Sinks for nitrogen inputs in terrestrial ecosystems: A meta-analysis of ¹⁵N tracer field studies

Effects of anthropogenic nitrogen (N) deposition and the ability of terrestrial ecosystems to store carbon (C) depend in part on the amount of N retained in the system and its partitioning among plant and soil pools. We conducted a meta-analysis of studies at 48 sites across four continents that used enriched ¹⁵N isotope tracers in order to synthesize information about total ecosystem N retention (i.e., total ecosystem ¹⁵N recovery in plant and soil pools) across natural systems and N partitioning among ecosystem pools. The greatest recoveries of ecosystem ¹⁵N tracer occurred in shrublands (mean, 89.5%) and wetlands (84.8%) followed by forests (74.9%) and grasslands (51.8%). In the short term (<1 week after ¹⁵N tracer application), total ecosystem ¹⁵N recovery was negatively correlated with fine-root and soil ¹⁵N natural abundance, and organic soil C and N concentration but was positively correlated with mean annual temperature and mineral soil C:N. In the longer term (3-18 months after ¹⁵N tracer application), total ecosystem ¹⁵N retention was negatively correlated with foliar natural-abundance ¹⁵N but was positively correlated with mineral soil C and N concentration and C: N, showing that plant and soil natural-abundance ¹⁵N and soil C:N are good indicators of total ecosystem N retention. Foliar N concentration was not significantly related to ecosystem ¹⁵N tracer recovery, suggesting that plant N status is not a good predictor of total ecosystem N retention. Because the largest ecosystem sinks for ¹⁵N tracer were below ground in forests, shrublands, and grasslands, we conclude that growth enhancement and potential for increased C storage in aboveground biomass from atmospheric N deposition is likely to be modest in these ecosystems. Total ecosystem ¹⁵N recovery decreased with N fertilization, with an apparent threshold fertilization rate of 46 kg N·ha^-1·yr ^-1 above which most ecosystems showed net losses of applied 15N tracer in response to N fertilizer addition.