The Influence of Various Forms of Nitrogen Fertilization And

agronomy

Article The Inﬂuence of Various Forms of Nitrogen Fertilization and Meteorological Factors on Nitrogen Compounds in Soil under Laboratory Conditions

Ruta¯ Dromantiene˙ 1, Irena Pranckietiene˙ 1, Darija Jodaugiene˙ 1,* and Aurelija Paulauskiene˙ 2

1 Institute of Agroecosystems and Soil Sciences, Vytautas Magnus University Agriculture Academy Studentu˛11, Akademija, LT-53361 Kaunas, Lithuania; [email protected] (R.D.); [email protected] (I.P.) 2 Institute of Agricultural and Food Sciences, Vytautas Magnus University Agriculture Academy Studentu˛ 11, Akademija, LT-53361 Kaunas, Lithuania; [email protected] * Correspondence: [email protected]

Received: 20 November 2020; Accepted: 18 December 2020; Published: 20 December 2020

Abstract: Nitrogen is one of the main factors that shapes soil fertility and the productivity of crops, although its abundance can also cause damage to the environment. The aim of this study is to evaluate the influences of different forms of nitrogen fertilizers, soil temperature, and precipitation + on the changes of nitrogen compounds (N-NH4 , N-NO3−, and Nmin) in two soil layers. Two pot experiments are performed, involving simulated precipitation levels of 10- and 20 mm. Urea and ammonium nitrate fertilizers are used for fertilization. The soil samples are stored in pots in a climate chamber at different temperatures of 5, 10, 15, and 20 ◦C. After seven days, the changes of + nitrogen compounds (N-NH4 , N-NO3−, and Nmin) in 0–15 and 15–30 cm soil layers are analyzed. + This study shows that the amount of N-NH4 nitrogen in the soil depends on the fertilizer form and + soil temperature. In the temperature range of 5–20 ◦C, significantly more N-NH4 nitrogen is present + in urea-fertilized soil. The migration of N-NH4 into the deeper 15–30 cm soil layer at both the 10- and 20-mm simulated precipitation levels is negligible. The N-NO3− contents in the 0–15 cm soil layer in the temperature range of 5–20 ◦C are 1.7–2.3 times lower in the urea-fertilized soil than in the ammonium nitrate-fertilized soil at a 10-mm simulated precipitation level and 1.6–2.2 times lower at 20 mm. The Nmin contents in soil are directly dependent on the fertilizer form and soil temperature for both levels of simulated precipitation.

Keywords: nitrogen; ammonium nitrate; urea; temperature; soil moisture

1. Introduction Anthropogenic activities have doubled the amount of reactive nitrogen circulating on Earth, leading to excess nutrient transfer to freshwater ecosystems and ultimately causing the eutrophication of freshwater and marine aquatic ecosystems [1,2]. Agriculture is considered one of the sources of nitrogen pollution (ammonia (NH3) and nitrous oxide (N2O) to air, and nitrate (N-NO3−) to surface and ground water) [3–5]. The adoption of more stringent environmental standards and increasing nitrogen fertilizer prices oblige researchers, fertilizer manufacturers, and agricultural producers to look for rational and sustainable solutions to reduce nitrogen emissions [5–7], which is also directly related to nitrogen losses from mineral fertilizers. Mineral nitrogen losses from the soil are caused by the unbalanced and untimely fertilization of crops, improper choices for fertilizer forms, and other factors [8–12]. Nitrogen losses from the soil and mineral nitrogen fertilizers directly depend on the intensity of nitrogen compound transformation in the soil. It has been documented that the changes of

Agronomy 2020, 10, 2011; doi:10.3390/agronomy10122011 www.mdpi.com/journal/agronomy Agronomy 2020, 10, 2011 2 of 12 the ammonium nitrogen and nitrate nitrogen forms depend on the crop growth season, fertilization intensity, fertilizer form, precipitation rate, soil temperature, etc. [11–13]. Precipitation directly alters N cycling and transformation [14]. Changes in these rates are also influenced by the soil temperature, because it affects the activity of urease, nitrifier communities, and nitrification rate in the soil [15,16]. For example, some studies have identified the stimulating effects of soil temperature [17,18], whereas a few other studies have shown that warming can suppress ammonification [19] or that the temperature dependency of ammonification is not significant [20]. According to Russell et al. [21] for sandy soil, the maximum rates of nitrification decreased as the temperature was reduced from 20 ◦C to 5 ◦C. Tripolskaja and Verbyliene˙ [22] suggest that precipitation during the warm period of the year promotes the leaching and migration of chemical elements from topsoil. Nitrates are especially dangerous to the environment because they are not absorbed by the soil sorption complex, meaning that excessive amounts of nitrates may be leached [23,24]. Haberle et al. [25] found that about 35–70% of nitrates can be leached into soil layers deeper than 1 m in certain conditions. Gallucci et al. [26] showed that the losses of N from fertilizer range from 20 to 30%, depending of the N sources, soil moisture, and pH. In recent years, N losses from fertilization practices have become an issue due to increased nitrate leaching [27]. + Our study hypothesizes that research into the changes in ammonium nitrogen (N-NH4 ), + nitrate nitrogen (N-NO3-), and mineral nitrogen forms (N-NH4 and N-NO3−, hereinafter Nmin) in soil, resulting from the complex effects of fertilizer forms, soil temperatures, and precipitation, will provide knowledge and contribute to a deeper understanding about mineral nitrogen dynamics in soil and crop fertilization modeling possibilities to ensure optimal nitrogen supply for crops and minimize environmental nitrogen pollution in warming climatic conditions. Specifically, the current study aims to evaluate the impact of different forms of nitrogen fertilizers, + soil temperature, and precipitation levels on the changes of nitrogen compounds (N-NH4 , N-NO3−, and Nmin) at soil layers of 0–15 and 15–30 cm.

