Recycling Reservoir Sediments and Rice Husk for Sustainable Rice Seedling Production

Abstract

Amending reservoir sediments with organic matter provides a sustainable alternative to conventional rice (Oryza sativa L.) seedling substrates, simultaneously reducing dependence on agricultural soils and promoting the recycling of dredged sediments and agricultural by-products. Preliminary tests showed that adding rice husk (RH) improved the porosity and water retention of the sediments while preventing surface cracking. This study further examined the effects of RH and rice husk biochar (RHB) on sediment fertility and rice seedling growth. Seedlings were grown for 15 days in a fine- or coarse-texture sediment amended with 0, 5, 10, or 20% (w/w) RH or RHB. A 10% amendment was identified as the optimal ratio for promoting seedling growth (increasing ca. 20% biomass). Nitrogen (N) availability was the primary factor influencing seedling performance, outweighing the effects of salinity and phosphorus availability. Compared with RH, RHB amendment resulted in lower substrate available N, likely due to greater losses through denitrification and ammonia volatilization, leading to reduced growth. In contrast, RH amendment maintained higher levels of available N, resulting in greater shoot biomass and higher leaf chlorophyll concentrations. Overall, amending reservoir sediments with 10% RH provides the most effective substrate formulation, offering a practical and sustainable strategy for rice seedling production.

1. Introduction

Rice (Oryza sativa L.) is commonly cultivated via either direct seeding or transplanting methods [1]. The transplanting method involves raising seedlings in a nursery before transferring them into a puddled and flooded field [1]. The transplanting process is widely adopted [2] and often mechanized, requiring the use of plastic trays filled with soils for seedling cultivation [1]. The substrate is typically sourced from the topsoil of farmlands, as these soils are nutrient-rich and conducive to producing high-quality rice seedlings [3]. In addition to farmland soils, topsoils from mountainous regions are also excavated for use in nursery substrates [4]. However, the continuous extraction of soils from agricultural and mountainous regions is considered unsustainable due to the resulting soil degradation and environmental harm. As a solution, alternative nursery substrates need to be developed for the transplanting method to mitigate the negative impacts [5,6].

One potential alternative is the reuse of dredged reservoir sediments. In regions characterized by active tectonic activity and intense monsoon or typhoon rainfall, reservoir sedimentation can progress rapidly, leading to severe storage loss and reduced service lifespan [7,8]. In such contexts, for example, in Taiwan, annual storage loss of individual reservoirs may range from 77 × 10³ to 6600 × 10³ m³, equivalent to approximately 1% of their total capacity [7]. To maintain water storage functions, continuous dredging is often required [9], generating substantial volumes of sediment waste that pose further management and environmental challenges.

Reservoir sediments, originating from upstream soils, lose their soil structure and become dispersive during river transport. As these sediments are carried downstream, particles of various sizes are deposited in sediment storage dams and reservoirs. Consequently, coarser particles settle in the upstream sediment storage dam, while finer particles accumulate near the main reservoir dam [9]. This process results in reservoir sediments having excessive clay or sand fractions, depending on where they are dredged. Dispersive clay can impede seedling root development and limit water and air permeability, while dispersive sand lacks sufficient water-holding capacity and fertility. These characteristics can make reservoir sediments unsuitable for healthy rice seedling growth, as confirmed by our preliminary study. Therefore, this research aims to address these challenges by amending reservoir sediments with organic materials and exploring their potential use as a substrate for growing rice seedlings.

Materials commonly used in amending agricultural soils include organic materials such as livestock manure, compost, plant residues, biochar, and mineral amendments [10]. Organic amendments are known to increase soil organic matter, which enhances soil structure, water retention, and nutrient availability [11]. A review of 17 long-term field experiments (>9 years) shows that the application of organic materials, including manure, compost, and straw, significantly improves soil water availability [12]. Therefore, amending reservoir sediments with organic materials is expected to improve their structure, water retention, and nutrient availability, promoting the growth of rice seedlings.

Using agricultural wastes as the source of organic amendment materials not only reduces the environmental burden but also recycles valuable organic material, contributing to more sustainable agricultural practices [13,14]. Rice husk (RH) and rice straw, produced from rice production, are dominant agricultural wastes globally [15]. In this study, RH was selected to amend reservoir sediments for use as rice seeding substrates due to their lightweight nature and minimal need for preprocessing, such as cutting, making them a practical option for reuse. Meanwhile, incorporating RH into rice seedling substrates may help recycle nutrients in RH originally coming from rice field soils back into the fields. Rice straw was initially considered but excluded from this study due to its higher preprocessing requirements and reported adverse effects on seedling mat cohesion [16]. In addition to RH, rice husk biochar (RHB) was also included in the evaluation as a potential amendment material. Biochar, a product of pyrolysis under oxygen-limited conditions [17], is recognized as a good soil amendment for ameliorating soil acidity [18] and for improving soil nutrition and water retention capability [10]. The effect of amending biochars on increasing soil water contents was reported to be more significant in the coarse-textured soils than in the fine-textured soils [19,20]. As a potential carbon-negative material, RHB also contributes to climate-smart agriculture [21].

