The effect of lichen‐dominated biological soil crusts on growth and physiological characteristics of three plant species in a temperate desert of northwest China

Biocrusts (biological soil crusts) cover open spaces between vascular plants in most arid and semi‐arid areas. Information on effects of biocrusts on seedling growth is controversial, and there is little information on their effects on plant growth and physiology. We examined impacts of biocrusts on growth and physiological characteristics of three habitat‐typical plants, Erodium oxyrhynchum, Alyssum linifolium and Hyalea pulchella, growing in the Gurbantunggut Desert, northwest China. The influence of biocrusts on plant biomass, leaf area, leaf relative water content, photosynthesis, maximum quantum efficiency of PSII (F_v/F_m), chlorophyll, osmotic solutes (soluble sugars, protein, proline) and antioxidant enzymes (superoxide dismutase, catalase, peroxidase) was investigated on sites with or without biocrust cover. Biomass, leaf area, leaf water content, photosynthesis, F_v/F_m and chlorophyll content in crusted soils were higher than in uncrusted soils during early growth and lower later in the growth period. Soluble sugars, proline and antioxidant enzyme activity were always higher in crusted than in uncrusted soils, while soluble protein content was always lower. These findings indicate that biocrusts have different effects on these three ephemeral species during growth in this desert, primarily via effects on soil moisture, and possibly on soil nutrients. The influence of biocrusts changes during plant development: in early plant growth, biocrusts had either positive or no effect on growth and physiological parameters. However, biocrusts tended to negatively influence plants during later growth. Our results provide insights to explain why previous studies have found different effects of biocrusts on vascular plant growth.     

C4 plants in the vegetation of Mongolia: their natural occurrence and geographical distribution in relation to climate

The natural geographical occurrence, carbon assimilation, and structural and biochemical diversity of species with C₄ photosynthesis in the vegetation of Mongolia was studied. The Mongolian flora was screened for C₄ plants by using ¹³C/¹²C isotope fractionation, determining the early products of ¹⁴CO₂ fixation, microscopy of leaf mesophyll cell anatomy, and from reported literature data. Eighty C₄ species were found among eight families: Amaranthaceae, Chenopodiaceae, Euphorbiaceae, Molluginaceae, Poaceae, Polygonaceae, Portulacaceae and Zygophyllaceae. Most of the C4 species were in three families: Chenopodiceae (41 species), Poaceae (25 species) and Polygonaceae, genus Calligonum (6 species). Some new C₄ species in Chenopodiaceae, Poaceae and Polygonaceae were detected. C₄ Chenopodiaceae species make up 45% of the total chenopods and are very important ecologically in saline areas and in cold arid deserts. C₄ grasses make up about 10% of the total Poaceae species and these species naturally concentrate in steppe zones. Naturalized grasses with Kranz anatomy,of genera such as Setaria, Echinochloa, Eragrostis, Panicum and Chloris, were found in almost all the botanical-geographical regions of Mongolia, where they commonly occur in annually disturbed areas and desert oases. We analyzed the relationships between the occurrence of C₄ plants in 16 natural botanical-geographical regions of Mongolia and their major climatic influences. The proportion of C₄ species increases with decreasing geographical latitude and along the north-to-south temperature gradient; however grasses and chenopods differ in their responses to climate. The abundance of Chenopodiaceae species was closely correlated with aridity, but the distribution of the C₄ grasses was more dependent on temperature. Also, we found a unique distribution of different C₄ Chenopodiaceae structural and biochemical subtypes along the aridity gradient. NADP-malic enzyme (NADP-ME) tree-like species with a salsoloid type of Kranz anatomy, such as Haloxylon ammodendron and Iljinia regelii, plus shrubby Salsola and Anabasis species, were the plants most resistant to ecological stress and conditions in highly arid Gobian deserts with less than 100 mm of annual precipitation. Most of the annual C₄ chenopod species were halophytes, succulent, and occurred in saline and arid environments in steppe and desert regions. The relative abundance of C₃ succulent chenopod species also increased along the aridity gradient. Native C₄ grasses were mainly annual and perennial species from the Cynodonteae tribe with NAD-ME and PEP-carboxykinase (PEP-CK) photosynthetic types. They occurred across much of Mongolia, but were most common in steppe zones where they are often dominant in grazing ecosystems. Received: 17 March 1999 / Accepted: 1 November 1999     

