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Methane (CH4) is the second most important atmospheric greenhouse gas (GHG) and forest soils are a significant sink for atmospheric CH4. Uptake of CH4 by global forest soils is affected by nitrogen (N) deposition; clarifying the effect of N deposition helps to reduce uncertainties of the global CH4 budget. However, it remains an unsolved puzzle why N input stimulates soil CH4 uptake in some forests while suppressing it in others. Combining previous findings and data from N addition experiments conducted in global forests, we proposed and tested a "stimulating‐suppressing‐weakened effect" ("three stages") hypothesis on the changing responses of soil CH4 flux (RCH4) to N input. Specifically, we calculated the response factors (f) of RCH4 to N input for N‐limited and N‐saturated forests across biomes; the phased changes in f values supported our hypothesis. We also estimated the global forest soil CH4 uptake budget to be approximately 11.2 Tg yr−1. CH4 uptake hotspots were predominantly located in temperate forests. Furthermore, we quantified that the current level of N deposition reduced global forest soil CH4 uptake by ∼3%. This suppression effect was more pronounced in temperate forests than in tropical or boreal forests, likely due to differences in N status. The proposed "three stages" hypothesis in this study generalizes the diverse effects of N input on RCH4, which could help improve experimental design. Additionally, our findings imply that by regulating N pollution and reducing N deposition, soil CH4 uptake can be significantly increased in the N‐saturated forests in tropical and temperate biomes. Plain Language Summary: Methane is an important greenhouse gas. Forest soils can absorb methane from the atmosphere and mitigate its warming effect. Meanwhile, forests suffer from high atmospheric nitrogen deposition, yet the effect of nitrogen on the methane uptake by forest soils remains unclear. Using data from global nitrogen addition experiments, we validated a "stimulating‐suppressing‐weakened effect" ("three stages") hypothesis, which could explain the diverse responses of soil methane flux to nitrogen input observed in different forests. On this basis, we quantified that nitrogen deposition decreased global forest soil methane uptake by approximately 3%. Our findings also imply that by regulating nitrogen pollution, soil methane uptake can be significantly increased in the nitrogen‐saturated forests in tropical and temperate biomes, potentially mitigate global warming. Key Points: A "three stage hypothesis" was developed to generalize the diverse responses of forest soil CH4 flux to N inputCH4 uptake by global forest soils was estimated to be 11.2 Tg yr−1, with N deposition suppressing ∼3% of this uptakeEffective regulation to reduce N deposition would promote CH4 uptake by N‐saturated forests and mitigate global warming [ABSTRACT FROM AUTHOR]

Nitrogen oxides (NOx) play an important role for atmospheric chemistry and radiative forcing. However, NOx emissions from the vast northern circumpolar permafrost regions have not been studied in situ due to limitations of measurement techniques. Our goals were to validate the offline analytical technique, and based on this, to widely quantify in situ NOx emissions from peatlands in the southern Eurasian permafrost region. To this end, we conducted a comparison of online and offline flux measurements in 2018 and 2019 using the synthetic air flushing, steady‐state opaque chamber method. With differences in annual average and cumulative fluxes less than 0.1 μg N m−2 h−1 and 0.01 kg N ha−1 year−1, the online and offline fluxes were in good agreement, demonstrating the feasibility of conducting offline measurements in remote regions without power supply. The flux measurements over 2 years showed obvious NOx emissions of 0.05–0.14 and 0.13–0.30 kg N ha−1 year−1 in the hollow and hummock microtopography of permafrost peatlands, respectively. The rapid expansion of alder (Alnus sibirica) in the peatlands induced by permafrost degradation significantly increased soil mineral N contents and NOx emissions depending on the age of alder (0.64–1.74 and 1.44–2.20 kg N ha−1 year−1 from the alder forests with tree ages of 1–10 years and 11–20 years, respectively). Alder expansion also intensively altered the thermal state of permafrost including the sharp increases of soil temperatures during the non‐growing season from October to April and active layer thickness. This study provides the first in situ evidences of NOx emissions from the northern circumpolar permafrost regions and uncovers the well‐documented expansion of alders can substantially stimulate NOx emissions and thus, significantly affect air quality, radiative forcing, and ecosystem productivity in the pristine regions. [ABSTRACT FROM AUTHOR]

The assessment of cropland carbon and nitrogen (C and N) balances plays a key role in identifying cost-effective mitigation measures to combat climate change and reduce environmental pollution. In this paper, a biogeochemical modelling approach is adopted to assess all C and N fluxes in a regional cropland ecosystem of Thessaly, Greece. Additionally, the estimation and quantification of the modelling uncertainty in the regional inventory are realized through the propagation of parameter distributions through the model, leading to result distributions for modelling estimations. The model was applied to a regional dataset of approximately 1000 polygons, deploying model initializations and crop rotations for the five major crop cultivations and for a time span of 8 years. The full statistical analysis on modelling results (including the uncertainty ranges given as ± values) yields for the C balance carbon input fluxes into the soil of 12.4 ± 1.4 t C ha -1 yr -1 and output fluxes of 11.9 ± 1.3 t C ha -1 yr -1 , with a resulting average carbon sequestration of 0.5 ± 0.3 t C ha -1 yr -1. The averaged N influx was 212.3 ± 9.1 kg N ha -1 yr -1 , while outfluxes of 198.3 ± 11.2 kg N ha -1 yr -1 were estimated on average. The net N accumulation into the soil nitrogen pools was estimated to be 14.0 ± 2.1 kg N ha -1 yr -1. The N outflux consists of gaseous N fluxes composed of N 2 O emissions of 2.6 ± 0.8 kg N 2 O–N ha -1 yr -1 , NO emissions of 3.2 ± 1.5 kg NO–N ha -1 yr -1 , N 2 emissions of 15.5 ± 7.0 kg N 2 –N ha -1 yr -1 and NH 3 emissions of 34.0 ± 6.7 kg NH 3 –N ha -1 yr -1 , as well as aquatic N fluxes (only nitrate leaching into surface waters) of 14.1 ± 4.5 kg NO 3 –N ha -1 yr -1 and N fluxes of N removed from the fields in yields, straw and feed of 128.8 ± 8.5 kg N ha -1 yr -1. [ABSTRACT FROM AUTHOR]