This study aimed to develop and assess a sustainable nursery substrate by amending reservoir sediments with RH and RHB. It was hypothesized that these amendments can improve the water and air permeability issues caused by poor soil structure of the sediments, effectively transforming the sediments into a suitable substrate for growing rice seedlings and, therefore, replacing traditional farmland and mountainous soils used in the transplanting method of rice cultivation. This study first assessed the structural response of reservoir sediments, compared to agricultural soil, under surface irrigation. Subsequent pot experiments explored various amendment methods, focusing on the effects of amendment ratio, RH particle size (whole vs. powder), and the type of amendment material (RH vs. RHB). The findings offer insights into developing alternative substrates to grow healthy rice seedlings while recycling agricultural by-products and reducing reliance on unsustainable soil sources.

2. Materials and Methods

2.1. The Preparation of Reservoir Sediments and Amendment Materials

Reservoir sediments were collected from the Shimen Reservoir (24°48′40″ N 121°14′45″ E), located in the Dahan River in Taiwan, which generates approximately 1 million m³ of sediments annually—the highest among Taiwanese reservoirs. Fine-textured sediments (FS) were dredged from near the dam body, while coarse-textured sediments (CS) were collected from the Amping sediment storage dam (24°50′02″ N 121°16′08″ E), located at the upstream of the Shimen Reservoir. Additionally, a nursery soil (NS) was collected from a nursery farm of rice seedlings for comparative purposes. The sediments and soils were air-dried and sieved through a 2 mm mesh before use. Their physicochemical properties are presented in

Table 1. Physiochemical properties of the fine and coarse sediments and nursery soil.

Properties	FS	CS	NS
Soil texture	Clay	Sandy loam	Clay
Sand (%)	0.0	73.0	15.5
Silt (%)	37.5	16.0	25.5
Clay (%)	62.5	11.0	59.0
pH	7.9	7.7	5.3
EC (dS m−1)	1.40	0.98	0.10
CEC (cmolc kg−1)	7.46	5.36	7.86
Organic C (%)	1.1	0.6	0.4
Total N (g kg−1)	230	190	80
Total P (mg kg−1)	607	533	410
Total K (g kg−1)	25.4	23.9	12.5
Available N (mg kg−1)	18.3	35.5	7.1
Available P (mg kg−1)	3.14	0.87	0.69
Exchangeable K (mg kg−1)	234	156	283
Exchangeable Na (mg kg−1)	16.4	7.1	14.1
Exchangeable Ca (g kg−1)	1.82	1.20	0.25
Exchangeable Mg (g kg−1)	1.62	1.08	1.08

, and the corresponding analytical procedures are described in Section 2.4.

The RH was purchased from a rice milling plant in Taoyuan City, Taiwan. To produce RHB, the RH was packed into stainless steel cylinder containers and sealed with two layers of aluminum foil to isolate the contents from the air. The containers were then heated at 500 °C in a furnace for 30 min. The basic properties of RH and RHB were analyzed using the same methods applied to soil samples, and the results are presented in Table S1. The FTIR spectra of the RH and RHB are shown in Figure S1, with the corresponding analytical procedures described in Text S1.

2.2. Preliminary Experiments

Two preliminary experiments were conducted to evaluate the feasibility of using reservoir sediments as nursery substrates. The first experiment examined the physical properties of FS and CS (without amendments) compared to NS. The original sediments and the soil, as well as their counterparts amended with 10% (w/w) RH, were placed into nursery trays. Water was then sprinkled over the substrates to saturate them, simulating the typical irrigation method commonly used in rice-seedling nurseries. The watered substrates were placed in a greenhouse under room temperature and uncontrolled humidity conditions for one day. Surface cracks were subsequently observed and recorded.

The second preliminary experiment evaluated the changes in soil physical properties, including available water content and soil porosity, right after amending the sediments with the ground and unground (whole) RH at ratios of 0%, 7.6%, 10%, and 20% (w/w). No fertilizers were added. Each treatment was triplicated, with sample allocation following a completely randomized design (CRD).

2.3. Rice Seedling Cultivation

This experiment aimed to investigate the interactive effects of RH or RHB amendment and sediment texture on the growth of rice seedlings and nutrient supply from the substrates. Ground RH or RHB was incorporated into the FS and CS at 0%, 5%, 10%, and 20% (dry matter weight basis, w/w). The substrates were then added with 0.08 g urea, 0.12 g Na₂HPO₄, and 0.16 g KCl per tray, providing an N:P₂O₅:K₂O ratio of 1.7:0.8:2.4. Nursery soil (NS) with the identical fertilizer additions but without amendment served as the control. Each 100 g portion of amended substrates and NS was placed into nursery trays (dimensions: 8.5 × 8.5 × 4 cm³). Each treatment was triplicated, and the trays were arranged following a Completely Randomized Design.

Rice seeds (Oryza sativa L. var. Tainan 11) were first washed with distilled water, sterilized with 1% NaOCl solution for 15 min and 75% ethanol for 5 min, and thoroughly rinsed with distilled water. The sterilized seeds were germinated at room temperature for 5 days. Eight grams of seeds with embryonic roots measuring 1–3 mm in length were selected and transplanted into each tray. The rice seedlings were grown under submerged conditions at 26 ± 1 °C for 15 days, which is considered a suitable duration for transplanting according to the International Rice Research Institute. An artificial LED light source (RGB) provided 12 h of light daily, with an illuminance of 2000–5000 lx. Seedling growth parameters, including aboveground height, dry matter, and chlorophyll concentration, were measured to evaluate the amendment effects.

