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Global climate warming may induce a positive feedback through increasing soil carbon (C) release to the atmosphere. Although warming can affect both C input to and output from soil, direct and convincing evidence illustrating that warming induces a net change in soil C is still lacking. We synthesized the results from field warming experiments at 165 sites across the globe and found that climate warming had no significant effect on soil C stock. On average, warming significantly increased root biomass and soil respiration, but warming effects on root biomass and soil respiration strongly depended on soil nitrogen (N) availability. Under high N availability (soil C:N ratio < 15), warming had no significant effect on root biomass, but promoted the coupling between effect sizes of root biomass and soil C stock. Under relative N limitation (soil C:N ratio > 15), warming significantly enhanced root biomass. However, the enhancement of root biomass did not induce a corresponding C accumulation in soil, possibly because warming promoted microbial CO2 release that offset the increased root C input. Also, reactive N input alleviated warming‐induced C loss from soil, but elevated atmospheric CO2 or precipitation increase/reduction did not. Together, our findings indicate that the relative availability of soil C to N (i.e., soil C:N ratio) critically mediates warming effects on soil C dynamics, suggesting that its incorporation into C‐climate models may improve the prediction of soil C cycling under future global warming scenarios. [ABSTRACT FROM AUTHOR]

Purpose: Fine roots play an essential role in global carbon cycles, but phenological variations in root function and metabolism are poorly understood. To illustrate the dynamics of fine root function and metabolism in the field, we partitioned root respiration (Rr) into growth (Rg), maintenance (Rm), and ion uptake (Rion) components using a modified traditional model. Methods: A year-round experiment was conducted in a young larch-dominated forest regrowing on bare soil. Soil respiration was measured with a chamber method and partitioned into Rr and heterotrophic respiration by trenching. Fine root biomass and production were measured simultaneously. Using the field data, the model was parameterized, and Rr was further partitioned. Results: Annually, Rr (210–253 g C m−2 yr−1) accounts for 45–47% of the total soil respiration. The contribution of fine root Rg, fine root Rm, coarse root Rm, and fine root Rion were 26–40, 46–51, 10–16, and 12%, respectively. The Rg contribution showed a clear seasonal variation, with a peak in mid-spring and a minimum in early fall, mainly because of different seasonality between fine root production and soil temperature. Conclusion: The model parameters were consistent with those from our previous study conducted by the same method in the same site. Thus, we believe that our approach was robust under a relatively simple condition. However, our growth respiration parameter resulting from only field data was much higher than those from laboratory experiments. To further improve our understanding of root respiration, more field data should be accumulated. [ABSTRACT FROM AUTHOR]

An accurate assessment of root respiration in mine reclaimed soil is important for effectively evaluating mining area ecosystems. This study investigated dynamic changes in root respiration and the contribution of root respiration to total soil respiration (Rr/Rt ratio) during the non-growing season in mine reclaimed soil, with different covering-soil thicknesses. According to the covering-soil thicknesses, the study area was divided into four sites: 10–25 cm (site A), 25–45 cm (site B), 45–55 cm (site C), and 55–65 cm (site D). From November 2017 to April 2018 (except February in 2018), the soil respiration, root respiration, temperature at 5 cm, water content, and root biomass were measured. The results show that soil temperature and root respiration exhibited similar diurnal and monthly variations. The root respiration is strongly influenced by soil temperature during the non-growing season, with an exponential and positive relationship (P < 0.001). Root respiration varies with the covering-soil thickness and is greatest with a covering-soil thickness of 25–45 cm. The Rr/Rt ratio also exhibits monthly variations. During the non-growing season, the mean value of the Rr/Rt ratio is 51.15% in mine reclaimed soil. The study indicates that root respiration is the primary source of soil respiration and is an important factor for estimating the potential emission of soil CO2 from mine reclaimed soil at the regional scale. [ABSTRACT FROM AUTHOR]

Nitrogen (N) deposition and grazing (G) are two disturbances of grassland ecosystems and these can greatly affect soil respiration. To date, few studies have investigated this topic in sandy grasslands. Here, we conducted a manipulative experiment in a sandy grassland in Inner Mongolia, China in the year 2013 and 2014, to investigate the effects of N addition and grazing on soil respiration. The results showed that N addition significantly accelerated soil respiration in both 2013 and 2014. The respiration enhancement under N addition varied during the growing season, being larger in early May 2014 (0.40 μmol·m−2·s−1) and early July 2014 (1.29 μmol·m−2·s−1) than in October 2014 (0.03 μmol·m−2·s−1). Grazing plots yielded a lower cumulative soil respiration rate than the control plots (−5,68% in 2013 and −1.07% in 2014), although the difference was not significant. In this study, soil temperature rather than soil moisture was the main factor affecting the temporal variation in soil respiration during the growing season. The inter-plot analysis showed that the soil ammonium nitrogen (NH4+–N) content, C:N ratio, and root biomass were significantly correlated with soil respiration, indicating that soil respiration under N-addition or grazing treatment could be affected by changing soil microhabitat conditions, thus affecting soil respiration. [ABSTRACT FROM AUTHOR]

