Effects of elevated CO2, temperature and N fertilization on nitrogen fluxes in a temperate grassland ecosystem

The soil nitrogen cycle was investigated in a pre‐established Lolium perenne sward on a loamy soil and exposed to ambient and elevated atmospheric CO₂ concentrations (350 and 700 μL L^?1) and, at elevated [CO₂], to a 3 °C temperature increase. At two levels of mineral nitrogen supply, N– (150 kgN ha^?1 y^?1) and N+ (533 kgN ha^?1 y^?1), ¹⁵N‐labelled ammonium nitrate was supplied in split applications over a 2.5‐y period. The recovery of the labelled fertilizer N was measured in the harvests, in the stubble and roots, in the macro‐organic matter fractions above 200 μm in size (MOM) and in the aggregated organic matter below 200 μM (AOM). Elevated [CO₂] reduced the total amount of N harvested in the clipped parts of the sward. The harvested N derived from soil was reduced to a greater extent than that derived from fertilizer. At both N supplies, elevated [CO₂] modified the allocation of the fertilizer N in the sward, in favour of the stubble and roots and significantly increased the recovery of fertilizer N in the soil macro‐organic matter fractions. The increase of fertilizer N immobilization in the MOM was associated with a decline of fertilizer N uptake by the grass sward, which supported the hypothesis of a negative feedback of elevated [CO₂] on the sward N yield and uptake. Similar and even more pronounced effects were observed for the native N mineralized in the soil. At N–, a greater part of the fertilizer N organized in the root phytomass resulted in an underestimation of N immobilized in dead roots and, in turn, an underestimation of N immobilization in the MOM. The 3 °C temperature increase alleviated the [CO₂] effect throughout much of the N cycle, increasing soil N mineralization, N derived from soil in the harvests, and the partitioning of the assimilated fertilizer N to shoots. In conclusion, at ambient temperature, the N cycle was slowed down under elevated [CO₂], which restricted the increase in the aboveground production of the grass sward, and apparently contributed to the sequestration of carbon belowground. In contrast, a temperature increase under elevated [CO₂] stimulated the soil nitrogen cycle, improved the N nutrition of the sward and restricted the magnitude of the soil C sequestration.     

Effects of N-fertilisation on CH4 oxidation and production, and consequences for CH4 emissions from microcosms and rice fields

The world's growing human population causes an increasing demand for food, of which rice is one of the most important sources. In rice production nitrogen is often a limiting factor. As a consequence increasing amounts of fertiliser will have to be applied to maximise yields. There is an ongoing discussion on the possible effects of fertilisation on CH₄ emissions. We therefore investigated the effects of N‐fertiliser (urea) on CH₄ emission, production and oxidation in rice microcosms and field experiments. In the microcosms, a substantial but short‐lived reduction of CH₄ emission was observed after N‐addition to 43‐d‐old rice plants. Methane oxidation increased by 45%, demonstrated with inhibitor measurements and model calculations based on stable carbon isotope data (δ¹³CH₄). A second fertilisation applied to 92‐d‐old plants had no effect on CH₄ emission rates. The positive effect of additional N on methanotrophic bacteria was also found in vitro for potential CH₄ oxidation rates in soil and root samples from the microcosm and field experiments, indicated by elevated initial oxidation rates and reduced lag‐phases. Fertilisation did not affect methane production in the microcosms. In the field, the effects were diverse: methane production was inhibited in the topsoil, but stimulated instead in the bulk soil. Stimulation occurred probably in the anaerobic food chain at the level of hydrolytic or fermenting bacteria, because acetate, a methanogenic precursor, increased simultaneously. Combining field, microcosm and laboratory experiments we conclude that any agricultural treatment improving the N‐supply to the rice plants will also be favourable for the CH₄ oxidising bacteria. However, N‐fertilisation had only a transient influence and was counter‐balanced in the field by an elevated CH₄ production. A negative effect of the fertilisation was a transient increase of N₂O emissions from the microcosms. However, integrating over the season the global warming potential (GWP) of N₂O emitted after fertilisation was still negligible compared to the GWP of emitted CH₄.     

