Responses of Soil Water, Temperature, and Yield of Apple Orchard to Straw Mulching and Supplemental Irrigation on China’s Loess Plateau

Abstract

The combination effect of straw mulching and supplemental irrigation on the soil water and heat, and the growth and productivity of mature apple trees on the Loess Plateau of China remains unclear. Field experiments were conducted in 2022 and 2023 to evaluate the combined effect of straw mulching and supplemental irrigation (two irrigation types, ring and double-row drip irrigation, and three irrigation levels: high, medium, and low irrigation level) on the soil water and temperature, growth, yield, and water productivity (WP) of a mature apple orchard. Local rainfed cultivation was used as the control (CK). The results showed that straw mulching increased soil moisture by 2.4–6.5% compared to the CK treatment. Supplemental irrigation significantly increased soil moisture in the 0–60 cm layer by 3.0–8.1%, and its effect increased with an increasing irrigation level. Straw mulching significantly reduced soil temperature by 7.8% compared to the CK treatment. Supplemental irrigation significantly increased the new shoot length and stem thickness of apple trees. Under straw mulching, a medium supplemental irrigation level significantly increased both apple yield and WP compared to the CK treatment. In this study area, it is recommended to choose a combination of straw mulching and a medium supplemental irrigation level.

1. Introduction

Apples (Malus domestica Borkh.) are adapted to semi-arid and sub-humid dry areas in warm temperate zones. China is the world’s largest apple producer, accounting for nearly half of total global apple production [1]. The Loess Plateau, which has suitable climate and soil, sufficient light, and other natural and ecological conditions for apple growth, is one of the two largest apple-producing areas in China [2]. However, this region faces a number of water resource challenges common to other semi-arid regions. These include the realities of increasing water scarcity and competition for water due to economic growth, population expansion, and climate change [3]. Consequently, the quality and yield of apples are influenced by water scarcity, with problems such as poor quality, varying fruit sizes, and fluctuating yields. Additionally, there are problems such as seasonal drought and low water productivity in this region. There is therefore an urgent need to improve water productivity, and to reduce non-beneficial water uses in this region. Therefore, water conservation measures should be taken in order to solve these problems mentioned above.

Straw mulching is widely used as a water conservation technique in farming systems [4]. Straw mulching acts as a barrier to runoff and intercepts raindrops, protecting the soil from splashing, particle detachment, and the clogging of surface soil pores, thereby contributing to the minimization of soil erosion [5]. Straw mulching can improve soil physical properties [2], increase the soil water holding capacity and water use efficiency [4,6], and adjust soil surface temperature [7,8]. Furthermore, straw mulching can improve the soil fertility and crop yield [9]. As a result, straw mulching is often adopted in water-scarce regions for crop production, both in rainfed and irrigated areas, owing to its potential to enhance water use efficiency and save water. Scientists have been exploring the effects of straw mulching on soil water and temperature and crop yield; however, most previous studies have focused on crop yield, such as maize [10], wheat [11], pea, and soybean [6]. Some studies have also evaluated the influence of straw mulching on the soil water, temperature, and yield of fruit orchards [4,12,13,14].

In this semi-arid plateau region, apple orchards are mainly located in rainfed areas with no irrigation facilities. However, precipitation in this region has a sharply uneven spatial–temporal distribution, and seasonal droughts are frequent, both of which lead to the phenomenon that precipitation does not meet the water demand of fruit trees [9]. This phenomenon eventually seriously affects the quality and yield of apple orchards in this region. Therefore, it is urgent to take measures to improve the water use efficiency [15,16]. Supplemental irrigation, especially during critical crop growth stages, is an efficient measure to improve water productivity for crop yield. Similarly, relevant research about the effects of supplementary irrigation on plant yields mainly focuses on crops [17,18]. Few studies have evaluated the influence of supplemental irrigation on apple trees in this rainfed area [19,20]. Foreign scholars have carried out relevant research in similar areas [21,22,23,24]. For example, Espinoza-Meza et al. (2023) reported that irrigation scheduling based on 100% ET_a and Ψ_stem close to −1.5 MPa could be a suitable strategy for microsprinkler-irrigated apple (cv. Fuji) orchards growing in Mediterranean climate areas [21]. Previous studies are of great significance to this study. However, these studies did not compare the effects of different drip irrigation methods on apple yields. Moreover, there are no studies on the impact of ring drip irrigation on apple yields.

In general, the research about the effects of water conservation measures on apple yield has focused on surface mulching or supplemental irrigation. However, the research about the impact of the combination of straw mulching and supplemental irrigation on apple yield is relatively weak. Some studies have explored the relationship between the combination of straw mulching and supplemental irrigation and apple yield [25]. For example, Liao et al. (2021) reported that straw mulching has the potential to increase apple yields on the Loess Plateau by improving the soil environment and regulating the growth and physiology of apple trees [25]. However, these studies about the effects of the combination of straw mulching and supplemental irrigation on yield focus on young apple trees (<10 a), and relevant research on mature apple trees (10–20 a) remains unknown. However, some scholars have found that as the age of fruit trees increases, soil moisture deficit becomes more serious [26,27]. Therefore, considering the possible divergent perspectives regarding the regulatory impact of the combination of straw mulching and supplemental irrigation on apple trees of different ages, additional research is warranted to validate these findings in diverse ages of apple trees on the Loess Plateau, China.

