Separation of heterotrophic and autotrophic contributions to soil CO2 efflux under simulated dormancy conditions

Background/Question/Methods

Soil CO₂ efflux is the largest efflux in terrestrial ecosystems and therefore an important metric of ecosystem change. Carbon cycling research has increased over the past 20 years, but most research focuses on growing season data, even in areas where growing season is less than half the year; thus, leaving gaps of knowledge in dormant season biogeochemical cycling. For example, little is known about the primary contributors to soil respiration (i.e., heterotrophic and autotrophic) under dormant conditions. We hypothesized that heterotrophic respiration would have higher soil CO₂ efflux than autotrophic respiration under simulated dormant conditions. To quantify the contributions of autotrophic and heterotrophic respiration to total soil CO₂ efflux under simulated winter conditions, we designed an experiment with the following treatments; combination autotrophic-heterotrophic respiration (AHR), heterotrophic respiration (HR), autotrophic respiration (AR), and no organism respiration (NR, autoclaved soil). Seedlings and soil were placed in specially designed chambers that allowed for measurement of soil CO₂ efflux and were stored in a cold room at 4¢ª C under fluorescent grow-lights (8/16 light cycle). Soil CO₂ efflux measurements were taken after seedlings and soils were placed into the chambers. Soil nutrients and microbial densities were measured for autoclaved and non-autoclaved soils.

Results/Conclusions

Preliminary data suggests that heterotrophic respiration (1.88 µmol CO₂ m^-2 s^-1) treatments were higher than both the autotrophic respiration (0.6313 µmol CO₂ m^-2 s^-1) and no organism respiration (0.25 µmol CO₂ m^-2 s^-1) treatments on day one of dormancy measurements. Treatments were not significantly different on day four, but the same trend was observed. Autoclaving soils did not alter soil pH or soil carbon, but nutrient concentrations were significantly increased. For example, soil nitrogen and phosphorus levels increased 2.75 and 1.45 times, respectively. Autoclaving soils did not remove all soil microbes, but did significantly decrease fungal and bacterial densities, 80% and 90%, respectively. Lower microbial densities and accompanying soil CO₂ effluxes support the hypothesis that the majority of winter soil CO₂ efflux is heterotrophic. Further, soil CO₂ efflux measurements in the field appear to be comparable to heterotrophic respiration treatments in the lab. Both suggest that, while less than summer effluxes, winter soil effluxes may account for 5-20% of the annual efflux. Our results suggest a potential method to separate the contributors of soil respiration during dormancy, but more data needs to be collected to support our interpretations and to facilitate translation of lab measurements to field conditions.