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Microbial carbon-use efficiency (CUE) in soils captures carbon (C) partitioning between anabolic biosynthesis of microbial metabolites and catabolic C emissions (i.e. respiratory C waste). The use of C for biosynthesis provides a potential for the accumulation of microbial metabolic residues in soil. Recognised as a crucial control in C cycling, microbial CUE is implemented in the majority of soil C models. Due to the models' high sensitivity to CUE, reliable soil C projections demand accurate CUE quantifications. Current measurements of CUE neglect microbial non-growth metabolites, such as extracellular polymeric substances (EPS) or exoenzymes, although they remain in soil and could be quantitatively important. Here, we highlight that disregarding non-growth anabolism can lead to severe underestimations of CUE. Based on two case studies, we demonstrate that neglecting exoenzyme and EPS production underestimates CUE by more than 100% and up to 30%, respectively. By incorporating these case-specific values in model simulations, we observed that the model projects up to 34% larger SOC stocks over a period of 64 years when non-growth metabolites are considered for estimating CUE, highlighting the crucial importance of accurate CUE quantification. Our considerations outlined here challenge the current ways how CUE is measured and we suggest improvements concerning the quantification of non-growth metabolites. Research efforts should focus on (i) advancing CUE estimations by capturing the multitude of microbial C uses, (ii) improving techniques to quantify non-growth metabolic products in soil, and (iii) providing an understanding of dynamic metabolic C uses under different environmental conditions and over time. In the light of current discussion on soil C stabilisation mechanisms, we call for efforts to open the 'black box' of microbial physiology in soil and to incorporate all quantitative important C uses in CUE measurements. [Display omitted] • Microbial carbon-use efficiency (CUE) is a crucial control of soil carbon cycling. • Current CUE measurements disregard carbon used for non-growth anabolism. • Current measurements of ' apparent ' CUE underestimate ' actual' CUE. • Underestimations of CUE can be large, with consequences for soil carbon projections. • To advance our understanding, we need to open the 'black box' of CUE. [ABSTRACT FROM AUTHOR]

Mechanisms controlling carbon stabilisation in soil and its feedback to climate change are of considerable importance. Microbial substrate-use efficiency is an important property during decomposition of soil organic matter. It determines the allocation of substrate towards biosynthetic stabilisation of carbon and for respiratory losses into the atmosphere. Previously, it was observed that substrate-use efficiency declines with an increase in temperature and that it varies across organic substrates. Yet, our mechanistic understanding of processes causing the temperature sensitivity of substrate-use efficiency is limited. Changes in substrate-use efficiency could be triggered by (i) shifts in the active components of microbial communities, (ii) changes in microbial physiology within the same community, or (iii) a combination of both. In the present study, we evaluated the link between microbial community composition and substrate-use efficiency, combining measurements of carbon mineralisation and microbial energetics. We found only minor shifts in microbial community composition, despite large differences in substrate-use efficiencies across incubation temperatures and substrate additions. We conclude that short-term changes in substrate-use efficiency were mainly caused by changes in microbial physiology, but emphasize that future studies should focus on resolving long-term trade-offs between physiological and community influences on substrate-use efficiency. [ABSTRACT FROM AUTHOR]

Microbial substrate use efficiency is an important property in process-based soil organic matter models, but is often assumed to be constant in mechanistic models. However, previous studies question if a constant efficiency is appropriate, in particular when evaluating carbon (C) cycling across temperatures and various substrates. In the present study, we evaluated the relation between substrate use efficiency, microbial community composition and substrate complexity in contrasting long-term management regimes (47-49 years of either arable, ley farming, grassland, or forest systems). Microbial community composition was assessed by phospholipid fatty acid analysis and three indices of substrate use efficiencies were considered: (i) thermodynamic efficiency, (ii) calorespirometric ratio, and (iii) metabolic quotient. Three substrates, d-glucose, l-alanine, or glycogen, varying in complexity, were added separately to soils, and heat production as well as C mineralization was determined over a 32-h incubation period at 12.5 °C. Microbial communities from forest systems were most efficient in utilizing substrates, supporting our hypothesis that maturing ecosystems become more efficient. These changes in efficiency were linked to microbial community composition with fungi and Gram-negative bacteria being important biomarkers. Despite our initial hypothesis, complex substrate such as glycogen was utilized most efficiently. Our findings emphasize that differences in land use management systems as well as the composition of soil organic matter need to be considered when modelling C dynamics in soils. Further research is required to establish and evaluate appropriate proxies for substrate use efficiencies in various ecosystems. [ABSTRACT FROM AUTHOR]

