Understanding nitrogen (N) cycling in different ecosystems is crucial to predicting and mitigating the global effects of altered N inputs. Although wetlands have always been assumed to differ largely from terrestrial ecosystems in N cycling, evidence from direct comparison from the field along wide environmental gradients is lacking. Here, we hypothesized strong coupling of plant and soil δ¹⁵N in terrestrial ecosystems due to lower N inputs and losses but weak coupling of plant and soil δ¹⁵N in wetlands because of higher N inputs and losses.

We performed a large-scale field investigation on 26 pairs of herbaceous wetland and terrestrial sites across China covering 21 degrees of latitude and determined natural abundance of nitrogen isotopes (δ¹⁵N) in soils and leaves of 346 dominant and subordinate plant species. We analysed the relationships between leaf and soil δ¹⁵N and their drivers including plant functional types in these two types of ecosystems.

Plant functional types including mycorrhizal type and N2-fixing status had consistently significant influences on leaf δ¹⁵N in herbaceous wetland and terrestrial ecosystems. Leaf δ¹⁵N increased significantly with soil δ¹⁵N within and across mycorrhizal types in both ecosystems, and, as hypothesized, the relationships were stronger and steeper in terrestrial than in wetland ecosystems. Moreover, leaf and soil δ¹⁵N were positively and significantly correlated within both N₂-fixers and non-fixers in terrestrial ecosystems and within only non-N₂-fixers in wetlands. At the community level, we also found more highly significant relationships between leaf and soil δ¹⁵N in terrestrial than in wetland ecosystems. Besides plant functional types, climatic and soil factors contributed to the variation in leaf δ¹⁵N in both ecosystems.

Synthesis. Weaker relationships between plant and soil δ¹⁵N in wetlands at species and community levels supports the hypothesis that larger N inputs and losses lead to weaker coupling in the plant-soil systems in wetlands than in terrestrial ecosystems. This provides strong evidence from a large spatial scale for contrasting N cycling in these two types of ecosystems regardless of plant functional type in terms of nutrient uptake strategy. Our findings add to our predictive power of ecosystem N dynamics under environmental changes, e.g. land-use changes and elevated N inputs.

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