Effect of Deep Straw Return under Saline Conditions on Soil Nutrient and Maize Growth in Saline–Alkali Land

Abstract

To clarify the effect of tillage methods on saline–alkali land improvement and maize growth in cropland salinized to different degrees, we set up two treatments (shallow rotation (15 cm depth; CK) and deep straw return (35 cm depth; DPR)) in land characterized by three different salinization degrees and analyzed the effects of the two treatments on soil nutrient content, salinity index, chlorophyll fluorescence, growth status, and yield at three salinization levels. The results show that (1) compared with CK, alkaline N, total N, Olsen P, exchangeable K, and organic matter in saline soils were all significantly improved, and total salt and pH values were reduced by 34.01–50.79% and 2.56–7.54%, respectively, under deep straw return conditions, representing the largest values in moderately saline–alkali land. (2) Compared with CK, chlorophyll fluorescence was significantly improved, and maximum photochemical efficiency (Fv/Fm), photochemical quenching (qP), and effective quantum yield of PSII (ΦPSII) were significantly increased by 8.09–15.41%, 9.13–17.93%, and 38.79–70.83% following deep straw return treatment; these increases were the largest ones observed in moderately saline–alkali land. (3) Deep straw return promoted the growth of maize and significantly increased the yield of maize. Plant height, leaf area index, and yield increased the most in moderately saline–alkali land and increased by 6.84–21.79%, 0.59–2.28 units, and 12.78–28.07%, respectively. The yield increased by 33.89 %, which was mainly due to the increase in 1000-grain weight. The results provide a theoretical basis for taking straw return measures to improve soil and increase maize yield in saline–alkali land.

1. Introduction

The Yellow River irrigation area is a maize-dominant production area in Inner Mongolia. As the amount of Yellow River water decreases year by year, the traditional method of “washing and draining salt by large amounts of water” is being more and more restricted. Therefore, choosing a simple, efficient, convenient, and economically sustainable method to improve saline soil has become urgent in agricultural development in the Yellow River irrigation area. Salinity stress is one of the environmental limiting factors in agricultural production in arid and semiarid regions [1,2]. For this reason, arable lands partially or entirely lose their fertility. According to statistics, approximately 20% of cultivated and 33% of irrigated agricultural lands have been subjected to salinity stress [3]. Under saline stress, plants mostly exhibit dwarfism, stunted development, leaves turning yellow, reduced leaf area, and significantly reduced biomass, and these conditions may even cause plant death [4,5]. Salinity stress seriously affects individual plant morphological development. For example, plants respond to stress by reducing the rate of leaf area expansion and eventually terminate leaf expansion with the increase in salinity [6]. The reduction in leaf area expansion results in smaller photosynthetic area, weaker carbon assimilation, reduced organic matter production, and growth inhibition. Under salt stress, photosynthesis is directly and indirectly affected, so growth is significantly inhibited [7].

Numerous studies have shown that straw return can increase the yield, reduce soil salinity and pH, and improve soil fertility [8,9,10,11,12,13]. Deep straw return was shown to be even more helpful in improving soil fertility. The research study by Dikgwatlhe et al. showed that soil organic carbon content following deep straw return was higher than that obtained with other tillage measures [14]. Deep straw return also significantly improved Olsen P content in soil [15]. The results of domestic and foreign research showed that straw returning effectively reduced the environmental pollution and soil water evaporation, improved moisture holding capacity, prevented soil salt from moving upwards, and improved soil quality [16], and these effects were shown to be persistent, so this technology has broad application prospects. Both abiotic and biotic stress factors can reduce photosynthetic activity and the growth rate of plants. Therefore, it is essential to examine the changes in plant photosynthetic activity under stress [17]. Chlorophyll fluorescence has been reported as the most modern and reliable technique for measuring photosynthetic activity [18]. According to Acosta-Motos et al., photochemical and non-photochemical quenching parameters such as maximum quantum yield, photochemical quenching, non-photochemical quenching, and effective quantum yield of photochemical energy conversion in PSII could be used for detecting salt stress [19]. Zivcak et al. found that abiotic stress not only caused structural damage to PSII but also affected the process of photosynthetic electron transport, maximum photochemical efficiency (Fv/Fm), and quantum yield of PSII (ΦPSII) [20].

