Enzymatic biofilm detachment causes a loss of aggregate stability in a sandy soil
Abstract. The stability of soil aggregates against shear and compressive forces as well as water caused dispersion is an integral marker of soil quality. High stability results in less soil compactibility and erodibility, enhanced water retention, a dynamic water transport and aeration regime, increased rooting depth and protection of soil organic matter (SOM) against microbial degradation. For decades the importance of biofilm extracellular polymeric substances (EPS) regarding aggregate stability has been canonical because of its distribution, geometric structure and ability to link primary particles. However, experimental proof is still missing. This lack is mainly due to methodological reasons. Thus, the objective of this work is to develop a method of enzymatic biofilm detachment for studying the effects of EPS on soil aggregate stability. The method combines an enzymatic pre-treatment with different activities of α-glucosidase, β-galactosidase, DNAse and lipase, which preserves aggregate structure, with a subsequent sequential ultrasonic treatment for disaggregation and density-fractioning. Soil organic carbon (SOC) releases of treated samples were compared to an enzyme-free control. To test the effectivity of biofilm detachment the ratio of bacterial DNA from sessile and suspended cells after enzymatic treatment was measured by quantitative real-time PCR. Although the enzyme treatment was not sufficient for total biofilm removal, our results confirm, that EPS stabilizes soil aggregates predominantly by a strong intra-aggregate fixation, and enzymatic biofilm digestion caused a shift of occluded particulate organic matter (POM) to more fragile binding patterns. This suggests that an effect of agricultural practices on soil microbial populations could influence aggregate stability and thereby soil quality.