Greenhouse gas budget (CO2, CH4 and N2O) of intensively managed grassland following restoration

Meta-analysis shows the impacts of ecological restoration on greenhouse gas emissions

International initiatives set ambitious targets for ecological restoration, which is considered a promising greenhouse gas mitigation strategy. Here, we conduct a meta-analysis to quantify the impacts of ecological restoration on greenhouse gas emissions using a dataset compiled from 253 articles. Our findings reveal that forest and grassland restoration increase CH4 uptake by 90.0% and 30.8%, respectively, mainly due to changes in soil properties. Conversely, wetland restoration increases CH4 emissions by 544.4%, primarily attributable to elevated water table depth. Forest and grassland restoration have no significant effect on N2O emissions, while wetland restoration reduces N2O emissions by 68.6%. Wetland restoration enhances net CO2 uptake, and the transition from net CO2 sources to net sinks takes approximately 4 years following restoration. The net ecosystem CO2 exchange of the restored forests decreases with restoration age, and the transition from net CO2 sources to net sinks takes about 3-5 years for afforestation and reforestation sites, and 6-13 years for clear-cutting and post-fire sites. Overall, forest, grassland and wetland restoration decrease the global warming potentials by 327.7%, 157.7% and 62.0% compared with their paired control ecosystems, respectively. Our findings suggest that afforestation, reforestation, rewetting drained wetlands, and restoring degraded grasslands through grazing exclusion, reducing grazing intensity, or converting croplands to grasslands can effectively mitigate greenhouse gas emissions.

Changes in the soil fungal communities of steppe grasslands at varying degradation levels in North China

The grasslands in North China are rich in fungal resources. However, the knowledge of the structure and function of fungal communities and the role of microbial communities in vegetation restoration and succession are limited. Thus, we used an Illumina HiSeq PE250 high-throughput sequencing platform to study the changing characteristics of soil fungal communities in degraded grasslands, which were categorized as non-degraded (ND), lightly degraded, moderately degraded, and severely degraded (SD). Moreover, a correlation analysis between soil physical and chemical properties and fungal communities was completed. The results showed that the number of plant species, vegetation coverage, aboveground biomass, and diversity index decreased significantly with increasing degradation, and there were significant differences in the physical and chemical properties of the soil among the different degraded grasslands. The dominant fungal phyla in the degraded grassland were as follows: Ascomycota, 44.88%-65.03%; Basidiomycota, 12.68%-29.91%; and unclassified, 5.51%-16.91%. The dominant fungi were as follows: Mortierella, 6.50%-11.41%; Chaetomium, 6.71%-11.58%; others, 25.95%-36.14%; and unclassified, 25.56%-53.0%. There were significant differences in the microbial Shannon-Wiener and Chao1 indices between the ND and degraded meadows, and the composition and diversity of the soil fungal community differed significantly as the meadows continued to deteriorate. The results showed that pH was the most critical factor affecting soil microbial and fungal communities in SD grasslands, whereas soil microbial and fungal communities in ND grasslands were mainly affected by water content and other environmental factors.

A Beginner's Guide to Eddy Covariance: Methodology and Its Applications to Photosynthesis

The eddy covariance technique, commonly applied using flux towers, enables the investigation of greenhouse gas (e.g., carbon dioxide, methane, nitrous oxide) and energy (latent and sensible heat) fluxes between the biosphere and the atmosphere. Through measuring carbon fluxes in particular, eddy covariance flux towers can give insight into how ecosystem scale photosynthesis (i.e., gross primary productivity) changes over time in response to climate and management. This chapter is designed to be a beginner's guide to understanding the eddy covariance method and how it can be applied in photosynthesis research. It introduces key concepts and assumptions that apply to the method, what materials are required to set up a flux tower, as well as practical advice for site installation, maintenance, data management, and postprocessing considerations. This chapter also includes examples of what can go wrong, with advice on how to correct these errors if they arise. This chapter has been crafted to help new users design, install, and manage the best towers to suit their research needs and includes additional resources throughout to further guide successful eddy covariance research activities.

Divergent impacts of VPD and SWC on ecosystem carbon-water coupling under different dryness conditions

Ecosystem water use efficiency (WUE) is an indicator of carbon-water interactions and is defined as the ratio of gross primary productivity (GPP) to evapotranspiration (ET). However, it is currently unclear how WUE responds to atmospheric and soil drought events in terrestrial ecosystems with different dryness conditions. Additionally, the contributions of GPP and ET to the WUE response remain poorly understood. Based on measurements from 26 flux tower sites distributed worldwide, the binning method and random forest model were employed to separate the sensitivities of daily ecosystem WUE, GPP, and ET to vapor pressure deficit (VPD) and soil water content (SWC) under different dryness conditions (dryness index = potential evapotranspiration/precipitation, DI). Results showed that the sensitivity of WUE to VPD was negative at humid sites (DI < 1), while the sensitivity of WUE to SWC was positive at arid sites (DI > 2). Furthermore, the contribution of GPP to VPD-induced WUE variability was 63 % at humid sites, and the contribution of ET to SWC-induced WUE variability was 68 % when SWC was less than the 60th percentile at arid sites. Consequently, one increasing VPD-induced decrease in GPP was generally linked to a decrease in WUE at humid sites, and one drying soil moisture-caused decrease in ET was linked to a WUE increase under low SWC conditions at arid sites. Finally, VPD had a stronger effect on WUE than SWC when VPD was less than the 90th percentile or SWC was greater than the 50th percentile. Our findings underscore the importance of considering ecosystem dryness when investigating the impacts of VPD and SWC on ecosystem carbon-water coupling.

N2O and CH4 fluxes from intensively managed grassland: The importance of biological and environmental drivers vs. management

Agriculture is the main contributor to anthropogenic nitrous oxide (N2O) and methane (CH4) emissions. Therefore, mitigation options are urgently needed. In contrast to carbon dioxide, eddy covariance measurements of N2O and CH4 fluxes are still scarce, and thus little is known how environmental and biotic drivers as well as management affect the net N2O and CH4 exchange in grasslands. Thus, we investigated the most important drivers of net ecosystem N2O and CH4 fluxes in a temperate grassland, and continued a N2O mitigation experiment (increased clover proportion vs. fertilization with slurry). Random forest gap-filling models were able to capture intermittent emission peaks, performing better for half-hourly N2O than for CH4 fluxes. The unfertilized clover parcel (parcel B) continued to show lower N2O emissions (4.4 and 2.7 kg N2O-N ha-1 yr-1) compared to the fertilized parcel (parcel A; 6.9 and 5.9 kg N2O-N ha-1 yr-1) for 2019 and 2020, respectively. Tier 1 nitrogen (N) emission factors of 2.6 % and 1.9 % were observed at the fertilized parcel during the study period. Lower soil N concentrations indicated a lower N leaching risk at the clover than at the fertilized parcel. Annual CH4 emissions (including periods with sheep grazing) were similar from both parcels, and ranged from 25 to 38.5 kg CH4-C ha-1. The most important drivers of both N2O and CH4 fluxes were lagged precipitation and water filled pore space, but also management (for N2O from parcel B; CH4 from parcel A). Biotic variables such as vegetation height and leaf area index were important predictors for the N2O exchange, while grazing temporarily increased CH4 emissions. Overall, reducing N fertilization and increasing the legume proportion were effective N2O reduction measures. In particular, adjusting N fertilization to plant N demands can help to avoid high N2O emissions from grasslands.

