Zootechnical and Municipal Solid Waste Digestates: Effects on Soil Nitrogen Mineralization and Kinetics

Abstract

Soil fertilization with fertilizers derived from renewable sources is a topic of great interest in terms of the sustainable management of organic waste. To optimize the management of nitrogen supplied to the soil with digestates, it is necessary to deepen knowledge on the process of mineralization of organic nitrogen over time. In this research, a laboratory incubation system was utilized to study the impact of various digestate sources on nitrogen mineralization processes in soils and nitrogen mineralization kinetics. Six types of digestates of different origins and composition were administered to soil and the soil samples were placed under controlled conditions. The release of N was determined by measuring ammonium-N and nitrate-N concentrations in leachates during a 12-week period of incubation. The nonlinear regression technique was used to fit the cumulative leaching of total N to the Stanford and Smith first-order kinetic model during the incubation period. The results showed that the differences between digestates, nitrogen and organic carbon concentration, and C/N ratio influenced both ammonification and nitrification processes in the soil and the nitrogen mineralization kinetics. The processing of the statistical data highlighted that the potentially mineralizable nitrogen (MPN) followed first-order kinetics.

1. Introduction

Among the main objectives of the eco-sustainable management of the agricultural environment is reducing the use of chemical fertilizers by replacing them with bio-based fertilizers. This strategy aligns with the European Commission’s guidelines for sustainable digestate management in the Circular Economy Package [1]. Utilizing fertilizers produced from anaerobic digestion by-products presents a significant opportunity to promote sustainability within the circular economy framework [2,3]. Biowaste can undergo composting or anaerobic digestion to stabilize organic matter and produce bio-based fertilizers [4,5,6]. Optimizing the processes of conversion of waste into resources is essential to generate high-quality by-products suitable for soil fertilization [7,8].

Anaerobic digestion is a biotechnology for converting agricultural biomass into valuable products such as biogas and digestate, a renewable fertilizer [9,10]. This process helps extract nutrients like nitrogen and phosphorus from organic matter. Anaerobic digestate’s use as organic fertilizer or soil amendment appears to be the optimal choice for nutrient recycling. The digestate is usually composed of a solid and a liquid fraction which differ in chemical composition and therefore in fertilizer value. The solid fraction is comparable to a slow-release organic fertilizer; the liquid fraction is rich in readily assimilable nitrogen, as well as phosphorus, potassium, and other nutrients, and is therefore intended as a replacement for mineral fertilizers [11].

The main benefits deriving from the agricultural use of digestates can be summarized as the provision of organic matter and nutrients to the soil and in a reduction in the use of energy-intensive mineral fertilizers [12,13,14,15]. In Europe, the placing on the market of fertilizers derived from digestates of different origins and compositions is regulated by Regulation (EC) 2019/1009 [16]. The quality and potential applications of digestate primarily depend on the source material. Our study concerns the agricultural use of digestate derived from the organic fraction of mixed municipal solid waste (OFMSW) and could be considered preparatory to the evaluation of the use of digestate from future mixed plants with different percentages of OFMSW. Nitrogen pools from organic waste undergo biological decomposition in soil, which affects the temporal rates of nitrogen available to plants. Other factors can influence the N mineralization process in soil such as temperature, water content, soil properties, and the nature and chemical characteristics of organic material. Among the latter, the ratio between carbon and nitrogen (C:N) represents an important indicator of N mineralization dynamics, allowing the prediction of the N mineralization trend. Low and high soil C:N ratio values normally favor N mineralization and immobilization processes, respectively. Based on some studies, the threshold of the C:N ratio between immobilization and mineralization seems to lie between 14:1 and 19:1 for a wide range of organic amendments [17]. Knowledge of the availability of nitrogen resulting from mineralization processes over time is essential to improve its nutritional effectiveness.