Rhizosphere respiration (from roots and rhizosphere microbes utilizing root-derived carbon (C)) is a significant component of soil respiration, and rhizosphere priming effect (RPE, change in soil C decomposition by the presence of living roots in the rhizosphere) is crucial for regulating soil C decomposition. However, the relationships between rhizosphere respiration and RPE and root traits (such as biomass and nitrogen (N) concentration) across plant species and growing conditions are not fully resolved. In this study, we investigated rhizosphere respiration rate and RPE of three grassland species (a grass, a forb and a legume) using a continuous isotope-labeling technique. We found a significantly positive relationship between root-mass-specific rhizosphere respiration rate and root N concentration across the three species and two types of mesocosms (small pots and large buckets), and the scaling exponents were approximately one (indicating isometric scaling). Further, soil organic matter (SOM) decomposition rate was not statistically different between the planted treatments and the unplanted control, suggesting insignificant RPE at the early flowering stage (90 days after seeding) for these three species. Likely, respiration from rhizodeposition (root inputs to soil during the 90-day labeling period) was not included in SOM decomposition by the isotope labeling method, which underestimates SOM decomposition and may partly contribute to the lack of RPE of the three species. Overall, our results show that rhizosphere respiration rate is scalable with root N concentration across different plant species and growing conditions, while RPE of these grassland species at the early flowering stage was insignificant. • We used a continuous 13C-labeling method to non-destructively measure rhizosphere respiration. • We also quantified the rhizosphere priming effect on soil organic matter decomposition. • The experiment included three grassland species and two container types. • Mass-specific rhizosphere respiration rate was positively related to root N concentration. • Rhizosphere priming effect was insignificant at early flowering stage for these three species. [ABSTRACT FROM AUTHOR]

The method of component integration was tested in the course of studies in typical biogeocenoses of the north of Western Siberia. This method as applied for separate horizons made it possible not only to estimate the contribution of different sources to the total efflux of CO2 from soils but also to evaluate the participation of horizons in soil respiration. Root respiration contributed to about 14 ± 3% of the CO2 efflux from the permafrost-affected peat soil (Hemic Cryic Histosol) and to about 29 ± 6% of the CO2 efflux from the tonguing podzol (Glossic Albic Rustic Podzol (Arenic)). Histosol was characterized by a higher storage of total (organic and inorganic) carbon (26 kg C/m2), extractable carbon (Cextr) (32 g C/m2), and microbial carbon (Cmic) (43 g C/m2). Its upper horizon (TO) contained about 50% of the total soil storage of Cmic and 100% of the total storage of roots (up to 300 g/m2). It had the maximum activity of microbiota and contributed to more than 70% of the total efflux of CO2 from the active layer (on average, 35 cm in thickness) of Cryic Histosol. Podzol was characterized by the low storage of total carbon (6 kg C/m2), considerable storage of Cmic (38 g C/m2), and much higher microbial activity. This circumstance, together with a tenfold higher storage of the roots (up to 3000 g C/m2) and favorable temperature regime specified high values of the gross production of CO2 and its emission from the Podzol surface exceeding the corresponding values for the Cryic Histosol by three–five times. The mineral profile of Podzol contributed to about 20% of the total efflux of CO2 from this soil (on average, about 38 cm in thickness). [ABSTRACT FROM AUTHOR]

Tallgrass prairie restorations can quickly accrue organic C in soil and biomass, but the rate of C accumulation diminishes through time and is highly variable among more mature prairies. Long-term soil organic carbon ( SOC) accumulation in prairies has been linked to edaphic factors such as soil texture, soil moisture, and SOC content, but it is unclear how these factors affect the ecosystem processes that are responsible for observed differences in C accumulation rates in older prairies. We measured belowground plant and SOC pools and fluxes within 27-36-year-old restored tallgrass prairies in order to quantify total C storage, determine the net ecosystem production of C ( NEP-C), and explore which edaphic factors influence the ecosystem processes responsible for divergent NEP-C. We found that 11% of organic C was stored in biomass, and we estimate that one-third of post-restoration C sequestration has occurred in biomass, thereby highlighting biomass as a large but often overlooked C pool. Belowground biomass and soil C pools were notably smaller than those reported for remnant prairie, suggesting that future belowground C accumulation could still occur. During this study, the prairies appeared to be a net source of C, although the range of NEP-C values encompassed zero. Sand content positively affected NEP-C via increased belowground biomass production-C inputs, and SOC negatively affected NEP-C due to increased soil respiration C outputs. However, soil moisture had a smaller negative effect on soil respiration, indicating that both SOC and soil moisture play important roles in determining prairie C balance. [ABSTRACT FROM AUTHOR]

