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Article

Nitrogen-Reduction in Intensive Cultivation Improved Nitrogen Fertilizer Utilization Efficiency and Soil Nitrogen Mineralization of Double-Cropped Rice

by
Zhuo Luo
1,2,
Haixing Song
1,2,*,
Min Huang
3,
Zhenhua Zhang
1,2,
Zhi Peng
1,4,
Tao Zi
5,
Chang Tian
1,2,* and
Mamdouh A. Eissa
6
1
Hunan Provincial Key Laboratory of Farmland Pollution Control and Agricultural Resources Use, Hunan Provincial Key Laboratory of Plant Nutrition in Common University, College of Resources and Environment, Hunan Agricultural University, Changsha 410128, China
2
National Engineering Research Center for Efficient Utilization of Soil and Fertilizer Resources, Changsha 410128, China
3
State Key Laboratory of Hybrid Rice, Hunan Agricultural University, Changsha 410128, China
4
Yueyang Agricultural Sciences Institute, Yueyang 414000, China
5
Hunan Biological and Electromechanical Polytechnic, Changsha 410127, China
6
Department of Soils and Water, Faculty of Agriculture, Assiut University, Assiut 71526, Egypt
*
Authors to whom correspondence should be addressed.
Agronomy 2022, 12(5), 1103; https://doi.org/10.3390/agronomy12051103
Submission received: 28 March 2022 / Revised: 15 April 2022 / Accepted: 28 April 2022 / Published: 30 April 2022
(This article belongs to the Section Soil and Plant Nutrition)
Browse Figures
Versions Notes

Abstract

:
Under the current rice cropping system, excessive nitrogen application has become a major issue that needs to be changed, and nitrogen reduction has become a hot research topic in recent years. The use of optimum planting density is becoming a common agronomic management system in addition to nitrogen reduction, especially under double cropping rice systems. In this paper, changes in rice yield, nitrogen-use efficiency (NUE) and net N mineralization under dense planting with a reduced nitrogen rate (DPRN) were studied. By comparing DPRN with high-nitrogen sparse planting (SPHN), we found that the population tiller number (tiller number per unit area) increased by 9–27% under DPRN cultivation. Nitrogen accumulation under DPRN treatment of double-cropped rice was basically stable. NUE under DPRN was significantly higher by 1.3–22.7% compared to SPHN. The partial factor productivity of applied N (PFPN) was significantly higher than that of SPHN, with an increase of 4.3–22.8%. The net N mineralized of double-cropped rice under DPRN increased at different stages, and the increase in late-season rice (LSR) was greater than that of early-season rice (ESR). The highest net N mineralized in double cropping rice at different stages was found in the dense planting treatment (DP) and N2 (120 kg N h−1). In conclusion, DPRN cultivation of double-cropped rice could be accepted as a proper management strategy for reducing nitrogen input, improving NUE and promoting soil nitrogen mineralization under given conditions.

1. Introduction

As one of the essential staple cereal crops around the world, rice supplies over 21% of the calorific needs of the world’s population [1,2]. The area of rice cultivated in China exceeds 30 million hectares, comprising approximately 30% of the total area of cultivated crops, and the total yield was more than 200 million tons, comprising more than 40% of the total grain yield [3]. However, in recent years, China’s rice planting area has been shrinking, the aging of personnel involved in agricultural production has increased and the cost of agricultural labor has risen rapidly [4]. In order to improve the unit yield of rice and reduce the labor cost, it has become a common practice for farmers to increase the amount of fertilization and reduce the frequency of fertilization, even as a large amount of base fertilizer at one time [5]. Excessive application of nitrogen fertilizer may lead to a significant increase in the yield but will reduce the efficiency of NUE [6]. In addition, with the increasing amount of nitrogen applied, problems, such as overgrowth, late maturity, low seed setting rate, reduced resistance to diseases and insects and lodging resistance, have appeared, resulting in a significant decline in rice yield [7]. Rice yield is closely correlated with NUE [8]; thus, it is important to take the most proper effective practices to improve NUE and, consequently, ensure a high rice yield [9].
The trend of high nitrogen input and sparse planting of rice in rural China is becoming increasingly prominent [10,11]. Planting density and the rate of nitrogen fertilizer application are two vital factors that influence NUE and the production of rice [12,13]. Sparse planting of rice often requires substantial nitrogen fertilizer, because under the condition of sparse planting, yield stability is not determined by the population of rice but by the individual rice. Farmers often hope to use higher nitrogen fertilizer inputs to promote the individual advantages of rice in each stage to compensate for the shortage of basic seedlings [14,15]. Excessive application of nitrogen fertilizer not only reduces nitrogen-use efficiency but also causes environmental problems such as water eutrophication, air pollution and soil degradation, which has an adverse impact/with adverse effects on biodiversity and human health, and even poses a great challenge to the nitrogen cycle [16,17,18]. The severity of nitrogen overuse does not mean that a single nitrogen reduction method can be applied directly employed. Previous studies have shown that blindly reducing the application of N fertilizer will lead to the lack of nitrogen required for crop growth and, hence, a decrease in yield. In addition, long-term nitrogen deficiency will lead to the obvious degradation of soil fertility [19,20]. Therefore, it is very important to explore an appropriate nitrogen reduction method while balancing the environment and production requirements.
Under the cultivation measures of sparse planting and high nitrogen (SPHN), reducing the amount of nitrogen fertilizer is conducive to improving the utilization efficiency of nitrogen fertilizer, but it cannot effectively guarantee the crop yield. A single nitrogen reduction measure may reduce the crop yield, seed setting rate, quality and stress resistance [20]. On the basis of reducing nitrogen application, increasing reasonably close planting agronomic measures to ensure the stability of yield is considered to be an effective management measure. Agronomic measures by increasing planting density can help rice gain population dominance and reduce the adverse effects of density and nitrogen [15,21,22]. Most studies were limited to changes in rice yield and production composition under dense planting with a reduction in nitrogen rate (DPRN). However, there is a lack of research on the combination of changes in the agronomic characteristics of rice and soil nitrogen supply.
This study aimed to investigate the differences between DPRN and SPHN in the yield, NUE and the agronomic traits based on field and microplot experiments. Combined with the change law of soil nitrogen supply under DPRN, the effects of nitrogen accumulation change with double cropping rice under DPRN on these indexes were analyzed. Furthermore, considering that the inherent factors of double-cropped rice may influence the experimental results, a three-year field trial was conducted in Santang County to investigate the annual variation in yield and the agronomic and environmental impacts of rice seasons (i.e., early-season rice and late-season rice) and cultivars (i.e., hybrid and conventional rice). In 2020, micro-areas with different fertilization gradients were established to explore the soil nitrogen supply potential and response to rice nitrogen demand by measuring the changes in mineralized nitrogen in farmland soil with different fertilization rates. Considering that the longer growth cycle of late-season rice (LSR) compared with/relative to early-season rice (ESR) and the fertility demand of conventional rice relative to hybrid rice is small, it was expected that conventional LSR would be better adapted to the DPRN pattern and more effective at improving NUE and exploring soil nitrogen supply potential to compensate for the adverse effects of reducing nitrogen fertilizer, thus stabilizing or even increasing the yield. In addition, we expected that DPRN treatment would help to improve the population agronomic traits and NUE of double cropping rice, promote soil nitrogen mineralization in rice fields and, thus, achieve the stability of nitrogen accumulation and yield of aboveground populations. Therefore, we chose a typical double cropping rice planting area to carry out the experiment and verified our hypothesis by comparing the differences between DPRN and SPHN in order to explore the law of nitrogen accumulation and soil nitrogen mineralization of double cropping rice under DPRN.

