Relationships of Nitrous Oxide Emissions to Fertilizer Nitrogen Recovery Efficiencies in Rain-fed and Irrigated Corn Production Systems: Data Review

Relationships between cumulative nitrous oxide (N₂O) emissions and N recovery efficiency (NRE) during corn production remain largely unknown, perhaps because N₂O emission and NRE are seldom measured and/or reported for the same experiments. Appropriate regression models were applied to published N₂O emission data (treatment mean N₂O; n = 338) collected at sites where corn whole-plant N uptake at maturity (published and unpublished) was also determined. The data came from rainfed and irrigated corn cropping systems in four U.S. states and two Canadian provinces.

Our primary objective was to assess the relationships between growing season cumulative N₂O emission and total aboveground N uptake (NU), NRE (% N uptake relative to applied N) or net N balance (NB; difference between the grain/total above ground N removal at harvest and applied + rotation N) in the context of the 4R N management factors (rate, source, timing, and placement). Cumulative N₂O emissions were calculated from thrice-weekly, twice-weekly, weekly, and sometimes bi-weekly N₂O emissions measured following N application(s).

The relationships between cumulative N₂O and total NU, NRE, and NB ranged widely and the strength (regression r²) of the relationships depended on how cumulative N₂O emissions were expressed (whether area-based, yield-based, or as relative N₂O - % of site-year maximum). Across locations and N sources (predominantly UAN, UAN+nitrapyrin, urea, SuperU, ESN), the relationships between area-scaled N₂O and either NU or NRE were generally weak (r² = <0.04). However, positive relationships existed between cumulative N₂O and N rate (r²=0.40).

Overall, across N rates, sources and timings, the regression models indicated that area-scaled N₂O response to N management systems was more related to NB than to any other plant N factor. Net NB accounted for 19% of the overall 29% of N₂O emission variability that was explained by corn-plant-N based parameters. Despite weak relationships between N₂O and NRE when averaged across management systems, the relationship between N uptake-scaled N₂O and NRE improved when N rate was below 200 kg N ha^-1 (r² = 0.17) instead of above 200 kg N ha^-1.
We conclude that if 4R N management improves whole-plant N uptake and grain N removal, the smaller soil mineral N pool that remains should reduce seasonal N₂O emissions. Most enhanced-efficiency N sources also reduced N₂O emissions whether or not plant N uptake improved. However, substantially more N₂O emission research that simultaneously targets whole-plant N responses to N management systems is needed before more definitive conclusions are possible.