Effective Control of the Nitrogen Gap—Higher Yields and Reduced Environmental Risk

1. Nitrogen Gap—Hierarchy of Factors Driving Nitrogen Management

The world’s growing demand for food cannot be met without the consumption of fertilizer nitrogen (N_f). The standard plant production strategy is not effective in terms of yield and environmental risks [1,2]. The key goal of sustainable agriculture intensification is to optimize the efficiency of production measures [3]. This assumption should be based on two pillars. The first is the maximum yield, or rather the maximum attainable yield (Y_attmax), considered in the following descending order: geographical region, field and homogenous field unit [1]. The second pillar is the size of the nitrogen gap (NG), i.e., the amount of N_f not converted into yield during a single growing season [4]. The most important challenge for the farmer during the next step is to recognize, identify and then prioritize the actual hierarchy of factors affecting the NG. The size of the NG depends on two factors:, climatic and soil conditions, considered the basic factors in crop production [5]. The first, fully natural, is a production factor, completely independent of the farmer’s activity. It is of key importance for determining the set of crop plants typical for a precisely defined geographic area. The homogenous climatic area is determined on the basis of the annual and seasonal course of temperatures, rainfall, etc. [6]. The second key factor is related to inherent soil fertility, which is modified by farmer activity, e.g., the application of fertilizers [7].

Determining the actual environmental framework for crop production in a specific field is the only basis for determining the conditions for N management. Therefore, the crop production factors responsible for N efficiency in the soil/crop system must first be divided into necessary and sufficient, i.e., dominant and supporting N activity. The necessary condition results from the hierarchy of nutrients in the processes of plant growth that determine the formation of the yield structure. The growth rate of crop plants depends on the resources of nitrate nitrogen N-NO₃ in the rooted zone of the soil [1,5]. A necessary factor is the N-NO₃ pool in the critical phases of yield formation of the grown crop. The productive efficiency of N (NE) depends not only on these resources, but also the factors directly responsible for the nitrate ion (NO₃⁻) uptake:

$$\mathrm{N}\mathrm{E}=\frac{\mathrm{N}-{\mathrm{N}\mathrm{O}}_3}{\mathrm{C}+\left(\mathrm{P}+\mathrm{M}\mathrm{g}\right)}$$

A sufficient physiological condition is related to the supply of nutrients responsible for the uptake of NO₃⁻ ions. This absolute set of factors includes phosphorus (P) and magnesium (Mg). The former represents metabolic energy (ATP—adenosine triphosphate) required for the transport of NO₃⁻ ions through the plasma membrane and its subsequent transportation and transformation within the plant. ATP transformation processes take place only in the presence of Mg ions (Mg²⁺). This process cannot be carried out without an accompanying cation (C). Potassium (K⁺) is accumulated in crop plants in the highest amounts [8].

In practice, the necessary condition only concerns the size of the N-NO₃ soil pool. Sufficient conditions are factors which are responsible for its size during the growing season of a specific crop. In agricultural practice, the N-NO₃ pool consists of its resources found in the soil at the beginning of the growing season, the N-NH₄ reservoir subjected to nitrification, N-NH₄ released during the growing season from organic N, as well as the applied N fertilizers [1,5]. The condition of sufficient soil fertility refers to the resources of the available nutrient other than N in the plant’s rooting zone that support the N-NO₃ uptake and transformation in the plant [8]. The supply of these nutrients to the plant root determines both the uptake and utilization of inorganic N present in the soil–plant continuum during the growing season. The limited nature of these resources forces the farmer to replenish them periodically. The primary goal of the farmer’s action is to control the yield-forming effects of N. This set of factors/activities includes: (i) selection of used varieties, (ii) crop plant protection, (iii) in-season N_f fertilization (topdressing), (iv) foliar fertilization (macro- and micro-nutrient application) and (v) foliar application of growth stimulants. Through their impact on the action of N in the plant, these factors are responsible for the degree of yield component development, ultimately determining the yield, the efficiency of N use, and thus the N pressure on the environment.

