The Fate of Soil OC in the Marine Environment: Examples from the Rapidly Eroding Landscapes of Two New Zealand North Island Rivers

Abstract:

Approximately 10% of the OC lost from soils as a result of land use has been argued to be delivered to the ocean (Lal 2003). The fate of this OC is highly dependent on the organic geochemical composition of the soil pool and the nature of the marine environment that receives it. The conversion of bush to pastureland via burning in the Waipaoa and Waiapu watersheds increased erosion rates by an order of magnitude. Surface and bank erosion, coupled with landsliding and gullying deliver OC to the rivers. Visual observations, sediment budgets, C-isotope (¹²C, ¹³C, ¹⁴C) mass balances and biomarker analyses all indicate that the OC is a mixture of recent plant debris, charcoal, aged soil C (< 18 kyrs old) and Cretaceous – Neogene sedimentary rock-derived C. The vastly different ages of the OC pools might be expected to lead to different reactivities and fates in the seabed.

Nearshore wave-driven deposition-resuspension cycles winnow fines from sands in water depths ~<50 m. The sand-sized sedimentary OC is dominated by rock C. Younger fractions of soil C are transported primarily as fines to deeper water. Marine OC is added to the fine-grained sediments as they encounter zones of primary production.

Dissolved inorganic C (DIC) within the interstitial (pore) waters of the marine sediments is a mixture of seawater DIC and benthic respired C. The C-isotopic composition of the DIC reflects its source. Stable isotope and radiocarbon measurements indicate that contemporary terrestrial C3 plant OC oxidation dominates respiration on the Waiapu shelf nearshore (~60 m). Marine OC is preferentially oxidized at water depths >80 m. The rock-derived C does not seem to be oxidized on the shelf or upper slope.

A comparison of riverine particulate organic C (POC) with shelf depocenter OC concentrations suggest the Waipaoa and Waiapu soil C burial efficiencies are ~50 and 85% respectively. This does not consider the fate of soil C dispersed beyond the depocenter where preservation efficiencies are expected to be lower because of greater exposure times to O₂ at the sediment-water interface. Nevertheless, these small rivers are more efficient at the sequestration of soil C than some tropical counterparts (e.g. Amazon and Fly) in which extensive oxidation of the terrestrial OC has been documented.