A ¹⁵N-Tracing Study to Explore the Coupling Effects of Biochar and Nitrogen Fertilizer on Tomato Growth, Yield, Nitrogen Uptake and Utilization, and the Rhizosphere Soil Environment under Root-Divide Alternative Irrigation

Abstract

To investigate the coupling effects of biochar and nitrogen fertilizer on tomato growth, nitrogen uptake and utilization (NUU), and the soil environment, a pot experiment was conducted using ¹⁵N-tracing technology from March to July 2021 and from September 2021 to January 2022. Three biochar application rates (B0, B1, and B2; 0, 3, and 6 t/hm², respectively) and three nitrogen levels (N1, N2, and N3; 150, 300, and 450 kg/hm², respectively) were set up. The results show that the growth, yield, rate of ¹⁵N uptake, nitrogen derived from soil (Ndfs), total nitrogen (TN), ¹⁵N utilization, and recovery rate of tomatoes were improved under biochar application, but nitrogen derived from fertilizer (Ndff) gradually decreased. A Pearson correlation analysis showed that the ¹⁵N uptake, Ndfs, TN, rhizosphere soil organic matter, soil organic carbon, and TN were significantly positively correlated with the yield and lycopene content of tomatoes. The comprehensive benefit to the tomatoes was evaluated based on combination weighting with the help of the technique for order preference by similarity to ideal solution (TOPSIS). This indicates that the best planting mode was the B2N2 treatment, with a biochar rate of 6 t/hm² and nitrogen levels of 300 kg/hm², under the alternative partial root-zone irrigation.

1. Introduction

There are many important variables that affect crop yield and quality, such as fertilizers, irrigation methods, and soil. Nitrogen fertilizer could be lost through various ways, such as runoff [1], volatilization [2], nitrification, and denitrification [3], and improper application may cause economic losses and environmental pollution. Nitrogen fertilizer consumption accounted for over 35% of fertilizer consumption across the world [4], while the utilization rate is only around 30% in China [5]. So, calculating the nitrogen fertilizer utilization rate (NUR), clarifying the sources and destinations of nitrogen fertilizers, and optimizing nitrogen management levels are of great significance. Alternative partial root-zone irrigation is an efficient water-saving irrigation method. By artificially maintaining partial soil dryness, a portion of the root system is kept in a relatively dry soil environment, thus controlling the stomatal aperture of the leaves, reducing water transpiration losses, enhancing the root’s ability to absorb moisture, and improving the crop irrigation water-use efficiency (IWUE). Previous research conducted on apples [6], tomatoes [7], and oranges [8] has shown that alternative partial root-zone irrigation improved IWUE and nutrient utilization efficiency compared to traditional deficit irrigation [9,10]. However, some studies have also indicated that it reduced crop yield and quality to some extent [11]. Hence, it is crucial to find soil amendments that can simultaneously enhance the yield, quality, IWUE, and nutrient utilization efficiency of crops.

Biochar is currently widely used as a soil amendment and is produced by the high-temperature pyrolysis of plant waste under anoxic conditions [12], which not only contains certain nutrients (C, N, K, Ca, etc.), but also has the characteristics of a large specific surface area, high porosity, and strong adsorption [13]. Li et al. [14] indicated that biochar changed the soil composition, affecting soil water and heat dynamics and contributing to crop growth. Research has also demonstrated that biochar improved the quality indicators of tomatoes, such as vitamins (CV_C) and soluble solids, and enhanced the water status within tomato plants, as well as their leaf gas exchange rate, plant biomass, and growth. Therefore, biochar serves as an excellent soil amendment [15,16]. Akhtar et al. [17] showed that biochar application was a new method to improve IWUE and tomato yield under alternative partial root-zone irrigation; however, there is still no consistent conclusion on the source and destination of nitrogen during tomato growth. Therefore, it is crucial to find a suitable method to elucidate the source and destination of nitrogen during tomato growth.

The ¹⁵N-tracing method is a reliable isotope-tracing technique that can accurately distinguish the source and destination of nitrogen fertilizers [18]. Li et al. [19] studied the nitrogen transfer pathway of maize in three different soils by using ¹⁵N-isotope tracing technology in a two-year field experiment and concluded that loam soil had the highest maize yield and nitrogen accumulation, followed by clay soil; sand soil had the lowest. Guo et al. [20] proposed that plastic-film mulching improved the yield and nitrogen recovery rate of maize and reduced nitrogen fertilizer loss. When the nitrogen level was 300 kg/hm², the maize yield and nitrogen utilization efficiency were the highest. However, under alternate partial root-zone irrigation, the coupling impact of biochar and nitrogen on the nitrogen source and utilization of plants is still unclear.

The aim of this study was to determine the optimal biochar and nitrogen fertilizer coupling mode that can provide a theoretical basis for improving the water and nitrogen fertilizer utilization efficiency, fruit yield, and quality, and for achieving sustainable soil utilization under alternate partial root-zone irrigation.

The main objectives of this study were as follows: (1) to explore the coupling effects of different biochar and nitrogen fertilizer application modes on tomato growth, quality, and soil environment under alternative partial root-zone irrigation; (2) to reveal the effects of biochar and nitrogen fertilizer coupling on nitrogen utilization and the nitrogen transfer pathway using the ¹⁵N-isotope-tracing method; (3) to explore the relationship between the root zone soil environment and the yield and quality of tomatoes using Pearson correlation analysis; and (4) to comprehensively evaluate tomato growth and nitrogen fertilizer utilization benefits.

2. Materials and Methods

2.1. Experimental Site

A two-season pot experiment was conducted in a greenhouse (104°23′46′′ E, 36°5′19′′ N, altitude 1869.4 m) in Weiling Township, Guya Mountain, Qilihe District Lanzhou City, Gansu province, China, from March to July 2021 (2021S) and from September 2021 to January 2022 (2021A). The greenhouse is in the east–west direction, with natural light and a wall thickness of about 70 cm, covered with polyethylene film on the top. The climate of the experimental region is a typical temperate continental climate, with cold winters and hot summers, and an average annual rainfall of 367 mm, average annual evaporation of 1180 mm, average annual temperature of 11.2 °C, and a frost-free period of 160 days.