2. Special Issue: General Topics

2.1. High- and Low-Input Strategies for Different Winter Wheat Production Conditions

The direction of wheat grain use depends on its crude protein content. The standardized protein content in high-quality flour is determined by both the wheat variety and the appropriate dose of N_f applied [9]. The study presented in the paper by Klikocka and Szczepaniak [10] clearly showed that the production of winter bread wheat, taking into account both the grain yield and its quality, is profitable providing a high dose of N_f and full fungicide protection. On the fungicide-protected variant, a grain yield of 12.3 t ha⁻¹ was achieved with an optimal N_f of 231 kg N ha⁻¹. At the same time, the protein content increased in accordance with the applied N_f dose, reaching almost 14% (13.7%, treatment with 240 kg N ha⁻¹). The yield of crude protein followed a linear pattern in response to increasing N_f doses. The grain yield in the variant without fungicide protection reached a maximum of 9.5 t ha⁻¹ with an optimal N_f of only 68 kg N ha⁻¹. For this N_f dose, the crude protein content was below 11% (exactly 10.7%), while the standardized requirement for high-quality bread wheat was 12.5% [11].

It is well-documented that bread wheat fertilized with high doses of N_f becomes more susceptible to pathogen attacks, resulting in yield losses ranging from 20% to 30% [12]. In the presented case, production losses concern both grain (−2.8 t ha⁻¹, i.e., 23%) and protein content (−3 p.ps.) [10]. Replacing organic fungicides with inorganic ones, considering the pressure of pathogens on the growth and yield of winter wheat, is a major challenge for both researcher and farmers. The article by Grzebisz et al. [13] is devoted to this issue. This study was based on the assumption that organic fungicides can be substituted by copper and magnesium phosphites (Cu-Phi and Mg-Phi). The yield loss resulting from this exchange was 3.6 (30.5%) and 1.0 (12.5%) t ha⁻¹ of grain during favorable and unfavorable years, respectively. The lack of plant protection with fungicides resulted in a much greater pressure of pathogens. The plant disorder was manifested by a significant decrease in the value of indicators of N management during the grain filling period (BBCH 75) such as SPAD (708 vs. 581; 315 vs. 156) and the content of N in leaves (2.5 vs. 2.2 and 1.5 vs. 1.1 g kg⁻¹) during the grain filling period, which consequently led to a decrease in the thousand-grain weight (51 vs. 42.2 and 41 vs. 39.5 g, respectively, for protected and non-protected winter bread wheat).

The economic analysis clearly showed that a high profitability of wheat production in fertile soil is achieved by using two management strategies [10,11]. The first is high-input and based on high doses of N_f and full fungicide protection. These two studies clearly documented and showed that the production of winter bread wheat focused on high-quality grain is profitable for farmers, provided that it is carried out using integrated production technology, which includes the following:

• High level of soil fertility, high content of soil nutrients supporting N_f use;
• A leafy fore-crop, ensuring both a good supply of nutrients and decreasing the pressure of pathogens;
• Sufficiently high N_f dose to ensure both a high grain yield and high content of crude proteins;
• A sufficiently high level of fungicidal protection, ensuring a high efficiency of the applied N_f.

However, this production strategy is effective under one more key condition: high prices of bread wheat [14]. Alternatively, a low-input winter wheat production strategy assumes relatively small N_f doses and no or strongly reduced fungicide protection. Grain yields, as documented in the presented studies, can be high or even very high under the above-indicated production conditions, but the grain will not meet the quality standards.

2.2. The Principle of Right Timing—The Critical Cereal/Grain Window

Of the four rules of crop plant fertilization, the most important concerns the timing of N_f application (the right time principle) [15]. In the case of cereals, the critical period of yield formation extends from the beginning of the stem elongation to the beginning of the milk phase, which determines grain density (the number of grains per unit area). The beginning of this long period determines the number of fertile shoots (ears), while the end determines the number of grains in the ear. The critical period of ear formation includes the booting and flowering phases, which can be termed the critical cereal/grain window (CC/GW). In the first phase, the growing ear competes with the stem, while in the second, fertile flowers compete with each other for assimilation [16,17]. The basic challenge for farmers during this period is to recognize the plant’s demand for nutrients. This is a necessary step to correct the nutritional status of seed plants through top-dressing or foliar fertilization. CC/GW is also the growing period that allows for a reliable grain yield forecast. The point is to select the appropriate part of the plant in order to make a reliable assessment of its nutritional status, as discussed in the paper by Grzebisz et al. [18]. The grain yields (GYs) of winter wheat obtained in the study were very high, significantly responding to increasing rates of fertilizer nitrogen (N_f), reaching the maximum yield of 11.3 t ha⁻¹ in 2014 (N_f rate of 209 kg N ha⁻¹), 13.7 t ha⁻¹ in 2015 and 8.6 t ha⁻¹ in 2016 (N rate of 240 kg N ha⁻¹, respectively, in 2015 and 2016). The content of crude protein (CP), wet gluten (GL), and wet gluten yield (GLY) increased linearly in accordance with the N_f rates. The most reliable GY prediction, using a simple regression model, i.e., path analysis, was obtained on the basis of the nutrient content in all wheat leaves at the beginning of booting (94%) and at full flowering (95%). The advantage of the booting stage as a diagnostic part of wheat was due to the fact that the accuracy of the GL prediction at this stage, regardless of the plant part, exceeded 99%.

