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Citation: Terrero Turbí, M.A.; Gómez
Garrido, M.; El bied, O.; Cuevas
Bencosme, J.G.; Cano, Á.F.
Preliminary Results on the Use of
Straw Cover and Effective
Microorganisms for Mitigating GHG
and Ammonia Emissions in Pig Slurry
Storage Systems. Agriculture 2024,14,
1788. https://doi.org/10.3390/
agriculture14101788
Academic Editors: Alessandro
D’Emilio and Brad Ridoutt
Received: 9 August 2024
Revised: 30 September 2024
Accepted: 10 October 2024
Published: 11 October 2024
Copyright: © 2024 by the authors.
Licensee MDPI, Basel, Switzerland.
This article is an open access article
distributed under the terms and
conditions of the Creative Commons
Attribution (CC BY) license (https://
creativecommons.org/licenses/by/
4.0/).
agriculture
Article
Preliminary Results on the Use of Straw Cover and Effective
Microorganisms for Mitigating GHG and Ammonia Emissions in
Pig Slurry Storage Systems
Martire Angélica Terrero Turbí*, Melisa Gómez Garrido, Oumaima El bied , JoséGregorio Cuevas Bencosme
and Ángel Faz Cano
Sustainable Use, Management and Reclamation of Soil and Water Research Group, Agronomic Engineering
Department, Technical University of Cartagena, Paseo Alfonso XIII, 48, 30203 Cartagena, Spain;
melisa.gomez@upct.es (M.G.G.); oumaima.elbied@upct.es (O.E.b.); jose.cuevas@upct.es (J.G.C.B.);
angel.fazcano@upct.es (Á.F.C.)
*Correspondence: angelica.terrero@upct.es
Abstract: Spain has been the largest pork producer in the EU in recent years, leading to significant
pig slurry (PS) production that requires proper management to prevent environmental impacts. The
objectives of this study were to quantify greenhouse gas (GHG) and ammonia emissions and to
characterize the PS in storage pond systems. A straw cover pond (SP) and addition of effective
microorganisms (EMs) in a biological pond (BP) were used to treat the slurries. During two periods
(autumn and spring), PS was characterized and GHG (CO
2
, CH
4
, N
2
O) and NH
3
emissions were
measured with a dynamic chamber. After 5 weeks of storage, BP achieved a reduction of 96% for
CO
2
, 98% for CH
4
and 59% for NH
3
compared to the control pond (CP) in spring, while SP presented
a 74% reduction for CO
2
in autumn, and 60% and 97% reductions for CH
4
and NH
3
, respectively,
in spring. Additionally, the PS samples showed a decreasing trend for EC, dry matter, COD, BOD
5
,
total N, NH
4+
-N, Org.-N, NO
3−
-N, and PO
43−
during both seasons. This preliminary study shows
promise in reducing GHG/NH
3
emissions and improving PS properties, but further replication
is recommended. Varying straw cover thickness, optimizing EM dose, and a pH reduction may
enhance outcomes.
Keywords: GHG emission; ammonia; pig slurry storage; emission-mitigation techniques; straw cover;
effective microorganism
1. Introduction
The pig sector in Spain has experienced a significant growth in recent years, becoming
one of the most important industries in the European Union (millions of animals): 32.6 in
2020, 34.4 in 2021, 34.1 in 2022 and 33.8 in 2023, according to the Ministry of Agriculture,
Fisheries and Food. However, this growth has also led to an increase in waste generation,
which indicates growing environmental and management challenges. The waste generated
by the pig industry has a significant impact on the atmosphere, contributing to greenhouse
gas (GHG) emissions and climate change. One of the main wastes generated in the pig
sector is manure (solid and liquid), which contains high concentrations of nitrogen and
organic matter. If pig waste is not managed properly, it can generate emissions of ammonia
(NH
3
), a gas that contributes to the formation of fine particles and acid rain according to the
Ministry for the Ecological Transition and the Demographic Challenge. Furthermore, the
anaerobic decomposition of manure in storage and treatment systems can produce large
amounts of methane (CH
4
), which accounts for 6% of anthropogenic emissions. Methane
is a greenhouse gas that is 25 times more potent than carbon dioxide (CO
2
) in terms of
global warming capacity over a 100-year period [
1
,
2
]. To mitigate greenhouse gas emissions
and reduce the environmental impact of the pig farming industry, technologies based on
Agriculture 2024,14, 1788. https://doi.org/10.3390/agriculture14101788 https://www.mdpi.com/journal/agriculture
Agriculture 2024,14, 1788 2 of 17
best available techniques (BAT) have been implemented during the last 10 years. These
techniques represent the most efficient and sustainable methods for reducing emissions
and minimizing the environmental impact of pig slurry storage systems: 0% without crust
and uncovered (reference), 80% rigid cover, 40% natural crust, 40% straw, 60% geometric
pieces, 50% acidification (pH 6). One of the most used techniques is the rigid or waterproof
cover, which is placed over manure storage systems to prevent the release of gases into the
atmosphere, although for farms with a large lagoon surface, they may present prohibitive
prices [
3
]. This cover prevents the release of methane and ammonia into the atmosphere;
additionally, it blocks the entry of oxygen, promoting anaerobic conditions that encourage
methane production beneath the cover. This methane production could be reduced by
lowering the pH. Another mitigation technique is the use of flexible covers, which consists
of a tarp or membrane that is placed over the manure to seal the storage system. This
cover prevents oxygen from entering and reduces ammonia volatilization, although it may
increase methane production. The reduction in NH
3
volatilization with straw covering is
likely attributed to the straw’s ability to adsorb and absorb ammonia. Also, the effectiveness
of this process is significantly influenced by the size of the straw used. Natural crusting is
another technique used to reduce greenhouse gas emissions. It involves allowing a layer
of crust to form on the surface of the manure, which acts as a physical barrier that limits
the release of gases. This crust can also help reduce water evaporation, thus improving the
efficiency of using nutrients present in the manure.
The use of chopped straw as manure cover has been shown to be economic and
effective in reducing greenhouse gas emissions. Physical methods such as plastic covers,
fibers, biochar, wood chips, straw, and oil layers have been explored for controlling methane
and odor emissions. Slurry covers serve as both physical barriers and absorbents for
gaseous emissions. Additionally, they help reduce turbulence and the ebullition process
associated with methane production in the slurry [
4
,
5
]. Furthermore, the use of geometric
pieces, such as sheets or meshes, in manure storage systems can improve the efficiency
of gas capture and reduce their release to the atmosphere [
6
]. These geometric pieces
help maximize the contact surface between the manure and the cover, thereby reducing
gas leaks. Slurry acidification has also been used as an emissions mitigation technique.
This process involves the addition of acids, such as sulfuric acid or hydrochloric acid, to
manure to reduce its pH and limit ammonia volatilization. Manure acidification with
chemical additives can also improve the availability of nutrients for later applications as
fertilizer in agriculture. In addition to the techniques mentioned, the use of biological
additives such as microorganisms has also proven to be effective in mitigating odors and
emissions in the pig industry, as microorganisms promote the biodegradation process
of organic matter present in slurry at notable concentrations [
7
,
8
]. Microorganisms can
be used to promote the aerobic decomposition of manure, which reduces the production
of ammonia [
9
]. These microorganisms can be added to the manure storage system or
applied directly to the soil during fertilization. Effective microorganisms consist of a
multiculture of coexisting microorganisms of lactic acid bacteria, phototrophic bacteria
and yeasts, not genetically modified and non-pathogenic, responsible for synergistic and
non-harmful microbial activities [
10
]. It is a consortium that, utilizing sunlight and heat, is
characterized by accelerating the degradation of complex proteins and carbohydrates from
the organic matter of pig slurry, with the consequent reduction of bad odors, proliferation of
undesirable insects and emissions [
10
]. Although the benefits of adding organic substances
for ammonia emission and odor control in slurry have been demonstrated, this technology
is not well explored and there is a lack of information about mitigation of emissions [8].
It is important to highlight that the implementation of these emission-mitigation
techniques requires an integrated approach adapted to the specific characteristics of each
pig farm. In addition, aspects such as the proper management of manure storage systems,
ventilation control in swine facilities, and efficient management of the waste generated
must be considered [5,9].
Agriculture 2024,14, 1788 3 of 17
The rise in livestock production in Spain has led to an increase in the generation of pig
waste, consequently producing significant environmental impacts, especially in terms of
GHG emissions (CH
4
, CO
2
and N
2
O) and polluting gases (NH
3
). Therefore, the adoption
of technologies based on best or emerging practices becomes imperative to prevent the
formation of pollutant gases that may be released into the atmosphere. Most farmers store
slurry in ponds and recycle it agronomically, while zero treatment is common on most
private farms in Spain. Consequently, farmers with limited resources currently have the
need to implement economically and easily deployable technologies on their farms to
reduce emissions generated on-site.
