Optimization of Controlled Water and Nitrogen Fertigation on Greenhouse Culture of Capsicum annuum

This study investigated the effects of different combinations of irrigation and nitrogen levels on the growth of greenhouse sweet peppers, assessing yield, quality, water use efficiency (WUE), and partial factor productivity from applied N (PFPN). By using controlled drip irrigation, the optimal conditions for efficient, large-scale, high-yield, and high quality production of sweet peppers in Northwest China were determined. Using the local conventional irrigation and nitrogen regime as a control (105% ET0, N: 300 kg·hm−2), three alternative irrigation levels were also tested, at 90%, 75%, and 60% ET0. These were combined with nitrogen levels at 100%, as the control, and 75%, 50%, and 25%, resulting in 16 combination treatments. The results show that different supplies of water and nitrogen nutrition had a significant impact on the growth, yield, WUE, PFPN, and quality of fruit. The treatments of W0.90N0.75, W0.90N0.50, W0.75N0.75, and W0.75N0.50 can better maintain the “source-sink” relationship of peppers. They increased the economic yield, WUE, and PFPN. A principal component analysis was performed to evaluate indicators of fruit quality, revealing that the treatment of W0.75N0.50 resulted in the best fruit quality. For greenhouse sweet peppers produced in Northwest China, the combination of W0.90N0.75 resulted in the highest economic yield of 34.85 kg·hm−2. The combination of W0.75N0.75 had the highest WUE of 16.50 kg·m−3. The W0.75N0.50 combination treatment had the highest fruit quality score. For sustainable ecological development and in view of limited water resources in the area, we recommend the W0.75N0.50 combination treatment, since it could obtain the optimal fruit quality, while its economic yield and WUE were 9% and 4% less than the maximum, respectively. This study provides a theoretical basis for the optimal management of water and nitrogen during production of greenhouse sweet peppers in Northwest China.


Introduction
The ever-increasing imbalance between supply and demand of agricultural water has forced adjustments of modern agricultural irrigation practices [1]. Crop yields generally benefit from short growth cycles and the selection of highyield varieties that undergo rapid development, strategies that have effectively increased the income of farmers. However, in order to maximize yield, excessive irrigation and fertilization are often required, resulting in overuse and serious runoff of fertilizer. Other negative effects are more frequent plant diseases and pests [2], a reduction of product quality, less efficient water and fertilizer utilization [3], and damage to local soil and water environments [4].
Water and nitrogen are critical factors determining crop yield and quality [5]. Appropriate irrigation technologies and nitrogen application strategies can lead to high yields and growth efficiencies of crops. Compared to conventional furrow irrigation, fertigation (the injection of fertilizer solution into the soil) effectively reduces surface runoff, evaporation between plants, and deep percolation [6]. At the same time, the drip irrigation technology with small flow can lower the downward migration rate of nitrogen into the soil and reduce nutrient loss [7]. As a result, a suitable nutrient 2 The Scientific World Journal microenvironment in the crop root zone can be achieved, promoting absorption of nutrients into the crops.
C. annuum (L. var. grossum), a member of the Solanaceae, produces fruits with a high nutritional value. Its effective culture is strongly dependent on the environment [8], as the plant requires a constant supply of water and nutrients [9]. The availability of nitrogen is an important determinant of crop yield and quality, by directly affecting photosynthesis and the accumulation, transfer, and distribution of biomass [10]. Previously, the studies of water-fertilizer management mainly focused on irrigation methods [11], irrigation amount and frequency [12], and various types of fertilizers [13][14][15].
Lately, an improved fertigation technique was developed that applies nutrients with water to the crops. Compared to classical fertigation, this new technique significantly increases yield, water use efficiency (WUE), and nutrient use efficiency [16][17][18][19][20][21][22]. For instance, compared to conventional furrow irrigation, properly controlled fertigation increases the yield of C. annuum and enhances WUE [11,23]. It was experimentally shown that carefully determined fertilizer quantities could ensure high yield of C. annuum, at the same time reducing production costs [13]. Another study reported that compared to conventional furrow irrigation, an optimized fertigation strategy could increase the WUE of C. annuum by 95%, saving water by 34% and fertilizer by 20% [15].
Few studies have investigated the comprehensive influence of a water-nitrogen coupling effect on the growth, yield, and quality of C. annuum, and a quantitative index remains to be determined. Particularly, fruit quality as a function of the water-nitrogen ratio has rarely been reported. Therefore, the experiments presented here aimed at enhancing the utilization of water and nitrogen as well as promoting yield and quality of C. annuum crops. Using an automatically controlled irrigation and fertilization system, plot tests were performed in a sunlight greenhouse to optimize water and nitrogen management and make full use of water-nitrogen synergistic effects. This work provides technical support for application of automatic water and nitrogen management systems in C. annuum cultivation under protected conditions and offers a scientific basis for reference of high quality, high efficiency, and large-scale production.

