Organic amendments as strategy to increase organic matter in particle-size fractions of a semi-arid soil

The distribution of organic matter (OM) in soil particle-size fractions is a key process in the dynamic and persistence of soil organic carbon. In order to study the distribution of organic amendments among the different soil particle-size fractions in a semi-arid, degraded soil (a sandy loam Haplic Regosol), a 9-month-incubation experiment was carried out after the application of composted organic amendments (90 Mg ha−1) differing in nature (farmyard manure, cFYM; vine pruning wastes, cPW, and sewage sludge, cSS). A physical fractionation based on the separation of soil according to particle size was carried out and chemical and microbiological parameters were measured both in the whole soil and in the particle-size fractions. After amendment, all the particle-size fractions in the amended soils showed higher organic carbon (Corg) and N concentration than the control soil. At the end of the experiment, the amount of Corg in the silt-sized (2–63 μm) and clay-sized (0.1–2 μm) fractions did not show significant differences among amended soils, indicating that the accumulation and protection of the OM in the fine fractions of this semi-arid soil are not affected by the nature of the composted amendment. Furthermore, the fine fractions (silt and clay) of the amended soils showed intermediate C/N ratios, suggesting that the organic compounds of these fractions are derived from both microorganisms and organic amendments. At the end of the experiment, the soils amended with cPW showed the highest Corg content in the whole soil and in the coarse sand fraction (200–2000 μm). In contrast, soils amended with cSS showed the highest Corg loss among treated soils and showed in general the highest microbial and enzymatic activities in the coarse sand fraction. These results highlight the importance of the recalcitrance of the amendment (high C/N ratio) to avoid fast degradation of non-protected OM in the coarse sand fraction. In conclusion, the type of the composted organic amendment did not influence the OM accumulation in fine fractions of the studied semi-arid soil, but was determinant for the degradation of OM in the coarse sand fractions.

► Organic amendments promote the association of OM with soil fine fractions.
► The amendment type is not determinant for the accumulation of OM in fine fractions.
► The stability of the amendment is important for C accumulation in coarse sand.
► Fine fractions accumulate both microbial- and organic amendment-derived compounds.
► Semi-arid soils with composted pruning wastes had the lowest Corg loss.