SOM, Nitrogen and Climate Change
Temperature controls on soil N2O efflux
Author: Lisa Tiemann
Temperature responses of denitrifying microbes likely play a governing role in the production and consumption of N2O. We investigated temperature effects on denitrifier communities and their potential to produce N2O and N2 by incubating grassland soils collected in multiple seasons at four temperatures with 15N-enriched NO3– for ~24h, and quantifying relative abundance of genes responsible for N2O production (cnorB) and reduction (nosZ). In all seasons, net N2O production was positively linked to incubation temperature, with highest estimates of net and gross N2O production in late spring soils. N2O dynamics were tightly coupled to changes in denitrifier community structure, which occurred on both seasonal and incubation time scales. We observed increases in nosZ abundance with increasing incubation temperature after 24 h, and relatively larger increases in cnorB abundance from winter to late June. The difference between incubation and in situ temperature was a robust predictor of cnorB:nosZ. These data provide convincing evidence that short-term increases in temperature can induce remarkably rapid changes in community structure that increase the potential for reduction of N2O to N2, and that seasonal adaptation of denitrifying communities is linked to seasonal changes in potential N2O production, with warmer seasons linked to large increases in N2O production potential. Both short- and longer-term community dynamics are tightly coupled to soil N2O dynamics. This work helps explain observations of high spatial and temporal variation in N2O effluxes, and highlights the importance of temperature as an influence on denitrification enzyme kinetics, denitrifier physiology and community adaptations, and associated N2O efflux and reduction.
Soil moisture variability controls C and N flows through microbial biomass
Grassland ecosystems contain ~12% of global soil organic carbon (C) stocks and are located in regions where global climate change will likely alter the timing and size of precipitation events, increasing soil moisture variability. In response to increased soil moisture variability and other forms of stress, microorganisms can induce ecosystem-scale alterations in C and N cycling processes through alterations in their function. We explored the influence of physiological stress on microbial communities by manipulating moisture variability in soils from four grassland sites in the Great Plains, representing a precipitation gradient of 485 to 1003 mm y-1. Keeping water totals constant, we manipulated the frequency and size of water additions and dry down periods in these soils by applying water in two different, two-week long wetting-drying cycles in a 72 day laboratory incubation. To assess the effects of the treatments on microbial community function, we measured C mineralization, N dynamics, extracellular enzyme activities (EEA) and a proxy for substrate use efficiency. In soils from all four sites undergoing a long interval (LI) treatment for which added water was applied once at the beginning of each two-week cycle, 1.4 to 2.0 times more C was mineralized compared to soils undergoing a short interval (SI) treatment, for which four wetting events were evenly distributed over each two-week cycle. A proxy for carbon use efficiency (CUE) suggests declines in this parameter with the greater soil moisture stress imposed in LI soils from all four different native soil moisture regimes. A decline in CUE in LI soils may have been related to an increased effort by microbes to obtain N-rich organic substrates for use as protection against osmotic shock, consistent with EEA data. These results contrast with similar in situ studies of response to increased soil moisture variability and may indicate divergent autotrophic vs. heterotrophic responses to increased moisture variability. Increases in microbial N demand and decreases in microbial CUE with increased moisture variability observed in this study, regardless of the soils‚Äô site of origin, imply that these systems may experience enhanced heterotrophic CO2 release and declines in plant-available N with climate change. This has particularly important implications for C budgets in these grasslands when coupled with the declines in net primary productivity reported in other studies as a result of increases in precipitation variability across the region.