The effects of landfill leachate irrigation on the physio-chemical properties of Oxford clay and physiology of coppiced Salix viminalis

The irrigation of Salix species with landfill leachate, when grown on landfill sites, is a financially attractive solution to leachate treatment and landfill site remediation. Detailed experiments were carried out in the summers of 2001 and 2002 in which Salix viminalis, growing in lysimeters, received >100mm per week irrigation with landfill leachate of electrical conductivity 12-13 DS.m-1. The aim was to investigate the physiological effects of heavy leachate irrigation and determine a simple mechanism to relate these effects to salt loading. Incorporation of a temporal factor into EC (S.m-1.day) resulted in reliable correlations between EC days and salt addition (R2 = 0.96), soil matric potential (R2 = 0.89), foliar gas exchange (R2 > 0.72) and leaf area (R2 = 0.78). Persistent leachate treatment resulted in extensive defoliation but a 236% enhancement of dry weight relative to water only irrigation. Foliar gas exchange of leachate irrigated plants was suppressed more rapidly than in water deprived treatment, although the response to leachate and droughting was broadly similar. Destructive analysis of lysimeters from each treatment revealed diverse physio-chemical soil conditions. The leachate irrigated treatment showed elevated concentrations of soil nitrate (336 fold), ammonium (30 fold) and chloride (10 fold) relative to the water only irrigation treatment. All irrigation treatments had properties which were indicative of a soil which is inhibitory to liquid and gas movement as shown by oxygen diffusion rate (< 20 g.cm-2.s-1) and redox potential (> 273 mV). After four weeks irrigation with bore water or under natural precipitation a reduction of soil salts to the original levels was recorded.