Simulated rain addition modifies diurnal patterns and temperature sensitivities of autotrophic and heterotrophic soil respiration in an arid desert ecosystem

The timing and magnitude of rainfall events in arid and semiarid regions are expected to change dramatically in future decades, which will likely greatly affect regional carbon cycles. To understand how increases in rainfall affect the diurnal patterns and temperature sensitivities (Q₁₀) of soil respiration (R_S) and its key components (i.e. heterotrophic respiration (R_H) and autotrophic respiration (R_A)), we conducted a manipulative field experiment in a desert ecosystem of Northwest China. We simulated five different scenarios of future rain regimes (0%, 25%, 50%, 75% and 100% increase over local annual mean precipitation) each month from May to September in 2009. We measured R_S and R_H every three hours on 6 and 16 days after the rain addition, and estimated R_A by calculating the difference between R_S and R_H. We found that rain addition significantly increased the daily mean R_S and its components on the two measurement days during the growing season. However, the diurnal pattern was different between the two respiration components. Rain addition significantly increased the daily Q₁₀ value of R_H but suppressed that of R_A on Day 6. Rain addition had no influence on daily Q₁₀ value of both respiration components on Day 16 when soil moisture was lower. In addition, we observed significantly higher daily Q₁₀ of R_H than R_A under all five rain addition treatments, indicating that microbial respiration is more temperature sensitive than root respiration in a short-time scale in this desert ecosystem. Thus, partitioning soil respiration into its two components, and analyzing the differential responses of R_H and R_A to future climate changes should be considered for more accurate predictions of soil respiration and regional carbon cycle in these arid and semiarid regions.     

Long-term (42 years) effect of thinning on soil CO2 emission in a mixed broadleaved-Korean pine (Pinus koraiensis) forest in Northeast China

Thinning is an important forest management practice that has great potential to influence regional soil carbon storage and dynamics.The present study measured soil respiration(RS,the efflux of CO2 emitted)and its two components(heterotrophic(RH)and autotrophic(RA)respiration)from soil 42 years after thinning in comparison to un-thinning(control).Autotrophic respiration was significantly greater in the thinning plot,approximately 44%higher compared to the control,while both RSand RHwere slightly,but not significantly,higher in the thinning plot.Higher fine root biomass might have contributed to the higher RAin the thinning plot.Both RSand RHshowed clear soil temperature-dependent seasonal patterns,whereas RAwas less responsive to changes in temperature,especially within one specific season.The annual and season-specific temperature sensitivities of RSand RHwere lower in the thinning plot,specifically during the mid-growing season.Furthermore,variations in the season-specific temperature sensitivity of RSand RHwere less intense in the thinning plot.We conclude that forest thinning can reduce the temperature sensitivity of RSand RHduring the mid-growing season and increase soil CO2 emission in the long term.     

Temporal variations of soil respiration at multiple timescales in a spruce-fir valley forest,northeastern China

Purpose

There is a paucity of data regarding the multiple timescale variations of heterotrophic respiration (R _H) and autotrophic respiration (R _A) as well as the primary controlling factors. The objective of this study is to find the temporal variations of total soil respiration (R _S) and its components, revealing the driving factors at different timescales.

Materials and methods

A trenching method was used to distinguish R _S, R _H, and R _A in a spruce-fir valley forest in northeastern China. We used the closed dynamic chamber method to measure the soil respiration rate. Analyses of R _S, R _H, and R _A in relation to biotic and abiotic factors were conducted to realize the temporal variations at different timescales.

