How bacteria remediate soil nitrate for sustainable crop production

Soil secondary salinization has attracted increasing public concern in greenhouse cultivation, and the dominant salt ion is nitrate. In this study, the Bacillus megaterium NCT-2 microbial agent was tested to remediate nitrate for sustainable production of agriculture. To understand the mechanism of microbial agents on remediating nitrate and their contribution to sustainable production, a laboratory 15N tracing study was carried out to quantify the simultaneously operating gross N transformation rates and associated them with copy numbers of key genes in the nitrogen metabolism. The NCT-2 strain showed strong nitrate assimilation (99%) and weak dissimilatory nitrate reduction to ammonium (DNRA) (1%). Mineralization of recalcitrant organic N dominated internal NH4+ production. Ammonium immobilization by NCT-2 strain occurred mainly into the recalcitrant organic N pool (60–67%). No nitrification activity (neither heterotrophic or autotrophic) was detectable in NCT-2 strain. Also the strain did not denitrify and was not involved in nitrous oxide emission. The study showed that NCT-2 strain is able to effectively remediate excessive nitrate in secondary salinized soil without causing any additional N losses, which is extremely conducive to sustainable production of agriculture. With this study the theoretical basis is provided to further develop suitable management tools in using Bacillus megaterium NCT-2 to increase the nitrogen use efficiency in greenhouse cultivation.