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

• Published in: Agriculture, ecosystems & environment ecosystems & environment, 2023-10, Vol.356, p.108616, Article 108616
• Main Authors: Ghotbi, Mitra, Taghizadeh-Mehrjardi, Ruhollah, Knief, Claudia, Ghotbi, Marjan, Kent, Angela D., Horwath, William R.
• Format: Article
• Language: English
• Subjects: Accelerated decomposition; Conventional vs. conservation tillage; Digital mapping; Soil depth profiles; Soil organic matter turnover; Tracing erosion
• ISSN: 0167-8809; 1873-2305
• DOI: 10.1016/j.agee.2023.108616

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. [Display omitted] •Accelerated SOM dynamics and erosion led to patchy spatial distribution of δ13C.•Agronomic practices' impacts on SOM dynamics were contingent on geomorphic position.•Reduced tillage was insufficient for soil stabilization along slope position.•The depositional site displayed a wider range of δ13C and δ15N compared to upslope.•Long-term erosion/deposition slowed δ13C enrichment trends across soil depths.