Approximately 1 billion ha of the global land surface is currently salt-affected, representing about 7% of the earth's land surface. Whereas most of it results from natural geochemical processes, an estimated 30% of irrigated lands globally are salt-affected through secondary human-induced salinization. Application of lower quality, alternative irrigation water is further threatening expansion of the areal extent of soil salinity, in addition to climate change causing increases of salt-water intrusion in coastal areas and increasing crop water requirements. The reduced availability of freshwater resources for irrigation, the continued reduction of the world's cultivated agricultural area by land degradation and urbanization, in conjunction with a growing world population further complicates the problem seeking sustainable solutions. This scoping review prioritizes critical knowledge gaps and makes recommendations for 10 priorities in soil salinity research toward a sustainable and productive agricultural system for a food-secure future world. We also include basin-specific case studies that illustrate progress of the world's major irrigated areas in addressing impacts of soil salinization. By identifying research priorities, we seek to accelerate enhanced research funding to bring new knowledge and innovative solutions toward mitigation of soil salinity impacts. We further want to inspire the science community to develop new directions in salinity research.

The potential of “biosaline agriculture” relies on easy-to-apply tools to select plant genotypes that are best adapted to saline conditions. We aimed to determine the effects of salinity-sodicity on the functional response of alfalfa varieties by evaluating instantaneous vs integrated plant-based measurements for the selection of alfalfa cultivars in biosaline agriculture. Functional responses of three alfalfa varieties were evaluated in a greenhouse study under different saline-sodic conditions. Physiological parameters included instantaneous (gas exchange and chlorophyll fluorescence) vs time-integrated (carbon isotope discrimination -∆13C- and nitrogen isotope composition -δ15N-; specific leaf weight and chlorophyll content) measurements. From all assessed physiological traits, only δ15N was able to effectively discriminate among genotypes WL656HQ, PGI908S and SW8421S, and showed the highest correlation with biomass production at all experimental stages. On average, the δ15N increased by a factor of 3.3 as salinity increased from the non-saline control treatment (ECiw ~ 0.4 dS m−1) to the highest salinity level (ECiw ~ 10.0 dS m−1) indicating that biological N fixation was significantly limited by salinity. Specific leaf weight was also significantly correlated with dry matter although to a much lesser extent than was δ15N. δ15N was found to be the best proxy for assessing alfalfa varieties for their adaptation potential in saline conditions. This parameter was able to discriminate alfalfa’s functional response within a narrow range of irrigation water salinity. δ15N was also capable of differentiating between alfalfa varieties classified as tolerant to salinity defined at a particular plant growth stage, making this an excellent tool for genotypic selection.

Olive trees are iconic to the Mediterranean landscape and in recent times, have expanded to other regions across the globe that share similar climatic conditions. Olive oil production benefits from irrigation, but with a changing climate and uncertainty in precipitation patterns, wastewaters will likely play a larger role supplementing irrigation water requirements. However, due to their relatively poor quality, wastewaters present challenges for sustained long-term use in olive production. Wastewaters include all effluents from municipalities, agricultural drainage, animal production facilities, agricultural processing and industrial processes. This review focuses on potential opportunities and limitations of sustaining olive oil production in the Mediterranean region using wastewater of various sources. The primary challenges for using such wastewaters include concerns related to salinity, sodicity, metals and trace elements, nutrients, organics, and pathogens. Organics and plant nutrients in the effluents are typically beneficial but depend on dosages. Many studies have shown that saline wastewaters have been successfully used to irrigate olives in Greece, Israel, Italy, Jordan and Tunisia. Still, olive varieties and rootstocks have different tolerances to salinity and could respond differently and oil quality may improve or be compromised. Salts and trace elements need to be monitored in plants and soil to make sure accumulation does not continue from year to year and that soil physical conditions are not affected. Some food industries generate effluents with suitable characteristics for irrigation but one must balance the benefits (e.g. addition of nutrients), detriments (e.g. addition of salts or other limiting chemicals) and costs when determining the feasibility and practicality of reuse. Long-term accumulation of trace elements and metals will likely limit the feasibility of using industrial-originating effluents without treatment processes that would remove the toxic constituents prior to reuse. Therefore, untreated wastewaters from the many industries have limited long-term potential for reuse at this time. Application of olive mill wastewater may be agronomically and economically beneficial, particularly as a local disposal solution, but there are concerns associated with high-concentrations of polyphenols that may be phytotoxic and toxic to soil microbial populations. With regards to human safety, risk of contamination of table olives and olive oil is very low because irrigation methods deliver water below the canopy, fruits are not picked from the ground, processing itself eliminates pathogens and the irrigation season typically ends days or weeks before the harvest (depending on the climate condition). Finally, considering physiological, nutritional and intrinsic characteristics of this species, it is clear that olive trees are appropriate candidates for the reuse of recycled water as an irrigation source.

