The nitrogen-fixing potential of plant communities depends on climate and land management

Abstract

Aim: The occurrence, relative richness and abundance of N-fixing plants are the main features of the N-fixing potential of vascular plant communities. We aimed to unveil how climate, biome type and land management regime affect the N-fixing potential of vegetation. Since the world is witnessing an invincible transition of all those environmental factors during the last century, we examined the global growth trends of N-fixers throughout recent history. Location: Global. Taxon: Vascular plant species. Materials and Methods: We conducted a meta-analysis of publicly available vegetation data from 290 communities. Combined with the species' N-fixation capability, we examined the occurrence, abundance and richness of N-fixing vascular species across varying climate conditions, land use regimes and potential habitat types. Results: There are relatively more N-fixing vascular plants in warmer and drier climatic conditions. Management of communities by cutting woody plants, grazing and mowing hay increases the number and abundance of N-fixing species. The effect of management is pronounced in wet conditions, where a dense canopy would otherwise emerge. As a result, semi-natural grasslands exhibit higher N-fixer occurrence than natural grasslands and non-tropical forests, cessation of management results in a temporary increase in the abundance of N-fixers. Repeated observations of sites indicated that the global occurrence of N-fixers increased slightly in managed communities from 1906 to 2018, but there were no other temporal trends. Main conclusions: The overall number of N-fixers in plant communities is low. The occurrence, relative richness and abundance of N-fixing species are higher in sparse vegetation in dry climates and in extensive management where biomass is removed from communities. A probable explanation is the high light availability in dry vegetation and due to biomass removal, which provides a competitive advantage to light-demanding N-fixers. A lack of consistent temporal trends suggests that global change is not yet an active driver of N-fixing potential in plant communities.