Application of an augmented nitrification assay to elucidate the effects of a spring barley crop and manures on temporal variations in rates

Abstract

The effects of crop plants and farmyard or poultry manure applications on temporal variations in nitrification rates were measured in a field experiment. In order to elucidate factors which may have been governing such rates, an augmented nitrification assay was applied. The basis of the assay was to measure nitrification rates under circumstances where substrate, i.e. ammonium-ion, and water and spatial constraints had been removed. Nitrification rates showed marked temporal variation, of over one order of magnitude, throughout the growing season. Nitrification rates were also similarly increased when substrate and spatial constraints were removed, but distinct temporal variations still persisted. The pattern of such variations varied according to assay conditions in the augmented nitrification assay. Barley plants had a statistically significant effect on nitrification rates, positive early in the growing season and negative at the end. Manures stimulated nitrification, with poultry manure having a greater effect than farmyard manure, and there was evidence for a relationship between heterotrophic and autotrophic activity. Factors other than ammonium-ion concentration and water or spatial restrictions must also regulate nitrification rates in mineral soils; these could include population size or interactions.