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Article

Seasonal Dynamics of Soil Respiration and Its Autotrophic and Heterotrophic Components in Subtropical Camphor Forests

by
Ping He
1,2,
Wende Yan
1,3,*,
Yuanying Peng
4,
Junjie Lei
1,
Wei Zheng
5,
Yi Zhang
1,
Yaqin Qi
6 and
Xiaoyong Chen
7,*
1
College of Life Science and Technology, Central South University of Forestry and Technology, Changsha 410004, China
2
Forestry Bureau of Hunan Province, Changsha 410004, China
3
National Engineering Laboratory for Applied Forest Ecological Technology in Southern China, Changsha 410004, China
4
College of Arts and Sciences, Lewis University, Romeoville, IL 60446, USA
5
Guangxi Forestry Research Institute, Nanning 530002, China
6
Aerospace Information Research Institute, Chinese Academy of Sciences, Beijing 100094, China
7
College of Arts and Sciences, Governors State University, University Park, IL 60484, USA
*
Authors to whom correspondence should be addressed.
Forests 2023, 14(12), 2397; https://doi.org/10.3390/f14122397
Submission received: 3 November 2023 / Revised: 25 November 2023 / Accepted: 5 December 2023 / Published: 8 December 2023
(This article belongs to the Section Forest Soil)
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Abstract

:
On a global scale, soil respiration (Rs), representing the CO2 flux between the soil surface and the atmosphere, ranks as the second-largest terrestrial carbon (C) flux. Understanding the dynamics between Rs and its autotrophic (Ra) and heterotrophic (Rh) components is necessary for accurately evaluating and predicting global C balance and net ecosystem production under environmental change. In this study, we conducted a two-year root exclusion experiment in subtropical China’s Camphor (Cinnamomum camphora (L.) Presl.) forests to assess seasonal changes in Ra and Rh and their relative contributions to Rs. Additionally, we examined the influence of environmental factors on the dynamics of Ra, Rh, and Rs. Our results showed that seasonal mean Rs values were 2.88 µmol m−2 s−1, with mean Ra and Rh of 1.21 and 1.67 µmol m−2 s−1, respectively, in the studied forests. On an annual basis, the annual values of mean Rs in the studied forests were 405 ± 219 g C m−2 year−1, with Rh and Ra accounting for 240 ± 120 and 164 ± 102 g C m−2 year−1, respectively. The seasonal mean ratio of Rh to Rs (Rh/Rs) was 58%, varying from 45 to 81%. Seasonal changes in Rs and Rh were strongly correlated with soil temperature but not soil water content. Both Rh and Rs increased exponentially with the average soil temperature measured in the topsoil layer (about 5 cm), with Q10 values of 2.02 and 1.73 for Rh and Rs, respectively. Our results suggest that the composition and activity of soil microbes and fauna play a primary role in releasing carbon flux from soil to the atmosphere in the studied forest ecosystems.

