Drought and fire affect soil CO2 efflux and use of non-structural carbon by roots in forests of southern Amazonia

Abstract

Drought and fire events reduce the ability of tropical forests to cycle and store carbon. However, the combined effects of drought and fire on soil CO₂ efflux and subsurface carbon cycling remain poorly understood, particularly in the Amazon. Here we evaluated how multiple burns and extreme drought events affect soil CO₂ efflux and the use of non-structural carbon (NSC) in the roots of forest species in southern Amazonia. We studied one intact forest (control) and another burned annually from 2004 until 2010 (burned). We monitored: a) soil CO₂ efflux every three months between 2009 and 2012 (n = 25); b) the litterfall (n = 21); c) the fine roots production (n = 16); and d) estimated the age of NSC used in the production of fine roots from radiocarbon (samples from 2009 to 2011; n = 15). Multiple fires in the burn plot reduced soil CO₂ efflux by 18.7 % compared to the control plot, and altered the relationships between litter production, soil temperature and soil CO₂ efflux. After the 2010 drought, soil CO₂ efflux in 2011 in the control plot was reduced by 17 %. Relatively freshly fixed C was used to produce new fine roots in 2009 (<2 years), but the age of C used to grow new roots in 2010 and 2011 increased to 2–4 years old, especially in the burned plot. Our results suggest that fire and drought events reduce soil CO₂ efflux and root growth, with post-disturbance root growth likely relying on stored non-structural carbohydrates (NSC).