Proton release by N2‐fixing plant roots: A possible contribution to phytoremediation of calcareous sodic soils

With a world‐wide occurrence on about 560 million hectares, sodic soils are characterized by the occurrence of excess sodium (Na⁺) to levels that can adversely affect crop growth and yield. Amelioration of such soils needs a source of calcium (Ca²⁺) to replace excess Na⁺ from the cation exchange sites. In addition, adequate levels of Ca²⁺ in ameliorated soils play a vital role in improving the structural and functional integrity of plant cell walls and membranes. As a low‐cost and environmentally feasible strategy, phytoremediation of sodic soils — a plant‐based amelioration — has gained increasing interest among scientists and farmers in recent years. Enhanced CO₂ partial pressure (P_CO2) in the root zone is considered as the principal mechanism contributing to phytoremediation of sodic soils. Aqueous CO₂ produces protons (H⁺) and bicarbonate (HCO₃^‐). In a subsequent reaction, H⁺ reacts with native soil calcite (CaCO₃) to provide Ca²⁺ for Na⁺ Ca²⁺ exchange at the cation exchange sites. Another source of H⁺ may occur in such soils if cropped with N₂‐fixing plant species because plants capable of fixing N₂ release H⁺ in the root zone. In a lysimeter experiment on a calcareous sodic soil (pH_s = 7.4, electrical conductivity of soil saturated paste extract (EC_e) = 3.1 dS m^‐1, sodium adsorption ratio (SAR) = 28.4, exchangeable sodium percentage (ESP) = 27.6, CaCO₃ = 50 g kg^‐1), we investigated the phytoremediation ability of alfalfa (Medicago sativa L.). There were two cropped treatments: Alfalfa relying on N₂ fixation and alfalfa receiving NH₄NO₃ as mineral N source, respectively. Other treatments were non‐cropped, including a control (without an amendment or crop), and soil application of gypsum or sulfuric acid. After two months of cropping, all lysimeters were leached by maintaining a water content at 130% waterholding capacity of the soil after every 24±1 h. The treatment efficiency for Na⁺ removal in drainage water was in the order: sulfuric acid > gypsum = N₂‐fixing alfalfa > NH4NO3‐fed alfalfa > control. Both the alfalfa treatments produced statistically similar root and shoot biomass. We attribute better Na+ removal by the N₂‐fixing alfalfa treatment to an additional source of H⁺ in the rhizosphere, which helped to dissolve additional CaCO₃ and soil sodicity amelioration.     

Driving forces for sodium removal during phytoremediation of calcareous sodic and saline–sodic soils: a review

Abstract. Sodic and saline–sodic soils are characterized by the occurrence of sodium (Na⁺) at levels that result in poor physical properties and fertility problems, adversely affecting the growth and yield of most crops. These soils can be brought back to a highly productive state by providing a soluble source of calcium (Ca²⁺) to replace excess Na⁺ on the cation exchange complex. Many sodic and saline–sodic soils contain inherent or precipitated sources of Ca²⁺, typically calcite (CaCO₃), at varying depths within the profile. Unlike other Ca²⁺ sources used in the amelioration of sodic and saline‐sodic soils, calcite is not sufficiently soluble to effect the displacement of Na⁺ from the cation exchange complex. In recent years, phytoremediation has shown promise for the amelioration of calcareous sodic and saline–sodic soils. It also provides financial or other benefits to the farmer from the crops grown during the amelioration process. In contrast to phytoremediation of soils contaminated by heavy metals, phytoremediation of sodic and saline–sodic soils is achieved by the ability of plant roots to increase the dissolution rate of calcite, resulting in enhanced levels of Ca²⁺ in soil solution to replace Na⁺ from the cation exchange complex. Research has shown that this process is driven by the partial pressure of CO₂ (P_CO2) within the root zone, the generation of protons (H⁺) released by roots of certain plant species, and to a much smaller extent the enhanced Na⁺ uptake by plants and its subsequent removal from the field at harvest. Enhanced levels of P_CO2 and H⁺ assist in increasing the dissolution rate of calcite. This results in the added benefit of improved physical properties within the root zone, enhancing the hydraulic conductivity and allowing the leaching of Na⁺ below the effective rooting depth. This review explores these driving forces and evaluates their relative contribution to the phytoremediation process. This will assist researchers and farm advisors in choosing appropriate crops and management practices to achieve maximum benefit during the amelioration process.     