Results and discussion

Only R _S and R _H showed a distinct diurnal variation and soil temperature (T _S) can explain 68 and 59 % of the daily variation, respectively. R _S, R _H, and R _A showed a pronounced, single peak curve seasonally, and T _S can explain 11–95 % of the seasonal variation. Soil moisture (W _S) maintained at a relatively high level and was not related to R _S, R _H, or R _A on a seasonal scale, and there was no significant relationship between the seasonal R _S, R _A, and root biomass. However, for 5 years, only the mean R _A of the growing season was significantly related to the mean W _S, which can explain 39 % of the inter-annual variation of R _A. The annual variations of litterfall and the relative growth rate of stems were not related to R _S, R _H, or R _A. The contribution of R _H to R _S was larger, and the temperature sensitivity was 2.01–3.71 for R _S, 1.90–3.08 for R _H, and 2.20–5.65 for R _A.

Conclusions

R _S, R _H, and R _A show different temporal variations at multiple timescales. When W _S is not restricted, T _S is the primary driving factor of daily and seasonal variation of R _S and R _H. In this site, R _H accounts for a large proportion of R _S and plays a crucial role in determining the magnitude and temporal variation of R _S.

     

Different plant covers change soil respiration and its sources in subtropics

Heterotrophic soil respiration (R _H) and autotrophic soil respiration (R _A) by a trenching method were monitored in four vegetation types in subtropical China from November 2011 to October 2012. The four vegetation types included a shrubland, a mixed-conifer, a mixed-legume, and a mixed-native species. The average R _H was significantly greater in soils under the mixed-legume and the mixed-native species than in the shrubland and the mixed-conifer soils, and it affected the pattern of soil total respiration (R _S) of the four soils. The change in R _H was closely related to the variations of soil organic C, total N and P content, and microbial biomass C. The R _A and the percentage of R _S respired as R _A were only significantly increased by the mixed-native species after reforestation. Probably, this depended on the highest fine root biomass of mixed-native species than the other vegetation types. Soil respiration sources were differently influenced by the reforestation due to different changes in soil chemical and biological properties and root biomass.     

Components,drivers and temporal dynamics of ecosystem respiration in a Mediterranean pine forest

To investigate the climate impacts on the different components of ecosystem respiration, we combined soil efflux data from a tree-girdling experiment with eddy covariance CO₂ fluxes in a Mediterranean maritime pine (Pinus pinaster) forest in Central Italy. 73 trees were stem girdled to stop the flux of photosynthates from the canopy to the roots, and weekly soil respiration surveys were carried out for one year. Heterotrophic respiration (R_H) was estimated from the soil CO₂ flux measured in girdled plots, and rhizosphere respiration (R_Ab) was calculated as the difference between respiration from controls (R_S) and girdled plots (R_H).Results show that the R_S dynamics were clearly driven by R_H (average R_H/R_S ratio 0.74). R_H predictably responded to environmental variables, being predominantly controlled by soil water availability during the hot and dry growing season (May–October) and by soil temperature during the wetter and colder months (November–March). High R_S and R_H peaks were recorded after rain pulses greater than 10 mm on dry soil, indicating that large soil carbon emissions were driven by the rapid microbial oxidation of labile carbon compounds. We also observed a time-lag of one week between water pulses and R_Ab peaks, which might be due to the delay in the translocation of recently assimilated photosynthates from the canopy to the root system. At the ecosystem scale, total autotrophic respiration (R_At, i.e. the sum of carbon respired by the rhizosphere and aboveground biomass) amounted to 60% of ecosystem respiration. R_At was predominantly controlled by photosynthesis, and showed high temperature sensitivity (Q₁₀) only during the wet periods. Despite the fact that the study coincided with an anomalous dry year and results might therefore not represent a general pattern, these data highlight the complex climatic control of the respiratory processes responsible for ecosystem CO₂ emissions.     