Source-sink relations in Lolium perenne L. as reflected by carbohydrate concentrations in leaves and pseudo-stems during regrowth in a free air carbon dioxide enrichment (FACE) experiment*

The effect of an elevated partial pressure of CO₂ (p_CO2) on carbohydrate concentrations in source leaves and pseudo-stems (stubble) of Lolium perenne L. (perennial ryegrass) during regrowth was studied in a regularly defoliated grass sward in the field. The free air carbon dioxide enrichment (FACE) technology enabled natural environmental conditions to be provided. Two levels of nitrogen (N) supply were used to modulate potential plant growth. Carbohydrate concentrations in source leaves were increased at elevated p_CO2, particularly at low N supply. Elevated leaf carbohydrate concentrations were related to an increased structural carbon (C) to N ratio and thus reflected an increased C availability together with a N-dependent sink limitation. Immediately after defoliation, apparent assimilate export rates (differences in the carbohydrate concentrations of young source leaves measured in the evening and on the following morning) showed a greater increase at elevated p_CO2 than at ambient p_CO2; however, replenishment of carbohydrate reserves was not accelerated. Distinct, treatment-dependent carbohydrate concentrations in pseudo-stems suggested an increasing degree of C-sink limitation from the treatment at ambient p_CO2 with high N supply to that at elevated p_CO2 with low N supply. During two growing seasons, no evidence of a substantial change in the response of the carbohydrate source in L. perenne to elevated p_CO2 was found. Our results support the view that the response of L. perenne to elevated p_CO2 is restricted by a C-sink limitation, which is particularly severe at low N supply.     

Elevated pCO2 Affects C and N Metabolism in Wild Type and Transgenic Tobacco Exhibiting Altered C/N Balance in Metabolite Analysis

Abstract: Diurnal changes in starch, sugar and amino acid concentrations in source leaves, sink leaves and roots of tobacco plants were determined. In addition to wild type tobacco, transformed plants deficient in root nitrate reductase and exhibiting decreased rates of growth were employed. Further, the growth rates of tobacco plants were modulated by exposure to elevated pCO₂. From the diurnal alterations in metabolite concentrations, the daily turnover of starch and amino N was estimated in order to: (i) elucidate whether turnover rates can be related to growth rates, and (ii) identify individual amino compounds with the potential to indicate nitrogen fluxes and the C/N status of plants. Elevated pCO₂ increased growth rates and daily turnover of starch in both wild type and transformed plants, indicating enhanced rates of photosynthesis. In wild type plants, elevated pCO₂ increased the turnover of amino N, notably glutamine and alanine, in mature source leaves, indicating enhanced nitrate reduction. By contrast, amino N turnover in source leaves of transformed plants was not affected by elevated pCO₂, although nitrate reduction was presumably enhanced. Apparently, export of amino N was increased from the source leaves of transformed plants. This assumption was supported by a significantly increased turnover of amino N in young sink leaves compared to mature source leaves, indicating a preference for acropetal amino N allocation and import into the young leaves of the transformed plants. Further, elevated pCO₂ increased the allocation of leaf‐derived amino N to the roots of transformed plants. This led to increased levels of amino compounds during the entire day, notably glutamate, but did not affect root growth of the transformed plants. The suitability of individual amino compounds as markers for major N fluxes, such as nitrate reduction, photorespiration, and amino N export and import is discussed.     

Nitrous oxide emissions from grass swards during the eighth year of elevated atmospheric pCO2 (Swiss FACE)

