Tracing Water Movement, using Tritium, in a Peaty Gley Soil under Sitka Spruce

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Tracing Water Movement, using Tritium, in a Peaty Gley Soil under Sitka Spruce R. BOGGIE AND A.H. KNIGHT The Macaulay Institute for Soil Research, Craigiebuckler, Aberdeen AB9 2QJ.

1 Tracing Water Movement, using Tritium, in a Peaty Gley Soil under Sitka Spruce R. BOGGIE AND A.H. KNIGHT The Macaulay Institute for Soil Research, Craigiebuckler, Aberdeen AB9 2QJ. SUMMARY Tritiated water was used as a tracer to investigate the movement of soil water in a peaty gley soil planted with Sitka spruce which has reached 'windthrow' stage. The effects of three drainage treatments (different depths and spacings of ditches) were examined. Downward and lateral movement of tritium was observed in both the surface peaty horizon and in the mineral subsoil, with no evidence of either an impermeable horizon or a seepage layer. It was concluded, however, that at this site drainage treatment had less influence on water movement than ground topography. INTRODUCTION A stability problem exists in many large afforested areas where trees have reached windthrow susceptibility stage and, because of soil conditions, have poorly developed root systems. Shallow rooting nearly always occurs where the soil is wet and badly aerated (Armstrong et al, 1976), although the presence of an iron pan or indurated layer may also restrict root development. Large areas of peaty gley soils in the south of Scotland and north of England planted with Sitka spruce (Picea sitchensis (Bong.) Carr.), now around thirty years of age, are beginning to suffer from early wind-throw. These forests were mostly planted following superficial ploughing and minimal provision of deep drains. While this provided adequate conditions for establishment and growth in the early years, the very restricted root systems which developed have proved incapable of securely anchoring the trees to the ground. It has been shown that for crops with 15-18m top height every metre increase in height increases total volume yield by over 10 per cent (Savill et al., 1974). It is obvious that there is considerable economic incentive to improve the stability of a vulnerable crop reaching this stage of development. The only way of doing this is to extend the tree roots further into the soil which, in the case of peaty gley soils, means improving the aeration status of the profile. Fraser (1962,1967) has shown that deeper rooting and greater resistance to wind-throw on peaty gley soils may sometimes be achieved by improved drainage. In order to investigate the effect of installing new ditches in an established forest, the Forestry Commission laid out an experiment in 1965 combining different ditch depths and spacings, in Kershope Forest, Cumbria, a typical Sitka spruce forest on peaty gley soil. It was thought that an early indication of the efficacy of these ditches might be obtained by investigating the movement of soil water within the experimental area by a method which employs tritium as a tracer (Knight, et al 1972). Forestry, Vol. 53, No

2 180 Forestry TABLE 1. Site details of plots. Ditch depth Ditch spacing. Slope Angle of Ploughing Angle of Ditches (cm) (m) /upec ^er cent) to contour to contour SE SE NE " " " Site description The experimental area comprises several large plots, each approximately one hectare, in which ditches at different spacings and depths have been dug. Water movement was investigated in three of the drainage treatments (Table 1). Unfortunately the area is not uniform: both aspect and angle of slope vary between plots and so also does the depth of the peaty overburden of the soil. The surface of the soil is very uneven both because of the original strip ploughing and the more recent deposition of spoil from the ditches. The latter is particularly evident in Plot 1 where the ditches are closest together. The crop of Sitka spruce, planted in 1948, is uniform over the area and although the rides between plots are grassy there is virtually no ground vegetation under the trees. Most of the trees are between 12 and 15m high with an average breast height diameter of 15-17cm; a few attain a diameter of about 25cm. The plots are at an altitude of 200m O.D. and the annual rainfall is about 1500mm, distributed evenly over the year. The soil is a typical upland surface water gley (peaty gley) developed on Carboniferous clay till. The needle litter layer is underlain by 15-80cm of dark brown amorphous peat-like material with a loss on ignition of about 90 per cent. Below this, a slightly humose transitional horizon is evident in places but is generally absent, there being a sharp transition from the organic to the mineral horizons in which the average clay content (International System) is over 30 per cent. The total pore volume of the organic horizon varies between 80 and 90 per cent and in the clay subsoil from below to 50 per cent. The clay is plastic, very stiff and without much structure. METHOD In every experimental plot, eight tritium placement sites (each 1.0m X 0.5m) were selected and aligned with their long axes in the direction of the original plough furrows. The placement sites were positioned approximately mid-way between the ditches. At four of the sites in each plot, mci of tritium in 31 of water were sprinkled on the surface of the ground, allowed to soak in and a similar amount of plain water added. At the other four sites, the litter and peat horizons were first removed in blocks and the tritiated water applied to the surface of the mineral layer without further water being added. The peat was then carefully replaced and the surface restored as well as possible. Six weeks after application, which was in late spring, soil samples were taken