Rhizospheric and heterotrophic respiration of a warm-temperate oak chronosequence in China

Plot trenching and root decomposition experiments were conducted in a warm-temperate oak chronosequence (40-year-old, 48-year-old, 80-year-old, and 143-year-old) in China. We partitioned total soil surface CO₂ efflux (R_S) into heterotrophic (R_H) and rhizospheric (R_R) components across the growing season of 2009. We found that the temporal variation of R_R and R_H can be well explained by soil temperature (T₅) at 5 cm depth using exponential equations for all forests. However, R_R of 40-year-old and 48-year-old forests peaked in September, while their T₅ peaks occurred in August. R_R of 80-year-old and 143-year-old forests showed a similar pattern to T₅. The contribution of R_R to R_S (RC) of 40-year-old and 48-year-old forests presented a second peak in September. Seasonal variation of R_R may be accounted for by the different successional stages. Cumulative R_H and R_R during the growing season varied with forest age. The estimated R_H values for 40-year-old, 48-year-old, 80-year-old and 143-year-old forests averaged 431.72, 452.02, 484.62 and 678.93 g C m⁻², respectively, while the corresponding values of R_R averaged 191.94, 206.51, 321.13 and 153.03 g C m⁻². The estimated RC increased from 30.78% in the 40-year-old forest to 39.85% in the 80-year-old forest and then declined to 18.39% in the 143-year-old forest. We found soil organic carbon (SOC), especially the light fraction organic carbon (LFOC), stock at 0-10 cm soil depth correlated well with R_H. There was no significant relationship between R_R and fine root biomass regardless of stand age. Measured apparent temperature sensitivity (Q₁₀) of R_H (3.93 ± 0.27) was significantly higher than that of R_R (2.78 ± 0.73). Capillary porosity decreased as stand age increased and it was negatively correlated to cumulative R_S. Our results emphasize the importance of partitioning soil respiration in evaluating the stand age effect on soil respiration and its significance to future model construction.     

Intensive management enhances mycorrhizal respiration but decreases free-living microbial respiration by affecting microbial abundance and community structure in Moso bamboo forest soils

Intensive management is known to markedly alter soil carbon (C) storage and turnover in Moso bamboo forests compared with extensive management. However, the effects of intensive management on soil respiration (R_S) components remain unclear. This study aimed to evaluate the changes in different R_S components (root, mycorrhizal, and free-living microorganism respiration) in Moso bamboo forests under extensive and intensive management practices. A 1-year in-situ microcosm experiment was conducted to quantify the R_S components in Moso bamboo forests under the two management practices using mesh screens of varying sizes. The results showed that the total R_S and its components exhibited similar seasonal variability between the two management practices. Compared with extensive management, intensive management significantly increased cumulative respiration from mycorrhizal fungi by 36.73%, while decreased cumulative respiration from free-living soil microorganisms by 8.97%. Moreover, the abundance of arbuscular mycorrhizal fungi (AMF) increased by 43.38%, but bacterial and fungal abundances decreased by 21.65% and 33.30%, respectively, under intensive management. Both management practices significantly changed the bacterial community composition, which could be mainly explained by soil pH and available potassium. Mycorrhizal fungi and intensive management affected the interrelationships between bacterial members. Structural equation modeling indicated that intensive management changed the cumulative R_S by elevating AMF abundance and lowering bacterial abundance. We concluded that intensive management reduced the microbial respiration-derived C loss, but increased mycorrhizal respiration-derived C loss.     

Separation of soil respiration into CO2 emission sources using 13C natural abundance in a deciduous broad-leaved forest in Japan