2. Materials and Methods

2.1. Study Site and Experimental Design Two two-factor pot experiments that differed in the amount of simulated precipitation were conducted at Vytautas Magnus University Agriculture Academy (Lithuania) in 2018. Over a period of 7 days, the first pot received a simulated amount of precipitation of 10 mm, and the second pot received 20 mm. The experiments were replicated twice in the same year and conditions. In terms of the two-factor design, factor A included different forms of fertilizers, namely, a control (without fertilizers), ammonium nitrate (NH4NO3), and urea (CO(NH2)2. For factor B, this included varying soil temperatures of 5, 10, 15, and 20 ◦C. The experimental treatments were arranged in four replicates with a completely randomized block design. The experiment involved the simulation of soil temperature and moisture, which are relevant factors during plant growth and the fertilization period. The experiments were carried out in a controlled climate chamber (RUMED 1301). The soil from the 0–30 cm layer of Luvisol soil [28] was used, characterized by a loamy texture (15.1% clay, 44.5% silt, and 40.4% sand). Before the experiment, the pHKCl value of the topsoil was determined to be 6.8, the concentration of available P2O5 was 1 1 162 mg kg− , the available K2O was 188 mg kg− , and the organic carbon content was 1.8%. The mineral 1 + 1 1 nitrogen content was 9.8 mg kg− (N-NH4 —0.72 mg kg− ; N-NO3−—9.08 mg kg− ). The soil was homogenized and moistened to a 25% humidity level, with a water capacity of 46.2%. The experimental soil samples were placed into cylindrical pots that were 10.5 cm in diameter and 3 30 cm in height. The formed soil density was close to the equilibrium density of the loam of 1.40 Mg m− . Nitrogen fertilizers of different forms were applied to the soil surfaces of each pot. The nitrogen rate 1 was 90 kg ha− . The experiment was performed for 7 days at 70% air humidity. Simulated precipitation levels of 10 and 20 mm were applied in equal parts on the first and fifth days after fertilizer application. Agronomy 2020, 10, 2011 3 of 12

The soil moisture levels were the following: (1) Before the first irrigation—25.0%; after the first irrigation—26.7% (precipitation of 10 mm) and 28.3% (precipitation of 20 mm); (2) before the second irrigation—24.0–26.0% (precipitation of 10 mm) and 25.6–27.6% (precipitation of 20 mm); after the second irrigation—25.7–27.7% (precipitation of 10 mm) and 28.9–30.9% (precipitation of 20 mm). The distribution of simulated precipitation (water) was performed manually using a Stihl Sg 11 sprayer, having estimated the volume of water sprayed with one click. Watering was provided early in the morning. All pots received the same volume of water per event. The volume of water applied was determined according to the average meteorological data of the second to third ten-day period of March and the first ten-day period of April in the 2013–2017 period (Kaunas meteorological station, Lithuania 2013–2017), i.e., the beginning of the winter wheat and winter oilseed rape growing season in the northern part of the temperate climate zone (coordinates 54◦5303.26” north latitude, 23◦50033.25” east longitude), which is when the highest rates of nitrogen are used. On the eighth day after the start of the experiment, the soil samples were taken from the 0–15 and + 15–30 cm soil layers for analysis and the determination of the N-NH4 , N-NO3−, and Nmin contents.

2.2. Analytical Methods

Soil pH, available P2O5 and K2O, the concentration of organic C and N, and contents of mineral + nitrogen (N-NH4 and N-NO3−) were analyzed in the Agrochemical Research Laboratory of the Lithuanian Research Centre for Agriculture and Forestry. The topsoil was analyzed for pHKCl as measured in 1N KCl extractions (potentiometric method). The available phosphorus (P2O5) and available potassium (K2O) were measured by the Egner–Riehm–Domingo (A-L) method, and organic carbon (C) was determined by the wet oxidation method. The percentages of sand, silt, and clay (soil texture) were determined using the pipette method. + The Nmin contents in the soil samples were determined with two methods, namely, N-NH4 by the spectrophotometric method, and N-NO3− by the ionometric method. The chemicals used in this study were of an analytical grade. Chemical analyses were performed in three replications.

2.3. Statistical Analysis A two-way statistical analysis of the experimental data was carried out with TIBCO Statistica version 7 (TIBCO Software, USA). The research data were evaluated using an analysis of variance (ANOVA) method. The significance of data was determined by Fisher’s least significant difference (LSD) test with a significance level of p 0.05. There were significant interactions between different ≤ forms of fertilizers (factor A) and soil temperatures (factor B), and the averages of factors A and B are not presented. Correlation and regression analyses were performed to determine the strengths and characters of the relationships between variables at a probability level of 95% [29,30].

3. Results and Discussion

The concentrations of Nmin in soil solutions are variable and dynamic [31]. Li et al. [32] suggested that mineral nitrogen rates are positively correlated with soil temperature and moisture levels. Guntinas et al. [33] reported that 25 ◦C was the maximum value for sensitivity to temperature. The data + from this study define the variation of N-NH4 nitrogen and N-NO3− contents in the arable layer of soil of a certain temperature when fertilizing with different forms of nitrogen fertilizers at the precipitation levels of 10 and 20 mm. Experiment 1. In the 0–15 cm soil layer, under the simulated precipitation level of 10 mm, + the experimental findings show significantly higher amounts of N-NH4 nitrogen in the urea-fertilized soil compared with the ammonium nitrate-fertilized soil (Figure1). This was due to the amide nitrogen + (hereinafter N-NH2−) form present in urea, part of which was transformed into N-NH4 , and a small part was converted into N-NO3− within 7 days of the experiment. This is based on the fact that the + rapid hydrolysis of urea in the soil can result in a high content of N-NH4 ions, which slows down the + nitrification process [34]. The scientific literature indicates that the transformation of N-NH4 into Agronomy 2020, 10, x FOR PEER REVIEW 4 of 12

[36] show that nitrification is limited at 4 °C, extensive at 9 °C, and essentially complete after 48–68 days at 18 °C. As the soil temperature increased from 5 °C to 15 °C, both the N-NH4+ and N-NO3− levels in the soil consistently increased, regardless of the form of the fertilizer. For urea fertilization, the highest N-NH4+ content and significant increase was determined at a

