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. Generally, microbial communities in microbial hotspots are dominated by fast-growing r-strategists, which produce enzymes with higher activities but lower affinity to substrate. However, the extent to which the functional properties of soil microorganisms are different in the hotspots formed in soils with contrasting fertility remains unclear.
Research group in Institute of Applied Ecology, CAS along with research group in University of G？ttingen, Germany took advantage of a new technology to visualize the distribution patterns of microbial hotspots in rhizospheric soil, and collected the hotspots samples as well as bulk soil based on the zymography of leucine aminopeptidase.
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. 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.
These results indicated that the differences in microbial growth strategy between rhizosphere hotspots and bulk soil were dependent on soil fertility. Affinity of hydrolytic enzyme systems to substrate was mainly modulated by plant, whereas potential enzymatic activity was driven by both plant and soil quality. This research could take a step forward in understanding the complex interactions in plant-soil-microorganisms.
This research was published in Soil Biology &Biochemistry with the title of “Microbial growth and enzyme kinetics in rhizosphere hotspots are modulated by soil organics and nutrient availability”.
This research was supported by the National Natural Science Foundation of China and the National Key Research and Development Program of China.
Fig. 1 Examples of maize roots grown in rhizoboxes (center) and zymographs; showing spatial distribution of enzyme activities: (a) β-glucosidase, and (b) leucine aminopeptidase in the fertile soil; (c) β-glucosidase, and (d) leucine aminopeptidase in the poor soil, and (e) the sampling scenario using wet needle (Image by TIAN Peng).
Fig.2 Conceptual graph showing changes of microbial activities and functions in the hotspots as affected by soil fertility. Vertical and horizontal red arrow indicate increase and no change of microbial kinetics and functions in the hotspots compared to bulk soil, respectively. Red gradient arrow indicates increasing trend, blue gradient arrow indicates decreasing trend, gray arrow indicates no change along soil fertility (Image by TIAN Peng).
Email: yueqian@iae.ac.cn
Publication Name: TIAN Peng et al.