Emissions of N₂O were measured during the growth season over a year from grass swards under ambient (360 μL L^?1) and elevated (600 μL L^?1) CO₂ partial pressures at the Free Air Carbon dioxide Enrichment (FACE) experiment, Eschikon, Switzerland. Measurements were made following high (56 g N m^?2 yr^?1) and low (14 g N m^?2 yr^?1) rates of fertilizer application, split over 5 re‐growth periods, to Lolium perenne, Trifolium repens and mixed Lolium/Trifolium swards. Elevated pCO₂ increased annual emissions of N₂O from the high fertilized Lolium and mixed Lolium/Trifolium swards resulting in increases in GWP (N₂O emissions) of 179 and 111 g CO₂ equivalents m^?2, respectively, compared with the GWP of ambient pCO₂ swards, but had no significant effect on annual emissions from Trifolium monoculture swards. The greater emissions from the high fertilized elevated pCO₂Lolium swards were attributed to greater below‐ground C allocation under elevated pCO₂ providing the energy for denitrification in the presence of excess mineral N. An annual emission of 959 mg N₂O‐N m^?2 yr^?1 (1.7% of fertilizer N applied) was measured from the high fertilized Lolium sward under elevated pCO₂. The magnitude of emissions varied throughout the year with 84% of the total emission from the elevated pCO₂Lolium swards measured during the first two re‐growths (April–June 2001). This was associated with higher rainfall and soil water contents at this time of year. Trends in emissions varied between the first two re‐growths (April–June 2001) and the third, fourth and fifth re‐growths (late June–October 2000), with available soil NO₃^? and rainfall explaining 70%, and soil water content explaining 72% of the variability in N₂O in these periods, respectively. Caution is therefore required when extrapolating from short‐term measurements to predict long‐term responses to global climate change. Our findings are of global significance as increases in atmospheric concentrations of CO₂ may, depending on sward composition and fertilizer management, increase greenhouse gas emissions of N₂O, thereby exacerbating the forcing effect of elevated CO₂ on global climate. Our results suggest that when applying high rates of N fertilizer to grassland systems, Trifolium repens swards, or a greater component of Trifolium in mixed swards, may minimize the negative effect of continued increasing atmospheric CO₂ concentrations on global warming.     

The effects of nitrogen fertilisation and elevated CO2 on the lipid biosynthesis and carbon isotopic discrimination in birch seedlings (Betula pendula)

The effects of nitrogen (N) fertilisation and elevated [CO₂] on lipid biosynthesis and carbon isotope discrimination in birch (Betula pendula Roth.) transplants were evaluated using seedlings grown with and without N fertiliser, and under two concentrations of atmospheric CO₂ (ambient and ambient+250 μmol mol^-1) in solar dome systems. N fertilisation decreased n-fatty acid chain length (18:0/16:0) and the ratios of α-linolenate (18:2)/linoleate (18:1), whereas elevated [CO₂] showed little effect on n-fatty acid chain length, but decreased the unsaturation (18:2+18:1)/18:0. Both N fertilisation and elevated [CO₂] increased the quantity of leaf wax n-alkanes, whilst reducing that of n-alkanols by 20–50%, but had no simple response in fatty acid concentrations. ¹³C enrichment by 1–2.5‰ under N fertilisation was observed, and can be attributed to both reduced leaf conductance and increased photosynthetic consumption of CO₂. Individual n-alkyl lipids of different chain length show consistent pattern of δ¹³C values within each homologue, but are in general 5–8‰ more depleted in ¹³C than the bulk tissues. Niether nitrogen fertilisation and elevated CO₂ influenced the relationship between carbon isotope discrimination of the bulk tissue and the individual lipids. This revised version was published online in June 2006 with corrections to the Cover Date.     

Effects of enhanced atmospheric CO2 and nutrient supply on the quality and subsequent decomposition of fine roots of Betula pendula Roth. and Picea sitchensis (Bong.) Carr.

Fine root litter derived from birch (Betula pendula Roth.) and Sitka spruce (Picea sitchensis (Bong.) Carr.) plants grown under two CO₂ atmospheric concentrations (350 ppm and 600 ppm) and two nutrient regimes was used for decomposition studies in laboratory microcosms. Although there were interactions between litter type, CO₂/fertiliser treatments and decomposition rates, in general, an increase in the C/N ratio of the root tissue was observed for roots of both species grown under elevated CO₂ in unfertilized soil. Both weight loss and respiration of decomposing birch roots were significantly reduced in materials derived from enriched CO₂, whilst the decomposition of spruce roots showed no such effect. A parallel experiment was performed using Betula pendula root litter grown under different N regimes, in order to test the relationship between C/N ratio of litter and root decomposition rate. A highly significant (p<0.001) negative correlation between C/N ratio and root litter respiration was found, with an r²=0.97. The results suggest that the increased C/N ratio of plant tissues induced by elevated CO₂ can result in a reduction of decomposition rate, with a resulting increase in forest soil C stores.     

