Nitrous oxide emissions from soil amended with glucose,alfalfa, or corn residues

Abstract

Nitrous oxide (N₂O) emissions result from the nitrification and denitrification processes, the latter strongly affected by soil organic carbon (C) derived from plant residues. This study addressed two questions: (1) does plant residue C become less available to denitrifiers after a period of aerobic incubation, and (2) do plant residues with smaller particle sizes provide C for higher rates of N₂O production due to a faster decomposition rate? Nitrous oxide fluxes from soil amended with alfalfa or corn residues, or glucose were measured in the laboratory using a gas flow‐through chamber system. Soil amended with these C substrates was also subjected to a 5‐d aerobic preincubation treatment. The significance of particle size on C availability was studied by comparing N₂O released from soil amended with ground (particle size <1 mm) and large pieces (5‐cm lengths) of alfalfa residues. A 5‐d aerobic preincubation of soil amended with plant residues resulted in reduced N₂O production during a subsequent anaerobic period. Results suggested that, due to consumption of the most available substrate, remaining C in plant residues is less available to denitrifiers after a period of aerobic incubation. Higher N₂O losses were found with large alfalfa particles than with ground alfalfa.     

Biodegradation of sunflower hulls with different nitrogen sources under mesophilic and thermophilic incubations

The decomposition of sunflower hulls and mixtures of sunflower hulls and other crop residues was examined under mesophilic (25°C) and thermophilic (50°C) temperatures during a 45-day incubation. Four treatments were set up: sunflower hull, sunflower hull+5% alfalfa, sunflower hull+5% vetch, sunflower hull+0.1% urea, to asses the efficiency of composting. Changes in total organic C (TOC), oxidizable C, N, pH, electrical conductivity (EC), and different fibre fractions—cellulose, hemicellulose and lignin—were determined. TOC decreased by 13–21% at 25°C and 25–41% at 50°C and the largest loss of C was for hulls amended with vetch residues and urea. Loss of oxidizable C was not affected by either the temperature or the treatments. The largest loss of cellulose occurred at 50°C in all the treatments. The hemicellulose content of the sunflower hulls alone and in the treatment with urea was significantly different with respect to the other treatments, whereas lignin content showed either a small increase (1.6% in sunflower hulls amended with alfalfa at 50°C and 1.8% in sunflower hulls with urea at 25°C) or a decrease of between 2.5% and 19% in the other treatments. The C/N ratio never fell below 50:1 and the highest decrease was for the thermophilic incubation. Increases in EC and pH values accounted for 3 and 1.5 units, respectively, and occurred after a 45-day incubation for mesophilic and thermophilic conditions. In general the incubation at 50°C facilitated the decomposition of sunflower hulls with high C/N ratios and little N addition.     

The addition of Propionibacterium freudenreichii to Raclette cheese induces biochemical changes and enhances flavor development

Two mixtures of Propionibacterium freudenreichii commercial strains were tested as adjunct cultures in pasteurized milk Raclette cheese to investigate the ability of propionibacteria (PAB) to enhance flavor development. Cheese flavor was assessed by a trained sensory panel, and levels of free amino acids, free fatty acids, and volatile compounds were determined. The PAB level showed a 1.4 log increase within the ripening period (12 weeks at 11 degrees C). Eye formation, which was not desired, was not observed in PAB cheeses. PAB fermented lactate to acetate and propionate and produced fatty acids by lipolysis, branched chain volatile compounds derived from isoleucine and leucine catabolism and some esters. One of the experimental cheeses received the highest scores for odor and flavor intensity and was characterized by higher frequencies of detection for some minor notes ("propionic"and "whey" odor, "sweet" taste). PAB can therefore be considered as potential adjunct cultures to enhance or modify cheese flavor development.     

