|Grassland is one of the most sensitive terrestrial ecosystem types in terms of response to global climate change. The environmental effects of long-term atmospheric nitrogen (N) deposition on the structure and functioning of grassland ecosystems have attracted much attention. Effects of simulated N deposition on the structure and productivity of plant communities have been explored in many studieshowever, its effects on soil microbial nutrient utilization processes and plant-microbe interactions have remained underexplored. Meanwhile, previous studies often experimented N addition with levels higher than current and future projected N deposition rates, producing highly uncertain predictions of ecosystem structure and functioning. In particular, it remains unclear whether the environmental effects of atmospheric N deposition can be alleviated in the special regional environment of arid and semi-arid grassland ecosystems ‒ where the growth of plants is more strongly limited by N and the shortage of water resources and alkaline soil suppresses plant and soil microbial activities. In this study, 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-2yr-1), in a semiarid grassland on the Loess Plateau of China. Based on eight years of experimental monitoring, we evaluated the effects of N deposition on species diversity, productivity and stability of plant communities, and the mechanisms underlying the changes in plant community stability. Additionally, we investigated the effects of N deposition on soil physicochemical and microbial properties and plant-microbe interactions. We also studied the effects of N deposition on soil microbial nutrient utilization strategy and its regulation on soil carbon and nitrogen mineralization processes. The results of this dissertation study have important theoretical and practical significance for understanding the influence of atmospheric N deposition on the structure and functioning of grassland ecosystem and the scientific management and sustainable utilization of grassland nutrients.
The main results are as follows:
1. N addition altered community diversity, reduced species richness, evenness, diversity, and dominance. With the increase of N addition, the aboveground biomass of plant communities increased first and then decreased, while the stability of plant communities first decreased and then increased. Both the aboveground biomass and stability of plant communities showed non-linear responses to N addition. The nonlinear change in community stability was positively correlated with species asynchrony, species richness, and species diversity, as well as the stability of dominant species and the stability of the grass functional group. The current level of atmospheric N deposition had no significant influence on the stability of plant communities.
2. N addition changed soil physiochemical properties and microbial nutrient utilization characteristics. With increasing N addition, soil pH slightly decreased and soil organic carbon (SOC) increased at the later stagein contrast, soil total nitrogen (TN) barely changed. The ratios of dissolved organic carbon (DOC) and dissolved organic nitrogen (DON) in soils first increased and then decreased. The ratios of DOC and dissolved organic phosphorus (DOP) in soils decreased significantly in 2014, but DON:DOP varied among years. The change of soil resource stoichiometry resulted in the enhancement of soil microbial C limitation, and thus weakened soil microbial respiration. The effects of N addition were much larger on fungal community compositions than on bacteria compositions, which significantly increased soil fungal richness and changed community composition. However, it had no significant effect on the diversity of soil bacteria and its community compositions.
3. N addition indirectly affected plant species richness, soil microbial respiration, and their interactions by changing soil physiochemical properties. Reduced plant richness and soil microbial respiration were associated with the changes in soil dissolved inorganic nutrients (NO3--N、NH4+-N and available phosphorous), soil total nutrients, DOC, and soil water content (SWC), rather than DOP and pH. In addition, aboveground biomass had a significantly negative effect on plant richness, whereas this effect became somewhat positive via the pathway of soil microbial respiration path. These results indicate that changes in plants and microbes are mainly due to changes in soil resources rather than pH when ecosystems are exposed to low levels of atmospheric N deposition and further enhanced plant-microbe interactions and consequently, impacted plant community composition.
4. N addition altered resource stoichiometry while soil microbes still maintained stronger stoichiometric homeostasis, which resulted in changes in the stoichiometric imbalances between soil microbial communities and their resources. All stoichiometric imbalances, including C:N, C:P, and N:P imbalances, increased up to intermediate doses and then decreased. These nonlinear responses implied that increasing N addition enhanced microbial C limitation rather than P limitation. Data on microbial adaptive responses to resource stoichiometric imbalances revealed that, under C limitation, soil microbial communities regulated their ecoenzyme production and threshold element ratios (TER) to maintain stoichiometric homeostasis. Additionally, we found that the N-induced reduction of soil microbial respiration was directly linked to increasing TER but indirectly linked to soil ecoenzyme stoichiometry and microbial biomass stoichiometry. These results suggest that coordinated regulation of microbial biomass stoichiometry and soil enzyme stoichiometry can lead to higher C use efficiency (CUE) and lower nutrient use efficiency, further lowering soil microbial respiration.
5. N addition, season, and their interactions had significant effects on soil carbon mineralization (Cmin) rates. In contrast, we observed a significant seasonal effect on the soil nitrogen mineralization (Nmin) rate only. Across all seasons, DOC, SWC, catalase, urease, sucrase, MBC, SOC:TN, and TN were the most pivotal predictors of the soil Cmin rate. Comparatively, catalase, MBC, DOC, NO3--N, urease, TN, NH4+-N, SWC, and MBC:MBN ratio were the dominant drivers of soil Nmin rate. The identified potential drivers that regulated the soil Cmin and Nmin rates in responses to N addition varied seasonally. With elevated N addition, the soil Cmin and Nmin rates became decoupled, which was a consistent relationship among most seasons.