Characteristics of DOM exported in hillslope lateral subsurface flow
Indices of aromaticity (SUVA254) of old-growth DOM export were 14% higher (3.46 vs. 3.00) and humification index (HIX) was nearly twice that of DOM exported from the second-growth forest (12.24 vs. 6.63). The DOM exported from the old-growth hillslope had a mean fluorescence index (FI) of 1.14 compared to 1.52 from the second-growth hillslope. Fluorescence regional integration modelling (FRI) confirms the patterns shown by the EEMs indices (Fig 2). FDOM corresponding to amino acids (RI and RII) and microbial exudates (RIV) were higher in exports from the second-growth hillslope. Conversely, plant-derived fulvic- and humic-like hydrophobic organic acids (RIII and RV) were higher in exports from the old-growth hillslope.
Indices of FDOM chemical character (FI and HIX) were similar in exports from the two hillslopes at the onset of subsurface flow but diverged over time (Fig 3a and 3b). Across the subsurface flow season, FI decreased by half the possible range in the old-growth hillslope. The proportion of complex organic acids in FDOM exports (based on HIX), increased 3-fold during the flow period. In contrast, the chemical character of FDOM exported from the second-growth hillslope remained relatively uniform (Fig 3a and 3b).