2.2. Experimental Materials

The investigated soil was classified as yellow cultivated loessial soil, and the specific soil physical and chemical properties are shown in

Table 1. Soil physical and chemical properties before the experiment.

Season	pH	Total Nitrogen (g/kg)	Organic Matter (g/kg)	Organic Carbon (g/kg)	Bulk Density (g/cm3)	Water-Holding Capacity of Soil (%)
2021S	7.84	1.061	16.12	9.503	1.35	25.3
2021A	7.99	1.034	16.03	9.411	1.36	24.8

. The tomato variety used in the experiment was “Zhongyan 958F1”. The diameter and height of the pots were 30 cm and 34 cm. Each pot was filled with 22 kg of dry soil, passed through a 2 mm sieve, and 6 holes were set in the bottom of the pot. The plastic film was covered in the middle of the pot to prevent the water from interpenetrating the sides. A V-shaped gap was cut out in the middle of the film to plant tomatoes.

During the experiment, we first placed the V-shaped gap and sieved dry soil into the pots, and then tomato plants with four leaves were transplanted into the pot. One tomato seedling was transplanted into each pot after 7 days, and 2000 mL water was irrigated during the rejuvenation period. The ¹⁵N fertilizer used in the experiment was ¹⁵N-labeled urea with 10.10% abundance and 40% nitrogen mass fraction provided by the Shanghai Research Institute of Chemical Industry. The biochar used in the experiment was provided by the Shandong Composite Fertilizer Production Company, and the raw material was wheat straw, which cracked under the conditions of high temperature and hypoxia at 6°C. The specific physical and chemical properties of biochar are shown in

Table 2. Basic physical and chemical properties of biochar.

PH	Bulk Density (g/cm3)	Specific Surface Area (m2/g)	Total Porosity (%)	Cation Exchange Capacity (mol/kg)	Total Nitrogen (g/kg)	Fixed Carbon (g/kg)
10.23	0.19	9	67.03	60.8	13.17	400

.

2.3. Experimental Design

The experiment was conducted under alternate partial root-zone irrigation. Three biochar rates (0, 3, 6 t/hm², recorded as B0, B1, and B2, respectively) and three nitrogen fertilizer levels (150, 300, 450 kg/hm², denoted as N1, N2, and N3, respectively, equivalent to 1.06 g/pot, 2.21 g/pot, and 3.18 g/pot, respectively) were set for a total of nine treatments. Each treatment had 18 replicates, for a total of 162 pots. The levels of biochar and nitrogen fertilizer applied are shown in

Table 3. Experimental design.

Treatments	Biochar Application Rates (t/hm2)	Nitrogen Application Rates (kg/hm2)
B0N1	0	150
B0N2	0	300
B0N3	0	450
B1N1	3	150
B1N2	3	300
B1N3	3	450
B2N1	6	150
B2N2	6	300
B2N3	6	450

. The irrigation frequency was once every 2 days, alternating with partial root-zone irrigation. A Φ-20 evaporating dish was used for calculating the irrigation volume, using the following formula:

I = Kp·S·Ep

(1)

where I is the irrigation amount, mm; Kp is the crop-evaporation dish coefficient, 0.7; S is the pot area, 700 cm²; and Ep is the evaporation amount of a 20 cm standard evaporating dish between two irrigation intervals, mm.

The growth period of the tomatoes was divided into the seedling period, flowering period, and maturity period. ¹⁵N fertilizer was applied at a mass ratio of 1:2:2 in the seedling period, flowering period, and maturity stage, respectively. In the seedling period, ¹⁵N fertilizer was applied along with the base fertilizers. ¹⁵N fertilizer was applied along with irrigation water 10 days before sampling in the flowering period and maturity period. The basal fertilizer under all treatments was applied according to the levels of local farmers (94.5 kg/hm² phosphorus fertilizer and 97.5 kg/hm² potassium fertilizer).

2.4. Indices and Measurement Methods

2.4.1. Plant Growth Indicators

The plant height and stem diameter were measured every 2 days during tomato growth, which was related to the irrigation frequency. At the beginning of the experiment, three irrigation frequencies were selected, namely 1 day, 2 days, and 4 days. We found that the experimental effect of 4 days was the worst, compared to 1 day and 2 days. However, 1-day irrigation increased the investment time and manpower. Therefore, we selected 2 days for the irrigation frequency and measured the plant height and stem diameter every 2 days. The plant height was measured from the bottom of the stem with a meter ruler. The stem diameter was measured with an electronic caliper (0.01 mm) at the tomato stem base using the cross method [21].

At the end of the experiment, three representative tomato plants and soil samples were selected for each treatment. Tomato plants were divided into roots, stems, leaves, and fruits. Plant samples were rinsed and put into self-sealing bags, then brought back to the laboratory together with soil samples. Plant samples were weighed and put into an oven at 105 °C for 30 min to inactivate enzyme activity and inhibit enzymatic reactions. Finally, they were dried in a 75° oven to a constant value, and the dry mass of each tomato organ was weighed.

2.4.2. Fruits’ Yield and Quality

Three tomato plants were randomly selected from each treatment and fruits picked at maturity. They were weighed and recorded using an electronic scale (0.01 g). The average value calculated for each treatment is the yield per plant.

Using the second ear of mature tomatoes to determine the quality, five fresh fruits of uniform maturity were selected for each treatment. The soluble sugars were determined using the Anthrone method. The titratable acid was determined by the 0.1 mol/L NaOH titration method. The soluble solids were measured using a refractometer. The vitamin C was measured using the 2,6-dichlorophenol indophenol titration method. The soluble proteins were measured by Coomassie Brilliant Blue G-250 staining. The lycopene was determined with an EV300PC ultraviolet visible spectrophotometer (Thermo Fisher, Waltham, MA, USA) [22].

