Submitted to: Soil Science Society of America Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/1/1997
Publication Date: N/A
Citation: Interpretive Summary: Longevity of carbon sources in soil and the accretion of carbon in organic matter have a strong impact on the global carbon balance; and also a major impact on productivity and long term soil quality. Methods to measure and study soil organic carbon cycling are indeed difficult. Abundance of natural carbon isotopes is a new method to trace the decomposition of soybean and corn separately. Decomposition of corn and soybean residues could be distinguished after 10 years although the total amount of soil carbon remained unchanged. A new equilibrium of soil carbon had not yet been reached although there have been soybean-corn sequences since about 1920 on this land in southwestern Minnesota. This information will be used by ecologists and others to evaluate carbon and organic matter reactions in intensively managed corn-soybean production.
Technical Abstract: Carbon flow regulates partitioning between soil organic C (SOC) and CO2. We assessed SOC dynamics using natural 13C abundance in corn (C4 species)- soybean (C3 species) sequences. 15 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 samples from 0 to 15-cm and 15 to 30-cm depths, and shoot samples from all treatments were analyzed for total organic C and delta 13C in soil. Measured delta 13C, C residue additions, and SOC integrated into a two-pool model evaluated differences in C dynamics between corn and soybean. Total SOC was similar across all treatments after 10 yr, however, significant differences in soil delta 13C occurred between continuous corn and continuous soybean (delta 13C=-17.2 vs -18.2 ppt). Soil delta 13C were less negative with depth. SOC decay rates 0.011/y for C4-derived C and 0.007/y for C3-derived C; both were 5 percent lower than other studies in the Corn Belt. Humification rates of 0.16/y for corn and 0.11/y for soybean were similar to other studies. Equilibrium levels of SOC were less than current SOC values and indicated that SOC would decline an additional 7 to 18 percent with current C inputs from either corn or soybean, respectively. Annual C additions for SOC mainte- nance averaged 5.6 Mg C/ha. Controlled variation in natural 13C abundance in corn-soybean rotations over a 10-yr period traced C dynamics precisely.