Nitrogen Cycle in Farming Systems

1. Nitrogen Cycle on the Farm: Challenges for the Farmer

The challenge for people currently living on Earth is to develop a food production strategy to cover the food gap and at the same time maintain or even improve the soil use production potential [1]. Meeting these threshold conditions depends on the assumptions of unique activities, i.e., adapting food production strategies to local climatic and soil conditions [2]. The first and absolutely basic limitation determining food production is arable land, or rather, fertile land resources. As is commonly known, arable soil resources are limited. Highly fertile soils are already being exploited. The potential use of soil currently covered with primary virgin forest cannot be considered at all due to its importance for climate balance [3]. Therefore, a key but multi-level requirement for the food production sector to cover the food gap is to make progress in yields. In the first case, this refers to crops from the group of staple foods, such as cereals, legumes, and root and tuber crops [4].

The progress in breeding that has occurred over the last 100 years is currently not sufficiently exploited by farmers. Moreover, the expected increases in yields of major crops from breeding progress, including cereals, maize and soybean, are lower than necessary to cover the future yield gap [5]. At this point, it is necessary to return to the order, or better yet, the hierarchy of factors ruling crop production [6]. Crop variety represents a group of defining factors and is at the forefront of a farmer’s choice. The effective use of a given variety on a farm depends largely on the management of N resources, both mineral fertilizers and their recycled by-products. A farmer should consider internal N turnover or its recycling in several mutually complementary areas of required activity on a farm, as outlined below:

(1)

The optimization of the use of N fertilizer (N_f), including nutrients supporting N;

(2)

Nitrogen use efficiency (NUE);

(3)

Residual nitrogen left in the soil after harvesting non-legume plants;

(4)

Residual nitrogen left in the soil after harvesting legumes;

(5)

Harvest residues as direct sources of energy for soil microorganisms:

(6)

Recycled crop residues as energy sources for microorganisms:

• Farmyard manure fermentation;
• Biogas.

The implementation of the above-mentioned areas of farmer activity is essential to reduce N loss, regardless of its chemical form, to the environment.

2. Special Issue: General Topics

2.1. Nitrogen Hot Spots on the Farm: Supporting Nutrients

The first and most critical factor affecting N productivity, regardless of the plant, is the critical period of its growth, which is related to the development of its yield components. The farmer, without such a recognition, is unable to determine the right rate and time of N_f application [7]. However, these treatments are not enough to optimize a plant’s N economy during the growing season. The identification of so-called hot spots in N management on a farm, including cropland, was the focus of the first two articles of this Special Issue [8,9]. In these articles, the authors discuss four main aspects of N management on a farm.

The uptake of N by a plant root depends on the concentration of inorganic N forms (mineral N, N_min; nitrate, NO₃⁻; and ammonium, NH₄⁺) in the soil solution. This a necessary condition for supplying crop plants with N. The efficiency of this process, which determines the rate of plant growth, is controlled by three main groups of factors. As is known, N_min distribution is not homogenous in the soil profile. In addition, nitrates are susceptible to leaching from upper layers. In the humus soil layer, which is naturally rich in humus, the rate of organic N mineralization and, consequently, the rate of ammonia nitrification, determines the nitrate resources to crop plants. The farmer’s main task is to identify N_min resources. This is a critical task for the farmer which is necessary to determine the amount of N_f used in fertilizers. The basic factor controlling these processes and, at the same time, the rate of root growth, is soil pH. Lime and/or CaSO₄ × 2H₂O are two soil amendments that control both Al³⁺ toxicity and resources of calcium (Ca), a nutrient required to initiate root growth. The effective uptake of N requires a balanced supply of other nutrients. Efficient N uptake requires sufficiently high amounts of available K, P, and Mg. The supply of P and K to plants depends on the root system size. Therefore, the farmer should control both the content of available nutrients and the factors limiting the growth of roots.

The primary source of N for crop plants is N₂. However, only legumes are equipped with an N₂ fixation mechanism. The efficiency of this process is strongly dependent on the soil pH and the content of available Ca, S, and Fe, as well as K, and P, Mg. Non-legume plants should be fed with N fertilizers to produce yield. The primary N contained in the crop plant is is partly subject to secondary circulation, which is called recycling. It is recycled directly in the field through crop residues as well as cover crops. In this way, the farmer can control the post-harvest N_min resources that may be lost from the field. No less important is the recycling of N in crop by-products (straw) by animals whose end product is manure.