Responses of community traits and soil characteristics of Achnatherum inebrians-type degraded grassland to grazing systems in alpine meadows on the Qinghai-Tibet Plateau

Introduction

Scientific grazing management is of great significance for the ecological health and sustainable use of alpine meadows.

Methods

To explore appropriate management methods of alpine grasslands of the Qinghai-Tibet Plateau degraded by Achnatherum inebrians (Hance) Keng ex Tzvele presence, we studied the effects of different grazing systems on the A. inebrians population, grassland vegetation community traits, soil characteristics and soil microbial community structure for cold- season grazing plus supplementary feeding pasture (CSF) and four-season open public pasture (FOP) in Tianzhu County, Gansu Province.

Results

Compared with FOP, the CSF site showed significantly inhibited reproduction of A. inebrians, especially the crown width, seed yield and number of reproductive branches per plant were as high as 50%, significantly increased the aboveground biomass of edible forage and soil water content by 57% and 43-55%, better soil nutrients, and significantly reduced soil bulk density by 10- 29%. Different grazing systems affected the composition and diversity of soil microbial communities, with a greater effect on fungi than on bacterial flora. The most abundant phyla of bacteria and fungi were Proteobacteria and Ascomycota for CSF (by 30-38% and 24-28%) and for FOP (by 67-70% and 68-73%), and the relative abundance and species of bacterial and fungal genera were greater for CSF than FOP. The α-diversity indexes of fungi were improved, and the β-diversity of fungi was significant difference between CSF and FOP. However, the grazing utilization time was prolonged in FOP, which reduced the diversity and abundance of soil bacteria and increased soil spatial heterogeneity. The use of A. inebrians-type degraded grassland in the cold season, and as a winter supplementary feeding and resting ground, could effectively inhibit expansion of A. inebrians, promote edible forage growth, enhance grassland productivity and community stability, and improve soil structure.

Discussion

The results guide healthy and sustainable utilization of A. inebrians-type degraded grassland in the Qinghai-Tibet Plateau.

Changes in grassland soil types lead to different characteristics of bacterial and fungal communities in Northwest Liaoning, China

Introduction

Soil microbial communities are critical in regulating grassland biogeochemical cycles and ecosystem functions, but the mechanisms of how environmental factors affect changes in the structural composition and diversity of soil microbial communities in different grassland soil types is not fully understood in northwest Liaoning, China.

Methods

We investigated the characteristics and drivers of bacterial and fungal communities in 4 grassland soil types with 11 sites across this region using high-throughput Illumina sequencing.

Results and Discussion

Actinobacteria and Ascomycota were the dominant phyla of bacterial and fungal communities, respectively, but their relative abundances were not significantly different among different grassland soil types. The abundance, number of OTUs, number of species and diversity of both bacterial and fungal communities in warm and temperate ecotone soil were the highest, while the warm-temperate shrub soil had the lowest microbial diversity. Besides, environmental factors were not significantly correlated with soil bacterial Alpha diversity index. However, there was a highly significant negative correlation between soil pH and Shannon index of fungal communities, and a highly significant positive correlation between plant cover and Chao1 index as well as Observed species of fungal communities. Analysis of similarities showed that the structural composition of microbial communities differed significantly among different grassland soil types. Meanwhile, the microbial community structure of temperate steppe-sandy soil was significantly different from that of other grassland soil types. Redundancy analysis revealed that soil total nitrogen content, pH and conductivity were important influencing factors causing changes in soil bacterial communities, while soil organic carbon, total nitrogen content and conductivity mainly drove the differentiation of soil fungal communities. In addition, the degree of connection in the soil bacterial network of grassland was much higher than that in the fungal network and soil bacterial and fungal communities were inconsistently limited by environmental factors. Our results showed that the microbial community structure, composition and diversity of different grassland soil types in northwest Liaoning differed significantly and were significantly influenced by environmental factors. Microbial community structure and the observation of soil total nitrogen and organic carbon content can predict the health changes of grassland ecosystems to a certain extent.

The potential to increase grassland soil C stocks by extending reseeding intervals is dependent on soil texture and depth

Grasslands account for ∼30% of global terrestrial carbon (C), of which most is stored in soils and provide important ecosystem services including livestock and forage production. Reseeding of temporary grasslands on a 5-year cycle is a common management practice to rejuvenate sward productivity and reduce soil compaction, but is physically disruptive and may reduce soil organic carbon (SOC) stocks. However, research to date is limited, which impacts on the ability to optimise grassland management for climate change mitigation. To determine whether extending the time interval up to 20 years between grassland reseeding can increase stable SOC stocks, a soil survey was conducted across three UK grassland chrono-sequences comprising 24 fields on contrasting soil types. We found that grassland SOC stocks (39.8-114.8 Mg C ha^-1) were higher than co-located fields in arable rotations (29.3-83.2 Mg C ha^-1) and the relationship with grassland age followed a curvilinear relationship with rapid SOC stock accumulation in the year following reseeding (2.69-18.3 Mg C ha^-1 yr^-1) followed by progressively slower SOC accumulation up to 20 years. Contrary to expectation, all grasslands had similar soil bulk densities and sward composition questioning the need for traditional 5-year reseeding cycles. Fractionation of soils into stable mineral associated fractions revealed that coarse textured grassland topsoils (0-15 cm) were near-saturated in C irrespective of grassland age whilst loam soils reached saturation ∼10 years after reseeding. Fine-textured topsoils and subsoils (15-30 cm) of all textures were under saturated and thus appear to hold the most potential to accrue additional stable C. However, the lack of a relationship between C saturation deficit and grassland age in subsoils suggests that more innovative management to promote SOC redistribution to depth, such as a switch to diverse leys or full inversion tillage may be required to maximise subsoil SOC stocks. Taken together our findings suggest that extending the time between grassland reseeding could temporarily increase SOC stocks without compromising sward composition or soil structure. However, detailed monitoring of the trade-offs with grassland productivity are required. Fine textured soils and subsoils (15-30 cm) have the greatest potential to accrue additional stable C due to under saturation of fine mineral pools.

Non-native Brachiaria humidicola with biological nitrification inhibition capacity stimulates in situ grassland N2O emissions

Introduction

Brachiaria humidicola, a tropical grass, could release root exudates with biological nitrification inhibition (BNI) capacity and reduce soil nitrous oxide (N2O) emissions from grasslands. However, evidence of the reduction effect in situ in tropical grasslands in China is lacking.

Methods

To evaluate the potential effects of B. humidicola on soil N2O emissions, a 2-year (2015–2017) field experiment was established in a Latosol and included eight treatments, consisting of two pastures, non-native B. humidicola and a native grass, Eremochloa ophiuroide, with four nitrogen (N) application rates. The annual urea application rates were 0, 150, 300, and 450 kg N ha−1.

Results

The average 2-year E. ophiuroides biomass with and without N fertilization were 9.07–11.45 and 7.34 t ha−1, respectively, and corresponding values for B. humidicola increased to 31.97–39.07 and 29.54 t ha−1, respectively. The N-use efficiencies under E. ophiuroide and B. humidicola cultivation were 9.3–12.0 and 35.5–39.4%, respectively. Annual N2O emissions in the E. ophiuroides and B. humidicola fields were 1.37 and 2.83 kg N2O-N ha−1, respectively, under no N fertilization, and 1.54–3.46 and 4.30–7.19 kg N2O-N ha−1, respectively, under N fertilization.

