There is a growing concern about the risk of contamination of waters, soils and agricultural products, in the rapid urbanising areas in Southeast (SE) Asia due to heavy, or inappropriate, use of organic wastes, fertilisers, pesticides, and poor quality irrigation water. In the EU-funded RURBIFARM project, ‘Sustainable farming at the rural-urban interface -an integrated knowledge based approach for nutrient and water recycling in small-scale farming systems in peri-urban areas of China and Vietnam', the use and recycling of nutrients in organic and mineral fertilizers and irrigation waters in small-scale vegetable based peri-urban farming systems will be evaluated and provide consumer and producer organizations and local government with relevant science to help them find and negotiate solutions. Several factors, including crop product quality, long term soil quality and fertility, pollution and potential pollution of surface waters and groundwater will be used for the evaluation.

In this paper we will present the results from a case study on nutrient (N, P and K) and trace metal (Cu and Zn) cycling in two peri-urban villages, located up streams and one downstream Hanoi City, Vietnam. In the study, input and output flows have been monitored and quantified at field level during 12 months (Sept 2003-August 2004). One site was located in Bang B village, Hoang Liet commune, Thanh Tri district, 10 km south of Hanoi centre (N 20° 57,17'- 20° 57,41', E 105° 49,04'-105° 49,44'), downstream of industries and urban settlement, where mainly wet vegetables are cultivated. The other site was in Phuc Ly village, Minh Khai commune, Tu Liem district, 10 km west of Hanoi centre (N 21° 04,375', E 105° 45,170'), where dry vegetables are the main crop. The two sites have different sources of irrigation water, Bang B receives wastewater from Hanoi City and in Phuc Ly the irrigation water originates from the Red River. At each of study sites, two plots were chosen that have different history in terms of cropping and types of vegetables that are grown.

Figure 1. Inputs and outputs of N and P, kg ha^-1 yr^-1, for the different plots at the Bang B (BB) and Phuc Ly (PL) villages. The number above the bar is the input - output net balance.

The net balance for N showed a surplus from 85 to 882 kg ha^-1 yr^-1, and for P and K the surplus ranged between 109-196 and 20-306 kg ha^-1 yr^-1, respectively (Fig. 1). The mass balances for Cu and Zn varied from 256-2763, and 1873-8927 g ha^-1 yr^-1, respectively. In Bang B village input via irrigation water was significant for all elements, except P where mineral P fertilizer was the most importance source. It was a significant difference in the amount of irrigation water being applied to the study plots related to differences in altitude on the field which highly influenced the inputs of N, K, Cu and Zn. In the dry vegetable systems in Phuc Ly, mineral fertilizers were the main N and P sources whereas organic fertiliser accounted for the major inputs of K, Cu and Zn. The nutrient use efficiency was significantly higher for N and K compared with P. The high net balances for the study elements indicate a high risk for soil accumulation as well as risks for losses to the external environmental. The study showed that wastewater irrigation can contribute to high inputs, but that excess use of organic and mineral fertilisers can be a major threat to the soil and water environment.