Effects of controlled-release coated urea (CRCU) on soil microbial biomass N in paddy fields examined by the 15N tracer technique

Experiments were conducted in paddy fields at Shiga and Chiba Prefectures to study the effects of controlled-release coated urea (N-LP100) on soil microbial biomass and N uptake of rice plants by the 15N-tracer technique, during one cropping season. Three field fertilizer treatments (Zero N: 0 kgN ha−1, 15N-LP100: 64 kg N ha−1and 15NH4Cl: 100 kg N ha−1) were set-up in the Shiga field experiment. After transplanting in the paddy fields at Shiga and Kashiwa (Chiba), a number of rice hills with standard growth were selected randomly and enclosed by polyacryl-plastic frames designated as microplots. 15N-LP100 (64 kg N ha−1) was applied in the Shiga and Kashiwa microplot experiments and the Shiga field experiment as deep-side placement (5 cm away from rice hill and 5 cm soil depth). Total N uptake of rice plants was analyzed in the course of plant growth. In addition, soils from the field fertilizer treatment plots and microplots (divided into 11 blocks) were taken and analyzed for microbial biomass N (BN) and biomass 15N (B15N). The results indicated that; (1) Plant N uptake from basal-applied fertilizers at the end of the study in the Shiga field experiment was 71.9 and 26.0% for 15N-LP100 and 15NH4Cl, respectively. In the Kashiwa microplot experiment, plant N uptake from applied 15N-LP100 was 51.2% at 67 days after transplanting (DAT) (2) Throughout the cropping season, BNwas the highest, intermediate and the lowest for 15NH4Cl, 15N-LP100 and Zero N field experimental plots in the Shiga experiment, respectively. (3) In the micro-plot experiments, BNand B15Nwere generally higher in the soil block with deep-side application of 15N-LP100 compared with the other soil blocks. The deep-side placement of 15N-LP100 ensured a high efficiency of utilization of its N by rice plants. The method of 15N-LP100-placement also affected the spatial heterogeneity of microbial biomass N in the microplots.