Elucidating the microbial resuscitation cascade in biological soil crusts following a simulated rain event

Biological soil crusts (biocrusts) are photosynthetic mats formed through an association of prokaryotic and eukaryotic microorganisms with soil particles. Biocrusts are found in virtually any terrestrial ecosystem where vascular plant coverage is abiotically limited, with drylands comprising the primary habitat for them. We studied the dynamics of the active bacterial community in two biocrusts from an arid and a hyperarid region in the Negev Desert, Israel, under light‐oxic and dark‐anoxic incubation conditions after simulated rainfall. We used H₂¹⁸O for hydrating the crusts and analysed the bacterial community in the upper and lower parts of the biocrust using an RNA‐stable isotope probing approach coupled with 454‐pyrosequencing. In both biocrusts, two distinct bacterial communities developed under each incubation condition. The active anaerobic communities were initially dominated by members of the order Bacillales which were later replaced by Clostridiales. The aerobic communities on the other hand were dominated by Sphingobacteriales and several Alphaproteobacteria (Rhizobiales, Rhodobacterales, Rhodospirillales and Rubrobacteriales). Actinomycetales were the dominant bacterial order in the dry crusts but quickly collapsed and accounted for < 1% of the community by the end of the incubation. Our study shows that biocrusts host a diverse community whose members display complex interactions as they resuscitate from dormancy.     

Distribution of greenhouse gases in hyper-arid and arid areas of northern Chile and the contribution of the high altitude wetland microbiome (Salar de Huasco,Chile)

Northern Chile harbors different bioclimatic zones including hyper-arid and arid ecosystems and hotspots of microbial life, such as high altitude wetlands, which may contribute differentially to greenhouse gases (GHG) such as carbon dioxide (CO₂), methane (CH₄) and nitrous oxide (N₂O). In this study, we explored ground level GHG distribution and the potential role of a wetland situated at 3800 m.a.s.l, and characterized by high solar radiation <?1600 W m^?2, extreme temperature ranges (?12 to 24 °C) and wind stress (<?17 m s^?1). The water source of the wetland is mainly groundwater springs, which generates streams and ponds surrounded by peatlands. These sites support a rich microbial aquatic life including diverse bacteria and archaea communities, which transiently form more complex structures, such as microbial mats. In this study, GHG were measured in the water and above ground level air at the wetland site and along an elevation gradient in different bioclimatic areas from arid to hyper-arid zones. The microbiome from the water and sediments was described by high-throughput sequencing 16S rRNA and rDNA genes. The results indicate that GHG at ground level were variable along the elevation gradient potentially associated with different bioclimatic zones, reaching high values at the high Andean steppe and variable but lower values in the Atacama Desert and at the wetland. The water areas of the wetland presented high concentrations of CH₄ and CO₂, particularly at the spring areas and in air bubbles below microbial mats. The microbial community was rich (>?40 phyla), including archaea and bacteria potentially active in the different matrices studied (water, sediments and mats). Functional microbial groups associated with GHG recycling were detected at low frequency, i.e., <?2.5% of total sequences. Our results indicate that hyper-arid and arid areas of northern Chile are sites of GHG exchange associated with various bioclimatic zones and particularly in aquatic areas of the wetland where this ecosystem could represent a net sink of N₂O and a source for CH₄ and CO₂.     

