Deep Autotrophic Soil Respiration in Shrubland and Woodland Ecosystems in Central New Mexico

Quantifying the controls on soil respiration is important for understanding ecosystem physiology and for predicting the response of soil carbon reservoirs to climate change. The majority of soil respiration is typically considered to occur in the top 20–30 cm of soils. In desert soils, where organic matter concentrations tend to be low and plants are deeply rooted, deeper respiration might be expected. However, little is known about the depth distribution of respiration in dryland soils. Here we show that the average depth of soil respiration between pulse precipitation events is almost always greater than 20 cm and is frequently greater than 50 cm in two central New Mexico desert shrublands. The average depth of soil respiration in a pi?on-juniper woodland was shallower, between 5 and 40 cm. In the shrublands, 8‰ seasonal variations in the carbon isotope composition of soil-respired CO₂ (δ¹³C_r-soil) that correlate with vapor pressure deficit support root/rhizosphere respiration as the dominant source of soil CO₂. Such deep autotrophic respiration indicates that shrubs preferentially allocate photosynthate to deep roots when conditions near the surface are unfavorable. Therefore, respiration rates in these soils are not necessarily correlated with root biomass. The δ¹³C_r-soil values provide no evidence for CO₂ evolved from soil inorganic carbon. Our results also suggest that organic carbon cycling is rapid and efficient in these soils and that the δ¹³C value of CO₂ respired from soils in much of the southwestern US, and perhaps in other semiarid regions, varies seasonally by at least 4‰.     

Short-Term Response of Soil Enzyme Activity and Soil Respiration to Repeated Carbon Nanotubes Exposure

Nanomaterials such as single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs) may repeatedly enter the soil environment with unknown adverse consequences. To provide the information on the effects of repeated exposure of CNTs, we determined the response of soil enzyme activity and soil basal respiration (SBR) through a two-week incubation of farmland soil repeatedly treated with different concentrations of CNTs (100, 200, 500 mg kg^?1 for SWCNTs and 100, 500, 1000 mg Kg^?1 for MWCNTs). The activities of catalase, alkaline phosphatase, and invertase and SBR were measured after one- and two-time treatments. The repeated contamination of SWCNTs and MWCNTs repressed the activity of alkaline phosphatase and invertase in the 14-day incubation. Alkaline phosphatase and invertase were more sensitive indicators of CNTs’ contamination than catalase and soil basal respiration. High concentration of the SWCNTs stimulated SBR while the lower concentration suppressed SBR. The recurred exposure of SWCNTs and MWCNTs repressed the activity of catalase and invertase. The obtained results indicated that the soil microorganisms were suppressed under repeated pollution, as suggested by the same suppressed response of SBR between SWCNTs and MWCNTs treatment, except for the concentration of 500 mg kg^?1.     

Responses of Soil,Heterotrophic, and Autotrophic Respiration to Experimental Open-Field Soil Warming in a Cool-Temperate Deciduous Forest

How global warming will affect soil respiration (R _S) and its source components is poorly understood despite its importance for accurate prediction of global carbon (C) cycles. We examined the responses of R _S, heterotrophic respiration (R _H), autotrophic respiration (R _A), nitrogen (N) availability, and fine-root biomass to increased temperature in an open-field soil warming experiment. The experiment was conducted in a cool-temperate deciduous forest ecosystem in northern Japan. As this forest is subjected to strong temporal variation in temperature, on scales ranging from daily to seasonal, we also investigated the temporal variation in the effects of soil warming on R _S, R _H, and R _A. Soil temperature was continuously elevated by about 4.0°C from 2007 to 2014 using heating wires buried in the soil, and we measured soil respiratory processes in all four seasons from 2012 to 2014. Soil warming increased annual R _S by 32–45%, but the magnitude of the increase was different between the components: R _H and R _A were also stimulated, and increased by 39–41 and 17–18%, respectively. Soil N availability during the growing season and fine-root biomass were not remarkably affected by the warming treatment. We found that the warming effects varied seasonally. R _H increased significantly throughout the year, but the warming effect showed remarkable seasonal differences, with the maximum stimulation in the spring. This suggests that warmer spring temperature will produce a greater increase in CO₂ release than warmer summer temperatures. In addition, we found that soil warming reduced the temperature sensitivity (Q ₁₀) of R _S. Although the Q ₁₀ of both R _H and R _A tended to be reduced, the decrease in the Q ₁₀ of R _S was caused mainly by a decrease in the response of R _A to warming. These long-term results indicate that a balance between the rapid and large response of soil microbes and the acclimation of plant roots both play important roles in determining the response of R _S to soil warming, and must be carefully considered to predict the responses of soil C dynamics under future temperature conditions.     

