Tillage and Nitrogen Effects on Soil Organic Matter Fractions in Wheat‐based Systems

Management practices that alter plant residue production and distribution influence SOC (soil organic carbon) dynamics. The objectives of this study were to investigate the impacts of tillage, cropping system, and N fertilization on SOC and soil N pools through physical fractionation of a central Texas soil after 20 yr. Nitrogen fertilization and no‐tillage (NT) significantly increased wheat ( Triticum aestivum L.) straw yield. Compared with conventional tillage (CT), SOC under NT in surface (0–5‐cm) samples was 38, 69, and 68% greater for continuous wheat (CW), wheat–soybean [ Glycine max (L.) Merr.]–sorghum [ Sorghum bicolor (L.) Moench] rotation (SWS), and double‐cropped wheat–soybean (WS), respectively. The greatest SOC was observed in WS under NT with N fertilization, and the lowest occurred in CW under CT without N. Increased cropping intensity increased SOC compared with monoculture. Nitrogen fertilization only significantly increased SOC sequestration under NT. No‐tillage increased SOC concentration in all physical size fractions compared with CT. Increased cropping intensity and N fertilization significantly increased SOC sequestration in most size fractions only under NT. Intraparticulate organic matter C (IPOM‐C) was proportionally more affected by tillage than total SOC, indicating that this fraction was more sensitive to management. Carbon concentrations in all size fractions were significantly correlated with each other as well as SOC. Our results indicated that NT associated with enhanced cropping intensity and N fertilization sequestered greater SOC and soil total N.