Effects of Nitrogen Form on Root Activity and Nitrogen Uptake Kinetics in Camellia oleifera Seedlings

Abstract

This study investigated the effects of nitrogen form on root activity and nitrogen uptake kinetics of Camellia oleifera Abel. seedlings, providing a scientific basis for improving nitrogen use efficiency and scientific fertilization in C. oleifera production. Taking one-year-old C. oleifera cultivar ‘Xianglin 27’ seedlings as subjects, 8 mmol·L⁻¹ of nitrogen in varied forms (${\mathrm{NO}}_3^{-}$:${\mathrm{NH}}_4^{+}$ = 0:0, 10:0, 7:3, 5:5, 3:7, 0:10) was applied in this study as the treatment conditions to investigate the effects of different nitrogen forms on root activity and nitrogen uptake kinetics in C. oleifera seedlings. Comparing the performance of nutrient solutions with different ${\mathrm{NO}}_3^{-}$:${\mathrm{NH}}_4^{+}$ ratios, the results showed that a mixed nitrogen source improved the root activity of C. oleifera seedlings based on total absorption area, active absorption area, active absorption area ratio, specific surface area, and active specific surface area. When ${\mathrm{NO}}_3^{-}$:${\mathrm{NH}}_4^{+}$ = 5:5, the total absorption area and active absorption area of the seedling roots reached the maximum. The results of uptake kinetic parameters showed that Vmax ${\mathrm{NH}}_4^{+}$ > Vmax ${\mathrm{NO}}_3^{-}$ and Km ${\mathrm{NO}}_3^{-}$ > Km ${\mathrm{NH}}_4^{+}$, indicating that the uptake potential of ammonium–nitrogen by C. oleifera seedlings is greater than that of nitrate–nitrogen. The conclusion was that compared to either ammonium– or nitrate–nitrogen, the mixed nitrogen source was better for promoting the root activity of C. oleifera seedlings, and the best nitrate/ammonium ratio was 5:5.

1. Introduction

Camellia oleifera is a woody edible oil tree species in China and mainly distributed in red soil regions which are characterized by soil depletion and nutrient deficiency. It is widely cultivated in south–central and southern China since their growing commercial, medic, cosmetic and ornamental values in recent years provide an important guarantee for a targeted poverty alleviation strategy in China [1]. The genome of oiltea-camellia is very complex and not well explored. Recently, genomes of three oiltea-camellia species were sequenced and assembled [2,3,4]; multi-omic [1,5,6] studies of oiltea-camellia were carried out and provided a better understanding of this important woody oil crop.

Nitrogen is an essential macronutrient for plant growth and development and has an irreplaceable role in plant life [7]. Nitrogen deficiency is an important factor limiting the growth and fructification of C. oleifera [8], which can be resolved by the scientific application of nitrogen fertilizers. Plants have a higher demand for nitrogen than for other elements, and the two main forms of nitrogen that can be absorbed and used by plants are ammonium–nitrogen (${\mathrm{NH}}_4^{+}$-N) and nitrate–nitrogen (${\mathrm{NO}}_3^{-}$-N). Although it is volatile, ${\mathrm{NH}}_4^{+}$-N can be adsorbed and fixed by soil; thus, it is not easily lost via leaching. In comparison, ${\mathrm{NO}}_3^{-}$-N is non-volatile but is prone to leaching and denitrification losses, which affects the use efficiency of nitrogen fertilizers. Selective uptake characteristics for different nitrogen forms and the kinetic characteristics of nitrogen uptake by roots are important factors affecting nitrogen use efficiency in plants [9,10]. The kinetic approach is an effective method to study the nutrient uptake characteristics of plant roots and to identify the differences between different plant species [11]. In the early 1950s, Epstein et al. first applied the kinetic equation of enzymatic reactions to the study of nutrient ion uptake by plant roots [12], and in the mid-to-late 1970s, Barber et al. further modified the uptake kinetic equation and proposed the concept of critical concentration or minimum equilibrium concentration [13]. The Michaelis kinetic equation is a kinetic equation that can quantitatively characterize plant roots’ nutrient uptake. This equation was used to investigate the effects of environmental conditions on nutrient uptake by plants [14]. The characteristic parameters obtained from this equation have important applications in comparing the barren resistance of different plant species (or varieties) [15,16]. To date, kinetic studies of nutrient ion uptake have been largely limited to crops and fruit trees [12,14,17,18]. There are few studies on nutrient ion uptake kinetics by roots of woody plants [19], and no relevant studies on C. oleifera have been reported.

