Effects of soil moisture, physical, and chemical characteristics on abiotic nitric oxide production
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The objective of this study was to examine the effect of soil water content, and other physical and chemical factors, on the abiotic component of nitric oxide (NO) production in laboratory studies using soils from agricultural fields in Minnesota, California, and Connecticut. In all soils, gross NO production decreased with increasing gravimetric water content (theta) in nitrite (NO2-)-amended sterilized soils. The rate coefficient describing nitrous acid (HNO2)-mediated NO production (kp) also decreased with increasing theta in both gamma-irradiated and autoclaved soils. Significant correlations were found between ln kp and several soil properties including: content of silt, clay, total carbon, total N, and extractable iron, and an estimate of the cation exchange capacity of the clay fraction. Multiple regression models incorporating these variables explained 85-93% of the variance in ln kp. The relationships obtained suggest that the mechanism of abiotic NO production is primarily mediated at the soil solution-surface interface. These findings provide consistent evidence of a previously unrecognized mechanism by which soil water content can affect NO production by mediating a chemical process. Application of a dynamic process model indicated that the simulated variation in NO emissions as a consequence of this effect is comparable to the variation observed in previous studies of NO emissions. Comparison of soils from two different long-term tillage studies also indicated that reduced pH in no-till systems may lead to greater NO emissions for a given level of NO2- accumulation. Overall, these results suggest that current views of controls over N oxide gas emissions may need to be revised to include abiotic reactions, in addition to microbial and physical processes, as yet another category of factors that is highly sensitive to soil water content.