| Agricultural ecosystem in the Qinghai-Tibetan Plateau is unique and fragile.Soil quality in the Plateau not only directly decides food security and sustainable development in this region,but also affects stabilization of ecosystem,closely relates to role performance of ecological defense and safety control that the Plateau plays.Soil carbon is key indicator of soil quality for the size and change of pools of soil organic and inorganic decide the direction in which soil quality will develop.Studying character and evolving dynamic of cropland soil quality and carbon pool in the Plateau is significantly meaningful to recognize the role of the Plateau in global carbon cycle.This dissertation studied character and change of soil quality(through soil physical,chemical,and microbiological properties)to evaluate soil quality comprehensively and ascertain key factor of soil quality;studied change and dynamic of soil organic and inorganic carbon and their sequestrating mechanism based on a consecutive 26-year field experiment locates in eastern agricultural area of the Qinghai-Tibetan Plateau.Cropland soil profile sampling campaign in nine counties and one city,eastern agricultural area of the Plateau was carried out to investigate change of soil carbon pool under situation of long-term fertilization and cultivation,with data collected during the 1980 s by the Second State Soil Survey of China,and observational data from field surveys in this area conducted in recent years.Soil organic and inorganic carbon pool stocks were estimated and their temporal and spatial variations were revealed.Furthermore,impacting factors of soil carbon sequestration in the Plateau were also studied.The main results and conclusions are as follows:(1)Cropland soil physical,chemical and microbiological properties changed after long-term fertilization and cultivation in the Qinghai-Tibetan Plateau.Effect of long-term different fertilization on soil properties was significant,straw return based on chemical fertilization contributed more to improvement of soil properties than only chemical fertilization.Compared to solely chemical fertilization,straw return amendment based on chemical fertilization increased topsoil >0.25 mm aggregate by 33.4-123.4%,soil water content by 4.7-13.5%,decreased soil bulk density by 1.2-7.1% and decreased soil compact degree.Straw return based on chemical N,P fertilizer significantly increased soil CEC,organic matter,light fraction of organic matter,available and total N,P,K content;additional application of K chemical fertilizer and straw return based on chemical N,P fertilizer,increased soil N and K content,especially increased available K content 2.6 times than that in the NP treatment.Additional application of chemical K fertilizer,or combination of straw return and chemical N,P,K fertilizer,increased soil fertility poorly,even deceased soil organic matter,light fraction of organic matter and P content.Based on chemical N,P chemical fertilizer,additional application of chemical K fertilizer improved availability of Cu and Fe,straw return improved availability of Zn and Mn.Straw return amendment with chemical fertilizer increased soil alkaline phosphatase,urase,saccharase,catalase activity and cultivable microorganism population.Based on chemical fertilizer,half amount of straw return increased bacterium population,fungus and actinomycetes populations increased with increase of straw return.Different fertilization changed soil microbial property,and altered seasonal character of soil microorganism.Additional chemical K fertilization perhaps impact soil microbial property negatively.(2)Minimum data set of soil quality evaluation in eastern agricultural area of the Qinghai-Tibetan Plateau was established.The MDS included available N,total P,Cu,Zn,alkaline phosphatase,urase,saccharase,clay,p H,and CEC,totally 10 indicators,which were key impact factor to soil quality in this region.The weight order of above 10% indicator in weight from high to low was: total P>Zn>Cu>available N>saccharase>CEC.Soil quality index(SQI)of different fertilization,NP,NP+50%ST,NP+100%ST,NPK,NPK+50%ST,NPK+100%ST was 47.9,49.5,50.0,47.8,51.6 and 52.6 respectively.