Tracing nitrogen transfer from aboveground and belowground cover crop residues to subsequent grain crops over the winter season

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Spencer M. Heuchan and Hugh A.L. Henry, Biology, University of Western Ontario, London, ON, Canada, Claudia Wagner-Riddle, Environmental Sciences, University of Guelph, Guelph, ON, Canada

Background/Question/Methods
Effective nitrogen (N) fertilizer management is crucial for the reduction of environmental pollutants associated with agricultural systems. 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, and by increasing plant biodiversity. However, the overall success of cover crops in reducing N losses and benefiting the yield of the subsequent grain crop is contingent on the release of N from decomposing cover crop residues being well-synchronized with the grain crop’s N demand. The efficiency of this N transfer may be influenced by variation in winter temperatures, which can affect the survival of cover crops and the timing of the decomposition of their residues. Few data are available to establish how cover crops alter the dynamics of N losses vs. N transfer to the grain crop over the winter. It is also unclear how the aboveground residues of cover crops contribute to this N transfer relative to the belowground components.
Our objective was to quantify N transfer from cover crops to the subsequent corn crop over winter. We tested both cover crops monocultures (oats, red clover) and cover crop mixtures (3-way, 10-way). Stable 15N labelled fertilizer tracer was used to isolate the N contributions from the above and belowground components of the cover crop residue by reciprocally exchanging the aboveground biomass from microplots enriched in 15N label with that of paired unlabeled plots. Total 15N enrichment in the subsequent corn residue and kernels was used to determine N transfer from the cover crops.
Results/Conclusions
With the exception of the 10-way cover crop mixture, which established poorly in the fall, all of the 15N-labelled cover crop aboveground residues transferred to unlabeled plots increased the 15N-enrichment in the corn kernels significantly, with the clover and oat monocultures providing the greatest enrichment. For the belowground 15N-labelled cover crop plots, only the clover monoculture provided significant 15N-enrichment of the corn kernels. However, the latter was approximately five times higher than the enrichment provided by the 15N-labelled cover crop aboveground residues. Overall, our results indicate that cover crop monocultures were more effective at transferring 15N to the subsequent corn crop, primarily because the cover crop mixtures contained some species that established poorly, thus reducing the overall cover crop productivity (and capacity for 15N retention and transfer) in the mixed plots.