Relative responses of soil and root respiration to soil warming suggest little soil C loss after the initial two years
The magnitude of future climate change will depend in part on feedbacks between warming and ecosystem carbon balance, yet these feedbacks are still imperfectly understood. With regard to representations in earth systems sub-models, clear knowledge gaps exist regarding the long-term effects of climate change on plant tissue respiration and microbially mediated soil organic matter transformations. We have been investigating warming-induced changes in soil respiration and its components (roots, mycorrhizae, other microbes) in a soil warming by moisture addition experiment in a Michigan sugar maple forest since 2010. Experimental treatments included four combinations of soil warming and moisture addition: control, soil warming (+4 to 5 oC above ambient), water addition (+30% of average ambient growing season precipitation); and warming + water addition. Soil and root respiration were measured approximately monthly during the growing season from initiation of soil warming in September, 2010, through September, 2014. Objectives were to determine how changes over time in root and microbial respiration contributed to observed responses of soil respiration to warming and the implications of these responses for the rate and duration of declines in soil C content.
The warming-induced enhancement of soil respiration declined from nearly 60% at the initiation of the experiment to less than 10% over the last three years. Specific root respiration partially acclimated to warmer soil, but remained elevated in treatments with soil heating throughout the experiment. Enhancement of root respiration under warming averaged 30% during the last two years of treatment. Root biomass was unchanged, thus root respiration at the ecosystem level was similarly elevated. Because root respiration remained significantly elevated in experimentally warmed treatments, the decline in the treatment response of soil respiration to only 10% more than the control indicates that respiration associated with microbial decomposition of soil organic matter and/or mycorrhizal activity has decreased over time in warmed soil. We have previously observed declines of both mycorrhizal and decomposer activity in sugar maple forests in response to long-term simulated atmospheric N deposition, and we hypothesize that greater soil N availability in warmer soil has similarly suppressed microbial activity as a secondary effect of soil warming. This suggests the possibility that soil C is no longer declining at these sites, after only several years of warming, in contrast to model simulations, which sometimes show declining soil C for many years.