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.

Biochar has the potential for improving soil properties and supporting ecological functions, but it has negative impacts on soil organisms in some cases. This study aimed to assess the effect of biochar application at rates of 0 (B0), 5 Mg ha⁻¹ (B5), 20 Mg⁻¹ (B20), and 40 Mg ha⁻¹ (B40) on soil nematode community under upland and flooded conditions in a short-term microcosm experiment. After biochar application, soil was incubated for 2 to 8 weeks and nematodes were identified for community composition, trophic structures, functional guilds, maturity index and metabolic footprints. The chemical properties of the soils were also analyzed. General linear model revealed that biochar increased soil pH, EC, NO₃⁻-N, available phosphorus, total C, and C/N ratio, particularly in the highest application rate and shifted the composition of nematodes. The greatest abundances of omnivores (Mesodorylaimus, Thornenema), predator (Nygolaimus) and functional guilds of cp5 were observed in B5, resulting in greatest structure footprint and composite footprint, (omnivorous + predator) footprint and total biomass in B5. While abundances of nematodes tended to decrease with the biochar applicate rates, the abundance of Prismatolaimus was the highest in B40. During the 8-week incubation period, the abundances of Achromadora, Alaimus, Aporcelaimellus, Cryptonchus, Mononchus, and Tobrilus remained stable in upland conditions. Under flooded conditions, the abundances of almost all taxa were markedly lower than those under upland conditions irrespective of biochar application, except for Acrobeloides, Alaimus, Aphelenchoides, and Ditylenchus. We highlighted that 5 Mg ha⁻¹ of rice husk biochar can be the optimum in shaping the nematode community.

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.

Mung bean residues stimulate the hatching of soybean cyst nematode (SCN). In our previous study, combined incorporation of mung bean residues and biochar into soil can be effective in suppression of the soybean cyst nematode (SCN), Heterodera glycines, in the upper layer soil. However, there are no data available as to whether such effects are transmissible, and could for example be manifest in subsoil zones where such incorporation is confined to topsoils, via water-based pathways. We evaluated the effects of leachate passage from a biochar-amended soil in an upper soil zone to a lower zone in a microcosm-based system, upon a range of physicochemical properties and density of SCN. Disturbed soil was filled in a total of 9 cylindrical cores with two layers. The upper layer (0–15 cm) was amended with biochar at rates equivalent to 0, 0.3% or 1.8%, with bulk density set at of 1.1 g cm⁻³. The lower layer (15–25 cm) without biochar amendment was compacted to 1.2 g cm⁻³. Mung beans were grown for two weeks and incorporated into the upper layer. Water was surface-applied to the cores 4, 6, and 8 weeks after mung bean incorporation. After 16 weeks, the upper and lower layer soils were separately collected and assayed. The presence of biochar in the upper layer reduced the abundance of free-living nematodes, mainly bacterivorous, but increased that of a predator genus Ecumenicus in this zone. In the lower layer of soil under a biochar-amended upper layer, available P and soluble cations were increased as were abundances of total nematodes including Ecumenicus, resulting in greater maturity index, basal and structure indices. Notably, SCN density was decreased in lower zones by more than 90% compared to zero-biochar controls. This demonstrates that the effects of biochar upon soil properties, including impacts on biota and plant pathogens, are transmissible.