Decomposition of paper de-inking sludge in a sandpit minesoil during its revegetation

Paper de-inking sludge was used as an organic amendment for revegetating an abandoned sandpit in Québec, Canada. In situ patterns of sludge decomposition and of total nitrogen and phosphorus dynamics were characterized in a litter bag study. In a one-time operation, sludge was applied at a rate of 0 or 105 Mg dry matter ha−1, along with N at 3, 6 or 9 kg Mg−1 sludge and P at 0.5 or 1.0 kg Mg−1 sludge. Sludge and fertilizers were incorporated into the top 0.21 m of the minesoil and tall wheatgrass (Agropyron elongatum (Host) Beauv.) was seeded. Mass loss was well described by a double exponential model when cumulative degree-days (sum of daily temperature above 0°C) were used as the independent variable. Fifty-one percent of the initial material decomposed with a half life of 0.4 yr, whereas the remaining material had a much slower rate of decay with a half life of 13 yr. The large size and slow decomposition rate of the recalcitrant pool of this material were attributed to the high lignin content and the presence of clay in the sludge. Both N and P in decomposing sludge presented a short accumulation phase followed by a long release phase which likely contributed to the successful revegetation of this disturbed sandpit site.