2. Materials and Methods

2.1. Experimental Site

In 2017 to 2019, a field trial was established in Santang County, Hunan Province, China (112°29′ E; 26°52′ N), and a microplot test with different fertilization gradients was established in the same plot in 2020. Before the test set-up, 20 cm depth soil samples were collected from the cultivated layer, and the physicochemical properties were determined: total N using the Kjeldahl N method; available N using alkaline hydrolysis diffusion; available P using colorimetry following extraction with 0.5 mol L−1 NaHCO3 (pH = 8.5); available K by flame photometry following extraction with 1 mol L−1 CH3COONH4 (pH = 7.0); pH by the use of deionized water to remove CO2 (1:1 soil/water, w/v); soil organic matter was determined by the potassium dichromate external heating method. The soil texture was clay loam with total N of 1.69 g kg−1; available N of 122.38 mg kg−1; available P of 21.46 mg kg−1; available K of 211.82 mg kg−1; pH of 5.62; organic matter of 31.44 g kg−1. The growth period of ESR was from April to July and that of LSR was from July to November. The ESR cultivars were Zhuliangyou819 (ZLY819) and Zhongjiazao17 (ZJZ17), and the LSR cultivars were Taiyou390 (TY390) and Xiangwanxian13 (XWX13). The rice cultivars of this experiment were provided by the State Key Laboratory of Hybrid Rice, Hunan Agricultural University. Climatic data for daily air temperature (°C) and precipitation (mm) in each experimental site were obtained from a local meteorological bureau (Figure 1).

2.2. Experimental Design

A three-year field trial was conducted in a split-plot design with nitrogen fertilizer and planting density treatment as the main plot and cultivar as the secondary zone. Considering the nitrogen fertilizer and planting density, this experiment included DPRN and SPHN treatments with three replicated plots (30 m2 per plot). Based on the results of previous research and local habitual fertilization, we set two treatments: DPRN and SPHN, where the SPHN treatment was the habitual treatment, namely, the control group; the nitrogen application amount of the SPHN treatment came from the comprehensive consideration of local soil fertility levels. In the DPRN treatment, the nitrogen fertilizer for ESR and LSR was applied at 120 kg N ha−1, and the planting density was 364,000 plants ha−1 (spacing: 25 × 11 cm). Considering the SPHN treatment, the urea nitrogen fertilizer application was 150 kg N ha−1 at a density of 286,000 plants ha−1 (spacing: 25 × 14 cm). In the DPRN treatment, the application rate of base fertilizer (the day before transplanting), tiller fertilizer (7 days after transplanting), and panicle fertilizer (the day before heading) of the ESR and LSR were 60, 36 and 24 kg N ha−1, respectively. In the SPHN treatment, the amount of base fertilizer, tiller fertilizer and panicle fertilizer of the ESR and LSR were 75, 45 and 30 kg N ha−1, respectively. Base fertilizer was applied after paddy cultivation and before rice transplanting.
Phosphorus fertilizer (33 kg P h−1, fused calcium–magnesium phosphate) was applied as the base fertilizer. Potassium fertilizer (125 kg K h−1; KCl) was applied as the base fertilizer and spike fertilizer at a ratio of 1:1. In 2020, the blank control unfertilized (0N) area in the original test area was divided into 5 equal parts of microplots (12 m2 per plot), and the nitrogen application rate of each microplot area was as following, N0: 0; N1: 90; N2: 120; N3: 150; N4: 180 kg h−1. Each microplot was separated by ridge and covered with plastic film to separate water and fertilizer.

