FIELD STUDIES OF THE SOIL ATMOSPHERE

Abstract

SummaryA study was made of the concentrations of ethylene and oxygen in the soil atmosphere under field conditions at three contrasting sites. The duration of the experiments ranged from 6 weeks to 9 months. A clear relationship between high moisture content and both the production of ethylene and the depression of oxygen levels was found in a sandy loam, although there was great variability between replicate sampling points. Concentrations of oxygen in a heavy clay soil remained below 10 per cent (v/v) throughout the winter and spring, followed by a gradual rise to near‐atmospheric levels. Concentrations of ethylene were not correlated with those of oxygen, except in the winter, but rose logarithmically with soil temperature during the spring, increasing by twenty times over the range 4‐1 I °C. There was no significant difference between plots which had been subjected to different drainage treatments, and again there was great variability from point to point. In a more permeable clay soil, however, the levels of oxygen were higher, and were related to the proximity to drains and the height of the water table, but not to the levels of ethylene. The concentration of ethylene showed a similar logarithmic increase with temperature to that observed in the heavy clay, and exceeded I ppm throughout the period May‐July.The factors which have the greatest influence on the concentrations of ethylene in the soil were considered to be temperature, depression of oxygen concentration, the availability of substrates for microbial activity and the effect of soil moisture content on the air‐filled porosity of the soil. Concentrations of ethylene greatly in excess of those which, under some circumstances, inhibit root growth, are likely to be of widespread occurrence in heavy soils during the spring and early summer and in light soils when unusually wet.