Effects of combined application of organic and inorganic fertilizers on the uptake and utilization of topdressed nitrogen in a black soil paddy field under controlled irrigation

To elucidate the absorption and utilization of topdressing nitrogen during the late growth stage of rice when organic fertilizers substitute for part of the basal fertilizers under water-saving irrigation, a combined approach of field plot and micro-plot experiments was employed. The ¹⁵N tracer technique was utilized to label the applied topdressing fertilizers (tillering and panicle fertilizers). Conventional flooding irrigation with conventional nitrogen application (CK) served as the control. Under water-saving irrigation, five gradients of organic fertilizer and other nitrogen substitute basal fertilizers were set up, which were conventional application of chemical nitrogen fertilizer (CF0), organic fertilizer instead of 20% basal fertilizer (CF1), organic fertilizer instead of 40% basal fertilizer (CF2), organic fertilizer instead of 60% basal fertilizer (CF3) and 80% basal fertilizer (CF4).This study investigated the absorption, utilization, and translocation of topdressing nitrogen by rice under different proportions of organic fertilizer replacing basal fertilizers in the water-saving irrigation system, as well as the distribution of topdressing nitrogen in various plant organs at the rice maturity stage. Experimental results demonstrate that replacing 60% of the basal fertiliser with organic fertiliser under controlled irrigation (CF3) leads to significant improvements in several key growth and yield parameters of rice. Specifically, this substitution markedly enhances the above-ground dry matter accumulation, boosts total nitrogen uptake, and increases grain yield at the physiological maturity stage of the crop.Furthermore, under controlled irrigation regimes, the amount of topdressing nitrogen absorbed by rice plants varied between 26.20 and 33.89 kg/hm² across treatments with different organic fertiliser substitution ratios. This range represents a substantial increase of 2.14 to 9.83 kg/hm² compared to the conventional control (CK). Notably, the CF3 treatment exhibited the most pronounced enhancement, showing a 29.36% greater nitrogen absorption relative to the treatment without organic substitution (CF0).In terms of nitrogen utilisation efficiency, the efficiency of topdressing nitrogen application ranged from 43.74% to 61.62% among the various organic substitution treatments. Importantly, under identical nitrogen application levels, controlled irrigation consistently resulted in higher nitrogen use efficiency compared to conventional flooding irrigation. This underscores the synergistic benefits of combining controlled irrigation with partial organic fertiliser substitution in optimising nitrogen management for rice production.Regarding the contribution of topdressing nitrogen to total nitrogen accumulation, values ranged from 14.21% to 16.12% under controlled irrigation with organic manure substitution, representing a 0.71% - 2.62% increase relative to CK. However, no significant difference was detected between the two irrigation modes under conventional nitrogen application. Under controlled irrigation conditions, substituting 60% of basal fertilizers with organic fertilizer significantly enhances the translocation contribution of topdressing nitrogen to grain nitrogen accumulation, improves nitrogen fertilizer use efficiency, and modifies the distribution pattern of topdressing nitrogen across different rice organs.Correlation analysis revealed significant positive relationships (P < 0.05) between topdressing nitrogen utilization efficiency and both its contribution to grain translocation and its accumulation percentage in organs relative to total nitrogen. In summary, replacing 60% of the basal fertiliser with organic fertiliser under controlled irrigation (CF3) offers advantages such as high yields and efficient utilisation of topdressing fertilisers, making it the optimal ratio for organic fertiliser substitution. The results of this study can provide a theoretical basis and technical reference for further improving the utilization rate of fertilizer nitrogen in paddy fields in the black soil region of cold areas.