REVIEW: Time lag between photosynthesis and carbon dioxide efflux from soil: a review of mechanisms and controls

CO₂ efflux from soil depends on the availability of organic substances respired by roots and microorganisms. Therefore, photosynthetic activity supplying carbohydrates from leaves to roots and rhizosphere is a key driver of soil CO₂. This fact has been overlooked in most soil CO₂ studies because temperature variations are highly correlated with solar radiation and mask the direct effect of photosynthesis on substrate availability in soil. This review highlights the importance of photosynthesis for rhizosphere processes and evaluates the time lag between carbon (C) assimilation and CO₂ release from soil. Mechanisms and processes contributing to the lag were evaluated. We compared the advantages and shortcomings of four main approaches used to estimate this time lag: (1) interruption of assimilate flow from leaves into the roots and rhizosphere, and analysis of the decrease of CO₂ efflux from soil, (2) time series analysis (TSA) of CO₂ fluxes from soil and photosynthesis proxies, (3) analysis of natural δ¹³C variation in CO₂ with photosynthesis‐related parameters or δ¹³C in the phloem and leaves, and (4) pulse labeling of plants in artificial ¹⁴CO₂ or ¹³CO₂ atmosphere with subsequent tracing of ¹⁴C or ¹³C in CO₂ efflux from soil. We concluded that pulse labeling is the most advantageous approach. It allows clear evaluation not only of the time lag, but also of the label dynamics in soil CO₂, and helps estimate the mean residence time of recently assimilated C in various above‐ and belowground C pools. The impossibility of tracing the phloem pressure–concentration waves by labeling approach may be overcome by its combination with approaches based on TSA of CO₂ fluxes and its δ¹³C with photosynthesis proxies. Numerous studies showed that the time lag for grasses is about 12.5±7.5 (SD) h. The time lag for mature trees was much longer (～4–5 days). Tree height slightly affected the lag, with increasing delay of 0.1 day m^?1. By evaluating bottle‐neck processes responsible for the time lag, we conclude that, for trees, the transport of assimilates in phloem is the rate‐limiting step. However, it was not possible to predict the lag based on the phloem transport rates reported in the literature. We conclude that studies of CO₂ fluxes from soil, especially in ecosystems with a high contribution of root‐derived CO₂, should consider photosynthesis as one of the main drivers of C fluxes. This calls for incorporating photosynthesis in soil C turnover models.     

Influence of interactions between litter decomposition and rhizosphere activity on soil respiration and on the temperature sensitivity in a subtropical montane forest in SW China

Aims

The aims were to identify the effects of interactions between litter decomposition and rhizosphere activity on soil respiration and on the temperature sensitivity of soil respiration in a subtropical forest in SW China.

Methods

Four treatments were established: control (CK), litter removal (NL), trenching (NR) and trenching together with litter removal (NRNL). Soil CO₂ efflux, soil temperature, and soil water content were measured once a month over two years. Soil respiration was divided into four components: the decomposition of basic soil organic matter (SOM), litter respiration, root respiration, and the interaction effect between litter decomposition and rhizosphere activity. A two-factor regression equation was used to correct the value of soil CO₂ efflux.

Results

We found a significant effect of the interaction between litter decomposition and rhizosphere activity (R _INT) on total soil respiration, and R _INT exhibited significant seasonal variation, accounting for 26 and 31 % of total soil respiration in the dry and rainy seasons, respectively. However, we found no significant interaction effect on the temperature sensitivity of soil respiration. The temperature sensitivity was significantly increased by trenching compared with the control, but was unchanged by litter removal.

Conclusions

Though the interaction between litter decomposition and rhizosphere activity had no effects on temperature sensitivity, it had a significant positive effect on soil respiration. Our results not only showed strong influence of rhizosphere activity on temperature sensitivity, but provided a viable way to identify the contribution of SOM to soil respiration, which could help researchers gain insights on the carbon cycle.     

Responses of CO2 efflux from an alpine meadow soil on the Qinghai Tibetan Plateau to multi-form and low-level N addition

Aims

To assess the effects of atmospheric N deposition on the C budget of an alpine meadow ecosystem on the Qinghai–Tibetan Plateau, it is necessary to explore the responses of soil-atmosphere carbon dioxide (CO₂) exchange to N addition.

