Comparative effects of application of coated and non-coated urea in clayey and sandy paddy soil microcosms examined by the 15N tracer technique

Nitrogen fertilizer and soil types exert an impact on plant and soil microbial biomass (SMB). A N-15 tracer experiment was conducted to compare the effects of the application of controlled-release coated urea (CRCU) and urea on SMB in gley (clayey) and sandy paddy soils. The fertilizers were applied at the rate of 8 g N m(-2) for CRCU as deep-side placement and 10 g N m(-2) for urea mixed into soil or applied into floodwater. The soil type and soil layer (surface: few millimeter depth of surface soil to include benthic algae; subsurface: 1 to 20 cm depth), but not the fertilizer type, affected the amount of microbial biomass N (B-N) On an area basis, subsurface soil layers contained about 2-3 times the amount of BN in the surface layers. The seasonal average B-N amount i.e. at 1 to 20 cm depth, in the gley soil was 1.67 g N m(-2), compared to 1.20 g N m(-2) for the sandy soil. The proportion of BN in total soil N was significantly influenced by the soil type and soil layer, and was higher for the surface layers of both soils and subsurface layer of the sandy soil than for the subsurface layer of gley soil. Soil type, soil layer, and fertilizer type significantly influenced the amount of microbial biomass N-15 (B-15N) Unlike B-N, the amount of B-15N was significantly higher in the surface (11.9-177.3 mg N m(-2)) than in the subsurface soil layers (4.8-83.6 mg N m(-2)), especially with urea application between 60 and 120 DAT (days after transplanting). At 30 DAT, the subsurface layer of the sandy soil showed a higher B-15N (218 mg N m(-2)) amount than the surface layer (133.4 mg N m(-2)). Sandy soil (4.8-218 mg N m(-2)) and urea (6.2-218 mg N m(-2)) induced a larger increase of the amount of B15N than the gley soil (6.2-83.6 mg N m(-2)) and CRCU (4.8-40 mg N m(-2)). Again, the sandy soil, surface soil layers, and urea induced a higher proportion (%) of B-15N in B-N than the gley soil, subsurface soil layers, and CRCU, respectively. The soil type affected B-N more than the fertilizer type, which showed only minimal differences. However, N-15 analysis revealed the existence of greater differences in the effects of CRCU and urea on the B-15N amount. Sandy soil and the application of urea led to a higher microbial N immobilization than the gley soil and CRCU application, respectively.