Long-Term C and N Cycling in Response to Changes in Harvest Intensity and N Fertilization.

The dryland crop residue (DCR) study at Lethbridge was devised to investigate carbon and nitrogen cycling under rain-fed spring wheat as influenced by changes in above-ground residues annually returned to the soil.  Since adoption of combine harvesters in the 1940's, annual crop production in western Canada has been based on the harvest and export of cereal grains and oilseeds.  The stems, leaves and chaff (viz. ‘crop residues') typically were returned to the land where they were grown and the seeds were harvested and exported from the region.  Since the 1990's, however, pressures to expand the harvest to include some portion of the non-seed components have increased with demands for livestock bedding and feed, and more recently, biofuels and other bio-products.  Consequently, this experiment was initiated in 1999 to assess the consequences of crop residue removal for soil quality, and to evaluate the long-term retention of plant carbon and nitrogen as soil organic matter.  Spring wheat was grown continuously in a study with three contrasting residue inputs (removed, returned or supplemented), two fertilizer N levels (0 or 45 kg N ha^-1 yr^-1), and six replications.  Without fertilizer, grain yields were roughly 60% of those receiving fertilizer N, indicating that soil N was insufficient.  Above-ground residues yields also increased with N fertilization, but residue manipulation treatments had an even greater influence on the amounts returned to the soil.  Changes in soil C and N stocks were quantified after the treatments had been in place for ten years.  Soil C and N increased in response to increased inputs of above-ground crop residues, but the vast majority of the residue C presumably was decomposed and emitted back to the atmosphere as CO₂.  This study is providing valuable data for testing assumptions in models of C and N cycling, and to evaluate the potential for harvesting greater portions of above-ground plant production.