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Article

Effects of Mung Bean Residue Return and Biochar Amendment Combined with Reduction in Inorganic Fertilizer on Rice Yield and Nutrient Uptake: A Case Study in Mekong Delta, Vietnam

by
Doan Thi Truc Linh
1,2,
Chau Minh Khoi
2,
Tran Van Dung
2,
Tran Huynh Khanh
2,
Nguyen Van Sinh
2,
Nguyen Thi Kim Phuong
2,
Huynh Mach Tra My
1,2 and
Koki Toyota
1,*
1
Graduate School of Bio-Applications and Systems Engineering, Tokyo University of Agriculture and Technology, Tokyo 184-8588, Japan
2
Faculty of Soil Science, Can Tho University, Campus II, Can Tho 900100, Vietnam
*
Author to whom correspondence should be addressed.
Agronomy 2025, 15(2), 278; https://doi.org/10.3390/agronomy15020278
Submission received: 24 December 2024 / Revised: 17 January 2025 / Accepted: 21 January 2025 / Published: 23 January 2025
(This article belongs to the Section Soil and Plant Nutrition)
Browse Figures
Versions Notes

Abstract

:
This study aimed to assess the co-incorporation of mung bean residue and rice husk biochar with reduced NPK fertilizer rates on rice yield and nutrient uptake in the subsequent rice crop. A field experiment was conducted in five treatments (T1 to T5). In the spring–summer (SS) of 2023, rice was cultivated and its straw was burned (T1), while mung bean was cultivated and its residue was incorporated (T2 to T5). In the next summer–autumn crop (SA), rice was cultivated with different levels of inorganic fertilizers. T1 was added with the conventional fertilizer (95 kg N, 45 kg P2O5, and 35 kg K2O ha−1). T2 included the same amount of NPK fertilizer as T1 with mung bean incorporation. In T3 to T5, rice husk biochar was amended at a rate of 10 Mg ha−1 before SA. In T3, inorganic fertilizers were reduced by 30% of N and 50% of P2O5 and K2O; in T4, by 15% of N, 30% of P2O5 and K2O; and in T5, by 15% of N only. The rice grain yield was 26.6–35.3% significantly higher in T3, T4, and T5 compared to T1. P accumulation in straw and grain was significantly higher in T3, T4, and T5 than in T1. Furthermore, K accumulation in grain was markedly higher in T3, T4, and T5. No significant differences were observed in any of the soil chemical properties among treatments at harvest in SA. This study highlights that the combination of residue incorporation and biochar may substitute a part of chemical fertilizers and contribute to more sustainable rice production.

1. Introduction

Vietnam has become one of the world’s biggest exporters of rice (Oryza sativa L.) and thus made an important contribution to global food security [1]. Under the intensive rice monoculture system, rice yield was stagnant and had even declined [2]. Moreover, continuous intensive rice cropping deprives a high level of nutrients from soils, hence causing soil nutrient deficiency [3]. For instance, Dobermann et al. [4] showed that the K balance was negative at most experimental sites, with an average net removal of 34–63 kg K season−1. Zhang et al. [5] found that imbalanced fertilization may influence nutrient uptake from the soil, directly impacting the crop yield.
Many previous studies, including our recent study, have shown that incorporating green manure significantly increases enzyme activities such as those of β-glucosidase, alkaline phosphatase, C mineralization, labile C, and exchangeable K in paddy soils [6,7]. In addition, the application of green manure to paddy soil increased the soil organic matter content and nutrient retention [8]. It is well known that leguminous green manure provides N to soils through biological N fixation and the conversion of organic N into mineral forms available for subsequent crops [9]. A previous study showed that groundnut residue of 5 Mg dry ha−1 improved subsequent rice production [10]. Despite the proven benefits of green manure in many regions such as South China [11], South Africa [12], and Kenya [13], this practice is not popular amongst smallholder farmers in Vietnam. For example, in mung bean cultivation, most farmers remove its stems and leaves after harvesting the seeds, in spite of the fact that leguminous crop residue incorporation could play a significant role in the supply of mineral N to the soil [14,15].
Rice husk is one of the most widely available agricultural wastes in many rice-producing countries of the world [16]. It serves as a nutrient-rich soil amendment, and notably, after undergoing combustion or pyrolysis, it is transformed into a readily available nutrient source [17]. In rice production, a sustainable approach is converting rice husk residue into biochar or ash and applying it back to paddy fields as a soil amendment. Phuong et al. [18] observed that rice husk biochar contained a large amount of P and K and its application provided soluble P and K which are available for crops. Our previous study also found that repeated application of rice husk biochar enhanced the available P and then the rice grain yield in acid paddy fields in Vietnam [19].
Nowadays, agricultural production involves substantial costs, particularly in the use of chemical fertilizers, which have a significant impact on greenhouse gas emission [20]. It is imperative to explore cost-effective alternatives, such as recycled agricultural residues like rice husk or green manure, with lower costs and less environmental impact. We hypothesized that the co-incorporation of mung bean residue and rice husk biochar would provide plant available N, P, and K in the soil and thus their incorporation may replace a part of the inorganic fertilizer requirement of the subsequent crop. The objective of this study was to assess the influence of the co-incorporation of mung bean and rice husk biochar combined with reduced inorganic fertilizer on the rice yield and nutrient uptake by a rice crop.

