Tillage and cropping intensification effects on soil aggregation: Temporal dynamics and controlling factors under semiarid conditions

During decades, in semiarid agroecosystems of the Ebro valley, intensive soil tillage and low crop residue input has led to a loss of soil structure. Conservation tillage and cropping intensification can improve soil structure in these areas. The objective of this study was to determine the influence of three different tillage systems (conventional tillage, CT; reduced tillage, RT; and no-tillage, NT) under two cropping systems (barley-fallow rotation, CF; and continuous barley, CC) on soil aggregation dynamics during two consecutive growing seasons (2003-2004 and 2004-2005). At the same time, the role that different soil and climatic factors play on soil aggregation in these semiarid areas was studied. Soil samples were collected at the soil surface (0-5 cm depth) from a long-term tillage experiment with a loamy soil (Xerollic Calciorthid). Two aggregation indexes were studied: dry aggregate size distribution and water aggregate stability from both air-dried and field-moist macroaggregates. A decrease in tillage intensity resulted in a higher mean size of dry aggregates and a greater water aggregate stability in both cropping systems particularly under NT. During the whole experiment, the dry aggregate size distribution (measured as the mean weight diameter, MWD) and the water stability of field-moist and air-dried soil aggregates (WASAD and WASFM, respectively) were greater under NT than under RT and CT due to a higher soil organic matter content under NT. Intensification of cropping system resulted in a greater water aggregate stability (both WASAD and WASFM) but it did not have any effect in the MWD. Differences among tillage treatments were more pronounced under the CC system than under the CF rotation due to the lower soil organic matter content and microbial biomass when long-fallowing was used. Variations in soil aggregation dynamics during the cropping season were mainly affected by crop growth and the associated activity of soil microorganisms. These findings indicate that the use of alternative management practices as NT and CC are viable strategies to improve soil aggregation from semiarid Ebro valley.