In this study, different ratios of nitrate–/ammonium–nitrogen were applied to C. oleifera seedlings to investigate the root activity and root uptake kinetics under different nitrogen forms; these results would provide a theoretical basis for the fertilization of C. oleifera seedlings.

2. Materials and Methods

2.1. Overview of the Experimental Field

The experimental field is located at the experimental station of the National Oil-tea Camellia Engineering and Technology Research Center, at 113°01′ E, 28°06′ N, and 80–100 m above sea level. It belongs to a subtropical monsoon climate, with an annual average temperature of 16.8–17.3 °C, annual average rainfall of 1422 mm, a frost-free period of 275 days, and annual average relative humidity of 80%. The soil classifies as Quaternary red soil with a pH between 4.5 and 5.5, an organic matter content of 41.01 g·kg⁻¹, a total nitrogen content of 2.68 g·kg⁻¹, a total phosphorus content of 0.61 g·kg⁻¹, and a total potassium content of 4.53 g·kg⁻¹.

2.2. Experimental Materials

The C. oleifera cultivar ‘Xianglin 27’ one-year-old seedlings were used for the experiment. C. oleifera seeds were collected in October 2018, and uniform, full-grained, and disease-free seeds were selected for stratification. In March 2019, seeds with consistent germination states were selected and sowed in containers with a height of 12 cm and an upper diameter of 8 cm. The cultivation substrate consisted of yellow subsoil, perlite, and peat at a volume ratio of 3:1:1. The substrate had a pH of 5.88, and the concentrations of ${\mathrm{NH}}_4^{+}$-N and ${\mathrm{NO}}_3^{-}$-N were 0.92 and 2.34 mg·L⁻¹, respectively. Slow seedling was performed for 3 months after planting seeds in nutrition cups, and seedlings with consistent growth conditions and no pests or diseases were selected for fertilization in June 2019.

2.3. Experimental Design

Fertilizers were applied via liquid irrigation, and a modified Hoalgland nutrient solution (nitrogen-free) was added to ensure that the seedlings grew under normal nutritional conditions. The formulation of the nutrient solution is: K₂SO₄ 261.39 mg·L⁻¹, KH₂PO₄ 136.09 mg·L⁻¹, CaCl₂ 221.98 mg·L⁻¹, MgSO₄·7H₂O 246.47 mg·L⁻¹, MnSO₄·H₂O 1.54 mg·L⁻¹, H₃BO₃ 2.86 mg·L⁻¹, ZnSO₄·7H₂O 0.22 mg·L⁻¹, CuSO₄·5H₂O 0.08 mg·L⁻¹, Na₂MoO₄·2H₂O 0.02 mg·L⁻¹, and FeSO₄·7 H₂O 20 mg·L⁻¹. In all nutrient solutions, 7 μmol·L⁻¹ of the nitrification inhibitor dicyandiamide (C₂H₄N₄) was added for nitrification inhibition.

The experiment employed a completely randomized block design. According to a previous study [20], six treatments (

Table 1. Nitrogen ratio under different treatments used in this study.