Discussions

According to the results, B. humidicola cultivation increased soil N2O emissions, especially under N fertilization. This is because B. humidicola exhibited the more effective stimulation effect on N2O production via denitrification primarily due to increased soil organic carbon and exudates than the inhibition effect on N2O production via autotrophic nitrification. Annual yield-scaled N2O emissions in the B. humidicola treatment were 93.02–183.12 mg N2O-N kg−1 biomass, which were significantly lower than those in the E. ophiuroides treatment. Overall, our results suggest that cultivation of the non-native grass, B. humidicola with BNI capacity, increased soil N2O emissions, while decreasing yield-scaled N2O emissions, when compared with native grass cultivation.

Alfalfa-grass mixtures reduce greenhouse gas emissions and net global warming potential while maintaining yield advantages over monocultures

Improving forage productivity with lower greenhouse gas (GHG) emissions from limited grassland has been a hotspot of interest in global agricultural production. In this study, we analyzed the effects of grasses (tall fescue, smooth bromegrass), legume (alfalfa), and alfalfa-grass (alfalfa + smooth bromegrass and alfalfa + tall fescue) mixtures on GHG emissions, net global warming potential (Net GWP), yield-based greenhouse gas intensity (GHGI), soil chemical properties and forage productivity in cultivated grassland in northwest China during 2020-2021. Our results demonstrated that alfalfa-grass mixtures significantly improved forage productivity. The highest total dry matter yield (DMY) during 2020 and 2021 was obtained from alfalfa-tall fescue (11,311 and 13,338 kg ha^-1) and alfalfa-smooth bromegrass mixtures (10,781 and 12,467 kg ha^-1). The annual cumulative GHG emissions from mixtures were lower than alfalfa monoculture. Alfalfa-grass mixtures significantly reduced GHGI compared with the grass or alfalfa monocultures. Furthermore, results indicated that grass, alfalfa and alfalfa-grass mixtures differentially affected soil chemical properties. Lower soil pH and C/N ratio were recorded in alfalfa monoculture. Alfalfa and mixtures increased soil organic carbon (SOC) and soil total nitrogen (STN) contents. Importantly, alfalfa-grass mixtures are necessary for improving forage productivity and mitigating the GHG emissions in this region. In conclusion, the alfalfa-tall fescue mixture lowered net GWP and GHGI in cultivated grassland while maintaining high forage productivity. These advanced agricultural practices could contribute to the development of climate-sustainable grassland production in China.

Greenhouse gas fluxes (CO2, N2O and CH4) of pea and maize during two cropping seasons: Drivers, budgets, and emission factors for nitrous oxide

Agriculture contributes considerably to the increase of global greenhouse gas (GHG) emissions. Hence, magnitude and drivers of temporal variations in carbon dioxide (CO2), nitrous oxide (N2O) and methane (CH4) fluxes in croplands are urgently needed to develop sustainable, climate-smart agricultural practices. However, our knowledge of GHG fluxes from croplands is still very limited. The eddy covariance technique was used to quantify GHG budgets and N2O emission factors (EF) for pea and maize in Switzerland. The random forest technique was applied for gap-filling N2O and CH4 fluxes as well as to determine the relevance of environmental, vegetation vs. management drivers of the GHG fluxes during two cropping seasons. Environmental (i.e., net radiation, soil water content, soil temperature) and vegetation drivers (i.e., vegetation height) were more important drivers for GHG fluxes at field scale than time since management for the two crop species. Both crops acted as GHG sinks between sowing and harvest, clearly dominated by net CO2 fluxes, while CH4 emissions were negligible. However, considerable N2O emissions occurred in both crop fields early in the season when crops were still establishing. N2O fluxes in both crops were small later in the season when vegetation was tall, despite high soil water contents and temperatures. Results clearly show a strong and highly dynamic microbial-plant competition for N driving N2O fluxes at the field scale. The total loss was 1.4 kg N2O-N ha-1 over 55 days for pea and 4.8 kg N2O-N ha-1 over 127 days for maize. EFs of N2O were 1.5 % (pea) and 4.4 % (maize) during the cropping seasons, clearly exceeding the IPCC Tier 1 EF for N2O. Thus, sustainable, climate-smart agriculture needs to consider crop phenology and better adapt N supply to crop N demand for growth, particularly during the early cropping season when competition for N between establishing crops and soil microorganisms modulates N2O losses.

Nitrous oxide flux observed with tall-tower eddy covariance over a heterogeneous rice cultivation landscape

Although croplands are known to be strong sources of anthropogenic N₂O, large uncertainties still exist regarding their emission factors, that is, the proportion of N in fertilizer application that escapes to the atmosphere as N₂O. In this study, we report the results of an experiment on the N₂O flux in a landscape dominated by rice cultivation in the Yangtze River Delta, China. The observation was made with a closed-path eddy covariance system on a 70-m tall tower from October 2018 to December 2020 (27 months). Temperature and precipitation explained 78% of the seasonal and interannual variability in the observed N₂O flux. The growing season (May to October) mean flux (1.14 nmol m^-2 s^-1) was much higher than the median flux found in the literature for rice paddies. The mean N₂O flux during the observational period was 0.90 ± 0.71 nmol m^-2 s^-1, and the annual cumulative N₂O emission was 7.6 and 9.1 kg N₂O-N ha^-1 during 2019 and 2020, respectively. The corresponding landscape emission factor was 3.8% and 4.6%, respectively, which were much higher than the IPCC default direct (0.3%) and indirect emission factors (0.75%) for rice paddies.

High denitrification potential but low nitrous oxide emission in a constructed wetland treating nitrate-polluted agricultural run-off

Constructed wetlands (CW) can efficiently remove nitrogen from polluted agricultural run-off, however, a potential caveat is nitrous oxide (N₂O), a harmful greenhouse gas and stratospheric ozone depleter. During five sampling campaigns, we measured N₂O fluxes from a 0.53 ha off-stream CW treating nitrate-rich water from the intensively fertilized watershed in Rampillon, France, using automated chambers with a quantum cascade laser system, and manual chambers. Sediment samples were analysed for potential N₂ flux using the HeO₂ incubation method. Both inlet nitrate (NO₃^-) concentrations and N₂O emission varied significantly between the seasons. In the Autumn and Winter inlet concentrations were about 11 mg NO₃^--N L^-1, and < 6.5 mg NO₃^--N L^-1 in the Spring and Summer. N₂O emission was highest in the Autumn (mean ± standard error: 9.7 ± 0.2 μg N m^-2 h^-1) and lowest in the Summer (wet period: 0.2 ± 0.3 μg N m^-2 h^-1). The CW was a very weak source of N₂O emitting 0.32 kg N₂O-N ha^-1 yr^-1 and removing around 938 kg NO₃^--N ha^-1 yr^-1, the ratio of N₂O-N emitted to NO₃^--N removed was 0.033%. The automated and manual chambers gave similar results. From the potential N₂O formation in the sediment, only 9% was emitted to the atmosphere, the average N₂ N ₂O ratio was high: 89:1 for N₂-N_potential: N₂O-N_potential and 1353:1 for N₂-N_potential: N₂O-N_emitted. These results indicate complete denitrification. The focused principal component analysis showed strong positive correlation between the gaseous N₂O fluxes and the following environmental factors: NO₃^--N concentrations in inlet water, streamflow, and nitrate reduction rate. Water temperature, TOC and DOC in the water and hydraulic residence time showed negative correlations with N₂O emissions. Shallow off-stream CWs such as Rampillon may have good nitrate removal capacity with low N₂O emissions.