The WUE was calculated according to the following equation [23]:

WUE = Y/ET

(2)

where the WUE is the water-use efficiency, kg/m³; Y is the yield, kg/plant; and ET is the irrigation amount, m³.

2.4.3. Nitrogen Content

The total nitrogen content (TN) of the tomato plants was measured using a Kjeldahl nitrogen analyzer. The abundance of ¹⁵N in plants was measured using a Finnigan DELTA V Advantage isotope ratio mass spectrometer (Thermo Fisher, Waltham, MA, USA).

2.4.4. Nitrogen Utilization Indicators

The calculation formulas for nitrogen utilization indicators are as follows:

Ndff = (¹⁵N₁ − ¹⁵N₀)/ (¹⁵N₂ − ¹⁵N₀) × 100%

(Ndfs) = 1 − Ndff

¹⁵NUO = K × Ndff

¹⁵NU = ¹⁵NUR + ¹⁵NUS + ¹⁵NUL + ¹⁵NUF

Ndfso = K × Ndfs

Ndfsa = NdfsaR + NdfsaS + NdfsaL + NdfsaF

¹⁵NUr = (¹⁵NUR/TN) × 100%

¹⁵NRR = (¹⁵NSR + ¹⁵NUO)/TN × 100%

The explanation of formula symbols are shown in

Table 4. Explanation of formula symbols.

Symbol	Representative Meaning	Unit
15N0	Abundance of 15N in natural	%
15N1	Abundance of 15N in sample	%
15N2	Abundance of 15N in labeled urea	%
Ndff	Nitrogen derived from fertilizer	%
Ndfs	Nitrogen derived from soil	%
15NUO	15N uptake capacity of each organ	g/plant
K	Mass ratio of TN in each organ	/
15NU	15N uptake capacity	g/plant
15NUR, 15NUS, 15NUL, 15NUF	15N uptake capacity by roots, stems, leaves, fruits	g/plant
Ndfso	Absorbed capacity of nitrogen derived from soil by tomato organs	g/plant
Ndfsa	Capacity of nitrogen derived from soil absorbed	g/plant
NdfsaR, NdfsaS, NdfsaL, NdfsaF	Capacity of nitrogen derived from soil absorbed by roots, stems, leaves, fruits	g/plant
15NUr	15N utilization rate of tomato	%
15NUR	15N uptake rate	%
TN	Total nitrogen content	g/plant
15NRR	15N recovery rate of tomato	%
15NSR	Residue of soil 15N	g/plant

.

2.4.5. Soil Nutrient Content

The soil samples were air-dried and protected from natural light for measurement.

The soil pH was measured using a pH meter (Lei Magnet Company in Shanghai, China, model: PHS-3C). The soil organic matter was determined by the potassium dichromate volumetric method. The soil organic carbon was measured by phosphoric acid bath heating and the potassium dichromate oxidation method. The TN of soil was determined using the Kjeldahl method [24].

2.4.6. Date Analysis

The data processing, calculation, and chart making were completed using Excel 2019 and Origin 9.0. Significant difference analyses among the different treatments and comprehensive benefit analysis based on the technique for order preference by similarity to the ideal solution (TOPSIS) were carried out using SPSS 24.0.

3. Results

3.1. Coupling Effect of Biochar and Nitrogen Fertilizer Application on Tomato Growth and Yield

The plant height, stem diameter, dry matter mass, and fruit yield increased with the increasing biochar rates under the same N application level in both seasons (

Table 5. Coupling effects of biochar and nitrogen fertilizer on tomato plant height, stem diameter, dry matter mass, and yield in two growing seasons.

Treatment	2021 S					2021 A
	Plant Height (cm)	Stem Diameter (mm)	Dry Matter Mass (g/plant)	Yield (kg)	Irrigation Water-Use Efficiency (kg/m3)	Plant Height (cm)	Stem Diameter (mm)	Dry Matter Mass (g/plant)	Yield (kg)	Irrigation Water-Use Efficiency (kg/m3)
B0 N1	76.8 f	8.32 g	114.52 g	1.95 g	63.37 g	70.2 f	7.95 g	111.54 f	1.69 f	63.68 g
B0 N2	77.6 e	8.54 fg	122.42 e	2.20 de	71.49 d	71.6 e	8.12 f	120.56 de	1.94 d	73.10 c
B0 N3	80.6 d	8.75 de	116.75 fg	2.06 f	66.94 f	72.9 d	8.41 e	116.15 e	1.79 ef	67.45 f
B1 N1	78.8 f	8.81 ef	118.73 f	2.14 e	69.54 e	73.6 d	8.57 d	117.93 e	1.83 e	68.95 e
B1 N2	83.5 b	9.13 bc	133.26 b	2.43 c	78.32 c	74.4 d	8.62 d	131.72 c	2.11 bc	79.50 bc
B1 N3	83.9 b	9.19 bc	128.25 c	2.37 c	77.02 c	75.9 bc	8.79 b	125.93 d	2.06 c	77.62 bc
B2 N1	82.4 c	8.92 cd	123.35 d	2.23 de	72.47 d	75.2 c	8.71 c	123.67 d	1.92 d	72.34 d
B2 N2	84.9 a	9.36 ab	144.96 a	2.61 a	84.82 a	76.6 b	8.85 b	139.95 a	2.29 a	86.28 a
B2 N3	85.8 a	9.58 a	136.01 b	2.49 b	80.92 b	78.1 a	9.03 a	135.16 b	2.15 b	81.01 b

Note: (a, b, c, d, e, f, and g) significant differences among the same indexes of different treatments at the p = 0.05 level. The same applies below.

