Managing Global Resources for a Secure Future

Organic phosphorus (P) accounts in average to 43% of total P in agricultural soils and have limited availability to crops. The accumulation of organic P in soils is generally assumed to be predominantly a legacy of past P fertilizer application in excess of plant requirement. Many attempts have been made to increase organic P availability to crops with the use of phosphatase enzymes. Enzyme biotechnologies can improve P availability without affecting the stabilized soil organic carbon and therefore represent an ideal approach to mobilizing a significant part of the residual soil organic P without causing loss of carbon to the atmosphere. Among the used phosphatases, phytases have been widely proposed as the best candidate enzymes due to their usual broad substrate specificity and their ability to start the stepwise dephosphorylating of phytate molecules, which are among the most recalcitrant forms of organic P present in soils. Several enzyme delivery systems have been proposed among them: a) microbial inoculants (eg. rhizosphere phosphobacteria and mycorrhiza); b) biostimulation of native microbes for increased P solubilization and mineralization; c) enzyme amendments (eg. immobilised phytases); and d) root exudation of phosphatase enzymes and other complementary traits such as exudation of organic acids. The biostimulation of native microbial traits is still not well demonstrated and microbial inoculants tend to have low survival and/or insufficient expression of P mineralization traits in soil environments. The direct soil amendment with stabilized phytases and the heterologous expression of microbial phytases in plant roots have shown some promising results and also many failures in improving organic P bioavailability to crops. This presentation will summarize the main challenges and achievements during the author’s last eight years of research on improving the understanding of the complex interrelation of the factors affecting organic P mobilization and phytase performance in soil environments.