The above-belowground coupling of the C cycle: fast and slow mechanisms of C transfer for root and rhizomicrobial respiration

© 2016 Springer International Publishing SwitzerlandBackground and aims: The coupling of photosynthesis with belowground processes appears to be much faster than the time needed for assimilate translocation with the phloem flow. Pressure/concentration waves have been hypothesized to release belowground C already present in the phloem, resulting in a very fast feedback of rhizosphere processes to photosynthesis changes. We evaluate the speed of aboveground-rhizosphere coupling under maize by two mechanisms: pressure/concentration waves and direct phloem transport. Methods: We combined two isotopic approaches: 1) the speed of direct phloem transport was evaluated by labeling shoots in 14CO2 and tracing 14C in the nutrient solution and in the CO2 flux, 2) pressure/concentration waves were evaluated by labeling the solution with [13C] glucose and tracing the isotope dilution during photoassimilation. Results: 14C shoot labeling of maize plants showed that 12 h were needed for 14C to peak in root-derived CO2. In contrast, in the solution labeling approach, CO2 flux increased within 2 h after switching on the light. Pressure/concentration waves contributed 5 % to diurnal respiration efflux and affected only root respiration. Root exudation was independent of the fast mechanism of above-belowground coupling. Conclusions: Photosynthesis affected root and rhizomicrobial respiration on variable time-scales: root respiration within the first 2 h by pressure/concentration waves, whereas rhizomicrobial respiration may depend on internal circadian cycles in regulating exudation rather than on light directly

The above-belowground coupling of the C cycle: fast and slow mechanisms of C transfer for root and rhizomicrobial respiration

© 2016, Springer International Publishing Switzerland.Background and aims: The coupling of photosynthesis with belowground processes appears to be much faster than the time needed for assimilate translocation with the phloem flow. Pressure/concentration waves have been hypothesized to release belowground C already present in the phloem, resulting in a very fast feedback of rhizosphere processes to photosynthesis changes. We evaluate the speed of aboveground-rhizosphere coupling under maize by two mechanisms: pressure/concentration waves and direct phloem transport. Methods: We combined two isotopic approaches: 1) the speed of direct phloem transport was evaluated by labeling shoots in 14CO2 and tracing 14C in the nutrient solution and in the CO2 flux, 2) pressure/concentration waves were evaluated by labeling the solution with [13C] glucose and tracing the isotope dilution during photoassimilation. Results: 14C shoot labeling of maize plants showed that 12 h were needed for 14C to peak in root-derived CO2. In contrast, in the solution labeling approach, CO2 flux increased within 2 h after switching on the light. Pressure/concentration waves contributed 5 % to diurnal respiration efflux and affected only root respiration. Root exudation was independent of the fast mechanism of above-belowground coupling. Conclusions: Photosynthesis affected root and rhizomicrobial respiration on variable time-scales: root respiration within the first 2 h by pressure/concentration waves, whereas rhizomicrobial respiration may depend on internal circadian cycles in regulating exudation rather than on light directly

Carbon allocation and carbon isotope fluxes in the plant-soil-atmosphere continuum: a review

