Structure and activity of the nitrate-reducing community in the rhizosphere of Lolium perenne and Trifolium repens under long-term elevated atmospheric pCO.

Rhizosphere soil was sampled in monocultures of Lolium perenne and Trifolium repens in June and October 2002, at two different nitrogen fertilisation levels (14 and 56 g N m(-2) year(-1)) and under two pCO(2) atmospheres (360 and 600 ppmv) at the Swiss FACE (Free Air Carbon dioxide Enrichment) site. Directly extracted soil DNA was analysed with restriction fragment length polymorphism (PCR-RFLP) by use of degenerated primers for the narG gene encoding the active site of the membrane-bound nitrate reductase. The corresponding enzyme activity of the nitrate reductase was determined colorimetrically after 24 h of anaerobic incubation. The narG PCR-RFLP fingerprints showed that the structure of the nitrate-reducing community was primarily affected by season and pH of the sampling site, whereas CO(2) enrichment, plant species or fertiliser treatment had no apparent effect. In contrast, the nitrate reductase activity responded to N fertilisation, CO(2) enrichment and plant species in October, whereas in June drought stress most likely kept the enzyme activity at a low level in all treatments. Apparently, the respiratory nitrate-reducing community adapted to different treatments primarily by altered enzyme activity.