Stanford and Smith (1972) [18] long-term aerobic incubation is a laboratory method used for the estimation of MPN. The method estimates the amount of N that is likely to be released in mineral form from soil organic substrates in a 32-week period. Many studies have used aerobic incubation for the calibration of chemical methods used to estimate N supply from SOM mineralization. However, the long-term aerobic incubation method has been modified by Benedetti [19] to reduce the test duration and to evaluate the initial N-ready flux after the preparation and handling of soil samples [20].

Empirical kinetics models capable of describing nitrogen release curves over time are very useful for summarizing and understanding the dynamics for implementing the most favorable soil management practices [21]. Various mathematical equations described in the literature (exponential, hyperbolic, parabolic, logistic, etc.) have been used as empirical models to describe nitrogen mineralization curves, providing a good fit of data and simplifying the process to just a few parameters.

Among these, the single exponential Stanford and Smith model [18] has largely been used to describe nitrogen and carbon mineralization resulting from the decomposition of various organic substrates in soil [22,23,24,25] as well as the dynamics of nitrogen from slow-release fertilizers [26]. Some implementations of the Stanford and Smith model [18] were proposed by Molina et al. in 1980 [27] and Inubushi et al. in 1985 [28] as double exponential models to identify and quantify different N pools in organic substrates. However, the over-parameterization of these models is often not justified when using a data set limited to a few points. In this case, the use of models with more than three parameters is not very suitable [29]. The only two parameters of the single exponential model representing the potentially mineralizable nitrogen and the mineralization kinetic constant summarize the information contained in the data and are very useful in scientific studies and practical interpretations [19]. Furthermore, simple first-order models generally fit well the kinetics of N mineralization from organic amendments that do not cause N immobilization [30].

The present study was designed to investigate the following: (i) the effect of digestate amendment on soil ammonification, nitrification, and nitrogen mineralization and (ii) the nitrogen mineralization kinetics in different treatments over time.

2. Materials and Methods

2.1. Soil Characterization

The soil was sampled in an experimental area annexed to CREA-AA—Rome and dedicated to the research. The soil was collected from the surface layer (0–0.30 m) and subsequently oven-dried at 105 °C prior to analysis. The soil was analyzed before adding different substrates. The main chemical properties of the soil were established using the Official Methods of the Ministry of Agriculture (Italy) [31]. Moisture is determined by weighing a known quantity of a sample of material as such, placing it in an oven at 105 °C until the weight is constant (24 h), and weighing the sample again. Soil pH was determined by means of a glass electrode using a water-to-soil ratio (v/w) of 2.5:1; particle size was assessed through a sedimentation procedure; total soil organic carbon and nitrogen were measured using RC-612 carbon analyzer and FP-528 nitrogen analyzer (Leco Corporation, St. Joseph, MI, USA) respectively; cation exchange capacity (CEC) was determined through the ammonium acetate procedure; and exchanged Ca, K, Mg, and Na were determined by ICP (Inductively Coupled Plasma-OES) (Thermo Jarrell-Ash Corporation, Watertown, MA, USA). Total CaCO₃ was determined using the Dietrich–Frühling calcimeter. Available phosphorus was measured using the Olsen method through a spectrophotometer. The analyzed values were subsequently referred to dry soil at 105 °C using the appropriate conversion factors. The soil was A1-Fluventic Xerochrepts (USDA) with a clay loam texture, sub-alkaline (pH 7.5), not saline (EC_1:2.5 ms cm⁻¹ 0.213), and had a favorable supply of total organic carbon (TOC: 2.11%) and macro-elements (total N: 0.18%; P₂O₅: 86.56 mg kg⁻¹; K₂O: 805.37 mg kg⁻¹), a C/N ratio of 11.7, and C.E.C. of 25.48 meq 100 g⁻¹; exchangeable cations—Ca: 22.20 meq 100 g⁻¹, Mg: 1.32 meq 100 g⁻¹, K: 1.71 meq 100 g⁻¹, Na: 0.11 meq 100 g⁻¹, total CaCO₃: 3%.