The terrestrial carbon (C) cycle has received increasing interest over the past few decades, however, there is still a lack of understanding of the fate of newly assimilated C allocated within plants and to the soil, stored within ecosystems and lost to the atmosphere. Stable carbon isotope studies can give novel insights into these issues. In this review we provide an overview of an emerging picture of plant-soil-atmosphere C fluxes, as based on C isotope studies, and identify processes determining related C isotope signatures. The first part of the review focuses on isotopic fractionation processes within plants during and after photosynthesis. The second major part elaborates on plant-internal and plant-rhizosphere C allocation patterns at different time scales (diel, seasonal, interannual), including the speed of C transfer and time lags in the coupling of assimilation and respiration, as well as the magnitude and controls of plant-soil C allocation and respiratory fluxes. Plant responses to changing environmental conditions, the functional relationship between the physiological and phenological status of plants and C transfer, and interactions between C, water and nutrient dynamics are discussed. The role of the C counterflow from the rhizosphere to the aboveground parts of the plants, e.g. via CO<sub>2</sub> dissolved in the xylem water or as xylem-transported sugars, is highlighted. The third part is centered around belowground C turnover, focusing especially on above- and belowground litter inputs, soil organic matter formation and turnover, production and loss of dissolved organic C, soil respiration and CO<sub>2</sub> fixation by soil microbes. Furthermore, plant controls on microbial communities and activity via exudates and litter production as well as microbial community effects on C mineralization are reviewed. A further part of the paper is dedicated to physical interactions between soil CO<sub>2</sub> and the soil matrix, such as CO<sub>2</sub> diffusion and dissolution processes within the soil profile. Finally, we highlight state-of-the-art stable isotope methodologies and their latest developments. From the presented evidence we conclude that there exists a tight coupling of physical, chemical and biological processes involved in C cycling and C isotope fluxes in the plant-soil-atmosphere system. Generally, research using information from C isotopes allows an integrated view of the different processes involved. However, complex interactions among the range of processes complicate or currently impede the interpretation of isotopic signals in CO<sub>2</sub> or organic compounds at the plant and ecosystem level. This review tries to identify present knowledge gaps in correctly interpreting carbon stable isotope signals in the plant-soil-atmosphere system and how future research approaches could contribute to closing these gaps

Does long-term warming affect C and N allocation in a Mediterranean shrubland ecosystem? Evidence from a¹³C and¹⁵N labeling field study

© 2017 In the Mediterranean basin the effects of climate warming on ecosystem functioning will strongly depend on the warming intensity directly but also on its effects on evapotranspiration and nutrient cycling. Climate manipulation experiments under field conditions are a source of unique empirical evidence regarding climate-related modifications of biotic processes. A field night-time warming experiment, simulating the predicted near-future increase in ambient temperatures (+0.3 up to 1 °C), was established in a Mediterranean shrub community located in Porto Conte (Italy) in 2001. After 11 years of continuous treatment, we labeled the dominant shrub Cistus monspeliensis with 13 CO 2 and studied the dynamics of the label allocation between aboveground and belowground pools and fluxes in warmed and ambient plots within 2 weeks of the chasing period. The interactions between C and N metabolism were assessed by parallel labeling of soil with K 15 NO 3. Most of the assimilated 13 C was respired by Cistus shoots (28–51%) within two weeks. Cistus under warming respired more 13 C label and tended to allocate less 13 C to leaves, branches and roots. The higher C and N content in microbial biomass in warming plots, combined with the higher N content in plant tissues and soil, evidenced a greater N mobilization in soil and a better nutrient status of the plants as compared to the ambient treatment. Acceleration of N cycling is probably responsible for higher respiratory C losses, but combined with the reduction in the number of frost days, should also positively affect plant photosynthetic performance. We conclude that, although Cistus plants are already growing in conditions close to their thermal optimum, long-term warming will positively affect the performance of this species, mainly by reducing the nutrient constraints. This positive effect will highly depend on the frequency and amount of rain events and their interactions with soil N content

Carbon allocation and carbon isotope fluxes in the plant-soil-atmosphere continuum: A review

