Pushing the envelope: do narrowly and widely distributed Eucalyptus species differ in response to climate warming?

Contemporary climate change will push many tree species into conditions that are outside their current climate envelopes. Using the Eucalyptus genus as a model, we addressed whether species with narrower geographical distributions show constrained ability to cope with warming relative to species with wider distributions, and whether this ability differs among species from tropical and temperate climates. We grew seedlings of widely and narrowly distributed Eucalyptus species from temperate and tropical Australia in a glasshouse under two temperature regimes: the summer temperature at seed origin and +3.5°C. We measured physical traits and leaf-level gas exchange to assess warming influences on growth rates, allocation patterns, and physiological acclimation capacity. Warming generally stimulated growth, such that higher relative growth rates early in development placed seedlings on a trajectory of greater mass accumulation. The growth enhancement under warming was larger among widely than narrowly distributed species and among temperate rather than tropical provenances. The differential growth enhancement was primarily attributable to leaf area production and adjustments of specific leaf area. Our results suggest that tree species, including those with climate envelopes that will be exceeded by contemporary climate warming, possess capacity to physiologically acclimate but may have varying ability to adjust morphology.

Acid rain reduced soil carbon emissions and increased the temperature sensitivity of soil respiration: A comprehensive meta-analysis.

Soil respiration the second-largest carbon flux in terrestrial ecosystems, has been extensively studied across a wide range of biomes. Surprisingly, no consensus exist on how acid rain (AR) impacts the spatiotemporal pattern of soil respiration. Therefore, we conducted a meta-analysis using 318 soil respiration and 263 soil respiration temperature sensitivity (Q10) data points obtained from 48 studies to assess the impact of AR on soil respiration components and their Q10. The results showed that AR reduced soil total respiration (Rt) and soil autotrophic respiration (Ra) by 7.41 % and 20.75 %, respectively. As the H+ input increased, the response rates of Ra to AR (RR-Ra) and soil heterotrophic respiration (Rh) to AR (RR-Rh) decreased and increased, respectively. With increased AR duration, the RR-Ra increased, whereas the RR-Rh did not change. AR increased the Q10 of Rt (Rt-Q10) and Rh (Rh-Q10) by 1.92 % and 9.47 %, respectively, and decreased the Q10 of Ra (Ra-Q10) by 2.77 %. Increased mean annual temperature, mean annual precipitation, and initial soil organic carbon increased the response rate of Ra-Q10 to AR (RR-Ra-Q10) and decreased the response rate of Rh-Q10 to AR (RR-Rh-Q10). However, as the AR frequency and initial soil pH increased, both RR-Ra-Q10 and RR-Rh-Q10 also increased. In summary, AR decreased Rt but increased Q10, likely due to soil acidification (soil pH decreased by 7.84 %), reducing plant root biomass (decreased by 5.67 %) and soil microbial biomass (decreased by 5.67 %), changing microbial communities (increased fungi to bacteria ratio of 15.91 %), and regulated by climate, vegetation, soil and AR regimes. To the best of our knowledge, this is the first study to reveal the large-scale, varied response patterns of soil respiration components and their Q10 to AR. It highlights the importance of applying the reductionism theory in soil respiration research to enhance our understanding of soil carbon cycling processes with in the context of global climate change.

Flooding Length Mediates Fencing and Grazing Effects on Soil Respiration in Meadow Steppe.

Grassland management affects soil respiration (Rs, consists of heterotrophic respiration and autotrophic respiration) through soil micro-ecological processes, such as hydrothermal, plant root, organic carbon decomposition and microbial activity. Flooding, an irregular phenomenon in grasslands, may strongly regulate the response of soil respiration and its components to grassland management, but the regulatory mechanism remains unclear. We conducted a 3-year experiment by grassland management (fencing and grazing) and flooding conditions (no flooding (NF), short-term flooding (STF) and long-term flooding (LTF)) to study their effects on Rs and its components in a meadow steppe in the Hui River basin of Hulunbuir. We found differences in the patterns of Rs and its components under grassland management and flooding conditions. In 2021-2023, the temporal trends of Rs, heterotrophic respiration (Rh) and autotrophic respiration (Ra) were generally consistent, with peaks occurring on days 190-220, and the peaks of grazing were higher than that of fencing. In NF, Rs of grazed grassland was significantly higher than that of fenced grassland in 2021-2022 (p < 0.05). In STF and LTF, there was no significant difference in Rs between fenced and grazed grassland (p > 0.05). The dependence of Rs on soil temperature (ST) decreased with increasing flooding duration, and the dependence of Rs on ST of grazed grassland was higher than fenced grassland under NF and STF, but there was no difference between fenced grassland and grazed grassland under LTF. In addition, Rh was more sensitive to ST than Ra. This may be due to the different pathways of ST effects on Rs under grazing in different flooding conditions. Our study indicates that the effect of flooding on Rs is the key to the rational use of grassland under future climate change. To reduce regional carbon emissions, we recommend grazing on flooding grassland and fencing on no-flooding grassland.

