Characteristics And Mechanisms Of Soil Carbon And Nitrogen Coupling During The Anthropogenic Transition From Desert Steppe To Shrubland

Transition from grassland to shrubland is considered as a typical regime shifts of terrestrial ecosystem,which has an important impact on the carbon and nitrogen cycles and microorganisms in native grassland ecosystems.Semi-arid desert grasslands are often limited by soil carbon and nitrogen elements,so soil carbon and nitrogen availability becomes a critical factor for vegetation restoration in desert grasslands.However,the introduction of shrub might lead to the increased complexity of coupling state between soil carbon and nitrogen.Therefore,understanding the characteristics of soil carbon,nitrogen and moisture variation,their coupling and driving mechanisms during the transition from desert grassland to shrubland is an important step to conserve the stability and sustainability of native desert grassland ecosystem.Based on the practical needs of vegetation restoration in desert grasslands and ecological problems of shrub and guided by theory of ecosystem state and transition theory,shrub encroachment process theory and ecological stoichiometry theory,this experiment carried out at the grassland-shrubland mosaics over the past 40 years(desert grassland,grassland edge,shrubland edge and shrubland)in the eastern Ningxia desert grassland area by using macroscopic and microscopic research techniques and methods under the condition of field observation and laboratory simulation incubation,such as GIS and high-throughput sequencing,to investigate the soil carbon,nitrogen,and moisture pattern,carbon and nitrogen conservation and coupling characteristics,and microbial response through different spatial scale(the landscape scale,community scale,microsite scale and root scale).We also used ecological stoichiometry to try to explore the state,causes and trends of carbon and nitrogen coupling in multi-scale.This paper aimed at providing scientific basis and data support for the sustainability of anthropogenic vegetation restoration model in semi-arid desert grasaland.The main findings are as follows:(1)Soil moisture,organic carbon and total nitrogen contents and their spatial distribution patterns in landscape scale were changed during the anthropogenic state transition from desert grassland to shrubland.According to observation for soil carbon,nitrogen,and moisture of 43 grassland-shrubland mosaics covering about 3000 km2,0～200 cm soil water content significantly decreased by 46.13%with desert grassland to shrubland transition,but increased the spatial autocorrelation of soil moisture,mainly caused by structural factors such as topography and climate.In addition,the anthropogenic desert grassland-shrubland state transition increased the 0～100 cm soil organic carbon content by 18.32%,but had no significant effect on the total nitrogen content,and the spatial autocorrelation of both decreased,which was controlled by stochastic factors such as livestock grazing and mowing.(2)The anthropogenic transition over the last 40 years has led to variation of microbial mineralization,accelerating decomposition of soil organic matter and therefore leading to a decline of soil quality in shrubland.The anthropogenic transition from desert grassland to shrubland over the past 40 years has led to a decrease in soil carbon fraction,some nitrogen fraction and enzyme activities involved in carbon and nitrogen decomposition at the community scale from 0 to 20 cm.ultimately leading to a significant decrease in soil quality.In situ mineralization experiment in the field showed that soil organic carbon and microbial biomass decreased significantly by 49.18%and 44.61%,respectively,while their conversion rates did not change significantly,mainly because soil microbial quotien remained stable.However,the conversion rate of soil total nitrogen increased significantly by 20.90%,while the net mineralization rate of soil(net ammonification rate and net nitrification rate)decreased significantly by 5.07 times,mainly due to the decrease of soil carbon to nitrogen ratio.Soil organic and inorganic nitrogen contents were the main driving factors affecting soil carbon and nitrogen coupling during in situ mineralization.These results suggested that the microbial mineralization process experienced a shift from a slow-re lease type in desert grasslands to an eager type(rapid depletion)in shrublands.consuming large amounts of substrate in initial phrase and then resulting in a negative mineralization rate.