Abstract

In determining the soil and ecosystem carbon balance, it is necessary to distinguish between autotrophic respiration and heterotrophic respiration. We attempted to measure the contribution of CO₂ emissions from plant roots (R_RHI), from soil organic matter (R_SOM), and from litter (R_L) to CO₂ emissions from the forest floor (soil respiration; R_S) in a deciduous forest of oak (Quercus serrata Thunb.) and hornbeams (Carpinus laxiflora Sieb. et Zucc. Bl., Carpinus tschonoskii Maxim. and Carpinus japonica Bl.) on Andosols in Japan, using a ¹³C natural abundance technique. The ¹³C natural abundances of roots (δ_RHI), litter (δ_L) and SOM (δ_SOM) in the surface soil were ?28.9, ?30.1 and ?24.3‰, respectively. This means that the differences between δ_SOM and δ_RHI are large enough to calculate the contributions of R_RHI, R_SOM and R_L to R_S based on the mass balance of the CO₂ isotope ratios. R_RHI and R_SOM had close relationships with soil temperature, and R_L was influenced by soil temperature and moisture. In summer, under high soil temperatures, R_RHI and R_SOM were the predominant sources of R_S and the proportion of R_RHI to R_SOM to R_L was 51:44:5. In winter, R_L was predominant and the proportion of R_RHI to R_SOM to R_L was 20:11:69. The estimated annual emissions of R_RHI, R_SOM and R_L were 1.45, 2.10 and 1.30 Mg C ha^?1, respectively; thus, the proportion of R_RHI to R_SOM to R_L was 30:43:27 on a whole-year basis.     

Effect of grazing intensity and soil characteristics on soil organic carbon and nitrogen stocks in a temperate long-term grassland

The effects of different grazing pressures (GPs) on soil properties are not sufficiently understood. The objectives were to analyse the effects of three different extensive GPs on stocks of soil organic C and total N, soil microbial biomass C, basal respiration and mineral N in three different soil depths of a long-term pasture in Central Germany (FORBIOBEN field trial). No significant (p ≤ 0.05) effects of GP on weighted stocks of soil organic C, total N, soil microbial biomass C, mineral N and basal respiration rate were observed, suggesting that the C and N cycles are coupled in the three grazing treatments. Oxalate soluble Fe contents explained a marked part of the variation of soil organic C (multiple linear regression: R² = 0.64) and total N contents (R² = 0.64) in the soils, whereas almost all of the variability of soil microbial biomass C contents and basal respiration was explained by soil organic C contents. Overall, variabilities of soil organic C and N contents were largely explained by oxalate soluble Fe contents, whereas grazing intensity did not affect the C and N dynamics.     

Net ecosystem carbon exchange for Bermuda grass growing in mesocosms as affected by irrigation frequency

Intensification of grazed grasslands following conversion from dryland to irrigated farming has the potential to alter ecosystem carbon (C) cycling and affect components of carbon dioxide (CO₂) exchange that could lead to either net accumulation or loss of soil C. While there are many studies on the effect of water availability on biomass production and soil C stocks, much less is known about the effect of the frequency of water inputs on the components of CO₂ exchange. We grew Bermuda grass (Cynodon dactylon L.) in mesocosms under irrigation frequencies of every day (I₁ treatment, 30 d), every two days (I₂ treatment, 12 d), every three days (I₃ treatment, 30 d), and every six days (I₆ treatment, 18 d, after I₂ treatment). Rates of CO₂ exchange for estimating net ecosystem CO₂ exchange (F_N), ecosystem respiration (R_E), and soil respiration (R_S) were measured, and gross C uptake by plants (F_G) and respiration from leaves (R_L) were calculated during two periods, 1–12 and 13–30 d, of the 30-d experiment. During the first 12 d, there were no significant differences in cumulative F_N (mean ±standard deviation, 61 ±30 g C m^-2, n = 4). During the subsequent 18 d, cumulative F_N decreased with decreasing irrigation frequency and increasing cumulative soil water deficit (W), with values of 70 ±22, 60 ±16, and 18 ±12 g C m^-2 for the I₁, I₃, and I₆ treatments, respectively. There were similar decreases in F_G, R_E, and R_L with increasing W, but differences in R_S were not significant. Use of the C₄ grass growing in a C₃-derived soil enabled partitioning of R_S into its autotrophic (R_A) and heterotrophic (R_H) components using a ¹³C natural abundance isotopic technique at the end of the experiment when differences in cumulative W between the treatments were the greatest. The values of R_H and its percentage contributions to R_S (43% ±8%, 42% ±8%, and 8% ±5% for the I₁, I₃, and I₆ treatments, respectively) suggested that R_H remained unaffected across a wide range of W and then decreased under extreme W. There were no significant differences in aboveground biomass between the treatments. Nitrous oxide (N₂O) emission was measured to determine if there was a trade-off effect between irrigation frequency and increasing W on net greenhouse gas emission, but no significant differences were found between the treatments. These findings suggest that over short periods in well-drained soil, irrigation frequency could be managed to manipulate soil water deficit in order to reduce net belowground respiratory C losses, particularly those from the microbial decomposition of soil organic matter, with no significant effect on biomass production and N₂O emission.     

