Community-wide consistence in plant N acquisition during post-agricultural succession in a karst area, southwest China

Strategies of plant N acquisition among coexisting species are critical for many aspects of ecosystems with differential uptake of various N forms as one of these strategies, but much debate still exists. In the present study, strategic N uptake of nitrate and ammonium by 30 dominant species were investigated over two post-agricultural succession sequences in a calcareous karst area, southeast China. Each sequence includes grassland, shrubland, secondary forest and primary forest. Proportional contribution of nitrate and ammonium to plant N was estimated using isotopic mixing calculation. N uptake preference for the two N forms was assessed by comparing assimilation and availability of the N forms. The proportional contribution to plant N was much greater for nitrate than ammonium for the investigated species except three species (Monochoria vaginalis, Boehmeria dolichostachya and Miscanthus floridulus) for which the difference was not significant. Correlation analysis revealed that the contribution to plant N by a particular N form was linearly and positively related to its concentrations in soil. The proportional contribution to plant N by nitrate and ammonium increased logarithmically and decreased exponentially, respectively, as soil nitrate to ammonium ratio increased. N form uptake preference varied according to communities, but was consistent for the coexisting species within each community. The results indicate that the coexisting species share a similar strategy in their uptake of nitrate and ammonium during post-agricultural succession in the calcareous karst region, southwest China.