Effects of grazing and soil micro-climate on decomposition rates in a spatio-temporally heterogeneous grassland

Grazing and seasonal variation in precipitation and temperature are important controls of soil and plant processes in grasslands. As these ecosystems store up to 30% of the world’s belowground carbon (C), it is important to understand how this variability affects mineral soil C pools/fluxes, and how C cycling might be affected by changes in precipitation and temperature, due to climate change. The aim of this study was to investigate the effects of grazing and differences in soil temperature and moisture on standard organic matter (OM) decomposition rates (cotton cloth) incubated in the top 10 cm soil of grasslands with variable topography in Yellowstone National Park (YNP) during the 2004 growing season. Grazing did not affect soil temperature, moisture, cotton cloth decomposition rates, soil bulk density, soil C and N concentrations, or soil C:N ratios. However, a large spatio-temporal variability in decomposition was observed: cotton cloth decomposition was positively related to soil moisture and soil C and N concentrations, and negatively to soil temperature. Highest decomposition rates were found in wetter slope bottom soils [season averages of decomposition given as rate of decomposition (cotton rotting rate = CRR) = 23–26%] and lower rates in drier, hill-top soils (season averages, CRR = 20%). Significantly higher decomposition rates were recorded in spring, early summer and early fall when soils were moist and cool (spring, CRR = 25%; early summer, CRR = 26%; fall, CRR = 20%) compared to mid-summer (CRR = 18%) when soils were dry and warm. Our findings suggest that climate-change related decreases in precipitation and increases in temperature predicted for North American grasslands would decrease soil OM decomposition in YNP, which contrasts the general assumption that increases in temperature would accelerate OM decomposition rates.     

Increases in soil respiration following labile carbon additions linked to rapid shifts in soil microbial community composition

Organic matter decomposition and soil CO₂ efflux are both mediated by soil microorganisms, but the potential effects of temporal variations in microbial community composition are not considered in most analytical models of these two important processes. However, inconsistent relationships between rates of heterotrophic soil respiration and abiotic factors, including temperature and moisture, suggest that microbial community composition may be an important regulator of soil organic matter (SOM) decomposition and CO₂ efflux. We performed a short-term (12-h) laboratory incubation experiment using tropical rain forest soil amended with either water (as a control) or dissolved organic matter (DOM) leached from native plant litter, and analyzed the effects of the treatments on soil respiration and microbial community composition. The latter was determined by constructing clone libraries of small-subunit ribosomal RNA genes (SSU rRNA) extracted from the soil at the end of the incubation experiment. In contrast to the subtle effects of adding water alone, additions of DOM caused a rapid and large increase in soil CO₂ flux. DOM-stimulated CO₂ fluxes also coincided with profound shifts in the abundance of certain members of the soil microbial community. Our results suggest that natural DOM inputs may drive high rates of soil respiration by stimulating an opportunistic subset of the soil bacterial community, particularly members of the Gammaproteobacteria and Firmicutes groups. Our experiment indicates that variations in microbial community composition may influence SOM decomposition and soil respiration rates, and emphasizes the need for in situ studies of how natural variations in microbial community composition regulate soil biogeochemical processes.     

Landscape patterns of litter decomposition in alpine tundra

A two-year study of the decomposition of alpine avens (Acomastylis rossii) foliage in alpine tundra of the Front Range of Colorado demonstrated a strong landscape-mediated effect on decay rates. Litter on sites with intermediate amounts of snowpack decayed more rapidly than litter on sites with larger or smaller amounts of snow. Annual decay constants (k-values) of this foliage ranged from-0.33 in dry tundra to-0.52 in moist tundra to-0.47 in the wettest habitat. No site differences in mass loss of litter were detected until late winter-early spring of the first year of decomposition, when significantly faster decomposition was observed for litter beneath the snowpack. In spite of obvious landscape-related patterns in rates of litter decomposition, total microarthropod densities in the top 5 cm of soil did not differ among habitats. However, the relative abundance of the oribatid and prostigmatid mites did vary significantly across the landscape in relation to the moisture gradient. Oribatid mites comprised a greater proportion of total mites in wetter areas. Microarthropod densities and composition, as well as patterns of decomposition, were compared with previous alpine, as well as arctic tundra, studies. The effects of soil invertebrates on decomposition rates in the alpine were evaluated with a mushroom litterbag decomposition experiment. Naphthalene was used to exclude fauna from a subset of litterbags placed in mesic and xeric habitats. Mushrooms without naphthalene additions decayed significantly faster in the mesic sites. Densities of invertebrates were also greater on mushrooms in these mesic sites. Mushrooms placed in xeric sites generally lacked fauna. Thus, both the activities and the composition of the detritus-based food web appear to change substantially across the moisture gradient found in alpine tundra.     

