Soil Temperature Sensitivity Research Articles

Temperature sensitivity of soil organic carbon decomposition increased with mean carbon residence time: Field incubation and data assimilation.

Temperature sensitivity of soil organic carbon (SOC) decomposition is one of the major uncertainties in predicting climate-carbon (C) cycle feedback. Results from previous studies are highly contradictory with old soil C decomposition being more, similarly, or less sensitive to temperature than decomposition of young fractions. The contradictory results are partly from difficulties in distinguishing old from young SOC and their changes over time in the experiments with or without isotopic techniques. In this study, we have conducted a long-term field incubation experiment with deep soil collars (0-70cm in depth, 10cm in diameter of PVC tubes) for excluding root C input to examine apparent temperature sensitivity of SOC decomposition under ambient and warming treatments from 2002 to 2008. The data from the experiment were infused into a multi-pool soil C model to estimate intrinsic temperature sensitivity of SOC decomposition and C residence times of three SOC fractions (i.e., active, slow, and passive) using a data assimilation (DA) technique. As active SOC with the short C residence time was progressively depleted in the deep soil collars under both ambient and warming treatments, the residences times of the whole SOC became longer over time. Concomitantly, the estimated apparent and intrinsic temperature sensitivity of SOC decomposition also became gradually higher over time as more than 50% of active SOC was depleted. Thus, the temperature sensitivity of soil C decomposition in deep soil collars was positively correlated with the mean C residence times. However, the regression slope of the temperature sensitivity against the residence time was lower under the warming treatment than under ambient temperature, indicating that other processes also regulated temperature sensitivity of SOC decomposition. These results indicate that old SOC decomposition is more sensitive to temperature than young components, making the old C more vulnerable to future warmer climate.

Effects of warming and nitrogen fertilization on GHG flux in an alpine swamp meadow of a permafrost region

Uncertainties in the seasonal changes of greenhouse gases (GHG) fluxes in wetlands limit our accurate understanding of the responses of permafrost ecosystems to future warming and increased nitrogen (N) deposition. Therefore, in an alpine swamp meadow in the hinterland of the Qinghai-Tibet Plateau, a simulated warming with N fertilization experiment was conducted to investigate the key GHG fluxes (ecosystem respiration [Re], CH4 and N2O) in the early (EG), mid (MG) and late (LG) growing seasons. Results showed that warming (6.2 °C) increased the average seasonal Re by 30.9% and transformed the alpine swamp meadow from a N2O sink to a source, whereas CH4 flux was not significantly affected. N fertilization (4 g N m−2 a−1) alone had no significant effect on the fluxes of GHGs. The interaction of warming and N fertilization increased CH4 uptake by 69.6% and N2O emissions by 26.2% compared with warming, whereas the Re was not significantly affected. During the EG, although the soil temperature sensitivity of the Re was the highest, the effect of warming on the Re was the weakest. The primary driving factor for Re was soil surface temperature, whereas soil moisture controlled CH4 flux, and the N2O flux was primarily affected by rain events. The results indicated: (i) increasing N deposition has both positive and negative feedbacks on GHG fluxes in response to climate warming; (ii) during soil thawing process at active layer, low temperature of deep frozen soils have a negative contribution to Re in alpine ecosystems; and (iii) although these alpine wetland ecosystems are buffers against increased temperature, their feedbacks on climate change cannot be ignored because of the large soil organic carbon pool and high temperature sensitivity of the Re.

Seasonal differences in soil respiration and methane uptake in rubber plantation and rainforest