Methods

Based on a multi-form, low-level N addition experiment, soil CO₂ effluxes were monitored weekly using the static chamber and gas chromatograph technique. Soil variables and aboveground biomass were measured monthly to examine the key driving factors of soil CO₂ efflux.

Results

The results showed that low-level N input tended to decrease soil moisture, whereas medium-level N input maintained soil moisture. Three-year N additions slightly increased soil inorganic N pools, especially the soil NH ₄ ⁺ -N pool. N applications significantly increased aboveground biomass and soil CO₂ efflux; moreover, this effect was more significant from NH ₄ ⁺ -N than from NO ₃ ^? -N fertilizer. In addition, the soil CO₂ efflux was mainly driven by soil temperature, followed by aboveground biomass and NH ₄ ⁺ -N pool.

Conclusions

These results suggest that chronic atmospheric N deposition will stimulate soil CO₂ efflux in the alpine meadow on the Qinghai–Tibetan Plateau by increasing available N content and promoting plant growth.     

Carbon fluxes in coniferous and deciduous forest soils

Aims

Our aims were to identify responsible factors for the site-to-site variability in soil CO₂ efflux and to assess the sources of soil CO₂ of different forest types on a regional scale.

Methods

Soil CO₂ effluxes were measured over 1–4 years in four coniferous and three deciduous forests of Bavaria, Germany, and related to climate, soil properties and forest productivity. Total belowground carbon allocation (TBCA) was assessed using soil CO₂ effluxes and aboveground litterfall. Additionally, CO₂ production of organic layers was examined over 10 months under constant conditions in an incubation experiment.

Results

Annual soil CO₂ effluxes were not different among the forest sites, but predicted effluxes at a given temperature of 10°C revealed some significant differences and correlated with the phosphorous stock of the organic layers. The incubation study indicated 50% faster decomposition of organic layers from deciduous than from coniferous forests. TBCA related to soil CO₂ efflux was smaller in the deciduous than in the coniferous forests. The ratio of TBCA to soil CO₂ efflux was positively correlated with the C stock of organic layers.

Conclusions

Our results suggest that marked differences in site characteristics have little impact on soil CO₂ effluxes at the regional scale, but the contribution of soil CO₂ sources varies among the forest types.     

Elevated CO2 temporally enhances phosphorus immobilization in the rhizosphere of wheat and chickpea

Aims

The efficient management of phosphorus (P) in cropping systems remains a challenge due to climate change. We tested how plant species access P pools in soils of varying P status (Olsen-P 3.2–17.6 mg?kg^?1), under elevated atmosphere CO₂ (eCO₂).

Methods

Chickpea (Cicer arietinum L.) and wheat (Triticum aestivum L.) plants were grown in rhizo-boxes containing Vertosol or Calcarosol soil, with two contrasting P fertilizer histories for each soil, and exposed to ambient (380 ppm) or eCO₂ (700 ppm) for 6 weeks.

Results

The NaHCO₃-extractable inorganic P (Pi) in the rhizosphere was depleted by both wheat and chickpea in all soils, but was not significantly affected by CO₂ treatment. However, NaHCO₃-extractable organic P (Po) accumulated, especially under eCO₂ in soils with high P status. The NaOH-extractable Po under eCO₂ accumulated only in the Vertosol with high P status. Crop species did not exhibit different eCO₂-triggered capabilities to access any P pool in either soil, though wheat depleted NaHCO₃-Pi and NaOH-Pi in the rhizosphere more than chickpea. Elevated CO₂ increased microbial biomass C in the rhizosphere by an average of 21 %. Moreover, the size in Po fractions correlated with microbial C but not with rhizosphere pH or phosphatase activity.

Conclusion

Elevated CO₂ increased microbial biomass in the rhizosphere which in turn temporally immobilized P. This P immobilization was greater in soils with high than low P availability.     

Model-assisted evaluation of crop load effects on stem diameter variations and fruit growth in peach

Key message

The paper identifies and quantifies how crop load influences plant physiological variables that determine stem diameter variations to better understand the effect of crop load on drought stress indicators.