Carbon exchange in biological soil crust communities under differential temperatures and soil water contents: implications for global change

Biological soil crusts (biocrusts) are an integral part of the soil system in arid regions worldwide, stabilizing soil surfaces, aiding vascular plant establishment, and are significant sources of ecosystem nitrogen and carbon. Hydration and temperature primarily control ecosystem CO₂ flux in these systems. Using constructed mesocosms for incubations under controlled laboratory conditions, we examined the effect of temperature (5–35 °C) and water content (WC, 20–100%) on CO₂ exchange in light (cyanobacterially dominated) and dark (cyanobacteria/lichen and moss dominated) biocrusts of the cool Colorado Plateau Desert in Utah and the hot Chihuahuan Desert in New Mexico. In light crusts from both Utah and New Mexico, net photosynthesis was highest at temperatures >30 °C. Net photosynthesis in light crusts from Utah was relatively insensitive to changes in soil moisture. In contrast, light crusts from New Mexico tended to exhibit higher rates of net photosynthesis at higher soil moisture. Dark crusts originating from both sites exhibited the greatest net photosynthesis at intermediate soil water content (40–60%). Declines in net photosynthesis were observed in dark crusts with crusts from Utah showing declines at temperatures >25 °C and those originating from New Mexico showing declines at temperatures >35 °C. Maximum net photosynthesis in all crust types from all locations were strongly influenced by offsets in the optimal temperature and water content for gross photosynthesis compared with dark respiration. Gross photosynthesis tended to be maximized at some intermediate value of temperature and water content and dark respiration tended to increase linearly. The results of this study suggest biocrusts are capable of CO₂ exchange under a wide range of conditions. However, significant changes in the magnitude of this exchange should be expected for the temperature and precipitation changes suggested by current climate models.     

Making a living while starving in the dark: metagenomic insights into the energy dynamics of a carbonate cave

Carbonate caves represent subterranean ecosystems that are largely devoid of phototrophic primary production. In semiarid and arid regions, allochthonous organic carbon inputs entering caves with vadose-zone drip water are minimal, creating highly oligotrophic conditions; however, past research indicates that carbonate speleothem surfaces in these caves support diverse, predominantly heterotrophic prokaryotic communities. The current study applied a metagenomic approach to elucidate the community structure and potential energy dynamics of microbial communities, colonizing speleothem surfaces in Kartchner Caverns, a carbonate cave in semiarid, southeastern Arizona, USA. Manual inspection of a speleothem metagenome revealed a community genetically adapted to low-nutrient conditions with indications that a nitrogen-based primary production strategy is probable, including contributions from both Archaea and Bacteria. Genes for all six known CO₂-fixation pathways were detected in the metagenome and RuBisCo genes representative of the Calvin–Benson–Bassham cycle were over-represented in Kartchner speleothem metagenomes relative to bulk soil, rhizosphere soil and deep-ocean communities. Intriguingly, quantitative PCR found Archaea to be significantly more abundant in the cave communities than in soils above the cave. MEtaGenome ANalyzer (MEGAN) analysis of speleothem metagenome sequence reads found Thaumarchaeota to be the third most abundant phylum in the community, and identified taxonomic associations to this phylum for indicator genes representative of multiple CO₂-fixation pathways. The results revealed that this oligotrophic subterranean environment supports a unique chemoautotrophic microbial community with potentially novel nutrient cycling strategies. These strategies may provide key insights into other ecosystems dominated by oligotrophy, including aphotic subsurface soils or aquifers and photic systems such as arid deserts.     

