1. Introduction
Rainfall in the Loess Plateau, west Shanxi Province, China, is limited and unevenly distributed. Fruit–crop intercropping can reduce evaporation and promote rainwater retention and infiltration, which improves the soil water environment. However, intercropping transpiration may increase the soil water consumption and intensify interspecific water competition, which reduces the system productivity and the economic benefits [1,2]. There are many types of intercropping systems used In the Loess region, but the most widely used is the apple–soybean intercropping model, because an agroforestry system with leguminous crops as the intercropping material can not only improve water utilization but also improve nitrogen uptake, which is beneficial for apple growth. However, fruit trees and crops have different degrees of resource competition due to ecological niche overlap. This competition is mainly manifested as water-resource competition [3]. In order to alleviate water competition among species, some fruit farmers collect rain using ridgeways. However, ridgeway harvesting is limited by the temporal and spatial distribution of rainfall and rainfall intensity and is also affected by the underlying surface of the ridge. When there has been no effective rainfall for a long time, a water deficit in the intercropping system will occur. This water deficit leads to more intense interspecific water competition between the apple and soybean, which results in poor growth of the soybean, lower yields, and reduced economic benefits. Long-term interspecific competition for water resources will aggravate the soil nutrient deficits and soil degradation, which will ultimately affect the sustainable development of the agricultural and forestry economy across the whole region.
Micro-irrigation is an irrigation method in which the water and nutrients required for crop growth are uniformly and accurately transported to the soil near the roots of crops at a small flow rate through a pipeline system and an irrigator installed on the final-stage pipeline. The water and nutrients are dispensed according to the water and fertilizer requirements of the crops [4,5,6]. This system significantly reduces irrigation water losses by applying appropriate irrigation according to the water requirements and by only wetting the soil near the root zone [7,8]. At the same time, the overlap among different plant niches can be reduced, which alleviates the water competition between species in the intercropping system. Previous studies have compared the effects of drip irrigation and traditional flood irrigation on soil water use during intercropping and the results showed that compared to flood irrigation, drip irrigation, a micro-irrigation method, can significantly improve the water-use efficiency (WUE) of intercropping systems, alleviate interspecific competition within the intercropping system to some extent, and improve crop yields [9,10,11]. However, different micro-irrigation methods will affect the distribution of water and fertilizer in intercropping systems and there is a cost input associated with installing and maintaining the pipeline layout. Micro-irrigation may also affect the WUE and economic benefits of the intercropping system due to differences among the water delivery pipes, irrigators, and water supply modes. At present, the differences in the water-use characteristics and the economic benefits under different micro-irrigation methods in fruit–crop intercropping systems are not clear. Therefore, to further optimize the WUE of an apple–soybean intercropping system and obtain greater economic benefits, this study selected three different micro-irrigation methods: drip irrigation under a mulch, subsurface seepage irrigation, and bubbler irrigation. These three micro-irrigation methods were combined with different irrigation levels. Then the water and root distributions of the intercropping system at different distances from the apple trees and at different soil depths were analyzed to evaluate the overall economic benefits. The results provide a theoretical basis for further optimizing the water use and economic benefits of the apple–soybean intercropping system.
2. Materials and Methods
2.1. Study Area
The study area was located in Jixian County (110°26′28″–111°07′21″ E, 35°53′13″–36°21′03″ N), Linfen City, Shanxi Province, China (Figure 1), and is a typical loess gully region with poor soil and organic matter content levels below 1%. It has a temperate continental climate. The annual average precipitation is 576 mm and 61.8% occurs between June and August; the annual average evaporation is 1723.9 mm; the annual average frost-free period is about 170 days; the average temperature is 10 °C; and the average accumulated temperature is 3357.9 °C. The average soil bulk density in the 0–60 cm layer is 1.26 g/cm3 and the average field water capacity is 27.05%. During the growth period for soybean, the temperature and rainfall generally show a trend of first rising and then decreasing from the podding stage to the filling stage (Figure 2). The average temperature during the growth period is 19.5 °C and the total rainfall is 195 mm. It is mainly fall during the branching and filling stages and accounts for 75.6% of the total rainfall over the growth period. The total irrigation amounts for the W1 (60% field capacity (Fc)), W2 (75% Fc), and W3 (90% Fc (W30) treatments were 21.57 mm, 21.56 mm, and 20.34 mm, respectively, at the podding stage. During the filling stage, the total irrigation amount was 23.54 mm in W1, 26.52 mm in W2, and 22.02 mm in W3.
2.2. Experimental Material
The apple–soybean intercropping system in the Yonggu Village, Jichang Township, and Ji County was used in this study. The apple variety, Yanfu No. 8, was planted in 2018 and did not set fruit during the trial. The apple trees followed a north–south pattern with a row spacing of 5 m × 5 m, a north–south crown width of 1.8 m, an east–west crown width of 1.3 m and a crown height of 2.7 m. The soybean variety was Jindou 37 with a row spacing of 0.3 m × 0.5 m and a plant spacing of 0.5 m in a north–south direction. The sowing date was 10 May 2022. Each cell contained one fruit tree and eight rows of soybeans and the cell area was 17 m2 (Figure 3). Since the apple plants were yet to reach the fruit set stage, the growth period of the intercropping system was based on the soybean growth period. The soybean branching stage, podding stage, filling stage, and maturing stage were 5/28–6/18, 6/19–7/25, 7/26–9/5, and 9/6–9/22, respectively.
