Apparent Respiratory Quotient Research Articles

The Apparent Respiratory Quotient of Soils and Tree Stems and the Processes That Control It

AbstractThe CO2/O2 fluxes ratio (apparent respiration quotient [ARQ]) measured in soils and plants contains valuable information about the respiratory‐substrate stoichiometry and biotic and abiotic non‐respiratory processes. We investigated ARQ variability by measurements in soil pore space air, and in headspace air from incubations of bulk‐soil and tree stem tissues (both fresh and 24‐hr stored tissues) in 10 measurement campaigns over 15 months in a Mediterranean oak forest. Mean (range) ARQ values were: soil air, 0.76 (0.60–0.92); bulk soil, 0.75 (0.53–0.90); fresh stem tissues, 0.39 (0.19–0.70); and stored stem tissues, 0.68 (0.42–1.08). The variability in tree stems was assumed to be controlled by CO2 re‐fixation that lowered ARQ from 1.0, the value expected for carbohydrate respiration in plants. We estimate that the values of the stored tissues represent better stem metabolism since the fresh‐tissue results contained a signal of wound‐response O2 uptake that further lowered ARQ. The mean bulk‐soil ARQ (0.75) was considerably lower than expected by soil organic matter (SOM) stoichiometry (0.95). This lower value might represent the stoichiometry of the SOM sub‐pool that supports respiration, and/or oxidative depolymerization that increases O2 fluxes. Abiotic O2 uptake was demonstrated to reduce bulk‐soil ARQ down to 0.37 and consume Fe2+, but estimated to have small effect under typical respiration rates. Soil‐air ARQ was usually higher than bulk‐soil ARQ and lower than root ARQ (which, when measured, ranged from 0.73 to 0.96), demonstrating the potential of ARQ to partition the autotrophic and heterotrophic sources of soil respiration. The limitations of this partitioning method are discussed.

Apparent respiratory quotient observed in headspace of static respirometers underestimates cellular respiratory quotient of pear fruit

Abstract A three-compartment non-equilibrium gas transport model of ‘Conference’ pear fruit under controlled atmosphere (CA) storage was developed. The model fruit tissue consists of cells, in which the concentrations of respiratory gasses can show gradients, and intercellular space, in which gasses are uniformly distributed. Non-equilibrium of gas concentrations in the cell compartment and intercellular space is assumed. A respiration model based on Michaelis-Menten respiration kinetics without inhibition of respiration by CO2 and incorporating down-regulation of the maximal O2 consumption rate in response to O2 was developed. Conversion of CO2 dissolved in the cell compartment to hydrogen carbonate at a constant pH of 5.0 was included. The model was validated based on experimental data of ‘Conference’ pear fruit during a complete depletion experiment starting from 3.58 mol m−3 O2 and 0.00 mol m−3 CO2. Model predictions match experimental observations well. Gas concentrations in the cell compartment were found to be in equilibrium with the gas concentrations in the intercellular space. The model was used to calculate apparent respiration rates and RQ as if measured in the storage headspace. Apparent values were compared to actual values in the fruit cells and it was found that apparent respiration rates and RQ, calculated based on headspace measurements, underestimated the actual respiration rate and respiratory quotient in the fruit cells. Relative differences of 4 %, 41 % and 41 % were found for the apparent O2 consumption rate, CO2 production rate and RQ, respectively. This affects the design of commercial RQ based DCA systems.

