Abstract
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 = 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 = 0.68, p < 0.0001), than in temperate sites (τ = 0.23, p = 0.02). The only significant correlations between terrestrial- and water-based metrics were those of net nitrification (τ = 0.48, p = 0.01) and N mineralization (τ = 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 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.
Highlights
Nitrogen (N) limitation to primary production is widespread in both terrestrial and aquatic ecosystems, and variation in N availability drives differences in ecosystem properties across space and time (Vitousek and Howarth, 1991; Elser et al, 2007; LeBauer and Treseder, 2008)
We found that neither soil solution nor stream dissolved inorganic to organic nitrogen (DIN) : DON were correlated with mean annual temperature, precipitation, elevation or N deposition (p > 0.05)
We suggest that future research utilize metaanalysis techniques to look at how soil C : N changes across ecosystem gradients, and whether or not it agrees with latitudinal patterns observed for soil δ15N and stream DIN : DON (Martinelli et al, 1999; Brookshire et al, 2011)
Summary
Nitrogen (N) limitation to primary production is widespread in both terrestrial and aquatic ecosystems, and variation in N availability drives differences in ecosystem properties across space and time (Vitousek and Howarth, 1991; Elser et al, 2007; LeBauer and Treseder, 2008). Quantifying N availability over timescales that are relevant to ecosystems is nontrivial. Short timescale measurements of N availability in soil are common (e.g. inorganic N pools, N mineralization and nitrification rates; Binkley and Hart, 1989; Sparks et al, 1996), but such short-term proxies are influenced by both short and long-term drivers, and it is difficult to know whether short-term proxies can be used to infer N status (i.e. the relative abundance of plant available N) over long timescales in relatively undisturbed ecosystems. Land-based investigations of N cycling commonly measure soil extractable N, N mineralization, and nitrification, which give a snapshot of N status over minutes to days (Binkley and Hart, 1989; Robertson et al, 1999). Repeated measurements give longer timescale information, but even the longest studies are short relative to ecosystem development
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