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Climate change currently manifests in upward and northward shifting treelines, which encompasses changes to the carbon (C) and nitrogen (N) composition of organic inputs to soils. Whether these changed inputs will increase or decrease microbial mineralisation of native soil organic matter remains unknown, making it difficult to estimate how treeline shifts will affect the C balance. Aiming to improve mechanistic understanding of C cycling in regions experiencing treeline shifts, we quantified priming effects in soils of high altitudes (Peruvian Andes) and high latitudes (subarctic Sweden), differentiating landcover types (boreal forest, tropical forest, tundra heath, Puna grassland) and soil horizons (organic, mineral). In a controlled laboratory incubation, soils were amended with substrates of different C:N, composed of an organic C source at a constant ratio of 30% substrate-C to microbial biomass C, combined with different levels of a nutrient solution neutral in pH. Substrate additions elicited both positive and negative priming effects in both ecosystems, independent from substrate C:N. Positive priming prevailed above the treeline in high altitudes and in mineral soils in high latitudes, where consequently climate change-induced treeline shifts and deeper rooting plants may enhance SOM-mineralisation and soil C emissions. However, such C loss may be compensated by negative priming, which dominated in the other soil types and was of larger magnitude than positive priming. In line with other studies, these results indicate a consistent mechanism linking decreased SOM-mineralisation (negative priming) to increased microbial substrate utilisation, suggesting preferential substrate use as a potential tool to support soil C storage. Graphical abstract

There is an emerging consensus that microbial necromass carbon is the primary constituent of stable soil carbon, yet the controls on the stabilization process are unknown. Prior to stabilization, microbial necromass may be recycled by the microbial community. We propose that the efficiency of this recycling is a critical determinant of soil carbon stabilization rates. Here we explore the controls on necromass recycling efficiency in 27 UK grassland soils using stable isotope tracing and indicator species analysis. We found that recycling efficiency was unaffected by land management. Instead, recycling efficiency increased with microbial growth rate on necromass, and was highest in soils with low historical precipitation. We identified bacterial and fungal indicators of necromass recycling efficiency, which could be used to clarify soil carbon stabilization mechanisms. We conclude that environmental and microbial controls have a strong influence on necromass recycling, and suggest that this, in turn, influences soil carbon stabilization. Microbial necromass recycling, which can influence soil carbon stabilization, is controlled by microbial growth and precipitation, as indicated by stable isotope tracing and indicator species analysis in a range of UK grasslands.

Greenhouse gas (GHG) fluxes from grasslands are affected by climate warming and agricultural management practices including nitrogen (N) fertiliser application and grazing. However, the interactive effects of these factors are poorly resolved in field studies. We used a factorial in situ experiment - combining warming, N-fertiliser and above-ground cutting treatments - to explore their individual and interactive effects on plant-soil properties and GHG fluxes in a temperate UK grassland over two years. Our results showed no interactive treatment effects on plant productivity despite individual effects of N-fertiliser and warming on above- and below-ground biomass. There were, however, interactive treatment effects on GHG fluxes that varied across the two years. In year 1, warming and N-fertiliser increased CO2 and reduced N2O fluxes. N-fertilised also interacted with above-ground biomass (AGB) removal increasing N2O fluxes in year one and reducing CO2 fluxes in year two. The grassland was consistently a sink of CH4; N-fertilised increased the sink by 45% (year 1), AGB removal and warming reduced CH4 consumption by 44% and 43%, respectively (year 2). The majority of the variance in CO2 fluxes was explained by above-ground metrics (grassland productivity and leaf dry matter content), with microclimate (air and soil temperature and soil moisture) and below-ground (root N content) metrics also significant. Soil chemistry (soil mineral N and net mineralisation rate), below-ground (specific root length) and microclimate (soil moisture) metrics explained 49% and 24% of the variance in N2O and CH4 fluxes, respectively. Overall, our work demonstrates the importance of interactions between climate and management as determinants of short-term grassland GHG fluxes. These results show that reduced cutting combined with lower inorganic N-fertilisers would constrain grassland C and N cycling and GHG fluxes in warmer climatic conditions. This has implications for strategic grassland management decisions to mitigate GHG fluxes in a warming world.

