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/lp/springer_journal/soil-respiration-of-the-dahurian-larch-larix-gmelinii-forest-and-the-PSogVt3LdL
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- Springer Journals
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- Copyright © 2017 by The Author(s)
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- Science, Humanities and Social Sciences, multidisciplinary; Science, Humanities and Social Sciences, multidisciplinary; Science, multidisciplinary
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- 2045-2322
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- 10.1038/s41598-017-03325-4
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Abstract
www.nature.com/scientificreports OPEN Soil Respiration of the Dahurian Larch (Larix gmelinii) Forest and the Response to Fire Disturbance in Da Received: 26 January 2017 Xing’an Mountains, China Accepted: 26 April 2017 Published: xx xx xxxx 1 1 1 2 3 Tongxin Hu , Long Sun , Haiqing Hu , David R. Weise & Futao Guo Despite the high frequency of wildfire disturbances in boreal forests in China, the effects of wildfires on soil respiration are not yet well understood. We examined the effects of fire severity on the soil respiration rate (Rs) and its component change in a Dahurian Larch (Larix gmelinii) in Northeast China. The results showed that Rs decreased with fire burning severity. Compared with the control plots, Rs in the low burning severity plots decreased by 19%, while it decreased by 28% in the high burning severity plots. The Rs decrease was mainly due to a decreased autotrophic respiration rate (Ra). The temperature sensitivity (Q ) of Rs increased after the low severity fire disturbances, but it decreased after the high severity fire disturbance. The Rs were triggered by the soil temperature, which may explain most of the Rs variability in this area. Our study, for the first time, provides the data-based foundation to demonstrate the importance of assessing CO fluxes considering both fire severity and environmental factors post-fire in boreal forests of China. −1 1 The soil respiration rate (R s) is the second largest carbon flux (80–98 Pg C·yr ) in terrestrial ecosystems . Soil respiration is the sum of soil autotrophic respiration (Ra) from plant metabolic activity and soil heterotrophic respiration (Rh) from the decomposition of organic material by microbes . The amount of CO released by soil respiration is more than ten times that released by global fossil fuel combustion ; Slight changes in soil respiration may therefore influence the global carbon balance . Understanding the mechanisms and potential changes of soil and CO exchange as a function of soil respiration is key to learning about the forest ecosystem response to global climate change . Recent soil respiration research has focused on the effects of disturbances on soil respiration, such as pre- 5, 6 scribed burning, litter thinning, harvesting removal, nitrogen addition and land use management . However, despite the global importance of this process, there is still a lack of understanding of the variability of soil respira- tion in high-latitude boreal ecosystems . The boreal forests of Asia, Europe, and North America contain approx- imately 40% of the global soil organic carbon, roughly the same value as atmospheric carbon, making the global boreal ecosystem the largest terrestrial organic carbon pool . Boreal forest carbon sequestration and emission is largely determined by forest fire disturbances , and the frequent and severe forest fires significantly ae ff ct the carbon balance in these ecosystems . The fire return interval and fire severity have increased significantly over the past few decades . The carbon loss in boreal forest soil caused by fire disturbance is not only an important factor in determining forest carbon balance but also a point of uncertainty in global carbon assessment . Much of 13, 14 this uncertainty stems from the high degree of soil heterogeneity , as well as the complex interactions between differences in the soil environment characteristics and forest fire . The fire duration and severity and the meteor - ological condition post-fire can also significantly influence soil respiration aer fir ft e disturbance, which can last a few months to a few years . Therefore, understanding the regime of soil respiration and its ae ff cting factors aer ft fire disturbance can enhance the accuracy of estimating soil respiration in boreal forest ecosystems. Fire can increase