Sugarcane (Saccharum officinarum), is one of the world’s largest crops.
Worldwide sugarcane occupies an area of 26 million ha with a total production of 1833 million metric tons (FAO). The world demands for sugar and ethanol are the primary drivers of sugarcane cultivation. Considering the importance in global agriculture and the water demands of the crop special attention to improve water productivity and the sustainability of sugar cane plantations is urgently needed to cope with the consequences of climate change and predicted water scarcity.
Freshwater availability
Although sugar cane is C4 –Plant with a high photosynthetic efficiency, it still needs about 1500-2000mm/ha/year. The WWF Report “Agricultural Water Use and River Basin Conservation” considered the impact of water use by global agricultural commodities in nine large river basins and points out that sugar cane is one of the four thirstiest crops which stand out for priority attention. Together, cotton, rice, sugar cane and wheat account for 58% of the world's irrigated farmland. Sugar cane is a deep-rooted crop, which remains in the soil all year round and is able to extract soil water to depths well below one meter and can influence river flows as it intercepts run-off from the catchment into rivers and taps into ground water resources (WWF 2005). Several severe impacts of sugar cane cultivation on freshwater availability are known and presented in the mentioned WWF Report.
In the Indian state of Maharashtra for example, sugar cane covers just three percent of the land but uses 60 percent of the state irrigation supply and is a cause of substantial groundwater withdrawals. Sugarcane cultivation is the largest water consumer in the Indus Basin, Pakistan. There the construction of dams and irrigation systems lead to a 90 percent reduction in the amount of freshwater reaching the Indus Delta with tremendous consequences for the mangrove forest, leading to a decline in fish and shrimp which contribute to the livelihoods of thousands.
Soil degradation
Soil degradation caused by erosion and compaction is among the major problems linked to sugarcane cultivation. Erosion rates up to 30 t of soil/ha/yr for sugarcane fields in Sao Paulo state are reported in Martinelli (2008). Sugarcane is currently grown on many steep slopes and hillsides, leading to high rates of soil erosion resulting from the increased rates of water runoff on sloping land. It is recommended that cane should not be grown on slopes greater than eight percent (WWF 2003).
The used management practices in sugar cane fields lead to extensive areas of bare soil which are highly vulnerable to erosion processes, mostly during initial process of land use conversion when grasses are killed to prepare for the planting of sugarcane, then again in the period between crop harvesting and regrowth. Soils remain bare for several months when sugarcane stalks are replaced with new ones every 5–6 years.
The use of agricultural machinery results in soil compaction and destroys soil physical properties such as porosity and density, which in turn decreases water infiltration and further enhances soil erosion (Martinelli 2008).
Deterioration of aquatic systems
The sedimentation of wetlands, streams and reservoirs is a negative effect of the high erosion rates in sugarcane fields. Reservoirs loss water-holding capacity and contaminants like pesticides are transported through the aquatic system. For example, sugar cane farming contributes to a sediment export rate of 180,000 tonnes of fine sediment/year in the lower South Johnstone River in Australia (WWF 2003).
Nitrogen pollution
Pollution of aquatic and marine ecosystems from nitrogen (N) fertiliser is an issue facing many agricultural regions around the world. The overuse of nitrogen fertilizers is typical in sugar cane fields. Excess nutrients accumulate in the environment, and because of the high mobility of N, much of the excess is transported to aquatic systems.
The Increased application of N fertilisers in Australia has led to acidification, contamination of ground and surface water and enhanced greenhouse gas emission. Elevated concentrations of dissolved inorganic N have been detected in rivers draining to the Great Barrier Reef and are one of the reasons for the declining coral cover (Webster et al. 2012, Martinelli 2008).
Environmental consequences of sugarcane burning
Sugarcane burning is a common crop management practice. In many sugar producing countries, the cane fields are burnt immediately before harvesting for easier cutting, post-harvest cultivation and pest control. Sugarcane burning increases soil temperature, decreases soil water content, microbial activity, bulk density and, consequently, leads to soil compaction, higher surface water runoff, and soil erosion. Sugarcane burning was determined as the most likely source of air pollution in the state of Sao Paulo, Brazil. Substantially elevated levels of carbon monoxide and ozone in the atmosphere have been found around sugarcane fields.