Confounding Effects of Soil Moisture on the Relationship Between Ecosystem Respiration and Soil Temperature in Switchgrass

Understanding the response of ecosystem respiration (ER) to major environmental drivers is critical for estimating carbon sequestration and large-scale modeling research. Temperature effect on ER is modified by other environmental factors, mainly soil moisture, and such information is lacking for switchgrass (Panicum virgatum L.) ecosystems. The objective of this study was to examine seasonal variation in ER and its relationship with soil temperature (T _s) and moisture in a switchgrass field. ER from the nighttime net ecosystem CO₂ exchange measurements by eddy covariance system during the 2011 and 2012 growing seasons was analyzed. Nighttime ER ranged from about 2 (early growing season) to as high as 13 μmol m^?2 s^?1 (peak growing period) and showed a clear seasonality, with low rates during warm (>30 °C) and dry periods (<0.20 m³ m^?3 of soil water content). No single temperature or moisture function described variability in ER on the seasonal scale. However, an exponential temperature–respiration function explained over 50 % of seasonal variation in ER at adequate soil moisture (>0.20 m³ m^?3), indicating that soil moisture <0.20 m³ m^?3 started to limit ER. Due to the limitation of soil–atmosphere gas exchange, ER rates declined markedly in wet soil conditions (>0.35 m³ m^?3) as well. Consequently, both dry and wet conditions lowered temperature sensitivity of respiration (Q ₁₀). Stronger ER–T _s relationships were observed at higher soil moisture levels. These results demonstrate that soil moisture greatly influences the dynamics of ER and its relationship with T _s in drought prone switchgrass ecosystems.     

Effects of Forest Age on Soil Autotrophic and Heterotrophic Respiration Differ between Evergreen and Deciduous Forests

We examined the effects of forest stand age on soil respiration (SR) including the heterotrophic respiration (HR) and autotrophic respiration (AR) of two forest types. We measured soil respiration and partitioned the HR and AR components across three age classes ∼15, ∼25, and ∼35-year-old Pinus sylvestris var. mongolica (Mongolia pine) and Larix principis-rupprechtii (larch) in a forest-steppe ecotone, northern China (June 2006 to October 2009). We analyzed the relationship between seasonal dynamics of SR, HR, AR and soil temperature (ST), soil water content (SWC) and normalized difference vegetation index (NDVI, a plant greenness and net primary productivity indicator). Our results showed that ST and SWC were driving factors for the seasonal dynamics of SR rather than plant greenness, irrespective of stand age and forest type. For ∼15-year-old stands, the seasonal dynamics of both AR and HR were dependent on ST. Higher Q₁₀ of HR compared with AR occurred in larch. However, in Mongolia pine a similar Q₁₀ occurred between HR and AR. With stand age, Q₁₀ of both HR and AR increased in larch. For Mongolia pine, Q₁₀ of HR increased with stand age, but AR showed no significant relationship with ST. As stand age increased, HR was correlated with SWC in Mongolia pine, but for larch AR correlated with SWC. The dependence of AR on NDVI occurred in ∼35-year-old Mongolia pine. Our study demonstrated the importance of separating autotrophic and heterotrophic respiration components of SR when stimulating the response of soil carbon efflux to environmental changes. When estimating the response of autotrophic and heterotrophic respiration to environmental changes, the effect of forest type on age-related trends is required.     

Impact of Environmental Factors and Biological Soil Crust Types on Soil Respiration in a Desert Ecosystem

The responses of soil respiration to environmental conditions have been studied extensively in various ecosystems. However, little is known about the impacts of temperature and moisture on soils respiration under biological soil crusts. In this study, CO₂ efflux from biologically-crusted soils was measured continuously with an automated chamber system in Ningxia, northwest China, from June to October 2012. The highest soil respiration was observed in lichen-crusted soil (0.93±0.43 µmol m⁻² s⁻¹) and the lowest values in algae-crusted soil (0.73±0.31 µmol m⁻² s⁻¹). Over the diurnal scale, soil respiration was highest in the morning whereas soil temperature was highest in the midday, which resulted in diurnal hysteresis between the two variables. In addition, the lag time between soil respiration and soil temperature was negatively correlated with the soil volumetric water content and was reduced as soil water content increased. Over the seasonal scale, daily mean nighttime soil respiration was positively correlated with soil temperature when moisture exceeded 0.075 and 0.085 m³ m⁻³ in lichen- and moss-crusted soil, respectively. However, moisture did not affect on soil respiration in algae-crusted soil during the study period. Daily mean nighttime soil respiration normalized by soil temperature increased with water content in lichen- and moss-crusted soil. Our results indicated that different types of biological soil crusts could affect response of soil respiration to environmental factors. There is a need to consider the spatial distribution of different types of biological soil crusts and their relative contributions to the total C budgets at the ecosystem or landscape level.     

