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SOIL & CROP MANAGEMENT

No-Till Corn after Bromegrass: Effect on Soil Carbon and Soil Aggregates

^a USDA/ARS, Soil-Plant-Nutrient Res. Unit., NRRC, Bldg D, Ste. 100, 2150 Centre Ave., Fort Collins, CO 80526
^b USDA/ARS, Agroecosystem Management Research Unit, 120 Keim Hall
^c USDA/ARS, Grain, Forage and Bioenergy Research Unit, 314 Biochemistry Hall, Univ. of Nebraska, Lincoln, NE 68583
^d (NRCS retired), 151 East Hill Church Road, Addison, NY 14801

^* Corresponding author (ronald.follett{at}ars.usda.gov).

Grasslands in the Conservation Reserve Program (CRP) in the USA may be converted to grain crops for bioenergy. The effect of no-till conversion of a smooth bromegrass (Bromus inermis Leyss) grassland to no-till corn (Zea mays L.) production on soil organic carbon (SOC) in the western Corn Belt was monitored for over 6 yr. A different ¹³C/¹²C isotope signature is imparted to SOC by C4 plants including corn versus C3 plants such as bromegrass. Changes in C isotope ratios in SOC in three soil depths (0- to 5-, 5–10, and 10–30 cm) by particle size was also monitored during 6.5 yr of no-till corn production at two different N levels (60 and 120 kg ha^–1). Soil was collected eight times during the study from the 0- to 5- and 5- to 10-cm depths, and at four of these times from the 10- to 30-cm depth from each of the N rate replicates. Because fertilizer N had no significant effect over years on any of the aboveground biomass production variables, the data from both N treatments was combined for regression analysis to determine the effects of years of no-till corn production on SOC variables. Total SOC did not change significantly at any depth during the study, but there was a significant change in the source of the SOC. Total C4-C increased over this time, while C3-C decreased in the 0- to 5- and 5- to 10-cm depth, while neither changed in the 10- to 30-cm depth. In the 0- to 5- and 5- to 10-cm depths, largest loss of C3-C was from 2-mm aggregates, while largest increases in C4-C were in the 1-, 0.5-, 0.25-, and 0.125-mm aggregates. If CRP grasslands are converted to grain crop production, the data from this study strongly support the use of no-till farming practices as a method of conserving the SOC that was sequestered during the time period that the land was in the CRP.

Abbreviations: C3-C, carbon from cool season (C3) plants • C4-C, carbon from warm season (C4) plants • CRP, Conservation Reserve Program • MSC, minimal amounts of annual source C inputs • PDSI, Palmer Drought Stress Index • SOC, soil organic carbon

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¹ Trade and company names are included for the benefit of the reader and do not imply endorsement or preferential treatment of the product by the authors or the USDA.

Received for publication April 4, 2008.

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