2.3. Sampling and Measurement

One row was fixed per treatment from 10 days after seedling transplantation to elongation stage, and its position was marked. The number of tillers was counted every 5 days. At the stages of jointing, heading and filling, 20 flag leaves (or the strongest leaves in the absence of flag leaves) were randomly selected from each plot, and the SPAD value of the leaves was measured using a chlorophyll meter (SPAD-502Plus, KONICA MINOLTA, INC., Tokyo, Japan). Five representative rice plants were taken from each plot at the stages of jointing, heading and filling, and all leaves were cut off to determine the leaf area by the coefficient method (leaf area = leaf length × leaf width × 0.75). The leaf area index (LAI) was then calculated.
The aboveground parts of five rice plants were taken from each plot during tillering, jointing, heading, grouting and maturity stages, and then washed and divided into stems, leaves and spikes. Samples were heated at 105 °C for 30 min to inactivate the enzymes and then dried to a constant weight in an oven at 65 °C and weighed. The nitrogen content was determined using a Kjeldahl nitrogen analyzer (VAP50), and the total nitrogen accumulation was calculated from dry matter accumulation. At the maturity stage of the ESR and LSR, a 5 m2 area was harvested from the center of plots to determine the yield (avoiding the sampling area), which was converted to a 14% moisture content yield. The samples were manually threshed, and solid and empty grains were separated by a wet cleaning method. Three sets of 30 g samples were randomly collected and counted from the solid grains. Solid and empty seeds were dried to a constant weight at 70 °C, and the total seed count, firmness rate, and 1000 seed weight per spike were calculated. NUE is equal to the difference between N absorption and N application divided on the amount of applied N. AEN was the difference between the yield of the N application area minus the yield of non-N application area divided by the amount of N application. PFPN was the yield of N application treatment divided by the amount of N application. NUE and differentiate between the different N-use efficiencies were calculated as previously described by Xie et al. [15]. Mixed soil samples were collected from five nitrogen-containing gradient microregions including jointing (JT), booting (BT), heading (HD), grain-filling period (GFP) and maturity (MA) by the five-point sampling method. The retrieved soil samples were air-dried and screened through 20 meshes. Five grams of soil samples were weighed. Then, the 5 g of air-dried soil sample were measured for the initial amount of NH4-N using an automated colorimeter (QuickChem 8000 FIA+, Lachat Instruments, Loveland, CO, USA). Soil mineralization N was determined as follows: 5 g of air-dried soil sample was weighed, 5 mL of distilled water was added, then was cultured anaerobically at 40 °C in an incubator for two weeks. After two weeks, NH4-N was extracted with 2 mol L−1 potassium chloride and then measured using an automated colorimeter [23].

2.4. Data Analysis

A summary was performed using Microsoft Excel 2016 (Redmond, Washington, DC, USA). Analysis of variance (ANOVA) was performed using the DPS software (v.9.01). The least significance (LSD) test at the 5% significance level was used to compare the mean of each variable and histograms using GraphPad Prism7 (San Diego, CA, USA).

3. Results

3.1. Rice Growth Status and Grain Yield

The number of tillers in the DPRN treatment was slightly higher than that in SPHN treatment in the three years of double cropping rice (Figure 2).
Under different fertilizer density treatments, the interannual change of ESR had little difference, while that of LSR had a large difference. There was a significant difference between DPRN and SPHN in tiller number at early tillering stage, but the difference gradually decreased at the late tillering stage. In terms of the growth and development time of the double cropping rice, in the first the amount of tillering increased then decreased and remained stable. The tiller of early rice in all three years peaked at approximately 20 days of transplanting, and late rice peaked at approximately 20–25 days. When tillering was completed, the differences among treatments were significantly increased except for 17 year double-cropping rice, and the tiller numbers of ESR and LSR under DPRN were 9–24% and 10–27% higher than those of SPHN, respectively. On the whole, except the jointing stage of LSR XWX13 in 2018, the value of chlorophyll SPAD of double-cropped rice under DPRN treatment was lower than that of SPHN (Figure 3). However, only some treatments in each year showed significant differences.
Except that, the leaf area index of ESR ZLY819 in the heading stage under DPRN in 2018 was markedly higher than that of SPHN, while there was no significant difference among other treatments (Figure 4).
In terms of years, the leaf area index of different rice seasons and varieties of double cropping rice under DPRN treatment had no significant change compared with SPHN. The dry matter accumulation of double cropping rice treated with DPRN was higher than that treated with SPHN (Figure 5). As far as ESR is concerned, there were significant differences among treatments at the tillering and heading stages of ZLY819 in 2018; the tillering and heading stages of ZJZ17 in 2018; and the tillering stage, jointing stage and heading stage of ZJZ17 in 2019. From the perspective of LSR, the difference between the treatments of the tillering stage of TY390 in 2017 and the heading stage, filling stage and maturity stage of XWX13 in 2019 reached a significant level. Dry matter accumulation at maturity period under DPRN was higher than SPHN, but the only the difference between treatments of LSR XWX13 in 2019 reached a significant level. Compared with the SPHN, the dry matter accumulation of DPRN increased by 6.4–23.4%, and the dry matter accumulation of late rice was higher than that of early rice.
The effective panicles of double cropping rice under different treatments, except for ESR in 2017, was higher in DPRN than SPHN, and the difference between LSR TY390 in 2017 and ESR and LSR in 2019 was significant (Table 1).
The number of grains per panicle among different treatments was not significantly varied, except for the DPRN treatment of ESR ZJZ17 in 2019 which tended to be lower than the SPHN treatment. There was no difference in the seed setting rate among different treatments, but from the numerical point of view, the average seed setting rate of ESR in three years (88.1%) was higher than that of LSR in three years (78.2%). There was no significant difference in the seed setting rate among different rice seasons and varieties (Table 1). At the same time, the difference was not significant in 1000 grain weight among different treatments and different rice seasons, and the yield of conventional varieties was higher than the hybrid varieties. The difference among different treatments was not significant, but the three-year average yield of LSR XWX13 was 5715 kg ha−1, lower than the average 7192 kg ha−1, with a decrease of 20.54%. Variation in yield increments as affected by different rice seasons and varieties was not obvious.

3.2. Nitrogen Accumulation in Plant and NUE

Under DPRN treatment, the nitrogen accumulation of 2017 ESR ZLY819 at maturity and 2017 ESR ZJZ17 at the jointing stage was significantly lower than that of SPHN (Figure 6).
In terms of ESR, under DPRN treatment, the nitrogen accumulation at the jointing stage of ZLY819 in 2018 and the tillering stage of ZJZ17 in 2019 was significantly higher than that of SPHN. In terms of LSR, under DPRN treatment, the nitrogen accumulation at the tillering stage of TY390 in 2017; the jointing stage and the maturity stage of TY390 in 2018; the tillering stage, heading stage and maturity stage of XWX13 in 2018 was apparently higher than that of SPHN. At the maturity stage, the nitrogen accumulation of DPRN treatment, in general, was not significantly different than that of SPHN, and the difference among different rice seasons and varieties was not significant.
NUE and PFPN showed an increasing trend (Table 2). The NUE of DPRN was higher than SPHN; however, only significant differences existed between LSR treatments in 2017 and 2018; the difference between different rice seasons and varieties was not significant.
The PFPN of DPRN was higher than SPHN. Except for ESR ZLY819 in 2017 and ESR ZLY819 and ZJZ17 in 2018, the differences among other treatments were significant; the difference between different rice seasons and cultivated varieties did not reach a significant level. There was no obvious rule for the change of AEN under different treatments, and there were no significant differences among the other treatments except for the ESR ZJZ17 in 2017 and the LSR XWX13 in 2019. There was no significant change in the N harvest index under different treatments.

