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Nitrogen Leaching from Soils Cropped with Sugarcane under the Humid Tropical Climate of Mauritius, Indian Ocean¹

ABSTRACT

The significance of N leaching in the N economy of sugarcane (Saccharum sp.) and as a hazard to the environment in Mauritius was not known with certainty. A lysimeter study was therefore initiated to investigate leaching of N from soils planted with hybrid sugarcane and fertilized with either ¹⁵N-labeled (NH₄)₂SO₄ or NaNO₃ at the rate of 100 kg N/ha. The results obtained showed that the amount of N leached by any one rainfall of sufficient magnitude to give rise to percolates depended more upon the duration and intensity of the drying step preceeding the rainfall than on the leachate volume. In consequence, N leached from any one soil by an outburst of rain was greater at the drier site (Reduit, 1550 mm rain/yr) than at the wet site (Belle Rive, 3700 mm rain/yr). However, the cumulative N loss over 1 yr was not invariably higher at Reduit than at Belle Rive because frequent though shorter and less intense drying and wetting cycles may be cumulatively just as effective in mobilizing soil N than the less frequent but longer and more intense drying at Reduit. In addition, at both sites more N was leached from the soil with a higher organic matter content.

Though the soils were able to retain NO₃^– by adsorption, the mobility of NO₃^– in the soils used was primarily reduced by biological immobilization. As a result, in spite of an annual rainfall exceeding 3000 mm at Belle Rive, N leaching was not more extensive than that reported for soils of the temperate regions. Instead, cations such as K⁺ and Ca²⁺ were more readily leached than N. This draws attention to the fact that the need for management practices to attenuate leaching losses of cations from soils located in a climatic environment similar to that of Mauritius may be more acute than the need for measures to minimize N leaching.

Key Words: nitrate adsorption • biological immobilization • cation leaching • environmental pollution

¹ Research supported by the Int. Atomic Energy Agency through research contract 1822/GS.

² Scientific Officer and Head, respectively, Agric. Chem. Div., Sugar Industry Res. Inst., Reduit, Mauritius.

Received for publication October 11, 1983.