Long-term effects of tillage systems and rotations on soil structural stability and organic carbon stratification in semiarid central Spain

Under semiarid Mediterranean climatic conditions, soils typically have low organic matter content and weak structure resulting in low infiltration rates. Aggregate stability is a quality indicator directly related to soil organic matter, which can be redistributed within soil by tillage. Long-term effects (1983-1996) of tillage systems on water stability of pre-wetted and air dried aggregates, soil organic carbon (SOC) stratification and crop production were studied in a Vertic Luvisol with a loam texture. Tillage treatments included conventional tillage (CT), minimum tillage (MT) and zero tillage (ZT) under winter wheat (Triticum aestivum L.) and vetch (Vicia sativa L.) rotation (W-V), and under continuous monoculture of winter wheat or winter barley (Hordeum vulgare L.) (CM). Aggregate stability of soil at a depth of 0-5 cm was much greater when 1-2 mm aggregates were vacuum wetted prior to sieving (83%) than when slaked (6%). However, slaking resulted in tillage effects that were consistent with changes in SOC. Aggregate stability of slaked aggregates was greater under ZT than under CT or NIT in both crop rotations (i.e., 11% vs. 3%, respectively). SOC under ZT tended to accumulate in the surface soil layer (0-5 and 5-10 cm) at the expense of deeper ones. At depths of 10-20 and 20-30 cm no differences in SOC were encountered among tillage systems, but CT exhibited the highest concentration at 30-40 cm depth. Nevertheless, when comparisons were made on mass basis (Mg ha(-1)), significant differences in stocked SOC were observed at depths of 0-10 and 0-20 cm, where ZT had the highest SOC content in both rotations. The stock of SOC to a depth of 40 cm, averaged across crop rotations, was greater under ZT (43 Mg ha(-1)) than under CT (41 Mg ha(-1)) and MT (40 Mg ha-1) although these figures were not significantly different. Likewise, no significant differences were encountered in the stock of SOC to a depth of 40 cm among crop rotations (i.e., 42 Mg ha(-1) for W-V vs. 40 Mg ha(-1) for CM). Crop production with wheat-vetch and continuous cereal showed no differences among tillage systems. Yields were strongly limited by the environmental conditions, particularly the amount of rainfall received in the crop growth season and its distribution. Similar yield and improved soil properties under ZT suggests that it is a more sustainable system for the semiarid Mediterranean region of Spain. (C) 2002 Elsevier Science B.V. All rights reserved.