Next Article in Journal
The Sustainable Development and Strategic Approaches for Contemporary Higher Education
Next Article in Special Issue
Assessment of the Red Seaweed Gelidium sesquipedale By-Products as an Organic Fertilizer and Soil Amendment
Previous Article in Journal
COVID-19 Lockdown Stress and the Mental Health of College Students: A Cross-Sectional Survey in China
Previous Article in Special Issue
Fertiliser Effect of Ammonia Recovered from Anaerobically Digested Orange Peel Using Gas-Permeable Membranes
 
 
Font Type:
Arial Georgia Verdana
Font Size:
Aa Aa Aa
Line Spacing:
Column Width:
Background:
Article

Nitrogen Availability in Organic Fertilizers from Tannery and Slaughterhouse By-Products

Department of Agricultural and Food Sciences, Alma Mater Studiorum-University of Bologna, Viale G. Fanin 40, 40127 Bologna, Italy
*
Author to whom correspondence should be addressed.
Sustainability 2022, 14(19), 12921; https://doi.org/10.3390/su141912921
Submission received: 29 August 2022 / Revised: 23 September 2022 / Accepted: 8 October 2022 / Published: 10 October 2022
(This article belongs to the Special Issue Organic Fertilizers: Applications and Research)

Abstract

:
Agriculture can play a primary role in the context of nutrients recovery by promoting the use of organic fertilizers (OFs). In order to use them efficiently, it is necessary to predict the nitrogen (N) bioavailability, which is a challenging matter due to the different physical-chemical characteristics of commercially available OFs. This study aims to evaluate hot-water extractable N as a rapid and cheap chemical indicator of bioavailable N. The trial was conducted on nine animal-based OFs obtained from different raw materials and treatment processes. They were fully characterized and the bioavailable N was determined by a 7-week soil incubation experiment. The results showed that hot-water extractable N underestimated bioavailable N in the case of leather meal based OFs; however, a significant linear regression fitting was achieved (R2 = 0.53). The C:N ratio was also assessed, which showed a negative correlation (−0.87) and a better linear regression fitting (R2 = 0.76) with the bioavailable N, but manifested some limitations in the prediction of leather meal based products. This experiment showed that both hot-water extractable N and C:N ratio can provide useful information for farmers in managing this class of OFs.

1. Introduction

The use of organic fertilizers (OFs) represents a common agricultural practice to increase both nutrients and organic matter (OM) content in the soil [1]. This class of fertilizers contributes to alleviation of problems related to desertification and soil erosion, improving physical, chemical and biological properties of depleted soils by supplying OM [2,3,4,5]. They also play an important role in the reduction of environmental impact, minimizing the use of chemical fertilizers, which create concerns both in their (mis)use and in their production [3,4,5,6]. Furthermore, using OFs partially permits solving problems related to the disposal of biomasses, usually destined to landfill or incineration creating potential risks for the environment [7].
Despite their “green attitude”, OFs, as well as the chemical ones, must be managed with awareness, since excessive and misleading use could lead to environmental contamination or pollution due to the dispersion of nutrients in other parts of the ecosystem [8]. In particular, nitrogen (N) represents the most crucial macronutrient for plant nutrition. Still, at the same time, the amount that is not absorbed by plants impacts not only on the soil itself, leading to acidification process, but also on the other environmental compartments in terms of coastal eutrophication, air pollution, increase of greenhouse gas emissions, and loss of biodiversity [9,10,11,12].
The OFs are typically divided, by source, into: (i) animal-based, such as blood meal, fish meal, leather meals, horn and hoofs meal, slaughterhouse by-products, and manure and (ii) plant-based, such as crop residues and seaweed extracts. Unfortunately, we have only partial data about the amount of global animal by-products available for OFs production; for example, in the European Union and UK about 1.4 billion tonnes of manure are generated annually [13], and the global slaughterhouse by-products are estimated at about 150 million tonnes per year [14]. Therefore, considering that the global production of meat is projected to progressively grow until 2050 [15], we expect that the amount of animal by-products available for OFs productions will increase. The recycling of this huge amount of materials containing N in agriculture as fertilizers is an opportunity, but also a matter of concern considering the lack of knowledge about the availability of N to plants.
Organic N fertilizers are generally constituted by two different pools of N consisting in inorganic (Nin) and organic (Norg) forms (Figure 1). While the Nin compounds, ammonium (NH4+) and nitrate (NO3), are readily available to plants [16], the Norg component is characterized by a wide spectrum of organic substances [16], which, according to their chemical characteristics, have a different behavior in the soil and can be readily as well as scarcely bioavailable. In fact, the Norg fraction can be immediately available or speedily mineralizable by microorganisms (i.e., amino acids and small peptides), or recalcitrant and slowly mineralizable (i.e., polypeptides and heterocyclic N compounds). For these reasons, OFs management is still a challenge for agronomists and the lack of knowledge of potentially mineralizable organic N (N0) and of the mineralization rate of Norg leads to low OFs’ N efficiency use. Since these pools are strictly related to the raw materials and the transformation process from which OFs derive, the amount of N0 as well as the rate at which it becomes bioavailable in soils (Figure 1) is highly affected by OFs’ origin [17].
In order to optimize N efficiency, it is therefore important to know the amount of Nin, as well as of N0, since these pools are potentially available for crops and their sum can be defined as bioavailable N (Nbav). While Nin is obtainable by straightforward chemical analysis (i.e., extraction with 1 M KCl and determination of ammonium and nitrate in the extract), the determination of N0 requires a long-time incubation essay, resulting in a time- and reagents-consuming experiment [18,19]. Therefore, developing a rapid method capable of giving a reliable indication of the N0 content in both soils and fertilizers has always aroused great interest. For instance, several authors [20,21,22] found satisfying methods to estimate the organic N mineralization capacity of the soil through a single chemical extraction using hot water, potassium permanganate and warm water, respectively. Dell’Abate et al. [23] conducted a study on leather meal-based fertilizers, determining the faster and slower soluble N pools using phosphate buffer extractions and correlating them with the fertilizers’ particle sizes. Cassity-Duffey et al. [17] stated that the total N (Ntot) content of the fertilizers could be used as an estimate of the N0, while according to other authors [24,25] the C:N ratio is the most indicative parameter.
The aims of this study were (i) to determine the bioavailable N in organic fertilizers from animal by-products subjected to different treatment such as anaerobic digestion, thermobaric hydrolysis, chemical and biological stabilization, (ii) to determine the hot-water extractable N in the organic fertilizers, and (iii) to study the relation between bioavailable N and chemical indicators of N availability, thus assessing the possibility to predict the bioavailable N starting from these indicators.

2. Materials and Methods

2.1. General Description of Organic Fertilizers

Nine organic fertilizers (OFs) obtained from the manufacturer or from the market were codified by an increasing numbering from OF01 to OF09 (Table 1). OF01 and OF07 were anaerobic digestates obtained from slaughterhouse by-products, and were classified as nitrogen-phosphorus (NP) OFs. Similarly, OF02 was a NP OF derived from chemical stabilized tannery sludge mixed with meat and bone meals. OF03 and OF05 were N OFs resulting from mixing chemical stabilized tannery sludge and leather meal. OF04, OF08 and OF09 were N OFs obtained from leather by-products after thermobaric hydrolysis. OF06 was a bio-solid derived from the biological stabilization of tannery sludge.

