Active carbon pools and enzyme activities in soils amended with de-inking paper sludge

Application of paper mill wastes generally improves soil organic matter content, biological activity and physical properties. However, the impact of large application rates (>50 Mg ha(-1)) on soil microflora and their activity has not been assessed. A field study was undertaken on a well-drained clay loam and a poorly drained silty clay loam amended with de-inking paper sludge (DPS) at rates of 0 (control), 50 or 100 Mg ha(-1). K(2)SO(4)-extractable C (C(ext)), soil water content (SWC), microbial biomass C (MBC) and different enzyme activity rates were periodically measured in soil during 1075 d following DPS incorporation. Compared with control soils, C(ext) content increased by 100 to 200%, and soil water content increased by 35% following incorporation of DPS at 100 Mg ha(-1). Those differences decreased in time as DPS decomposed. Soil MBC increased proportionally with the rate of DPS amendment and was about twice the amount in soils amended with 100 Mg ha(-1) compared with the control. Microbial quotient (ratio of MBC to total soil organic C) was greater in DPS-amended than in control soils until day 370, reflecting the input of labile C from DPS. Compared with the control, fluorescein diacetate hydrolysis and alkaline phosphatase activity rates increased by 40 to 100% when adding 50 Mg DPS ha(-1). However, the rates were similar for 50 and 100 Mg DPS ha(-1). We concluded that DPS promoted microbial growth and activity in the soil by improving C and water availability. However, levelling off of enzyme activity at a DPS loading rate above 50 Mg ha(-1) could reflect changes in soil microbial community, or some kinetic interference or nutrient deficiency induced by excessive C input.