2.2. Digestate Characterization

The Italian Composting Consortium (CIC) provided three types of zootechnical digestates and three types of organic fraction municipal solid waste (OFMSW) digestates. Approximately 5 L of each organic residue was collected, thoroughly mixed, and stored in 0.25–0.50 L plastic bottles at −20 °C until use.

The six types of digestates were as follows: digested pig slurry as such (PS), digested pig slurry–solid fraction (PF), digested bovine manure (BM), digested OFMSW (DO), dried digested OFMSW (DD), and digested and composted OFMSW (DC). The digestates were analyzed for their chemical–physical characteristics through an accredited private laboratory in accordance with Regulation (EC) 2019/1009 [16] and the Legislative Decree 29 April 2010 n. 75 and subsequent amendments (Italian Law) [32,33]. The parameters analyzed included total solids, volatile solids, total organic carbon, total nitrogen, organic nitrogen, total phosphorus, total potassium, total heavy metals, and pH (

Table 1. Chemical characteristics of digestates.

Parameter	PS	PF	BM	DO	DD	DC
Moisture %	95.5	71.9	92.9	72.7	16.3	36.8
pH	8.3	8.6	7.8	8.5	8.4	8.0
Total solids %	97.4	76.4	94.9	82.4	47.9	67.3
Volatile solids %	2.6	23.6	5.1	17.6	52.1	32.7
N tot. %	8.2	2.2	5.6	4.3	3.9	2.2
Organic N %	2.2	1.2	2.4	3.0	3.1	2.1
Inorganic N %	6.0	1.0	3.2	1.3	0.8	0.1
Organic C %	24.7	41.8	37.5	33.8	23.3	48.6
C/N ratio	3	19	7	8	6	22
P (P2O5) %	3.1	1.9	1.1	3.6	3.4	0.7
K2O %	7.0	1.0	6.2	0.7	0.8	1.3
Cu mg kg−1	101	29	67	93	102	56
Zn mg kg−1	812	206	326	310	344	182
Pb mg kg−1	<LOQ	<LOQ	<LOQ	2	0.5	3
Cr mg kg−1	<LOQ	<LOQ	<LOQ	12	9	5
Cd mg kg−1	<LOQ	<LOQ	<LOQ	0.4	0.1	0.1
Ni mg kg−1	<LOQ	<LOQ	<LOQ	8	5	7
Hg mg kg−1	<LOQ	<LOQ	<LOQ	<LOQ	<LOQ	<LOQ

Except for pH, all data are expressed in dry matter. LOQ = limit of quantification (0.05 mg kg⁻¹).

). The different digestates showed a different organic carbon and nitrogen content both in organic and total form. Consequently, the C/N ratios also differed in the various substrates, varying from 3 (PS) to 22 (DO).

Organic C was mainly recovered in the pig slurry–solid fraction and in the composted form of OFMSW digestate. In the pig digestate, the solid–liquid separation process also influenced the concentration of total N and the C/N ratio.

2.3. Soil Incubation Procedure

The soil incubation procedure was performed according to the Benedetti method [19]. Soil microcosms under standardized conditions and in the absence of plants were set up to evaluate the effect of different digestate amendments on soil ammonification, nitrification, and nitrogen mineralization compared to fertilization with synthetic nitrogen fertilizer (ammonium sulfate—AS). The microcosm incubation test took place under controlled temperature and humidity conditions using Büchner funnels, on the bottom of which a Whatman^® filter paper No. 4 (Merck Life Science Limited, Maharashtra, India) was placed. In detail, air-dried soil (50 g) was mixed with quartz sand at a 1:1 ratio (weight/weight) and added with fertilizers (ammonium sulfate or digestates) with a dose equivalent to 250 mg N kg⁻¹ of soil. The mixtures (soil, quartz sand, and fertilizers) were then transferred to the Büchner funnels. The moisture content of the soil was adjusted to 60% of its water-holding capacity by adding deionized water (WHC) (pF = 2.5), and the soil was then incubated at 30 °C. The Büchner funnels were covered with flexible perforated Parafilm ^®M sealing film and weighed every 3 d to monitor soil moisture. During the incubation time, distilled water was added to keep the soil at 60% WHC.

