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Experimental nutrient additions accelerate terrestrial carbon loss from stream ecosystems

Carbon kicked out by nutrients Excess nutrients added to streams result in net carbon loss from aquatic ecosystems. Nitrogen and phosphorus are known to fuel increases in algal carbon. Now, Rosemond et al. show that nutrients stimulate losses of terrestrially derived carbon (e.g., from twigs and leaves). The authors monitored several multiyear experiments on headwater forest streams in the United States. Some of these streams had extra nitrogen and phosphorus added at levels that are now common in many streams and lakes. To successfully manage river ecosystems, we need to take into account nutrient pollution effects on multiple carbon pathways. Science, this issue p. 1142 Terrestrial carbon is rapidly lost from stream ecosystems as a result of nutrient enrichment. Nutrient pollution of freshwater ecosystems results in predictable increases in carbon (C) sequestration by algae. Tests of nutrient enrichment on the fates of terrestrial organic C, which supports riverine food webs and is a source of CO2, are lacking. Using whole-stream nitrogen (N) and phosphorus (P) additions spanning the equivalent of 27 years, we found that average terrestrial organic C residence time was reduced by ~50% as compared to reference conditions as a result of nutrient pollution. Annual inputs of terrestrial organic C were rapidly depleted via release of detrital food webs from N and P co-limitation. This magnitude of terrestrial C loss can potentially exceed predicted algal C gains with nutrient enrichment across large parts of river networks, diminishing associated ecosystem services.