Methane oxidation potentials and fluxes in agricultural soil:Effects of fertilisation and soil compaction

We have studied the inhibiting effect offertilisation and soil compaction on CH₄oxidation by measuring gas fluxes and soil mineral Ndynamics in the field, and CH₄ oxidation rates inlaboratory-incubated soil samples. The fertilisationand soil compaction field experiment was establishedin 1985, and the gas fluxes were measured from 1992 to1994. Methane oxidation was consistently lower infertilised than in unfertilised soil, but thereapparently was no effect of repeated fertiliseradditions on the fertilised plots. The measuredmineral N in fertilised and unfertilised soil showedlarge differences in NH₄ ⁺ concentrationsjust after fertilisation, but the levels rapidlyconverged because of plant uptake and nitrification.The CH₄ oxidation rate did not reflect thesecontrasting mineral N patterns, suggesting that theCH₄ oxidation capacity remaining in the soil thathad been fertilised since 1985 was largely insensitiveto ammonia in the new fertiliser. Thus, competitiveinhibition by ammonia may have been involved in theearly stage of the field fertiliser experiment, butthe CH₄ oxidation remaining after 7 to 9 years ofcontinued fertilisation seems not to have beenaffected by ammonia. The substrate affinity of theCH₄-oxidizing microflora appeared to be the samein both the fertilised soil and the unfertilisedcontrol, as judged from the response to elevatedCH₄ concentrations (52 µl l^–1) inlaboratory incubations. Soil compaction resulted in apersistent reduction of CH₄ influx, also seen inlaboratory incubations with sieved (4-mm mesh) soilsamples. Since the sieving presumably removesdiffusion barriers created by the soil compaction, thefact that compaction effects persisted through thesieving may indicate that soil compaction has affectedthe biological potential for CH₄ oxidation in thesoil.     

Carbon‐13 input and turn‐over in a pasture soil exposed to long‐term elevated atmospheric CO2

The impact of elevated CO₂ and N‐fertilization on soil C‐cycling in Lolium perenne and Trifolium repens pastures were investigated under Free Air Carbon dioxide Enrichment (FACE) conditions. For six years, swards were exposed to ambient or elevated CO₂ (35 and 60 Pa pCO₂) and received a low and high rate of N fertilizer. The CO₂ added in the FACE plots was depleted in ¹³C compared to ambient (Δ? 40‰) thus the C inputs could be quantified. On average, 57% of the C associated with the sand fraction of the soil was ‘new’ C. Smaller proportions of the C associated with the silt (18%) and clay fractions (14%) were derived from FACE. Only a small fraction of the total C pool below 10 cm depth was sequestered during the FACE experiment. The annual net input of C in the FACE soil (0–10 cm) was estimated at 4.6 ± 2.2 and 6.3 ± 3.6 (95% confidence interval) Mg ha^{? 1} for T. repens and L. perenne, respectively. The maximum amount of labile C in the T. repens sward was estimated at 8.3 ± 1.6 Mg ha^{? 1} and 7.1 ± 1.0 Mg ha^{? 1} in the L. perenne sward. Mean residence time (MRT) for newly sequestered soil C was estimated at 1.8 years in the T. repens plots and 1.1 years for L. perenne. An average of 18% of total soil C in the 0–10 cm depth in the T. repens sward and 24% in the L. perenne sward was derived from FACE after 6 years exposure. The majority of the change in soil δ¹³C occurred in the first three years of the experiment. No treatment effects on total soil C were detected. The fraction of FACE‐derived C in the L. perenne sward was larger than in the T. repens sward. This suggests a priming effect in the L. perenne sward which led to increased losses of the old C. Although the rate of C cycling was affected by species and elevated CO₂, the soil in this intensively managed grassland ecosystem did not become a sink for additional new C.     

The use of different soil nitrogen sources by young Norway spruce plants

 Three-year-old Norway spruce trees were planted into a low-nitrogen mineral forest soil and supplied either with two different levels of mineral nitrogen (NH₄NO₃) or with a slow-release form of organic nitrogen (keratin). Supply of mineral nitrogen increased the concentrations of ammonium and nitrate in the soil solution and in CaCl₂-extracts of the rhizosphere and bulk soil. In the soil solution, in all treatments nitrate concentrations were higher than ammonium concentrations, while in the soil extracts ammonium concentrations were often higher than nitrate concentrations. After 7 months of growth, ¹⁵N labelled ammonium or nitrate was added to the soil. Plants were harvested 2 weeks later. Keratin supply to the soil did not affect growth and nitrogen accumulation of the trees. In contrast, supply of mineral nitrogen increased shoot growth and increased the ratio of above-ground to below-ground growth. The proportion of needle biomass to total above-ground biomass was not increased by mineral N supply. The atom-% ¹⁵N was higher in younger needles than in older needles, and in younger needles higher in plants supplied with ¹⁵N-nitrate than in plants supplied with ¹⁵N-ammonium. The present data show that young Norway spruce plants take up nitrate even under conditions of high plant internal N levels. Received: 1 April 1998 / Accepted: 9 October 1998     

