Agronomic Performances of Fragrant Rice Cultivars under Different Vermicompost Rates

Abstract

Vermicompost is an organic fertilizer with high microbiological activity. However, the application of vermicompost in fragrant rice production and the effects of vermicompost rates on fragrant rice performances have not been reported. The present study conducted a field experiment with two cropping lugs to investigate the agronomic performances of fragrant rice cultivars under different vermicompost rates. Five vermicompost rates, i.e., 2.250 (Ver1), 3.000 (Ver2), 3.750 (Ver3), 4.8750 (Ver4), and 6.000 (Ver5) t ha⁻¹, were adopted and a treatment without any fertilizer applied was taken as control (CK). The results showed that the application of vermicompost significantly increased the grain yield of fragrant rice, while the highest or equally highest yield was recorded in the Ver2 treatment. Similar trends were observed in dry matter weight at 80 and 100 days after transplanting. However, the vermicompost didn’t have remarkable effects on chlorophyll content and grain 2-acetyl-1-pyrroline (the key component of fragrant rice aroma) content. The effects of different vermicompost rates on grain quality characters were unstable and irregular across the cropping lugs, although Ver1 and Ver2 treatment exhibited relatively good grain quality with lower amylose content and higher protein content. In the present study, the recommended amount of vermicompost was 3.0 t ha⁻¹ in fragrant rice production.

1. Introduction

In recent years, global warming and soil degradation have become serious environmental problems and brought a series of negative influences to agricultural development [1]. The food demand is increasing due to the ever-increasing population, and it is a challenge to produce more cereal in a deteriorating environment. Guided by conventional concepts such as “the more fertilization, the greater the yield,” farmers always choose to apply large amounts of fertilizer to gain high crop yields [2]. However, the large application of chemical fertilizer has accelerated the nitrogen cycle rate and affected the soil nitrogen fixation capacity, carbon/nitrogen ratio, physical and chemical environment, and the number and community diversity of soil microorganisms [3,4]. Bossolani et al. [5] demonstrated that higher inputs of nitrogen fertilizers and poor soil management would intensify soil acidification. In order to save and improve the quality of cultivated land and ensure food security, more and more governments, and scientists have begun to pay attention to the development of sustainable agriculture.

Fragrant rice is a special rice type with good grain quality and characteristic aroma and is popular with consumers [6]. Previous studies have evidenced that 2-acetyl-1-pyrroline (2-AP) is the key component of fragrant rice aroma [7,8,9]. The yield formation and 2-AP content of fragrant rice are affected by paddy environments and crop management. For example, the study by Mo et al. [10] revealed that the nitrogen input during the booting stage had substantial effects on grain yield and 2-AP content. The study by Luo et al. [11] showed that the application of selenium fertilizer increased 2-AP content and improved grain quality [12]. Our previous attempts have found that the application of vermicompost as nursery substrate was able to promote the growth of fragrant rice seedlings, and vermicompost could be used as fertilizer to enhance the early growth of fragrant rice through pot experiments. However, the vermicompost application in fragrant rice production and the effects on yield formation and aroma of fragrant rice has not been reported.

Vermicompost is an organic fertilizer with high microbiological activity, produced by the processing of biological waste through the concerted action of earthworms and microorganisms. The study by Zhang et al. [13] showed that vermicompost application reduced cucumber incidence (Cucumis sativus L.) of Fusarium wilt and improved soil quality. Sahin et al. [14] indicated that the application of vermicompost could promote the phosphorus uptake of maize (Zea mays L.). The research by Cai et al. [15] discovered that the application of vermicompost could improve coastal saline soil quality by increasing the stability of soil aggregates and reducing soil electrical conductivity. In agricultural production, vermicompost application has direct and indirect effects on crops and improves soil properties, leading to a long-term increase in soil sustainability.

Therefore, the present study was conducted with a field experiment in Guangdong province (a major rice-producing province in South China) to investigate the effects of different vermicompost rates on yield formation, grain quality characters, and 2-AP content of fragrant rice cultivars. Our findings would provide new information and guidance for applying vermicompost as fertilizer in fragrant rice production.