Soil Fluid Biogeochemical Response to Climatic Events

AbstractPredicting fluid biogeochemistry in the vadose zone is difficult because of time‐dependent variation in multiple controlling factors, such as temperature, moisture, and biological activity. Furthermore, soils are multicomponent, heterogeneous porous media where manifold reactions may be affecting solution chemistry. We postulated that ecosystem‐scale processes, such as carbon fixation and ecohydrologic partitioning, control subsurface biogeochemical reactions, including mineral weathering. To test this hypothesis, we applied a novel “instrumented pedon” research approach. Analysis of the data streams demonstrates the interactions between pulsed wetting events and biogeochemical processes in the soil profile, and along groundwater flow paths. Rapid wetting front propagation into dry soil resulted in a pulsed increase in CO2 partial pressure in deeper soil layers, whereas wetting front propagation into a premoistened soil profile showed the opposite effect. The apparent respiratory quotient (ARQ), calculated from CO2 and O2 fluxes, deviated from expected oxidative ratios particularly during soil wetting events. These deviations were correlated in time with pore water geochemical responses, revealing that a fraction of the respired CO2 was consumed locally in pulsed silicate weathering events that accompanied wetting‐front propagation. However, most of this CO2 was dissolved in the soil pore water and transported downgradient, and along the soil‐bedrock interface, where a portion of it was further consumed in silicate weathering reactions, and another portion was degassed to the atmosphere. These results highlight the tight coupling that exists between physical, biological, and chemical processes, on event time scales, during incremental co‐evolution of the critical zone, particularly in water‐limited systems.

Development and deployment of a field-portable soil O2 and CO2 gas analyzer and sampler.

Here we present novel method development and instruction in the construction and use of Field Portable Gas Analyzers study of belowground aerobic respiration dynamics of deep soil systems. Our Field-Portable Gas Analysis (FPGA) platform has been developed at the Calhoun Critical Zone Observatory (CCZO) for the measurement and monitoring of soil O2 and CO2 in a variety of ecosystems around the world. The FPGA platform presented here is cost-effective, lightweight, compact, and reliable for monitoring dynamic soil gasses in-situ in the field. The FPGA platform integrates off-the-shelf components for non-dispersive infrared (NDIR) CO2 measurement and electro-chemical O2 measurement via flow-through soil gas analyses. More than 2000 soil gas measurements have been made to date using these devices over 4 years of observations. Measurement accuracy of FPGAs is consistently high as validated via conventional bench-top gas chromatography. Further, time series representations of paired CO2 and O2 measurement under hardwood forests at the CCZO demonstrate the ability to observe and track seasonal and climatic patterns belowground with this FPGA platform. Lastly, the ability to analyze the apparent respiratory quotient, the ratio of apparent CO2 accumulation divided by apparent O2 consumption relative to the aboveground atmosphere, indicates a high degree of nuanced analyses are made possible with tools like FPGAs. In sum, the accuracy and reliability of the FPGA platform for soil gas monitoring allows for low-cost temporally extensive and spatially expansive field studies of deep soil respiration.

Soil CO2 and O2 Concentrations Illuminate the Relative Importance of Weathering and Respiration to Seasonal Soil Gas Fluctuations

Soil CO2 and O2 cycles are coupled in some processes (e.g., respiration) but uncoupled in others (e.g., silicate weathering). One benchmark for interpreting soil biogeochemical processes affected by soil pCO2 and pO2 is to calculate the apparent respiratory quotient (ARQ). When aerobic respiration and diffusion are the dominant controls on gas concentrations, ARQ equals 1; ARQ deviates from 1 when other processes dominate soil CO2 and O2 chemistry. Here, we used ARQ to understand lithologic, hillslope, and seasonal controls on soil gases at the Susquehanna Shale Hills Critical Zone Observatory in central Pennsylvania. We measured soil pCO2 and pO2 at three depths from the soil surface to bedrock across catenas in one shale and one sandstone watershed over three growing seasons. We found that both parent lithology and hillslope position significantly affect soil gas concentrations and ARQ. Soil pCO2 was highest (>5%) and pO2 was lowest (<16%) in the valley floors. Controlling for depth, pCO2 was higher and pO2 was lower across all sites in the sandstone watershed. We attribute this pattern to higher macroporosity in sandstone lithologies, which results in greater root respiration at depth. We recorded seasonal variation in ARQ at all sites, with ARQ rising above 1 during July through September, and dipping below 1 in the early spring. We hypothesize that this seasonal fluctuation arises from anaerobic respiration in reducing microsites July through September when the soils are wet and demand for O2 is high, followed by oxidation of reduced species when the soils drain and re‐oxygenate. We estimate that this anaerobic respiration in microsites contributes 36 g C m−2 yr−1 to the soil C flux. Our results provide evidence for a conceptual model of metal cycling in temperate watersheds and point to the importance of anaerobic respiration to the carbon flux from forest soils.Core Ideas Hillslope position and lithology affect soil CO2 and O2 in humid temperate forests. The ratio of CO2 and O2 (ARQ) fluctuates over the growing season. The ARQ fluctuations indicate seasonal metal redox cycling at all hillslope positions. Anaerobic respiration is important to soil CO2 flux during the late growing season.