2.2. Characterization of Organic Fertilizers

All the OFs were ground in a ball mill (Mixer mill MM400, Retsch GmbH, Haan, Germany) and sieved at 0.5 mm before analysis. The chemical reaction (pH) was measured in 1.5:25 mass-to-water ratio using a platinum electrode (5261 electrode, Crison Instruments SA, Alella, Spain) connected to a pH-meter (Compact titrator, Crison Instruments SA, Alella, Spain). The electrical conductivity (EC) was measured in 1:5 mass-to-water ratio using an electrical conductivity meter (Radiometer Analytical CDM 210, Hach Company, Loveland, CO, USA) equipped with a conductivity cell (Radiometer Analytical CDC 749, Hach Company, Loveland, CO, USA). Total solids (TS) and ashes were determined reaching the constant weight at 105 and 650 °C, respectively, using an oven (FD 23 model, Binder GmbH, Tuttlingen, Germany). Total organic carbon (TOC) was determined according to the dichromate method (Springer and Klee, 1954). Total Kjeldahl nitrogen (TKN) was determined after acid digestion with sulfuric acid and selenium-potassium persulfate as catalyzer, using a Kjeldahl automatic instrument (KjelFlex K360, BUCHI Labortechnik AG, Flawil, Switzerland). The ammonium N (NH4+-N) was determined according to the 920.03 A.O.A.C. method [26]. Briefly, OFs were extracted with 1 M potassium chloride (KCl) at 1:50 mass-to-volume ratio for 2 h, then an aliquot of filtrate (20 mL) was dispensed into a digestion tube and analyzed in the Kjeldahl automatic instrument after adding 1 g of magnesium oxide (MgO). The nitrate N (NO3-N) was determined with the same method, after reduction with 0.5 g of Devarda alloy (Carlo Erba, Italy). The total organic nitrogen (Norg) was calculated by the difference between TKN and NH4+-N. The total N was considered equal to TKN because the NO3-N was under the detection limit of the method for all the OFs. Total phosphorus (P) was determined by an inductively coupled plasma optical emission spectrometer (Spectro Arcos ICP-OES Analyzer, Spectro Analytical Instrument GmbH, Kleve, Germany) after flame digestion with 96% nitric acid.

2.3. Soil Description and Characterization

The soil used in the incubation test was sampled from the surface layer (0–0.2 m) of a cultivated field from the University of Bologna experimental farm located in the southern part of Po Valley, Italy (45.53° N, 11.38° E, 28 m a.s.l.). Once arrived in the laboratory, the soil was air dried, sieved at 2 mm and cleaned from plant debris. The main soil physical and chemical properties were determined according to the Soil Science Society of America methods [27] and the results are shown in Table 2. The soil was classified as a fine silty, mixed mesic Udic Ustochrept [28], and it was a sandy-loam soil characterized by a sub-alkaline pH with an average content of TOC and TN.

2.4. Incubation of Organic Fertilizers into Soil

Aliquots of 200 g of dry soil were placed into cylindrical plastic pots and pre-incubated for 15 days in controlled conditions of temperature (20 °C) and moisture (30% of full water holding capacity) in a growth chamber in the dark. During this period, moisture was checked and restored when needed. OFs were added to the pots in a weight equivalent to 100 mg N kg−1 of dry soil (ds), corresponding approximately to 250–300 kg N ha−1. The test was carried out in triplicate, according to a complete randomized design. In addition to the nine OFs, a negative control (CK) with no fertilizer added was included in the trial. The experiment, conducted under the same temperature and moisture conditions as the pre-incubation, lasted 7 weeks. A weekly soil sampling was carried out to determine the inorganic N (Nin) according to the ISO 14256-2 protocol [29]: aliquots of 5 g of soil were extracted in 50 mL of 1 M KCl for one hour, then the filtrate solution was analyzed by means of an automated flow analyzer (AutoAnalyzer 3, Bran Luebbe GmbH, Norderstedt, Germany).
The data obtained from the analysis of Nin were used to calculate the mineralized organic N (Nmin) using the following equation:
Nmin (OF,t) = [(Nin)t=n − (Nin)t=0]OF − [(Nin)t=n]CK,
where t is the sampling time, OF is the organic fertilizers added, CK is the control treatment.
The Nmin values calculated for each product and for each sampling time were processed through a first order kinetic model:
Nmin = N0 · (1 − e−kt),
where N0 is the potentially mineralizable organic N, k is the first order kinetic constant and t is the time. This equation provided qualitative and quantitative information on N mineralization, both in quantity and kinetic terms and it widely used in similar cases [19].
Finally, using the data obtained from Equations (1) and (2), it was possible to calculate the N fraction made bioavailable by the OFs added to the soil (Nbav). It corresponds to the sum of the Nin at time zero and the N0:
Nbav (OF) = [(Nin)t=0]OF − [(Nin)t=0]CK + N0 (OF),

2.5. Hot-Water Extractable Nitrogen

The determination of hot-water extractable nitrogen in OFs was performed following the procedure adopted by Curtin et al. [21] to evaluate the soil nitrogen availability. Briefly, 5 g of product was added to 100 mL of deionized water and incubated in a water bath at 80 °C under agitation for 16 h. The sample was subjected to centrifugation (6000 rpm, 15 min, 20 °C) and filtration using Whatman No. 42 paper filters. The filtered extract was analyzed for Ntot, NH4+-N and NO3-N as described in Section 2.2.
The hot-water extractable N was then calculated by discriminating the hot-water total nitrogen (HWTN) and the hot-water organic nitrogen (HWON), according to the following equations:
HWTN (% Ntot) = (TNHW/Ntot) × 100,
HWON (% Norg) = (TONHW/Norg) × 100,
where TNHW (hot-water total N) = (TNHW + NO3-NHW) and TONHW (hot-water total organic N) = (TNHW − NH4+-NHW); TNHW, NO3-NHW and NH4+-NHW are the total N, the nitrate N and the ammonium N extracted in hot-water; Ntot and Norg are the total N and the organic N of OFs.

2.6. Data Handling and Statistical Analysis

The data obtained were handled and analyzed using the environment for statistical computing R [30]. The first order kinetic model (Equation (2)) has two unknown parameters (N0 and k), which are estimated using nonlinear regression procedure with the Marquardt–Levenberg algorithm and iterative method to find the parameters to minimize the residual sum of squares [31]. In some cases, manual adjustment of initial values was needed to obtain sensible results. The good-of-fitness of nonlinear models was estimated by the Pseudo-R2 (P-R2), calculated using the follow equation: P-R2 = 1 − (RSS/TSS), were RSS is residual sum of squares and TSS is the total sum of squares [32]. The half-time of organic N mineralization (t1/2) was determined using the following equation: t1/2 = Ln(2)/k. The relationships between N mineralization (N0 and Nbav) vs. hot-water extractable N (HWON and HWTN) and C:N ratio were assessed using simple linear regression analysis. The coefficient of determination (R2 = SSreg/TSS, where SSreg is the sum of squares explained by regression and TSS is the total sum of squares) was calculated as an indicator of goodness of fit.