Amelioration of calcareous saline sodic soils through phytoremediation and chemical strategies

Abstract. The worldwide occurrence of saline sodic and sodic soils on more than half a billion hectares warrants attention for their efficient, inexpensive and environmentally acceptable management. These soils can be ameliorated by providing a source of calcium (Ca²⁺) to replace excess sodium (Na⁺) from the cation exchange sites. Although chemical amendments have long been used to ameliorate such soils, the chemical process has become costly during the last two decades in several developing countries. As a low‐cost and environmentally acceptable strategy, the cultivation of certain salt tolerant forage species on calcareous sodic and saline sodic soils, i.e. phytoremediation, has gained interest among scientists and farmers in recent years. In a field study conducted at three calcareous saline sodic sites (pH_s=8.1–8.8, EC_e=7.8–12.5 dS m^–1, SAR=30.6–76.1) in the Indus Plains of Pakistan, we compared chemical and phytoremediation methods. There were four treatments; two involved plants: Kallar grass (Leptochloa fusca (L.) Kunth), and sesbania (Sesbania bispinosa (Jacq.) W. Wight). The other two treatments were uncropped: soil application of gypsum and an untreated control. All treatments were irrigated with canal water (EC=0.22–0.28 dS m^–1). The plant species were grown for one season (5–6 months). Sesbania produced more forage yield (34 t ha^–1) than Kallar grass (23 t ha^–1). Phytoremediation and chemical treatments resulted in similar decreases in soil salinity and sodicity, indicating that phytoremediation may replace or supplement the more costly chemical approach. The soil amelioration potential of sesbania was similar to that of the Kallar grass, which suggests that moderately saline sodic calcareous soils can be improved by growing a forage legume with market value.     

SOIL PROPERTY CHANGES FOLLOWING IRRIGATION WITH COALBED NATURAL GAS WATER: ROLE OF WATER TREATMENTS,SOIL AMENDMENTS AND LAND SUITABILITY

Saline‐sodic water is a by‐product of coalbed natural gas (CBNG) production in the Powder River Basin of Wyoming, USA and is being beneficially used in places as irrigation water. This study evaluated effects of 2 years of natural precipitation on soil properties of a hay field after the cessation of managed irrigation with CBNG water. The hay field had been irrigated with only CBNG water [CBNG(NT)], CBNG water amended with gypsum [CBNG(G)] or gypsum plus sulfur via a sulfur burner [CBNG(GSB)] in combination with soil amendments—gypsum ( +G ), elemental sulfur ( +S ), and both ( +GS ). Results indicated that infiltration rates were the lowest on fields irrigated with CBNG(NT), followed by CBNG(G) and CBNG(NT) +G treatments (12·2, 13·2, and 13·5 cm h⁻¹, respectively). The CBNG(GSB) +GS treatment had the highest infiltration rates (33·5 cm h⁻¹). By the second year, salinity and sodicity of treated soils had decreased in the A‐horizon of most CBNG‐water irrigated plots, whereas in Bt₁‐ and Bt₂‐horizons salinity generally decreased but sodicity increased; S and GS soil amended plots had higher profile salinities compared with NT and G soil treatments. Although Na⁺ leaching was observed in all fields that received soil and/or water amendments, CBNG(GSB) +GS plots had the lowest sodicity in the A‐ and Bt₁‐horizons. Effective managed irrigation requires knowledge of site‐specific soil properties, plant suitability, water chemistry, and amendments that would be needed to treat the CBNG waters and soils. This study indicates the greatest success was realized when using both soil and water amendments. Copyright © 2011 John Wiley & Sons, Ltd.     

