Nitrogen (N) is one of the most important nutrients for rice growth [1
]. Application of nitrogen fertilizer has been an effective way of improving rice yields [2
]. China is the world’s largest consumer of nitrogen fertilizer, accounting for 30% of the world’s total nitrogen consumption. In China, split application of conventional urea (CU) was still the most widely uesd method in rice production [3
]. In this method, CU is typically divided into three or four broadcast applications. Split CU applications require extra time and labor, which will unlikely meet the high-efficiency demands of modern rice production.
Controlled release nitrogen fertilizer (CRNF) was designed to have a release pattern that matched a crop’s pattern of N demand [4
]. The advantage of CRNF is that it reduces the number of fertilizer applications from the traditional multiple applications of nitrogen, which saves time and reduces labor [5
]. However, because there are several peaks of N demand throughout the growing season of rice, a single application of a CRNF that generally presents a pattern of N release resembling an “S” or “J” curve with only a single peak of nitrogen release would not be able to meet the N requirements of rice [6
]. Another disadvantage of CRNF is that CRNFs have been considered too expensive for use in rice production, especially in developing countries.
Recently, a better fertilizer management strategy has been developed using CRNF, which is the combined applications of CRNF and urea as a basal fertilizer. This strategy has made it possible to use CRNF in rice production [7
]. Several researchers found that the combined use of CRNF and CU as basal fertilizer could meet rice N demands [8
]. At the same time, the amount of CRNFs could be reduced to save the costs of purchased fertilizers [6
]. However, reported effects of combined use of CRNF and CU as basal fertilizer in increasing rice yield have varied. Some research has indicated that compared to CU alone, a mixture of CRNF and CU as basal fertilizer could enhance yield due to larger leaf area index [10
] and higher photosynthetic potential [11
] in rice. Moreover, the mix of CRNF and CU as basal fertilizer has reduced greenhouse gas emissions, including release of methane and nitrous oxide [12
]. However, some studies found that basal application of a combination of CRNF and CU had no advantage in rice production compared with split applications of CU [13
]. Hu et al. [15
] showed that lodging and delayed ripening may occur with application of CRNF and CU as a basal fertilizer in rice production. Discrepancies between results of various studies may result from that rice yield was strongly affected by the synchronicity of CRNF release and N requirement characteristics of rice.
Various types of rice are characterized by different N requirements throughout growth to obtain high grain yields. A large amount of nitrogen needs to be absorbed to promote more effective panicles and achieve production of more panicles in the early stage for early rice, [16
]. The rice was absorbed N evenly at each stage of N demand to achieve higher numbers of both panicles and spikelets for middle rice [17
]. Higher N uptake occurring in the middle and late growth stages can increase the dry matter accumulation after heading for late rice, which reportedly is beneficial to the production of grain yield [18
]. It can be seen that yields were greater when the release of CRNF is synchronized with the critical periods of nitrogen requirement of rice [19
]. Therefore, to effectively use CRNFs in rice production, it is important to develop a one-time application of a CRNF formula that can synchronize N fertilizer release rate with the pattern of N requirements according to the N uptake characteristics of different types of rice.
The Yangtze River Delta region is the largest rice-producing region and an important commercial grain base in China [20
]; the rice planting area and yield also rank at the top compared to other regions in China. Late japonica rice is one of the main types of rice grown in this region due to the greater potential yields. Late japonica rice has a longer growing period, usually more than 155 days. Its duration of N uptake from transplanting to the jointing stage was long, and it needs to absorb greater amounts of N in the middle and late periods of growth. In the Yangtze River Delta, a serious labor shortage has occurred recently with urbanization. Thus, for this area of rice production, finding a suitable formula of a one-time application of CRNF to meet N demands according to the critical periods of N uptake by late Japonica rice is more urgently needed in order to maximize agronomic benefits as well as to minimize time and labor costs. Therefore, in this study, a field experiment testing with various combinations of CRNF was conducted to determine the optimal formula appropriate for the late japonica rice grown in the Yangtze River Delta. The main objectives of this study were: (1) evaluate the effects of different combinations of CRNF and CNF on rice grain yield, N uptake and N-use efficiency; and (2) determine the optimum CRNF formula and provide empirical evidence that can realize the dual benefits of cost savings and rice yield increases to support the use of this fertilization strategy to grow late japonica rice in the Yangtze River Delta of China.
2. Materials and Methods
2.1. Experimental Location
Field experiments were conducted at Ningbo City (29°47′ N, 121°53′ E), Zhejiang Province, China, during the rice growing seasons of 2018 and 2019. The field soil was blue purple clay with the following properties: organic matter 37.57 g kg−1, total N 1.7 g kg−1, available P 6.51 mg kg−1, and available K 173.22 mg kg−1.
2.2. Plant Materials, Growth Conditions, and Treatments
Late japonica rice cultivars Jia67 and Jia58, the most popular rice cultivar grown in the Yangtze River Delta, were used as materials. Seedlings were sown on 26 May in 2018 and 27 May in 2019 with a seeding rate of 120 g of dry seeds per plastic plate. Seedlings were mechanically transplanted in hills on 15 June in both 2018 and 2019. Hill spacing was 11.7 cm × 30 cm with four seedlings per hill. The average times from transplanting to the jointing and heading stages for the two late japonica rice were 45 and 77 days, respectively.
