Diel mineralization patterns of standing-dead plant litter:: Implications for CO2 flux from wetlands

We examined the effects of environmental conditions on the microbially mediated CO2 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 CO2 evolution rates. In the laboratory, microbial assemblages responded rapidly to controlled additions of water, with large increases in CO2 evolution occurring within five minutes after wetting of litter (e.g., leaf blades: 10-295 mug CO2-C.(g ash-free dry mass [AFDM])(-1).h(-1). Under field conditions, CO2 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 CO2 evolution over a 24-h cycle ranged from 5 to 223 mug CO2-C.(g AFDM)(-1).h(-1) for leaf blades and from 10 to 155 mug CO2-C.(g AFDM)(-1).h(-1) for leaf sheaths. Maximum rates of CO2 evolution from sheaths were consistently lower than those for leaf blades (by similar to25%), but were typically an order of magnitude higher than those observed from culm litter (e.g., 1.0-18 mug CO2-C.(g AFDM)(-1).h(-1) over a diel cycle in August) exposed to identical environmental conditions. Much of the differences in maximum CO2 evolution rates from different litter types were related (r(2) = 0.72) to differences in litter associated fungal biomass (leaf blades 34-74 mg (fry mass/g AFDM, leaf sheaths 16-67 mg dry mass/g AFDM, and culms 2-7 mg dry rnass/g AFDM), which was estimated from litter ergosterol concentrations. Based on measured stocks of standing-dead plant litter, estimated daily CO2 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.