Potential for denitrification in cereal soils of northern Australia after legume or grass–legume pastures

A period of legume pastures can lead to a significant accumulation of soil nitrogen and organic carbon that could influence soil N transformations, including soil denitrification. A temperature-controlled (30°C) pot study was conducted to determine the effect of the addition of legume pasture residues on the denitrification in soils from three sites which had supported 4–20 y of legume pastures. The denitrification after 12 d of incubation was measured from the % loss of 15N-nitrate (applied at a rate of 40 kg N ha−1) which was quantified by mass balance and daily gaseous emissions of N2 and N2O. Without added residues, soil after 20 y of Mitchell grass–naturalised medic showed the highest potential for denitrification (53%, 1.8 kg N ha−1 d−1) and soil after 4 y of lucerne pasture had the lowest (36%, 1.2 kg N ha−1 d−1) while soil after 4 y of snail medic lost 46% (1.5 kg N ha−1 d−1) of applied 15N, as quantified by mass balance. When three times the quantity of above-ground residues was added, denitrification loss was ∼90% (∼3.0 kg N ha−1 d−1). Denitrification losses of the three soils were significantly correlated with the amounts of soil water-soluble C (WSC). With added residues, soil tended to emit N gases immediately after the soil was waterlogged. However, no emissions from soil without added residues were detected during the 2–3 d after waterlogging. Nitrogen losses measured by gas emissions were within the range from 78 to 136% of that measured by mass balance. Losses determined by the two methods were significantly correlated (R2=0.67). The quantity and time pattern of N2O emission bore no significant relationship to the total gaseous emission and constituted only a small part (0.8–4.1%) of total N gases emitted. After a period of legume pasture, soil is likely to lose a substantial amount of N by denitrification and the addition of plant material can promote denitrification if soil is subjected to heavy rainfalls during summer months.