Seasonal Dynamic Of Soluble Organic Nitrogen In Alpine Meadow Soil

Soil soluble organic nitrogen (SON) is an important component of nitrogen pool in terrestrial ecosystems. Since SON has strong mobility and nutrient availability, it plays an important role in the absorption and utilization by plants and soil microorganisms, nitrogen mineralization and leaching as well as other aspects in ecosystems. Currently the study on the content, composition and seasonal dynamics of soil SON is limited, and previous SON research has been focused mainly on temperate forest ecosystems. The report on SON in alpine meadow soil is scarce. In this study, the alpine meadow soil that was dominated by Polygonum viviparum L., Kobresia graminifolia C. B. Clarke and Elymus nutans Griseb., separately, was used as research objects. The content, composition and seasonal dynamics of SON was qualitatively and quantitatively investigated. This research would provide a scientific basis for the mechanisms on plants uptake of organic nitrogen in alpine meadow, and help to explanation the ecological significance of SON. The main findings of the study are as follows:1. The SON content in the three soil extracts was in the range of 279.35-882.77 mg/kg, The percentage in the total soluble nitrogen pool was 94.1%-99.9%, and in the total soil nitrogen pool was 5.2%-20.0%. The content of SON in the three soils was 20 times higher than that of inorganic nitrogen at the same time period (p< 0.01). The seasonal dynamics of SON in the three soils were summer> autumn> spring, The SON content in the three soils showed little differences at the same time period. The content of SON may be influenced by many factors such as soil properties, vegetation types, soil microbes, climate.2. The composition of SON in the three soils was different. The proportion of hydrophobic and hydrophilic components in SON was closely related with soil extraction method. In the KC1 extracts of K.graminifolia and E.nutans soil, the hydrophobic compounds account for a larger proportion in SON, whereas the opposite result was found in the H2O extracts. Both the KC1 and H2O extracts contained similar amount of hydrophilic and hydrophobic compounds in SON from P.viviparum soil. The results showed that using salt solution was more efficient in the extraction of hydrophobic compounds than that of water solution.3. Total amino acid contents in the three soils exhibited similar seasonal dynamics were summer> autumn> spring. The total amino acid content in Kgraminifolia soil among the three soils was the highest. The tatol amino acid content in E.nutans soil could reach a quarter of the inorganic nitrogen content. The proportion of total amino acids in SON pool was small, probably because of its fast turnover rate and less accumulation in soil.The main conclusions of the study are summarized as follows:Soil nitrogen is abundant in the alpine meadow ecosystems, and the size of SON pool is much larger than that of inorganic nitrogen pool. Amino acids accounts for a certain portion in SON pool, which means that some low molecular weight SON may be directly absorbed by plants, and SON may play an important role in nitrogen cycling in alpine meadow ecosystems. The composition of SON is primarily related to its source. The content and composition of SON may be influenced by biotic and abiotic factors, within which soil properties, vegetation types and the proportion of fungi in soil microbe communities may be the dominant roles in the alpine meadow ecosystem.