Soil Fauna Effects On Chemical Compounds In Foliar Litters During Decomposition In Seven Ecosystems Along An Altitudinal Gradient In The Minjiang River Basin

Decomposition of foliar litter is an important ecological process linking carbon and nutrient cycling on plant-soil interface in terrestrial ecosystems.Numerous studies have greatly advanced our knowledge that some macro-soil fauna physically breaks and phytophagous soil fauna feeds on litter materials and some saprophagous soil fauna impacts microbial activity;all of these effects may alter the direction and magnitude of litter decomposition process.These findings suggest that effects of soil fauna are limited by inherent habitats and ambient environment,so will faunal effect vary between the litters with labile or recalcitrant components?Further,will such an effect vary with changes in climate?Both questions are still poorly understood.Here we hypothesized that a)Faunal effect is greater for labile compounds and minor for recalcitrant compounds in litter,and b)such effects decrease with increasing altitude.The altitudinal interval is greater than 3500 m in the Minjiang River Basin,which shows an obvious vertical zonality with changes in climate along the altitudinal gradient.We conducted an in situ litterbag experiment with two mesh sizes(0.04 mm to exclude soil fauna and 3 mm to allow soil fauna accesses in)to evaluate faunal effects on contents and remaining levels of water-(total,dissolved organic carbon and total dissolved nitrogen),organic-and acid-soluble substances and acid-unhydrolysable residues in 13 dominant foliar litters in 7 corresponding ecosystems(2 dominant species each with an exception of cypress in two ecosystems)at different altitudes.The studied ecosystems and foliar litters were mid-subtropical forest(453 m a.s.l.,camphor(Cinnamomum longepaniculatum)and pine(Pinus massoniana)),northern subtropical forest(945 m a.s.l.,oak(Quercus acutissima)and cedar(Cryptomeria fortunei)),arid valley(1563 m a.s.l.,planted cypress(Cupressus chengiana)and a shrub(Campylotropis macrocarpa)),forest-valley ecotone(2158 m a.s.l.,natural cypress(Cupressus chengiana)and oak(Quercus baronii)),subalpine forest(3028 m a.s.l.m birch(Betula albosinensis)and fir(Abies faxoniana)),alpine forest(3593 m a.s.l.,another cypress(Sabina salturaria)and willow(Salix paraplesia))and alpine meadow(3989 m a.s.l.,a forb(Ajania nubigena)and a grass(Carex atrofusca)).Our three-year decomposition results showed that:After 2-3 years of decomposition,water soluble substances(WSS)in foliar litters remained 0-39%across species,and soil fauna accelerated WSS decomposition.Both WSS content and remaining were negatively correlated with mass remaining.Faunal effect on WSS remaining was positively correlated with faunal effect on mass remaining,but such an effect had less relationship with soil fauna community.WSS contents in foliar litters in high altitude ecosystems(subalpine forest,alpine forest and alpine meadow)increased with increasing altitude.Faunal effects on WSS contents and remaining infoliar litters among different ecosystems were only significant at the end of the first year,and such an effect was higer in low altitude ecosystems.Dissolved organic carbon(DOC),which was accelerated by soil fauna,in foliar litters remained 2-28%and total dissolved nitrogen(TDN)remained 6-174%across species at the end of this experiment.DOC concentration and remaining and TDN remaining were negatively correlated with mass remaining.Faunal effects on both DOC and TDN remaining were positively correlated with such an effect on mass remaining,and such effects were positively correlated with group density,Shannon-Weiner diversity,Pielou evenness and Margalef richness indexes of soil fauna but negatively correlated with Simpson dominance index.Overall,DOC concentrations in foliar litters were higher in high altitude ecosystems.Faunal effect on DOC concentration in foliar litter was sifnificant different in the first year,and such an effect was higer in low altitude ecosystems.Organic-soluble substances(OSS)in foliar litters remained 5-52%across species at the end of this experiment,and were accelerated by soil fauna.Both OSS content and remaining were negatively correlated with mass remaining.Faunal effects on both OSS content and remaining were positively correlated with such an effect on mass remaining,but were negatively correlated with group density of soil fauna.Overall,both OSS contents and remaining in foliar litters were higher in high altitude ecosystems.Faunal effects on OSS remaing foliar litter were strongest in arid valley.Acid-soluble substances(ASS)in foliar litters remained 19-64%across species at the end of this experiment.Both ASS content and remaining were negatively correlated with mass remaining.Faunal effect on ASS remaining was positively correlated with such an effect on mass remaining and Pielou evenness and Margalef richness indexes of soil fauna,but negatively correlated with group density,individual density and Simpson dominance index of soil fauna.Overall,ASS contents in foliar litters increased and then decreased with increasing altitude,and highest in subalpine forest.Faunal effect on ASS remaining in foliar litter was only significant different in growing season of the first year,and such an effect was lower in mid-subtropical than other ecosystems.Acid-unhydrolysable residues(AUR)in foliar litters remained 57-175%across species at the end of this experiment,and were accelerated by soil fauna.AUR content and remaining were positively and negatively,respectively,correlated with mass remaining.Faunal effect on AUR remaining was positively correlated with such an effect on mass remaining,but was negatively correlated with group density and individual density of soil fauna.Overall,AUR contents were lower but remaining levels were higher in foliar litters in high altitude ecosystems.Faunal effects on AUR contents were higher in arid valley and alpine meadow.Direct effect of soil fauna community on litter decomposition was greater than faunal effect on chemical compounds and then indirectly impacts it,and group density of soil fauna was the primary factor in regulating decomposition of foliar litter in this study.OSS and DOC concentrations that were affected by soil fauna have greater interpretability and AUR has lowest interpretability in decomposition of foliar litters.Faunal effects on chemical compounds in foliar litters varied in different ecosystems(altitudes).First,faunal effects on labile compounds with high decomposability in explaining litter decomposition greatly varied among different ecosystems.For instance,faunal effect on WSS content increased litter decomposition in alpine meadow but decreased it in arid valley and subalpine forest.Faunal effect on DOC increased litter decomposition in forest-arid valley ecotone and alpine meadow,but such an effect on TDN decreased it in subalpine forest.Second,faunal effects on OSS and ASS with medium decomposability increased litter decomposition in subalpine forest in low altitude and in alpine meadow in high altitude,but decreased litter decomposition in other ecosystems.Third,faunal effects on recalcitrant compounds with low decomposability decreased litter decomposition in subalpine forest in low altitude and in alpine meadow in high altitude,but increased litter decomposition in other ecosystems.In summary,soil fauna increased WSS,DOC and OSS decomposition and decreased AUR decomposition in foliar litters,but such effects on TDN and ASS varied at different stages.Faunal effect was higher for labile compounds and such an effect greatly varied among different ecosystems,but this effect was lower for recalcitrant compounds and such an effect showed homogeneity in mid-altitude ecosystems.Community structure of soil fauna was the dominant factor in regulating litter decomposition,and chemical compounds that altered by soil fauna had lower interpretability in litter decomposition.These results advanced our understanding of faunal effect on litter decomposition and its temporal and spatial variabilities.