Effects of urea topdressing time on yield, nitrogen utilization, and quality of mechanical direct-seeding hybrid indica rice under slow-mixed fertilizer base application

Introduction The use of controlled-release nitrogen (N) fertilizers has been shown to improve yield and N-use efficiency (NUE) in mechanical transplanted rice. However, the fertilizer requirements for mechanical direct-seeding rice differ from those for mechanical transplanted rice. The effects of controlled-release fertilizers on yield, NUE, and quality in mechanical direct-seeding rice are still unknown. Methods Hybrid indica rice varieties Yixiangyou 2115 and Fyou 498 were used as test materials, and slow-mixed N fertilizer (120 kg hm-2) as a base (N1), N1+urea-N (30 kg hm-2) once as a base (N2), N1+urea-N (30 kg hm-2) topdressing at the tillering stage (N3), N1+urea-N (30 kg hm-2) topdressing at the booting stage (N4) four N fertilizer management to study their impact on the yield, NUE and quality of mechanical direct-seeding rice. Results and discussion Compared with Yixiangyou 2115, Fyou 498 significantly increased photosynthetic potential, population growth rate, root vigor, and N transport rate by 3.34–23.88%. This increase further resulted in a significant improvement in the yield and NUE of urea-N topdressing by 1.73–5.95 kg kg-1. However, Fyou 498 showed a significant decrease in the head rice rate and taste value by 3.34–7.67%. All varieties were treated with N4 that significantly increase photosynthetic potential and population growth rate by 15.41–62.72%, reduce the decay rate of root vigor by 5.01–21.39%, promote the N transport amount in stem-sheaths (leaves) by 13.54–59.96%, and then significantly increase the yields by 4.45–20.98% and NUE of urea-N topdressing by 5.20–45.56 kg kg-1. Moreover, the rice processing and taste values were optimized using this model. Correlation analysis revealed to achieve synergistic enhancement of high-yield, high-quality, and high-NUE in rice, it is crucial to focus on increasing photosynthetic potential, population growth rate, and promoting leaf N transport. Specifically, increasing the contribution rate of N transport in stem-sheaths is the most important. These findings offer an effective N management strategy for 4R nutrient stewardship (right source, right method, right rate and right timing) of mechanical direct-seeding hybrid indica rice.


Introduction:
The use of controlled-release nitrogen (N) fertilizers has been shown to improve yield and N-use efficiency (NUE) in mechanical transplanted rice.However, the fertilizer requirements for mechanical direct-seeding rice differ from those for mechanical transplanted rice.The effects of controlledrelease fertilizers on yield, NUE, and quality in mechanical direct-seeding rice are still unknown.
Methods: Hybrid indica rice varieties Yixiangyou 2115 and Fyou 498 were used as test materials, and slow-mixed N fertilizer (120 kg hm -2 ) as a base (N 1 ), N 1 +urea-N (30 kg hm -2 ) once as a base (N 2 ), N 1 +urea-N (30 kg hm -2 ) topdressing at the tillering stage (N 3 ), N 1 +urea-N (30 kg hm -2 ) topdressing at the booting stage (N 4 ) four N fertilizer management to study their impact on the yield, NUE and quality of mechanical direct-seeding rice.
Results and discussion: Compared with Yixiangyou 2115, Fyou 498 significantly increased photosynthetic potential, population growth rate, root vigor, and N transport rate by 3.34-23.88%.This increase further resulted in a significant improvement in the yield and NUE of urea-N topdressing by 1.73-5.95kg kg -1 .However, Fyou 498 showed a significant decrease in the head rice rate and taste value by 3.34-7.67%.All varieties were treated with N 4 that significantly increase photosynthetic potential and population growth rate by 15.41-62.72%,reduce the decay rate of root vigor by 5.01-21.39%,promote the N transport amount in stem-sheaths (leaves) by 13.54-59.96%,and then significantly increase the yields by 4.45-20.98%and NUE of urea-N topdressing by 5.20-45.56kg kg -1 .Moreover, the rice processing and taste values were optimized using this model.Correlation analysis revealed to achieve synergistic enhancement of high-yield, high-quality, and high-NUE in rice, it is crucial to focus on