Comparison of CO<sub>2</sub> and O<sub>2</sub> fluxes demonstrate retention of respired CO<sub>2</sub> in tree stems from a range of tree species

Abstract. The ratio of CO2 efflux to O2 influx (ARQ, apparent respiratory quotient) in tree stems is expected to be 1.0 for carbohydrates, the main substrate supporting stem respiration. In previous studies of stem fluxes, ARQ values below 1.0 were observed and hypothesized to indicate retention of respired carbon within the stem. Here, we demonstrate that stem ARQ < 1.0 values are common across 85 tropical, temperate, and Mediterranean forest trees from nine different species. Mean ARQ values per species per site ranged from 0.39 to 0.78, with an overall mean of 0.59. Assuming that O2 uptake provides a measure of in situ stem respiration (due to the low solubility of O2), the overall mean indicates that on average 41 % of CO2 respired in stems is not emitted from the local stem surface. The instantaneous ARQ did not vary with sap flow. ARQ values of incubated stem cores were similar to those measured in stem chambers on intact trees. We therefore conclude that dissolution of CO2 in the xylem sap and transport away from the site of respiration cannot explain the low ARQ values. We suggest refixation of respired CO2 in biosynthesis reactions as possible mechanism for low ARQ values.

A considerable fraction of soil-respired CO2 is not emitted directly to the atmosphere

Soil CO2 efflux (Fsoil) is commonly considered equal to soil CO2 production (Rsoil), and both terms are used interchangeably. However, a non-negligible fraction of Rsoil can be consumed in the subsurface due to a host of disparate, yet simultaneous processes. The ratio between CO2 efflux/O2 influx, known as the apparent respiratory quotient (ARQ), enables new insights into CO2 losses from Rsoil not previously captured by Fsoil. We present the first study using continuous ARQ estimates to evaluate annual CO2 losses of carbon produced from Rsoil. We found that up to 1/3 of Rsoil was emitted directly to the atmosphere, whereas 2/3 of Rsoil was removed by subsurface processes. These subsurface losses are attributable to dissolution in water, biological activities and chemical reactions. Having better estimates of Rsoil is key to understanding the true influence of ecosystem production on Rsoil, as well as the role of soil CO2 production in other connected processes within the critical zone.

Ratio of CO2 and O2 as index for categorising soil biological activity in sugarcane areas under contrasting straw management regimes

This study was developed in a sugarcane area under contrasting management regimes defined by mechanical green harvesting (GH) and burning harvesting (BH) to test the hypotheses that the ratio of carbon dioxide (CO2) and oxygen (O2), known as the apparent respiratory quotient (ARQ), can be used to categorise soil biological activity. The study aimed to (i) examine the profile and relationship between the CO2 flux (FCO2) and the O2 flux (FO2) in a sugarcane area under mechanical harvesting with straw burning (BH) and mechanical harvesting with maintenance of straw (GH), considering soil moisture; (ii) and suggest the use of ARQ as an index for categorising the biological activity of soils. Our results showed consistently lower FCO2 for soil moisture in the range of 6.0–8.6% for both management regimes. The soil moisture increments triggered a decrease in FO2 and an increase in FCO2 and ARQ. The FCO2 and FO2 were positively correlated under BH. The BH yielded a cumulative CO2 emission of 53.68% higher than for GH. Overall, our findings revealed that soil moisture affected the O2 uptake and CO2 emission profile of soil, limiting O2 uptake and increasing CO2 releases for water-filled porosity below 70%. The GH management system, which incorporates sugarcane residues into the superficial layer of the soil, can help protect against soil erosion. The ARQ can be used as an index to categorise biological activity in soil, where ARQ values close to 1 can be considered a reflection of aerobic activity with balance between CO2 production and O2 consumption.

