Estimating the impact of changing fertilizer application rate, land use, and climate on nitrous oxide emissions in Irish grasslands

Aim

This study examines the impact of changing nitrogen (N) fertilizer application rates, land use and climate on N fertilizer-derived direct nitrous oxide (N₂O) emissions in Irish grasslands.

Methods

A set of N fertilizer application rates, land use and climate change scenarios were developed for the baseline year 2000 and then for the years 2020 and 2050. Direct N₂O emissions under the different scenarios were estimated using three different types of emission factors and a newly developed Irish grassland N₂O emissions empirical model.

Results

There were large differences in the predicted N₂O emissions between the methodologies, however, all methods predicted that the overall N₂O emissions from Irish grasslands would decrease by 2050 (by 40–60 %) relative to the year 2000. Reduced N fertilizer application rate and land-use changes resulted in decreases of 19–34 % and 11–60 % in N₂O emission respectively, while climate change led to an increase of 5–80 % in N₂O emission by 2050.

Conclusions

It was observed in the study that a reduction in N fertilizer and a reduction in the land used for agriculture could mitigate emissions of N₂O, however, future changes in climate may be responsible for increases in emissions causing the positive feedback of climate on emissions of N₂O.      

Effects of nitrate concentration on the denitrification potential of a calcic cambisol and its fractions of N2, N2O and NO

Background and aims

The direct measurement of denitrification dynamics and its product fractions is important for parameterizing process-oriented model(s) for nitrogen cycling in various soils. The aims of this study are to a) directly measure the denitrification potential and the fractions of nitrogenous gases as products of the process in laboratory, b) investigate the effects of the nitrate (NO ₃ ^? ) concentration on emissions of denitrification gases, and c) test the hypothesis that denitrification can be a major pathway of nitrous oxide (N₂O) and nitric oxide (NO) production in calcic cambisols under conditions of simultaneously sufficient supplies of carbon and nitrogen substrates and anaerobiosis as to be found to occur commonly in agricultural lands.

Methods

Using the helium atmosphere (with or without oxygen) gas-flow-soil-core technique in laboratory, we directly measured the denitrification potential of a silt clay calcic cambisol and the production of nitrogen gas (N₂), N₂O and NO during denitrification under the conditions of seven levels of NO ₃ ^? concentrations (ranging from 10 to 250 mg N kg^?1 dry soil) and an almost constant initial dissolved organic carbon concentration (300 mg C kg^?1 dry soil).

Results

Almost all the soil NO ₃ ^? was consumed during anaerobic incubation, with 80–88 % of the consumed NO ₃ ^? recovered by measuring nitrogenous gases. The results showed that the increases in initial NO ₃ ^? concentrations significantly enhanced the denitrification potential and the emissions of N₂ and N₂O as products of this process. Despite the wide range of initial NO ₃ ^? concentrations, the ratios of N₂, N₂O and NO products to denitrification potential showed much narrower ranges of 51–78 % for N₂, 14–36 % for N₂O and 5–22 % for NO.

Conclusions

These results well support the above hypothesis and provide some parameters for simulating effects of variable soil NO ₃ ^? concentrations on denitrification process as needed for biogeochemical models.     

Nitrous oxide emissions and nitrate leaching from a rain-fed wheat-maize rotation in the Sichuan Basin, China

Aims

A 3-year field experiment (October 2004–October 2007) was conducted to quantify N₂O fluxes and determine the regulating factors from rain-fed, N fertilized wheat-maize rotation in the Sichuan Basin, China.

Methods

Static chamber-GC techniques were used to measure soil N₂O fluxes in three treatments (three replicates per treatment): CK (no fertilizer); N150 (300 kg N fertilizer ha^?1 yr^?1 or 150 kg N?ha^?1 per crop); N250 (500 kg N fertilizer ha^?1 yr^?1 kg or 250 kg N?ha^?1 per crop). Nitrate (NO ₃ ^? ) leaching losses were measured at nearby sites using free-drained lysimeters.

Results

The annual N₂O fluxes from the N fertilized treatments were in the range of 1.9 to 6.7 kg N?ha^?1 yr^?1 corresponding to an N₂O emission factor ranging from 0.12 % to 1.06 % (mean value: 0.61 %). The relationship between monthly soil N₂O fluxes and NO ₃ ^- leaching losses can be described by a significant exponential decaying function.

