LEACHING OF PHOSPHORUS FERTILISER APPLIED ON
CUTAWAY PEATLAND FORESTS RECENTLY ESTABLISHED IN
CENTRAL IRELAND.
RENOU, F.1, S. JONES2 AND E.P. FARRELL3
1,2,3
Forest Ecosystem Research Group, Department of Environmental Resource Management, University
College Dublin, Belfield, Dublin 4, Ireland.
1
Tel: 00-353-1-7067673, 2Tel: 00-353-1-7067719, 3Tel: 00-353-1-7067616
1,2,3
Fax: 00-353-1-7061102
E-mails: 1 [email protected], 2 [email protected], 3 [email protected]
Keywords: cut-over peatland forestry, phosphatic fertilization, leaching
Introduction
In Ireland, 8.1% of peatlands are being mined for fuel and horticultural products. In
the next three decades, it is estimated that a total of 80,000 ha of bogs (mostly raised bogs)
will come out of production. Their subsequent development will be one of the greatest
reclamation ventures in Europe. In the context of Irish governmental policy to double the
proportion of land under forest to 17% (1.2 million hectares) by 2035, a major contender for
this considerable cutaway land surface is commercial forestry. It is expected that up to 50,000
ha of cutaway peatlands could become available for afforestation.
Background information
Large-scale afforestation of milled, cutaway peatlands began in 1988. These milledcutaway bogs have considerable forestry potential, but inadequate research into establishment
and management techniques has been carried out on this particular site type. A research
programme, known as BOGFOR, has been established to focus on the specific problems
encountered in establishing a forest plantation on milled-cutaway peatland in central Ireland.
One aspect of this multi-task research programme concerns the environmental issues of this
afforestation scheme.
The development of such forest-crop ecosystems ought to be productive and above all
sustainable. A prerequisite for the satisfactory development of such a forest-crop is that
drainage and nutrient status are taken care of. The amount of mineral nutrients in peat soils is
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quite low. Phosphorus and potassium fertilisation is essential on young cutaway peatland
plantations to maintain tree growth at acceptable rates. Experiments monitoring tree
productivity and nutrient concentrations in tree foliage have shown that the growth response
peaked at 30-60 kg P/ha (Carey et al. 1985). The current recommended practice in Ireland is
to apply 42 kg of P/ha (350 kg of granulated rock phosphate) at the establishment of the trees.
In 1994, the Forest Ecosystem Research Group undertook an intensive survey of
3,000 ha of plantations established on cutaway peatland of the Midlands in 1989 (Jones &
Farrell, 1997). The results of foliar analysis suggest that phosphorus deficiency was
widespread in the plantations. This was unexpected as phosphate fertiliser applied after tree
planting was considered to be sufficient to sustain the tree crop. In the areas where
phosphorus was deficient, little or no vegetation grew, which would lead to the conclusion
that fertiliser had not been applied, had been washed away in heavy rainfall or had been
leached out.
Unlike mineral soils, in which phosphorus movement is limited, cutaway peat has a
very low capacity to sorb phosphorus. It is well established that the retention of phosphorus
is closely connected with levels of iron and aluminium. More recently, it has been
demonstrated that this connection is also present in peat soils (Kaila 1959, Bloom 1981,
Nieminen & Jarva 1996). The concentrations of soluble iron and aluminium ions – which are
able to form insoluble complexes with phosphates – are often very low in peat, which
suggests that P fertiliser application might lead to an increase of phosphate in soil water. In
the case where phosphatic fertiliser is applied at planting, the young trees may not be
physiologically able to respond fast enough to take up all these available nutrients. The
virtual absence of other vegetation (at least during the first year after plantation) means that
the overall uptake by vegetation is minimal and that most of this available phosphorus is very
mobile and predisposed to be leached. The high runoff and erosion risk present on cutaway
peatlands (Collins & Cummins, 1996) also accelerates the movement of this mobile
phosphorus within the landscape.
The interaction of cutaway peatland plantations with the environment and particularly
the possibility of losses of applied phosphorus from these ecosystems to drainage is
important for two reasons: (1) to assess the potentially deleterious effects (eutrophication)
that phosphorus losses may have on the ecology of downstream water bodies in the Midlands
(this could be critical for tourism and especially fishing to develop in this area; (2) the
potential inefficient use of phosphatic fertilisers essential to the establishment of a forest crop
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on cutaway bog may result in the poor tree growth and unproductive stands making the
proposed afforestation programme uneconomical.
Objectives
This long-term study was undertaken to investigate the movement of applied
phosphatic fertiliser on cutaway peatland plantations. The experiment endeavours to
determine:
-the qualitative contribution of surface runoff and/or vertical leaching to the removal of
phosphorus from the root-zone and to the possible contamination of waterbodies by
phosphorus fertiliser;
-temporal trends in the concentration of phosphorus in the leachates and the reasons for them.
