Soil Organic Matter Management: The Roles of Residue Quality in C Sequestration and N Supply - Sustainable Management of Soil Organic Matter - page 178

matrix, providing still more charged surfaces and greater fertility. It is these
organic complexes that coat soil particles and give the dark earths their
distinctive coloration (Glaser, 1999). The spatial associations of the
two types of dark earths - TP zones embedded within TM - support the
hypothesis outlined above and suggest a pre-European landscape of
long-term settlements and associated fields.
However, we must ask why are the distinctive qualities of high fertility,
dark colour and high SOM so persistent in both types of dark earth, despite
the severe tropical weathering effects of high temperature and high rainfall,
and even long after cultural manipulations have ceased. They persist, we
believe, primarily because of the coupled attributes of high biological
activity and high nutrient-retaining capacity. Recent research has demon-
strated the long-lasting influence of ash deposition in terms of increased pH
(de Moraes, 1996) and higher concentrations of nutrients such as Ca and
Mg (Ludwig et al ., 1999). Moreover, black carbon is notoriously long lived,
and not entirely inert as is often assumed (Glaser, 1999). Most telling,
perhaps, is the nature of the humus in dark earth soils. Zech et al . (1990)
compared TP with an Oxisol developed from the same parent material
and found stable, high molecular weight, fused aromatic ring structures in
the dark earths, as opposed to the readily degraded polysaccharides of the
Oxisol. They identified the source of the stable organics in the TP as lignin
and concluded that the process responsible was the mineralization and
humification of large amounts of nutrient-rich organic materials by cultur-
ally stimulated microbial activity. Apparently, at some threshold level of
biotic activity and soil nutrient retention status, dark earth attains the
capacity to perpetuate - even regenerate - itself thus behaving more like a
living 'super' organism than an inert mineral. Local farmers in our study
area have also noted the dark earths' unique ability to maintain fertility
under intensive cultivation, and those who dig it up for sale as potting
soil have learned that it will regenerate itself if some of the organic-rich,
biologically active dark layer is left intact.
Implications for Tropical Soil Management
The benefits of low input farming technologies in the tropics are becoming
increasingly apparent when compared with high-tech agriculture with
its expensive chemical inputs and associated environmental costs. Ash
contributes important nutrients, increases the cation exchange capacity and
encourages microbes by suppressing the activity of toxic Al. The Kayapó
people of Brazil have even developed specially formulated ashes for their
specific nutritive value (Hecht and Posey, 1989). Empirically measured
benefits of mulching include increased fertility, reduced soil temperature,