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\begin{document}

\title{Effect of tree species and substrate properties on organic phosphorus
forms in afforested technosols}



\author[1]{Marcin Chodak}%
\author[1]{Katarzyna Sroka}%
\author[2]{Marcin  Pietrzykowski}%
\affil[1]{AGH University of Science and Technology}%
\affil[2]{University of Agriculture in Kraków}%


\vspace{-1em}



  
  \date{May 24, 2024}


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\selectlanguage{english}
\begin{abstract}
Planted vegetation is considered one of the most important soil-forming
factors in the reclamation of degraded post-mining lands for forestry.
The objective of this study was to compare the effect of N-fixing tree
species and non-N-fixing species on the organic phosphorus (P
\textsubscript{org}) forms in technosols developing from various
substrates. Samples were taken from the uppermost layer (depth 0 -- 5
cm) of technosols afforested with black locust ( \emph{Robinia
pseudoaccaccia}), black alder ( \emph{Alnus glutinosa}), silver birch (
\emph{Betula pendula}) and Scots pine ( \emph{Pinus sylvestris}).
Samples of the tree foliage and the O horizons were taken as well. The
studied technosols developed from Quaternary sands (Sands), fly ashes
after lignite combustion (Ashes) and Miocene clays (Clays). The soil
samples were measured for the contents of labile (P
\textsubscript{Olab}) and moderately labile organic phosphorus (P
\textsubscript{Omod}), phosphorus contained in fulvic and humic acids (P
\textsubscript{fulv} and P \textsubscript{hum}, respectively) and
residual organic phosphorus (P \textsubscript{Ores}). The foliage and O
horizon samples were measured for the concentrations of C, N and P. The
N-fixing trees had higher P concentration in the foliage than
non-N-fixing species. However, in the O horizon the highest P
concentration was determined under birch and not under N-fixers. The
effect of tree species on the organic P (P \textsubscript{org})
concentrations in the mineral soil was limited with significantly lower
P \textsubscript{org} concentrations under pine. Soil under this species
contained less P \textsubscript{fulv}, P \textsubscript{hum} and P
\textsubscript{Ores}. However, the percentages of P
\textsubscript{Olab}, P \textsubscript{Omod}, P \textsubscript{fulv}, P
\textsubscript{hum} and P \textsubscript{Ores} in soil P
\textsubscript{org} were nearly the same under all tree species. The
largest effect on the P \textsubscript{org} concentration and on the
contents of particular P \textsubscript{org} fractions was from the
substrate type. Sands contained much less P \textsubscript{org} than the
Clays and Ashes but their percentage of P \textsubscript{Olab} in P
\textsubscript{org} was much higher than in two other substrates. We
conclude that N-fixing trees do not affect the concentration of labile
organic P and the major factor controlling this P \textsubscript{org}
fraction is the soil substrate quality.%
\end{abstract}%



\sloppy


\textbf{INTRODUCTION}

Post-industrial wastelands are often reclaimed for forestry (H\selectlanguage{ngerman}üttl 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 (\emph{Robinia pseudoacacia} ) and black alder
(\emph{Alnus glutinosa} )) and non-N-fixing species (silver birch
(\emph{Betula pendula} ) and Scots pine (\emph{Pinus sylvestris} )) on
the forms of organic P in the uppermost horizons of reclaimed technosols
developing from various parent materials.

\textbf{MATERIALS AND METHODS}

Study sites

The study was carried out in forest stands growing on technosols
developing from three different types of substrates referred to as
Clays, Sands and Ashes. Samples of Clays were taken at the external
spoil heaps of the open-pit lignite mine Turów (number of sampling
sites, n = 12) and the open-pit sulphur mine near Piaseczno (n = 12).
Clays sampled at Turów and Piaseczno represented acid and alkaline
materials, respectively. Sands were sampled in the reclaimed areas of
the sand quarry Szczakowa (n = 17) and on the external spoil heap of the
open-pit lignite mine Be\selectlanguage{polish}ł\selectlanguage{english}chat\selectlanguage{ngerman}ów (n = 7) while Ashes were taken at the
combustion waste disposal site Lubie\selectlanguage{polish}ń \selectlanguage{english}(n = 24). Geographic localization
and basic data on climate, parent material and initial fertilization of
the sampling sites are given in Table 1.