). Under the biochar application, the plant heights increased by 6.50~11.72% (2021S) and 7.13~11.25% (2021A), and the stem diameters increased by 0.69~15.14% (2021S) and 1.90~13.58% (2021A) compared to B0. With the increase in nitrogen fertilizer, the dry matter mass of the tomatoes increased first and then decreased under the same biochar rate. B2N2 obtained the maximum root, stem, leaf, and fruit dry matter masses of 3.83 g, 36.10 g, 41.86 g, and 63.17 g in 2021S, as well as 3.69 g, 35.59 g, 40.88 g, and 59.79 g in 2021A, respectively. The maximum yield was obtained under the B2N2 treatment, and was 2.61 kg/plant in 2021S and 2.29 kg/plant in 2021A. The lowest yield was obtained under B0N1, and was 1.95 kg/plant in 2021S and 1.69 kg/plant in 2021A.

3.2. Coupling Effect of Biochar and Nitrogen Fertilizer on Nitrogen Derived from Fertilizer Applied to Various Organs of Tomatoes

Both biochar and nitrogen fertilizer affected the fertilizer contribution rate in each organ of tomatoes (Figure 1). Under the same biochar rate, the fertilizer contribution rate in each organ of the tomato increased with the increase in nitrogen fertilizer rates. Nitrogen derived from the fertilizer decreased with biochar rates. The highest amount of nitrogen derived from fertilizer was obtained under B0N3, in which the root, stem, leaf, and fruit levels of nitrogen derived from fertilizer were 15.09%, 13.06%, 18.77%, and 16.30% in 2021S, and 15.65%, 12.75%, 18.89%, and 16.20% in 2021A, respectively. The lowest nitrogen levels derived from fertilizer were obtained under B2N1, in which the roots, stems, leaves, and fruit levels of nitrogen were 10.72%, 8.83%, 12.39%, and 10.75% in 2021S, and 9.96%, 9.52%, 12.26%, and 10.72% in 2021A, respectively. The differences in the roots, stems, leaves, and fruits between B0N3 and B2N1 were significant (p < 0.05), and were 4.37%, 4.23%, 6.38%, and 5.55% in 2021S, and 5.69%, 3.23%, 6.63%, and 5.48% in 2021A, respectively. The fertilizer contribution rate under all treatments was leaf > fruit > root > stem, indicating that the growth of fruits and leaves was more vigorous at maturity.

3.3. Coupling Effect of Biochar and Nitrogen Fertilizer on the Nitrogen Accumulation of Tomatoes

Biochar application increased the nitrogen fertilizer uptake of tomatoes, soil nitrogen, TN, the proportion of fertilizer nitrogen uptake, ¹⁵N utilization, and ¹⁵N recovery under the same nitrogen fertilizer levels in the pot experiment (

Table 6. Coupling effect of biochar and nitrogen fertilizer on nitrogen accumulation.

Season	Treatment	15 N Uptake Capacity (g/plant)	Uptake of Nitrogen Derived from Soil (g/plant)	TN Uptake (g/plant)	15 N TN Uptake (%)	15 N Utilization Rate (%)	15 N Recovery Rate (%)
2021 S	B0 N1	298.2 g	1.918 h	2.216 h	13.45 e	28.13 b	60.37 f
	B0 N2	442.3 e	2.511 f	2.953 f	14.97 cb	20.86 d	52.17 g
	B0 N3	534.3 d	2.739 e	3.273 e	16.32 a	16.80 f	49.74 h
	B1 N1	325.6 f	2.268 g	2.594 g	12.57 f	30.71 a	68.45 c
	B1 N2	530.6 d	3.134 c	3.665 c	14.49 dc	25.03 c	63.76 d
	B1 N3	614.6 c	3.375 b	3.992 b	15.41 b	19.33 e	61.73 e
	B2 N1	337.5 f	2.924 d	3.270 d	10.58 g	31.84 a	75.53 a
	B2 N2	650.4 b	3.986 a	4.636 a	14.02 d	30.68 a	72.71 b
	B2 N3	699.2 a	4.028 a	4.727 a	14.79 c	21.99 d	69.46 c
2021 A	B0 N1	286.5 h	1.888 h	2.175 h	13.18 d	27.03 c	56.87 d
	B0 N2	403.7 e	2.352 f	2.755 f	14.65 b	19.04 f	45.99 g
	B0 N3	495.1 d	2.547 e	3.042 e	16.28 a	15.57 h	42.42 h
	B1 N1	320.0 g	2.286 g	2.606 g	12.28 e	30.19 a	64.38 b
	B1 N2	473.7 d	3.021 c	3.495 c	13.56 c	22.35 d	54.31 e
	B1 N3	576.9 c	3.271 b	3.848 b	14.99 b	18.14 g	52.08 f
	B2 N1	348.5 f	2.859 d	3.208 d	10.87 f	30.88 a	72.66 a
	B2 N2	614.1 b	4.079 a	4.693 a	13.09 cd	28.97 b	64.39 b
	B2 N3	662.2 a	4.033 a	4.695 a	14.10 bc	20.82 e	57.61 c

Note: (a, b, c, d, e, f, g and h) significant differences among the same indexes of different treatments at the p = 0.05 level.

