PARENT SESSION
Oral Session #40: Carbon sequestration and flux.
Presiding: G. Koch
Tuesday, August 6. 1:00 PM to 4:45 PM. Gila Meeting Room, TCC.

Are eroding landscapes C sources or sinks: testing the "dynamic equilibrium" hypothesis in highly erodible New Zealand pasture.

Baisden, W.^*,1, Parfitt, Roger¹, Trustrum, Noel¹, ¹ Landcare Research, Palmerston North, New Zealand

ABSTRACT- Erosion has been hypothesized to establish a dynamic equilibrium that results in a C sink from ongoing recovery of C in eroded soils combined with ongoing C accumulation in sediments. Given this hypothesis, full accounting for erosion and soil conservation in the C budgets of dynamic landscapes may represent a significant challenge. Potential difficulties include indirect effects (e.g., burial or nutrient dynamics) on the balance of plant production and decomposition of soil organic matter (SOM). As a method for evaluating the "dynamic equilibrium" hypothesis, we examine the burial and availability of eroded nutrients using a combination of field data and models for landsliding terrains on soft bedrock. Landslides represent a model system for studying erosional effects on C and N budgets because they offer the ability to study easily identifiable events representing a known proportion of the landscape. We therefore investigate the C and N balance of pastoral land on soft-rock landscapes in New Zealand that undergo both infrequent shallow landslides and frequent surface erosion. Soil cores driven to the bedrock interface indicate that 23- and 37-year-old landslides have ~50% and 28% less soil C and N, respectively, than cores from uneroded sites on similar slopes and aspects. This represents the removal of 4–9 kgC/m² from landslide scars. However, full accounting must consider SOM recovery as well as the sequestration of eroded C in terrestrial or marine sediments. Recovery of the SOM occurs over 50+ years near the soil surface, and whole soil columns are unlikely to recover completely over hundreds of years. Models accounting for N input/output budgets, such as CENTURY, appear to capture this recovery. Storage of landslide debris on land, and estimates of the marine bioavailability of eroded soil C, suggest that erosion can have a positive or negative effect on atmospheric C balance.

KEY WORDS: Kyoto Protocol, carbon, nitrogen, erosion