Carbon pools and fluxes along an environmental gradient in northern Arizona

Carbon pools and fluxes were quantified along an environmentalgradient in northern Arizona. Data are presented on vegetation, litter, andsoil C pools and soil CO₂ fluxesfrom ecosystems ranging from shrub-steppe through woodlands to coniferousforest and the ecotones in between. Carbon pool sizes and fluxes in thesesemiarid ecosystems vary with temperature and precipitation and are stronglyinfluenced by canopy cover. Ecosystem respiration is approximately 50percent greater in the more mesic, forest environment than in the dryshrub-steppe environment. Soil respiration rates within a site varyseasonally with temperature but appear to be constrained by low soilmoisture during dry summer months, when approximately 75% of totalannual soil respiration occurs. Total annual amount of CO₂ respired across all sites ispositively correlated with annual precipitation and negatively correlatedwith temperature. Results suggest that changes in the amount and periodicityof precipitation will have a greater effect on C pools and fluxes than willchanges in temperature in the semiarid Southwestern United States.     

Arctic mosses govern below-ground environment and ecosystem processes

Mosses dominate many northern ecosystems and their presence is integral to soil thermal and hydrological regimes which, in turn, dictate important ecological processes. Drivers, such as climate change and increasing herbivore pressure, affect the moss layer thus, assessment of the functional role of mosses in determining soil characteristics is essential. Field manipulations conducted in high arctic Spitsbergen (78° N), creating shallow (3 cm), intermediate (6 cm) and deep (12 cm) moss layers over the soil surface, had an immediate impact on soil temperature in terms of both average temperatures and amplitude of fluctuations. In soil under deep moss, temperature was substantially lower and organic layer thaw occurred 4 weeks later than in other treatment plots; the growing season for vascular plants was thereby reduced by 40%. Soil moisture was also reduced under deep moss, reflecting the influence of local heterogeneity in moss depth, over and above the landscape-scale topographic control of soil moisture. Data from field and laboratory experiments show that moss-mediated effects on the soil environment influenced microbial biomass and activity, resulting in warmer and wetter soil under thinner moss layers containing more plant-available nitrogen. In arctic ecosystems, which are limited by soil temperature, growing season length and nutrient availability, spatial and temporal variation in the depth of the moss layer has significant repercussions for ecosystem function. Evidence from our mesic tundra site shows that any disturbance causing reduction in the depth of the moss layer will alleviate temperature and moisture constraints and therefore profoundly influence a wide range of ecosystem processes, including nutrient cycling and energy transfer.     

Annual variation in soil respiration and its components in a coppice oak forest in Central Italy