). Herein, the ¹⁵N uptake of tomatoes was the largest under B2N3 and the smallest under B0N1. Under the N3 nitrogen fertilizer level, the B2 increased ¹⁵N uptake by 30.85% and 13.76% in 2021S, and by 33.47% and 14.78% in 2021A, compared to the B0 and B1 treatments, respectively. B2N3 absorbed the highest amount of nitrogen derived from soil and obtained the highest TN uptake: 4.028 g/plant and 4.727 g/plant in 2021S, and 4.033 g/plant and 4.695 g/plant in 2021A, respectively. The lowest nitrogen level derived from soil and TN uptake were obtained under B0N1: 1.918 g/plant and 2.216 g/plant in 2021S, and 1.888 g/plant and 2.175 g/plant in 2022A, respectively. There were significant differences between B2N3 and B0N1 in both the nitrogen derived from the soil and TN uptake (p < 0.05). In addition, the plant uptake of the ¹⁵N fertilizer increased with both the nitrogen fertilizer levels and biochar rates, and reached the maximum value under B2 and N3. The proportion of ¹⁵N uptake to the total N uptake gradually decreased with the increased biochar rates. Both the ¹⁵N utilization and ¹⁵N recovery rates decreased with the increased nitrogen fertilizer levels. The B2N1 treatment had the highest ¹⁵N recovery rates of 75.53% in 2021S and 72.66% in 2021A. The B0N3 treatment had the lowest ¹⁵N recovery rates of 49.74% in 2021 and 42.42% in 2022 under the same nitrogen fertilizer level. The B1 and B2 treatments increased ¹⁵N utilization by 2.53~9.82% in 2021S and by 2.57~9.93% in 2021A, compared to B0.

3.4. Coupling Effect of Biochar and Nitrogen Fertilizer on the Quality of Tomatoes

The soluble sugars, sugar/acid ratio, vitamin C, and soluble proteins of tomato fruits were positively correlated with the biochar rates (

Table 7. Coupling effect of biochar and nitrogen fertilizer on tomato fruit quality.

Season	Treatment	Flavor Quality				Nutritional Quality
		Soluble Sugar (%)	Titration Acid (%)	Sugar/Acid Ratio	Solvable Solids (%)	Vitamin C (mg·100/g)	Soluble Protein (mg/g)	Lycopene (mg/kg)
	B0 N1	3.611 f	0.356 g	10.143 d	5.47 f	35.256 h	0.936 g	28.34 h
	B0 N2	3.694 e	0.359 fg	10.290 c	5.68 d	35.797 g	1.043 e	29.15 g
	B0 N3	3.602 f	0.362 f	9.950 f	5.36 g	35.133 i	0.885 h	28.73 g
	B1 N1	3.759 d	0.373 d	10.078 e	5.92 b	37.082 e	1.052 de	31.52 e
2021 S	B1 N2	3.857 c	0.399 b	9.667 h	6.19 a	37.158 d	1.107 c	37.63 a
	B1 N3	3.715 de	0.406 a	9.150 i	5.76 c	36.141 f	0.984 f	35.87 c
	B2 N1	3.943 b	0.368 e	10.715 b	5.71 cd	38.065 b	1.124 b	30.55 f
	B2 N2	4.162 a	0.372 de	11.188 a	5.86 b	38.373 a	1.165 a	36.94 b
	B2 N3	3.721 de	0.381 c	9.766 g	5.59 e	37.734 c	1.061 d	33.26 d
	B0 N1	3.583 de	0.361 f	9.925 b	5.38 e	34.796 e	0.794 e	30.35 h
	B0 N2	3.609 d	0.372 e	9.702 c	5.46 e	34.929 e	0.805 de	32.66 f
	B0 N3	3.499 e	0.379 e	9.232 f	5.26 f	33.865 f	0.771 f	31.43 g
	B1 N1	3.716 c	0.395 cd	9.408 e	5.88 b	35.176 d	0.887 c	34.74 d
2021 A	B1 N2	3.867 b	0.403 c	9.596 d	6.08 a	36.124 c	1.016 b	37.51 a
	B1 N3	3.665 cd	0.425 a	8.624 h	5.57 d	35.063 d	0.813 d	36.97 b
	B2 N1	3.851 b	0.383 d	10.055 b	5.64 c	36.635 b	0.964 c	33.27 e
	B2 N2	3.926 a	0.388 d	10.119 a	5.81 b	37.918 a	1.085 a	36.59 b
	B2 N3	3.684 c	0.412 b	8.942 g	5.51 d	35.883 c	0.921 c	35.76 c

Note: (a, b, c, d, e, f, g, h and i) significant differences among the same indexes of different treatments at the p = 0.05 level.

) and show a trend of increasing first and then decreasing with the nitrogen fertilizer levels, while the maximum values were found under the B2N2 treatment. The titratable acidity first increased and then decreased with the biochar rates, and was positively correlated with the nitrogen fertilizer levels, while B1N3 obtained the highest titratable acidity of the fruits. B1N2 obtained the highest soluble solids and lycopene contents. The soluble sugar content of the fruits under the B2N2 treatment reached the maximum levels, which were 4.162% in 2021S and 3.926% in 2021A. When the nitrogen level was 300 kg/hm², compared to the B0 treatment, B1 and B2 increased the soluble sugar content of the fruits by 4.41% and 12.67% in 2021S, and by 7.15% and 8.78% in 2021A, respectively. The sugar/acid ratio of the B2N2 treatment increased by 22.27% in 2021S and by 17.34% in 2021A, compared to that of the B1N3 treatment. The vitamin C content under the B2N2 treatment increased by 9.22% in 2021S and 11.97% in 2021A compared to the B0N3 treatment, respectively. When the nitrogen fertilizer level was 450 kg/hm², compared to that of the B0 treatment, the titratable acid contents of B1 and B2 increased by 5.25% and 12.15% in 2021S and by 8.71% and 12.14% in 2021A, respectively. Compared to B0N3, the soluble solid content under the B1N2 treatment increased by 15.49% in 2021S and by 15.59% in 2021A. Compared to the B0N1 treatment, the lycopene content under the B1N2 treatment increased by 32.78% in 2021S and by 23.59% in 2021A.

3.5. Coupling Effect of Biochar and Nitrogen Fertilizer on the Rhizosphere Soil Environment of Tomatoes

The soil pH value, organic matter content, organic carbon content, and TN were positively correlated with the biochar rates (

Table 8. Coupling effects of biochar and nitrogen fertilizer on the rhizosphere soil environment of tomatoes.

