Long-term agricultural practice effects on carbon and nitrogen isotopes of soil organic matter fractions

Understanding the effects of long-term traditional and alternative agricultural management practice effects on carbon (C) and nitrogen (N) cycling and storage within particulate organic matter (POM) and light fractions (LF) within various soil aggregate-size classes can be illuminated by isotopic C-13/C-12 (delta C-13) and N-15/N-14 (delta N-15) differences. The objective of this study was to evaluate the effects of residue level, residue burning, tillage, and irrigation on delta C-13 and delta N-15 values of the bulk-soil, macro- (>250 mu m) and micro-aggregate-(53-250 mu m), coarse- (>250 mu m), and fine- (53-250 mu m) POM, and coarse- and fine-LF in the top 10 cm following 13 yr of consistent management in a wheat (Triticum aestivum L.)-soybean [Glycine max (L.) Merr.] double-crop system on a silt-loam soil in eastern Arkansas. Various treatment combinations affected (p < .05) delta C-13 values within the bulk-soil and fine-POM, as well as delta N-15 values within the bulk-soil, macro-aggregate, coarse-LF, and fine-LF fractions. Averaged across all other field treatments, macro-aggregate delta N-15 was greater (p < .01) in the no-tillage (NT)-low- (3.23%) compared with NT-high-residue (3.05%) and CT-high- and low-residue combination, which did not differ and averaged 3.11%, indicating that more labile residue can be achieved in the NT-high-residue treatment combination. Results showed significant variations in aggregate-associated delta C-13 and delta N-15, as affected by long-term residue and water management practices that would otherwise not have been evident from simple, bulk-soil analysis or a short-term field study.