SEASONAL VARIATION OF SOIL INORGANIC NITROGEN IN AN ULTlSOL AS AFFECTED BY LAND USE

The importance of soil inorganic nitrogen (NH/-N plus N03--N) in plant nutrition andconcerns over the environmental impact of nitrate leaching and N20 emissions hasfocused attention on inorganic N in soils. Inorganic N in soils can originate from manysources including fertilizer additions and mineralization of organic N from soil organicmatter, crop residues/plant litter and organic wastes. This study was conducted in anUltisol in the low country wet zone of Sri Lanka to determine influence of land use onseasonal variation of soil inorganic N. Five contrasting land use types (mahoganywoodlot/forested area, grass, legume, cinnamon and vegetable cropping) wereperiodically monitored during the March-June, 2003 for soil inorganic N. Composite soilsamples were taken to 0-15cm depth from four separate blocks of each land use type.Sampling was done initially at weekly and later at biweekly intervals.Soil samples were analyzed for inorganic N and moisture content was measuredgravimetrically for each sample. Soil from forested area contained the highest content ofsoil inorganic N (39 mg N kg" soil) on average during the entire sampling period whilethe other land use types showed similar (around 25 mg N kg" soil) results. No distinctrelationship could be observed between the inorganic N and moisture content in soil. Theaverage ~ +-N content observed throughout the sampling period was higher in the soilfrom the forested area compared to all other land use types indicating the status of Nmineralization in soil. Vegetable field had relatively high average content of N03--N insoil compared to mahogany/forest and other land use types. Results revealed thatinorganic N in soil is highly variable during the season and all land use types had moreNH/-N than N03--N in soil at any given time.