• Structure liming modified macropore networks, but not mesopore networks. • Structure lime tended to reduce macroporosity, macropore diameters and connectivity. • Structure liming tended to reduce readily dispersible soil. • Liming effects on pore networks differed substantially between sites. • Although quantitatively different, effects generally pointed in the same direction. Structure liming aims to improve soil structure (i.e., the spatial arrangement of particles and pores) and its stability against external and internal forces. Effects of lime application on soil structure have received considerable interest, but only a few studies have investigated effects on macro- and mesopore networks. We used X-ray computed tomography to image macropore networks (ø ≥ 0.3 mm) in soil columns and mesopores (ø ≥ 0.01 mm) in soil aggregates from three field sites with (silty) clay soils after the application of structure lime (3.1 t ha−1 or 5 t ha−1 of CaO equivalent). Segmented X-ray images were used to quantify soil porosity and pore size distributions as well as to analyse pore architecture and connectivity metrics. In addition, we investigated the amount of readily dispersible soil particles. Our results demonstrate that structure liming affected both, macropore networks and amounts of readily dispersible soil to different degrees, depending on the field site. Significant changes in macropore networks and amounts of readily dispersible soil after lime application were found for one of the three field sites, while only some indications for similar changes were observed at the other two sites. Overall, structure liming tended to decrease soil macroporosity and shift pore size distribution from larger (ε >1.0 mm) and medium sized macropores (ε 0.3–1.0 mm) towards smaller macropores (ε 0.1–0.3 mm). Furthermore, liming tended to decrease the critical and average pore diameters, while increasing the surface fractal dimension and specific surface area of macropore network. Structure liming also reduced the amounts of readily dispersible soil particles. We did not find any changes in mesopore network properties within soil aggregates or biopore networks in columns and aggregates. The effects of lime on macropore networks remain elusive, but may be caused by the formation of hydrate phases and carbonates which occupy pore space. [ABSTRACT FROM AUTHOR]

Soil organic carbon (SOC) is a substantial source of atmospheric CO 2 , but also a large cause of uncertainties in Earth-system models. A principal control on soil CO 2 release is the carbon-use efficiency (CUE) of microbial communities, which partitions the carbon (C) allocation between biosynthetic stabilization and CO 2 respiration during SOC decomposition. In Earth-system models, CUE is commonly considered as a constant, although it should be susceptible to environmental factors such as temperature. We explored CUE across a set of land-uses and temperatures, and we show the hitherto neglected phenomenon that land-use can alter the temperature response of CUE. In arable soils, CUE was constant over a temperature range between 5 and 20 °C, but it decreased with temperature in ley farming, grassland, and forest soils at temperatures above 12.5 °C. The decrease in CUE was strongest for forest soils. Implementing our findings into a soil-C model revealed substantial differences in projected SOC losses: Assuming an increase of mean annual temperature of 2 or 4 °C, soils were projected to lose up to 6 or 15% of their current SOC, respectively, until they reach a new steady-state. These projections varied among land-uses. Our findings confront the current representation of CUE in global C models and challenges C sequestration strategies based on land-use changes, because land-uses such as e.g. forest ecosystems with current high C storage may lose substantially more C than agricultural soils due to strong declines of CUE. Image 1 • Temperature responses of carbon-use efficiency (CUE) differed across land-uses. • CUE decreased strongly in ley farming, grassland, and forest soils beyond 12.5 °C. • Land-use specific CUE temperature responses have profound impact on SOC projection. • Novel consumption-based thermodynamic approach ensures similar microbial workloads. • Microbial community composition was not a major driver of differences in CUE. [ABSTRACT FROM AUTHOR]

