Respiratory Carbon Loss Research Articles

Simple and Accurate Representation of Cumulative Nighttime Leaf Respiratory CO2 Efflux.

Leaf respiratory carbon loss decreases independent of temperature as the night progresses. Detailed nighttime measurements needed to quantify cumulative respiratory carbon loss at night are challenging under both lab and field conditions. We provide a simple yet accurate approach to represent variation in nighttime temperature-independent leaf respiratory CO2 efflux in environments with both stable and fluctuating temperatures, which requires no detailed measurements throughout the night. We demonstrate that the inter- and intraspecific variation in the cumulative leaf respiratory CO2 efflux at constant temperature, at any length of night, scales linearly with the inter- and intraspecific variation in initial measurement of leaf respiratory CO2 efflux at the same temperature at the beginning of the night. This approach informs large-scale predictions of cumulative leaf respiratory CO2 efflux, which is needed to understand plant carbon economy in global change studies as well as in global modeling and eddy covariance monitoring of the land-atmosphere exchange of CO2.

Unexpected sustained soil carbon flux in response to simultaneous warming and nitrogen enrichment compared with single factors alone.

Recent observations document that long-term soil warming in a temperate deciduous forest leads to significant soil carbon loss, whereas chronic soil nitrogen enrichment leads to significant soil carbon gain. Most global change experiments like these are single factor, investigating the impacts of one stressor in isolation of others. Because warming and ecosystem nitrogen enrichment are happening concurrently in many parts of the world, we designed a field experiment to test how these two factors, alone and in combination, impact soil carbon cycling. Here, we show that long-term continuous soil warming or nitrogen enrichment when applied alone followed the predicted response, with warming resulting in significant soil carbon loss and nitrogen fertilization tending towards soil carbon gain. The combination treatment showed an unanticipated response, whereby soil respiratory carbon loss was significantly higher than either single factor alone, but without a concomitant decline in soil carbon storage. Observations suggest that when soils are exposed to both factors simultaneously, plant carbon inputs to the soil are enhanced, counterbalancing soil carbon loss and helping maintain soil carbon stocks near control levels. This has implications for both atmospheric CO2 emissions and soil fertility and shows that coupling two important global change drivers results in a distinctive response that was not predicted by the behaviour of the single factors in isolation.

Higher Grain-Filling Rate in Inferior Spikelets of Tolerant Rice Genotype Offset Grain Yield Loss under Post-Anthesis High Night Temperatures

Increased nighttime respiratory losses decrease the amount of photoassimilates available for plant growth and yield. We hypothesized that the increased respiratory carbon loss under high night temperatures (HNT) could be compensated for by increased photosynthesis during the day following HNT exposure. Two rice genotypes, Vandana (HNT-sensitive) and Nagina 22 (HNT-tolerant), were exposed to HNT (4 °C above the control) from flowering to physiological maturity. They were assessed for alterations in the carbon balance of the source (flag leaf) and its subsequent impact on grain filling dynamics and the quality of spatially differentiated sinks (superior and inferior spikelets). Both genotypes exhibited significantly higher night respiration rates. However, only Nagina 22 compensated for the high respiration rates with an increased photosynthetic rate, resulting in a steady production of total dry matter under HNT. Nagina 22 also recorded a higher grain-filling rate, particularly at 5 and 10 d after flowering, with 1.5- and 4.0-fold increases in the translocation of 14C sugars to the superior and inferior spikelets, respectively. The ratio of photosynthetic rate to respiratory rate on a leaf area basis was negatively correlated with spikelet sterility, resulting in a higher filled spikelet number and grain weight per plant, particularly for inferior grains in Nagina 22. Grain quality parameters such as head rice recovery, high-density grains, and gelatinization temperature were maintained in Nagina 22. An increase in the rheological properties of rice flour starch in Nagina 22 under HNT indicated the stability of starch and its ability to reorganize during the cooling process of product formation. Thus, our study showed that sink adjustments between superior and inferior spikelets favored the growth of inferior spikelets, which helped to offset the reduction in grain weight under HNT in the tolerant genotype Nagina 22.