3. Results

3.1. Characterization of Organic Fertilizers

The main characteristics of the OFs used are reported in Table 3. In general, the physical-chemical characteristics of OFs strictly reflected the raw materials and the production processes undergone. The OFs obtained from leather by-products (OF04, OF08, OF09) have acidic pH (4.3–4.7), probably due to the use of acids (i.e., sulfuric acid) in the tanning treatment and in the leather by-products thermobaric hydrolysis [33]. The remaining OFs showed a sub-alkaline pH, ranging from 7.3 to 8.5, in agreement with previous research with similar materials [34,35,36]. The EC ranged between 0.9 to 5.3 dS m−1, which represent typical values for this type of organic fertilizers [34]. Clearly, the EC depends on total soluble salts present in OFs, and the fact that OF04 showed the highest value is probably linked to the use of huge quantities of salts (i.e., NaCl) during the tanning process. However, high EC values were recorded in OFs subjected to chemical stabilization (OF02, OF03 and OF05).
All samples were in dried form and contained approximately 90% of total solids (TS). Differently, the volatile solids (VS) were found in a wide range, from 44 to 91%. In particular, OFs derived from leather (OF04, OF08, OF09) and slaughterhouse by-products (OF01, OF07) showed higher VS, up to 70%, clearly due to the high content of OM of the raw materials. The percentage of ashes was inversely proportional to the content of VS, and was higher in OFs derived from tannery sludge (OF02, OF03, OF05 and OF06), probably due to inorganic compounds (i.e., inorganic salts) present in these materials.
Total organic carbon (TOC) of OFs ranged from 14 to 47% (Table 3). This large range of TOC is due to the different chemical characteristics of raw materials used for OFs production [34,35,36] and represents an important indicator for OFs quality. Total Kjeldahl nitrogen (TKN), as well as total organic nitrogen (Norg), was significantly higher in OFs made from leather by-products (OF04, OF08 and OF09), presenting values ranging from 13 to 15%, clearly due to the large amounts of animal proteins (or polypeptides) in the raw material. The lowest value of TKN was found in OF06 (2.78%), the only sample obtained from tannery sludge that does not include the addition of other raw materials, such as OF02, OF03, and OF05. As expected, the higher the TKN the lower the C:N ratio, which showed values approximately of 3 for OF04, OF08 and OF09, and of 12 for the OF06.
While the nitrate N (NO3-N) content was below the detection limit of the method (<100 ppm), the ammonium N concentration (NH4+-N) ranged from 0.03 to 0.60%. In this case, there was no discrimination based on raw materials; the differences probably depend from the specific productive processes used. In any case, the amount of inorganic (or ammonium) N was a small fraction of TKN and the organic N was always higher than 95% of TKN.

3.2. Incubation of Organic Fertilizers in the Soil

The dynamics of NH4+-N showed significant differences only at the beginning of incubation (t0) since this form of N is immediately released in soil by the input of OFs (Figure 2a and Table 4). In fact, OF02 and OF07 presented the highest values of NH4+-N released, being the two characterized by the highest percentage of NH4+-N content (Table 3). In the subsequent samplings, all OFs showed values close to zero and equal to the untreated soil (CK) demonstrating that NH4+-N was subjected to soil nitrification process.
Consequently, analysis of nitrate manifested an opposite trend to that of ammonium (Figure 2b). In fact, an increase was observed in the first period, from the beginning to the 14th day, followed by a general stabilization phase until the end of the incubation. In particular, OF08 was the sample that released the most nitrate N, showing values above 100 mg N kg−1 d.s. starting from the fourteenth day. OF09 and OF02 showed approximately the same trend, achieving values around 90 mg N kg−1 d.s. in the same period, followed by a slight decrease of OF02 in the last two samplings. A value of 75 mg N kg−1 d.s. was reached and exceeded by both OF04 and OF03 during the incubation. In particular, the fertilizer deriving from leather meal (OF04) showed slightly higher values than OF03 in the last three sampling dates (t35, t42, t49). OF07 displayed an increase between the 3rd and the 4th week of incubation (respectively t21 and t28), reaching values above 60 mg N kg−1 d.s., as well as OF05 and OF01. OF06 was the sample that presented the lowest values (50 mg N kg−1 d.s.), but showed a positive trend compared to the unfertilized control.
Figure 3 shows the curves of mineralization of the organic N during the incubation period. The asymptote of these first kinetic order curves is equal to N0, the potentially mineralizable organic N, results for which are reported in Table 4.
As expected, the kinetic curves are very similar to those obtained with the NO3-N analysis, since the NH4+-N release showed no differences between the fertilized samples and the unfertilized control, except for t0. OF08 and OF09 showed the greater ability to mineralize, reaching values of mineralization of 63 and 54% of the total N added, respectively (Table 4). The mineralization percentage ranged from 29.6 to 36% for OF04, OF02, OF05 and OF03, while OFs deriving from anaerobic digestion, OF01 and OF07, attained 16 and 13%, respectively (Table 4). As confirmed by the NO3-N analysis, OF06 is the one that manifested the lowest ability to mineralize the organic N (6.8%). These results were quite expected, as OF01, OF07 and OF06 showed a consistent loss of organic matter during the digestion and the biological treatment.

3.3. Hot-Water Extractable Nitrogen

The results of HWON and HWTN are reported in Table 5. The values of HWON ranged between 10.5 (OF06) and 46.3% (OF02) of Norg, a wide range which, however, agrees with the results of N0 previously reported (Table 4). The higher HWON was observed for OF02, OF03, OF04, OF08 and OF09 with values ranging between 35 to 45% of Norg. These OFs were characterized by leather and bone meals as raw materials and chemical or thermobaric hydrolysis as transformation process (Table 1). Otherwise, OF01 and OF06 showed the lowest HWON, ranging between 10 to 20% of Norg. These fertilizers were made from slaughterhouse by-products and tannery sludge, respectively. The other OFs, from the same by-products, showed an intermediate HWON, with values ranging from 25 to 30% of Norg. The values of HWTN were slightly higher than HWON (Table 5), but closely related. Only OF02 and OF07 showed a difference between HWTN and HWON higher than 5%, due to their greater Nin content (Table 4).

