| The Loess Plateau is a key ecological security barrier in China.After the implementation of a series of ecological restoration projects,the ecological environment of the region has been greatly improved,the vegetation coverage has been significantly increased,and the ecological function has been enhanced to a great extent.In the past decades,scientists have conducted a series of studies on climate change,plant community succession,soil physicochemical properties,conservation and restoration models on soil and water,as well as related ecosystem service functions in the region,but little attention has been paid to below-ground biodiversity,especially soil fauna diversity.Soil fauna is an important component of terrestrial ecosystems and plays a key role in the process of ecosystem material cycling and energy flow.Due to its high sensitivity to environmental changes,it is also an important indicator factor.Thus,the study selected the soil fauna of the Loess Plateau grassland as the research object,and conducted a field survey on the composition,distribution and diversity of soil fauna in the region.Meanwhile,combining the data of environmental factors such as climate,vegetation,soil,etc.,using analysis methods such as ANOVA,regression analysis,redundancy analysis and path analysis,the writer revealed the relationship between soil fauna and environmental factors,and explored the main controlling factors affecting the distribution of soil fauna as well as their contribution.Based on the geostatistical spatial analysis method,the writer simulated and predicted the spatial distribution characteristics of soil fauna community in the grassland of the Loess Plateau,and then screened the optimal model for revealing the spatial distribution of soil fauna.Eco-morphologic indices(EMIs)were performed based on the morphological types of soil fauna,so as to evaluate the soil health of the Loess Plateau grassland from the perspective of soil fauna.Meanwhile,based on the number of soil fauna and the carbon content of their organisms,the writer estimated their carbon density and reserves.The main research are as follows:(1)The soil fauna captured in the Loess Plateau region belonged to 3 phylums,10classes,32 orders and 131 families.The average density of soil fauna in the grassland of the Loess Plateau reached 20731.85 ind./m~2 in the depth range from the litter horizon to the30 cm beneath the earth.At the order level,the main dominant groups were Oribatida(30.72%),Mesostigmata(29.18%)and Collembola(22.94%).At the family level,the Rhodacaridae(12.29%)was the dominant group.In the horizontal direction,the number of individuals,groups,functional groups of soil fauna decreased with increasing latitude,while only some of the indicators showed a significant linear relationship with longitude.In the vertical direction,the distribution of soil fauna community characteristics showed obvious"epimerization"characteristics,and the diversity of soil fauna decreased with the increase of depth.Soil fauna Shannon–Wiener index(H)and Margalef index(D)showed negative correlations with latitude and depth,while Simpson index(C)had the opposite trend.The similarity indices of soil fauna communities varied considerably in different region ranging from very dissimilar to very similar.(2)In the Loess Plateau region,soil(SWC,TN and AP,etc.),litter(LBM,LOC,and LTN,etc.),and climatic(TEM and PRE)factors showed a decreasing trend from the south to north latitude and a significant negative correlation in some layers,whereas SBD showed the opposite trend.Community characteristics such as soil fauna density,number of taxa and diversity index increased and showed significant positive correlations with increases in SWC,SOC,TN,TP,AP,LBM,LOC,TEM and PRE,while decreasing and showing significant negative correlations with increases in SBD and p H.From the litter horizon to the 30 cm beneath the earth,the number of environmental factors that have a significant relationship with soil fauna was decreasing and the influence on external environmental factors was gradually weakening.Multiple linear stepwise regression models indicated that PRE was the main indicator for explaining soil fauna community characteristics in general and in the layer,while common indicators selected for optimal modeling of deeper soil fauna were LTP and p H,etc.Based on environmental factors and soil fauna community,it concluded that PRE is the most important single factor influencing soil fauna communities,while soil is a key set of factors influencing soil fauna distribution.(3)The soil fauna of grassland on the Loess Plateau showed a spatial zonal distribution pattern decreasing from southeast to northwest in both the litter layer and different soil layers.Multiple linear regression and geographically weighted regression were suitable for modeling and predicting the spatial distribution of soil fauna in the shallow or surface layer,while the kriging method was more suitable for soil fauna in the deep layer.In addition,all the prediction methods showed that the effect on the low value zone of soil fauna distribution was better and low prediction in the high value zone of soil fauna distribution.(4)Soil health of grassland soils in the Loess Plateau based on soil fauna showed a gradual decrease from southeast to northwest in both horizontal directions,and an overall trend of gradual decrease from top to bottom in the vertical direction.FAI values were significantly negatively correlated with SBD and p H,and significantly positively correlated with each of the remaining indicators such as SWC,LTN,and PRE,with soil fauna community characteristics(Density and Group)having the highest relative importance for changes in FAI indices in each layer.(5)The carbon density of soil fauna organisms in the grasslands of the Loess Plateau had a spatial distribution pattern that shows a gradual decrease from southeast to northwest and a gradual decrease with depth.Overall,the highest value of soil fauna biomass carbon density reached about 41.16 g/m~2 in the Fufeng sample site,and the lowest value was about0.04 g/m~2 in the Yinchuan sample site.The preliminary estimation of the overall average soil faunal biomass carbon density in the grassland ecosystem of the Loess Plateau reached9.31 g/m~2,storing a total of 2.41 Mt of soil faunal biomass carbon,which accounted for about 7.53%,1.15%and 0.28%of the aboveground biomass carbon,belowground biomass carbon and soil organic carbon(0-100 cm)storage in the grasslands of the Loess Plateau,respectively.The total biomass carbon of Hemiptera taxa was the highest,reaching about0.50 Mt,while the total biomass carbon of Pauropoda taxa was the lowest(<0.01 Mt).In summary,community characteristics such as the number of individuals,taxa,and functional taxa of grassland soil fauna in the Loess Plateau decreased with increasing latitude and depth.On the regional scale of the Loess Plateau,the spatial distribution pattern of soil fauna showed a spatial zonal distribution pattern decreasing from southeast to northwest,and PRE was the most important factor influencing the distribution of soil fauna communities.Soil health and soil fauna biomass carbon density of grasslands in the Loess Plateau based on soil fauna showed a spatial distribution pattern that gradually decreased from southeast to northwest and from top to bottom.Soil fauna community characteristics were the most direct factor influencing the spatial distribution of carbon density in soil fauna organisms.The results of the study can help to enrich the basic research data of soil fauna in the Loess Plateau,help to expand the methods and directions of soil fauna ecology research,and contribute to biodiversity conservation,regional ecological management and long-term sustainable development of the Loess Plateau region. |
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