Soil salinity may damage crop production. Besides proper management of irrigation water, salinity reduction can be achieved through soil amendment. The objectives of this study were to evaluate the effects of rice husk biochar and compost amendments on alleviation of salinity and rice growth. Field experiments were conducted at two salt-affected paddy rice fields located in distinct sites for five continuous crops. Treatments, with four replicates, consisted of continuous three rice crops per year (RRR), two rice crops rotated with fallow in spring–summer crop (FRR), FRR plus compost at 3 Mg ha⁻¹ crop⁻¹ (FRR + Comp), and biochar at 10 Mg ha⁻¹ crop⁻¹ (FRR + BC). Salt contents and hydraulic properties of soils, plant biomass, and plant uptake of cations were investigated. Soil bulk density (BD), exchangeable sodium (Na⁺), and exchangeable sodium percentage (ESP) were reduced remarkably by biochar application. Biochar application significantly increased other soil properties including total porosity, saturated hydraulic conductivity (Ksat), soluble and exchangeable potassium (K⁺), K⁺/Na⁺ ratio, available P, and total C. Compost application also improved BD, total porosity, and available P, but not exchangeable Na⁺ and ESP. Total aboveground biomass of rice showed a trend of FRR + BC > FRR + Comp > FRR > RRR. Relatively higher K⁺ uptake and lower Na⁺ uptake in rice straw in FRR + BC resulted in a significant two times higher K⁺/Na⁺ ratio over other treatments. Our results highlight that biochar amendment is a beneficial option for reducing ESP and providing available K⁺ and P under salinity-affected P-deficient conditions, hence improving straw biomass.

Acid sulfate soil (ASS) has major problems related to phosphorus deficiency and high potential for N₂O emissions, as well as strong acidity. The objective of this study was to evaluate the effects of rice husk biochar and compost on P availability and greenhouse gas (GHG) emissions in ASS in in vitro incubation studies. An ASS was amended with two types of rice husk biochar (at rates of 0 g kg⁻¹, 20 g kg⁻¹, and 50 g kg⁻¹, equivalent to 0 Mg ha⁻¹, 20 Mg ha⁻¹, and 50 Mg ha⁻¹, assuming that bulk density was 1 g cm⁻³ and evenly applied for 10 cm in depth) and compost (at rates of 0 g kg⁻¹, 10 g kg⁻¹, and 20 g kg⁻¹, equivalent to 0 Mg ha⁻¹, 10 Mg ha⁻¹, and 20 Mg ha⁻¹) and incubated. Application of compost increased labile P by 100% and 200% at rates of 10 g kg⁻¹ and 20 g kg⁻¹, respectively. Both biochars showed an increase in NaHCO₃-soluble inorganic P by 16% to 30%, decreases in NaOH-soluble inorganic P and NaHCO₃-soluble organic P. N₂O emissions were significantly decreased by 80% by a biochar with a higher surface area and higher NH₄⁺ adsorption capacity at a rate of 50 g kg⁻¹ as compared with those in un-amended soil. In contrast, compost amendment at a rate of 10 g kg⁻¹ significantly increased N₂O emission by 150%. These results suggest that in ASS, whilst compost is more effective in improving P availability, biochar is more effective in mitigating GHG emissions, emphasizing that fundamental characteristics of organic amendments influenced the outcomes in terms of desirable effects.

