The potential of a salt-tolerant plant (Distichlis spicata cv. NyPa Forage) to treat effluent from inland saline aquaculture and provide livestock feed on salt-affected farmland

Abstract

Dryland salinity is a major problem affecting food production from agricultural land in Australia and throughout the world. Although there is much interest in using saline groundwater to grow marine fish on salt-affected farmland, the disposal of nutrient enriched, saline aquaculture effluent is a major environmental problem. We investigated the potential of the salt-tolerant NyPa Forage plant (Distichlis spicata L. Greene var. yensen-4a) to trap nutrients from saline aquaculture effluent and subsequently to provide a fodder crop for livestock. Sub-surface flow wetlands containing NyPa Forage were constructed and their efficacy in removing total nitrogen, ammonia, nitrite/nitrate, total phosphorus and orthophosphate was monitored under different levels of nutrients and salinity. The wetlands removed 60 -90% of total nitrogen loads and at least 85% of ammonia, nitrite/nitrate, total phosphorus and orthophosphate loads, with greater efficiency at high nutrient and low salinity levels. The above-ground yield, sodium, crude protein (CP) and in vitro dry matter digestibility (DMD) of NyPa Forage plants were measured after fertilisation with different nutrient levels and cropping at different frequencies. Yield of plants increased with increased nutrient, while nutritive value was greater when nutrients were applied but did not differ among nutrient levels. Yield was not affected by cropping frequency, but nutritive value was greatest when plants were cropped at intervals of 21 or 42 days. At optimum nutrient addition and cropping levels, the plants had a mean CP content of 16.7 % and an in vitro DMD of 67.6 %, equivalent to an energy value of 9.5 MJ kg -1 . Assuming an equivalent fibre content and voluntary food intake as grass hay, and no accumulation of other toxic minerals, these nutritive values would be sufficient for maintenance or moderate liveweight gains in dry adult sheep or cattle. Keywords:salinity; inland saline aquaculture; effluent; constructed wetlands; saltland pasture; agri-aquaculture 3 IntroductionSecondary salinisation, in which human activities lead to high concentrations of ions (principally sodium and chloride) in the landscape, is a major environmental problem affecting freshwater and soil in many arid and semi-arid areas of the world (Ghassemi et al., 1995;Rozema and Flowers, 2008). Secondary salinisation occurs because of rising water tables, as a result of irrigation (irrigation salinity) or land clearing (dryland salinity). It is a particular problem in economically less-developed countries (e.g. India, Tunisia, Morocco, Peru and Bolivia) although more-developed regions, such as Australia, may also be badly affected (Rozema and Flowers, 2008). In Australia, the widespread clearing of native vegetation has resulted in approximately 5.7 M ha of agricultural land being adversely affected by dryland salinity, and this figure may grow to 17 M ha by 2050 (National Land and Water Resources Audit, 2001).The salinisation of agricultural land in arid regions is largely irreversible because ...

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