Assessing environmental phosphorus status of soils in Mauritius following long-term phosphorus fertilization of sugarcane

The intensive phosphorus (P) fertilization of sugarcane (Saccharum hybrid sp.) for more than 60 years has built up the P status of soils in Mauritius. While this accumulation of P is desirable from an agronomic perspective, there is growing concern about its possible impact on the quality of surface waters. To ensure no impairment of surface water quality by P from the soil in Mauritius, the critical soil P level which might cause an undesirable concentration of P (>0.03 mg L−1) in surface runoffs during rainfall events was determined using the rainfall simulation technique. The study showed a strong linear correlation (r2 = 0.92) between the flow weighted concentration of orthophosphate P in the runoff waters and the 0.01 M calcium chloride extractable P (0.01 M CaCl2-P) in the surface soils. It is desirable from a practical viewpoint that any assessment of environmental P risk be based on the same soil test that is routinely utilized in fertilizer recommendations for crop P requirements. The current agronomic soil P test in Mauritius involves an extraction with 0.1 M sulphuric acid (0.1 M H2SO4). The relationship between the 0.01 M CaCl2-P and 0.1 M H2SO4-P was best represented by a quadratic model (r2 = 0.87). More importantly it was found that the limit of 0.03 mg P L−1 (as total P) in surface runoffs, a concentration below which surface waters would remain uncontaminated by algal blooms would be exceeded if 0.1 M H2SO4-P is greater than 160 mg kg−1. In other words, this means that from an environmental viewpoint, soils with more than 160 mg kg−1 0.1 M H2SO4-P may impair surface water quality. Application of this soil critical P threshold showed that 28% of sugarcane fields in Mauritius had P levels above this environmental threshold. However as other variables such as field topography and cultural practices also influence P transport to waters, impairment of water quality should not be attributed solely to the high soil P status of the sugarcane soils.