon ( SOC), nutrients and black carbon ( BC) in soil as fire was routinely used in coppiced woodland to clear land. We further hypothesized that these changes in soil properties would result in increased biodiversity. To test these hypotheses, we sampled the surface soil (0-5, 5-10 and 10-20 cm) from a chronosequence of forest sites found in the Siegerland (Germany) that had been coppiced and burned 1, 2, 3.5, 6, 8, 11 and 17 years before present. Mature beech and spruce forests (i.e., >60 years) were also sampled as reference sites: to provide a hint of what might occur in the absence of human intervention. We measured stocks of SOC, BC, NO3-N, P, K, Mg, as well as cation exchange and water-holding capacity, and we mapped plant composition to calculate species richness and evenness. The results showed that coppicing in combination with burning soil and litter improved soil nutrient availability, enhanced biodiversity and increased SOC stocks. The SOC stocks and biodiversity were increased by a factor of three relative to those in the mature beech and spruce forests. This study shows that traditional coppicing practice may facilitate net C accrual rates of 20 t ha−1 yr−1 and maintain high biodiversity, indicating that aspects of traditional practice could be applied in current forest management to foster biodiversity and to mitigate climate change. [ABSTRACT FROM AUTHOR]

Variations in soil moisture and surface roughness are major obstacles for the proximal sensing of soil organic C (SOC) using visible and near-infrared spectroscopy (VIS-NIRS). We gained a significant improvement of SOC prediction under field conditions with a stepwise approach. This comprised of (i) the estimation of these disturbing factors and (ii) the subsequent use of this information in multivariate SOC prediction. We took 120 surface soil samples (SOC contents 6.55-13.40 g kg-1) from a long-term trial near Bonn, Germany. To assess soil moisture, we recorded VIS-NIR spectra on <2-mm sieved disturbed samples at seven different moisture levels (air-dried to 30% w/w). The impact of roughness on VIS-NIRS performance was studied with undisturbed samples (air-dried and at different moisture levels), which were scanned with a laser profiler after fractionation into six aggregate size classes. The results confirmed that it was possible to include VIS-NIRS based assessments of soil moisture [R²adj = 0.96; root mean square error of cross validation (RMSEcv) = 1.99% w/w] into the prediction of SOC contents for sieved samples <2 mm (R²adj = 0.81-0.94; RMSEp = 0.41-0.72 g SOC kg-1). However, for rough soil surfaces, SOC contents were overestimated, and the prediction of roughness indices using VIS-NIRS failed. Fortunately, surface roughness did not impair the VIS-NIRS assessment of soil moisture. Hence, we could directly estimate moisture via VIS-NIRS in undisturbed field samples and then incorporate this information into a moisture-dependent prediction of SOC contents. This provided accurate SOC estimates for field-moist, undisturbed samples (R²adj = 0.91). Deviations from the reference method (elemental analysis) were below 2 g SOC kg-1. [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]

Aim Concern about climate change, with the subsequent emergence of carbon markets and policy initiatives such as REDD (reducing carbon emissions by decreasing deforestation and forest degradation), have focused attention on assessing and monitoring terrestrial carbon reserves. Most effort has focused on above-ground forest biomass. Soil has received less attention despite containing more carbon than above-ground terrestrial biomass and the atmosphere combined. Our aim was to explore how well soil carbon concentration could be estimated on three continents from existing climate, topography and vegetation-cover data. Location Peru, Brazil, Argentina, Australia, China. Methods Soil carbon concentration and leaf area index ( LAI) as well as GIS-derived climate and topography variables for 65 temperate and 43 tropical, forest and woodland ecosystems, were either directly measured or estimated from freely available global datasets. We then used multiple regressions to determine how well soil carbon concentration could be predicted from LAI, climate and topography at a given site. We compared our measurements with top soil carbon estimates from the Food and Agriculture Organization of the United Nations ( FAO) harmonized world soil map. Results Our empirical model based on estimates of temperature, water availability and plant productivity provided a good estimate of soil carbon concentrations ( R2 = 0.79). In contrast, the values of topsoil carbon concentrations from the FAO harmonized world soil map correlated poorly with the measured values of soil carbon concentration ( R2 = 0.0011). Main conclusions The lack of correlation between the measured values of soil carbon and the values from the FAO harmonized world soil map indicate that substantial improvements in the production of soil carbon maps are needed and possible. Our results demonstrate that the inclusion of freely available GIS data offers improved estimates of soil carbon and will allow the creation of more accurate soil carbon maps. [ABSTRACT FROM AUTHOR]

