The Impact of Exogenous Organic Matter on Wheat Growth and Mineral Nitrogen Availability in Soil

Abstract

Application of exogenous organic matter (EOM) to soil enriches it with micro- and macro-elements necessary for the proper growth and yield of crops. One of these elements is nitrogen, which is a major nutrient affecting crop production worldwide. Therefore, the aim of our study was to assess the impact of various EOM treatments (with and without mineral fertilization) on wheat yield characteristics and the dynamics of mineral nitrogen release. This study was conducted as a pot experiment using three soils characterized by different physicochemical properties, which were collected from the Polish–Czech Republic transboundary area. A spring wheat (the Tybalt cultivar) was selected as the model test plant. The EOMs tested in the experiment included three soil amendments (animal meal, industrial compost, and digestate) characterized by different potential impacts on plant growth and development. The efficiency of the selected amendments was analyzed in two doses, set at 50% and 100% mineral nitrogen ratios (equivalent to 70 and 140 kg ha⁻¹, respectively). The content of mineral nitrogen (N-NH₄⁺ and N-NO₃⁻) in soils before sowing and after harvesting, and the quality and biomass of the wheat yield were determined. The application of an entire N rate in the form of EOM did not cause any decrease in the wheat yields or a clear diversification of the wheat biomass. However, the appropriate selection of rates and fertilizer combinations resulted in an increased amount of available nitrogen being introduced into the soil (a 9–31% and 17–38% increase of N-NH₄⁺ in soils before sowing and after harvesting, respectively, and a 4–63% and 10–34% increase of N-NO₃⁻ in soils before sowing and after harvesting, respectively), which resulted in an increase in grain weight, reflecting yield and grain quality (from 2% to 12% higher grain weight compared to the control). The applied EOMs were characterized by readily transforming forms of organic nitrogen into N-NH₄⁺ and further increasing the speed of its conversion into N-NO₃⁻, indicating the capacity of these treatments to act as substitutes for synthetic nitrogen fertilizers.

1. Introduction

Soil organic matter (SOM), which is the arguably the most complex and least understood aspect of soil, determines its physical, chemical, and biological properties, as well as its functions in the environment [1]. SOM improves soil fertility, increases crop yields, and positively affects food production, water storage, the retention of xenobiotics, the development of soil microorganisms, and nutrient cycling [1,2,3]. However, the SOM concentration in agricultural soils is frequently decreased due to management practices and/or environmental conditions [2]. Generally, SOM dynamics depend on the balance between the soil’s inputs and outputs [4,5,6]. Numerous studies have reported that organic inputs, reduced tillage, increased rotation intensity, and balanced fertilization may increase soil organic carbon sequestration [3,7,8,9,10]. To increase organic matter accumulation or prevent its loss in agricultural soils, additional inputs of organic materials should be applied. Organic amendments of soil have been suggested as viable measures for increasing organic carbon amounts via the physicochemical protection of SOM through processes of organomineral complexation [4,11]. These materials, referred as exogenous organic matter (EOM), can be derived from various sources and often arise as byproducts of various processes, such as processes of the agricultural industry and intensive livestock farming. These materials include seaweed, bone meal, biochar, humic substances, compost, digestate, fly ash, and others [4,5,6,9,10,12]. Organic matter derived from these materials and processes is characterized by diverse properties (decomposition rate, availability of nutrients, and sorption properties) due to the different formation processes and the diversity of the feedstock materials. Therefore, the persistence in and impact of these materials on the soil environment are also diverse [3,4,8,9,10,12]. The high interest in applying organic amendments to agricultural soils as partial or full substitutes for inorganic fertilizers is stimulated by a desire to manage nutrients more cost-effectively [12]. Quilty and Cattle [12] noted that the widespread introduction of synthetic inputs in developed countries during the 21st century limited the use of various forms of EOM to maintain proper soil fertility and crop yields. Despite the descriptive research on the possible effects of various EOM applications on soil properties and crop quality [4,6,7,8,9,10,11,13,14], there has been little scientific investigation into the utility of these products in broadacre farming systems. The transformations towards a low-carbon economy and the decrease in availability of macro- and micronutrient resources [15] have increased the importance of recycling nutrients originating from agricultural, industrial, and municipal waste for agronomic purposes. In addition, recent EU strategies seek to reduce nutrient losses in agriculture and the use of synthetic fertilizers by at least 50% by 2030, while ensuring no deterioration in soil fertility [16]. For this reason, EOMs may become increasingly prominent soil and crop husbandry inputs, especially EOMs with multiple nutritional functions in soils and crops.

