Fertilization effects on soil organic matter turnover and corn residue C storage

Soil organic matter turnover is influenced by N; thus long-term fertilization of corn (Zea mays L,) may significantly affect soil organic matter levels. Effects of fertilization on soil organic matter turnover and storage of residue C under continuous corn were evaluated using soils from a long-term field experiment in Ontario, Total organic C and natural C-13 abundance measurements indicated that fertilized soils had more organic C than unfertilized soils, the difference accounted for by more C-4-derived C in the fertilized soils. About 22 to 30% of the soil C in the plow layer had turned over and was derived from corn in the fertilized soils; in unfertilized soils only 15 to 20% was derived from corn. Assuming that organic matter turnover follows first-order kinetics, the half-life of C-4-derived C in the surface 10 cm of both soils was the same, about 19 yr. Natural C-13 abundance measurements and estimates from a soil organic matter model indicate that 10 to 20% of the added residue C was retained in the soil. Fertilized soils had more light fraction (LF) C than unfertilized soils. More than 70% of the C in the LF of fertilized soils was derived from corn; in unfertilized soils only 41% was derived from corn. The half-life of C-3-derived C in the LF was shorter than 10 yr, These results indicate that adequate fertilization increases crop yields, in turn leading to greater C storage, and that fertilization does not significantly alter the rate of turnover of native soil organic matter.