Effects of tillage, crop rotation and N application rate on labile and recalcitrant soil carbon in a Mediterranean Vertisol

The quantification of labile and recalcitrant fractions of organic C could provide valuable information in the study of soil organic carbon (SOC) changes in agrosystems. Few studies have determined the effects of farming practices on the recalcitrant and labile fractions of SOC at depth in a Mediterranean Vertisol. Our objective was to determine the effects of tillage, crop rotation and N fertilization on labile and recalcitrant SOC fractions and characterize δ13C in a soil profile (0-120 cm) from a long-term experiment established in 1986 on rainfed Mediterranean Vertisols in southern Spain. The following treatments were studied: conventional tillage (CT) vs. no-tillage (NT); three crop rotations (wheat [Triticum aestivum L.]-chickpea [Cicer arietinum L.], wheat-sunflower [Helianthus annuus L.] and wheat-faba-bean [Vicia faba L.]); and two N fertilizer application rates (0 and 100 kg N ha−1). The SOC contents of the soil samples from five soil layers (0-15, 15-30, 30-60, 60-90 and 90-120 cm) were determined. Throughout the experiment, the SOC content was greater in surface than in deep layers. The NT resulted in a greater SOC content than CT (10.7 Mg ha−1 and 8 Mg ha−1, respectively) in the most superficial soil layer. The SOC content was greater in the recalcitrant fraction than in the labile fraction (62% and 38% of total SOC, respectively). The recalcitrant SOC fraction was greater under CT than under NT. In the uppermost 15 cm, the labile organic C fraction was greater in the NT treatments than in the CT treatments because tillage resulted in a greater mineralization of the least stable forms of SOC. The influence of N rate and crop rotation on SOC was very low. The stable C isotopic composition (δ13C) was greater at depth than at the surface for both total SOC and the recalcitrant fraction. The labile fraction had more 13C than the recalcitrant fraction. The δ13C value was greater under CT than under NT for both soil fractions. Tillage system exerted a notable influence in both soil fractions and therefore in the organic matter quality since the labile fraction is an indicator of this quality.