Impacts of Nitrogen Sources and Rates on Soil Chemical Attributes and Potato Nutrition and Yield in a Subtropical Environment

doi:10.21203/rs.3.rs-3228913/v1

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Impacts of Nitrogen Sources and Rates on Soil Chemical Attributes and Potato Nutrition and Yield in a Subtropical Environment

https://doi.org/10.21203/rs.3.rs-3228913/v1

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The nutritional status of the plants, soil chemical attributes prior to vine killing, and the classified yield of tubers may characterize soil-plant interactions with nitrogen (N) side-dressing for potato cultivars, indicating the most appropriate source and rate at sidedressing in subtropical environment. Two experiments with potato cultivars Agata and Atlantic were carried out with the aim of evaluating the effect of N sources (urea, ammonium sulfate and calcium nitrate) and rates (70, 140 and 210 kg ha^− 1) on sidedressing, plus a zero-N control treatment, in the southern Brazil. Discrete variations on nutrients in diagnostic leaves and on soil chemical attributes in the Agata experiment were not reflected in the maximum average increase of 69.0% (14.2 Mg ha^− 1) in total tuber yields obtained with 152 kg N ha^− 1 as ammonium sulfate or calcium nitrate, which did not differ in AE’s. For the Atlantic cultivar, urea rates were practically inert in soil and plant attributes, and the ammonium sulfate rates acidified the soil and increased their Al³⁺ contents in the ridges, reduced Ca and Mg foliar concentrations and did not increase marketable tuber yields. Calcium nitrate increased foliar N, Ca and Mg concentrations, increasing marketable tuber yields by 26.6% (7.3 Mg ha^− 1) with 116.7% higher AE than other N sources. In subtropical acidic inceptisols with potentially available Al³⁺, 153 kg N ha^− 1 as ammonium sulfate or calcium nitrate for cultivar Agata and 152 kg N ha^− 1 as calcium nitrate for cultivar Atlantic are the best options for sidedressing fertilization.

Solanum tuberosum L.

mineral nutrition

soil fertility

agronomic efficiency

N management

With a global production of 359 million tonnes of tubers in 2020, potato (Solanum tuberosum L.) is the third most important food crop and could become one of the most advantageous in terms of food security when maximum cereal crop yields are close to their limit (Dongyu 2022; FAOSTAT 2022). However, in a scenario of climate change, global warming could harm potato crops in regions of higher latitudes, although it should favour potato cultivation in subtropical regions in winter (Hijmans 2003; Bender and Sentelhas 2020). Potato production has stood out in tropical and subtropical countries, including Brazil (Haverkort and Struik 2015; Lizana et al. 2020; FAOSTAT 2022). In these environments, improvements in crop management, such as the application of nitrogen (N), are more important than advances in genetic parameters (Kleinwechter et al. 2016).

Nitrogen is the nutrient most taken up by the main potato cultivars planted in Brazil, mostly (> 80%) during the tuberization phase (Fernandes et al. 2011, 2021; Silva et al. 2020). Thus, it is a key nutrient increasing tuber yields in regions with subtropical climates in the Northern Hemisphere (Zotarelli et al. 2014; Makani et al. 2020) and Brazilian tropical environments (Fontes et al. 2010; Souza et al. 2019; Fernandes et al. 2021). In addition, supplying N in the most demanding phase of the crop has been the best strategy to obtain higher tuber yields and to avoid N losses (Zotarelli et al. 2014; Rens et al. 2016; Souza et al. 2019; Fernandes et al. 2022). In Brazil, the Agata and Atlantic cultivars are the main potato cultivars produced for the fresh market and as chips, respectively, and differ in growth (Fernandes et al. 2010; Silva et al 2013), nutrient requirements (Fernandes et al. 2011; Silva et al. 2020), nutrient utilization efficiencies (Fernandes and Soratto 2013) and tuber yield in response to N fertilization (Fontes et al. 2010; Oliveira et al. 2020). In addition, these cultivars present different behaviours in dry matter partitioning to the tubers according to the N source (Silva et al. 2013).

The application of the most suitable source and rate of N in sidedressing must be precise and synchronized with a short period of maximum N uptake by the potato crop, which is the initial stage of tuberization (Fernandes et al. 2011). The application of granulated ammonium nitrate during preplanting, of nitrate, urea or liquid ammonium during the initiation of potato tubers (Zotarelli et al. 2014; Rens et al. 2016), of granulated ammonium nitrate dissolved in water for drip irrigation according to nutritional demands (Makani et al. 2020) and of urea divided into four applications during the crop cycle (Liu et al. 2021) are examples of N management with sources, rates and application times of N in subtropical sandy soils in the Northern Hemisphere. In Brazil, sidedressing N fertilization for potato crops is usually carried out immediately before the hilling operation, approximately 30 days after planting (DAP) (Souza et al. 2019; Yagi et al. 2020), and urea or ammonium sulfate (AMS) is commonly used (Souza et al. 2019; Fernandes et al. 2021). Fertilizer incorporation into the soil by adding N fertilizers with or without other nutrients, such as potassium (K) and chlorine (Cl), is a common practice in Brazil and can affect both the dynamics and the availability of the main inorganic forms of N, as well as the soil chemical attributes of the ridge microenvironment.

