Cropping System and Nitrogen Effects on Mollisol Organic Carbon

Time, fertilizer, tillage, and cropping systems may alter soil organic carbon (SOC) levels. Our objective was to determine the effect of long-term cropping systems and fertility treatments on SOC. Five rotations and two N fertility levels at three Iowa sites (Kanawha, Nashua, and Sutherland) maintained for 12 to 36 yr were evaluated. A 75-yr continuous corn (Zea mays L.) site (Ames) with a 40-yr N-P-K rate study also was evaluated. Soils were Typic and Aquic Hapludolls and Typic Haplaquolls. Four-year rotations consisting of corn, oat (Avena saliva L.), and meadow (alfalfa [Medicago sativa L.], or alfalfa and red clover [Trifolium pratense L.]) had the highest SOC (Kanawha, 32.1 g/kg; Nashua, 21.9 g/kg; Sutherland, 27.9 g/kg). Corn silage treatments (Nashua, < 18.9 g/kg; Sutherland, <23.2 g/kg) and no-fertilizer treatments (Kanawha, 25.3 g/kg; Nashua, <20.9 g/kg; Sutherland, <23.5 g/kg) had the lowest SOC. A corn-oat-meadowmeadow rotation maintained initial SOC (27.9 g/kg) after 34 yr at Sutherland. Continuous corn resulted in loss of 30% of SOC during 35 yr of manure and lime treatments. SOC increased 22% when N-P-K treatments were imposed. Fertilizer N, initial SOC levels, and previous management affected current SOC levels. Residue additions were linearly related to SOC (Ames, r = 0.40; Nashua, r = 0.82; Sutherland, r = 0.89). All systems had 22 to 49% less SOC than adjacent fence rows. Changing cropping systems to those that conserve SOC could sequester as much as 30% of C released since cropping began, thereby increasing SOC. T ATMOSPHERIC CC>2 CONCENTRATION has gained much attention for its potential contribution to global warming. Agriculture affects atmospheric COz concentrations through consumption of fossil fuels, clearing of forested lands for food production (U.S. Congress, 1991; Wallace et al., 1990), and alteration of SOC levels by agricultural management practices. Agricultural fuel consumption and N fertilizer production release 35.4 Tg C yr~' into the atmosphere. Based on current production practices, the Council for Agricultural Science and Technology (1992) estimates another 2.7 Tg C yr~' are released into the atmosphere from cultivated soils in the USA alone. Changes in SOC can be attributed to crop species grown, cropping systems (including rotations), residue management practices, fertilizer applications, tillage practices, and other management factors (Havlin et al., 1990; Unger, 1968). Anderson et al. (1990), Bauer and Black (1981), and Havlin et al. (1990) independently showed that SOC losses were directly related to tillage intensity. Manure applications modified the tillage-SOC relation, increasing SOC even with high-intensity conventional tillage (Anderson et al., 1990). Crop rotations may retard SOC losses relative to those observed in C.A. Robinson, Div. of Agriculture, West Texas A&M Univ., WTAMU Box 998, Canyon, TX 79016; and R.M. Cruse and M. Ghaffarzadeh, Dep. of Agronomy, Agronomy Hall, Iowa State Univ., Ames, IA 50011. Iowa State Univ. Journal Paper no. J-15950 of the Iowa Agriculture and Home Economics Exp. Stn. Project no. 2556. Received 29 Aug. 1994. *Corresponding author ([email protected]). Published in Soil Sci. Soc. Am. J. 60.264-269 (1996). continuous corn systems (Odell et al., 1984). Rasmussen and Rohde (1988) reported linear increases in SOC with applied N fertilizer, and noted that crop residue has a positive impact on SOC. Rasmussen and Parton (1994) reported that the rate of SOC change was directly related to C input from crop residues and amendments. Mann (1986) reported that SOC losses from a number of cultivated soils averaged <20% of initial levels, and that quantity and direction of change were related to initial SOC. Mann (1986) noted the greatest change in the first 20 yr. Lucas et al. (1977) suggested that more than 60 yr were required to approach new steady-state conditions on Michigan soils. Changes in SOC quantity following implementation of a new management practice, and the time required to achieve new SOC equilibria, will be ecosystem dependent because climate, parent materials, topography, biotic factors, and time affect soil equilibria (Jenny, 1941). The recent conversion of cropland to perennial grass cover through the Conservation Reserve Program increased SOC in the Great Plains and potentially sequestered C (Gebhart et al., 1994). Kern and Johnson (1993) suggested that increasing conservation tillage to 76% of planted cropland would change agricultural systems from C sources to C sinks. A concomitant conversion to cropping systems that conserve, or increase, SOC could also help move agriculture from C source to C sink. The impact of long-term soil and crop management on SOC, and subsequently on sustainability of the soil resource, is not clear. The objective of this study was to determine the effects of several Iowa long-term cropping systems and fertilizer applications on SOC of Typic Haplaquolls and Typic Hapludolls. METHODS AND MATERIALS Cropping Systems Studies Several long-term rotation-fertility studies exist in Iowa. Three sites were chosen for study: North Central Research Center at Kanawha, Northwest Research Center at Sutherland, and Northeast Research Center at Nashua. The study at Kanawha was established in 1954 on a Webster clay loam (fineloamy, mixed, mesic Typic Haplaquoll), with a mean particlesize distribution of 21.9% sand, 44.9% silt, and 33.2% clay. The study at Sutherland was established in 1956 on a Galva silty clay loam (fine-silty, mixed, mesic Typic Hapludoll), with a mean particle-size distribution of 2.6% sand, 62.2% silt, and 35.2% clay. The study at Nashua was established in 1979 on Kenyon (fine-loamy, mixed, mesic Typic Hapludoll) and Readlyn (fine-loamy, mixed, mesic Aquic Hapludoll) loams, with a mean particle-size distribution of 31.9% sand, 45.6% silt, and 22.4% clay. Mean annual precipitation is 813 mm at Kanawha, 711 mm at Sutherland, and 825 mm at Nashua. The experimental design at Kanawha and Sutherland was a split-split plot design, replicated twice with crop year (first, second, third, or fourth year of rotation) as the main plot, crop rotation (corn, corn-soybean [Glycine max (L.) Merr.], corn-corn-oat-meadow, etc.) as the subplot, and N fertilizer