Conclusions

The N₂O emission factor obtained in our study was somewhat lower than the current IPCC default emission factor (1 %). Nitrate leaching, through removal of topsoil NO ₃ ^? , is an underrated regulating factor of soil N₂O fluxes from cropland, especially in the regions where high NO ₃ ^- leaching losses occur.     

Contribution of 15N-labelled leaf litter to N turnover, nitrous oxide emissions and N sequestration in a beech forest during eleven years

Aims

Decomposition of leaf litterfall plays a major role for nitrogen (N) dynamics in soils. However, little is known as to which extent beech leaf litter contributes to N turnover and nitrous oxide (N₂O) emissions within one decade after litterfall.

Methods

In 1997, we exchanged recently fallen leaf litter by ¹⁵N-labelled litter in a beech stand (Fagus sylvatica) at the Solling, Germany. Measurements were conducted 2–3 and 10–11 years after litter exchange.

Results

Two years after litter exchange, 92 % of added ¹⁵N was recovered in the surface 10 cm of the soil. The labelled N was primarily found in the upper part of the F layer of the moder type humus. Eleven years after litter exchange, 73 % of the added ¹⁵N was lost and the remaining 27 % was mainly recovered in the lower part of the F layer indicating N sequestration. The remaining leaf litter N was subject to measurable N mineralisation (2–3 % of litter N) and N₂O production (0.02 %). Between 0.3 % (eleventh year) and 0.6 % (second year) of total annual N₂O emissions were attributed to beech leaf litter of a single year.

Conclusions

Most of the annual N₂O emissions (1.33–1.54 kg N ha^?1 yr^?1) were probably derived from older soil N pools.     

Liebig’s law of the minimum applied to a greenhouse gas: alleviation of P-limitation reduces soil N2O emission

Background and aims

Emission of the greenhouse gas (GHG) nitrous oxide (N₂O) are strongly affected by nitrogen (N) fertilizer application rates. However, the role of other nutrients through stoichiometric relations with N has hardly been studied. We tested whether phosphorus (P) availability affects N₂O emission. We hypothesized that alleviation of plant P-limitation reduces N₂O emission through lowering soil mineral N concentrations.

Methods

We tested our hypothesis in a pot experiment with maize (Zea mays L.) growing on a P-limiting soil/sand mixture. Treatment factors included P and N fertilization and inoculation with Arbuscular Mycorrhizal Fungi (AMF; which can increase P uptake).

Results

Both N and P fertilization, as well as their interaction significantly (P?<?0.01) affected N₂O emission. Highest N₂O emissions (2.38 kg N₂O-N ha^?1) were measured at highest N application rates without P fertilization or AMF. At the highest N application rate, N₂O fluxes were lowest (0.71 kg N₂O-N ha^?1) with both P fertilization and AMF. The N₂O emission factors decreased with 50 % when P fertilization was applied.

Conclusions

Our results illustrate the importance of the judicious use of all nutrients to minimize N₂O emission, and thereby further underline the intimate link between sound agronomic practice and prudent soil GHG management.     

Role of maize stover incorporation on nitrogen oxide emissions in a non-irrigated Mediterranean barley field

Aims

Agricultural soils in semiarid Mediterranean areas are characterized by low organic matter contents and low fertility levels. Application of crop residues and/or manures as amendments is a cost-effective and sustainable alternative to overcome this problem. However, these management practices may induce important changes in the nitrogen oxide emissions from these agroecosystems, with additional impacts on carbon dioxide emissions. In this context, a field experiment was carried out with a barley (Hordeum vulgare L.) crop under Mediterranean conditions to evaluate the effect of combining maize (Zea mays L.) residues and N fertilizer inputs (organic and/or mineral) on these emissions.

Methods

Crop yield and N uptake, soil mineral N concentrations, dissolved organic carbon (DOC), denitrification capacity, N₂O, NO and CO₂ fluxes were measured during the growing season.

Results

The incorporation of maize stover increased N₂O emissions during the experimental period by c. 105 %. Conversely, NO emissions were significantly reduced in the plots amended with crop residues. The partial substitution of urea by pig slurry reduced net N₂O emissions by 46 and 39 %, with and without the incorporation of crop residues respectively. Net emissions of NO were reduced 38 and 17 % for the same treatments. Molar DOC:NO ₃ ^? ratio was found to be a robust predictor of N₂O and NO fluxes.

