Earthworm population dynamics as influenced by the quality of exogenous organic matter

We conducted a replicated field plot experiment to investigate the influence of five exogenous organic materials (farmyard manure, cattle slurry and three types of compost), as well as mineral fertilization and two unfertilized control treatments (with and without a crop) on the number, biomass and species composition of earthworms (Lumbricidae) in an arable soil. A crucial feature of the experimental design was that the same mass of exogenous organic C was applied in each of the five organic treatments (1500–4000 kg C ha−1), making it possible to interpret observed effects in terms of the quality of the organic amendments. Earthworms were sampled twice per year using a combination of mustard extraction and handsorting. Two and a half years after the first of four organic matter applications, the farmyard manure and cattle slurry treatments had the largest earthworm abundance of about 800–900 individuals m−2 with 120–140 g biomass m−2. The unamended controls had the smallest earthworm number (about 150 individuals m−2), while the three compost treatments had intermediate values (400–500 individuals m−2). Since the mass of exogenous organic C applied was the same in the five organic treatments and since final soil organic C contents under these treatments were statistically similar (p>0.05), we hypothesize that the observed large differences in earthworm abundance between the farmyard manure and cattle slurry treatments on the one hand and the three compost treatments on the other hand were caused by differences in the chemical properties, and hence nutritional value for earthworms, of the applied amendments. We propose that manure and slurry provided earthworms with larger amounts of available C sources for direct consumption (including polysaccharides and proteinaceous materials) than the composted organic materials which were more humified and stabilized as a result of extended microbial aerobic degradation.