Diurnal variations in soil nitrous oxide (N­2O) emissions have been documented for nearly four decades yet consensus on their significance is still lacking. Resolving this question is important as soil N2O emissions have some of the highest uncertainties in national greenhouse gas inventories. A major challenge for understanding diurnal variation is that conventional measurements rarely operate at temporal frequencies that can observe and report this phenomenon. Some higher frequency studies have observed daytime peaking of soil N­2O emissions and often ascribe it to the diurnal oscillation of soil temperature. However, night-time peaking and irregular diurnal N­2O patterns have also been reported in a number of studies.To investigate the prevalence and characteristics of diurnal N­2O variability, we systematically reviewed published studies that measured N2O at high temporal frequencies (≥ 5 times/day). We identified 46 published studies covering cropland, grassland and forest soils; and extracted sub-daily N­2O flux data and other soil parameters, yielding 286 individual days of data. Diurnal variability of N­2O emissions were found in ~80% of the data, with ~60% peaking during the day and ~20% at night. Diurnal N­2O patterns were associated with non-diurnal factors including soil texture and land use but the relationship between soil temperature and N­2O flux was inconsistent, with strong positive correlations (R > 0.7) only found in one-third of the datasets.This talk explores the implications of the review results on the time of sampling using conventional approaches (single time-point flux measurements), and the potential drivers of diurnal N­2O variations for future research. In addition, this talk will also introduce a novel automated measurement technique allowing flux measurements at high temporal resolutions and how its application could enable experimental investigations of potential drivers of diurnal N­2O variability.

The sensitivity of peatland carbon (C) fluxes to changes in climate and hydrology are uncertain due to the complex interactions between plants and peat properties. In this study we examine how peat cores taken from under three plant functional types (PFT) (bryophyte, graminoid and ericoid) differ in their biotic and abiotic properties and how this indirectly modulates the response of C fluxes to environmental change. Peat cores taken from under three PFTs had their aboveground vegetation removed to exclude direct plant-mediated effects, and were incubated in a temperature x water table factorial experiment at 12, 14 and 16 degrees C (air temperature) with the water table level -25, -15 or -5 cm below the peat surface. Carbon dioxide (CO2) and methane (CH4) fluxes were measured over 11 months. Emissions of CO(2)and CH(4)increased with temperature, with strong positive (CH4) and negative (CO2) interactions with increasing water table level. There were significant effects of removed PFT on the environmental sensitivity of CH4, but not CO(2)fluxes. CH(4)emissions were greatest in peat with graminoid PFT removed at the warmest temperature but these indirect effects were not explained by peat abiotic or biotic properties, which did not differ between PFTs. These results show that climate change-induced expansion of graminoids in northern peatlands will have direct and indirect effects on C fluxes and the stability of peatland C stores. These responses will be determined by the interactive effects of vegetation composition, hydrology and warming on methane-cycling microbial communities. Highlights Peatland carbon flux strength under a changing climate is influenced by PFT. Peat from under graminoid PFT emits more methane than peat from under bryophyte or ericoid PFT. Prior PFT cover influenced methane emissions, but did not affect peat abiotic or biotic properties. Increases in graminoid cover with climate change could indirectly increase peatland methane fluxes.

Manual measurements of nitrous oxide (N2O) emissions with static chambers are 2Department of Environment and Geography, University of York, Heslington, commonly practised. However, they generally do not consider the diurnal variability of N2O flux, and little is known about the patterns and drivers of such variability. We systematically reviewed and analysed 286 diurnal data sets of N2O fluxes from pub- lished literature to (i) assess the prevalence and timing (day or night peaking) of diurnal N2O flux patterns in agricultural and forest soils, (ii) examine the relationship between N2O flux and soil temperature with different diurnal patterns, (iii) identify whether non-diurnal factors (i.e. land management and soil properties) influence the occur- rence of diurnal patterns and (iv) evaluate the accuracy of estimating cumulative N2O emissions with single-daily flux measurements. Our synthesis demonstrates that diur- nal N2O flux variability is a widespread phenomenon in agricultural and forest soils. Of the 286 data sets analysed, ~80% exhibited diurnal N2O patterns, with ~60% peak- ing during the day and ~20% at night. Contrary to many published observations, our analysis only found strong positive correlations (R > 0.7) between N2O flux and soil temperature in one-third of the data sets. Soil drainage property, soil water-filled pore space (WFPS) level and land use were also found to potentially influence the occur- rence of certain diurnal patterns. Our work demonstrated that single-daily flux meas- urements at mid-morning yielded daily emission estimates with the smallest average bias compared to measurements made at other times of day, however, it could still lead to significant over- or underestimation due to inconsistent diurnal N2O patterns. This inconsistency also reflects the inaccuracy of using soil temperature to predict the time of daily average N2O flux. Future research should investigate the relationship between N2O flux and other diurnal parameters, such as photosynthetically active radiation (PAR) and root exudation, along with the consideration of the effects of soil moisture, drainage and land use on the diurnal patterns of N2O flux. The information could be incorporated in N2O emission prediction models to improve accuracy.

