Decoupling shredder activity and physical abrasion in leaf litter decomposition process: experiments in the Torna-stream (Hungary) affected by red sludge spill

Abstract

Leaf litter represents a significant energy source for headwater ecosystems. Its breakdown includes dissolution, microbial activity, fragmentation by shredders, and physical abrasion. As a consequence of red sludge spill that took place in October 2010 in Hungary, the fauna of Torna-stream was killed, offering a “natural laboratory” to measure mass loss without shredder activity. Decomposition of nine leaf litter types was investigated with litter-bag method in 2011 (no macroinvertebrates; post-disaster experiment, POST), 2012 (after macroinvertebrate recolonization, at two sites: POSTRE: impacted site and POSTREF: reference site) in the Torna-stream. In the POST it was possible to measure leaf mass loss without shredder activity and to separate the leaf decomposition rates for each contributor of decomposition with a newly developed model. The model simulates the mass loss through three processes: physical abrasion and drifting, leaching and microbial mineralization, and macroinvertebrate consumption. In 2011, physical abrasion, microbial decomposition, and leaching accounted for 10–50% of initial mass loss, while in the POSTRE and in POSTREF for only 5–47%. The rate of litter decomposition driven by shredders in POST was between 5 and 44%, and in the POSTRE and POSTREF it accounted for 24–90%. The most apparent result of the experiments described in this paper is that the presence or absence of macroinvertebrates determines the decay characteristics and underlines the key role of macroinvertebrates in leaf litter decomposition.