In the context of the severe reduction in arable land area and continuous increase in maize demand, improving soil fertility and enhancing salt tolerance in maize are effective measures for improving maize yield. Soil chemical properties could directly reflect the quality of soil. Many studies have shown that deep straw returning could significantly improve the soil chemical properties of saline–alkali land. Concomitantly, the sensitivity of plants to salt stress varies with the degree of soil salinization. The effects of different salinity on maize growth and chlorophyll fluorescence parameters have been studied. Research has shown that chlorophyll fluorescence could be used to detect the harmful effects of salinity stress on plants [21]. The effects of deep straw return on soil chemistry and maize chlorophyll fluorescence properties in different saline–alkali soils have not been studied in detail. Further findings on the synchronous effects of straw return on soil and corn growth and development in saline land are lacking. Accordingly, the main purpose of this study was to investigate the improvement in soil chemical properties after deep straw return in different saline–alkali soils and the effect on maize growth. Therefore, maize was planted in different saline–alkali soils to compare the soil chemical properties and chlorophyll fluorescence characteristics of maize following different treatments, in order to compare the effect of deep straw return on different saline–alkali soils. The aim was to reveal the mechanism of deep straw return in improving salt tolerance and promoting growth of maize in arable land of different saline degrees and provide a theoretical basis for taking straw return measures to improve soil and increase maize yield in saline–alkali land.

2. Materials and Methods

2.1. General Situation of Test Area

The experiment was conducted in Yonglian Village (41°46′9.84″ N and 108°0′33.11″ E), Wayao Village (40°35′16.56″ N and 110°47′50.88″ E), and Ershisi Qingdi Village (40°24′39.11″ N and 110°39′0.40″ E) in 2021, respectively. The annual accumulated temperature values for maize growth were 3128.5 °C in Yonglian Village, 3130.9 °C in Wayao Village, and 3189.2 °C in Ershisi Qingdi Village. Annual rainfall values during the growing season were 246.4 mm, 351.9 mm, and 358.2 mm, respectively.

The previous crop was always maize. The basic conditions of the experimental sites are shown in

Table 1. Basic conditions of experimental sites.

Location	pH	EC1:5 (mS/cm)	Soil Salt Content (g kg−1)	Organic Matter (g kg−1)	Olsen P (mg kg−1)	Exchangeable K (mg kg−1)	Available N (mg kg−1)	Total N (g kg−1)
S1	8.7	1.28	2.57	14.79	15.53	221.14	39.11	0.515
S2	9.5	2.21	3.63	5.46	5.0	176	28.49	0.367
S3	9.0	2.85	5.76	8.42	2.9	155	56.53	0.507

. According to the soil salinity grading method devised by Wang et al. in China (1.5–3, 3–5, and >5 g·kg⁻¹ of total salinity for low, medium, and high salinity, respectively) [22], Yonglian Village has lightly saline soils (S1); Wayao Village has moderately saline soils (S2); and Ershisi Qingdi Village has heavily saline soils (S3).

2.2. Experimental Design and Field Management

The experiment adopted a large-area comparison design. Treatments consisted of two soil tillage methods: deep straw return (35 cm depth; DPR) and shallow rotation (15 cm depth; CK). The experimental areas were 666.7 m². Tillage and straw return were carried out after the last season of corn harvest, and straw was crushed and returned to the field. The application rate of straw was 12 t/hm². Maize varieties were DK159 and JSH257, which are popular local varieties. Three replicates were used in this experiment, and the planting density was 75,000 plants hm². Row spacing was 60 cm. Plant spacing was 22.23 cm. Before sowing, 600 kg/hm² compound fertilizer (28-12-10) was applied as the base fertilizer with film mulching. Other management procedures followed typical field production practices.

2.3. Measurement Indicators and Methods

2.3.1. Soil Nutrient Index

On the sowing date and during the maturity stage, 0–40 cm soil samples were taken from each treatment area. After soil samples were ventilated and dried in a cool place, they were crushed to pass through a 0.15–0.25 mm soil sieve. Soil samples of different particle sizes were used to determine the essential soil nutrients according to the measurement requirements [23].

(1) Organic matter was determined using the potassium dichromate titration method. (2) Soil total N was determined using a Kjeldahl nitrogen analyzer (K-9840; Jinan, China) and the semi-micro Kjeldahl method. (3) Olsen P was determined using the NaHCO₃ (0.5 mol/L) Mo-Sb colorimetric method. (4) Exchangeable K was determined using the flame photometric method. (5) Available N was determined using the alkaline hydrolysis diffusion–absorption method.

2.3.2. Soil pH and Total Salt Content

Soil samples were taken from five soil layers, i.e., 0–10 cm, 10–20 cm, and 20–30 cm, at each test site, respectively, at sowing, before and after irrigation during the spatulation period, and in the harvest period. Soil pH was determined using an acidity meter. Soil total salinity was determined using the conductivity method.

2.3.3. Indicators of Morphological Characteristics of Maize

The following stem indicators were measured during the silking stage: (1) Plant height and ear height were measured using a steel ruler to measure the distance. (2) Green leaf area (LA) per plant and LAI were measured using the following formula [24]:

$$LA={{\displaystyle \sum }}_i^{n}L\times W\times 0.75$$

(1)

$$LAI=\frac{LA\times PD}{10,000}$$

(2)

where the L is leaf length; W is maximum leaf width; n indicates the leaf number, with i = 1, 2, 3; and PD is plant density (75,000 plants ha⁻¹).

