Persistent effects of subsoil compaction on pore size distribution and gas transport in a loamy soil

The ever-increasing weight of agricultural machines exacerbates the risk of subsoil compaction, a condition believed to be persistent and difficult to alleviate by soil tillage and natural loosening processes. However, experimental data on the persistency of subsoil compaction effects on soil pore functioning are scarce. This study evaluated and quantified persistent effects of subsoil compaction on soil pore structure and gas transport processes using intact cores taken at 0.3, 0.5, 0.7 and 0.9 m depth from a loamy soil in a compaction experiment in southern Sweden (Brahmehem Farm). The treatments included four repeated wheelings with ∼10 Mg wheel loads. Water retention characteristics (WRC), air permeability (ka) and gas diffusivity (Ds/Do) were measured. A dual-porosity model fitted the WRC well, and there was a reduction in the volume of macropores >30 μm in compacted compared with control soil for all soil depths. Averaged for all sampling depths and also for some individual depths, both ka and Ds/Do were significantly reduced by compaction. Gas transport measurements showed that the experimental soil was poorly aerated, with local anoxic conditions at water regimes around field capacity in all plots and depths, but with significantly higher percentage anoxia in compacted soil. Our main findings were that: (1) commonly used agricultural machinery can compact the soil to 0.9 m depth, (2) the effect may persist for at least 14 years, and (3) important soil functions are affected.