The effect of some carbon substrates on denitrification rates and carbon utilization in soil

Summary Soil was amended with a variety of carbon sources, including four soluble compounds (glucose, sucrose, glycerol and mannitol) and two plant residues (straw and alfalfa).. Potential denitrification rates, measured both as N₂O accumulation and NO₃ ^– disappearance, were compared, and the predicted values of available C, measured as CO₂ production and water-extractable C, were assessed.The two measures of denitrification agreed well although N₂O accumulation was, found to be most sensitive. Soil treated with the four soluble C compounds resulted in the same rate of denitrification although glycerol was not as rapidly oxidized. Alfalfa-amended soil produced a significantly higher rate of denitrification than the same amount of added straw. CO₂ evolution was found to be a good predictor of denitrification over the first 2 days of sampling, but neither measure of available substrate C correlated well with denitrification rate beyond 4 days, when NO₃ ^– was depleted in most treatments. The data with alfalfa-amended soil suggested that denitrifiers used water-extractable C. materials produced by other organisms under anaerobic conditions.     

Effect of temperature on the rate limiting step in the methanogenic degradation pathway in rice field soil

Temperature is an important factor controlling CH₄ production in rice field soils. However, it is unknown which step in the methanogenic degradation of organic matter is the limiting one that is controlled by temperature. Soil slurries prepared from Italian rice field soil were anaerobically incubated in the dark at six different temperatures between 10 and 37 °C until quasi-steady state was reached. Then, the potential and actual rates of polysaccharide hydrolysis and of CH₄ production from different immediate (acetate, H₂) and distal (glucose, propionate) methanogenic substrates were determined. Potential activities of exo-glucanase and glucosidase were always higher than the actual rates of polysaccharide hydrolysis indicating that the availability of the polysaccharide substrate was limiting at all temperatures. The actual rates of CH₄ production were always lower than those predicted from glucose release during polysaccharide hydrolysis indicating that a substantial amount of the released glucose was assimilated into microbial biomass. Addition of the different methanogenic substrates stimulated CH₄ production at all temperatures >10 °C, but only at >20 °C to values higher than rates of polysaccharide hydrolysis. Under steady state conditions, however, hydrolysis of organic polymers was the rate-limiting step at all temperatures >10 °C.     

Denitrification,N2-fixation and fermentation during anaerobic incubation of soils amended with glucose and nitrate

Nitrate and glucose additions were investigated for their role in the C and N dynamics during anaerobic incubation of soil. A gas-flow soil core method was used, in which the net production of N₂, N₂O, NO, CO₂, and CH₄ under a He atmosphere could be monitored both accurately and frequently. In all experiments clayey silt loam soil samples were incubated for 9 days at 25 °C. Addition of nitrate (50 mg KNO₃-N kg^-1 soil) had no effect on total denitrification and CO₂ production rates, while the N₂O/N₂ ratio was affected considerably. The cumulative N₂O production exceeded the cumulative N₂ production for 6 days in the treatment with nitrate addition, compared to 1.2 days in the unamended treatment. Glucose addition stimulated the microbial activity considerably. The denitrification rates were limited by the growth rate of the denitrifying population. During denitrification no significant differences were observed between the treatments with 700 mg glucose-C kg^-1 and 4200 mg glucose-C kg^-1, both in combination with 50 mg KNO₃-N kg^-1. The N₂ production rates were remarkably low, until NO _inf3 ^sup- exhaustion caused rapid reduction of N₂O to N₂ at day 2. During the denitrification period 15–18 mg N kg^-1 was immobilised in the growing biomass. After NO _inf3 ^sup- shortage, a second microbial population, capable of N₂-fixation, became increasingly important. This change was clearly reflected in the CO₂ production rates. Net volatile fatty acid (VFA) production was monitored during the net N₂-fixation period with acetate as the dominant product. N₂-fixation faded out, probably due to N₂ shortage, followed by increased VFA production. In the high C treatment butyrate became the most important VFA, while in the low C treatment acetate and butyrate were produced at equal rates. During denitrification no VFA accumulation occurred; this does not prove, however, that denitrification and fermentation appeared sequentially. The experiments illustrate clearly the interactions of C-availability, microbial population and nitrate availability as influencing factors on denitrification and fermentation.Dedicated to Professor J. C. G. Ottow on the occasion of his 60th birthday     

