The abandonment of agricultural and pasture lands in tropical regions raises questions about possible changes in soil organic matter dynamics and nutrient availability impacting ecosystem productivity, carbon stocks and global biogeochemical cycles. The transition from human land uses to secondary forests can alter plant community composition, soil physicochemical properties, and bacterial and fungal communities. Fungal communities can be classified into two major functional groups that affect nutrient cycling in soils and plant productivity: mycorrhizal and saprotrophic fungi. Shifts in fungal functional groups in soil may lead to changes in carbon stocks, but few studies have measured how fungal communities change with tropical forest succession. The objective of this research is to understand how fungal communities change across forest successional stages recovering from pasture use and to relate fungal functional groups with carbon stocks in soils. We collected soils across a successional chronosequence in Puerto Rico and measured soil organic carbon, extracted and amplified DNA targeting ITS2 gene to characterize fungal communities, and cleared and stained roots to assess mycorrhizal fungi colonization.
Results/Conclusions
We found that mycorrhizal fungi colonization increased at early stages of forest succession on former pastures. This observed increase could be explained by greater nutrient demand in early growing stages. Young secondary forests in our sites have lower leaf litter mass and reduced concentrations of surface soil exchangeable ions as measured by soil resins, indicating potential limitations of available nutrients. Also, we found that carbon concentration in soil increase with the conversion of pastures to secondary forests and with forest age. Older forests have faster litter decomposition and greater nitrogen mineralization rates. Given these findings, we would expect greater abundance of saprotrophic fungi in older forests which we will determine by molecular techniques. An increase in saprotrophic fungi in older successional stages may lead to competition with mycorrhizal fungi, which has been shown to result in greater soil organic matter decomposition. Understanding fungal shifts in post-agricultural forests will improve predictions of the response of carbon stocks and biogeochemical processes to land-use change in tropical ecosystems.