Assessing the Suitability of Sediment Soil to Be Reused by Different Soil Treatments for Forest Agriculture

Abstract

In order to determine the suitability of reusing the sediment deposited in the upper part of the dam as soil for growing forest products, a total of eight treatment plots were formed by mixing cultivated soil, compost, and biochar with the sediment in a volume ratio. Generally, the soil fertility of SS100 was relatively lower than that of CS, with Av. P₂O₅ being only 22%. SS70 and SS50 increased soil physicochemical properties including OM, exchangeable cations, CEC, and BS by 1.1–2.9 times more than SS100, thus, A. scaber Thunb. treated in SS70 and SS50 showed increased photosynthetic parameters including A, V_cmax, J_max, and the growth characteristics, especially in the aboveground part, were 20% and 31% higher than the SS100, respectively. Furthermore, A. scaber in CS, SS50, Bc10, and Comp had higher PI_abs, DFI_abs, and SFI_abs while maintaining lower V_K/V_J after 10 days of drought stress, Comp and Bc10, in particular, had a high Y(NPQ) and a low Y(NO). Bc5 and Bc10, revealed no discernible differences in soil physicochemical properties, nonetheless, A. scaber in Bc10 demonstrated relatively high drought tolerance. Overall, CS, SS50, and Comp had relatively well-balanced plant growth, and drought tolerance was found to be higher in CS, Comp, SS50, and Bc10, which is thought to have higher water-holding capacity and soil fertility. As a result, if appropriate treatment methods are established, such as mixing sediment soil with cultivated soil at a one-to-one ratio or adding compost, increase the public value of forests by promoting activities such as watershed conservation, soil runoff prevention, and reducing the financial burden associated with sustainable forestry management and SS50 is recommended as the most cost-effective method.

1. Introduction

Mountainous areas in Korea are naturally characterized by numerous steep slopes and valleys and are subjected to frequent erosions, such as soil runoff, caused by concentrated rainfall in summer [1,2]. Water erosion, the leading cause of soil loss in the Republic of Korea, is a type of splash erosion in which soil aggregates are destroyed when raindrops fall and hit the ground. [3]. Surface water runoff, which does not penetrate into the soil and flows along the surface, has been identified as a factor that reduces soil fertility and productivity while also destroying the ecosystem [4]. According to a preliminary survey of topsoil erosions, the amount of soil loss due to erosion averages approximately 3.3 kg·m⁻²·yr⁻¹ in more than 30% of the national territory, with significant loss due to natural and artificial factors caused by arable land activities [5].

Highland fields in the Republic of Korea are sandy weathered granite soils with weak cohesion [6] that lead to soil loss due to rainfall and nonpoint pollutant leakage which are serious issues during farming activities [4]. Furthermore, farmers frequently extract topsoil from mountainous areas adjacent to highland cropland in order to increase cultivated area and then divert it to their fields, resulting in accelerated soil loss [7].

To reduce nonpoint source pollution and improve soil water quality, the Ministry of Environment in Korea has implemented muddy water reduction projects to limit soil loss. Similarly, the Ministry of Agriculture, Food, and Rural Affairs in Korea is conducting a field-based project to maintain agricultural productivity, as well as a project to reduce soil loss and soil nutrient leakage [8]. Additionally, the Korea Forest Service is conducting research projects on improving forest road management and limiting soil collection, to conserve forest resources [9]. The second comprehensive nonpoint source management plan was developed in consultation with the related ministries, and topsoil was included as a management item [5,10].

On another side, due to frequent localized torrential rains, excessive sediment deposition can exceed the dam’s stable water level, resulting in landslides and flooding in the surrounding areas [11]. To prevent sediment runoff in an impervious dam, a large sediment area directed toward the water surface is required [12]; however, the valleys in the Republic of Korea are narrow and have severe curvatures, resulting in critical spatial restrictions [13]. Measures to improve the structure of check dams, such as permeable and partially permeable dams have been reported to address this issue [12,14]; nonetheless, studies on methods for recycling soil deposited on top of check dams are still scarce.

The check dams deposit the lost forest soil from the upper part [15], although studies on soil loss reduction overlook the recycling method of previously accumulated sediment soil, and studies on lost forest soil usage have been limited to the use of topsoil when primarily restoring through buried seeds [16,17,18]. The upstream side of the forest has relatively uncontaminated natural soils, yet on the downstream side, excess nutrients or pollutants from the cultivation area make it uneasy to recycle [19], and the evaluation of the value of sediment soil has hardly been carried out.

Aster scaber is a perennial plant belonging to the family Asteraceae [20]. Having a distinct taste and aroma, its leaves and stems have been used as raw vegetables since olden history, and the production happens in a variety of environments, including forest, facility, and open field cultivation [21]. A. scaber is rich in flavonoids and saponins, its flower ears and stems have essential oil components, and its roots contain functional ingredients such as coumarin, sapogenin, shionon, alkaloid, squalene, friedelin, and α-spinasterol [22]. Additionally, it is a healthy food with a high biological value, rich in calcium, iron, and vitamin A and is used as a herbal medicine. Recently, pharmacological effects in lowering cholesterol and anticancer have been associated with this herb [22,23]. Nonetheless, it has been reported that A. scaber is a nutrient-demanding species, with increased yields and growth at higher nitrogen fertilization rates [24].

