EFFECTS OF EARTHWORMS ON NITROGEN MINERALIZATION

The influence of earthworms (Lumbricus terrestris and Aporrectodea tub erculata) on the rate of net N mineralization was studied, both in soi l columns with intact soil structure (partly influenced by past earthw orm activity) and in columns with sieved soil. Soil columns were colle cted from a well drained silt loam soil, and before the experiment all earthworms present were removed. Next, either new earthworms (at the rate of five earthworms per 1200 cm(3), which was only slightly higher than field numbers and biomass) were added or they were left out. At five points in time, the columns were analyzed for NH4+, NO3-, and mic robial biomass in separate samples from the upper and lower layers of the columns. N mineralization was estimated from these measurements. T he total C and N content and the microbial biomass in the upper 5 cm o f the intact soil columns was higher than in the lower layer. In the h omogenized columns, the C and N content and the microbial biomass were equally divided over both layers. In all columns, the concentration o f NH4+ was small at the start of the experiment and decreased over tim e. No earthworm effects on extractable NH4+ were observed. However, wh en earthworms were present, the concentration of NO3- increased in bot h intact and homogenized cores. The microbial biomass content did not change significantly with time in any of the treatments. In both intac t and homogenized soil, N mineralization increased when earthworms wer e present. Without earthworms, both type of cores mineralized comparab le amounts of N, which indicates that mainly direct and indirect biolo gical effects are responsible for the increase in mineralization in th e presence of earthworms. The results of this study indicate that eart hworm activity can result in considerable amounts of N being mineraliz ed, up to 90 kg N ha(-1) year(-1), at the density used in this experim ent.