Multi-year comparisons of shoot and root decomposition dynamics of Italian ryegrass (Lolium multiflorum) under soybean cropping

Abstract

Cover crop residue (shoot or root) decomposition regulates nutrient cycling and soil organic matter in crop rotation. This study examined the decomposition and nutrient release of Italian ryegrass (Lolium multiflorum, IRG) shoot and root residues incorporated into soil under an IRG-soybean (Glycine max L.) rotation for three years. IRG shoot and root litterbags were incorporated into the soil, and retrieved over 140 days. Dry matter, C, and N fitted asymptotic single-exponential models to residue remaining, and rise-to-maximum models to cumulative C and N release. Initial residue quality and weather covariates supported interpretation. Shoot biomass ranged from 4.43 to 6.92 t ha-1 , which was approximately 2.7-5.3 times greater than root. Moreover, shoot had higher nitrogen concentrations (12.8-20.1 g kg-1) and consistently exhibited lower C/N ratio and lignin/N ratios. The IRG shoot residues decomposed 2.2-2.5 times faster and released up to average 60 kg N ha-1 within 40 days, matching the initial N demand of the subsequent crop. In contrast, root residues decomposed dry matter and C slowly, released negligible N, but contributed to sustained C (129 kg ha-1), indicating potential for soil organic matter stabilization. These results suggest the shoot residue serves as an immediate N source for the subsequent crop and the roots contribute to long-term soil C sequestration in the field. In tilled IRG-soybean systems, incorporating both plant parts have impacts: shoot residues supply starter N and root residues add a slower, more persistent C input that supports soil organic matter accumulation.