Animal manure application and soil organic carbon stocks: a meta‐analysis

The impact of animal manure application on soil organic carbon (SOC) stock changes is of interest for both agronomic and environmental purposes. There is a specific need to quantify SOC change for use in national greenhouse gas (GHG) emission inventories. We quantified the response of SOC stocks to manure application from a large worldwide pool of individual studies and determined the impact of explanatory factors such as climate, soil properties, land use and manure characteristics. Our study is based on a meta‐analysis of 42 research articles totaling 49 sites and 130 observations in the world. A dominant effect of cumulative manure‐C input on SOC response was observed as this factor explained at least 53% of the variability in SOC stock differences compared to mineral fertilized or unfertilized reference treatments. However, the effects of other determining factors were not evident from our data set. From the linear regression relating cumulative C inputs and SOC stock difference, a global manure‐C retention coefficient of 12% ± 4 (95% Confidence Interval, CI) could be estimated for an average study duration of 18 years. Following an approach comparable to the Intergovernmental Panel on Climate Change, we estimated a relative SOC change factor of 1.26 ± 0.14 (95% CI) which was also related to cumulative manure‐C input. Our results offer some scope for the refinement of manure retention coefficients used in crop management guidelines and for the improvement of SOC change factors for national GHG inventories by taking into account manure‐C input. Finally, this study emphasizes the need to further document the long‐term impact of manure characteristics such as animal species, especially pig and poultry, and manure management systems, in particular liquid vs. solid storage.     

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.     

Effects of straw carbon input on carbon dynamics in agricultural soils: a meta‐analysis

Straw return has been widely recommended as an environmentally friendly practice to manage carbon (C) sequestration in agricultural ecosystems. However, the overall trend and magnitude of changes in soil C in response to straw return remain uncertain. In this meta‐analysis, we calculated the response ratios of soil organic C (SOC) concentrations, greenhouse gases (GHGs) emission, nutrient contents and other important soil properties to straw addition in 176 published field studies. Our results indicated that straw return significantly increased SOC concentration by 12.8 ± 0.4% on average, with a 27.4 ± 1.4% to 56.6 ± 1.8% increase in soil active C fraction. CO₂ emission increased in both upland (27.8 ± 2.0%) and paddy systems (51.0 ± 2.0%), while CH₄ emission increased by 110.7 ± 1.2% only in rice paddies. N₂O emission has declined by 15.2 ± 1.1% in paddy soils but increased by 8.3 ± 2.5% in upland soils. Responses of macro‐aggregates and crop yield to straw return showed positively linear with increasing SOC concentration. Straw‐C input rate and clay content significantly affected the response of SOC. A significant positive relationship was found between annual SOC sequestered and duration, suggesting that soil C saturation would occur after 12 years under straw return. Overall, straw return was an effective means to improve SOC accumulation, soil quality, and crop yield. Straw return‐induced improvement of soil nutrient availability may favor crop growth, which can in turn increase ecosystem C input. Meanwhile, the analysis on net global warming potential (GWP) balance suggested that straw return increased C sink in upland soils but increased C source in paddy soils due to enhanced CH₄ emission. Our meta‐analysis suggested that future agro‐ecosystem models and cropland management should differentiate the effects of straw return on ecosystem C budget in upland and paddy soils.     

Effect of ammonium and nitrate application on the NO and N2O emission out of different soils

The effect of nitrate and ammonium application (0, 50, 100 and 150 mg N kg^-1 soil) was studied in an incubation experiment. Four Belgian soils, selected for different soil characteristics, were used. The application of both nitrate and ammonium caused an increase of the NO and N₂O emission. The NO production from nitrate and ammonium was found to be of the same order of magnitude. At low pH the NO production was found to be highest from nitrate, at higher pH values the production was found to be higher from ammonium. This seems to be the result of the negative effect of low pH on nitrification.The ANOVA analysis was carried out to separate the effect of the form of nitrogen, quantily of N applied and soil characteristics. The total production of NO was found to depend for 97% on the soil characteristics and for 3% on the quantity of N added. The total N₂O production depended for 100% on the soil characteristics.Stepwise regression analysis showed that the total NO production was best predicted by a combination of the factors CaCO₃ content and NH₄ ⁺ concentration in the soil. Total N₂O production was best described by a combination of CaCO₃, water soluble carbon (WSC) and sand-content.The N₂O/NO ratio was found to be highly variable, indicating that their productions react differently to changes in conditions, or are partly independent.It may be concluded that to NO and N₂O from soils both nitrification and denitrification may be equally important, their relative importance depending on local conditions such as substrate availability, water content of the soil etc. However, the NO production seems to be more nitrification dependent than the N₂O production. ei]{gnE}{fnMerckx}{edSection editor}     