Black carbon (BC) is the product of incomplete burning processes and a significant component of the passive soil organic carbon (SOC) pool. The role of BC in the global carbon cycle is still unclear. This study aimed to quantify and characterize BC in major grassland ecosystems of the world. Twenty-eight representative soil profiles (mainly Mollisols) were sampled in the Russian Steppe, the U.S. Great Plains, the Argentinian Pampa, the Manchurian Plains in China, and the Chemozem region in central Germany. Black carbon contents were estimated using benzene polycarboxylic acids (BPCA) as a molecular marker, and indications about the origin of the BC were derived from bulk and compound-specific δ13C analyses and radiocarbon dating of bulk soil organic matter (SOM). Our findings suggest that between 5% and 30% of SOC stocks consist of BC. Maximum BC contributions to SOC frequently were found at deeper parts of the A horizon with 14C ages younger than 7000 years BP; that is, incorporation of C as charred particles accompanied ecosystem development since the mid-Holocene. Most of this BC formed from local vegetation, as indicated by a 13C isotope signature similar to that of bulk SOM. At some sites, also nonlocal sources contributed to soil BC, e.g., fossil fuel BC inputs at the German sites. Black carbon stocks were highest in Chernozems and lowest in Kastanozems. The Russian Steppe and Chinese Manchurian sites stored about 3-4 times more BC (around 3 kg m-2) than did the other sites because of thicker A horizons that were rich in BC. On a global scale, we estimate that steppe ecosystems contain between 4 and 17 Pg BC. [ABSTRACT FROM AUTHOR]

Anthropogenic perturbations have profoundly modified the Earth's biogeochemical cycles, the most prominent of these changes being manifested by global carbon (C) cycling. We investigated long-term effects of human-induced land-use and land-cover changes from native tropical forest (Kenya) and subtropical grassland (South Africa) ecosystems to agriculture on the dynamics and structural composition of soil organic C (SOC) using elemental analysis and integrated 13C nuclear magnetic resonance (NMR), near-edge X-ray absorption fine structure (NEXAFS) and synchrotron-based Fourier transform infrared-attenuated total reflectance (Sr-FTIR-ATR) spectroscopy. Anthropogenic interventions led to the depletion of 76%, 86% and 67% of the total SOC; and 77%, 85% and 66% of the N concentrations from the surface soils of Nandi, Kakamega and the South African sites, respectively, over a period of up to 100 years. Significant proportions of the total SOC (46–73%) and N (37–73%) losses occurred during the first 4 years of conversion indicating that these forest-and grassland-derived soils contain large amounts of labile soil organic matter (SOM), potentially vulnerable to degradation upon human-induced land-use and land-cover changes. Anthropogenic perturbations altered not only the C sink capacity of these soils, but also the functional group composition and dynamics of SOC with time, rendering structural composition of the resultant organic matter in the agricultural soils to be considerably different from the SOM under natural forest and grassland ecosystems. These molecular level compositional changes were manifested: (i) by the continued degradation of O-alkyl and acetal-C structures found in carbohydrate and holocellulose biomolecules, some labile aliphatic-C functionalities, (ii) by side-chain oxidation of phenylpropane units of lignin and (iii) by the continued aromatization and aliphatization of the humic fractions possibly through selective accumulation of recalcitrant H and C substituted aryl-C and aliphatic-C components such as (poly)-methylene units, respectively. These changes appeared as early as the fourth year after transition, and their intensity increased with duration of cultivation until a new quasi-equilibrium of SOC was approached at about 20 years after conversion. However, subtle but persistent changes in molecular structures of the resultant SOM continued long after (up to 100 years) a steady state for SOC was approached. These molecular level changes in the inherent structural composition of SOC may exert considerable influence on biogeochemical cycling of C and bioavailability of essential nutrients present in association with SOM, and may significantly affect the sustainability of agriculture as well as potentials of the soils to sequester C in these tropical and subtropical highland agroecosystems. [ABSTRACT FROM AUTHOR]

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.

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]

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.

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]

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]

Crop rotations encompassing flooded rice and an upland crop are commonly found in large parts of South and East Asia. However, also rice farmers in Southeast Asia increasingly switch from double-cropping paddy rice to one non-flooded crop–such as maize–in the dry season. We hypothesized that introducing maize (maize–paddy rice, M-MIX) into a double paddy rice (R-WET) cropping system will increase carbon (C) release from rice roots into the rhizosphere and the dissolved soil C pool. To test this hypothesis we assessed the kinetics of C release by the rice plants in a hydroponic greenhouse experiment, and used these data for interpreting their C release in field experiments using 13 C pulse labeling of rice plants. In the greenhouse we observed that rice roots released 13 C labeled dissolved organic carbon (DOC) for 21 days with a mean residence time (MRT) of 19 days after exposure to a 13 CO 2 pulse. The MRT of labeled dissolved inorganic carbon (DIC) released from rice roots was only 2 days. In the field 13 CO 2 pulse labeling increased the 13 C excess of rhizosphere soil up to 0.7 ± 0.2 mg 13 C kg −1 in R-WET and 0.9 ± 0.3 mg kg −1 in M-MIX. The 13 C signature of bulk soil remained unaffected. DOC concentrations in R-WET were significantly higher than in M-MIX during the mature grain stage of the rice plants. Nevertheless, the 13 C excess in DOC transiently increased by only 0.5 μg L −1 after labeling in 13 cm depth in one of three lysimeters previously cropped with maize (M-MIX), while no labeled DOC was detected in 13 cm depth of the R-WET lysimeters and in 60 cm depth of both treatments. In contrast, the 13 C excess of DIC increased by 42.4–93.1 μg L −1 a few days after labeling with a MRT of 53–66 days in both treatments. Considering the results of the greenhouse experiment, this suggests a rapid mineralization of labeled rhizodeposits in the field and an effective transient storage of CO 2 produced by respiration in soil water. [ABSTRACT FROM AUTHOR]

