The occurrence and possible sources of nitrite in a grazed, fertilized, grassland soil

Concentrations of NO2−N in land drainage and river waters in Northern Ireland in recent years have frequently exceeded EEC guide values. Very little information exists to indicate if and when NO2− accumulates in soil solution, and whether NO2− from the soil profile is the source of NO2− in drainage and river waters. The occurrence of NO2− in the field was studied and laboratory incubation experiments carried out to determine the possible sources of NO2− in grassland soil. Field studies were carried out to determine the occurrence and spatial variability of NO2− in a grazed, grassland soil. Plots receiving either 100 or 500 kg N ha−1 yr−1 were systematically sampled in May and October 1992. Concentrations of NO2− in soil were highly variable and ranged from 0 to 2.747 μg N g−1, the data being significantly skewed to the right. Correlation matrices and stepwise multiple regression analyses showed relationships between NO2− and a number of soil variables. Nitrite appeared to be related to variables which indicated its occurrence as a result of nitrification of either fertilizer- or urine-derived NH4+. Nitrate was repeatedly correlated to NO2− concentrations, suggesting that both nitrification and nitrate reduction may be responsible for NO2− formation. Spatially, nitrite occurred at random, basic geostatics producing only one variogram, showing an increase in NO2− concentrations with an increase in distance between sampling points. There was no pattern to the distribution of NO2− with depth, indicating differences in the ratios of the rates of NO2− production and consumption. Numbers of NH3-oxidizers were consistently higher than numbers of NO2−-oxidizers, with some degree of variation between samples. The microbial aspects of NO2− formation are discussed, including partial recycling of NO2− via the NO3− pool, and possible causes of NO2− accumulation due to the inhibition of NO2−-oxidizing bacteria. Laboratory incubation studies were carried out in which measurable NO2− flushes were induced. Increasing soil pH and NH4+ concentrations produced large NO2− flushes, which peaked after about 17 days of incubation, then rapidly declined. Soil incubated with urea produced NO2−N concentrations equivalent to those encountered in the field, suggesting that NH4+ oxidation accounts for a significant proportion of NO2− formed in this soil.