1. Introduction

Soil CO2 efflux (FCO2), often referred to as soil respiration (Rs), is the second-largest carbon (C) flux between terrestrial ecosystems and the atmosphere. On a global scale, land plants absorb approximately 120 Pg (1015 g) of C per year through the photosynthetic process. Concurrently, the Rs process releases around 68–98 Pg of C back into the atmosphere annually [1,2,3]. Therefore, Rs is a critical component of the global C cycle, significantly affecting global climate [4,5]. In terrestrial ecosystems, Rs is the result of soil autotrophic respiration (Ra, mainly from roots and associated rhizosphere respiration) and soil heterotrophic respiration (Rh, from microbes and soil fauna respiration) [6,7]. Ra is primarily influenced by root growth and productivity, photosynthesis capacity, C substrate availability, soil organic matter, and nutrient contents [4,7,8]. Since different C sources, biological processes, and metabolic pathways are involved in Rs components, the feedback of the Ra and Rh components to environmental changes varies. Hence, partitioning Rs into Ra and Rh components is important. It can provide insight into the C cycle and sequestration in terrestrial ecosystems under natural and anthropogenic disturbances [4,8].
Numerous studies have focused on partitioning Rs, yet significant uncertainty and variability persist in estimates within forest ecosystems [9,10]. For example, we summarized and evaluated the advantages and disadvantages of three commonly used methods for partitioning Rs into Ra and Rh components in plant communities [11]. The ratio of Rh to Rs varied from 10 to 90% in terrestrial ecosystems, depending on vegetation types and seasonal variations [12]. On average, Ra contributed 45.8% in forest ecosystems and 60.4% in non-forest ecosystems to Rs [13]. Based on soil FCO2 data from 54 forest sites, we summarized that Ra and Rh were approximately evenly partitioned, ranging between 50 and 60% [4]. Recent studies found that in longleaf pine forests, Rh dominated Rs, with an annual ratio of Rh to Rs ranging from 66 to 96% [14,15]. In addition, the Ra, Rh, and Rs processes are largely regulated by environmental factors, primarily soil temperature (Tsoil) and soil water content (Wsoil) [16]. Rs exhibits a distinct seasonal pattern, primarily controlled by Tsoil [17], and the annual pattern of precipitation indirectly influences the interannual variation of Rs by affecting Wsoil in subtropical forests [18]. Thus, significant variations in the proportions of Ra and Rh components to Rs highlight the need for further research to better understand the mechanisms that regulate Ra, Rh, and Rs dynamics in forest ecosystems.
Among the methods used to partition Ra and Rh, studies have used trenching method as a root exclusion method to separate Ra and Rh from Rs [19,20]. Previous studies showed that the trenching technique is easy to use in field conditions, adaptable to various circumstances, yields reasonable values, and produces comparable partitioning results with other methods [9,20]. Nevertheless, the trenching method used to separate the contribution of Ra and Rh components to total Rs has limitations [21]. The literature extensively discusses major shortcomings associated with this method, including (1) the potential influence of newly deceased fine and coarse roots [22]; (2) disturbance effects caused by the act of trenching [23]; (3) alterations in soil water regimes resulting from the artifacts of the trenching treatment [24]. Significantly, recent research has highlighted that trenching artifacts may lead to increased soil water content due to reduced water uptake and elevate the relative proportion of Rh to Rs due to inputs from newly severed dead roots [25].
Subtropical evergreen broad-leaved forests in Southern China are globally significant biomes that play a crucial role in C cycling and sequestration at regional, national, and global scales [26,27]. The total net ecosystem productivity (NEP) in East Asian subtropical forests has been 0.72 ± 0.08 Pg C year−1, accounting for 8% of the global forest NEP [28]. Camphor (Cinnamomum camphora (L.) Presl.) forests are a significant part of evergreen broad-leaved forests in this region. This species contains volatile chemical compounds in all plant parts, which have allelopathic effects on certain plant species and natural habitats [29]. In Camphor forests, numerous studies have examined the characteristics of the Rs process, but little is known about how it contributes to Ra and Rh components in these forest ecosystems. The purpose of the current study was to examine the contribution patterns of Ra and Rh components of Rs in a Camphor forest ecosystem. We hypothesized that (a) Rh would contribute more to Rs than Ra based on the findings from our previous experiments and other studies in subtropical forests; (b) the relative proportions of Rh and Ra to Rs would change following seasonal variations in the study region’s weather conditions. The specific objectives of this project were: (1) to quantify the seasonal and annual fluxes of Rs, Ra, and Rh; (2) to explore the respective contributions of Ra and Rh components to Rs; (3) to examine the relationships between Tsoil and Wsoil and Rs and its components.

