1. Introduction
Wolfberry (
Lycium barbarum L.) is a perennial deciduous shrub belonging to the Solanaceae family. It has a rich cultivation history spanning over a millennium in China. Renowned for its robust traits, such as salt and alkali resistance, adaptability to infertile soils, drought tolerance, and high economic returns [
1], wolfberry is a significant agricultural commodity. The arid and semi-arid regions of Northwest China have emerged as primary production hubs for this remarkable crop owing to favorable climatic conditions characterized by ample sunlight and notable day–night temperature fluctuations [
2]. As the market demand for wolfberry continues to surge and local efforts to harness saline–alkali lands intensify, the cultivation footprint of wolfberry has consistently expanded. However, the current state of wolfberry cultivation in China remains in a developmental phase, marked by monocropping, meager vegetation cover, severe soil evaporation, and extensive field management practices. In tandem, these factors exacerbate secondary soil salinization. Intercropping systems featuring the harmonious coexistence of woody plants and grasses have emerged as a flagship approach to eco-agriculture [
3]. They stand out as a means to effectively curtail soil evaporation, elevate water utilization efficiency, and bolster economic return [
4]. Alfalfa (
Medicago sativa L.), celebrated as the “forage king” and “feed queen”, emerged as a natural choice for intercropping with wolfberry. Its robust regenerative abilities, palatability, and rich reservoir of high-quality crude protein render it an essential part of wolfberry-based agroforestry systems [
5]. Thus, investigation of the water-saving mechanisms within the wolfberry–alfalfa intercropping system is pivotal for the sustainable evolution of agroforestry practices.
Agroforestry systems that combine woody and herbaceous plants are widely implemented across various regions, including Europe, South America, the Saharan region of Africa, and South Asia. These systems exhibit diverse forms owing to variations in climatic conditions and management practices [
6,
7,
8]. Within agroforestry systems, the interplanting of herbaceous vegetation between woody plants in alleys optimizes land and solar resource utilization. This multifaceted approach not only enhances land coverage but also mitigates soil evaporation, fostering improved vegetation structure and soil enrichment strategies in line with the adage, “grasses to ameliorate the soil, and grasses to stimulate the trees” [
8,
9,
10]. The interaction between these two components in agroforestry systems introduces variations in soil moisture, soil evaporation, and crop transpiration. These changes are influenced by factors such as the choice of intercropped vegetation, prevailing climatic conditions, soil texture, and field management practices. Several studies suggest that, although herbaceous plants in agroforestry systems consume some soil moisture, they also contribute significantly to increased rainfall infiltration, reduced soil evaporation, and minimized soil erosion, ultimately resulting in stable or even augmented soil moisture levels [
11,
12]. For instance, in the case of guinea grass (
Panicum maximum Jacq.) intercropped with four tree species at varying row spacings, medium row spacing consistently demonstrated the highest soil moisture levels, accompanied by optimal growth and yields [
13]. Wen et al. [
14] observed that intercropping of
Alpinia oxyphylla Miq. (a medicinal herb) in oak plantations exhibited variable effects on water consumption rates and uptake across different soil layers, ultimately achieving an equilibrium in soil moisture distribution and overall soil moisture enhancement. In a semi-arid region of Liaoning, China, Bai et al. [
15] reported that apricot (
Prunus armeniaca L.) orchard intercropping systems positively influenced soil moisture conditions compared with sole cropping. Nonetheless, Oswald et al. [
16] found that intercropping grass-legume mixtures with young peach (
Prunus persica (L.) Batsch) trees in southern France led to more severe water stress than sole cropping, particularly during early growth when soil cover fabric was used. However, judicious intercrop cutting effectively mitigates water stress during mid- to late-growth stages [
17]. Similarly, Kurt et al. [
18] documented that in a switchgrass (
Panicum virgatum L.) and juvenile loblolly pine (
Pinus taeda L.) intercropping system, competition exerted a greater negative impact than positive effects, resulting in reduced soil moisture and decreased yields for both switchgrass and juvenile loblolly pine when compared with sole cropping. In jujube (
Ziziphus jujuba Mill.) orchards on the Chinese Loess Plateau, intercrop species and field management practices significantly influence the root distribution of agroforestry systems, thereby affecting the soil moisture conditions [
19]. These findings underscore the need to consider various factors when evaluating the impact of agroforestry systems on soil moisture dynamics, emphasizing the necessity of context-specific approaches to optimize water resource management within agroforestry practices.