Intensifying forest disturbance regimes are likely to impact heavily on future carbon (C) budgets of forest ecosystems. Our understanding of how forest disturbance affects the sources of soil CO2 efflux ( Fs) is, however, poor. This may lead to uncertainties over future C sink estimates of forest ecosystems and associated feedbacks to the atmosphere., We investigated the impact of forest windthrow on the heterotrophic and autotrophic sources of Fs, underlying biotic and abiotic drivers (i.e. plant community composition, soil organic matter ( SOM) properties and soil microclimate), and consequences for soil organic carbon ( SOC) stocks in situ along a disturbance chronosequence in the European Alps. This chronosequence facilitated the study of temporal changes in the above parameters between the third and sixth years after windthrow., Along the chronosequence, structural equation modelling revealed that soil temperature, soil moisture, SOM properties and plant community composition explained 90% of the variation in Fs. While no direct interactions among plants and SOM properties could be determined, plants significantly affected soil microclimate. Windthrow had no obvious effect on Fs because reduced autotrophic soil respiration ( Ra) was offset by a ∼60% increase in heterotrophic soil respiration ( Rh), principally due to increased soil temperatures. Ra after wind-throw was dominated by grasses and herbs rather than trees; however, a high abundance of ectomycorrhizal fungi suggests an important indirect tree contribution to post-windthrow Ra. SOC stocks significantly declined over the post-windthrow period., Our results show that Rh was by far the dominant source of Fs after forest windthrow. As C losses from Rh and SOC stocks were in the same order of magnitude, this study demonstrates that post-windthrow declines in SOC stocks were mainly driven by a temperature-related increase in Rh., A is available for this article. [ABSTRACT FROM AUTHOR]

Soil respiration is a major exchange channel of carbon between terrestrial ecosystem and atmospheric environment; it plays a vitally important role in global carbon cycle and carbon balance. Under the background of global change, the issue of acid rain caused by anthropogenic activities has been serious increasingly; and the effects of increased acid rain on soil respiration have attracted increasing attention all over the world in recent years. Acid rain leads to soil acidification, and it then affects the metabolic activity of soil microbes, root growth, and litter decomposition, etc.; which finally affects soil respiration. This paper reviews the progresses of studies on the effects of simulated acid rain on soil respiration in forest and cropland ecosystem,and there are three kinds of results for the effects: depression, promotion, and no effect. The different results are affected by experimental, biological and non-biological factors, including the experimental acidity and duration of simulated acid rain, vegetation type, plant growth season, vegetation succession phases, and physical and chemical properties of soil, etc. Both the low and high intensity of acid rain tends to reduce the temperature sensitivity (Q10) of soil respiration. This paper also analyzes the potential mechanisms of how acid rain affects the soil respiration in ecosystem from four key biological factors affecting soil respiration, including photosynthesis, litter, microbes and root biomass. However, the process of the effects of acid rain on soil respiration is complex, leading to some uncertainties of the response mechanism of soil respiration to acid rain. Accordingly, some deficiencies for the current studies are summarized, and then four directions are proposed to be considered in the future study: (1) The studies on the responses of soil respiration to acid rain in different ecosystems; (2) The studies on the responses of different components of soil respiration to acid rain; (3) The studies on the combined effects of acid rain and other external factors; (4) The studies on the responses of biological factors that are relevant to soil respiration to acid rain. [ABSTRACT FROM AUTHOR]

Aims: Subtropical ecosystems are receiving unprecedented changes in temperature as a consequence of anthropogenic activities, which potentially affects soil respiration ( R ) and carbon (C) sequestration. Due to the large amounts of C store and cycle in subtropical forests, investigations about how R and C sequestration respond to warming will be critical for our understanding of future global-scale climate and biogeochemical cycling. Methods: In this study, we transferred soil samples and plant seedlings collected from a mixed forest to the growth chambers in two sites (300 m and 30 m a.s.l.), which induced an artificial warming of ca. 1 °C between the two corresponding forest mesocosms. We tested whether the modification of abiotic factors induced by the downward translocation could alter R and soil C sequestration. We also investigated the effects on the biotic factors by including root biomass and soil microbial biomass. Results: Our results showed that R was greater in the warm site than in the control site, which were related to the higher aboveground biomass, litterfall and root biomass. R showed a significantly positive exponential relationship with soil temperature. The downward translocation tended to decrease soil C sequestration, which was attributed to the decreased C use efficiency of soil microorganisms and increased root growth under downward translocation. Conclusion: R responded strongly to downward translocation, suggesting that climate warming exacerbated R and tended to reduce soil C sequestration. The ability of subtropical forests to act as CO sink may be reduced under climate warming. [ABSTRACT FROM AUTHOR]

Aims: The objectives of this study were to determine the spatial structure of soil respiration (Rs) in a naturally-regenerated longleaf pine forest and to assess the ecological factors affecting the spatial variability in Rs. Methods: Soil respiration, soil temperature (Ts), and soil moisture were repeatedly measured over 6 days in summer 2012 in 3 semi-independent plots. Edaphic, forest floor, and root variables were measured. Diameters of 338 trees were mapped. Spatial analysis and regression were applied. Results: Soil respiration was spatially autocorrelated across plots (66-92 m), but not within plots (6-34 m). Spatial distributions of Rs were relatively stable from morning through early evening and were decoupled from temporal variation of Ts. Ecological covariates (e.g., soil moisture, bulk density and carbon, litter mass, understory cover, roots, nearby trees) related to the spatial variability in Rs; however, models varied between plots. Conclusions: This study shows the importance of stationary plant and soil factors in determining the spatial, temperature-independent distribution of Rs in a heterogeneous forest. We suggest the need for a better understanding of the complex interactions between the heterotrophic, autotrophic, and physical processes driving Rs in order to better model forest carbon budgets. [ABSTRACT FROM AUTHOR]

