1. Introduction
Soil salinization is a global environmental issue that has led to vast tracts of land becoming barren, severely impacting food security and the livelihoods of farmers. Globally, regions such as the Central Asian steppes, the Middle East, and parts of South America have also faced the devastating effects of soil salinization, exacerbating food insecurity and economic instability [
1]. Currently, approximately 9.5 × 10
9 ha of land worldwide is affected by varying degrees of salinization, accounting for about 10% of the global arable land area, with an annual increase of 1 × 10
6 to 2.5 × 10
6 ha [
2,
3]. These barren lands particularly threaten agricultural productivity in semi-arid and arid regions where food production heavily relies on irrigation and soil health maintenance. In China, approximately 50 million hectares of saline–alkali soil is predominantly distributed across the northeastern, northern, northwestern, and coastal regions, presenting substantial challenges to sustainable agricultural development. Among these regions, the Hetao Plain stands out due to its long history of agriculture and fertile soils, serving as a vital agricultural hub and a significant producer of staple crops in the country [
4]. Secondary salinization, a process largely driven by human-induced factors such as inefficient irrigation practices, has exacerbated soil degradation in many agricultural regions. In the Hetao Plain, this issue is particularly pronounced, with severe soil salinization resulting from a combination of high evaporation rates, over-irrigation, and poor drainage systems. These factors not only compromise soil health but also threaten the region’s ability to continue functioning as a major producer of staple crops. Similarly, in countries like India and Pakistan, salinization has significantly reduced the productivity of historically fertile regions [
5,
6]. This trend has also been observed in the Nile Delta, where soil degradation has threatened food production in one of the world’s most densely cultivated areas [
7]. Northern Ningxia, with an average annual precipitation of only 200 mm, experiences extreme aridity due to evaporation rates as high as 2000 mm. This imbalance in water availability and demand creates a challenging environment for agricultural production, particularly in saline–alkali soils that require precise water management to prevent salinization [
8]. Additionally, the region’s limited water resources heighten the need for efficient irrigation methods to sustain crop yields in this harsh climate. Unlike other regions, the secondary salinization process in northern Ningxia’s soils is driven by a combination of natural factors, such as high evaporation rates, and human activities, including long-term reliance on inefficient irrigation practices. Being adjacent to the Yellow River, the local area benefits from convenient irrigation. Prolonged and continuous surface irrigation with Yellow River water raises the groundwater level, causing salt accumulation on the soil surface through evapotranspiration, leading to severe secondary soil salinization [
9]. Extensive moderately to severely saline–alkali soils in Ningxia have been abandoned, significantly reducing arable land resources, lowering crop yields, and hindering high-quality development and ecological restoration. The loss of productivity in these saline–alkali soils has constrained sustainable agriculture in the Yellow River Basin, where soil degradation further complicates efforts to restore land for productive use [
10]. Soil salinization results from the combined effects of water and salt and is closely related to irrigation methods, cultivation practices, climatic conditions, soil types, and other factors [
11].
The essence of soil improvement measures for saline–alkali soil lies in regulating soil water–salt movement, accelerating the downward leaching of soil salts, and impeding upward salt migration to the soil surface due to evapotranspiration [
12]. Traditional hydraulic engineering improvement measures often involve using flood irrigation to flush out salts, a practice that not only wastes valuable water resources but also increases environmental risks such as groundwater contamination, nutrient runoff, and ecosystem degradation [
13]. The limited availability of this resource in northern Ningxia makes these traditional methods economically unsustainable, driving the need for more efficient irrigation strategies that conserve water while preventing soil degradation. Farmers in Ningxia have long relied on surface irrigation from the Yellow River, which has contributed to low water use efficiency, secondary soil salinization, and declining productivity. Given the region’s high evaporative demand and limited freshwater resources, surface irrigation practices have also exacerbated the region’s vulnerability to droughts, further limiting agricultural production. The inefficient use of water not only raises farming costs but also increases the environmental footprint of irrigation, pushing for a shift towards more sustainable techniques. Therefore, it is urgent to research field irrigation technology to support agricultural development in Ningxia. Drip irrigation with integrated water and fertilizer application can achieve salt flushing via high-frequency, precise irrigation, allowing water and nutrients to infiltrate the soil uniformly. This process reduces subsurface soil solution percolation and surface runoff while locally leaching salts, creating a suitable microenvironment near the drip emitter with ample water and a low salt content. In Israel, drip irrigation has been successfully implemented to reclaim saline soils, significantly enhancing water use efficiency and crop productivity in arid regions [
14]. Similarly, Mexico’s adoption of drip irrigation in its arid northern regions has led to marked improvements in soil health and agricultural yields [
15]. This technology enhances water use efficiency and crop yields [
16] and has become one of the most effective methods for developing low-productivity farmland, widely used on saline–alkali soil at home and abroad [
12].
