Mineralization of Farm Manures and Slurries under Aerobic and Anaerobic Conditions for Subsequent Release of Phosphorus and Sulphur in Soil

Abstract

A good understanding of nutrient release from manure or compost after application through mineralization is important to assure meeting the nutrient demand of crops, to secure timely fertilizer application and to enhance nutrient use efficiency. The current study was done to evaluate phosphorus (P) and sulphur (S) release patterns from different types of manures viz. cow dung, cow dung slurry, tricho-compost, vermicompost, poultry manure, poultry manure slurry and mungbean residues. The mineralization study was performed under aerobic (field capacity) and anaerobic (waterlogging) conditions for 180 days at 25 ± 1 °C in the laboratory. The release of P and S showed the highest values within 75–180 and 75–150 days, respectively, and was always higher in aerobic conditions than in anaerobic conditions. The first-order kinetic cumulative model was a good fit for mineralization, which was significantly influenced by manure type, soil moisture level and incubation period. Poultry manure slurry exerted the highest P and S release under both moisture conditions. Both slurries showed higher potential mineralization, with a lower rate constant for these elements compared to that in their manure states. Hence, appropriate manures should be chosen and applied in the proper quantity to provide exact amounts of nutrients, to increase crops nutrient use efficiency and to formulate correct fertilizer recommendations.

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.

2. Materials and Methods

2.1. Soil Sample Collection and Preparation

The soils samples used for this study were collected at a depth of 0–15 cm from the Field Laboratory of Soil Science Department, Bangladesh Agricultural University (BAU), Mymensingh—2202. Geographically, the soil sampling site lies between 24°75′ north altitude and 90°50′ east longitude at a height of 18 m above sea level, and the experimental soil was characterized as Non-Calcareous Dark Grey Floodplain soil (Aeric Haplaquept) belonging to Sonatola series under AEZ 9 (Old Brahmaputra Floodplain) [19]. Soil sampling was done from different areas where crop plants were usually grown under no experimental trial and thoroughly mixed to make composite samples. Each composite sample contained six simple soil samples. Composite samples were transported to the laboratory and air dried on a brown paper. After being air dried, soil samples were crushed and passed through sieves with a 2 mm mesh in order to remove unwanted materials such as crop debris. Sieved soils were kept in polyethylene bags and incubated for 21 days under aerobic conditions at 25 °C temperature. All soil samples were stored in a dry and cool place prior to incubation study.

2.2. Analysis of Initial Soil Samples

The initial soil samples were used to determine the physicochemical properties of the soil including textural class, bulk density, cation exchange capacity (CEC), water holding capacity, soil pH, organic carbon (OC), total N, exchangeable potassium (K), available P and available S (as presented in

Table 1. General (physical and chemical) characteristics of the soil.

Soil Characteristics	Values
Particle size distribution (USDA system)
% Sand (0.2–0.05 mm)	1.4
% Silt (0.05–0.002 mm)	80
% Clay (<0.002 mm)	18.6
Textural class	Silt loam
Water holding capacity (%)	51.5
Bulk density (g cm−3)	1.33
Organic C (%)	2.15
Cation exchange capacity (cmol kg−1)	12.1
pH	6.5
Total N (%)	0.119
Available P (mg kg−1)	9.1
Exchangeable K (cmol kg−1)	0.12
Available S (mg kg−1)	25.3

). Physicochemical properties were measured as they change with land use [20]. The hydrometer method [21] was employed to determine the particle size analysis and Marshall’s Triangular Coordinates (USDA system) were used to determine the textural class of the soil. The bulk density of the soil was measured by the core sampler method [22]. The pH of the samples was assessed in a soil:water ratio of 1:2.5 with a glass electrode pH meter [23]. The CEC and OC were estimated by the sodium saturation method [24] and the Walkley and Black method [25], respectively. Total N was determined by semi-micro Kjeldahl method [26], and available P was measured according to Olsen method [27]. Analysis of exchangeable K was based on the NH₄OAc (1 N) extraction method at pH 7 with a flame photometer [28]. Available S was assessed by a 0.15% extractant solution of CaCl₂ following turbidity measurement with a spectrophotometer [29]. The water holding capacity of the soil was measured by the gravimetric method following standard protocol [30].

