Current and relic carbon using natural abundance carbon-13 1

The role of agricultural practices on soil carbon (C) dynamics is critical to improved soil management. Our main objective was to examine the C interactions resulting from crop changes under different tillage and residue treatments. Tillage choices were no-till (NT), moldboard plow (MB), and chisel plow (CH); residue options were residue harvested (h) and residue returned (r); and N fertilization rates were 0 and 200 kg N ha, applied annually only during the corn (Zea mays L.) (C4 plant) years. Soil samples were taken from a long-term experiment representing 13 years of continuous corn followed by 4 years of soybean [Glycine max (L.) Merr.] (C3 plant). Samples taken at 5-cm depth increments to either 45 cm (yr-13) or 30 cm (yr-17) and at 15-cm depth increments thereafter, down to 90 cm, were analyzed for C and natural abundance C (δC). Specific objectives were to show: (i) differences in soil organic carbon (SOC) and δC caused by crop change; (ii) distributions of SOC and δC with depth at yr-13 and yr-17; and (iii) how much of the SOC, after 4 years of soybean, was current C (soybean-derived) and how much was relic C (after 13 yr of corn and earlier grass crops). There were three depth zones: 0 to 20, 20 to 40, and 40 to 90 cm. SOC decreased with depth, but in the 20- to 40-cm zone the decrease was relatively rapid. Values of δC increased with depth, except at depths greater than 40 cm when δC tended to decrease. Differences in SOC and δC caused by crop change generally occurred in the upper 20 cm. Decreases in SOC and δC under NT and CH as a result of crop change occurred at the very surface, whereas under MB, the decrease occurred down to 25 cm. The contribution of soybean to SOC ranged from −6 to 27%. When residues were returned, SOC in the 0- to 5-cm depth increased under NT (about 26%) and CH (about 19%). Under MB, the measured contribution of soybean at any 5-cm increment within the upper 25 cm was less than 8%; however, at the 25- to 30-cm depth with residue harvested, the contribution was greater than 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. A positive interaction between N application and corn residues returned showed that N could not sustain SOC without residue return. This interaction carried a residual through the soybean period, yet no N was applied to soybean. An overall relation between yr-13 and yr-17 suggested a 24% reduced SOC after soybean.