Conclusions

The main effect of the interaction between crop residue and N fertilizer application occurred in the medium term (4–6 month after application), enhancing N₂O emissions and decreasing NO emissions as consequence of residue incorporation. The substitution of urea by pig slurry can be considered a good management strategy since N₂O and NO emissions were reduced by the use of the organic residue.     

Nitrous oxide (N<Subscript>2</Subscript>O) and subsequent open-label SSRI treatment of adolescents with depression (NOTAD): study protocol for a randomised controlled trial

Background

The first line of pharmacological treatment for severe depressive disorders in young people is selective serotonin reuptake inhibitors (SSRIs). However, beneficial clinical effects are rarely observed before several weeks into treatment. Nitrous oxide (N₂O) has a long-standing safety record for pain relief and has been used in adults and young people. In adults with severe treatment-resistant depression, a single dose of N₂O had significant antidepressant effects, with maximum antidepressant effects observed 24 h after administration. However, the antidepressant effects of N₂O have never been investigated in adolescents with a confirmed diagnosis of depression in a prospective trial. The aims of this study are to (1) investigate whether a single inhaled N₂O administration leads to antidepressant effects in adolescents with depression at 24 h, (2) determine whether combined N₂O and SSRI administration (commenced after N₂O intervention) provides a clinically significant improvement in mood over and above the benefits from SSRI administration alone, and, (3) investigate whether the effect seen following N₂O administration can be used as a predictor of SSRI treatment response.

Methods/design

In this study, we will use a single-blind, randomised, placebo-controlled design. Patients aged between 12 and 17 years with major depressive disorder will be recruited. This study will consist of two phases: phase A and phase B. During phase A, participants will be randomised to receive either inhaled N₂O or placebo (air) for 1 h. In phase B, participants will receive open-label pharmacological treatment with the SSRI fluoxetine and will be followed over a 12-week period. Participants will undertake mood assessments at 2 and 24 h after N₂O or placebo administration (phase A) and weekly during the 12-week follow up in phase B.

Discussion

We expect an antidepressant effect from a single dose of inhaled N₂O compared with placebo at 24 h after administration. Additionally, we expect that subjects treated with N₂O will also show greater improvements than the placebo group after 6 and 12 weeks into fluoxetine treatment because of potential additive antidepressant effects. Such findings would be of clinical importance because currently children and adolescents often do not experience any symptom alleviation for several weeks following the initiation of SSRIs.

Trial registration

Australian and New Zealand Clinical Trials Registry, ACTRN12616001568404. Registered on 14 November 2016.

     

Soil moisture effects on gross nitrification differ between adjacent grassland and forested soils in central Alberta, Canada

Background and aims

Changes in soil moisture availability seasonally and as a result of climatic variability would influence soil nitrogen (N) cycling in different land use systems. This study aimed to understand mechanisms of soil moisture availability on gross N transformation rates.

Methods

A laboratory incubation experiment was conducted to evaluate the effects of soil moisture content (65 vs. 100% water holding capacity, WHC) on gross N transformation rates using the ¹⁵N tracing technique (calculated by the numerical model FLUAZ) in adjacent grassland and forest soils in central Alberta, Canada.

Results

Gross N mineralization and gross NH ₄ ⁺ immobilization rates were not influenced by soil moisture content for both soils. Gross nitrification rates were greater at 100 than at 65% WHC only in the forest soil. Denitrification rates during the 9 days of incubation were 2.47 and 4.91 mg N kg^-1 soil d^-1 in the grassland and forest soils, respectively, at 100% WHC, but were not different from zero at 65% WHC. In the forest soil, both the ratio of gross nitrification to gross NH ₄ ⁺ immobilization rates (N/IA) and cumulative N₂O emission were lower in the 65 than in the 100% WHC treatment, while in the grassland soil, the N/IA ratio was similar between the two soil moisture content treatments but cumulative N₂O emission was lower at 65% WHC.

Conclusions

The effect of soil moisture content on gross nitrification rates differ between forest and grassland soils and decreasing soil moisture content from 100 to 65% WHC reduced N₂O emissions in both soils.     

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

Aims

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

Methods

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

Results

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

Conclusions

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

Nitrous oxide emissions during the non-rice growing seasons of two subtropical rice-based rotation systems in southwest China

Background and aims

High nitrous oxide (N₂O) emissions may occur during the non-rice growing season of Chinese rice-upland crop rotation systems. However, our understanding of N₂O emission during this season is poor due to a scarcity of available field N₂O measurements.