It should also be noted that livestock farmers are subject to increasingly restrictive
regulations and greater environmental control of farms from public administrations. It
is essential to continue researching and promoting the implementation of these emission-
mitigation technologies in the pig farming sector, in order to achieve more sustainable and
environmentally friendly production. The objectives of this study were: (a) to quantify
CH
4
, N
2
O, CO
2
and NH
3
gas emissions using two emission-mitigation techniques in pig
slurry storage (barley straw cover, which is a BAT, and biological additive application,
which is an emerging technique) and (b) to analyze the physical, chemical and biological
properties of slurry during two periods of storage in the autumn and spring seasons in a
semi-arid Mediterranean climate.
This study represents a preliminary investigation, as the results obtained need to be
confirmed through further replications. While the findings suggest promising outcomes,
especially regarding the reduction of GHG/NH
3
emissions and the improvement of PS
properties, additional trials are necessary to validate the consistency and reliability of these
results. Replicating the experiments under varying conditions and over longer periods
will provide a more comprehensive understanding of the effectiveness of the treatment
techniques, and will ensure that the observed effects are not influenced by specific seasonal
or experimental variables.
2. Materials and Methods
2.1. Study Site and Description
The experiment was carried out in the municipality of Fuente Álamo (Murcia Region,
Spain), which represents 80% of the intensive livestock farming (289 pig farms) in the
catchment area to the Mar Menor basin. The pig farm where the study was conducted had
an intensive production system with an annual census of 15,000 pigs, which theoretically
produce about 32,250 m
3
of slurry per year (Spanish Royal Decree 306/2020). Two periods
were tested from 2022 to 2023, the first one from September to November 2022 (Autumn),
and the second one from April to June 2023 (Spring).
2.2. Experimental Design and Procedure
The storage duration was set at 3 months for each test period. This meant that, for
both test periods, the PS was stored and monitored over a consistent period of 3 months.
Portable storage ponds were used to conduct the testing; these were circular portable pools
(ref. 84265026, Bestway
®
, Nanjing, China) with a structure of steel and polyvinyl chloride
(PVC), placed outside the pig farm trying to simulate real-scale conditions. Each portable
pond had a total capacity of 15 m
3
, although the useful volume of pig slurry was 13 m
3
,
with a depth of 1 m to accomplish the requirements established by the VERA test protocol
regarding technologies for reduction of gaseous emissions from stored manure.
To ensure consistency and accuracy in the experiment, a specific method was employed
to fill the ponds with slurry obtained from a pig farm. A tanker truck was used to transport
the slurry directly from the farm to the experimental pools. The truck’s capacity was
divided into three equal parts, and the slurry was distributed evenly among the three
ponds. Each pond received exactly one-third of the total volume of slurry, ensuring that
they were filled simultaneously and uniformly.
Agriculture 2024,14, 1788 4 of 17
After filling, an initial analysis was conducted to verify the homogeneity of the slurry
in each pond. This analysis confirmed that the slurry was consistent across all ponds,
validating the effectiveness of the filling method. By ensuring that each pond contained
slurry of the same composition, the researchers could confidently proceed with the study,
knowing that any observed differences in the results would be due to the experimental
variables rather than inconsistencies in the slurry itself.
In this work, two pig slurry treatment techniques were thoroughly investigated: the
placement of a barley straw cover (SP) over the slurry storage pond and a biological additive
based on the addition of effective microorganisms (BP). These methods were evaluated for
their effectiveness in mitigating emissions and improving slurry characteristics. In addition
to these treatment techniques, a control pond (CP) was also installed to provide a baseline
for comparison (Figure 1).
Agriculture 2024, 14, x FOR PEER REVIEW 4 of 19
After filling, an initial analysis was conducted to verify the homogeneity of the slurry
in each pond. This analysis confirmed that the slurry was consistent across all ponds,
validating the effectiveness of the filling method. By ensuring that each pond contained
slurry of the same composition, the researchers could confidently proceed with the study,
knowing that any observed differences in the results would be due to the experimental
variables rather than inconsistencies in the slurry itself.
In this work, two pig slurry treatment techniques were thoroughly investigated: the
placement of a barley straw cover (SP) over the slurry storage pond and a biological
additive based on the addition of effective microorganisms (BP). These methods were
evaluated for their effectiveness in mitigating emissions and improving slurry
characteristics. In addition to these treatment techniques, a control pond (CP) was also
installed to provide a baseline for comparison (Figure 1).
Figure 1. Control storage pond (CP) after filling.
The barley straw used as a cover technique came from a feed factory for ruminant
animals. It is a fibrous byproduct with low nutritional value and is chopped during its
processing to achieve a particle size of around 8–10 cm with a characteristic yellow color.
A thickness of 20–25 cm of barley straw cover was manually spread over the pond at the
beginning of each period and the coverage was maintained until the end of the experiment
(Figure 2).
Figure 2. Manual covering of the storage pond with barley straw.
2.3. Effective Microorganism Preparation and Application
The biological additive consisted of the use of EM, primarily composed of a
consortium with lactic acid bacteria (Lactobacillus spp.), phototrophic bacteria
(Rhodopseudomonas spp.) and yeast (Saccharomyces spp.). This product is commonly used
for the treatment and maintenance of water, based on a selected community of beneficial
microorganisms from various families that work in synergy. Figure 3 shows a schematic
Figure 1. Control storage pond (CP) after filling.
The barley straw used as a cover technique came from a feed factory for ruminant
animals. It is a fibrous byproduct with low nutritional value and is chopped during
its processing to achieve a particle size of around 8–10 cm with a characteristic yellow
color. A thickness of 20–25 cm of barley straw cover was manually spread over the pond
at the beginning of each period and the coverage was maintained until the end of the
experiment (Figure 2).
Agriculture 2024, 14, x FOR PEER REVIEW 4 of 19
After filling, an initial analysis was conducted to verify the homogeneity of the slurry
in each pond. This analysis confirmed that the slurry was consistent across all ponds,
validating the effectiveness of the filling method. By ensuring that each pond contained
slurry of the same composition, the researchers could confidently proceed with the study,
knowing that any observed differences in the results would be due to the experimental
variables rather than inconsistencies in the slurry itself.
In this work, two pig slurry treatment techniques were thoroughly investigated: the
placement of a barley straw cover (SP) over the slurry storage pond and a biological
additive based on the addition of effective microorganisms (BP). These methods were
evaluated for their effectiveness in mitigating emissions and improving slurry
characteristics. In addition to these treatment techniques, a control pond (CP) was also
installed to provide a baseline for comparison (Figure 1).
Figure 1. Control storage pond (CP) after filling.
The barley straw used as a cover technique came from a feed factory for ruminant
animals. It is a fibrous byproduct with low nutritional value and is chopped during its
processing to achieve a particle size of around 8–10 cm with a characteristic yellow color.
A thickness of 20–25 cm of barley straw cover was manually spread over the pond at the
beginning of each period and the coverage was maintained until the end of the experiment
(Figure 2).
Figure 2. Manual covering of the storage pond with barley straw.
2.3. Effective Microorganism Preparation and Application
The biological additive consisted of the use of EM, primarily composed of a
consortium with lactic acid bacteria (Lactobacillus spp.), phototrophic bacteria
(Rhodopseudomonas spp.) and yeast (Saccharomyces spp.). This product is commonly used
for the treatment and maintenance of water, based on a selected community of beneficial
microorganisms from various families that work in synergy. Figure 3 shows a schematic
Figure 2. Manual covering of the storage pond with barley straw.
2.3. Effective Microorganism Preparation and Application
The biological additive consisted of the use of EM, primarily composed of a consortium
with lactic acid bacteria (Lactobacillus spp.), phototrophic bacteria (Rhodopseudomonas spp.)
and yeast (Saccharomyces spp.). This product is commonly used for the treatment and
Agriculture 2024,14, 1788 5 of 17
maintenance of water, based on a selected community of beneficial microorganisms from
various families that work in synergy. Figure 3shows a schematic representation of the
procedure followed to prepare the mixture of EM. The properties that characterize the EM
formula are as follows: lactic acid bacteria, 1.5
×
10
6
CFU mL
−1
; photosynthetic bacteria,
1.2
×
10
6
CFU mL
−1
; yeast, 7
×
10
5
CFU mL
−1
; pH, 3.3; and density, 1.012 g cc
−1
. The
addition of EM was carried out according to a structured pattern. Initially, a dose of
0.5 L m−3
was administered, followed by a maintenance schedule in which a subsequent
dose of 1.0 L m
−3
was added every 15 days. This consistent regimen ensured the continued
effectiveness of the treatment throughout the study period.
