Decomposition of de-inking paper sludge in agricultural soils as characterized by carbohydrate analysis

Increasing amounts of de-inking paper sludge (DPS) are available from paper mills, and could be used to improve soil fertility because of their high organic matter content. Our aim was to use chemical fractionation and carbohydrate characterization to determine the transformation and decay rates of DPS in different soils when large loading rates are applied. DPS was added to a well-drained silty clay loam (Typic Dystrochrept) and a poorly-drained clay loam (Typic Humaquept) at rates of 0 (control), 50 or 100 Mg dry matter ha−1. Soil samples were obtained periodically during 726 days after sludge incorporation. Soil organic matter was fractionated into hot-water extractable (HWC), mild-acid extractable (MAC) and strong-acid extractable carbohydrates (SAC), and acid-resistant carbon (ARC). The MAC fraction mostly contained hemicellulosic sugars, whereas SAC fraction included most cellulosic glucose. The contribution of microbial saccharides to the different carbohydrate fractions increased during DPS decomposition. The carbohydrate composition indicated that the chemical fractions reflected the net balance between disappearance of sludge carbohydrates and appearance of newly synthesized microbial carbohydrates. The MAC, SAC and ARC fractions in DPS-amended soils, had relative degradabilities of SAC > MAC > ARC. The sludge used, appeared to decompose according to a two-phase pattern, with an initial rapid-decay phase mostly determined by SAC and ARC disappearance (mean residence time 0.1 and 0.3 year, respectively), and a second slow-decay phase: largely characterized by ARC disappearance (mean residence time 8.5 years). DPS decomposed more slowly at the highest application rate, presumably because the capacity of soil microbes to decompose C was temporarily limited by nutrient deficiency. Chemical fractionation and carbohydrate analysis proved useful to study quantitatively and qualitatively the decomposition and transformation of wood-derived residues in agricultural soils.