A partial nitrification of NH₄⁺-N, resulting from sidedressing N application or from the mineralization of soil organic matter, concentrates NH₄⁺-N and NO₃⁻N at the top of the ridge right after hilling, increasing the NO₃⁻-N levels at the base of the ridge until the plants desiccate (Zebarth and Milburn 2003). NH₄⁺-N should be the form of N metabolically preferred by potato plants, as it does not need to be reduced to be assimilated, but it is harmful to plant growth when available in high concentrations and exclusively at any stage of development (Davis et al. 1986). Potato plants tolerate and increase the dry matter of stems, leaves, and tubers in a less wide range with NH₄⁺-N than with NO₃⁻-N (Cao and Tibbitts 1998). Furthermore, the pH decreases and the electrical conductivity of the medium increases with the contribution of NH₄⁺-N in relation to NO₃⁻-N (Serio et al. 2004). At moderate concentrations, NH₄⁺-N favours shoot growth before tuber initiation (Gao et al. 2014), stimulates tuber bulking (Qsaki et al. 1995) and earlier tuber initiation (Qiqige et al. 2017), while NO₃⁻-N promotes the branching and growth of stolons (Qsaki et al. 1995), which swell or form tubers earlier (Gao et al. 2014) and in greater quantity than plants fed with NH₄⁺-N (Serio et al. 2004; Gao et al. 2014; Qiqige et al. 2017).

NH₄⁺-N uptake decreases calcium (Ca) concentrations in the shoots of potato plants compared to NO₃⁻-N nutrition (Polizotto et al. 1975; Davis et al. 1986; Cao and Tibbitts 1993), which motivates an additional supply of Ca to prevent possible deficiencies (Cao and Tibbitts 1993). In this sense, calcium nitrate (CN) has been evaluated as a source of Ca for potato plants with supplementary input of N to isolate the effect of Ca (Simmons and Kelling 1987; Giletto et al. 2022), as well as a source of N in split applications for plant emergence and tuber initiation stages in sandy soil in temperate climates (Bundy et al. 1986). According to the latter authors, CN did not increase total and graded tuber yields compared to similarly split applications of AMS, urea, and ammonium nitrate over five years, but on average over the years, AMS increased total tuber yields by 3.5 Mg ha^− 1 (6.2%) in relation to the average of the other N sources. Given the above, we hypothesize that in soils of medium texture in a subtropical environment, the effects of ammoniacal, nitric, and amidic sources for sidedressing N fertilization of potato crops can be distinct and vary according to the potato cultivar, with these effects being observed in tuber yield and size, plant nutritional status, and soil chemical attributes.

The objective of this study was to evaluate the effects of sidedressing N sources and rates on soil fertility, plant nutrition, and the tuber yield and size of Agata and Atlantic potato cultivars in subtropical conditions of southern Brazil.

2.1 Site, Experimental Design and Crop Management

The study was carried out during the 2020–2021 spring-summer season in a commercial potato field in Porto Amazonas, Paraná State, southern Brazil (25°34'06''S; 49°55'54''W; altitude of 817 m). Two experiments were conducted concurrently in adjacent areas, one with Agata and the other with the Atlantic cultivar. The soil is classified as a Typic Inceptisol (Soil Survey Staff 2014). Representative soil samples from each experimental area were collected before the beginning of the experiments and subjected to chemical (Pavan et al. 1992) and granulometric (EMBRAPA 1997) analyses (Table 1). The climate in the region is humid subtropical CWb (mild summer, evenly distributed rainfall and no dry season), according to the Köppen classification system. Daily climatological data for the experimental period were recorded at a meteorological station approximately 1.5 km from the experimental areas (Figure S1).

Table 1

Chemical and textural characteristics of the soils before the potato planting, according to cultivars Agata and Atlantic.
Soil attribute	Agata cultivar	Atlantic cultivar
pH (CaCl₂)	5.3	5.1
C_org, g dm^–3	21	20
P_mehlich−1, mg dm^–3	17	16
H + Al, cmol_c dm^–3	5.5	6.0
Ca²⁺, cmol_c dm^–3	8.7	7.2
Mg²⁺, cmol_c dm^–3	1.6	1.5
K⁺, cmol_c dm^–3	0.7	0.6
Al³⁺, cmol_c dm^–3	0.2	0.2
SO₄²⁻-S, cmol_c dm^–3	4.1	4.1
CEC, cmol_c dm^–3	15.7	15.3
Base saturation, %	65	61
B, mg dm^–3	0.22	0.21
Cu, mg dm^–3	1.24	1.34
Fe, mg dm^–3	86.5	57.0
Mn, mg dm^–3	3.80	7.60
Zn, mg dm^–3	0.20	0.38
Sand, g kg^–1	525	520
Silt, g kg^–1	152	149
Clay, g kg^–1	323	331
C_org, organic carbon; Ca²⁺,Mg²⁺, and K⁺ indicate basic exchangeable cations; CEC, cation exchange capacity.

In both experiments, the treatments were composed of the combination of three N rates (70, 140, and 210 kg N ha^–1) with three N fertilizer sources [urea (U), 45% N; ammonium sulfate (AMS), 20% N + 24% S; and calcium nitrate (CN), 16% N + 19% Ca, sidedressed immediately before hilling, in addition to a control without sidedressed N fertilization. The treatments were arranged in a 3×3 + 1 factorial scheme, with a randomized block design and four replications. Cultivated with black oats (Avena strigosa Schreber) before the implementation of the experiments, the areas were desiccated (1500 g ha^− 1 glyphosate), and the soil was prepared one week before planting the seed tubers with subsoiling, ploughing and leveling harrowing. In this process, no correction was made for soil acidity with limestone due to base saturation being higher than 60%, a critical value for limestone application according to regional criteria (NEPAR-SBCS 2017).