3.3. Variation Law of Net Mineralized Nitrogen in Farmland Soil

The variation in soil net mineralized nitrogen under different treatments fluctuated with different periods and reached the peak at the rice maturity stage. In the BT stage of ESR, the difference between treatments reached a significant level (Figure 7). The amount of soil net mineralized nitrogen under different treatments of LSR increased with different stages and reached the highest value at rice ripening period. The amount of net mineralized nitrogen under late rice DPRN treatment was higher than that under SPHN, and the difference was significant in the BT, HD, GFP and MA stages (Figure 7).
Under different nitrogen application rates, most of the ESR and LSR treatments increased first and then decreased (Table 3). In terms of the nitrogen application rate, the difference between BT, HD and GFP treatments of LSR was significant under N0 nitrogen application rate, but the change rule among treatments was not obvious on the whole. Under the N1 nitrogen application rate, the difference between the GFP of early rice and JT, BT, HD, GFP and MA of late rice reached a significant level, indicating that the DP’s net mineralized nitrogen content was higher than SP’s. Under the N2 application rate, net mineralized N in the JT, BT and HD stages of early rice and DP treatment was apparently higher than that in SP treatment (Table 3). Under the N3 application rate, the net mineralized nitrogen content of JT, BT and HD of early rice and DP of JT, BT, GFP and MA of late rice was obviously higher than that of SP. Under the N4 application rate, the net N mineralization of BT, HD of early rice and MA of late rice under the DP treatment was evidently higher than that of the SP treatment. In terms of stages, the net mineralized nitrogen in the JT, BT and HD stages of ESR and the BT, HD, GFP and MA stages of LSR under the DP treatment reached the maximum under the N2 application rate. The net mineralized nitrogen of early rice at the GFP stage reached the maximum under the N4 application rate. The net mineralized nitrogen of ESR at the MA and LSR at the JT stage reached the maximum under the N3 application rate.

4. Discussion

4.1. Stable Yield Mechanism for Double-Cropped Rice under DPRN

From the tiller data over three years, the tiller number of double cropping rice under DPRN treatment was higher than that of SPHN in general, and the tiller number increased in the early stage, reaching the maximum value at 20–25 days and then gradually decreasing, which is also in harmony with previous studies [24]. After the peak tiller period, double cropping rice will gradually reduce ineffective tillers to ensure the growth and development of effective tillers [25,26]. At the completion of tillering, the number of tillers of double cropping rice under DPRN was 9–27% higher than that of SPHN. The previous studies showed that the tillering of the rice population was regulated by density and nitrogen application [12]. A reduction in the nitrogen application rate will reduce the survival rate of tillers, but the increase in the rice planting density will increase the number of tillers, and the increase in the tiller base will make up for the adverse effect of the reduction in the survival rate, which also explains the reason why DPRN treatment maintained a high number of tillers [12,27]. The chlorophyll SPAD value of double cropping rice under DPRN treatment was lower than that of the SPHN treatment, but there was no distinct change between the two years. Relevant studies show that there is an obvious correlation between the content of chlorophyll and the amount of nitrogen application in rice [28]. The decrease in nitrogen application often means a decrease in chlorophyll [29]. In our experiment, compared with the control treatment, the chlorophyll SPAD value of individual rice plants decreased after nitrogen reduction by 20% in the DPRN treatment, and the difference was significant in individual periods, but on the whole, the impact on the overall yield formation was limited. In addition, DPRN treatment increased the planting density, while reducing nitrogen, which alleviated this adverse effect. Relevant research conclusions show that appropriate nitrogen reduction measures will not affect the yield formation [4,30].
Except for the early rice in 2017, the leaf area index (LAI) of double-harvest rice showed an obvious upward trend in the jointing–heading stages and a downward trend in the heading–filling stages. The reason for this may be that the transport of nutrients was more obvious in the later stage of rice growth, while the absorption of nutrients by double cropping rice was depressed, and a large number of nutrients in leaves were transported to grains during the grain-filling stage; as a result, the leaf area index showed an obvious downward trend [31]. Under DPRN treatment, there was no obvious difference in LAI between double cropping rice and SPHN treatment. From different rice seasons, there was no distinct difference between early and late rice. From the perspective of varieties, hybrid rice performs better. Earlier research shows that there was an obvious correlation between nitrogen reduction and rice leaf area index [32]. The reduction in nitrogen application will affect the healthy growth of rice so as to reduce the number of leaves and leaf area of rice individuals; therefore, a single nitrogen reduction measure is not conducive to the stable yield of rice [33]. However, reasonably close planting of rice can offset this adverse effect. Close planting leads to intensified competition between individuals in light and growth space, but the increase in density also improves the utilization of weak light and growth space by rice population so as to improve the utilization efficiency of light, space and nitrogen fertilizer, obtaining population advantages, offsetting the adverse effects of N reduction and close planting and stabilizing the yield [1,34,35]. Double-harvest rice’s dry matter accumulation in each period increased rapidly in the early stage but slowly in the middle and late stage. From different treatments, the dry matter quality of early and late rice under DPRN was better than that of SPHN, but the difference was obvious in some stages. In terms of maturity, the difference between treatments at mature XWX13 reached a significant level only in 2019, indicating that DPRN treatment can only maintain the dry matter accumulation of the rice population. Previous investigations showed that the dry matter accumulation of double cropping rice is the cornerstone of double cropping rice yield formation, and higher dry matter accumulation often means high yield [36]. Therefore, the DPRN treatment in the present experiment did not obtain the advantage of high yield compared with SPHN (the control group), from the yield data of different treatments, which also verifies this conclusion. There was no significant difference in dry matter accumulation among the different rice seasons. However, the dry matter accumulation of hybrid rice was higher than those of conventional rice, which depends more on the advantage of hybrid rice, and the individual advantage is more reflected after close planting [37]. Under DPRN, the reduction in the nitrogen application rate had an impact on the nitrogen accumulation of double cropping rice, but on the whole, the dry matter accumulation of double cropping rice under the two treatments did not reflect any significant changes in nitrogen. DPRN could maintain the population nitrogen accumulation of double cropping rice. Double cropping rice is suitable for different rice seasons and varieties. From previous studies, the nitrogen accumulation of double cropping rice was positively correlated with the amount of applied nitrogen [38]. A reduction in N application often means a reduction in nitrogen accumulation, but reasonable agronomic measures, such as dense planting, enhanced light and fractional fertilization, will promote the nitrogen accumulation of double cropping rice [39]. In addition, the nitrogen remaining in the soil after years of cultivation and the nitrogen supplement way of the soil itself, for example, soil nitrogen mineralization, will affect the population nitrogen accumulation of double cropping rice, which also explains the reason why the nitrogen accumulation of double cropping rice does not decrease under DPRN [40,41].

