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 ecosystem tracer 15N recoveries 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 to 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 tracer 15N were belowground 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 tracer 15N in response to N fertilizer addition.