Combining high biomass cover crops with in situ organic mulches may achieve adequate weed control for no-till production, but the persistence and nutrient release rates from cover crops and mulches is unknown. This article describes carbon and nitrogen mineralization rates from three organic mulches (mimosa (Albizia julibrissin Durazz.), lespedeza (Lespedeza cuneata (Dum. Cours.) G. Don)), and oat (Avena sativa L.) straw) and one summer cover crop (soybean (Glycine max (L.) Merr.)). The experiment used litterbag methodology on a Wickham fine sandy loam in Tallassee, AL, USA. Litterbags containing the equivalent of 6.7 Mg ha-1 were placed on the soil surface (to represent conservation tillage) or buried at 10 cm depth (to represent conventional tillage) in October 2007 and retrieved periodically up to a year later. Soybean residue mineralized N at similar rates regardless of placement, but more N was potentially available from surface-placed residues than buried residues, showing that surface placed residues may act as a slow-release N fertilizer. Likewise, C was mineralized more rapidly from buried residues than surface-placed residues. The results quantify the increased amount of C that is sequestered when these residues are used in conservation tillage compared to conventional tillage. This research implies that the use of biomass cover crops and in situ organic mulches could have a measurable impact on carbon retention rates, which will reduce the amount of carbon being released into the atmosphere.