Effect of pig slurry additions on the organic carbon of calcareous soils

The effect of pig slurry additions at various rates over two years on the organic carbon in two calcareous soils with silt-loam and sandy-loam textures was studied in a greenhouse experiment. The organic carbon content in fresh pig slurries was very low, most of it being in a water-soluble form. There was, in general, a predominance of the fulvic-like fraction as opposed to the humic-like fraction. Because of the low level of organic carbon in the fresh slurry, the increases found in the soil were detected only with the highest doses, the equivalent of 800 and 1000 m3 ha−1 year−1, and after 14 months of additions. With a longer period of treatment there were significant increases using low rates. Thus, after two years, increases were detected with additions from 200 to 400 m3 ha−1 year−1, depending on the soil characteristics. In order to reach the same addition of organic carbon, a higher level of slurry was required in the sandy-loam soil than in the silt-loam soil, because in the former the organic matter mineralization was quicker and also the losses through leaching of water-soluble carbon were higher.The authors wish to thank 'The Instituto de fomento de la Region de Murcia' for the financial support awarded to Dr M. P. Bernal.Peer reviewe