Carbon‐13 and Nitrogen‐15 Natural Abundance in Crop Residues and Soil Organic Matter

Abstract

AbstractSpatial variability in the 13C and 15N natural abundance of plant residues (δ13Cresidue and δ15Nresidue) and soil (δ13Csoil and δ15Nsoil) at the landscape scale remains largely unexplored. This 2‐yr study assessed landscape‐scale patterns and investigated topographic and soil factors that control spatial variability. Low values of δ13Cresidue were associated primarily with the lower level and footslope elements, with significantly (P ≤ 0.001) higher (less negative) values in the upper level and shoulder elements. Strong similarities in the spatial patterns observed for δ13Cresidue and δ13Csoil indicate that the 13C content of soil organic matter is largely dependent on δ13Cresidue. Although the δ13C of the crop residues exhibited marked year‐to‐year variability, which reflected genetic differences and variations in growing season precipitation and perhaps N‐fertilization regime, landscape‐scale patterns for δ13Cresidue were similar in both years. These data support the hypothesis that δ13Cresidue varies in response to environmental conditions and also suggest that topographic or hydrologic controls are at work. The presence of a landscape‐scale pattern in δ13Cresidue and δ13Csoil may be expected to have a confounding effect on studies that employ the 13C natural abundance method to detect changes in plant species composition or to estimate the rate of soil organic matter turnover. Spatial patterns for δ15Nresidue and δ15Nsoil indicated that 15N was distributed in an essentially random pattern.