Soil carbon and nitrogen cycling along an urban-to-remote gradient in humid tropical forests

Background/Question/Methods

           Tropical forests in and around urban centers face a suite of anthropogenic disturbances that may be muted or absent in more remote forests. For example, nitrogen (N) deposition, local warming, runoff, and exotic plant species are all factors that likely affect urban forests more than their remote counterparts. In tropical regions, N deposition in particular is increasing rapidly and is likely to affect ecosystem dynamics differently than in Northern sites. Here, I characterize soil carbon (C) and N cycling in tropical forest stands along an urban-to-remote gradient where N deposition is thought to vary spatially. The main questions driving this study were: (a) Do forests in closer proximity to urban areas experience elevated N availability relative to remote sites? (b) Is there evidence that increased N availability alters soil C cycling and storage in urban-proximate tropical forests? I measured soil C and N content, mineral N pools, total dissolved N (TDN), dissolved organic C (DOC), pH, microbial biomass, and the activities of decomposition enzymes in an urban watershed and in two remote watersheds in Puerto Rico to address these questions.

Results/Conclusions

           Forest stands in the urban watershed had elevated levels of soil nitrate (NO₃^-) relative to the two remote watersheds, while extractable DOC and TDN were both lower in the urban watershed. Soil pH was significantly higher and more variable, ranging from 4.5 to 8.5 across nine urban forest stands, whereas the two remote watersheds had soil pH ranging from 4.4 to 5.2. Dissolved organic C and TDN declined with increases in pH across the three watersheds (R² = 0.64 and 0.48 respectively, n = 48, p < 0.05). Microbial biomass was not different among the three study areas, but several microbial enzyme activities were lower in urban forest soils relative to the two remote watersheds, including phosphorous-acquiring phosphatase, N-acquiring NAGase, and oxidative enzymes (which degrade complex C compounds). Soil moisture was a strong predictor of soil enzyme activities in general, and average soil moisture was significantly lower in the urban watershed than in the remote sites. Together, these data indicate a significant effect of proximity to urban development on forest soil C and N cycling, with changes in soil pH and moisture of particular importance.