Phosphorus addition prevent plant species richness loss with nitrogen deposition in Inner Mongolia grassland
Increased atmospheric nitrogen (N) deposition due to excessive application of N fertilizer and combustion of fossil fuels has decreased plant species richness in grassland ecosystems. Several possible mechanisms have been proposed to explain the species loss with increased N deposition, e.g. competitive exclusion, preemption of light or space, and metal toxicity (Al3+, Mn2+ and Fe3+). Based on ecological stoichiometric theory, it is hypothesized that imbalanced nutrient conditions (especially N and phosphorus (P)) should be an important mechanism. Yet, this mechanism has never been tested. We conducted a N (0.2 mol N m-2 yr-1) and P (0, 0.05, 0.1, 0.2, 0.3, 0.4 mol P m–2 yr-1) addition experiment and a P (0.05 mol m-2 yr-1 P) addition experiment since 2006. Species composition and biomass data were collected each year in August to explore the dynamics of plant species richness and composition under imbalanced and balanced N and P conditions. We also did a long-term monitoring experiment to explore the dynamic of plant species richness in a native grassland undergoing natural N deposition.
The results of N and P addition experiment showed that N addition decreased plant species richness significantly. Similarly, the long-term data showed that with natural N deposition, species richness decreased significantly in the past 30 years. However, the results showed P addition increased species richness in grassland with both added N and natural N deposition. Nitrogen addition decreased relative biomass of species with foliar N:P > 12 and increase relative biomass of species with foliar N:P < 12, but P addition showed opposite trends. Nitrogen addition increased soil N:P significantly. However, with the addition of P soil N:P decreased significantly, i.e. P addition returned imbalanced N:P back to normal conditions. Thus, our results indicate that P addition counters plant species richness loss with N deposition in Inner Mongolia grassland, suggesting that the imbalanced N and P condition may result in the biodiversity loss in grassland.