Soil ammonium accumulation after sod cutting hampers the restoration of degraded wet heathlands

Summary

In the late 1980’s, recognition of the deterioration of wet heaths and mat-grass Nardus stricta swards led to the introduction of restoration measures in the Netherlands. Sod-cutting was one technique used. Whilst in a few cases this led to successful restoration of species-rich wet heath, in many the result was species-poor dwarf shrub vegetation with few endangered heathland plant species. Accumulation of ammonium in the soil (which inhibits germination) was suspected as a possible reason for poor restoration success. A study was therefore undertaken to look at the effects of sod-cutting on soil ammonium concentration and effects on seed germination and seedling survival.

Study sites: Two wet heathlands, the ‘Leemputten’ nature reserve (8 km east of Ermelo) and ‘Havelte-Oost’ in the central Netherlands were selected.

Sod-cutting: In spring 2000, permanent plots (sod-removal and control) were randomly established. At Leemputten sods were cut in April from six purple moor-grass Molinia caerulea-dominated 2.5 m x 2.5 m plots. Havelte-Oost was divided into two zones, one M.caerulea-dominated and one cross-leaved heath Erica tetralix-dominated. Sods were cut in May from four 2 m x 3 m plots.

Soil and groundwater: Soil samples from the upper 10 cm of soil were collected monthly (April 2000 to August 2001. Groundwater level was recorded using piezometers on the same dates that soil samples were collected.

Germination and survival experiments: Two endangered wet heath plants, meadow thistle Cirsium dissectum and devils’-bit scabious Succisa pratensis, were used to test the effects of ammonium (NH₄)2SO₄ concentration (five replicates; five concentrations i.e. 0 , 100, 250, 500 and 1000 μmol/l sprayed weekly) on germination and survival. One hundred PVC cylinders (15 cm diameter, 20 cm high), were filled with heathland soil (pH 4.3) in a glass house. Twenty seeds of Cirsium were placed on the soil surface in 25 cylinders and, 20 Succisa seeds similarly in 25. Germination was followed over 82 days commencing 30 June 2000. On the soil surface of 50 cylinders, two seedlings of either Cirsium or Succisa were placed. The same ammonium solution treatments were applied. Survival was recorded over 10 weeks.

Germination in the field was quantified during the growing season, with the presence and cover of seedlings estimated in a 1 m x 1 m quadrat within each sod-cut plot using the Braun-Blanquet approach.

Soil ammonium concentrations: In Leemputen, ammonium concentrations were similar in the control and sod-cut plots two weeks after cutting. Subsequently concentrations in the sod-cut plots increased strongly with significant differences from June 2000 until May 2001, reaching maximum values of 572 and 327 μmol/kg dry soil at the end of February 2001 in the 0-5 cm and 6-10 cm layers respectively. No increase in the untreated control plots was recorded. In the Molinia-dominated plots of Havelte-Oost similar differences arose, but with a sharp decline in ammonium concentrations in November, possibly due to heavy rainfall. In the Erica-dominated zone, ammonium concentrations were much lower than in the Molinia-dominated plots but concentrations were generally higher in the sod-cut plots.

Groundwater: The groundwater table at Leemputten was more or less stable, ranging from 10 to 2 cm below the soil surface during spring and summer respectively. In Havelte-Oost much larger fluctuations were found approximately 90 cm to 10 cm below the soil surface.

Germination in the field: Maximum seedling cover reached 38% at Leemputten (August 2001) but only 13% and 4% for the Molinia- and Erica-dominated plots respectively at Havelte-Oost. The number of species that germinated in the sod-cut plots was small with the greatest diversity (seven species) found at Leemputten. In both areas only one target species, oblong-leaved sundew Drosera intermedia, was found commonly in the sod-cut plots; the average number of target species was less than two. The largest number of seedlings could be attributed to the occurrence of four common non-target species.

Germination: The cumulative germination percentage and survival of Cirsium seeds in the control was 60%, which was significantly higher than the 1000 μmol treatment (approx. 36%) with the 100, 250 and 500 μmol treatments with intermediate effects. At the end of the experiment there was a highly significant negative correlation between Cirsium germination and ammonium concentration.

In general germination of Succisa was very low. The maximium (15%) was recorded in the control treatment, followed by the 250, 100, and 1000 μmol treatments. None germinated in the 500 microM treatment. At the end of the experiment there was a significant negative correlation between Succisa germination and ammonium concentration i.e. Succisa germination was reduced as ammonium concentrations increased.

Seedling survival: There was a similar pattern found for seedling survival as germination. For Cirsium the 0 and 100 μmol treatments had the highest survival (60 and 50% respectively) compared with the higher concentrations 250, 500 and 1000 (20, 30 and 10% respectively). Likewise a similar relationship was found for Succisa.

Ammonium concentrations: The ammonium concentrations in the soil moisture samples, averaged over the duration of the experiment, differed significantly between treatments. The highest concentration was found in the 1000 μmol treatment (252 μmol/kg dry soil), significantly higher than the 500 microM treatment (144 μmol/kg dry soil). Both had higher ammonium concentrations than those of the control, 100 and 250 treatments (47, 45 and 70 μmol/kg dry soil).

Conclusions: Following sod-cutting on the two heaths, germination and survival of target species was low. High ammonium concentrations followed sod-cutting and germination experiments suggest that these low rates are at least in part due to soil ammonium accumulation. Thus undertaken on its own under these conditions, sod-cutting appears not to be a successful restoration technique.


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