Differential responses of production and respiration to temperature and moisture drive the carbon balance across a climatic gradient in New Mexico

Southwestern North America faces an imminent transition to a warmer, more arid climate, and it is critical to understand how these changes will affect the carbon balance of southwest ecosystems. In order to test our hypothesis that differential responses of production and respiration to temperature and moisture shape the carbon balance across a range of spatio‐temporal scales, we quantified net ecosystem exchange (NEE) of CO₂ and carbon storage across the New Mexico Elevational Gradient, which consists of six eddy‐covariance sites representing biomes ranging from desert to subalpine conifer forest. Within sites, hotter and drier conditions were associated with an increasing advantage of respiration relative to production such that daily carbon uptake peaked at intermediate temperatures – with carbon release often occurring on the hottest days – and increased with soil moisture. Across sites, biotic adaptations modified but did not override the dominant effects of climate. Carbon uptake increased with decreasing temperature and increasing precipitation across the elevational gradient; NEE ranged from a source of ～30 g C m^?2 yr^?1 in the desert grassland to a sink of ～350 g C m^?2 yr^?1 in the subalpine conifer forest. Total aboveground carbon storage increased dramatically with elevation, ranging from 186 g C m^?2 in the desert grassland to 26 600 g C m^?2 in the subalpine conifer forest. These results make sense in the context of global patterns in NEE and biomass storage, and support that increasing temperature and decreasing moisture shift the carbon balance of ecosystems in favor of respiration, such that the potential for ecosystems to sequester and store carbon is reduced under hot and/or dry conditions. This implies that projected climate change will trigger a substantial net release of carbon in these New Mexico ecosystems (～3 Gt CO₂ statewide by the end of the century), thereby acting as a positive feedback to climate change.     

Growing season net ecosystem CO2 exchange of two desert ecosystems with alkaline soils in Kazakhstan

Central Asia is covered by vast desert ecosystems, and the majority of these ecosystems have alkaline soils. Their contribution to global net ecosystem CO₂ exchange (NEE) is of significance simply because of their immense spatial extent. Some of the latest research reported considerable abiotic CO₂ absorption by alkaline soil, but the rate of CO₂ absorption has been questioned by peer communities. To investigate the issue of carbon cycle in Central Asian desert ecosystems with alkaline soils, we have measured the NEE using eddy covariance (EC) method at two alkaline sites during growing season in Kazakhstan. The diurnal course of mean monthly NEE followed a clear sinusoidal pattern during growing season at both sites. Both sites showed significant net carbon uptake during daytime on sunny days with high photosynthetically active radiation (PAR) but net carbon loss at nighttime and on cloudy and rainy days. NEE has strong dependency on PAR and the response of NEE to precipitation resulted in an initial and significant carbon release to the atmosphere, similar to other ecosystems. These findings indicate that biotic processes dominated the carbon processes, and the contribution of abiotic carbon process to net ecosystem CO₂ exchange may be trivial in alkaline soil desert ecosystems over Central Asia.     

西北干旱荒漠区生态系统可持续管理理念与模式

结合对西北干旱区水资源短缺、资源开发中的生态与经济矛盾突出、气候变暖可能加剧干旱区荒漠化以及生态水权与长效保障机制缺失等问题的分析诊断,探讨了荒漠区生态系统可持续管理面临的问题与难点,分析提出了荒漠生态系统植被保育的生态水位与生态阈值,阐述了荒漠-绿洲过渡带生态融合以及绿洲生态系统安全的生态防护梯度等干旱区生态系统管理理念;结合干旱区极端环境自然条件,在多年现场试验基础上,研发集成了退化群落改造与生态多样性构建技术、植物群落结构优化配置、组装与生态融合技术、生态系统恢复水分利用与生态自维持技术以及胡杨萌蘖更新技术等多种适宜荒漠植被恢复与重建的技术模式,并进行了成功实践和试验示范,为干旱荒漠区生态系统可持续管理提供了重要科技支撑。     

Invasive species and climate change: <Emphasis Type="Italic">Conyza canadensis</Emphasis> (L.) Cronquist as a tool for assessing the invasibility of natural plant communities along an aridity gradient