One of the essential nutrients for plant growth, development, and reproduction is nitrogen. Despite being one of the most abundant elements, nitrogen deficiency is one of the most common nutritional problems affecting plants worldwide [9,17,18,19,20]. Nitrogen plays an important role in the processes of protein, nucleotide, nucleic acid, alkaloid, and chlorophyll synthesis and is a key factor in the photosynthesis cycle [17,19]. Moreover, nitrogen stimulates the growth of the aboveground parts of plants via its regulatory functions in use of potassium and phosphorus and the assimilation of other nutrients. The majority of soil nitrogen (ca. 99%) occurs in complexes of organic nitrogen compounds, mainly in the organic matter fractions [21]. However, these forms of nitrogen may not be entirely usable by plants. Mineral forms of nitrogen constitute only 1–5% of the total nitrogen concentration in the soil [21,22]. These forms mainly include N-NH₄⁺ and N-NO₃⁻, which are forms the most easily assimilated by plants [9,20]. The organic nitrogen accumulated in the soil undergoes complex and dynamic transformations. The transformation of organic nitrogenous compounds in the soil is related to potentially mineralizable nitrogen or the maximum amount of inorganic nitrogen that can be formed during EOM decomposition [11,18,21]. This involves mineralization and immobilization processes related to the ratio of carbon to nitrogen (C/N) in the organic matter and also to the type of organic amendment [18,23]. The rate of mineralization strictly depends on soil and environmental conditions (moisture content, temperature, amount of oxygen, and pH) and the quality of the EOM [11,23]. The addition of EOMs enhances hydrolyzable and nonhydrolyzable N contents in soil. Chakraborty et al. [24] established that the total hydrolyzable N status declines with the application of inorganic N fertilizer alone and increases with the conjunctive use of inorganic N fertilizer and manure. The application of EOMs at different stages of decomposability not only affects the patterns of N availability, but also influences the distribution of the N-NH₄^- and N-NO₃⁺ forms in soils [25]. The main differences in the efficiency of nitrogen input into the soil via different EOMs are due to the total C and N contents in the EOMs. Nevertheless, the transformation of organic and inorganic nitrogen in soil is mainly governed by the physicochemical and biological properties of the soil. As a result of the impact of soil properties on the N-conversion processes, especially nitrification, mineral forms of nitrogen are formed and released into the soil in the form of N-NO₃⁻ [18,22,23]. N-NO₃⁻ and N-NH₄⁺ are directly available to plants but can also be leached from the soil to the groundwater or emitted to the atmosphere in the form of N₂, NO, or N₂O [17,21,26].

The management of nutrients derived from all available sources is noted to be the one of the main objectives of sustainable agriculture [27], along with the protection of water against excessive nitrogen inflow from mineral fertilizers applied to soil, in the EU Nitrates Directive (ND; 91/676/EEC). To this end, along with the deficiency of organic matter content in soils, appropriate sources of fertilizer should be sought to adequately fulfill the needs of agriculture. Furthermore, although numerous scientific studies have independently discussed the issue of the impact of mineral or organic fertilization on nitrogen content in soils, there is a lack of key information on the potential synergistic effects of these fertilizers. Therefore, the aim of our research was to assess the impact of EOM additions (with and without mineral fertilization) to soil on crop characteristics, and the dynamics of mineral nitrogen release under the applied soil amendments.

2. Materials and Methods

2.1. Characteristics of Soils and Exogenous Organic Matter (EOM) Amendments

The soils for the study were collected from three locations in the Poland–Czech Republic transboundary area, including soils that developed from different parent rock materials and in different agroecological habitats. The soils sampled from Dlouha Ves (silty clay loam; Site 1 soil), Nowa Wieś (sandy loam; Site 2 soil), and Pastuchów (silt loam; Site 3 soil) (Figure 1) were characterized by diverse physicochemical properties of their dry matter (

Table 1. Selected physical and chemical properties of the soil materials used in the experiment.

Soil Origin	Description	Particle Size Distribution			pH	TC	TOC	TN
		2.0–0.05 mm	0.05–0.002 mm	<0.002 mm
Dlouha Ves	Site 1 soil	10	58	32	7.00	19.3	11.9	1.72
Nowa Wieś	Site 2 soil	70	28	2	5.78	7.7	4.5	0.70
Pastuchów	Site 3 soil	27	67	6	6.88	11.4	7.8	1.14

2.0–0.05 mm: sand fraction (%), 0.05–0.002 mm: silt fraction (%), <0.002 mm: clay fraction (%), TC: total carbon (g kg⁻¹), TOC: total organic carbon (g kg⁻¹), TN: total nitrogen (g kg⁻¹).

).

The exogenous organic matter (EOM) treatments applied in the experiment included three types of soil amendment (animal meal, industrial compost, and digestate) characterized by different properties, processes of production, and substrates, which were hypothesized to have an impact on plant growth and development:

• Animal meal—organic fertilizer produced from animal waste according to Regulation (EC) No 1069/2009 of the European Parliament and the Council of Europe. Registered as an organic fertilizer under number MBN-NP-7–6 (70% organic matter content, 90% dry matter) with a high content of nitrogen, phosphorus, and macro-elements.
• Industrial compost—organic fertilizer obtained from municipal biodegradable waste, e.g., grass, leaves, sawdust, and weeds (including municipal sewage sludge) (organic matter content 23%, dry matter 60%).
• Digestate—obtained from the largest producer of French fries (McCain’s Sp. z o.o. in Strzelin) and made from potato starch, utilizing materials from the potato industry. The batch included potato peelings and residues from the production of French fries, all of them fermented and then dewatered in a biogas plant (organic matter content 69%, dry matter content 14%).