Abstract

Stem diameter (D ^stem) variations have extensively been applied in optimisation strategies for plant-based irrigation scheduling in fruit trees. Two D ^stem derived water status indicators, maximum daily shrinkage (MDS) and daily growth rate (DGR), are however influenced by other factors such as crop load, making it difficult to unambiguously use these indicators in practical irrigation applications. Furthermore, crop load influences the growth of individual fruits, because of competition for assimilates. This paper aims to explain the effect of crop load on DGR, MDS and individual fruit growth in peach using a water and carbon transport model that includes simulation of stem diameter variations. This modelling approach enabled to relate differences in crop load to differences in xylem and phloem water potential components. As such, crop load effects on DGR were attributed to effects on the stem phloem turgor pressure. The effect of crop load on MDS could be explained by the plant water status, the phloem carbon concentration and the elasticity of the tissue. The influence on fruit growth could predominantly be explained by the effect on the early fruit growth stages.     

Phloem transport in Picea abies (L.) Karst. in mid-winter

Summary Translocation of ¹⁴C assimilates was studied on four different transport systems of Picea abies branches after induced activation in January. ¹⁴CO₂ assimilation of terminal shoots for 48 h at 25° C resulted in phloem loading and basipetal transport of ¹⁴C photosynthate into the following, older shoot generations. ¹⁴C import was enhanced, when these older shoot generations were kept in the dark. Microautoradiographs of the labelled terminal shoots showed that ¹⁴C assimilates were exported from needles via sieve elements of the leaf traces and loaded into the latest increment of the axial secondary phloem. No ¹⁴C label appeared in the obliterated sieve cells or in the tracheids. In addition, ¹⁴C photosynthate accumulated densely in the chlorophyllous cells of the cortex and in cells of the resin ducts, indicating certain sink activity. In the darkened 2-year-old shoot, imported ¹⁴C photosynthate was concentrated in the functional secondary phloem, while some ¹⁴C label was unloaded into the latest xylem increment. When 6-year-old shoots were exposed to ¹⁴CO₂ for 48 h in the light, ¹⁴C assimilates accumulated in the phloem of the leaf trace and in the latest increment of the axial secondary phloem. However, a substantial amount of radioactivity was unloaded into ray cells and phloem parenchyma cells. Thus, the presence of functioning phloem in needles and twigs of P. abies during winter allows long-distance translocation and radial distribution of assimilates according to existing source-sink relations.     

Components of forest soil CO2 efflux estimated from Δ14C values of soil organic matter

Aims

The partitioning of the total soil CO₂ efflux into its two main components: respiration from roots (and root-associated organisms) and microbial respiration (by means of soil organic matter (SOM) and litter decomposition), is a major need in soil carbon dynamics studies in order to understand if a soil is a net sink or source of carbon.

Methods

The heterotrophic component of the CO₂ efflux was estimated for 11 forest sites as the ratio between the carbon stocks of different SOM pools and previously published (Δ¹⁴C derived) turnover times. The autotrophic component, including root and root-associated respiration, was calculated by subtracting the heterotrophic component from total soil chamber measured CO₂ efflux.

Results

Results suggested that, on average, 50.4 % of total soil CO₂ efflux was derived from the respiration of the living roots, 42.4 % from decomposition of the litter layers and less than 10 % from decomposition of belowground SOM.

Conclusions

The Δ¹⁴C method proved to be an efficient tool by which to partition soil CO₂ efflux and quantify the contribution of the different components of soil respiration. However the average calculated heterotrophic respiration was statistically lower compared with two previous studies dealing with soil CO₂ efflux partitioning (one performed in the same study area; the other a meta-analysis of soil respiration partitioning). These differences were probably due to the heterogeneity of the SOM fraction and to a sub-optimal choice of the litter sampling period.     

Effects of tree species composition on the CO2 and N2O efflux of a Mediterranean mountain forest soil

Background and Aims

Tree species composition shifts can alter soil CO₂ and N₂O effluxes. We quantified the soil CO₂ and N₂O efflux rates and temperature sensitivity from Pyrenean oak, Scots pine and mixed stands in Central Spain to assess the effects of a potential expansion of oak forests.

Methods

Soil CO₂ and N₂O effluxes were measured from topsoil samples by lab incubation from 5 to 25 °C. Soil microbial biomass and community composition were assessed.

Results

Pine stands showed highest soil CO₂ efflux, followed by mixed and oak forests (up to 277, 245 and 145 mg CO₂-C m^?2 h^?1, respectively). Despite contrasting soil microbial community composition (more fungi and less actinomycetes in pine plots), carbon decomposability and temperature sensitivity of the soil CO₂ efflux remain constant among tree species. Soil N₂O efflux rates and its temperature sensitivity was markedly higher in oak stands than in pine stands (70 vs. 27 μg N₂O-N m^?2 h^?1, Q₁₀, 4.5 vs. 2.5).

