Opening the black box: deciphering carbon and nutrient flows in Terra Preta

Summary

The soils of the Amazon region are usually unfertile, due to the high decomposition rate of organic carbon (C), rapid losses of nitrogen (N) and potassium (K) through leaching, and rapid phosphorus (P) fixation to (hydr-) oxides of iron (Fe) and aluminium (Al). Consequently, these soils (oxisols, ultisols) are known for their low suitability for agricultural production purposes. However, relatively fertile, pH-neutral soils also occur in the Amazonian lowlands. Such soils of high fertility are remnants of ancient pre-Columbian inhabitants. They are known as terra preta de índio or Amazonian Dark Earths (ADE) (Woods and McCann 1999). Terra preta contain up to eight times more carbon than adjacent soils. Furthermore, available and total nitrogen is two to eight times higher and there is up to 1,000 times more available phosphorus and up to ten times more total phosphorus than in adjacent soils (Lehmann et al. 2003). Five years after Wim Sombroek, who can be considered the founder of research on terra mulata, passed away, his legacy is still alive in Wageningen, the city where he spent much of his career. Anthrosols are common in many areas in the Netherlands and, consequently, much research has been done on these soils in Wageningen. Sombroek's parents used to farm so called eerdgronden (Plaggen soils). The soil formation history of ADE shows similarities to these eerdgronden and both types are classified into the FAO-category of man-made anthrosols. To compare, P levels of eerdgronden (currently under forest) vary from 375 to 3,000 mg kg−1 (TW Kuyper, unpublished data, 2008) and ADE from 73 to 8,800 mg kg−1 (e.g. Costa and Kern 1999; Glaser 1999; Madari et al. 2003; Ruivo et al. 2003; Lehmann et al. 2003; Schaefer et al. 2004), whereas adjacent soils (non man-made) contain on average 150–300 mg kg−1 (Guttmann et al. 2006). C levels in Northern European sandy anthrosols vary from 1.3% to 2.8% (Blume and Leinweber 2004) which is lower than C levels in ADE which are between 2.8% and 9.0% (Costa and Kern 1999; Glaser 1999; Lehmann et al. 2003; Madari et al. 2003).