Impacts of natural factors and farming practices on greenhouse gas emissions in the North China Plain: A meta‐analysis

Requirements for mitigation of the continued increase in greenhouse gas (GHG ) emissions are much needed for the North China Plain (NCP ). We conducted a meta‐analysis of 76 published studies of 24 sites in the NCP to examine the effects of natural conditions and farming practices on GHG emissions in that region. We found that N₂O was the main component of the area‐scaled total GHG balance, and the CH ₄ contribution was <5%. Precipitation, temperature, soil pH , and texture had no significant impacts on annual GHG emissions, because of limited variation of these factors in the NCP . The N₂O emissions increased exponentially with mineral fertilizer N application rate, with y  =  0.2389e^0.0058x for wheat season and y  =  0.365e^0.0071x for maize season. Emission factors were estimated at 0.37% for wheat and 0.90% for maize at conventional fertilizer N application rates. The agronomic optimal N rates (241 and 185 kg N ha^?1 for wheat and maize, respectively) exhibited great potential for reducing N₂O emissions, by 0.39 (29%) and 1.71 (56%) kg N₂O‐N ha^?1 season^?1 for the wheat and maize seasons, respectively. Mixed application of organic manure with reduced mineral fertilizer N could reduce annual N₂O emissions by 16% relative to mineral N application alone while maintaining a high crop yield. Compared with conventional tillage, no‐tillage significantly reduced N₂O emissions by ~30% in the wheat season, whereas it increased those emissions by ~10% in the maize season. This may have resulted from the lower soil temperature in winter and increased soil moisture in summer under no‐tillage practice. Straw incorporation significantly increased annual N₂O emissions, by 26% relative to straw removal. Our analysis indicates that these farming practices could be further tested to mitigate GHG emission and maintain high crop yields in the NCP .     

Initial nitrous oxide,carbon dioxide,and methane costs of converting conservation reserve program grassland to row crops under no‐till vs. conventional tillage

Around 4.4 million ha of land in USDA Conservation Reserve Program (CRP) contracts will expire between 2013 and 2018 and some will likely return to crop production. No‐till (NT) management offers the potential to reduce the global warming costs of CO₂, CH₄, and N₂O emissions during CRP conversion, but to date there have been no CRP conversion tillage comparisons. In 2009, we converted portions of three 9–21 ha CRP fields in Michigan to conventional tillage (CT) or NT soybean production and reserved a fourth field for reference. Both CO₂ and N₂O fluxes increased following herbicide application in all converted fields, but in the CT treatment substantial and immediate N₂O and CO₂ fluxes occurred after tillage. For the initial 201‐day conversion period, average daily N₂O fluxes (g N₂O‐N ha^?1 d^?1) were significantly different in the order: CT (47.5 ± 6.31, n = 6) ? NT (16.7 ± 2.45, n = 6) ? reference (2.51 ± 0.73, n = 4). Similarly, soil CO₂ fluxes in CT were 1.2 times those in NT and 3.1 times those in the unconverted CRP reference field. All treatments were minor sinks for CH₄ (?0.69 ± 0.42 to ?1.86 ± 0.37 g CH₄–C ha^?1 d^?1) with no significant differences among treatments. The positive global warming impact (GWI) of converted soybean fields under both CT (11.5 Mg CO₂e ha^?1) and NT (2.87 Mg CO₂e ha^?1) was in contrast to the negative GWI of the unconverted reference field (?3.5 Mg CO₂e ha^?1) with on‐going greenhouse gas (GHG) mitigation. N₂O contributed 39.3% and 55.0% of the GWI under CT and NT systems with the remainder contributed by CO₂ (60.7% and 45.0%, respectively). Including foregone mitigation, we conclude that NT management can reduce GHG costs by ~60% compared to CT during initial CRP conversion.     

