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Title: A Simple Three Pool Model Accurately Describes Patterns of Long Term Litter Decomposition in Diverse Climates

Author
item ADAIR, E - U OF MN, ST. PAUL, MN
item PARTON, W - COLO ST U, FT. COLLINS,
item Del Grosso, Stephen - Steve
item SILVER, W - UC, BERKELEY, CA
item HARMON, M - OR ST U, CORVALLIS, OR
item HALL, S - NATURE CONS. WENATCHEE, W
item BURKE, I - COLO ST U, FT. COLLINS, C
item HART, S - NO AZ U, FLAGSTAFF, AZ

Submitted to: Global Change Biology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/4/2007
Publication Date: 7/24/2008
Citation: Adair, E.C., Parton, W.J., Del Grosso, S.J., Silver, W.L., Harmon, M.E., Hall, S.A., Burke, I.C., Hart, S.C. 2008. A Simple Three Pool Model Accurately Describes Patterns of Long Term Litter Decomposition in Diverse Climates. Global Change Biology 14:2636-2660.

Interpretive Summary: Natural terrestrial ecosystems exchange huge amounts of carbon with the atmosphere. Annual carbon dioxide (CO2) emissions from the decomposition of plant and soil organic matter exceed those resulting from fossil fuel combustion. Without the added CO2 from fossil fuel combustion, however, emissions from decomposition are roughly balanced by uptake of CO2 by plants via photosynthesis. The process of plant uptake of atmospheric carbon is better understood than that of CO2 release from decomposition. A better understanding of what controls decomposition is needed because relatively small changes in CO2 emissions from this source could have a significant impact on atmospheric CO2 levels. As plant litter decomposes and releases CO2 to the atmosphere, the mass of remaining litter gradually decreases. Modeling this litter mass loss quantifies how key factors control decomposition rates and should lead to improved estimates of CO2 emissions from decomposition. To develop a model that predicts litter mass loss, we used the Long Term Intersite Decomposition Experiment Team (LIDET) data set. The LIDET experiment consisted of burying litter bags in various ecosystems throughout the world and weighing the remaining litter every year during a 10 year period. In most of the systems studied, the rate of mass loss was highest after litter burial and then gradually decreased as the supply of easily decomposed compounds diminished. Litter mass loss was best represented by a three pool negative exponential model. The first being a rapidly decomposing labile pool, the second an intermediate pool representing free and lignin encrusted cellulose, and the third a recalcitrant pool. The effect of temperature on decomposition was tested using a set of climate decomposition indices that are the product of monthly water stress and temperature equations. This generally worked well, but we observed some systematic deviations from model predictions. Fine root and aboveground material decomposed at notably different rates, depending on the stage of decomposition. The model did not duplicate the pattern of litter mass loss through time observed at some sites. Despite these limitations, this model explained about 68% of the variability in long term global decomposition patterns for a wide array of litter types, using relatively minimal climatic and litter quality data.

Technical Abstract: The ability of ecosystems to sequester carbon (C) is largely dependent on how global changes in climate will alter the balance between rates of decomposition and net primary production. The response of primary production to changes in climate has been examined using reasonably well-validated mechanistic models, but the same is not true for decomposition, a primary source of CO2 to the atmosphere. We used the Long Term Intersite Decomposition Experiment Team (LIDET) data set in combination with model selection techniques to choose and parameterize a model to represent dynamics of litter mass loss. Litter mass loss was best represented by a three pool negative exponential model, with a rapidly decomposing labile pool, an intermediate pool representing free and lignin encrusted cellulose, and a recalcitrant pool. The initial lignin to nitrogen (N) ratio of litter defined the size of the labile and intermediate pools, and lignin content determined the size of the recalcitrant pool. The decomposition rate of all pools was modified by climate, but the intermediate pool’s decomposition rate was also controlled by amounts of litter cellulose and lignin (an indicator of amount of lignin-encrusted cellulose). We tested the effect of temperature on decomposition using a set of climate decomposition indices (CDIs) that are the product of monthly water stress and temperature functions. The temperature effect was best represented by the Lloyd and Taylor (1994) variable Q10 function. Although our model explained nearly 70% of the variation in the LIDET data set, we observed systematic deviations from model predictions. Fine root and aboveground material decomposed at notably different rates, depending on the stage of decomposition. Decomposition in certain site- or ecosystem-specific environmental conditions was not well represented by our model; this included roots in very wet and very cold soils, and aboveground litter in N-rich and arid sites. Despite these limitations, our model may still be extremely useful for global modeling efforts, since it was able to accurately (R2 = 0.6804) describe general patterns of long term global decomposition for a wide array of litter types, using relatively minimal climatic and litter quality data. [GRACENet Publication].