Can management compensate for atmospheric nutrient deposition in heathland ecosystems?

Summary

Throughout Europe atmospheric nutrient deposition, has contributed to widespread changes in the structure and function of many heathland ecosystems. Increased nitrogen inputs, in particular, have been suggested as one possible cause of the replacement of heather Calluna vulgaris by grasses like wavy hair-grass Deschampsia flexuosa. Heathland management, primarily aimed at the prevention of invasion by scrub and trees, also plays an important role in regulating or reducing impacts caused by increased atmospheric nutrient loads. This study investigated the effectiveness of three management techniques, mowing, prescribed burning and sod-cutting, on removal of nutrient elements from the different heathland compartments and the effects of leaching on the nutrient levels.

Study area: The study was undertaken at Lueneburg Heath (Lüneburg Heide) nature reserve Lower Saxony, north-west Germany (105 m a.s.l.). The site has the largest complex of heathlands (about 5000 ha) in north-west Germany. Prevailing soils are nutrient-poor podzols with topsoil pH ranging between 3.3 and 3.5.

Management: Management measures (treatments) were carried out during the winter of 2001–02. The effectivness of mowing, prescribed burning and sod-cutting, on removal of nutrients from the different heathland compartments was compared:

Mowing - primarily applied to heather Calluna vulgaris dominated stands in order to initiate its regeneration. Above-ground biomass was cut with a mower at a 10 cm height.

Prescribed burns - undertaken in fine weather, burns were of low-intensity characterized by 'low temperature fires'. Burns removed above-ground biomass, while the organic layers, as for mowing, remained untouched. Burning was comparable to mowing in that it was primarily applied to heather dominated stands in order to initiate regeneration from seed and root stocks.

Sod-cutting - the above-ground biomass and the organic layers were completely removed and the A-horizon was partially removed with a rotary hoe. Sod-cutting was applied to stands where wavy hair-grass Deschampsia flexuosa had partially replaced dwarf shrubs such as Calluna.

Sample plots: Twelve 20 × 40 m² plots were randomly established (four replicates per treatment) within a 100 ha area of heathland. Each was divided into two 20 × 20 m² subplots. A treatment was carried out in one subplot and the second served as an untreated control. The treatment applied was appropriate to dominant species composition i.e. subplots selected for mowing or burning were heather-dominated average vegetation covers were: mown subplots - heather 80%, Poaceae 10%, cryptogams 63%; burned subplots - heather 56%, Poaceae 19%, cryptogams 42%); subplots selected for sod-cutting were grass dominated: heather 46%, graminoid species 50% and cryptogams 59%. The age of heather in plots ranged from 10 to 15 years.

Determination of atmospheric nutrient deposition: Atmospheric nutrient deposition was analysed with 12 bulk deposition samplers installed in the area were the sample plots were situated. Samples were collected biweekly from winter 2001 to winter 2002, starting immediately after the management treatments took place (see orignal paper for methods used for this and other nutrient analyses).

Determination of nutrient loss by leaching: Nutrient loss by leaching was determined by means of a lysimeter and tension-controlled porous cup soil water samplers. Soil water samplers were installed at depths of 100 cm and samples were taken simultaneously and at the same intervals as deposition samples. To avoid effects of soil disturbance on nutrient measurements, samplers were installed 4 months prior to the experimental heathland management. The mean annual nutrient output rate was calculated for each treatment and the corresponding controls.

Nutrient stores in above-ground biomass and soil: Nutrient stores in the above-ground biomass, organic layer and A-horizon were determined in the treated subplots before and immediately after the management measures took place. Prior to chemical analyses, all samples of above-ground biomass, organic layer and A-horizon, were oven dried at 105 °C and weighed. N, C, Ca, K, Mg and P content was determined.

Calculation of increased leaching rates: After application of the treatments, leaching rates increased in comparison with the controls, as expected. Vegetation removal led to increases in both the amount of percolating water and in the quantities of leached nutrients. Leaching rates were expected to be particularly high in sod-cut plots compared with mown and burned plots because of complete vegetation removal and partial removal of the humus horizons. To calculate an approximate value for the quantities of increased nutrient loss by leaching as a result of the treatments, a linear decrease of leaching rates was assumed (as a result of the development of the vegetation) until leaching rates would theoretically return to levels as (i.e. for mown and burned subplots after 5 years, sod-cut subplot after 15 years). It was therefore assumed that leaching rates would decrease by 1/5 per year on the mown and burned subplots and by 1/15 per year on the sod-cut subplots.

Calculation of nutrient balances: The ratio of the net output of nutrients (as a result of the treatments applied) and the annual net input was calculated. This ratio thus described the period of time (in years) in which the quantities of nutrients removed as a result of a particular treatment were equivalent to atmospheric nutrient inputs.

Atmospheric deposition: Atmospheric nutrient deposition was similar between the 12 samplers, thus deposition was considered equal for all plots. N input was 22.8 kg/ha/year. P deposition rates were below the analytically detectable threshold value (0.0326 mg/L), i.e. below 0.5 kg/ha/year.

Pre-management above-ground biomass: Dwarf shrub (mostly heather) biomass achieved the highest values in the subplots assigned to mowing or burning (c.12,000 kg/ha dry weight). Poaceae achieved the highest biomass values in the sod-cut subplots (over 2,000 kg/ha). Mowing and burning had little effect on Poaceae or cryptogam biomass, nutrient removal was therefore mainly the result of heather biomass reduction. Sod-cutting removed the entire above-ground biomass.

Nutrients in the above-ground biomass and soils: Stores of N in the above-ground biomass were highest in mown (221.5 kg/ha) and lowest in sod-cut (121.6 kg/ha) pre-management subplots. For the two dominant species, N and P contents for heather were 1.03% and 0.07%, respectively, and for wavy hair-grass 2.11% and 0.13%.

Mowing reduced N in the above-ground biomass by about 44%, and burning by about 53%. Ca, Mg, K and P in the organic layer of the burned subplots increased significantly as a result of ash deposition.

As expected, nutrient removal was highest in the sod-cut subplots. N in the organic layer (completely removed) amounted to 935 kg/ha. In the A-horizon (partly removed) 627 kg N/ha were removed corresponding to 36% of the total N store in the A-horizon.

Nutrient output by leaching: Leaching of N was significantly increased in the burned and sod-cut subplots. The N output during the first year after sod-cutting increased by about 4 kg/ha. The differences in N leaching rates between the burned subplots and their controls was 2.2 kg/ha/year, and for the mown subplots and their controls 1 kg/ha/year. Quantities of leached P were close to the analytically detectable threshold value, i.e. leaching rates were thus below 0.5 kg/ha/year in all subplots.

Nutrient balance and theoretical effective period: The application of mowing and prescribed burning removed quantities of N corresponding to about 5 years of atmospheric input. The theoretical effective period (TEP) for N for sod-cutting was much longer at around 90 years. However, if maximum deposition rates of N for the region were assumed (as shown by another study to be approximately twice that found in this experiment) values for TEP would be halved. TEP for P was highest for sod-cutting and lowest for prescribed burning.

Conclusions: In order to maintain a diverse structure and appropriate nutrient levels on heathlands, the authors recommend periodic high-intensity management, e.g. sod-cutting, combined with lower-intensity measures e.g. prescribed burning. This is considered best means of avoiding an increasing P shortage. The knowledge of plant N:P ratios could be helpful to managers, as they indicate whether plant growth tends to be limited by N, P or both.


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