2.3. Experimental Design
The experiment was conducted from April to September 2022 and there were two factors: micro-irrigation method and irrigation level. The micro-irrigation treatments were drip irrigation under a mulch (D), subsurface seepage irrigation (S), and bubbler irrigation (Y). The layout is shown in Figure 4. Based on the range of soil moisture contents suitable for apple and soybean, three upper irrigation levels were set. The three levels were a planned average mass moisture content in the wet soil layer of 60% (W1, low water), 75% (W2, medium water), and 90% (W3, high water) of the field water capacity (Fc). All experimental treatments were applied as mulch treatments.
There were nine experimental treatments in total and one no-flooding treatment (GL), which used the furrow-and-ridge rainwater harvesting technique. Each treatment was replicated three times, making a total of 30 test plots. Irrigation was carried out during the soybean podding and bulging stages after there was no effective precipitation for seven consecutive days. The soil water content was measured before watering and the irrigation quota was set based on the difference between the measured soil water content and the target moisture content. Drip irrigation was laid out in each row of soybeans that was subjected to that treatment with a belt spacing of 0.5 m. Underground seepage irrigation was laid out for each row of soybeans that was subject to that treatment with a belt spacing of 0.5 m and a burial depth of 30 cm. Finally, bubbler irrigation was laid out for each row of soybeans that was subject to that treatment with 20 irrigators at a spacing of 0.5 m between irrigators. All pipes and irrigators were laid near the roots of the crop. The buried drip irrigation pipe had an inner diameter of 16.0 mm, a wall thickness of 0.8 mm, and a drip head flow rate of 2.0 L/h. The under-the-membrane drip irrigation pipe had an inner diameter 16.0 mm, a wall thickness of 0.5 mm, and a drip head flow rate of 2.7 L/h. The spring-fed irrigator height was 20.0 cm and the outlet flow rate was 4.5 L/h. In this experiment, the irrigation quota for dryland crops was used to calculate the irrigation amount using the following formula:
M = 10γH(θw − θ0)(1)
where M is the irrigation amount (mm); γ is the soil bulk density in the moist layer of the soil; H is the depth of the planned moist layer in the soil (cm); θw is the target gravimetric water content of the soil; and θ0 is the soil mass water content at the time of measurement. The H values were 20, 40, and 60 cm at the branching, podding, and maturing stages, respectively.Before soybean sowing, a biochar fertilizer was applied as a base fertilizer and the application amount was 1200 kg/ha. In addition, compound fertilizer (N-P-K ratio was 15-5-25) was applied to each treatment at the pod-setting stage. The application method was to spread the fertilizer before rainfall or irrigation and the topdressing amount was 500 kg/ha.
2.4. Measurements
2.4.1. Soil Water Content Measurement
Time-domain reflectometry (TDR) probes (TRIME-PICO-IPH, IMKO, Ettlingen, Germany) were used to measure the soil water content (SWC) after sowing at observation points set at 0.3, 0.8, 1.4, and 2.0 m from the apple tree row (Figure 2) and at soil depths of 0–10, 10–20, 20–30, 30–40, 40–60, 60–80, 80–100, and 100–120 cm. Calibration was carried out using the drying method.
2.4.2. Apple and Soybean Root Growth and Distribution
Root sampling was carried out with a root auger during each soybean growing season. The samples were taken at 0.3 m, 0.8 m, 1.4 m, and 2.0 m from the apple trunks and soybean stems at a depth of 20 cm. The apple and soybean roots were collected separately. Then the roots were cleaned and the root lengths, root diameters, and root dry weights were determined. The root length and root surface area were analyzed using the WinRHIZO root-scanning system.
The root biomass density was calculated using the following equation:
RDW = W1/Vs(2)
where RDW is the root biomass density (cm/m3), W1 is the root length (cm), and Vs is the soil volume (m3).2.4.3. Soybean Yield Measurement
The soybean seeds in the sample squares were harvested and dried at 105 °C and then at 75 °C to a constant weight. They were left to cool and were then weighed. The yields were averaged and converted into hectare yields (extrapolation of yields from sample plots to the whole trial site).
2.4.4. Economic Indicator Measurement
The input amounts were aggregated, based on the local purchase price of soybeans in the current year converted into the value of production per hectare, to obtain the net income per hectare to input–output ratio based on the value of production per hectare.
