Effects of CaO on the Clonal Growth and Root Adaptability of Cypress in Acidic Soils

Abstract

Cypress (Cupressus funebris Endl.) is a major tree species planted for forestland restoration in low-fertility soil and in areas where rocky desertification has occurred. Calcium (Ca) fertilizer can adjust the pH of soil and has an important effect on the growth of cypress. Soil and water losses are serious in Southern China, and soil acidification is increasing, which results in high calcium loss. However, the adaptability of cypress clones to different concentrations of calcium in acidic soils has not been studied. In this investigation, a potted-plant experiment was set up with three concentrations of calcium oxide (CaO) fertilizer (0, 3, and 6 g·kg⁻¹) added under local soil conditions with 0 and 3 g·kg⁻¹ nitrogen (N), phosphorus (P), and potassium (K) fertilizer. The effects of CaO on the growth, root development, and nutrient uptake and utilization efficiency of cypress clones were analyzed. The growth, root development, and nutrient absorption and utilization of cypress differed when calcium fertilizer was applied to acidic soils with different degrees of fertility. In the soil with 0 g·kg⁻¹ NPK fertilizer, the 3 and 6 g·kg⁻¹ CaO treatments significantly increased the clonal growth of cypress seedling height, basal diameter, and dry-matter weight. In addition, the length, surface area, and volume of the roots less than 2.0 mm of root diameter also significantly increased, indicating that the fine cypress roots were somewhat able to adapt to differing Ca levels under lower fertility conditions. Moreover, the efficiency of N, P, and Ca accumulation was highest in the 3 g·kg⁻¹ CaO treatment. After adding 3 g·kg⁻¹ CaO fertilizer to the soil with 3 g·kg⁻¹ NPK fertilizer, only the root dry-matter weight increased significantly, indicating that root development (including root length, surface area, and volume) in the D1–D3 diameter classes (≤1.5 mm in diameter) was significantly elevated. When CaO application reached 6 g·kg⁻¹, the seedling height, basal diameter, and dry-matter weight of each organ decreased, as did the length, surface area, and volume of the roots in the all diameter classes, indicating that the addition of excessive CaO to fertile soil could inhibit the growth and root development of cypress. In Ca-deficient low-quality acidic soils, adding CaO fertilizer can promote the development of fine roots and the uptake and utilization of N, P, and Ca. The results of this study provide a basis for determining the optimal fertilization strategy when growing cypress in acidic soils in Southern China.

1. Introduction

In the subtropics of China, fast-growing plantations are primarily distributed in large areas of hills and mountains, and these areas are mostly covered by low-fertility acidic red soils. Due to heavy rainfall and strong weathering in Southern China, soil acidification is serious, and this condition results in soil and water loss, a deficiency of soil nutrients and a decline in soil fertility [1]. Damaged land is difficult to recover, which results in difficulty in tree survival and low vegetation coverage [2]. Areas where acid deposition occurs are continuously expanding along with a serious loss of base ions, especially calcium (Ca²⁺), in the soil [3]. Ca²⁺ is an essential nutrient required for plant growth and development and plays a role in maintaining the stability of plant cell walls, cell membranes, and membrane-bound proteins; modulating inorganic ion transport; and regulating various enzyme activities [4,5,6,7].

Applying calcium fertilizer and other elements, such as nitrogen (N), phosphorous (P), and potassium (K), to acidic soil helps regulate the soil pH value and also plays a role in calcium supplementation, which can significantly increase the content of available calcium in soil [8,9]. Calcium application can modulate intracellular Ca²⁺ levels, promote seedling growth and root development, and enhance plant stress resistance [10,11]. In recent years, higher yield and quality and higher concentrations of nutrient elements have been obtained by the application of chelated microfertilizers rather than simple chemical fertilizers [12]. Data have shown that high Ca fertilized Douglas-fir ((Mirb.) Franco) seedlings had greater new-needle biomass and growth than low Ca fertilized seedlings. High Ca availability also led to higher foliar membrane-associated Ca, Ca-pectate, and Ca-oxalate than low Ca seedlings [13]. However, numerous fertilization experiments have been carried out in low-fertility acidic red soils to determine optimal fertilization regimes for forest trees, and the issue remains a hot topic [14].