In this study, it is hypothesized that soil water deficit exists in the mature apple orchards on the China’s Loess Plateau under rainfed conditions, and the detrimental effects could be minimized in the mature apple orchards, provided that appropriate management measures (e.g., straw mulching and supplemental irrigation) are selected based on precipitation and soil water conditions. Considering these knowledge gaps, field experiments were conducted on the Loess Plateau of China in the 2022 and 2023 growing seasons of apple trees to evaluate the combined effect of straw mulching and supplemental irrigation on the soil temperature, soil water, soil water consumption, growth, yield, and water productivity of apple trees. Therefore, our objectives were as follows: (i) to investigate the effects of straw mulching and supplemental irrigation on soil water and temperature; (ii) to determine the influence of straw mulching and supplemental irrigation on soil water consumption; and (iii) to evaluate the effects of straw mulching and supplemental irrigation on the physiological growth, yield, and water productivity of apple trees. The above research results are expected to provide theoretical basis and technical support for surface mulching and supplemental irrigation management in apple orchards on the Loess Plateau, China.

2. Materials and Methods

2.1. Experimental Site

The field experiments were conducted at the Shishanwan Experimental Station (36°00′~36°13′ N, 110°31′~110°56′ E, altitude: 1338 m) of the National Field Scientific Observatory of Forest Ecosystems in Ji County, Shanxi Province, China, during the apple growing seasons of 2022 and 2023 (Figure 1).

This area is a typical hilly-gully region of the Loess Plateau and has a semi-arid climate. The climate is temperate continental monsoon with an annual average temperature of 10.5 °C, and annual average evapotranspiration of 1724 mm. The average number of annual sunshine hours is 2309.6 h. The annual average frost-free period is around 170 days.

The annual average precipitation is 576 mm, over 65% of which falls from June to September. Furthermore, in this study area, groundwater is more than 30 m below the ground level, with no replenishment effect on the designed soil profile.

The main vegetation types are naturally restored Quercus wutaishanica Mayr, Populus davidiana, etc., pure forests of Robinia pseudoacacia L., Pinus tabuliformis Carr., Platycladus orientalis, etc., and mixed forests of Pinus sylvestris × Sophora japonica.

The soil texture is categorized as sandy loam, with the average proportions of sand, silt, and clay in the 0–60 cm soil layer measuring 53.4 ± 1.36%, 41.3 ± 2.14%, and 5.3 ± 1.39%, respectively. Soil bulk density in the first 0–60 cm soil layer is 1.31 g/cm³. The field capacity and wilting point are 27.0% and 6.0%, respectively, for the effective rooting depth (1 m). The soil pH in the apple orchard is 8.2. The basic properties of the soil profile at the beginning of the study are presented in

Table 1. Physical and chemical properties of soil.

Indicator	Soil Bulk Density (g/cm3)	Field Capacity (%)	pH	Organic Matter (%)	Total Nitrogen (mg/kg)	Available Phosphorus (mg/kg)	Available Potassium (mg/kg)
	1.31	27.0	8.2	0.75	512.21	10.13	248.26

.

2.2. Experimental Design

Apple trees (Malus domestica Borkh.) were planted in 2006 at a density of 625 plants ha⁻¹, with plant spacing and row spacing of 4.0 m and 4.0 m, respectively. According to the climatic pattern of ‘short-branch Red Fuji’ apples, phenological stages were divided into four stages: the bud development and flowering (BBCH 00–19), the leaf expansion (BBCH 31–35), the fruit expanding (BBCH 71–75), and the fruit maturing (BBCH 81–89) [28]. Agronomic management including weeding, insecticide spraying, pruning flowers, and fruit thinning were conducted according to the local standardized orchard.

The experiments were conducted from April to September for each year. Two experimental factors, including surface mulching and drip irrigation, were considered. One type of surface mulching was set: straw mulching (SM). Two types of surface drip irrigation were considered: ring and double-row drip irrigation. Three supplemental irrigation levels of 85% (I1), 75% (I2), and 55% (I3) of field capacity (θ_f) (referred to as high, medium, and low irrigation levels, respectively) were used. Six treatments were considered, including no straw mulching + ring drip irrigation + high irrigation level (D1I1), no straw mulching + two-row drip irrigation + high irrigation level (D2I1), straw mulching + ring drip irrigation + high irrigation level (SMD1I1), straw mulching + ring drip irrigation + medium irrigation level (SMD1I2), straw mulching + ring drip irrigation + low irrigation level (SMD1I3), and straw mulching + no drip irrigation (SM). Additionally, conventional rainfed apple trees were used as the control (CK), totaling seven treatments (Figure 2). Each treatment had three replications. In total, 21 experimental plots were created. Each replication had three apple trees. The agronomic management measures of each plot were the same except for the straw mulching. For the SM treatment, the total thickness of the straw mulching layer was around 10 cm, and the straw was taken from the maize field near the apple orchards.

2.3. Irrigation and Fertilization

Two types of drip irrigation, including ring and double-row drip irrigation, were considered in the field experiments. Different arrangements for driplines were designed for the two types of drip irrigation. Specifically, for the double-row drip irrigation, driplines with an inner diameter of 16 mm were placed between every two rows of apple trees, and the distance between dripline and apple tree was around 40 cm. However, for the ring drip irrigation, driplines were arranged into a ring that circled the apple tree, which had a radius of 40 cm. The emitter spacing and discharge was 30 cm and 2 L/h, respectively. Additionally, driplines for the combined treatments of straw mulching and supplemental irrigation were arranged in the form of ring drip irrigation and were placed under the straw mulching layer.

An individual irrigation system, including a ball valve (32 mm inner diameter, Suzhou Niuwei Valve Co. Ltd., Suzhou, China), a water meter (Dipuer, SDB-002, Shandong Kaili Meter Technology Co., Ltd., Linyi, China), and a pressure gauge (Y-100, Hangzhou Fuyi Meter Co. Ltd., Hangzhou, China), was installed for each experimental plot to record the volume applied during an irrigation event and control the inlet pressure. For all irrigation events, the inlet pressure was maintained at a fixed value of 0.1 MPa.

