Annual Rs Research Articles

Short-term phosphorus addition increases soil respiration by promoting gross ecosystem production and litter decomposition in a typical temperate grassland in northern China

Anthropogenic activities have increased phosphorus (P) deposition and affect ecosystem carbon cycles. Although soil respiration (Rs) is a vital part of terrestrial C fluxes, the responses of Rs to altered substrate supply, inputs, and decomposition rates under P addition are not fully understood. Here, we conducted a 2-year field experiment with P fertilization rates ranging from 0 to 12.5 g P m−2 yr−1 to explore the effects on the Rs of a temperate grassland in Inner Mongolia, northern China. P addition increased Rs and annual Rs ranged from 413.04 to 938.83 g C m−2 yr−1. Soil heterotrophic respiration (Rh) increased after P addition, and its contribution to Rs ranged from 65.5% to 71.4%, possibly due to the increased litter input and microbial biomass. P addition increased soil P availability and the net N mineralization rate, thereby mitigating microbial nutrient limitations, and increased the maximum net photosynthetic rate of the dominant species and gross ecosystem production, which was strongly correlated with Rs. P addition increased litter quantity and improved litter quality, with decreased litter C:N and C:P ratios, and generally increased the activities of the soil enzymes β-1,4-glucosidase, polyphenol oxidase, leucine aminopeptidase, and alkaline phosphatase. This accelerated litter decomposition and increased Rh. Soil autotrophic respiration reached a maximum at 2.5 g P m−2 yr−1, possibly due to the decreased transportation of photosynthate to roots at high P. Our results suggest that Rs in temperate grassland responds sensitively to P addition and highlight the regulation of Rs by biotic factors under P fertilization.

Improved soil characteristics in the deeper plough layer can increase grain yield of winter wheat

In the North China Plain (NCP), soil deterioration threatens winter wheat (Triticum aestivum L.) production. Although rotary tillage or plowing tillage are two methods commonly used in this region, research characterizing the effects of mixed tillage on soil characteristics and wheat yield has been limited. A fixed-site field trial was carried out during 2011–2016 to examine the impacts of three tillage practices (5-year rotary tillage with maize straw removal (RT); 5-year rotary tillage with maize straw return (RS); and annual RS and with a deep plowing interval of 2 years (RS/DS)) on soil characteristics and root distribution in the plough layer. Straw return significantly decreased soil bulk density, increased soil organic carbon (SOC) storage and SOC content, macro-aggregate proportion (R0.25) and its stability in the plough layer. The RS/DS treatment significantly increased the SOC content, total nitrogen (TN), and root length density (RLD) in the 10–40 cm layer, and enhanced the proportion of RLD in the 20–30 and 30–40 cm layers. In the 20–30 and 30–40 cm layers, an increase in SOC and TN could lead to higher grain production than commensurate increases in the surface layer, resulting in a sustainable increase in grain yield from the RS/DS treatment. Thus, the RS/DS treatment could lead to high productivity of winter wheat by improving soil characteristics and root distribution at the deeper plough layer in the NCP.

Soil respiration dynamics in forage-based and cereal-based cropping systems in central Italy

ABSTRACT: Studies that have investigated soil carbon dynamics under Mediterranean conditions are scarce and fragmented and contrasting results have often been reported. This study aimed to fill some gaps in our knowledge by: (i) determining annual dynamics of total (RS) and heterotrophic (RH) soil respiration; (ii) estimating annual cumulative RS and RH; and (iii) investigating the relationships between RS and RH and soil temperature and water content. The study was carried out in central Italy, for a plain and a hilly site, with the focus on two main cropping systems: an alfalfa-based forage system and a wheat-based rotation system. RS and RH showed different dynamics, with spatial and temporal variability across these sites. Estimated annual cumulative RS fluxes were 8.97 and 7.43 t C ha–1 yr–1 for the plain and hilly alfalfa-based sites, respectively, and 4.67 and 5.22 t C ha–1 yr–1 for the plain and hilly wheat-based sites, respectively. The RH components of RS were 4.26 and 3.52 t C ha–1 yr–1 for the plain and hilly alfalfa-based sites, respectively, and 3.89 and 2.45 t C ha–1 yr–1 for the plain and hilly wheat-based sites, respectively. A model with a combination of soil temperature and soil water content explained 43 % to 49 % and 33 % to 67 % of the annual variation of RS and RH, respectively. These findings help to extend our knowledge of Mediterranean cropping systems, although further studies are needed to clarify the effects of management practices on the modelling of soil respiration efflux.