Each of the applied EOM amendments was analyzed for the content of the most prominent macro-elements (

Table 2. Chemical composition of the applied EOMs.

Type of EOM	C	N	C/N	Mg	Na	K	Ca	P
	g kg−1			mg kg−1
Animal meal	401	84	4.8	2110	7911	5717	84,208	64,231
Industrial compost	179	23	7.8	4557	407	10,605	10,061	7535
Digestate	407	69	5.9	5054	3708	17,681	12,338	27,905

Mg: magnesium, Na: sodium, K: potassium, Ca: calcium, P: phosphorus.

) that constitute the main enzyme cofactors of many catalytic reactions that occur in the soil environment. The concentrations of magnesium (Mg), sodium (Na), potassium (K), calcium (Ca), and phosphorus (P) were used to determine the level of other plant nutrients.

2.2. Description of the Pot Experiment

The efficiency of the selected EOM amendments was tested in two doses at 50% and 100% nitrogen ratios (equivalent to 70 and 140 kg ha⁻¹, respectively). In the control samples, mineral fertilization only was applied in the form of a calcium nitrate (Ca(NO₃)₂) solution at a concentration equivalent to 140 kg ha⁻¹. A scheme featuring the combinations of soil fertilizers used in this experiment is presented in

Table 3. Characteristics of combinations of soil additives used in the experiment.

Soil Additive	Organic Fertilization: EOM	Mineral Fertilization: Ca(NO3)2
Control	0 N ratio	100% N ratio
Animal meal	50% N ratio	50% N ratio
Animal meal	100% N ratio	0 N ratio
Industrial compost	50% N ratio	50% N ratio
Industrial compost	100% N ratio	0 N ratio
Digestate	50% N ratio	50% N ratio
Digestate	100% N ratio	0 N ratio

. The experiment was carried out in triplicate for each combination in 3 L pots with holes at the bottom enabling drainage. Before the experiment, bulk soils were sieved through a 4 mm sieve. A 3.5 kg quantity of moist soils was weighed and thoroughly mixed with soil amendments prior to transferring to the soils to pots. Soil moisture was then brought to a level equivalent to a 60% field water-holding capacity (FWHC, determined using Richard´s chambers) in each pot. The study was run in a greenhouse under controlled conditions (supplemental light, 27/20 °C day/night temperatures, 16/8 h day/night length).

After mixing the soils with EOMs and fertilizers, the pots were left in the greenhouse for 4 weeks to enable the reaction of soil amendments with the soil. After this period, a spring wheat (the Tybalt cultivar) was seeded into the pots at 15 seeds per pot. This cultivar is the highest-yielding spring wheat cultivar, distinguished by its exceptional adaptation to different cultivation conditions and agrotechnical levels. At the two-leaf stage, 10 plants were left in each pot (after plant thinning). The soil moisture content was maintained at a level of 60% FWHC through regular weighing of the pots and addition of deionized water as needed until harvest. Plants were harvested at full maturity, and the obtained biomass yield was separated into grain and straw. The biomasses of the yield and grains (determined as the thousand grain weight; TGW), as well as the proportion of grain in the yield, were determined for the collected plant material (all parameters were determined in dry matter). Homogenous soil samples were collected 4 weeks after soil mixing with EOMs (right before plant seeding) and after wheat harvest (4 months after seeding) to analyze the available forms of ammonium nitrogen (N-NH₄⁺) and nitrate nitrogen (N-NO₃⁻) and thereby assess the effect of the applied EOMs on the dynamics of N transformation in the soils.

2.3. Determination of the Physicochemical Properties of Soils and EOMs

The measured basic physicochemical soil properties included the following: pH in 1 mol KCl, clay (particle size < 0.002 mm), silt (fraction 0.002–0.05 mm), sand (particle size 0.05–2.0 mm), total carbon (TC), and total organic carbon (TOC) contents (determined in dry matter). The pH was measured potentiometrically in a 1:2.5 (m V⁻¹) soil suspension in 1 mol L⁻¹ KCl solution (PNISO10390, 1997). The clay, silt, and sand contents were analyzed via the aerometric method (PN R-04032, 1998), while the TC and total nitrogen (TN) contents in soils and EOMs were determined using a Vario Macro Cube CN elemental analyzer (Elementar Analysensysteme GmbH, Langenselbold, Germany) after dry combustion. The TOC content was determined after sulfochromic oxidation, followed by titration of excess K₂Cr₂O₇ with FeSO₄(NH₄)₂SO₄∙6H₂O (PN-ISO 14235, 2003).