The N content in leaves at the onset of the booting phase increased with the increasing N_f doses, which indicates the state of N unsaturation of the plant. The balanced N status was achieved at the full flowering phase and resulted from the application of N_f at the early booting phase (N dose of 40–80 kg ha⁻¹). This treatment significantly increased the content of GL. This specific N tendency confirms the important role of late-season N application to bread wheat. The threshold N content in wheat leaves at the beginning of the booting phase, associated with the highest grain yield and GL content, assumed above 10 t ha⁻¹, was 2.7% to 3.8%. This study clearly highlighted the important role of nutrients other than N in maintaining N homeostasis during CC/GW. The content of N, K, Fe, Zn and Cu in leaves stabilized starting from the N_f dose of 160 kg N ha⁻¹. Micronutrients, including Zn and Cu, turned out to be crucial for GY, GL and GLY content. The obtained results clearly confirm that the game for the yield and grain quality of winter wheat takes place within the critical cereal/grain window.

2.3. Sulfur as a Nitrogen Use Efficiency Enhancer—The Case of Potatoes

Due to their high nutritional value, potato tubers are an important source of food in many regions of the world.

Unfortunately, yields harvested by farmers under natural rainfall conditions are far from the yield potential of this crop [19]. The quantitative and qualitative response of potato to N, P and K, and the interactions between these nutrients are well documented [20]. However, the use of sulfur (S) in potato is still under study [21]. It is well known that the yield potential of crops cannot be realized when soil available sulfur is deficient, regardless of the soil’s abundance in other nutrients, as documented for many crops [22,23].

The use of S in potato focuses on many issues, but the most important is the increase in tuber yield in response to the nutrient dose. However, the hypothetical yield increase cannot be considered without assessing the impact of S on N_f efficiency. Sulfur doses tested in field experiments range from a dozen to several dozen kg S ha⁻¹ [24]. The development of effective S fertilization systems requires a combination of the S dose and the S carrier in relation to the N_f dose, as discussed in the article by Potarzycki and Wendel [25].

In their study, a dose of 35 kg S ha⁻¹ was applied in two different S forms, i.e., elemental sulfur (acronym S0) and calcium sulfate (CaSO₄ × 2H₂O; CaS) against the background of S control (without the use of S) and N doses from 0 to 150 kg ha⁻¹, increasing every 30 kg ha⁻¹. The use of S, regardless of its chemical form, resulted in a significant, although small (+5.3%), increase in the yield of tubers. The maximum tuber yields were 51.1; 53.7 and 53.9 t ha⁻¹ for NPK, NPK-S0 and NPK-CaS, respectively. The optimal N_f dose was 121, 134 and 106 kg ha⁻¹. Therefore, the unit N_f productivity for these N_f rates was 422 (100%), 401 (95%) and 508 (120%) kg tubers per kg of N_f, respectively. The difference of 28 kg N ha⁻¹ clearly indicates superiority of NPK-CaS over NPK-S0. Theoretically, this difference equals to 14.2 t ha⁻¹ of tubers. Classic N_f efficiency indicators, such as agronomic efficiency, partial factor productivity, physiological efficiency, showed a significant response to the applied dose of S, regardless of the weather in subsequent years of the study. All these three indicators achieved higher values for CaS than for S0. This was strongly manifested for treatments fertilized with N_f doses from 0 to 120 kg N ha⁻¹. Above this N_f dose, the applied elementary sulfur (S⁰) showed a greater influence on the effectiveness of N_f than CaS. One of the most important observations is that nitrogen recovery (R) exceeded 100%, as documented on plots fertilized with N_f from 0 to 60 kg N ha⁻¹. The applied S, as indicated by its unit productivity, was effectively used by potatoes. The nutrient that limited potato yield in the studied case was calcium. For this reason, under the study conditions, calcium sulfate turned out to be the optimal S carrier.

