Effects of Long-Term Storage of Pig Slurry on Nitrogen Loss in Different Seasons and Storage Conditions

Guangyin Chen^1,*, Jinzhu Dong¹, Pei Wu¹, Enhui Wang¹, Jiawei Zheng¹, Xu Liu¹

Published in Journal of the ASABE 67(3): 641-648 (doi: 10.13031/ja.15677). Copyright 2024 American Society of Agricultural and Biological Engineers.

¹    School of Environment and Ecology, Anhui Normal University, Wuhu, Anhui, China.

^*    Correspondence: 2016073@ahnu.edu.cn

The authors have paid for open access for this article. This work is licensed under a Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License https://creative commons.org/licenses/by-nc-nd/4.0/

Submitted for review on 20 May 2023 as manuscript number NRES 15677; approved for publication as a Research Article by Associate Editor Dr. Xiaoyu (Iris) Feng and Community Editor Dr. Kati Migliaccio of the Natural Resources & Environmental Systems Community of ASABE on 5 February 2024.

Citation: Chen, G., Dong, J., Wu, P., Wang, E., Zheng, J., Liu, X. (2024). Effects of long-term storage of pig slurry on nitrogen loss in different seasons and storage conditions. J. ASABE, 67(3), 641-648. https://doi.org/10.13031/ja.15677

Highlights

• The physicochemical properties of the pig slurry in different seasons were different, the contents of COD and total nitrogen of the pig slurry in the summer were significantly higher than the pig slurry in the other seasons.
• The slurry temperature varied greatly in different seasons, which affected the changes of indicators in pig slurry during long-term storage.
• Storing pig slurry under sealed conditions was significantly more beneficial in reducing N loss, which was 47%–56% lower than that in the open treatments.

Abstract. Pig slurry (PS) can be applied to farmland only in certain periods during the year, so long-term storage is required before field application. The effects of the long-term storage of the PS on N loss in different seasons and under different storage conditions (open and sealed storage) were investigated in this study. The physicochemical properties and slurry temperature varied greatly in different seasons. The electrical conductivity, chemical oxygen demand, ammonium nitrogen, nitrate nitrogen, total nitrogen (TN), and heavy metals (Cu, Zn, Pb, As, and Cd) decreased after 150 days of storage, while the pH value increased, and all these indicators (except for the heavy metals) were affected by the slurry temperature, exhibiting a larger decrease at higher slurry temperatures. The decrease in the N loss was 47%–56% lower in the sealed treatments than in the open treatments. The As, Pb, and Cd contents of the stored PS (except for the AU-OP treatment) met the requirements specified for GB/T 40750-2021. Overall, storing PS under sealed and lower temperature conditions was conducive to reducing N loss and improving the N content of the stored PS.

Keywords.Heavy metal, Nitrogen loss, Open storage, Pig slurry, Sealed storage, Temperature.

China is the largest pig-breeding country (with 453 million pigs in 2022, NBS, 2023) in the world and accounts for 37% of global pork consumption (FAO, 2022). Thus, a large amount of pig slurry (PS) is produced every year and needs to be treated. Land application is the most important method of dealing with livestock manure (GOSC, 2017; MARA, 2017, 2019) since there are 120 million hectares of farmlands that require fertilization (Kong, 2014; Xin and Li, 2018).

Ammonia (NH₃), a common gas released from livestock and poultry manure, has significant effects on human health and environmental protection (Balsari et al., 2007; Dai and Blanes-Vidal (2013); Sanchis et al., 2019). Approximately 80%–90% of global NH₃ emissions came from agriculture (Xu et al., 2019), and 64% of agricultural NH₃ emissions were associated with livestock production (Sanchis et al., 2019; Steinfeld et al., 2006). Anderson et al. (2003) reported that NH₃ emissions from barns and slurry storage are the primary sources of the total ammonia emissions from livestock activities. For an integrated crop and livestock systems, the key stage for reducing NH₃ emissions is slurry storage (Zhou et al., 2022; Vayssières and Rufino, 2012).