Agriculture 2024, 14, x FOR PEER REVIEW 5 of 19
representation of the procedure followed to prepare the mixture of EM. The properties
that characterize the EM formula are as follows: lactic acid bacteria, 1.5 × 106 CFU mL−1;
photosynthetic bacteria, 1.2 × 106 CFU mL−1; yeast, 7 × 105 CFU mL−1; pH, 3.3; and density,
1.012 g cc−1. The addition of EM was carried out according to a structured paern. Initially,
a dose of 0.5 L m−3 was administered, followed by a maintenance schedule in which a
subsequent dose of 1.0 L m−3 was added every 15 days. This consistent regimen ensured
the continued effectiveness of the treatment throughout the study period.
Figure 3. Scheme of the preparation procedure for effective microorganisms (EMs).
2.4. Pig Slurry Parameters and Methodology
Slurry samples were collected during the 5-week testing periods, coinciding with the
gas measurements. In each experimental period, three replicates of the same pond were
taken through homogenization. All slurry samples were carefully collected in a sterile
container, preserved at 4 °C and transported to the laboratories for analysis.
The properties analyzed in slurry were pH, electrical conductivity (EC), dry matter,
chemical oxygen demand (COD), biological oxygen demand after 5 days (BOD5), total
nitrogen (Total N), ammoniacal nitrogen (NH4+-N), organic nitrogen, (Org.-N), nitrogen as
nitrites (NO2−-N), nitrogen as nitrates (NO3−-N), phosphate ion (PO43−) and potassium ion (K+).
The pH and EC were measured in situ using HANNA multiparameter equipment (ref.
HI98194, made for Hanna Instruments, Inc., Woonsocket, RI, USA). For dry matter
determination, 100 g of slurry was weighed in a capsule and then dried in an oven at 105 °C
until constant weight for at least 24 h. The COD was determined via photometric analysis of
the chromium (III) concentration after 2 h of oxidation with potassium dichromate/sulfuric
acid and silver sulfate at 148 °C (Macherey–Nagel GmbH & Co., KG, Nanocolor Test; ref. 985
028/29, Weilheim, Germany) according to German standard methods [11–13]. The BOD5 was
determined using the Oxitop WTW equipment and measured with a manometer (Darmstadt,
Germany). Total N was calculated from the sum of Kjeldahl N, N-NO3− and N-NO2-. Kjeldahl
N content was measured using a modified Kjeldahl method [14]: 1 mL of pig slurry was used
for digestion and the form NH4+–N was determined via steam distillation, followed by
titration with HCl 0.1 N. Kjeldahl N encompassed both Org.-N and NH4+–N. Finally, NH4+–N
was determined with the previous methodology but without digestion. N-NO3−, N-NO2−,
PO43− and K+ were determined by an ionic chromatography technique (Methrom, 861
Advanced Compact IC) after sample preparation.
2.5. Methodology for Measuring Gaseous Emissions
Measurements of emissions for CH4, CO2 N2O, and NH3 were conducted using a
dynamic chamber. The floating dynamic chamber used in this study was a rectangular
PVC open boom chamber coupled with a base of polyethylene to keep it floating.
Considering the VERA test protocol requirements [15], it is necessary to confine a
sampling area of 0.5 m2 minimum under hood/canopy to quantify emissions; therefore,
the dynamic chamber used in this study has an effective coverage area of 0.564 m2.
Figure 3. Scheme of the preparation procedure for effective microorganisms (EMs).
2.4. Pig Slurry Parameters and Methodology
Slurry samples were collected during the 5-week testing periods, coinciding with the
gas measurements. In each experimental period, three replicates of the same pond were
taken through homogenization. All slurry samples were carefully collected in a sterile
container, preserved at 4 ◦C and transported to the laboratories for analysis.
The properties analyzed in slurry were pH, electrical conductivity (EC), dry mat-
ter, chemical oxygen demand (COD), biological oxygen demand after 5 days (BOD
5
),
total nitrogen (Total N), ammoniacal nitrogen (NH
4+
-N), organic nitrogen, (Org.-N), ni-
trogen as nitrites (NO
2−
-N), nitrogen as nitrates (NO
3−
-N), phosphate ion (PO
43−
) and
potassium ion (K+).
The pH and EC were measured in situ using HANNA multiparameter equipment
(ref. HI98194, made for Hanna Instruments, Inc., Woonsocket, RI, USA). For dry matter
determination, 100 g of slurry was weighed in a capsule and then dried in an oven at 105
◦
C
until constant weight for at least 24 h. The COD was determined via photometric analysis of
the chromium (III) concentration after 2 h of oxidation with potassium dichromate/sulfuric
acid and silver sulfate at 148
◦
C (Macherey–Nagel GmbH & Co., KG, Nanocolor Test; ref.
985 028/29, Weilheim, Germany) according to German standard methods [
11
–
13
]. The
BOD
5
was determined using the Oxitop WTW equipment and measured with a manometer
(Darmstadt, Germany). Total N was calculated from the sum of Kjeldahl N, N-NO
3−
and
N-NO
2
-. Kjeldahl N content was measured using a modified Kjeldahl method [
14
]: 1 mL
of pig slurry was used for digestion and the form NH
4+
–N was determined via steam
distillation, followed by titration with HCl 0.1 N. Kjeldahl N encompassed both Org.-N and
NH
4+
–N. Finally, NH
4+
–N was determined with the previous methodology but without
digestion. N-NO
3−
, N-NO
2−
, PO
43−
and K
+
were determined by an ionic chromatography
technique (Methrom, 861 Advanced Compact IC) after sample preparation.
2.5. Methodology for Measuring Gaseous Emissions
Measurements of emissions for CH
4
, CO
2
N
2
O, and NH
3
were conducted using a
dynamic chamber. The floating dynamic chamber used in this study was a rectangular PVC
open bottom chamber coupled with a base of polyethylene to keep it floating. Considering
the VERA test protocol requirements [
15
], it is necessary to confine a sampling area of
0.5 m2
Agriculture 2024,14, 1788 6 of 17
minimum under hood/canopy to quantify emissions; therefore, the dynamic chamber used
in this study has an effective coverage area of 0.564 m2.
The methodology using floating dynamic chamber [
16
] represents a relatively straight-
forward approach for directly measuring greenhouse gases emitted from storage systems.
Although it is frequently used, there is a lack of information about studies related to this
type of method. This chamber comprises an inlet and an outlet port, pivotal for collecting
representative samples of inlet and outlet gases during the testing period using driving
and extraction equipment.
The fundamental principle of this technique involves isolating a section of the surface
where the slurry is stored and monitoring the change in gas concentration within the
chamber over time. A simple or multigas analyzer for continuous gas measurement is
connected to the dynamic chamber. The gases are introduced into the analyzer through a
tube, and the internal pump extracts the gas sample through the instrument, displaying the
measurements on the device. The analyzer measures and analyzes an infrared spectrum of
the gas samples using a photoacoustic sensor based on an optical microphone patented by
GASERA (model equipment: GASERA ONE Ltd., Turku, Finland). The flow rate for each
gas was determined using a mass balance as described in the following equation:
F=
Cout −Cin
Ab Ai ·Vi (1)
where Fis the flux with dynamic chamber in kg (gas) ha
−1
h
−1
, and Cout and Cin represent
the time-averaged gaseous concentration of the gas (kg m
−3
) in the outlet and inlet air,
respectively. Ai denotes the cross-sectional area of the inlet (m
2
), Vi indicates the measured
wind speed at the tunnel inlet (m s
−1
), and Ab represents the source surface area covered
by the tunnel canopy (m2) [15,17,18].
The sampling period and frequency of gas emissions were chosen considering and
adapting to the recommendations given by VERA test protocol from storage systems.
The emissions of CH
4
, CO
2
N
2
O, and NH
3
were measured every 15 days, 5 weeks in
autumn 2022 and 5 weeks in spring 2023. The concentrations of each gas were measured
by positioning the floating dynamic chamber over the PS storage ponds. The initial
concentration was measured at T1 = 0 min, when the dynamic chamber was placed over
the slurry and after 30 min of placing the chamber, T2 = 30 min, the average emission rates
over 30 min were calculated according to Equation (1) for each gas using air flow rates and
gas concentrations recorded as 3 replicates at 10 min intervals.
2.6. Statistical Analysis
The collected data were analyzed using the statistical software package SPSS (IBM
SPSS Statistics, Version 26). One-way analysis of variance (ANOVA) was employed to
assess the impacts of each treatment on slurry composition and gaseous emissions from
storage. Statistical significance (p< 0.05) of the differences between treatment means was
determined through Tukey’s HSD (honestly significant difference) test; parameters of the
PS for treatments were also analyzed.
3. Results
3.1. Results of Treatments on Pig Slurry
The results of the physical, chemical and biological analyses of the PS storage ponds
are presented below for the autumn (W1-W5) and spring (W6-W10) seasons (Table 1).
