Information about the mineralisation rates and effects on soil microorganisms must be obtained prior to the rational use of organic wastes in agriculture or forestry. The objective of this work was to study the mineralisation of two manures derived from the solid phase of pig slurries and the effects on the soil microbial biomass of an agricultural soil. Samples of this soil were mixed at two different rates with two manures derived from the solid phase of pig slurry (composted, CSP, and non-composted, NSP), and then were incubated during 163 days. Carbon mineralised from manures was fitted to first-order kinetic model, and small differences were found between manures despite the composting of one of them. Approximately 45% of the C added was mineralised in the experimental period. The soil microbial biomass C (C(mic)) was increased by the amendments according to the application rate. The sudden increases of the qCO(2) in the treated samples were ephemeral. The most appreciable differences between these manures were those related with net N mineralisation, being greater in the NSP-treated samples. The application of the solid phase of pig slurries, composted or not, could be a feasible practice to enhance in a short-term the microbial biomass of agricultural soils. In order to avoid an excessive release of inorganic N, the use of composted materials is preferred.