COS 264-2 - Biotic and abiotic constraints on soil microbial responses to moisture

Background/Question/Methods

Soil microbial activity releases about 60 billion tons of carbon into the atmosphere each year, almost double the CO2 emissions from human activity. We know that respiration rates are tied to soil moisture. Although we generally understand how microbial communities respond to drying and wetting of soils, the specifics regarding variability of microbial responses to soil moisture seen among ecosystems is left unknown. In this project, we tested the hypothesis that environmental selection for slow-growing bacteria prevents dryland microbial communities from increasing respiration activity when artificially providing moisture. To test this hypothesis, we conducted a three-part experiment: First, we characterized the responses of dryland and wetland soil microbial respiration activity to varying moisture levels. Microbes from dry sites were far less responsive to changes in soil moisture than microbes from wet sites. Second, we conducted reciprocal transplant experiments by crossing microbial origin with soil type. We found that bacterial communities from the dryland site were able to respire at nearly the same rates as wetland communities when placed in identical conditions. The third part of the experiment was to measure respiration activity of dryland and wetland microbial communities to nutrient supplementation of carbon, nitrogen, and phosphorus.

Results/Conclusions

Under no soil moisture treatment were the dry microbial communities able to respire at the same rate as the wet microbial communities. There must be a fundamental constraint in either the biotic community or abiotic environment that dictates how microbial communities respond to water. To test whether the biotic community constrained respiration, we cross inoculated wet and dry soils with wet and dry bacterial communities. Surprisingly we found that microbial community origen did not affect respiration. In other words, dry communities placed into wet site soils respired as much as wet communities in wet site spoils, and vise versa. Combined, this suggests that nighter soil moisture nor the biotic community constrains respiration. Thus we tested whether soil carbon, nitrogen, or phosphorus limited microbial respiration. We found that soil carbon was by far the most limiting in both dry and wet sites. Taken together our results demonstrate that soil carbon is going to be primary determinant to how soil bacterial communities respond to drought.