In order to investigate the annual variation of soil respiration and its components in relation to seasonal changes in soil temperature and soil moisture in a Mediterranean mixed oak forest ecosystem, we set up a series of experimental treatments in May 1999 where litter (no litter), roots (no roots, by trenching) or both were excluded from plots of 4 m². Subsequently, we measured soil respiration, soil temperature and soil moisture in each plot over a year after the forest was coppiced. The treatments did not significantly affect soil temperature or soil moisture measured over 0–10 cm depth. Soil respiration varied markedly during the year with high rates in spring and autumn and low rates in summer, coinciding with summer drought, and in winter, with the lowest temperatures. Very high respiration rates, however, were observed during the summer immediately after rainfall events. The mean annual rate of soil respiration was 2.9 µ mol m^?2 s^?1, ranging from 1.35 to 7.03 µmol m^?2 s^?1. Soil respiration was highly correlated with temperature during winter and during spring and autumn whenever volumetric soil water content was above 20%. Below this threshold value, there was no correlation between soil respiration and soil temperature, but soil moisture was a good predictor of soil respiration. A simple empirical model that predicted soil respiration during the year, using both soil temperature and soil moisture accounted for more than 91% of the observed annual variation in soil respiration. All the components of soil respiration followed a similar seasonal trend and were affected by summer drought. The Q₁₀ value for soil respiration was 2.32, which is in agreement with other studies in forest ecosystems. However, we found a Q₁₀ value for root respiration of 2.20, which is lower than recent values reported for forest sites. The fact that the seasonal variation in root growth with temperature in Mediterranean ecosystems differs from that in temperate regions may explain this difference. In temperate regions, increases in size of root populations during the growing season, coinciding with high temperatures, may yield higher apparent Q₁₀ values than in Mediterranean regions where root growth is suppressed by summer drought. The decomposition of organic matter and belowground litter were the major components of soil respiration, accounting for almost 55% of the total soil respiration flux. This proportion is higher than has been reported for mature boreal and temperate forest and is probably the result of a short‐term C loss following recent logging at the site. The relationship proposed for soil respiration with soil temperature and soil moisture is useful for understanding and predicting potential changes in Mediterranean forest ecosystems in response to forest management and climate change.     

Landscape and Ecosystem-Level Controls on Net Carbon Dioxide Exchange along a Natural Moisture Gradient in Canadian Low Arctic Tundra

Our understanding of the controls and magnitudes of regional CO₂ exchanges in the Arctic are limited by uncertainties due to spatial heterogeneity in vegetation across the landscape and temporal variation in environmental conditions through the seasons. We measured daytime net ecosystem CO₂ exchange and each of its component fluxes in the three major tundra ecosystem-types that typically occur along natural moisture gradients in the Canadian Low Arctic biweekly during the full snow-free season of 2004. In addition, we used a plant-removal treatment to compare the contribution of bulk soil organic matter to total respiratory CO₂ loss among these ecosystems. Net CO₂ exchange rates varied strongly, but not consistently, among ecosystems in the spring and summer phases as a result of ecosystem-specific and differing responses of gross photosynthesis and respiration to temporal variation in environmental conditions. Overall, net carbon gain was largest in the wet sedge ecosystem and smallest in the dry heath. Our measures of CO₂ flux variation within each ecosystem were frequently most closely correlated with air or soil temperatures during each seasonal phase. Nevertheless, a particularly large rainfall event in early August rapidly decreased respiration rates and stimulated gross photosynthetic rates, resulting in peak rates of net carbon gain in all ecosystems. Finally, the bulk soil carbon contribution to total respiration was relatively high in the birch hummock ecosystem. Together, these results demonstrate that the relative influences of moisture and temperature as primary controls on daytime net ecosystem CO₂ exchange and its component fluxes differ in fundamental ways between the landscape and ecosystem scales. Furthermore, they strongly suggest that carbon cycling responses to environmental change are likely to be highly ecosystem-specific, and thus to vary substantially across the low arctic landscape. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Soil respiration of forest ecosystems in Japan and global implications

Within terrestrial ecosystems, soil respiration is one of the largest carbon flux components. We discuss the factors controlling soil respiration, while focusing on research conducted at the Takayama Experimental Site. Soil respiration was affected by soil temperature, soil moisture, rainfall events, typhoons, and root respiration. We consider the temporal and spatial variability of soil respiration at the Takayama Experimental Site and review the variability of annual soil respiration in Japanese forests. In the 26 compiled studies, the values of annual soil respiration ranged from 203 to 1,290 g C m⁻² year⁻¹, with a mean value of 669 g C m⁻² year⁻¹ (SD=264, CV=40). We note the need for more studies and data synthesis for the accurate prediction of soil respiration and soil carbon dynamics in Japanese forests. Finally, several methods for measuring soil respiration rates are compared and the implications of soil respiration rates for global climate change are discussed.     