Unequal impacts of unidirectional freeze-thaw on different soil layers and aggregate sizes: Evidenced by microbial communities and CO2 emissions

Currently available investigations often fail to adequately recognize that soil does not freeze or thaw instantly nor uniformly, but unidirectionally and progressively. Unidirectional freeze-thaw can re-organize soil physio-, chemical- and biological-properties across layers as well among aggregates, thus posing far-reaching yet undervalued impacts on soil carbon biogeochemical cycling, especially in eroding settings. In this study, a Mollisol was refilled into soil columns, and respectively experienced complete freeze-thaw (i.e., no layer distinguished), and progressive freeze-thaw (i.e., from outer layer T1, to inner core T3). After thawing, all soils were fractionated into six size classes, and each size class was then incubated at 10℃ over 7 days to measure daily CO2 emission rate. Our results show that: (1) The soil microbial biomass carbon (SMBC) was generally reduced by 25 % after freeze-thaw, but only the finest aggregates ≤ 32 μm maintained comparable CO2 emission rates. (2) The inner core T3 thawed last and survived with significantly more SMBC and CO2, but lower respiratory quotient (qCO2), than the outer layer T1 thawed first. (3) The fine aggregates survived with more SMBC and total CO2 emissions, but significantly lower qCO2 and smaller ratios of fungal to bacterial biomass carbon, than the coarse aggregates. The disproportionally less respiratory carbon losses (i.e., lower qCO2) in T3 was probably because the lagged freezing and thus weakened water-ice phase changes in the inner core helped to better preserve the soil microbes. The fungal-dominant coarse aggregates were very responsive to freezing and evidently more respiratory active, whereas the fine aggregates were more resilient as the enhanced freezing point depression and the thus prolonged unfrozen state, fostering a better microbial survival with bacteria-dominant communities and robust basal respiration. Although limited to controlled simulation, this study may help to disentangle the role of unidirectional freeze-thaw in perturbing slope-scale soil carbon biogeochemical cycling.

Effect of Secondary Paper Sludge on Physiological Traits of Lactuca sativa L. under Heavy-Metal Stress.

To eliminate the negative effect of soil contamination with heavy metals on plant growth and crop yield, different methods and techniques are the subject of discussion and study. In this study, we aimed to evaluate the effect of secondary pulp and paper-mill sludge application to soil on the response of the main physiological processes such as the growth, photosynthesis, and respiration of lettuce (Lactuca sativa L.) plants to soil contamination with Pb. For the pot experiment, Pb was added to sandy loam soil at concentrations of 0, 50, and 250 mg Pb(NO3)2 per kg of the soil, and secondary sludge was added to a 0, 20, or 40% sludge solution during each plant watering. The Pb-mediated change in plant biomass allocation, decrease in the photosynthetic rate, increase in leaf respiration rate, and the degree of light inhibition of respiration were closely associated with increases in both root and shoot Pb content. For the Pb-free soil condition, secondary sludge application contributed to the allocation of plant biomass towards a greater accumulation in the shoots than in the roots. Although stomatal opening was not affected by either Pb or sludge, sludge application increased photosynthetic CO2 assimilation regardless of soil Pb content, which was associated with an increase in the electron-transport rate and carboxylase activity of Rubisco. Soil contamination with Pb significantly increased the ratio of respiration to photosynthesis, reflecting a shift in the carbon balance toward carbon losses in the leaves, but sludge application modified the coupling between the processes with a decrease in the proportion of respiratory carbon losses. The sludge-mediated recovery of the physiological processes of L. sativa reflected an increase in plant tolerance to soil contamination with heavy metals, the formation of which is associated with plant and soil adjustments initiated by secondary sludge application.

An assessment of ozone risk for date palm suggests that phytotoxic ozone dose nonlinearly affects carbon gain