Each plot was composed of four 5-m-long rows spaced by 0.8 m, where the area corresponding to the two centermost rows less than 0.5 m from each end was used for plant and soil evaluations to avoid the border effect. The furrows were mechanically opened with a furrower, and the planting fertilizers were applied manually and incorporated into the furrows in all plots with a hoe at an average depth of 0.15 m. The simple superphosphate fertilizer and the multinutrient formula in the same granule (YaraBasa® Soberano™) containing 6% N, 12% P, 5% K, 8% Ca, 8.4% S, 0.03% B, 0.05% Mn, and 0.1% Zn were mixed for initial fertilization, providing 40 kg N ha^− 1, 183 kg P ha^− 1, 33 kg K ha^− 1, 349 kg Ca ha^− 1, 218 kg S ha^− 1, 0.201 kg B ha^− 1, 0.335 kg Mn ha^− 1, and 0.670 kg Zn ha^− 1 according to the results of chemical soil analyses (Table 1), recommendations for potato cultivation (Soratto et al. 2016; NEPAR-SBCS 2017), meeting the demands of S by potato plants (Souza et al. 2019), and transferring the amounts of K to be applied from planting to sidedressing prior to hilling along with N fertilization (Yagi et al. 2020). Thus, along with the studied N treatments, 91 kg K ha^− 1 (KCl, 50% K) was also applied in sidedressing in all plots, according to regional recommendations (NEPAR-SBCS 2017).

Seed tubers of the potato cultivars Agata and Atlantic, weighing approximately 35 g and with diameters between 30 and 40 cm, were planted with a spacing of 0.30 m between tuber seeds on October 19, 2020, with subsequent application of pesticides authorized for the crop. The use of agrochemicals was guided by pest management recommendations commonly used in Brazil (Lopes et al. 2016), using drones with pesticide applications in an ultralow volume system. The furrows were then closed, and the plants emerged late, at 24 and 35 DAP, respectively, for the Agata and Atlantic cultivars, probably due to the rain and the subsequent minimum temperatures (Figure S1). Supplementary irrigation was not necessary due to the adequate distribution of rainfall during the experiment (Figure S1), with 595 mm throughout the entire crop cycle and 480 mm (80.7%) after the application of treatments and hilling. At 40 DAP, N treatments were manually applied, plus KCl in all plots, prior to the hilling operation. At 60 DAP, Mn, Cu, Zn, and B foliar applications were performed using a drone at the respective rates of 165, 55, 42, and 150 g ha^− 1.

2.2. Sampling and Analysis

Plant nutritional status was evaluated from diagnostic leaf samplings at 34 and 30 days after emergence (DAE) in experiments with the Agata and Atlantic cultivars. The third expanded leaf (leaflets and petioles) from the apex of 12 plants selected randomly from each plot was collected. The leaves were washed in deionized water and dried in an oven with forced air circulation at 65°C for 72 h, ground in a 1.0 mm sieve and then chemically analysed according to the procedures described in Malavolta et al. (1997). After nitric-perchloric digestion (HNO₃ + HClO₄), the concentrations of P, Ca, Mg, S, Cu, Mn and Zn were determined by atomic absorption spectrophotometry, and those of K were determined by flame photometry. N concentrations were determined from extraction with sulfuric acid (H₂SO₄) and quantification by the semimicro-Kjeldahl method. After the dry digestion of leaf samples at 550°C and dilution with dilute HCl, B concentrations were determined colorimetrically using the azomethine-H method.

After the desiccation of the plants, soil samples were taken (0-0.2 m) by collecting 12 simple samples per sample in each plot in the planting furrows in the intervals between the potato plants. Soil samples were air-dried, sieved and chemically analysed using methods described in Pavan et al. (1992). The active acidity (pH) was estimated potentiometrically with a CaCl₂ solution of 0.01 mol L^− 1 (ratio 1:2.5; soil:CaCl₂ solution), the potential acidity (H + Al) by the SMP buffer method and the exchangeable acidity (Al³⁺) with extraction with 1 mol L^− 1 KCl solution (1:10 ratio; soil:KCl solution) and titration with 0.0125 mol L^− 1 NaOH. Exchangeable Ca and Mg (Ca²⁺ and Mg²⁺) were extracted with 1 mol L^− 1 KCl and quantified by atomic absorption spectrophotometry. Exchangeable K (K⁺) was extracted with Mehlich-1 solution and determined by flame photometry. The cation exchange capacity (CEC) was calculated as the sum of the concentrations of H + Al, Ca²⁺, Mg²⁺, and K⁺. The base saturation index (i.e., Ca²⁺, Mg²⁺, and K⁺ saturation) in the ion exchange complex was calculated according to the formula for base saturation (%) = ((Ca²⁺ + Mg²⁺ + K⁺)/CEC)*100.

Prior to harvesting, the plots were thinned and standardized to 12 plants per row in the two central rows in both experiments, excluding the plants at the ends of the rows. Harvesting was carried out at 150 DAP in a semimechanized manner with a tractorized mat puller followed by manual collection. According to the transverse diameter, the tubers were classified as marketable (> 45 mm) and nonmarketable (< 45 mm) and were then weighed. The total tuber yield (TTY) was obtained by adding the yields of marketable (MTY) and nonmarketable (NMTY) tubers.