Due to the continuous production of animal slurry and the need for timely application to meet crop nutrient requirements, a significant amount of slurry needs to be stored for a relatively long period before being spread on the land. During the storage of animal slurry, the proteins in the animal slurry are converted into ammonium nitrogen by microorganisms, and the alkaline environment of animal slurry is conducive to NH₃ emissions, leading to a large amount of nitrogen (N) loss. Kupper et al. (2020) reported that the baseline NH₃ emissions per area are 0.15 g m^-2 h^-1 when PS is stored in lagoons and 0.24 g m^-2 h^-1 when PS is stored in tanks. Dexter et al. (2014) reported that 62% of the initial N in dairy effluent is lost after 81 days of open-air, rain-sheltered storage in open buckets (each containing 7 L of amended effluent). The loss of gaseous N strongly reduces the value of animal slurry fertilizer and causes a series of environmental problems (Kai et al., 2008; Sørensen and Amato, 2002). Therefore, research in many countries explored the lessening of environmental issues associated with slurry by focusing on NH₃ emission mitigation (Galloway et al., 2003).

In addition, the composition of the PS in different seasons is different. The average N content, biochemical oxygen demand (BOD), phosphorus (P) content, and dry mass content tend to decrease systematically from the spring to the winter. The highest correlation coefficient, which indicates a significant interdependency among the variables tested, has been consistently found for the chemical oxygen demand (COD) and BOD (Kowalski et al., 2013). The composition and storage temperature of the PS are the two most important factors that affect N loss during the slurry storage process (Balsari et al., 2007). Popovic and Jensen (2012) reported that the dry matter, volatile fatty acid (VFA), total N (TN), and ammonium contents of the PS decreased during the storage process. Petersen et al. (2013) observed that NH₃ emissions were significantly lower in winter than in summer. Balsari et al. (2006) observed that the NH₃ emission from PS storage ranged between 0.98 and 2.68 g NH₃/m² and decreased with increasing storage temperature. A simple method of avoiding NH₃ emissions is to create conditions that minimize the NH₃ content relative to the NH₄⁺ content, namely, by lowering the pH of the slurry (Fangueiro et al., 2015). Studies have confirmed that NH₃ emissions are directly related to the pH of the slurry (Vandre and Clemens, 1997; Dai and Blanes-Vidal, 2013). Studies have shown that NH₃ emissions are directly related to the pH of the slurry. Furthermore, pH levels tend to increase (or decrease) as temperatures rise. Therefore, there is a need for further investigation into the potential impact of storage temperature on N loss during PS storage. Although previous studies (Viguria et al., 2015; Kupper et al., 2020) have surveyed N loss from a storage tank or lagoon in different seasons, N loss during long-term storage of the PS in different seasons has rarely been studied. In addition, there are three main storage methods for pig slurry in China: lagoon, concrete tank, and black film, and no systematic study has been conducted on the effectiveness of this method in reducing N loss.

In this study, the PS was stored in different seasons (spring, summer, autumn, and winter) and under different closure conditions (open and sealed) for 150 days. During the experiments, the pH, electrical conductivity (EC), TN, ammonium nitrogen (NH₄⁺-N), nitrate nitrogen (NO₃^--N), and heavy metals (Cu, Zn, Pb, As, and Cd) were measured. The objectives of this study were (a) to quantify the N loss from the PS stored in different seasons and (b) to compare the effects of sealed storage on reducing N loss during the long-term storage of the PS.

Materials and Methods

Pig Slurry

The PS used in the experiments was obtained from a pit in a pig farm located in Funan County, Anhui Province, China. On this pig farm, the floor of the pigsty was cleaned with freshwater. The PS for the experiments was collected on 21 September 2020 (autumn), 27 December 2020 (winter), 21 March 2021 (spring), and 2 July 2021 (summer). The collected PS was stored at 4°C for less than 24 hours. Before conducting the experiment, the collected PS was sieved through a 10-mesh sieve to remove large particles and ensure homogeneity. The physicochemical properties of the PS obtained during different seasons are presented in table 1.

Experimental Design

Two series of experiments were conducted to evaluate the effects of the sealing conditions of the PS and the storage season on the PS storage process and N loss. The experiments were conducted in spring (SP) (from 22 March to 19 August 2021), summer (SU) (from 3 July to 30 November 2021), autumn (AU) (from 22 September 2020 to 19 February 2021), and winter (WI) (from 28 December 2020 to 27 May 2021) for 150 days of storage. Furthermore, the storage conditions of the PS included open (OP) and sealed (SE); therefore, eight treatments were set up, denoted as treatments SP-OP, SP-SE, SU-OP, SU-SE, AU-OP, AU-SE, WI-OP, WI-SE. Three replicates of each treatment were set up.