Soil solutions were collected using MicroRhizon samplers (0.15-μm pore size; Rhizosphere Research Products, Wageningen, the Netherlands) on 0, 5, 10, and 15 days after sowing the seeds (DAS). Approximately 2 mL of solution was drawn each time using clean, acidified syringes and then filtered through 0.25-μm syringe filters. The filtrates were stored at 4 °C and analyzed within 2 days for pH, redox potential (Eh), electrical conductivity (EC), dissolved organic carbon (DOC), and inorganic N species (ammonium, nitrate, and nitrite). All the measurements were performed in triplicate.

2.4. Soil and Soil Solution Analyses

Soil texture was analyzed using the hydrometer method [22]. Substrate porosity was calculated as $\left(1-{\displaystyle \frac{B_d}{P_d}}\right)\times 100\%$, where the Bd and Pd stands for the bulk density and particle density of the substrates. For the measurement of Bd, each of the substrates was loaded into a stainless-steel cylinder (height = 50 mm; diameter = 55 mm); artificial compaction was avoided during the process. Pd was measured using the pycnometer method [23]. Available water capacity was determined as the difference in soil water contents between field capacity (30 kPa) and permanent wilting point (1500 kPa) [24]. Soil pH and EC were measured in a 1:1 soil-to-water mixture using a combined pH electrode and an EC electrode, respectively, connected to a benchtop meter (Laqua F-74BW, HORIBA Scientific, Kyoto, Japan). Soil organic carbon was analyzed using the Walkley-Black oxidation method [25]. Total N and carbon (C) contents were analyzed using a CHNS/O analyzer (2400 II, Perkin Elmer, Waltham, MA, USA). Total and available phosphorus (P) contents were quantified with the Molybdenum-Blue colorimetric method (at 889 nm) following an aqua-regia acid digestion and a 0.5 M NaHCO₃ extraction, respectively [26]. Cation exchange capacity and exchangeable potassium (K), sodium (Na), calcium (Ca), and magnesium (Mg) were determined using the ammonium acetate method (pH 7) [27]. The K, Na, Ca, and Mg concentrations in the extracts were analyzed using an atomic absorption spectrometer (AAnalyst 200, Perkin Elmer, Waltham, MA, USA). Exchangeable NH₄⁺ was analyzed using the Na-salicylic colorimetric method at 653 nm using a UV-Vis spectrophotometer (Specord 50 Plus, Analytik Jena GmbH+Co. KG, Jena, Germany) [28], following the extraction with 2 M KCl (soil: solution = 1g:10mL) for 30 min at room temperature and filtration through Whatman No. 42 filter paper. Soil available N (NH₄⁺ + NO₃⁻ + NO₂⁻) was extracted using 2 M KCl (soil/solution = 3g: 20 mL) at 100 °C for 4 h and analyzed by the Kjeldahl distillation method [29].

Soil solution pH, EC, and Eh were measured using a combined pH electrode, an EC electrode, and an ORP electrode, respectively, connected to a benchtop meter (Laqua F74BW, HORIBA Scientific, Kyoto, Japan). The concentration of DOC was analyzed using a TOC analyzer (Analytik Jena GmbH+Co. KG, Jena, Germany; sample volumes: 500 μL; furnace temperature: 800 °C; integration time: 300 s). The P concentration was quantified with the Molybdenum-Blue colorimetric method [26]. Ammonium concentrations were analyzed using the Na-salicylic colorimetric method [28]. The concentrations of nitrite and nitrate were determined using the Griess colorimetric method at 543 nm with VCl₃ as the reduction agent [30,31].

2.5. Plant Analyses

Seedling heights were measured on DASs 4, 9, and 14; however, only the result at DAS 14 was presented. As shown in a preliminary test (Figure S2), the nutrient dependency shifted from seeds to substrates after DAS 10. Therefore, the seedling height on DAS 14 was a more suitable indicator of substrate effect. Root and shoot samples were harvested at the end of the pot experiment (DAS 15). Roots were carefully washed with ultrapure water to remove adhering soil and then oven-dried at 60 °C for 24 h to determine root dry weight. Shoots were oven-dried at 60 °C for 24 h for dry weight analysis. For leaf chlorophyll analysis, 0.02 g of fresh leaves was cut into small pieces and immersed in 5 mL of 95% ethanol in a 15 mL capped, light-impermeable tube for 48 h. The extracted solution was filtered using an Advantec^® 5A filter paper (pore size: 7 μm; Adventec MFS Inc., Dublin, CA, USA), and the absorbance of the filtrate was measured at 664.1 nm (A664.1) and 648.6 nm (A648.6) using a UV-VIS spectrophotometer (Specord 50 Plus, Analytik Jena GmbH+Co. KG, Jena, Germany). The concentrations of chlorophyll a (C_a) and b (C_b) were calculated using Equations (1) and (2):

$$c_a\left({\displaystyle \frac{\mu g}{mL}}\right)=13.36A_{664.1}-5.19A_{648.6}$$

