Nitrogen transfer from cover crops to the subsequent grain crop and the influence of variability in winter conditions

Spencer Heuchan, The University of Western Ontario

Abstract

Cover crops, which are used to provide ground cover after the harvest of the grain crop, can potentially improve the sustainability of agroecosystems by reducing nutrient losses. However, few data are available to document the extent to which cover crops improve both the retention of soil nitrogen (N) and the transfer of this N to the grain crop. The efficiency of this N transfer may be further influenced by variation in winter soil temperature; for example, reduced snow cover can increase the frequency and intensity of soil freezing, which can affect the survival of cover crops and the timing of the decomposition of their residues. I quantified N transfer from cover crops (legumes, non-legumes and mixtures) to the subsequent corn crop using ¹⁵N tracer. Residue swapping was used to isolate the individual contributions of the aboveground and belowground N components of the cover crops. N transfer responses to soil temperature variability over the winter were examined via snow removal and pulsed warming from overhead heaters. My results revealed that the belowground N pool contributed substantially more to N retention and N transfer than the aboveground N pool. However, less than 10% of the ¹⁵N added to the soil was transferred to the corn, while the majority remained in the soil. In addition, increased soil freezing reduced the effectiveness of the cover crops in transferring N to the corn, with legumes being more susceptible to N losses than non-legumes. Year-to-year variability in cover crop establishment and productivity also had strong effects on the effectiveness of the cover crops in retaining and transferring N. Overall, my results reveal that while the scavenging of N by cover crops after harvest of the main crop may not provide a substantial contribution of N to the grain crop the following year, this scavenging may be important for reducing N losses to the surrounding environment.