| Litter is the dead,above and belowground,organic material of plants.In terrestrial ecosystems,more than 50% net primary productivity return to soil in the form of plant litter.Hence,plant litter accumulation and decomposition are the key processes in mediating nutrient cycling and energy flow in ecosystems.Previously,most of study of litter decomposition are focused on forest ecosystems,relatively little is known about how plant litter response to the changes in environmental factors,as well as the role and mechanisms of plant litter in mediating nutrient dynamics in grassland ecosystems.Moreover,as an important type of human disturbance,little is known about how livestock grazing can affect plant litter accumulation and decomposition in grasslands.In this study,we aim to evaluate how changes in habitat under different climate conditions in the large scale,and how livestock grazing at both of the local and large scale,can influence the litter decomposition processes of the dominant grass Leymus chinensis in grasslands of northern China.We asked four specific questions: 1)how different grazing regime(sheep vs.cattle grazing)can affect litter accumulation? 2)how livestock grazing can affect litter decomposition by changing litter quality and soil abiotic and biotic conditions? 3)how livestock under different climatic conditions at the large scale can affect litter decomposition? 4)The relative importance of the several abiotic and biotic factors in controlling the decomposition rate of plant litter.We found that: 1)At the local scale,we found that livestock grazing can significantly lower the accumulation of plant litter at soil surface.We also found that different grazing regime could have different influences on litter decomposition processes.Compared to the control treatments,sheep grazing significantly increased litter decomposition rate.Cattle grazing tended to enhance litter decomposition compared to the control treatments,but not significantly.2)livestock grazing could affect litter decomposition in two pathways,the first one is via changing soil environments,the second one is via changing the quality(C:N)of plant litter itself.Our study confirms the importance of these two pathways in grazing regulation of plant litter decomposition.On the one hand,we used the litter with same quality to test how grazing-induced changes in soil conditions influence litter decomposition.We found that grazing significantly changed the physiochemical properties of soils,particularly those of soil moisture,soil C content,soil N content,and soil microbe biomass.Importantly,our regression analyses showed that the litter decomposition rate is closely related the changes in soil conditions in the plots.On the other hand,using the plant litter collected from areas with different grazing animals,we found that grazing-induced changes in plant litter quality(C:N)will also have important influences on litter decomposition.Litter from cattle grazed areas have a significant lower N content compared to the litter from sheep grazed and control areas.We also found a significant relationship between litter decomposition and litter quality in the plots.3)Under the effect of spatial heterogeneity,the climate,vegetation and soil environment of different habitats affect the decomposition of litter.Our study found that cattle grazing can significantly reduce soil water content and increase soil bulk density in different habitats,soil nutrient content and soil microbial responses to grazing are also extremely sensitive.In the areas with lower soil nutrients,grazing significantly increased soil C content,soil N content and soil microbial biomass carbon.In contrast,grazing significantly reduced soil C content,soil N content and soil microbial biomass carbon in areas with rich soil nutrients.There is a significant coupling effect between soil C and N content.We found that the effects of grazing on litter decomposition also have similar trends.However,grazing can significantly changed the C/N ratio of the remaining litter.As the decomposition accelerates,the C/N ratio of the remaining litter increases,which means the C and N content of the litter are coupled-related during the decomposition.In addition,there was a significant interaction between different habitats and grazing treatments on soil environment and litter decomposition.Taken together,our study showed that,livestock grazing plays a very important role in the regulation of litter.One the one hand,grazing changed soil conditions,such as moisture,N content,and microbe biomass,therefore influence litter decomposition.Livestock at different body sizes can play a unique role in the decomposition of litter by affecting the soil environment.On the other hand,grazing can affect litter decomposition via changing the quality(C:N)of litter.Different livestock exert different impacts on litter quality,which probably due to the differences in their diet selection and feeding behaviors.Besides,at the large scale,we found that grazing has different regulation on litter decomposition in habitats under different climatic conditions,and this regulation is directly related to soil nutrient content in the habitat.Under the effect of spatial heterogeneity,the climate,vegetation and soil environment in different habitats affect the decomposition of litter.In the decomposition process,there are a number of coupling effects exist,such as the interactions between climate factors,between soil C and N content,and between litter C and N content.Moreover,there was a significant interaction between different habitats and grazing treatments on soil environment and litter decomposition.In summary,we found that at different scale,the same ecological processes(litter decomposition)are regulated by different factors(climate and grazing),and their underlying mechanisms are complex.We need more study in this field to reveal the underlying mechanism of these litter decomposition patterns.Our study may have important implications for the prediction of nutrient dynamics,as well as grassland productivity in northern China. |
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