Integrated soil and water management in acid sulphate soils : balancing agricultural production and environmental requirements in the Mekong Delta, Viet Nam

The objectives of this study in the Mekong delta, Vietnam, were: (1) to obtain a better understanding of the effects of soil physical properties and flow types on solute transport in ASS emphasing aluminum; (2) to quantify environmental hazards resulting from amelioration activities in acid sulphate soils (ASS); and (3) to identify measures which can increase agricultural production and reduce negative environmental side effects.This study was conducted on ASS in the Mekong delta, Vietnam and consisted of 5 experiments starting in the dry season and ending at the end of the flood season. All experiments were carried out under field conditions. Transport of soluble aluminum was investigated for different types of water flow, which are typical for each season such as capillary rise, bypass flow and runoff.During the dry season, soluble aluminum was accumulated in topsoil layers by capillary rise. Effects of land management methods on accumulation of aluminum was the main focus in this period: Plowing (P1) and mulching (Ml), compared with non-plowing (P0) and non-mulching (M0). Experiments were conducted in lysimeters, and under field conditions. In both experiments, topsoils were treated with P1M1, P1M0, P0M1, and P0M0. Three levels of ground water (GWL: 30, 60, and 90 cm below the ground surface) were maintained in the undisturbed soil columns in the lysimeters. Aluminum accumulation increased with increased evaporation. Under field conditions, where ground water levels were monitored but not controlled, mulching treatments gave a significantly lower aluminum accumulation as compared with the non-mulching treatments, whereas plowing did not result in a significant decrease of this accumulation. Rainfall during the first 3 weeks of the rainy season caused the ground water to rise rapidly while its aluminum concentration increased. This increased the soluble aluminum concentrations in the topsoils and eliminated the leaching effects of earlier land management practices.During the rainy season, the study was focused on aluminum transport with bypass flow and runoff in and on raised beds, which are constructed by soil materials excavated from adjacent lateral ditches with the objective to avoid flooding and to enhance leaching of soil. This is a very common technique to grow upland crops in ASS. Therefore, a better understanding of leaching processes in raised beds is needed to properly assess management options for ASS. Three types of raised beds, which are commonly constructed in the Mekong delta, were studied. In the low raised beds only topsoil material was used to construct the bed. In the high type both top soil and the jarosite layer were used. In the "traditional" raised beds, pyritic material was also found on top of the beds. The amount of runoff increased with cumulative rainfall due to a decrease of infiltration rates and saturated hydraulic conductivities. Due to surface crusting, traditional beds gave the highest runoff amounts among the three types. Concentrations of aluminum in bypass flow were consistently higher than in runoff In low and high beds, amounts of aluminum in bypass flow were also higher than in runoff, whereas in traditional bed-types it was slightly lower. However, the negative impacts on the surrounding surface water was not significantly different for the three types of beds. Therefore, the low bed type is the most desirable from an agricultural production point of view, because less effort is needed in construction.Pore system distribution can play a very important role in determining water flows in and on the raised beds and as a consequence, on the effectiveness of leaching toxic substances. Thus, field and laboratory studies were carried out to quantify the effects of soil physical properties and bypass flow on leaching processes of new, 1-year old and 2-year old raised beds for yam and pineapple cultivation. Water-conducting pores were characterized using Methylene Blue. Number, area, and perimeter of water- conducting pores at 10-cm depth intervals of six 1 x 1m subplots were investigated. Undisturbed 20 cm x 25 cm soil cores were subjected to three 30 mm h -1rains in 30 minutes. Volume, aluminum and sulphate concentration of outflows were monitored. Due to consolidation, the area and perimeter of water-conducting pores in 2-year old pineapple beds had decreased to about one third, and bypass flow rates to about 80% of those in newly constructed beds. Consolidation, however, did not affect macropore network geometry in yam beds because they were subjected to annual tillage and yam tubers were uprooted regularly. Al and SO42-concentrations in the outflows of newly constructed and 1-year old raised beds were higher in pineapple, while those in 2-year raised beds were higher in yam.A side effect of leaching of ASS may be the pollution of surrounding waters. In order to obtain a proper assessment of this problem, the concentration and the amount of aluminum in water leaching from ASS during cultivation of rice, pineapple and yam were investigated. The fields have been reclaimed for 2 months, 1 and 2 years, respectively. Pineapple and yam were cultivated on raised beds. Values of pH in drainage water ranged from 2.9 to 3.9 and aluminum concentration from 3 to 13 mmol(+) l -1. Mean monthly aluminum concentrations in the water discharged from pineapple and yam raised beds was about 3 times higher than from rice fields. Monthly total amount of aluminum released by the raised beds could be as high as 16,690 mol ha -1, and was 3 to 5 times higher than that from rice fields. Consolidation and crust formation in pineapple beds reduced the concentration and amount of aluminum released as the beds grew older. In June, leaching from ASS was most hazardous to the environment due to a combination of highest total aluminum released to the canal network and a relatively low river discharge.At the flood recession period, the effectiveness of flood water (in combination with harrowing) in flushing out toxic substances from the top soil of ASS was investigated. Three experimental sites with slight, moderate and strong acidity were selected. Treatments were the number of harrowings (one: H1 and three times: H3) and the number of flushings (one: F1 and three times: F3).Three times harrowing in combination with three times flushing was the most effective in leaching acid in the most acid soils. When flushing once, the number of harrowings had no effect. The quantity of aluminum adsorbed on the soil exchange complex was not affected by different harrowing and flushing treatments. F3H3 also gave a significantly higher rice yield as compared with other treatments. The flood recession period is the most appropriate moment for flushing topsoils for rice cultivation, which has a high water requirement.