Root-temperature effects on competition for nodule occupancy between two Rhizobium meliloti strains

Summary Axenically grown alfalfa (Medicago sativa L. var. Peace) was simultaneously inoculated with Canadian commercial Rhizobium meliloti strains NRG-185 and BALSAC. The plants were grown for 7 weeks in sealed units at five different root temperatures (8°, 13°, 17°, 21°, and 25°C) and at a relatively constant air temperature (24°–30°C). Nodule occupancy by each strain was determined by enzyme-linked immunosorbent assay (ELISA). Nitrogenase activity, nodule fresh weight, and plant dry weight were also measured. The lowest root-temperature regime (8°C) resulted in substantially lower nodule numbers and weights, and plant dry weights, than the higher temperature regimes. Development of nitrogenase activity was completely inhibited at 8°C. The immunoassay of nodule-strain occupancy showed markedly different strain-nodulation responses to the various root-temperature regimes. At 8°C, 63% of nodules were occupied by both strains. Dual strain occupancy decreased from 63% to 2% with increasing root-growth temperature, while the proportion of nodules containing only strain NRG-185 increased from 9% to 75%. Nodules containing only strain BALSAC remained relatively constant at 25% from 8° to 21°C, decreasing slightly at 25°C. The results suggest that root-environment temperatures during the period of nodule formation may have major differential effects on the success of competing rhizobial strains. If this is so, then selection of Rhizobium strains with enhanced low-temperature nodulation capabilities should be possible.     

Short-term temperature dependence of heterotrophic soil respiration after one-month of pre-incubation at different temperatures

Quantification of microbial activities involved in soil organic carbon (SOC) decomposition is critical for the prediction of the long-term impact of climate change on soil respiration (SR) and SOC stock. Although the temperature sensitivity of SR is especially critical in semi-arid regions, such as North West Tunisia, where the SOC stock is low, little research has been carried out in these environments. More needs to be known about factors, such as SOC availability that influence temperature sensitivity. In this study, soil samples were incubated with and without glucose addition for 28 days after a 28-day pre-incubation period. Pre-incubation and incubation was carried out at 20 °C, 30 °C, 40 °C and 50 °C. Respiration measurements were taken with temperature, glucose addition and incubation time as independent variables. The highest pre-incubation temperature reduced the temperature sensitivity of SR during the subsequent incubation period, both with and without glucose addition. Soil samples pre-incubated at 50 °C had the lowest SR at all subsequent incubation temperatures and the lowest temperature sensitivity of SR, even after glucose addition. However, after glucose addition, the effect of a high pre-incubation temperature on soil respiration lasted only two days. Measuring the water-soluble carbon (WSC) in soil samples suggested that the high pre-incubation temperature may have killed part of the microbial biomass, modified microbial communities or solubilized SOC. For quantifying the possible effect of global warming, in particular heat waves, on soil respiration in the soil studied, the results indicate a moderate response of soil respiration to temperature at high temperatures, as shown by Q₁₀ close to 1.7, even in the range 40-50 °C.     

