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

A laboratory experiment was designed to investigate the degradation pattern s of leaves from 12 forest and shrub species typical of Mediterranean ecosy stems by solid-state C-13 NMR. The spectral data have been compared with th ose for the major organic fractions, and elementary composition in three tr ansformation stages (zero time, intermediated and advanced (168 d)). The pl ant material in general showed a selective depletion of lipid and water-sol uble products and a concentration in acid-insoluble residue (Klason lignin fraction), but the increasing percentage of total alkyl carbons (not observ ed in pine leaves) suggests that recalcitrant aliphatic material accumulate s 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 r esults for the different plants suggested some general transformation trend s simultaneous to specific biodegradation patterns. The non-ameliorant, soi l acidifying species (i.e. those a priori considered to favor the accumulat ion 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 t he 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, f or example, (i) accumulation of recalcitrant, nonhydrolyzable alkyl and aro matic structures (Retama, Genista); (ii) enrichment of resistant O-alkyl st ructures such are stable fractions of carbohydrate and tannins (Pinus, Call una); and (iii) accumulation of aliphatic extractives with the lowest stabi lization of protein in resistant forms (Arctostaphylos, Ilex). In particula r, in the acidifying species, the spectral patterns suggest that the appare nt stability of the aromatic domain is compatible with selective preservati on of tannins together with aliphatic structures, Such specific tendencies are also illustrated by the difference spectra (0 vs 168 d) which suggest t hat early humification processes are highly heterogeneous and distinct rath er 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. (C) 2000 Elsevier Science Ltd. Al l rights reserved.