Combined application of strong alkaline materials and specific organic fertilizer accelerates nitrification process of a rare earth mining soil

Abstract

The extensive usage of ammonium sulfate as the leaching agent to extract rare earth elements led to widespread ammonia nitrogen (NH4+-N) pollution in the tailing soils of ion-adsorbed rare earth deposits in southern China. However, the cost-effective technologies to tackle with the long-term retention of NH4+-N in the rare earth mining soil have been largely unresolved. In this study, we developed a cost-effective approach to activate soil nitrification by the co-application of alkaline materials and organic fertilizer. The co-application of 0.3 % of organic fertilizer and 0.1 % ∼ 0.2 % of CaO or MgO or Mg(OH)2 stimulated a soil NH4+-N decrease rate of 2.01–7.58 mg kg−1 d−1 and a soil NO3−-N accumulation rate of 1.56–7.09 mg kg−1 d−1. Noting that only if the soil pH was elevated to 7.81–9.00, the NH4+-N decrease rate and NO3−-N accumulation rate were dependent on the proton consumption capacity of the alkaline materials. The application of CaCO3 could not stimulate soil nitrification possibly due to the soil pH was uncapable to be elevated to above 7.68. The qPCR, amplicon sequencing, and nitrification inhibitor batch incubation results demonstrated that organic fertilizer supplied active ammonia-oxidizing bacteria Nitrosomonas europaea. The proliferation of Nitrosomonas europaea in the alkaline materials and organic fertilizer co-applied soil was responsible for the soil nitrification. Furthermore, the application of commercial denitrifying bacteria inoculum promoted the removal of accumulated NO3−-N. The findings of this study provide a lost-cost technology to remove NH4+-N from the rare earth mining soil.