Fate of Nitrogen Inputs to Terrestrial Ecosystems.

Nitrogen (N) is an essential nutrient for all living organisms and often limits net primary productivity in terrestrial ecosystems. Humans have applied a great deal of ingenuity to surmounting this limitation, and have now doubled global N inputs to terrestrial ecosystems. Recent technical innovations in the use of ¹⁵N stable isotopic tracers at the ecosystem scale have produced a wealth of data on the fate of N inputs to terrestrial ecosystems. Although this analytically intensive technique has been used at over 20 sites world wide, there have been few attempts at cross-site synthesis. To date, two syntheses have been carried out across N-polluted temperate forest ecosystems, but there have been no syntheses across ecosystem types or across latitudes.  In this study, we synthesized ecosystem-scale ¹⁵N tracer experiments across a wide geographic range of ecosystem types using meta-analysis tools. Sites included temperate forests of the northeastern and western United States, Canada, South America and Europe, as well as grasslands of California, tundra of Alaska and alpine areas of the American West. We found that across sites, the amount of added ¹⁵N that was retained in the ecosystem on an annual time scale was 78.5%, with relatively little variation amongst ecosystems types, including forests, wetlands, shrublands and grasslands. Retention of ¹⁵N declined with fertilizer addition (51.2% ¹⁵N recovery for fertilized sites and 76.5% for sites not receiving nitrogen fertilizer), increased foliar %N (regression P = 0.06) and increased fine root d¹⁵N (regression P = 0.05). These results suggest that a significant amount of nitrogen deposited onto ecosystems is retained within a year of addition, but that increasing amounts of N loading significantly decreases the retentive capacity of a variety of ecosystems.