Impact of Long-Term Conservation Tillage Cropping Systems on Soil Hydraulic Properties in the Pacific Northwest.

Soil erosion driven by water and wind has long been a serious problem in sustaining air, soil, and water resources in the Pacific Northwest United States. Erosion processes, however, can be manipulated through tillage and crop residue management as a consequence of altering the physical attributes of soil that govern soil particle detachment  by water and wind. A long-term, field-scale, cropping systems experiment was initiated at Ralston, WA in 1995 to identify alternative cropping systems that would reduce windblown dust emissions from the winter wheat/summer fallow system conventionally used in the low and intermediate precipitation zones of the Columbia Plateau. The randomized, complete-block experiment included four cropping systems, three of which included: 1) winter wheat/summer fallow (SF), 2) spring wheat/chemical fallow (ChF), and 3) continuous, direct-seed, spring barley/spring wheat (NTSW). In 2006 and ten years after the experiment was initiated, we assessed the impact of these cropping systems on soil hydraulic properties; namely, infiltration, water retention, and saturated hydraulic conductivity (Ks). These properties were assessed in the upper 5cm of the silt loam soil profile in both spring, after sowing spring wheat or during fallow, and autumn, after harvest of spring wheat or sowing winter wheat. Infiltration was measured using a double-ring infiltrometer while water retention was assessed in-situ by measuring the temporal variation in soil water content following ponded infiltration. Saturated hydraulic conductivity was determined on extracted soil cores (7 cm in diameter) using the falling-head method. Overall, infiltration and Ks were greater for NTSW and SF as compared with ChF; this was likely due to ChF having a higher bulk density than NTSW and SF. Water retention characteristics indicated that SF and ChF retained more water near saturation than NTSW. In addition, water holding capacity was greater for SF and ChF than for NTSW. These retention characteristics suggest a larger pore size distribution for SF and ChF than for NTSW. Our results indicate that hydraulic properties of a silt loam can be modified by long term management in the intermediate precipitation zone of the Columbia Plateau.