Agronomy15 °C soil2020 temperature., 10, 2011 This was likely determined by an increase in both the mineralization4 of of 12 organic matter and the intensity of hydrolysis of N-NH2− at a higher soil temperature. This agrees with the research data documented by Rochette et al. [37], Abalos et al. [38], and Zhang et al. [12], N-NOwhich3 suggest− can take that about both one increased week (at intensive a temperature organic of 23 matter◦C) [35 mineralization]. Studies by Canadian and the researchersactivity of [Uro36] showbacteria that are nitriﬁcation associated iswith limited a higher at 4 ◦ soilC, extensive temperature. at 9 ◦ C, and essentially complete after 48–68 days at 18 ◦C.

mg kg−1 mg kg−1 N-NO − N-NH + 3 60 4 60 48.2f 50 50 42.0e 41.8e 45.2f 40 40 25.9d 27.1d 30 9.31e 6.15c 30 18.1c 6.90d 12.70g 14.03h 18.5c 19.1c 20 4.03b7.07d 10.30f 20 10.7a 12.2a 16.0b 10 1.29a 1.40a 1.32a 1.83a 10 0 0 5 101520 5101520 Soil temperature oC Soil temperature oC Control NH4NO3 CO(NH2)2 Control NH4NO3 CO(NH2)2

(a) (b)

Figure 1. TheThe influences influences of of nitrogen nitrogen fertilizer fertilizer forms forms and and ambient ambient temperatures temperatures on onthe the changes changes of ( ofa) + (N-NHa) N-NH4+ and4 and(b) N-NO (b) N-NO3− in the3− in0–15 the cm 0–15 soil cm layer soil with layer a withsimulated a simulated precipitation precipitation of 10 mm. of 10Values mm. Valuesfollowed followed by different by di ffletterserent are letters significantly are significantly different diff (perent ≤ 0.05) (p based0.05) on based Fisher’s on Fisher’s LSD test. LSD test. ≤ + AsIn our the soilexperiment, temperature at a increased20 °C soil from temperature, 5 ◦C to 15 the◦C, content both the of N-NH N-NH4 4and+ was N-NO significantly3− levels lower in the soilthan consistently that at 15 °C. increased, This difference regardless may of thehave form been of determined the fertilizer. by increased nitrogen losses due to + volatilizationFor urea in fertilization, the form of the ammonia highest (NH N-NH3) 4andcontent a more and intensive significant nitrification increase process. was determined The second at astatement 15 ◦C soil is temperature.corroborated by This the was data likely of this determined study, where by anthe increase content inof bothN-NO the3− at mineralization 20 °C was 4.6% of organichigher than matter at 15 and °C. the intensity of hydrolysis of N-NH2− at a higher soil temperature. This agrees withWhen the research fertilizing data with documented ammonium by nitrate, Rochette the et highest al. [37 ],N-NH Abalos4+ content et al. [ 38in], the and soil Zhang was also et al. found [12], whichat 15 °C. suggest In this case, that boththe increase increased in the intensive N-NH4 organic+ content matter can be mineralizationattributed to the and more the intensive activity release of Uro bacteriaof mineral are nitrogen associated compounds with a higher due to soil the temperature. naturally occurring processes related to soil organic matter + mineralizationIn our experiment, [39,40] and at athe 20 less◦C soilintensive temperature, nitrification the contentprocess ofthan N-NH at 204 °C.was This significantly is supported lower by thanthe data that of at our 15 ◦study,C. This as dia ffsignificanterence may decrease have been in N-NH determined4+ and an by increase increased of N-NO nitrogen3− were losses recorded due to volatilizationat 20 °C soil temperature in the form (Figure of ammonia 1). The (NH reduction3) and a in more N-NH intensive4+ content nitrification at 20 °C can process. also be The attributed second statementto the higher is corroborated possibility for by nitrogen the data loss of this due study, to volatilization, where the content as research of N-NO shows3− atthat 20 nitrogen◦C was4.6% loss higherfrom ammonium than at 15 ◦nitrateC. when applied to the soil surface (broadcast) can be up to 2% [41]. The lowest + amountsWhen of fertilizingN-NH4+ were with present ammonium in the nitrate, soil at the5 °C. highest Changes N-NH in the4 content N-NH4+ in content the soil when wasalso fertilizing found + atwith 15 ◦ammoniumC. In this case, nitrate the increasewere similar in the to N-NH those4 obtainedcontent canwith be urea attributed fertilization. to the more intensive release of mineralThe regression nitrogen compoundsanalysis of the due experimental to the naturally data occurring shows that processes in the 0–15 related cm layer, to soil the organic dependence matter mineralizationof N-NH4+ on soil [39, 40temperature] and the less (in intensive the range nitrification of 5–20 °C) process for both than urea- at 20 and◦C. Thisammonium is supported nitrate- by + thefertilized data of soil our can study, be asdescribed a significant by the decrease following in N-NH quadratic4 and anequations: increase ofyCO(NH2)2 N-NO =3 −−were3.424 recorded+ 1.683x at− + 200.093◦Cx soil2 and temperature yNH4NO3 = −3.135 (Figure + 1.68371). Thex − 0.0603 reductionx2. In in both N-NH cases,4 thecontent relationships at 20 ◦C between can also the be attributedindicators towere the very higher strong possibility (R2 = 0.980 for and nitrogen R2 = 0.902, loss due respectively) to volatilization, and significant as research (p ≤ shows0.05). that nitrogen loss fromThe ammonium intensity nitrate of transformation when applied of tolow-mobility the soil surface N-NH (broadcast)4+ due to cannitrification be up to into 2% [ very41]. The mobile lowest N- + + amountsNO3− has ofan N-NH influence4 were on presentnitrogen in fertilizer the soil atuse 5 ◦effiC. Changesciency and in theenvironmental N-NH4 content quality, when especially fertilizing in withcases ammoniumwhen the likelihood nitrate were of N-NO similar3− losses to those due obtained to leaching with is urea high fertilization. and plant uptake is low (i.e., due to smallThe root regression systems, analysis high fertilizer of the experimental rates, or ex datacess showsrainfall). that In in urea-fertilized the 0–15 cm layer, soil, thethe dependenceamounts of + ofN-NO N-NH3− in4 theon soil0–15 temperature cm layer, regardless (in the range of ofthe 5–20 soil◦ C)temperature, for both urea- were and significantly ammonium nitrate-fertilizedlower and were soil can be described by the following quadratic equations: y = 3.424 + 1.683x 0.093x2 and 1.7–2.3 times lower compared to the ammonium nitrate-fertilizedCO(NH2)2 soil− (Figure 1). This− was largely y = 3.135 + 1.6837x 0.0603x2. In both cases, the relationships between the indicators were NH4NO3 − − very strong (R2 = 0.980 and R2 = 0.902, respectively) and significant (p 0.05). + ≤ The intensity of transformation of low-mobility N-NH4 due to nitrification into very mobile N-NO3− has an influence on nitrogen fertilizer use efficiency and environmental quality, especially in cases when the likelihood of N-NO3− losses due to leaching is high and plant uptake is low (i.e., due to small root systems, high fertilizer rates, or excess rainfall). In urea-fertilized soil, the amounts of Agronomy 2020, 10, x FOR PEER REVIEW 5 of 12