2.5. Data Processing and Statistical Analysis
2.5.1. Soil Water Content Calculation
The average value was calculated using the three measurements outlined below and multiplied by the dry soil capacity to obtain the soil mass water content, which was calculated using the following formula:
SWC = γ × (W1 + W2 + W3)/3(3)
where SWC is the soil water content; W is the mass water content; W1 is the volumetric water content obtained from the first measurement; W2 is the volumetric water content obtained from the second; W3 is the volumetric water content obtained from the third; and γ is the soil capacity.2.5.2. Water Consumption Calculation
The soybean water consumption at each growth stage was calculated following the water-balance principle using the formula below:
ET = I + P + U − R − F ± ΔW(4)
where ET is the water consumption at the growth stage (mm); I represents the amount of irrigation at a particular growth stage (mm); P is the effective precipitation at that stage (mm); U is the amount of groundwater recharge (mm); R is the surface runoff (mm); F is the deep seepage; and ΔW is the difference (mm) between the water content in the 0–120 cm soil water storage layer at the beginning and the end of the growth stage. Since the terrain of the plots used in this experiment was flat, the surface runoff was considered to be zero. Groundwater recharge at the 30 m underground water depth was also considered to be zero and deep leakage is not considered in drip irrigation and was considered to be zero.2.5.3. Water-Use Efficiency
The WUE of the intercropping system was based on the soybean yield and calculated using the following formula:
WUE = GY/ET(5)
where WUE is the water-use efficiency (kg/m3) and GY is the soybean grain yield (kg/hm2).2.5.4. Economic Benefit Evaluation
Hectare Value Of Output: the sum of the value of each type of product output per hectare in an intercropping system over a given period of time;
Net Income Per Hectare: the net value of production per hectare of plot in an intercropping system over a given period of time;
Input–output Ratio: the ratio of the output value of the intercropping system to all inputs during the cultivation process.
In this study, the AHP-fuzzy comprehensive evaluation method was adopted to evaluate the economic efficiency of the intercropping system and the hierarchical analysis method (AHP) was used in the index weight calculation to determine the weights of each level of the indexes. A 1–9 ratio scaling method (Table 1) was introduced and the importance of each indicator was first rated using importance weighting analysis. Then the ratings for each indicator based on expert opinion were judged and compared to establish a judgment matrix for weight calculation (Table 2). The maximum characteristic roots and eigenvectors of the weight-judgment matrix were used to calculate the specific weight values for each level of indicators. The final weights for each indicator were determined after a comprehensive calculation of each indicator. Then, based on the calculated weight values and actual value of each level of indicators, a suitable evaluation method was selected for the economic efficiency evaluation [12,13].
The weights in the table were calculated using the square root method (Table 2) and the judgment matrix consistency test was calculated using the formula:
CR = CI/RI(6)
where CR is the stochastic consistency ratio of the judgment matrix, RI is the average random consistency index of the judgment matrix, and CI is a general consistency indicator for the judgment matrix, which is calculated asCI = (λmax − n)/(n − 1)(7)
where n is the order of the judgment matrix and λmax is the maximum characteristic root of the judgment matrix.All raw data were calculated and collated using Microsoft Excel 2010 software (Microsoft, Redmond, WA, USA) and the soil water content and apple–soybean root system markers between treatments were plotted using Origin 2021 (Origin Lab, Northampton, MA, USA) software. The three values for SWC measured vertically and horizontally were averaged, the three factors affecting SWC (micro-irrigation method, distance from the tree, and amount of irrigation) were then analyzed with an analysis of variance (ANOVA) using SPSS 27 software (IBM, Chicago, Il, USA), and the significance of the index data was tested with the least significant difference (LSD) at the 5% level.
3. Results
3.1. Spatial Distribution of Soil Water
In the horizontal direction (Figure 5), the average soil water content in the 0–120 cm soil layer increased at first, then decreased, and then increased as the distance from the trees increased and the trend was most obvious at the flowering and podding stage. Under the different micro-irrigation methods and irrigation volumes, most of the lowest SWCs appeared at 0.3 m away from the tree and the SWCs only slightly varied under the different treatments. The overall order for the soil water contents at the different growth stages was flowering and podding stage > bulging stage > mature stage > branching stage and the soil water content at the bulging stage increased by 13.6% compared to the other growth stages, with the exception of the flowering and podding stages. Under the same irrigation method, the soil water content increased as the irrigation level rose, especially in subsurface seepage irrigation. There was also an increase at the flowering and podding stage and in the soil water content, which increased by 18.6% and 4.0% in W2S and W3S, respectively, compared to W1S. Under the different micro-irrigation methods at the same irrigation level, the soil water content with bubbler irrigation was mostly higher than that of the other two micro-irrigation methods, with a maximum increase of 17.5%. The soil water content of the micro-irrigation treatment was significantly higher than that of the GL treatment, with a maximum increase of 36.1% compared to the GL treatment.
In the vertical direction (Figure 6), SWC in the 0–120 cm soil layer during the different growth periods generally increased at first, then decreased, and then increased with the increase in soil depth and most of the inflection points were at the 30–40 cm and 80–90 cm soil depths. The soil water content in the 0–40 cm soil layer changed sharply, ranging from 8.2% to 63.0%. The soil water content in the 60–120 cm soil layer tended to be stable and increased uniformly with the increase in depth. Over the whole growth period, the maximum soil water content occurred in the 100–120 cm soil layer of the W3Y treatment. In the 0–120 cm soil layer, the greater the irrigation amount, the gentler the change in the soil moisture. The soil moisture change in the W1Y treatment was the most severe and that in the W3Y treatment was the gentlest. With regards to the different micro-irrigation methods, subsurface seepage irrigation had a considerable effect on the soil moisture in the 70–90 cm soil layer and the soil moisture increased gradually when the depth exceeded 90 cm. The effect of bubbler irrigation on the soil moisture in the 10–40 cm layer was obvious and the soil water at a 40 cm depth showed an obvious downward trend, whereas the soil water at an 80 cm depth began to rise. The overall change in the soil moisture with drip irrigation under mulch between 0 and 120 cm depth was small. The analysis of variance showed that the growth stage had a very significant effect on the soil moisture content in a vertical direction (p < 0.01). The soil moisture content of the different treatments was lowest at the maturity stage and highest at the flowering and podding stages.