The experiments were conducted at the lab scale with 800 mL (height 11.6 cm, diameter 10.2 cm) of the PS placed in a 1000 mL glass container (height 17.3 cm, outer diameter 10.6 cm, inner diameter 10.2 cm). The open storage treatments were conducted as follows: a 15-cm silicone rubber tube was inserted into the PS for sampling. To avoid solid particles being removed, the end of the silicone tube inserted into the PS was positioned at approximately one-third of the total height of the PS and wrapped with gauze, and the other end was placed outside the mouth of the container. Then, the mouth of the glass container was covered with gauze, and the gauze was secured with a rubber band to avoid material falling into the slurry (Chen et al., 2023). The sealed storage treatment was conducted as follows: the mouth of the glass container was sealed with a butyl rubber stopper. There were two outlets in the stopper. One of the outlets was used for discharging gas, and the other one was used for sampling. Then, all the glass containers were placed outdoors in an area away from rain and direct sunlight to simulate the slurry storage process for 150 days. We used a syringe to aspirate the supernatant of the pig slurry. During the experiment, samples were collected on days 0 and 150, and the pH and EC values of the samples were measured immediately. The contents of COD, TN, NH₄⁺-N, and NO₃^--N of the samples were measured within 24 hours. The heavy metal contents (Cu, Zn, Pb, As, and Cd) of the samples were measured within one month. Each sample volume was around 25 mL. Meanwhile, a test glass container containing 800 mL of PS was prepared and placed under the same conditions for the daily determination of the slurry temperature. The air and PS temperatures were measured from September 22, 2020, to January 10, 2021, and from March 7 to September 21, 2021. PS temperatures were recorded at 5 cm depth using a mercury thermometer at 7:00, 14:00, and 22:00 each day. Considering the highest temperature measured each day as the highest temperature of the pig slurry on that day, and the lowest temperature measured as the lowest temperature of the pig slurry, the data on ambient temperature were obtained from the meteorological reports of the Meteorological Bureau of Wuhu City, Anhui Province, China.

Chemical Analysis Methods and Calculations

The physicochemical properties of the collected PS (pH, EC, COD, TN, and NH₄⁺-N, NO₃^–-N) obtained during the experiment process were analyzed according to standard methods (APHA, 2005) in triplicate. The TP and SP contents in PS were determined via potassium persulfate digestion-molybdenum antimony anti-spectrophotometry according to the proper Chinese standards (SEPA, 2002). The As content of the PS was determined using atomic fluorescence spectrometry (AFS-9700, Haiguang, China), and the Cu, Zn, Pb, and Cd contents were determined using flame atomic absorption spectrometry (AA6800, Shimadzu, Japan).

After the experiments, the N loss from the PS in the sealed and open treatments was calculated to assess the effect of the storage conditions and storage season on the N conservation. The N loss from the PS after 150 days of storage was calculated using the formula provided by Chen et al. (2023).

Data Processing and Statistical Analysis

All the experimental data were processed using Excel 2016 (Microsoft, Redmond, WA, USA), and the figures were plotted using Origin 2017 (OriginLab, Northampton, MA, USA). The statistical analyses were conducted using SPSS 24.0 (IBM SPSS Statistics for Windows, Released 2016, Version 24.0; IBM Corp., Armonk, NY, USA). The results of the N loss were analyzed using one-way analysis of variance (ANOVA) to evaluate the effects of the long-term storage of the PS on N loss in different seasons and under different storage conditions. In addition, the Tukey test was performed to determine the differences between the treatments following each ANOVA, with P < 0.05 considered to be statistically significant.

