Oxygen Uptake and Nitrification at Various Moisture Levels by Soils and Mats From Irrigated Pastures

Summary The oxygen uptake and nitrification rates of the organic layer (mat) and underlying soil from two irrigated white clover ( Trifolium repens L.) pastures produced on different soil types, were measured at moisture tensions ranging from pF o to 4.2. The highest rate of oxygen uptake was at pF 2 for the mats and at pF 2.8 for the soils. The highest rate of nitrification was at pF 2.8 for both mats and soils. The rate of oxygen uptake/g organic C at moisture tensions greater than pF 2 was of the same order of magnitude for both mats and soils, despite their having widely different organic C contents. This was probably due to the organic matter in both mats and soils having similar C/N ratios. At moisture tensions of pF 2 and less, the oxygen uptake/g organic C was usually greater for the mats than for the soils and this reflected the differences in physical characteristics of the two horizons. There was a greater production of nitrates in the mats than in the soils, at all moisture tensions. At low moisture tensions, there was no significant decrease in nitrates for the mats. Poorly aerated conditions prevailed at low moisture tensions in both soils and this coincided with low uptake of oxygen, respiratory quotients well in excess of unity, and a decrease in nitrate content, probably indicating denitrification.