Linking microbial community structure and function with tropical forest recovery

Soil microorganisms regulate fundamental biochemical processes in soil organic matter (SOM) transformations and soil organic carbon (SOC) storage and are thus, important drivers for ecosystem processes and biogeochemical cycles. In order to predict how land cover change affects belowground carbon storage, an understanding of how forest floor and soil microbial communities respond to changes in vegetation, and the consequences for SOM formation and stabilization, is fundamental. Using a well-replicated, long-term successional chronosequence, I investigated the effects of natural post-agricultural forest regeneration on microbial communities and belowground C cycling in southeastern Puerto Rico. My primary objectives included: (1) characterizing microbial community composition and activity during 90-years of forest recovery on former pastures, (2) investigating links between microbial community structure, function and SOM pools, and (3) identifying direct links between microbial community composition and microbial functional gene diversity. Results show that forest successional stage best predicts microbial community structure in this landscape. At the same time, microbial community structure and activity varied by season and year, stressing the importance of a multiple, temporal, sampling strategy when investigating microbial community dynamics. This study also revealed the importance of mineral interactions in defining the relationship between soil aggregates, microbial communities and SOM storage in highly weathered tropical soils. The fungal -to-bacterial ratio decreased with diminishing aggregate size, while the ratio between gram-positive and gram-negative bacteria increased in the silt and clay fractions. Differences in microbial composition among soil aggregates may influence SOC mineralization and storage as these microbial functional groups utilize different sources of SOC. The fungal-to-bacterial ratio was also important in shaping microbial functional gene diversity of genes involved in carbon, nitrogen and phosphorus cycling, linking microbial composition to functional potential in SOM transformations. This data supports a direct link between aboveground and belowground biotic community structures and highlights the importance of long-term repeated sampling of microbial communities in dynamic ecosystems. As more regions in the tropics experience post-agricultural reforestation, understanding patterns in belowground community structure and function can improve predictions of the fate of ecosystem carbon with an increase in forest cover.