Chemical characterization of dissolved organic matter released by limed and unlimed forest soil horizons

Liming of forest soils, currently practiced in Europe, may be necessary in the future in North America to counteract calcium (Ca) depletion due to whole tree harvesting. With current concerns over increasing carbon dioxide (CO2) levels in the atmosphere, the effects of forest practices such as liming on carbon (C) cycling in forest ecosystems has been receiving increasing attention. This laboratory study investigated the effect of an increase in pH on dissolved organic matter (DOM) in water extracts of organic horizons. Organic horizons were collected from nine forested sites, some predominantly hardwood and some predominantly softwood stand types. The soils were chemically characterized. Water extracts of limed and unlimed treatments of nine organic horizons were analyzed for pH, Al, Fe, Ca, C, and DOM negative charge density. Total luminescence fluorescence spectra were also obtained for each extract. The addition of lime (CaCO3) to organic horizons caused an average of 55% more C to be released than in unamended samples. Liming caused no significant changes in negative charge density of the DOM. Liming decreased organically-complexed aluminum (Al) and increased organically-complexed Ca. There was a negative correlation between Al concentration of extracts and fluorescence intensity. Chemical changes in the DOM released by liming were suggested by decreased fluorescence emission and excitation wavelengths due to liming. Hardwood samples did not respond differently than softwood samples with respect to DOM concentration, functional group concentration, or spectroscopic characteristics. However, there was a suggestion that particular tree species, such as cedar, may have unique effects on C concentrations and DOM chemistry.