Tropospheric ozone (O3) is a significant phytotoxic air pollutant that has a negative impact on plant carbon gain. Although date palm (Phoenix dactylifera L.) is a globally important crop in arid or semi-arid regions, so far O3 risk assessment for this species has not been reported. This study estimated leaf- and plant-level photosynthetic CO2 uptake for understanding how elevated levels of O3 affects date palm biomass growth. Ozone risks to date palm plants were assessed based on exposure- (AOT40) or flux-based indices (Phytotoxic Ozone Dose, PODy, where y is a threshold of uptake). For this purpose, plants were exposed to three levels of O3 [ambient air, AA (45 ppb as daily average); 1.5 × AA; 2.0 × AA] for 92 days in an O3 Free-Air Controlled Exposure facility. According to the model simulations, the negative effects of O3 on plant-level net photosynthetic CO2 uptake were attributed to reduced gross photosynthetic carbon gain and increased respiratory carbon loss. Season-long O3 exposure and elevated temperatures promoted the negative O3 effect because of a further increase of respiratory carbon loss, which was caused by increased leaf temperature due to stomatal closure. POD1 nonlinearly affected the photosynthetic CO2 uptake, which was closely related to the variation of dry mass increment during the experiment. Although the dose-response relationship suggested that a low O3 dose (POD1 < 5.2 mmol m−2) may even positively affect photosynthetic CO2 uptake in date palms, stomatal O3 uptake at the current ambient O3 levels has potentially a negative impact on date palm growth. The results indicate 5.8 mmol m−2 POD1 or 21.1 ppm h AOT40 as critical levels corresponding to a 4% reduction of net CO2 uptake for date palm, suggesting that this species can be identified as a species moderately sensitive to O3.

Leaf physiological and morphological constraints of water-use efficiency in C3 plants.

The increasing evaporative demand due to climate change will significantly affect the balance of carbon assimilation and water losses of plants worldwide. The development of crop varieties with improved water-use efficiency (WUE) will be critical for adapting agricultural strategies under predicted future climates. This review aims to summarize the most important leaf morpho-physiological constraints of WUE in C3 plants and identify gaps in knowledge. From the carbon gain side of the WUE, the discussed parameters are mesophyll conductance, carboxylation efficiency and respiratory losses. The traits and parameters affecting the waterside of WUE balance discussed in this review are stomatal size and density, stomatal control and residual water losses (cuticular and bark conductance), nocturnal conductance and leaf hydraulic conductance. In addition, we discussed the impact of leaf anatomy and crown architecture on both the carbon gain and water loss components of WUE. There are multiple possible targets for future development in understanding sources of WUE variability in plants. We identified residual water losses and respiratory carbon losses as the greatest knowledge gaps of whole-plant WUE assessments. Moreover, the impact of trichomes, leaf hydraulic conductance and canopy structure on plants' WUE is still not well understood. The development of a multi-trait approach is urgently needed for a better understanding of WUE dynamics and optimization.

Sodium Lignosulfonate Effect on Physiological Traits of Cucumis sativus L. Seedlings

The application of pulp and paper mill sludge to agricultural soils is commonly considered as a strategy to improve soil properties, promote plant growth, and reduce the demand for costly chemical fertilization. The aim of this study was to evaluate if sodium lignosulfonate (sLS), one of the sludges of pulp production, may affect the biomass production, the respiration (R) and net CO2 assimilation rate (An) at the leaf level, and the content and accumulation of trace elements in the leaves of cucumbers grown under a sufficient nutrient supply or soil nutrient deficit. A pot culture experiment was conducted using sLS application rates of 0, 1.0, 2.5, 5.0, and 10 vol% to sandy loam soil. The decline in nutrient availability caused an increase in the R/An ratio and dramatically depressed biomass accumulation. The leaf Fe, Ni, Cr, Co, Al, and Pb contents were lower under low nutrient availability than under sufficient nutrient supply. Although sLS was not very effective in lessening the negative effect of nutrient deficiency on biomass accumulation, it reduced respiratory carbon losses and cell membrane permeability in the leaves of cucumbers grown under nutrient deficit. The reduction in the toxic level of leaf Mn in seedlings grown under sufficient nutrient availability and the toxic level of leaf Fe under a nutrient deficit might also be considered as a positive effect of the sLS application to sandy soil.

Drought and warming alter gross primary production allocation and reduce productivity in a widespread pasture grass

Carbon allocation determines plant growth, fitness and reproductive success. However, climate warming and drought impacts on carbon allocation patterns in grasses are not well known, particularly following grazing or clipping. A widespread C3 pasture grass, Festuca arundinacea, was grown at 26 and 30°C in controlled environment chambers and subjected to drought (65% reduction relative to well-watered controls). Leaf, root and whole-plant carbon fluxes were measured and linked to growth before and after clipping. Both drought and warming reduced gross primary production and plant biomass. Drought reduced net leaf photosynthesis but increased the leaf respiratory fraction of assimilated carbon. Warming increased root respiration but did not affect either net leaf photosynthesis or leaf respiration. There was no evidence of thermal acclimation. Moreover, root respiratory carbon loss was amplified in the combined drought and warming treatment and, in addition to a negative carbon balance aboveground, explained an enhanced reduction in plant biomass. Plant regrowth following clipping was strongly suppressed by drought, reflecting increased tiller mortality and exacerbated respiratory carbon loss. These findings emphasize the importance of considering carbon allocation patterns in response to grazing or clipping and interactions with climatic factors for sustainable pasture production in a future climate.

