Effects of calcium silicate treatment on the composition of forest floor organic matter in a northern hardwood forest stand

Calcium amendment can help improve forest sustainability in stands that have been impacted by chronic acid deposition. An important component of this improvement is the stimulation of the microbial activity that supports ecosystem nutrient cycling processes. To test the hypothesis that Ca treatment alters the structure and solubility of organic matter substrates, an important driver of microbial activity, we investigated the effect of wollastonite (CaSiO₃) treatment on soil organic matter (SOM) and hot-water-extractable organic matter (HWEOM). We found a decrease in the HWEOM content of forest floor soils within 2 years of treatment with a high dosage of wollastonite (4,250 kg Ca/ha), but not at a low dosage (850 kg Ca/ha). High-dosage treatment did not reduce the biodegradability of HWEOM. Hence, a high dose of CaSiO₃appears to reduce the solubility of organic matter in the forest floor but not the bioavailability of the extracted SOM. Nuclear magnetic resonance spectroscopy revealed no significant changes in the O-alkyl C content of SOM in response to wollastonite addition, but a reduction in the O-alkyl C content of HWEOM suggests that the extractability of carbohydrate structures was reduced by added CaSiO₃. Phosphorous treatment, when performed in combination with Ca, also decreased the O-alkyl C content of HWEOM, but had no effect when performed without Ca. The reduced solubility of SOM after Ca treatment may have been the result of bridging between Ca²⁺and negatively charged sites on SOM, as suggested in other studies. Also, high concentrations of Si in soil solution, due to dissolution of the wollastonite, likely resulted in oversaturated conditions with respect to SiO₂or kaolinite, perhaps leading to co-precipitation of soluble organic matter. Overall, our results suggest that added Ca and/or Si may react with SOM to reduce the accessibility of labile C forms to soil microbes.