Experimental Materials
The experiments were conducted in a test area with the Guanzhong plain, China, located at 108 ∘ 04 E and 34 ∘ 20 N. The work was performed between April and July 2014, in a sunlight greenhouse belonging to the Key Laboratory of the Ministry of Education for Agricultural Water and Soil Engineering in Arid Area, Northwest Agriculture and Forestry University, China. The greenhouse windows face south and north, with vents at the top and southern-facing bottom of the greenhouse. The test facility has an altitude of 521 m and warm temperature due to a local semihumid climate. The annual average temperature is 13 ∘ C with annual average precipitation from 550 to 600 mm. The greenhouse measures 76 m in length, 7.5 m in width, and 2.8 m in height. Heavy soil (1% sand, 72% silt, and 27% clay) was used, with details on its physical and chemical properties shown in Table 1. The soil bulk density was measured by dry weight. Soil samples were collected from a depth between 0 and 80 cm (every 20 cm) with a ring knife (diameter: 5 cm, height: 5 cm). A small weather station (HOBO Event Logger, Onset Computer Corporation, USA) was set up inside the greenhouse. The atmospheric pressure, temperature, photo synthetically active radiation (PAR), relative humidity, and meteorological factors such as solar radiation were recorded every 10 minutes. The obtained measurements confirmed that these parameters were comparable for all plots.

Fertigation
Equipment. An online irrigation fertilizer applicator (NETAJET 3G INLINE, NETAFIM, Israel) was employed for fertigation, which can precisely apply and control fertilization based on the amount of irrigation water. The flow rate supported by this system is from 0.5 to 20 m 3 ⋅h −1 . Venturi-type applicators were used, equipped with an optical fertilizer meter with a flow rate capacity from 30 to 300 L⋅h −1 . Both fertilizer and acid solutions can be used. In addition, the system includes electrical conductivity (EC) and pH measurement and control modules. Drip irrigation pipe (inner diameter: 8 mm) was employed using drip laterals with inline emitter, distanced at 30 cm between emitter, providing a flow rate of 2 L⋅h −1 for each emitter and pipe working pressure of 0.3 MPa.

C. annuum Type and Fertilizer
Used. American C. annuum "Marco" (Capsicum annuum L. var. grossum Marcomi F1) was used, as this type is particularly suitable for greenhouse production. The long, lantern shaped fruit is green when young and turns red when ripe. The fertilizer used in the experiment contains urea, calcium superphosphate (Ca(H 2 PO 4 ) 2 ⋅2H 2 O), and potassium chloride (KCl).

Experimental Design.
Two experimental variables were studied: the amount of irrigation water and the amount of nitrogen fertilizer. Based on the reference crop evapotranspiration (ET 0 ), four irrigation levels were tested, at 105% ET 0 (W 1.05 ), 90% ET 0 (W 0.90 ), 75% ET 0 (W 0.75 ), and 60% ET 0 (W 0.60 ). Based on the locally recommended nitrogen application amount of 300 kg⋅hm −2 (300 kg⋅hm −2 , N 1.00 ), three alternative regimes were tested with 75% N (225 kg⋅hm −2 , N 0.75 ), 50% (150 kg⋅hm −2 , N 0.50 ), and 25% (75 kg⋅hm −2 , N 0.25 ). The test was designed to assess all 16 possible combinations of these variables, with three identical plots for each situation, reaching a total of 48 plots. Cultivation in furrows covered with film was adopted, which is typically used by local farmers. Each ridge had a height of 25 cm and width of 75 cm. The top of the ridge was flat with spacing of 50 cm between ridges, plants were spaced at 45 cm, and rows were separated by 30 cm. Individual planting was used with a planting density of 31,000 plants per hm 2 . Each plot consists of one ridge with two plant rows. A drip lateral pipe was installed in the middle of two plant rows, so that one pipe controlled two rows. Each plot was 6.70 × 1.25 m, with a The Scientific World Journal 3 To prevent the interaction of water and fertilizer between neighboring plots, the plots were separated by embedding plastic foil 1 m deep into the soil.