(1)

$$c_b\left({\displaystyle \frac{\mu g}{mL}}\right)=27.43A_{648.6}-8.12A_{664.1}$$

(2)

2.6. Statistics

Analysis of variance (ANOVA) was conducted to evaluate the effects of treatments on the response variables. A two-way ANOVA model (y∼T + A + T × A) was applied to assess the effects of sediment texture (T), amendment (A), and their interaction (T × A). All factors were treated as fixed in the model. To further examine the amendment effects within each sediment texture group, one-way ANOVA (y∼A) was performed, followed by Least Significant Difference (LSD) post hoc multiple comparison tests (α = 0.05). Principal Component Analysis (PCA) was performed to show the difference in soil solution composition among different amendment groups using the ‘prcomp’ function in the R software. The eigenvalue and eigenvector of each principal component are presented in Table S2. All the statistical analyses were performed using the R software v. 3.3.1 with the Agricolae package.

3. Results

3.1. Preliminary Experiments

The first preliminary experiment tested the responses of the sediments in trays after surface irrigation. The water was sprinkled over the substrates to saturate them, simulating the typical irrigation method commonly used in rice-seedling nurseries. The sediments without amendment, whether fine or coarse in texture, exhibited visible cracking after surface irrigation (Figure 1a,b), whereas no cracking was observed in the control, i.e., NS (Figure 1c). Cracking was most significant in FS, but unexpectedly also occurred in CS (Figure 1b). Amending 10% RH significantly prevented the cracking in both FS and CS (Figure 1d,e).

The second preliminary experiment evaluated the changes in soil physical properties, including available water content and soil porosity, after amending the sediments with the ground and unground (whole) RH at ratios of 0%, 7.6%, 10%, and 20% (w/w). The results showed a significant (p < 0.05) interaction between sediment texture and RH particle size (whole vs. ground) on the available water content and porosity of the substrates (

Table 2. Soil available water content and porosity of sediments amended with ground/non-ground rice husks at different amendment ratios.

Treatments			Substrate Available Water Content (%)	Substrate Porosity (%)
Sediments	Amendment	Addition Rate (w/w)
FS	-	0%	14.6 ± 3.3 c	47.5 ± 0.5 d
	Whole RH	7.6%	22.6 ± 0.8 ab	61.2 ± 2.0 b
		10%	21.7 ± 0.2 ab	64.4 ± 3.3 b
		20%	22.2 ± 0.2 ab	69.1 ± 4.3 a
	Ground RH	7.6%	14.0 ± 5.0 c	46.7 ± 0.5 d
		10%	18.6 ± 2.8 bc	48.0 ± 0.6 d
		20%	24.7 ± 1.3 a	55.6 ± 0.4 c
CS	-	0%	1.5 ± 2.4 E	13.7 ± 2.3 D
	Whole RH	7.6%	12.8 ± 0.1 AB	30.8 ± 2.3 BC
		10%	12.2 ± 0.5 BC	31.2 ± 7.3 BC
		20%	7.4 ± 0.3 D	69.1 ± 0.4 A
	Ground RH	7.6%	11.7 ± 0.2 BC	14.9 ± 6.6 D
		10%	10.9 ± 0.2 C	27.1 ± 6.8 C
		20%	14.2 ± 0.5 A	36.7 ± 6.1 B
ANOVA test	Sediment texture (T)		<0.001 ***	<0.001 ***
	Amendment (A)		<0.001 ***	<0.001 ***
	T × A		<0.001 ***	<0.001 ***

The symbol ‘***’ indicate statistical significances of the ANOVA test at p-value and <0.001, respectively. The different lowercase and uppercase letters in the same column indicate a significant difference between the treatment groups of fine and coarse sediment, respectively, in the LSD multiple comparison analysis at α = 0.05.

). In both FS and CS, amending either ground or whole RH resulted in significantly greater soil available water content, with the effect more significant in CS than in FS (Table 2). Notably, in CS, amending 20% whole RH decreased available water content (7.4%) as compared to amending 10% whole RH (12.2%) (Table 2). On the contrary, amending 20% of ground RH in CS increased the water content in the 10% group from 10.9% to 14.2% (Table 2). In FS, amending both ground and whole RH at different ratios increased the available water content (Table 2). Both whole and ground RH amendments enhanced soil porosity, but to different extents (Table 2). In both FS and CS, amending with the whole RH resulted in significantly higher porosity (Table 2). In contrast, the effect of ground RH on porosity became significant only at amendment levels exceeding 20% in FS and 10% in CS (Table 2). Overall, amending with either whole or ground RH in FS improved both porosity and available water content. However, in CS, while the whole RH at high ratios increased porosity, it tended to reduce soil available water content. In contrast, ground RH at high ratios improved both available water content and porosity.

3.2. Rice Seedling Cultivation

3.2.1. Rice Seedling Growth

The amendment of ground RH and RHB, as well as sediment texture, significantly influenced the growth of rice seedlings, including both seedling height and shoot dry matter (

Table 3. Rice seedling height, shoot dry matter, and chlorophyll concentration.

