Introduction
Phosphorus (P) is considered an
essential nutrient for all living organisms as a structural component of
tissues and (together with nitrogen and potassium) represents the main nutrient
for crop development and growth. Phosphorus is responsible for vital functions
influencing seed germination, seedling establishment, and root, shoot, flower
and seed development as well as physiological changes including photosynthesis,
respiration and nitrogen fixation processes (Michigami
2013). P is applied in the form of processed phosphate as salt granules
that are dissolved in soil pore water and increases P uptake by plants (Dodds et al. 2009). Commercial mineral
P fertilizers are produced from the limited resources of phosphate rock (around
80–90% of the yearly mined phosphate is used to produce phosphate fertilizer).
However, natural resources of this mineral are being depleted as a result of
intensive exploitation which is an effect of demographic and economic factors (Cordell et al. 2011). Considering the
increased consumption of phosphate fertilizers, and the depletion of its
reserves, attention should be focused on finding alternative P sources in a
circular re-use system. Peak phosphorus demand will occur between 2030 and
2040; therefore, the EU Raw Minerals Initiative has classified phosphorus as a
critical raw material (Cordell et al.
2011; EC 2014; Jama-Rodzeńska et al. 2021). The high degree of
dependence of agriculture on P and additionally the growing problem of
eutrophication of inland and shore waters due to P infiltration due to food
production chains has generated renewed and urgent interest in the concept of
closing the P cycle by recovering and recycling P in a circular economy (Kasprzyk and Gajewska 2019).
One solution
that is adopted by the circular economy is the use of sewage sludge as a rich
nutrient source which includes phosphorus for the production of phosphate
fertilizers. Wastewater treatment plants are producing increasing amounts of
sewage sludge n has been found to cause severe groundwater contaminate (Egle et al. 2016; Jama-Rodzeńska et al. 2021).
The concept of P fertilizer production is consistent with a circular economy
that relies on the re-use of waste on valuable products, thus minimizing the
amount of waste generated and reducing environmental pollution (Egle et al. 2016). However, increased
waste productioon leading to increased P concentrations causing eutrophication (Schindler et al. 2016). Uncontrolled
eutrophication
leads to undesirable changes in the natural environment: excessive plant
production, fish death, and algae blooms (Schindler
et al. 2016).
Recovery of
P by precipitation of struvite (MgNH4PO4 6H2O)
is of great interest. Struvite is characterized by a theoretical P content
(12.6% dry weight – DW) that is similar to that of single super phosphate, and
it has been shown to be an effective P fertilizer, especially in acidic
environments (Muys et al. 2021).
This phosphorus fertilizer is a struvite product obtained from anaerobically
digested sewage sludge in communal sewage treatment plants (De-Bashan and Bashan 2004). Most experiments
are devoted to the processes of phosphorus recovery, focusing on a variety of
technologies depending on the initial material type (sludge, sewage and ash),
environmental impact and economic aspects. Struvite is characterized by
potential efficiency savings and environmental advantages compared to conventional
fertilizers because of its low degree of solubility (Cabeza et al. 2011; Talboys et al. 2016).
The objective of this study was to evaluate the
potential of Phosgreen-struvite (STR) as a source of
P and to compare it to commercial P fertilizer for the cultivation of the test
plant on deacidified peat and mineral soil. Primary studies were performed to
select horticultural media appropriate for phosphorus fertilization and lettuce
(the test plants) cultivation in subsequent experiments.
Materials and Methods
Experiment
setup and establishment
Pot experiments under controlled
(greenhouse) conditions were conducted in 2020 at the Research and Education
Station in Psary, Department of Horticulture, Wroclaw University of
Environmental and Life Sciences. The experiment was performed in two series:
May–June and August–September 2020. The experiments examined the efficiency of
Phosgreen compared to triple superphosphate (a commercial fertilizer).
The triple
superphosphate (SUP) used in this research was supplied by Ampol-Merol as an
enriched fertilizer with lime, containing 40% mineral phosphate and 10%
water-soluble CaO, recommended as a standard P fertilizer to be applied to all
crops. The Phosgreen fertilizer was produced by the Krevox European
Environmental Centre (KREVOX Sp. z o.o.) working on an Ostara license.
