Decreased carbon limitation of litter respiration in a mortality-affected pinon-juniper woodland
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 pinon-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 pinon mortality effects on C and water limitation of microbial respiration, we applied field amendments (sucrose and water) to two pinon-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 pinon-juniper woodland is water- and labile C-limited in both the litter layer and mineral soil; (2) pinon mortality reduces the C limitation of litter respiration; and (3) pinon 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 pinon mortality may decrease labile C limitation of litter respiration, at least during the first growing season following mortality.