Net soil–atmosphere fluxes mask patterns in gross production and consumption of nitrous oxide and methane in a managed ecosystem

Abstract. Nitrous oxide (N₂O) and methane (CH₄) are potent greenhouse gases that are both produced and consumed in soil. Production and consumption of these gases are driven by different processes, making it difficult to infer their controls when measuring only net fluxes. We used the trace gas pool dilution technique to simultaneously measure gross fluxes of N₂O and CH₄ throughout the growing season in a cornfield in northern California, USA. Net N₂O fluxes ranged 0–4.5 mg N m⁻² d⁻¹ with the N₂O yield averaging 0.68 ± 0.02. Gross N₂O production was best predicted by net nitrogen (N) mineralization, soil moisture, and soil temperature (R² = 0.60, n = 39, p< 0.001). Gross N₂O reduction was correlated with the combination of gross N₂O production rates, net N mineralization rates, and CO₂ emissions (R² = 0.74, n = 39, p< 0.001). Overall, net CH₄ fluxes averaged −0.03 ± 0.02 mg C m⁻² d⁻¹. The methanogenic fraction of carbon mineralization ranged from 0 to 0.27 % and explained 40 % of the variability in gross CH₄ production rates (n = 37, p< 0.001). Gross CH₄ oxidation exhibited a strong positive relationship with gross CH₄ production rates (R² = 0.67, n = 37, p< 0.001), which reached as high as 5.4 mg C m⁻² d⁻¹. Our study is the first to demonstrate the simultaneous in situ measurement of gross N₂O and CH₄ fluxes, and results highlight that net soil–atmosphere fluxes can mask significant gross production and consumption of these trace gases.