Phosphorus recovery consisted of phosphorus mineral precipitation as struvite
from municipal sewage sludge (magnesium ammonium phosphate hexahydrate, MgNH4PO4
6H2O).
The chemical
composition of Phosgreen granules is as follows: > 99% struvite (NH4MgPO4.6H2O)
equivalent to 12% P (28% P2O5). Struvite was crushed to a
small particle size and mixed with growing media. The content of heavy metals
in both STR and SUP is presented in Table 1 and this indicates that STR
contained a lower amount of Cu, Zn, Pb and Cd. Struvite has a lower heavy metal
content than that of triple superphosphate: 93 Cu, 98 Zn, 92 Pb and 98% (the
percentages refer to the content of the respective heavy metals in triple
superphosphate).
Two
horticultural media were used in the greenhouse experiment: low phosphorus
content mineral soil (MS) and deacidified
peat (DP) (Table 2). A low fertile soil was sampled from the tillage layer one
month before starting experiment for chemical analysis (depth 0–25 cm), from
fields belonging to the Research and Education Station in Psary, Wroclaw University of
Environmental and Life Sciences. After shredding larger lumps, the dried
soil was not sieved so as not to destroy its structure. The chemical
composition of the soil is presented in Table 2. Chemical analysis was
performed at the Chemical and Agricultural Station in Wroclaw according to
applicable methods. The deacidified peat substrate was characterized by a
standard nutrient concentration which was modified by adding mineral fertilizer
to satisfy lettuce nutrient demand.
The
following mineral fertilizers were used once before the experiment started,
precisely mixed with horticultural media: ammonium nitrate (AN), potassium
sulfate (PS), triple superphosphate (SUP) and struvite (STR). For controls (C),
only AN and PS, and no phosphorus fertilizer (SUP and STR) were used. Doses of
fertilizers for media (in mg/L) were calculated on the basis of the elemental
content of the peat/soil and the nutritional needs of lettuce, and these were
as follows (Table 3).
Deacidified
peat (DP) and mineral soil (MS) along with fertilizers were prepared two weeks
before lettuce sowing. Lettuce seeds were sown directly into pots (using the
same procedure for series I and II) with soil and peat in the first decade of
May (series I) and the second decade of August 2020 (series II). Butter lettuce
of the Omega variety was used in the experiment.
The experiment was conducted in three repetitions
with two orders: phosphorus fertilizers (control, superphosphate and struvite -
Phosgreen) and various horticultural media (DP, MS). Experimental series I was
harvested on 23/06/2020 and the second series during the second week of
September (12/09/2020). During lettuce vegetation, observations were made for
the occurrence of pests, diseases and weeds. Decis Mega 50 EW (0.15 l×ha-1)
was sprayed for Frankliniella. Weeds
were removed manually during the experimental period in both series of the experiment. The plants were
watered every morning and evening using an adjustable stowage line.
Biometric measurements and chemical analysis
After
harvesting, biometric measurements of the lettuce were performed. These included
the rosette weight, number of lettuce leaves and width of rosette. Lettuce
rosettes were weighed and the fresh biomass (g) was determined as an average
value from 12 heads. The dry biomass weight was determined by drying samples
(specific weight, 200–300 g of fresh mass) 105°C for 4 h and then at 60°C for
48 h. Nutrient content in peat, soil and lettuce was determined after
extraction with acetic acid (0.03 M). Chemical analyses of P and Mg content in
plant material were carried out colorimetrically: P with ammonium
vanadomolybdate and magnesium with titanium yellow. Uptake of phosphorus and
magnesium was calculated on the basis of the mass of the lettuce and the
content of these macronutrients. The same elements were determined in DP and MS
using the above methods. pH measured in water suspension (soil-to-water ratio
of 1:2., peat-to-water ratio 1:2). Heavy metals contents were determined using
the ICP-MS method in an earlier prepared solution with perchloric acid (after
sample digestion in 70% HClO4).
