Effect of Conocarpus erectus biochar on some physical and mechanical properties of a calcareous soil under corn cultivation

Introduction

Biochar is a potential soil amendment which produced by pyrolyzing waste organic materials. Biochar with improving soil quality indicators could increase soil sustainability. Amending soil with biochar not only improve soil quality but also stimulate plant growth. So far, most studies have investigated the potential impacts of biochar on soil fertility, soil biota, soil chemical properties, soil greenhouse gas emissions, and remediation of contaminated soils. Comparatively, a very small number of research has been carried out on the implications of biochar application on soil's physical and mechanical properties on the field scale and in the presence of plants. The aim of this study was to investigate the effect of Conocarpus erectus biochar as a modifier on some mechanical properties of soil (shear strength, coefficient of linear extensibility (COLE), liquid limit (LL), plastic limit (PL) moistures and plasticity index) as well as some physical properties includes soil porosity, soil moisture retention (FC, PWP and AWC), air capacity, and bulk density.

Materials and Methods

For this purpose, an experiment was conducted in a completely randomized block design with three replications. The treatments including biochar at three levels (0, 3 and 6-ton ha-1) were added into a calcareous soil. The biochar was produced from Conocarpus erectus wood through the slow pyrolysis process at 550 °C. Before being applied to soil plots, the biochar was crushed to pieces smaller than 0.5 cm. The biochar was mixed to around 20 cm soil depth and soil moisture was kept at 70% of field capacity for three months. The corn plant was then planted and harvested after three months. Then soil samples were collected and used for physical and mechanical experiments. Some physical and mechanical properties of soil include shear strength (SS), coefficient of linear extensibility (COLE), liquid limit (LL), plastic limit (PL) moistures, plasticity index (PI), bulk density (BD), porosity, soil air capacity (SAC), field capacity (FC) and permanent wilting point (PWP) moisture were measured. The surface functional groups analysis of the biochars were detected using Fourier transform infrared spectroscopy (FTIR). Furthermore, the surface morphology of bulk biochar was portrayed by a scanning electron microscope (SEM).

Results and Discussion

The results of the analysis of variance (ANOVA) indicated that The addition of Conocarpus erectus biochar had a significant effect on the soil physical properties (P<1%). the results revealed that the biochar significantly enhanced soil porosity, air capacity, and moisture content at FC and PWP. While diminished soil bulk density (P <0.01). The results of the mean comparison test of the effect of the biochar on soil porosity, air capacity, FC, PWP moisture and plant water available content (AWC) showed that although the treatments of 3 and 6 ton-ha-1of biochar did not have a significant difference, they had a significant difference with the control treatment (P<1%). The increase of FC, PWP, and plant available water by the rising amount of biochar was attributed to the porosity of the biochar particles. The results of SEM images clearly revealed that synthesized biochar is a porous material that significantly can enhance the total soil porosity and water retention capacity. Furthermore, In the FTIR spectra of the synthesized biochar functional groups such as carboxylic acid, phenolic, ketone, ester and, amine was detected. The findings of the analysis of variance (ANOVA) also show a significant effect of Conocarpus erectus biochar on the soil mechanical properties (shear strength, coefficient of linear extensibility (COLE), liquid limit (LL), plastic limit (PL) moistures and plasticity index (PI) (P<1%). Moreover, the results of the mean comparison test revealed that 3 and 6 ton-ha-1of biochar treatments had a significant difference with the control treatment (P<1%); The difference between 3 and 6 ton-ha-1of biochar was also significant (P<1%). Application of the biochar increased LL, PL, and PI; while, diminished shear strength and COLE index. In the treatments of 3 and 6-ton ha-1of the biochar, the amount of LL increased by 40.32 and 77.74%, respectively, and PL increased by 40.8 and 70%, respectively, compared to the control treatment. Furthermore, the value of PI enhanced by 38.33 and 71.66% in the biochar treatments of 3 and 6-ton ha-1 compared to the control treatment. While, the amount of shear resistance in the treatments of 3 and 6-ton ha-1of the biochar decreased by 23.94 and 34.75%, respectively, compared to the control. The amount of decrease in COLE index at the 3 and 6-ton ha-1of the biochar compared to the control was 20.28 and 36.95%, respectively. The results also revealed that the application level of 6% biochar treatment increased the amount of porosity, SAC, FC, PWP and AWC by 70, 13.7, 6.2, 5 and 8 percent, compared to the control treatment. In the clay loam soil texture, the application of the biochar reduced COLE index significantly; therefore, biochar has the potential to improve the mechanical characteristics of expandable soils.

Conclusion

This study showed that the biochar of Conocarpus as a suitable modifier improves the quality of the physical and mechanical properties calcareous soils. According the findings, it can be concluded that biochar by reducing soil bulk density, and shear strength, increasing porosity, water retention, plant available water, and air capacity, and improving soil consistency (Atterberg Limits) can provide suitable conditions for plant growth. The application of biochar not only can have positive effects on the transport of nutrient elements, gases, heat, and water movement, in soil but also by increasing soil porosity and water retention capacity provides beneficial conditions for plant growth. Therefore, in arid and semi-arid regions such as Khuzestan province, which is facing the problem of lack of water resources and organic matter, biochar can be a useful solution. Overall, the use of biochar of Conocarpus could improve soil physical and mechanical properties at the field scale but long-term studies in different soils under plant cultivation are needed for a better understanding of its performance as a soil amendment.