The root is an important organ in plants for resource acquisition, and the spatiotemporal distribution of plant roots determines the quantities of water and nutrients absorbed for photosynthesis and harvest products [15,16]. Additionally, the root acts as a supporting organ that allows a plant to be fixed in the ground for a long time, ensuring normal growth and development [17]. Under different growth conditions, functional attributes such as root number and morphology show differential responses to changes in underground resources. For example, fine roots with diameters ≤1.5 mm are key for nutrient uptake, accounting for over 80% of the total root length and total root surface area [18]. When the soil environment is altered, the rate at which fine roots lengthen changes, and the mean root diameter increases or decreases in a short period of time; the duration of such changes varies [19]. In contrast, roots with a diameter of >1.5 mm mainly play a role in transport and support, and they constitute relatively low proportions of the total root length and surface area. Especially for tall arbores, root growth and development status determine forest tree growth status in subsequent years or even decades.

Cypress (Cupressus funebris Endl.) is highly adaptive, featuring a developed root system, strong resistance to drought stress, and a strong capacity for self-repair [20]. Thus, it is also a major tree species for afforestation and forestland restoration under low-fertility site conditions. However, most subtropical regions have low-fertility acidic soils; only the residues (5–10%) of dissolved limestone matter can form soil parent material, the soil layer is shallow, and calcium loss is severe [2]. Previous research has found substantial variation in the root length of cypress when the soil environment is disturbed; this effect is long-lasting and especially impactful on the number, morphology, and function of fine roots [21]. There are few reports on the synergistic relationships between calcium (Ca) and cypress growth, root development, and nutrient uptake; however, these studies are of great significance for understanding the development of cypress forests in Ca-deficient acidic soil [22,23].

The adaptability of plants to the soil Ca environment is related to the absorption, transport, and accumulation of nutrient elements [8,9]. Considering the differences in the influence of Ca²⁺ on root traits and plant growth, adding Ca fertilizer to acidic soil can increase the soil Ca content [14,24]. We hypothesized that (i) the basal diameter, seedling height, and dry matter of cypress would respond differently to Ca fertilizer, and soil fertility may modulate the effect of Ca fertilizer [25]; and (ii) the morphological characteristics of fine roots differ greatly between fertile and poor soils. The ratio of nutrient uptake in cypress might differ with Ca addition because fine roots are the most active part of the root system in nutrient uptake and transport [26,27]. The objectives of this study were to explore (1) the ground diameter and seedling height growth of cypress clones in response to Ca²⁺ fertilizer; (2) the effects of Ca²⁺ addition on the roots of different diameter classes in terms of length, surface area, and volume; and (3) the variation in N, P, and Ca accumulation efficiencies of cypress under different nutrient conditions.

2. Materials and methods

2.1. Study Site and Selection of Materials

The experiment was conducted in a greenhouse at Laoshan Forestry Farm in Zhejiang Province, China. One-year-old cutting seedlings of cypress were planted as the experimental material. The semi-lignified branches used for cutting came from elite individual plants of clone 1 (fast height growth) and clone 2 (slow height growth) in the full-sib progeny. For each clone, robust cutting seedlings aged 1 year were selected. At the time of planting, seedlings were selected based on plant height (5.15 ± 0.05 cm) and ground diameter (0.17 ± 0.01 cm). Then, they were planted in containers that were 30 cm in height and 20 cm in diameter. The potting soil was the local acidic soil, and the soil layer was 0–20 cm thick. This soil is a red acid soil typical of subtropical areas in China. Soil pH value was determined by Potentiometric method. Total nitrogen was determined by Kjeldahl method, and available nitrogen was determined by alkaline hydrolysis method. Extraction of readily available phosphorus was determined by sodium bicarbonate molybdenum antimony colorimetric method. The available potassium was extracted with 1 mol·L⁻¹ neutral ammonium acetate and measured by flame photometer. Organic matter was determined by Potassium dichromate external heating method. The exchangeable Ca and Mg were determined by EDTA volumetric method. The physicochemical properties of the soil are provided in

Table 1. Texture and chemical properties of potted soil.

