The Responses of Molinia Caerulea and Erica Tetralix to Soil Aeration and Related Factors: II. Gas Concentrations in Soil Air and Soil Water

Abstract

Previous studies (Sheikh 1969) described the growth of Molinia caerulea and Erica tetralix on some wet-heath sites at Bramshill Forest, Hampshire, which were described and named Valley-bog, Central associes, wetter Molinietum and drier Molinietum. Both species grew most slowly on the Valley-bog site, and best on the drier Molinietum. In the case of Molinia, differences of growth among sites appeared to be due to different nutrient conditions (see also Loach 1966) since the differences almost disappeared when nutrients were added to experimental plots. In the case of Erica tetralix, however, additional nutrients had no effect in overcoming the poor growth on the Valley-bog site and it was decided to investigate the aeration factor. This paper examines the variation of soil aeration, and related factors, at different depths and times on four sites, as a preliminary to controlled experiments on the significance of the aeration factor. Russell (1952) has emphasized the need of characterizing soil aeration by parameters that are of significance to plant growth. Parameters that describe, or allow the calculation of, conditions at the interface between the plant root and the soil system are an excellent means to this end. In soil, active plant roots are probably covered by a film of water. This is particularly true in the case of the roots growing in wet soils such as those studied in the present work. The concentration of gases at the root surface will depend upon the composition of the soil air, the transfer of gases across the gas-liquid interface, their diffusion across the liquid phase, and the rates of chemical and biological processes involving these gases in the soil. The effects of these various factors will be reflected in concentration gradients from the atmosphere above the soil to the root surfaces, first through the gaseous phase in the soil and then across the liquid phase. It would seem, therefore, that measurement of the concentration of gases in the liquid phase, although not identical with conditions at the root surfaces, does at least indicate these conditions better than the more widely employed analyses of samples of the gaseous phase. Studies on root distribution (Sheikh & Rutter 1969) showed that E. tetralix was a more shallow-rooting species than Molinia caerulea, but even in the latter about 950/ of the roots were found in the first 25 cm of soil on the sites studied. In view of this, analyses of gases in the soil were made mainly in the first 25 cm, although samples from 60 cm, i.e. below the permanent water-table, were also analysed.