2. Materials and Methods

2.1. Plant Materials, Site Conditions, and Treatment Description

A field experiment was conducted in Huangjiashan village (22.62° N, 111.57° E), Luoping town, Yunfu City, Guangdong Province, China, during two rice-growing lugs (early lug and late lug) of 2021. The site enjoys a subtropical monsoon climate. The experimental soil was sandy loam consisting of 16.65 g kg⁻¹ organic matter, 2.08 g kg⁻¹ total nitrogen, 1.74 g kg⁻¹ total phosphorus, and 10.35 g kg⁻¹ total potassium with a pH of 5.80. Seeds of two fragrant rice genotypes, Xiangyaxiangzhan (Xiangsimiao126 × Xiangyaruanzhan, bred by Taishan Institute of Agricultural Sciences, Jiangmen City, Guangdong Province, China) and Meixiangzhan-2 (Lemont × Fengaozhan, bred by the Rice Research Institute, Guangdong Academy of Agricultural Sciences, Guangzhou City, Guangdong Province, China), were used as plant materials in the present study. The two cultivars are conventional rice and are widely planted in South China for fragrant rice production [16]. The fragrant rice was sown into seedbed in March, transplanted in April, and harvested in July in the early lug. It was sown into seedbed in July, transplanted in August, and harvested in November in the late lug. The daily temperature between transplanting and harvest in both cropping lugs is shown in Figure 1. Five vermicompost rates were adopted as 2.250 (Ver1), 3.000 (Ver2), 3.750 (Ver3), 4.8750 (Ver4), and 6.000 (Ver5) t ha-1 in the present experiment with 2.250 t ha⁻¹ as basal, and others were all applied at three days after transplanting. The treatment without any fertilizer was taken as a control (CK). The applied vermicompost was manufactured by Hubei Tianshenjia Biological Environmental Protection Technology Co., Ltd, Wuxue City, Hubei Province, China, consisting of 34.90% organic matter, 1.48% total nitrogen, 1.21% P₂O₅, 0.90% K₂O, and with a pH of 7.60. The treatments were arranged in a randomized complete block design in triplicate with a net plot size of 8 × 4 m. The paddy field was kept flooded for most of the growth period until ten days before the harvest, and weeds, diseases, and insects were strictly controlled throughout both cropping lugs.

2.2. Determination of Plant Growth Parameters

The height of fragrant rice plants was measured at 3, 20, 40, 60, and 80 days after transplanting. To record total aboveground biomass, i.e., dry matter accumulation, at 20, 40, 60, 80, and 100 days after transplanting, five representative plants were collected at each plot, and then the plants’ samples were oven-dried at 60 °C until a constant weight (about 48 h). SPAD meter ‘SPAD-502’ (Konica Minolta, Tokyo, Japan) was used for precise, rapid, and non-destructive estimation of leaf chlorophyll contents at 20, 40, 60, and 80 days after transplanting.

2.3. Determination of Yield and Yield Components

At harvest, the effective panicle number of ten representative plants was recorded in each plot, and rice grains were harvested from three sampling areas (1.00 m²) in each plot and then manually threshed to determine the grain yield with the standard grain moisture content adjusted to 14%. Three representative plants in each plot were also collected to measure the 1000-grain weight, seed-setting rate (the ratio of filled grain number to total grain number), and grain number per panicle.

2.4. Determination of Grain Quality Characters

Before grain quality evaluation, the grain samples were air-dried to 12–13% of moisture content and stored for at least three months. 210 g rice grains from each treatment were taken from storage, and the brown rice rate was measured and calculated with a rice huller (Jiangsu, China). The milled rice and head rice rates were measured and calculated using a Jingmi testing rice grader (Zhejiang, China). The chalky rice rate and chalkiness degree of head rice were estimated by scanner (MRS-9600TFUL, Shanghai Zhongjing Technology Co., Ltd, Shanghai, China) and a rice appearance quality analysis and detection system (Hangzhou Wanshen Testing Technology Co., Ltd, Hangzhou, Zhejiang, China). The crude protein and amylose content were determined using the near-infrared grain analyzer (Infratec-1241, FOSS-TECATOR, Hoganas, Sweden).