Carbon balance in grapevines (Vitis viniferaL.): effect of environment, cultivar and phenology on carbon gain, losses and allocation

Background and Aims Measuring the carbon assimilation and respiration during vine phenology can provide an understanding of the dynamics of carbon fluxes from different organs and their relationship. Most field studies to date do not consider the respiratory losses of different plant organs and their variability under environmental, genetic and phenological changes. The aim of this study was to investigate the effect of genotype and water regime on carbon assimilation, respiration and allocation during vine phenology. Methods and Results Field trials were carried out during 2013 and 2014 to study the effect of genotype and water status on carbon assimilation, respiratory losses from leaves, shoots, fruits and roots during the vine phenological cycle, and on biomass production. Carbon respiration varied during plant phenology and represented a significant proportion of the total vine carbon assimilation. The integrated carbon respiratory loss in leaves, fruits and roots was greater in irrigated vines than in non-irrigated vines. Tempranillo recorded the highest carbon assimilation, leaf and stem respiration, as well as the highest above-ground biomass. Garnacha showed a higher root respiration loss and allocated more biomass to the permanent organs. Accumulation of above-ground biomass was influenced by plant carbon budgets during the growing season. Conclusions Vine phenology, cultivar and plant water status affected carbon assimilation, carbon loss and carbon allocation. Non-irrigated vines had a higher respiratory carbon loss in respect to the total carbon assimilation by photosynthesis. Above- and below-ground carbon fluxes were coupled during vine phenology. Significance of the Study The present work illustrates the importance of respiratory processes on the carbon balance and the relationship among different carbon balance components during vine phenology.

‘Breathing Out’ under Heat Stress—Respiratory Control of Crop Yield under High Temperature

Respiration and photosynthesis are indispensable plant metabolic processes that are affected by elevated temperatures leading to disruption of the carbon economy of the plants. Increasing global temperatures impose yield penalties in major staple crops that are attributed to increased respiratory carbon loss, through higher maintenance respiration resulting in a shortage of non-structural carbohydrates and an increase in metabolic processes like protein turnover and maintenance of ion concentration gradients. At a cellular level, warmer temperatures lead to mitochondrial swelling as well as downregulation of respiration by increasing the adenosine triphosphate:adenosine diphosphate (ATP:ADP) ratio, the abscisic acid-mediated reduction in ATP transfer to the cytosol, and the disturbance in a concentration gradient of tricarboxylic acid (TCA) cycle intermediates, as well as increasing lipid peroxidation in mitochondrial membranes and cytochrome c release to trigger programmed cell death. In this review, we discuss the mechanistic insight into the heat stress-induced mitochondrial dysfunction that controls dark respiration in plants. Furthermore, the role of hormones in regulating the network of processes that are involved in retrograde signaling is highlighted. We also propose different strategies to reduce carbon loss under high temperature, e.g., selecting genotypes with low respiration rates and using genome editing tools to target the carbon-consuming pathways by replacing, relocating, or rescheduling the metabolic activities.

Stem respiration and growth in a central Amazon rainforest

Key messageAnnual stem CO2 efflux increases with stem wood production rates and are inhibited by daily moisture stress.Tropical forests cycle a large amount of CO2 between the land and atmosphere, with a substantial portion of the return flux due tree respiratory processes. However, in situ estimates of woody tissue respiratory fluxes and carbon use efficiencies (CUEW) and their dependencies on physiological processes including stem wood production (Pw) and transpiration in tropical forests remain scarce. Here, we synthesize monthly Pw and daytime stem CO2 efflux (ES) measurements over 1 year from 80 trees with variable biomass accumulation rates in the central Amazon. On average, carbon flux to woody tissues, expressed in the same stem area normalized units as ES, averaged 0.90 ± 1.2 µmol m−2 s−1 for Pw, and 0.55 ± 0.33 µmol m−2 s−1 for daytime ES. A positive linear correlation was found between stem growth rates and stem CO2 efflux, with respiratory carbon loss equivalent to 15 ± 3% of stem carbon accrual. CUEW of stems was non-linearly correlated with growth and was as high as 77–87% for a fast-growing tree. Diurnal measurements of stem CO2 efflux for three individuals showed a daytime reduction of ES by 15–50% during periods of high sap flow and transpiration. The results demonstrate that high daytime ES fluxes are associated with high CUEW during fast tree growth, reaching higher values than previously observed in the Amazon Basin (e.g., maximum CUEW up to 77–87%, versus 30–56%). The observations are consistent with the emerging view that diurnal dynamics of stem water status influences growth processes and associated respiratory metabolism.

