Nitrogen and phosphorus transport by surface runoff from a silty clay loam soil under sugarcane in the humid tropical environment of Mauritius

The off-site movement of nitrogen (N) and phosphorus (P) has been extensively investigated in soils of the temperate region. Studies under the high rainfall and different soil conditions in the humid tropical countries are however, few and non-existent for sugarcane (Saccharum hybrid sp.), an important crop in the humid tropics. To close that gap in the existing knowledge, the transport of N and P by surface runoff was studied in Mauritius over a 2-year period (1998-1999) in a silty clay loam soil having slopes ranging from 5 to 12% and receiving more than 3000 mm rain annually. The data obtained from five runoff events in a nested catchment, comprising a 500 m(2) plot and 2.5, 13 and 54 ha subcatchments, showed that transformation and dissipation of the inorganic N and P rapidly reduced the concentration of mineral N and P in the runoff soil extraction zone. This, coupled with the long time lag (6 weeks) between fertilizer application and occurrence of the first runoff resulted in insignificant loads of P and N being moved from the plot and subcatchments during each runoff event. Consequently, the total N and P transported by surface runoff over the study period were agronomically not important. While less than I kg total P ha(-1) was lost, the N load transported off-field by surface runoff ranged from 2 to 7 kg ha(-1) only. The transport of N and particularly P was intimately linked to that of sediment in the runoff water. Although insignificant from an agronomic viewpoint, the N and P moved could raise N and P concentrations in runoffs to levels above the values generally proposed as guidelines for degraded waters and is therefore an environmental concern. All the data obtained in the present study therefore showed that N and P movement by surface runoff from soils in the humid tropics is not different from movement in temperate region soils. Innovative and practical measures adopted for minimizing N and P contamination of water from agricultural lands in temperate environment should thus, be equally effective when adopted in the humid tropics. (C) 2002 Elsevier Science B.V. All rights reserved.