Full inversion tillage during pasture renewal to increase soil carbon storage: New Zealand as a case study

As soils under permanent pasture and grasslands have large topsoil carbon (C) stocks, the scope to sequester additional C may be limited. However, because C in pasture/grassland soils declines with depth, there may be potential to sequester additional C in the subsoil. Data from 247 continuous pasture sites in New Zealand (representing five major soil Orders and ~80% of the grassland area) showed that, on average, the 0.15-0.30 m layer contained 25-34 t ha^-1 less C than the top 0.15 m. High-production grazed pastures require periodic renewal (re-seeding) every 7-14 years to maintain productivity. Our objective was to assess whether a one-time pasture renewal, involving full inversion tillage (FIT) to a depth of 0.30 m, has potential to increase C storage by burying C-rich topsoil and bringing low-C subsoil to the surface where C inputs from pasture production are greatest. Data from the 247 pasture sites were used to model changes in C stocks following FIT pasture renewal by predicting (1) the C accumulation in the new 0-0.15 m layer and (2) the decomposition of buried-C in the new 0.15-0.30 m layer. In the 20 years following FIT pasture renewal, soil C was predicted to increase by an average of 7.3-10.3 (Sedimentary soils) and 9.6-12.7 t C ha^-1 (Allophanic soils), depending on the assumptions applied. Adoption of FIT for pasture renewal across all suitable soils (2.0-2.6 M ha) in New Zealand was predicted to sequester ~20-36 Mt C, sufficient to offset 9.6-17.5% of the country's cumulative greenhouse gas emissions from agriculture over 20 years at the current rate of emissions. Given that grasslands account for ~70% of global agricultural land, FIT renewal of pastures or grassland could offer a significant opportunity to sequester soil C and offset greenhouse gas emissions.

Clonal integration in Phagmites australis mitigates effects of oil pollution on greenhouse gas emissions in a coastal wetland

Clonal integration, i.e., resource sharing within clones, enables clonal plants to maintain biomass production when ramets (asexual individuals) under stress are connected to those not under stress. Oil pollution can strongly reduce biomass production, and connected ramets within clones may experience different levels of oil pollution. Therefore, clonal integration may help plants maintain biomass production despite oil pollution. Because biomass production is often negatively correlated with greenhouse gas emissions, we hypothesized that oil pollution would increase greenhouse gas emissions and that clonal integration would reduce such an effect. We tested these hypotheses in a coastal wetland dominated by the rhizomatous grass Phragmites australis near a major site of oil production in the Yellow River Delta in China. We applied 0, 5, or 10 mm crude oil per year for two years in plots within stands of P. australis and tested effects of severing rhizomes connecting ramets inside and outside a plot (i.e. preventing clonal integration) on biomass production, soil chemistry and greenhouse gas emissions. When severed, ramets inside plots with no added oil produced about 220 g aboveground biomass m^--2 over the second growing season, and plots absorbed about 500 g total CO₂ equivalents m^-2. Adding 10 mm oil per year reduced aboveground biomass by about 30%, and caused plots to emit about 800 g CO₂ equivalents m^-2. Leaving ramets connected to those outside plots eliminated the negative effects of oil pollution on biomass production, and caused plots given 10 mm oil per year to emit about 50% fewer total CO₂ equivalents. We conclude that oil pollution can increase greenhouse gas emissions and clonal integration can reduce the effect of oil pollution on biomass production and greenhouse gas emissions. Our study provides the first experimental evidence that clonal integration in plants can reduce greenhouse gas emissions.

Impacts of Clear-Cutting of a Boreal Forest on Carbon Dioxide, Methane and Nitrous Oxide Fluxes

The 2015 Paris Agreement encourages stakeholders to implement sustainable forest management policies to mitigate anthropogenic emissions of greenhouse gases (GHG). The net effects of forest management on the climate and the environment are, however, still not completely understood, partially as a result of a lack of long-term measurements of GHG fluxes in managed forests. During the period 2010–2013, we simultaneously measured carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O) fluxes using the flux-gradient technique at two clear-cut plots of different degrees of wetness, located in central Sweden. The measurements started approx. one year after clear-cutting, directly following soil scarification and planting. The study focused on robust inter-plot comparisons, spatial and temporal dynamics of GHG fluxes, and the determination of the global warming potential of a clear-cut boreal forest. The clear-cutting resulted in significant emissions of GHGs at both the wet and the dry plot. The degree of wetness determined, directly or indirectly, the relative contribution of each GHG to the total budgets. Faster establishment of vegetation on the wet plot reduced total emissions of CO2 as compared to the dry plot but this was partially offset by higher CH4 emissions. Waterlogging following clear-cutting likely caused both plots to switch from sinks to sources of CH4. In addition, there were periods with N2O uptake at the wet plot, although both plots were net sources of N2O on an annual basis. We observed clear diel patters in CO2, CH4 and N2O fluxes during the growing season at both plots, with the exception of CH4 at the dry plot. The total three-year carbon budgets were 4107 gCO2-equivalent m−2 and 5274 gCO2-equivalent m−2 at the wet and the dry plots, respectively. CO2 contributed 91.8% to the total carbon budget at the wet plot and 98.2% at the dry plot. For the only full year with N2O measurements, the total GHG budgets were 1069.9 gCO2-eqvivalents m−2 and 1695.7 gCO2-eqvivalents m−2 at the wet and dry plot, respectively. At the wet plot, CH4 contributed 3.7%, while N2O contributed 7.3%. At the dry plot, CH4 and N2O contributed 1.5% and 7.6%, respectively. Our results emphasize the importance of considering the effects of the three GHGs on the climate for any forest management policy aiming at enhancing the mitigation potential of forests.

Applicability of a gas analyzer with dual quantum cascade lasers for simultaneous measurements of N2O, CH4 and CO2 fluxes from cropland using the eddy covariance technique

We evaluated the applicability of a closed-path gas analyzer with two mid-infrared quantum cascade lasers (QCLs) for simultaneous measurement of nitrous oxide (N₂O), methane (CH₄) and carbon dioxide (CO₂) fluxes from a cropland using the eddy covariance (EC) technique. The measurements were carried out in a typical vegetable field in the subtropical China during the wintertime, when the gas fluxes are at their lowest level in the year. A new approach was proposed to optimize the determination of lag times between the wind and gas concentration data, which was proven efficient to increase the reliability of the measured fluxes when the gas exchanges are weak. The dual-QCL analyzer showed a median precision (1σ) of 0.14 nmol mol^-1 for N₂O, 3.3 nmol mol^-1 for CH₄ and 0.36 μmol mol^-1 for CO₂ at sampling frequency of 10 Hz under the field conditions. Such precisions are better than, or comparable with, those of other commonly used closed-path or open-path gas analyzers, which are capable of measuring ony one or two ot the three gases. The detection limit of the EC system for measuring half-hourly fluxes were 0.05 nmol m^-2 s^-1 for N₂O, 1.12 nmol m^-2 s^-1 for CH₄ and 0.14 μmol m^-2 s^-1 for CO₂. The results showed that 100% of the N₂O, 85% of the CH₄ and 96% of the CO₂ fluxes were larger than the above detection limits. This study suggests that the EC technique using a closed-path gas analyzer with two quantum cascade lasers is qualified for reliable and simultaneous measurements of N₂O, CH₄ and CO₂ fluxes from a subtropical cropland throughout the year. Moreover, EC method based on this type of gas analyzer provides an additional option for long-term and simultaneous flux measurements of the three greenhouse gases in a wide range of agricultural and natural ecosystems.

