The impact of alders (Alnus spp.) on the physico-chemical properties of technosols on a lignite combustion waste disposal site

Combustion waste disposal sites typically display unfavourable conditions for revegetation and are nitrogen deficient, therefore the introduction of alders may be promising due to their nitrogen-fixing abilities. We investigated the impact of alder species (Alnus glutinosa, A. incana and A. viridis) after ten years of growth on the physico-chemical properties (pH, SOC and nutrient content, sorption complex parameters) of combustion waste technosols. The experiment was fully randomized with 24 study plots (area 72 m2) and included a combination of 3 alder species × 2 different soil treatments: CCW + L (combustion waste with lignite culm amendment in planting hole) and CCW (pure combustion waste with no amendment). Pre-treatment preparation of the disposal site included start-up mineral fertilisation (N: 60, P: 36 and K: 36 kg ha−1) and initial stabilisation by hydro-seeding with sewage sludge (4 Mg ha−1 dry mass) and a mixture of grasses (Dactylis glomerata L. and Lolium multiflorum Lam.) (200 kg ha−1). The obtained results indicated that over the long term the lignite culm addition did not affect soil properties, whereas the effect of alder species was evident. The accumulation of litter in the O horizon ranged from 2.9 Mg ha−1 to 3.6 Mg ha−1 (dry mass). Litter pH was slightly acidic (pH in KCl 6.3) to neutral (pH in KCl 6.7), which is favourable for strongly alkaline combustion wastes. The O horizon under A. incana typically displayed the highest content of N, P and K, whereas the highest N content (1.57 g kg−1) in the mineral horizon (0-5 cm) was under the black alder. The SOC content in the 0-5 cm mineral horizon ranged from 18.60 g ha−1 to 28.34 g kg−1, respectively, for A. viridis and A. glutinosa. The obtained results corroborated findings of a previous study on alder growth on combustion waste disposal sites and indicated usefulness of alders, especially A. glutinosa, for biological reclamation of this type of barrens.