Soil seed bank composition in relation to the above-ground vegetation in fertilized and unfertilized hay meadows on a Somerset peat moor

Published source details Kirkham F.W. & Kent M. (1997) Soil seed bank composition in relation to the above-ground vegetation in fertilized and unfertilized hay meadows on a Somerset peat moor. Journal of Applied Ecology, 34, 889-902.
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Summary

In hay meadows in southwest England, fertilizer treatments have been shown to have significant effects on abundance of individual plant species and species diversity in the above-ground vegetation. This study investigated soil seed bank composition in relation to the above-ground vegetation in fertilized and unfertilized hay meadows on a Somerset peat moor, and implications of these findings for restoration of high diversity grasslands on previously fertilized meadows.

Study area: The study was carried out in hay meadows on a peat moor in Somerset in the south-west of England (51º 12'N, 2º49'W) in 1991. The plant communities corresponded to MG5 (crested dog's-tail Cynosurus cristatus-black knapweed Centaurea nigra meadow and pasture) and MG8 (Cynosurus cristatus-marsh marigold Caltha palustris flood pasture) of the UK National Vegetation Classification (Rodwell 1992).

Experimental design: Study plots had either never received inorganic fertilizers or received inorganic nitrogen (N), phosphorous (P) and potassium (K) each year over the previous 5 years. Traditional management each year (hay cutting in July followed by aftermath grazing) had been maintained.

Six sets of 12, 2 x 4 m plots (72 in total) were established with one set within each of three unfertilized controls, and in each of three plots that had received 200 kg N/ha/yr from 1986 to 1990. Meadows that had received repeated applications of inorganic fertilizers during the preceeding 5 years were compared to unfertilized ones.

Seed bank: Assessments were made of the soil seed bank on 19 March 1991 (20, 5-cm diameter, 5-cm deep soil cores/plot). These soil samples were bulked, placed in seed trays and watered as required to keep them moist. Over the subsequent 19 months seedlings were identified, counted and removed at about monthly intervals.

Plant cover: In the plots above-ground vegetation cover was assessed in May 1991 within four 50 x 50 cm quadrats/plot.

A substantial number of seeds of species that normally germinate soon after shedding (i.e. type I species) survived in the soil until March before germinating. This was probably due to a rapid change from very dry to waterlogged soil in the autumn, with inhibited germination. Type I species accounted for a disproportionately large fraction of total ground cover (average 42% cover) relative to the number of species involved (18% of total), compared with type III (35% cover; 32% of species) and type IV (18% cover; 28% pf species) species groups.

The type I group showed the closest correspondence between vegetation cover and seed bank abundance, type IV (long-term persistent seed bank) species showed the least.
Abundance was fairly even between grasses (44%) and forbs (48%) in terms of ground cover.

Conclusions: Five years of fertilizer application increased the balances of those species that flourish and are more competitive in higher nutrient soils but reduced the abundance of a larger number. These effects were mirrored by differences in seed bank composition. The effects of fertilizer application were greatest within species that normally germinate soon after being shed. Fertilizers caused a 4-fold shift in the balance of species in the seed bank towards those that were previously identified as responding positively to N in terms of vegetation cover, with a smaller concomitant shift from type IV species to type III.

Where the aim is to restore high species diversity to previously fertilized meadows, these results show the importance of reducing soil fertility and encouraging seed production of those species that have declined.

References

Rodwell J. (1992) British Plant Communities Vol.3. Grassland and Montane Communities. Cambridge University Press, Cambridge, UK.

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