1. Introduction
Rice (
Oryza sativa L.), with around 161 million ha [
1] grown globally, is one of the most significant agricultural crops in the world [
2]. Over half of the world’s population bases its food security on rice [
3]. In 2018, around 782 million tons of paddy rice were produced globally [
3]. The Guilan province in Iran is one of the country’s key areas of rice production due to its abundant water, rich soil, and high humidity [
4]. Rice is grown on more than 205,000 ha in this region, accounting for 35.8 percent of Iran’s total paddy fields [
5].
Conventional agriculture requires a wide range of external inputs to sustain output and profit [
6]. Chemical fertilizers and insecticides, in particular, can boost food production while efficiently decreasing crop pests and illnesses to satisfy the rising food demand linked with population expansion [
7,
8]. In addition, the overuse of chemical fertilizers and pesticides results in a number of environmental problems, such as greenhouse gas (GHG) emissions, nitrate leaching, water eutrophication, soil acidification, groundwater pollution, biological diversity loss, soil degradation, and toxicity across the food chain [
6,
9,
10,
11,
12,
13]. Due to the high proportions of non-renewable resources used in agriculture (such as fuel for machinery), as well as the fact that the production of frequently toxic chemicals causes environmental dangers, it is essential to pay special attention to these issues. To summarize, the production of agriculture is currently facing the enormous challenge of feeding a rising population [
14,
15]. The agricultural sector contributes 10–20% of worldwide GHGs [
16], while paddy fields are the second-largest source of methane (CH
4) generation [
17]. The emission of GHGs such as carbon dioxide (CO
2), CH
4, and dinitrogen monoxide (N
2O) have a considerable influence on global warming, eutrophication, human toxicity, and ozone layer depletion [
18,
19]. He et al. (2018) stated that organic farming methods are seen as an effective way to reduce negative environmental consequences [
20]. Organic farming, which prohibits the use of synthetic agrochemicals to ensure long-term sustainability in agricultural systems and biodiversity protection, is frequently touted as an essential alternative to conventional farming. The results of some research showed that organic farming methods cause a lower toxicity and eutrophication per unit of product [
19,
21,
22,
23,
24]. Due to its capacity to enhance soil fertility, the use of organic fertilizer may be a sustainable way to raise grain production [
25,
26,
27,
28,
29]. For the development of sustainable cropping, it is essential to identify the factors that are affecting the crop’s reaction to organic amendment [
21,
22,
23].
The assessment of environmental risks and health damages of rice production can be done by life cycle assessment (LCA) [
2]. The LCA technique considers all input and output sources of the whole production system and can be therefore used to analyze the probable impact of agricultural production on the environment from raw material extraction to final product production and disposal [
23,
24]. Throughout a product’s life cycle, LCA develops an inventory of relevant environmental flows and assesses any potential environmental consequences linked with those flows [
26]. The method examines additional environmental consequences, such as acidification, environmental toxicity, resource depletion, and so on, to reduce environmental impacts at various phases of the production process of various systems of farming [
27]. LCA also considers how emissions could affect human health [
28]. As a result, the usage of LCA in agricultural research is comprehensive and trustworthy for analyzing production systems [
29]. Recently, LCA has been used in several studies throughout the world to analyze different cropping systems [
30]. Mansoori et al. (2012), who compared organic and traditional rice cultivation systems, reported that the conventional system had larger energy inputs, with fuel and electricity accounting for the largest share [
31]. Several studies have shown that yields in organic rice farming may be quite varying and have increased as a result of improved organic fertilizer or manure application technology [
19,
30,
31,
32,
33]. The results of Jirapornvaree et al. (2021) show that the yields of organic jasmine rice cultivation were much greater than those of chemical (traditional) jasmine rice production. They stated that for supporting sustainable agriculture, organic rice production is an alternative approach [
34]. Jiang et al. (2021) conducted a life cycle assessment of wheat production and stated that the environmental impacts of wheat production greatly improved by changing the conventional strategy to a manure compost strategy, especially with a biochar-amended manure compost strategy [
35]. Only a few LCA research studies have been conducted on rice production in Iran, and the existing studies have mostly concentrated on traditional rice [
36,
37,
38,
39,
40], paddy size [
41], crop rotation [
42], and organic rice [
43]. So far, there has been no LCA study comparing organic and conventional rice production in Iran. Further, it is important to highlight the barriers in the role of organic fertilizers such as cattle manure and vermicompost in rice production. Therefore, the main goals of this study were to investigate the impacts of vermicompost and cattle manure as organic fertilizers in organic farming and further to compare the environmental impact assessment of conventional and organic farming to recognize the environmental damage of each system of farming.