Treatment	2021 S				2021 A
	Organic Matter (g/kg)	Organic Carbon (g/kg)	TN (g/kg)	pH	Organic Matter (g/kg)	Organic Carbon (g/kg)	TN (g/kg)	pH
B0 N1	18.46 e	10.688 f	1.305 f	7.76 bc	18.31 f	10.616 f	1.311 f	7.75 d
B0 N2	18.59 d	10.775 e	1.431 ef	7.71 cd	18.51 e	10.684 ef	1.422 d	7.69 ef
B0 N3	18.61 d	10.794 e	1.440 e	7.72 c	18.57 e	10.691 e	1.431 b	7.67 f
B1 N1	18.91 c	10.895 d	1.434 d	7.78 cd	18.86 d	10.882 d	1.339 e	7.79 bc
B1 N2	19.04 c	10.994 c	1.465 c	7.76 de	18.98 c	10.974 c	1.464 c	7.76 cd
B1 N3	19.09 c	11.025 c	1.533 b	7.74 ef	19.05 c	11.015 b	1.485 b	7.71 e
B2 N1	19.45 b	11.203 b	1.447 d	7.82 a	19.04 cd	11.027 b	1.450 c	7.85 a
B2 N2	19.67 a	11.394 a	1.472 c	7.79 ab	19.38 b	11.258 a	1.469 bc	7.81 b
B2 N3	19.71 a	11.442 a	1.604 a	7.78 b	19.46 a	11.264 a	1.545 a	7.78 c

Note: (a, b, c, d, e and f) significant differences among the same indexes of different treatments at the p = 0.05 level.

). With the increase in the nitrogen fertilizer levels, the soil pH value gradually decreased, while the soil organic matter content, organic carbon content, and TN content gradually increased. Under the same nitrogen fertilizer level, compared to B0, B1 and B2 increased the soil organic matter by 2.56~6.25% in 2021S and by 2.68~5.07% in 2021A, the organic carbon content by 1.94~6.00% in 2021S and by 2.51~5.37% in 2021A, the TN by 2.38~11.39% in 2021S and by 2.14~10.60% in 2021A, and the pH value by 0.26~0.78% in 2021S and by 0.52~1.56% in 2021A. The coupling effects of biochar and nitrogen fertilizer improved the soil acidity, and the pH varied within 0.3 units compared to the original soil, maintaining a relatively stable value.

3.6. Effect of Nitrogen Uptake and Soil Environment on Yield and Quality of Tomatoes

Under alternate partial root-zone irrigation, the ¹⁵N uptake, the uptake of nitrogen derived from soil, and the TN uptake were significantly positively correlated with the tomato yield, titratable acid, and lycopene of the fruits (Figure 2a). Among these, the maximum correlation coefficient of the nitrogen derived from soil uptake between the TN uptake and the tomato yield was 0.75, followed by 0.69 between the ¹⁵N uptake and yield, indicating that the nitrogen content of the tomatoes had a larger correlation with the yield. Under the alternate partial root-zone irrigation, the contents of soil organic matter, organic carbon, and TN in the root zone were significantly correlated with the yield and lycopene of the fruits (Figure 1b). There were significant correlations among the pH, organic matter, and organic carbon contents with the soluble sugars, vitamin C, and soluble proteins of the tomato fruits (Figure 2b).

3.7. Evaluating the Comprehensive Benefits of Tomatoes Using the TOPSIS Method

The comprehensive benefits of tomato cultivation needed to be evaluated through multiple indicators, as each indicator had a different impact on the fruit quality and each indicator also interacted with the others. Thus, it is important to evaluate the comprehensive benefits of tomatoes for tomato production. The entropy weight method is commonly used in multi-attribute decision-making, which can objectively determine the importance of a certain indicator. The analytic hierarchy process (AHP) can subjectively determine the importance of each indicator. In this study, the tomato comprehensive benefit evaluation and analysis model was used (Figure 3). Evaluation target layer A was the comprehensive benefit of tomatoes, and layer B included flavor quality (B1), health quality (B2), nitrogen utilization (B3), and growth benefit (B4). Evaluation target layer P included soluble sugar (P1), titratable acid (P2), sugar/acid ratio (P3), soluble solids (P4), Vc (P5), soluble protein (P6), lycopene (P7), ¹⁵N utilization rate (P8), ¹⁵N recovery rate (P9), TN uptake (P10), fruits yield (P11), and WUE (P12). The entropy weight method and the AHP were used to comprehensively determine the combined weights of various indicators for calculating the comprehensive benefits of tomatoes. Finally, the comprehensive benefits of tomatoes were evaluated with the help of the technique for order preference by similarity to ideal solution (TOPSIS).

In

Table 9. Weights of comprehensive benefit indicators of tomatoes based on the analytical hierarchy process.

Layer	Judgment Matrix					Partial Weight	Final Weight	Parametric Test
B-P	Index	B1	B2	B3	B4	γi	μi	CR = 0.004 λmax = 4.014
	B1	1.000	1.301	0.802	1.021	0.253	0.253
	B2	0.769	1.000	0.830	1.109	0.229	0.229
	B3	1.247	1.205	1.000	1.224	0.289	0.289
	B4	0.979	0.902	0.817	1.000	0.229	0.229
B-P	Index	P1	P2	P3	P4	γi	μi	CR = 0.014 λmax = 4.049
	P1	1.000	1.514	1.496	1.289	0.320	0.081
	P2	0.661	1.000	1.502	1.223	0.258	0.065
	P3	0.668	0.666	1.000	1.351	0.211	0.053
	P4	0.776	0.818	0.740	1.000	0.024	0.052
B-P	Index	P5	P6		P7	γi	μi	CR = 0.000 λmax = 3.000
	P5	1.000	1.042		0.995	0.337	0.077
	P6	0.960	1.000		0.947	0.323	0.074
	P7	1.005	1.056		1.000	0.340	0.078
B-P	Index	P8	P9		P10	γi	μi	CR = 0.000 λmax = 3.000
	P8	1.000	1.021		1.135	0.350	0.101
	P9	0.979	1.000		1.104	0.342	0.099
	P10	0.881	0.906		1.000	0.309	0.089
B-P	Index	P11		P12		γi	μi	CR = 0.000 λmax = 2.000
	P11	1.000		1.045		0.511	0.117
	P12	0.957		1.000		0.489	0.112

Note: CR is the consistency parameter in the judgment matrix. If CR < 0.1, the consistency met the requirements. λ Max is the maximum eigenvalue. B, and P represent the hierarchical structure of growth efficiency indicators of tomatoes.

