Plant communities as drivers of soil respiration: pathways, mechanisms, and significance for global change

摘要

Understanding the impacts of plant community characteristics on soil carbondioxide efflux (R) is a key prerequisite for accurate prediction of thefuture carbon (C) balance of terrestrial ecosystems under climate change.However, developing a mechanistic understanding of the determinants of R iscomplicated by the presence of multiple different sources of respiratory Cwithin soil – such as soil microbes, plant roots and their mycorrhizalsymbionts – each with their distinct dynamics and drivers. In this review,we synthesize relevant information from a wide spectrum of sources toevaluate the current state of knowledge about plant community effects onR, examine how this information is incorporated into global climate models,and highlight priorities for future research. Despite often large variationamongst studies and methods, several general trends emerge.Mechanisms whereby plants affect R may be grouped into effects on belowgroundC allocation, aboveground litter properties and microclimate. Withinvegetation types, the amount of C diverted belowground, and hence R, may becontrolled mainly by the rate of photosynthetic C uptake, while amongstvegetation types this should be more dependent upon the specific Callocation strategies of the plant life form. We make the case that plantcommunity composition, rather than diversity, is usually the dominantcontrol on R in natural systems. Individual species impacts on R may belargest where the species accounts for most of the biomass in the ecosystem,has very distinct traits to the rest of the community and/or modulates theoccurrence of major natural disturbances. We show that climate vegetationmodels incorporate a number of pathways whereby plants can affect R, but thatsimplifications regarding allocation schemes and drivers of litterdecomposition may limit model accuracy. We also suggest that under a warmerfuture climate, many plant communities may shift towards dominance by fastgrowing plants which produce large quantities of nutrient rich litter. Wherethis community shift occurs, it could drive an increase in R beyond thatexpected from direct climate impacts on soil microbial activity alone.We identify key gaps in knowledge and recommend them as priorities forfuture work. These include the patterns of photosynthate partitioningamongst belowground components, ecosystem level effects of individual planttraits, and the importance of trophic interactions and species invasions orextinctions for ecosystem processes. A final, overarching challenge is howto link these observations and drivers across spatio-temporal scales topredict regional or global changes in R over long time periods. A moreunified approach to understanding R, which integrates information about planttraits and community dynamics, will be essential for better understanding,simulating and predicting patterns of R across terrestrial ecosystems and itsrole within the earth-climate system.