BayCEER Workshop 2021

C and N isotopes for tracing erosion and estimating soil organic matter turnover in Upper Eastern Ghana hilly farmland

Farming on hillslopes can drastically affect soil organic matter (SOM) loss. Most hillslope studies to date have been primarily focused on soil movement to characterize SOM turnover under erosive conditions. This study aims at characterizing the integrated impacts of natural erosion and accelerated erosion on SOM turnover by employing soil δ¹⁵N and δ¹³C changes. Except for plowing, individual agricultural practices (conventional vs. reduced tillage, cover crop and residue removal vs. cover crop and residue incorporation, and soil fertilization with 0, 40, and 80 kg ha^-1 nitrogen) caused no significant shifts in δ¹⁵N and δ¹³C values of alluvial and colluvial deposits but in total N %, C: N ratio of the deposits. Topography and tillage interaction significantly altered soil ¹³C values, due possibly to the high contribution of old SOM to the soil C pool and ¹³C fractionation. Despite 30 years of groundnut (C₃) cultivation in upslope plots, and rice, millet-sorghum-sorghum, millet-sorghum, millet, and millet-sorghum-groundnut (C₄ and C₃) sequences in footslope plots, a slight δ¹³C variation (0.46‰) was detected between upslope (-18.96‰) and footslope (-18.50‰) plots. This implies the alleviation impact of erosion following the adaptation of upslope ¹³C values into the deposition site. Unlike soil δ¹³C, δ¹⁵N signatures were strongly correlated with spatial distances and enriched at footslope along with enhancement of total C%, total N%, and soil organic matter% (SOM%) at the depositional site. This study has implications for sequestering SOM by foreseeing potent SOM turnover under erosion and accelerated erosion.