Effect of olive kernel biochar suspension on physical properties and loss of an erosion-prone soil

Main portion of organic wastes usually is burned or left in fields or landfills, which leads to pollution of soil, water, and air. The use of organic wastes for biochar production can be considered as a solution for the above-mentioned problems. Biochar is a rich carbon material which is produced via pyrolysis of various biomasses in anaerobic or limited oxygen condition. Inspire of world attention to biochar application in soil, effects of particle size, and method of biochar application has also been studied in a few pieces of research. Biochar is mixed with surface soil without the selection of a given class of particle size in most researches. Furthermore, olive kernel is considered as the main waste in agro-industrial integration, which often has no special use and is left in nature. Therefore, the use of olive kernel as a feedstock for biochar production can be a great approach for the management of these wastes and improvement of soil quality. Therefore, the purpose of this research is: 1- biochar production of olive kernel and its characterization and 2- A study of biochar addition effects with different particle sizes as the suspension on soil physical and erosion related properties of an erosion- prune soil.

An erosion-prone slit clay soil was sampled as undisturbed via metal cylinders (diameter and height of 25 and 15 cm, respectively) from 0 to 10 cm of soil located at Rudbar, Southern of Guilan, Biochar was produced of olive kernel at 650 C○ and 2 hours and 45 minutes. Biochar yield was calculated based on produced biochar per unit weight of feedstock (olive kernel). pH and electrical conductivity of olive kernel was determined at 1:5 (olive kernel powder : water) and 1:20 (biochar : water). Carbon, hydrogen, and nitrogen content and Fourier Transform Infrared Spectroscopy of olive kernel and biochar were assessed through Elemental Analysis and Fourier Transform Infrared Spectroscopmeter, respectively. Biochar was milled, and with particle size of 53-250 and 250-500 micron at 1 and 2 weight percent as a water suspension was injected to cylinder containing undisturbed soil. Four cylinders without biochar was also considered as a control treatment. Soil cylinders were located at a greenhouse at 20-25 C○ for 10 months (300 days) and their moisture was held approximately at 70 percent of field capacity. At the end of the tenth-mont of incubation, samples were provided of cylinders, and some soil properties and soil loss were studied.

The studied soil had a silt clay texture. Containing an abundant amount of silt can increase sensitivity of soil to surface crust formation. Carbon and hydrogen content was increased and decreased 44 and 69 percent, respectively as a result of olive kernel transformation to biochar. Result of Fourier transform infrared spectroscopy also showed olive kernel transformation to biochar has led to decrease of volatile organic components and increase of carbon and nitrogen content.
Results of this study showed that biochar with coarser particle size (250-500 micron) led to more increase of hydraulic conductivity compared to ones with finer particle size (53-250 micron). Although, biochar application result in aggregate stability improvement and runoff production decrease, biochar with particle size of 53 to 250 and 250 to 500 micron had no significant difference compared to each other. Runoff delay time and sediment yield were more and less, respectively in all biochar treatments in comparison with control (without biochar). The most increase of runoff delay time (5.65 minutes) was also was observed at 2 percent biochar application level of 250 to 500 micron. Biochar with particle size of 53 to 250 and 250 to 500 micron result in more decrease of sediment yield at 1 and 2 percent biochar application levels, respectively.

Generally, the results of this research showed that the transformation of organic wastes such as olive kernel can be considered as a useful approach to manage this kind of wastes. Olive kernel biochar with having less hydrogen to carbon molar ratio compared to feedstock (olive kernel) has long-term persistence in soil.  Furthermore, olive kernel biochar application as a suspension in an erosion-prone clay slit has positive effects on improvement of aggregation and hydraulic conductivity, and potential erosion decline.  Therefore, biochar production of olive kernel and its application as a suspension not only prevent soil degradation in time of application in soil but also lead to long-term carbon sequestration, improvement of important properties influencing soil quality, and sustainable use of unused wastes. A Study of biochar particle size and application-level also showed a specific type of biochar does not exert similar effects on different properties of a given soil, and particle size and level of biochar application must prescribe according to the desired goal.