Concentrations of Corg, Ntand Porg in mineral soil
Concentrations of Corg, Nt and Porg in the mineral soil were significantly higher under deciduous trees compared with Scots pine (except for Corg which did not differ significantly between pine and birch) but there was no difference in Pt concentration between the studies species. The ultimate source of P in soil is the parent material as airborne inputs are usually negligible (Tipping et al., 2014). Similar Pt concentrations under the studied tree species indicate that within every substrate type particular tree species grew on soils developed from materials with similar phosphorus abundance. Hence, we can exclude a bias in the concentrations of organic P fraction resulting from planting any of the species on sites less abundant in this element.
The Porg concentration in the mineral soil under Scots pine was lower than under other tree species. Organic P in soil is mainly of plant origin (Spohn and Stendahl, 2022). Therefore, we presume that low Porg concentrations in the mineral soil under Scots pine were due to low input of organic P as indicated by low concentration of P in pine needles and in the O horizon. Lower Porg concentrations under Scots pine were mainly due to lower concentrations of relatively stable and recalcitrant fractions of organic phosphorus (Pfulv, Phum and POres). However, the concentrations of labile and moderately labile organic P fractions were lower under Scots pine than under other species as well (although the difference was not statistically significant). When the concentrations of particular P fractions were expressed as the percentage of Porg(Figure S1) there were nearly no significant differences between the studied species. This suggests that the tree species did not affect the quality of organic P in the mineral soil and that differences observed in the concentration of Porg and its fractions between Scots pine and other tree species resulted from lower organic P input under this species. This conclusion is supported also by lower Corg and Nt concentrations in the mineral soil under Scots pine compared with other species. Tree species are known to differently affect organic matter distribution in the soil profile (Gruba and Socha, 2019) and coniferous trees have been described to contain less organic C in the mineral soil than deciduous trees (Peng et al., 2020).
The remaining tree species did not differ in terms of Porg concentration in the mineral soil despite significant differences in P concentrations observed in the foliage and in the O horizon. They had also similar concentrations of POlab, POmod, Pfulv and POres expressed either in absolute values or as percentages of Porg. This indicates that deciduous tree species - both N-fixing and non-N-fixing – have similar effect on the concentration of Porg and its fractions in the mineral soil. However, we cannot rule out that large variability in the properties of parent materials included in our study (e.g. pH, texture) obscured the tree species effect in the mineral soil. Spohn and Stendahl (2022) reported no substantial difference in the C-to-Porg ratios in the mineral soil under different tree species and concluded that Porg concentrations were strongly affected by soil texture. On the contrary, Redel et al. (2008) found significant differences in soil P fractions under evergreen and deciduous forests growing on the same soil type in Chile.
Substrate type proved to be more important factor affecting the concentrations of Porg and Pt as well as Corg and Nt than tree species. Sands contained significantly less of all these elements and had significantly lower Corg-to-Porg than Clays and Ashes. Fine textured materials with large specific surface area and high charge density are capable of sorbing more organic compounds than coarse textured ones as they have better ability to build-up of organo-mineral complexes resistant to microbial degradation (Franzluebbers et al., 1996; Hassink, 1997; Müller and Höper, 2004). In line with our results Spohn and Stendahl (2022) found significantly lower organic P concentrations and higher Corg-to-Porgratios in coarse textured forest soils compared to fine textured ones. Similarly, Chodak and Niklińska (2010) reported that coarse textured sands contained significantly less Corg an Nt than loamy sands. Next to the texture some other properties of parent materials such as initial concentrations of elements or pH also could have played a role in the observed differences in Corg, Nt and Porgconcentrations between various substrate types. Ashes in our study contained less clay particles than Sands, yet they contained significantly more Corg, Nt and Porg. However, Ashes had higher percentage of silt particles that also contribute to stabilization of organic compounds in soil (Hassink, 1997), contained more P-rich minerals as indicated by their larger Pt concentrations and more Corg originating from unburnt lignite residues (Świątek et al., 2019). Significant difference in pH of Sands and Ashes also could affect the ability of these two substrate types to stabilize organic matter (Garrido and Matus, 2012; Rasmussen et al., 2018).
Classification of organic P availability based on chemical solubility has been criticized because various fractions haven’t been correlated to P availability for plants and it was proven that plants can acquire P from various organic P fractions including those regarded as stable (Turner et al., 2005; Fox et al., 2011). Nevertheless the existing fractionation schemes remain useful in studying factors affecting development and transformations of soil P pools over time (DeBruler et al., 2019). In our study concentrations of particular fractions of organic P depended on the substrate type. When concentrations of POlab, POmod, Pfulv, Phum and POres were expressed in absolute values (µg g-1) the observed differences followed the trend observed for Porg, with Sands having the lowest concentrations of all fractions except for POlab. However, very different pattern was revealed when the organic P fractions were expressed as percentage of Porg. Sands had the highest relative content of POlab and the lowest of POres among the studied substrate types, Ashes had the highest share of Pfulv while in Clays the organic P was dominated by POres which constituted more than a half of organic P pool. The observed differences in relative contents of particular organic P fractions resulted apparently from parent material properties. Clays being rich in minerals with large specific surface area and high number of exchange sites retained considerable part of P-containing organic molecules in recalcitrant fraction (POres). On the contrary, Sands provided much less exchange sites and large part of organic P entering the mineral soil remained in labile fraction. High share of POlab in the Porg and low Corg-to-Porg ratio of Sands could be important factors contributing to better P supply of trees growing on this substrate type indicated by higher concentration of P and lower N-to-P ratio in the foliage. It is known that soil microbial biomass plays a central role in cycling and transformations of soil organic P (Bünemann, 2015). However, soil microorganisms are usually C-limited and even in the P-poor soils the mineralization of organic P is mainly driven by microbial C demand (Heuck et al., 2015). Sands contained less Corg than Clays and Ashes but their organic matter was richer in organic P (as indicated by Corg-to-Porg ratios) and contained relatively large pool of labile organic matter (extractable with NaHCO3) rich in phosphorus. Rapid microbial mineralization of this fraction might have resulted in high P release to soil solution while low content of clay minerals in Sands prevented adsorption of the released mineral P (Gèrard, 2016).