Soil aggregation and biochemical properties following the application of fresh and composted organic amendments

The aim of this study was to evaluate the effects of fresh and composted paper sludge on macroaggregate stability of a silt loam under field conditions, and to assess the possible role of carbohydrate fractions and humic substances. The treatments included fresh paper mill sludge (PMS) and its compost (CPMS) applied at a rate of 40 Mg ha(-1) with or without a mineral N fertilizer (120 kg N ha(-1)), N fertilizer only (recommended rate of 160 kg N ha(-1)), and an unamended control. Measurements of total and amino sugars and humic substances were made on slaking-resistant aggregates 2 yr after the last of three successive annual applications of the treatments. Compared with the treatments that received no organic amendment, the PMS and CPMS applications increased macroaggregate stability by an average of 45%. The effects of fresh vs. composted amendments on soil macroaggregates and their organic C contents were similar but differences in C composition were observed. Humic acid content of aggregates >2 mm was significantly higher (50%) with CPMS than PMS, although part of this effect could be attributed to the slightly greater C application rate with CPMS. Conversely, glucosamine content, an indicator of fungi abundance, was significantly greater following PMS than CPMS application. We concluded that microorganisms, in particular fungi, were a more important factor of stable macroaggregation in the soil amended with fresh sludge, while humic substances played a greater role in compost-amended soil. These effects were long lasting in the field since they were still noticeable 2 yr after the last application.