Nitrogen (N) deposition and fertilization, which represent key sources of N input in many terrestrial ecosystems, influence all levels of the ecosystem and involve complex mechanisms. Quantitative and modelling approaches can be used to understand this complexity. In this study, we carried out in situ N addition experiments in a Stipa krylovii steppe in northern China. We evaluated the effects of N addition on plant diversity and productivity under two scenarios (fertilization and simulated increased N deposition) using a structural equation model (SEM). N addition had direct effects on community weighted means (CWM) of plant functional traits and soil properties but had indirect effects on community structure. The changes in community structure and soil properties caused by N addition decreased plant diversity, whereas productivity remained relatively stable and was mainly controlled by changes in community structure. The changes in soil properties and plant diversity caused by N addition had little effect on productivity or soil pH. We conclude that the changes in plant diversity and productivity with increased N input in the S. krylovii steppe were mainly due to differences in growth responses of different species to increased N and the resulting community responses, such as changes in community structure. The results of the present study provide a theoretical basis for grassland management and conservation in the wake of global environmental change.
Keywords: Nitrogen deposition; Plant diversity; Plant functional traits; Productivity; Soil properties; Stipa krylovii steppe.
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