While surface runoff is the main focus in the environmental monitoring experiment described
below, the role of vertical leaching is being investigated in a second section of the experiment
involving the set up of fertiliser trials in different peat types in order to establish to what
extent the inputs, concentrations, effects and trends of phosphorus can be modified. This
section is not dealt with in this paper.
Materials and methods
The regular measurement of phosphorus concentrations in runoff water collected from
newly planted cutaway bogs forms the basis of this experiment.
Experimental site
The experiment was set up on ‘Tumduff East’ cutaway bog belonging to the Lough
Boora Parkland, located 20 km from Tullamore, Co. Offaly, Ireland, 53°13’N, 7°42’E, 300m.
The experimental area has a known history of intensive drainage and peat harvesting using
the milling process. It consists of a reed swamp peat deposit directly overlying a blue/grey
silty clay. Due to the calcareous nature of this sub-peat mineral soil, a sufficient depth of peat
must be left in these areas to permit their afforestation by conifers. The thickness of the peat
layer left at Tumduff East varies but remains over the whole area greater than 60 cm. The socalled Phragmites peat or reed swamp peat type is very dense and poorly aerated. These
attributes along with active drainage and cambered surfaces make this area prone to
important surface runoff while annual precipitations average c. 1,000mm.
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Methods
In order to investigate the course and extent of phosphorus loss to drainage via
surface runoff, long-term collectors were designed to allow surface runoff water and eroded
sediments to be collected as it moves freely into the drains (Figure 1). The site consists of 22
bays (each 15m wide and 382m long) separated by a drain. The division of the area into two
comparable plots, A and B of c. 5 hectares each, permitted the investigation of the potential
impact of cultivation on phosphorus movement. Area A was left intact while area B was
cultivated (ripped and disced). Both experimental areas were planted on flat ground with
Norway spruce (Picea abies (L.) Karst.) in May 1999. Rock phosphate (Gafsa origin) was
applied by hand, following strips, at the rate of 175kg of rock phosphate/ha, i.e. 25kg of
P/hectare, which is half of the rate applied at establishment. A second application of rock
phosphate at the same rate will be applied in 2001. This split into two applications follows a
new recommendation by the Irish forestry board for afforestation on cutaway peatlands. In
total, 14 collectors have been set up to allow replicates. Surface runoff water and sediments
are collected weekly. Measurements started on both sites in May 1999, immediately after
plantation and 10 weeks before fertiliser was applied.
Forms of phosphorus in water are determined by analytical methods based around:
-Whether the phosphorus is in a dissolved or particulate form. This is differentiated by
0.45µm membrane filtration.
-Whether or not the phosphorus is molybdate (Mo) reactive. This is determined
colorimetrically with the Mo blue method by using ascorbic acid as the reductant (Murphy &
Riley 1962). As a general rule, the molybdate (Mo) reactive phosphorus is considered to be a
free and biologically available form of phosphorus.
Total phosphorus is determined on the sediment samples using the perchloric acid
digest method (Zasoski et al. 1977) and concentrations are then measured using the ICP
(Inductively Coupled Plasma).
Results and discussion
During the first 10 weeks of sampling before fertilisation (Figure 2), phosphorus
concentrations in runoff water remained low with a mean concentration of 26 µg/L. This
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figure agrees with concentrations that have been reported from unfertilised drained peatland
soil water: 20-30µg/L (Kenttämies 1981, Paavilainen & Päivänen 1995).
The application of phosphorus led to an immediate increase of P concentrations in the runoff
water with mean concentration (over a period of seven months) of 318 µg/L and
concentrations exceeding 2mg/L on occasion.
The difference in phosphorus concentrations measured in the cultivated area and noncultivated is so far insignificant.
Soluble and total P content were similar. This does not necessarily indicate that no
phosphorus was lost to drainage water in particular form, as particles greater than 450 micron
are hold in the nets collecting sediments and which are currently being analysed.
Fertiliser-induced leaching of phosphorus from peatlands drained for forestry has
been the subject of a range of studies in Scandinavia and Scotland (Särkkä 1970, Karsisto &
Ravela 1971, Kenttämies 1981, Malcom & Cuttle 1983, Ahti 1984, Ahti et al. 1998), and the
results have shown that phosphorus content in runoff increased after fertilisation. Kenttamies
reports an increase of phosphorus concentrations from 18µg/L of P before fertilisation to a
mean value of 128µg/L of P. The highest monthly mean was 300µg/L and the duration of the
leaching of phosphorus-fertiliser from a drained peatland may exceed 10 years.
Long term monitoring is necessary to form a clear picture of phosphorus movement
via runoff as rock phosphate constantly releases phosphorus over a relatively long period.
Seasonal variations are also predictable. Cold temperatures restrict the dissolution of the
fertiliser. A very wet and mild winter however would allow a considerable dissolution of the
fertiliser without any uptake by the vegetation.
In addition to this long term monitoring, further results from the fertiliser trials
assessing various fertiliser application procedures on different site types from the perspective
of minimising the loss of applied phosphorus from the root-zone, will contribute to the
establishment of management guidelines for the development of productive and sustainable
forest-crop ecosystems on Irish cutaway peatlands.
