Soil erosion in New Zealand is sinking carbon (C) at 3 million tonnes per year
Over two hundred million tonnes of sediment are lost from New Zealand to the ocean every year, primarily during storms. Intense rainfall erodes soil from steep slopes and this ends up in waterways as sediment. Floodwaters carry the sediment to the ocean and deposit it on the ocean floor. As the sediment contains about 2.4% organic C, this means a total of 4.8 million tonnes of C is exported to the ocean from New Zealand each year. This is a lot of C to lose – equivalent to 70% of New Zealand’s total use of C through the burning of fossil fuels in 1990.
This loss of C initially appears like a bad news story. But John Dymond of Landcare Research has discovered otherwise. He estimates that soils recovering from erosion are sequestering C on average at nearly the same rate that C is lost through erosion. Assuming most (~80%) of the sediment is buried permanently on the ocean floor, erosion is responsible for a net sink from the atmosphere of 3.1 million tonnes of C.
CO2 from the atmosphere is sequestered to soil organic C in soils recovering from erosion, and then transported after a new cycle of erosion to the ocean, where it is buried.
New Zealand chose not to opt for full carbon accounting in its Kyoto Protocol negotiations, so the significant C sink in soil is excluded from eligibilty for carbon credits. Further, most of the erosion in the North Island was caused by the clearing of native forests over 100 years ago, so it is well outside the 1990 baseline set in the Protocol. However, if New Zealand ever decides to negotiate a full C accounting method, then soil erosion could possibly qualify as a major C sink.
Dr Dymond strongly cautions against thinking soil erosion could be considered a good thing and we could do with some more of it. Soil erosion impacts on water quailty and reduces productivity. A certain base level of natural erosion is beneficial through a greenhouse gas perspective, only when the soils can sequester C as fast as they lose it through erosion. Excessive erosion however, is dangerous and can lead to complete collapse of some landscapes, where there is no soil recovery (such as the gullied landscapes in the soft-rock hill country in the Gisborne district). If these landscapes were reforested to reduce erosion then the net C sink for New Zealand would be even greater.