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

Abstract

Abstract 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 13C/12C (δ13C) and 15N/14N (δ15N) differences. The objective of this study was to evaluate the effects of residue level, residue burning, tillage, and irrigation on δ13C and δ15N values of the bulk‐soil, macro‐ (>250 μm) and micro‐aggregate‐(53–250 μm), coarse‐ (>250 μm), and fine‐ (53–250 μm) 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) δ13C values within the bulk‐soil and fine‐POM, as well as δ15N values within the bulk‐soil, macro‐aggregate, coarse‐LF, and fine‐LF fractions. Averaged across all other field treatments, macro‐aggregate δ15N 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 δ13C and δ15N, 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.