Ssj50095 1168..1177

Winter cover crops have the potential to increase soil organic C in the corn (Zea mays L.)–soybean [Glycine max (L.) Merr.] rotation in the upper Midwest. Management effects on soil C, however, are often difficult to measure because of the spatial variation of soil C across the landscape. The objective of this study was to determine the effect of oat (Avena sativa L.), rye (Secale cereale L.), and a mixture of oat and rye used as winter cover crops following soybean on soil C levels over 3 yr and both phases of a corn–soybean rotation using terrain attributes as covariates to account for the spatial variability in soil C. A field experiment was initiated in 1996 with cover crop treatments, both phases of a corn–soybean rotation, and a controlled-traffic notill system. Oat, rye, and oat–rye mixture cover crop treatments were overseeded into the soybean phase of the rotation in late August each year. Cover crop treatments were not planted into or after the corn phase of the rotation. Soil C concentration was measured on 450 samples taken across both rotation phases in a 7.62-m grid pattern in the late spring of 2000, 2001, and 2002. Slope, relative elevation, and wetness index (WI) were used as covariates in the analysis of variance to remove 77% of the variation of soil C caused by landscape driven patterns of soil C. Soil C concentrations were 0.0023 g C g soil higher in 2001 and 0.0016 g C g soil higher in 2002 than in 2000. The main effects of cover crops were not significant, but the interaction of cover crops and rotation phase was significant. The rye cover crop treatment had 0.0010 g C g soil higher soil C concentration than the no-covercrop control in the soybean phase of the rotation, which included cover crops, but had 0.0016 g C g soil lower C concentrations than the control in the corn phase of the rotation, which did not have cover crops. Using terrain covariates allowed us to remove most of the spatial variability of soil C, but oat and rye cover crops planted every other year after soybean did not increase soil C concentrations averaged over years and rotation phases. ONE APPROACH for offsetting emissions of greenhouse gases from agricultural systems is to employ management practices that increase soil C. Winter cover crops have the potential to increase soil organic C in agricultural soils (Karlen and Cambardella, 1996; Lal et al., 1998; Jarecki and Lal, 2003). In general, soil C storage increases when inputs of plant biomass to the soil are greater than C losses through decomposition, erosion, and leaching (Paustian et al., 1997; Huggins et al., 1998). Winter cover crops have been used successfully to increase soil C in parts of the USA with mild winters (Beale et al., 1955; Patrick et al., 1957; Utomo et al., 1987; Utomo et al., 1990; Kuo et al., 1997; Nyakatawa et al., 2001; Sainju et al., 2002). In some of these studies, the cover crop residues were incorporated with tillage (Beale et al., 1955; Patrick et al., 1957; Kuo et al., 1997; Sainju et al., 2002). Eckert (1991) in Ohio, however, was not able to detect an increase in soil C with a rye cover crop in no-till. Similarly, Utomo et al. (1990) observed no change in soil C with a rye cover crop in either no-till or conventional tillage, but measured an increase with a hairy vetch (Vicia villosa Roth) cover crop in no-till. Mendes et al. (1999) found that red clover (Trifolium pratense L.) or triticale (3 Triticosecale Wittmack) winter cover crops did not increase soil C in a tilled vegetable production system. Although not often used in a no-till corn–soybean rotation, winter cover crops would increase inputs of plant biomass to the soil and would have the potential to increase soil C in this rotation. In the upper Midwest, however, the cover-crop growing season between harvest and planting in a corn–soybean rotation is cold and short. To address this problem, Johnson et al. (1998) successfully established oat and rye cover crops by overseeding into soybean in late August before leaf drop and were able to produce substantial biomass in a no-till corn– soybean rotation in Iowa. Similar, attempts to overseed oat and rye cover crops into corn were not successful (T.C. Kaspar, unpublished data, 1998). Thus, the ability of small grain winter cover crops to increase or maintain soil C levels in corn–soybean rotations in the upper Midwest needs to be evaluated. Progress in understanding soil C dynamics and in developing management practices, like cover crops, to increase or maintain soil C has been limited by the long time-frame required to observe changes in soil C content. Part of the difficulty in measuring changes in soil C is caused by the temporal and spatial variability of soil C levels in agricultural fields (Ellert et al., 2001; Janzen et al., 2002). Soil C varies from year-to-year as a result of weather-affected changes in crop residue inputs or decomposition of residues and organic matter (Campbell et al., 2000; Janzen et al., 2002). Campbell et al. (2005) showed the soil C in a long-term wheat–fallow rotation varied by up to 13% over a 22-yr period because of weather-affected changes. Additionally, differences in soil C across a field are often greater than the expected response of soil C to management practices (Ellert et al., 2001; Janzen et al., 2002). For example, soil C varies with topography (Schimel et al., 1985; MoorT.C. Kaspar, T.B. Parkin, D.B. Jaynes, C.A. Cambardella, and D.W. Meek, USDA-ARS, National Soil Tilth Lab., Ames, IA 50011; Y.S. Jung, Div. of Biological Environment, Kangwon National Univ., Chunchun, Korea Joint contribution from USDA-ARS and Kangwon National Univ. Received 29 Mar. 2005. *Corresponding author