Nutrient Elements	Texture	Soil Type	Total N (g·kg−1)	Total P (g·kg−1)	Hydrolytic N (mg·kg−1)	Available K (mg·kg−1)	Available P (mg·kg−1)	Organic Matter (g·kg−1)	Exchange Ca (mg·kg−1)	Exchange Mg (mg·kg−1)	pH Value
Average content	light loam	red acid soil	0.75 ± 0.09	0.32 ± 0.05	53.5 ± 4.70	18.50 ± 1.12	0.99 ± 0.14	15.8 ± 1.89	128 ± 12.5	9.24 ± 0.85	4.65 ± 0.21

.

2.2. Experimental Design

The controlled-release fertilizer used in the experiment was a nursery fertilizer (APEX). NPK fertilizer was added at 0 and 3 g per kg of soil to simulate low-fertility and fertile soil, respectively. For Ca²⁺ fertilization, CaO was added at 0, 3, and 6 g per kg of soil. Both the NPK fertilizer and CaO were mixed with the soils, stirred uniformly, and placed into containers. The experiment involved 6 treatments: (1) 0 g·kg⁻¹ soil CaO, (2) 3 g·kg⁻¹ soil CaO, (3) 6 g·kg⁻¹ soil CaO, (4) NPK fertilizer (3 g·kg⁻¹ soil) + 0 g·kg⁻¹ soil CaO, (5) NPK fertilizer (3 g·kg⁻¹ soil) + 3 g·kg⁻¹ soil CaO, and (6) NPK fertilizer (3 g·kg⁻¹ soil) + 6 g·kg⁻¹ soil CaO. The experiment used a completely randomized block design. Twenty cutting seedlings were planted per treatment per clone, with three replicates each; therefore, 720 potted seedlings were planted. All seedlings were maintained in a greenhouse, under conventional management.

2.3. Measurement Indices

The experiment started on 2 April 2018. Seedlings were harvested on 23 November, and height and ground diameter were measured for all plants. Whole plants were collected and divided into roots, stems, and leaves, with each organ harvested separately. First, the roots were separated from the soil, washed with deionized water, and stored. Root diameter was classified as follows: class D1 (root diameter range: 0–0.5 mm), class D2 (0.5–1.0 mm), class D3 (1.0–1.5 mm), class D4 (1.5–2.0 mm), and class D5 (>2.0 mm) [9]. The root length, surface area, and root volume for each diameter class were measured by using the image analysis software WinRHIZO Pro STD1600 + (Regent Instruments, Quebec City, QC, Canada). Next, the roots, stems, and leaves were deactivated in an oven at 105 °C for 30 min and then dried at 80 °C until a constant weight was achieved to obtain the dry biomass of each part. The N content of each organ was measured by using a FOSS (Foss Analytical A/S, Hillerød, Denmark) nitrogen analyzer. The P content was measured by molybdenum antimony anticolorimetry [28]. The Ca content was measured by atomic absorption spectrophotometry [29]. The N, P, and Ca contents were multiplied by the dry biomass of the whole plant to obtain the N, P, and Ca accumulation. N accumulation efficiency = dry biomass accumulation of whole plant/N uptake of whole plant (g·mg⁻¹); P and Ca accumulation efficiencies were calculated by following the same method used for N accumulation efficiency.

2.4. Data Analysis

One-way analysis of variance (ANOVA) was used to test the significance of differences in seedling growth, root morphological characteristics, and nutrient accumulation efficiency in fertile and low-fertility soils. Two-way analysis of variance was used to examine the differences among clones and calcium treatments under two kinds of soil conditions. Duncan’s minimum significant difference method was used to assess the significance of differences among treatments, and the significance level was set at 0.05. All statistical analyses were performed by using IBM SPSS Statistics 22.0 (IBM Corp, Armonk, NY, USA).

3. Results

3.1. Effects of Soil Fertility and Calcium Fertilizers on Seedling Height, Ground Diameter, and Dry Biomass

In soil with 0 g·kg⁻¹ NPK fertilizer, there were significant differences in seedling height, ground diameter, and dry biomass in the roots, stems, and leaves among the CaO treatments (p < 0.01). Treatment with 3 g·kg⁻¹ CaO significantly promoted seedling height growth, ground diameter, and dry biomass in the roots, stems, and leaves, resulting in values 49.9%, 25.6%, 39.4%, 51.2%, and 50.1% higher than those with 0 g·kg⁻¹ soil CaO. In contrast, the seedling height, ground diameter and dry biomass of roots, stems, and leaves were lower with 6 g·kg⁻¹ CaO than with 3 g·kg⁻¹ soil CaO (

Table 2. Seedling height, ground diameter, and dry matter of cypress clones affected by the CaO level with and without NPK fertilization. Asterisks * and double asterisks ** indicate significant differences at p < 0.05 and p < 0.01, respectively.