4. Discussion
The potential of global warming, which is expressed based on the carbon dioxide equivalent (kg CO
2 eq), illustrates the role of GHG emissions for climate change. Global warming is the most important parameter in the climate change category. According to the results, CO
2 emissions in organic rice were 32.7% lower than in conventional rice, and the amounts of N
2O and CH
4 emissions in organic rice were 152.2% and 122.7%, respectively, higher than in conventional rice. He et al. (2018) stated that significant greenhouse gas emissions in organic systems were linked to the usage of a lot of organic manure. They also stated that after the conversion to organic farming, the systems need some time to reach a new stable state [
20]. According to Hokazono and Hayashi (2015), in the organic rice system, direct emissions and field operations significantly contribute to the environmental impacts [
33]. Similar research studies showed that the usage of chemical fertilizers (especially urea) and fossil fuels had the greatest impact on global warming potential and GHG emissions in other crops [
38,
39].
According to the findings of Hokazono and Hayashi (2012), paddy agricultural flooding contributes significantly to GHG emissions [
49]. Fertilizer use in agriculture has an influence on the environment, since nitrogen in fertilizers contributes to N
2O, nitrogen oxides (NOx), and NH
3 [
41,
42] emissions. Fertilizers release N
2 because of the process of nitrification followed by denitrification, which is created by microbial interactions in the soil under aerobic and anaerobic conditions [
52]. Fertilizer application dosages and their impacts on yields have a large influence on GHG emissions. Changing crop management to organic fertilizers might have the opposite impact due to this relationship. For example, Saber et al. (2020) and Harun et al. (2021) estimated that moving from conventional to organic farming reduces the carbon footprint of rice by two and three times, respectively [
3,
53]. According to Meng et al. (2014) and Mungkung et al. (2019), organic rice cultivation is promoted because of the supposed decreased risks associated with eliminating chemical inputs, as well as the potential to lessen environmental consequences such as global warming and ecological destruction [
54,
55]. Organic farming systems use biological and physical controls instead of synthetic fertilizer and prevent groundwater pollution, increase microbial biodiversity, and are less reliant on external inputs [
56,
57,
58]. The main sources of N
2O emissions to the atmosphere are nitrification proceeded by denitrification processes carried out by soil microorganisms, though these emissions can change based on agricultural waste management, soil tillage, nitrogen fertilizer used, and climate conditions [
50].
According to
Table 4, although there was no significant difference between organic and conventional system of farming, organic rice had lower (PMFP) values than conventional rice. The conventional farming strategy relies on a chemical technology in which an extra supplement is applied to improve productivity. Several studies have shown that these practices affect the ecosystem and also have impacts on human health [
2,
46,
50,
51]. Agrochemical contamination has a long-term impact on humans, food chains, and the environment. One of the issues with use of pesticide, especially in rice farming, is the migration of pesticide into surface water after treatment. If drinking water is obtained from surface water, pesticides can harm aquatic ecosystems and human health [
59,
60,
61]. Contrary to that, organic agricultural techniques have been shown to be a potential way to reduce negative environmental effects [
20]. One of the most important environmental stressors that causes pathogenicity is the inorganic respiration index, which is described as the influence of respiratory minerals on human health. The inorganic respiration index is influenced by field operations, fossil fuel use, and nitrogen fertilizer use [
61,
62,
63]. Darzi-Naftchali et al. (2022) found that on the carcinogenic illnesses index, urea fertilizer had the greatest impact [
16]. In rice cropping systems, excessive urea fertilizer application and inadequate nitrogen usage efficiency may exacerbate these negative environmental consequences [
64]. According to Darzi-Naftchali et al. (2022), the use of diesel-based fossil fuels had the greatest influence on the ionizing radiation potential, followed by the use of triple superphosphate and urea fertilizers [
16]. Agricultural machinery that operates on diesel fuel can exacerbate ozone layer depletion. The consumption of butachlor herbicides is also a source of ozone depletion. Because of the chlorine in its composition, this herbicide has high destructive power and is categorized as a category one danger for water resources and ozone depletion [
61,
62,
63]. Organic rice cultivation is a long-term rice-growing system that uses no chemicals and has the potential to decrease ecological issues [
62,
63].
