Factors influencing the release of the biological nitrification inhibitor 1,9-decanediol from rice (Oryza sativa L.) roots

Aims: Root exudates of rice (Oryza sativa L.) can inhibit nitrification in Nitrosomonas bioassays, and 1,9-decanediol was recently identified as an important new biological nitrification inhibitor (BNI) from rice. However, the release characteristics of 1,9-decanediol have not been studied. The present study was designed to identify the major factors influencing the release of 1,9-decanediol from rice roots. Methods: Rice plants were hydroponically grown in controlled environment chambers for 6 weeks, and root exudates were collected. Responses of exudate release to nitrogen form and concentration, pH, aeration, and bacterial inoculation were explored. The pH of root exudates, collected under different nitrogen-provision regimes, was determined, and 1,9-decanediol levels in exudates were monitored. Results: Ammonium (NH ₄ ⁺ ) and low pH in the root environment stimulated the release of 1,9-decanediol from rice roots. When only a part of the root system was exposed to NH ₄ ⁺ , the secretion of 1,9-decanediol was triggered in the whole root system. Aeration of the root environment significantly enhanced 1,9-decanediol release. The presence of two major nitrifiers (Nitrosomonas europaea and Nitrosomonas stercoris) in the root medium stimulated release of 1,9-decanediol, whereas denitrifiers had no effect. Conclusions: Our results demonstrate that the release of 1,9-decanediol is enhanced by low to moderate concentrations of NH ₄ ⁺ (≤1.0 mM), low pH, and aeration of the rhizosphere. Our study provides the first evidence of significant 1,9-decanediol secretion induced by nitrifying bacteria.