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The rhizosphere reflects a sphere of high substrate input by means of rhizodeposits. Active microorganisms and extracellular enzymes are known to be responsible for substrate utilization in soil, especially in rooted soil. We tested for microbial- and enzyme activities in arable soil, in order to investigate the effects of continuous input of easily available organics ( e.g., root-exudates) to the microbial community. In a field experiment with maize, rooted and root-free soil were analyzed and rhizosphere processes were linked to microbial activity indicators such as specific microbial growth rates and kinetics of six hydrolytic extracellular enzymes: β-glucosidase, β-cellobiohydrolase, β-xylosidase, acid phosphatase, leucine- and tyrosine-aminopeptidase. Higher potential activities of leucine-aminopeptidase (2-fold) for rooted vs. root-free soil suggested increased costs of enzyme production, which retarded the specific microbial growth rates. Total microbial biomass determined by the substrate-induced respiration technique and dsDNA extraction method was 23% and 42% higher in the rooted surface-layer (0-10 cm) compared to the root-free soil, respectively. For the rooted soil, potential enzyme activities of β-glucosidase were reduced by 23% and acid phosphatase by 25%, and increased by 300% for β-cellobiohydrolase at 10-20 cm depth compared to the surface-layer. The actively growing microbial biomass increased by the 17-fold in rooted soil in the 10-20 cm layer compared to the upper 10 cm. Despite the specific microbial growth rates showing no changes in the presence of roots, these rates decreased by 42% at 10-20 cm depth compared to the surface-layer. This suggests the dominance in abundances of highly active but slower growing microbes with depth, reflecting also their slower turnover. Shifts in microbial growth strategy, upregulation of enzyme production and increased microbial respiration indicate strong root effects in maize planted soil. [ABSTRACT FROM AUTHOR]

Microbial and enzyme functioning depends on the quality of substrates, which strongly differ in bare soil and in the hotspots of microbial activity such as the rhizosphere and detritusphere. We established a field experiment to determine the effects of contrasting substrate quality, namely, soil organic matter, maize shoot litter (detritusphere) and maize rhizodeposits (rhizosphere) on microorganisms and their extracellular enzymes in an arable soil. Kinetic parameters (Vmax and Km) of four hydrolytic extracellular enzymes: β-cellobiohydrolase, β-glucosidase, acid phosphate and β-xylosidase were analyzed in 0e10 and 10e20 cm to elucidate the effects of substrate content on substrate affinity and catalytic efficiency (Vmax/Km). Living roots increased microbial biomass by 179% and microbial respiration by 100% compared to fallow soil. Lower enzyme affinities to substrates (e.g. 93% for β-glucosidase) in rooted soil pointed to the domination of r-strategists, which are favored in the decomposition of labile organics common in the rhizosphere. No differences in catalytic properties of cellulolytic enzymes were detected between bulk and litter-treated soil, indicating the recalcitrance of organics in both treatments. The rhizosphere and detritusphere effects on enzyme kinetics were negligible in 10e20 cm, except β-glucosidase. The reduction of Km of all enzymes in 10e20 cm versus the upper 10 cm indicated increasing substrate affinity with depth. Nonetheless, the catalytic efficiency increased from 0 to 10 to 10e20 cm (e.g. up to 420% for acid phosphatase), reflecting changes in properties and functioning of enzymatic systems. This pointed to a shift towards a more K-selected microbial community with higher affinity and more efficient substrate utilization. It also indicated the microbial adaptation to decreasing substrate contents with depth by altered enzyme functioning. Overall, the catalytic properties of cellulolytic enzymes were much more strongly affected by plants (substrate quality in the rhizosphere and detritusphere compared to bare fallow) than by depth (substrate content). [ABSTRACT FROM AUTHOR]

The competition of microbial cells for nutrients (e. g. N or P) and carbon (C) infers the survival of the fittest principle favored by natural selection, depending on soil properties and nutrient availability. Therefore, the functional properties of dominating population, i.e. growth rate, carbon use efficiency, enzymes production and maintenance requirements are dependent on both soil nutrition potential and on the quantity and quality of the substrate input, implying different responses in microbial growth strategies. Up to now it is uncertain, how soil enzyme systems respond during the growth and the retardation of microorganisms in soils of contrasting nutritional status. We determined microbial growth by kinetic approach in high resolution, via substrate-induced respiratory response of microorganisms, enabling the estimation of total and growing biomass of the microbial community. Glucose and a combination of glucose and yeast were used as easily available substrates. Time series of potential hydrolytic extracellular enzyme activities were determined in order to elucidate enzyme production strategies of microorganisms at various stages of microbial growth. We expected faster growth and greater proportion of actively growing microbial biomass in soil amended with yeast extract + glucose versus sole glucose amendment, because a broader community including auxotrophic microorganisms gets activated by specific growth factors and metabolites from microbial necromass (yeast extract).In face of a smaller proportion of actively growing microbial biomass up to 32% faster specific growth rate of microorganisms was found in nutrient poor than in nutrient rich soil when the substrate combination was added. The proportion of growing microbial biomass was only 8% higher in nutrient-rich than in nutrient-poor soil with the addition of glucose. Substrate specific responses of enzyme activities indicated contrasting substrate utilization strategies during microbial growth. For the combined substrate amendment, the cross correlation of acid phosphatase between the two sites was highly significant, indicating harmonized time constraints of acid phosphatase during the phase of unlimited microbial growth. Similar relation was found for β-glucosidase during exponential growth, reflecting similar enzymatic substrate utilization irrespective of soil nutritional status. We conclude that the fast growing microbial community members activated by yeast amendment contribute significantly to an increased enzyme activity during the phase of exponential growth and thus strongly increase the SOM decomposition and nutrient mining from SOM. [ABSTRACT FROM AUTHOR]