Recycling of nutrient-loaded biochars produced from agricultural residues as soil promoters for Gomphrena growth

Abstract

Biochar has been recognized as a potential media for soil amendment regarding its high surface area and retention capacity to slowly release nutrients to soils. However, the recycling of biochar after domestic water treatment towards agricultural application is still not well known. Therefore, this research studied the role of nutrient-loaded biochars produced from agricultural residues after canal water treatment as soil promoters for Gomphrena growth. Corncob, coconut husk, coconut shell and rice straw derived biochars were separately produced in a kiln (~378 °C) (namely CC, CH, CS and RS, respectively) and a pyrolysis reactor (500 °C) (namely CC-P, CH-P, CS-P and RS-P, respectively). The CH biochar was further modified with chitosan to improve its surface properties (labeled as CHC). The CH and CHC biochars after canal water treatment at lab and pilot scales are labeled as CH-column, CHC-column, CH-pilot and CHC-pilot, respectively. The loaded and unloaded biochars were further added in aquaculture sediment and loamy soil at 0.4, 0.7 and 1% mass ratio for Gomphrena growth. From the results, biochars amended in soil and sediment significantly improved seed germinations of Gomphrena, compared to control treatments. RS 0.4% amended in soil and sediment showed the highest seedling height (~2.5 cm) among all biochars, in accordance with its releases of K+, PO43- and NO3- into solution at high concentrations. Gomphrena growth in sediment amended with CH-column 1.0% biochar was comparable to unloaded biochar, indicating that loaded biochar can provide nutrients without harming the plant. In addition, chitosan modification induced higher plant growth in sediment amended with CHC-column 1.0% than with unmodified biochar. Gomphrena germination was also improved in CH-pilot and CHC-pilot biochars amended in sediments with maximum seedling heights of 3.5 and 4.2 cm, respectively. This is likely due to the abilities of CH-pilot and CHC-pilot biochars to release N (NH4+, NO3-) and total P of 0.106 and 0.111 mgN/L, and 0.770 and 0.637 mgP/L, respectively. This study revealed that the nutrient-loaded biochars can be used to sustain soil fertility through gradual releases of nutrients and thus promote the recycling of agricultural residues.