Climate and litter quality controls on decomposition: An analysis of modeling approaches

Four mathematical models simulated decay of two litter types of contrasting quality over a 2-year period at four sites in North America. The litter ty pes were Drypetes glauca and Triticum aestivum, representing litter with hi gh and low nitrogen:lignin ratios, respectively. The field sites were an Ar ctic tussock tundra (Alaska, United States), a warm desert (New Mexico, Uni ted States), a temperate deciduous forest (New York, United States) and a t ropical rain forest (Puerto Rico). Models captured the overall patterns of site and litter quality controls on decomposition; both simulated and obser ved mass losses were higher in warm, moist environments (both forests) than in cold (tundra) or dry sites (desert), and simulated and observed decay w as more rapid for Drypetes than Triticum. However, predictions tended to un derestimate litter mass loss in the tropical forest and overestimate decay in the desert and tundra, suggesting that site controls in model formulatio ns require refinement for use under such a broad range of conditions. Also, predicted nitrogen content of litter residues was lower than observed in D rypetes litter and higher than observed for Triticum. Thus mechanisms descr ibing loss of nitrogen from high-quality litter and nitrogen immobilization by low-quality litter were not captured by model structure. Individual mod el behaviors revealed different sensitivities to controlling factors that w ere related to differences in model formulation. As these models represent working hypotheses regarding the process of litter decay, results emphasize the need for greater resolution of climate and litter quality controls. Re sults also demonstrate the need for finer resolution of the relationships b etween carbon and nitrogen dynamics during decomposition.