The energy crop Miscanthus x. giganteu s is a deep rooting perennial rhizomatous C4 grass with great biomass production, even under temperate German climate conditions. Accordingly we hypothesized that this crop may accumulate great amounts of carbon in soil, particularly in deeper soil layers. We sampled several former C3-derived arable fields that had been cropped with Miscanthus for 0–19 years. We were able to trace the origin and turnover of soil organic C (SOC) on the basis of natural 13 C/ 12 C abundance measurements. The analysis was performed on bulk soil samples and on particle-size fractions that are known to comprise SOC of different availability for decay. Miscanthus -derived C accumulated at a rate of 1800 kg ha −1 y −1 down to a soil depth of 100 cm. Only about 50% of this C accrual occurred in the surface soil (0–10 cm). The C accumulation differed among size fractions. Miscanthus -derived C in the coarse-POM fraction increased rapidly during the first years of Miscanthus cultivation until a steady state was reached after approximately seven years. The stocks of Miscanthus -derived C associated with the clay fraction increased at a rate of 230 kg ha −1 y −1 in 0–5 cm, 45 kg ha −1 y −1 in 20–30 cm and 38 kg ha −1 y −1 in 50–75 cm. The C accumulation rate decreased with increasing soil depth. In particular, Miscanthus -derived C associated with the clay fraction led to increasing SOC stocks, even below the former Ap; that is, below a depth that would respond sensitively to a future land use change. [ABSTRACT FROM AUTHOR]

• Combined effect of water deficit and livestock stocking density on soil carbon stocks are studied. • The interaction effect of livestock stocking density and water deficit on soil carbon stocks is strong. • High stocking density is associated with a larger soil carbon stock than is a low stocking density when the water deficit is low. • Both larger-scale variation and short-range variation in soil carbon stock is accounted for by water deficit and stocking density. Livestock integrated land use systems are considered as viable options for enhancing soil organic carbon (SOC) sequestration in a changing climate. This study assessed the influence of water deficit and livestock stocking density on soil carbon stocks. A total of 101 matching data were extracted from map layers of water deficit and livestock stocking density for C storage in Arenosols of Omusati and Otjozondjupa regions of Namibia. Maps for water deficit and livestock stocking density were obtained from national databases. The SOC data were arranged into four treatment combinations represented by two levels of water deficit and two levels of livestock stocking density. Linear mixed models (LMM) were then used to evaluate differences in SOC stocks in response to livestock stocking density and water deficit. Results showed that there was a significant interaction (p = 0.013) between the effects of livestock stocking density and water deficit on C-stocks. In conditions where the water deficit was small, the SOC stock was larger under more intensive grazing. Whilst in conditions where water deficit was large, the SOC stock was larger under less intensive grazing. Furthermore, the difference between SOC stock at large and small water deficits was larger under more intensive grazing. This shows that the impacts of a changing climate, and changes in the intensity of grazing must be considered together to predict effects on the SOC stock. [ABSTRACT FROM AUTHOR]

The Atacama Desert frequently serves as model system for tracing life under extremely dry conditions. We hypothesized that traces of life in the Atacama Desert follow distinct micro- and macro-scale gradients such as soil depth and elevation, respectively. Different depth intervals of surface soils (0–1, 1–5, and 5–10 cm) were sampled at five sites along an elevational transect near the Quebrada Aroma, spanning from the hyperarid core of the desert towards the arid Western Andean Precordillera (1300 to 2700 m a.s.l.), and from one additional site in the hyperarid core near Yungay. We determined the contents of major elements, pedogenic minerals and oxides, organic carbon (OC), and its δ13C and δ15N isotopic composition. The presence of living microorganisms was assessed by cultivation, and bacterial community composition was analyzed based on 16S rRNA gene sequencing. Additional information about past and present plant and microbial life was obtained from lipid biomarker analysis. We did not detect consistent micro-scale distributions for most of these proxies within the soils. However, concentrations of OC and of long-chain, plant wax-derived n -alkanes increased in soils along the aridity gradient towards the wetter sites, indicating the presence of past life at places presently not covered by vegetation. Likewise, bacterial abundance and diversity decreased as hyperaridity increased and the microbial community composition changed along the transect, becoming enriched in Actinobacteria. The distributional patterns of phospholipid fatty acids (PLFAs) confirmed the larger bacterial diversity at the higher, more humid sites compared to the drier ones. Archaeal isoprenoid glycerol dialkyl glycerol tetraethers (isoGDGTs) and bacterial branched (br)GDGTs, which can also indicate past life, did not follow a clear elevational trend and were absent at the driest site. Taken together, plant-derived and microbiological markers follow primarily the macro-scaled elevation and aridity gradient. Viable bacteria are present even at the driest sites, while detected biomolecules also indicate past life. The detection of past plant life in nowadays apparently lifeless regions suggests that conditions for life were less hostile in former times. • Traces of past plant and microbial life detected in hyperarid Atacama Desert soils. • Higher microbial diversity and abundance at more humid sites / higher elevation. • Bacterial and plant-derived lipid biomarkers follow aridity gradient. • No consistent patterns of soil properties or biomolecules with soil depth. [ABSTRACT FROM AUTHOR]

