Ed Barrett-Lennard

Salinity is a major problem in the irrigated zones of central and southern Iraq. We investigated biomass production from five salt-tolerant forage species, represented by 15 introduced accessions and 3 local accessions, during two successive summer growing seasons. Species included pearl millet (Pennisetum glaucum), sorghum (Sorghum bicolour), guar (Cluster bean; Cyamopsis tetragonoloba), cowpea (Vigna unguiculata) and sesbania (Sesbania aculeata and S. sesban). The research site was located in the Al-Dujaila irrigation project area in Wasit. The soil had a silty clay texture with moderate salinity (ECe 10-20 dS/m). The site was irrigated with water (ECw 1.1 dS/m) from the Tigris River on a 3 to 8-day basis. We hypothesised that there would be variation in biomass production, salt accumulation and crude protein both between and within species. In addition, we thought that the legumes would be less productive than grasses as they tend to be more sensitive to salinity. Over 2 experimental seasons, the plants produced an average shoot dry mass of 12.1 t/ha with a range for accessions of 3 to 35 t/ha. There was significant variation both within and between species, supporting the first hypothesis. None of the plants accumulated excessive salt (ash < 12% DM); crude protein was variable (6 to 12%), but higher in two legumes – cowpea and sesbania. Furthermore, there was no evidence that grasses were more productive than legumes, with sesbania and cowpea achieving the highest mean biomass production across the years. We conclude that moderately saline irrigated areas of Iraq can sustain high levels of biomass production for livestock although protein may be limited.

Secondary salinity (salinity induced by human activity) is a major world problem. Hydrological imbalances in landscapes cause watertables to rise, drawing salt from deeper in the soil profile to the soil surface, where the combined stresses of salinity and waterlogging threaten the growth of crops and pastures. The reversal of salinity by the broad-scale reintroduction of perennial plants back into landscapes may not be economically feasible, but watertables may be at least partly directly drawn-down by the growth of salt tolerant plants (halotypes) on saltland. This review summarises data from Australia in which the water use by perennial halotypes has been sufficient to facilitate the growth of the less salt tolerant under-storey legumes balansa clover (Trifolium michelianum) and burr medic (Medicago polymorpha). Although the halophytes may have undesirable nutritive value traits, these plants, mixed with the under-storey species are then able to be utilised profitably by grazing sheep.

The large areas of saltland pastures in South West Australia offer opportunities for carbon (C) sequestration through re-vegetation. An investigation was carried out to quantify the amount of C sequestered in above- and below-ground biomass for wavy leaf saltbush (Atriplex undulata) at a saline site east of Wickepin with average EM38 readings of ~400 mS/m (in the vertical mode). The preliminary results showed that the above ground and below ground biomass was higher under ungrazed than grazed management systems. The amount of above-ground biomass was 10.8 t/ha (4.01 t carbon /ha) for ungrazed saltbush plants and 3.6 t/ha (1.34 t C/ha) for grazed plants. Similarly, root biomass in the ungrazed system had higher biomass [2.7 t/ha (1.02 t C kg/ha)] compared with the grazed field [1.03 t/ha or 0.38 t C/ha]. Saltbush clearly has the potential to sequester a substantial amount of carbon and further research is underway to determine how amounts of carbon sequestered vary with species, soil type, site salinity and hydrology.

Salt tolerant herbaceous plants potentially fill a number of vital roles in an Australian landscape increasingly affected by salinity,. In some cases, these are required simply for amenity use, which directly reflects their environmental and lifestyle values, but also contributes indirectly to the economy through industries such as tourism, mining, and aquaculture, and through coastal urban development. In other cases, they must also be able to contribute to agricultural production, usually via grazing animals. Although not previously a focus of national policies on salinity, coastal areas require salt tolerant species (particularly grasses) for a diversity of uses. Examples include beach protection, rehabilitation of degraded sites, stabilisation of banks on saltwater aquaculture ponds, roadside plantings through estuaries, grassing of public foreshore parks (including some constructed on compacted marine sediment), and high quality turf for golf courses and home lawns. Mine revegetation (e.g. in the Bowen Basin) sometimes requires the use of salt tolerant species, as does the stabilisation of banks on tailings dams. Some salt tolerant plants (e.g. grasses, saltbushes) produce palatable forage, and so can be used to revegetate privately and publicly owned farmland increasingly affected by dryland salinity in inland Australia. For grazing use in particular, suitable plants should also cover a range of salt tolerance, from halophytic species for the worst affected sites through to less tolerant but more productive ones for moderately saline land. As yet, we do not have anything like the range of salt tolerant herbaceous plants (grasses, legumes, saltbushes, other forbs), both native and exotic, needed to tackle the prevention and amelioration of soil salinity effectively across the diversity of environment and uses required. in Australia