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Climate warming may be exacerbated if rising temperatures stimulate losses of soil carbon to the atmosphere. Despite a plethora of studies stimulated by this alarming perspective, the direction and magnitude of this so-called carbon-climate feedback remain uncertain. Part of the uncertainty is due to lack of knowledge of the thermal adaptation of the physiology and composition of soil microbial communities. Here, we tested the hypothesis that microbial communities adapt to long-term warming with a shift in their composition and their temperature response that are consistent with evolutionary theory. We used the unique combination of state-of-the-art macromolecular rate theory (MMRT) to describe the temperature response of microbial respiration and metabarcoding to characterise the microbial community composition of soils sampled along transects of increasing distance from a geothermally active zone in New Zealand. Along those transects, the mean annual temperature of the soil increased dramatically (by up to 15oC), and this concurred with shifts in microbial communities composition, with thermophile and thermotolerant genera increasing in relative abundance with increasing ambient temperature. However, the ambient temperature had no detectable influence on the MMRT parameters or the relative temperature sensitivity of decomposition (Q10). MMRT parameters were, however, strongly correlated with soil carbon concentration and carbon:nitrogen ratio. Our findings suggest that, while long-term warming selects for warm-adapted taxa, substrate quality and quantity exert a stronger influence than temperature in selecting for distinct thermal traits. The results have major implications for our understanding of the role of soil microbial processes in the long-term effects of climate warming on soil carbon dynamics and will help increase confidence carbon-climate feedback projections.

Climate warming may be exacerbated if rising temperatures stimulate losses of soil carbon to the atmosphere. The direction and magnitude of this carbon-climate feedback are uncertain, largely due to lack of knowledge of the thermal adaptation of the physiology and composition of soil microbial communities. Here, we applied the macromolecular rate theory (MMRT) to describe the temperature response of the microbial decomposition of soil organic matter (SOM) in a natural long-term warming experiment in a geothermally active area in New Zealand. Our objective was to test whether microbial communities adapt to long-term warming with a shift in their composition and their temperature response that are consistent with evolutionary theory of trade-offs between enzyme structure and function. We characterized the microbial community composition (using metabarcoding) and the temperature response of microbial decomposition of SOM (using MMRT) of soils sampled along transects of increasing distance from a geothermally active zone comprising two biomes (a shrubland and a grassland) and sampled at two depths (0?50 and 50?100 mm), such that ambient soil temperature and soil carbon concentration varied widely and independently. We found that the different environments were hosting microbial communities with distinct compositions, with thermophile and thermotolerant genera increasing in relative abundance with increasing ambient temperature. However, the ambient temperature had no detectable influence on the MMRT parameters or the relative temperature sensitivity of decomposition (Q10). MMRT parameters were, however, strongly correlated with soil carbon concentration and carbon:nitrogen ratio. Our findings suggest that, while long-term warming selects for warm-adapted taxa, substrate quality and quantity exert a stronger influence than temperature in selecting for distinct thermal traits. The results have major implications for our understanding of the role of soil microbial processes in the long-term effects of climate warming on soil carbon dynamics and will help increase confidence in carbon-climate feedback projections. Made available in DSpace on 2021-12-02T12:00:26Z (GMT). No. of bitstreams: 1 gcb-2021.pdf: 1272291 bytes, checksum: 7b68a15c95c3c82ebedcd10f8f5efeb6 (MD5) Previous issue date: 2021

SUMMARY Miniswards of Lolium perenne, Festuca rubra, Agrostis casteUana and Poa trivialis were grown in a sand-limestone mixture and fed with a complete nutrient solution containing 3 mol m−3 NH4NO3. The miniswards were cut weekly to a height of 4 cm until a steady state of growth, based on weight of clippings removed, was achieved. After clipping (day 0) the nutrient solution was replaced by one in which all the nitrogen (N) was enriched with 15N to 5 atom per cent, but was otherwise identical. This allowed discrimination of remobilization from current root uptake when considering the supply of N for regrowth of laminae. Destructive harvests were made over the following 28-d period, while unharvested plants continued to receive a weekly clip. The proportion of the total uptake of labelled N which appeared in the clipped material was of the order L. perenne > P. trivialis > A. castellana > F. rubra. The change in unlabelled N in the roots plus leaf bases over time was modelled as a proportion of the amount present on day 0. A simple exponential model with a non-zero asymptote was found to model the data; the asymptote was assumed to represent the proportion of unlabelled N not readily available for remobilization. Species differences in the asymptotes were found, with F. rubra having a significantly larger asymptote than A. castellana and P. trivialis, which in turn both had significantly larger asymptotes than L. perenne. All species used both root uptake and remobilization to supply N for post-defoliation regrowth of laminae. However, the relative contribution of each source was species-dependent. The proportion of N in the clipped material derived from remobilization was significantly greater for F. rubra than A. castellana, which in turn had a significantly greater proportion derived from remobilization than P. trivialis and L. perenne.

SUMMARY Four-year-old seedlings of Picea sitchensis (Bong.) Carr. were grown in sand culture throughout 1989 and irrigated with nutrient solutions containing either 1.0 mol N m−3 (Low N) or 6.0 mol N m−3 (High N), to precondition their growth and capacity for N storage. During 1990 N enriched with 15N was supplied, either from 15 March to 27 June, or 28 June to 20 November. Recovery of unlabelled N was used to determine the storage and immobilization of N for foliage growth, and the partitioning of labelled N taken up during the two periods was measured. Initial growth of trees in 1990 was unaffected by the current N supply and determined only by the N supplied the previous year. High N throughout increased the number of needles grown in 1990 compared to low N-treated trees, but had little effect on the dry weight of individual needles. When preconditioned with High N, trees responded to Low N in 1990 by a reduction in needle dry weight, without altering the number of needles produced. Low N trees supplied with High N in 1990 responded by increasing both needle numbers and dry weight, compared with trees supplied with Low N throughout. The amount of unlabelled N remobilized to foliage growth in 1990 was unaffected by the current N supply but reflected the amount of N in store, as determined by the N supply the previous year. The majority of N was remobilized from the 1989 foliage and none from roots. Partitioning of labelled N taken up during 1990 altered during the year, with a greater proportion of N taken up after 28 June recovered in the roots in all treatments, due to root growth as opposed to allocation of N to storage during the autumn, since root N concentrations fell between 17 June and the final harvest on 20 November.

SUMMARY Nodulated plants of Ulex europaeus L. (gorse) were grown over two seasons in sand. Nutrients were supplied as solutions containing either 0-25 mol m-3 NO3->, 5·0 mol m-3 NO3, 0·25 mol m-3 NH4-+ or 5·0 mol m-3 NH4+, as Ca(NO3)2 or (NH4)2SO4. In the first season all nitrogen (N) was enriched with 15N to 5 atom %. In the second season half the plants received the same solution, while the remaining plants received a similar solution in terms of form and amount of nitrogen but at natural abundance. Throughout the second season eight destructive harvests were performed. Plants were separated into roots, nodules, previous season's shoot, new shoot and flowers, and then analysed for total N and 15N content. This allowed estimates to be made of total root N uptake, N2 fixation and the contribution of re-mobilization to the nitrogen content of new shoot growth in the second season. The main source of N was N2 fixation for plants receiving a low supply of N and root uptake for plants receiving a high supply of N. Plants supplied high NH4+ stored more N over winter than did plants given other treatments. The proportion of over-wintering stores that were re-mobilized to support spring shoot growth was unaffected by the capacity of the plants for N2 fixation.

