Nitrous oxide and nitric oxide budgets
Balancing nitrous oxide sources and sinks to develop a global nitrous oxide budget (Table 20.2) has been a major challenge for researchers. Soils from natural and agricultural ecosystems account for more than half of global nitrous oxide emissions. The remaining nitrous oxide is mostly produced in the oceans, by fossil fuel combustion and industrial processes, and from biomass burning (e.g., forest fires). Nitrous oxide is removed from the atmosphere mostly by photochemical reactions in stratosphere, which is a slow process: the average residence time of an N2O molecule is more than 100 years. Soil microbes can remove small amounts of atmospheric nitrous oxide under some circumstances, but this is usually counterbalanced by much greater rates of nitrous oxide production.
A relatively consistent annual increase in atmospheric N2O accumulation (~4 Tg year–1 of N2O-N) has occurred over the last several decades. This observed increase agrees with the difference between our global estimates of sources (~17 Tg N year–1) and sinks of nitrous oxide (13 Tg N year–1; Table 20.2). Thus, at the global scale, our “bottom-up” estimates of nitrous oxide emissions from multiple sources and removal pathways approximately match “top down” observations of atmospheric nitrous oxide. However, at smaller scales these estimates can disagree considerably. One widely used approach for estimating bottom-up emissions involves emissions factors, for which a given fraction of nitrogen inputs (e.g., 1% of synthetic fertilizer) or ecosystem nitrogen pools (e.g., 1% crop residues) is predicted to be released from soil as nitrous oxide. The United States Corn Belt is a region of intensive agriculture that receives large inputs of reactive nitrogen from fertilizer and biological nitrogen fixation. Top-down measurements in this region show much greater atmospheric nitrous oxide concentrations than can be accounted for by soil nitrous oxide emissions estimated from emissions factors. Small streams that drain from agricultural watersheds can produce very high nitrous oxide emissions from denitrification and could thus account for at least part of this missing source (Griffis et al., 2017). However, long-term measurements have also shown that high nitrous oxide emissions from agricultural soils also contribute to regional underestimates by default emissions factors (Gillette et al., 2018). Improving our capacity to estimate nitrous oxide emissions at the site and regional scales remains an active area of research that is critical to inform policy and management for decreasing the emissions of this potent greenhouse gas.
In contrast to nitrous oxide, nitric oxide is a very short-lived compound in the atmosphere (hours–weeks), and its budget is even harder to estimate than for nitrous oxide. Based on extrapolation from field measurements of soil nitric oxide emissions, global nitric oxide emissions from soil average 21 Tg NO-N year–1 but with large uncertainty spanning 4−10 Tg NO-N year–1 (Davidson and Kingerlee, 1997).