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

Abstract

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.

Roots are key to increasing the mean residence time of organic carbon entering temperate agricultural soils

“…Interestingly, Sanderman et al (2003) also compared the MRT OC estimates derived from eddy covariance measurement with four other common methods, namely laboratory incubations, flux chamber measurements, radiocarbon uptake and changes in stable isotope ( 13 C) natural abundance after C3 to C4 vegetation change. Only the latter method, that is, estimating MRT OC from changes in δ 13 C, yielded significantly higher values (>100 years), which is in line with other studies (Balesdent et al, 1990; Rasse et al, 2006; Schneider et al, 2020). It would appear that turnover times calculated by the stable isotope method refer to the pool that is ultimately measured as SOC, that is, OC measured in the bulk mineral soil <2 mm.…”

“…Including or excluding this very fast cycling fraction from the estimate has a strong effect on MRT, as recently also elaborated by Luo et al, submitted. When only accounting for the proportion of retained OC (10–20% of the actual total input), the estimated MRT of SOC in croplands would have been 108–216 years in this study, which is well in line with studies using the 13 C approach with C3–C4 vegetation changes (Sanderman et al, 2003; Schneider et al, 2020). The MRT OC values presented here should thus be interpreted as the MRT of OC entering the soil, not OC that has been retained in the soil and is measured as SOC.…”

“…In contrast, climate variables were not important in explaining the variability of MRT OC at the scale of Germany, with 'pedoclimatic zones' being the only indirectly climate-related variable among the 10 most important variables in the model for all sites and model for croplands only.4 | DISCUSS ION4.1 | Mean residence time of organic carbon entering German agricultural soilsuptake and changes in stable isotope ( 13 C) natural abundance after C3 to C4 vegetation change. Only the latter method, that is, estimating MRT OC from changes in δ 13 C, yielded significantly higher values (>100 years), which is in line with other studies(Balesdent et al, 1990;Rasse et al, 2006;Schneider et al, 2020). It would appear…”

“…Other soils, such as Chernozems, can also have high proportions of centennially stable SOC that are different in OC quality and might have entered the soil under contrasting environmental conditions (Franko & Merbach, 2017), potentially also as charred organic matter (Ponomarenko & Anderson, 2001). Furthermore, other OC inputs such as allochthonous OC, for example in colluvial soils, may be of a different quality to the current OC input (Schneider et al, 2020). The MRT OC estimated in this study is thus unlikely to reflect exactly the average time that the entering C will remain in the soil.…”

“…The analysis was restricted to topsoils (0-30 cm) since one of the critical assumptions of this MRT OC approach is that SOC is autochthonous, that is, it has been entirely built up in situ by the estimated carbon inputs. This is not always correct for topsoils, but is even more problematic for subsoils, which often contain allochthonous, translocated, relictic or geogenic OC (Kalks et al, 2020;Schneider et al, 2020). As mentioned above, SOC in soils with a C:N ratio >13, which have been classified as 'black sands' (Poeplau et al, 2020;Vos et al, 2018), were excluded since they constitute an extreme example of relictic SOC.…”