Understanding the effects of forest management practices (e.g. thinning) on soil respiration ( R s ) is crucial for the accurate estimation of forest carbon budget. However, little is known about the response of R s to forest thinning in the subalpine region and its linkage to changes in environmental factors induced by thinning. We aimed to quantify the response of R s rate to various gap sizes following thinning treatments, and to explore the relationships between R s and soil temperature and moisture and other biophysical factors in the different gap sizes. We applied the thinning by simulating gap formation (four gap sizes at 0, 74, 109 and 196 m 2 ) in a 26-year old spruce plantation in the eastern Tibetan Plateau. We measured R s monthly before (July to November 2008) and after (December 2008 to June 2012) thinning, as well as monthly soil temperature and moisture and other biophysical factors. Thinning tended to decrease fine root biomass, litterfall, soil extractable C, and increased soil temperature and soil moisture. The change in soil temperature and moisture depended on the time after thinning and the size of forest gap. We found that R s showed an immediate decrease in initial stage after thinning, followed by a gradual increase with understory development towards the level at the control plot. Overall, thinning decreased R s rate by 14.9%, 15.8% and 25.8% in the small, intermediate and large gap, respectively, as compared to the control. We concluded that the decrease in R s rates by thinning in a spruce plantation was driven by the decline in tree root biomass and reduction in soil labile C. The positive effect of soil temperature elevation under thinning on R s was masked by other factors, and the development of understory vegetation after thinning gradually offset the thinning-induced R s reduction. Our results suggest the need to consider a set of abiotic and biotic factors induced by forest thinning intensity on R s rates in modeling the response of soil C cycling to forest management practices. [ABSTRACT FROM AUTHOR]

Soil respiration ( R S ) is one of the largest fluxes of net ecosystem exchange and is related to both soil climate and vegetatively driven substrate supply at various spatial and temporal scales. Relationships between the intra-annual variation in R S and abiotic and biotic variables were examined across diverse longleaf pine ( Pinus palustris Mill.) forests to better understand factors related to R S in these low density, spatially heterogeneous forests. Soil respiration, soil temperature, soil moisture, litter mass, size and proximity of nearby trees, understory cover, and root biomass were measured over 13 months in four longleaf pine forests varying in age from 5 to 87 years. The exponential relationship between R S and soil temperature accounted for the majority of the intra-annual variation in R S with a corresponding temperature sensitivity ( Q 10 ) of 2.18. Soil moisture affected the R S -temperature relationship by dampening R S and Q 10 during times of extremely dry soil conditions, as defined by soil moisture ⩽50% of the texture-derived wilting point, but volumetric soil moisture did not directly correlate with R S . The intra-annual variation in temperature-normalized R S was negatively related to the distance to nearest tree and positively related to pine root biomass, but not related to litter mass, understory cover, or stand structural variables such as stand age, basal area, or tree density. Annual R S estimates ranged from 12.0 Mg ha - 1 of C in the 5-year-old stand with mostly grass stage seedlings to 13.9 Mg ha - 1 of C in the dense 21-year-old stand. This study contributes to our understanding of carbon fluxes across diverse longleaf pine ecosystems and indicates the importance of climate in determining the carbon sink potential of southeastern longleaf pine forests. [ABSTRACT FROM AUTHOR]

Aim: This study aims to improve our understanding of temporal variations and controlling factors of soil respiration (R) and its components (microbial respiration or R and root respiration or R) in an Asian tropical seasonal forest at diurnal, seasonal and annual timescales in relation to biotic and abiotic controls. Methods: R was studied by the trenching method in a seasonal dry diptercarp forest, western Thailand. An automated soil chamber system was used to produce hourly data of R during 2008-2011. Analysis of R in relation to both biotic and abiotic factors was carried out to understand its temporal variations at different timescales. Results: R was the main contributor to overall magnitude and variability of R. Soil temperature alone was the main driver of diurnal variation, while the combination of soil moisture and soil temperature determined the seasonal variations. The amount of R was also related to the fine root (<2 mm) and microbial biomass at seasonal timescales. Due to the small inter-annual variations in soil temperature and moisture, total soil respiration among the 4 years was not significantly different ( p < 0.05). The annual totals for R during 2008-2011 were 3.20, 3.89, 3.52, 4.14 kgCO m years, respectively. The 4-year average ratio of R (trenched) to R (untrenched) was 66 ± 4 %. Conclusion: R plays a crucial role in determining the magnitude (large ratio between R and R) and temporal variations of R. In this forest ecosystem, high seasonal variations in R were observed and were mainly attributed to the response of R to moisture. [ABSTRACT FROM AUTHOR]