2. Materials and Methods

2.1. Materials Description

A field experiment was performed in Thoi Lai district, Can Tho city (N10.021424–E105.491912), which is a major rice-producing region in the Mekong Delta, Vietnam. The soil texture is clay with 0.3% of sand, 30.2% of silt, and 69.5% of clay. Before the start of the experiment, the soil was acidic (pHH2O = 4.6) and contained 0.62 mS cm−1 of EC, 27.2 g kg−1 of total C, 2.7 g kg−1 of total N, 23.4 mg kg−1 of NH4–N, 10.8 mg kg−1 of NO3–N, 18.7 mg kg−1 of available P (Bray 2), 0.057 cmolc kg−1 of exchangeable K, 6.21 cmolc kg−1 of exchangeable Ca, and 19.5 cmolc kg−1 of cation exchangeable capacity. These soil characteristics are generally fit for rice cultivation, since paddy rice has been cultivated for more than 20 years. However, the levels of inorganic N, available P, and exchangeable K were relatively low, which may limit crop productivity.
A commercial rice husk biochar was provided by Mai Anh Co., Dong Thap, Vietnam. Some properties of biochar were 7.7 pHH2O, 4.1 mS cm−1 of EC, 471 g kg−1 of total C, 4.72 g kg−1 of total N [18], 1.5 g kg−1 of labile C, and 7.4 g kg−1 of total K.

2.2. Experimental Designs

An experiment was conducted in two crop seasons: spring–summer (SS) from March to June 2023 and summer–autumn (SA) from June to September 2023. The field experiment was designed as a randomized complete block design, including 5 treatments with 4 replicated plots. Each plot covered an area of 12 m2 (3 m × 4 m). Buffer zones were 30 cm between plots and each plot was separated with plastic sheets (25 cm height) to avoid cross-contamination. The five treatments consisted of two rice crops (T1) in SS and SA and mung bean in SS and rice in SA (T2, T3, T4, and T5). In T3, T4, and T5, rice husk biochar was applied with different amounts of chemical fertilizers, as described below in Section 2.2.3 and in Figure 1.

2.2.1. Preceding Rice and Mung Bean Crops

In SS, rice was grown only in T1 and mung bean was grown in T2 to T5. In T1, Oryza sativa L. OM 380 seeds were directly sown at a rate of 150 kg ha−1. According to the local recommended dosage, 100% of the recommended inorganic fertilizer was surface-applied at rates of 95, 45, and 35 kg ha−1 for N, P2O5, and K2O, respectively, for the rice crop. During the rice cropping season, the soil remained submerged in water, and irrigation was halted 15 days before the harvest. On 10 June 2023, the aboveground biomass was harvested in triplicate from a subplot measuring 1 m × 1 m. The yields were 4.02 Mg dry ha−1 for rice grain and 5.26 Mg dry ha−1 for rice straw. Then, rice straw was burned on the field. According to farmers’ practices, rice straw incorporation in the field requires time for decomposition before sowing the next rice crop. However, this can delay the cropping schedule, where three rice crops are grown annually. Removing straw from the field also requires significant labor. Consequently, farmers often choose to burn the straw as a practical and time-saving solution.
In T2 to T5, mung bean seeds (‘DX-208’, Southern Seed Joint Stock Company, Ho Chi Minh City, Vietnam) were directly sown at a rate of 20 kg ha−1. A basal fertilization, equivalent to 30 kg N ha−1, 40 kg P2O5 ha−1, and 60 kg K2O ha−1, was surface-applied according to the local recommendation. The mung bean seed was cultivated to maturity to harvest the seeds’ beans. The aboveground parts of the mung bean plants were harvested on 14 May 2023 within the T2, T3, T4, and T5 treatments (192 m2 total) and they were separated into stems, leaves, and beans in pods. The total dry biomass of mung bean residue after removing the beans in pods was 1.6 Mg dry ha−1 and was incorporated as described below.

2.2.2. Residue Management During Pre-Rice Phase

In T1, in accordance with farmers’ practices, rice straw harvested in SS was burned on 12 June 2023. Subsequently, water was introduced into the rice field for 5 days. Following this flooding phase, the soil underwent ploughing as part of the preparation for planting the subsequent rice crop.
Mung bean residue (stems and leaves) was cut using a handheld lawn mower in T2, T3, T4, and T5 and incorporated into a depth of 0 to 10 cm using a hoe on 24 May 2023. All roots were also returned to their respective mung bean plots. However, mung bean residue was taken from triple subplots (1 m × 1 m) in T3, T4, and T5 and used for the measurement of moisture content, biomass yield, and chemical properties. The mung bean residue had a C:N ratio of 22:1 with 17 g N kg−1 and 373 g C kg−1, with 9.8 g kg−1 of labile C.