Temporal variation in soil respiration and its sensitivity to temperature along a hydrological gradient in an alpine wetland of the Tibetan Plateau

Wetlands are predicted to experience lowered water tables due to permafrost degradation in the Tibetan Plateau. These changes may affect carbon cycle processes such as soil respiration (Rs). However, the magnitude, patterns and controls of Rs remain poorly understood in alpine wetlands with their distinct hydrological regimes. Here, we conducted a field study on Rs from 2012 to 2014 in three alpine ecosystems on the Tibetan Plateau—fen, wet meadow and meadow—with soil water decreases along hydrological gradients. From 2012 to 2014, the annual Rs was 128.9–193.3 g C m−2yr−1, 281.5–342.9 g C m−2yr−1, and 663.4–709.1 g C m−2yr−1 for the fen, wet meadow, and meadow, respectively. An abrupt increase in CO2 emissions was caused by the spring thawing of the frozen soil in the fen and wet meadow, contributing 20.4–37.6% and 13.2–17.4%, respectively, to the annual Rs. The diurnal variation in the Rs was site specific among the three ecosystems, with one peak at 1300 h in the fen and meadow and two peaks at 1300 h and 1900 h in the wet meadow. The temperature-independent components of the diurnal variation in Rs were generally explained by photosynthetically active radiation in the fen and wet meadow, but not in the meadow. The temperature sensitivity of the Rs (unconfounded Q10) varied significantly among the three ecosystems, with the highest values occurring in the wet meadow, implying that permafrost thaw-induced wetland drying from the fen to the wet meadow could enhance the response of CO2 emissions to climate warming but that further drying from the wet meadow to the meadow probably weakens the effect of warming on the Rs. Our study emphasized the important role of the hydrological regime in regulating the temporal variation in Rs and its response to climate warming.

Spatial Predictions and Associated Uncertainty of Annual Soil Respiration at the Global Scale

AbstractSoil respiration (Rs), the soil‐to‐atmosphere CO2 flux produced by microbes and plant roots, is a critical but uncertain component of the global carbon cycle. Our current understanding of the variability and dynamics is limited by the coarse spatial resolution of existing estimates. We predicted annual Rs and associated uncertainty across the world at 1‐km resolution using a quantile regression forest algorithm trained with observations from the global Soil Respiration Database spanning from 1961 to 2011. This model yielded a global annual Rs estimate of 87.9 Pg C/year with an associated global uncertainty of 18.6 (mean absolute error) and 40.4 (root mean square error) Pg C/year. The estimated annual heterotrophic respiration (Rh), derived from empirical relationships with Rs, was 49.7 Pg C/year over the same period. Predicted Rs rates and associated uncertainty varied widely across vegetation types, with the greatest predicted rates of Rs in evergreen broadleaf forests (accounting for 20.9% of global Rs). The greatest prediction uncertainties were in northern latitudes and arid to semiarid ecosystems, suggesting that these areas should be targeted in future measurement campaigns. This study provides predictions of Rs (and associated prediction uncertainty) at unprecedentedly high spatial resolution across the globe that could help constrain local‐to‐global process‐based models. Furthermore, it provides insights into the large variability of Rs and Rh across vegetation classes and identifies regions and vegetation types with poor model performance that should be prioritized for future data collection.

