Local response of bacterial densities and enzyme activities to elevated atmospheric CO2 and different N supply in the rhizosphere of Phaseolus vulgaris L.

Altered flux of labile C from plant roots into soil is thought to influence growth and maintenance of microbial communities under elevated atmospheric CO2 concentrations. We studied the abundance and function of the soil microbial community at two levels of spatial resolution to assess the response of microorganisms in the rhizosphere of the whole root system and of apical root zones of Phaseolus vulgaris L. to elevated CO2 and high or low N supply.At the coarser resolution, microbial biomass C, basal respiration and phospholipid fatty acid (PLFA) patterns in the rhizosphere remained unaffected by elevated CO2, because the C flux from the whole root system into soil did not change [as shown by Haase, S., Neumann, G., Kania, A., Kuzyakov, Y., Römheld, V., Kandeler, E., 2007. Elevation of atmospheric CO2 and N-nutritional status modify nodulation, nodule carbon supply, and root exudation of Phaseolus vulgaris L. Soil Biology & Biochemistry 39, 2208–2221]. At a higher spatial resolution, more low-molecular-weight compounds were released from apical root zones under elevated CO2. Thus, at an early stage of plant growth (12 days after sowing), elevated CO2 induced an increase of enzyme activities (xylosidase, cellobiosidase and leucine-aminopeptidase) in the rhizosphere soil of apical root zones. At later stages of plant growth (21 days after sowing), however, enzyme activities (those above as well as N-acetyl-β-glucosaminidase and phosphatase) decreased under elevated CO2. The abundance of total and denitrifying bacteria in the rhizosphere soil of apical root zones was unaffected by CO2 elevation or N supply. Plant age seemed to be the main factor influencing the density of the bacterial community. In conclusion, the soil microbial community in the apical root zone responded to elevated CO2 by altered enzyme regulation (production and/or activity) and not by greater bacterial abundance.