1. Introduction
With COVID-19 coming to an end, China has moved into a ‘post-epidemic period’. However, it does not mean that there are no new challenges anymore. Due to the outbreak of COVID-19, various industries in China have taken a hard hit, not only in terms of macroeconomic implications but also in personal lives at the microlevel, especially employment and basic living standards [
1]. As an international agricultural power with a significant proportion of agriculture, China’s farmers are reliant only on crop cultivation. Consequently, it is difficult to restore economic income to the pre-epidemic level. Over the past few decades, China’s animal husbandry has maintained a steady pace of growth, driving the increase in income earned by farmers and the improvement in dietary structure. Aside from promoting the rapid recovery of local economies, the fast shift of animal husbandry from an intensive farming model to a large-scale farming model also meets the needs of agriculture for sustainability [
2].
Nevertheless, new environmental issues have arisen from the growth of large-scale livestock and poultry farming. Composed of animal manure and urine, the livestock excreta (LE) has increased significantly, causing a widespread concern [
3]. Despite various treatments carried out for LE in China, its impact on the environment remains. According to the ‘Second National Pollution Source Census Plan’ published by the State Council of China in 2017, agriculture continues to be the primary contributor to pollution due to its extensive coverage and remote geographical locations [
4,
5].
The treatment of livestock waste is conducted primarily through composting and anaerobic digestion recovery methods. Nevertheless, it is challenging to determine the processing time accurately, which makes it difficult to estimate the precise completion time [
6]. Also, it is often the case that substantial labor and financial investments lead to relatively low economic profits [
7]. From a short-term perspective, organic fertilizers have a less significant impact on crop yield than inorganic fertilizers [
8]. However, in the long run, large-scale farmers are more willing to utilize organic fertilizers and increase the intensity of application, as opposed to small-scale farmers. Notably, the application of organic fertilizers by farmers is influenced by their heterogeneity [
9,
10]. Moreover, the application of organic fertilizer is more effective than the use of inorganic fertilizer in reducing greenhouse gas emissions [
11].
As the largest producer of agricultural waste across the globe, China faces significant challenges in agricultural waste management. China is projected to generate approximately 3.8 billion tons of livestock and poultry manure annually, with a comprehensive utilization rate of less than 60% [
12]. Additionally, nearly 900 million tons of straw are produced in the country each year, of which around 200 million tons go to waste. Meanwhile, roughly 260 million tons of vegetable tails are discarded [
13]. As the main organic waste, animal manure contains various essential nutrients required for crop growth. However, there are now various obstacles in China to the large-scale conversion of livestock waste into organic fertilizer, including diverse livestock species, the variations in environmental conditions, the differences in the quality of management personnel, and the regional disparities in livestock waste management. Reportedly, animal husbandry production has become one of the “three most important factors of the world’s most serious environmental problems” [
14].
The pollution caused by LE involves four main aspects [
15]. Firstly, it affects the atmosphere by releasing various foul smelling gases during the accumulation process, such as hydrogen sulfide, ammonia, mercaptans, phenols, and indole. Ranking second among the six major public hazards, foul odor pollution causes severe environmental pollution and poses a threat to human health. This dual pollution affects both the atmosphere and the release of detrimental gases. Secondly, the impact on water bodies is significant. The release of livestock excreta (LE) not only leads to widespread water pollution but also jeopardizes the quality of drinking water and disrupts the daily lives of residents who rely on those water sources without proper treatment [
16]. Third is the impact on soil. Due to the relatively simple composting method used in China, the rapid accumulation and fermentation of LE would occur around the farmland. However, LE is likely to contaminate farmland soil due to the lack of expertise in accumulation. By blocking the gaps present between soil particles, it results in soil acidification, a decline in microbial activity, and soil compaction [
17]. Ultimately, crop yield drops and farmers’ income is affected. Additionally, LE contains plenty of pathogenic bacteria and harmful insect eggs [
18]. If improperly handled, it causes the growth and spread of these pathogens, thus threatening biological health [
19].
In some studies, the significant contribution of nitrous oxide (N
2O) emissions, from grazing land to global warming, has been highlighted, including pastures and large-scale breeding farms [
20,
21]. It has been revealed that the nitrogen deposition from livestock excrement dictates the spatiotemporal pattern of global change [
22]. The microlevel aspects of livestock excreta (LE) have been examined by some scholars, such as deodorization bacteria and the fecal wastewater treatment industry. In spite of this, there are still few studies exploring its different environmental impacts across China [
23,
24,
25,
26,
27,
28]. Weiqing Bao et al. [
29] adopted mathematical models and other methods to explore the biogas and production potential of LE, with useful insights gained for this study.
This article focuses on using more accurate mathematical models to estimate the emissions of livestock excreta in China’s livestock industry. This study analyzes the spatial relationship between LE production in different provinces and selects appropriate models, including both parametric and non-parametric models, to predict LE production in the next decade. Additionally, this article proposes two hypotheses based on previous research. The first hypothesis suggests that China’s LE emissions may exhibit spatial clustering characteristics. The second hypothesis predicts that by 2031, China’s LE emissions will surpass 10% of the levels recorded in 2021. By demonstrating these assumptions, this article aims to provide valuable insights and serve as a reference for future research.