Biological Soil Crust Microsites Are the Main Contributor to Soil Respiration in a Semiarid Ecosystem

Biological soil crusts (BSCs) are a key biotic component of dryland ecosystems worldwide. However, most studies carried out to date on carbon (C) fluxes in these ecosystems, such as soil respiration, have neglected them. We conducted a 3.5-year field experiment to evaluate the spatio-temporal heterogeneity of soil respiration in a semiarid Stipa tenacissima steppe and to assess the contribution of BSC-dominated areas to the annual soil respiration of the whole ecosystem. We selected the six most frequent microsites in the study area: Stipa tussocks (ST), Retama sphaerocarpa shrubs (RS), and open areas with very low (<5% BSC cover, BS), low, medium and high cover of well-developed BSCs. Soil respiration rates did not differ among BSC-dominated microsites but were significantly higher and lower than those found in BS and ST microsites, respectively. A model using soil temperature and soil moisture accounted for over 85% of the temporal variation in soil respiration throughout the studied period. Using this model, we estimated a range of 240.4–322.6 g C m⁻² y⁻¹ released by soil respiration at our study area. Vegetated (ST and RS) and BSC-dominated microsites accounted for 37 and 42% of this amount, respectively. Our results indicate that accounting for the spatial heterogeneity in soil respiration induced by BSCs is crucial to provide accurate estimations of this flux at the ecosystem level. They also highlight that BSC-dominated areas are the main contributor to the total C released by soil respiration and, therefore, must be considered when estimating C budgets in drylands.     

Effect of Simulated Climate Change on Soil Respiration in a Mediterranean-Type Ecosystem: Rainfall and Habitat Type are More Important than Temperature or the Soil Carbon Pool

Soil respiration (R _S) is known to be highly sensitive to different environmental factors, such as temperature, precipitation, and the soil carbon (C) pool. Thus, the scenario of global change expected for the coming decades might imply important consequences for R _S dynamics. In addition, all of these factors may have an interactive effect, and the consequences are often confounded. We performed a field experiment to analyze the effect of soil moisture and habitat type on R _S in a Mediterranean-type ecosystem by simulating three possible climate scenarios differing in the precipitation amount during summer (drier, wetter, and current precipitation pattern) in the main successional habitats in the area (forest, shrubland, and open habitat). We also considered other factors that would affect R _S, such as the soil C pool and microbial biomass. By the use of structural-equation modeling (SEM), we disentangled the interactive effects of the different factors affecting R _S. A higher simulated precipitation boosted R _S for the different habitats across the sampling period (14.6% higher respect to control), whereas the more severe simulated drought reduced it (19.2% lower respect to control), a trend that was similar at the daily scale. Temperature had, by contrast, scant effects on R _S. The SEM analysis revealed a positive effect of moisture and canopy cover on R _S, whereas the effect of temperature was weaker and negative. Soil C pool and microbial biomass did not affect R _S. We conclude that the precipitation changes expected for the coming decades would play a more important role in controlling R _S than would other factors. Thus, the projected changes in the precipitation pattern may have much more profound direct effects on R _S than will the projected temperature increases.     

Temperature Increases Soil Respiration Across Ecosystem Types and Soil Development,But Soil Properties Determine the Magnitude of This Effect

Ecosystems - Soil carbon losses to the atmosphere, via soil heterotrophic respiration, are expected to increase in response to global warming, resulting in a positive carbon-climate feedback....     

Consequences of Cool-Season Drought-Induced Plant Mortality to Chihuahuan Desert Grassland Ecosystem and Soil Respiration Dynamics