soil hydrophobicity, which may indirectly control the rates of decomposition by reducing the soil moisture infiltration and increasing the surface runoff . Wildfires can also ae ff ct soil respiration by reducing 1 2 College of Forestry, Northeast Forestry University, Harbin, 150040, China. USDA Forest Service, PSW Research Station, 4955 Canyon Crest Drive, Riverside, CA, 92507, USA. College of Forestry, Fujian Agriculture and Forestry University, Fuzhou, 350002, China. Long Sun and Haiqing Hu contributed equally to this work. Correspondence and requests for materials should be addressed to L.S. (email: sunlong365@126.com) or H.H. (email: hhq@nefu.edu.cn) Scientific Repo R ts | 7: 2967 | DOI:10.1038/s41598-017-03325-4 1 www.nature.com/scientificreports/ 5, 18 vegetation cover and surface albedo, which increases soil temperatures and decomposition rates . Forest fires can oxidize part to all forest vegetation, which in turn ae ff cts soil temperature, moisture, microbial activity, and root composition, and then significantly ae ff cts soil respiration . Additionally, the Q value, the factor by which soil respiration is multiplied when the temperature increases by 10 degrees, also varies with forest fire severity . Generally, the Q increases with decreasing temperature and increasing moisture on large scales and is also dependent on the substrate quality and availability . The quantity and quality of detritus and root on the ground 22, 23 18, 24, 25 and underground varies with the duration and severity of forest fires . Several studies have explored the effects of fire disturbance on the component of soil respiration (microbial and root respiration), but there is much uncertainty that requires further study. The previous results indicated that the quantitative relationship between soil temperature moisture and the change in soil respiration components is the key to understanding the response of forest ecosystems to fire disturbance . The Daxing’an Mountains are the largest area of boreal forests in China. The dominant vegetation is the Dahurian Larch (Larix gmelinii) forest, which accounts for 70% of the total forest area in the Daxing’an Mountains . It is the southern edge of the Eurasian boreal forest, a cold-temperate forest transition zone, which is very sensitive to rapid climate changes . The Daxing’an Mountains have the highest incidence of forest fires in China. In total, 1,614 fires occurred from 1965 to 2010. The effect of fire disturbance on the carbon cycle in a Dahurian Larch (Larix gmelinii) forest will help to elucidate the role played by the boreal forest of China in the process of global carbon balance. However, the Dahurian Larch (Larix gmelinii) forest of China has received only limited attention, and there is much uncertainty about soil respiration following fire disturbance in the back- ground of global climate change. This study aims to (1) quantify the soil autotrophic and heterotrophic respiration of the Dahurian Larch (Larix gmelinii) forest during the growing season, (2) compare the effects of different severities of forest fire on soil res- piration, and (3) identify the factors influencing soil respiration changes aer fir ft es. Results Effects of fire disturbance on soil respiration and environmental factors. Statistically significant seasonal variations of Rs were observed in all three types of plots (control, low, and high burning severity) dur- ing the growing season (P < 0.05), and the overall trend of Rs increased from early May to late July and then decreased until late September. The seasonal trend of R s in all three types of plots showed a single peak curve (Fig. 1a). The mean values of R s in the control, low, and high burning severity plots was 5.29 ± 0.48, 4.31 ± 0.36, −2 −1 and 3.79 ± 0.25 μmol CO ·m s , respectively. Compared with the control plots, the average Rs in the low and high burning severity plots decreased by approximately 19% and 28%, respectively. The R s in the high burning severity plot was significantly lower than that in the control plot (P < 0.05). The maximum soil temperature occurred in July, while the minimum value was obtained in May. The sea- sonal dynamic variations of T were significantly observed in all three types of plots (Fig. 1b) (P < 0.05). The mean soil temperatures in the control, low, and high burning severity plots were 10.95 ± 1.43, 11.42 ± 1.2, and 13.54 ± 1.8 °C, respectively. There was no significant difference in the seasonal dynamic of soil moisture in the three types of plots (P > 0.05) (Fig. 1c). The