2.5. Determination of 2-AP Content

At harvest, fresh and mature grains were collected and stored at −80 °C for determination of 2-AP content. The contents of 2-AP and other volatile components were determined according to the methods of Liu et al. [17]. In brief, grain samples were ground into a powder with liquid nitrogen. Powder sample (1.0 g) for each treatment in triplicate was added into 10 mL dichloromethane and ultrasonically extracted for 240 min. The supernatant was filtered by using a filter membrane. The final determination was carried out using GCMS-QP 2010 Plus (Shimadzu Corporation, Tokyo, Japan). The 2-AP content was expressed as μg kg⁻¹.

2.6. Statistic Analysis

Analysis of variance was performed with Statistix 8.1 (Analytical Software, Tallahassee, FL, USA), and the means of treatments were compared based on the least significant difference (LSD) test at the 0.05 probability level. The figures were made using SigmaPlot 12.5 (Systat Software Inc., California, CA, USA).

3. Results

3.1. Grain Yield and Yield-Related Traits

The grain yield and yield-related traits of fragrant rice cultivars under different application rates of vermicompost are shown in

Table 1. Effects of different vermicompost rates on yield formation of fragrant rice cultivars.

Lug	Cultivar	Treatment	Effective Panicle Number (Plant−1)	Grain Number (Panicle−1)	Seed-Setting Rate (%)	1000-Grain Weight (g)	Grain Yield (t ha−1)
Early lug
	Meixiangzhan-2
		CK	17.10 ± 1.73 a	131.33 ± 0.58 b	70.24 ± 1.82 a	19.07 ± 0.42 c	4.79 ± 0.52 b
		Ver1	18.10 ± 3.14 a	144.00 ± 3.00 a	68.79 ± 1.51 a	18.83 ± 0.21 c	5.55 ± 0.08 a
		Ver2	18.80 ± 2.09 a	142.67 ± 1.15 a	67.98 ± 0.50 a	20.15 ± 0.41 a	6.03 ± 0.45 a
		Ver3	17.80 ± 2.20 a	146.00 ± 4.00 a	69.93 ± 1.42 a	20.27 ± 0.24 a	5.57 ± 0.36 a
		Ver4	18.10 ± 1.91 a	145.33 ± 3.79 a	70.41 ± 0.97 a	19.90 ± 0.26 ab	5.83 ± 0.46 a
		Ver5	18.00 ± 3.16 a	145.00 ± 4.36 a	68.44 ± 1.68 a	19.40 ± 0.24 bc	5.64 ± 0.12 a
	Xiangyaxiangzhan
		CK	16.90 ± 2.13 d	146.67 ± 4.93 a	69.47 ± 0.66 a	19.13 ± 0.31 d	5.62 ± 0.35 b
		Ver1	18.10 ± 2.77 cd	145.67 ± 2.89 a	69.97 ± 2.03 a	20.17 ± 0.12 a	5.89 ± 0.51 a
		Ver2	21.60 ± 4.01 a	143.33 ± 4.16 a	68.72 ± 0.65 a	19.53 ± 0.15 bc	6.51 ± 0.44 a
		Ver3	19.80 ± 2.98 abc	145.33 ± 4.04 a	70.40 ± 0.79 a	19.83 ± 0.25 ab	6.15 ± 0.17 a
		Ver4	18.50 ± 2.84 bcd	146.00 ± 2.64 a	68.80 ± 1.24 a	20.00 ± 0.17 a	6.12 ± 0.55 a
		Ver5	21.00 ± 3.02 ab	145.33 ± 3.79 a	70.29 ± 1.82 a	19.33 ± 0.21 cd	6.28 ± 0.50 a
Late lug
	Meixiangzhan-2
		CK	13.90 ± 2.60 b	105.00 ± 13.75 b	71.46 ± 6.17 ab	16.43 ± 0.36 d	3.14 ± 0.28 c
		Ver1	17.00 ± 2.11 a	110.00 ± 17.69 ab	77.85 ± 5.08 a	17.81 ± 0.79 bc	3.58 ± 0.36 bc
		Ver2	16.70 ± 4.97 ab	128.67 ± 14.47 ab	70.84 ± 5.91 a	19.64 ± 0.59 a	4.61 ± 0.42 a
		Ver3	17.70 ± 2.00 a	131.67 ± 10.12 a	66.91 ± 3.54 b	20.04 ± 0.61 a	4.43 ± 0.59 a
		Ver4	17.40 ± 2.63 a	130.00 ± 8.54 a	66.23 ± 5.13 b	18.52 ± 0.55 b	3.99 ± 0.41 ab
		Ver5	19.30 ± 4.43 a	126.67 ± 6.03 ab	67.83 ± 3.64 b	16.78 ± 0.8 cd	3.88 ± 0.62 abc
	Xiangyaxiangzhan
		CK	16.40 ± 2.27 c	101.67 ± 6.66 a	65.96 ± 6.72 ab	16.84 ± 0.62 a	3.26 ± 0.33 b
		Ver1	17.90 ± 2.64 bc	98.33 ± 9.29 a	69.44 ± 3.92 a	16.96 ± 0.09 a	3.35 ± 0.40 b
		Ver2	19.30 ± 5.54 abc	101.33 ± 5.77 a	70.67 ± 4.06 a	17.17 ± 0.22 a	4.24 ± 0.28 a
		Ver3	19.80 ± 3.49 ab	108.67 ± 8.5 a	63.08 ± 4.09 ab	17.05 ± 0.09 a	3.53 ± 0.42 b
		Ver4	20.90±2.64 ab	104.00±4.00 a	58.79±5.15 bc	17.45±0.62 a	3.74±0.35 ab
		Ver5	22.00±2.66 a	84.33±9.71 b	51.41±3.97 c	17.46±0.55 a	3.19±0.32 b