This research is part of a larger investigation into reusing plans for the soil that is constantly lost near highland fields and deposited on the upper portion of check dams. We assume that the value of the sediment soil can be increased by adding appropriate soil amendments, which may improve fertility as well as plant growth in terms of increased belowground biomass, resulting in improved drought tolerance. The study aims to assess the feasibility of reusing sediment soils as a sustainable and cost-effective approach in forest-agriculture and long-term water resource management.

2. Materials and Methods

2.1. Experimental Design

2.1.1. Soil and Environmental Conditions

In 2022, we collected SS from the accumulated soil on the upper part of the dam (37°33’51.6” N, 128°50’19.1” E) in Songhyeon, Wangsan, Gangneung, Republic of Korea (Figure 1) after removing gravel with a 10 mm sieve. Meanwhile, CS was gathered from highland vegetable farmland (37°33’54.7” N, 128°50’04.2” E) located 392 m upstream from the area where SS was collected (Figure 1). The initial soil improvement treatment for the experiment was classified into eight; SS100 (SS 100%), SS70 (SS 70% + CS30%), SS50 (SS 50% + CS 50%), CS (CS 100%), Bc5 (Bc 5% + SS 95%), Bc10 (Bc 10% + SS 90%), Comp10 (Comp 10% + SS 90%), and Comp20 (Comp 20% + SS 80%). However, more than 80% of Comp20 withered away within 30 days after the treatment; thus, the physiological experiment could not be performed. All treatments were weighted as a percentage of volume and laid out in a randomized block design with five replications, three plants per treatment; thus, a total of fifteen plants were placed per treatment. SS70 and SS50 were made by mixing SS and CS, whereas Bc (Soilmate, K biochar, Jeonju, Korea) and compost (Seobu livestock poultry compost, Egg Plus, Pocheon, Korea) are commercially available in Korea. On 20 April 2022, Aster scaber sown in March 2022 was transplanted to a circular pot with a height of 20 cm and a diameter of 15 cm after completing the soil treatment. Then, the samples were grown in the greenhouse at the Korea National University of Agriculture and Fisheries until 20 July 2022. For the growth experiments, from the fifteen plants per treatment, five specimens from each treatment were randomly selected at the very beginning of the study, and no physiological experiments were carried out to minimize damage to the leaves. Thus, ten plants were used for physiological experiments, and five specimens out of ten were randomly selected for the physiological experiments.

From 10 May to 5 August 2022, a thermo-humidity meter (HOBO H08-004-02, Onset Corporation; Bourne, MA, USA) was installed at a height of 1 m above the ground to determine the main environmental factors. Moreover, every day at 9 AM, the soil water content (SWC) was measured with a portable soil moisture meter (PMS-714, Lutron Electronic Enterprise Co., Ltd., Taipei, Taiwan), and when SWC fell below 10%, 500 mL per pot was watered.

During the experimental period, the average temperature was 26.5 °C, with a maximum range of 42 °C and a minimum range of 9.9 °C, and the relative humidity was 42.2% on average, ranging from 98.7% to 15.0%.

2.1.2. Physicochemical Properties of Soil

On 20 July 2022, we gathered 200 g of air-dried and sieved (2 mm) soil for each treatment, and the analysis method of the soil collected by these methods was in accordance with the Soil and Plant Analysis Method [25] of the National Institute of Agricultural Science of the Rural Development Administration.

The pH was measured with a pH meter by extracting a ratio of soil and distilled water of 1:5, and the organic matter (OM) was analyzed by the Tyurin method. The available phosphoric acid (Av. P₂O₅) was measured by the Lancaster method, and cations such as substituted potassium, calcium, and magnesium were extracted with 1 M NH₄OAc and measured with an atomic absorption spectrophotometer (Analytik Jena AG Nov AA-300, Germany). The total nitrogen content (T-N) was analyzed by the Kjeldahl method using Foss Kjeltec 2200. The percentage of base saturation (BS) was computed by dividing the sum of exchangeable cations by the cation exchange capacity (CEC) [26].

2.1.3. Photosynthetic Responses and Chlorophyll Contents

We measured the photosynthetic response to soil treatments once a month from May to July, on the 19th and the 20th, using a portable photosynthesis meter (Li-6800, Li-Cor Inc., Lincoln, NE, USA). The photosynthetic photon flux density was set at 1200 mol·m⁻²·s⁻¹ using the LED light source attached to the photosynthetic meter. We measured the net photosynthetic rate (A), the stomatal transpiration rate (E), and the stomatal conductance (g_s). Subsequently, we calculated the instantaneous transpiration efficiency (ITE) and the intrinsic water use efficiency (WUEi). Furthermore, we computed the maximum carboxylation rate (V_cmax) and the maximum electron transfer rate (J_max) in mesophyll cells by analyzing the CO₂ response curve (A–Ci) with CO₂ concentrations ranging from 0 to 1400 µmol·m⁻²·s⁻¹ [27]. As standard measurement conditions, the airflow into the chamber was kept at 600 µmol·s⁻¹ and the temperature at 25 ± 1 °C. SPAD-502 m (Minolta Co. Ltd., Osaka, Japan) was used to measure chlorophyll contents at 10 repetitions per treatment.

