Homogenization of detrital leachate in an old-growth coniferous forest, OR: DOC fluorescence signatures in soils undergoing long-term litter manipulations

Abstract

Characterizing the relationship between plant detrital inputs and the resulting dissolved organic carbon (DOC) leachate is vital to understanding the ultimate fate of root carbon, fallen wood and needles in forested watersheds. Similarly, elucidating chemical differences in the soil DOC pool may help to explain which DOC fractions are sorbed to mineral surfaces and contribute to accumulation of soil organic carbon, are respired as CO2, or are exported to nearby catchments. In order to test the hypothesis that soils with different detrital inputs impart a detectable signal on DOC in mineral soil, soil solution DOC was sampled from the Detrital Input and Removal Treatment (DIRT) plots located in the H.J. Andrews Experimental Forest, OR. Multiple types of fresh litter extracts, along with lysimeter and soil extracts from DIRT treatment plots were characterized using UV-Vis and fluorescence spectroscopy coupled with the Cory and McKnight (Environ Sci Technol 39:8142–8149, 2005) parallel factor analysis (PARAFAC) model. Principal component analysis of 13 unique fluorophores distinguished using PARAFAC show that litter and soil extracts (Douglas-fir needles, wood of decomposition Class 2, Class 3 and Class 5, O-horizon, and 0–5 cm A-horizon) each have distinct fluorescence signatures. However, while litter-leached DOC chemistry varies by litter type, neither lysimeter-collected DOC or soil extracts in the DIRT plots show statistically significant differences in fluorescence signatures among treatments, even after 17 years of litter manipulations. The lack of observed differences among DIRT treatments suggests that both abiotic interactions and microbial activity effectively homogenize organic carbon constituents within the dissolved pool. Minor but observable changes in PARAFAC components and optical indices during a 1-month biodegradation incubation of litter and soil extracts indicate that while biodegradation significantly alters DOC chemistry, abiotic mechanisms are also critical to DOC transformation in these soils with high sorption capacity. Although leachates from different plant detrital sources have distinct carbon chemical signatures, these DOC signatures are effectively homogenized after passage through mineral soil. These results highlight the dominant role of both biotic and abiotic interactions in controlling the chemistry of DOC in shallow soils.