Two field experiments,in the south of Italy,were established in 2009 to study and characterize SOM during transition to organic farming.Experiments included a cereal/leguminous rotation fertilized with permitted amendments with three field replicates.A sequential fractionation procedure was used to separate different SOM fractions:light fraction (LF),two size classes of particulate organic matter (POM),mobile humic acid (MHA) and Ca-bound humic acid (CallA).Isolated fractions were quantified and analyzed for their C and N content and carbohydrates and amino compounds composition.The masses of the isolated fractions increased during 2-year course,with noticeable increases in LF and POM.Moreover LF and POM were found more responsive than MHA to treatment and crop.The xylose/mannose ratio explained that MHA-carbohydrates were mainly of microbial origin while LF-and POM-carbohydrates were of plant origin.Amino compounds constituted up to 30% of total soil N and were found to be more responsive to seasonal variation than to agronomic practices.
Department of Biology and Chemistry of Agro-Forestry and Environment,University of Bari,Italy;USDA-A USDA-ARS-National Laboratory for Agriculture and the Environment,Ames,IA,USA Department of Biology and Chemistry of Agro-Forestry and Environment,University of Bari,Italy Research Unit for Cropping Systems in Dry Environments(CRA-SCA).Bari,Italy Research Unit for the Study of Cropping Systems(CRA-SSC).Metaponto,Italy