Foliar Litter Decomposition In An Alpine Forest Streams

As one of the most important components of a forest meta-cosystem,forest stream plays a key role in transporting carbon and nutrients and in exchanging information between individual ecosystems within the forest meta-ecosystem,and contributes to the maintainence of ecosystem funcitions such as water conservation,detritus-based wood webs,and biodiversity.Forest streams are also crucial components of biogeochemical cycles across the landscape.Plant litter is the dominant common source of energy and nutrients for heterotrophic communities in forest streams,and foliar litter constitutes a majorty of these plant litters.Litter decomposition in aquatic and terrestrial ecosystems shares common abiotic and biotic drivers such as climate,the quality traits of plant litter,and ambient nutrient availabilities,but litter decomposition in streams can also be influenced by several fundamental differences from terrestrial systems which preclude generalizations,including temperature ranges,which are buffered in streams;water availability,not limiting in streams;oxygen levels,occasionally limiting in streams but not in the superficial soil layer of terrestrial systems;abrasion by sediment transport;and unidirectional transport of organic matter in streams.In addition,recent studies suggested that local-scale environmental factors may be much more important than we previously thought,indicating that litter decomposition and the associated processes of nutrient release and component degradation may be significantly impacted by local-scale environmental factors that are represented by different types of ecosystems.However,studies focusing on litter decomposition in streams are much rare than in terrestrial ecosystems,and even much fewer studies have bring litter decompositionin aquatic and terrestrial ecosystems into a single decomposition framework.Alpine forests in the upper reaches of the Minjiang River are typical cold ecosystems experiencing considerable seasonal freezing and thawing events,and seasonal changes are associated with distinct changes in environmental conditions.Such typical environmental conditions will be useful to test the effects of local-scale environmental factors on litter decomposition process.Thus,a two-year field litter decomposition experiment was conducted in an alpine forest meta-ecosystem in the upper reaches of the Minjiang River.Litter accumulated mass loss,decomposition constant(k value),decomposition extent(limit value),nutrient release dynamics,lignin and cellulose degradations,and water soluble components dynamics from the foliar litter of four dominant species(Salix paraplesia,Rhododendron lapponicum,Sabina saltuaria,and Larix mastersiana)that vary widely in chemical traits were investigated for two years.Meanwhile,to better understand litter decomposition pattern in stream and the similarities and differences compared with other types of ecosystems,we investigated the decomposition pattern of these four litter species in forest floor(terrestrial ecosystem),riparian zone(forest wetland,stable aquatic ecosystem),and river(lotic ecosystem).The results suggested that:(1)during the two-year decomposition period,litter mass loss were significantly higher than in forest floor and riparian zone,but lower than in river.Litter decomposition constant and limit value vaired significantly among different types of ecosystems,and showed a general trend of river>stream>riparian zone>forest floor.Litter mass loss,k value,and limit value were significantly affected by litter species,ecosystem type,decomposition time and their interactions,and were significantly correlated with litter initial lignin concentration,lignin/N ratio,P,Mn,Ca,and Mn concentrations.These results indicated that stream can remarkably promote litter decomposition comparied with terrestrial ecosystems,and local-scale environmental factors represented by different ecosystem type have important influences on litter decomposition.Meanwhile,in addition to the concentrations of lignin,N,P,and their ratios,litter Ca,Mg,and Mn concentrations have significant influences on litter decomposition rate and extent.In addition,the varing litter decomposition rate indicates that material cycling and energy flow in forest meta-ecosystem may be organized hierarchically;(2)during litter decomposition,the loss rates of macronutrient C,N,P,K,Ca,and Mg were significantly(P<0.001)affected by litter species,ecosystem type,decomposition time and their interactions,but showed similar release patterns in different types of ecosystems.Carbon,N,P,and K generally showed release pattern during the two-year decomposition period,while Ca and Mg sbowed release or immobilization pattern depending on litter species and ecosystem type.The loss rates of C,N,P,K,Ca,and Mg were significantly modulated by environmental factors,litter initial quality,and quality dynamics(C quality,nutrients,and stoichiometry.Specifically,the effects of environmental factors were most obvious in forest floor ecosystems.Litter quality dynamics showed higher influences on macronutrient loss rate than initial quality,environmental factors and microbial diversity,and C quality and nutrient were the domiant index.Carbon quality represents water soluble C(WSC)concentration,and nutrient mainly represents the concentrations of P,Ca,Mg,and Mn.As a result,the release pattern of macronutrients C,N,P,K,Ca,and Mg were mainly affected by the concentrations of WSC,P,Ca,Mg,and Mn dynamics during litter decomposition.The influence of microbial diversity had significant or insignificant influences on macronutrient release pattern depending on ecosystem type,and even the significant effects were much lower than environtal factors or litter quality dynamics;(3)the loss rates of micronutrients Na,Fe,Mn,Zn,and Cu during litter decompositioin were significantly affected(P<0.001)by litter species,ecosystem type and decomposition time,and showed similar release patterns in different types of ecosystems.These micronutrients all generally showed an immobilization pattern during litter decomposition,although their loss rate may vary among different litter species,ecosystem type,or decomposition time.The impacts of