The predicted reduction in precipitation in the eastern Mediterranean due to climate change may expose the natural plant communities to invasive species. We assessed whether natural plant communities along an aridity gradient in Israel were resistant to invasion by considering differences in abiotic conditions and community characteristics in these regions. We considered Conyza canadensis as a model plant as it is a common invader in the region. We examined the mechanisms and functional traits of both the plant communities and C. canadensis that promote or discourage invasion. Study sites represented a rainfall gradient with four ecosystem types: mesic Mediterranean, Mediterranean, semiarid and arid. Our results showed that the mechanisms of community invasion resistance varied along the aridity gradient. At the arid and semiarid sites, water deficiency impaired the establishment of C. canadensis. At the mesic Mediterranean site, plant competition had a negative effect on C. canadensis performance, thus greatly reducing the likelihood of its establishment. We conclude that a decrease in regional precipitation due to climate change may not affect intrinsic resistance characteristics of natural plant communities to invasion in the area.     

An unfortunate alliance: Native shrubs increase the abundance,performance, and apparent impacts of Bromus tectorum across a regional aridity gradient

Interspecific facilitation contributes to the assembly of desert plant communities. However, we know little of how desert communities invaded by exotic species respond to facilitation along regional-scale aridity gradients. These measures are essential for predicting how desert plant communities might respond to concomitant plant invasion and environmental change. Here, we evaluated the potential for Bromus tectorum (a dominant invasive plant species) and the broader herbaceous plant community to form positive associations with native shrubs along a substantial aridity gradient across the Great Basin, Mojave, and San Joaquin Deserts in North America. Along this gradient, we sampled metrics of abundance and performance for B. tectorum, all native herbaceous species combined, all exotic herbaceous species combined, and the total herbaceous community using 180 pairs of shrub and open microsites. Across the gradient, B. tectorum formed strong positive associations with native shrubs, achieving 1.6–2.2 times greater abundance, biomass, and reproductive output under native shrubs than away from shrubs, regardless of relative aridity. In contrast, the broader herbaceous community was not positively associated with native shrubs. Interestingly, increasing B. tectorum abundance corresponded to decreasing native abundance, native species richness, exotic species richness, and total species richness under but not away from shrubs. Taken together, these findings suggest that native shrubs have considerable potential to directly (by increasing abundance and performance) and indirectly (by increasing competitive effects on neighbors) facilitate B. tectorum invasion across a large portion of the non-native range.     

An evaluation of abiotic carbon sinks in deserts

Recent field studies have reported anomalous CO₂ uptake using eddy‐covariance techniques in arid and semiarid ecosystems. The rates of CO₂ uptake are incompatible with changes in situ of organic carbon pools. Here, I examine several potential mechanisms of abiotic CO₂ uptake in arid and semiarid soils: atmospheric pressure pumping, carbonate dissolution, and percolation of soil water through the vadose zone. Each mechanism is deemed inadequate to explain the observations of the eddy‐covariance systems, which must now be questioned for their accuracy in desert ecosystems.     

黄土高原藓结皮土壤呼吸速率对降雨量变化的响应

全球气候变化加剧背景下，干旱和半干旱地区的降雨模式将进一步改变，其造成的土壤水分波动是引起土壤呼吸动态变化的重要因素，但生物结皮土壤呼吸响应降雨模式变化继而影响陆地生态系统碳源/汇功能的机制尚不明确。针对黄土高原风沙土发育的藓结皮，以自然降雨量为对照，分别进行幅度为10%、30%、50%的模拟增雨和减雨处理，并利用便携式土壤碳通量分析仪(LI-8100A)测定了模拟增减雨后的藓结皮土壤呼吸速率，对比分析了其对降雨量变化的响应及机制。结果表明：(1)整个实验周期(2018和2019)增雨和减雨分别显著提高(增幅分别为17.9%—48.2%和27.1%—54.2%)和降低了(降幅分别为1.8%—26.8%和5.2%—20.8%)土壤含水量，但对土壤温度的影响不显著；(2)增雨抑制了藓结皮土壤呼吸速率(降幅分别为7.8%—31.7%和14.7%—39.4%),且随梯度增大抑制作用越明显；减雨则取决于减雨梯度，减雨10%和30%会促进土壤呼吸速率(增幅分别为27.5%、9.6%和23.6%、9.7%)而减雨50%具有抑制作用(降幅分别为15.6%和18.5%)。不同实验周期和不同降雨处理间藓结...     

