Acid deposition strongly influenced element fluxes in a forested karst watershed in the upper Yangtze River region, China

The hydrological fluxes and associated element input-output budgets were investigated from July 2007 to August 2008 in a forested karst watershed in Guizhou, southwest China. Banning logging in the watershed (mountain closure) has been in practice as an effort to reduce soil erosion and its impact to Yangtze River. The watershed lies in an area heavily affected by acid deposition. Therefore, specific objective of this study was to investigate the impact of acid deposition on elemental fluxes in the forested karst watershed. Weekly bulk precipitation, throughfall, and stemflow samples were collected from two forest plots. Volume and chemistry of streamflow out of the watershed were continuously monitored to determine the concentration and fluxes of elements. The bulk precipitation had low pH value of 4.8 as annual volume-weighted mean and high SO42--S and Ca2+ concentrations. Concentrations of elements were higher in throughfall or stemflow than the bulk precipitation. Forested karst watershed had low streamflow but high water discharge through deep seepage, occupying 4.0% and 40.4% of the annual bulk precipitation, representatively. Streamflow had high and constant pH value of 8.3 as annual mean, and was dominated by Ca2+ and Mg2+. The annual net input of SO42--S into the watershed was greater than 40 kg S ha−1 year−1. The strikingly high rate of output of Ca2+ and Mg2+, greater than 250 kg ha−1 year−1 and 150 kg ha−1 year−1, representatively, reflected the fast weathering of carbonaceous rocks, and the strong neutralizing capacity of weathering to acid deposition in the karst watershed. This research indicates that acid deposition strongly influenced the export of nutrients out of the karst watershed and can accelerate transformation of the local land form.