Can we see the nitrate from the trees? Long-term linkages between tropical forest productivity and stream nitrogen concentrations

High abundance of trees capable of biological N-fixation (henceforth “N-fixers”) in tropical forests has been hypothesized to drive higher stream nitrate (NO 3 ) concentrations compared to temperate counterparts. However, to date there have been no empirical linkages of stream NO 3 concentrations with the productivity of tropical forests. Here, we combined three unique long-term datasets from La Selva Biological Station, Costa Rica: 21 years of (1) mean annual stream NO 3 -N concentrations in six stream sites within the same watershed, (2) annual growth of trees, and (3) annual leaf litterfall. We hypothesized that years of greater growth of N-fixer tree species and of greater leaf litterfall would be correlated with higher stream water NO 3 -N concentrations. We also hypothesized that landscape position mediates these relationships, with growth of N-fixer trees on adjacent slopes being more strongly correlated to stream NO 3 -N than the growth of such trees on upland plateau sites. We found that mean annual stream NO 3 -N concentrations were consistently high (160–260 µg L −1 ). There was substantial interannual variation in leaf litterfall (inter-year range: 5.4 to 8.1 Mg ha −1 year −1 ), growth of N-fixers (inter-year range: 1.2 to 2.2 Mg ha −1 year −1 ), and growth of all other tree species (inter-year range: 2.1 to 3.2 Mg ha −1 year −1 ). To assess stream NO 3 -N relationships with forest productivity, we used water conductivity to account for dilution resulting from variable discharge. We found that NO 3 -N concentrations were positively related to the annual growth of the N-fixers on nearby slopes, and were negatively correlated with annual leaf litterfall. Stream NO 3 -N concentrations were not related to the growth of N-fixers or other tree species in the more removed plateau areas. Using a mass balance, we estimated that symbiotic N fixation can account for 7–29% of NO 3 export. Both the growth of adjacent N-fixers and landscape-wide leaf litterfall are important drivers of the inter-annual variability of stream NO 3 -N concentrations. Our results suggest that predicted changes in precipitation extremes due to climate change will alter N dynamics in tropical forests both directly, by altering discharge and export, and indirectly, by altering N-fixer tree productivity.