Editing IPCC:AR6/WGII/Chapter-12 (section)

=== 12.6.2 Anthropogenic Soils, an Option for Mitigation and Adaptation to Climate Change in Central and South America. Learning from the “Terras Pretas de Índio” in the Amazon ===

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Amazonian dark earths (ADEs), also known as Terras Pretas de Índio, are anthropogenic soils derived from the activities associated with the settlements and agricultural practices of pre-Hispanic societies in the Amazon (Woods and McCann, 1999; [[#Lehmann--2003|Lehmann et al., 2003]] ; [[#Sombroek--2003|Sombroek et al., 2003]] ). Most of the ADEs identified so far are 500 to 2500 years old ( [[#de%20Souza--2019|de Souza et al., 2019]] ). According to [[#Maezumi--2018a|Maezumi et al. (2018a)]] , polyculture agroforestry allowed for the development of complex societies in the eastern Amazon around 4500 years ago. Agroforestry was combined with the cultivation of multiple crops and the active and progressive increase in the proportion of edible plant species in the forest, along with hunting and fishing. The formation of ADEs as a result of these activities served as the basis for a food production system that supported a growing human population in the area ( [[#Maezumi--2018a|Maezumi et al., 2018a]] ).

ADEs are the result of the accumulation and incomplete combustion of waste materials such as ceramic artefacts and organic residues from harvesting, weeding, food processing (including cooking) and other activities ( [[#Lima--2002|Lima et al., 2002]] ; [[#Hecht--2003|Hecht, 2003]] ; [[#Kämpf--2003|Kämpf et al., 2003]] ). ADEs are characterised by their increased fertility in relation to adjacent soils, with high contents of organic carbon (C) (mainly as charcoal) as well as inorganic nutrients, especially phosphorus (P) and calcium (Ca) and high carbon/nitrogen ratios ( ''high confidence'' ) (Moline and Coutinho, 2015; [[#Alho--2019|Alho et al., 2019]] ; [[#Barbosa--2020|Barbosa et al., 2020]] ; [[#Pandey--2020|Pandey et al., 2020]] ; [[#Soares--2021|Soares et al., 2021]] ; [[#Zhang--2021|Zhang et al., 2021]] ). They also exhibit a high cation exchange capacity and moisture retention, among other properties ( [[#Hecht--2003|Hecht, 2003]] ; [[#Kämpf--2003|Kämpf et al., 2003]] ; [[#Falcão--2009|Falcão et al., 2009]] ). Charcoal content is a key indicator of pre-Hispanic fire activity and sedentary occupation, which is evidence of the anthropic origin of these soils ( ''high confidence'' ) ( [[#Hecht--2017|Hecht, 2017]] ; [[#Maezumi--2018b|Maezumi et al., 2018b]] ; [[#Alho--2019|Alho et al., 2019]] ; [[#Barbosa--2020|Barbosa et al., 2020]] ; [[#Iriarte--2020|Iriarte et al., 2020]] ; [[#Montoya--2020|Montoya et al., 2020]] ; [[#Shepard--2020|Shepard et al., 2020]] ).

Accumulation of organic residues and low-intensity fire management are recognised key elements in ADE formation. ADEs originating around settlements show a relatively high density of ceramic artefacts and are called terras pretas ''.'' They present a higher content of calcium and phosphorus than those originating from agricultural activities, which are known as ''terras mulatas'' ( [[#Hecht--2003|Hecht, 2003]] ).

There is a robust and growing body of research from various disciplines that assigns a high relevance to ADEs in the region. It has been shown through archaeological and palaeoclimatic data that Amazonian societies that based their agricultural management on Terras Pretas de Índio were more resilient to the changing climate due to increased soil fertility and water retention capacity ( [[#de%20Souza--2019|de Souza et al., 2019]] ). Additionally, low organic carbon degradability over long time periods, associated with high contents of charcoal or pyrogenic carbon, makes these soils an important C sink ( ''medium confidence'' : ''robust evidence, medium agreement'' ) ( [[#Lehmann--2003|Lehmann et al., 2003]] ; [[#Guo--2016|Guo, 2016]] ; [[#Trujillo--2020|Trujillo et al., 2020]] ), which is particularly relevant in an area like the Amazon, that could change from a net carbon sink to a net carbon source as a consequence of anthropogenic climate change ( [[#Maezumi--2018b|Maezumi et al., 2018b]] ).

The Indigenous agricultural practices that led to ADEs are thought to be associated with a more sedentary agricultural model than the current slash-and-burn and shifting cultivation practices. Although this is a controversial topic, as the precise definitions of slash and burn and shifting cultivation are presently under discussion ( [[#Hecht--2003|Hecht, 2003]] ), several present-day local and Indigenous agricultural practices, including in-field burning and nutrient additions from food processing and residue management, have been recognised as promoting high organic carbon and nutrient soil contents similar to those found in ADEs ( [[#Hecht--2003|Hecht, 2003]] ; Winklerprins, 2009).

At present, ADEs are estimated to cover up to 3.2% of the Amazon basin and are highly valued for their persistent fertility, and they have become a key resource for sustainable agriculture for Amazon communities in a climate-change context ( [[#Altieri--2013|Altieri and Nicholls, 2013]] ; [[#Maezumi--2018a|Maezumi et al., 2018a]] ; [[#de%20Souza--2019|de Souza et al., 2019]] ). Based on the lessons learned from the Terras Pretas de Índio, some researchers have proposed the development of technologies to promote a new generation of anthropogenic soils (e.g., Kern et al. 2009; Lehmann 2009; Schmidt et al. 2014; Bezerra et al. 2016; Kern et al. 2019). Among the technologies based on ADE findings, biochar, obtained by the slow pyrolysis of agricultural residues, is the most explored application found in the literature ( [[#Mohan--2018|Mohan et al., 2018]] ; [[#Matoso--2019|Matoso et al., 2019]] ; [[#Amoah-Antwi--2020|Amoah-Antwi et al., 2020]] ). The dual purpose of increased soil fertility and carbon sequestration is considered an important goal in connection with developing sustainable agriculture in a climate-change context ( [[#Kern--2019|Kern et al., 2019]] ).

Preservation of the practices and knowledge associated with these soils is vital for sustainable agriculture in a climate-change scenario in the Amazon. It will greatly contribute to the preservation of valuable IK as well as the contribution to the development of new adaptation and mitigation technologies, among other unexplored solutions.

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