Variation Characteristics and Expression State of Nitrogen and Phosphorus Metering Ratio of Rice in Black Soil under Film Mulching and Irrigation Methods

Abstract

In order to investigate the effects of film mulching and water-saving methods on soil inorganic nitrogen, nitrogen content of rice organs, nitrogen-phosphorus metering ratio, and nutrient limitation during rice yield formation, two water-saving irrigation methods and black biodegradable film mulching were adopted. The effects of water-saving film mulching on NH₄⁺-N and NO₃⁻-N in 0 to 60 cm soil, nitrogen accumulation and ratio, and nitrogen-phosphorus metering ratio in rice organs were analyzed. The nitrogen-phosphorus-limiting state of rice growth under water-saving film mulching was determined. The results showed that water-saving and film mulching methods could weaken the leaching of NH₄⁺-N and NO₃⁻-N and enhance the nitrogen uptake of rice. The water-saving method of mulching could reduce the metering ratio of nitrogen and phosphorus in rice organs. The panicle maturity process had been in a state of nitrogen limitation, and the effect was gradually enhanced. The nitrogen and phosphorus metering ratio of panicles was positively correlated with NH₄⁺-N accumulation in the 0 to 60 cm soil layer and nitrogen accumulation of rice organs, and negatively correlated with soil NO₃⁻-N accumulation under film mulching and water-saving methods. The correlation between stems and leaves’ nitrogen and phosphorus metering ratios and influencing factors was basically opposite to that of panicles’ nitrogen and phosphorus metering ratio. This study can provide a reference for the formulation of a fertilization system under film mulching in the black soil region of northeast China.

1. Introduction

Black soil is an important soil resource that is well known for its good physical and chemical properties and high organic matter content [1,2]. As one of the three black soil regions in the world, the black soil region in Northeast China is an important grain production base, mainly planting rice, corn, and other food crops [3]. However, the successive occurrence of soil degradation and water and fertilizer loss in the black soil region has had an impact on crop growth [4,5]. Inorganic nitrogen in soil is an important source of nitrogen in plants, and the nitrogen content in plant organs is the main factor that determines the relationship between nitrogen and phosphorus in plants [6]. It greatly affects the growth effect of crops [7,8,9]. Nitrogen and phosphorus are important nutrient elements that limit the growth of crops. Because of the relationship between nitrogen-containing protein and phosphorus-containing RNA in plants, the ratio of nitrogen and phosphorus (N/P) can characterize the nutrient limitation of plants and play an important role in maintaining the sustainability and productivity of ecosystems [10,11]. It is of great practical significance to study the variation characteristics of N/P for judging the main nutrient elements that limit plant growth and their dynamic change process.

It has been found that different agricultural measures can affect the N/P crop. Nitrogen availability is the key factor affecting the relationship between nitrogen and phosphorus in crops. Nitrogen application rate will change the content of inorganic nitrogen in soil and the law of nitrogen uptake by crops, and then affect the N/P in crops [6]. The dynamic change of soil moisture in the water-saving irrigation method affects the transport of soil nitrogen and other nutrients, which leads to a change in the N/P of plants [12,13]. Plastic film mulching can also effectively reduce the N/P and balance the relationship between nitrogen and phosphorus [14,15]. In addition, rice-wheat rotation and tillage measures also affect the N/P of plants [16,17]. In black soil paddy fields in Northeast China, water-saving irrigation has attracted much attention because of the serious shortage of water resources. However, water-saving irrigation reduces the amount of irrigation water, resulting in soil water and heat being affected, which may lead to slow crop growth and a final yield decline [18]. The effect of increasing temperature and preserving water by plastic film mulching can make up for the shortage of water-saving irrigation methods, and the use of biodegradable film is also a hot issue in agricultural measures research at present. Research on the effects of water-saving irrigation and biodegradable film mulching on N/P of rice plants has been carried out; however, the research on the mechanism of the combination of water-saving irrigation and biodegradable film on N/P of rice is not in-depth.

