1. Introduction
Agriculture is a key economic engine in Bangladesh, accounting for 14.23% of the country’s gross domestic product (GDP) [
1] and employing 41% of the workforce [
2]. Agriculture in this nation has evolved throughout time, with crop production skyrocketing as a result of new technology, mechanization, increased chemical usage, higher cropping intensity, adoption of contemporary varieties (high-yielding varieties and hybrids) and cultivation of high-biomass-potential crops, among other factors. Although these changes have had many positive effects, there have also been many negative effects, including topsoil depletion, nutrient mining, ground and surface water contamination, continued neglect of farm labour living and working conditions, increasing production costs and the disintegration of economic and social conditions in rural communities [
3]. To meet the demands of an ever-increasing population and for food security reasons, our farmers must employ innovative and sustainable agricultural practices and technologies. There is a growing realization that adopting ecological and sustainable farming practices can help to reverse the declining trend in crop productivity and contribute to environmental protection [
4]. Because soil is a delicate and living medium, it has to be maintained and nourished in order to maintain its long-term production and stability. It is not sufficient to just increase the use of inorganic fertilizers to prevent organic matter depletion and nutrient mining; organic sources of plant nutrients such as cow dung, chicken manure, bioslurry, compost, green manure and other organic sources of plant nutrients are also important. Organic manures such as crop wastes, animal manure and green manure have a direct influence on soil organic matter content, which can enhance physical, chemical and fertility characteristics; boost microbial activity and reduce metal toxicity through complexation with metals of a contaminated soil [
5]. The mineralization of soil organic matter releases large amounts of nitrogen (N), P, and S, as well as a smaller quantity of micronutrients [
6]. To achieve better and long-term soil fertility and crop output, a strategy of regular organic matter addition and balanced fertilizer management, which includes both organic and inorganic fertilizers, is required.
Without sacrificing output or increasing the danger of contamination, the correct amount of manure may be determined effectively from precise measurements of nutrient mineralization. The quickness with which organic materials mineralize and liberate the nutrients found in them determines their usefulness as a fertilizer [
7]. The amount of manure that should be put into the soil is determined by its composition, the availability of nutrients in the soil, the crop produced and the surrounding conditions [
8]. When manure is utilized as a source of a certain nutrient, it is necessary to understand the mineralization rate under field circumstances. The mineralization of organic materials is aided by climatic circumstances, notably high temperatures, which are prevalent in Bangladesh for the majority of the year, and usually excessive soil tillage. When organic manure is utilized as a source of nutrients, understanding the mineralization of nutrient elements in the soil is crucial for forecasting their availability [
9]. The amount of nutrients released into the soil for the first crop may be calculated using mineralization data, and the residual effects of applied organic nutrient sources on the next crop can be simply determined. Incorporating organic manures of various qualities also influences nutrient availability and necessitates manipulation to maximize nutrient release and coordinate with crop needs. As a result, determining the rate of net mineralization of compost in soil is critical for optimizing the use of compost and supplementing some of the chemical fertilizers required throughout the plant growth phase [
10]. Thus, understanding the mineralization process and nutrient availability in various organic manures is critical for avoiding nutrient deficiency, maintaining proper soil fertility and improving successful crop production using integrated nutrient management, which will reduce the use of inorganic fertilizers and save our environment.
