The Effects And Mechanisms Of Livestock Grazing On Soil Carbon Sequestration In Major Grasslands Of Northern China

Grassland,defined as permanent herbaceous plant communities,covers approximately 25% of the terrestrial surface.Grassland ecosystem provides vital ecosystem services role in carbon sequestration and climate regulation,and have substantial ecological and economic value.Grassland management is one of the key issues in grassland ecosystem research,as well as the main technical ways to maintain sustainable grassland production.How to manage grassland scientifically and improve the carbon sequestration function of grassland ecosystem plays an important role in regulating carbon cycle and climate change.As grazing by domestic livestock currently occupies 26% of terrestrial land cover,and it has an important regulatory role in the structure,process(diversity,nutrient cycling),and functions(productivity,stability)of grassland ecosystems.Empirical evidence has identified livestock grazing to have complex effects on the function of grassland soil carbon sequestration.The otherness effect of grazing on grassland soil carbon sequestration patterns may depend on climatic factors,soil characteristics,topography,plant community composition,grazing history,grazing intensity,etc.In terms of grazing modes,the effect of grazing livestock species and assemblage on grassland soil carbon sequestration may be one of the important factors affecting grassland ecosystem carbon sequestration.In order to effectively improve the carbon sequestration function of grassland,how to select grazing species and assemblage are still very weak in different grassland types.In this study,we investigated the impacts of herbivore species identity(NG,no grazing;SG,sheep grazing;CG,cattle grazing;and MG,mixed grazing of sheep and cattle),on the function of soil carbon sequestration(soil carbon inputs,carbon outputs,carbon fractions,and carbon storage)across three different grassland types(MS,meadow steppe;TS,typical steppe;and DS,desert steppe)in temperate steppes of Eastern Eurasia.We explored the response mechanism of grassland ecosystem carbon sequestration to grazing.It is expected to provide certain theoretical and practical guidance for the rational utilization of grassland grazing to achieve the future goal of“carbon neutrality”.We reported the main results and conclusions as follows:(1)Livestock grazing had a strong effect on soil carbon input(plant and microbial sources),and the effect was dependent on herbivore species,assemblage,and grassland types.The soil carbon input from plant sources mainly depended on the belowground plant biomass.In mesic grasslands(i.e.,meadow steppe and typical steppe),large-sized herbivores(cattle)grazing and mixed grazing significantly enhanced belowground plant biomass by 28.10%-116.70%.However,small-sized herbivores(sheep)grazing and mixed grazing dramatically increased belowground plant biomass by 41.27% and 30.40%.Soil carbon input from microbial sources may be derived by microbial biomass,in the high plant diversity grassland(i.e.,meadow steppe and desert steppe),cattle grazing and mixed grazing significantly increased soil microbial biomass carbon content by 18.37%-88.80%.By contrast,in the low plant diversity grassland(i.e.,typical grassland),cattle grazing and mixed grazing significantly reduced soil microbial biomass carbon content by 11.70%-39.57%.The effects of grazing on grassland carbon input were mainly attributed to the changes in plant community element content and plant community diversity.It also suggested that the change of plant community caused by grazing livestock might be the key factor of carbon input in grassland ecosystem and affect carbon sequestration in grazing system.(2)The effects of herbivores significantly reduced soil respiration rate,and soil respiration rate and accumulation were affected by herbivores and grassland types.In all grassland types,the effect of cattle grazing on soil respiration rate was significantly greater than that of sheep grazing.The effects of livestock grazing on soil respiration showed similar trends in the simulated rising temperature mode,decreasing temperature mode,short-term and long-term incubation experiment.However,the main control factors of soil respiration were different in grazing and no grazing plots.Our results showed that soil respiration was mainly affected by soil biotic factors(i.e.,microbial abundance and complexity)in no grazing plots.By contrast,the effect of the soil respiration rate is mainly affected by soil abiotic factors(i.e.,carbon quality)in grazing plots.Moreover,the influencing factors of soil respiration were also diverse in different grassland types.In the meadow steppe,it is mainly affected by soil abiotic factors,and in the typical steppe,it is mainly controlled by soil biotic factors.This is caused by the disparate habitat conditions(plant diversity and soil types)of different grassland types.