Aeration of upland soils under afforestation

Abstract

The relationships between soil aeration and other physical properties of two upland soils prone to waterlogging, a peaty gley in the Kielder forest region of Northumberland (Falstone forest) and a deep peat in Eddleston forest of the Borders region of Scotland, were investigated.

The investigations examined the soil atmosphere composition, moisture potential, depth to water table, temperature and rainfall/ throughfall throughout 1978 and 1979 at Falstone, and during 1979 at Eddleston. Measurements were made under stands of Picea sitchensis and Pinus contorta, and neighbouring areas of the original vegetation. Samples removed from the soil pore space were analysed for oxygen and carbon dioxide concentrations by gas chromatography.

Oxygen concentrations in the soil were found to be greaterunder the tree species than the unplanted areas, at all depths, duringthe moisture deficit period. During the moisture deficit period thesoil dried to a greater depth under the trees, and to lower matricpotentials above the water table. The oxygen concentrations were found3 -3to be near atmospheric (0.21 m m ) when the matric potential fell below3 -3-5kPa, and could be as low as 0.01m m , or even zero, at higher potentials. The oxygen concentrations were also found to decrease, and carbon dioxide concentrations increase, with increasing depth under all species corresponding to increasing matric potential. Statistical analysis revealed a small but significant improvement in aeration under Pinus contorta over Picea sitchensis on both soil types. It was also revealed that lower oxygen concentrations and higher carbon dioxide concentrations could be expected for matric potentials above about -5kPa, when soil temperatures were at their maximum (about 10°C). Thesetrends were observed in both soils and during both years of study.

The moisture release characteristics, bulk density and porevolume fraction of the soils were also measured, and increasing oxygenconcentrations over a range of matric potentials from 0 to -lOkPa foundto correlate with an increase in gas filled pore volume.

Gaseous diffusion through peat cores was measured over a range2 16of 0 to -lOkPa matric potential and the equation D/Dq = 0.65 e ‘ found to describe the increase in diffusivity (D/D ) with gas filled pore space (£g )-

The respiration rate of peat cores was found to vary randomly over a range of gas filled pore volume fractions (from 0 to about 0.4) but increase with increasing temperature from 5°C to 30°C, having a Q of about 3.

An investigation into root growth rate of Pinus contorta andPicea sitchensis cuttings grown in a range of carbon dioxide concentrations3 -3(0.01, 0.06, 0.12 and 0.18 m m ), showed no reduction in growth rate3 -3when oxygen concentrations were 0.21 m m , but growth ceased when oxygen3 _3concentrations were 0.01 m m and the carbon dioxide concentration greater than this value.