No.	m(${\mathbf{NO}}_3^{\mathbf{-}}$-N)/m(${\mathbf{N}\mathbf{H}}_4^{+}$-N)	Total Nitrogen/mmol·L−1	Nitrogen Sources/mmol·L−1
			NaNO3	(NH4)2SO4
A0(CK)	0:0	0	0	0
A1	10:0	8	8	0
A2	7:3	8	5.6	1.2
A3	5:5	8	4	2
A4	3:7	8	2.4	2.8
A5	0:10	8	0	4

), including five experimental groups at a nitrogen level of 8.0 mmol·L⁻¹ with different ratios of nitrogen forms ([m(${\mathrm{NO}}_3^{-}$-N)/m(${\mathrm{NH}}_4^{+}$-N)] = 10:0, 7:3, 5:5, 3:7, 0:10), and a control group (no nitrogen fertilization) were set up. For each treatment, three replicates, each containing 200 seedlings, were set up, as shown in Table 1. The first fertilization was carried out in mid-June, followed by 10 fertilizer applications at one-week intervals. Each seedling was thoroughly irrigated with 300 mL solution each time. The solution collected in trays was used for re-irrigation to ensure no nitrogen loss. The experiments were conducted in a greenhouse with a luminance of 6000–8000 lux, a temperature of 20.0–25.0 °C, and a humidity of 80%–85%. In addition to fertilization, other care and maintenance measures, such as normal watering and weed clearing, were performed.

2.4. Experimental Methods

The methylene blue method was used to determine root absorption activity [21]. The kinetic characteristics of the uptake of ${\mathrm{NO}}_3^{-}$-N and ${\mathrm{NH}}_4^{+}$-N solutions by seedlings were determined by the conventional depletion method from late July to the middle of August, which was the most active stage of seedlings’ growth. The content of ${\mathrm{NO}}_3^{-}$-N was determined using the colorimetric method [22], while that of ${\mathrm{NH}}_4^{+}$-N was determined using the indophenol blue method [23]. Immediately after the uptake kinetics test, the C. oleifera seedlings were taken out and weighed after removing water from the root surface with absorbent paper.

2.5. Experimental Materials

The roots’ kinetic uptake parameters were calculated using the method described by Hua Haixia and Zhai Mingpu. The maximum uptake rate V_max and the Michaelis constant K_m were calculated, and α = V_max/K_m [24,25]. Data were processed and statistically analyzed using Excel 2007 and SPSS25.0 software, and significant differences between treatments were compared using one-way analysis of variance (ANOVA) and the least significant difference (LSD) test (p < 0.05). Histograms were plotted with GraphPad Prism 8.0 and line graphs were plotted with OriginPro 8.5.1.

3. Results

3.1. Effects of Nitrogen Form on the Root Activity of C. oleifera Seedlings

3.1.1. Effect of Nitrogen Form on the Total Absorption Area of C. oleifera Seedling Roots

For simplicity, the nitrogen source treatments used in this study [m(${\mathrm{NO}}_3^{-}$-N)/m(${\mathrm{NH}}_4^{+}$-N)] = 10:0, 7:3, 5:5, 3:7, 0:10 will be referred to as A1–A5, respectively, and the control treatment will be referred to as A0. With the increasing proportion of ${\mathrm{NH}}_4^{+}$-N, the total absorption area of the roots first increased and then decreased. This shows that the mixed-nitrogen-source treatment (A2–A4) had greater root absorption than the control (A0), which had a greater root absorption than the single-nitrogen-source treatment (A1, A5). For the single-nitrogen-source treatments, the total-nitrate treatment (A1) had a greater absorption than the total-ammonium treatment (A5). The total root absorption area of ${\mathrm{NO}}_3^{-}$:NH+ 4 = 5:5, A3 treatment reached a maximum of 1.48 cm², which was 43.69% higher than the control, while the total absorption area of the total-ammonium (A5) treatment was the smallest at 0.85 cm², which was 17.48% lower than the control.

