A positive feedback to climate change: The effect of temperature on the respiration of key wood‐decomposing fungi does not decline with time

Heterotrophic soil microorganisms are responsible for ~50% of the carbon dioxide released by respiration from the terrestrial biosphere each year. The respiratory response of soil microbial communities to warming, and the control mechanisms, remains uncertain, yet is critical to understanding the future land carbon (C)‐climate feedback. Individuals of nine species of fungi decomposing wood were exposed to 90 days of cooling to evaluate the medium‐term effect of temperature on respiration. Overall, the effect of temperature on respiration increased in the medium term, with no evidence of compensation. However, the increasing effect of temperature on respiration was lost after correcting for changes in biomass. These results indicate that C loss through respiration of wood‐decomposing fungi will increase beyond the direct effects of temperature on respiration, potentially promoting greater C losses from terrestrial ecosystems and a positive feedback to climate change

Drying and rewetting conditions differentially affect the mineralization of fresh plant litter and extant soil organic matter

Drought is becoming more common globally and has the potential to alter patterns of soil carbon (C) storage in terrestrial ecosystems. After an extended dry period, a pulse of soil CO2 release is commonly observed upon rewetting (the so-called ‘Birch effect’), the magnitude of which depends on soil rewetting frequency. But the source and implications of this CO2 efflux are unclear. We used a mesocosm field experiment to subject agricultural topsoil to two distinct drying and rewetting frequencies, measuring Birch effects (as 3-day cumulative CO2 efflux upon rewetting) and the overall CO2 efflux over the entire drying-rewetting cycle. We used 14C-labelled wheat straw to determine the contribution of fresh (recently incorporated) plant litter or extant soil organic matter (SOM) to these fluxes, and assessed the extent to which the amount of soil microbial biomass + K2SO4-extractable organic C (fumigated-extracted C, FEC) before rewetting determined the magnitude of Birch effect CO2 pulses. Our results showed a gradual increase in SOM-derived organic solutes within the FEC fraction, and a decrease in soil microbial biomass, under more extreme drying and rewetting conditions. But, contrary to our hypothesis, pre-wetting levels of FEC were not related to the magnitude of the Birch effects. In the longer term, rewetting frequency and temperature influenced the overall (31-day cumulative) amount of CO2–C released from SOM upon rewetting, but the overall 14CO2–C respired from fresh straw was only influenced by the rewetting frequency, with no effect of seasonal temperature differences of ∼15 °C. We conclude that the mineralization of fresh plant litter in soils is more sensitive to water limitations than extant SOM in soils under drying-rewetting conditions. Moreover, we found little evidence to support the hypothesis that the availability of microbial and soluble organic C before rewetting determined the magnitude of the Birch effects, and suggest that future work should investigate whether these short-term CO2 pulses are predominantly derived from substrate-supply mechanisms resulting from the disruption of the soil organo-mineral matrix

Microbially mediated mechanisms underlie soil carbon accrual by conservation agriculture under decade-long warming

Increasing soil organic carbon (SOC) in croplands by switching from conventional to conservation management may be hampered by stimulated microbial decomposition under warming. Here, we test the interactive effects of agricultural management and warming on SOC persistence and underlying microbial mechanisms in a decade-long controlled experiment on a wheat-maize cropping system. Warming increased SOC content and accelerated fungal community temporal turnover under conservation agriculture (no tillage, chopped crop residue), but not under conventional agriculture (annual tillage, crop residue removed). Microbial carbon use efficiency (CUE) and growth increased linearly over time, with stronger positive warming effects after 5 years under conservation agriculture. According to structural equation models, these increases arose from greater carbon inputs from the crops, which indirectly controlled microbial CUE via changes in fungal communities. As a result, fungal necromass increased from 28 to 53%, emerging as the strongest predictor of SOC content. Collectively, our results demonstrate how management and climatic factors can interact to alter microbial community composition, physiology and functions and, in turn, SOC formation and accrual in croplands.</p

