The use of organic amendments, including livestock slurry, in intensive grassland soils may result in a temporary increase or decrease in soil organic matter (SOM) mineralisation through microbial activity (positive or negative priming effect, PE). However, the effects of applying organic amendments of different complexity on the activity and composition of the microbial community, as well as on PE, in temperate grassland soils remain unclear. In this study, we hypothesised that the complexity of carbon (C) compounds added to the soil via livestock slurry and in carbohydrate amendments would significantly affect both the partitioning of C and the composition of the soil microbial community. We also hypothesised that applying slurry that had received a biological additive would significantly alter the partitioning of C within the soil and the soil microbial community, compared to an unamended slurry. Microbial community structure, based on phospholipid fatty acid (PLFA) analysis, microbial respiration, and uptake into microbial biomass of 14C-glucose, 14C-glucose-6-phosphate (G6P), and 14C-cellulose, alongside livestock slurry amended or not amended with a biological additive, were investigated in laboratory incubations to test these hypotheses. Both 14C mineralisation and 14C biomass uptake declined significantly as the complexity of the carbohydrate compound added to soil increased (glucose > G6P > cellulose), with the incorporation of C into microbial biomass exceeding respiration of C as CO2 for each compound. The soil microbial community structure was dominated by bacteria over fungi in all treatments through time, with a prevalence of gram-negative over gram-positive bacteria. Our data indicate that adding slurry to soil alongside carbohydrate compounds decreased microbial biosynthesis and the cumulative respiration of carbohydrate compounds, compared to treatments in which slurry was not applied. Our study also indicates that the use of biological additives during slurry storage increased 14C biomass uptake following application of slurry to agricultural grasslands, thus suggesting that biological additives have the potential to enhance soil fertility.