IONIC DISPLACEMENT AND RECLAMATION OF SALINE‐SODIC SOILS USING CHEMICAL AMENDMENTS AND CROP ROTATION

Biological, chemical and bio‐chemical strategies have been tested in the past for reclamation of saline‐sodic and sodic soils. The efficiency of two crop rotations (rice‐wheat and Sesbania‐wheat) alone or in combination with either gypsum (CaSO₄.2H₂O) or sulfuric acid (H₂SO₄) was tested for ionic displacement from four saline‐sodic soils. Pure gypsum was applied at 50 per cent of soil gypsum requirement at the time of planting rice and Sesbania, whereas 95 per cent pure sulfuric acid was added at 50 per cent soil gypsum requirement as one‐third applications by mixing with the first three irrigations. The rice crop biomass decreased at a soil saturation extract electrical conductivity (EC_e) of 8 dS m⁻¹, whereas wheat and Sesbania were influenced at a sodium adsorption ratio (SAR) of ≥40. Gypsum treatment helped the crops flourish well at these EC_e and SAR levels. The infiltrated volume of water dropped with decrease in EC_e : SAR ratio of soils and increase in crop biomass production. Crops rotation treatments alone helped leach sodium (Na⁺) and other ions successfully at SAR ≤ 21 but were less effective at SAR ≥ 40 at which point plants growth was also curtailed. Gypsum and H₂SO₄ treatments significantly aided leaching of Na⁺ and other ions with water at SAR ≥ 40 under both the crop rotations. Hence, crops effectively reclaimed soil at low sodicity level, whereas at high SAR, chemical amendments are obligatory in order to reclaim soils. This study also suggests that the required dose of H₂SO₄ should be applied with pre‐planting irrigation for better yield of the first crop. Copyright © 2011 John Wiley & Sons, Ltd.     

Sodicity‐induced land degradation and its sustainable management: problems and prospects

Currently at least 20 per cent of the world's irrigated land is salt‐affected. However, projections of global population growth, and of an increased demand for food and fibre, suggest that larger areas of salt‐affected soil will need to be cropped in the future. About 60 per cent of salt‐affected soils are sodic, and much of this land is farmed by smallholders. Ameliorating such soils requires the application of a source of calcium (Ca²⁺), which replaces excess sodium (Na⁺) at the cation exchange sites. The displaced Na⁺ is then leached from the root zone through excess irrigation, a process that requires adequate flows of water through the soil. However, it must now be recognized that we can no longer conduct sodic soil amelioration and management solely with the aim of achieving high levels of crop productivity. The economic, social, and environmental impacts of different soil‐amelioration options must also be considered. A holistic approach is therefore needed. This should consider the cost and availability of the inputs needed for amelioration, the soil depth, the level to which sodicity needs to be reduced to allow cropping, the volume and quality of drainage water generated during amelioration, and the options available for drainage‐water disposal or reuse. The quality and cost of water available for post‐amelioration crops, and the economic value of the crops grown during and after amelioration should also be taken into account, as should farmers' livelihoods, the environmental implications of amelioration (such as carbon sequestration), and the long‐term sustainable use of the ameliorated site (in terms of productivity and market value). Consideration of these factors, with the participation of key stakeholders, could sustainably improve sodic soil productivity and help to transform such soils into a useful economic resource. Such an approach would also aid environmental conservation, by minimizing the chances of secondary sodicity developing in soils, particularly under irrigated agriculture. Copyright © 2006 John Wiley & Sons, Ltd.     

Amelioration strategies for sodic soils: a review

Sodic soils are characterized by the occurrence of excess sodium (Na⁺) to levels that can adversely affect soil structure and disturb availability of some nutrients to plants. Such changes ultimately affect crop growth and yield. There are large areas of the world that exist under sodic soils and need attention for efficient, inexpensive and environmentally feasible amelioration. Sodic soil amelioration involves increase in calcium (Ca²⁺) on the cation exchange sites at the expense of Na⁺. The replaced Na⁺ together with excess soluble salts, if present, is removed from the root zone through infiltrating water as a result of excessive irrigations. Records nearly a century old reveal the use of water, crop, chemical amendment, electric current, and tillage as amelioration tools for such soils. Among the amelioration strategies, chemical amendments have an extensive usage. Owing to gradual increases in amendment cost in some parts of the world during the last two decades, this amelioration strategy has become cost‐intensive, particularly for the subsistence farmers in developing countries. In the meantime, phytoremediation with low initial investment has emerged as a potential substitute of chemical amelioration. Phytoremediation works through plant root action that helps dissolve native soil calcite (CaCO₃) of low solubility to supply adequate levels of Ca²⁺ for an effective Na⁺−Ca²⁺ exchange without the application of an amendment. Although significant progress has been achieved in improving amelioration methods, a great deal of work remains to analyse the economics of such methods with focus on (1) the long‐term sustainability of the amelioration projects and (2) the consequences of amelioration for the farmer himself, other growers and society as a whole. Computer modelling may help assess economic viability of different soil amelioration methods to extend results broadly to other similar locations. In addition, computer modelling to stimulate movement and reactions of salts in sodic soils has been a potentially useful complement to experimental data. However, such models need evaluation under field conditions. Copyright © 2001 John Wiley & Sons, Ltd.     