Introduction
China is currently undergoing critical transformation from traditional to modern agriculture (Cheng et al., 2023).Although the mechanization rate for plowing and harvesting is high, the rice (Oryza sativa L.) machine planting segment lags behind at a rate of less than 50% (National Bureau of Statistics of China, 2022).To improve this situation, China has increased its support for mechanical transplanted rice.However, although machine transplanting technology has significant advantages over traditional seedling raising and hand transplanting, the laborious process of seedling raising and management for mechanical transplanted rice, coupled with the high labor intensity of centralized seedling transport and transplanting, still results in high overall cost (Zhong et al., 2021;He et al., 2023).For largescale production, direct seeding using rice machines is the most convenient method for rice cultivation (Guo et al., 2023a).Mechanical direct-seeding eliminates the need for raising, transporting, and transplanting seedlings, resulting in improved production efficiency (Farooq et al., 2011).This approach also boosts mechanized planting and is an efficient method for largescale rice production (Farooq et al., 2011;Yang et al., 2023).However, rice varieties suitable for mechanical direct-seeding in China's major rice-producing regions are not appropriate (Sun et al., 2022;Guo et al., 2023a).To achieve high-yield and highefficiency cultivation, it is further essential to integrate agricultural machinery and agronomy deeply (Yang et al., 2022).However, research on the theory of high-quality and high-yield cultivation of mechanically direct-seeding rice is still relatively inadequate.
Nitrogen (N) fertilizers are key for boosting rice production.However, N fertilizers are prone to volatilization and leakage.In China, the N use efficiency (NUE) of rice is low at approximately 30-35%, which is considerably lower than the global average in developed countries (46%) (Peng et al., 2009;Sun et al., 2023a).The primary cause is excessive fertilizer application, which results in diminishing returns.Nitrification and denitrification are significant contributors to low NUE (Chen et al., 2022).To address these issues, measures such as the 4R nutrient stewardship concept (right source, right rate, right time, right place) (IPNI, 2012) promoted by the International Plant Nutrition Institute (IPNI), N fertilizer management (Yokamo et al., 2023;Sun et al., 2023a;Sun et al., 2023b), soil testing and formulated fertilizer application (Chen et al., 2021), and leaf color diagnosis of SPAD meter (Peng et al., 1996) have been implemented by numerous scholars, resulting in positive outcomes.It is crucial to adopt practices to ensure that the N release rate aligns with the crop fertilizer requirements.The development and application of controlled-release fertilizers have garnered increased interest and research due to their potential to enhance N use and production efficiency, while saving time and labor (Ke et al., 2018;Lyu et al., 2021a;He et al., 2023).This area remains a focus of research, with the literature predominantly focusing on hand-and machinetransplanted rice (Ke et al., 2018;Lyu et al., 2021a;Yu et al., 2022).However, there is limited research on the effects of controlledrelease fertilizers in mechanically direct-seeding rice.This study primarily examined the various types, optimal application amounts, and methods of side deep fertilization, as well as the use of slow-mixed fertilizers and other relevant factors related to slow-controlled-release fertilizers (Ke et al., 2018;Wu et al., 2021;Sun et al., 2023b).These factors were investigated under a onetime basal application.However, inconsistencies were observed in the study due to variations in controlled-release fertilizers (Wu et al., 2021;Lyu et al., 2021a), nutrient release timing (Cheng et al., 2022), and supporting application techniques (Hou et al., 2021;He et al., 2023).During the later stages, the rate and intensity of controlled-release fertilizers did not meet the immediate requirements of the heavy panicle hybrid rice and super rice varieties.These varieties require increased tillers, grains per panicle, 1000-grain weight, and single panicle weight (Jiang et al., 2016;Sun et al., 2022;Li et al., 2023).Currently, there is limited research on the optimal period for applying N fertilizer during the grain-filling stage of mechanical direct-seeding of heavy panicle hybrid rice.It is uncertain whether adjusting the timing of N fertilizer topdressing to match the growth and development characteristics of the plant will cause improved yield, NUE, and rice quality compared with the basal application of a controlled-release fertilizer.
Based on our previous research (Sun et al., 2022;Guo et al., 2023a;Sun et al., 2023b), we selected heavy panicle super-hybrid rice varieties for this study.We investigated the effects of base application of slow-mixed fertilizer and topdressing of conventional N fertilizer on photosynthetic production and N-use characteristics under the condition of mechanical direct seeding.This study systematically examined the yield and quality of direct-seeding rice, along with its physiological mechanisms.This study proposes an optimal management mode for combining controlled-release N fertilizers with mechanical direct-seeding rice.It also suggests technical regulations for improving quality, yield, and NUE.These findings provide a theoretical and practical basis for implementing high-quality, high-yield, and high-efficiency 4R nutrient stewardship (right source, right method, right rate and right timing) technologies for heavy panicle mechanical directseeding hybrid indica rice.

Study site and materials
Field experiments were conducted in Chongzhou (103°38′E, 30°3 3′N), Sichuan Province, China, in 2021 and 2022.The soils samples (0-20 cm) were analyzed physicochemical characteristics before initiation of the experiments (Table 1).The study site has a subtropical monsoon humid climate, and the rainfall, sunshine hours, and average temperature, during the rice growing season (May to October) were 924.10 mm, 836.05 h, and 22.77°C in 2021 and 894.20 mm, 762.41 h, and 23.02°C in 2022, respectively.This study used Yixiangyou 2115 (growth periods 145.2 d, female parent Yixiang 1A and male parent Yahui 2115) and Fyou 498 (growth periods 144.5 d, female parent FS3A and male parent Shuhui 498), two representative hybrid indica rice cultivars bred by Sichuan Agricultural University that are widely planted in South China.A slow-mixed basal fertilizer was applied, including 120 kg hm -2 N fertilizer comprising polymer-coated controlled-release urea-N (90 kg hm -2 ) and conventional urea-N (30 kg hm -2 ), 75 kg hm -2 P 2 O 5 , and 150 kg hm -2 K 2 O, provided by Yantai Longdeng Fertilizer Co., Ltd.The N-release characteristics of the controlled-release N immersed in water at pH 7 and 25°C and the cumulative release rate of N reached 87.67% within 78 d shown in Figure 1 as described by Tomaszewska and Jarosiewicz, 2002.

Experimental design and field management
The experiments used a randomized design with two varieties and four strategies for managing N fertilizer.The comprehensive N fertilizer management mode (Table 2) were as follows: (1) slowmixed N fertilizer (120 kg hm -2 ) as a base (N 1 , as control, in order to further calculate the NUE of urea-N topdressing); (2) N 1 +urea-N (30 kg hm -2 ) once as a base (N 2 ); (3) N 1 +urea-N (30 kg hm -2 ) topdressing on 32d after sowing at the tillering stage (N 3 ); (4) N 1 +urea-N (30 kg hm -2 ) topdressing on 93d after sowing at the booting stage (N 4 ).Rice seeds were sown directly using a 2BDS-6 hand-held rice precision hill-direct-seeding machine (Guilin Hightech Zone Kefeng Machinery Co., Ltd.) on May 14th for both years.The row spacing and plant spacing were 25 cm and 20 cm, respectively, with a sowing amount of 30.0 kg hm -2 (4-6 seeds per hole) and density of 200,000 holes hm -2 .Each treatment had three replicates, with a plot area of 40.8 m 2 (8.5 m length and 4.8 m width).To prevent water and fertilizer from mixing, the plastic film was wrapped around ridges (40 cm wide and 30 cm high) constructed between the plots.All treatments used a highefficiency alternation-irrigation technique (Sun et al., 2012).Chemical pesticides have been used to prevent yield loss and experimental errors caused by insects, diseases, and weeds.