CO2 uptake is offset by CH4 and N2O emissions in a poplar short‐rotation coppice

The need for renewable energy sources will lead to a considerable expansion in the planting of dedicated fast‐growing biomass crops across Europe. These are commonly cultivated as short‐rotation coppice (SRC), and currently poplar (Populus spp.) is the most widely planted. In this study, we report the greenhouse gas (GHG) fluxes of carbon dioxide (CO₂), methane (CH₄) and nitrous oxide (N₂O) measured using eddy covariance technique in an SRC plantation for bioenergy production. Measurements were made during the period 2010–2013, that is, during the first two rotations of the SRC. The overall GHG balance of the 4 years of the study was an emission of 1.90 (±1.37) Mg CO₂eq ha^?1; this indicated that soil trace gas emissions offset the CO₂ uptake by the plantation. CH₄ and N₂O contributed almost equally to offset the CO₂ uptake of ?5.28 (±0.67) Mg CO₂eq ha^?1 with an overall emission of 3.56 (±0.35) Mg CO₂eq ha^?1 of N₂O and of 3.53 (±0.85) Mg CO₂eq ha^?1 of CH₄. N₂O emissions mostly occurred during one single peak a few months after the site was converted to SRC; this peak comprised 44% of the total N₂O loss during the two rotations. Accurately capturing emission events proved to be critical for deriving correct estimates of the GHG balance. The nitrogen (N) content of the soil and the water table depth were the two drivers that best explained the variability in N₂O and CH_4, respectively. This study underlines the importance of the ‘non‐CO₂ GHGs’ on the overall balance. Further long‐term investigations of soil trace gas emissions should monitor the N content and the mineralization rate of the soil, as well as the microbial community, as drivers of the trace gas emissions.     

Terrestrial N2O emissions and related functional genes under climate change: A global meta‐analysis

Nitrous oxide (N₂O) emissions from soil contribute to global warming and are in turn substantially affected by climate change. However, climate change impacts on N₂O production across terrestrial ecosystems remain poorly understood. Here, we synthesized 46 published studies of N₂O fluxes and relevant soil functional genes (SFGs, that is, archaeal amoA, bacterial amoA, nosZ, narG, nirK and nirS) to assess their responses to increased temperature, increased or decreased precipitation amounts, and prolonged drought (no change in total precipitation but increase in precipitation intervals) in terrestrial ecosystem (i.e. grasslands, forests, shrublands, tundra and croplands). Across the data set, temperature increased N₂O emissions by 33%. However, the effects were highly variable across biomes, with strongest temperature responses in shrublands, variable responses in forests and negative responses in tundra. The warming methods employed also influenced the effects of temperature on N₂O emissions (most effectively induced by open‐top chambers). Whole‐day or whole‐year warming treatment significantly enhanced N₂O emissions, but daytime, nighttime or short‐season warming did not have significant effects. Regardless of biome, treatment method and season, increased precipitation promoted N₂O emission by an average of 55%, while decreased precipitation suppressed N₂O emission by 31%, predominantly driven by changes in soil moisture. The effect size of precipitation changes on nirS and nosZ showed a U‐shape relationship with soil moisture; further insight into biotic mechanisms underlying N₂O emission response to climate change remain limited by data availability, underlying a need for studies that report SFG. Our findings indicate that climate change substantially affects N₂O emission and highlights the urgent need to incorporate this strong feedback into most climate models for convincing projection of future climate change.     

Effects of biochar application on soil greenhouse gas fluxes: a meta‐analysis

Biochar application to soils may increase carbon (C) sequestration due to the inputs of recalcitrant organic C. However, the effects of biochar application on the soil greenhouse gas (GHG) fluxes appear variable among many case studies; therefore, the efficacy of biochar as a carbon sequestration agent for climate change mitigation remains uncertain. We performed a meta‐analysis of 91 published papers with 552 paired comparisons to obtain a central tendency of three main GHG fluxes (i.e., CO₂, CH₄, and N₂O) in response to biochar application. Our results showed that biochar application significantly increased soil CO₂ fluxes by 22.14%, but decreased N₂O fluxes by 30.92% and did not affect CH₄ fluxes. As a consequence, biochar application may significantly contribute to an increased global warming potential (GWP) of total soil GHG fluxes due to the large stimulation of CO₂ fluxes. However, soil CO₂ fluxes were suppressed when biochar was added to fertilized soils, indicating that biochar application is unlikely to stimulate CO₂ fluxes in the agriculture sector, in which N fertilizer inputs are common. Responses of soil GHG fluxes mainly varied with biochar feedstock source and soil texture and the pyrolysis temperature of biochar. Soil and biochar pH, biochar applied rate, and latitude also influence soil GHG fluxes, but to a more limited extent. Our findings provide a scientific basis for developing more rational strategies toward widespread adoption of biochar as a soil amendment for climate change mitigation.     