3.1.2. Effect of Nitrogen Form on the Active Absorption Area of C. oleifera Seedling Roots

As presented in Figure 1, the active absorption area of the roots also showed an increase, followed by a decreasing trend with increasing proportions of ${\mathrm{NH}}_4^{+}$-N. Mixed-nitrogen-source treatment (A2–A4) had a greater active absorption area compared with the control (A0), which had a greater active absorption area than the single-nitrogen-source treatment (A1, A5). For the single-nitrogen-source treatments, A1 showed a greater active absorption area than A5. The active absorption area reached a maximum of 1.00 m² when using the A3 treatment, which was 78.57% higher than that of the control. The total-ammonium treatment resulted in the smallest active absorption area at 0.45 cm², which was 19.64% lower than the control. These results indicate that mixed nitrogen sources can significantly increase the active absorption area, and both total-nitrate and total-ammonium treatments will lower the active absorption area.

3.1.3. Effect of Nitrogen Form on the Active Absorption Area Ratio of C. oleifera Seedling Roots

As shown in Figure 2, the active absorption area ratio tended to decrease, then increase, and then decrease with treatments A0–A5. Mixed-nitrogen-source treatments (A3, A4) had greater active absorption than the control (A0), which had greater active absorption than the single-nitrogen-source treatments (A1, A5). For the single-nitrogen-source treatments, A5 had greater active absorption than A1. The active absorption area ratio reached a maximum of 79.72% at ${\mathrm{NO}}_3^{-}$:NH+ 4 = 3:7, which was 46.36% higher than that of the control, followed by that of A3 treatment with an active absorption area ratio that was 34.07% higher than the control’s. There was no significant difference between A4 and A3 treatments, and both resulted in a significantly higher active absorption area ratio than the other treatments. The smallest active absorption area ratio, at 53.00%, was associated with the A5 treatment, which was 2.70% lower than that of the control. These results indicate that mixed nitrogen sources (A3 and A4) can significantly increase the active absorption area ratio.

3.1.4. Effect of Nitrogen Form on the Specific Surface Area of C. oleifera Seedling Roots

The effect of nitrogen form on the specific surface area of C. oleifera seedling roots is shown in Figure 3. After treatments with nitrogen sources of different forms and ratios, the specific surface area of the roots showed a first increasing, then decreasing, and then increasing trend with an increasing proportion of ${\mathrm{NH}}_4^{+}$-N. Specifically, seedlings had a greater root surface area after mixed-nitrogen-source treatment (A2, A4) compared with control (A0), which had a greater root surface area compared with seedlings after single-nitrogen-source treatment (A1, A5). Regarding the single-nitrogen-source treatments, A5 showed greater root surface area than A1. The root-specific surface area reached a maximum of 0.98 cm²·cm⁻³ in the A2 treatment, which was 30.67% higher than that of the control, followed closely by the A4 treatment. The root-specific surface area after the A2 treatment was significantly higher than those with single-nitrogen-source treatments (A1 and A5), but showed no significant differences compared to the other treatments. The A1 treatment led to the smallest root-specific surface area of 0.53 cm²·cm⁻³, which was 29.33% lower than that of the control. These results indicate that both total-nitrate and total-ammonium treatments will significantly reduce the specific surface area of C. oleifera seedlings.

3.1.5. Effect of Nitrogen Form on the Active Specific Surface Area of C. oleifera Seedling Roots

As shown in Figure 4, with the increased ratio of ${\mathrm{NH}}_4^{+}$-N, the active specific surface area of the roots showed a first increasing and then decreasing trend. The active specific surface area of the roots reached a maximum value of 0.66 cm²·cm⁻³ under A4 treatment, which was 65.00% higher than the control. There was no significant difference between A4 and the other mixed-nitrogen-source treatments (A2 and A3), but the active specific surface area achieved by the A4 treatment was significantly higher than those by the single-nitrogen-source treatments (A1 and A5), and the control (A0). The active specific surface area was the smallest, at 0.27 cm²·cm⁻³, when A1 treatment was applied, which was 32.50% lower than the control. The results indicate that mixed nitrogen sources can significantly increase the proportion of active specific surface area, and both total-nitrate and total-ammonium treatments will reduce the active specific surface area of C. oleifera seedling roots.

