| The implementation of“Grain for Green”Project has a profound impact on soil organic carbon（SOC）pool and cycles in the Loess Plateau region,China.Under the background of“carbon peaking and carbon neutrality”goals of China,it is of great significance to accurately evaluate the carbon sequestration in the project.Biological soil crusts（Biocrusts）are extensively distributed on the surface of the revegetated grasslands and largely regulate the C cycle in the ecosystems.Grazing disturbance can affect the soil carbon（C）cycle by affecting the plant,litter,biocrust community structure and coverage,and soil physical and chemical properties,ultimately regulating the SOC sequestration in the revegetated grasslands.However,the previous studies on grazing disturbance effects on the C cycle of grassland ecosystems often ignored the biocrusts.To date,the effect and its mechanism of grazing disturbance on SOC sequestration in biocrusts in the revegetated grasslands were not clear,which was the weak point in the study on the role of biocrusts in soil C cycle,and also hindered the accurate assessment of the C sequestration in the“Grain for Green”Project.Therefore,with the revegetated grasslands from croplands excluded from grazing as the control,the study was carried out in the controlled fenced grazing grasslands and long-term free grazing grasslands.Starting from the effects of grazing disturbance on land surface characteristics and soil physical and chemical properties,this study analyzed the effects of grazing disturbance on SOC and its labile component,explored the effects and mechanisms of grazing disturbance on SOC stability of biocrusts,focused on analyzing the effects of grazing disturbance on biocrust photosynthesis and soil respiration,evaluated the effect of grazing disturbance on SOC sequestration in the revegetated grasslands and revealed the mechanism,by using field survey,in situ monitoring and laboratory analysis,etc.The main conclusions are as follows:1.Grazing disturbance significantly affected the coverage,community structure and biomass of biocrust in the revegetated grasslands,and the influence degree was related to grazing intensity and duration,precipitation and soil texture.After 2 years of grazing disturbance,the biocrust coverage in the revegetated grasslands decreased,and when the grazing intensity was≥3.0 goat·ha-1,the biocrust coverage was significantly reduced by 37.24%-57.76%compared with that before grazing.The effect of long-term grazing on biocrust coverage was related to precipitation,soil texture and grazing intensity.There was no significant difference in biocrust coverage between long-term grazing and enclosure in the areas with less precipitation and coarse-textured soil,while it decreased by 35.49%-57.81%under long-term grazing compared with that under enclosure in the areas with high precipitation.After 2 years of grazing disturbance,there was no significant change in biocrust community structure.Long-term grazing alted biocrust community structure,with an increase in the relative cover of cyanobacteria but a decrease in the relative cover of mosses,and the influence degree was related to grazing intensity.Grazing decreased biocrust biomass.After 2 years of grazing disturbance,when the grazing intensity was≥6.7 goat·ha-1,cyanobacteria biomass decreased significantly by 42.42%compared with that before grazing.The biomass of cyanobacteria and mosses in the revegetated grasslands under grazing were significantly lower than those under enscosure in the areas with less precipitation,while they did not change significantly in other areas.2.Grazing disturbance significantly affected the bulk density and C/N ratio in the biocrust soil layer and its underlying soil layers,and the degree of the effects were related to grazing intensity and duration,precipitation and soil texture.After 2 years of grazing disturbance,the soil bulk density in the biocrust layer and 0-5 cm soil layer in the returned grassland increased by 3.90-19.12%compared with that before grazing.The degree of long-term grazing affecting soil bulk density was related to soil texture.Except the area with coarse-textured soil,long-term grazing increased soil bulk density in the biocrust layer and the soil layer beneath the biocrusts by 6.10%-62.61%,compared with the enclosure.After 2 years of grazing disturbance,when the grazing intensity was 2.2-3.0 and>6.7 goat·ha-1,the soil C/N ratios increased in the biocrust layer and 2-10 cm soil layer.The effect of long-term grazing on soil C/N ratio was related to precipitation.The soil C/N ratio did not change significantly with grazing intensity in the areas with higher precipitation.With the increasing grazing intensity,soil C/N ratios of the biocrust layer and 0-10 cm soil layer showed a trend of increasing first and then decreasing in the areas