2.3.4. Chlorophyll Fluorescence Parameters

Chlorophyll fluorescence parameters were measured using a portable, non-modulated fluorimeter Plant Efficiency Analyzer (Handy PEA; Hansatech Instruments, Kings Lynn, UK) during the spatulation stage. Closed clamps were used on each spike leaf to create dark acclimation conditions for 30 min before using the instrument. The minimum fluorescence (Fo) was measured when the leaves had adapted to the dark, and the maximum fluorescence (Fm) was measured after using the saturation pulse light for 0.8 s on these leaves. Then, using natural light as the actinic light, the saturation pulse for annihilation analysis was turned on; the fluorescence yield (F′) was determined at a random time; and the maximum fluorescence yield (F′m) was measured under light adaptation. PSII potential activity (Fv/Fo), maximum photochemical efficiency (Fv/Fm), photochemical quenching coefficient (qP), non-photochemical quenching coefficient (qN), and actual photochemical efficiency (ΦPSII) were calculated according to Rohacek’s method [25].

$${\mathrm{F}}_v∕{\mathrm{F}}_0=\frac{{\mathrm{F}}_{\mathrm{m}}-{\mathrm{F}}_0}{{\mathrm{F}}_0}$$

(3)

$${\mathrm{F}}_v∕{\mathrm{F}}_{\mathrm{m}}=\frac{{\mathrm{F}}_{\mathrm{m}}-{\mathrm{F}}_0}{{\mathrm{F}}_{\mathrm{m}}}$$

(4)

$$\mathrm{qP}=\frac{{\mathrm{F}}_{\mathrm{m}}^{\prime }- {\mathrm{F}}^{\prime }}{{\mathrm{F}}_{\mathrm{m}}^{\prime }-{\mathrm{F}}_0^{\prime }}$$

(5)

$$\mathrm{qN}=\frac{{\mathrm{F}}_{\mathrm{m}}-{\mathrm{F}}_{\mathrm{m}}^{\prime }}{{\mathrm{F}}_{\mathrm{m}}-{\mathrm{F}}_0^{\prime }}$$

(6)

$$\mathsf{\Phi }\mathrm{PSII}=\frac{{\mathrm{F}}_{\mathrm{m}}^{\prime }-{\mathrm{F}}_{\mathrm{s}}}{{\mathrm{F}}_{\mathrm{m}}^{\prime }}$$

(7)

2.3.5. Yield and Yield Components

In the physiological maturity stage, two rows in the middle of the measured production area were selected, and all plants in these rows were harvested after the removal of side plants. The number of harvested ears was counted. Ten plants with uniform ear growth were selected for the determination of ear rows, row grains, 1000-grain weight, and grain water content (measured with an LDS-1G moisture content detector), which were converted into maize yield (converted into hectare yield with 14% water content).

2.4. Statistical Analysis

Data recording and organization were performed using Microsoft Excel 2019. Analysis of variance, t-test at p < 0.05, and principal component analysis on different treatment groups were carried out using SPSS 25.0 (IBM, New York, NY, USA), and Origin 2021 (OriginLab, Northampton, MA, USA) software was used to plot graphs.

3. Results

3.1. Effect of Deep Straw Return on Soil Chemical Properties of Saline Land

3.1.1. Analysis of Variance for Soil Chemical Properties

The analysis of variance showed that the effects of different locations and tillage practices on alkaline N, total N, Olsen P, exchangeable K, organic matter, pH, and total salinity were significant at p < 0.05 (

Table 2. Variance analysis of soil physical and chemical properties with each treatment (F-value).

Source of Variation	Available N	Total N	Olsen P	Exchangeable K	Organic Matter	pH	Soil Salt Content
Location (L)	267.07 **	491.19 **	6.36 *	90.85 **	171.75 **	21.64 **	10.07 **
Tillage method (T)	18.17 **	6.92 *	14.68 **	15.65 **	19.06 **	70.42 **	45.28 **
L × T	0.16	0.28	0.10	0.01	6.57 *	10.95 **	1.90

Note: * and ** indicates significant difference (p < 0.05 and p < 0.01)

). Moreover, organic matter and pH were significantly different according to location × tillage.

3.1.2. Effect of Deep Straw Return on Soil Nutrient Characteristics of Saline Land

As shown in

Table 3. Effects of deep straw return on soil nutrient characteristics.