The slope for the SWC change with the distance from the apple trees in the soybean treatments was analyzed (Table 3). The soil water content was generally positively correlated with the distance from trees during the whole growth period except for individual treatments. The only exceptions to this were the W1S, W1Y, and W2Y treatments. Furthermore, it was significantly or extremely significantly positively correlated with the increase in the distance from trees during the branching period. There was some negative correlation between W1 (60% Fc) and W2 (75% Fc) in the bubbler irrigation treatment at the podding stage and filling stage. Under the different micro-irrigation methods, the overall slope for drip irrigation under a mulch was greatest and all the slopes were generally positive. The only exceptions were some negative values that appeared under bubbler irrigation. The slope for drip irrigation under a mulch and subsurface seepage irrigation at the different growth stages increased before the decrease with the increase in irrigation amount and the highest slope values mostly appeared when the W2 treatment was applied. Under bubbler irrigation, the slope showed an upward trend over all the growth stages as the irrigation amount increased and the maximum value appeared in the W3 treatment. The slope data for the different distances from the trees and soil water content showed that the competition between species was greatest when drip irrigation under a mulch was used. The incline of the negative slope was the highest under bubbler irrigation, indicating that the water use and water consumption by the crops under bubbler irrigation were different from those under the other treatments.
3.2. Effects of Micro-Irrigation Water Regulation on the Vertical Root Distributions of Apple and Soybean
The vertical distribution characteristics of the apple and soybean root length densities (Figure 7) showed that the total apple root length density reached a maximum and minimum under W3Y and W2S, respectively. Under bubbler irrigation, the apple root length density increased with the increase in irrigation amount and W3Y increased by 33.6% and 69.0% compared to W1 and W2, respectively. Compared to bubbler irrigation, the soybean roots under subsurface seepage irrigation and drip irrigation under a mulch were mainly distributed in the 0–20 cm layer, especially in W1D and W1S. Except for the W2S treatment, the apple roots were mainly distributed in the 20 cm–60 cm soil layer, accounting for 51.1–82.0% of the total root length density in the 0–80 cm soil layer. The root distribution quantity in the vertical direction of the W3 irrigation treatment significantly increased by 30.8%, 84.1%, and 146.5% compared to GL, respectively, whereas the W1 and W2 irrigation volumes did not significantly increase the root density. The root density of W3Y soybean was significantly greater than that of the other treatments. Except for W3S, the soybean roots were mainly distributed in the 0–40 cm soil layer and accounted for 52.1–97.1% of the total root length density in the 0–80 cm soil layer. There were fewer roots in the 60–80 cm soil layer. Under the same irrigation treatment, the root length density of soybean under bubbler irrigation was greatest with increases of 9.2% to 331.4% compared to subsurface seepage irrigation and drip irrigation under mulch. This indicated that bubbler irrigation could effectively promote the growth and development of crop roots and had a more significant effect on the root density than the other two micro-irrigation methods.
3.3. Effects of Micro-Irrigation Regulation on Yield and Water Use in the Intercropping System
As can be seen from Table 4, irrigation effectively improved the above-ground dry-matter quality of soybean, among which the underground seepage irrigation treatment produced the greatest improvement. The W3 irrigation treatment increased the above-ground dry matter by 16.5% and 23.7% compared to W1 and W2, respectively. Under the same irrigation treatment, subsurface seepage irrigation increased the above-ground dry matter by 4.2–16.4% compared to the other two micro-irrigation methods. There were no significant changes in the underground dry-matter mass with irrigation method and irrigation (p > 0.05) and the underground biomass under W2 irrigation was the largest under the same micro-irrigation method. The yields with irrigation treatment were higher than that of GL. The maximum yield of the W2D treatment increased by 71.1% compared to the GL treatment and the minimum yield of the W1D treatment increased by 11.3% compared to the GL treatment. Micro-irrigation had no significant effect on the soybean yield under the different irrigation volumes and the order was W2 > W3 > W1 under the same method of micro-irrigation. The order for the different micro-irrigation methods was as follows: drip irrigation under a mulch > bubbler irrigation > underground seepage irrigation. The overall water consumption increased with the increase in irrigation amount and the maximum water consumption value occurred in W3S. The irrigation treatment increased the water consumption by 19.0–177.0% compared to GL. Except for the W2 treatment, there were significant differences in water consumption among the other treatments (p < 0.01). The water-use efficiency decreased with the increase in irrigation amount under the same micro-irrigation method and was smaller than in the GL treatment. The WUE under bubbler irrigation was higher than for the other two micro-irrigation methods. The lowest WUE occurred under W3S and was 56.0% lower than that of GL. The W1Y treatment produced the greatest WUE among the treatments, but it still decreased by 1.6% compared to GL. The analysis of variance showed that micro-irrigation had no significant effects on the above-ground dry-matter mass, underground dry-matter mass, yield, and WUE (p > 0.05), but had extremely significant effects on the water consumption (p < 0.01). The irrigation amount had no significant effect on the underground dry-matter quality (p > 0.05), but had a significant effect on the above-ground dry-matter quality (p < 0.05) and had extremely significant effects on the yield, water consumption, and WUE (p < 0.01).
