Reviews and syntheses: Measuring ecosystem nitrogen status - A comparison of proxies

Abstract

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 δ15N, soil δ15N, 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 τ) was between net nitrification and N mineralization (τ Combining double low line 0.71, p < 0.0001). Net nitrification and N mineralization were each correlated with foliar and soil δ15N ( p < 0.05). Foliar and soil δ15N were more tightly correlated in tropical sites (τ Combining double low line 0.68, p < 0.0001), than in temperate sites (τ Combining double low line 0.23, p Combining double low line 0.02). The only significant correlations between terrestrial- and water-based metrics were those of net nitrification (τ Combining double low line 0.48, p Combining double low line 0.01) and N mineralization (τ Combining double low line 0.69, p Combining double low line 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 δ15N and stream DIN : DON, despite the fact that both have been used to infer spatially or temporally integrated N status. Given that both soil δ15N and stream DIN : DON are used to infer long-term N status, their lack of correlation in watersheds merits further investigation.