SOURCES OF NITRIC-OXIDE AND NITROUS-OXIDE FOLLOWING WETTING OF DRY SOIL

When dry soil is wetted, significant fluxes of NO and N2O have been reported. The sources of these gases have been difficult to identify because a number of biological and abiological processes can produce both gases. Soil sampled from an annual grassland in central California at the end of the dry season was wetted to soil water contents above and below field capacity. Production of NO and N2O began within minutes of adding water. Below field capacity, the net production of NO exceeded N2O, whereas above field capacity the production of N2O exceeded NO. Production of NO was inhibited by acetylene (C2H2) at both water contents, indicating that the NO source was dependent on nitrification. In contrast, N2O production was inhibited by C2H2 only when soil water was below field capacity, indicating that denitrification was the dominant source when soil water was above field capacity. Addition of NO2- also stimulated NO production in both sterile and nonsterile soil, suggesting that abiological reactions could contribute significantly to NO emissions. Production of NO2- and H+ by NH4+-oxidizing bacteria, followed by chemodenitrification of a small fraction of the NO2- produced, could account for the observed NO fluxes from dry and moist soil. Nitrifying and denitrifying bacteria appear to be well adapted to surviving extreme drought and to becoming active with minutes of wetting dry soil.