Significantly more carbon is stored in the world's soils--including peatlands, wetlands and permafrost--than is present in the atmosphere. Disagreement exists, however, regarding the effects of climate change on global soil carbon stocks. If carbon stored belowground is transferred to the atmosphere by a warming-induced acceleration of its decomposition, a positive feedback to climate change would occur. Conversely, if increases of plant-derived carbon inputs to soils exceed increases in decomposition, the feedback would be negative. Despite much research, a consensus has not yet emerged on the temperature sensitivity of soil carbon decomposition. Unravelling the feedback effect is particularly difficult, because the diverse soil organic compounds exhibit a wide range of kinetic properties, which determine the intrinsic temperature sensitivity of their decomposition. Moreover, several environmental constraints obscure the intrinsic temperature sensitivity of substrate decomposition, causing lower observed 'apparent' temperature sensitivity, and these constraints may, themselves, be sensitive to climate.

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Temperature sensitivity of soil carbon decomposition and feedbacks to climate change

Microbes are the unseen majority in soil and comprise a large portion of lifes genetic diversity. Despite their abundance, the impact of soil microbes on ecosystem processes is still poorly understood. Here we explore the various roles that soil microbes play in terrestrial ecosystems with special emphasis on their contribution to plant productivity and diversity. Soil microbes are important regulators of plant productivity, especially in nutrient poor ecosystems where plant symbionts are responsible for the acquisition of limiting nutrients. Mycorrhizal fungi and nitrogenfixing bacteria are responsible for c. 5‐20% (grassland and savannah) to 80% (temperate and boreal forests) of all nitrogen, and up to 75% of phosphorus, that is acquired by plants annually. Free-living microbes also strongly regulate plant productivity, through the mineralization of, and competition for, nutrients that sustain plant productivity. Soil microbes, including microbial pathogens, are also important regulators of plant community dynamics and plant diversity, determining plant abundance and, in some cases, facilitating invasion by exotic plants. Conservative estimates suggest that c. 20 000 plant species are completely dependent on microbial symbionts for growth and survival pointing to the importance of soil microbes as regulators of plant species richness on Earth. Overall, this review shows that soil microbes must be considered as important drivers of plant diversity and productivity in terrestrial ecosystems.

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The unseen majority: Soil microbes as drivers of plant diversity and productivity in terrestrial ecosystems

There is growing recognition that classifying terrestrial plant species on the basis of their function (into 'functional types') rather than their higher taxonomic identity, is a promising way forward for tackling important ecological questions at the scale of ecosystems, landscapes or biomes. These questions include those on vegetation responses to and vegetation effects on, environmental changes (e.g. changes in climate, atmospheric chemistry, land use or other disturbances). There is also growing consensus about a shortlist of plant traits that should underlie such functional plant classifications, because they have strong predictive power of important ecosystem responses to environmental change and/or they themselves have strong impacts on ecosystem processes. The most favoured traits are those that are also relatively easy and inexpensive to measure for large numbers of plant species. Large international research efforts, promoted by the IGBP–GCTE Programme, are underway to screen predominant plant species in various ecosystems and biomes worldwide for such traits. This paper provides an international methodological protocol aimed at standardising this research effort, based on consensus among a broad group of scientists in this field. It features a practical handbook with step-by-step recipes, with relatively brief information about the ecological context, for 28 functional traits recognised as critical for tackling large-scale ecological questions.

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A handbook of protocols for standardised and easy measurement of plant functional traits worldwide

I. CONTEXT * The Ecosystem Concept * Earth's Climate System * Geology and Soils * II. MECHANISMS * Terrestrial Water and Energy Balance * Carbon Input to Terrestrial Ecosystems * Terrestrial Production Processes * Terrestrial Decomposition * Terrestrial Plant Nutrient Use * Terrestrial Nutrient Cycling * Aquatic Carbon and Nutrient Cycling * Trophic Dynamics * Community Effects on Ecosystem Processes * III. PATTERNS * Temporal Dynamics * Landscape Heterogeneity and Ecosystem Dynamics * IV. INTEGRATION * Global Biogeochemical Cycles * Managing and Sustaining Ecosystem * Abbreviations * Glossary * References

Principles of Terrestrial Ecosystem Ecology

Understanding and predicting ecosystem functioning (e.g., carbon and water fluxes) and the role of soils in carbon storage requires an accurate assessment of plant rooting distributions. Here, in a comprehensive literature synthesis, we analyze rooting patterns for terrestrial biomes and compare distributions for various plant functional groups. We compiled a database of 250 root studies, subdividing suitable results into 11 biomes, and fitted the depth coefficient β to the data for each biome (Gale and Grigal 1987). β is a simple numerical index of rooting distribution based on the asymptotic equation Y=1-βd, where d = depth and Y = the proportion of roots from the surface to depth d. High values of β correspond to a greater proportion of roots with depth. Tundra, boreal forest, and temperate grasslands showed the shallowest rooting profiles (β=0.913, 0.943, and 0.943, respectively), with 80-90% of roots in the top 30 cm of soil; deserts and temperate coniferous forests showed the deepest profiles (β=0.975 and 0.976, respectively) and had only 50% of their roots in the upper 30 cm. Standing root biomass varied by over an order of magnitude across biomes, from approximately 0.2 to 5 kg m-2. Tropical evergreen forests had the highest root biomass (5 kg m-2), but other forest biomes and sclerophyllous shrublands were of similar magnitude. Root biomass for croplands, deserts, tundra and grasslands was below 1.5 kg m-2. Root/shoot (R/S) ratios were highest for tundra, grasslands, and cold deserts (ranging from 4 to 7); forest ecosystems and croplands had the lowest R/S ratios (approximately 0.1 to 0.5). Comparing data across biomes for plant functional groups, grasses had 44% of their roots in the top 10 cm of soil. (β=0.952), while shrubs had only 21% in the same depth increment (β=0.978). The rooting distribution of all temperate and tropical trees was β=0.970 with 26% of roots in the top 10 cm and 60% in the top 30 cm. Overall, the globally averaged root distribution for all ecosystems was β=0.966 (r 2=0.89) with approximately 30%, 50%, and 75% of roots in the top 10 cm, 20 cm, and 40 cm, respectively. We discuss the merits and possible shortcomings of our analysis in the context of root biomass and root functioning.