A farmer has many diagnostic tools to control the state of the N economy on their farm. The basic tool is N balance, either for each individual field or for the entire farm. The N balance for a given field has a diagnostic value only when it takes into account not only N introduced in fertilizers or manure but also the N_min soil resources at the beginning and the end of the growing season. The new diagnostic solution proposed by the authors is the concept of the Nitrogen Gap (NG). This concept is based on the assumption that in well-defined soil and climatic conditions, the effect of N_f is limited by agrotechnical factors. The formation of an NG or, more precisely, its size, is an indicator of the ineffectiveness of a farmer’s actions in N management.

2.2. Microorganisms Activity: Availability of Soil Nitrogen

The inorganic N pool (N_min) constitutes 1(2)% of the total organic N (N_org) content in mineral soil. The balance of N_min during the growing season depends on two groups of factors: uptake by plants as a physiological sink and the activity of microorganisms, a factor determining the release of N_min from N_org soil resources [10]. The N_min release rate is strongly dependent on both the biodegrability of N_org and environmental factors affecting microbial activity. These issues are discussed in detail in this section [11,12,13].

The total soil N_org content allows for only an approximate estimate of the N pool. This N pool consists of fractions with varying degrees of mineralization. Depending on the procedure used, three key N fractions can be distinguished: (i) easily available (EA), (ii) readily available (RA), and (iii) resistant to mineralization (R). A study on the mineralization potential of soil N_org (NMP) was carried out on soil continuously fertilized for 26 years with mineral fertilizers (NP and NPK) and with the addition of manure (M + NPKM; 1.5 M + NPK) and soil not fertilized at all (CK) [11]. The content of the labile N pool (the sum of EA and RA) increased in the following order: CK < NP. < NPK < M + NPK < 1.5 M + NPK. The reverse order was obtained for the resistant N pool. The rate of N mineralization was significantly related to the soil microbial biomass, indicating a significant role of soil microbial activity.

The effect of manure on the rate of N mineralization and its impact on plants is observed by farmers. The stimulating role of manure on the content of available N is a result of two complementary factors. First, manure contains N in various chemical forms, including urea and uric acid. Secondly, there are microorganisms in the soil that contain the enzyme urease (ureolytic bacteria). This issue has been further investigated in China by examining the activity, abundance, and genetic diversity of soil ureolytic bacteria [12]. The study was carried out in two contrasting climatic zones (temperate—North China; tropical—South China), differing in soil type (Udic Mollisols and Agri-Udic Ferrosol, respectively). The differences in soil characteristics between both sites were large. The total C (TC) content in the manure control plot (CK) was almost four times higher in North China than in South China. The total N (TN) content showed the same trend, but the difference between both sites was considerably smaller. As a result, the C/N ratio was much narrower in South China. Manure application significantly increased TC and TN in both sites. In South China, it increased by 260% compared to the CK plot. The urease activity increased in response to manure application in both sites, but it was significantly higher in South China. In North China, a site naturally rich in humus (Udic Mollisols), a variation in the ureolytic community was observed (Adonis, 32.2%), but no change in the abundance of microorganisms was observed compared to CK. A quite different response to manure application was recorded in South China, a site naturally poor in humus. The variance in the ureolytic community was high (Adonis, 83.4%). Moreover, the abundance of this community doubled. The increase in urease activity significantly affected the content of ammonium N. At the same time, it showed a strong association with the abundance of the ureC gene and the 16SrRNA gene. Moreover, urease activity showed dependence on the total P content.