Patterns of Below- and Aboveground Biomass in Eucalyptus populnea Woodland Communities of Northeast Australia along a Rainfall Gradient

In vegetated terrestrial ecosystems, carbon in below- and aboveground biomass (BGB, AGB) often constitutes a significant component of total-ecosystem carbon stock. Because carbon in the BGB is difficult to measure, it is often estimated using BGB to AGB ratios. However, this ratio can change markedly along resource gradients, such as water availability, which can lead to substantial errors in BGB estimates. In this study, BGB and AGB sampling was carried out in Eucalyptus populnea-dominated woodland communities of northeast Australia to examine patterns of BGB to AGB ratio and vertical root distribution at three sites along a rainfall gradient (367, 602, and 1,101 mm). At each site, a vegetation inventory was undertaken on five transects (100 × 4 m), and trees representing the E. populnea vegetation structure were harvested and excavated to measure aboveground and coarse-root (diameter of at least 15 mm) biomass. Biomass of fine and small roots (diameter less than 15 mm) at each site was estimated from 40 cores sampled to 1 m depth. The BGB to AGB ratio of E. populnea-dominated woodland plant communities declined from 0.58 at the xeric end to 0.36 at the mesic end of the rainfall gradient. This was due to a marked decline in AGB with increased aridity whereas the BGB was relatively stable. The vertical distribution of fine roots in the top 1 m of soil varied along the rainfall gradient. The mesic sites had more fine-root biomass (FRB) in the upper soil profile and less at depth than the xeric site. Accordingly, at the xeric site, a much larger proportion of FRB was found at depth compared to the mesic sites. The vertical distribution patterns of small roots of the E. populnea woodland plant communities were consistently )-shaped, with the highest biomass occurring at 15–30-cm depth. The potential significance of such a rooting pattern for grass–tree and shrub–tree co-existence in these ecosystems is discussed. Overall, our results revealed marked changes in BGB to AGB ratio of E. populnea woodland communities along a rainfall gradient. Because E. populnea woodlands cover a large area (96 M ha), their contribution to continental-scale carbon sequestration and greenhouse gas emission can be substantial. Use of the rainfall-zone-specific ratios found in this study, in lieu of a single generic ratio for the entire region, will significantly improve estimates of BGB carbon stocks in these woodlands. In the absence of more specific data, our results will also be relevant in other regions with similar vegetation and rainfall gradients (that is, arid and semiarid woodland ecosystems).     

Carbon balance,productivity, and water use of cold-winter desert shrub communities dominated by C3 and C4 species

Summary Common generalizations concerning the ecologic significance of C₄ photosynthesis were tested in a study of plant gas exchange, productivity, carbon balance, and water use in monospecific communities of C₃ and C₄ salt desert shrubs. Contrary to expectations, few of the hypotheses concerning the performance of C₄ species were supported. Like the C₃ species, Ceratoides lanata, the C₄ shrub, Atriplex confertifolia, initiated growth and photosynthetic activity in the cool spring months and also exhibited maximum photosynthetic rates at this time of year. To compete successfully with C₃ species, Atriplex may have been forced to evolve the capacity for photosynthesis at low temperatures prevalent during the spring when moisture is most abundant. Maximum photosynthetic rates of Atriplex were lower than those of the C₃ species. This was compensated by a prolonged period of low photosynthetic activity in the dry late summer months while Ceratoides became largely inactive. However, the annual photosynthetic carbon fixation per ground area was about the same in these two communities composed of C₃ and C₄ shrubs. The C₄ species did not exhibit greater leaf diffusion resistance than the C₃ species. The photosynthesis/transpiration ratios of the two species were about the same during the period of maximum photosynthetic rates in the spring. During the warm summer months the C₄ species did have superior photosynthesis/transpiration ratios. Yet, since Ceratoides completed a somewhat greater proportion of its annual carbon fixation earlier in the season, the ratio of annual carbon fixation/transpiratory water loss in the two communities was about the same. Atriplex did incorporate a greater percentage of the annual carbon fixation into biomass production than did Ceratoides. However, this is considered to be a reflection of properties apart from the C₄ photosynthetic pathway. Both species displayed a heavy commitment of carbon to the belowground system, and only about half of the annual moisture resource was utilized in both communities.     

