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 (NO3) concentrations compared to temperate counterparts. However, to date there have been no empirical linkages of stream NO3 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 NO3-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 NO3-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 NO3-N than the growth of such trees on upland plateau sites. We found that mean annual stream NO3-N concentrations were consistently high (160-260 mu g 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 NO3-N relationships with forest productivity, we used water conductivity to account for dilution resulting from variable discharge. We found that NO3-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 NO3-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 NO3 export. Both the growth of adjacent N-fixers and landscape-wide leaf litterfall are important drivers of the inter-annual variability of stream NO3-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.

Automated summary

A study in La Selva Biological Station showed that trees capable of biological N-fixation in tropical forests lead to higher stream nitrate concentrations compared to temperate forests. The study also found a correlation between the productivity of tropical forests and stream nitrate concentrations, as well as the influence of landscape position on this relationship.