Mechanisms to mitigate global climate change by sequestering carbon (C) in different 'sinks' have been proposed as at least temporary measures. Of the major global C pools, terrestrial ecosystems hold the potential to capture and store substantially increased volumes of C in soil organic matter (SOM) through changes in management that are also of benefit to the multitude of ecosystem services that soils provide. This potential can only be realized by determining the amount of SOM stored in soils now, with subsequent quantification of how this is affected by management strategies intended to increase SOM concentrations, and used in soil C models for the prediction of the roles of soils in future climate change. An apparently obvious method to increase C stocks in soils is to augment the soil C pools with the longest mean residence times (MRT). Computer simulation models of soil C dynamics, e.g. RothC and Century, partition these refractory constituents into slow and passive pools with MRTs of centuries to millennia. This partitioning is assumed to reflect: (i) the average biomolecular properties of SOM in the pools with reference to their source in plant litter, (ii) the accessibility of the SOM to decomposer organisms or catalytic enzymes, or (iii) constraints imposed on decomposition by environmental conditions, including soil moisture and temperature. However, contemporary analytical approaches suggest that the chemical composition of these pools is not necessarily predictable because, despite considerable progress with understanding decomposition processes and the role of decomposer organisms, along with refinements in simulation models, little progress has been made in reconciling biochemical properties with the kinetically defined pools. In this review, we will explore how advances in quantitative analytical techniques have redefined the new understanding of SOM dynamics and how this is affecting the development and application of new modelling approaches to soil C.

Soils store at least twice as much carbon (C) as plant biomass 1 , and, each year, soil microbial respiration releases ~60 Pg of C to the atmosphere as carbon dioxide (CO 2 ) 2 .In the short term, soil microbial respiration increases exponentially with temperature 3 , and thus models predict that warming-induced increases in CO 2 release from soils could represent an important positive feedback to 21 st century climate change 4 . However, the magnitude of this feedback remains uncertain, not least because the adaptation of soil microbial communities to changing temperatures has the potential to either substantially decrease ('compensatory adaptation' 5-7 ) or substantially increase ('enhancing adaptation' 8,9 ) warming-induced C losses. By collecting contrasting soils along a climatic gradient from the Arctic to the Amazon, we undertook the first global analysis of the role microbial thermal adaptation plays in controlling rates of CO 2 release from soils. Here we show that, enhancing adaptation was between three and ten times more common than compensatory adaptation. Furthermore, the strongest enhancing responses were observed in soils with high C contents and from cold climates; enhancing thermal adaptation increased the temperature sensitivity of respiration in these soils by a factor of 1.4. This suggests that the substantial stores of C present in organic and high-latitude soils may be more vulnerable to climate warming than currently predicted. Text:Short-term experiments have demonstrated that the rate of microbial respiration in soil increases exponentially with temperature, and this general relationship has been used in parameterising soil C and Earth system models 4,10 . However, plant physiologists have demonstrated that short-term measurements are inadequate for representing the dynamic response of plant respiration to changes in temperature. In plants, thermal acclimation, defined as the "subsequent adjustment in the rate of respiration to compensate for an initial change in temperature" 11 greatly reduces the impact of temperature change on plant respiration in the medium-to long-term, with major consequences for modelling C-cycle feedbacks to climate change 12 . In soil there is growing evidence of the potential for a similar compensatory effect through microbial adaptation to temperature 13 ('compensatory adaptation': defined here to include the potential for physiological acclimation, adaptation within populations, and changes in microbial community size and structure). However, it is unclear if microbial community-level responses should always be compensatory. In fact, responses that enhance the direct and instantaneous effect of temperature changes on soil respiration ('enhancing adaptation') have also been observed 8,9,14 . To date there has been no large-scale evaluation of the role of microbial adaptation in controlling the temperature sensitivity of soil respiration. This lack of understanding adds considerable uncertainty to predictions of the magnitude and direction of carbon-cycle feedb...

Running headerChange-induced feedbacks in the Arctic-alpine Word count8158 words within the continuous narrative 4 AbstractGlobal environmental change, related to climate change and the deposition of airborne Ncontaining contaminants, has already resulted in shifts in plant community composition among plant functional types in arctic and temperate alpine regions. In this paper, we review how key ecosystem processes will be altered by these transformations, the complex biological cascades and feedbacks that may result, and some of the potential broader consequences for the earth system. Firstly, we consider how patterns of growth and allocation, and nutrient uptake, will be altered by the shifts in plant dominance. The ways in which these changes may disproportionately affect the consumer communities, and rates of decomposition, are then discussed. We show that the occurrence of a broad spectrum of plant growth forms in these regions (from cryptogams to deciduous and evergreen dwarf shrubs, graminoids and forbs), together with hypothesized low functional redundancy, will mean that shifts in plant dominance result in a complex series of biotic cascades, couplings and feedbacks which are supplemental to the direct responses of ecosystem components to the primary global change drivers. The nature of these complex interactions is highlighted using the example of the climate-driven increase in shrub cover in low arctic tundra, and the contrasting transformations in plant functional composition in mid-latitude alpine systems. Finally, the potential effects of the transformations on ecosystem properties and processes which link with the earth system are reviewed. We conclude that the effects of global change on these ecosystems, and potential climate-change feedbacks, can not be predicted from simple empirical relationships between processes and driving variables. Rather, the effects of changes in species distributions and dominances on key ecosystem processes and properties must also be considered, based upon best estimates of the trajectories of key transformations, their magnitude and rates of change.5

The agronomic and pulping performance of transgenic trees with altered lignin has been evaluated in duplicated, long-term field trials. Poplars expressing cinnamyl alcohol dehydrogenase (CAD) or caffeate/5-hydroxy-ferulate O-methyltransferase (COMT) antisense transgenes were grown for four years at two sites, in France and England. The trees remained healthy throughout the trial. Growth indicators and interactions with insects were normal. No changes in soil microbial communities were detected beneath the transgenic trees. The expected modifications to lignin were maintained in the transgenics over four years, at both sites. Kraft pulping of tree trunks showed that the reduced-CAD lines had improved characteristics, allowing easier delignification, using smaller amounts of chemicals, while yielding more high-quality pulp. This work highlights the potential of engineering wood quality for more environmentally benign papermaking without interfering with tree growth or fitness.