For parameters such as EC, dry matter, COD, BOD
5
, total N, NH
4+
-N, Org.-N, NO
3−
-
N, and PO
43−
, a generally decreasing trend was observed as sampling progressed. The
remaining parameters showed either an increasing trend or minimal variation between
the first and last sampling. For pH, significant differences were observed between the
different treatments (CP, SP, and BP). In autumn, the BP treatment exhibited the highest pH
values, which increased progressively over time. The CP and SP treatments showed lower
pH values compared to BP, with significant differences between them (except for W3). In
Agriculture 2024,14, 1788 7 of 17
spring, pH values were more stable and similar across treatments, with no statistically
significant differences. These results suggest that the BP treatment, which involves the
use of bacteria, had a notable impact on the pH of pig slurry, progressively increasing
it. These findings are consistent with previous studies. Regarding EC, autumn results
for the CP and SP treatments showed similar and stable values over time, while the BP
treatment exhibited a significant decrease in EC during the final measurement. In spring,
the CP and SP treatments again presented similar and stable EC values, whereas the BP
treatment showed a slight decrease compared to previous measurements. These results
suggest that the CP and SP treatments did not significantly affect the EC of the slurry, as
values remained relatively constant over time. In contrast, the BP treatment showed a
decrease in EC, especially in the final measurement. However, a more detailed statistical
analysis is necessary to confirm these differences and assess their significance. Previous
studies have also examined the influence of different treatments on the EC of pig slurry,
supporting these findings. For dry matter, the CP and SP treatments showed similar and
stable values over time, while the BP treatment demonstrated a decreasing trend from W3
to W5, potentially due to changes in the composition and water content of the slurry. In
spring, the CP and SP treatments continued to present stable dry matter values, while the
BP treatment showed variability, with a significant decrease in W5 (also observed for CP).
These results suggest that the CP and SP treatments did not significantly impact the dry
matter content of pig manure, as values remained relatively constant over time.
Regarding biodegradability parameters, the SP and BP treatments showed similar
and stable COD values over time during autumn, while the CP treatment exhibited some
variability in COD values, though they generally remained in a lower range. In spring,
all treatments showed a significant decrease in COD values by W5. The BP treatment
demonstrated minimal variability in COD values, which may indicate stability in the
composition and organic matter content of the slurry. The BOD
5
parameter showed
variable values over time for both the CP and SP treatments, with SP showing the lowest
values from W3 to W5. In spring, the trend was similar to that observed in autumn, with
BOD5values decreasing as the experiment progressed.
Regarding total N, a decreasing trend was observed for both stations as the weekly
sampling progressed. In autumn, no significant differences in total N values were noted
between treatments, except for W2. However, in spring, the SP and BP treatments showed
similar and stable values for total nitrogen, while the CP treatment displayed slightly lower
values. These results suggest that the treatments had a limited effect on the total N content
of pig slurry, as the values remained relatively constant over time. The NH
4+
-N fraction did
not show significant differences between treatments in most weeks, except in W2, where
the CP treatment had a significantly higher value than the other treatments. In spring, some
significant differences were observed between treatments. Notably, in W8, all treatments
showed an increase in NH
4+
-N values, which was significant for SP and BP. These results
suggest that treatments such as straw application and the addition of EM can influence
NH
4+
-N concentration in pig slurry during certain weeks, although the effects are not
consistent throughout the entire period studied. For Org.-N, no clear trend was observed
during autumn. However, a decreasing trend was evident in spring, with significantly
lower values recorded for W10 in both the SP and BP treatments. During spring (W6–W8;
W10), the SP and BP treatments showed significantly higher values than the CP treatment.
These results suggest that the application of a straw crust (SP) and the addition of EM
(BP) may influence the concentration of Org.-N in pig slurry during certain weeks, though
the effects are not consistent throughout the entire study period. The recorded results for
NO
2−
-N were very low and did not provide substantial information for the study. Similarly,
NO
3−
-N values were also low across both stations, without a clear trend. Recorded values
for NO
2−
-N ranged between 0 g/L (SP-W7; CP-W7, and BP-W8) and 0.020 g/L (SP-W1),
while NO3−-N values ranged between 0.007 g/L (SP-W9) and 0.048 mg/L (BP-W1).
Agriculture 2024,14, 1788 8 of 17
Table 1. Means and standard deviation values for the evolution of physicochemical and biological parameters in control, straw, and biological ponds of raw pig
slurry (n = 3).
Parameter
(*)
Treatment
(
**
)Autumn 2022 (***)Spring 2023 (***)
W1 W2 W3 W4 W5 W6 W7 W8 W9 W10
pH
CP 7.23
±
0.06
a
7.41
±
0.05
a
7.67
±
0.02
a
7.79
±
0.01
b
8.13
±
0.15
b
7.32
±
0.01 ns 7.68
±
0.02
b
7.71
±
0.02
c
7.94
±
0.01
b
8.11
±
0.02 ns
SP 7.46
±
0.02
b
7.52
±
0.01
b
7.61
±
0.04
a
7.67
±
0.01
a
7.74
±
0.01
a
7.31
±
0.03 ns 7.49
±
0.08
a
7.56
±
0.02
a
7.76
±
0.04 ab 7.74
±
0.47 ns
BP 7.61
±
0.05
c
7.68
±
0.02
c
7.81
±
0.02
b
8.38
±
0.02
c
8.61
±
0.01
c
7.30
±
0.01 ns 7.63
±
0.04
b
7.65
±
0.01
b
7.71
±
0.12 ab 8.04
±
0.02 ns
EC (dS
m−1)
CP 22.21
±
0.40 ns 22.91
±
0.36 ns 24.76
±
0.13
b
25.62
±
0.64
b
25.48
±
0.04
c
22.09
±
0.02 ab 24.81
±
0.12
b
21.56
±
7.46 ns 26.78
±
0.01
b
23.55
±
0.06
b
SP 22.54
±
0.31 ns 22.52
±
0.26 ns 22.41
±
0.20
a
22.07
±
1.39
a
22.41
±
0.13
b
23.20
±
0.52
b
23.46
±
0.42
a
23.99
±
0.11 ns 24.07
±
0.04
a
20.17
±
1.19
a
BP 22.50
±
0.09 ns 22.42
±
0.03 ns 22.25
±
0.14
a
22.29
±
0.14
a
19.71
±
