Vertically sediment samples were taken using a 4-cm diameter PVC corer up to approximately 10-cm depth.In the laboratory, they were mixed, homogenized, and dried in a convection oven at 70 °C until they were
completely dry. The sediment was collected belowground biomass or gravel. The sediment samples were digested by EPA method 200.2 (USEPA 1991a, b) and analyzed by atomic absorption spectrometry (PerkinElmer Analyst 800). The pH of the sediment was measured with an Orion pH meter. Organic matter was determined by weight loss on ignition at 550 °C for 3 h (Wetzel 1975).