Microdialysis as an in situ technique for sampling soil enzymes

Abstract

Soil extracellular enzyme activity (EEA) represents a critical bottleneck in the release of bioavailable nutrients from organic materials. However, quantifying spatial and temporal dynamics of EEA remains challenging. Techniques which measure the activity of, or directly sample free enzymes in situ may assist in understanding the short-term exoproteomic responses of microbes and roots to substrates, but few tools exist to explore EEA with minimal disturbance. We explore the potential of in situ microdialysis to directly sample soil enzymes, measuring their activity using a modified enzyme assay. We hypothesise that the technique's bias towards free solutes will also allow differentiation of free and stabilised enzyme pools. As little is known about the efficiency of microdialysis to sample enzymes from soil, recovery of a protease standard was quantified from solution and soil, finding that enzyme recovery is hindered at lower soil moisture contents. We further measured the response of native protease activity after the addition of soybean litter to clay and sandy soils, finding microdialysis observed greater EEA in litter-amended treatments than controls in both soil types. In comparison, EEA as measured by conventional extraction-incubation methods was only greater in amended clay soils. In a final experiment, hydrolytic enzyme activity of free and stabilised clay soil fractions were estimated using microdialysis. Free enzymes contributed 9% of total hydrolytic activity in soil without litter, increasing to 46% in litter-amended soil, suggesting fresh litter promoted a transient increase in the production of free exoenzymes by soil microbes. In contrast, the addition of litter had no significant effect on stabilised EEA. In spite of the obvious challenges involved in applying microdialysis as a method for soil protein sampling, this method offers new possibilities for investigating challenging spatial and temporal aspects of enzyme dynamics and protein availability in soils.