Effects of afforestation on soil CH4 and N2O fluxes in a nsubtropical karst landscape

Afforestation is of importance for terrestrial carbon sequestration as well as soil and water conservation in karst landscapes. However, few studies have evaluated the effects of afforestation on soil CH₄ and N₂O emissions in subtropical karst areas. Thus, a year-round field experiment was conducted to quantify the effects of afforestation on soil CH₄ and N₂O fluxes from a subtropical karst landscape in South China. In this study, soil CH₄ and N₂O fluxes were simultaneously monitored using static chamber-gas chromatography from three paired sites, including a cropland site (SC) and adjacent sites at two stages of afforestation, a shrubland (SD) and a woodland (AF). The results showed that annual soil CH₄ uptake for SC, SD, and AF sites were 1.53 ± 0.20 kg C ha^-1 yr^-1, 2.90 ± 0.20 kg C ha^-1 yr^-1, and 5.68 ± 0.18 kg C ha^-1 yr^-1, respectively. Afforestation (i.e., SD and AF sites) significantly increased soil CH₄ uptake compared with the adjacent cropland. Annual soil N₂O fluxes for SC, SD, and AF sites were 2.38 ± 0.17 kg N ha^-1 yr^-1, 0.94 ± 0.14 kg N ha^-1 yr^-1, and 0.47 ± 0.01 kg N ha^-1 yr^-1, respectively. Afforestation significantly decreased soil N₂O fluxes compared with the adjacent cropland. The effects of afforestation on soil CH₄ and N₂O fluxes in the present study were mainly attributed to changes in soil characteristics, such as temperature and moisture, as these were significantly correlated with soil CH₄ and N₂O fluxes across different experimental sites. The present study highlights that afforestation is an effective land use management practice to mitigate non-CO₂ greenhouse gas emissions from subtropical karst landscapes in South China.

When Does Vapor Pressure Deficit Drive or Reduce Evapotranspiration?

Increasing vapor pressure deficit (VPD) increases atmospheric demand for water. While increased evapotranspiration (ET) in response to increased atmospheric demand seems intuitive, plants are capable of reducing ET in response to increased VPD by closing their stomata. We examine which effect dominates the response to increasing VPD: atmospheric demand and increases in ET or plant response (stomata closure) and decreases in ET. We use Penman-Monteith, combined with semiempirical optimal stomatal regulation theory and underlying water use efficiency, to develop a theoretical framework for assessing ET response to VPD. The theory suggests that depending on the environment and plant characteristics, ET response to increasing VPD can vary from strongly decreasing to increasing, highlighting the diversity of plant water regulation strategies. The ET response varies due to (1) climate, with tropical and temperate climates more likely to exhibit a positive ET response to increasing VPD than boreal and arctic climates; (2) photosynthesis strategy, with C3 plants more likely to exhibit a positive ET response than C4 plants; and (3) plant type, with crops more likely to exhibit a positive ET response, and shrubs and gymniosperm trees more likely to exhibit a negative ET response. These results, derived from previous literature connecting plant parameters to plant and climate characteristics, highlight the utility of our simplified framework for understanding complex land-atmosphere systems in terms of idealized scenarios in which ET responds to VPD only. This response is otherwise challenging to assess in an environment where many processes coevolve together.

Constraining N cycling in the ecosystem model LandscapeDNDC with the stable isotope model SIMONE

The isotopic composition (ic) of soil nitrogen (N) and, more recently, the intramolecular distribution of ¹⁵ N in the N₂ O molecule (site preference, SP) are powerful instruments to identify dominant N turnover processes, and to attribute N₂ O emissions to their source processes. Despite the process information contained in the ic of N species and the associated potential for model validation, the implementation of isotopes in ecosystem models has lagged behind. To foster the validation of ecosystem models based on the ic of N species, we developed the stable isotope model for nutrient cycles (SIMONE). SIMONE uses fluxes between ecosystem N pools (soil organic N, mineral N, plants, microbes) calculated by biogeochemical models, and literature isotope effects for these processes to calculate the ic of N species. Here, we present the concept of SIMONE, apply it to simulations of the biogeochemical model LandscapeDNDC, and assess the capability of ¹⁵ N-N₂ O and, to our knowledge for the first time, SP, to constrain simulated N fluxes by LandscapeDNDC. LandscapeDNDC successfully simulated N₂ O emission, soil nitrate, and ammonium, as well as soil environmental conditions of an intensively managed grassland site in Switzerland. Accordingly, the dynamics of ¹⁵ N-N₂ O and SP of soil N₂ O fluxes as simulated by SIMONE agreed well with measurements, though ¹⁵ N-N₂ O was on average underestimated and SP overestimated (root-mean-square error [RMSE] of 8.4‰ and 7.3‰, respectively). Although ¹⁵ N-N₂ O could not constrain the N cycling process descriptions of LandscapeDNDC, the overestimation of SP indicated an overestimation of simulated nitrification rates by 10-59% at low water content, suggesting the revision of the corresponding model parameterization. Our findings show that N isotope modeling in combination with only recently available high- frequency measurements of the N₂ O ic are promising tools to identify and address weaknesses in N cycling of ecosystem models. This will finally contribute to augmenting the development of model-based strategies for mitigating N pollution.

Perennial forb invasions alter greenhouse gas balance between ecosystem and atmosphere in an annual grassland in China

Grassland ecosystems are sensitive to invasions by plants from other functional groups which can alter soil greenhouse gas (GHG) fluxes. However, the effects of plant invasion on net GHG exchanges between soils and the atmosphere, plant production, and global warming potential (GWP) of annual grasslands is poorly understood. To evaluate the impacts of perennial forb invasions on GHG budgets of an annual grassland in China, we measured soil carbon dioxide (CO₂), methane (CH₄), and nitrous oxide (N₂O) fluxes over two years in replicated invaded (dominated by Alternanthera philoxeroides or Solidago canadensis) and non-invaded (dominated by the annual grass Eragrostis pilosa or the annual forb Sesbania cannabina) field sites. On average, soil CO₂ and N₂O emissions from invaded sites were 30% and 76% higher, respectively, relative to sites dominated by native species. Emissions of N₂O and CO₂ were especially high in Solidago and Alternanthera dominated sites, respectively. Soil CH₄ emissions did not vary with plant species. On average, total biomass C of invaded sites was higher than that of the native dominated sites but this reflected the high C in Solidago dominated sites. Global warming potential (GWP) was increased by Alternanthera invasions and decreased by Solidago invasions. Plant invasions affected GWP of these annual grasslands through higher emissions of some GHGs but also sometimes higher biomass C. Together, this suggests that perennial forb invasions could change the net source or sink role of annual grasslands for GHG budgets, but the effects on GWP vary among species depending on GHG responses and C storage.