In this paper, rice in the black soil of Heilongjiang Province was taken as the research object. By adopting the combination of water-saving irrigation and biodegradable film mulching, the changes in soil inorganic nitrogen, nitrogen content, and N/P of rice organs and the correlation between N/P of plants and nitrogen in soil and plants were studied and analyzed. The purpose of this paper is to: (1) study the nitrogen transport law of soil and rice organs under film mulching and water-saving methods; (2) reveal the mechanism of the effect of water-saving methods with film mulching on the N/P in rice organs; and (3) assess the nitrogen-phosphorus limitation in the process of rice yield formation under film mulching and water-saving methods by the N/P.

2. Materials and Methods

2.1. Experimental Site

The experiment was carried out at the rice irrigation experimental station (127°40′45″ E, 46°57′28″ N) in Heilongjiang Province. The experimental station is located in the core area of the black soil in the cold region of China [19], and the climate characteristics belong to the cold temperate continental monsoon climate, with obvious seasonal changes. The annual average temperature is 3.6 °C, the precipitation is 543.5 mm, the average relative humidity is 67%, the annual sunshine hours are 2682.4 h, the annual accumulated temperature is 2755 °C, and the frost-free period is 143 d. The pH value of the soil in the experimental site was 6.73, the soil bulk density was 1.21 g/cm³, the mass ratio of organic matter was 25.68 g/kg, the mass ratio of total nitrogen was 15.1 g/kg, the mass ratio of total phosphorus was 15.61 g/kg, the mass ratio of total potassium was 19.86 g/kg, and the mass ratio of alkali hydrolyzable nitrogen was 148.27 mg/kg.

2.2. Field Management and Experiment Treatments

In this experiment, two water-saving irrigation methods—ridge irrigation and controlled irrigation—and the combination of these two irrigation methods and black biodegradable film mulching with black and white were used. And this is the second year of the continuous water-saving and biodegradable film mulching experiment. The traditional irrigation (CK) was used as the control, and a total of five treatments were found. Each treatment had 3 replicates and a total of 15 experimental plots, each with an area of 100 m² (10 m × 10 m), and they were arranged randomly. The waterproof shed film was laid between plots to prevent the exchange of water. The rice variety used in this study was Longqingdao32, with a planting density of 25 cm × 16.7 cm and 3 plants per hole. The film is mainly made of PBAT and PPC; the thickness is 9.2 μm and the width is 100 cm. Under the condition of CK, except for the natural drying in the yellow maturity period, the field surface of other rice growth stages always maintains a 3 to 5 cm water layer. Under controlled irrigation conditions, except for the natural drying in the yellow maturity period, the soil moisture content of other rice growing periods was always maintained between 85% and 100% of the saturated soil moisture content. Before rice transplanting, the land surface was completely covered with mulch film, and the edge of the mulch film was compacted with soil. Under ridge irrigation conditions, trenches 15 cm deep and 20 cm wide were dug every 180 cm. Except for the natural drying in the yellow maturity period, the fields of other rice growth periods always kept water in the trench, and the film was moist. Before rice transplanting, the flat land between the two ditches was completely covered with mulch film, and the edge of the film was compacted with soil. Trenches were not covered with mulch film. The fertilizer of each treatment was applied as base fertilizer at one time before film covering, and the application amount was N 110 kg/hm², P₂O₅ 45 kg/hm², and K₂O 80 kg/hm² [20]. Experimental treatments are shown in

Table 1. Treatments.

Treatment	Irrigation Method	Whether to Covered
CK	Traditional irrigation	No
DM	Ridge irrigation	No
DB	Ridge irrigation	Yes
TM	Controlled irrigation	No
TB	Controlled irrigation	Yes

.

2.3. Observations and Measurements

(1)

Soil nitrogen content

At the tillering stage (TS), jointing and booting stage (JS), heading and flowering stage (HF), and milky filling stage (RS) of rice, soil samples were sampled by the five-point sampling method in each plot, and the sampled soil layers were 0 to 20 cm, 20 to 40 cm, and 40 to 60 cm, respectively. Fresh soil samples were stored in the refrigerator. A continuous flow analyzer (sensitivity 0.001 AUFS, Seal Analytical GmbH, Norderstedt, Germany) was used to determine the content of NH₄⁺-N and NO₃⁻-N in the soil sample.

