Sl. Lachnicht et Pf. Hendrix, Interaction of the earthworm Diplocardia mississippiensis (Megascolecidae)with microbial and nutrient dynamics in a subtropical Spodosol, SOIL BIOL B, 33(10), 2001, pp. 1411-1417
Direct and indirect effects of earthworm feeding and activity can alter soi
l microbial and nutrient dynamics. Little is known about influence of nativ
e North American earthworms on these dynamics. We investigated effects of n
ative D. mississippiensis activity on total and soluble C, N and microbial
biomass pools in Spodosols from the Apalachicola National Forest in North F
lorida, USA. Soil native to D. mississippiensis was collected and reconstru
cted by horizon (A, E, B-h) in laboratory chambers and incubated with earth
worms at zero, low or high density, respectively 0, three, or six individua
ls, 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 f
rom 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 B-h horizons aver
aged 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 signi
ficantly increased over two fold in the E with high earthworm density. Micr
obial 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 in
creased with earthworm density. Net N mineralization increased significantl
y 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 nutri
ents indicated that greater turnover and mineralization Occurred with earth
worm activity. SIR indicated earthworm activity induced microbial respirati
on 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 t
he 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 ac
tivity. Direct and indirect effects on nutrient dynamics by earthworms were
induced by casting activity and were associated with passage of material t
hrough the gut (direct effect) and nutrient and microbial enrichment throug
h the mixing of soil horizons (indirect effect). This study established the
importance of D. mississippiensis to nutrient availability in its native s
oil. (C) 2001 Elsevier Science Ltd. All rights reserved.