2. Materials and Methods

2.1. Study Site

The experimental site is located in Tianjiling National Park in Changsha, Hunan province, China, at coordinates 113°02′–01′ E and 28°06′–07′ N. This region features a low mountain and hill terrain, with elevations ranging from 46 to 114 m above sea level and slopes varying from 5° to 20°. The site experiences a typical monsoon subtropical climate, characterized by a mean annual temperature of 17.2 °C, with the lowest monthly mean air temperature in January at 4.7 °C and the highest in July at 29.4 °C. The mean annual rainfall is 1422 mm, falling primarily between April and August. Annual relative humidity averages above 80%.
The dominant tree species in the experimental area included Camphor (Cinnamomum camphora (L.) Presl.), Chinese sweet gum (Liquidambar acalycina), Chinese fir (Cunninghamia lanceolata (Lamb.) Hook.), Masson pine (Pinus massoniana Lamb.), and slash pine (Pinus elliottii). The soil beneath these forests has been classified as a typical clay-loam red soil developed from slate parent rock, corresponding to Alliti-Udic Ferrosols as per the World Reference Base for Soil Resources (CRG-CST 2001). The soil has an acidic pH, with an average of 5.0 in the surface layer (0–10 cm) and a soil organic C content of 19.77 ± 0.68 mg g−1 at a depth of 10 cm.
For this study, the selected Camphor forests were established as pure forests in 1990, commencing with an initial tree density of 1600 trees per hectare. The mean diameter at breast height (DBH) was 14.9 cm, and the mean tree height was 12.6 m. These Camphor forests were in a young stage of growth and development. The understory plant species at the study site consisted of Sassafeas tsumu Hemsl.; Clerodendron cyrtophyllum Turcz; Cinnamomum camphora; Symplocos caudata Wall. ex A. DC.; Lophantherum gracile Brengn.; Nephrolepis auriculata Trimen; Miscanthus floridulus Warb; and Phytolacca acinosa Roxb.

2.2. Experiment Design

Soil FCO2 measurements in the 20-year-old Camphor forests began in August 2010 and were routinely conducted for two years. The experiment was conducted following a completely random design (CRD). Three 20 × 20 m sites were established within the study area’s Camphor forests. Each site was divided into six square plots, each with a side length of 2 m (area 4 m2). Among these plots, three plots were randomly selected for trenched treatments, and the remaining three were designated as un-trenched treatments. This arrangement resulted in three pairs of trenched and un-trenched plots within each forest site.
The sites and plots were chosen based on their relatively homogeneous topography within the Camphor forests. To minimize the potential influence of tree proximity on soil FCO2 rate measurements, the selected plots were positioned near the center of the tree lines within the forests. These plots represented factors within the experiment, with trenched plots devoid of living roots and un-trenched plots serving as the control, representing intact areas with living roots. The trenched plot was a cubic block with a narrow ditch, approximately 0.2 m wide, excavated to a depth of 0.8 m along the four sides of the square. This depth extended below the rooting zone, where minimal root presence was observed [29]. The excavated trenches excluded live tree roots. To create a barrier, we placed several 2 mm thick polyethylene plastic sheets around the trenches, extending them to the trench’s depth. Afterward, we backfilled the trenches with excavated soil, carefully removing herbaceous vegetation from the trenched plots by hand throughout the study to minimize soil disturbance.
Each trenched plot was equipped with a PVC respiration collar measuring 10.5 cm in diameter and 4.5 cm in height, inserted at approximately 2 cm into the soil. These collars were installed at least one week before the first measurement of Rs and remained in place throughout the experiment. To mitigate both root decay and soil disturbance effects resulting from trenching and the use of flux chambers, the trenched plots were established two months before the experiment, and the PVC collars were inserted into the soil at least one week before the initial Rs measurement, where they remained for the entire study duration. The un-trenched plot was located 35 m away from the trenched plot and remained undisturbed, with no excavation or removal of herbaceous vegetation. In each un-trenched plot, a PVC respiration collar was installed for soil FCO2 measurements.