3.2. Effects of Nitrogen Form on the Nitrogen Uptake Kinetics of C. oleifera Seedling Roots

3.2.1. Effect of Nitrogen Form on ${\mathrm{NO}}_3^{-}$ Uptake Kinetics of C. oleifera Seedling Roots

Figure 5 shows the concentration curves of ${\mathrm{NO}}_3^{-}$ at different uptake times in the ${\mathrm{NO}}_3^{-}$ depletion test of C. oleifera seedlings treated with different nitrogen forms. The kinetic response patterns of ${\mathrm{NO}}_3^{-}$-N uptake by seedlings subjected to different nitrogen treatments showed no significant difference. With the extension of the absorption time, the ${\mathrm{NO}}_3^{-}$ concentration in the fertilizer solution gradually decreased. Specifically, the ${\mathrm{NO}}_3^{-}$ concentration decreased rapidly within the first nine hours, with the most dramatic drop observed within the first two hours. From the ninth hour onwards, the ${\mathrm{NO}}_3^{-}$ concentration remained almost constant. Via fitting, the ${\mathrm{NO}}_3^{-}$ depletion equations for treatments with different nitrogen forms and ratios were listed in

Table 2. Kinetic parameters of ${\mathrm{NO}}_3^{-}$ uptake by roots of C. oleifera seedlings.

Treatment	Equation	R2	Vmax (umol·g−1·h−1)	Km (mmol·L−1)	α
A0	Y = 0.0013x2 − 0.044x + 2.0386	0.9917	16.10	1.76	0.009
A1	Y = 0.0014x2 − 0.0462x + 2.0373	0.9911	17.52	1.75	0.010
A2	Y = 0.0009x2 − 0.0432x + 2.0290	0.9847	16.24	1.54	0.011
A3	Y = 0.0014x2 − 0.0531x + 2.0414	0.9947	17.57	1.64	0.011
A4	Y = 0.0009x2 − 0.0441x + 2.0424	0.9858	16.29	1.56	0.011
A5	Y = 0.0013x2 − 0.0436x + 2.0525	0.9950	15.62	1.79	0.009

. The kinetic parameters of ${\mathrm{NO}}_3^{-}$ uptake by the roots were calculated according to the fresh weights of the roots of C. oleifera seedlings in different treatments.

As shown in Table 2, the Vmax values of different treatments in descending order, are A3 > A1 > A4 > A2 > A0 > A5. The Vmax of the A3 treatment was the highest, at 17.57μmol·g⁻¹·h⁻¹, indicating that the A3 treatment had the highest intrinsic potential for ${\mathrm{NO}}_3^{-}$ uptake. The A1 treatment came in second place, with a Vmax of 17.52 μmol·g⁻¹·h⁻¹, followed by the other mixed-nitrogen-source treatments (A4 and A2). The A5 treatment resulted in the lowest Vmax, which was lower than that of the control. These results indicate that the presence of ${\mathrm{NH}}_4^{+}$ accelerates the uptake of ${\mathrm{NO}}_3^{-}$; however, when ${\mathrm{NH}}_4^{+}$ exceeds a certain percentage, the ${\mathrm{NO}}_3^{-}$ uptake rate decreases.

The Km values of different treatments followed a descending order, with A5 > A0 > A1 > A3 > A4 > A2, i.e., seedlings treated with mixed nitrogen sources (A2–A4) had a lower Km compared with seedlings treated with a total-nitrate source (A1), which had a lower Km than the control seedlings (A0), which, in turn, had a lower Km than total-ammonium-treated seedlings (A5). This indicates that adding a certain proportion of ${\mathrm{NH}}_4^{+}$ can increase the affinity of C. oleifera roots with ${\mathrm{NO}}_3^{-}$.

