Monitoring of Nitrogen Leaching from a Tropical Paddy in Thailand

B. K Pathak, F. Kazama, I. Toshiaki


A study on monitoring of nitrogen (N) leaching was conducted in acid- sulfate 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 (NO-
3-N), ammonium nitrogen (NH4
+-N) and total Kjeldahl nitrogen (TKN)
to groundwater was found 0.04, 0.11 and 0.17 kg ha-1d-1 respectively. It was also observed
that fertilizer application increased NO-
3-N concentration at five-fold in groundwater.
Although, the measured NO-
3-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.

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