FLOW OF MICROBIALLY FIXED NITROGEN IN A MODEL ECOSYSTEM

The preceding observations showed that both Azotobacter and a mixed flora with Azotobacter are about equally capable of fixing atmospheric nitrogen. During subsequent transfer of the nitrogen there was a distinct differentiation between movement of the newly fixed nitrogen and the nitrogen that was already present in tissues introduced in the beginning of the experiment. Pathways and transfer coefficients of both N¹⁴ and N¹⁵ were different in the presence of the different microbial populations. Turnover was more complex and in general slower in the presence of a mixed population than with pure Azotobacter. The lower transfer coefficients in the mixed populations may reflect more recycling within these populations than in pure cultures of Azotobacter. Denitrification losses upon acidification were negligible for N¹⁴ but appreciable for N¹⁵. The effect of acidification tended to be greater in systems with pure cultures of Azotobacter than in systems with a mixed culture.