4. Discussion
In this study, the two hypotheses proposed in the introductory section were explored. Based on our results, the first hypothesis was supported to some extent, while the second hypothesis was rejected.
The estimation of LE emissions from China’s livestock industry indicates that the peak emissions occurred in 2005 at 3082.758 Mt/Year, which contradicts the findings of Peidong Z [
40]. This discrepancy is suspected to be a result of data deviations, although there were no significant differences in the overall trend. The study identified four distinct stages of LE emissions in China, representing the historical trajectory of emissions in the country. The growth rate and efforts to address environmental issues varied across these stages. These findings highlight the need for continued efforts by China in the post-COVID-19 era to balance economic development with the sustainability of the livestock industry [
41]. This suggests the importance of raising awareness and investing more in the management and mitigation of the environmental impact. Importantly, the COVID-19 pandemic did not have a significant impact on the growth of LE emissions, indicating that other factors play a more significant role. Additionally, the emissions from China’s commercial-scale husbandry feedlots (CSHF) were found to be consistent with normal LE emissions, suggesting that other types of livestock production systems also contribute to the growth and impact of LE emissions in China [
42].
In terms of LE emissions from different livestock categories, cattle are considered the most significant contributors to LE-FM, followed by pigs. Poultry accounted for the largest proportion of LE-DM emissions. This pattern was confirmed in the study by Zhang T [
43], which suggested that the specific moisture content in their manure might be responsible for it. The results also showed a significant upward trend in total LE emissions from poultry in recent years. These findings highlight the variation in the contributions and trends between different livestock species in terms of LE emissions. Considering the prominence of cattle and the increasing significance of poultry, China should implement targeted measures to mitigate their environmental impact by controlling LE emissions from these two livestock types [
44].
When investigating the spatial clustering characteristics of LE emissions across Chinese provinces, no significant spatial clustering patterns were observed, leading to the rejection of the second hypothesis proposed in this study. However, based on the results of global autocorrelation, there is a possibility that high–high clustering features may emerge in regions like Shandong and Sichuan in the future. This highlights the need for further investigation in future studies. To address this, China should implement rational planning and distribution strategies for LE emissions in spatial terms, which are essential for effectively assimilating and disposing of LE. One of the key innovations in this study was the prediction of LE emissions, which was crucial to verify the second hypothesis. Three prediction models were trained and fitted based on error rates, goodness of fit, and RMSE, and the BP-RE prediction model was adopted to forecast China’s LE emissions for the next ten years. By 2031, FM is projected to increase by 24.53% compared to 2021, while DM is expected to decrease by 28.06%. Under CSHF conditions, both FM and DM are predicted to rise by 11.16% and 2.05%, respectively. These findings indicate a change in livestock structure from 2021 to 2031, with a significant decline in free-range poultry possibly contributing to this phenomenon. Additionally, this change can be attributed to the increasing level of industrialization [
45]. Therefore, China needs to innovate and improve the methods of handling LE-FM, while also increasing its utilization rate through techniques like composting and anaerobic digestion [
46]. Furthermore, it is necessary to enhance the existing approaches to managing LE-DM, incorporating CSHF principles to improve FM and DM treatment under such conditions.
The limitations of this study are primarily related to the estimation of LE, which is based on formulas derived from previous research. This approach may introduce biases in the collection and calculation of data. Additionally, the analysis only considered one specific spatiotemporal model which may yield different results compared to other models. These limitations should be considered in future research.
5. Conclusions
According to the findings, the emissions of LE in China reached 3082.758 Mt/year for FM and 691.951 Mt/year for DM by 2005. The lowest emissions were recorded in 2008, with FM reaching 1508.665 Mt/year and DM reaching 380.970 Mt/year. In this study, the emissions of LE in China over the past 20 years have been estimated.
Judging from the results of global correlation analysis, there was no spatial correlation present among various provinces in China regarding LE emissions. However, the findings from local autocorrelation analysis indicated a strong likelihood of spatial positive correlation in China’s LE in the future, particularly in the regions like Shandong and Sichuan with high clustering. After thorough comparison and analysis of the prediction models, it was found out the BP-RE model performed best in predicting the emissions of LE in China. According to the projected outcome, the total amount of FM (fecal matter) in LE in China will reach 2313.949 billion tons by 2031, up by 24.53% compared to 2021. Conversely, DM (dung matter) emissions are projected to decrease by 28.06%, reaching 344.740 Mt by 2031. Under the conditions of CSHF (current stable housing facilities), the emissions of FM and DM are projected to reach 894.289 Mt and 233.913 Mt, respectively, up by 11.16% and 2.05%, respectively. These projections highlight the potential changes in the future demand of China for different types of livestock.