Rubber plantations expanded remarkably in South-East Asia, while the impact of this land use change on soil carbon dynamics and greenhouse gases emissions has not been sufficiently understood. We measured monthly soil CO2 fluxes during one year as well as CH4 fluxes during the rainy season in secondary rainforest, 9 and 22 year-old rubber monoculture and 22-year-old rubber-tea intercropping in Xishuangbanna, Southwest China. Our aim was to assess the impact of the land use change on soil carbon fluxes and quantify the factors determining the difference in the carbon fluxes. A linear mixed effect model was used in studying the soil temperature and moisture variation and temperature sensitivity (Q10) of soil respiration.The temporal pattern of soil respiration distinctly differed between sites during the rainy season: rainforest maintained a high soil respiration rate, while soil respiration became suppressed (by up to 69%) during the most moist period in rubber plantations. Rainforest soils thus emitted the highest amount of CO2 with an annual cumulative flux of 8.48±0.71MgCha−1yr−1, compared to 6.75±0.79, 5.98±0.42 and 5.09±0.47MgCha−1yr−1 for mature rubber, rubber-tea intercropping, and young rubber, respectively. Additionally, the soil CH4 uptake was stronger in rainforest than in rubber plantations during the wet period. Soil temperature was the main factor explaining the overall seasonal variation of soil respiration. Adding a quadratic soil moisture term into the model accounted for moisture effects, identified moisture tipping points, and improved temperature sensitivity assessment when high soil moisture suppressed soil respiration under rubber. Temperature sensitivity of soil respiration was higher for rainforest soil compared to rubber plantations, Q10 values were 3.1 for rainforest and 1.7, 2.2 and 2.4 for mature rubber, rubber-tea intercropping and young rubber respectively.Converting rainforest to rubber plantations tended to reduce soil CO2 emissions and weakened CH4 uptake especially during the very wet period. The altered condition of soil aeration under converted land appears to have a pronounced impact on processes of carbon fluxes from the soil and thus mitigates the positive feedback of climate change given the large area of cultivated rubber.

Carbon quality and nutrient status drive the temperature sensitivity of organic matter decomposition in subtropical peat soils

Estimates of gaseous carbon (C) fluxes in wetlands are heavily based on temperature. However, isolating specific effects of temperature on anaerobic C processing from other controls (C quality and nutrients) has proven difficult. Here, we test the hypothesis that temperature sensitivity of soil organic matter (SOM) decomposition is more influenced by C quality than nutrient availability in subtropical freshwater, sawgrass (Cladium jamaicense)-based peats. Carbon age (characterized by depth: 0–10 and 10–20 cm) was used as a surrogate of C quality while two sites were selected with contrasting levels of nutrient (P) availability. In anaerobic laboratory incubations temperature was increased in 5 °C steps to assess the proportion of C available at a given temperature (i.e. thermo-labile C) as productions of gaseous (CO2 and CH4) and dissolved organic C (DOC) fractions. Thermo-labile C increased 3.1–3.6 times from 15 °C to 30 °C in all soils. Disproportionate increase in the production of gaseous forms versus DOC as well as CH4:CO2 was observed with warming. Observed Q10 values followed the trend of CH4 (~14) ≫ CO2 (~2.5) > DOC (~1.7) and temperature sensitivity was more dependent on C quality than nutrient availability over the entire temperature range. Spectral analysis indicated more bio-available DOC production at higher temperature. Regression analysis also indicated that C quality primarily influenced SOM decomposition at lower temperature, while at higher temperature nutrient limitation dominantly controlled SOM decomposition. These findings confirm the role of C quality in temperature sensitivity of warm peat soils, but also indicate an increased importance of nutrient limitation at higher temperature.

Temperature sensitivity of soil microbial communities: An application of macromolecular rate theory to microbial respiration

AbstractThere is compelling evidence that microbial communities vary widely in their temperature sensitivity and may adapt to warming through time. To date, this sensitivity has been largely characterized using a range of models relying on versions of the Arrhenius equation, which predicts an exponential increase in reaction rate with temperature. However, there is growing evidence from laboratory and field studies that observe nonmonotonic responses of reaction rates to variation in temperature, indicating that Arrhenius is not an appropriate model for quantitatively characterizing temperature sensitivity. Recently, Hobbs et al. (2013) developed macromolecular rate theory (MMRT), which incorporates thermodynamic temperature optima as arising from heat capacity differences between isoenzymes. We applied MMRT to measurements of respiration from soils incubated at different temperatures. These soils were collected from three grassland sites across the U.S. Great Plains and reciprocally transplanted, allowing us to isolate the effects of microbial community type from edaphic factors. We found that microbial community type explained roughly 30% of the variation in the CO2production rate from the labile C pool but that temperature and soil type were most important in explaining variation in labile and recalcitrant C pool size. For six out of the nine soil × inoculum combinations, MMRT was superior to Arrhenius. The MMRT analysis revealed that microbial communities have distinct heat capacity values and temperature sensitivities sometimes independent of soil type. These results challenge the current paradigm for modeling temperature sensitivity of soil C pools and understanding of microbial enzyme dynamics.

