Abstract
Exchangeable cations and soil nutrients leach out/lost in the raised bed soils system through irrigation water and rainwater. Cover crops or crop residue mulching are considered sustainable agricultural measures because they prevent soil degradation and compaction and increase the soil ecological diversity. However, the impact of these on soil quality in fruit orchards in the Vietnamese Mekong Delta is not well studied. The present work aimed to assess the effect of cover crops (grass, pinto peanut) and rice straw mulching on change in soil chemical properties and productivity of pomelo. The 3-year (2019–2021) field experiment was conducted at Phu Huu Commune, Chau Thanh District, Hau Giang Province, Vietnam. Four treatments, control (no-cover crop), grass cover crop, mulching with rice straw (MRS), and pinto peanut cover crop were used. Soil samples were collected from the depth of 0–20 and 20–40 cm for analyzing soil pH, total nitrogen (TN), total phosphorus (TP), available phosphorus (AP), and soil organic matter (SOM). Results showed that MRS or cover crop by pinto peanut significantly decreased soil acidity in the topsoil layer. Soil fertility properties (TN, TP, AP, and SOM) at the depth 0–20 cm were greatly improved after the application of the mulched rice straw and cover crop by legume, thus improving pomelo yield. Further study is required to understand the effects of the cover crop with pinto peanut and rice straw mulching on the physical and biological properties of soil as well as nutrient uptake of pomelo orchards.
1 Introduction
The Vietnamese Mekong Delta (VMD) soils were formed and developed during the Holocene Epoch [1]. According to Minderhoud et al. [2], the VMD is also located higher than 0.8 m from the sea level. Therefore, to avoid waterlogging during the rainy season, farmers had to raise bed construction for the cultivated upland crop [3]. The raised bed technique is used to reclaim lowland clay soils with shallow water tables. Unlike traditional rice-based systems, which now provide a fairly low income, raised bed systems allow the cultivation of cash crops with high added value, such as fruits or vegetables [3].
Horticulture plays a vital role in improving the farmer’s livelihood as well as in reducing the poverty in this area [4]. Notably, citrus is one of the most crucial fruit crops due to its high economic value. In 2019, the citrus cultivation area in the VMD was about 35,500 ha with an estimated production of 500,000 tones, occupying about 50% of the total region and 65% of total production in Vietnam [5]. Pomelo (Citrus grandis Osbeck) is an important citrus crop with respect to the economy as well as nutrition. It is well known for the vitamin C, polysaccharides, and phytochemicals present in the fruit [6]. However, pomelo yield in the VMD has decreased significantly in recent years [7].
The aging of raised bed soil is considered a key factor affecting growth and citrus yield [8], because exchangeable cations (potassium, magnesium, and calcium) easily move out of the soil surface during rainfall and irrigation in the old-raised bed systems [8]. Besides, the elevation of raised beds may increase soil erosion. In recent years, soil erosion and degradation in the raised bed system are key environmental problems in the Mekong Delta of Vietnam [9], especially in citrus orchards, leading to the loss of available nutrients through runoff, which is a major driver for soil fertility reduction [10].
Soil conservation measures (cover crops and mulching) are considered the best solution to reduce sediment runoff [11]. Soil quality indicators and soil fertility were greatly enhanced due to soil conservation practices [12,13]. According to Fang [14], total nitrogen (TN) and total phosphorous reduced significantly after the implementation of soil conservation practices. In addition, soil compaction greatly decreased after using permanent cover for soil conservation [15].
Cover crops are the best way to support soil microflora growth and activity, which directly affect nutrition mineralization in soil [16]. Furthermore, cover crops may reduce the effect of raindrops splattering on the ground leading to less erosion [17]. The loss of sediment and nutrients also decreases due to cover crops, thus improving soil quality fertility, resulting in increased crop yield [18]. Pinto peanut (Arachis pintoi) is among the leguminous crops, which are used for ground cover in the agricultural systems, because they are easy to cultivate and have the ability of biological nitrogen fixation [19].
