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).