Detritivore conversion of litter into faeces accelerates organic matter turnover

Abstract

Litter-feeding soil animals are notoriously neglected in conceptual and mechanistic biogeochemical models. Yet, they may be a dominant factor in decomposition by converting large amounts of plant litter into faeces. Here, we assess how the chemical and physical changes occurring when litter is converted into faeces alter their fate during further decomposition with an experimental test including 36 combinations of phylogenetically distant detritivores and leaf litter of contrasting physicochemical characteristics. We show that, across litter and detritivore species, litter conversion into detritivore faeces enhanced organic matter lability and thereby accelerated carbon cycling. Notably, the positive conversion effect on faeces quality and decomposition increased with decreasing quality and decomposition of intact litter. This general pattern was consistent across detritivores as different as snails and woodlice, and reduced differences in quality and decomposition amongst litter species. Our data show that litter conversion into detritivore faeces has far-reaching consequences for the understanding and modelling of the terrestrial carbon cycle.

Synergistic interactions between detritivores disappear under reduced rainfall

“…Such a reduction in the contribution of diverse detritivore communities to decomposition under future rainfall patterns could slow carbon release from litter. This could have important consequences for soil carbon storage, but an improved understanding of detritivore contribution to soil organic matter formation has yet to emerge (Joly et al 2020) to predict the consequences on global carbon cycling.…”

“…This litter consisted in 0.465 g C g -1 litter and 0.0089 g N g -1 litter, for a C:N ratio of 52.0. Litter of this tree species is characterized as relatively recalcitrant, with high C:N ratio and tannin concentrations, and low concentration of water-soluble compounds and capacity to retain water (Joly et al 2020). We used decomposing leaf litter rather than freshly senesced litter because detritivores prefer feeding on partially decomposed litter (David and Gillon, 2002).…”

“…Litter-feeding soil invertebrates (detritivores hereafter) play a major role in organic matter turnover by consuming large amounts of decomposing litter (García-Palacios et al 2013) and returning most of it to the soil as feces (David 2014). This conversion of litter into feces consisting of a myriad of fecal particles facilitates further leaching and microbial activity, thereby accelerating decomposition (Joly et al 2020). Despite the importance of this pathway, its response to expected changes in rainfall patterns is difficult to predict.…”

Synergistic interactions between detritivores disappear under reduced rainfall

“…This litter consisted in 0.465 g C/g litter and 0.0089 g N/g litter, for a C:N ratio of 52.0. Litter of this tree species is characterized as relatively recalcitrant, with high C:N ratio and tannin concentrations, and low concentration of water‐soluble compounds and capacity to retain water (Joly et al 2020). We used decomposing leaf litter rather than freshly senesced litter because detritivores prefer feeding on partially decomposed litter (David and Gillon 2002).…”

“…Such a reduction in the contribution of diverse detritivore communities to decomposition under future rainfall patterns could slow carbon release from litter. This could have important consequences for soil carbon storage, but an improved understanding of detritivore contribution to soil organic matter formation has yet to emerge (Joly et al 2020) to predict the consequences on global carbon cycling. However, our focus on a single and rather recalcitrant litter prevents generalizing our findings to litter of different quality.…”