Nitrogen deposition enhances plant-microbe interactions in a semiarid grassland: The role of soil physicochemical properties

Abstract Rapidly increasing atmospheric nitrogen (N) deposition may affect plants, microbes, and their interactions by changing soil physicochemical properties. A few studies have explored how the integrated plant-soil-microbe system responds to elevated N deposition; however, the experimental N loads used in these studies are generally higher than current and future N deposition rates. Thus, these results could generate highly uncertain predictions of the plant-soil-microbe system in response to N deposition because high N addition levels always trigger environmental stress (e.g., soil acidification). We manipulated a long-term experiment, consisting of six N addition levels (0, 1.15, 2.30, 4.60, 9.20, and 13.80 g N m−2 yr−1), in a semiarid grassland in northern China. Our results showed that N addition caused a significant negative relationship between dissolved organic carbon (DOC) and inorganic N, indicating that N addition enhanced C limitation of the soil microbial community and this effect triggered significant decreases microbial respiration. We also demonstrated that N addition significantly decreased soil fungal richness but had no effect on soil bacterial richness. Additionally, aboveground and belowground biomass were significantly increased with N addition, with the highest value at 9.20 N m−2 yr−1 treatment. Relative to control treatment, plant richness was not significantly changed when the N addition rates were lower than 4.6 g N m−2 yr−1 but then significantly decreased. Reduced plant richness and soil microbial respiration were associated with the changes in dissolved inorganic nutrients, soil total nutrients, DOC, as well as soil water content rather than dissolved organic phosphorus and pH. Importantly, aboveground biomass presented a significant negative effect on plant richness, whereas this effect was positive via the pathway of soil microbial respiration. These results show that the current simulated N deposition rates would weaken the linkage between plant productivity and richness but enhance plant-microbe interactions by modifications of soil physicochemical properties other than soil pH, and further impact plant composition.