Salt-tolerant forage cultivation on a saline-sodic field for biomass production and soil reclamation

Chemical reclamation of sodic and saline-sodic soils has become cost-intensive. Cultivation of plants tolerant of salinity and sodicity may mobilize the CaCO₃ present in saline-sodic soils instead of using a chemical approach. Four forage plant species, sesbania (Sesbania aculeata), kallar grass (Leptochloa fusca), millet rice (Echinochloa colona) and finger millet (Eleusine coracana), were planted in a calcareous saline-sodic field (EC_e = 9·6–11·0 dS m⁻¹, SAR = 59·4–72·4). Other treatments included gypsum (equivalent to 100 per cent of the gypsum requirement of the 15 cm soil layer) and a control (no gypsum or crop). The crops were grown for 5 months. The performance of the treatments in terms of soil amelioration was in the order: Sesbania aculeata ≅ gypsum > Leptochloa fusca > Echinochloa colona > Elusine coracana > control. Biomass production by the plant species was found to be directly proportional to their reclamation efficiency. Sesbania aculeata produced 32·3 Mg forage ha⁻¹, followed by Leptochloa fusca (24·6 Mg ha⁻¹), Echinochloa colona (22·6 Mg ha⁻¹) and Eleusine coracana (5·4 Mg ha⁻¹). Sesbania aculeata emerged as the most suitable biotic material for cultivation on salt-affected soils to produce good-quality forage, and to reduce soil salination and sodication processes.     

Degradation processes and nutrient constraints in sodic soils

Accumulation of excess sodium (Na⁺) in a soil causes numerous adverse phenomena, such as changes in exchangeable and soil solution ions and soil pH, destabilization of soil structure, deterioration of soil hydraulic properties, and increased susceptibility to crusting, runoff, erosion and aeration, and osmotic and specific ion effects on plants. In addition, serious imbalances in plant nutrition usually occur in sodic soils, which may range from deficiencies of several nutrients to high levels of Na⁺. The structural changes and nutrient constraints in such soils ultimately affect crop growth and yield. The principal factor that determines the extent of adverse effects of Na⁺ on soil properties is the accompanying electrolyte concentration in the soil solution, with low concentration promoting the deleterious effects of exchangeable Na⁺ even at exchangeable sodium percentage (ESP) levels less than 5. Consequent to an increase in the use of poor quality waters and soils for crop production, the problems of sodic soils can be expected to increase in future. The mechanisms that explain sodic behaviour can provide a framework in which slaking, swelling and dispersion of clay together with nutrient constraints in sodic soils may be assessed so that the practices to manage such soils can be refined for long‐term sustainable agriculture. Copyright © 2002 John Wiley & Sons, Ltd.     

Hydraulic conductivity of calcareous soils as affected by salinity and sodicity. II. effect of gypsum application and flow rate of leaching solution

Abstract

Saline‐sodic irrigation water, coupled with low annual rainfall and high evapotranspiration in the arid and semi‐arid regions have resulted in accumulation of soluble salts in the soil solution and of cations (especially sodium ions) on exchange sites, which can alter the structure and, consequently, affect the soil hydraulic conductivity (HC). Among the different factors, the amount of gypsum applied and the flow rate of leaching solution are major factors influencing the HC of the soil in the presence of saline sodic solutions. The study was initiated to improve the understanding of swelling and dispersion processes (as two major mechanisms responsible for reduction in HC) in response to saline‐sodic conditions, in particular, the role of gypsum application and the flow rate of leaching solutions. The study was conducted in a series of two leaching experiments. In the first set, different rates of gypsum (i.e., 0, 10, 20, and 30 tons ha^‐1) were mixed with 4 soil samples and leached with a saline‐sodic solution of concentration of 100 meq (NaCl+CaCl₂)L^‐1 with sodium adsorption ratio (SAR) 20 and the base flow rate (BFR) of 15 mL min^‐1. In the second set of experiments, the same soils treated with the same gypsum level and the same leaching solutions as in the first set, but leached with the BFR of 5 mL min^‐1 instead of the BFR of 15 mL min^‐1. In general, the gypsum application modified the suppressing effect of salinity and sodicity on the HC values of the tested soils, and the effects were more pronounced for higher rates of gypsum applied. However, increase in the BFR from 5 to 15 mL min^‐1, significantly masked this recovering effect of gypsum application, and the effect was reflected in both swelling and dispersion processes.     