A global analysis of root distributions for terrestrial biomes

This study summarizes the responses of soil and plant processes to experimental manipulation of air temperatures and nutrients in three subarctic ecosystems at Abisko, N Sweden and compares the treatment effects across a wider latitudinal gradient ranging from the temperate zone in the U.K. to the High Arctic in Svalbard. The temperature enhancement of 2-4?C is within the most commonly predicted range of temperature increase in the Arctic if atmospheric C02 doubles, and the fertilizer addition simulated the expected increase of nutrient mineralization rates in warmer arctic soils. Soil nutrient mineralization was not, however, affected by the temperature increase, probably because the soil warming was slight, and the soils were even cooled in some circumstances. Fertilizer application increased the N and P content in soil microbial biomass indicating a substantial potential for microbial nutrient immobilization. Both the temperature enhancement and the application of fertilizer caused responses in plant growth and reproductive output. The responses to fertilizer application were often stronger than those to temperature. Responses to temperature were greatest at the sites closest to the northern or upper climatic distributional limits for the species studied. For instance, boreal dwarf shrubs responded strongly at a forest understorey site which was close to their distributional limit, although this site did not experience extreme climatic conditions in comparison to the other sites along the gradient. Lowr arctic shrubs and dwarf shrubs did not respond at a subarctic tree-line site, whereas all species investigated hitherto responded by increased growth at the climatically most harsh site at a high altitude fellfield. Reproductive output was increased by temperature perturbation. Vegetatively, plants responded to enhanced temperature with greater branching or shoot formation, and although individual leaves and branches also grew more in most cases, their response to temperature manipulation was less marked. Nutrient concentration declined in species which showed enhanced growth in re sponse to increased temperature, probably because constrained nutrient uptake due to the lack of increased soil nutrient mineralization resulted in low availability of plant nutrients. The integrated response of plants and microbes is discussed in the context of possible sink to source status for C02 in a warmer Arctic.

Direct and indirect effects of increasing temperatures on subarctic ecosystems

Understanding of controls on primary production and net ecosystem production in the Arctic, and on variation in the responsiveness of different arctic ecosystems to climate change and disturbance, has advanced rapidly over the past two decades. The aim of this chapter is to review and summarize these more recent advances, starting in the mid-1970s. Considerable progress has been made in understanding the controls over primary production and its variation across the Arctic landscape. The greatest future need is to understand how long-term changes in primary production are linked to soil processes. One specific need is for new data on root biomass, root production, and the partitioning of belowground respiration into root and soil components. Basic measurements of net ecosystem production (NEP) are also lacking from many kinds of Arctic ecosystems, and information is needed to delineate more clearly the landscape patterns of NEP and its environmental correlations. Controls on soil organic matter accumulation and carbon–nutrient interactions in Arctic soils are another key to improving our understanding. Perspectives on the role of temperature in Arctic ecosystems have changed especially dramatically in the past three decades, from an early focus on adaptations to a low-temperature environment and direct effects of low temperature on individual plants and species. Great progress has been made in understanding indirect effects of temperature and interactions of temperature with other environmental variables in control of plant processes.

Productivity of Arctic Ecosystems

An assessment of the impacts of changes in climate and UV-B radiation on Arctic terrestrial ecosystems, made within the Arctic Climate Impacts Assessment (ACIA), highlighted the profound implications of projected warming in particular for future ecosystem services, biodiversity and feedbacks to climate. However, although our current understanding of ecological processes and changes driven by climate and UV-B is strong in some geographical areas and in some disciplines, it is weak in others. Even though recently the strength of our predictions has increased dramatically with increased research effort in the Arctic and the introduction of new technologies, our current understanding is still constrained by various uncertainties. The assessment is based on a range of approaches that each have uncertainties, and on data sets that are often far from complete. Uncertainties arise from methodologies and conceptual frameworks, from unpredictable surprises, from lack of validation of models, and from the use of particular scenarios, rather than predictions, of future greenhouse gas emissions and climates. Recommendations to reduce the uncertainties are wide-ranging and relate to all disciplines within the assessment. However, a repeated theme is the critical importance of achieving an adequate spatial and long-term coverage of experiments, observations and monitoring of environmental changes and their impacts throughout the sparsely populated and remote region that is the Arctic.

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Uncertainties and recommendations.

https://bioone.org/journals/ambio-a-journal-of-the-human-environment/volume-33/issue-7/0044-7447-33.7.386/Key-Findings-and-Extended-Summaries/10.1579/0044-7447-33.7.386.pdf

Sven Jonasson

Papers

Direct and indirect effects of increasing temperatures on subarctic ecosystems

Productivity of Arctic Ecosystems

Uncertainties and recommendations.

Key findings and extended summaries.

Global Warming and Terrestrial Ecosystems