4.2. Main Mechanism of NUE Enhancement in Double-Cropped Rice under DPRN

NUE under DPRN treatment was higher than SPHN, and the difference between treatments of LSR was greater than that of ESR, indicating that the NUE of LSR increased more significantly under the DPRN cultivation mode, which was also in line with our preset; that is, late rice was more suitable for DPRN cultivation mode. An appropriate DPRN cultivation mode is conducive to rice to obtain population advantage so as to avoid the risk caused by the change in nitrogen application [35]. The PFPN of DPRN treatment was apparently higher than that of SPHN treatment, indicating that the PFPN of double cropping rice was significantly improved when the fertilization was reduced by 20%. The reduction in fertilizer did not mean a reduction in double cropping rice yield but improved fertilizer utilization efficiency while ensuring the stability of the yield. The data over three years proved the possibility of DPRN cultivation at the experimental site. From previous studies, a reduction in nitrogen application in double cropping rice often meant that the nitrogen-use efficiency was improved. However, it is not advisable to blindly reduce nitrogen fertilizer application and ignore the yield formation of double cropping rice [4,35]. DPRN cultivation not only improves NUE and PFPN but also ensures the stability of yield. There was no significant change in N harvest index among different treatments, indicating that there was no significant change in the population nitrogen accumulation of double cropping rice under DPRN cultivation. In conclusion, the improvement of nitrogen-use efficiency of double cropping rice is an important factor to maintain the nutritional growth demand of double cropping rice population under DPRN.

4.3. Changes in the Net Mineralized Nitrogen in Double Cropping Paddy Soil under DPRN

Under DPRN treatment, the change in soil net mineralized nitrogen of ESR showed a fluctuating trend while that of LSR showed an increasing trend. Although both ESR and LSR reached the maximum at maturity, the law was obviously different. Compared with LSR, in the growth cycle of ESR, due to the small fertilizer demand of ESR, when the fertilizer demand meets the nitrogen demand of ESR in a certain period, the demand for other nitrogen sources will be reduced, which also means that the nitrogen supply potential of soil itself cannot be fully released. When the demand for nitrogen in a certain period of ESR exceeds the supply of fertilizer, the demand for other nitrogen sources will increase greatly, and soil mineralized nitrogen will fill this gap [41,42]. Because the fertilizer demand of LSR is greater than that of ESR, and the soil temperature in the growth period of LSR is higher, the net mineralized nitrogen in each period of LSR soil shows a gradual increasing trend, and the difference between treatments from BT to MA is significant. The results of soil net mineralized nitrogen under different fertilization and density treatments showed that the two density cultivation methods (i.e., DP and SP) achieved the maximum net mineralized nitrogen in N2 (120 kg N ha−1) and N3 (150 kg N ha−1), and the net mineralized nitrogen did not increase with the increase in nitrogen application. Previously, many researchers showed that the soil net mineralized nitrogen increased with the increase in the N application rate, and when the N application rate reached a certain extent, it would inhibit the soil nitrogen mineralization rate [43,44,45,46,47]. In the present experiment, we set-up two treatments of DPRN and SPHN by setting cross-treatments with different nitrogen application rates and different densities; it was verified that under the treatment of DP density, double cropping rice mostly achieved the maximum net mineralized nitrogen under the N2 and N3 application rates in different periods. This shows that after DPRN, the nitrogen supply potential of soil is released, which will promote soil nitrogen mineralization, ensure the nitrogen supply of double cropping rice population and maintain the stability of the yield.

5. Conclusions

In this research, DPRN cultivation can stabilize the yield, improve the nitrogen-use efficiency and increase the net N mineralized. Under DPRN cultivation conditions, the yield of hybrid rice and conventional rice in different rice seasons could keep stable when nitrogen application was reduced by 20% (from 150 to 120 kg N ha−1). Compared with SPHN, the tillering number, dry matter accumulation and effective panicle number of the population under DPRN increased, and nitrogen accumulation of the population remained stable at the maturity stage. Both NUE and PFPN increased, and the effect of nitrogen partial productivity was more obvious. Under DPRN, the amount of net mineralized N in soil of double cropping rice fields reached the maximum, which provided a sufficient nitrogen supply basis for double cropping rice. DPRN cultivation can maintain the stability of double cropping rice yield and can be used for field management in the southern region of Hunan Province (Santang County, Hengyang, China) where the experimental site was located for a long time. The research conclusion of this study has a long time span, and an in-depth study on soil nitrogen mineralization can provide theoretical support for DPRN cultivation of double cropping rice and even more crops. However, this study has not been set-up in different regions. Therefore, the researchers expect to carry out extensive DPRN cultivation research in more regions in order to strengthen the wide adaptability of DPRN cultivation technology.

