Earthworms secrete granules of calcium carbonate. These are potentially important in soil biogeochemical
cycles and are routinely recorded in archaeological studies of Quaternary soils. Production rates of calcium
carbonate granules by the earthworm Lumbricus terrestris L. were determined over 27 days in a range of
soils with differing chemical properties (pH, organic matter content, water holding capacity, bulk composition,
cation exchange capacity and exchangeable cations). Production rate varied between soils, lay in
the range 0–0.043 mmolCaCO3 (0–4.3 mg) earthworm−1 d−1 with an average rate of 8 × 10−3 mmolCaCO3
(0.8 mg) earthworm−1 d−1 and was significantly correlated (r = 0.68, P ≤ 0.01) with soil pH. In a second
experiment lasting 315 days earthworms repeatedly (over periods of 39–57 days) produced comparable
masses of granules. Converting individual earthworm granule production rates into fluxes expressed
on per hectare of land per year basis depends heavily on estimates of earthworm numbers. Using values
of 10–20 L. terrestris m−2 suggests a rate of 18– 3139 molCaCO3 ha−1 yr−1. Data obtained from
flow-through dissolution experiments suggest that at near neutral pH, granule geometric surface areanormalised
dissolution rates are similar to those for other biogenic and inorganic calcium carbonate.
Fits of the data to the dissolution relationship r = k(1 − ˝)n where r = dissolution rate, k = a rate constant,
˝ = relative saturation and n = the reaction order gave values of k = 1.72 × 10−10 mol cm−2 s−1 and n = 1.8
for the geometric surface area-normalised rates and k = 3.51 × 10−13 mol cm−2 s−1 and n = 1.8 for the BET
surface area-normalised rates. In 196 day leaching column experiments trends in granule dissolution rate
referenced to soil chemistry corresponded to predictions made by the SLIM model for dissolution of limestone
in soil. If soil solution approaches saturation with respect to calcium carbonate granule dissolution
will slow or even stop and granules be preserved indefinitely. Granules have the potential to be a small
but significant component of the biogeochemical cycling of C and Ca in soil
