Decreased carbon limitation of litter respiration in a mortality-affected piñon–juniper woodland

Abstract

Microbial respiration depends on microclimatic variables and carbon (C) substrate availability, all of which are altered when ecosystems experience major disturbance. Widespread tree mortality, currently affecting piñon–juniper ecosystems in southwestern North America, may affect C substrate availability in several ways, for example, via litterfall pulses and loss of root exudation. To determine piñon mortality effects on C and water limitation of microbial respiration, we applied field amendments (sucrose and water) to two piñon–juniper sites in central New Mexico, USA: one with a recent (< 1 yr), experimentally induced mortality event and a nearby site with live canopy. We monitored the respiration response to water and sucrose applications to the litter surface and to the underlying mineral soil surface, testing the following hypotheses: (1) soil respiration in a piñon–juniper woodland is water- and labile C-limited in both the litter layer and mineral soil; (2) piñon mortality reduces the C limitation of litter respiration; and (3) piñon mortality enhances the C limitation of mineral soil respiration. Litter respiration at both sites responded to increased water availability, yet surprisingly, mineral soil respiration was not limited by water. Consistent with hypothesis 2, C limitation of litter respiration was lower at the recent mortality site compared to the intact canopy site. Applications to the mineral soil showed evidence of reduction in CO₂ flux on the girdled site and a non-significant increase on the control. We speculate that the reduction may have been driven by water-induced carbonate dissolution, which serves as a sink for CO₂ and would reduce the net flux. Widespread piñon mortality may decrease labile C limitation of litter respiration, at least during the first growing season following mortality

Evidence for biological denitrification inhibition (BDI) by plant secondary metabolites

“…These effects were not linked to abiotic factors known to be important in regulating denitrification (Wallenstein et al ., ), such as humidity or nitrate availability, which were identical in all samples (data not shown). However, microbial (aerobic or anaerobic) respiration is highly dependent on carbon availability, and under limiting conditions, carbon amendments often lead to increases in DEA and SIR (Tenuta et al ., ; Berryman et al ., ). To avoid the potentially masking effect of C input on denitrification, Schimann et al .…”

Preface: Impacts of extreme climate events and disturbances on carbon dynamics

“…They found that litter respiration responded to water availability at both treatment and control sites, and that soil respiration decreased at the site with experimental mortality. These results demonstrate ecosystem-level consequences of tree mortality that differs as a function of water availability (Berryman et al, 2013). Yue et al (2013) compared observations from post-fire vegetation trajectories in the boreal forest with simulations from the process-based ORCHIDEE (Organizing Carbon and Hydrology In Dynamic Ecosystems) vegetation model and supported the notion that the increase in atmospheric CO 2 concentrations and vegetation recovery were jointly responsible for current carbon-sink conditions.…”

“…Berryman et al (2013) tested the impacts of experimental pinyon pine (Pinus edulis Englem.) mortality on microbial respiration.…”

Response to Reviewer #2

“…Disturbance-induced tree mortality regulates the forest carbon balance, but tree mortality and its carbon consequences are not well represented in ecosystem models (Bond-Lamberty et al, 2015). Bond-Lamberty et al (2015) tested whether three ecosystem models -the classic bigleaf model Biome-BGC (BioGeochemical Cycles) and the gap-oriented models ZELIG, a gap model, and ED (ecosystem demography) -could reproduce the resilience of forest ecosystems to moderate disturbances. The models replicated observed declines in aboveground biomass well but could not fully capture observed post-disturbance carbon fluxes.…”