Simulation of Effects of Soils,Climate and Management on N<Subscript>2</Subscript>O Emission from Grasslands

Nitrous oxide (N₂O) is a potent greenhouse gas with a high contribution from agricultural soils and emissions that depend on soil type, climate, crops and management practices. The N₂O emissions therefore need to be included as an integral part of environmental assessments of agricultural production systems. An algorithm for N₂O production and emission from agricultural soils was developed and included in the FASSET whole-farm model. The model simulated carbon and nitrogen (N) turnover on a daily basis. Both nitrification and denitrification was included in the model as sources for N₂O production, and the N₂O emissions depended on soil microbial and physical conditions. The model was tested on experimental data of N₂O emissions from grasslands in UK, Finland and Denmark, differing in climatic conditions, soil properties and management. The model simulated the general time course of N₂O emissions and captured the observed effects of fertiliser and manure management on emissions. Scenario analyses for grazed and cut grasslands were conducted to evaluate the effects of soil texture, climatic conditions, grassland management and N fertilisation on N₂O emissions. The soils varied from coarse sand to sandy loam and the climatic variation was taken to represent the climatic variation within Denmark. N fertiliser rates were varied from 0 to 500 kg N ha⁻¹. The simulated N₂O emissions showed a non-linear response to increasing N rates with increasing emission factors at higher N rates. The simulated emissions increased with increasing soil clay contents. N₂O emissions were slightly increased at higher temperatures, whereas increasing annual rainfall generally lead to decreasing emissions. Emissions were slightly higher from grazed grasslands compared with cut grasslands at similar rates of total N input (fertiliser and animal excreta). The results indicate higher emission factors and thus higher potentials for reducing N₂O emissions for intensively grazed grasslands on fine textured soils than for extensive cut-based grasslands on sandy soils.     

Yield response of Lolium perenne swards to free air CO2 enrichment increased over six years in a high N input system on fertile soil

After a step increase in the atmospheric partial pressure of CO₂ (pCO₂), the availability of mineral N may be insufficient to meet the plant's increased demand for N. Over time, however, the ecosystem may adapt to the new conditions, and a new equilibrium may be established in the fluxes of C and N. This would result in a higher dry mass (DM) yield response of the plants to elevated pCO₂. The effect of elevated atmospheric pCO₂ (60 Pa pCO₂) was studied in Lolium perenne L. swards with two N fertilization treatments (14 and 56 g m^?2 y^?1) in a six‐year FACE (Free Air Carbon dioxide Enrichment) experiment. In the high N treatment, the input of N with fertilizer considerably exceeded the export of N with the harvested plant material in both CO₂ treatments leading to an apparent net input of N into the ecosystem. Accordingly, the proportion of harvested N derived from ¹⁵N labelled fertilizer N, applied throughout the experiment (< 6 years), increased over the years. Under these high N conditions, the annual DM yield response of the Lolium perenne sward to elevated pCO₂ increased (from 7% in 1993 to 25% in 1998). In parallel, the response of N yield to elevated pCO₂ increased, and the initially negative effect of elevated pCO₂ on specific leaf area (SLA) disappeared. The high N input system seemed to overcome in part an initially limiting effect of N on the yield response to elevated pCO₂ within a few years. In contrast, there was no apparent net input of N into the ecosystem in the low N treatment, because N fertilization just compensated the export of N with the harvested plant material. Accordingly, the proportion of harvested N yield, derived from fertilizer N, which was applied throughout the experiment, remained low. At low N, the availability of mineral N strongly limited plant growth and yield production in both CO₂ treatments; the low yields of DM and N, the low concentration of N in the plant material, and the low SLA reflected this. Although the plants grew under the same environmental conditions and the same management treatment as plants in the high N treatment, the response of DM yields to elevated pCO₂ in the low N treatment remained weak throughout the experiment (5% in 1993 and 9% in 1998). The results are discussed in the context of the sizes of the different N pools in the soil, the allocation of N within the plant and the possible effects on temporal immobilization, and the availability of mineral N for yield production as affected by elevated pCO₂ and N fertilization.     

