Soil Quality Assessment after Weed-Control Tillage in a No-Till Wheatâ•fiFallow Cropping System

retaining crop residue on the soil surface to reduce erosion create an environment favorable for downy brome Adoption of reduced-tillage fallow systems in the western USA is because tillage operations are shallow (or eliminated) limited by winter annual grass weeds such as downy brome (Bromus and downy brome seeds are not buried deep enough to tectorum L.). Moldboard plowing is an effective means of controlling downy brome in winter wheat (Tritcum aestivum L.)–fallow systems. prevent seedling emergence. Moldboard plowing is an The purpose of this study was to assess the influence of plowing and effective means of controlling downy brome in a winter secondary tillage operations, for the purpose of weed control, on soil wheat–fallow system (Wicks, 1984). Plowing kills growquality attributes of a silt loam soil that had been cropped in a subing downy brome plants and buries seed at depths suffitill or no-till (NT) winter wheat–fallow system for more than 20 yr. cient to prevent their emergence after germinating. Compared with undisturbed NT, downy brome populations in plowed Seeds at a soil depth of 8 cm or greater generally germiNT decreased 97 and 41% in the first and third crops following tillage, nate but do not emerge (Wicks et al., 1971). Most downy respectively. Wheat yields in plowed NT treatments were 30 and 9% brome seed in soil germinates in the first year, and less greater in the first and third crops following tillage, respectively, than 2% of the seed remains viable in the soil after compared with undisturbed NT. Soil quality indicators assessed were 3 yr (Wicks, 1997); however, some studies have shown organic C (OC), total N, inorganic N, pH, electrical conductivity, bulk density, water infiltration rate, and pore-size distribution. Five years seeds to remain viable for up to 5 yr (Morrow and after tillage, soil OC decline in the 0to 7.5-cm depth was 20% in Stahlman, 1984). plowed compared with undisturbed NT; however, OC increased 15% Tillage and cropping system effects on soil chemical in the 7.5to 15-cm depth and was not different in the 0to 30-cm and physical characteristics have been documented in depth. Total soil N followed similar trends. Soil inorganic N in plowed many long-term studies (Fenster and Peterson, 1979; NT decreased 37%, and soil pH increased 9%, compared with undisVoorhees and Lindstrom, 1984; Wicks et al., 1988; Hill, turbed NT, at the 0to 7.5-cm depth. Occasional tillage with the 1990; Dalal et al., 1991; Karlen et al., 1991; Lyon et al., moldboard plow in a reducedor no-tillage management system will 1997). It was observed in these studies that frequent use help control winter annual grass weeds, while retaining many of the of the moldboard plow causes declines in soil organic soil quality benefits of conservation-tillage management. C (OC), decreases in soil structure and aggregation, reductions in water infiltration rates, and increases in soil erosion by wind and water. In two studies of wheat– A of no-till (NT) management systems by fallow systems in western Nebraska, Doran et al. (1998) winter wheat producers in Nebraska and other reported surface (0 to 7.5 cm) soil OC declines of 27 and parts of the western USA is currently limited by winter 40% for conventional tillage with a spring moldboard annual grasses such as downy brome (Wicks, 1984, 1997; plow (PL), and 4 and 20% for NT after 11 yr of cropping Lyon and Baltensperger, 1995; Rasmussen, 1995; Wiese following sod breakout. After 27 yr at these same two et al., 1995). In Nebraska, winter wheat yields were sites, surface OC declines were 41 and 48% for PL, and depressed 30% by downy brome densities of 11 to 18 and 21% for NT. However, some OC was redistrib22 plants m22; in Oregon, downy brome densities of uted to the depth of plowing due to physical movement ,160 plants m22 reduced yields by 40%; and in Idaho, and perhaps due to enhanced root growth. Thus, dewheat yields were reduced by up to 40% with 55 to clines in soil OC in the 0to 30.5-cm depth after 27 yr 110 plants m22 (Wicks, 1984). Selective control of downy were only 11.7 to 14.4% for PL, and 7.0 to 11.7% for brome in winter wheat is difficult since both species NT at these same sites. have similar life cycles. Crop rotations with at least 2 yr There is limited information on the effects of infrebetween winter wheat crops are proven effective for quent, intermittent, or periodic moldboard plowing or downy brome control if plants are kept from producing other inversion tillage used in conjunction with a conserseed during the non-wheat periods (Lyon and Baltenvation-tillage or no-tillage management system. In the sperger, 1995). Crop rotation, however, is not practiced sub-humid climate of Michigan, Pierce et al. (1994) studby many producers due to considerations of climate, ied whether soil properties developed under NT in a machinery, and/or the economics of these management continuous corn cropping system and on a Capac loam systems. Conservation-tillage practices that emphasize soil (fine-loamy, mixed, mesic Aeric Endoaqualf) were retained or reestablished after a single moldboard plowT.A. Kettler and J.W. Doran, USDA–ARS, 119 Keim Hall, Lincoln, ing. In the year after plowing, bulk density, total porosNE 68583-0934; D.J. Lyon, Panhandle Research and Extension Cenity, and microporosity (,24 mm pore radii) were deter, 4502 Ave. I, Scottsbluff, NE 69361; W.L. Powers, Univ. of Necreased, while macroporosity (.24 mm pore radii) braska, 131 Keim Hall, Lincoln, NE 68583; and W.W. Stroup, Univ. of Nebraska, 103 Miller Hall, Lincoln, NE 68583. Journal Ser. 12494 of the Agric. Res. Div. , Univ. of Nebraska, Lincoln, NE. Received Abbreviations: EC, electrical conductivity; NT, no-till; OC, organic 20 Jan. 1999. *Corresponding author (tkettler@unlinfo.unl.edu). carbon; PL, spring moldboard plow; PSD, pore-size distribution; ST, sub-till. Published in Soil Sci. Soc. Am. J. 64:339–346 (2000).