Plant growth is usually constrained by the availability of nutrients, water, or temperature, rather than photosynthetic carbon (C) fixation. Under these conditions leaf growth is curtailed more than C fixation, and the surplus photosynthates are exported from the leaf. In plants limited by nitrogen (N) or phosphorus (P), photosynthates are converted into sugars and secondary metabolites. Some surplus C is translocated to roots and released as root exudates or transferred to root-associated microorganisms. Surplus C is also produced under low moisture availability, low temperature, and high atmospheric CO2 concentrations, with similar below-ground effects. Many interactions among above- and below-ground ecosystem components can be parsimoniously explained by the production, distribution, and release of surplus C under conditions that limit plant growth. Non

International audience; Evaluation of the temperature sensitivity of soil organic matter (SOM) decomposition is critical for forecasting whether soils in a warming world will lose or gain carbon and, therefore, accelerate or mitigate climate warming. It is usually described, using Arrhenius kinetics, as increasing with the stability of the substrate in laboratory conditions, where substrate availability is non-limiting and where chemical recalcitrance, therefore, predominantly regulates stability. However, conditions of non-limiting subtrate availability are rare in the undisturbed soil, where physicochemical protection of substrates may control their stability. The aim of this study was to assess the temperature sensitivity of decomposition of SOM with contrasting stability in the field. Our conceptual approach was based on in situ measurements of soil CO2 efflux at a range of temperatures from root exclusion plots of increasing age (1 month and three decades) and, therefore, with SOM of increasing stability. From a set of short-term measurements in spring, using diurnal temperature variation, the relative temperature sensitivity of SOM decomposition decreased significantly (p < 0.0001) with increasing SOM stability, and was weak (Q(10) < 1.3) in long-term root exclusion plots. This result was confirmed in a similar set of short-term measurements repeated later in the year, in summer, as well as from an analysis perfomed at the seasonal timscale. We provide direct field evidence that the temperature sensitivity of SOM decomposition decreases with increasing stability, in direct contrast with Arrhenius kinetics prediction, and therefore show that stability of SOM in the field cannot be the sole result of chemical recalcitrance. We conclude that the physicochemical protection of SOM, which controls SOM stability in the field, constrains the temperature sensitivity of SOM decomposition under field conditions. (C) 2019 The Author(s).

© 2020 The Authors Is persistent soil organic matter (SOM), characterised by an old age and long-turnover time, more or less sensitive to changes in temperature than fast-cycling, recent SOM? Largely due to our limited understanding of the mechanisms of SOM formation, this question remains controversial. Laboratory incubation studies, through sieving the soil, may create conditions in which substrate accessibility is modified. The recent recognition of SOM accessibility as a defining factor of SOM persistency calls into question conclusions from these studies. Previously, in a study using root exclusion plots of increasing age, we showed in the field that the temperature sensitivity of SOM decomposition decreased with increasing persistence of SOM (Moinet et al., 2020), in opposition to many laboratory incubation studies. Here we sampled soils from the same root exclusion plots and conducted a laboratory incubation experiment to test the hypotheses that (i) the relationship between temperature sensitivity and SOM persistence is inverted as compared to the field, and (ii) the discrepancy is due to sieving the soil. We showed that, in the laboratory, the relationship was indeed inverted, with the temperature sensitivity being higher for the old root exclusion plots. However, sieving the soil at 2 mm did not affect estimates of the temperature sensitivity of SOM decomposition, suggesting that discrepancies between field and laboratory estimates are unlikely to stem from artificially modified substrate accessibility due to sieving.

Herbivory is one of the most globally distributed disturbances affecting carbon (C)-cycling in trees, yet our understanding of how it alters tree C-allocation to different functions such as storage, growth or rhizodeposition is still limited. Prioritized C-allocation to storage replenishment vs growth could explain the fast recovery of C-storage pools frequently observed in growth-reduced defoliated trees. We performed continuous 13C-labeling coupled to clipping to quantify the effects of simulated browsing on the growth, leaf morphology and relative allocation of stored vs recently assimilated C to the growth (bulk biomass) and non-structural carbohydrate (NSC) stores (soluble sugars and starch) of the different organs of two tree species: diffuse-porous (Betula pubescens Ehrh.) and ring-porous (Quercus petraea [Matt.] Liebl.). Carbon-transfers from plants to bulk and rhizosphere soil were also evaluated. Clipped birch and oak trees shifted their C-allocation patterns above-ground as a means to recover from defoliation. However, such increased allocation to current-year stems and leaves did not entail reductions in the allocation to the rhizosphere, which remained unchanged between clipped and control trees of both species. Betula pubescens and Q. petraea showed differences in their vulnerability and recovery strategies to clipping, the ring-porous species being less affected in terms of growth and architecture by clipping than the diffuse-porous. These contrasting patterns could be partly explained by differences in their C cycling after clipping. Defoliated oaks showed a faster recovery of their canopy biomass, which was supported by increased allocation of new C, but associated with large decreases in their fine root biomass. Following clipping, both species recovered NSC pools to a larger extent than growth, but the allocation of 13C-labeled photo-assimilates into storage compounds was not increased as compared with controls. Despite their different response to clipping, our results indicate no preventative allocation into storage occurred during the first year after clipping in either of the species.

Laboratory studies have shown that priming effects, caused by inputs of carbon into the rhizosphere, can change the rate of soil organic matter (SOM) decomposition and could have significant impacts on soil carbon cycling. However, there have been few studies in field conditions because of experimental constraints but field data are needed to improve models that forecast the effects of climate change on SOM decomposition rates and the impact of these changing rates on atmospheric CO2 concentration. In 2009 a fire at a Eucalyptus forest site in Australia killed all standing vegetation. Trenched plots were installed in 2010, approximately 12 months after the fire, and were maintained plant-free for the subsequent year. In 2011, after forest re-growth outside the trenched plots, we compared SOM decomposition rates in the presence of plants (rooted plots) and in the absence of plants (trenched plots) using a natural abundance stable carbon isotope technique with minimal disturbance of the soil. We then compared our results to those obtained in another study conducted at the same time and the same plots using laboratory incubations of sieved soil samples. There was no difference in SOM decomposition rates between the trenched and the rooted plots estimated using our non-disruptive technique. In contrast, laboratory incubations of sieved soils highlighted a two-fold increase in SOM decomposition rates in the rooted plots compared with rates from the trenched plots. Our results suggest that rhizosphere priming may not actively influence soil carbon turnover in the undisturbed soil environment and question conclusions from laboratory incubation studies. We attribute the different findings from laboratory and field studies to the physical disturbance of the soil involved in laboratory incubations causing the release of previously protected substrates, making them available for decomposition.