Immobilisation, remineralisation and residual effects in subsequent crops of dairy cattle slurry nitrogen compared to mineral fertiliser nitrogen

About 50–60% of dairy cattle slurry nitrogen is ammonium N. Part of the ammonium N in cattle slurry is immobilised due to microbial decomposition of organic matter in the slurry after application to soil. The immobilisation and the remineralisation influence the fertiliser value of slurry N and the amount of organic N that is retained in soil. The immobilisation and the remineralisation of 15 N-labelled dairy cattle slurry NH₄-N were studied through three growing seasons after spring application under temperate conditions. Effects of slurry distribution (mixing, layer incorporation, injection, surface-banding) and extra litter straw in the slurry on the plant utilisation of labelled NH₄-N from slurry were studied and compared to the utilisation of ¹⁵N-labelled mineral fertiliser. The initial immobilisation of slurry N was influenced by the slurry distribution in soil. More N was immobilised when the slurry was mixed with soil. Surface-banding of slurry resulted in significant volatilisation losses and less residual ¹⁵N in soil. Much more N was immobilised after slurry incorporation than after mineral fertiliser application. After 2.5 years the recovery of labelled N in soil (0–25 cm) was 46% for slurry mixed with soil, 42% for injected slurry, 22% for surface-banded slurry and 24% for mineral fertiliser N. The total N uptake in a ryegrass cover crop was 5–10 kg N/ha higher in the autumn after spring-application of cattle slurry (100–120 kg NH₄-N/ha) compared to the mineral fertiliser N reference, but the immobilised slurry N (labelled N) only contributed little to the extra N uptake in the autumn. Even in the second autumn after slurry application there was an extra N uptake in the cover crop (0–10 kg N/ha). The residual effect of the cattle slurry on spring barley N uptake was insignificant in the year after slurry application (equivalent to 3% of total slurry N). Eighteen months after application, 13% of the residual ¹⁵N in soil was found in microbial biomass whether it derived from slurry or mineral fertiliser, but the remineralisation rate (% crop removal of residual ¹⁵N) was higher for fertiliser- than for slurry-derived N, except after surface-banding. Extra litter straw in the slurry had a negligible influence on the residual N effects in the year after application. It is concluded that a significant part of the organic N retained in soil after cattle slurry application is derived from immobilised ammonium N, but already a few months after application immobilised N is stabilised and only slowly released. The immobilised N has negligible influence on the residual N effect of cattle slurry in the first years after slurry application, and mainly contributes to the long-term accumulation of organic N in soil together with part of the organic slurry N. Under humid temperate conditions the residual N effects of the manure can only be optimally utilised when soil is also covered by plants in the autumn, because a significant part of the residual N is released in the autumn, and there is a higher risk of N leaching losses on soils that receive cattle slurry regularly compared to soils receiving only mineral N fertilisers.     

Environmental Effects over the First 2½ Rotation Periods of a Fertilised Poplar Short Rotation Coppice

A short rotation coppice (SRC) with poplar was established in a randomised fertilisation experiment on sandy loam soil in Potsdam (Northeast Germany). The main objective of this study was to assess if negative environmental effects as nitrogen leaching and greenhouse gas emissions are enhanced by mineral nitrogen (N) fertiliser applied to poplar at rates of 0, 50 and 75 kg N ha^?1 year^?1 and how these effects are influenced by tree age with increasing number of rotation periods and cycles of organic matter decomposition and tree growth after each harvesting event. Between 2008 and 2012, the leaching of nitrate (NO₃ ^?) was monitored with self-integrating accumulators over 6-month periods and the emissions of the greenhouse gases (GHG) nitrous oxide (N₂O) and carbon dioxide (CO₂) were determined in closed gas chambers. During the first 4 years of the poplar SRC, most nitrogen was lost through NO₃ ^? leaching from the main root zone; however, there was no significant relationship to the rate of N fertilisation. On average, 5.8 kg N ha^?1 year^?1 (13.0 kg CO_2equ) was leached from the root zone. Nitrogen leaching rates decreased in the course of the 4-year study parallel to an increase of the fine root biomass and the degree of mycorrhization. In contrast to N leaching, the loss of nitrogen by N₂O emissions from the soil was very low with an average of 0.61 kg N ha^?1 year^?1 (182 kg CO_2equ) and were also not affected by N fertilisation over the whole study period. Real CO₂ emissions from the poplar soil were two orders of magnitude higher ranging between 15,122 and 19,091 kg CO₂ ha^?1 year^?1 and followed the rotation period with enhanced emission rates in the years of harvest. As key-factors for NO₃ ^? leaching and N₂O emissions, the time after planting and after harvest and the rotation period have been identified by a mixed effects model.     

