Monitoring, Reclamation and Management of Salt-Affected Lands

1. Introduction

Soil is the foundation of agriculture, and the world’s farmers depend on soil to produce about 95% of the food we eat. Globally, saline soils cover more than 833 million hectares of land (8.7 per cent of the Earth’s surface), most of which is found in naturally arid or semi-arid zones in Africa, Asia, and Latin America. Between 20 and 50 per cent of irrigated soils on all continents are hypersaline, which means that more than 1.5 billion people around the world face major challenges in food production due to soil degradation. The causes of rapid soil salinization are diverse and include poor management practices, excessive or inappropriate fertilization, deforestation, rising sea levels, shallow water tables affecting root zones, and the intrusion of seawater into groundwater used for irrigation. Climate change further exacerbates these risks. By the end of this century, the global arid land area may increase by 23%, primarily concentrated in developing countries.

This Special Issue focuses on the monitoring, reclamation, and management of salt-affected soil. It discusses water and salt regimes, carbon and nutrient cycling, and fertility improvement, with a particular focus on soil, water, and salt and associated agricultural and ecological issues in coastal saline–alkaline land. This Special Issue has aroused widespread interest among researchers and includes a total of 28 related academic papers.

2. Overview of This Special Issue

This Special Issue includes twenty-eight original contributions focused on the monitoring, reclamation, and management of salt-affected soils. The included studies mainly comprise research by Chinese universities and research institutions, with some contributions from international institutions. The twenty-eight articles in this Special Issue can be divided into five categories: category A, “Salt-affected soil remediation”; category B, “Rational fertilization in saline farmland”; category C, “Salt sources and treatments in saline agriculture”; category D, “Salinity monitoring and spatial distribution”; category E, “Impact of soil salinity stress on plant growth and nutrient element availability”; and category F, “Ecosystem strategies for sustainable saline agriculture development”.

In category A, “Salt-affected soil remediation”, Mengzhu Li et al. (contribution 1) studied various straw mulching techniques with indoor soil column tests and field tests to screen optimal mulching measures for salt and vapor suppression in saline soils. Ji Qi et al. (contribution 2) investigated the various ridging configurations affecting soil salinity in the Yellow River Delta. The study showed that a triangular ridge fostered a favorable soil environment for crop growth. Meng xiao et al. (contribution 3) analyzed the addition of fulvic acid to organic fertilizer, which increased the content of soil organic matter and promoted salt ion leaching. Yunpeng Sun et al. (contribution 4, 5) reported two studies on field experiments in coastal soil with the addition of biomaterials. Biochar, fulvic acid, and Bacillus subtilis showed good effects on soil salinity amelioration and fertility improvement. Xiangping Wang et al. (contribution 6) analyzed the reduction in soil salinity and improvement in barley–maize growth in newly reclaimed coastal lands resulting from the utilization of manure plus plastic film mulch.

In category B, “Rational fertilization in saline farmland”, Junyao Liu et al. (contribution 7) analyzed the effects of using organic fertilizer with ryegrass–alfalfa rotation and compound measures on lowering salinity and boosting fertility in coastal salinized soils. Haogeng Zhao et al. (contribution 8) reported that low-nitrogen microbial fertilizers led to more ammonium nitrogen in the soil, whereas a high-nitrogen fertilizer application resulted in higher nitrate nitrogen in the soil, leading to nitrogen leaching in saline soils. Panbo Deng et al. (contribution 9) used organic fertilizer with Ganoderma Iucidum residue for ameliorating the physical and chemical properties of saline–alkaline soils and adjusting microbial communities. Yunpeng Sun et al. (contribution 10) found that nitrogen fertilizer application could ameliorate salt damage and promote the growth of wheat.

In category C, “Salt sources and treatments in saline agriculture”, Sourour Mzahma (contribution 11) studied membrane technologies aimed at decreasing the salt content of textile effluents for agricultural reuse. Bakytzhan Amralinova et al. (contribution 12) investigated the composition of Burabay rock, which releases salt ions into a lake. The deposition could be developed to produce valuable components.

