TURNOVER OF SOIL ORGANIC-MATTER AND STORAGE OF CORN RESIDUE CARBON ESTIMATED FROM NATURAL C-13 ABUNDANCE

Total organic C and natural C-13 abundance were measured in a forest s oil and a soil under corn (Zea mays L.) to assess management-induced c hanges in the quantity and initial source of organic matter. The total mass of organic C in the cultivated soil was 19 % lower than in the f orest soil. It was estimated that after 25 yr of continuous corn, 100 Mg C ha(-1) was returned to the soil as residues, of which only 23 Mg ha(-1) remained in the soil; 88% of the remaining corn-derived C (C-4- derived C) was in the plow layer. About 30% of the soil organic C in t he plow layer (0-27 cm) was derived from corn. Assuming first order ki netics, the half-life of C-3-derived C in the 0- to 15-cm layer was 13 yr. The half-life of C-3-derived C in the 0- to 30-cm layer, which in cluded organic C below the plow layer, was 24 yr. Mineralization of th e light fraction (LF) was faster than that of organic matter associate d with particle-size fractions. More than 70% of the LF had turned ove r since the start of corn cropping, and 45% of organic matter in the s and fraction comprised corn residue. The half-life of C-3-derived C in the LF was 8 yr. The mineralization of C from native organic matter a ssociated with the coarse silt fraction was the slowest of all particl e-size fractions.