The gradual increase in the amount of land and water resources affected by salt in arid and semi-arid regions requires strategies to optimize the use of these marginal-quality resources. Recent field and greenhouse experiments have demonstrated the potential of growing certain 'pre-selected' varieties of alfalfa in highly saline conditions. A greenhouse study was conducted to determine the impact of irrigation with saline groundwater on alfalfa growth and production in saline-sodic soils. The sustainability of the system in terms of forage yield and quality was also evaluated. The study included three varieties of alfalfa (Medicago sativa, vars. SW8421S, PGI908S and WL656HQ) planted in pots filled with saline-sodic soil (Calcic Haplosalids) collected on the island of Lanzarote (Spain) and irrigated for 18 months with increasingly saline water. Although the yield of the alfalfa varieties was reduced by an average of 7, 20, 31 and 46% as the salinity of the irrigation water increased from 0.4 dS m(-1) to 2.5,5.0, 7.5 and 10.0 dS m(-1), respectively, their relative salt tolerance, based on the average electrical conductivity of the saturated soil extract (ECe), was much higher than those established in the literature. Based on their nutritional quality, all alfalfa varieties are categorized as 'supreme' quality, with metabolizable energy (ME) values in excess of 10 MJ kg(-1). Moreover, no detriment to quality was observed at the higher levels of irrigation water salinity. Mineral composition analysis revealed S, K and B levels near or above the established maximum tolerable levels (MTLs) suggesting that this forage could only be safely consumed by ruminants over the long term if combined with other forages with lower mineral content. (C) 2017 Elsevier B.V. All rights reserved.

•Review of world-wide recycled waste water studies on citrus.•Literature review on citrus tolerance to salinity, specific ions and boron.•Role of scion and rootstock for controlling tolerance to sodium, chloride and boron.•Leaf boron concentrations, toxicities, symptoms and interactions.•Boron tolerance guidelines including relations between soil solution B and irrigation water B.

The sensitivity of wild rice (Zizania palustris) to sulfide is not well understood. Because sulfate in surface waters is reduced to sulfide by anaerobic bacteria in sediments and historical information indicated that 10 mg/L sulfate in Minnesota (USA) surface water reduced Z. palustris abundance, the Minnesota Pollution Control Agency established 10 mg/L sulfate as a water quality criterion in 1973. A 21-d daily-renewal hydroponic study was conducted to evaluate sulfide toxicity to wild rice and the potential mitigation of sulfide toxicity by iron (Fe). The hydroponic design used hypoxic test media for seed and root exposure and aerobic headspace for the vegetative portion of the plant. Test concentrations were 0.3, 1.6, 3.1, 7.8, and 12.5 mg/L sulfide in test media with 0.8, 2.8, and 10.8 mg/L total Fe used to evaluate the impact of iron on sulfide toxicity. Visual assessments (i.e., no plants harvested) of seed activation, mesocotyl emergence, seedling survival, and phytoxicity were conducted 10 d after dark-phase exposure. Each treatment was also evaluated for time to 30% emergence (ET30), total plant biomass, root and shoot lengths, and signs of phytotoxicity at study conclusion (21 d). The results indicate that exposure of developing wild rice to sulfide at >= 3.1 mg sulfide/L in the presence of 0.8 mg/L Fe reduced mesocotyl emergence. Sulfide toxicity was mitigated by the addition of Fe at 2.8 mg/L and 10.8 mg/L relative to the control value of 0.8 mg Fe/L, demonstrating the importance of iron in mitigating sulfide toxicity to wild rice. Ultimately, determination of site-specific sulfate criteria taking into account factors that alter toxicity, including sediment Fe and organic carbon, are necessary. (C) 2017 SETAC.