Fermentation of cottonseed and other feedstuffs in cattle rumen fluid

Bovine rumen fluid was fermented anaerobically over 48 h with cottonseed, corn, alfalfa, or a mixture of these substrates in anaerobic mineral buffer. Samples taken at different incubation times were derivatized with n-butanol and subjected to gas chromatography and mass spectroscopy. No unusual fermentation end-products from the cottonseed substrate were detected. Cottonseed supported rumen fermentation at levels comparable to those of the other substrates. Major components were usually found in the decreasing order of acetate, propionate, butyrate, and valerate, although acetate and propionate concentrations decreased late in the alfalfa and mixed-feed fermentations, eventually allowing butyrate concentrations to exceed those of propionate. As expected, lactate was produced in high concentrations when corn was fermented. The minor components 2-methylpropionate, 2- and 3-methylbutyrate, phenylacetate, phenylpropionate, and caproate also accumulated, with their relative concentrations varying with the substrate. Succinate was produced in substantial amounts only when corn and alfalfa were fermented; it did not accumulate when cottonseed was the substrate. Samples containing cottonseed were derivatized and subjected to reversed-phase high-performance liquid chromatography, revealing that gossypol concentrations did not change during fermentation.     

Volatile compounds of wines produced by cells immobilized on grape skins

A biocatalyst was prepared by immobilization of Saccharomyces cerevisiae cells on grape skins. Repeated batch fermentations were conducted using the immobilized biocatalyst as well as the free yeast cells at 25, 20, 15, and 10 degrees C. The major volatile byproducts were determined by GC, whereas the minor volatile constituents were extracted in dichloromethane and analyzed by HRGC-MS. The qualitative profiles of the wines produced were similar in every case. Immobilized cells gave wines with higher contents of ethyl and acetate esters that increased with temperature decreases from 25 to 15 degrees C. The amount of volatile alcohols was more pronounced in wines produced by free cells and decreased dramatically at low fermentation temperatures (10 degrees C).     

Investigation of volatiles evolution during the alcoholic fermentation of grape must using free and immobilized cells with the help of solid phase microextraction (SPME) headspace sampling

A biocatalyst was prepared by immobilization of Saccharomyces cerevisiae strain AXAZ-1 on delignified cellulosic material (DCM). Repeated batch fermentations were conducted using these biocatalysts and free cells, separately, at temperatures of 20, 15, and 10 degrees C. Solid phase microextraction (SPME) was used in monitoring the formation of volatile alcohols, acetate esters, and ethyl esters of fatty acids. The kinetics of volatile production were similar for free and immobilized cells. In all cases immobilized cells showed a better rate of volatile production, which was directly connected to sugar consumption. The main difference observed was in propanol production, which increased with temperature decrease for the immobilized cells, whereas it remained constant for the free ones. In the case of immobilized cells significant amounts of esters were also produced. It is well-known that esters contribute to the fruity aroma of wine. It was also established that SPME is a very sensitive, accurate, and reliable technique and can be used without any reservation in the characterization of volatile constituents of wine.     

Relationships between soil moisture content and nitrous oxide production during nitrification and denitrification

Summary The effect of soil water content [60%–100% water-holding capacity (WHC)] on N₂O production during autotrophic nitrification and denitrification in a loam soil was studied in a laboratory experiment by selectively inhibiting nitrification with a low C₂H₂ concentration (2.1 Pa). Nitrifiers usually produced more N₂O than denitrifiers. During an initial experimental period of 0–6 days the nitrifiers produced more N₂O than the denitrifiers by a factor ranging from 1.4 to 16.5, depending on the water content and length of incubation. The highest N₂O production rate by nitrifiers was observed at 90% WHC, when the soil had become partly anaerobic, as indicated by the high denitrification rate. At 100% WHC there were large gaseous losses from denitrification, while nitrification losses were smaller except for the first period of measurement, when there was still some O₂ remaining in the soil. The use of 10 kPa C₂H₂ to inhibit reduction of N₂O to N₂ stimulated the denitrification process during prolonged incubation over several days; thus the method is unsuitable for long-term studies.     

Studies on the bacteria isolated anaerobically from paddy field soil IV. Model experiments on the production of branched-chain fatty acids

Based on the fact that the clostridia isolated from paddy field soil were mostlyproteolytic and that they produced some branched-chain volatile fatty acids on peptone media, the occurrence and the substrates for the production of such acids in paddy soil were investigated. The results obtained are as follows:

• 1) It was revealed that two branched-chain volatile fatty acids, iso-valeric and iso-butyric acids, were detected In fresh paddy field soil.

