Abstract: This study explored the temporal variation of dissolved organic matter (DOM) in postfire forest floor as burned stands recovered over time. Such knowledge would help us understand the role of forest floor in the fire climax forest ecosystem and mechanism of the lasting impact of fire disturbance on terrestrial ecosystem. The unburned and burned forest floors of Larix gmelinii with varied post-fire time (in years) were studied with focus on Oi horizon, Oe horizon, and moss layer. DOM was extracted using the water extraction method. Dissolved organic carbon (DOC) content, UV visible spectrum parameters, and fluorescence spectrum parameters of DOM were measured, and then possible differences among forest stand restoration stages were analyzed. The DOC content, UV-Vis absorbance and HIX_b in Oe horizon and moss layer, and a(355) in moss layer exhibited a continuous decreasing pattern in the early and middle restoration stages, which began to increase in the late restoration stage. The a(355), UV-Vis absorbance, SUVA₂₅₄, and HIX_b in Oi horizon and the DOC content in Oe horizon decreased continuously in the early restoration stage but increased in the middle and late restoration stages. The DOC content in moss layer decreased in the middle restoration stage but increased in the late restoration stage. The SUVA₂₅₄ increased in the early restoration stage but decreased in the middle restoration stage. The SUVA₂₈₀ in Oi horizon increased continuously in the early and middle restoration stages but decreased in the late restoration stage. The SUVA₂₈₀ in Oi horizon and moss layer and the SUVA₂₅₄ in moss layer decreased continuously after fire, but the decline was slowing down over time. The FI in forest floor and a(355) in Oe horizon were not significantly affected by fire disturbance. The decomposition rate of the Oi horizon was higher than that of the Oe horizon in post fire forest floor. However, the decomposition rate varied over time, showing decrease first and then increase in both horizons. Fire disturbance had significant effects on DOM characteristics of Larix gmelinii forests. In the burned area, the DOC content, humification degree, aromatization degree, the number of unsaturated conjugated double bonds and mean relative molecular weight of DOM in Oe horizon and moss layer all initially decreased and then increased. By contrast, the average molecular weight of DOC in Oi horizon increased first and then decreased. Along with the restoration, the variation trends of DOC content, a(355), SUVA₂₅₄, SUVA₂₈₀, HIX_b and FI of Oi horizon were different from those of Oe horizon, while the variation trends of SUVA₂₅₄ and SUVA₂₈₀ in Oi horizon were opposite to those in Oe horizon. Fire disturbance did not affect DOM source and autotrophic characteristics of Larix gmelinii forest floor. The magnitude of the effects of restoration duration on forest floor chemical dynamics was strong for Oi horizon, moderate for Oe horizon, and weak for moss layer.

Key words: Larix gmelinii, burned area, dissolved organic matter, time-scale, UV visible spectrum, fluorescence spectrum.