Sugar Cane and Climate Change – Impacts of Sugar Cane Plantations in Brazil
Benefits of increasing sugar-cane area through the expansion into existing agriculture are described in Loarie et al. 2001. After quantifying the direct climate effects of sugar cane expansion in Brazil, they conclude that “in settings where demands on food security do not preclude biomass agriculture, conversion of crop and pasture land to sugar cane leads to substantial cooling effects by altering surface albedo and evapotranspiration and reinforces the indirect climate benefits of this land-use option”. On the other hand, sugar-cane production is nitrogen-intensive and emissions of the greenhouse gas nitrous oxide are increased especially in environments with high temperatures and soil water content (Thorbun et al. 2010; Martinelli et al. 2008).
Recommendations for sustainable sugar cane production
- Efficient Irrigation Systems (Drip, sprinkler and alternate furrow irrigation)
The majority of Sugar Cane Production needs some form of irrigation. Considering that the highest yields are reported from desert countries with good irrigation infrastructure, improving the water – productivity in irrigation schemes is crucial to meet the challenges of rising water scarcity in the future. Inman Bamber 2005 points out that commonly used irrigation criteria for sugarcane (in Australia) could lead to a greater water use than necessary. Water application in sugarcane cultivation in the Amaravathy River Basin, India, was 28 percent higher than the recommended levels reports the WWF (2003). The implementation of efficient irrigation systems to increase Water Use Efficiency is crucial to tackle those problems.
- Breeding programs to increase crop water use efficiency:
As the ENSO Variability is predicted to increase and many growing regions of sugar cane are under ENSO-influences, the question arises whether developing drought tolerant and water use-efficient sugarcane varieties may reduce possible productivity losses. Inman-Bamber (2005) gives an overview of the sugar cane water relations and points out where knowledge must be strengthened. Some detailed work on 13C discrimination indicated that variation in transpiration use efficiency could be exploited in sugarcane breeding programs, and clear benefits appear from breeding cultivars with improved drought-tolerance and water use efficiency. The work on water-stress thresholds for irrigation is promising for reductions in water use and sugar cane quality. Inman-Bamber 2012 conducted an assessment of the impact of specific plant traits on biomass yield to support breeding programs. The best traits to consider for selection of sugarcane clones in water-limited environments in the tropics and subtropics are increased rooting depth, increased intrinsic water use efficiency and reduced conductance leading to increased transpiration efficiency.
Further recommendations include:
The adoption of management practices to reduce Nitrogen losses in surface water plays a major role in combating the degradation of ecosystems. Webster et al. 2012 gives an example of lower N management practices compared to the conventional fertilizer N application rate to sugarcane in the catchment of the Great Barrier Reef. Moreover, Reducing N fertilizer applications would help to reduce N2O emissions.
- Biological control and Integrated Pest Management
- Proper planning (IWM), Environmental risk assessments
- Improvement of land-use practices, Anti-Erosion measures
- Protection of streams and riparian ecosystems
- Reduction of sugarcane burning practices (Martinelli et al. 2008)
References
- Crutzen, P. J. et al (2008): N2O release from agro-biofuel production negates global warming reduction by replacing fossil fuels. Atmos. Chem. Phys., 8, 389–395
- Inman-Bamber, N.G., Lakshmananb, P., Park, S. (2012): Sugarcane for water-limited environments: Theoretical assessment of suitable traits. Field Crops Research 134 95–104
- Inman-Bamber, N.G., Smith , D.M. (2005): Water relations in sugarcane and response to water deficits. Field Crops Research 92 185–202
- Loarie, S. R et al. (2011): Direct impacts on local climate of sugar-cane expansion in Brazil. Nature Clim. Change 1, 105–109
- Martinelli, L. A., Filoso, S. (2008): Expansion of sugarcane ethanol production in Brazil: Environmental and social challenges. Ecological Applications, 18(4) 885–898
- Thorburn, P. J. et al. (2010): Using the APSIM model to estimate nitrous oxide emissions from diverse Australian sugarcane production systems. Agriculture, Ecosystems and Environment 136, 343–350
- Webster, A.J. et al. (2012): Reducing dissolved inorganic nitrogen in surface runoff water from sugarcane production systems. Marine Pollution Bulletin 65 128–135
- WWF (2003): Agricultural Water Use and River Basin Conservation, A summary report compiled and edited by Tim Davis, DJEnvironmental, UK
- WWF (2005): Sugar and the Environment. http://wwf.panda.org/?22255/Sugar-and-the-Environment-Encouraging-Better-Management-Practices-in-Sugar-Production-and-Processing#