Irrigation and Fertilization.
On January 6, 2014, seeds were soaked in water for 12 hours, followed by incubation in 1% copper sulfate solution for 5 min, and subsequently washed with water. The seeds were then placed into a thermostat for germination from 25 to 30 ∘ C for ten days.
The budding seeds were sowed in a hotbed. Five weeks later, the greenhouse was disinfected as follows. For each cubic meter of soil, 5 g sulfur, 0.1 g 80% insecticide (dichlorvos, 2,2dichlorovinyl dimethyl phosphate), and 10 g saw dust were mixed uniformly and ignited. After sealing the greenhouse overnight, the greenhouse was vented and sealed again. The elevated temperature of the greenhouse was continued for 48 hours. On April 1 and 2, the soil was prepared and basic fertilizer (phosphate) was applied. The seedlings, with 1 heart and from 8 to 12 leaves, were planted on the next day and uprooted on July 23. To ensure a proper seedling survival rate, 40 mm planting water was applied. Beginning from April 19, irrigation was conducted every five days. During the whole growth period, the total irrigation amounts for W

Chlorophyll Content.
At the same time points that dry weight was determined, 0.1 g leaves were picked from randomly chosen plants for each plot. The third new leaf from the heart was selected, which grows rapidly. The amount of chlorophyll, comprised of chlorophyll a and chlorophyll b, was measured by a UV-Vis spectrophotometer (EV300PC, Thermo Fisher, USA) using an extraction method previously described [24].

Fruit Yield.
During the ripening stage, red fruits in each test plot were picked every 10 days and weighed. The yields from each pick were added together to obtain the economic yield, and this was converted to t⋅hm −2 . Three plants from each plot were marked and weighed, and the average yield per plant was calculated.

Fruit Quality.
Ripe fruits with similar development characteristics were picked in each plot. The content of soluble solids in the fruits was measured as previously described [25] using an RHBO-90 hand refractometer (LINK, Co. Ltd., Taiwan China). The capsaicin content was measured by high performance liquid chromatography, and the vitamin C content was obtained by spectrometry using the molybdenum blue colorimetric method [24]. The soluble sugar content was measured by sulfuric acid anthrone colorimetry, and nitrate content was obtained using a UV-Vis spectrophotometer [19].

Water Use Efficiency.
The water content in the soil was determined with a TDR moisture meter [26] and calibrated by traditional drying method. Two days before and after the test, the water content in the soil was measured every 10 cm up to a depth of 80 cm. The evapotranspiration (ET, mm) of the plants at various stages was calculated by a water balance based on the reference [26]. There was no precipitation in greenhouse. The deep percolation and runoff were considered negligible, since the amount of water each time was less (the maximum value was about 24.1 mm). The equation used to calculate ET is where is the irrigation amount (mm) and Δ is the water variation (mm) in the initial and final soil.

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The Scientific World Journal The irrigation amount can be calculated as follows: where is the crop coefficient which was based on FAO56 [27] and ini , mid , and end were 0.60, 1.05, and 0.90, respectively.
In the greenhouse evaporation and heat transfer still occur, even in the absence of wind, since the air boundary layer is a nonneutral stable layer. ET 0 was calculated according to the modified Penman-Monteith equation for a sunlight greenhouse, as published previously [28]. Meteorological data for calculating ET 0 were taken from the weather station inside the greenhouse.
where ET 0 is the referenced crop evapotranspiration is the actual vapor pressure (kPa), Δ is the slope of saturated vapor pressure curve (kPa⋅ ∘ C −1 ), is the dry wet constant (kPa⋅ ∘ C −1 ), and is the average temperature at 2 m ( ∘ C).
The water use efficiency WUE is calculated as follows [29]: where is the yield in kg⋅hm −2 .
2.4.6. Partial Factor Productivity from Applied Nitrogen. The partial factor productivity from applied nitrogen (PFPN) can be calculated as follows [30]: where is the total mass of applied nitrogen (kg⋅hm −2 ).

Data Analysis. Statistical analysis software including
Excel 2010 and SPSS Statistics 18.0 was used to analyze the experimental data. Duncan's new multiple range test method was employed for multiple comparisons. If a significant difference was observed ( < 0.05), Tukey HSD comparison was adopted. All figures were plotted using Origin 8.0 software.