3.4. Correlation and Linear Regression between Indicators of Available Nitrogen

The data concerning indicators of N availability in OFs were analyzed for correlation using the Pearson coefficient. The results obtained are shown in Table 6. The N0, Nbav, HWON and HWTN were significantly and positively correlated (p-value < 0.001). On the other hand, N0 and Nbav were significantly but negatively correlated with C:N ratio (p-value < 0.05).
The capacity of HWON, HWTN and C:N ratio to estimate N0 and Nbav was assessed using the linear regression approach. The results are shown in Figure 4 and Figure 5. The HWON showed a significant (p-value = 0.039) linear regression with the N0 (Figure 4a) and a R2 = 0.478. Even if the slope of the straight line was 1.065 ± 0.420 (p-value = 0.039), with a quite perfect linearity between the two indicators, the samples OF08 and OF09 deviated from linearity and fell outside the 95% confidence limit. HWTN and Nbav showed similar results (Figure 4b), with a significant linearity (R2 = 0.53, p-value = 0.027), a slope of 0.921 ± 0.328 (p-value = 0.027) and the same samples outside of the 95% confidence limits. In both cases the estimated intercept was not significantly different from zero (respectively: −2.5 ± 14.4 (p-value = 0.87) and 6.1 ± 12.1 (p-value = 0.63)).
The C:N ratio showed (Figure 5a,b) a significant linearity with N0 (R2 = 0.75, p-value = 0.003) and Nbav (R2 = 0.76, p-value = 0.002). The slope was negative in both cases and significantly different from zero: −5.12 ± 1.13 (p-value = 0.003) and −4.61 ± 0.97 (p-value = 0.002), respectively, for N0 and Nbav. The intercepts were higher than zero and corresponded to 64.7 ± 8.0 (p-value <0.001) and 67.8 ± 6.9 (p-value <0.001) for N0 and Nbav, respectively.

4. Discussion

4.1. Characterization and Soil Incubation of the Organic Fertilizers

After the physical-chemical characterization and incubation in soil, it is possible to classify the studied animal-based OFs in four groups: (i) leather meal from thermobaric hydrolysis, (ii) slaughterhouse by-products from anaerobic digestion, (iii) tannery sludge + animal based by-products from chemical stabilization, and (iv) tannery sludge from biological stabilization. In fact, this confirms that the source materials and treatment processes largely affected OFs characteristics and the N release in the soil.
The OFs obtained from leather meals (OF04, OF08 and OF09) showed higher Ntot (12–14%) than other OFs, in agreement with the greater protein content of leather [23,33,37]. These OFs are characterized also by lower C:N ratio (3–3.5:1), higher volatile solids (77–88%) and lower ash contents (7.5–10%) than other OFs. Soil incubation indicated that this class of OFs mineralized more organic N, with a N0 ranging from 36 to 63% of Ntot, resulting in 40–64% of bioavailable N (Nbav). The rate of mineralization was also higher than in other OFs studied, with a half-life time of 3.6–5.8 days at the experimental conditions applied in this study. Practically, the release of the bioavailable N in soil from these OFs proceeded intensely and quickly, similar to inorganic N fertilizers [23,38], due to the thermobaric hydrolysis that guaranteed a high availability of Norg for soil microorganisms, and to the lower C:N ratio that results in a lower N use efficiency by soil microorganisms and consequently a net N release into the soil [39].
Slaughterhouse by-products (OF01 and OF07) after anaerobic digestion have been poorly studied as fertilizers [14,40], and they are characterized by a relatively high Ntot (4–5%) and low C:N ratio (8–9:1). Therefore, it is not surprising that these OFs released as Nbav only a quarter of the Ntot added to the soil. As previously discussed [17,39], the net mineralization of Norg in soil depends on the C:N ratio and on the quality or availability of OM for soil microorganisms. Regarding this class of OFs, a large part of the Nbav was directly released as Nin probably due to anaerobic digestion, which is well known to generate digestates that often contain important amounts of ammonium N [35,41,42].
The tannery sludge-based OFs (OF02, OF03 and OF05) showed a Ntot content similar to the slaughterhouse by-products (3.5–5%), but with a lower C:N ratio (5–8:1). These OFs were obtained by mixing tannery sludge with other animal-based fertilizers (bone, meat and leather meals), thus increasing the Ntot. This reduced the C:N ratio compared to a pure tannery sludge from biological stabilization (OF06), which instead showed the lowest Ntot (2.5%) and the highest C:N ratio (16:1). The tannery sludge has been proposed as organic N fertilizer considering its interesting N content [43,44,45], but it is often mixed with other OFs to meet commercial and agronomical requirements. Indeed, mixing tannery sludge with meat, bone and leather meal makes it possible to modulate Ntot and the C:N ratio of the final OF. This induces a distinct behavior of the OFs in soil and a different release of bioavailable N: for example, OF02 characterized by lower C:N ratio (5.2:1), showed higher Norg mineralization (36% of Ntot). The Nbav released by this group of OFs showed a wide range of values (34–52%) which is largely due to the different Nin content. Indeed, the high Nin (16%) in OF02 corresponded to the high Nbav (52%), and this probably derived from the raw materials (i.e., meat meal) or from the hydrolysis during the transformation process.
The characteristics of pure tannery sludge (OF06) were less interesting from the agronomical point of view, with the lower Ntot (2.5%), the higher C:N ratio (16:1) and the lower volatile solids (40%), compared to the other OFs. OF06 released only the 16% of Ntot in available form, a value much lower compared to those obtained testing sewage sludge deriving from different raw materials [46,47,48]. This outcome is probably related to the more intensive biological treatment to which the tannery wastewater was subjected [49] due to the high load of OM and stabilizing compounds, such as chromium, aluminum and glutaraldehyde [45,50]. Practically, this product is more like an amendment than a OF; indeed, the mineralization of organic N in soil was lower than for the other OFs (7%). For these reasons, the tannery sludge is generally mixed with other OFs to make it a marketable OF [51].

4.2. Relation between Indicators of Available Nitrogen

Three indicators were used to estimate the available N in animal-based OFs: HWON, HWTN and C:N ratio. All the indicators are relatively easy to determine and standardized methods are available, making them a valid alternative to estimate the bioavailable N in OFs.
However, in some cases these indicators under- or overestimate the bioavailable N. In particular, OF08 and OF09, based on leather meal, mineralized the highest amount of organic N, but these results do not agree with those found in the analysis of HWON and HWTN that underestimate N0 and Nbav. Conversely, in the case of OF02 and OF03, based on tannery sludge mixed with other materials, the hot-water indicators slightly underestimate only the Nbav. The reason for these discrepancies seems to be connected to the quality of Norg, which appears, in some cases, to be more hot-water extractable than mineralizable, and in other cases less so. In the case of OFs based on leather meal, the hot water may not be a strong enough extractant to guarantee the optimal solubility of collagen, the main component of leather meal [23], whereas it is well hydrolyzed by collagenase and protease in the soil [52]. In the case of tannery sludge, the hot water seems to extract the organic N fraction that is less accessible to or less degradable by the soil microbiota.
The C:N ratio of OFs was, as expected [17,25,42], a good indicator of organic N mineralization in soil. The C:N ratio was negatively correlated with bioavailable N; in fact, fertilizers with a low ratio induce high bioavailable N in soil [39]. The linear regression analysis between bioavailable N and C:N ratio was more significant (R2 = 0.75) than that of hot-water indicators. Interestingly, the linear models estimate a maximum available N for the animal-based OFs near to 65–68% of Ntot (Figure 5, calculate for C:N = 0), and a C:N ratio limit between mineralization-immobilization of N near to 12–15:1 (Figure 5, calculate for N0 and Nbav = 0), in agreement with findings of other authors [17,25]. Indeed, a C:N ratio less than 15–25:1 generally results in net N mineralization, whereas a C:N ratio greater than 15–25:1 can lead to net N immobilization [25,53,54], and all OFs studied fall in the first case. The C:N ratio is, unlike HWON and HWTN, an eco-stoichiometric indicator or an indicator that predicts the N availability on the basis of N and C use efficiency by soil microorganisms. When the C:N ratio is less than 15–25:1, the soil microorganisms face a substrate with an excess of N with respect to their needs, and this N is released in the soil as inorganic (available for plant) form. Nevertheless, this is valid only theoretically, as in real cases the quality of the OM added with the OF and the presence of other elements or compounds able to inhibit or stimulate the microbial metabolism can modulate the N mineralization. For example, the OFs based on leather meal (OF04, OF08 and OF09) have a similar C:N ratio, 3–3.5:1, but show three different Nbav, 40.3% (OF04), 64.5% (OF08) and 54.6% (OF09), that probably are due to differences in the treatment process (i.e., more or less hydrolysis conditions) or to a residual presence of stabilizing agents used for tanning the leather.

