A thesis submitted in partial fulfillment of the requirements for the degree of Master of Engineering

Thesis (M.Eng.) - Asian Institute of Technology, 2002

An assessment of N dynamics was conducted in the central region of Thailand. The objective of study was to assess the N mass balance, nitrate leaching to groundwater and N loss to the atmosphere from a tropical paddy. The study period was 87 days. Daily measurement of evapotranspiration, ponding depth, irrigation water, drainage water and precipitation were performed in the field. From the water balance, percolation water was determined. N fertilizers were applied at three different doses and total amount was 100 kg N ha-1. Vacuum lysimeters were installed to collect the soil water solution. The soil and water samples were collected at different time intervals and N concentrations were measured. N dynamics was analyzed daily to determine the relationship between daily N available and N loss to the atmosphere and water. It was observed that loss of N through the emission into the atmosphere and outflow to the water from the field corresponded to 36.08% of total applied N. This indicates that more than one third of fertilizer input is responsible for environmental pollution if the same conditions prevail. Decision-makers are recommended to formulate strategy and plan to reduce the loss of N. The observed average N03-N leached to the groundwater was 0.04 kg ha-1 d- 1. It was also observed that fertilizer application induced a five-fold increase of N03- N concentration. This indicates that groundwater aquifers under paddy fields are vulnerable due to N03-N leaching.