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Soil & Water Management & Conservation

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

^a Great Plains Systems Res. Unit, USDA-ARS, P.O. Box E, Fort Collins, CO 80522
^b Dep. of Soil and Crop Science, Colorado State Univ., Fort Collins, CO 80523

^* Corresponding author (Lucretia.Sherrod{at}colostate.edu)

Previous studies of no-till management in the Great Plains have shown that increased cropping intensity increased soil organic carbon (SOC). The objectives of this study were to (i) determine which soil C pools (active, slow, and passive) were impacted by cropping intensity after 12 yr of no-till across potential evapotranspiration (PET) and slope position gradients; (ii) relate C pool sizes to the levels found in total SOC; and (iii) determine C pool sizes relative to C levels found in a grass treatment (G). Cropping systems were wheat (Triticum aestivum)-fallow (WF), wheat-corn (Zea mays L.)-fallow (WCF), wheat-corn-millet (Panicum miliaceum)-fallow (WCMF), and continuous cropping (CC) at three PET sites in Colorado. Active C (Soil microbial biomass C [SMBC]); and slow pool C (particulate organic matter C; POM-C) increased as cropping intensity increased, dependent on PET. Passive C (mineral associated organic C [MAOC]) was strongly influenced by a site-by-slope position interaction but not by cropping system. Toeslope soils had 35% higher POM-C compared with summits and sideslopes. All C pools were strongly correlated with total SOC, with the variability decreasing as C pool turnover time increased. Carbon pool sizes in cropping systems relative to levels found in G were independently influenced by cropping system. The highest were found in the CC system, which had 91, 78, and 90% of the amounts of C found in the perennial G system in the active, slow, and passive C pools, respectively.

Abbreviations: CC, continuous cropping • C_MIN, carbon mineralized • CRP, Conservation Reserve Program • G, grass • LSD, least significant difference • MAOC, mineral-associated organic carbon • MAT, mean annual temperature • OPE, open pan evaporation • PET, potential evapotranspiration • POM-C, particulate organic matter carbon • SMBC, soil microbial biomass C • SOC, soil organic carbon • SOM, soil organic matter • WCF, wheat–corn–fallow • WCMF, wheat–corn–millet–fallow • WF, wheat–fallow

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