Irrigation amount was determined using the irrigation water upper and lower limit method. An irrigation event was conducted when soil water content was below the lower limit. Specifically, when soil water content in the 0–100 cm depth for the D1I1 treatment was lower than 70% field capacity (θ_f), an irrigation event was carried out. The irrigation amount was calculated by the formula:

$$W=10\times \left({\theta }_{\mathrm{m}\mathrm{a}\mathrm{x}}-{\theta }_{\mathrm{D}1\mathrm{I}1}\right)\times r\times H$$

(1)

where W is the irrigation amount (mm); θ_max is the upper limit of irrigation (%), i.e., 85% θ_f; θ_D1I1 is the gravimetric water content (%) before irrigation for the D1I1 treatment, which was determined by Trime-T3 tube TDR system (TRIME-PICO-IPH, IMKO, Ettlingen, Germany); r is the soil bulk density (g/cm³); and H is the planned wetting depth (cm), which was set as 90 cm.

In order to study the responses of apple yield to different supplemental irrigation levels, the irrigation for the I2 and I3 treatments was applied on a date similar to the I1, but the irrigation amount was applied at a rate of 75% and 55% of the I1. This irrigation schedule resulted in a total irrigation of 67.25, 45.93, 29.43 mm for the three irrigation levels of I1, I2, and I3, respectively.

In 2022 and 2023, there were 4 and 3 irrigation events, respectively, all occurring at BBCH 00–75, with a total irrigation volume of 108.1 and 76.9 mm, respectively. Additionally, due to the effects of rainfall, snow, and freezing during the budding and flowering periods, irrigation was concentrated at the leaf expansion (BBCH 31–35) and the fruit expanding (BBCH 71–75) stages.

Basal fertilization was adopted according to the local fertilization practice. All the experimental plots were fertilized with 240 kg/hm² of compost, 120 kg/hm² of urea, 180 kg/hm² of fungal fertilizer, and 120 kg/hm² of micronutrients as a basal fertilizer in mid-March, followed by a further application of 480 kg/hm² of high-potash fertilizer in the second half of June (leaf expansion stage, BBCH 31–35). In early September (fruit maturing stage, BBCH 81–89), a water-soluble fertilizer was applied.

2.4. Field Measurements

2.4.1. Meteorological Measurements

An automatic weather station equipped with a data logger (CR1000, Campbell Scientific Inc., Logan, UT, USA) was installed in an open area approximately 300 m from the apple orchards for measuring rainfall (TE525-L, Texas Electronics, Dallas, TX, USA), temperature and humidity (HMP50, Vaisala, Helsinki, Finland), wind speed and wind direction (Model 03001, RM Young, Traverse City, MI, USA), and solar radiation (LI-200SA, LI-COR Inc., Lincoln, NE, USA). A single rainfall greater than 5 mm was regarded as the effective rainfall (Figure 3).

There were 10 and 24 effective rainfall events in 2022 and 2023, respectively (

Table 2. Effective rainfall during the different growth stages of apple trees.

Year	Growth Stage	Effective Rainfall
		Events	Amount (mm)
2022	Bud development and flowering (BBCH 00–19) Leaf expansion (BBCH 31–35)	3	33.0
	Fruit expanding (BBCH 71–75)	1	34.4
	Fruit maturing (BBCH 81–89)	5	129.2
	Whole growth season	10	207.6
2023	Bud development and flowering (BBCH 00–19)	6	69.9
	Leaf expansion (BBCH 31–35)	3	23.6
	Fruit expanding (BBCH 71–75)	12	190.9
	Fruit maturing (BBCH 81–89)	3	61.1
	Whole growth season	24	345.5

). The effective rainfall amount was 207.6 and 345.5 mm in 2022 and 2023, respectively, with the majority of the rainfall occurring during the period of fruit expanding (BBCH 71–75), which accounted for 62.2% and 55.3% of the total annual rainfall amount, respectively.

2.4.2. Soil Water and Temperature Measurements

During the growing season, volumetric water content at 20 cm intervals within 0–200 cm soil layers was measured using the TRIME-tube system (Trime-T3, IMKO Ltd., Ettlingen, Germany), which was installed below the driplines. In normal circumstances, soil water measurements were conducted at 7 day intervals. Additionally, volumetric water content was determined before and after each irrigation event, and after the effective rainfall. In this study, it is defined that soil is in a state of water deficit when the volumetric soil content is lower than 60% field capacity (i.e., 60%θ_f).

For each plot, soil temperature at 5, 10, 15, 20, and 25 cm depths were determined using a right-angle ground thermometer (YF-303, Henan Yunfei Technology Development Co. Ltd., Zhengzhou, China) at 15 day intervals. For each day, soil temperature measurements were conducted at 8:00, 10:00, 12 00, 14:00, and 16:00, respectively.

2.4.3. Apple Growth and Yield Measurements

Three new shoots were selected for each direction of east, south, west, and north of the apple tree, totaling twelve new shoots. The length and diameter of the new shoots were measured using a soft tape measure and Vernier calipers during the reproductive period.

The leaf area index (LAI) was measured using the SunScan Plant Canopy Analyzer (Delta-T Devices, Cambridge, UK) at 15 day intervals. To reduce the influence of strong light, measurements of the LAI were conducted before sunrise or after sunset.

For each treatment, three apple trees were selected for determining apple yield. For each apple tree, four directions (i.e., east, west, south, and north) were selected, and three apples were randomly selected for determining single fruit weight for each direction. Additionally, the number of fruits was counted. The apple yield for each treatment was determined by multiplying the average single fruit weight by the number of fruits.

