Agricultural practices for crop residue transformation into soil organic matter in cold humid temperate agroecosystems

Understanding how agricultural practices affect crop residue retention in soil organic matter (SOM) fractions is necessary to apply strategies that maintain or increase the SOM level in cold humid temperate agroecosystems. This review describes how SOM fractions, i.e. physically uncomplexed, aggregate-occluded and mineral-associated SOM, are influenced by crop residue inputs, considering that the amount of residue and its residence time within each fraction is impacted by climate, edaphic properties and residue characteristics. We also investigate how agricultural practices may improve residue retention. In cold humid temperate regions, crop residues can account for 17-62% of the light fraction (as physically uncomplexed SOM), 20-60% of aggregate-occluded SOM and a considerable amount of mineral-associated SOM, depending on the time of year when samples are collected. Crop residue retention within each fraction is strongly influenced by clay content and mineralogy. Physico-chemical characteristics of crop residues affect their transformation into SOM fractions. Agricultural practices within a cropping system, such as tillage, fertilization and residue management, determine the amount and timing of residue introduction to the agroecosystem, as well as the quantity of residues that are retained in the soil. Practices leading to residue retention within mineral-associated SOM, which has the longest residence time, are important to sustain the SOM pool. We recommend agricultural practices that build and sustain the aggregate-occluded and mineral-associated SOM reserves, including tillage practices that promote residue retention, judicious fertilization and continuous cropping.