Individual study: Effects of vegetation management and raising the water table on nutrient dynamics and vegetation change in a wet grassland
Ommes M.J.M, Olff H. & Altena H.J. (1996) Effects of vegetation management and raising the water table on nutrient dynamics and vegetation change in a wet grassland. Journal of Applied Ecology, 33, 576-588
Species-rich grassland communities have become rare in Western Europe because of agricultural intensification including fertilizer application, frequent cutting and lowering of the water table. The results of a restoration experiment carried out on grassland on peaty, heavy clay in the Netherlands are described here.
The experiment was under taken on a 13 ha improved field near Wageningen (52°54'N, 53°8'E), a formerly species-rich wet rush Juncus and purple moor-grass Molinia caerulea dominated pasture. After 1945, it was drained and annual applications of NPK (1968 to 1978) were made to convert it to high productivity (but low diversity) grassland.
Restoration started in 1978. Fertilizer application ceased and thereafter vegetation was mown and removed each June and September. In 1985, the field was divided into 1.5-2 ha areas in which ground water levels could be manipulated independently. Of the two reported here, one was raised to its former level ('wet field') and the other was left as it was ('dry field').
Management: In 1987, 9 years after cessation of fertilizer application, three management practices were implemented in the wet and dry fields (treatments (i) and (ii) five replicate 100 m² blocks; sod cutting one plot of 375 m² in each field):
i) cutting and removal of the hay in June and September (RR);
ii) cutting in June and September but mulching the cut biomass and leaving in situ (M M);
iii) sod removal (to 5 cm depth) in 1985, and thereafter cutting and removal of the hay in June and September (RS).
An analysis was made of the effect of the treatment regime and of raising the water table on nutrient availability, tissue nutrient concentration, dynamics of species and plant species replacement.
Water level differences: Over 4 years, the average water level difference between the wet and dry fields was 20-30 cm, the wet field being totally saturated in winter and spring. Raising the water level resulted in a more rapid establishment of species indicative of wet conditions, some of which invaded from nearby ditches.
No trend was discernible in soil pH, total C, N and P in the RR treatment; extractable P and K decreased sharply in the field with the raised groundwater level.
Biomass and nutrients Nine years after fertilizer application ceased, dry matter production had fallen from 10-11 to 6-7 t/ha/year. In the subsequent 5 years of the experiment it declined to 5-6 t/ha/year when all cut biomass was removed. and to about 4 t/ha/year after sod removal. Mulching caused an increase to 11 t/ha/year. No effect was seen of the raised water level.
The dry matter yield of the first June cut in the RR treatment decreased. The availability of K and to some extent of P appeared more important than N availability in explaining the decrease in dry matter production. The tissue nutrient concentrations were not influenced by the water table height.
Sod removal to a depth of 5 cm resulted in the lowest productivity and the lowest tissue concentrations of P, while tissue concentrations of N and It were not affected.
Conclusions: Raising the water table resulted in plant species indicative of wet conditions becoming dominant, independently of vegetation management. However, the removal of nutrients resulted in the appearance of smaller species, indicative of lower P and K availability.
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