Wednesday, August 4, 2010 - 8:20 AM

OOS 26-2: Soil warming and N2O production and consumption: Differential enzymatic responses?

Sharon A. Billings and Lisa K. Tiemann. University of Kansas


Investigators appreciate the important role that nitrate and moisture availability can play in governing net N2O production from soils. However, a large knowledge gap remains surrounding the drivers of soil N2O consumption and the role of microbial adaptation to changing environmental conditions in governing both N2O production and consumption. Net N2O soil efflux can be correlated with temperature, but little is known about the influence of temperature on gross rates of N2O production vs. consumption. Further, we do not understand how microbial communities responsible for these processes adapt or acclimate to soil warming. To investigate whether temperature alters the denitrifier-mediated fate of nitrate lost via N2O or N2, and if any such effect changes across seasons, we incubated soil collected in three seasons at four temperatures with and without 15N-labeled nitrate for 26 hours. Incubations were conducted in an anaerobic environment flushed with helium to permit detection of N2O and N2, and those gases’ δ15N.

Temperature positively influenced CO2 production resulting from anaerobic processes. Maximum values of net N2O production were positively influenced by temperature, but the maximum rate of net N2O production occurred relatively early in warmer incubations. We also observed greater N2O:N2 ratios early in the incubations at warmer temperatures.  Isotope data are consistent with these trends. For those soils receiving the 15N label, differences in δ15N2O between early and late in the incubations were increasingly negative, and differences in δ15N2 increasingly positive, as temperature increased. These data provide convincing evidence that increasing temperatures induced denitrifying communities to perform complete denitrification (i.e. consumption of gross N2O production into N2) to a greater degree, and to permit release of a relatively smaller proportion of the nitrate they consumed as N2O. Soil collected across seasons, as soils warm naturally, will provide insight into the role of microbial adaptation to temperature regime as an influence on these processes. These data suggest that the suite of enzymes responsible for N2O production and the one enzyme responsible for its consumption may exhibit differential temperature sensitivities in their production and expression. Additionally, temperature may exert varying selective forces on microbial populations such that gene copy numbers and expression levels are promoted with warming. Though we do not know the extent to which these mechanisms operate with warming, these data indicate the importance of warming as a factor governing the net fluxes of N2O from soil, vs. its transformation into N2.