Location	Growth Period	Tillage Method	Soil Depth	Available N (mg kg−1)	Total N (g kg−1)	Olsen P (mg kg−1)	Exchangeable K (mg kg−1)	Organic Matter (g kg−1)
S1	Before sowing	CK	0–20	59.62 ± 2.14	0.93 ± 0.09	25.21 ± 3.91	175.47 ± 11.81	20.83 ± 2.37
			20–40	32.90 ± 1.05	0.78 ± 0.04	17.24 ± 1.39	151.73 ± 2.84	13.65 ± 1.46
		DPR	0–20	66.90 ± 2.29 *	0.97 ± 0.13	26.74 ± 1.15	183.01 ± 1.65	21.86 ± 0.45
			20–40	35.16 ± 1.29	0.84 ± 0.04	18.95 ± 0.44	167.20 ± 4.64 **	14.32 ± 1.06
	Maturity	CK	0–20	36.92 ± 1.76	1.23 ± 0.09	21.22 ± 3.87	199.99 ± 10.55	14.99 ± 0.18
			20–40	31.67 ± 4.66	1.07 ± 0.04	14.48 ± 4.57	188.03 ± 9.39	12.88 ± 0.20
		DPR	0–20	37.58 ± 0.84	1.27 ± 0.13	24.13 ± 2.21	201.35 ± 12.28	23.72 ± 2.02 **
			20–40	33.67 ± 3.15	1.13 ± 0.04	15.37 ± 0.18	194.40 ± 4.77	13.39 ± 0.03 *
S2	Before sowing	CK	0–20	43.86 ± 0.82	0.89 ± 0.03	16.59 ± 0.20	153.68 ± 9.97	15.78 ± 1.60
			20–40	20.58 ± 1.23	0.67 ± 0.07	14.81 ± 0.12	141.62 ± 0.58	12.44 ± 0.89
		DPR	0–20	45.75 ± 1.40	0.92 ± 0.11	19.08 ± 0.87 **	163.18 ± 4.01	16.41 ± 1.78
			20–40	23.03 ± 0.88 *	0.69 ± 0.09	16.05 ± 0.37 **	154.37 ± 2.77 **	13.00 ± 1.18
	Maturity	CK	0–20	33.83 ± 2.72	1.16 ± 0.13	24.81 ± 1.39	167.35 ± 11.71	24.91 ± 0.04
			20–40	23.77 ± 0.73	1.00 ± 0.03	16.27 ± 1.17	162.61 ± 8.81	13.10 ± 0.22
		DPR	0–20	41.68 ± 2.64 *	1.26 ± 0.04	28.93 ± 1.05 *	172.80 ± 4.62	25.30 ± 0.29
			20–40	25.14 ± 0.93	1.05 ± 0.06	16.46 ± 0.14	166.64 ± 10.97	13.56 ± 0.33
S3	Before sowing	CK	0–20	25.08 ± 0.88	0.35 ± 0.02	13.14 ± 1.44	144.00 ± 1.69	11.37 ± 0.26
			20–40	18.20 ± 0.70	0.32 ± 0.02	8.89 ± 0.66	119.56 ± 8.85	10.48 ± 0.16
		DPR	0–20	28.34 ± 1.01 *	0.35 ± 0.01	14.91 ± 0.79	155.40 ± 1.82 **	11.91 ± 0.18 *
			20–40	20.08 ± 0.70 *	0.34 ± 0.02	9.66 ± 1.81	121.61 ± 8.49	10.84 ± 0.14 *
	Maturity	CK	0–20	24.50 ± 4.68	0.60 ± 0.02	30.48 ± 3.65	176.70 ± 6.84	11.67 ± 0.11
			20–40	15.89 ± 2.18	0.57 ± 0.03	15.21 ± 0.52	146.81 ± 3.40	10.32 ± 0.67
		DPR	0–20	27.75 ± 2.24	0.65 ± 0.02 *	35.12 ± 5.64	179.42 ± 4.30	12.22 ± 0.37
			20–40	17.28 ± 0.17	0.59 ± 0.03	17.40 ± 0.27 **	162.37 ± 6.27 *	10.98 ± 0.53

Note: * indicates significant difference in the same group (p < 0.05), and ** indicates highly significant difference in the same group (p < 0.01) (same below).

, the soil nutrient contents following DPR increased compared with CK. At the three test locations, alkaline N, total N, Olsen P, exchangeable K, and organic matter showed an increasing trend under deep straw return conditions. Alkaline N, total N, Olsen P, exchangeable K, and organic matter content increased by 1.81–23.18%, 1.92–8.93%, 1.17–16.62%, 0.68–16.60%, and 1.58–58.20%. At test location S1, organic matter was improved in the maximum range, and alkaline N, total N, available P, and exchangeable K showed the most significant increase in S3. Soil nutrients at different test locations could be ordered as S1 > S3 > S2.

3.1.3. Effect of Deep Straw Return on Soil Salinity Characteristics of Saline Land

As can be seen in

Table 4. Effects of deep straw return on soil pH value of saline land.