Agronomydetermined2020 ,by10, 2011the composition of the fertilizer and the longer time taken for the conversion of5 of N- 12 NH2− to N-NO3−. The experimental data show that the soil temperatures of 15 °C and 20 °C had the greatest influence on the amount of nitrate nitrogen, both when fertilizing with ammonium nitrate N-NO3− in the 0–15 cm layer, regardless of the soil temperature, were significantly lower and were and urea. In these cases, the N-NO3− levels in the soil were significantly higher than at 5 °C and 10 1.7–2.3 times lower compared to the ammonium nitrate-fertilized soil (Figure1). This was largely °C. determined by the composition of the fertilizer and the longer time taken for the conversion of N-NH2− The change of N-NO3− as influenced by soil temperature (in the range of 5–20 °C) can be to N-NO3−. The experimental data show that the soil temperatures of 15 ◦C and 20 ◦C had the greatest described by the following linear equations: yCO(NH2)2 = 14.377 + 0.617x (R2 = 0.828; p ≤ 0.05) and yNH4NO3 influence on the amount of nitrate nitrogen, both when fertilizing with ammonium nitrate and urea. = −38.8 + 0.44x (R2 = 0.960; p ≤ 0.05). In these cases, the N-NO3− levels in the soil were significantly higher than at 5 ◦C and 10 ◦C. The N-NH4+ contents in the 15–30 cm soil layer in all cases were significantly lower than those in theThe 0–15 change cm soil of layer N-NO (Figures3− as influenced1 and 2). The by correlation soil temperature analysis (inof the the experimental range of 5–20 data◦C) suggests can be y x R2 p thatdescribed at the bysimulated the following precipitation linear level equations: of 10 mm,CO(NH2)2 there were= 14.377 no significant+ 0.617 ((p ≤ 0.05)= 0.828; relationships0.05) 2 ≤ and yNH4NO3 = 38.8 + 0.44x (R = 0.960; p 0.05). between N-NH4+ content in the 0–15 and 15–30 cm layers. The low migration may have been caused −+ ≤ The N-NH4 contents in the 15–30 cm soil layer in all cases were significantly lower than those in by the ability of soil to absorb N-NH4+ ions [42], peculiarities of nitrogen compounds transformation atthe different 0–15 cm temperatures soil layer (Figures [43], low1 and infiltrati2). Theon correlation of precipitation, analysis and of other the experimental factors [22]. data suggests that at the simulated precipitation level of 10 mm, there were no significant (p 0.05) relationships When fertilizing with urea, N-NH4+ levels in the 15–30 cm soil layer increased significantly with + ≤ anbetween increasing N-NH temperature4 content in (Figure the 0–15 2); and however, 15–30 cmthe layers.maximum The lowwas migrationreached at may a soil have temperature been caused of + 15by °C. the Similar ability ofdata soil were to absorb obtained N-NH in 4theions 0–15 [42 cm], peculiaritiessoil layer (Figure of nitrogen 1). compounds transformation at different temperatures [43], low infiltration of precipitation, and other factors [22].

mg kg−1 −1 + mg kg N-NO − 20 N-NH4 20 3 14.1f 13.1e 11.7c 13.3e 12.5d 15 15 10.6b 13.5e 12.5d 10.1a 9.8a 10.8b 10.3b 10 10 0.89b 0.89b 1.17f 1.23f 5 0.64a 0.71a 0.85b 1.20f 5 0.74a 1.05d 1.64g 1.11e 0 0 5 101520 5101520 Soil temperature oC Soil temperature oC Control NH4NO3 CO(NH2)2 Control NH4NO3 CO(NH2)2

(a) (b)

