Origin of carbon in agricultural soil profiles deduced from depth gradients of C:N ratios, carbon fractions, δ13C and δ15N values

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), delta C-13 and delta N-15 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 C-4 plant) in crop rotations and soil delta C-13 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.

Automated summary

German agricultural soils store a significant amount of organic carbon, with varied sources including fresh photosynthates and organic fertilizers. Machine learning is effective in analyzing depth profiles of organic matter and understanding the influence of different variables on soil carbon content. Crop rotation, geographical location, and management practices all play important roles in the turnover rates and distribution of organic carbon in agricultural soils.