The effect of biochar on concentration of DTPA-extractable zinc from acidic and alkaline soils in one year incubation period

Zinc (Zn) is an essential element for plants growth at low concentrations and at high concentrations, it acts as a heavy metal and soil pollutant. Biochar is used to improve soil quality, plant growth and also to reduce the availability of heavy metals in contaminated soils. The biochar behavior in soil and its effect on Zn availability rely on the feedstock nature and pyrolysis temperature. This study aimed to investigate the effects of produced biochars on Zn bioavaiability in two acidic and alkaline soils with different levels of applied Zn, during one year-long incubation.

The experiment was conducted as a factorial split arrangement in a completely randomized design in two acidic and alkaline soils, with two factors including biochar types at 9 levels and extraction times at 12 levels with two replications. Four types of biochar were produced from rice straw (RB) and apple wood waste (WB) biomasses at two pyrolysis temperatures (300 and 600 °C). Two acidic (pH=5.8) and alkaline (pH=8.1) soils were collected and treated with 3 levels of Zn (0, 10, and 200 mg kg-1) from zinc sulfate (ZnSO4.7H2O) source. Biochars were added to soils in two doses (1 and 4 % w/w) and incubated at around FC moisture condition for 360 days at 25±2◦C. The pH, EC, moisture content and DTPA extractable-Zn were measured in the studied soils at 12 designated extraction times (0.25, 1, 3, 5, 15, 30, 60, 90, 120, 180, 270 and 360 day).

In acidic soil and Zn level of 200 mg kg-1, levels of 1 and 4 % biochars caused the significant decrease in DTPA-Zn concentration and the elapsing of time had a significant effect on the reduction of DTPA-Zn concentration and maximum decrease (48 %) was observed in the treatment of 4% RB600 and 360th day (p<0.05). At the Zn levels of 0 and 10 mg kg-1, in acidic soil, DTPA-Zn concentration was significantly increased in 4% RB300, 1% RB600, and 4% RB600 treatments, and passage of time had a decreasing effect on it and DTPA-Zn concentration significantly reduced with time in 4% WB300 treatment compared to the control (without biochar). A significant increase of pH in acidic soil was observed in 4% RB600 and 4% WB600 treatments during the 360 days of incubation while the significant decrease was showed in 4% WB300 treatment. In alkaline soil and under Zn=0, the maximum increment of DTPA-Zn concentration compared to the control (without biochar) was obtained in 4% RB600 treatment but the elapsing of time had a significant reduction effect on it. In alkaline soil at Zn level of 10 mg kg-1, only the 4% WB300 treatment could significantly decrease the concentration of DTPA-Zn over time, but in the same soil at the level Zn of 200 mg kg-1, the significant decrease of DTPA-Zn concentration was observed at the 4% RB300, 1% RB600, 4% RB600 and WB300 4% treatments. A significant decrease in alkaline soil pH was observed in the 4% RB300 and 4% WB300 treatments and an increase in electrical conductivity (EC) in both acidic and alkaline soils was observed in rice straw-derived biochars treatments.

Although the application of rice straw derived biochar (pyrolysis at 600 °C) decreased the availability of Zn in both acid and alkaline soils with the high level of Zn (200 mg kg-1), it did not have a negative effect on Zn availability in normal levels of Zn (0 and 10 mg kg-1) and even increased the concentration of DTPA-Zn in both acidic and alkaline soils under without Zn application conditions.