5. Conclusions

The results of this research can inform farmers and fertilizer producers about the N availability in organic fertilizers from animal by-products and the relationship with chemical indicators of N availability. It was demonstrated that it is difficult to predict N mineralization solely on the basis of their total N content; in fact, raw materials and treatment processes influence the ability of OFs to release N in inorganic forms. The hot-water extractable N proved to be a good indicator of Nbav, but underestimated it in the case of OFs based on leather meal. The C:N ratio provided the best linear fitting with Nbav, but also showed some limitations in estimating the Nbav.
This study has demonstrated that OFs from animal by-products are potentially useful as organic N fertilizers. However, to use them consciously and efficiently, the evaluation of C:N ratio and the hot-water extractable N analysis should be considered, as they are rapid and inexpensive methods that can provide a reliable indication of N availability in the soil.

Author Contributions

Conceptualization, L.C. and S.R.; methodology, G.D.B., L.C. and S.R.; formal analysis, G.D.B. and S.R.; data curation, G.D.B., L.C. and S.R.; writing—original draft preparation, L.C. and S.R.; writing—review and editing, C.C., M.M. and S.R.; supervision and resources, C.C. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Not applicable.

Acknowledgments

The authors wish to thank for technical support: Paola Gioacchini and Andrea Simoni (DISTAL, University of Bologna) for soils and organic fertilizers analysis, and Maurizio Quartieri (DISTAL, University of Bologna) for inorganic nitrogen analysis.

Conflicts of Interest

The authors declare no conflict of interest.

