The nitrogen cycle is undergoing rapid transformation globally, with some of the most rapid changes occurring in tropical regions. Despite the obvious and significant physical linkages between watersheds and the streams that drain them, there have been few attempts to understand commonalities or differences in underlying biogeochemical responses to additional N inputs in the aquatic and terrestrial portions of the landscape. Here we examine the response of tropical landscapes to long-term increases in nitrogen inputs, including both aquatic and terrestrial systems. We use data collected during a long-term N addition experiment to two tropical forest sites in Puerto Rico, where we have measured basal area growth, organic carbon in soil solution, and soil stocks of carbon in fertilized and unfertilized plots. We have also measured net primary productivity and nitrogen retention in streams with a range of N concentrations associated with variation in land use and inputs of treated sewage effluents.
Results/Conclusions
In both forests and streams, primary productivity was insensitive to N addition. In contrast, there was a very strong connection of the N cycle to carbon availability in both forest soils and streams. N fertilization increased total C storage in soils, but labile C fractions declined. Continued input of N to these tropical forests may dramatically alter carbon storage and carbon quality. In streams elevated N levels are associated with an increased efficiency of nitrate uptake, and nitrate uptake was positively correlated with DOC levels. Controls on nitrogen flux in streams thus appear to be fundamentally different across a tropical landscape than they are at the continental scale, where increased nitrate levels result in declining efficiency of nitrate uptake. These data point to an emerging paradigm that the nitrogen cycle in tropical landscapes differs fundamentally from the nitrogen cycle of temperate landscapes.