Reclamation of a coastal reclaimed tidal land soil by gypsum and rice straw

This study was to evaluate the application effect of gypsum and rice straw on the saline–sodic soils of coastal reclaimed tidal lands during newly reclamation process by leaching method. Soil used in the lysimeter experiment is classified as Greyic Hydragric Anthrosols (Sodic Arenic). The experiment was carried out over a period of 1 year from May 2014 to July 2015 inside a plastic film house. Saturated electrical conductivity (EC_e) and exchangeable sodium percentage (ESP) values of soils in gypsum treatment and gypsum with rice straw treatment then reached the desalinization criteria (EC_e = 4.0 dS m^?1, ESP = 15%) after the five or six pore volumes leaching cycles. However, in spite the 10 pore volumes leaching cycles in control and rice straw alone treatment, the values did not reach the desalinization criteria. The results showed that effectiveness of improvement of sodicity and salinity in coastal reclaimed tidal land soils was in the order of gypsum = gypsum + rice straw > rice straw = control. Our study has shown that gypsum alone treatment or gypsum with rice straw treatment in a coastal reclaimed tidal land soils improves their physicochemical characteristics.     

Sodicity Intensifies the Effect of Salinity on Grain Yield and Yield Components of Wheat

ABSTRACT

This study reports the relationship of the leaf ionic composition with the grain yield and yield components of wheat in response to salinity x sodicity and salinity alone. The study was conducted in soil culture in pots with three treatments including control (ECe 2.6 dS m^{? 1} and SAR 4.53), salinity (ECe 15 dS m^{? 1} and SAR 9.56), and salinity x sodicity (ECe 15 dS m^{? 1} and SAR 35). The soil was treated before being put in the pots and the pots were arranged in a completely randomized factorial arrangement with five replications. The seeds of three wheat genotypes were sown directly in the pots and the study was continued till the crop maturity. At booting stage, the leaf second to the flag leaf of each plant was collected and analyzed for sodium (Na⁺), potassium (K⁺), and chloride (Cl^?). At maturity, plants were harvested and data regarding grain yield and yield components were recorded. This study shows that salinity and sodicity in combination decreases the grain yield of wheat more than the salinity alone with a greater difference in the sensitive genotype. This study also shows that as for salinity, the maintenance of lower Na⁺ and higher K⁺ concentrations and higher K⁺: Na⁺ ratio in the leaves relates positively with the better development of different yield components and higher grain yield in saline sodic soil conditions. Although, the leaf Cl^? concentration was increased significantly by salinity as well as salinity x sodicity and would have affected the growth and yield, yet it does not seem to determine the genotypic tolerance or sensitivity to either salinity or salinity x sodicity.     

Implication of Gypsum Rates to Optimize Hydraulic Conductivity for Variable‐Texture Saline–Sodic Soils Reclamation

Sodium (Na⁺) dominated soils reduce saturated hydraulic conductivity (K_s) by clay dispersion and plugging pores, while gypsum (CaSO₄•2H₂O) application counters these properties. However, variable retrieval of texturally different saline–sodic soils with gypsum at soil gypsum requirement (SGR) devised to define its quantity best suited to improve K_s, leach Na⁺ and salts. This study comprised loamy‐sand (LS), sandy loam (SL), and clay loam (CL) soils with electrical conductivity of saturation extract (EC_e) of ~8 dS m⁻¹, sodium adsorption ratio (SAR) of ~44 (mmol L⁻¹)^1/2 and exchangeable sodium of ~41%, receiving no gypsum (G₀), gypsum at 25% (G₂₅), 50% (G₅₀) and 75% (G₇₅) of SGR. Soils packed in lysimeters were leached with low‐carbonate water [EC at 0·39 dS m⁻¹, SAR at 0·56 (mmol L⁻¹)^1/2 and residual sodium carbonate at 0·15 mmol_c L⁻¹]. It proved that a rise in gypsum rate amplified K_s of LS ≫ SL > CL. However, K_s of LS soil at G₂₅ and others at G₇₅ remained efficient for salts and Na⁺ removal. Retention of calcium with magnesium (Ca²⁺ + Mg²⁺) by LS and SL soils increased by G₅₀ and decreased in G₇₅, while in CL, it also increased with G₇₅. The enhanced Na⁺ leaching efficiency in LS soil with G₂₅ was envisaged by water stay for sufficient time to dissolve gypsum and exchange and leach out Na⁺. Overall, the superiority of gypsum for LS at G₂₅, SL at G₅₀ and CL at G₇₅ predicted cost‐effective soil reclamation with a decrease in EC_e and SAR below 0·97 dS m⁻¹ and 5·92 (mmol L⁻¹)^1/2, respectively. Copyright © 2015 John Wiley & Sons, Ltd.     