Effects of N addition and precipitation reduction on soil respiration and its components in a temperate forest

As the important pathway for soil carbon (C) fluxes from terrestrial ecosystems to the atmosphere, soil respiration (Rs) is strongly regulated by soil nitrogen (N) and water availability. Knowledge of the effect of soil N and moisture availability on Rs and its components (heterotrophic respiration (Rh) and autotrophic respiration (Ra)) is important understand soil C responses to climate change. Therefore, an 8-year field manipulation experiment to investigate the responses of Rs and its components to a reduction in the growing season precipitation amount with or without N addition was conducted in northern China. The results show that the average annual Rs, Rh, and Ra rates increased on average by 47%, 94%, and 38%, respectively, under the N addition treatments; increased on average by 18%, 53%, and 24%, respectively, under the precipitation reduction treatments; and increased on average by 5%, 4%, and 6%, respectively, under their interaction. The Rs and Rh were positively correlated with fine root biomass and microbial biomass, whereas the Ra was only related to the fine root biomass. Nitrogen addition and precipitation reduction significantly increased the Rs, Rh and Ra, possibly due to increased root biomass and microbial biomass. The Rs and Rh were not significantly affected by the interaction of N addition and precipitation reduction, whereas their combined effects significantly decreased the Ra. The Rs and Ra were also negatively correlated with soil total P concentration. The N addition significantly decreased the contribution of Rh to Rs compared with that of the control and precipitation reduction treatments, and precipitation reduction only significantly decreased the contribution of Rh to Rs compared with that of the control treatment in 2017. The results suggest that the N addition played a more important role than precipitation reduction in changing the contribution of Rh and Ra to Rs. Moreover, the N addition and precipitation reduction significantly increased the Q10 value of the Rs and Rh, suggesting that the N addition and precipitation reduction may alter the response of soil C emission to global climate warming to a certain degree. However, the study highlights the crucial importance of the impacts of N addition, precipitation reduction, and their interaction on Rs and its components and could improve predictions of the future states of the soil C cycle in response to global climate change.

Temporal changes of soil respiration in a subalpine meadow in the Heihe River Basin, Northwest China

Although soil respiration (Rs), as the largest flux of CO2 from terrestrial ecosystems to the atmosphere, has been measured and simulated widely, its drivers on different temporal scales in high-altitude regions are not well known. These driving factors were analysed at a site upstream of the Heihe River Basin based on measurements carried out using a field chamber over 3 years during three stages of the 5 cm soil layer: freezing and thawing (FT), completely frozen (CF), and completely thawed (CT). The analysis covered hysteresis with reference to soil respiration, photosynthesis, diurnal changes in soil temperature during CT in the growing season, and the contribution of carbon emissions outside the growing season (winter) to the total annual Rs. Results show that 1) the drivers of Rs on the diurnal scale during FT, CF, and CT were totally different: soil temperature (T5) and moisture (W5) at 5 cm depth for FT; air temperature near soil surface (Ta) and soil water content at 20 cm depth (W20) for CF, and T5 and photosynthetically active radiation (PAR) for CT; 2) at the diurnal scale during CT, Rs took 1.48–6.16 h less to reach its peak value than what T5 did, while 0.84–2.81 h more than PAR did; 3) cumulative Rs in winter was 132.31 g cm−2, accounted for approximately 16% of the annual carbon efflux from the subalpine meadow. Results suggest that the contribution of winter to the annual total Rs was large and should be considered in assessing carbon budget in high-altitude regions.

Nitrogen depositions increase soil respiration and decrease temperature sensitivity in a Moso bamboo forest

Nitrogen (N) deposition plays an important role in regulating forest productivity and microbial biomass and activities, ultimately influencing soil respiration (Rs). However, the effects of increasing atmospheric N depositions on Rs in subtropical Moso bamboo forests remain poorly understood. Here, we conducted a 4-year field experiment in a subtropical Moso bamboo forest to quantify the effect of simulated N depositions at four rates (0, 30, 60 and 90 kg N ha−1 yr-1) on Rs. The mean Rs rate of the control was 353.17 ± 53.23 mg CO2 m-2 h-1 or 30.75 ± 2.38 t CO2 ha-1 yr-1. Soil respiration showed significantly higher sensitivity (Q10) to soil temperature than to air temperature, and the Rs rate was significantly positively related to soil microbial biomass carbon, soil temperature, and NO3-. In response to N addition treatments of 30, 60, and 90 kg N ha-1 yr-1, the mean annual Rs increased by approximately 45.7%, 37.7%, and 13.0%, respectively, compared with the control. Nitrogen depositions decreased the temperature sensitivity of Rs, leading to predictions that they may be able to mitigate the priming effects of future climate warmings on Rs in Moso bamboo forests in the coming decades. Combined models based on the significant relationships between Rs rates, daily mean air temperatures, and hourly soil temperatures at a depth of 5 cm may reliably and feasibly estimate annual soil CO2 efflux. On average, soil emitted 470 kg CO2 ha-1 yr-1 per 1 kg N ha-1 yr-1 added, which declined when N addition surpassed the N saturation threshold of 60 kg N ha-1 yr-1. Our findings provide a method for estimating annual soil CO2 efflux and new insights into the effects of N deposition rates on soil CO2 efflux in Moso bamboo forests.