+ Figure 2. The influenceinfluence ofof nitrogen nitrogen fertilizers fertilizers and and ambient ambient temperature temperature on theon changesthe changes of (a )of N-NH (a) N-NH4 and4+ (andb) N-NO (b) N-NO3− in3 the− in 15–30 the 15–30 cm soil cm layer soil withlayer a with simulated a simulate precipitationd precipitation amount amount of 10 mm. of Values10 mm. followed Values followedby different by lettersdifferent are letters significantly are significantly different( differentp 0.05) based(p ≤ 0.05) on Fisher’sbased on LSD Fisher’s test. LSD test. ≤ When fertilizing with urea, N-NH + levels in the 15–30 cm soil layer increased significantly with In the 15–30 cm soil layer, ammonium4 nitrate fertilization tended to increase the N-NH4+ content withan increasing an increasing temperature soil temperature (Figure2); up however, to 20 °C, the while maximum urea fertilization was reached tended at a soil to temperatureincrease the N- of 15 C. Similar data were obtained in the 0–15 cm soil layer (Figure1). NH◦4+ content with an increasing soil temperature up to 15 °C. Significantly higher levels of this + compoundIn the 15–30were found cm soil in layer, the soil ammonium at 15 and nitrate 20 °C compared fertilization with tended those to present increase at the 5 and N-NH 10 4°C.content + withIn an this increasing soil layer, soil both temperature with urea up and to 20 ammonium◦C, while urea nitrate fertilization fertilization, tended the to correlations increase the between N-NH4 content with an increasing soil temperature up to 15 C. Significantly higher levels of this compound N-NH4+ and soil temperature were very strong (R2◦ = 0.904 and R2 = 0.827, respectively), but not weresignificant found (p in ≥ the0.05). soil at 15 and 20 ◦C compared with those present at 5 and 10 ◦C. In this soil layer, both with urea and ammonium nitrate fertilization, the correlations between The content of N-NO3− in the 15–30 cm soil layer increased with an increasing soil temperature, N-NH + and soil temperature were very strong (R2 = 0.904 and R2 = 0.827, respectively), but not both with4 urea and ammonium nitrate fertilization. At 15 and 20 °C, significantly more N-NO3− was significant (p 0.05). present in the≥ urea-fertilized soil, while at 5 and 10 °C, significantly higher N-NO3− contents were The content of N-NO in the 15–30 cm soil layer increased with an increasing soil temperature, present in the ammonium3 nitrate-fertilized− soil. In both cases, the relationships between N-NO3− in theboth 15–30 with ureacm andsoil ammoniumlayer and the nitrate ambient fertilization. temperature At 15 andwere 20 strong◦C, significantly (R2 = 0.753 more and N-NO R2 = 3−0.894,was presentrespectively), in the but urea-fertilized not significant soil, (p while≥ 0.05). at 5 and 10 ◦C, significantly higher N-NO3− contents were presentExperiment in the ammonium 2. N transformations nitrate-fertilized in agricultural soil. In both soils cases, and the leaching/migration relationships between may N-NO differ3− underin the 2 2 15–30different cm water soil layer regimes and the[8,13]. ambient When temperature simulating the were 20 strong mm precipitation (R = 0.753 andlevelR in= the0.894, upper respectively), (0–15 cm) but not significant (p 0.05). + soil layer in the soil temperature≥ range of 5–20 °C, significantly more N-NH4 was present in the urea- fertilizedExperiment soil compared 2. N transformations with the ammonium in agricultural nitrate-fe soilsrtilized and soil leaching (Figure/migration 3). Similar may trends diff wereer under also determineddifferent water at the regimes 10 mm [8 ,precipitation13]. When simulating level (Figure the 201). mm precipitation level in the upper (0–15 cm) soil layer in the soil temperature range of 5–20 C, significantly more N-NH + was present in the The highest and most significantly different N-NH◦ 4+ content in the soil, compared4 to other cases, wasurea-fertilized observed soilin comparedthe urea-fertilized with the ammonium soil at 20 nitrate-fertilized°C. Meanwhile, soil having (Figure simulated3). Similar the trends 10 weremm also determined at the 10 mm precipitation level (Figure1).

Agronomy 2020, 10, x FOR PEER REVIEW 6 of 12 precipitation amount, the content of N-NH4+ in soil at this temperature was significantly lower when compared with the soil at 15 °C. In the first experiment, the reduction can be attributed to a higher possibility of nitrogen loss due to volatilization, and in the second experiment it can be attributed to the lower nitrogen loss due to volatilization, as higher precipitation amounts accelerate urea dissolution and soaking in soil [37,44]. At a soil temperature of 15 °C, the N-NH4+ content was significantly lower than at 10 °C; however, the content of N-NO3− in the soil for this treatment was 1.2 time higher, indicating different intensities for the mineralization process. The correlation analysis of the data showed that in both urea-fertilized and ammonium nitrate-fertilized soil at the 20 mm precipitation level, the relationships of N-NH4+ with the ambient temperature were moderate (R2 = 0.609 and R2 = 0.353, respectively), but significant (p ≤ 0.05), while at the 10 mm precipitation level, Agronomythese relationships2020, 10, 2011 were very strong (R2 = 0.980 and R2 = 0.902, respectively) and significant (p ≤ 0.05).6 of 12

−1 −1 − mg kg + mg kg N-NO3 60 N-NH4 60 51.9h 53.0h 45.6g 50 50 42.3f 32.2e 40 40 31.6e 30 30 18.8c 21.9d 8.38f 20 12.26h 10.90g 14.06i 20 14.0b 15.9b 12.5ab 4.47c 6.75e 4.97d 5.23d 10.6a 10 1.39b 1.41b 1.11a 1.31a 10 0 0 5101520 5101520 Soil temperature oC Soil temperature oC Control NH4NO3 CO(NH2)2 Control NH4NO3 CO(NH2)2

(a) (b)