Can highly saline irrigation water improve sodicity and alkalinity in sodic clayey subsoils?

Purpose

The concept of irrigating crops with saline irrigation water is not new, but impacts of this practice on soil properties remain debatable, particularly the use of highly saline water. In this work, key soil chemical properties were assessed to a depth of 300 cm following 2.5 years of application of highly saline irrigation to a sodic texture-contrast soil (Brown Sodosol) in south-eastern Tasmania, Australia.

Materials and methods

Control plots (rainfall only) were compared to irrigation treatments of low (0.8 dS/m) and high salinity (16 dS/m) waters at application rates of both 200 and 800 mm/year.

Results and discussion

Whilst significant increases in both electrical conductivity and chloride concentration occurred throughout the soil profile in the high salinity treatment, these values were well below those of the irrigation water, indicating effective deep leaching. In the upper soil profile, 0–50 cm, of the high salinity treatments both the exchangeable Na⁺ and its ratio to total base cations (ESP) were significantly increased whilst the lower soil profile between 50 and 200 cm, was improved via both reduced alkalinity and sodicity. Leaching of the exchangeable base cations Ca²⁺, Mg²⁺ and K⁺ was significant in the upper soil profile (0–50 cm). As expected, the low salinity treatment (0.8 dS/m) had minimal impacts on soil chemical properties. The upper topsoil (0–10 cm) total organic carbon was significantly reduced in the high salinity plots and was negatively correlated with Cl^? concentration.

Conclusions

The data confirms the general concerns about application of saline irrigation, namely increased whole profile salinisation and upper soil profile (0–50 cm) sodicity, but they also show unexpected and desirable reductions in the lower soil profile (>?50 cm) alkalinity and sodicity. It appears the Na⁺ ions present in the saline waters led to differential leaching of base cations from the rooting zone, especially Ca²⁺ which then ameliorate the alkalinity and sodicity deeper in the soil profile (>?50 cm). Thus, surface application of gypsum may help sustain the application of highly saline waters; alternatively, subsurface irrigation above the sodic clayey subsoils could be trailed.

     

盐分对土壤呼吸和有机质动力学的影响与土壤电导率相关，而与渗透势的关系更为密切

Osmotic potential (OP) of soil solution may be a more appropriate parameter than electrical conductivity (EC) to evaluate the effect of salts on plant growth and soil biomass.However,this has not been examined in detail with respect to microbial activity and dissolved organic matter in soils with different texture.This study evaluated the effect of salinity and sodicity on respiration and dissolved organic matter dynamics in salt-affected soils with different texture.Four non-saline and non-sodic soils differing in texture (S-4,S-13,S-24 and S-40 with 4％,13％,24％ and 40％ clay,respectively) were leached using combinations of 1 mol L-1 NaC1 and 1 mol L-1 CaC12 stock solutions,resulting in EC (1:5 soil:water ratio) between 0.4 and 5.0 dS m-1 with two levels of sodicity (sodium absorption ratio (SAR) ＜ 3 (non-sodic) and 20 (sodic),1:5 soil:water ratio).Adjusting the water content to levels optimal for microbial activity,which differed among the soils,resulted in four ranges of OP in all the soils:from-0.06 to--0.24 (controls,without salt added),-0.55 to-0.92,-1.25 to-1.62 and-2.77 to-3.00 Mpa.Finely ground mature wheat straw (20 g kg-1) was added to stimulate microbial activity.At a given EC,cumulative soil respiration was lower in the lighter-textured soils (S-4 and S-13) than in the heavier-textured soils (S-24 and S-40).Cumulative soil respiration decreased with decreasing OP to a similar extent in all the soils,with a greater decrease on Day 40 than on Day 10.Cumulative soil respiration was greater at SAR =20 than at SAR ＜ 3 only at the OP levels between-0.62 and-1.62 MPa on Day 40.In all the soils and at both sampling times,concentrations of dissolved organic C and N were higher at the lowest OP levels (from-2.74 to-3.0 MPa) than in the controls (from-0.06 to-0.24 MPa).Thus,OP is a better parameter than EC to evaluate the effect of salinity on dissolved organic matter and microbial activity in different textured soils.     