Treatments		Height of Rice Seedling (cm)	Shoot Dry Matter (g pot−1)	Leaf Chlorophyll (a + b) (g kg-dw−1)
Sediments/Soils	Amendment Ratios and Materials
NS	-	11.06 ± 0.36	1.25 ± 0.07	14.5 ± 2.44
FS	+0% RH/RHB	11.87 ± 0.32 c	1.22 ± 0.09 bc	16.6 ± 2.2 b
	+5% RH	12.30 ± 0.11 bc	1.39 ± 0.04 ab	15.3 ± 0.5 b
	+10% RH	12.13 ± 0.30 c	1.46 ± 0.13 a	15.0 ± 1.7 b
	+20% RH	12.06 ± 0.12 c	1.34 ± 0.03 abc	18.3 ± 2.0 a
	+5% RHB	12.78 ± 0.60 ab	1.33 ± 0.07 abc	15.9 ± 1.8 c
	+10% RHB	13.34 ± 0.41 a	1.47 ± 0.05 a	13.9 ± 0.9 c
	+20% RHB	12.21 ± 0.40 bc	1.16 ± 0.23 c	12.3 ± 0.70 c
CS	+0% RH/RHB	10.78 ± 0.77 CD	0.98 ± 0.09 C	16.3 ± 2.3 B
	+5% RH	12.27 ± 0.48 BC	1.26 ± 0.09 AB	18.3 ± 2.5 A
	+10% RH	11.99 ± 0.83 BCD	1.25 ± 0.12 AB	18.6 ± 2.1 A
	+20% RH	10.69 ± 0.65 D	1.37 ± 0.07 A	16.8 ± 2.2 AB
	+5% RHB	14.18 ± 0.17 A	1.13 ±0.11 ABC	15.6 ± 1.2 C
	+10% RHB	13.16 ± 0.58 AB	1.11 ± 0.16 ABC	16.8 ± 1.3 C
	+20% RHB	11.17 ± 1.85 CD	1.06 ± 0.31 BC	15.9 ± 2.7 C
ANOVA test	Sediment texture (T)	0.108	<0.001 ***	0.390
	Amendment (A)	<0.001 ***	0.005 **	<0.001 ***
	T × A	0.029 *	0.327	0.030 *

The symbols ‘*’, ‘**’, and ‘***’ indicate statistical significances of the ANOVA test at p-value < 0.05, <0.01, and <0.001, respectively. Each treatment has three replicates. The different lowercase and uppercase letters in the same column indicate a significant difference between the treatment group of fine and coarse sediment, respectively, in the LSD multiple comparison analysis at α = 0.05.

). For seedling height, the interaction between sediment texture and amendment ratio was statistically significant (Table 3). In CS, amending with either RH or RHB increased seedling height, with the positive effect being significant at amendment levels of 5% and 10% (Table 3). Comparatively, in FS, the amendment with RH showed no significant effect on seedling height (Table 3). However, using FS amended with RH resulted in seedling heights closer to the optimum seeding height. Amending with RHB in FS appeared to increase seedling height, but similar to CS, the positive effect was only significant at 5% and 10% amendment ratios (Table 3). The effect on shoot dry matter was significantly increased when amending with RH in both FS and CS (Table 3). In contrast, amending with RHB tended to decrease shoot dry matter in both FS and CS, with the decrease being statistically significant at a 20% amendment ratio (Table 3). Similarly to the effect on shoot dry matter, leaf chlorophyll concentrations increased with RH amendment, while they decreased with RHB amendment. Overall, while RHB amendment increased the seedling heights, it tended to reduce shoot dry matter and leaf chlorophyll concentration in both FS and CS, particularly at the higher amendment ratio (20%). In contrast, RH amendment consistently increased shoot dry matter across all treatments.

3.2.2. Soil Solution Analyses

The EC values of the soil solutions, an indicator of salinity, increased with the addition of RH and RHB, but the increase was more pronounced in RH-treated groups (Table S3). The salinity increased to a greater extent in the RH groups than in the RHB groups, with a statistically significant increase observed only at the 20% RH amendment ratio, from 1.19 to 3.28 dS m⁻¹ in FS and from 2.44 to 4.90 dS m⁻¹ in CS at DAS 0 (Table S3). The increased EC values were more significant in the CS than in the FS. Thus, amending 20% RH could result in salt stress to the seedlings in both FS and CS, and amending RHB is less likely to cause salt stress than amending RH.

The pH values in the soil solution in both FS and CS were significantly decreased by the amendment of RH but increased by the amendment of RHB (

Table 4. Volatilized NH₃ gas from the RH/RHB-amended fine sediment.

Experimental Batch	Rate of Gas Volatilization (mg-N L−1 h−1)
	FS + 20% RH + NPK Fertilizer	FS + 20% RHB + NPK Fertilizer
System recovery rate	58.9%	58.9%
Batch 1–3 h †	0.0164	0.1026
Batch 2–5 h †	0.0408	0.1207

(n = 3; ^† 3 h and 5 h represent 3 h and 5 h as the length of the running time of the apparatus).

