Even Amazon forests with no record of fire carry its legacy deep in their soils

Posted by Ted Feldpausch

8 August 2017

Forests across the Amazon Basin that have no known history of fire still hold the carbon left by fires long past. About one tenth of the carbon in their soils is pyrogenic carbon, the charcoal-like residue of burning, and half of it lies more than half a metre below the surface.

These are the findings of the first field-based estimate of pyrogenic carbon across the Amazon Basin, led by Nina Koele at the Universidade do Estado de Mato Grosso (UNEMAT) with Ted Feldpausch at the University of Exeter and partners at James Cook University, the University of Leeds and INPA.

Key findings

Fire has left its mark on soils that look untouched. In 37 one-hectare forest plots across the basin, chosen because they had no known recent fire or human disturbance, pyrogenic carbon was present in every soil analysed.
It is a large carbon store. Amazon forest soils hold about 2.8 billion tonnes of pyrogenic carbon in the top metre, and 1.1 billion tonnes in the top 30 cm. That is about three times what earlier global budgets assumed: they put the pyrogenic share of tropical forest soil carbon at 1%, while across the basin it averages about 3% in the top 30 cm.
Most of it is deep. Pyrogenic carbon makes up a growing share of soil carbon the deeper you go. Half of it lies below about 58 cm on average, so a survey must reach at least 71 cm to capture even half, and 127 cm to capture three quarters. Most soil carbon surveys stop at 30 cm.
It is a long-term store. Deep pyrogenic carbon is protected by clay and iron and aluminium minerals and by low oxygen, and it cycles over thousands of years. Ferralsols, the most widespread Amazon soils, hold the most.

Why it matters

Fire is becoming more frequent across the Amazon. To understand what fire does to forest soils today, and what deforestation puts at risk, we first need to know how much fire-derived carbon the soils already contain and where it sits. This study provides that baseline. Because so much of the store lies below the depths that inventories and carbon projects usually measure, it is largely invisible to the accounts used to manage soil carbon.

The study also sets out a practical sampling design: three samples per hectare are enough to capture the variation within a plot, while sampling to 1 m rather than 30 cm is what it takes to capture more than half of the pyrogenic carbon in a 2 m profile.

Reference

Koele, N., Bird, M., Haig, J., Marimon, B.H., Marimon, B.S., Phillips, O.L., de Oliveira, E.A., Quesada, C.A. & Feldpausch, T.R. (2017) Amazon Basin forest pyrogenic carbon stocks: first estimate of deep storage. Geoderma 306: 237–243. doi.org/10.1016/j.geoderma.2017.07.029 (open access)

Policy brief

We distilled the study for soil inventory agencies, carbon standard bodies and carbon modellers. It makes three recommendations: sample soil carbon to at least 1 m, with three samples per hectare; measure the pyrogenic fraction rather than assuming a fixed 1% share; and represent pyrogenic carbon in models as its own slow pool, to depth.

Read the full policy brief below:

Amazon pyrogenic carbon policy brief (PDF)Download

See also the companion brief on the fire history of the same forests: Amazon forest fire: rare for millennia, absent for 800 years.

Funding: CAPES Science without Borders (PVE 177/2012), NERC (NE/N011570/1), CNPq (PELD 403725/2012-7; PPBio 457602/2012-0) and the University of Exeter.