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Background: There is considerable uncertainty about the actual size of the global soil organic carbon (SOC) pool and its spatial distribution due to insufficient and heterogeneous data coverage.Aims: We aimed to assess the size of the German agricultural SOC stock and develop a stratification approach that could be used in national greenhouse gas reporting.Methods: Soils from a total of 3104 sites, comprising 2234 croplands, 820 permanent grasslands and 50 sites with permanent crops (vineyards, orchards) were sampled in a grid of 8 × 8 km to a depth of 100 cm in fixed depth increments. In addition, a decade of management data was recorded in a questionnaire completed by farmers. Two different approaches were used to stratify cropland and grassland mineral soils and derive homogeneous groups: stratification via soil type (pedogenesis) and via SOC‐relevant soil properties.Results: A total of 146 soils were identified as organic soils, which stored by far the highest average SOC stock of 528 ± 201 Mg ha−1 in 0–100 cm depth. Of the mineral soils, croplands and permanent crops stored on average 61 ± 25 and 62 ± 25 Mg ha−1 in 0–30 cm (topsoil) and 35 ± 30 and 44 ± 28 Mg ha−1 in 30–100 cm (subsoil), while permanent grasslands stored significantly more SOC (88 ± 32 and 47 ± 50 Mg ha−1 in topsoil and subsoil). Overall, topsoils stored 67 ± 14% and subsoils 33 ± 14% of total SOC stocks. Soil C:N ratio, clay content and groundwater level were major factors that explained the spatial variability of SOC stocks in mineral soils. Accordingly, Podzols, Gleysols and Vertisols were found to have the highest SOC stocks.Conclusions: Stratification via soil properties yielded the most comparable cropland and grassland strata and is thus preferable for estimating land‐use change effects, e.g., for greenhouse gas inventories.In total, 2.5 Pg C are stored in the upper 100 cm of German agricultural soils, making them the largest organic carbon pool in terrestrial ecosystems of Germany. This bares a large responsibility for the agricultural sector and society as a whole to maintain and, if possible, enhance this pool.

Aims Root-restricting layers pose a barrier to vertical root elongation. The German Agricultural Soil Inventory was used to assess the extent, cause and effect of root-restricting layers in German agricultural soils. Methods The following causes for root restriction were considered: bedrock, rock fragments, cementation, compactness, sandy subsoil, anoxia and acidity. Threshold values for restricted root growth were extracted from the literature and validated using root counts of winter wheat and permanent grassland. The effect of management-induced compaction in cropland was quantified using machine learning. Results In 71% of all agricultural soils, potential rooting was restricted to less than 100 cm depth. Compactness was the most common cause of root restriction, affecting 51% of cropland and 32% of grasslands. It was estimated that agricultural management explained 27% of all compacted cropland, while the remaining 73% has always been compacted as a result of pedogenic causes. Root-restricting soil layers decreased the yield of winter wheat significantly. Conclusions In view of potential rooting being restricted on more than half of Germany's agricultural land and this study's results suggesting that root-restricting soil layers have a direct impact on crop yield, there is considerable potential in the melioration of affected sites.

The ratio of soil organic carbon stock (SOC) to annual carbon input gives an estimate of the mean residence time of organic carbon that enters the soil (MRTOC). It indicates how efficiently biomass can be transformed into SOC, which is of particular relevance for mitigating climate change by means of SOC storage. There have been few comprehensive studies of MRTOC and their drivers, and these have mainly been restricted to the global scale, on which climatic drivers dominate. This study used the unique combination of regional‐scale cropland and grassland topsoil (0–30 cm) SOC stock data and average site‐specific OC input data derived from the German Agricultural Soil Inventory to elucidate the main drivers of MRTOC. Explanatory variables related to OC input composition and other soil‐forming factors were used to explain the variability in MRTOC by means of a machine‐learning approach. On average, OC entering German agricultural topsoils had an MRT of 21.5 ± 11.6 years, with grasslands (29.0 ± 11.2 years, n = 465) having significantly higher MRTOC than croplands (19.4 ± 10.7, n = 1635). This was explained by the higher proportion of root‐derived OC inputs in grassland soils, which was the most important variable for explaining MRTOC variability at a regional scale. Soil properties such as clay content, soil group, C:N ratio and groundwater level were also important, indicating that MRTOC is driven by a combination of site properties and OC input composition. However, the great importance of root‐derived OC inputs indicated that MRTOC can be actively managed, with maximization of root biomass input to the soil being a straightforward means to extend the time that assimilated C remains in the soil and consequently also increase SOC stocks.

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.

Aims In many agricultural soils in Germany, elongation of deep roots is restricted by compactness, anoxia and acidity. This study examined the adaptation and melioration strategies of farmers who cultivate sites with such root-restricting layers (RRLs). Methods The German Agricultural Soil Inventory was evaluated with respect to land use and crop rotations on sites with and without RRLs. The likelihoods of deep tillage, drainage and liming and the feasibility of biological melioration (bio-drilling) were predicted using soil, geology, climate and socioeconomic data. Results Anoxic and acidic sites were preferentially used as grassland. Cropland with RRLs was often dominated by maize instead of wheat. About 54% of agricultural land in Germany was limed, 45% drained, 6% deep chiselled and 5% deep ploughed. The abundance of biopores was positively related to silt content and pH, but negatively related to rock content. Conclusions Deep tillage is not very popular for alleviating soil compactness, but bio-drilling could be used to facilitate deeper rooting in loamy and well-aerated soils with low rock fragment contents and pH values >5. Waterlogged soils could be meliorated by improved drainage and extreme acidity by enhanced liming practices. However, many farmers preferred grassland use as opposed to meliorating RRLs. Keywords Subsoiling. Deep ripping. Earthworm burrow. Hardpan. Land improvement. Soil profile modification Abbreviations RMSE root mean square error RRLs root-restricting soil layers R 2 coefficient of determination SOC soil organic carbon

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