Effect of biochar and irrigation water salinity on soil chemical properties after wheat harvest

  Salinity is one of the major environmental stresses negatively influence the agricultural production. The saline soils usually have poor soil structure due to lack of organic matter. Therefore, application of organic matter improves soil aggregate stability, increase soil porosity and enhance soil water holding capacity. Biochar is a carbon rich product, which is produced by burning crop residues and animal manure, under high temperature and limited oxygen conditions. Biochar, when used as a soil amendment, increase soil fertility and improve soil quality. Therefore, the aim of this study was to investigate the effect of different levels of irrigation water salinity and wheat straw biochar on some of soil chemical properties after wheat harvest. A greenhouse experiment was conducted based on completely randomized design with four saline irrigation levels (0.5, 5, 7 and 9 dS m-1) and four levels of biochar (0, 25, 50 and 75 Mg ha-1) with three replications. The soil texture used in experiment was sandy loam with bulk density of 1.53 g cm-3, saturated paste extract electrical conductivity of 0.66 dS m-1 and volumetric soil water content of 21 and 8 % at field capacity and permanent wilting point, respectively. The biochar was produced from wheat straw at 500oC and under no oxygen conditions. After harvesting the wheat (Shiraz cv.), the chemical parameters of soil (such as sodium, calcium and potassium concentration, soil electrical conductivity in the saturation paste extract and sodium absorption ratio) were measured in the depths of 0-10 and 10-20 cm. Statistical analysis was performed using SAS software. The means were compared using Duncan’s multiple range test at the 5% level of probability. In general, the results showed that the concentration of sodium, potassium and calcium, electrical conductivity and sodium absorption ratio with application of maximum level of biochar (75 Mg ha-1) increased by 1.1 and 143.8, 2.2, 2.1 and 0.8 times in comparison with no biochar application in the 0-10 cm soil depth. In the same soil depth (0-10 cm), the latter parameters with the application of the maximum level of water salinity (9 dS m-1) were enhanced by 14.8 and 1.6, 8.6, 2.1 and 5.4 times in comparison with 0.5 dS m-1 salinity. The concentration of measured ions, soil electrical conductivity and sodium absorption ratio were higher at 0-10 cm compared with those in 10-20 cm due to soil evaporation from soil surface and higher water holding capacity in top layer of soil due to biochar application. Moreover, increase in soil electrical conductivity in both layers by application of biochar was due to high saturated paste extract electrical conductivity of the used biochar (7.5 dS m-1) in comparison with the electrical conductivity of the soil (0.7 dS m-1) and also accumulation of ions in the soil, as water was not drained out of the pots. Although, biochar increased soil salinity and other soil chemical properties due to its salinity, increase in biochar application higher than 25 Mg ha-1 reduced the negative effects of water salinity, to some extent. On the other hand, as potassium increased in the biochar treatments, biochar as k additive can be used in soil with potassium deficiency. Therefore, according to biochar ability as soil amendment, application of biochar (with low salinity) at lower levels of 25 Mg ha-1 is recommended, if it is economically justified.