Recycling pulp mill sludge to volcanic soil: a column leaching study

A leaching experiment was conducted to determine the influence of four successive applications of pulp mill sludge on some parameters of an Andisol. Undisturbed soil columns were treated with pulp mill sludge (0 to 30 Mg ha(-1)) every three months during one year with a hydraulic regime of 1200 mm per year. Leachates of each treatment were analyzed periodically and, at the end of the period, columns were fractionated at three depths (0-20, 20-40 and 40-60 cm) and soil chemical parameters were analyzed. Sludge added successively to Andisol increased the nitrate concentration in leachates. After the fourth application of 30 Mg ha(-1) sludge, nitrate leaching was about 1.7 times higher than in the control column, however, this level is still lower than the limit under Chilean regulations. Low ammonium concentrations were observed in leachates as a result of nitrification processes and retention by clay and organic matter in the Andisol. In fractionated soil columns, pH, organic matter, total nitrogen and phosphorus increased as the sludge rate increased, with the highest values found in the upper part of the columns. The concentration of nitrate and ammonium in soil columns followed an inverse pattern: while ammonium concentration was the highest at 0 to 20 cm, nitrate concentration was the lowest in that fraction; sludge addition caused a linear increase in ammonium and nitrate content at the three depths analyzed.