Key words :
The Link Between Agricultural Erosion and Global Warming
19 Nov, 2007 10:37 am
Carbon emissions are of great concern worldwide because they trap heat in the Earth's atmosphere and are a major cause of global climate change. We need to know where and how much carbon is being released or captured in order to develop sensible and cost-effective measures to curb climate change. Unfortunately, there are still a lot of unknowns in the global carbon cycle, especially on land.
In our study, we estimated the net amount of carbon being captured into the soil and the net amount of carbon being released into the atmosphere during the last 50 years as a result of erosion. We used a by-product of nuclear weapons testing, caesium-137, that is present throughout the world to track the movement of soil around the agricultural landscapes. This allowed us to predict how much carbon would be expected to be found in areas of soil erosion and deposition. By comparing these predictions with measured amounts of carbon in 1400 soils we could identify which soils had released carbon versus which ones had captured carbon.
Our results showed that in landscapes subject to soil erosion, erosion acts like a conveyor belt, excavating subsoil, passing it through surface soils and burying it in hill-slope hollows. During its journey, the soil absorbs carbon from plant material, and this becomes buried within the soil in depositional areas. At sites of erosion, carbon is exported so there is less remaining carbon available for release. At the same time, plants that continue to grow at sites of erosion contribute carbon to the soil, leading to the replenishment of the carbon stocks. We estimated that approximately 26% of the eroded carbon is replenished.
The proportion of eroded carbon that is replaced is similar to the magnitude of the “active” carbon pool, which turns over within years to decades, and we suggest that only this pool undergoes most rapid replacement. At sites of deposition, the addition of sediment and soil to the surface leads to the gradual burial of carbon below the surface. As carbon is further buried down in the soil profile, it is moved from the area of most microbial activities (and hence large carbon release) to an area where there is less microbial activity (and hence reduced carbon release). Therefore, our results suggested that the buried carbon was actually stable and almost no carbon was lost to the atmosphere. Erosion and deposition, therefore, lead to more carbon being removed from the atmosphere than is emitted; creating what can be described as a ‘sink’ of atmospheric carbon.
Using models, we extrapolated these findings to a global scale and found that this sink of atmospheric carbon represents the equivalent of around 1.5% of annual fossil fuel emissions. This finding challenges previous assessments that erosion represents an additional release of carbon to the atmosphere equivalent to 10% of annual fossil fuel emissions. On the other hand, the amount of carbon taken out of the atmosphere and mixed into the soil is also dramatically less beneficial than some previous studies had estimated. Why is our estimate so much lower? There are two reasons here. One is that past estimates have tended to assume that all the eroded carbon found in deposition areas has been replaced by new carbon on the slopes, whereas for our data we find this figure around about only 26% of the carbon being replaced. The other reason is that we’ve estimated rather lower rates of soil erosion on agricultural land, reflecting improvements in the modeling of erosion rates at the global scale.
This new insight into the effect of erosion on the carbon cycle is essential for sound management of agricultural soils. If previous assessments that erosion causes a high level of carbon emissions to the atmosphere had been correct, then erosion control could have been used to offset fossil fuel emissions. These results show that erosion control should be pursued for its environmental and agronomic benefits but will not play a role in the currently needed potentials to offset fossil fuel emissions. Finally, our results indicate that large amounts of carbon (16-21 Pg) have been buried in shallow deposits within agricultural watersheds during the last 50 years. The stability of this large pool under present and future climatic disturbances remains highly uncertain.
1. K. Van Oost, T. A. Quine, G. Govers, S. De Gryze, J. Six, J. W. Harden, J. C. Ritchie, G. W. McCarty, G. Heckrath, C. Kosmas, J. V. Giraldez, J. R. Marques da Silva, and R. Merckx. The Impact of Agricultural Soil Erosion on the Global Carbon Cycle. Science, 26 October 2007: 626-629. Abstract available here.