Ecology and Evolution: Biology 301 -- Wethey
Department of Biological Sciences
University of South Carolina

Nutrient Cycling

Hubbard Brook Experimental Forest photos:
• Site description
• This site has been studied since 1963, and inputs, output, and cycling of water and nutrients have been determined using rain gauges, and weirs to measure streamflow. Because bedrock is close to the surface, almost all groundwater comes out as streamflow.
• Clearcutting and herbicide treatment in mid 1965 increased streamflow 39% in first year and 28% in second year. Stream water increased acidity 5-fold (pH 5.1 to 4.3), and streamwater concentrations of Ca²⁺ and Mg²⁺ increased 400%, K⁺ increased 1560% after clearcutting. Losses of Nitrate were 41 times higher than normal following deforestation. The graph is a plot of nitrate export in the stream water. The horizontal axis is plotted in units of months since June 1963. Total inorganic ion export was 15 times higher than from undisturbed adjacent watersheds. Inputs of nitrate were 5 kg /hectare/yr, and outputs were 150 kg/hectare/yr, indicating it would take at least 60 years to replenish the losses from the first 2 years after deforestation.
• Clearcutting stopped the tight recycling of nutrients that normally occurs in forests, so nutrients washed out of the system.

  Another watershed was experimentally strip-cut sequentially by thirds (25m-wide strips, parallel to the contours) in the years 1970, 1972, and 1974, and the trees were removed by rubber-tired skidders. A forested buffer strip was left along the lower third of the stream channel. This watershed lost less nitrate than did the clearcut.

  The retention of nutrients in the system is dependent on recycling by organisms.

• Similar results occurred when the Romans built the Via Cassia in 71BC alongside the Lago di Monterosi near Rome. The change in nutrient runoff caused by deforestation can be seen in the sediments of the lake.