2.1.4. Growth Characteristics

To determine the difference in growth by soil treatments following all the physiological experiments, five samples per treatment were dried at 80 °C using a dryer (DS-80-5, Dasol Scientific Co. Ltd., Gyeonggi-do, Korea), and the dry weights of the aboveground (leaves and shoots) and the belowground (roots) parts were measured on 20 July 2022. Based on the dry weight of the leaves, shoots and roots, we calculated the shoot/root (S/R) ratio, the leaf weight ratio (LWR), and the root weight ratio (RWR).

2.1.5. Chlorophyll a Fluorescence

Visible damage was shown after 10 days of water outage; thus, DS was treated for a total of ten days from July 20 (DS-0) to July 30 (DS-10), 2022. To compare the vitality of photosystem II (PS II) and the photosynthetic apparatus, we performed OKJIP and image fluorescence analyses. A plant efficiency analyzer (Hansatech Instrument Ltd., King’s Lynn, UK) was used on DS-1, DS-5, and DS-10, with five repetitions per treatment. The light intensity of 3500 m⁻²s⁻¹ was irradiated for 1 s to the leaves that were dark-adapted for 20 min, and we measured the chlorophyll fluorescence intensity at 50 µs (stage O), 300 µs (stage K), 2 ms (stage J), 30 ms (stage I), and 500 ms (stage P) [28,29].

Furthermore, on July 30, the MAXI version of MAGING-PAM (IMAG-K7 by Walz GmbH, Effeltrich, Germany) was used to obtain chlorophyll fluorescence images of plants exposed to drought after 10 days of water outage. The measurement head consisting of the LED-Array Illumination Unit IMAG-MAX/L and the CCD (charged coupled device) camera IMAG-MAX/K4 was mounted on the IMAG-MAX/GS stand. The actinic light (550 µmol·m⁻²·s⁻¹) with light intensity was turned on with saturated flashing (1450 µmol·m⁻²·s⁻¹) applied every 20 s. Dark adaptation was performed for more than 20 min before the measurement, with a black fabric covering the measuring head to avoid external light input during the sample stage. All experiments were repeated three times per treatment. The measured image was defined as an area of interest using ImagingWin v2.41 software, where we computed the maximum quantum yield of PS II photochemistry measured in the dark-adapted state (Fv/Fm), the PS II actual photochemical quantum yield (Y(II)), the quantum yield of regulated energy dissipation in PS II (Y(NPQ)), and the quantum yield of nonregulated energy dissipation in PS II (Y(NO)) [30,31,32].

2.2. Statistical Analysis

SPSS Statistics program 19.0 (SPSS Inc., Chicago, IL, USA) was used for statistical analysis of the experimental results. The photosynthetic response was performed using an analysis of variance based on repeated measurements, whereas the growth and chlorophyll fluorescence responses were performed using a one-way analysis of variance. DMRT (Duncan’s Multiple Range Test) was used for the post hoc analysis, and for all tests, statistical significance was set at p < 0.05.

3. Results

3.1. Physicochemical Properties of Soil

Table 1. Soil properties of different soil treatments.

Treatment	Soil Texture (%)			pH	EC	OM	CEC	Av. P2O5	T-N	Exchangeable Cations (cmol·kg−1)				BS
	Sand	Silt	Clay	(1:5)	(dS·m−1)	(g·kg−1)	(cmolc·kg−1)	(mg·kg−1)	(%)	Ca++	Mg++	K+	Na+	(%)
SS100	93.5	6.2	0.3	6.4	0.09	2.81	5.66	105.43	0.02	0.17	2.57	0.38	0.01	55.3
SS70	90.8	8.3	0.9	6.8	0.19	4.62	6.32	172.94	0.04	0.27	3.47	0.49	0.01	67.1
SS50	86.7	10.2	3.1	7.1	0.30	5.95	7.14	308.78	0.05	0.42	4.19	0.52	0.01	72.0
CS	80.9	14.9	4.2	7.1	0.41	7.86	8.62	482.92	0.08	0.86	4.94	0.6	0.02	74.5
Bc5	93.5	6.4	0.1	6.7	0.11	3.72	5.54	119.08	0.02	0.19	2.86	0.47	0.03	64.1
Bc10	93.4	5.0	1.6	6.3	0.08	5.57	6.32	106.90	0.02	0.21	2.73	0.53	n.d.	54.9
Comp	92.6	4.8	2.6	7.5	1.35	13.00	6.90	873.59	0.1	2.48	6.53	1.84	0.44	163.6

pH = level of alkalinity/acidty; EC—electrical conductivity; OM—organic matter; CEC—cation exchange capacity; Av. P₂O₅—available phosphorus; T–N—Total Nitrogen; BS—base saturation.

depicts the changes in soil physicochemical properties as a result of the forest SS improvement methods. SS100 exhibited very low clay characteristics, with sand, silt, and clay ratios of 93.5%, 6.2%, and 0.3%, respectively, whereas the soil acidity (pH), electrical conductivity (EC), organic matter (OM), available phosphorus pentoxide (Av. P₂O₅), total nitrogen (T–N), and exchangeable cation (Ca²⁺, Mg²⁺, K⁺, and Na⁺) showed the lowest tendency among the treatments. CS had a lower sand content of approximately 10%, but a higher silt and clay contents. Soil chemical properties such as pH, OM, T–N, and exchangeable cations showed a significantly higher tendency than SS. Particularly, EC and Av. P₂O₅ were 4.6 times and calcium was 5.1 times higher in the CS.