, the judgment matrix was constructed by comparing two factors pairwise, and the local and final weights between each indicator were calculated. The consistency test coefficients were CR < 0.1, indicating that the consistency met the requirements. In

Table 10. Weights of comprehensive benefit indicators of tomatoes based on the entropy weight method.

Index	Soluble Sugar	Titration Acid	Sugar/Acid Ratio	Soluble Solids	Vitamin C	Soluble Protein
2021S	0.0119	0.0116	0.0187	0.0102	0.0059	0.0419
2021A	0.0067	0.0118	0.0130	0.0096	0.0050	0.0664
Index	Lycopene	15N Utilization Rate	15N Recovery Rate	TN Uptake	Yield	IWUE
2021S	0.0891	0.2735	0.1059	0.3355	0.0479	0.0480
2021A	0.0250	0.2888	0.1253	0.3012	0.0737	0.0738

, the objective weights of each evaluation indicator were calculated using the entropy weight method (EWM) based on the tomato quality indicators, nitrogen utilization, and growth benefits, while the indicator matrix was constructed [25]. Simultaneously, to achieve more scientific and reasonable weights, the combined weights were calculated using AHP and EWM. Finally, the combined weight and TOPSIS method were used for obtaining the comprehensive benefit evaluation results of tomatoes. The combination weight calculation method was as follows, and the results are shown in

Table 11. Weights of comprehensive benefit indicators of tomatoes based on combination weighting.

Index	Soluble Sugar	Titration Acid	Sugar/Acid Ratio	Soluble Solids	Vitamin C	Soluble Protein
2021S	0.0097	0.0074	0.0101	0.0053	0.0046	0.0312
2021A	0.0054	0.0077	0.0069	0.0049	0.0038	0.0489
Index	Lycopene	15N Utilization Rate	15N Recovery Rate	TN Uptake	Yield	IWUE
2021S	0.0697	0.2785	0.1053	0.0431	0.0565	0.3785
2021A	0.0194	0.2913	0.1234	0.0656	0.0860	0.3366

.

$$W_i=({\theta }_i\times {\mu }_i)/({\displaystyle \sum _{i=1}^n{\theta }_i\times {\mu }_i})(n=1,2,\dots 12)$$

(3)

Here, Wi is the index combination weight; θi is the analytic hierarchy process for calculating the weights; and μ is the weight calculated by the i-entropy weight method.

From the data of the experiments, the B1 and B2 were closer than B0 (

Table 12. Evaluation results and ranking based on TOPSIS method under each treatment.

Time	Treatment	D+	D-	Si	Rank
2021S	B0N1	0.0318	0.0239	0.4290	7
	B0N2	0.0372	0.0127	0.2548	8
	B0N3	0.0424	0.0139	0.2461	9
	B1N1	0.0246	0.0353	0.5898	4
	B1N2	0.0202	0.0303	0.6003	3
	B1N3	0.0332	0.0272	0.4508	6
	B2N1	0.0219	0.0397	0.6446	2
	B2N2	0.0043	0.0455	0.9137	1
	B2N3	0.0242	0.0314	0.5642	5
2021A	B0N1	0.0539	0.0305	0.3613	7
	B0N2	0.0586	0.0184	0.2386	8
	B0N3	0.0561	0.0144	0.2045	9
	B1N1	0.0426	0.0408	0.4891	4
	B1N2	0.0373	0.0345	0.4807	5
	B1N3	0.0436	0.0348	0.4437	6
	B2N1	0.0301	0.0525	0.6354	2
	B2N2	0.0115	0.0630	0.8455	1
	B2N3	0.0324	0.0526	0.6188	3

). The highest value was obtained under the B2N2 treatment, followed by B2N1. The B0N3 treatment had the lowest score. In addition, biochar rates of 6 t/hm² and a nitrogen fertilizer level of 300 kg/hm² were beneficial for the growth and quality of tomatoes, concurrently reducing the fertilizer loss, achieving the highest comprehensive benefit of tomatoes, and obtaining a higher ¹⁵N utilization efficiency, yield, and WUE.

4. Discussion

Alternative partial root-zone irrigation improved the crop yield, WUE, and root growth [26,27] compared to traditional irrigation. In this study, the dry matter mass of tomatoes increased with the biochar and nitrogen fertilizer application rates. Under the same nitrogen fertilizer level, the plant height, stem diameter, and dry matter mass of the tomatoes improved with the biochar rates, which is similar to the previous research results obtained by Guo et al. [28]. Biochar application under the same irrigation and fertilization conditions was more beneficial for promoting crop growth and dry matter. Under the same biochar rate, the dry matter mass of tomatoes was not positively correlated with the nitrogen fertilizer levels, reaching the maximum dry matter mass of tomatoes under a nitrogen level of 300 kg/hm². It can be concluded that excessive nitrogen input inhibited tomato growth under alternative partial root-zone irrigation, while the appropriate biochar–nitrogen fertilizer coupling mode could promote the growth and dry matter accumulation of tomatoes. As for tomato fruit yield, the treatments applying biochar showed a trend of increasing first and then decreasing with increasing nitrogen fertilizer levels. Sha et al. [29] proposed that tomato fruit yield also increased with the nitrogen fertilizer level, which was below 600 kg/hm². This result may be explained by the fact that biochar applied with N fertilizer fixed more nutrients near the root zone of the crops, forming a great rhizosphere soil environment to promote crop growth and development and improving ¹⁵N utilization, thus maintaining high yield under the reduced N application.

