NZeem - Erosion carbon study

In this case study erosion models, including the New Zealand Empirical Erosion Model (NZeem), are used to estimate the net carbon transfer between the atmosphere and soils due to erosion in New Zealand.

Soil erosion in New Zealand exports much sediment and particulate organic carbon (POC) to the sea. The influence of this carbon export on carbon transfers between soils and the atmosphere has been largely unknown. The models are used to estimate the spatial distribution of erosion rates, which are then combined with a digital map of soil organic carbon (SOC) content to produce the spatial distribution of carbon erosion.

Methodology

This study considered that the base processes driving this system were removal of organic carbon by erosion, redistribution and burial on the landscape, transport to and burial by the sea or release to the atmosphere, and sequestration of atmospheric CO2 by soils recovering from erosion. Therefore the modelling approaches aimed to quantify both the carbon content of soils and the amount of soils leaving the landscape and being captured by the ocean due to erosion.

Erosion modelling was based on 89 types of land which were categorised based on rock type, landform, slope and rainfall and as such the nature of erosion that was occurring. NZeem® was used to model the erosion rate throughout New Zealand based on these land types; the total erosion rate was estimated from three factors: erosion terrain; mean annual rainfall and land cover. The net transfer of carbon between the atmosphere and soils (due to erosion) was estimated for each erosion terrain.

The study assumed a high burial efficiency of erosion carbon at sea at around 80%, meaning that carbon exports to the sea were largely buried permanently. The budgets for each erosion terrain were amalgamated for the North and South Islands separately; this was due to the fact that erosive processes in the north were largely dominated by human induced processes while in the south the system was dominated by natural erosion.

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Outputs

The North Island of New Zealand is estimated to export 1·9 (with uncertainty of −0·5 and +1·0) million tonnes of POC per year to the sea and to sequester 1·25 (−0·3 /+0·6) million tonnes of carbon per year from the atmosphere through regenerating soils. The South Island of New Zealand is estimated to export 2·9 (−0·7/+1·5) million tonnes of POC per year and to sequester approximately the same amount. Assuming exported carbon is buried at sea with an efficiency of 80% gives New Zealand a net carbon sink of 3·1 (−2·0/+2·5) million tonnes per year; which is equivalent to 45% of New Zealand's fossil fuel carbon emissions in 1990. The net sink primarily results from a conveyor belt transfer of carbon from the atmosphere to soils regenerating from erosion to the sea floor where carbon is permanently buried. The net sink due to soil erosion can be further increased by reforestation of those terrains where erosion is excessive and there is no carbon recovery in the soils.

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Associated Models

New Zealand Empirical Erosion Model (NZeem®)

NZeem® partitions the New Zealand landscape according to the factors controlling erosion: rock type, land form (especially slope angle), rainfall and land cover.

References

Dymond, John R. (2010) Soil erosion in New Zealand is a net sink of CO2. In: Earth Surface Processes and Landforms 35(15), 1763–1772. (URL: http://onlinelibrary.wiley.com/doi/10.1002/esp.2014/abstract)