Previous studies have predominantly concentrated on the effects of single-drip irrigation systems with integrated water and fertilizer applications on soil amelioration and crop yield improvement in saline–alkali soils [
17,
18,
19]. While effective, these methods often overlook the intricate dynamics of soil water and salt distribution under varying irrigation patterns, and their long-term impacts on crop physiology, nutrient uptake, and soil sustainability. Traditional surface irrigation methods, such as flood irrigation, have been widely used due to their simplicity and cost-effectiveness, particularly in regions with access to ample water resources. However, these methods suffer from inefficiencies, including excessive water use, poor distribution uniformity, and a tendency to exacerbate secondary salinization through the accumulation of salts near the soil surface. In contrast, drip irrigation, especially when integrated with fertigation, offers precise water and nutrient delivery directly to the root zone, reducing water wastage and limiting salt buildup in the topsoil. However, one drawback of drip irrigation is the high initial installation cost and the need for maintenance to prevent emitter clogging. Recent studies from Australia and the Middle East [
20,
21] have shown that adjustments in irrigation regimes—especially under fluctuating climatic conditions—can profoundly impact soil salinity levels and crop productivity. Building on these findings, the present study takes a more comprehensive approach by examining the long-term effects of various irrigation strategies on soil water–salt dynamics, emphasizing their cumulative impact on crop development, soil health, and overall productivity.
Unlike previous research, which has primarily focused on short-term effects, this study provides a comprehensive evaluation of the long-term interactions between soil water, salt, and crop growth under different irrigation practices. By addressing the complex relationships between water–salt movement, nutrient bioavailability, and crop performance over three consecutive years, this research fills a critical knowledge gap. The findings will contribute to the development of more efficient irrigation techniques that can mitigate soil salinity, enhance water use efficiency, and promote sustainable agricultural production in saline–alkali soils. Ultimately, this study aims to provide a robust framework for optimizing irrigation practices in degraded lands, particularly in the Hetao Irrigation Area.
In this study, a three-year field experiment was conducted on saline–alkali soil in northern Ningxia to investigate the effects of varying irrigation quotas on soil water dynamics, salt content, and water utilization in maize (Zea mays L.). The research focuses on the integrated application of water and fertilizers and their subsequent impact on soil salinity and crop performance. Maize was selected for its significance as a staple and forage crop in arid and semi-arid regions, its inherent tolerance to saline conditions, and its potential to produce high yields when subjected to optimized irrigation regimes. By assessing the long-term effects of different irrigation methods on soil water–salt interactions and maize growth over three consecutive years, this study seeks to offer theoretical insights and technical guidance for the sustainable development and efficient utilization of drip-irrigated saline–alkali soils. The findings are expected to enhance our understanding of water and salt movement under drip irrigation, providing a solid foundation for future practices in water conservation, salt management, and yield optimization in saline–alkali agricultural landscapes, particularly in the Hetao Irrigation Area.
4. Discussion
Three consecutive years of field trials demonstrated significant differences in soil water content under different irrigation patterns. Drip irrigation consistently resulted in higher soil moisture compared to traditional flood irrigation, with moisture levels increasing as irrigation quotas were raised, aligning with findings by Liang et al. [
28]. Moreover, similar findings have been reported in semi-arid regions of Africa, where drip irrigation has been shown to enhance water retention in highly saline–alkali soil [
29]. Likewise, studies in India’s semi-arid regions have demonstrated increased crop yields with drip irrigation despite challenging climatic conditions [
30]. These findings suggest that the effectiveness of drip irrigation extends beyond China, reinforcing its potential for broader agricultural applications globally. The increased quotas under drip irrigation effectively replenished water losses in shallow soil layers due to high evapotranspiration, maintaining higher moisture levels. Moreover, drip irrigation promoted uniform and stable water infiltration, enhancing moisture accumulation in deeper soil layers [
31,
32]. This deeper infiltration is influenced by several factors, including soil texture, structure, and hydraulic conductivity, which allow for gradual percolation and moisture retention at depth. These processes are optimized by drip irrigation’s ability to deliver water directly to the root zone, reducing surface evaporation and preventing soil compaction, thereby sustaining higher moisture content in the sub soil. In the Middle East, the implementation of drip irrigation systems has not only improved water distribution but also enhanced agricultural sustainability under severe water scarcity [
33]. Conversely, traditional flood irrigation involves large irrigation volumes per application, leading to the frequent lateral movement of irrigation water on the surface and increasing the susceptibility of subsurface flow to clogging in upper soil layers, disrupting the soil structure and hindering water infiltration, and resulting in uneven soil moisture distribution [
34].