The utilization of mulching materials is a soil conservation measure that decreases soil erosion, enhances soil fertility, and improves crop growth and productivity [20,21]. Rice straw mulching is a vital soil management technique of covering the soil surface to regulate soil temperature and decrease soil loss [22]. Moreover, it has a positive effect on increasing the soil organic matter (SOM) and microflora diversity, resulting in an improved soil-integrated fertility index [23].
We hypothesize that in a raised bed of VMD, the exchangeable cations in the soil are susceptible to leaching due to tropical conditions, resulting in highly acidic soils, while over-exploitation also leads to low fertility. Our previous study (unpublished data) showed that farmers in VMD use rice straw, legumes, and grass to cover the topsoil of their orchards. However, there have been no studies evaluating and comparing the effectiveness of the three conservation methods on the raised bed soil. Therefore, this study aimed to evaluate how conservation practices (rice straw mulch, leguminous, and non-leguminous cover crops) are used to improve soil acidity and nutrients as well as pomelo yield.
2 Materials and methods
2.1 Study site, soil, and climate
The field experiment was conducted in a pomelo orchard from January 2018 to January 2021. The pomelo orchard is located in Phu Huu commune, Chau Thanh district, Hau Giang province, Vietnam (9°54′30.3″N, 105°51′06.7″E).
The initial soil physicochemical properties, before conducting an experiment, are presented in Table 1. Soil analyses indicated that the topsoil (0–20 cm) and subsoil (20–40 cm) layers were acidic (pH = 5.0–5.22), medium in available phosphorus (AP) (22.5–24.6 mg P kg−1), low in TN (0.12–0.15%), low in SOM (1.28–1.46%), low in total phosphorus (TP) (0.10–0.12%), and silty clay in texture (46.5–48.0% clay, 1.50–2.20% sand, and 49.5–52.0% silt).
Initial soil quality before the experiment
| Property | Value | |
|---|---|---|
| 0–20 cm | 20–40 cm | |
| pHH2O (1:2.5) | 5.00 | 5.22 |
| PBray-2 (mg P kg−1) | 22.5 | 24.6 |
| TP (%) | 0.12 | 0.10 |
| TN (%) | 0.15 | 0.12 |
| SOM (%) | 1.46 | 1.28 |
| Sand (%) | 1.50 | 2.20 |
| Silt (%) | 52.0 | 49.5 |
| Clay (%) | 46.5 | 48.3 |
| Soil textural class | Silty clay | Silty clay |
The average monthly rainfall from 2019 to 2021 at the study site was 175 mm, of which September usually had the highest rainfall (470 mm) and March had the lowest (10 mm). The meteorological data of the study location were presented in detail in our previous study [24].
2.2 Experimental design
The field trial was conducted in a randomized complete block design with four replications using the following groups: (1) control (no-cover crop); (2) cover crop by grass (CCG); (3) mulching with rice straw (MRS); and (4) cover crop by pinto peanut (CCP). In the present study, a “Da Xanh” 5-year-old pomelo orchard was used for trial. The fruit diameter of “Da Xanh” pomelo is from 20 to 25 cm, its average weight is 1.0–2.0 kg, and its skin is green. The distance between pomelo plants was 4.0 m × 4.5 m. The total area used for the experiment was about 1,500 m2. All trees in the present study took carefully with conventional pesticides for pests and disease control as well as treated with chemical fertilizers. The fertilization rates were determined based on the recommendations of Can Tho University, Vietnam. Each tree received 800 g N, 500 g P, and 700 g K annually. All treatments followed the no-till practice.
The herbicide NIPHOSATE 480SL, used in the control plots, was a commercial product of Nicotex Co., Ltd., Vietnam. The concentration of glyphosate IPA salt was 480 g per liter. The herbicide was applied at a dose of 2,500 ml ha−1. It was sprayed when the weeds grew to 8–10 cm (approximately 5 leaves). We used a hand sprayer (Mitsuyama TL-767) for the herbicide.