The α values of different treatments followed the order A3 = A4 = A2 > A1 > A0 = A5, i.e., mixed-nitrogen-source treatments (A2–A4) produced greater α values in the C. oleifera seedlings compared to the total-nitrate treatment (A1), which were greater than those in the control treatment (A0), which, in turn, were greater than the ones resulting from the total-ammonium treatment (A5). The mixed-nitrogen-source treatments (A2–A4) had the largest α value of 0.011, while the single-nitrogen-source treatments (A1, A5) had the smallest α value of 0.009, indicating that the presence of a certain percentage of ${\mathrm{NH}}_4^{+}$ ions can increase the uptake rate of ${\mathrm{NO}}_3^{-}$ by the root.

3.2.2. Effect of Nitrogen Form on ${\mathrm{NH}}_4^{+}$ Uptake Kinetics of C. oleifera Seedling Roots

Figure 6 and Figure 7 show the curves of ${\mathrm{NH}}_4^{+}$ concentration at different uptake times in the ${\mathrm{NH}}_4^{+}$ depletion test of C. oleifera seedlings treated with different nitrogen forms. The kinetic response patterns of ${\mathrm{NH}}_4^{+}$-N uptake by seedlings subjected to different treatments showed no statistical difference. In all samples, the ${\mathrm{NH}}_4^{+}$ concentration decreased rapidly in the first 8 h, with the most obvious change observed within the first hour. The ${\mathrm{NH}}_4^{+}$ concentration remained almost unchanged after 8 h. Via fitting, the ${\mathrm{NH}}_4^{+}$ depletion equations for treatments with different nitrogen forms and ratios were listed in

Table 3. Kinetic parameters of ${\mathrm{NH}}_4^{+}$ uptake by roots of C. oleifera seedlings.

Treatment	Equation	R2	Vmax (umol·g−1·h−1)	Km (mmol·L−1)	α
A0	Y = 0.0032 x2 − 0.0888x + 2.0549	0.9889	32.49	1.59	0.020
A1	Y = 0.0037x2 − 0.0993x + 2.0744	0.9946	35.01	1.57	0.022
A2	Y = 0.0034x2 − 0.0971x + 2.0632	0.9927	33.95	1.54	0.022
A3	Y = 0.0057x2 − 0.1361x + 2.0988	0.9924	45.07	1.49	0.030
A4	Y = 0.0041x2 − 0.1111x + 2.0825	0.9947	39.35	1.52	0.026
A5	Y = 0.0032x2 − 0.0793x + 2.0401	0.9676	29.62	1.67	0.018

. The kinetic parameters of ${\mathrm{NH}}_4^{+}$ uptake by the roots were calculated according to the fresh weights of the roots of C. oleifera seedlings in different treatments.

As shown in Table 3, the Vmax values of different treatments followed a descending order, with A3 > A4 > A1 > A2 > A0 > A5. The A3 treatment has the highest Vmax of 45.07 μmol·g⁻¹·h⁻¹, indicating that the A3 treatment had the highest intrinsic potential for ${\mathrm{NH}}_4^{+}$ uptake. The A3 treatment was followed by the A4 treatment with a Vmax of 39.35 μmol·g⁻¹·h⁻¹, and then by the A1 treatment. This indicates that the addition of ${\mathrm{NO}}_3^{-}$ accelerated ${\mathrm{NH}}_4^{+}$ uptake. As for single-nitrogen-source treatments, the total-nitrate treatment (A1) had a higher Vmax than the total-ammonium treatment (A5), i.e., the ${\mathrm{NH}}_4^{+}$ uptake rate was higher in the total-nitrate treatment.

The Km values of different treatments were in the descending order of A5 > A0 > A1 > A2 > A4 > A3, showing that mixed-nitrogen-source treatments (A2–A4) had a lower Km value than the total-nitrate treatment (A1), which had a lower Km value than control (A0) treatment, which, in turn, had a lower Km than the total-ammonium treatment (A5). This indicates that the involvement of a certain percentage of ${\mathrm{NO}}_3^{-}$ can enhance the affinity of C. oleifera roots with ${\mathrm{NH}}_4^{+}$.

