Structure and hydraulic properties of the boreal clay soil under differently managed buffer zones

Vegetated buffer zones (BZs) between arable fields and bodies of water are commonly established to reduce erosion and run-off of particle-bound nutrients. Functioning of a BZ depends on soil structure, as it is important for water infiltration. Therefore, it is vital to understand how varying management practices affect soils of BZs. We studied the structural and hydraulic properties of three differently managed BZs established in a boreal Vertic Stagnic Cambisol (clay, 51%). The three management practices for vegetation were as follows: natural with no treatment, harvested yearly and grazed by cattle. We used bulk density and macroporosity, together with a pore geometry index (air permeability per unit air-filled porosity), to describe the soil structural properties. Hydraulic properties were measured at different length scales by means of an aggregate sorptivity test, saturated hydraulic conductivity of the core samples and field-saturated hydraulic conductivity. Vegetation management markedly affected the physical properties in the top 5cm of the soil. Properties were least favourable for infiltration at the grazed site, with the greatest bulk density, least macroporosity and hydraulic conductivity or greatest pore tortuosity. In general, spatial variation in zones with restricted and good hydraulic conductivity together with reduced aggregate sorptivity in the deeper horizons made the soil prone to preferential flow when initially dry. Prolonged wetness, on the other hand, reduced saturated hydraulic conductivity significantly, resulting in surface run-off. Harvesting was considered the best management practice due to its inherent capacity for reducing the soil nutrient content and because it has minor implications for soil physical properties.