References

  1. Luo, G.; Li, L.; Friman, V.-P.; Guo, J.; Guo, S.; Shen, Q.; Ling, N. Organic Amendments Increase Crop Yields by Improving Microbe-Mediated Soil Functioning of Agroecosystems: A Meta-Analysis. Soil Biol. Biochem. 2018, 124, 105–115. [Google Scholar] [CrossRef]
  2. Carbonell, G.; de Imperial, R.M.; Torrijos, M.; Delgado, M.; Rodriguez, J.A. Effects of Municipal Solid Waste Compost and Mineral Fertilizer Amendments on Soil Properties and Heavy Metals Distribution in Maize Plants (Zea Mays L.). Chemosphere 2011, 85, 1614–1623. [Google Scholar] [CrossRef] [PubMed]
  3. Spångberg, J.; Hansson, P.-A.; Tidåker, P.; Jönsson, H. Environmental Impact of Meat Meal Fertilizer vs. Chemical Fertilizer. Resour. Conserv. Recycl. 2011, 55, 1078–1086. [Google Scholar] [CrossRef]
  4. Svanbäck, A.; McCrackin, M.L.; Swaney, D.P.; Linefur, H.; Gustafsson, B.G.; Howarth, R.W.; Humborg, C. Reducing Agricultural Nutrient Surpluses in a Large Catchment—Links to Livestock Density. Sci. Total Environ. 2019, 648, 1549–1559. [Google Scholar] [CrossRef]
  5. Zhang, G.; Sun, B.; Zhao, H.; Wang, X.; Zheng, C.; Xiong, K.; Ouyang, Z.; Lu, F.; Yuan, Y. Estimation of Greenhouse Gas Mitigation Potential through Optimized Application of Synthetic N, P and K Fertilizer to Major Cereal Crops: A Case Study from China. J. Clean. Prod. 2019, 237, 117650. [Google Scholar] [CrossRef]
  6. Ott, C.; Rechberger, H. The European Phosphorus Balance. Resour. Conserv. Recycl. 2012, 60, 159–172. [Google Scholar] [CrossRef]
  7. Sharma, B.; Vaish, B.; Singh, U.K.; Singh, P.; Singh, R.P. Recycling of Organic Wastes in Agriculture: An Environmental Perspective. Int. J. Environ. Res. 2019, 13, 409–429. [Google Scholar] [CrossRef]
  8. Song, K.; Xue, Y.; Zheng, X.; Lv, W.; Qiao, H.; Qin, Q.; Yang, J. Effects of the Continuous Use of Organic Manure and Chemical Fertilizer on Soil Inorganic Phosphorus Fractions in Calcareous Soil. Sci. Rep. 2017, 7, 1164. [Google Scholar] [CrossRef] [Green Version]
  9. Billen, G.; Garnier, J.; Lassaletta, L. The Nitrogen Cascade from Agricultural Soils to the Sea: Modelling Nitrogen Transfers at Regional Watershed and Global Scales. Phil. Trans. R. Soc. B 2013, 368, 20130123. [Google Scholar] [CrossRef]
  10. Galloway, J.N.; Aber, J.D.; Erisman, J.W.; Seitzinger, S.P.; Howarth, R.W.; Cowling, E.B.; Cosby, B.J. The Nitrogen Cascade. BioScience 2003, 53, 341–356. [Google Scholar] [CrossRef]
  11. Lassaletta, L.; Billen, G.; Grizzetti, B.; Anglade, J.; Garnier, J. 50 Year Trends in Nitrogen Use Efficiency of World Cropping Systems: The Relationship between Yield and Nitrogen Input to Cropland. Environ. Res. Lett. 2014, 9, 105011. [Google Scholar] [CrossRef]
  12. Sutton, M.A.; Bleeker, A. The Shape of Nitrogen to Come. Nature 2013, 494, 435–437. [Google Scholar] [CrossRef] [PubMed] [Green Version]
  13. Köninger, J.; Lugato, E.; Panagos, P.; Kochupillai, M.; Orgiazzi, A.; Briones, M.J.I. Manure Management and Soil Biodiversity: Towards More Sustainable Food Systems in the EU. Agric. Syst. 2021, 194, 103251. [Google Scholar] [CrossRef]
  14. Limeneh, D.Y.; Tesfaye, T.; Ayele, M.; Husien, N.M.; Ferede, E.; Haile, A.; Mengie, W.; Abuhay, A.; Gelebo, G.G.; Gibril, M.; et al. A Comprehensive Review on Utilization of Slaughterhouse By-Product: Current Status and Prospect. Sustainability 2022, 14, 6469. [Google Scholar] [CrossRef]
  15. Bustillo-Lecompte, C.F.; Mehrvar, M. Slaughterhouse Wastewater Characteristics, Treatment, and Management in the Meat Processing Industry: A Review on Trends and Advances. J. Environ. Manag. 2015, 161, 287–302. [Google Scholar] [CrossRef]
  16. Whalen, J.K.; Thomas, B.W.; Sharifi, M. Novel Practices and Smart Technologies to Maximize the Nitrogen Fertilizer Value of Manure for Crop Production in Cold Humid Temperate Regions. In Advances in Agronomy; Elsevier: Amsterdam, The Netherlands, 2019; Volume 153, pp. 1–85. ISBN 978-0-12-817404-3. [Google Scholar]
  17. Cassity-Duffey, K.; Cabrera, M.; Gaskin, J.; Franklin, D.; Kissel, D.; Saha, U. Nitrogen Mineralization from Organic Materials and Fertilizers: Predicting N Release. Soil Sci. Soc. Am. J. 2020, 84, 522–533. [Google Scholar] [CrossRef]
  18. Stanford, G.; Smith, S.J. Nitrogen Mineralization Potentials of Soils. Soil Sci. Soc. Am. J. 1972, 36, 465–472. [Google Scholar] [CrossRef]
  19. Honeycutt, C.W.; Zibilske, L.M.; Clapham, W.M. Heat Units for Describing Carbon Mineralization and Predicting Net Nitrogen Mineralization. Soil Sci. Soc. Am. J. 1988, 52, 1346–1350. [Google Scholar] [CrossRef]
  20. Bremner, J.M.; Keeney, D.R. Steam Distillation Methods for Determination of Ammonium, Nitrate and Nitrite. Anal. Chim. Acta 1965, 32, 485–495. [Google Scholar] [CrossRef]
  21. Curtin, D.; Wright, C.E.; Beare, M.H.; McCallum, F.M. Hot Water-Extractable Nitrogen as an Indicator of Soil Nitrogen Availability. Soil Sci. Soc. Am. J. 2006, 70, 1512–1521. [Google Scholar] [CrossRef]
  22. Hussain, F.; Malik, K.A.; Azam, F. Evaluation of Acid Permanganate Extraction as an Index of Soil Nitrogen Availability. Plant Soil 1984, 79, 249–254. [Google Scholar] [CrossRef]
  23. Dell’Abate, M.T.; Benedetti, A.; Trinchera, A.; Galluzzo, D. Nitrogen and Carbon Mineralisation of Leather Meal in Soil as Affected by Particle Size of Fertiliser and Microbiological Activity of Soil. Biol. Fertil. Soils 2003, 37, 124–129. [Google Scholar] [CrossRef]
  24. Delin, S.; Stenberg, B.; Nyberg, A.; Brohede, L. Potential Methods for Estimating Nitrogen Fertilizer Value of Organic Residues: Methods for Estimating Nitrogen Fertilizer Value of Organic Residues. Soil Use Manag. 2012, 28, 283–291. [Google Scholar] [CrossRef]
  25. Lazicki, P.; Geisseler, D.; Lloyd, M. Nitrogen Mineralization from Organic Amendments Is Variable but Predictable. J. Env. Qual. 2020, 49, 483–495. [Google Scholar] [CrossRef] [PubMed]
  26. A.O.A.C. Official Methods of Analysis, 15th ed.; Association of Official Analytical Chemist: Washington, DC, USA, 1990. [Google Scholar]
  27. Methods of Soil Analysis: Part 3 Chemical Methods; Sparks, D.L.; Page, A.L.; Helmke, P.A.; Loeppert, R.H.; Soltanpour, P.N.; Tabatabai, M.A.; Johnston, C.T.; Sumner, M.E. (Eds.) SSSA Book Series; Soil Science Society of America, American Society of Agronomy: Madison, WI, USA, 1996; ISBN 978-0-89118-866-7. [Google Scholar]
  28. Soil Survey Staff. Soil Survey Staff. Soil Taxonomy: A Basic System of Soil Classification for Making and Interpreting Soil Surveys. In U.S. Department of Agriculture Handbook, 2nd ed.; Natural Resources Conservation Service: Washington, DC, USA, 1999. [Google Scholar]
