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The application of organic substrates can affect soil respiration, dehydrogenase (DH-ase) activity, dissolved organic carbon (DOC), and humic acid (HA) fractions. This study aimed to evaluate the effects of five organic substrates in the organic fractions of degraded alluvial soil, acid sulfate soil, and sandy soils and the physicochemical properties of the soil. Soil samples were amended at a rate of 5 tons ha−1 with (1) water hyacinth compost, (2) sugarcane filter cake compost, (3) biogas sludge-rice straw compost, (4) vermicompost, and (5) sludge. The results showed that soil respiration and DH-ase activity increased rapidly within the first 5 days of incubation, while the concentrations of DOC and HA decreased throughout the incubation period. The highest respiration and DH-ase activity occurred after the application of vermicompost. DOC was found to be the highest in soils amended with sugarcane. The highest concentration of HA was observed with the application of sugarcane residues, regardless of the type of soil. The application of water hyacinth and biogas sludge stimulated cumulative HA only in the acid sulfate soil, while vermicompost improved HA only in the degraded soil. The largest stimulation in respiration and DH-ase activity was observed in degraded and sandy soils, regardless of the type of amendment. In the acid sulfate soil (3.7 mg·C·g−1), larger amounts of DOC and HA were observed than in both degraded (1.7 mg·C·g−1) and sandy soils (1 mg·C·g−1). However, DH-ase activity was the lowest in acid sulfate soil.

This study investigated the effect of upland crop rotation on soil bacterial community in bulk soil and rice straw residues in the alluvial soils. Soil samples and rice straw residues in two crop rotation models including triple rice and rice-upland crops were collected for incubation under anaerobic and aerobic conditions. Data were analyzed from Denaturing Gradient Gel Electrophoresis band patterns. The results showed that the composition and diversity of communities colonizing the rice straw residues differed from those inhabiting the bulk soil. The bacterial community composition and diversity were only moderately affected by rice straw residues in the bulk soil. Especially, this study indicated that the composition of the bacterial community associated with the bulk soil and rice straw residues was dynamic in two incubation conditions (aerobic or anaerobic) and the different crop rotation models. The findings of this study demonstrated that the bacterial diversity was not affected by the difference in continuous paddy rice cultivation compared to the upland crop rotation system.

The rice crop intensification led to soil degradation and yield decline in the Mekong Delta, Vietnam. Therefore, it is necessary to classify soil fertility and identify the constraints for proper soil use. The soil legend classified by World Reference Based was converted to the Fertility Capability Classification system. The soil constraints and recommendations for appropriate services are suggested. The findings indicate seven primary soil categories, ten diagnostic layers, three diagnostic features, and three diagnostic materials in the rice soils of the Mekong Delta. Eleven soil constraints were found, most of which were associated with acid sulfate and saline soils. These constraints included low acidity, strong acidity, low available P, high P fixation, high potential Fe toxicity, slightly actual acid sulfate and shallow potential acid sulfate, deep potential acid sulfate, slight salinity, and strong salinity, low mineral supply capacity, limited organic carbon content, and low nutrient retention capacity. Reclamation of acid sulfate and saline soils by leaching soil toxicity and boosting soil nutritional status with organic matter and P, K treatment were recommended for degraded, acid sulfate, and saline soils.

Advocating proper phosphorus (P) fertilisation is necessary to save this limited natural resource and to save the investment in rice cultivation. This study aimed to evaluate changes in phosphorus availability, total phosphorus in soil, phosphorus buffering capacity, and phosphorus saturation in the long-term phosphorus fertilisation in the paddy rice system. Soil samples were collected in the harvest stage after seven consecutive crops over three years at Can Tho city, Vietnam. The applied phosphorus fertiliser rates were: no phosphorus fertilisation (P0), 17.4 kg P/ha (P17.4), and 26.2 kg P/ha as farmer's practice (P26.2). The results showed that the soil phosphorus buffering capacity in P0, P17.4 and P26.2 treatments was 9.49, 9.08 and 9.04 mg/kg, respectively. The degree of phosphorus saturation of P17.4 and P26.2 treatments ranged from 17.7% to 25.5%, showing the medium to high risk of phosphorus leaching. This study indicated that the application of phosphorus rate higher than 17.4 kg P/ha might result in the reduced soil phosphorus buffering capacity in the intensive rice cropping system in the Vietnamese Mekong Delta region. Our results implied that the application of a rate lower than 17.4 kg P/ha/crop could be extended to the other rice-growing (double/triple rice) areas in the Vietnamese Mekong Delta region or other paddy rice on alluvial soils in Asia.

Knowledge on bioavailable ammonium sensu strictu (i.e., immediately available for cellular uptake) in soil is required to understand nutrient uptake processes in microorganisms and thus of vital importance for plant production. We here present a novel ammonium biosensor approach based on the lithoautotrophic ammonia-oxidizing bacterium Nitrosomonas europaea transformed with a luxAB sensor plasmid. Bioluminescence-based ammonium detection was achieved within 10 min with a quantification limit in liquid samples of ∼20 μM and a linear response range up to 400 μM. Biosensor and conventional chemical quantification of ammonium in soil solutions agreed well across a range of sample and assay conditions. The biosensor was subsequently applied for a solid phase-contact assay allowing for direct interaction of biosensor cells with soil particle-associated (i.e., exchangeable plus fixed) ammonium. The assay successfully quantified bioavailable ammonium even in unfertilized soil and demonstrated markedly higher ratios of bioavailable ammonium to water- or 2 M KCl-exchangeable ammonium in anoxic soil than in corresponding oxic soil. Particle-associated ammonium contributed by at least 74% and 93% of the total bioavailable pool in oxic and anoxic soil, respectively. The N. europaea biosensor should have broad relevance for environmental monitoring of bioavailable ammonium and processes depending on ammonium bioavailability. [ABSTRACT FROM AUTHOR]