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Soil Respiration as an Index of Soil Aeration

Dep. of Soil and Water Science, Univ. of Arizona, Tucson, AZ 85721

* Corresponding author.

ABSTRACT

The accurate determination of available soil O₂ is problematic due to the dynamic interaction of physical and biological soil properties. This study was conducted to evaluate combined effects of soil bulk density, water content, and organic and inorganic amendments on available soil O₂, using CO₂ evolution from soil respiration as an index. Optimal levels of soil aeration vary with respiratory O₂ consumption, which is influenced by soil amendments. Comparisons were made between Pima clay loam [fine-silty, mixed (calcareous), thermic, Typic Torrifluvent] amended with inorganic fertilizer or anaerobically digested liquid sewage sludge at equivalent N loading rates. Pima soil was amended with inorganic fertilizer, artificially compacted to bulk densities of 1.1, 1.4, and 1.6 Mg m^–3 and maintained at gravimetric water contents of 0.10, 0.20, and 0.24 kg kg^–1. Soil samples were placed in air-tight mason jars fitted with rubber serum stoppers for gas sampling. Carbon dioxide evolution from microbial respiration was monitored for an incubation period of 18 to 26 d. Aeration became limiting to microbial respiration at a bulk density of 1.6 Mg m^–3 and 0.24 kg kg^–1 water content (97% saturation). In sewage-sludge-amended soil compacted to a bulk density of 1.4 Mg m^–3 and maintained at a range of water contents from 0.09 to 0.30 kg kg^–1, aeration became limiting to microbial respiration at a water content of 0.26 kg kg^–1 (77% saturation). These data illustrate the limitation of using percent saturation or water-filled pore space as an index characterizing soil aeration, particularly when soils of differing bulk densities are compared. Respiration rates in the inorganic-fertilizer- and sludge-amended soil samples were equivalent.

Received for publication October 28, 1988.