Most of the benefits from conservation tillage are attained by maintaining crop residues on the soil surface. However, the effectiveness of crop residues depends on their persistence in time and maintenance of sufficient residue cover can become difficult, especially when a long-fallow period is involved. In this study, we evaluate the effects of conventional tillage (CT) and two conservation tillage systems (reduced tillage, RT, and no-tillage, NT), under both continuous cropping (CC) and cereal-fallow rotation (CF), on the dynamics of surface barley residues during four fallow periods in a dryland field of semiarid Aragon. The CC system involves a summer fallow period of 5-6 months and the CF rotation a long-fallow of 17-18 months. Results indicate that the lack of residue-disturbing operations in NT makes this practice the best strategy for fallow management. With this tillage system, the soil surface still conserved a residue cover of 10-15% after long-fallowing and percentages of standing residues ranging from 20 to 40% of the total mass after the first 11-12 months. In both CT and RT, primary tillage operations had the major influence on residue incorporation, with percentages of cover reduction of 90-100% after mouldboard ploughing (CT) and 50-70% after chiselling (RT). Two decomposition models were tested, the Douglas-Rickman and the Steiner models. Our data indicate that the Steiner model described more accurately the decline of surface residue mass over the long-fallow period in the NT plots. Measured and predicted data indicate that, under NT, 80-90% of the initial residue mass is lost at the end of fallow and that 60-75% of this loss occurs during the first 9-10 months. Finally, the mass-to-cover relationship established in this study for barley residues could be used to predict soil cover from flat residue mass through the fallow period by using a single A(m) coefficient (0.00208 ha kg(-1)). (C) 2004 Elsevier B.V. All rights reserved.