(3)Livestock grazing significantly affected soil carbon fractions(physical,chemical,and biological fractions),and this effect was strongly dependent on herbivore species identity,grassland types,and their interactive effects.For physical carbon fractions,in the meadow steppe and typical steppe,cattle grazing and mixed grazing significantly increased by 5.93%-59.49%,while in desert steppe,sheep grazing and mixed grazing significantly increased by 17.29%-35.14%.For chemical and biological carbon fractions,cattle grazing and mixed grazing had the greatest impact in all grassland types.The variation of soil carbon fractions influenced by grazing had different effects on predicting the variability of soil organic carbon,and the physical carbon fractions could best predict the change of soil organic carbon.The variation of carbon fractions caused by grazing was strongly correlated with plant and edaphic factors,and the correlation of each factor and physical carbon fractions,chemical carbon fractions was stronger than that of biological carbon fractions.Different soil carbon fractions were affected by plant and edaphic factors.The physical carbon fractions were mainly influenced by plant carbon input,the chemical carbon fractions were mainly controlled by soil carbon output,and biological carbon fractions were mainly driven by edaphic factors.(4)The effects of herbivores grazing on shallow layer of soil carbon storage depended on herbivore species,assemblage,grassland type,and their interactive effects.Although the grazing intensity was similar,the grazing activity of herbivore-specific species and assemblage showed distinct effects on soil carbon storage in various grassland types.Specifically,in mesic grasslands(i.e.,meadow steppe and typical steppe),large-sized herbivores(cattle)grazing enhanced soil carbon storage by 16.29%-20.80%,but no effects for small-sized herbivores(sheep)grazing.By contrast,small-sized herbivores grazing dramatically increased soil carbon storage by 21.42% and 31.55%,while no effects for large-sized herbivores grazing in xeric grassland(i.e.,desert steppe).Importantly,mixed grazing of cattle and sheep consistently increased soil carbon storage 11.41%-30.37% across entire grassland types.These results were mainly determined by the belowground plant biomass,plant community composition,and soil bulk density.The differential and idiosyncratic herbivore effects on soil C might be associated with the following three mechanisms: first,various foraging mode in sheep and cattle resulted in different root growth;second,various diet selection in sheep and cattle changed differently plant community composition;third,different travelling and trampling by sheep and cattle led to different soil properties.Based on the main results and conclusions,we gained the mechanisms about grazing affected carbon sequestration in grassland ecosystems.Our study confirmed that the effects of livestock grazing on grassland carbon sequestration were complex and varied.The effects were not only dependent on grassland types,but also affected by livestock types and assemblage.The change of plant community caused by herbivores was the key factor of soil carbon input and affected the carbon sequestration function of grazing system.Livestock grazing reduced soil carbon output and was beneficial to soil carbon sequestration.The change of soil carbon output mainly depends on the effects of herbivores and grassland types on soil carbon quality and microbe.Specifically,in mesic grasslands(i.e.,meadow steppe and typical steppe),large-sized herbivores increased the belowground carbon input to compensate for the loss of aboveground carbon input through foraging stimulation,and the livestock maintained carbon sequestration by reducing carbon output through reducing carbon quality.In xeric grassland(i.e.,desert steppe),the effect of small-sized herbivores foraging stimulus on belowground carbon input was stronger.The effect of livestock grazing on soil carbon sequestration in different grassland types is the comprehensive effect of foraging,trampling,and excretion on plant community and soil characteristic.Our findings suggest that adopting specific-sized herbivore stewardship in different grasslands could assist with enhancing ecosystem C functionality and services;large-sized herbivores and small-sized herbivores are suitable for grazing in mesic grasslands and xeric grasslands,respectively.Moreover,mixed grazing with diverse herbivores may be the optimal grazing mode in most grasslands in northern China,in terms of mitigating soil C emissions and climate change.This study has important implications for grassland management and our findings support the “diversifying livestock” hypothesis and verified the adaptability of the “carbon quality temperature” hypothesis in grazing system.This study provides new thinking for grassland management.The carbon sequestration function of grassland ecosystem is not only dependent on environmental factor,but also affected by human activity such as livestock grazing.Therefore,diversified livestock grazing is crucial for maintaining and improving the carbon sequestration function of grassland ecosystems.This study has certain theoretical significance and practical guiding value for grazing system management.