Plant, soil microbial and soil inorganic nitrogen responses to elevated CO2: a study in microcosms of Holcus lanatus

The impact of elevated atmospheric CO₂ concentrations on the nitrogen cycle was evaluated in a 2-month experiment in monospecific grassland microcosms (Holcus lanatus L.) grown on reconstituted grassland soil. The responses of the N pools in the plants, soil, and soil microbes were studied. The impact of high CO₂ on key stages of the N cycle, especially nitrification and denitrification processes, were also measured. Our study showed a strong plant response to high CO₂: total biomass increased by 76% (P < 0.001) and root length density increased by 77% (P = 0.010). However, total plant N was not significantly modified by high CO₂, because the percent N in the plant decreased by 40% (P < 0.001). We observed a large decrease in soil NO₃^– concentration under elevated CO₂ (–50%; P = 0.002). Soil ammonium concentrations were much less affected by CO₂ enrichment, and only in resin bags (–8%, P = 0.019). Soil nitrifying enzyme activity (NEA) had a tendency to increase (+17%; P = 0.061) and denitrifying enzyme activity (DEA) decreased (-12%; P = 0.013). We found evidence of increased microbial N sink (microbial N increased by 17%, P = 0.004). This and other studies suggest that rising CO₂ often reduces soil nitrate concentrations, which may lead to decreased nitrate leaching. Elevated CO₂ led to environmental conditions that were less favourable for denitrification in our study.     

Direct evidence that symbiotic N2 fixation in fertile grassland is an important trait for a strong response of plants to elevated atmospheric CO2

Although legumes showed a clearly superior yield response to elevated atmospheric pCO₂ compared to nonlegumes in a variety of field experiments, the extent to which this is due to symbiotic N₂ fixation per se has yet to be determined. Thus, effectively and ineffectively nodulating lucerne (Medicago sativa L.) plants with a very similar genetic background were grown in competition with each other on fertile soil in the Swiss FACE experiment in order to monitor their CO₂ response. Under elevated atmospheric pCO₂, effectively nodulating lucerne, thus capable of symbiotically fixing N₂, strongly increased the harvestable biomass and the N yield, independent of N fertilization. In contrast, the harvestable biomass and N yield of ineffectively nodulating plants were affected negatively by elevated atmospheric pCO₂ when N fertilization was low. Large amounts of N fertilizer enabled the plants to respond more favourably to elevated atmospheric pCO₂, although not as strongly as effectively nodulating plants. The CO₂‐induced increase in N yield of the effectively nodulating plants was attributed solely to an increase in symbiotic N₂ fixation of 50–175%, depending on the N fertilization treatment. N yield derived from the uptake of mineral N from the soil was, however, not affected by elevated pCO₂. This result demonstrates that, in fertile soil and under temperate climatic conditions, symbiotic N₂ fixation per se is responsible for the considerably greater amount of above‐ground biomass and the higher N yield under elevated atmospheric pCO₂. This supports the assumption that symbiotic N₂ fixation plays a key role in maintaining the C/N balance in terrestrial ecosystems in a CO₂‐rich world.     

Denitrification and N2O emissions from a UK pasture soil following the early spring application of cattle slurry and mineral fertiliser