mean soil moisture in the control, low, and high burning severity plots was 18% ± 3%, 20% ± 5%, and 19% ± 2%, respectively. The seasonal variation of R s was closely related to the soil temperature changes at a 5-cm depth rather than to the soil moisture. The change in Rs was consistent with soil temperature, whereas soil moisture did not show a close relationship with Rs (Fig. 1). Effects of fire disturbance on soil respiration components. Seasonal patterns of Rh and Ra were similar to that of Rs (Fig. 2a,b). Both Rh and Ra increased from early May to late July. The annual mean R h in −2 −1 the control, low, and high burning severity plots was 3.93 ± 0.71, 3.04 ± 0.81, and 3.05 ± 0.26 μmol CO ·m s , respectively. No significant difference between the R h in the control plot and that in the high and low burning severity plots (P > 0.05) was found. The annual mean R a in the control, low, and high burning severity plots was −2 −1 1.36 ± 0.24, 1.26 ± 0.48, 0.74 ± 0.31 μmol CO ·m s , respectively. Compared with the control and low burning severity plots, the annual mean Ra in the high burning severity plot significantly decreased by approximately 46% and 41%, respectively. e Th average RC (R a:Rh) in the control, low, and high burning severity plots was approxi- mately 27%, 29%, and 19%, respectively (Fig. 2c). Relationships between soil respiration and environmental factors. The exponential regression model with soil temperature as a single controlling factor of Rs and Rh were significant for the control, low and high burning severity plots and explained approximately 50~60% and 34%~60% of variation in Rs and Rh, respectively (Table 1). The models that t fi soil temperature, soil moisture, and their interaction could explain approximately 50~67% and 43~60% of the variability in Rs and Rh, respectively, in different fire burning severity plots (Table 1). Although soil temperature and soil moisture together can improve the correlation coefficients (R ) of the Rs and Rh regression models in all types of plots, the soil temperature is still the dominant factor con- trolling the variability of Rs and Rh in this region. Figure 3 and Table 1 illustrated the relationship between the Rs and Rh and the soil temperature at the depth of 5 cm and the regression equations in the three types of plots, respectively. Soil respiration increased exponentially with soil temperature. Compared with the control plot, the low burning severity plot had a higher Q aer fir ft e disturbance, while the high burning severity plot had a decreased Q . The Q in the control and low burning 10 10 severity plot was 1.4~1.5 times higher than that in the high burning severity plot (Table 1). Effects of fire disturbance on annual C efflux. e m Th ean annual C efflux of R h in the control, low and −2 high burning severity plots during the 2010–2012 periods was 735 ± 261, 533 ± 172 and 428 ± 19 g C·m (Fig. 4), Scientific Repo R ts | 7: 2967 | DOI:10.1038/s41598-017-03325-4 2 www.nature.com/scientificreports/ Figure 1. Seasonal variations of (a) the total soil respiration rates (Rs), (b) soil temperature and (c) soil moisture in the control, low, and high burning severity plots. The data were the average values in 2010, 2011, and 2012. The error bars represent the standard deviation. and was approximately 68%, 74%, and 75% of the mean annual C efflux of R s, respectively. The mean annual C efflux of R a in the high burning severity plot was significantly lower than that in the control plot (P < 0.05, Fig. 4). Discussion Our study first quantified the effects of fire severity on the soil respiration rates and its component change in a Dahurian Larch (Larix gmelinii) forest in Northeast China. The R s in the Dahurian Larch (Larix gmelinii) for- est decreased with an increased fire burning severity. The R a after high burning severity fires decreased, which decreased the Rs. Our results indicated that wildfires still ae ff cted the Ra aer 5–7 y ft ears of fire disturbances. The soil respiration after fire disturbance in the Da Xing’an Mountains was triggered by a higher soil temperature, which can explain a large fraction of soil respiration and its component variability. −2 −1 The mean Rs in our study sites (5.29 ± 0.48 μmol CO ·m s ) was higher than that (2.32–3.88 μmol −2 −1 28, 29 CO ·m s ) in the temperate Dahurian Larch (Larix gmelinii) forest and was in the range of the results in −2 −1 29–32 33–35 other boreal forests (1.14–14.0 μmol CO ·m s ) . Several studies have indicated that the Rh accounts Scientific Repo