The α values of ${\mathrm{NH}}_4^{+}$ uptake by the seedlings were ranked as A3 > A4 > A2 = A1 > A0 > A5, with the mixed-nitrogen-sources (A2–A4) and total-nitrate treatments (A1) producing greater α values of ${\mathrm{NH}}_4^{+}$ uptake compared with control (A0), which had a greater α value of ${\mathrm{NH}}_4^{+}$ uptake than the total-ammonium treatment (A5). This indicates that involving a certain percentage of ${\mathrm{NO}}_3^{-}$ can accelerate ${\mathrm{NH}}_4^{+}$ uptake by the root.

4. Discussion

4.1. Effects of Nitrogen Form on the Root Activity of C. oleifera Seedlings

Root activity is an important indicator reflecting the nutrient uptake capacity. The total and active absorption area of the root can reflect the strength of root activity and, to some extent, the thickness, branching, and root hair volume of the root [26]. Study of nitrogen form supply on root respiration of walnut seedlings showed that the root respiration rate of walnut seedlings, as well as soluble sugar and starch content, significantly higher than other nitrogen forms treatment when the ratio of ammonium nitrogen to nitrate nitrogen was 50:50 [27]. A study on the effects of exogenous nitrogen forms on cucumber growth showed that the equal amount of ammonium–nitrogen and nitrate–nitrogen promoted the growth of branches, leaves and roots of cucumber seedlings, as well as having enhanced tolerance to sub-low temperatures [28]. In this study, when the ratio of ammonium–nitrogen to nitrate–nitrogen was 50:50, the roots of C. oleifera seedlings showed the strongest uptake ability of water and nutrient elements, i.e., the highest root activity, which was consistent with the findings in the other species mentioned above. The root activity was the lowest in the total-ammonium treatment, indicating that the C. oleifera root has poor adaptability to a pure ${\mathrm{NH}}_4^{+}$-N environment. Therefore, mixed nitrogen forms can improve root activity, thus enhancing nitrogen nutrient uptake by the plants.

4.2. Effects of Nitrogen Form on ${\mathrm{NO}}_3^{-}$ and ${\mathrm{NH}}_4^{+}$ Uptake by Roots of C. oleifera Seedlings

Plants have different uptake and transport pathways for ${\mathrm{NH}}_4^{+}$-N and ${\mathrm{NO}}_3^{-}$-N. Extensive studies have shown that high concentrations of ${\mathrm{NH}}_4^{+}$ inhibit ${\mathrm{NO}}_3^{-}$ uptake, which may be due to the inhibition of gene expression for ${\mathrm{NO}}_3^{-}$ carrier protein synthesis, as the presence of ${\mathrm{NH}}_4^{+}$ affects the environment in which ${\mathrm{NO}}_3^{-}$ carriers are located on the cell membrane [29]. In this study, low concentrations of ${\mathrm{NH}}_4^{+}$ accelerated the uptake of ${\mathrm{NO}}_3^{-}$-N by the roots of C. oleifera seedlings. This is likely because a certain concentration of ${\mathrm{NH}}_4^{+}$ induces H⁺ secretion by roots, promoting ${\mathrm{NO}}_3^{-}$/H⁺ cotransport, thus accelerating ${\mathrm{NO}}_3^{-}$ uptake [30]. However, as the ${\mathrm{NH}}_4^{+}$ concentration gradually increased, the Vmax for ${\mathrm{NO}}_3^{-}$-N uptake decreased rapidly, which was confirmed by the ${\mathrm{NO}}_3^{-}$-N uptake characteristics of C. oleifera seedlings in this study. This conclusion is consistent with findings in other plants, such as citrange and banana plants [31,32].