Oxygen consumption rates and respiratory carbon losses in three species of copepods(Acartia clausi, Calanus helgolandicus and Limnocalanus macrurus) during starvation

Calanoid copepods with different life cycle strategies demonstrate varied velocity in the utilization of energy resources during starvation. This study analyzes the influence of multiday starvation on the rate of total, basal and active energy metabolism in adult females of two Marmara Sea and one Baltic Sea species of copepods; epiplanktonic Acartia clausi unable to accumulate lipid reserves and mesoplanktonic Calanus helgolandicus with minor lipid reserves in the oil sac (approximately 7% of body volume) and the Baltic Sea copepod Limnocalanus macrurus in quasi-diapausing state with large lipid reserves in the oil sac (approximately 19% of body volume). Initially, total weight specific respiration rates were about 0.12 μg C per μg C body weight per day (d$^{-1}$) for both Marmara Sea species at 20 °C and 0.079 d$^{-1}$ in the Baltic L. macrurus at 14 °C, decreasing significantly by 1.6-2.5 times with starvation, while basal metabolic rates, measured in the same anesthetized individuals, was nearly constant during starvation. In this regard, the active metabolic rates, defined as the difference between the respiration rate of active and anesthetized individuals, decreased during starvation by 1.7 times in C. helgolandicus and by 4.7 and 6 times in A. clausi and L. macrurus, respectively. In A. clausi and C. helgolandicus with minimal lipid reserves, total daily energy expenditure (5.1% of initial body carbon content) was twice as high as in L. macrurus (2.2% of initial body carbon content daily), which has a large oil sac. Overall, the trend demonstrates that despite different energy reserves, a decrease in the total energy metabolism in copepods occurs due to a reduction in energy expenditure for locomotor activity.

Darkness and low-light alter reserve mobilization during the initial growth of soybean (Glycine max (L.) Merrill)

Light is both a source of energy and a developmental signal, and therefore has a major impact upon initial seedling growth. This is particularly the case for species where photosynthesis in the cotyledons makes a significant contribution to carbon balance. In addition to containing reserves of proteins, lipids and carbohydrates the cotyledons of soybean (Glycine max (L.) Merrill) maintain sufficient photosynthetic activity to counterbalance respiratory carbon loss. Based on the hypothesis that altered light regime would affect seedling photosynthesis and development, we investigated the impact of shading and darkness on the reserves present in different soybean seedling organs. Plants were grown until the VC/V1 stage under normal light conditions (500 µmol m−2 s−1, 12/12 h) before being divided into three groups; normal light, low light (50 µmol m−2 s−1, 12/12 h) and darkness. After 3 days plants were harvested at three time points over the course of a day/night cycle and seedling organs analysed separately. Low light and darkness led to partial etiolation and the consumption of carbohydrates and fatty acids in several organs. Effects on cotyledon reserve mobilisation were modest, as darkness and low light did not lead to decreased cotyledon mass. However, metabolism in the epicotyl was more strongly perturbed, with the accumulation of branched chain and aromatic amino acids together with sugars that may be required to drive organ elongation. Darkness also served to abolish diurnal alterations in starch and asparagine concentrations detected in control plants. Overall, the data reinforce the flexibility of seedling metabolism, revealing similarities to the responses of adult plants to carbon starvation .

Effects of climate warming on the production of the pioneer moss Racomitrium japonicum: seasonal and year-to-year variations.

