Subsurface tile drainage systems with drainspacings of 15 m in 0.4 ha and 25 m in 3.2 ha wereinstalled at the farmers' field in 1986 and 1987,respectively, to study their effect on the reclamationof the coastal saline sodic clay soils. The system'sperformance in terms of the changing physical andchemical properties of the soil and rice yield wascontinuously monitored for a decade. Field datasuggested the possibility of adopting wider drainspacings and thus, drainage system with 35 and 55 mspacings was laid in 1997 in a 4 ha area. On theseinstallations the losses of NH₄ ⁺-N throughsub-surface drainage effluent were estimated. Thearea under 25 m drain spacing was the control with nocrops, fertilization and irrigation. Analysis ofwater samples collected daily for 10 days startingfrom 40 DAT from the drain laterals revealed thatthere were no trace of NH₄ ⁺-N in theeffluent from 15 and 25 m drain spacings. However,the effluent from 35 and 55 m spacings contained anaverage of 6.704 mg L⁻¹ and 4.205 mg L⁻¹ of NH₄ ⁺-N, respectively, before irrigation and2.438 and 1.650 mg L⁻¹ after irrigation. Themagnitudes of the losses of NH₄ ⁺-N duringthe crop season were 6.43 kg ha⁻¹ in 35 m spacingwith a drainage rate of 5.6 mm d⁻¹ and 2.14 kgha⁻¹ in 55 m spacing with a drainage rate of 3.5 mm d⁻¹. The rice yield was 6.5 Mg ha⁻¹ in15 m drain spacing where no ammonium losses throughsubsurface drainage effluent occurred. The rice yieldsunder 35 and 55 m drain spacings were 1.9 and 1.8 Mgha⁻¹, respectively. The poor yield was due tosignificant loss of ammonium form of nitrogen throughthe drainage effluent and lesser availability of totalnitrogen to the plants. The plant uptake of nitrogen in the unreclaimed area with 55 m spacing was half ofthat in the reclaimed area with 15 m spacing.

Most livestock wastewaters treated in constructed wetlands are typically rich in ammonium N. The objective of this study was to evaluate the soil-water ammonium distribution and the diffusive flux through the soil-water interface. Wetland system 1 (WS1) was planted to rush and bulrushes, and wetland system 2 (WS2) was planted to bur-reed and cattails. Nitrogen was applied at a rate of 2.5 g m-2 d-1. Interstitial soil water was sampled at 9, 24, 50, and 70 m from the inlet. In both wetlands, we found that NH4+ diffusion gradient and N losses were highest in the wetland system with lowest water depth. From other studies, we knew that shallower depths may have promoted a more effective interfacing of nitrifying and denitrifying environments. In turn, this N reduction in the water column may be the reason for steady NH4+-N upward diffusion fluxes. The assumed mechanism for N removal has been nitrification and denitrification but ammonia volatilization could also have occurred. Although diffusion may explain a significant portion of the material transport between the soil-water interface, the large differences in concentrations between outlet and inlet need further explanation.