Surface evaporation moves the soil solution upward, driven by capillary action, which draws water and dissolved salts toward the surface through the narrow pores in the soil matrix. This capillary movement enhances salt accumulation in the upper soil layers, particularly in arid regions. Simultaneously, transpiration by plants causes water uptake through the roots, which concentrates salts around the root zone. The process of ion exchanges further influences salt movement, as cation exchange sites in the soil attract and retain positively charged ions, such as Na
+ and calcium Ca
2+, affecting the redistribution of salts within the soil profile. This combination of capillary action and ion exchange contributes to saline–alkali stress, ultimately affecting crop growth [
35]. Consistent with the findings of Dong et al. [
36], we found that soil salinity under drip irrigation was significantly lower than that under traditional flood irrigation in the 0–40 cm soil layer (
Figure 3). This is because salts in the soil solution are dissolved by percolation water under drip irrigation, and with frequent high-frequency subsurface infiltration, a desalination effect is formed in the surface soil layer. In the 60–100 cm soil layer, salt movement may have been impeded because of increased soil moisture and reduced porosity, resulting in the gradual accumulation and an overall higher vertical distribution pattern of salt in the lower layers. This occurs because higher bulk density decreases soil porosity, limiting the movement of water and dissolved salts. The reduced pore space restricts both water infiltration and salt leaching, which slows down the downward migration of salts and causes their retention in these deeper layers. As the soil becomes more compact, it further obstructs the vertical salt movement, leading to greater accumulation of salts in specific soil horizons [
37]. It is noteworthy that the surface soil salinity under CK was over double that under high-volume drip irrigation, indicating that flood irrigation failed to form a uniformly effective percolation zone to remove soil salts despite its high irrigation quota, resulting in a poor desalination effect in the surface soil layer, consistent with the findings of Fan et al. [
38]. Over time, the accumulation of salts in deeper soil layers poses significant risks to both crop productivity and soil health. As salts continue to build up in the subsoil, they can create osmotic stress, reducing the availability of water to plant roots and leading to inhibited root growth and reduced nutrient uptake. This can result in stunted crops, lower yields, and poor plant health. In addition, persistent salt accumulation can lead to the degradation of soil structure, reducing soil permeability and porosity, which in turn hampers root penetration and water infiltration. Over the long term, these conditions can contribute to soil salinization, making the land less arable and decreasing its overall agricultural productivity.
The desalination effect was most significant in the W3 treatment, whereas the W1 treatment showed lower desalination in the 40–100 mm soil layer, suggesting that lower drip irrigation quotas can leach surface soil salts but have poor desalination effects on deep soil layers. In addition, during the first and second years of the experiment, it could be observed that the desalination effect in the 0–60 cm soil layer under the drip treatment was not significant, but significant soil migration to the deeper soil layer could be observed. By the third year of the experiment (September 2023) when the salinity of the surface soil (0–20 cm) under drip irrigation treatment was significantly lower than that in 2021, the salinity gradually migrated downward to the 40–100 cm soil layer. Therefore, in future efforts to improve irrigation in saline–alkali soil, it may be necessary to appropriately increase irrigation quotas based on soil texture, soil type, and salinity to ensure the effective desalination of deeper soils. Environmental studies in Australia also support this, where large-scale drip irrigation projects have led to substantial reductions in salinity levels in irrigated fields [
39].
Soil conductivity changes can be used to characterize soil salinity [
40]. This study demonstrates that under drip irrigation conditions, soil salinity gradually migrated to deeper soil layers with the infiltration of irrigation water, and the distribution of salts gradually transitioned from “surface aggregation” to “bottom accumulation” (
Figure 3,
Table 3). Additionally, in the 40–60 cm soil layer, downward salt movement may have been hindered by factors such as excessive bulk density in deep soil, reduced porosity, and proximity to the groundwater level. Anions in the soil under drip irrigation gradually migrate to deeper soil layers with the percolation water (
Figure 4), possibly because soil colloids carry negative charges because of cation exchange and the deprotonation of surface hydroxyl groups. In the process of cation exchange, Na
+ and Ca
2+ exhibit differential movement. Na
+, with their smaller charge and hydration radius, are weakly adsorbed by soil colloids, and are therefore more mobile, allowing them to migrate downward more readily with percolating water. In contrast, Ca
2+, which have a larger charge and hydration radius, are more strongly attracted to the negatively charged sites on soil colloids, leading to greater retention in the upper soil layers. This difference in cation exchange behavior contributes to the distinct movement patterns of Na
+ and Ca
2+ within the soil profile [
41].