Results and Discussion

Changes in Air and Pig Slurry Temperatures

The temperature was different in different seasons and was also an important factor affecting N loss during PS storage (fig. 1). In China, from January 11 to March 6, 2021, was winter vacation, and it was also a longer vacation because of the COVID-19 pandemic. As can be seen, the trends of the minimum temperature of the PS, the maximum temperature of the PS, the minimum air temperature, and the maximum air temperature were similar. The air and PS temperatures decreased gradually after September 22, 2020, and the lowest temperature was reached in January, which was the coldest time in the region. Then, the temperature increased gradually, and the highest temperature was reached from mid-July to mid-August and then decreased. We also found that the PS temperature was always lower than the air temperature. The size of the container for PS storage has a large effect on the slurry temperature and the N loss from PS. Viguria et al. (2015) compared the PS temperature and ammonia emissions from lagoon A (with 1000 m³ capacity and 440 m² surface area) and lagoon B (with 768 m³ capacity and 300 m² surface area) and showed that the PS temperature and ammonia emissions from lagoon B were higher than those from lagoon A. The change in the PS temperature directly affects the microbial activity, organic matter degradation, and N loss in the PS.

Figure 1. Variation of air and pig slurry temperatures during the experiment.

The statistical results of the PS temperature are shown in table 2. As can be seen, the highest PS temperature of 32°C occurred in summer, while the lowest PS temperature of 0°C occurred in autumn and winter. The maximum average PS temperature of 26°C occurred in the summer, while the minimum average PS temperature of 0°C occurred in the winter. In addition, the average PS temperature was higher in spring than in autumn. The results of the temperature measurements showed that the PS temperature in different seasons was different, and it changed greatly, which would affect N loss from the PS during the storage process.

Changes in the Physicochemical Properties  of Pig Slurry During Storage

The physicochemical propertiesof the PS measured before and after storage are presented in table 3.

COD Concentration

As can be seen from table 3, the COD concentrations of all the treatments were significantly lower after 150 days of storage. The maximum reduction of 89% for the open treatments was obtained for treatment SU-OP, while the maximum reduction of 75% for the sealed treatments was obtained for SP-SE. In addition, the minimum reduction of 66% for the open treatments was obtained for WI-OP, while the minimum reduction of 31% for the sealed treatments was obtained for WI-SE. The initial COD concentration was different in different seasons. The highest PS COD concentration of 18.46 g/L occurred in winter, while the lowest COD concentration of 5.75 g/L occurred in summer. This was not consistent with the results of Kowalski et al. (2013), which may be related to different farming methods.

pH Value

After 150 days of storage, the pH value of the PS was significantly higher (table 3), which promoted N loss (Viguria et al., 2015). According to Petersen et al. (2012) and Hjorth et al. (2015), the pH value increases due to microbial activity, hydrolysis of volatile fatty acids, mineralization of organic nitrogen, and dissolution of carbonates. Moreover, Regueiro et al. (2016) reported that the outputs of acid hydrolysis were small sugars and inorganic soluble C, which tend to increase the pH value. In addition, the pH values of the stored PS in all the treatments were greater than 8.0 (weakly alkaline), which was conducive to NH₃ emissions. The pH values of the PS in the open treatments were higher than those in the sealed treatments, which may have led to higher N loss. Moreover, the higher degree of organic matter decomposition (the result of COD content) in open storage implied that more organic nitrogen was mineralized to ammonium nitrogen, which promoted ammonia volatilization, and this may be one of the reasons for its higher N losses than in sealed storage.

EC

As can be seen from table 3, the EC values of all the treatments (except for SP-SE and SU-SE) decreased, and the maximum decrease of 52% occurred for SU-OP, indicating that storing PS at higher temperatures and under open conditions was conducive to decreasing the EC. The EC values of the PS after sealed storage were much higher than those after open storage, which were consistent with the results of the COD concentration.

Nitrogen Transformation During Long-Term Slurry Storage

The NH₄⁺-N, NO₃^–-N, and TN contents of the PS after 150 days of storage are presented in table 4.

