Poultry-manure biochar and phosphorus availability: influence of magnesium enrichment and bacterial solubilization

Abstract

Biochar is a stable and recalcitrant byproduct of pyrolysis, and its applications are related primarily to soil conditioning. Producing biochar from an organic residue such as poultry manure is an alternative to increase the fertilizer value, increase nutrient recycling, and prevent nutrient losses by decreasing nutrient availability in water. A low water availability is important specially to enhance phosphorus (P) use efficiency in soils, especially in tropical soils due to their high affinity to clay mineral fraction (Fe and Al oxides and hydroxides). However, Ca content in poultry manure is relatively high (~10%), and with pyrolysis, P availability decreases due to thermal transformation. Moreover, is it necessary to develop methods to increase P availability from poultry manure biochar without increasing P availability in water. For instance, chemical modifications by introducing additives such as magnesium (Mg) in the feedstock and/or inoculation with phosphate- solubilizing bacteria may improve the biochar's ability to supply P in soil. Thus, the objectives of this study were: (1) To determine how pyrolysis temperature and increasing the ratio of Mg/Ca affects P solubility; (2) To evaluate how changes in P species and their crystallinity affect the extent to which phosphate-solubilizing bacteria (PSB) increase P solubilization; (3) To evaluate how maize plants growth and P uptake in shoots are affected with P fertilization using biochar and Mg- enriched biochar at increasing pyrolysis temperature and bacterial inoculation. For this, Mg modified-biochars were produced in a laboratory-scale pyrolysis system, performed with a heating rate of 10 oC/min to their target temperature and a holding period of 30 min. After the production, biochar was completely characterized, and P availability (formic and citric acid 2%) and P speciation by using XANES spectroscopy were studied. In vitro and in vivo experiments were performed, and P solubilization, maize plant growth, and P uptake in plant shoots were determined. With Mg addition, P availability increased up to a pyrolysis temperature of 600 °C compared to the pristine biochar. Adding Mg to poultry manure biomass prevented or decreased the formation of more crystalline minerals, such as hydroxyapatite and pyrophosphate. As for PSB, the P source influenced specific organic acid production, but not necessarily with increased P solubilization. The strain Pseudomonas sp. released higher amounts of P from Mg-biochar at 350 °C (in vitro condition). Similarly, Mg-enriched biochar (350 °C) and PSB inoculation increased P uptake and accumulation in maize shoot tissues. Thus, adding Mg prior to the pyrolysis of poultry manure is an attractive alternative to increase its fertilizer value for further plant growth. Future studies should focus on comprising the P transformations in biochar produced at higher pyrolysis temperatures (>350 °C) and how to enhance the potential of Mg enrichment and PSB inoculation on P availability in soil.