Impact of Land Cover on Temperature and Moisture Sensitivity of Soil Organic Matter Mineralization in Subtropical Southeastern China

Abstract: Understanding the temperature and moisture sensitivity of soil organic matter (SOM) mineralization variations with changes in land cover is critical for assessing soil carbon (C) storage under global change scenarios. We determined the differences in the amount of SOM mineralization and the temperature and moisture sensitivity of soils collected from six land-cover types, including an orchard, a cropland, and four forests, in subtropical southeastern China. The responses of SOM mineralization to temperature (5, 10, 15, 20, and 25°C) and moisture (30%, 60%, and 90% of water-holding capacity [WHC]) were investigated by placing soil samples in incubators. Soil C mineralization rate and cumulative C mineralization were higher in orchard and cropland soils than in other forest soils. With increasing temperature, soil C mineralization rates and cumulative C mineralization increased with the rise of WHC. The temperature sensitivity of soil C mineralization was not affected by land-cover type and incuba...

A warmer climate reduces the bioreactivity of isolated boreal forest soil horizons without increasing the temperature sensitivity of respiratory CO2 loss

Changes in the carbon (C) balance of boreal forest ecosystems may impact the global C cycle and climate. The degree to which antecedent temperature regime and mineral protection of soil organic matter (OM) influence the temperature response of boreal soil C pools remains unknown, however. To investigate these phenomena on time scales relevant to anthropogenic climate change, we quantified the temperature response of four soil C pools (L, F and H organic horizons and B mineral horizon) within soil profiles collected from replicated sites representing two regions along a climate transect (“regional warming”) during a 480-day incubation at 5 and 15 °C (“experimental warming”). We hypothesized that 1) warmer region soils would exhibit reduced bioreactivity, a measure of C lability assessed via cumulative soil C mineralization, relative to colder region soils, paralleling a decrease in bioreactivity with depth in both regions, and 2) temperature sensitivity of C mineralization (denoted as Q10) would increase with decreasing bioreactivity congruent with the “C quality-temperature” (CQT) hypothesis, with a smaller effect in mineral soil where physico-chemical protection likely occurs. Cumulative C mineralization decreased from surface L to deeper horizons and from the cold to warm region for organic F and H horizons only. This decrease in soil bioreactivity with depth was paralleled by an increase in Q10 with depth as expected, except in mineral soil where Q10 was similar to or lower relative to the overlying organic layer. The lower bioreactivity in F and H horizons of the warm relative to the cold region was not, however, associated with a greater Q10. A warmer regional climate in these otherwise similar forests thus resulted in reduced bioreactivity of isolated soil C pools without increasing the temperature sensitivity of soil C mineralization. This suggests that assumptions about temperature sensitivity of C mineralization based on the propensity for isolated organic C pools to undergo mineralization may not be valid in some organic-rich, boreal forest soils.

Temperature sensitivity of soil and root respiration in contrasting soils

Positive relationships between temperature and soil respiration rate are widely observed, but it remains unclear if the relationships are due to increases in soil organic matter mineralisation (R om ), or in root and rhizosphere respiration (R roots ), or increases in both. This study aims to determine the relative sensitivity of R om and R roots to temperature in soils with differing properties. Taking advantage of the difference in stable carbon isotopic composition provided by C3 and C4 plants, we partitioned soil respiration into R om and R roots for two soils with contrasting clay mineralogy, pH and carbon content over a 24 °C temperature range (from 12 to 36 °C). The Chromosol (dominated by illite, with near neutral pH and low organic carbon content) showed an increase in the proportion of R om with temperature, indicating an increase in the decomposition of soil organic carbon. In contrast, the Ferrosol (dominated by hematite and goethite, with acidic pH and high organic carbon) showed no change in the proportion of R om with warming, and a negative priming effect at the highest temperature. The observed positive priming effect for the Chromosol and a negative priming effect for the Ferrosol are consistent with contrasting mineralogy, reflecting the relatively weaker bond strength between soil carbon and illites in the Chromosol compared to the Ferrosol.

