13C NMR assessment of decomposition patterns during composting of forest and shrub biomass

Abstract

A laboratory experiment was designed to investigate the degradation patterns of leaves from 12 forest and shrub species typical of Mediterranean ecosystems by solid-state 13C NMR. The spectral data have been compared with those for the major organic fractions, and elementary composition in three transformation stages (zero time, intermediated and advanced (168 d)). The plant material in general showed a selective depletion of lipid and water-soluble products and a concentration in acid-insoluble residue (Klason lignin fraction), but the increasing percentage of total alkyl carbons (not observed in pine leaves) suggests that recalcitrant aliphatic material accumulates in the course of the 168 d incubation, when the total weight losses were up to 660 g kg-1. This contrasts with the fact that the concentration of extractable alkyl C (i.e. the lipid fraction) decreased in all cases. The results for the different plants suggested some general transformation trends simultaneous to specific biodegradation patterns. The non-ameliorant, soil acidifying species (i.e. those a priori considered to favor the accumulation of humus with low biological activity) have high initial concentrations of extractives, alkyl structures and comparatively lower percentages of O-alkyl structures. The decay process in these species is not associated to the increase of the alkyl-to-O-alkyl ratio, which is shown by the ameliorant species. Superimposed on these major trends, the biomass of the different plants underwent divergent paths in the course of composting, leading to, for example, (i) accumulation of recalcitrant, nonhydrolyzable alkyl and aromatic structures (Retama, Genista); (ii) enrichment of resistant O-alkyl structures such are stable fractions of carbohydrate and tannins (Pinus, Calluna); and (iii) accumulation of aliphatic extractives with the lowest stabilization of protein in resistant forms (Arctostaphylos, Ilex). In particular, in the acidifying species, the spectral patterns suggest that the apparent stability of the aromatic domain is compatible with selective preservation of tannins together with aliphatic structures. Such specific tendencies are also illustrated by the difference spectra (0 vs 168 d) which suggest that early humification processes are highly heterogeneous and distinct rather than the selective degradation of lipid and water-soluble fractions and carbohydrates, and they may include stabilization of tannins and aliphatic (cutin- and protein-like) macromolecules.