Rock Fragments Control Size and Saturation of Organic Carbon Pools in Agricultural Topsoil

Rock fragments (RF) may severely reduce the amount of fine earth (FE) per soil volume. Here we quantified the effect of variable RF on spatial patterns of bulk soil organic carbon (SOC), particulate organic matter (POM) of three size classes, nonparticulate organic matter (nonPOM), and black carbon (BC). One hundred and twenty-nine soil samples (0-30 cm) were taken in a regular grid on an arable field nearby Selhausen (Germany). The weakly inclined site featured a strong gradient in RF, soil types covered Alfic Eutrochepts, as well as Typic and Albaquic Hapludalfs. In addition to C-pool patterns and RF we determined texture, Fe oxides, soil moisture, as well as hillslope and elevation above sea level. Additionally, Cs-137 measurements were conducted to indicate soil erosion. The reduction of SOC stocks by increasing amounts of RF on a percentage basis was smaller than attributable to mere dilution effects. Multiple regression analysis indicated BETA values of 0.65 to 0.73 between RF and FE contents of BC, nonPOM, and the finest POM fraction. The C contents of the coarse POM fractions were positively correlated with the amount of RF (R-2 = 0.72-0.78) and their spatial patterns were assumed to be caused by disproportionate input of plant litter to a reduced solum when the amount of RF increased. Statistical structure analysis and variography pointed to hierarchical C saturation of nonPOM and the finest POM fraction, which was additionally regulated by Fe oxide contents. Only the spatial distribution of BC (BETA = -0.15) was affected by erosion.