Multifactor controls on terrestrial N ₂O flux over North America from 1979 through 2010

Nitrous oxide (N ₂O) is a potent greenhouse gas which also contributes to the depletion of stratospheric ozone (O ₃). However, the magnitude and underlying mechanisms for the spatiotemporal variations in the terrestrial sources of N ₂O are still far from certain. Using a process-based ecosystem model (DLEM - the Dynamic Land Ecosystem Model) driven by multiple global change factors, including climate variability, nitrogen (N) deposition, rising atmospheric carbon dioxide (CO ₂), tropospheric O ₃ pollution, N fertilizer application, and land conversion, this study examined the spatial and temporal variations in terrestrial N ₂O flux over North America and further attributed these variations to various driving factors. From 1979 to 2010, the North America cumulatively emitted 53.9 ± 0.9 Tg N ₂O-N (1 Tg = 10 ¹² g), of which global change factors contributed 2.4 ± 0.9 Tg N ₂O-N, and baseline emission contributed 51.5 ± 0.6 Tg N ₂O-N. Climate variability, N deposition, O ₃ pollution, N fertilizer application, and land conversion increased N ₂O emission while the elevated atmospheric CO ₂ posed opposite effect at continental level; the interactive effect among multiple factors enhanced N ₂O emission over the past 32 yr. N input, including N fertilizer application in cropland and N deposition, and multi-factor interaction dominated the increases in N ₂O emission at continental level. At country level, N fertilizer application and multi-factor interaction made large contribution to N ₂O emission increase in the United States of America (USA). The climate variability dominated the increase in N ₂O emission from Canada. N inputs and multiple factors interaction made large contribution to the increases in N ₂O emission from Mexico. Central and southeastern parts of the North America - including central Canada, central USA, southeastern USA, and all of Mexico - experienced increases in N ₂O emission from 1979 to 2010. The fact that climate variability and multi-factor interaction largely controlled the inter-annual variations in terrestrial N ₂O emission at both continental and country levels indicate that projected changes in the global climate system may substantially alter the regime of N ₂O emission from terrestrial ecosystems during the 21st century. Our study also showed that the interactive effect among global change factors may significantly affect N ₂O flux, and more field experiments involving multiple factors are urgently needed.