[Temporal-spatial variations of total nitrogen in the degraded grassland of Three-River Headwaters region in Qinghai Province].

Based on the data of the second soil survey and field sampling in 2009 and 2010, temporal-spatial variations of total nitrogen (TN) in the degraded grassland 0-10 cm, 10-20 cm and 30-50 cm beneath the surface soil at the Three-River Headwaters region in Qinghai Province for a 30-year period (1980-2010) were evaluated using geostatistics and geographic information system (GIS). After exclusion of the outliers, the results showed a downward trend from the surface to the bottom in the mean TN values measured in the samples collected during the two periods. For the same soil layers, the average TN contents and the coefficient of variation in 1980 were higher than those in 2010. The TN contents of the two periods showed a log-normal distribution. Semivariograms analysis of the experiments indicated that the nugget effect in the same soil layer was lower in 2010 than in 1980; suggesting that the spatial distribution autocorrelation of TN in the Three-River Headwaters region in Qinghai Province was strengthened and structural factors played a more and more important role on the spatial distribution of TN. The results of ordinary kriging showed that there were regional differences in variations of the total nitrogen content. There were significant decreases in the southern, central and eastern regions, while the increase mainly occurred in the western areas.