PNNL

Abstract

Upland forest soils are typically major atmospheric carbon dioxide (CO2) sources and methane (CH4) sinks, but the contributions of root and microbial processes, as well as their separate temporal responses to environmental change, remain poorly understood. This two-year study was conducted in a temperate, deciduous forest located on the western shore of the Chesapeake Bay in Maryland, USA. We used high-resolution temporal CO2 and CH4 flux measurements, exclusion-source partitioning, and an ecosystem-scale flooding experiment to understand how carbon (C) fluxes, and their separate root and microbial sources, respond to seasonal and episodic environmental change. We show that the root-and-rhizosphere component of soil CO2 and CH4 flux is significant and that its dependence on soil temperature and volumetric water content (VWC) influences soil C dynamics at seasonal timescales. Experimental flooding shows that the responses of CO2 and CH4 flux to episodic moisture change were driven by suppression of soil heterotrophs, while root respiration did not respond to transient hydrologic disturbance. Methane uptake responded strongly to episodic inundation, reinforcing the important role of soil moisture in the short-term control of the forest soil CH4 sink. The weak control of seasonal soil temperature and VWC on CH4 uptake contrasts with the clear seasonality of CH4 uptake and short-term effects of experimental inundation. We suggest that CH4 consumption in the long term may be determined by vegetation, soil nutrients, microbial communities, or other factors correlated with seasonal changes. Our results indicate that the root and microbial sources of both CO2 and CH4 flux respond differently in timing and magnitude to seasonal and episodic environmental change.