Soil and vegetation sampling

Samples representing the organic horizon (O) and the uppermost mineral
soil (0 -- 5 cm) were taken in late August and early September 2019 from
sampling plots (10\selectlanguage{ngerman}×10 m) covered by pure stands of black locust
(\emph{Robinia pseudoacacia} ), black alder (\emph{Alnus glutinosa} ),
Scots pine (\emph{Pinus sylvestris} ) and silver birch (\emph{Betula
pendula} ). The age of forest stands was 18 -- 20 years for those
growing on Ashes, 30 -- 35 years for the stands growing on Sands and 38
-- 44 years for the stands growing on Clays. At each sampling site one
composite sample was taken from the O horizon and from the mineral soil.
Each composite sample consisted of five subsamples (area of each
subsample = 1.0 m\textsuperscript{2} for the O horizon and 0.16
m\textsuperscript{2} for the mineral soil) located at the corners and in
the middle of the sampling plot. The samples were sieved (10 mm mesh for
the O horizon and 2 mm mesh for the mineral soil). The mineral soil
samples were divided into two parts. One part was air-dried and used for
physical, physicochemical, and chemical analyses, and the other one was
stored field-moist at 4°C and used for organic P fractionation. The
samples of O horizon were air-dried and used for chemical analyses.

Leaves and needles were taken from trees growing within the sampling
sites in direct vicinity of the O horizon and mineral soil sampling
plots following Rautio and Fürst (2013) methodology. Within each
sampling site the foliage material was taken from five trees. Leaves and
needles were collected from the upper third of the crown with SW
exposition. In case of Scots pine current year needles were collected.
The collected leaves and needles from each sampling site were combined
to produce a composite sample representative for the site. The foliage
samples were dried to constant weight at 40 °C, homogenized and analyzed
for C and nutrient contents.

Chemical analyses of mineral soils, O horizons and tree foliage

The mineral soil, O horizon and foliage samples were analyzed for total
carbon and total nitrogen concentration using LECO
TruMac\textsuperscript{®} CNS analyzer. The inorganic C content in
mineral soil samples was calculated based on the carbonate content
measured with the gas-volumetric Scheibler method. The organic C
(C\textsubscript{org}) content was then calculated as a difference
between the total C and the inorganic C. Total phosphorus
(P\textsubscript{t}) was determined colorimetrically by the molybdate
blue method from \emph{aqua regia} extracts by Varian CARY 300 Conc
UV--Visible Spectrophotometer.

The texture of mineral soil samples was determined by hydrometer using
Bouyoucos method. The pH of the samples was measured in
H\textsubscript{2}O (pH\textsubscript{H2O}) and 1 M KCl solution
(pH\textsubscript{KCl}) (soil/liquid ratio 1:5, w/v) at 20° C using a
digital pH meter (CPC-411, ELMETRON). Data on the texture and pH of the
studied soils are presented in supplementary material (Table S1).

Fractionation of organic phosphorus

Organic P fractionation was carried out according to Bowman and Cole
(1978) method modified by Ivanoff et al. (1998), and described in Zhang
and Kovar (2009) wherein determination of microbial P was excluded from
the fractionation scheme. Briefly, 1 g of soil was sequentially
extracted with 0.5 M NaHCO\textsubscript{3} at pH = 8.5, 1.0 M HCl, and
0.5 M NaOH. The 0.5 M NaHCO\textsubscript{3} extract contained labile P
fraction (P\textsubscript{Olab}) and the 1.0 M HCl extract moderately
labile P (P\textsubscript{Omod}). The 0.5 M NaOH extract was acidified
to pH = 1.0 to distinguish P contained in fulvic (P\textsubscript{fulv})
and humic acids (P\textsubscript{hum}). The sample residue after
sequential extraction was ashed at 550 °C and extracted with
concentrated 1M H\textsubscript{2}SO\textsubscript{4} to determine
residual, non-labile organic P fraction (P\textsubscript{Ores}). The
extracts were centrifuged at 4000~rpm for 10~min, filtered and measured
for inorganic P (IP) and total P (TP) by the molybdate colorimetric
method. Total P (TP) in the extracts was determined after digestion with
2.5 M H\textsubscript{2}SO\textsubscript{4}~and potassium persulfate.
Organic P in the extracts was calculated as the difference between TP
and IP.

Statistical analysis

Two-way analysis of variance (ANOVA) followed by Student-Newman-Keuls
test (p \textless{} 0.05) was used to test for the effect of tree
species, and substrate type on the concentrations of organic P forms in
the studied technosols. Prior to ANOVA the dependent variables were
tested for distribution normality (Shapiro-Wilk test, p \textless{}
0.05) and homoscedasticity (Levene's test, p \textless{} 0.05) and
transformed (logarithmic, square root or power transformation) if
necessary.