Abiotic determinants of the fynbos/succulent karoo boundary,South Africa

This study investigates the influence of texture, soil moisture and nutrient status on the growth and survival of seedlings of two typical fynbos (Leucadendron pubescens and Passerina vulgaris) and succulent karoo (Ruschia spp.) species, which grow in the boundary zone between these two vegetation types. Seedlings of each species were grown in shalederived and sandstone‐derived soils and under xeric and mesic regimes. Under the xeric regime, the shale‐derived and sandstone‐derived soils represented fine and coarse‐textured soils, respectively. Under the mesic regime, the same soils represented nutrient‐rich and nutrient‐poor soils, respectively. The seedlings of both fynbos species died rapidly under the xeric regime, irrespective of soil type. In contrast, the succulent karoo seedlings survived for over 77 days without water. Under mesic conditions, the fynbos seedlings grew faster than the succulent karoo seedlings, irrespective of soil type. Fynbos seedlings appear to be directly limited by the environment (moisture and salinity), whereas succulent karoo seedlings may be limited by interactions with other plants.     

Soil animal responses to moisture availability are largely scale,not ecosystem dependent: insight from a cross‐site study

Climate change will result in reduced soil water availability in much of the world either due to changes in precipitation or increased temperature and evapotranspiration. How communities of mites and nematodes may respond to changes in moisture availability is not well known, yet these organisms play important roles in decomposition and nutrient cycling processes. We determined how communities of these organisms respond to changes in moisture availability and whether common patterns occur along fine‐scale gradients of soil moisture within four individual ecosystem types (mesic, xeric and arid grasslands and a polar desert) located in the western United States and Antarctica, as well as across a cross‐ecosystem moisture gradient (CEMG) of all four ecosystems considered together. An elevation transect of three sampling plots was monitored within each ecosystem and soil samples were collected from these plots and from existing experimental precipitation manipulations within each ecosystem once in fall of 2009 and three times each in 2010 and 2011. Mites and nematodes were sorted to trophic groups and analyzed to determine community responses to changes in soil moisture availability. We found that while both mites and nematodes increased with available soil moisture across the CEMG, within individual ecosystems, increases in soil moisture resulted in decreases to nematode communities at all but the arid grassland ecosystem; mites showed no responses at any ecosystem. In addition, we found changes in proportional abundances of mite and nematode trophic groups as soil moisture increased within individual ecosystems, which may result in shifts within soil food webs with important consequences for ecosystem functioning. We suggest that communities of soil animals at local scales may respond predictably to changes in moisture availability regardless of ecosystem type but that additional factors, such as climate variability, vegetation composition, and soil properties may influence this relationship over larger scales.     

干旱半干旱区不同环境因素对土壤呼吸影响研究进展

土壤呼吸是全球陆地生态系统碳循环的重要环节,也是全球气候变化的关键生态过程。阐明和探讨影响土壤呼吸的各类环境因素,对准确评估陆地生态系统碳收支具有重要意义。干旱半干旱区是陆地生态系统的重要组成部分,研究该区域影响土壤呼吸的环境因素有助于深刻了解干旱半干旱区土壤碳循环过程。就土壤温度、土壤水分、降水、土壤有机质等非生物因子及植被类型、地上、地下生物量、土壤凋落物等生物因子两个方面对土壤呼吸的影响进行了综述。以干旱半干旱区的研究进展为主要论述对象,在上述因素中重点阐述了土壤温度、水分及其耦合作用下土壤呼吸的响应,并就土壤呼吸的Q10值及各影响因素间的交互作用进行归纳总结。在此基础上,说明了土壤温度和水分是影响干旱半干旱区土壤呼吸的主要因素。为了更准确的估算干旱半干旱区土壤呼吸速率,综合分析多种因子的交互影响,提出目前土壤呼吸研究存在的问题和今后重点关注的方向:1)不同尺度下干旱半干旱区土壤呼吸的研究;2)荒漠生态系统土壤呼吸研究;3)非生长季土壤呼吸研究;4)多因素协同作用土壤呼吸模型建立;5)测量方法的改进与完善。     

Representative estimates of soil and ecosystem respiration in an old beech forest

