Impact of crop residues on nutrient availability in conservation tillage systems
Crop residue management is a key consideration when attempting to optimize fertility in conservation tillage systems. Major factors affecting the impact of crop residues on nutrient availability include the chemical composition of the residue [e.g. carbon (C) to nitrogen (N) ratio], residue placement, fertilizer placement in relation to residue and time. Greater surface accumulation of crop residues in reduced and no-till systems tends to slow decomposition of N-poor residues such as cereal straw, and crop N supply during the current year can be reduced by immobilization if the straw is incorporated close to the time of high crop demand. Similarly, placement of fertilizer directly in the surface straw residue can reduce fertilizer use efficiency due to greater immobilization. Greater immobilization in reduced and no-till systems can enhance the conservation of soil and fertilizer N in the long term, with higher initial N fertilizer requirements decreasing over time because of 1) reduced losses by erosion and 2) the build-up of a larger pool of readily mineralizable organic N. For N-rich residues, such as legumes, volatilization losses may be greater when these residues are left on the surface than when incorporated into soil. Leaching of soluble phosphorus and sulfur compounds from standing and surface-placed crop residues into the mineral soil below may be a significant pathway for recycling of these elements in no-till systems. Greater coverage of the soil surface by crop residues can increase soil moisture and affect soil biological activity related to nutrient turnover. Future research should address how above- and below-ground decomposition processes differ for a wider range of crop residues and nutrients, emphasizing both short and long-term nutrient recycling. Key words: Crop residue, no-till, nutrient availability, nutrient cycling