Methods

Using the static manual chamber-GC technique, seasonal N₂O emissions during the non-rice growing season were simultaneously measured at two adjacent rice-wheat and rice-rapeseed fields in southwest China for three consecutive annual rotation cycles (May 2005 to May 2008).

Results

Compared to the control, N fertilizer applications significantly enhanced soil N₂O emissions from both wheat and rapeseed systems. Seasonal cumulative N₂O fluxes from wheat systems were on average 2.6 kg N ha^?1 for the recommended practice (RP [150 kg N ha^?1]) and 5.0 kg N ha^?1 for the conventional practice (CP [250 kg N ha^?1]). Lower N₂O emissions were observed from the adjacent rapeseed systems. Average cumulative seasonal N₂O fluxes from rapeseed were 1.5 and 2.2 kg N ha^?1 for the RP and CP treatments, respectively. The first 3 weeks after N fertilization were the “hot moment” of N₂O emissions for both the wheat and rapeseed systems. The lowest yield-scaled N₂O fluxes for wheat were obtained at the RP treatment (mean: 0.81 kg N Mg^?1) while for rapeseed the CP treatment produced the lowest yield-scaled fluxes (mean: 0.79 kg N Mg^?1). On average, the direct N₂O emission factors (EF_d) for the wheat system (1.76 %) were over two times higher than for the rapeseed system (0.73 %).

Conclusions

Intercropping of rapeseed tends to result in lower N₂O emissions than wheat for rice-upland crop rotation systems of southwest China, indicating that either the N fertilization or the cropping system need to be considered not only for improving the estimate of regional and/or national N₂O fluxes but also for proposing the climate-smart agricultural management practice to reduce N₂O emissions from agricultural soils.     

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

Background and Aims

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

Methods

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

Results

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

Conclusions

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

Effects of elevated CO2 and O3 on N-cycling and N2O emissions: a short-term laboratory assessment

Background and aims

Elevated atmospheric CO₂ (eCO₂) and tropospheric O₃ (eO₃) can alter soil microbial processes, including those underlying N₂O emissions, as an indirect result of changes in plant inputs. In this study, effects of eCO₂ and eO₃ on sources of N₂O in a soybean (Glycine max (L.) Merr.) agroecosystem in Illinois (SoyFACE) were investigated. We hypothesized that increases in available C and anaerobic microhabitat under eCO₂ would stimulate N₂O emissions, with a proportionally larger increase in denitrification derived N₂O (N₂O_D) compared to nitrification plus nitrifier denitrification derived N₂O (N₂O_N+ND). We expected opposite effects under eO₃.

Methods

Isotopically labeled ¹⁵NH ₄ ¹⁴ NO₃ and ¹⁴NH ₄ ¹⁵ NO₃ were used to evaluate mineral N transformations, N₂O_D, and N₂O_N+ND in a 12-day incubation experiment.

Results

We observed minimal effects of eCO₂ and eO₃ on N₂O emissions, movement of ¹⁵?N through mineral N pools, soil moisture content and C availability. Possibly, altered C and N inputs by eCO₂ and eO₃ were small relative to the high soil organic C content and N-inputs via biological N₂-fixation, minimizing potential effects of eCO₂ and eO₃ on N-cycling.

Conclusion

We conclude that eCO₂ and eO₃ did not affect N₂O emissions in the short term. However, it remains to be tested whether N₂O emissions in SoyFACE will be unaltered by eCO₂ and eO₃ on a larger temporal scale under field conditions.     

Inundation strongly stimulates nitrous oxide emissions from stems of the upland tree Fagus sylvatica and the riparian tree Alnus glutinosa

Background and aims

Nitrous oxide (N₂O) and methane (CH₄) can be emitted from surfaces of riparian plants. Data on the emission of these greenhouse gases by upland trees are scarce. We quantified CH₄ and N₂O emissions from stems of Fagus sylvatica, an upland tree, and Alnus glutinosa, a riparian tree.

Methods

The gas fluxes were investigated in mesocosms under non-flooded control conditions and during a flooding period using static chamber systems and gas chromatographic analyses.

