Application of different organic residues and their biochars for amending a saline-sodic soil

Soil salinity and sodicity is a world-wide problem of irrigated farmlands that threaten soil quality and crop productivity in arid and semi-arid regions. Soil salinity refers to the accumulation of salt in the plant root zone which reduces plant growth. The use of saline and sodic groundwater for irrigation purposes deteriorates soil quality, causing clay dispersion, and reduction in hydraulic conductivity and plant available water. It also negatively affects plant physiological parameters such as reduction of grain yield, shoot and root length, biomass, rate of photosynthesis, leaf water potential, and increases proline content. Therefore research on possible amendments for the soils in these systems is necessary. Different organic and inorganic amendments are used to address salinity problems in agricultural soils. Common organic amendments include animal manures, agricultural residues, green wastes and biochars, and the inorganic amendments include gypsum, zeolite and super absorbents. In this study, the application of biochars produced at pyrolysis temperatures of 300 and 500 °C and their feedstocks as well as a superabsorbent (SA) on the leaching of salts through the soil columns were investigated.

The experiment was carried out in the Irrigation Laboratory, Department of Irrigation, College of Agriculture, Isfahan University of Technology, Iran, in 2018. The experiment was conducted as a completely randomized design with three replications. A saline-sodic soil (0–30 cm layer) was sampled from the Rodasht Soil Salinity Research Center, located in the east of Isfahan, Iran. The soil was classified as Typic Haplosalids, soil texture was silt loam and comprised 610, 180 and 210 g kg-1 silt, sand and clay, respectively. Five combinations of sugarcane bagasse, rice husk, cow manure, and pine wood were considered and pyrolyzed at 300 °C and 500 °C under oxygen-limited conditions (continuous inflow of nitrogen) for 2 hours (slow pyrolysis) in a pyrolysis chamber. A control with no amendment (CT), a super absorbent (Stockosorb) (SA), feedstocks (BWM, RWM, WM, BWMR and BWR), and their biochar produced at 300 and 500 °C were considered. The five combinations were pyrolyzed at two temperatures of 300 and 500 °C to obtain the biochar samples. Then the feedstocks and their biochars, as well as the SA sample, were applied to the saline-sodic soil and incubated in sealed plastic bags at a water content of 80% of the soil plastic limit, and room temperature (25 ± 1°C) for 40 days. After the incubation, the amended soils were packed into soil columns (PVC pipes) of 15 cm diameter and 50 cm height, and leached with 4 pore volumes (PV) of irrigation water. The PV was determined using the porosity and volume of the soil column. A constant water height of 5.0 cm was held on the soil surface by a mariotte bottle. The saturated hydraulic conductivity (KS) of the soil and the leachate electrical conductivity (EC) were measured during the leaching period at every 1/3 PV leaching. After leaching, the soil samples were carefully collected from the columns. Then the soluble ion concentrations (Mg2+ and Ca2+) were determined.

At a given period, there was a lower EC of the leachate for the BWR (sugarcane bagasse, rice husk and pine wood) treatment compared to the other amendments. The KS of soil increased significantly with the application of all amendments, which could be related to the improvement of soil structure, and increased porosity and leaching of sodium. The lowest KS was observed in control, which could be related to exchangeable sodium between the solution and the soil exchange sites, and soil dispersion. The biochar treatments had less Mg2+/Ca2+ ratio than their feedstocks and subsequently, the soil aggregate stability increased in the biochar treatments. Soil quality is influenced by the physical properties of amendments that are applied to the soil. It is important to consider the pH and EC of biochar for soil amendment. The BWR combination had the lowest EC and pH compared to other combinations. Also, the BWR had the highest C/N ratio compared to other combinations, subsequently, the KS increased. Overall, this study suggested that the use of BWR300 (the biochar of BWR compound produced at pyrolysis temperature of 300 °C) and SA amendments might be suitable for the improvement of saline-sodic soils in arid and semi-arid regions. Further experiments are needed to determine the rates of amendments for different soil types and water qualities in the long–term periods and large-scale applications in saline-sodic conditions.