The patchiness of soil 13C versus the uniformity of 15N distribution with geomorphic position provides evidence of erosion and accelerated organic matter turnover

Farming on hillslopes often affects the accumulation and loss of soil organic matter (SOM) depending on slope position and cropping patterns. Most hillslope studies focus on soil movement to characterize SOM turnover under erosive conditions. In this study, we trace erosion and characterize agronomic practices erosive impacts on SOM translocation and transformation along geomorphic positions. To achieve this, we assessed the horizontal distribution (upper 15 cm) and vertical distribution (to 100 cm profiles) of soil 815N and 813C isotope abundance individually. We mapped the spatial distribution of 813C, 815N, and SOM turnover indices as a novel approach to tracing erosion and degradation of SOM in the field. Except for tillage (conventional vs. reduced tillage), other individual agricultural practices (residue removal with no cover crop vs. retaining residuals, cover cropping, and fertilizer 0, 40, and 80 kg ha-1 nitrogen) caused no significant shifts in 815N and 813C values in topsoil (0-15 cm). Among the evaluated factors, topography and depth predicted soil 815N and 813C profiles. Trends in 813C vs. 815N showed a wider range of 813C values in topsoil of upslope plots under reduced tillage, while in the depositional location, conventional tillage had the same effect. This suggests erosion under reduced tillage occurred. Erosion and accelerated decomposition gradually slowed 813C enrichment with soil depth. Digital soil mapping approach depicted low continuity of 813C vs. high continuity of 815N with geomorphic position We attributed the intermediate 813C values, and steeper slope of 813C against logarithm of soil organic carbon (SOC) across the slope to erosion and high SOM turnover, particularly of recently added plant inputs. Current results support the prediction of intensive vs. conservation practices' effects on upslope soil stability and the fate of SOM in both topsoil and at depth of sloping farmlands.

Automated summary

Farming on hillslopes affects soil organic matter accumulation and loss, depending on slope position and cropping patterns. In this study, erosion and agronomic practices' impacts on soil organic matter translocation and transformation were examined. The spatial distribution of 813C, 815N, and SOM turnover indices were mapped to trace erosion and degradation of SOM in the field.