AUTHOR=Omonode Rex A. , Halvorson Ardell D. , Gagnon Bernard , Vyn Tony J. 

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

JOURNAL=Frontiers in Plant Science

VOLUME=8

YEAR=2017

URL=https://www.frontiersin.org/journals/plant-science/articles/10.3389/fpls.2017.01080

DOI=10.3389/fpls.2017.01080

ISSN=1664-462X

ABSTRACT=<p>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 (N<sub>2</sub>O) emission during crop production. Understanding the relationships between N<sub>2</sub>O 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 N<sub>2</sub>O 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<sub>(GNU)</sub>] and TNU [NNB<sub>(TNU)</sub>], and surplus N (SN). The relationship between N<sub>2</sub>O and N application rate was sigmoidal with relatively small emissions for N rates &lt;130 kg ha<sup>−1</sup>, and a sharp increase for N rates from 130 to 220 kg ha<sup>−1</sup>; on average, N<sub>2</sub>O increased linearly by about 5 g N per kg of N applied for rates up to 220 kg ha<sup>−1</sup>. Fairly strong and significant negative relationships existed between N<sub>2</sub>O and NRE when management focused on N application rate (<italic>r</italic><sup>2</sup> = 0.52) or rate and timing combinations (<italic>r</italic><sup>2</sup> = 0.65). For every percentage point increase, N<sub>2</sub>O decreased by 13 g N ha<sup>−1</sup> in response to N rates, and by 20 g N ha<sup>−1</sup> for NRE changes in response to rate-by-timing treatments. However, more consistent positive relationships (<italic>R</italic><sup>2</sup> = 0.73–0.77) existed between N<sub>2</sub>O and NNB<sub>(TNU)</sub>, NNB<sub>(GNU)</sub>, and SN, regardless of rate and timing of N application; on average N<sub>2</sub>O emission increased by about 5, 7, and 8 g N, respectively, per kg increase of NNB<sub>(GNU)</sub>, NNB<sub>(TNU)</sub>, and SN. Neither N source nor placement influenced the relationship between N<sub>2</sub>O 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 N<sub>2</sub>O. However, N<sub>2</sub>O 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 N<sub>2</sub>O should include N uptake parameter measurements to better understand N<sub>2</sub>O emission relationships to plant NRE and N uptake.</p>