Leaf area index
At the jointing, heading, and maturity stages, we obtained five holes from the representative rice plants in each plot based on the average number of tillers.We measured the leaf area of the rice plants at each growth stage using a CID-203 leaf area analyzer (CID Company, USA).The leaf area index (LAI) was calculated using the method reported by Liu et al. (2022).

Biomass accumulation
Five holes were sampled from each plot, representing rice plants with average tillers at the heading and maturity stages.The samples were divided into four parts: the stem sheaths, leaves, panicles, and roots.The samples were then exposed to 105°C for 40 min and subsequently dried at 80°C until they reached a constant weight (Guo et al., 2023a).The total biomass accumulation was calculated as the sum of the dry matter accumulation of the four plant parts.

Root vigor
As mentioned in Section 2.3.2, the method of Ramasamy et al. (1997) was used.Fresh roots (2.0 g) from each plot sample were transferred into a 100 mL flask.Then, 25.0 mL of 50.0 mg L -1 a-NA and 25.0 mL of phosphate buffer (0.1 mol L -1 , pH 7.0) were added.After filtration for 2h at 25°C in a closed shaker, 2.0 mL of NaNO 2 (100.0 mg L -1 ) and 1.0mL of sulfonamide (1.0%) were added to the

Year
Organic matter (g kg -1 ) Total N (g kg -1 ) filtrate.Color was determined using a Shimadzu-1700 spectrophotometer (Japan) at 510 nm.The results were expressed as mg a-NA g -1 DW h -1 .

Nitrogen content
As stated in Section 2.3.2, each part was crushed and sieved separately through an 80-mesh sieve.The N content of each part was determined using the Kjeldahl method (Kjeltec-8400; FOSS, Hillerd, Denmark), as described by Yoshida et al. (1976).

Grain yield and its components
The number of effective panicles per plant was determined by examining 50 holes in rice plants in each plot at maturity.From each plot, 10 holes of rice plants were selected based on the average tillers to examine the total grains per panicle, number of full grains, and 1000-grain weight.The seed-setting percentages were also calculated (Guo et al., 2023a).The grain yield was determined by harvesting each 12.0 m² plot without border plants and adjusting it to a standard moisture content of 13.5% using a grain moisture meter (PM-8188-A, Kett Electric Laboratory, Tokyo, Japan).

Grain quality measurements
Approximately 1000 g of grains were harvested from each plot and naturally dried in the shade for three months to stabilize their physicochemical properties.The grains were then analyzed for quality after being passed through a dehusker and polished.A 250gram sample was separated into broken and unbroken grains, and the brown rice, milled rice, and head rice rates were expressed as percentages of the total 250-gram rice grains (Zhang et al., 2008).For each sample, 1000 whole milled rice grains were randomly selected and scanned to create a digital image.This process was repeated three times.Image analysis software (JMWT-12, Dongfujiuheng Instrument Technology Co., Ltd., Beijing, China) was used to determine the rates of chalky grains and the degree of chalkiness (Guo et al., 2023b).The taste of cooked rice was measured using a Satake Rice Taste Analyzer (STA1A type, Satake, Japan) (Shi et al., 2022).30.0 g of milled rice were weighed and placed in a stainless steel tank.Water was added at a rice-to-water ratio of 1:1.4,and the mixture was soaked for 30 min.The mouth of the tank was wrapped with filter paper and the tank was placed in a steam electric rice cooker.The rice was steamed for 30 min and then cooled for 2 h.After cooling, 7.0 g of rice was weighed at 25°C and placed into a special rice press instrument to form a rice cake.The rice cakes were then placed in a taste analyzer for testing.

Indicator calculation
As mentioned in Section 2.3 measurement terms, the calculation and definition of the following parameters are based on the method of Sun et al. (2023a).
Population photosynthetic potential from jointing to heading stage (PP; ×10 4 m 2 d hm -2 ) = 1/2 (the leaf area at the jointing stage + the leaf area at the heading stage) × (the time at the heading stage-the time at the jointing stage) Dry matter transport rate in stem sheaths (DTR; %) = (dry matter weight in stem sheaths at the maturity stage-dry matter weight in stem sheaths at the heading stage)/dry matter weight in stem sheaths at the heading stage × 100 Dry matter transport contribution rate in stem sheaths (DCR; %) = (dry matter weight in stem sheaths at the maturity  N 1 : slow-mixed N fertilizer (120 kg hm -2 ) as a base; N 2 : N 1 +urea-N (30 kg hm -2 ) one-time as a base; N 3 : N 1 +urea-N (30 kg hm -2 ) topdressing at tillering stage (32d after sowing); N 4 : N 1 +urea-N (30 kg hm -2 ) topdressing at booting stage (93d after sowing).

Statistical analysis
Data were analyzed using Microsoft Excel 2010.Analysis of variance (ANOVA) was performed using the statistical program SPSS 18.0 (SPSS Statistics, SPSS Inc., Chicago, IL, USA).Graphs were generated using SigmaPlot 10.0 (Systat Software Inc., Chicago, IL, USA).The treatment means were tested using the least significant difference (LSD) test (P < 0.05).Principal component analysis was conducted using Origin 2021 (OriginLab Corp., Northampton, MA, USA).