(2)

Biomass, nitrogen, and phosphorus content of rice plants

At TS, JS, HS, and RS, rice samples from three holes were randomly collected in each plot, washed with water and removed roots, divided into stems, leaves, and panicles, and stored at 120 °C for 30 min, dried at 80 °C and weighed, crushed, and bagged. The solution to be measured was obtained after sterilization by the H₂SO₄-H₂O₂ method, and the nitrogen and phosphorus contents of plants were determined by a continuous flow analyzer.

(3)

Grain yield

At maturity, nine holes of rice plants were selected from each plot. The effective panicle number, the number of grains per spike, and the thousand-grain weight were determined after air-drying to a constant weight, and the theoretical rice yield was calculated according to the rice planting density.

2.4. Statistical Analysis

Calculation formula for inorganic nitrogen accumulation in soil:

$$N_{ai}=0.1D{P}_{b}C$$

(1)

where N_ai is inorganic nitrogen accumulation (kg·hm⁻²); D is the thickness of soil layer (cm); P_b is soil bulk density (g·cm⁻³); C is inorganic nitrogen content in a soil layer (mg·kg⁻¹); i is different kinds of inorganic nitrogen; when h and o are taken, NH₄⁺-N and NO₃⁻-N are represented, respectively.

Calculation formula for nitrogen accumulation in different organs of rice:

$$N_{aj}=N_j{M}_j$$

(2)

where N_aj is nitrogen accumulation in different organs of rice (kg·hm⁻²); N_j is nitrogen content in organs of rice (%); M_j is dry matter quantity of rice organs (kg·hm⁻²); j is different organs of rice; stems, leaves, and panicles are represented respectively when s, l, and p are taken. The same as below.

Calculation formula of N/P in different organs of rice:

$${(N/P)}_j=N_j/P_j$$

(3)

where (N/P)_j is N/P in different organs of rice; N_j and P_j are the nitrogen and phosphorus contents of rice organs (%).

The accumulation of NH₄⁺-N and NO₃⁻-N in 0–20 cm, 20–40 cm, and 40–60 cm soil layers (N_ah1, N_ah2, N_ah3, and N_ao1, N_ao2, N_ao3), nitrogen accumulation, and N/P in stems, leaves, and panicles of rice (N_as, N_al, N_ap, and N/P_s, N/P_l, and N/P_p) were calculated by Formulas (1)–(3).

The average value of each index was used, and Excel 2010 and IBM SPSS Statistics 25 were used for related data processing and significance analysis, and Origin 2022 software was used for graphing.

3. Results

3.1. Soil Inorganic Nitrogen Accumulation under Different Treatments

Under different treatments, changes in NH₄⁺-N accumulation in different soil layers at rice growth stages are shown in Figure 1. The accumulation of NH₄⁺-N in the 20 to 60 cm soil layer of CK treatment was significantly higher than that of other treatments at different growth stages (p < 0.05); under the 0 to 20 cm soil layer, the accumulation of DB treatment was 3.035% to 17.694% higher than that of other treatments. The NH₄⁺-N accumulation in each treatment showed a decreasing trend with an increase in soil depth.

Under the same irrigation method, the NH₄⁺-N accumulation in the 0 to 20 cm soil layer with film mulching treatment was significantly higher than that without film mulching treatment (p < 0.05); however, in the 20 to 40 cm soil layer, the accumulation of no mulching treatment was higher under ridge irrigation and mulching treatment was higher under controlled irrigation; there was no significant difference except for a few treatments (p < 0.05); the accumulation of the 40 to 60 cm soil layer without mulching was higher than that of mulching treatments, and the difference was significant except for a few treatments (p < 0.05). Analysis shows that mulching treatment can change NH₄⁺-N accumulation in different soil layers; the accumulation of NH₄⁺-N in the shallow soil layer was greater, and the accumulation of NH₄⁺-N in the deep layer of soil was less. With the change of growth stage, the difference in NH₄⁺-N accumulation in the 0 to 60 cm soil layer with and without mulching treatments was 1.042% to 26.207%, 2.770% to 28.815%, 0.470% to 23.198%, and 1.074% to 23.250%. The effect of film mulching on the soil NH₄⁺-N accumulation showed a trend of first increasing, then decreasing, and then gradually flattening, which may be related to the gradual degradation of film, which led to the weakening of the effect on soil water and heat [21,22].