Abstract Crop rotations with maize and flooded rice lead to temporally aerobic soil conditions. This promotes the development of desiccation cracks in the soil, which can act as preferential flow pathways for water and solutes. We hypothesized that these cracks are enriched with organic carbon (C), plant nutrients and microbial residues (amino sugars) and that they can thus also serve as hot spots of fertilizer and C cycling. To test this hypothesis, we applied 13C-labelled rice straw and 15N-labelled urea to rice fields of the International Rice Research Institute (Los Baños, Philippines). We then traced the fate of the 13C-labelled rice straw and the 15N-labelled urea in crack and bulk soil and in microbial residues in two approaches, i) in the short term, i.e., 24 h after application of straw and fertilizer jointly with a dye tracer (Brilliant Blue) prior to maize seeding in the paddy - maize cropping system, as well as ii) in the long-term, i.e., during one year and for three different crop rotations (continuous paddy rice, paddy rice – maize, and paddy rice – maize with straw mulching and cover cropping). The short-term analyses of the dye tracer depth profiles showed that flow path areas decreased with increasing depth. A typical impermeable plough pan was not identified. Instead, we observed rapid infiltration of irrigation water down to 60 cm soil depth. The dyed flow paths were enriched in organic C (+12%) and plant nutrients (N: +21%, Ca2+: +59%, K+: +39%, Mg2+: +39%) relative to the bulk soil. The labelled straw and fertilizer quickly reached 60 cm depth with the dye tracer. We could not identify elevated microbial biomass along the flow paths, however, we did find larger microbial activities along the cracks in the long-term experiment than in the surrounding bulk soil. The increased activity fostered microbial uptake of fertilizer 15N along the cracks, which was detected mainly for fungal residues and only in the trials receiving straw (crack soil: 0.6 ± 0.1 mg glucosamine-15N kg soil−1, bulk soil: 0.2 ± 0.1 mg glucosamine-15N kg soil−1). We conclude that analysis of homogenized bulk soil samples can underestimate C and nutrient availability, as well as their microbial processing in paddy rice soils, when crack systems are not considered. Highlights • Straw application delays the appearance of desiccation cracks in paddy soils. • Flow paths in paddy rice soils are enriched with carbon and plant nutrients. • Intensified microbial activity increases fertilizer-N immobilization in soil cracks. [ABSTRACT FROM AUTHOR]

Enhancing global soil organic carbon storage by 4 per mille (‰) per annum would be enough to halt current net greenhouse gas emissions, but this goal seems lofty for conventional agriculture, which frequently results in soil organic carbon and nitrogen losses. Replacing mineral nitrogen with organic nitrogen sources may benefit soil carbon and nitrogen cycling in agricultural soils, but long-term effects are yet to be clearly demonstrated, especially in soils of high natural fertility. Here we report the effects of 34 years of legumes (crimson clover, fava beans) and non-legumes (maize) in rotation combined with different fertilization regimes (no fertilization, PK fertilization, NPK fertilization) on soil carbon and nitrogen storage throughout the uppermost meter of the soil profile. Fava beans did not enhance profile carbon storage. However, fava beans induced positive effects on subsoil nitrogen cycling, with lower subsoil nitrogen densities indicating lower nitrogen leaching potential. Incorporating a clover green mulch every 4 years enhanced organic carbon storage by an average of 4.1‰ per annum down the full meter of soil compared with a conventional maize rotation, but only combined with phosphorus and potassium fertilization. The enhancement was detected below the plough-horizon, indicating that merely sampling topsoil is insufficient to assess soil carbon dynamics in these arable soils. In contrast, maize contributed only a small portion to SOC, with subsoil C contributions negligible. These results indicate that a careful combination of long-term, site-adapted crop and fertilization management strategies can help enhance SOC storage in naturally fertile soils without apparent C deficit. [ABSTRACT FROM AUTHOR]