Production of greenhouse‐grown biocrust mosses and associated cyanobacteria to rehabilitate dryland soil function

Mosses are an often‐overlooked component of dryland ecosystems, yet they are common members of biological soil crust communities (biocrusts) and provide key ecosystem services, including soil stabilization, water retention, carbon fixation, and housing of N₂ fixing cyanobacteria. Mosses are able to survive long dry periods, respond rapidly to precipitation, and reproduce vegetatively. With these qualities, dryland mosses have the potential to be an excellent dryland restoration material. Unfortunately, dryland mosses are often slow growing in nature, and ex situ cultivation methods are needed to enhance their utility. Our goal was to determine how to rapidly produce, vegetatively, Syntrichia caninervis and S. ruralis, common and abundant moss species in drylands of North America and elsewhere, in a greenhouse. We manipulated the length of hydration on a weekly schedule (5, 4, 3, or 2 days continuous hydration per week), crossed with fertilization (once at the beginning, monthly, biweekly, or not at all). Moss biomass increased sixfold for both species in 4 months, an increase that would require years under dryland field conditions. Both moss species preferred short hydration and monthly fertilizer. Remarkably, we also unintentionally cultured a variety of other important biocrust organisms, including cyanobacteria and lichens. In only 6 months, we produced functionally mature biocrusts, as evidenced by high productivity and ecosystem‐relevant levels of N₂ fixation. Our results suggest that biocrust mosses might be the ideal candidate for biocrust cultivation for restoration purposes. With optimization, these methods are the first step in developing a moss‐based biocrust rehabilitation technology.     

Energy crosstalk between photosynthesis and the algal CO2-concentrating mechanisms

Microalgal photosynthesis is responsible for nearly half of the CO₂ annually captured by Earth’s ecosystems. In aquatic environments where the CO₂ availability is low, the CO₂-fixing efficiency of microalgae greatly relies on mechanisms – called CO_2-concentrating mechanisms (CCMs) – for concentrating CO₂ at the catalytic site of the CO₂-fixing enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco). While the transport of inorganic carbon (C_i) across membrane bilayers against a concentration gradient consumes part of the chemical energy generated by photosynthesis, the bioenergetics and cellular mechanisms involved are only beginning to be elucidated. Here, we review the current knowledge relating to the energy requirement of CCMs in the light of recent advances in photosynthesis regulatory mechanisms and the spatial organization of CCM components.     

Effects of an increase in summer precipitation on leaf, soil, and ecosystem fluxes of CO2 and H2O in a sotol grassland in Big Bend National Park, Texas