Soil moisture plays a pivotal role in agroecosystems and is primarily influenced by two main consumption pathways: soil evaporation and crop transpiration. The introduction of intercropping into these systems reshapes the spatial distribution of plant canopies, thereby exerting a profound influence on the partitioning of soil evaporation and crop transpiration [
20]. Notably, intercropping often features taller crops, which, owing to their increased stature, can absorb more solar radiation, consequently leading to higher rates of transpiration when compared with shorter companion crops. For instance, a study conducted in the Hetao Irrigation District of Inner Mongolia revealed that maize (
Zea mays L.), a tall crop, exhibited a substantial increase in transpiration, ranging from 29.3% to 42.1%, compared with shorter tomato plants [
19]. This difference in transpiration is predominantly attributed to belowground resource competition, with the root system of maize demonstrating a water uptake capacity 7.3 times greater than that of tomatoes (
Solanum lycopersicum L.) [
21]. To better understand the nuances of soil moisture dynamics, Wang et al. [
22] used an enhanced modeling approach to simulate transpiration allocation within agroforestry systems. Their findings indicated that apple trees (
Malus pumila Mill.), with their greater stature, consumed 33.3–48.0% more water than cocksfoot grass (
Dactylis glomerata L.). However, it is crucial to emphasize that the effect of intercropping on soil moisture is not universally consistent. For instance, in wheat (
Triticum aestivum L.) and maize intercropping, adequate underground interaction between crop roots is conducive to reducing water consumption. Increasing maize planting density is beneficial to reduce the water consumption of the long-term subsurface complete interaction between wheat and maize [
23]. Within the agroforestry systems (crop + shrub) situated in central and southern Senegal, the introduction of shading mechanisms reduces direct solar exposure to the soil surface. After rainfall, there is an initial increase in the soil moisture content, followed by a period of pronounced soil evaporation, with a subsequent gradual decline in the rate of evaporation [
24]. In semi-arid tropical grasslands where taller trees predominate, the higher proportion of above-ground woody structures reduces evaporation due to “savings” from reduced solar exposure, significantly surpassing the “losses” from tree transpiration. In contrast, agroforestry systems, such as orchards, which are characterized by densely planted, shorter trees, experience a counterbalancing effect due to canopy interception and increased transpiration. Consequently, these dynamics can offset the favorable microclimatic effects. In pursuit of maximum productivity and economic returns, farmers tend to favor economically profitable fruit trees and similar choices [
25]. It is important to emphasize that soil surface water evaporation is a pivotal component of soil moisture balance, particularly in areas with sparse vegetation, where young trees may struggle to provide complete ground cover. This results in substantial soil moisture loss through ineffective evaporation mechanisms [
26]. Although intercropping has the potential to reduce soil evaporation within tree rows, it may also intensify soil transpiration [
27]. Radersma and Ong [
28] conducted research in Kenya and reported a 35% reduction in soil evaporation within agroforestry systems compared with bare soil. However, Wang et al. [
29] found that intercropping forage grass with jujube orchards in the hilly Loess Plateau partially offset reduced soil evaporation and increased infiltration, resulting in a certain degree of negative impact on soil moisture conditions. Effective field management practices such as timely grass-cutting have been identified as viable solutions for mitigating this impact. In conclusion, the perspectives of researchers, both nationally and internationally, regarding the influence of intercropping systems on soil water content (SWC), soil evaporation, and crop transpiration remain diverse. These disparities emphasize the necessity of region-specific analyses that account for factors such as climate conditions, crop types, and field management practices. Importantly, the distinctive wolfberry–alfalfa intercropping system, particularly in the arid regions of Northwestern China, is not well-represented in the literature. Therefore, it is imperative to elucidate the spatial distribution of soil moisture within the wolfberry–alfalfa intercropping system, quantify soil evaporation and crop transpiration, explore their intricate interactions, and uncover the mechanisms governing soil-crop water regulation within the soil–plant–atmosphere continuum (SPAC). This undertaking will establish a robust theoretical foundation underpinning rational irrigation practices and sustainable advancement of the wolfberry–alfalfa intercropping system.
Therefore, the aims of this study were to (1) utilize the SIMDualKc (Simulation Dual Crop Coeffcient) model for simulation and validation to demonstrate its applicability in the study area and determine crop coefficients for each growth stage; (2) define the soil evaporation dynamics during the crop growth stages and quantitatively assess the influence of intercropping on soil evaporation; (3) compare the spatiotemporal variations in soil moisture between intercropping and sole cropping; and (4) determine the mechanisms governing soil–plant water dynamics and quantitatively assess the water-saving benefits of intercropping by evaluating crop transpiration across growth stages and their proportion to evaporation, in conjunction with the interactive impacts on soil evaporation and moisture. Through a comprehensive analysis of the water-saving mechanisms at play, we hope to provide valuable insights to the scientific community and promote the adoption of eco-friendly and economically viable agroforestry practices.