Nitrogen (N) deposition to soils is globally rising, but its effect on soil organic carbon (SOC) turnover is still uncertain. Moreover, common theories of stoichiometric decomposition and microbial N mining predict opposing effects of N supply on SOC turnover. We hypothesized that the effect of N deposition on SOC turnover depends on initial soil nutrient conditions. Thus, we sampled tropical forests and rubber gardens with pronounced gradients of nutrient availability from the topsoil to the deep subsoil (up to 400 cm) and measured substrate-induced respiration (SIR) for 30 days in four treatments (C, CN, CP, CNP additions). A natural 13 C abundance approach was conducted to quantify priming effects (PE) of the added C on SOC mineralization. For this purpose we assessed the 13 CO 2 isotope composition after adding a C4 sugar to the C3 soil; to correct for isotopic fractionation a treatment with C3 sugar additions served as control. We found that nutrient additions to topsoil did neither alter cumulative CO 2 release within 30 days (SIR acc ) nor PE (PE = 1.6, i.e., sugar additions raised the release of SOC-derived CO 2 by a factor of 1.6). In the upper subsoil (30–100 cm), however, both CN and CP additions increased SIR acc (by 239% and 92%, respectively) and the PE (PE = 5.2 and 3.3, respectively) relative to the treatments that received C only (PE = 1.7), while CNP additions revealed the largest increase of SIR acc (267%) and PE (PE = 6.0). In the deep subsoil (>130 cm depth), only the CNP addition consistently increased SIR acc (by 871%) and PE (PE = 5.2) relative to only C additions (PE = 2.0). We conclude that microbial activity was not limited by nutrients in the topsoil but was co-limited by both N and P in the subsoil. The results imply that microbes mine nutrients from previously unavailable pools under the conditions that 1) deficiency actually exists, 2) co-limitation is alleviated, and 3) nutrient reserves are present. Yet, as opposed to microbial nutrient mining theories, we showed that the subsoil PE is highest when nutrient supply matches microbial demand. As a result also N deposition might exert variable effects on SOC turnover in tropical soils: it might have no effect in nutrient-rich topsoils and in co-limited subsoils without P reserves but might increase SOC turnover in co-limited subsoils with potentially acquirable P reserves. [ABSTRACT FROM AUTHOR]

The ongoing conversion of natural ecosystems of the tropics into agricultural land usually results in a loss of soil organic carbon (SOC), the extent of which being largely affected by climate and soil order. We assumed that i) SOC losses are not restricted to the surface soil but extend into the subsoil, but that ii) the rate of SOC losses is controlled by interactions of SOC with Fe oxides. Hence, we sampled Plinthosols down to one meter (if feasible) that had been converted from native savannah into cropland 0 to 29 years ago in the Dano district (Southwest Burkina-Faso). Beside the assessment of SOC stocks, we also determined the proportion of surface (0–10 cm) SOC remaining after Fe oxide removal as well as its distribution among the following particle-size fractions: 2000–250 μm (coarse sand-sized SOC; POM1), 250 μm–53 μm (fine-sand-sized SOC; POM2), 53 μm–20 μm (very fine sand-sized SOC; POM3), and <20 μm (nonPOM). We found that the soils lost 24 t C ha −1 from the upper 10 cm and 49 t C ha −1 from the upper 30 cm, respectively, after 29 years. SOC losses extended also into the subsoil, and, on the average, 0.7–19.5 t C ha −1 was lost from the 30–100 cm depth interval. The loss rates of SOC were similar to those reported for other soil types. Rapid losses of SOC occurred from all POM fractions as well as from the Fe-oxide pool, which, on average, contained about 16% of total SOC stock. We conclude that even if Fe oxides as well as other silt- and clay-sized organomineral complexes delay losses of SOC relative to POM, they failed to efficiently protect SOC from decomposition in these Plinthosols. [ABSTRACT FROM AUTHOR]

Decomposition of soil organic matter (SOM) is regulated by microbial activity, which strongly depends on the availability of carbon (C) and nitrogen (N). Yet, the special role of N on soil organic carbon (SOC) mineralization is still under discussion. The recent concept of microbial N mining predicts increasing SOC mineralization under N-deficiency, which is in contrast to the generally accepted stoichiometric decomposition theory. Following this concept we hypothesized that spatio-temporal patterns of microbial activity are controlled by SOC and N contents, but that microorganisms maintain their functionality to mineralize C under conditions of N deficiency because of microbial N mining. To test this hypothesis, we added glucose to an arable soil that had experienced increasing losses of C 3 -derived SOM after one, three, and seven years of bare fallow and measured spatio-temporal patterns of substrate-induced respiration (SIR). The SIR measurements were performed with and without additions of mineral N. Selected samples were treated with C 4 sugar in order to trace the source of CO 2 emissions (sugar vs. SOC-derived) by natural 13 C abundance measurements. Sugar additions were repeated after the first SIR experiment to derive information on changing N availability. The results showed that spatial patterns of SIR were not consistently regulated by SOC and N. On a temporal scale, the maximum microbial growth peak declined by 47% from one year bare fallow to seven years bare fallow but soils often developed a second growth phase in the 7th year of fallow. Intriguingly, the maximum microbial growth peak increased again when N was added together with the glucose and no second growth peak occurred. A similar effect was observed after repeated sugar additions but without N additions. The 13 C experiment revealed a slightly higher contribution of SOC-derived CO 2 in N-deficient samples (16.7%) than in N-fertilized samples (14.6%). We conclude that the first SIR peak was related to the supply of immediately available N while the second growth phase indicated a delayed release of N, due to N mining from SOM. Hence, microbes were able to compensate for initial N limitation and there was no significant change in the overall substrate-induced CO 2 release with proceeding time under fallow. [ABSTRACT FROM AUTHOR]