++ Figure 3. TheThe influence influence of nitrogen fertilizers andand ambient temperaturetemperature onon changeschanges ofof ((aa)) N-NH N-NH44 and − (b) N-NO 3− inin the the 0–15 0–15 cm cm soil soil layer layer with with a a simulated simulated pr precipitationecipitation amount amount of of 20 20 mm. mm. Values Values followed followed by different different letters are significantlysignificantly didifferentfferent ((pp ≤ 0.05) based on Fisher’sFisher’s LSD test. ≤ + AtThe the highest simulated and most 20 mm significantly precipitation different level, N-NH the amount4 content of N-NO in the3 soil,− tended compared to increase to other with cases, an wasincreasing observed ambient in the urea-fertilizedtemperature, both soil atfor 20 the◦C. urea Meanwhile,-fertilized having and ammonium simulated the nitrate-fertilized 10 mm precipitation soil. + Theamount, N-NO the3− contentcontent of was N-NH 1.84 timesin soil lower at this in temperature the urea-fertilized was significantly soil than lowerin the whenammonium compared nitrate- with fertilizedthe soil at soil. 15 ◦C. In In both the firstcases, experiment, the highest the N-NO reduction3− contents can be were attributed recorded to a at higher soil temperatures possibility of nitrogenof 15 °C andloss 20 due °C to and volatilization, differed by a and factor in theof 1.6 second due to experiment the form of it the can fertilizer. be attributed At the to soil the temperatures lower nitrogen of 5loss and due 10 to°C, volatilization, the amounts of as this higher compound precipitation differed amounts by 2.3 acceleratetimes and urea2.1 times, dissolution respectively. and soaking in + soil [At37, 44the]. 20 At mm a soil simulated temperature precipitation of 15 ◦C, the level, N-NH the4 relationshipscontent was of significantly the N-NO lower3− content than with at 10 soil◦C; temperaturehowever, the (in content the range of N-NO of 5–203− in°C) the in soilthe 0–15 for this cm treatment soil layer wasfor both 1.2 time urea-fertilized higher, indicating and ammonium different nitrate-fertilizedintensities for the soil mineralization were very strong process. (R2 = The 0.91 correlation and R2 = 0.96) analysis and significant of the data (p showed ≤ 0.05), that and incan both be describedurea-fertilized by the and following ammonium linear nitrate-fertilized equations: yCO(NH2)2 soil at = the13.605 20 mm+ 0.9988 precipitationx and yNH4NO3 level, = the38.63 relationships + 0.767x. + 2 2 of N-NHThe N-NH4 with4+ thecontents ambient in the temperature 15–30 cm soil were layer moderate varied (veryR = unevenly0.609 and betweenR = 0.353 both, respectively), unfertilized soilbut significantand ammonium (p 0.05), nitrate-fertilized while at the 10or mmurea-fertiliz precipitationed soil. level, When these fertilized relationships with urea, were the very contents strong 2 ≤2 (R = 0.980+ and R = 0.902, respectively) and significant (p 0.05). of N-NH4 at 5 and 10 °C did not differ significantly (Figure≤ 4). The highest content was found at a soil temperatureAt the simulated of 15 °C, 20 mmand when precipitation the temp level,erature the rose amount to 20 of°C, N-NO the content3− tended of N-NH to increase4+ decreased with significantly.an increasing Similar ambient changes temperature, in the N-NH both4 for+ content the urea-fertilized in the 15–30 cm and soil ammonium layer were nitrate-fertilizedalso observed at thesoil. 10 The mmN-NO precipitation3− content level, was although 1.8 times depending lower in on the the urea-fertilized temperature (Figures soil than 1 inand the 2). ammonium nitrate-fertilizedWhen fertilizing soil. In with both ammonium cases, the highest nitrate, N-NO the3 −contentcontents of wereN-NH recorded4+ was atthe soil highest temperatures at a soil of 15temperature◦C and 20 of◦C 5 and °C, diwhileffered at bysoil a temperatures factor of 1.6 due of 10 to and the form15 °C, of it thewas fertilizer. significantly At the lower. soil temperatures A negligible increaseof 5 and 10in ◦thisC, the form amounts of nitrogen of this compoundwas observed differed at 20 by 2.3°C timeswhen andcompared 2.1 times, with respectively. 15 °C. Such a distributionAt the 20of mmdata simulatedis explained precipitation by the regularities level, the of soil relationships temperature-related of the N-NO processes3− content occurring with soil in thetemperature soil [17,19]. (in At the 5 range°C, the of higher 5–20 ◦ C)N-NH in the4+ content 0–15 cm in soil the layer ammonium for both nitrate-fertilized urea-fertilized and soil ammonium may have nitrate-fertilized soil were very strong (R2 = 0.91 and R2 = 0.96) and significant (p 0.05), and can be been caused by the slow activity of Nitrosomonas and Nitrobacter bacteria [15,16]. With≤ an increasing soildescribed temperature, by the following the amount linear equations:of this compoundyCO(NH2)2 =in13.605 most +cases0.9988 decreased,x and yNH4NO3 as most= 38.63 of+ it0.767 wasx. + transformedThe N-NH into4 contentsN-NO3−; inhowever, the 15–30 a cmvery soil negligible layer varied increase very unevenly in the N-NH between4+ nitrogen both unfertilized content observedsoil and ammonium at 20 °C may nitrate-fertilized have been caused or urea-fertilized by the intensification soil. When of organic fertilized matter with mineralization. urea, the contents In + theof N-NH urea-fertilized4 at 5 and soil, 10 ◦theCdid relationship not differ between significantly the N-NH (Figure4+ 4content). The highest in the 15–30 content cm was soil found layer atand a + soil temperature of 15 ◦C, and when the temperature rose to 20 ◦C, the content of N-NH4 decreased + significantly. Similar changes in the N-NH4 content in the 15–30 cm soil layer were also observed at the 10 mm precipitation level, although depending on the temperature (Figures1 and2). + When fertilizing with ammonium nitrate, the content of N-NH4 was the highest at a soil temperature of 5 ◦C, while at soil temperatures of 10 and 15 ◦C, it was significantly lower. A negligible increase in this form of nitrogen was observed at 20 ◦C when compared with 15 ◦C. Such a distribution of data is explained by the regularities of soil temperature-related processes occurring in the soil [17,19]. + At 5 ◦C, the higher N-NH4 content in the ammonium nitrate-fertilized soil may have been caused by the slow activity of Nitrosomonas and Nitrobacter bacteria [15,16]. With an increasing soil temperature, the amount of this compound in most cases decreased, as most of it was transformed into N-NO3−; Agronomy 2020, 10, 2011 7 of 12

+ however, a very negligible increase in the N-NH4 nitrogen content observed at 20 ◦C may have been Agronomy 2020, 10, x FOR PEER REVIEW 7 of 12 caused by the intensification of organic matter mineralization. In the urea-fertilized soil, the relationship between the N-NH + content in the 15–30 cm soil layer and the soil temperature was strong (R2 = 0.552), the soil temperature4 was strong (R2 = 0.552), but not significant (p ≤ 0.05), whereas in the ammonium but not significant (p 0.05), whereas in the ammonium nitrate-fertilized soil, this relationship was nitrate-fertilized soil, ≤this relationship was both very strong (R2 = 0.902) and significant (p ≤ 0.05). both very strong (R2 = 0.902) and significant (p 0.05). ≤

mg kg−1 + mg kg−1 N-NO − N-NH4 3 16.7f 20 20 15.0e 12.6b 14.6de 15.0e 14.2cd 13.6c 15 15 11.2a 12.0b 10.8a 10.6a 10.7a 10 10 1.04cd 1.25e 1.01b 1.13d 1.08d 5 0.76a 5 1.00bc 0.84a 0.83a 1.03c 0.96bc 1.05bc 0 0 5101520 5101520 Soil temperature oC Soil temperature oC Control NH4NO3 CO(NH2)2 Control NH4NO3 CO(NH2)2

(a) (b)

+ Figure 4. The influenceinfluence of of nitrogen nitrogen fertilizers fertilizers and and ambient ambient temperature temperature on theon changesthe changes of (a )of N-NH (a) N-NH4 and4+ (andb) N-NO (b) N-NO3− nitrogen3− nitrogen in the in 15–30 the 15–30 cm soil cm layer soilwith layer a simulatedwith a simulated precipitation precipitation of 20 mm. of Values20 mm. followed Values followedby different by lettersdifferent are letters significantly are significantly different ( pdifferent0.05) based(p ≤ 0.05) on Fisher’sbased on LSD Fisher’s test. LSD test. ≤

