Soil quality assessment after weed-control tillage in a no-till wheat-fallow cropping system

Adoption of reduced-tillage fallow systems in the western USA is limited by winter annual grass weeds such as downy brome (Bromus tectorum L.). Moldbo ard plowing is an effective means of controlling downy brome in winter whea t (Tritcum aestivum L.)-fallow systems. The purpose of this study was to as sess the influence of plowing and secondary tillage operations, for the pur pose of weed control, on soil quality attributes of a silt loam soil that h ad been cropped in a sub-till or no-till (NT) winter wheat-fallow system fo r more than 20 yr. Compared with undisturbed NT, downy brome populations in plowed NT decreased 97 and 41% in the first and third crops following till age, respectively. Wheat yields in plowed NT treatments were 30 and 9% grea ter in the first and third crops following tillage, respectively, compared with undisturbed NT. Soil quality indicators assessed were organic C (OC), total N, inorganic N, pH, electrical conductivity, bulk density, water infi ltration rate, and pore-size distribution. Five years after tillage, soil O C decline in the 0- to 7.5-cm depth was 20% in plowed compared with undistu rbed NT; however, OC increased 15% in the 7.5- to 15-cm depth and was not d ifferent in the 0- to 30-cm depth. Total soil N followed similar trends. So il inorganic N in plowed NT decreased 37%, and soil pH increased 9%, compar ed with undisturbed NT, at the 0- to 7.5-cm depth. Occasional tillage with the moldboard plow in a reduced- or no-tillage management system will help control winter annual grass weeds, while retaining many of the soil quality benefits of conservation-tillage management.