Effects Of Soil Acidification On Ecosystm Multifunctionality In A Semi-arid Grassland On The Loess Plateau

Terrestrial ecosystems worldwide have experienced or are experiencing dramatic increases in acid deposition and anthropogenic reactive nitrogen(N)inputs,largely caused by fossil fuel combustion and agricultural fertilization.The acid deposition and anthropogenic reactive nitrogen inputs often decrease soil pH and result in multiple ecological effects,including changes in plant and microbial communities and other soil properties.Those changes may further influence the ability of ecosystem to maintain multiple functions and services simultaneously,known as ecosystem multifunctionality.However,how the changes of soil pH may affect ecosystem multifunctionality has rarely been studied,especially in fragile and sensitive ecosystems,such as grasslands on the Loess Plateau in northwestern China.To address this knowledge gap,we conducted a 2-yr field experiment consisting of five levels of acid addition rates(0,0.68,1.69,10.81 and 27.04 mol H+m-2)in a semi-arid grassland on the Loess Plateau and examined the responses of soil properties,plants,soil organisms as well as 19 ecosystem functions.We classified these 19 ecosystem functions into five functional scenarios including 1)functions relating to primary productivity only;2)functions relating to carbon cycling only;3)functions relating to nitrogen cycling only;4)functions relating to phosphorus cycling only and 5)all 19 ecosystem functions.Then,we used multi-model inference approach,classification random forest analysis,and structural equation modeling to assess the relative importance of soil properties,plants and microbial communities in driving the ecosystem multifunctionality as well as those functional scenarios.We found that acid addition(the decreased of soil pH)altered soil properties,plant communities,microbial properties,and reduced multiple ecosystem functions and ecosystem multifunctionality significantly.The decreased soil pH drove the changes of multifunctionality in two ways.On the one hand,soil pH indirectly drove the multifunctionality by altering plant communities,microbial properties and soil properties.On the other hand,it directly drove the changes of the multifunctionality.And,the direct effects of soil pH were the dominant pathway to drive the multifunctionality,followed by the intermediary effects of increased concentrations of soil Al3+ ions and decreased concentrations of soil cations(K+,Ca2+,Mg2+and Na+).However,the strengths of direct and indirect effects of soil pH on multifunctionality varied when different functional scenarios were examined.For instance,specially,the direct effects of decreasing soil pH on EMF,EMF-productivity and EMF-C accounted for 85.1%,41.8%and 48.3%of their total effects,respectively.The total effects of Al3+ ions on EMF-productivity and EMF-C were greater than the direct effects of soil pH.Species abundance had a non-negligible effect on primary productivity.These results indicated that soil pH plays an important role in driving ecosystem multifunctionality in a semi-arid grassland on the Loess Plateau.Our study can help advance the understanding to the effects of global change factors(e.g.nutrient enrichment and acid deposition)on ecosystem multifunctionality via alteration in soil pH and provides new insights to better develop sustainable strategies for ecosystem management.