Statistical
analysis
Data from independent morphological
measures, lettuce rosette mass and chemical analysis (P, Mg, Cu, Zn, Cd, Pb)
were subjected to Anova/Manova statistical analysis in Statistica software
(version 13.1, StatSoft, Poland). The level of significance was determined as P < 0.05. One-way and two-way analysis of variance was performed
to determine the effects of horticultural media and P fertilizer on selected
morphological traits, biomass and chemical analysis of lettuce. Homogeneous groups
were determined using a post hoc test (Tukey test at P < 0.05). Names of homogeneous
groups were determined from the smallest to the largest value.
Results
Effect of struvite fertilization on biometric traits
The
statistical analysis of the results obtained in the research showed a
significant increase in lettuce mass for rosettes fertilized with SUP and STR
compared with controls (Table 4, Fig. 1). The mass of lettuce leaves after
struvite fertilization increased by 54 and 66% after SUP fertilization compared
to controls. Lettuce mass was also dependent on the horticultural media. A
significantly higher lettuce mass was observed on the deacidified peat (DP). The number of leaves
after STR fertilization was comparable to that for lettuce fertilized with SUP.
Significant differences were also observed after struvite application on the
peat. On the basis of interaction of factors, the width of rosette showed
comparable results on peat fertilized with either struvite or SUP. The width of
lettuce was 48% greater than that of controls. Interaction between factors
significantly affected this trait under study where SUP and STR were applied on
deacidified peat (DP). The number of leaves was therefore dependent on
phosphorus fertilization, horticultural media and interaction between the
examined factors and presented promising results with struvite fertilization
with peat.
Effect of
struvite fertilization on P and Mg content and uptake by tests plants
Phosphorus content and uptake was
significantly dependent on phosphorus fertilization and significantly higher
after struvite fertilization compared to controls, but lower or comparable to
that with superphosphate (Table 5). These results clearly indicate that STR is
just as effective as SUP in providing P to lettuce (Table 5).
Phosphorus
fertilization contributed significantly to P content increase and its uptake. A
significantly higher content of P was observed with superphosphate
fertilization; however, struvite caused a 28% increase in this nutrient
compared with controls. Regarding P content and uptake, struvite was as
effective as triple superphosphate. Analyzing the interaction between factors,
significantly more phosphorus was found in lettuce leaves in peat medium after
superphosphate application, as well as P uptake. In turn, Mg content strictly
depended on horticultural media and series (Table 5).
Effect of
struvite fertilization on P and Mg changes in horticultural media
Phosphorus
fertilization had no significant influence on pH, P and Mg content. A significantly
higher content of P was observed on the peat (127.92 mg dm-3) compared to soil (24.91 mg dm-3).
Interaction between horticultural media x phosphorus fertilizer also shaped the
content of P. A significantly greater value of P was stated in deacidified peat
fertilized with struvite. Contradictory results were stated in the case of Mg
content: a significantly higher content in the soil compared to a lower value
in the peat (Table 6).
Effect of
struvite fertilization on heavy metal content in test plants
Phosphorus fertilization and the selected horticultural
media had no significant impact on Zn, Pb and Cd content (Table 7). Cu content
was significantly dependent on phosphorus fertilizer and horticultural media.
Significantly higher Cu was noted after SUP and STR fertilization. A
significantly higher content of Cu was found in lettuce leaves in mineral soil
(1.90 mg/kg) than in peat (0.90 mg/kg). Interaction
between factors with significant results was noted in the case of Zn, with the
highest values for heavy metals in deacidified peat fertilized with SUP and STR.
The content of Pb and Cd was not detectable in the lettuce leaves (Table 7).