All analyses were performed with Statgraphics Centurion XVII software
(Statistical Graphics Corporation).

\textbf{Results}

Chemical properties of foliage and O horizons

The highest concentration of N was measured in the leaves of black
locust (33.7 mg g\textsuperscript{-1}), followed by black alder (28.1 mg
g\textsuperscript{-1}), silver birch (18.6 mg g\textsuperscript{-1}) and
Scots pine (12.1 mg g\textsuperscript{-1}) (Table 2). The leaves of
black alder had the highest P concentration (1492 \selectlanguage{greek}µ\selectlanguage{english}g
g\textsuperscript{-1}) significantly higher than in the leaves of birch
(1276 \selectlanguage{greek}µ\selectlanguage{english}g g\textsuperscript{-1}) and the Scots pine needles (1009 \selectlanguage{greek}µ\selectlanguage{english}g
g\textsuperscript{-1}). The P concentration in the foliage of black
alder (1401 \selectlanguage{greek}µ\selectlanguage{english}g g\textsuperscript{-1}) was significantly higher than in
Scots pine needles but not than in the silver birch leaves (Table 2).
The leaves of N-fixing species were characterized by significantly lower
C-to-N and C-to-P ratios but significantly higher N-to-P ratio than the
foliage of non-N-fixing trees. The largest C-to-N and C-to-P ratios and
the smallest N-to-P ratio were determined in the Scots pine needles.

The O horizons under both N-fixing species contained more N than the O
horizons under both non-N-fixing trees (Table 3). However, this was not
the case for P. The highest concentration of P in the O horizons was
determined under silver birch (851 \selectlanguage{greek}µ\selectlanguage{english}g g\textsuperscript{-1}) followed by
black locust (808 \selectlanguage{greek}µ\selectlanguage{english}g g\textsuperscript{-1}). The P concentration under
alder was significantly lower (747 \selectlanguage{greek}µ\selectlanguage{english}g g\textsuperscript{-1}) compared
with birch but higher than under pine (649 \selectlanguage{greek}µ\selectlanguage{english}g g\textsuperscript{-1}).
The C-to-N ratio was significantly lower and the N-to-P ratio
significantly higher under N-fixing trees than under both non-N-fixing
species. However, the C-to-P ratio was significantly higher under Scots
pine and black alder than under birch and black locust.

C\textsubscript{org}, N\textsubscript{t}, P\textsubscript{t} and
P\textsubscript{org} in mineral soil

There were no significant differences in P\textsubscript{t} contents in
mineral soil under the studied tree species (Table 4). However, the soil
under Scots pine had lower concentration of C\textsubscript{org} (37.7
mg g\textsuperscript{-1}); N\textsubscript{t} (1.38 mg
g\textsuperscript{-1}) and P\textsubscript{org} (149.7 \selectlanguage{greek}µ\selectlanguage{english}g
g\textsuperscript{-1}) than the soils under all remaining tree species
(46.1 mg g\textsuperscript{-1} -- 49.7 mg g\textsuperscript{-1} for
C\textsubscript{org}, 2.00 mg g\textsuperscript{-1} -- 2.66 mg
g\textsuperscript{-1} for N\textsubscript{t} and 206.9 \selectlanguage{greek}µ\selectlanguage{english}g
g\textsuperscript{-1} -- 211.9 \selectlanguage{greek}µ\selectlanguage{english}g g\textsuperscript{-1} for
P\textsubscript{org}) (Table 4). Tree species did not affect soil
C\textsubscript{org}-to-P\textsubscript{org} ratio. The
N\textsubscript{t}-to-P\textsubscript{org} ratio was the largest under
black locust (12.7) but the difference was significant only compared
with Scots pine (8.5).

The contents of C\textsubscript{org}, N\textsubscript{t},
P\textsubscript{t}, P\textsubscript{org} as well as
C\textsubscript{org}-to-P\textsubscript{org} ratio depended on substrate
type. The lowest contents of these elements were measured in Sands and
the highest in Clays (although for P\textsubscript{t} the difference
between Clays and Ashes was not significant). The
C\textsubscript{org-}to-P\textsubscript{org} ratio was the lowest in
Sands and the largest in Clays (Table 4). However, there was no effect
of substrate type on N\textsubscript{t}-to-P\textsubscript{org} ratio.

