A nutrient balance analysis of the sustainability of a composite swiddening agroecosystem in Vietnam's Northern Mountain Region

Abstract

This paper reports some results of the first three years of an on-going research project on nutrient balances in a composite swidden agroecosystem. In a composite swiddening system, households simultaneously cultivate both swidden fields on hill slopes and paddy fields in the valleys. The study is being carried out in Ban Tat, a small settlement of Da Bac Tay ethnic minority people in Hoa Binh Province in Vietnam's Northern Mountain Region. Nutrient inputs and outputs for a swidden field on a hill slope and a wet rice field in the valley below it were recorded and nutrient balances for each subsystem calculated. The swidden had large negative balances for N, P, and K in all years of the experiment whereas in the paddy field only K was in serious deficit. Nutrients lost from the swidden field constituted a major source of inputs into the paddy field. This suggests that the sustainability of wet rice agriculture in the valleys is heavily dependent on interactions with the hill slopes. At the level of the composite swidden system as a whole, nutrient inputs and outputs were not in balance. Outputs of N exceeded inputs by 159 kg/ha and outputs of K exceeded inputs by 867 kg/ha. Only P showed a positive balance with inputs exceeding outputs by 220 kg/ha. Comparison of the nutrient balances for the swidden and the wet rice field revealed that the negative imbalance of nutrient inputs to outputs was much greater in the swidden field than in the paddy field. Thus, if the farmers at Ban Tat relied exclusively on swiddens to meet their food requirements, their agricultural system would be much less sustainable than it is now. This confirms the main hypothesis of this research that it is the fact that most households simultaneously cultivate both wet rice fields and swiddens that explains the relatively high sustainability of the land use system in Ban Tat.