In the CCG treatment, Asiatic dayflower (Commelina communis) was used as cover crop. Asiatic dayflower was planted by cutting the seedling into cuttings of approximately 20 cm and spread evenly on the orchard surface and watered regularly. After a month, these grass cuttings grew roots and when 30 cm tall, the tops were cut (leaving only the base about 10 cm long) using Honda Grass Cutter GX35.
Pinto peanut was grown during January and July each year in CCP plots. Pinto peanut cuttings were prepared when pinto peanuts are in the cake stage, when leaves begin to turn yellowish, about 30–40 cm long. Pinto peanut was cultivated in a row; each cluster consisting of 2–3 cuttings spaced 10–15 cm apart. The cuttings were filled with soil thoroughly, pressed firmly for fast rooting, and irrigated lightly for enough moisture.
Rice straw was mulched two times per year in October and March in MRS treatment. A 2–2.5 cm layer of rice straw mulch was spread under the pomelo canopy, keeping the mulch 50–60 cm away from the trunks. The mulch was spread out far enough from the base of the plant to cover the entire root system. The total rice straw used for the experiment was 5.5 t ha−1 year−1.
2.3 Soil sampling and analysis
To determine the chemical properties of the soil, soil samples were collected at the same time at the pomelo harvesting stage (January 2019, 2020, and 2021). In each treatment, a soil auger (diameter 3 cm) was used to collect five soil cores from the depths of 0–20 and 20–40 cm. Soil samples were collected around the canopy of trees. The soil was divided into subsamples of about 500 g after samples from the same depth were blended into one composite sample per depth. All soil samples were air-dried, crushed, and sieved using a 2 mm sieve to make samples suitable for laboratory analysis [25].
Soil pH in the 1:2.5 solution (soil:water) was determined using a digital pH meter [26]. Soil organic carbon was determined using the wet-oxidation method described by Walkley and Black [27]. Total N was determined using the semi-micro Kjeldahl method [28]. The levels of total P present in the soil samples were estimated using the ammonium phosphomolybdate method [29]. The available P was determined using the Bray-2 method [30].
2.4 Yield component and fruit productivity
The total number of fruits per tree was counted during each treatment at the harvest stage. After that, weight of all fruit per trees were measured by “Nhon Hoa” scale. The mean weight of fruit was determined based on the total weight (kg) of fruits per tree divided by the total number of fruits. Fruit yield (FY) (t ha−1) was estimated using the number of fruits per tree multiplied by the mean weight of fruit and then by the plant density. In the current study, the pomelo density was estimated to be 555 trees ha−1.
2.5 Statistical analysis
SPSS software (version 20.0) was used for data analysis. Variance analysis was used to compare the differences between means among the treatments using the Duncan test with statistical levels at 5 and 1%. The relationships between soil properties were determined by Pearson’s correlation coefficient.
3 Results
3.1 Impact of soil conservation practices on soil properties
3.1.1 Surface layer (0–20 cm)
Soil acidity and fertility in the topsoil layer significantly improved after implementing soil conservation measures during the 3-year experiment. In particular, MRS and CCP enhanced soil pH by about 0.52 and 0.74 units, respectively (Table 2) compared to the initial soil pH (Table 1). Soil pH also increased in the CCG treatment, but it was lower when compared with CCP and MRS. Moreover, the nutrients (TN, TP, and AP) in soil conservation practices were greater compared with no cover crop. The average TN, TP, and AP concentrations in MRS and CCP increased by 0.11, 0.11, and 0.018% respectively, when compared with the control treatment. Similarly, SOM was significantly ameliorated by the use of soil conservation practices. Although CCG improved soil fertility, the effect was lower when compared with MRS and CCP. Hence, cover crop by legume and MRS were the best measures for increasing the soil quality indicators to a depth of 0–20 cm.
