Preparation Of Fast Pyrolysis Biochars And Their Applications In Desert Soils

Biochar is a carbonaceous residue derived from pyrolysis of biomass. It’s characteristics vary with production technologies such as slow or fast pyrolysis, production conditions such as pyrolysis temperature, residence time and feedstock used. Recently, biochar is proved a tool to sequester carbon and reduce greenhouse gases emission from the soil to atmosphere. Hence, biochar can be used to mitigate global environmental change. Biochar addition to conventional agricultural soils has been suggested to increase soil fertility and crop productivity. This study investigated the effect of pyrolysis temperature on yield and properties of biochar, effect of pyrolysis temperature and different levels of biochar on plant growth and properties of desert soils.Biochar was produced through a fast pyrolysis using pine saw dust at five different pyrolysis temperatures i.e.,400,500,600,700and800℃. In addition, the fast pyrolysis of poplar sawdust was also conducted at a pyrolysis temperature of400℃only. The product distribution during the fast pyrolysis was determined on a wet basis. The biochars made at different temperatures and feedstock were characterized using proximate and ultimate analyses, pH, electrical conductivity (EC), cation exchange capacity (CEC) and essential plant nutrients detection. Furthermore, we conducted the Brunauer-Emmett-Teller (BET) surface area and porosity and particle size distribution analyses to explore the ability of different biochars to hold moisture. In addition, to understand the surface chemistry and surface morphology of biochars, we also conducted the Fourier transformation infrared (FTIR), the X-ray Diffraction (XRD) and the Scanning Electron Microscope (SEM) imaging analyses.In this study we used two soil materials, first was taken from the Kubuqi desert of China and the second from the Thar desert of Pakistan. Biochars of different temperatures (i.e.,400,500,600,700, and800℃) were mixed in the soil of Kubuqi desert with1%by mass (20t ha-1). The treatments were referred as T-400, T-500, T-600, T-700, and T-800, respectively. Meanwhile, the biochar made through fast pyrolysis of poplar sawdust at400℃was mixed in the Kubuqi desert soil or in Thar desert soil with four levels of0,15,22and45t ha-1. The treatments were named as KB-0, KB-15, KB- 22and KB-45or TR-0, TR-15, TR-22and TR-45, respectively. Sorghum was grown as a test crop in all treatments under lab experiments for eight weeks. The effect of different biochars on plant growth, dry matter yield (DMY) and water consumption was monitored under the control and biochar-amended soils. To understand the effects of biochars on hydraulic properties of the desert soils, we computed the water-holding capacity (WHC), water-retention capacity (WRC), hydraulic conductivity and water-use efficiency (WUE) of the soil. After plant growth experiments, the chemical analyses of the soil were also conducted to evaluate the influence of biochars on soil fertility.Biochar yield decreased from55%to15.70%while syngas yield increased from25.00%to63.60%with increasing pyrolysis temperature (400to800℃). The maximum yield of bio-liquid,28.30%was obtained at a pyrolysis temperature of500℃. The pH (6.35to9.31), EC (0.57to2.44dS m-1), ash content (2.20to8.80%) total carbon content (51.71to77.46%), fixed carbon (31.40to87.77%) C/N ratio (64.86to161.37) and higher heating value (20.40to28.71MJ kg-1) increased while, the carbon conversion efficiency (71.41to30.55), volatile matter (VM)(72.00to20.19%), and CEC (27.50to23.60cmol kg-1) of biochar decreased with increasing pyrolysis temperature (400to800℃). The biochar made at a pyrolysis temperature of700℃had maximum content of essential plant nutrients such as Phosphorus (P) and Potassium (K). The BET surface area and pore volume of the biochar increased from3.02to8.93m2g-1and0.010to0.048m3g-1, respectively. Hence the WHC of biochar significantly increased from3.77to6.68g g-1with increasing pyrolysis temperature from400to800℃. The yield of poplar sawdust-derived biochar was51%and the biochar was acidic with a pH of4.21. This biochar had a54.32%carbon,19.23%fixed carbon,3.27%ash content and a C/N ratio of72.84. The surface properties such as BET surface area and WHC were almost similar to the pine sawdust-derived biochars.Due to large surface area and small particle sizes, the biochar particles filled the pore spaces of soil; hence, it reduced soil hydraulic conductivity and improved WHC and WRC. Related to the control, sorghum DMY yield increased by19%and32%, soil WHC increased by14%and57%, soil hydraulic conductivity reduced by15%and42%, soil WRC improved by16%and59%under T-400and T-700, respectively. Due to higher plant growth under biochar-amended soil, sorghum net WUE increased by52%and74%under T-400and T-700, respectively.In case of biochar application rates, related to the control, soil WHC increased by11%,22%,30%,10%,14%and27%, soil WRC increased by17%,28%,50%,16%,27%and66%while, soil hydraulic conductivity decreased by0%,30%,37%,4%,7%and16%under KB-15, KB-22, KB-45, TR-15, TR-22and TR-45, respectively. Soil pH decreased by0.33,0.61,0.92,0.38,0.79and0.95units, Soil total carbon increased by7%,11%,25%,6%,8%and20%, total K increased by25%,37%,56%,31%,41%and74%and total P increased by40%,70%,83%,30%,68%and73%under KB-15, KB-22, KB-45, TR-15, TR-22and TR-45, respectively. Sorghum DMY improved by16%,19%,18%and22%, WUE increased by25%,40%,27%and41%under KB-15, KB-22, TR-15and TR-45, respectively. In contrast, sorghum DMY decreased by24%and27%, and WUE also decreased by6%and8%under KB-45and TR-45, respectively. Biochar amendment improved soil hydraulic properties hence, at15to22t ha-1biochar rate, the sorghum DMY and WUE improved. The plant growth and DMY declined at higher application rate (45t ha-1) of biochar mainly due to increase in soil C/N ratio and loss of some micro plant nutrients such as Cu, Fe and Mn. Hence, sorghum WUE decreased at45t ha-1biochar application rate.Finally, it is concluded that, pyrolysis temperature for biochar preparation should not exceed700℃to produce high quality biochar suitable for soil amendment. The addition of fast pyrolysis biochar made from sawdust improved the quality of desert soils and enhanced plant growth and improved WUE. Higher pyrolysis temperature biochar had higher potential to improve desert soil quality than lower pyrolysis temperature biochar. The biochar made at a pyrolysis temperature of700℃with field application rates of15to22t ha-1is suitable for improving the quality of desert soils.