2.5. Data Analysis

2.5.1. Actual Evapotranspiration (ET_a)

The ET_a is calculated using the formula:

$${ET}_{\mathrm{a}}=∆W+I+P+G-D-R$$

(2)

where ET_a is actual evapotranspiration (mm); ΔW is the difference in water storage in the 0–200 cm depth at the beginning and at the end of the growth period (mm); I is the irrigation amount (mm); P is the effective rainfall (mm); G is the groundwater recharge (mm); D is the deep seepage (mm); and R is the surface runoff (mm). In this study area, the soil layer is deep and the groundwater depth is below 30 m, and the G can be regarded as 0; the emitter discharge is small, and the D can be regarded as 0; and the terrain is flat, and the R can be regarded as 0. Therefore, the formula is simplified as:

$${ET}_{\mathrm{a}}=∆W+I+P$$

(3)

2.5.2. Water Productivity

The water productivity (WP) is calculated as:

$$WP=Y/{ET}_{\mathrm{a}}$$

(4)

where Y is the apple yield (t/hm²).

2.6. Statistical Analysis

The experimental data were analyzed by one-way analysis of variance (ANOVA) using the IBM SPSS statistical software package (version 27.0, SPSS Inc., Chicago, IL, USA). Multiple comparisons were performed using the least significant difference (LSD) method at a significance level of p < 0.05. Figures were generated using Origin software (v. Pro 2024, OriginLab Corp., Northampton, MA, USA).

3. Results

3.1. Soil Water Content (SWC)

For a given treatment, an increase in SWC was observed after an irrigation or rainfall event, and then it decreased gradually due to apple tree water consumption or soil evaporation. Straw mulching slightly improved soil water conditions when compared to the CK treatment. Specifically, the SM treatment slightly increased SWC in the 0–120 cm depth by 2.4–6.5% compared to the CK treatment (Figure 4), although the difference was insignificant. Moreover, the increase in SWC for the SM treatment mostly focused on the topsoil layer (i.e., 0–20 cm depth) compared to the CK treatment. Additionally, as the growth period of apple trees progressed, the difference in SWC between the SM and CK treatments became smaller (Figure 4).

As shown in Figure 4, supplemental irrigation significantly improved SWC in the 0–60 cm layer, which increased with an increasing supplemental irrigation level. Specifically, the SMD1I1, SMD1I2, and SMD1I3 treatments significantly increased SWC by 8.1%, 5.5%, and 3.3%, respectively, compared to the CK treatment. Additionally, the SMD1I1, SMD1I2, and SMD1I3 treatments significantly increased SWC by 5.1%, 2.6%, and 1.3%, respectively, compared to the SM treatment.

Figure 5 shows the SWC in the 120–200 cm layer. SWC increased with an increasing supplemental irrigation level. Supplemental irrigation significantly improved SWC in the 180–200 cm layer compared to the CK treatment. The SMD1I1, SMD1I2, and SMD1I3 treatments significantly increased SWC by 42.2%, 34.4%, and 24.8%, respectively, compared to the CK treatment. Moreover, the SMD1I1, SMD1I2, and SMD1I3 treatments significantly increased SWC by 13.9%, 7.6%, and 1.4%, respectively, compared to the SM treatment.

3.2. Soil Temperature

During the two apple growth seasons, soil temperature in all observation layers varied consistently, showing a decreasing and then an increasing trend (Figure 6 and Figure 7). Soil temperature in 2022 was lower than that in 2023 during the budding and flowering periods, but higher than that in 2023 during the leaf expansion period. There was no obvious difference in soil temperature during the latter two growth periods. Straw mulching significantly reduced soil temperature compared to unmulched treatments (Figure 6). Specifically, the SM treatment decreased soil temperature at the 5–25 cm soil layer by 7.8% compared to the CK treatment during stage I (p < 0.05) (

Table 3. Variation in soil temperature during the growth seasons in 2022 and 2023.

Treatment	2022				2023
	Ⅰ	Ⅱ	Ⅲ	Ⅳ	Ⅰ	Ⅱ	Ⅲ	Ⅳ
D1I1	16.53 ± 1.83 a	25.77 ± 1.72 ab	20.75 ± 0.87 a	18.51 ± 0.48 ab	18.37 ± 3.01 a	23.39 ± 2.91 ab	23.06 ± 1.69 a	20.56 ± 0.76 ab
D2I1	16.73 ± 1.73 a	25.68 ± 1.81 ab	20.78 ± 0.8 a	18.55 ± 0.5 ab	18.59 ± 2.91 a	23.35 ± 2.43 ab	23.09 ± 1.69 a	20.61 ± 0.71 ab
SM	15.17 ± 1.59 b	24.95 ± 1.56 c	21.06 ± 0.92 a	19.08 ± 0.62 a	16.85 ± 2.5 b	22.5 ± 2.11 b	23.56 ± 2.2 a	20.97 ± 0.82 a
SMD1I1	15.13 ± 1.04 b	25.25 ± 1.56 ac	20.82 ± 1.16 a	18.56 ± 0.47 ab	16.81 ± 1.89 b	22.96 ± 2.85 ab	23.14 ± 1.9 a	20.63 ± 0.79 ab
SMD1I2	15.2 ± 1.18 b	25.22 ± 2.27 c	20.83 ± 1.44 a	18.82 ± 0.49 a	16.89 ± 2.14 b	22.56 ± 2.66 b	23.26 ± 2.01 a	20.94 ± 0.91 a
SMD1I3	15.23 ± 1.19 b	25.04 ± 1.33 c	20.97 ± 1.16 a	18.91 ± 0.5 a	16.92 ± 2.21 b	22.76 ± 2.49 b	23.28 ± 1.71 a	21.01 ± 0.87 a
CK	16.35 ± 1.69 a	26.55 ± 2.01 a	21.32 ± 1.36 a	18.53 ± 0.49 b	18.16 ± 2.96 a	24.13 ± 3.58 a	23.57 ± 2.47 a	20.41 ± 0.74 b
	ANOVA (F)
SM	5.1 **	2.2 *	0.2 ns	2.8 *	5.0 **	2.4 *	0.6 ns	2.3 *
DII1/D2I1	0.2 ns	0.5 ns	0.4 ns	0.2 ns	0.2 ns	0.5 ns	0.4 ns	0.2 ns
SMDII1/I2/I3	6.4 **	2.9 *	0.4 ns	2.2 *	5.0 **	2.7 *	0.4 ns	2.5 *

Different lowercase letters in the same column indicate the significant difference of mulching or irrigation treatment (p < 0.05); * means significant difference at p < 0.05; ** means significant difference at p < 0.01; and ns means no significant difference. Stage I–IV: the bud development and flowering (BBCH 00–19), the leaf expansion (BBCH 31–35), the fruit expanding (BBCH 71–75), and the fruit maturing (BBCH 81–89).