Same letters after the values indicate not significant differences between treatments (p < 0.05).

. The highest or equally highest grain yield was recorded in Ver2 treatment, which was 6.03 and 6.51 t ha⁻¹ for Meixiangzhan-2 and Xiangyaxiangzhan in the early lug, and 4.61 and 4.24 t ha⁻¹ for Meixiangzhan-2 and Xiangyaxiangzhan in the late lug, respectively. The highest or equally highest effective panicle number per plant was also recorded in Ver2 treatment.

3.2. Plant Height, Chlorophyll Content, and Dry Matter Accumulation

The plant height, chlorophyll content (SPAD value), and dry matter weight of fragrant rice plants at different growth periods are shown in Figure 2, Figure 3 and Figure 4, respectively. There was no substantial difference among all treatments in plant height for both cultivars in each stage except at 40 and 60 days after transplanting for Xiangyaxiangzhan in the early lug. As for SPAD values, there was no significant difference among all treatments at most periods, either. In the early lug, when compared with CK, Ver2 treatment significantly increased dry matter weight at 80 and 100 days after transplanting by 63.70 and 87.98% for Meixiangzhan-2, and 28.58 and 51.65% for Xiangyaxiangzhan, respectively. In the late lug, 111.16 and 68.32% higher dry matter weights at 100 days after transplanting were recorded in the Ver2 treatment than CK for Meixiangzhan-2 and Xiangyaxiangzhan, respectively.

3.3. Grain Quality Characters

The grain quality characters of fragrant rice cultivars under different applied vermicompost rates are shown in

Table 2. Effects of different vermicompost rates on grain quality characters of fragrant rice cultivars.

