Impact of Upstream Soil Nutrient Losses on Soil Fertility in Paddy Fields

This paper reports the results of a five-year ongoing research project examining nutrient balances in a 'composite swidden' agroecosystem, which integrates permanent wet-paddy rice fields in valley bottoms with rotating swidden plots on hillslopes. The study is being carried out in Tat hamlet, a small settle - ment of the Da Bac Tay ethnic minority in the mountains of Hoa Binh province, northern Vietnam. Nutrient flows (total and available nitrogen, phosphorus and potassium) from the upland to the paddy field were found to be directly proportional to the amount of rainfall and prevailing soil surface/crop - ping conditions. The flows increased between the years 2000 and 2001 due to high rainfall and then decreased sharply thereafter as the land was left fallow. On average, the inflow of nutrients in available forms to the three upper paddy fields was equivalent to additions of as much as 453, 58 and 1,455 kg/ha of nitrogen, phosphorus and potassium respectively. Potentially the amount of total nutrients supplied to the paddy fields could be even larger, but in practice farmers do not divert all upland discharge into the paddy fields. As a consequence of runoff and sediment from the uplands, the soil texture in the paddy fields was also altered. The net nutrient flow balance varied considerably between years. In the first year of testing nitrogen flow balances were positive for both total and available nitrogen but in subsequent years these balances became negative. Net flow phosphorus balances showed little variation. Potassium net flow balances, although mostly negative at first, had become positive by the last year. Thus, the data provided evidence for substantial amounts of nutrient inflow to the paddy field from runoff from upland fields as had been hypothesised, but also showed that nutrient losses, especially of potassium, were quite large and appeared to offset the inflow of nutrients. Water outflows also carried nutrients from applied manure and fertilisers, and it was not possible to distinguish these from the inflow nutrients from upland fields. Further studies and subsequent suitable management measures are required to prevent nutrient loss from paddy fields due to runoff flows.