). Similarly, the SMD1I1, SMD1I2, and SMD1I3 treatments decreased soil temperature at the 5–25 cm soil layer by 8.0%, 7.5%, and 7.3%, respectively, compared to the CK treatment (p < 0.05) during stage I (Table 3). Additionally, we found that the difference in soil temperature between the SM and CK treatments was decreased as the growth period progressed. Both in 2022 and 2023, the results also indicate that the influence of straw mulching on soil temperature was significant during the growth period except for stage III.

Without straw mulching, supplemental irrigation did not significantly affect soil temperature in 2022 or 2023. For instance, the soil temperature for the D1I1 and D2I1 treatments was slightly higher than that for the CK treatment; however, the difference was not significant (p > 0.05). With straw mulching, soil temperature under different levels of supplemental irrigation also did not significantly differ during the growth period (p > 0.05). The results also indicate that the influence of supplemental irrigation on soil temperature was insignificant at the 0.05 level during the growth period.

3.3. Apple Tree Growth

During the two apple growth seasons, straw mulching significantly increased new shoot length and thickness compared to unmulched treatments (

Table 4. Effects of mulching and supplemental irrigation on new shoot growth and LAI.

Treatment	2022			2023
	New Shoot Length	New Shoot Thickness	LAI	New Shoot Length	New Shoot Thickness	LAI
D1I1	44.86 ± 7.08 a	6.92 ± 0.97 ab	1.46 ± 0.16 a	46.67 ± 8.88 a	6.57 ± 0.75 b	1.52 ± 0.04 abc
D2I1	41.84 ± 7.22 ab	6.53 ± 0.98 bc	1.47 ± 0.18 a	42.66 ± 9.53 ab	6.47 ± 0.88 bc	1.62 ± 0.1 a
SM	41.06 ± 8.69 ab	6.88 ± 1.22 ab	1.34 ± 0.27 a	43.04 ± 11.35 ab	6.05 ± 1 c	1.27 ± 0.12 c
SMD1I1	40.99 ± 8.52 ab	6.28 ± 0.79 c	1.47 ± 0.26 a	42.99 ± 11.01 ab	5.53 ± 0.76 d	1.34 ± 0.09 bc
SMD1I2	37.98 ± 6.92 b	6.57 ± 1.17 bc	1.39 ± 0.28 a	39.87 ± 8.94 b	6.18 ± 1.14 bc	1.54 ± 0.24 ab
SMD1I3	43.68 ± 8.81 a	7.21 ± 1.24 a	1.64 ± 0.16 a	47.66 ± 13.85 a	7.12 ± 1.21 a	1.6 ± 0.14 a
CK	27.55 ± 9.47 c	6.18 ± 1.2 c	1.42 ± 0.12 a	28.29 ± 9.06 c	5.29 ± 1.11 d	1.39 ± 0.12 abc
	ANOVA (F)
SM	18.1 **	14.2 **	0.1 ns	14.5 **	14.1 **	0.1 ns
DII1/D2I1	15.3 **	0.9 ns	0.2 ns	16.4 **	0.3 ns	0.1 ns
SMDII1/I2/I3	13.4 **	0.4 ns	0.3 ns	14.7 **	0.1 ns	0.3 ns

Different lowercase letters in the same column indicate the significant difference of mulching or irrigation treatment (p < 0.05); ** means significant difference at p < 0.01; and ns means no significant difference.

). Specifically, the SM treatment increased new shoot length and thickness by 49.0% and 11.3%, respectively, compared to the CK treatment in 2022. In 2023, the increase in new shoot length and thickness for the SM treatment was 52.1% and 14.4%, respectively, compared to the CK treatment (p < 0.05) (Table 4). Additionally, the SMD1I1, SMD1I2, and SMD1I3 treatments increased new shoot length by 48.8%, 37.9%, and 58.5%, respectively, compared to the CK treatment in 2022 (p < 0.05). Similarly, in 2023, the new shoot length for the SMD1I1, SMD1I2, and SMD1I3 treatments was 52.0%, 40.9%, 68.5% higher than that for the CK treatment, respectively (Table 4). During the two apple growth seasons, straw mulching did not significantly affect the LAI. Additionally, we found that the difference in new shoot length between the SM and CK treatments was increased as the growth period progressed. The results also indicate that the influence of straw mulching on new shoot length and thickness was significant at the 0.01 level, while the influence of straw mulching on the LAI was insignificant during the growth period.

Without straw mulching, supplemental irrigation significantly increased new shoot length and thickness. For instance, the new shoot length for the D1I1 and D2I1 treatments was 62.8% and 51.9% higher than that for the CK treatment in an average of two years (p < 0.05). Without straw mulching, supplemental irrigation did not significantly affect the LAI (p > 0.05). With straw mulching, the new shoot length and thickness for different levels of supplemental irrigation were significantly different (p < 0.05), while the LAI under different levels of supplemental irrigation did not significantly differ (p > 0.05). Over two years, the results indicate that the influence of supplemental irrigation on new shoot length and thickness and the LAI was insignificant at the 0.05 level.

3.4. Actual Evapotranspiration of Apple Orchards

The actual evapotranspiration (ET_a) of apple orchards in 2023 ranged from 315.7 to 662.8 mm. The ET_a of apples at different growth stage was in the following order: III > I > IV > II (

Table 5. Evapotranspiration during the growth period of apple orchards.

