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.     

Calcium carbonate solubilization through H‐proton release from some legumes grown in calcareous saline‐sodic soils

Increasing levels of CO₂ and H⁺ proton in the rhizosphere from some legumes may play an important role in calcite dissolution of calcareous salt affected soils. Soils planted with white and brown varieties of cowpea (Vigna unguiculata L.) and hyacinth bean (Dolichos lablab L.) relying on either fertilizer N (KNO₃) or N‐fixation were compared against soils to which gypsum was applied and a control without plants and gypsum application to study the possibility of Ca²⁺ release from calcite and Na⁺ leaching. As compared to plants relying on inorganic N, leachates from all pore volumes (0·5, 1·0, 1·5, 2·0 pore volume) in lysimeters planted with N‐fixing hyacinth bean contained significantly higher concentrations of HCO with lower concentrations from lysimeters planted with white cowpea relying on N‐fixation. However, the lowest concentrations of HCO were recorded in the gypsum and control treatments. In initial leaching, lysimeters planted with N‐fixing plants maintained similar leachate Ca²⁺ and Na⁺ concentrations compared to gypsum amended soils. However, gypsum amended soils were found to have a prolonged positive effect on Na⁺ removal. It might be concluded that some legumes that are known to fix N in calcareous salt affected soils may be an alternative ameliorant to the extremely expensive gypsum through calcite solubilization and a consequent release of Ca²⁺. Copyright © 2009 John Wiley & Sons, Ltd.     

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.     

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.     

Reclamation and salt leaching efficiency for tile drained saline‐sodic soil using marginal quality water for irrigating rice and wheat crops

Due to increased population and urbanization, freshwater demand for domestic purposes has increased resulting in a smaller proportion for irrigation of crops. We carried out a 3‐year field experiment in the Indus Plains of Pakistan on salt‐affected soil (EC_e 15·67–23·96 dS m⁻¹, pH_s 8·35–8·93, SAR 70–120, infiltration rate 0·72–0·78 cm h⁻¹, ρ _b 1·70–1·80 Mg m⁻³) having tile drainage in place. The 3‐year cropping sequence consisted of rice (Oryza sativa L.) and wheat (Triticum aestivum L.) crops in rotation. These crops were irrigated with groundwater having electrical conductivity (EC) 2·7 dS m⁻¹, sodium adsorption ratio (SAR) 8·0 (mmol L⁻¹)^1/2 and residual sodium carbonate (RSC) 1·3 mmol_c L⁻¹. Treatments were: (1) irrigation with brackish water without amendment (control); (2) Sesbania (Sesbania aculeata) green manure each year before rice (SM); (3) applied gypsum at 100 per cent soil gypsum requirement (SGR) and (4) applied gypsum as in treatment 3 plus sesbania green manure each year (GSM). A decrease in soil salinity and sodicity and favourable infiltration rate and bulk density over pre‐experiment levels are recorded. GSM resulted in the largest decrease in soil salinity and sodicity. There was a positive relationship between crop yield and economic benefits and improvement in soil physical and chemical properties. On the basis of six crops, the greatest net benefit was obtained from GSM. Based on this long‐term study, combined use of gypsum at 100 per cent soil gypsum requirement along with sesbania each year is recommended for soil amelioration and crop production. Copyright © 2010 John Wiley & Sons, Ltd.     

Reclamation of saline‐sodic soil under a rice–wheat system by horizontal surface flushing

Saline sodic soil with a high content of soluble carbonates is one of the important agricultural soils on the Central Indo‐Gangetic plains and elsewhere. Conventional reclamation procedures using gypsum application followed by vertical leaching (GC) is uneconomic; high ECe and precipitation of applied gypsum, reacting with soluble carbonates, reduce the efficacy of gypsum in these soils. This paper reports results from a project designed to evaluate reclamation by irrigation of the ploughed soil and turning of soil with a power tiller followed by flushing of standing water after 24 h, a second flushing after 7 days and subsequent application of gypsum and vertical leaching (GF2). Average rice and wheat production after GF2 significantly increased (25 and 62%, respectively) over the conventional practice. Compared with conventional treatment, GF2 significantly reduced the ECe and SAR of the soil and improved physical properties such as ζ‐potential, dispersible clay content, water stable aggregates expressed as MWD, and saturated hydraulic conductivity. Split application of gypsum between flushing (GF1/2 and GF2/3) gave similar results to GF2 in terms of soil amelioration and crop production.     

