Nitrogen use efficiency for green onion (Allium fistulosum) in sands of South Central Coastal Vietnam using 15N-labelling

Truc T T Do1, Richard W Bell2, Nga P N Doan3, Surender Mann2

1 The Institute of Agricultural Sciences for southern Vietnam, 121 Nguyen Binh Khiem St., district 1, Ho Chi Minh city, 70000, Vietnam. http://iasvn.org. [email protected]

2 School of Veterinary and Life Sciences, Murdoch University, 90 South Street, Murdoch, 6150, Western Australia, Australia.

3 Center for Nuclear Techniques, Ho Chi Minh city, Vietnam.

Abstract

The recovery of nitrogen (N) from N fertilizer is generally poor. Increasing N use efficiency (NUE) on sands is particularly challenging due to low nutrient storage and high percolation rate of water through the root zone.  The objectives of the experiment were to assess the change in fertilizer NUE for onion with clay or sugarcane residue amendments, and using 15N labelled urea to determine the recovery of N in the plant-soil system in a deep sand. The experiments were carried out in Ninh Thuan province, Vietnam  under flood or sprinkler irrigation. The 15N-labelled urea was applied to 0.72 m2 mini-plots at 134 kg N/ha and 10.16 % N atom excess. Clay-rich soil or bentonite were applied to raise clay content to 25 g/kg  and sugarcane residues were applied at 30 t/ha.

Fertilizer supplied 47.5 to 50.5 % the onion N demand, however, this represented only 3.8 to 19 % of fertilizer N applied. Sugarcane residue was more effective than clay and bentonite additions while sprinkler irrigation increased NUE compared to flood irrigation. After harvesting onion, 19 to 24 % of fertiliser N was found in 0-20 cm top soil. Despite the increases in NUE with sugarcane residue, clay-rich soil or bentonite, 63 to 73 % of fertiliser N was lost from the soil-plant system. In addition to using sprinkler irrigation on sands, we suggest that adding clay and organic materials in combination with postponed application of N fertiliser or fertigation may be needed to further increase soil N retention and fertilizer NUE.