The salinity of agricultural soil leads to a decline in crop yields. Therefore, a question arises with respect to the extent to which this factor affects nitrification and denitrification, key components of the N cycle. This issue was studied in the soils of the Yellow River Estuary (China), evaluating the response of nitrifying and denitrifying microorganisms to a salinity gradient [13]. It was clearly found that increased salinity significantly reduced the abundance of nitrifiers. However, the abundance of ammonia-oxidizing Archaea (AOA, Nitrobacter) were more susceptible to the increase in soil salinity than ammonia-oxidizing bacteria (AOB, Nitrospira), although no significant impact of salinity on the abundance of denitrifier communities was found. These studies showed that the abundance of both groups of microorganisms depends not only on the direct effect of salinity; the indirect effect of salinity due to physicochemical changes in the soil should also be considered. The variability in the nitrifier community was associated with variability in total N (TN), total sulfur (TS), and the C/N ratio. The set of factors affecting the group of denitrifier was TS and available K (AK).

2.3. Nitrogen Lost to the Environment

A natural feature of the N cycle is the presence of inorganic N as nitrates and gases (ammonia and N oxides). Human activity has accelerated these processes, seriously disturbing biochemical balances in the environment [14]. Several aspects of these complex issues are addressed in four articles.

Snow, or more precisely snow cover, is the main source of water in temperate regions of the world. An increase in temperature causing snow melting can lead not only to water loss but also to N losses. According to an experiment carried out in northeast China, N loss was greater with a greater snow water equivalent (SWE), combined with increasing slope steepness. An important factor inducing water losses on a steep slope is the rate of the frozen soil thawing, which is decisive for water infiltration into the soil [15]. The study found that N losses were significantly reduced when snow cover was low and slope steepness was also low. The soil on such a slope thaws quickly, which consequently increases its infiltration capacity. Under such conditions, water infiltration into the soil increases, reducing the loss of both water and N.

Drainage ditches are natural components of agricultural fields in humid climates. At the same time, they are natural reservoirs of water, sediment, and N flowing from fields. Experimental studies simulating the interaction of water–sediment–plant systems made it possible to determine the roles of plants and microorganisms in purifying water from inorganic N [16]. The content of nitrate N during the experimental period, extending from April to October, followed the quadratic regression model, with the lowest value in the summer months (June–August). This trend was in accordance with the course of temperature. This factor significantly affected the purification rate for both plants and microorganisms. The critical temperature of 22 °C significantly reduced or even stopped the activity of both groups of organisms. The average purification efficiency of N by plants during the growing season was 14%, while for microorganisms, it was 20%. The observed migration of ammonium N into the bottom soil layers with time created favorable conditions for its subsequent nitrification. This tendency was indicated by the highest content of nitrates in the layer just above the bottom soil layer. The observed N migration processes created conditions for nitrate denitrification.

The process indicated above is intensified in rice paddy soils. This issue was examined in a short-term experiment [17]. The emission of N₂O from the soil results from both nitrification and denitrification processes. The dominance of one of these processes depends on the current state of the water-filled pore space (WFPS). In soil with a WFPS, more than 80(90)% dominates denitrification. N₂O emissions, regardless of the type of paddy soil (hydromorphic, Hydragric Anthrosols, or Gleyic Anthrosols), were almost negligible when the WPFS was maintained at 60%. An increase in the WPFS to 100% or even its doubling to 200% of the WPFS, regardless of the soil, accelerated the rate of N₂O production. In well-structured hydromorphic soil, the maximum rate of N₂O emission was twice as low compared to poorly structured gleyed soil. The content of nitrates was higher in gleyed paddy soils. Moreover, in both soils, it decreased with soil depth, consequently decreasing the amount of N₂O. The same trend was observed for the abundance of denitrification genes (nirK and nosZ). At the same time, a strong correlation was found between the content of Fe²⁺ and Fe³⁺ with the content of nitrates, as well as with the abundance of nitrifiers, the AOB amoA and nirK genes, and denitrifiers, the nosZ genes. These dependencies highlight the specific impact of the content of Fe²⁺ and Fe³⁺ on both the distribution of nitrifying and denitrifying microorganisms and, consequently, on N₂O emission from the paddy soil.

