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

Introduction
Biochar is a potential soil amendment produced by pyrolyzing waste organic materials. Biochar with improving soil quality indicators could increase soil sustainability. Amending soil with biochar enhances soil quality and stimulates 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 minimal 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. This study aimed to investigate the effect of Conocarpus erectus biochar as a modifier on some mechanical properties of soil (shear strength (SS), coefficient of linear extensibility (COLE), liquid limit (LL), plastic limit (PL) moistures, and plasticity index (PI)) as well as some physical properties includes soil porosity, soil moisture retention (field capacity (FC), permanent wilting point (PWP), and plant water available content (AWC)), soil air capacity (SAC), and bulk density (BD).
 
Materials and Methods
The research experiment was conducted in a completely randomized block design with three replications. The treatments including biochar at three levels (zero, three, and six 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 SS, COLE, LL), PL moistures, PI, BD, porosity, SAC, FC, and PWP moisture were measured. The surface functional groups analysis of the biochars was 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's physical properties (P< 0.01). 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 amount of soil porosity, air capacity, FC, PWP moisture, and AWC in the treatments of three and six ton ha^-1 of biochar had a significant difference with the control treatment (P< 0.01). The increase of FC, PWP, and plant available water by raising the amount of biochar was attributed to the porosity of the biochar particles. The results of SEM images revealed that synthesized biochar is a porous material that significantly can enhance the total soil porosity and water retention capacity. Furthermore, the FTIR spectra of the synthesized biochar functional groups such as carboxylic acid, phenolic, ketone, ester, and, amine were detected. The findings of the ANOVA also show a significant effect (P< 0.01) of Conocarpus erectus biochar on the soil mechanical properties (SS, COLE, LL, PL moistures and PI). Moreover, the results of the mean comparison test revealed that three and six ton ha^-1 of biochar treatments had a significant difference with the control treatment (P< 0.01); The difference between three and six ton ha^-1 of biochar was also significant (P< 0.01). Application of the biochar increased LL, PL, and PI; while diminished shear strength and COLE index. In the treatments of three and six ton ha^-1 of 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 was enhanced by 38.33 and 71.66% in the biochar treatments of three and six ton ha^-1 compared to the control treatment. While, the amount of shear resistance in the treatments of three and six ton ha^-1 of the biochar decreased by 23.94 and 34.75%, respectively, compared to the control. The amount of decrease in COLE index at the three and six ton ha^-1 of the biochar compared to the control was 20.28 and 36.95%, respectively. The results also revealed that the application level of six ton ha^-1 biochar treatment increased the amount of porosity, SAC, FC, PWP, and AWC by 70, 13.7, 6.2, 5, and 8 %, compared to the control treatment. The application of biochar reduced the 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 of calcareous soils. According to 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 has 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 valuable soil amendment. 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.