2.2.3. Cultivation and Harvest of Succeeding Rice in SA

In SA, a rice variety (OM 380) was sown on 24 June 2023 at a rate of 150 kg ha−1 for T1 to T5. Rice was cultivated with different amounts of inorganic and organic fertilizer application (Table 1). T1 received 100% of the recommended inorganic fertilizers (95 kg N, 45 kg P2O5, and 35 kg K2O ha−1), without mung bean and rice husk biochar (T1: 95N–45P2O5–35K2O). T2 underwent application of the same inorganic fertilizer dose as T1, with mung bean residue return (T2: 95N–45P2O5–35K2O + MB). Rice husk biochar was amended in T3, T4, and T5 at a rate of 10 Mg ha−1 with different amounts of inorganic fertilizers. Biochar was spread on the soil surface, and then thoroughly mixed in a depth of 0 to 10 cm using a hoe 4 days before sowing. In this study, 1.6 Mg of mung bean residue provided 28 kg of total N, 60 kg of total K2O, and 7 kg of total P2O5. In addition, 10 Mg of biochar provided of 47.2 kg of total N, 89.6 kg of total K2O, and 21.9 kg of total P2O5. Based on this nutrient contribution, different reduction rates of inorganic fertilizer were tested. For instance, in T3, inorganic fertilizers were reduced by 30% of N and 50% of P2O5 and K2O (T3: 67N–23P2O5–18K2O + MB + BC). In T4, inorganic fertilizers were reduced by 15% of N and 30% of P2O5 and K2O (T4: 81N–32P2O5–25K2O +MB + BC). In T5, inorganic fertilizers were reduced by 15% of only N (T5: 81N–45P2O5–35K2O + MB + BC). The application of inorganic fertilizer was carried out in the following: 20% of the total N was applied 7 days after sowing, 40% of the total N and 50% of the total K were applied 20 days after sowing, and the remaining N and K were applied 40 days after sowing. Urea (46% N) and potassium chloride (60% K2O) were used as the N and K sources, respectively. P fertilizer (superphosphate, 16% P2O5) was applied once before sowing. Manual weeding and the application of herbicides were conducted following the farmer’s conventional practices. Insecticides were applied according to the farmer’s practice to manage pests during the crop growth phase. Throughout the rice cropping season, the soil remained submerged in water, and irrigation was halted 15 days before the harvest. Rice was harvested on 19 September 2023.

2.3. Soil and Plant Sampling

2.3.1. Soil Sampling

Soil samples were collected at a depth of 0 to 15 cm during three distinct time points: (1) prior to SS (T1 to T5) on 12 March 2023, (2) around 10 days after straw burning (T1) and approximately one month after the incorporation of mung bean residue (T2 to T5) on 21 June 2023, and (3) at the harvest of SA (T1 to T5) on 19 September 2023. Five soil cores (diameter: 3 cm) were randomly taken from each plot and combined to create a composite sample. Additionally, undisturbed soil samples were obtained using 100 cm3 cores at the depth of 0–15 cm to measure bulk density. Subsequently, the soil samples were air-dried and sieved through a 0.5 mm screen for labile C, total C, total N, and a 2 mm screen for pH, EC, NH4-N, NO3-N, available P, CEC, exchangeable Ca, and K.

2.3.2. Plant Sampling

In SS, mung bean plants were sampled at maturity in triplicate from sub-plots of 1 m × 1 m in total area of T3, T4, and T5 treatments for the determination of biomass including stems and leaves. In SS, mung bean plants were sampled at maturity from three subplots of T3, T4, and T5 each consisting of 1 m × 1 m for the determination of biomass including stems and leaves. Plants were cut at the ground level and separated into straw and grain stems, leaves and beans in pods. Then, they were separately dried for 3 days in an oven at 60 °C to determine the aboveground biomass yield (stems and leaves). The dried samples were ground (<1 mm) with a crusher from Kinematica and used for total C, total N, total P, and total K measurements, as described below.
In SA, to determine rice grain and straw yields, plant samples were collected from an area of 5 m2 (2.5 m × 2 m) in each plot, oven-dried at 60 °C, and weighed for biomass. Then, all plant samples were ground (<1 mm) with the crusher and used for analysis of the total N, total P, and total K contents, as described below.

