Decay Rates and Semi-stable Fraction Formation after 12 years of Foliar Litter Decomposition in Canadian Forests

Litter decay in early and midphases of decomposition have been shown to highly influenced by climate and substrate quality, however factors affecting decay during the late semi-stable phase are less well understood. The Canadian Intersite Decomposition Experiment (CIDET) was established in 1992 with the objective of providing data on the long-term rates of litter decomposition and nutrient mineralization for a range of forested ecoclimatic regions in Canada. Such data were needed to help verify models used for national C accounting, as well as aid in the development of other soil C models. CIDET examined the annual decay, over a 12-year period, of 10 standard foliar litters and 2 wood substrates at 18 forested upland and 3 wetland sites ranging from the cool temperate to subarctic regions, a nearly 20oC span in temperature. On a subset of sites and litter types, changes in litter C chemistry over time were also determined. Over the first 6 years, C/N ratio and iron increased, NMR showed an overall decline in O-alkyl C (carbohydrates) and increase in alkyl, aromatic, phenolic, and carboxyl C. Proximate analysis showed the acid unhydrolyzable residue (AUR) increases, but true lignin did not accumulate, in contrast to the conceptual ligno-cellulose model of decomposition. Litter decay during first phase was related to initial litter quality (AUR and water soluble extract), winter precipitation, but not temperature, suggesting the importance of leaching during this phase. Decay rate “k” during the mid phase was related to temperature, initial litter quality (AUR and AUR/N), summer precipitation, but not soil N. In most cases decay had approached an asymptote before end of experiment. Although annual temperature was the best single predictor for 12-year asymptotes, summer precipitation and forest floor pH and C/N ratio were the best set of combined predictors. The changes in the decay factors during different phases may explain some of the discrepancies in the relative importance of factors eg. temperature, which affect decomposition. Initial litter quality will have little influence during the later phase as chemical properties of well decayed litter converge. Since site factors other than climate were related to the size of the semi-stable fraction, this may reflect the role site chemistry and biota on decay rates during the semi-stable phase. Even with 12 years of data, results were not sufficient to determine late phase decay rates suggesting either even longer-term studies or other experimental approaches will be needed to determine the fate and factors affecting rates of organic matter sequestration and loss.