2.4. Determination of the Ammonium Nitrogen (N-NH₄⁺) and Nitrate Nitrogen (N-NO₃⁻) Concentrations

The analyses of N-NH₄⁺ and N-NO₃⁻ were performed via 24 h soil extraction (soil to liquid ratio = 1:10) with 1% potassium sulfate (K₂SO₄). The extracts were analyzed using flow colorimetry with QuAAtro39 Seal Analytical equipment. The method was based on continuous segmented flow analysis and microflow hydraulics with high sampling rates. The procedure for the determination of nitrate was based on the ISO standard method (ISO 13395:1996), involving the nitrate’s reduction to nitrite at pH = 7.5 in a copperized cadmium reduction coil. This reaction occurs under acidic conditions with sulfanilamide to form a diazo compound, which then couples with N-1-naphthylethylenediamine dihydrochloride to form a reddish-purple color that is measured at 520–560 nm. The procedure for the determination of ammonium was based on the ISO standard method (ISO 11732:2005). The sample was reacted with salicylate and dichloroisocyanuric acid to produce a blue compound measured at 660 nm in the presence of nitroprusside as a catalyst. To the control the quality of the analysis results, a Reagecon (no. IC-GLO-7-100 and no. ICA-TG-45) was used. The limit of quantification (LOQ) was 0.5 mg kg⁻¹, while the repeatability of the results was ensured with an accuracy of 5%.

2.5. Statistics

The obtained data were evaluated using the STATISTICA program (StatSoft Ver. 13.1). The differences between groups of variables were tested via a parametric comparison using ANOVA and supported by Tukey’s test. The significant differences were analyzed at the level of significance of p < 0.05 and are appropriately marked herein with letters (lowercase letters in the figures indicate differences between the EOM amendments’ impacts in individual soils, while the uppercase letters in the tables indicate differences between the soils with the same EOM amendments). The results were assessed based on the mean values of a minimum of three replicates. The outliers were discarded based on a chi-squared test to avoid interference with the interpretation of the results.

3. Results and Discussion

3.1. The Effect of Exogenous Organic Matter (EOM) on the Mineral Nitrogen (N-NH₄⁺ and N-NO₃⁻) Content in Soils

The total nitrogen contents in the EOMs applied to the soils were 83.6 g kg⁻¹ for animal meal, 11.5 g kg⁻¹ for industrial compost, and 69.0 g kg⁻¹ for digestate (Table 2). Generally, the content of N-NH₄⁺ depended on the individual soils’ properties. The N-NH₄⁺ exhibited significantly diversified concentrations (CoV = 36%) in the Site 2 soil (

Table 4. Effect of soil properties on ammonium nitrogen (N-NH₄⁺) content.

	Control	Animal Meal 50%	Industrial Compost 50%	Digestate 50%	Animal Meal 100%	Industrial Compost 100%	Digestate 100%
	Soil before sowing (mg kg−1)
Site 1 soil	<0.5 A	<0.5 A	<0.5 A	<0.5 A	<0.5 A	<0.5 A	<0.5 A
Site 2 soil	2.3 B	3.2 B	3.0 B	2.7 B	1.5 B	1.2 B	1.8 B
Site 3 soil	<0.5 A	<0.5 A	<0.5 A	<0.5 A	<0.5 A	<0.5 A	<0.5 A
	Soil after harvesting (mg kg−1)
Site 1 soil	<0.5 A	<0.5 A	<0.5 A	<0.5 A	<0.5 A	<0.5 A	<0.5 A
Site 2 soil	4.1 B	4.0 B	4.5 B	4.9 B	4.7 B	5.4 B	3.9 B
Site 3 soil	<0.5 A	<0.5 A	<0.5 A	<0.5 A	<0.5 A	<0.5 A	<0.5 A

Uppercase letters indicate significant differences between soils based on Tukey’s test. Means marked with the same letter did not differ significantly between soils (p < 0.05; n = 4).

, Figure 2a,b), while in the other cases (Site 1 soil and Site 3 soil) it remained below 0.5 mg kg⁻¹ (<LOQ) both before sowing and after harvesting. The N-NH₄⁺ concentration in the Site 2 soil (sandy loam) was diversified and significantly higher after harvesting (39–5.4 mg kg⁻¹) than in soils before plant sowing (1.2–3.2 mg kg⁻¹) (Table 4). Moreover, before sowing, no significant differences were observed between the EOMs, but each EOM’s impact on the N-NH₄⁺ content was different from the levels observed in the control soil (2.3 mg kg⁻¹) treated only with mineral nitrogen (Figure 2a). Animal meal and industrial compost at ratios of 100% caused a 34% and 49% decrease in the content of available N-NH₄⁺, respectively, while animal meal in combination with mineral fertilization increased the content of nitrogen by 38%. The concentration of N-NH₄⁺ in soils after harvesting was relatively comparable (CoV = 17%) regardless of the amendments used (Figure 2b). This suggests that the release of N-NH₄⁺ from EOMs is plant-and time-driven. N being delivered entirely via EOMs resulted in low ammonia availability for a short period after soil fertilization, regardless of the type of EOM. During plant growth, the differences between the soils fertilized with calcium nitrate and those fertilized with EOMs practically vanished due to the increasing release of ammonia from the EOMs.

