Saturday, 15 July 2006

Cropping Intensity and Tillage effects on Carbon Sequestration and Enzyme Activities.

Maysoon M. Mikha, Verónica Acosta-Martínez, and Merle F. Vigil. ARS-USDA, 40335 County Road GG, Akron, CO 80720

The predominant cropping system in the Central Great Plains, winter Wheat-Fallow (W-F) rotation, is not sustainable. Decreases in Soil Organic Matter (SOM) content are associated with tillage and W-F. Intensive cropping with reduced tillage and fallow are practices that provide more residues, which may increase SOM and improve soil quality parameters. Our study evaluated selected soil properties related to soil quality in research plots established in 1990 on a Weld loam (fine, smectitic, mesic aridic Paleustolls). Several rotations with various cropping intensities were evaluated. We investigated the effects of cropping intensity on soil C, soil aggregate-size distribution, particulate organic matter, soil microbial communities, and enzymatic activities of C and N cycling. Soil samples were fractionated into two groups of aggregate size: macroaggregates (>250 mm) and microaggregates (<250 mm) by wet sieving. Continuous cropping increase soil total C compared with W-F rotation. Relative to W-F conventional tillage, continuous cropping increased soil total C by an average of 0.27 Mg C ha-1 yr-1 to the depth of 15 cm. Aggregate-size distribution was affected by tillage but not by cropping intensity. Microbial Biomass C (MBC) was higher with continuous cropping compared with W-F. Enzyme activity of b-glucosaminidase, b-glucosidase, and a-galactosidase were affected by rotation phase and fallow frequency. Principal Component Analyses (PCA) of Fatty Acids Methyl Ester (FAME) profiles demonstrated shifts in the microbial community structure between the alternative systems compared with W-F rotation. Higher fungal populations were found with reductions in fallow and significant correlations were found between fungal FAME indicator (18:2ω6c) and b-glucosaminidase activity. For this soil, fourteen years of no-tillage and continuous cropping with reduced fallow frequency have a positive effect on nutrient cycling, soil C sequestration, and soil biochemical functioning.

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