R ts | 7: 2967 | DOI:10.1038/s41598-017-03325-4 3 www.nature.com/scientificreports/ Figure 2. Seasonal variations of (a) the soil heterotrophic respiration rates (Rh), (b) the soil autotrophic respiration rates (Ra) and (c) the RC (Ra:Rs) in the control, low, and high burning severity plots. The data were the average values in 2010, 2011, 2012. The error bars represent the standard deviation. for approximately 50%–68% of Rs across different forest ecosystems, which was broadly similar to our study (68~70%). The reduction of soil respiration by fires depends on the fire severity and duration . Our results confirmed this finding and indicated that the soil respiration decreased with an increased fire severity. The effects of fire dis- 34, 37 turbance on soil respiration can also be influenced by the proportion of Ra and Rh . In our study, the different severity fires had no significant effect on the Rh. Fire disturbance, however, significantly decreased the Ra. In the boreal forest, the Rs was reduced by approximately 53%~67% in the first and second year aer t ft he fire disturbance due to removal of the decomposed humic materials . This leads to a reduction in R h after a fire disturbance 32 25 because of the litter and surface organic carbon reduction aer t ft he fire disturbance . However, Hicke et al. and Muñoz-Rojas et al. found that the heterotrophic soil respiration rates increased aer ft fire disturbance due to the decomposition of a large amount of combustible substances. Fire removes the vegetation cover and resets the vegetation succession , and a high quality and quantity of detritus increases the decomposition rates by microbes in the early stage after fire disturbance . However, due to a lower net primary productivity during the process of initial system recovery, the Rh begins to decrease after two years of fire disturbances. The R h after five years of fire disturbances decreases to the level before fire disturbance . Although fire may restrain Ra due to the root mortality, this effect is usually shrouded by the short-term increase of R h aer a fir ft e disturbance because the R h contributes the majority of Rs . Compared with a low severity fire disturbance, a high severity fire disturbance Scientific Repo R ts | 7: 2967 | DOI:10.1038/s41598-017-03325-4 4 www.nature.com/scientificreports/ Site Respiration α β ε ω Q R Rs 0.569 ± 0.059 0.086 ± 0.005 b b 2.36 0.6 Rs 0.164 ± 0.082 0.094 ± 0.005 1.747 ± 0.266 a 0.67 Control Rh −0.143 ± 0.1 0.109 ± 0.008 b b 2.97 0.6 Rh −0.143 ± 0.1 0.109 ± 0.008 a a 0.6 Rs −0.025 ± 0.073 0.104 ± 0.006 b b 2.83 0.59 Rs −0.025 ± 0.073 0.104 ± 0.006 a a 0.59 Low Rh −0.310 ± 0.09 0.10 ± 0.007 b b 2.72 0.55 Rh −0.337 ± 0.89 0.082 ± 0.009 a 0.095 ± 0.031 0.57 Rs −0.060 ± 0.086 0.077 ± 0.006 b b 2.01 0.5 Rs −0.063 ± 0.083 0.061 ± 0.006 a 0.145 ± 0.026 0.52 High Rh −0.109 ± 0.112 0.069 ± 0.007 b b 1.99 0.34 Rh −0.839 ± 0.173 0.081 ± 0.007 2.726 ± 0.516 a 0.43 Table 1. Summary of the regression models of soil respiration against soil temperature (Ln(Rs) = α + β × T) and the best-fit regression models of soil respiration against soil temperature and soil moisture (Ln(Rs) = α + β × T + ε × W + ω × T × W), where T is the soil temperature (°C), W is the soil moisture (%), and T × W is the interaction effect of T and W. Note that this variable of the model was not significant in an ANOVA (at the P = 0.05 level), and this variable was not included in the model. Figure 3. Seasonal variation of total soil respiration rates (Rs) (a), heterotrophic respiration rates (Rh) (b) against soil respiration at a 5 cm depth for the control, low, and high burning severity plots. 17, 40 significantly ae ff cts the R a, which may result from the more serious damage of larch roots by high severity fires . Burke et al. indicated that soil respiration did not significantly change after fires in Northern Canada but declined aer ft two years of fires and then recovered to the pre-fire level aer s ft even years of fire disturbances. The high burning severity fires still significantly ae ff cted the Rs aer fi ft ve years of fire disturbances. The decrease in Rs Scientific Repo R ts | 7: 2967 | DOI:10.1038/s41598-017-03325-4 5 www.nature.com/scientificreports/ Figure 4. Comparison of the mean annual C efflux in the control, low and high burning severity plots. Rs, Rh, and Ra represent the total soil respiration, heterotrophic and autotrophic respiration, respectively. The error bars represent the standard deviation, and the different lowercase letters are significant at