3 Boggie and Knight Tracing water movement under Sitka spruce 181 from within the placement sites and analysed for tritium. Where placement had been made on the surface a half-cylinder sampler 2cm diameter was used to take samples in 5cm steps down to the mineral horizon. Two cores per site were required to provide a large enough sample for analysis. Using a screw auger of the same diameter two samples of the underlying mineral material were taken at 0-10cm and 10-20cm, measured below the top of the mineral layer. At sites where tritium was applied to the interface between the peat and mineral soil the peat cores were divided equally, providing an 'upper' and 'lower' peat sample. The mineral soil was sampled at 0-10cm and 10-20cm as before. Samples were placed in polythene bags, sealed and transported to the laboratory for analysis. Soil water was extracted from most of the samples by squeezing or centrifuging, in the case of a few samples which were very dry, water was extracted after a known volume of inactive water had been added, and, knowing the moisture content of the sample, this dilution was taken into account in subsequent calculations. The water extracts were analysed for tritium using a liquid scintillation spectrometer. A second sampling was carried out eight weeks after the first and in addition samples were taken from outside the placement sites at 0.5m and 1.0m distance downslope, and at 0.5m upslope of each placement site. RESULTS Table 2 shows the percentage recoveries, within the treated rectangles, of the radioactive water added per unit area. Where the tritiated water was applied to the surface, the greatest amount accounted for six weeks later was the 37 per cent in Plot 1, whereas, when placement was made on the peat: mineral interface, recovery was as high as 77 per cent (Plot 3). By the time of the second sampling the quantity of accountable activity within each treatment area was considerably reduced, the reduction being approximately two fold where tritium was applied to the surface and four fold where it had been applied to the mineral horizon. The relative magnitudes of recovery between the three drainage treatments were the same at both sampling dates. TABLE 2. Percentages of applied activity recovered at each plot at the two sampling dates 1 st Sampling 2nd Sampling Plot Placed on: peat surface peat: mineral interface

4 182 Forestry TABLE 3. Counts per minute (X 10' 3 Jper unit volume of soil at the first sampling, samples taken from centres of treated areas. Placement on peat Horizon: Peat Mineral Placement on peat mineral interface Horizon: Peat Mineral 0-5 cm 5-10 cm cm cm cm cm Upper Lower Upper Lower Upper Lower Peat in Plots 2 and 3 only 20 cm and 15 cm deep respectively. Plots Table 3 shows the distribution of activity in the profiles 6 weeks after placement. Where placement of tritium had been made on the surface, the maximum concentration of activity was always in the 5-10cm layer, and the greatest downward movement into the mineral soil occurred where the peat horizon was thinnest. Where placement was at the top of the mineral soil, maximum activity was recorded in the upper mineral horizon. There was also substantial activity in the huiiiic horizons above the placement levels. An isopleth diagram (Fig. 1) illustrates the downward and lateral distribution of activity in the soil profiles in Plots 1, 2 and 3 at the second sampling, 14 weeks after application. Because of the considerable range and variation in the total amounts of activity between plots it was convenient to use geometric contour intervals (1, 2,4, 8 etc) of counts per minute (X10~ 3 ) per unit volume of soil. By this time, even when placement had been made on the peat surface, it is evident that in Plots 2 and 3 the zones of maximum activity were located in the mineral horizons, both within the treated rectangles and at 0.5m and 1.0m downslope. In Plot 1, however, where the peaty horizon is deeper, the zone of maximum activity has not penetrated the mineral soil, but is spread over several layers (10-30cm) which differed from each other by less than 10 (X10" 3 ) counts per minute. Where placement was at the peat: mineral interface, maximum activity in Plots 2 and 3 still occurred in the same upper mineral layers as at the first sampling, although in Plot 1 the zone of maximum activity had moved to a slightly lower level. Lateral movement in the mineral soil in Plot 3 is particularly obvious.

Boggie and Knight Tracing water movement under Sitka spruce 183 DISCUSSION Tritiated water applied to the surface or placed at depth in a soil can move or be lost by several processes.

