Abstract
Nitrous oxide (N2O) is a major greenhouse gas and cultivated soils are the most important anthropogenic source. N2O production and consumption are known to occur at depths below the A or Ap horizon, but their magnitude in situ is largely unknown. At a site in SW Michigan, USA, we measured N2O concentrations at different soil depths and used diffusivity models to examine the importance of depth-specific production and consumption. We also tested the influence of crop and management practices on subsurface N2O production in (1) till versus no-till, (2) a nitrogen fertilizer gradient, and (3) perennial crops including successional vegetation. N2O concentrations below 20 cm exceeded atmospheric concentrations by up to 900 times, and profile concentrations increased markedly with depth except immediately after fertilization when production was intense in the surface horizon, and in winter, when surface emissions were blocked by ice. Diffusivity analysis showed that N2O production at depth was especially important in annual crops, accounting for over 50% of total N2O production when crops were fertilized at recommended rates. At nitrogen fertilizer rates exceeding crop need, subsurface N2O production contributed 25–35% of total surface emissions. Dry conditions deepened the maximum depth of N2O production. Tillage did not. In systems with perennial vegetation, subsurface N2O production contributed less than 20% to total surface emissions. Results suggest that the fraction of total N2O produced in subsurface horizons can be substantial in annual crops, is low under perennial vegetation, appears to be largely controlled by subsurface nitrogen and moisture, and is insensitive to tillage.
Highlights
Nitrous oxide (N2O) is a major greenhouse gas responsible for stratospheric ozone depletion
Total annual N2O emissions interpolated from chamber measurements and calculated from soil N2O concentration profiles were correlated for the three experiments (Figure 8)
Results suggest that subsurface sources of N2O are important in annual cropping systems whether rainfed or irrigated, and that subsurface conditions should be included when designing practices to minimize greenhouse gas emissions from agricultural soils
Summary
Nitrous oxide (N2O) is a major greenhouse gas responsible for stratospheric ozone depletion. N2O can be produced and consumed at depths deeper than the A or Ap horizons (Clough and others 2006), though little is known about the quantitative importance of such production. Evidence for significant subsurface N2O production is largely inferential, consisting mainly of sharp increases in N2O concentrations with depth. Van Groenigen and others (2005a) and Goldberg and Gebauer (2009) observed N2O concentrations 20–30 times those of the free atmosphere at subsurface sampling points of 50–90 cm. N2O dissolved in groundwater can substantially exceed atmospheric equilibrium concentrations (McGill and others 2018). Rates of subsurface N2O production have not been measured in situ
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