Spatial-Temporal Changes of Soil Respiration across China and the Response to Land Cover and Climate Change

Soil respiration (Rs) plays an important role in the carbon budget of terrestrial ecosystems. Quantifying the spatial and temporal variations in Rs in China at the regional scale helps improve our understanding of the variations in terrestrial carbon budgets that occur in response to global climate and environmental changes and potential future control measures. In this study, we used a regional-scale geostatistical model that incorporates gridded meteorological and pedologic data to evaluate the spatial Rs variations in China from 2000 to 2013. We analysed the relationship between Rs and environmental factors, and suggest management strategies that may help to keep the terrestrial carbon balance. The simulated results demonstrate that the mean annual Rs value over these 14 years was 422 g/m2/year, and the corresponding total amount was 4.01 Pg C/year. The Rs estimation displayed a clear spatial pattern and a slightly increasing trend. Further analysis also indicated that high Rs values may occur in areas that show a greater degree of synchronicity in the timing of their optimal temperature and moisture conditions. Moreover, cultivated vegetation exhibits higher Rs values than native vegetation. Finally, we suggest that specific conservation efforts should be focused on ecologically sensitive areas where the Rs values increase significantly.

Patterns of soil respiration and its temperature sensitivity in grassland ecosystems across China

Abstract. Soil respiration (Rs), a key process in the terrestrial carbon cycle, is very sensitive to climate change. In this study, we synthesized 54 measurements of annual Rs and 171 estimates of Q10 value (the temperature sensitivity of soil respiration) in grasslands across China. We quantitatively analyzed their spatial patterns and controlling factors in five grassland types, including temperate typical steppe, temperate meadow steppe, temperate desert steppe, alpine grassland, and warm, tropical grassland. Results showed that the mean (±SE) annual Rs was 582.0±57.9 g C m−2 yr−1 across Chinese grasslands. Annual Rs significantly differed among grassland types, and was positively correlated with mean annual temperature, mean annual precipitation, soil temperature, soil moisture, soil organic carbon content, and aboveground biomass, but negatively correlated with soil pH (p<0.05). Among these factors, mean annual precipitation was the primary factor controlling the variation of annual Rs among grassland types. Based on the overall data across Chinese grasslands, the Q10 values ranged from 1.03 to 8.13, with a mean (±SE) of 2.60±0.08. Moreover, the Q10 values varied largely within and among grassland types and soil temperature measurement depths. Among grassland types, the highest Q10 derived by soil temperature at a depth of 5 cm occurred in alpine grasslands. In addition, the seasonal variation of soil respiration in Chinese grasslands generally cannot be explained well by soil temperature using the van't Hoff equation. Overall, our findings suggest that the combined factors of soil temperature and moisture would better predict soil respiration in arid and semi-arid regions, highlight the importance of precipitation in controlling soil respiration in grasslands, and imply that alpine grasslands in China might release more carbon dioxide to the atmosphere under climate warming.