Treatment	Different Growth Period
	Ⅰ	Ⅱ	Ⅲ	Ⅳ	Total
SM	101.96 ± 31.08 bc	45.96 ± 31.08 cd	158.76 ± 31.08 c	68.86 ± 31.08 bc	375.54 ± 142.02 ab
D1I1	135.49 ± 34.92 ab	109.39 ± 34.92 a	251.59 ± 34.92 b	102.39 ± 34.92 ab	598.87 ± 151.65 ab
D2I1	122.18 ± 48.27 ab	98.08 ± 48.27 a	262.86 ± 48.27 b	89.08 ± 48.27 ab	572.19 ± 200.05 ab
SMD1I1	129.98 ± 37.44 ab	73.98 ± 37.44 abc	313.5 ± 37.44 a	96.88 ± 37.44 ab	614.34 ± 156.96 ab
SMD1I2	147.62 ± 28.47 a	91.62 ± 28.47 ab	309.08 ± 28.47 a	114.52 ± 28.47 a	662.82 ± 97.07 a
SMD1I3	80.05 ± 62.25 c	24.05 ± 62.25 d	164.7 ± 62.25 c	46.95 ± 62.25 c	315.73 ± 271.2 b
CK	110.7 ± 23.21 abc	54.7 ± 23.22 bcd	167.5 ± 23.22 c	77.6 ± 23.21 abc	410.51 ± 87.16 ab
		ANOVA (F)
SM	8.7 ns	8.7 ns	8.7 ns	8.7 ns	1.9 ns
D1I1/D2I1	18.2 ns	49.0 **	145.9 **	18.1 ns	1.7 ns
SMD1I1/I2/I3	29.0 ns	45.7 **	89.4 **	19.3 ns	1.8 ns

Different lowercase letters in the same column indicate the significant difference of mulching or irrigation treatment (p < 0.05); ** means significant difference at p < 0.01; and ns means no significant difference. Stage I–IV: the bud development and flowering (BBCH 00–19), the leaf expansion (BBCH 31–35), the fruit expanding (BBCH 71–75), and the fruit maturing (BBCH 81–89).

). Straw mulching reduced the ET_a compared to unmulched treatments, although the influence of straw mulching on the ET_a was insignificant (p > 0.05) (Table 5). Specifically, the SM treatment decreased the ET_a by 7.9%, 16.0%, 4.9%, and 11.3% at stages I, II, III, and IV, respectively, compared to the CK treatment (p > 0.05) (Table 5). The results also indicate that the influence of straw mulching on the ET_a was insignificant at the 0.05 level during the growth period.

Without straw mulching, supplemental irrigation did not significantly affect the ET_a at stages I and IV; however, supplemental irrigation significantly increased the ET_a at stages II and III. For instance, the ET_a for the D1I1 and D2I1 treatments was slightly higher than that for the CK treatment at stage I; however, the difference was not significant (p > 0.05). With straw mulching, the ET_a under different levels of supplemental irrigation significantly differed during the growth period (p < 0.05). Specifically, the ET_a at stage I for the SMD1I1 and SMD1I2 treatments was 62.4% and 84.4% higher than that for the SMD1I3 treatment, respectively (p < 0.05). The results also indicate that the influence of supplemental irrigation on the ET_a was significant at the 0.01 level at stage II (the leaf expansion (BBCH 31–35)) and III (the fruit expanding (BBCH 71–75)); the effect of supplemental irrigation on the ET_a was insignificant at the 0.05 level at stage I (the bud development and flowering (BBCH 00–19)) and IV (the fruit maturing (BBCH 81–89)).

3.5. Apple Yield and Water Productivity

Average apple yield for all of the treatments in 2023 was 13.6% higher than that in 2022. Straw mulching slightly increased apple yield compared to the CK treatment (Figure 8). In contrast, without straw mulching, supplemental irrigation significantly increased apple yield. For instance, in 2023, the apple yield for the D1I1 and D2I1 treatments was 25.8% and 18.6% higher than that for the CK treatment. Similarly, in 2022, the D1I1 and D2I1 treatments increased apple yield by 6.9% and 6.6%, respectively, compared to the CK treatment. Additionally, in 2022, the SMD1I1, SMD1I2, and SMD1I3 treatments significantly increased apple yield by 35.8%, 30.8%, and 15.8%, respectively, compared to the CK treatment. Similarly, in 2023, apple yield for the SMD1I1, SMD1I2, and SMD1I3 treatments was 33.9%, 29.3%, and 19.9% higher than that for the CK treatment, respectively (Figure 8). Moreover, apple yield increased with an increased supplemental irrigation level. During the two apple growth seasons, the results also indicate that the influence of straw mulching on apple yield was insignificant at the 0.05 level, while the influence of supplemental irrigation on apple yield was significant at the 0.05 level.

Similar to the results of apple yield, straw mulching did not significantly affect the water productivity (p > 0.05) (Figure 8). Specifically, in 2023, the SM treatment increased water productivity by 27.6% compared to the CK treatment; however, the difference between the two treatments was insignificant (p > 0.05) (Figure 8). In contrast, without straw mulching, supplemental irrigation significantly increased water productivity. For instance, the water productivity for the D1I1 and D2I1 treatments was 42.2% and 46.9% higher than that for the CK treatment in 2022 (p < 0.05). In 2023, water productivity for the D1I1 and D2I1 treatments was 42.2% and 47.2% higher than that for the CK treatment (p < 0.05). Similarly, the SMD1I1, SMD1I2, and SMD1I3 treatments significantly increased water productivity by 48.6%, 55.5%, and 66.3%, respectively, compared to the CK treatment in 2022 (p < 0.05). In 2023, the SMD1I1, SMD1I2, and SMD1I3 treatments significantly increased water productivity by 50.5%, 55.0%, and 66.4%, respectively, compared to the CK treatment in 2023 (p < 0.05) (Figure 8). Moreover, water productivity decreased with an increased supplemental irrigation level. During the two apple growth seasons, the results also indicate that the influence of straw mulching on water productivity was insignificant at the 0.05 level, while the influence of supplemental irrigation on water productivity was significant at the 0.05 level.

