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

Abstract

Concentrations of NO 2 −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 NO 2 − accumulates in soil solution, and whether NO 2 − from the soil profile is the source of NO 2 − in drainage and river waters. The occurrence of NO 2 − in the field was studied and laboratory incubation experiments carried out to determine the possible sources of NO 2 − in grassland soil. Field studies were carried out to determine the occurrence and spatial variability of NO 2 − 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 NO 2 − 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 NO 2 − 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 NH 4 +. Nitrate was repeatedly correlated to NO 2 − concentrations, suggesting that both nitrification and nitrate reduction may be responsible for NO 2 − formation. Spatially, nitrite occurred at random, basic geostatics producing only one variogram, showing an increase in NO 2 − concentrations with an increase in distance between sampling points. There was no pattern to the distribution of NO 2 − with depth, indicating differences in the ratios of the rates of NO 2 − production and consumption. Numbers of NH 3-oxidizers were consistently higher than numbers of NO 2 −-oxidizers, with some degree of variation between samples. The microbial aspects of NO 2 − formation are discussed, including partial recycling of NO 2 − via the NO 3 − pool, and possible causes of NO 2 − accumulation due to the inhibition of NO 2 −-oxidizing bacteria. Laboratory incubation studies were carried out in which measurable NO 2 − flushes were induced. Increasing soil pH and NH 4 + concentrations produced large NO 2 − flushes, which peaked after about 17 days of incubation, then rapidly declined. Soil incubated with urea produced NO 2 −N concentrations equivalent to those encountered in the field, suggesting that NH 4 + oxidation accounts for a significant proportion of NO 2 − formed in this soil.