Tea plants prefer ammonium N as the dominant source of N, so nitrates are potentially vulnerable to denitrification. In fact, tea plantations emit greater amounts of N oxides than soil under other crops [18]. The age of tea plantation stands located in different climatic regions of China (subtropical and tropical) was the key factor evaluated in a soil incubation experiment [19]. At both sites, the production of nitrates, as indicated by the net nitrification rate and the content of N-NO₃, was the highest in the 15-year-old stand, provided that the WFPS was 50%. However, it significantly decreased when the WFPS increased to 80%. The rates of NO and N₂O production in the subtropical site followed the pattern found for N-NO₃. In the tropical soil, the highest flaxes of both gases were recorded in 30-year-old plantations for treatments with a WFPS of 50%. The main reasons for the observed differences were soil characteristics. The soil in the subtropical region was richer in soil organic matter (SOM) and total N compared to the tropical soil. Soil geochemical conditions in both sites significantly affected NO and N₂O emissions. In the subtropical zone, this was associated with the content of available potassium. However, in the tropics, the amount of N emission depended on SOM, pH, and the content of available phosphorus.

2.4. Farmers’ Actions Supporting N Productivity

In crop production, different nitrogen carriers are used, both mineral and organic, including biofertilizers. This section will discuss two articles presenting different approaches to the optimization of fertilizer nitrogen use [20,21].

One of the options for optimizing the effect of N is to use so-called slow-release N fertilizers. A classic example is polymer-coated urea. The effect of this type of N carrier was tested in maize cultivated in two soils with different geo-physical characteristics (sandy loam with good drainage, Eutrochrepts soil; clay-loam with imperfect drainage, typic Endoaquolls) [20]. Both sites were fertile, as evidenced by high yields in the control N plots (approximately 7 t ha⁻¹). In such sites, a high efficiency of N_f cannot be expected. This conclusion was confirmed by yields that increased progressively with N rates but only slightly exceeded 10 t ha⁻¹. The impact of Urea:PCU blends on grain yield became weaker as the PCU share increased. At the same time, the amount of residual N in the soil increased. This increase was recorded for N doses of 100 kg N ha⁻¹, mainly in lighter soil. Therefore, optimizing the N dose is more important for both yields and environment than the chemical composition of the applied N fertilizer.

In organic farming, strategies for feeding crops with N are based on original, site-specific agronomic solutions. One option is intercropping cereals with legumes. The action of legumes can be supported by the simultaneous use of biofertilizer. This interactive solution was tested in spring barley intercropped with red clover and the use of biofertilizers [21]. The key components of each bio-treatment were nitrogen-fixing bacteria (Azospirillum lipoferum and Azotobacter chroococcum), phosphorus-releasing bacteria (Bacillus megaterium var. phosphaticum and Arthrobacter agilis), and plant growth-promoting rhizobacteria (PGPR (Bacillus subtilis, Bacillus amyloliquefaciens, and Pseudomonas fluorescens). The interactional effects of both experimental factors were revealed through increases in the protein yield. The greatest yield was recorded in the treatment with red clover and biofertilizer containing N-fixing bacteria and PGPR. The protein yield in this particular treatment, compared to the absolute control, doubled (223%), and compared to clover alone, it increased by 66%.

2.5. Nitrogen Management and Recommnedations

Nitrogen management on a farm cannot be limited to the concept of four laws [7]. This includes the evaluation of N resources in the soil and N release from organic carriers (manure) used by farmers. A farmer’s activities related to canopy architecture cannot be omitted. This topic is discussed in four articles in the Special Issue devoted to the N cycle on the farm.

Determining the optimum dose of fertilizer N (N_f) is a significant challenge for farmers. It was assumed that the combination of the N_f rate and plant density in double-cropped rice (early-season and late-season) significantly affects both N productivity and grain yield [22]. The impact of initial plant density on N accumulation by rice during the growing season was more pronounced for late-season than early-season rice. Low-density rice usually accumulated a smaller amount of N. As a consequence, the rice biomass responded strongly to the supply of N. However, reductions in N_f and planting density did not change the yield, regardless of the variety (conventional vs. hybrid). The higher yield was a consequence of higher productivity of N_f, as evidenced by N efficiency indices. At the same time, the N harvest index (HI) did not reveal any sensitivity to the studied factors. The lack of impact of experimental factors on HI resulted from a greater transfer of N, accumulated in the vegetative parts of rice before flowering, to the developing panicle and grains during the grain filling period.