Agronomic efficiency (AE) was calculated as a measure of the yield gained (marketable and total tuber yield) over the control (without sidedressing N fertilization) in relation to the N applied [AE (kg tuber increased kg^− 1 N applied) = (yield with sidedressing N application – yield without sidedressing N application)/sidedressing N application rate].

2.3. Statistical Analyses

A combined analysis of variance using the two experiments (cultivars) resulted in significant interactions for almost all measured variables; therefore, individual analyses of variance were separately performed for each cultivar using Agroestat software (Barbosa and Maldonado 2015). The blocks and all of the block interactions were considered random effects. The sidedressing N rate and source were considered fixed effects. To maintain a balanced experimental design, the data from the control (without sidedressing N fertilization) were excluded from the tests of the main effect of the N source and N rate × N source interaction in a (3×3) + 1 factorial scheme. The effect of the quantitative factor was evaluated by polynomial regression analyses independently of the analysis of variance (Finkelman et al. 1977; Piepho and Edmondson 2018); therefore, the effects of N rates were split between the three N sources for all dependent variables. For this purpose, the control (without sidedressing N fertilization) was considered the zero rate common for all independent variables. Significant (p ≤ 0.05) regression equations between the N rates and dependent variables were plotted, and the most significant polynomial equation was selected between linear or quadratic functions.

The N source means were compared using Tukey’s test (p < 0.05), according to the interaction (p < 0.05) between the factors in the F test of the analysis of variance.

3.1. Plant Nutritional Status

There were no differences between the effects of the sidedressing N sources on leaf N concentrations in both experiments (Table S1). There was no significant effect of the rates of each N source on the N concentrations in leaves of the Agata cultivar (Fig. 1a). In the Atlantic potato experiment, mean leaf N concentrations increased linearly and quadratically with rates of urea-N and CN-N, respectively (Fig. 2a). The respective maximum increments under an applied N rate of 210 kg N ha^− 1 were 15.2% and 16.5% in relation to the control treatment.

The leaf Ca concentrations of the Agata cultivar were 25.6% lower on average under urea compared to the averages obtained with AMS and CN (Table S1), whose rates decreased and linearly increased the leaf Ca concentrations, respectively (Fig. 1a). With 210 kg N ha^− 1 as CN, a 12.4% increase in the Ca concentration in the leaves was obtained, while with AMS, there was a decrease of 18.9%. In leaves of Atlantic potato plants, Ca concentrations differed between the three N sources, with higher concentrations in response to CN, intermediate concentrations in response to urea and lower concentrations in response to AMS (Fig. 2b). Linear decreases with CN-N rates and quadratic decreases with AMS-N and urea-N rates were observed for leaf Ca concentrations, i.e., 14.0% with 210 kg N ha^− 1, 37.3% with 175 kg N ha^− 1 and 12.1% with 100 kg N ha^− 1. In both potato cultivars, leaf Mg concentrations were lower with urea compared to top dressing with AMS and CN (Table S1), by an average 14.0% for Agata and 15.9% for Atlantic (Table S1). The quadratic effects of the CN-N and AMS-N rates on the Mg concentrations in the Atlantic potato leaves were antagonistic, showing maximum and minimum concentrations of 4.8 and 3.6 g kg^− 1 with 95 and 108 kg N ha^− 1, respectively (Fig. 2c).

Among the N sources, the concentrations of Mn in the leaves of the Agata cultivar did not follow a definite trend, sometimes showing higher concentrations with urea compared to CN (rate of 70 kg N ha^− 1) and sometimes higher concentrations with AMS compared to urea (rate of 140 kg N ha^− 1) (Table S1; Fig. 1). Similarly, Zn concentrations in leaves of the Agata cultivar showed no definite trend at these intermediate N rates. The rates of sidedressing N as AMS linearly increased the B concentrations in the leaves of the Atlantic cultivar (Fig. 2d), which increased by an average of 8.4% in relation to the other N sources (Table S1). Significant interactions between N sources and rates were observed for leaf Mn and Zn concentrations in the Agata experiment and for Mn concentrations in diagnostic leaves in the 'Atlantic' potato experiment (Table S1, Figs. 1 and 2). The Mn concentrations in the Agata potato leaves did not fit any polynomial regression model (Fig. 1c). In the Atlantic cultivar, there were quadratic effects of AMS-N and CN-N rates on leaf Mn concentrations (Fig. 2e). Among the N sources, the concentrations of Mn in the leaves of the Agata cultivar did not follow a definite trend at the intermediate rates, sometimes presenting higher concentrations (42.6%) with urea in relation to CN (70 kg N ha^− 1) and sometimes higher concentrations (43.7%) with AMS in relation to urea (140 kg N ha^− 1). Similarly, the Zn concentrations in the leaves of the Agata cultivar followed the same undefined trend at these intermediate N rates (Fig. 1d).

3.2. Soil Chemical Attributes

The chemical attributes of the soil in the ridge at the desiccation time of the 'Agata' potato plants were not influenced by the treatments (Table S2), except the Al³⁺ contents, which increased linearly with the rates of N in the form of AMS (Fig. 3a), as well as in the soil samples collected in the desiccation period of the 'Atlantic' potato experiment (Fig. 3b). In both experiments, Al³⁺ contents were higher with AMS in relation to the other N sources (which did not differ), i.e., an average of 107.0% higher in the experiment with Agata and 72.9% higher in the experiment with Atlantic (Table S2). In the experiments with the Agata and Atlantic cultivars, the Al³⁺ contents increased to 0.38 and 0.24 cmol_c dm^− 3, respectively, at a rate of 210 kg ha^− 1 AMS-N. In the experiment with Atlantic, in addition to the effect on the Al³⁺ content, there was also an effect of the N sources on the pH (CaCl₂) of the soil and the interaction of the N sources and rates on the Mg²⁺ content of the soil (Table S2).

