Nitrogen rates shaping rice yield and groundwater nitrate in the Jamuna River basin of north central Bangladesh

Abstract Background Understanding how varying nitrogen fertilizer rates influence both rice productivity and groundwater nitrate contamination is critical for ensuring sustainable crop production and environmental protection in the Jamuna River Basin of north‐central Bangladesh. Therefore, a field experiment was conducted at Paikortoli, Kajipur, Sirajganj (24.6694° N, 89.6511° E) to quantify nitrate concentrations in urea‐amended paddy soils and determine the optimal urea dose for minimizing groundwater contamination while maximizing rice yield. This experiment was laid out in a randomized complete block design (RCBD) with three replications. Our study examined the effects of four treatments: control (no urea), 75%, 100%, and 125% of the recommended fertilizer dose (RFD) on rice growth, yield, nutrient uptake, and nitrate leaching using piezometer‐based groundwater monitoring. Uniform management practices were followed across all the experimental plots. Results Nitrogen fertilizer application significantly improved growth parameters such as plant height, panicle length, total and effective tillers, and number of filled grains compared to control. The grain and straw yields also showed a significant increase compared to control, with the highest grain (4.76 t/ha) and straw (5.48 t/ha) yields recorded in the 125% RFD treatment, though yields at 75% and 100% RFD were statistically similar. Groundwater nitrate concentration responded significantly to different fertilizer rates and sampling time, remaining the lowest on day 1 and peaking consistently on day 5 after each split application. The highest nitrate concentrations in groundwater reached 6.8, 9.4, and 12.2 ppm following the first, second, and third splits, respectively, with the highest levels always observed under 125% RFD. Elevated urea application rate led to greater nitrate leaching. Despite slightly lower yield than the 125% RFD treatment, the 75% RFD dose showed a significant decrease in nitrate leaching in groundwater without compromising the yield significantly. Conclusions Therefore, the findings suggest that excessive nitrogen use accelerates groundwater nitrate contamination in this floodplain ecosystem, while a 75% RFD rate offers an optimal balance between yield performance and the risk of groundwater contamination.