The limited availability of N-NH₄⁺ in the soils before sowing most likely resulted from their partial binding to the soil sorption complexes, especially those of clay minerals [20]. Soil minerals exert significant direct and indirect influences on the supply and availability of nitrogen. The main processes involved in the release and fixation of nitrogen include adsorption–desorption processes. Adsorption reactions involving minerals are often more important in controlling nitrogen availability than the release of the element itself. Jilling et al. [28] stated that the variable charges of clay minerals with low cation-exchange capacity and low crystallinity (e.g., Fe oxides) give such minerals a very high affinity to nitrogen under acid pH soil conditions, which may limit the nitrogen’s availability to plants. Kothawala and Moore [29] observed that different forest soils (including Podzols, Brunisols, Luvisols, Gleysols, and organic soil) from Canada showed different degrees of dissolved nitrogen adsorption (N-NH₄⁺ and N-NO₃) due to differences in their mineralogical compositions. The nitrogen adsorbed by clay minerals in soil is a major contributing factor to soils’ N depletion and losses, especially in subsoil horizons, where the types of minerals favor N retention.

Moreover, low nitrogen availability could be caused by leaching or gaseous losses to the atmosphere through denitrification. Brady and Weil [30] suggested that understanding the nitrogen cycle can provide insight into plant–nutrient relationships, as well as nutrient-management decisions.

The low ammonia contents of the soils in our study before wheat seeding might indicate a relatively slow mineralization of organic N shortly after mixing EOMs with soils connected to an apparently intensive nitrification process, especially in the soils with a neutral pH (Site 1 and Site 3 soils) observed in our study.

Chang et al. [20] and Chen et al. [9] noted that N-NO₃⁻ ions are more labile than N-HH₄⁺ ions. Therefore, when assessing the environmental effects of fertilization, special attention should be paid to the nitrate form of nitrogen. The present experiment indicated that nitrogen transformations in soil occur quickly and that their N-NO₃⁻ forms are much more accessible than their N-NH₄⁺ ones. The obtained results indicate that the main differentiating factors are related to soil properties (

Table 5. Effect of soil properties on nitrate nitrogen (N-NO₃⁻) content.

	Control	Animal Meal 50%	Industrial Compost 50%	Digestate 50%	Animal Meal 100%	Industrial Compost 100%	Digestate 100%
	Soil before sowing (mg kg−1)
Site 1 soil	144.0 A	88.3 B	92.2 B	110.5 A	89.8 C	69.6 A	114.8 A
Site 2 soil	87.3 B	96.4 B	162.7 A	97.8 A	122.7 B	61.7 A	115.7 A
Site 3 soil	104.1 B	129.9 A	89.4 B	128.6 A	139.7 A	64.3 A	113.0 A
	Soil after harvesting (mg kg−1)
Site 1 soil	23.9 A	23.7 A	19.6 A	22.6 A	25.0 A	21.4 A	25.1 A
Site 2 soil	4.0 B	4.2 B	6.2 B	1.9 B	3.2 B	4.9 C	6.4 B
Site 3 soil	16.2 A	17.2 A	15.9 A	15.3 A	21.5 A	15.1 B	26.5 A

Uppercase letters indicate significant differences between soils based on Tukey’s test. Means marked with the same letter did not differ significantly (p < 0.05).

, Figure 3).

The amount of available N-NO₃⁻ was mainly dependent on the presence of plants and was several times higher in soils before sowing than after harvesting. The average concentration of N-NO₃⁻ before seeding in the control soils varied significantly: 87.3 mg kg⁻¹ < 104.3 mg kg⁻¹ < 144.0 mg kg⁻¹, respectively, for Site 2 soil < Site 3 soil < Site 1 soil. The same trend was observed for the control soils after harvesting (4.0 mg kg⁻¹ < 16.2 mg kg⁻¹ < 23.9 mg kg⁻¹). Thus, in all cases, the highest content of available N-NO₃⁻ forms was recorded in the clay-rich soils. This confirms that soil properties have a significant impact on nitrogen content, which was also supported by the differences between soils with the same combination of applied fertilizers (Figure 3a). Generally, the soils with applied mixed fertilization (50% organic + 50% mineral fertilization) exhibited the greatest variety, especially with the contribution of animal meal and industrial compost, but without consistency for the soil with the highest N-NO₃⁻ content. Animal meal significantly reduced the availability of N-NO₃⁻ in Site 1 soil and Site 2 soil compared to Site 3 soil, while the industrial compost application induced the highest N-NO₃⁻ availability in the Site 2 soil. Moreover, the animal meal at a 100% application rate also yielded higher N-NO₃⁻ availability in Site 3 soil. Under digestate application, the differences between soils were not significant at either 50% or 100% fertilization ratios. Additionally, the influence of industrial compost at a 100% ratio on N-NO₃⁻ content was comparable across all soils, leading to the most homogeneous results.

The solid forms of N derived from the applied EOMs constituted an unavailable form to plants until they were processed by enzymatic cleavage into smaller soluble units. The dissolved organic nitrogen pool may have been larger after the addition of some EOMs due to the high amounts of N forms that neither plants nor microorganisms can utilize. Moreover, current evidence suggests [9,20] that most dissolved organic nitrogen in soil solutions has a high-molecular-weight recalcitrant nature, while roots possess only the capacity to take up low-molecular-weight nitrogen forms (e.g., urea, amino acids, polyamines, and small polypeptides).