Halophytes have been considered as potential crops for the reuse of saline drainage water (DW) in the western portion of California's San Joaquin Valley. This management strategy can reduce drainage volumes through plant water consumption and concentrate salts and other contaminants prior to discharge of the final effluent into a solar evaporator. A field study was conducted in order to assess the performance of six halophytes species Salicornia bigelovii, Atriplex lentiformis, Distichlis spicata, Spartina gracilis, Allenrolfea occidentalis and Bassia hyssopifolia, under long-term irrigation (4-6 years) with saline (Na-sulfate dominated) agricultural DW. The suitability of the halophytes was evaluated in terms of biomass production, water consumption, mineral composition and nutritional quality as animal fodder. Results indicate that all species grew well under highly saline-sodic soil conditions (average ECe, = 28.6 dS m(-1); SAR = 39.4), with average standing biomass ranging between 3.8 and 17.4 tons dry matter (DM) ha(-1) depending on species. Under frequent irrigation in drainage lysimeters, daily evapotranspiration (ET) rates for the halophytes were 1.02-1.18 times higher than reference ET (ET0). For S. bigelovii daily ET rates were similar to that of a non-halophytic grass, Festuca arundinacea, irrigated with fresh water (7.5 mm day(-1) vs. 7.4 mm day(-1). Mineral composition and forage quality data indicate several drawbacks associated with the utilization of DW-irrigated halophytes as forages. All species had metabolizable energy (ME) values lower than 7 MJ kg(-1) DM, the minimum value considered to be acceptable quality for most classes of ruminant animals. Additionally, halophyte tissues contained high levels of salts (total ash content ranged between 6 and 52%), and accumulated Na+, Cl-, SO42-, NO3-, B and Se to levels close or above the maximum tolerable levels (MTL). Halophytes tested in this study can be classified as very low quality forage for which long-term grazing is not recommended. However, they could be used as a fodder supplement, if rations can be controlled. (C) 2013 Elsevier B.V. All rights reserved.

Soil pH is known to influence many important biochemical processes in plants and soils, however its role in salinity—boron interactions affecting plant growth and ion relations has not been examined. The purpose of this research was to evaluate the interactive effects of salinity, boron and soil solution pH on broccoli (Brassica oleracea L.) growth, yield, consumptive water use and shoot-boron accumulation.

In the western San Joaquin Valley (SJV) of California, re-use of saline-sodic drainage water (DW) for the irrigation of salt tolerant forages and field crops is an important strategy for drainage water reduction and to extend irrigation water supplies. Drainage reuse designs require estimates of crop water use under saline conditions, so this study employed two methods to estimate the actual evapotranspiration (ETa) of saline-irrigated forages. Using sand-filled drainage lysimeters, ET was measured for ‘Jose’ tall wheatgrass (TWG; Thinopyrum ponticum var. ‘Jose’) creeping wildrye (CWR; Leymus triticoides, var. ‘Rio’) and Paspalum (Paspalum vaginatum, var. ‘SeaIsle’) irrigated with saline DW with an electrical conductivity (EC) averaging 13.2dSm−1. Annual ET in 2005 (20 January to 28 December) was 1470mm for TWG, 1376mm for CWR, and 1275mm for Paspalum. Comparing to standardized reference evapotranspiration (ETo), the annual (ETa/ETo)lys ratios in 2005 were 0.98 for TWG, 0.92 for CWR, and 0.85 for Paspalum. Forage ET was also estimated by a surface renewal (SR) method in large pastures of TWG and CWR irrigated with saline DW. For TWG, the seasonal (ETa/ETo)SR in 2005 was 0.94, slightly lower than the ratio of 0.98 determined by lysimetry. For CWR, the seasonal (ETa/ETo)SR was only 0.73 in 2005; however, in 2006 it was 1.03 which was closer to the ratio of 0.92 determined by lysimetry in 2005. Conditions were less uniform in the CWR pasture as compared to the TWG pasture which may have caused the greater variability in (ETa/ETo)SR for this forage. These findings, along with earlier work, demonstrate that tall wheatgrass, creeping wildrye and Paspalum are suitable candidates for DW re-use systems for the western SJV due to their high ET under saline irrigation and adequate dry matter production and forage quality.

