Biological N2 fixation represents the major source of N input in agricultural soils including those in arid regions. The major N2-fixing systems are the symbiotic systems, which can play a significant role in improving the fertility and productivity of low-N soils. The Rhizobium-legume symbioses have received most attention and have been examined extensively. The behavior of some N2-fixing systems under severe environmental conditions such as salt stress, drought stress, acidity, alkalinity, nutrient deficiency, fertilizers, heavy metals, and pesticides is reviewed. These major stress factors suppress the growth and symbiotic characteristics of most rhizobia; however, several strains, distributed among various species of rhizobia, are tolerant to stress effects. Some strains of rhizobia form effective (N2-fixing) symbioses with their host legumes under salt, heat, and acid stresses, and can sometimes do so under the effect of heavy metals. Reclamation and improvement of the fertility of arid lands by application of organic (manure and sewage sludge) and inorganic (synthetic) fertilizers are expensive and can be a source of pollution. The Rhizobium-legume (herb or tree) symbiosis is suggested to be the ideal solution to the improvement of soil fertility and the rehabilitation of arid lands and is an important direction for future research.

Rhizobium-Legume Symbiosis and Nitrogen Fixation under Severe Conditions and in an Arid Climate

/pdf/responses-of-woody-plants-to-flooding-and-salinity-4f68qmbyey.pdf

Responses of woody plants to flooding and salinity

In Victoria, Australia, both dryland salinity and salinity in irrigation regions are serious agricultural problems. One option to control the latter is to pump groundwater to maintain it below the surface. However, this leaves a saline wastewater for disposal, probably into local streams or wetlands. This review of the salt sensitivity of the biota of Australian streams and wetlands gives information of interest to those responsible for developing controls on these discharges. The review addresses the lethal and sub-lethal effects of salinity on microbes (mainly bacteria), macrophytes and micro-algae, riparian vegetation, invertebrates, fish, amphibians, reptiles, mammals, and birds. Data suggest that direct adverse biological effects are likely to occur in Australian river, stream and wetland ecosystems if salinity is increased to around 1 000 mg L−1. The review highlights a general lack of data on the sensitivity of freshwater plants and animals to salinity increases.

A review of the salt sensitivity of the Australian freshwater biota

Soil salinity (high levels of water-soluble salt) and sodicity (high levels of exchangeable sodium), called collectively salt-affected soils, affect approximately 932 million ha of land globally. Saline and sodic landscapes are subjected to modified hydrologic processes which can impact upon soil chemistry, carbon and nutrient cycling, and organic matter decomposition. The soil organic carbon (SOC) pool is the largest terrestrial carbon pool, with the level of SOC an important measure of a soil's health. Because the SOC pool is dependent on inputs from vegetation, the effects of salinity and sodicity on plant health adversely impacts upon SOC stocks in salt-affected areas, generally leading to less SOC. Saline and sodic soils are subjected to a number of opposing processes which affect the soil microbial biomass and microbial activity, changing CO fluxes and the nature and delivery of nutrients to vegetation. Sodic soils compound SOC loss by increasing dispersion of aggregates, which increases SOC mineralisation, and increasing bulk density which restricts access to substrate for mineralisation. Saline conditions can increase the decomposability of soil organic matter but also restrict access to substrates due to flocculation of aggregates as a result of high concentrations of soluble salts. Saline and sodic soils usually contain carbonates, which complicates the carbon (C) dynamics. This paper reviews soil processes that commonly occur in saline and sodic soils, and their effect on C stocks and fluxes to identify the key issues involved in the decomposition of soil organic matter and soil aggregation processes which need to be addressed to fully understand C dynamics in salt-affected soils. © 2009 The Authors. Journal compilation

Soil carbon dynamics in saline and sodic soils: a review.

Physiological ecology of natural regeneration of harvested and disturbed forest stands: implications for forest management

The response of 11 Casuarina species to salinity (NaCl) at germination (0, 20, 50, 200, 400 mM) and/or as seedlings (0, 75, 150 mM) was studied. Both the final germination percentage and germination rate were reduced in all species by increasing salinity. Treatment of seedlings tended to depress growth, increase the concentrations of Na and Cl in the woody stem and shoot tip and decrease K concentrations in the root and shoot. The species exhibited a wide range of responses to salinity, with some (e.g. C. equisetifolia) showing little growth reduction and no visible injury symptoms, and others (e.g. C. inophloia) showing greatly reduced growth accompanied by shoot tip chlorosis and/or death at 150 mM NaCl. Na and C1 concentrations in the shoot tip in sensitive species treated at 150 mM NaCl were 6-10 times higher than those of controls. In the more resistant species estimated shoot tip concentrations of these ions in plants treated at 150 mM NaCl were approximately the same as or less than those of the external medium. Na and C1 concentrations in the root were higher in these species than in the more sensitive species.

Germination, Growth and Mineral Ion Concentrations of Casuarina Species under Saline Conditions

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Germination, Growth and Mineral Ion Concentrations of Casuarina Species under Saline Conditions