P and N concentrations in foliage and the O horizon
As expected the foliage of N-fixing tree species contained significantly more N than the foliage of Scots pine and silver birch (Table 2). The measured N concentrations in leaves of black alder and black locust represented average or relatively high values compared to data reported for these species growing at the natural and the reclaimed mine sites (Rodríguez-Barrueco et al., 1984; Còte et al., 1989; Kuznetsova et al., 2011; Deng et al., 2019; González et al., 2020; Woś et al., 2020). The N-fixing trees contained also significantly more P in the foliage than non-N-fixing species. However, the concentrations of P in the foliage of all the studied species were relatively low compared with literature data. For instance, P concentrations in leaves of black locust growing in a plantation on Loess Plateau, China varied from 1720 µg g-1 to 2170 µg g-1 (Deng et al., 2019) and of black locust growing in the areas affected by the industrial emissions from 1680 µg g-1 to 2530 µg g-1 (Tzvetkova and Petkova, 2015). Even higher P concentration (3410 µg g-1) was reported in the leaves of black locust growing in the reclaimed Green Valley coal mine, Indiana, USA (Jensen et al., 2010). The P concentration in black alder leaves in our study was close to the lower range of values reported by Kuznetsova et al. (2011) for the leaves of 2 – 7 years old black alders (1500 µg g-1 – 1900 µg g-1) and by Rodríguez-Barrueco et al. (1984) for the leaves of 40-50 years old alders sampled in summer (1600 µg g-1). However, it was distinctly lower than values reported by Temperton et al. (2003) for the leaves of 4 years old black alders growing on the sites after oil shale mining reclaimed for forestry (2200 µg g-1 – 4100 µg g-1). Concentration of P in the foliage of Scots pine represented the very low end of the range of values observed in needles of this species at various sites in Europe (Oleksyn et al., 2003; Fäth et al., 2019) and was well below the threshold value (1300 µg g-1; Göttlein, 2015). Similarly, the concentration of P in the leaves of silver birch was distinctly lower than values measured in afforested arable fields in Estonia (3890 µg g-1; Uri et al., 2007) and Finland (2650 µg g-1; Saramäki and Hytönen, 2004). Assessment of nutrient supply for trees includes not only the concentrations of particular elements in the foliage but also their ratio as the elements are required in appropriate proportions (Ingestad 1987; Knecht and Göranson, 2004; Marschner, 2012). The N-to-P ratio of 14 has been used as a general indicator of either P (>14) or N (<14) limitation for plant’s growth (Knecht and Göranson, 2004). The N-to-P ratios observed in our study for all the species except Scots pine were higher than 14. Low concentrations of P in the foliage and high N-to-P ratios indicate insufficient P supply for all the studied tree species at all of the studied substrate types. Considering relatively high N concentrations and high N-to-P ratios in the foliage of the studied species (except for Scots pine) it seems that phosphorus and not nitrogen is the most limiting element in the studied technosols. Similarly, Manimel Wadu et al. (2017) identified insufficient P availability as a major long-term risk for ecosystem development at reclaimed sites after oil sands extraction in Canada.
Litterfall is the major source of organic matter accumulated in the O horizon. However, P concentrations in this horizon under the studied tree species did not correspond to P concentrations in the foliage. The highest P concentrations were found not under black alder but under silver birch followed by black locust. We presume that lower P concentration in O horizon under alder resulted from larger resorption of P from leaves prior to abscission. Còte and Dawson (1991) reported that black alder was able to resorb large amount of P from senesced leaves and that P resorption efficiency was particularly high at sites with low P availability. The P resorption efficiency of black alder was larger than efficiency of black locust and several non-N-fixing species growing on the swamp forest sites (Sürmen et al., 2014). Differences in the N and P resorption efficiency between the studied species may explain also the observed pattern in the N-to-P ratios in the O horizon. Nitrogen-fixing plants are less efficient at resorbing nitrogen than non-nitrogen-fixing plants but the opposite is true for P resorption (Stewart et al., 2008). Hence, the O horizons under N-fixers had higher N-to-P ratios than non-N-fixing species. Furthermore, black locust has been described as more efficient in N resorption from leaves than various alder species (Stewart et al., 2008). This together with higher P resorption efficiency of black alder compared with black locust (Sürmen et al., 2014) may explain significantly larger N-to-P ratio in the soil O horizon under black alder.
Low P concentration in the O horizon under Scots pine resulted probably from the low P concentration in green needles and significant P resorption from the senesced foliage. Blanco et al. (2007) reported high P resorption efficiency (49.6% – 54.0%) in Scots pines growing at two sites with different fertility in Pyrenees, Spain but noticed that the P resorption efficiency did not depend on soil P concentration. The highest C-to-N and C-to-P ratios of the O horizon under Scots pine resulted probably from chemical properties of pine needles. Similarly, Spohn and Stendahl (2022) reported higher C-to-N and C-to-P ratios in organic horizons under Scots pine stands compared to other tree species growing in Swedish forests and attributed this to elemental ratios of leave/needle litter.