Résumé
1. In many terrestrial ecosystems, detritivorous soil organisms ingest
large amounts of leaf litter returning most of it to the soil as faeces.
Such conversion of leaf litter into faeces may stimulate decomposition by
increasing the surface area available for microbial colonization. Yet,
experimental support for either the outcome or the mechanism of these
conversion effects is lacking. 2. Based on the hypothesis that the
identity of plant species from which leaf litter is transformed into
faeces has a critical role in how faeces decomposition proceeds, we
collected faeces of the widely abundant millipede Glomeris marginata fed
with leaf litter from seven distinct tree species. We compared the
physical and chemical characteristics and the rates of carbon (C) and
nitrogen (N) loss between litter and faeces. 3. We found that after 100
days of exposure under controlled conditions, C loss was on average higher
in faeces (40%) than in litter (26.6%), with a significant increase for
six out of the seven species. Concurrently, N dynamics switched from a net
immobilisation (7.7%) in litter to a net release (14.6%) in faeces, with a
significant increase for five out of the seven species. 4. Litter
conversion into faeces generally homogenised differences in physical and
chemical characteristics among species. Despite such homogenisation,
variability in rates of faeces C and N loss among species was similar
compared to leaf litter, but correlated with a different set of traits.
Specifically, faecal pellet C loss was positively related to compaction
(decreased specific area and increased density of faecal pellets), and
both C and N loss from faecal pellets were positively related to
fragmentation (increased specific area and perimeter of particles within
faecal pellets). 5. We conclude that litter fragmentation and compaction
into detritivore faecal pellets leads to substantially enhanced
decomposition, with a particularly strong impact on N dynamics that
changed from immobilisation to net release depending on litter species.
Moreover, litter quality control on decomposition is reshuffled by litter
conversion into faeces. In ecosystems with high detritivore abundance,
this so far largely overlooked pathway of organic matter turnover may
strongly affect ecosystem C and N cycling.