Changes in soil respiration components and their specific respiration along three successional forests in the subtropics

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

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

Impact of Natural Forest Succession on Changes in Soil Organic Carbon in the Polish Carpathian Mountains

The main driver of the Carpathian landscape is the process of natural forest succession on the semi-natural meadows unique to the region. Moreover, these semi-natural mountain meadows contribute to ecosystem services, although increasing forest areas are recommended by current international policy agendas. The purpose of this study was to examine the impact of natural forest succession in the Polish part of Carpathian on changes in soil organic carbon and assess the influence of different soil properties on organic carbon content across three land uses. Soil samples were taken from 10 transects consisting of semi-natural mountain meadow, natural successional forest and old-growth forest, selected in three Polish Carpathian national parks. Measurements of organic carbon, dissolved organic carbon, microbial properties, such as microbial respiration, and enzyme activities were made; additionally, biochemical indicators were calculated. To describe the influence of measured soil parameters and calculated indicators of soil organic carbon changes, the organic carbon dependent variable regression equations across all studied soils and for the individual land use and examined layers were evaluated. The overall regression equation indicated that changes in organic carbon general to all investigated soils depended on microbial biomass carbon content, microbial quotient, dissolved organic carbon content and metabolic quotient. The regression models obtained for the individual land use variants and soil layers explained 77% to 99% of the variation in organic carbon. Results showed that natural forest succession caused a decrease in microbial biomass carbon content, and successional forest soils characterized less efficient use of organic substrates by microbial biomass.

Different responses of soil respiration to environmental factors across forest stages in a Southeast Asian forest

Soil respiration (SR) in forests contributes significant carbon dioxide emissions from terrestrial ecosystems and is highly sensitive to environmental changes, including soil temperature, soil moisture, microbial community, surface litter, and vegetation type. Indeed, a small change in SR may have large impacts on the global carbon balance, further influencing feedbacks to climate change. Thus, detailed characterization of SR responses to changes in environmental conditions is needed to accurately estimate carbon dioxide emissions from forest ecosystems. However, data for such analyses are still limited, especially in tropical forests of Southeast Asia where various stages of forest succession exist due to previous land-use changes. In this study, we measured SR and some environmental factors including soil temperature (ST), soil moisture (SM), and organic matter content (OM) in three successional tropical forests in both wet and dry periods. We also analyzed the relationships between SR and these environmental variables. Results showed that SR was higher in the wet period and in older forests. Although no response of SR to ST was found in younger forest stages, SR of the old-growth forest significantly responded to ST, plausibly due to the nonuniform forest structure, including gaps, that resulted in a wide range of ST. Across forest stages, SM was the limiting factor for SR in the wet period, whereas SR significantly varied with OM in the dry period. Overall, our results indicated that the responses of SR to environmental factors varied temporally and across forest succession. Nevertheless, these findings are still preliminary and call for detailed investigations on SR and its variations with environmental factors in Southeast Asian tropical forests where patches of successional stages dominate.

Soil respiration variation along an altitudinal gradient in the Italian Alps: Disentangling forest structure and temperature effects

On the mountains, along an elevation gradient, we generally observe an ample variation in temperature, with the associated difference in vegetation structure and composition and soil properties. With the aim of quantifying the relative importance of temperature, vegetation and edaphic properties on soil respiration (SR), we investigated changes in SR along an elevation gradient (404 to 2101 m a.s.l) in the southern slopes of the Alps in Northern Italy. We also analysed soil physicochemical properties, including soil organic carbon (SOC) and nitrogen (N) stocks, fine root C and N, litter C and N, soil bulk densities and soil pH at five forest sites, and also stand structural properties, including vegetation height, age and basal area. Our results indicated that SR rates increased with temperature in all sites, and 55–76% of SR variability was explained by temperature. Annual cumulative SR, ranging between 0.65–1.40 kg C m^-2 yr^-1, decreased along the elevation gradient, while temperature sensitivity (Q10) of SR increased with elevation. However, a high SR rate (1.27 kg C m^-2 yr^-1) and low Q10 were recorded in the mature conifer forest stand at 1731 m a.s.l., characterized by an uneven-aged structure and high dominant tree height, resulting in a nonlinear relationship between elevation and temperature. Reference SR at 10°C (SR_ref) was unrelated to elevation, but was related to tree height. A significant negative linear relationship was found between bulk density and elevation. Conversely, SOC, root C and N stock, pH, and litter mass were best fitted by nonlinear relationships with elevation. However, these parameters were not significantly correlated with SR when the effect of temperature was removed (SR_ref). These results demonstrate that the main factor affecting SR in forest ecosystems along this Alpine elevation gradient is temperature, but its regulating role can be strongly influenced by site biological characteristics, particularly vegetation type and structure, affecting litter quality and microclimate. This study also confirms that high elevation sites are rich in SOC and more sensitive to climate change, being prone to high C losses as CO₂. Furthermore, our data indicate a positive relationship between Q10 and dominant tree height, suggesting that mature forest ecosystems characterized by an uneven-age structure, high SR_ref and moderate Q10, may be more resilient.

