Can Perennial Bioenergy Cropping Systems Promote Negative N2O Fluxes?

Efforts to mitigate nitrous oxide (N2O) emissions from agricultural soils have thus far focused on reducing fluxes from high-emitting systems such as fertilized row crops. An alternative approach may be to promote N2O consumption (or negative N2O fluxes) in those systems where it occurs. Negative N2O emissions can be difficult to identify in situ because fluxes tend to be small and statistically indistinguishable from zero, and thus effectively ignored. They may be especially frequent in low-emitting systems such as perennial bioenergy feedstocks. Automated flux chambers deployed in situ offer the opportunity to better resolve the potential for negative fluxes with near-continuous measurements and low, empirically defined analytical error. Here, we used sub-daily N2O flux data from 2012 to 2017 under nine different bioenergy cropping systems in southwest Michigan USA to explore the frequency and magnitude of negative emissions. Daily, weekly, monthly, and annual flux measurements were calculated individually for positive and negative sub-daily data after first determining a minimum flux detection limit of -1.53 and +1.46 g N2O-N ha-1 d-1. Of 37,000 sub-daily fluxes, ~30% were statistically negative, ranging from -1.53 g N2O-N ha-1 d-1. The weather transitions lowered the magnitude of negative N2O fluxes. Minimum daily temperature and volumetric soil moisture content (0-10 cm depth) together explained 45% of negative N2O fluxes. The variability of N2O fluxes and the preceding seven days' average precipitation were strongly associated. Higher plant diversity and lower inputs also promoted the frequency and magnitude of negative fluxes. Results suggest a potential for designing bioenergy cropping systems with a capacity to consume atmospheric N2O.