Leaching and reclamation of calcareous saline‐sodic soil by moderately saline and moderate‐SAR water using gypsum and calcium chloride

Lysimeter experiments were conducted with sandy‐clay‐loam soil to study the efficiency of two amendments in reclaiming saline‐sodic soil using moderately saline and SAR (sodium‐adsorption ratio) irrigation water. Gypsum obtained from industrial phosphate by‐products and reagent grade Ca chloride were applied to packed soil columns and irrigated with moderately saline (ECe = 2.16 dS m^–1), moderate‐SAR water (SAR = 4.8). Gypsum was mixed with soil prior to irrigation at application rates of 5, 10, 15, 20, 25, and 32 Mg ha^–1, and Ca chloride was dissolved directly in leaching water at application rates of 4.25, 8.5, 12.75, 17.0, and 21.25 Mg ha^–1, respectively. The highest application rate in both amendments resulted in 96% reduction of total Na in soil. The hydraulic conductivity (HC) of soils receiving gypsum increased in all treatments. The highest HC value of 6.8 mm h^–1 was obtained in the highest application rate (32 Mg ha^–1), whereas the lowest value of 5.2 mm h^–1 was observed with the control treatment. Both amendments were efficient in reducing soil salinity and sodicity (exchangeable‐sodium percentage, ESP); however, Ca chloride was more effective than gypsum as a reclaiming material. Exchangeable Na and soluble salts were reduced with gypsum application by 82% and 96%, and by 86% and 93% with Ca chloride application, respectively. Exchangeable Ca increased with increasing amendment rate. Results of this study revealed that sodium was removed during cation‐exchange reactions mostly when the SAR of effluent water was at maximum with subsequent passage of 3 to 4 pore volumes. Gypsum efficiently reduced soil ESP, soil EC, leaching water, and costs, therefore, an application rate of 20 Mg ha^–1 of gypsum with 3 to 4 pore volumes of leaching water is recommended for reclaiming the studied soil.     

SOIL SALINITY AND SODICITY APPRAISAL BY ELECTROMAGNETIC INDUCTION IN SOILS IRRIGATED TO GROW COTTON

In the Far West Texas region in the USA, long‐term irrigation of fine‐textured valley soils with saline Rio Grande River water has led to soil salinity and sodicity problems. Soil salinity [measured by saturated paste electrical conductivity (EC_e)] and sodicity [measured by sodium adsorption ratio (SAR)] in the irrigated areas have resulted in poor growing conditions, reduced crop yields, and declining farm profitability. Understanding the spatial distribution of EC_e and SAR within the affected areas is necessary for developing management practices. Conventional methods of assessing EC_e and SAR distribution at a high spatial resolution are expensive and time consuming. This study evaluated the accuracy of electromagnetic induction (EMI), which measures apparent electrical conductivity (EC_a), to delineate EC_e and SAR distribution in two cotton fields located in the Hudspeth and El Paso Counties of Texas, USA. Calibration equations for converting EC_a into EC_e and SAR were derived using the multiple linear regression (MLR) model included in the EC_e Sampling Assessment and Prediction program package developed by the US Salinity Laboratory. Correlations between EC_a and soil variables (clay content, EC_e, SAR) were highly significant (p ≤ 0·05). This was further confirmed by significant (p ≤ 0·05) MLRs used for estimating EC_e and SAR. The EC_e and SAR determined by EC_a closely matched the measured EC_e and SAR values of the study site soils, which ranged from 0·47 to 9·87 dS m⁻¹ and 2·27 to 27·4 mmol^1/2 L^−1/2, respectively. High R² values between estimated and measured soil EC_e and SAR values validated the MLR model results. Results of this study indicated that the EMI method can be used for rapid and accurate delineation of salinity and sodicity distribution within the affected area. Copyright © 2012 John Wiley & Sons, Ltd.     