Author Contributions

Z.L., H.S., M.H. and C.T., conceptualized the project, investigated, collected and analyzed the original draft of the data; H.S. and C.T., project administration and supervision; Z.L., graph editing; Z.L., H.S., M.A.E. and C.T., review and editing; Z.Z., Z.P. and T.Z., field and lab help. All authors have read and agreed to the published version of the manuscript.

Funding

This research was supported by a project supported by the National Key Research and Development Program of China (2017YFD0301503).

Data Availability Statement

All data generated or analyzed during this study are included in this published article.

Acknowledgments

We thank Wei Cao, Ming Zhang and Yu Tan for help with conducting the field experiments and related measurements. We are grateful to Yinhang Xia and Shifu He for the timely help with data analysis.

Conflicts of Interest

The authors declare no conflict of interest.

References

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Figure 1. Daily temperature and precipitation of the entire rice growing season in Santang from March to November.
Figure 1. Daily temperature and precipitation of the entire rice growing season in Santang from March to November.
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Figure 2. Tiller dynamic of double-cropped rice under different treatments from 2017 to 2019. DPRN represents dense planting with a reduced nitrogen rate; SPHN represents sparse planting with a high nitrogen rate; (a) represents early–season rice in 2017; (b) represents late–season rice in 2017; (c) represents early−season rice in 2018; (d) represents late−season rice in 2018; (e) represents early−season rice in 2019; (f) represents late−season rice in 2019.
Figure 2. Tiller dynamic of double-cropped rice under different treatments from 2017 to 2019. DPRN represents dense planting with a reduced nitrogen rate; SPHN represents sparse planting with a high nitrogen rate; (a) represents early–season rice in 2017; (b) represents late–season rice in 2017; (c) represents early−season rice in 2018; (d) represents late−season rice in 2018; (e) represents early−season rice in 2019; (f) represents late−season rice in 2019.
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Figure 3. Chlorophyll content (SPAD) of double-cropped rice under different treatments from 2017 to 2019. Data points represent treatment means (n = 3), and different letters indicate significant differences (* p < 0.05). DPRN represents dense planting with a reduced nitrogen rate; SPHN represents sparse planting with a high nitrogen rate. ZLY 819 (hybrid rice) and ZJZ17 (conventional rice) belong to early-season rice cultivars; TY 390 (hybrid rice) and XWX 13 (conventional rice) belong to late-season rice cultivars; (a) represents early−season rice in 2017; (b) represents late−season rice in 2017; (c) represents early−season rice in 2018; (d) represents late−season rice in 2018; (e) represents early−season rice in 2019; (f) represents late−season rice in 2019.
Figure 3. Chlorophyll content (SPAD) of double-cropped rice under different treatments from 2017 to 2019. Data points represent treatment means (n = 3), and different letters indicate significant differences (* p < 0.05). DPRN represents dense planting with a reduced nitrogen rate; SPHN represents sparse planting with a high nitrogen rate. ZLY 819 (hybrid rice) and ZJZ17 (conventional rice) belong to early-season rice cultivars; TY 390 (hybrid rice) and XWX 13 (conventional rice) belong to late-season rice cultivars; (a) represents early−season rice in 2017; (b) represents late−season rice in 2017; (c) represents early−season rice in 2018; (d) represents late−season rice in 2018; (e) represents early−season rice in 2019; (f) represents late−season rice in 2019.
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Figure 4. Leaf area index (LAI) of double-cropped rice under different treatments from 2017 to 2019. Data points represent treatment means (n = 3), and different letters indicate significant differences (p < 0.05). DPRN represents dense planting with a reduced nitrogen rate; SPHN represents sparse planting with a high nitrogen rate. ZLY 819 (hybrid rice) and ZJZ17 (conventional rice) belong to early-season rice cultivars; TY 390 (hybrid rice) and XWX 13 (conventional rice) belongs to late-season rice cultivars. JT represents the jointing stage; HD represents the heading stage; GFP represents the grain-filling period; (a) represents early−season rice in 2017; (b) represents late−season rice in 2017; (c) represents early−season rice in 2018; (d) represents late−season rice in 2018; (e) represents early−season rice in 2019; (f) represents late−season rice in 2019.
Figure 4. Leaf area index (LAI) of double-cropped rice under different treatments from 2017 to 2019. Data points represent treatment means (n = 3), and different letters indicate significant differences (p < 0.05). DPRN represents dense planting with a reduced nitrogen rate; SPHN represents sparse planting with a high nitrogen rate. ZLY 819 (hybrid rice) and ZJZ17 (conventional rice) belong to early-season rice cultivars; TY 390 (hybrid rice) and XWX 13 (conventional rice) belongs to late-season rice cultivars. JT represents the jointing stage; HD represents the heading stage; GFP represents the grain-filling period; (a) represents early−season rice in 2017; (b) represents late−season rice in 2017; (c) represents early−season rice in 2018; (d) represents late−season rice in 2018; (e) represents early−season rice in 2019; (f) represents late−season rice in 2019.