Understanding of soil carbon dynamics after establishment of alley-cropping systems is crucial for mitigation of greenhouse CO gas. This study investigates soil CO fluxes in an alley-cropping system composed of tree strips of black locust ( Robinia pseudoacacia L.) and poplar ( Populus nigra × P. maximowiczii, Max 1) trees and adjacent to them crop strips ( Lupinus/Solarigol). Soil CO flux was measured monthly over a period from March to November 2012, using a LI-COR LI-8100A automated device. Concurrently with CO flux measurements, soil and air temperature, soil moisture, microbial C and hot water-extractable C were determined for the soils nearby soil collars. Root biomass was determined to a depth of 15 cm. In all sampling areas, soil CO flux increased from May to July, showing a significant positive correlation with air and soil temperature, which can be a reflection of increase in photosynthesis, and therefore supply of carbohydrates from leaves to rhizosphere, over the warm summer months. A positive correlation between CO flux and soil moisture over the warm period indicates an enhancing role of soil moisture on microbial mineralization and root respiration. Average CO flux values observed over March-November period did not differ significantly between sampling areas, showing 2.5, 3.2, and 2.9 µmol m s values for black locust, poplar and crops, respectively. Significantly higher CO flux values over the summer period in trees could be attributed to the higher photosynthetic activity and higher root density compared to crops. [ABSTRACT FROM AUTHOR]

Abstract: Soil respiration accounts for much of the CO2 released from terrestrial ecosystems into the atmosphere. Although respiration depends on temperature, the relationship between respiration and temperature may vary among soils. Here, we measured soil respiration and soil temperature in chronosequences of reclaimed and unreclaimed post-mining sites (10–50-year-old coal mining heaps near Sokolov, the Czech Republic) to determine the major factors affecting temperature-dependent soil respiration. Soil respiration was repeatedly measured in situ during 2011 and 2012 at five reclaimed sites (planted with alder) and five unreclaimed sites (overgrown with willow, birch, and aspen). In addition, spatial heterogeneity was assessed by repeatedly measuring soil respiration at 30 permanent points at one 28-year-old site (the “30-point” site) in 2007–2008. In all sites root biomass, soil carbon (C) content, soil pH, and the thickness of Oe layer were also measured. In the chronosequences and 30-point site, the relationship between soil respiration and temperature increased with soil C content; soil respiration was unrelated to temperature if soil C content was <9%. The increase in respiration with temperature was enhanced by a thick Oe layer and by high root biomass. Soil respiration at reclaimed sites increased with site age to age 30 years and then decreased. The decrease in respiration at the older sites was associated with a decrease in soil temperature (associated with increased shading). Respiration at unreclaimed sites increased with age and was usually lower than in reclaimed alder plantation of similar ages. [Copyright &y& Elsevier]

Background and aims: Lateral tree-scale variability in plantations should be taken into account when scaling up from point samples, but appropriate methods for sampling and calculation have not been defined. Our aim was to define and evaluate such methods. Methods: We evaluated several existing and new methods, using data for throughfall, root biomass and soil respiration in mature oil palm plantations with equilateral triangular spacing. Results: Three ways of accounting for spatial variation within the repeating tree unit (a hexagon) were deduced. For visible patch patterns, patches can be delineated and sampled separately. For radial patterns, measurements can be made in radial transects or a triangular portion of the tree unit. For any type of pattern, including unknown patterns, a triangular sampling grid is appropriate. In the case studies examined, throughfall was 79 % of rainfall, with 95 % confidence limits being 62 and 96 % of rainfall. Root biomass and soil respiration, measured on a 35-point grid, varied by an order of magnitude. In zones with steep gradients in parameters, sampling density has a large influence on calculated mean values. Conclusions: The methods defined here provide a basis for representative sampling and calculation procedures in studies requiring scaling up from point sampling, but more efficient methods are needed. [ABSTRACT FROM AUTHOR]

• Heavy thinning reduced soil respiration (SR) and its components in the sixth to eighth year after thinning. • Thinning improved the temperature sensitivity for heterotrophic respiration (HR). • Thinning reduced the temperature sensitivity for autotrophic respiration (AR). • Decreases in SR, HR, and AR were related to microbial biomass and fine root biomass. Mounting evidence has indicated that forest thinning would increase soil respiration (SR) in the early stage (c. < 5 years) after thinning. However, the responses of SR and its components to different thinning intensities in the long-term have not been sufficiently studied. In April 2010, four levels of thinning intensities including control (CK, no thinning), light thinning (LT, 20% basal are removed), moderate thinning (MT, 40% basal area removed), and heavy thinning (HT, 60% basal area removed) were conducted in a Chinese pine (Pinus tabulaeformis) plantation in northern China. SR and its components were measured monthly in growing seasons (May to October) from 2016–2018. After 6–8 years of thinning, on average, HT significantly reduced SR by 24.56%, compared to the control. Declines in SR were ascribed to the decreased heterotrophic respiration (HR) and autotrophic respiration (AR) in HT plots, because heavy thinning inhibited soil nutrients, microbial biomass carbon (MBC), and fine root biomass (FRB). Thinning increased temperature sensitivity (Q 10) of HR (CK = 2.24, LT = 2.58, MT = 2.82, HT = 2.51), but decreased the Q 10 values of AR (CK = 2.76, LT = 1.14, MT = 1.82, HT = 2.23). Soil moisture had a positive relationship with HR but did not correlate with AR (expect for MT). In addition, two-factor models combining soil temperature and soil moisture explained 40–64% and 1–12% of variation in the HR and AR, respectively. The effects of thinning on soil respiration and its components varied substantially over time. Our study highlights that fine root and microbial biomass should be incorporated into biogeochemical models when accurately predicting long-term effects of thinning on soil respiration and its components. [ABSTRACT FROM AUTHOR]