• 2) The paddy soil supplemented with peptone solution produced significant amounts of branched-chain volatile fatty acids-iso-butyric and iso-valeric acidsin addition to straight-chain volatile fatty acids-acetic, propionic, and butyric acids. When glucose was added instead of peptone, not branched-chain fatty and propionic acids were detected but only acetic and butyric acids. These facts indicate that the branched-chain fatty acids may originate from amino acids.

• 3) When a pair of amino acids, such as proline and leucine or proline-and valine, were supplemented in addition to peptone, the production of iso-valeric or iso-butyric acids was increased, respectively. Even In the case of glucose, the supplementation of a pair of amino acids supported the production of branched-chain fatty acids. These facts indicate that the Stickland reaction may take place in paddy field soil.

• 4) All the strains of strict anaerobes isolated from the soil and supplemented with peptone formed iso-valeric and iso-butyric acids in addition to two or three kinds of volatile fatty acids when they were cultured in medium containing peptone. All of them were classified as belonging to genus Clostridium. No strains of facultative anaerobes isolated from the same soil produced any branched-chain fatty acids, but produced only acetic acid.

• 5) In conclusion, these results reveal that the branched-chain fatty acid in paddy field soil is produced only by clostridia through the Stickland reaction.

     

Loss of ripening capacity of pawpaw fruit with extended cold storage

The fruit ripening traits of pawpaw [ Asimina triloba (L.) Dunal] were examined after harvest and after cold storage at -2, 2, 4, and 6 degrees C for up to 12 weeks. Generally, fruits stored at 2-4 degrees C for 4 weeks ripened normally, but those stored at -2 degrees C did not ripen normally, those stored at 6 degrees C were overripe, and by 6-8 weeks those stored at 2-4 degrees C had a lower respiration rate and ethylene production, lower firmness, and lower pH than fruit cold-stored for 4 weeks or less. These changes, and the occasional development of brown discoloration in the pulp once the fruits were moved back to room temperature, were evidence of chilling injury by 6 weeks. After harvest and through 4 weeks of cold storage, the main volatile compounds produced by fruit were methyl and ethyl octanoates and hexanoates. Volatile production significantly increased >5-fold in fruit ripened for 72 h after harvest or after removal from up to 4 weeks of cold storage. Fruit cold-stored for 6 weeks or more produced fewer total volatiles and esters but increased levels of such off-flavor compounds as ethyl acetate, ethyl propionate, and hexanoic and decanoic acids. Alcohol acyltransferase (AAT) activity declined in cold-stored fruit but was not correlated with either total volatile production or total ester production. Alcohol dehydrogenase activity did not change during ripening after harvest or cold storage. Lipoxygenase activity was highest just after harvest or after 2 weeks of cold storage, but was low by 4 weeks. Thus, ripening of pawpaw fruit seems to be limited to 4 weeks at 2-4 degrees C with loss of ability to continue ripening and chilling injury symptoms evident at colder temperatures and after longer periods of cold storage.     

Factors limiting denitrification in a Mediterranean riparian forest

In situ denitrification (DNT) and denitrification enzyme activity (DEA) were measured in a Mediterranean riparian forest soil during two periods under contrasting soil moisture conditions in order to investigate the factors that affect denitrification through the year. Results showed that in summer, soil moisture limited denitrification throughout the entire soil profile, whereas in winter, anaerobic conditions in the soil were more favourable for denitrifiers. The potential for denitrification was larger at shallow depths (<30 cm), and neither nitrate nor organic carbon limited denitrification significantly. Some denitrification was measured during winter at depths below 30 cm, suggesting that a reduction of groundwater nitrate could occur in some areas of this riparian forest during the wet period. In summer, low denitrification, together with high mineralization rates, brought about an increase of soil N, which could be leached to the stream channel during rainfall events. This study suggests that Mediterranean riparian soils act as sources or sinks of dissolved nitrogen depending on the period of the year.     

