Impact of earthworm casts on soil pH and calcium carbonate in black truffle burns

Granules of calcium carbonate are known to be produced by the calciferous glands of earthworms, and may have a measurable impact on the soil; however, little is known of their dynamics. Earthworms are often found in Tuber melanosporum burns, and this truffle is closely linked to calcium carbonate and soil pH. The present work shows that in weakly calcareous soil burns with high earthworm (Prosellodrilus sp.) activity, earthworm casts showed a significant increase of 0.2 units of pH and 66 % more total calcium carbonate than soils in the burns, although the casts were produced from these same soils. Analysis of the soil carbonate fractions (active carbonate versus calcium carbonate greater than 50 μm in size) showed that the origin of the increase in the total calcium carbonate content of the Prosellodrilus sp. casts cannot be explained by the levels of original carbonate existing in the soils in the study, although it can be explained by the synthesis of calcite granules by earthworms. These results obtained in the field with Prosellodrilus sp. confirm Canti's (2009) experiments in the laboratory using δ13C concerning the synthesis of calcite granules by Lumbricus terrestris. Both these results and other works indicate that production of calcite granules by Prosellodrilus sp., L. terrestris, Aporrectodea longa, A. trapezoides and Anisochaetae sp. has the ability to increase soil pH and calcium carbonate; additionally Lambkin et al. (2011) have found that higher soil pH and carbonates can cause an increase in the production of calcite granules by L. terrestris. All these results, therefore, point to a feedback process, scope of which is a function of the different soil types and earthworm species. In summary, different earthworm species may have a significant and positive impact on soil pH and net recarbonation of T. melanosporum burns, and the importance of both soil properties justifies the development of further experiments aimed at incorporating vermiculture into the truffle cultivations