Various combinations of CRNFs were tested in determining the best combination as a one-time application of fertilizer to replace the traditional fertilization methods that require more labor and time. There were two types of CRNFs with short release periods (40 and 60 days) and three types of CRNFs with long controlled-release periods (80, 100, and 120 days). One short CRNF was combined with one long CRNF to establish the six treatment combinations listed in Table 1
. Short and long CRNFs were mixed in a ratio of 1:4, and then they were mixed with a CU as a one-time application of basal fertilizer. Treatments were applied at a rate of 270 kg ha−1
N in a CRNF to CU ratio of 5:5. The control treatment (CK) consisted of a CU that was divided and applied at four different times: 35% as a basal application, 35% at 7 days after transplanting, 15% as a spikelet promotion fertilizer when the rice had four leaves that had not appeared, and 15% as a spikelet development fertilizer when the rice had two leaves that had not appeared. The amount of CU used as basal fertilizer was the same amount used in all CRNF treatments. All N fertilizer types and rates are listed in Table 1
. Other fertilizers consisting of 150 kg ha−1
P (superphosphate) and 150 kg ha−1
K (KCl) were also incorporated into soil prior to transplanting. Treatments were applied to 25 m2
(5 m × 5 m) plots with three replications for each treatment. Each plot was separated by a soil ridge (35 cm wide and 20 cm high) and covered with plastic film. The experimental field was flooded post-transplantation and remained flooded until 7 days before the maturity stage. Insect pests, pathogens, and weeds were controlled using common chemical treatments.
2.3. Sampling and Measurements
All rice plants in an 8 m2 area in the middle of each plot were hand-harvested at maturity, and the grain yield was weighed. The final grain yield was adjusted to 14% moisture content. Plants covering an area of 1 m2 (excluding the border rows) in each plot were collected to determine the number of panicles per square meter. All plants from 30 hills in each plot were collected to determine the number of spikelets per panicle, filled-grain percentage, and 1000-grain weight.
Leaf area index (LAI) and above-ground biomass accumulation were determined at the jointing, heading and maturity stages. Plants from five hills were sampled from each plot and all samples were separated into green leaf, stem (internode plus sheath) and panicle tissues. Green leaf area was measured with a leaf area meter (LI-3100, LI-COR, USA). Samples of each plant part were then dried separately and weighed to determine total aboveground biomass per unit area per plot. Each component of the rice plants was bagged and oven-dried separately at 105 °C for 30 min and then at 80 °C to a constant weight. Nitrogen concentrations were determined by semi-micro-Kjeldahl digestion [21
]. Accumulation of N in the plant was calculated by multiplying the N concentration (%) by the plant total biomass.
The amount of CU used as a basal fertilizer is the same in all CRNF treatments. The nitrogen cumulative release rate of each treatment was determined by the N release curve of CRNF. The actual N release rate of CRNF in the field plot was measured by using the buried mesh bag method [22
]. The fertilizer weighed (10 ± 0.01) g was placed into a nylon mesh bag (12 cm × 8 cm) with a hole diameter of 1.0 mm, strung onto a line and then buried in field. The bags were buried 5–8 cm below the soil surface. Three bags were sampled every ten days. CRNF was removed from each bag and rinsed with distilled water before being placed in a vacuum oven at 60 °C for 72 h. After drying, the particles were weighed to determine the weight of the remaining CRNF and then ground to pass through a 0.25 mm sieve to determine the residual N content by using the Kjeldahl digestion method [21
2.4. Equations and Data Analysis
The percentage of cumulative N release from the CRNF, photosynthetic potential, crop growth rate, N accumulation, and N-use efficiency were calculated using the following formulas:
Cumulative N release percentage (%) = (β1 × M1 − β2 × M2)/(β1 × M1) × 100%,
where β1 = weight of CRNF before burial, M1 = N content of CRNF before burial, β2 = weight of CRNF after burial, M2 = N content of CRNF after burial.
Decreasing rate of leaf area × 104 m2 d−1 = (L1 − L2)/(t2 − t1) and photosynthetic potential (×104 m2 d ha−1) =1/2(L1 + L2) × (t2 − t1),
where L1 and L2 are the first and second measurements of LAI, respectively, and t1 and t2 represent the first and second times (day) of measurements, respectively.
Crop growth rate (g m−2 d−1) = (W2 − W1)/(t2 − t1),
where W1 and W2 are the first and second measurements of above-ground biomass accumulation, respectively, and t1 and t2 represent the first and second times (day) of measurement, respectively.
N accumulation (kg ha−1) = above-ground biomass accumulation × N content.
N accumulation in the panicle after heading (kg ha−1) = nitrogen accumulation at maturity stage − nitrogen accumulation at heading stage.
Apparent recovery efficiency of N fertilizer (%) = [N accumulation in N-application plots − N accumulation in N-omission plots (kg)]/amount of applied N fertilizer (kg) × 100.
Internal N-use efficiency = grain yield (kg)/N accumulation in a plant (kg).
Agronomic N-use efficiency = [grain yield in N-application plots − grain yield in N-omission plots (kg)]/amount of applied N fertilizer (kg).
Physiological N-use efficiency = [grain yield in N-application plots − grain yield in N-omission plots (kg)]/[N accumulation in N-application plots − N accumulation in N-omission plots (kg)].
Data were analyzed by ANOVA with SPSS 13.0 for Windows. The statistical model included sources of variation due to year, rice cultivar, treatment and the interactions of year × rice cultivar, year × treatment, and year × rice cultivar × treatment. The means were compared by the least significant difference test at the 0.05 probability level.