Influence of Water-Nitrogen Regimes on Dry Matter
Content of C. annuum. Growth of C. annuum plants was followed during the course of the experiment by determining dry matter content (DM) at various time points, as shown in Figure 1. The DM initially increased rapidly, followed by a slower increase at later stages. Fifty-four days after planting, DM starts to show increasing differences between the tested conditions. As expected, DM is affected by the amount of N supplied to the plants. According to the slopes of the curves (Figure 1), under the same irrigation regime, the increase of DM is more rapid at N 0.75 and N 0.50 than at N 1.00 , while N 0.25 produced the slowest increase in dry weight.

Influence of Water-Nitrogen Regimes on Chlorophyll
Content. Chlorophyll content in the leaves was determined to provide a measure of the growth status of the plants. Chlorophyll is required for photosynthesis to produce sugars that enable growth, but a high chlorophyll content in the leaves can be disadvantageous for fruit production, when the plants favor growth of parts other than fruit.
During growth, the overall chlorophyll content in the leaves first increased, followed by a decrease (Figure 2). Comparing conditions with the same irrigation level (individual curves within a panel) showed that the chlorophyll content increased with nitrogen levels. However, when the nitrogen level was kept constant (comparing curves with the same color), an increase in irrigation resulted first in an increase and then in a decrease of the chlorophyll content in leaves. According to the slope of the curve obtained, W 0.90 and W 0.75 had the largest influence on chlorophyll content compared to W 1.05 . Particularly between days 54 and 61 a rapid increase was observed. This phase corresponds to the flowering, fruiting, and reproductive stages of the plant. The products of photosynthesis are mainly reserved for growth of stems, leaves, and fruits, and these plant parts may compete with each other. Under various irrigation levels, control fertilizer level N 1.00 resulted in a high leave chlorophyll content during the entire fruiting stage (Figure 2). Under this nitrogen supply level, a large number of small fruits developed which, after The Scientific World Journal thinning, resulted in fewer nutrients being distributed to the fruits, allowing the stem and leaves to grow vigorously. In contrast, the condition W 0.60 N 0.25 produced fewer fruits, suggesting that low water and nitrogen supplies resulted in an imbalance of resources, accelerating leaf aging and resulting in low chlorophyll content. As can be seen in Figures 2(b) and 2(c), under conditions W 0.90 N 0.75 , W 0.90 N 0.50 , W 0.75 N 0.75 , and W 0.75 N 0.50 the chlorophyll content increased fastest to give maximum levels at day 54, indicating that limited water stress and nitrogen application can promote the allocation of nutrients to the fruits and this can accelerate fruit growth.
Towards the end of the fruiting stage the chlorophyll content of leaves was shown to increase, independent of the fertigation regime ( Figure 2). This is because the perennial plant quickly develops to the next growth period after ripe fruit has been picked.

Influence of Water-Nitrogen Regimes on Fruit Yield and Water
Usage. The influence of various irrigation and nitrogen supply combinations on economic yield, WUE, and PFPN is shown in Table 2. As can be seen, water and nitrogen levels have a significant ( < 0.01) impact on fruit yield, WUE, and PFPN.
Under the same irrigation condition, an increase of nitrogen level resulted in an initial increase in the economic yield and WUE, followed by decrease, while PFPN decreased   In combination, these results indicate that moderate water supply (W 0.90 and W 0.75 ) and limited nitrogen application (N 0.75 and N 0.50 ) promote the forming of the fruit resulting in higher yields, while improving WUE. Overly high levels of water (W 1.05 ) and nitrogen (N 1.00 ) supply can promote plant growth to a certain extent, but excessive growth results in reduced economic yields. Strong water stress (W 0.60 ) and nitrogen stress (N 0.25 ) are insufficient for optimal plant growth, leading to low DM ( Figure 1)