Long-Term Nitrogen Addition Decreases Soil Carbon Mineralization in an N-Rich Primary Tropical Forest

Anthropogenic elevated nitrogen (N) deposition has an accelerated terrestrial N cycle, shaping soil carbon dynamics and storage through altering soil organic carbon mineralization processes. However, it remains unclear how long-term high N deposition affects soil carbon mineralization in tropical forests. To address this question, we established a long-term N deposition experiment in an N-rich lowland tropical forest of Southern China with N additions such as NH4NO3 of 0 (Control), 50 (Low-N), 100 (Medium-N) and 150 (High-N) kg N ha−1 yr−1, and laboratory incubation experiment, used to explore the response of soil carbon mineralization to the N additions therein. The results showed that 15 years of N additions significantly decreased soil carbon mineralization rates. During the incubation period from the 14th day to 56th day, the average decreases in soil CO2 emission rates were 18%, 33% and 47% in the low-N, medium-N and high-N treatments, respectively, compared with the Control. These negative effects were primarily aroused by the reduced soil microbial biomass and modified microbial functions (e.g., a decrease in bacteria relative abundance), which could be attributed to N-addition-induced soil acidification and potential phosphorus limitation in this forest. We further found that N additions greatly increased soil-dissolved organic carbon (DOC), and there were significantly negative relationships between microbial biomass and soil DOC, indicating that microbial consumption on soil-soluble carbon pool may decrease. These results suggests that long-term N deposition can increase soil carbon stability and benefit carbon sequestration through decreased carbon mineralization in N-rich tropical forests. This study can help us understand how microbes control soil carbon cycling and carbon sink in the tropics under both elevated N deposition and carbon dioxide in the future.

Influences of temperature and moisture on abiotic and biotic soil CO2 emission from a subtropical forest

BACKGROUND

Soil CO₂ efflux is considered to mainly derive from biotic activities, while potential contribution of abiotic processes has been mostly neglected especially in productive ecosystems with highly active soil biota. We collected a subtropical forest soil to sterilize for incubation under different temperature (20 and 30 °C) and moisture regimes (30%, 60 and 90% of water holding capacity), aiming to quantify contribution of abiotic and biotic soil CO₂ emission under changing environment scenarios.

MAIN FINDINGS

Results showed that abiotic processes accounted for a considerable proportion (15.6-60.0%) of CO₂ emission in such a biologically active soil under different temperature and moisture conditions, and the abiotic soil CO₂ emission was very likely to derive from degradation of soil organic carbon via thermal degradation and oxidation of reactive oxygen species. Furthermore, compared with biotically driving decomposition processes, abiotic soil CO₂ emission was less sensitive to changes in temperature and moisture, causing reductions in proportion of the abiotic to total soil CO₂ emission as temperature and moisture increased.

CONCLUSIONS

These observations highlight that abiotic soil CO₂ emission is unneglectable even in productive ecosystems with high biological activities, and different responses of the abiotic and biotic processes to environmental changes could increase the uncertainty in predicting carbon cycling.

