Regulation Of Livestock Grazing On The Response Of Grassland Photosynthetic Carbon Input To The Addition Of Water And Nitrogen

Grassland ecosystem as an important part of the global ecosystem, its ecosystemphotosynthetic carbon input to play an important role in regulating the balance of the globalcarbon cycle. Water is a major limiting factor for primary productivity in arid and semi-aridsteppe ecosystem. Moreover, primary productivity of grasslands is also affected by the lack ofsoil nitrogen. Global changes pattern suggests that both precipitation and nitrogen depositionwill be increased for next several decades in grassland zone of northern China. Grazing is themost important land use type for grassland ecosystem; its effects on grassland carbon cyclingdepend highly on grazing intensity. It is likely that different livestock feed intensity mayaffect the response of grassland photosynthetic carbon input to changes in key resources. Weestablished a manipulated grazing intensity platform in the steppe grassland, which isdominated by Leymus chinensis, of northeast China. We applied water and nitrogen additiontreatments in both grazed and non-grazed plots and measured vegetation composition andecosystem carbon fluxes. The major findings and conclusions are:(1) Nitrogen fertilization promoted L. chinensis growth, increased L. chinensis density,and enhanced ecosystem photosynthetic carbon input, but the strength of impact decreasedwith increasing grazing intensity. We observed significant N addition effects on height,density, cover and aboveground biomass in L. chinensis in both non-grazed and light grazedplots. There were no significant N addition effects on important value, plant diversity, leaf netphotosynthetic rate, stomatal conductance, leaf transpiration rate, leaf water use efficiencyand ecosystem water use efficiency in either non-grazed and grazed sites. In non-grazed plots,N addition significantly enhanced gross ecosystem productivity, ecosystem evapotranspirationand net ecosystem CO2exchange. In low grazing intensity plots, there were no significantdifferences in gross ecosystem productivity, net ecosystem CO2exchange and ecosystemevapotranspiration between the N addition subplots and control subplots.(2) The effects of water addition on ecosystem photosynthetic carbon input showed nodifferences among the examined grazing intensity. Moreover, we observed that there were nowater addition effects on height, density, important value and coverage of L. chinensis andplant species diversity. At leaf level, there were no differences in net assimilation rate,stomatal conductance, transpiration rate, and leaf water use efficiency. The observed no wateraddition effects on vegetation composition, leaf and ecosystem levels photosynthetic carboninput may attribute to the high amount of precipitation in the experimental year. (3) Compared to the water addition treatment, the impacts of nitrogen fertilization ongrassland ecosystem photosynthetic carbon input were much greater. There were nosignificant differences in L. chinensis height, density, important value, coverage andaboveground biomass between the N addition and N plus water addition plots, which suggestthat there were no interactive effects between N addition and water addition. Similar patternswere detected in leaf and ecosystem level photosynthetic carbon assimilation.The results showed that livestock grazing could regulate the response of grassland to keyglobal change factors, such as the examined enhancement in nitrogen and water. There weresignificant impacts of N addition on vegetation composition, density, height and biomass of L.chinensis, as well as carbon assimilation at either leaf or ecosystem level. The observed noeffects of water addition may attribute to the high background water availability as the studiedarea received much more rain during the experimental year relative to average precipitationvalue. Overall, in-depth study of the regulating effects of livestock grazing on the response ofgrassland ecological functions is required for the prediction of the grasslandmulti-functionality under future environmental changes.