Grain density (GD, the number of grains per unit area, mostly expressed as the number of grains per spike or m⁻²) is the key yield-forming factor affecting grain yield. In modern intensive agriculture, using the yield potential of the grown variety to maximize GD is a challenge for the farmer [23]. The use of fungicides can be an important factor in realizing this goal [24]. The interaction of increasing N doses and the use of fungicides was studied in winter wheat [25]. These studies clearly showed that the critical phase of yield formation of winter wheat begins at the onset of the stem elongation phase. Grain yield depended significantly on the N accumulated in the wheat canopy at the beginning of stem elongation and on its mass in the wheat stem at the beginning of heading. In contrast, GD depended on the same set of variables plus the N amount in the ears (EN_a), which significantly responded to the N_f dose. The N accumulated in wheat before flowering acted as a buffer stabilizing the ear N, ultimately affecting the grain yield. This conclusion was supported by the positive, linear relationship of both GD and GY with the change in the N mass in the wheat stem during the grain-filling period. Leaves were an important but not dominant source of N for growing grains. It can be concluded that the degree of N remobilization and its subsequent transfer to developing grains depends on GD.

The supply of N to a seed plant during the critical stages of yield formation is the result of two key variabilities. The basic variable is the growth and development of the plant, which implies seasonal variability in the demand for N. The second characteristic is the spatial variability in the N supply to plants in the field [26]. The first issue can be solved by determining the crop biomass and content of N or using remote sensing techniques [27]. Both methods were used to determine N accumulation by winter wheat in four critical stages of yield formation (in the beginning and at the end of stem elongation–BBCH 31 and BBCH 39, respectively), full heading (BBCH 55), and full flowering (BBCH 65) [28]. Sensor and satellite spectral data were evaluated based on chemometric N data (laboratory analysis), and crop biomass measured directly in the field. The best assessment of N status using winter wheat and spectral data was obtained for BBCH 39 and BBCH 55. Spectral data were found to be a useful diagnostic tool for determining the N status of winter wheat during the cardinal phase of yield formation.

A farmer cannot reliably determine N_f without knowing the amount of inorganic N (N_min) in the soil at the beginning of the growing season and its release from soil N resources during the growing season of a given crop [10]. The second issue was the subject of a long-term study on the applicability of two calculation procedures (Dungungsempfehlungs- und Bilanzierungssystem–Bestandesführung, BEFU; Candy Carbon Balance, CCB) [29]. The first procedure is based on the N balance, and the second one is based on the turnover of organic matter in the soil [30,31]. The authors proposed to calculate the recommended N_f dose, assuming the threshold N value for a given crop. This base N value includes (depending on the procedure) the following components: (i) the N_min quota at the beginning of the growing season, (ii) the amount of N released from the soil (N mineralization, calculated from a model CCB), (iii) the N supply from organic fertilizers (calculated from a model CCB), and (iv) the N supply from other sources. The estimation of the N_f dose, excluding sugar beets, was more reliable using the CCB procedure. The best estimate of N_f was obtained for maize. The advantage of the CCB over BEFU resulted from the implementation of data on the amount of inorganic N released from the soil during the growing season of a given crop into the calculation procedure. Moreover, the CCB procedure enables a long-term estimation of the N supply from applied organic fertilizers. In this way, without risking a decrease in yields, the excess of residual N can be reduced.

3. Conclusions

Effective N management on a farm is a significant challenge for the farmer. This requires the skillful use and processing of the available knowledge in this area into targeted skills, enabling effective control of the N cycle. These activities must be oriented toward both obtaining high yields and, at the same time, reducing agricultural pressure on the environment. The series of articles presented clearly shows that one of the basic N resources that requires control and quantification is the soil. Organic fertilizers, which can partially replace mineral N fertilizers, are additional important sources of N. However, their effect should be taken into account over a longer period of time than the year of application. The use of bio-fertilizers containing microorganisms capable of N₂ fixing from the atmosphere is another option for the farmer. During the growing season of the cultivated crop, the farmer may use diagnostic tools that enable the reliable estimation of its nitrogen status. The rational use of N resources by the farmer allows for a simultaneous reduction in the demand of the cultivated crops for mineral N fertilizers and a reduction in inorganic N pools that are susceptible to leaching or emission into the atmosphere.