When soil erodes into a watercourse it causes big problems. Water with high levels of sediment needs more treatment to make it safe for drinking, the sediment fills up reservoirs and reduces the performance of hydroelectric power plants, and murky mud-filled water is harmful for a number of species. Normally, vegetation helps to hold sediment in place, but as more land is converted to farmland the amount of bare soil is increasing, and greater amounts of sediment are washing into streams, rivers and lakes.

Soy is an important ingredient in many consumer products and many multinational companies buy soy in bulk. In order to better understand the impact that agriculture is having on soil erosion, Rebecca Chaplin-Kramer from Stanford University, US, and her colleagues compared four soy-growing regions of the world: Iowa, US; Mato Grosso, Brazil; Jiangxi, China and Heilongjiang, China. The sites had differing landscapes and climates, with both the Chinese sites being hillier than the Brazilian and US sites. Meanwhile, rainfall was high at Mato Grosso and Jiangxi.

By modelling a hypothetical corporate supply chain and taking into account climate, soil-type, topography and land-use, the researchers simulated the amount of sediment erosion at each location associated with producing the same amount of soy. The results showed that for each hectare of habitat converted, Jiangxi exports on average more than ten times the sediment compared to Mato Grosso or Iowa, and four times that of Heilongjiang. Much of this variation was due to the differences between the landscapes (with Jiangxi being wetter and having steeper terrain, so more prone to erosion), but a significant part was also due to the proximity of the agricultural land to watercourses. Non-cultivated areas next to streams can dramatically reduce erosion, retaining as much as 80 to 90% of the soil.

In some cases, proximity to a stream turned out to be more important than differences in climate and topography, with the model showing that conversion of land next to a stream in Iowa would produce more soil erosion than converting the equivalent land in Jiangxi.

"A place much less inherently prone to erosion, like Iowa where the climate is temperate and land is relatively flat, can actually have worse sediment problems depending on where habitat is converted," explained Chaplin-Kramer, whose findings were published in Environmental Research Letters (ERL). This means that sometimes agricultural expansion can be less harmful at erosion-prone sites if they happen to have more undeveloped land situated far from watercourses.

This research was supported by Unilever, with the aim of helping to improve its assessments and develop more robust approaches to addressing environmental impacts when sourcing ingredients for products.

"For water quality and soil erosion in particular, we've shown that it's important not only to implement best practices, but to especially focus them where they're needed most – on the slopes and soil types most vulnerable to erosion, and in the locations where the sediment, if it erodes, is most likely to reach the nearest stream," said Chaplin-Kramer.

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