Mechanisms of cover crop-derived carbon sequestration in winter wheat fields: Insights from 13C labeling

Abstract

Cover cropping is an effective agricultural management strategy for enhancing soil organic carbon (SOC) sequestration and mitigating climate change. However, the contribution of different cover crop species to individual carbon (C) fractions in soil remains unclear. An in-situ decomposition experiment using 13C-labeled residues of soybean (SB) or sudangrass (SG), along with a control with no residue (CK), was designed to explore the dynamics of residue decomposition, distribution of cover crop-derived C into aggregate-protected and -unprotected C, and the sequestration mechanisms of these fractions. The aggregate-protected C included intra-aggregate particulate organic C (iPOC) and mineral-associated organic C (MAOC), and aggregate-unprotected C included coarse particulate organic C (cPOC) and free fine particulate organic C (fPOC). The amount and rate of cover crop residue C mineralization were greater in SB than in SG across all wheat-growing stages. The SB increased large macroaggregate (>2 mm) compared to SG during the early wheat growth stages. The aggregate-protected C fractions were greater in SB and SG than CK at the pre-sowing, tillering, and heading stages. The 13C labeling indicated that C sequestration occurred primarily as aggregate-protected C, predominantly as MAOC. The recovery efficiencies of cover crop-derived C into soil C fractions fell below 0 % at green-up and jointing stages. At maturity stage, the cumulative C recovery rate of cover crop-derived C into SOC was greater in SB (16.3 %) than in SG (8.76 %). Correlation analysis indicated that cover cropping promoted SOC sequestration primarily and directly by increasing the aggregate-protected C. Structural equation model analysis suggested that SG sequestered C into soils primarily by increasing cPOC and iPOC, In contrast, SB sequestered C by increasing cPOC, iPOC, and MAOC. This study elucidates the dynamic effects of cover cropping on soil C during wheat growth and the distinct C sequestration mechanisms in legume and non-legume systems.