Measurement strategies to account for soil respiration temporal heterogeneity across diverse regions

Soil respiration (Rs) rates fluctuate daily and seasonally; therefore, the timing of measurements is critical when estimating the daily mean and scaling up to annual Rs rates. Temporal fluctuations also vary with climate and biome, yet the current recommendation for when to measure Rs (e.g., 09:00 to 12:00) has not been evaluated for different climates, biomes, and seasons. To provide more refined recommendations for measuring Rs, we: 1) analyzed the diurnal and seasonal fluctuations of Rs and tested the accuracy of typical measurement practices under different climates and biomes, and 2) identified the measurement frequency necessary for different climates and biomes to achieve certain levels of accuracy in estimating annual Rs. Across biomes, diurnal variation in Rs is considerable in spring and summer, moderate in autumn, and minimal in winter, and closely related with soil temperature. Based on these diurnal patterns, the best measurement time for estimating the daily mean Rs was 10:00 in all climates and biomes, which is within the recommended range of 9:00 and 12:00 previously identified for temperate forests. Measurements made between 20:00 and 23:00 also accurately estimated the daily mean Rs. Regions with high plant coverage over the year have lower seasonal variation and require less measurement frequency. For global scale estimates, Rs needs to be measured once per day to attain an accuracy of ±10% of the Rs population mean with 95% confidence, and once per month to achieve ±30% with a confidence of 80%. Results from this study provide guidelines that reduce measurement frequency while retaining reasonable accuracy for better Rs estimates using manual chamber systems.

Response of soil respiration and its components to experimental warming and water addition in a temperate Sitka spruce forest ecosystem

Future climate change is expected to alter the terrestrial carbon cycle through its impact on soil respiration. In this study, we determined the responses of soil respiration and its components to experimental warming with or without water addition. A replicated in situ heating (∼2 °C above ambient soil temperatures) and water addition (170 mm in total each year) experiment was carried out for the first time in a temperate plantation forest of Sitka spruce over the period 2014–2016. Rh was measured inside deep collars (35 cm deep) that excluded root growth, while Rs was measured using the static chamber approach and near-surface collars (5 cm deep) and Ra calculated by subtracting Rh from total soil respiration (Rs). Experimental warming significantly increased Rs, Rh and Rh/Rs, but had no effect on Ra. In contrast, none of the respiration components were affected by water addition. Warming increased annual Rh by 62% but had no effect on Ra. Overall, warming did not significantly increase annual Rs. Warming showed a stronger impact on Rs in the non-growing season but had a smaller impact in the growing season. Warming increased Ra in the non-growing season but decreased it in the growing season. The effects of warming on Rh were similar for the two periods. Our results highlight the differential response of Ra and Rh to warming, which was mediated by water addition or season. For this and other similar forest sites that don’t experience water limitation, global warming may have a positive feedback on atmospheric CO2 concentrations through enhanced soil respiration.

Effect of precipitation on soil respiration in a temperate broad-leaved forest

BackgroundFor understanding and evaluating a more realistic and accurate assessment of ecosystem carbon balance related with environmental change or difference, it is necessary to analyze the various interrelationships between soil respiration and environmental factors. However, the soil temperature is mainly used for gap filling and estimation of soil respiration (Rs) under environmental change. Under the fact that changes in precipitation patterns due to climate change are expected, the effects of soil moisture content (SMC) on soil respiration have not been well studied relative to soil temperature. In this study, we attempt to analyze relationship between precipitation and soil respiration in temperate deciduous broad-leaved forest for 2 years in Gwangneung.ResultsThe average soil temperature (Ts) measured at a depth of 5 cm during the full study period was 12.0 °C. The minimum value for monthly Ts was − 0.4 °C in February 2015 and 2.0 °C in January 2016. The maximum monthly Ts was 23.6 °C in August in both years. In 2015, annual precipitation was 823.4 mm and it was 1003.8 mm in 2016. The amount of precipitation increased by 21.9% in 2016 compared to 2015, but in 2015, it rained for 8 days more than in 2016. In 2015, the pattern of low precipitation was continuously shown, and there was a long dry period as well as a period of concentrated precipitation in 2016. 473.7 mm of precipitation, which accounted for about 51.8% of the precipitation during study period, was concentrated during summer (June to August) in 2016. The maximum values of daily Rs in both years were observed on the day when precipitation of 20 mm or more. From this, the maximum Rs value in 2015 was 784.3 mg CO2 m−2 h−1 in July when 26.8 mm of daily precipitation was measured. The maximum was 913.6 mg CO2 m−2 h−1 in August in 2016, when 23.8 mm of daily precipitation was measured. Rs on a rainy day was 1.5~1.6 times higher than it without precipitation. Consequently, the annual Rs in 2016 was about 12% higher than it was in 2015. It was shown a result of a 14% increase in summer precipitation from 2015.ConclusionsIn this study, it was concluded that the precipitation pattern has a great effect on soil respiration. We confirmed that short-term but intense precipitation suppressed soil respiration due to a rapid increase in soil moisture, while sustained and adequate precipitation activated Rs. In especially, it is very important role on Rs in potential activating period such as summer high temperature season. Therefore, the accuracy of the calculated values by functional equation can be improved by considering the precipitation in addition to the soil temperature applied as the main factor for long-term prediction of soil respiration. In addition to this, we believe that the accuracy can be further improved by introducing an estimation equation based on seasonal temperature and soil moisture.

