Soil Organic Carbon Pools After 12 Years in No-Till Dryland Agroecosystems

the slow pool, with decadal turnover times, while 5% of the SOC is found in the rapidly cycling active fraction, Previous studies of no-till management in the Great Plains have with turnover times ranging from hours to months (Folshown that increased cropping intensity increased soil organic carbon (SOC). The objectives of this study were to (i) determine which soil lett, 2001; Shaffer et al., 2001; Burke et al., 1997; Parton C pools (active, slow, and passive) were impacted by cropping intensity et al., 1988). The POM-C fraction has been reported to after 12 yr of no-till across potential evapotranspiration (PET) and estimate the slow turnover pool (Cambardella and Elliott, slope position gradients; (ii) relate C pool sizes to the levels found 1992), while methods that assess microbially respired in total SOC; and (iii) determine C pool sizes relative to C levels found CO2–C estimate the active fraction of SOC (Davidson in a grass treatment (G). Cropping systems were wheat (Triticum et al., 1987; Franzluebbers et al., 1996, 2000). Carbon aestivum)-fallow (WF), wheat-corn (Zea mays L.)-fallow (WCF), mineralized (CMIN) after rewetting air-dry soil during a wheat-corn-millet (Panicum miliaceum)-fallow (WCMF), and contin3-d incubation has been correlated with SMBC, explaining uous cropping (CC) at three PET sites in Colorado. Active C (Soil 86% of the variability (Franzluebbers et al. 1996, 2000). microbial biomass C [SMBC]); and slow pool C (particulate organic Climate and soil texture strongly influence the dymatter C; POM-C) increased as cropping intensity increased, dependent on PET. Passive C (mineral associated organic C [MAOC]) was namics of SOM with C losses due to cultivation increasstrongly influenced by a site-by-slope position interaction but not by ing with precipitation and decreasing with soil clay concropping system. Toeslope soils had 35% higher POM-C compared tent (Burke et al., 1989). Predicted Great Plains regional with summits and sideslopes. All C pools were strongly correlated patterns in SOC show higher levels in the cooler Northwith total SOC, with the variability decreasing as C pool turnover ern Great Plains and lower in the warmer Southeast time increased. Carbon pool sizes in cropping systems relative to Great Plains (Burke et al., 1989, 1995). Analysis of clilevels found in G were independently influenced by cropping system. matic and textural controls on SOC by Parton et al. The highest were found in the CC system, which had 91, 78, and 90% (1987) indicate that temperature is a direct control on of the amounts of C found in the perennial G system in the active, SOC decomposition, while soil texture controls both the slow, and passive C pools, respectively. formation and decomposition rates of the active and slow SOC pools. Hook and Burke (2000) suggest that soil texture has a major impact over the biogeochemical A factor in maintaining long-term production processes, which is a reason why topographic influences in the semiarid Great Plains is protection and/or are observed in SOC pools. improvement in the soil organic matter (SOM) content. Besides the limits imposed by climate, topography, Carbon inputs via crop production are influenced by and soil texture, the particular management system also factors such as climate and topography, as well as soil has a major impact on the quantity and quality of SOC factors such as texture, structure, and fertility (Campbell found within an agroecosystem. In dry regions such as et al., 1991; Lal et al., 1999). These factors set the producthe Great Plains, conversion to no-till management has tion limits of a particular agroecosystem, and thus indiincreased SOM in surface soils with the most dramatic rectly control soil C inputs. effects occurring in cropping systems without summer Soil organic matter is not physically and biochemically fallow (Campbell and Zentner, 1993; Bremer et al., 1995; homogeneous. To better understand the mechanisms by Potter et al., 1998; Bowman et al., 1999; Campbell et which C is lost or stored in terrestrial systems, SOC has al., 2000; Sherrod et al., 2003). Publications that report been conceptually separated into various pools. Terresresponses in SOC pools to management changes across trial ecosystem models have been employed to study cropping system intensity gradients, soils, and climate the impacts of management and/or climate change on are few in number. This paper reports data from a study SOC turnover under different climates, topographies, initiated in 1985 in eastern Colorado to evaluate the and management. One example of a terrestrial model is effects of cropping intensity on production, water-use CENTURY, which partitions SOC (Parton et al., 1988). efficiency, and soil physical and chemical properties Most of the organic C in soil (60–70%) resides in the across PET and topographic gradients. A number of passive pool, with turnover times ranging from centuries publications have reported the effects of cropping intento millennia. Approximately 20 to 40% of SOC is in sity, slope position, and PET gradient on soil C and N properties. Wood et al. (1990) reported a 61% increase L.A. Sherrod and L.R. Ahuja, Great Plains Systems Res. Unit, USDAin CMIN during a 30-d incubation (surface 0 to 5 cm of ARS, P.O. Box E, Fort Collins, CO 80522; G.A. Peterson and D.G. Westfall, Dep. of Soil and Crop Science, Colorado State Univ., Fort Abbreviations: CC, continuous cropping; CMIN, carbon mineralized; Collins, CO 80523. Received 7 Oct. 2003. *Corresponding author CRP, Conservation Reserve Program; G, grass; LSD, least significant ([email protected]). difference; MAOC, mineral-associated organic carbon; MAT, mean annual temperature; OPE, open pan evaporation; PET, potential evapoPublished in Soil Sci. Soc. Am. J. 69:1600–1608 (2005). Soil & Water Management & Conservation transpiration; POM-C; particulate organic matter carbon; SMBC, soil microbial biomass C; SOC, soil organic carbon; SOM, soil organic doi:10.2136/sssaj2003.0266 © Soil Science Society of America matter; WCF, wheat–corn–fallow; WCMF, wheat–corn–millet–fallow; WF, wheat–fallow. 677 S. Segoe Rd., Madison, WI 53711 USA 1600 Published online August 25, 2005