May 30, 2012
Deep soils that strongly bind phosphorus fertilizer buffer streamwater against changes to dissolved nutrient concentrations across wide areas of the Amazon rainforest that have been recently converted to intensive soybean agriculture, Brown-MBL student Shelby Riskin showed in new research. Riskin, who defended her PhD dissertation on May 30, spent the last five years comparing soils and waters in small forest and soybean watersheds in the Brazilian state of Mato Grosso, where the area of soybean farms has exploded since the early 2000s.
One of Riskin’s main findings is that the flow of water out of small streams in soybean fields increases four-fold because of the loss of evapotranspiration through the leaves of the original forest trees. But that change was not accompanied by increased “flashiness,” or flows of streamwater during storms. That’s because even under intensive cropping, the high permeability of soils allows water to infiltrate directly to groundwater without running overland. So even though more water reaches streams, it arrives in a very even flow controlled by release from groundwater. Because soils over the majority of the new soybean farming region are very deep as well as highly permeable, this allows water to contact soils along very long pathways that further buffer streamwater against changes in dissolved nutrients. “Soils play a critical role in regulating the way that intensive farming systems affect surface waters,” Riskin said, “and those effects may not follow the same patterns that we observe in intensively-farmed areas of temperate regions.”
Riskin analyzed the potential role of soils in controlling both the need for fertilizer and impacts to surface waters in the world’s three largest intensive soybean farming systems in Brazil, Iowa and Argentina. Compared with Iowa and Argentina, the large amounts of phosphorus fertilizer used to overcome the low native fertility of Brazilian Amazon rainforest soils are less likely to move to stream and cause nutrient overenrichment and algae blooms that compromise water quality because the Amazon soils bind phosphorus so tightly and permeable soils resist erosion. But the flip side to all that phosphorus retention is that Amazon soybeans will need high rates of fertilization for many years into the future because soils bind phosphorus so tightly it becomes relatively unavailable to the soybean crop. As crop agriculture on cleared rainforest soils expands around the world on high phosphorus-binding soils, the global demand for phosphorus fertilizer is likely to soar. Because relatively few countries control the vast majority of mineable phosphorus fertilizer is mined in relatively few countries (Morocco, China and Algeria have the world’s largest reserves).
Riskin worked at Tanguro Ranch, a 200,000-acre soybean farm in the headwater region of the Xingu River watershed in central Brazil. She was advised by Chris Neill at MBL and Stephen Porder at Brown. Her work was supported by grants from the National Science Foundation and the Mellon Foundation, a Watson Fellowship from MBL and a Cogut Fellowship from the Center for Latin American and Caribbean Studies at Brown.