2.3. Field Measurements

Soil FCO2 rates were measured in the field biweekly from August 2010 to August 2012 using a portable infrared gas analyzer (LI-COR 8100, LI-COR Inc., Lincoln, NE, USA) equipped with a chamber. During measurements, the respiration collar was sealed with a soil chamber connected to the infrared gas analyzer. All measurements were conducted between 10:00 a.m. and 2:00 p.m. to avoid diurnal fluctuations. For data analysis, we used the mean value of the two measurements per plot.
Soil FCO2 rates were expressed as µmol CO2 m−2 s−1. Measurements from trenched plots represented Rh due to root exclusion, while measurements from un-trenched plots reflected the total Rs, including both Ra and Rh. As a result, Ra can be estimated by subtracting Rh from Rs [29,30].
During each soil FCO2 measurement, Tsoil was monitored using a soil thermocouple probe (LI-COR 8100-09 TC, LI-COR Inc., Lincoln, NE, USA) inserted into the soil at a depth of 5 cm below the surface. We also measured Wsoil (volumetric soil water content, %) in the topsoil layer (0–5 cm) using an ECH2O EC-5 soil moisture sensor (METER Environment, formerly Decagon Devices, Inc., Pullman, WA, USA) [8]. Both Tsoil and Wsoil measurements were obtained outside the PVC collars.

2.4. Data Analysis

We assessed the differences in soil FCO2 between trenched and un-trenched plots using analysis of variance (ANOVA). To meet the normality and homoscedasticity assumptions of ANOVA, the original Rs and Rh data were log-transformed. A repeated two-way ANOVA was applied to assess the effects of treatments and monitoring time on soil FCO2 rates, Tsoil, and Wsoil. The ratios Ra/Rs and Rh/Rs were used to represent the respective contributions of the Ra and Rh components. All statistical analyses were performed with a significance level set at p < 0.05 using SAS statistical software (Version 8, SAS Institute Inc., Cary, NC, USA, 1999–2001). Nonlinear regression analysis was employed to model the relationship between Rs and Tsoil and Wsoil. To assess the temperature sensitivity of Rs, we calculated the Q10 index, defined as the difference in respiration rates over a 10 °C interval, using the following equation:
Q10 = e10b
where, b is the constant fitted into Equation (1).

3. Results

During the two-year study period, soil respiration (Rs) rates were significantly lower in the trenched plots than in the control plots of the Camphor forests (p < 0.05). Rs rates ranged from 0.61 to 3.55 µmol m−2 s−1 in trenched plots and from 0.73 to 5.85 µmol m−2 s−1 in un-trenched plots (Figure 1). On average, soil FCO2 rates were reduced by approximately 60% in trenched plots (1.67 ± 0.13 µmol m−2 s−1, Mean ± SD) compared to un-trenched plots (2.88 ± 0.09 µmol m−2 s−1) (Table 1).
Throughout the two-year study, there was significant seasonal variability in soil FCO2 rates. The mean monthly contributions of each Rs component varied, with Ra/Rs contributing between 25.5 and 51.4% to Rs (Table 2).
The monthly relative proportion of Ra to Rs was consistently below 50% for all months throughout the year, except in September (Table 2). In addition, the ratio of Ra/Rs reached its maximum in summer and autumn, and its minimum in winter. On average, the ratio of Ra/Rs was lower than that of Rh/Rs for all four seasons, with a difference of about 10% in summer and autumn, 40% in winter, and 30% in spring at the study site (Table 2). While Tsoil exhibited significant variation throughout the study, there were no notable differences in Tsoil between trenched and un-trenched plots (p > 0.05). However, trenching had statistically significant effects on Wsoil (p < 0.005). In general, the soil was generally dry during the autumn and winter seasons and wetter in the spring and summer.
The maximum and minimum Tsoil values were 26.1 and 26.3 °C in July 2011, and 3.9 and 3.9 °C in January 2011 for trenched and un-trenched plots, respectively (Figure 1). The mean values of Wsoil were consistently higher in trenched plots than in un-trenched plots, with an average value of 29.9 and 26.4% in trenched and un-trenched plots, respectively (Figure 1). The peak value of Wsoil occurred in June 2010 at 37.9 and 33.6%, whereas the minimum value was recorded in September 2011 at 15.1 and 13.9% for trenched and un-trenched plots (Figure 1). Soil FCO2 rate was significantly correlated with Tsoil (p < 0.0001) (Figure 2), but not with Wsoil (p > 0.05) (Figure 3). Instantaneous soil FCO2 rates were exponentially related to Tsoil, and the corresponding Q10 was 1.73 for trenched plots and 2.02 for un-trenched plots.