Location	Growth Period	Tillage Method	pH Value			Soil Salt Content (g kg−1)
			0–10	10–20	20–30	0–10	10–20	20–30
S1	Before sowing	CK	8.08 ± 0.40	8.05 ± 0.34	8.02 ± 0.28	4.22 ± 0.12	3.57 ± 0.54	2.93 ± 0.95
		DPR	7.80 ± 0.05	7.64 ± 0.14	7.48 ± 0.24	2.57 ± 0.36 **	2.51 ± 0.06 *	2.45 ± 0.24
	Before irrigation	CK	8.72 ± 0.02	8.70 ± 0.11	8.79 ± 0.12	8.06 ± 1.45	6.68 ± 1.34	5.30 ± 1.23
		DPR	8.62 ± 0.10	8.68 ± 0.13	8.65 ± 0.24	3.90 ± 2.80	3.41 ± 1.68	2.93 ± 0.56 *
	7 days after irrigation	CK	8.68 ± 0.01	8.57 ± 0.07	8.47 ± 0.13	2.88 ± 0.75	2.93 ± 0.42	3.29 ± 0.03
		DPR	8.32 ± 0.12 **	8.36 ± 0.04 **	8.39 ± 0.05	2.15 ± 1.16	2.72 ± 0.59	2.99 ± 0.08 **
	Maturity	CK	9.51 ± 0.47	9.49 ± 0.18	9.59 ± 0.31	3.68 ± 1.99	2.91 ± 1.00	2.14 ± 0.02
		DPR	9.39 ± 0.06	9.41 ± 0.40	9.32 ± 0.32	1.48 ± 0.39	1.48 ± 0.22	1.49 ± 0.04 **
S2	Before sowing	CK	8.28 ± 0.03	8.24 ± 0.03	8.24 ± 0.02	12.24 ± 8.37	8.41 ± 4.66	4.58 ± 0.95
		DPR	8.25 ± 0.08	8.23 ± 0.27	8.18 ± 0.51	3.63 ± 0.71	3.46 ± 0.77	3.28 ± 0.83
	Before irrigation	CK	9.28 ± 0.44	9.29 ± 0.30	9.31 ± 0.15	10.18 ± 9.08	6.70 ± 4.92	4.28 ± 0.30
		DPR	8.20 ± 0.32 *	8.30 ± 0.22 **	8.40 ± 0.13 **	5.22 ± 0.47	4.75 ± 0.38	3.22 ± 0.77
	7 days after irrigation	CK	8.71 ± 0.74	8.60 ± 0.53	8.50 ± 0.32	5.40 ± 5.15	4.23 ± 0.64	4.92 ± 0.56
		DPR	7.33 ± 0.31 *	7.30 ± 0.23 *	7.27 ± 0.15 **	3.53 ± 0.73	3.73 ± 3.06	2.05 ± 0.95 *
	Maturity	CK	10.01 ± 0.85	9.89 ± 0.61	9.77 ± 0.37	7.11 ± 2.30	5.15 ± 1.34	3.20 ± 0.36
		DPR	9.54 ± 0.40	9.50 ± 0.30	9.47 ± 0.20	2.01 ± 1.89 *	1.52 ± 1.18 *	1.02 ± 0.47 **
S3	Before sowing	CK	8.89 ± 0.11	8.85 ± 0.15	8.82 ± 0.20	4.58 ± 1.36	4.32 ± 0.98	4.07 ± 0.59
		DPR	8.67 ± 0.10	8.55 ± 0.14	8.43 ± 0.17	2.97 ± 1.87	3.03 ± 1.59	3.10 ± 1.32
	Before irrigation	CK	9.70 ± 0.08	9.72 ± 0.12	9.74 ± 0.15	6.87 ± 1.95	5.89 ± 1.45	4.92 ± 0.93
		DPR	9.50 ± 0.24	9.58 ± 0.12	9.66 ± 0.01	6.57 ± 0.04	5.05 ± 0.05	3.52 ± 0.13
	7 days after irrigation	CK	8.43 ± 0.07	8.42 ± 0.10	8.41 ± 0.13	5.72 ± 0.04	7.68 ± 0.16	9.64 ± 0.35
		DPR	8.06 ± 0.21 *	8.02 ± 0.10 **	7.99 ± 0.01 **	4.3 ± 1.22	4.27 ± 1.01 **	4.24 ± 0.81 **
	Maturity	CK	9.15 ± 0.11	9.27 ± 0.15	9.39 ± 0.19	5.00 ± 3.14	4.85 ± 2.57	4.70 ± 2.00
		DPR	8.82 ± 0.11 *	8.96 ± 0.15	9.10 ± 0.20	2.29 ± 0.68	2.55 ± 0.55	2.81 ± 0.41

Note: * indicates significant difference in the same group (p < 0.05), and ** indicates highly significant difference in the same group (p < 0.01).

