不同退化程度高寒草原土壤肥力变化特征

Characteristics of soil fertility in alpine steppes at different degradation grades

就藏北退化高寒草原土壤肥力的变化等进行了研究。结果表明：（1）高寒草原土壤物理性质的变化对土壤化学、生物学性质具有重要的调控作用,土壤生物学性质对土壤肥力的演变则具有关键影响。（2）随草地退化程度的提高,2-10cm土层土壤容重均呈不同程度的下降,土壤孔度、土壤含水量则分呈显著的增、减趋势,但草地退化对土壤含水量的影响更为显著;高寒草原土壤中,〉0.25mm的水稳性团粒与土壤含水量呈极显著正相关,有机质对改善土壤结构、提高土壤含水量则具有极为显著的促进作用。（3）轻度退化草地土壤有机质、腐殖质与土壤全氮、磷、钾含量均呈不同程度的提高,中度、严重退化草地则呈相反趋势;土壤有效氮、磷、钾含量在总体上随草地退化的加剧而呈下降趋势;腐殖质碳占有机碳比重、HA-C占腐殖质碳比重、HA/FA则均随草地退化的加剧而呈明显上升;（4）不同程度退化草地2-10cm土层微生物（细菌、真菌、放线菌）数量、微生物生物量（碳、氮）、土壤酶（纤维素酶、脲酶、碱性磷酸酶）活性等在总体上与有机质的变化趋势相一致;BC/BN与TC/TN间呈极显著正相关（r=0.9370＊＊,p≤0.01）,轻度、中度退化草地BC/TC、BN/TN均呈明显上升,仅严重退化草地呈下降趋势。5）高寒草原微生物量、土壤酶活性与土壤有机质、全氮、有效氮、有效钾含量间均呈极显著或显著正相关,与有效磷则均呈不同程度的负相关。

Characteristics of soil fertility variation and their relationships were studied in Stipa purpurea steppes,the most representative alpine steppes in northern Tibet.Based on the same parent material（lake sediments）and soil texture（sandy soil）during the rapid growing period of steppe plants,we selected four sampling areas located at the altitude of 4550-4740 m with a distance of 40-50 km away from each other.In each sampling area,according to the classification standard of steppe degradation,we investigated four kinds of steppes（no degradation,slightly degraded,moderately degraded and seriously degraded）with an area of one hectare each.All the sampling sites（3 sites per area）were chosen randomly in our study.Soil samples in 0-10 and 11-20 cm layers were collected by a soil auger（4 cm inside diameter）in steppes with different degradation grades in each sampling area.Sol samples used for measuring indexes concerned with soil microorganisms were from 2-10 and 11-20 cm layers.At each sampling site,ten soil cores were mixed thoroughly to form one composite sample for chemical and biological analyses related to soil fertility. The results were listed as follows：（1）Under the ecological conditions of the Northern Tibet Plateau,steppes degradation had consistent effects on soil physical,chemical and biological fertilities.These soil physical,chemical and biological fertilities were associated with each other and set the direction of soil fertility evolvement in the alpine steppes.（2）The degradation of soil fertility in alpine steppes was mainly depended on the changes of soil biological properties while it was also regulated by the variations of soil physical factors,such as soil structure,soil bulk density and soil moisture.In general,soil biological,physical and chemical fertilities in the slightly degraded steppe showed increasing trends at various levels because of the decline in soil moisture content and soil frozen status induced by slight soil desertification.（3）With increased steppe degradation grades,soil bulk density at both 0-10 and 11-20 cm layers showed a decreasing trend（the difference between slightly degraded steppe and normal steppe was not significant）,while soil porosity and moisture content increased or decreased significantly although steppe degradation influenced soil moisture content more than soil porosity.In alpine steppe soils,water stable aggregates（〉0.25 mm）content positively and significantly correlated with soil moisture content,indicating that organic matter played an important role in the improvement of soil structure and the increase in soil moisture content.（4）Soil organic matter content as well as total N,P and K contents were increased in slightly degraded steppe but decreased in moderately and seriously degraded steppes compared with no degradation steppe.Soil available N,P and K showed a decreasing trend with steppe degradation.The proportions of soil humus C to soil organic C,HA-C to soil humus C and the ratio of HA/FA improved with the increase in steppe degradation grades.（5）The quantities of soil microorganisms（e.g.bacteria,fungi and actinomycetes）and soil microbial biomass（BC and BN）,and the activities of soil enzymes（cellulase,urease and alkaline phosphatase）in 2-10 cm layer of different degraded steppe soils showed the same trends with soil organic matter content,in general.There was a positive relationship between the ratios of BC/BN and TC/TN（r=0.937 0＊p≤0.01）,and the ratos of BC/BN and TC/TN increased largely in the slightly and moderately degraded steppes and decreased only in seriously degraded steppe.（6）There were significantly（p≤0.05 or 0.01）positive relationships between soil microbial biomass,the activities of three mentioned soil enzymes and soil organic matter,total N,available N,available K contents in alpine steppe soils.In contrast,we found negative relationships between soil microbial biomass or the activities of soil enzymes and soil available P content.