). The amendment of RH decreased the pH from 7.60 to 6.97 in FS and from 7.85 to 7.20 in CS at DAS 0 (Table S3). In contrast, the amendment of RHB increased the pH from 7.60 to 8.12 in FS and from 7.85 to 8.39 in CS at DAS 0 (Table S3). At the end of the experiment (DAS 15), the pH values in both sediments amended with RH were no different than those of the control groups. However, the pH values of the RHB groups remained significantly higher than those of the control groups, indicating an alkaline environment throughout the experiment period. In contrast, the Eh values were decreased by the amendment of RH but increased by the amendment of RHB at DAS 0 (Table S3). Throughout the experiment, the Eh values became highly variable, as reflected by the large standard deviation at the end of the experiment, with no clear pattern emerging (Table S3). Concurrently, RH reduced Eh below 300 mV in higher amendment rates, suggesting more anaerobic conditions, while RHB increased initial Eh values above 400 mV, indicating relatively aerobic conditions.

The concentrations of DOC in the soil solution collected on DAS 0 were significantly increased in the RH-treated groups, whereas the concentrations remained no different in the RHB-treated groups (Table S4). For the P concentration, represented by orthophosphate, adding either RH or RHB increased the P concentrations in the soil solution of both sediments, but to a greater extent in the RHB-treated groups than in the RH-treated groups (Table S4).

Ammonium was the dominant mineral N species (>90%) in the soil solutions in the sediments amended with either RH or RHB across the whole growing period (Tables S5–S8). In soil solutions, RH significantly increased the ammonium concentrations in both sediments (Table S4). In contrast, RHB significantly decreased the ammonium concentrations in both sediments (Table S4). Similarly to the results in the soil solutions, the result of soil extractable available N (NO₂⁻ + NO₃⁻ + NH₄⁺) showed similar trends to the concentrations of exchangeable NH₄⁺ in the RH groups (Figure 2a,c), indicating that the N in RH treatments primarily existed in the form of NH₄⁺. However, in both FS and CS, the amendment of RHB resulted in decreasing available N (Figure 2b,d).

3.2.3. PCA of the Soil Solution Chemistry

A PCA using the above nutrient-associated data, including ammonium, nitrate + nitrite (NOx), orthophosphate (Soluble P), DOC, DOC/total soluble N (C/N ratio), pH, and Eh in the soil solution, was performed to show the most determinative factors that associate with the difference in rice seedling growth between the RH and RHB groups. The result showed that the soil solution characteristics of the RH-amended sediments, indicated by the green circle, were significantly shifted from the control groups and the RHB groups, indicated by the red and blue circles, respectively (Figure 3a). The difference was dominantly driven by soluble NH₄⁺, DOC concentration, and C/N ratio in the soil solutions. In contrast, amending RHB, as indicated by the blue circle, did not drive as significant a difference as amending RH in those indicators in the soil solution (Figure 3a). The difference in the soil solution composition increased with the amendment ratios, which is indicated by the darker blue circle (control) compared to the lighter blue circle (with amendment) (Figure 3b). The impact of sediment texture was relatively mild, as indicated by the well-distributed data points in each group and the overlapping circles representing the coarse and fine sediment (Figure 3c).

4. Discussion

4.1. The Optimal Amendment Ratio and Material

In the first preliminary experiment, the significant surface cracks in the sediments without amendment after sprinkler irrigation suggested the necessity of amending the organic material (Figure 1). In the second preliminary experiment, amending either whole or ground RH improved both substrate porosity and available water content (Table 2). However, in CS, while amending the whole RH at higher ratios increased porosity, it tended to reduce soil available water content. Therefore, a 10% amendment ratio appeared to be a suitable amendment ratio for both whole and ground RH and in both CS and FS.

In the rice-seedling cultivation experiment, the responses of seeding growth were compared between the amendments of RH and RHB. The patterns of seedling height appeared to be different from those of shoot dry matter (Table 3). The amendment of RH tended to increase shoot dry matter, while the seedling height was either maintained or slightly lower than the optimal seedling height (120mm; [16]) (Table 3). In contrast, the amendment RHB appeared to increase seedling heights but significantly decrease shoot dry matter and leaf chlorophyll (a + b), particularly at a high amendment ratio (20%) (Table 3). The reduced chlorophyll concentrations in the RHB-amended treatments suggest decreased photosynthetic efficiency, which could explain the decline in shoot dry matter observed in these groups. Overall, an amendment ratio of 10% appears to be the most effective for promoting positive seedling growth, because this ratio can significantly increase the dry matter of the seedlings, while avoiding the negative effects on seedling height and chlorophyll concentration that may occur if an excessive amount is applied, regardless of sediment texture or amendment type. This result is consistent with the previous findings [16], which reported optimal amendment ratios of 10–30% for various organic materials (e.g., cow dung, poultry litter, rice bran, and vermicompost) in sandy loam and clay loam soils during wet seasons, and 10–20% for most materials during dry seasons. These results support the general suitability of a 10% amendment ratio for improving seedling performance in both soils and sediments.