2.3. Collection of Different Manures and Determination of Their Chemical Composition

The present investigation utilized seven categories of organic fertilizers viz. cow dung (CD), cow dung slurry (CDSL), poultry manure (PM), poultry manure slurry (PMSL), tricho-compost (TC), vermicompost (VC) and mungbean residues (MR). CD and PM were obtained from the dairy and poultry farm of BAU. VC and TC were supplied by Gono Kallyan Swabolambi Sangstha (GKSS) located in Bogra (NGO). CDSL and PMSL were collected from a biogas plant situated at Boira village, Mymensingh Sadar Upazila. MR was obtained from the current experimental plot located at the BAU farm. CD and PM were prepared after fermentation for 6 and 1 months in piles, respectively. VC was prepared from vermicast and organic sources viz. fermented CD (75%) and kitchen wastes (25%) in a tank after 3 months of decomposition. TC was prepared in a similar tank using Trichoderma sp. with organic materials such as PMSL and molasses after 3 months of decomposition. MR was prepared after drying (for 2 weeks) and cutting into small pieces before application. Total N and OC content in manure samples were determined following a similar protocol as described in Section 2.2. Total P and S in initial manure samples were measured by colorimetric and turbidimetric methods, respectively, with a spectrophotometer [31] after digestion with di-acid mixture (HNO₃–HClO₄ 3:1) as suggested by Piper [32].

2.4. Experimental Setup

The mineralization of P and S in different categories of manures was assessed under controlled conditions (aerobic and anaerobic) based on the destructive sampling procedure. A series of plastic pots (pipes or cups) were filled with 100 g air-dried soil. The air-dried manures (CD, CDSL, PM, PMSL, TC, VC and MR) were mixed with soil at 20 t per ha or 20 Mg per ha (1 g manure was mixed with 100 g soil) (see Table S1 provided as Supplementary Material). The sets without adding organic manures served as control treatments. The moisture status of both manure-amended and unamended soil was maintained at anaerobic condition (with 1 cm of standing water above the soil) and at field capacity (25% moisture; 50% pore space filled with water). The containers were placed in an incubation room and incubated for the duration of 180 days at 25 °C temperature. Three replications were used for each treatment.

2.5. Organic Fertilizer Incubation in Soil

For measuring P and S mineralization, PVC pipes with an inner diameter of 5.5 cm and height of 15 cm were used for aerobic incubation of dry soil whereas plastic cups with an inner diameter of 7 cm and height of 12 cm were used for anaerobic incubation of moist soil for better water management. All the containers were placed in a dark place for incubation at room temperature (25 ± 2 °C) for the period of 180 days. Para film was used to wrap the pots to reduce subsequent water loss, and then the pots were shifted to an air-conditioned room with a proper aeration system. The soil moisture content was recorded at 7-day intervals. The weight reduction in terms of evaporation was diminished by supplying adequate quantity of deionized water and properly mixed with soil to achieve specific moisture content (soil saturation and field capacity of soil). The total number of PVC pipes or plastic cups filled up for this incubation (aerobic and anaerobic) study was 432. The destructive sampling method was followed in this experiment. The 1st sampling was performed at just the next day of incubation, and this assessment was utilized as the preliminary content of soils. The preliminary weights of the PVC pipes along with soil were also measured.