Lug	Cultivar	Treatment	Brown Rice Rate (%)	Milled Rice Rate (%)	Head Rice Rate (%)	Crude Protein (%)	Amylose (%)	Chalky Rice Rate (%)	Chalkiness (%)
Early lug
	Meixiangzhan-2
		CK	76.19 ± 0.37 bc	67.59 ± 0.36 c	51.45 ± 1.49 b	7.17 ± 0.06 b	18.17 ± 0.50 a	12.00 ± 4.36 bc	2.52 ± 1.14 bc
		Ver1	75.91 ± 0.28 cd	68.73 ± 0.45 ab	55.35 ± 1.61 a	7.47 ± 0.06 a	17.50 ± 0.36 c	4.00 ± 1.00 d	0.68 ± 0.26 c
		Ver2	75.38 ± 0.28 d	68.22 ± 0.31 bc	50.68 ± 0.50 b	7.37 ± 0.06 a	16.50 ± 0.17 d	9.00 ± 1.00 cd	2.33 ± 0.73 c
		Ver3	76.72 ± 0.10 ab	67.66 ± 0.63 c	49.80 ± 1.37 bc	6.57 ± 0.06 e	17.70 ± 0.17 bc	18.33 ± 1.53 ab	5.20 ± 1.61 ab
		Ver4	76.83 ± 0.59 a	69.45 ± 0.25 a	47.90 ± 1.67 c	6.77 ± 0.06 d	18.13 ± 0.23 ab	19.33 ± 3.21 ab	5.54 ± 3.10 a
		Ver5	76.59 ± 0.13 ab	68.5 ± 0.30 b	49.60 ± 0.46 bc	6.90 ± 0.00 c	17.87 ± 0.15 abc	21.67 ± 7.64 a	5.17 ± 1.43 ab
	Xiangyaxiangzhan
		CK	75.17 ± 0.82 a	64.44 ± 0.71 a	56.74 ± 0.48 a	7.50 ± 0.00 b	17.97 ± 0.21 a	5.33 ± 1.53 c	1.11 ± 0.61 a
		Ver1	74.77 ± 0.75 a	63.90 ± 1.19 a	54.26 ± 1.98 b	8.07 ± 0.06 a	16.77 ± 0.12 d	6.00 ± 2.00 bc	1.52 ± 1.02 a
		Ver2	74.60 ± 0.62 a	62.18 ± 0.30 a	47.95 ± 1.07 d	8.00 ± 0.00 a	16.60 ± 0.1 d	7.00 ± 2.00 bc	1.27 ± 0.37 a
		Ver3	74.74 ± 0.81 a	63.67 ± 0.74 a	46.50 ± 0.60 de	7.33 ± 0.06 c	17.37 ± 0.12 b	8.33 ± 3.06ab	2.10 ± 1.08 a
		Ver4	72.90 ± 1.65 b	61.48 ± 1.41 bc	51.32 ± 1.25 c	7.23 ± 0.06 cd	17.17 ± 0.25 bc	10.00 ± 3.61a	1.90 ± 1.70 a
		Ver5	74.13 ± 0.63 ab	60.20 ± 0.85 c	44.99 ± 0.71 e	7.20 ± 0.10 d	16.80 ± 0.26cd	7.67 ± 3.21abc	1.65 ± 1.06 a
Late lug
	Meixiangzhan-2
		CK	76.50 ± 0.18 d	63.07 ± 0.49 b	55.28 ± 1.76 c	7.53 ± 0.21 a	18.20 ± 1.08 a	3.00 ± 3.46 ab	0.45 ± 0.68 ab
		Ver1	77.63 ± 0.22 a	63.12 ± 0.04 b	58.09 ± 0.84 ab	7.70 ± 0.00 a	18.07 ± 0.06 a	2.67 ± 1.15 ab	0.58 ± 0.11 ab
		Ver2	76.57 ± 0.20 cd	62.99 ± 0.26 b	50.89 ± 0.87 d	7.57 ± 0.06 a	18.37 ± 0.15 a	1.00 ± 0.00 b	0.12 ± 0.03 b
		Ver3	76.95 ± 0.34 c	64.20 ± 0.31 a	59.13 ± 0.42 a	7.60 ± 0.00 a	18.37 ± 0.60 a	1.33 ± 0.58 ab	0.20 ± 0.10 b
		Ver4	77.44 ± 0.41 ab	64.40 ± 0.36 a	57.08 ± 0.40 bc	7.23 ± 0.06 b	18.27 ± 0.46 a	5.67 ± 4.62 a	0.89 ± 0.74 ab
		Ver5	77.00 ± 0.38 bc	63.14 ± 0.52 b	56.08 ± 0.98 c	6.77 ± 0.06 c	18.77 ± 0.06 a	4.33 ± 1.15 ab	1.08 ± 0.48 a
	Xiangyaxiangzhan
		CK	73.41 ± 0.97 ab	55.92 ± 0.56 a	46.66 ± 5.63 a	8.67 ± 0.06 b	17.13 ± 0.15 b	2.00 ± 1.73 a	0.38 ± 0.33 a
		Ver1	72.11 ± 2.79 b	55.63 ± 1.77 a	48.12 ± 1.38 a	8.97 ± 0.06 a	17.03 ± 0.15 b	2.67 ± 1.15 a	0.54 ± 0.50 a
		Ver2	73.85 ± 0.94 ab	54.63 ± 2.47 a	44.57 ± 0.99 ab	7.20 ± 0.00 cd	18.20 ± 0.62 a	3.33 ± 4.93 a	0.57 ± 0.73 a
		Ver3	73.58 ± 0.28 ab	54.59 ± 0.61 a	44.85 ± 0.60 ab	7.03 ± 0.12 e	18.20 ± 0.17 a	7.33 ± 5.69 a	1.42 ± 1.26 a
		Ver4	75.30 ± 0.35 a	50.88 ± 1.08 b	41.11 ± 0.31 bc	7.30 ± 0.00 c	18.00 ± 0.17 a	4.67 ± 7.23 a	0.63 ± 1.06 a
		Ver5	74.59 ± 0.70 ab	51.13 ± 1.15 b	36.48 ± 1.24 c	7.10 ± 0.10 de	18.20 ± 0.20 a	5.67 ± 6.43 a	1.26 ± 1.16 a

