Evaluation of select biochars and clays as supports for phytase to increase the fertilizer potential of animal wastes

Abstract

Manures may contain considerable amounts of organophosphates (org-P) that must be enzymatically converted to inorganic phosphate (i.e., PO4-P) to be plant available. Although adding enzymes into manures can facilitate mineralization of org-P to PO4-P, enzymes that are not immobilized are easily lost through leaching, degradation, or denaturation. In this study, the immobilization of enzymes onto nine different biochar surfaces was explored. Phytase, which mineralizes a main class of org-P, was used as the model enzyme. Immobilization methods included covalent grafting accomplished by the carbodiimide crosslinker method and physical sorption. The results showed that physisorption was as effective as grafting for loading phytase to the biochars. Phytase loading after mixing 0.1 g biochar and 2 mg phytase correlated positively with biochar C:H ratio (an indicator of aromatic content) suggesting the importance of the hydrophobic effect. An increase in pH led to a decrease in phytase loading consistent with repulsion between negatively charged sites on phytase and the increasing negative charge on biochar. Less than 4% of the immobilized phytase leached after sequential extractions over seven days using manure dissolve organic matter solutions. However, the activity of immobilized phytase decreased markedly compared to the free state phytase. The specific activity of immobilized phytase was two orders of magnitude lower than that of free phytase at pH 5 and 7. Nevertheless, results showed that deactivation of phytase by biochars were reversible once the phytase was detached from the surfaces. Compared to the biochars, clay minerals (montmorillonite, kaolinite and hematite) tended to have greater loading rates and higher phytase activity. Composting manures with coamendments of biochar and minerals may enhance both short- and long-term P mineralization potential.