Phosphorus Sorption Behavior of Torrefied Agricultural Byproducts under Sonicated Versus Non-Sonicated Conditions

The application of agrichar derived from organic feedstocks has the potential to improve soil fertility and crop production by providing major nutrients like phosphorus (P) to the crop, and in some cases also reducing P leaching. The effect of agrichar on the leaching of P in soils is not uniform and can vary depending on the type of agrichar and amount applied. The objective of this study was to (i) evaluate the behavior of four agrichars (rice hulls RH, palm fronds PF, horse bedding HB, and bagasse BG) for their ability to retain/release P and (ii) determine the effect of wet sonication on P sorption behavior. The feedstocks were torrefied at 500 °C using a top-lit updraft gasifier and used in multiple batch incubation experiments to evaluate equilibrium phosphorus concentration, maximum P sorption capacity (Smax), and adsorption/desorption potential. Both, RH (12.6 g kg-1) and HB (11.5 g kg-1) contained 10-15 times higher total P concentration than PF and BG, rendering RH and HB as potentially suitable products to be used as soil amendments. However, this initial P content of the agrichar seemed to have an overriding effect on the P sorption behavior of the agrichar. PF had Smax of 676 (±127) mg kg-1 for the non-sonicated agrichar, and 237 (±91) mg kg-1 following sonication. There was significant increase in CEC for PF and BG agrichar upon sonication. PF agrichar increased from 27 (±3) cmolc kg-1 to 41 (±4) cmolc kg-1 with sonication. Similarly, BG agrichar CEC increased from 21 (±2) cmolc kg-1 to 45 (±5) cmolc kg-1 with sonication. Initial total P and Ca concentration and their ratios was found to play significant roles on P adsorption and desorption. Major cations like Ca, Fe, Al and Mn are found to act as active sorption site for phosphorus controlling its fate and mobility along with compounding effects of other physicochemical parameters.