| A sharp increase in greenhouse gases induced by human activities has formed the greenhouse effect.Global warming will then lead to global precipitation pattern change.At the same time,excessive application of nitrogen fertilizers and fossil fuel combustion have continuously increased nitrogen input into terrestrial ecosystems.Nitrogen enrichment,global warming,and precipitation alteration will directly affect plant physiological processes,and indirectly affect plant growth and community composition by changing soil nutrients and water content.These changes will subsequently change ecosystem functioning.Understanding global change effects can provide theoretical support for making strategies to sustainably manage ecosystem under the scenario of global change.The Loess Plateau is the largest loess accumulation area in the world.The main vegetation type is grassland.It is a key area for animal husbandry production and ecological protection in China.However,it has been over-developed by humans for a long time.Its low vegetation coverage and serious soil erosion have made it a fragile ecosystem,which is extremely sensitive to environmental changes.Here,using a typical semi-arid grassland in the Loess Plateau,we mainly studied the composition and structure of plant community,functional diversity and aboveground net primary productivity under five-year treatments of nitrogen addition,warming and precipitation alteration.We examined plant functional traits(including leaf area,specific leaf area,leaf dry matter content,maximum photosynthetic rate per unit area of the leaf,and intrinsic water use efficiency)and investigated community composition in 2019.We found that:(1)Nitrogen addition significantly increased soil inorganic nitrogen content(increased by 128%),but significantly reduced the canopy light transmittance(decreased by 25.81%).Under warming,the soil moisture significantly reduced by 3.72%.As the rainfall increased,soil available nitrogen showed a significant downward trend,and the soil water content increases significantly.Nitrogen addition and warming significantly increased soil water content by 9.6%under the treatment of reduced rainfall.(2)Nitrogen addition,warming and precipitation alteration significantly changed the composition of plant communities.Nitrogen addition significantly reduced the species richness via affecting graminoid and forb(decreases 13.33%).Under warming,the richness of forb increased when water was more sufficient,which in turn promoted community richness increase.The abundance of the community significantly decreased by 14.67%under warming.Community structure was affected by the interaction of temperature,nitrogen addition and precipitation alteration.Under increased rainfall,the negative effects of nitrogen addition on forb abundance got alleviated,leaving the overall community abundance unchanged.(3)Community weighted mean of functional traits changed significantly.Under nitrogen addition,plant leaf area,specific leaf area and height increased significantly(67.38%,15.18%,27.13%,respectively),intrinsic water use efficiency decreased significantly by 18.75%.Plants adopted resource acquisition strategies in response to nitrogen addition.Under warming,leaf dry matter content increased significantly by 9.52%,and the plants adopted a resource-conservative strategy.When the rainfall decreased,leaf area increased significantly by 20.45%.Nitrogen addition and rainfall changes have an interactive effect on leaf maximum photosynthetic rate per area.Except for intrinsic water use efficiency and leaf maximum photosynthetic rate per area,the community function dispersion of other traits did not change.The intrinsic water use efficiency significantly trended to be same under the treatment of nitrogen addition.Warming would make the maximum photosynthetic rate per unit area of the leaf significantly diverge.(4)Warming significantly reduced the above-ground biomass by 14.90%.Leaf area and the maximum photosynthetic rate per unit area of the leaf could explain biomass change under warming.When the rainfall decreased,leaf dry matter content could be used to explain aboveground biomass change under nitrogen addition.Under the natural rainfall treatment,nitrogen addition mainly increased community height to increase aboveground biomass.In conclusion:(1)Nitrogen addition,warming and precipitation alteration mainly changed the community structure by affecting the relative composition of graminoids and forbs;(2)Plants adopted resource acquisition strategies responding to enriched nutrients,but adopted resource conservative strategies under warming to copy with stressful temperature.(3)The mass ratio hypothesis can explain the change of the net primary productivity of the ecosystem under warming and the interaction of nitrogen addition and rainfall change.This study is of great practical significance for understanding the response of ecosystem to global change and developing strategies for semi-arid grassland ecosystem management. |
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