Saturday, 15 July 2006

Soil Microbial Communities Associated with Anthropogenic Dark Earths (Terra Preta) and Black Carbon Particles.

Julie Grossman1, Brendan O'Neill1, José Elias Gomes2, Siu Mui Tsai2, Biqing Liang1, Johannes Lehmann1, and Janice Thies1. (1) Cornell Univ, Dept of Crop and Soil Science, Bradfield Hall, Ithaca, NY 14853, (2) Centro de Energia Nuclear na Agricultura (CENA), Av. Centenário, 303, Piracicaba, 13400-970, Brazil

Soil microorganisms are important in the global carbon (C) cycle for the role they play in decomposing and stabilizing organic matter in soil. Anthropic Terra Preta (TP) soils were developed between 500 and 6,500 years before present by indigenous pre-Colombian Indians and are characterized by unusually high nutrient availability, cation exchange capacity and soil C contents dominated by incompletely combusted biomass (Black Carbon; BC). In contrast to most tropical soils, TP soils have extremely persistent soil organic carbon due to the high proportion of BC. We hypothesized that: (i) soil microbial communities in TP soils are distinct from those present in mineralogically-similar adjacent background soils, and that (ii) BC found in TP soils provides a habitat conducive to supporting microbial life. Soils were collected from four TP sites within the central Amazon basin and compared to adjacent background soils (Oxisols, Spodosols or Ultisols) of identical mineralogy. Given that less than 1% of the soil microbial community is likely accessible via traditional culturing techniques, we used two culture-independent DNA fingerprinting techniques (DGGE and T-RFLP) to characterize the soil microbial communities in each soil. Our most striking findings were that both DNA fingerprinting methods showed TP communities to be similar to each other regardless of site; and that communities were distinct from those in the adjacent background soils. We observed a greater richness in Bacterial as compared with Archaeal communities. In both cases communities from background soils were radically divergent from those in TP soils, which in the case of Archaea diverged by over 90%. Sequencing of taxa unique to particular samples showed that both TP and background soils contain organisms that are taxonomically distinct from those found in sequence databases. Most sequences obtained were novel and matched those in databases at less than 97% similarity. Several sequences obtained only from TP soils grouped with the Verrucomicrobia sp. at 93% similarity. Proteobacteria and Cyanobacteria sp. were found only in background soils, and Pseudomonas, Acidobacteria, and Flexibacter sp. were common to both soil types. NEXAFS analysis showed that BC particles are highly oxidized near their surface and have an adsorbed surface layer, suggesting the surfaces are associated with microbial or plant material. Confocal microscopy images also demonstrated the presence of live microorganisms on BC surfaces, indicating that BC can serve as a habitat for microbial life, despite its chemical recalcitrance. Combined, these data show that composition of both Bacteria and Archaea communities was strongly affected by soil characteristics of Terra Preta. In the future, we plan to identify organisms on BC surfaces and determine if their presence is unique to soils with high BC concentrations.

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