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Respiratory Activity and Oxygen Distribution in Natural Aggregates in Relation to Anaerobiosis

Departamento de Suelos, Facultad de Agronomia, Universidad de Buenos Aires, 1417 Buenos Aires, Argentine

Pierre Renault^*

INRA, Unité de Science du Sol, Domaine Saint-Paul, Site Agroparc, 84914 Avignon Cédex 9, France

*Corresponding author (pierre.renault{at}avignon.inra.fr).

ABSTRACT

A reliable estimate of the anoxic regions in soil is essential in predicting the activity of microbial processes such as denitrification. In this study, two models were tested that have been previously proposed to estimate anaerobiosis within saturated aggregates. The first model assumes O₂ consumption is constant as long as O₂ is present; in the second model, O₂ consumption depends on O₂ concentration. The test was based on the overall respiration of natural aggregates of different sizes placed at different O₂ concentrations. Some physical, chemical, and biological properties of the aggregates were also analyzed to verify the basic assumptions of the models. This analysis included direct measurements of O₂ concentration within aggregates. The second model better described aggregate respiration, but some discrepancies were found between data and theory. Oxygen profiles indicate a steep fall of O₂ near the aggregate surface. This suggests that the potential of respiration was not homogeneously distributed as assumed by the model, but decreased toward the center of the aggregates. The coefficient of variation of the respiration of 4-mm aggregates placed at 0.21 m³ O₂ m^–3 was approximately 40%. This interaggregate variability must be incorporated into the model to prevent underestimation of the real and potential rates of respiration. Physical heterogeneities affected O₂ transport within aggregates and induced irregularities in the O₂ distribution. However, there was no evidence of any effect of such heterogeneities on the aggregate respiration.

Received for publication October 4, 1995.

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