Under the same mulching conditions, with the increase in soil depth, the accumulation of NH₄⁺-N in ridge irrigation treatments was 1.422% to 4.669%, 0.761% to 7.912%, and −8.358% to 6.636% higher than that in controlled irrigation treatments, respectively. Compared with controlled irrigation, the NH₄⁺-N accumulation in ridge irrigation without film mulching was higher in the 0 to 40 cm soil layer and lower in the 40 to 60 cm soil layer. This may be because the soil water movement under ridge irrigation was mainly the horizontal movement of water in the ditch, and the vertical movement degree of water under controlled irrigation was stronger than that under ridge irrigation, and the water movement drove the nitrogen movement, which led to more nitrogen vertical movement in controlled irrigation. Under mulching treatments, the accumulation of NH₄⁺-N in the 0 to 60 cm soil layer of ridge irrigation was higher than that of controlled irrigation, which may be related to the increase in soil temperature under film mulching [21].

Figure 2 shows the changes in soil NO₃⁻-N accumulation in different soil layers of different treatments. During the whole growth period, in the 0 to 20 cm soil layer, the accumulation of NO₃⁻-N under CK treatment was significantly lower than that under other treatments except for a few treatments and growth periods (p < 0.05); in the 20 to 40 cm and 40 to 60 cm soil layers, the accumulation of CK treatment was −0.249% to 17.379% and 37.603% to 69.513% higher than that of other treatments; the difference was significant except for a few treatments (p < 0.05). The results showed that water conservation with and without film mulching can effectively reduce the vertical migration of NO₃⁻-N and weaken the sedimentation of NO₃⁻-N. With the increase in soil depth, the accumulation of NO₃⁻-N in the soil increased at first and then decreased in other treatments, except that the NO₃⁻-N accumulation decreased gradually in the mulching treatment.

Under the same irrigation methods, the accumulation of NO₃⁻-N in the 0 to 60 cm soil layer with mulching treatments was higher than that with no mulching treatments, and the difference was significant except for a few treatments and growth periods (p < 0.05). The vertical variation of NO₃⁻-N accumulation under film mulching treatment was different from that under no film mulching treatment and gradually decreased with the increase in soil depth. Film mulching could change the law of NO₃⁻-N accumulation in different soil layers. With the change in growth period, the difference in NO₃⁻-N accumulation between the 0 to 60 cm soil layer with and without film mulching was 4.348% to 19.664%, 3.829% to 12.150%, 2.032% to 16.276%, and 1.228% to 21.263%. Except for a few periods and soil layers, the effect of film mulching treatment on NO₃⁻-N accumulation in the 0 to 60 cm soil layer gradually weakened with the change of time.

Under the same film mulching conditions, the NO₃⁻-N accumulation of 0 to 20 cm of soil under ridge irrigation was 2.262% to 7.797%, 1.326% to 3.411%, and 1.647% to 5.217% higher than that under controlled irrigation, except that the NO₃⁻-N accumulation of 0 to 20 cm of soil under TM treatment was higher than that under DM treatment at HF. Compared with controlled irrigation, the effect of ridge irrigation on NO₃⁻-N accumulation decreased at first and then increased with the increase in soil depth.

3.2. Nitrogen Absorption of Rice under Different Treatments

Figure 3 shows the difference in nitrogen accumulation in different growth stages of rice under different treatments. The nitrogen accumulation of CK treatment was 10.440% to 37.776% lower than that of water-saving treatments with mulching (p < 0.05); however, there was no significant difference between CK treatment and water-saving treatments without mulching except for a few treatments and growth periods (p < 0.05).