, soil pH did not change too much among soil layers, and total soil salinity showed a decreasing trend with the increase in soil layer depth. DPR reduced soil pH by 2.56–7.54% and total soil salinity by 34.01–50.79% compared with CK. Before sowing, with DPR, total salinity at 0–20 cm soil depth was lower than with CK (by 34.40%) in S1, and total salinity at 20–30 cm soil depth was significantly lower than with CK (by 16.38%). With DPR, the pH at 0–20 cm soil depth was significantly lower than with CK (by 3.30%) on the 7th day after irrigation (p < 0.05) in S1. The pH with DPR was lower than with CK (by 9.77–11.64% before irrigation and 14.47–15.84% on the 7th day after irrigation) in S2. In S3, total salinity and pH with DPR were reduced by 41.75% and 4.71%, respectively, on the 7th day after irrigation. On the whole, S2 had the best improvement after DPR treatment, followed by S1, while S3 had the worst improvement.

3.2. Effect of Deep Straw Return on Chlorophyll Fluorescence Characteristics of Maize for Improving Saline Land

Fluorescence parameters are variables or constants and are used to describe the photosynthetic and physiological status and mechanism of the plant. They are often used to study the relationship between photosynthesis in the plant body and its environment of action. As shown in

Table 5. Effects of deep straw return on the chlorophyll fluorescence parameters of maize in saline land.

Location	Variety	Tillage Method	Fv/Fo	Fv/Fm	qP	qN	ΦPSⅡ
S1	DK159	CK	1.73 ± 0.36 b	0.61 ± 0.07 b	0.89 ± 0.13 a	0.56 ± 0.11 a	0.37 ± 0.04 b
		DPR	2.38 ± 0.33 a,b	0.7 ± 0.03 a	0.94 ± 0.06 a	0.42 ± 0.34 a	0.51 ± 0.14 a,b
	JSH257	CK	2.36 ± 0.5 a,b	0.7 ± 0.04 a	0.87 ± 0.1 a	0.65 ± 0.21 a	0.37 ± 0.13 b
		DPR	2.58 ± 0.46 a	0.71 ± 0.03 a	0.98 ± 0.06 a	0.44 ± 0.11 a	0.57 ± 0.04 a
S2	DK159	CK	2.25 ± 0.42 b	0.75 ± 0.02 a	0.88 ± 0.05 a,b	0.41 ± 0.11 a	0.46 ± 0.01 b
		DPR	3.69 ± 0.13 a	0.9 ± 0.16 a	0.97 ± 0.06 a	0.21 ± 0.15 a	0.69 ± 0.03 a
	JSH257	CK	2.08 ± 0.37 b	0.74 ± 0.03 a	0.78 ± 0.08 b	0.51 ± 0.12 a	0.36 ± 0.06 c
		DPR	3.73 ± 1.09 a	0.82 ± 0.26 a	0.98 ± 0.02 a	0.38 ± 0.24 a	0.69 ± 0.01 a
S3	DK159	CK	2.74 ± 0.74 b	0.7 ± 0.01 b	0.8 ± 0.29 a	0.26 ± 0.11 a	0.52 ± 0.31 a
		DPR	4.23 ± 0.61 a	0.76 ± 0.04 a	0.98 ± 0.01 a	0.16 ± 0.07 a	0.77 ± 0.06 a
	JSH257	CK	2.33 ± 0.12 b	0.7 ± 0 b	0.94 ± 0.01 a	0.23 ± 0.04 a	0.61 ± 0.02 a
		DPR	4.45 ± 0.51 a	0.76 ± 0.02 a	0.99 ± 0.01 a	0.12 ± 0.08 a	0.79 ± 0.01 a
Source of variation
Location (L)			14.66 **	5.19 *	0.18	12.22 **	12.29 **
Variety (V)			0.24	0.01	0.18	0.94	0.06
Tillage method (T)			50.56 **	6.01 *	10.3 **	7.47 *	38.16 **
L × V			0.88	0.71	1.00	0.88	0.78
L × T			5.55 **	0.36	0.26	0.16	0.74
V × T			0.14	0.61	0.03	0.01	0.22
L × V × T			0.76	0.17	1.18	0.11	0.42

Note: Different lowercase letters of the same place in the same column indicate significant differences. * and ** indicates significant difference (p < 0.05 and p < 0.01).

, the PSII energy conversion potential (Fv/Fo) and the effective quantum yield of PSII (ΦPSII) were extremely significantly different among locations, between tillage practices, and considering locations × tillage practices. The maximum photochemical efficiency (Fv/Fm) was significantly different among locations and between tillage practices (p < 0.05). Non-photochemical quenching (qN) was extremely different among locations and between tillage practices (p < 0.01).

Compared with CK, with DPR, Fv/Fo, Fv/Fm, qP, and ΦPSII were increased by 23.45–72.68%, 8.09–15.41%, 9.13–17.93%, and 38.79–70.83%, respectively; however, qN decreased by 28.65–43.14 % with DPR compared with CK. Fv/Fo significantly increased by 71.66 % and 72.68 % with DPR compared with CK in S2 and S3. With DPR, Fv/Fm significantly increased by 8.57% compared with CK in S3. With DPR, ΦPSII significantly increased by 70.83% compared with CK in S2. On the whole, Fv/Fo and qN had the largest values in S3, and Fv/Fm, qP and ΦPSII had the largest increases in S2.