In category D, “Salinity monitoring and spatial distribution”, Ying Song et al. (contribution 13) identified the spatial and temporal characteristics of cultivated saline lands in coastal areas and provided decision-making support for strengthening the dynamic monitoring and regulation of agricultural soils. Yingxuan Ma et al. (contribution 14) monitored soil salinity using soil measurement data and Sentinel-2 remote sensing data. The results are of great significance to understanding the salinization situation in the Weigan River–Kuqa River Delta Oasis. Wenxiu Li et al. (contribution 15) studied the results of water and salt distribution in a column experiment under three different tillage methods. Ying Song et al. (contribution 16) investigated soil salinity in the spring and summer in a coastal area using a geographic information system and groundwater modeling system. Mengyao Wang et al. (contribution 17) clarified the key mechanisms of the natural regeneration of M. alba for the transformation of a declining R. pseudoacacia plantation. Their study found that the germination ability of maternal trees in saline–alkali land was higher than that in non-saline–alkali land under salt stress.

In category E, “Impact of soil salinity stress on plant growth and nutrient element availability”, Sharaf M. Al-Tardeh et al. (contribution 18) designed a biological experiment to assess the effects of various salinity levels on seed germination using ten certified Palestinian barley cultivars. Some cultivars showed moderate to high resilience to salinity, reaching >80% seed germination at 120 mM NaCl. Chunyan Yin et al. (contribution 19) carried out a split-plot experiment with straw return and various nitrogen application rates to study the nitrification and mineralization of nitrogen in saline soil. They found that straw return and a reduced nitrogen application to saline soil can retain more ammonium nitrogen, thus inhibiting the nitrification of soil nitrogen. Raphaela Revorêdo Bezerra et al. (contribution 20) found that the water efficiency and production components of coriander plants were affected by increases in electrical conductivity in the nutrient solution. Kongming Zhu et al. (contribution 21) determined the ¹⁵N uptake rates in various parts of mature wheat plants and residual ¹⁵N in three different saline soils, as well as ¹⁵N loss in soil–wheat systems, using the ¹⁵N labeled urea N tracing method in the Yellow River Delta. An increase in soil salinity inhibited wheat uptake and soil residues and intensified the losses from ¹⁵N fertilizer. Wenping Xie et al. (contribution 22) found significant correlations between soil salinity and other environmental factors. In their study, soil salinity and alkaline phosphatase activity were the main influencing factors of soil available phosphorus.

Finally, in category F, “Ecosystem strategies for sustainable saline agriculture development”, Adil K. Salman et al. (contribution 23) investigated the impact of osmotic potential on salt-affected soils. The study found a strong correlation between osmotic potential and evapotranspiration. Qianwen Du et al. (contribution 24) found that a BP neural network model could simulate and predict dissolved N₂O concentrations in overlying water under a saline–alkali environment. Luofan Li (contribution 25) analyzed the dynamic degree, landscape type transfer matrix, and landscape indices of landscape types in the Yellow River Delta region. Their results showed that the areas of construction land, salt fields, and breeding ponds in this region had significantly increased. Their study can guide a more sustainable use of land resources. Jian Liu (contribution 26) provide a reference for the scientific evaluation of ecosystem service value (ESV) for saline–alkali lands, as well as a basis for the rational development and utilization of Huanghe Island. Yueying Wang et al. (contribution 27) solved the problem of the regulation of abiotic factors on plant aboveground biomass and diversity on the slopes of coastal agricultural ditches through field surveys of vegetation at different sampling sites. Simeng Chne et al. (contribution 28) investigated different irrigation and drainage practices in a land development project and examined their effects on ecosystem service value in the Yellow River Delta. They revealed that the “Pipeline irrigation + concealed pipe” irrigation and drainage model increased the area of cultivated land by 4.04 km².

3. Research Progress on Salt-Affected Land: Key Scientific Issues

3.1. Salt-Affected Soil Remediation

Salt-affected soils are defined by elevated concentrations of soluble salts, which can significantly impair their physical and chemical properties. Salt-affected soils exhibit several physical and chemical characteristics that render them unsuitable for conventional agricultural practices. High concentrations of salts can lead to osmotic stress in plants, inhibiting water uptake even in well-watered conditions. This stress can result in stunted growth, reduced crop yields, and even plant death [1]. The significance of remediating salt-affected soils needs more attention, particularly in the context of global food security and environmental sustainability. Soil amendments play a crucial role in enhancing the quality of salt-affected soils [2]. Among the various amendments available, gypsum (calcium sulfate) and organic matter are two of the most effective in addressing issues related to soil salinity, particularly in displacing sodium ions and improving soil structure and fertility [3]. Another integral element of an integrated management strategy is crop rotation. Selecting salt-tolerant crops as part of a well-planned rotation can help mitigate salinity levels while enhancing soil structure and fertility. The cultivation of diverse crops not only disrupts pest and disease cycles but also encourages a healthier soil microbiome, which contributes to nutrient cycling and organic matter improvement [4]. Cover crops can also be integrated into the rotation system, providing ground cover that reduces evaporation and helps maintain moisture levels while sequestering salts in their biomass. When these crops are terminated or incorporated into the soil, they contribute organic matter, further enhancing the soil’s structure and eventual fertility.

