Water availability is the primary driver of microbial function in a tropical rainforest soil

Background/Question/Methods

Given the current pace of anthropogenic environmental change, soil microbes are likely to experience simultaneous changes in soil temperature, moisture, and resource stoichiometry. Research in plant and animal communities has shown that organisms respond differently to changes in abiotic stress vs. resource supply, and that abiotic stress can alter or even reverse the outcome of resource competition. To test how soil microbes respond to the combination of altered abiotic stress and resources, we examined microbial functional responses to soil moisture and nutrient availability, tracking field patterns and manipulating conditions in lab incubations for soils from an old-growth tropical forest. We studied two soil types (Inceptisols and Ultisols) that have the same parent material but differ in the relative availabilities of phosphorus and nitrogen. In the field, we tracked moisture, labile nutrient pools, and fungal biomass over eighteen months. For the incubation, we subjected soils to two levels of soil moisture (control and 20% reduction) and two levels of labile phosphorus (control and 0.5 μg P/g weekly) in a full factorial design and monitored soil respiration and the activity of three enzymes involved in carbon, nitrogen, and phosphorus cycling over six weeks. 

Results/Conclusions

In the field, we found that soil moisture was the single best predictor of fungal biomass, explaining 42% of the variation both spatially and temporally. Consistent with the field observations, mild reductions in soil moisture substantially reduced fungal biomass and soil enzyme activity in the lab incubations. Reducing soil moisture in the incubation experiment also significantly affected patterns of soil respiration, although the magnitude and direction of change depended on soil type. Phosphorus addition had no significant impact on any soil variable measured. Soils from this tropical forest were quite sensitive to mild stress imposed by a slight reduction in soil moisture, but more resilient to changes in nutrient supply. This suggests that changes in rainfall in the tropics are likely to have a larger impact on soil microbial activity compared to changes in resources, at least within the ranges observed here. Further experiments will explore the dynamics of soil microbial responses to a broader range of soil moistures and examine how microbial community composition relates to the observed functional responses.