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.     

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.     

Conversion of coastal wetlands,riparian wetlands,and peatlands increases greenhouse gas emissions: A global meta‐analysis

Land‐use/land‐cover change (LULCC) often results in degradation of natural wetlands and affects the dynamics of greenhouse gases (GHGs). However, the magnitude of changes in GHG emissions from wetlands undergoing various LULCC types remains unclear. We conducted a global meta‐analysis with a database of 209 sites to examine the effects of LULCC types of constructed wetlands (CWs), croplands (CLs), aquaculture ponds (APs), drained wetlands (DWs), and pastures (PASs) on the variability in CO₂, CH₄, and N₂O emissions from the natural coastal wetlands, riparian wetlands, and peatlands. Our results showed that the natural wetlands were net sinks of atmospheric CO₂ and net sources of CH₄ and N₂O, exhibiting the capacity to mitigate greenhouse effects due to negative comprehensive global warming potentials (GWPs; ?0.9 to ?8.7 t CO₂‐eq ha^?1 year^?1). Relative to the natural wetlands, all LULCC types (except CWs from coastal wetlands) decreased the net CO₂ uptake by 69.7%?456.6%, due to a higher increase in ecosystem respiration relative to slight changes in gross primary production. The CWs and APs significantly increased the CH₄ emissions compared to those of the coastal wetlands. All LULCC types associated with the riparian wetlands significantly decreased the CH₄ emissions. When the peatlands were converted to the PASs, the CH₄ emissions significantly increased. The CLs, as well as DWs from peatlands, significantly increased the N₂O emissions in the natural wetlands. As a result, all LULCC types (except PASs from riparian wetlands) led to remarkably higher GWPs by 65.4%?2,948.8%, compared to those of the natural wetlands. The variability in GHG fluxes with LULCC was mainly sensitive to changes in soil water content, water table, salinity, soil nitrogen content, soil pH, and bulk density. This study highlights the significant role of LULCC in increasing comprehensive GHG emissions from global natural wetlands, and our results are useful for improving future models and manipulative experiments.     

A global meta‐analysis of soil organic carbon response to corn stover removal

Corn (Zea mays L.) stover is a global resource used for livestock, fuel, and bioenergy feedstock, but excessive stover removal can decrease soil organic C (SOC) stocks and deteriorate soil health. Many site‐specific stover removal experiments report accrual rates and SOC stock effects, but a quantitative, global synthesis is needed to provide a scientific base for long‐term energy policy decisions. We used 409 data points from 74 stover harvest experiments conducted around the world for a meta‐analysis and meta‐regression to quantify removal rate, tillage, soil texture, and soil sampling depth effects on SOC. Changes were quantified by: (a) comparing final SOC stock differences after at least 3 years with and without stover removal and (b) calculating SOC accrual rates for both treatments. Stover removal generally reduced final SOC stocks by 8% in the upper 0–15 or 0–30 cm, compared to stover retained, irrespective of soil properties and tillage practices. A more sensitive meta‐regression analysis showed that retention increased SOC stocks within the 30–150 cm depth by another 5%. Compared to baseline values, stover retention increased average SOC stocks temporally at a rate of 0.41 Mg C ha^?1 year^?1 (statistically significant at p < 0.01 when averaged across all soil layers). Although SOC sequestration rates were lower with stover removal, with moderate (<50%) removal they can be positive, thus emphasizing the importance of site‐specific management. Our results also showed that tillage effects on SOC stocks were inconsistent due to the high variability in practices used among the experimental sites. Finally, we conclude that research and technological efforts should continue to be given high priority because of the importance in providing science‐based policy recommendations for long‐term global carbon management.     