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Figure 5. Dry matter weight of double-cropped rice under different treatments from 2017 to 2019. Data points represent treatment means (n = 3), and different letters indicate significant differences (* p < 0.05). DPRN represents dense planting with a reduced nitrogen rate; SPHN represents sparse planting with a high nitrogen rate. ZLY 819 (hybrid rice) and ZJZ17 (conventional rice) belong to early-season rice cultivars; TY 390 (hybrid rice) and XWX 13 (conventional rice) belong to late-season rice cultivars. TR represents the tillering stage; JT represents the jointing stage; HD represents the heading stage; GFP represents the grain-filling period; MA represents the maturity stage; (a) represents early−season rice in 2017; (b) represents late−season rice in 2017; (c) represents early−season rice in 2018; (d) represents late−season rice in 2018; (e) represents early−season rice in 2019; (f) represents late−season rice in 2019.
Figure 5. Dry matter weight of double-cropped rice under different treatments from 2017 to 2019. Data points represent treatment means (n = 3), and different letters indicate significant differences (* p < 0.05). DPRN represents dense planting with a reduced nitrogen rate; SPHN represents sparse planting with a high nitrogen rate. ZLY 819 (hybrid rice) and ZJZ17 (conventional rice) belong to early-season rice cultivars; TY 390 (hybrid rice) and XWX 13 (conventional rice) belong to late-season rice cultivars. TR represents the tillering stage; JT represents the jointing stage; HD represents the heading stage; GFP represents the grain-filling period; MA represents the maturity stage; (a) represents early−season rice in 2017; (b) represents late−season rice in 2017; (c) represents early−season rice in 2018; (d) represents late−season rice in 2018; (e) represents early−season rice in 2019; (f) represents late−season rice in 2019.
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Figure 6. Nitrogen accumulation of double-cropped rice under different treatments from 2017 to 2019. Data points represent treatment means (n = 3), and different letters indicate significant differences (* p < 0.05). DPRN represents dense planting with a reduced nitrogen rate; SPHN represents sparse planting with a high nitrogen rate. ZLY 819 (hybrid rice) and ZJZ17 (conventional rice) belong to early-season rice cultivars; TY 390 (hybrid rice) and XWX 13 (conventional rice) belong to late-season rice cultivars. TR represents the tillering stage; JT represents the jointing stage; HD represents the heading stage; MA represents the maturity stage; (a) represents early−season rice in 2017; (b) represents late−season rice in 2017; (c) represents early−season rice in 2018; (d) represents late−season rice in 2018; (e) represents early−season rice in 2019; (f) represents late−season rice in 2019.
Figure 6. Nitrogen accumulation of double-cropped rice under different treatments from 2017 to 2019. Data points represent treatment means (n = 3), and different letters indicate significant differences (* p < 0.05). DPRN represents dense planting with a reduced nitrogen rate; SPHN represents sparse planting with a high nitrogen rate. ZLY 819 (hybrid rice) and ZJZ17 (conventional rice) belong to early-season rice cultivars; TY 390 (hybrid rice) and XWX 13 (conventional rice) belong to late-season rice cultivars. TR represents the tillering stage; JT represents the jointing stage; HD represents the heading stage; MA represents the maturity stage; (a) represents early−season rice in 2017; (b) represents late−season rice in 2017; (c) represents early−season rice in 2018; (d) represents late−season rice in 2018; (e) represents early−season rice in 2019; (f) represents late−season rice in 2019.
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Figure 7. Net soil mineralized nitrogen cultured for two weeks at different stages of double cropping rice under different treatments. Data points represent treatment means (n = 3), and different letters indicate significant differences (* p < 0.05). DPRN represents dense planting with a reduced nitrogen rate; SPHN represents sparse planting with a high nitrogen rate; JT represents the jointing stage; BT represent the booting stage; HD represents the heading stage; GFP represents the grain-filling period; MA represents the maturity stage.
Figure 7. Net soil mineralized nitrogen cultured for two weeks at different stages of double cropping rice under different treatments. Data points represent treatment means (n = 3), and different letters indicate significant differences (* p < 0.05). DPRN represents dense planting with a reduced nitrogen rate; SPHN represents sparse planting with a high nitrogen rate; JT represents the jointing stage; BT represent the booting stage; HD represents the heading stage; GFP represents the grain-filling period; MA represents the maturity stage.
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Table
Table 1. The effective spike, grains per spike, seed setting rate (%) 1000 grain weight, and the grain yield of double-cropped rice under different treatments from 2017 to 2019.
Table 1. The effective spike, grains per spike, seed setting rate (%) 1000 grain weight, and the grain yield of double-cropped rice under different treatments from 2017 to 2019.
YearsSeasonCultivarTreatmentEffective
Spike m−2		Grains Per Spike −1Seed Setting
Rate (%)		1000 Grain Weight (g)Grain Yield
(kg ha–1)
2017ESRZLY819DPRN258.2 a88.0 a87.1 a24.9 a7230 a
SPHN309.5 a91.2 a86.5 a25.3 a7370 a
ZJZ17DPRN244.8 a119.1 a88.6 a24.8 a7530 a
SPHN251.4 a127.6 a90.4 a25.9 a7720 a
