The Effect of Biochar of Date Palm's Leaves and Pistachio Harvesting wastes on the growth of maize (Zea mays L.) and heavy metals concentrations

The application of biochar in soil as a method for disposal of organic wastes from environment has been considered by environmental scientists in recent years, due to the unique properties of these components. Biochar is a carbon-rich compound that is produced by burning different types of organic wastes under anaerobic or limited supply of oxygen, which called pyrolysis. Biochar due to its physicochemical properties such as porous structure, expanded specific surface area, high organic carbon content, active functional groups, and also high cation-exchange capacity could able to stabilize organic and mineral compounds. Many studies showed that the biochars enhance soil fertility and improve plant growth but if we want to recommend or apply a specific biochar as an amendment of soil, it's necessary to know about the effects of this biochar on the soil properties and growth of plant. So, the aim of this study was to find out the effect of two biochar (biochar of Date Palm's Leaves (DPL biochar) and biochar of Pistchio Harvesting wastes (PW biochar)) on the growth and heavy metals concentrations of Maize (Zea mays L.) under two different soil textures (Sandy and Sandy Loam).

This study was conducted in a greenhouse condition on the growth of maize in two types of soil (Sandy and Sandy Loam) with application of 5 levels (0, 1, 2, 3 and 5% w/w) of two different types of biocahr (DPL biochar and PW biochar). Maize were cultivated in treatments for 38 days and at harvesting the shoot and root dry weight and shoot height were measured. Also, the concentration of heavy metals (including Fe, Zn, Cu, Mn, Ni, Pb, and Cd) in plant shoots were evaluated.

The result showed that the growth of maize severely decreased due to the application of the biochar and the negative effect of PW biochar was more than DPL biochar. Meanwhile, the negative effect of PW biochar on plant growth in sandy soil was more than other one (i.e. Sandy Loam soil), which medium (2 and 3% w/w) and high (5% w/w) levels of this biochar caused the plant to stop growing. Also application of 5% of DPL biochar in Sandy Loam soil caused in a decrease of about 19, 69 and 72% in plant height, shoot dry weight and root dry weight of maize in compared with control (without biochar application in this soil), respectively and these ratios were about 15, 44 and 31% with application of 3% DPL biochar; while with application of 3% of PW biochar in sandy loam soil has decreased plant height, shoot dry weight and root dry weight of maize about 17, 53 and 37%, in compared to control respectively. These results approved the greater negative effect of PW biochar on plant growth. Assessment of soil salinity as the application of different levels of biochars showed that these materials increased salinity and thus had a negative effect on plant growth. In overall, the results of this study showed that the use of different biochars have different effects on plant growth, since most of biochars have high salinity, coarse-textured soils could more affected by salinity, because of the lower water holding capacity of this soils. Since, biochar is a stable substance, the results of the concentration of elements in the shoot of plants showed that the concentration of most elements not significantly affected by the application of biochar, however the increase in Fe concentration in sandy soil due to application of PW biochar, also Mn uptake in the effect of applying 1% of DPL biochar was observed. On the other hand, the results of this part of the research showed that DPL biochar at higher levels has even reduced the concentration of Mn in the plant. The results of this section also showed that the application of biochar in sandy loam soil, although it was significant on the concentration of heavy metals Pb and Cd in the plant and had slightly increased them, but their concentration was less than critical levels (dangerous) for human health.

The effect of biochar on improving plant growth can be greatly influenced by the combined effect of biochar properties and soil conditions. The results showed that despite the many benefits of the soil application of biochar in the different scientific literatures, it is necessary to study the effect of biochar on soil properties and plant growth before applying any type of biochar in the soil.