Humification (HF) is the natural process of converting bioorganic matter into humic substances (humus, humate, humic acid, fulvic acid, and humin) via geo-microbiological mechanisms under aerobic and/or anaerobic conditions. Humic substances (HSs) and their composition and concentrations mostly determine the basic properties of soils and play an important role in regulating the growth of plants and soil microorganisms and the accumulation and migration of metal ions, radionuclides, and ecotoxicants in soils. Various processes designed for the humification (HF) of animal husbandry wastes, primarily bird droppings, reduce their volumes, solve environmental problems, and make it possible to obtain products with artificially formed humic substances (HSs) as analogues of natural HSs, usually extracted from fossil sources (coal and peat).
1. Introduction
Humification (HF) is the natural process of converting bioorganic matter into humic substances (humus, humate, humic acid, fulvic acid, and humin) via geo-microbiological mechanisms under aerobic and/or anaerobic conditions
[1][2][3][4]. Humic substances (HSs) and their composition and concentrations mostly determine the basic properties of soils and play an important role in regulating the growth of plants and soil microorganisms
[5][6] and the accumulation and migration of metal ions, radionuclides, and ecotoxicants in soils
[7]. It is possible to regulate these processes by changing primarily the concentration of HSs in soils due to their introduction
[6]. Actively used and damaged soils necessitate the constant introduction of HS in significant quantities in order to restore them
[8], while the presence of main the components of HSs (humic acids (HAs) and fulvic acids (FAs)) and their quantitative ratios predetermine the functions of HSs as soil structurators involved in the regulation of soil humidity and air and water permeability. Currently, the raw materials for the commercial production and widespread use of HSs are mainly peat and coal, from which HSs are extracted
[9][10]. In nature, the formation of HSs occurs as a result of the HF of bioorganic (mainly plant) residues. However, various methods similar to natural ones are being developed for the “artificial” production of HSs from organic wastes accumulated mainly in agriculture (plant waste and animal excrement). These wastes are generated annually in significant quantities on livestock and poultry farms in various countries around the world (
Table 1)
[11][12][13][14][15][16][17][18][19][20][21][22][23]. Their conversion into HS resources containing simultaneous sources of carbon (C), nitrogen (N), phosphorus (P), and microelements allows us to simultaneously solve the problem of obtaining HSs similar to natural ones and significantly reducing the volumes of waste. In addition, a number of methods for the HF of these wastes allow, together with the production of HSs, for the destruction of various micro-pollutants contained in them (pesticides, mycotoxins, microplastics, pharmaceutical pollutants, etc.)
[24][25][26][27]. However, the initial composition of wastes and the applied HF methods not only lead to the acquisition of various products (
Figure 1) but also HSs with different compositions and properties, widening the range of fields of their potential use instead of natural HSs.
Figure 1.
The scheme used in this research for a comparative analysis of the main HF processes of AW under different conditions and the HS resources obtained in them.
Table 1.
Annual amounts of animal waste produced in various countries.
Country/Reference |
Animal Wastes [References] |
AP * |
USA |
Dairy manure [11] |
24,000 |
China |
Livestock manure [12] |
3800 |
Chicken manure [13] |
155.0 |
Brazil |
Cattle manure [14] |
1900 |
EU |
Farm manure [15] |
1200 |
France |
Farm manure [16] |
214.3 |
Germany |
Farm manure [16] |
175.7 |
The United Kingdom |
Farm manure [16] |
112.0 |
Spain |
Farm manure [16] |
108.3 |
Bangladesh |
Cow manure [17] |
102.6 |
Poland |
Farm manure [16] |
91.3 |
Italy |
Farm manure [16] |
89.4 |
India |
Poultry manure [18] |
38.0 |
Malaysia |
Chicken manure [19] |
23.1 |
Serbia |
Farm manure [16] |
18.6 |
Greece |
Farm manure [16] |
16.9 |
Belgorod Region, Russia |
Total manure [20] |
14.2 |
Turkey |
Chicken manure [21] |
11.0 |
Canary Islands |
Livestock manure [22] |
0.5 |
Malta |
Farm manure [16] |
0.3 |
South Africa |
Cattle manure [23] |
0.1 |