Microbial necromass dominates soil organic matter. Recent research on necromass and soil carbon storage has focused on necromass production and stabilization mechanisms but not15 on the mechanisms of necromass retention. We present evidence from soil incubations withstable-isotope labelled necromass that abiotic adsorption may be more important than biotic immobilization for short-term necromass retention. We demonstrate that necromass adsorbs not only to mineral surfaces, but may also interact with other necromass. Furthermore, necromass cell chemistry alters necromass-necromass interaction, with more bacterial tracer retained when there is yeast necromass present. These findings suggest that the adsorption and abiotic interaction of microbial necromass and its functional properties, beyond chemical stability, deserve further investigation in the context of soil carbon sequestration.

This study evaluates the dynamics of soil organic carbon (SOC) underperennial crops across the globe. It quantifies the effect of change fromannual to perennial crops and the subsequent temporal changes in SOCstocks during the perennial crop cycle. It also presents an empiricalmodel to estimate changes in the SOC content under crops as a functionof time, land use, and site characteristics. We used a harmonised globaldataset containing paired-comparison empirical values of SOC andincluding different types of perennial crops (perennial grasses, palms,and woody plants) with different end-uses: bioenergy, food, other bioproducts, and short rotation coppice crops. Salient outcomes include: a20-year period encompassing a change from annual to perennial cropsled to an average 20% increase in SOC at 0-30 cm (6.0 ± 4.6 Mg ha-1gain) and a total of 10% increase over the 0-100 cm soil profile (5.7 ±10.9 Mg ha-1). A change from natural pasture to perennial cropdecreased SOC stocks by 1% over 0-30 cm (-2.5 ± 4.2 Mg ha-1) and10% over 0-100 cm (-13.6 ± 8.9 Mg ha-1). The effect of a land usechange from forest to perennial crops did not have significant impacts,probably due to the limited number of plots; but the data indicated thatwhile a 2% increase in SOC was observed at 0-30 cm (16.81 ± 55.1 Mgha-1), a decrease of 24% was observed at 30-100 cm (-40.1 ± 16.8 Mgha-1); perennial crops generally accumulate SOC through time,especially woody crops; and temperature was the main driver explainingdifferences in SOC dynamics, followed by crop age, soil bulk density, claycontent and depth. We present empirical evidence showing that the FAOperennialization strategy is reasonable, underscoring the role ofperennial crops as a useful component of climate change mitigationstrategies.

Soil organic matter (SOM) is an indicator of sustainable land management as stated in the global indicator framework of the United Nations Sustainable Development Goals (SDG Indicator 15.3.1). Improved forecasting of future changes in SOM is needed to support the development of more sustainable land management under a changing climate. Current models fail to reproduce historical trends in SOM both within and during transition between ecosystems. More realistic spatio-temporal SOM dynamics require inclusion of the recent paradigm shift from SOM recalcitrance as an 'intrinsic property' to SOM persistence as an 'ecosystem interaction'. We present a soil profile, or pedon-explicit, ecosystem-scale framework for data and models of SOM distribution and dynamics which can better represent land use transitions. Ecosystem-scale drivers are integrated with pedon-scale processes in two zones of influence. In the upper vegetation zone, SOM is affected primarily by plant inputs (above- and belowground), climate, microbial activity and physical aggregation and is prone to destabilization. In the lower mineral matrix zone, SOM inputs from the vegetation zone are controlled primarily by mineral phase and chemical interactions, resulting in more favourable conditions for SOM persistence. Vegetation zone boundary conditions vary spatially at landscape scales (vegetation cover) and temporally at decadal scales (climate). Mineral matrix zone boundary conditions vary spatially at landscape scales (geology, topography) but change only slowly. The thicknesses of the two zones and their transport connectivity are dynamic and affected by plant cover, land use practices, climate and feedbacks from current SOM stock in each layer. Using this framework, we identify several areas where greater knowledge is needed to advance the emerging paradigm of SOM dynamics-improved representation of plant-derived carbon inputs, contributions of soil biota to SOM storage and effect of dynamic soil structure on SOM storage-and how this can be combined with robust and efficient soil monitoring.

