Isotopic CO₂ measurements of soil respiration over conventional and no-till plots in fall and spring

Soils and vegetation are the second largest global carbon reservoir. Carbon storage in soil can be increased by increasing carbon inputs and/or reducing decomposition rates. Reduced tillage practices have the potential to increase soil carbon storage by reducing decomposition of soil organic matter and/or crop residue. Isotope values ([formula removed]C) of soil respiration can help infer the contribution of soil carbon pools to the soil CO₂flux, providing insight into the effects of tillage on residue decomposition. The objectives of this paper were to: (1) measure the isotope signatures of respired CO₂ from conventional tillage and no-till plots; (2) compare and contrast the two treatments in the fall and in the following spring before planting; and (3) compare the [formula removed] C of soil respiration of the experimental plots with the surrounding region. Isotope soil CO₂ fluxes were measured using tunable diode laser spectroscopy and micrometeorological methods in a field experiment in Ontario, Canada. Measurements were made in fall following corn (a C4 plant) harvest and again in spring prior to soybean emergence. Data were used to compute the [formula removed] C value of respired CO₂ using both the flux ratio and Keeling plot methods. The large concentration footprint prevented the Keeling plots from discerning the treatment effect. In the conventional till plots, the respired [formula removed] C value showed a stronger C4 signature ( - 16.7 ± 2.5 [per thousand] ) compared to the no-till field ( - 20.2 ± 2.7 [per thousand] ) which had no soil incorporation of above-ground crop residue. This indicates more rapid decomposition of the 'new' residue in the conventional tillage treatment. Both treatments showed a decrease in the isotope ratio during the spring measurements ( - 20.6 ± 3.7 [per thousand] and - 24.2 ± 3.8 [per thousand] for the conventional and no-till plots, respectively) which shows a depletion of the labile C4 substrate and a shift in respired substrate towards the soil C3 organic matter over the fall to spring period. For the fall, we estimate that 57 and 25% of the CO₂ flux originated from crop residue for the conventional and no-till systems, while in the spring the proportions had decreased to 22 and 0%, respectively.