3.4. Effects of Micro-Irrigation Regulation on Economic Benefits in the Intercropping System
The weight of each index was obtained through the analytical hierarchy process (Table 5). The fuzzy evaluation values and rankings for each index of economic benefit for each treatment were obtained using the comprehensive evaluation method based on fuzzy mathematics (Table 6). As can be seen from Table 6, the W1D treatment has the highest economic benefits, followed by the W2D and W2Y treatments. The fuzzy evaluation values for the three treatments are greater than 0.4 and the lowest is the W3D treatment with an evaluation value of only 0.05. The order for the different micro-irrigation methods was drip irrigation under a mulch > bubbler irrigation > underground seepage irrigation. Except for drip irrigation, the economic benefits of the different irrigation treatments were in the order W2 > W1 > W3. The highest net income occurred under W1D, with an increase of 116.2%, while the lowest net income occurred under W3D, where it decreased by 96.8% compared to GL. In general, the net income per hectare showed a downward trend as the irrigation amount increased. Under the same micro-irrigation method, the input–output ratio decreased as the irrigation amount increased and the maximum value occurred under the W1D treatment, which increased by 9.8%–109.2% compared to the other treatments. The output value per hectare treated with irrigated water was higher than that for GL. The maximum output value per hectare under drip irrigation was achieved under W2 and the lowest occurred under W3 with values of 71.2% and 11.3% higher than GL, respectively. The economic benefit analysis showed that the W1D treatment achieved the highest net income and input–output ratio per hectare. In addition, drip irrigation under a mulch combined with W1 (60% Fc), W2 (75% Fc) under film, and bubbler irrigation combined with W2 (75% Fc) also produced large increases in economic benefits.
4. Discussion
4.1. Effects of Different Micro-Irrigation Water Regulation Systems on the Spatial Distribution of Soil Water
The results showed that the spatial distribution of soil water was significantly different under the different micro-irrigation treatments. In the horizontal direction, the average soil moisture content in the 0–120 cm soil layer generally increased at first, then decreased, and then increased with increasing distance from the trees. This result is caused by the extension distance of the crown width and the root systems of young fruit trees. From 0 to 0.3 m away from the tree, the roots were densely distributed, but as the distance from the tree gradually increased, the root density gradually decreased. However, from 0.3 to 0.8 m away from the tree, the shading effect of the tree crown effectively reduces the ground evaporation, leading to a recovery in the soil moisture at this distance [14,15,16]. Except for between specific individual treatments, the general soil moisture content with bubbler irrigation was higher than that of the other two micro-irrigation methods, mainly because the large discharge by the single irrigator in bubbler irrigation is related to the large and abundant overall discharge, which promotes horizontal migration and the vertical infiltration of water [17,18]. In the vertical direction, the soil moisture content increased at first, then decreased, and then increased and the soil moisture content in the 0–40 cm soil layer changed considerably. This may be because the fine roots of apple are mainly distributed in the 0–40 cm layer while those of soybean are mainly distributed in the 0–20 cm layer [19]. As a result, the soil moisture in the first 40 cm soil layer changes dramatically and increases after it exceeds the 20 cm depth. The wetting depth of the three micro-irrigation methods was mainly distributed within the 0–60 cm layer, whereas the deep soil moisture change was small as the soil depth increased. This led to an upward trend in the soil moisture within the 60–120 cm soil layer. The change in the soil moisture became more pronounced as the irrigation amount decreased. The reason is that as the irrigation amount decreases, the change in soil moisture due to rainfall becomes larger and changes with the intensity of rainfall [20]. The soil water content and the slope of the line with distance from the trees indicates the difference in water competition ability between species. The greater the slope, the greater the difference. The overall slope for drip irrigation under mulch is high and positive and the negative slope of subsurface seepage irrigation was the most pronounced. This is due to the fact that the soil water evaporation is lower under subsurface seepage irrigation than with mulched drip irrigation, while mulched drip irrigation can better replenish the surface soil water, promote the growth and development of intercropped crops, and improve the absorption and utilization of soil water by crops [21,22,23]. Among the different micro-irrigation methods, the negative slope for bubbler irrigation was the most pronounced and the slope for bubbler irrigation increased with the increase in irrigation amount over the whole growth period. This trend is related to the targeting of gusset irrigation and the large discharge. The gusset irrigation outlet has a high quantity ratio with the number of soybean plants [17], which increases the soil water content, reduces water competition between intercropped crops and fruit trees, decreases the stress effect of fruit trees on crops, further promotes the growth and development of intercropped crops, and improves the competitiveness of soybeans. This suggests that the system could considerably reduce interspecific competition between apple and soybean compared to subsurface seepage irrigation and drip irrigation under mulch. In conclusion, bubbler irrigation can greatly improve water distribution, increase soil water content, and reduce water competition in the intercropping system.