Respiration has been proposed to be the main determinant of the carbon balance in European forests and is thus essential for our understanding of the carbon cycle. However, the choice of experimental design strongly affects estimates of annual respiration and of the contribution of soil respiration to total ecosystem respiration. In a detailed study of ecosystem and soil respiration fluxes in an old unmanaged deciduous forest in Central Germany over 3 years (2000–2002), we combined soil chamber and eddy covariance measurements to obtain a comprehensive picture of respiration in this forest. The closed portable chambers offered to investigate spatial variability of soil respiration and its controls while the eddy covariance system offered continuous measurements of ecosystem respiration. Over the year, both fluxes were mainly correlated with temperature. However, when soil moisture sank below 23 vol.% in the upper 6 cm, water limitations also became apparent. The temporal resolution of the eddy covariance system revealed that relatively high respiration rates occurred during budbreak due to increased metabolic activity and after leaf fall because of increased decomposition. Spatial variability in soil respiration rates was large and correlated with fine root biomass (r ² = 0.56) resulting in estimates of annual efflux varying across plots from 730 to 1,258 (mean 898) g C m⁻² year⁻¹. Power function calculations showed that achieving a precision in the soil respiration estimate of 20% of the full population mean at a confidence level of 95%, requires about eight sampling locations. Our results can be used as guidelines to improve the representativeness of soil respiration measurements by nested sampling designs, being applied in long-term and large-scale carbon sequestration projects such as FLUXNET and CarboEurope.     

Moisture Effects on Temperature Sensitivity of CO<Subscript>2</Subscript> Exchange in a Subarctic Heath Ecosystem

Carbon fluxes between natural ecosystems and the atmosphere have received increased attention in recent years due to the impact they have on climate. In order to investigate independently how soil moisture and temperature control carbon fluxes into and out of a dry subarctic dwarf shrub dominated heath, monoliths containing soil and plants were incubated at three different moisture levels and subjected to four different temperature levels between 7 and 20 °C. Ecosystem CO₂ exchange was monitored continuously day and night during the 16 to 18 days that each of three experiments lasted. Additionally, the carbon allocation pattern of the plants was investigated by labelling monoliths with ¹⁴CO₂ followed by harvest of above and below ground plant parts. The results revealed that the three different soil moisture levels caused distinctly differing levels of CO₂ fluxes. Also, both carbon fixation calculated as gross ecosystem production (GEP) and carbon release measured as ecosystem respiration (ER) increased with increasing temperatures, with ER increasing faster than GEP. Hence, short term carbon loss from the ecosystem accelerated with raised temperatures. Temperature sensitivity of the ecosystem was dependent on the soil moisture level, shown by differing Q10 values of both GEP and ER at different soil moisture levels. It is therefore highly important to take soil moisture levels into consideration when evaluating responses of ecosystem carbon balance to changes in temperature. The greatest C fixation took place via the two most dominant species of the ecosystem, Vaccinium uliginosum and Empetrum hermaphroditum, with the former being responsible for the different size of C fixation at the three moisture levels.     

Plants Mediate the Sensitivity of Soil Respiration to Rainfall Variability

Soil respiration from grasslands plays a critical role in determining carbon dioxide (CO₂) feedbacks between soils and the atmosphere. In these often mesic systems, soil moisture and temperature tend to co-regulate soil respiration. Increasing variance of rainfall patterns may alter aboveground–belowground interactions and have important implications for the sensitivity of soil respiration to fluctuations in moisture and temperature. We conducted a set of field experiments to evaluate the independent and interactive effects of rainfall variability and plant–soil processes on respiration dynamics. Plant removal had strong effects on grassland soils, which included altered CO₂ flux owing to absence of root respiration; increased soil moisture and temperature; and reduced availability of dissolved organic carbon (DOC) for heterotrophic respiration by microorganisms. These plant-mediated effects interacted with our rainfall variability treatments to determine the sensitivity of soil respiration to both moisture and temperature. Using time-series multiple regression, we found that plants dampened the sensitivity of respiration to moisture under high variability rainfall treatments, which may reflect the relative stability of root contributions to total soil respiration. In contrast, plants increased the sensitivity of respiration to temperature under low variability rainfall treatment suggesting that the environmental controls on soil CO₂ dynamics in mesic habitats may be context dependent. Our results provide insight into the aboveground–belowground mechanisms controlling respiration in grasslands under variable rainfall regimes, which may be important for predicting CO₂ dynamics under current and future climate scenarios.     

