Physiological responses of red maple saplings to sub-irrigation with an untreated municipal landfill leachate

An experiment was undertaken to examine the response of hydroponically-grown red maple ( Acer rubrum L.) saplings to a series of four flooding (sub-irrigation) treatments distributed over a 25-day period with an untreated (saline) municipal solid waste landfill leachate or deionized water. Net photosynthesis rates measured for water-treated saplings rapidly declined to 62% of the levels measured in untreated (control) saplings, but returned to pre-treatment levels with subsequent flooding treatments. Net photosynthesis rates measured for leachate-treated saplings decreased to about 50% of the levels measured for control saplings over the 25-day treatment period, and remained suppressed. Loss of turgor in leaves and a iron-oxyhydroxide plaque on root surfaces were also observed. Reasons proposed for this acute and apparently irreversible response to leachate exposure include: (i) extreme root anaerobiosis conditions caused by root system flooding and exacerbated by a high chemical oxygen demand leachate; (ii) increased root-soil interface resistance to transpiration water flow (osmotic potential gradient, iron oxyhydroxide plaque); (iii) metabolic intolerance to high solute concentrations in plant tissue; and (iv) exposure to potentially toxic volatile organic compounds. Water sub-irrigation had virtually no effect on nutrient and non-nutrient element concentrations in foliage or on the spectral reflectance characteristics of the leaves. Leachate treatment decreased the foliar content of many plant macro- and micro-nutrients significantly, and shifts in spectral reflectance patterns indicated declining plant vigour. Certain chemical constituents present in high concentrations in the leachate irrigant and which can be phytotoxic, such as Cl, accumulated to a significant degree in leachate-treated plant tissue.