0.04
a
21.78
±
0.71
a
24.66
±
0.05
b
25.96
±
0.01 ns 26.72
±
0.06
b
22.82
±
0.87
b
Dry matter
(%)
CP 16.79
±
0.81 ns 16.53
±
0.57 ns 15.21
±
0.67
b
13.25
±
0.60
a
16.01
±
0.05
a
17.78
±
0.19
a
15.41
±
0.39 ns 15.78
±
0.07
b
15.33
±
0.09
b
14.75
±
0.15
b
SP 14.34
±
2.53 ns 14.27
±
1.60 ns 14.39
±
0.26 ab 14.62
±
0.27
b
16.56
±
0.40
a
17.16
±
0.12
b
15.53
±
0.24 ns 14.36
±
0.52
a
13.35
±
0.24
a
13.53
±
0.16
a
BP 14.45
±
0.89 ns 14.10
±
0.30 ns 13.58
±
0.39
a
16.38
±
0.43
c
19.46
±
0.11
b
15.76
±
0.04
c
15.31
±
0.06 ns 15.41
±
0.23
b
15.81
±
0.18
c
14.37
±
0.28
b
COD (g
L−1)
CP 47.87
±
2.29
b
55.00
±
6.56
b
22.07
±
0.64
b
21.20
±
0.20
b
15.73
±
0.31
c
32.50
±
0.50
b
21.00
±
0.00 ns 26.00
±
0.00
b
17.50
±
0.50
b
10.90
±
0.20
b
SP 25.37
±
1.39
a
22.37
±
0.71
a
17.47
±
0.90
a
13.27
±
0.81
a
11.07
±
0.64
a
26.00
±
1.00
a
22.50
±
1.50 ns 20.50
±
1.50
a
11.00
±
0.00
a
7.90
±
0.10
a
BP 22.74
±
1.00
a
21.27
±
0.23
a
18.70
±
0.10
a
13.90
±
0.82
a
14.10
±
0.10
b
27.00
±
1.00
a
22.50
±
0.50 ns 28.00
±
2.00
b
22.00
±
1.00
c
14.05
±
0.05
c
BOD
5
(g O
2
L−1)
CP 11.99
±
0.69
b
6.44
±
0.16
a
9.21
±
0.11
b
9.07
±
0.10
c
4.48
±
0.06
c
12.00
±
0.36
b
11.68
±
0.08
b
12.98
±
0.03
c
9.41
±
0.26
b
2.54
±
0.09
b
SP 10.37
±
0.52
a
9.52
±
0.14
b
6.40
±
0.40
a
2.60
±
0.06
a
2.65
±
0.08
a
11.44
±
0.08
a
11.10
±
0.11
a
8.20
±
0.06
a
2.67
±
0.15
a
1.99
±
0.02
a
BP 10.04
±
0.47
a
10.27
±
0.25
c
10.69
±
0.12
c
3.32
±
0.20
b
3.74
±
0.07
b
11.11
±
0.02
a
13.07
±
0.08
c
10.60
±
0.21
b
10.27
±
0.37
c
3.44
±
0.09
c
Total N (g
L−1)
CP 3.67
±
0.76 ns 3.52
±
0.54
b
2.94
±
0.11 ns 2.52
±
0.73 ns 2.72
±
0.66 ns 2.66
±
0.11
a
2.33
±
0.07
a
2.33
±
0.06
a
2.70
±
0.21 ns 1.73
±
0.06 ns
SP 2.53
±
0.59 ns 2.38
±
0.28
a
2.09
±
0.50 ns 2.60
±
0.34 ns 2.18
±
0.50 ns 2.95
±
0.05
b
2.48
±
0.06
b
2.59
±
0.14
b
2.61
±
0.08 ns 1.59
±
0.09 ns
BP 2.92
±
0.24 ns 2.56
±
0.13
a
1.91
±
0.66 ns 2.43
±
0.38 ns 1.80
±
0.16 ns 2.87
±
0.04
b
2.47
±
0.04 ab 3.24
±
0.05
c
2.50
±
0.17 ns 1.72
±
0.20 ns
NH4+-N (g
L−1)
CP 2.59
±
0.30 ns 2.49
±
0.21
b
2.27
±
0.24 ns 1.94
±
0.37 ns 2.09
±
0.54 ns 1.66
±
0.19 ns 1.81
±
0.06
b
1.85
±
0.09
a
1.74
±
0.18 ab 1.22
±
0.21 ns
SP 2.31
±
0.48 ns 2.11
±
0.07
a
1.71
±
0.50 ns 2.35
±
0.45 ns 1.35
±
0.41 ns 1.68
±
0.07 ns 1.60
±
0.12
a
1.94
±
0.13 ab 1.88
±
0.08
b
1.25
±
0.05 ns
BP 2.45
±
0.13 ns 2.20
±
0.12 ab 1.29
±
0.47 ns 1.49
±
0.51 ns 1.36
±
0.26 ns 1.81
±
0.14 ns 1.56
±
0.04
a
2.11
±
0.03
b
1.56
±
0.06
a
1.07
±
0.11 ns
Org.-N (g
L−1)
CP 1.03
±
0.53 ns 0.99
±
0.42
b
0.63
±
0.12 ns 0.54
±
0.37 ns 0.60
±
0.45 ns 0.96
±
0.18 ns 0.49
±
0.08
a
0.44
±
0.14
a
0.92
±
0.36 ns 0.49
±
0.15 ns
SP 0.18
±
0.11 ns 0.22
±
0.25
a
0.34
±
0.18 ns 0.23
±
0.22 ns 0.79
±
0.75 ns 1.24
±
0.03 ns 0.84
±
0.07
b
0.62
±
0.13
a
0.72
±
0.12 ns 0.33
±
0.14 ns
BP 0.42
±
0.24 ns 0.32
±
0.10 ab 0.58
±
0.93 ns 0.90
±
0.27 ns 0.40
±
0.36 ns 1.02
±
0.12 ns 0.87
±
0.04
b
1.09
±
0.06
b
0.91
±
0.12 ns 0.62
±
0.30 ns
NO2−-N (g
L−1)
CP 0.003
±
0.001
a
0.003
±
0.001
a
0.002
±
0.000 ns 0.003
±
0.001 ns 0.002
±
0.000 ns 0.007
±
0.004 ns 0.000
±
0.000 ns 0.001
±
0.002 ns 0.003
±
0.000 ns 0.002
±
0.002 ns
SP 0.020
±
0.008
b
0.015
±
0.002
b
0.003
±
0.000 ns 0.000
±
0.000 ns 0.002
±
0.000 ns 0.005
±
0.000 ns 0.015
±
0.013 ns 0.003
±
0.000 ns 0.003
±
0.000 ns 0.003
±
0.000 ns
BP 0.003
±
0.000
a
0.003
±
0.000
a
0.002
±
0.001 ns 0.003
±
0.000 ns 0.003
±
0.000 ns 0.007
±
0.002 ns 0.011
±
0.014 ns 0.000
±
0.000 ns 0.002
±
0.002 ns 0.003
±
0.000 ns
NO3−-N (g
L−1)
CP 0.043
±
0.004
b
0.041
±
0.002
b
0.038
±
0.001 ns 0.039
±
0.004 ns 0.023
±
0.001 ns 0.026
±
0.001 ab 0.031
±
0.001 ns 0.036
±
0.000 ns 0.035
±
0.000
b
0.015
±
0.001 ab
SP 0.018
±
0.004
a
0.028
±
0.003
a
0.043
±
0.005 ns 0.022
±
0.001 ns 0.039
±
0.002 ns 0.024
±
0.001
a
0.022
±
0.006 ns 0.029
±
0.001 ns 0.007
±
0.000
a
0.010
±
0.001
a
BP 0.048
±
0.014
b
0.045
±
0.007
b
0.045
±
0.004 ns 0.036
±
0.012 ns 0.041
±
0.002 ns 0.025
±
0.000
b
0.027
±
0.005 ns 0.036
±
0.001 ns 0.037
±
0.001
b
0.018
±
0.002
b
PO43−(mg
L−1)
CP 449.6
±
143.8
b
413.8
±
101.5
b
209.9
±
13.5
a
277.9
±
13.6 ns 228.2
±
44.1 ab 898.3
±
139.6
b
212.8
±
3.8 ab 290.2
±
4.8
a
281.8
±
5.5
b
220.0
±
6.3
a
SP 204.7
±
23.5
a
236.4
±
21.6
a
325.2
±
55.0
b
207.7
±
8.5 ns 324.3
±
57.5
b
400.5
±
30.7
a
290.5
±
55.7
b
321.9
±
36.8 ab 209.3
±
4.9
a
199.0
±
0.7
a
BP 351.8
±
71.4 ab 313.1
±
36.6 ab 315.7
±
28.0
b
262.0
±
55.5 ns 209.1
±
11.6
a
459.1
±
55.3
a
193.4
±
14.9
a
352.7
±
4.9
b
312.5
±
4.2
c
583.4
±
25.2
b
K+(mg
L−1)
CP 1.846.0
±
230.1 ns 1.966.9
±
167.7 ns 2.155.4
±
39.3
b
2.432.2
±
68.4
b
2.733.1
±
72.7
c
2.096.6
±
31.7 ns 2.451.4
±
51.5 ns 2.772.7
±
53.0
b
2.975.6
±
57.4
b
3.096.9
±
137.0
b
SP 1.780.0
±
16.6 ns 1.858.8
±
24.0 ns 1.963.9
±
59.6
a
2.225.9
±
10.4
a
1.992.1
±
60.0
a
2.144.7
±
12.2 ns 2.243.3
±
225.6 ns 2.492.7
±
95.5
a
2.461.2
±
54.3
a
2.247.8
±
35.1
a
BP 1.745.6
±
26.6 ns 1.851.9
±
36.2 ns 2.021.3
±
18.4
a
2.291.2
±
26.1
a
2.253.6
±
29.8
b
2.115.8
±
2.8 ns 2.495.6
±
124.0 ns 2.906.2
±
98.6
b
3.183.9
±
75.4
c
2.995.1
±
70.4
b
(
*
)
EC: electrical conductivity; COD: chemical oxygen demand; BOD
5
: biological oxygen demand after 5 days; Total N: total nitrogen; NH
4+
-N: ammoniacal nitrogen; NO
2−
-N: nitrogen
as nitrites; NO
3−
-N: nitrogen as nitrates; PO
43−
: phosphates; K
+
: potassium ion.
(
**
)
CP: Control Pond; SP: Straw Pond; BP: Biological Pond.
(
***
)
W: week. Different letters indicate
significant differences between the treatment and the control (p < 0.05). ns: no significant differences.
Agriculture 2024,14, 1788 9 of 17
For PO
43−
, a significant decrease in CP concentrations was observed between the
first and final sampling at both stations. Phosphate values for SP decreased in autumn
but increased in spring, with the most notable variation occurring during the spring
season. This may be due to the presence of the straw cover, which could influence PO
43−
concentrations. In the BP treatment, values were significantly higher compared to CP and
SP in W3 and W8-W10, suggesting that treatment with EM may have a variable impact on
PO
43−
levels over time. Finally, K
+
concentrations ranged from 1745.6 mg/L (BP-W1) to
3183.9 mg/L (BP-W9). For both stations and all treatments, an increase in K
+
concentrations
were observed as the weekly sampling progressed, with CP and BP treatments showing
significantly higher concentrations compared to SP.
