Microbial responses to an imposed gradient of water availability in wet tropical forest soils

Background/Question/Methods

Water availability strongly affects belowground ecosystem processes in wet tropical forests, where large changes in precipitation regime are predicted in the coming century.  Rates of ecosystem functions such as decomposition and soil respiration are assumed to increase linearly with soil moisture, or to exhibit a parabolic relationship reflecting the balance between drought and soil anoxia. However, theory predicts that soil microbial communities should exhibit thresholds in community structure along gradients of abiotic stress, which may yield non-linear functional responses. The goal of this study was to determine whether native soil moisture regime influences the shape of microbial responses to an imposed gradient of water availability. In a wet tropical forest ecosystem, we characterized microbial biomass and community structure of three soil types: pasture soils that experience frequent wet-dry cycles, forest soils that are constantly wet, and forest soils collected from beneath rain-out shelters installed four months prior. In a laboratory microcosm experiment, we incubated soils of each type at one of five soil moisture treatments representing a gradient of decreasing water availability. We measured soil respiration, microbial biomass, and fungal abundance in each microcosm over the course of a month-long incubation. 

Results/Conclusions

Although differences in microbial biomass and community function among forest, pasture, and rain-out shelter plots were observed in the field, soil type did not affect the qualitative response to decreased water availability in the lab incubation. Respiration rates decreased by over 40% in the driest soil moisture treatment (P = 0.035), whereas total microbial biomass was unaffected by water availability. However, soil fungal biomass in the two driest moisture treatments decreased significantly in comparison with wetter treatments (P < 0.05, Tukey’s HSD test), suggesting a non-linear shift in the fungal:bacterial ratio along the water availability gradient. We conclude that fungal communities in this wet tropical forest ecosystem exhibit little resilience to changes in water availability, especially at low soil moistures rarely observed in the field. Because lowered fungal abundance accompanies strong decreases in soil respiration rates, declines in soil fungal biomass under predicted drier climates may have large implications for ecosystem carbon storage.