Organic P fractions in mineral soil

Tree species did not affect the contents of P\textsubscript{Olab} and
P\textsubscript{Omod} in the uppermost layers of the studied soils
(Figure 1). However, the tree species effect was observed for the
remaining organic P fractions. The lowest contents of
P\textsubscript{fulv}, P\textsubscript{hum} and
P\textsubscript{Ores}were measured in soils under Scots pine (31.4 \selectlanguage{greek}µ\selectlanguage{english}g
g\textsuperscript{-1}, 19.8 \selectlanguage{greek}µ\selectlanguage{english}g g\textsuperscript{-1} and 57.4 \selectlanguage{greek}µ\selectlanguage{english}g
g\textsuperscript{-1}, respectively) (Figure 1). The soils under the
remaining tree species did not differ in terms of P\textsubscript{fulv}
(39.5 \selectlanguage{greek}µ\selectlanguage{english}g g\textsuperscript{-1} -- 40.5 \selectlanguage{greek}µ\selectlanguage{english}g g\textsuperscript{-1}),
P\textsubscript{hum} (30.7 \selectlanguage{greek}µ\selectlanguage{english}g g\textsuperscript{-1}-- 41.5 \selectlanguage{greek}µ\selectlanguage{english}g
g\textsuperscript{-1}) and P\textsubscript{Ores} (76.9 \selectlanguage{greek}µ\selectlanguage{english}g
g\textsuperscript{-1}-- 81.6 \selectlanguage{greek}µ\selectlanguage{english}g g\textsuperscript{-1}) contents. When
the contents of particular organic P fractions were expressed as
percentage of P\textsubscript{org} no significant differences between
the tree species were observed (Figure S1).

Concentrations of all organic P fractions (except P\textsubscript{Olab})
depended on the substrate type (Figure 1). The highest contents of
P\textsubscript{Omod} were measured in Ashes (38.7 \selectlanguage{greek}µ\selectlanguage{english}g
g\textsuperscript{-1}), followed by Sands (20.7 \selectlanguage{greek}µ\selectlanguage{english}g
g\textsuperscript{-1}) and Clays (13.6 \selectlanguage{greek}µ\selectlanguage{english}g g\textsuperscript{-1}). Sands
were characterized by significantly lower
P\textsubscript{org}concentrations (106.5 \selectlanguage{greek}µ\selectlanguage{english}g g\textsuperscript{-1};
Table 4) and significantly lower contents of P\textsubscript{fulv},
P\textsubscript{hum} and P\textsubscript{Ores} fractions (20.8 \selectlanguage{greek}µ\selectlanguage{english}g
g\textsuperscript{-1}, 19.1 \selectlanguage{greek}µ\selectlanguage{english}g g\textsuperscript{-1} and 21.5 \selectlanguage{greek}µ\selectlanguage{english}g
g\textsuperscript{-1}, respectively) than two other substrate types
(Figure 1). Clays had the highest P\textsubscript{org} concentration
(272.1 \selectlanguage{greek}µ\selectlanguage{english}g g\textsuperscript{-1}; Table 4) resulting from the highest
concentration of P\textsubscript{Ores} (143.1 \selectlanguage{greek}µ\selectlanguage{english}g g\textsuperscript{-1}).
Ashes contained significantly less of this fraction (56.8 \selectlanguage{greek}µ\selectlanguage{english}g
g\textsuperscript{-1}) than Clays but the contents of
P\textsubscript{fulv} and P\textsubscript{hum} were similar for both
substrate types (46.0 \selectlanguage{greek}µ\selectlanguage{english}g g\textsuperscript{-1} -- 46.6 \selectlanguage{greek}µ\selectlanguage{english}g
g\textsuperscript{-1} for P\textsubscript{fulv} and 37.4 \selectlanguage{greek}µ\selectlanguage{english}g
g\textsuperscript{-1} -- 40.0 \selectlanguage{greek}µ\selectlanguage{english}g
g\textsuperscript{-1}P\textsubscript{hum}) (Figure 1). The
C\textsubscript{org}-to-P\textsubscript{org} ratio depended on substrate
type with Clays exhibiting the highest (271), Ashes intermediate (225)
and Sands the lowest (166) value (Table 4). The effect of substrate type
was evident also when the contents of particular organic P fractions
were expressed as percentage of P\textsubscript{org.} Sands had the
largest share of P\textsubscript{Olab,,} Ashes of
P\textsubscript{fulv}while Clays of P\textsubscript{Ores} (Figure S1).