Yield and yield components
The impact of varying N fertilizer management on grain yield and its components in mechanical direct-seeding hybrid indica rice is significant.The interaction effect of the two factors on grain yield, filled spikelets, total spikelets, and seed-setting rate was also significant.The trend observed in the two-year experiments was consistent (Table 3).Under different varieties and N fertilizer management, the impact of N fertilizer on yield was significantly greater than variety.The yields of the N 2 , N 3 , and N 4 treatments increased by 4.45%, 13.29%, and 20.98%, respectively, compared to the N 1 treatment, with the highest yield observed in the N 4 treatment.Fyou 498 showed a greater increase under N 2 and N 3 treatments compared to Yixiangyou 2115.This suggests that varieties with a large panicle type and high sink capacity, such as Fyou 498, should be treated with late booting fertilizer besides the base application of slow-mixed fertilizer to exploit their highyield potential.
Table 3 show that N fertilizer management resulted in higher numbers of filled spikelets, total spikelets, and filled grains than the effects of variety differences.However, the number of effective panicles and 1000-grain weight showed opposite trends.This suggests that a suitable combination of variety and N fertilizer regulation can adjust the yield components of mechanical directseeding hybrid indica rice, ultimately promoting yield.The mean values of the yield components for Fyou 498 were significantly higher than those for Yixiangyou 2115 among the different varieties, except for the number of effective panicles and 1000-grain weight.Under the application of slow-release fertilizer combined with urea-N topdressing, the number of effective panicles initially increased and then decreased with a delay in the urea-N fertilizer application time.The yield components of different varieties also increased with the delay in the urea-N fertilizer application time.The filled spikelets, total spikelets, filled grains, and 1000-grain weight of the two varieties under N 4 treatment were higher than those under the other treatments.This compensated for the deficiency of the effective panicle number and was a significant factor in achieving a high yield under N 4 treatment.

Leaf area index and population photosynthetic potential
At both the jointing and heading stages, the LAI of Fyou 498 was significantly higher than Yixiangyou 2115 by 9.61-13.91%and 8.67-13.35%,respectively.This trend was consistent across both years of experimentation (Figures 2A, B, D, E).For the same variety under different N fertilizer management, LAI at the jointing and heading stages showed an initial increase, followed by a decrease.Compared to the N 1 treatment, the average values of the N 2 , N 3 , and N 4 treatments for the two varieties during the jointing stage (N 4 treatment without topdressing urea-N fertilizer during this time) were significantly higher by 22.62%, 30.15%, and 20.63%, respectively.At the heading stage, the LAI of the N 2 , N 3 , and N 4 treatments increased significantly by 21.45%, 31.03%, and 26.20%, respectively.The LAI in the N 4 treatment showed the most significant increase.From the jointing to heading stage, the population photosynthetic potential of Fyou 498 was 5.01%-10.49%higher than Yixiangyou 2115 (Figures 2C, F).The photosynthetic potential of the population under the same variety increased significantly from the jointing to the heading stage with the postponement of urea-N fertilizer application.Compared with the N 1 treatment, the N 2 , N 3 , and N 4 treatments increased significantly by 15.07%, 35.43%, and 47.09%, respectively.

Dry matter accumulation and translocation and population growth rate
The impact of variety and N fertilizer on the indices of dry matter accumulation and transport, as well as the population growth rate from the heading to the maturity stage, was significant.The interaction effect of the two factors on the amount of dry matter accumulation, transport rate, and contribution rate in stem sheaths, as well as the population growth rate, was also significant (Table 4).Furthermore, the accumulation and translocation of dry matter, as well as the population growth rate from the heading to maturity stage, were significantly more affected by the management of N fertilizer than by the differences between the varieties.The amount of dry matter accumulated in the stem sheaths of Fyou 498 was 4.46% higher than Yixiangyou 2115 at the heading stage, and 2.42% higher at the maturity stage.The dry matter accumulation, dry matter transport rate, dry matter transport contribution rate, and population growth rate of Fyou 498 from the heading to maturity stage were 8.83%, 5.48%, 7.09%, and 8.40% higher than those of Yixiangyou 2115,   respectively.The indices of dry matter accumulation and population growth rate increased to varying degrees with the postponement of the urea-N topdressing time under the same variety.Compared to the N 1 treatment, the N 2 , N 3 , and N 4 treatments significantly increased the dry matter accumulation in stem sheaths during the heading and maturity stages by 3.04-40.43%and 17.31-53.50%,respectively.Furthermore, the dry matter accumulation in plants and population growth rate from the heading to maturity stage were significantly increased by 27.18-78.55%and 26.69-62.72%,respectively.Compared with the N 1 treatment, the dry matter transport rate and contribution rate in stem sheaths from heading to maturity among the N 2 , N 3 , and N 4 treatments were significantly reduced by 6.95-8.77%and 2.57-6.18%.However, the dry matter transport rate and contribution rate in stem sheaths from heading to maturity increased by 1.18-1.82%and 3.11-3.62%with the postponement of the urea-N fertilizer application time in the N 2 , N 3 , and N 4 treatments.