Water for irrigation is a major limitation to agricultural production in many parts of the world. Use of waters with elevated levels of salinity is one likely option to meet the supply of increased demands. The sources of these waters include drainage water generated by irrigated agriculture, municipal wastewater, and poor quality groundwater. Soil salinity leaching requirements that were established several decades ago were based on steady-state conditions. Recently transient-state models have been developed that potentially can more correctly predict the dynamics of the chemical–physical–biological interactions in an agricultural system. The University of California Center for Water Resources appointed a workgroup to review the development of steady-state analyses and transient-state models, and to determine whether the current recommended guidelines for leaching requirement based on steady-state analyses need to be revised. The workgroup concludes that the present guidelines overestimate the leaching requirement and the negative consequences of irrigating with saline waters. This error is particularly large at low leaching fractions. This is a fortuitous finding because irrigating to achieve low leaching fractions provides a more efficient use of limited water supplies.

Two plant stress factors, salinity and high levels of boron, often co-occur in natural and agricultural environments. Many investigations have been conducted to document the influence of the combined stresses on crop growth and yield. Only limited information, however, is available concerning the combined effects of the two stresses on mineral ion uptake and partitioning to shoot organs and tissues. Data for this study were obtained from an experiment conducted in greenhouse sand cultures with two water types: (1) a chloride-dominated system, and (2) sulfate-dominated waters characteristic of those present in the San Joaquin Valley of California. Each saline composition treatment was tested at three salinity levels (2, 12, 18dSm−1) and three B concentrations (0.046, 1.11, 2.22mM; 0.5, 12, 24mgL−1). The gradient in B distribution in individual leaves sampled midseason was steep, with less boron accumulating in petioles than in the blades. The highest B concentrations (>100mmolBkg−1 dry weight) were found in leaf margins of plants grown in the low salinity-high boron treatments. These leaves were cupped upward, an unusual visual symptom characteristic of B toxicity. At final harvest, concentrations of B, Ca, Mg, Na, K, and Cl were highest in the oldest leaves on the broccoli shoots, decreasing acropetally to the heads. Total-P, however, was highest in broccoli heads, decreasing in the older tissues. Leaf-B accumulation was more closely related to salinity rather than to the concentration of Cl in the irrigation waters. Analysis of variance indicated that substrate B had little effect on Cl accumulation in salt-stressed broccoli leaves regardless of water type. No clear cut relationships were evident that would explain the reciprocal effects of B and Cl on crop yields.

Soil salinity is a major factor relating microbial communities to environmental stress in the microbial selection process as stress can reduce bacterial diversity. In the San Joaquin Valley (SJV) of California, the problem of increasing salinity and consequently, decreasing crop productivity, due to reuse of saline drainage water are major concerns. An experiment was conducted in a closed, recirculating volumetric lysimeter system (VLS) consisting of 24 experimental plant growth units to determine the interactive effects of salinity, boron and pH on rhizosphere and non-rhizosphere microbial composition of cucumber (Cucumis sativus L. cv. Seminis Turbo hybrid). Plants in the VLS were irrigated from individual reservoirs containing a modified half-strength Hoagland's nutrient solution combined with salinity, boron (B), and pH treatments. The results indicated that salinity and pH were the most influential factors affecting the growth of plants and the effect of boron on the plant was more severe under slightly acidic conditions. Total bacterial DNA was extracted from rhizosphere and non-rhizosphere samples, and a 236-bp DNA fragment in the V3 region of the small subunit ribosomal RNA genes of eubacteria was amplified. The 16S rRNA and the products were subjected to denaturing gradient gel electrophoresis (DGGE) and sequencing. Analyses of bacterial diversity showed that the effects of salinity, boron, and pH were more severe on the rhizosphere bacterial population during the first week of growing cucumber, with decreasing impacts with plant growth. However, there was no salinity–B–pH interaction effects on plant biomass, but the effects were seen in the number of heterotrophic bacteria in the rhizosphere and on species richness and diversity during week seven of the study. These suggest that the effects of salinity–B–pH interactions may influence microorganisms first before plants and may pose long term effects on soil quality.