Response of nitrogen cycling to simulated climate change: differential responses along a subalpine ecotone

In situ nitrogen (N) transformations and N availability were examined over a four‐year period in two soil microclimates (xeric and mesic) under a climate‐warming treatment in a subalpine meadow/sagebrush scrub ecotone. Experimental plots that spanned the two soil microclimates were exposed to an in situ infrared (IR) climate change manipulation at the Rocky Mountain Biological Laboratory, near Crested Butte, Colorado. Although the two microclimates did not differ significantly in their rates of N transformations in the absence of heating, they differed significantly in their response to increased IR. Under a simulated warming in the sagebrush‐dominated xeric microclimate, gross N mineralization rates doubled and immobilization rates increased by up to 60% over the first 2 years of the study but declined to predisturbance rates by the fourth year. This temporal pattern of gross mineralization rates correlated with a decline in SOM. Concurrently, rates of net mineralization rates in the heated plots were 60% higher than the controls after the first year. There were no differences in gross or net nitrification rates with heating in the xeric soils. In contrast to the xeric microclimate, there were no significant effects of heating on any N transformation rates in the mesic microclimate. The differing responses in N cycling rates of the two microclimate to the increased IR is most certainly the result of differences in initial soil moisture conditions and vegetation type and cover.     

土壤呼吸作用时空动态变化及其影响机制研究与展望

测定不同陆地生态系统土壤呼吸速率及其时空波动, 阐明其影响因子, 对于全球碳素平衡预算和全球变化潜在效应估计是最为基本的数据。然而, 有关土壤呼吸作用变异性及其影响因素的知识仍存在局限性, 一些关键的过程和机制还有待阐明。该文综述了近年来土壤呼吸作用时空动态规律、影响机制和模拟方面的研究进展, 指出环境因子和生物因子共同驱动着土壤呼吸作用的时间动态变化; 土壤呼吸作用在不同时间尺度上还具有明显的空间异质性, 这主要是植被覆盖、根系分布、主要的环境因素和土壤特性空间分布的异质性造成的。生物因子是影响土壤呼吸作用时空动态变化的主要因素之一。然而, 目前所使用的土壤呼吸作用经验模型通常利用土壤温度、土壤湿度或者两者的交互作用模拟土壤呼吸作用动态变化, 但没有考虑生物因子的影响, 这可能会导致明显的偏差和错误。因此, 为了精确估算土壤呼吸作用, 必须解决土壤呼吸作用小尺度上的空间变异性; 加强不同时间尺度上生物要素对土壤呼吸作用动态变化的影响研究; 除了气候因子外, 土壤呼吸作用经验模型应该纳入生物因子等其它影响因素作为变量, 用以提高模型模拟的正确性和准确性。     

Annual soil respiration in broadleaf forests of northern Wisconsin: influence of moisture and site biological,chemical, and physical characteristics

Soil temperature and moisture influence soil respiration at a range of temporal and spatial scales. Although soil temperature and moisture may be seasonally correlated, intra and inter-annual variations in soil moisture do occur. There are few direct observations of the influence of local variation in species composition or other stand/site characteristics on seasonal and annual variations in soil moisture, and on cumulative annual soil carbon release. Soil climate and soil respiration from twelve sites in five different forest types were monitored over a 2-year period (1998–1999). Also measured were stand age, species composition, basal area, litter inputs, total above-ground wood production, leaf area index, forest floor mass, coarse and fine root mass, forest floor carbon and nitrogen concentration, root carbon and nitrogen concentration, soil carbon and nitrogen concentration, coarse fraction mass and volume, and soil texture. General soil respiration models were developed using soil temperature, daily soil moisture, and various site/soil characteristics. Of the site/soil characteristics, above-ground production, soil texture, roots + forest floor mass, roots + forest floor carbon:nitrogen, and soil carbon:nitrogen were significant predictors of soil respiration when used alone in respiration models; all of these site variables were weakly to moderately correlated with mean site soil moisture. Daily soil climate data were used to estimate the annual release of carbon (C) from soil respiration for the period 1998–1999. Mean annual soil temperature did not differ between the 2 years but mean annual soil moisture was approximately 9% lower in 1998 due to a summer drought. Soil C respired during 1998 ranged from 8.57 to 11.43 Mg C ha⁻¹ yr⁻¹ while the same sites released 10.13 and 13.57 Mg C ha⁻¹ yr⁻¹ in 1999; inter-annual differences of 15.41 and 15.73%, respectively. Among the 12 sites studied, we calculated that the depression of soil respiration linked to the drought caused annual differences of soil respiration from 11.00 to 15.78%. Annual estimates of respired soil C decreased with increasing site mean soil moisture. Similarly, the difference of respired carbon between the drought and the non-drought years generally decreased with increasing site mean soil moisture.     