Changes in NH₄⁺-N Content

The NH₄⁺-N contents of the PS in the open treatments decreased by 64%–96%, while the NH₄⁺-N contents of the PS in the sealed treatments decreased by -26%–18%, indicating that sealed storage greatly reduced NH₃ emissions during the storage of the PS compared with open storage. After 150 days of storage, the NH₄⁺-N contents in the PS stored under open conditions decreased by 92%, 96%, 64%, and 65% in spring, summer, autumn, and winter, respectively, compared to day 0. Conversely, the NH₄⁺-N contents under sealed conditions decreased by 7%, 10%, 18%, and increased by 26% in the respective seasons. In the experiments, the storage period of the PS was 150 days, spanning two seasons, so the reduction in the NH₄⁺-N content of the stored PS was not always consistent with the average temperature of each season (table 2). In addition, storing PS at a lower temperature was conducive to the preservation of NH₄⁺-N in the stored PS (Popovic and Jensen, 2012). Moreover, some of the NH₄⁺-N may be converted into NO₃^–-N (Gooddy et al., 1998), leading to an increase in the NO₃^--N content of the stored slurry. Due to the conversions between different forms of nitrogen during slurry storage, the reduction of the NH₄⁺-N content of the stored PS does not directly reflect the N loss during slurry storage.

Changes in NO₃^--N Content

The NO₃^--N contents of the original PS and stored PS were much lower than the NH₄⁺-N contents. The NO₃^--N contents of all the treatments were significantly lower after 150 days of storage. The NO₃^--N contents of the open treatments (except SU-OP) were higher than those of the sealed treatments. The NO₃^--N contents of the PS in the open treatments and sealed treatments decreased by 25%–84% and 40%–82%, respectively, indicating that sealed storage improved the reduction of the NO₃^--N content during the storage of the PS compared with open storage, which may be due to nitrification (Nodar et al., 1992). The maximum reductions of 84% and 82% of the NO₃^--N contents of the PS in the open treatments and sealed treatments were obtained for treatments SU-OP and SP-SE. Furthermore, the minimum reductions of 25% and 40% of the NO₃^--N contents of the PS in the open treatments and sealed treatments were obtained for treatments AU-OP and AU-SE, respectively.

Changes in TN Content

As can be seen from table 4, the TN contents of the PS in all the treatments decreased greatly. After 150 days of storage, the TN contents of the open treatments and sealed treatments had decreased by 67%–87% and 20%–47%, respectively, indicating that storing PS under sealed conditions was conducive to reducing N loss and increasing the nitrogen content of the stored PS. After 150 days of storage, the TN contents in the slurry stored under open conditions decreased by 79%, 87%, 67%, and 62% in spring, summer, autumn, and winter, respectively, compared to day 0. Conversely, the TN contents under sealed conditions decreased by 24%, 47%, 20%, and 20% in the respective seasons. In addition, the decrease in the TN content of the PS was 40%–55% lower in the sealed treatments than in the open treatments. The results showed that a higher storage temperature leads to a larger reduction in the TN content of the stored PS (Ding et al., 2016; Popovic and Jensen, 2012).

N Loss

It was difficult to estimate the N loss from the changes in the NH₄⁺-N, NO₃^--N, or TN content alone due to changes in the slurry volume resulting from sampling, evaporation, and nitrogen conversion during slurry storage. Considering the changes in the TN content, volume, and evaporation of slurry, the N loss from the PS after 150 days of storage was calculated using equation 1:

        (1)

where

Click or tap here to enter text. = N loss of the stored PS (%)

Click or tap here to enter text. = TN content of the PS on the sampling day (mg/L)

Click or tap here to enter text. = sample volume of the PS (L)

n = total number of sampling events, n was 8 in this experiment

Click or tap here to enter text. = remaining volume of PS on day 150 (L)

Click or tap here to enter text. = TN content of the PS on day 0 (mg/L)

Click or tap here to enter text. = total volume of PS on day 0 (L).

After 150 days of open storage, the N losses from the slurry stored under open conditions in spring, summer, autumn, and winter were 75%, 82%, 71%, and 57%, respectively; while the N losses from the slurry stored for 150 days in spring, summer, autumn, and winter under sealed conditions were 19%, 35%, 14%, and 9%, respectively (fig. 2). The N loss from the PS stored in different seasons varied greatly. Popovic and Jensen (2012) reported that the N loss of PS was affected by temperature, exhibiting higher losses at higher storage temperatures. Moreover, the N losses from the sealed treatments were significantly lower than those from the corresponding open treatments, indicating that storing PS under sealed conditions was conducive to reducing N loss. The N losses from SP-OP, SU-OP, and AU-OP were much higher than those reported by Dexter et al. (2014), i.e., 62% (after 81 days of storage). This may be related to the storage duration (Dewes et al., 1990). Although the N losses from the sealed treatments were significantly lower than those from the corresponding open treatments, the maximum N loss of 35% from the sealed treatments was obtained for treatment SU-SE and was much higher than the value of 12% reported by Jin et al. (2012) for a hydraulic residence time of 15 days, whereas the storage period in our tests was 150 days.