2.4. Analysis of Physical and Chemical Properties of Soil and Plant

Physical properties: Soil textural analysis was performed by the Robinson pipette method [21] and was classified using the United States Department of Agriculture/Soil Taxonomy Texture. Soil bulk density was determined by oven-drying undisturbed soil samples at 105 °C [22].
Chemical properties: Soil pH and EC with a ratio of 1:5 (soil:deionized water) were measured using a pH meter (Horiba pH, HORIBA Advanced Techno Co., Ltd., Kyoto, Japan) and an EC meter (Horiba EC, HORIBA Advanced Techno Co., Ltd., Kyoto, Japan), respectively. Soil pH and electrical conductivity (EC) were measured in a 1:5 (soil:deionized water) slurry using a Horiba pH meter and a Horiba EC meter, respectively. Total C and N were determined using a CN corder apparatus (MT-700, Yanaco Co., Tokyo, Japan). Labile C was extracted using 0.02 M KMnO4 solution for 2 min and measured with spectrometry by the method of [23]. Nitrate and ammonium were extracted using 2 M of KCl solution and measured with spectrometry at 220 nm and 650 nm, respectively [24]. The available P was determined by the Bray 2 method [25]. The effective cation exchangeable capacity was determined using un-buffered BaCl2 as an exchanging extracting reagent and subsequent re-exchange of Ba2+ with Mg2+ [26]. Furthermore, the obtained BaCl2 extracts were analyzed by flame photometry (Flame Photometers, BWB) for exchangeable K and Ca.
Analysis of N, P, and K in straw and grain rice: For rice gain and rice straw, 0.3 g of the plant sample was placed in an Erlenmeyer flask and mixed with 3 mL of a digestion solution which was composed of 18 mL of deionized water, 100 mL of sulfuric acid, and 6 g of salicylic acid. After further digestion with H2O2, the samples were analyzed to determine the total N, total P, and total K contents. The concentration of N and P was determined at 650 nm [24] and 880 nm [25], respectively. The K content was measured using an atomic absorption spectrophotometer.

2.5. Calculation of N, P, and K Accumulation in Rice Grain, Straw, and Total Biomass

N, P, and K accumulation in grain and straw was calculated by multiplying the N, P, and K contents with the straw and grain yield in kg ha−1. The total N, P, and K accumulation by whole biomass was obtained by summing up the accumulation by grain and straw and was expressed in kg ha−1.

2.6. Calculation of N, P, and K Balance

The apparent nutrient balance represents the difference between nutrient input entering the farming system and nutrient output removed by the crop in aboveground biomass. It was calculated in kg ha−1 crop−1 using the following equation [27].
Apparent nutrient balance = ∑N input − ∑N output
where N input represents the input nutrients of N, P, or K (kg ha−1 crop−1) supplied to the field through mung bean, biochar, and inorganic fertilizer. N output refers to the nutrient accumulation of N, P, or K (kg ha−1 crop−1) by total rice straw and grain biomass.

2.7. Statistical Analysis

A normality test was conducted on the data for every parameter. A t-test was conducted to compare the chemical properties of soil collected at two time points: (1) at the harvest of rice or mung bean during the spring–summer season and (2) 10 days after rice straw burning or 1 month after mung bean incorporation. Significant differences between treatments were determined by one-way ANOVA followed by Duncan’s test. All statistical analyses were performed using SPSS software (Version 20).

3. Results

3.1. Effects of Residue Management (Straw Burning and Mung Bean Residue Incorporation) in Spring–Summer Crop on the Soil Chemical Properties

One month after mung bean incorporation, a significant increase was only observed in exchangeable K (Table 2). Soil pH, EC, labile C, available P, inorganic N, and exchangeable Ca did not exhibit significant differences during this period. Likewise, these chemical properties remained unchanged after straw burning (Table 2).

3.2. Effects of Co-Incorporation of Mung Bean Residue and Rice Husk Biochar on Rice Grain and Straw Yields, Nutrient Accumulation, and Nutrient Balance in the Rice Crop in Summer–Autumn

No significant difference was observed in straw yield among the treatments (Figure 2). However, the rice grain yield was 32%, 26.6%, and 35.3%, significantly higher in T3, T4, and T5, respectively, than in T1. Similarly, the aboveground biomass was significantly higher by 22.2–28.7% in T3, T4, and T5 than in T1. No significant differences in rice grain yield and aboveground biomass were observed in T3, T4, and T5, in spite of the fact that the amounts of P and K fertilizers were 30% to 50% lower in T3 and T4 than in T5. Overall, the rice grain yield and total aboveground biomass were significantly increased by co-incorporation of mung bean residue and rice husk biochar with reducing chemical fertilizer.
The N accumulation in grain, straw, and aboveground biomass did not show significant differences among treatments. Compared with T1, P accumulation was significantly higher in T3, T4, and T5 in both straw (33.8–75.3%) and grain (33.4–60.5%) than that in T1. However, the P accumulation of total aboveground biomass was significantly higher in T3 and T5 by 52.4% and 59.1%, respectively, than that in T1. The K accumulation of grain in T3, T4, and T5 was higher by 99.2%, 55.9%, and 92,4% than that in T1. There was no significant difference in K accumulation in straw and total aboveground biomass among the treatments.
The nutrient balance analysis revealed distinct trends for N, P, and K across treatments. The N balance ranged from 1.02 to 59.4 kg N ha−1 crop−1, P balance from –13.2 to 36.6 kg P2O5 ha−1 crop−1, and K balance from –95.7 to 26.3 kg K2O ha−1 crop−1. T1, the control without mung bean or biochar, exhibited a net negative N balance, while T2, incorporating mung bean residues, improved the N balance relative to T1, although this difference was not statistically significant. The most positive N balance in T4 (Figure 3a) was significantly higher than that in T5. In terms of P, all treatments exhibited negative balances, with no significant differences among the treatments (Figure 3b). Noticeably, T1 and T2 had significant negative balances due to insufficient K2O inputs, while T3, T4, and T5 significantly improved the balances with mung bean incorporation and biochar amendment, compared to T1 and T2.

