Microbial Growth and Carbon Use Efficiency in the Rhizosphere and Root-Free Soil

Plant-microbial interactions alter C and N balance in the rhizosphere and affect the microbial carbon use efficiency (CUE)–the fundamental characteristic of microbial metabolism. Estimation of CUE in microbial hotspots with high dynamics of activity and changes of microbial physiological state from dormancy to activity is a challenge in soil microbiology. We analyzed respiratory activity, microbial DNA content and CUE by manipulation the C and nutrients availability in the soil under Beta vulgaris. All measurements were done in root-free and rhizosphere soil under steady-state conditions and during microbial growth induced by addition of glucose. Microorganisms in the rhizosphere and root-free soil differed in their CUE dynamics due to varying time delays between respiration burst and DNA increase. Constant CUE in an exponentially-growing microbial community in rhizosphere demonstrated the balanced growth. In contrast, the CUE in the root-free soil increased more than three times at the end of exponential growth and was 1.5 times higher than in the rhizosphere. Plants alter the dynamics of microbial CUE by balancing the catabolic and anabolic processes, which were decoupled in the root-free soil. The effects of N and C availability on CUE in rhizosphere and root-free soil are discussed.     

Soil respiration under elevated CO2 and its partitioning into recently assimilated and older carbon sources

Plant and Soil - Efflux of soil CO2 (soil respiration) plays a crucial role in the global carbon cycle and may be strongly altered by global change. In this study, we measured soil respiration in...     

The influence of acid irrigation and liming on the soil microbial biomass in a Norway spruce (Picea abies [L.] K.) stand

Over a period of three years (1990–1992) microbial biomass-C (C_mic), CO₂ evolution, the C_mic:C_org ratio and the metabolic quotient for CO₂ (qCO₂) were determined in a Norway spruce stand (Höglwald) with experimentally acid-irrigated and limed plots since 1984. A clear relationship between soil pH and the level of microbial biomass-(C_mic) was noted, C_mic increasing with increasing soil pH in O_h or A_h horizons. More microbial biomass-C per unit C_{org} (C_mic:C_org ratio) was detected in limed plots with elevated pH of O_h or A_h horizons as compared to unlimed plots with almost 3 times more C_mic per unit C_org in the limed O_h horizon. Differences here are significant at least at the p=0.05 level. The positive effects of liming (higher pH) on the C_mic:C_org ratio was more pronounced in the upper horizon (O_h)). The total CO₂ evolution rate of unlimed plots was only half of that noted for limed plots which corresponded to the low microbial biomass levels of unlimed plots. The specific respiratory activity, qCO₂, was similar and not significantly different between the unlimed control plot and the limed plot.Acid irrigation of plots with already low pH did not significantly affect the level of microbial biomass, the C_mic:C_org ratio or qCO₂. An elevated qCO₂ could be seen, however, for the limed + acid irrigated plot. The biomass seemed extremely stressed, showing with 3.8 g CO₂-C mg_-1 C_mic h_-1 (O_h) the highest qCO₂ value of all treatments. This was interpreted as a reflection of the continuous adaptation processes to the H⁺ ions by the microflora. The negative effect of acid irrigation of limed plots was also manifested in a decreased C_mic:C_org ratio.