Conclusions

Conversion of pine to oak forests in the region will likely decrease soil CO₂ effluxes due to decreasing SOC contents on the long run and will likely enhance soil N₂O effluxes. Our results present only a seasonal snapshot and need to be confirmed in the field.     

Annual variation in soil respiration and its component parts in two structurally contrasting woody savannas in Central Brazil

Background and aims

Due to the high spatial and temporal variation in soil CO₂ efflux, terrestrial carbon budgets rely on a detailed understanding of the drivers of soil respiration from a diverse range of ecosystems and climate zones. In this study we aim to evaluate the independent influence of vegetation structure and climate on soil CO₂ efflux within cerrado ecosystems.

Methods

We examine the seasonal and diel variation of soil CO₂ efflux, including its autotrophic and heterotrophic components, within two adjacent and structurally contrasting woody savannas in central Brazil.

Principle results

We found no significant difference in the annual soil CO₂ efflux between the two stands (p?=?0.53) despite a clear disparity in both LAI (p?<?0.01) and leaf litterfall (p?<?0.01). The mean annual loss of carbon from the soil was 17.32(±1.48) Mg C?ha^?1 of which approximately 63% was accounted for by autotrophic respiration. The relative contribution of autotrophic respiration varied seasonally between 55% in the wet season to 79% of the total soil CO₂ efflux in the dry season. Furthermore, seasonal fluctuations of all the soil respiration components were strongly correlated with soil moisture (R ²?=?0.79–0.90, p?<?0.01).

Conclusions

Across these two structurally distinct cerrado stands, seasonal variations in rainfall, was the main driver of soil CO₂ efflux and its components. Consequently, soil respiration within these ecosystems is likely to be highly sensitive to any changes in seasonal precipitation patterns.     

Effect of sand-stabilizing shrubs on soil respiration in a temperate desert

Aims

Explore how soil CO₂ efflux and its components change after moving sand dunes are stabilized with shrubs, and how abiotic factors affect those components at different scales.

Methods

Soil CO₂ efflux from a sand-stabilized area was compared to that from moving sand dunes in the Tengger Desert. To partition rhizosphere respiration (R_R) from soil basal respiration (R_B), a root-isolation plot was established.

Results

Compared to moving sand dunes, total soil respiration (R_T) in the sand-stabilized area increased 3.2 fold to 0.28?±?0.08 μmol CO₂ m^-2?s^-1, two thirds of which was from R_B. Shrub patchiness produced spatial variation in soil respiration, whereas temporal dynamics of soil respiration were affected mainly by soil water content. Shallow soil water content (0–20 cm) influenced R_T and R_B, whereas deep soil water content (30–210 cm) influenced R_R and the ratio R_R/R_T. During most of the year when soil water content was below field capacity, diurnal changes in soil respiration were partially decoupled from soil temperature but could be modeled using soil temperature and photosynthetic active radiation.

Conclusions

Sand-dune stabilization increased soil respiration, and increased R_B from biological soil crust and altered soil properties such as increased soil organic matter contributed more than increased R_R from increased shrubs.     

Respiratory C fluxes and root exudation differ in two full-sib clones of Pinus taeda (L.) under contrasting fertilizer regimes in a greenhouse

Aims

We investigated whether changes in respiratory C fluxes, soil CO₂ efflux, or root exudate quantity or quality explained differences in growth rates between closely related clones of Pinus taeda (L.).

Methods

A factorial design with two clones, fertilized and control treatments, and four sequential harvests was installed in a greenhouse for 121 days.

Results

The two clones did show significant differences in respiratory C fluxes, soil CO₂ efflux, and root exudation quantity and quality. While the clones also differed in growth rates, the C fluxes assessed in this paper did not explain how seedlings were able to allocate more C to stem growth in the months following fertilizer application. Changes in root exudation were not consistent with reduced heterotrophic soil CO₂ efflux, which does not appear to be a plant-mediated process.

Conclusions

These results indicate that if single genotypes are deployed over large land areas in plantations, dramatic differences between clonal plant-soil interactions may require consideration in ecosystem C budgets. Further, the range of belowground fluxes observed implies that genotype-specific C allocation may make some clones better able to exploit a given resource environment than others.     