Overall, the percentage of sand in SS70 and SS50 decreases as the CS mixing portion increases, while silt increases by 1.3 and 1.6 times, and clay increases by three and ten times, respectively. The soil chemistry parameters pH, EC, OM, CEC, Av. P₂O₅, T-N, exchangeable cations, and BS show an overall increase of 1.1–2.9 times in SS70 and SS50 compared to SS100. Furthermore, the higher soil physicochemical properties in SS50 compared to SS70 can be interpreted as an increase in soil fertility as the percentage of CS increases, and particularly for SS50, the soil physicochemical properties approach those of CS.

The Bc treatments (Bc5 and Bc10) did not differ significantly from the SS100 in terms of overall soil physicochemical properties such as soil texture, pH, and Av. P₂O₅, whereas OM in the Bc10 was approximately twice as high as in the SS treatment, due to a slight increase. In the Comp treatment, the clay content increased compared to the SS100, whereas BS was 163.6%, resulting in the highest pH, EC, OM, Av. P₂O₅, T–N, and exchangeable cations of all treatments. Especially, EC, Av. P₂O₅, and T–N increased 15.0, 8.3, and 5.0 times, respectively, whereas, among the exchangeable cations, Ca²⁺and Na⁺ increased 14.6 and 44.0 times, respectively.

3.2. Photosynthetic Responses and Chlorophyll Contents

As shown in

Table 2. Repeated measures analysis of variance of photosynthetic characteristics as affected by different soil treatments.

Month	Treatment	E	A	gs	ITE	WUEi	SPAD
		(mmol·m−2·s−1)	(µmol·m−2·s−1)	(mmol·m−2·s−1)	(µmol·mmol−1)	(µmol·mmol−1)
May	SS100	1.7(0.7)	5.7(0.3)	101.6(48.5)	3.8(1.3)	63.6(23.8)	36.0(5.0)
	SS70	2.4(0.4)	9.9(0.4)	149.0(29.0)	4.3(0.6)	68.0(11.0)	34.9(3.2)
	SS50	1.6(0.3)	8.6(1.3)	92.1(17.4)	5.6(0.2)	94.1(5.6)	35.8(3.5)
	CS	1.8(0.2)	7.9(1.3)	108.6(15.0)	4.4(0.5)	72.7(8.6)	32.0(3.9)
	Bc5	2.6(0.3)	8.5(1.0)	166.6(19.9)	3.3(0.1)	51.0(1.6)	35.1(2.8)
	Bc10	2.4(0.4)	9.1(1.9)	154.4(29.2)	3.8(0.6)	59.8(10.5)	34.2(3.7)
	Comp	1.5(0.5)	9.5(1.6)	91.4(32.2)	6.4(0.9)	108.4(19.1)	33.2(3.0)
June	SS100	3.4(0.1)	11.8(1.5)	228.5(8.1)	3.5(0.3)	51.4(5.2)	37.9(4.4)
	SS70	3.0(0.9)	10.5(1.0)	205.2(74.0)	3.6(0.7)	54.8(15.0)	37.3(5.5)
	SS50	1.9(0.5)	10.0(1.2)	115.6(34.3)	5.4(0.7)	89.7(14.2)	42.7(2.8)
	CS	2.2(0.4)	10.0(1.9)	135.8(32.0)	4.6(0.5)	74.9(10.4)	37.1(3.4)
	Bc5	2.6(0.9)	11.6(1.7)	167.5(69.5)	4.8(1.1)	75.8(22.9)	35.1(1.4)
	Bc10	3.3(0.1)	12.0(1.2)	225.3(10.3)	3.6(0.3)	53.3(3.7)	36.7(2.6)
	Comp	3.0(1.1)	13.9(2.2)	208.5(95.6)	4.8(1.0)	72.9(21.3)	34.7(3.2)
July	SS100	3.7(0.5)	12.1(1.5)	278.8(50.9)	3.3(0.7)	44.7(11.1)	41.9(6.5)
	SS70	3.3(0.9)	12.4(2.0)	253.8(81.3)	3.8(0.8)	51.1(11.8)	41.2(3.6)
	SS50	3.2(1.4)	13.4(2.6)	229.5(126.9)	4.6(1.1)	66.2(22.2)	41.5(5.8)
	CS	3.3(0.9)	14.4(0.5)	235.6(78.9)	4.6(1.1)	65.4(20.1)	47.7(4.9)
	Bc5	3.6(1.4)	10.8(1.7)	260.4(110.9)	3.3(1.0)	46.2(17.0)	37.2(7.2)
	Bc10	2.0(0.2)	8.7(0.4)	135.8(5.1)	4.3(0.5)	64.1(5.0)	42.1(5.0)
	Comp	4.0(0.9)	14.6(2.0)	310.5(91.3)	3.8(1.1)	50.2(16.8)	39.0(4.5)
Between subjects
Treatment (T)		<0.049	0.055	<0.047	<0.017	<0.025	<0.001
Within subjects
Month (M)		<0.000	< 0.000	<0.000	<0.034	<0.00	<0.000
M × T		<0.019	< 0.001	0.051	<0.013	<0.008	<0.001

E—stomatal transpiration rate; A—net photosynthetic rate; g_s—stomatal conductance; ITE –instantaneous transpiration efficiency; WUE_i—intrinsic water use efficiency.