Greenhouse gas fluxes from an Australian subtropical cropland under long‐term contrasting management regimes

The long‐term effects of conservation management practices on greenhouse gas fluxes from tropical/subtropical croplands remain to be uncertain. Using both manual and automatic sampling chambers, we measured N₂O and CH₄ fluxes at a long‐term experimental site (1968–present) in Queensland, Australia from 2006 to 2009. Annual net greenhouse gas fluxes (NGGF) were calculated from the 3‐year mean N₂O and CH₄ fluxes and the long‐term soil organic carbon changes. N₂O emissions exhibited clear daily, seasonal and interannual variations, highlighting the importance of whole‐year measurement over multiple years for obtaining temporally representative annual emissions. Averaged over 3 years, annual N₂O emissions from the unfertilized and fertilized soils (90 kg N ha^?1 yr^?1 as urea) amounted to 138 and 902 g N ha^?1, respectively. The average annual N₂O emissions from the fertilized soil were 388 g N ha^?1 lower under no‐till (NT) than under conventional tillage (CT) and 259 g N ha^?1 higher under stubble retention (SR) than under stubble burning (SB). Annual N₂O emissions from the unfertilized soil were similar between the contrasting tillage and stubble management practices. The average emission factors of fertilizer N were 0.91%, 1.20%, 0.52% and 0.77% for the CT‐SB, CT‐SR, NT‐SB and NT‐SR treatments, respectively. Annual CH₄ fluxes from the soil were very small (?200–300 g CH₄ ha^?1 yr^?1) with no significant difference between treatments. The NGGF were 277–350 kg CO₂‐e ha^?1 yr^?1 for the unfertilized treatments and 401–710 kg CO₂‐e ha^?1 yr^?1 for the fertilized treatments. Among the fertilized treatments, N₂O emissions accounted for 52–97% of NGGF and NT‐SR resulted in the lowest NGGF (401 kg CO₂‐e ha^?1 yr^?1 or 140 kg CO₂‐e t^?1 grain). Therefore, NT‐SR with improved N fertilizer management practices was considered the most promising management regime for simultaneously achieving maximal yield and minimal NGGF.     

Soil nitrous oxide emissions following crop residue addition: a meta‐analysis

Annual production of crop residues has reached nearly 4 billion metric tons globally. Retention of this large amount of residues on agricultural land can be beneficial to soil C sequestration. Such potential impacts, however, may be offset if residue retention substantially increases soil emissions of N₂O, a potent greenhouse gas and ozone depletion substance. Residue effects on soil N₂O emissions have gained considerable attention since early 1990s; yet, it is still a great challenge to predict the magnitude and direction of soil N₂O emissions following residue amendment. Here, we used a meta‐analysis to assess residue impacts on soil N₂O emissions in relation to soil and residue attributes, i.e., soil pH, soil texture, soil water content, residue C and N input, and residue C : N ratio. Residue effects were negatively associated with C : N ratios, but generally residue amendment could not reduce soil N₂O emissions, even for C : N ratios well above ca. 30, the threshold for net N immobilization. Residue effects were also comparable to, if not greater than, those of synthetic N fertilizers. In addition, residue effects on soil N₂O emissions were positively related to the amounts of residue C input as well as residue effects on soil CO₂ respiration. Furthermore, most significant and stimulatory effects occurred at 60–90% soil water‐filled pore space and soil pH 7.1–7.8. Stimulatory effects were also present for all soil textures except sand or clay content ≤10%. However, inhibitory effects were found for soils with >90% water‐filled pore space. Altogether, our meta‐analysis suggests that crop residues played roles beyond N supply for N₂O production. Perhaps, by stimulating microbial respiration, crop residues enhanced oxygen depletion and therefore promoted anaerobic conditions for denitrification and N₂O production. Our meta‐analysis highlights the necessity to connect the quantity and quality of crop residues with soil properties for predicting soil N₂O emissions.     

