Highest potential of subtracting nitrogen fertilizer through addition of urease inhibitor NBPT in double-cropping paddy fields

【Objectives】The addition of urease inhibitor is proved to be an effective measure to improve the efficiency of mineral fertilizer. Addition of urease inhibitor N-(n-butyl) thiophosphoric triamideNBPT］ at a rate of 10 g/kg of urea has been widely used in researches. From the point of the high urea input in present rice production, the highest potential of saving nitrogen fertilizer through adding 1% of NBPT in urea was studied using a field experiment in paddy soils, and the mechanism of saving fertilizer and synergism from urease inhibitor were investigated. 【Methods】A field experiment was conducted in a double-cropping paddy field in middle and lower reaches of the Yangtze River area, Southern China. Urea was mixed with urease inhibitor NBPT at a rate of 10 g/kg urea, five urea levels: N 90, 112.5, 135, 157.5 and 180 kg/ha were designed and recorded as U1, U2, U3, U4 and U5 in turn, plus a no N control and a normal urea of N 180 kg/ha in the experiment. A completely randomized design with seven treatments and three replicates was as follows, 1) CK (no N control); 2) U1+UI; 3) U2+UI; 4) U3+UI; 5) U4+UI; 6) U5+UI; 7) U5. The subtracting ratio of N fertilizer for U1, U2, U3, U4 and U5 is 50%, 37.5%, 25%, 12.5% and 0% respectively. The activities of urease and nitrate reductase, the contents of NH+4-N, NO-3-N and microbial biomass carbon (MBC) and microbial biomass nitrogen (MBN) in soil were analyzed in tillering and booting stages. The rice yield and N efficiency were investigated. 【Results】1) With the addition of UI, the highest yield was obtained at N rate of 135 kg/ha. Compared with the normal urea N rate of 180 kg/ha, the grain yields were increased by 8.54% and 12.87% in early and late rice, the recovery of applied N improved by 6.78% and 9.46%, respectively, and 25% of N fertilizer could be saved as a result. 2)The addition of NBPT in urea notably reduced urease activity, and soilNH+4-N content at the tillering stage and increased soilNH+4-N content at the booting stage, while there were no obvious effects on nitrate reductase activity (NRA), soilNO-3-N content and MBC and MBN. There was no significant difference in urease activity between with and without NBPT at the booting stage. Therefore, the urease inhibitor NBPT was effective before the booting stage and had less long term negative influence on soil environment. 3)The stepwise regression analysis reveals that the grain yield of rice is significantly associated withNH+4-N content in soil at the tillering and booting stages, especially, the latter. However, other properties in soil have no obvious effects on the grain yield. 【Conclusions】The potential of saving N fertilizer from NBPT is outstanding in double-cropping paddy fields. Compared with the normal urea, N fertilizer is saved by 25% when NBPT is used. NBPT slows down the hydrolytic action of urea and dramatically improves soilNH+4-N content in the booting stage, which appears to be the dominant factor of saving N fertilizer. The above results suggest that NBPT could remain adequate N resources at late growth stage for rice and serve as a urease inhibitor for developing slow release fertilizer in paddy soils.Objectives The addition of urease inhibitor is proved to be an effective measure to improve the efficiency of mineral fertilizer. Addition of urease inhibitor N(n-butyl) thiophosphoric triamide NBPT at a rate of 10 g/kg of urea has been widely used in researches. From the point of the high urea input in present rice production, the highest potential of saving nitrogen fertilizer through adding 1% of NBPT in urea was studied using a field experiment in paddy soils, and the mechanism of saving fertilizer and synergism from urease inhibitor were investigated. Methods: A field experiment was conducted in a double-cropping paddy field in middle and lower reaches of the Yangtze River area, Southern China. Urea was mixed with urease inhibitor N(n-butyl) thiophosphoric triamide NBPT at a rate of 10 g/kg urea, five urea levels: 90, 112.5, 135, 157.5 and 180 kgN/ha were designed and recorded as U1, U2, U3, U4 and U5 in turn, plus a no N control and a normal urea of 180 kg N/ha in the experiment. A completely randomized design with seven treatments and three replicates was as follows: 1) CK (no N control), 2) U1+UI, 3) U2+UI, 4) U3+UI, 5) U4+UI, 6) U5+UI, 7) U5. The subtracting ratio of N fertilizer for U1, U2, U3, U4 and U5 is 50%, 37.5%, 25%, 12.5% and 0% in turn. The activities of urease and nitrate reductase, the contents ofNH+4-N,NO-3-N and microbial biomass carbon (MBC) and microbial biomass nitrogen (MBN) in soil were analyzed in tillering and booting stages.. The rice yield and N efficiency were investigated. Results (1) With the addition of UI, the highest yield was obtained at N rate of 135 kgN/ha. Compared with the normal urea rate of 180 kg N/ha, the grain yields are increased by 8.54% and12.87% in early and late rice, the recovery of applied N improved by 6.78% and 9.46%, respectively, and 25% of N fertilizer can be saved as a result. (2)The addition of NBPT in urea notably reduces urease activity , and soilNH+4-N content at the tillering stage and increases soil NH4+-N content at the booting stage, while there are no obvious effects on nitrate reductase activity (NRA), soilNO-3-N content and microbial biomass carbon (MBC) and microbial biomass nitrogen (MBN). There is no significant difference of urease activity between urea with and without NBPT at the booting stage. Therefore, the urease inhibitor NBPT is effective before the booting stage and has less long term negative influence on soil environment. (3)The stepwise regression analysis reveals that the grain yield of rice is significantly associated withNH+4-N content in soil at the tillering and booting stages, especially, the latter. However, other properties in soil have no obvious effects on the grain yield.