Increased water retention in the rhizosphere allows for high phosphatase activity in drying soil

Background and aims Soil drying negatively impacts several rhizosphere processes, but plant roots are capable of alleviating changes in rhizosphere water content by releasing mucilage. We propose that enhanced water retention in the rhizosphere due to mucilage and microbial extracellular polysaccharides allows for fast diffusion of enzymes and substrates, and thus high enzyme activity in the vicinity of roots. Methods To assess the effect of diffusion on enzyme activity, the relation between phosphatase activity and volumetric soil water content (VWC) was quantified in sterile soil. Then, barley plants were grown in rhizoboxes and subjected to a drying cycle, while VWC and phosphatase activity were monitored by neutron radiography and soil zymography. Results The relation between phosphatase activity and VWC was well described by a diffusion model (R-2 = 0.64), demonstrating the importance of diffusion for enzyme activity. This finding was confirmed in the experiment with plants where phosphatase activity strongly decreased upon soil drying. Enzyme activity decreased less in the rhizosphere than in the bulk soil: the ratio between phosphatase activity in the rhizosphere and bulk soil was 10 when the soil was close to saturation and 63 when the soil contained 5% water. The relationship between phosphatase activity and local soil WC were well fitted by the diffusion model (rhizosphere: R-2 = 0.54, bulk: R-2 = 0.63), emphasizing the effect of diffusion on soil enzyme activity. Conclusions Our results indicate that soil VWC has a significant effect on soil enzyme activity by increasing diffusion of enzymes and substrate. The higher retention of water in the rhizosphere maintains high enzyme activity around roots in drying soils, which might be beneficial for plant nutrient acquisition.