Each treatment was replicated three times.

2.4. Nitrogen Extraction from Soil and Analysis

The mineral nitrogen initially present (Time 0) was washed out with 100 mL of 0.01 M CaCl₂. After 1, 2, 4, 8, and 12 incubation weeks, the released N (Nrel) was leached as NO₃⁻-N and NH₄⁺-N with 500 mL of 0.01 mol L⁻¹ CaSO₄ solution. To avoid nutrient-limiting effects, after each leaching, a nutrient solution without N containing 0.002 mol L⁻¹ CaSO₄, 0.002 mol L⁻¹ MgSO₄, 0.005 mol L⁻¹ Ca(H₂PO₄)₂, and 0.002 5 mol L⁻¹ K₂SO₄ was added to the mixtures. The leachates were filtered by Whatman No. 42 filter paper and analyzed. The mineral forms of nitrogen in the leached solution were determined according to Wall [34] for ammonium and according to Kamshake [35] for nitrates plus nitrites by an automatic continuous flow analyzer (Technicon II autoanalyzer—Bran & Luebbe, Sydney, Australia). The nitrogen concentrations were referred to soil dried at 105 °C.

2.5. Statistical Analysis

The concentrations of the mineral forms of N detected at each incubation week as NO₃⁻-N and NH₄⁺-N and the cumulative Nrel recovered at the end of the experiment (12 weeks) were analyzed by one-way analysis of variance (ANOVA). The homogeneity of variances was tested using Levene’s test. Significant differences between treatments were assessed by means of Tukey’s Honest Significant Difference (HSD) test at a p value ≤ 0.05 when the F test of ANOVA was significant (α level = 0.05). The ANOVA and Tukey tests were performed using JASP statistical software [36].

Cumulative Nrel was fit to the Stanford and Smith [18] first-order model as follows:

Nrel = N₀ (1 − e^−kt)

(1)

where Nrel (mg N kg⁻¹ soil) is the cumulative N released from the soil treated with the different N sources at time t (week), N₀ (mg N kg⁻¹ soil) is the size of potentially mineralizable N, and k (week⁻¹) is the first-order rate constant. All data as triplets were used in the regression analysis, although the results are reported as mean values. Estimates of the N₀ and k values of the different N sources were deemed significantly different (α = 0.05) if the 95% confidence intervals did not overlap. The half-life t_1/2 (week), the time required to mineralize half of the potentially mineralizable N₀ obtained from Model (1), was calculated using the following equation [37]:

t_1/2 = ln (2)/k

(2)

The reparameterization of Model (1) by replacing K = t_1/2/ln(2) does not change either the estimated values of the parameters or the chosen good fit indices.

The quality of fit of the model was assessed using the Root Mean Square Error (RMSE) and the Adjusted R squared (R²_Adj.) statistical indices. Nonlinear curve fitting was performed through a specific library of the R software Version 4.1.3 (“nls.multstart” Version 1.3.0) [38].

3. Results and Discussion

3.1. Nitrogen Mineralization Processes

The concentrations of NH₄⁺-N appeared to be influenced by the substrate and incubation time (

Table 2. Concentrations of NH₄⁺-N (mg·kg⁻¹ dry soil).

NH4+-N
Week	1		2	4		8	12
AS	81.93	c	6.60	2.20	a	3.60	40.00	d
PS	50.33	bc	6.40	8.00	a	0.53	5.27	ab
PF	16.27	ab	4.00	15.67	a	2.93	3.27	a
BM	32.33	ab	7.13	9.53	a	1.80	7.00	ab
DO	2.67	a	9.73	34.93	b	3.93	10.27	ab
DD	2.20	a	2.87	0.00	a	2.07	13.73	bc
DC	25.33	ab	3.87	0.00	a	3.40	22.13	c
ANOVA F test	***		ns	***		ns	***

Means with different letters within a column are statistically different according to Tukey’s test. ns = not significant; (***) p ≤ 0.001.

