Dynamics of fine root carbon in Amazonian tropical ecosystems and the contribution of roots to soil respiration

Susan Trumbore,Daniel C Nepstad,Plínio Barbosa De Camargo,Teresa Restom,Luiz A Martinelli,Enir Salazar Da Costa,David Ray,Whendee Silver

doi:10.1111/j.1365-2486.2005.001063.x

Abstract

AbstractRadiocarbon (14C) provides a measure of the mean age of carbon (C) in roots, or the time elapsed since the C making up root tissues was fixed from the atmosphere. Radiocarbon signatures of live and dead fine (<2 mm diameter) roots in two mature Amazon tropical forests are consistent with average ages of 4–11 years (ranging from <1 to >40 years). Measurements of 14C in the structural tissues of roots known to have grown during 2002 demonstrate that new roots are constructed from recent (<2‐year‐old) photosynthetic products. High Δ14C values in live roots most likely indicate the mean lifetime of the root rather than the isotopic signature of inherited C or C taken up from the soil.Estimates of the mean residence time of C in forest fine roots (inventory divided by loss rate) are substantially shorter (1–3 years) than the age of standing fine root C stocks obtained from radiocarbon (4–11 years). By assuming positively skewed distributions for root ages, we can effectively decouple the mean age of C in live fine roots (measured using 14C) from the rate of C flow through the live root pool, and resolve these apparently disparate estimates of root C dynamics. Explaining the 14C values in soil pore space CO2, in addition, requires that a portion of the decomposing roots be cycled through soil organic matter pools with decadal turnover time.

Highlights

Roots are an essential yet poorly understood component of terrestrial ecosystems
As in temperate forests, several hypotheses may explain the fact that the D14C values of live fine roots are much greater than expected: (1) roots may live a long time; (2) roots may grow from C that has been stored in the tree for some time; (3) roots may take up C from the soil subsequent to or during initial growth; and (4) the separation of live roots from dead roots may err and r 2006 Blackwell Publishing Ltd, Global Change Biology, 12, 217–229 include old, dead roots within what is identified as the live root pool
Estimates of the turnover time of C in forest fine roots are substantially shorter (1–3 years) than the age of standing fine root C stocks obtained from radiocarbon (4–11 years)

Summary

Introduction

Roots are an essential yet poorly understood component of terrestrial ecosystems. They play an important role in the carbon (C) cycle by contributing a significant fraction of ecosystem net primary production (Nadelhoffer & Raich, 1992; Vogt et al, 1996). Few data exist on the distribution and dynamics of fine roots in tropical forests. A recent review by Gill & Jackson (2000) cited only five studies of root dynamics from broadleaf tropical forests (precipitation 41000 mm yrÀ1). Methods of measuring fine root dynamics are hampered by large spatial variability and the difficulties of quantifying root biomass and turnover (reviewed by Aber et al, 1985; Vogt et al, 1998; Eissenstat et al, 2000; Pregitzer, 2002)

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Discussion

Conclusion