Fertile topsoil is probably not eroding from farmlands at the alarming rate that it's widely believed to be, suggests a study in the 20 August issue of Science. The results, some of the first precise measurements of a notoriously difficult to measure problem, challenge previous reports that soil erosion is becoming as serious as it was during the Dust Bowl six decades ago.
Stanley Trimble, of the University of California in Los Angeles, has assembled 140 years-worth of information about sediment buildup and erosion throughout a watershed, made up of the Coon Creek river and its tributaries, in a region of Wisconsin farmland. Because the journey of a sediment particle through a watershed is so confoundingly complex (it may be waylaid many times, washing up on a stream's banks or sandbars whenever the water slows down and lets the particle sink), it takes a long-term study like this one to make a reliable assessment of just how much soil is actually eroding from croplands.
Trimble's results show that, overall, soil erosion in this watershed has been steadily decreasing since the 1930s and is now a mere 6 percent of what it was during the Dust Bowl years. Ultimately, the message from Coon Creek is that soil conservation efforts seem to be working. Since the 1980s, more farmers have been trying new techniques, for example, tilling the soil as little as possible, or fitting crops to the contours of the land.
These findings stand out in sharp relief from several other reports on soil erosion published since the 1970s claiming that increasingly aggressive farming practices have been causing extensive amounts of topsoil to vanish from croplands--in some cases at a rate matching that of the 1930s. The costs of this alleged problem include cleaning up waterways and aquifers polluted by eroded soil. They also include the extra fertilizer and irrigation (which both pose environmental problems of their own) that farmers need to compensate for the poor quality of the soil left behind.
"But you can't make or destroy matter, and very little soil is actually soluble. What I'm trying to do is ask, 'how can we have a lot of erosion?'" said Trimble. "To account for a lot of erosion, you have to account for a lot of sediment."
Previously, scientists have determined erosion rates by studying soil movement over one particular patch of land and then using a theoretical model to calculate soil loss for a whole watershed. But Trimble took the reverse approach. If more soil were really eroding from the croplands around Coon Creek, he reasoned, then it should show up in various spots throughout the watershed as well. Instead he found that the buildup of sediment in the watershed has been progressively slowing down.
"We found that much of the sediment in Coon Creek doesn't move very far, and that it moves in very complex ways," he said. For example, even while sediment was restlessly shifting its location throughout the watershed, the amount of sediment actually leaving the system and flowing down the Mississippi river (long thought to be a key indicator of how much erosion was taking place on land) has stayed roughly the same over the last 140 years. It appears that watersheds are even more intricate than scientists have imagined, and that existing models of erosion--the basis for recent claims about the dire straits of agricultural soil--need to be re-evaluated.
Just as the conservation efforts set in place in the mid-1980s seem to be slowing erosion, the Coon Creek record also shows a sharp drop in the levels of sediment piling up on the river bottom after 1938, once the newly established Soil Conservation Service began taking measures to protect cropland soil. That year, Trimble's predecessors began the Coon Creek study in order to see what the effects of these measures would be. The oldest layers of sediment they studied appeared to have been deposited in the 1850s. By World War II, when the study was abandoned, the geologists had a record of almost 90 years.
Trimble revived the study in 1973 and carefully surveyed the watershed, looking for changes in elevation caused by sediment buildup since the 1930s. From these results, he proposed that erosion was on the decline (See Science, 9 October, 1981, p. 181). But the 40 years between the Dust Bowl and the 1970s weren't enough to be certain that his information was typical of the watershed's behavior over time. In the current Science paper, Trimble has now extended the record up to 1993, allowing him to propose with much more confidence that farmlands in temperate, humid areas are not, in fact, suffering greatly from soil erosion.
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