Shrub expansion in the Arctic may induce large-scale carbon losses due to changes in plant-soil interactions

Background Tall deciduous shrubs are increasing in range, size and cover across much of the Arctic, a process commonly assumed to increase carbon (C) storage. Major advances in remote sensing have increased our ability to monitor changes aboveground, improving quantification and understanding of arctic greening. However, the vast majority of C in the Arctic is stored in soils, where changes are more uncertain. Scope We present pilot data to argue that shrub expansion will cause changes in rhizosphere processes, including the development of new mycorrhizal associations that have the potential to promote soil C losses that substantially exceed C gains in plant biomass. However, current observations are limited in their spatial extent, and mechanistic understanding is still developing. Extending measurements across different regions and tundra types would greatly increase our ability to predict the biogeochemical consequences of arctic vegetation change, and we present a simple method that would allow such data to be collected. Conclusions Shrub expansion in the Arctic could promote substantial soil C losses that are unlikely to be offset by increases in plant biomass. However, confidence in this prediction is limited by a lack of information on how soil C stocks vary between contrasting Arctic vegetation communities; this needs to be addressed urgently

Whole-crown 13C-pulse labelling in a sub-arctic woodland to target canopy-specific carbon fluxes

Climate change-driven increases in plant productivity have been observed at high northern latitudes. These trends are driven, in part, by the increasing abundance of tall shrub and tree species in arctic ecosystems, and the advance of treelines. Higher plant productivity may alter carbon (C) allocation and, hence, ecosystem C cycling and soil C sequestration. It is important to understand the contributions that the newly established canopy forming overstorey species makes to C cycling in these ecosystems. However, the presence of a dense understorey cover makes this challenging, with established partitioning approaches causing disturbance and potentially introducing measurement artefacts. Here, we develop an in situ whole-crown 13C-pulse labelling technique to isolate canopy C fluxes in areas of dense understorey cover. The crowns of five mountain birch (Betula pubescens ssp. czerepanovii) trees were provided with a 13CO2 pulse using portable field equipment, and leaf samples were collected from neighbouring con-specific trees and hetero-specific understorey shrubs on days 1–10 and 377 post-crown labelling. We found effective and long-term enrichment of foliage in labelled trees, but no evidence of the 13C-signal in con- or hetero-specific neighbouring trees or woody shrubs. This method is promising and provides a valuable tool to isolate the role of canopy tree species in ecosystems with dense understorey cover.Output Status: Forthcoming/Available Onlin

Resistance of subarctic soil fungal and invertebrate communities to disruption of below-ground carbon supply

The supply of recent photosynthate from plants to soils is thought to be a critical mechanism regulating the activity and diversity of soil biota. In the Arctic, large-scale vegetation transitions are underway in response to warming, and there is an urgent need to understand how these changes affect soil biodiversity and function. We investigated how abundance and diversity of soil fungi and invertebrates responded to a reduction in fresh below-ground photosynthate supply in treeline birch and willow, achieved using stem girdling. We hypothesised that birch forest would support greater abundance of ectomycorrhizal (ECM) fungal species and fauna than willow shrubs, and that girdling would result in a rapid switch from ECM fungi to saprotrophs as canopy supply of C was cut, with a concomitant decline in soil fauna. Birch forest had greater fungal and faunal abundance with a large contribution of root-associated ascomycetes (ericoid mycorrhizal fungi and root endophytes) compared to willow shrub plots, which had a higher proportion of saprotrophs and, contrary to our expectations, ECM fungi. Broad-scale soil fungal and faunal functional group composition was not significantly changed by girdling, even in the third year of treatment. Within the ECM community, there were some changes, with genera that are believed to be particularly C-demanding declining in girdled plots. However, it was notable how most ECM fungi remained present after 3 years of isolation of the below-ground compartment from contemporary photosynthate supply. Synthesis. In a treeline/tundra ecosystem, distinct soil communities existed in contrasting vegetation patches within the landscape, but the structure of these communities was resistant to canopy disturbance and concomitant reduction of autotrophic C inputs