4. Discussion

Trenched plots in Camphor forests exhibited a substantial reduction in soil respiration rates (Rs) over two years compared to un-trenched control plots. On average, Rs rates in trenched plots decreased by approximately 42% compared to control plots. Similar observations of Rs reduction due to root exclusion were reported in other studies. For instance, a nearby Chinese fir forest showed a 28% decrease in Rs in trenched plots than in un-trenched plots. One study observed a 39% reduction in Rs rates one year after trenching in a lowland tropical forest [31]. In a 30-year-old beech stand, there was a decrease of around 36% in the annual soil carbon efflux observed in the trenched plots compared to control plots. Additionally, in subtropical evergreen broad-leaved forests, trenching reduced soil FCO2 by approximately 17% over a three-year period [32]. This reduction in annual soil FCO2 in trenched plots was primarily associated with root exclusion, as Ra is a significant component of total soil respiration in forest ecosystems [29]. These findings highlight the consistent impact of root exclusion on reducing soil respiration rates in various forest types, emphasizing the importance of considering root contributions in assessing soil C dynamics [33]. The results from the current study indicate a relative contribution of 42% for Ra to Rs in evergreen broad-leaved Camphor forests, which aligns well with the ranges previously reported for subtropical forests (Table 3).
Further research indicates that both Ra and Rh are influenced by temperature and precipitation. Studies analyzing global patterns found that an increase in mean annual temperature led to higher Ra and Rh rates, with increases of 12.9 and 16.1 g C m−2 year−1, respectively, for every 1 °C rise [38]. Ra was found to increase by 44.5 g C m−2 year−1 for every 100 mm increase in mean annual precipitation (MAP) when MAP was <1000 mm, while Rh increased linearly by 15.0 g C m−2 year−1 for every 100 mm increase in MAP [38]. The study suggested that the fractional contribution of Ra to Rs may be greater in boreal forests than in temperate forests, reflecting regional differences in ecosystem dynamics [39]. These study findings contribute to our understanding of the variation in Ra and Rh contributions to Rs, highlighting a multitude of factors influencing these dynamics with the potential to exhibit regional distinctions [40].
Bond–Lamberty et al. [1] established a relationship between Ra and Rs, expressed as RC = −0.66 + 0.16 × ln (Rs), indicating that Ra contribution may vary depending on Rs. By employing this model, we estimated the root contribution (RC) of our study site. The calculated RC value averaged 30.1%, ranging from 17.6 to 37.0% across the Camphor forests under investigation. These estimated values were slightly lower than our field measurements, where the RC averaged 41.9% with a range of 25.5–51.4% (Table 2). It is worth noting that this variation may be attributed to limitations in the data sources used to develop the model. The data sources were primarily derived from 54 forest sites, with a significant focus on boreal and temperate forests, a minimal representation of tropical forests, and none from subtropical regions [41]. As a result, the RC–RS relationship, while potentially reliable globally, may show significant deviations at a local scale [42]. This finding may be due to a myriad of biotic and abiotic factors, including Tsoil, Wsoil, soil nutrients, soil microbial composition, tree species, and forest types, which can have specific influences on Ra at local or site-specific scales [21]. Therefore, Ra–Rs relationships developed at a regional level may not provide precise estimates of the respective contributions of Ra and Rh components within a specific site [4,38,39]. Ra appeared to be predominantly governed by physiological activities associated with root growth [9], below-ground C allocation [39], and phenological characteristics of tree species [37]. On the other hand, Rh appeared to be primarily regulated by substrate availability and biophysical conditions within the soil [43]. This seasonal pattern aligns with findings that reported a similar trend in Ra dynamics, reaching its highest value in late July due to maximal fine-root biomass and living fungal biomass during the summer and autumn [44]. During the growing season, Ra comprises both maintenance respiration and growth respiration, whereas, in the dormant season, Ra primarily consists of maintenance respiration [45]. Additionally, the positive relationship between maintenance respiration and temperature can lead to higher maintenance respiration during summer (the growing season) when temperatures are elevated, in contrast to winter (the dormant season) when temperatures are lower [46].