Grasslands support large populations of herbivores and store up to 30% of the world's soil carbon (C). Thus, herbivores likely play an important role in the global C cycle. However, most studies on how herbivory impacts the largest source of C released from grassland soils-soil carbon dioxide (CO) emissions-only considered the role of large ungulates. This ignores all other vertebrate and invertebrate herbivores and their collective effects on ecosystem properties. We progressively excluded large, medium, and small vertebrates and invertebrates from two subalpine grasslands (productive, heavily grazed short-grass; less productive, lightly grazed tall-grass) using size-selective fences, assessed the impact on soil CO emissions and related biotic and abiotic variables. Exclusion resulted in significant changes in soil CO emissions in both vegetation types. Short-grass soil CO emissions progressively increased when large and medium mammals were excluded. However, no difference was detected among plots were all or no herbivores grazed. In contrast, tall-grass soil CO emissions were not affected by mammal exclusion, but excluding all herbivores lead to reduced emissions. Soil micro-climatic parameters best predicted the patterns of soil CO emissions in short-grass vegetation, whereas root biomass was the best predictor of CO release in tall-grass vegetation. Our results showed that diverse herbivore communities affect soil respiration differently than assumed from previous studies that only excluded large ungulates. Such information is important if we are to understand how changes in herbivore species composition-as could happen through altered management practices, extinction or invasion-impact grassland C storage and release. [ABSTRACT FROM AUTHOR]

Natural rubber is a valuable source of income in many tropical countries and rubber trees are increasingly planted in tropical areas, where they contribute to land-use changes that impact the global carbon cycle. However, little is known about the carbon balance of these plantations. We studied the soil carbon balance of a 15-year-old rubber plantation in Thailand and we specifically explored the seasonal dynamic of soil CO 2 efflux ( FS) in relation to seasonal changes in soil water content ( WS) and soil temperature ( TS), assessed the partitioning of FS between autotrophic ( RA) and heterotrophic ( RH) sources in a root trenching experiment and estimated the contribution of aboveground and belowground carbon inputs to the soil carbon budget. A multiplicative model combining both TS and WS explained 58 % of the seasonal variation of FS. Annual soil CO 2 efflux averaged 1.88 kg C m −2 year −1 between May 2009 and April 2011 and RA and RH accounted for respectively 63 and 37 % of FS, after corrections of FS measured on trenched plots for root decomposition and for difference in soil water content. The 4-year average annual aboveground litterfall was 0.53 kg C m −2 year −1 while a conservative estimate of belowground carbon input into the soil was much lower (0.17 kg C m −2 year −1). Our results highlighted that belowground processes (root and rhizomicrobial respiration and the heterotrophic respiration related to belowground carbon input into the soil) have a larger contribution to soil CO 2 efflux (72 %) than aboveground litter decomposition. [ABSTRACT FROM AUTHOR]

Background and aims: The response of soil respiration (SR) to elevated CO is driven by a number of processes and feedbacks. This work aims to i) detect the effect of elevated CO on soil respiration during the second rotation of a short rotation forest, at two levels of N availability; and ii) identify the main drivers behind any changes in soil respiration. Methods: A poplar plantation (POP-EUROFACE) was grown for two rotations of 3 years under elevated CO maintained by a FACE (Free Air CO Enrichment) technique. Root biomass, litter production and soil respiration were followed for two consecutive years after coppice. Results: In the plantation, the stimulation of fine root and litter production under elevated CO observed at the beginning of the rotation declined over time. Soil respiration (SR) was continuously stimulated by elevated CO, with a much larger enhancement during the growing (up to 111 %) than in the dormant season (40 %). The SR increase at first appeared to be due to the increase in fine root biomass, but at the end of the 2nd rotation was supported by litter decomposition and the availability of labile C. Soil respiration increase under elevated CO was not affected by N availability. Conclusions: The stimulation of SR by elevated CO was sustained by the decomposition of above and belowground litter and by the greater availability of easily decomposable substrates into the soil. In the final year as elevated CO did not increase C allocation to roots, the higher SR suggests greater C losses from the soil, thus reducing the potential for C accumulation. [ABSTRACT FROM AUTHOR]