Inconsistent autotrophic respiration but consistent heterotrophic respiration responses to 5-years nitrogen addition under natural and planted Pinus tabulaeformis forests in northern China

Increased atmospheric N deposition is known to have significant effects on soil respiration (Rs) in natural and planted forest ecosystems. Responses of Rs to such N enrichment has been widely investigated in forest ecosystems, both in natural and planted forests. As natural and planted forests differ in many aspects (e.g. species composition, forest management and succession stage, soil properties etc.), the responses of Rs in natural and planted forests to N addition may be different. However, so far, few studies have made a direct comparison between natural versus planted forests. To fill this gap, we have examined how autotrophic (Ra), heterotrophic (Rh) and total Rs respond to experimental N addition in a natural and a planted Pinus tabulaeformis forests in northern China. Three levels of N addition (CK = 0, LN = 50 and HN = 100 kg N ha−1 yr.−1) were applied monthly over five growing seasons. Soil respiration and its components, soil properties, microbial biomass (MBC), enzymes activity and fine root biomass were measured. After 5-years of N addition, our results showed that: (1) for all three treatments, mean annual Rs of the planted forest were significantly higher than that of the natural forest; (2) in the natural forest, mean annual Rh was reduced by 16.8 and 28.3% under LN and HN treatments, respectively, whereas in the planted forest, mean annual Rh was reduced by 14.4 and 18.3% under LN and HN treatments, respectively. (3) mean annual Ra was increased by 47.6 and 59.5% under LN and HN, respectively, in the natural forest. In contrast, in the planted forest, LN and HN both enhanced Ra to the same rate. In both natural and planted forests, the inhibition of Rh was largely associated with the decreased microbial biomass C (MBC) and reduced activity of the cellulose degrading enzyme. However, inconsistent patterns of Ra to N addition of different intensities in the natural vs. planted forests might be due to root ammonium toxicity under high N-availability scenarios. We demonstrated that the natural and planted forests may differ in their Rs responses to N addition, depending on different responses of Ra. Our results may have potential implications for forest management and predicting forest ecosystem carbon balance under future N scenarios.

Characteristics of accumulated soil carbon and soil respiration in temperate deciduous forest and alpine pastureland

