Seasonal and experimental effects on microbial composition and dynamics in a tropical secondary forest in the eastern Amazon, Brazil

Tropical secondary forests are an increasingly important land cover in the Brazilian Amazon, with 30 to 50% of the deforested area of the Brazilian Amazon in some stage of abandonment. This study investigated water and nutrient constraints on microbial dynamics and nutrient availability in a tropical secondary forest in the Eastern Amazon with manipulative experiments---dry-season irrigation and bi-weekly litter removal---using frequent sampling to capture seasonal and intra-annual fluctuations in Castanhal, Para, Brazil. Irrigation did not consistently alter microbial dynamics, except for lowered NH4+ availability and fungal densities, and increased phosphatase activity. Litter removal decreased microbial biomass C and P, N-mineralization, phosphatase activity and NH4+ availability but increased NO3- availability. Intra-annual variability was mainly driven by wet-up events in the dry season that were not minimized by continuous irrigation (except for NH4+ availability), suggesting either that seasonal drought may not constrain the availability of nutrients or that irrigation was insufficient to cause a more significant effect. These results confirm the critical role of litterfall in tropical forest nutrient cycling and the importance of fluctuations in soil moisture status to nutrient availability. How these belowground results interact with aboveground processes including C uptake, is a fertile area for future research and modeling.