Soil microorganisms act as gatekeepers for soil–atmosphere carbon exchange by balancing the accumulation and release of soil organic matter. However, poor understanding of the mechanisms responsible hinders the development of effective land management strategies to enhance soil carbon storage. Here we empirically test the link between microbial ecophysiological traits and topsoil carbon content across geographically distributed soils and land use contrasts. We discovered distinct pH controls on microbial mechanisms of carbon accumulation. Land use intensification in low-pH soils that increased the pH above a threshold (~6.2) leads to carbon loss through increased decomposition, following alleviation of acid retardation of microbial growth. However, loss of carbon with intensification in near-neutral pH soils was linked to decreased microbial biomass and reduced growth efficiency that was, in turn, related to trade-offs with stress alleviation and resource acquisition. Thus, less-intensive management practices in near-neutral pH soils have more potential for carbon storage through increased microbial growth efficiency, whereas in acidic soils, microbial growth is a bigger constraint on decomposition rates., Land use intensification could modify microbial activity and thus ecosystem function. Here, Malik et al. sample microbes and carbon-related functions across a land use gradient, demonstrating that microbial biomass and carbon use efficiency are reduced in human-impacted near-neutral pH soils.

The introduction of Registration, Evaluation and Authorisation of Chemicals (REACH), requires companies to register and risk assess all substances produced or imported in volumes of >1 tonne per year. Extrapolation methods which use existing data for estimating the effects of chemicals are attractive to industry, and comparative data are therefore increasingly in demand. Data on natural toxic chemicals could be used for extrapolation methods such as read-across. To test this hypothesis, the toxicity of natural chemicals and their synthetic analogues were compared using standardised toxicity tests. Two chemical pairs: the napthoquinones, juglone (natural) and 1,4-naphthoquinone (synthetic); and anthraquinones, emodin (natural) and quinizarin (synthetic) were chosen, and their comparative effects on the survival and reproduction of collembolans, earthworms, enchytraeids and predatory mites were assessed. Differences in sensitivity between the species were observed with the predatory mite (Hypoaspis aculeifer) showing the least sensitivity. Within the chemical pairs, toxicity to lethal and sub-lethal endpoints was very similar for the four invertebrate species. The exception was earthworm reproduction, which showed differential sensitivity to the chemicals in both naphthoquinone and anthraquinone pairs. Differences in toxicity identified in the present study may be related to degree of exposure and/or subtle differences in the mode of toxic action for the chemicals and species tested. It may be possible to predict differences by identifying functional groups which infer increased or decreased toxicity in one or other chemical. The development of such techniques would enable the use of read-across from natural to synthetic chemicals for a wider group of compounds.

Global energy demand is increasing as greenhouse gas driven climate change progresses, making renewable energy sources critical to future sustainable power provision. Land-based wind and solar electricity generation technologies are rapidly expanding, yet our understanding of their operational effects on biological carbon cycling in hosting ecosystems is limited. Wind turbines and photovoltaic panels can significantly change local ground-level climate by a magnitude that could affect the fundamental plant-soil processes that govern carbon dynamics. We believe that understanding the possible effects of changes in ground-level microclimates on these phenomena is crucial to reducing uncertainty of the true renewable energy carbon cost and to maximise beneficial effects. In this Opinions article, we examine the potential for the microclimatic effects of these land-based renewable energy sources to alter plant-soil carbon cycling, hypothesise likely effects, and identify critical knowledge gaps for future carbon research.

Soil enzymes are catalysts of organic matter depolymerisation, which is of critical importance for ecosystem carbon (C) cycling. Better understanding of the sensitivity of enzymes to temperature will enable improved predictions of climate change impacts on soil C stocks. These impacts may be especially large in tropical montane forests, which contain large amounts of soil C. We determined the temperature sensitivity (Q 10) of a range of hydrolytic and oxidative enzymes involved in organic matter cycling from soils along a 1900 m elevation gradient (a 10 °C mean annual temperature gradient) of tropical montane forest in the Peruvian Andes. We investigated whether the activity (V max) of selected enzymes: (i) exhibited a Q 10 that varied with elevation and/or soil properties; and (ii) varied among enzymes and according to the complexity of the target substrate for C-degrading enzymes. The Q 10 of V max for β-glucosidase and β-xylanase increased with increasing elevation and declining mean annual temperature. For all other enzymes, including cellobiohydrolase, N-acetyl β-glucosaminidase and phosphomonoesterase, the Q 10 of V max did not vary linearly with elevation. Hydrolytic enzymes that degrade more complex C compounds had a greater Q 10 of V max, but this pattern did not apply to oxidative enzymes because phenol oxidase had the lowest Q 10 value of all enzymes studied here. Our findings suggest that regional differences in the temperature sensitivities of different enzyme classes may influence the terrestrial C cycle under future climate warming.