Given that rice husk biochar has been shown to modulate salinity in salt-affected acid soils, the objective of this study was to investigate the effects of organic amendment of salinized acid soils on P fractions, enzyme activities, and associated rice yield. Four treatments, viz. Rice–Rice–Rice, [RRR]; Fallow–Rice–Rice, [FRR]; Fallow–Rice–Rice + 3 Mg ha⁻¹ of compost [FRR + Comp]; and Fallow–Rice–Rice + 10 Mg ha⁻¹ of biochar [FRR + BC] were established at Ben Tre and Kien Giang sites, Viet Nam, over six consecutive crops. Soil properties at harvest of the sixth crop showed that there were diverse patterns of fractionation between P forms with respect to treatment. Overarchingly, biochar increased labile and moderately labile inorganic P and organic P by 30% to 70%, respectively, whilst compost had a relatively modest effect on these pools. Soil phosphatase activities at crop tillering increased following the FRR + Comp and FRR + BC treatments compared with those in RRR, except for acid phosphatase at Ben Tre. At harvest, there were no significant differences between the enzyme activities among the treatments. Rice yield was positively correlated with the more labile forms of P, soil C, and acid phosphatase activity. In the absence of organic amendments, there was no effect of triple versus double rice crops being grown in one-year cycle. Repeated application of biochar (10 Mg ha⁻¹ × 5 times) showed potential to increase grain yields and total soil C in salt-affected acid soils, via modulation of P transformations to more plant-available forms.

Avoidance of intensive rice cultivation (IRC) and soil amendments are potential practices to enhance soil properties. There is only limited information on the effects of reduced IRC and its mixture with compost or silicate fertilizer (Si) on the soil nematode community in salt–affected soils. This study aimed to assess the shifts of soil nematode community by reducing a rice crop from triple rice system (RRR) to a double rice system and mixed with compost or Si in paddy fields in acid sulfate soil (ASS) and alluvial soil (AL) in the Mekong Delta, Vietnam. Field experiments were designed with four treatments in four replicates, including RRR and a proposed system of double–rice followed by a fallow (FRR) and with 3 Mg ha^–1 crop⁻¹ compost or 100 kg ha^–1 crop⁻¹ Si. Soils were collected at harvest after the 2 year experiment, reflecting the fifth and third consecutive rice crop in RRR and FRR system, respectively. Results showed that reduced IRC gave a significant reduction in abundance of plant–parasitic nematodes (PPN), dominated by Hirschmanniella and increased abundance bacterivorous nematodes when mixed to compost and silicate fertilizer in ASS. In addition, reduced IRC increased nematode biodiversity Hill’s indices and reduced herbivorous footprint in ASS. Proposed system having compost or Si had strongly increased in bacterivorous and omnivorous footprints. Particularly, reduced IRC mixture with Si increased abundance of Rhabdolaimus, Mesodorylaimus and Aquatides, metabolic footprints (structure footprint, bacterivorous, omnivorous and predator) and diversity Hill’s N1 index in ASS. Our results highlighted that reduced IRC was a beneficial practice for decreasing abundance of PPN in salt-affected soils and increasing abundance of FLN in ASS. IRC mixture with compost or Si had potential in structuring the nematode communities with increasing biodiversity, trophic structure, and metabolic footprints.

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.

Vietnam's Mekong River Delta (MRD), where rice is the dominant crop, is increasingly impacted by salinity intrusion, highlighting the need for alternative cropping options. This study evaluated the growth and yield performance of quinoa, beetroot, and maize under three irrigation salinity levels (0, 2 and 4 g/L), with and without rice straw mulch (7 t/ha), in greenhouse conditions representative of the MRD dry season. Agronomic traits, physiological parameters, and changes in soil, including electrical conductivity (ECe), soluble sodium (Sol-Na +), and exchangeable sodium percentage (ESP), were assessed. Results showed that quinoa demonstrated the greatest salinity tolerance, maintaining stable growth and yield under 4 g/L saline irrigation and soil ECe exceeding 15 dS/m. Beetroot's yield was not significantly different under 2 g/L saline irrigation with straw mulching. Maize was highly sensitive to salinity and environmental stress, failing to complete its growth cycle under high heat and humidity, even in non-saline conditions. Across treatments, rice straw mulching significantly reduced soil ECe, Sol-Na + , and ESP, and improved crop performance under saline irrigation. Overall, quinoa and beetroot, especially when combined with mulching, offer promising alternatives for dry-season cropping in saline-prone areas of the MRD. In contrast, maize cultivation requires improved soil and environmental management under such conditions.