CO2 efflux from decomposing leaf litter stacks is influenced by the vertical distribution of leaf litter water content and its temporal variation

We evaluated the temporal changes in the vertical distribution of leaf litter respiration (R-LL) due to changes of leaf litter water content (W-LL) within the leaf litter layer (L layer) using in situ direct measurements. To investigate the vertical distribution of R-LL and W-LL within the leaf litter layer over a time-series, we measured the R-LL and W-LL of 10 separate layers in vertical leaf litter stacks fixed to the forest floor using wire pins. Measurements were conducted for 30 stacks in a temperate broad-leaved secondary forest between May 2008 and January 2009. In the warm season, high R-LL was observed at high W-LL, while low R-LL was observed at low W-LL. R-LL was comparatively lower during the cool season than during the warm season regardless of the magnitude of W-LL. The temperature sensitivity of R-LL differed depending on W-LL; temperature increases under low-moisture conditions had small effects on R-LL, while under higher-moisture conditions, R-LL was more responsive to temperature increases. Small differences in position within the leaf litter stack affected the vertical variation of W-LL and, consequently, there was large distribution in R-LL during the wet period and small distribution in R-LL (totally small values) during the dry period. Thus, CO2 efflux from the net L layer was strongly affected by R-LL distribution induced by the local wetting within the L layer. In estimating CO2 efflux from the L layer using water content of the L layer, monitoring of the water content of the L layer, which takes into account the vertical distribution in W-LL within the L layer, is necessary.