Long‐term no‐till and stover retention each decrease the global warming potential of irrigated continuous corn

Over the last 50 years, the most increase in cultivated land area globally has been due to a doubling of irrigated land. Long‐term agronomic management impacts on soil organic carbon (SOC) stocks, soil greenhouse gas (GHG) emissions, and global warming potential (GWP) in irrigated systems, however, remain relatively unknown. Here, residue and tillage management effects were quantified by measuring soil nitrous oxide (N₂O) and methane (CH₄) fluxes and SOC changes (ΔSOC) at a long‐term, irrigated continuous corn (Zea mays L.) system in eastern Nebraska, United States. Management treatments began in 2002, and measured treatments included no or high stover removal (0 or 6.8 Mg DM ha^?1 yr^?1, respectively) under no‐till (NT) or conventional disk tillage (CT) with full irrigation (n = 4). Soil N₂O and CH₄ fluxes were measured for five crop‐years (2011–2015), and ΔSOC was determined on an equivalent mass basis to ~30 cm soil depth. Both area‐ and yield‐scaled soil N₂O emissions were greater with stover retention compared to removal and for CT compared to NT, with no interaction between stover and tillage practices. Methane comprised <1% of total emissions, with NT being CH₄ neutral and CT a CH₄ source. Surface SOC decreased with stover removal and with CT after 14 years of management. When ΔSOC, soil GHG emissions, and agronomic energy usage were used to calculate system GWP, all management systems were net GHG sources. Conservation practices (NT, stover retention) each decreased system GWP compared to conventional practices (CT, stover removal), but pairing conservation practices conferred no additional mitigation benefit. Although cropping system, management equipment/timing/history, soil type, location, weather, and the depth to which ΔSOC is measured affect the GWP outcomes of irrigated systems at large, this long‐term irrigated study provides valuable empirical evidence of how management decisions can impact soil GHG emissions and surface SOC stocks.     

Phenological response to climate change in China: a meta‐analysis

The change in the phenology of plants or animals reflects the response of living systems to climate change. Numerous studies have reported a consistent earlier spring phenophases in many parts of middle and high latitudes reflecting increasing temperatures with the exception of China. A systematic analysis of Chinese phenological response could complement the assessment of climate change impact for the whole Northern Hemisphere. Here, we analyze 1263 phenological time series (1960–2011, with 20+ years data) of 112 species extracted from 48 studies across 145 sites in China. Taxonomic groups include trees, shrubs, herbs, birds, amphibians and insects. Results demonstrate that 90.8% of the spring/summer phenophases time series show earlier trends and 69.0% of the autumn phenophases records show later trends. For spring/summer phenophases, the mean advance across all the taxonomic groups was 2.75 days decade^?1 ranging between 2.11 and 6.11 days decade^?1 for insects and amphibians, respectively. Herbs and amphibians show significantly stronger advancement than trees, shrubs and insect. The response of phenophases of different taxonomic groups in autumn is more complex: trees, shrubs, herbs and insects show a delay between 1.93 and 4.84 days decade^?1, while other groups reveal an advancement ranging from 1.10 to 2.11 days decade^?1. For woody plants (including trees and shrubs), the stronger shifts toward earlier spring/summer were detected from the data series starting from more recent decades (1980s–2000s). The geographic factors (latitude, longitude and altitude) could only explain 9% and 3% of the overall variance in spring/summer and autumn phenological trends, respectively. The rate of change in spring/summer phenophase of woody plants (1960s–2000s) generally matches measured local warming across 49 sites in China (R = ?0.33, P < 0.05).     

A meta‐analysis of fertilizer‐induced soil NO and combined NO+N2O emissions

Soils are among the important sources of atmospheric nitric oxide (NO) and nitrous oxide (N₂O), acting as a critical role in atmospheric chemistry. Updated data derived from 114 peer‐reviewed publications with 520 field measurements were synthesized using meta‐analysis procedure to examine the N fertilizer‐induced soil NO and the combined NO+N₂O emissions across global soils. Besides factors identified in earlier reviews, additional factors responsible for NO fluxes were fertilizer type, soil C/N ratio, crop residue incorporation, tillage, atmospheric carbon dioxide concentration, drought and biomass burning. When averaged across all measurements, soil NO‐N fluxes were estimated to be 4.06 kg ha^?1 yr^?1, with the greatest (9.75 kg ha^?1 yr^?1) in vegetable croplands and the lowest (0.11 kg ha^?1 yr^?1) in rice paddies. Soil NO emissions were more enhanced by synthetic N fertilizer (+38%), relative to organic (+20%) or mixed N (+18%) sources. Compared with synthetic N fertilizer alone, synthetic N fertilizer combined with nitrification inhibitors substantially reduced soil NO emissions by 81%. The global mean direct emission factors of N fertilizer for NO (EF_NO) and combined NO+N₂O (EF_c) were estimated to be 1.16% and 2.58%, with 95% confidence intervals of 0.71–1.61% and 1.81–3.35%, respectively. Forests had the greatest EF_NO (2.39%). Within the croplands, the EF_NO (1.71%) and EF_c (4.13%) were the greatest in vegetable cropping fields. Among different chemical N fertilizer varieties, ammonium nitrate had the greatest EF_NO (2.93%) and EF_c (5.97%). Some options such as organic instead of synthetic N fertilizer, decreasing N fertilizer input rate, nitrification inhibitor and low irrigation frequency could be adopted to mitigate soil NO emissions. More field measurements over multiyears are highly needed to minimize the estimate uncertainties and mitigate soil NO emissions, particularly in forests and vegetable croplands.     

