Abstract

Combined hood and disk infiltrometer experiments were performed in conjunction with the laboratory measurement of soil water retention to quantify the impact on soil hydraulic properties of a mixed cropping rotation compared with pasture. The two sites were in adjacent fields on the same soil type. One site had been cropped for 2 yr. The other site had been under extensively grazed pasture for 2 yr. Our hypothesis was that the soil structure under the cropped treatment represented an initial stage and the soil structure under the pasture a final stage of a mixed pasture–cropping rotation cycle. The measured data were incorporated into an existing pore-space evolution model describing the temporal change in the pore-size distribution. The saturated and near-saturated hydraulic conductivities of the cropped soil were up to four times higher than those of the pasture soil and the amount of flow-active macropores were approximately 80% larger under the cropping than under pasture. This can be attributed to the loosening by tillage of the cropped soil before our measurements. The observed high infiltration rates under cropping could indicate that water flow took place between the aggregates rather than through the soil matrix. The lower conductivities and smaller amount of flow-active pores in the pasture soil than the cropped soil can be seen as a collapse of interaggregate pores after tillage. A reasonable prediction of the pore-size distribution dynamics resulting from different management practices was only possible when this loss of pores due to collapse was considered. For this purpose, a degradation term describing the time-dependent loss of macropores after tillage was incorporated in the model.

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