The effect of organic amendment, water content, and oxygen on the incorporation of 15N2 by some agricultural and forest soils

Abstract

Thirty samples of nine soil profiles were variously treated and incubated in aerobic and in anaerobic 15N-labelled atmospheres in the laboratory. The addition of glucose to agricultural soils stimulated nitrogen fixation, especially under anaerobic conditions, but glucose had little or no consistent effect in most forest soils. The addition of straw litter to aerobically incubated waterlogged agricultural soils caused increased fixation. In general, however, and especially in forest soils, there was no consistent effect of either litter addition or waterlogging. It may be noted that in many cases greater fixation occurred in waterlogged soil–litter mixtures when incubated anaerobically than when incubated aerobically. About 80% of all treatments showed greater fixation under anaerobic than under aerobic conditions. Nitrogen incorporation during 28 days at 25 °C ranged from 0 to 70 μg N/g dry weight. Estimates on an area basis for a 17-cm profile depth ranged from 2 to 200 kg N fixed/ha per year.Azotobacter was confined to agricultural soils above about pH 6.5. Clostridium was widely distributed and usually abundant even in soils with pH values as low as 3.3. "Oligonitrophiles" were abundant but their significance remains uncertain.