Proton release by N 2 ‐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 (Ca 2+ ) to replace excess Na + from the cation exchange sites. In addition, adequate levels of Ca 2+ 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 CO 2 partial pressure (P CO2 ) in the root zone is considered as the principal mechanism contributing to phytoremediation of sodic soils. Aqueous CO 2 produces protons (H + ) and bicarbonate (HCO 3 ‐ ). In a subsequent reaction, H + reacts with native soil calcite (CaCO 3 ) to provide Ca 2+ for Na + Ca 2+ exchange at the cation exchange sites. Another source of H + may occur in such soils if cropped with N 2 ‐fixing plant species because plants capable of fixing N 2 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, CaCO 3 = 50 g kg ‐1 ), we investigated the phytoremediation ability of alfalfa ( Medicago sativa L.). There were two cropped treatments: Alfalfa relying on N 2 fixation and alfalfa receiving NH 4 NO 3 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 = N 2 ‐fixing alfalfa > NH4NO3‐fed alfalfa > control. Both the alfalfa treatments produced statistically similar root and shoot biomass. We attribute better Na+ removal by the N 2 ‐fixing alfalfa treatment to an additional source of H + in the rhizosphere, which helped to dissolve additional CaCO 3 and soil sodicity amelioration.