BackgroundFor various reasons such as agricultural and economical purposes, land-use changes are rapidly increasing not only in Korea but also in the world, leading to shifts in the characteristics of local carbon cycle. Therefore, in order to understand the large-scale ecosystem carbon cycle, it is necessary first to understand vegetation on this local scale. As a result, it is essential to comprehend change of the carbon balance attributed by the land-use changes. In this study, we attempt to understand accumulated soil carbon (ASC) and soil respiration (Rs) related to carbon cycle in two ecosystems, artificially turned forest into pastureland from forest and a native deciduous temperate forest, resulted from different land-use in the same area.ResultsRs were shown typical seasonal changes in the alpine pastureland (AP) and temperate deciduous forest (TDF). The annual average Rs was 160.5 mg CO2 m− 2 h− 1 in the AP, but it was 405.1 mg CO2 m− 2 h− 1 in the TDF, indicating that the Rs in the AP was lower about 54% than that in the TDF. Also, ASC in the AP was 124.49 Mg C ha− 1 from litter layer to 30-cm soil depth. The ASC was about 88.9 Mg C ha− 1, and it was 71.5% of that of the AP. The temperature factors in the AP was high about 4 °C on average compared to the TDF. In AP, it was observed high amount of sunlight entering near the soil surface which is related to high soil temperature is due to low canopy structure. This tendency is due to the smaller emission of organic carbon that is accumulated in the soil, which means a higher ASC in the AP compared to the TDF.ConclusionsThe artificial transformation of natural ecosystems into different ecosystems is proceeding widely in the world as well as Korea. The change in land-use type is caused to make the different characteristics of carbon cycle and storage in same region. For evaluating and predicting the carbon cycle in the vegetation modified by the human activity, it is necessary to understand the carbon cycle and storage characteristics of natural ecosystems and converted ecosystems. In this study, we studied the characteristics of ecosystem carbon cycle using different forms in the same region. The land-use changes from a TDF to AP leads to changes in dominant vegetation. Removal of canopy increased light and temperature conditions and slightly decreased SMC during the growing season. Also, land-use change led to an increase of ASC and decrease of Rs in AP. In terms of ecosystem carbon sequestration, AP showed a greater amount of carbon stored in the soil due to sustained supply of above-ground liters and lower degradation rate (soil respiration) than TDF in the high mountains. This shows that TDF and AP do not have much difference in terms of storage and circulation of carbon because the amount of carbon in the forest biomass is stored in the soil in the AP.

Effects of forest cover types and environmental factors on soil respiration dynamics in a coastal sand dune of subtropical China

Trees on sand dunes are more sensitive to environmental changes because sandy soils have extremely low water holding capacity and nutrient availability. We investigated the dynamics of soil respiration (R s) for secondary natural Litsea forest and plantations of casuarina, pine, acacia and eucalyptus. Results show that significant diurnal variations of R s occurred in autumn for the eucalyptus species and in summer for the pine species, with higher mean soil respiration at night. However, significant seasonal variations of R s were found in all five forest stands. R s changed exponentially with soil temperatures at the 10-cm depth; the models explain 43.3–77.0% of R s variations. Positive relationships between seasonal R s and soil moisture varied with stands. The correlations were significant only in the secondary forest, and the eucalyptus and pine plantations. The temperature sensitivity parameter (Q 10 value) of R s ranged from 1.64 in casuarina plantation to 2.32 the in secondary forest; annual R s was highest in the secondary forest and lowest in the pine plantation. The results indicate that soil temperatures and moisture are the primary environmental controls of soil respiration and mainly act through a direct influence on roots and microbial activity. Differences in root biomass, quality of litter, and soil properties (pH, total N, available P, and exchangeable Mg) were also significant factors.

Underestimation of soil respiration in a desert ecosystem

Soil respiration (Rs), conventionally considered as net soil CO2 flux, is an important link in the terrestrial carbon cycle. However, existing estimates of Rs in drylands may be erroneous, because of the abiotic soil CO2 flux. This may seriously hamper our understanding of the carbon cycle of desert ecosystems. In this study, we monitored CO2 flux of natural and sterilized soils, and obtained the net and abiotic soil CO2 flux in a desert ecosystem in Ningxia, China, and then computed Rs by using these data. Daily Rs was much larger than the net soil CO2 flux both in the growing and non-growing season. Further analysis indicated that Rs was controlled by soil temperature (Ts) over the annual cycle, and an exponential model could provide a good prediction of Rs. During the growing season, Rs increased significantly with rising soil water content (VWC) (P<0.05). A bivariate (Ts and VWC) exponential–power model had a better performance for predicting Rs in the growing season than a Ts-only model. However, during the non-growing season, this model failed to simulate Rs. Our results suggest that Rs is greatly underestimated, and the exponential model is more applicable for annual Rs prediction.