CO2 production, dissolution and pressure dynamics during silage production: multi-sensor-based insight into parameter interactions

Silage is a critical global feedstock, but is prone to aerobic deterioration. The dominant mechanism of O2 transport into silage remains unresolved. Here, multiple sensors tracked O2 and CO2, gas pressure (ΔP) between internal silage and ambient air, pH and silage temperature (Tsi) during the ensilage of maize and ryegrass. We report the first observation that CO2 produced from microbial respiration was partially dissolved in silage water, with evidence of negative or positive ΔP depending on the changing balance between CO2 production and dissolution. The ΔP < 0 reflected an apparent respiratory quotient (RQ) < 1. Net CO2 production was much greater in anaerobic fermentation stage than in initial aerobic phase or later aerobic feed-out phase. O2 transport into silage is intimately linked to the dynamics of net CO2, ΔP, microbial activity, pH and Tsi. These results suggested that both gas diffusion (based on Fick’s law) and advective transfer (Darcy’s law) play equally important roles in governing the complex temporal progression of inward and outward gas fluxes to and from the silage interior. Even though low pH suppressed microbial activity and supported aerobic stability, the negative ΔP increased the risk of O2 entry and aerobic deterioration during feed-out phase.

Temperature Dependence of Apparent Respiratory Quotients and Oxygen Penetration Depth in Contrasting Lake Sediments

AbstractLake sediments constitute an important compartment in the carbon cycle of lakes, by burying carbon over geological timescales and by production and emission of greenhouse gases. The degradation of organic carbon (OC) in lake sediments is linked to both temperature and oxygen (O2), but the interactive nature of this regulation has not been studied in lake sediments in a quantitative way. We present the first systematic investigation of the effects of temperature on the apparent respiratory quotient (RQ, i.e., the molar ratio between carbon dioxide (CO2) production and O2 consumption) in two contrasting lake sediments. Laboratory incubations of sediment cores of a humic lake and an eutrophic lake across a 1–21°C temperature gradient over 157 days revealed that both CO2 production and O2 consumption were positively, exponentially, and similarly dependent on temperature. The apparent RQ differed significantly between the lake sediments (0.63 ± 0.26 and 0.99 ± 0.28 in the humic and the eutrophic lake, respectively; mean ± SD) and was significantly and positively related to temperature. The O2 penetration depth into the sediment varied by a factor of 2 over the 1–21°C temperature range and was significantly, negatively, and similarly related to temperature in both lake sediments. Accordingly, increasing temperature may influence the overall extent of OC degradation in lake sediments by limiting O2 supply to aerobic microbial respiration to the topmost sediment layer, resulting in a concomitant shift to less effective anaerobic degradation pathways. This suggests that temperature may represent a key controlling factor of the OC burial efficiency in lake sediments.

Low sediment‐water gas exchange in a small boreal lake

AbstractBoreal lake sediments are carbon sources by producing CO2. CO2 flux from sediments is partly controlled by turbulence in the water column, which is not given the same attention as CO2 production rates in current estimates of CO2 fluxes from sediments. We quantified the in situ CO2 flux across the sediment‐water interface in a small (0.07 km2) lake in Sweden by measuring the in situ O2 flux with the Eddy Correlation (EC) method and using the apparent respiratory quotient (CO2 production:O2 consumption) derived from sediment incubations. We demonstrate that median CO2 flux estimated by EC was ~70% smaller than estimated by sediment incubations with artificial water mixing (1.0 × 10−2 and 3.6 × 10−2 µmol C m−2 s−1, respectively). Additionally, we show that inducing artificial mixing of supernatant water in the incubation experiment has a positive effect on observed fluxes, enhancing CO2 flux by ~30% compared to not mixing supernatant water. We suggest that the difference between the methods is due to the strong artificial water mixing in sediment incubations compared to the turbulent mixing in this small lake. Additionally, low O2 supply to sediment aerobic heterotrophic microbes during extended periods of low water currents can inhibit respiration and thus CO2 production. These findings suggest that the sediment contribution to total lake CO2 emission might currently be overestimated for small boreal lakes. Care should be taken when upscaling sediment CO2 flux derived from incubation experiments to entire basins of small lakes, as incubation experiments are unlikely to accurately mimic in situ bottom water currents and gas exchange.

