Characteristics of dissolved organic matter (DOM) in aqueous extracts of litter may provide early indications of alteration of soil organic matter due to disturbances. Aqueous extracts of litter (pine and understory components) were prepared from two young ponderosa pine (Pinus ponderosa Doug. ex Laws.) plantations in California. Treatments were vegetation control (understory removal by herbicides), full nutrient fertilization, and control (untreated). Early changes due to management practices shown by spectroscopic techniques were not revealed by standard chemical analysis of DOM concentrations, pH or C/N ratios. Fourier-transform infrared spectroscopy (FTIR) showed that DOM from both treatments at the warmer site (Whitmore) had relatively more COOH, phenolics, and aliphatics compared with controls, for both pine and understory components. At the cooler site (Chester) similar trends were observed only for understory DOM. Ultraviolet-visible spectroscopy (UV-VIS) showed that the ratio of absorptions at 465 vs. 665 nm (E4/E6 ratio) was higher for vegetation-control DOM at both sites (6.7), compared with controls (5.9). Fertilization increased E4/E6 ratios at Whitmore only. The E4/E6 ratio was higher at Whitmore (6.7) than at Chester (5.1). The DOM from treated plots had smaller or less condensed chromophores, and relatively higher COOH, phenolics, and aliphatics, compared with the control. These effects were not as pronounced at Chester. At Whitmore warmer temperatures and increased water availability resulting from understory removal, and increased nutrient supply resulting from fertilization, may enhance the activity of decomposers. Total nutrients in litter increased with fertilization and decreased with understory removal. However, at Whitmore, understory removal increased the nutrients stored specifically in pine litter.
Research supported by the Agric. Exp. Stn., Univ. of California, Berkeley, Project CA-B-SSC-5607-H, and Extramural Cooperative Research Agreement between Univ. of California, Berkeley, and the Research Station, USDA Forest Service, Project PSW-91-0024CA, and the National Agricultural Pesticide Impact Assessment Program, no. PSW-29.