1. Substrates for Promoting the Restoration of Slate Waste Tips in Wales
Julie Williamson, Edwin Rowe, Mark Nason, Davey Jones, John Healey and Peter Holliman
Institute of Environmental Science, University of Wales Bangor, Deiniol Road, Bangor, Gwynedd, WALES. LL57 2UW
Introduction
We address best management practices for restoration at Europe’s largest slate quarry, Penrhyn, N. Wales. Penrhyn is on the boundary of the Snowdonia National Park and adjoins both broadleaf woodland and dry acid heathland ecosystems: both are targeted in Penrhyn’s restoration strategy.
Experimental Design, Materials and Methods
Sustainable plant establishment on slate requires:
Accumulation of organic matter (a reservoir for nutrients and water)
Active soil microbial population (to enhance mineralisation of organic-bound nutrients)
Hypotheses
Applying NPK fertiliser alone would provide only a short-term, luxuriant nutrient supply and would not sustain an active microbial population
Amendments of high organic C content relative to N and P would provide for the C-requirement of an active heterotrophic microbial population and a slower release source of N and P for plant uptake.
A field experiment was set up in Spring 2000 to test the effects of different slate amendments on plant establishment. Tree species of local provenance were planted in substrates which differed in levels of water and/or nutrient management in a 6x3x3 factorial design.
Tree species:
Oak, alder, rowan, birch, gorse, willow
Water retentive treatments:
None (i.e. slate)
Slate + polyacrylamide (PAM) gel
Subsoil (boulder clay)
Subsoil from below dry acid heath on site was spread over slate to a depth m: it differed from slate predominantly in total C, N, stone content and pH.
Fertiliser treatments:
No fertiliser
NPK fertiliser
Digested sewage cake + deinked paper pulp mix
The mix gave a C:N ratio between 15 – 20, to allow for slow release of N through microbial mineralisation. NPK fertiliser was applied at a rate to deliver N equivalent to that assumed to be mineralised from the sewage-paper mix in the first year.
Pocket Planting
PAM gel and NPK treatments were placed at the base of 3 litre pockets (Fig. 1) whilst the sewage-paper mix treatment was mixed with the contents of the pocket prior to backfilling and inserting the tree plug or willow cutting. Plants were watered, ‘mulched’ with slate and fenced.
Results and Discussion
Fewer plants died in subsoil than in slate or slate+PAM gel (Table 1). Whilst PAM gel had been used at the recommended rate, we found that once fully wetted, it moved upwards pushing the tree plug out of its pocket (Plate 1). This caused some early tree mortalities.
Fewer plants died in the sewage-paper mix than in no fertiliser or NPK treatments (Table 1). This suggests that the nutrient content of sewage-paper mix was not as important to initial survival as its potential for water retention.
Table 1. Initial tree mortality (numbers of dead trees) two months after planting. (450 trees per treatment).
Water retention agent Fertiliser type
Slate Slate+PAM Subsoil 2 test No fertiliser Sewage-paper mix NPK 2 test
P< P<0.01
Plate 1. A newly-planted tree plug displaced from its pocket by swelling PAM gel.
Destructive sampling of some of the pockets was undertaken within two weeks of assessing tree mortalities, to establish baseline levels of substrate physical, chemical and biological properties.
Neither subsoil nor PAM gel treatments significantly improved soil moisture retention when tested across all fertiliser treatments (Table 2). However, sewage-paper mix increased moisture retention greatly and a significant interaction between water retention and fertiliser type was detected. Excluding sewage-paper mix treatment from analysis, both slate+PAM and subsoil treatments did significantly improve water retention (data not shown). This may explain the lower tree mortality seen in subsoil.
Table 2. Selected properties of slate waste and subsoil four weeks after amendment with water retention agents and fertiliser (means of 9 replicates per treatment).
ns P>0.05 * P<0.05 ** P<0.01 *** P<0.001
Conclusions
Planting in subsoil reduced early mortality in trees due to its water retention capacity. Subsoil is often widely available at quarry sites and should be stored separately to overburden.
PAM gels improved moisture retention but methods for application need to be refined to avoid tree plug displacement.
The sewage-paper mix provided moisture retention, nutrients and organic matter. Both components of the mix were locally available and free. It is too early to say whether it can sustain plant and microbial establishment.
Acknowledgements
The project is funded for three years by the European Commission Directorate General for Environment, which will match industry spend. Our industry partners are Alfred McAlpine Slate, Slate Ecology Company, Wales; Dan Morrissey Ltd., Ireland; Pizzaras-Villar del Rey, Spain.
Fig. 1. Schematic diagram of pocket planting design on slate waste.