| Nitrogen deposition,one of the global climate change factors,has significantly affected the structural function and stability of grassland ecosystems.Current research on nitrogen deposition mainly focuses on simulating through nitrogen addition within fenced grasslands.However,more than half of the world’s grasslands once have or are currently experiencing grazing.Therefore,in order to fully reveal the response of grassland ecosystems to nitrogen deposition,it is necessary to investigate the effects of N addition on grasslands with different grazing intensities.In this study,we investigated the response mechanism and regularity of grassland plant biomass,plant diversity and soil physicochemical properties to nitrogen addition at different grazing intensities at the global scale through Meta-analysis.The main results are as follows:1.Aboveground biomass increased significantly with the increase of nitrogen application rate,but with the increase of grassland grazing intensity,the average percentage increase of aboveground biomass gradually decreased.After nitrogen addition,the aboveground biomass of the community increased significantly on average by 40.1%,28.6% and 22.9% on the no-grazing,light-grazing and mediumgrazing grasslands,respectively,but there was no significant change in the heavygrazing grassland.In addition,following nitrogen addition,community belowground biomass increased significantly only in heavy grazing.2.Different effects of nitrogen addition on different functional groups under different grazing intensities.Nitrogen addition significantly increased grass biomass,but the average rate of increase in grass biomass decreased with increasing grazing intensity.The response of Forbs biomass to nitrogen addition was opposite to that of grass biomass,the average rate of increase in forbs biomass was higher in grassland with higher grazing intensity.In addition,nitrogen addition significantly decreased legumes biomass in no-grazing and light-grazing grasslands,but there was no significant change in legumes biomass in medium-grazing and heavy-grazing grasslands.3.Nitrogen addition significantly decreased soil p H,and significantly increased soil available nitrogen.Whether in the no-grazing grassland or grazing grassland,the soil p H decreased significantly with the increase of nitrogen application rate and the duration of the experiment,while the soil available nitrogen and soil nitrate nitrogen increased significantly with the increase of nitrogen application rate.In addition,grazing inhibited the promoting effect of nitrogen addition on soil ammonium nitrogen.4.Nitrogen addition significantly increased soil organic carbon on no-grazing grassland,and the percentage change increased with the duration of the experiment,and at high nitrogen application rate,the response of soil organic carbon to the duration of the experiment was more obvious.However,grazing inhibited the promoting effect of nitrogen addition on soil organic carbon,resulting in no significant effect of nitrogen addition on soil organic carbon in grazing grassland.Nitrogen addition had little effect on soil stoichiometric ratio,and only significantly increased soil C:P ratio in the nograzing grassland.5.Nitrogen addition significantly reduced plant diversity,and its negative effects increased with increasing nitrogen application rate and experimental duration.However,the mean reduction in plant diversity weakened as grassland grazing intensity increased.Grazing effectively slowed the decreasing trends of Shannon-Wiener diversity index,Pielou evenness,and Simpson diversity index along nitrogen application rates.In addition,the negative effects of nitrogen addition on Shannon-Wiener diversity index,Pielou evenness,and Simpson diversity index were smaller in grasslands with high average annual precipitation than in grasslands with low average annual precipitation.6.Grazing significantly altered the relationship between changes in aboveground biomass and species richness.There was a significant negative correlation between aboveground biomass effect value and species richness effect value in no-grazing grassland,but there was no significant relationship between the two in grazing grassland.Soil acidification due to nitrogen addition resulted in a decrease in species richness in both no-grazing and grazing grasslands.The results suggest that soil acidification plays a more important role in the loss of grassland species,especially in grazing grasslands,than changes in aboveground biomass under nitrogen addition. |
