Amazon Basin forest pyrogenic carbon stocks: First estimate of deep storage

Nina Koele,Michael Bird,Jordahna Haig,Ben Hur Marimon-Junior,Beatriz Schwantes Marimon,Oliver L Phillips,Edmar A De Oliveira,C.A Quesada,Ted R Feldpausch

doi:10.1016/j.geoderma.2017.07.029

Abstract

Amazon Basin forest soils contain considerable soil organic carbon stocks; however, the contribution of soil pyrogenic carbon (PyC) to the total is unknown. PyC is derived from local fires (historical and modern) and external inputs via aeolian deposition. To establish an initial estimate of PyC stocks in non-terra preta forest with no known history of fire, to assess site and vertical variability, as well as to determine optimal sampling design, we sampled 37 one hectare forest plots in the Amazon Basin and analysed PyC via hydrogen pyrolysis of three individual samples per plot and of bulked samples to 200cm depth. Using our data and published total organic carbon stocks, we present the first field-based estimate of total PyC stock for the Amazon Basin of 1.10 Pg over 0–30cm soil depth, and 2.76 Pg over 0–100cm soil depth. This is up to 20 times higher than previously assumed. Three individual samples per 1ha are sufficient to capture the site variability of PyC in our plots. PyC showed significant, large-scale variability among plots. To capture 50% of the PyC in 200cm soil profiles, soil must be sampled to a depth of at least 71cm. PyC represents a significant (11%) portion of total organic carbon in soil profiles 0–200cm depth. This finding highlights the potentially important role that historical fire has played in modifying soil C stocks. Our data suggest that PyC is an important carbon pool for long-term storage, involved in millennial scale biogeochemical cycling, particularly in the subsurface soil.

Highlights

Amazon forest soils represent a large soil carbon pool, containing approximately 36.1 Pg C in the upper 30 cm and 66.9 Pg C in the upper 1 m (Batjes and Dijkshoorn, 1999)
TOC and pyrogenic carbon (PyC) decrease by 99% from 0 to 5 cm to 150–200 cm soil depth (Fig. 2, Table 1)
PyC stock estimates for the 0–30 cm depth interval sampled in the Amazon Basin forest soils vary per soil type, from 26 Tg for Arenosols, to as high as 246 Tg for Ferralsols, and from 102 Tg for Plinthosols to 645 Tg for Ferralsols in the 0–100 cm depth interval (Table 2)

Summary

Introduction

Amazon forest soils represent a large soil carbon pool, containing approximately 36.1 Pg C in the upper 30 cm and 66.9 Pg C in the upper 1 m (Batjes and Dijkshoorn, 1999). A substantial part of this carbon pool may exist as pyrogenic carbon (PyC) resulting from biomass burning. A fraction of PyC can quickly be lost from the soil through erosion in steep slopes (Rumpel et al, 2006, 2009) and decomposition (Kuzyakov et al 2009; Bird et al, 2015). Anthropogenic Amazon Dark Earths, referred to as Terra Preta de Índio, show significantly increased fertility through PyC additions (Glaser 2007); it is unknown if PyC in natural forest soils in the Amazon Basin affects biomass and vegetation dynamics. Knowledge on PyC in global carbon stocks and cycling is needed for monitoring and management of soil organic carbon under land-use change and climate change Evaluating the impact of PyC on natural forest soil dynamics, requires first quantifying PyC stocks in ecosystems globally

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