Influence of Water-Nitrogen
Regimes on the Quality of the Produce. The influence of water and nitrogen on the content of soluble sugar, capsaicin, vitamin C (V c ), nitrates, and soluble solids in the fruits is shown in Table 3. The tested regimes had a strong impact on the quality indexes ( < 0.01), and V c content was also significantly affected ( < 0.05). Comparing increasing nitrogen levels with a constant water supply, the content of soluble sugars, V c , and soluble solids first increased and then decreased, while capsaicin and nitrates contents both increased. Under the same nitrogen level, an increase of irrigation resulted in a decrease in soluble sugar and nitrates content, while V c content first increased and then decreased, and capsaicin and soluble solids contents both increased. The contents of soluble sugars, capsaicin, V c , and soluble solids provide important indexes of fruit quality, as they determine the nutritional value and flavor. A lower nitrate content of vegetables is generally preferred, whereas greenhouse cultures have a higher nitrogen content than openair cultures (Liao et al., 2011). Five indexes were calculated to assess the nutritional value of the produced fruit: soluble sugar ( 1 ), capsaicin ( 2 ), V c ( 3 ), nitrates ( 4 ), and soluble solids ( 5 ) using SPSS 18 software. The calculated contribution ratio for each index is as follows: soluble sugar: 42.851%, capsaicin: 28.923%, V c : 20.569%, nitrates: 5.779%, and soluble solids: 1.879%. The former three indexes contribute 92.342% of the total index. Thus, these three main contents were used, and the corresponding characterization values produced were 1 = 2.143, 2 = 1.446, and 3 = 1.028, respectively.
By calculation, the main content can be expressed as follows: Using the ratio of each characterization value to the sum of the values as a weighing factor, a comprehensive evaluation function was established that calculated the quality of the produced fruits, given as = 0.464 1 + 0.313 2 + 0.223 3 . Higher scores calculated with this function indicate better fruit quality.

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The Scientific World Journal The results (Table 4) show that, under the same nitrogen level, W 0.75 resulted in the highest average score, followed by W 0.90 and W 1.05 , while W 0.60 produced the lowest score. Under the same irrigation condition, N 0.50 gave the highest score, followed by N 0.75 and N 1.00 (N 0.25 was the lowest). These results again indicate that moderate irrigation (W 0.90 and W 0.75 ) and nitrogen (N 0.75 and N 0.50 ) levels are favorable for nutrients absorption into the fruits. Particularly, the soluble sugar and V c content in the fruit can be increased using these regimes.
The conditions were ranked for the obtained scores, which placed the control at the 11th position.