).

In the presence of some substrates, the ammonium concentrations were high after the first week of incubation and then decreased in the second one. Soil fertilization with the inorganic fertilizer AS or with PS digestate, characterized by a high percentage of mineral nitrogen compared to total N (73%) and a low C/N (C/N = 3, Table 1), probably activated the microbial processes responsible for the rapid mineralization of organic nitrogen and triggered a strong priming effect [39]. This phenomenon was less evident in BM, DC, and PF. In the case of the soil amended with digested OFMSW as such (DO), the ammonification processes increased after four and twelve weeks of incubation. A similar trend occurred in the presence of PF digestate; however, the times when the phenomenon manifested changed (i.e., in the first and fourth weeks). It is highlighted that the highest ammonium concentrations have similar values to each other in both DO and PF; there does not seem to be an obvious priming effect. It should be highlighted that PF has a C/N ratio equal to 19, which does not favor the rapid mineralization of soil organic matter. In the case of DO (C/N = 8), this digestate has a much higher % concentration of organic nitrogen (3.0%) compared to PF (1.2%) and a lower percentage of inorganic N compared to total N (30%). This condition could justify the trend in NH₄⁺ concentration like PF (with two peaks) but with a different time. At the eighth week, all the ammonium values were low and then increased at the following percolation (twelfth week) with significantly different values in the case of AS, DC, and DO. Finally, there is the case of soil mixed with dried digested OFMSW (DD), which maintained low ammonium values until the eighth week of incubation and grew in the twelfth one. In a soil incubation test with different organic materials, Marzi [40] highlighted the discontinuous trend of the ammonification process with incremental and decremental trends, attributing the alternation of ammonification and immobilization phenomena not only to the C/N ratio of the substrate added to the soil but also to the form of nitrogen predominantly present (mineral or organic) compared to the total form.

The type of digestates administered to the soil also influenced the NO₃⁻-N concentrations (

Table 3. Concentrations of NO₃⁻-N (mg·kg⁻¹ dry soil).

NO3−-N
Week	1		2		4		8	12
AS	42.93	a	75.33	c	68.67	b	39.67	10.53
PS	35.73	a	56.20	b	28.87	ab	33.07	18.20
PF	46.87	ab	9.87	a	0.00	a	32.93	21.87
BM	37.73	a	56.40	b	18.73	a	49.07	10.47
DO	94.53	c	9.87	a	19.00	a	52.87	20.33
DD	75.60	bc	0.00	a	13.40	a	50.73	24.53
DC	91.27	bc	0.00	a	15.47	a	39.20	16.33
ANOVA F test	***		***		**		ns	ns

Means with different letters within a column are statistically different according to Tukey’s test. ns = not significant. (**) p ≤ 0.01; (***) p ≤ 0.001.

).

In the case of soil fertilized with OFMSW digestates and the pig digestate solid fraction (PF), the highest concentrations of NO₃⁻-N were analyzed after the first week of incubation with DO ≥ DC > DD > PF.

In the presence of ammonium sulfate (AS) or pig digestate as such (PS), only a single incremental period of NO₃⁻-N production was observed, which reached a maximum after 15 days of incubation. Subsequent values decreased until the 12th week. Even soil treated with bovine digestate (BM) shows a peak in the nitrification process after the first week. However, like the other digestates used in this experiment, it also presents another one after 8 weeks of incubation. The phenomena of nitrogen immobilization by telluric microorganisms appear to prevail between the second and fourth weeks of incubation depending on the different types of digestates, as the nitrate concentrations are lower than those measured in the previous period. At the eighth week of incubation, the analyzed concentrations of NO₃⁻-N reveal a second incremental period following this order: DO ≥ DD ≥ BM > DC ≈ PF ≈ PS. During the timeframe of the nitrate decrement, there was probably a reduction in NO₃⁻-N likely caused by the absorption of nitrogen by the developing microbial population. The next increment in NO₃⁻-N suggests that nitrification was suppressed during this timeframe, as Marzi [34] observed. At the end of the incubation period, all values decreased. The main cause of this alternation could be attributable not only to the different digestates’ organic C and N (organic or mineral) concentrations but also to the different degrees of stability of the organic matter brought to the soil.