Discussion
Table 1: Selected heavy metals
content in SUP and STR
|
P fertilizer |
Heavy
metals content (mg kg-1) |
|||
|
|
Cu |
Zn |
Pb |
Cd |
|
SUP |
23.8 ± 4.8 |
213 ± 43 |
1.75 ± 0.35 |
10.7 ± 2.1 |
|
STR |
1.66 ± 0.33 |
3.73 ± 0.75 |
< 0.125 |
< 0.125 |
Results are presented as
a mean ± standard deviation
Table 2: Chemical composition of mineral soil (MS)/peat (DP) used
in greenhouse experiment
|
Specification |
Units |
Value of available
nutrients (MS) |
Value of available
nutrients (DP) |
|
pH in the water |
- |
8.1 |
5.6 |
|
salinity |
g NaCl dm-3 |
0.2 |
1.4 |
|
available nitrogen |
mg dm-3 |
- |
230 |
|
nitrate nitrogen |
mg dm-3 |
24 |
- |
|
phosphorus |
mg dm-3 |
63 |
180 |
|
potassium |
mg dm-3 |
81 |
230 |
|
calcium |
mg dm-3 |
4278 |
- |
|
magnesium |
mg dm-3 |
126 |
150 |
|
sodium |
mg dm-3 |
11 |
- |
|
chlorides |
mg dm-3 |
12 |
- |
Table 3: Doses of fertilizers
used in the experiment (mg/L)
|
Source of fertilizer |
Peat media |
Soil media |
|
AN |
294 |
294 |
|
PS |
400 |
300 |
|
SUP |
300 |
150 |
|
STR |
500 |
250 |
Table 4: Effect of applied
phosphorus fertilizers on selected features of lettuce
|
Experiment factor |
Selected
measurement |
||
|
|
Mass of rosette (g) f.m. |
Number of leaves (pcs) |
Width of rosette (cm) |
|
Phosphorus fertilizer |
|||
|
Control (C) |
56.21a |
11.18 a |
18.53 a |
|
SUP |
93.34 b |
22.17 b |
26.58 b |
|
STR |
87.10 b |
21.66 b |
27.41b |
|
P value |
P < 0.001*** |
P < 0.01** |
P < 0.001*** |
|
Horticultural
media |
|||
|
DP |
92.20 a |
21.14 b |
22.69 a |
|
MS |
65.65 b |
14.93 a |
25.69 a |
|
P value |
P < 0.001*** |
P < 0.01** |
0.1791 |
|
Phosphorus
fertilizer X Horticultural media |
|||
|
DP x C |
49.98 a |
7.74 a |
9.83 a |
|
DP x SUP |
120.72 c |
29.60 c |
29.08 b |
|
DP x STR |
105.89 bc |
27.90 c |
29.16 b |
|
MS x C |
62.45 ab |
14.62 b |
27.33 b |
|
MS x SUP |
65.96 ab |
14.75 b |
24.08 b |
|
MS x STR |
68.31 ab |
15.41b |
25.66 b |
|
P value |
P < 0.001*** |
P < 0.001*** |
P < 0.001*** |
c = control; SUP = superphosphate;
STR = struvite; DP = deacidified peat; MS = mineral soil
* Analysis of variance
at Significance at P < 0.05
** Analysis of variance
at Significance at P < 0.01
*** Analysis of variance
at Significance at P < 0.001
Means for factors. Different letters indicate significant differences
between factors (Tukey’s multiple range test)
The results obtained from the above
experiment confirm previous studies, which have shown significantly improved
growth and yield of plants fertilized with STR (Cabeza
et al. 2011; Wen et al. 2019). Ricardo et al. (2009) claimed that the fresh mass of head
lettuce was significantly influenced by the P source. According to Wen et
al. (2019), struvite is a promising P fertilizer for cabbage cultivation;
however, the soil type plays an important role. In our study, lettuce rosette
weight also varied significantly different horticultural media. Significantly
higher values for the examined parameters were obtained on deacidified peat,
with the exception of the width of the rosette. According to Min et al. (2019), struvite is an
effective fertilizer to cultivate chilli pepper and cucumber; however, struvite
inhibited the growth of these vegetable crops, with the exception of chili pepper, at doses in excess of
the standard dose. This was visible as yellowing or browning of leaf edges.
Other experiments, in turn, have indicated a positive impact of struvite
fertilization on grasses, vegetables, corn, and fruits compared to conventional water-soluble
fertilizers (Liu et al. 2011; Latifian
et al. 2012; Talboys et al. 2016). According to Plaza et al. (2007), a pot experiment
conducted in P-poor loamy sand soil with struvite and single superphosphate
application demonstrated an increase in the yield of the dry matter of ryegrass.
