Many tropical forests will experience less rainfall with climate change, yet the implications for their biogeochemical cycles remain poorly understood. Changes in soil organic matter are of particular concern because tropical forest soils store approximately 30% of global soil carbon (C). Insights into how soil C may change in diverse tropical forests following shifts in rainfall can be obtained by studying soil C storage over gradients of rainfall, seasonality, and soil fertility. We used density fractionation to examine the quantity and composition of soil organic carbon (SOC) pools in wet and dry seasons in 13 forested sites in Panama spanning gradients in rainfall (1809–3000 mm/yr) and fertility (e.g. 0.21–22.8 mg P/kg resin extractable P). We predicted that free-debris SOC would increase during the dry season as it is thought to be derived mainly from leaf and root litter, occluded-debris SOC would increase during the wet season when aggregate formation is increased, and mineral-associated SOC would have muted seasonality relative to the debris fractions.
Results/Conclusions
Free-debris SOC was greater in the dry season and had a fertility*rainfall interaction in which it tended to increase in fertile sites with greater rainfall but decrease in infertile sites with greater rainfall. Occluded-debris SOC did not vary seasonally, but was greater on average and varied more widely with rainfall in fertile sites than infertile sites. Mineral-associated SOC also did not vary seasonally but decreased in sites with greater rainfall, although it was lowest at an infertile low rainfall site. Dissolved organic carbon (DOC) associated with each fraction was greater in the wet season for all fractions, and also responded to complex interactions of season, fertility, and rainfall. Overall, these results suggest that changing patterns of rainfall in tropical forests will alter the physical nature of SOC depending on baseline rainfall and soil fertility.