Contrasting nitrogen cycling between herbaceous wetland and terrestrial ecosystems inferred from plant and soil nitrogen isotopes across China

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-N₂-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 support 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, for example, land-use changes and elevated N inputs.