Tillage leads to unnaturally accelerated decomposition and loss of organic material
"Using tillage depletes organic matter. Each time the soil is tilled, oxygen is stirred into it, stimulating microbial action to decompose organic matter at an accelerated rate. As a matter of fact, when a woodland is cleared and planted or a prairie is plowed, most of the organic matter that was built over hundreds of years is lost within 10 years of tillage. Combining frequent tillage with farming practices that leave little plant residue for soil microbes to eat (such as burning or removing crop residues) will lead to the depletion of organic matter."
Source: U.S. Department of Agriculture, Natural Resources Conservation Service, “Healthy Soils Are: High in Organic Matter,” 1, accessed July 30, 2014, http://nrcspad.sc.egov.usda.gov/DistributionCenter/pdf.aspx?productID=1024.