The temperature responses of soil respiration in deserts: a seven desert synthesis

The temperature response of soil respiration in deserts is not well quantified. We evaluated the response of respiration to temperatures spanning 67°C from seven deserts across North America and Greenland. Deserts have similar respiration rates in dry soil at 20°C, and as expected, respiration rates are greater under wet conditions, rivaling rates observed for more mesic systems. However, deserts differ in their respiration rates under wet soil at 20°C and in the strength of the effect of current and antecedent soil moisture on the sensitivity and magnitude of respiration. Respiration increases with temperature below 30°C but declines for temperatures exceeding 35°C. Hot deserts have lower temperature sensitivity than cold deserts, and insensitive or negative temperature sensitivities were predicted under certain moisture conditions that differed among deserts. These results have implications for large-scale modeling efforts because we highlight the unique behavior of desert soil respiration relative to other systems. These behaviors include variable temperature responses and the importance of antecedent moisture conditions for soil respiration.     

Partitioning sources of soil-respired CO2 and their seasonal variation using a unique radiocarbon tracer

Soil respiration is derived from heterotrophic (decomposition of soil organic matter) and autotrophic (root/rhizosphere respiration) sources, but there is considerable uncertainty about what factors control variations in their relative contributions in space and time. We took advantage of a unique whole‐ecosystem radiocarbon label in a temperate forest to partition soil respiration into three sources: (1) recently photosynthesized carbon (C), which dominates root and rhizosphere respiration; (2) leaf litter decomposition and (3) decomposition of root litter and soil organic matter >1–2 years old. Heterotrophic sources and specifically leaf litter decomposition were large contributors to total soil respiration during the growing season. Relative contributions from leaf litter decomposition ranged from a low of ～1±3% of total soil respiration (6± 3 mg C m^?2 h^?1) when leaf litter was extremely dry, to a high of 42±16% (96± 38 mg C m^?2 h^?1). Total soil respiration fluxes varied with the strength of the leaf litter decomposition source, indicating that moisture‐dependent changes in litter decomposition drive variability in total soil respiration fluxes. In the surface mineral soil layer, decomposition of C fixed in the original labeling event (3–5 years earlier) dominated the isotopic signature of heterotrophic respiration. Root/rhizosphere respiration accounted for 16±10% to 64±22% of total soil respiration, with highest relative contributions coinciding with low overall soil respiration fluxes. In contrast to leaf litter decomposition, root respiration fluxes did not exhibit marked temporal variation ranging from 34±14 to 40±16 mg C m^?2 h^?1 at different times in the growing season with a single exception (88±35 mg C m^?2 h^?1). Radiocarbon signatures of root respired CO₂ changed markedly between early and late spring (March vs. May), suggesting a switch from stored nonstructural carbohydrate sources to more recent photosynthetic products.     

Linking forest shade tolerance and soil moisture in North America

Increasing occurrence of droughts is a major environmental concern, however its consequences on forested ecosystems are not fully understood at the landscape level. Here we link the forest shade tolerance index to soil moisture in the North America using the U.S. and Quebec forest inventories. We report a significant decrease of shade tolerance index along the hydric–mesic–xeric soil transition in most of the area considered except three subtropical/tropical ecoregions of the Southeastern U.S. We conclude that droughts may alter forest succession, and in particular decrease the role of forest gap dynamics and dominance of the shade-tolerant species in mature forests.