Climate,duration, and N placement determine N2O emissions in reduced tillage systems: a meta‐analysis

No‐tillage and reduced tillage (NT/RT) management practices are being promoted in agroecosystems to reduce erosion, sequester additional soil C and reduce production costs. The impact of NT/RT on N₂O emissions, however, has been variable with both increases and decreases in emissions reported. Herein, we quantitatively synthesize studies on the short‐ and long‐term impact of NT/RT on N₂O emissions in humid and dry climatic zones with emissions expressed on both an area‐ and crop yield‐scaled basis. A meta‐analysis was conducted on 239 direct comparisons between conventional tillage (CT) and NT/RT. In contrast to earlier studies, averaged across all comparisons, NT/RT did not alter N₂O emissions compared with CT. However, NT/RT significantly reduced N₂O emissions in experiments >10 years, especially in dry climates. No significant correlation was found between soil texture and the effect of NT/RT on N₂O emissions. When fertilizer‐N was placed at ≥5 cm depth, NT/RT significantly reduced area‐scaled N₂O emissions, in particular under humid climatic conditions. Compared to CT under dry climatic conditions, yield‐scaled N₂O increased significantly (57%) when NT/RT was implemented <10 years, but decreased significantly (27%) after ≥10 years of NT/RT. There was a significant decrease in yield‐scaled N₂O emissions in humid climates when fertilizer‐N was placed at ≥5 cm depth. Therefore, in humid climates, deep placement of fertilizer‐N is recommended when implementing NT/RT. In addition, NT/RT practices need to be sustained for a prolonged time, particularly in dry climates, to become an effective mitigation strategy for reducing N₂O emissions.     

Methane and nitrous oxide exchange in differently fertilised grassland in southern Germany

We examined the effect of fertilisation (200 kg cattle slurry-N ha^–1 year^–1) on the exchange of N₂O and CH₄ in the soil–plant system of meadow agroecosystems in southern Germany. From 1996 to 1998, we regularly determined the gas fluxes (closed chamber method) and associated environmental parameters. N₂O and CH₄ fluxes were not significantly affected by fertilisation. N₂O fluxes at the unfertilised and fertilised plots were small, generally between 50 and –20 g N m^–2 h^–1. We identified some incidents of N₂O uptake. CH₄-C fluxes ranged from 1.3 to –0.2 mg m^–2 h^–1 and were not significantly different from 0 at both plots. We budgeted an annual net emission of 15.5 and 29.6 mg m^–2 N₂O-N and an annual CH₄-C net emission of 184.2 and 122.7 mg m^–2 at the unfertilised and fertilised plots, respectively. Apparently, rapid N mineralization and uptake in the densely rooted topsoil prevents N losses and the inhibition of CH₄ oxidation.     