Soil Respiration Characteristics in Three Shrublands along an Elevation Gradient in a Semiarid Loess Plateau Mountain Area

Shrub is one of the major vegetation types distributed mostly in the mountainous area in China, and its vegetation carbon storage is approximately one-third of both forests and grasslands. It is essential to investigate how soil temperature (T_s) and soil water content (W_s) affect soil respiration (R_s) in this ecosystem. The purpose of this study was to understand the correlations of R_s with T_s, W_s, and other factors in the shrubs. In the current study, R_s was characterized in three shrublands (hereafter, shrub 1, shrub 2, and shrub 3, respectively) located in different elevations over a 4-year period at a biweekly interval in the eastern Loess Plateau (Shanxi province) of China. Our results showed that the trend of seasonal change of R_s was controlled mainly by T_s and W_s. The measured mean R_s over 4 years was 3.64 ± 2.83 (mean ± S.D.), 2.69 ± 2.05, and 4.41 ± 3.28 μmol carbon dioxide (CO₂) m^?2 s^?1 for shrubs 1, 2, and 3, respectively, exhibiting an increase trend with elevation increment. Over the season, R_s illustrated a significant change depending on the variation of T_s and W_s, with larger values appearing in summer when both T_s and W_s were high, and smaller values in winter or in summer whenever W_s was low. An exponential model (R_s = a e ^bTs) fitted well the relation between R_s and T_s for shrub 3, whereas linear (R_s = a W_s + b) and power (R_s = a W_s ^b) models of R_s to W_s fitted well for shrub 1. This indicated that at a lower elevation, W_s had a greater effect on R_s than that at a higher elevation. The reverse trend was true between R_s and T_s, i.e., at a higher elevation T_s had a greater effect on R_s than that at a lower elevation. The calculated Q₁₀ values of 1.61, 3.03, and 3.73 for shrubs 1, 2, and 3 increased to 2.25, 3.63, and 4.07, respectively (when the data in low W_s conditions were excluded from the analysis), showing that Q₁₀ increased with elevation increment. Furthermore, three two-variable models, one linear (R_s = a (T_s W_s) + b), and two nonlinear (R_s = a T_s ^b W_s ^c and R_s = a e^bTs W_s ^c), were also well developed to predict the dependency of R_s on both T_s and W_s. Our research results might have important implications for the estimation of soil carbon emissions of the shrublands in this region.     

The effect of boreal forest composition on soil respiration is mediated through variations in soil temperature and C quality

Getting a better understanding of CO₂ efflux from forest soils is critical for increasing our comprehension of the global C cycle. We examined the influence of two common boreal tree species, either in pure stands (BS = black spruce; TA = trembling aspen) or in mixtures (MW = BS + TA mixedwood), on total (R_S), heterotrophic (R_H) and autotrophic soil respiration (R_A) and their relationship with soil temperature and moisture, distance to the nearest tree, labile and total soil organic C (SOC), and root content. Stand-specific soil respiration–temperature models were developed to estimate annual soil CO₂ efflux. Soil temperature was the main factor explaining R_S and its components, followed by labile and total SOC. These three variables were significantly affected by forest composition, while no difference in soil moisture, distance to the nearest tree and root content was observed between stand types. A reciprocal forest floor transplant experiment showed that the influence of stand types on mineral soil temperature was due to a difference in light penetration rather than forest floor characteristics. Annual R_S and R_H were significantly greater in MW and TA than in BS, whereas annual R_A was greater in BS and MW than in TA. Temperature sensitivity (Q₁₀) of both R_S and R_H was significantly higher in BS than in MW and TA, suggesting that CO₂ efflux from BS soils could be increased more under climate warming than that from the other stand types. Our results show evidence that boreal forest composition affects soil CO₂ efflux and that litter quality is not the only factor explaining the differences between stand types. The influence of forest composition on soil CO₂ efflux would be mediated through effects on soil temperature as well as on factors affecting the accumulation and the quality of SOC.     

