Soil structure rehabilitation of arable soil degraded by compaction

Soil degradation, especially compaction, due to wheeling with heavy machinery is a problem on arable land, which may rise in the future due to increasing weight. In a combined laboratory and field experiment, the rehabilitation of soil structure was investigated. We tried to differentiate between the rehabilitating effects of intrinsic soil processes and that of biological activity, especially earthworm burrowing. Therefore, undisturbed soil monoliths were taken in spring 1997 from arable land, which was compacted in spring 1995. All monoliths were taken from an agricultural field under conventional tillage (CT) or conservation tillage (CS). One part of the field was compacted six times by repeated wheeling with 5 Mg wheel load at one date in spring 1995, another part remained uncompacted. After defaunation, part of these soil monoliths had been inoculated with individuals of anecic Lunbricus terrestris, or endogeic Aporrectodea caliginosa, or remained uninoculated and were stored for 6 months. Using X-ray computed tomography for analysis, the structural parameters total void length, total void volume, tortuosity, and continuity of the monoliths were quantified. In the case of earthworms, the development of void length and void volume over the investigated time was calculated. Intrinsic soil processes have a strong influence on soils, which can obviously lead to a "natural compaction", as shown for the 1997 uncompacted monoliths. The reduction in total void length, total void volume, and tortuosity appears to be intensified by tillage operations and field traffic. Compacted soil monoliths showed a clear reduction in the parameters in 1995 after the compaction event, but showed an increase in 1997. This suggests a considerable soil structure rehabilitation during this time. This rehabilitation might be more the result of intrinsic soil processes and biological activity than of soil tillage, especially plowing. The increase in the parameters' values was obviously higher in soil from CS plots. Depending on their ecology, burrowing activity of lumbricid worms can be affected or not by a 2-year-old soil compaction. Burrowing activity of anecic L. terrestris seems not to be affected negatively, whereas endogeic A. caliginosa appears to have a reduced burrowing activity in formerly compacted soil. The effect of tillage systems on earthworm burrowing parameters appears to be minor. The combination of intrinsic soil processes and earthworm burrowing activity has a positive effect on soil structure rehabilitation after a heavy compaction event. But it has to be underlined that the effects of the compaction event are visible in earthworm burrowing activity even 2 years later. (C) 2002 Elsevier Science B.V. All rights reserved.