Soil Organic Matter Management: The Roles of Residue Quality in C Sequestration and N Supply - Sustainable Management of Soil Organic Matter - page 177

These results support our classification of two types of ADE. The initial
groupings based upon field and chemical criteria appear to be sound
when their NIR spectra are compared with those of other samples from
known contexts. Moreover, specific bandwidths - hence specific organic
compounds - contribute markedly toward the group placement. We feel
confident that NIRS can be applied fruitfully to a number of pedogenic,
agronomic and cultural questions relating to Amazonian black earths. With
a robust calibration, NIRS holds great potential for grouping samples on
the basis of their spectral characteristics and then processing large numbers
with a high degree of reliability as to their group placement. Aside from
the difficulties in collecting samples in the field, a major stumbling block
to studies of these anthrosols has been the high price of quantitative
procedures. Consequently, only a relatively few samples on any project
routinely are subjected to detailed analysis. With appropriate controls,
NIRS would greatly alleviate this problem.
Discussion
We have identified two distinct types of dark earth, but how were they
formed? Also, especially in the case of TM, how do we know that they are
anthropogenic? Our analyses have shown that TP samples are high in
organic carbon - and thus organic matter content - and also have greatly
elevated concentrations of Ca and P, elements strongly associated with
human habitation. These characteristics, and the abundance of cultural
materials, attest to the importance of household waste deposition (e.g.
food processing, bones, blood, faeces, urine) in the formation of TP. It is
incorrect, however, to view these soils as middens.
In the case of TM, the samples contain only slightly more Ca and P
than the background sands and clays, though organic matter concentra-
tions are equal to or even greater than those in TP. The consistently
high organic concentrations in TM samples, regardless of texture, parent
material or geomorphic context, strongly suggest an anthropogenic origin.
However, in situ habitation waste deposition is clearly not a likely source.
The most plausible explanation is that organic content was elevated
through long-term soil management practices (especially mulching and
burning) under intensive agriculture. Though perhaps temporarily
reducing near-surface soil biota, fire more importantly contributes charcoal
and ash, which affect the soil in two important ways. First, these products
of incomplete combustion provide charged surfaces largely absent from
both the highly weathered clays and sands in the local soils, thereby
increasing nutrient retention capacity. The charcoal and ash also increase
soil pH, suppressing Al activity toxic to soil biota. Increased microbiological
activity adds colloidal-sized organic decomposition products to the soil