Phosphorus release behaviors of poultry litter biochar as a soil amendment

- The predominant portion of P in poultry litter biochar is water insoluble.
- Poultry litter P was immobilized by forming Ca/Mg (pyro)phosphates in biochar.
- Release of P from biochar was slower and steadier than from raw poultry litter.
- Soil pH greatly influenced the P release patterns of poultry litter biochar.
- Soil amendment with biochar instead of poultry litter would reduce P runoff risks.

Phosphorus (P) may be immobilized and consequently the runoff loss risks be reduced if poultry litter (PL) is converted into biochar prior to land application. Laboratory studies were conducted to examine the water extractability of P in PL biochar and its release kinetics in amended soils. Raw PL and its biochar produced through 400 °C pyrolysis were extracted with deionized water under various programs and measured for water extractable P species and contents. The materials were further incubated with a sandy loam at 20 g kg− 1 soil and intermittently leached with water for 30 days. The P release kinetics were determined from the P recovery patterns in the water phase. Pyrolysis elevated the total P content from 13.7 g kg− 1 in raw PL to 27.1 g kg− 1 in PL biochar while reduced the water-soluble P level from 2.95 g kg− 1 in the former to 0.17 g kg− 1 in the latter. The thermal treatment transformed labile P in raw PL to putatively Mg/Ca phosphate minerals in biochar that were water-unextractable yet proton-releasable. Orthophosphate was the predominant form of water-soluble P in PL biochar, with condensed phosphate (e.g., pyrophosphate) as a minor form and organic phosphate in null. Release of P from PL biochar in both water and neutral soils was at a slower and steadier rate over a longer time period than from raw PL. Nevertheless, release of P from biochar was acid-driven and could be greatly promoted by the media acidity. Land application of PL biochar at soil pH-incorporated rates and frequency will potentially reduce P losses to runoffs and minimize the adverse impact of waste application on aquatic environments.