2.6. Analysis of Soil Samples after Manure Application

Soil sampling was done from each replicate through destructive methods by taking out soil from every tube at 15, 30, 45, 60, 75, 90, 150 and 180 days after the application of organic manure. The P availability in soil was determined by the NaHCO₃ (0.5 M) extraction method at pH 8.5 [27]. A CaCl₂ extractant solution (0.15%) was used to quantify available S in soil followed by measurement of the turbidity with a spectrophotometer [29,33]. The obtained data were adjusted for moisture content and presented based on an oven-dry basis.

2.7. Statistical Analysis

The data were subjected to analysis of variance (ANOVA) technique using Statistix 10 software package by repeated measures design. Mineralization kinetics were determined using Sigma-Plot 14.0 software. For the repeated measure analysis, treatments were taken as the between-subject factor and time as the within-subject factor. Non-linear regression was used to fit the mineralization data into first-order kinetic model using the Levenberg–Marquardt algorithm. Post hoc tests were performed to separate differences among the modelled value of potentially mineralized P, P₀, S, S₀ and rate constant k using the Tukey–Kramer multiple comparison. All statistical analyses were considered significant at P < 0.05, unless otherwise mentioned.

3. Results

3.1. Chemical Compostion of the Manures

The chemical properties of all types of organic fertilizers are demonstrated in

Table 2. Organic manures used in the mineralization study with chemical composition.

Manure	C (%)	N (%)	P (%)	S (%)	C:N	C:P	C:S
CD	33.14 ± 1.91	1.27 ± 0.07	0.50 ± 0.03	0.28 ± 0.02	26.2 ± 1.51	66.8 ± 3.86	118.4 ± 6.84
CDSL	20.88 ± 1.21	1.90 ± 0.11	1.23 ± 0.07	0.48 ± 0.03	11.0 ± 0.64	17.0 ± 0.98	43.5 ± 2.51
PM	33.54 ± 1.94	3.08 ± 0.18	2.33 ± 0.13	0.56 ± 0.03	10.9 ± 0.63	14.4 ± 0.83	59.9 ± 3.46
PMSL	22.34 ± 1.29	2.69 ± 0.16	2.49 ± 0.14	0.69 ± 0.04	8.3 ± 0.48	9.0 ± 0.52	32.4 ± 1.87
TC	19.41 ± 1.12	1.32 ± 0.08	1.76 ± 0.10	0.57 ± 0.03	14.7 ± 0.85	11.0 ± 0.64	34.1 ± 1.97
VC	22.81 ± 1.32	1.15 ± 0.07	0.52 ± 0.03	0.42 ± 0.02	19.9 ± 1.15	43.9 ± 2.53	54.3 ± 3.14
MR	45.60 ± 2.63	1.29 ± 0.07	0.45 ± 0.03	0.46 ± 0.03	35.4 ± 2.04	101.3 ± 5.85	99.1 ± 5.72

Figures in the column of the table indicate mean ± standard deviation.

. The C (%) in the manures ranged from 19.41 to 45.60. The highest C (%) was observed in MR and the lowest in TC (Table 2). Similarly, N (%) of the manures ranged from 1.15 to 3.08 exhibiting the highest value in PM and the lowest in VC (Table 2). The manures had a wide range of P concentration (0.50%, 1.23%, 2.33%, 2.49%, 1.76%, 0.52% and 0.45% in CD, CDSL, PM, PMSL, TC, VC and MR, respectively) (Table 2). Likewise, S concentration (%) in the manures ranged from 0.28 to 0.69. The highest S (%) was observed in PMSL and the lowest in CD (Table 2). The ranges of C:N, C:P and C:S were from 8.3 to 35.4, 9.0 to 101.3 and 32.4 to 118.4, respectively (Table 2).

3.2. Phosphorus Release Pattern from Mineralization of OM in Soils

3.2.1. Total P Release

PMSL and PM showed clearly distinct and better performance with respect to total P release (Figure 1a,b). Next to PM, the manures TC, CDSL and VC performed well under both aerobic and anaerobic moisture conditions. MR and CD had the least performance in relation to total available P release in soil over the moisture contents and incubation periods.