In both the urea-fertilizedurea-fertilized andand ammoniumammonium nitrate-fertilizednitrate-fertilized soil samples, significantlysignificantly higher levels of of N-NO N-NO33−− nitrogennitrogen in in the the 15–30 15–30 cm cm soil soil layer layer were were recorded recorded at atsoil soil temperatures temperatures of 15 of 15and and 20 °C20 ◦(FigureC (Figure 4).4 ).This This indicates indicates an an intensive intensive process process of of nitrification nitrification under under these conditions, whichwhich isis corroborated by the research findingsfindings ofof otherother researchersresearchers [[15,16].15,16]. At lower soil temperatures (5(5 andand 1010 ◦°C),C), the content of N-NON-NO33− inin the the soil soil was was on on average average 21% 21% lower inin the the urea-fertilized urea-fertilized soil soil and and 25% lower25% lower in the ammoniumin the ammonium nitrate-fertilized nitrate-fertilized soil when soil compared when withcompared that at with 15 and that 20 at◦C. 15 At and a simulated 20 °C. At precipitation a simulated levelprecipitation of 20 mm, level the contentof 20 mm, of N-NO the content3− in the of soil, N- NOregardless3− in the of soil, the fertilizerregardless applied, of the wasfertilizer about applied, 10% higher was thanabout that 10% at higher the 10 mmthan precipitation that at the 10 level. mm precipitationThe regression level. analysis showed that at the 20 mm precipitation level, the relationships between N-NOThe3− regressionin the 15–30 analysis cm soil showed layer th andat at the the soil20 mm temperature precipitation (in level, the range the relationships of 5–20 ◦C) between for the 2 2 N-NOurea-fertilized3− in the and15–30 ammonium cm soil layer nitrate-fertilized and the soil treatments temperature were (in both the strongrange (ofR 5–20= 0.840 °C) and forR the= urea-0.805, fertilizedrespectively) and andammonium significant nitrate-fertil (p 0.05),ized and maytreatments be described were both by the strong following (R2 = 0.840 quadratic and R equations:2 = 0.805, ≤ 2 2 respectively)yNH4NO3 = 6.025 and+ significant1.1035x 0.0318(p ≤ 0.05),x and andy CO(NH2)2may be described= 6.2625 + by0.9367 the followingx 0.0277 xquadratic. equations: − − + yNH4NO3Zhang = 6.025 et al.+ 1.1035 [45] andx − 0.0318 Shavivx2 and and Hagin yCO(NH2)2 [46 =] 6.2625 suggest + that0.9367 thex − ratio 0.0277 ofx N-NH2. 4 to N-NO3− is of a greatZhang significance et al. [45] and and can impactShaviv plantand Hagin growth. [46] The suggest highest that grain the yield ratio andof N-NH total yield4+ to N-NO were obtained3− is of a + withgreat asignificance 25:100 mixture and can of N-NH impact4 plant/N-NO growth.3− nitrogen. The highest The findings grain yield of the and current total yield study were suggest obtained that withthis ratioa 25:100 is closer mixture to optimalof N-NH (28–404+/N-NO/60–72%)3− nitrogen. with The urea findings fertilization of the current than with study ammonium suggest that nitrate this ratiofertilization is closer (9–17 to/ 87–91%).optimal (28–40/60–72%) with urea fertilization than with ammonium nitrate fertilizationThe content (9–17/87–91%). of Nmin in the 0–15 cm soil layer was directly dependent on the fertilizer form, i.e., onThe their content chemical of Nmin composition in the 0–15 andcm soil soil layer temperature was directly at both dependent simulated on precipitationthe fertilizer levelsform, i.e., (10 onand their 20 mm). chemical Significantly composition higher and amounts soil temperature of Nmin atat theboth simulated simulated 10 precipitation mm precipitation levels level(10 and were 20 mm).found Significantly in the ammonium higher nitrate-fertilizedamounts of Nmin at soil the with simulated a steady 10 increasemm precipitation in soil temperature level were (Tablefound1 in). theDepending ammonium on the nitrate-fertilized soil temperature, soil thewith content a steady of Nincreasemin in thein soil ammonium temperature nitrate-fertilized (Table 1). Depending soil was 1.4–1.8on the soil times temperature, higher than the that content in the urea-fertilizedof Nmin in the ammonium soil. The smallest nitrate-fertilized differences soil in the was amounts 1.4–1.8 oftimes this highercompound than resultingthat in the from urea-fertilized different forms soil. ofThe fertilizers smallest weredifferences determined in the atamounts 15 and of 20 this◦C. Thecompound lowest resultingamounts offrom Nmin differentin the soilforms were of determinedfertilizers were at 5 de andtermined 10 ◦C. at 15 and 20 °C. The lowest amounts of Nmin in the soil were determined at 5 and 10 °C. Several researchers, including Kandeler et al. [47], Tripolskaja et al. [48], and Zhang et al. [12], have suggested that this is related to the activity of bacteria involved in the processes of nitrogen transformation and organic matter mineralization at higher soil temperatures. This is in line with Cookson et al. [49], who presented data from laboratory research suggesting that the concentration of Nmin in soil significantly (p ≤ 0.05) increases at a 15 °C soil temperature. In the urea-fertilized soil, the relationship between the Nmin content and ambient temperature (in the range of 5–20 °C) was described by a quadratic equation here, while in the ammonium nitrogen-fertilized soil, this

Agronomy 2020, 10, 2011 8 of 12

Table 1. The eﬀect of nitrogen fertilizer forms, ambient temperature, and amounts of precipitation on

the changes of Nmin in the 0–30 cm soil layer.

