Natural organochlorine (Clorg) is ubiquitous in soil humus, but the distribution and cycling of different Cl species during the humification of plant material is poorly understood. Our X-ray spectromicroscopic studies indicate that the distributions of Clorg and inorganic Cl-(Clinorg) in oak leaf material vary dramatically with decay stage, with the most striking changes occurring at the onset of weathering. In healthy or senescent leaves harvested from trees, Clinorg occurs in sparsely distributed, highly localized "hotspots" associated with trichomes as well as in diffuse concentration throughout the leaf tissue. The Clinorg associated with trichomes exists either in H-bonded form or in a solid salt matrix, while the Clinorg in diffuse areas of lower Cl concentration appears exclusively in H-bonded form. Most solid phase Clinorg leaches from the leaf tissue during early weathering stages, whereas the H-bonded Clinorg appears to leach away slowly as degradation progresses, persisting through advanced weathering stages. In unweathered leaves, aromatic and aliphatic Clorg were found in rare but concentrated hotspots. In weathered leaves, by contrast, aromatic Clorg hotspots are prevalent, often coinciding with areas of elevated Fe or Mn concentration. Aromatic Clorg is highly soluble in leaves at early weathering stages and insoluble at more advanced stages. These results, combined with optical microscopy, suggest that fungi play a role in the production of aromatic Clorg in weathering leaf material. Aliphatic Clorg occurs in concentrated hotspots in weathered leaves as well as in diffuse areas of low Cl concentration. The distribution and speciation of Cl in weathering oak leaves depicted by this spectromicroscopic study provides new insight into the formation and cycling of Clorg during the decay of natural organic matter.
All Science Journal Classification (ASJC) codes
- Geochemistry and Petrology