, A, E, and g_s of each treatment, except for Bc10, increased from June to July 2022 compared to May 2022. As for the Comp, A was found the highest among all treatments.

In SS100, A of 5.7 µmol·CO₂·m⁻²·s⁻¹, which was noticeably lower among the treatments in May 2022, greatly increased around June–July 2022, showing similar levels with other treatments, and similarly, E and g_s showed common trends. A, E, and g_s gradually increased with the growth period in SS70 and SS50, especially in July, when A of SS70 was approximately 16.1% lower than the CS, whereas, SS50 showed relatively smaller differences by approximately 7.5%. Conversely, in the case of CS, A, E, and g_s were rather low in May 2022 and gradually increased, reaching 14.4 µmol·CO₂·m⁻²·s⁻¹ for A around July 2022, matching a similar level as Comp.

WUE_i of all treatments, except for Bc5 and Bc10, tend to decrease gradually with the growth period, in particular, Comp showed the highest variability as ITE and WUE_i decreased by approximately 50% in July 2022 compared to May 2022. CS, conversely, showed the smallest variation in WUE_i depending on the growth period.

In July, the SPAD value of CS was 47.7, which was the highest in the overall trend, and while the other treatments showed a difference of 6.0–23.1% from May to July, CS showed a difference of approximately 49.1%, which was the most pronounced not only in the months but also among treatments.

Following the soil treatment method, V_cmax and J_max showed relatively similar trends as A. In May and July 2022, SS100 had the lowest values, which tended to rise as the CS ratio increased, especially in July 2022. In the case of SS70, CS, and Comp, V_cmax was 53–56 µmol·m⁻²·s⁻¹, and J_max was 108–110 µmol·m⁻²·s⁻¹, with SS100 and Bc5 showing 60.3–69.9% and 52.1–54.9%, respectively (Figure 2).

As a result of the analysis of variance by repeated measurements, the interaction of the soil treatment method with the growth period resulted in significant differences for all the indicators except for A (p > 0.05), whereas only the mutual factors of the growth period and the treatment groups showed significant differences. On the other hand, the interactions between the growth period and soil treatment methods of V_cmax and J_max, which represent photosynthetic capacity, were significant in all treatments.

3.3. Growth Characteristics

Depending on the soil treatment method regarding the growth characteristics of A. scaber (Figure 3), CS had the highest aboveground dry weight (69%). Moreover, the aboveground growth in SS70 and SS50 was 20% and 31% higher than in SS100, respectively, whereas Bc5 and Bc10 had the lowest rates. In Comp, the aboveground dry weight did not differ significantly from SS100, whereas, the belowground dry weight increased significantly and was approximately 3.6 times greater than SS100. Consequently, Comp had the highest total dry weight, followed by CS > SS50 > SS70 > Bc5, yet no discernible difference was detected between Bc10 and SS100 (p > 0.05). Moreover, in most treatments, Comp had a lower S/R ratio and LWR than SS100, whereas CS had higher S/R and LWR than RWR when compared with all treatments.

3.4. Chlorophyll a Fluorescence under Drought Condition

In accordance with soil treatment methods, the photosynthetic capacity and the physiological sensitivity to DS were investigated using chlorophyll a fluorescence, and related parameters are represented in

Table 3. Summary of chlorophyll fluorescence parameters.

Parameters	Description
VJ	Relative variable fluorescence at the J-step
VK	Relative variable fluorescence at the k-step
Fv/Fm	Maximum quantum yield of PSII photochemistry measured in the dark-adapted state
Y(II)	PSII actual photochemical quantum yield
Y(NPQ)	Quantum yield of regulated energy dissipation in PSII
Y(NO)	Quantum yield of non-regulated energy dissipation in PSII
ΦEO	Probability that an absorbed photon leads to electron transport further than QA-
ΨEO	Probability that an electron moves further than QA-
ABS/RC	Absorption flux per reaction center
TRo/RC	Trapping of electrons per reaction center
ETo/RC	Electron flux per reaction center beyond QA-
DIo/RC	Energy dissipation flux per reaction center
REo/RC	Electron transport flux until PSI acceptors per reaction center
PIabs	Performance index on absorption basis.
DFIabs	Driving force on absorption basis.
SFIabs	The structure function index on absorption basis.

.

Figure 4 shows the change in energy flux per reaction center (RC) (ABS/RC, DIo/RC, TRo/RC, ETo/RC, and REo/RC) as well as the quantum yield of each photochemical step (ΦEo and ΨEo) of each treatment compared to SS100 as the DS treatment period is prolonged [28,33].

On the first day of drought treatment (DS-1), ABS/RC, DIo/RC, and TRo/RC in Bc5 and Bc10 were slightly higher than SS100, and this tendency was prominent in Comp around the DS-5. However, on DS-10, ABS/RC, DIo/RC, TRo/RC, ETo/RC, and REo/RC tended to be lower than SS100 in all treatments except SS70, whereas ΦEo and ΨEo showed an opposite trend. However, the difference was significantly noticeable in DIo/RC and REo/RC. In SS70, REo/RC decreased slightly when compared with SS100, but other indicators were almost identical between SS70 and SS100. Conversely, SS100 exhibits a relatively high energy flux in DS-10 with relatively low efficiency, implying that it does not properly resolve excessive energy accumulation in DS.