Tracing the history of industrial fossil fuel combustion on the carbon stock in soil is challenging, since it is mixed with other soil organic carbon (SOC). Isolation of black carbon (BC) yields a mixture of diagenetic fossil C and pyrogenic BC from biomass and fossil fuel combustion. We investigated the degree to which fossil fuel combustion emissions have contributed to SOC and BC and how deposition dynamics have changed in the last century. As only biomass-derived BC contains 14 C, we determined the content and 14 C signature of the benzene polycarboxylic acid (BPCA) fraction as the product of total BC oxidation. From an isotopic mass balance model, the proportion of fossil BC deposition and its contribution to SOC was calculated. Soil samples were taken from 1958–2002 from a long term agricultural field experiment in Halle, Germany; the area represents one of the most productive lignite mining areas in the world. Between 1958 and 1971, total BC content increased significantly (1.9 ± 0.1–2.30 ± 0.06 g/kg soil), and the BC-specific radiocarbon content decreased from 29.6 ± 0.4 to 26.9 ± 0.2 pMC, corresponding to 1.35 ± 0.07 and 1.8 ± 0.08 g/kg BC from diagenetic C and fossil fuel combustion (denoted here as fossil BC). We infer an increase in fossil BC content of 30% within this timespan, with a net input rate of 9 g/m 2 yr. In the 1970s, after railway electrification, reduction of lignite mining and related heavy industries, fossil BC deposition to soil was no longer significant. Overall, about 120 g/m 2 fossil BC was deposited between 1958–1971, with 390 g/m 2 before 1958. Fossil BC contributed ca. 75% of total BC and 15% SOC to arable soil, without any significant change in the following 30 yr (1971–2002). [ABSTRACT FROM AUTHOR]

The use of charcoal as a soil amendment is currently of great interest to sequester carbon and improve soil fertility, however, studies of sites where charcoal amendments to the soil have been made many years ago are lacking at the moment. In this study we investigated historical charcoal production sites in Germany that have not been in use for >60 years, and evaluated the effects of the former charcoal inputs on soil and vegetation parameters relative to those of adjacent, unamended areas. Surface soil samples (0-5, 5-20 cm) were taken from five sites located on extremely acidic (Siegerland, pH 3.8-4.1) and base rich soils (Eifel, pH 4.8-5.3) in species poor (Luzulo-Fagetum) and species rich (Hordelymo-Fagetum) beech forests, respectively. We determined stocks of black carbon (BC) and natural soil organic carbon (SOC=total C minus BC) as well as of soil nutrient stocks (NO3-N, P, K, Mg), cation exchange capacity and water holding capacity, and we mapped plant composition to calculate richness and evenness. The results showed that historical charcoal production sites were enriched with BC and also exhibited increased stocks of natural SOC and total N possibly due to enhanced stabilization of natural SOC by the charcoal. The availability of nitrate-nitrogen, phosphate and potassium was increased when the charcoal was added to the base rich soils and less so when charcoal was added to the extremely acidic soils. Plant biodiversity was not different between the sites of historical charcoal addition and the reference sites. We conclude that charcoal additions may increase soil carbon storage capacity over prolonged periods of time without negatively affecting plant ecological interactions over the long term. [ABSTRACT FROM AUTHOR]

Abstract: Termite activities are known to significantly influence small-scale soil properties in tropical savannas. The lateral and vertical extent of the alterations to the nest''s surrounding, and particularly resulting impacts on diagnostic soil horizons remain largely unresolved until today. We examined the effects of mound-building termites on soil genesis and constitutive chemical soil properties in and below their nests. Two transects to a soil depth of 100cm were dug below three younger mounds of Cornitermes silvestrii (the primary nest builder), three older mounds in which C. silvestrii had died out and which were secondarily colonized mainly by Nasutitermes kemneri, and three reference sites in the Brazilian Cerrado. The samples were characterized by standard procedures for soil classification; in addition, phosphorus extractions were conducted on selected samples using NaHCO3 for labile P forms, and concentrated HCl for stable P forms. This data set was then used to build calibration models for the prediction of labile and stable inorganic (Pi) and organic (Po) P forms, as well as for contents of organic carbon (OC), for the remaining samples applying mid-infrared spectroscopy in combination with partial least squares regression (MIRS-PLSR). We can show that the termite influence on the soil was sufficiently large to change diagnostic characteristics of the soils under termite mounds. The MIRS-PLSR predictions were suitable for quantifying organic carbon and most of the labile and stable phosphorus fractions. They showed an enrichment of OC, NaHCO3-Po and NaHCO3-Pi contents in nests inhabited by primary and secondary termites by factors of 1.6–2.0 and 1.4–1.5, respectively. The soils surrounding the nests had higher contents of OC and NaHCO3-P under both nest types vertically down to 30cm below the lower nest border, and OC and NaHCO3-Pi contents were elevated at minimum to a lateral distance of 60cm away from the nest border. As the pattern of HClconc-Pi, which comprised 95% of total P, showed no variations, we conclude that the higher NaHCO3-Pi amount was formed in termite nests by changing the availability of the more stable HClconc-Pi. In contrast to the contents, the OC and NaHCO3-P stocks below the mounds inhabited by primary termites were comparable to those inhabited by secondary ones, because the bulk density of the secondarily inhabited nests was elevated. This was due to a transport of clay-rich material from the subsurface argic horizons into the nests. Here, the secondary termites even reverted the lessivation observed in the reference soils and under mounds inhabited by primary termites, thus causing the soil types to change from Alisols and Acrisols to the properties of Umbrisols. [Copyright &y& Elsevier]

