Achieving lower nitrogen balance and higher nitrogen recovery efficiency reduces nitrous oxide emissions in North America’s maize cropping systems

Abstract

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 (N2 O) emission during crop production. Understanding the relationships between N2 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 N2 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(GNU) ] and TNU [NNB(TNU) ], and surplus N (SN). The relationship between N2 O and N application rate was sigmoidal with relatively small emissions for N rates <130kg ha−1, and a sharp increase for N rates from 130 to 220kg ha−1; on average, N2 O increased linearly by about 5g N per kg of N applied for rates up to 220kg ha−1. Fairly strong and significant negative relationships existed between N2 O and NRE when management focused on N application rate (r2 = 0.52) or rate and timing combinations (r2 = 0.65). For every percentage point increase, N2 O decreased by 13g N ha−1 in response to N rates, and by 20g N ha−1 for NRE changes in response to rate-by-timing treatments. However, more consistent positive relationships (R2 = 0.73–0.77) existed between N2 O and NNB(TNU), NNB(GNU), and SN, regardless of rate and timing of N application; on average N2 O emission increased by about 5, 7, and 8g N, respectively, per kg increase of NNB(GNU), NNB(TNU), and SN. Neither N source nor placement influenced the relationship between N2 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 N2 O. However, N2 O reduction benefits of optimum N rate-by-timing practices were achieved most consistently with management systems that reduced NNBthrough an increase of grain Nremoval or total plant Nuptake relative to the total fertilizer N applied to maize. Future research assessing crop or N management effects on N2 Oshould include Nuptake parameter measurements to better understand N2 O emission relationships to plant NRE and N uptake.