3.2. GHG (CO2, CH4, and N2O), and Ammonia (NH3) Emissions
The results for GHG and ammonia emissions are presented in Figures 4–7to assess the
impact of treatments and study periods (autumn from W1–W5 and spring from W6–W10).
Each figure provides a comprehensive summary of the gases studied for the three ponds
(CP, SP and BP). Additionally, Figure 8displays the mean values and standard deviation of
ambient temperature and humidity in the study area. Tables 2and 3show the percentages
of reductions in emissions, comparing the SP and BP treatments to CP at the end of each
period (W5 and W10, for autumn and spring).
Agriculture 2024, 14, x FOR PEER REVIEW 11 of 19
3.2. GHG (CO
2
, CH
4
, and N
2
O), and Ammonia (NH
3
) Emissions
The results for GHG and ammonia emissions are presented in Figures 4–7 to assess
the impact of treatments and study periods (autumn from W1–W5 and spring from W6–
W10). Each figure provides a comprehensive summary of the gases studied for the three
ponds (CP, SP and BP). Additionally, Figure 8 displays the mean values and standard
deviation of ambient temperature and humidity in the study area. Tables 2 and 3 show
the percentages of reductions in emissions, comparing the SP and BP treatments to CP at
the end of each period (W5 and W10, for autumn and spring).
Figure 4. Average CO
2
emission (g CO
2
m
−2
day
−1
) for each measurement week (W1–W5: autumn;
W6–W10: spring). CP: Control Pond; SP: Straw Pond; and BP: Biological Pond. Different leers
indicate significant differences (p < 0.05) between treatments.
Figure 5. Average CH
4
emission (g CH
4
m
−2
day
−1
) for each measurement week (W1–W5: autumn;
W6–W10: spring). CP: Control Pond; SP: Straw Pond; and BP: Biological Pond. Different leers
indicate significant differences (p < 0.05) between treatments.
Figure 4. Average CO
2
emission (g CO
2
m
−2
day
−1
) for each measurement week (W1–W5: autumn;
W6–W10: spring). CP: Control Pond; SP: Straw Pond; and BP: Biological Pond. Different letters
indicate significant differences (p< 0.05) between treatments.
Table 2. Percentage reduction (%) calculated in week 5 in the treated pond compared to the control
pond in autumn.
Autumn
Gas (g m−2d−1)Control
Pond
Straw
Pond
1Red (%)
Biological
Pond
1Red
(%)
CH40.00280 0.00567 - 0.00030 89
CO20.14108 0.03653 74 0.01283 91
N2O 0.00052 0.00044 14 0.00016 69
NH30.00122 0.00048 61 0.00121 1
1Percentage reduction = 100 −((treatment/control) ×100); (-): with no reduction.
Agriculture 2024,14, 1788 10 of 17
Table 3. Percentage reduction (%) calculated in week 5 in the treated pond compared to the control
pond in spring.
Spring
Gas (g m−2d−1)Control
Pond
Straw
Pond
1Red
(%)
Biological
Pond
1Red
(%)
CH40.47727 0.19255 60 0.00859 98
CO22.72718 2.11327 23 0.10739 96
N2O 0.00047 0.00371 - 0.00003 93
NH30.09454 0.00284 97 0.03890 59
1Percentage reduction = 100 −((treatment/control) ×100); (-): with no reduction.
Agriculture 2024, 14, x FOR PEER REVIEW 11 of 19
3.2. GHG (CO
2
, CH
4
, and N
2
O), and Ammonia (NH
3
) Emissions
The results for GHG and ammonia emissions are presented in Figures 4–7 to assess
the impact of treatments and study periods (autumn from W1–W5 and spring from W6–
W10). Each figure provides a comprehensive summary of the gases studied for the three
ponds (CP, SP and BP). Additionally, Figure 8 displays the mean values and standard
deviation of ambient temperature and humidity in the study area. Tables 2 and 3 show
the percentages of reductions in emissions, comparing the SP and BP treatments to CP at
the end of each period (W5 and W10, for autumn and spring).
Figure 4. Average CO
2
emission (g CO
2
m
−2
day
−1
) for each measurement week (W1–W5: autumn;
W6–W10: spring). CP: Control Pond; SP: Straw Pond; and BP: Biological Pond. Different leers
indicate significant differences (p < 0.05) between treatments.
Figure 5. Average CH
4
emission (g CH
4
m
−2
day
−1
) for each measurement week (W1–W5: autumn;
W6–W10: spring). CP: Control Pond; SP: Straw Pond; and BP: Biological Pond. Different leers
indicate significant differences (p < 0.05) between treatments.
Figure 5. Average CH
4
emission (g CH
4
m
−2
day
−1
) for each measurement week (W1–W5: autumn;
W6–W10: spring). CP: Control Pond; SP: Straw Pond; and BP: Biological Pond. Different letters
indicate significant differences (p< 0.05) between treatments.
Agriculture 2024, 14, x FOR PEER REVIEW 12 of 19
Figure 6. Average N
2
O emission (mg N
2
O m
−2
day
−1
) for each measurement week (W1–W5: autumn;
W6–W10: spring). CP: Control Pond; SP: Straw Pond; and BP: Biological Pond. Different leers
indicate significant differences (p < 0.05) between treatments.
Figure 7. Average NH
3
emission (mg NH
3
m
−2
day
−1
) for each measurement week (W1–W5: autumn;
W6–W10: spring). CP: Control Pond; SP: Straw Pond; and BP: Biological Pond. Different leers
indicate significant differences (p < 0.05) between treatments.
Figure 6. Average N
2
O emission (mg N
2
O m
−2
day
−1
) for each measurement week (W1–W5: autumn;
W6–W10: spring). CP: Control Pond; SP: Straw Pond; and BP: Biological Pond. Different letters
indicate significant differences (p< 0.05) between treatments.
Agriculture 2024,14, 1788 11 of 17
Agriculture 2024, 14, x FOR PEER REVIEW 12 of 19
Figure 6. Average N
2
O emission (mg N
2
O m
−2
day
−1
) for each measurement week (W1–W5: autumn;
W6–W10: spring). CP: Control Pond; SP: Straw Pond; and BP: Biological Pond. Different leers
indicate significant differences (p < 0.05) between treatments.
Figure 7. Average NH
3
emission (mg NH
3
m
−2
day
−1
) for each measurement week (W1–W5: autumn;
W6–W10: spring). CP: Control Pond; SP: Straw Pond; and BP: Biological Pond. Different leers
indicate significant differences (p < 0.05) between treatments.
Figure 7. Average NH
3
emission (mg NH
3
m
−2
day
−1
) for each measurement week (W1–W5:
autumn; W6–W10: spring). CP: Control Pond; SP: Straw Pond; and BP: Biological Pond. Different
letters indicate significant differences (p< 0.05) between treatments.
Agriculture 2024, 14, x FOR PEER REVIEW 13 of 19
Figure 8. Average Ambient Temperature (°C) and Humidity (%) for each measurement week (W1–
W5: autumn; W6–W10: spring). The average values for each week were calculated based on the
averages of 3 ponds.
Table 2. Percentage reduction (%) calculated in week 5 in the treated pond compared to the control
pond in autumn.
Aut um n
Gas (g m
−2
d
−1
) Control
Pond
Straw
Pond
1
Red (%) Biological
Pond
1
Red
(%)
CH
4
0.00280 0.00567 - 0.00030 89
CO
2
0.14108 0.03653 74 0.01283 91
N
2
O 0.00052 0.00044 14 0.00016 69
NH
3
0.00122 0.00048 61 0.00121 1
1
Percentage reduction = 100 − ((treatment/control) × 100); (-): with no reduction.
Table 3. Percentage reduction (%) calculated in week 5 in the treated pond compared to the control
pond in spring.
Spring
Gas (g m
−2
d
−1
) Control
Pond
Straw
Pond
1
Red
(%)
Biological
Pond
1
Red
(%)
CH
4
0.47727 0.19255 60 0.00859 98
CO
2
2.72718 2.11327 23 0.10739 96
N
2
O 0.00047 0.00371 - 0.00003 93
NH
3
0.09454 0.00284 97 0.03890 59
1
Percentage reduction = 100 − ((treatment/control) × 100); (-): with no reduction.
4. Discussion
4.1. Effects of Treatments on the Physical, Chemical and Biological Properties of Pig Slurry
The pH gradually increased in the CP treatment over the weeks in both seasons, with
initial values of 7.23 in autumn and 7.32 in spring, reaching 8.13 and 8.11, respectively, by
the end of the seasons. These results align with previous studies, which reported an
increase in pH in pig manure treated with organic maer, likely due to bacterial activity
during organic maer decomposition and the breakdown of volatile fay acids [4,19,20].
