Distribution of organic carbon in humic and granulodensimetric fractions of soil as influenced by tillage and crop rotation

It is widely believed that soil disturbance by tillage is a primary cause of the loss of soil organic carbon (SOC) and that substantial SOC sequestration can be accomplished by conversion from conventional ploughing to reduced tillage. The objective of our study was to find alterations of the organic C content in soil humic and granulodensimetric fractions depending on soil tillage and crop rotation. The field experiment was carried out at the Joniskelis Experimental Station of the Lithuanian Institute of Agriculture on a drained clay loam Endocalcari-Endohypogleyic Cambisol (CMg-n-w-can). Two technologies - reduced tillage (RT) and conventional tillage (CT) - were compared in crop rotations with different proportions of overwintering and spring crops (0%, 25%, 50%, 75%, and 100% overwintering crops). The results of 2004-2006 are presented. Tillage had a greater influence than crop rotation on all soil C fractions. RT promoted the formation of all fractions of humic acids and FA1 and FA3 fractions of fulvic acids in the entire plough layer. Increasing the proportion of overwintering crops in the rotation to 100% tended to strengthen this effect. The C content in particulate organic matter (POM), light fraction (LF), and both clay-sized sub-fractions, expressed per unit mass of soil, significantly increased under RT in the top 15 cm of soil. The introduction of overwintering crops into the rotation and increasing their proportion had a significant positive influence on C content in POM and LF in the whole plough layer.