Compositional characteristics of organic matter and its water-extractable components across a profile of organically managed soil

• Effect of compost addition on soil organic matter (SOM) is limited to the top 30 cm.
• SOM is enriched by hydrophilic components with the increase in depth.
• Hydrophobic SOM components are contributed by added compost.
• Compost application increases contents of water-extractable humic-like constituents.
• Different water-extractable components of SOM show similar depth distributions.

Soil organic matter (SOM) plays a dominant role in the functionality of agricultural soils and particularly so in organic farming. Yet, there is limited knowledge on the effect of organic management in which the soil is subjected to incorporation of a variety of organic residues on the composition of SOM and water-extractable organic matter (WEOM) in top and sub-surface soil layers. The general objective of this study was to quantify depth-related changes in the composition of SOM and WEOM in an organically managed soil subjected to multiple compost applications, using spectroscopic techniques requiring no or minimal soil sample pre-treatment. We collected soil samples across the top 60 cm on October 2012 from an existing field experiment initiated in late 2009, which was organically fertilized by means of compost and green manure incorporation. Compost was applied at levels of 0 (control), 20, 40 and 60 m3 ha− 1, with the control treatment being fertilized with urea and amended with green manure. The collected samples were then used to characterize (i) SOM by FTIR absorbance associated with hydrophilic SOM functional groups and aliphatic CHs, total organic C and N contents, and (ii) WEOM by dissolved organic C (DOC) concentration, UV–VIS absorbance and fluorescent components identified following parallel factor analysis. In general, for all the studied attributes the core data tended to (i) increase after compost addition, although differences among the compost doses could not always be identified, and (ii) decrease with soil depth for all compost doses as well as the control treatment. Compost addition enriched soil by hydrophobic organic matter and water-extractable aromatic and, specifically, humic-like components. In the compost-amended soil, SOM became depleted of hydrophilic groups and enriched by hydrophobic aliphatic CH-rich substances. However, the content of hydrophilic organic matter in SOM was elevated with increasing depth.