The rates of CN-N and AMS-N both quadratically increased and decreased the soil pH (CaCl₂) values up to the rate of 75 kg N ha^− 1, with maximum and minimum values of 5.2 and 4.9, respectively. At rates of 70 and 140 kg N ha^− 1, the pH (CaCl₂) under AMS decreased by 0.3 (4.8%) and 0.4 (7.5%), respectively, in relation to the means obtained in response to urea and CN, which did not differ (Fig. 4d). Similarly, base saturation values also decreased quadratically with AMS-N rates, reaching a minimum value of 56% at 89 kg N ha^− 1 and then increasing to 89 kg ha^− 1 at 65%. The Mg contents also fit a quadratic model with the AMS-N rates, decreasing by up to 0.6 cmol_c dm^− 3 with 71 kg ha^− 1 of N but with a subsequent increase of up to 1.7 cmol_c dm^− 3 with the maximum rate of N applied as sidedressing, a value 81.9% higher than the average of urea and CN at 210 kg N ha^− 1.

The K⁺ contents in the soil increased with the application of urea at a rate of 70 kg N ha^− 1 followed by a reduction at rates of 140 and 210 kg N ha^− 1. In relation to the control treatment, there was an increase of 28.5% in K+ (0.77 cmol_c dm^− 3 with 130 kg ha^− 1 urea-N; Fig. 3e) in addition to decreases of 8.3% (55% base saturation with 95 kg ha^− 1 AMS-N; Fig. 3b) and 39.0% in Mg²⁺ (0.59 cmol_c dm^− 3 with 74 kg ha^− 1 AMS-N; Fig. 3f). With 210 kg N ha^− 1 applied as sidedressing, however, there were increases of 95.2% in Mg²⁺ (1.9 cmol_c dm^− 3; Fig. 3f), where AMS increased the Mg contents by 55.6% and 134.7% in relation to CN and urea, respectively (Fig. 3f).

3.3. Tuber Yield and Agronomic Efficiency

Nonmarketable, marketable and total potato tuber yields in the experiments with the Agata and Atlantic cultivars were not influenced by N sources or by the N source × N rate interactions (Table S3). However, the potato tuber yields of the Agata and Atlantic cultivars responded distinctly to the N rates applied with the sources tested (Fig. 4).

The total tuber yields of the Agata potato cultivar increased linearly with rates of urea-N and quadratically with rates of AMS-N and CN-N, resulting in maximum yields of 40.7, 35.5 and 34.2 Mg ha^− 1 at the respective rates of 210, 152 and 152 kg N ha^− 1 (Fig. 4a) and in AE values of 194, 234 and 225 kg tubers kg^− 1 applied N. The yields of marketable tubers varied similarly to the total yields, linearly for the urea-N rates and quadratically for the AMS-N and CN-N rates, resulting in maximum yields of 30.1, 25.0 and 24.2 Mg ha^− 1 at respective rates of 210, 161 and 145 kg N ha^− 1 (Fig. 4c) and AE values of 143, 155 and 167 kg tubers kg^− 1 of applied N (Fig. 5). The yields of nonmarketable tubers increased linearly up to 14.5 Mg ha^− 1 with urea-N rates of 210 kg ha^− 1 N and did not vary (p > 0.05) with rates of N in the form of AMS and CN (Fig. 4e).

In the Atlantic potato experiment, total tuber yields increased linearly with AMS rates and quadratically with CN rates, resulting in maximum yields of 35.7 and 38.2 Mg ha^− 1 at rates of 210 and 130 kg N ha^− 1 (Fig. 4b) and in AE values of 170 and 294 kg tubers kg^− 1 of applied N. Only the CN rates affected the yields of marketable tubers, which in a quadratic manner resulted in a maximum yield of 34.8 Mg ha^− 1 with 129 kg N ha^− 1 (Fig. 4d) and in an AE of 270 kg tubers kg^− 1 applied N. There was no effect of rate for any N source on nonmarketable tuber yields (Fig. 4e). Tuber yields did not change linearly or quadratically as a function of the urea-N rates applied (Figs. 4b, d, e).

There was an interaction between N sources and rates for AE (Table 2) in terms of TTY (Fig. 5), as well as an effect of N rates on AE in terms of MTY for the Agata cultivar (Table 2). With 70 kg N ha^− 1, AMS presented an AE that was 70.4% greater than that of urea, with no differences between these N sources and CN (Fig. 5). This same N rate presented an AE that was 105.9% higher in terms of the Agata potato marketable tuber yield compared to the average of the other N rates. For the Atlantic cultivar, the CN presented AE values that were 117.1% and 116.7% higher in terms of the respective marketable and total tuber yield in relation to the means obtained with AMS and urea, which did not differ from each other (Table 2).