Same letters after the values indicate not significant differences between treatments (p < 0.05).

. When compared with CK, Ver4 and Ver5 treatments significantly reduced head rice rate for both cultivars in both cropping lugs. The lowest or equally lowest amylose content was recorded in Ver1 treatment. When compared with CK, Ver1 and Ver2 treatments significantly increased crude protein content by 4.19 and 2.79% for Meixiangzhan-2, and 7.56 and 6.67% for Xiangyaxiangzhan, respectively in the early lug. Higher chalky rice rates and chalkiness were recorded in Ver4 and Ver5 treatments than CK across lugs.

3.4. Grain 2-AP Content

The grain 2-AP content at harvest of fragrant rice cultivars is shown in Figure 5. For Meixiangzhan-2, there was no significant difference among all treatments in 2-AP content in both lugs. For Xiangyaxiangzhan, there was no significant difference among all CK, Ver1, and Ver2 treatments, while, when compared with CK, Ver5 treatment significantly reduced 2-AP content by 11.48 and 13.06% in early and late lugs, respectively.

4. Discussion

As an organic fertilizer, vermicompost has proven its applied potential in the cultivation of some crops [18]. For example, the study by Lin et al. [19] showed that the application of vermicompost could substantially improve the growth and quality of tobacco (Nicotiana tabacum L.). The study by Wang et al. [20] revealed that the application of vermicompost could reduce the incidence rate of Fusarium wilt in tomato (Lycopersicon esculentum) and promote the soil microbial community structure. The present study firstly applied vermicompost as fertilizer in fragrant rice production and tried to find a suitable applied amount. Our results showed that fragrant rice yield varied with the different vermicompost rates, and the application of vermicompost had substantial effects in increasing the grain yield. In two cropping lugs, the highest or equally highest grain yield was recorded in the Ver2 treatment, which applied vermicompost at 3.000 t ha⁻¹. The increment in grain yield was attributed to the improvement in dry matter accumulation. Dry matter weight is the direct result of the photosynthesis of rice plants [21], and previous studies also revealed that dry matter accumulation during the growing period is crucial to yield formation of rice [22]. However, we observed that vermicompost didn’t substantially affect chlorophyll content (SPAD value), which indicated that vermicompost didn’t influence the biosynthesis of chlorophyll. During the growth of rice plants, the photosynthetic matter production depends on photosynthetic rate and leave area, while the photosynthetic rate is related to stomatal conductance and chlorophyll content [23]. Thus, we deduced that vermicompost influenced dry matter accumulation by altering stomata condition and leaf area index, but more studies need to be conducted to determine the photosynthetic properties of fragrant rice under vermicompost application.