Soil aggregates may stabilize carbon at mineral surfaces and in the interior, but resolving such micro-scale carbon (C) turnover at a scale of soil aggregates <2 mm is challenged by C contaminations during sample preparation such as from resin embedding. Here we introduce a novel C-free embedding method using silica gel for water glass formation, and applied it to soil aggregates from a C3/C4 vegetation change soil chronosequence (4, 10 and 19 years of Miscanthus cropping on former C3 soil) for subsequent δ13C and C turnover analyses using laser ablation isotope ratio mass spectrometry (LA-IRMS). We hypothesized that C-free embedding allows for the first time the comparison of C turnover at soil aggregates both in their center and at their outer surface. We found that using water glass embedding enabled δ13C analyses via LA-IRMS in all parts of the sample, free of any contaminations. There was a large micro-scale heterogeneity in δ 13C signals within aggregates, which increased with cropping duration (total range −5.5 to –41.5‰). Noteworthy, after 19 years Miscanthus materials were still found preferentially at soil aggregate surfaces and hardly in interior parts, documenting slow aggregate turnover but also success of the embedding technique for future micro-scale C dynamic analyses in environmental samples. • Novel water-glass embedding reduces contamination for δ13C analyses. • Micro-scale δ13C analyses now possible until contact with embedding medium. • δ13C in soil aggregates revealed large microscale heterogeneity. • C residence time ranged between 1 and 119 years within soil aggregates <2000 μm. [ABSTRACT FROM AUTHOR]

The global decline in invertebrate diversity requires urgent conservation interventions. However, identifying priority conservation areas for invertebrates remains a significant challenge. We hypothesized that aligning the conservation of invertebrate biodiversity with climate change mitigation ofer offers a solution. As both soil carbon storage and invertebrate biodiversity are positively influenced by plant diversity and productivity, a positive correlation can also be expected between SOC and invertebrate biodiversity. Drawing on >10,000 invertebrate observations organized into functional groups, and site-specific soil organic Carbon (SOC) measurements from Patagonia, the Peruvian Andes, and montane tropical rainforest, we examined the role of climate, soil, topographical position and land use for prediction of invertebrate biodiversity. We found that taxonomic and functional invertebrate diversity and abundance closely correlate with SOC stocks within ecosystems. Topographical position of sites, which was partly associated with SOC, was also important, whereas land use was of subordinate importance. We conclude that recent advances in predicting and mapping SOC can guide the identification of habitats within landscapes with high biodiversity and conservation value for invertebrates. Our findings stress the importance of linking global climate change mitigation initiatives that aim to preserve and restore SOC to efforts aimed at improving the conservation of invertebrates and the ecosystem services they provide, for the realization of mutual climate and biodiversity benefits. • Identifying habitats with high invertebrate biodiversity is challenging due to the abundance and diversity of the group. • Identifying areas that need to be prioritised in conservation planning is thus a serious challenge. • Within landscapes the sites holding the highest biodiversity values tend to be those where soil carbon is highest. • This offers opportunities to consolidate global SOC preservation efforts with the conservation of invertebrates. [ABSTRACT FROM AUTHOR]