Sugar beet (Beta vulgaris L.) response to diammonium phosphate and potassium sulphate under saline–sodic conditions

Salinity and sodicity are prime threats to land resources resulting in huge economic and associated social consequences in several countries. Nutrient deficiencies reduce crop productivity in salt‐affected regions. Soil fertility has not been sustainably managed in salt‐affected arid regions. Few researchers investigated the crop responses to phosphorus and potassium interactions especially in saline–sodic soils. A research study was carried out to explore the effect of diammonium phosphorus (DAP) and potassium sulphate (K₂SO₄) on sugar beet (Beta vulgaris L.) grown in a saline–sodic field located in Kohat district of Pakistan. The crop was irrigated with ground water with EC_iw value of 2.17–3.0 dS/m. Three levels each of K₂O (0, 75 and 150 kg/ha) as K₂SO₄ and P₂O₅ (0, 60 and 120 kg/ha) as DAP were applied. The application of P significantly affected fresh beet and shoot yield while K fertilizers had significant effect on fresh beet yield and ratio of beet:shoot, while non‐significant effects on the fresh shoot were observed. The application of K₁ and K₂ promoted sugar beet shoot yield by 49.2 and 49.2% at P₁ and 64.4 and 59.7% at P₂, respectively over controls. In comparison with controls, fresh beet yield was increased (%) by 15 and 51, 45 and 84, and 50 and 58 for corresponding K₁ and K₂ at P₀, P₁ and P₂, respectively. Addition of P₁ and P₂ increased beet yield by 37 and 47% over control. The shoot [P] (mmol/kg) were achieved as 55.2, 73.6 and 84.3 at P₀, P₁ and P₂, respectively. The shoot [Mg] and [SO₄] tended to decrease with increasing P levels, while [SO₄] was markedly reduced at P₂. The effect of P on leaf [Na] was non‐significant, but increasing levels of K decreased [Na] substantially at P₀ and P₁, but there was no difference in the effect of K level on [Na] at P₂. Consequently, K application reduced leaf Na:K ratios. Fresh shoot yield was weakly associated with leaf [P] (R² = 0.53). The leaf Na:K ratio showed a negative relationship (R² = 0.90) with leaf [K]. A strongly positive relationship (R² = 0.75) was observed between leaf [K] and fresh beet yield. The addition of K₂SO₄ also enhanced [SO₄] and SO₄:P ratios in leaf tissues. The ratio of Na:K in the shoot decreased with increasing K application. These results demonstrated that interactions of K and P could mitigate the adverse effects of salinity and sodicity in soils. This would contribute to the efficient management of soil fertility system in arid‐climate agriculture.     

Effects of consecutive applications of gypsum in equal,increasing, and decreasing quantities on soil hydraulic conductivity of a saline‐sodic soil

The objective of this study was to determine the effects of consecutive application of gypsum dissolved in leaching water on hydraulic conductivity of a saline‐sodic soil. Drainage type plastic columns with a 10 cm diameter were used in this laboratory experiment. Soil depth within columns was 30 cm with an average bulk density of 1.38 g cm^–3. Leaching water was applied in six equal portions. Total gypsum was applied at 1, 3, and 5 portions after dissolving in leaching water. In dissolution, equal (1.273 + 1.273 + 1.273 Mg ha^–1), increasing (0.637 + 1.273 + 1.910 Mg ha^–1) and decreasing (1.910 + 1.273 + 0.637 Mg ha^–1) quantities of gypsum were used. Results were compared with the control treatment, in which total amount of gypsum were mixed with surface layer of soil column before leaching. Hydraulic conductivity of soil increased in all treatments. The maximum hydraulic conductivity value was obtained at consecutive application of gypsum at decreasing quantities.     

Long‐term changes in the floristic composition and soil characteristics of reclaimed sodic land during eco‐restoration

The study was conducted under the “Uttar Pradesh Sodic Lands Reclamation Project” to examine changes that occurred in the reclaimed sodic land in two districts of Uttar Pradesh, India. The study focuses on long‐term seasonal changes in the floral diversity and soil characteristics of the reclaimed sodic land over a period of 10 y. The changes in the floristic composition, plant density, and soil characteristics (microbial biomass carbon [MBC], pH, exchangeable‐sodium percentage (ESP), and electrical conductivity) were compared among the different study plots after different years of sodic‐land reclamation. The study plots comprised reclaimed land with rice–wheat cultivation; semireclaimed land under rice cultivation only and nonreclaimed barren sodic land. There was a significant variation in the floristic composition of the three study plots. Dominance in the floristic composition was shifted from monocotyledonous weeds in the nonreclaimed sodic land to dicotyledonous weeds in the reclaimed land after 10 y of reclamation. Among the soil characteristics, the most remarkable changes were observed in soil MBC and ESP during the course of sodic‐land reclamation. Soil MBC increased up to 480% and ESP values decreased up to 79% in the reclaimed plots with reference to the nonreclaimed plots. The soil amelioration was more pronounced in the upper layer (0–30 cm) as compared to the lower layer (below 30 cm depth). A positive significant correlation was revealed between soil MBC and floristic composition of the reclaimed plots. These changes in floristic composition and soil characteristics could be used as good indicators of the eco‐restoration of the sodic lands. The present study provides useful insights in understanding the temporal progress of eco‐restoration in the reclaimed sodic lands.     

