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 soil and a soil under corn (Zea mays L.) to assess management-induced changes 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 forest 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 the plow layer (0-27 cm) was derived from corn. Assuming first order kinetics, 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 included organic C below the plow layer, was 24 yr. Mineralization of the light fraction (LF) was faster than that of organic matter associated with particle-size fractions. More than 70% of the LF had turned over since the start of corn cropping, and 45% of organic matter in the sand 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 associated with the coarse silt fraction was the slowest of all particle-size fractions.