Nitrogen Mobilization by Earthworms in Limed Peat Soils Under Picea sitchensis

1. Liming (at 2.5 or 10 t ha-1) of peat taken from under a Picea sitchensis stand reduced mortality and weight-loss of five earthworm species, measured over 8 weeks in the laboratory. In the limed treatments, least population biomass was lost by Aporrectodea caliginosa, A. longa and Lumbricus terrestris. A. caliginosa was the only species to gain any weight in any treatment. 2. All earthworm species increased the amount of N released in leachates from limed and unlimed peat. Nitrate-N release was substantially greater (by up to 50-fold) in treatments with lime and earthworms than with earthworms alone. 3. Aporrectodea caliginosa was the most suitable species of those tested for inoculation into limed deep peat soils under P. sitchensis to improve rates of N mineralization. 4. Aporrectodea caliginosa was successfully introduced into limed peat profiles in field lysimeters under a stand of N-deficient P. sitchensis. Total earthworm biomass was sustained over the 12 months of the experiment in the limed treatments, whereas only 4% remained in the treatment without lime. Earthworm cocoons were produced in the limed treatments only. 5. Aporrectodea caliginosa in limed soil significantly increased leachate N release (to 60 kg ha-1year-1) compared with untreated (19 kg ha-1year-1), and limed soils (36 kg ha-1year-1). Nitrogen release was mainly as NO3 --N. Ammonium-N and NO3 --N mobilized by addition of earthworms plus lime appeared to be taken up by living roots of P. sitchensis. This increased mineralization can be expected to lead to enhanced tree growth.