Previous studies have demonstrated that both Tsoil and Wsoil are crucial factors controlling Ra, Rh, and Rs [47,48]. Soil CO2 effluxes closely followed seasonal and diurnal variations in Tsoil, as indicated by our findings (Figure 1). Tsoil accounted for over 80% of the seasonal variation in soil FCO2 in the Camphor forest, showcasing a strong correlation between soil FCO2 and Tsoil. This observation aligns with the results of previous studies [29]. However, it is worth noting that most Rs–Tsoil relationships may not accurately reflect the actual temperature response of Rs. Therefore, these temperature response functions are likely inadequate for predicting the effects of climate change on Rs [49]. In addition, climate change is expected to affect water availability by comprehensively altering the amount, distribution, and frequency of precipitation and evaporation [50]. To gain a better understanding of Rs in changing environments, considering both biotic and abiotic interactions is essential [49].
In our experiment, we observed that Wsoil consistently remained higher in trenched plots compared to control plots (Figure 1). This finding is likely attributable to trenching, which increased Wsoil by reducing evapotranspiration [30] and root transpiration [29]. Notably, we observed that the correlations between soil FCO2 and Wsoil were not statistically significant (p > 0.05), which is consistent with previous research on Chinese fir forests [29], an old-growth coniferous forest [51], and boreal forests [30]. In reality, the soil FCO2–WSoil relationship is complex, and the influence of Wsoil on soil FCO2 rates is often modulated by the Tsoil–soil FCO2 relationship under a threshold value of Wsoil [48]. When the threshold value of Wsoil is reached, it creates conditions in the soil that promote the diffusion of both oxygen and soluble substrates, thereby enhancing soil FCO2 rates [52]. However, if Wsoil falls significantly below or rises above this threshold value, it can impede biological processes and alter the relationship between WSoil and soil FCO2. For instance, it was reported that when soil exceeded 0.11 m3 m−3, soil FCO2 rates were positively correlated with Tsoil in a temperate Douglas fir forest, but when Wsoil was below this threshold, the soil FCO2-Tsoil relationship became largely decoupled [53]. Additionally, one of the authors of this study conducted research in a wet–dry savanna in Northern Australia and observed similar trends in the Tsoil–soil FCO2 relationship and the threshold value of Wsoil [54]. In this wet–dry savanna, the threshold value of Wsoil was about 0.07 m3 m−3, with soil FCO2 rates showing a significant positive correlation with Tsoil when Wsoil was above this threshold and a weak relationship when Wsoil was below 0.07 m3 m−3 [43]. This weak relationship between soil FCO2 and Tsoil under lower Wsoil conditions can be attributed to limitations in the soluble substrate [52,54]. Furthermore, the status of Wsoil directly affects the composition and activity of the soil microbial community, which can significantly influence the Tsoil–soil FCO2 relationship [55]. Different microbial communities have distinct optimal Wsoil conditions for their survival, growth, and development. Changes in Wsoil conditions can create different habitats for soil microbial communities, which directly impacts Rh and Rs [56]. A meta-analysis indicates that the response of organism respiration to water stress varies widely across functional types, such as soil fauna, bacteria, and fungi [57].
Although we did not observe a tight relationship between soil FCO2 and Wsoil in the present study, it is plausible that Wsoil indirectly affects soil FCO2 rates through Q10 regulation [53]. We found that the temperature sensitivity of Rs was reduced in trenched plots (Q10 = 1.73) compared to un-trenched plots (Q10 = 2.02) in the current study. This observation suggests that the temperature sensitivity of Rh was less pronounced than that of Rs. Such findings indirectly support the conclusion that Q10 values derived from field measurements, including Ra, could potentially overestimate the response of Rh to temperature changes on a future, warmer Earth [38].