3.2.2. Net P Release

There was a remarkable variation in net P release between moist and saturated soil conditions, and aerobic condition showed higher P release compared to anaerobic condition (Figure 1c,d). From CD, CDSL, PM, PMSL, TC, VC and MR through mineralization, the maximum P release was 1.6, 3.0, 5.0, 5.4, 4.2, 3.1 and 1.6 mg P g⁻¹ manure, respectively under the aerobic soil condition whereas under the anaerobic condition the highest release of P from the manures was 0.7, 1.6, 4.3, 4.4, 2.0, 2.8, 1.1 and 0.7 mg P g⁻¹ manure, respectively. Phosphorus release progressively increased with time and reached its peak within 45–150 days of incubation for all the manures, and thereafter it gradually declined until the incubation time. Over the incubation periods and soil moisture levels, PMSL mineralized the highest and released the maximum amount of P among all the manures. This slurry attained the highest P value at the 75th day and then decreased slowly under the aerobic condition whereas under the anaerobic condition this slurry reached the maximum value at the 150th day and thereafter it decreased drastically. MR had the lowest net P release from mineralization over the incubation periods and soil moisture conditions.

3.2.3. Cumulative P Release

The pattern of cumulative P release from the mineralization of different manures in the soils across the moisture regimes and incubation times is shown in Figure 1e,f. The cumulative P release was higher in the aerobic condition than in the anaerobic condition and showed a wide variation among the types of manures. The lowest and the highest cumulative P releases were found in PMSL and MR, respectively, under both aerobic and anaerobic conditions. In addition, cumulative P release from slurry of either CD or PM was always higher compared to that in their manure states. After 180 days of incubation, the cumulative P release trend under both aerobic and anaerobic condition followed the order: PMSL > PM > TC > CDSL > VC > CD > MR.

3.2.4. Cumulative Mineralization Kinetic Model and Rates for P

The model for interpreted mineralization varied with the different types of organic manures and sources for P. The net cumulative mineralization data for all types of manures fitted strongly to the first-order exponential kinetic model [P_t = P₀ (1 − e^−kt)], where P_t is the cumulative amount of mineralized P over time t, P₀ is defined as the potentially mineralized P, k is the first-order rate constant of mineralization of P₀ and t is the incubation time. PMSL showed the highest potential P mineralization (P₀) at both moisture levels. When CD and PM are compared to their slurries, PMSL and CDSL had always higher P₀ compared to that in their manure states, respectively (

Table 3. Parameter values for the P mineralization kinetics of different manures.

Manures	Aerobic Incubation				Anaerobic Incubation
	P0 (mg/g Manure)	k (mg/g Manure/Day)	R2 adj *	F	P0 (mg/g Manure)	k (mg/g Manure/Day)	R2 adj *	F
CD	1.38	0.003	0.505	9.166 (P < 0.019)	0.63	0.009	0.479	8.346 (P < 0.020)
CDSL	2.73	0.004	0.268	3.932(P < 0.088)	1.18	0.011	0.281	4.126 (P < 0.080)
PM	4.54	0.004	0.155	2.468 (P < 0.160)	3.09	0.004	0.768	27.36 (P < 0.001)
PMSL	5.31	0.004	0.214	3.174 (P < 0.118)	3.28	0.006	0.642	15.37 (P < 0.006)
TC	4.09	0.005	0.320	4.763 (P < 0.065)	1.96	0.007	0.523	9.767 (P < 0.016)
VC	2.72	0.002	0.601	13.06 (P < 0.009)	0.94	0.011	0.820	37.39 (P < 0.001)
MR	2.25	0.001	0.894	68.24 (P < 0.160)	0.23	0.006	0.506	9.191 (P < 0.019)

Here, P₀—potentially mineralizable pool of P; k—mineralization constant rate; F—F value of ANOVA table; P—probability level; R² adj *—R² adjusted; R² adj * value: 0.0 to 0.2—very weak fit, 0.2 to 0.4—weak fit, 0.4 to 0.7—moderate fit, 0.7 to 0.9—strong fit, 0.9 to 1.0—very strong fit.