Influence of soil conservation measures on soil chemical properties in the topsoil layer (0–20 cm)
| Years | Treatments | pHH2O (1:2.5) | TN (%) | TP (%) | SOM (%) | AP (mg kg−1) |
|---|---|---|---|---|---|---|
| 2019 | Control | 5.08c ± 0.04 | 0.14b ± 0.04 | 0.13c ± 0.03 | 1.41c ± 0.04 | 22.8c ± 0.96 |
| CCG | 5.25b ± 0.08 | 0.18a ± 0.02 | 0.19b ± 0.03 | 1.53b ± 0.07 | 32.0b ± 2.94 | |
| MRS | 5.36a ± 0.07 | 0.22a ± 0.03 | 0.21ab ± 0.01 | 1.65a ± 0.06 | 36.7a ± 2.29 | |
| CCP | 5.38a ± 0.16 | 0.22a ± 0.03 | 0.23a ± 0.02 | 1.64a ± 0.06 | 36.1a ± 1.80 | |
| P value | ** | ** | * | ** | ** | |
| 2020 | Control | 5.04c ± 0.04 | 0.13c ± 0.02 | 0.15b ± 0.01 | 1.45c ± 0.04 | 21.5c ± 0.73 |
| CCG | 5.33b ± 0.08 | 0.20b ± 0.02 | 0.22a ± 0.02 | 1.58b ± 0.03 | 34.6b ± 0.86 | |
| MRS | 5.48a ± 0.11 | 0.24a ± 0.02 | 0.24a ± 0.03 | 1.84a ± 0.08 | 40.5a ± 1.17 | |
| CCP | 5.53a ± 0.20 | 0.25a ± 0.02 | 0.24a ± 0.02 | 1.83a ± 0.06 | 42.1a ± 2.45 | |
| P value | ** | ** | ** | ** | ** | |
| 2021 | Control | 4.97c ± 0.06 | 0.14c ± 0.02 | 0.15c ± 0.01 | 1.46c ± 0.05 | 21.8c ± 1.10 |
| CCG | 5.37b ± 0.14 | 0.22b ± 0.02 | 0.25b ± 0.03 | 1.71b ± 0.03 | 38.9b ± 0.98 | |
| MRS | 5.52a ± 0.04 | 0.29a ± 0.02 | 0.30a ± 0.02 | 2.00a ± 0.13 | 43.4a ± 2.26 | |
| CCP | 5.74a ± 0.13 | 0.32a ± 0.03 | 0.32a ± 0.02 | 2.03a ± 0.06 | 43.0a ± 3.15 | |
| P value | ** | ** | ** | ** | ** |
The different letters indicate the significant differences among treatments at P < 0.05 (*) and P < 0.01 (**). CCG — cover crop by grass; MRS — mulching with rice straw; CCP — cover crop by pinto peanut.
3.1.2 Subsurface layer (20–40 cm)
Table 3 shows the soil chemical properties at a depth of 20–40 cm in a pomelo orchard under different soil conservation practices. Soil pH, soil nutrients (TN, TP, and AP), and SOM content did not improve by the CCG, MRS, or CCP. The average pH, TN, TP, SOM, and AP values in the subsoil layer were 5.26, 0.14, 0.13, 1.27, and 0.025%, respectively. Moreover, soil fertility properties in the depth of 20–40 cm did not affect by conservation practices during 3 years (2019, 2020, and 2021) of the study.
