INTRODUCTION
Post-industrial wastelands are often reclaimed for forestry (Hüttl and Weber, 2001; Sheoran et al., 2010; Macdonald et al., 2015; Pietrzykowski, 2019). In this way of reclamation planted trees are to transform the uppermost layer of barrens into functional soils. However, a common feature of the substrates forming the uppermost layer of post-industrial barrens is low concentration of essential nutrients, in particular nitrogen and phosphorus (Zipper et al., 2011; Krzaklewski et al., 2012; Cross et al., 2019; Manimel Wadu et al., 2017). Insufficient nutrient contents in the dumped or excavated materials may hinder growth of the introduced vegetation (Sheoran et al., 2010; Pietrzykowski, 2019). In order to alleviate N deficiency in developing technosols planted are tree species capable of forming a symbiosis with N-fixing bacteria (hereafter referred to as ‘N-fixers’). Owing to the symbiosis with N-fixing bacteria these species may enrich the developing soils in nitrogen (Zipper et al., 2011; Vlachodimos et al., 2013; Bissonette et al., 2014; Pietrzykowski et al., 2015). However, the role of N-fixers in alleviating P deficiency has been less recognized although it is known that certain N-fixing plants can acquire P from compounds that are unavailable for other plant species (Hinsinger, 2001; Hinsinger et al., 2011) and in this way intensify P cycling in the forming soils.
Vegetation is one of the major factors affecting transformations and cycling of soil P (Hinsinger, 2015). Plants take up phosphorus in mineral forms and deposit P-containing organic residues that become substrates for a variety of soil organic P forms (Hinsinger, 2015). In the uppermost soil horizons the organic P may constitute more than 50% of total P (Sulieman and Mühling, 2021). Organic P is not directly plant available, however there are several mechanisms that enable plants accessing organic P (Richardson et al., 2009). Chodak et al. (2021) reported that N-fixing tree species (black locust and black alder) improved capacity of technosols to release P by increasing the activity of phosphatases and enhancing microbial uptake of P. Organic P in soils occurs in a large variety of forms that differ in chemical composition, reactivity and plant availability (Turner et al., 2005). Recent studies suggest that organic P may be an important source of P for plants (Sulieman and Mühling, 2021). However, the effect of N-fixing plant species on the forms of organic phosphorus in soil is unclear and the published results are inconsistent. Increased contents of available P were reported for some N-fixing tree species and but not for others (Wang et al., 2010; Orczewska et al., 2012). Discrepant results might have been due to differences in the soil properties that obscured the effect of trees.
The objective in this study was to compare the effect of N-fixing tree species (black locust (Robinia pseudoacacia ) and black alder (Alnus glutinosa )) and non-N-fixing species (silver birch (Betula pendula ) and Scots pine (Pinus sylvestris )) on the forms of organic P in the uppermost horizons of reclaimed technosols developing from various parent materials.