). CD showed the lowest P₀ value under the aerobic condition, but in the anaerobic situation, MR had the lowest P₀. The rate constant (k) value was found to be higher in slurries compared to that in their original manures.

3.2.5. Phosphorus Release from Manures

The release of P from different manure sources is presented in

Table 4. Maximum release of net total available P from different sources of manure during incubation period under aerobic and anaerobic soil.

Organic Manure	P Added (mg 100 g−1 Soil)	P Release in 180 Days (mg g−1 Manure)		% P Release
		Aerobic	Anaerobic	Aerobic	Anaerobic
CD	5.0	1.6 ± 0.2d	0.7 ± 0.1e	32.5 ± 3.8b	14.3 ± 1.7ef
CDSL	12.3	3.0 ± 0.4c	1.6 ± 0.2d	24.4 ± 2.8c	12.9 ± 1.5f
PM	23.3	5.0 ± 0.6a	4.3 ± 0.5b	21.4 ± 2.5cde	18.5 ± 2.1cdef
PMSL	24.9	5.4 ± 0.6a	4.4 ± 0.5ab	21.7 ± 2.5cde	17.8 ± 2.1cdef
TC	17.6	4.2 ± 0.5b	2.8 ± 0.3c	24.1 ± 2.8cd	16.0 ± 1.9def
VC	5.2	3.1 ± 0.4c	1.1 ± 0.1de	60.1 ± 6.9a	21.5 ± 2.5cde
MR	4.5	1.6 ± 0.2d	0.7 ± 0.1e	35.6 ± 4.1b	14.4 ± 1.7ef

Figures having common letters do not differ significantly at 5% level of significance. Comparisons made between column 3 and 4 and column 5 and 6. Figures in the column (except second column) of the table indicate mean ± standard deviation.

. Once mixed with soil, available P was augmented from 4.5 to up to 24.9 mg P 100 g⁻¹ in amended soil. The P release responded to the amount of P added. In 180 days, P release ranged from 1.6 to 5.4 mg P g⁻¹ manure and from 0.7 to 4.4 mg P g⁻¹ manure under aerobic and anaerobic condition, respectively. In both of the moisture conditions, PMSL released the maximum amount of P. The minimum amount of P was released from CD (aerobic condition) and MR (both aerobic and anaerobic conditions). Again, % P release from the manures varied from 21.4 to 60.1 under the aerobic situation but from 12.9 to 21.5 under the anaerobic condition.

3.2.6. Relation between Net Available P and Manure C/P Ratio

Net available P accumulation, calculated as the difference in NaHCO₃-extractable P between control and manure-amended soils was inversely related to the residue C/P ratio and decreased exponentially with it, recorded after 180 d of incubation under aerobic condition (Figure 1g,h). This relationship indicates that where the C/P ratio is higher, the P availability is lower in soil and vice versa. In our study, it is evident that PMSL, which had the lowest C/P ratio, showed the highest P availability, and similarly MR, which had the highest C/P ratio, demonstrated the lowest P availability in the soil.

3.3. Sulphur Release Pattern from Mineralization of OM in Soils

3.3.1. Total S Release

The total S release pattern was influenced by the types of manure, soil moisture levels and number of incubation days. Release of total available S increased with the application of manures and crop residue. Sulphur release from PMSL was the highest among the manures over the moist (FC level) and saturated (1 cm waterlogging) conditions (Figure 2a,b). In the aerobic condition, PM, TC and MR recorded near about similar amounts of available S. Under both aerobic and anaerobic situations, CD recorded the lowest amount of available S.

