Seasonal and site-scale variation in soil biogeochemistry and microbial activity across four lowland tropical forests

Lee H. Dietterich, Amanda Longhi Cordeiro and Daniela Cusack, Ecosystem Science & Sustainability, Colorado State University, Fort Collins, CO, Stephany S. Chacon, Climate and Ecosystem Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA, Nicholas J. Bouskill, Earth and Environmental Sciences, Lawrence Berkeley National Laboratory, Berkeley, CA

Background/Question/Methods
Many tropical forests are predicted to experience changes in precipitation as climate change continues. These changes are expected to influence coupling between water, nutrient, and biomass dynamics in these forests. Here we investigate seasonal changes in soil hydro-biogeochemistry in four tropical humid forests in Panama, varying in precipitation and soil fertility and including a rainfall reduction experiment, to inform how future changes in precipitation may affect tropical forests. We hypothesized that (1) soil nutrients accumulate during the dry season due to moisture constraints on plant and microbial uptake, and (2) soil respiration, microbial biomass, and enzyme activities increase during the wet season as moisture availability increases. To address this, we monitored soil hydro-biogeochemistry on fine timescales over the course of 2-3 years. These measures included soil moisture and temperature (hourly), soil respiration (monthly), and soil chemistry and nutrient availability, microbial biomass, and enzyme activities (quarterly).
Results/Conclusions
Soluble total organic C, total dissolved N, and resin-extractable P increased in the dry season, in most sites approximately doubling from the wet season to the dry season. Soil respiration, pH, and microbial biomass C and N increased in the wet season on average for soil respiration by 2-35%, for pH by 0.18-0.91 pH units, and for microbial biomass C and N by 25-40% compared to the dry season. These patterns support our hypotheses of greater nutrient accumulation in the dry season and greater microbial growth and activity in the wet season. Seasonal patterns in microbial P varied by site: microbial P was on average 18.5% greater in the wet season in three of our sites, but was 43.8% greater in the dry season in our wettest, lowest-fertility site. Phosphorus acquisition enzyme activities varied inversely with soil resin P availability across sites, suggesting that greater investment in P acquisition is necessary in more P-limited soils. Otherwise, C and N acquisition enzyme activities had complex, interacting effects with site and season, suggesting widespread substrate specificity in microbial metabolism of soil organic matter. We conclude that accumulation of soil nutrients during the dry season supports wet season microbial growth and respiration.