Changes in Concentrations of Malodorous Compounds During Controlled Aeration Composting

Effects of composting on odorous chemicals in dairy manure were investigated in replicated pilot-scale studies. Three 16-day composting trials were conducted, using 205 L vessels containing 83 kg mass. Fresh or 12-day-aged manure, from lactating cows, was mixed with sawdust (3man:1saw w/w). Vessels were either aerated continuously with high (2.3 kg/hr)/low (0.8 kg/hr) air flow controlled by thermostats or intermittently on a 5 min high air flow/55 min off clock controlled cycle. Six vessel replicates were conducted on each manure/air flow treatment combination. Temperatures, air flow, O₂ consumption, and CO₂ production were recorded every 10 minutes. Trapped NH₃ emissions were determined daily. Exhaust air was passed through water-cooled condensers to analyze emitted volatiles, and condensate volume, pH, and volatile fatty acids (VFAs) were quantified at 12 or 24 hour intervals. Solids were collected from each vessel initially, at remix at the end of day 7, and at the end of each trial (day 16) for analysis of moisture, pH, ash, C/N, and odorants. Phenolics and indolics were extracted with ethyl ether. VFAs were recovered with pH 2.0 water. Analysis was by flame or mass selective detection gas chromatography. Temperature increased most rapidly in continuously aerated vessels yet maintained a lower mean temperature (49 vs 58°C) than intermittent aeration. Both returned to near ambient temperature by day 16. Continuous aeration nearly doubled (11 vs 18 L) the amount of condensate released over 16 days. Fresh manure/sawdust mixes contained 6553, 795, 77, 51, 19, and 17μg/g of acetate, propionate, isobutyrate, isovalerate, phenol, and p-cresol. Aged manure mixes contained 9350, 3397, 2810, 445, 285, 441, 34, 176, and 18 μg/g acetate, propionate, butyrate, valerate, isobutyrate, isovalerate, phenol, p-cresol, and skatole, as well as a number of C₁₁ to C₁₇ fatty acids. Both aeration methods maintained conditions that resulted in the destruction of most of the odorous chemicals studied in the composting mass in 7 days with only small quantities of acetate, isobutyrate, and skatole present by the end of day 16. Continuous aeration, as opposed to intermittent, more than doubled (115 vs 55 g) the emissions of NH3-N and increased the emissions of VFAs in condensate four-fold. Whereas, limited aeration did not destroy the odorants as rapidly, they remained in the compost until destruction.     

Influence of fungal-soil water interactions on phytophthora root rot of alfalfa

Summary Phytophthora root rot of alfalfa (Medicago sativa L.) is a serious problem in wet soils. This disease is caused by Phytophthora megasperma f. sp. medicaginis. The influence of soil-water interactions with P. megasperma f. sp. medicaginis and other factors on the severity of phytophthora root rot of mature alfalfa plants (10–12 weeks) was studied in greenhouse experiments. Severe and reproducible root rot was produced by subsurface (3–4 cm) placement of mycelial suspension. Soil saturation 3 days prior to inoculation followed by alternating 3-day wet (soil saturation) and 4-day dry (surface watering once a day) moisture regimes (for 30–40 days following inoculation) resulted in severe root damage.The severity of root rot was greater when the inoculation was done at an ambient temperature of 20°C than at 15°C. Water quality (tap water or deionized distilled water) had no effect on severity of infection. The isolates PT 78-3 (Minnesota) and TN-2 (Maryland) were equally effective in terms of severity of damage.The impact of excess soil water stress (described above) alone on the shoot and root dry weight as well as on shoot symptoms was similar to that of root rot stress. However, root symptoms showed a marked difference. A close examination of root symptoms is highly recommended to differentiate clearly the plant injury due to root rot from that due to excess soil water stress.     