The cumulative mineralized nitrogen concentrations over twelve weeks of incubation under controlled temperature and humidity conditions are shown in Figure 1. The administration of N to the soil in totally inorganic form (AS) strongly stimulated the soil organic matter mineralization. In fact, in this case, the highest cumulative concentration of N mineralized was analyzed, with a significant difference from all the other treatments. The pig digestate as such (PS) had a low C/N (3) ratio. This condition directs the microorganisms’ ammonifiers and nitrifiers towards rapid mineralization of the organic matter of the soil. The pig digestate solid fraction (PF) had a higher C/N ratio (19), suggesting slower mineralization. Bovine digestate (BM) is characterized by a C/N ratio of 7 (Table 1). Its mixing with the soil positively stimulates the mineralization processes of organic nitrogen in the soil. This phenomenon is more evident in DO (C/N = 8). In fact, the quantity of nitrogen mineralized in DO is significantly higher than PF (C/N = 19). The nitrogen mineralization in soil treated with different digestates was probably influenced by the digestates’ C/N ratios but also by the percentage of organic/inorganic nitrogen and by the level of degradability of organic matter added to the soil [39,41,42].

3.2. Kinetics of Nitrogen Mineralization

The first-order kinetics model (Model (1)) showed, in general, quite a good fit for the cumulative data to the total inorganic nitrogen (Figure 2,

Table 4. Potentially mineralizable nitrogen N₀ (mg kg⁻¹ of dry soil) in the fertilizers and mineralization rate constant K (week⁻¹) estimated by nonlinear regression; calculated half-life t_1/2 (week) of N₀ and fitting results assuming the first-order kinetics model.

Substrates	Parameters	Estimate		SE	95% CI		RMSE	R2(Adj.)
					Lower Limit	Upper Limit
AS	N0	354.17	a	19.55	312.74	395.61	40.83	0.907
	k	0.41	a	0.07	0.27	0.56
	t1/2	1.69	a	0.28	1.08	2.28
PS	N0	233.00	bc	16.82	197.34	268.66	37.27	0.837
	k	0.46	a	0.10	0.24	0.68
	t1/2	1.51	a	0.35	0.78	2.25
PF	N0	143.82	d	10.00	122.62	165.03	19.01	0.868
	k	0.36	a	0.07	0.21	0.51
	t1/2	1.93	a	0.39	1.11	2.77
BM	N0	226.49	bc	13.02	198.90	254.01	25.60	0.910
	k	0.38	a	0.06	0.24	0.51
	t1/2	1.82	a	0.31	1.18	2.51
DO	N0	253.32	b	8.83	234.61	272.03	15.45	0.970
	k	0.32	a	0.03	0.26	0.39
	t1/2	2.17	a	0.21	1.70	2.59
DD	N0	178.01	cd	14.98	146.27	209.76	21.99	0.868
	k	0.27	a	0.06	0.15	0.39
	t1/2	2.55	a	0.56	1.39	3.76
DC	N0	188.06	bcd	14.04	158.31	217.82	34.50	0.777
	k	0.58	a	0.15	0.26	0.90
	t1/2	1.20	a	0.31	0.54	1.85

Note: Estimated values of N₀, k, and t_1/2 with the same lowercase letter (bold for N₀, normal for k, and italics for t_1/2, respectively) indicate that their 95% confidence intervals overlap. Abbreviations: SE = standard error of the estimated parameters; CI = confidence intervals of the estimated parameters; RMSE = Root Mean Square Error; R²_(Adj.) = adjusted R².

