Monitoring of Nitrogen Leaching from a Tropical Paddy in Thailand

A study on monitoring of nitrogen (N) leaching was conducted in acidsulfate clayey soil in the central region of Thailand. The objective was to measure the N leaching to shallow groundwater from a tropical paddy field to assess the consequences of agricultural management practices. Mass balance approach was used to estimate the contribution of nitrogen fertilizer to the water environment. The N inputs to and outputs from field were measured by direct method. Inputs of N to the site came from commercial fertilizer, precipitation, irrigation water and soils. Outputs of N from the site were leached to groundwater, harvested crops, in surface runoff, soils and loss from the field. Leaching loss was calculated from daily fluxes of water percolation and soil water N concentrations extracted by vacuum lysimeter. Based on three month observation, average leaching of nitrate nitrogen (NO3-N), ammonium nitrogen (NH4-N) and total Kjeldahl nitrogen (TKN) to groundwater was found 0.04, 0.11 and 0.17 kg had respectively. It was also observed that fertilizer application increased NO3-N concentration at five-fold in groundwater. Although, the measured NO3-N concentration in groundwater was below the threshold value of WHO drinking water standard (10 mg/l), such increment may pose a serious threat during the dry season when the groundwater recharge is very low. Furthermore, nitrogen mass balance result showed that loss of N inputs as outflow to the water and atmosphere were from the 19 % and 13.6% of total applied N respectively which indicates fertilizer input was responsible for water pollution. In this study, minor components such as groundwater input, mineralization and input from the atmosphere through biological fixation were ignored due to lack of available data where it should be considered.