Under the same irrigation conditions, the nitrogen accumulation of rice under film mulching treatments was 45.247% and 56.514%, 17.977% and 8.164%, 21.134% and 13.510%, 13.478% and 9.591% higher than that under no film mulching treatment in each growth period, respectively. Except for TM and TB treatments at JS, the difference between mulching treatment and no mulching treatment was significant in the whole growth stage (p < 0.05). The results showed that film mulching could effectively enhance the ability of rice to absorb nitrogen and improve nitrogen accumulation in rice plants, and the promotion effect was the best at TS. This may be because film mulching promoted the utilization of soil water and photosynthesis in plants and then improved transpiration efficiency, which was beneficial to the transport and accumulation of nitrogen in the organs of rice through soil water.

Under the same mulching condition, the difference in nitrogen accumulation between the two water-saving irrigation methods showed that the ridge irrigation was significantly higher than the controlled irrigation at TS and RS (p < 0.05). The law of JS and HF is opposite; however, the difference is not significant (p < 0.05). This may be because the soil moisture under controlled irrigation treatments was higher and more evenly distributed than that under ridge irrigation treatments, which could more easily meet the requirements of water absorption by rice and then promote the absorption and utilization of nitrogen by rice. However, at TS, rice organs were not fully developed; therefore, it was more dependent on temperature. Ridge irrigation increased the surface area of soil exposed to air and then increased the surface temperature of the soil, which was more beneficial for rice to absorb nitrogen. During the RS, the water naturally dried down. Because of the accumulated water in the ditch, the soil water content in ridge irrigation was maintained at a higher level for a longer time than that in controlled irrigation, which led to the higher nitrogen absorption of rice in ridge irrigation.

The difference in nitrogen distribution ratio in different organs of rice under different treatments at different growth stages is shown in Figure 4. With the change in growth period, the proportion of nitrogen distribution in the organs of rice changed; the proportion of nitrogen distribution in stems first increased and then decreased; the proportion of nitrogen distribution in leaves continued to decrease; and the proportion of nitrogen distribution in panicles gradually increased.

At TS and JS, the nitrogen distribution ratio of stems with mulching was 8.275% to 19.413% and 1.819% to 3.697% higher than that without mulching under the same irrigation methods, respectively. In the early stages of rice growth, the stems could absorb more nitrogen and transfer it to the leaves. At HF, the difference in nitrogen distribution ratio between mulching treatments and no mulching treatments was between 0.999% and 2.798%; at RS, the firm mulching treatments were 4.706% to 5.312% higher than the non-film mulching treatment. At HF, the panicle organs began to absorb nitrogen, but due to the short development time, the amount of nitrogen absorbed was low, and there was little difference among the treatments; when reaching RS, panicles of rice were basically complete, and the nitrogen distribution ratio of panicles under mulching treatments was higher. The results indicated that film mulching was beneficial to nitrogen absorption and utilization in the early growth stage of rice, promoted nitrogen transport to panicles in the late growth stage, and improved the final grain yield and quality of rice.

Under the same conditions of mulching, the difference in nitrogen distribution ratio between the two irrigation methods was 0.711% to 10.427% and 0.868% to 1.010% at TS and JS. In the early stages of growth, the influence of different irrigation methods on the nitrogen distribution ratio of stems gradually weakened with time. The proportion of nitrogen distribution in panicles of ridge irrigation was 0.506% to 1.112% higher than that of controlled irrigation at RS, and the difference was small, which indicated that the effects of the two irrigation methods on panicles were basically the same.