PI_abs, DFI_abs, and SFI_abs represent the vitality indices of photosynthetic apparatus, and V_K/V_J is a well-known indicator for assessing DS [33,34,35]. As shown in Figure 5, overall, in DS-10 with severe DS, SS100 and SS70 had the lowest PI_abs, DFI_abs, and SFI_abs under any treatment, whereas V_K/V_J had the highest indices. Additionally, CS had the highest vitality indices (PI_abs, DFI_abs, SFI_abs) across all treatments, followed by SS50, Bc10, and Comp.

Figure 6 presents chlorophyll a fluorescence images, and Figure 7 demonstrates the changes in each parameter based on the soil treatment method on DS-10. Fv/Fm was relatively high in CS and low in Bc5, whereas Y(II) was relatively high in Bc5, Bc10, and SS50, with lower tendencies in SS100 and SS70. Y(NPQ) had a slightly different trend, being the lowest in SS70, contrary to Y(NO). In general, relatively high Y(II) and Y(NPQ) were shown in both Bc and Comp treatments.

4. Discussion

4.1. Physicochemical Properties of Soil

The sediment soil (SS100) deposited along the valley of the forest has a very high sand content, with a pH of 6.4, which is considered to be affected by soil loss through forest erosion in relation to the weak acidity of Korean forest soils with granite and granite gneiss as base materials [26]. Moreover, OM, Av. P₂O₅, T–N, CEC, and BS were found to be the lowest among the treatments, in particular, OM, T–N, and Av. P₂O₅, which was only 22–36% when compared to CS. Additionally, Ca²⁺ was found to be extremely deficient among exchangeable cations. Nutrient retention in the soil is dependent on CEC and BS [36]. When CS was mixed with SS, the content of sand decreased, whereas the content of silt, clay, OM, and the exchangeable cations increase. Therefore, CEC and BS gradually increase [36], and these findings lead to an increase in soil fertility. In SS50, BS reached a similar level as CS. Bc is known to improve soil properties such as soil pH, nutrient retention, water content, and porosity [37,38,39], and Laird et al. [39] reported that Bc increases OM contents of soil by 1.5 times. In this experiment, OM in Bc5 and Bc10 increased slightly, but the soil physicochemical properties were not significantly affected when compared with other treatments.

According to Lee and Koo [26], the average CEC and T–N of Korean forest soils collected from 65 national locations were 18.2 cmolc·kg⁻¹ and 0.17%, respectively, indicating that none of the soil improvement treatments, including SS, met this standard.

Comp with 10% of poultry manure showed high nutrient content and pH in all indicators except CEC, and the BS was 163.6%, which is three times higher than SS100. In the field cultivated with OM, BS appeared up to 109.7%, which has been reported to be primarily due to exchangeable K⁺ [40], whereas in the case of Comp, BS was higher and Ca²⁺ concentration was a larger factor. Na⁺ content, in particular, was up to 44 times higher than in other treatments. In general, livestock manure can have a high salt content; in the case of poultry, the salt content is known to be 0.52%, which is lower than cow’s manure but higher than hog manure [41]. Due to its low solubility and high soil absorption, phosphorus (P) is a major limiting factor for plant growth in the ecosystem and can be considered to be an important fertilizer component for crops [42]. Because livestock manure has a greater capacity to supply P₂O₅ than chemical fertilizers, using Comp as a source of P₂O₅ in addition to fertilizing nitrogen (N) was suggested as an appropriate fertilization strategy [43]. Moreover, Comp showed a markedly high level of Av. P₂O₅, which was 8.3 times higher than SS100 and 1.8 times higher than CS.

4.2. Photosynthetic Responses and Chlorophyll Contents

A and chlorophyll contents of A. scaber increased while WUE_i decreased, as the growth period lengthened. Particularly, the photosynthetic indicators in June 2022 significantly increased compared to May 2022, indicating a relatively inconspicuous trend in the treatments. Additionally, in Bc5 and Bc10, A tended to relatively decrease in July 2022 compared to May and June 2022, where Bc10 showed the greatest decrease in July.

In May 2022, when compared with other treatments, SS100 had the lowest A, which was a relatively unfavorable condition for growth due to the low photosynthetic rate in the early growing period. Particularly, SS50, CS, and Comp had higher A than SS100, but g_s and E were similar or slightly lower. These findings indicate that the CO₂ supply flowing into the intercellular space via the stomatal opening and closing is not significantly different from SS100, whereas the photosynthesis remains relatively high. Considering the soil characteristics of SS50, CS, and Comp, which showed higher OM, CEC, and BS than other treatments (Table 1), it is clear that fine levels of soil nutrient conditions enable increasing photosynthetic rates during the early growth period.

WUE_i is an important criterion for evaluating plant transpiration and photosynthesis as it compares the efficiency of plant CO₂ fixation to the rate of water loss through transpiration [44]. Except in June 2022, SS100 and Bc5 had lower WUE_i and ITE compared to other treatments. By contrast, SS50 and CS showed a distinctly high tendency, indicating that WUE_i and photosynthetic rate are increased, which can help to maintain a relatively high A even in water-stress circumstances like drought [35].

Alternatively, both V_cmax and J_max are important parameters in determining the photosynthetic capacity referring to the electron transfer efficiency that regulates the rate of photosynthetic carbon assimilation and regeneration of RuBP based on Rubisco activity [35,45]. The concentration of Rubisco in the leaves is affected by N in the soil, which is an important limiting factor for V_cmax [46,47]. Consequently, the photosynthetic rate of plants has a high correlation with the N content of the leaves and the N uptake of the plants [48].