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 emissions under no‐till farming in China: a meta‐analysis

No‐till (NT) practices are among promising options toward adaptation and mitigation of climate change. However, the mitigation effectiveness of NT depends not only on its carbon sequestration potential but also on soil‐derived CH₄ and N₂O emissions. A meta‐analysis was conducted, using a dataset involving 136 comparisons from 39 studies in China, to identify site‐specific factors which influence CH₄ emission, CH₄ uptake, and N₂O emission under NT. Comparative treatments involved NT without residue retention (NT0), NT with residue retention (NTR), compared to plow tillage (PT) with residue removed (PT0). Overall, NT0 significantly decreased CH₄ emission by ~30% (P < 0.05) compared to PT0 with an average emission 218.8 kg ha⁻¹ for rice paddies. However, the increase in N₂O emission could partly offset the benefits of the decrease in CH₄ emission under NT compared to PT0. NTR significantly enhanced N₂O emission by 82.1%, 25.5%, and 20.8% (P < 0.05) compared to PT0 for rice paddies, acid soils, and the first 5 years of the experiments, respectively. The results from categorical meta‐analysis indicated that the higher N₂O emission could be mitigated by adopting NT within alkaline soils, for long‐term duration, and with less N fertilization input when compared to PT0. In addition, the natural log (lnR) of response ratio of CH₄ and N₂O emissions under NT correlated positively (enhancing emission) with climate factors (temperature and precipitation) and negatively (reducing emission) with experimental duration, suggesting that avoiding excess soil wetness and using NT for a long term could enhance the benefits of NT. Therefore, a thorough understanding of the conditions favoring greenhouse gas(es) reductions is essential to achieving climate change mitigation and advancing food security in China.     

Benefits and trade‐offs of replacing synthetic fertilizers by animal manures in crop production in China: A meta‐analysis

Recycling of livestock manure to agricultural land may reduce the use of synthetic fertilizer and thereby enhance the sustainability of food production. However, the effects of substitution of fertilizer by manure on crop yield, nitrogen use efficiency (NUE), and emissions of ammonia (NH₃), nitrous oxide (N₂O) and methane (CH₄) as function of soil and manure properties, experimental duration and application strategies have not been quantified systematically and convincingly yet. Here, we present a meta‐analysis of these effects using results of 143 published studies in China. Results indicate that the partial substitution of synthetic fertilizers by manure significantly increased the yield by 6.6% and 3.3% for upland crop and paddy rice, respectively, but full substitution significantly decreased yields (by 9.6% and 4.1%). The response of crop yields to manure substitution varied with soil pH and experimental durations, with relatively large positive responses in acidic soils and long‐term experiments. NUE increased significantly at a moderate ratio (<40%) of substitution. NH₃ emissions were significantly lower with full substitution (62%–77%), but not with partial substitution. Emissions of CH₄ from paddy rice significantly increased with substitution ratio (SR), and varied by application rates and manure types, but N₂O emissions decreased. The SR did not significantly influence N₂O emissions from upland soils, and a relative scarcity of data on certain manure characteristic was found to hamper identification of the mechanisms. We derived overall mean N₂O emission factors (EF) of 0.56% and 0.17%, as well as NH₃ EFs of 11.1% and 6.5% for the manure N applied to upland and paddy soils, respectively. Our study shows that partial substitution of fertilizer by manure can increase crop yields, and decrease emissions of NH₃ and N₂O, but depending on site‐specific conditions. Manure addition to paddy rice soils is recommended only if abatement strategies for CH₄ emissions are also implemented.     

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 .     

Assessing the performance of the photo‐acoustic infrared gas monitor for measuring CO2, N2O,and CH4 fluxes in two major cereal rotations

Rapid, precise, and globally comparable methods for monitoring greenhouse gas (GHG) fluxes are required for accurate GHG inventories from different cropping systems and management practices. Manual gas sampling followed by gas chromatography (GC) is widely used for measuring GHG fluxes in agricultural fields, but is laborious and time‐consuming. The photo‐acoustic infrared gas monitoring system (PAS) with on‐line gas sampling is an attractive option, although it has not been evaluated for measuring GHG fluxes in cereals in general and rice in particular. We compared N₂O, CO_2, and CH₄ fluxes measured by GC and PAS from agricultural fields under the rice–wheat and maize–wheat systems during the wheat (winter), and maize/rice (monsoon) seasons in Haryana, India. All the PAS readings were corrected for baseline drifts over time and PAS‐CH₄ (PCH₄) readings in flooded rice were corrected for water vapor interferences. The PCH₄ readings in ambient air increased by 2.3 ppm for every 1000 mg cm^?3 increase in water vapor. The daily CO₂, N₂O, and CH₄ fluxes measured by GC and PAS from the same chamber were not different in 93–98% of all the measurements made but the PAS exhibited greater precision for estimates of CO₂ and N₂O fluxes in wheat and maize, and lower precision for CH₄ flux in rice, than GC. The seasonal GC‐ and PAS‐N₂O (PN₂O) fluxes in wheat and maize were not different but the PAS‐CO₂ (PCO₂) flux in wheat was 14–39% higher than that of GC. In flooded rice, the seasonal PCH₄ and PN₂O fluxes across N levels were higher than those of GC‐CH₄ and GC‐N₂O fluxes by about 2‐ and 4fold, respectively. The PAS (i) proved to be a suitable alternative to GC for N₂O and CO₂ flux measurements in wheat, and (ii) showed potential for obtaining accurate measurements of CH₄ fluxes in flooded rice after making correction for changes in humidity.     

