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 in which several metrics of nitrogen availability have been measured. Our metrics included: 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. Not surprisingly, the strongest correlation (Kendall's tau) was between net nitrification and N mineralization (τ = 0.61, p < 0.0001). Net nitrification was correlated with foliar and soil δ15N (p < 0.05), while net N mineralization was correlated with soil δ15N but not foliar δ15N. Foliar and soil δ15N were correlated across tropical sites (τ = 0.68, p < 0.0001), but not in temperate sites (τ = 0.02, p > 0.05). To our surprise, the only significant correlation we found between terrestrial- and water-based metrics was that of net N mineralization with stream DIN : DON (τ = 0.62, p = 0.004). Given both soil δ15N and stream DIN : DON are used to infer long-term N status, their lack of correlation in watersheds merits further investigation.