水分和温度对旱地红壤硝化活力和反硝化活力的影响

采集第四纪红色粘土发育和第三纪红砂岩发育的红壤,分别在4C冰箱内保存(O),室温下湿润(M)和淹水(F)培养110天后测定硝化细菌、反硝化细菌、硝化势、反硝化势和反硝化酶活性。结果表明,低温有利于保持硝化细菌和反硝化细菌的数量,但显著抑制它们的硝化和反硝化活力。湿润有利于保持硝化细菌的硝化活力,而淹水则有利于保持反硝化细菌的反硝化活力,但均不利于硝化细菌和反硝细菌的存活。由此说明,不同的研究目的和需要测定的项目,应采用不同的土壤样本保存方法。     

中温和高温环境下乙酸和丙酸厌氧发酵产甲烷动力学特征

采用不同浓度的乙酸和丙酸在中高温下进行厌氧发酵批次试验,采用修正的Gompertz模型和产甲烷的一级动力学模型分析,研究酸浓度和温度对发酵产气动力学的影响。研究表明,当乙酸和丙酸浓度较低时降解较快,高浓度酸抑制产气。乙酸在中温条件下降解较快,质量浓度为5 000 mg/L时中温反应有最大产甲烷速率101 mL/d;质量浓度为10 000 mg/L时高温条件下有最大产甲烷速率77 mL/d,随酸浓度增加,最大产甲烷速率减小,高温反应器对酸的耐受度较高。丙酸在高温条件下更易降解,浓度为4 000 mg/L时,中高温反应均有最大产气速率：78 mL/d（中温）和96 mL/d（高温）。另外,高浓度乙酸和丙酸厌氧降解产气具有滞后性,且随酸浓度的增加滞后期延长,降解过程受到抑制,一级动力学常数减小。温度对厌氧降解的影响大于酸浓度对厌氧降解的影响。     

Why does temperature affect relative uptake rates of nitrate, ammonium and glycine: A test with Eucalyptus pauciflora

Few studies have examined how temperature affects uptake of nitrate, ammonium and amino acids from soil. This study tests the hypothesis that cool temperatures favour uptake of the amino acid glycine while warm temperatures favour uptake of inorganic forms of N such as nitrate. We used glasshouse-grown ectomycorrhizal seedlings of the sub-alpine tree species Eucalyptus pauciflora Sieber ex Spreng. Seedlings were grown in soil (humic umbrosol, from species' habitat) that was dominated by amino acids and ammonium with only small amounts of nitrate. To examine if root physiology affects temperature responses of N uptake, we measured uptake from ¹⁵N-labelled hydrosolutions containing equimolar 100 μmol L⁻¹ mixtures of ammonium, nitrate and glycine at temperatures from 5 to 35 °C. We also examined if the effect of temperature on uptake of N forms was due to plant-microbe competition by following the fate of equimolar amounts of labelled ammonium, nitrate and glycine injected into the soil at temperatures of 5 °C and 25 °C. Hydrosolution experiments showed that uptake of glycine was favoured by warm temperatures and inorganic N by cool temperatures. In contrast, when ¹⁵N was injected into soil the uptake of glycine was favoured by low temperatures and nitrate by warm temperatures. At 25 °C, glycine was 17% of the N taken up from soil and nitrate was 51%; whereas at 5 °C glycine was 30% of the N taken up from soil and nitrate was 23%. Microbes were better competitors than seedlings for all forms of N, but temperature did not affect microbial preference for the different N forms. Hence, while microbes limit N available for plant uptake, they do not seem to be the cause of the greater plant uptake of glycine at cool temperatures and nitrate at warm temperatures. Intact uptake of glycine by plants was suggested by the positive relationship between uptake of ¹³C and ¹⁵N and detection by GC-MS of intact , ¹⁵N glycine molecules in roots. In conclusion, uptake of glycine is favoured by cool temperatures and nitrate by warm temperatures, but this is apparently not a function of root physiology or competition with soil microbes.