Similarly, in our study peat use and STR and SUP contributed to a higher mass
of leaves; however, higher values for mass were obtained using peat as the
medium (Table 4). These results are similar to those obtained by Reza et al. (2019) who claimed that the
yield of Sudan grass was significantly higher on the struvite application than
on the control treatment. However, they did not find differences between the
struvite and superphosphate-treated plants. Bonvin
et al. (2015) also received higher yields of ryegrass (Lolium multiflorum var. Gemini) fertilized with struvite compared with controls. Szymanska et al. (2019) also stated that
struvite was more effective compared with commercial phosphorus fertilizers
because of the presence of magnesium and the synergistic impact of the P and Mg
ratio in STR.
Table 5: Effect of phosphorus fertilization on
content and uptake of selected elements by lettuce
|
Experiment factor |
Dry mass (%) |
P content mg 100 g-1
DM |
P uptake mg per
rosette DM |
Mg content/mg 100 g-1
DM |
Mg uptake mg per
rosette DM |
|
Phosphorus
fertilizers |
|||||
|
C |
6.21a |
241. 81a |
7.66a |
195.83a |
6.77a |
|
SUP |
5.46a |
365.41b |
19.31b |
169.16a |
8.20a |
|
STR |
5.94a |
310.20ab |
17.10ab |
186.66a |
9.16a |
|
P value |
0.5483 |
P < 0.001*** |
P < 0.01** |
0.5016 |
0.4621 |
|
Horticultural media |
|||||
|
DP |
6.40a |
325.40 a |
19.62b |
156.38a |
8.23a |
|
MS |
5.34a |
286.22 a |
9.77a |
211.38b |
7.86a |
|
P value |
0.0573 |
0.1871 |
P < 0.01** |
P < 0.001*** |
0.8177 |
|
Phosphorus
fertilizer X Horticultural media |
|||||
|
Control DP |
7.54a |
200.75a |
7.02a |
193.33ab |
7.32a |
|
SUP DP |
5.44a |
414.33d |
27.25c |
122.50a |
7.90a |
|
STR DP |
6.21a |
361.12 cd |
24.58bc |
153.33ab |
9.45a |
|
Control MS |
4.88a |
282. 87abc |
8.31a |
198.33ab |
6.22a |
|
SUP MS |
5.47a |
316.50bcd |
11.38ab |
215.83b |
8.51a |
|
STR MS |
5.68a |
259.29 ab |
9.63a |
220.00b |
8.86a |
|
P value |
0.0982 |
P < 0.001*** |
P < 0.01** |
0.0704 |
0.9086 |
c = control; SUP = superphosphate;
STR = struvite; DP = deacidified peat; MS = mineral soil
*Significance at P < 0.05
**Significance at P < 0.01
***Significance at P < 0.001
Means for factors. Different letters indicate significant differences
between factors (Tukey’s multiple range test)
Table 6: Peat/soil pH, phosphorus and magnesium content in the tested horticulture
media under phosphorus fertilization
|
Experiment factor |
pH |
P content mg dm-3 |
Mg content mg dm-3 |
|
Phosphorus
fertilizer |
|||
|
Control (C) |
6.08 a |
78.79 a |
24.08 a |
|
SUP |
6.45 a |
62.41 a |
29.66a |
|
STR |
6.34 a |
90.75 a |
33.81 a |
|
P value |
0.0749 |
0.4977 |
0.5047 |
|
Horticultural media
(B) |
|||
|
DP |
6.20 a |
127.92 a |
20.73 a |
|
MS |
6.38 a |
24.91b |
37.73 b |
|
P value |
0.2020 |
P < 0.001*** |
P < 0.01** |
|
Horticultural
media x phosphorus fertilizer |
|||
|
Control DP |
6.03 a |
118.16 b |
17.91a |
|
SUP DP |
6.41a |
106.50 b |
20.41a |
|
STR DP |
6.16 a |
164.50 c |
23.87 a |
|
Control MS |
6.13 a |
39.41 a |
30.25 a |
|
SUP MS |
6.49 a |
18.33 a |
38.91 a |
|
STR MS |
6.51a |
17.00 a |
43.75 a |
|
P value |
0.6420 |
P < 0.001*** |
0.8753 |
c = control; SUP = superphosphate;
STR = struvite; DP = deacidified peat; MS = mineral soil
*Significance at P < 0.05
**Significance at P < 0.01
***Significance at P < 0.001
Means for factors. Different letters indicate significant differences
between factors (Tukey’s multiple range test)
Different
results related to P uptake under STR fertilization were obtained by Ricardo et al. (2009) who found that
struvite contributed to increased P uptake by lettuce compared with SUP. The
maximum P uptake (Ricardo et al. 2009)
for lettuce was 18.6 ± 1.2 mg kg-1 DM and 18.4 ± 1.8 mg kg-1 DM,