the 95% level. may stem from the reduction in fine root biomass and canopy removal aer s ft everal years of burning . The loss of Ra is due to the plant death and the decrease in the soil organic carbon (SOC) quality aer s ft everal years of fire disturbances . −2 42 The annual soil C efflux in this area was consistent with other studies (511 to 1300 g C m ) . The low and high severity fires cause the approximately 33% to 47% decrease in the annual C efflux. The annual soil C efflux in the Dahurian Larch (Larix gmelinii) forest of Northeast China decreased with an increased fire burning sever - ity. Wildfires caused uncertainties in the estimation of the annual C efflux after fire disturbance in this area. Therefore, we need to monitor the long-term change in CO fluxes after fire disturbance. The long-term effects of wildfire on the R s depend on fire severity, fire duration and forest management measures, which will directly inu fl ence the post-fire vegetation restoration, the recovery of microbial populations and the variability of the 32, 43 quantity and quality of SOC . Fire can cause an imbalance of soil carbon in boreal forest ecosystems. If the frequency and severity of fires continue to increase, the permafrost in boreal forests will degrade and enhance decomposition, which will cause a transition of the boreal forest ecosystems from a net C sink to a source . Our findings indicated that the forest soil aer hig ft h severity fires continues to be a C source aer 5–7 y ft ears of fires. Soil temperature and moisture strongly influence soil respiration changes , and soil temperature is the main factor controlling soil respiration during the growing season . Due to the reciprocal interactions effect between soil temperature and moisture, it is difficult to control and distinguish their effects under field conditions . Our results suggested that the high Rs is generally triggered by the high temperature during fires. The high soil tem- perature in high burning severity sites was due to the reduction of vegetation cover aer fir ft e and the increasing exposure of the soil surface to solar radiation and the decrease in transpiration, limiting the cooling effect of latent energy in the fire burning area . The variation of soil temperature at a 5-cm depth was consistent with the R s. In contrast, the soil moisture did not show a close relationship with Rs (Fig. 1). The soil moisture did not signifi- cantly change during the growing season because of the saturated soil moisture in this area. Recently, a few studies suggested that soil moisture can significantly affect the R s only when the soil is in an extreme environment . Although the exponential model of the interaction of soil temperature and moisture was the best-fitted curve to explain the Rs in the control and high burning severity sites, temperature is still the dominant factor to explain the variation of the Rs in this area (Table 1). The Rs exponentially increased with an increased soil temperature at a 5-cm depth by a Q of 2.36, which is within the range of results from other studies. e Th Q in Dahurian Larch (Larix gmelinii) forest during the grow- 50, 51 ing season ranged from 1.5 to 5.7 . Our results showed that a high burning severity decreased the Q aer a ft fire disturbance. The Q in the control and low burning severity sites was 1.4~1.5 times higher than that in the high burning severity sites. According to the recent studies, the Q not only reflects the soil respiration sensitivity to temperature but also expresses the combined response to fluctuations in temperature, root biomass, moisture conditions, and substrate quality . The variation of the Q after fire disturbance may result from the effects of fires on root material because low and moderate severity fires provide more labile carbon in burnt soil versus sta- ble carbon, which will accelerate the root assimilate SOC in the carbon, while high burn severity fires will destroy 52, 53 root structures and cause the loss of the labile fraction of SOC in the atmosphere . The Q of Ra was higher 54 37 than that of Rh in boreal forests , which highlights the importance of the Ra in regulating the Q of Rs . The higher root respiration may accelerate the soil organic matter decomposition rate . The decomposition of more recalcitrant soil organic matter may cause the high Q . Therefore, further studies are necessary to explore the effect of fires on the Q in root and rhizosphere respiration. This can be used to understand not only the response of soil respiration