Predicted reductions of cool-season rainfall may expand and accelerate drought-induced plant mortality currently unfolding across the Southwest US. To assess how repeated plant mortality affects ecosystem functional attributes, we quantified net ecosystem CO₂ exchange (NEE), ecosystem respiration (R _eco), and gross ecosystem photosynthesis (GEP) responses to precipitation (P) at a semidesert grassland over spring (Feb 1–Apr 30) and summer (June 15–Oct 1) plant-active periods across eight years, including two with distinct patterns of extensive species-specific mortality. In addition, we quantified daily soil respiration (R _soil) in high- (56–88%) and low-mortality (8–27%) plots the summer following the most recent mortality event. Plant mortality coincided with severely dry cool-season conditions (Dec 1–Apr 30). We found a positive relationship between springtime P and GEP, and that springtime conditions influenced GEP response to summer rainfall. High springtime R _eco/GEP ratios followed plant mortality, suggesting increased available carbon after mortality was rapidly decomposed. R _soil in low-mortality plots exceeded high-mortality plots over drier summer periods, likely from more root respiration. However, total cumulative R _soil did not differ between plots, as variation in surviving plant conditions resulted in high and low C-yielding plots within both plot types. Vegetation status in high C-yielding R _soil plots was similar to that across the grassland, suggesting R _soil from such areas underlay higher R _eco. This, coupled to springtime drought constraints to GEP, resulted in positive NEE under summer P accumulations that previously supported C-sink activity. These findings indicate that predicted lower cool-season precipitation may strongly and negatively affect summer season productivity in these semiarid grasslands.     

Nitrification and Autotrophic Nitrifying Bacteria in a Hydrocarbon-Polluted Soil

In vitro ammonia-oxidizing bacteria are capable of oxidizing hydrocarbons incompletely. This transformation is accompanied by competitive inhibition of ammonia monooxygenase, the first key enzyme in nitrification. The effect of hydrocarbon pollution on soil nitrification was examined in situ. In a microcosm study, adding diesel fuel hydrocarbon to an uncontaminated soil (agricultural unfertilized soil) treated with ammonium sulfate dramatically reduced the amount of KCl-extractable nitrate but stimulated ammonium consumption. In a soil with long history of pollution that was treated with ammonium sulfate, 90% of the ammonium was transformed into nitrate after 3 weeks of incubation. Nitrate production was twofold higher in the contaminated soil than in the agricultural soil to which hydrocarbon was not added. To assess if ammonia-oxidizing bacteria acquired resistance to inhibition by hydrocarbon, the contaminated soil was reexposed to diesel fuel. Ammonium consumption was not affected, but nitrate production was 30% lower than nitrate production in the absence of hydrocarbon. The apparent reduction in nitrification resulted from immobilization of ammonium by hydrocarbon-stimulated microbial activity. These results indicated that the hydrocarbon inhibited nitrification in the noncontaminated soil (agricultural soil) and that ammonia-oxidizing bacteria in the polluted soil acquired resistance to inhibition by the hydrocarbon, possibly by increasing the affinity of nitrifying bacteria for ammonium in the soil.     

Soil Respiration in Different Agricultural and Natural Ecosystems in an Arid Region

The variation of different ecosystems on the terrestrial carbon balance is predicted to be large. We investigated a typical arid region with widespread saline/alkaline soils, and evaluated soil respiration of different agricultural and natural ecosystems. Soil respiration for five ecosystems together with soil temperature, soil moisture, soil pH, soil electric conductivity and soil organic carbon content were investigated in the field. Comparing with the natural ecosystems, the mean seasonal soil respiration rates of the agricultural ecosystems were 96%–386% higher and agricultural ecosystems exhibited lower CO₂ absorption by the saline/alkaline soil. Soil temperature and moisture together explained 48%, 86%, 84%, 54% and 54% of the seasonal variations of soil respiration in the five ecosystems, respectively. There was a significant negative relationship between soil respiration and soil electrical conductivity, but a weak correlation between soil respiration and soil pH or soil organic carbon content. Our results showed that soil CO₂ emissions were significantly different among different agricultural and natural ecosystems, although we caution that this was an observational, not manipulative, study. Temperature at the soil surface and electric conductivity were the main driving factors of soil respiration across the five ecosystems. Care should be taken when converting native vegetation into cropland from the point of view of greenhouse gas emissions.     

Organic Matter and Water Addition Enhance Soil Respiration in an Arid Region

Climate change is generally predicted to increase net primary production, which could lead to additional C input to soil. In arid central Asia, precipitation has increased and is predicted to increase further. To assess the combined effects of these changes on soil CO₂ efflux in arid land, a two factorial manipulation experiment in the shrubland of an arid region in northwest China was conducted. The experiment used a nested design with fresh organic matter and water as the two controlled parameters. It was found that both fresh organic matter and water enhanced soil respiration, and there was a synergistic effect of these two treatments on soil respiration increase. Water addition not only enhanced soil C emission, but also regulated soil C sequestration by fresh organic matter addition. The results indicated that the soil CO₂ flux of the shrubland is likely to increase with climate change, and precipitation played a dominant role in regulating soil C balance in the shrubland of an arid region.     