3.3.2. Net S Release

There was a considerable difference in net S release between moist and saturated soil conditions. The aerobic condition showed higher S release compared to the anaerobic condition (Figure 2c,d). Similar to P, the highest net S release was observed with PMSL over the soil moisture levels. Sulphur release progressively increased with number of incubation days and reached its peak within 60–180 days under the aerobic condition but in 45–60 days under the anaerobic condition. Under both moisture conditions, CD had the least performance in terms of net S release over the incubation periods.

3.3.3. Cumulative S Release

The cumulative S release from the mineralization of various manures in soils that differed in moisture status and incubation period is shown in Figure 2e,f. Much higher cumulative S release was noted in the aerobic condition in comparison to that in the anaerobic condition. The highest cumulative S release was found in PMSL whereas the lowest cumulative S release was found in CD under both aerobic and anaerobic conditions. In the aerobic condition, the cumulative S release values from mineralization of TC, VC, PM, CDSL and MR were almost similar throughout the incubation period. After 180 days of incubation, the cumulative S release trend under the anaerobic condition followed the order: PMSL > MR > TC > VC > PM > CDSL > CD.

3.3.4. Cumulative Mineralization Kinetic Model and Rates for S

Net cumulative mineralization data for S fitted strongly to the first-order kinetic model, S_t = S₀ (1 − e−^kt) where S_t is the cumulative amount of mineralized S over time t, S₀ is defined as the potentially mineralized S, k is the first-order rate constant of mineralization of S₀ and t is the incubation time. Potentially mineralizable S (S₀) content was found to be higher in PMSL under both aerobic and anaerobic conditions. PMSL showed the highest potential S mineralization (S₀) at both moisture levels. In the aerobic condition, both CDSL and PMSL showed a higher S₀ value compared to that in their original manure, but in the anaerobic condition, only PMSL had a higher S₀ than PM (

Table 5. Parameter values for the S mineralization kinetics of different manures.

Manures	Aerobic Incubation				Anaerobic Incubation
	S0 (mg/g Manure)	k (mg/g Manure/Day)	R2 adj *	F	S0 (mg/g Manure)	k (mg/g Manure/Day)	R2 adj *	F
CD	1.82	0.0020	0.809	34.57 (P < 0.001)	0.32	0.0037	0.753	25.39 (P < 0.002)
CDSL	3.2	0.0049	0.731	22.79 (P < 0.002)	0.32	0.0074	0.739	23.67 (P < 0.002)
PM	4.76	0.0015	0.787	30.51 (P < 0.001)	0.51	0.0066	0.616	13.85 (P < 0.007)
PMSL	7.02	0.0011	0.755	25.65 (P < 0.002)	0.89	0.0046	0.592	12.60 (P < 0.009)
TC	4.54	0.0055	0.069	1.596 (P < 0.247)	0.65	0.0081	0.265	3.89 (P < 0.063)
VC	2.77	0.0086	0.819	37.21 (P < 0.001)	0.44	0.0150	0.327	4.89 (P < 0.063)
MR	4.21	0.0083	0.389	6.102 (P < 0.043)	0.65	0.0048	0.363	5.56 (P < 0.050)

Here, S₀—potentially mineralizable pool of S; k—mineralization constant rate; F—F value of ANOVA table; P—probability level; R² adj *—R² adjusted; R² adj * value: 0.0 to 0.2—very weak fit, 0.2 to 0.4—weak fit, 0.4 to 0.7 –moderate fit, 0.7 to 0.9—strong fit, 0.9 to 1.0—very strong fit.

). For CDSL and PMSL, the mineralization rate constant was always lower, and the exponentially mineralizable S content was always higher compared to that in their original state, which indicated that a slower rate, but a higher amount, of S mineralization occurred in CDSL and in PMSL.

3.3.5. Sulphur release from Manures

Table 6. Maximum release of net total available S from different types of organic manures during incubation period under aerobic and anaerobic soil moisture conditions.