The maize varieties used in this experiment were suitable for both silage and forage. Parameters such as plant height, stem thickness, and aboveground biomass represent the basic growth status of maize, with aboveground biomass indicating the total yield for silage. In this study, plant height and stem thickness of maize were assessed under different irrigation methods (
Table 5). Although there were no significant differences in plant height and stem thickness across treatments, this is likely because drip irrigation enhances grain filling and kernel development by providing consistent moisture to the root zone, thereby increasing the overall grain weight and spike yield, which contribute significantly to the aboveground biomass. Drip irrigation significantly increased the aboveground biomass and grain yield of maize in the second year of the experiment compared to conventional flood irrigation. This aligns with the findings of Suat et al. [
42], which showed that maize yield increased with irrigation. Similarly, our results indicate that maize grain yield increased with higher drip irrigation quotas, with the most substantial yield enhancement observed under W3 drip irrigation. Drip irrigation offers notable potential economic benefits through improved water-use efficiency and increased crop yield. By delivering water directly to the root zone, it minimizes waste through evaporation and runoff, leading to significant water savings compared to conventional flood irrigation. This reduction in water usage translates into lower input costs, while the higher yields and improved crop quality observed under drip irrigation contribute to enhanced profitability for farmers. In California, USA, the widespread adoption of drip irrigation led to significant water savings [
43]. Furthermore, the ability of drip irrigation to deliver water-soluble fertilizers more precisely allows for better nutrient uptake by the crops, promoting consistent growth and reducing the likelihood of nutrient leaching, thus enhancing crop quality parameters such as kernel size, starch content, and plant robustness. Moreover, the environmental impact of drip irrigation is also worth noting. Compared to conventional flood irrigation, drip irrigation reduces the volume of water applied, decreasing energy consumption and thus contributing to lower greenhouse gas emissions. The overall improvement in water and nutrient use efficiency lessens the risk of soil degradation, waterlogging, and contamination of surrounding water bodies, making it a more sustainable option. Additionally, the enhanced water efficiency and increased yield potential of drip irrigation enable faster cost recovery, as the higher productivity and quality of the maize produced can offset the initial investment in the irrigation infrastructure. In this way, drip irrigation not only improves maize yield, but also offers long-term economic and environmental benefits [
44].
Water plays a critical role in plant growth, and enhancing water use efficiency in arid regions is pivotal for improving agricultural productivity [
45]. In this study, the W1 treatment demonstrated the highest water use efficiency under drip irrigation, significantly surpassing the CK treatment, which exhibited almost four times the surface evaporation of W1 (
Table 6). The increased efficiency directly contributed to corn yield improvements: W1, W2, and W3 treatments achieved yield increases of 9.64–19.84%, 9.08–15.71%, and 10.94–21.90%, respectively, over CK. These gains are attributed to the ability of drip irrigation to minimize water loss through evaporation and runoff, ensuring better hydration of the root zone and promoting more consistent plant growth. This also reduced drought stress and improved nutrient uptake, ultimately enhancing overall crop performance. Conventional flood irrigation, by contrast, applies large volumes of water across the field, resulting in extensive surface evaporation and surface water runoff, which degrades soil structure over time. This can lead to the formation of cracks, facilitating the upward movement of salts to the surface during winter, thereby re-salinizing soils [
46]. Drip irrigation, however, delivers water in precise, small amounts, preventing surface water accumulation and maintaining soil structure. The integration of fertilization with high-frequency (HF) drip irrigation also improved nutrient delivery, reducing phosphorus fixation and nitrogen volatilization, which further enhanced water and nutrient use efficiency [
47]. Despite these benefits, the study does have limitations. The specific characteristics of saline soils, such as their heterogeneous texture and the tendency for salts to accumulate in deeper layers, can complicate long-term management. Moreover, the results may vary under different climate conditions or soil types, indicating the need for further research to explore these variables and refine drip irrigation strategies for broader applications.