Even in the case of A. scaber, CS, SS50, and Comp showed the highest V_cmax and J_max when compared to SS100, Bc5, and Bc10, which showed a similar percentage in T–N content (Table 1). Consequently, the improvement of the soil nutrient conditions can help to improve the photosynthetic capacity by increasing the Rubisco activity for CO₂ fixation and the electron efficiency that affects the regeneration of RuBP.

Many studies have found that soil fertilization has a tendency to improve the photosynthetic rate and capacity [42,49,50]. Moreover, the microorganism activity is stimulated in soils rich in OM, increasing decomposition rates and assisting plants in utilizing nutrients [51]. This is consistent with previous findings that improved nutrient conditions, such as CEC and cation concentrations, promote the physiological metabolic activities such as the supply of inorganic elements required for photosynthesis and enzyme production [42,50,51].

Consequently, regardless of the growth period, Comp had the highest A, V_cmax, and J_max. Alternatively, the photosynthetic reaction in CS and SS50 was generally high in the treatment with high soil fertility, such as a large increase in photosynthetic indicators similar to Comp around July 2022. Even so, in SS100, the photosynthetic reaction, such as A, was lowest during the early growth period, but this tendency was alleviated when mixed with CS.

4.3. Growth Characteristics

The aboveground and total dry weights increased significantly (p < 0.05) as the mixing ratio of CS to SS increased. For the belowground dry weight, the growth was greater in the treatments mixed with CS (SS70, SS50) compared with SS100. These findings were consistent with the trend of improving soil physicochemical properties such as clay content, EC, OM, Av. P₂O₅, CEC, and BS. Improving soil physicochemical properties is known to increase the activity of effective microorganisms and soil nutrient availability, thereby promoting aboveground crop growth [42,52]. Additionally, Choi et al. (2009) reported that A. scaber treated with 120 kg·ha⁻¹ nitrogen yielded 88.0% more than the control that did not receive any fertilization, implying that as a leafy forest product, increased nitrogen content in aboveground growth can be directly related to growth and yield. Such findings were also found in the other forest crops e.g., Ligularia fischeri Turcz. and Allium microdictyon Prokh. [24,53].

According to Sorensen and Lamb [54], if too much Bc is added to the soil, crops may grow slowly due to nutrient adsorption. Yun et al. [55] reported that the crop’s yield decreased when >1000 kg per 10 a was applied in onion cultivation. In this study, Bc treatments reduced LWR while increasing RWR, indicating a tendency to significantly reduce the aboveground growth. In Bc10, OM increased by 98% when compared to SS100 (Table 1), but the aboveground dry weight decreased by 40%, and there was no significant difference in total dry weight due to slightly higher root growth (p > 0.05). Furthermore, as shown in Table 2, Bc treatments had a relatively high A in their early growth stage but started to decrease in July 2022 when grown for more than 3 months; specifically, Bc10, which had a high Bc input, showed a large decrease. Consequently, Bc, which is applied to SS for soil improvement, requires more investigations on the appropriate amount of application. It is thought that growth may be reduced when the amount of applications exceeds 10% of the total volume.

The total dry weight of Comp was twice as high as SS100 and >30% higher than CS. Compared to other treatment areas, S/R and LWR were low, whereas RWR showed the highest value, with the distribution of materials injected into the belowground area being greater than the aboveground part. Hence, the growth and development of these roots are believed to be beneficial for healthy crop production in the long-term perspective and increasing susceptibility to drought. However, considering that the majority of the 20% level of Comp treatment prepared for the experiment withered within 30 days (data not presented), the excessive Comp application to SS may increase the physiological damage; hence, caution is required.

Consequently, mixing CS with SS can promote aboveground growth. When 10% of poultry manure was mixed, it had a greater effect on the belowground growth promotion than the aboveground, whereas Bc did not result in a significant increase in growth characteristics.

4.4. Chlorophyll a Fluorescence under Drought Conditions

An increase in ABS/RC indicates that RC has been deactivated, whereas an increase in DIo/RC indicates that an excess stored energy has been released, such as heat [34]. ABS/RC and DIo/RC tended to be relatively high in Bc5, Bc10, and Comp until DS-5, but the fluorescence yields of ΦEo and ΨEo were not significantly different from SS100. ΦEo and ΨEo represent the full-energy transfer rate after QA⁻ and efficient regeneration of QA⁻, the initial electron acceptor, respectively [56,57], indicating that the regeneration of QA⁻ for the smooth photosynthetic process in the electron transport chain and the energy transfer in the subsequent steps are well done. This shows that Bc5, Bc10, and Comp reduce RC damage during the initial DS, resulting in RC inactivation, and increase heat emission resulting in stable energy transfer [33]. Furthermore, in DS-10, which has the most severe DS, fluorescence yields such as has ΦEo and ΨEo in CS, SS50, and Comp were higher than SS100, whereas the energy fluxes such as ABS/RC, DIo/RC, TRo/RC, ETo/RC, and REo/RC were lower, indicating a response to DS. Notably, DIo/RC and REo/RC tended to be significantly lower than the captured energy (TRo/RC) and the energy flow transferred to the electron transport system (ETo/RC), indicating that CS, SS50, and Comp induce photosynthetic apparatus stabilization by limiting the thermal energy dissipation and the energy transferred to PS I (REo/RC) in response to DS [28,29,33,35].

