Low levels of fixed nitrogen required for isolation of free-living N2-fixing organisms from rice roots
ISOLATION and counting of N2-fixing bacteria from the natural environment have until now been conducted on selective N-free media. It is likely that some N2-fixing bacteria are overlooked by using this procedure. We now report that most of the bacteria present in rice roots are N2-fixers, but that they require a supply of mineral or organic N for growth. In the presence of organic nitrogen, nitrogenase activity is detected in these organisms. The requirement for organic nitrogen for the detection of nitrogenase is also true for N2-fixing cultures of free-living rhizobia1-3.