Soils can be sources or sinks of carbon depending on the balance between carbon inputs from plants and losses from the decomposition of soil organic matter (SOM). A good understanding of the temperature sensitivity of SOM decomposition is critical for forecasting whether soils in a warming world will lose or gain carbon, and therefore accelerate or mitigate the rate of increasing atmospheric carbon dioxide (CO2) concentration. We provide new evidence to show that the response of SOM decomposition to temperature may be constrained by substrate availability to microbial decomposers. We used laboratory incubations of a grassland soil to compare the temperature sensitivity of SOM decomposition with unmodified substrate availability with that of the same soil in which substrate availability was reduced by adding allophane, a clay-size mineral with a high capacity for binding SOM. In the soil with no added allophane, the decomposition rate increased about 7-fold over the temperature range from 1 to 40 °C. With added allophane, decomposition rate increased only about 3-fold over the same temperature range. We then used a non-disruptive, natural abundance isotopic technique at our field site to partition total soil respiration into CO2 efflux from newly released, 13C-depleted SOM (root respiration and rhizosphere decomposition) from CO2 efflux from older 13C-enriched SOM from the decomposition of more stable SOM. We found no increase in the decomposition rate of the 13C-enriched pool of SOM between 11 and 28 °C. That finding contrasts with most previous studies that have generally reported strong increases in SOM decomposition with temperature. We hypothesised that the large temperature sensitivity observed in laboratory incubations was due to substrate becoming readily available as a result of the disturbance involved in collecting soil samples. In undisturbed field conditions, the limiting step for the decomposition of the more stable SOM pool may be the rate at which decomposable substrate becomes available for decomposition. Our findings will have important implications for the feedbacks between soil carbon storage and the rate of increase in atmospheric CO2 concentration mediated by global warming.

To be economically and environmentally sound, the nitrogen (N) supply in orchards needs to be finely tuned according to crop needs. This requires knowledge of the amount and the dynamics of N uptake by roots and of its allocation. This paper reports two experiments, carried out on the same set of trees, in order to: 1) study the uptake dynamics of N derived from fertiliser (N dff ) and its allocation to tree organs and, 2) quantify its N remobilization in spring from storage organs to new growth. The study was carried out using field-grown adult orange trees (cv. Tarocco nucellare grafted on Troyer Citrange), one of the most important fruit tree crops in the Mediterranean area. Nitrogen was supplied as ammonium nitrate in 10 applications with equal doses evenly distributed from March to November 2009. Four groups of 5 trees each received 15 N-enriched N starting from March, July, September, and November, respectively, while they received unlabelled N in the remaining periods. The results show that trees absorbed roughly 30% of the fertiliser N. Fertiliser-N use efficiency (NUE) was generally low in spite of the fact that readily available N sources were regularly supplied and the trees were irrigated; the later the fertiliser N was supplied, the lower was the NUE. Uptake rate was rather constant from April to November, but relatively less N was partitioned to fruits when the fertiliser was supplied later in the season. Remobilization of N from one-year-old leaves provided more than 60% of the N required by shoots in spring. By the end of April, most of the N had already been transferred from storage to growing organs. We estimated that, during the vegetative season, trees absorbed 94 kg N ha −1 from the soil and half of this amount derived from fertiliser. Spring remobilisation of winter stored N contributed for additional 40 kg N ha −1 to the N needs.

Increasing soil carbon stocks in agricultural grasslands has a strong potential to mitigate climate change. However, large uncertainties around the drivers of soil respiration hinder our ability to identify management practices that enhance soil carbon sequestration. In a context where more intense and prolonged droughts are predicted in many regions, it is critical to understand how different management practices will temper drought-induced carbon losses through soil respiration. In this study, we compared the impact of changing soil volumetric water content during a drought on soil respiration in permanent grasslands managed either as grazed by dairy cows or as a mowing regime. Across treatments, root biomass explained 43% of the variability in soil respiration (p &lt, 0.0001). Moreover, analysis of the isotopic composition of CO2 emitted from the soil, roots, and root-free soil suggested that the autotrophic component largely dominated soil respiration. Soil respiration was positively correlated with soil water content (p = 0.03) only for the grazed treatment. Our results suggest that the effect of soil water content on soil respiration was attributable mainly to an effect on root and rhizosphere activity in the grazed treatment. We conclude that farm management practices can alter the relationship between soil respiration and soil water content.

Irrigated agriculture is conducted on approximately 257 million hectares worldwide and continues to expand, particularly in arid to semi-arid regions. Applications of water containing dissolved calcium and bicarbonate ions cause the precipitation of calcium carbonate in the soil and the release of carbon dioxide into the air. However, the contribution of inorganic C to CO2 emissions from the soil is rarely considered. Using a short-term incubation technique developed to examine changes in mineralizable organic C pools, we found that soils beneath drip emitters in an irrigated apple orchard released CO2 from both organic and inorganic C. Soils under drippers had higher concentrations of carbonates than soils that had not received direct inputs of irrigation water. The quantity of carbonates detected in the soil under the drippers at this site was small but may be greater on sites using irrigation water with higher concentrations of Ca2 + and HCO3−. Furthermore, site productivity may be reduced by unfavourable physical and chemical changes caused by carbonate deposition within the small soil volume occupied by tree roots in micro-irrigated orchards with dwarfing rootstocks. In order to better understand the implications for site productivity and for global C flux of carbonate precipitation in micro-irrigated systems, future work is required to quantify CO2 emissions during irrigation, and to characterize soil chemical and physical properties through the soil profile.

Applications of nitrogen to vineyard foliage or soil at veraison can improve grape juice yeast assimilable nitrogen concentrations and may prevent the excessive vine growth, delayed maturity, and adverse changes in fruit properties sometimes associated with high applications of N earlier in the growing season. However, the consequences of late-season foliar- and soil-applied nitrogen for grape juice yeast assimilable nitrogen (YAN) and, specifically, grape juice amino acid profiles have rarely been directly compared. Over two years in drip-irrigated Merlot and Pinot gris vineyards, grape juice amino acid concentrations were measured from vines to which urea had been applied three times around veraison at 3.8 g N/vine to either the foliage or the soil surface. Foliar-applied urea (applied as a 2% w/v solution) was usually more effective at boosting grape juice ammonium and amino acid concentrations, although soil-applied urea improved some grape juice amino acids at the Pinot gris site. Changes in the amino acid profiles of grape juice, observed in response to foliar N applications but not soil N applications, may have implications for wine quality. Applications of 15N-labeled urea at the Pinot gris site demonstrated that a greater percentage of fertilizer N was incorporated into grape juice amino acids when urea was applied to the foliage than when it was applied to the soil surface. Late-season foliar applications of urea are a reliable, efficient, and effective method of improving grape juice YAN. Further work is required to examine how treatment effects vary among sites and cultivars under different management practices and to understand the implications of altered grape juice amino acid profiles for wine quality.