Temperature ( C) in the 0–15 cm Soil Temperature ( C) in the 0–15 cm Soil Fertilizer ◦ ◦ Layer (Factor B) Layer (Factor B) (Factor A) 5 10 15 20 5 10 15 20 Simulated precipitation at 10 mm Control 12.0a 13.6b 17.3c 20.9d 10.7a 11.5b 14.6de 14.1d NH4NO3 46.0i 48.7lj 54.5k 54.4k 11.5b 12.6c 14.5de 13.7d CO(NH2)2 25.2e 31.2f 39.9h 37.4g 10.5a 11.2ab 15.3e 14.2de Simulated precipitation at 20 mm Control 12.0a 15.4bc 17.0c 13.8ab 11.8ab 11.5a 15.7ef 15.2de NH4NO3 46.8g 52.4h 56.9i 58.2i 12.4b 13.6c 17.7g 16.0ef CO(NH2)2 27.1d 34.1e 42.4f 46.2g 11.4a 12.7b 16.1f 14.7d Values followed by different letters are significantly different (p 0.05) based on Fisher’s LSD test. ≤

Several researchers, including Kandeler et al. [47], Tripolskaja et al. [48], and Zhang et al. [12], have suggested that this is related to the activity of bacteria involved in the processes of nitrogen transformation and organic matter mineralization at higher soil temperatures. This is in line with Cookson et al. [49], who presented data from laboratory research suggesting that the concentration of N in soil significantly (p 0.05) increases at a 15 C soil temperature. In the urea-fertilized soil, min ≤ ◦ the relationship between the Nmin content and ambient temperature (in the range of 5–20 ◦C) was described by a quadratic equation here, while in the ammonium nitrogen-fertilized soil, this relationship was defined by a linear equation. In both cases, the relationships are strong (R2 = 0.906 and R2 = 0.711). Urea-fertilization at temperatures above 15 ◦C resulted in lower Nmin contents in the soil due to nitrogen loss in the form of N-NH3. This agrees with the findings of Li et al. [50], Pan et al. [51], and Rochette et al. [37], who suggest that the volatilization of N-NH3 from urea has been known to be directly dependent on the ambient temperature, soil moisture, and other factors. Gardinier et al. [52] and Sommer and Hutchings [53] have also indicated that nitrogen losses in the form of ammonium begin at ambient temperatures above 15.5 ◦C. The data from our study show that the content of Nmin at 5–10 ◦C is significantly higher in ammonium nitrogen-fertilized soil, and no significant differences caused by the fertilizer form were found at soil temperatures of 15 and 20 ◦C. In all cases, the amount of Nmin in the 15–30 cm soil layer was significantly lower than in the 0–15 cm layer, possibly due not only to the slow migration of Nmin resulting from the low precipitation amount, but also due to lower soil biological activity with an increasing soil depth [47]. The findings of the second experiment (simulated precipitation level of 20 mm) show that the content of Nmin in the 0–15 and 15–30 cm soil layers increased with an increasing ambient temperature for both the ammonium nitrate-fertilized and urea-fertilized soils. In both cases, the relationships 2 2 between Nmin in the soil and soil temperature were strong (R = 0.782 and R = 0.818, respectively). Given the fact that the amount of nitrogen (as an active ingredient) applied with fertilizers was the same, the increase in Nmin in the ammonium nitrate-fertilized soil was only possible due to the mineralization of soil organic matter, whereas in the urea-fertilized soil, this increase was possible due to amide + nitrogen (N-NH2−) transformation to N-NH4 . The former has been confirmed by the Nmin changes in the control treatment soil, i.e., at 20 ◦C, the Nmin nitrogen content was 54.4% higher than at 5 ◦C (estimated 0–30 cm soil layer data). The effect of soil temperature on the mineralization process in the soil has been documented by Hagemann et al. [13], Wang et al. [11], and Zhang et al. [12]. It is also important to note that when fertilizing with urea, the content of Nmin in the 0–30 cm soil layer (i.e., all considered layers) was on average 1.4 times lower when compared with the ammonium nitrate fertilization. Firstly, this could be explained by the incomplete hydrolysis of urea in 7 days and, secondly, by different mineralization intensities via the influence of different fertilizers. More comprehensive research is needed to confirm these assumptions, especially the latter one. Agronomy 2020, 10, 2011 9 of 12

4. Conclusions

+ The longer that soil nitrogen can remain in the ammonium (N-NH4 ) ion form, the lower the chances of nitrogen losses through leaching or denitrification are. The regularities and intensities of + the transformation of amide and low-mobility ammonium nitrogen (N-NH4 ) into highly mobile nitrate nitrogen (N-NO3−) affect the nitrogen fertilizer use efficiency, along with the quality of the environment, especially in the cases where the likelihood of nitrate nitrogen loss due to leaching is high and plant uptake is low. + The content of ammonium nitrogen (N-NH4 ) over seven days in a soil temperature range of 5–20 ◦C was significantly higher in the urea-fertilized soil than in the ammonium nitrate-fertilized soil + here. The highest amounts of ammonium nitrogen (N-NH4 ) were recorded in the soil at 15 ◦C, while at 20 ◦C the contents significantly decreased due to more intensive nitrification processes, which resulted in significant increases in the nitrate nitrogen contents and possibly more intensive volatilization in the + form of NH3. When fertilizing with urea, the ratio of N-NH4 and N-NO3− was closer to the optimal ratio (25/75%) when compared with the ammonium nitrate fertilization. The migration of ammonium + nitrogen (N-NH4 ) into the deeper 15–30 cm layer under the influence of the precipitation levels of 10 and 20 mm was negligible. The amounts of nitrate nitrogen (N-NO3−) in the 0–15 cm soil layer in the soil temperature range of 5–20 ◦C were 1.7–2.3 times lower when fertilizing with urea than when fertilizing with ammonium nitrate at a simulated precipitation level of 10 mm, and 1.6–2.2 times lower at a simulated precipitation level of 20 mm. With increasing soil temperatures, the amounts of nitrate nitrogen (N-NO3−) in the 0–15 and 15–30 cm soil layers increased.

Author Contributions: Conceptualization, R.D. and I.P.; methodology, D.J.; software, I.P.; validation, R.D., I.P, D.J., and A.P; formal analysis, I.P.; investigation, R.D.; resources, A.P.; data curation, D.J.; writing—original draft preparation, R.D. and I.P.; writing—review and editing, R.D., I.P, D.J., and A.P.; visualization, R.D.; supervision, D.J.; All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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