Soil moisture constrains fire effects on microbial functional communities in a California annual grassland

Background/Question/Methods

Grassland fire is an influential ecosystem disturbance that may change and reallocate plant biomass, alter nutrient cycling, and potentially influence soil microbial communities. The frequency and intensity of grassland fire is likely to increase under regional and global climate change. However, so far, it is still unclear how soil microbial communities are influenced by the interactions of fire and other climate change factors. Here, we examined fire effects on soil microbial communities by a functional gene based microarray technique, GeoChip 4.6, under different combinations of global change conditions (CO₂, C; temperature, T; precipitation, P; and nitrogen, N) after one growing season following a single, low-severity burning treatment at the Jasper Ridge Global Change Experiment. 

Results/Conclusions

Results show that the microbial functional genes significantly shifted when the fire co-occurred with certain combinations of other treatments (split-plot multivariate permutation ANOVA, p < 0.05 under elevated N, PN, CP, CTP and TPN, p < 0.1 under elevated T, P, CT, CN and TN), indicating strong interactive effect of fire and other global change factors on soil microbial functional communities. Specifically, 34 % of the genes involved in carbon, nitrogen, phosphorous and sulfur cycling had increased abundance (paired t-test, p < 0.05, the same for other mentioned gene abundance tests) by fire when extra precipitation was added during the growing season following the fire, but 24 % of these genes decreased in abundance by fire under elevated temperature, which possibly contributed to observed lowered soil moisture in burn plots. In contrast, only less than 0.03 % genes showed abundance change by the single treatment of fire, although fire was likely to increase soil ammonia content. This could be due to carbon, phosphorous and other substrate limitations and/or the decrease in soil moisture caused by fire. Consistently, fire effect on microbial functional genes was trivial under nitrogen deposition. These results indicates the importance of soil moisture in constraining fire effects on soil microbial communities and potentially nutrient cycling bioprocesses in this Mediterranean climate mediated California grassland.