The Mekong Delta is the most important rice‐ and shrimp‐producing region for food and economic security in Vietnam. Rice‐shrimp farming is practised where salinity fluctuates substantially between wet and dry seasons. Research points to several potential risk factors for rotational systems, but how these link directly to both rice and shrimp production remains poorly quantified for systems that stock and harvest animals year‐round. We examined water and soil quality of 18 rice‐shrimp‐crab ponds, in which shrimp and crab are grown in both wet and dry seasons, in the Cà Mau Province of Vietnam. Multiple lines of evidence indicated that environmental conditions experienced by both rice and shrimp were suboptimal and contributed to low yields and survival. Year‐round cropping of shrimp and crab was associated with sustained suboptimal salinity, intensified by drought, for the wet‐season cultivation of rice. Although rice seedlings were sown in all 18 ponds, only three had a harvestable crop. Low shrimp production and survival was associated with sustained suboptimal water temperatures (too high), salinity (too high in the dry season and too low in the wet season) and dissolved oxygen concentrations (too low). Food availability and quality may also have affected shrimp production. Improving productivity of rice‐shrimp‐crab ponds in the study region may require (1) separation of rice and shrimp crops and improving efficiency of soil washing practices such that salinity conditions are more suitable for each when grown, and (2) management intervention to increase oxygenation of water, and the availability and quality of food for shrimp.

A field experiment was conducted to study the effects on yield and soil properties of the continuous application of rice straw compost to an alluvial soil in the Mekong Delta, Vietnam. Fourteen rice crops, two crops per year, were grown by direct seeding of the crop. There were seven treatments: F0C) (no fertilizer and compost), F0C+ (compost with no fertilizer), F20C+, F40C+, F60C+, F80C+ (20, 40, 60 and 80% of the fertilizer application rate with compost, respectively), F100C) (full strength fertilizer application as N:P 2 O 5 :K 2 O at 100:30:30 kg ha )1 in the dry season and 80:30:30 kg ha )1 in the wet season without compost). Compost prepared from the rice straw was applied at 6 Mg ha )1 (fresh weight) at the cultivation of each crop. The F0C+ treatment showed higher yield than the F0C) treatment by 0.68 Mg ha )1 on average in the wet season, but not in the dry season. The rice yield in F0C) declined at a rate of 0.163 Mg ha )1 year )1 in the wet season, but there was no decline in rice yield in F0C+. In treatments with compost, the yield reached its plateau at F40, suggesting that compost could replace part of the fertilizer. Although there were no significant differences in the total C concentrations in the soil among the treatments, even after 12 consecutive crops, soil penetration resistance appeared to be lower with compost than without compost. This long-term field trial showed that the continuous application of rice straw compost has some positive effects on rice yield as well as on soil physical properties.

The aim of this study was to isolate and select salt tolerant bacteria having both functions in phosphorus solubilization and phosphatase synthesis from rice-shrimp farming soil in saline areas in Mekong River Delta of Vietnam. Phosphorus solubilizing bacteria were isolated on National Botanical Research Institute's Phosphate (NBRIP) agar medium containing 1% NaCl and the activity of phosphatase enzyme was determined by disodium p-nitrophenyl phosphate reagent method at a wavelength of 420 nm. The result showed that from 15 saline soil samples, a total of 95 strains of phosphorus solubilizing bacteria were isolated and 19 of them showed their good phosphorus solubilization. The results about phosphatase activities of these 19 strains illustrated that TBT5-3 bacterial strain was the highest phosphatase producing strain with an amount of 0.377 U/mL after 10 days of incubation. This strain showed its best phosphatase producing capacity when cultured in the liquid culture medium containing pH 5, 1% NaCl, glucose and urea under the shaking speed of 120 rpm. Based on 16S rRNA gene sequence analysis this phosphatase synthesizing bacterial strain was genetically identified as species of Bacillus sp. TBT5-3 since 100% of this train sequence is affiliated with Bacillus megaterium.

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