4.2. Effects of Different Micro-Irrigation Regulation Methods on Root Distribution in the Intercropping System
The results from this study suggested that the fine roots of apple were mainly distributed in the 20–60 cm layer and the fine roots of soybean were mainly distributed in the 0–40 cm layer. This suggests that the root overlap within the intercropping system was mainly concentrated in the 20–40 cm layer, which agreed with the apple–soybean intercropping system results reported by Sun Yubu et al. [24] in the Loess region, Shanxi Province, China. The root distribution within the intercropping system showed an obvious increase as the irrigation amount increased. As the irrigation amount increased, the bubbler irrigation may have promoted the vertical infiltration of water and thus promoted root development in the intercropping system [17]. In contrast to bubbler irrigation, the root densities with underground seepage irrigation and drip irrigation under mulch were mainly distributed in the 0–20 cm layer. The reason is that the water discharge location and water discharge from the drip heads of these two micro-irrigation methods lead to different depths of moist soil and moist soil ranges compared to bubbler irrigation. Subsurface seepage irrigation and drip irrigation under mulch have strong wetting abilities toward the surface soil, which increases the water distribution in the surface soil and promotes root growth and development in the 0–20 cm layer [24,25]. The irrigation treatments increased the root length density compared to the GL treatment, mainly because irrigation increases the water distribution range in soil, the SWC, the water absorption range of roots, and the growth and development of roots [25,26]. The root length density of soybean under bubbler irrigation was greatest at the same irrigation amount. The reason is that compared to the other two micro-irrigation methods, under the same irrigation amount, more gravity water is formed in the ground at the initial stage of bubbler irrigation, the downward penetration rate of water in the soil is faster, the lateral expansion ability of soil water is stronger, and the increased soil water range in the root zone and non-root zone is larger. In addition, under irrigation, the average soil moisture content across the whole soil mass was the highest and the water distribution was more uniform [17,27]. In conclusion, irrigation can effectively promote the growth and distribution of crop roots. To a certain extent, the greater the irrigation amount, the better the growth and development of crop roots. Among the different micro-irrigation methods, bubbler irrigation increased the root density of the intercropping system to a greater extent than subsurface seepage irrigation and drip irrigation under a mulch. It can also promote the growth of deep crop roots by deepening the moisture depth of the soil layer.
4.3. Effects of Different Micro-Irrigation Regulation Methods on Water-Use Characteristics and the Economic Benefits of the Intercropping System
The results showed that the yields produced by the irrigation treatments were higher than that of the GL treatment and the soybean yield at the W2 irrigation level was higher than that at the W3 and W1 irrigation levels when the micro-irrigation methods were the same. The reason is that the W1 irrigation treatment water deficit was more serious and subsequently restricted the growth and development of soybean [28]. Appropriate levels of water stress can play a positive role in crop growth and development and the formation of later yields [29,30]. The different micro-irrigation methods had no significant effects on the above-ground dry-matter quality, underground dry-matter quality, yield, and WUE, but had very significant effects on the water consumption. The reason is that the different micro-irrigation methods had different water delivery and water consumption forms. This may have led to different soil water distributions, which affected the soil water loss and water absorption by the crops. The irrigation amount had a significant impact on the yield, water consumption, and WUE. It increases the soil water content, thus promoting the growth and development of crop roots and the absorption of soil water by crops. This means that, in general, water consumption increases with the increase in irrigation amount. There was a negative correlation between the WUE and water consumption and it decreased with the increase in irrigation amount. Among the different micro-irrigation methods, bubbler irrigation had the highest WUE, which is due to the rapid increase in the soil water matrix potential after bubbler irrigation, which would accelerate the infiltration of irrigation water and reduce ineffective water evaporation [27].
This study found that increasing the irrigation amount, especially to the W3 level (90% Fc irrigation level) significantly reduced the economic benefits, mainly because the increase in the irrigation amount increased the irrigation cost, which significantly decreased the economic benefits of the intercropping system [31]. Among the different micro-irrigation methods, drip irrigation under a mulch had the greatest economic benefits, which is because the coupling effect of water and fertilizer under film drip irrigation were synergistic. However, bubbler irrigation will cause partial nutrient leaching due to the larger water output of the irrigator [17], which indirectly reduces the economic benefits of the intercropping system. In conclusion, the yield of an intercropping system can be effectively improved by using the W2 irrigation level. Among the different micro-irrigation methods, the yield order was drip irrigation under a mulch > bubbler irrigation > underground seepage irrigation and bubbler irrigation had the highest WUE. Therefore, in order to effectively improve the WUE of the intercropping system and obtain higher economic benefits, it is recommended that drip irrigation under mulch or bubbler irrigation combined with the W2 irrigation level should be adopted when there are adequate water resources, whereas drip irrigation under film combined with the W1 (60% Fc) irrigation level should be adopted when water resources are relatively scarce.