The use of biogeochemical models to evaluate mitigation of greenhouse gas emissions from managed grasslands

Simulation models quantify the impacts on carbon (C) and nitrogen (N) cycling in grassland systems caused by changes in management practices. To support agricultural policies, it is however important to contrast the responses of alternative models, which can differ greatly in their treatment of key processes and in their response to management. We applied eight biogeochemical models at five grassland sites (in France, New Zealand, Switzerland, United Kingdom and United States) to compare the sensitivity of modelled C and N fluxes to changes in the density of grazing animals (from 100% to 50% of the original livestock densities), also in combination with decreasing N fertilization levels (reduced to zero from the initial levels). Simulated multi-model median values indicated that input reduction would lead to an increase in the C sink strength (negative net ecosystem C exchange) in intensive grazing systems: -64 ± 74 g C m^-2 yr^-1 (animal density reduction) and -81 ± 74 g C m^-2 yr^-1 (N and animal density reduction), against the baseline of -30.5 ± 69.5 g C m^-2 yr^-1 (LSU [livestock units] ≥ 0.76 ha^-1 yr^-1). Simulations also indicated a strong effect of N fertilizer reduction on N fluxes, e.g. N₂O-N emissions decreased from 0.34 ± 0.22 (baseline) to 0.1 ± 0.05 g N m^-2 yr^-1 (no N fertilization). Simulated decline in grazing intensity had only limited impact on the N balance. The simulated pattern of enteric methane emissions was dominated by high model-to-model variability. The reduction in simulated offtake (animal intake + cut biomass) led to a doubling in net primary production per animal (increased by 11.6 ± 8.1 t C LSU^-1 yr^-1 across sites). The highest N₂O-N intensities (N₂O-N/offtake) were simulated at mown and extensively grazed arid sites. We show the possibility of using grassland models to determine sound mitigation practices while quantifying the uncertainties associated with the simulated outputs.

Maximizing the information obtained from chamber-based greenhouse gas exchange measurements in remote areas

Measurements of greenhouse gas (GHG) fluxes, particularly methane (CH4) and nitrous oxide (N2O) in mountain ecosystems are scarce due to the complexity and unpredictable behavior of these gases, in addition to the remoteness of these ecosystems. In this context, we measured CO2, CH4, and N2O fluxes in four semi-natural pastures in the Pyrenees to investigate their magnitude and range of variability. Our interest was to study GHG phenomena at the patch-level, therefore we chose to measure the gas-exchange using a combination of a gas analyzer and manual chambers. The analyzer used is a photoacoustic field gas-monitor that allows multi-gas instantaneous measurements. After implementing quality control and corrections, data was of variable quality. We tackled this by categorizing data as to providing quantitative or only qualitative information: •50% and 59% of all CH4 and N2O data, respectively, provided quantitative information above the detection limit.•We chose not to discard data providing only qualitative information, because they identify highest- and lowest-flux peak periods and indicate the variability of the fluxes, along different altitudes and under different climatic conditions.•We chose not to give fluxes below detection limit a quantitative value but to acknowledge them as values identifying periods with low fluxes.

Greenhouse gas fluxes over managed grasslands in Central Europe

Central European grasslands are characterized by a wide range of different management practices in close geographical proximity. Site-specific management strategies strongly affect the biosphere-atmosphere exchange of the three greenhouse gases (GHG) carbon dioxide (CO₂ ), nitrous oxide (N₂ O), and methane (CH₄ ). The evaluation of environmental impacts at site level is challenging, because most in situ measurements focus on the quantification of CO₂ exchange, while long-term N₂ O and CH₄ flux measurements at ecosystem scale remain scarce. Here, we synthesized ecosystem CO₂ , N₂ O, and CH₄ fluxes from 14 managed grassland sites, quantified by eddy covariance or chamber techniques. We found that grasslands were on average a CO₂ sink (-1,783 to -91 g CO₂  m^-2  year^-1 ), but a N₂ O source (18-638 g CO₂ -eq. m^-2  year^-1 ), and either a CH₄ sink or source (-9 to 488 g CO₂ -eq. m^-2  year^-1 ). The net GHG balance (NGB) of nine sites where measurements of all three GHGs were available was found between -2,761 and -58 g CO₂ -eq. m^-2  year^-1 , with N₂ O and CH₄ emissions offsetting concurrent CO₂ uptake by on average 21 ± 6% across sites. The only positive NGB was found for one site during a restoration year with ploughing. The predictive power of soil parameters for N₂ O and CH₄ fluxes was generally low and varied considerably within years. However, after site-specific data normalization, we identified environmental conditions that indicated enhanced GHG source/sink activity ("sweet spots") and gave a good prediction of normalized overall fluxes across sites. The application of animal slurry to grasslands increased N₂ O and CH₄ emissions. The N₂ O-N emission factor across sites was 1.8 ± 0.5%, but varied considerably at site level among the years (0.1%-8.6%). Although grassland management led to increased N₂ O and CH₄ emissions, the CO₂ sink strength was generally the most dominant component of the annual GHG budget.

Consensus, uncertainties and challenges for perennial bioenergy crops and land use

Perennial bioenergy crops have significant potential to reduce greenhouse gas (GHG) emissions and contribute to climate change mitigation by substituting for fossil fuels; yet delivering significant GHG savings will require substantial land-use change, globally. Over the last decade, research has delivered improved understanding of the environmental benefits and risks of this transition to perennial bioenergy crops, addressing concerns that the impacts of land conversion to perennial bioenergy crops could result in increased rather than decreased GHG emissions. For policymakers to assess the most cost-effective and sustainable options for deployment and climate change mitigation, synthesis of these studies is needed to support evidence-based decision making. In 2015, a workshop was convened with researchers, policymakers and industry/business representatives from the UK, EU and internationally. Outcomes from global research on bioenergy land-use change were compared to identify areas of consensus, key uncertainties, and research priorities. Here, we discuss the strength of evidence for and against six consensus statements summarising the effects of land-use change to perennial bioenergy crops on the cycling of carbon, nitrogen and water, in the context of the whole life-cycle of bioenergy production. Our analysis suggests that the direct impacts of dedicated perennial bioenergy crops on soil carbon and nitrous oxide are increasingly well understood and are often consistent with significant life cycle GHG mitigation from bioenergy relative to conventional energy sources. We conclude that the GHG balance of perennial bioenergy crop cultivation will often be favourable, with maximum GHG savings achieved where crops are grown on soils with low carbon stocks and conservative nutrient application, accruing additional environmental benefits such as improved water quality. The analysis reported here demonstrates there is a mature and increasingly comprehensive evidence base on the environmental benefits and risks of bioenergy cultivation which can support the development of a sustainable bioenergy industry.

Soil Moisture-Temperature Coupling in a Set of Land Surface Models

The land surface controls the partitioning of water and energy fluxes and therefore plays a crucial role in the climate system. The coupling between soil moisture and air temperature, in particular, has been shown to affect the severity and occurrence of temperature extremes and heat waves. Here we study soil moisture-temperature coupling in five land surface models, focusing on the terrestrial segment of the coupling in the warm season. All models are run off-line over a common period with identical atmospheric forcing data, in order to allow differences in the results to be attributed to the models' partitioning of energy and water fluxes. Coupling is calculated according to two semiempirical metrics, and results are compared to observational flux tower data. Results show that the locations of the global hot spots of soil moisture-temperature coupling are similar across all models and for both metrics. In agreement with previous studies, these areas are located in transitional climate regimes. The magnitude and local patterns of model coupling, however, can vary considerably. Model coupling fields are compared to tower data, bearing in mind the limitations in the geographical distribution of flux towers and the differences in representative area of models and in situ data. Nevertheless, model coupling correlates in space with the tower-based results (r = 0.5-0.7), with the multimodel mean performing similarly to the best-performing model. Intermodel differences are also found in the evaporative fractions and may relate to errors in model parameterizations and ancillary data of soil and vegetation characteristics.

