Dynamics of dissolved organic carbon in forested and disturbed catchments, Westland, New Zealand: 1. Maimai

Abstract

Dissolved organic carbon (DOC) dynamics were investigated in 2 undisturbed catchments (beech‐podocarp‐hardwood forest) and 6 catchments which had been clear‐cut 8–10 years previously and planted with pine. Over a 4‐month sampling period, DOC concentrations in an undisturbed catchment averaged 1.4, 16.0, 356.0, and 55.7 mg/L in precipitation, throughfall, stemflow, and surface organic horizon, respectively. DOC concentrations decreased to averages of 11.8 and 4.5 mg/L in the subsoil and stream, respectively. The annual flux of DOC was estimated as an input through precipitation of 3.4 g/m2, 83.6 g/m2 through the organic horizon, and an export of 6.8 g/m2 in the stream. The pronounced decrease in DOC concentration and flux in the subsoil horizon is related to the high concentrations of amorphous, oxalate‐extractable Fe, and Al with high DOC‐adsorption capacities. In streams draining the 8 catchments, discharge‐weighted DOC concentrations ranged from 4.5 to 10.2 mg/L. DOC and log discharge showed a significant positive relationship and the rising limb of the storm hydrograph contained higher DOC concentrations than the falling limb, at similar discharges. Export of DOC in 1986 ranged from 8 to 21 g/m2 and was related to the type of disturbance: the lowest values were recorded in undisturbed catchments and the highest in catchments that had been clear‐cut and the slash had not been burnt. Organic debris in the stream channel appear to be important sources of stream DOC and probably account for the higher DOC concentrations in the disturbed catchments, 8–10 years after disturbance.