Effect of wheat roots on denitrification at varying soil air-filled porosity and organic-carbon content

Summary The effect of the air-filled porosity and organic-matter content of the soil on denitrification with non-limiting NO₃^– concentrations was studied in unplanted pots and in pots sown to wheat. Four organic-C levels were established by using pure and mixed soil material from a Bt horizon with 0.12% organic-C and an A_p horizon with 1.31% organic C from a mollic luvisol. A range of air-filled porosities from 3% to 25% during denitrification assays was obtained by varying soil compaction. Beyond a 10% to 12% threshold of air-filled porosity the denitrification rates were at an insignificant and constant level in planted as well as in unplanted soil for all organic-C contents. Below this threshold denitrification increased exponentially with decreasing air-filled porosity. In planted soil the excess of denitrification over that of unplanted soil was inversely related to air-filled porosity. This rhizosphere effect on denitrification, which was confined to air-filled porosities lower than 10%–12%, became significantly greater with increasing soil organic-C content. The findings indicate that root dependent respiration amplifies O₂ depletion in the rhizosphere and may accelerate the onset of denitrification in planted soil.