Effect of soil conservation practices on soil quality in the subsoil layer (20–40 cm)
| Years | Treatments | pHH2O (1:2.5) | TN (%) | TP (%) | SOM (%) | AP (mg kg−1) |
|---|---|---|---|---|---|---|
| 2019 | Control | 5.25 ± 0.05 | 0.13 ± 0.03 | 0.11 ± 0.01 | 1.25 ± 0.11 | 25.0 ± 1.78 |
| CCG | 5.30 ± 0.05 | 0.14 ± 0.02 | 0.12 ± 0.03 | 1.25 ± 0.21 | 26.1 ± 1.46 | |
| MRS | 5.30 ± 0.12 | 0.14 ± 0.03 | 0.14 ± 0.03 | 1.24 ± 0.10 | 24.8 ± 3.12 | |
| CCP | 5.27 ± 0.18 | 0.13 ± 0.02 | 0.12 ± 0.03 | 1.26 ± 0.09 | 24.3 ± 2.57 | |
| P value | ns | ns | ns | ns | ns | |
| 2020 | Control | 5.25 ± 0.05 | 0.13 ± 0.03 | 0.12 ± 0.01 | 1.29 ± 0.03 | 24.8 ± 1.35 |
| CCG | 5.29 ± 0.11 | 0.14 ± 0.02 | 0.14 ± 0.02 | 1.27 ± 0.15 | 25.0 ± 2.89 | |
| MRS | 5.25 ± 0.16 | 0.16 ± 0.03 | 0.15 ± 0.04 | 1.30 ± 0.11 | 25.3 ± 3.23 | |
| CCP | 5.28 ± 0.15 | 0.15 ± 0.04 | 0.15 ± 0.02 | 1.26 ± 0.07 | 24.6 ± 2.01 | |
| P value | ns | ns | ns | ns | ns | |
| 2021 | Control | 5.20 ± 0.08 | 0.14 ± 0.03 | 0.13 ± 0.03 | 1.28 ± 0.10 | 24.3 ± 1.02 |
| CCG | 5.22 ± 0.09 | 0.14 ± 0.03 | 0.15 ± 0.03 | 1.31 ± 0.11 | 24.5 ± 2.87 | |
| MRS | 5.25 ± 0.07 | 0.15 ± 0.02 | 0.15 ± 0.04 | 1.25 ± 0.12 | 26.0 ± 0.93 | |
| CCP | 5.32 ± 0.12 | 0.17 ± 0.05 | 0.14 ± 0.03 | 1.30 ± 0.12 | 26.0 ± 1.21 | |
| P value | ns | ns | ns | ns | ns |
ns — not significant; CCG — cover crop by grass; MRS — mulching with rice straw; CCP — cover crop by pinto peanut.
3.2 Effect of soil conservation measures on yield of pomelo
The effects of conservation measures on yield component and productivity of fruit are presented in Table 4. The number of fruits per tree in the treatments with conservation practices CCP and MRS between 2019 and 2021 was significantly higher than those without it. The number of fruits tended to increase from 2019 to 2021 in all treatments. An increasing number of fruits per tree resulted in improving the FY. The average productivity of pomelo increased to 5.0 t ha−1 in the CCP and MRS in the 3 consecutive years of research. CCG also improved the FY of pomelo about 2.0 t ha−1. However, there was no statistically significant difference in pomelo productivity between the control and CCG. Table 4 indicates that there was no difference in the weight of fruit in the 3 consecutive years of research, with the average being 1.47 kg per fruit.