Impact of understory vegetation on soil carbon and nitrogen dynamic in aerially seeded Pinus massoniana plantations

Understory vegetation plays a vital role in regulating soil carbon (C) and nitrogen (N) characteristics due to differences in plant functional traits. Different understory vegetation types have been reported following aerial seeding. While aerial seeding is common in areas with serious soil erosion, few studies have been conducted to investigate changes in soil C and N cycling as affected by understory vegetation in aerially seeded plantations. Here, we studied soil C and N characteristics under two naturally formed understory vegetation types (Dicranopteris and graminoid) in aerially seeded Pinus massoniana Lamb plantations. Across the two studied understory vegetation types, soil organic C was significantly correlated with all measured soil N variables, including total N, available N, microbial biomass N and water-soluble organic N, while microbial biomass C was correlated with all measured variables except soil organic C. Dicranopteris and graminoid differed in their effects on soil C and N process. Except water-soluble organic C, all the other C and N variables were higher in soils with graminoids. The higher levels of soil organic C, microbial biomass C, total N, available N, microbial biomass N and water-soluble organic N were consistent with the higher litter and root quality (C/N) of graminoid vegetation compared to Dicranopteris. Changes in soil C and N cycles might be impacted by understory vegetation types via differences in litter or root quality.

Verification of micrometeorologically determined nitrous oxide fluxes following controlled release from pasture

We have developed a high-precision micrometeorological system capable of measuring emissions of nitrous oxide (N2O) from up to four adjacent pasture plots. The system can be used to compare the influence of environmental factors and management practice on N2O emissions at the paddock scale. The system is capable of determining a minimum detectable N2O difference of the order of 40 pmol/mol, with an ability to resolve flux differences among plots of ~26 µg (N2O-N)/m2.h. So as to independently verify the emission estimates of the micrometeorological system, we developed a calibrated N2O-release system and compared known release rates with the micrometeorological flux estimates. Adjustable release rates up to the equivalent average surface flux of ~500 µg (N2O-N)/m2.h were achieved using mass flow-controlled input of pure N2O in a compressed air stream over two 1.5-ha plots upwind of flux-measurement masts. The comparison of network release rate with measured emission rate was quite variable and complicated by a significant and varying background emissions of N2O from the soil. For optimal steady-wind cases, the ratio of uncorrected measured flux to known release, including the estimated background, was of the order of 0.4–0.5; this ratio is likely to be influenced by the turbulent Schmidt number. Flux estimates for uncorrected flux gradient and WindTrax backward Lagrangian Stochastic method (which includes Schmidt correction) agreed well with a ratio of 0.54. The experiment highlighted the need for accurate estimates of gas eddy diffusivity in the micrometeorological gradient or difference-based flux measurement of N2O.

Conversion from rice to vegetable production increases N2O emission via increased soil organic matter mineralization

The conversion from rice to vegetable production widely occurs in China. However, the effects of this conversion on N₂O emission and the underlying mechanisms are not well understood. In the present study, 12 rice paddies (R) were selected and half of them converted to vegetable fields (V) with the following treatments: rice paddies without N-fertilizer (R-CK), rice paddies with conventional N-fertilizer (R-CN), converted vegetable fields without N-fertilizer (V-CK), and converted vegetable fields with conventional N-fertilizer (V-CN) in a randomized block design with 3 replicates. N₂O emissions were measured with static chambers from December 2012 to December 2015. Within each V-CN plot, a root exclusion subplot was established to measure soil heterotrophic respiration (CO₂ effluxes), a proxy for soil organic matter mineralization. Conversion of rice paddies to vegetable production dramatically increased N₂O emissions. The three-year cumulative N₂O emissions were 0.59, 1.90, 55.50 and 160.14kg N ha^-1 for R-CK, R-CN, V-CK and V-CN, respectively. The annual N₂O emissions from vegetable fields ranged between 5.99 and 113.45kg N ha^-1yr^-1, with substantially higher emissions in the first year. N₂O fluxes from V-CN were significantly and positively related to CO₂ fluxes and inorganic N concentrations. The linear relationship between natural logarithms of N₂O and CO₂ fluxes was stronger and the regression coefficient higher in the first year, showing the dependence of N₂O on soil organic matter mineralization. These results suggest that soil organic matter and N mineralization contributes significantly to N₂O emission following conversion of rice paddies to vegetable production.

Interactive impacts of nitrogen input and water amendment on growing season fluxes of CO2, CH4, and N2O in a semiarid grassland, Northern China

Nitrogen and water are two important factors influencing GHG (primarily CO₂ - carbon dioxide; CH₄ - methane, and N₂O - nitrous oxide) fluxes in semiarid grasslands. However, the interactive effects of nitrogen and water on GHG fluxes remain elusive. A 3-year (2010-2012) manipulative experiment was conducted to investigate the individual and interactive effects of nitrogen and water additions on GHG fluxes during growing seasons (May to September) in a semiarid grassland in Northern China. Accumulated throughout growing seasons, nitrogen input stimulated CO₂ uptake by 3.3±1.0gCm^-2 (gN)^-1, enhanced N₂O emission by 1.2±0.3mgNm^-2 (gN)^-1, and decreased CH₄ uptake by 5.2±0.9mgNm^-2 (gN)^-1; water amendment stimulated CO₂ uptake by 0.2±0.1gCm^-2 (mmH₂O)^-1 and N₂O emission by 0.2±0.02mgNm^-2 (mmH₂O)^-1, decreased CH₄ uptake by 0.3±0.1mgCm^-2 (mmH₂O)^-1. A synergistic effect between nitrogen and water was found on N₂O flux in normal year while the additive effects of nitrogen and water additions were found on CH₄ and CO₂ uptakes during all experiment years, and on N₂O emission in dry years. The nitrogen addition had stronger impacts than water amendment on stimulating CH₄ uptake in the normal year, while water was the dominant factor affecting CH₄ uptake in dry years. For N₂O emission, the N-stimulating impact was stronger in un-watered than in watered plots, and the water-stimulating impact was stronger in non-fertilized than in fertilized treatments in dry years. The interactive impacts of nitrogen and water additions on GHG fluxes advance our understanding of GHG fluxes in responses to multiple environmental factors. This data source could be valuable for validating ecosystem models in simulating GHG fluxes in a multiple factors environment.

Microbial N Transformations and N2O Emission after Simulated Grassland Cultivation: Effects of the Nitrification Inhibitor 3,4-Dimethylpyrazole Phosphate (DMPP)

Grassland cultivation can mobilize large pools of N in the soil, with the potential for N leaching and N₂O emissions. Spraying with the nitrification inhibitor 3,4-dimethylpyrazole phosphate (DMPP) before cultivation was simulated by use of soil columns in which the residue distribution corresponded to plowing or rotovation to study the effects of soil-residue contact on N transformations. DMPP was sprayed on aboveground parts of ryegrass and white clover plants before incorporation. During a 42-day incubation, soil mineral N dynamics, potential ammonia oxidation (PAO), denitrifying enzyme activity (DEA), nitrifier and denitrifier populations, and N₂O emissions were investigated. The soil NO₃^- pool was enriched with ¹⁵N to trace sources of N₂O. Ammonium was rapidly released from decomposing residues, and PAO was stimulated in soil near residues. DMPP effectively reduced NH₄⁺ transformation irrespective of residue distribution. Ammonia-oxidizing archaea (AOA) and bacteria (AOB) were both present, but only the AOB amoA transcript abundance correlated with PAO. DMPP inhibited the transcription of AOB amoA genes. Denitrifier genes and transcripts (nirK, nirS, and clades I and II of nosZ) were recovered, and a correlation was found between nirS mRNA and DEA. DMPP showed no adverse effects on the abundance or activity of denitrifiers. The ¹⁵N enrichment of N₂O showed that denitrification was responsible for 80 to 90% of emissions. With support from a control experiment without NO₃^- amendment, it was concluded that DMPP will generally reduce the potential for leaching of residue-derived N, whereas the effect of DMPP on N₂O emissions will be significant only when soil NO₃^- availability is limiting.