5. Conclusions
This study showed that the soil water content in a fruit–soybean intercropping system had obvious spatial distribution differences when different micro-irrigation water control methods were used. Bubbler irrigation could effectively increase the SWC and reduce the intensity of water change in the vertical direction compared to the other two micro-irrigation methods at the same irrigation amount. The root distribution of the intercropping system significantly increased as the irrigation amount increased. Among the different micro-irrigation methods, bubbler irrigation had the most obvious effect on promoting root development in the intercropping system. The yields under the irrigation treatments were higher than that under the GL treatment. The yields at the W2 irrigation level were higher than at the W1 and W3 irrigation levels. The effect of drip irrigation under a mulch was greater than that under the other two micro-irrigation methods and the yield reached a maximum under the W2D treatment. The water consumption increased as the irrigation amount increased and the WUE of bubbler irrigation was relatively greater than that of the other two micro-irrigation methods. The different micro-irrigation methods produced no significant differences in yield and WUE, but had extremely significant effects on the water consumption, whereas the irrigation amount had extremely significant effects on the yield, water consumption, and WUE. However, increasing the irrigation amount will increase the irrigation cost, thus reducing the economic benefits. Drip irrigation under a mulch produced the greatest economic benefit. Therefore, to effectively improve the WUE of the intercropping system and obtain higher economic benefits, it is recommended that drip irrigation under mulch or bubbler irrigation combined with the W2 irrigation level should be adopted when water resources are adequate whereas drip irrigation under film combined with the W1 irrigation level should be adopted when water resources are relatively scarce.
Conceptualization, H.D. and R.W.; methodology, L.C.; software, C.X.; validation, R.W.; formal analysis, L.W.; investigation, X.W. and M.Z.; resources, X.W.; data curation, H.D.; writing—original draft preparation, H.D.; writing—review and editing, H.D. and R.W.; visualization, H.D.; supervision, R.W.; project administration, R.W.; funding acquisition, National Key Research and Development Program of China and National Natural Science Fund. All authors have read and agreed to the published version of the manuscript.
The data presented in this study are available on request from the corresponding author. The data are not publicly available due to ethical restrictions.
We are grateful for the support from the Forest Ecosystem Studies, National Observation and Research Station, Jixian, Shanxi, China.
The authors declare no conflict of interest.
Footnotes
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Enlarge this image.Figure 2. Precipitation and temperature during co-growth period of the apple and soybean. Notes: W1, W2, and W3 are the irrigation volumes for the three irrigation gradients; all meteorological data are for the year 2022.
Enlarge this image.Figure 5. Dynamics of soil water content levels during the soybean reproductive period. Note: * indicates significant correlation (p < 0.05); ** is highly significant (p < 0.01); ns is insignificant correlation (p > 0.05). IM is irrigation method and IV is irrigation volume.
Enlarge this image.Figure 6. Vertical variation of soil water content during soybean growth period. Note: ** is highly significant (p < 0.01); ns is insignificant correlation (p > 0.05). Fe is the fertility period and H is the soil depth.
Enlarge this image.Figure 7. Soybean and apple vertical root length density diagram. Note: * indicates significant correlation (p < 0.05); ** is highly significant (p < 0.01);ns is insignificant correlation (p > 0.05). IM is irrigation method and IV is irrigation volume.
Hierarchical analysis 1–9 digital scale.
| Scale | Meaning |
|---|---|
| 1 | Two-factor comparison: Ui and Uj are equally important |
| 3 | Ui is slightly more important than Uj |
| 5 | Ui is significantly more important than Uj |
| 7 | Ui is strongly more important than Uj |
| 9 | Ui is extremely more important than Uj |
| 2, 4, 6, 8 | Median of two adjacent scales |
| Countdown | Factor Uj compared to Ui, i.e., Uji = 1/Ui |
Judgement matrix construction table.
| Scale | U1 | U2 | …… | Un | Weighting |
|---|---|---|---|---|---|
| U1 | U11 | U12 | …… | U1n | W1 |
| U2 | U21 | U22 | …… | U12 | W2 |
| . | . | . | …… | . | . |
| . | . | . | …… | . | . |
| . | . | . | …… | . | . |
| Um | Um1 | Um2 | …… | Umn | Wm |
Slope analysis of soil water content and distance from tree at each growth stage.
| Treatment | W1S | W2S | W3S | W1D | W2D | W3D | W1Y | W2Y | W3Y | GL |
|---|---|---|---|---|---|---|---|---|---|---|
| Branching stage | −0.031 ** | 0.782 ** | 0.317 ** | 0.850 ** | 1.069 ** | 0.376 ** | −0.619 ** | 0.544 ns | 0.909 ** | 0.172 * |
| Podding stage | 0.223 ** | 0.850 ** | −0.491 * | 0.734 * | 0.670 ** | 0.452 * | −0.361 * | −0.044 ** | 0.650 ** | 0.104 ns |
| Filling stage | 0.369 ** | 0.633 ** | 0.263 * | 0.613ns | 0.920 ** | 0.474 ** | −0.261 ** | −0.023 * | 0.896 ** | 0.048 ns |
| Mature stage | −0.208 * | 0.429 * | 0.818 ns | 1.171 ** | 0.175 ** | 0.104 * | 1.179 ** | 0.623 ns | 1.295 ** | 0.172 ** |
Note: * is significant correlation (p < 0.05); ** is highly significant correlation (p < 0.01); ns is insignificant correlation (p > 0.05).