NPK Fertilizer Treatment	Trait	CaO Treatment			F Value
		0 g·kg−1	3 g·kg−1	6 g·kg−1	CaO	Clones	Clones × CaO
NPK fertilizer 0 g·kg−1 soil	Seedling height (cm)	29.67 ± 2.56	44.49 ± 3.87	33.17 ± 3.28	8.26 **	5.57 **	3.54 *
	ground diameter (cm)	4.57 ± 0.35	5.74 ± 0.51	4.89 ± 0.38	15.65 **	7.21 **	4.07 **
	Root dry matter (g)	2.84 ± 0.21	3.96 ± 0.36	3.43 ± 0.34	7.58 **	5.53 **	2.43 *
	Stem dry matter (g)	2.09 ± 0.18	3.16 ± 0.34	2.56 ± 0.31	3.59 *	5.32 **	1.46
	Leaf dry matter (g)	4.19 ± 0.36	6.29 ± 0.61	5.33 ± 0.42	6.41 **	4.34 **	2.89 *
NPK fertilizer 3 g·kg−1 soil	Seedling height (cm)	48.67 ± 5.17	49.70 ± 4.88	51.36 ± 5.04	0.59	8.65 **	2.45 *
	Ground diameter (mm)	6.02 ± 0.58	6.38 ± 0.54	6.16 ± 0.57	1.57	12.04 **	4.03 **
	Root dry matter (g)	3.91 ± 0.34	5.48 ± 0.52	4.56 ± 0.34	2.74 *	6.81 **	2.22 *
	Stem dry matter (g)	3.76 ± 0.29	4.88 ± 0.39	4.38 ± 0.42	1.20	10.04 **	2.51 *
	Leaf dry matter (g)	7.74 ± 0.59	8.75 ± 0.81	8.59 ± 0.78	0.97	8.12 **	2.81 *

). As shown in Figure 1, both clones performed best at 3 g·kg⁻¹ CaO. Moreover, seedling height growth, ground diameter, and dry biomass in the roots, stems, and leaves were significantly different between the clones (Table 2). The seedling height, ground diameter, and dry biomass in the roots, stems, and leaves of clone 1 were significantly higher than those of clone 2, and the mean values of these parameters were 15.7%, 83.6%, 10.4%, 40.3%, and 60.8% higher for clone 1 than for clone 2, respectively (Figure 2).

In soil with 3 g·kg⁻¹ NPK fertilizer, treatment with 3 g·kg⁻¹ CaO significantly increased the biomass of roots, resulting in a 40.1% increase for the clones relative to the root biomass under 0 g·kg⁻¹ soil CaO. In contrast, the root biomass was lower than the control level when the CaO content reached 6 g·kg⁻¹ (Figure 1 and Table 2). The seedling height, ground diameter, and dry biomass of the roots, stems, and leaves of clone 1 were significantly higher than those of clone 2, and the mean values of these parameters were 30.1%, 70.3%, 87.2%, 86.1%, and 64.3% higher for clone 1 than for clone 2, respectively (Figure 2).

3.2. Effects of Soil Conditions and Calcium Fertilizers on Root Growth and Development

In soil with 0 g·kg⁻¹ NPK fertilizer, the root length, root surface area and root volume of the diameter classes D1–D4 but not D5 differed significantly among the CaO treatments. In the 3 g·kg⁻¹ CaO treatment, the root length, root surface area, and root volume significantly increased. In the 3 g·kg⁻¹ CaO treatment compared with the 0 g·kg⁻¹ CaO treatment, for roots in the D1-D4 diameter classes, the root length increased by 32.8%, 24.3%, 35.3%, and 59.5%, respectively; the root surface area increased by 17.9%, 46.8%, 20.2%, and 72.6%, respectively; and the root volume increased by 39.1%, 35.1%, 37.2%, and 53.2%, respectively. With the application of 6 g·kg⁻¹ CaO, the root length, surface area, and volume in the D1–D4 diameter classes were lower than those in the 3 g·kg⁻¹ CaO treatment, although the differences were non-significant. Furthermore, the values of these parameters were significantly higher in the 6 g·kg⁻¹ CaO treatment than in the 0 g·kg⁻¹ CaO treatment, indicating that the fine roots (<2.0 mm) of cypress were somewhat able to adapt to the Ca level under low-fertility conditions (

