Chemistry of Tension Lysimeter Water and Lateral Flow in Spruce and Hardwood Stands

Abstract

AbstractThe concentrations of H+, NO−3, SO2−4, soluble Al, and undissociated acidity were measured in throughfall, soil solution, and streamflow during five rainfall events on two steeply sloping forested sites in the Great Smoky Mountains. The sources and sinks of these chemical species on a spruce‐birch (Picea rubens Sarg.‐Betula lenta L.) site were compared to those on a hardwood site. The chemistry of water collected in zero‐tension lateral flow collectors during storm events was contrasted with water collected at corresponding depths in tension lysimeters. Tension lysimeter samples from the spruce site had much higher concentrations than those from the hardwood site of H+ (3–7 ×), SO2−4 (1.5–2 ×), Al (2–5 ×), and undissociated acidity (1.5–2 ×). Baseflow from a first‐order stream draining the spruce site was significantly higher in Al and H+ than hardwood site baseflow. On both sites the largest sources of H+ and undissociated acidity were throughfall (incident precipitation + canopy sources) and the O horizon; on the spruce site the O horizon was the predominant source of each. Aluminum and SO2−4 also increased dramatically with passage through the O horizons, especially on the spruce site. Only the spruce site generated appreciable lateral flow. Nitrate, undissociated acidity, and Al concentrations in spruce site lateral flow were not significantly (p <0.05) different from those in corresponding tension lysimeter samples. Electrical conductivity and concentrations of H+ and SO2−4, however, were higher in lateral flow from spruce site B horizons than in corresponding tension lysimeter water. Reduced soil contact time and contact surface area were hypothesized reasons for the ineffectiveness of these B horizon sinks with regard to rapid lateral flow. Lateral flow pathways on the spruce site may have also bypassed sinks for Al and undissociated acidity that exist in the lower mineral horizons and bedrock zone. Stream pH depressions during stormflow may be related to rapid subsurface flow and interflow emanating from these acidic spruce‐birch sites.