environmental factors on micronutrient loss rates were most manifest in forest floor ecosystem,and had a higher effect on Zn loss rate than litter quality dynamics.Initial litter quality has significant and relatively high effects on Mn loss rate in all the types of ecosystems,but had very low effects on Na,Fe,Zn,and Cu loss rates.The effects of nutrient on Na,Fe,Mn,Zn,and Cu loss rates in forest floor,stream,and riparian zone were relatively higher than other factors,while stoichiometry had a higher influences in river ecosystem.Similarly,microbial diversity still showed weak influence on micronutrient loss rate compared with litter quality dynamics;(4)during the two years's incubation,Al and heavy metal Pb,Cd,and Cr indecomposing foliar litter all showed a general pattern of immobilization,indicating a similar release pattern in streams and other types of ecosystems.The loss rates of Pb,Cd,Cr,and Al were significantly influenced by litter species,ecosystem type,decomposition time and their interactions.Environmetal factor effects on Pb,Cd,Cr,and A1 in forest floor were much higher than in stream,river,and riparian zone,probably because changes in environmental conditions such as temperature and moisture as a result of freeze-thaw cycles could be buffered in aquatic ecosystems.In forest floor,stream,and riparian zone ecosystems,Pb and Cd loss rates were mainly influenced by nutrient,while in river were stoichiometry.In contrast,Cr loss rate in forest floor,stream,and riparian zone were mainly mediated by stoichiometry,but nutrient in river.Apart from the significantly effects of stoichiometry on Al loss rate,the effects of litter initial quality,C quality,nutrient and stoichiometry all showed relatively high influences on A1 loss rate in forest floor,stream,and riparian zone;(5)compared with terrestrial ecosystem(forest floor),stream significantly stimulated the degradation of lignin and cellulose,and the loss rates of lignin and cellulose in different types of ecosystems showed a general order of river>stream>riparian zone>forest floor.In contrast with traditional view that lignin concentration accumulates in the early stage of litter decomposition,our results suggested that lignin can be degraded from the beginning of decomposition.The significant differences in loss rates of lignin and cellulose among different types of ecosystems indicated that local-scale environmental factors can have remarkably influences on lignin and cellulose degradation,and mainly through temperature and ambient nutrients availability.Via SEM analysis we found that litter initial quality,nutrient,and stoichiometry all had relatively high influences on lignin loss rate,especially stoichiometry.While C quality appeared to be the dominant factor influenceing cellulose degradation.Microbial diversity showed very weak influences on lignin and cellulose loss rate.(6)foliar litter total water soluble components(TWS),WSC,water soluble N(WSN),and water soluble P(WSP)all showed substantial loss during the two-year decomposition.In stream,riparian zone,and river ecosystems,a substantial proportion of TWS,WSC,WSN,and WSP released in the early decomposition phase,and then released slowly with the proceeding of decomposition.While in forest floor,the loss rate of TWS,WSC,WSN,and WSP were lower in the early phase,but increased significantly at the end of the first decomposition year(first growing season).Litter species,ecosystem type,decomposition period and their interactions showed significant(P<0.001)influences on the loss rates of water soluble components.Carbon quality and stoichiometry had most significantly effects on WTS and WSC compared with other factors,but had insignificant effects in river.The loss rate of WSN was mainly influenced by nutrient and stoichiometry,while WSP loss rate was stoichiometry and C quality.More specifically,foliar litter WSC,P,Ca,Mg,and Mn concentrations showed significant influences on TWS,WSC,WSN,and WSP during litter decomposition.Summarily,compared with forest floor,stream significantly stimulated foliar litter decomposition.Litter accumulated mass loss,decomposition constant and decomposition extent showed a general order of river>stream>riparian zone>forest floor.The significant differences in litter decomposition rate and extent among different types of ecosystems suggested that material cycling and energy flow may be organized hierarchically in forest meta-ecosystem,which need further indepth investigating.Althgouh significant differencs were obseroved among different types of ecosystems for each element,but general patters were obtained across ecosystems,i.e.,macronutrients of C,N,P and K generally released,Ca and Mg released or immobilized depending on ecosystem type and litter species,micronutrients of Na,Fe,Mn,Zn,and Cu generally immobilized and as well as A1 and heavy metal Pb,Cd,and Cr.The general pattern for element dynamics during litter decomposition would be useful for the development of commom models across terrestrial and aquatic ecosystems,and also be important for better understanding element flow and information exchange among the individual ecosystems within a forest meta-ecosystem.In addition,in contrast with traditional view,our results suggested that lignin can be degraded from the beginning of decomposition,and streams can significantly stimulate lignin and cellulose degradation compared with forest floor.These results would be useful for better understanding the degradation of recalcitrant components during litter decomposition and for development of related models.The release of water soluble components was much rapid in stream,riparian zone,and river,which prioved potential mechanisms for the fast decomposition pattern in lotic ecosystems.Overall,through comparing litter decomposition pattern among forest floor,stream,riparian zone and river,we summarized foliar litter decomposition pattern in alpine forest streams and its similarities and differences with other types of ecosystems,aiming to provide base scientific data for litter decomposition model development across terrestrial and aquatic ecosystems,and for further indepth research on material cycling and energy flow among individual ecosystems within a forest meta-ecosystem.