Utility of 13C for ecosystem carbon turnover estimation in grazed mixed grass prairie

Differences in 13C inputs between two adjacent Canadian mixed grass prairie communities were used to estimate turnover of two soil organic matter fractions with increasing depth below the surface. Partial cultivation and abandonment of a C4-dominated grassland ca. 1930, and natural recovery to a native C3-dominated grassland changed the 13C/12C ratio (reported in units of δ13C‰) of plant residue inputs to soil organic matter. After 70 years, macro-organic matter carbon (MOMC) differed up to 5.6‰, and fine soil organic carbon (FSOC) by 2.9‰ near the surface; differences declined to a depth of 1 m. Mean residence time estimates differed among fractions from a short 35 years for MOMC 0–5 cm, to a longer 413 years for FSOC 5–10 cm. The background carbon pool on this site was substantial, and the minimum change in soil carbon of 10.29 Mg C ha−1 over 70 years represented less than 7% of the carbon pool to a 1-m depth. Considerable vertical and horizontal variation in soil organic carbon mass and 13C content were observed. Variation due to small sample size was a more substantial limitation than cumulative analytical variation for calculating turnover estimates >500 years or at depths >20 cm. Corrections for bias were also necessary since imperfect physical separation of soil organic matter fractions, or CO32− contamination, were issues.