Achieving Lower Nitrogen Balance and Higher Nitrogen Recovery Efficiency Reduces Nitrous Oxide Emissions in North America's Maize Cropping Systems.

Few studies have assessed the common, yet unproven, hypothesis that an increase of plant nitrogen (N) uptake and/or recovery efficiency (NRE) will reduce nitrous oxide (NO) emission during crop production. Understanding the relationships between NO emissions and crop N uptake and use efficiency parameters can help inform crop N management recommendations for both efficiency and environmental goals. Analyses were conducted to determine which of several commonly used crop N uptake-derived parameters related most strongly to growing season NO emissions under varying N management practices in North American maize systems. Nitrogen uptake-derived variables included total aboveground N uptake (TNU), grain N uptake (GNU), N recovery efficiency (NRE), net N balance (NNB) in relation to GNU [NNB] and TNU [NNB], and surplus N (SN). The relationship between NO and N application rate was sigmoidal with relatively small emissions for N rates <130 kg ha, and a sharp increase for N rates from 130 to 220 kg ha; on average, NO increased linearly by about 5 g N per kg of N applied for rates up to 220 kg ha. Fairly strong and significant negative relationships existed between NO and NRE when management focused on N application rate ( = 0.52) or rate and timing combinations ( = 0.65). For every percentage point increase, NO decreased by 13 g N ha in response to N rates, and by 20 g N ha for NRE changes in response to rate-by-timing treatments. However, more consistent positive relationships ( = 0.73-0.77) existed between NO and NNB, NNB, and SN, regardless of rate and timing of N application; on average NO emission increased by about 5, 7, and 8 g N, respectively, per kg increase of NNB, NNB, and SN. Neither N source nor placement influenced the relationship between NO and NRE. Overall, our analysis indicated that a careful selection of appropriate N rate applied at the right time can both increase NRE and reduce NO. However, NO reduction benefits of optimum N rate-by-timing practices were achieved most consistently with management systems that reduced NNB through an increase of grain N removal or total plant N uptake relative to the total fertilizer N applied to maize. Future research assessing crop or N management effects on NO should include N uptake parameter measurements to better understand NO emission relationships to plant NRE and N uptake.