Table 3. Root length, surface area, and volume of cypress clones with different diameters affected by CaO fertilization rates under the treatments with and without NPK fertilization. Asterisks * and double asterisks ** indicate significant differences at p < 0.05 and p < 0.01, respectively.

NPK Fertilizer Treatment	Trait	Diameter Class (mm)	CaO Treatment			F Value
			0 g·kg−1	3 g·kg−1	6 g·kg−1	CaO	Clones	Clones × CaO
NPK fertilizer 0 g·kg−1 soil	Root length (cm)	0–0.5 (D1)	1093.24 ± 89.21	1451.61 ± 100.36	1302.28 ± 98.65	6.11 **	1.46	1.27
		0.5–1.0 (D2)	458.41 ± 31.41	569.99 ± 50.24	566.18 ± 60.04	4.16 **	2.45 *	1.54
		1.0–1.5 (D3)	92.93 ± 10.02	125.71 ± 10.32	114.12 ± 8.97	5.88 **	3.90 **	1.87
		1.5–2.0 (D4)	21.70 ± 2.14	25.53 ± 2.07	23.21 ± 1.97	7.07 **	3.18*	1.37
		>2.0 (D5)	15.64 ± 1.85	18.52 ± 1.58	17.15 ± 2.04	1.21	2.03	1.17
	Root Surface area (cm2)	0–0.5 (D1)	117.38 ± 17.21	138.49 ± 15.47	129.98 ± 16.04	5.32 **	3.12 *	1.77
		0.5–1.0 (D2)	87.63 ± 8.36	128.69 ± 13.04	112.37 ± 12.11	4.97 **	2.43 *	1.54
		1.0–1.5 (D3)	38.02 ± 3.21	45.69 ± 4.23	42.97 ± 3.04	6.45 **	2.76 *	1.52
		1.5–2.0 (D4)	7.96 ± 0.47	13.74 ± 0.98	10.91 ± 0.75	7.03 **	0.98	1.04
		>2.0 (D5)	15.91 ± 1.02	20.46 ± 1.68	16.46 ± 1.47	1.63	0.34	2.68 *
	Root volume (cm3)	0–0.5 (D1)	0.90 ± 0.08	1.25 ± 0.11	1.14 ± 0.09	4.93 **	1.76	7.75 **
		0.5–1.0 (D2)	1.64 ± 0.12	2.22 ± 0.09	1.98 ± 0.10	5.08 **	4.43 **	0.88
		1.0–1.5 (D3)	0.98 ± 0.06	1.35 ± 0.07	1.22 ± 0.06	7.12 **	6.75 **	1.43
		1.5–2.0 (D4)	0.40 ± 0.02	0.61 ± 0.03	0.54 ± 0.03	7.54 **	0.81	1.60
		>2.0 (D5)	1.52 ± 0.11	2.31 ± 0.16	1.99 ± 0.16	2.32	0.61	2.20
NPK fertilizer 3 g·kg−1 soil	Root length (cm)	0–0.5 (D1)	1485.46 ± 105.24	1749.83 ± 135.36	1395.10 ± 101.24	3.27 *	5.67 **	1.36
		0.5–1.0 (D2)	981.03 ± 85.31	1147.28 ± 91.04	863.67 ± 74.36	7.31 **	7.22 **	5.67 **
		1.0–1.5 (D3)	205.25 ± 18.36	232.34 ± 17.65	218.88 ± 19.32	6.45 **	5.16 **	5.49 **
		1.5–2.0 (D4)	40.48 ± 3.65	45.28 ± 4.02	37.95 ± 3.21	2.09	9.26 **	1.24
		>2.0 (D5)	22.71 ± 2.11	28.28 ± 2.05	25.02 ± 2.24	1.53	4.89 **	0.98
	Root Surface area (cm2)	0–0.5 (D1)	136.62 ± 9.57	169.39 ± 14.03	149.19 ± 12.32	5.52 **	7.14 **	1.65
		0.5–1.0 (D2)	197.27 ± 15.36	225.99 ± 19.65	191.71 ± 17.65	9.23 **	5.05 **	1.88
		1.0–1.5 (D3)	79.78 ± 6.32	85.60 ± 7.05	81.28 ± 6.95	6.77 **	6.28 **	3.08 *
		1.5–2.0 (D4)	20.10 ± 1.83	24.09 ± 2.04	20.61 ± 1.76	1.68	8.16 **	4.73 **
		>2.0 (D5)	23.73 ± 1.51	29.72 ± 2.38	25.37 ± 2.01	0.98	4.78 **	1.69
	Root volume (cm3)	0–0.5 (D1)	1.18 ± 0.11	1.60 ± 0.13	1.27 ± 0.10	9.32 **	8.01 **	4.79 **
		0.5–1.0 (D2)	3.14 ± 0.39	4.22 ± 0.32	3.54 ± 0.23	5.11 **	5.44 **	2.04
		1.0–1.5 (D3)	2.17 ± 0.19	2.51 ± 0.21	2.40 ± 0.21	7.02 **	8.26 **	3.14 *
		1.5–2.0 (D4)	0.86 ± 0.07	1.03 ± 0.11	0.90 ± 0.06	1.43	10.31 **	6.70 **
		>2.0 (D5)	2.75 ± 0.21	3.46 ± 0.28	2.83 ± 0.20	1.58	12.21 **	1.59