Global climate models predict that in the next century precipitation in desert regions of the USA will increase, which is anticipated to affect biosphere/atmosphere exchanges of both CO₂ and H₂O. In a sotol grassland ecosystem in the Chihuahuan Desert at Big Bend National Park, we measured the response of leaf-level fluxes of CO₂ and H₂O 1 day before and up to 7 days after three supplemental precipitation pulses in the summer (June, July, and August 2004). In addition, the responses of leaf, soil, and ecosystem fluxes of CO₂ and H₂O to these precipitation pulses were also evaluated in September, 1 month after the final seasonal supplemental watering event. We found that plant carbon fixation responded positively to supplemental precipitation throughout the summer. Both shrubs and grasses in watered plots had increased rates of photosynthesis following pulses in June and July. In September, only grasses in watered plots had higher rates of photosynthesis than plants in the control plots. Soil respiration decreased in supplementally watered plots at the end of the summer. Due to these increased rates of photosynthesis in grasses and decreased rates of daytime soil respiration, watered ecosystems were a sink for carbon in September, assimilating on average 31 mmol CO₂ m⁻² s⁻¹ ground area day⁻¹. As a result of a 25% increase in summer precipitation, watered plots fixed eightfold more CO₂ during a 24-h period than control plots. In June and July, there were greater rates of transpiration for both grasses and shrubs in the watered plots. In September, similar rates of transpiration and soil water evaporation led to no observed treatment differences in ecosystem evapotranspiration, even though grasses transpired significantly more than shrubs. In summary, greater amounts of summer precipitation may lead to short-term increased carbon uptake by this sotol grassland ecosystem.     

A Synthesis of Climate and Vegetation Cover Effects on Biogeochemical Cycling in Shrub-Dominated Drylands

Semi-arid and arid ecosystems dominated by shrubs (“dry shrublands”) are an important component of the global C cycle, but impacts of climate change and elevated atmospheric CO₂ on biogeochemical cycling in these ecosystems have not been synthetically assessed. This study synthesizes data from manipulative studies and from studies contrasting ecosystem processes in different vegetation microsites (that is, shrub or herbaceous canopy versus intercanopy microsites), to assess how changes in climate and atmospheric CO₂ affect biogeochemical cycles by altering plant and microbial physiology and ecosystem structure. Further, we explore how ecosystem structure impacts on biogeochemical cycles differ across a climate gradient. We found that: (1) our ability to project ecological responses to changes in climate and atmospheric CO₂ is limited by a dearth of manipulative studies, and by a lack of measurements in those studies that can explain biogeochemical changes, (2) changes in ecosystem structure will impact biogeochemical cycling, with decreasing pools and fluxes of C and N if vegetation canopy microsites were to decline, and (3) differences in biogeochemical cycling between microsites are predictable with a simple aridity index (MAP/MAT), where the relative difference in pools and fluxes of C and N between vegetation canopy and intercanopy microsites is positively correlated with aridity. We conclude that if climate change alters ecosystem structure, it will strongly impact biogeochemical cycles, with increasing aridity leading to greater heterogeneity in biogeochemical cycling among microsites. Additional long-term manipulative experiments situated across dry shrublands are required to better predict climate change impacts on biogeochemical cycling in deserts.     

Dryland biological soil crust cyanobacteria show unexpected decreases in abundance under long‐term elevated CO2

Biological soil crusts (biocrusts) cover soil surfaces in many drylands globally. The impacts of 10 years of elevated atmospheric CO₂ on the cyanobacteria in biocrusts of an arid shrubland were examined at a large manipulated experiment in Nevada, USA. Cyanobacteria‐specific quantitative PCR surveys of cyanobacteria small‐subunit (SSU) rRNA genes suggested a reduction in biocrust cyanobacterial biomass in the elevated CO₂ treatment relative to the ambient controls. Additionally, SSU rRNA gene libraries and shotgun metagenomes showed reduced representation of cyanobacteria in the total microbial community. Taxonomic composition of the cyanobacteria was similar under ambient and elevated CO₂ conditions, indicating the decline was manifest across multiple cyanobacterial lineages. Recruitment of cyanobacteria sequences from replicate shotgun metagenomes to cyanobacterial genomes representing major biocrust orders also suggested decreased abundance of cyanobacteria sequences across the majority of genomes tested. Functional assignment of cyanobacteria‐related shotgun metagenome sequences indicated that four subsystem categories, three related to oxidative stress, were differentially abundant in relation to the elevated CO₂ treatment. Taken together, these results suggest that elevated CO₂ affected a generalized decrease in cyanobacteria in the biocrusts and may have favoured cyanobacteria with altered gene inventories for coping with oxidative stress.