Interactions between nitrogen and phosphorus in modulating soil respiration: A meta-analysis.

BACKGROUND: Economic and social development worldwide increases the input of nutrients, especially nitrogen (N) and phosphorus (P), to soils. These nutrients affect soil respiration (Rs) in terrestrial ecosystems. They may act independently or have interactive effects on Rs. The effect of N and P on Rs and its components (autotrophic respiration [Ra] and heterotrophic respiration [Rh]), however, either individually or together, is poorly understood. We performed a meta-analysis of 130 studies to examine the effects of different fertilization treatments on Rs and its components across terrestrial ecosystems. RESULTS: Our results showed that (1) The impact of fertilizer addition on Rs varies among different fertilizer types. N addition reduced Rs and Rh significantly but did not affect Ra; P addition had no significant effect on Rs, Rh, and Ra; NP addition increased Rs significantly but did not affect Rh and Ra. (2) Ecosystem type, duration of fertilization, fertilization rate, and fertilizer form influenced the response of Rs and its components to fertilizer application. (3) Based on our study, the annual average temperature may be a driving factor of Rs response to fertilizer addition, while soil total nitrogen may be an important predictor of Rs response to fertilizer addition. CONCLUSION: Overall, our study highlights the complex and multifaceted nature of the response of soil Rs and its components to fertilizer application, underscoring the importance of considering multiple factors when predicting and modeling future Rs and its feedback to global change.

Contextualized response of carbon-use efficiency to warming at the plant and ecosystem levels.

Carbon-use efficiency (CUE) has been widely used as a constant value in many earth system models to simulate how assimilated C is partitioned in ecosystems, to estimate ecosystem C budgets, and investigate C feedbacks to climate warming. Although correlative relationships from previous studies indicated that CUE could vary with temperature, and relying on a fixed CUE value could cause large uncertainty in model projections, however, due to the lack of manipulative experiment, it remains unclear how CUE at the plant (CUEp) and ecosystem (CUEe) levels respond to warming. Based on a 7-year manipulative warming experiment in an alpine meadow ecosystem on the Qinghai-Tibet Plateau, we quantitatively distinguished various C flux components of CUE, including gross ecosystem productivity, net primary productivity, net ecosystem productivity, ecosystem respiration, plant autotrophic respiration, and microbial heterotrophic respiration and explored how CUE at different levels responded to climate warming. We found large variations in both CUEp (0.60 to 0.77) and CUEe (from 0.38 to 0.59). The warming effect on CUEp was positively correlated with ambient soil water content (SWC) and the warming effect on CUEe was negatively correlated with ambient soil temperature (ST), but was positively correlated with warming-induced changes in ST. We also found that the direction and magnitude of the warming effects on different CUE components scaled differently with changes in the background environment, which explained the variation in CUE's warming response under environmental changes. Our new insights have important implications for reducing modelling uncertainty of ecosystem C budgets and improving our ability to predict ecosystem C-climate feedbacks under climate warming.

[Effects of Short-Term Nitrogen and Phosphorus Addition on Soil Respiration Components in a Subalpine Grassland of Qilian Mountains].