The effects of soil treatment on N-NO₃⁻ availability after plant harvest were slightly different (Figure 3b). The ability of a plant to capture nitrogen from the soil depends on the soil type, environment, and plant species. The use of nitrogen by plants involves several steps, including uptake, assimilation, translocation, and, when the plant is ageing, recycling and remobilization. For crops, the nitrogen use efficiency factor is defined as the grain yield per unit of nitrogen available from the soil, including nitrogen fertilizer. In our study, the content of nitrates sharply decreased during plant growth in all soils, regardless of the fertilization regime (Table 5). Similarly to the N-NH₄⁺ results, the differences in the concentration of N-NO₃⁻ stimulated by various fertilizers were much less visible than those induced by the type of soil. None of the soils with applied combinations of fertilizers were statistically different from the control after harvest in any analyzed soils. This means that during one vegetation period, nitrogen was sufficiently released from the tested organic soil amendments to feed the wheat plants. The influence of soil type (Site 1 soil vs. Site 2 soil vs. Site 3 soil) was observed in the case of industrial compost fertilization at a 100% ratio and digestate at a 50% ratio. The observed decrease in N-NO₃⁻ content after the growing season was the highest in the Site 2 soil, with the lowest pH, averaging 95%, while in the Site 3 and Site 1 soils, it was 82% and 76%, respectively. The lowest N-NO₃⁻ content in the Site 2 soil can be attributed to the most intensive N-NO₃⁻ acquisition by wheat and the apparently less intensive transformation of N-NH₄⁺ to N-NO₃⁻ through a nitrification process at the lowest pH. The latter hypothesis is supported by the highest ammonia content being present in the Site 2 soil. Nagele and Conrad [31] observed that the release of nitrogen increased after fertilization and strongly decreased with the increasing pH of acidic soil adjusted to 6.5. The authors suggested that soil pH controls—for example—microbial populations or enzyme activities involved in nitrification and denitrification. Neina [32] stated that mineralization processes occur most intensively in soils with pH values between 6.5 and 8.

Compared to an N dose entirely from mineral fertilization, much lower contents of N-NO₃⁻ were recorded shortly after soil fertilization for all EOM variants in the Site 1 soil, which had the finest texture (Figure 3a). This can be interpreted as a positive effect of mineral N substitution by EOMs due to the reduced risk of nitrate leaching from the soil. No such general effects were observed in coarser soils (Site 2 soil and Site 3 soil); however, there was much less intensive release of N-NO₃⁻ into the soil solution from the industrial compost at a 100% ratio than in the case of the controls. Conversely, animal meal induced a greater short-term release of N-NO₃⁻ than was observed for 100% mineral fertilization (Figure 3a).

The present study showed that the concentrations of N-NO₃⁻ and N-NH₄⁺ in soil after harvest and before sowing differ significantly. This difference was likely caused by the direct use of nitrogen derived from the organic matter mineralization by wheat. The transformation and availability of mineral nitrogen depends on the organic matter mineralization processes that constitute the main source and pathways of nutrient uptake by plants [9,11,20,33,34]. Organic matter also helps to retain nutrient cations at exchange sites, promotes soil structure, and improves the water-holding capacity, among other potential agronomic benefits [1]. Thus, EOM amendments also improve many other soil physicochemical properties, significantly benefiting the soil and its environment [1,10]. Nevertheless, these amendments can differ in organic matter composition (e.g., C:N), thereby affecting the rate of mineralization and susceptibility to microbial decomposition [9,20]. The organic amendments selected for our study differed in the quality of the organic matter, as expressed by the carbon to nitrogen ratio. The highest C:N parameter (C:N = 7.8) was noted for the industrial compost, while the lowest values were found for the digestate (C:N = 5.9) and animal meal (C:N = 4.8). These relations reflect only the availability of N-NO₃⁻, especially in the Site 2 soil before sowing. Janssen [35] indicated that the mineralization of organic amendments is primarily related to the microbial conversion of the organic matter. Thus, some of the nitrogen that is present in the converted organic material is used by microorganisms and some is released (mineralized) as inorganic nitrogen. If the converted organic matter is characterized by a low nitrogen content (high C:N ratio), the amount of nitrogen that can be converted may be too low and nitrogen deficiencies may occur among microorganisms, resulting in the secondary uptake of inorganic nitrogen forms and their immobilization, resulting in lower N availability to plants.

Generally, the ability of microorganisms to effectively ensure the transformation of organic compounds (the decomposition, mineralization, and immobilization of nutrients) is dependent on their quick reactions to the substrate input. Soil microorganisms, as well as plants, produce enzymes and other secretions that contribute to the overall biological activity, as these microorganisms are closely involved in the catalytic reactions essential for the decomposition of organic matter, mineralization, and nutrient uptake [11,20,36]. Therefore, significant differences in the concentrations of particular nitrogen forms were observed between samples collected before seeding and after harvest (Figure 2 and Figure 3). The ammonia content increased during plant growth, whereas the N-NO₃⁻ concentration sharply decreased.