Eight years of variations in ecosystem respiration over a residue-incorporated rotation cropland and its controlling factors

The carbon dioxide emissions from cropland play important roles in the regional carbon budget. In this study, continuous measurements of the ecosystem respiration (RE) were obtained using the eddy covariance technique in a winter wheat-summer maize double cropping agroecosystem mainly between 2004 and 2012 in order to identify the among-year variations in RE and the related factors responsible. The annual RE, estimated by Lloyd and Taylor model, which was the most accurate, was 1866.4 ± 105.75 g C m^-2 year^-1 and it ranged from 1650.68 g C m^-2 year^-1 to 1945.57 g C m^-2 year^-1 during the eight years. The seasonal RE values were 867.98 ± 125.24 g C m^-2 year^-1 and 890.55 ± 131.34 g C m^-2 year^-1 for wheat and maize, respectively. Additionally, crop residue carbon ranged from 322.73 g C m^-2 year^-1 in 2012 and 453.49 g C m^-2 year^-1 in 2007. Correlation analysis indicated that the interannual variations in wheat and maize RE were correlated with the seasonal mean soil water content (W-W_s) and maximum leaf area index (W-LAI_max) of wheat, and seasonal mean air temperature of maize (S-T_a), respectively. A rest method was attempted to investigate whether these relationships were occasional or inevitable. The rests of RE, i.e. the difference between simulated and observed RE values, were significantly influenced by LAI of wheat and hourly T_a of maize season but not by hourly W_s of maize season, indicating that the influence of W-LAI_max and S-T_a on RE were inevitable outcomes and that of W-W_s on wheat RE was occasional. So we suggested that one should not confirm the controlling factors of interannual variations in carbon fluxes just from simple relationships, which may be statistical coincidences and do not correlated with biotical processes.

Soil respiration in a subtropical forest of southwestern China: Components, patterns and controls

Partitioning the components of soil respiration is crucial to understand and model carbon cycling in forest ecosystems. In this study, total soil respiration (R_S), autotrophic respiration (R_A), heterotrophic respiration (R_H), litter respiration (R_L), litterfall input and environmental factors were synchronously monitored for 2 years in a subtropical Michelia wilsonii forest of southwestern China. R_H rates were often higher than R_A rates during the two years except for the middle growing season (from July to September). The mean rate of Rs, R_A, R_H and R_L was 1.94 μmol m^-1 s^-1, 0.85 μmol m^-1 s^-1, 1.09 μmol m^-1 s^-1 and 0.65 μmol m^-1 s^-1, respectively, during the 2-year experiment. Annual CO₂ emission derived from R_A, R_H and R_L was 3.26 Mg C ha^-1 a^-1, 4.67 Mg C ha^-1 a^-1 and 2.61 Mg C ha^-1 a^-1, respectively, which accounted for 41.4%, 58.6% and 32.9% of R_S. Over the experimental period, the ratio of R_A/R_S increased with soil temperature but the opposite was true for R_H/R_S and R_L/R_S. The Q₁₀ value was 2.01, 4.01, 1.34 and 1.30, respectively, for R_S, R_A, R_H and R_L. Path analysis indicated that environmental variables and litterfall production together explained 82.0%, 86.8%, 42.9% and 34.7% variations of monthly fluxes of R_S, R_A, R_H and R_L, respectively. Taken together, our results highlight the differential responses of the components of R_S to environmental variables.

Thinning but not understory removal increased heterotrophic respiration and total soil respiration in Pinus massoniana stands

Quantifying soil respiration (R_s) and its components [autotrophic respiration (R_a) and heterotrophic respiration (R_h)] in relation to forest management is vital to accurately evaluate forest carbon balance. Thus, R_s, R_a, and R_h were continuously monitored from November 2013 to November 2016 in Pinus massoniana forests subjected to four different management practices in China. We hypothesized that understory removal and thinning decrease R_a and R_h and thus R_s, and these decreases will change with time following UR and thinning. Mean values of R_s, R_a, and R_h in light thinned plots (LT=15% of tree basal area thinned) and heavily thinned plots (HT=70% of tree basal area thinned) were significantly higher than in control (CK) and understory removal plots (UR). The annual R_h/R_s ratio ranged from 58% to 70% across all treatments, and this ratio was significantly higher in HT and LT than in UR and CK. Only HT significantly increased soil temperature. Soil temperature could better explain R_h (R²=0.69-0.96) than R_a (R²=0.51-0.86). HT and LT increased Q₁₀ for both R_a and R_h, except for R_h in UR. Soil moisture content (W; %) was significantly higher in HT than in other treatments, but W had limited effects on soil respiration in that rain-rich subtropical China. This result suggests that global warming alone, or in combination with clear-cutting or canopy tree thinning will markedly increase soil heterotrophic respiration and thus the total soil CO₂ emission. To get firewood for local people and to reduce soil CO₂ emissions under global warming, canopy trees are needed to be protected and understory shrubs may be allowed to be used in the subtropical China.