3.3. Effects of Co-Incorporation of Mung Bean and Rice Husk Biochar on Chemical Properties of Soil at the Harvest of Rice Crop in Summer–Autumn

The soil pH across treatments ranged from 5.08 to 5.28, indicating slightly acidic conditions (Table 3). Soil pH, EC, labile C, available P, inorganic N, exchangeable K, and exchangeable Ca contents were not significantly affected by treatments at the end of the experiment.

4. Discussion

4.1. Response of Rice Yield to Mung Bean Residue Return and Rice Husk Biochar Amendment Combined with Reducing Inorganic Fertilizer

The total aboveground and grain rice biomass significantly increased in mung bean return and biochar amendment combined with a reducing inorganic fertilizer, compared with only inorganic fertilizer. Furthermore, under mung bean and rice husk biochar amendment, the treatment using 40% of N and 50% of P2O5 and K2O fertilizers did not result in a significant reduction in grain yield compared to the use of 80% of N and 100% of P2O5 and K2O. These results indicate that the co-incorporation of mung bean residue and rice husk biochar may replace a half of conventional chemical fertilizers. By contrast, only mung bean residue incorporation did not significantly enhance rice yield. Similar results have already been reported in the co-incorporation of rice straw and green manure [6,28] or of rice straw biochar and green manure [29]. We did not test the effect of only rice husk biochar application on the subsequent rice yield; however, previous studies have already indicated that the application of rice husk biochar at a rate of 10 Mg ha−1, the same rate as this study, did not enhance rice grain yield [18,30]. Therefore, our study emphasizes the importance of adequate organic amendments, such as mung bean residue and biochar, in enhancing rice yield. In summary, 1.6 Mg of mung bean residue provided 28 kg of total N, 60 kg of total K2O, and 7 kg of total P2O5. In addition, 10 Mg of biochar provided of 47.2 kg of total N, 89.6 kg of total K2O, and 21.9 kg of total P2O5. According to Xie et al. [29], the incorporation of legumes into cropping systems can significantly enhance N availability for subsequent crops. Hegde et al. [30] reported that the incorporation of a leguminous crop as green manure enhanced soil nutrition via the fixing of atmospheric N.
Our previous study highlighted that mung bean incorporation significantly improved enzyme activities and exchangeable K [7,31]. Moreover, this study showed that mung bean incorporation increased exchangeable K in the soil after 1 month (Table 1). Therefore, these amendments, when combined with reduced inorganic fertilizers, offer a sustainable approach to improving crop productivity without significantly reducing rice yield. For instance, the slow release of organic N in green manure would enable a greater fraction to be taken up by the crop and thus reduce N losses through leaching and run off, which could cause consequent contamination of waterways and aquifers. This strategy can reduce environmental impacts and contribute to long-term soil health.

4.2. Response of Nutrient Accumulation and Apparent Balance to Mung Bean Residue Return and Rice Husk Biochar Amendment Combined with Reducing Inorganic Fertilizer on Sustainable Agricultural Practice