Seasonal and diurnal dynamics of soil respiration fluxes in two typical forests on the semiarid Loess Plateau of China: Temperature sensitivities of autotrophs and heterotrophs and analyses of integrated driving factors

Partitioning of total soil respiration (R_T) into autotrophic (R_A) and heterotrophic (R_H) components was undertaken in two typical (natural and artificial) forests on the temperate, semiarid Loess Plateau of China, to determine and compare temperature sensitivities between the two components. The natural secondary forest was dominated by oak (Quercus liaotungensis) while the artificial forest was a plantation of black locust (Robinia pseudoacacia). Soil CO₂ efflux and different abiotic and biotic factors were measured during dormant and growing seasons. Temperature sensitivities of soil respiration components were investigated using the Q₁₀ function at diurnal and seasonal scales. The temperature sensitivities of autotrophic (R_A) and heterotrophic (R_H) respiration varied with the time scales (daily, seasonal, or annual) of the investigation, and were affected by other biological and environmental factors. The largest contribution of R_A to R_T was 46% in the oak forest and 60% in the black locust plantation during the growing season. During the dormant season it was as low as 12% in the oak forest and 6% in the black locust plantation. The Q₁₀ of R_A for the black locust plantation was higher than for the oak forest during the growing season, but was lower during the dormant season. The Q₁₀ of R_A in both forests was higher than that of R_H at both diurnal and seasonal scales. Multiple regression analyses suggested that photosynthesis is an important parameter in soil respiration studies and that a multiple-factor model may be more suitable during the annual periods.     

Spatial and seasonal variations in soil respiration in a temperate deciduous forest with fluctuating water table

Our objectives were to determine both spatial and temporal variations in soil respiration of a mixed deciduous forest, with soils exhibiting contrasting levels of hydromorphy. Soil respiration (R_S) showed a clear seasonal trend that reflected those of soil temperature (T_S) and soil water content (W_S), especially during summer drought. Using a bivariate model (RMSE=1.03), both optimal soil water content for soil respiration (W_SO) and soil respiration at both 10 °C and optimal soil water content (R_S10) varied among plots, ranging, respectively, from 0.25 to 0.40 and from 2.30 to 3.60 μmol m⁻² s⁻¹. Spatial variation in W_SO was related to bulk density and to topsoil N content, while spatial variation in R_S10 was related to basal area and the difference in pH measured in water or KCl suspensions. These results offer promising perspectives for spatializing ecosystem carbon budget at the regional scale.     

Study of soil respiration and fruit quality of table grape (Vitis vinifera L.) in response to different soil water content in a greenhouse

