PNNL

Abstract

Partitioning the soil surface CO2 flux (RS) flux is an important step in understanding ecosystem-level carbon cycling, given that RS is poorly constrained and its source components may have different responses to climate change. Trenched plots are a classic method of separating the RS source fluxes, but labor-intensive and may cause considerable disturbance to the soil environment. This study tested if various methods of plant suppression in trenched plots affected RS fluxes, quantified the RS response to soil temperature and moisture changes, and estimated the heterotrophic contribution to RS. It was performed in a boreal black spruce (Picea mariana) plantation, using a complete randomized design, during the 2007 growing season (May-November). Trenched plots had significantly lower RS than control plots, with differences appearing ~100 days after trenching; spatial variability doubled after trenching but then declined throughout the experiment. Most trenching treatments had significantly lower (by ~0.5 µmol m-2 s-1) RS than the controls, and there was no significant difference in RS among the various trenching treatments. Soil temperature at 2 cm explained more RS variability than did 10-cm temperature or soil moisture. Temperature sensitivity (Q10) declined in the control plots from ~2.6 (at 5 °C) to ~1.6 (at 15 °C); trenched plots values were higher, from 3.1 at 5 °C to 1.9 at 15 °C. We estimated RS for the study period to be 241±40 g C m-2, with roots contributing 64% of RS after accounting for fine root decay, and 293 g C m-2 for the entire year. These findings suggest that laborious hand weeding of vegetation may be usefully replaced by other methods, easing future studies of this large and poorly-understood carbon flux.