The mung bean residue return (T2) or co-incorporation of mung bean and biochar combined with 15% (T4 and T5) and 30% (T3) reducing N fertilizer did not have a significant effect on N accumulation in either rice grain or straw compared with only inorganic fertilizer. This outcome aligns with a previous study that found rapid N mineralization of green manure, resulting in similar N accumulation compared to 100% N inorganic [32]. Furthermore, Linh et al. [19] showed that N in biochar amendments might not play a major role in the increased grain yield. Similarly, Xie et al. [33] reported that low bio-availability of biochar N did not make a significant impact on rice production or N nutrition of rice. In fact, the total N input across treatments exceeded the total N output, and no significant differences were observed in soil inorganic N levels at harvest (Table 3). Thus, excluding the N contribution from biochar, mung bean could replace 30% of N from inorganic fertilizer without affecting N accumulation in rice grain. This aligns with sustainable agricultural practices by promoting the use of crop residue, which can mitigate environmental risks associated with excess chemical fertilizer application.
Regarding P, the highest P accumulation in total rice grain and straw was found in treatments incorporating mung bean residue and rice husk biochar amendments with 100% inorganic P (T5). Under the same conditions with mung bean residue and rice husk biochar amendments, there was no significant difference in P accumulation between treatments with 100% inorganic P and those with a 30% and 50% reduction in inorganic P. This finding is corroborated by Win et al. [34], who reported an increased P accumulation by biochar amendment and inorganic fertilizer compared with only inorganic fertilizer. As discussed above, 10 Mg of biochar and 1.6 Mg of mung bean residue provided 22 and 7 kg P2O5, respectively. In addition, our previous study showed that mung bean incorporation combined with biochar amendment increased alkaline phosphatase activity in paddy soil [7], which led to an influence on the hydrolysis of organic phosphate compounds, with a consequent increase in the available P in the soil and improved nutritional quality status of plants [35,36]. Noticeably, a negative P balance was observed in all treatments where P input was lower than P accumulation. The remaining available P content in the soil did not differ significantly among treatments; therefore, the negative P balance among the treatments may be due to high accumulation by crop biomass. This finding highlights the importance of not reducing P fertilizer application, even when using mung bean residue and biochar amendments. Our previous study [19] demonstrated that rice husk biochar application increased the rice yield via increasing the amount of available P, although the paddy field was amended with the conventional amount (60 kg P2O5 ha−1 crop−1) of P fertilizer. These results may indicate the importance of maintaining a P supply to productively sustain Vietnamese acid paddy soils with low P availability.
In terms of K, rice grain K accumulation was significantly higher under mung bean residue and rice husk biochar amendment compared to only inorganic fertilizer. Interestingly, the highest K accumulation in rice grain was observed in the treatment with a 50% reduction in inorganic fertilizer. These results were consistent with a finding from a previous study [37]. Similarly, biochar was found to contain exchangeable K, making it a promising alternative to chemical K fertilizer, as reported by previous studies [18,38,39]. Win et al. [34] found that K accumulation in straw was significantly increased by biochar amendment in paddy soil as compared with that in the treatment without biochar. Our study indicated that mung bean and rice husk biochar provided 60 and 90 kg K2O, respectively, which could partially replace chemical K fertilizer. Furthermore, Zhang et al. [18] found that rice husk biochar improved exchangeable K in soil and K accumulation in rice biomass. Noticeably, K balance was positive in all treatments except T1 and T2, with the highest K balances observed under the combined use of mung bean residues and rice husk biochar and reduced K inorganic fertilizer (T3, T4, and T5). Our result were consistent with previous studies [4,40], in which a negative K balance was observed under long-term chemical fertilization in triple-rice- and double-rice-based cropping systems. Therefore, these results underline the potential of biochar in improving K use efficiency, particularly in low-input systems, while also emphasizing the importance of adequate K fertilization for maintaining nutrient balance in intensive cropping systems.

5. Conclusions

The co-incorporation of mung bean residue and biochar significantly improved rice yield compared with only inorganic fertilizer. Moreover, this practice allowed for a 30% reduction in N and a 50% reduction in P2O5 and K2O applications without compromising rice yield. The co-incorporation of mung bean residue and biochar contributed significantly to P and K accumulation, showing a comparable performance to only inorganic fertilizer treatments. However, the reduced P inputs resulted in further negative P balances. Therefore, further studies are essential to assess overall economic efficiency and to monitor long-term soil fertility in a low-input system like 50% reduction in P2O5 for ensuring sustainable agricultural practices.

Author Contributions

D.T.T.L. conceptualized the study; T.H.K., D.T.T.L. and H.M.T.M. conducted the experiments and collected samples; D.T.T.L., N.V.S., N.T.K.P. and H.M.T.M. performed sample analysis and data analysis; D.T.T.L. and K.T. validated data; D.T.T.L. wrote the original draft of the paper; D.T.T.L., C.M.K., T.V.D., T.H.K., N.V.S., N.T.K.P., H.M.T.M. and K.T. reviewed, edited, and finalized the paper. All authors have read and agreed to the published version of the manuscript.

Funding

This study was funded by JICA scholarship project “Human Resources Development in science Technology and Innovation” (JFY 2020) to the first author.

Data Availability Statement

Data are contained within the article.

Acknowledgments

We sincerely thank the farmer Nguyen Thanh Nhan at Thoi Lai district, Can Tho city, and the two students Nguyen Thanh Dat and Pham Van Luong from the Faculty of Soil Science, for their support during the field experiments.

Conflicts of Interest

The authors declare that they have no conflicts of interest.