Results

Despite differences in the presence of an aerenchyma system, both tree species emitted N₂O and CH₄ from the stems. Flooding caused a dramatic transient increase of N₂O stem emissions by factors of 740 (A. glutinosa) and even 14,230 (F. sylvatica). Stem emissions of CH₄ were low and even deposition was determined (F. sylvatica controls). The results suggest that CH₄ was transported mainly through the aerenchyma, whereas N₂O transport occurred in the xylem sap.

Conclusions

For the first time it has been demonstrated that upland trees such as F. sylvatica clearly significantly emit N₂O from their stems despite lacking an aerenchyma. If this result is confirmed in adult trees, upland forests may constitute a new and significant source of atmospheric N₂O.     

Emissions of nitrous oxide from the leaves of grasses

Aims

Nitrous oxide (N₂O) emissions from pastoral agriculture are considered to originate from the soil as a consequence of microbial activity during soil nitrification and denitrification. However, recent studies have identified the plant canopy as a potentially significant source of N₂O emissions to the atmosphere. Understanding the extent and mechanisms of plant emissions may provide new mitigation opportunities as current options only target soil microbial processes.

Methods

We developed an experimental apparatus and protocol to partition N₂O emissions between the leaves of grasses and the soil and measured emissions from ten common grass species found in New Zealand pastures.

Results

The chamber design enabled us to identify measurable changes in N₂O concentration over a period of 1 h and to distinguish a range of emissions from 0.001 to 0.25 mg N₂O-N/m² leaf area/h. There was a 10-fold variation among species; Holcus lanataus, Lolium perenne and Paspalum dilatatum had the highest leaf N₂O emissions and Poa annua the lowest.

Conclusions

Grasses do emit N₂O from their leaves and the rate that this occurs varies among grass species. The emission does not appear to arise from formation of N₂O in plant leaves but more likely reflects transport of N₂O from the soil. Differences in emission rates appear to arise from a plant influence on the rate of formation of N₂O in the soil rather than the rate of transportation through the plant.     

Arbuscular mycorrhizas and their role in plant growth, nitrogen interception and soil gas efflux in an organic production system

Background and aims

Roots and mycorrhizas play an important role in not only plant nutrient acquisition, but also ecosystem nutrient cycling.

Methods

A field experiment was undertaken in which the role of arbuscular mycorrhizas (AM) in the growth and nutrient acquisition of tomato plants was investigated. A mycorrhiza defective mutant of tomato (Solanum lycopersicum L.) (named rmc) and its mycorrhizal wild type progenitor (named 76R) were used to control for the formation of AM. The role of roots and AM in soil N cycling was studied by injecting a ¹⁵N-labelled nitrate solution into surface soil at different distances from the 76R and rmc genotypes of tomato, or in plant free soil. The impacts of mycorrhizal and non-mycorrhizal root systems on soil greenhouse gas (CO₂ and ¹⁴⁺¹⁵N₂O and ¹⁵N₂O) emissions, relative to root free soils, were also studied.

Results

The formation of AM significantly enhanced plant growth and nutrient acquisition, including interception of recently applied NO ₃ ^? . Whereas roots caused a small but significant decrease in ¹⁵N₂O emissions from soils at 23?h after labeling, compared to the root-free treatment, arbuscular mycorrhizal fungi (AMF) had little effect on N₂O emissions. In contrast soil CO₂ emissions were higher in plots containing mycorrhizal root systems, where root biomass was also greater.

Conclusions

Taken together, these data indicate that roots and AMF have an important role to play in plant nutrient acquisition and ecosystem N cycling.     

Processes leading to N2O and NO emissions from two different Chinese soils under different soil moisture contents

Background and Aims

Great attention has been paid to N₂O emissions from paddy soils under summer rice-winter wheat double-crop rotation, while less focus was given to the NO emissions. Besides, neither mechanism is completely understood. Therefore, this study aimed at evaluating the relative importance of nitrification and denitrification to N₂O and NO emissions from the two soils at different soil moisture contents

Methods

N₂O and NO emissions during one winter wheat season were simultaneously measured in situ in two rice-wheat based field plots at two different locations in Jiangsu Province, China. One soil was neutral in pH with silt loam texture (NSL), the other soil alkaline in pH with a clay texture (AC). A ¹⁵?N tracer incubation experiment was conducted in the laboratory to evaluate the relative importance of nitrification and denitrification for N₂O and NO emissions at soil moisture contents of 40 % water holding capacity (WHC), 65 % WHC and 90 % WHC.