PI_abs represents the energy conservation efficiency in the electron transfer process, DFI_abs denotes the driving force of PS II, and SFI_abs reflects the structural and functional reactions of PS II that induce electron transport during photosynthesis, implying that these indicators are sensitive to environmental stress [28,29,35,58].

Comparing the DS sensitivity of A. scaber according to soil treatment, in DS-10, SS100 and SS70 showed significantly low vitality indexes and relatively high V_K/V_J. In particular, DFI_abs showed the greatest decrease, and SFI_abs showed a relatively small difference, implying that the driving force of PS II is greatly reduced compared to the damage to the structure and function of the photosynthetic apparatus due to DS in SS70 and SS100. Hence, recovery of potential photosynthetic capacity can be expected through improvements in environmental conditions such as reirrigation. Knowing that an increase in V_K/V_J is strongly related to the inactivity of the oxygen-evolving complex [29,33].

In contrast to SS100, CS, SS50, Bc10, and Comp showed high PI_abs, DFI_abs, and SFI_abs while maintaining low V_K/V_J, indicating relatively high drought tolerance. Given that the combination of Bc and soil improves soil structure, porosity, and water-holding capacity [37], it is believed that the proper mixing ratio that can help crop growth should be investigated while increasing the drought tolerance of SS. Furthermore, unlike SS50, SS70 lacked CS and thus was stressed with an equal amount; hence, these parameters suggest that an appropriate amount of CS to SS should be mixed to obtain the effects of mixing CS.

Y(II) expresses the result of the functional linkage of the photosynthetic apparatus and is directly related to the rate of CO₂ assimilation. The crop productivity under DS is known to be highly dependent on A and the performance of Y(II) [30,59]. When compared with other treatments, SS100 and SS70 had a relatively low Y(II), indicating that the photosynthetic CO₂ assimilation was severely suppressed under DS (Figure 7).

Furthermore, when exposed to various stresses such as drought, Y(NPQ) protects the photosynthetic apparatus by dispersing excess excitation energy. Y(NO) differs from Y(NPQ); an increase in Y(NO) indicates that PS II photochemical efficiency and function are declining [57,60]. Given that SS70, SS50, and SS100 had relatively low Y(NPQ) compared to Comp, it is clear that the process of dissipating excessive accumulated energy due to DS is inefficient. Moreover, because the accumulation of Y(NPQ) is dependent on the formation of a trans-thylakoid pH gradient, ATP formation leading to carbon assimilation is thought to be inhibited [29]. By contrast, Bc and Comp showed relatively high Y(NPQ) and low Y(NO), which increase nonphotochemical energy dissipation by heat to adapt to DS. Obviously, a reaction prevented the severe irreversible damage of PS II. These fluorescence image results reflect the health of stress sensitivity and function of plant leaves [59]. Alternatively, the tolerance to drought can be increased when Bc, CS, and Comp, are mixed with SS.

5. Conclusions

In comparison to CS, SS100 had relatively low levels of overall soil fertility, including OM, CEC, BS, and Av. P₂O₅, which led to the lowest photosynthetic parameters, such as A, V_cmax, and J_max, and, as a result, the lowest growth characteristics were represented among overall treatments. However, when SS was mixed with a specific amount of CS, the texture and fertility of the soil improved significantly; in particular, the physicochemical properties of the SS50 nearly reached the level of CS, and the aboveground part of the Aster scaber increased significantly. The texture of the soil in Bc and Comp did not change noticeably, but chemical property-related parameters in Comp such as EC, OM, Av. P₂O₅, T-N, Exchangeable cations, and BS increased significantly. A significant increase in the growth of the belowground part of the Comp is predicted to have increased drought tolerance.

Furthermore, as a result of comparing DS based on soil treatment methods, CS, SS50, BC10, and Comp showed higher PI_abs, DFI_abs, and SFI_abs while maintaining lower V_K/V_J, indicating a relatively higher drought tolerance. Bc10 and Comp, in particular, had a relatively high Y(NPQ) and low Y(NO), an increased nonphotochemical energy dissipation, and responded well to DS while maintaining photochemical efficiency.

Consequently, soil that is lost from forests and continues to accumulate in dams can be fully reused for forest crop production by using appropriate treatment methods, enabling sustainable forest management. Specifically, mixing SS and CS at 1:1 or treating with 10% Comp improved the physicochemical properties of the soil and created favorable growing conditions for Aster scaber, thereby improving both photosynthetic rate and photosynthetic capacity. Considering physiological resistance to drought, the best alternative may be a fair amount of compost. Although this study confirmed that plants can be grown when forest sediment soil is mixed with cultivated soil or fertilizer and reused, it is believed that the physicochemical properties of the soil vary depending on forest use, soil texture, slope, and forest floor in the upper part of the forest where soil runoff occurs the most, so further investigation and analysis should be replicated based on different soil and forest cases. Furthermore, the uptake and accumulation of soil substances in plants are contrary to the safety of forest crops grown for consumption or medicinal purposes, so future analysis based on the level of soil contamination is also considered an important factor. Thus, the data gathered is expected to increase the public value of forests through activities such as watershed conservation and soil runoff prevention, while also reducing the financial burden of sustainable forestry management.