The objective of this research was to determine characteristics of soil water extractable organic matter (WEOM) under different land uses in a subarctic environment. Soil (Volkmar, Aquic Eutrocrepts) samples were taken in October, 2005 from forested, agricultural, and Conservation Reserve Program (CRP) land. Fresh soil samples were sieved (<2 mm), and extracted with distilled water at a 1:10 of soil:water ratio. The extraction solutions were sequentially filtered through Millipore membranes of 1.6 μm, 0.45 μm, and 1 kD, and solution pH, EC, concentrations of Al3+, Fe3+, Ca2+, Mg2+, Cu2+, Zn2+, and K+, and soluble carbon concentrations were determined. The spectra of UV-Vis were measured for each fraction. The ratio of E4/E6 was calculated. Spectral slope coefficient was derived from model Aλ = A375eS(λ375- λ) + K.
The majority (>65%) of Al3+ and Fe3+ were in 0.45-1.6 μm size fraction in CRP and the forest soils, but most Al3+ and Fe3+ were in 1 kD - 0.45 μm fraction in agricultural soil. There were 57 to 72% Ca2+ in 1 kD - 0.45 μm fraction for CRP and forest soil in comparison with 68% Ca2+ in < 1 kD fraction for agricultural soil. The E4/E6 of WEOM was similar in <1.6 μm and <0.45 μm fractions among different land uses. But for the < 1 kD fraction, the E4/E6 ratio was apparently smaller in forest than in CRP and agricultural soils, demonstrating a high condensation of aromatic constituents. The spectral slope coefficient at 375 nm was higher for forest soil in <1 kD size fraction, reflecting the impact of substrate sources on the small size WEOM. In summary, land use affected trivalent and divalent cations in different size fractions of WEOM. The impact of land use on spectra properties of WEOM can only be found in < 1 kD size fraction.