). The nonlinear regression technique overall led to reliable estimates of the parameters N₀, k, and calculated t_1/2 (2). The rather small standard errors and the narrow confidence intervals demonstrate the accuracy and the ability of the statistical approach to summarize the kinetics of nitrogen mineralization through a few parameters to which biological significance could be attributed. The N₀ parameter represents the maximum amount of soil mineralized nitrogen at the end of the incubation time and provides useful information on soil nitrogen available to plants. The correlation coefficient between the N₀ estimates and the cumulative experimental data of N released at the end of the experiment (Figure 1) was 0.994, confirming the good predictive capacity of the model of this parameter. The N₀ values ranged from 354.17 mg kg⁻¹ of AS to 143.82 mg kg⁻¹ of PF as a substrate, which recovered only about 57% of the total N supplied. As expected, significant differences were found between the highest value of N₀ of the mineral fertilizer AS compared to all other digestates. Among the digestates, the highest values of the N₀ parameter were estimated in DO, PS, and BM (253.32, 233.00 and 226.49 mg kg⁻¹, respectively), statistically different from the lowest value of PF and corresponding to about the 90–100% of the total N supplied. Estimates of intermediate values of the N₀ parameter, not statistically different from each other, were obtained in DD and DC (178.01 and 188.06 mg kg⁻¹, respectively), indicating a potential N release of approximately 73% of the total N supplied.

The estimates of the parameter k and the related half-life t_1/2 are useful synthetic indicators of the speed of N released over time, indicating the soil’s nitrogen-supplying intensity. In particular, t_1/2 is a direct indicator of the time at which half of the estimated potential N₀ is released. Low values of t_1/2 are likely linked to the presence of labile and fast mineralizable N pools, well balanced with the organic carbon content. However, in our experiment, the estimated confidence intervals of these parameters always overlapped, thus showing no statistical differences between all treatments.

In other work, significative variances in k values were observed under different soil moisture and temperature conditions or related to different soil physicochemical characteristics such as soil texture, bulk density, pH, etc. [43]. In this paper, the soil moisture and temperature conditions were constant, so we hypothesized that, on the k parameter, the soil’s characteristics exerted more important influences than the added organic substrates. Nevertheless, we can observe that all substrates released on average half of the correlated N₀ over one to two weeks, followed by a subsequent phase of slow release with exponential behavior until the end of the incubation time. Overall, the nonlinear regression curves show good fit in all treatments (Figure 2), confirmed by the values of RMSE ranging from 15.45 in DO to 40.83 in AS and by the adjusted coefficients of determination (R² adj), with the highest value in DO (0.970) and the lowest in DC (0.777) (Table 4).

4. Conclusions

The results obtained both in the mineralization test and in the estimation of potentially mineralizable nitrogen highlight that prolonged mineral fertilization over time could lead to a depletion in the organic nitrogen content of soil due to the acceleration of the potential nitrogen mineralization process compared to soil treated with bio-based fertilizers. The different treatment processes of the digestates (dehydration, solid/liquid separation, or composting) influenced the chemical composition and, probably, the degree of organic matter stability. This had a different influence on the nitrogen mineralization kinetics and the quantity of mineralized nitrogen in the soil treated with zootechnical and OFMSW digestates, mitigating the risk of soil organic matter depletion and simultaneously supporting the nitrogen nutrition of crops.

The first-order kinetics model accurately simulated the N mineralization process of various organic substrates studied by providing useful information from scientific knowledge and potential implementations in more complex management models. The two parameters of the model showed different variability in the response to organic fertilizers, with N₀ being very sensitive to treatments while k was scarcely variable and probably more influenced by the soil characteristics, suggesting the need to carry out further studies on this topic. The valorization of agricultural utilization of digestates through further research and the adoption of models for potentially mineralizable N in fertilization plans could be valid aids for optimizing the use of digestates of different origins and compositions within a vision of a circular economy and sustainability applied to agriculture and some waste management.