Phosphorus is a key nutrient that influences plant development and production by engaging in a variety of metabolic and structural processes such as photosynthesis, respiration and protein synthesis. Low P availability in many locations can result in chronic P shortages in the soil, restricting crop production. Again, higher P fertilizer application in some agricultural soils results in greater run-off that can contribute to eutrophication in aquatic environments. When organic matter is introduced to soils, the mineralization process compensates for a large amount of the plant’s P need. Season, climate, soil organic matter, soil depth and the C/P ratio of organic matter, among other variables, influence the rate of phosphorus mineralization, according to Jalali et al. [
11]. Furthermore, Xiao et al. [
12] found that diverse biogeochemical characteristics such as soil moisture, organic matter and clay content have a substantial impact on the distribution and dynamics of P in soils. Soils with too much P are prone to leaching and run-off [
13]. Sulphur is another essential element for plants, since it aids in the production of chlorophyll and in protein synthesis, and its deficiency can have negative metabolic and visual repercussions. Because more than 95% of total S collected from manures, crop residues and fertilizers [
14] exists in organic form, organic S mineralization is considered a crucial source of S for plants [
15,
16]. Biological activity, organic compound types and soil physical and chemical characteristics all have a role in S mineralization [
17,
18]. In Bangladesh, the release kinetics of P and S in different soils after mineralization from various types of manures has not yet been thoroughly studied. Thus, there is currently a lack of data on the transformation of diverse organic sources of these nutrients in soils under aerobic and anaerobic conditions. In this study, we highlighted the mineralization potential of manures, which guided us to determine the time course and release patterns of P and S in soils after manure application and to find out the suitability of manures. The goal of this study was to look at the release kinetics of P and S in soils amended with different organic manures that varied in moisture levels over time, and to see whether these manures could be used as an organic source of nutrient supply and as an alternative to inorganic fertilizers in our farming system.
4. Discussion
Some soil characteristics, such as type, depth, temperature, moisture content, pH, C/N ratio and complex carbohydrate content influence the mineralization of organic manures in soil [
35]. Since most of these factors cannot be accurately predicted, only an approximation of manure nutrient mineralization following application is possible. The amount and kinds of organic materials added to the soil, as well as the complex interplay between soil physical, chemical and biological processes, as well as climatic conditions such as moisture, were found to have a substantial influence on P mineralization in this study (
Figure 1a–h). Phosphorus release from the mineralization of fresh PM, partially decomposed CD and rice straw occurred after 15 days of application and increased gradually over time, and CD and PM had greater P release from mineralization, according to Naher et al. [
36]. Phosphorus release from mineralization is influenced by soil characteristics as well as organic wastes, according to Meena et al. [
37]. Phosphorus is an immobile nutrient in soil, and mineralization of its organic sources is mediated by soil microbial biomass and variety. Its release after mineralization (net P) varied substantially over the incubation days in our research. Haque et al. [
38] similarly discovered that P release rose with time, peaked between the 4th and 6th week of incubation, and then steadily dropped till the incubation period ended. Pal et al. [
39] discovered that the net P release increased with increasing time intervals but did not reach a significant value during the incubation period, while the rate of P release from mineralization reduced as the incubation period lengthened. According to Meena et al. [
37], a decreased concentration of P in the residue delays the mineralization process because the C-to-P ratio in the soil is too large for proper mineralization. In our study, there was a significant influence of organic sources on cumulative P release after 180 days of incubation (
Figure 1e,f). The net cumulative P release from the mineralization of manures was described using a first-order exponential kinetic model that suited well. Haque et al. [
38] showed that the net P released from mineralization aligned well with the first-order kinetic model, similar to our findings.
The process of mineralization is a microbiological one. The mineralization process is split into three stages in aerobic conditions. Initially, surface bacteria and any interior bacteria typically present initiate mineralization; these organisms include, among others,
Bacillus,
Clostridium,
Proteus and lactic acid bacteria [
39]. Second, there is an intermediate stage when fermentation end-products and components were not utilized by the first decomposers.
Pseudomonas,
Acinetobacter,
Arthrobacter,
Enterobacter and some of the more specialized members of the genus
Bacillus are among the organisms involved in this process [
39]. Slow aerobic CO
2 release from the most refractory of the organic residues characterizes the last stage. In this case, the one-carbon-104 GEORGE HEGEMAN compound-oxidizing bacteria and other specialists have a role to play [
39]. Oxygen is not available in anaerobic conditions. Organic molecules degrade due to the activity of non-aerobic living organisms, yielding intermediate compounds such as methane, organic acid, hydrogen sulphide and various substrates.
Desulfovibrio desulfuricans,
Clostridium botulinum and other organisms are involved in this process [
39].
