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Abstract Carbon flow in terrestrial ecosystems regulates partitioning between soil organic C (SOC) and atmospheric CO 2 . Our objectives were to assess SOC dynamics using natural 13 C abundance in corn ( Zea mays L., a C 4 species)‐soybean Glycine max (L.) Merr., a C 3 species sequences. Fifteen treatments of continuous corn, continuous soybean, various sequences of corn and soybean, and fallow were initiated in 1981 at Lamberton, MN, on a Webster clay loam (fine‐loamy, mixed, mesic Typic Haplaquoll). In 1991, soil and aboveground shoot samples from all treatments were analyzed for total organic C and δ 13 C. Carbon inputs, δ 13 C, and SOC were integrated into a two‐pool model to evaluate C dynamics of corn and soybean. Total SOC was similar across all treatments after 10 yr; however, differences in soil δ 13 C occurred between continuous corn (δ 13 C = −17.2‰) and continuous soybean (δ 13 C = −18.2‰). Modeled C dynamics showed SOC decay rates of 0.011 yr −1 for C 4 ‐derived C and 0.007 yr −1 for C 3 ‐derived C, and humification rates of 0.16 yr −1 for corn and 0.11 yr −1 for soybean. Decay and humification rates were slightly lower than those found in other Corn Belt studies. Levels of SOC were predicted to decline an additional 7 to 18% with current C inputs from either corn or soybean, respectively. Annual C additions required for SOC maintenance averaged 5.6 Mg C ha −1 , 1.4 to 2.1 times greater than previously reported estimates. Controlled variation in natural 13 C abundance in corn‐soybean rotations during a 10‐yr period adequately traced C dynamics.
Huggins et al. (Thu,) studied this question.