IMPORTANCE

Residue incorporation following grassland cultivation can lead to mobilization of large pools of N and potentially to significant N losses via leaching and N₂O emissions. This study proposed a mitigation strategy of applying 3,4-dimethylpyrazole phosphate (DMPP) prior to grassland cultivation and investigated its efficacy in a laboratory incubation study. DMPP inhibited the growth and activity of ammonia-oxidizing bacteria but had no adverse effects on ammonia-oxidizing archaea and denitrifiers. DMPP can effectively reduce the potential for leaching of NO₃^- derived from residue decomposition, while the effect on reducing N₂O emissions will be significant only when soil NO₃^- availability is limiting. Our findings provide insight into how DMPP affects soil nitrifier and denitrifier populations and have direct implications for improving N use efficiency and reducing environmental impacts during grassland cultivation.

Micrometeorological measurements over 3 years reveal differences in N2 O emissions between annual and perennial crops

Perennial crops can deliver a wide range of ecosystem services compared to annual crops. Some of these benefits are achieved by lengthening the growing season, which increases the period of crop water and nutrient uptake, pointing to a potential role for perennial systems to mitigate soil nitrous oxide (N2 O) emissions. Employing a micrometeorological method, we tested this hypothesis in a 3-year field experiment with a perennial grass-legume mixture and an annual corn monoculture. Given that N2 O emissions are strongly dependent on the method of fertilizer application, two manure application options commonly used by farmers for each crop were studied: injection vs. broadcast application for the perennial; fall vs. spring application for the annual. Across the 3 years, lower N2 O emissions (P < 0.001) were measured for the perennial compared to the annual crop, even though annual N2 O emissions increased tenfold for the perennial after ploughing. The percentage of N2 O lost per unit of fertilizer applied was 3.7, 3.1 and 1.3 times higher for the annual for each consecutive year. Differences in soil organic matter due to the contrasting root systems of these crops are probably a major factor behind the N2 O reduction. We found that a specific manure management practice can lead to increases or reductions in annual N2 O emissions depending on environmental variables. The number of freeze-thaw cycles during winter and the amount of rainfall after fertilization in spring were key factors. Therefore, general manure management recommendations should be avoided because interannual weather variability has the potential to determine if a specific practice is beneficial or detrimental. The lower N2 O emissions of perennial crops deserve further research attention and must be considered in future land-use decisions. Increasing the proportion of perennial crops in agricultural landscapes may provide an overlooked opportunity to regulate N2 O emissions.

Methane and nitrous oxide exchange over a managed hay meadow

The methane (CH4) and nitrous oxide (N2O) exchange of a temperate mountain grassland near Neustift, Austria, was measured during 2010-2012 over a time period of 22 months using the eddy covariance method. Exchange rates of both compounds at the site were low, with 97% of all half-hourly CH4 and N2O fluxes ranging between ±200 and ±50 ng m-2 s-1, respectively. The meadow acted as a sink for both compounds during certain time periods, but was a clear source of CH4 and N2O on an annual timescale. Therefore, both gases contributed to an increase of the global warming potential (GWP), effectively reducing the sink strength in terms of CO2 equivalents of the investigated grassland site. In 2011, our best guess estimate showed a net greenhouse gas (GHG) sink of -32 g CO2 equ. m-2 yr-1 for the meadow, whereby 55% of the CO2 sink strength of -71 g CO2m-2 yr-1 was offset by CH4 (N2O) emissions of 7 (32) g CO2 equ. m-2 yr-1. When all data were pooled, the ancillary parameters explained 27 (42)% of observed CH4 (N2O) flux variability, and up to 62 (76)% on shorter timescales in-between management dates. In the case of N2O fluxes, we found the highest emissions at intermediate soil water contents and at soil temperatures close to 0 or above 14 °C. In comparison to CO2, H2O and energy fluxes, the interpretation of CH4 and N2O exchange was challenging due to footprint heterogeneity regarding their sources and sinks, uncertainties regarding post-processing and quality control. Our results emphasize that CH4 and N2O fluxes over supposedly well-aerated and moderately fertilized soils cannot be neglected when evaluating the GHG impact of temperate managed grasslands.

Measuring fluxes of trace gases and energy between ecosystems and the atmosphere - the state and future of the eddy covariance method

The application of the eddy covariance flux method to measure fluxes of trace gas and energy between ecosystems and the atmosphere has exploded over the past 25 years. This opinion paper provides a perspective on the contributions and future opportunities of the eddy covariance method. First, the paper discusses the pros and cons of this method relative to other methods used to measure the exchange of trace gases between ecosystems and the atmosphere. Second, it discusses how the use of eddy covariance method has grown and evolved. Today, more than 400 flux measurement sites are operating world-wide and the duration of the time series exceed a decade at dozens of sites. Networks of tower sites now enable scientists to ask scientific questions related to climatic and ecological gradients, disturbance, changes in land use, and management. The paper ends with discussions on where the field of flux measurement is heading. Topics discussed include role of open access data sharing and data mining, in this new era of big data, and opportunities new sensors that measure a variety of trace gases, like volatile organic carbon compounds, methane and nitrous oxide, and aerosols, may yield.

Perception, attitude and behavior in relation to climate change: a survey among CDC health professionals in Shanxi province, China

BACKGROUND

A better understanding of public perceptions, attitude and behavior in relation to climate change will provide an important foundation for government׳s policy-making, service provider׳s guideline development and the engagement of local communities. The purpose of this study was to assess the perception towards climate change, behavior change, mitigation and adaptation measures issued by the central government among the health professionals in the Centres for Disease Control and Prevention (CDC) in China.

METHODS

In 2013, a cross-sectional questionnaire survey was undertaken among 314 CDC health professionals in various levels of CDC in Shanxi Province, China. Descriptive analyses were performed.

RESULTS

More than two thirds of the respondents believed that climate change has happened at both global and local levels, and climate change would lead to adverse impacts to human beings. Most respondents (74.8%) indicated the emission of greenhouse gases was the cause of climate change, however there was a lack of knowledge about greenhouse gases and their sources. Media was the main source from which respondents obtained the information about climate change. A majority of respondents showed that they were willing to change behavior, but their actions were limited. In terms of mitigation and adaptation measures issued by the Chinese Government, respondents׳ perception showed inconsistency between strategies and relevant actions. Moreover, although the majority of respondents believed some strategies and measures were extremely important to address climate change, they were still concerned about economic development, energy security, and local environmental protection.

CONCLUSION

There are gaps between perceptions and actions towards climate change among these health professionals. Further efforts need to be made to raise the awareness of climate change among health professionals, and to promote relevant actions to address climate change in the context of the proposed policies with local sustainable development.