Yield component and fruit productivity as affected by soil conservation measures
| Years | Treatments | Number of fruits per tree | Mean weight of fruit (kg) | Fruit yield (t ha–1) | Compared with the control treatment | |
|---|---|---|---|---|---|---|
| t ha–1 | % | |||||
| 2019 | Control | 25.5b ± 1.05 | 1.52 ± 0.07 | 21.5b ± 1.64 | — | — |
| CCG | 27.4b ± 0.81 | 1.50 ± 0.08 | 22.8b ± 1.00 | 1.31 | 6.10 | |
| MRS | 31.7a ± 1.94 | 1.47 ± 0.07 | 25.9a ± 2.39 | 4.45 | 20.7 | |
| CCP | 31.4a ± 1.12 | 1.48 ± 0.09 | 25.8a ± 2.07 | 4.35 | 20.3 | |
| P value | ** | ns | * | |||
| 2020 | Control | 30.0c ± 1.92 | 1.49 ± 0.08 | 24.8b ± 1.31 | — | — |
| CCG | 33.9b ± 1.31 | 1.43 ± 0.10 | 26.8b ± 1.17 | 2.00 | 8.06 | |
| MRS | 36.5a ± 0.44 | 1.48 ± 0.08 | 29.9a ± 1.34 | 5.14 | 20.7 | |
| CCP | 36.9a ± 0.97 | 1.46 ± 0.07 | 29.8a ± 2.02 | 5.05 | 20.4 | |
| P value | ** | ns | ** | |||
| 2021 | Control | 34.3c ± 0.88 | 1.47 ± 0.07 | 27.8b ± 0.79 | — | — |
| CCG | 37.4b ± 1.33 | 1.46 ± 0.11 | 30.3ab ± 3.20 | 2.41 | 8.67 | |
| MRS | 40.8a ± 1.47 | 1.47 ± 0.08 | 33.3a ± 2.77 | 5.46 | 19.6 | |
| CCP | 41.2a ± 2.26 | 1.47 ± 0.04 | 33.5a ± 2.43 | 5.69 | 20.4 | |
| P value | ** | ns | * | |||
The different letters indicate the significant differences among treatments at P < 0.05 (*) and P < 0.01 (**); ns — not significant. CCG — cover crop by grass; MRS — mulching with rice straw; CCP — cover crop by pinto peanut.
3.3 Matrix correlation between soil chemical properties and FY
3.3.1 Depth of 0–20 cm
There was a significantly positive correlation between soil chemical properties (pH, TN, TP, SOM, and AP) and the number of fruits per tree (Table 5), with the correlation coefficient of r = 0.65, 0.74, 0.76, 0.80, and 0.70, respectively. A positive correlation was recorded between soil quality and FY, including pH and FY (r = 0.63), TN and FY (r = 0.69), TP and FY (r = 0.71), SOM and FY (r = 0.74), and AP and FY (r = 0.65). Meanwhile, there was no significant correlation between soil fertility parameters and an average weight of fruit.
Pearson correlation matrix between the soil quality properties of topsoil layer and fruit yield (n = 48)
| pH | TN | TP | SOM | AP | NF | WF | FY | |
|---|---|---|---|---|---|---|---|---|
| pH | 1 | 0.80** | 0.82** | 0.83** | 0.85** | 0.65** | −0.05 | 0.63** |
| TN | 1 | 0.86** | 0.86** | 0.87** | 0.74** | −0.15 | 0.69** | |
| TP | 1 | 0.86** | 0.87** | 0.76** | −0.15 | 0.71** | ||
| SOM | 1 | 0.85** | 0.80** | −0.15 | 0.74** | |||
| AP | 1 | 0.70** | −0.13 | 0.65** | ||||
| NF | 1 | −0.17 | 0.94** | |||||
| WF | 1 | 0.17 | ||||||
| FY | 1 |
** indicates significant difference at P < 0.01. pH, soil-to-water ratio (1:2.5); TN — total nitrogen; TP — total phosphorus; SOM — soil organic matter; AP — available phosphorus; NF — number of fruit per tree; WF — mean weight of fruit; FY — fruit yield.
Highly positive correlation coefficients were observed between pH and TN (r = 0.80), pH and TP (r = 0.82), pH and SOM (r = 0.83), and pH and AP (r = 0.85). Similar to pH, there was a significant correlation between TN and TP, SOM, and AP (r = 0.86, 0.86, and 0.87, respectively). The content of SOM was positively correlated to the concentration of AP (r = 0.87). However, our study did not find any significant relationships between the mean weight of fruit and soil chemical properties as well as FY (Table 5).
