Technical Comments

Comment on "Impacts of Fine Root Turnover on Forest NPP and Soil C Sequestration Potential"

Science  18 Jun 2004:
Vol. 304, Issue 5678, pp. 1745
DOI: 10.1126/science.1098080

Matamala et al. (1) recently highlighted the importance of estimating mean residence time (MRT) of fine root C for understanding soil C dynamics. Using isotopic signals of 13C from two CO2 experiments as a tracer, they estimated MRT of C through fine roots that ranged from 1.20 to 6.25 years. They obtained these MRT values by fitting an exponential equation to the 13C data with a one-pool model that assumed that newly synthesized C is immediately used for fine root growth.

Actually, however, photosyntheically fixed C is first incorporated into the plant C pool to mix with stored nonstructural carbohydrate (NSC), from which root growth draws C. Thus, the interpretation of isotope data needs to consider NSC storage (2) and to use a two-pool model—that is, one that includes both NSC and fine roots. We developed such a model (3) and estimated MRT of fine roots from the isotope data presented by Matamala et al. The resulting MRT values that we derive from this model are 2.37 years for pine roots of <1 mm, 2.01 years for pine roots of 1 to 2 mm, and 6.06 years for pine roots of 2 to 5 mm (4), shorter by 44.5%, 64.8%, and 3.0%, respectively, than the estimates in (1). For sweetgum roots of <1 mm and 1 to 2 mm, the two-pool model gives estimated MRTs of 1.19 and 1.32 years, respectively— 0.8% and 56.0% less than the Matamala et al. estimates (1).

Whereas the sums of MRT of fine roots and plant NSC estimated from our two-pool model are similar to the estimates from the one-pool model (1), separating plant NSC from fine root MRT will lead to estimates of soil C sequestration that are different from those of Matamala et al. (1). In addition, data points immediately after CO2 fumigation, which are absent for approximately 10 months of that study, are particularly important for estimation of MRT of plant NSC and need to be collected in future studies.

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