Nitrogen deficiency frequently occurs at agricultural soil because of NH3 volatilization to the environment which results in low urea-N use efficiency by rice plants. A pot experiment was conducted to assess the synergistic effects of a mixture of rice straw and rice husk biochars (RSRH) on (1) total N, soil exchangeable NH4+, and available NO3−, and (2) uptake of N, P, and K in rice plant. RSRH biochar at 5 and 10 t ha−1 had significantly minimized ammonia volatilization by 33.5% and 40.7%. Further, RSRH biochars also had significantly increased available NH4+, NO3−, available P, and available K in the soil over T1. In line to increase in soil nutrient availability, the rice plant height, tiller number, greenness, and panicle number were increased. This resulted in an increase of nutrient uptake, use efficiency, and dry matter production of the rice plant. RSRH biochar altered the soil environment by minimizing NH3 loss and increasing soil nutrients for efficient plant uptake.

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Weed has become one of the most serious problems in aerobic rice systems. Weed management in aerobic rice system needs a manipulation of optimum N fertilizer application to control the weed and simultaneously enhance aerobic rice growth performance. In this study, three types of N sources (urea, ammonium sulphate and ammonium nitrate) with concentration starting from (0, 60, 120, 180 and 240 kg ha -1 ) were evaluated for effective control of the bioassay species, Eleusine indica, while observing the rice injury level. The data on weed (emergence and shoot fresh weight), while rice growth (root length, shoot fresh weight, shoot height and leaf greenness) was collected in this study. It was found that urea at a higher application rate of 240 kg ha -1 strongly inhibit the emergence and shoot growth of E. indica by >78%. Conversely, ammonium sulphate and ammonium nitrate gave moderate inhibition (58-65% inhibition) on weed emergence and shoot growth at higher application rates of 240 kg ha -1 , respectively. Significant stimulation effect on rice root growth, shoot height, shoot fresh weight and leaf greenness was noticed at a low application rate of 60 kg ha -1 urea, while ammonium sulphate and ammonium nitrate only exerts its stimulation effect starting at 120 kg ha -1 . Stimulation effect on the rice growth was likely negligible at 0 and 4 DAS across urea, ammonium sulphate and ammonium nitrate application rates. However, the rice plants growth increases with increasing growth stage. The finding suggest that application of urea at 240 kg ha -1 starting from 8 DAS of rice growth stage was the most efficient application rate in inhibiting E. indica with the stimulation effect on the growth of rice seedlings.

Urea fertilizer as a nitrogen source is used widely and globally. However, N loss through ammonia volatilization from applied urea has become a major drawback to agriculture. A pot experiment was conducted to determine the effect of rice straw biochar on (1) total N, soil exchangeable NH4+, and available NO3− (2) uptake of N, P, and K in rice plants. The treatments evaluated were: (S: Soil only, U: soil + 175 kg ha−1 urea, B1: soil + 175 kg ha−1 urea + 5 t ha−1 rice straw biochar, B2: soil + 175 kg ha−1 urea + 10 t ha−1 rice straw biochar, CB1: 50% soil + 50% commercial biochar potting media and CB2: 100% commercial biochar potting media). The addition of rice straw biochar at 5–10 t ha−1 in the pot experiment significantly increased the soil total N availability by 33.33–46.67%. Treatments B1 and B2 also had significantly increased exchangeable NH4+, NO3−, P, and K in the soil over U. The soil availability nutrients increment in soil was attributed to the higher adsorption capacity of the rice straw biochar. Increment in soil nutrient availability such as N, P, and K increased the plant height, tiller number, greenness, and panicle number because of effective rice plant absorption. This resulted in dry matter production increment in line with plant nutrient uptake and use efficiency. Rice straw biochar at 5–10 t ha−1 can improve the productivity of rice plants by increasing N retention in soil.

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