COS 125-4 - Effects of partial throughfall exclusion on soil carbon cycling in four lowland tropical forests across rainfall and soil fertility variation

Background/Question/Methods

Humid tropical forests contain some of the largest soil organic carbon (C) stocks on Earth. Despite the importance of C storage in this biome, controls over variation in soil C stock sizes and depth distribution remain poorly understood. We hypothesized that soil C dynamics and related ecosystem processes are more resistant to drying in lower rainfall and infertile sites, where plant and microbial communities are more adapted to stress. We used a partial throughfall exclusion established in 2018 in four Panamanian forests that span 2350 mm to 3300 mm mean annual rainfall, and variation in soil fertility. We measured soil C storage inputs, losses, microbial community characteristics, and nutrient availability. The experiment excludes ~50% of throughfall from plots using clear roofing over 10 x 10 m plots, diverting moisture out past 50 cm-deep plastic-lined trenches (n = 4 per site, 32 plots total). Overall, soil C dynamics varied with rainfall and soil fertility across the four forests, and responded to throughfall exclusion.

Results/Conclusions

Soil respiration was lower in throughfall exclusion versus control plots, with interacting effects of site and season. This was similar to background patterns of decreased soil respiration during the natural dry season. Along with decreased soil respiration, the radiocarbon age of respired soil C was significantly older in throughfall exclusion versus control plots. The decline in soil respiration could be related to reduced fine root (< 2 cm diameter) production during, and/or to reduced microbial biomass, both of which appear to be declining with throughfall exclusion. For example, there was significantly less microbial biomass in throughfall exclusion versus control plots, including decreased fungal biomass. Also, microbial community composition converged toward a “drought microbiome”, whereby taxonomically similar bacteria were selected for by drying. At the same time, throughfall exclusion promoted accumulation of soil nitrogen during the dry season, and soil moisture data indicated reduced vertical flushing of soils during rainfall events in exclusion plots. These results suggest that drying in tropical forests is likely to alter soil C storage, and continued research will clarify the net balance between reduced soil respiration, reduced root inputs, and changes in microbial processing.