Table 2

Agronomic efficiency for marketable (MTY) and total (TTY) tuber yields of two potato cultivars, as affected by sidedressing nitrogen rates and sources, and analyses of variance.
Treatment	Agata cultivar		Atlantic cultivar
	MTY	TTY	MTY	TTY
	^{________________} (kg tuber increase kg^− 1 N applied) ^{________________}
Sidedressing N rate (kg ha^− 1)
70	157.3a	201.5	64.2	64.8
140	85.6b	104.3	45.5	48.0
210	67.2b	78.7	69.7	68.2
N source
Urea	101.0	121.3	38.4b	37.1b
Ammonium sulfate	109.9	143.9	47.7b	49.7b
Calcium nitrate	99.2	119.3	93.3a	94.2a
Source of variation	^{_____________________________________} p > F ^{_____________________________________}
N rate (NR)	< 0.001	< 0.001	0.304	0.384
N source (NS)	0.779	0.323	0.005	0.002
NS × NR	0.058	0.048	0.091	0.064
Values followed by the same letter in a column for each factor (N rate and N source) are not significantly different at p < 0.05 according to Tukey’s test.

The results of this study make evident the effects of sources and rates of N in sidedressing for potato crops as highly dependent on the soil‒plant interface. The plant nutritional status, soil chemical attributes in preharvest ridges, and tuber yields varied systematically and interrelatedly but distinctly among the potato cultivars. The Agata and Atlantic cultivars, the main cultivars planted in Brazil for the fresh market and frying in the form of chips, respectively (Fontes et al. 2010), have different nutrient requirements throughout their cycles, especially after the beginning of tuberization, highlighting the highest amounts of N and basic cations extracted by the Atlantic cultivar compared to Agata (Fernandes et al. 2011; Silva et al. 2020). This characteristic becomes relevant because, in addition to N transformations in the soil, the exports of basic cations are identified as the two main causes of acidification in subtropical soils (Dong et al. 2022).

Thus, the higher extractions of Ca, Mg and K by the Atlantic cultivar and the application of AMS rates in sidedressing may have been jointly determinant in the decreases in pH and base saturation in the ridges (Fig. 3), considering that these effects on soil chemical attributes were less intense with rates of AMS applied as sidedressing for Agata potato, except for the Al³⁺ concentration in the soil. Greater S extractions by the Atlantic cultivar, in relation to Agata (Fernandes et al. 2011; Silva et al. 2020), also justify a greater effect of AMS rates on soil Al³⁺ solubilization in the experiment with Agata potato, represented by the highest angular coefficient in the respective linear regression.

Soil acidification, particularly related to AMS application, can be attributed both to NH⁴⁺-N nitrification (Mizuno et al. 2000) and to the release of organic acids and protons in the root zone in response to NH⁴⁺-N uptake to maintain the internal ionic balance (Kissel et al 2020). One should also consider the greater affinity of sulfate anions (SO₄²⁻-S) than of NO₃⁻-N for basic cations in the formation of zero-charge ionic pairs susceptible to leaching (Foloni et al. 2006), in addition to the colloidal exchange complex in planting furrows. Furthermore, soil acidification under AMS-N rates in the Atlantic potato experiment may also have been responsible for the increases in the leaf B and Mn concentrations, similar to what was observed by Maier et al. (2005) and contrary to the results obtained by Braun et al. (2011) for leaf Mn concentrations, who observed a linear decrease in Mn concentrations in Atlantic cultivar tubers with AMS-N rates.

Although soil acidification with sidedressing AMS was identified only in the experiment with the Atlantic cultivar, the Al³⁺ contents increased linearly with rates of AMS-N in sidedressing in both experiments. However, it should be noted that these increases in Al³⁺ contents occurred even with the application of 889 kg ha^− 1 of agricultural gypsum in the planting furrows together with phosphate fertilization with single superphosphate. Consequently, the lower leaf Ca concentration in both potato cultivars and in the Mg concentration in Atlantic plants with higher rates of AMS may be related to the higher Al³⁺ ester provided by this treatment. Increased availability of Al³⁺ from the soil can also decrease concentrations and accumulations of Ca and Mg in shoots and roots of potato plants due to the competition of these cations for the same absorption sites in the roots (Lee 1971; Kock et al. 2020). Furthermore, NH₄⁺-N uptake decreases Ca and Mg concentrations in the shoots of potato plants compared to NO₃⁻-N nutrition (Polizotto et al. 1975; Davis et al. 1986; Cao and Tibbitts 1993) since the supply of NH4+-N inhibits the absorption of Ca and Mg by plants to maintain the internal ionic balance (Polizotto et al. 1975). The results obtained in the present study corroborate those obtained in a greenhouse by Maier et al. (2015), who also observed lower leaf concentrations of Ca and Mg and lower soil pH values after harvest in pot experiments with the Atlantic cultivar fertilized with AMS in relation to urea, ammonium nitrate and CN. Souza et al. (2021) also observed lower concentrations of Mg in tubers of the Agata cultivar fertilized with AMS compared to urea, both split at planting and as top dressing.

Decreases in leaf Ca concentrations in both potato cultivars according to the increase in sidedressing N rates, which were inverse in Agata plants and less intense in the Atlantic cultivar when both were fertilized with CN, can be attributed to a dilution effect of N fertilizers. However, even at the highest N rate applied, leaf Ca concentrations in both cultivars were above the lower limit of 7.5 g Ca kg^− 1 considered adequate (NEPAR-SBCS 2017). This difference between potato cultivars with the application of Ca with CN can be attributed to the lower accumulation of dry matter in the leaves of Agata plants than in those of Atlantic plants with the application of CN (Silva et al. 2013), justifying the absence of a dilution effect and increases in leaf Ca concentrations in the former. Furthermore, potato plants treated with NH₄⁺-N show greater leaf area (Qiqige et al. 2017) as well as lower concentrations of Ca and Mg in the shoot (Polizotto et al. 1986; Davis et al. 1986; Cao and Tibitts 1993, 1994) than those treated with NO₃⁻-N, agreeing with the more pronounced decreases in leaf concentrations or lower concentrations of Ca in both potato cultivars and Mg in the Atlantic cultivar with rates of AMS-N in relation to the other sources of N. Similar results to this study were obtained by Maier et al. (2015) in three experiments in a greenhouse, where CN application systematically provided higher leaf concentrations of Ca and Mg compared to urea, AMS and ammonium nitrate in Russet Burbank and Atlantic cultivars.