Methane and nitrous oxide emissions from rice paddy fields as affected by nitrogen fertilisers and water management

Methane and N₂O emissions affected by nitrogen fertilisers were measured simultaneously in rice paddy fields under intermittent irrigation in 1994. Ammonium sulphate and urea were applied at rates of 0 (control), 100 and 300 kg N ha^-1. The results showed that CH₄ emission, on the average, decreased by 42 and 60% in the ammonium sulphate treatments and 7 and 14% in the urea treatments at rates of 100 and 300 kg N ha^-1, respectively, compared to the control. N₂O emission increased significantly with the increase in the nitrogen application rate. N₂O emission was higher from ammonium sulphate treatments than from the urea treatments at the same application rate. A trade-off effect between CH₄ and N₂O emission was clearly observed. The N₂O flux was very small when the rice paddy plots were flooded, but peaked at the beginning of the disappearance of floodwater. In contrast, the CH₄ flux peaked during flooding and was significantly depressed by mid-season aeration (MSA). The results suggest that it is important to evaluate the integrative effects of water management and fertiliser application for mitigating greenhouse gas emissions in order to attenuate the greenhouse effect contributed by rice paddy fields.     

Residue incorporation depth is a controlling factor of earthworm‐induced nitrous oxide emissions

Earthworms can increase nitrous oxide (N₂O) emissions, particularly in no‐tillage systems where earthworms are abundant. Here, we study the effect of residue incorporation depth on earthworm‐induced N₂O emissions. We hypothesized that cumulative N₂O emissions decrease with residue incorporation depth, because (i) increased water filled pore space (WFPS) in deeper soil layers leads to higher denitrification rates as well as more complete denitrification; and (ii) the longer upward diffusion path increases N₂O reduction to N₂. Two 84‐day laboratory mesocosm experiments were conducted. First, we manually incorporated maize (Zea mays L.) residue at different soil depths (incorporation experiment). Second, ¹³C‐enriched maize residue was applied to the soil surface and anecic species Lumbricus terrestris (L.) and epigeic species Lumbricus rubellus (Hoffmeister) were confined to different soil depths (earthworm experiment). Residue incorporation depth affected cumulative N₂O emissions in both experiments (P < 0.001). In the incorporation experiment, N₂O emissions decreased from 4.91 mg N₂O–N kg^?1 soil (surface application) to 2.71 mg N₂O–N kg^?1 soil (40–50 cm incorporation). In the earthworm experiment, N₂O emissions from L. terrestris decreased from 3.87 mg N₂O–N kg^?1 soil (confined to 0–10 cm) to 2.01 mg N₂O–N kg^?1 soil (confined to 0–30 cm). Both experimental setups resulted in dissimilar WFPS profiles that affected N₂O dynamics. We also found significant differences in residue C recovery in soil organic matter between L. terrestris (28–41%) and L. rubellus (56%). We conclude that (i) N₂O emissions decrease with residue incorporation depth, although this effect was complicated by dissimilar WFPS profiles; and (ii) larger residue C incorporation by L. rubellus than L. terrestris indicates that earthworm species differ in their C stabilization potential. Our findings underline the importance of studying earthworm diversity in the context of greenhouse gas emissions from agro‐ecosystems.     

Mitigation of ammonia,nitrous oxide and methane emissions from manure management chains: a meta‐analysis and integrated assessment

Livestock manure contributes considerably to global emissions of ammonia (NH₃) and greenhouse gases (GHG), especially methane (CH₄) and nitrous oxide (N₂O). Various measures have been developed to mitigate these emissions, but most of these focus on one specific gas and/or emission source. Here, we present a meta‐analysis and integrated assessment of the effects of mitigation measures on NH₃, CH₄ and (direct and indirect) N₂O emissions from the whole manure management chain. We analysed the effects of mitigation technologies on NH₃, CH₄ and N₂O emissions from individual sources statistically using results of 126 published studies. Whole‐chain effects on NH₃ and GHG emissions were assessed through scenario analysis. Significant NH₃ reduction efficiencies were observed for (i) housing via lowering the dietary crude protein (CP) content (24–65%, compared to the reference situation), for (ii) external slurry storages via acidification (83%) and covers of straw (78%) or artificial films (98%), for (iii) solid manure storages via compaction and covering (61%, compared to composting), and for (iv) manure application through band spreading (55%, compared to surface application), incorporation (70%) and injection (80%). Acidification decreased CH₄ emissions from stored slurry by 87%. Significant increases in N₂O emissions were found for straw‐covered slurry storages (by two orders of magnitude) and manure injection (by 26–199%). These side‐effects of straw covers and slurry injection on N₂O emission were relatively small when considering the total GHG emissions from the manure chain. Lowering the CP content of feed and acidifying slurry are strategies that consistently reduce NH₃ and GHG emissions in the whole chain. Other strategies may reduce emissions of a specific gas or emissions source, by which there is a risk of unwanted trade‐offs in the manure management chain. Proper farm‐scale combinations of mitigation measures are important to minimize impacts of livestock production on global emissions of NH₃ and GHG.     

