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.

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.