Additionally, pH plays a critical role in NH
3
emissions: at pH levels between 6 and 7,
almost all ammoniacal nitrogen tends to remain in solubilized form (NH
4+
); at pH > 7, the
Figure 8. Average Ambient Temperature (
◦
C) and Humidity (%) for each measurement week (W1–
W5: autumn; W6–W10: spring). The average values for each week were calculated based on the
averages of 3 ponds.
4. Discussion
4.1. Effects of Treatments on the Physical, Chemical and Biological Properties of Pig Slurry
The pH gradually increased in the CP treatment over the weeks in both seasons, with
initial values of 7.23 in autumn and 7.32 in spring, reaching 8.13 and 8.11, respectively,
by the end of the seasons. These results align with previous studies, which reported an
increase in pH in pig manure treated with organic matter, likely due to bacterial activity
during organic matter decomposition and the breakdown of volatile fatty acids [
4
,
19
,
20
].
Additionally, pH plays a critical role in NH
3
emissions: at pH levels between 6 and 7,
almost all ammoniacal nitrogen tends to remain in solubilized form (NH
4+
); at pH > 7, the
balance shifts toward the non-ionized and volatile form (NH
3
); and at pH > 11, all nitrogen
is present as NH3[21–23]. In the present study, the predominant form was NH3.
Agriculture 2024,14, 1788 12 of 17
Regarding EC, the CP treatment showed higher values than the SP and BP treatments
during most weeks and seasons. These findings are consistent with previous studies
by Chen [
24
], which indicated that the addition of organic matter can increase the EC
of pig manure through organic matter decomposition, particularly in the SP treatment.
However, the SP treatment showed slightly lower EC values compared to the CP treatment,
possibly due to the lower amount of nutrients released by the straw in comparison to the
CP treatment. The BP treatment presented similar EC values to the SP treatment, which
may suggest that microbial activity did not significantly affect the EC of the pig manure.
Regarding dry matter, the BP treatment exhibited the highest values at the end of autumn,
followed by the CP and SP treatments. These results are consistent with those reported by
Vanderzaag [
4
], who observed an increase in the dry matter of pig manure with the addition
of straw. In this study, the use of straw in the SP treatment also contributed to an increase
in dry matter, though to a lesser extent than the BP treatment, and without a clear trend in
spring. All treatments showed a reduction in COD and BOD
5
values, confirming active
decomposition processes within the slurries. The SP and BP treatments had significantly
lower COD values than the CP treatment in autumn, while the BP treatment showed higher
values in spring, potentially indicating a reduction in biodegradable organic matter in pig
manure. These findings support the fact that temperature is a crucial factor in microbial
metabolism, as the metabolic activity of most microorganisms is enhanced with increasing
temperature and weakened with decreasing temperature [
25
,
26
]. The optimal temperature
range for aerobic microorganisms is between 10 and 35
◦
C, with temperatures below 10
◦
C
typically having a negative effect on biological treatment. In relation to BOD
5
, the CP and
BP treatments showed the highest values in most weeks and seasons.
The decrease in N fractions indicates that N emissions into the atmosphere occurred,
with NH
4+
-N being the most significant decline. The rise in pH favors NH
3
volatilization [
4
].
These results are consistent with earlier studies that observed a reduction in total N in
pig manure treated with organic matter and bacteria [
5
,
24
]. The SP treatment exhibited
slightly lower total N values than the CP treatment, likely due to the lower nutrient release
from straw compared to the CP treatment. The BP treatment showed total N values
similar to the SP treatment, suggesting that bacterial activity and the chosen dose did
not significantly affect the total N content of the pig manure. Concerning NH
4
+-N, the
CP treatment showed higher values than the SP and BP treatments in most weeks and
seasons, with a gradual decrease over time for all treatments. These results agree with
previous studies that also observed a reduction in NH
4+
-N in pig manure treated with
organic matter and bacteria [
9
]. The SP treatment presented slightly lower NH
4+
-N values
than the CP treatment, which may be due to the lower nutrient release from straw. The BP
treatment had NH
4+
-N values similar to the SP treatment, suggesting that the activity of
the effective microorganisms (EMs) did not significantly impact the NH
4+
-N content in the
pig manure. Regarding organic nitrogen (Org.-N), the CP and BP treatments showed the
highest values in most weeks and seasons, followed by the SP treatment. These results are
consistent with findings by Provolo [
27
], who reported an increase in Org.-N in pig manure
with bacterial application. The addition of straw to pig manure (SP) also contributed to an
increase in Org.-N, though to a lesser extent than the BP treatment. In terms of NO
2−
-N
and NO
3−
-N, the values were very low in all treatments, often close to zero in most weeks
and seasons. These results contradict previous studies that observed fluctuations in these
nitrogen fractions in pig manure treated with organic matter [
24
,
28
]. The lack of detection
may be due to simultaneous nitrification–denitrification processes.
Regarding PO
43−
, the CP treatment showed higher values than the SP and BP treat-
ments in most weeks and seasons, with a gradual decrease over time. The SP treatment
exhibited slightly lower PO
43−
values than the CP treatment (W1, W2, W4, W6, W9, and
W10), likely due to the lower nutrient release from straw compared to the CP treatment. The
BP treatment had PO
43−
values similar to the SP treatment, which could suggest that bacte-
rial activity did not significantly impact the PO
43−
content in pig manure. Fangueiro [
19
]
reported an increase in inorganic phosphate concentrations through more labile phospho-
Agriculture 2024,14, 1788 13 of 17
rus fractions when slurry is acidified; conversely, PO
43−
concentrations tend to decrease
under alkaline conditions. Concerning K
+
, the CP treatment presented higher values than
the SP and BP treatments in most weeks and seasons, with a gradual increase over time.
These findings align with previous studies that reported an increase in K
+
in pig manure
treated with organic matter and bacteria [9]. The SP treatment exhibited slightly lower K+
values than the CP treatment, potentially due to the lower nutrient release from straw. The
BP treatment had K
+
values similar to the SP treatment, suggesting that the activity of EM
did not significantly impact the K+content in pig manure.
4.2. Treatment Effects on GHG (CO2, CH4, and N2O), and Ammonia (NH3) Emissions
4.2.1. GHG Emissions
The pattern of CO
2
and CH
4
emissions in the three ponds was similar during autumn,
while lower fluxes were observed in spring. Previous research [
4
] observed this trend,
reporting CO
2
and CH
4
emissions over 5 times higher in autumn compared to spring.
During W1, steady-state conditions for PS storage led to no significant differences
(p< 0.05)
in CO
2
and CH
4
emissions between treatments and the control. However, during W2,
CO
2
and CH
4
emissions decreased in the SP treatment compared to the control. Emissions
increased again in W3, potentially due to the homogenization following the addition of
microorganisms to the pond [
9
], resulting in significant differences (p< 0.05) between the
BP treatment and both CP and SP treatments. This increase in BP could be attributed
to anaerobic decomposition of organic matter by methanogenic microorganisms, which,
along with sugar-fermenting bacteria, contribute to the production of these gases. While
effective microorganisms (EMs) have been widely studied in various contexts (such as
their properties, applications, and effects on manure or soil characteristics), there is limited
research on their ability to reduce emissions from PS storage using biological additives
based on EM [8].
The increase in these gases in W3 can also be explained for SP by the fermentation pro-
cess, where biomass is transformed into organic acids and other intermediate compounds
during acidogenesis, followed by methanogenesis, which leads to the production of CH
4
and CO
2
[
9
]. Berg [
29
] observed that methane emissions increased with straw cover, even
when combined with saccharose. However, other cover systems, such as Leca combined
with saccharose, reduced methane emissions by barely 10%. In W4 and W5, a decrease in
carbonic gases (methane and carbon dioxide) was observed in the ponds with treatments,
while CP showed a slight increase in W4, followed by a decrease in W5, similar to SP and
BP. In the BP pond, EMs could act as bioremediating agents, functioning as biocatalysts
in biochemical reactions with contaminants [
30
], facilitating environmental restoration by
enabling the degradation, alteration, immobilization, or removal of toxic compounds [
31
],
thereby contributing to emission reductions.
During the second period, emissions fluctuated significantly across all treatments.
Conflicting results have been found in other studies. Matulaitis [
32
] observed no significant
difference in CH
4
emissions between manure covered with straw and the control, while
Amon [
33
] and Berg [
29
] reported increased CH
4
emissions from slurry covered with
straw. However, our results, showing a 98% reduction in CH
4
for BP, are consistent with
Saufi [
8
], who reported a 27% reduction in CH
4
emissions with the addition of effective
microorganisms. These authors suggested that variations in microbial community structure
after the addition of EM, which displace harmful microbes with beneficial ones, could
explain the reduction in emissions, although no changes in PS characteristics were observed.