Oil palm plantations cover ≈14.6 million ha worldwide and the total area under cultivation is expected to increase during the 21st century . Indonesia and Malaysia together account for 87% of global palm oil production and the combined harvested area in these countries has expanded by 6.5 million ha since 1990. Despite this, soil C cycling in oil palm systems is not well quantified but such information is needed for C budget inventories. We quantified soil C storage (root biomass, soil organic matter ( SOM) and microbial biomass) and losses [potential soil respiration ( Rs) and soil surface CO2 flux ( Fs)] in mineral soils from an oil palm plantation chronosequence (11-34 years since planting) in Selangor, Malaysia. There were no significant effects of plantation age on SOM, microbial biomass, Rs or Fs, implying soil C was in dynamic equilibrium over the chronosequence. However, there was a significant increase in root biomass with plantation age, indicating a short-term C sink. Across the chronosequence, Rs was driven by soil moisture, soil particle size, root biomass and soil microbial biomass N but not microbial biomass C. This suggests that the nutrient status of the microbial community may be of equal or greater importance for soil CO2 losses than substrate availability and also raises particular concerns regarding the addition of nitrogenous fertilizer, i.e. increased yields will be associated with increased soil CO2 emissions. To fully assess the impact of oil palm plantations on soil C storage, initial soil C losses following land conversion (e.g. from native forest or other previous plantations) must be accounted for. If initial soil C losses are large, our data show that there is no accumulation of stable C in the soil as the plantation matures and hence the conversion to oil palm would probably represent a net loss of soil C. [ABSTRACT FROM AUTHOR]

Abstract: Close relationships among climatic factors and soil respiration (R s) are commonly reported. However, variation in R s across the landscape is compounded by site-specific differences that impede the development of spatially explicit models. Among factors that influence R s, the effect of ecosystem age is poorly documented. We hypothesized that R s increases with grassland age and tested this hypothesis in a chronosequence of tallgrass prairie reconstructions in central Iowa, U.S.A. We also assessed changes in root biomass, root ingrowth, aboveground net primary productivity (ANPP), and the strength of soil temperature and moisture in predicting R s. We found a significant increase in total growing season R s with prairie age (R 2 =0.79), ranging from 714gCm−2 in the youngest reconstruction (age 4) to 939gCm−2 in the oldest prairie (age 12). Soil temperature was a strong predictor of intra-seasonal R s among prairies (R 2 =0.78–0.87) but mean growing season soil temperature and moisture did not relate to total R s. The increase in R s with age was positively correlated with root biomass (r =0.80) and ANPP (r =0.87) but not with root ingrowth. Our findings suggest that growing season R s increases with tallgrass prairie age, root biomass, and ANPP during young grassland development. [ABSTRACT FROM AUTHOR]

Atmospheric CO2 ( Ca) concentration has increased significantly during the last 20 000 years, and is projected to double this century. Despite the importance of belowground processes in the global carbon cycle, community-level and single species root responses to rising Ca are not well understood. We measured net community root biomass over 3 years using ingrowth cores in a natural C3–C4 grassland exposed to a gradient of Ca from preglacial to future levels (230–550 μmol mol−1). Root windows and minirhizotron tubes were installed below naturally occurring stands of the C4 perennial grass Bothriochloa ischaemum and its roots were measured for respiration, carbohydrate concentration, specific root length (SRL), production, and lifespan over 2 years. Community root biomass increased significantly ( P<0.05) with Ca over initial conditions, with linear or curvilinear responses depending on sample date. In contrast, B. ischaemum produced significantly more roots at subambient than elevated Ca in minirhizotrons. The lifespan of roots with five or more neighboring roots in minirhizotron windows decreased significantly at high Ca, suggesting that after dense root growth depletes soil resource patches, plants with carbon surpluses readily shed these roots. Root respiration in B. ischaemum showed a curvilinear response to Ca under moist conditions in June 2000, with the lowest rates at Ca<300 μmol mol−1 and peak activity at 450 μmol mol−1 in a quadratic model. B. ischaemum roots at subambient Ca had higher SRLs and slightly higher carbohydrate concentrations than those at higher Ca, which may be related to drier soils at low Ca. Our data emphasize that belowground responses of plant communities to Ca can be quite different from those of the individual species, and suggest that complex interactions between and among roots and their immediate soil environment influence the responses of root physiology and lifespan to changing Ca. [ABSTRACT FROM AUTHOR]

The recent infestation of southern Appalachian eastern hemlock stands by hemlock woolly adelgid (HWA) is expected to have dramatic and lasting effects on forest structure and function. We studied the short-term changes to the carbon cycle in a mixed stand of hemlock and hardwoods, where hemlock was declining due to either girdling or HWA infestation. We expected that hemlock would decline more rapidly from girdling than from HWA infestation. Unexpectedly, in response to both girdling and HWA infestation, hemlock basal area increment (BAI) reduced substantially compared to reference hardwoods in 3 years. This decline was concurrent with moderate increases in the BAI of co-occurring hardwoods. Although the girdling treatment resulted in an initial pulse of hemlock needle inputs, cumulative litter inputs and O horizon mass did not differ between treatments over the study period. Following girdling and HWA infestation, very fine root biomass declined by 20–40% in 2 years, which suggests hemlock root mortality in the girdling treatment, and a reduction in hemlock root production in the HWA treatment. Soil CO2 efflux ( Esoil) declined by approximately 20% in 1 year after both girdling and HWA infestation, even after accounting for the intra-annual variability of soil temperature and moisture. The reduction in Esoil and the concurrent declines in BAI and standing very fine root biomass suggest rapid declines in hemlock productivity from HWA infestation. The accelerated inputs of detritus resulting from hemlock mortality are likely to influence carbon and nutrient fluxes, and dictate future patterns of species regeneration in these forest ecosystems. [ABSTRACT FROM AUTHOR]

