Effect of crop sequences on soil properties and runoff on natural-rainfall erosion plots under no tillage

The objectives of this work were: (i) to assess the effect of different crop sequences under NT in natural-rainfall erosion plots on different soil properties and runoff, taking as extreme and contrasting references a 10-year pasture and a tilled plot without vegetation, (ii) to analyze the effect on runoff of different categories by volume of rainfalls and (iii) to evaluate the relationship between the intensification of the crop sequence and runoff. The study was carried out between July 2006 and June 2007 (1574mm of rainfall and 25 runoff events) on six natural-rainfall runoff plots with 3.5% slope and Aquic Argiudoll (Luvic Phaeozem) soil. The treatments were: corn (C) and soybean (S) monocultures, wheat/soybean (W/S) and the W/S phase of a wheat/soybean-corn rotation (W/S-C), pasture (P), and tilled soil without vegetation (L). Surface saturated hydraulic conductivity (K hc) was determined with disk permeameters. Saturated hydraulic conductivity (K h), bulk density (BD), and pore-size distribution were measured in undisturbed soil cores from 0-0.04 and 0.04-0.08m soil layers. Cumulative and average runoff and the average runoff coefficient were analyzed while rainfalls were categorized by the magnitude of the rain event. An intensification sequence index (ISI) was calculated as the ratio between the number of months occupied by crops and the total number of months of the year. Surface K h under field and laboratory conditions (0-0.04m) was higher in P than in the other treatments (3.3- and 9-fold, respectively) and was similar between crop sequences. Cropping increased BD 26% as compared to P and L. In both layers, BD was negatively associated with K h and macroporosity, and showed no relation with micro- and mesoporosity. All cropping sequences had reduced macroporosity compared to P, without differences between them. L and S had a higher runoff coefficient than P (6.25-fold), W/S-C and W/S, while C showed an intermediate behavior. Treatments had different runoff coefficients during intermediate and small rainfalls, but not with >70mm rainfalls. Independently of the rainfall category, S had runoff coefficients similar to those of L, whereas P and crop rotations showed similar losses. Neither the physical nor the hydrological soil properties studied explained the variation in cumulative and average runoff coefficients or those obtained with different rainfall categories. The ISI allowed us to explain cumulative runoff variation, average runoff and average coefficient runoff (R ² =0.9). Water loss through runoff in our type of soil and climate conditions is more associated with the management of surface cover, mainly the number of month of the year occupied by crops, than with an improvement in the physical properties related to porosity and internal soil water movement.