Root vigor
The root vigor of Fyou 498 was significantly higher than Yixiangyou 2115 at both the heading (Figures 3A, D) and maturity stages (Figures 3B, E), with differences ranging from 4.80% to 18.15% and 8.76% to 23.88%, respectively.Furthermore, the root vigor decay rate from the heading to maturity stage (Figures 3C, F) was 1.76-7.08%lower in Fyou 498 than in Yixiangyou 2115.The results of the 2-year experiments showed consistent trends.In the same variety, postponing the application time of urea-N resulted in a significant increase in root vigor during the heading and maturity stages.Compared with the N 1 treatment, the root vigor at the heading stage increased significantly by 12.05-27.04%,39.07-53.36%,and 77.89-80.09% in the N 2 , N 3 , and N 4 treatments, respectively.At the maturity stage, root vigor increased significantly by 27.60-40.42%,65.91-76.26%,and 118.52-126.18% in the N 2 , N 3 , and N 4 treatments, respectively.Moreover, the decay rate of root vigor from the heading to maturity stage decreased with postponement of the urea-N fertilizer application time.Compared to the N 1 treatment, root vigor decay rate decreased significantly by 7.91-10.72%,11.05-15.07%,and 16.05-21.39%in the N 2 , N 3 , and N 4 treatments, respectively, from the heading to maturity stage.

Nitrogen transport and utilization
Table 5 shows that the effect of variety on the N transport contribution rate in stem sheaths from the heading to the maturity stage was not significant.However, the effects of variety and N fertilizer management on N translocation in stem sheaths (leaves) from heading to maturity and NUE (NAE and NPE) were significant.Furthermore, the interaction effect of these two factors on N transport in stem sheaths and leaves, as well as the transport rate from heading to maturity, was significant.Compared to Yixiangyou 2115, Fyou 498 exhibited an increase of 1.03-9.38% in the N transport rate and N transport contribution rate in stem sheaths from heading to maturity.Moreover, Fyou 498 showed an Effects of slow-mixed fertilizer base application combined with urea topdressing time on root vigor at the heading stage (A, D), root vigor at the maturity stage (B, E) and root vigor decay rate (C, F) from the heading to maturity stage of mechanical direct-seeding hybrid indica rice.N 1 : slowmixed N fertilizer (120 kg hm -2 ) as a base; N 2 : N 1 +urea-N (30 kg hm -2 ) one-time as a base; N 3 : N 1 +urea-N (30 kg hm -2 ) topdressing at the tillering stage (32 d after sowing); N 4 : N 1 +urea-N (30 kg hm -2 ) topdressing at the booting stage (93 d after sowing).Different lowercase letters indicate significant (P < 0.05) differences among N fertilizer treatments under the same variety.increase of 3.34-12.01%in the N transport rate and N transport contribution rate in leaves from heading to maturity.The NAE and NPE of Fyou 498 were significantly higher than those of Yixiangyou 2115, by 13.98% and 15.44%, respectively.For the same variety, the amount of N transported in stem sheaths (leaves) and the contribution rate of N transport in stem sheaths (leaves) increased to varying degrees from the heading to maturity stage, with a delay in the urea-N topdressing time.Compared to the N 1 treatment, the N 2 , N 3 , and N 4 treatments significantly increased the amount of N transported in stem sheaths (leaves) from the heading to maturity stage by 13.54-59.96%.Furthermore, the N transport contribution rate in stem sheaths (leaves) increased by 0.29-16.50%.However, the rate of N transport in stem sheaths (leaves) from the heading to maturity stage decreased by 2.13-25.27%when the urea-N topdressing was postponed.Moreover, both NAE and NPE increased significantly with a delay in the urea-N topdressing.

Rice quality
The 2-year experiments showed significant effects of variety and N fertilizer management on rice quality indicators.The interaction effect of the two factors only had a significant impact on chalkiness and chalky kernel rate (Table 6).Regarding the treatment of varieties and N fertilizer management, the rice quality indicators of the variety differences were significantly higher than those of N fertilizer management, except for the brown rice rate.Compared with Yixiangyou 2115, Fyou 498 exhibited a decrease in brown rice rate, milled rice rate, head rice rate, and taste value by 0.52-5.51%,1.53-3.40%,3.73-6.64%,and 5.05-7.67%,respectively.Furthermore, the chalkiness and chalky kernel rates increased by 1.63-4.91%and 4.37-13.17%,respectively.When comparing the same variety, brown rice, milled rice, head rice, chalkiness, and chalky grain rates increased to varying degrees with the postponement of the urea-N topdressing time.Compared to the N 1 treatment, the brown rice, milled rice, head rice, chalkiness, and chalky grain rates increased by 0.65-5.09%,0.67-3.75%,1.17-6.44%,0.21-3.48%,and 0.52-12.77% in the N 2 , N 3 , and N 4 treatments, respectively.Compared to the N 1 treatment, the taste values of the N 2 , N 3 , and N 4 treatments decreased significantly by 1.16-5.88%.However, the taste value increased significantly when the urea-N topdressing time was delayed.