3.3. Effect of Deep Straw Return on Agronomic Traits of Maize for Improving Saline Land

3.3.1. Variance Analysis of Agronomic Traits

The results of the multi-factor ANOVA of agronomic traits (

Table 6. Analysis of variance of maize agronomic traits following each treatment (F-value).

Source of Variation	Plant Height (cm)	Ear Height (cm)	Leaf Area Index
Location (L)	66.03 **	134.55 **	10.53 **
Variety (V)	0.42	17.62 **	0.30
Tillage method (T)	68.36 **	79.82 **	59.86 **
L × V	2.29	6.98 **	4.43 *
L × T	4.59 *	26.03 **	3.55 *
V × T	3.44	3.37	1.09
L × V × T	1.15	3.40 *	0.72

Note: * indicates significant difference in the same group (p < 0.05), and ** indicates highly significant difference in the same group (p < 0.01).

) show that plant height, ear height, and leaf area index were all highly significantly different among locations, between tillage methods, and considering locations × tillage method. Ear height was highly significantly different between varieties, and considering locations × varieties and locations × variety × tillage method (p < 0.05).

3.3.2. Effect of Deep Straw Return on Corn Plant Height and Ear Height

As can be seen in Figure 1, the average plant height and ear height following CK were 227.72 cm and 84.83 cm in all locations. Compared with CK, plant height following DPR significantly increased by 6.84–21.79% at each test location (p < 0.05). In S1 and S2, ear height following DPR significantly increased by 7.06–44.44% compared with CK. Thus, this illustrates that DPR could effectively increase the plant height and ear height of maize in saline land, and the effect on ear height was more obvious. DPR caused the largest increase in plant height in S1 and the largest increase in ear height in S2.

3.3.3. Effect of Deep Straw Return on Leaf Area Index of Maize

As can be seen in Figure 2, the leaf area index following DPR significantly increased by 0.59–2.28 units compared with CK at the three test locations (p < 0.05). The largest increase was observed in S2, with 84.49%, followed by S1, with 60.29%, and S3, with 31.89%.

3.4. Effect of Deep Straw Return on Maize Yield and Its Constituent Factors in Saline Land

The results of the multi-factor ANOVA of maize yield and its components (

Table 7. Analysis of variance of corn yield and its constituent factors following each treatment (F-value).

Source of Variation	Effective Panicles	Grain Number per Spike	1000-Grain Weight	Yield
Location (L)	516.62 **	20.94 **	32.90 **	47.39 **
Variety (V)	2971.75 **	311.52 **	11246.20 **	1245.84 **
Tillage method (T)	108.20 **	23.43 **	5.78 *	12.67 **
L × V	39.28 **	4.12 *	27.28 **	10.90 **
L × T	304.71 **	1.94	5.90 *	0.01
V × T	128.39 **	28.95 **	77.82 **	25.65 **
L × V × T	114.22 **	1.75	8.58 **	0.07

Note: * indicates significant difference (p < 0.05), and ** indicates highly significant difference (p < 0.01).

) show that effective panicles, grain number per spike, 1000-grain weight, and yield were all highly significantly different among locations, between varieties, between tillage practices, and considering locations × varieties and varieties × tillage practices. Effective panicles and 1000-grain weight were highly significantly different according to locations × tillage practices and locations × varieties × tillage practices (p < 0.05).

In Figure 3, it can be seen that DPR could increase the number of grains per ear compared with CK by 15.09%, 5.34%, and 55.93% at the three test locations, respectively. The 1000-grain weight following DPR increased by 15.99% compared with CK in S2 (p < 0.05), but it did not increase significantly in S1 or S3. Compared with CK, the yield following DPR significantly increased by 20.30%, 28.07%, and 12.78% at the three test locations. In general, S2 had the largest increase in yield.

3.5. Principal Component Analysis of Chlorophyll Fluorescence and Yield in Soils with Different Salt Concentrations

In order to better reflect the comprehensive indicators that play a dominant role in different degrees of salinity, a principal component analysis was conducted on 11 indicators of maize, including plant height, ear height, leaf area index, Fv/Fo, Fv/Fm, qP, qN, ΦPSII, and yield. The data were first standardized to derive the principal component characteristic roots, variance contribution rates, and principal component loading matrices. As can be seen in Figure 4, the cumulative contribution to the variance of the two principal components reached 90.0%, which indicates strong information representativeness. The first principal component consisted of Fv/Fo, Fv/Fm, qN, ΦPSII, and yield. The second principal component consisted of plant height, ear height, LAI, and qP.

Using the formula, the values of the principal component scores were normalized. As can be seen in

Table 8. Principal component scores and comprehensive evaluation of each index under different degrees of salinity.

