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Published December 1996 | Published
Journal Article Open

Substrate limitations for heterotrophs: Implications for models that estimate the seasonal cycle of atmospheric CO_2


We examine the sensitivity of the seasonal cycle of heterotrophic respiration to model estimates of litterfall seasonality, herbivory, plant allocation, tissue chemistry, and land use. As a part of this analysis, we compare heterotrophic respiration models based solely on temperature and soil moisture controls (zero‐order models) with models that depend on available substrate as well (first‐order models). As indicators of regional and global CO_2 exchange, we use maps of monthly global net ecosystem production, growing season net flux (GSNF), and simulated atmospheric CO_2 concentrations from an atmospheric tracer transport model. In one first‐order model, CASA, variations on the representation of the seasonal flow of organic matter from plants to heterotrophs can increase global GSNF as much as 60% (5.7 Pg C yr^(−1)) above estimates obtained from a zero‐order model. Under a new first‐order scheme that 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 models reflects the dynamics of labile litter fractions; specifically, the rapid decomposition of a pulse of labile leaf and fine root litter that enters the heterotrophic community primarily from the middle to the end of the growing season shifts respiration outside the growing season. From the perspective of a first‐order model, we then explore the consequences of land use change and winter temperature anomalies on the amplitude of the seasonal cycle of atmospheric CO_2. Agricultural practices that accelerate decomposition may drive a long‐term increase in the amplitude, independent of human impacts on plant production. Consideration of first‐order litter decomposition dynamics may also help explain year‐to‐year variation in the amplitude.

Additional Information

© 1996 American Geophysical Union. (Received December 13, 1995; revised June 17, 1996; accepted June 24, 1996.) Paper number 96GB01981. This research was funded in part by an NASA EOS/IDS grant to P. J. Sellers and H. A. Mooney and a DOE WESTGEC Grant to C. B. F. and J. A. Berry. J.T.R is supported on a NASA Earth System Science Fellowship. C.M.M. is supported on a DOE Global Change Fellowship. The complete model (in C with Unix C shell scripts and with documentation) is available upon request. We thank Peter Vitousek, Anne Ruimy and Geeske Joel for useful discussions and comments on the manuscript and Jasmin John for help with the tracer runs. This is C.l.W. D.P.B. publication number 1293.

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October 19, 2023