In order to investigate the effects of short-term nitrogen and phosphorus addition on soil respiration and its components in a subalpine grassland located on the Qilian Mountains, a random block design of nitrogen[10 g·(m2·a)-1, N], phosphorus[5 g·(m2·a)-1, P], nitrogen and phosphorus addition[10 g·(m2·a)-1N and 5 g·(m2·a)-1P, NP], the control (CK), and complete control (CK') was conducted from June to August 2019, and total soil respiration and its component respiration rates were measured. The results showed that nitrogen addition reduced soil total respiration and heterotrophic respiration rates at a lower rate than P addition[-16.71% vs. -19.20%; -4.41% vs. -13.05%], but the rate of decrease in autotrophic respiration was higher than that of P addition (-25.03% vs. -23.36%); N and P mixed application had no significant effect on soil total respiration rate. The total soil respiration rate and its components were significantly exponentially correlated with soil temperature, and the temperature sensitivity of soil respiration rate was decreased by nitrogen addition (Q10:-5.64%-0.00%). P increased Q10 (3.38%-6.98%), and N and P reduced autotrophic respiration rate but increased heterotrophic respiration rate Q10 (16.86%) and decreased total soil respiration rate Q10 (-2.63%- -2.02%). Soil pH, soil total nitrogen, and root phosphorus content were significantly correlated with autotrophic respiration rate (P<0.05) but not with heterotrophic respiration rate, and root nitrogen content was significantly negatively correlated with heterotrophic respiration rate (P<0.05). In general, autotrophic respiration rate was more sensitive to N addition, whereas heterotrophic respiration rate was more sensitive to P addition. Both N and P addition significantly reduced soil total respiration rate, whereas N and P mixture did not significantly affect soil total respiration rate. These results can provide a scientific basis for the accurate assessment of soil carbon emission in subalpine grassland.

Meta-analysis shows forest soil CO2 effluxes are dependent on the disturbance regime and biome type.

Forest soil CO2 efflux (FCO2 ) is a crucial process in global carbon cycling; however, how FCO2 responds to disturbance regimes in different forest biomes is poorly understood. We quantified the effects of disturbance regimes on FCO2 across boreal, temperate, tropical and Mediterranean forests based on 1240 observations from 380 studies. Globally, climatic perturbations such as elevated CO2 concentration, warming and increased precipitation increase FCO2 by 13% to 25%. FCO2 is increased by forest conversion to grassland and elevated carbon input by forest management practices but reduced by decreased carbon input, fire and acid rain. Disturbance also changes soil temperature and water content, which in turn affect the direction and magnitude of disturbance influences on FCO2 . FCO2 is disturbance- and biome-type dependent and such effects should be incorporated into earth system models to improve the projection of the feedback between the terrestrial C cycle and climate change.

Significant effects of precipitation frequency on soil respiration and its components-A global synthesis.

Global warming intensifies the hydrological cycle, which results in changes in precipitation regime (frequency and amount), and will likely have significant impacts on soil respiration (Rs ). Although the responses of Rs to changes in precipitation amount have been extensively studied, there is little consensus on how Rs will be affected by changes in precipitation frequency (PF) across the globe. Here, we synthesized the field observations from 296 published papers to quantify the effects of PF on Rs and its components using meta-analysis. Our results indicated that the effects of PF on Rs decreased with an increase in background mean annual precipitation. When the data were grouped by climate conditions, increased PF showed positive effects on Rs under the arid condition but not under the semi-humid or humid conditions, whereas decreased PF suppressed Rs across all the climate conditions. The positive effects of increased PF mainly resulted from the positive response of heterotrophic respiration under the arid condition while the negative effects of decreased PF were mainly attributed to the reductions in root biomass and respiration. Overall, our global synthesis provided for the first time a comprehensive analysis of the divergent effects of PF on Rs and its components across climate regions. This study also provided a framework for understanding and modeling responses of ecosystem carbon cycling to global precipitation change.

Canada Goldenrod Invasion Regulates the Effects of Soil Moisture on Soil Respiration.

Canada goldenrod (Solidago canadensis L.) is considered one of the most deleterious and invasive species worldwide, and invasion of riparian wetlands by S. canadensis can reduce vegetation diversity and alter soil nutrient cycling. However, little is known about how S. canadensis invasion affects soil carbon cycle processes, such as soil respiration, in a riparian wetland. This study was conducted to investigate the effects of different degrees of S. canadensis invasion on soil respiration under different moisture conditions. Soil respiration rate (heterotrophic and autotrophic respiration) was measured using a closed-chamber method. S. canadensis invasion considerably reduced soil respiration under all moisture conditions. The inhibition effect on autotrophic respiration was higher than that on heterotrophic respiration. The water level gradient affects the soil autotrophic respiration, thereby affecting the soil respiration rate. The changes in soil respiration may be related to the alteration in the effective substrate of the soil substrate induced by the invasion of S. canadensis. While the effects of S. canadensis invasion were regulated by the fluctuation in moisture conditions. Our results implied that S. canadensis invasion could reduce the soil respiration, which further potentially affect the carbon sequestration in the riparian wetlands. Thus, the present study provided a reference for predicting the dynamics of carbon cycling during S. canadensis invasion and constituted a scientific basis for the sustainable development and management of riparian wetlands invaded by alien plants.