Results

Higher N₂O emission rates in the AC soil than in the NSL soil were found both in the field and in the laboratory experiments; however, the differences in N₂O emissions between AC soil and NSL soil were smaller in the field than in the laboratory. In the latter experiment, nitrification was observed to be the more important source of N₂O emissions (>70 %) than denitrification, regardless of the soils and moisture treatments, with the only exception of the AC soil at 90 % WHC, at which the contributions of nitrification and denitrification to N₂O emissions were comparable. The ratios of NO/N₂O also supported the evidence that the nitrification process was the dominant source of N₂O and NO both in situ and in the laboratory. The proportion of nitrified N emitted as N₂O (P _N2O ) in NSL soil were around 0.02 % in all three moisture treatments, however, P _N2O in the AC soil (0.04 % to 0.10 %) tended to decrease with increasing soil moisture content.

Conclusions

Our results suggest that N₂O emission rates obtained from laboratory incubation experiments are not suitable for the estimation of the true amount of N₂O fluxes on a field scale. Besides, the variations of P _N2O with soil property and soil moisture content should be taken into account in model simulations of N₂O emission from soils.     

The effects of snowpack properties and plant strategies on litter decomposition during winter in subalpine meadows

Background and aims

Approximately 50 % of belowground organic carbon is present in the northern permafrost region and due to changes in climate there are concerns that this carbon will be rapidly released to the atmosphere. The release of carbon in arctic soils is thought to be intimately linked to the N cycle through the N cycle’s influence on microbial activity. The majority of new N input into arctic systems occurs through N₂-fixation; therefore, N₂-fixation may be the key driver of greenhouse gases from these ecosystems.

Methods

At Alexandra Fjord lowland, Ellesmere Island, Canada concurrent measurements of N₂-fixation, N mineralization and nitrification rates, dissolved organic soil N (DON) and C, inorganic soil N and surface greenhouse gas fluxes (CO₂, N₂O and CH₄) were taken in two ecosystem types (Wet Sedge Meadow and Dryas Heath) over the 2009 growing season (June-August). Using Structural Equation Modelling we evaluated the hypothesis that CO₂, CH₄ and N₂O flux are linked to N₂-fixation via the N cycle.

Results

The soil N cycle was linked to CO₂ flux in the Dryas Heath ecosystem via DON concentrations, but there was no link between the soil N cycle and CO₂ flux in the Wet Sedge Meadow. Methane flux was also not linked to the soil N cycle, nor surface soil temperature or moisture in either ecosystem. The soil N cycle was closely linked to N₂O emissions but via nitrification in the Wet Sedge Meadow and inorganic N in the Dryas Heath, indicating the important role of nitrification in net N₂O flux from arctic ecosystems.

Conclusions

Our results should be interpreted with caution given the high variability in both the rates of the N cycling processes and greenhouse gas flux found in both ecosystems over the growing season. However, while N₂-fixation and other N cycling processes may play a more limited role in instantaneous CO₂ emissions, these processes clearly play an important role in controlling N₂O emissions.     

N2 fixation of pea hypernodulating mutants is more tolerant to root pruning than that of wild type

Background and aims

As a legume, pea plant has the ability to symbiotically fix N₂. However, symbiotic N₂ fixation is very sensitive to environmental stresses that affect plant growth, and there is little knowledge on the impact of root pruning on N₂ fixation and plant growth.

Methods

In this study, we removed half of the nodulated roots of pea wild-type Frisson and hypernodulating mutants P64, P118, and P121. Dinitrogen fixation was measured using ¹⁵N labeling and carbon assimilation and partitioning between plant organs using ¹³C labeling.

Results

Root pruning decreased N₂ fixation by ?46 to ?79 % in wild-type and mutants. Pea mutant P118 had a lower decrease of specific activity of N₂ fixation (?17 %) than both wild-type and other mutants (?36 to ?62 %). For all genotypes, root pruning increased root and nodule sinks strengths for carbon. For P118 and for P121, this was associated to higher nodule growth than for control plants, as measured 8 days after root pruning.

Conclusion

This is the first analysis of N₂-fixing plant response to root pruning. Importantly, we showed that some hypernodulating mutant pea lines (P118 and to a lesser extent P121) withstood this stress better than wild-type did.     