5. Conclusions

In summary, this study has shown that Rh plays a significant role, contributing approximately 60% to the annual Rs in Camphor forests. Our estimates of the relative contributions of Rs components align with those reported in subtropical forests. Tsoil is the primary factor controlling the seasonal variability of Rs, Rh, and the Rh/Rs ratio. The proportion of Ra to Rs reaches its peak during the growing season and is at its lowest when the trees are dormant. Considering the significant concentration of the Rh component in the soil respiration (Rs) of the studied forests, the formulation of suitable management practices focusing on the biophysical environment and microbial community of soil in subtropical forests becomes imperative. Such practices can significantly help reduce CO2 emissions from soils, mitigating rising CO2 concentrations in the atmosphere.

Author Contributions

Conceptualization, P.H., W.Y. and X.C.; methodology, P.H., W.Y., Y.P. and X.C.; validation, J.L., W.Z., Y.Z. and Y.Q.; formal analysis, Y.P., J.L., W.Z., Y.Z. and Y.Q.; investigation, P.H., J.L. and W.Z.; resources, J.L., Y.Z. and Y.Q.; writing—original draft preparation, P.H. and Y.P; writing—review and editing, Y.P., J.L. and X.C.; supervision, W.Y. and X.C.; project administration, P.H. and W.Y.; funding acquisition, W.Y. and X.C. All authors have read and agreed to the published version of the manuscript.

Funding

The study was supported by the National Key Research and Development Program of China (2020YFA0608101), the Joint Funds of the National Natural Science Foundation of China (U21A20187), and a ‘Shu Ren Scholar’ plan of Central South University of Forestry and Technology. We would like to thank Cao Zhang, Wancai Wang, and Dongjun Zou for their field and laboratory measurements.

Data Availability Statement

The data presented in this study are available on request from the corresponding author.

Conflicts of Interest

The authors declare no conflict of interest.