with less precipitation.3.Grazing disturbance significantly affected SOC and easily oxidized organic carbon（EOC）content in the biocrust layer and its underlying soil layers,and the soil depth and degree of the effects were related to grazing intensity and precipitation.After 2 years of grazing disturbance,SOC and EOC contents in the biocrust layer decreased,and SOC in the 0-5 cm soil layer increased.However,grazing had no significant effect on EOC in the soil layer beneath the biocrusts.Compared with the enclosure,long-term grazing decreased SOC and EOC contents in 2-10 cm soil layer by 19.59%-40.23%and 19.89%-49.98%,respectively,in the area with less precipitation.SOC and EOC contents showed a trend of increasing first and then decreasing with the increase in grazing intensity in other areas.4.Under different precipitation and soil texture conditions,grazing disturbance affected SOC stability by affecting plant cover and biomass,biocrust coverage and community structure,and soil physical and chemical properties.Under grazing disturbance,the order of factors affecting soil organic carbon stability was soil porosity（-0.370）>biocrust community structure（0.295）>soil C/N ratio（0.265）>precipitation（-0.198）>soil texture（-0.135）>grazing intensity（-0.031）>plant cover（-0.016）.SOC stability in the soil layer beneath biocrusts was mainly regulated by altitude,plant aboveground biomass,biocrust coverage,and plant coverage.With increasing soil depth,the explanation of biocrust cover for SOC stability variation decreased from 6.7%to 1.7%.5.Grazing disturbance reduced biocrust photosynthetic rate and soil respiration rate by reducing biocrust coverage and biomass,and by regulating the labile C in soils.Under grazing disturbance,biocrust photosynthetic rate and soil respiration rate were linearly positively correlated with biocrusts coverage and biomass.Soil respiration rate was linearly positively correlated with soil microbial biomass C of biocrust layer and dissolved OC content of 0-2 cm soil layer.With increasing grazing intensity,biocrust photosynthetic rate and soil respiration rate decreased gradually.Compared with those under enclosure,biocrust photosynthetic rate and soil respiration rate under the grazing intensity more than 6.7goat·ha-1 decreased by 76.87%and 29.57%-34.26%,respectively.6.Grazing disturbance affected the land surface characteristics（litter,animal excrement,biocrusts）and SOC content in the revegetated grasslands,and then changed the amounts of C input and output in biocrusts,and finally affected the total SOC stocks（SOCS）in the uppermost 20 cm soil layer of biocrusts in the revegetated grasslands.Grazing disturbance regulated the amount of C input from litter decomposition and biocrust photosynthesis by affecting litter biomass and its decomposition rate,and biocrust coverage and biomass.Grazing increased the amount of C input from goat dung decomposition by increasing the amount of goat dung and its decomposition rate.Under grazing disturbance,according to the magnitude of influence of parameters on soil respiration,the order was soil temperature（0.688）,soil moisture（0.271）,SOC of 0～2 cm soil layer（0.151）,biocrust coverage（0.109）,grazing intensity（-0.090）,SOC of biocrust layer（-0.075）.After 2 years of grazing disturbance,SOCS in the uppermost 20 cm soil layer of biocrusts increased by 10.59～28.25%compared with that before grazing,and the increase rate of SOCS decreased with increasing grazing intensity.The SOCS increased the most under the grazing intensity of 2.2 goat·ha-1.Under long-term grazing,the SOCS in the uppermost 20 cm soil layer of biocrusts was related to grazing intensity.There was no significant difference in SOCS between grazing intensity of 3.3-4.7 goat·ha-1and enclosure in each area.In summary,in order to accurately assess the SOC sequestration effect in the revegetated grasslands on the Loess Plateau,this study explored the influence of grazing disturbance on SOC stability,and revealed the influence mechanism of grazing disturbance on the input and output of SOC in biocrusts,and evaluated the effect of grazing disturbance on SOC sequestration in biocrusts in the revegetated grasslands.The results showed that after 2 years of grazing disturbance,SOCS in the uppermost 20 cm soil layer of biocrusts increased by 10.59～28.25%compared with that before grazing.In addition,there was no significant difference in SOCS between long-term grazing intensity of 3.3-4.7 goat·ha-1 and enclosure on the Loess Plateau.This study lay a foundation for the comprehensive understanding of the effects of grazing on SOC cycle,and provide a basis for the accurate assessment of the accurate assessment of the C sequestration in the“Grain for Green”Project and the sustainable management of the revegetated grasslands. |
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