Environmental factors function as constraints on soil nitrous oxide fluxes in bioenergy feedstock cropping systems

Nitrous oxide (N ₂ O) is a potent greenhouse gas and major component of the net global warming potential of bioenergy feedstock cropping systems. Numerous environmental factors influence soil N ₂ O production, making direct correlation difficult to any one factor of N ₂ O fluxes under field conditions. We instead employed quantile regression to evaluate whether soil temperature, water-filled pore space (WFPS), and concentrations of soil nitrate (NO ₂ ^- ) and ammonium (NH ₄ ⁺ ) determined upper bounds for soil N ₂ O flux magnitudes. We collected data over 6 years from a range of bioenergy feedstock cropping systems including no-till grain crops, perennial warm-season grasses, hybrid poplar, and polycultures of tallgrass prairie species each with and without nitrogen (N) addition grown at two sites. The upper bounds for soil N ₂ O fluxes had a significant and positive correlation with all four environmental factors, although relatively large fluxes were still possible at minimal values for nearly all factors. The correlation with NH ₄ ⁺ was generally weaker, suggesting it is less important than NO ₂ ^- in driving large fluxes. Quantile regression slopes were generally lower for unfertilized perennials than for other systems, but this may have resulted from a perpetual state of nitrogen limitation, which prevented other factors from being clear constraints. This framework suggests efforts to reduce concentrations of NO ₂ ^- in the soil may be effective at reducing high-intensity periods—”hot moments”—of N ₂ O production.