Climate-smart agriculture aims amongst others at protecting and increasing soil organic carbon (SOC) stocks. The allocation of metabolised carbon (C) between soil microbial growth and respiration, i.e. C use efficiency (CUE) is crucial for SOC dynamics. We hypothesised that raising soil pH would alleviate CUE-limiting conditions and that liming could thus increase CUE, thereby supporting SOC accrual. This study investigated whether CUE can be manipulated by liming and how this might contribute to SOC stock changes. The effects of liming on CUE, microbial biomass C, abundance of microbial domains, SOC stocks and OC inputs were assessed for soils from three European long-term field experiments. Field control soils were additionally limed in the laboratory to assess immediate effects. The shift in soil pH H2O from 4.5 to 7.3 with long-term liming reduced CUE by 40 %, whereas the shift from 5.5 to 8.6 and from 6.5 to 7.8 was associated with increases in CUE by 16 % and 24 %, respectively. The overall relationship between CUE and soil pH followed a U-shaped (i.e. quadratic) curve, implying that in agricultural soils CUE may be lowest at pH H2O = 6.4. The immediate CUE response to liming followed the same trends. Changes in CUE with long-term liming contributed to the net effect of liming on SOC stocks. Our study confirms the value of liming as a management practice for climate-smart agriculture, but demonstrates that it remains difficult to predict the impact on SOC stocks due its complex effects on the C cycle. [Display omitted] • Liming altered CUE depending on the initial soil pH and the span of the pH shift. • At low pH, CUE declined with liming likely due to microbial community shifts. • At high pH, CUE increased with liming likely due to altered nutrient availability. • Liming increased microbial biomass C in all cases. • Changes in CUE with liming were not the primary cause for changes in SOC stocks. [ABSTRACT FROM AUTHOR]

The article titled "Gibbs energy or enthalpy—What is relevant for microbial C‐turnover in soils?" discusses the relationship between microbial carbon utilization and energy transformation in soil. The authors argue that a consolidated concept for coupled carbon use efficiency (CUE) and energy use efficiency (EUE) in soil systems requires the correct application of thermodynamic principles and state variables. They criticize the oversimplifications and inconsistencies in a previous review on the topic, pointing out errors in the understanding of Gibbs energy, the use of the NOSC approach, and the comparison of activation energy and reaction enthalpy. The authors conclude that the previous review's statements and equations lack validity and are not supported by data. The article is authored by Matthias Kästner, Thomas Maskow, Anja Miltner, Marcel Lorenz, Sören Thiele‐Bruhn, Tobias Bölscher, Sergey Blagodatsky, Thilo Streck, Holger Pagel, and Evgenia Blagodatskaya. [Extracted from the article]

Concerned decision makers increasingly pose questions as to whether current management practices are able to cope with climate change and increased climate variability. This signifies a shift in the framing of climate change from asking what its potential impacts are to asking whether it induces policy failure and unacceptable change. In this paper, we explore the background, feasibility, and consequences of this new framing. We focus on the specific situation in which a social-political threshold of concern is likely to be exceeded as a result of climate change, requiring consideration of alternative strategies. Action is imperative when such a situation is conceivable, and at this point climate change becomes particularly relevant to decision makers. We call this situation an "adaptation turning point." The assessment of adaptation turning points converts uncertainty surrounding the extent of a climate impact into a time range over which it is likely that specific thresholds will be exceeded. This can then be used to take adaptive action. Despite the difficulty in identifying adaptation turning points and the relative newness of the approach, experience so far suggests that the assessment generates a meaningful dialogue between stakeholders and scientists. Discussion revolves around the amount of change that is acceptable; how likely it is that unacceptable, or more favorable, conditions will be reached; and the adaptation pathways that need to be considered under these circumstances. Defining and renegotiating policy objectives under climate change are important topics in the governance of adaptation. [ABSTRACT FROM AUTHOR]

Soil structure influences water infiltration, aeration and root growth and, thereby, also the conditions for sustainable crop production. Our objective was to quantify the effects of different soil management methods, with a focus on liming, on the topsoil structure of clay soils in Sweden. We sampled 32 intact soil columns (18 cm high, 12.7 cm diameter) from an experimental field with four treatments: conventional tillage (CT), conventional tillage followed by liming (CTL), reduced tillage (RT) and unfertilized fallow (UF). The samples were taken in autumn after harvest, seven years after CaO was applied to the CTL plots. We also sampled in total 72 smaller columns (6 cm high, 6 cm diameter) from the same experiment and from two additional liming experiments where a mixture of Ca(OH)2 and CaCO3 had been applied three years prior to sampling. All columns were analysed using X-ray tomography. One soil aggregate (diameter approximately 5 mm) were isolated from each small column and analysed with the same method. Despite a relatively large number of replicates per treatment for the large columns, there were no significant differences between any of the investigated macropore network properties related to tilled treatments. It is possible that the effects of liming on soil structure was limited to a few years, which means that any effect would have diminished by the time of sampling. The UF treatment, in contrast, exhibited more vertically oriented macropores, which were also better connected compared to the other treatments. This confirms previous findings that tillage may disrupt the vertical continuity of macropore clusters. Liming had generally reduced total imaged porosity for pores >100 μm) for the smaller columns resulting in less connected pore networks. Liming had no significant effects on the pore networks (pores>11 μm) of the aggregates. [ABSTRACT FROM AUTHOR]