br Acknowledgments br Introduction Understanding the charact

Acknowledgments
Introduction Understanding the characteristics of the Amazonian Dark soils “Terras Pretas de Índio” (TPIs) is of ecological importance, because the TPI soils represent a residue-based model for tropical sustainable agriculture (Sombroek et al., 2003, Neves et al., 2003, Cohen-Ofri et al., 2006, Cohen-Ofri et al., 2007, Falcão et al., 2003, Glaser, 2007). The TPI sites are identified by the presence of ceramics and by their deep black horizons, generally down to 1m in depth (Fraser et al., 2011). The dark color comes from the high content of black carbon (BC), which are here defined as stable charcoal particles present in the terrestrial ecosystems (Lian et al., 2006, Liang et al., 2008). In the case of the TPI soil, the black carbon is pyrogenic, produced by the Indians when burning residues. Almost no black carbon or ceramics are detectable in immediately surrounding soils, already below 20cm in soil depth (Glaser et al., 2001). The TPI soils sequester up to 70 times more carbon than the surrounding soils, and it is chemically and microbially stable (Glaser et al., 2001, Steiner et al., 2004). Consequently, the microbial d-cycloserine australia in the TPI is different from those in the surrounding soils (Grossman et al., 2010, Glaser and Birk, 2012). The TPI pyrogenic black carbon (TPI-BC) is made of sp2-ordered carbon nanocrystallites with lateral dimensions of La∼3–8nm (Jorio et al., 2012, Ribeiro-Soares et al., 2013). Despite a structural complexity, the TPI-BC dimensionality (La) has been investigated as a critical parameter defining the stability vs. reactivity properties of the soil (Jorio et al., 2012, Ribeiro-Soares et al., 2013, Archanjo et al., 2014, Archanjo et al., 2015). To be able to reproduce charcoal structures similar to the TPI-BC, for the use as a soil conditioner, it is necessary to understand whether the TPI-BC structure (mostly La) found today was generated by the char and burning, or whether the time exposure to complex physical, chemical and biological degradation played a role. Some authors are discussing how the microbial communities inhabiting the soil may influence its structure (Kabir et al., 1998, Oehl et al., 2005, Talbot et al., 2008, Eilers et al., 2012). While the elemental composition of the TPI has been largely studied (Glaser and Birk, 2012, Schaefer et al., 2004, Kern and Kampf, 2005, Cunha et al., 2009), few works addressed the microbiological composition (Grossman et al., 2010, Glaser and Birk, 2012). The primary objective of this work is to detect changes in the critical parameter La of TPI-BC as a function of soil depth, crossing this information with the elemental and microbiological composition, from the surface down to 100cm in depth. Estimates point to a TPI formation rate of 1cm for every 10 years of Indian occupation (Smith, 1980). The comparative analysis among the black carbon properties from shallower (newer) and deeper (older) soil strata should provide information about the stability of their structures. The elemental composition of the soils has been measured and the microbial occurrence evaluated to test the influence of chemical elements and microorganisms as potential charcoal degradation agents. Due to the lack of information about the microbiological composition of the TPI soils, this part is discussed here in more details. The types of microorganisms present in TPI and the abundance of the associated fungi in the surface and deeper soil strata are determined. The central hypothesis of this work is that, despite differences in (1) the time permanence in the soil, (2) the elemental and (3) the microbiological soil compositions, the as produced TPI-BC maintained their basic structural properties, represented by a unique distribution of La values.
Material and methods
Results and discussion
Conclusions Raman spectroscopy-based structural analysis is capable of realizing the differences among black carbon structures from TPI, turf, peat, compost, nature forest, synthetic vegetal and activated commercial charcoals. However, the Raman spectroscopy analysis indicates that the nanostructure parameter La of the TPI-BC does not change with soil depth. On the other side, the elemental composition and the microbial abundance were found to exhibit a clear depth dependence. Based on these results, we propose that the nanostructure of the TPI-BC found today is similar to the form they were produced by the Indians, i.e. they have not experienced long-term degradation. This is a key aspect to synthesize this type of black carbon structure, since it suggests one has to reproduce the conditions utilized by the Indians, excluding complex unknown long-term physical–chemical–biological influences on the TPI-BC structure in the soil.