C-13 AND N-15 NATURAL-ABUNDANCE IN CROP RESIDUES AND SOIL ORGANIC-MATTER

Spatial variability in the C-13 and N-15 natural abundance of plant residues (deltaC-13residue and deltaN-15residue) and soil (deltaC-13soil and deltaN-15soil 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 deltaC-13residue were associated primarily with the lower level and footslope elements, with significantly (P less-than-or-equal-to 0.001) higher (less negative) values in the upper level and shoulder elements. Strong similarities in the spatial patterns observed for deltaC-13residue and deltaC-13soil indicate that the C-13 content of soil organic matter is largely dependent on deltaC-13residue. Although the deltaC-13 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 deltaC-13residue were similar in both years. These data support the hypothesis that deltaC-13residue varies in response to environmental conditions and also suggest that topographic or by hydrologic controls are at work. The presence of a landscape-scale pattern in deltaC-13residue and deltaC-13soil may be expected to have a confounding effect on studies that employ the C-13 natural abundance method to detect changes in plant species composition or to estimate the rate of soil organic matter turnover. Spatial patterns for deltaN-15residue and deltaN-15soil indicated that N-15 was distributed in an essentially random pattern.