مهدی رحمتی

To evaluate the effect of crop rotation on soil structural properties and aggregate fractal dimension, three locations were selected in Maragheh and Hashtroud areas, northwest of Iran. Four crop rotations were implemented and several soil physical and chemical properties were evaluated. In wheat-chickpea and wheat-fallow rotation treatments (T2 and T4) average bulk density (BD) of 1.155 g cm-3 was significantly (P< 0.01) lower than continuous wheat cropping (T1) and wheat-fallow-wheat-wheat-wheat (T3) treatments with BD of 1.29 g cm-3. Water aggregate stability (WAS) in T4 became significantly (P<0.01) greater than other rotations. Saturated water content (θs) of T1 and T3 (0.39 cm3cm-3) was significantly (P< 0.01) lower than T2 and T4 (nearly equal to 0.55 cm3cm-3). The lowest saturated hydraulic conductivity (KS) of about 2.0 cm h-1 occurred in T1 and T3, and the highest (5.27 cm h-1) in T4. Soil moisture content (θm) in T2 and T4 during GS21 growth stage significantly exceeded those of T1 and T3. Biological yields in T2 and T4 were nearly equal (3.05 t ha-1) and were significantly (P< 0.05) greater than T1 and T3 (1.5 t ha-1). During growth stages of GS54 and GS87, θm and other measured properties were not significantly affected by crop rotations. It appears that substitution of wheat-follow or wheat-chickpea rotations by continuous wheat cropping would gradually degrade the soils and thus should be abandoned.