Methane and nitrous oxide emissions from direct-seeded and seedling-transplanted rice paddies in southeast China

Background and aims

The rice production is experiencing a shift from conventionally seedling-transplanted (TPR) to direct-seeded (DSR) cropping systems in Southeast Asia. Besides the difference in rice crop establishment, water regime is typically characterized as water-saving moist irrigation for DSR and flooding-midseason drainage-reflooding and moist irrigation for TPR fields, respectively. A field experiment was conducted to quantify methane (CH₄) and nitrous oxide (N₂O) emissions from the DSR and TPR rice paddies in southeast China.

Methods

Seasonal measurements of CH₄ and N₂O fluxes from the DSR and TPR plots were simultaneously taken by static chamber-GC technique.

Results

Seasonal fluxes of CH₄ averaged 1.58 mg m^?2 h^?1 and 1.02 mg m^?2 h^?1 across treatments in TPR and DSR rice paddies, respectively. Compared with TPR cropping systems, seasonal N₂O emissions from DSR cropping systems were increased by 49 % and 46 % for the plots with or without N application, respectively. The emission factors of N₂O were estimated to be 0.45 % and 0.69 % of N application, with a background emission of 0.65 and 0.95 kg N₂O-N ha^?1 under the TPR and DSR cropping regimes, respectively. Rice biomass and grain yield were significantly greater in the DSR than in the TPR cropping systems. The net global warming potential (GWP) of CH₄ and N₂O emissions were comparable between the two cropping systems, while the greenhouse gas intensity (GHGI) was significantly lower in the DSR than in the TPR cropping systems.

Conclusions

Higher grain yield, comparable GWP, and lower GHGI suggest that the DSR instead of conventional TPR rice cropping regime would weaken the radiative forcing of rice production in terms of per unit of rice grain yield in China, and DSR rice cropping regime could be a promising rice development alternative in mainland China.     

Annual emissions of nitrous oxide and nitric oxide from rice-wheat rotation and vegetable fields: a case study in the Tai-Lake region, China

Background and aims

Knowledge on nitrous oxide (N₂O) and nitric oxide (NO) emissions from typical cropping systems in the Tai-Lake region is important for estimating regional inventory and proposing effective N₂O and NO mitigation options. This study aimed at a) characterizing the seasonal and annual emissions of both gases from the major cropping systems, and b) determining their direct emission factors (EF_ds) as the key parameters for inventory compilation.

Methods

Measurements of N₂O and NO emissions were conducted year-round in the Tai-Lake region using a static opaque chamber method. The measurements involved a typical rice-wheat rotation ecosystem and a vegetable field. The two types of croplands were subjected to both a fertilized treatment and a control treatment without nitrogen addition. In the rice-wheat ecosystem, N₂O emissions were measured throughout an entire year-round rotation spanning from June 2003 to June 2004, whereas NO emissions were measured only during the non-rice period. In the vegetable field, both N₂O and NO emissions were measured from November 2003 to November 2004.

Results

During the investigation period, the average cumulative N₂O and NO emissions under the fertilized conditions amounted to 3.80 and 0.80 (during the non-rice period for NO) kg?N?ha^?1, respectively, in the rice-wheat field, and 20.81 and 47.13?kg?N ha^?1, respectively, in the vegetable field. The average total N₂O and NO emissions under the control conditions were 1.39 and 0.29 (during the non-rice period for NO) kg?N?ha^?1, respectively, in the rice?wheat rotation, and 2.98 and 0.80?kg?N ha^?1, respectively, in the vegetable field. The direct emission factor (EF_d, which is defined as the loss rate of applied nitrogen via N₂O or NO emissions in the current season or year) of N₂O was annually determined to be 0.56?% in the rice-wheat field, while the seasonal EF_d of NO was 0.34?% during the non-rice period of the rotation cycle. In the vegetable field, the seasonal EF_ds of N₂O and NO varied from 0.15?% to 14.50?% and 0.80?% to 28.21?%, respectively, among different crop seasons; and the annual EF_ds were 1.38?% and 3.59?%, respectively.

Conclusions

This study suggests that conventional vegetable fields associated with intensive synthetic nitrogen application, as well as addition of manure slurry, may substantially contribute to the regional N₂O and NO emissions though they account for a relatively small portion of the farmlands in the Tai-Lake region. However, further studies to be conducted at multiple field sites with conventional vegetable and rice-based fields are needed to test this conclusion.