Characteristics of Soil Water-Soluble Organic C and N Under Different Land Uses in Alaska

Abstract

Land use conversion induces quantitative change of soil water-soluble organic matter (WSOM), but knowledge of such change is still limited. In this study, field moist and air-dried soils sampled from subarctic Alaska under three land use managements (i.e., forest, agriculture, and grassland converted from agricultural use and under a Conservation Reserve Program [CRP]) were extracted with deionized water and separated by filtration into different size fractions (2.5 μm, 0.45 μm, and 1 kDa). Water-soluble organic C (WSOC), water-soluble organic N, and fluorescence spectroscopy of each fraction were determined. There were few differences in quantitative data between samples from different land uses with air-dried samples, implying that air-dried samples were not suitable for characterizing the impact of soil management practices on soil WSOM. For field moist soil samples, the WSOC contents decreased in the order forest > CRP > agricultural land. Furthermore, WSOC was dominated by large (>0.45 μm) and small (<1 kDa) size molecules in CRP and forest soils, whereas small molecules predominated in agricultural soils. The WSOM of different size fractions and land use displayed three similar fluorophore components (two humic-like and a tyrosine-like), indicating that the impact of land use was mainly on the quantity, rather than on the composition, of WSOM. In conclusion, our data suggested that the long-term agricultural land use could lower the WSOM levels in soils; however, the decreasing trend could be reversed by conversion of agricultural land to grassland under conservation.