Joint control of seasonal timing and plant function types on drought responses of soil respiration in a semiarid grassland.

Globally, droughts are the most widespread climate factor impacting carbon (C) cycling. However, as the second-largest terrestrial C flux, the responses of soil respiration (Rs) to extreme droughts co-regulated by seasonal timing and PFT (plant functional type) are still not well understood. Here, a manipulative extreme-duration drought experiment (consecutive 30 days without rainfall) was designed to address the importance of drought timing (early-, mid-, or late growing season) for Rs and its components (heterotrophic respiration (Rh) and autotrophic respiration (Ra)) under three PFT treatments (two graminoids, two shrubs, and their combination). The results suggested that regardless of PFT, the mid-drought had the greatest negative effects while early-drought overall had little effect on Rh and its dominated Rs. However, PFT treatments had significant effects on Rh and Rs in response to the late drought, which was PFT-dependence: reduction in shrubs and combination but not in graminoids. Path analysis suggested that the decrease in Rs and Rh under droughts was through low soil water content induced reduction in MBC and GPP. These findings demonstrate that responses of Rs to droughts depend on seasonal timing and communities. Future droughts with different seasonal timing and induced shifts in plant structure would bring large uncertainty in predicting C dynamics under climate changes.

Extreme wet precipitation and mowing stimulate soil respiration in the Eurasian meadow steppe.

The imbalance of terrestrial carbon (C) inputs versus losses to extreme precipitation can have consequences for ecosystem carbon balances. However, the current understanding of how ecosystem processes will respond to predicted extreme dry and wet years is limited. The current study was conducted for three years field experiment to examine the effects of environmental variables and soil microbes on soil respiration (Rs), autotrophic respiration (Ra) and heterotrophic respiration (Rh) under extreme wet and dry conditions in mowed and unmowed grassland of Inner Mongolia. Across treatments (i.e. control, dry spring, wet spring, dry summer and wet summer), the mean of Rs was increased by 24.9 % and 24.1 % in the wet spring and wet summer precipitation treatments, respectively in mowed grassland. In other hand, the mean of Rs was decreased by -22.1 % and -3.5 % in dry spring and dry summer precipitation treatments, respectively in mowed grassland. The relative contribution of Rh and Ra to Rs showed a significant (p < 0.05) change among simulated precipitation treatments with the highest value (76.18 %) in wet summer and 26.41 % in dry summer, respectively under mowed grassland. Rs was significantly (p < 0.05) affected by the interactive effect of extreme precipitation and mowing treatments in 2020 and 2021. The effects of precipitation change via these biotic and abiotic factors explained by 52 % and 81 % in Ra and Rh, respectively in mowed grassland. The changes in microbial biomass carbon (MBC) and nitrogen (MBN) had significant (p < 0.05) direct effects on Rh in both mowed and unmowed grasslands. The influence of biotic and abiotic factors on Rs was stronger in mowed grasslands with higher standardized regression weights than in unmowed grassland (0.78 vs. 0.69). These findings highlight the importance of incorporating extreme precipitation events and mowing in regulating the responses of C cycling to global change in the semiarid Eurasian meadow steppe.

[Effect of Organic Material Amendments on Soil Respiration in Tobacco Fields of Central Henan].

A field experiment was conducted to study the effects of different organic material amendments on soil respiration in a flue-cured tobacco field. Five treatments were set up:no fertilizer (NF), chemical fertilizer (NPK), chemical fertilizer+ryegrass (NPKG), chemical fertilizer+wheat straw (NPKS), and chemical fertilizer+tobacco straw biochar (NPKB). The results showed that:① Compared with that under NPK, NPKG and NPKS decreased the temperature sensitivity (Q10) of total soil respiration and heterotrophic respiration, whereas NPKB increased the Q10 of heterotrophic respiration. The two-factor fitting model of soil respiration and soil hydrothermal factors accounted for 50%-80% of the variation in soil respiration. ② The addition of organic materials significantly increased the content of soil soluble organic carbon (DOC) and root dry matter. Soil heterotrophic respiration(Rh) was significantly positively correlated with DOC content, and soil autotrophic respiration(Ra) was significantly parabolically correlated with root biomass, with an R2 of 0.327-0.634. ③ Soil respiration increased first and then decreased during the tobacco growth period. Compared with that under the NF treatment, the NPK treatment significantly promoted soil respiration and its components. Compared with those of the NPK treatment, Rsrates were significantly increased by 20.08%, 10.32%, and 9.88% under the NPKG, NPKS, and NPKB treatments, respectively; Rh rate increased by 24.21%, 16.51%, and 11.68% respectively, and Ra rate was increased by 15.12% in the NPKG treatment. In summary, straw returning and biochar addition significantly increased Rh by increasing soil DOC, thereby promoting Rs. Incorporation of ryegrass not only increased the Rh but also increased Ra by promoting the growth and development of roots and therefore the Rs.