LSRTY390DPRN326.0 a144.2 a79.9 a26.3 a9010 a
SPHN238.1 b150.6 a79.2 a26.3 a9540 a
XWX13DPRN270.3 a114.3 a80.7 a32.5 a8770 a
SPHN223.8 a116.0 a75.6 a33.6 a8770 a
2018ESRZLY819DPRN366.1 a74.4 a87.5 a29.3 a5800 a
SPHN352.4 a84.4 a90.9 a29.4 a6600 a
ZJZ17DPRN322.4 a95.3 a84.3 a29.0 a5500 a
SPHN321.9 a97.2 a87.0 a28.6 a5920 a
LSRTY390DPRN431.5 a120.1 a81.9 a24.6 a8070 a
SPHN417.1 a119.8 a81.5 a24.4 a8170 a
XWX13DPRN394.7 a96.7 a81.1 a29.1 a6430 a
SPHN392.3 a85.1 a79.2 a29.2 a6270 a
2019ESRZLY819DPRN405.3 a128.0 a90.1 a24.1 a6900 a
SPHN316.5 b129.0 a91.7 a23.7 a7170 a
ZJZ17DPRN390.7 a133.6 b88.4 a23.5 a6070 a
SPHN291.7 b160.0 a85.7 a23.5 a6570 a
LSRTY390DPRN503.5 a151.8 a76.1 a22.3 a7770 a
SPHN432.8 b162.1 a71.6 a21.6 a8000 a
XWX13DPRN491.4 a142.1 a73.7 a25.3 a5600 a
SPHN397.5 b118.2 a77.6 a25.0 a5830 a
Data points represent treatment means (n = 3), and different letters indicate significant differences (p < 0.05). ESR represents early-season rice; LSR represents late-season rice; DPRN represents dense planting with a reduced nitrogen rate; SPHN represents sparse planting with a high nitrogen rate. Black bold represents a significant difference.
Table
Table 2. Nitrogen-use efficiency of double-cropped rice under different treatments from 2017 to 2019.
Table 2. Nitrogen-use efficiency of double-cropped rice under different treatments from 2017 to 2019.
YearsSeasonCultivarTreatmentNUE
(%)		PFPN
(kg kg−1)		AEN
(kg kg−1)		N Index of Harvest
2017ESRZLY819DPRN32.7 a59.6 a17.1 a0.83 a
SPHN23.9 a54.1 a9.7 a0.79 a
ZJZ17DPRN36.2 a62.1 a16.0 a0.83 a
SPHN20.1 a50.9 b7.4 b0.82 a
LSRTY390DPRN52.5 a75.1 a10.1 a0.65 a
SPHN29.8 b63.6 b14.9 a0.62 a
XWX13DPRN50.9 a73.1 a17.7 a0.57 a
SPHN28.3 b58.5 b14.9 a0.57 a
2018ESRZLY819DPRN30.8 a48.3 a21.8 a0.69 a
SPHN29.5 a44.0 a24.1 a0.69 a
ZJZ17DPRN37.3 a45.8 a23.8 a0.69 a
SPHN36.4 a39.5 a23.9 a0.72 a
LSRTY390DPRN53.7 a67.2 a12.5 a0.66 a
SPHN37.1 b54.4 b10.8 a0.67 a
XWX13DPRN43.7 a53.6 a9.1 a0.59 a
SPHN37.2 b41.7 b6.6 a0.59 a
2019ESRZLY819DPRN34.2 a71.7 a26.9 a0.74 a
SPHN31.0 a48.9 b23.3 a0.73 a
ZJZ17DPRN34.4 a67.8 a20.6 a0.74 a
SPHN32.9 a57.9 b20.7 a0.73 a
LSRTY390DPRN32.6 a68.6 a20.0 a0.75 a
SPHN31.0 a53.3 b18.0 a0.73 a
XWX13DPRN34.4 a57.4 a12.7 a0.66 a
SPHN32.9 a38.9 b8.9 b0.62 a
Data points represent treatment means (n = 3), and different letters indicate significant differences (p < 0.05). ESR represents early-season rice; LSR represents late-season rice; DPRN represents dense planting with a reduced nitrogen rate; SPHN represents sparse planting with a high nitrogen rate; NUE represents nitrogen-use efficiency for grain production; PFPN represents the partial factor productivity of applied nitrogen fertilizer; AEN represents the response agronomic efficiency of applied nitrogen. Black bold represents a significant difference.
Table
Table 3. Net soil mineralized nitrogen cultured for two weeks at different stages of double cropping rice under different fertilization rates (mg N kg–1 dry soil).
Table 3. Net soil mineralized nitrogen cultured for two weeks at different stages of double cropping rice under different fertilization rates (mg N kg–1 dry soil).
SeasonPeriodTreatmentN0N1N2N3N4
ESRJTDP68.8 a88.5 a114.5 a113.5 a89.5 a
SP63.0 a88.9 a107.4 b106.5 b91.8 a
BTDP117.6 a119.0 a152.7 a130.9 a125.3 a
SP101.2 a110.7 a128.8 b119.4 b116.6 b
HDDP71.3 a86.9 a97.1 a64.8 a80.1 a
SP74.8 a88.3 a46.7 b58.1 b57.3 b
GFPDP44.4 a53.5 a41.1 b70.8 a72.7 a
SP49.9 a36.7 b59.1 a70.7 a73.5 a
MADP71.1 a77.5 a85.2 b96.0 a75.1 a
SP72.7 a81.1 a87.3 a87.9 a73.9 a
LSRJTDP62.8 a58.2 b108.6 a160.8 a99.7 a
SP63.2 a115.6 a62.7 b101.3 b88.8 a
BTDP54.3 a56.1 a143.0 a105.4 a88.5 a
SP39.7 b45.1 b67.6 b79.6 b79.1 a
HDDP66.9 a64.9 a130.9 a124.1 a102.8 a
SP38.6 b51.7 b99.8 b129.7 a109.1 a
GFPDP41.1 b95.0 b153.0 a142.7 a150.0 a
SP87.6 a115.1 a108.9 b93.2 b153.0 a
MADP57.4 a110.2 a166.9 a118.2 a150.0 a
SP51.1 a68.8 b91.6 b98.6 b82.3 b
Data points represent treatment means (n = 3), and different letters indicate significant differences (p < 0.05). ESR represents early-season rice; LSR represents late-season rice; JT represents the jointing stage; BT represent the booting stage; HD represents the heading stage; GFP represents the grain-filling period; MA represents the maturity stage; DP represents dense planting; SP represents sparse planting. N0 represents 0 kg hm−2 nitrogen fertilizer; N1 represents 90 kg hm−2 nitrogen fertilizer; N2 represents 120 kg hm−2 nitrogen fertilizer; N3 represents 150 kg hm−2 nitrogen fertilizer; N4 represents 180 kg hm−2 nitrogen fertilizer. Black bold represents a significant difference.
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Luo, Z.; Song, H.; Huang, M.; Zhang, Z.; Peng, Z.; Zi, T.; Tian, C.; Eissa, M.A. Nitrogen-Reduction in Intensive Cultivation Improved Nitrogen Fertilizer Utilization Efficiency and Soil Nitrogen Mineralization of Double-Cropped Rice. Agronomy 2022, 12, 1103. https://doi.org/10.3390/agronomy12051103

AMA Style

Luo Z, Song H, Huang M, Zhang Z, Peng Z, Zi T, Tian C, Eissa MA. Nitrogen-Reduction in Intensive Cultivation Improved Nitrogen Fertilizer Utilization Efficiency and Soil Nitrogen Mineralization of Double-Cropped Rice. Agronomy. 2022; 12(5):1103. https://doi.org/10.3390/agronomy12051103

Chicago/Turabian Style

Luo, Zhuo, Haixing Song, Min Huang, Zhenhua Zhang, Zhi Peng, Tao Zi, Chang Tian, and Mamdouh A. Eissa. 2022. "Nitrogen-Reduction in Intensive Cultivation Improved Nitrogen Fertilizer Utilization Efficiency and Soil Nitrogen Mineralization of Double-Cropped Rice" Agronomy 12, no. 5: 1103. https://doi.org/10.3390/agronomy12051103

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