\textbf{Discussion}

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
(Rodr\selectlanguage{ngerman}íguez-Barrueco et al., 1984; Còte et al., 1989; Kuznetsova et al.,
2011; Deng et al., 2019; González et al., 2020; Wo\selectlanguage{polish}ś \selectlanguage{english}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 \selectlanguage{greek}µ\selectlanguage{english}g
g\textsuperscript{-1} to 2170 \selectlanguage{greek}µ\selectlanguage{english}g g\textsuperscript{-1} (Deng et al.,
2019) and of black locust growing in the areas affected by the
industrial emissions from 1680 \selectlanguage{greek}µ\selectlanguage{english}g g\textsuperscript{-1} to 2530 \selectlanguage{greek}µ\selectlanguage{english}g
g\textsuperscript{-1} (Tzvetkova and Petkova, 2015). Even higher P
concentration (3410 \selectlanguage{greek}µ\selectlanguage{english}g g\textsuperscript{-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 \selectlanguage{greek}µ\selectlanguage{english}g g\textsuperscript{-1} -- 1900 \selectlanguage{greek}µ\selectlanguage{english}g g\textsuperscript{-1}) and by
Rodr\selectlanguage{ngerman}íguez-Barrueco et al. (1984) for the leaves of 40-50 years old
alders sampled in summer (1600 \selectlanguage{greek}µ\selectlanguage{english}g g\textsuperscript{-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 \selectlanguage{greek}µ\selectlanguage{english}g g\textsuperscript{-1} --
4100 \selectlanguage{greek}µ\selectlanguage{english}g g\textsuperscript{-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; F\selectlanguage{ngerman}äth et al., 2019) and was well below the threshold value (1300 \selectlanguage{greek}µ\selectlanguage{english}g
g\textsuperscript{-1}; G\selectlanguage{ngerman}öttlein, 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 \selectlanguage{greek}µ\selectlanguage{english}g
g\textsuperscript{-1}; Uri et al., 2007) and Finland (2650 \selectlanguage{greek}µ\selectlanguage{english}g
g\textsuperscript{-1}; Saram\selectlanguage{ngerman}äki 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 (\textgreater{}14) or N
(\textless{}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.

Concentrations of C\textsubscript{org}, N\textsubscript{t}and
P\textsubscript{org} in mineral soil

Concentrations of C\textsubscript{org}, N\textsubscript{t} and
P\textsubscript{org} in the mineral soil were significantly higher under
deciduous trees compared with Scots pine (except for
C\textsubscript{org} which did not differ significantly between pine and
birch) but there was no difference in P\textsubscript{t} 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 P\textsubscript{t} 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 P\textsubscript{org} 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 P\textsubscript{org} 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
P\textsubscript{org} concentrations under Scots pine were mainly due to
lower concentrations of relatively stable and recalcitrant fractions of
organic phosphorus (P\textsubscript{fulv}, P\textsubscript{hum} and
P\textsubscript{Ores}). 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
P\textsubscript{org}(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 P\textsubscript{org}
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 C\textsubscript{org} and N\textsubscript{t}
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
P\textsubscript{org} 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
P\textsubscript{Olab}, P\textsubscript{Omod}, P\textsubscript{fulv} and
P\textsubscript{Ores} expressed either in absolute values or as
percentages of P\textsubscript{org}. This indicates that deciduous tree
species - both N-fixing and non-N-fixing -- have similar effect on the
concentration of P\textsubscript{org} 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-P\textsubscript{org} ratios in the mineral soil under different
tree species and concluded that P\textsubscript{org} 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 P\textsubscript{org} and P\textsubscript{t} as well as
C\textsubscript{org} and N\textsubscript{t} than tree species. Sands
contained significantly less of all these elements and had significantly
lower C\textsubscript{org}-to-P\textsubscript{org} 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
C\textsubscript{org}-to-P\textsubscript{org}ratios in coarse textured
forest soils compared to fine textured ones. Similarly, Chodak and
Nikli\selectlanguage{polish}ń\selectlanguage{english}ska (2010) reported that coarse textured sands contained
significantly less C\textsubscript{org} an N\textsubscript{t} 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 C\textsubscript{org},
N\textsubscript{t} and P\textsubscript{org}concentrations between
various substrate types. Ashes in our study contained less clay
particles than Sands, yet they contained significantly more
C\textsubscript{org}, N\textsubscript{t} and P\textsubscript{org}.
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
P\textsubscript{t} concentrations and more C\textsubscript{org}
originating from unburnt lignite residues (\selectlanguage{polish}Ś\selectlanguage{english}wi\selectlanguage{polish}ą\selectlanguage{english}tek 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
P\textsubscript{Olab}, P\textsubscript{Omod}, P\textsubscript{fulv},
P\textsubscript{hum} and P\textsubscript{Ores} were expressed in
absolute values (\selectlanguage{greek}µ\selectlanguage{english}g g\textsuperscript{-1}) the observed differences
followed the trend observed for P\textsubscript{org}, with Sands having
the lowest concentrations of all fractions except for
P\textsubscript{Olab}. However, very different pattern was revealed when
the organic P fractions were expressed as percentage of
P\textsubscript{org}. Sands had the highest relative content of
P\textsubscript{Olab} and the lowest of P\textsubscript{Ores} among the
studied substrate types, Ashes had the highest share of
P\textsubscript{fulv} while in Clays the organic P was dominated by
P\textsubscript{Ores} 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 (P\textsubscript{Ores}). 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 P\textsubscript{Olab} in the P\textsubscript{org} and low
C\textsubscript{org}-to-P\textsubscript{org} 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
(B\selectlanguage{ngerman}ünemann, 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
C\textsubscript{org} than Clays and Ashes but their organic matter was
richer in organic P (as indicated by
C\textsubscript{org}-to-P\textsubscript{org} ratios) and contained
relatively large pool of labile organic matter (extractable with
NaHCO\textsubscript{3}) 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).

