Sulfur-coated slow-release fertilizers comprise four nitrogen (N) products with distinct release periods. This study aimed to determine whether these products should be applied individually or in combination, whether they should be blended with quick-release N fertilizers, and how to optimize their application to synergistically improve rice yield and nitrogen use efficiency (NUE).

The field experiments were conducted in Yangzhou and Changzhou, Jiangsu Province from 2022 to 2024, using rice cultivars Nanjing 9108 and Nanjing 46 as test materials. The trials included comparisons of slow-release N products, screening of optimal product combinations, evaluations of fertilizer types, blending ratios of slow- and quick-release N fertilizers, and application timing of quick-release N, and optimization of slow-release N formulations. The effects of sulfur-coated slow-release fertilizer application methods on rice yield formation and nitrogen uptake and utilization were analyzed。

(1) The rice yield under the straw return treatment increased by 4.50% and 8.88% compared with the half-straw return and no-straw return treatments, respectively. Additionally, the nitrogen use efficiency (NUE) under the straw return treatment was 11.35% and 25.64% higher than that under the other two treatments, respectively. (2) The rice yield of the 60-day slow-release nitrogen product was 4.10%, 15.68%, and 19.09% higher than that of the 30-day, 90-day, and 120-day products, respectively, but 4.13% lower than that of the conventional precise fertilization. Overall, the NUE of the 60-day product exceeded that of other slow-release nitrogen products by an average of 8.68%. Under no-straw return and half-straw return conditions, the NUE was 9.88% and 0.93% higher than that under conventional precise fertilization, respectively; however, under full straw return, it was 4.57% lower. (3) The combination of slow-release nitrogen products in the ratio of 3+6+9 achieved the highest rice yield and NUE, yet both were lower than those of the conventional precise fertilization. (4) The treatment with a blending ratio of 7:3 (slow- to quick-release nitrogen) achieved the highest rice yield, nitrogen accumulation, and NUE, averaging 4.31%, 3.25%, and 6.15% higher than that under conventional precise fertilization, respectively. Under this treatment, applying quick-release fertilizer as tillering fertilizer (for medium-duration varieties) or panicle initiation fertilizer (for long-duration varieties) synergistically improved yield, nitrogen accumulation, and NUE. The yield increase was primarily attributed to a significant enhancement in spikelets per panicle on the basis of adequate panicle number. (5) When slow-release nitrogen products were blended at a ratio of 30-day:60-day:90-day = 1:4:1, rice yield, nitrogen accumulation, and NUE all achieved the highest values, averaging 3.76%, 5.66% and13.35% higher than conventional precise fertilization, respectively, which was closely associated with significant increases in leaf area index (LAI), sink capacity, and biomass yield.

A blended slow-release N fertilizer was formulated by combining three controlled-release fertilizer with dissolution periods of 30 d, 60 d, and 90 d at a 1:4:1 mass ratio. Application of this blended slow-release N with quick-release N at a 7:3 ratio—implemented through one-time basal application of slow-release fertilizer and single topdressing of quick-release N as either tillering fertilizer (for 150-day varieties) or flowering-promoting fertilizer (for >160-day varieties)—significantly increased grain yield by 3.76% and nitrogen use efficiency by 13.55% compared with conventional precision quantification. This field-validated protocol demonstrates sulfur-coated fertilizers enabled synergistic gains in rice productivity and nitrogen utilization efficiency.

The aim of this study was to explore and analyze the differences in yield, panicle differentiation and degeneration, soil microbial activity, and response of soil alkali-hydrolysable nitrogen to simplified panicle fertilization among different rice varieties under the condition of wheat straw return (SR).

As test materials, two cultivars were selected, including late maturing medium japonica rice Nanjing 9108 (NJ 9108) and indica rice Yangdao 6 (YD6). Rice yield, spikelet differentiation, and degeneration were evaluated under the interactive conditions of SR and simplified panicle fertilization (0∶0, no panicle fertilization; 2∶0, full application of spikelet-promoting fertilizer; 1∶1, equal application of spikelet-promoting and protecting fertilizers; 0∶2, full application of spikelet-protection fertilizer). The nutritional basis of rice spikelet differentiation and degeneration was analyzed in terms of nutrient release from straw decomposition, alterations in soil microbial communities and enzyme activity, and soil alkaline nitrogen content.

(1) After SR, the average yield of NJ 9108 and YD 6 increased by 4.2% and 3.2%, respectively. Under panicle fertilization treatments, the highest yield for NJ9108 and YD6 was achieved under the 2∶0 and 1∶1 treatment, respectively. After no wheat straw return treatment (NR), the yield of both varieties was highest under the 1∶1 treatment. The yield trend of panicle fertilization treatments was consistent within the range of 180-360 kg N·hm^-2. (2) After SR, the initial 0-30 days were a period of rapid decomposition, with the rapid release of carbon and nitrogen from the straw. At 30 days, the average number of bacteria, fungi, and actinomycetes in the soil increased by 179.2%, and the average activity of urease, acid phosphatase, and sucrase increased by 88.8%. During the period of 40 to 60 days, the straw's decomposition and carbon-nitrogen release rates diminished, and the number of microorganisms and enzyme activities decreased significantly. Decomposition and carbon-nitrogen release of straw essentially stalled between 60 and 90 days, while the number of microorganisms and enzyme activities decreased gradually. From 10 to 40 days after SR, the soil's alkali-hydrolysable nitrogen content decreased by an average of 4.8%, while it increased by an average of 5.2% between 50 and 90 days. (3) After SR, the increase in soil alkali-hydrolysable nitrogen caused an increase of 1.4% in the average number of spikelets differentiation, a decrease of 12.3% in the average number of spikelets degeneration, but an increase of 4.4% in the average number of surviving spikelets. Rice harvest increased primarily due to an increase in the number of spikelets per panicle (the number of surviving spikelets per panicle). After SR, the 2∶0 treatment reduced the rate of spikelet degeneration in NJ9108 and YD6 relative to NR by 23.5% and 7.6%, respectively. The number of spikelet differentiation and degeneration of NJ9108 increased by 8.9 and 5.7 spikelets per panicle under the 2∶0 treatment relative to the 1∶1 treatment, whereas Yangdao 6 increased by 6.8 and 11.6 spikelets per panicle, respectively. As the increase in the number of spikelets differentiation was greater than the increase in the number of spikelets degeneration, NJ9108 had the highest number of surviving spikelets under the 2∶0 treatment, while YD6 had the highest number under the 1∶1 treatment.

Wheat straw return increased rice yield, with NJ 9108 achieving the highest yield under the full application of spikelet-promoting fertilizer and Yangdao 6 achieving the highest yield under the equal application of spikelet-promoting and protecting fertilizers. Those factors that contributed to the increase in yield were mainly due to a significant reduction in the rate and number of spikelets degeneration and an increase in the number of spikelets per panicle. The decrease in the rate of spikelets degeneration for NJ 9108 was greater than that for YD6, which was the main reason for the difference in their response to different panicle fertilization treatments in terms of the number of surviving spikelets.