Summary The addition of organic wastes as a strategy to restore degraded soil in semiarid regions can increase soil aggregation and carbon fixation. These processes can be affected by both the characteristics of the organic amendments and the soil microbial community structure. This study examines the effect of single additions (150 t ha−1) of different organic residues for soil restoration (fresh and composted sewage sludge, vegetal residue and farmyard manure) on aggregation and carbon (C) retention in a semiarid degraded soil. Changes in the distribution of microbial biomass in soil aggregates were also evaluated. Sewage sludge compost was added at rates of 150 and 300 t ha−1 to determine the effect of the amount applied on the distribution processes. Large macroaggregates (2000–8000 μm), small macroaggregates (250–2000 μm), microaggregates (53–250 μm) and the < 53‐μm soil fraction were obtained, and organic carbon (Corg) concentration was determined in these fractions in addition to the total phospholipid‐derived fatty acids (PLFAs). Both the characteristics of the amendments and the amount applied affected soil aggregation and Corg concentration in the aggregate size classes. The manure and the compost added at 300 t ha−1 contributed to the fixation of stable Corg in soil aggregates to a greater extent than the fresh sludge, vegetal residue or compost added at 150 t ha−1. In all soil fractions, the total concentration of PLFAs was larger in restored soil than in the control soil, and increases depended on the aggregate size class and characteristics of the amendment. The largest microbial abundance occurred in both large and small macroaggregates. Highlights Effects of type and amount of amendment on aggregation and C fixation in semiarid soil are elucidated. We explored distribution of the microbial community in soil aggregates in response to new soil C pools. Carbon fixation and microbial abundance of aggregates are affected by type and amount of amendment. Restored and degraded soils differ in terms of the breakdown of new organic matter inputs.

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