Benefits of a Preceding Legume Crop for Soil Organic Matter and Microbial Dynamics During Wheat Residue Decomposition

Abstract

The decomposition of crop residues is essential for maintaining soil fertility and can be influenced by agronomic practices. This study assessed how preceding crops (pea or triticale) and fertilisation (mineral fertiliser, untreated digestate, dried acidified digestate, or none) affect wheat residue decomposition and soil biochemical dynamics. A field experiment used litter bags to track residue carbon (C) and nitrogen (N) release at four sampling times over 1 year. Changes in soil microbial biomass C (MBC), enzymatic activity (β-glucosidase, phenol oxidase, protease) and soil organic C (SOC) were monitored, linking biological changes to mineral-associated and particulate organic C (POC) dynamics. The preceding pea enhanced N release from wheat residue mainly by lowering the C:N ratio by 28%. It also contributed to more stable MBC and SOC levels with 78% and 86% smaller declines compared to triticale, respectively. Increased residue N inputs were related to positive changes in MBC, while POC was the main driver for SOC accumulation. The fertilisation effect was more limited, increasing residue quality and N release only for the triticale-preceded wheat. Digestates had no effect on MBC and SOC, potentially due to their low C inputs. Phenol oxidase and protease reflected differences in substrate quality and nutrient availability affected by management. β-glucosidase remained a key indicator of organic matter turnover, evidenced by its strong association with POC. Organic matter stabilisation was associated with enhanced MBC but was not directly promoted by the different microbial strategy caused by preceding pea. These findings enhance understanding of soil biological activity under field conditions, highlighting the long-lasting effect of legumes on soil functioning.