3.3. N/P of Rice under Different Treatments

As an important index to judge the growth and development of crops, the N/P can reflect the response of plant growth to environmental changes [23]. According to the “homeostasis theory”, there is a relatively stable N/P in each organ of the plant to maintain the dynamic balance of nutrient supply between the plant and the growing environment [24]. The N/P can be used as an evaluation index for plant health. Figure 5 shows the N/P in different organs of rice under different treatments. At TS and JS, the N/P in stems and leaves of the CK treatment was lower than that of water-saving irrigation without film mulching but higher than that with film mulching, and the difference was significant (p < 0.05). At HF and RS, the N/P in stems of CK treatment was lower than that of no film mulching treatments and higher than that of film mulching treatment, with a significant difference (p < 0.05); the N/P in leaves was higher than that of other treatments, and the overall difference was significant (p < 0.05); compared with other treatments, the N/P in panicles of CK treatment was the lowest at HF and the highest at RS; there was no significant difference except for a few treatments (p < 0.05).

Under the same irrigation methods, except that the leaves N/P of TB treatment was higher than that of TM treatment at RS, the stems, leaves, and panicles N/P of film mulching treatments were 41.334% to 47.341%, 1.523% to 21.023%, and 0.126% to 2.712% lower than that of no film mulching treatments at the whole growth stage, respectively. It indicated that film mulching could reduce the N/P in the organs of rice and promote the balance of nitrogen and phosphorus nutrition, which was consistent with the conclusion drawn by Lu et al. [14], and the effect on stems was the best. Under the same film mulching conditions, except for a few treatments and growth periods, the N/P in stems, leaves, and panicles under ridge irrigations was 0.413% to 4.468%, 0.827% to 3.595%, and 2.306% to 3.656% lower than that under controlled irrigation, respectively. This may be related to insufficient soil moisture under ridge irrigation compared with controlled irrigation. The soil environment for rice growing under ridge irrigation was better. At the same time, because there was less water, the oxygen and temperature in the soil were higher, which created a good environment for microbial activities in the soil. Acid substances produced by microbial respiration could promote phosphate fertilizer dissolution and absorption by plants [25]. The increase in nitrogen accumulation in rice would also promote the absorption and utilization of phosphorus by plants, which would lead to the lower N/P of plants under ridge irrigation. The results showed that the effect of film mulching on the N/P in stems and leaves was stronger than that of irrigation, and the effect on the N/P in panicles was weaker than that of irrigation.

Table 2. Correlation between rice yield and N/P of rice organs.

	Yield	N/P in Stems	N/P in Leaves	N/P in Panicles
Yield	1	−0.727	−0.404	−0.902 *
N/P in stems		1	−0.071	0.386
N/P in leaves			1	0.481
N/P in panicles				1

“*” is significant at p < 0.05 level.

shows the correlation between rice yield and the N/P of rice organs. The N/P of stems, leaves, and panicles was negatively correlated with rice yield; the correlation coefficients were −0.727, −0.404, and −0.902, respectively. The correlation between the N/P of panicles and rice yield was significant (p < 0.05), and the correlation coefficient is the highest; the correlation coefficient between the N/P of leaves and yield is the lowest. The results indicated that decreasing N/P in different organs of rice was beneficial to the growth and development of rice and then increased the final yield, which may be related to the fact that decreasing N/P could promote the balance of nitrogen and phosphorus elements in crops [14,26].

Liebig’s law of minimum holds that the growth and development of an organism will be limited by the nutrient with the smallest supply compared with the demand, and this nutrient will become the limiting nutrient of the organism [27]. The results showed that the optimum N/P of grain for the main cereal crops was between 4.2 and 6.7 [28]. In this study, the N/P of panicles under each treatment decreased gradually with the change of time (Figure 5), and the ratios were all less than 4.2. According to the law of minimum, the panicles of each treatment were limited by nitrogen in the growth process, and the limited effect gradually increased with the change of time. According to the previous analysis, compared with the CK treatment, the N/P in rice panicles of water-saving treatments with film mulching was higher at HF and lower at RS; therefore, the nitrogen limitation of film mulching and water-saving treatments was weaker at HF, while it was stronger at RS. This was due to the higher degradation rate of film and the larger surface area of soil exposed to the air at the RS, which made the oxygen content in the soil increase and the microbial activity increase, thus promoting the absorption of phosphorus by crops [25]. With the increasing phosphorus content in crops, the growth of rice was more limited by nitrogen. In this study, there was a significant negative correlation between rice yield and the N/P in rice panicles (Table 2). The effect of nitrogen restriction of panicles on the final yield formation at HF was stronger than that at RS under water-saving methods with film mulching.