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Agronomy 15 00278 g001
Figure 1. Experiment design in Thoi Lai district, Can Tho city, Vietnam. MB: mung bean incorporation, BC: rice husk biochar.
Figure 1. Experiment design in Thoi Lai district, Can Tho city, Vietnam. MB: mung bean incorporation, BC: rice husk biochar.
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Agronomy 15 00278 g002aAgronomy 15 00278 g002b
Figure 2. Total aboveground biomass (a), nitrogen accumulation (b), phosphorus accumulation (c), and potassium accumulation (d) in grain and straw of rice plants cultivated under co-incorporation of mung bean and rice husk biochar combined with reducing inorganic fertilizer. Error bars indicated the standard deviation of four replications (n = 4). Different uppercase letters indicated a significant difference of total aboveground nutrient or biomass among treatments by Duncan’s test at p < 0.05; Different lowercase letters on bars denote a significant difference of biomass or nutrient accumulation of straw and grain among the treatment by Duncan’s test at p < 0.05. T1: 95N–45P2O5–35K2O kg ha−1, without mung bean return and rice husk biochar amendment; T2: 95N–45P2O5–35K2O kg ha−1 plus mung bean return; T3: 67N–23P2O5–18K2O kg ha−1 plus mung bean return and rice husk biochar amendment; T4: 81N–32P2O5–25K2O kg ha−1 plus mung bean return and rice husk biochar amendment; T5: 81N–45P2O5–35K2O kg ha−1 plus mung bean return and rice husk biochar amendment.
Figure 2. Total aboveground biomass (a), nitrogen accumulation (b), phosphorus accumulation (c), and potassium accumulation (d) in grain and straw of rice plants cultivated under co-incorporation of mung bean and rice husk biochar combined with reducing inorganic fertilizer. Error bars indicated the standard deviation of four replications (n = 4). Different uppercase letters indicated a significant difference of total aboveground nutrient or biomass among treatments by Duncan’s test at p < 0.05; Different lowercase letters on bars denote a significant difference of biomass or nutrient accumulation of straw and grain among the treatment by Duncan’s test at p < 0.05. T1: 95N–45P2O5–35K2O kg ha−1, without mung bean return and rice husk biochar amendment; T2: 95N–45P2O5–35K2O kg ha−1 plus mung bean return; T3: 67N–23P2O5–18K2O kg ha−1 plus mung bean return and rice husk biochar amendment; T4: 81N–32P2O5–25K2O kg ha−1 plus mung bean return and rice husk biochar amendment; T5: 81N–45P2O5–35K2O kg ha−1 plus mung bean return and rice husk biochar amendment.
Agronomy 15 00278 g002aAgronomy 15 00278 g002b
Agronomy 15 00278 g003
Figure 3. Effect of co-incorporating mung bean and rice husk biochar combined with reducing inorganic organic fertilizer on nutrient balance of nitrogen (a), phosphorus (b), and potassium (c) in rice crop of summer–autumn. Different lowercase letters on bars denote a significant difference of nutrient balance among the treatment by Duncan’s test at p < 0.05. T1: 95N–45P2O5–35K2O kg ha−1, without mung bean return and rice husk biochar amendment; T2: 95N–45P2O5–35K2O kg ha−1 plus mung bean return; T3: 67N–23P2O5–18K2O kg ha−1 plus mung bean return and rice husk biochar amendment; T4: 81N–32P2O5–25K2O kg ha−1 plus mung bean return and rice husk biochar amendment; T5: 81N–45P2O5–35K2O kg ha−1 plus mung bean return and rice husk biochar amendment.
Figure 3. Effect of co-incorporating mung bean and rice husk biochar combined with reducing inorganic organic fertilizer on nutrient balance of nitrogen (a), phosphorus (b), and potassium (c) in rice crop of summer–autumn. Different lowercase letters on bars denote a significant difference of nutrient balance among the treatment by Duncan’s test at p < 0.05. T1: 95N–45P2O5–35K2O kg ha−1, without mung bean return and rice husk biochar amendment; T2: 95N–45P2O5–35K2O kg ha−1 plus mung bean return; T3: 67N–23P2O5–18K2O kg ha−1 plus mung bean return and rice husk biochar amendment; T4: 81N–32P2O5–25K2O kg ha−1 plus mung bean return and rice husk biochar amendment; T5: 81N–45P2O5–35K2O kg ha−1 plus mung bean return and rice husk biochar amendment.
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Table 1. Amounts of N, P, and K input via inorganic fertilizer, mung bean residue, and rice husk biochar in different treatments.
Table 1. Amounts of N, P, and K input via inorganic fertilizer, mung bean residue, and rice husk biochar in different treatments.
TreatmentsTotal N (kg N ha−1)Total P (kg P2O5 ha−1)Total K (kg K2O ha−1)
Inorg.MBRHBTotalInorg.MBRHBTotalInorg.MBRHBTotal
T1950095450045350035
T2952801284570523560095
T36728471422372252186090168
T48128471563272261256090175
T58128471564572274356090185
Inorg., inorganic fertilizer; MB, mung bean residue; RHB, rice husk biochar.
Table 2. Effect of straw burning and mung bean residue incorporation on the chemical properties of soil taken at harvest of rice or mung bean in spring–summer season (1st) and at 10 days after rice straw burning or at 1 month after mung bean incorporation (2nd).
Table 2. Effect of straw burning and mung bean residue incorporation on the chemical properties of soil taken at harvest of rice or mung bean in spring–summer season (1st) and at 10 days after rice straw burning or at 1 month after mung bean incorporation (2nd).
CropsTimepHH2O (1:5)EC (1:5)Labile
C		P Bray 2Inorganic NExchangeable KExchangeable Ca
mS cm−1g kg−1mg kg−1mg kg−1cmolc kg−1cmolc kg−1
Rice1st4.530.640.9719.027.00.076.2
2nd4.500.640.9719.127.60.076.3
T-testnsnsnsnsnsnsns
Mung bean1st4.880.590.9519.426.50.096.8
2nd4.800.570.9417.117.50.206.6
T-testnsnsnsnsns***ns
*** p < 0.001; ns, not significant.
Table 3. Effect of co-incorporating mung bean and rice husk biochar combined with reducing inorganic fertilizer on some chemical properties of soil after rice harvesting in summer–autumn.
Table 3. Effect of co-incorporating mung bean and rice husk biochar combined with reducing inorganic fertilizer on some chemical properties of soil after rice harvesting in summer–autumn.
TreatmentpH
(H2O)		EC
(1:5)		Labile
C		Bray 2
P		Inorganic NExchangeable KExchangeable Ca
mS cm−1g kg−1mg kg−1mg kg−1cmolc kg−1cmolc kg−1
T15.08 ± 0.100.23 ± 0.051.01 ± 0.1512.7 ± 5.627.5 ± 4.00.183 ± 0.0286.37 ± 0.44
T25.13 ± 0.100.19 ± 0.041.12 ± 0.0616.5 ± 7.929.1 ± 5.00.134 ± 0.0086.43 ± 0.37
T35.13 ± 0.260.30 ± 0.101.05 ± 0.0516.5 ± 5.932.4 ± 4.20.199 ± 0.0416.23 ± 0.25
T45.28 ± 0.220.22 ± 0.041.03 ± 0.0616.0 ± 5.428.1 ± 2.00.130 ± 0.0276.11 ± 0.20
T55.10 ± 0.320.29 ± 0.141.08 ± 0.0614.2 ± 4.128.6 ± 1.60.162 ± 0.0126.33 ± 0.43
Mean ± standard deviation (n = 4). T1: 95N–45P2O5–35K2O kg ha−1, without mung bean return and rice husk biochar amendment; T2: 95N–45P2O5–35K2O kg ha−1 plus mung bean return; T3: 67N–23P2O5–18K2O kg ha−1 plus mung bean return and rice husk biochar amendment; T4: 81N–32P2O5–25K2O kg ha−1 plus mung bean return and rice husk biochar amendment; T5: 81N–45P2O5–35K2O kg ha−1 plus mung bean return and rice husk biochar amendment.
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Linh, D.T.T.; Khoi, C.M.; Dung, T.V.; Khanh, T.H.; Sinh, N.V.; Phuong, N.T.K.; My, H.M.T.; Toyota, K. Effects of Mung Bean Residue Return and Biochar Amendment Combined with Reduction in Inorganic Fertilizer on Rice Yield and Nutrient Uptake: A Case Study in Mekong Delta, Vietnam. Agronomy 2025, 15, 278. https://doi.org/10.3390/agronomy15020278