4.2. The Dominant Factors That Drove the Difference Between Amending RH and RHB

The growth of genetically identical rice seedlings can be influenced by multiple environmental and edaphic factors, including salinity, alkalinity, temperature, moisture content, nutrient availability, CO₂ levels, and even environmental sound [32,33]. In this study, ambient factors, such as sound, air temperature, and CO₂ level, were likely homogeneous with the random allocation of the pots in a controlled environment room. Therefore, the primary differences between the RH and RHB treatments can be attributed to their effects on substrate properties, particularly salinity, pH, Eh, and nutrient availability.

The EC values of the soil solutions, an indicator of salinity, increased with the addition of RH and RHB, with a more pronounced rise in the RH-treated groups (Table S3). Excess salinity in soil may delay seed germination, inhibit seedling growth, and cause leaf chlorosis [33]. In this study, seedling growth rates were lower in the RH-amended treatments than in the RHB-amended ones across both sediments, suggesting that elevated salinity may have affected seedling height (Figure S3). As reported by Rhoades and Loveday [34], an EC value between 2 and 3 dS m⁻¹ is generally considered optimal for rice seedling growth, and the threshold salt tolerance for paddy rice is approximately EC = 3.0 dS m⁻¹ [35]. Therefore, the reduction in shoot dry matter in the RHB groups is unlikely to be caused by salinity, as their EC values remained within the tolerance range and were lower than those in the RH groups. In other words, if salinity were the limiting factor for shoot dry matter, the RH groups should have exhibited lower shoot dry matter than the RHB groups, which was not the case. Therefore, these results indicate that other mechanisms contributed to the observed differences.

Soil nutrient availability, another potential limiting factor to rice seedling growth, is strongly influenced by soil pH and Eh [36]. In both fine and coarse sediments, amendment with RH significantly decreased pH, whereas amendment with RHB increased it (Table S3). Conversely, Eh values decreased with RH addition but increased with RHB at the beginning of the experiment (Table S3). The availability of ammonium, nitrate, nitrite, and orthophosphate, which greatly affect seedling growth, is regulated by changes in pH and Eh. The PCA results revealed that the differences in the nutrient composition in the soil solution between amending RH and RHB were mainly driven by the difference in N availability in the soil solution, with RH exerting a greater influence than RHB relative to the control (Figure 3). This finding is consistent with the observed differences in shoot dry matter and leaf chlorophyll concentrations (Table 3), as chlorophyll content is closely correlated with leaf N concentration [37]. Therefore, the variation in available N among the amended sediments likely represents the key factor underlying the distinct growth responses of rice seedlings to RH and RHB amendments.

Although the PCA results indicate that N availability is the dominant factor differentiating rice seedling growth between the RH- and RHB-amended sediments, this interpretation is based on correlative relationships among the measured variables. Therefore, the observed association between N availability and seedling performance should not be interpreted as direct causality. Other unmeasured soil or biochemical factors may also have contributed to the observed differences. Accordingly, the causal inference drawn from the PCA should be treated with caution, and further controlled experiments are needed to confirm the mechanistic role of N in mediating seedling responses to RH and RHB amendments.

4.3. Mechanisms Underlying the Different N Supply Between the RH- and RHB-Amended Sediments

In both FS and CS, the amendment of RHB resulted in decreasing available N (Figure 2b,d). In FS, the proportion of NH₄⁺ in the available N decreased with higher RHB amendment ratios, suggesting that the amendment of RHB led to N transformation and/or loss. This may explain the observed reduction in leaf chlorophyll and shoot dry matter of seedlings grown in the RHB-amended sediments (Table 3), although some of these effects were not statistically significant. On the other hand, the available N did not decrease with the amendment of RH, and the concentration of NH₄⁺ in the soil solution increased (black bars in Figure 2a,c), suggesting enhanced mobility and accessibility of NH₄⁺ to the seedlings. This may explain the increased shoot dry matter and leaf chlorophyll observed in the RH-treated seedlings (Table 3).

Reactions in soil that can lead to the loss of available N include clay fixation, OM fixation, microbial immobilization, denitrification, NH₃ volatilization, and leaching. Among these mechanisms, clay fixation and leaching could be omitted because the clay mineral composition (both composed of chloride, kaolinite, and mica in CS and FS) and leaching were the same in RH and in RHB treatments. Microbial immobilization of N occurs when microbial assimilation is greater than the microbial mineralization of organic N. In general, microbial immobilization occurs when microbes can obtain less than 1 g N for every 24 g C [38,39]. The C/N ratios of the sediments increased with both the amendment of RH and RHB, whereas only when amending at 20% resulted in a C/N ratio higher than 24 (Table S7). This implies that the decreased available N in the RHB treatment was unlikely to result from microbial immobilization because the increased C/N ratios in the RH treatments did not reflect a decreased available N in the substrates.