Organic Manure	S Added (mg 100 g−1 Soil)	Maximum S Release (mg g−1 Manure)		% S Release
		Aerobic	Anaerobic	Aerobic	Anaerobic
CD	2.8	1.8 ± 0.2c	0.4 ± 0.04cd	62.5 ± 7.2b	13.2 ± 1.5c
CDSL	4.8	3.3 ± 0.4bc	0.3 ± 0.03d	68.5 ± 7.9ab	5.4 ± 0.6c
PM	5.6	4.0 ± 0.5b	0.5 ± 0.06cd	71.9 ± 8.3ab	8.9 ± 1.0c
PMSL	6.9	5.9 ± 0.7a	0.6 ± 0.07cd	85.1 ± 9.8ab	9.2 ± 1.1c
TC	4.7	4.2 ± 0.5b	0.4 ± 0.05cd	88.9 ± 9.5a	9.0 ± 1.0c
VC	3.4	2.9 ± 0.3bc	0.3 ± 0.04cd	86.2 ± 9.9a	9.8 ± 1.1c
MR	5.6	4.7 ± 0.4b	0.5 ± 0.05cd	84.5 ± 7.7ab	8.1 ± 0.9c

Figures having common letters do not differ significantly at 5% level of significance. Comparisons made between column 3 and 4 and column 5 and 6. Figures in the column (except the second column) of the table indicate mean ± standard deviation.

shows the release of S from different manures. When mixed with soil, available S was augmented from 2.8 to up to 6.9 mg S 100 g⁻¹ in amended soil. Sulphur release responded to the amount of P added. In 180 days, S release ranged from 1.8 to 5.9 mg S g⁻¹ manure in the aerobic condition and from 0.3 to 0.6 mg S g⁻¹ manure under the anaerobic condition. Poultry manure slurry released the highest amount of S under both the moisture conditions. The lowest amount of S was released from cow dung (aerobic condition) and cow dung slurry (anaerobic condition). Further, % S release from the manures ranged from 62.5 to 88.9 under the aerobic situation but from 5.4 to 13.2 under the anaerobic condition.

3.3.6. Relation between Net S Accumulation and Manure C/S Ratio

Net S accumulation after 180 days of incubation due to manure incorporation, calculated as the difference in CaCl₂-extractable S between manure-amended and unamended (control) soils, was proportionally related to the residue C/S ratio and increased linearly with it (Figure 2g,h). This relationship indicates that where the C/S ratio is higher, the availability of S is lower in soil and vice versa.

The C/P or C/S ratio is very important because the amount of P or S released is dependent on the C/P or C/S ratio of organic manures. The chemolithotroph microorganisms responsible for mineralization get their necessary energy through the decomposition of organic materials. Their activities depend on soil properties such as the moisture, temperature, pH and C/P or C/S ratio of organic manures. The optimum ratio for net mineralization is <200 [34]. If this ratio is <200, the net mineralization will be positive. If the ratio is above 400, the net mineralization becomes negative and immobilization occurs [34].

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 CO₂ 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 CO₂ during organic matter decomposition and cation (Ca²⁺) 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 S₀ 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 SO₄⁻²–S concentration over time, while liquid biogas residues had greater soil SO₄⁻²–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.

5. Conclusions

From this incubation experiment, we found that the first-order kinetic cumulative model could fit well with the observed P and S release data from mineralization in all the cases. The present study also suggests that the release of P and S was higher in the aerobic condition compared to that in the anaerobic condition. Release of P and S had peaks within 75–180 and 75–150 days, respectively. The slurry, especially PMSL, mineralized greater amount of soil P and S than other manures. Thus, the release of nutrient elements from manures after decomposition was significantly influenced by manure types, soil moisture conditions (aerobic and anaerobic) and incubation period. Therefore, organic matter of varying sources can be used to regulate the timing of nutrient availability and to estimate the best time for planting.