Ammonium losses through subsurface drainage effluent from rice fields of coastal saline sodic clay soils

Subsurface tile drainage systems with drain spacings of 15 m in 0.4 ha and 25 m in 3.2 ha were installed at the farmers' field in 1986 and 1987, respe ctively, to study their effect on the reclamation of the coastal saline sod ic clay soils. The system's performance in terms of the changing physical a nd chemical properties of the soil and rice yield was continuously monitore d for a decade. Field data suggested the possibility of adopting wider drai n spacings and thus, drainage system with 35 and 55 m spacings was laid in 1997 in a 4 ha area. On these installations the losses of NH4+-N through su b-surface drainage effluent were estimated. The area under 25 m drain spaci ng was the control with no crops, fertilization and irrigation. Analysis of water samples collected daily for 10 days starting from 40 DAT from the dr ain laterals revealed that there were no trace of NH4+-N in the effluent fr om 15 and 25 m drain spacings. However, the effluent from 35 and 55 m spaci ngs contained an average of 6.704 mg L-1 and 4.205 mg L-1 of NH4+-N, respec tively, before irrigation and 2.438 and 1.650 mg L-1 after irrigation. The magnitudes of the losses of NH4+-N during the crop season were 6.43 kg ha(- 1) in 35 m spacing with a drainage rate of 5.6 mm d(-1) and 2.14 kg ha(-1) in 55 m spacing with a drainage rate of 3.5 mm d(-1). The rice yield was 6. 5 Mg ha(-1) in 15 m drain spacing where no ammonium losses through subsurfa ce drainage effluent occurred. The rice yields under 35 and 55 m drain spac ings were 1.9 and 1.8 Mg ha(-1), respectively. The poor yield was due to si gnificant loss of ammonium form of nitrogen through the drainage effluent a nd lesser availability of total nitrogen to the plants. The plant uptake of nitrogen in the unreclaimed area with 55 m spacing was half of that in the reclaimed area with 15 m spacing.