Interaction of the earthworm Diplocardia mississippiensis (Megascolecidae) with microbial and nutrient dynamics in a subtropical Spodosol

Direct and indirect effects of earthworm feeding and activity can alter soil microbial and nutrient dynamics. Little is known about influence of native North American earthworms on these dynamics. We investigated effects of native D. mississippiensis activity on total and soluble C, N and microbial biomass pools in Spodosols from the Apalachicola National Forest in North Florida, USA. Soil native to D. mississippiensis was collected and reconstructed by horizon (A, E, Bh) in laboratory chambers and incubated with earthworms at zero, low or high density, respectively 0, three, or six individuals, for 28–31 days. Total C and N were measured by dry combustion methods. Soluble C and N and microbial biomass were measured in 0.5 K2SO4 extracts from non-fumigated and fumigated soil samples. Substrate induced respiration (SIR) was used with inhibitors to measure microbial respiration and shifts in bacterial and fungal components. Earthworm activity mixed soil horizons, largely through burrowing and casting. Total C in A and Bh horizons averaged 16.7 mg C g−1 soil and was significantly greater than in the E (2.6 mg C g−1 soil). Soil N concentration was highest in the A (0.73 mg g−1 soil) and lowest in the E (0.11 mg N g−1 soil). N concentration was significantly increased over two fold in the E with high earthworm density. Microbial biomass C was greatest in the A and E, averaging 8.6 mg g−1 C, and decreased with earthworm activity. In all horizons, soluble C and N also increased with earthworm density. Net N mineralization increased significantly with earthworm density and ranged from –0.22 to 68 mg g−1 N across all treatments and layers. Changes in proportion of soluble to total soil nutrients indicated that greater turnover and mineralization occurred with earthworm activity. SIR indicated earthworm activity induced microbial respiration in A and E horizons. Earthworm activity stimulated fungal and bacterial respiration, respectively, by 13 and 30% in the A and by 123% and 450% in the E. Although bacterial respiration was stimulated more than fungal there was no significant shift in B:F ratio, which ranged from 0.80 to 0.99. SIR did not provide evidence of any selective microbial effects of earthworm activity. Direct and indirect effects on nutrient dynamics by earthworms were induced by casting activity and were associated with passage of material through the gut (direct effect) and nutrient and microbial enrichment through the mixing of soil horizons (indirect effect). This study established the importance of D. mississippiensis to nutrient availability in its native soil.