Soil oxygen availability and biogeochemistry along rainfall and topographic gradients in upland wet tropical forest soils

We measured soil oxygen concentrations at 10 and 35 cm depths and indices o f biogeochemical cycling in upland forest soils along a rainfall and elevat ion gradient (3500-5000 mm y(-1); 350-1050 masl) and along topographic grad ients (ridge to valley, similar to 150 m) in the Luquillo Experimental Fore st, Puerto Rico. Along the rainfall gradient, soil O-2 availability decreas ed significantly with increasing annual rainfall, and reached very low leve ls (<3%) in individual chambers for up to 25 consecutive weeks over 82 week s of study. Along localized topographic gradients, soil O-2 concentrations were variable and decreased significantly from ridges to valleys. In the va lleys, up to 35% of the observations at 10-35 cm depth were <3% soil O-2. C ross correlation analyses showed that soil O-2 concentrations were signific antly positively correlated along the topographic gradient, and were sensit ive to rainfall and hydrologic output. Soil O-2 concentrations in valley so ils were correlated with rainfall from the previous day, while ridge sites were correlated with cumulative rainfall inputs over 4 weeks. Soils at the wettest point along the rainfall gradient had very high soil methane concen trations. (3-24%) indicating a strong influence of anaerobic processes. We measured net methane emission to the atmosphere at the wettest sites of the rainfall gradient, and in the valleys along topographic gradients. Other m easures of biogeochemical function such as soil organic matter content and P availability were sensitive to chronic O-2 depletion along the rainfall g radient, but less sensitive to the variable soil O-2 environment exhibited at lower elevations along topographic gradients.