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Background Food safety has become a major issue, with serious environmental pollution resulting from losses of nitrogen (N) fertilizers. N is a key element for plant growth and is often one of the most important yield-limiting nutrients in paddy soil. Urea-N immobilization is an important process for restoring the levels of soil nutrient depleted by rice production and sustaining productivity. The benefits of biochar application include improved soil fertility, altered N dynamics, and reduced nutrient leaching. However, due to high variability in the quality of biochar, the responses of N loss and rice productivity to biochar amendments, especially those prepared at different pyrolysis temperatures, are still unclear. The main objectives of the present study were to examine the effects of biochar prepared at different pyrolysis temperatures on fertilizer N immobilization in paddy soil and explore the underlying mechanisms. Methods Two biochar samples were prepared by pyrolysis of maize straw at 400 °C (B400) and 700 °C (B700), respectively. The biochar was applied to paddy soil at three rates (0, 0.7, and 2.1%, w/w), with or without N fertilization (0, 168, and 210 kg N ha–1). Pot experiments were performed to determine nitrous oxide (N2O) emissions and 15N recovery from paddy soil using a 15N tracer across the rice growing season. Results Compared with the non-biochar control, biochar significantly decreased soil bulk density while increasing soil porosity, irrespective of pyrolysis temperature and N fertilizer level. Under B400 and B700, a high biochar rate decreased N loss rate to 66.42 and 68.90%, whereas a high N level increased it to 77.21 and 76.99%, respectively. Biochar also markedly decreased N2O emissions to 1.06 (B400) and 0.75 kg ha−1 (B700); low-N treatment caused a decrease in N2O emissions under B400, but this decrease was not observed under B700. An application rate of biochar of 2.1% plus 210 kg ha−1 N fertilizer substantially decreased the N fertilizer-induced N2O emission factor under B400, whereas under B700 no significant difference was observed. Biochar combined with N fertilizer treatment decreased rice biomass and grain yield by an average of 51.55 and 23.90 g pot–1, respectively, but the yield reduction under B700 was lower than under B400. Conclusion Irrespective of pyrolysis temperature, biochar had a positive effect on residual soil 15N content, while it negatively affected the 15N recovery of rice, N2O emissions from soil, rice biomass, and grain yield in the first year. Generally, a high application rate of biochar prepared at high or low pyrolysis temperature reduced the N fertilizer-induced N2O emission factor considerably. These biochar effects were dependent on N fertilizer level, biochar application rate, and their interactions.

A field experiment was carried out in the years 2008–2011 in China to assess the nitrogen metabolism enzyme activities and photosynthetic characteristics in stable-yielding stay-green rice (Oryza sativa L.) cv. Shennong196. The results showed that higher levels of nitrogen content, nitrate reductase activity, and glutamine synthetase activity occurred in leaves of cv. Shennong196 compared with cv. Toyonishiki (control). Leaf color of cv. Shennong196 was positively correlated with nitrogen levels and nitrogen metabolism enzyme activities (P < 0.05). Superoxide dismutase activity and malondialdehyde contents were 18.53 unit/g fresh weight and 3.32 nmol/g, respectively, which were lower in flag leaves of cv. Shennong196 than cv. Toyonishiki. Cv. Shennong196 had a higher level of net photosynthetic rate, stomatal conductance, intercellular CO₂ concentration, and transpiration rate in flag leaves of diurnal variation of photosynthesis at the ripening stage. The high net photosynthetic rate in cv. Shennong196 was positively correlated with the stomatal density of flag leaves (P < 0.01). Considering the yield-increasing potential and to prevent premature senescence of crop, these traits of cv. Shennong196 are useful for improved rice cultivar.