4. Discussion

4.1. Effects of Straw Mulching and Supplemental Irrigation on Soil Water Content (SWC)

SWC is greatly affected by straw mulching, owing to the variations in soil hydrological properties induced by straw mulching [6]. We found that straw mulching (SM) had a significant increasing effect on soil water content, although this effect was weaker as the growing season progressed. For example, under the same irrigation level, SM treatment effectively increased the SWC in the 0–20 cm soil layer compared with the CK treatment (Figure 4). The reduced evaporation and increased rainfall infiltration could explain this phenomenon [8,29,30]. Specifically, straw mulching could improve soil structure, thereby facilitating water infiltration [8]. Moreover, straw mulching on the soil surface forms an insulating layer, which could effectively reduce or hinder the exchange of water by preventing solar radiation heat from directly reaching the soil surface [8]. This result is similar to the findings of many other studies [7,21,25]. For example, Liao et al. [8] revealed that mulching can improve soil water conditions by reducing evaporation and increasing water infiltration. Straw, as an organic mulch, forms a rainwater harvesting and absorption layer on the surface itself, and acts as an obstacle to improve the roughness of the surface. Thus, SM not only absorbs part of the rainfall, but also prevents the formation of surface ponding and runoff after rainfall [14,31,32]. Additionally, this increased effect of straw mulching on soil water condition was partly attributed to the high soil water holding capacity and water supply capacity of straw [25]. However, further research is needed on the effects of straw mulching on soil water holding capacity and water supply capacity.

The Loess Plateau region is experiencing increasingly severe droughts and rainfall does not meet the water demand of fruit trees. Moreover, soil moisture conditions become increasingly poor as fruit trees age. In this study, the apple trees were 17 years old, which makes them mature apple trees. Previous studies have reported that soil water depletion was observed under rainfed cultivation of fruit trees [19,33]. Thus, appropriate management measures (e.g., supplemental irrigation) need to be taken based on precipitation and soil water conditions. SWC is influenced by the type of drip irrigation. In this study, we found that drip irrigation type changes the soil water conditions. Specifically, the SWC in the 0–60 cm layer for the ring drip irrigation (D1I1) was slightly higher than that for the double-row parallel drip irrigation (D2I1). This was consistent with the results of a previous study conducted in the Xinjiang region wherein ring drip irrigation improved the irrigation uniformity [21]. One possible explanation for this phenomenon might be wetted soil volume. The shape of the wetted soil volume mostly overlapped with the root distribution of fruit trees, while a similar phenomenon was not observed in the double drip irrigation. Additionally, irrigation level also affects the SWC. We found that supplemental irrigation significantly improved soil moisture content in the 0–60 cm layer, which increased with an increased irrigation level (Figure 4). Additionally, we found that volumetric water content in the 0–200 cm layer for the CK treatment was 19.2% (Figure 4 and Figure 5), which was lower than the soil water deficit threshold (i.e., 60%θ_f = 21.2%), indicating that the mature apple orchard suffers from soil water deficit under rainfed conditions on China’s Loess Plateau. In contrast, volumetric water content in the 0–200 cm layer for the SM, SMD1I1, SMD1I2, SMD1I3, D1I1, and D2I1 treatments was 21.6%, 25.7%, 24.2%, 22.7%, 22.3%, and 22.0%, respectively, which were higher than 60%θ_f (Figure 4 and Figure 5). These results suggest that appropriate management measures (e.g., straw mulching and supplemental irrigation) could alleviate soil water deficit in the mature apple orchard on China’s Loess Plateau.

4.2. Effects of Straw Mulching and Supplemental Irrigation on Soil Temperature

Soil temperature affects crop growth and development and varies with solar radiation and atmospheric temperature as a process of energy absorption and release [34]. Straw forms a temperature isolation layer on the soil surface, which acts as a buffer and regulates changes in surface soil temperature, and at the same time straw reduces the fluctuation in soil temperature throughout the growing season, which is beneficial to the growth and development of the above-ground parts of the plant and the root system. Throughout the growth period, soil temperature showed an increasing trend and then a decreasing trend. In this study, soil temperature for the SM treatment was significantly lower than that for the CK treatment, and the difference gradually decreased as the growth stage progressed (Figure 6 and Figure 7, Table 2), which was consistent with the findings of Du et al. [35]. This might be due to the fact that SM could effectively retard the increase in soil temperature when soil temperature increased at the early stage, and thus the difference in soil temperature with the CK treatment was greater; however, SM could retard the decrease in soil temperature when soil temperature decreased at the late stage, so the difference with the CK treatment was reduced. On the other hand, straw decomposed into residues and organic matter, which weakened the mulching effect on soil temperature.

Additionally, an increase in canopy cover area and a decrease in bare soil area also reduced the mulching effect on soil temperature as the growth stage progressed [29]. Increased ground temperature tends to lead to earlier crop phenology, while decreased ground temperature tends often to result in later crop phenology [24]. Straw mulching can effectively reduce surface soil temperature during the early spring warm-up period, which helps to mitigate the effects of apple blossom frost [25]. However, it has also been suggested that straw and other organic matter mulching can cause surface soil temperature to continue to rise in early spring, which is unfavorable to root growth and development and affects crop growth [36]. In our study, supplemental irrigation did not significantly impact soil temperature compared to the CK treatment (Figure 6 and Figure 7, Table 2). In a previous study, Yang et al. [33] reported that irrigation did not have a significant effect on soil temperature, but under mulched conditions, higher irrigation reduced soil temperature accumulated near the surface because the insulating effect of the mulch and the larger canopy reduced the solar radiation reaching the ground.