Figure 2. The N loss of pig slurry after 150 days of storage.

Changes in Heavy Metal Contents of  Pig Slurry After Long-Term Storage

Pan et al. (2013) reported the heavy metal contents (Cr, Cu, Zn, As, Cd, and Pb) of a total of 126 pig manure samples from 21 large-scale pig farms in Shandong Province and found that the average Cr, Cu, Zn, As, Cd, and Pb contents of the pig manure were 12.30, 472.80, 1908.60, 36.50, 0.90, and 2.90 mg/kg, respectively. After 150 days of storage, the contents of all the detected heavy metals had significantly decreased (P < 0.05), and Pb and Cd were not detected in all the treatments, indicating that long-term storage is effective in reducing the contents of heavy metals in PS (table 5). This may be attributed to a large number of suspended solids either sinking or floating, with some heavy metal ions forming precipitates that settle at the bottom of the glass container. The sample taken on the 150th day of the experiment was the supernatant from the pig slurry, and there was no stirring prior to sampling. For both the open and sealed treatments, the Cu and Zn contents of the stored PS were significantly reduced (P < 0.05). The stored pig slurry needs to be mixed with water and returned to the field. Although there is no clear national standard for returning slurry to the field, the high content of Cu, Zn, etc. in the slurry needs to be paid attention to, and combined with the manure application standards, it should be uniformly implemented. The specific details will be studied in the next step.

Currently, there is no national standard in China for the heavy metal content limits of livestock slurry for farmland utilization. The limits of the heavy metal contents of water-rich/liquid fertilizer for application to farmland are presented in table 6. As can be seen, the limits of the As, Pb, and Cd contents are as follows: NY/T 2596-2014 > GB/T 40750-2021 > GB 5084-2021, and the differences are large. According to the high COD, TN, and TP contents of the PS, as well as the definitions of NY/T 2596-2014, GB/T 40750-2021, and GB 5084-2021 in the abovementioned standards, it is believed that the characteristics of the stored PS are closer to those of GB/T 40750-2021, so the limits of the heavy metal contents of the stored PS in this study refer to those in GB/T 40750-2021. After 150 days of storage, the As, Pb, and Cd contents all (except for AU-OP) met the limits specified for GB/T 40750-2021, indicating that the stored PS is safe and can be used as a type of fluid fertilizer for farmland.

Conclusions

The physicochemical properties of the PS stored in different seasons were different and varied greatly. The maximum TN, NO₃^--N, COD, TP, and SP contents of the PS occurred in winter. In addition, the slurry temperature varied greatly in different seasons, and the average slurry temperatures in spring, summer, autumn, and winter were 18°C, 26°C, 12°C, and 9°C, respectively. After 150 days of storage, the EC, COD, NH₄⁺-N, NO₃^--N, TN, and heavy metal (Cu, Zn, Pb, As, and Cd) contents of the stored PS had decreased while the pH value increased, and all the aforementioned indicators (except for the heavy metals) were affected by the slurry temperature, exhibiting higher reductions at higher slurry temperatures. However, the heavy metal contents of the stored PS were not significantly affected by the storage temperature. The N loss, TN content, and decrease in COD content were positively correlated with the slurry temperature. Moreover, storing PS under sealed conditions was significantly more conducive to reducing N loss and improving the NH₄⁺-N and TN contents of the stored PS. The N loss in the sealed treatments was 47%–56% lower than that in the open treatments. Therefore, storing PS under sealed and lower temperature conditions is recommended.

Acknowledgments

This work was funded by the Key Research and Development Projects in Anhui Province (201904a06020044) and the University Synergy Innovation Program of Anhui Province (GXXT-2019-010). The authors would like to take this opportunity to express our sincere appreciation for the support of the China Scholarship Council.

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