| Globally,grasslands are affected by climate change and unsustainable management practices which usually leads to transitions from stable,degraded and then to desertification.Spatial vegetation patch configurations are regarded as key indicators of such transitions.Understanding of the relationships between this grassland vegetation and its environment is key to vegetation restoration projects.Typical alpine grassland species patches were chosen across different soil and topographic conditions.Vegetation patch configurations such as patch numbers,perimeter and cover of each patch were measured along transects of each patch type.Using field surveys and multivariate statistical analysis we investigated the differences in vegetation biomass and distribution and soil properties of some typical alpine plant species patches along a range of topographic and other environmental conditions.Vegetation patchiness is common in degraded grasslands.Vegetation patchiness enhances the spatial heterogeneity of grassland soil organic carbon and total nitrogen.Stripped vegetation patches have a great impact on ecosystem carbon(C)and nitrogen(N)storage.Using field surveys,we equally examined the effects of vegetation patchiness on ecosystem carbon and nitrogen storage of ten alpine grass species patches.Grasslands contain substantial amount of soil organic carbon(SOC)and thus have a key function in global carbon cycle.Thus,knowledge on SOC contents and aggregate stability is vital in exploring their impact as a carbon(C)sink or source under climate change simulations.The research investigated the spatial variability of SOC and its fractions and stability of aggregates and their relationships with factors like vegetation,climate factors and soil conditions along an elevation gradient of the alpine meadow grassland(2480-3480m)on the Qilian Mountains of the Qinghai Tibetan Plateau.Grassland degradation is a major cause of soil erosion and environmental degradation worldwide.So far,few works have quantitatively analyzed how different levels of grassland degradation impacts soil organic carbon fractions and soil physical and chemical properties.This work endeavors to fill this knowledge gap by studying the impact of three different levels of severity of vegetation degradation of ten typical alpine grass species patches.The main results are as follows:(1)It was found that topographic conditions and soil properties,particularly soil moisture,explained most of the variation in spatial patch vegetation characteristics and thus controls vegetation restoration in the alpine grassland.The Kobresia humilis,Blysmus sinocompressus and Iris lactea patches under the drylands recorded smaller patch sizes,larger patch numbers,low connectivity and larger total perimeter per unit area.Generally,species within the high moisture sites recorded small patch numbers,large fraction of vegetation cover and small total perimeter per m~2.Patches in limited soil moisture areas recorded of patch configurations indicating they are unstable and undergoing degradation.(2)The results indicated ecosystem C,N and respiration were significantly higher in intact vegetation patches than the stripped vegetation patches.Also,stripped vegetation patches recorded higher quantities of soil gravel content than the intact patches.In Leymus secalinus and Koeleria pers grass species patches,soil approximately contributed about 62%and 65%respectively and vegetation about 38% and 35%respectively to ecosystem carbon and nitrogen storage whereas,in Stipa aliena and Leontopodium nanum grass species patches,close to 80% of ecosystem carbon and nitrogen were found in soil while close to 20%were stored in vegetation.Soil total phosphorus(TP),total potassium(TK),available phosphorus(AP),soil microbial biomass carbon(MBC)and soil microbial biomass nitrogen(MBN)were high in intact vegetation patches than the stripped vegetation patches.Ecosystem carbon and nitrogen were observed to have a significant correlation with soil gravel content and vegetation productivity.Stripped vegetation patches resulted in decreased plant biomass input and increased rate of soil erosion.(3)The results indicated the amount of SOC,water-soluble organic carbon(WSOC),readily oxidizable organic carbon(ROC),humic acid carbon(HAC),fulvic acid carbon(FAC),humin carbon(HUC),macro-aggregate and micro-aggregate organic carbon,mean weight diameter(MWD),geometric weight diameter(GWD),percentage of>0.25mm water stable aggregates(W0.25),and stability of water-stable aggregates ratio(WASAR)increased with increasing elevation,attaining a peak at 3080 or 3480m and afterwards decreased while that of structure deterioration rate(SDR)decreased with increasing elevation and got to a peak at3280m and increased afterwards.There were positive correlations between SOC contents,aggregate organic C,above ground biomass(AGB)and all the aggregate stability indices except SDR.The optimum temperature and moisture along the elevation gradient could account for the strong positive correlation of AGB with elevation,which thus impact the distribution of SOC and aggregate stability.The positive correlations between SOC contents and aggregate stability indices imply that both the resistant and labile forms of C were the major determinants of soil aggregate stability in the study.(4)The results showed that soil degradation increases with an increase in severity of vegetation degradation.Soil organic carbon and its fractions were low in the severely degraded grasslands as compared to the slightly and non-degraded grasslands.Soil physical and chemical properties were high in the non-degraded as compared to slightly and severely degraded grasslands.The less vegetation cover in the degraded grasslands leads to low amounts of litter and root biomass and hence increases soil erosion resulting in loss of soil fertility.In conclusion,patches in limited soil moisture areas recorded patch configurations indicating they are unstable and undergoing degradation.Grassland patchiness resulted in the decline of ecosystem carbon and nitrogen storage due to a reduction in vegetation input and an increase in soil erosion.Grasslands are likely to have a higher possibility of serving as C sink if the input of organic matter exceeds its output via sustainable management practices.The findings of this research show that global climate warming in the future could have a telling effect on SOC and soil aggregate stability by affecting the distribution of AGB.Due to the low structural stability of soils at the lower parts of the elevations,it is proposed that these areas should be barred from continuous grazing to enhance high grassland productivity and be able to cope with prospective climate change in this region.The strategies for restoring the degraded alpine grasslands should aim at enhancing vegetation,roots and litter cover in the top soil and to stabilize the soil structure and create an enabling environment for pioneering species to be established and facilitate the natural recovery of degraded alpine grasslands.These results would provide quantitative easy-to-use indicators for vegetation degradation and help in vegetation restoration projects. |
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