4.3. Effects of Straw Mulching and Supplemental Irrigation on Apple Yield

Apple yield is influenced by many environmental factors, in which soil water content and soil temperature are the limiting factors in rainfed regions. In this study, straw mulching slightly increased the apple yield, although this effect was insignificant (Figure 8). This was inconsistent with the findings of Suo et al. [14], who reported that straw mulching was an appropriate technique to improve the apple yield in the gully region of the Loess Plateau. One possible explanation for these discrepancies might be the degree of maintaining straw mulching. In the above-mentioned study, the straw was added to each plot in November for each year to maintain the thickness of straw mulching; however, in our study, maintaining the thickness of straw mulching was only conducted at the early stage, thereby reducing the degree of the effect of straw mulching on soil water and temperature, eventually leading to a lower difference in apple yield between the straw mulching and the CK treatments.

The Loess Plateau region is a typical supplemental irrigation area, as seasonal drought is frequent in this region. Wang et al. [27] reported that mature trees absorbed more water from soil layers than young trees during the growing period. Excessive consumption of deep soil water inevitably results in deep soil drying and severely threatens the sustainability of apple cultivation. Supplemental irrigation could alleviate the water stress in apple trees. We found that supplemental irrigation significantly increased apple yield. For instance, the apple yield for the D1I1 and D2I1 treatments was 18.8% and 12% higher than that for the CK treatment in 2022, and it was 25.8% and 18.6% higher than that for the CK treatment in 2023 (Figure 8). These results suggests that seasonal drought exists in mature trees under rainfed cultivation, and it is necessary to take actions (e.g., supplemental irrigation, mulching, and/or their combination) to reduce the proportion of deep soil water used by apple trees to prevent the development of dried soil layers. Furthermore, under the combined treatments of straw mulching and supplemental irrigation, apple yield for the medium irrigation level (SMD1I2 treatment) was significantly higher than that for the low irrigation level (SMD1I3 treatment), whereas the difference between the high and medium irrigation levels (SMD1I1 and SMD1I2 treatments) was not obvious (Figure 8).

4.4. Implications and Limitations for Apple Cultivation and Soil Water Conservation

China’s Loess Plateau is a major apple-cultivating region, but much of the Plateau is water-limited, and the expansion of apple growing is putting pressure on soil water resources. Excessive consumption of deep soil water inevitably results in deep soil drying and severely threatens the sustainability of apple cultivation [27]. Numerous studies have attempted to identify methods for improving apple yield. Accordingly, several approaches, such as surface mulching [8,14,26] and supplemental irrigation [15,19,24], have been adopted, but little attention has been paid to the effect of the combination of straw mulching and supplemental irrigation on the soil water, temperature, and yield of mature apple trees. We found that the combination of straw mulching and supplemental irrigation significantly increased apple yield compared to the straw mulching or supplemental irrigation treatment, suggesting that water stress exists for apple trees under rainfed cultivation. Moreover, the actions (e.g., supplemental irrigation, mulching, and/or their combination) that we take could reduce the proportion of deep soil water used by apple trees, and improve apple yield. Thus, our study could provide significant insights into the restoration of degraded apple orchards on China’s Loess Plateau.

There were four limitations of this study. Firstly, this study was only conducted for two years. For perennial fruit trees, it seems that the research period is a little short. Therefore, further research should be conducted considering the effects of climate change (e.g., rainfall characteristics and accumulated temperature), i.e., with a research period that exceeds three years. Secondly, this study did not show water deficit conditions in apple trees. Thus, it is important to diagnose crop water status based on canopy temperature as a function of straw mulching and supplemental irrigation. Thirdly, straw mulching and/or supplemental irrigation may change soil water availability and influence drought stress and the hydraulic traits of apple trees, and thus has the potential to affect the water utilization of apple trees. However, our knowledge on the effects of straw mulching and/or supplemental irrigation on the water utilization of apple trees is still limited, which will affect the evaluation of the yield and quality of apple trees. Fourthly, straw mulching mainly changes the growth and development of apple trees by adjusting soil water and heat conditions, thus influencing the rooting depth of apple trees. Therefore, more research is required on root growth and distribution, which are important to explain the responses of the yield and quality of apple trees.

5. Conclusions

Field experiments were conducted on China’s Loess Plateau to evaluate the effects of straw mulching and supplemental irrigation on the soil water, temperature, growth, and yield of mature apple orchards. The following conclusions were supported by this study:

(1)

Straw mulching increased soil moisture, whereas it decreased soil temperature in the topsoil layer. Moreover, as the growth period of apple trees progressed, the influence of straw mulching on soil moisture and temperature became smaller. Supplemental irrigation significantly increased soil moisture, and its effect increased with an increasing irrigation level. Soil moisture for the combination of straw mulching and supplemental irrigation was higher than that for straw mulching or supplemental irrigation.

(2)

Both straw mulching and supplemental irrigation significantly increased new shoot length and thickness compared to unmulched treatments. Moreover, the influence of the combination of straw mulching and supplemental irrigation on apple growth was greater than the impact of a single measure. Under straw mulching, a medium supplemental irrigation level significantly increased both apple yield and water productivity compared to the control.

(3)

Although soil water depletion in mature apple orchards is inevitable, the detrimental effects could be minimized during orchard development, provided that appropriate management measures (e.g., supplemental irrigation, surface mulching, and their combination) are selected based on precipitation and soil water conditions. In this study area, it is recommended to choose a combination of straw mulching and a medium supplemental irrigation level. These findings may provide a basis for evaluating the effect of straw mulching and supplemental irrigation on water productivity in dryland apple orchards.