Animal slurries are often applied to agricultural soils as a source of nutrients and organic matter to maintain or improve soil fertility. The degradation in soil of the slurries-derived carbon (C) and nitrogen (N) has been studied in previous works but there is still little information about the influence of their particle size fractions on such dynamics. Two aerobic incubations (126 days), using a sandy loamy soil amended with pig and dairy slurry and their relative four particle size fractions obtained by sieving (>2000 mu m, 2000-500 mu m, 500-100 mu m, <100 mu m), were simultaneously performed, to investigate the effects of the slurries composition and respective particle size fractions on the soil C and N mineralization/immobilization processes. Our results showed that the higher amounts of recalcitrant C were present in the grossest fractions of both slurries, while the finest fractions presented the highest amounts of ready degradable C, suggesting that the carbon speciation is related with the particle size of the fractions. In both slurries, the finest fractions (<100 mu m) led to the highest N availability values in soil, being the slurries particle size inversely correlated with the N availability. N availability in soil is also related with the C and N speciation in each slurry type. Results show that, for a same application rate, dairy slurry should be more efficient to increase soil OM content than pig slurry. However, pig slurry led to a higher availability of mineral N in soil (920 mg g(-1) of the total N applied against 420 mg g(-1) in dairy slurry). (c) 2012 Elsevier Ltd. All rights reserved.