Soil carbon stock change following afforestation in Northern Europe: a meta‐analysis

Northern Europe supports large soil organic carbon (SOC) pools and has been subjected to high frequency of land‐use changes during the past decades. However, this region has not been well represented in previous large‐scale syntheses of land‐use change effects on SOC, especially regarding effects of afforestation. Therefore, we conducted a meta‐analysis of SOC stock change following afforestation in Northern Europe. Response ratios were calculated for forest floors and mineral soils (0–10 cm and 0–20/30 cm layers) based on paired control (former land use) and afforested plots. We analyzed the influence of forest age, former land‐use, forest type, and soil textural class. Three major improvements were incorporated in the meta‐analysis: analysis of major interaction groups, evaluation of the influence of nonindependence between samples according to study design, and mass correction. Former land use was a major factor contributing to changes in SOC after afforestation. In former croplands, SOC change differed between soil layers and was significantly positive (20%) in the 0–10 cm layer. Afforestation of former grasslands had a small negative (nonsignificant) effect indicating limited SOC change following this land‐use change within the region. Forest floors enhanced the positive effects of afforestation on SOC, especially with conifers. Meta‐estimates calculated for the periods <30 years and >30 years since afforestation revealed a shift from initial loss to later gain of SOC. The interaction group analysis indicated that meta‐estimates in former land‐use, forest type, and soil textural class alone were either offset or enhanced when confounding effects among variable classes were considered. Furthermore, effect sizes were slightly overestimated if sample dependence was not accounted for and if no mass correction was performed. We conclude that significant SOC sequestration in Northern Europe occurs after afforestation of croplands and not grasslands, and changes are small within a 30‐year perspective.     

Spatially explicit estimates of N2O emissions from croplands suggest climate mitigation opportunities from improved fertilizer management

With increasing nitrogen (N) application to croplands required to support growing food demand, mitigating N₂O emissions from agricultural soils is a global challenge. National greenhouse gas emissions accounting typically estimates N₂O emissions at the country scale by aggregating all crops, under the assumption that N₂O emissions are linearly related to N application. However, field studies and meta‐analyses indicate a nonlinear relationship, in which N₂O emissions are relatively greater at higher N application rates. Here, we apply a super‐linear emissions response model to crop‐specific, spatially explicit synthetic N fertilizer and manure N inputs to provide subnational accounting of global N₂O emissions from croplands. We estimate 0.66 Tg of N₂O‐N direct global emissions circa 2000, with 50% of emissions concentrated in 13% of harvested area. Compared to estimates from the IPCC Tier 1 linear model, our updated N₂O emissions range from 20% to 40% lower throughout sub‐Saharan Africa and Eastern Europe, to >120% greater in some Western European countries. At low N application rates, the weak nonlinear response of N₂O emissions suggests that relatively large increases in N fertilizer application would generate relatively small increases in N₂O emissions. As aggregated fertilizer data generate underestimation bias in nonlinear models, high‐resolution N application data are critical to support accurate N₂O emissions estimates.     

When and where plant‐soil feedback may promote plant coexistence: a meta‐analysis

Plant‐soil feedback (PSF) theory provides a powerful framework for understanding plant dynamics by integrating growth assays into predictions of whether soil communities stabilise plant–plant interactions. However, we lack a comprehensive view of the likelihood of feedback‐driven coexistence, partly because of a failure to analyse pairwise PSF, the metric directly linked to plant species coexistence. Here, we determine the relative importance of plant evolutionary history, traits, and environmental factors for coexistence through PSF using a meta‐analysis of 1038 pairwise PSF measures. Consistent with eco‐evolutionary predictions, feedback is more likely to mediate coexistence for pairs of plant species (1) associating with similar guilds of mycorrhizal fungi, (2) of increasing phylogenetic distance, and (3) interacting with native microbes. We also found evidence for a primary role of pathogens in feedback‐mediated coexistence. By combining results over several independent studies, our results confirm that PSF may play a key role in plant species coexistence, species invasion, and the phylogenetic diversification of plant communities.