In arid and semi-arid ecosystems, salinization is a major threat to the productivity of agricultural land. The Gefara Plain located in the northwest of Libya; contain about 80% of the total agricultural activity. The high water requirements for the populations and agriculture are depleting the groundwater aquifer, resulting in intrusion of seawater in the first few kilometres along the coast. Due to increasing salinity in the groundwater used for irrigation, the soils of the Gefara Plain are becoming increasingly saline. The objective of this study is to investigate the effects of salinity on soil respiration in the Gefara Plain. The
rate of CO₂ evolution from soil is principally a function of soil microbial activity. This study investigated the sensitivity of these soils to increased salinity using soil respiration as an integrating measure of soil function. Soil was collected from four sites located in the Gefara Plain, Almaya, Janzur, Gargaresh and Tajura. The soils were shown to differ in salinity and the amount of organic matter in the soil. Soil collected from Tajura had the highest background salinity, and Janzur had the highest organic matter content. All of the soils had relatively low organic matter content, ranging between 0.49-1.25%. In vitro, addition of NaCl decreased soil CO₂ efflux in all soils, with the greatest decrease shown in
Tajura. The smallest decrease was shown in Janzur the soil with the highest organic matter content. Similar results were also shown for substrate induced respiration using glucose and glutamate. In a field investigation, in situ addition of NaCl showed the same sensitivity of the soils as in the laboratory investigation. In further laboratory investigations a greater decrease in soil CO₂ efflux was shown with NaCl compared to similar concentrations of Na₂SO₄ and KCl respectively. The cumulative rate of ¹⁴CO₂ of added ¹⁴C-labelled Lolium
perenne and ¹⁴C-labelled glucose to soils was decreased under effects of water containing different concentrations of NaCl at 20, 50, 70, 90, 150 and 200 mM compared to the control at any time of incubation in Tajura, Gargaresh, Almaya and Janzur respectively. To determine the effects of additional organic matter on the influence of salinity, and the decomposition of soil organic matter under salinity, investigations using coconut husk compost and ¹⁴C-labelled Lolium perenne were carried out. The coconut husk compost, addition to the soils increased pH, electrical conductivity and K⁺ concentrations in the soil solution. Addition of coconut husk compost both sterilized and unsterilized increased the
rate of soil CO₂ efflux, but did not change the relative sensitivity of the different soils to addition of NaCl. Similarly after addition of ¹⁴C-Iabelled Lolium perenne, again a similar sensitivity of the soils to NaCl was shown.
In further investigations attempts were made to separate mycorrhizal hyphal respiration from soil microbial heterotrophic respiration. Using cultures of wheat (Triticum aestivum L. var) grown with separate plant, mycorrhizal hyphal and soil microbial, as well as soil microbial only compartments, the effect of NaCl addition was investigated. Growth of external hyphae, and CO₂ evolution rate in the presence external hyphae were reduced by increased concentrations of NaCl to the soil compared to the control. In the mycorrhizal compartment the decrease in soil CO₂ efflux was proportional to the decrease in hyphal length. The tolerance to salt of extracted soil bacterial communities was investigated using an assay based on the leucine incorpation method to measure bacterial growth. Clear concentration-response relationships between microbial growth and soil salinity could be established, providing estimates of the salt tolerance. However, there was no relationship between in situ soil salinity and the salt tolerance of the soil bacterial communities, suggesting that other factors were more influential for the actively growing decomposer community in these soils.