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-NH
4+ depended on the individual soils’ properties. The N-NH
4+ exhibited significantly diversified concentrations (CoV = 36%) in the Site 2 soil (
Table 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-NH
4+ 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-NH
4+ 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-NH
4+, respectively, while animal meal in combination with mineral fertilization increased the content of nitrogen by 38%. The concentration of N-NH
4+ in soils after harvesting was relatively comparable (CoV = 17%) regardless of the amendments used (
Figure 2b). This suggests that the release of N-NH
4+ 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-NH
4+ 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-NH
4+ and N-NO
3) 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-NO
3− ions are more labile than N-HH
4+ 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-NO
3− forms are much more accessible than their N-NH
4+ ones. The obtained results indicate that the main differentiating factors are related to soil properties (
Table 5,
Figure 3).
The amount of available N-NO
3− 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-NO
3− 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-NO
3− 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-NO
3− content. Animal meal significantly reduced the availability of N-NO
3− in Site 1 soil and Site 2 soil compared to Site 3 soil, while the industrial compost application induced the highest N-NO
3− availability in the Site 2 soil. Moreover, the animal meal at a 100% application rate also yielded higher N-NO
3− 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-NO
3− 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-NO
3− 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-NH
4+ results, the differences in the concentration of N-NO
3− 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-NO
3− 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-NO
3− content in the Site 2 soil can be attributed to the most intensive N-NO
3− acquisition by wheat and the apparently less intensive transformation of N-NH
4+ to N-NO
3− 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-NO
3− 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-NO
3− 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-NO
3− than was observed for 100% mineral fertilization (
Figure 3a).
The present study showed that the concentrations of N-NO
3− and N-NH
4+ 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-NO
3−, 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-NO
3− 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-NH
4+. During organic matter decomposition, N-NH
4+ is released into the soil and converted into N-NO
3− 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 NO
3− ions. This is a two-step process during which bacteria (
Nitrosomonas) convert NH
4+ to nitrite (NO
2−), and then other bacteria (
Nitrobacter) convert the NO
2− to NO
3− [
20,
33]. This process requires well-aerated soil and occurs rapidly enough that one usually finds mostly NO
3−, rather than NH
4+, in soils during the growing season [
33,
37]. Thus, significantly higher amounts of N-NO
3− than N-NH
4+ were noted in our investigation (in both cases, both before and after plant harvest). The rapid transformation of organic nitrogen from EOMs into N-NH
4+ and its fast conversion into N-NO
3− 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-NO
3− is usually more subject to loss than N-NH
4+. 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-NO
3− 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-NO
3−. 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). 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-NO
3 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-NH
4+ than its N-NO
3− 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-NH
4+ 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.