Effect of Municipal Wastewater as a Wetland Water Source on Soil Microbial Activity

Microbial activity levels of two soil materials, excavated from a wetland and irrigated with municipal wastewater effluent or Missouri River water, were compared. The wastewater had twice the electrical conductivity and four times the sodium concentration as river water. We performed activity assays on the soils before leaching, immediately after leaching, and after harvesting plants. Gas chromatography was used to measure carbon dioxide (CO₂) evolved in soil samples incubated for 7 d. Activity was significantly reduced in preleached wastewater–irrigated soils compared with river water–irrigated soils. Immediately after leaching, activity significantly increased and was similar to river water–irrigated soils. Activity decreased slightly after plant harvest in postleached treatments. Increased activity after leaching may be related to decreased salinity and sodicity, which probably lowered osmotic pressure in the soil. Our study demonstrated that soil salinity and sodicity induced by wastewater irrigation decreased microbial activity, which may impact nutrient cycling and glycophytic vegetation communities in wetlands.     

Sodic Soil Reclamation Potential of Gypsum and Biocharadditions: Influence on Physicochemical Properties and Soil Respiration

Reclamation of sodic soils is proving increasingly vital as greater land area becomes salt-affected in the northern Great Plains of the United States. Flue gas desulfurization gypsum (FGDG) can be an agriculturally important resource for increasing land productivity through the amelioration of sodic soils. Biochar is also considered as an aid in reclaiming degraded soils. In this incubation study, two rates of FGDG (33.6 Mg ha^?1 and 66.2 Mg ha^?1), two rates of biochar made from sugar beet (Beta vulgaris L.) pulp (16.8 Mg ha^?1), and one rate of FGDG combined with one rate of biochar (33.6 Mg ha^?1 ea.) were applied to a sodic soil. Soil physicochemical properties, including cationic exchange, pH, electrical conductivity (EC_e), sodium adsorption ratio (SAR_e), total organic carbon (TOC), water retention, and soil respiration rate, were assessed during and at the end of the incubation period. Addition of FGDG to sodic soil increased EC_e from 3.5 to 8.4 dS m^?1 and decreased SAR_e from 16 to 9. Biochar addition to sodic soil increased TOC from 62.2 to 99.5 μg g^?1 and increased soil respiration rate (mg C kg^?1 soil day^?1) on every measurement period. When FGDG and biochar were both added to the sodic soil, TOC did not significantly improve; however, EC_e increased from 3.5 to 7.7 dS m^?1, SAR_e decreased from 16 to 9, and soil respiration rate increased for all measurements. The results confirm there is potential for FGDG and biochar to reclaim sodic soils alone, and applied in combination.     

Available N and P,Microbial Activity,and Biomass in Saline Sandy and Clayey Soils Amended with Residues of Wheat and Alfalfa

Salt accumulation has a negative effect on microorganisms, but plant residues may enhance the microbial activity and biomass. An experiment was conducted over 50 days to evaluate the effect of wheat and alfalfa residues on microbial activity and biomass and nitrogen (N) and phosphorus (P) availability in sandy and clayey soils at different salinity levels. Equivalent amounts of calcium (Ca⁺²) and sodium (Na⁺) salts were added to both soils. Values of electrical conductivity (EC_1:5), denoted S1, S2, and S3 in each soil, were 0.16, 1.10 and 1.98 dS m^?1 in the sand and 0.19, 0.82 and 1.75 dS m^?1 in the clay. Residues of wheat and alfalfa were added at 2% (w/w). Cumulative respiration and microbial biomass decreased with increasing salinity, but with residue addition they increased with a greater rate in amended sandy soil than in clay soil, with a more pronounced effect for alfalfa than for wheat residue. After 10 days, with wheat residue available N values were 113, 86, and 71 μg in the clay and 144, 114, and 94 μg g^?1 soil in the sand in S1, S2, and S3, respectively. Relative to wheat residue, alfalfa residue increased N availability by 9, 13, and 19% and 22, 24, and 24% in the clay and in the sand in S1, S2, and S3, respectively. Compared to the control, in the clay P availability increased by 33, 57, and 100% with wheat residue and by 58, 128, and 175% with alfalfa residue, whereas in the sand it increased by 92, 45, and 40% with wheat residue and by 130, 145, and 280% with alfalfa residue in S1, S2, and S3, respectively. Availability of N and P increased from day 10 to day 50 in both soils, but with different magnitudes. Residue addition can increase microbial activity and nutrient availability in saline soils, particularly in coarser textured soils.