| Exploring the response of nutrient cycling to increased nitrogen(N)deposition caused by human activities and climate warming in grassland,as well as understanding the underlying mechanisms,can help us better comprehend and predict the response strategies of terrestrial ecosystems to global changes in the future.However,previous studies have lacked multi-site investigations at regional or spatiotemporal scales,and the diverse experimental conditions have often led to disparate results.Therefore,in this study,N addition and experimental warming control devices were established along an aridity gradient in Inner Mongolia,encompassing desert,typical,and meadow grasslands.The study aimed to investigate the response of soil-plant nutrient concentration to N addition and experimental warming,and to examine the regulating effect of soil moisture on this response process.Additionally,validating the results obtained from Inner Mongolia grasslands by integrating experimental data on N addition and warming from global grassland,and further summarizing the general patterns of soil-plant nutrient concentration in response to N addition and experimental warming,with the goal of providing insights for understanding and predicting the dynamic changes in nutrient cycling in grassland ecosystems under N deposition and climate warming.The main findings of this study are as follows:(1)Nitrogen addition significantly increased the soil ammonium N by 51.0% and nitrate N by 204% in arid years of Inner Mongolia grasslands,and by 20.8% and 54.4% in wet years.However,the effect of experimental warming on soil N in both arid and wet years was not significant.The response ratios of soil N to N addition and warming showed a significant decreasing trend with increasing aridity index.Along the aridity gradient in Inner Mongolia,the aridity index,together with N addition,explained 72.8%,30.3%,and 14.1% of the variations in total N,nitrate N,and ammonium N in soil,respectively.Similarly,the aridity index,along with warming,explained 64.7%,7.7%,and 28.2% of the variations in total N,nitrate N,and ammonium N in soil,respectively.Global data analysis also yielded similar results to Inner Mongolia grasslands,showing that N addition significantly increased the soil ammonium N and nitrate N concentrations by 37.0% and 58.2%,respectively.In contrast to the results in Inner Mongolia grasslands,global data indicated that warming increased the soil nitrate N concentration by 19.7% in grasslands.This discrepancy may be attributed to the specific warming devices used in this study in Inner Mongolia grasslands,as the analysis of global data also indicated that warming devices significantly influenced the response of global grassland soil N to warming.Additionally,factors such as grassland type,functional group,experimental duration,mean annual temperature,and mean annual precipitation can also mediate the response of soil N to N addition and warming in grasslands.(2)Nitrogen addition did not significantly affect the total phosphorus(TP)and available phosphorus(AP)concentrations in both arid and wet years of Inner Mongolia grasslands.However,experimental warming significantly increased the AP in the soil.The response ratios of soil AP to N addition and warming also showed a decreasing trend with increasing aridity index.Along the aridity gradient in Inner Mongolia,the aridity index,together with N addition,explained 62.9% and 12.9% of the variations in TP and AP in soil,respectively.Similarly,the aridity index,along with warming,explained 54.2% and 17.5% of the variations in TP and AP in soil,respectively.In contrast to the results in Inner Mongolia grasslands,global data analysis indicated that N addition increased the AP by 14.2% and increased litter P by 15.5%,while it decreased microbial biomass P by 11.2%.Warming,on the other hand,decreased the AP and microbial biomass P by 6.7% and 10.5%,respectively,while increasing litter P by 46.2%.Different N addition rates,fertilizers,warming methods,warming magnitudes,and experimental durations can all have varying effects on the TP and AP in grassland ecosystems.(3)Nitrogen addition significantly increased the leaf N of both grasses and forbs by20.5% and 9.23%,respectively,in arid years of Inner Mongolia grasslands.In wet years,N addition increased the leaf N of grasses and forbs by 36.6% and 37.1%,respectively.However,the effect on leaf P was not significant.Along the aridity gradient in Inner Mongolia,the response ratios of leaf N and P to N addition and warming showed an increasing trend with increasing aridity index.The aridity index,together with N addition,explained 22.7% and 3.7% of the variations in leaf N and P along the aridity gradient,respectively.Similarly,the aridity index,along with warming,explained 33.0% and 5.2% of the variations in leaf N and P,respectively.Consistent with the results in Inner Mongolia grasslands,global data analysis showed that N addition significantly increased leaf N in grassland ecosystems,while the effect on leaf P was not significant.Specifically,N addition significantly increased green and senesced leaf N by 32% and 50%,respectively,with only a slight increase in senesced leaf P.The green leaf P did not show a significant change with N addition.Global data analysis also indicated that N addition only significantly altered the N-P coupling in green leaves,while there was no significant change in N-P coupling in senesced leaves.This finding may be closely related to the observed significant decrease in nutrient resorption efficiency of plant leaves under N addition in the global data analysisIn summary,this study used a combination of controlled experiments and data analysis to elucidate the effects of N addition and experimental warming on soil N,P,and plant leaf N-P characteristics in grassland ecosystems at a larger regional scale.The results indicate that N addition and warming increased the concentration of soil available nutrients in grassland,alleviating nutrient limitation in the short term and enhancing ecosystem productivity.The alleviation of nutrient limitation further promoted the acquisition of soil nutrients by grassland plants,resulting in increased leaf nutrient concentrations and altered leaf N-P coupling.However,this may disadvantage microorganisms in nutrient competition with plants,suppressing microbial assimilation and utilization of soil nutrients,and limiting microbial biomass and activity in the grassland ecosystem.Long-term N addition and warming may lead to more severe nutrient limitation in grassland ecosystems by altering the microbial community structure.Additionally,environmental and geographical factors complicate the response of soil-plant N-P characteristics to N addition and warming by regulating soil enzyme and microbial activity,as well as plant nutrient utilization strategies.Therefore,longterm field monitoring and the establishment of data-sharing networks may be more conducive to accurately understanding and monitoring the dynamics of nutrient cycling in grassland ecosystems under future global changes.The findings of this study provide valuable insights for understanding and predicting the responses of global grassland ecosystems to future N deposition and climate warming. |