The terrestrial carbon (C) cycle has received increasing interest over the past few decades, however, there is still a lack of understanding of the fate of newly assimilated C allocated within plants and to the soil, stored within ecosystems and lost to the atmosphere. Stable carbon isotope studies can give novel insights into these issues. In this review we provide an overview of an emerging picture of plant-soil-atmosphere C fluxes, as based on C isotope studies, and identify processes determining related C isotope signatures. The first part of the review focuses on isotopic fractionation processes within plants during and after photosynthesis. The second major part elaborates on plant-internal and plant-rhizosphere C allocation patterns at different time scales (diel, seasonal, interannual), including the speed of C transfer and time lags in the coupling of assimilation and respiration, as well as the magnitude and controls of plant-soil C allocation and respiratory fluxes. Plant responses to changing environmental conditions, the functional relationship between the physiological and phenological status of plants and C transfer, and interactions between C, water and nutrient dynamics are discussed. The role of the C counterflow from the rhizosphere to the aboveground parts of the plants, e.g. via CO2 dissolved in the xylem water or as xylem-transported sugars, is highlighted. The third part is centered around belowground C turnover, focusing especially on above- and belowground litter inputs, soil organic matter formation and turnover, production and loss of dissolved organic C, soil respiration and CO2 fixation by soil microbes. Furthermore, plant controls on microbial communities and activity via exudates and litter production as well as microbial community effects on C mineralization are reviewed. A further part of the paper is dedicated to physical interactions between soil CO2 and the soil matrix, such as CO2 diffusion and dissolution processes within the soil profile. Finally, we highlight state-of-the-art stable isotope methodologies and their latest developments. From the presented evidence we conclude that there exists a tight coupling of physical, chemical and biological processes involved in C cycling and C isotope fluxes in the plant-soil-atmosphere system. Generally, research using information from C isotopes allows an integrated view of the different processes involved. However, complex interactions among the range of processes complicate or currently impede the interpretation of isotopic signals in CO2 or organic compounds at the plant and ecosystem level. This review tries to identify present knowledge gaps in correctly interpreting carbon stable isotope signals in the plant-soil-atmosphere system and how future research approaches could contribute to closing these gaps

Effects of Mowing on Methane Uptake in a Semiarid Grassland in Northern China

Background: Mowing is a widely adopted management practice for the semiarid steppe in China and affects CH4 exchange. However, the magnitude and the underlying mechanisms for CH 4 uptake in response to mowing remain uncertain. Methodology/Principal Findings: In two consecutive growing seasons, we measured the effect of mowing on CH 4 uptake in a steppe community. Vegetation was mowed to 2 cm (M2), 5 cm (M5), 10 cm (M10), 15 cm (M15) above soil surface, respectively, and control was set as non-mowing (NM). Compared with control, CH4 uptake was substantially enhanced at almost all the mowing treatments except for M15 plots of 2009. CH4 uptake was significantly correlated with soil microbial biomass carbon, microbial biomass nitrogen, and soil moisture. Mowing affects CH 4 uptake primarily through its effect on some biotic factors, such as net primary productivity, soil microbial C\N supply and soil microbial activities, while soil temperature and moisture were less important. Conclusions/Significance: This study found that mowing affects the fluxes of CH4 in the semiarid temperate steppe of north China

The above-belowground coupling of the C cycle: fast and slow mechanisms of C transfer for root and rhizomicrobial respiration

© 2016 Springer International Publishing SwitzerlandBackground and aims: The coupling of photosynthesis with belowground processes appears to be much faster than the time needed for assimilate translocation with the phloem flow. Pressure/concentration waves have been hypothesized to release belowground C already present in the phloem, resulting in a very fast feedback of rhizosphere processes to photosynthesis changes. We evaluate the speed of aboveground-rhizosphere coupling under maize by two mechanisms: pressure/concentration waves and direct phloem transport. Methods: We combined two isotopic approaches: 1) the speed of direct phloem transport was evaluated by labeling shoots in 14CO2 and tracing 14C in the nutrient solution and in the CO2 flux, 2) pressure/concentration waves were evaluated by labeling the solution with [13C] glucose and tracing the isotope dilution during photoassimilation. Results: 14C shoot labeling of maize plants showed that 12 h were needed for 14C to peak in root-derived CO2. In contrast, in the solution labeling approach, CO2 flux increased within 2 h after switching on the light. Pressure/concentration waves contributed 5 % to diurnal respiration efflux and affected only root respiration. Root exudation was independent of the fast mechanism of above-belowground coupling. Conclusions: Photosynthesis affected root and rhizomicrobial respiration on variable time-scales: root respiration within the first 2 h by pressure/concentration waves, whereas rhizomicrobial respiration may depend on internal circadian cycles in regulating exudation rather than on light directly