Limited research has been conducted on the interactive effects of salinity and boron stresses on plants despite their common occurrence in natural systems. The purpose of this research was to determine and quantify the interactive effects of salinity, salt composition and boron on broccoli (Brassica oleracea L.) performance, particularly, element accumulation, ion interactions and boron uptake processes. A greenhouse experiment was conducted using a sand tank system where salinity-B treatment solutions were supplemented with a complete nutrient solution. Chloride-dominated salinity and salinity characteristic of California's San Joaquin valley (SJV), or sulfate-dominated, were tested at ECw (electrical conductivity of the irrigation water) levels of 2, 12 and 19dSm−1. Each salinity treatment consisted of boron treatments of 0.5, 12 and 24mgL−1. Salinity, regardless of salt composition, reduced shoot boron concentration at very high boron concentration (24mgL−1). However, increased salinity increased shoot boron concentration when external boron concentration was low (0.5mgL−1). Tissue Ca, Mg, Na, K, S and Cl concentrations were also affected by salinity level, chloride or sulfate salinity composition, and in some cases by substrate boron concentration. Calcium concentrations in shoots were greater for chloride-treated salinity as compared to SJV salinity-treated plants; magnesium concentrations trended opposite and were greater in those treated with SJV salinity. Chloride and sodium shoot concentrations both increased with salinity. Shoot chloride was greater with chloride substrate salinity and shoot sodium was greater with SJV substrate salinity. Using stable isotope analysis of solutions to separate transpiration from evapotranspiration (ET), we found that boron uptake and accumulation in the shoot was not simply the product of mass flow (solution concentration×cumulative transpiration), and the vast majority of the water lost from the tank system was by transpiration (>90%) regardless of treatment. Under low substrate boron, the levels of boron in broccoli shoots could be not accounted for by simple passive uptake and transport in the transpiration stream, which suggests that some energy-dependent process was also occurring. However, under high boron treatments, broccoli plants exhibited a mechanism that restricted boron uptake, transport and accumulation in the shoot.

A model was developed to predict rootzone salinity under different irrigation practices on different soil types, with similar rainfall but different monthly distributions. A rootzone daily water and salt balance was performed using eight scenarios: two soil types (coarse textured vs. fine textured), two multi-year series of actual rainfall data and two irrigation practices (surface with fixed number of irrigations and ET-based sprinkler irrigation). All factors influenced the mean electrical conductivity (EC) of the rootzone in the growing season (ECeS): (i) Surface irrigation led to lower ECeS than sprinkler irrigation; (ii) Winter-concentrated rainfall caused lower ECeS than rainfall distributed uniformly throughout the year; and (iii) Coarser-textured soil usually resulted in lower ECeS than the finer textured. The ECeS was related to the total precipitation of the hydrologic year and to the annual leaching fraction (LF) but surprisingly not to the seasonal LF. In most cases, the model predicted lower ECeS than the FAO steady-state approach. Therefore, considering these site-specific features could lead to lower leaching requirements and the safe use of higher salinity water.

Research addressing the interactive effects of the dual plant stress factors, excess boron and salinity, on crop productivity has expanded considerably over the past few years. The purpose of this research was to determine and quantify the interactive effects of salinity, salt composition and boron (B) on broccoli (Brassica oleracea L.) fresh head yield, biomass distribution and consumptive water use. A greenhouse experiment was conducted using a sand-tank system in which salinity–B treatment solutions were supplemented with a complete nutrient solution. Chloride-dominated salinity and salinity characteristic of California's San Joaquin valley (SJV), or sulfate-dominated, were tested at ECw levels of 2, 12 and 19dSm−1. Each salinity treatment consisted of boron treatments of 0.5, 12 and 24mgL−1. Plant head yield and shoot biomass were significantly reduced by both salinity and boron. Moreover, there was a significant salinity–boron interaction where increased boron was relatively less detrimental under saline conditions. These results occurred regardless of the salt solution composition (chloride or SJV). We found that an ‘interactive model’ better described our growth response than did a ‘single stressor yield model’. Salinity and boron also affected the distribution of shoot biomass. Regardless of salt type, as salinity increased, the fraction of biomass as leaf tissue increased while the biomass fraction as stems and particularly heads, decreased. However, an increase in B at low or high salinity with the SJV composition, decreased the head biomass fraction. This was not observed at moderate salinity, nor on any plants treated with Cl-dominated salinity. Cumulative evapotranspiration (ET) was also reduced by increased salinity but water use efficiency (WUE) was not. WUE was reduced by increased boron, but only at the low and high salinity levels.