Phosphorus release was understood by McDowell and Shapley [
40] as a function of P concentration and availability. According to Islas-Espinoza et al. [
41], P release was affected by the quantity of P added and the type of modification. The amount of P released from manure is determined by its mineralization pattern, which is influenced by a number of variables, including the C/P ratio [
42]. Materials with high P, humic compounds and a low C/P ratio, according to Gagnon and Simard [
43], release more P. In our research, it was found that PMSL with the lowest C/P ratio had the best P availability, whereas MR with the highest C/P ratio had the lowest P availability in soil. Because of its lower C/P ratio than other organic composts, PMSL is also predicted to degrade quickly and contribute to the available pool of P. According to Garg and Bahl [
44], the rise in P might be attributed to the release of a significant quantity of CO
2 during organic matter decomposition and cation (Ca
2+) complexation, which is primarily responsible for P fixation in alkaline and calcareous soils. Furthermore, organic manures increase phosphatase activity in the soil, resulting in increased P in the soil solution via mineralization/solubilization. Phosphorus solubilization, according to Meena and Biswas [
45], is caused by increased microbial biomass C, which promotes microbial activities, as well as enhanced phosphatase and dehydrogenase enzyme activities.
Sulphur release after mineralization in modified soils, according to some studies [
14,
18], is dependent on the S concentration of the decomposing materials. The quantity of S in PMSL was larger in this study than that in other manures (
Table 2). Therefore, S release was highest in PMSL-treated soils compared to that in soils modified with other manures. Sulphur release, on the other hand, was lowest in CD-treated soils, which contain the least amount of S. Again, S release increased over time with incubation, peaking in 60–180 days in aerobic conditions and in 45–60 days in anaerobic conditions. Our findings are similar to those of Moharana et al. [
9], who found increasing net S release levels as the incubation time progressed from 30 to 60 to 90 days, regardless of treatment. Slurry had a greater ability to provide nutrients to plants for a shorter period of time. According to Eriksen [
46], residues with the highest total S concentration had the fastest decomposable S. In wet and saturated conditions, CD and CDSL had the lowest S
0 values, respectively. There was no clear pattern in terms of rate constant (k) value; k value was sometimes greater in slurries and sometimes higher in their original manures. Slow release is shown by the lower rate constant values (k) observed with PMSL and PM.
According to Moharana et al. [
9], S release from mineralization of organic materials in soil is usually attributed to biological or biochemical processes regulated by microorganisms that result in the release of S as a by-product from ester sulphates via extracellular enzymatic hydrolysis. Singh et al. [
47] postulated that the fast release of sulphate following the addition of organic residues was related not only to the overall S levels but also to the form of S present in the tissues (soluble sulphate and readily degradable organic S forms). The extraction and hydrolysis of organic sulphates in the residues, not the S mineralization–immobilization cycling associated with the breakdown of residue C by heterotrophic bacteria, caused the negative connection as discovered by Niknahad-Gharmakher et al. [
48]. The chemical composition and C/S ratio of plant residues are significant factors for forecasting the kinetics of residue breakdown as well as the amount of sulphate available.
Sulphur fertilizer optimization necessitates a research of S transformation. Assefa et al. [
49] found that fertilizer treatments had a significant impact on soil SO
4−2–S concentration over time, while liquid biogas residues had greater soil SO
4−2–S content. Biostimulants reduce the need for fertilizers [
50,
51,
52]. Microbial needs for organic C to supply energy drive biological mineralization, as does S release as sulphate, which is a consequence of microbes breaking down organic molecules to get C for their energy metabolism.
Composition of manures, local management techniques in terms of treatment, storage and field application and ambient climatic conditions have a significant impact on nutrient mineralization. To maintain soil fertility, the most appropriate source of organic amendment should be chosen based on the farm’s needs. Soil rhizobacteria play a significant role in nutrient uptake [
53,
54,
55,
56,
57,
58]. The nutrient release pattern should be developed first considering the soil condition, and then there must be adequate coordination of nutrient input and crop demand based on the data obtained from farm manure mineralization research. The use of manure at the right time and in the right amount will prevent nutrient shortage or over usage in crop production, provide balanced fertilization and, ultimately, save our ecosystem.