Summary Labile soil organic matter (SOM) is often stabilized in soil, but is vulnerable to loss after soil disturbance. We used measurements of the 13carbon (13C) signature of soil-respired CO2 (δ13CO2) immediately after the disturbance caused by coring and sampling the soil, to assess labile SOM availability and potential vulnerability to loss. We incubated a range of pasture soils over 300 minutes and periodically measured δ13CO2. Strong temporal trends in δ13CO2 suggest that labile SOM became vulnerable to loss within minutes of soil disturbance. Equilibrium soil-respired δ13CO2 values were a function of the amount of labile SOM (measured as hot-water extractable C, HWEC), total soil C and soil protection capacity (measured as specific soil surface area, SSA). An independent experimental approach that immobilized labile SOM on to allophane (a clay mineral with a large active surface area) was used to assess the effect of SSA on equilibrium soil-respired δ13CO2. In this experiment δ13CO2 shifted progressively to more enriched values during the first six days of soil incubations after allophane addition, suggesting that labile SOM became less available after stabilization by allophane. At the same time there was a large reduction in HWEC when compared with the control treatments, also indicating limited availability of labile SOM. Further studies coupling the isotopic measurements with CO2 evolution rates are needed to test directly whether equilibrium δ13CO2 can reflect labile SOM vulnerability to loss.

Seed production by trees has consequences for regeneration, nutrient cycling, trophic interactions, invasive species impacts and various diseases influencing human health. Effective management of these consequences requires a mechanistic understanding of tree seeding patterns. It has long been known that inter-annual variability and synchrony in seed production relate to among year differences in climate. A central question to resolve is whether such seeding patterns relate to climate-driven resource dynamics. To answer this question, we first assess the role of resources (emphasising mineral nutrients and carbohydrates) during the reproductive cycle. We conclude that trees not only utilise remobilised resources stored at the beginning of a reproductive cycle but also resources taken up and recycled subsequently. Secondly, we consider resource dynamics in relation to climate-driven processes. A sequence of climate-resource relationships potentially influences the maximum seed production set at initiation as well as the level to which this maximum is achieved throughout the rest of the reproductive cycle. Thirdly, climate-resource relationships are used to appraise the assumptions of resource-based models commonly employed to enhance our understanding of tree seeding. We conclude that these models need to accommodate a wider set of processes including resource uptake and interrelationships among multiple resources throughout the reproductive cycle. Finally, we consider how climate-driven resource dynamics may have facilitated selection for mast seeding. We propose that climate-driven resource dynamics in an antecedent population could have synchronised seeding and directly advantaged individuals. These advantages were subsequently amplified through the economies of scale fitness benefits seen today in contemporary tree populations.

Soils are the largest pool of carbon (C) in terrestrial ecosystems with labile C being particularly vulnerable to loss. In this study we incubated a range of soils in both the short- (minutes) and long-term (months) to assess the loss of labile soil C by measuring the isotopic signature of soil respired CO 2 (∂ 13 CO 2 ). Strong temporal trends in ∂ 13 CO 2 values were observed following soil disturbance: ∂ 13 CO 2 rapidly changed from a range of −22.5 to −23.9‰ to −25.8 to −27.5‰ during short-term incubations and reverted back to the initial values in long-term incubations. The shifts in ∂ 13 CO 2 over the course of soil incubations were consistent with changes in labile C substrate utilization following the disturbance of sampling the soil. An independent experimental approach which immobilised labile soil C onto allophane and included chemical extractions as a measure of extractable C in soils also confirmed this interpretation. Collectively, these results indicate that the isotopic analysis of respired CO 2 can be a powerful technique which enables us to probe mechanisms and examine the consequences of disturbance on the labile component of soil C.

In the Okanagan Valley of British Columbia, low rates of nitrogen fertilizer are typically applied early in the growing season to prevent excessive vine growth, disease, and adverse changes in grape juice composition. As a consequence, grape juice yeast assimilable nitrogen (YAN) concentrations at harvest are often below the level considered sufficient to complete fermentation during wine-making, and require augmentation with additional nitrogen. Over a three year period at seven study sites planted to five winegrape varieties, late-season foliar applications of urea-N were investigated as a method for enhancing grape juice YAN. Foliar-applied solutions of urea at rates equivalent to 14–18 kg N/ha/yr (1% w/v, applied in one year only) or 28–36 kg N/ha/yr (2% w/v) caused significant improvements in grape juice YAN concentrations in six out of seven of the study sites each year, but there was no consistent pattern among years as to which study sites were most amenable to treatment. Little of the N applied in the foliar spray treatments appeared to be retained by the vines, and there were only few, small negative effects of treatment application on vine performance and juice quality. Thus, urea sprays applied to the foliage around the time of veraison show considerable promise as a supplement to more traditional soil fertilization programs on these and similar sites.

Impacts of elevated ground-level ozone (O(3)) on nitrogen (N) uptake and allocation were studied on mature European beech (Fagus sylvatica L.) and Norway spruce (Picea abies [L.] Karst.) in a forest stand, hypothesizing that: (i) chronically elevated O(3) limits nutrient uptake, and (ii) beech responds more sensitively to elevated O(3) than spruce, as previously found for juvenile trees. Tree canopies were exposed to twice-ambient O(3) concentrations (2 × O(3)) by a free-air fumigation system, with trees under ambient O(3) serving as control. After 5 years of O(3) fumigation, (15)NH(4)(15)NO(3) was applied to soil, and concentrations of newly acquired N (N(labelled)) and total N (N(total)) in plant compartments and soil measured. Under 2 × O(3), N(labelled) and N(total) were increased in the bulk soil and tended to be lower in fine and coarse roots of both species across the soil horizons, supporting hypothesis (i). N(labelled) was reduced in beech foliage by up to 60%, and by up to 50% in buds under 2 × O(3). Similarly, N(labelled) in stem bark and phloem was reduced. No such reduction was observed in spruce, reflecting a stronger effect on N acquisition in beech in accordance with hypothesis (ii). In spruce, 2 × O(3) tended to favour allocation of new N to foliage. N(labelled) in beech foliage correlated with cumulative seasonal transpiration, indicating impaired N acquisition was probably caused by reduced stomatal conductance and, hence, water transport under elevated O(3). Stimulated fine root growth under 2 × O(3) with a possible increase of below-ground N sink strength may also have accounted for lowered N allocation to above-ground organs. Reduced N uptake and altered allocation may enhance the use of stored N for growth, possibly affecting long-term stand nutrition.

This study examined the expression of genes in leaf tissue of greenhouse-grown potatoes to the form of N supplied in the context of development of a diagnostic tool for detection of N status. Potato cultivar ‘Shepody’ was grown over a two-week period at three stages of growth (tuber initiation, flowering and early tuber bulking) with three forms of N (NO3− only, NH4+ only, or 1:1 NO3−:NH4+) at two levels of N supply (0.75 or 7.5 mM). Leaf disks were collected and extracted for RNA, and 28 genes examined for expression using nCounter. Expression of ammonium transporter AT1 was increased when the N supply was reduced, regardless of N form, and was generally independent of growth stage. Expression of AT1 was less influenced by N form than currently used chemical or optical measures of plant N status. Expression of AT1 can be used as a quantitative indicator for plant N status of potato regardless of nitrogen form and crop growth stage.