FETP in conventional rice was 62.2% higher than in organic rice. The acidification impact is mostly caused by the emission of sulfur dioxide (SO
2), NH
3, and nitrogen dioxide (NO
2) into the air or soil, which are significantly reduced in the organic system compared to the conventional one. These results may be influenced by significant reductions in chemical inputs and diesel rates [
3]. The usage of fertilizers and chemical pesticides in agriculture has a direct relationship with soil toxicity. Because of the adsorption of particles to the soil surface, soil is an important pollution sink [
24,
62,
63,
64]. Leaching of soil pollutants and toxic elements lead to groundwater pollution. Therefore, high use of these chemicals threatens the lives of animal and plant species [
6,
9,
10,
11,
12,
13]. In general, organic rice reduced the negative environmental effects compared with conventional rice farming.
Organic farming systems generate less damage to marine and freshwater ecosystems per unit of rice production, owing to lower pesticide and synthetic fertilizer use. Utilizing nitrogen-fixing plants in a crop rotation can be a good way to avoid overuse of nitrogen in the production system [
65]. On the other hand, increasing resource usage efficiency is a long-term option for reducing agricultural output’s environmental impact, especially in rice production systems [
66].
MEP in the organic system was 85.37% higher than for conventional rice production. Fertilizer emissions of nitrogen and phosphorus are a key contribution to eutrophication and acidification effects related to rice cultivation, as well as a cause of climate change and field flooding-related CH
4 emissions [
29]. According to Darzi-Naftchali et al. (2022), in terms of eutrophication potential, phosphorus fertilizers and pesticides had a much larger percentage than other input sources. They also stated that the use of nitrogen fertilizer has a significant impact on the eutrophication of water sources [
16]. In accordance with
Table 3, the amount of phosphorous emissions to water for the organic system was higher than for the conventional system (0.85 kg ha
−1 and 0.64 kg ha
−1, respectively). The high AP content of the applied manure, which enhances the percentage of bioavailable P in the soil surface, especially in the first few days following fertilizer applications, might be the cause of this finding [
67]. In terms of nutritional levels and solubility, the impacts of organic replacements are obviously a major cause for worry for P loss through surface runoff [
68]. According to reports, phosphorous is the most important element in eutrophication intensification among other inputs [
37,
50,
51,
52,
53,
54,
55,
56]. In rice cultivation, one way for phosphorous to enter water sources is by leaching [
69,
70]. Since rice production is carried out in flooded conditions and leaching is common in rice production, improved fertilizer management is critical for reducing the effects of climate change, acidification, and freshwater eutrophication all at the same time [
71,
72,
73].
TAP in the organic system was 80.19% higher than in the conventional system. According to
Table 3, the amounts of ammonia emissions to water for organic and conventional systems were 18.67 kg ha
−1 and 5.04 kg ha
−1, respectively. This is consistent with the reports of Regina et al. (2013) and Schmidt Rivera et al. (2017), who claimed that ammonia (NH
3) emissions from fertilizer usage cause these effects [
52,
72].