Measuring the ratio of CO2efflux to O2influx in tree stem respiration

In recent studies, the ratio of tree stem CO2 efflux to O2 influx has been defined as the apparent respiratory quotient (ARQ). The metabolism of carbohydrates, the putative respiratory substrate in trees, is expected to yield an ARQ of 1.0. However, previous studies have reported ARQ values ranging between 0.23 and 0.90. These interesting results may indicate internal transport of respired CO2 within stems; yet no simple field applicable methods for ARQ measurement have been available. Here, we report on the assembly of a closed circulating system called 'Hampadah', which uses CO2 and O2 analyzers to measure air samples from stem chambers. We tested the performance of the Hampadah with samples from 36 trees (Tetragastris panamensis (Engl.) Kuntze). Additionally, we showed the feasibility of measuring ARQ directly from stem chambers, using portable CO2 and O2 sensors, in both discrete and continuous modes of operation. The Hampadah measurement proved to be consistent with CO2 gas standards (R2 = 0.999) and with O2 determined by O2/Ar measurements with a mass spectrometer (R2 = 0.998). The Hampadah gave highly reproducible results for ARQ determination of field samples (±0.01 for duplicates). The portable sensors measurement showed good correlation with the Hampadah in measuring CO2, O2 and ARQ (n = 5, R2 = 0.97, 0.98 and 0.91, respectively). We have demonstrated here that the Hampadah and the sensors' methods enable accurate ARQ measurements for both laboratory and field research.

Using O&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; to study the relationships between soil CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; efflux and soil respiration

Abstract. Soil respiration is the sum of respiration processes in the soil and is a major flux in the global carbon cycle. It is usually assumed that the CO2 efflux is equal to the soil respiration rate. Here we challenge this assumption by combining measurements of CO2 with high-precision measurements of O2. These measurements were conducted on different ecosystems and soil types and included measurements of air samples taken from the soil profile of three Mediterranean sites: a temperate forest and two alpine forests. Root-free soils from the alpine sites were also incubated in the lab. We found that the ratio between the CO2 efflux and the O2 influx (defined as apparent respiratory quotient, ARQ) was in the range of 0.14 to 1.23 and considerably deviated from the value of 0.9 ± 0.1 expected from the elemental composition of average plants and soil organic matter. At the Mediterranean sites, these deviations are explained as a result of CO2 dissolution in the soil water and transformation to bicarbonate ions in these high-pH soils, as well as by carbonate mineral dissolution and precipitation processes. Thus, a correct estimate of the short-term, chamber-based biological respiratory flux in such soils can only be made by dividing the measured soil CO2 efflux by the average (efflux-weighted) soil profile ARQ. Applying this approach to a semiarid pine forest resulted in an estimated short-term biological respiration rate that is 3.8 times higher than the chamber-measured surface CO2. The ARQ values often observed in the more acidic soils were unexpectedly low (&lt; 0.7). These values probably result from the oxidation of reduced iron, which has been formed previously during times of high soil moisture and local anaerobic conditions inside soil aggregates. The results reported here provide direct quantitative evidence of a large temporal decoupling between soil–gas exchange fluxes and biological soil respiration.

Internal respiration of Amazon tree stems greatly exceeds external CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; efflux

Abstract. Respiration in tree stems is an important component of forest carbon balance. The rate of CO2 efflux from the stem has often been assumed to be a measure of stem respiration. However, recent work in temperate forests has demonstrated that stem CO2 efflux can either overestimate or underestimate respiration rate because of emission or removal of CO2 by transport in xylem water. Here, we studied gas exchange from stems of tropical forest trees using a new approach to better understand respiration in an ecosystem that plays a key role in the global carbon cycle. Our main questions were (1) is internal CO2 transport important in tropical trees, and, if so, (2) does this transport result in net release of CO2 respired in the roots at the stem, or does it cause the opposite effect of net removal of stem-respired CO2? To answer these questions, we measured the ratio of stem CO2 efflux to O2 influx. This ratio, defined here as apparent respiratory quotient (ARQ), is expected to equal 1.0 if carbohydrates are the substrate for respiration, and the net transport of CO2 in the xylem water is negligible. Using a stem chamber approach to quantifying ARQ, we found values of 0.66 ± 0.18. These low ARQ values indicate that a large portion of respired CO2 (~ 35%) is not emitted locally, and is probably transported upward in the stem. ARQ values of 0.21 ± 0.10 were found for the steady-state gas concentration within the stem, sampled by in-stem equilibration probes. These lower values may result from the proximity to the xylem water stream. In contrast, we found ARQ values of 1.00 ± 0.13 for soil respiration. Our results indicate the existence of a considerable internal flux of CO2 in the stems of tropical trees. If the transported CO2 is used in the canopy as a substrate for photosynthesis, it could account for up to 10% of the C fixed by the tree, and perhaps serve as a mechanism that buffers the response of the tree to changing CO2 levels. Our results also indicate, in agreement with previous work, that the widely used CO2 efflux approach for determining stem respiration is unreliable. We demonstrate here a field applicable approach for measuring the O2 uptake rate, which we suggest to be a more appropriate method to estimate stem respiration rates.

