Atmospheric nitrogen deposition

David A. Wedin and David Tilman (Reports, 6 Dec.,p 1720) show that increased nitrogen inputs to terrestrial ecosystems might cause smaller increases in the capacity of those ecosystems to store carbon than expected. Their findings are important because nitrogen inputs have increased dramatically over the past decades through fertilizer production, cultivation of nitrogen-fixing legumes, and production of oxides of nitrogen associated with fossil-fuel burning (1). However, the simultaneous increase in atmospheric carbon dioxide (CO₂) concentrations caused by burning fossil fuels is likely to at least partially counteract the processes that limited carbon storage in Wedin and Tilman’s experiment. CO₂ enrichment generally increases the amount of carbon fixed by plants per unit of nitrogen taken up from the soil, particularly in carbon-3 (C³) species (2) such as those that invaded their nitrogen-enriched plots. Compared with the C⁴ species that thrived before nitrogen was added, the invading C³ species have relatively lower C-to-N ratios, limiting the amount of carbon stored in response to nitrogen input. However, with elevated CO₂ tending to increase the C-to-N ratio of these C³ plants, N and CO₂ enrichment in concert would likely cause greater C storage than observed by Wedin and Tilman.

Rising atmospheric CO₂ may also increase N inputs to terrestrial ecosystems, amplifying the direct human impact on the N cycle. CO₂ enrichment often increases the growth of plants housing N-fixing bacteria in their roots, and this stimulation is relatively larger than non-N-fixing plants (3). Thus, in addition to the direct anthropogenic stimulation of N inputs to terrestrial ecosystems through agriculture and fossil-fuel burning (1), humans may indirectly increase N inputs to terrestrial ecosystms by increasing atmospheric CO2 concentrations. The interaction between CO2 and N enrichment, as well as shifts in plant species, will likely influence future C storage by the terrestrial biosphere