Long-term fertilization of a boreal Norway spruce forest increases the temperature sensitivity of soil organic carbon mineralization.

Boreal ecosystems store one-third of global soil organic carbon (SOC) and are particularly sensitive to climate warming and higher nutrient inputs. Thus, a better description of how forest managements such as nutrient fertilization impact soil carbon (C) and its temperature sensitivity is needed to better predict feedbacks between C cycling and climate. The temperature sensitivity of in situ soil C respiration was investigated in a boreal forest, which has received long-term nutrient fertilization (22 years), and compared with the temperature sensitivity of C mineralization measured in the laboratory. We found that the fertilization treatment increased both the response of soil in situ CO2 effluxes to a warming treatment and the temperature sensitivity of C mineralization measured in the laboratory (Q10). These results suggested that soil C may be more sensitive to an increase in temperature in long-term fertilized in comparison with nutrient poor boreal ecosystems. Furthermore, the fertilization treatment modified the SOC content and the microbial community composition, but we found no direct relationship between either SOC or microbial changes and the temperature sensitivity of C mineralization. However, the relation between the soil C:N ratio and the fungal/bacterial ratio was changed in the combined warmed and fertilized treatment compared with the other treatments, which suggest that strong interaction mechanisms may occur between nutrient input and warming in boreal soils. Further research is needed to unravel into more details in how far soil organic matter and microbial community composition changes are responsible for the change in the temperature sensitivity of soil C under increasing mineral N inputs. Such research would help to take into account the effect of fertilization managements on soil C storage in C cycling numerical models.

Temperature sensitivity and carbon release in an acidic soil amended with lime and mulch

Lime is commonly applied on agricultural lands for ameliorating soil acidity. However, lime dissolution and its concomitant contribution to carbon dioxide (CO2) fluxes with the addition of organic residues at varying temperatures in acidic soils are not well known. We conducted laboratory incubation experiments for 96days (20+/−1°C and 40+/−1°C) to quantify the priming effects of lime (0.46 and 0.92% w/w) and mulch (0.5% w/w) additions on carbon (C) release in a Chromic Luvisol. The C released from lime (δ13C of −8.67‰), mulch (−13.02‰) and soil (−25.2‰) was quantified using their distinct δ13C values. Total lime derived C in soils without mulch constituted approximately 32% and 17% of the total C fluxes at 20°C and 40°C, respectively. During the 96-day incubation period, in the absence of mulch addition between 64% and 100% of the applied lime C was released as CO2 at both incubation temperatures. Furthermore, lime derived, mulch derived and soil derived C increased by 59, 48 and 284% respectively, when the incubation temperature was increased from 20°C to 40°C. These results suggest that mineralization of native soil C was more sensitive to temperature than the C released by lime dissolution and mulch mineralization. Temperature sensitivity of soil derived C was lower in treatments with mulch and lime addition compared to soil without C amendments. We obtained a Q10 value of 2.41±0.06 for the C released by the lime dissolution in the acidic soil. These findings are important for modelling the contribution of different C sources to atmospheric CO2 concentrations in soils, such as the acidic limed and mulched soils.

Responses of soil respiration and its temperature/moisture sensitivity to precipitation in three subtropical forests in southern China

Abstract. Both long-term observation data and model simulations suggest an increasing chance of serious drought in the dry season and extreme flood in the wet season in southern China, yet little is known about how changes in precipitation pattern will affect soil respiration in the region. We conducted a field experiment to study the responses of soil respiration to precipitation manipulations – precipitation exclusion to mimic drought, double precipitation to simulate flood, and ambient precipitation as control (abbr. EP, DP and AP, respectively) – in three subtropical forests in southern China. The three forest sites include Masson pine forest (PF), coniferous and broad-leaved mixed forest (MF) and monsoon evergreen broad-leaved forest (BF). Our observations showed that altered precipitation strongly influenced soil respiration, not only through the well-known direct effects of soil moisture on plant and microbial activities, but also by modification of both moisture and temperature sensitivity of soil respiration. In the dry season, soil respiration and its temperature sensitivity, as well as fine root and soil microbial biomass, showed rising trends with precipitation increases in the three forest sites. Contrarily, the moisture sensitivity of soil respiration decreased with precipitation increases. In the wet season, different treatments showed different effects in three forest sites. The EP treatment decreased fine root biomass, soil microbial biomass, soil respiration and its temperature sensitivity, but enhanced soil moisture sensitivity in all three forest sites. The DP treatment significantly increased soil respiration, fine root and soil microbial biomass in the PF only, and no significant change was found for the soil temperature sensitivity. However, the DP treatment in the MF and BF reduced soil temperature sensitivity significantly in the wet season. Our results indicated that soil respiration would decrease in the three subtropical forests if soil moisture continues to decrease in the future. More rainfall in the wet season could have limited effect on the response of soil respiration to the rising of temperature in the BF and MF.

