Current status of soil fertility and geochemical characteristics of a typical alpine wetland in Xizang, China

Abstract. The Qinghai-Xizang Plateau has owned a large number of various types of wetlands, which serve as major pastures for regional animal husbandry and water restoration. Among, Mitika Wetland (MW) is located in Lhari county in Nagqu city, southwest China's Xizang Autonomous Region, with an average elevation of 4,900 m.a.s.l.. It is known as the Mitika Wetland National Nature Reserve (MWNNR) in China and it is the first wetland in Xizang that has been included on the List of Wetlands of International Importance. MWNNR is also known as the headwater region of Lhasa River, the “mother river” of people in Lhasa city. Study on its current status of soil fertility and geochemical characterization is therefore crucial for better understanding of its future vagaries under the global changes.In this study, analysis of 10 soil physiochemical parameters and 37 elements were carried out for the evaluation of topsoil fertility and geochemical features of the MW. The non-parametric test results indicated that the contents of soil organic matter, total organic carbon (TOC), cation exchange capacity (CEC), soil moisture, soil bulk density and soil salinity were in large extent related to the soil type. In contrast, pH, contents of available potassium, available phosphorus, ammonia nitrogen were regardless with soil type. Comprehensive soil fertility coefficient (F) among the different soil types studied in the wetland was as following: alpine meadow soils (1.72)＞alpine shrub meadow soils (1.66)＝frigid desert soils (1.66)＞bog soils (1.56)＞skeletal soils (1.43). Except the alpine meadow soil belonged to fertility grade, the other types belonged to the general grade, as this is the common state on the Plateau. Despite the high elevation and hash climate, nevertheless, soil fertility level in MW is comparable to that of cultivated land or artificial afforestation areas in the lower altitude (<3000 m.a.s.l.) regions of Xizang. Combined results from comparison with reginal background value and source identification using multivariable analysis showed that the contents of most studied soil elements were equivalent to the background values, and their distribution have been greatly affected by the geological background. In general, this study shows that the soil fertility of the alpine wetland located in the remote northeastern Xizang with a high elevation, surprisingly, have an adequate supply of soil nutrients to the pastures and, in a large extent, still remain chemically undisturbed under global change and human activities. Obviously, the wetland has played a key role in ecologically secure of the Lhasa River catchments.

Keywords: Alpine Wetland, Ecologically Secure, Element’s background value, Source Identification, Multivariable Analysis