The ecology of earthworms and their impact on carbon distribution and chemical characteristics in soil.

Abstract

In many soils earthworms play a major role in the decomposition of organic matter and the recycling of nutrients. Earthworm populations in acidic soils tend to be small and this can result in a breakdown in nutrient cycling processes, and the development of a thick mat of undecomposed plant material. In this study earthworm communities were modified by the introduction of three species, representing different ecological groups, into enclosed boxes of limed and unlimed soil at a site in the Scottish borders. The survival of the introduced communities in these two soil types was examined and their effects on soil carbon, specifically its chemical characteristics and distribution through the soil profile examined. Liming resulted in the increase of one species only, Allolobophora chlorotica, and results indicate that this effected a reduction in the thickness of the litter layer present at the soil surface. This is not behaviour associated with this endogeic species in the UK, although similar observations for other endogeic species have been recorded in Australia and the Netherlands. Cross Polarization Magic Angle Spinning C NMR spectroscopy was used on soils collected from this field experiment, and on cast material collected in a laboratory microcosm experiment, to determine the chemical characteristics of carbon. This showed that the direct impact that earthworms have on the decomposition of organic material is small, and their important effects are in the incorporation, comminution and mediating further microbial decomposition of the organic material. Cast material contains a relatively smaller concentration of easily assimilated carbohydrate rich material than the soil, and a concomitant increase in less easily degradable carbon compounds and microbial metabolites. Any small differences between species can be related to the quality of organic matter the earthworm ingested. Microbial activity in casts was greater when the ingested organic material was of a high quality. In this study the conclusions are: • The liming of acidic soil does not result in an increase in the abundance of all earthworm species, and the results are dependant on intraspecific competition; • There was no significant impact on plant yields of an increased size of earthworm community; • Large size fractions from limed soil appear more decomposed due to an increase in earthworm, and therefore microbial, decomposition, and; • There is little difference in the chemical composition of cast material from different earthworm species, and what differences there are can be directly related to the quality of organic material ingested by the earthworm.