Decrease of soil organic matter stabilization with increasing inputs: Mechanisms and controls

Crop residue addition is a way to increase soil organic matter (SOM) level in croplands. However, organic matter input and SOM stocks are not linearly related. Consequently, adding high amounts of residues, such as straw, may increase SOM to only a small extent, and an alternative use of the residues may be justified. The objective of this study was to test how the level and type (above- or belowground) of residue addition affect SOM stabilization. We hypothesise that (1) root residues will be mineralised slower than leaf and stalk residues, (2) soil aggregate formation will increase with high additions, and (3) wheat residue addition will induce positive priming, with the magnitude depending on the residue level and type. Homogeneously 13C-labelled wheat residues (leaves, stalks, roots) were added to a silt-loam soil at levels of 1.40 and 5.04 g DM kg− 1 and CO2 release and δ13C signature were measured over 64 days at 20 °C. Water-stable macroaggregates (> 250 μm), microaggregates (53–250 μm) and silt plus clay size fractions (