Cattle trample the banks of streams. Other consequences of forest clearing are more subtle. Clearing alters the availability and proportion of nutrients in formerly forested soils and thus the likelihood that these nutrients will make their way into streams. This team is studying the effects of forest clearing for pasture on soil organic matter, nitrogen cycling and exchanges of trace gases with the atmosphere. The joint MBL-CENA group has identified several key changes in terrestrial ecosystems that are associated with the conversion of forest to pasture.
Pasture soils have higher pH levels because of the ash deposited after forest vegetation is burned.
They also have a higher availability of phosphorus, lower availability of inorganic nitrogen and lower production of nitrate than forest soils. According to our hypotheses, the relatively high availability of nitrogen and low availability of phosphorus in forest soils results in a high movement of nitrogen and low movement of phosphorus into streams. Because forest streams have high ratios of nitrogen to phosphorus, algal growth is limited by phosphorus where it is not already limited by low light levels in the shade of trees. After pastures are established, both soil nitrogen availability and the flow of nitrogen into streams decline. Because phosphates associated with iron and aluminum, which dominate those of tropical soils, are more soluble at high pH levels, we suspect that the higher pH of pasture soils leads to greater availability of soluble phosphorus.