Nitrogen and harvest effects on soil properties under rainfed switchgrass and no‐till corn over 9 years: implications for soil quality

Nitrogen fertilizer and harvest management will alter soils under bioenergy crop production and the long‐term effects of harvest timing and residue removal remain relatively unknown. Compared to no‐tilled corn (NT‐C, Zea mays L.), switchgrass (Panicum virgatum L.) is predicted to improve soil properties [i.e. soil organic C (SOC), soil microbial biomass (SMB‐C), and soil aggregation] due to its perennial nature and deep‐rooted growth form, but few explicit field comparisons exist. We assessed soil properties over 9 years for a rainfed study of N fertilizer rate (0, 60, 120, and 180 kg N ha^?1) and harvest management on switchgrass (harvested in August and postfrost) and NT‐C (with and without 50% stover removal) in eastern NE. We measured SOC, aggregate stability, SMB‐C, bulk density (BD), pH, P and K in the top 0–30 cm. Both NT‐C and switchgrass increased SMB‐C, SOC content, and aggregate stability over the 9 years, reflecting improvement from previous conventional management. However, the soils under switchgrass had double the percent aggregate stability, 1.3 times more microbial biomass, and a 5–8% decrease in bulk density in the 0–5 and 5–10 cm depths compared to NT‐C. After 9 years, cumulative decrease in available P was significantly greater beneath NT‐C (?24.0 kg P ha^?1) compared to switchgrass (?5.4 kg P ha^?1). When all measured soil parameters were included in the Soil Management Assessment Framework (SMAF), switchgrass improved soil quality index over time (ΔSQI) in all depths. NT‐C without residue removal did not affect ΔSQI, but 50% residue removal decreased ΔSQI (0–30 cm) due to reduced aggregate stability and SMB‐C. Even with best‐management practices such as NT, corn stover removal will have to be carefully managed to prevent soil degradation. Long‐term N and harvest management studies that include biological, chemical, and physical soil measurements are necessary to accurately assess bioenergy impacts on soils.     

Soil pH as the chief modifier for regional nitrous oxide emissions: New evidence and implications for global estimates and mitigation

Nitrous oxide (N₂O) is a greenhouse gas that also plays the primary role in stratospheric ozone depletion. The use of nitrogen fertilizers is known as the major reason for atmospheric N₂O increase. Empirical bottom‐up models therefore estimate agricultural N₂O inventories using N loading as the sole predictor, disregarding the regional heterogeneities in soil inherent response to external N loading. Several environmental factors have been found to influence the response in soil N₂O emission to N fertilization, but their interdependence and relative importance have not been addressed properly. Here, we show that soil pH is the chief factor explaining regional disparities in N₂O emission, using a global meta‐analysis of 1,104 field measurements. The emission factor (EF) of N₂O increases significantly (p < .001) with soil pH decrease. The default EF value of 1.0%, according to IPCC (Intergovernmental Panel on Climate Change) for agricultural soils, occurs at soil pH 6.76. Moreover, changes in EF with N fertilization (i.e. ΔEF) is also negatively correlated (p < .001) with soil pH. This indicates that N₂O emission in acidic soils is more sensitive to changing N fertilization than that in alkaline soils. Incorporating our findings into bottom‐up models has significant consequences for regional and global N₂O emission inventories and reconciling them with those from top‐down models. Moreover, our results allow region‐specific development of tailor‐made N₂O mitigation measures in agriculture.     