Drought and warming alter gross primary production allocation and reduce productivity in a widespread pasture grass.

Carbon allocation determines plant growth, fitness and reproductive success. However, climate warming and drought impacts on carbon allocation patterns in grasses are not well known, particularly following grazing or clipping. A widespread C3 pasture grass, Festuca arundinacea, was grown at 26 and 30°C in controlled environment chambers and subjected to drought (65% reduction relative to well-watered controls). Leaf, root and whole-plant carbon fluxes were measured and linked to growth before and after clipping. Both drought and warming reduced gross primary production and plant biomass. Drought reduced net leaf photosynthesis but increased the leaf respiratory fraction of assimilated carbon. Warming increased root respiration but did not affect either net leaf photosynthesis or leaf respiration. There was no evidence of thermal acclimation. Moreover, root respiratory carbon loss was amplified in the combined drought and warming treatment and, in addition to a negative carbon balance aboveground, explained an enhanced reduction in plant biomass. Plant regrowth following clipping was strongly suppressed by drought, reflecting increased tiller mortality and exacerbated respiratory carbon loss. These findings emphasize the importance of considering carbon allocation patterns in response to grazing or clipping and interactions with climatic factors for sustainable pasture production in a future climate.

Nitrogen concentration and physical properties are key drivers of woody tissue respiration.

BACKGROUND AND AIMS: Despite the critical role of woody tissues in determining net carbon exchange of terrestrial ecosystems, relatively little is known regarding the drivers of sapwood and bark respiration. METHODS: Using one of the most comprehensive wood respiration datasets to date (82 species from Australian rainforest, savanna and temperate forest), we quantified relationships between tissue respiration rates (Rd) measured in vitro (i.e. 'respiration potential') and physical properties of bark and sapwood, and nitrogen concentration (Nmass) of leaves, sapwood and bark. KEY RESULTS: Across all sites, tissue density and thickness explained similar, and in some cases more, variation in bark and sapwood Rd than did Nmass. Higher density bark and sapwood tissues had lower Rd for a given Nmass than lower density tissues. Rd-Nmass slopes were less steep in thicker compared with thinner-barked species and less steep in sapwood than in bark. Including the interactive effects of Nmass, density and thickness significantly increased the explanatory power for bark and sapwood respiration in branches. Among these models, Nmass contributed more to explanatory power in trunks than in branches, and in sapwood than in bark. Our findings were largely consistent across sites, which varied in their climate, soils and dominant vegetation type, suggesting generality in the observed trait relationships. Compared with a global compilation of leaf, stem and root data, Australian species showed generally lower Rd and Nmass, and less steep Rd-Nmass relationships. CONCLUSIONS: To the best of our knowledge, this is the first study to report control of respiration-nitrogen relationships by physical properties of tissues, and one of few to report respiration-nitrogen relationships in bark and sapwood. Together, our findings indicate a potential path towards improving current estimates of autotrophic respiration by integrating variation across distinct plant tissues.

A decreasing carbon allocation to belowground autotrophic respiration in global forest ecosystems.

