Coordination of aboveground and belowground responses to local-scale soil fertility differences between two contrasting Jamaican rain forest types

There is growing interest in understanding how declining soil fertility in the prolonged absence of major disturbance drives ecological processes, or ecosystem retrogression'. However, there are few well characterized study systems for exploring this phenomenon in the tropics, despite tropics occupying over 40% of the Earth's terrestrial surface. We studied two types of montane rain forest in the Blue Mountains of Jamaica that represent distinct stages in ecosystem development, i.e. an earlier stage with shallow organic matter and a late stage with deep organic matter (hereafter mull' and mor' stages). We characterized responses of soil fertility and plant, soil microbial and nematode communities to the transition from mull to mor and whether these responses were coupled. For soil abiotic properties, we found this transition led to lower amounts of both nitrogen (N) and phosphorus (P) and an enhanced N to P ratio. This led to shorter-statured and less diverse forest, and convergence of tree species composition among plots. At the whole community (but not individual species) level foliar and litter N and P diminished from mull to mor, while foliar N to P and resorption efficiency of P relative to N increased, indicating increasing P relative to N limitation. We also found impairment of soil microbes (but not nematodes) and an increasing role of fungi relative to bacteria during the transition. Our results show that retrogression phenomena involving increasing nutrient (notably P) limitation can be important drivers in tropical systems, and are likely to involve aboveground-belowground feedbacks whereby plants produce litter of diminishing quality, impairing soil microbial processes and thus reducing the supply of nutrients from the soil for plant growth. Such feedbacks between plants and the soil, mediated by plant litter and organic matter quality, may serve as major though often overlooked drivers of long term environmental change.