Sustained losses of bioavailable nitrogen from montane tropical forests

Tropical forests account for one third of terrestrial primary production and contribute significantly to the land carbon sink(1,2). The future of this sink relies critically on forest interactions with nutrient cycles(3-5). Humid montane tropical forests are often thought to be rich in phosphorus, but to contain low levels of bioavailable nitrogen(6). Here, we examine the concentration of dissolved nitrogen compounds and the isotopic composition of nitrate in streams in six well-characterized and phosphorus-rich montane forests(7) in Costa Rica, and in 55 montane forests across Central America and the Caribbean, using data collected between 1990 and 2008. We found high levels of nitrate in these streams, indicative of large losses of bioavailable nitrogen from these forests. We detected no trend in the concentration and isotopic signature of nitrate over the measurement period, implying that high levels of export are neither recent nor episodic. An analysis of the oxygen isotopic signature of stream nitrate showed that exports are sourced from the plant-soil system, rather than from atmospheric deposition that bypasses forest biota. Our findings indicate that nitrogen-rich conditions can develop irrespective of phosphorus availability at the ecosystem scale. We suggest that nitrogen availability may not limit plant growth, or its response to increasing atmospheric carbon dioxide levels, in many montane tropical forests.