Radiative forcing of Earth's atmosphere is increasing at unprecedented rates, largely because of increases in the greenhouse gases CO2, CH4, and N20 (1). Agriculture plays a major role in the global fluxes of each of these gases and has been promoted as a partial means for slowing further increases in radiative forcing through the potential for soil C sequestration in cropping systems under reduced tillage (2, 3) and organic (4) management regimes. Crop management also affects N20 and CH4 fluxes (5), and changes to these fluxes can also affect the impact of agriculture on radiative forcing (6), as can the use of CO2-producing crop subsidies such as fertilizer (7), lime, and fuel. A complete understanding of agriculture's impact on radiative forcing requires field-level analysis of all contributors to the net global warming potential (GWP) of these systems.During 1991-99, we measured N20 production, CH4 oxidation, and soil C sequestration in a replicated series of cropped and unmanaged ecosystems in the Midwest United States (8). We studied four com-wheatsoybean rotations managed (i) with conventional chemical inputs and tillage, (ii) with conventional inputs and no tillage, (iii) with reduced chemical inputs, and (iv) organically with no chemical inputs (9). The latter two treatments included a winter legume cover We also examined two perennial cropsalfalfa (Medicago sativa) and poplar (Populus sp.) trees-and four types of unmanaged vegetation, including a set of early successional ecosystems abandoned from agriculture in 1989, two types of midsuccessional systems, and a set of late successional forested sites. One of the two midsuccessional types was abandoned from agriculture around 1950, and another was established in 1959 by converting a small never-tilled woodlot to an annually mowed grassland (10). Productivity and soil properties of all sites are consistent with their management histories (Table 1).With the exception of our conventionally managed system, all of our cropping systems accumulated soil C over the decade since their establishment. The no-till system accumulated 30 g C m-2 year-1, which is an average value for no-till rotations in the Midwest United States (11), and the organicbased systems accumulated 8 to 11 g C m-2 year-1 (Table 1). Reduced tillage is the likely cause of C sequestration in the no-till system (12); in the low-input systems, both of which were plowed and cultivated as part of their normal management regime, sequestered C is likely the result of a winter cover crop that in two of three rotation years added an additional 1 to 2 megagrams ha-1 of unharvested plant biomass to soil.The poplar and alfalfa systems added 32 to 44 g C m-2 year-to the soil C pool, similar to that added by the no-till system, and 60 g C m-2 year-1 was sequestered in the early successional community abandoned from agricul-REPORTS ture in 1989 (Table 1). We detected minor soil C changes in the midsuccessional old field (about 0.9 g C m-2 year-) and no changes in the never-tilled successional or late succes...

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to year and from field to field (Lamb et al., 1997;Kravchenko and Bullock, 2000; Lark, 2001; Machado et al., The quantitative characterization of spatiotemporal variability in 2002;Kaspar et al., 2003). These variations are often crop grain yields is an important component for successful precisionassociated with the prevailing weather conditions during agriculture applications. The objective of this study was to analyze and quantify effects of management practices, topographical features, the growing season of each particular year, such as and weather conditions on spatial variability of crop yields. A onespring and summer precipitation (Kravchenko and Bulfactor randomized complete block design experiment with six replicalock, 2000; Jaynes and Colvin, 1997) or total growing tions was established at the Long Term Ecological Research site in degree days (Lamb et al., 1997), indicating that in wellsouthwest Michigan in 1988. The treatments used in this study were managed fields, moisture availability is often the main two treatments with conventional chemical inputs (chisel plow and yield-affecting factor. no-till) and two organic-based chisel-plowed treatments with a winter Not only the overall yields but also yield variability leguminous cover crop (low chemical input and zero chemical input). has been reported to vary from year to year depending The data consisted of corn (Zea mays L.)-soybean [Glycine max (L.) on weather conditions. Whelan and McBratney (2000) Merr.]-wheat (Triticum aestivum L.) yields collected via combine observed coefficients of variation of yield ranging from monitors from 1996 to 2001. We observed that stressful conditions, regardless of the stress origin, were associated with increase in the 13 to 83% for wheat and from 12 to 44% for sorghum overall yield variability (coefficient of variation) as well as the small-[Sorghum bicolor (L.) Moench] in two consecutive scale yield variability (variogram values at short lag distances and years. Porter et al. (1998) reported coefficients of variavariogram slopes near the origin) with yields probably being more tion from field plots for corn and soybean yields ranging sensitive to the small-scale variations in growth conditions due to soil from 2 to 19% over 10 studied years. Noticeable differand microtopographical differences. Coefficients of variation were as ences in spatial variability patterns were observed by high as 45% in years with low precipitation and as low as 14% in Jaynes and Colvin (1997) in a study of 6 yr of cornyears with above-average precipitation. During the years with low

sufficiently to survive the winter. Cover crop residue can modify the conditions under which weeds germinate Cover crops often reduce density and biomass of annual weeds in or regrow in the spring. Such effects could be due to no-till cropping systems. However, cover crops that over-winter also changes in soil temperature, increase in soil moisture, have the potential to reduce crop yield. Currently, there is an interest in annual medics (Medicago spp.) and other annual legumes that release of allelopathic chemicals, and physical impediwinter-kill for use as cover crops in midwestern grain cropping systems. ments to weed seedlings (Facelli and Pickett, 1991; Teas-A 2-yr study was conducted at East Lansing and the Kellogg Biological dale, 1996; Teasdale and Mohler, 1993). Station, Michigan, to investigate the influence of annual legume cover Many legume species that are used as cover crops in crops on weed populations. Two annual medic species [burr medic (M. no-till corn production are winter annuals or short-lived polymorpha cv. Santiago) and barrel medic (M. truncatula Gaertn. cv. perennials. In northern regions of the USA, over-win-Mogul)], berseem clover (Trifolium alexandrinum L. cv. Bigbee), tering species are normally established in the summer and medium red clover (Trifolium pratense L.) were no-till seeded or fall and accumulate most of their biomass when they as cover crops into winter wheat (Triticum aestivum L.) stubble in

A 5‐yr cropping system experiment was initiated in 1981 to study transition from a conventional agricultural system using pesticides and fertilizers to a low‐input system. The site was primarily Comly silt loam (fine‐loamy, mixed, mesic, Typic Fragiudalf) with 12% Berks shaly silt loam (loamy‐skeletal, mixed, mesic, Typic Dystrochrept), and a small area of Duffield silt loam (fine‐loamy, mixed, mesic, Ultic Hapludalf), in Berks County, eastern Pennsylvania. Three 5‐yr rotations were compared. A conventional corn (Zea mays L.)‐soybean [Glycine max (L.) Merr.] rotation (designated “conventional”) was compared to two low‐input rotations which utilized oat (Avena sativa L.), red clover (Trifolium pratense L.) and winter wheat (Triticum aestivum L.), in addition to corn and soybean. One low‐input rotation used cattle manure as a nutrient source and produced forage crops in addition to cash crops (designated “low‐input/livestock”), while the other used legume crops as a nutrient source, and produced a cash crop every year (designated “low‐input/cash grain”). Corn grain yields in the low‐input systems were 75% of conventional in 1981 to 1984, but yields were not significantly different in 1985. Weed competition and insufficient N limited low‐input corn yields during the first 4 yr. Soybean yields in the low‐input systems were equal to or greater than conventional all 5 yr. It is concluded that a favorable transition from input‐intensive cropping to low‐input systems is feasible, but only if crop rotations are used which include crops that demand less N and are competitive with weeds, such as small grain, soybean, or legume hay. Corn should be avoided for the first 3 to 4 yr.