Nitrogen fertilizer is one of the most important factors affecting crop growth and the development of crops, and the nitrogen accumulation and distribution were of great significance to the increase in the yield and quality of crops. In this study, the nitrogen derived from the fertilizer in tomato organs (root, stem, leaf, fruit) decreased gradually with the biochar application rates, indicating that biochar reduced the ¹⁵N abundance of each tomato organ. Biochar application increased the ¹⁵N uptake of tomato organs, which may indicate that biochar application increased the soil porosity, improved the soil structure, provided a better breeding environment for the survival of soil microorganisms, and improved soil fertility, thus promoting the absorption of more nutrients from the soil. The results from this study show that biochar application promoted the uptake of fertilizer nitrogen by tomatoes under the alternate partial root-zone irrigation procedure. Biochar increased the residue of fertilizer nitrogen in the soil and reduced the loss of fertilizer nitrogen. It seems possible that these results are due to the large specific surface area of biochar and the increased adsorption and fixation capacity of the soil to fertilizer, which increased the fertilizer retention in the soil. However, the loss of fertilizer nitrogen gradually increased with the increase in nitrogen application [30]. Meanwhile, this study also found that under alternate partial root-zone irrigation, high ¹⁵N recovery and utilization rates were obtained under nitrogen levels of 150 kg/hm² and 300 kg/hm². Excessive nitrogen was not helpful for improving nitrogen fertilizer utilization and recovery rates, which is similar to the research results of Liu et al. [31].

Proper biochar and nitrogen fertilizer application could improve tomato fruit quality. On the one hand, biochar with a high specific surface area, pore structure, certain mechanical strength, and small bulk density led to better aeration and water retention, reduced the leaching loss of water and nutrients, improved the activity of soil microorganisms, enhanced enzyme catalysis, provided a great soil environment for roots, and improved the efficiency of water and fertilizer utilization, which was conducive to the high fruit quality and yield of tomatoes [32]. On the other hand, alternate partial root-zone irrigation reduced the stomatal conductance and transpiration rate and improved the WUE and photosynthesis. Therefore, biochar did not significantly reduce the yield under water-saving conditions, but significantly increased fruit quality [33]. In addition, crop growth was closely related to the soil environment in the root zone. The continuous use of nitrogen fertilizer caused the soil pH value to drop by more than 1 unit [34], which might lead to soil acidification. In this study, compared to the soil pH value before the experiment, the changing range of the soil pH value under the biochar–nitrogen coupling application mode was within 0.3 units. The biochar–nitrogen coupling application mode kept the soil at a relatively stable pH value, indicating that the biochar–nitrogen coupling application mode did not significantly reduce the soil pH value and effectively prevented soil acidification caused by fertilization. Meanwhile, the biochar and nitrogen fertilizer coupling application effectively increased the contents of the soil’s organic matter, organic carbon, and TN under alternate partial root-zone irrigation. It may be that the soil benefitted from the biochar with a good pore structure and a large specific surface area. After being added to the soil, the biochar improved the microbial abundance and soil enzyme activity, which was rich in certain nutrients, further improving soil fertility in the root zone [35]. In addition, the inorganic nitrogen in the soil mainly originated from fertilizer nitrogen, so the content of the soil inorganic nitrogen increased with the nitrogen fertilizer levels, thereby increasing the TN content in the rhizosphere soil [36].

The agricultural production evaluation models played a positive guiding role in agricultural production, and effectively improved the economic benefits of agricultural production. The analytic hierarchy process, entropy weight method, and TOPSIS method are widely applied in agricultural production evaluation [37]. Yu et al. [38] used the TOPSIS method based on the entropy weight method to evaluate the experimental effect of nitrogen fertilizer levels under different irrigation upper limits in protected areas of Northeast China. Wang et al. [39] used the analytic hierarchy process and TOPSIS method to establish a comprehensive quality index and analyzed the rationality of the comprehensive quality index. In this study, the comprehensive benefit evaluation results of growth, nitrogen utilization, yield, and quality indicators using the TOPSIS method, based on combination weighting, showed that the yield and quality of tomato and nitrogen utilization improved under the biochar application rates of 6 t/hm², and the highest yield and quality of tomato and nitrogen utilization were obtained under nitrogen levels of 300 kg/hm². Reasonable nitrogen levels and biochar rates were conducive to promoting the growth of tomatoes, NUU, yield, and quality, and to obtaining the maximum benefits.

5. Conclusions

(1)

Biochar application promoted the growth and dry matter mass accumulation of tomatoes and increased the yield and WUE of tomatoes.

(2)

Under the biochar and nitrogen fertilizer coupling application mode, the nitrogen derived from the fertilizer was concentrated in the leaves and fruits of the tomatoes. Biochar application rates increased the uptake of ¹⁵N, the nitrogen derived from soil, the TN, and the proportion of fertilizer nitrogen, and increased the utilization rate and recovery rate of nitrogen fertilizer, while decreasing the nitrogen derived from fertilizer.

(3)

Biochar application increased the contents of the soil organic matter, organic carbon, and TN.

(4)

The uptake of nitrogen derived from soil and the TN, organic matter, organic carbon, and total nitrogen of the soil had a direct effect on the tomato yield, with significant correlations. B2N2 treatment with biochar rates of 6 t/hm² and a N level of 300 kg/hm² under split-root alternate irrigation are recommended.

The results from this study provide a theoretical basis for improving the water and nitrogen fertilizer utilization efficiency and fruit yield and quality, and for achieving sustainable soil utilization under alternate partial root-zone irrigation.