Implications of crop residue management and conservation tillage on soil organic matter

Under natural grassland or native prairie, aboveground residue or surface litter modifies the microenvironment. It promotes water infiltration and, by insulating the soil surface, moderates soil temperatures and limits evaporation. Root mass decomposes and transforms within the conditions created by surface litter. Together with root exudates, this below-ground residue or subsurface litter reacts with soil minerals to form aggregates, lower bulk density and increase water-holding capacity. Bringing such soils under cultivation leads to lower soil organic matter content, thereby increasing bulk density. The role of surface litter becomes even more important, as it affects wind and water erosion, reduces the impact of raindrops, prevents crusting, protects the soil from drying by sublimation, and captures snow. Management of crop residues depends on the role of the residue. A distinction must be made between above- and below-ground residues: their roles are distinctly different. Aboveground crop residue protects the soil and creates the conditions for below-ground residue to decompose and transform. These decomposition products, in turn, create favourable soil structure for plant growth. Research is needed on the effect of repeated harvesting of "excess" aboveground residues. Key words: Labile organic matter, resilience, resistance, surface litter, subsurface litter