Interactions between soil microbial communities, erodibility and tillage practices

Abstract

The soil biota are a vital component of belowground systems, driving many key processes such as nutrient cycling, underwriting soil structural integrity and providing crucial ecosystem services to the wider environment. In agricultural systems, tillage practices are known to impact upon both the soil biota and surface erosion processes, but little is understood about the relationships between these three factors. This work addresses this issue within the framework of an EU Life/ Syngenta project “Soil and water protection for northern and central Europe” (SOWAP). Within this component of the SOWAP programme, the influence of different soil management practices on the size and overall composition of the soil microbial community was determined and related to the propensity for erosion, at a variety of spatial scales. Microbial biomass and phenotypic structure, measured using phospholipid fatty acid (PLFA) analysis, were used to determine the effect tillage had on microbial communities at sites in Belgium, Hungary and the UK. The field sites were split into differing tillage practices on the same slope. Samples were taken prior to, and three years after, the adoption of inversion (conventional) and non-inversion tillage techniques. In addition, samples were taken periodically from two sites in the UK (Loddington, Leicestershire and Tivington, Somerset) to assess the temporal changes in microbial community size and structure under the tillage practices. Other soil, agronomic and ecological properties were measured at the field scale by SOWAP project partners. These field trials were supported by small plot rainfall simulations at the Loddington field site and by laboratory-based microcosm-scale studies using manipulated microbial communities and controlled rainfall, to further characterise microbial effects on soil erodibility. The results showed that across the European sites microbial community size was reduced in conventionally tilled soils. However there was no effect of tillage type on microbial biomass at the Tivington site after three years. Microbial community structure showed significant seasonal changes greater than those relatable to tillage type. It was notable that the fungal biomarker PLFA 18:2ω6 decreased in conventionally tilled soils. The small-scale experimentation using rainfall simulators and manipulated microbial communities was designed to specifically observe relationships between soil microbial communities, water movement and erodibility. These experiments showed that the presence of microbes in soils impacted upon both erosion processes and hydrological properties. There was a trend showing a decreased sediment concentration in runoff from soils containing a living microbial community. Propensity to runoff and infiltration was altered differentially as a result of microbial inocula derived from soils under different tillage practices. There was evidence that there was a specific and characteristic fraction of the microbial community susceptible to mobilisation by runoff and infiltrate waters, and hence potentially prone to relocation within the ecosystem. Linking the laboratory experiments to field rainfall simulations demonstrated the difficulty of controlling environmental variables, particularly at larger scales. Nevertheless, the same basic trends were observed at both laboratory and small plot scales.