and Figure 3). The root lengths of diameter classes D2, D3, and D4 of clone 1 were significantly higher (8.8%, 2.8%, and 9.5% higher, respectively) than those of clone 2. The root surface areas of diameter classes D1, D2, and D3 of clone 1 were significantly higher (15.1%, 7.2%, and 1.8% higher, respectively) than those of clone 2. The root volumes of diameter classes D2 and D3 of clone 1 were significantly higher (7.4% and 9.1% higher, respectively) than those of clone 2.

In the soil with 3 g·kg⁻¹ NPK fertilizer, adding 3 g·kg⁻¹ CaO significantly increased the root length, surface area, and volume in the D1–D3 diameter classes. In the 3 g·kg⁻¹ NPK fertilizer treatment compared with the 0 g·kg⁻¹ CaO treatment, for roots in the D1–D3 diameter classes, the root length increased by 17.8%, 16.9%, and 13.2%, respectively; the root surface area increased by 23.9%, 14.6%, and 7.3%, respectively; and the root volume increased by 35.6%, 34.4%, and 15.4%, respectively. The effect on roots in the D4 and D5 diameter classes was non-significant. With the application of 6 g·kg⁻¹ CaO, the root length, surface area, and volume in the D1–D5 diameter classes decreased (Table 3 and Figure 3). The root length, root surface area, and root volume of diameter classes D1–D5 were significantly different between the clones, and these parameters were significantly higher in clone 1 than in clone 2.

3.3. N, P, and Ca Accumulation Efficiencies

The t-test results showed that the N accumulation efficiency with 3 g·kg⁻¹ NPK fertilizer was significantly greater than that in low-fertility soil. In the 3 g·kg⁻¹ CaO and 6 g·kg⁻¹ CaO treatment groups, the Ca and P accumulation efficiency was significantly greater in lower fertility soil than in the soil with 3 g·kg⁻¹ NPK fertilizer. In soil with 0 g·kg⁻¹ NPK fertilizer, the two clones achieved their highest N, P, and Ca accumulation efficiencies under the 3 g·kg⁻¹ CaO treatment (Figure 4). When the soil was treated with 3 g·kg⁻¹ NPK fertilizer, the P accumulation efficiency in the cypress clones exhibited a downward trend with increasing CaO concentration, the P accumulation efficiency was 1.3% (3 g·kg⁻¹ CaO treatment group) and 7.5% (6 g·kg⁻¹ CaO treatment group) lower than that in the 0 g·kg⁻¹ CaO treatment group. However, the differences in P accumulation efficiency were not significant. The calcium accumulation efficiency was significantly higher in the 3 g·kg⁻¹ CaO and 6 g·kg⁻¹ CaO treatment groups than in the 0 g·kg⁻¹ CaO treatment group (Figure 4). As shown in Figure 4, the calcium accumulation efficiency was 25.0% (3 g·kg⁻¹ CaO treatment group) and 34.1% (6 g·kg⁻¹ CaO treatment group) higher than that in the 0 g·kg⁻¹ CaO treatment group. Moreover, with or without fertilization, the N and Ca accumulation efficiencies differed significantly between the clones.

