Assessment of the cycle and availability of major and trace alkali and alkaline earth metals in the coastal Carmel Ridge, eastern Mediterranean

Abstract

Major alkali and alkaline earth metals (Na, K, Mg, Ca) have been investigated extensively due to their role in soil, natural waters, living organisms, and agriculture, whilst their trace congeners (Li, Rb, Sr, Ba) are less well studied. Such information is of special interest for semi-arid Mediterranean ecosystems, in which these elements are believed to accumulate during prolonged dry seasons. However, data on the speciation, (bio)availability, and cycling of these metals are limited. We studied the fluxes of these metals in truly dissolved/reactive and ʻoperationally dissolvedʼ (microfiltrate, < 0.20 µm) forms in the sequence of ʻrainwater–throughfall water–litter leachateʼ in a restored mixed sclerophyll forest on the eastern Mediterranean coast. Truly dissolved metals were separated using an ion-exchange chromatography technique. The results suggest that decomposing plant litter followed by throughfall water are the main sources of the metals to underlying soils, whilst the contribution from rainwater is small, except for Na. Thereafter, the main flux of the water-mobile metals occurs within a semi-closed and rather self-sustaining soil–plant system. Only a portion of the water-mobile (< 0.20 µm) Na, K, Ca, and Mg was in truly dissolved/reactive form. The high-molecular-weight organic matter produced by leaf excretions in throughfall water and decomposing litter seems to play a major role in the speciation and transport of the metals, as well as in the mechanism of excretion of excess Na and Ca.