ABSTRACT

To understand the response of grape (Hutai No.8) quality and soil respiration (R_s) to different soil relative water contents (SRWCs), this study was designed with three soil moisture levels (A: 80–95%, B: 60–75%, and C: 40–55% of SRWC) for grape cultivation. Meanwhile, environmental factors, including air temperature (T_a), air relative humidity, and light intensity, were also recorded. The results showed the following: (1) Through the comprehensive analysis of fruit quality by the method of subordinate function, we concluded that the optimum soil moisture treatment was 60–75% SRWC, and the soluble sugars, proanthocyanidin, and resveratrol were most abundant. In addition, vitamin C (Vc) content was the largest under C treatment. (2) Photosynthetic characteristic under high soil moisture was better than those under low soil moisture condition during grape coloring periods, and it was largest under A treatment in 2015. R_s rate was in accordance with the trend of grape photosynthesis. High soil moisture could accelerate the photosynthetic rate of grape leaves and increase R_s. (3) Correlation analysis showed that higher soil moisture and air humidity and lower soil temperature (T_s) and T_a could promote the accumulation of more nutrients in grape berries; it also could increase photosynthetic rate and R_s during grape coloring periods. In conclusion, 60–75% SRWC was the optimum soil moisture condition, which could improve the nutrient contents and accumulate more bioactive substances. Of course, keeping a lower T_s and T_a, as well as higher air humidity, was also necessary.

Abbreviations: SRWC: soil relative water content; A, 90-95% SRWC; B, 70-75% SRWC; C, 40-55% SRWC; Rs: soil respiration; Ta: air temperature; Ts: soil temperature; OPC: proanthocyanidin; TSS: total soluble solids.     

Soluble organic C and N and their relationships with soil organic C and N and microbial characteristics in moso bamboo (Phyllostachys edulis) plantations along an elevation gradient in Central Taiwan

Purpose

Moso bamboo (Phyllostachys edulis), an important economic crop, is distributed from low- to medium-elevation mountains in Taiwan. Bamboo is a fast-growing herbaceous species with an extensive rhizome structure. With the hypothesis that the characteristics of soil organic matter and microbes might change after long-term bamboo plantation, we investigated different fractions of organic C and N as well as soil microbial biomass and activities in five moso bamboo plantations along an elevation gradient in Central Taiwan.

Materials and methods

Five soil samples (top 10 cm of soil) were collected from each bamboo plantation (600, 800, 1,000, 1,200, and 1,400 m above sea level (asl)) in January 2011. Soil was processed and analyzed for soil total C and N contents, biologically available C, potentially mineralizable N, soil microbial biomass and soil respiration (CO₂). Two extraction methods (2 M KCl and hot-water extraction) were used to estimate soil soluble organic C and N (S_bOC and S_bON) and soil inorganic N (NH₄ ⁺ and NO₃ ^?) concentrations to evaluate the relationship with soil organic matter and microbe characteristics in bamboo plantations.

Results and discussion

Soil total C and N contents as well as soil microbial biomass and soil respiration (CO₂) of the bamboo plantations increased along the elevation gradient. Temperature changes along elevation contributed to such variations observed among the selected bamboo plantations. The S_bON in hot-water extracts was highest in the 1,200-m plantation, then in the 1,400-m plantation, and lowest in the low-elevation plantations (600, 800, and 1,000 m). However, S_bON in 2 M KCl extracts did not differ by elevation. The S_bON was strongly correlated with soil total N in both 2 M KCl and hot-water extracts, but only S_bON in hot-water extracts was strongly correlated with microbial biomass N and potentially mineralizable N. S_bOC was strongly correlated with soil total C content, microbial biomass C, and biologically available C in both 2 M KCl and hot-water extracts.

Conclusions

Soil total C and N, S_bOC and S_bON, and microbial biomass characteristics increased in the moso bamboo plantations with increasing elevation. No altitudinal difference in specific soil respiration (CO₂) rate suggested that the enhanced potentially mineralizable N and soil respiration (CO₂) in the high-elevation plantations were associated with increased microbial biomass rather than microbial activities.     