Phosphorus supply enhances the response of legumes to elevated CO2 (FACE) in a phosphorus-deficient vertisol

Background & aims

Understanding the mechanism of how phosphorus (P) regulates the response of legumes to elevated CO₂ (eCO₂) is important for developing P management strategies to cope with increasing atmospheric CO₂ concentration. This study aimed to explore this mechanism by investigating interactive effects of CO₂ and P supply on root morphology, nodulation and soil P fractions in the rhizosphere.

Methods

A column experiment was conducted under ambient (350?ppm) (aCO₂) and eCO₂ (550?ppm) in a free air CO₂ enrichment (FACE) system. Chickpea and field pea were grown in a P-deficient Vertisol with P addition of 0–16?mg?P?kg^?1.

Results

Increasing P supply increased plant growth and total P uptake with the increase being greater under eCO₂ than under aCO₂. Elevated CO₂ increased root biomass and length, on average, by 16?% and 14?%, respectively. Nodule biomass increased by 46?% in response to eCO₂ at 16?mg P kg^?1, but was not affected by eCO₂ at no P supply. Total P uptake was correlated with root length while N uptake correlated with nodule number and biomass regardless of CO₂ level. Elevated CO₂ increased the NaOH-extractable organic P by 92?% when 16?mg P kg^?1 was applied.

Conclusion

The increase in P and N uptake and nodule number under eCO₂ resulted from the increased biomass production, rather than from changes in specific root-absorbing capability or specific nodule function. Elevated CO₂ appears to enhance P immobilization in the rhizosphere.     

Spatial distribution and turnover of root-derived carbon in alfalfa rhizosphere depending on top- and subsoil properties and mycorrhization

Aims

This study analyzed the extent to which root exudates diffuse from the root surface towards the soil depending on topsoil and subsoil properties and the effect of arbuscular mycorrhizal fungal hyphae on root-derived C distribution in the rhizosphere.

Methods

Alfalfa was grown in three-compartment pots. Nylon gauze prevented either roots alone or roots and arbuscular mycorrhizal fungal hyphae from penetrating into the rhizosphere compartments. ¹⁴CO₂ pulse labeling enabled the measurement of ¹⁴C-labeled exudates in dissolved (DOC) and total organic carbon (TOC) in the rhizosphere, distributed either by diffusion alone or by diffusion, root hair and hyphal transport.

Results

Root exudation and microbial decomposition of exudates was higher in the rhizosphere with topsoil compared to subsoil properties. Exudates extended over 28 mm (DOC) and 20 mm (TOC). Different soil properties and mycorrhization, likely caused by the low arbuscular mycorrhizal colonization of roots (13?±?4 % (topsoil properties) and 18?±?5 % (subsoil properties)), had no effect.

Conclusions

Higher microbial decomposition compensated for higher root exudation into the rhizosphere with topsoil properties, which resulted in equal exudate extent when compared to the rhizosphere with subsoil properties. Higher ¹⁴C activity used for labeling compared with previous studies enabled the detection of low exudate concentrations at longer distances from the root surface.     

Temperature sensitivity of soil carbon and nitrogen mineralization: impacts of nitrogen species and land use type

Background and aims

Climate warming, nitrogen (N) deposition and land use change are some of the drivers affecting ecosystem processes such as soil carbon (C) and N dynamics, yet the interactive effects of those drivers on ecosystem processes are poorly understood. This study aimed to understand mechanisms of interactive effects of temperature, form of N deposition and land use type on soil C and N mineralization.

Methods

We studied, in a laboratory incubation experiment, the effects of temperature (15 vs. 25 °C) and species of N deposition (NH₄ ⁺-N vs. NO₃ ^?-N) on soil CO₂ efflux, dissolved organic C (DOC) and N (DON), NH₄ ⁺-N, and NO₃ ^?-N concentrations using intact soil columns collected from adjacent forest and grassland ecosystems in north-central Alberta.

Results

Temperature and land use type interacted to affect soil CO₂ efflux, concentrations of DON, NH₄ ⁺-N and NO₃ ^?-N in most measurement times, with the higher incubation temperature resulted in the higher CO₂ efflux and NH₄ ⁺-N concentrations in forest soils and higher DON and NO₃ ^?-N concentrations in grassland soils. Temperature and land use type affected the cumulative soil CO₂ efflux, and DOC, DON, NH₄ ⁺-N and NO₃ ^?-N concentrations. The form of N added or its interaction with the other two factors did not affect any of the C and N cycling parameters.