Long‐term nutrient addition increases respiration and nitrous oxide emissions in a New England salt marsh

Salt marshes may act either as greenhouse gas (GHG) sources or sinks depending on hydrological conditions, vegetation communities, and nutrient availability. In recent decades, eutrophication has emerged as a major driver of change in salt marsh ecosystems. An ongoing fertilization experiment at the Great Sippewissett Marsh (Cape Cod, USA) allows for observation of the results of over four decades of nutrient addition. Here, nutrient enrichment stimulated changes to vegetation communities that, over time, have resulted in increased elevation of the marsh platform. In this study, we measured fluxes of carbon dioxide (CO₂), methane (CH₄) and nitrous oxide (N₂O) in dominant vegetation zones along elevation gradients of chronically fertilized (1,572 kg N ha^?1 year^?1) and unfertilized (12 kg N ha^?1 year^?1) experimental plots at Great Sippewissett Marsh. Flux measurements were performed using darkened chambers to focus on community respiration and excluded photosynthetic CO₂ uptake. We hypothesized that N‐replete conditions in fertilized plots would result in larger N₂O emissions relative to control plots and that higher elevations caused by nutrient enrichment would support increased CO₂ and N₂O and decreased CH₄ emissions due to the potential for more oxygen diffusion into sediment. Patterns of GHG emission supported our hypotheses. Fertilized plots were substantially larger sources of N₂O and had higher community respiration rates relative to control plots, due to large emissions of these GHGs at higher elevations. While CH₄ emissions displayed a negative relationship with elevation, they were generally small across elevation gradients and nutrient enrichment treatments. Our results demonstrate that at decadal scales, vegetation community shifts and associated elevation changes driven by chronic eutrophication affect GHG emission from salt marshes. Results demonstrate the necessity of long‐term fertilization experiments to understand impacts of eutrophication on ecosystem function and have implications for how chronic eutrophication may impact the role that salt marshes play in sequestering C and N.     

Long‐term research avoids spurious and misleading trends in sustainability attributes of no‐till

Agricultural management recommendations based on short‐term studies can produce findings inconsistent with long‐term reality. Here, we test the long‐term environmental sustainability and profitability of continuous no‐till agriculture on yield, soil water availability, and N₂O fluxes. Using a moving window approach, we investigate the development and stability of several attributes of continuous no‐till as compared to conventional till agriculture over a 29‐year period at a site in the upper Midwest, US. Over a decade is needed to detect the consistent effects of no‐till. Both crop yield and soil water availability required 15 years or longer to generate patterns consistent with 29‐year trends. Only marginal trends for N₂O fluxes appeared in this period. Relative profitability analysis suggests that after initial implementation, 86% of periods between 10 and 29 years recuperated the initial expense of no‐till implementation, with the probability of higher relative profit increasing with longevity. Importantly, statistically significant but misleading short‐term trends appeared in more than 20% of the periods examined. Results underscore the importance of decadal and longer studies for revealing consistent dynamics and emergent outcomes of no‐till agriculture, shown to be beneficial in the long term.     

Circulating trimethylamine N‐oxide and the risk of cardiovascular diseases: a systematic review and meta‐analysis of 11 prospective cohort studies

Circulating trimethylamine N‐oxide (TMAO), a canonical metabolite from gut flora, has been related to the risk of cardiovascular disorders. However, the association between circulating TMAO and the risk of cardiovascular events has not been quantitatively evaluated. We performed a systematic review and meta‐analysis of all available cohort studies regarding the association between baseline circulating TMAO and subsequent cardiovascular events. Embase and PubMed databases were searched for relevant cohort studies. The overall hazard ratios for the developing of cardiovascular events (CVEs) and mortality were extracted. Heterogeneity among the included studies was evaluated with Cochran's Q Test and I² statistics. A random‐effect model or a fixed‐effect model was applied depending on the heterogeneity. Subgroup analysis and meta‐regression were used to evaluate the source of heterogeneity. Among the 11 eligible studies, three reported both CVE and mortality outcome, one reported only CVEs and the other seven provided mortality data only. Higher circulating TMAO was associated with a 23% higher risk of CVEs (HR = 1.23, 95% CI: 1.07–1.42, I² = 31.4%) and a 55% higher risk of all‐cause mortality (HR = 1.55, 95% CI: 1.19–2.02, I² = 80.8%). Notably, the latter association may be blunted by potential publication bias, although sensitivity analysis by omitting one study at a time did not significantly change the results. Further subgroup analysis and meta‐regression did not support that the location of the study, follow‐up duration, publication year, population characteristics or the samples of TMAO affect the results significantly. Higher circulating TMAO may independently predict the risk of subsequent cardiovascular events and mortality.     

