Microbial growth and enzyme kinetics in rhizosphere hotspots are modulated by soil organics and nutrient availability

The input of labile organics by plant roots stimulates microbial activity and therefore facilitates biochemical process rates in the rhizosphere compared to bulk soil, forming microbial hotspots. However, the extent to which the functional properties of soil microorganisms are different in the hotspots formed in soils with contrasting fertility remains unclear. We identified the hotspots related to different levels of Zea mays L. root architecture by zymography of leucine aminopeptidase in two soils with contrasting fertility. The hotspots localized by tiny wet-needle approach around first- and second-order roots were compared for parameters of microbial growth and enzyme kinetics. The pattern of hotspot distribution was more dispersed and the hotspot area was one order of magnitude smaller around first-versus second-order roots. The specific microbial growth rate (μm) and biomass of active microorganisms were soil-specific, with no difference between the hotspots and bulk soil in the fertile soil. In contrast, in the soil poor in organic matter and nutrients, 1.2-fold higher μm and greater growing biomass were found in the hotspots versus bulk soil. Lower enzyme affinity (1.3–2.2 times higher Km value) of β-glucosidase and leucine aminopeptidase to the substrate was detected in the hotspots versus bulk soil, whereas only β-glucosidase showed higher potential enzyme activity (Vmax) in the hotspots, being 1.7–2.1 times greater than that in bulk soil. Notably, the activity of C-acquiring enzyme, β-glucosidase positively correlated with the biomass of actively growing microorganisms. The fertile soil, on the whole, showed greater Vmax and catalytic efficiency (Vmax/Km) and an approximately 2.5 times shorter substrate turnover time as compared to the poor soil. Therefore, we conclude that i) the differences in microbial growth strategy between rhizosphere hotspots and bulk soil were dependent on soil fertility; ii) affinity of hydrolytic enzyme systems to substrate was mainly modulated by plant, whereas potential enzymatic activity was driven by both plant and soil quality. © 2019 Elsevier Ltd