Unlike leaf Ca concentrations, the dilution effect does not justify the absence of treatment effects on leaf N concentrations in the Agata cultivar, as this cultivar has leaves with lower dry matter production compared to Atlantic potato leaves (Fernandes et al. 2010; Silva et al. 2013). In the present experiment with the Agata cultivar (Fig. 1a), only trends of linear increases in leaf N concentrations were identified for rates of urea-N (R² = 0.77; P = 0.094) and AMS (R² = 0.98; P = 0.138), as well as a quadratic effect for CN-N rates (R² = 0.99; P = 0.139) (data not shown). In this case, the assessment of the nutritional N status of the plants may have been hampered by the difference between the dates of emergence of the plants in the two experiments, by the volume of rainfall between the application of the treatments and the leaf sampling, and by the differences between the system roots of the studied cultivars.

Leaf samplings were carried out 18 and 28 days after the application of N as sidedressing in the respective experiments with cultivars Agata and Atlantic. During the period from the application of treatments to the collection of diagnostic leaves in both experiments, 154 mm of rainfall was recorded, of which 47.5% (73 mm) occurred in intense events in the five days following the application of treatments. This rainfall regime in these different time intervals for the nutritional diagnoses of the plants, associated with the smaller accumulation of roots of the Agata cultivar in relation to the Atlantic potato (Fernandes et al. 2010; Silva et al. 2013), justifies both the absence of variations in leaf N concentrations in the Agata cultivar and the occurrence of more significant variations in nutrient concentrations in the experiment with the Atlantic cultivar. The absence of differences between N sources in the leaf N concentrations of the Agata cultivar was also attributed to the short time interval between top dressing and leaf sampling by Souza et al. (2019) in the evaluation of AMS, urea, and AMS with nitrification inhibitors.

Except for the respective control treatments, the leaf N concentrations in both potato cultivars are within the range considered adequate, from 40 to 60 g kg^− 1 (NEPAR-SBCS 2017). However, the maximum leaf N concentrations observed in this study for the cultivars Agata (44.0 g kg^− 1 with 210 kg urea-N ha^− 1) and Atlantic (45.8 g kg^− 1 210 kg CN-N ha^− 1) are lower than those observed in studies that evaluated the application of N at planting in potato crops. For example, considering the application of urea at planting at the maximum rate used in the present study (210 kg N ha^− 1), leaf concentrations of 65.1 g N kg^− 1 were measured in Agata potato (Coelho et al. 2010) and of 48.9 and 50.6 g N kg^− 1 were measured in the diagnostic leaves of Agata and Atlantic potatoes, respectively (Oliveira et al. 2020).

Thus, later assessments of the N nutritional diagnosis of potato plants, in addition to the recommended standard time for the crop, are necessary for more accurate assessments in studies on top-dressing N fertilization for potato crops. The results obtained by Souza et al. (2019) corroborate this argument based on N concentrations in diagnostic leaves of the Agata cultivar in three experiments in three local years with collections 8, 10 and 20 days after the application of N. These authors measured leaf N concentrations of 45.3, 46.9, and 53.9 g kg^− 1 under AMS and of 43.9, 44.0, and 50.6 g kg^− 1 under urea. According to Fernandes et al. (2022), later samplings of diagnostic potato leaves after traditional sidedressing N fertilization are feasible and necessary to maintain N availability throughout tuberization to obtain higher tuber yields.

The responses in tuber yield as a function of N sources and rates in sidedressing were consistent with the interrelationships between plant nutritional diagnoses and soil chemical attributes, both influenced by the different nutritional requirements of the studied potato cultivars. Despite being less demanding in macro- and micronutrients (Fernandes et al. 2011; Silva et al 2020) and having a smaller root system compared to the Atlantic cultivar (Fernandes et al. 2010; Silva et al. 2013), the Agata cultivar has higher nutrient utilization efficiency, i.e., produces more fresh tubers with less absorbed nutrients (Fernandes and Soratto 2013). This is probably the justification for this potato cultivar to have shown a higher AE, in addition to more responses and greater magnitudes to the rates of N sources in sidedressing in relation to the Atlantic potato, e.g., maximum increases in the total tuber yields of Agata and Atlantic of 67.5% (14.3 Mg ha^− 1) and 55.9% (12.8 Mg ha^− 1) with AMS and of 69.3% (14.0 Mg ha^− 1) and 36.9% (10.3 Mg ha^− 1) with CN.

The effect of sidedressing N fertilization on the Agata potato crop was dependent on the N source as well as the applied rate. With higher AE in terms of marketable tuber yields and total tuber yields, 70 kg N ha^− 1 applied as AMS was advantageous, and between 140 and 210 kg N ha^− 1, the total yields or marketable tuber yields and their respective AEs were similar between N sources. However, with 210 kg N ha^− 1 applied as urea, there were still increases in tuber productivity, which did not occur with AMS and CN at this same rate. With this rate and source of N, there were increases in total productivity, of 14.7% (5.2 Mg ha^− 1) and 19.1% (6.5 Mg ha^− 1), and in marketable tubers, of 20.5% (5.1 Mg ha^− 1) and 24.2% (5.9 Mg ha^− 1), compared to the respective maximum tuber yields obtained with AMS and CN.

