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 δ¹⁵N and δ¹³C isotope abundance individually. We mapped the spatial distribution of δ¹³C, δ¹⁵N, 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 δ¹⁵N and δ¹³C values in topsoil (0–15 cm). Among the evaluated factors, topography and depth predicted soil δ¹⁵N and δ¹³C profiles. Trends in δ¹³C vs. δ¹⁵N showed a wider range of δ¹³C 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 δ¹³C enrichment with soil depth. Digital soil mapping approach depicted low continuity of δ¹³C vs. high continuity of δ¹⁵N with geomorphic position We attributed the intermediate δ¹³C values, and steeper slope of δ¹³C 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.