Sorption of sulfate and retention of cations in forest soils of Lien-Hua-Chi watershed in central Taiwan

In Taiwan, the average sulfate and nitrate deposited by acid rain as precipitation amount to 50 kg SO42− and 15–50 kg NO3− ha−1 year−1, respectively. The reactions of sulfate and nitrate with forest soils were not fully understood. Thus, we selected two forest pedons, namely the Dystrochrept and Hapludult of the Lien-Hua-Chi watershed in central Taiwan, to study their sorption and desorption of sulfate. Soil samples were leached with simulated acid rain solutions of various acidity and sulfate concentrations to assess their retention of cations and sulfate. The amounts of sorbed-SO42− of two pedons extracted with NaH2PO4 (0.016 M) solutions in the range 0.14–1.09 mmol kg−1 soil were higher than that of native water-soluble sulfate (i.e., extracted with H2O), in the range 0.15–0.41 mmol kg−1 soil. The amounts of native sorbed-SO42− of two pedons correlated significantly with the exchangeable Al (r = 0.91). The sorption of sulfate did not fit well the Langmuir equation. The native sulfate contents of all tested pedons were near the maximum sulfate sorption capacity. The sulfate sorption capacity and the amount of released Al correlated well with increasing acidity of simulated acid rain added. Introducing solutions of various acidity and sulfate concentrations into the leaching solution flowing through the soil column resulted in sharp increases in amount of sulfate and aluminum released in percolates after several pore volumes were replaced. High clay and sesquioxide contents at soil depths of 30–50 cm (Bw horizon) and 50–80 cm (BC) of pedon 1 retained part of the sulfate added serving as the sulfate sink. Released K, Mg and Ca showed two periods of higher leaching pattern, having a trend similar to that the electrical conductivity (EC) pattern. After leaching of simulated acid rain through the soil column, the soil exchangeable cations were decreased. The tested soils possess an acid-buffering capacity in pedon 1. Pedon 1 is composed of high clay and sesquioxide contents and possesses greater buffer capacity than that of pedon 2.