3.2. Rational Fertilization in Saline Farmland

Many fertilizers contain salts and can thus increase soil salinity. However, certain practices and types of fertilizers can help manage or reduce soil salinity. High salinity can hinder the availability of essential nutrients. Salts compete with plant roots for the uptake of nutrients, particularly macronutrients like potassium and calcium, which are vital for healthy growth and development. Potassium nitrate (KNO₃) is one of the most effective fertilizers for managing soil salinity. This nitrate-based fertilizer not only provides essential nutrients—potassium (K) and nitrogen (N)—but also has a low salt index compared to other commonly used fertilizers. Its unique chemical structure allows it to play a dual role in salinity management [5]. Moreover, bio-organic fertilizer is an effective saline–alkali soil conditioner [6]. Understanding the dynamic changes in fertilizers is the key to implementing suitable management options for re-building the fertility of saline soils.

3.3. Salt Sources and Treatments in Saline Agriculture

In agriculture, salt can originate from various sources and pose significant challenges for crop production [7]. (1) Many irrigation waters contain dissolved salts, which can accumulate in the soil over time, especially in arid regions. (2) Natural salts present in the soil can be mobilized through weathering processes. (3) Some fertilizers are salts themselves (e.g., ammonium nitrate), contributing to soil salinity. (4) In hot climates, evaporation can concentrate salts in the soil surface, leading to salinity issues.

3.4. Salinity Monitoring and Spatial Distribution

Predicting and monitoring soil salinity is important in order to take protective measures against further deterioration of the soil. Soil salinity monitoring and spatial distribution can be effectively achieved through various methods, primarily utilizing remote sensing, geographic information systems (GISs), and field sampling [8]. Tools like Landsat and Sentinel satellites provide multispectral images that can be analyzed to detect soil salinity. Specific indices, such as the Normalized Difference Vegetation Index (NDVI) and salinity index, can help identify salinized areas [9]. Collecting soil samples from various locations for laboratory analysis provides ground truth data that can validate remote sensing and GIS findings. In situ measurements of soil electrical conductivity (EC) can be used to estimate salinity levels directly [10]. Combining remote sensing, GIS, and field data creates a robust framework for monitoring soil salinity. This integrated approach allows for the identification of salinity hotspots and aids in effective land management practices.

3.5. Impact of Soil Salinity Stress on Plant Growth and Nutrient Element Availability

Soil salinization has become a severe problem affecting soil quality and restricting sustainable agricultural development around the world. High salinity creates an osmotic pressure that makes it difficult for plants to absorb water, leading to dehydration and reduced turgor pressure. Salinity reduces the water potential of the soil, creating a situation where water is less available for plant uptake. This condition leads to a reduction in turgor pressure within the plant cells, essential for maintaining structural integrity and growth. Excessive salts can lead to the accumulation of toxic ions (e.g., sodium and chloride), which disrupts cellular functions and can damage plant tissues [11]. High salinity can hinder the uptake of essential nutrients such as potassium, calcium, and magnesium. This is due to competition between sodium ions and these nutrients at the root level [12]. Under saline conditions, plants may experience diminished calcium ion availability, leading to a cascade of negative physiological effects impacting growth and development [13]. Prolonged exposure to soil salinity presents several critical challenges to agricultural systems, significantly affecting crop yield, food security, and soil health. Understanding these long-term implications is essential for developing effective management strategies and enhancing sustainable agricultural practices.

