DIEL MINERALIZATION PATTERNS OF STANDING‐DEAD PLANT LITTER: IMPLICATIONS FOR CO 2 FLUX FROM WETLANDS

We examined the effects of environmental conditions on the microbially mediated CO 2 evolution from standing‐dead litter (leaf blades, leaf sheaths, and culms) of the common reed, Phragmites australis (Cav.) Trin. ex Steud., in two temperate littoral freshwater marshes. Water availability was the major factor affecting CO 2 evolution rates. In the laboratory, microbial assemblages responded rapidly to controlled additions of water, with large increases in CO 2 evolution occurring within five minutes after wetting of litter (e.g., leaf blades: 10–295 μg CO 2 ‐C·(g ash‐free dry mass [AFDM]) −1 ·h −1 . Under field conditions, CO 2 evolution in the absence of precipitation exhibited a pronounced diel periodicity, with the highest rates occurring during periods of increased water availability resulting from a temperature‐induced rise in relative humidity (>95%) and corresponding litter water potential (>−2.0 MPa) during nighttime. For example, in October, rates of CO 2 evolution over a 24‐h cycle ranged from 5 to 223 μg CO 2 ‐C·(g AFDM) −1 ·h −1 for leaf blades and from 10 to 155 μg CO 2 ‐C·(g AFDM) −1 ·h −1 for leaf sheaths. Maximum rates of CO 2 evolution from sheaths were consistently lower than those for leaf blades (by ∼25%), but were typically an order of magnitude higher than those observed from culm litter (e.g., 1.0–18 μg CO 2 ‐C·(g AFDM) −1 ·h −1 over a diel cycle in August) exposed to identical environmental conditions. Much of the differences in maximum CO 2 evolution rates from different litter types were related ( r 2 = 0.72) to differences in litter associated fungal biomass (leaf blades 34–74 mg dry mass/g AFDM, leaf sheaths 16–67 mg dry mass/g AFDM, and culms 2–7 mg dry mass/g AFDM), which was estimated from litter ergosterol concentrations. Based on measured stocks of standing‐dead plant litter, estimated daily CO 2 flux from standing‐dead shoots ranged between 51 and 570 mg C/m 2 of wetland surface area. These values translate into a roughly estimated annual carbon mineralization equivalent to a mean of 8% (leaf blades), 29% (leaf sheaths), and 3% (culms) of net aboveground plant production. These data provide compelling evidence that microbial decomposition of plant litter in the aerial standing‐dead phase can contribute appreciably to overall carbon flux from marshes to the atmosphere, even in cool temperate climates, where most wetlands occur.