Rhizosphere allocation by canopy-forming species dominates soil CO2 efflux in a subarctic landscape

In arctic ecosystems, climate change has increased plant productivity. As arctic carbon (C) stocks are predominantly located below ground, the effects of greater plant productivity on soil C storage will significantly determine the net sink/source potential of these ecosystems, but vegetation controls on soil CO2 efflux remain poorly resolved. To identify the role of canopy‐forming species in below‐ground C dynamics, we conducted a girdling experiment with plots distributed across 1 km2 of treeline birch (Betula pubescens) forest and willow (Salix lapponum) patches in northern Sweden and quantified the contribution of canopy vegetation to soil CO2 fluxes and below‐ground productivity. Girdling birches reduced total soil CO2 efflux in the peak growing season by 53% ‐double the expected amount, given that trees contribute only half of the total leaf area in the forest. Root and mycorrhizal mycelial production also decreased substantially. At peak season, willow shrubs contributed 38% to soil CO2 efflux in their patches. Our findings indicate that C, recently fixed by trees and tall shrubs, makes a substantial contribution to soil respiration. It is critically important that these processes are taken into consideration in the context of a greening arctic since productivity and ecosystem C sequestration are not synonymous

Boreal permafrost thaw amplified by fire disturbance and precipitation increases

Permafrost soils store huge amounts of organic carbon, which could be released if climate change promotes thaw. Currently, modelling studies predict that thaw in boreal regions is mainly sensitive to warming, rather than changes in precipitation or vegetation cover. We evaluate this conclusion for North American boreal forests using a detailed process-based model parameterised and validated on field measurements. We show that soil thermal regimes for dominant forest types are controlled strongly by soil moisture and thus the balance between evapotranspiration and precipitation. Under dense canopy cover, high evapotranspiration means a 30% increase in precipitation causes less thaw than a 1 °C increase in temperature. However, disturbance to vegetation promotes greater thaw through reduced evapotranspiration, which results in wetter, more thermally conductive soils. In such disturbed forests, increases in precipitation rival warming as a direct driver of thaw, with a 30% increase in precipitation at current temperatures causing more thaw than 2 °C of warming. We find striking non-linear interactive effects on thaw between rising precipitation and loss of leaf area, which are of concern given projections of greater precipitation and disturbance in boreal forests. Inclusion of robust vegetation-hydrological feedbacks in global models is therefore critical for accurately predicting permafrost dynamics; thaw cannot be considered to be controlled solely by rising temperatures

Variable thermal plasticity of leaf functional traits in Andean tropical montane forests

Tropical montane forests (TMFs) are biodiversity hotspots and provide vital ecosystem services, but they are disproportionately vulnerable to climate warming. In the Andes, cold-affiliated species from high elevations are being displaced at the hot end of their thermal distributions by warm-affiliated species migrating upwards from lower elevations, leading to compositional shifts. Leaf functional traits are strong indicators of plant performance and at the community level have been shown to vary along elevation gradients, reflecting plant adaptations to different environmental niches. However, the plastic response of such traits to relatively rapid temperature change in Andean TMF species remains unknown. We used three common garden plantations within a thermosequence in the Colombian Andes to investigate the warming and cooling responses of key leaf functional traits in eight cold- and warm-affiliated species with variable thermal niches. Cold-affiliated species shifted their foliar nutrient concentrations when exposed to warming, while all other traits did not significantly change; contrastingly, warm-affiliated species were able to adjust structural, nutrient and water-use efficiency traits from acquisitive to conservative strategies in response to cooling. Our findings suggest that cold-affiliated species will struggle to acclimate functional traits to warming, conferring warm-affiliated species a competitive advantage under climate change