Crop species affect soil organic carbon turnover in soil profile and among aggregate sizes in a Mollisol as estimated from natural 13C abundance

Soil organic carbon (SOC) stock varies in soil profiles under different cropping systems, which is possibly due to the difference in SOC turnover rate. This study used a 22-year field experiment to determine the effect of crop species on SOC turnover rate in soil aggregates and profiles of a Mollisol. Soils were sampled from 2-m soil profiles in 1991 and 2012, from the plough layer regularly under continuous monocropping of maize, soybean and wheat. Natural C-13 abundance was determined to calculate the fraction of newly derived C in SOC in bulk soil and water-stable aggregates of different sizes. The SOC content in the plough layer tended to decrease under wheat and maize but did not change under soybean from 1991 to 2012. The soil delta C-13 abundance in bulk soil and all aggregate sizes increased in 0-1.8 m depth under maize but decreased in 0-0.8 m under soybean and wheat, with the variations being larger in the larger aggregates. The SOC turnover at 0-0.8 m depth was fastest under wheat, followed by soybean and slowest under maize. The SOC turnover was faster in the larger aggregates for all crops, with the variations among the crop species being larger in > 2 mm aggregates. Crop species affect the delta C-13 abundance and SOC turnover rate consistently in different aggregate sizes in the plough layer and in bulk soil within the rooting zone.