Effect of grazing enclosure on the priming effect and temperature sensitivity of soil C miner-alization in <I>Leymus chinensis</I> grasslands, Inner Mongolia, China

Effect of grazing enclosure on the priming effect and temperature sensitivity of soil C miner-alization in <I>Leymus chinensis</I> grasslands, Inner Mongolia, China

The Research on the Impact Factors of Low-Quality Stands Soil Respiration after Different Transformations in Greater Higgnan Mountains

Low-quality stands in Greater Higgnan Mountains were transformed by clear-cuttings with different area of forest gaps, then larch were planted after induced transformations. The LI-8150 multi-channel automated soil CO2 flux system was used to measure CO2 flux on soil surface. Changes of different soil respiration rates and influence factors were analyzed after different transformations. The results indicated that the soil respiration rates were all raised after different transformations compared with no interfered control plots. After analyzing different transformations comprehensively, it showed that the soil respiration rate performed a negative correlation with the soil density significantly, while the correlation with soil organic matter and litter weight in little decomposed was positive (p﹤0.05). The largest Q10 with forest gap area of 625 m2 was 3.561. Influenced by soil respiration rate, soil underground with depth of 10cm showed the strongest temperature sensitivity. The smallest Q10 with forest gap area of 900 m2 was 2.312, and temperature sensitivity of soil was the weakest.

Soil warming effect on net ecosystem exchange of carbon dioxide during the transition from winter carbon source to spring carbon sink in a temperate urban lawn

The significant warming in urban environment caused by the combined effects of global warming and heat island has stimulated widely development of urban vegetations. However, it is less known of the climate feedback of urban lawn in warmed environment. Soil warming effect on net ecosystem exchange (NEE) of carbon dioxide during the transition period from winter to spring was investigated in a temperate urban lawn in Beijing, China. The NEE (negative for uptake) under soil warming treatment (temperature was about 5°C higher than the ambient treatment as a control) was −0.71 μmol/(m2·sec), the ecosytem was a CO2 sink under soil warming treatment, the lawn ecosystem under the control was a CO2 source (0.13 μmol/(m2·sec)), indicating that the lawn ecosystem would provide a negative feedback to global warming. There was no significant effect of soil warming on nocturnal NEE (i.e., ecosystem respiration), although the soil temperature sensitivity (Q10) of ecosystem respiration under soil warming treatment was 3.86, much lower than that in the control (7.03). The CO2 uptake was significantly increased by soil warming treatment that was attributed to about 100% increase of a (apparent quantum yield) and Amax (maximum rate of photosynthesis). Our results indicated that the response of photosynthesis in urban lawn is much more sensitive to global warming than respiration in the transition period.

Effects of Precipitation Increase on Soil Respiration: A Three-Year Field Experiment in Subtropical Forests in China

BackgroundThe aim of this study was to determine response patterns and mechanisms of soil respiration to precipitation increases in subtropical regions.Methodology/Principal FindingsField plots in three typical forests [i.e. pine forest (PF), broadleaf forest (BF), and pine and broadleaf mixed forest (MF)] in subtropical China were exposed under either Double Precipitation (DP) treatment or Ambient Precipitation (AP). Soil respiration, soil temperature, soil moisture, soil microbial biomass and fine root biomass were measured over three years. We tested whether precipitation treatments influenced the relationship of soil respiration rate (R) with soil temperature (T) and soil moisture (M) using R = (a+cM)exp(bT), where a is a parameter related to basal soil respiration; b and c are parameters related to the soil temperature and moisture sensitivities of soil respiration, respectively. We found that the DP treatment only slightly increased mean annual soil respiration in the PF (15.4%) and did not significantly change soil respiration in the MF and the BF. In the BF, the increase in soil respiration was related to the enhancements of both soil fine root biomass and microbial biomass. The DP treatment did not change model parameters, but increased soil moisture, resulting in a slight increase in soil respiration. In the MF and the BF, the DP treatment decreased soil temperature sensitivity b but increased basal soil respiration a, resulting in no significant change in soil respiration.Conclusion/SignificanceOur results indicate that precipitation increasing in subtropical regions in China may have limited effects on soil respiration.

