Effect of palm leaf and lemon peel biochar on some physical and mechanical properties of a sandy loam soil

Introduction
Increasing agricultural production is necessary due to the growing population in order to ensure food security. Also, a large part of agricultural lands in arid and semi-arid climates face the limitation of carbon storage and nutrients. This limitation is more visible in coarse textured soils with low clay content, due to the inability to supply elements required for plant growth and the ability to retain water in the soil. Therefore, modifying the physical, chemical and biological characteristics of the soil is inevitable to achieve sustainable agriculture. In the meantime, with the high use of chemical fertilizers along with the excessive tillage activities while destroying the soil, management costs increase. Soil degradation is an increasing worldwide threat to the sustainability of agriculture. The use of organic amendments like biochar may prove a key for sustainable agriculture, as it could keep the carbon pool in soil over the long term, thus improving soil fertility and crop productivity, mitigating global climate change, and finally enhancing soil physicochemical quality. The ability of biochar to enhance the physical and mechanical properties of soils is dependent on the characteristics of biochar including its particle size, application rate, feedstock type, and pyrolysis conditions. This study aimed to assess the effect of applying different application rates, feedstock, and particle sizes of biochar on the soil’s physical and mechanical properties in sandy loam soil.
 
Materials and Methods
The study was conducted in a research greenhouse of Shiraz University in 2019. To investigate the effect of biochar feedstock, application rate, and particle size on soil bulk density, aggregate stability (mean weight diameter), penetration resistance, and shear strength in sandy loam soil, using two feedstock types (palm leaf biochar and lemon peel biochar) were pyrolyzed at a temperature of 500 °C for 3 h. Each biochar was fractioned by dry sieving into three sizes: 2-4, 0.8-2, and < 0.8 mm, and mixed with sandy loam soil at four application rates of 0.5, 1, 2, and 4 % (v/v) with zero application rates (control). The pots were incubated in standard conditions and water content was kept at near field capacity throughout the experiment for 15 months. In the production of biochar, plant residues of palm leaves and lemon peel were used, which are surplus plant wastes available in Fars province. The apparent specific gravity of the soil was measured by the conventional method of paraffin blocks. The statistical analysis of the results was done in order to check the influence of the studied treatments on the soil characteristics using SAS statistical software. The mean of the effect of each treatment separately and also the interaction of the effects of the investigated treatments (if significant) were compared using Duncan's multiple range test at the five percent probability level (p < 0.05).
 
Results and Discussion
This result indicated that applying biochar improved the physical properties of soil, including a significant decrease in soil bulk density from 5.4 to 19.8% by using application rates of 0.5 to 4% biochar, increase aggregate stability from 37.6 to 73.6% and increase shear strength from 3.2 to 15 by using application rates of 1 to 4% of biochar as compared to control. For a 4% biochar rate, penetration resistance increased by 5% as compared to control. The results show that lemon peel is more efficient in soil bulk density whereas palm leaf biochar was efficient in aggregate stability and shear strength (no significant difference in penetration resistance was observed with the application of different biochar sources). Moreover, irrespective of biochar sources, biochar with finer particle sizes (< 0.8 mm) improved aggregate stability and shear strength, when the biochar application rate was the same, but the most notable improvement in soil bulk density was observed at the coarse fractions (2-4 mm).
 
Conclusion
Biochar as a type of organic compound that has great compatibility with the environment, is of interest in advanced agriculture due to its stable carbon storage and its positive effects on the biochemical and physical characteristics of the soil. According to the results of this research, the use of biochar in soil had a significant effect on reducing the apparent specific mass of the soil and also a significant increase in the stability of soil grains and shear resistance, which improves the physical and mechanical properties of the soil can have an effect on protecting the soil against water and wind erosion. On the other hand, although at the application level of 4% biochar, the resistance to subsidence increased by a significant amount of 5%, but no significant changes were observed in this feature. Palm leaf biochar had a stronger role in increasing the stability of soil grains and shear resistance, on the other hand, lemon pomace biochar had a greater effect in reducing specific mass due to its structure similar to sand particles. Also, by increasing the levels of biochar to four percent as the most effective level of biochar application, the apparent specific mass values ​​decreased by 19.8% and the stability of soil grains and shear strength increased by 73.6% and 15% respectively. Also, the results showed that in general, by reducing the size of biochar particles from 4 mm to less than 0.8 mm, the weight average of soil grain diameter and shear strength increased significantly, but the average apparent specific mass increased with the increase of particle size (2-4 mm particles) reduced. Based on the above results, it can be concluded that choosing an optimal mode of sources, levels and size of biochar particles as an organic soil conditioner can lead to maximum productivity in agricultural production while reducing agricultural management costs. In this research, it was found that under the same conditions, the use of two percent palm leaf biochar with a particle size of less than 0.8 mm in sandy loam soil is useful. It should be kept in mind that the behavior of biochar is not only limited to the characteristics investigated in this research, and the change in the conditions of biochar production (thermal heating) causes a change in the characteristics of biochar. Therefore, it is suggested that in addition to the studied effects, the effects of characteristics based on the biochar production process, such as temperature and duration of thermodilution, should be investigated at different times on the physical, chemical and biological characteristics of different soils and in field conditions in order to obtain more comprehensive information on the optimal amounts of charcoal. Biologically, especially at the farm scale, to obtain soil amendment.