The ${\mathrm{NO}}_3^{-}$-N concentration in the environment can also affect the uptake capacity of plant roots. The presence of ${\mathrm{NO}}_3^{-}$-N promotes root growth, which, in turn, promotes the uptake and utilization of ${\mathrm{NH}}_4^{+}$-N by the underground part of the plant [33]. In this study, the Vmax of the total-nitrate treatment (A1) was lower than that of the A3 treatment but higher than all the other treatments, which may be because the root has sufficient ${\mathrm{NO}}_3^{-}$-N uptake carriers under pure ${\mathrm{NO}}_3^{-}$ supply [34]. In contrast, the Vmax of the total-ammonium treatment (A5) was lower than that of all the other treatments, which is consistent with the findings of Du Xvhua et al. on tea trees and Sun Minhong et al. on citrange [31,35].

The C. oleifera seedlings subjected to A3 treatment had higher Vmax and α values for the uptake rates of both ${\mathrm{NO}}_3^{-}$ and ${\mathrm{NH}}_4^{+}$ than those in the other treatments, indicating that C. oleifera seedlings have a strong preference and competitiveness for both ${\mathrm{NH}}_4^{+}$-N and ${\mathrm{NO}}_3^{-}$-N when applied in equal ratios.

By comparing the characteristic parameters of ${\mathrm{NH}}_4^{+}$-N and ${\mathrm{NO}}_3^{-}$-N uptake kinetics by C. oleifera seedlings treated with different nitrogen forms and ratios, it was found that Vmax ${\mathrm{NH}}_4^{+}$ > Vmax ${\mathrm{NO}}_3^{-}$, i.e., the uptake potential of C. oleifera seedlings for ${\mathrm{NH}}_4^{+}$-N, is greater than that for ${\mathrm{NO}}_3^{-}$-N. In addition, Km ${\mathrm{NO}}_3^{-}$ > Km ${\mathrm{NH}}_4^{+}$, indicating that C. oleifera seedlings have a higher affinity with ${\mathrm{NH}}_4^{+}$-N than with ${\mathrm{NO}}_3^{-}$-N. The results of the study show that C. oleifera seedlings prefer ${\mathrm{NH}}_4^{+}$-N, which is likely due to the long-term adaptation of C. oleifera to acidic soils. In acidic soils, ${\mathrm{NO}}_3^{-}$ tends to be rapidly reduced to ${\mathrm{NH}}_4^{+}$, and ${\mathrm{NH}}_4^{+}$ becomes the main nitrogen source [36]. However, upon ${\mathrm{NH}}_4^{+}$ uptake, the plants release H⁺ into the soil, which will increase the acidity of the soil. Favorable growth of C. oleifera seedlings requires soil with a suitable acidity (pH 4.5–6.0), as soil that is too acidic will inhibit seedling growth. During ${\mathrm{NO}}_3^{-}$ uptake, C. oleifera releases ${\mathrm{HCO}}_3^{-}$ into the soil and raises the pH of the soil. Therefore, applying mixed ${\mathrm{NH}}_4^{+}$-N and ${\mathrm{NO}}_3^{-}$-N at an appropriate ratio can stabilize the soil pH and promote the growth of C. oleifera seedlings. In addition, ${\mathrm{NO}}_3^{-}$ is easily mobile and has a high diffusion efficiency, as well as being prone to leaching loss, which reduces the uptake and utilization of ${\mathrm{NO}}_3^{-}$ by plants.5.

5. Conclusions

By comparing the characteristic parameters of different ratios of ammonium– and nitrate–nitrogen with either single ammonium– or nitrate–nitrogen, the mixed nitrogen source was better for promoting the root activity of C. oleifera seedlings, while both the total absorption area and active absorption area of the seedling roots were highest with the nitrogen source form of ${\mathrm{NO}}_3^{-}$:${\mathrm{NH}}_4^{+}$ = 5:5. Moreover, C. oleifera seedlings have a higher uptake potential and affinity with ${\mathrm{NH}}_4^{+}$-N than with ${\mathrm{NO}}_3^{-}$-N, indicating that C. oleifera prefers ${\mathrm{NH}}_4^{+}$-N.