3.3.2 Depth of 20–40 cm
Table 6 shows the relationship between soil quality in the subsoil layer (20–40 cm) and yield component and FY. There was no significant correlation between soil pH and soil nutrients (TN, TP, SOM, and AP). Soil pH was also not significantly related to NF (number of fruits per tree), WF (mean weight of fruit), and FY. There was no significant correlation between TN and other variables, except for TP (r = 0.37). A positive correlation was recorded between TP and NF (r = 0.40) and TP and FY (r = 0.41). The content of SOM and AP was not correlated with other soil chemical parameters as well as productivity of fruit. Similar to the Pearson correlation matrix of the topsoil layer, there was no relation between the mean weight of fruit and soil chemical properties at the depth of 20–40 cm (Table 6).
Pearson correlation matrix between the soil fertility properties of subsoil layer and fruit yield (n = 48)
| pH | TN | TP | SOM | AP | NF | WF | FY | |
|---|---|---|---|---|---|---|---|---|
| pH | 1 | 0.20 | 0.26 | −0.16 | 0.07 | −0.02 | 0.09 | 0.01 |
| TN | 1 | 0.37* | −0.10 | 0.22 | 0.26 | 0.05 | 0.28 | |
| TP | 1 | 0.14 | −0.07 | 0.40** | 0.01 | 0.41** | ||
| SOM | 1 | 0.34 | 0.08 | 0.08 | 0.10 | |||
| AP | 1 | 0.12 | 0.05 | 0.13 | ||||
| NF | 1 | −0.17 | 0.94** | |||||
| WF | 1 | 0.17 | ||||||
| FY | 1 |
* and ** indicate significant difference at P < 0.05 and P < 0.01, respectively. pH – soil-to-water ratio (1:2.5); TN – total nitrogen; TP – total phosphorus; SOM – soil organic matter; AP – available phosphorus; NF – number of fruits per tree; WF – mean weight of fruit; FY – fruit yield.
4 Discussion
According to Noland et al. [31] and Cruz et al. [32], soil conservation measures not only enhanced soil physicochemical properties but also improved crop yield; we discovered that there was an improvement in SOM and soil nutrients (TN, TP, and AP) in pomelo plots with conservation practices at a depth of 0–20 cm (Table 2). The amount of SOM, TN, TP, and AP in treatments (MRS and CCP) is the highest compared with CCG and control. In addition, soil acidity declined greatly after MRS and CCP treatments.
Several studies have indicated that cover cropping and mulching significantly increased SOM and soil nutrient concentration [32,33,34]. According to Kogel-Knabner [35], plant biomass (leave, root, and stem) is the major parent material for SOM formation. In agricultural systems, leguminous crops contribute positively to improving the natural carbon pool in the soil [36]. Through various ways including contributing plant biomass and dead roots and nodules of legume [37], legumes can increase SOM. As well as this SOM can improve soil acidity [36], these materials contribute to the increase of soil nutrients. Cover crops with legumes enhance soil chemical P availability and adjust the pH [37]. Reduction of soil acidity is considered a helpful factor to promote soil microorganism activity, resulting in increased nutrient cycling processes as well as nutrient mineralization [38,39].
Previous studies have indicated that rice straw mulching utilization greatly increased soil organic carbon [40,41,42]. Rice straw is a rich carbon source that has several macronutrients (N, P, and K) essential for crop development. The TN, TP, and AP contents rose significantly by rice straw mulching utilization as indicated by the results. Improvement of soil nutrients and SOM may be due to the relationship between nutrients and carbon content in rice straw. In our previous study, the average concentrations of N, P, and K in rice straw (n = 120) that was cultivated in VMD were 6.20, 2.70, and 16.7 g kg−1, respectively [43]. According to Hung et al. [44], rice straw contains carbon content ranging between 11.1 and 16.7%. Mulched rice straw increased soil pH by about 0.5 units in a 3-year field trial (Table 2). Samuel et al. [45] found that MRS increased pH about 1.6 unit when compared with the control treatment. The results of the study are in line with those of Yan et al. [46] and Fu et al. [47], stating that MRS enhances the soil health and available soil nutrients.