4. Discussion

The application of NPK fertilizer to soil can significantly increase the growth of seedlings, but the use of Ca fertilizer in production is often neglected. Moreover, excessive application of NPK fertilizer can lead to acidic soil; under these conditions, it can cause the loss of calcium from the soil, which seriously affects the absorption and utilization of Ca by plants [25]. Compared with the 0 g·kg⁻¹ CaO treatment, regardless of the application of NPK fertilizer, seedling height, basal diameter, and root development of cypress increased in the other CaO treatments (Table 2), indicating that the application of CaO fertilizer was able to promote the growth of cypress. Following treatment with Ca²⁺, the soil conditions were dramatically altered. The cation exchange capacity (CEC) and base saturation (BS) increased considerably after addition [14], which might promote the migration of N, P, and Mg plasma in the soil [30]. At the same time, adding an appropriate amount of calcium fertilizer can improve soil microbial activity [31], enhance the respiration ability of roots in the soil, and improve the absorption capacity of roots [32].

In this study, in the fertile soils, the seedling height, basal diameter, and stem and leaf dry biomass of cypress were not significantly different among the Ca²⁺ treatments, but the root dry-matter weight, root length, root surface area, and root volume in the D1–D3 diameter classes (≤1.5 mm) were significantly different among the CaO treatments (Table 2 and Table 3). When CaO application reached 6 g·kg⁻¹, the root length, surface area, and volume in the D1–D5 diameter classes decreased, indicating that the growth of the root system was inhibited, and the seedling height and basal diameter were also reduced. Although the accumulation efficiency of N and Ca was the highest in the 6 g/kg CaO treatment, the differences were not significant between CaO treatment. Moreover, the accumulation efficiency of P was reduced by the application of Ca, and the possible reasons for this finding are as follows: The orthophosphates in the soil are prone to chemical precipitation with ions, such as Al³⁺, Fe³⁺, and Ca²⁺, or to adsorption-fixation by soil, resulting in the poor mobility of P in the soil and difficulty in P uptake by plant roots [23]. The results showed that, in fertile soils, the use of appropriate amounts of Ca can create a favorable underground environment for the growth of plants, increase the pH of the soil, and promote the growth and development of fine roots (i.e., ≤1.5 mm in diameter), while excess Ca can inhibit plant growth.

In the low-fertility soil compared with the fertile soil, cypress seedling growth and root development show greater responsiveness to CaO. Under low-fertility conditions, the seedling height of cypress increased with the addition of an appropriate amount of Ca²⁺ (3 g·kg⁻¹), and the highest N, P, and Ca accumulation efficiencies were all achieved under the 3 g·kg⁻¹ Ca²⁺ treatment, with synergy between Ca²⁺ fertilizer and N and P in terms of accumulation efficiencies (Figure 4). However, when the Ca²⁺ concentration was increased, seedling growth of cypress clones decreased under the 6 g·kg⁻¹ Ca²⁺ treatment. These results indicate that the synergy showed a range of adaptation to the level of Ca²⁺ applied. That is, an appropriate amount of Ca²⁺ promoted plant N and P uptake, while an excessively high concentration of Ca²⁺ fertilizer inhibited uptake [33,34]. In a study conducted on coniferous species such as pine (Pinus massoniana Lamb.), favorable adaptation was also observed in the soil environment with Ca²⁺ supplied at 1 to 2 mmol·L⁻¹, while the plant height growth of pine seedlings decreased after the Ca²⁺ supply exceeded this concentration [35]. Therefore, full consideration should be given to the tolerance of tree species when applying Ca²⁺ to promote seedling growth.