3.4. Relationship between Soil-Crop Nitrogen Transport and N/P of Rice

The response pattern of N/P in different organs of rice to soil-crop nitrogen transport is shown in Figure 6. Except for 40 to 60 cm soil NH₄⁺-N accumulation, which had a positive correlation with the N/P of stems, 0 to 60 cm soil NH₄⁺-N accumulation had a negative correlation with the N/P of stems and leaves N/P, and a positive correlation with the N/P of panicles. Among them, 0 to 20 cm accumulation of NH₄⁺-N had a significant correlation with aboveground organs N/P of rice (p < 0.01), and 20 to 40 cm soil NH₄⁺-N accumulation had an extremely significant correlation with panicles N/P (p < 0.001). It can be seen from the figure that with the deepening of soil depth, the correlation degree between soil NH₄⁺-N accumulation and N/P of stems and leaves gradually decreased, and the correlation degree between soil NH₄⁺-N accumulation and N/P of panicles first increased and then decreased.

NO₃⁻-N accumulation in the 0 to 60 cm soil layer was positively correlated with the N/P of stems and leaves and negatively correlated with the N/P of panicles. The correlation between NO₃⁻-N accumulation in the 20 to 60 cm soil layer and N/P of leaves was significant (p < 0.05). With the increase in soil depth, the correlation between the accumulation of NO₃⁻-N and the N/P of stems and leaves first increased and then decreased, and the correlation between soil NO₃⁻-N accumulation and the N/P of panicles first decreased and then increased.

Nitrogen accumulation in stems and panicles was negatively correlated with the N/P of stems and leaves and positively correlated with the N/P of panicles. Nitrogen accumulation of leaves was positively correlated with N/P of aboveground organs; there was a significant correlation between nitrogen accumulation of panicles and N/P of stems (p < 0.05); and there was an extremely significant correlation between nitrogen accumulation of panicles and N/P of leaves and panicles (p < 0.001). Compared with stems and leaves, panicles had the strongest regulation on the N/P of the aboveground organs of rice.

4. Discussion

The water-saving method of film mulching can affect the field environment, such as soil microorganisms, nutrients, and organic matter, by adjusting soil water and heat [29,30,31], changing the nutrient content and nutrient balance in plants. In this study, the water-saving treatments with film mulching could improve the nitrogen accumulation in plants (Figure 3); this is related to the weakening of soil nitrogen leaching (Figure 1 and Figure 2), and it may also be related to the decrease of nitrogen loss caused by volatilization [31]. Film mulching treatment was beneficial to soil nitrogen accumulation; however, the effect of film mulching on soil nitrogen accumulation was weaker at the later stage of rice growth, which was different from the results of Ding et al. [32] The main reason for the difference was that film mulching had a large amount of degradation in this experiment, which increased the soil surface area exposed to air, and nitrogen leaching occurred more strongly during irrigation and rainfall, thus reducing the effect of film mulching on soil nitrogen accumulation [33]. The research of Wang et al. [34] can also prove this idea. However, the accumulation of NO₃⁻-N under ridge irrigation was higher than that under controlled irrigation, which was related to the oxygen content in the soil. Because of the uneven water distribution and more voids in the soil, the air in the upper part of the soil can easily enter the soil, which makes the nitrification reaction stronger under the condition of ridge irrigation; therefore, the accumulation of NO₃⁻-N in the soil under ridge irrigation is higher. Gu et al. [35] and Saglam et al. [36] found that in the later stage of growth, the effect of degradable film mulching on soil NO₃⁻-N accumulation in the 0–20 cm soil layer weakened, while the soil NO₃⁻-N accumulation in the 20–60 cm soil layer increased. This is different from the discovery that the effect of film mulching treatment on the accumulation of NO₃⁻-N in the 0–60 cm soil layer gradually weakened with time. The main reason is that the degradation of plastic film will accelerate the evaporation rate of water, and the lack of water will increase the irrigation times and irrigation amount; therefore, the oxygen amount in soil will not increase or even decrease because of the degradation of film, which will hinder the nitrification reaction.