Pioneer mosses are among the dominant vegetation in the early stages of xeric successions. Recent climate warming may have a significant effect on the productivity of these mosses, thereby affecting successional processes. In this study, we investigated the effects of temperature changes on the productivity of Racomitrium japonicum, a pioneer moss species commonly found on open ground in Japan. We examined the microclimate (moss temperature and water content) of a natural R. japonicum stand in Higashi-Hiroshima City, western Japan, and related them to the climate records of the weather station to create a model for estimating microclimate from past climatic data. We also examined the effects of environmental factors (light, temperature, and water) on photosynthesis in the laboratory to construct a production model. Using these models, we estimated the net primary production (NPP) over the last 10years (2009-2018) based on the climatic data (air temperature and precipitation) recorded at the weather station of Higashi-Hiroshima City. The estimated NPP showed negative values in summer, which indicated that respiratory carbon loss exceeded photosynthetic carbon gain. In contrast, NPP was positive in the spring and winter seasons throughout the 10years. Autumn NPP varied widely, showing both positive and negative values. The annual NPP also showed considerable year-to-year variations. Additionally, we examined the effects of temperature conditions on NPP assuming annual temperature changes of 0°C (present temperature), + 1°C, and + 2°C. The results indicated that NPP decreased with increasing temperature, except in the winter season. The findings of this study suggest that climate warming has a large impact on the NPP of R. japonicum; however, the impact can be both positive and negative depending on the season. The results also suggest that future climate warming is likely to decrease NPP on an annual basis.

It Is Hot in the Sun: Antarctic Mosses Have High Temperature Optima for Photosynthesis Despite Cold Climate.

The terrestrial flora of Antarctica’s frozen continent is restricted to sparse ice-free areas and dominated by lichens and bryophytes. These plants frequently battle sub-zero temperatures, extreme winds and reduced water availability; all influencing their ability to survive and grow. Antarctic mosses, however, can have canopy temperatures well above air temperature. At midday, canopy temperatures can exceed 15°C, depending on moss turf water content. In this study, the optimum temperature of photosynthesis was determined for six Antarctic moss species: Bryum pseudotriquetrum, Ceratodon purpureus, Chorisodontium aciphyllum, Polytrichastrum alpinum, Sanionia uncinata, and Schistidium antarctici collected from King George Island (maritime Antarctica) and/or the Windmill Islands, East Antarctica. Both chlorophyll fluorescence and gas exchange showed maximum values of electron transport rate occurred at canopy temperatures higher than 20°C. The optimum temperature for both net assimilation of CO2 and photoprotective heat dissipation of three East Antarctic species was 20–30°C and at temperatures below 10°C, mesophyll conductance did not significantly differ from 0. Maximum mitochondrial respiration rates occurred at temperatures higher than 35°C and were lower by around 80% at 5°C. Despite the extreme cold conditions that Antarctic mosses face over winter, the photosynthetic apparatus appears optimised to warm temperatures. Our estimation of the total carbon balance suggests that survival in this cold environment may rely on a capacity to maximize photosynthesis for brief periods during summer and minimize respiratory carbon losses in cold conditions.

Drought and recovery effects on belowground respiration dynamics and the partitioning of recent carbon in managed and abandoned grassland.

The supply of soil respiration with recent photoassimilates is an important and fast pathway for respiratory loss of carbon (C). To date it is unknown how drought and land‐use change interactively influence the dynamics of recent C in soil‐respired CO2. In an in situ common‐garden experiment, we exposed soil‐vegetation monoliths from a managed and a nearby abandoned mountain grassland to an experimental drought. Based on two 13CO2 pulse‐labelling campaigns, we traced recently assimilated C in soil respiration during drought, rewetting and early recovery. Independent of grassland management, drought reduced the absolute allocation of recent C to soil respiration. Rewetting triggered a respiration pulse, which was strongly fuelled by C assimilated during drought. In comparison to the managed grassland, the abandoned grassland partitioned more recent C to belowground respiration than to root C storage under ample water supply. Interestingly, this pattern was reversed under drought. We suggest that these different response patterns reflect strategies of the managed and the abandoned grassland to enhance their respective resilience to drought, by fostering their resistance and recovery respectively. We conclude that while severe drought can override the effects of abandonment of grassland management on the respiratory dynamics of recent C, abandonment alters strategies of belowground assimilate investment, with consequences for soil‐CO2 fluxes during drought and drought‐recovery.