Belowground autotrophic respiration (RAsoil) depends on carbohydrates from photosynthesis flowing to roots and rhizospheres, and is one of the most important but least understood components in forest carbon cycling. Carbon allocation plays an important role in forest carbon cycling and reflects forest adaptation to changing environmental conditions. However, carbon allocation to RAsoil has not been fully examined at the global scale. To fill this knowledge gap, we first used a Random Forest algorithm to predict the spatio-temporal patterns of RAsoil from 1981 to 2017 based on the most updated Global Soil Respiration Database (v5) with global environmental variables; calculated carbon allocation from photosynthesis to RAsoil (CAB) as a fraction of gross primary production; and assessed its temporal and spatial patterns in global forest ecosystems. Globally, mean RAsoil from forests was 8.9 ± 0.08 Pg C yr-1 (mean ± standard deviation) from 1981 to 2017 and increased significantly at a rate of 0.006 Pg C yr-2, paralleling broader soil respiration changes and suggesting increasing carbon respired by roots. Mean CAB was 0.243 ± 0.016 and decreased over time. The temporal trend of CAB varied greatly in space, reflecting uneven responses of CAB to environmental changes. Combined with carbon use efficiency, our CAB results offer a completely independent approach to quantify global aboveground autotropic respiration spatially and temporally, and could provide crucial insights into carbon flux partitioning and global carbon cycling under climate change.

Root Functional Trait and Soil Microbial Coordination: Implications for Soil Respiration in Riparian Agroecosystems.

Predicting respiration from roots and soil microbes is important in agricultural landscapes where net flux of carbon from the soil to the atmosphere is of large concern. Yet, in riparian agroecosystems that buffer aquatic environments from agricultural fields, little is known on the differential contribution of CO2 sources nor the systematic patterns in root and microbial communities that relate to these emissions. We deployed a field-based root exclusion experiment to measure heterotrophic and autotrophic-rhizospheric respiration across riparian buffer types in an agricultural landscape in southern Ontario, Canada. We paired bi-weekly measurements of in-field CO2 flux with analysis of soil properties and fine root functional traits. We quantified soil microbial community structure using qPCR to estimate bacterial and fungal abundance and characterized microbial diversity using high-throughput sequencing. Mean daytime total soil respiration rates in the growing season were 186.1 ± 26.7, 188.7 ± 23.0, 278.6 ± 30.0, and 503.4 ± 31.3 mg CO2-C m-2 h-1 in remnant coniferous and mixed forest, and rehabilitated forest and grass buffers, respectively. Contributions of autotrophic-rhizospheric respiration to total soil CO2 fluxes ranged widely between 14 and 63% across the buffers. Covariation in root traits aligned roots of higher specific root length and nitrogen content with higher specific root respiration rates, while microbial abundance in rhizosphere soil coorindated with roots that were thicker in diameter and higher in carbon to nitrogen ratio. Variation in autotrophic-rhizospheric respiration on a soil area basis was explained by soil temperature, fine root length density, and covariation in root traits. Heterotrophic respiration was strongly explained by soil moisture, temperature, and soil carbon, while multiple factor analysis revealed a positive correlation with soil microbial diversity. This is a first in-field study to quantify root and soil respiration in relation to trade-offs in root trait expression and to determine interactions between root traits and soil microbial community structure to predict soil respiration.

Xylem transport of root-derived CO2 caused a substantial underestimation of belowground respiration during a growing season.

Previous research has indicated that a potentially large portion of root-respired CO2 can move internally through tree xylem, but these reports are relatively scarce and have generally been limited to short observations. Our main objective was to provide a continuous estimate of the quantity and variability of root-respired CO2 that moves either internally through the xylem (FT ) or externally through the soil to the atmosphere (FS ) over most of a growing season. Nine trees were measured in a Populus deltoides stand for 129 days from early June to mid-October. We calculated FT as the product of sap flow and dissolved [CO2 ] in the xylem (i.e., [CO2 *]) and calculated FS using the [CO2 ] gradient method. During the study, stem and soil CO2 concentrations, temperature, and sap flow were measured continuously. We determined that FT accounted for 33% of daily total belowground CO2 flux (i.e., FS  + FT ; FB ) during our observation period that spanned most of a growing season. Cumulative daily FT was lower than FS 74% of the time, equivalent to FS 26% of the time, and never exceeded FS . One-third of the total CO2 released by belowground respiration over most of the growing season in this forest stand followed the FT pathway rather than diffusing into the soil. The magnitude of FT indicates that measurements of FS alone substantially underestimate total belowground respiration in some forest ecosystems by systematically underestimating belowground autotrophic respiration. The variability in FT observed during the growing season demonstrated the importance of making long-term, high-frequency measurements of different flux pathways to better understand physiological and ecological processes and their implications to global change.

Major and persistent shifts in below-ground carbon dynamics and soil respiration following logging in tropical forests.