Maintaining and increasing soil organic carbon stocks in grasslands is essential for sustainable productivity and to offset anthropogenic carbon emissions. Direct measurements of net ecosystem carbon dioxide exchange, FN, can be used to detect whether an ecosystem is a net sink or a source of carbon. However partitioning heterotrophic respiration, RH, from ecosystem respiration, RE, is needed to determine the impacts of land-use and global change on soil organic carbon stocks. We extracted intact soil cores from an intensively grazed dairy farm to establish mesocosms growing in controlled conditions and we subjected them to low and high nitrogen treatments (100 and 400 kgN ha1 y− 1, respectively). After concurrent clipping and addition of nitrogen, we measured the timing for the recovery of FN and its components, photosynthesis, A, and ecosystem respiration, RE, by measuring them daily. Subsequently, we measured RH from the same mesocosms seven days after the treatments were applied, using a non-disruptive, natural abundance carbon isotope technique. To test the significance of the presence of living roots when measuring RH, we compared the results obtained from the isotopic approach to those obtained from a root exclusion technique, which involved removing the roots from the mesocosms. As the plants grew after clipping, FN decreased (increasing net CO2 uptake) exponentially to mean (± standard error) steady-state values of 1.11 ± 0.26 μmol m− 2 s− 1 (net source) and − 0.19 ± 0.33 μmol m− 2 s− 1 (near neutral) for the low and high nitrogen treatments, respectively. When measured using the isotopic approach, RH increased by 60%, from 1.26 ± 0.29 μmol m− 2 s− 1 in the low, to 2.06 ± 0.55 μmol m− 2 s− 1 in the high nitrogen treatment. Thus, addition of the high nitrogen resulted in an increase in soil organic carbon loss concurrently with an increase in net uptake of carbon by the ecosystem in the high nitrogen treatment compared with the low nitrogen treatment. When measured in the absence of living roots using the root exclusion technique, RH was overall much higher than the value obtained with the isotopic technique (4.34 ± 0.13 μmol m− 2 s− 1), indicating an apparent negative rhizosphere priming effect. Furthermore, when using the root exclusion technique, no difference was found between the nitrogen treatments, suggesting that the presence of roots mediated the response of heterotrophic respiration to addition of nitrogen. These results highlight the need to include measurements of changes in RH alongside measurements of FN in non-disturbed ecosystems to interpret the processes regulating the effects of management practices on long-term changes in soil organic carbon stocks.

Near-infrared reflectance spectroscopy (NIRS) is frequently used for the assessment of key nutrients of forage or crops but remains underused in ecological and physiological studies, especially to quantify non-structural carbohydrates. The aim of this study was to develop calibration models to assess the content in soluble sugars (fructose, glucose, sucrose) and starch in foliar material of Eucalyptus globulus. A partial least squares (PLS) regression was used on the sample spectral data and was compared to the contents measured using standard wet chemistry methods. The calibration models were validated using a completely independent set of samples. We used key indicators such as the ratio of prediction to deviation (RPD) and the range error ratio to give an assessment of the performance of the calibration models. Accurate calibration models were obtained for fructose and sucrose content (R2 > 0.85, root mean square error of prediction (RMSEP) of 0.95%–1.26% in the validation models), followed by sucrose and total soluble sugar content (R2 ~ 0.70 and RMSEP > 2.3%). In comparison to the others, calibration of the starch model performed very poorly with RPD = 1.70. This study establishes the ability of the NIRS calibration model to infer soluble sugar content in foliar samples of E. globulus in a rapid and cost-effective way. We suggest a complete redevelopment of the starch analysis using more specific quantification such as an HPLC-based technique to reach higher performance in the starch model. Overall, NIRS could serve as a high-throughput phenotyping tool to study plant response to stress factors.

International audience; Although N storage determines early spring growth in trees, the usefulness of autumn N supply remains unclear as N uptake decreases in autumn, but could be restored earlier in spring to compensate for low N cycling. We intended here to evaluate the effects of autumn N supply on N uptake, storage and cycling, and spring growth. Four levels of N fertilisation were applied to 1-year-old peach trees, between the end of shoot growth and leaf fall. In spring, N supply was 15 N labelled. Organ dry weights and concentrations of 14 N, 15 N, starch and soluble sugars were evaluated after the first growth flush. Bud development had previously been described in the same trees by Jordan et al. (Trees-Struct Func 23:235–245, 2009). Fertilisation promoted autumn N uptake, spring N uptake and growth up to a threshold level, since no differences were evidenced between the three highest N treatments. The variability in tree 14 N contents was related to the number of phytomers per tree in autumn, i.e. to tree size. In spring, the depletion of the perennial structures was independent of treatment, indicating a complete mobilisa-tion of the N stores. Spring growth was related to the amounts of cycling N, and spring N uptake was in turn proportioned to shoot and fruit growth. The lower N uptake of the N limited trees was not due to a C shortage since these trees displayed the highest starch concentrations. We conclude that a moderate autumn fertilisation improved spring growth and fruit production (Jordan et al. in Trees-Struct Func 23:235–245, 2009) and that a deficit of N storage could not be compensated for by an increase in spring N uptake.

Application of organic materials to agricultural soils offers the potential to divert these from conventional waste streams and to reduce the use of mineral fertilisers for crop production. In addition to direct impacts of organic materials on soil physico-chemical conditions, such amendments may also affect the structure and functioning of soil food webs and the coupling of plant- and soil biological activity. In this study we investigated the impact of cattle slurry and municipal green compost applications on soil microbial and nematode communities. Specifically, we applied steady-state 13C-labelling to quantify the impacts of the amendments on transfer of plant-derived C through components of the soil food web and to determine the activity of the soil biota in mineralising amendment- and soil organic matter derived-C. Both the slurry and compost amendments significantly promoted barley N-acquisition and growth. The municipal compost amendment, but not slurry application, strongly affected soil microbial community structure and increased nematode abundance, relative to unamended soil. This was associated with increased flux of plant-derived C through bacterial (particularly Gram negative) populations in compost-amended soil and was associated with increased mineralisation of amendment- and soil organic matter-derived substrates. For slurry-amended soils, the amendment did not alter soil community structure and mineralisation of soil C was not coupled to the flux of plant-derived C through below-ground pools. The results are indicative of N-supply to barley in compost-amended soil being dependent on the functioning of the soil food web (including C-flux to it from living roots), whereas N-supply from slurry was independent of this coupling between plant and soil processes. We suggest that plant/food web interactions are important targets for development of effective management strategies for organic waste applications to agricultural soils in the context of optimising the balance between nutrient supply to crops and the susceptibility of nutrients to losses from soil.

Methanotrophs use methane (CH(4)) as a carbon source. They are particularly active in temperate forest soils. However, the rate of change of CH(4) oxidation in soil with afforestation or reforestation is poorly understood. Here, soil CH(4) oxidation was examined in New Zealand volcanic soils under regenerating native forests following burning, and in a mature native forest. Results were compared with data for pasture to pine land-use change at nearby sites. We show that following soil disturbance, as little as 47 years may be needed for development of a stable methanotrophic community similar to that in the undisturbed native forest soil. Corresponding soil CH(4)-oxidation rates in the regenerating forest soil have the potential to reach those of the mature forest, but climo-edaphic fators appear limiting. The observed changes in CH(4)-oxidation rate were directly linked to a prior shift in methanotrophic communities, which suggests microbial control of the terrestrial CH(4) flux and identifies the need to account for this response to afforestation and reforestation in global prediction of CH(4) emission.