In conventional system of rice, high doses of phosphate, potassium, and nitrogen produced from synthetic fertilizers contribute to the terrestrial ecotoxicity potential due to the release of NH
3, SO
2, and NO
x into the soil [
50]. Terrestrial ecotoxicity potential (TETP) caused by inorganic substance deposition in the atmosphere allows one to examine the environmental effect of sulfates, nitrates, and phosphates, as well as their subsequent deposition, which can lower soil pH [
71]. Increased leaching and the production of acidic chemicals in water sources are caused by a rise in anthropogenic sulfur and nitrogen emissions. In sensitive catchments, this sediment heads to negative environmental consequences on aquatic habitats, as well as ecological loss or extinction. Therefore, as a result of the existence of shallow groundwater and surface water bodies in rice farming, pollution from sulfur and nitrogen sources should be decreased in areas that are vulnerable to acidification [
16].
As shown in
Table 4 and
Figure 3, organic rice had lower values of agricultural land occupation potential (LOP) than conventional rice (0.04 m
2a crop eq and 0.02 m
2a crop eq for conventional and organic rice, respectively). In this study, the grain yields of organic rice were 4000 (kg ha
−1) and for conventional rice 3290 (kg ha
−1); thus, the organic rice grain yield was 14.6% higher than the conventional rice grain yield. Therefore, to produce the same amount of paddy, less land is required. One of the factors contributing to the higher yield is the higher amount of organic matter content in organic farming. In fact, organic matter that is accumulated in the soil will help to improve the yield in an organic farming system [
34]. Meta-analysis research carried out by Liu et al. (2021), who studied the response of rice yield and agronomic characters to organic fertilization, stated that the use of organic fertilizer improved N absorption by rice by 5.2% on average across all studies [
45]. Similarly, when organic fertilizer was applied to rice, it increased P and K uptake by 7.2% and 10.5%, respectively, as compared to the control. In addition, when organic fertilizer was utilized, rice’s physiological N, P, and K usage efficiency increased by 6.2%, 4.6%, and 3.7%, respectively. They also stated that in terms of organic fertilizer types, using animal manure increased rice production more than applying biochar or crop straw. Specifically, animal manure treatment enhanced rice output by 10.5%, whereas organic fertilization using biochar or crop straw increased rice grain yield by only 7.2% and 8.3%, respectively [
45]. When organic fertilizer is applied, soil organic carbon, one of the key measures of soil fertility, considerably rises, mostly due to the high carbon content of organic fertilizers [
45,
55,
74,
75,
76]. More importantly, the increase in nutrients that are accessible with the application of organic fertilizer is greater than the increase in total nutrients for each mineral element, indicating that the increase in nutrients that are available promote the improvement of nutrient use efficiency [
45]. The significant increase in plant-available nutrients might be related to the increased soil microbial community activity, which is necessary for nutrient cycling and transformation, and hence improves nutrient availability [
72,
73,
74,
75,
76]. The application of organic fertilizers may change the architecture of the rice root system, promoting nutrient absorption from the soil, because they include amino acids and other physiologically active chemicals [
77]. Organic fertilization could also improve the physical properties of paddy soil and help rice roots grow deeper into the soil, resulting in the absorption of more nutrients [
74,
75,
76]. In general, it can be stated that the use of organic fertilizers compared to chemicals leads to increased yields of rice, and as a result, land occupation potential in organic farming is less than in conventional farming.
The FFP in conventional rice was 16.67% higher than in organic rice. The overuse of non-renewable resources for energy production and fuel significantly increase the risk of climate change. Diesel is a common fuel for agricultural equipment and is an expensive and non-renewable source [
77,
78]. The use of lower amounts of fertilizers and diesel fuel, as well as ecological methods for disease and insect prevention, could help to lessen the negative environmental impact. The production of fertilizers, which uses a lot of coal or natural gas as a source of hydrogen (to synthesize ammonia), increases CO
2 emissions [
78,
79]. Further, around 1.3–1.8% of the world’s fossil fuel consumption is attributable to the manufacturing of nitrogen fertilizers [
50]. According to previous studies, non-renewable inputs have been shown to account for 60–80 % of the overall costs of agricultural crops, with chemicals such as fertilizers and insecticides accounting for 25–97 % of all non-renewable inputs [
80,
81,
82,
83,
84]. Around 14% of the world’s ammonia production is based on coal gasification, 77% on natural gas reforming, and 9% on the partial oxidation of heavy hydrocarbon fractions and oil products [
50].