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

Abstract

Abstract. Understanding the impacts of plant community characteristics on soil carbon dioxide efflux (R) is a key prerequisite for accurate prediction of the future carbon (C) balance of terrestrial ecosystems under climate change. However, developing a mechanistic understanding of the determinants of R is complicated by the presence of multiple different sources of respiratory C within soil -such as soil microbes, plant roots and their mycorrhizal symbionts -each with their distinct dynamics and drivers. In this review, we synthesize relevant information from a wide spectrum of sources to evaluate the current state of knowledge about plant community effects on R, examine how this information is incorporated into global climate models, and highlight priorities for future research. Despite often large variation amongst studies and methods, several general trends emerge.Mechanisms whereby plants affect R may be grouped into effects on belowground C allocation, aboveground litter properties and microclimate. Within vegetation types, the amount of C diverted belowground, and hence R, may be controlled mainly by the rate of photosynthetic C uptake, while amongst vegetation types this should be more dependent upon the specific C allocation strategies of the plant life form. We make the case that plant community composition, rather than diversity, is usually the dominant control on R in natural systems. Individual species impacts on R may be largest 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 the occurrence of major natural disturbances. We show that climate vegetation models incorporate a number of pathways whereby plants can affect R, but that simplifications regarding allocation schemes and drivers of litter decomposition may limit model accuracy. We also suggest that under a warmer future climate, many plantCorrespondence to: D. B. Metcalfe (daniel.metcalfe@slu.se) communities may shift towards dominance by fast growing plants which produce large quantities of nutrient rich litter. Where this community shift occurs, it could drive an increase in R beyond that expected from direct climate impacts on soil microbial activity alone.We identify key gaps in knowledge and recommend them as priorities for future work. These include the patterns of photosynthate partitioning amongst belowground components, ecosystem level effects of individual plant traits, and the importance of trophic interactions and species invasions or extinctions for ecosystem processes. A final, overarching challenge is how to link these observations and drivers across spatio-temporal scales to predict regional or global changes in R over long time periods. A more unified approach to understanding R, which integrates information about plant traits and community dynamics, will be essential for better understanding, simulating and predicting patterns of R across terrestrial ecosystems and its role within the earthclimate system.

Effects of nitrogen fertilization on the forest floor carbon balance over the growing season in a boreal pine forest

“…Our results suggest that such a plant community shift could reduce understorey GPP, and therefore partly offset the beneficial effects of forest fertilization in terms of enhanced tree C uptake (De Vries et al, 2006). Integration of such longer-term, indirect effects of environmental changes on ecosystem C storage via shifts in plant community composition are poorly constrained in global models compared to the direct impacts of abiotic factors such as temperature and moisture (Ostle et al, 2009;Metcalfe et al, 2011). Therefore, further work linking together community ecology and C cycling are required to improve model predictions of forest C sequestration in a changing world.…”

Plant functional groups regulate soil respiration responses to nitrogen addition and mowing over a decade

“…For example, changes in plant community structure under long-term warming have been found to be an important driver of soil respiration responses (Niu, Sherry, Zhou, & Luo, 2013;Xu, Shi, et al, 2015). Plant species generally differ in productivity, canopy structure and litter quality (Metcalfe, Fisher, & Wardle, 2011;Xu, Shi, et al, 2015), which affects the soil's physicalchemical properties (Aponte, Garcia, & Maranon, 2012), root biomass and microbial communities (Kiikkila, Kanerva, Kitunen, & Smolander, 2014;Xu, Shi, et al, 2015), and ultimately the soil autotrophic and heterotrophic respiration. Two common plant functional groups (PFGs, grass and forb) in the temperate grasslands have contrasting responses to N enrichment (Niu et al, 2010;Tian et al, 2016;Yang et al, 2012).…”

Vegetation and elevation influence the timing and magnitude of soil CO<sub>2</sub> efflux in a humid, topographically complex watershed

“…Vegetation also exerts controls on production of CO 2 through root respiration in the soil and through complex mycorrhizal associations that can mediate the response of soil CO 2 production to rain pulse events (Vargas et al, 2010). Finally, vegetation also elicits feedbacks on the abiotic aspects of a system, including the soil moisture and soil temperature regimes, further impacting biogeochemical cycling (Wullschleger et al, 2002;Metcalfe et al, 2011;Vesterdal et al, 2012).…”