Testing the application of an agronomic concept to microbiology: A degree-day model to express cumulative co2 emission from soils

Although much research has been carried out, there is still no consensus about the temperature dependence of CO2 emissions from soil organic carbon (SOC) decomposition. The temperature sensitivity of soil CO2 emissions varies with the time and the temperature levels used in laboratory experiments. Although combined models have been used to take account of the incubation time and temperature to describe the decomposition of SOC, the factors temperature and time in these models are still independent. These models were fitted to CO2 data obtained from parallel laboratory incubations. In this study, sequential incubations were carried out for 2 months at temperatures between 20°C and 50°C, assuming that (1) the sequence of temperature levels did not affect the soil CO2 emissions and (2) the CO2 emissions depended only on the temperature sum accumulated by the soil. A degree-day model, which is commonly used in agronomy, was applied to predict soil CO2 emission variations with time and temperature. The results showed that, for any sequence of temperature levels, the accumulation of degree-days explained the cumulative CO2 emissions during two months laboratory incubation over the 20–40°C range, where 6% of SOC was emitted as CO2. However, at 50°C, soil CO2 emissions were higher than predicted by the degree-day model. This underestimation of soil CO2 emissions lasted for one month after the soil had been at 50°C. These results suggest that, for the range of incubation temperatures (20–40°C) and time tested (56 days), or 2000 degree-days, (i) the degree-day model is valid only between 20 and 40°C, (ii) the main determinant of soil temperature sensitivity is the amount of labile carbon rather than microbial adaptation of soil respiration to temperature.

Soil temperature and moisture sensitivities of soil CO 2 efflux before and after tillage in a wheat field of Loess Plateau, China

As a conventional farming practice, tillage has lasted for thousands of years in Loess Plateau, China. Although recent studies show that tillage is a prominent culprit to soil carbon loss in croplands, few studies have investigated the influences of tillage on the responses of soil CO 2 efflux (SCE) to soil temperature and moisture. Using a multi-channel automated CO 2 efflux chamber system, we measured SCE in situ continuously before and after the conventional tillage in a rain fed wheat field of Loess Plateau, China. The changes in soil temperature and moisture sensitivities of SCE, denoted by the Q 10 value and linear regression slope respectively, were compared in the same range of soil temperature and moisture before and after the tillage. The results showed that, after the tillage, SCE increased by 1.2-2.2 times; the soil temperature sensitivity increased by 36.1%–37.5%; and the soil moisture sensitivity increased by 140%–166%. Thus, the tillage-induced increase in SCE might partially be attributed to the increases in temperature and moisture sensitivity of SCE.

Effects of nitrogen fertilization on soil respiration in temperate grassland in Inner Mongolia, China

Nitrogen addition to soil can play a vital role in influencing the losses of soil carbon by respiration in N-deficient terrestrial ecosystems. The aim of this study was to clarify the effects of different levels of nitrogen fertilization (HN, 200 kg N ha(-1) year(-1); MN, 100 kg N ha(-1) year(-1); LN, 50 kg N ha(-1) year(-1)) on soil respiration compared with non-fertilization (CK, 0 kg N ha(-1) year(-1)), from July 2007 to September 2008, in temperate grassland in Inner Mongolia, China. Results showed that N fertilization did not change the seasonal patterns of soil respiration, which were mainly controlled by soil heat-water conditions. However, N fertilization could change the relationships between soil respiration and soil temperature, and water regimes. Soil respiration dependence on soil moisture was increased by N fertilization, and the soil temperature sensitivity was similar in the treatments of HN, LN, and CK treatments (Q (10) varied within 1.70-1.74) but was slightly reduced in MN treatment (Q (10) = 1.63). N fertilization increased soil CO2 emission in the order MN > HN > LN compared with the CK treatment. The positive effects reached a significant level for HN and MN (P 0.05). The pairwise differences between the three N-level treatments were not significant in either year (P > 0.05).