Use of an in-season measure of crop N sufficiency to guide fertilizer management is one approach to match the supply of N to the crop N demand. This study examined use of gene expression in leaf tissue of field-grown potatoes for use in assessment of potato N sufficiency. Potato cultivar ‘Shepody’ was grown with six fertilizer N rates (0–250 kg N ha–1). Leaf disks were collected weekly for quantification of the expression of N uptake/transport, N assimilation, and amino acid metabolism genes in leaf tissue by nCounter. Many of the genes evaluated were responsive to crop N supply, but the response varied widely among sampling dates. The exception was an ammonium transporter gene (AT1) which was highly expressed, was relatively consistent across sampling dates, was closely related to root zone soil nitrate concentration across N rates and sampling dates, and was highly negatively correlated with total tuber yield. The level of expression of AT1 in leaf tissue was as good as or better than conventional chemical or optical measures of potato N sufficiency in the current study.

34 páginas, 2 tablas, 4 figuras, [EN] Current knowledge on tree carbon (C) allocation to wood is particularly scarce in plants subjected to disturbance factors, such as browsing, which affects forest regeneration worldwide and has an impact on the C balance of trees. Furthermore, quantifying the degree to which tree rings are formed from freshly assimilated vs. stored carbohydrates is highly relevant for our understanding of tree C allocation. We used (13)C labelling to quantify seasonal allocation of stored C to wood formation in two species with contrasting wood anatomy: Betula pubescens Ehrh. (diffuse-porous) and Quercus petraea [Matt.] Liebl. (ring-porous). Clipping treatments (66% shoot removal, and unclipped) were applied to analyse the effect of browsing on C allocation into tree rings, plus the effects on tree growth, architecture, ring width and non-structural carbohydrates (NSCs). The relative contribution of stored C to wood formation was greater in the ring-porous (55-70%) than in the diffuse-porous species (35-60%), although each species followed different seasonal trends. Clipping did not cause a significant depletion of C stores in either species. Nonetheless, a significant increase in the proportion of stored C allocated to earlywood growth was observed in clipped birches, and this could be explained through changes in tree architecture after clipping. The size of C pools across tree species seems to be important in determining the variability of seasonal C allocation patterns to wood and their sensibility to disturbances such as browsing. Our results indicate that the observed changes in C allocation to earlywood in birch were not related to variations in the amount or concentration of NSC stores, but to changes in the seasonal availability of recently assimilated C caused by modifications in tree architecture after browsing., S.P. was funded by an MEC-SEUI postdoctoral fellowship and a JAE-Doc (CSIC) contract. This study was partly funded by projects CGL2008-04847-C02-01/BOS (MCI-Spain) and GA-LC- 011/2008 (DGA-La Caixa). E.P., A.S., A.H. and P.M. thank the Scottish Government’s Rural and Environmental Research and Analysis Directorate for funding.

We identified the role of various soil parameters and root density as drivers of soil respiration (Rs) in an apple orchard, measured during different periods of the year and at a range of distances from trees, in plots with a different history of nutrient supply. Rs was measured in April, May, August and December and studied in relation to soil temperature and moisture, total soil C and N, as well as to fine root density and medium-, and large-sized root density and root N concentration. The study also aimed to partition Rs by applying the root regression technique. Rs ranged from 0.06 in December to 1.49 g CO2 m−2 h−1 in August. Average soil temperature alone explained up to 71% of the annual variability of Rs, while soil water content was negatively correlated to Rs. Fertilization, soil C and N concentration and root N had negligible effects on Rs. Fine root density, but not medium- and large-sized root density, contributed to explaining part of the yearly variability of Rs and proved to be a good predictor in December, when the statistical significance of the regression made it possible to estimate the autotrophic component of Rs as being about 35% of total soil respiration.

The δ13C of the soil surface efflux of carbon dioxide (δ13CRS) has emerged as a powerful tool enabling investigation of a wide range of soil processes from characterising entire ecosystem respiration to detailed compound-specific isotope studies. δ13CRS can be used to trace assimilated carbon transfer below ground and to partition the overall surface efflux into heterotrophic and autotrophic components. Despite this wide range of applications no consensus currently exists on the most appropriate method of sampling this surface efflux of CO2 in order to measure δ13CRS. Here we consider and compare the methods which have been used, and examine the pitfalls. We also consider a number of analysis options, isotope ratio mass spectrometry (IRMS), tuneable diode laser spectroscopy (TDLS) and cavity ring-down laser spectroscopy (CRDS). δ13CRS is typically measured using chamber systems, which fall into three types: closed, open and dynamic. All are imperfect. Closed chambers often rely on Keeling plots to estimate δ13CRS, which may not be appropriate without free turbulent air mixing. Open chambers have the advantage of being able to maintain steady-state conditions but analytical errors may become limiting with low efflux rates. Dynamic chambers like open chambers are complex, and controlling pressure fluctuations caused by air movement is a key concern. Both open and dynamic chambers in conjunction with field portable TDLS and CRDS analysis systems have opened up the possibility of measuring δ13CRS in real time permitting new research opportunities and are on balance the most suited to this type of measurement.

New platforms allow quantification of gene expression from large, replicated experiments but current sampling protocols for plant tissue using immediate flash freezing in liquid nitrogen are a barrier to these high-throughput studies. In this study, we compared four sampling methods for RNA extraction for gene expression analysis: (1) the standard sampling method of flash freezing whole leaves in liquid nitrogen immediately upon removal from the plant; (2) incubation of excised leaf disks for 2 min at field temperature followed by flash freezing; (3) incubation of excised leaf disks for 1 h on ice followed by flash freezing; and (4) incubation of excised leaf disks for 1 h at field temperature followed by flash freezing. Gene expression analysis was done for 23 genes using nCounter, and normalization of the data was done using the geometric mean of five housekeeping genes. Quality of RNA was highest for protocol A and lowest for protocol D. Despite some differences in RNA quality, gene expression was not significantly different among protocols A, B, and C for any of the 23 genes. Expression of some genes was significantly different between protocol D and the other protocols. This study demonstrates that when sampling leaf disks for gene expression analysis, the time between tissue removal from the plant and flash freezing in liquid nitrogen can be extended. This increase in time allowable during sampling provides greater flexibility in sampling large replicated field experiments for statistical analysis of gene expression data.

Quantifying the loss of soil carbon through respiration has proved difficult, due to the challenge of measuring the losses associated with the turnover of soil organic matter (SOM) as distinct from autotrophic components. In forest ecosystems the δ13C value of respiration from turnover of SOM (δ13CRSOM) is typically 2–4‰ enriched compared with that from roots and associated microbes (δ13CRROOTS), with that from the litter (δ13CRLITTER) lying between the two. We measured soil respiration at 50 locations in a forest soil and then used differences in isotopic signatures to quantify the proportion of soil respiration arising from the turnover of SOM (fRSOM) at a subset of 30 locations, chosen randomly. The soil surface CO2 efflux was collected using an open chamber system supplied with CO2-free air and the δ13C signature (δ13CRS) measured, giving a mean (±SD) value across the site of −26.1 ± 0.58‰. The values of δ13CRROOTS, δ13CRLITTER and δ13CRSOM were measured at each location by incubation of roots, litter and root-free soil and collection of the CO2 for isotopic analysis. δ13CRSOM became progressively depleted with length of incubation (1.5‰ after 8 h), so CO2 was collected after 20 min. The mean value of δ13CRLITTER was −27.2 ± 0.68 ‰, which was indistinguishable from δ13CRROOTS of −27.6 ± 0.51‰, while δ13CRSOM was −25.1 ± 0.88‰. δ13CRROOTS and δ13CRSOM measured at each location were used as the end points of a two component mixing model to calculate fRSOM, giving a mean value for fRSOM of 0.61 ± 0.28. It was not possible to estimate fRSOM using the total C contents of the roots and soil which were significantly depleted in 13C in comparison to their respired CO2. However, at seven locations the δ13CRS was slightly enriched compared with δ13CRSOM (mean 0.3‰), which was not considered significantly different so fRSOM was constrained to 1.0. If these seven rings were excluded mean fRSOM was 0.49 ± 0.20. We have shown the possibility of using natural abundance 13C discrimination to quantify fRSOM in a forest soil with an input of carbon only from C3 photosynthesis.