Different Effects of Spring and Summer Droughts on Ecosystem Carbon and Water Exchanges in a Semiarid Shrubland Ecosystem in Northwest China

Ecosystems - Extensive revegetation and conservation programs have been initiated in semiarid desert areas in northern China for over 10&nbsp;years. However, our knowledge on how drought...     

Vegetation Affects the Relative Abundances of Dominant Soil Bacterial Taxa and Soil Respiration Rates in an Upland Grassland Soil

Plant-derived organic matter inputs are thought to be a key driver of soil bacterial community composition and associated soil processes. We sought to investigate the role of acid grassland vegetation on soil bacterial community structure by assessing bacterial diversity in combination with other soil variables in temporally and spatially distinct samples taken from a field-based plant removal experiment. Removal of aboveground vegetation resulted in reproducible differences in soil properties, soil respiration and bacterial diversity. Vegetated soils had significantly increased carbon and nitrogen concentrations and exhibited higher rates of respiration. Molecular analyses revealed that the soils were broadly dominated by Alphaproteobacterial and Acidobacterial lineages, with increased abundances of Alphaproteobacteria in vegetated soils and more Acidobacteria in bare soils. This field-based study contributes to a growing body of evidence documenting the effect of soil nutrient status on the relative abundances of dominant soil bacterial taxa, with Proteobacterial taxa dominating over Acidobacteria in soils exhibiting higher rates of C turnover. Furthermore, we highlight the role of aboveground vegetation in mediating this effect by demonstrating that plant removal can alter the relative abundances of dominant soil taxa with concomitant changes in soil CO₂-C efflux.     

Climate Cycles, Geomorphological Change, and the Interpretation of Soil and Ecosystem Development

We evaluated changes in temperature and precipitation associated with climate change, subsidence, and erosion on a chronosequence of sites across Hawaii. The sites range in age from 0.3 to 4100 ky, and the current temperature and precipitation are similar at all sites. Interpretations of fossil pollen records suggest that cooler, dryer conditions prevailed in windward Hawaii during the last glacial period. If the previous glacial periods were similar, the 20-, 150-, and 1400-ky-old sites would have spent 60% or more of their development under relatively cool and dry conditions, whereas the 0.3- and 2.1-ky-old sites have experienced only the warmer, wetter climate of the present interglacial. Subsidence and erosion have also affected the temperature and precipitation of these sites over time; in the past, some of them have been in the dry air above the trade wind inversion or in the lee of larger mountains. Combining these components of change, we estimate that the average temperature over the history of Pleistocene-aged sites (20, 150, and 1400 ky) was up to 2.2°C cooler and that the average precipitation was only about 50% of current values. Under current conditions, it would take only 230 ky for as much water to leach through the 1400-ky-old site as we calculate has leached over 1400 ky. Incorporating more reasonable assumptions about environmental history has the potential to allow more powerful interpretations of chronosequence data and thereby improve the predictive potential of models of soil and ecosystem development. Received 21 December 1999; accepted 1 June 2000.     

Interactions Among Abiotic Drivers,Disturbance and Gross Ecosystem Carbon Exchange on Soil Respiration from Subtropical Pine Savannas

Globally, soil CO₂ efflux rates (F_s) have been linked to changes in soil water content (SWC), rainfall and temperature and/or productivity. However, within an ecosystem, F_s can vary based on site structure and function, which can be affected by a combination of abiotic and biotic factors. This becomes particularly important when an ecosystem is faced with disturbances, such as drought or fire. Site-specific compensatory responses to disturbances may therefore alter C mineralization, as well as root respiration. Hence, single location F_s estimates may not be a representative for ecosystems across their distributional ranges. We conducted a 6-year study along an edaphic moisture gradient of longleaf pine ecosystems that were maintained with prescribed fire, using eddy covariance and soil respiration measurements to address how F_s varies with changes in ecosystem structure and function, as well as disturbances. Lower air temperatures (T_air) decreased F_s at all sites, but that response was also affected by productivity and SWC. Productivity significantly altered F_s rates at all sites, especially when we accounted for changes in temperature and SWC. Plant regrowth post-fire temporarily increased F_s (10–40%), whereas drought reduced F_s at all sites. Our results show that site productivity, F_s and the degree to which ecosystems adapt to climate variations and disturbance can be site specific. Hence, model forecasting of carbon dynamics would strongly benefit from multi-location measurements of F_s across the distributional range of an ecosystem.     

Watershed-scale Land Use Change Increases Ecosystem Metabolism in an Agricultural Stream

Ecosystems - Stream metabolism, in the form of gross primary production (GPP) and ecosystem respiration (ER), is an important metric of stream ecosystem function, given GPP and ER are integrative...