2.4. Soil Nitrogen Nitrification and N₂O Emission

Inorganic nitrogen in the soil plays a dual role. On the one hand, its compounds are a key source of N to crop plants. On the other hand, they are food for microorganisms, and the transformation products can be beneficial for crop production or may pose a threat to the environment [26]. The initial, primary stage of inorganic N transformation into nitrate (NO₃⁻) is the oxidation of ammonia (NH₃) into nitrite (NO₂⁻). This process is carried out by several groups of microorganisms, but ammonia-oxidizing bacteria (AOB) and ammonia-oxidizing archaea (AOA) predominate. Ammonia-oxidizing microorganisms are common in the environment, including soil, in which the AOA population significantly, often many times, exceeds the AOB population [27]. Both microorganism groups are highly sensitive to environmental factors, including soil pH, ammonia concentration, temperature and soil moisture. The prevailing opinion is that in soil, the activity of AOB increases at high ammonia concentrations and AOA at low ammonia concentrations [27].

Vegetables are a group of food of which the production is gradually increasing globally, as family income increases. The highest consumption of vegetables per capita is recorded in East Asia. China takes second place in the world, with first being Croatia [28,29]. Vegetable production requires large inputs, including fertile soil or gardening substrates, water and fertilizers. In China, the most intensive vegetable production, including greenhouse production, is carried out along the middle and lower reaches of the Yangtze River [30]. The nitrogen doses used are large, reaching up to several hundred kg ha⁻¹, which creates a potential threat to the environment through greenhouse gas emissions. According to Zhong et al. [30], in soils where vegetables are grown and fertilized with large doses of nitrogen, AOB plays a greater role in ammonia oxidation than AOA.

Dong et al. [31] confronted the conclusion of Zhong et al. [30], assessing the role of both groups of microorganisms in N₂O emissions in five selected subregions of the middle and lower reaches of the Yangtze River. The authors documented that the ammonia oxidation process is responsible for N₂O production in greenhouse vegetable fields, accounting for 88–97% of the total N₂O emissions. These studies found that AOAs were much more efficient in N₂O emission, accounting for 46–82%, in comparison to AOBs, of which the contribution was approximately 50% of total N₂O emissions. In all soil samples, regardless of the studied site, the abundance of AOA-amoA functional genes was significantly higher compared to the abundance of AOB-amoA ones. The impact of the tested nitrification inhibitors on this soil trait was highly variable depending on the field location. A positive correlation was found between N₂O emissions induced by AOA, and soil pH and the content of N-NH₄ (R² = 0.59; 0.51, respectively). The relationships between N₂O emissions resulting from AOB activity and both soil characteristics were slightly weaker, but only for pH (R² = 0.40; 0.51). These relationships clearly confirm the well-known fact that the key factor determining nitrate reduction is the amount of available substrate [32].

3. Conclusions

The prevailing inertia of the current crop production processes results to a much greater extent from activities focused on environmental threats from N_f rather than maximizing production profits, i.e., the yield being considered a physiological N sink. Maximizing its capacity reduces inorganic N resources in the soil. Therefore, a high N sink capacity (yield and N content) in the currently grown crop is a necessary condition to obtain high N_f productivity, which reduces the dispersion of N compounds into the environment. Therefore, a well-determined N_f dose is a key challenge for farmers. Most crops, except legumes and oilseeds, produce carbohydrates. These so-called starch crops do not require high doses of N_f to produce high yields. Theoretically, these crops should be fertilized in accordance with the Law of the Minimum, ensuring the highest N_f efficiency. Farmers’ efforts, apart from the sink capacity increase, should also be oriented on other activities relating to the N cycle within the soil/crop system. Other growth factors should therefore not be subjected to the rules of the Minimum law. In fact, they should not limit the growth rate and yield of the grown crop plant. Nitrogen, or more precisely, nitrate nitrogen, should be considered the only growth factor. An effective system for controlling the content of nitrates in soil and the emission of nitrogen oxides should primarily focus on the content of inorganic N substrates (organic N, N-NH₄ and N-NO₃) in the soil. At the second stage, this system should focus on identifying and controlling environmental factors that determine the activity of nitrifiers and denitrifiers.