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

Mitra Ghotbi1, Knief Claudia2, Horwath William R.3
1 University of California Davis, Plant and Environmental Sciences Building, Dept. Land, Air & Water Resources and University of Bonn, Institute of Crop Science and Resource Conservation, Molecular Biology of the Rhizosphere
2 University of Bonn, Institute of Crop Science and Resource Conservation, Molecular Biology of the Rhizosphere
3 University of California Davis, Plant and Environmental Sciences Building, Dept. Land, Air & Water Resources

P 3.1 in What goes around comes around - Biogeochemical cycling of Iron, Sulfur & Carbon in the Environment

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.



Keywords: soil δ13C and δ15N, SOM turnover, eroding site, topography, conventional management practices
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