Alkaline soils in Libyan Sahara and Murray-Darling Basin, Australia: characteristics, carbon geochemistry and environmental issues

Alkaline soils in Libyan Sahara and Murray-Darling Basin, Australia: characteristics, carbon geochemistry and environmental issues

The geochemical characteristics of alkaline soils in the Fezzan Basin in the Sahara Desert (Libya) and the Murray-Darling Basin in Australia were studied with emphasis on carbon geochemistry and the environmental and agricultural implications of these soils. In addition, microcosm experiments were conducted to evaluate the potential of a highly alkaline soil for carbon sequestration through both soil carbonation and biomass production.

The results show that in the Fezzan area, 0.7% carbon was stored in the topsoil with approximately 1/3 being inorganic carbon and 2/3 being organic carbon. The fine-grained soil fraction contained 2.13% of iron and 252 mg/kg of phosphorus, suggesting that this area could be an important source of ocean iron and phosphorus. Manganese and strontium were identified as the major chemical pollutants that are likely to dominate in the dusts formed in this area. The soils were generally alkaline and saline, which require appropriate amendment in order to develop desert agriculture using the groundwater resources from the Man-made River Project.

In the Murray Darling Basin, the SiO2/Al2O3 and most elements in the soils between the upper catchment and the lower catchment were similar, suggesting that the surface soil materials in the lower catchment tended to be of upper catchment origin. However, there were marked differences in pH, electrical conductivity, sodium adsorption ratio, exchangeable sodium percentage, inorganic carbon, and 87Sr/86Sr ratio of the acetic acid-extractable fraction between the upper catchment soils and the lower catchment soils.

This is attributable to the catchment processes, which drive the re-distribution of readily movable bicarbonates of basic metals within the catchment. The elevated pH in the lower catchment zone inhibits the growth of plants, resulting in lower soil organic carbon content, as compared to the upper catchment zone. Given the relatively low outward discharge rate, large amounts of carbonates were deposited within the lower catchment zone. The strontium isotopic signatures obtained from this study suggests a significant contribution of silicate rock-originated Sr towards the composition of soil strontium in these soils, inferring that silicate rock-originated Ca might play an important role in the formation of carbonate minerals in the lower catchment zone, and consequently contribute significantly to CO2 sequestration.

It appears that calcium and strontium behaved differently in the investigated highly alkaline soil system in the upper catchment of Murray Darling Basin. There was also a marked difference in geochemical behaviour between the two elements during the course of atmospheric transport from the ocean surface to the land surface. Therefore, 87Sr/86Sr provided no reliable indication of the Ca source to form pedogenetic carbonates in the investigated alkaline soils. This raises concerns over the suitability of strontium isotopic signatures in tracing Ca source for pedogenetic carbonation in alkaline soils.

The microcosm experiments showed that application of gypsum resulted in an increase in inorganic carbon and a decrease in organic carbon. The addition of talc did not significantly enhance carbonate formation. Soluble CaCl2 and MgCl2 did not have statically significant better effects on soil carbonation, as compared to gypsum. The one-year growth experiment using five widely cultivated pasture grasses revealed that accumulation of carbonates following gypsum application could be inhibited by plant growth; the organic acids secreted from plant roots were likely to facilitate soil carbonate dissolution. In comparison with pedogenic carbonation, carbon sequestration by biomass production was much more evident. However, the biomass carbon gain varied markedly among the five species with Digitaria eriantha showing the highest biomass carbon gain. This further enhanced the accumulation of soil organic carbon. At the end of the experiment, an estimated CO2 sequestering capacity of 93 t/ha was achieved. The research findings have implications for cost–benefit analysis of alkaline soil reclamation projects.