According to García-Ruiz et al. [37], the activity of hydrolytic soil enzymes significantly increases under the regular application of organic fertilizers. Therefore, soil microorganisms interact with plants and affect the contents of available nutrients and their cycling. Allison and Vitousek [34], Bila et al. [36], Franco-Otero et al. [38], and Cayuela et al. [39] analyzed the biomasses of microorganisms and their enzymatic activities in soils amended with digestate, animal meal, and industrial compost and noted that soil microorganisms showed varying degrees of sensitivity to the application of EOM. Enzyme activity levels (cellulase, acidic and alkaline phosphatases) were generally stimulated by the highest doses of EOM, specifically C. Microbial biomass is not only an agent of nutrient mineralization but also represents an important labile pool of other essential plant nutrients, of which the activity in soil controls excessive losses of nitrogen [20,23,40].

Nitrogen supplied to the soil by different types of EOMs occurs mainly in organic complexes, such as N-NH₄⁺. During organic matter decomposition, N-NH₄⁺ is released into the soil and converted into N-NO₃⁻ by the nitrification processes caused by different groups of microorganisms [11,33]. Ammonium ions not immobilized or taken up by higher plants are usually converted rapidly to NO₃⁻ ions. This is a two-step process during which bacteria (Nitrosomonas) convert NH₄⁺ to nitrite (NO₂⁻), and then other bacteria (Nitrobacter) convert the NO₂⁻ to NO₃⁻ [20,33]. This process requires well-aerated soil and occurs rapidly enough that one usually finds mostly NO₃⁻, rather than NH₄⁺, in soils during the growing season [33,37]. Thus, significantly higher amounts of N-NO₃⁻ than N-NH₄⁺ were noted in our investigation (in both cases, both before and after plant harvest). The rapid transformation of organic nitrogen from EOMs into N-NH₄⁺ and its fast conversion into N-NO₃⁻ indicates the potential of EOMs to be effective nitrogen fertilizers, as well as their capacity to substantially replace synthetic N fertilizers. Nitrogen use efficiency is one of the most important factors to consider when evaluating the agronomical value of an organic amendment [11].

Despite the many benefits related to nitrogen fertilization, the nitrogen cycle includes several routes through which plant-available nitrogen can be lost from the soil. N-NO₃⁻ is usually more subject to loss than N-NH₄⁺. Significant loss pathways include leaching, denitrification, volatilization, and crop removal. Vos and Mackerron [26] showed that nitrogen losses outside the growing season are more dangerous for the environment, as such losses leach deep into the soil profile and pose a risk to groundwater. Nitrogen leaching is more significant in coarse soils and depends mainly on the amount of nitrogen in the applied fertilizers (mainly mineral), as well as the prevailing weather conditions, such as precipitation [17,21,26,33]. Hence, the benefits of using EOMs related to the benefits of supplying organic matter are additionally strengthened by the slower release of N-NO₃⁻ from organic materials than from mineral fertilizers.

3.2. The Effect of Exogenous Organic Matter (EOM) on the Wheat Biomass Yield

Available forms of nitrogen, regardless of their origin and form, are fundamental in shaping the volume and quality of crop harvests. The present experiment showed that the yield of wheat (straw and grain) grown in the studied soils significantly depended on the EOM additives and mineral fertilization applied (Figure 4). In some cases, EOM amendments reduced the wheat yield. Industrial compost (with a 50% and 100% nitrogen ratio) was the most restrictive for plant biomass development. This compost caused decreases in the yield of 27%, 33%, and 43% when N was applied entirely as EOM, and of 12%, 8%, and 25% for a 50% ratio of N as EOM, respectively, for Site 1 soil, Site 2 soil, and Site 3 soil. This effect might have been related to the slower rate of N release from industrial compost, as observed in the extractability of N-NO₃⁻. The addition of animal meal or digestate negatively or positively affected the plant growth, depending on the type of soil. Site 1 soil exhibited the most positive wheat response to EOM addition, but the differences in the obtained yield were not significant compared to the controls. In contrast to Site 1 and Site 2 soils, significant negative effects of EOM addition using animal meal and digestate were observed in the Site 3 soil (at both 100% and 50% doses). In Site 2 soils, the yields did not statistically differ from those observed for the control treatments.

The obtained total yields of wheat grain largely reflected the total biomass results (r = 0.92, p < 0.05). The thousand grain weight (TGW) is the basic parameter characterizing the quality of grain yields. TGW depends mainly on meteorological conditions, the occurrence of diseases, macro-element fertilization, and the physicochemical properties of soil. In the present experiment, optimal soil moisture and plant health were ensured, so the only factor affecting TGW was the availability of fertilizer components. The calculated average TGW parameter was the highest for Site 1 soil and slightly lower for Site 2 and Site 3 soils (35.61 g, 39.43 g, and 30.61 g, respectively) (

Table 6. Impact of fertilization regime on grain quality and contribution of grain to total above-ground plant biomass.