Ecosystem carbon and water cycling from a sky island montane forest

“Sky islands” are characteristic of sequential mountain-valley terrain where mountains form an island archipelago rising from surrounding valleys of desert “sea”. At high elevations in the Madrean sky islands of the southwestern United States (USA) and Mexico, mixed evergreen conifer forests occur near the latitudinal extent of their distribution. This setting provides a unique opportunity to explore the ecosystem response to warmer and drier conditions that are forecasted to become more common throughout the species range. Accordingly, this work used the eddy covariance method to quantify carbon and water cycling dynamics from a Madrean sky island forest ecosystem for nine years between 2009 and 2018. The forest functioned as net sink of carbon dioxide throughout the year, which resulted in more carbon sequestration than other monitored montane forests in the continental western USA. Sustained forest activity was made possible by the combination of mild winter temperatures and a bimodal precipitation regime that delivered moisture during both summer and winter. Seasonally, gross primary production (GPP) was temperature limited in winter and could become moisture limited during the dry early summer period depending on antecedent snowmelt moisture. Ecosystem respiration was more sensitive than GPP to moisture availability throughout the rest of the non-winter period. Forecasted warming could thus stimulate forest carbon gains during the winter and either increase or decrease respiratory carbon loss during summer as a function of moisture. Overall, a metric of snow aridity that included snow depth and potential evapotranspiration was the best predictor of the warm season carbon balance (R2 = 0.86). The seasonally dissimilar impacts of warming and drying identified by this work inform current understanding of how climate change and/or variability may affect forest water and carbon cycling dynamics throughout the montane forest biome.

Effects of temperature and salinity on respiratory losses and the ratio of photosynthesis to respiration in representative Antarctic phytoplankton species.

The Southern Ocean (SO) is a net sink for atmospheric CO2 whereby the photosynthetic activity of phytoplankton and sequestration of organic carbon (biological pump) plays an important role. Global climate change will tremendously influence the dynamics of environmental conditions for the phytoplankton community, and the phytoplankton will have to acclimate to a combination of changes of e.g. water temperature, salinity, pH, and nutrient supply. The efficiency of the biological pump is not only determined by the photosynthetic activity but also by the extent of respiratory carbon losses of phytoplankton cells. Thus, the present study investigated the effect of different temperature and salinity combinations on the ratio of gross photosynthesis to respiration (rGP/R) in two representative phytoplankton species of the SO. In the comparison of phytoplankton grown at 1 and 4°C the rGP/R decreased from 11.5 to 7.7 in Chaetoceros sp., from 9.1 to 3.2 in Phaeocystis antarctica strain 109, and from 12.4 to 7.0 in P. antarctica strain 764, respectively. The decrease of rGP/R was primarily dependent on temperature whereas salinity was only of minor importance. Moreover, the different rGP/R at 1 and 4°C were caused by changes of temperature-dependent respiration rates but were independent of changes of photosynthetic rates. For further interpretation, net primary production (NPP) was calculated for different seasonal conditions in the SO with specific combinations of irradiance, temperature, and salinity. Whereas, maximum photosynthetic rates significantly correlated with calculated NPP under experimental ‘Spring’, ‘Summer’, and ‘Autumn’ conditions, there was no correlation between rGP/R and the respective values of NPP. The study revealed species-specific differences in the acclimation to temperature and salinity changes that could be linked to their different original habitats.

Arabidopsis seedlings display a remarkable resilience under severe mineral starvation using their metabolic plasticity to remain self-sufficient for weeks.

During the life cycle of plants, seedlings are considered vulnerable because they are at the interface between the highly stress tolerant seed embryos and the established plant, and must develop rapidly, often in a challenging environment, with limited access to nutrients and light. Using a simple experimental system, whereby the seedling stage of Arabidopsis is considerably prolonged by nutrient starvation, we analysed the physiology and metabolism of seedlings maintained in such conditions up to 4weeks. Although development was arrested at the cotyledon stage, there was no sign of senescence and seedlings remained viable for weeks, yielding normal plants after transplantation. Photosynthetic activity compensated for respiratory carbon losses, and energy dissipation by photorespiration and alternative oxidase appeared important. Photosynthates were essentially stored as organic acids, while the pool of free amino acids remained stable. Seedlings lost the capacity to store lipids in cytosolic lipid droplets, but developed large plastoglobuli. Arabidopsis seedlings arrested in their development because of mineral starvation displayed therefore a remarkable resilience, using their metabolic and physiological plasticity to maintain a steady state for weeks, allowing resumption of development when favourable conditions ensue.