3.6. Ecosystem Strategies for Sustainable Saline Agriculture Development

Sustainable saline agricultural practices provide significant benefits in terms of biodiversity preservation and carbon sequestration, both crucial elements in efforts to combat climate change and enhance ecosystem resilience [14]. Sustainable saline agriculture promotes the cultivation of a variety of salt-tolerant crops, known as halophytes, which can thrive in environments with high salinity. This practice enhances the diversity of plant species in saline ecosystems, contributing to overall biodiversity. The development and cultivation of these salt-tolerant varieties must be coupled with the preservation of traditional biodiversity. For instance, utilizing indigenous plants that have co-evolved with local saline conditions can strengthen ecosystem resilience and adaptability [15].

4. Prospects

The increasing problem of soil salinization is seen as a major challenge for the environment and economy globally, and it is expected to worsen due to projected climate change [16]. Soil salinity problems can arise in various climates, including both natural and human-influenced conditions, but are most common in dry areas where there is not enough rainfall to wash away salt build-up from the plant’s root zone, whether from irrigation or rain. To establish a sustainable and productive agricultural system that ensures food security for the future, it is essential to prioritize certain key areas of saline agriculture. (1) Soil salinity mapping: The rising extent of soil salinity poses a significant worldwide risk to agricultural production, as soil degradation hinders efforts to achieve food security. The expansion of saline soils is also attributable to the increased utilization of marginal waters for irrigation purposes. The reduction in the availability of freshwater has prompted the adoption of treated wastewater or low-salinity water for irrigation, thereby contributing to the expansion of saline soils [17]. We urge that priority be given to the development of remote sensing instruments for future satellite missions that focus on observing spatial and temporal changes in land degradation, including soil erosion and salinity, on a global scale. (2) Improving soil salinity management practices: Optimal soil salinity management is key to sustaining irrigated agriculture. Salinity management options have expanded beyond the provision of essential field drainage to include new technologies in irrigation practices, drainage, soil and crop monitoring, and model prediction. The advancement of precision irrigation (PI) is largely contingent upon the development of cost-effective technologies that integrate soil moisture, salinity, and nutrient measurements within a cloud-based multi-sensor platform [18]. (3) Reassessment of crop salt tolerance: The tolerance of crops to soil salinity varies considerably. The physiological response of crops to salinity is associated with two processes: osmotic and specific ion effects. Key genes for control of Na⁺ and Cl uptake from a saline soil, their transport throughout the plant, and their sequestration within cells have been identified [19]. Although soil saturation extracts were of considerable value in the past, they do not necessarily represent in situ root zone salinity. Furthermore, there is significant uncertainty regarding how the plant integrates multiple stresses across the rooting zone and during its growing season, representing a significant knowledge gap [20]. (4) Improved understanding of the combined and interactive effects of crop drought and salinity stress: Drought and salinity are the two most prevalent abiotic stressors in agricultural crops, and their simultaneous occurrence is relatively common in irrigated fields. Despite their simplicity, empirical reduction functions are useful tools for studying the impacts of drought and salinity stresses on large-scale hydrological models. Currently, detailed process-based approaches are available, which can be used to improve our understanding of how combined water and salinity stresses impact plant transpiration and growth [21,22]. (5) Effects of salinity and sodicity on soil physical properties: In many salt-affected regions where sodium salts are prevalent, salinity and sodicity are inter-related, yet they exert distinct effects on the soil. There is a paucity of research quantifying dynamic soil hydraulic properties, including soil water retention, hydraulic conductivity, solute transport, and soil aeration properties and processes, as influenced by changing soil salinity and sodicity. This is particularly evident when considering the complex nature of soil structure degradation, which is controlled by numerous additional soil properties beyond soil salinity and sodicity alone [23,24]. (6) Limitations and opportunities of using non-conventional water sources for irrigation: It is imperative that irrigated agriculture expands, and that new water sources, which were previously deemed “marginal” (e.g., saline, treated wastewaters, and desalinated water), are employed to meet the growing demands of the future. It is inevitable that the projected intensification of irrigated agriculture in areas utilizing marginal quality water will affect pre-existing fragile environments and threaten the overall sustainability and functionality of these agro-ecosystems [25]. The challenge for the future is to devise strategies that increase food production while simultaneously preserving soil ecological functionality, minimizing human health risks and promoting the sustainable use of our land and water resources for agricultural use [26].

The identification of knowledge gaps and priority needs in saline agriculture will facilitate the allocation of research funding for the advancement of knowledge and innovative solutions. Furthermore, it is our intention to encourage the scientific community to pursue new avenues of salinity research that address the gaps identified by our synthesis.