respectively. In our experiment, the content of P was much higher, ranging from
20 mg per kg D.M. (control DP) to 41.4 mg per kg D.M. (SUP DP). Everaert et al. (2018) achieved a higher
phosphorus uptake by plants fertilized with ammonium phosphate compared with
struvite. In the present experiment, P uptake was comparable for both SUP
fertilization and struvite fertilization. In turn, Johnston and Richards (2003) presented no differences in P uptake
from struvite and monocalcium phosphate in ryegrass cultivation.
According to
Worwąg (2018), physicochemical
properties of soil such as pH and P content increased with increasing doses of
struvite. In our study, P content increased under STR fertilization; however,
there were no significant differences. Talboys et
al. (2016) conducted research examining the impact of struvite on
soil pH. In their study, 2 days after struvite application, soil pH increased
from pH 5.5 to 6.0 and 6.5 to pH 6.9–7.1. Rahman
et al. (2014) also concluded that struvite increased soil pH in
acidic soil. Our study did not confirm this statement. According to Vogel et al. (2017), more P is left in
soil after struvite fertilization, an observation which is also confirmed %201-7_files/image002.png)
in our study where peat was used.
According to
Wen et al. (2019), the heavy metal
concentrations in vegetables after struvite fertilization were lower compared
to maximal contaminant levels for Chinese national food safety standards
(GB2762-2017). According to Latifian et al.
(2012), struvite had a significantly lower content of heavy metals,
apart from iron, compared to commercial NPK fertilizer. The results presented
in Uysal et al. (2010) are in
agreement with those of our study (Table 1); that struvite is characterized by
low heavy metals content. it is probable that this is caused by specific
structure of struvite that prevents absorption of metal ions into its
well-defined crystal structure.
Conclusion
Phosgreen recovered from wastewater
treatment plants was used in lettuce greenhouse production. The value of
Phosgreen as fertilizer was evaluated by comparing it with a commercial
phosphorus fertilizer in controlled conditions based on experimental results.
It was revealed that the rosettes mass as well as the number of leaves of the
lettuce and the width of the lettuce rosette were comparable to results
achieved with superphosphate fertilization. It was found that struvite fertilization
contributed to comparable P uptake by lettuce to that reported with commercial
phosphorus fertilizer. Phosphorus fertilization did not contribute to a
significant increase in P and Mg content in the horticultural media or to an
increase in pH. Deacidified
peat was chosen as a substrate for further study with Phosgreen. In
addition, neither Pb nor Cd was detected in struvite pots and results
comparable to those achieved with superphosphate in terms of Cu content were
noted.
Acknowledgments
This research work was funded Wroclaw University of
Environmental and Life Sciences, Innovative Scientist, N060/0011/20”.
Author’s
Contributions
Anna Jama-Rodzeńska: Conceptualization,
resources, validation, formal analysis, investigation, visualization, formal
analysis,writing original draft, review and editing.
Conflicts of Interest
“The authors declare no conflict of
interest”.
Data Availability
The reported data can be made available upon
requesting to the corresponding author, Dr inż. Anna Jama-Rodzeńska.
All the data is alredy reported in this manuscript.
Ethics Approval
Not applicable
Funding Source
This research was funded by Wroclaw
University of Environmental and Life Sciences, Innovative scientist,
N060/0011/20.
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