Reviews and syntheses: measuring ecosystem nitrogen status - a comparison of proxies

There are many proxies used to measure nitrogen (N) availability in watersheds, but the degree to which they do (or do not) correlate within a watershed has not been systematically addressed. We surveyed the literature for intact forest or grassland watersheds globally, in which several metrics of nitrogen availability have been measured. Our metrics included the following: foliar delta N-15, soil delta N-15, net nitrification, net N mineralization, and the ratio of dissolved inorganic to organic nitrogen (DIN : DON) in soil solution and streams. We were particularly interested in whether terrestrial and stream based proxies for N availability were correlated where they were measured in the same place. Not surprisingly, the strongest correlation (Kendall's tau) was between net nitrification and N mineralization (tau = 0.71, p < 0.0001). Net nitrification and N mineralization were each correlated with foliar and soil delta N-15 (p < 0.05). Foliar and soil delta N-15 were more tightly correlated in tropical sites (tau = 0.68, p < 0.0001), than in temperate sites (tau = 0.23, p = 0.02). The only significant correlations between terrestrial-and water-based metrics were those of net nitrification (tau = 0.48, p = 0.01) and N mineralization (tau = 0.69, p = 0.0001) with stream DIN : DON. The relationship between stream DIN : DON with both net nitrification and N mineralization was significant only in temperate, but not tropical regions. To our surprise, we did not find a significant correlation between soil delta N-15 and stream DIN : DON, despite the fact that both have been used to infer spatially or temporally integrated N status. Given that both soil delta N-15 and stream DIN : DON are used to infer long-term N status, their lack of correlation in watersheds merits further investigation.