Competition for nitrogen between plants and microorganisms in grasslands: effect of nitrogen application rate and plant acquisition strategy

Several studies have investigated how nitrogen (N) addition changes N competition between focal plant species and soil microorganisms; still, the impact on community-level plant-microbial N competition and the underlying mechanisms remain unclear. We conducted a short-term (4 h) N-15 labeling experiment in an alpine meadow subjected to 7 years of NH4NO3 additions (0, 5, 10, and 15 g N m(-2) year(-1)), by monitoring changes in soil properties (e.g., pH, Al3+, NH4+, NO3-), microbial biomass (MB), plant community composition, root traits (e.g., root length, root area, specific root length), as well as the plant (nine focal species and at the community level) and microbial N uptake. Change in the N competition between the nine focal plant species and microorganisms following N addition depended on the species. At the community level, the N addition rate did not affect plant-microbial competition for NH4+ and NO3- (P > 0.05). Nitrogen addition directly decreased NH4+ competition (beta = - 0.700) but indirectly increased because of improved plant uptake due to increased N availability (beta = 1.214). Competition for NO3- was dependent on microbial uptake (beta = - 0.953) and was influenced by opposing effects of increased N availability (beta = 1.342) and reduced MB (beta = - 0.439). Thus, the effects of increased soil N availability and suppressed MB on plant and microbial N competition offset each other, while the plant community had a negligible impact. Such responses should be taken into account for better predictions of the effect of N addition on net primary productivity and ecosystem stability.