Conclusions

Temperature and land use type were dominant factors affecting soil C loss, with the soil C in grassland soils more stable and resistant to temperature changes. The lack of short-term effects of the deposition of different N species on soil C and N mineralization suggest that maybe there was a threshold for the N effect to kick in and long-term experiments should be conducted to further elucidate the species of N deposition effects on soil C and N cycling in the studied systems.     

Genetic variation in plant below-ground response to elevated CO2 and two herbivore species

Aims

It is unclear how changing atmospheric conditions, including rising carbon dioxide concentration, influence interactions between above and below-ground systems and if intraspecific variation exists in this response.

Methods

We assessed interactive effects of atmospheric CO₂ concentration, above-ground herbivory, and plant genotype on root traits and mycorrhizal associations. Plants from five families of Asclepias syriaca, a perennial forb, were grown under ambient and elevated atmospheric CO₂ concentrations. Foliar herbivory by either lepidopteran caterpillars or phloem-feeding aphids was imposed. Mycorrhizal colonization, below-ground biomass, root biomass, and secondary defensive chemistry in roots were quantified.

Results

We observed substantial genetic variation among A. syriaca families in their mycorrhizal colonization levels in response to elevated CO₂ and herbivory treatments. Elevated CO₂ treatment increased root biomass in all genetic families, whereas foliar herbivory tended to decrease root biomass. Root cardenolide concentration and composition varied greatly among plant families, and elevated CO₂ treatment increased root cardenolides in two of the five plant families. Moreover, herbivores differentially affected the composition of cardenolides expressed below ground.

Conclusions

Increased atmospheric CO₂ has the potential to influence interactions among plants, herbivores and mycorrhizal fungi and intraspecific variation suggests that such interactions can evolve.     

Sucrose Transport and Phloem Unloading in Stem of Vicia faba: Possible Involvement of a Sucrose Carrier and Osmotic Regulation

Stems of Vicia faba plants were used to study phloem unloading because they are hollow and have a simple anatomical structure that facilitates access to the unloading site. After pulse labeling of a source leaf with ¹⁴CO₂, stem sections were cut and the efflux characteristics of ¹⁴C-labeled sugars into various buffered solutions were determined. Radiolabeled sucrose was shown to remain localized in the phloem and adjacent phloem parenchyma tissues after a 2-hour chase. Therefore, sucrose leakage from stem segments prepared following a 75-minute chase period was assumed to be characteristic of phloem unloading. The efflux of ¹⁴C assimilates from the phloem was enhanced by 1 millimolar p-chloromercuribenzene sulfonic acid (PCMBS) and by 5 micromolar carbonyl cyanide m-chlorophenly hydrazone (CCCP). However, PCMBS inhibited and CCCP enhanced general leakage of nonradioactive sugars from the stem segments. Sucrose at concentrations of 50 millimolar in the free space increased efflux of [¹⁴C]sucrose, presumably through an exchange mechanism. This exchange was inhibited by PCMBS and abolished by 0.2 molar mannitol. Increasing the osmotic concentration of the efflux medium with mannitol reduced [¹⁴C]sucrose efflux. However, this inhibition seems not to be specific to sucrose unloading since leakage of total sugars, nonlabeled sucrose, glucose, and amino acids from the bulk of the tissue was reduced in a similar manner. The data suggest that phloem unloading in cut stem segments is consistent with passive efflux of sucrose from the phloem to the apoplast and that sucrose exchange via a membrane carrier may be involved. This is consistent with the known conductive function of the stem tissues, and contrasts with the apparent nature and function of unloading in developing seeds.     

Greenhouse gas emissions from forestry in East Norway

Purpose

So far no calculations have been made for greenhouse gas (GHG) emissions from forestry in East Norway. This region stands for 80 % of the Norwegian timber production. The aim of this study was to assess the annual GHG emissions of Norwegian forestry in the eastern parts of the country from seed production to final felling and transport of timber to sawmill and wood processing industry (cradle-to-gate inventory), based on specific Norwegian data.

Methods

The life cycle inventory was conducted with SimaPro applying primary and secondary data from Norwegian forestry. GHG emissions of fossil-related inputs from the technosphere were calculated for the functional unit of 1 m³ timber extracted and delivered to industry gate in East Norway in 2010. The analysis includes seed and seedling production, silvicultural operations, forest road construction and upgrading, thinning, final felling, timber forwarding and timber transport on road and rail from the forest to the industry. Norwegian time studies of forestry machines and operations were used to calculate efficiency, fuel consumption and transport distances. Due to the lack of specific Norwegian data in Ecoinvent, we designed and constructed unit processes based on primary and secondary data from forestry in East Norway.

