Temperature effects on δ13C of soil-respired CO2 from pastoral soil

<p>With the bulk of ecosystem carbon stored below-ground, terrestrial ecosystems may become a net source of carbon if rising soil temperatures cause previously recalcitrant soil carbon to be metabolized by soil microbes and released to the atmosphere as CO₂. Soil respiration rate increases dramatically with temperature, so long as soil water content is high and carbon substrates are available. However, it is difficult to ascertain if the CO₂ released comes from higher rates of root or microbial respiration of liable or recalcitrant soil carbon. Isotopes of carbon, both radioactive and stable, may provide a tool to understand the source of belowground respiration. We conducted experiments to determine the utility of stable carbon isotopes at natural abundance to partition below-ground respiration into autotrophic and heterotrophic components with increasing temperature in intact soil columns extracted from a pasture ecosystem. Autotrophic respiration contributed between 47 and 76% of total below ground respiration, with error estimates of up to ± 12%. No significant differences in partitioning were observed with increasing temperature, suggesting that (for this ecosystem) root and soil microbial respiration were equally sensitive to temperature. However, we did find that CO₂ respired by soil both with and without live roots was more depleted in ¹³C as soil temperature increased, while δ¹³C of CO₂ respired by roots alone was insensitive to temperature. This suggests that either substrates for microbial respiration change with temperature (i.e. more ¹³C enriched substrates are used at lower temperatures) or that soil microbial respiratory isotopic fractionation effects are temperature dependent. Under constant 20°C soil temperature day and night, respired CO₂ was 0.4‰ more depleted in ¹³C in the dark when no live roots were present and 1‰ more depleted in the dark with live roots. If temperature in the dark was lowered to 10°C after a 20°C light period no significant differences were found between light and dark, suggesting that the increase in ¹³C with lower temperature effectively cancelled out the decrease in ¹³C in the dark. These results suggest that field studies employing stable carbon isotopes to partition below ground respiration must consider temperature and diurnal effects on δ¹³C of CO₂.