Residual effects of compaction on the subsoil pore system—A functional perspective

Subsoil compaction caused by heavy traffic affects the soil pore system, resulting in long‐term damage to soil functions. The study contrasted two treatments from compaction experiments conducted at three different sites in Denmark: non‐trafficked control soil and soil subjected to four annual traffic events (2010–2013) with a wheel load of 58 to 78 kN. A cover crop of fodder radish ( Raphanus sativus L.) was grown in half of the initial experimental plots after completion of the compaction treatments (2013 and onwards). In the spring of 2017, undisturbed soil cores were sampled at 0.3 and 0.5 m depth. The air‐filled porosity ( ε a ), air permeability ( k a ) and gas diffusivity ( D s /D o ) were quantified for samples equilibrated to –100 hPa matric potential. Soil pore structural indicators were estimated from the combination of ε a , k a , and D s /D o . The ratio of non‐Darcian to Darcian k a ( R ) was also used as a pore morphology indicator. For all sites and depths, compaction reduced ε a , D s /D o , k a‐Darcy , PO1 ( k a‐Darcy / ε a ) and the effective radius ([(8 k a‐Darcy )/ D s /D o ] 0.5 ) compared to control soil ( p  < 0.05). The Buckingham‐ X variable relating D s /D o and ε a tended to be smaller for compacted soil, significantly for one of the sites. Compacted soils were also characterised by a significantly smaller R ‐ratio at high levels of k a‐Darcy  (> 32 μm 2 ), but also by having a tendency for the R ‐ratio to decrease rapidly with increasing pore air velocity compared to the control. The results reflect a compaction‐induced reduction in the number of marginal pores connected to large arterial pores, promoting a simple pore system formed by continuous vertical pores. The compaction effect was not affected by the cover crop. Neither natural recovery nor fodder radish‐induced mitigation of soil compaction was evident for the studied soils.