Relationships between photosynthetic production, root vigor, N transport, yield, rice quality, and NUE
Principal component analysis (PCA) was conducted to analyze the relationship between photosynthetic production, root vigor, and N translocation and grain yield, total spikelets, NUE, head rice rate, and taste value under different varieties and N application management (Figure 4).Under various N fertilizer treatments, Yixiangyou 2115 and Fyou 498 had principal components 1 and 2, explaining 92.2% and 93.1% of the total changes in grain yield, total spikelets, NAE, NPE, head rice rate, and taste value, respectively.The relationship between these variables suggested that grain yield, NAE, NPE, and taste value of the two varieties were correlated positively with LAI at the jointing stage, LAI at the heading stage, photosynthetic potential from the jointing to heading stage, root vigor at the maturity stage, dry matter transport rate and dry matter transport contribution rate in stem sheaths from the heading to maturity stage, population growth rate from the heading to maturity stage, N transport amount and N transport contribution rate in stem sheaths from the heading to maturity stage, N transport amount and N transport contribution rate in leaves from the heading to maturity stage (Figure 4).However, they correlated negatively with the root vigor of decay rate from the heading to maturity stage, N transport rate in stem sheaths from heading to maturity stage, and N transport rate in leaves from heading to maturity stage (Figure 4).The total explained amount of the two rice varieties under different N fertilizer treatments (Figure 4), compared to Yixiangyou 2115, can better explain the synergistic enhancement of high yield, good quality, and high NUE in rice, including grain yield, rice quality, and NUE for Fyou 498.The correlation analysis in Figure 5 indicates that the grain yield, total spikelets, NAE, NPE, head rice rate, and taste value of both varieties were significantly positively correlated with the photosynthetic potential from the jointing to heading stage, dry matter transport contribution rate in stem sheaths from the heading to maturity stage, N transport contribution rate in stem sheaths from the heading to maturity stage, and N transport amount in N 1 : slow-mixed N fertilizer (120 kg•hm -2 ) as a base; N 2 : N 1 +urea-N (30 kg•hm -2 ) one-time as a base; N 3 : N 1 +urea-N (30 kg•hm -2 ) topdressing at the tillering stage (32d after sowing); N 4 : N 1 +urea-N (30 kg•hm -2 ) topdressing at the booting stage (93d after sowing).V, Variety; N, N fertilizer treatment; V×N, cultivar and N fertilizer treatment interaction.*, P < 0.05; **, P < 0.01.Different lowercase letters indicate significant (P < 0.05) differences among N fertilizer treatments under the same variety.
leaves from the heading to maturity stage.The correlations between the N transport contribution rate in stem sheaths from the heading to maturity stage and the photosynthetic potential from the jointing to heading stage, population growth rate from the heading to maturity stage, and N transport amount in leaves from the heading to maturity stage were highly significant.This indicates that increasing the N transport contribution rate in stem sheaths from the heading to maturity stage can synergistically enhance the high yield, high quality, and high NUE of mechanical direct-seeding hybrid indica rice.Furthermore, when considering each variety, the effects of different N fertilizer management on the taste regulation of the high taste value Yixiangyou 2115 were significantly less Heat map of person correlation in grain yield, rice quality and NUE with photosynthetic production, root vigor and N transport.The number of samples is 24 in the analysis of each indicator under every variety.GY, grain yield; TS, total spikelets; NAE, N agronomic efficiency of urea-N topdressing; NPE, N recovery efficiency of urea-N topdressing; HRR, head rice rate; TV, taste value; LAIJ, LAI at the jointing stage; LAIH, LAI at the heading stage; PP, photosynthetic potential from the jointing to heading stage; RTMS, root vigor at the maturity stage; DCRT, root vigor of decay rate from the heading to maturity stage; DTR, dry matter transport rate in stem sheaths from the heading to maturity stage; DCR, dry matter transport contribution rate in stem sheaths from the heading to maturity stage; PGR, population growth rate from the heading to maturity stage; NSTA, N transport amount in stem sheaths from the heading to maturity stage; NSTR, N transport rate in stem sheaths from heading to maturity stage; NSCR, N transport contribution rate in stem sheaths from the heading to maturity stage; NLTA, N transport amount in leaves from the heading to maturity stage; NSTR, N transport rate in leaves from heading to maturity stage; NSCR, N transport contribution rate in leaves from the heading to maturity stage.*, P < 0.05; **, P < 0.01.pronounced than those of the low-taste value Fyou 498.This suggests that there is significant potential for improving the taste value of low-taste value varieties through urea-N topdressing under slow-mixed fertilizer-based application.

Discussion
4.1 Effect of slow-mixed fertilizer base application combined with available N fertilizer on rice yield formation Selecting appropriate rice varieties and optimizing N fertilizer management are crucial for regulating rice yield (Jiang et al., 2016;Li et al., 2023).The most important factors affecting yield were the number of effective panicles, number of filled spikelets, and 1000grain weight.Different conclusions have been drawn regarding how to balance the relationships between these factors under varying cultivation conditions (Guo et al., 2023a;Sun et al., 2023a).Previous studies have shown that the productivity of rice varieties primarily depends on the total number of spikelets, which is the product of the number of effective panicles and number of grains per panicle (Wei et al., 2016;Lyu et al., 2021a;Li et al., 2023).The study results indicate that the N 1 treatment, which involves using urea-N fertilizer as a base fertilizer without topdressing in the later stage and varieties with excessive total spikelets, can lead to issues such as unfilled grains, decreased seed-setting rate, and reduced 1000-grain weight.This is compared to the N 3 and N 4 treatments (Table 3).Previous studies have suggested that onetime basal application of controlled-release fertilizers and formulations can increase rice yield and efficiency.This method is effective for increasing rice yield and efficiency.This method also regulates the amount of fertilizer used (Deng et al., 2021;Hou et al., 2021;He et al., 2023).However, this study suggests that slow-release fertilizers should be applied as a single basal application.If combined with urea-N in the later stage, the 'sink' capacity of the variety should be taken into consideration.In this study, Fyou 498 had a significantly higher total spikelet count than Yixiangyou 2115, but Yixiangyou 2115 had a 1000-grain weight >36.0 g (grain length 7.60 mm, grain width 2.65 mm, and length-width ratio 2.87 of single grain).The 1000grain weight indirectly reflects the rice's 'sink' capacity based on a certain amount of total spikelets.Both varieties in this study had a large 'sink' capacity.Therefore, to promote a significant increase in grain yield (Table 3), it is necessary to consider the combined application of urea-N fertilizer in the later stage, based on the application of slow-mixed fertilizer.
Rice yield is closely related to the dynamics of population tillers, photosynthetic characteristics, material accumulation, and transport capacity (Sun et al., 2012;Guo et al., 2023a).Superhigh-yield rice is characterized by a lower number of tillers in the early growth stage but a higher percentage of productive tillers.LAI and dry matter accumulation exhibited slow growth in the early stage, moderate growth during the heading stage, and a significant increase after the heading stage.The population growth rate was high, and 70-80% of the grain yield was achieved during the late growth stage (Cheng et al., 2022;Liu et al., 2022;Li et al., 2023).The study showed that using slow-mixed fertilizer combined with urea-N fertilizer N 3 treatment can increase the photosynthetic potential from the jointing to heading stage and the root vigor decay rate from the heading to maturity stage.This is important for ensuring a high population growth rate and dry matter quality in the late growth stage, resulting in a high yield and efficiency of directseeding rice.These findings further enriched and improved the results of previous studies (Wang et al., 2021;Cheng et al., 2022;Liu et al., 2022;Guo et al., 2023a).This study confirmed that rice varieties can contribute to high yield and efficiency (Meng et al., 2022).The study found that Fyou 498 had a significantly higher population photosynthetic potential, population growth rate, and root vigor than Yixiangyou 2115 during the main growth stages (Figures 2, 3; Table 4).This study found that rice varieties and slowmixed fertilizer base application with urea-N topdressing had significant effects on the photosynthetic characteristics, dry matter accumulation, and transport, and root vigor of mechanical directseeding rice.When applying controlled-release fertilizer, it is important to consider the combination of improved varieties and cultivation methods.This study was conducted based on a previous study that found the optimal amount of topdressing N fertilizer for direct-seeding rice in the latter stage to be 20% of the total N application.Increasing the proportion of postponed N fertilizer beyond 20% to 40-60% of the total amount significantly reduces the population quality of direct-seeding rice and increases the lodging index, resulting in yield reduction (Wu et al., 2020;Sun et al., 2022).Therefore, this study did not consider increasing the amount of N fertilizer postponement.