Soil	EOM	TGW (g)	Proportion of Grains in the Yield (%)
Site 1 soil	Control	37.13 d	54
Site 1 soil	Animal meal 50%	33.13 abcd	46
Site 1 soil	Industrial compost 50%	37.75 d	43
Site 1 soil	Digestate 50%	36.88 d	46
Site 1 soil	Animal meal 100%	36.38 cd	46
Site 1 soil	Industrial compost 100%	36.63 bcd	44
Site 1 soil	Digestate 100%	34.50 d	53
Site 2 soil	Control	28.50 a	45
Site 2 soil	Animal meal 50%	29.13 ab	43
Site 2 soil	Industrial compost 50%	32.13 abcd	51
Site 2 soil	Digestate 50%	29.00 ab	46
Site 2 soil	Animal meal 100%	28.38 a	49
Site 2 soil	Industrial compost 100%	30.25 ab	50
Site 2 soil	Digestate 100%	28.63 ab	43
Site 3 soil	Control	31.88 abcd	46
Site 3 soil	Animal meal 50%	30.63 abc	45
Site 3 soil	Industrial compost 50%	28.75 ab	41
Site 3 soil	Digestate 50%	29.63 ab	40
Site 3 soil	Animal meal 100%	29.88 ab	50
Site 3 soil	Industrial compost 100%	30.50 abc	44
Site 3 soil	Digestate 100%	33.00 abcd	47

TGW: thousand grain weight, EOM: exogenous organic materials; lowercase letters indicate significant differences based on Tukey’s test across all combinations. Means marked with the same letter did not differ significantly (p < 0.05).

). The TGW values did not differ significantly from those of the control soil or with the applied dose of fertilization (a 50% and 100% N ratio), while there was an effect of the interaction between the type of soil and the type of EOM. Animal meal (a 50% N ratio) produced a significantly lower grain weight for wheat in the Side 1 soil, while industrial compost (a 50% N ratio) produced more efficient grain growth in the Site 2 soil.

The proportion of grains in the total yield was comparable for all tested soils (47%, 47%, and 45%, respectively, for Site 1 soil, Site 2 soil, and Site 3 soil). This means that the EOMs did not cause significant changes in the proportions of individual elements of plant growth (straw to grain). Some differences observed were not consistent across the range of soils.

The obtained results suggest a reduced plant availability of nitrogen in soil treated with industrial compost compared to other soil amendments, especially in the early phases of growth, as confirmed by the lower N-NO₃ content in soil before plant seeding. Nitrogen provided as industrial compost resulted in the lowest biomass yield, which may be related to the compost production process and the substrates used for this production [11,34]. Most of the components used for the production of compost, such as leaves, corn stalks, straw, wood chips, and other organic materials, vary greatly in the amount of their available nitrogen forms, as well as the other accompanying substances that affect plant growth and development. The composting process stimulates greater content of N-NH₄⁺ than its N-NO₃⁻ form [34,41,42]. Frequently, not all the nitrogen applied via EOMs is immediately available to crops [11], and residual nitrogen may be released into the soil over several years [41,42]. The amount of plant-available nitrogen is influenced by the nitrogen source, timing of application, soil conditions, and temperature. The first year of available organic nitrogen is estimated at 33%, and N-NH₄⁺ available in the first vegetation period can range from 15% to 75%, depending on the application timing [41,42]. For proper plant cultivation, there is a need to define the forms of plant-available nitrogen derived from EOMs. The added benefits of using EOMs include the provision of exogenous carbon to soils and a reduction in overall synthetic N use in agriculture.

4. Conclusions

In the present experiment, nitrogen applied via the simultaneous application of organic and mineral fertilizer or organic fertilizer alone did not cause a significant decrease in wheat yields compared to the nitrogen applied in a mineral form, except for a slight reduction in yield after a single application of industrial compost. The appropriate selection of rates and fertilizer combinations increased the amount of available nitrogen in the soils, thus conditioning the quality of wheat yields (i.e., mass of the grains). However, this research demonstrated that the extent of these changes depends mainly on the soil’s physicochemical properties (pH, texture, the content of organic matter, and the process of its mineralization). Regardless, the sampling time and added EOM and N-NH₄ content were substantial only in the sandy soil with a slightly acidic pH, where the additions were weakly adsorbed, and the nitrification process was apparently less intensive. Nitrates were, in general, highest in the soil with a rich clay fraction, but their content was also strongly affected by the type of soil amendment.

Overall, the applied EOMs were characterized by a rapid transformation of organic nitrogen into N-NH₄⁺ and its fast conversion into N-NO₃⁻, indicating the capacity of these amendments to partially replace synthetic nitrogen fertilizers as an element of the circular economy. This was supported by the fact that no symptoms of N deficiency or loss of total yield were recorded, except for one EOM source, even in variants where mineral N was fully replaced by EOM sources. The use of industrial compost as an organic fertilizer in intensive cereal production should be supplemented by an appropriate mineral N source. Soil nitrogen mineralization represents one of the most important factors to consider when evaluating the agronomical value of an organic amendment. Considering both proper nitrogen management and the simultaneous enrichment of soils with organic matter, the use of exogenous materials seems to be a very effective alternative to mineral fertilization. To provide a full picture of the efficiency of using N from EOMs and to address the additional benefits of soil enrichment with organic matter, there is a need to expand our data with further studies under field conditions. To maximize the agronomic productivity of crops and minimize the losses of nutrients, more information is needed on the long-term effects of EOMs on increases in soil nutrient fertility for sustainable crop production.