Apparent dryland salinity on the uplands of southeastern Australia; quantification of biotic and abiotic indicators, causes, mechanisms, processes and effects

Abstract

Secondary dryland salinity in Australia has been a major environmental concern for a number of decades, yet aspects remain controversial. These include the processes which induce salinised soils, the environmental impacts of salinity, and the way in which it is mapped and managed. Dryland salinity has been almost universally attributed to rising saline groundwater caused by excess water accumulation in the landscape following European settlement and tree clearing. However, there is a body of evidence that instead suggests increased soil salinisation in SE Australia is attributable to localized surface water problems associated with soil and vegetation degradation. The ‘Rising Groundwater Model’ has been widely accepted as the paradigm for understanding, mapping and monitoring dryland salinity. However, little quantitative research has been undertaken to understand the mechanisms and processes that cause secondary dryland salinity in the uplands of south eastern Australia. Further, there is little research that demonstrates adverse impacts of secondary salinity on terrestrial endemic biota even though it is listed as a threatening process to biodiversity. This research tested the applicability of an alternative ‘Surface Water Model’ to explain outbreaks of salinity or soil surface degradation in this region. This research investigated the effects of the joint phenomenon of soil and vegetation degradation and elevated salinity levels on soil biotic and abiotic parameters. Field research was conducted at ten box/gum grassy woodland sites in the agricultural zone of the Southern Tablelands of NSW. A holistic suite of metrics, including soil physical, chemical, hydrological and biological attributes, were assessed in the field and laboratory; geophysical surveys (EM31/EM38) and various fauna and flora surveys were performed. Results indicated that degraded soil surfaces were generally small in area and localized. These surfaces had highly variable soil EC levels (often very low), and were associated with in situ synergistic factors related to in situ soil and vegetation degradation. Some surfaces had accumulated NaCl, but many also had other, both toxic and low cation and anion levels particularly reduced levels of Ca, Fe, N, SOM and SOC. Extreme pH levels and other soil physical, chemical and biological impacts were also common. It is concluded that elevated soil salinity levels are a symptom of soil and vegetation degradation, not the cause. It was found that the predominant water movement in these landscapes occurred as overland runoff and surficial lateral interflow above the clay-dominant B horizon. There was no biological, pedological, geophysical or hydrological evidence of groundwater being a major factor for elevated soil surface salinity levels. Evidence suggests that these degraded ecosystems are relatively stable but urgently require nutrient/SOM input. Many endemic fauna and flora species flourish at highly degraded and salinised sites; tolerating elevated and fluctuating salinity levels, at all life cycle stages, which may effectively increase the gamma biodiversity in these grassy woodlands. No evidence was found to suggest that biodiversity is suffering from rising saline groundwater or elevated soil salinity levels per se, or that elevated salinity levels favour exotic species. It is therefore problematical to directly link soil salinity per se with ecological stress, as many other synergistic factors are involved and are more significant for degraded soils. Management decisions based on reducing the soil surface evaporation potential on site is the most coherent approach. Management activities should focus on stock grazing exclusion, soil amelioration and revegetation activities using endemic species, rather than focusing on excess deep landscape water management with hybrids and exotic plants. The present use of AEM for mapping dryland salinity in upland environments is therefore questionable. Show more