In the present study, the effects of vermicompost rates on yield-related traits and grain quality characters of fragrant rice were unstable and irregular; especially, the application with high vermicompost rates negatively affected the grain quality because the chalky rice rate and chalkiness in Ver4 and Ver5 treatments were higher than Ver1 and Ver2 treatments, and even than CK. Furthermore, we observed that the grain yield of fragrant rice didn’t increase with the increment of vermicompost rate. Those phenomena might be attributed to low nitrogen, potassium, and potassium contents in vermicompost. The nitrogen input in the present study varied from 33.3 to 88.8 kg ha⁻¹, while nitrogen input was normally between 120.0 and 240.0 kg ha⁻¹ in conventional rice production [13,23,24]. However, the increasing vermicompost rate in fragrant rice production for higher yield and stable yield-related traits seems impracticable. On the one hand, increasing the vermicompost rate would increase the cost of fragrant rice production. On the other hand, the results of the present study showed that grain yield did not increase with the increases of vermicompost rate, and two reasons could explain this. One is the high pH of vermicompost. The pH of vermicompost is 7.60, whilst the pH of paddy soil in the present site is 5.80, indicating that largely applying vermicompost would change soil acidity. The study by Huang et al. [25] revealed that soil pH is the major limiting factor for rice productivity, with rice plants preferring a pH below 7 and high pH values being unfavorable to the growth of rice, with the rice plant being prone to symptoms of lack of elements, growth obstruction, young leaves turning yellow, dry or scorched leaf edges, capillary root rot, and other phenomena under high soil pH conditions. An early study also showed that soil pH conditions have substantial effects on growth and yield formation of rice [26]. However, the response of fragrant rice performances to soil pH has not been reported, and thus, more studies should be carried out to investigate the effects of soil pH on agronomic traits and physiological properties of fragrant rice plants. The other reason is the high carbon/nitrogen ratio of vermicompost. It is well known that nitrogen plays an important part in the growth and development of rice, and rice yield normally increases with the increasing nitrogen input [27,28]. The study by Qi et al. revealed that soil total nitrogen and carbon/nitrogen ratio had remarkable effects on the yield formation of rice. Kurai et al. [29] demonstrated that it was difficult for rice plants to produce high yield under low nitrogen conditions. In general, Ver1 and Ver2 treatments exhibited better agronomic performances with higher grain yield and lower chalkiness of fragrant rice cultivars in the present study.

2-AP is the key component of fragrant rice aroma, and its biosynthesis is very complicated in fragrant rice plants [10]. The improvement in 2-AP content would benefit fragrant rice production. However, the results of our study showed that the application of vermicompost didn’t significantly increase grain 2-AP content. Moreover, high vermicompost rates (Ver4 and Ver5 treatments) reduced 2-AP content for Xiangyaxiangzhan. The results were inconsistent with our previous experiment with fragrant rice seedlings [8], which showed that the application of vermicompost increased the 2-AP content of leaves. The differences indicated that the 2-AP biosynthesis in grains might be different from in leaves, or the 2-AP biosynthesis is different in fragrant rice in the different growth periods. Moreover, the insignificant effects on 2-AP content under vermicompost application might be attributed to low nitrogen content in vermicompost. Previous studies revealed that the content of 2-AP is closely related to nitrogen and more input of nitrogen during the growing period would increase grain 2-AP content [11,30]. Therefore, we wondered that a combination with vermicompost and nitrogen fertilizer, such as urea, might be effective to increase 2-AP content, but more studies are required to be conducted to verify this hypothesis.

5. Conclusions

Application of vermicompost substantially increased grain yield and enhanced dry matter accumulation of fragrant rice plants. Considering that the highest or equally highest grain yield was recorded in Ver2 treatment, the recommended amount of vermicompost was 3.0 t ha⁻¹ in fragrant rice production. The chlorophyll and grain 2-AP contents were not significantly and positively affected by vermicompost. In order to better apply vermicompost in rice production and investigate the mechanism of vermicompost on rice performance, more studies should be conducted at multiple levels, such as physiology and microbiology.