  29. ISO Standard No 14256-2; International Organization for Standardization Soil Quality—Determination of Nitrate, Nitrite and Ammonium in Field-Moist Soils by Extraction with Potassium Chloride Solution—Part 2: Automated Method with Segmented Flow Analysis. ISO: Geneva, Switzerland, 2005.
  30. R Core Team. R: A Language and Environment for Statistical Computing; R Foundation for Statistical Computing: Vienna, Austria, 2022. [Google Scholar]
  31. Benbi, D.K.; Richter, J. A critical review of some approaches to modelling nitrogen mineralization. Biol. Fertil. Soils 2002, 35, 168–183. [Google Scholar] [CrossRef]
  32. Schabenberger, O.; Pierce, F.J. Contemporary Statistical Models for the Plant and Soil Sciences; CRC Press: Boca Raton, FL, USA, 2001. [Google Scholar]
  33. Ciavatta, C.; Manoli, C.; Cavani, L.; Franceschi, C.; Sequi, P. Chromium-Containing Organic Fertilizers from Tanned Hides and Skins: A Review on Chemical, Environmental, Agronomical and Legislative Aspects. JEP 2012, 3, 1532–1541. [Google Scholar] [CrossRef] [Green Version]
  34. Kataki, S.; Hazarika, S.; Baruah, D.C. Assessment of By-Products of Bioenergy Systems (Anaerobic Digestion and Gasification) as Potential Crop Nutrient. Waste Manag. 2017, 59, 102–117. [Google Scholar] [CrossRef] [Green Version]
  35. Moukazis, I.; Pellera, F.-M.; Gidarakos, E. Slaughterhouse By-Products Treatment Using Anaerobic Digestion. Waste Manag. 2018, 71, 652–662. [Google Scholar] [CrossRef]
  36. Prado, J.; Ribeiro, H.; Alvarenga, P.; Fangueiro, D. A Step towards the Production of Manure-Based Fertilizers: Disclosing the Effects of Animal Species and Slurry Treatment on Their Nutrients Content and Availability. J. Clean. Prod. 2022, 337, 130369. [Google Scholar] [CrossRef]
  37. Corte, L.; Dell’Abate, M.T.; Magini, A.; Migliore, M.; Felici, B.; Roscini, L.; Sardella, R.; Tancini, B.; Emiliani, C.; Cardinali, G.; et al. Assessment of Safety and Efficiency of Nitrogen Organic Fertilizers from Animal-Based Protein Hydrolysates-a Laboratory Multidisciplinary Approach. J. Sci. Food Agric. 2014, 94, 235–245. [Google Scholar] [CrossRef]
  38. Cayuela, M.L.; Velthof, G.L.; Mondini, C.; Sinicco, T.; van Groenigen, J.W. Nitrous Oxide and Carbon Dioxide Emissions during Initial Decomposition of Animal By-Products Applied as Fertilisers to Soils. Geoderma 2010, 157, 235–242. [Google Scholar] [CrossRef]
  39. Mooshammer, M.; Wanek, W.; Hämmerle, I.; Fuchslueger, L.; Hofhansl, F.; Knoltsch, A.; Schnecker, J.; Takriti, M.; Watzka, M.; Wild, B.; et al. Adjustment of Microbial Nitrogen Use Efficiency to Carbon:Nitrogen Imbalances Regulates Soil Nitrogen Cycling. Nat. Commun. 2014, 5, 3694. [Google Scholar] [CrossRef] [PubMed] [Green Version]
  40. Bhunia, S.; Bhowmik, A.; Mukherjee, J. Use of Rural Slaughterhouse Wastes (SHWs) as Fertilizer in Agriculture: A Review. In Proceedings of the 2019 International Conference on Energy Management for Green Environment (UEMGREEN), Kolkata, India, 25–27 September 2019; pp. 1–6. [Google Scholar]
  41. Alburquerque, J.A.; de la Fuente, C.; Bernal, M.P. Chemical Properties of Anaerobic Digestates Affecting C and N Dynamics in Amended Soils. Agric. Ecosyst. Environ. 2012, 160, 15–22. [Google Scholar] [CrossRef]
  42. Sigurnjak, I.; De Waele, J.; Michels, E.; Tack, F.M.G.; Meers, E.; De Neve, S. Nitrogen Release and Mineralization Potential of Derivatives from Nutrient Recovery Processes as Substitutes for Fossil Fuel-Based Nitrogen Fertilizers. Soil Use Manag. 2017, 33, 437–446. [Google Scholar] [CrossRef]
  43. Araujo, A.S.F.; de Melo, W.J.; Araujo, F.F.; Van den Brink, P.J. Long-Term Effect of Composted Tannery Sludge on Soil Chemical and Biological Parameters. Environ. Sci. Pollut. Res. 2020, 27, 41885–41892. [Google Scholar] [CrossRef]
  44. Giacometti, C.; Cavani, L.; Gioacchini, P.; Ciavatta, C.; Marzadori, C. Soil Application of Tannery Land Plaster: Effects on Nitrogen Mineralization and Soil Biochemical Properties. Appl. Environ. Soil Sci. 2012, 2012, 395453. [Google Scholar] [CrossRef] [Green Version]
  45. Guo-Tao, F.; Zhi-Hua, S.; Shuqing, L.; Hui, C. The Production of Organic Fertilizer Using Tannery Sludge. J. Am. Leather Chem. Assoc. 2013, 108, 189–196. [Google Scholar]
  46. Antil, R.S.; Bar-Tal, A.; Fine, P.; Hadas, A. Predicting Nitrogen and Carbon Mineralization of Composted Manure and Sewage Sludge in Soil. Compos. Sci. Util. 2011, 19, 33–43. [Google Scholar] [CrossRef]
  47. Grigatti, M.; Cavani, L.; di Biase, G.; Ciavatta, C. Current and Residual Phosphorous Availability from Compost in a Ryegrass Pot Test. Sci. Total Environ. 2019, 677, 250–262. [Google Scholar] [CrossRef]
  48. Sciubba, L.; Cavani, L.; Negroni, A.; Zanaroli, G.; Fava, F.; Ciavatta, C.; Marzadori, C. Changes in the Functional Properties of a Sandy Loam Soil Amended with Biosolids at Different Application Rates. Geoderma 2014, 221–222, 40–49. [Google Scholar] [CrossRef]
  49. Zhao, J.; Wu, Q.; Tang, Y.; Zhou, J.; Guo, H. Tannery Wastewater Treatment: Conventional and Promising Processes, an Updated 20-Year Review. J. Leather Sci. Eng. 2022, 4, 10. [Google Scholar] [CrossRef]
  50. Sundar, V.J.; Gnanamani, A.; Muralidharan, C.; Chandrababu, N.K.; Mandal, A.B. Recovery and Utilization of Proteinous Wastes of Leather Making: A Review. Rev. Env. Sci. Biotechnol. 2011, 10, 151–163. [Google Scholar] [CrossRef]
  51. Tahiri, S.; De La Guardia, M. Treatment and Valorization of Leather Industry Solid Wastes: A Review. J. Am. Leather Chem. Assoc. 2009, 104, 52–67. [Google Scholar]
  52. Harper, E. Collagenases. Annu. Rev. Biochem. 1980, 49, 1063–1078. [Google Scholar] [CrossRef] [PubMed]
  53. Calderón, F.J.; McCarty, G.W.; Reeves, J.B. Analysis of Manure and Soil Nitrogen Mineralization during Incubation. Biol. Fertil. Soils 2005, 41, 328–336. [Google Scholar] [CrossRef]
  54. Gale, E.S.; Sullivan, D.M.; Cogger, C.G.; Bary, A.I.; Hemphill, D.D.; Myhre, E.A. Estimating Plant-Available Nitrogen Release from Manures, Composts, and Specialty Products. J. Environ. Qual. 2006, 35, 2321–2332. [Google Scholar] [CrossRef] [PubMed]
Figure 1. Schematic representation of the bioavailable fraction of nitrogen in organic fertilizers (Ntot = total N; Nin = inorganic N; Norg = organic N; N0 = potentially mineralizable organic N; Nbav = bioavailable N).
Figure 1. Schematic representation of the bioavailable fraction of nitrogen in organic fertilizers (Ntot = total N; Nin = inorganic N; Norg = organic N; N0 = potentially mineralizable organic N; Nbav = bioavailable N).
Sustainability 14 12921 g001
Figure 2. Inorganic N release during the incubation of organic fertilizers in the soil: (a) ammonium N, and (b) nitrate N (ds = data expressed on dry soil basis).
Figure 2. Inorganic N release during the incubation of organic fertilizers in the soil: (a) ammonium N, and (b) nitrate N (ds = data expressed on dry soil basis).
