Water-filled microbially habitable pores: Relation to denitrification

Abstract

The onset of a decline in net N mineralization, primarily due to denitrification, has been related to water contents, θW, or water-filled porosities, fW, but values of these characteristics vary among soils thus limiting their use as diagnostic criteria. An implicit assumption in the use of these characteristics is that water-filled pores of all sizes are habitable by microorganisms participating in denitrification. However, microorganisms are excluded from small pores, and the volume fraction of these pores varies with soil structure. The objective of this study was to determine if variation in the volume fraction of water-filled microbially habitable pores, θMHP, contributes to the variation in denitrification in soils of different structure. Data were used from studies with and without growing maize (Zea maize L.) plants. Variation in soil structure was achieved by using soils of different texture and organic carbon contents that were packed to two different levels of relative compaction. At the onset of a decrease in net N mineralization, values of θMHP exhibited less variability among soils than either θW or fW. The θMHP will be of greatest value as a diagnostic criterion for the decline in net N mineralization in soils exhibiting variation in the volume fraction of pores ≤ 4 µm diameter. Key words: Denitrification, habitable pore space, soil structure