Earthworm populations in a cool and wet district as affected by tractor traffic and fertilisation

In a field trial (1985–1996) on a sandy loam soil the effects of tractor traffic and fertilisation on earthworm populations were investigated. The tractor traffic treatments compared normal farm practice (`normal') with reduced tractor traffic (`low'). In the fertilisation treatments, three different cattle-manure management methods were compared. The cattle manure was applied as diluted slurry, as aerated slurry, or as separated manure with the composted faeces being applied to tilled land, while the urine was applied to ley. The different cattle manure management methods were compared at two different application levels. In addition, there were an unfertilised control and a higher application level with diluted slurry and mineral fertiliser treatments. The endogeic species, Aporrectodea caliginosa and A. rosea constituted 67% and 7% of the population density, respectively, and 78% and 3% of the mass in the period 1993–1996. The epigeic species, Lumbricus rubellus, constituted 25% of the population density and 19% of the mass. The population density, but not the species composition, was strongly affected by tractor traffic. In the years 1987–1989, there was an average of 160 earthworms/m2 after normal, and 680 earthworms/m2 after low, tractor traffic. During the experimental period, the earthworm population decreased in treatments with low tractor traffic. Important reasons for the reduction in earthworm population are likely to be soil acidification and reduced manure application. In 1995, the differences in population density and mass between treatments with normal, and low, tractor traffic were no longer statistically significant (140 and 250 earthworms/m2, respectively). In 1996, when there was no tractor traffic or fertilisation carried out, the corresponding population densities were 200 and 280 earthworms/m2. The population density and mass of earthworms were highest in manured soil. Increasing the amount of slurry applied was associated with a higher earthworm population in some years and a lower population in others. Increasing slurry application was most favourable for endogeic species and most destructive for epigeic species. Neither the average total population density nor mass of earthworms was affected by whether the manure was treated as slurry, or separated. However, more earthworms were found after addition of solid manure than of urine. Aerated slurry did not result in a higher earthworm population than diluted slurry.