Respiratory gas exchanges of termites from the Sabah (Borneo) assemblage

Abstract.Oxygen uptake and carbon dioxide release at 28°C were determined in worker castes of twenty‐six species of forest termites from the Danum Valley Conservation Area, south‐east Sabah, by Warburg manometry.Metabolic rate varied inversely with body weight in a suite of soil‐, wood/soil‐ and wood‐feeding species, giving a slope (in a log–log plot) of – 0.63. However, a number of large species, actively foraging forms such as Macrotermes malaccensis, M. gilvus, Havilanditermes atripennis and Hospitalitermes hospitalis, but also the wood‐feeding Schedorhinotermes sarawakensis, showed an oxygen consumption greater than expected for their body weight. Rates of methane emission were above 0.100 μmol g–1 h–1 in seventeen species, with very high fluxes in two wood/soil‐feeders, Termes borneensis (0.546 ± 0.163 μmol g–1 h–1) and Prohamitermes mirabilis (0.303 ± 0.123 μmol g–1 h–1). Of the fifteen remaining species, seven were soil‐feeders, five were wood‐feeders, two were wood/litter‐feeders and a single species fed on lichen and moss. Low or negligible CH4 emissions (&lt; 0.100 μmol g–1 h–1) were observed in three other species, all wood‐feeders.An apparent respiratory quotient (RQapp) was calculated using xCO2 and xO2 (corrected for methane emission, but not hydrogen). Mean RQapp was at or above 1.00 in eleven species and between 0.95 and 1.00 in a further six species, the two sets of species together representing all trophic groups, including lichen‐feeders. This is argued to be consistent with carbohydrate being the principal substrate supporting respiration.

Alkalinities of Maine lakes: Are they really changing?

Alkalinity changes between 1938–1944 and 1985 were evaluated for Maine lakes (zmax ≳ 15 m) in granitic watersheds. Modern (late summer 1985) Gran alkalinities for the hypolimnion were compared with historical alkalinities computed using: urn:x-wiley:00243590:media:lno19903561238:lno19903561238-math-0001 where pH(T) is the historic reported colorimetric pH at each hypolimnetic sampling depth and temperature, pK'1(T) the temperature‐dependent negative logarithm of the first dissociation constant for carbonic acid, and H2CO3* represents initial CO2 at last spring overturn, plus the change in CO2 associated with the hypolimnetic oxygen deficit developed over summer (estimated from the historic oxygen data). With modern data for representative Maine lakes and historical (1932) data on dissolved gases reported for northern Wisconsin lakes, the apparent respiratory quotient relating ΔCO2 to ΔO2 = 0.92. The above model yields alkalinity estimates with a relative root‐mean‐square (RMS) error of ∼15%.The alkalinities of the Maine lakes have increased by 15–50 µeq liter−1 (&gt;30%) since the original surveys. Lake alkalization is not the result of differences in antecedent weather, since abnormally dry weather predating 1942 would have acted to elevate alkalinities.