Nitrogen fertilization stimulated soil heterotrophic but not autotrophic respiration in cropland soils: A greater role of organic over inorganic fertilizer

Nitrogen (N) enrichment may have considerable effects on soil carbon (C) fluxes. However, the responses of soil respiration (Rs) and especially its heterotrophic (Rh) and autotrophic (Ra) components to N fertilization remain controversial, and evidence on the impacts of N form and addition rate is lacking. We conducted a field experiment in a maize cropland in northeast China to investigate the responses of Rs, Rh, and Ra to different inorganic (IN) and/or organic (ON) N fertilization regimes, including no N addition (CK) and five N-fertilized treatments with a gradient ratio of IN to ON at 4:0 (IN4), 3:1 (IN3), 2:2 (IN2), 1:3 (IN1), and 0:4 (IN0). Annual Rs was higher in the N-fertilized treatments than CK, but only significantly so for IN1. Fertilization increased Rh from 118 to 123–149 g C m−2 with significant effects observed in all ON-fertilized treatments. However, fertilization did not affect Ra which varied at a range of 63–71 g C m−2. Rh was suppressed by excessive supply of ammonium and nitrate which was more effectively increased by IN than ON fertilization, but always increased with increasing extractable organic N and dissolved organic C which were higher in the treatments applied with more ON. Accordingly, a greater role of ON over IN fertilization was found in stimulating Rh. Rs (2.76–3.81) and Rh (2.67–3.28) had higher Q10 values than Ra (1.51–2.05). Application of N fertilizer, especially IN, enhanced the Q10 value of Ra, but decreased those of Rs and Rh. Unexpectedly, grain yield and aboveground biomass were reduced by IN fertilization, but increased with increasing ON fertilizer application rate. Overall, our findings highlight the significance of the form and addition rate of N fertilizer on soil C cycling and its feedback to climate change under N enrichment.

Long-term chamber measurements reveal strong impacts of soil temperature on seasonal and inter-annual variation in understory CO2 fluxes in a Japanese larch (Larix kaempferi Sarg.) forest

To understand climate change’s effect on understory CO2 flux components, we established automated chambers in a 56-year-old Japanese larch (Larix kaempferi Sarg.) forest in central Japan for long-term continuous measurements. Between 2006 and 2013, annual CO2 fluxes ranged from 7.0 to 8.4tCha−1yr−1 for soil respiration (Rs), 5.7–6.8tCha−1yr−1 for heterotrophic respiration (Rh), 9.3–10.7tCha−1yr−1 for total understory respiration (Ru), 2.6–3.5tCha−1yr−1 for understory gross primary production (GPPu), and 6.1–7.6tCha−1yr−1 for net understory CO2 exchange (NUE). Mean annual soil temperature (MATs), especially in spring, was positively related to annual Rs, Rh, Ru, and NUE. Based on the inter-annual relationship between MATs and annual understory CO2 effluxes, a 1°C MATs increase was estimated to increase annual effluxes by 25.1% for Rs and Rh, 14.4% for Ru, and 23.9% for NUE. The growing season CO2 flux components were weakly associated with soil moisture. However, during a short dry period in the summer of 2013, we observed a strong relationship between soil moisture and understory Rs, Rh, and Ru. GPPu was primarily controlled by the understory light intensity; GPPu during growing season increased where the canopy was disturbed by typhoons in the early growing seasons in 2007 (+9.8%) and 2012 (+24.4%) as well as during the 2013 growing season (+12.2%) due to a short drought. Understory CO2 effluxes in this larch forest will likely increase under global warming.

Potential bias of daily soil CO2 efflux estimates due to sampling time

Soil respiration (Rs) has been usually measured during daylight hours using manual chambers. This approach assumes that measurements made during a typical time interval (e.g., 9 to 11 am) represent the mean daily value; locally, this may not always be correct and could result in systematic bias of daily and annual Rs budgets. We propose a simple method, based on the temporal stability concept, to determine the most appropriate time of the day for manual measurements to capture a representative mean daily Rs value. We introduce a correction factor to adjust for biases due to non-optimally timed sampling. This approach was tested in a semiarid shrubland using 24 hr campaigns using two treatments: trenched plots and plots with shrubs. In general, we found optimum times were at night and potential biases ranged from −29 to + 40% in relation to the 24 hr mean of Rs, especially in trenched plots. The degree of bias varied between treatments and seasons, having a greater influence during the wet season when efflux was high than during the dry season when efflux was low. This study proposes a framework for improving local Rs estimates that informs how to decrease temporal uncertainties in upscaling to the annual total.