to temperature but also the mechanisms behind Rs following fire. Our finding can provide a scientific basis for the post-fire vegetation restoration in the Daxing’an Mountains. The results of this research further demonstrate that the boreal forest ecosystems of China, particularly in the background of global climate change, are areas sensitive to temperature change. Future efforts are required to fully understand the longer-term variations in soil respiration and its component changes following extreme climate events such as high burning, severe wildfires in Northeast China. Scientific Repo R ts | 7: 2967 | DOI:10.1038/s41598-017-03325-4 6 www.nature.com/scientificreports/ Methods Study area. The research area is located at the Daxing’an Mountains, Nanweng River Forest Ecological Station, Northeast China (51°05′07″N–51°39′24″N, 125°07′55″E–125°50′05″E). The elevation in this area ranges from 500 m to 800 m. The climate is a cold temperate continental monsoon zone. The average annual temperature is −3 °C. There are approximately 2500 annual sunshine hours, and the frost-free period is approximately 90 to 100 days. The annual precipitation is 350 mm to 500 mm. The zonal soil is Podzol. The dominant herb species include Lespedeza bicolour Turcz., Rosa davurica Pall., Vaccinium vitis-idaea L., Rhododendron Simsii Planch., Calamagrostis angustifolia Kom., and Maianthemum bifolium. Stand selection and the definition of fire severity. In April 2006, forest fires were caused by lightning in the Songling forest bureau (Nanweng River Forest Ecological Station) of the Daxing’an Mountains, China. The total burned area was approximately 15 × 10 ha. In the fire disturbance area, we classified the fire burning sever - 55–57 ity according to the depth of the burned organic soil, which is commonly used in boreal forest ecosystems . Moreover, we also referred to the consumption of the aboveground biomass, tree mortality, and the bark char height to define the fire severity, which would help us to understand the fire damage to the forest for each burning severity. For the high burning severity, the depth of the burned organic soil was 15.0 ± 1.4 cm; the understor y shrubs, litter, and duff layers were completely burned out; the bark char height was 2.5–5.5 m; and the tree mortal- ity was approximately 85%. For the low burning severity, the depth of the burned organic soil was 3.8 ± 0.6 cm, the approximately 25% of the understory shrubs was burned, the bark char height was 1.8–2.4 m, and approximately 20% of the trees died. We selected three replicated plots in each fire severity area to conduct our investigation and selected the nearby unburnt area as the control plots to compare the results with the burned plots. Nine plots (3 plots for high burning severity +3 plots for low burning severity +3 plots as the unburnt control) were selected in our research. The size of each plot was 400 m (20 m × 20 m), and all plots were established in October 2009. Soil respiration measurement. The Rs was measured by using an Li-8100-103 and Li-8100 portable auto- matic measuring system for soil carbon flux (Li-Cor, Inc., Lincoln, NE, USA). Five polyvinylchloride (PVC) soil rings (inner diameter 19 cm, height 7 cm) were randomly laid in each plot. Fifteen soil rings (5 soil rings × 3 replicate plots) measuring Rs were in each fire burning severity plot. The PVC ring remained in the same position throughout the measurement period. The trench method was used to measure the soil heterotrophic respiration (Rh) . Four 50 cm × 50 cm quadrats were established 2–3 m outside each plot. A trench (45–50 cm depth) was dug in each quadrat, and all roots were removed from the trench severing connections between the plant roots and the trench cross-section. A double-layered plastic cloth was laid in the cross-section of each trench to prevent connection between the trench and any plant roots, and the soil was then replaced. A PVC soil ring was placed in the centre of each quadrat following the same method as above. The CO flux of trenched quadrat PVC rings was treated as Rh, including microbial, soil faunal respiration, and CO emitted by soil organic matter decomposition, while that in non-trenched plots was treated as Rs. The dier ff ence of between R s and Rh was assumed to be Ra. A total of twelve Rh soil rings (4 quadrats ×3 replicate plots) was used to measure the Rh in each fire burning severity. The R a account for the ratio of Rs defined as RC (R a/Rs) was used to represent the relative contribution of root respiration