LOSS OF NITROGEN IN COMPACTED GRASSLAND SOIL BY SIMULTANEOUS NITRIFICATION AND DENITRIFICATION

The soils of mid-Wales in grazed permanent pasture usually exhibit sta gnogley features in the top 4-10 cm even though on sloping sites, they are freely drained. Nitrogen is often poorly recovered under these co nditions. Our previous studies suggest that continuing loss of availab le N through concurrent nitrification and denitrification might provid e an explanation for poor response to fertilizer N. The work described was designated to further test this proposition. When NH4+-N was appl ied to the surface of intact cores, equilibrated at -5kPa matric poten tial, about 70% of NH4+-N initially present was lost within 56 days of incubation. Study of different sections of the cores showed a rise in NO3- level in the surface 0-2.5 cm soil layer but no significant chan ges below this depth. The imbalance between NO3- accumulation and NH4 disappearance during the study indicated a simultaneous nitrification and denitrification in the system. Furthermore. the denitrification p otential of the soil was 3-4 times greater than nitrification potentia l so no major build-up of NO3- would be expected when two processes oc cur simultaneously in micro-scale. When nitrification was inhibited by nitrapyrin, a substantial amount of NH4+-N remained in the soil and p ersisted till the end of the incubation. The apparent recovery of appl ied N increased and of the total amount of N applied, 50% more was rec overed relative to without nitrapyrin. It appears that addition of nit rapyrin inhibited nitrification, and consequently denitrification, by limiting the supply of NO3- for denitrifying organisms. Emission of N2 O from the NH4+ amended soil cores further confirmed that loss of appl ied N was the result of both nitrification and denitrification, which occurred simultaneously in adjacent sites at shallow depths. This N lo ss could account for the poor response to fertilizer N often observed in pastoral agriculture in western areas of the UK.