\textbf{Conclusions}

Black locust and black alder contained significantly more P in the
foliage than silver birch and Scots pine indicating higher P uptake by
the N-fixing tree species. However, the P concentrations in the O
horizon followed a different trend with the highest concentration
measured under silver birch and not under the N-fixers. We presume that
this was due to more efficient P resorption from leaves by the N-fixing
species (in particular by black alder). The effect of tree species on
P\textsubscript{org} concentrations in the uppermost mineral soil was
limited. Only Scots pine exhibited significantly lower
P\textsubscript{org} concentrations in the mineral soil probably due to
lower concentration of P in pine needles and the O horizon and slower
decomposition rate of pine litter. The other tree species studied did
not differ in the P\textsubscript{org} concentrations in the mineral
soil. Tree species did not affect the concentrations of organic P
fractions with different lability as well. The concentrations of fulvic,
humic and residual P were lower in the mineral soil under Scots pine
only because there was generally less organic P under this tree. The
percentages of labile, moderately labile, fulvic, humic and residual
fractions in soil P\textsubscript{org} were nearly the same under all
tree species. The largest effect on the
P\textsubscript{org}concentration in the mineral soil as well as on the
contents of its particular fractions was from the substrate type. Sands
contained much less P\textsubscript{org} than Clays and Ashes. However,
the percentage labile P in P\textsubscript{org} was much higher there
resulting in better P supply for plants. We conclude that N-fixing trees
do not affect the concentration of labile organic P as the major factor
controlling this P\textsubscript{org} fraction is the soil substrate
quality.

\textbf{Acknowledgements}

The study was financed by The National Science Centre, Poland, grant No.
2018/31/B/ST10/01626.

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\textbf{Figure captions}

Figure 1. Results of two-way ANOVA for the concentrations of labile
organic P (P\textsubscript{Olab}), moderately labile organic P
(P\textsubscript{Omod}), organic P contained in fulvic acids
(P\textsubscript{fulv}) and humic acids (P\textsubscript{hum}) and
residual organic P (P\textsubscript{Ores}) in mineral soils (0 -- 5 cm)
of technosols under different tree species. Studied factors include tree
species (black locust, black alder, Scots pine, silver birch) and
substrate class (Clays, Ashes, Sands). Whiskers indicate standard
errors. Bars sharing the same letter do not differ significantly (p
\textless{} 0.05, Student-Newman-Keuls test).

\textbf{Hosted file}

\verb`Figure 1 LDD.docx`  available at \url{https://authorea.com/users/496878/articles/578100-effect-of-tree-species-and-substrate-properties-on-organic-phosphorus-forms-in-afforested-technosols}

\textbf{Hosted file}

\verb`Tables LDD.docx`  available at \url{https://authorea.com/users/496878/articles/578100-effect-of-tree-species-and-substrate-properties-on-organic-phosphorus-forms-in-afforested-technosols}

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