Integrated diurnal soil respiration model during growing season of a typical temperate steppe: Effects of temperature, soil water content and biomass production

Soil respiration was measured with the enclosed chamber method in an ungrazed Leymus chinensis steppe during the growing seasons of 2001 and 2002. Soil respiration rate (R_S) was significantly influenced by air temperature (T) at the diurnal scale, and could be described by Van't Hoff's equation (R_S = R₁₀ exp(β(T − 10))). At the seasonal scale, the normalized soil respiration rate at 10 °C (R₁₀) was mainly controlled by soil water content (R² = 0.717, P < 0.001), while the sensitivity of soil respiration to temperature (Q₁₀) was partially affected by absolute growth rate (R² = 0.482, P = 0.004). Thus, soil respiration could be described as R_S = (20.015W − 84.085) (0.103AGR + 1.786)^(T−10)/10 during the growing seasons, integrating soil water content (W) and absolute growth rate (AGR) into the temperature-dependent soil respiration equation. It was validated by the observed soil respiration rates in this study (R² = 0.890, P < 0.001) and observations from near-field experiment (R² = 0.687, P = 0.011). It implied that accurately evaluating annual soil respiration should include the effects of plant biomass production and other abiotic factors besides air temperature.     

Real-Time Soil CO2 Respiration Rate Determination and the Comparison between the Infrared Gas Analyzer and Microrespirometer (MicroRes®) Methods

Soil carbon dioxide (CO₂) respiration is one of the important soil health parameters that provides a general assessment of soil microbial activity and soil quality. Soil respiration rates, however, have not been widely applied in soil testing protocols mainly because the traditional methods are either inconvenient, technically cumbersome or too expensive. Currently, only two methods are available for a true real-time soil respiration rate determination (<2 h): the infrared gas analyzer (IRGA) and the microrespirometer (MR or MicroRes^®) methods. We analyzed the real-time soil respiration rates of 20 soil samples from fifteen states after various periods of incubation using the IRGA method and the MR method. The measured soil respiration rates ranged from 0.4 µL CO₂/h/g to 9.0 µL CO₂/h/g. Both methods show precision in soil respiration determinations (CV = 12.7% and 11.9%, respectively). Comparison of the results between the IRGA and MR methods indicates high degrees of agreement (r² = 0.914). This study shows that the MR method is a simpler and more cost-effective alternative for real-time soil respiration rate determinations.     

Weathered Fly Ash Does Not Affect Soil And Biosolid Carbon Mineralization

Fly ash and biosolid wastes can be mixed and applied to soil as a means of disposal. A significant decline in soil respiration following waste application indicates restricted activities of functional microbial populations. Weathering decreases salinity and neutralizes alkalinity in fly ash, but there is little information on the effects of unweathered fly ash and biosolid mixtures on soil carbon (C) mineralization. The objective of this study was to determine the effects of a weathered fly ash–limestone scrubber residue (LSR) mixed with an aerobically digested biosolid on soil respiration in a laboratory incubation study. Biosolids significantly increased carbon dioxide (CO₂) production (p < 0.05), but up to 6.75% (w/w) fly ash did not. Mean total C mineralization was 770 mg CO₂‐C kg^?1 soil in the control and 3,810 mg CO₂‐C kg^?1 soil in the 6.75% (w/w) biosolid treatment. Fly ash with neutral pH and low salinity appears unlikely to affect soil and biosolid C mineralization.     

Litter contribution to diurnal and annual soil respiration in a tropical montane cloud forest

Respiration of CO₂ from soils (R_s) is a major component of the carbon cycle of ecosystems, but understanding is still poor of both the relative contributions of different respiratory sources to R_s, and the environmental factors that drive diurnal variations in R_s. We measured total and litter-free R_s at half-hourly intervals over full 24 h periods, and thereafter twice a month for 10 months in a tropical montane cloud forest (TMCF) in Peru. Total R_s declined by about 61% during the night as a result of variations in respiration rate in the litter, which were partly correlated with the soil surface air temperature. Most of the diurnal variation of R_s in this TMCF appears to be driven by respiration in the litter layer, which contributed 37% to the total soil CO₂ efflux. Total R_s rates at this particular site would have been overestimated by 60% if derived from daytime measurements that had not been corrected for diurnal variations in R_s.