However, the use of soil conservation measures did not improve soil chemical properties at the subsoil layer (Table 3). This might have been due to the no-till system adopted in the present research decreasing transportation and accumulation of SOM and soil nutrients to the deeper layer (20–40 cm). Besides, the short duration experiment may be a reason leading to the soil quality not being affected even after conservation measures. This result is in agreement with the study conducted by Zhou et al. [40], reporting that soil fertility was not affected by rice straw mulching at a depth of 20–30 cm. Another study revealed that cover crop did not improve soil organic carbon in the subsoil layer [48].
There was a correlation between soil chemical properties in the topsoil layer and numbers of fruit per tree and FY (Table 5). These results might be due to the decomposition of green manure (biomass and root of legume and straw), which provide plenty of nutrients required for pomelo growth as well as increase the fruit set ratio, that led to an increased number of fruits per tree and pomelo productivity. There was a strong correlation between SOM, TN, and AP and citrus yield, considered as key factors for determining and assessing soil health in citrus orchards [49,50].
In the current study, treating with straw mulching and CCP markedly enhanced the effective number of fruits per tree (Table 4). As a result, the yield of fruit increased significantly when compared with the control treatment. Pomelo yield had a strong relationship with the numbers of fruit (Tables 5 and 6, r = 0.94). Several studies have reported that legume cover crops and mulching with straw substantially enhanced FY [51,52,53]. According to Acharya et al. [54], oat as a cover crop increased sorghum grain yield from 33 to 97%, when compared with no cover crop treatment. Another study also indicated that legume cover crop significantly improved wheat yield [55].
MRS in the current study increased FY to 20% compared with the control. The study agreed with the results reported by Noor et al. [56]; they concluded that maize yield and dry biomass increased by 18 and 20%, respectively, after being treated with straw mulching. A similar result also showed that straw mulching increased maize yield to 12.4% when compared with no mulching [57]. According to Hadaro et al. [58] and Retta et al. [59], soil conservation practices utilization positively affected soil fertility and soil nutrients availability, resulting in improved soil productivity, which directly influences the crop yield. The results were in agreement with the reports of Giacalone et al. [53], Scavo et al. [60], and Webber et al. [61]. They demonstrated that cover crop and mulching increased soil fertility and fruit crops compared with the control.
5 Conclusion
The results indicated that the use of soil conservation practices (cover crop by legume and MRS) improved soil quality in the surface layer, resulting in increased pomelo productivity up to 20% compared with the control treatment. However, soil fertility in the subsurface layer did not improve after the use of conservation measures in the 3 years of consecutive experiments. A positive correlation between soil quality indicators (SOM, TN, and AP) and FY was also noted. Therefore, they are considered factors improving pomelo productivity. These results are useful for the improvement of soil health as well as enhancing FY in citrus orchards cultivated in soils of the Mekong Delta, Vietnam. Thus, we recommend using cover crops with legumes or rice straw as the best option to increase soil fertility and FY. Further research is needed on the role of soil conservation practices in reducing soil compaction and nutrient loss under raised bed conditions.
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Funding information: The authors state that no funding is involved.
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Author contributions: N.N.H. and T.V.D. conceptualized and designed the experiment; L.V.D. conducted the experiments, collected, and analyzed samples; L.V.D., N.P.N., and T.V.D. analyzed the data and wrote the article – original draft; T.V.D., N.N.H., L.V.D., and N.P.N. reviewed, edited, and approved the final article. All authors have read and agreed to the published version of the article.
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Conflict of interest: The authors state no conflict of interest.
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Data availability statement: All data generated or analyzed during this study are included in this published article
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