In-season chemical or optical measures of crop N status can be effective tools in optimizing potato fertilizer N management. The feasibility of using a gene expression as an alternative approach for early detection of potato nitrate deficiency was examined using three potato cultivars (Shepody, Russet Norkotah, and Red Pontiac) with abundant (7.5 mM NO3), limited (0.75 mM NO3) or deficient (0 mM NO3) nitrate supply in nutrient culture over a 7 d period. RNA was extracted from the last fully expanded leaf and quantified using realtime RT-qPCR. Reduced nitrate supply had no measurable effect on shoot dry weight or leaf chlorophyll concentration, but decreased petiole nitrate concentration. Under deficit nitrate supply, down-regulation of nitrate reductase and nitrite reductase was measured within 3 days for all cultivars, and down-regulation of asparagine synthetase was measured in two cultivars. Nitrate supply had no effect on expression of ammonium transporter. In this experimental system, plant gene expression markers detected a reduction of nitrate supply prior to measureable reductions in plant growth or in N status measured using common chemical or optical methods.

In grazed pastures, soil pH is raised in urine patches, causing dissolution of organic carbon and increased ammonium and nitrate concentrations, with potential effects on the structure and functioning of soil microbial communities. Here we examined the effects of synthetic sheep urine (SU) in a field study on dominant soil bacterial and fungal communities associated with bulk soil and plant roots (rhizoplane), using culture-independent methods and a new approach to investigate the ureolytic community. A differential response of bacteria and fungal communities to SU treatment was observed. The bacterial community showed a clear shift in composition after SU treatment, which was more pronounced in bulk soil than on the rhizoplane. The fungal community did not respond to SU treatment; instead, it was more affected by the time of sampling. Redundancy analysis of data indicated that the variation in the bacterial community was related to change in soil pH, while fungal community was more responsive to dissolution of organic carbon. Like the universal bacterial community, the ureolytic community was influenced by the SU treatment. However, different taxa within the ureolytic bacterial community responded differentially to the treatment. The ureolytic community comprised of members from a range of phylogenetically different taxa and could be used to measure the effect of environmental perturbations on the functional diversity of natural ecosystems.

For soils in carbon balance, losses of soil carbon from biological activity are balanced by organic inputs from vegetation. Perturbations, such as climate or land use change, have the potential to disrupt this balance and alter soil-atmosphere carbon exchanges. As the quantification of soil organic matter stocks is an insensitive means of detecting changes, certainly over short timescales, there is a need to apply methods that facilitate a quantitative understanding of the biological processes underlying soil carbon balance. We outline the processes by which plant carbon enters the soil and critically evaluate isotopic methods to quantify them. Then, we consider the balancing CO(2) flux from soil and detail the importance of partitioning the sources of this flux into those from recent plant assimilate and those from native soil organic matter. Finally, we consider the interactions between the inputs of carbon to soil and the losses from soil mediated by biological activity. We emphasize the key functional role of the microbiota in the concurrent processing of carbon from recent plant inputs and native soil organic matter. We conclude that quantitative isotope labelling and partitioning methods, coupled to those for the quantification of microbial community substrate use, offer the potential to resolve the functioning of the microbial control point of soil carbon balance in unprecedented detail.

Does plant diversity drive soil microbial diversity in temperate, upland grasslands? Plants influence microbial activity around their roots by release of carbon and pot studies have shown an impact of different grass species on soil microbial community structure. Therefore it is tempting to answer yes. However, evidence from field studies is more complex. This evidence is reviewed at three different scales. First, studies from the plant community scale are considered that have compared soil microbial community structure in pastures of different vegetation composition, as a consequence of pasture improvement. These show fungi dominating the biomass in unimproved pastures and bacteria when lime and fertilizers have been applied. Secondly, evidence for interactions between individual grass species and soil microbes is discussed at the level of the rhizosphere, by considering both pot experiments and field studies. These have produced contrasting and inconclusive results, often due to spatial heterogeneity of soil properties and microbial communities. In particular, increased soil pH and fertility in urine patches and other nutrient cycling processes interact to increase the spatially complexity of soil microbial communities. Finally three studies which have measured microbial community structure in the rhizoplane are considered. These show that bacterial diversity is not directly related to plant diversity, although fungal diversity is. In addition, the soil fungal community has been demonstrated to have an effect upon the composition of the bacterial community. We suggest that while current vegetation influences fungal communities (particularly mycorrhizae) and litter inputs fungal saprotrophs, bacterial community structure is influenced more by the quality or composition of soil organic matter, thereby reflecting carbon inputs to the soil over decades.

Nitrogen (N) withdrawn from leaves before abscission can help to supply N requirements in plants of nutrient poor habitats. Besides N shortage, Mediterranean Quercus seedlings must face water and light stresses. However, there is little information on the influence of these stresses in the nitrogen resorption efficiency (NRE) at leaf level, and none at canopy level. We tested in two separated experiments how changes in water and light availability affect NRE and its components at both levels in seedlings of two evergreen oaks [Quercus coccifera L. and Quercus ilex subsp. ballota (Desf.) Samp] and in a semi-deciduous one (Quercus faginea Lam.). In the summer drought experiment seedlings were left to dehydrate to −2.5 and to −0.5 MPa (water stress and control, respectively) before watering. In the light experiment seedlings were grown at 100, 20 and 5% of full sunlight. The leaf abscission pattern was monitored and N content, N loss and NRE were calculated in the two peaks of leaf abscission (spring and late summer). After one year of treatments summer drought had little effect on N resorption and its components at both leaf and canopy levels. Moderate shade increased NRE at leaf level in Q. faginea but this response vanished at canopy level. N loss at the leaf level was unaffected. Deep shade decreased N lost at the canopy level in spring but increased it in late summer. N resorption and N losses at the canopy level were lower at late summer than in spring, due to fewer leaves falling. This study highlights the importance of the scale on the study of nitrogen dynamics (leaf vs. whole canopy), as the scaling factor (amount of leaf shedding) also responds to environmental factors, either enhancing or reversing the effects found at leaf level.