• Soil colloids promote the stability of microaggregate occlusion in macroaggregates. • Microaggregate reaggregation is affected by colloidal iron and organic carbon. • Less microaggregate reaggregation with reduced organic carbon concentrations in Luvisols. • Smaller-sized microaggregate structures in presence of colloidal-sized iron. Several beneficial soil functions are linked to aggregates, but how the formation and stability depend on the presence of colloidal- and nanosized (1000–1 nm) bulding blocks is still understood poorly. Here, we sampled subsites from an arable toposequence with 190 and 340 g kg−1 clay, and isolated small soil microaggregates (SMA; <20 µm) from larger macroaggregate units (>250 µm) using an ultrasonic dispersion energy of 60, 250, and 440 J mL−1, respectively. We then allowed these small SMA to reaggregated after chemical removal of organic carbon (OC) as well as of Fe- and Al (hydr)oxides, respectively. The size distribution of the reaggregated small SMA and fine colloids (<0.45 µm) was analyzed via laser diffraction and asymmetric flow field-flow fractionation coupled to inductively coupled plasma mass spectrometry and organic carbon detection, respectively. We found elevated amounts of both finer colloids and stable SMA at subsites with higher clay contents. The size distribution of small SMA was composed of two distinctive fractions including colloids and larger microaggregates with an average size of 5 µm. The removal of Fe with Dithionite-Citrate-Bicarbonate (DCB) shifted the size of the small SMA to a larger equivalent diameter, while removal of OC with NaOCl reduced it. After three wetting and drying cycles, the concentration of colloids declined, whereas the small SMA without chemical pre-treatments reaggregated to particles with larger average diameters up to 10 µm, with the size depending on the clay content. Intriguingly, this gain in size was more pronounced after Fe removal, but it was not affected by OC removal. We suggest that Fe (hydr)oxides impacts the stability of small SMA primarily by being present in small-sized pores and thus cementing the aggregates to smaller size. In contrast, the effect of OC was restricted to the size of colloids, gluing them together to small SMAs within defined size ranges when OC was present but releasing these colloids when OC was absent. [ABSTRACT FROM AUTHOR]

Although carbon (C) stored deep in soils of tree-dominated land use systems in the tropics represents a large reservoir of organic matter its vulnerability to land use change has been hardly assessed. To fill this gap, we sampled Acrisols down to 3 m under three different land use systems; namely, recent cacao agroforestry (<2 years old; not deeper than 1 m), young and old rubber gardens (<10 years and >50 years), and secondary forest (>50 years) all located in Kapuas Hulu regency, West Kalimantan, Indonesia. We then assessed soil organic carbon (SOC) stocks as well as C accumulated in above- and belowground biomass, litter and dead wood debris at the soil surface. The amount of C stored in soils to a depth of 1 m exceeded the amount stored in living biomass (Σ C stored in roots, understorey and overstorey) strongly in the cacao agroforestry systems (69 Mg SOC vs. 12 Mg biomass-C ha−1), slightly in young rubber gardens (85 Mg SOC vs. 69 Mg C ha−1), but not in old rubber gardens (87 Mg SOC vs. 200 Mg C ha−1) and secondary forests (65 Mg SOC vs. 138 Mg C ha−1). Additionally in the older systems, up to 140 Mg C ha−1 (old rubber gardens) and 116 Mg C ha−1 (secondary forest) were found in soils to a depth of 3 m, thus raising soil C stocks by 60 to 80% relative to C stored in upper soil (0 to 1 m). We conclude that (1) the form of land use and land use change can substantially affect C stocks in living biomass, with aboveground biomass in old rubber gardens comparable to that of secondary forests; and (2) that land use change can reduce SOC in topsoil, but that substantial C stocks found in deep (down to 3 m) subsoil remain stable. • Old secondary forests stored up to 140 Mg C ha−1 in soils to a depth of 3 m • Carbon stored up to a depth of 1 m varied between 65 and 87 Mg C ha−1 • Above ground biomass varied strongly between forest types and age. • Living and dead biomass stored between 9 and 200 Mg C ha−1 [ABSTRACT FROM AUTHOR]

The mean residence time of organic matter in soil ranges from days to millenia. Although chemical and physical processes are known to control the storage of organic matter in soil, the role of spatial arrangements in soil remains largely unknown due to the lack of in-situ techniques operating on the micro-scale. We asked whether deterministic patterns in element arrangement occur in large and small soil microaggregates (250-53 and 53-20 µm), and how these contribute to the storage of organic matter. To do so, we studied surface soils with increasing clay contents (sandy to loamy Luvisols, Germany) and subjected 60 individual aggregates to elemental mapping by electron probe micro analysis (EPMA), which recorded C, N, P, Al, Fe, Ca, K, Cl, and Si contents at micrometer scale resolution. We further developed the first laser-ablation isotope ratio mass spectrometry technique in soil science detecting d13C composition on the micro-scale (LA-IRMS) and employed this to trace micro-gradients in undisturbed soil and soil microaggregate samples. We found a pronounced heterogeneity in aggregate structure and composition, which was not reproducible for different microaggregates from the same soil fraction, and which was largely independent from clay content in soil. However, neighborhood analyses revealed close spatial correlations between organic matter debris (C:N app. 100:10) and microbial organic matter (C:N app. 10:1), indicating a spatial relationship between source and consumer. There was no systematic relationship between soil minerals and organic matter, suggesting that well-established macroscale correlations between pedogenic oxides and clay with soil organic matter storage do not apply to soil microaggregates. From first applications of LA-IRMS to soil we found also a high d13C variability of up to 9 per mill along spatial gradients of less than 300 µm, suggesting the appearance of very small hotspots of isotope enrichment and organic matter turnover by metabolic processes. [ABSTRACT FROM AUTHOR]