Soil Organic Carbon and Isotope Composition Response to Topography and Erosion in Iowa

Soil redistribution (erosion and deposition) can greatly affect the fate of soil organic carbon (SOC) in agroecosystems. Landscape topography is one of the key factors controlling erosion processes and creating spatial variability in SOC. We combined carbon (C) isoscape (isotopic landscape) analysis, historic orthophoto interpretation, cesium ( 137 Cs) inventory measurement, and digital terrain analysis to quantify SOC dynamics and soil redistribution relationship and their responses to landscape topography in an Iowa cropland field with soybean/maize (C3/C4) rotation. The historic orthophotos and 137 Cs were used to reflect soil redistribution before and after the 1960s, respectively. Topography‐based models were developed to simulate 137 Cs inventory, SOC density, and C isotopes using stepwise principal component regression. Spatial patterns of SOC were similar to soil erosion/deposition patterns with high SOC density in depositional areas and low SOC density in eroded areas. Soil redistribution, SOC density, and isotopic signature of SOC (δ 13 C) were highly correlated with topographic metrics, suggesting that topographic heterogeneity drove the spatial variability in erosion and SOC dynamics. Considering the isotopic composition of SOC, C3‐derived SOC density was strongly controlled by topographic metrics, but C4‐derived SOC density showed weaker expression of spatial pattern and poor correlation to topographic parameters. The resulting topography‐based stepwise principal component regression models captured more than 60% of the variability in SOC density, δ 13 C , and C3‐derived SOC density but could not reliably predict C4‐derived SOC density. Our results indicate that exploring C isotopes in response to soil erosion is important to understand the fate of eroded SOC within croplands under C3/C4 cultivation.