Sulfur Cycling in the Rainforest of Puerto Rico

The effect of climate on the terrestrial sulfur (S) cycle has received little scientific attention, despite the importance of the S geochemical cycle on earth. Here we focus on a wet end-member ecosystem, exploring S cycling at established CZEN sites in the Luquillo Experimental Forest of Puerto Rico (~3.5 m of mean annual precipitation). Our research traces the major S sources to the ecosystem and assesses the degree to which this S has been biologically cycled before being leached out of the soils. In summer 2010 we collected horizon-based soil samples, six soil cores down to 1.5 m, and representative plant litter samples at two sites: one on granitic parent material (Rio Icacos) and one on meta-basalt (Bisley). At each site we sampled along a topographic gradient from ridgetop to the adjacent drainages. We have also been collecting monthly pore water samples and rainwater. In our analyses, we assume (as is the case for C and N) that soil S pools are largely at steady state, representing the balance between inputs and losses, and that, similarly, the δ34S value of soil S (mostly organic) represents the balance between that of the inputs and the S isotope composition of the various losses. Our data reveals a broad range of δ34S values in local bulk deposition. Correcting for seasalt using Cl shows that more than half of the sulfate in rainwater each month is of a non-seasalt origin with an isotopic value ranging from 2.0% to 11.6%. Bulk soil δ34S (average of 17.9%) falls within the range of bulk deposition (δ34S of 11.3-18.5%), suggesting that most, if not all, of the S here is of atmospheric origin. There is no significant difference between soil and plant bulk δ34S, implying little to no fractionation during plant uptake. Additionally, there is no apparent trend of isotope ratios with depth. We found that, on average, the soil on granitic parent material has more sulfate in the upper 50 cm compared to the basaltic soil, but the situation reverses below 50 cm. The carbon content of the two soils is similar, however the basaltic soil has significantly more chloride and nitrate throughout the profile and more bulk S in the upper 10 cm. All soils sampled on the ridgetop and hillslope show similar S content. Topography seems to make a difference only at the foot of the slope, close to a stream on the basaltic bedrock, where sulfate concentrations are significantly lower than at all other sites. Based on our data so far, it appears that soils and plants retain the isotope signal of S inputs, which indicates that the losses (which must equal inputs in these largely steady state situations) must lack significant isotopic fractionation. From an N isotope perspective, little apparent soil isotope change was viewed as suggestive of ecosystems that were highly conservative of N due to high biological demand (Amundson et al. 2003. GBC 17:1 1031). However, we will further explore the significance of the S isotope ratios by examining pore water chemistry and the parallel N cycle and its isotopes. Overall, the abiotic environment (rainfall, parent material and topography) plays a greater role in determining the S content and stable isotope ratios of soil S than the biological processes occurring in these soils.