Conclusion
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While monitoring the movement of phosphatic fertiliser applied during the
establishment of plantation on cutaway peatlands, it was shown that phosphorus
concentrations measured in the runoff water had significantly increased after fertilisation.
However, while phosphorus is leaving the root-zone and running into the drains, it is
not determined in this experiment where the phosphorus is ultimately lost. The phosphorus
may not reach a P-sensitive water body. As runoff moves through the landscape and toward
water bodies, there is generally a progressive dilution of P through the addition of water.
Secondly, the effect of subpeat mineral soil (rich in calcium which is a phosphorus-holding
mineral) through its occurrence in the drains can have a strong effect on the retention of
phosphorus. The vegetation growing at the bottom of the drains (including filamentous algae
which have grown during the summer after fertilisation) may also have an important role to
play in the sorption of this soluble phosphorus.
If loss of phosphorus to drainage water is accepted as a natural consequence of this
type of forest management, considerations must be given to the possible effects of such a
nutrient on the ecology of water bodies downstream and to the solutions to alleviate these
effects if they are assessed to be noxious. It is suggested that a micro-catchment experiment
(and ultimately at subcatchment scale) should be set up in order to obtain a quantitative
estimate of the rate and amount of phosphorus loss to drainage water, to monitor the
streamwater quality above and below the experimental plots and finally to assess different
measures mentioned above which could minimise the loss of applied nutrients to surface
water runoff.
Acknowledgements
The financial support provided by the Forest Service (the Irish Department of Marine and
Natural Resources) and the European Union is gratefully acknowledged as well as the
assistance of Coillte Teoranta (the Irish Forestry Board) and Bord na Móna p.l.c..
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References
Ahti, E. 1984. Fertiliser-induced leaching of phosphorus and potassium for peatlands drained
for forestry. Proceedings 7th International Peat Congress, Dublin, Ireland, 18-23 June
1984. Vol iii, 153-63.
Ahti, E., Vuollekoski, M. & Joensuu, S. (1998) Quality of runoff water from old ditch
networks in Finnish peatlands forests. In: R. Sopo (ed.) The spirit of Peatlands:
Proceedings of the International Peat Symposium in Jyvaskylam Finland, 7-9 Sept
1998: 70-72.
Aro, L & Kaunisto, S. (1998) Forestry use of peat cutaway areas in Finland. In: R. Sopo,
(ed.) The spirit of Peatlands: Proceedings of the International Peat Symposium in
Jyvaskyla, Finland, 7-9 Sept 1998: 185-186.
Bloom, P.R. 1981. Phosphorus adsorption by an Aluminium-peat complex. Soil Sci. Soc. Am.
J., 45: 267-72.
Carey, M.L., Hammond, R.F. & McCarthy, R.G. (1985) Plantation forestry on cutaway
raised bogs and fen peats in Republic of Ireland. Irish Forestry, Vol 42 (2): 106-22.
Collins, J. F. & Cummins, Thomas (eds) (1996). Agroclimatic Atlas of Ireland. Working
Group on Applied Meteorology, University College Dublin, Ireland. 190 pp.
Jones, S. & Farrell, T. (1997) Survey of plantation forests on Bord na Móna cutaway bog.
Final Report. Forest Ecosystem Research Group, Report Number 20. Department of
Environmental Resource Management, University College Dublin, Dublin. 180 pp.
Kaila, A., 1959. Retention of phosphate by peat samples. Journal Agr. Soc. Finland, 31: 21525.
Karsisto, K. & Ravela, H. (1971) Washing away of phosphorus and potassium from areas
drained for forestry and topdressed at different time of the year. Suo, 22 (3-4): 39-46.
Kenttamies, K. 1981. The effects on water quality of forest drainage and fertilisation in
peatlands. Pub. Water Res. Inst. National Board on Waters Finl., 43: 24-31.
Malcom D.C. & Cuttle, S.P. (1983) The Application of fertilisers to drained peat: nutrient
losses in Drainage. Forestry, 56 (2): 155-73.
Murphy, J. & Riley, J.P. (1962) A modified single solution method for the determination of
phosphate in natural waters. Anal. Chim. Acta, 27: 31-6.
Nieminen, M. & Jarva, M. (1996) Phosphorus adsorption by peat from drained mires in
Southern Finland. Scand. J. For. Res., 11: 321-6.
Paavilainen, E. & Päivänen, J. (1995) Peatland Forestry: Ecology and Principles. Ecological
Studies 111. Springer - Verlag, Berlin. 248 pp.
Särkkä, M. 1970. On the influence of forest fertilisation on water courses. Suo, 21(3-4): 6774.
Zasoski, R.J. & Burau, R.G. (1977) A rapid nitric-perchloric acid digestion method for multielement tissue analysis. Comm. Soil. Sc. And Plant Anal., 8: 425-36.
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Fertilisation
Figure 2: Phosphorus concentrations +/- standard deviation of n+2 observation
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Molybdate Reactive Phosphorus (mg/L)
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