Despite variations in measurements during W6–W10, the mean values of CH
4
and
CO
2
emissions for the SP and BP ponds were lower than those for CP by the end of this
period. SP and BP treatments showed notable effectiveness, with CH
4
reductions of 60%
and 98%, respectively, and significant differences (p< 0.05) compared to CP. CO
2
emissions
were reduced by 23% and 96% for SP and BP, respectively, compared to CP by the end of
the period. For BP, the biological additives containing microbial strains and enzymes likely
facilitated the biodegradation of organic materials in animal slurry. The decomposition
Agriculture 2024,14, 1788 14 of 17
of organic matter by methanogenic bacteria under anaerobic conditions in SP may have
contributed to CH
4
and CO
2
emissions in both cases [
18
]. Nykänen [
7
] concluded that
emissions of contaminant compounds were reduced through biological processes when
sufficient fermentable carbohydrate substrates were added to swine manure. Van Vliet [
34
]
also observed a significant reduction in total nitrogen after six weeks of incubation with EMs.
Similarly, our study found a reduction in total N after 5 weeks, which may have contributed
to the reduction in emissions. Moreover, covers can influence gas transport from the liquid
to the atmosphere, reducing emissions through gas adsorption. The permeability and
thickness of the covers impact these processes by altering the gas contact time within the
cover [
4
,
35
]. Previous researchers reported that adsorption processes in SP [
4
,
24
,
30
] and
absorption by EM in BP [
30
,
31
] could contribute to the reduction of contaminants such as
carbon, total N, and metals, which is linked to reduced emissions.
Nitrous oxide (N
2
O) emissions result from a complex combination of reactions occur-
ring under specific conditions that may explain the minimal impact of livestock farming on
these emissions [
36
], as PS is often managed in liquid form, and nitrification is inhibited
under anaerobic conditions, preventing significant N
2
O emissions. Amon [
33
] highlighted
that N
2
O emissions during storage are influenced by the nitrogen and carbon content of
the manure, storage duration, and treatment type. Significant uncertainties are associated
with default emission factors. Berg [
29
] suggested that N
2
O emissions could be caused
by encrustation on the surface of the slurry or cover material, and to prevent this, cover
materials could be combined with lactic acid or saccharose. Vanderzaag [
4
] suggested
that microbial production of N
2
O could be related to various microsites with different
redox potentials within the covers, potentially explaining the peak observed in SP during
W9-W10 in spring, showing a significant difference (p< 0.05) compared to CP and BP.
Overall, N
2
O emissions were quantitatively low, as expected, in both autumn and
spring (Figure 6) across all ponds. Calvet [
36
] noted that measuring and controlling N
2
O
is difficult because the conditions for its formation (via denitrification processes) occur
sporadically in short-duration emission peaks, making it challenging to track. Additionally,
N
2
O is emitted in small quantities, but its high global warming potential (298 times that of
CO
2
) means that even minimal changes in N
2
O emissions can have significant implications.
In all cases, mean values were below 2 mg m
−2
d
−1
, except for peaks during W9–W10. Our
results are consistent with previous studies [
17
,
33
], where no significant N
2
O emissions
were found from any pond, whether treated or control. Vanderzaag [
37
] noted that the
dynamic open-chamber technique used in this study is generally less sensitive compared to
closed-chamber techniques, which rely on headspace accumulation to detect concentration
increases. Consequently, emission rates and differences between treatments might have
been detected using closed-chamber techniques or higher precision instruments. Despite
this, the dynamic chamber method for measuring gas emissions is strongly recommended
by VERA (Verification of Environmental Technologies for Agricultural Production), an
international organization dedicated to testing and verifying environmental technologies
used in agricultural and livestock operations.
4.2.2. NH3Emissions
During livestock management, ammonia is a gas released during the decomposition
of manure, primarily generated from the breakdown of urea excreted in urine. Urea in
urine is the main form of nitrogen elimination in PS, although the proportion of nitrogen
excreted in urine largely depends on nutritional factors. NH
3
emissions during autumn
showed a decreasing trend, except for W3, in all ponds (Figure 7). The spike in W3 may
be explained by the conversion of NH
4+
to NH
3
due to the disruption of the acid–base
equilibrium, associated with pH values greater than 7.5, as well as environmental factors
such as a temperature of approximately 28
◦
C and 53% humidity (Figure 8). By the end of
this period, no significant effect was observed in BP compared to CP, while SP achieved a
61% reduction in NH
3
emissions. Some authors [
24
] have suggested that covers may affect
Agriculture 2024,14, 1788 15 of 17
gas production by altering slurry pH, which is a crucial factor in reducing NH
3
emissions.
This effect was observed in our study for SP compared to CP at the end of this period.
An inconsistent pattern of NH
3
behavior was observed during the spring, with a peak
in SP and a less pronounced peak in BP during W7 and W9, compared to the control. The
increased NH
3
volatilization in SP could be attributed to the lack of adsorption/absorption
by the straw layer. The thickness of the straw layer is also an important factor that directly
affects adsorption efficiency. However, there are few studies investigating the impact of
cover layer thickness on emission reduction [
24
]. The EM used in BP could contribute
to NH
3
reduction due to the bio-augmentation effect, where the added microorganisms
multiply and outcompete other microbes [
8
,
38
]. Nevertheless, further research is needed to
determine the optimal EM dosage for maximum effectiveness.
By the end of spring, significant differences (p< 0.05) were observed between the
treatment techniques, with NH
3
reductions of 97% for SP and 59% for BP, compared to
CP. These percentages are consistent with those reported by Vanderzaag [
4
], who achieved
a 97% reduction using a 30 cm cover, similar to our study. Guarino [
39
] also reported
ammonia reductions of 34.2% and 86% with straw covers of 7 cm and 14 cm, respectively.
5. Conclusions
The study observed that there was no clear pattern of behavior during either assess-
ment period. Despite this, the treatment techniques applied (straw cover and biological
additive) to the storage pond have proven to be effective compared to the control, mitigat-
ing emissions and improving the properties of pig slurry. Considering long-term storage, a
significant reduction in gas emissions has been observed. Reduction percentages of 96% for
CO
2
, 98% for CH
4
and 59% for NH
3
were achieved in the storage pond with the addition
of effective microorganisms in spring. The storage pond covered with barley straw showed
a 74% reduction for CO
2
in autumn while, in spring, reductions of 60% and 97% for CH
4
and NH
3
, respectively, were observed. Results also indicated that experimental values
in the slurries from the straw cover and biological additives ponds were similar to those
from the control pond except for Org.-N values, which were higher. The technique of
using straw cover had a minimal impact on the analytical properties of the slurry, and the
dose of effective microorganisms used appeared to be insufficient for the experimental
periods. Alkalinization of the PS was detected, which explains the decrease in the fractions
of N (especially NH
4+
-N) and PO
43−
, along with an increase in K
+
. This preliminary
study underscores the need for further replications to confirm the findings and validate
the effectiveness of the treatment techniques across different conditions. Future research
should focus on testing varying straw cover thicknesses, effective microorganism doses,
and their combination with pH reduction to more effectively reduce emissions and improve
PS properties.
Author Contributions: Conceptualization, M.A.T.T. and M.G.G.; methodology, M.A.T.T. and M.G.G.;
validation, M.A.T.T., M.G.G. and O.E.b.; formal analysis, M.A.T.T.; investigation, M.A.T.T., M.G.G.,
O.E.b., J.G.C.B. and Á.F.C.; resources, M.A.T.T., M.G.G., O.E.b., J.G.C.B. and Á.F.C.; data curation,
M.A.T.T. and M.G.G.; writing—original draft preparation, M.A.T.T., M.G.G. and O.E.b.;
writing—review and editing,
M.A.T.T., M.G.G., O.E.b. and Á.F.C.; visualization, M.A.T.T., M.G.G.
and O.E.b.; supervision, Á.F.C. All authors have read and agreed to the published version of
the manuscript.
Funding: This research was funded by the town hall of Fuente Álamo, Murcia, Spain. Project
reference: CUE-AYTO FUENTE ÁLAMO 2020-T.
Institutional Review Board Statement: Not applicable.
Data Availability Statement: The data presented in this study are confidential and part of a finished
project. This publication is the first associated with this project. Access to the complete project data is
restricted to authorized personnel only due to privacy and confidentiality considerations. For further
inquiries or potential data access requests, please contact the corresponding author.
Agriculture 2024,14, 1788 16 of 17
Conflicts of Interest: The authors declare no conflicts of interest. The funders had no role in the design
of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or
in the decision to publish the results.
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