PARENT SESSION
Contributed Oral Session 155: Biogeochemistry: Soil Carbon Dynamics
Friday, August 12, 8:00 AM - 11:30 AM, Meeting Room 516 A, Level 5, Palais des congrès de Montréal

Soil carbon turnover in native subtropical tree plantations in Australia: Comparing stable carbon isotope measurements and the CENTURY Soil Organic Matter Model.

Richards, Anna^{*,1, 2}, Schmidt, Susanne¹, Dalal, Ram^{2, 3}, ¹ University of Queensland, Brisbane, Queensland, Australia² Cooperative Research Centre for Greenhouse Accounting, Canberra, ACT, Australia³ Queensland Department of Natural Resources and Mines, Brisbane, Queensland, Australia

ABSTRACT- Soil organic matter (SOM) is one of the largest near-surface carbon (C) stores on earth, and 30% of global SOM is stored under tropical and subtropical forests. Reforestation of sub/tropical regions could yield a large C sink, an important consideration for Kyoto protocol related mechanisms for reducing the immediate impact of the greenhouse effect. However, little is known about rates of C turnover and C sequestration upon reforestation of sub/tropical soils. We studied soil organic C (SOC) input from hoop pine plantations (Araucaria cunninghamii, an Australian rainforest gymnosperm) on former seasonally dry rainforests. SOC isotope measurements (¹³C) under rainforest and kikuyu pastures were used to calculate loss of rainforest SOC after forest clearing, assuming a single C pool exponential decay function. From these rates we could predict input of hoop pine SOC in a 63 year plantation chronosequence. In a 50 y plantation, 45% of SOC originated from rainforest and this C did not decline further in older plantations. In a 63 y plantation, hoop pine and rainforest derived SOC were 55.5 and 50.0 t ha^-1, respectively. A calibrated version of the CENTURY SOM model was used to simulate plantation SOC. Compared to the exponential decay calculation, the CENTURY model simulated lower hoop pine SOC inputs, but similar loss of rainforest derived SOC in the 63 y plantation sequence. The model outputs suggest that most rainforest C was lost from the "active SOC pool" (2.36 t ha^-1 in rainforest, 0.09 t ha^-1 C in 63 y old plantation) and replaced by hoop pine C. After 63 years the "slow SOC pool" had lost 0.8 t ha^-1 of rainforest SOC and gained 0.08 t ha^-1 of hoop pine SOC. We conclude that SOC input under hoop pine plantations reaches steady-state after 50 years. Although total SOC levels were higher under the original rainforest (171.36 t ha^-1 C compared to 128.42 t ha^-1 C in 50 y old plantation), very little of this C was lost from the "slow SOC pool," an important long-term C store. Compared to other subtropical land uses, our study is unique in quantifying the SOC input from native forest plantations over long time scales. These plantations have the potential to sequester considerable amounts of C into soil stores as well as above ground biomass.

Key words: soil organic matter, carbon, modeling, Araucaria cunninghamii