The root system determines plant water and nutrient uptake and is closely related to plant traits, such as height and growth rate [36,37], and a reasonable root configuration can provide the plant with a larger absorption area. As the key parts of plants for nutrient uptake, fine roots feature small diameters and low lignification levels, with high sensitivity to changes in soil nutrients [38]. The fine roots in the D1–D3 diameter classes usually consist of non-lignified components (e.g., cortical tissue); these roots are mainly involved in the acquisition and absorption of water, nutrients, and other soil resources and are classified as absorbing roots. The roots in the D4 and D5 diameter classes are usually composed of lignified components (e.g., secondary xylem) and are mainly used for transport and storage [26,27,36,39]. Compared with those of coarse roots, the greater length and surface area of fine roots enable plants to respond to changes in the soil environment more easily [40]. The quantity of absorbing roots and the root-length density were significantly correlated with the available nutrients in the soil. Specific root length is the ratio of root length to biomass. The greater the advantage afforded by higher specific root length to the plant in obtaining water and nutrients is, the larger the number of fine roots and the smaller the root diameter [41]. Fine roots of plants in classes D1–D3 (diameter ≤1.5 mm) accounted for more than 96.6% of the total root length and over 88% of the root surface area (Table 3). In soil treated with 3 g·kg⁻¹ NPK fertilizer, fine root diameters ≤1.0 mm accounted for 89.1%, 90.7%, and 88.9% of the total root length and accounted for 70.2%, 73.9%, and 71.8% of the total surface area across the three CaO treatments (0, 3, and 6 g·kg⁻¹), respectively (Table 3 and Figure 3). The corresponding proportions of roots with diameters ≤ 1.0 mm were even higher in the low-fertility soil, reaching 92.1%, 93.4%, and 92.5% of the total root length and accounting for 75.8%, 77.1%, and 76.1% of the total surface area across the three CaO treatments (0, 3, and 6 g·kg⁻¹), respectively. These results indicate that cypress can adjust the morphology of its fine roots to adapt to different CaO environments. In resource-poor locations, increasing the number and longevity of fine roots may be an optimal option. It can improve fine root turnover efficiency, maximize resource acquisition efficiency, improve seedling potential adaptability, and balance tolerance with competitiveness in adversity [15,42]. As expected, when the site conditions were relatively infertile, cypress formed more roots with diameters ≤ 1.0 mm. Fine, long, fast-growing, and absorbent roots can improve the root distribution and foraging accuracy, allowing plants to quickly obtain the nutrients and water needed for growth. Furthermore, fine roots can spread to fill the soil space, obtaining more soil nutrients and water resources [43].

The main functional unit of a root for nutrient uptake is close to the root-tip region [44,45]. The process of calcium uptake by roots is active or passive, depending on the concentration of calcium in the soil. With a low external Ca concentration, the absorption of Ca by roots has been found to be almost completely determined by the metabolic capacity, while Ca absorption capacity under a high external Ca concentration was not correlated with the metabolic capacity of roots but showed a linear relationship with the transpiration rate [46]. Root metabolism is closely related to root activity, and calcium absorption can be promoted by improving root activity. In plant roots, Ca²⁺ can enter epidermal cells and root hairs directly through Ca²⁺ channels. Various Ca²⁺ channels have been found in root cells, such as voltage-dependent Ca²⁺ channels, plasma membrane stretching activated Ca²⁺ channels, and second-messenger-molecule-activated Ca²⁺ channels [47]. These findings will provide insight into the study of the Ca²⁺ dependence, mobility, and absorption dynamics of cypress in a variety of environments in later stages of development.

5. Conclusions

Cypress roots have a strong ability to adapt to different environments. In Ca-deficient low-quality acidic soils, fine roots (<2.0 mm) were somewhat able to adapt to differing Ca levels, and adding 3 g·kg⁻¹ CaO fertilizer promoted the development of fine roots and the uptake and utilization of N, P, and Ca. When there were sufficient levels of N, P, and K in the soil, the promoting effect of CaO was not obvious. Adding excessive CaO inhibited growth and root development. The results of this study provide a basis for determining the optimal fertilization strategy for growing cypress in acidic soils in Southern China.