to soil respiration. The soil respiration rate was measured monthly from May to September in the years 2010–2012. The meas- urement time lasted approximately two minutes for each soil respiration ring. Each measurement was conducted from 9:00 AM to 11:00 AM for a total of 81 (45 non-trenched soil rings for Rs +36 trenched soil rings for Rh) measurements within two days. Soil temperature and soil moisture. e s Th oil temperature (T ) and soil moisture (W) were measured by using a temperature probe (Licor p/n8100–201) and soil volumetric water content probe (ECH20 EC-5; p/n 8100- 202) at a depth of 5 cm. e Th measurements of soil temperature and moisture synchronized with the measurement of the soil respiration. Statistical analysis. The data were processed and analysed using SPSS 19.0 statistical software (SPSS Institute, Inc., Chicago, IL, USA). Differences in variables between the burning and control plots were tested by analysis of variance (ANOVA), and comparisons between means were performed with the least-significant differ - ences (LSD) test. All statistical analyses were performed with a significance level of 0.05. Soil respiration model. e fi Th tting model for the soil respiration rate and soil temperature in the growing season was developed by an exponential model. The goodness-of-fit of the models were quantified using the coefficient of determination (R ) and residual analyses. The regression model between soil respiration and soil temperature is shown as Eq. 1 : β×T (1) Re s =× α −2 −1 where Rs is the soil respiration (µmol CO ·m s ), T is the soil temperature at a depth of 5 cm (°C), and α and β are regression coefficients. At a daily time step, we developed an exponential model that was used to describe the effects of soil tempera- ture and soil moisture on soil respiration. Logarithmic transformation of Rs was required to achieve linearity and homoscedasticity. The regression model is shown as Eq. 2: Ln() Rs =+ αβ ×+ TW εω × +×× TW (2) where Ln (Rs) is the logarithmic transformation of Rs that was applied to achieve linearity and homoscedastic- ity; T is soil temperature at −5 cm (°C); W is the soil moisture at −5 cm (%); T × W is the interaction effect of T Scientific Repo R ts | 7: 2967 | DOI:10.1038/s41598-017-03325-4 7 www.nature.com/scientificreports/ and W; and α, β, ε, and ω are regression coefficients. A stepwise regression procedure was performed to remove insignificant terms (P = 0.05). The estimation of the annual C efflux. We measured the soil respiration in the non-growing season from October 2011 to April 2012, which accounted for 11% of the total annual soil respiration in this area. er Th efore, we assumed that soil respiration during the non-growing season contributed 11% of the annual C efflux in all −2 20 plots. 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Separating root and soil microbial contributions to soil respiration: A review of methods and observations. Biogeochemistry. 48, 115–146 (2000). 59. Lloyd, J. & Taylor, J. A. On the temperature dependence of soil respiration. Functional Ecology. 8, 315–323 (1994). Acknowledgements This research was financially supported by the National Natural Science Foundation (No. 31470657; No. 31070544), Fundamental Research Funds for the Central Universities (No. 2572015DA01) and the CFERN & GENE Award Funds for Ecological Papers. We also greatly appreciate the staff of the Nanwenghe Forest Ecological Research Station for their field assistance. Author Contributions Tongxin Hu compiled this manuscript, performed the analysis, and generated the figures and table in the main text. Haiqing Hu contributed to the design of this study. Long Sun revised the manuscript and contributed to the discussion of the results and to the manuscript refinement. David R. Weise provided laboratory assistance. Futao Guo contributed to the discussion of the manuscript format. Additional Information Competing Interests: The authors declare that they have no competing interests. Publisher's note: Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. Scientific Repo R ts | 7: 2967 | DOI:10.1038/s41598-017-03325-4 9 www.nature.com/scientificreports/ Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Cre- ative Commons license, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons license and your intended use is not per- mitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/. © The Author(s) 2017 Scientific Repo R ts | 7: 2967 | DOI:10.1038/s41598-017-03325-4 10
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