Effect of different micro-irrigation methods on dry-matter accumulation, yield, and water-use efficiency in intercropping system.
| Treatment | Dry-Matter Accumulation | Production/(kg·hm2) | Water Consumption/(mm) | Water-Use Efficiency/(kg·hm2·mm−1) | |
|---|---|---|---|---|---|
| Ground Section/(g) | Lower Ground/(g) | ||||
| W1S | 101.69 ± 19.35 bc | 34.41 ± 2.32 ab | 1704.96 ± 498.33 de | 226.17 ± 8.17 f | 7.54 ± 1.96 abc |
| W2S | 118.47 ± 50.34 ab | 35.33 ± 2.00 ab | 2216.25 ± 480.82 abcd | 325.88 ± 6.32 de | 6.80 ± 1.31 bc |
| W3S | 125.80 ± 39.74 a | 34.78 ± 2.50 ab | 1862.60 ± 623.38 bcde | 446.91 ± 7.73 a | 4.17 ± 1.25 d |
| W1D | 100.49 ± 34.09 bc | 35.43 ± 2.74 ab | 1700.10 ± 368.44 de | 234.35 ± 5.80 f | 7.25 ± 1.40 abc |
| W2D | 104.18 ± 20.11 ab | 36.69 ± 2.83 a | 2613.05 ± 922.72 a | 319.91 ± 4.31 e | 8.17 ± 2.74 ab |
| W3D | 115.90 ± 34.34 ab | 35.50 ± 1.28 ab | 2390.94 ± 1039.15 abc | 434.63 ± 1.82 b | 5.50 ± 2.15 cd |
| W1Y | 97.58 ± 15.31 bc | 36.24 ± 3.40 ab | 1788.14 ± 640.41 cde | 191.95 ± 2.62 g | 9.32 ± 2.97 a |
| W2Y | 104.82 ± 23.63 ab | 37.32 ± 14.99 a | 2467.66 ± 669.20 ab | 330.12 ± 3.15 d | 7.48 ± 1.81 abc |
| W3Y | 108.09 ± 19.18 ab | 34.57 ± 2.02 ab | 2363.97 ± 650.28 abc | 425.41 ± 1.59 c | 5.56 ± 1.36 cd |
| GL | 80.55 ± 20.46 c | 31.52 ± 1.18 b | 1527.47 ± 429.90 e | 161.32 ± 4.55 h | 9.47 ± 2.50 a |
| Significance analysis (F-value) | |||||
| IM | 2.350 ns | 0.562 ns | 2.220 ns | 28.415 ** | 2.687 ns |
| IV | 4.416 * | 0.790 ns | 9.889 ** | 4086.486 ** | 15.894 ** |
| IW × IV | 0.391 ns | 0.186 ns | 0.579 ns | 600.739 ** | 1.152 ns |
Note: * indicates significant correlation (p < 0.05); ** is highly significant (p < 0.01);ns is insignificant correlation (p > 0.05). IM is irrigation method and IV is irrigation volume. (a,b,c,d,e,f,g,h) represent the label of significant difference.
Matrix for calculating the weights of economic efficiency indicators and the results of the calculation.
| Economic Benefits | Hectare Net Income | Input–Output Ratio | Hectares of Output | Normalised Weights |
|---|---|---|---|---|
| Hectare net income | 0.57 | 0.57 | 0.57 | 0.5714 |
| Input–output ratio | 0.29 | 0.29 | 0.29 | 0.2857 |
| Hectares of output | 0.14 | 0.14 | 0.14 | 0.1429 |
Economic efficiency evaluation results and ranking.
| Data Averages | Post-Standardised Data | |||||||
|---|---|---|---|---|---|---|---|---|
| Hectare Net Income | Input–Output Ratio | Hectares of Output | Hectare Net Income | Input–Output Ratio | Hectares of Output | Fuzzy Assessment Values | Ranking | |
| W1S | 4846.9720 | 1.6520 | 12,275.712 | 0.53 | 0.58 | 0.04 | 0.257 | 7 |
| W2S | 6383.8400 | 1.6670 | 15,957.000 | 0.70 | 0.60 | 0.58 | 0.353 | 4 |
| W3S | 1061.6500 | 1.0860 | 13,410.720 | 0.11 | 0.08 | 0.21 | 0.061 | 9 |
| W1D | 9074.3000 | 2.1150 | 17,214.768 | 1.00 | 1.00 | 0.77 | 0.522 | 1 |
| W2D | 9044.5400 | 1.9260 | 18,813.960 | 1.00 | 0.83 | 1.00 | 0.512 | 2 |
| W3D | 132.7400 | 1.0110 | 12,240.720 | 0.00 | 0.01 | 0.04 | 0.005 | 10 |
| W1Y | 5993.3480 | 1.8710 | 12,874.608 | 0.66 | 0.78 | 0.13 | 0.334 | 5 |
| W2Y | 7624.7620 | 1.7520 | 17,767.152 | 0.84 | 0.67 | 0.85 | 0.428 | 3 |
| W3Y | 5313.8040 | 1.4540 | 17,020.584 | 0.58 | 0.41 | 0.74 | 0.298 | 6 |
| GL | 4197.7840 | 1.6170 | 10,997.784 | 0.46 | 0.55 | 0.00 | 0.227 | 8 |
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