Discussion
Adjustment of water and fertilizer supplies is the basis of optimizing agricultural practices and facility management. Proper water management and nitrogen control can improve crop growth significantly, resulting in increased economic yields, more efficient water use, and higher quality produce with lower investment costs and higher output. Conversely, poor management of water and fertilizer can lead to increased costs, wasted use of water and nitrogen resources, and negative effects on the leaf area index of crops as well as final yields [31]. Nitrogen is of particular importance, as it directly affects vegetable growth and fruit development. Proper waternitrogen management can improve the photosynthetic assimilation of the plants and the quality of the produce [10]. The fertigation technique can provide crops with optimal supplies of water and nutrients [17,[19][20][21]32]. Based on previous research, we assessed in detail the influence of water and nitrogen supplies on the growth, photosynthesis, economic yield, WUE, PFPN, and quality of C. annuum fruit in order to define the optimal conditions for greenhouse culture of this economically important produce.
The results have identified that conditions of water levels W 0.90 and W 0.75 (90 and 75% ET 0 , resp.), in combination with nitrogen levels N 0.75 and N 0.50 (225 and 150 kg⋅hm −2 , resp.), provide an optimal window. A moderate water stress and limited nitrogen supplies promote the growth and development of fruit and result in a favorable increase of chlorophyll in the leaves, which in turn is responsible for an increase in DM. These results are in accordance with previously recorded observations [11,23,33]. Under the test conditions, the optimized water and nitrogen levels avoid excessive water and fertilizer use, while supporting proper growth and development. In contrast to our findings, Ayodele and colleagues concluded that the DM content of The Scientific World Journal 9 C. annuum positively correlates with nitrogen level supplies [34]. However, these authors tested much lower, suboptimal nitrogen levels (between 0∼75 kg⋅hm −2 ), so that any increase will be positive. This has also been observed by others [35].
Here, we compared nitrogen levels from 150 to 225 kg⋅hm −2 , which covered the complete range from suboptimal to overfertilization. Candido et al. compared four nitrogen levels (0, 100, 200, and 300 kg⋅hm −2 ) under 100% ET water level and showed that the aboveground biomass, individual fruit mass, and fruit thickness increased first and then decreased with an increase of nitrogen level [36]. Their study resulted in optimal fruit indexes at a nitrogen level of 200 kg⋅hm −2 , which is comparable to our findings, though our results indicate an optimal window instead of absolute values, which is of more practical use for farmers. The optimal water supply has also been studied by Gupta and coworkers, who compared 100% ET, 80% ET, and 60% ET in combination with three NPK levels (150 : 90 : 60 kg⋅hm −2 , at 100, 80, and 60%). Their results indicated that, under the same water level, the DM of C. annuum increased with nitrogen level, while 80% ET water level was favorable for fruit growth, giving an optimal combination of 80% ET and 80% NPK to maximize DM [33]. Our results corroborate these findings and also show the beneficial effects of a limited water stress. For sunflowers it was shown that severe drought can greatly reduce the DM, but under proper water levels, the crop growth rate can be increased by higher nitrogen levels [37]. However, for that crop the consumption of nitrogen did not change the relative growth rate and net absorption rate, consistent with findings we report here. Likewise, in previous studies concerning watermelon and muskmelon [21,38], it was concluded that moderate water and nutrient conditions are best for vegetable growth.
Our results show that nitrogen fertilizer corresponding with 150-225 kg⋅hm −2 N in combination with irrigation conditions representing from 75% to 90% ET 0 results in high individual plant and economic yields, while outside this range too much or too little water and nitrogen result in negative effects. This conclusion is consistent with previous works [10,14,39]. When zooming in on the partial factor productivity from applied nitrogen (PFPN), under the same water level, this factor decreases with an increase of nitrogen. Conversely, under the same nitrogen level, the PFPN increases first and then decreases with water supply, consistent with conclusions obtained by others [39]. Economic yields of C. annuum under test conditions have been reported as 27.29∼65.69 t⋅hm −2 with a WUE of 14.72∼32.90 kg⋅m −3 and as 29.72∼46.54 t⋅hm −2 with a WUE of 7.76∼10.71 kg⋅m −3 [40]. Yields as 21.01∼35.30 t⋅hm −2 with a WUE of 4.7∼7.9 kg⋅m −3 [23] or economic yields of 14.6∼50.3 t⋅hm −2 [37], WUE of 7.8∼12.3 kg⋅m −3 [41], or 4.1∼6.7 kg⋅m −3 [42] have also been reported. Compared to these published results, the economic yield under optimal conditions as determined here (31.63∼34.85 t⋅hm −2 with a WUE of 15.03∼16.50 kg⋅m −3 ) had greatly improved. In part, this may be due to the type of C. annuum used, while the fact that experiments were conducted in a protected environment may also have helped. However, the positive effect of the online intelligent irrigation fertilizer applicator cannot be ignored, which, when set correctly, increases the utilization efficiency of both water and fertilizer.
After comprehensive consideration of economic yield, water-nitrogen use efficiency, and fruit quality, it was concluded that W 0.90 N 0.75 resulted in the highest economic yield, with slightly reduced water-nitrogen use efficiency and quality. Although W 0.75 N 075 resulted in the highest WUE with economic yield comparable to W 0.90 N 0.75 , its nitrogen use efficiency was poor. W 0.75 N 0.50 resulted in lower economic yields. Compared to locally applied treatment W 1.05 N 1.00 our optimal condition could increase yields by 15.22% with WUE improved by 52.06%. At the same time, fruits produced under W 0.75 N 0.50 had excellent scores for contents of capsaicin, V c , and soluble solids, while the nitrates mass fraction was lower than the standard limit. In addition, the soluble sugar mass fraction was high, ensuing tasteful fruit.
In this study, principal component analysis was used to analyze the main factors affecting fruit quality. The results (Table 4) show that conditions W 0.90 , W 0.75 , N 0.75 , and N 0.50 , resulted in fruit of good quality. W 0.75 N 0.50 , W 0.90 N 0.50 , and W 0.75 N 0.75 produce the top 3 rankings, while W 0.60 N 0.25 ranks last. W 0.75 N 0.75 has the best water-nitrogen coupling effect, and W 0.60 N 0.25 confines the absorption of nutrients into the fruits, resulting in poor fruit quality. Other studies have also shown that proper water-nitrogen supply cannot only promote plant growth and fruit development [19] but also enhances fruit quality with no apparent reduction in yield [21,43,44]. These results are consistent with our work. Thus, it is plausible to improve the mass fraction of nutrients in C. annuum by adjusting water-nitrogen application. Moreover, an intelligent irrigation fertilizer can precisely apply and control fertilization based on the amount of irrigation water, providing accurate technical parameters for water and fertilizer integrated large-scale cultivation of C. annuum.

Conclusions
Experimental culture of C. annuum with precisely dosed water and nitrogen supplies in a greenhouse located in the northwest of China identified an optimal window between 75% and 90% ET 0 and between 50% and 75% of conventionally used nitrogen fertilizer, resulting in an increase of economic yields of over 20%, with a simultaneous increase in DM, PPFN, and fruit quality, and an improved WUE. These insights are extremely valuable for farming practices.

Conflicts of Interest
The authors declare that they have no conflicts of interest.