Effect of Aporrectodea trapezoides activity on seedling growth of Pseudotsuga menziesii, nutrient dynamics and microbial activity in different forest soils

Abstract

The effect of the endogeic earthworm, Aporrectodea trapezoides on the growth of Douglas-fir seedlings grown in three different soils was studied. Soils from pure and mixed stands of Douglas-fir and paper birch were used. Seedlings were grown in earthworm-worked and control soils for 10 months in the laboratory. At 5 and 10 months, differences in plant growth, available N and P, mycorrhizal colonization, microbial functional diversity and microbial activity were assessed. Douglas-fir seedling root biomass was greater in worm-worked compared to control soils. Height of seedlings varied significantly during the 5–8 months period of growth with taller seedlings in worm-worked soils. Foliar calcium levels of seedlings were higher at 10 months in worm-worked soils compared to the control. Effects of A. trapezoides on pH were unclear due to a worm×soil type interaction. Worm-worked mineral soil generally had higher organic matter levels compared to control soils but few differences were apparent in the forest floor layer. Average available NO 3 − and –NH 4 + levels were higher in worm-worked forest floor at 10 months yet there were no differences in the mineral soil at this time. The effect of A. trapezoides on mycorrhizal colonization of Douglas-fir seedling roots differed with the different soil types whereby seedlings grown in pure and mixed birch soils were less colonized compared to the control. Average total bacterial activity, substrate richness and diversity were consistently higher in worm-worked forest floor and mineral soil compared to the control treatment. The trend in microbial respiration in worm-worked forest floor and mineral soil was not clear while effects on microbial biomass in the mineral soil were confounded by a worm×soil type interaction. The metabolic quotient was generally lower in worm-worked soils compared to control soils. Douglas-fir seedlings grown in pure worm-worked fir soils were often affected differently or more dramatically compared to seedlings grown in pure birch or mixed soils. The results of the experiment indicate that A. trapezoides has the potential to enhance Douglas-fir seedling growth.