施硅对增温稻田CH4和N2O排放的影响

夜间增温幅度大于白天是气候变暖的显著特征。夜间增温影响水稻生产及CH4和N2O排放。硅是作物有益元素,施硅可提高产量,减少稻田CH4排放。增温或施硅单因子对稻田CH4和N2O排放影响已有报道,但二者耦合如何影响水稻生产及稻田CH4和N2O排放,尚不清楚。通过田间模拟试验,研究了夜间增温下施硅对水稻生长、产量及温室气体持续增温/冷却潜势和排放强度的影响。采用铝箔反光膜夜间(19:00-6:00)覆盖水稻冠层进行模拟夜间增温试验。增温设2水平,即常温对照(CK)和夜间增温(NW);施硅量设2水平,即Si0(不施硅)和Si1(钢渣硅肥,200 kgSiO2/ha)。结果表明,施硅可缓解夜间增温对水稻根系活力的抑制作用,降低夜间增温对水稻地上部、地下部干重和产量的抑制作用。夜间增温显著提高CH4累计排放量,而施硅显著降低CH4累计排放量。夜间增温下施硅处理稻田CH4累计排放量在分蘖期、拔节期、抽穗-扬花期和灌浆成熟期比未施硅处理分别低48.12%、49.16%、61.59%和39.13%。夜间增温或施硅均促进稻田N2O排放,夜间增温下施硅在上述生育期以及全生育期的累计排放量依次比对照高78.17%、51.45%、52.01%、26.14%和40.70%。研究认为,施硅可缓解夜间增温对稻田综合增温潜势和排放强度的促进作用。     

Night and day: short‐term variation in nitrogen chemistry and nitrous oxide emissions from streams

1. Diel variation in metabolism contributes to variation in oxygen (O₂) concentrations in streams. This variation in O₂ and other parameters (e.g. pH) can in turn affect the rates of microbial nitrogen (N) processing, concentrations of nitrogenous solutes and production of the greenhouse gas nitrous oxide (N₂O). We investigated diel variability in emissions of N₂O and the magnitude of short‐term variability in N solutes across 10 streams. 2. Nitrous oxide fluxes varied on average 2.3‐fold over diel cycles. Concentrations would be underestimated by sampling around noon, but N₂O fluxes would not show a consistent bias. Time‐weighted mean daily N₂O flux was strongly related to nitrate concentration (r² = 0.58). Diel patterns in N₂O and dissolved N species were often complex (rather than simple sinusoidal curves), probably reflecting complex underlying processes. 3. Reliance on samples obtained around noon would overestimate daily mean nitrate concentrations by 5% and underestimate ammonium by 32% (average bias across all streams and dates). 4. Dissolved organic N did not show consistent day–night variation. However, the magnitude of diel variability was similar to that observed for dissolved inorganic N. Organic and inorganic N concentrations were often similar. Both appear to be dynamic components of stream N budgets. 5. The Intergovernmental Panel on Climate Change (IPCC) relies upon an emission factor to estimate indirect agricultural N₂O emissions from streams and ground water. The measured emission factor (defined as the ratio of concentrations of N₂O‐N to ‐N) was typically below the recently revised IPCC default figure. Measured values varied on average 1.8‐fold over approximately 24‐h periods and were slightly higher at night than by day. The emission factor was actually highest in streams that were net sinks for N₂O, highlighting a conceptual problem in the current IPCC method. 6. Typical sampling programmes rely on daytime‐only sampling, which might cause bias in results. In our study streams, the bias was generally small. Diel variation in nitrate concentrations was related to mean temperature; variation in ammonium and N₂O concentrations was greatest at low concentrations of nitrite and ammonium.