5 Boggie and Knight Tracing water movement under Sitka spruce 183 DISCUSSION Tritiated water applied to the surface or placed at depth in a soil can move or be lost by several processes. A certain amount of self diffusion of water takes place as well as mass movement due to water flow; some activity will be translocated along roots and by micro-organisms. The biggest loss is to the atmosphere through evaporation from the surface and by transpiration of that taken up by the roots. The amounts of activity accountable at the two samplings was surprisingly high (Table 2) considering the poor site conditions. The ground surface within the ' forest was very uneven, making uniform placement of the tritium extremely difficult, but in spite of this the proportion of applied activity recovered after six weeks from the surface placement was nearly twice what was recovered elsewhere from deep peat with heath vegetation (Knight et al, 1972). On the whole, more was recovered where it had been applied at depth and covered over, the loss to the atmosphere in this case being chiefly by transpiration. Again, taking into account the site conditions, agreement between replicates was remarkably good. The use of radioactive labelled water in the investigation of ground water movement has again proved extremely sensitive and as applicable in a peaty gley soil under trees as it was in unplanted deep peat. Samples taken 0.5m up slope from the placement sites contained less than 10 per cent of the tritium in those from downslope locations, indicating that although some general dispersal of activity from the centre occurs, there is also a positive, directional movement. From the distribution of tritium at both sampling dates (Table 3, Fig. 1) it is apparent that there is movement both downwards in the profiles and laterally downslope. The zones of maximum activities increased in depth with time and although movement was slow there was nowhere any indication of an impermeable layer. Neither was the tritium held completely in the peaty horizons when applied to the surface, nor did it remain on the surface horizons of the underlying material when it was placed at the organic-mineral interface. There is nothing to suggest a seepage line at this interface. The evidence (Fig. 1) suggests that more lateral movement of water takes place in the upper horizons of the mineral soil than in the peaty overburden. It has not been possible to relate moisture movement, as may be inferred from the distribution of tritium, to drainage intensity. Plot 1 is the most intensely drained and Plot 2 least (Table 1), but unfortunately drainage was not the only variable. The depth of peat in Plot 1 is 30cm, in Plot 2 it is 20cm and in Plot 3 it is 15cm. The clay subsoil is also slightly variable in composition. The degree of slope, the aspect, the angle of the original ploughing to the slope and the directions of the subsequent deep ditches all differed between plots. The condition of the soil surface due to deposition of ditch spoil has already been mentioned. The deep ditches were installed in order to assess the effects and interactions of spacing and depth on soil conditions and eventually on tree growth. However, observations of a large number of borehole water-levels failed to reveal any drainage effect and although significant differences in gravimetric moisture contents between

6 restry.oxfordjournals.org/ at Penn State University (Paterno Lib) on September 15, 2016 Plot 2 Plot m 1.0 m «^ ~ Position of tritium applied Peat: mineral boundary Fig. 1. Isopleth diagrams showing radioactivity at different depths in the three plots where placement of tritium had been on the surface and at the peat: mineral interface. Contour intervals are at 1, 2,4, 8,16, 32, 64, 128 and 256 counts per minute (X10" J ). The positions where tritium was applied and the peat: mineral soil interfaces are shown.

Boggie and Knight Tracing water movement under Sitka spruce 185 plots were detected they were not correlated with drainage intensity either (Pyatt, 1973), the most intensively drained plot actually

7 Boggie and Knight Tracing water movement under Sitka spruce 185 plots were detected they were not correlated with drainage intensity either (Pyatt, 1973), the most intensively drained plot actually being the wettest. The combined results therefore confirm that in this experimental lay-out deep drainage did little to influence the moisture regime which was very much controlled by the nature of the soil and site features. It may very well be that deep drainage is not effective in peaty gley soils in upland areas and that a drainage system, such as the large ridge and furrow technique of Read et al (1973), to encourage rapid run off of surface water would be more beneficial. ACKNOWLEDGEMENTS We wish to thank the Forestry Commission for providing the facilities and opportunity of carrying out this investigation, and in particular to staff members Dr. D.G. Pyatt for providing information on the soils and to Mr. S.A. Neustein for valuable discussion at an early stage. We are grateful to Mr. R.A. Robertson for reading the manuscript, to Dr. H.G. Miller and Mr. A.T. Nicol for preparing the figure, and to Mr. H. Shepherd for supervising and carrying out much of the laboratory work. REFERENCES Armstrong, W., Booth, T.C., Priestley, P. and Read, D.J The relationship between aeration, stability and growth of Sitka spruce (Picea sitchensis (Bong.) Carr.) on upland Peaty Gleys. /. appl Ecol. 13, Fraser, A.I The soil and roots as factors in tree stability. Forestry 35, Fraser, A.I. and Gardiner, J.B.H Rooting and stability in Sitka spruce. Bull. For. Comma., Lond.. Knight, A.H., Boggie, R. and Shepherd, H The effect of ground water level on water movement in peat: a study using tritiated water. /. appl. Ecol. 9, Pyatt, D.G Physical and mechanical properties of soil types. Rep. Forest Res., Lond. 1973, Read, D.J., Armstrong, W., Weatherall, J The effects of cultivation treatment on water potential and soil aeration in wet heathland with special reference to afforestation./, appl. Ecol. 10, Savill, P.S., Dickson, D.A. and Wilson, W.T Effects of ploughing and drainage on growth and root development of Sitka spruce on deep peat in Northern Ireland. Proc. Int. Symp. Forest Drainage, Finland, p

HAVE YOU NOTICED at construction sites how a

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