AMA Style

Linh DTT, Khoi CM, Dung TV, Khanh TH, Sinh NV, Phuong NTK, My HMT, Toyota K. Effects of Mung Bean Residue Return and Biochar Amendment Combined with Reduction in Inorganic Fertilizer on Rice Yield and Nutrient Uptake: A Case Study in Mekong Delta, Vietnam. Agronomy. 2025; 15(2):278. https://doi.org/10.3390/agronomy15020278

Chicago/Turabian Style

Linh, Doan Thi Truc, Chau Minh Khoi, Tran Van Dung, Tran Huynh Khanh, Nguyen Van Sinh, Nguyen Thi Kim Phuong, Huynh Mach Tra My, and Koki Toyota. 2025. "Effects of Mung Bean Residue Return and Biochar Amendment Combined with Reduction in Inorganic Fertilizer on Rice Yield and Nutrient Uptake: A Case Study in Mekong Delta, Vietnam" Agronomy 15, no. 2: 278. https://doi.org/10.3390/agronomy15020278

APA Style

Linh, D. T. T., Khoi, C. M., Dung, T. V., Khanh, T. H., Sinh, N. V., Phuong, N. T. K., My, H. M. T., & Toyota, K. (2025). Effects of Mung Bean Residue Return and Biochar Amendment Combined with Reduction in Inorganic Fertilizer on Rice Yield and Nutrient Uptake: A Case Study in Mekong Delta, Vietnam. Agronomy, 15(2), 278. https://doi.org/10.3390/agronomy15020278

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