Biological Nitrogen Fixation by Native Microorganisms in a Waste-Amended Paddy Soil

Abstract

ABSTRACT Biological nitrogen (N2) fixation (BNF) is an important aspect of sustainable and environmentally-friendly food production and long-term crop productivity. It has been established that free-living N2-fixing microbes and Endophytic bacteria play a vital role in the maintenance of soil fertility in flooded rice culture. The potential use of these microorganisms as a natural biofertilizer under an acidic paddy soil was evaluated in this study. Three soil amendments (composted rice straw, sewage sludge, and NPK) and three combinations of N2 fixers [bacteria, blue-green algae (BGA), and BGA + bacteria] were studied for rice (Oryza sativa L.) in a pot experiment. The soil pH which was moderated by the soil amendments had a significant effect on the potential of N2 fixation by BGA and bacteria. Pots amended with straw manure exhibited a lower N2-fixing ability than those amended with sludge. Nitrogen mineralization rate was significantly affected by both type of soil amendments and microorganisms. The NH4-N increased with composted straw application, but the utilization of nitrogen in the rice culture inoculated with bacteria was delayed. BGA + bacteria treatment showed higher N2-fixing rate and proportion of plant nitrogen derived from N2 fixation (Pfix) than cultures inoculated separately with bacteria or BGA. A negative correlation was found between the Pfix and the shoot N uptake under the different soil amendments. The results show that free-living N2-fixing microbes in an acid soil are governed not only by the soil nitrogen content but also by the type of soil amendment. The addition of sludge to BGA + bacteria inoculated soil increased BNF and consequently could be beneficial to rice production especially in an acidic soil.