Comparison of soil porosity structure under conventional and reduced tillage

The soil porosity structures under conventional (CT) and reduced tillage (RT) were compared on a Luvisol (Belgium) on a field experiment initiated in 2003. The total porosity n was computed from the bulk density (BD) and the microporosity structure was analysed by mercury intrusion porosimetry (MIP) in the range 0.003 to 73μm. It was presented in two forms: (i) cumulative pore volume vs equivalent pore radius r, from which four classes of porosity were defined: r < 0.2μm (microporosity); 0.2 ≤ r < 9µm (mesoporosity); 9 ≤ r < 73µm (MIP macroporosity); r ≥ 73μm (macroporosity); (ii) pore-size distribution (PSD). Besides the MIP measurements, the intrinsic behaviour of soil samples was investigated in one-dimensional compression tests.
At 0.10m depth, n was 7% lower under RT than CT and corresponded mainly to a reduction of macroporosity r ≥ 73 μm which corresponds to pores in which water movement is important (P<0.05). The plough pan structure under CT was clearly different from other layers. It presented a higher precompression stress (Pc>160kPa) related to an increased proportion of small voids. When converting CT to RT, this compacted layer was still persistent after 10 years at 0.30m depth. With BD reaching 1.7Mgm-3, this layer could restrict the gas/water fluxes with negative environmental consequences. In the subsoil, n was similar under CT and RT (44%) but the porosity structure of RT was more favourable than under CT. Indeed, the macroporosity r ≥ 73 μm was 10% higher under RT than CT and the radius of the more represented pores was increased (3.2μm in RT versus 2.7μm in CT). This suggested that process of recovering the textural porosity due to long-term climatic and biological processes had begun in the subsoil of RT.