Possible increases in the availability of NH₄⁺-N at the beginning of tuberization and of NO₃⁻N at the end of the cycle with the application of a high rate of urea-N in sidedressing may have provided the respective filling and late initiation of tubers of the Agata cultivar. Regarding the application of urea in sidedressing, 137.2% and 91.2% increases were observed in marketable and nonmarketable tuber yields, respectively, with 210 kg N ha^− 1. The hydrolysis of urea to NH₄⁺-N is fast and occurs within 5 days, while most nitrification occurs 28 to 56 days after the addition of urea depending on temperature (MacLean and McRae 1987). The release of NH₄⁺-N or ammonia (NH₃) occurred with the application of urea-N according to pH conditions and altered the dry matter partitioning of the Agata and Atlantic cultivars in a hydroponic system, stimulating vegetative growth to the detriment of tuberization (Silva et al. 2013). The Agata cultivar has early to very early maturity (ECPD 2021), and fertilization with NH₄⁺-N at the beginning of tuberization causes the plants to develop tubers early (Qiqige et al. 2017). In response, N nutrition must be precise and in a suitable form because while the supply of NO₃⁻-N favours the growth of shoots and the initiation of stolons and stems, that of NH₄⁺-N stimulates tuber filling (Osaki et al. 1995a) but can also be toxic to potato plants (Cao and Tibbitts 1998) or harmful to plant growth at any stage of development in excessive concentrations (Polizotto et al. 1975; Davis et al. 1986).

In the experiment with the Atlantic cultivar, the application of CN-N was the best option for sidedressing N fertilization because it resulted in more than twice the AE in marketable and total tuber yields in relation to the other N sources due to the deleterious effects of AMS-N at the soil‒plant interface and the absence of responses in tuber yields with urea-N rates.

The supply of NO₃⁻-N with CN-N requires a higher energy cost for its reduction in plants, decreasing shoot growth, contrary to what occurs with urea-N (Silva et al. 2013). After absorption, NO₃⁻-N is initially distributed to the leaves, where it is transformed into soluble amino acids, mainly asparagine and aspartic acid and glutamine and glutamic acid, which are then stored in petioles and stems and subsequently increase the number of branches and stolons and tuber growth (Osaki et al. 1995b) and the number of tubers per plant (Qiqige et al. 2017; Osaki et al. 2012; Gao et al. 2014). Bundy et al. (1996) observed the superiority of AMS in total tuber yields in two out of five years of study and in the average of these five years in relation to urea, CN and ammonium nitrate. According to these same authors, the leaching of NO₃⁻-N with CN or the losses of nonhydrolysed urea or of NO₃⁻-N also by leaching due to faster nitrification with urea than with AMS, or the losses of NH₃ with urea, all associated with the superficial rooting of the plants, justified the obtained results.

This study demonstrated that the effects of sidedressing N sources and rates on potato crops under subtropical conditions were soil‒plant interface dependent and differed between potato cultivars due to their specific nutritional requirements. For Agata potato cultivation, the best source of N was dependent on the applied rate. Only sidedressing AMS showed higher AE compared to urea in total tuber yields with up to 70 kg N ha^− 1 and a rate of maximum technical efficiency of 152 kg N ha^− 1. However, increases in marketable and nonmarketable tuber yields were still obtained with up to 210 kg urea-N ha^− 1, indicating that this source and rate of N are indicated for the Agata cultivar. For the Atlantic cultivar, there was a lack of responses in tuber yields with urea-N rates, and sidedressed AMS-N fertilization was show to be harmful in increasing the exchangeable acidity of the soil, consequently worsening the plant Ca and Mg status. In contrast, the CN did not present negative effects on soil chemical attributes, favoured plant nutrition in terms of Ca and Mg, and provided the highest AE in total and marketable tuber yields, which reached a maximum with 130 kg CN-N ha^− 1, indicating that this N source and rate is suitable for the Atlantic cultivar.

Acknowledgments We are thankful to the potato grower (Alexandre Dzierwa), who provided the area for this investigation. We thank the National Council for Scientific and Technological Development (CNPq) for granting awards for excellence in research to the thirth author.

Authors’ contributions Conceptualization, Methodology, and Data Curation: R.Y., B.B. and D.S.A. Formal analysis and Investigation: R.Y. and R.P.S. Writing, Original Draft: R.Y. Writing, Review and Editing: R.Y., B.B., R.P.S., T.R.S.C. and D.S.A. Supervision: R.Y.

Data availability The datasets analyzed during the current study are available from the corresponding author on reasonable request.

Conflict of interest The authors declare no competing interests.

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Impacts of Nitrogen Sources and Rates on Soil Chemical Attributes and Potato Nutrition and Yield in a Subtropical Environment

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Abstract

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1. Introduction

2. Materials and methods

2.1 Site, Experimental Design and Crop Management

2.2. Sampling and Analysis

2.3. Statistical Analyses

3. Results

3.1. Plant Nutritional Status

3.2. Soil Chemical Attributes

3.3. Tuber Yield and Agronomic Efficiency

4. Discussion

5. Conclusions

Declarations

References

Supplementary Files

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