Effects of nitrogen deposition with phosphorus addition on desert steppe plant communities

Nitrogen (N) and phosphorus (P) enrichments are expected to increase above-ground net primary productivity (ANPP), alter plant community composition, and accelerate biodiversity losses. However, how N and P synergistically affect plant communities and the underlying mechanisms remain elusive. Here, a 2-year in situ experiment with five levels of N and two levels of P treatments in an arid ecosystem was conducted to examine the independent and interactive effects of N and P additions on plant community structure and ANPP and their driving factors. The analysis of variance showed that nutrient enrichment significantly increased ANPP by a maximum value of 93% at N (15 g N m−2 yr−1) treatment and a maximum of 95% at N-P (10 g N m−2 yr−1, 10 g P2O5 m−2 yr−1) treatments. N addition alone significantly decreased species richness and dominance but increased evenness and diversity. The principal component analysis indicated that dominant species of plant communities gradually shifted from a perennial species Stipa breviflora to an annual species Tribulus terrestris under N and P enrichments. The nonmetric multi-dimensional scaling ordination revealed that N addition significantly promoted the divergence in species composition, whereas P enrichment relieved this change. The structural equation modelling illustrated that N and P additions affected ANPP mainly via modifying plant community composition and structure. This study demonstrates that the potential mechanism of adverse changes in a vulnerable ecosystem is related to the alterations in plant functional groups mainly due to divergent responses of dominant species to nutrient enrichment. This should be considered when assessing and modelling the ecological and evolutionary trajectories of terrestrial ecosystems under the conditions of global environmental changes.