Treatment		y1	y2	y	Overview
Location	Tillage Method
S1	CK	−1.29	−0.08	−1.49	5
	DPR	−0.72	1.45	0.55	3
S2	CK	−0.42	−1.14	−1.52	6
	DPR	0.84	0.71	1.58	2
S3	CK	0.24	−1.03	−0.69	4
	DPR	1.36	0.09	1.58	1

, the comprehensive evaluation of the indicators when considering deep straw return showed that the different treatment locations could be ordered as S3 > S2 > S1. According to the comprehensive evaluation, S2 and S3 had the highest scores.

4. Discussion

Many studies have pointed out that deep straw return could effectively improve soil fertility, maize growth, and maize yield [26,27,28,29,30]. Soil pH and total salinity are often used as indicators to evaluate the effectiveness of saline land improvement. Tan et al. showed that straw return could effectively reduce soil pH and regulate soil acidity [31]. In this study, we found that total salt content following deep straw return could significantly decrease 7 days after irrigation in different saline–alkaline land. All soil salinity characteristics showed the best improvement in moderately saline land, but the soil pH of lightly saline land and soil total salt of heavily saline land were less reduced. Li et al. showed that corn straw returning could increase the content of organic matter in the soil, improve its chemicals, and increase its soil fertility, thus improving saline land [32]. Moreover, in this study, it was found that at the three test locations, soil alkaline N, total N, exchangeable K, and organic matter content were improved. Deep straw return had the greatest effect on soil exchangeable K content, but it had the least effect on soil available N content. It was similar to the result obtained by Tan et al. in non-saline land, which means that deep straw return is still an effective way to increase soil fertility in saline land [31]. It was probably due to straw return having significantly lowered soil pH, thus increasing the solubility of soil nutrients such as nitrogen, phosphorus, and potassium [33]. The increase in organic matter following treatment with deep straw return might have been due to the greater viscosity of saline–alkali land, which makes it easy to store water, so the degree of straw decomposition was greater. Moreover, deep straw return allowed straw to be present in deep soil areas, so the fusion between corn straw and soil was better. Finally, soil organic matter increased. However, no specific reason for it was found in this study, so it needs to be confirmed with further experiments. In addition, this study found that deep straw return had different effects on different levels of saline soil land; the effect on moderately saline land was the best, followed by lightly saline land, while the effect of heavily saline land was the worst.

Soil salinity adversely affects crops through two processes: osmotic stress and ionic toxicity [34]. These processes affect photosynthesis and dry matter accumulation and then reduce grain number and grain weight, eventually leading to decreased grain yield in maize [35,36]. These processes also restrain electron transport and reaction centers at PSII sites and disrupt the exoxygenation complex, making Fv/Fm significantly decrease [37,38]. In this study, deep straw return could significantly increase plant height, ear height, and leaf area index in maize in saline soil. This may have been due to the fact that straw returning can improve the soil environment, which in turn can promote the growth and development of plants and increase plant height and ear height [39]. In this study, we found that maize yield was significantly improved after deep straw return, and the yield increase extent in moderately saline land showed the highest degree of improvement, followed by lightly saline land, while the increase extent in heavily saline land was small. At the same time, deep straw return in moderately saline–alkali land increased Fv/Fm, qP, and ΦPSII the most, and the increase in yield might have been related to this. A further analysis of yield composition showed that the increase in yield was mainly attributed to the increase in 1000-grain weight. However, Zhang et al. concluded that the decrease in salinity affected the numbers of ears and grains, and grain weight, thus increasing maize yield [40]. This is different from the conclusion of this experiment. The reason might be that the above study was performed in coastal saline soil and that saline–alkali land improvement measures increased soil Ca²⁺ and raised effective spike number and grain number per spike. Therefore, we still need to carry out further research on the effect of maize yield components in soil salt ions. Overall, deep straw return in moderately saline–alkali land could improve soil and increase soil nutrients to the greatest extent, thereby improving the photosynthetic capacity of maize, such as increasing Fv/Fm, qP, and ΦPSII; promoting maize growth; and increasing yield.

5. Conclusions

Deep straw return could significantly improve soil properties and promote maize growth and development. It had different effects on soils with different degrees of salinization. In moderately saline–alkali land, the contents of total N, available N, and Olsen P increased the most; furthermore, soil pH and total salt content decreased the most. In mildly saline–alkaline land, the increase in organic matter content was the largest. In severely saline–alkaline land, the increase in exchangeable K content was the largest. The improvement in soil properties significantly enhanced the chlorophyll fluorescence parameters of maize. Fv/Fm, qP, and ΦPSII showed the largest increase in moderately saline–alkali land. Fv/Fo and qN increased the most in severely saline–alkali land. Further, maize plant height, ear height, and leaf area index also increased the most in moderately saline–alkali land. Thus, deep straw return had the best effect on the improvement in moderately saline–alkali land, which caused the largest increase in 1000-grain weight, which increased the yield.