Total denitrification and nitrous oxide (N₂O) losses were measured from three contrasting dairy management systems representing good commercial practice (system 1), production maintained but with reduced N losses (system 2); and nitrate leaching less than 50 mg L^-1 but with reduced production (system 3). Measurements were made following mineral fertiliser application and from two plot experiments where four treatments were applied: control, NH₄NO₃ at 60 kg N ha^-1, cattle slurry applied to the surface (equivalent to 45 kg N ha^-1), and cattle slurry injected. Despite low soil temperatures (<6 °C) and low rainfall (<3 mm), total denitrification and N₂O losses peaked at 56 and 16 g N ha^-1 d^-1, respectively. Total denitrification losses decreased: system 1 system 2 > system 3, whereas N₂O losses decreased: system 2 > system 3 > system 1. Total denitrification losses tended to decrease with decreasing fertiliser application rate, whereas fertiliser application rate was not the sole determinant of the N₂O loss. The system 3 field was injected with cattle slurry for 2 yr, system 2 received some slurry by injection and system 1 received slurry to the surface. Thus, the amount, timing and method of previous cattle slurry application was important in determining the loss following subsequent fertiliser application. For the plot experiments, total denitrification and N₂O losses decreased in the order: slurry injected > mineral fertiliser > slurry applied to the surface > control for 5 days following application. However, 16 and 19 days after application, N₂O losses above the control were measured from plots that had received cattle slurry. It was inferred that the application of cattle slurry to the pasture soil stimulated greater N₂O production and increased losses over a longer time period compared with mineral fertiliser additions.     

Increased nitrate availability in the soil of a mixed mature temperate forest subjected to elevated CO2 concentration (canopy FACE)

In a mature temperate forest in Hofstetten, Switzerland, deciduous tree canopies were subjected to a free‐air CO₂ enrichment (FACE) for a period of 8 years. The effect of this treatment on the availability of nitrogen (N) in the soil was assessed along three transects across the experimental area, one under Fagus sylvatica, one under Quercus robur and Q. petraea and one under Carpinus betulus. Nitrate, ammonium and dissolved organic N (DON) were analysed in soil solution obtained with suction cups. Nitrate and ammonium were also captured in buried ion‐exchange resin bags. These parameters were related to the local intensity of the FACE treatment as measured from the ¹³C depletion of dissolved inorganic carbon in the soil solution. Over the 8 years of experiment, the CO₂ enrichment reduced DON concentrations, did not affect ammonium, but induced higher nitrate concentrations, both in soil solution and resin bags. In the nitrate captured in the resin bags, the natural abundance of the isotope ¹⁵N increased strongly. This indicates that the CO₂ enrichment accelerated net nitrification, probably as an effect of the higher soil moisture resulting from the reduced transpiration of the CO₂‐enriched trees. It is also possible that N mineralization was enhanced by root exudates (priming effect) or that the uptake of inorganic N by these trees decreased slightly as the result of a reduced N demand for fine‐root growth. In this mature deciduous forest, we did not observe any progressive N limitation due to elevated atmospheric CO₂ concentrations; on the contrary, we observed an enhanced N availability over the 8 years of our measurements. This may, together with the global warming projected, exacerbate problems related to N saturation and nitrate leaching, although it is uncertain how long the observed trends will last in the future.     

Genotype-specific response of a lycaenid herbivore to elevated carbon dioxide and phosphorus availability in calcareous grassland

Effects of elevated CO₂ and P availability on plant growth of the legume Lotus corniculatus and consequences for the butterfly larvae of Polyommatus icarus feeding on L. corniculatus were investigated in screen-aided CO₂ control chambers under natural conditions on a calcareous grassland in the Swiss Jura mountains. Elevated CO₂ conditions and P fertilisation increased the biomass production of L. corniculatus plants and affected the plant chemical composition. CO₂ enrichment increased the C/N ratio and sugar concentration and decreased the N and P concentrations. C- and N-based allelochemicals (cyanoglycosides, total polyphenols and condensed tannins) were only marginally affected by CO₂ enrichment. P fertilisation increased the specific leaf area and concentrations of water, N, sugar and P, while the C/N ratio and the concentration of total polyphenols decreased. Furthermore, P availability marginally enhanced the effect of elevated CO₂ on the total dry mass and sugar concentration while the opposite occurred for the total polyphenol concentration. The changes in food-plant chemistry as a result of P fertilisation positively affected larval mass gain and accelerated the development time of P. icarus. Only a marginal negative effect on larval mass gain was found for CO₂ enrichment. However, we found genotype-specific responses in the development time of P. icarus to elevated CO₂ conditions. Larvae originating from different mothers developed better either under elevated CO₂ or under ambient CO₂ but some did not react to CO₂ elevation. As far as we know this is the first finding of a genotype-specific response of an insect herbivore to elevated CO₂ which suggests genetic shifts in insect life history traits in response to elevated CO₂.