Plants link atmospheric and soil carbon pools through CO2 fixation, carbon translocation, respiration and rhizodeposition. Within soil, microbial communities both mediate carbon-sequestration and return to the atmosphere through respiration. The balance of microbial use of plant-derived and soil organic matter (SOM) carbon sources and the influence of plant-derived inputs on microbial activity are key determinants of soil carbon-balance, but are difficult to quantify. In this study we applied continuous 13C-labelling to soil-grown Lolium perenne, imposing atmospheric CO2 concentrations and nutrient additions as experimental treatments. The relative use of plant- and SOM-carbon by microbial communities was quantified by compound-specific 13C-analysis of phospholipid fatty acids (PLFAs). An isotopic mass-balance approach was applied to partition the substrate sources to soil respiration (i.e. plant- and SOM-derived), allowing direct quantification of SOM-mineralisation. Increased CO2 concentration and nutrient amendment each increased plant growth and rhizodeposition, but did not greatly alter microbial substrate use in soil. However, the increased root growth and rhizosphere volume with elevated CO2 and nutrient amendment resulted in increased rates of SOM-mineralisation per experimental unit. As rhizosphere microbial communities utilise both plant- and SOM C-sources, the results demonstrate that plant-induced priming of SOM-mineralisation can be driven by factors increasing plant growth. That the balance of microbial C-use was not affected on a specific basis may suggest that the treatments did not affect soil C-balance in this study.

We used natural gradients in soil and vegetation δ 13 C signatures in a savannah ecosystem in Texas to partition soil respiration into the autotrophic (Ra) and heterotrophic (Rh) components. We measured soil respiration along short transects from under clusters of C 3 trees into the C 4 dominated grassland. The site chosen for the study was experiencing a prolonged drought, so an irrigation treatment was applied at two positions of each transect. Soil surface CO 2 efflux was measured along transects and CO 2 collected for analysis of the δ 13 C signature in order to: (i) determine how soil respiration rates varied along transects and were affected by localised change in soil moisture and (ii) partition the soil surface CO 2 efflux into Ra and Rh, which required measurement of the δ 13 C signature of root- and soil-derived CO 2 for use in a mass balance model. The soil at the site was unusually dry, with mean volumetric soil water content of 8.2%. Soil respiration rates were fastest in the centre of the tree cluster (1.5 ± 0.18 μmol m −2 s −1 ; mean ± SE) and slowest at the cluster–grassland transition (0.6 ± 0.12 μmol m −2 s −1 ). Irrigation produced a 7–11 fold increase in the soil respiration rate. There were no significant differences ( p > 0.5) between the δ 13 C signature of root biomass and respired CO 2 , but differences ( p 2 and soil when sampled at the edge of the clusters and in the grassland. Therefore, end member values were measured by root and soil incubations, with times kept constant at 30 min for roots and 2 h for soils. The δ 13 C signature of the soil surface CO 2 efflux and the two end member values were used to calculate that, in the irrigated soils, Rh comprised 51 ± 13.5% of the soil surface CO 2 efflux at the mid canopy position and 57 ± 7.4% at the drip line. In non-irrigated soil it was not possible to partition soil respiration, because the δ 13 C signature of the soil surface CO 2 efflux was enriched compared to both the end member values. This was probably due to a combination of the very dry porous soils at our study site (which may have been particularly susceptible to ingress of atmospheric CO 2 ) and the very slow respiration rates of the non-irrigated soils.

Carbon dioxide respired by soils comes from both autotrophic and heterotrophic respiration. 13C has proved useful in differentiating between these two sources, but requires the collection and analysis of CO2 efflux from the soil. We have developed a novel, open chamber system which allows for the accurate and precise quantification of the delta13C of soil-respired CO2. The chamber was tested using online analyses, by configuring a GasBench II and continuous flow isotope ratio mass spectrometer, to measure the delta13C of the chamber air every 120 s. CO2 of known delta13C value was passed through a column of sand and, using the chamber, the CO2 concentration stabilized rapidly, but 60 min was required before the delta13C value was stable and identical to the cylinder gas (-33.3 per thousand). Changing the chamber CO2 concentration between 200 and 900 micromol.mol(-1) did not affect the measured delta13C of the efflux. Measuring the delta13C of the CO2 efflux from soil cores in the laboratory gave a spread of +/-2 per thousand, attributed to heterogeneity in the soil organic matter and roots. Lateral air movement through dry sand led to a change in the delta13C of the surface efflux of up to 8 per thousand. The chamber was used to measure small transient changes (+/-2 per thousand) in the delta13C of soil-respired CO2 from a peaty podzol after gradual heating from 12 to 35 degrees C over 12 h. Finally, soil-respired CO2 was partitioned in a labelling study and the contribution of autotrophic and heterotrophic respiration to the total efflux determined. Potential applications for the chamber in the study of soil respiration are discussed.

We examined experimentally the effect of competition from Calluna vulgaris or Molinia caerulea on Betula pubescens saplings subjected to simulated mammalian browsing damage. We tested three hypotheses: 1. that responses to competition will follow the balanced growth hypothesis with the predominant competition effect being below-ground, resulting in increased allocation to roots; 2. that above-ground competition effects from C. vulgaris are greater than M. caerulea, resulting in differences in growth allocation; 3. that, when only B. pubescens saplings are damaged, herbivory reduces its ability to compete with both species. We grew B. pubescens saplings in the presence of below- or above- and below-ground interactions from C. vulgaris or M. caerulea. Saplings were also subjected to simulated browsing by clipping (50% of current year's growth), either pre-senescence or at bud-burst. We measured the morphology and dry mass allocation response of the saplings one year after the browsing was applied. Competition reduced sapling dry mass by approximately 50%, but C. vulgaris reduced dry mass to a greater extent than M. caerulea. This difference was due to a smaller impact of M. caerulea shoots than C. vulgaris shoots on birch growth. Saplings compensated for browsing damage, resulting in no difference in dry mass one year after damage. However, sapling morphological responses to browsing damage were dependent on the competing species. Despite the large competitive effect of below-ground interactions, saplings did not increase allocation to root growth as predicted. Additionally, in response to above-ground interactions from M. caeruela, saplings increased allocation to root growth relative to those growing with C. vulgaris. As such, growth responses to competition were not explained by the balanced growth hypothesis. This study highlights the importance of patterns, as well as intensity, of competition in determining plant responses to inter-specific interactions.

A central aim in ecology is to understand the relation between organism diversity and ecosystem functioning. We investigate this relation using a generic individual-based modelling framework described in part 1, in which individuals within a community are characterised by physiological traits and interact within a spatially structured environment. We explore the effect of intraspecific variation among individuals on community-scale productivity in a range of homogeneous and heterogeneous environments. We show that diversity among individuals has a significant, and in most but not all cases positive, impact on community productivity. At low levels of resource the persisting plants giving highest productivity have slowest uptake rates and longest times to reproduction. In contrast, at high levels of resource the persisting plants giving highest productivity have highest uptake levels and shortest time to reproduction. Thus, the individuals’ trait distributions defining community functioning emerge, after many iterations of the model, from the surviving individuals. We, therefore, show that different mechanisms, as evidenced in the surviving individuals’ traits, increase community productivity in different environmental contexts, and so demonstrate the importance of accounting for diversity at the scale of the individual. Consequently, progress may be made in linking diversity and function by considering small, manageable systems and making physiological measurements on individuals.