Results and discussion

GHG emissions from forestry in East Norway amounted to 17.893 kg CO₂-equivalents per m³ of timber delivered to industry gate in 2010. Road transport of timber accounted for almost half of the total GHG emissions, final felling and forwarding for nearly one third of the GHG emissions. Due to longer road transport distances, pulpwood had higher impact on the climate change category than saw timber. The construction of forest roads had the highest impact on the natural land transformation category. The net CO₂ emissions of fossil CO₂ corresponded to 2.3 % of the CO₂ sequestered by 1 m³ of growing forest trees and were compared to a calculation of biogenic CO₂ release from the forest floor as a direct consequence of harvesting.

Conclusions

Shorter forwarding and road transport distances, increased logging truck size and higher proportion of railway transport may result in lower emissions per volume of transported timber. A life cycle assessment of forestry may also consider impacts on environmental categories other than climate change. Biogenic CO₂ emissions from the soil may be up to 10 times higher than the fossil-related emissions, at least in a short-term perspective, and are highly dependent on stand rotation length.     

Mind the gap: non‐biological processes contributing to soil CO2 efflux

Widespread recognition of the importance of soil CO₂ efflux as a major source of CO₂ to the atmosphere has led to active research. A large soil respiration database and recent reviews have compiled data, methods, and current challenges. This study highlights some deficiencies for a proper understanding of soil CO₂ efflux focusing on processes of soil CO₂ production and transport that have not received enough attention in the current soil respiration literature. It has mostly been assumed that soil CO₂ efflux is the result of biological processes (i.e. soil respiration), but recent studies demonstrate that pedochemical and geological processes, such as geothermal and volcanic CO₂ degassing, are potentially important in some areas. Besides the microbial decomposition of litter, solar radiation is responsible for photodegradation or photochemical degradation of litter. Diffusion is considered to be the main mechanism of CO₂ transport in the soil, but changes in atmospheric pressure and thermal convection may also be important mechanisms driving soil CO₂ efflux greater than diffusion under certain conditions. Lateral fluxes of carbon as dissolved organic and inorganic carbon occur and may cause an underestimation of soil CO₂ efflux. Traditionally soil CO₂ efflux has been measured with accumulation chambers assuming that the main transport mechanism is diffusion. New techniques are available such as improved automated chambers, CO₂ concentration profiles and isotopic techniques that may help to elucidate the sources of carbon from soils. We need to develop specific and standardized methods for different CO₂ sources to quantify this flux on a global scale. Biogeochemical models should include biological and non‐biological CO₂ production processes before we can predict the response of soil CO₂ efflux to climate change. Improving our understanding of the processes involved in soil CO₂ efflux should be a research priority given the importance of this flux in the global carbon budget.     

Management of nitrogen fertilizer application, rather than functional gene abundance, governs nitrous oxide fluxes in hydroponics with rockwool

Aims

Nitrous oxide (N₂O) is a strong greenhouse effective gas (GHG); the primary human source of N₂O is agricultural activities. Excessive nitrogen (N) fertilization of agricultural land is now widely recognized as a major contributor. In soil, the microbial processes of nitrification and denitrification are the principal sources of N₂O. However, it remains poorly understood how conventional hydroponics influences GHG emission. Here, we compared GHG fluxes from soil and rockwool used for hydroponics under identical nutrient conditions.

Methods

Tomato plants (Solanum lycopersicum, momotaro) were grown in soil or by hydroponics using rockwool. In situ emissions of CH₄, CO₂, and N₂O, and the abundance of genes involved in nitrification and denitrification were measured during cultivation.

Results

Hydroponics with rockwool mitigated CO₂ emission by decreasing the microbial quantity in the rhizosphere. Dilution of the nutrient solution significantly decreased N₂O emission from rockwool. Although proliferation of nitrifiers or denitrifiers in the rhizosphere did not induce N₂O emission, reuse or long-term use of rockwool induced a 3.8-fold increase in N₂O emission.

Conclusions

Our data suggest that hydroponics has a lower environmental impact and that adequate fertilizer application, rather than bacterial control, governs N₂O fluxes in hydroponic cultivation using rockwool.