Denitrification occurs, primarily in anaerobic conditions, when NOx (NO₂⁻ + NO₃⁻) acts as an electron acceptor in microbial respiration and is transformed into N₂O or N₂ gases [40]. Although NH₄⁺ was the dominant N species in the soil solutions and the soil extracts in all the groups (Figure 2), approximately 30% of the available N was not in the form of NH₄⁺ in the RHB-amended FS (Figure 2b). This suggests that some of the N species may have been converted into NOx, which is more prone to denitrification [41]. The potential N species transformation, facilitated by the RHB amendment, can be further supported by the changes in soil Eh throughout the experiment. The initial Eh values in the RHB treatments were all above ca. 400 mV, whereas the Eh values of RH treatments dropped to less than 300 mV when amended more than 10%, creating relatively anaerobic conditions early in the experiment (Table S3). The Eh values of RHB treatment dropped to less than ca. 300 mV later in the experiment (Table S3), implying that, compared to the amendment of RH, amending RHB appeared to delay the consumption of oxygen and, in turn, created relatively aerobic conditions at the beginning. The aerobic conditions facilitate the occurrence of nitrification, transforming the ammonium to NOx. This can be evidenced by the significantly higher nitrate concentration in the soil solution amended with RHB than that amended with RH (Table S5). The nitrate is susceptible to further denitrification and N loss via gases [41]. Therefore, denitrification may be one of the factors contributing to the reduced available N in the RHB-treated sediments.

Ammonia gas volatilization, another mechanism of N loss, is a process in which ammonium is converted to ammonia and lost as a gas. This process is influenced by soil pH, texture, moisture, and temperature [40], and it increases significantly when the initial pH of the substrate rises from 6.5 to 8.5 [42]. The soil pH in the FS and CS amended with RHB increased from 7.60 and 7.85 to 8.12 and 8.39, respectively (Table 4). Therefore, the loss of the available N in the RHB groups was likely attributed to ammonia gas loss. To further test this, the FS was amended with 20% RH or RHB, along with the same fertilizers used in the seeding cultivation experiment, and placed in sample flasks connected to a custom ammonia gas collection apparatus (Figure S4). The ammonia gas was collected in 0.01 N H₂SO₄ solution in the collection flasks, and the concentration of NH₄⁺ in the H₂SO₄ solution was determined using the Na-salicylate colorimetric method. The results showed that during the first 3 to 5 h of incubation, the FS amended with 20% RHB produced approximately 0.11 mg-N L⁻¹ h⁻¹, while the 20% RH treatments produced about 0.03 mg-N L⁻¹ h⁻¹ (Table 4). The RHB treatments released about three times more NH₃ gas than the RH treatments at the start of the incubation, supporting the hypothesis that ammonia gas loss in the RHB groups was due to the increase in pH. Some might argue that ammonia fixation is also a possible pathway to N loss because it is also enhanced by the increased soil carbon and pH [43]. However, the increased ammonia gas loss from the RHB-amended sediments contradicts the idea that the reduced available N in these treatments was due to increased ammonia fixation.

Overall, the amendment of RHB led to distinct changes in N availability and speciation, as reflected by the reduced chlorophyll concentration and shoot biomass of rice seedlings compared with the RH treatments. The increase in pH and initial Eh in RHB-amended sediments suggested the promotion of aerobic conditions that favor nitrification, followed by potential denitrification and gaseous nitrogen loss. Moreover, the incubation experiment confirmed that ammonia volatilization increased substantially under RHB amendment, likely driven by elevated pH. These findings collectively indicate that both gaseous N losses and altered N transformation pathways contributed to the decline in available N in the RHB treatment. Nevertheless, most of these interpretations are based on correlative evidence linking soil chemical changes to plant growth responses rather than on direct causal proof. Therefore, while the results strongly suggest RHB-induced alterations in N cycling and loss, further mechanistic studies, such as isotopic tracing or process-specific assays, are needed to substantiate these proposed mechanisms.

4.4. Technical Limitations and Practical Considerations

The findings of this study demonstrate that dredged reservoir sediment, after proper amendment, can serve as an effective alternative substrate for rice seedling cultivation. A simple improvement by incorporating natural agricultural residues, such as 10% RH, is sufficient to enhance its properties for replacing conventional agricultural soil. However, for practical implementation and large-scale application, several technical constraints and considerations remain. When used as a seedling substrate, the sediment requires adequate pretreatment, including air-drying and crushing. Comparatively, conventional soil also requires air-drying but can be readily used afterward owing to its well-structured nature. In contrast, sediment typically lacks such structural integrity and requires an additional crushing process. This step necessitates suitable crushing equipment and may increase operational costs. In addition, the high moisture content of dredged sediment can raise transportation and storage costs. Nonetheless, these expenses could be reduced through localized or on-site processing.

5. Conclusions

Amending reservoir sediments, regardless of sediment texture, is necessary before being used as the alternative rice seedlings mat substrate, as their inherently poor physical structure leads to inadequate water infiltration and surface cracking that compromise the mat rolling quality. A 10% (w/w) amendment with ground RH is the most suitable and broadly effective ratio, enhancing seedling growth without negatively impacting the water-holding capacity of the sediments. Although RHB increased seedling height, it significantly reduced leaf chlorophyll concentration and dry matter content, particularly at higher amendment levels (20% w/w), likely due to increased N loss through denitrification and ammonia volatilization. Therefore, RH is a more suitable amendment material than RHB for improving seedling mat substrates derived from reservoir sediments. From a practical perspective, the use of reservoir sediments requires basic pretreatment, such as air-drying and crushing, to ensure uniform texture and ease of handling. While these steps may increase processing costs, localized treatment can reduce logistical burdens. With appropriate management, amended sediments offer a promising and sustainable alternative to conventional soil resources for rice seedling cultivation.