Soil respiration is the largest carbon efflux from the terrestrial ecosystem to the atmosphere, and selective logging influences soil respiration via changes in abiotic (temperature, moisture) and biotic (biomass, productivity, quantity and quality of necromass inputs) drivers. Logged forests are a predominant feature of the tropical forest landscape, their area exceeding that of intact forest. We quantified both total and component (root, mycorrhiza, litter, and soil organic matter, SOM) soil respiration in logged (n = 5) and old-growth (n = 6) forest plots in Malaysian Borneo, a region which is a global hotspot for emission from forest degradation. We constructed a detailed below-ground carbon budget including organic carbon inputs into the system via litterfall and root turnover. Total soil respiration was significantly higher in logged forests than in old-growth forests (14.3 ± 0.23 and 12.7 ± 0.60 Mg C ha-1  year-1 , respectively, p = 0.037). This was mainly due to the higher SOM respiration in logged forests (55 ± 3.1% of the total respiration in logged forests vs. 50 ± 3.0% in old-growth forests). In old-growth forests, annual SOM respiration was equal to the organic carbon inputs into the soil (difference between SOM respiration and inputs 0.18 Mg C ha-1  year-1 , with 90% confidence intervals of -0.41 and 0.74 Mg C ha-1  year-1 ), indicating that the system is in equilibrium, while in logged forests SOM respiration exceeded the inputs by 4.2 Mg C ha-1  year-1 (90% CI of 3.6 and 4.9 Mg C ha-1  year-1 ), indicating that the soil is losing carbon. These results contribute towards understanding the impact of logging on below-ground carbon dynamics, which is one of the key uncertainties in estimating emissions from forest degradation. This study demonstrates how significant perturbation of the below-ground carbon balance, and consequent net soil carbon emissions, can persist for decades after a logging event in tropical forests.

The response of soil respiration to precipitation change is asymmetric and differs between grasslands and forests.

Intensification of the Earth's hydrological cycle amplifies the interannual variability of precipitation, which will significantly impact the terrestrial carbon (C) cycle. However, it is still unknown whether previously observed relationship between soil respiration (Rs ) and precipitation remains applicable under extreme precipitation change. By analyzing the observations from a much larger dataset of field experiments (248 published papers including 151 grassland studies and 97 forest studies) across a wider range of precipitation manipulation than previous studies, we found that the relationship of Rs response with precipitation change was highly nonlinear or asymmetric, and differed significantly between grasslands and forests, between moderate and extreme precipitation changes. Response of Rs to precipitation change was negatively asymmetric (concave-down) in grasslands, and double-asymmetric in forests with a positive asymmetry (concave-up) under moderate precipitation changes and a negative asymmetry (concave-down) under extreme precipitation changes. In grasslands, the negative asymmetry in Rs response was attributed to the higher sensitivities of soil moisture, microbial and root activities to decreased precipitation (DPPT) than to increased precipitation (IPPT). In forests, the positive asymmetry was predominantly driven by the significant increase in microbial respiration under moderate IPPT, while the negative asymmetry was caused by the reductions in root biomass and respiration under extreme DPPT. The different asymmetric responses of Rs between grasslands and forests will greatly improve our ability to forecast the C cycle consequences of increased precipitation variability. Specifically, the negative asymmetry of Rs response under extreme precipitation change suggests that the soil C efflux will decrease across grasslands and forests under future precipitation regime with more wet and dry extremes.

Phosphorus addition increased carbon partitioning to autotrophic respiration but not to biomass production in an experiment with Zea mays.

Plant carbon (C) partitioning-the relative use of photosynthates for biomass production, respiration, and other plant functions-is a key but poorly understood ecosystem process. In an experiment with Zea mays, with or without arbuscular mycorrhizal fungi (AMF), we investigated the effect of phosphorus (P) fertilization and AMF on plant C partitioning. Based on earlier studies, we expected C partitioning to biomass production (i.e., biomass production efficiency; BPE) to increase with increasing P addition due to reduced C partitioning to AMF. However, although plant growth was clearly stimulated by P addition, BPE did not increase. Instead, C partitioning to autotrophic respiration increased. These results contrasted with our expectations and with a previous experiment in the same set-up where P addition increased BPE while no effect on autotropic respiration was found. The comparison of both experiments suggests a key role for AMF in explaining these contrasts. Whereas in the previous experiment substantial C partitioning to AMF reduced BPE under low P, in the current experiment, C partitioning to AMF was too low to directly influence BPE. Our results illustrate the complex influence of nutrient availability and mycorrhizal symbiosis on plant C partitioning.