Earthworms increase forest litter mass loss irrespective of deposited compounds -- A field manipulation experiment in subtropical forests

Abstract

Earthworms modulate the carbon and nitrogen cycling in terrestrial ecosystems, their effect may be affected by deposited compounds due to human activity such as industrial emissions. However, studies investigating how deposited compounds affect the role of earthworms in carbon cycling such as litter decomposition are lacking, although they are important for understanding the influence of deposited compounds on ecosystems and the bioremediation by applying earthworms. For this, we performed a 365-day in situ litterbag decomposition experiment in a deciduous (Quercus variabilis) and coniferous (Pinus massoniana) forest in southeast China. We manipulated nitrogen (N), sodium (Na) and polycyclic aromatic hydrocarbon (PAH) deposited compounds during litter decomposition with and without earthworms (Eisenia fetida). After one year, N, Na and PAH compounds all slowed down litter mass loss, with the effects of Na being the strongest. By contrast, E. fetida generally increased litter mass loss and their positive effects were uniformly maintained irrespective of the type of deposited compounds. Further, the pathways earthworms increasing litter mass loss varied among the types of deposited compounds and forests. As indicated by structural equation modeling, earthworms maintained their positive effects and mitigated the negative effects of deposited compounds by directly increasing litter mass loss and indirectly increasing soil pH and microbial biomass. Overall, the results indicate that the acceleration of earthworms on litter mass loss is not affected by deposited compounds, with the pathways of earthworms increasing litter mass loss varying among the types of deposited compounds and forests. This suggests that the effects of atmospheric deposited compounds and earthworms on terrestrial ecosystem processes need to be taken into account because earthworms may cancel out the detrimental influence of deposited compounds on litter decomposition.

Earthworms neutralize the influence of components of particulate pollutants on soil extracellular enzymatic functions in subtropical forests

“…As epigeic earthworm species E. fetida does not form permanent burrows and therefore its effect on soil porosity is limited ( Brown, 1995 ). However, the addition of E. fetida increased litter mass loss irrespective of the types of particulate components ( Yang et al, 2023 ). Presumably, processing of litter by E. fetida facilitated microbial litter decomposition and mitigated detrimental effects of PAHs on soil EFs and microbial activity ( Klamerus-Iwan et al, 2015 ).…”

“…In the N and Na addition treatments, earthworms weakened the correlation between soil EFs and litter decomposition. This suggests that increased nutrient supply due to earthworms ( Marhan & Scheu, 2006 ; Yang et al, 2023 ) may have reduced the need for nutrients from decomposing litter by microorganisms and thereby reduced the secretion of extracellular enzymes and litter decomposition. By contrast, with the addition of PAHs earthworms strengthened the correlation between litter N loss and soil EF-N.…”

“…In forests, earthworms may affect soil pH and microbial activity via gut and cast-associated processes ( Devliegher & Verstraete, 1995 ; Drake & Horn, 2007 ). In particular, earthworms may increase the rate of soil nutrient cycling and microbial biomass by accelerating litter decomposition, and forming biopores and microbial hotspot areas ( Hoang et al, 2016 ; Yang et al, 2023 ). Since particulate pollutants detrimentally affect microbial activity by promoting soil acidification and leaching of nutrients ( Lu et al, 2014 ; Yang et al, 2023 ), earthworms may attenuate their detrimental influence by increasing soil pH and improving nutrients availability.…”

“…In particular, earthworms may increase the rate of soil nutrient cycling and microbial biomass by accelerating litter decomposition, and forming biopores and microbial hotspot areas ( Hoang et al, 2016 ; Yang et al, 2023 ). Since particulate pollutants detrimentally affect microbial activity by promoting soil acidification and leaching of nutrients ( Lu et al, 2014 ; Yang et al, 2023 ), earthworms may attenuate their detrimental influence by increasing soil pH and improving nutrients availability. As the abundance of earthworms varies among different types of forests, it may be necessary to investigate their effects at varying density ( Cortez, 1998 ; Szlavecz et al, 2011 ).…”

“…As the abundance of earthworms varies among different types of forests, it may be necessary to investigate their effects at varying density ( Cortez, 1998 ; Szlavecz et al, 2011 ). Further, the effects of N, Na and PAHs on soil functions may vary with forest types ( Lin et al, 2017 ; Ji et al, 2020 ; Yang et al, 2023 ). Therefore, to understand interactions between earthworms and pollutants in different forests at close to natural settings, experimental studies need to mimic the density of earthworms in the respective forests.…”