Earthworms and nitrogen retention in northern hardwood forests

Background/Question/Methods

Invasion of forest soils by exotic earthworm species has attracted significant attention in soil and ecosystem ecology over recent decades. Earthworms significantly affect physical, chemical and biological properties of soils, with significant effects on the movement and retention of water, carbon and plant nutrients. Their effects on multiple aspects of the soil ecosystem greatly complicates evaluation of their effects on specific parameters and as a result, there have been conflicting results in studies of earthworm effects on ecosystem structure and function.  There is particular uncertainty relating to the effects of earthworm invasions on nitrogen (N) cycling and retention. While several studies have shown marked stimulation of N cycling and loss by earthworms, others have found that earthworms stimulate nitrogen retention and/or reduce nitrogen availability or loss. The effects of earthworms on N dynamics are of critical important to forest productivity, as N is an important limiting nutrient in temperate forests. Nitrogen retention also underlies the development of N saturation in forests exposed to high rates of atmospheric deposition, and to the delivery of N to receiving waters sensitive to N-induced eutrophication.

Results/Conclusions

We have been investigating the effects of exotic earthworm invasion on northern hardwood forest ecosystems since 1998.  In our first series of experiments, we found that earthworms significantly reduced soil carbon stocks, but had no effect on soil nitrogen. There was a marked reduction in soil C:N ratio, but no increase net mineralization or nitrification rates or nitrate leaching  which was surprising given previous studies showing that C:N is a strong controller of N losses in northern hardwood forest ecosystems.  Tracer-level additions of ¹⁵N and Br^- suggested that earthworms do not necessarily accelerate the movement of water and nitrogen through soil profiles. Resampling the original plots in a second study revealed that soil C pools and C:N ratios were still significantly lower (30 – 45%) in earthworm-invaded than no-worm sites, but soil N was also lower, by 6 – 27%, or 190 to 870 kg N/ha.  We added ¹³C and ¹⁵N-labelled sugar maple litter to our plots and found that after two years, total recovery of both ¹³C and ¹⁵N was much higher in the no worm (100%) than the earthworm plots (42%).  We hypothesize that variation in winter climate explains the different results that we have obtained in our previous studies and is the key regulator of earthworm effects on forest ecosystem nitrogen losses.