Impact of earthworms (Diplocardia: Megascolecidae) on cycling and uptake of nitrogen in coastal plain forest soils from northwest Florida, USA

The ecology of earthworms native to North America is poorly understood. Yet, in parts of the southeastern United States, these earthworms (principally Diplocardia mississippiensis) are common and are harvested from forest soils for a fish-bait industry. We conducted an experiment, using intact soil cores from Apalachicola National Forest in northwest Florida, to assess impacts of different earthworm densities on microbial, plant, and soil pools of C and N. Treatments consisted of a factorial arrangement of earthworm abundance (0, 2, and 6 earthworms per core) and plant associations (no plant, Aristida stricta plugs, and Pinus palustris seedlings). Each core received (NH4)(2)HPO4 fertilizer. After 110 d, increased earthworm, abundance resulted in significantly (p<0.0001) increased N concentration in plant tissues (5.54, 7.98, and 9.44 mg N g tissue(-1) for A. stricta shoots; 3.38, 5.85, and 14.69 mg N g tissue(-1) for P. palustris stems; and 6.86, 8.07, and 11.06 mg N g tissue(-1) for P. palustris leaves for 0, 2 and 6 earthworms, respectively). Though a trend of increasing dry mass accumulation with increasing earthworm abundance existed for all plants, the differences were significant only between the 0 and 6 earthworm treatments for A. stricta shoots (12.78 and 20.66 g dry mass individual(-1), respectively). Microbial biomass C was negatively related to earthworm abundance across plant treatments (p<0.001) in the 0-3 cm depth of soil. Our results indicate that earthworms may be more important for plant nutrition and nutrient cycling in these systems than previously assumed. (C) 1998 Elsevier Science B.V.