Submitted to: Soil Science
Publication Type: Peer reviewed journal
Publication Acceptance Date: 2/5/2002
Publication Date: N/A
Citation: Interpretive Summary: Agricultural practices play a significant role in the storage and release of carbon and hence have an important role for atmospheric carbon dioxide concentrations. We are attempting to understand some of the aspects of soil carbon dynamics in a long-term corn-soybean experiment with different tillage, residue and nitrogen fertilizer treatments, in south central Minnesota. We analyzed the carbon content and the naturally occurring stable carbon isotope ratio (C-13/C-12) of soil samples taken at the end of 13 years of continuous corn and the end of 4 years of continuous soybean. Results showed that there was a loss of soil organic carbon in the upper 15 cm under all tillage systems when soybean was the crop, especially at 0- to 5-cm depth when residues were returned under no-till. Appreciable contribution of soybean residues to soil organic carbon (current C) of 15% and greater was obtained only at the 0- to 5-cm depth with residues returned under no-till and chisel plowing. Under moldboard plowing with residues removed from the plots, soybean roots appeared to increase carbon at the 25- to 30-cm depth. The rest of the carbon was mainly pre-soybean (relic C). Findings on the soil organic carbon storage of corn versus soybean and the turnover rate of soil organic carbon which could be estimated from the current C provide valuable information to environmental scientists, engineers and consultants concerning present global carbon budgets.
Technical Abstract: Soil samples were taken from a long-term experiment on tillage, residue and N management representing 13 yr of continuous corn (C4) followed by 4 yr of soybean (C3). Tillage treatments were no-till (NT), moldboard plow (MB) and chisel plow (CH); residue treatments were residue harvested (h) and residue returned (r). Samples were taken at 5-cm depth increments to either 45 cm (yr-13) or 30 cm (yr-17) and 15-cm depth increments thereafte down to 90 cm. They were analyzed for C and delta **13C to examine the C dynamics as a result of crop change. Specific objectives were: to show the changes in SOC and delta**13C due to crop change; to show the distribution of SOC and delta **13C with depth at yr-13 and yr-17; and to show how much of the SOC, after 4 yr of soybean, was current C (soybean-derived) or relic C (pre-soybean). Distribution with depth showed decreasing SOC; there is a zone between 20 to 40 cm where the decrease was relatively rapid. Values of delta **13C increased with depth, however, at depths greater than 40 cm there was a tendency of delta 13C to decrease. The contribution of soybean to SOC ranged from -8 to 27 %. Appreciable contribution occurred at the 0- to 5-cm depth when residues were returned under NT (about 26 %) and CH (about 19 %). Under MB, the measured contribution of soybean in the upper 25 cm was < 8 %; however, at the 25- to 30-cm depth with residue harvested, the contribution was > 14 %. Except for the very surface of NTr and CHr and at the 25- to 30-cm depth under MBh, the SOC within the profile was predominantly relic C.