**2. Problem statement**

**1. Introduction**

224 Organic Fertilizers - From Basic Concepts to Applied Outcomes

opment [10-12].

The world population is expected to reach more than 9 billion people by mid‐century, creat‐ ing enormous pressure over the global food supply. Concurrently in the food chain, meat production is an activity that causes the greatest environmental impact due to the inefficiency of the transformation of nonusable parts for direct consumption in reusable by‐products [1]. Thus, the amount of waste to be recycled and reused for various purposes will increase significantly, negatively contributing to environmental sustainability due to its disposal in the environment in an inappropriate manner and thereby generating increased emissions of greenhouse gases (GHGs) [2-4]. This change may trigger an increase in the planet's average temperature by up to 5.8°C over the next 100 years [5]. It is estimated that alterations in soil management provide about 20% of the total emissions of greenhouse gases (GHGs) [6].

Global meat consumption is estimated to increase by 72% between 2000 and 2030, and much of this increase will be due to the consumption of poultry and pork [7]. Around 96 and 113 million tons of poultry and pork, respectively, are produced in the world. The Americas are responsible for 43.7% of the world's chicken production and 17.4% of pork production [8].

The United States accounts for 17% of the world's poultry meat production, China accounts for 13% and Brazil, becoming the third largest producer, accounts for 12%. China accounts for 60% of pork production, followed by the United States (10.5%), Russia (5.5%), Spain (3.9%) and Brazil (3.71%) [8]. The waste from slaughterhouses of poultry, pork and cattle has caused serious environmental consequences due to its improper disposal in the environment [2-4].

Poultry and pork production will generate 121 and 509 million tons of carbon dioxide, respectively, via carbon (C‐CO2) equivalent until 2020, with a prospective increase of 47% in 2030 [7]. The use of organic waste from the meat processing industry could increase the potential for soil carbon (C) drain and promote reduction in GHG emissions compared with industrial fertilizers derived from fossil fuels, thereby minimizing its environmental impact [9]. In addition to reducing its environmental impact, the organic waste produced in the slaughtering system is an organic fertilizer option for soil due to the presence of essential nutrients for plant growth and mainly due to its high content of organic matter, which acts positively on physical, chemical and biological soil properties, thus promoting plant devel‐

The use of industrial organic waste in combination with crop residues that return to the soil increases the C accumulation rate in the long term [13]. The C accumulation potential in the soil is governed by many factors, such as climate and soil type [14, 15], crop systems [16], soil

Thus, industrial organic waste presents several benefits regarding soil quality improvement and agronomic production increase [20]. However, the potential that these residues have to promote C compensation to the soil‐plant‐atmosphere system has been scarcely explored

management, including conservational systems [17, 18], and soil fertilization [19].

compared with the use of industrial mineral fertilizers.

The organic residues used in agriculture as fertilizers often originate from three main activities: agricultural, urban and industrial. Among agricultural residues, manure (cattle, porcine and poultry) is the most commonly used. With regard to the organic residues from urban activities, the products generated from composting of urban garbage and sewage sludge are the most used ones [21, 22]. Lastly, the waste generated in the food processing industry is the most used in the production of organic fertilizers.

The residues generated from chicken and poultry slaughterhouses have been causing serious pollution problems to the soil, surface water and groundwater. According to COWI Consulting Engineers and Planners AS [23] and Matos [24], 20% and 30% of chicken and swine weight, respectively, are considered inedible (blood, feathers, hairs, nails, fat, etc.). Part of the residues generated is destined for industrial purposes (e.g., animal food production), and approxi‐ mately 20-22% are discarded in the environment.

Several reports have demonstrated the benefits of using organic fertilizers to the chemical properties of soil. According to Rasmussen and Collins [25], the use of organic fertilizers in agriculture aims to increase the soil organic matter content. The soil organic matter plus the clay soil content form an absorption complex that increases soil chemical properties. The complex, in this way, is capable of retaining the nutrients as nitrogen that would be eventually leached. Organic fertilizers also add micronutrients and macronutrients to the soil. Therefore, organic fertilizers increase the soil cation exchange capacity (CEC), provide better water retention, create complex toxic elements [26-29] and determine the biological and physical qualities of soil.

Marchesini et al. [30] reported crop yield increases provided by the use of organic fertilizers, which are more persistent despite presenting lower and slower effects compared with synthetic mineral fertilizers (SMFs). This could be due to their lower and progressive nutrient release and plant root system development.

However, it is important to emphasize that since agricultural soils can be considered a destination for residue waste, we must respect the limits imposed by legislation, avoiding overpowering the soil's capacity [29, 31, 32]. Although the practice can work in ameliorating soil conditions, it can also cause contamination, consequently affecting crop yield and quality [33].

Synthetic fertilizers that come from nonrenewable sources are commonly used in agriculture. In 2012, Brazil consumed more than 29 million tons of industrial fertilizers [34]. Therefore, lowering their use by replacement with organic residues in this way can positively contribute to environmental conservation.

In the search for more sustainable practices, correct management of organic residues in agriculture is an important process to promote their environmental, social and economic benefits. Exploring the potential of organic residues can promote their proper destination, increase soil quality and promote economic benefits.

In this light, the specific objectives of this research were (a) to assess the contribution of slaughterhouse organic residues (SORs) from poultry and porcine activities to carbon (C) alterations as well as (b) to study crop performance under a no‐till system with organic residue applications with and without synthetic mineral fertilizers.
