Nutrient enrichment can reduce ecosystem stability, typically measured as the temporal stability of productivity that has multiple underlying mechanism including species resistance and resilience to nutrient pulses and the resulting compositional change. Moreover, nutrient enrichment can alter plant-soil interactions (e.g. mycorrhizal symbiosis) that determine plant productivity and diversity. Thus, it is likely that nutrient enrichment and interactions between plants and their soil communities co-determine the stability in plant community composition and productivity. Yet our understanding as to how nutrient enrichment affects the multiple facets of ecological stability and the role of above-belowground interactions are still lacking.

We tested how mycorrhizal suppression and phosphorus (P) addition influenced functional and compositional stability of plant community in a three-year field study. Here functional stability is the temporal community variance in primary productivity; compositional stability is represented by compositional resistance, turnover, species extinction and invasion.

Compared with mycorrhizal suppression, the intact AM fungal communities reduced community variance in primary productivity by reducing species synchrony at high levels of P addition. Species synchrony and population variance were linearly associated with community variance when mycorrhiza were not suppressed, while these relationships were decoupled or weakened by mycorrhizal suppression. The intact AM fungal communities promoted the compositional resistance of plant communities by reducing compositional turnover, but this effect was suppressed by P addition. P addition increased the number of species extinctions and thus promoted compositional turnover.

Our study shows P addition and AM fungal communities can jointly and independently modify the various components of ecosystem stability in terms of plant community productivity and composition.

We used a two-factor randomized block design containing all combinations of two levels of fungicide treatment (natural and fungicide treatment) and 6 levels of P addition (0, 2.4, 4.7, 9.4, 19.0 and 37.8 g P₂O₅ m^-2·year^-1), for a total of 12 treatments. In May 2010, we established 10 blocks and all treatments were randomly assigned to 12 plots (2 × 2 m) in each block. Block arrangement took into account the slope at the study sites. There were 2-m gaps between each plot and block to reduce potential effects of treatments on each other. 

Above-ground biomass, cover and species richness were measured at the peak of plant biomass production in the middle of August from 2010 to 2012. Species richness and cover was measured non-destructively in a permanent quadrat (1 × 1 m) in each plot (where no other sampling took place), established in May 2010. One subplot (0.5 × 1 m) was used to estimate above-ground biomass in each plot every year, with a new subplot selected every year for this purpose. Shoots of each species were cut at the soil surface and oven dried at 65°C for 72 hours, before being weighed. We recorded shoot dry weight of each species separately. Net biomass production (productivity) was calculated as the sum of shoot dry weights of all plant species in each subplot and expressed as g m^-2.