(3)Soil organic carbon(SOC)in plough layer in cropland of the eastern Qinghai-Tibetan Plateau took an increasing trend after more than 20 years' cultivation and fertilization,however the total pool of SOC(PSOC)decreased due to decrease of cropland area.Topsoil organic carbon in 316 profile samples in different soil types,subtypes of nine counties and one city's cropland in the 1980 s was 1.6-66.4g/kg,averaged 10.6g/kg.Topsoil organic carbon of 421 cropland samples in Huangzhong County in 2006-2007 was 3.0-35.2g/kg,averaged 12.25g/kg,which was increased by 16.0% than that in the 1980 s.Topsoil organic carbon in different treatments of the long-term field experiment increased by 10.4-34.0% in 2014 than in initiating year(1992).PSOC was estimated 21.9Tg in the 19980 s on cropland area of 711751.6ha.PSOC was estimated 1.9Tg in 2006-2007 in Huangzhong County on cropland area of 55266.7 ha,which was significantly lower than 4.0 Tg in the 1980 s on area of 124880.7ha.Soil organic carbon density(SOCD)in 0-100 cm depth-profile in cropland of eastern Qinghai-Tibetan Plateau was 63.60-367.36 Mg/ha,and soil inorganic carbon density(SICD)was 200-225 Mg/ha.On area of 333000 ha and in 0-100 depth cropland soil in eastern Qinghai-Tibetan Plateau,PSOC was estimated 44.26 Tg,PSIC was estimated 553.8 Tg,which was 12.5 times of PSOC.Fertilization increased SOC significantly(p<0.01),especially straw return based on chemical fertilizer contributed more to SOC,increased SOC with increase of straw return amount.Increase of SOC caused by chemical fertilizer was 10.4-17.4%,and increase of SOC caused by straw return based on chemical fertilizer was 21.2-34.1%.Annual increasing rate of SOC due to chemical fertilization was 0.10-0.41 Mg/ ha/yr and annual increasing rate of SOC due to straw return amendment with chemical fertilizer was 0.22-0.85 Mg/ ha/yr.SIC was not affected by long-term different fertilization(p>0.05).(4)There were significant difference of SOC and SIC between different depth of profile soil(p<0.05).SOC showed surface accumulation in plough layer,decreased gradually with increase of soil depth in 0-40 cm layer,reached bottom at 40-60 cm layer,and then tended to keep steady with little rise.SIC increased with increase of soil depth in 0-60 cm layer,and peaked at 40-60 cm layer,then reduced gradually in further depth.Effect of long-term different fertilization on SOC in plough layer was greatly significant(p<0.01),on SOC in further depth was not significant,on SIC was not significant in any layer.(5)Distribution of SOC in cropland of eastern Qinghai-Tibetan Plateau was not even.On the whole,SOC was higher in the north and the surrounding area,lower in the south and the middle area.Datong County,northeastern Huzhu County,southern Huangyuan County and northern Huangzhong County were high SOC distributing area,then middle and south of Huzhu County,Ledu County,Hualong County,northwest of Huangyuan County,west of Datong County followed.Minhe County,northeast of Xunhua County,Pingan County,north of Huangzhong County were low SOC area.The order of the soil types in the region with respect to SOC concentration distribution was Chernozems >Gleysols >Anthrosols >Luvisols >Cambisols >Kastanozems >Calcisols.The order of SOC distribution with respect to altitude and topography were high elevation > low elevation >middle elevation,high mountain >river basin > middle hill.Cropland SIC distribution in this region was extremely uneven and showed converse distribution character.(6)Content of macro-water-stabled aggregate(larger than 0.25mm)was 51.68%,and micro-water-stabled aggregate(smaller than 0.25mm)was 48.32% in plough layer(0-20cm)in the long-term field experiment.Micro-water-stabled aggregate was about 2 times of macro-aggregate in 20-40 cm layer,which made the micro-aggregate predominant in the whole soil structure in this layer.Macro water-stabled aggregate mainly was >5mm,0.5-0.25 mm,1-0.5mm aggregate.Total C content was higher in macro-aggregate than in micro-aggregate,was higher in plough layer than in 20-40 cm layer.The order of total C content distribution in macro-aggregate in plough layer from high to low was 2-1mm >5-2 mm >1-0.5 mm >0.5-0.25 mm >above 5mm and was converse in 20-40 cm layer.Total C mainly accumulated in >5mm,0.5-0.25 mm,1-0.5mm aggregates,where were main arena for soil carbon sequestration.Significant positive correlations(p<0.05)were found between glomalin-related soil protein(GRSP)and SOC,macro-water-stabled aggregate(WSA-R0.25).This indicated that GRSP might play an important role in soil aggregate construction,SOC protection and accumulation.Long-term chemical fertilization increased micro-aggregate and decrease macro-aggregate,straw return based on chemical fertilizer reversed this trend,and contributed more to SOC sequestration.Main impacting factors of SOC sequestration in the Qinghai-Tibetan Plateau were fertilization and cultivation,change of land use,soil type,unique climate and geography. |
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