(3)The anthropogenic transition changed soil-microbial-extracellular enzyme involving carbon,nitrogen,and phosphorus stoichiometry characteristics and microbial metabolic limitations,with accompany with alteration of diversity of soil bacterial and fungal communities by reducing soil microbial biomass nitrogen and organic carbon content.The anthropogenic transition from desert grassland to shrubland imbalance soil elemental homeostasis and exacerbating the relative microbial carbon limitation and nitrogen limitation in shrubland by changing soil-microbial-extracellular enzyme C-N-P stoichiometry ratios and nutrient use efficiencies In addition,the anthropogenic transition also increased the species richness and diversity of soil bacterial and fungal communities significantly.And the relative abundance of dominant soil bacteria and fungi,with significant increases in the relative abundance of Aspergillus phylum,Bacteroidetes phylum,and Cysticercus phylum in bacteria communities,and significant decreases in the relative abundance of Acidobacter phylum,Methylomirabilota phylum,and Streptomyces phylum,Peridiomycetes phylum in fungi communities.The decrease in soil microbial biomass nitrogen and organic carbon content had impacts on the structure of soil bacterial and fungal communities.(4)The anthropogenic transition from desert steppe to shrubland accelerates the "plantmicrobe-soil" positive feedback better for shrub encroachment under drought conditions.The anthropogenic transition increased soil carbon and nitrogen acquisition enzyme activity and microbial respiration.Soil carbon and nitrogen acquisition enzyme activities were enhanced even under low moisture pulse treatments,and especially in the shrubland microbial respiration rates significantly increased,indicating that soil microorganisms have adapted to the drier soil condition caused by precipitation decrease.The shrub litter treatment increased soil carbon and nitrogen acquisition enzyme activity,nutrient content,microbial biomass and its conversion rate,and net mineralization rate,indicating that soil microorganisms responded more positively to shrub litter input.Thus,shrub anthropogenic conversion have been forming a positive feedback mechanism between "pant and soil".helping microorganisms to retain certain characteristics to improve their adaptation in drier environments caused by rainfall decrease and vegetation shift.(5)Soil C:N ratio with anthropogenic state transition tended to decrease with spatial scale increase,but extents of coupling between soil carbon and nitrogen at multiple scales were significantly different.Soil carbon all coupled with soil nitrogen with some extent at multiple scales in this area experiencing anthropogenic state transition.At landscape scale soil carbon and nitrogen coupling were influenced mainly by slope orientation and soil water content.Alkaline phosphatase,inorganic nitrogen and microbial biomass nitrogen had effects on the C:N coupling at community scale.Soil pH,L-leucine aminopeptidase and β-1,4-glucosidase did so at patch scale.And L-leucine aminopeptidase,microbial biomass nitrogen and pH imposed impact on soil C:N coupling at root scale.The C:N ratio at the landscape scale,community scale,patch scale,and root scale showed an increasing trend with decreasing scale.In summary,anthropogenic state transition from desert grassland to shrubland over past 40 years has a positive or neutral effect on soil carbon and nitrogen at the landscape scale,but a negative effect on soil carbon and nitrogen content in shrubland planted more than 30a at the community scale,suggesting that decrease in perennial herbaceous plants diversity and water loss due to shrub massive introduction has been leading to a dramatic decrease in soil quality.According to results from incubation experiment of water and litter addition and variation in soil bacterial and fungal communities,microorganisms may have further adapted to the water-scarce environment in the shrubland and shrub litter,suggesting a positive "plant-soil" feedback mechanism of soil microorganisms in response to shrub transition and rainfall reduction.In addition,soil C:N ratio decreases with anthropogenic state transition from desert steppe to shrubland,but the relationships between soil carbon and nitrogen were linearly or nonlinearly coupled at different scales,and tended to increase as the scale decreases.Due to the reduction of soil moisture caused by shrub introduction and its importance to vegetation restoration in desert grassland area,even though soil carbon and nitrogen still responded to changes in above-ground vegetation with stable stoichiometric characteristics and no decoupling process of soil carbon and nitrogen occurred,the introduction of shrub as a vegetation restoration implement should be treated with caution.