Effects of soil water content on growth,tillering, and manganese uptake of lowland rice grown in the water‐saving ground‐cover rice‐production system (GCRPS)

A 2‐year field experiment and a pot experiment were carried out to compare Mn uptake, tillering, and plant growth of lowland rice grown under different soil water conditions in the ground‐cover rice‐production system (GCRPS) in Beijing, North China. The field experiment was conducted in 2001 and 2002, including two treatments: lowland‐rice variety (Oryza sativa L. spp. japonica) grown under thin (14 μm) plastic‐film soil cover (GCRPS_plastic) at 80%–90% water‐holding capacity (WHC) and traditional lowland rice (paddy control) grown with 3 cm standing‐water table. The pot experiment was conducted in a greenhouse with four treatments: (1) traditional lowland rice: paddy control; (2) GCRPS, water‐saturated soil: GCRPS_saturated; (3) GCRPS at 90% water‐holding capacity (WHC): GCRPS_90%WHC; and (4) GCRPS at 70% WHC: GCRPS_70%WHC. Results of the field experiment showed that dry‐matter production, number of tillers, as well as N and Mn concentrations in rice shoots of GCRPS were significantly lower than in paddy control, while there was no significant difference in shoot Fe, Cu, Zn, and P concentration and nematode populations. In the pot experiment, shoot Mn concentration significantly decreased with decreasing soil water content, while soil redox potential increased. Shoot–dry matter production and tiller number of GCRPS_saturated were significantly higher than in other treatments. Significant correlations were observed between the shoot Mn concentration and tiller number at maximum tillering stage in the field and pot experiment, respectively. We therefore conclude that the limitation of Mn acquisition might contribute to the growth and yield reduction of lowland rice grown in GCRPS. The experiment provides evidence that GCRPS_plastic combined with nearly water‐saturated soil conditions helps saving water and achieving optimum crop development without visual or latent Mn deficiency as observed under more aerobic conditions.     

Effects of aerobic conditions in the rhizosphere of rice on the dynamics and availability of phosphorus in a flooded soil – A model experiment

The secretion of O₂ by rice roots results in aerobic conditions in the rhizoshere compared to the bulk flooded soil. The effect of this phenomenon on the adsorption/desorption behavior and on the availability of phosphorus (P) in a flooded soil was investigated in a model experiment. An experimental set‐up was developed that imitates both O₂ release and P uptake by the rice root. The results showed that O₂ secretion significantly reduced P adsorption/retention and increased P desorption/release in the “rhizosphere” soil, compared to the anaerobic bulk soil. The P uptake by an anion exchange resin from both unfertilized and P‐amended soil was significantly increased. The results confirm that the O₂ secretion is an important mechanism to enhance P availability and P uptake of rice under flooded conditions, where the “physico‐chemical” availability of P in the anaerobic bulk soil is strongly reduced. The decrease of P availability in the P‐amended flooded bulk soil was mainly associated with the almost complete transformation of the precedingly enriched Al‐P fraction into Fe‐bound P with extremely low desorption/release characteristics during the subsequent flooding.     

Organic phosphorus compounds as a phosphorus source for higher plants through the activity of phosphatases produced by plant roots and microorganisms

Summary The efficiency of phosphatases produced by clover, barley, oats and wheat was investigated in soils treated with sodium glycerophosphate, lecithin and phytin. Root exudates of aseptically grown clover were also examined for the breakdown of different organic P compounds in order to test the efficiency of plant-produced phosphatases. In general, the plants were able to use P from all the organic sources used in the study almost as efficiently as inorganic sources. Dry-matter yield, P uptake, acid and alkaline phosphatase activity and microbial population were increased in all the P treatments. Organic P enhanced alkaline phosphatase activity. Lecithin increased fungal, and phytin bacterial growth. There was no alkaline phosphatase activity in the asepticallly grown clover root exudates. Phosphatase released in aseptic culture after 4 weeks of clover growth was able to efficiently hydrolyse sodium glycerophosphate, lecithin and phytin. The amount of organic P hydrolysed in this and in the soil experiment surpassed plant uptake by a factor of 20. This suggests that the limiting factor on plant utilization of organic P is the availability of hydrolysable organic P sources.     

Relation between soil P test values and mobilization of dissolved and particulate P from the plough layer of typical Danish soils from a long‐term field experiment with applied P fertilizers

Accumulation of phosphorus (P) in agricultural topsoils can contribute to leaching of P which may cause eutrophication of surface waters. An understanding of P mobilization processes in the plough layer is needed to improve agricultural management strategies. We compare leaching of total dissolved and particulate P through the plough layer of a typical Danish sandy loam soil subjected to three different P fertilizer regimes in a long‐term field experiment established in 1975. The leaching experiment used intact soil columns (20 cm diameter, 20 cm high) during unsaturated conditions. The three soils had small to moderate labile P contents, expressed by water‐extractable P (3.6–10.7 mg/kg), Olsen P (11–28 mg/kg) and degree of P saturation (DPS) (25–34%). Mobilization of total dissolved P (TDP) increased significantly (P < 0.05) from the intact soil columns with increasing labile P, whereas the increase in particulate P (PP) with increasing labile P content was modest and statistically insignificant. We found concentrations up to 1.5 mg TP/L for the plough layer of this typical Danish sandy loam soil. This highlights that even a moderate labile P content can be a potential source of TDP from the plough layer, and that a lower concentration margin of optimum agronomic P levels should be considered.