SUBSTRATE LIMITATIONS FOR HETEROTROPHS - IMPLICATIONS FOR MODELS THATESTIMATE THE SEASONAL CYCLE OF ATMOSPHERIC CO2

We examine the sensitivity of the seasonal cycle of heterotrophic resp iration to model estimates of litterfall seasonality, herbivory, plant allocation, tissue chemistry, and land use. As a part of this analysi s, we compare heterotrophic respiration models based solely on tempera ture and soil moisture controls (zero-order models) with models that d epend on available substrate as well (first-order models). As indicato rs of regional and global CO2 exchange, we use maps of monthly global net ecosystem production, growing season net flux (GSNF), and simulate d atmospheric CO2 concentrations from an atmospheric tracer transport model. In one first-order model, CASA, variations on the representatio n of the seasonal flow of organic matter from plants to heterotrophs c an increase global GSNF as much as 60% (5.7 Pg C yr(-1)) above estimat es obtained from a zero-order model. Under a new first-order scheme th at includes separate seasonal dynamics for leaf litterfall, fine root mortality, coarse woody debris, and herbivory, we observe an increase in GSNF of 8% (0.7 Pg C yr(-1)) over that predicted by the zero-order model. The increase in seasonality of CO2 exchange in first-order mode ls reflects the dynamics of labile litter fractions; specifically, the rapid decomposition of a pulse of labile leaf and fine root litter th at enters the heterotrophic community primarily from the middle to the end of the growing season shifts respiration outside the growing seas on. From the perspective of a first-order model, we then explore the c onsequences of land use change and winter temperature anomalies on the amplitude of the seasonal cycle of atmospheric CO2. Agricultural prac tices that accelerate decomposition may drive a long-term increase in the amplitude, independent of human impacts on plant production. Consi deration of first-order litter decomposition dynamics may also help ex plain year-to-year variation in the amplitude.