Temporal and spatial variations of soil organic and total nitrogen in the Songnen Plain maize belt

Temporal and spatial variations of soil organic matter(SOM) and total nitrogen(TN) contents in the Songnen Plain maize belt for a 25-year period(1980—2005) were evaluated using geostatistics and geographic information system(GIS).Results show that SOM and TN contents were log-normally distributed after excluding the outliers.The average content of SOM in 1980 and 2005 was 2.14% and 2.54%,respectively.The average content of TN was 0.12% in two periods.By performing experimental semivariograms analysis,SOM and TN were best fitted to Gaussian models in 1980 and 2005.The maximum correlative distance of major SOM and TN in contents was 532.6 km and 776.1 km in 1980,but it reduced to 269.7 km and 242.1 km in 2005 respectively.The spatial variability of the contents of SOM and TN was mainly affected by structural factors in 1980.SOM had a moderate spatial variability in 2005 and TN had a weaker spatial autocorrelation compared with that in 1980.The maps of SOM and TN were interpolated using the method of ordinary kriging.Their spatial distributions were very similar to the and content area in two periods,and had obvious geographical distribution with a high content in the central region and a low content in the marginal region.SOM contents increased in different extents in the upland,paddy land,woodland and grassland during 1980—2005,and TN contents increased in the paddy land and woodland.However,TN contents of the upland and grassland remained essentially unchanged.