In this study, the ripening process of panicles in each treatment was limited by nitrogen, which was different from the research results of Gu et al. [15], mainly because the difference between the amount of fertilizer applied and the proportion of fertilizer elements led to the different absorption of nutrients by plants, thus changing the limiting conditions of the plant growth process. When determining the nitrogen and phosphorus limiting states in rice development and maturation, this paper uses the N/P of rice panicles to judge the limiting problem, and the organs most closely related to them may change for different field management measures and crops. The representative organs that judge the restricted state also change. Kou et al. [37] and Zhang et al. [13] believe that roots are more susceptible to the influence of water-saving irrigation treatment; therefore, the nitrogen and phosphorus restriction status of crops can be judged by root N/P. However, Lu et al. [38] found that the N/P of leaves could determine the restriction of crops when irrigation changes. Therefore, it is of great significance that, through a large number of comparative experiments and N/P of rice organs with film mulching and water-saving treatment, we explore the organs that are most sensitive to film mulching and water-saving treatment and determine the representative organs to judge the problem of nitrogen and phosphorus restriction for related research on the nutrient influencing factors of rice growth and development under film mulching and water-saving treatment.

In this study, the effects of soil-crop nitrogen transport on rice nitrogen-phosphorus stoichiometric ratio and nitrogen-phosphorus limitation in rice final yield formation were studied. However, the nitrogen cycle process included not only nitrogen fixation, nitrogen leaching, and nitrogen absorption but also nitrification and denitrification, ammonia volatilization, ammonia oxidation, and other processes [39]. All these processes would affect the N/P of rice; therefore, in future research, it is necessary to explore the influence mechanism of the water-saving method with film mulching on rice N/P by measuring the nitrogen cycle process under the water-saving and film mulching methods. There are three major life elements in the ecosystem, which are carbon, nitrogen, and phosphorus, and the functions of carbon, nitrogen, and phosphorus in crops are closely coupled. The metrological characteristics of the three elements play an important role in maintaining the balance of elements in life and the stability of the ecosystem [11,40]. Therefore, it is necessary to calculate the metering ratio of carbon and nitrogen and carbon and phosphorus in rice under film mulching and the water-saving method and measure indexes such as carbon and phosphorus absorption of crops so as to reveal the response mechanism of rice nutrient utilization and crop growth to the water-saving method with film mulching.

5. Conclusions

The film mulching and water-saving methods could effectively reduce the leaching intensity of NH₄⁺-N and NO₃⁻-N and increase the accumulation in the 0 to 20 cm soil layer; however, this effect was weakened in the late growth stage of rice. Water-saving treatments with film mulching could effectively improve the nitrogen absorption intensity of rice, increase the nitrogen ratio of stems in the early stage of rice growth, and promote the transportation of nitrogen to panicles in the late stage of rice growth. Except for the N/P in panicles at HF, the water-saving method with film mulching could effectively reduce the N/P in all rice aboveground organs. The ripening of panicles was limited by nitrogen, and the limiting effect gradually increased with time. Rice was more sensitive to nitrogen-limiting intensity at HF than at RS. Under film mulching and water-saving methods, the N/P in stems and leaves had a negative correlation with the accumulation of NH₄⁺-N in 0 to 60 cm soil and the nitrogen accumulation of stems and panicles and a positive correlation with the accumulation of NO₃⁻-N in soil and the nitrogen accumulation of leaves. The N/P in panicles was positively correlated with NH₄⁺-N accumulation in 0 to 60 cm soil and nitrogen accumulation in aboveground organs but negatively correlated with the accumulation of NO₃⁻-N in soil. Under film mulching and water-saving methods, rice maturation is more limited by nitrogen, which can increase the proportion of nitrogen fertilizer application and provide a reference for the formulation of fertilization systems under film mulching in the northeast black soil region of China.