Management practices differently affect particulate and mineral-associated organic matter and their precursors in arable soils

Our understanding of soil organic matter (SOM) formation and stabilization mechanisms has evolved recently. The SOM is considered as a continuum from coarse fresh and decomposing plant residues (particulate organic matter, >53 mu m; POM) to fine and presumably more stable mineral-associated organic matter (fine organic matter, <53 mu m; FOM), which represents the major portion of total SOM. Current theoretical models call for a prominent role of microbial by-products in FOM formation. However, there is still relatively little field-based evidence to support these theoretical models, especially in arable soils. We examined surface (0-10 cm) soil quality indicators over 7 years in two Brunisols with contrasting textures (silty clay and sandy loam) under twenty different management systems, combining two tillage regimes (minimum tillage vs inversion tillage), two crop residue management (residue retained vs harvested) and five different fertilizer sources (no-N control, complete mineral fertilization, liquid dairy manure, liquid swine manure, solid poultry manure). Surface soil quality indicators included soil organic carbon (SOC), POM-C, FOM-C, microbial biomass-C (MBC), plant- and microbe-derived carbohydrates, and mean-weight diameter of water-stable aggregates (MWD). An univariate statistical analysis of these indicators was combined with a principal component analysis (PCA) to explore the relationships among soil quality indicators and SOC dynamics. In both soil types, POM-C was closely related to MWD and plant-derived carbohydrates, whereas FOM-C was closely related to microbial amino-sugars. These results support the recent conceptual models mainly attributing the formation of FOM-C to microbial by-products. We found that management practices that favored POM-C formation, such as reduced tillage and residue retention, were particularly efficient to increase surface SOC content, irrespective of soil texture. By contrast, manuring had a pronounced effect on microbial biomass, which resulted in the accumulation of microbial amino sugar and SOC, in the silty clay soil only. Use of N-rich organic amendments with low lignin content, such as manures, could therefore be an efficient way to foster the formation of stable SOC through the accumulation of microbe-derived FOM precursors in arable soils where clay content is sufficient to allow for the stabilization of microbial by-products.