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.
1Department of High Latitude Agriculture, School of Natural Resources and Agricultural Sciences, University of Alaska, Fairbanks, Alaska, USA.
2USDA-ARS, Southern Regional Research Center, New Orleans, Louisiana, USA.
Address for correspondence: Dr. Mingchu Zhang, University of Alaska, Fairbanks, AL 99775, USA. E-mail: firstname.lastname@example.org
Financial Disclosures/Conflicts of Interest: None Reported.
Received July 11, 2012.
Accepted for publication November 14, 2012.