Vegetation Degradationon Mechanism Of The Desert Steppe In Semi-arid Region

Desertification is one of the most severe types of land degradation in the world.In this study,we used a space-for-time method to quantify the impact of five different desertification regimes?potential?PD?,light?LD?,moderate?MD?,severe?SD?,and very severe?VSD??on a desert steppe ecosystem in northern China to examine the change characteristics of productivity of vegetation,community structure,soil physical and chemical property and to determine the mechanism underlying the effects of desertification on productivity and community structure.Our results revealed the mechanism soil degradation with desertification.Our study accessed the carbon and nitrogen storage,and clarified the carbon and nitrogen loss of desert steppe with desertification.Our results revealed the mechanism of productivity and community structure degradation with desertification.The main conclusions of our study were showed below:?1?Desertification lead to desiccation and erosion of the soilWhen compared VSD with PD,Shannon-Wiener index decreased from 1.55 to 0.18,evenness index decreased from 0.5 to 0.26,richness index decreased from 23 to 2,and ground coverdecreased from 74.02%to 6.63%.The biomass of grasslands suffering light desertification was 23%higher than that of grasslands with potential for desertification.As desertification proceeded,the soil bulk density and coarse sand content of grasslands increased but their soil moisture and clay-silt content tended to decrease most significantly.The desertification of grasslands caused the soil to form a coarse texture and thus increase its bulk weight,with the consequence that soil moisture and silt content tended to decrease.As desertification intensified,these changes made the soil more susceptible to erosion,so that,directly or indirectly,the productivity of such grasslands inevitably must decrease.Therefore,the desertification of steppes involves progressively less retention of moisture and fine soil particles,leading to desiccation and erosion of the soil,changes in plant species,and inevitable loss of productivity.?2?Desertification decreased the carbon and nitrogen storage of desert steppe ecosystemThe C and N content in different stages of desertification were significantly different,while there was no obvious variation of C and N in different plant components as desertification progressed.Changes in soil C and N were not in accordance with plant succession,with the soil being more sensitive to desertification than the ground vegetation.When the VSD stage was compared with the PD stage,desertification resulted in the total C and N storage in plants decreasing by 97.3%and 96.8%,respectively,and in the 0–40 cm soil layer decreasing by 58.5%,and 76.0%,respectively.The highest C and N storage levels in the desert steppe ecosystem were 1291.93 g m^-2,and 142.10 g m^-2,and the lowest levels were 505.14 and 33.41 g m^-2.C and N losses through desertification were 786.79 and 108.69g m^-2,respectively.?3?Desertification decreased thebiomass of desert steppe ecosystemOur results showed that desertification decreased biomass in the desert steppe as a result of direct changes to soil physical properties,which can directly affect soil chemical properties.The directly affect of physical properties to biomass is 68%,and the indirectly affect is 12%.Therefore,intensive grassland management to improve soil quality may result in the long-term preservation of ecosystem functions and services.?4?Desertification and nitrogen addition decrease beta diversity cause species homogenizationDesertification decreased beta diversity among communities,thus homogenizing the species composition.Further,the homogenization under desertification was driven mainly by decreases in the species turnover component of beta diversity.In contrast,nitrogen addition did not affect turnover component of beta diversity but increased the nestedness component.The contrasting effects of nitrogen addition and desertification on the species turnover and nestedness components of beta diversity suggest that the decrease in species diversity due to desertification will be mitigated by nitrogen addition.Overall,our findings provide evidence that the effects of desertification and N addition on beta diversity are different.Understanding these details about impacts on species composition may provide guidelines for combating the ecological damage caused by desertification and nitrogen enrichment.?5?Deterministic process is a major driving factor in determining species diversity variation,however,stochastic processes cannot be disregarded as a factor in community composition determinationDesertification was a major driver of local environmental heterogeneity,which also resulted in decreased soil nutrients and led to increased turnover in a heterogeneous environment;however,spatial turnover of species decreased with desertification intensify.Desertification decreases resource availability,which causes species loss and reduced total beta diversity.Those desertification effects,therefore,had a homogenizing effect on the community.However,stochastic processes cannot be disregarded as a factor in community composition determination.Overall,these results indicated that the study of desertification effects on beta diversity may add to our understanding of the deterministic and stochastic processes that create and maintain biodiversity.This is crucial in order to identify the relative importance of deterministic versus stochastic processes.