The effect of stand age on soil respiration and its components was studied in a first rotation Sitka spruce chronosequence composed of 10-, 15-, 31-, and 47-year-old stands established on wet mineral gley in central Ireland. For each stand age, three forest stands with similar characteristics of soil type and site preparation were used. There were no significant differences in total soil respiration among sites of the same age, except for the case of a 15-year-old stand that had lower soil respiration rates due to its higher productivity. Soil respiration initially decreased with stand age, but levelled out in the older stands. The youngest stands had significantly higher respiration rates than more mature sites. Annual soil respiration rates were modelled by means of temperature-derived functions. The average 10 value obtained treating all the stands together was 3.8. Annual soil respiration rates were 991, 686, 556, and 564 g C m−2 for the 10-, 15-, 31-, and 47-year-old stands, respectively. We used the trenching approach to separate soil respiration components. Heterotrophic respiration paralleled soil organic carbon dynamics over the chronosequence, decreasing with stand age to slightly increase in the oldest stand as a result of accumulated aboveground litter and root inputs. Root respiration showed a decreasing trend with stand age, which was explained by a decrease in fine root biomass over the chronosequence, but not by nitrogen concentration of fine roots. The decrease in the relative contribution of autotrophic respiration to total soil CO2 efflux from 59.3% in the youngest stand to 49.7% in the oldest stand was explained by the higher activity of the root system in younger stands. Our results show that stand age should be considered if simple temperature-based models to predict annual soil respiration in afforestation sites are to be used. [ABSTRACT FROM AUTHOR]

The loss of carbon through root respiration is an important component of grassland carbon budgets. However, few data are available concerning the contribution of root respiration to total soil respiration in grasslands in China. We investigated seasonal variations of soil respiration rate, root biomass, microbial biomass C and organic C content of the soil in a semi-arid Leymus chinensis (Trin.) Tzvel. grassland of northeast China during the 2002 growing season (from May to September). The linear regression relationship between soil respiration rate and root biomass was used to determine the contribution of root respiration to total soil respiration. Soil respiration rate ranged from 2.5 to 11.9 g C/m2 per d with the maximum in late June and minimum in September. The microbial biomass C and organic C content of the soil ranged from 0.3 to 1.5 g C/m2 and from 29 to 34 g C/kg respectively. Root biomass had two peaks, in early June (1.80 kg/m2) and mid-August (1.73 kg/m2). Root respiration rate peaked in mid-August (6.26 g C/m2 per d), whereas microbial respiration rate peaked in late June (7.43 g C/m2 per d). We estimated that the contribution of root respiration to total soil respiration during the growing season ranged from 38% to 76%. (Managing editor: Ya-Qin Han) [ABSTRACT FROM AUTHOR]

Research Highlights: Tree decline can alter soil carbon cycling, given the close relationship between primary production and the activity of roots and soil microbes. Background and Objectives: We studied how tree decline associated to old age and accelerated by land-use change and increased drought in the last decades, affects soil properties and soil respiration (Rs). Materials and Methods: We measured Rs over two years around centennial European beech (Fagus sylvatica L.) trees representing a gradient of decline in a sub-Mediterranean forest stand, where the number of centennial beech trees has decreased by 54% in the last century. Four replicate plots were established around trees (i) with no apparent crown dieback, (ii) less than 40% crown dieback, (iii) more than 50% crown dieback, and (iv) dead. Results: Temporal variations in Rs were controlled by soil temperature (Ts) and soil water content (SWC). The increase in Rs with Ts depended on SWC. The temperature-normalized Rs exhibited a parabolic relationship with SWC, suggesting a reduced root and microbial respiration associated to drought and waterlogging. The response of Rs to SWC did not vary among tree-decline classes. However, the sensitivity of Rs to Ts was higher around vigorous trees than around those with early symptoms of decline. Spatial variations in Rs were governed by soil carbon to nitrogen ratio, which had a negative effect on Rs, and SWC during summer, when drier plots had lower Rs than wetter plots. These variations were independent of the tree vigor. The basal area of recruits, which was three times (although non-significantly) higher under declining and dead trees than under vigorous trees, had a positive effect on Rs. However, the mean Rs did not change among tree-decline classes. These results indicate that Rs and related soil physico-chemical variables are resilient to the decline and death of dominant centennial trees. Conclusions: The development of advanced regeneration as overstory beech trees decline and die contribute to the Rs homeostasis along forest succession. [ABSTRACT FROM AUTHOR]