Effect Of Grazing On Soil Organic Carbon Stability Of Revegetated Grasslands In The Hilly Loess Plateau Region,China

Grassland soil carbon pool is played an important role in the soil carbon pool of terrestrial ecosystem.It is of great significance to study the change and mechanism of grassland soil organic carbon stability for elucidating the carbon cycle process of grassland ecosystem,increasing the function of soil carbon sink and guiding the sustainable utilization of grassland resources.Since the implementation of the returning farmland to forest（grass）in the Hilly Loess Plateau region,a large area of grasslands has been formed.Grazing is one of the main agricultural activities which affects the stability of grassland soil organic carbon directly or indirectly in this area.At present,there are few studies focus on the influence of grazing intensities on soil organic carbon stability in this area,and the sustainable management of revegetated grassland for carbon sequestration and emission reduction lacks the necessary scientific basis.Therefore,the object of this study was the revegetated grasslands in the Hilly Loess Plateau region,and the enclosed grassland was selected as the control.Short-term fenced grazing monitoring(2.2,3.0,4.2,6.7,16.7 goat·hm^-2 five grazing intensities were setting,representing ultra-light,light,moderate,heavy and ultra-heavy grazing intensity respectively)and long-term natural grazing plot investigation(three grazing intensities were divided according to goat dung density,and three grazing intensities were fitted according to the relationship between goat dung density and grazing intensity,which were 1.5～2.6,3.2～4.9,5.9～8.7 goat·hm^-2,recorded as G1,G2 and G3).Combined with the incubation experiment of soil organic carbon mineralization under laboratory goat excreta and litter addition,the effect and mechanism of grazing intensity on soil organic carbon content and stability were explored.The main conclusions are as follows:（1）Grazing reduced the aboveground biomass of plants.Under light grazing intensity(3 goat·hm^-2),the biomass of litter significantly increased or remained unchanged.The influence of grazing on soil physical and chemical properties（soil bulk density,p H,total nitrogen content and C/N）is closely related to grazing duration and grazing intensity.Under short-term grazing,the aboveground biomass of plants decreased by an average of 39.9%compared with the control,and under light grazing intensity(≤3 goat·hm^-2),the litter biomass increased by 105.1%compared with the control.The soil bulk density in 0～5cm soil layer significantly increased under ultra-heavy grazing intensity(16.7 goat·hm^-2).The total nitrogen（TN）content significantly increased by 450.9%in 0～2 cm soil layer under heavy grazing intensity(6.7 goat·hm^-2).The soil TN content significantly increased by139.4%～267.2%in 2～5 cm soil layer under 2.2～6.7 goat·hm^-2 grazing intensity.The soil C/N significantly decreased in 2～5 cm soil layer under 2.2～6.7 goat·hm^-2 grazing intensity.The soil C/N significantly decreased in 5～10 cm soil layer under moderate grazing intensity(4.2 goat·hm^-2).Under long-term grazing,the aboveground biomass decreased by 47.4%on average compared with the control.The soil bulk density significantly increased in 0～10 cm soil layer under G2 and G3 grazing intensity(≥3.2 goat·hm^-2).The soil p H significantly increased in 5～10 cm soil layer under G3 intensity(≥5.9 goat·hm^-2).The TN content significantly decreased in 5～20 cm soil layer under G3 grazing intensity.（2）Under light grazing intensity(3 goat·hm^-2),the soil organic carbon content did not change significantly,but the microbial biomass carbon ratio increased which the turnover rate of organic carbon quickly.Under short-term grazing,The soil organic carbon（SOC）content increased in 0～10 cm soil layer under light grazing(3 goat·hm^-2).The microbial biomass carbon ratio（MBC/SOC）increased in 0～10 cm soil layer under ultra-light and light grazing intensity(≤3 goat·hm^-2).Under long-term grazing,there was no significant change of SOC content in 0～10 cm soil layer under G2 grazing intensity(3.2～4.9 goat·hm^-2).And the MBC/SOC significantly increased by 0.8%～2.0%.（3）The effects of increased input of excreta on the soil labile organic carbon ratio was greater than that of litter,which increased the mineralization rate and total mineralization amounts of soil organic carbon,and accelerated the turnover rate of soil organic carbon.The addition of goat dung and litter increased soil MBC/SOC,and the increasing effects of goat dung addition on MBC/SOC were higher than the decreasing effects of litter addition.The addition of goat urine mainly increased soil dissolved organic carbon ratio（DOC/SOC）.The mineralization rate of soil organic carbon was divided into rapid release,slow release and relatively stable stages after the addition of excreta and litter.With the increasing of goat dung addition amounts,the increasing effect of the average mineralization rate in each stage and total mineralization amounts of soil organic carbon were(0.29～2.63,0.13～1.64 and 0.11～0.85 g·kg^-1·d^-1,12.03～111.51 g·kg^-1)higher than the decreasing effect of the litter addition amounts reduction(1.10～0.14,0.37～0.09 and 0.28～0.07 g·kg^-1·d^-1,31.72～6.39 g·kg^-1).（4）The relationship between soil organic carbon and plant biomass,soil physical and chemical properties under long-term grazing was more complex than that under short-term grazing.The influence factors of soil organic carbon stability under long-term and short-term grazing are different.Under short-term grazing,Soil organic carbon（SOC）content in 0～10 cm soil layer was affected by soil total nitrogen（TN）.Soil MBC/SOC was mainly affected by litter biomass（LB）.Soil DOC/SOC was mainly affected by SOC and C/N.Soil EOC/SOC has no significant correlation with the selected factors,which may be affected by other factors.Under long-term grazing,SOC content was comprehensively affected by plant biomass（LB and aboveground biomass AGB）and soil physical and chemical properties（TN,clay and C/N）.The DOC/SOC is mainly affected by SOC and clay content.MBC/SOC and EOC/SOC may be affected by other factors.In summary,light grazing intensity(≤3 goat·hm^-2)can maintain the surface litter biomass and soil organic carbon content in 0～10 cm soil layer,and increase the ratio of labile organic carbon（MBC/SOC）.The input of animal excreta increases the activity of soil organic carbon and promotes the turnover and utilization of organic carbon.The above results provide a theoretical basis for the appropriate utilization of revegetated grasslands.