Changes in Labile Fractions of Soil Organic Matter During the Conversion to Organic Farming

Organic farming can overcome the environmental consequences of intensive conventional farming. The objective of the work was to investigate the changes in labile soil organic matter (SOM) fractions during the conversion from conventional to organic farming in two Italian sites, namely Foggia (FG) and Metaponto (MT), which differed mainly in initial soil organic carbon (SOC) content. Fields were cultivated with lentil and wheat in rotation and treated with either compost or nitrogen or phosphorus (N/P) fertilizers in three field replicates. The SOM was sequentially fractionated into light fraction (LF), particulate organic matter (POM), and mobile humic acid (MHA) fraction. Isolated fractions were quantified and analyzed for C and N contents. Although total SOC responded to the fertilization treatments, the LF and POM fractions were yet more responsive. The MHA represented on average of 15% of SOC at both sites; however, the LF represented only 5–6% of the total SOC but was the most responsive to changes in soil management. Compost application contributed significantly greater quantities of LF, POM, and MHA than did the N/P fertilizers application. The initial SOC content can play an important role in determining the impacts of introducing organic farming practices on SOM fractions. Although both sites had an initial low SOC content, the MT site, with a lower SOC content, showed a substantial fractional C increment as compared to the FG site.