Effect of slow-mixed fertilizer base application combined with available N fertilizer on NUE and rice quality
High-NUE rice varieties, types of N fertilizer, and N fertilizer management practices are closely related to improving both NUE and rice quality (Li et al., 2023;Sun et al., 2023b).In this study, different varieties and slow-mixed basal fertilizer application with urea-N fertilizer management were compared.It was found that Fyou 498 significantly increased the transport of N and the contribution of stem sheaths (leaves) from heading to maturity compared to Yixiangyou 2115.Furthermore, Fyou 498 showed higher NAE and NPE values (Table 5), indicating synergistic characteristics of high yield and NUE (Koutroubas and Ntanos, 2003;Sun et al., 2017).However, the head rice rate and taste value of Fyou 498 were significantly lower than those of Yixiangyou 2115, and the chalky kernel rate significantly increased.This study found that the genetic difference in rice quality among varieties was significantly higher than N fertilizer (Table 6).This suggests that although the varieties were high yielding and highly efficient, they were not necessarily of high quality.Therefore, it is necessary to increase the screening of highquality, high-yield, and high-efficiency varieties suitable for mechanization.This finding complements the previous and our research results (Li et al., 2023;Yuan et al., 2023).Previous studies have concluded that the application of controlled-release N fertilizer and optimal N fertilizer operation can increase the N transport rate in plants, promoting N absorption and utilization in rice (Deng et al., 2021;Lyu et al., 2021a;Cheng et al., 2022).However, this study demonstrates that the N transport rate in stem sheaths (leaves) decreases to varying degrees with the delay of the N-topdressing period under the slow-mixed fertilizer base application.This finding is in contrast with the results of previous studies (Deng et al., 2021;Cheng et al., 2022).Although the amount of N transported in the stem sheaths (leaves) and the contribution rate of N transport in the stem sheaths (leaves) increased significantly during the topdressing time with N fertilizer in the experiments, the effect of urea-N fertilizer was significantly improved.However, the proportion of N retained in the stem sheaths (leaves) remained relatively high (Table 5).Further research is needed to improve the rate of N transport in vegetative organs during the filling stage of slow-mixed fertilizer combined with urea-N application.
Research findings on rice quality differences between varieties are more consistent due to their genetic background (Lin et al., 2011).However, the effect of N fertilizer management on rice quality characteristics remains a topic of debate (Cao et al., 2017;Shi et al., 2022;Guo et al., 2023b).Some studies have suggested that using controlled-release N fertilizer, increasing the N application amount, or delaying the application of N fertilizer under the same N application rate can reduce chalkiness and improve the overall eating quality of rice (Yuan et al., 2023;Guo et al., 2023b).Some studies have shown that increasing or delaying the application of N fertilizer can increase the chalkiness of rice (Cao et al., 2017).However, this study found that the effect of combined urea-N on the quality characteristics of direct-seeding rice under slow-mixed fertilizer-based application contradicted previous research (Zhang et al., 2008;Cao et al., 2017;Lyu et al., 2021b).The brown rice, milled rice, head rice, and taste values of direct-seeding rice improved with a delay in the N-topdressing period.The combined application of urea-N under slow-mixed fertilizer-based application may moderately increase grain plumpness (Sun et al., 2017;Sun et al., 2023b).However, delaying the application of urea-N under slow-mixed fertilizer base application worsens the degree of chalkiness and increases the rate of chalky grains.Further research is needed to determine whether the delayed release rate of controlled-release N fertilizer (Cheng et al., 2022) or the moderate delay of urea-N application affects the grain-filling rate (Sun et al., 2023b), starch anabolism (Yuan et al., 2023), and amylopectin chain length distribution (Guo et al., 2023b).
4.3 Mechanism of slow-mixed fertilizer base application combined with available N fertilizer synergistic improvement of grain yield, rice quality, and NUE Previous studies have shown the types and release rates of controlled-release N fertilizers as well as their effectiveness in increasing rice yield through one time basal application (Ke et al., 2018;Wu et al., 2021).These studies also investigated the physiological regulatory mechanisms under different soil types, varieties, and planting methods (Lyu et al., 2021b;Cheng et al., 2022;Meng et al., 2022;Ishfaq et al., 2023).However, there are few studies on the combined application of urea-N fertilizers under slow-mixed fertilizer base applications, especially in mechanical direct-seeding rice.This study aimed to investigate the synergistic effects of urea-N fertilizers on yield, rice quality, and NUE.However, the mechanism underlying this process has rarely been reported.The mechanism for the synergistic improvement of yield, rice quality, and NUE in direct-directing rice by slow-mixed fertilizer basal application with urea-N fertilizer (N 3 treatment) in this experiment is as follows: different varieties can improve the photosynthetic potential from the jointing to heading stage, optimize the LAI of the population, promote dry matter accumulation and transport during the grain-filling stage, improve the population growth rate from the heading to the maturity stage, and then optimize the yield components, which is an important reason for the final increase in yield.Applying urea-N fertilizer during the booting stage is optimal for slow-mixed fertilizer application.This application can increase the root vigor decay rate from heading to maturity and promote N transport from the stem sheath (leaves) to the panicle during the filling stage.This is the main reason for improving the NUE of urea-N topdressing and enhancing the rice processing and eating quality.It can be used as an important method to achieve high NUE and high-quality coordination in mechanical direct-seeding hybrid indica rice.
Selecting appropriate indicators to evaluate grain yield, rice quality, and NUE is crucial for evaluating the physiological ecology of high-yield, high-quality, and NUE crops (Zhang et al., 2013;Sun et al., 2017;Sun et al., 2023b).The use of principal components and correlation analysis is shown in Figures 4, 5.This study suggests that increasing the population's photosynthetic potential from the jointing to heading stage, promoting N transport in leaves from the heading to maturity stage, and increasing the N transport contribution rate in stem sheaths from the heading to maturity stage, can be used as an evaluation index for the simultaneous improvement of high yield, high quality, and NUE in direct-seeding rice.These findings provide another important way to achieve coordination and unification of high yield, high quality, and NUE in direct-seeding rice.