The effect of restricted air supply on some insects which infest grain

SUMMARYIf insect pests of grain develop in a strictly airtight container all are killed by exhaustion of oxygen when the concentration of this gas falls to approximately 2%. If the walls of the container permit limited access of oxygen from the atmosphere by diffusion, at a rate dependent on the difference between the internal and external concentrations, the insect population may also be killed, but this will depend on the amount of diffusion and on the level of the original population. A method for investigating this in the laboratory using polythene diaphragms to admit oxygen is described. A population of six adult Sitophilus granarius per pound of wheat in a container admitting 7·5 mg. of oxygen per litre per day at zero internal concentration will not be killed immediately but will be reduced to a very low level and may eventually die out after many months. In the same container a smaller number of insects may survive sufficiently long to increase in numbers to a level at which they also are killed. At a critical population level (about twelve adults per pound) there may occur sharp fluctuations in oxygen concentration, and presumably of insect activity, which may or may not lead to death. A population initially much greater than twelve adults per pound will certainly be killed although this may take a little longer than in an airtight container.If the container is substantially full of wheat having a moisture content near 14% the apparent respiratory quotient of the insects is about 0·69 but if the proportion of the air space occupied by wheat is reduced, the apparent respiratory quotient is increased until it is approximately unity when the volume of wheat is about 5% of the volume of the container. It is concluded that carbon dioxide is adsorbed by wheat in proportion to the concentration of the gas.

EXPERIMENTS ON THE AIRTIGHT STORAGE OF DAMP GRAIN

Airtight storage as a means of preventing deterioration of damp grain was studied both on a laboratory scale and in 10‐ton bins, at grain moisture contents from 17 to 24%.Except when containers leaked, there was no development of mould, and the grain was bright and free‐flowing, even after prolonged storage, at high moisture content. The grain remained mould‐free after a bin was opened several times and small quantities of grain run out in cool weather. Such grain, removed from hermetic conditions and stored in sacks in an unheated building, showed no mould development for several weeks.With prolonged hermetic storage or at high moisture content the grain developed a sour‐sweet smell and taste which when extreme were not entirely removed by subsequent airing or drying.There was no spontaneous heating; the grain in the 10‐ton bins reflected the mean temperature of the surroundings.The oxygen in the intergranular air was reduced to a low level and replaced by carbon dioxide within a few days or weeks, according to the moisture content and temperature of the grain. At moisture contents of 16% and above, appreciable positive pressure developed inside the containers. The carbon dioxide concentration rose to 90–95% at grain moistures of 22–24%, 70–75% at 19%, 50% at 18%, and 35–40% at about 17% moisture content. The change from aerobic to anaerobic activity was marked by a reduction in the rate of production of carbon dioxide. With grain of less than 14% moisture content, the rate of respiration was slow, the carbon dioxide concentration being only about 2% after 18 months' storage. As long as oxygen remained, the apparent respiratory quotient was consistently between 0.6 and 0.7, whatever the moisture content of the grain.This work formed part of the programme of work of the Pest Infestation Laboratory.

Respiration of Isolated Nerve.

Forty-three experiments on the rate of respiration of the lateral line nerve of the dogfish have been carried out by the tonometric method during the first 27 hours after isolation. Fenn's statement that the rate of oxygen consumption of these nerves at 22.0°C. increases rapidly several hours after removal from the body was confirmed. This increase which begins 6 or more hours after isolation may reach a value nearly twice that of the initial rate of oxygen consumption at the end of 27 hours. During this time the rate of carbon dioxide production remains relatively constant. The apparent respiratory quotient, therefore, falls from 0.82 to 0.50. Determinations of the carbon dioxide binding power of these isolated nerves indicated that this property is relatively constant during the first 12 hours. From 12 to 32 hours after isolation there is an enormous increase in the amount of carbon dioxide bound. This value becomes 500 volumes per cent at the end of 30' hours. Assuming the applicability of the Henderson-Hasselbalch equation, the pH is calculated to have increased from about 6.6 during the first 12 hours to 8.4 at 32 hours. This would indicate a liberation of approximately 16×10-5 molar equivalents of base. Calculation of the true respiratory quotient gives values which increase from about 0.82 during the first 6 hours to 5.80 at the end of 24 hours. It is possible that this high carbon dioxide production may be due to enzymatic or bacterial splitting of urea which is present in large amounts. Experiments will be carried out to determine the effect of bacterial contamination and the source of the base liberated.