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Forests 14 02397 g001
Figure 1. Seasonal changes in soil temperature at 5 cm soil depth, topsoil soil water content at the 5 cm layer, and soil respiration rate in trenched and un-trenched plots in the Camphor forest during the study period. Error bar indicates standard error ± s.e.
Figure 1. Seasonal changes in soil temperature at 5 cm soil depth, topsoil soil water content at the 5 cm layer, and soil respiration rate in trenched and un-trenched plots in the Camphor forest during the study period. Error bar indicates standard error ± s.e.
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Forests 14 02397 g002
Figure 2. The relationships between soil respiration rates and soil temperature (Tsoil) in un-trenched plots (A) and trenched plots (B) in the Camphor forest over the study period.
Figure 2. The relationships between soil respiration rates and soil temperature (Tsoil) in un-trenched plots (A) and trenched plots (B) in the Camphor forest over the study period.
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Forests 14 02397 g003
Figure 3. The relationships between soil respiration rates and soil water content (Wsoil) in un-trenched plots (A) and trenched plots (B) in the Camphor forest over the study period.
Figure 3. The relationships between soil respiration rates and soil water content (Wsoil) in un-trenched plots (A) and trenched plots (B) in the Camphor forest over the study period.
Forests 14 02397 g003
Table 1. Annual mean soil CO2 efflux (FCO2) rates, soil temperature (Tsoil), and soil water content (Wsoil) from trenched and un-trenched plots in Camphor forests during the study period.
Table 1. Annual mean soil CO2 efflux (FCO2) rates, soil temperature (Tsoil), and soil water content (Wsoil) from trenched and un-trenched plots in Camphor forests during the study period.
Time
(Year)		TreatmentSoil FCO2 Rate
(µmol m−2 s−1)		Tsoil
(°C)		Wsoil
(%)
2010–2011Trenched1.77 ± 0.12 a16.86 ± 0.07 a32.26 ± 2.12 a
Un-trenched3.09 ± 0.09 b16.88 ± 0.09 a28.41 ± 1.86 b
2011–2012Trenched1.56 ± 0.15 a16.11 ± 0.06 a27.45 ± 1.75 a
Un-trenched2.67 ± 0.10 b16.09 ± 0.19 a24.45 ± 1.95 b
AverageTrenched1.67 ± 0.13 a16.49 ± 0.06 a29.85 ± 1.94 a
Un-trenched2.88 ± 0.09 b16.49 ± 0.14 a26.43 ± 1.90 b
Note: Values are presented as mean ± standard deviation. Distinct letters within the same column and year indicate significant differences (p < 0.05).
Table 2. Average monthly patterns of Ra, Rh, and Rs (µmol m−2 s−1), and a relative proportion of Ra component to Rs (%) in studied forests over the 2-year study period.
Table 2. Average monthly patterns of Ra, Rh, and Rs (µmol m−2 s−1), and a relative proportion of Ra component to Rs (%) in studied forests over the 2-year study period.
MonthRaRhRsRa/Rs
January0.3540.7181.07133.0
February0.2870.7711.05827.1
March0.3000.9591.25923.8
April1.4531.7853.23744.9
May1.5372.3373.87439.7
June2.5682.7045.27248.7
July1.7832.4774.26041.9
August1.9502.5774.52743.1
September1.6501.5583.20751.4
October1.4451.5743.01947.9
November0.8321.3242.15538.6
December0.4131.2071.62025.5
Note: Ra, autotrophic respiration; Rh, heterotrophic respiration; Rs, total soil respiration.
Table 3. Comparison of Ra component contribution (%) to Rs in different subtropical forest types.
Table 3. Comparison of Ra component contribution (%) to Rs in different subtropical forest types.
Forest TypeRa/Rs
Mean (Range)		References
Camphor forest41.9 (19.0–55.0)This study
Chinese fir forest (5 years old)27.1[34]
Chinese fir forest (22 years old)32.6 (13.3–55.7)[29]
Chinese fir forest40.3[35]
Natural evergreen forest47.8[35]
Broadleaf and needle leaf mixed forest26.75[36]
Bamboo forest10.98[36]
Monsoon evergreen broad-leaf forest (about 400 years old)22.1–35.4[37]
Pine forest (about 60 years old)18.1–26.1[37]
Pine and broad-leaf mixed forest (~60 years old)20.0–29.1[37]
Evergreen broad-leaved forest (20–120 years old)21.4–32.3[34]
Moist forest33[32]
Note: Ra: autotrophic respiration, Rs: total soil respiration.
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He, P.; Yan, W.; Peng, Y.; Lei, J.; Zheng, W.; Zhang, Y.; Qi, Y.; Chen, X. Seasonal Dynamics of Soil Respiration and Its Autotrophic and Heterotrophic Components in Subtropical Camphor Forests. Forests 2023, 14, 2397. https://doi.org/10.3390/f14122397

AMA Style

He P, Yan W, Peng Y, Lei J, Zheng W, Zhang Y, Qi Y, Chen X. Seasonal Dynamics of Soil Respiration and Its Autotrophic and Heterotrophic Components in Subtropical Camphor Forests. Forests. 2023; 14(12):2397. https://doi.org/10.3390/f14122397

Chicago/Turabian Style

He, Ping, Wende Yan, Yuanying Peng, Junjie Lei, Wei Zheng, Yi Zhang, Yaqin Qi, and Xiaoyong Chen. 2023. "Seasonal Dynamics of Soil Respiration and Its Autotrophic and Heterotrophic Components in Subtropical Camphor Forests" Forests 14, no. 12: 2397. https://doi.org/10.3390/f14122397

APA Style

He, P., Yan, W., Peng, Y., Lei, J., Zheng, W., Zhang, Y., Qi, Y., & Chen, X. (2023). Seasonal Dynamics of Soil Respiration and Its Autotrophic and Heterotrophic Components in Subtropical Camphor Forests. Forests, 14(12), 2397. https://doi.org/10.3390/f14122397

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