Conclusions
The effects of different varieties and slow-mixed fertilizer basal application with urea-N topdressing on the photosynthetic characteristics, dry matter accumulation, and transport, root vigor, NUE, yield, and rice quality of mechanical direct-seeded rice were significant.In this experiment, under the N application level of 150 kg hm -2 , the combination of slow-mixed fertilizer (N 120 kg hm -2 ) basal application and booting stage urea-N fertilizer (N 30 kg hm -2 ) topdressing significantly improved the photosynthetic potential of different varieties from the jointing to heading stage, the population growth rate and the N transport amount of leaves from the heading to maturity stage.It especially improved the N transport contribution rate in the stem sheaths, achieving the effect of increasing yield.Simultaneously, it improved the NUE of the N fertilizer topdressing and the processing and eating quality of rice synergistically.The best slow-mixed fertilizer basal application with urea-N fertilizer topdressing achieved a synergistic improvement in grain quality, yield, and NUE of direct-seeding rice.

FIGURE 1 N
FIGURE 1N cumulative release rate curve of controlled-release N fertilizer.
FIGURE 2Effects of slow-mixed fertilizer base application combined with urea topdressing time on LAI at the jointing stage (A, D), LAI at the heading stage (B, E) and photosynthetic potential (C, F) from the jointing to heading stage of mechanical direct-seeding hybrid indica rice.N 1 : slow-mixed N fertilizer (120 kg hm -2 ) as a base; N 2 : N 1 +urea-N (30 kg hm -2 ) one-time as a base; N 3 : N 1 +urea-N (30 kg hm -2 ) topdressing at the tillering stage (32 d after sowing); N 4 : N 1 +urea-N (30 kg hm -2 ) topdressing at the booting stage (93 d after sowing).Different lowercase letters indicate significant (P < 0.05) differences among N fertilizer treatments under the same variety.

FIGURE 4
FIGURE 4Principal component analysis of grain yield, rice quality and NUE with photosynthetic production, root vigor and N transport.The number of samples is 24 in the analysis of each indicator under every variety.GY, grain yield; TS, total spikelets; NAE, N agronomic efficiency of urea-N topdressing; NPE, N recovery efficiency of urea-N topdressing; HRR, head rice rate; TV, taste value; LAIJ, LAI at the jointing stage; LAIH, LAI at the heading stage; PP, photosynthetic potential from the jointing to heading stage; RTMS, root vigor at the maturity stage; DCRT, root vigor of decay rate from the heading to maturity stage; DTR, dry matter transport rate in stem sheaths from the heading to maturity stage; DCR, dry matter transport contribution rate in stem sheaths from the heading to maturity stage; PGR, population growth rate from heading to maturity stage; NSTA, N transport amount in stem sheaths from the heading to maturity stage; NSTR, N transport rate in stem sheaths from heading to maturity stage; NSCR, N transport contribution rate in stem sheaths from the heading to maturity stage; NLTA, N transport amount in leaves from the heading to maturity stage; NSTR, N transport rate in leaves from the heading to maturity stage; NSCR, N transport contribution rate in leaves from the heading to maturity stage.

TABLE 1
Average values for selected soil characteristics of composite topsoil samples in the field experiments.

TABLE 2
The comprehensive N fertilizer management mode (kg hm -2 ).

TABLE 5
Effects of slow-mixed fertilizer base application combined with urea topdressing time on N translocation in stem-sheaths and leaves from the heading to maturity stage and NUE of mechanical direct-seeding hybrid indica rice.

TABLE 6
Effects of slow-mixed fertilizer base application combined with urea topdressing time on rice quality of mechanical direct-seeding hybrid indica rice.