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.

Automated summary

Long-term traditional and alternative agricultural management practices, as well as different treatment combinations, have significant effects on carbon and nitrogen cycling and storage in soil. Isotopic analysis can reveal these effects. The study shows that long-term residue and water management practices can alter the isotopic composition of particulate organic matter and light fractions in soil, which may not be apparent from simple bulk soil analysis or short-term field studies.