Aggregation and Organic Matter Decomposition in Soils Amended with De‐Inking Paper Sludge

Abstract

De‐inking paper sludge (DPS) has been traditionally disposed of by burning or landfilling, but could be used as an organic amendment in agricultural soils. Our objective was to assess the impact of DPS incorporation on organic matter and aggregation of a clay loam (Typic Dystrochrept) and a silty clay loam (Typic Humaquept). Whole soil C, particulate (>53 μm) and light fraction (density <1.8 Mg m−3) C, and water‐stable aggregation were measured periodically during a 3‐yr period after a single application of DPS at rates of 0 (control), 50, and 100 Mg ha−1 Microscopic observations of water‐stable aggregates were also performed. Adding DPS increased whole soil C content, which remained greater than in the control for the duration of the study. After 2 yr, about 40% of the initial material remained in the soil. The proportion of residual C attributed to DPS and present in the particulate fraction remained constant at 70 to 90% during the first 2 yr of the study, whereas the proportion of residual C present in the light fraction decreased from >95% for fresh DPS to <50% after 2 yr. One year after incorporation of DPS, the proportion of water‐stable aggregates >1 mm was 2 to 6 times larger in amended soils than in the control. This effect was still statistically significant after 3 yr. Microscopic observations revealed that DPS formed into clusters of wood fibers which became encrusted with mineral particles. We hypothesized that this encrustation provided physical protection to the decaying DPS which remained particulate (>53 μm) in size and progressively densified to >1.8 Mg m−3 As a result, water‐stable macroaggregates were formed with DPS as a central core.