Effects Of Grazing On Phosphorus Balance In A Typical Steppe Of Inner Mongolia

Livestock grazing is the most important and direct way to use grassland ecosystems.Grazing intensity(GI)is a key factor for nutrient input and output of grassland ecosystems.Phosphorus(P)is an essential element for plant growth and reproduction involving in the physiological and biochemical processes of organisms in many forms.Most studies focus on the carbon and nitrogen cycling,however,the process and mechanism of P balance remain understudied,especially for effects of grazing on P balance in grassland ecosystems.This study was carried out at long term grazing experiment in Inner Mongolia Grassland Ecosystem Research Station the Chinese Academy of Science.This study carried out in a randomized block design with 2 topography and 7 grazing intensities,including 14 grazing treatments in total.P concentration,P pool and P turnover of different grassland components were studied for 2 years.Main results as follows:(1)At the depth of 0-20cm,grazing had significant effects on soil total P concentration and P pool,and topography was also the main factor affecting the soil total P concentration and P pool.Soil total P concentration and P pool decreased(P<0.05)with an increase in GI in the flat grazing system,whereas they increased(P<0.05)slightly with an increase in GI in the slope grazing system.Soil total P concentration and P pool were higher(P<0.001)in the flat than in the slope.GI had no effects on the spatial heterogeneous of soil total P concentration,whereas the effects of spatial heterogeneous was greater than grazing treatment.The spatial heterogeneous of the flat grazing system was greater than the slope.GI and topography had significant effects on soil available-P(P<0.05),it increased with GI in two years.Soil available-P was higher(P<0.001)in the flat than in the slope.(2)Grazing had little effect on P concentration of the plant community(P>0.05),but the aboveground biomass P pool was influenced(P<0.001),and the differences was due to the different contribution of biomass.The aboveground biomass P concentration and P pool were also impacted by the year and topography(P<0.001).Grazing had significant effects on aboveground biomass P pool in 2 years in the flat and the slope system(P<0.05).Aboveground biomass P pool was hump-shaped in 2016 while decreased in 2017 with an increase in GI.In flat and slope system,aboveground biomass P concentration and P pool were higher(P<0.001)in 2016 than in 2017,the differences was caused by the P concentration of two years.(3)Grazing had little effect on litter P concentration(P>0.05),but litter P pool decreased(P<0.01)with an increase in GI,and the differences was due to the different contribution of biomass.Litter P concentration was affected by the topography(P<0.001)but litter P pool was not impacted(P>0.05).(4)Grazing had little effects on P concentration of live root,dead root and BNPP(P>0.05),but their P pool decreased(P<0.001)with GI increased,this trends was caused by the contribution of biomass is greater than the P concentration.The topography affected P concentration of live root(P<0.01)but did not impact P concentration of dead root and BNPP(P>0.05).P concentration of live root was higher(P<0.01)in the flat than in the slope.Neither P pool of live root or dead root was affected by the topography and the year(P>0.05),but P pool of BNPP differed(P<0.05)between years.(5)GI had little effect on litter residual rate(P>0.05),but P release decresed with an increase in GI due to litter mass decreased(P<0.001).Litter residual rate and P release was not affected(P>0.05)by the topography,whereas root litter residual rate and P release were affected by both GI and topography of three decomposition time(P<0.05).Root decomposition rate was affected by GI and topography(P<0.05).Root litter residual rate was hump-shaped with GI increased in the flat grazing system,while increased with an increase in GI in the slope grazing system.The effects of grazing on root litter P release was consistent in the flat or slope system,grazing decreased the root P release throuth decreased the root biomass.The decomposition rate of sheep feces was not influenced by GI(P>0.05),but P release of sheep feces altered due to feces was increased by grazing.P release of sheep feces was greater in the flat than in the slope(P<0.001).(6)P resorption proficiency and efficiency was affectedby GI(P<0.05).P resorption proficiency and efficiency decreased with an increase in GI in the flat topography(P<0.05),but was hump-shaped with an increase in GI in the slope(P<0.01).(7)Feed intake and excrement of sheep were hump-shaped or increased with an increase in GI.Feed intake was the highest in the heavy grazing but decreased in GI=9.0 sheep ha-1.Live mass gain per sheep and live mass gain per ha had the opposite trend.Live mass gain per sheep was the highest in light and moderate grazing,while live mass gain per ha was the highest in heavy grazing(P<0.05).In this study,the optimun GI to get live mass gain and keep grassland healthy was 6.0 sheep ha-1.GI also influenced the distribution of rodents,the dominate rodent species was Cricetulus barabensis with small body in low GI,and Spermophilus dauricus increased with an increase in GI.The effect of GI on insect biomass varied annually(P<0.001),which decreased with an icrease in GI in 2016 but was not affected in 2017.In summary,grazing had important influences on P cycling in the typical grassland ecosystem.Grazing accelerated P cycling and affected the ecosystem sustainability.Light grazing did not demage the grassland ecosystem,but animal products and the economic benefits were low.Heavy grazing increased animal products and herders' incomes but increased soil erosions and nutrents losses,lowered grassland function and sustainability.Therefore,optimum GI is crucial for maintaining ecological function and pruductivity of grassland ecosystems.