Effect on incrassation of mattic epipedon and pasture nutrient-moisture using by grazing in alpine Kobresia meadow

The distribution of N-P and the moisture variation caused by incrassation of the mattic epipedon together with its relationship to pasture degradation by grazing in alpine Kobresia meadow were studied at the Haibei ecosystem research station,Chinese Academy of Sciences.From 2005 to 2007,the plot pasture was maintained with a relatively stable grazing intensity and had formed obvious differences in surface character and species component.As grazing intensity increased the Cyperaceae gradually replaced the Gramineae as the dominant community,the depth of mattic epipedon increased,driven by the special biological characteristics of Cyperaceae(high ratio of under/aboveground). It developed from 4.03±0.49 cm in the Stipa sliena+Festuca ovina+K.humilis community to 10.1±0.38 cm in the K.parva community.Meanwhile the N-P nutrient amount needed by the system rose rapidly from 11.26 to 22.94 g/m2 and from 0.81 to 1.63 g/m2 respectively.The nutrients were mostly used for root growth(2/3 or so) which became biologically fixed and accumulated in the soil.The ratio of saturation was reduced from 2.18 mm/min in the S.sliena+F.ovina+K.humilis community to 0.37 mm/min in the K.parva community due to compaction and incrassation,especially of the dead-aging mattic epipedon,which resulted in a considerable decrease in the ability to accept precipitation.The rapid growth and quantity of roots caused an increase in the water requirement,so that soil moisture was gradually reduced with the smallest amplitude reaching 44.9%.The maladjustment of nutrient supply and demand together with drought enhancement caused by the thickening mattic epipedon maybe one reason for degradation in alpine Kobresia meadows.