Effects of root diameter and root nitrogen concentration on in situ root respiration among different seasons and tree species

AbstractKnowledge of root respiration is a prerequisite for a better understanding of ecosystem carbon budget and carbon allocation. However, there are not many relevant data in the literature on direct measurements of in situ root respiration by root chamber method. Furthermore, few studies have been focused on the effects of root diameter (Dr) and root nitrogen concentration (Nr) on in situ root respiration among different seasons and tree species. To address these goals, we used a simplified root‐chamber system to measure in situ root respiration rates of Acacia crassicarpa and Eucalyptus urophylla in subtropical plantations of south China. We found that the species and season variation in root respiration were affected by Dr and Nr. Also, the root respiration per unit dry mass (Rr, nmol CO2 g−1 s−1) and root respiration per unit N (Rn, nmol CO2 g N−1 s−1) were affected by Dr and Nr. The Rr, Rn, Nr and soil temperature sensitivity (Q10) of Rr for the two species significantly decreased with an increase of Dr. The Rr of the two species showed significant an inter‐seasonal and diurnal pattern, and this trend decreased with increasing Dr. Both the Rr and Q10 of the two species increased with increasing Nr. The Dr and Nr explained 54 and 52% of the observed variation in Rr for A. crassicarpa, and 65 and 70% for E. urophylla. The Rr, Nr, and Q10 of A. crassicarpa were significantly higher than those of E. urophylla. Our results indicated that root respiration was dependent on Dr and Nr, and this dependence varied with season and plant species.

Characteristics of the temperature coefficient, Q 10, for the respiration of non-photosynthetic organs and soils of forest ecosystems

The temperature coefficient, Q10 (fractional change in rate with a 10°C increase in temperature) describes the temperature sensitivity of soils, roots, and stems, as well as their possible performance in global warming processes. It is also a necessary parameter for the estimation of total CO2 efflux from each element. A number of studies have focused on Q10 values to date; however, their conclusions are not universal and do not always agree. A review of these reported Q10 values therefore becomes necessary and important for a global understanding of the temperature sensitivity of different forest types and elements. The aims of our present paper are, first, to find the frequency distribution pattern of soils, roots, and stems (branches) and compare their temperature sensitivity; then, to find the Q10 differences between conifer and deciduous tree species and the effect of methodology on Q10 values; finally we want to give a perspective on future Q10-related studies. We found that most Q10 values of each element were concentrated in a relatively narrow range despite a total data distribution over quite a wide range. For soil respiration, the median Q10 value was 2.74 and the center of the frequency distribution was between 2.0 and 2.5 with a percentage of 23%. Most of the data (>80%) were within the range from 1.0 to 4.0. The median Q10 value for root respiration was 2.40 and the center of the frequency distribution was from 2.5 to 3.0 with a percentage of 33%. Most of the results (>80%) ranged from 1.0 to 3.0. For stem respiration, the median Q10 value was 1.91 and the frequency distribution was concentrated between 1.5 and 2.0. Over 90% of the data ranged from 1.0 to 3.0. Obvious differences in Q10 value were found between different elements, stem 0.10). CO2 analytical methods (soda lime absorption method, IRGA (Infra-read gas analysis), and chromatograph analysis) and root separation methods (excised root and trenched box) slightly affected the Q10 values of soil and root respiration (p>0.10), but an in vitro measurement of stem respiration yielded a significantly higher Q10 value than an in vivo method (p<0.05). In general, although the Q10 values of non-photosynthetic organisms stayed within a relatively conservative range, considerable variation between and within elements were still detectable. Accordingly, attention should be paid to the quantitative estimation of total CO2 efflux by Q10-related models. In future studies, the biochemical factors and the environmental and biological factors controlling respiration should be emphasized for precise estimation of total CO2 efflux. The difficulty is how to clarify the underlying mechanism for fluctuations of Q10 values for one specific habitat and element (e.g. temperature acclimation or adaptation of Q10 values) and then allow the Q10 values to be more conservative for representation of temperature sensitivity in global warming processes.