Developing an index for phosphorus loss from sugarcane soils in Mauritius
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Date
2015-01
Authors
Mardamootoo, Tesha
Journal Title
Journal ISSN
Volume Title
Publisher
University of the Free State
Abstract
Sugarcane is the major crop cultivated in Mauritius and currently occupies some 53,500 hectares of land with an average cane productivity of 74 t ha-1. Phosphorus (P) fertilization on deficient soils can increase annual cane production up to 24 t ha-1. Currently the sugarcane industry consumes some 3,350 tonnes of P2O5 in the form of soluble mineral fertilisers with an additional 1,110 tonnes P2O5 coming from filter muds. While the application of P to agricultural land is essential in maintaining crop productivity, non-point sources of P leaving the agricultural landscape can cause accelerated eutrophication of surface waters, thereby impairing water quality. Although these losses may not always be of economic importance to farmers, only small amounts of P can trigger eutrophic conditions in freshwaters. Past studies in Mauritius have shown that runoff P concentrations of limnological significance (> 0.1 mg P L-1) can occur, particularly during high flow events. Hence, this study was initiated to provide a better understanding of P mobilization from cane fields and to integrate factors influencing P movement into a P risk assessment tool. To achieve these objectives, rainfall surface runoff simulations were conducted on 20 sites representing the main soils under which sugarcane is cultivated. The tests were done on runoff plots (2.1m by 0.75m in duplicate) at three slope categories (0 to 8%, 8 to 13%, 13 to 20%) and under three different rainfall intensities (50, 100, 150 mm hr-1) for a duration of 30 minutes each. In an attempt to evaluate the impact of continuous fertilizer application on the environmental soil P status of sugarcane soils in Mauritius, it was found that 0.01M CaCl2-P of surface soils was linearly correlated to dissolved runoff P losses (r2= 0.92). Thus simple laboratory soil extractions with 0.01M CaCl2 is a suitable estimate for dissolved P losses when field experimentation is not possible. Since the use of the routine agronomic soil test (0.1M H2SO4 soil extraction) for sugarcane in Mauritius provided a more accessible analytical tool for P management, the relationship between the 0.01M CaCl2-P and 0.1M H2SO4-P was established and it was found that soils with 0.1M H2SO4-P above 160 mg kg-1 can potentially impair runoff water quality. While soil testing provides a reliable pointer of desorbable P in surface soils, it gives no indication of its potential for transport during runoff and erosion.
Results from the simulation studies showed that runoff and erosion potential varied across the different soil types. It was also observed that with increasing rainfall intensities and field slopes, P mobilisation was enhanced due to increases in runoff and erosion rates. The results further showed that total runoff P was more strongly correlated with suspended sediments (r2=0.92) present in runoff waters than with runoff volume (r2=0.49) indicating that a greater proportion of the P transported in runoff occurred mostly as particulate P rather than dissolved P. Actually, about 90% of total P loss in runoff waters was mobilised in particulate forms regardless of field soil type, rainfall intensity and field slope. Using these research findings and historical data, the P index was developed to rank site vulnerability to P loss by accounting for source (dissolved P, particulate P, P application rate, method of application and application timing) and transport factors (soil erosion, surface runoff potential and precipitation) such that site-specific management practices can be implemented to critical source areas to minimize offsite P export. The proposed improved management practices to reduce P loss from fields in the P index include terracing, construction of diversions, field borders, field strips, grassed waterways, forest buffers and herbaceous cover. Sensitivity analysis and edge-of-plot field testing were used to assess the behavior and performance of the P index. The results indicated that further evaluations at a watershed scale would be more insightful about the strengths and weaknesses of the P index as a risk assessment tool. Besides, further evaluations of this tool will eventually lead to improvements in estimating the impacts of agricultural P management on downstream water quality.
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Keywords
Critical source areas, Diffuse pollution, Eutrophication, Phosphorus losses, Sensitivity analysis, Simulated rainfall, Soil erosion, Soil phosphorus testing, Surface runoff, Topography, Fertilizers -- Mauritius, Sugarcane -- Mauritius, Soils -- Phosphorus content -- Mauritius, Phosphatic fertilizers -- Mauritius, Thesis (Ph.D. (Soil, Crop and Climate Sciences))--University of the Free State, 2015