Sustainability 14 12921 g002
Figure 3. First order kinetic model fitting of organic N mineralized in the soil incubated with the organic fertilizers.
Figure 3. First order kinetic model fitting of organic N mineralized in the soil incubated with the organic fertilizers.
Sustainability 14 12921 g003
Figure 4. Linear regression between: (a) potentially mineralizable organic N (N0) with hot-water organic N (HWON), and (b) bioavailable N (Nbav) with hot-water total N (HWTN). The straight lines indicate the linear regression model and the dotted lines the 95% confidence limits.
Figure 4. Linear regression between: (a) potentially mineralizable organic N (N0) with hot-water organic N (HWON), and (b) bioavailable N (Nbav) with hot-water total N (HWTN). The straight lines indicate the linear regression model and the dotted lines the 95% confidence limits.
Sustainability 14 12921 g004
Figure 5. Linear regression between: (a) potentially mineralizable organic N (N0) with C:N ratio, and (b) bioavailable N (Nbav) with C:N ratio. The straight lines indicate the linear regression model and the dotted lines the 95% confidence limits.
Figure 5. Linear regression between: (a) potentially mineralizable organic N (N0) with C:N ratio, and (b) bioavailable N (Nbav) with C:N ratio. The straight lines indicate the linear regression model and the dotted lines the 95% confidence limits.
Sustainability 14 12921 g005
Table 1. Codes, raw materials and transformation processes of the organic fertilizers collected.
Table 1. Codes, raw materials and transformation processes of the organic fertilizers collected.
CodeRaw MaterialsTransformation Process
OF01Slaughterhouse by-productsAnaerobic digestion
OF02Tannery sludge + meat and bone mealsChemical stabilization
OF03Tannery sludge + leather mealChemical stabilization
OF04Leather mealThermobaric hydrolysis
OF05Tannery sludge + leather mealChemical stabilization
OF06Tannery sludgeBiological stabilization
OF07Slaughterhouse by-productsAnaerobic digestion
OF08Leather mealThermobaric hydrolysis
OF09Leather mealThermobaric hydrolysis
Table 2. Main characteristics of the soil used in the trial.
Table 2. Main characteristics of the soil used in the trial.
ParametersUnitValue
Texture (U.S.D.A.) sandy-loam
Sandg kg1700
Siltg kg−1260
Clayg kg−1140
pH (in water) 7.8
Electrical conductivitydS m−10.17
Cation exchange capacitycmol+ kg−123.4
Total carbonatesg CaCO3 kg−165
Total organic carbong C kg−121.6
Total nitrogeng N kg−12.15
C:N ratio 10.1
Available phosphorousmg P kg−164
Exchangeable calciummg Ca kg−15150
Exchangeable potassiummg K kg−1430
Exchangeable magnesiummg Mg kg−1280
Exchangeable sodiummg Na kg−132
Table 3. Main physical-chemical characteristics of the organic fertilizers (fw = fresh weight, dw = dry weight).
Table 3. Main physical-chemical characteristics of the organic fertilizers (fw = fresh weight, dw = dry weight).
ParametersOF01OF02OF03OF04OF05OF06OF07OF08OF09
pH7.77.97.64.37.58.57.34.74.3
Electrical cond. (dS m−1)0.95.14.45.33.532.752.52.43.8
Total solids (% fw)90.791.593.496.693.390.388.584.691.7
Volatile solids (% dw)834357.690.755.144.781.491.189.4
Ash (% dw)175742.49.344.955.318.68.910.6
Total organic C (% dw)44.214.331.646.234.932.440.247.647.1
Total Kjeldahl N (%)4.474.115.7414.84.272.785.0315.713.5
C:N ratio95.25.53.18.412833.5
Ammonium N (% dw)0.170.510.140.250.10.070.600.050.03
Nitrate N (% dw)<0.01<0.01<0.01<0.01<0.01<0.01<0.01<0.01<0.01
Organic N (% dw)4.33.65.614.64.172.714.4315.713.5
Table 4. Fitting of inorganic N release in the soil after incubation with organic fertilizers: Nin is the inorganic N at the beginning of incubation, N0 is the potentially mineralizable organic N, Nbav is the sum of Nin and N0, k is the first order kinetic constant, t1/2 is the half-time of organic N mineralization and P-R2 is the Pseudo R-squared coefficient of the first order kinetic model. Data are reported as mean ± error standard from the mean.
Table 4. Fitting of inorganic N release in the soil after incubation with organic fertilizers: Nin is the inorganic N at the beginning of incubation, N0 is the potentially mineralizable organic N, Nbav is the sum of Nin and N0, k is the first order kinetic constant, t1/2 is the half-time of organic N mineralization and P-R2 is the Pseudo R-squared coefficient of the first order kinetic model. Data are reported as mean ± error standard from the mean.
FertilizersNin
(%Ntot)
N0
(%Ntot)
Nbav
(%Ntot)
k
(days−1)
t1/2
(days)
P-R2
OF017.9 ± 4.116.1 ± 1.024.00.177 ± 0.0663.90.887
OF0216.7 ± 5.536.1 ± 1.452.80.165 ± 0.0354.20.959
OF034.8 ± 2.729.6 ± 1.934.40.162 ± 0.0564.30.895
OF044.3 ± 2.936.0 ± 2.240.30.110 ± 0.0235.80.951
OF052.8 ± 1.532.0 ± 1.034.80.173 ± 0.0359.80.962
OF067.3 ± 3.26.8 ± 0.614.10.294 ± 0.252280.766
OF0712.0 ± 0.313.1 ± 1.425.10.037 ± 0.0208.80.865
OF081.5 ± 0.563.1 ± 0.364.60.211 ± 0.0163.60.996
OF090.5 ± 0.254.1 ± 0.654.60.260 ± 0.0103.90.999
Table 5. Results of hot-water organic N (HWON) and hot-water total N (HWTN) analysis of the organic fertilizer.
Table 5. Results of hot-water organic N (HWON) and hot-water total N (HWTN) analysis of the organic fertilizer.
FertilizersHWON
(%Norg)
HWTN
(%Ntot)
OF0116.919.2
OF0246.353.9
OF0339.741.5
OF0443.144.1
OF0531.132.4
OF0610.510.9
OF0728.635.7
OF0839.640.8
OF0934.836.0
Table 6. Correlation between indicators of bioavailable nitrogen in organic fertilizers.
Table 6. Correlation between indicators of bioavailable nitrogen in organic fertilizers.
CorrelationPearson CoefficientDegree of Freedomp-Value
N0 vs. Nbav0.9627<0.001
HWON vs. HWTN0.9807<0.001
Nbav vs. HWTN0.72870.027
N0 vs. HWON0.69170.039
Nbav vs. C:N ratio−0.87370.002
N0 vs. C:N ratio−0.86370.003
Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Share and Cite

MDPI and ACS Style

Rapisarda, S.; Di Biase, G.; Mazzon, M.; Ciavatta, C.; Cavani, L. Nitrogen Availability in Organic Fertilizers from Tannery and Slaughterhouse By-Products. Sustainability 2022, 14, 12921. https://doi.org/10.3390/su141912921

AMA Style

Rapisarda S, Di Biase G, Mazzon M, Ciavatta C, Cavani L. Nitrogen Availability in Organic Fertilizers from Tannery and Slaughterhouse By-Products. Sustainability. 2022; 14(19):12921. https://doi.org/10.3390/su141912921

Chicago/Turabian Style

Rapisarda, Salvatore, Giampaolo Di Biase, Martina Mazzon, Claudio Ciavatta, and Luciano Cavani. 2022. "Nitrogen Availability in Organic Fertilizers from Tannery and Slaughterhouse By-Products" Sustainability 14, no. 19: 12921. https://doi.org/10.3390/su141912921

Note that from the first issue of 2016, this journal uses article numbers instead of page numbers. See further details here.

Article Metrics

Back to TopTop