Although more common in certain agronomic crops, ST is an emerging practice in vegetable production (Hoyt, 1999). A narrow strip (15 to 30 cm depending on equipment and crop) is tilled into otherwise undisturbed soil and a crop is seeded or planted into this strip. Soil between rows (BR) is left undisturbed, which may reduce the potential for erosion and maintain soil quality—advantages that ST provides compared with CT. Strip tillage also offers advantages compared with no-till—it offers a better seedbed for the crop IR and helps to warm and dry soil in the spring, which is important in geographic locations with cool, wet springs like Michigan (Mochizuki et al., 2007). Because of more flexible planting and harvest dates, vegetable fields offer more opportunities to integrate cover crops into rotations. Cover crop residues may help ameliorate some of the negative effects of disturbance IR by adding organic matter; BR, the residue remains as surface mulch, which may help retain soil moisture (Mochizuki et al., 2007; Wilhoit et al., 1990), prevent germination and emergence of weed seeds (Teasdale and Mohler, 1993), and further protect against erosion.
Tillage occurs IR in both ST and CT fields, although interactions with untilled BR areas may influence IR soil temperature, moisture, and IN dynamics in ST. These different growing conditions may result in improved crop growth and yield. For example, strip width influenced in-row soil temperature—with 15-cm wide strips, IR soil temperature was 1 °C cooler at night compared with IR soil temperature with full-width tillage or ST with 30-cm strips (Mochizuki et al., 2007). For warm-season vegetable crops in northern areas, lower soil temperatures associated with reduced tillage systems with cover crop residue can decrease yields or delay maturity. However, for cabbage—a cool-season crop—reductions in soil temperature during the hottest part of the growing season may be beneficial. BR soil temperature is generally lower in ST compared with CT (Licht and Al-Kaisi, 2005; Overstreet and Hoyt, 2008), whereas soil moisture is typically higher in this location (Hoyt and Konsler, 1988).
Differences between IR and BR in ST may be heightened when cover crops are used. Surface mulches tend to hold more soil moisture and further decrease soil temperature (Wagner-Riddle et al., 1997); incorporated residues also help retain more soil moisture. If the BR area can act as a soil moisture reservoir, more moisture may be available to crops grown with ST—indeed, higher yields of transplanted cabbage with ST were attributed to higher moisture availability in a dry year (Wilhoit et al., 1990). Characterization of soil temperature and moisture changes is important for understanding the direct impact of ST on crops as well as the effects of ST on soil biological and chemical processes, which affect crop growth.
Strip tillage and cover crops may also influence crop yields through changes in soil N dynamics. Tillage typically increases N mineralization, resulting in a flush of plant-available N (Calderon et al., 2000). Incorporating a non-legume cover crop like oats tends to decrease mineralization and increase immobilization, making less inorganic N available to the following crop, at least temporarily (Cheshire et al., 1999). Burying oat straw residue through tillage led to faster decomposition than leaving it on the surface, although surface oat straw residue immobilized less N than incorporated residue (Mulvaney et al., 2010). To our knowledge, no studies have examined soil N dynamics in ST vegetable systems, particularly those with cover crops. Tillage studies in agronomic crops have often included ST treatments but have not examined soil N dynamics in IR and BR areas separately (see Sainju and Singh, 2008). For example, combined over IR and BR areas, ST soils from 0 to 15 cm with an overwintering rye cover crop had a net gain in N over three years in a cotton/sorghum rotation, whereas soils with only weed cover and no cover crops over the winter lost N over this period (Sainju and Singh, 2008).
Strip till systems are characterized by distinct zones with different expected rates of N mineralization (Luna et al., 2012). Compared with CT, ST is likely to result in reduced initial N availability in the untilled BR zone as a result of both lower temperatures and lack of aeration from tillage. However, with non-legume cover crops, lack of incorporation in the BR zone of ST may reduce initial N immobilization relative to CT (Cheshire et al., 1999). The net effect of these two mechanisms is difficult to predict. The IR zone is tilled in both ST and CT, so smaller differences in N availability might be expected compared with the BR zone. To add to the complexity, N dynamics of ST may be influenced by movement between BR and IR zones of both biotic factors influencing mineralization rates and of soluble N along soil moisture gradients. Overstreet and Hoyt (2008) hypothesized a “radius of influence” in ST systems from IR into BR; they found, for example, that microbial biomass N and carbon were intermediate at the strip edge—higher than BR but lower than IR.
Delayed mineralization of cover crop residues in ST BR areas, combined with movement of soluble N from the BR zone to the IR zone, may result in better synchrony of N supply and crop demand under ST compared with CT when cover crops are used. This effect would be most pronounced where soil moisture content was low in the IR zone relative to the BR zone and where N was largely in the nitrate form. Strip tillage in combination with surface residues may also reduce N losses through leaching and runoff, resulting in greater N availability to the crop (Al-Kaisi and Licht, 2004).
Because of the aforementioned differences in IR growing conditions, yield differences may be expected when crops are produced with ST and cover crops compared with those grown in CT without cover crops. Yields of potato (Solanum tuberosum L.) and sweetpotato [Ipomoea batatas (L.) Lam] (Hoyt and Monks, 1996), pumpkin in one year (Cucurbita pepo L.) (Rapp et al., 2004), and sweet corn (Zea mays L.) (Luna and Staben, 2002) produced with ST were similar to, or greater than, yields produced with CT. Yields of transplanted cabbage after a winter rye (Secale cereale L.), barley (Hordeum vulgare L.), or wheat (Triticum aesthivum L.) cover crop were similar between ST and CT (Hoyt et al., 1996; Wilhoit et al., 1990). Cabbage yield and quality, measured as head width and length, core width and length, and overall head appearance, were similar between tillage treatments that included rototilling and different widths of zone tillage, a form of ST (Mochizuki et al., 2007). With CT, cabbage yield was increased after an oat cover crop (Franczuk et al., 2010) but lower after a sorghum–sudangrass (Sorghum bicolor × S. bicolor var. sudanense) cover crop (Finney et al., 2009).
The primary objectives of this experiment were to evaluate the impacts of ST and oat cover crop residue on soil temperature and moisture, IN content, and cabbage yield. A secondary objective, not reported here, was to evaluate the effects of ST and oat residue on weed suppression before cabbage planting and weed/cabbage competition. We anticipated that, compared with CT, the IR areas in ST plots would have: 1) lower soil temperature and higher soil moisture, particularly where oat cover crop residue was present; 2) improved synchrony of N availability and crop N demand; and hence 3) equivalent or higher cabbage yields.
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