Loss of nitrogen in compacted grassland soil by simultaneous nitrification and denitrification

The soils of mid-Wales in grazed permanent pasture usually exhibit stagnogley features in the top 4–10 cm even though on sloping sites, they are freely drained. Nitrogen is often poorly recovered under these conditions. Our previous studies suggest that continuing loss of available N through concurrent nitrification and denitrification might provide an explanation for poor response to fertilizer N. The work described was designated to further test this proposition. When NH ₄ ⁺ –N was applied to the surface of intact cores, equilibrated at –5kPa matric potential, about 70% of NH ₄ ⁺ –N initially present was lost within 56 days of incubation. Study of different sections of the cores showed a rise in NO ₃ ^- level in the surface 0–2.5 cm soil layer but no significant changes below this depth. The imbalance between NO ₃ ^- accumulation and NH ₄ ⁺ disappearance during the study indicated a simultaneous nitrification and denitrification in the system. Furthermore, the denitrification potential of the soil was 3–4 times greater than nitrification potential so no major build-up of NO ₃ ^- would be expected when two processes occur simultaneously in micro-scale. When nitrification was inhibited by nitrapyrin, a substantial amount of NH ₄ ⁺ –N remained in the soil and persisted till the end of the incubation. The apparent recovery of applied N increased and of the total amount of N applied, 50% more was recovered relative to without nitrapyrin. It appears that addition of nitrapyrin inhibited nitrification, and consequently denitrification, by limiting the supply of NO ₃ ^- for denitrifying organisms. Emission of N₂O from the NH ₄ ⁺ amended soil cores further confirmed that loss of applied N was the result of both nitrification and denitrification, which occurred simultaneously in adjacent sites at shallow depths. This N loss could account for the poor response to fertilizer N often observed in pastoral agriculture in western areas of the UK.