Soil microbial nitrate (NO3−) immobilization plays a vital role in enhancing the nitrogen (N) retention in the subtropical montane agricultural landscapes. However, how and why the potential microbial NO3− immobilization and the relative contribution of fungi and bacteria vary across different land use types remain still unclear in the subtropical mosaic montane agricultural landscapes. Thus, in the present study, soil gross microbial NO3− immobilization rates as well as the respective contribution of fungi and bacteria were determined throughout the whole soil profiles for three land use types (woodland, orchard, and cropland) by using the 15N tracing and amino sugar–based stable isotope probing (Amino sugars-SIP) techniques. The soil gross microbial NO3− immobilization rates in woodland soils were significantly higher than those in cropland and orchard soils across different soil layers (p < 0.05), and those of topsoil were significantly higher than those for subsoils (e.g., 20–40 cm) across different land use types (p < 0.05). Soil microbial biomass C (MBC) and N (MBN), organic C (SOC), total N (TN), and dissolved organic C (DOC) contents and C/N ratios were closely associated to gross microbial NO3− immobilization rates. Fungi played a greater role than bacteria in immobilizing soil NO3− in woodland and orchard soils, but the opposite occurred in cropland soils that over 85% of the variations in fungal and bacterial NO3− immobilization rates could be explained by their respective phospholipid fatty acid–derived (PLFA-derived) biomass. The present study indicated that afforestation may be effective to enhance soil NO3− retention in alkaline soils, thereby likely decreasing the risk of NO3− losses in subtropical mosaic montane agricultural landscapes through enhancing the soil NO3− immobilization by both fungi and bacteria. [ABSTRACT FROM AUTHOR]

Denitrification plays a critical role in soil nitrogen (N) cycling, affecting N availability in agroecosystems. However, the challenges in direct measurement of denitrification products (NO, N2O, and N2) hinder our understanding of denitrification N losses patterns across the spatial scale. To address this gap, we constructed a data‐model fusion method to map the county‐scale denitrification N losses from China's rice fields over the past decade. The estimated denitrification N losses as a percentage of N application from 2009 to 2018 were 11.8 ± 4.0% for single rice, 12.4 ± 3.7% for early rice, and 11.6 ± 3.1% for late rice. The model results showed that the spatial heterogeneity of denitrification N losses is primarily driven by edaphic and climatic factors rather than by management practices. In particular, diffusion and production rates emerged as key contributors to the variation of denitrification N losses. These findings humanize a 38.9 ± 4.8 kg N ha−1 N loss by denitrification and challenge the common hypothesis that substrate availability drives the pattern of N losses by denitrification in rice fields. [ABSTRACT FROM AUTHOR]

Maintaining or even increasing crop yields while reducing nitrous oxide (N2O) emissions is necessary to reconcile food security and climate change, while the metric of yield‐scaled N2O emission (i.e., N2O emissions per unit of crop yield) is at present poorly understood. Here we conducted a global meta‐analysis with more than 6000 observations to explore the variation patterns and controlling factors of yield‐scaled N2O emissions for maize, wheat and rice and associated potential mitigation options. Our results showed that the average yield‐scaled N2O emissions across all available data followed the order wheat (322 g N Mg−1, with the 95% confidence interval [CI]: 301–346) > maize (211 g N Mg−1, CI: 198–225) > rice (153 g N Mg−1, CI: 144–163). Yield‐scaled N2O emissions for individual crops were generally higher in tropical or subtropical zones than in temperate zones, and also showed a trend towards lower intensities from low to high latitudes. This global variation was better explained by climatic and edaphic factors than by N fertilizer management, while their combined effect predicted more than 70% of the variance. Furthermore, our analysis showed a significant decrease in yield‐scaled N2O emissions with increasing N use efficiency or in N2O emissions for production systems with cereal yields >10 Mg ha−1 (maize), 6.6 Mg ha−1 (wheat) or 6.8 Mg ha−1 (rice), respectively. This highlights that N use efficiency indicators can be used as valuable proxies for reconciling trade‐offs between crop production and N2O mitigation. For all three major staple crops, reducing N fertilization by up to 30%, optimizing the timing and placement of fertilizer application or using enhanced‐efficiency N fertilizers significantly reduced yield‐scaled N2O emissions at similar or even higher cereal yields. Our data‐driven assessment provides some key guidance for developing effective and targeted mitigation and adaptation strategies for the sustainable intensification of cereal production. [ABSTRACT FROM AUTHOR]

Anthropogenic nitrogen (N) loading alters soil ammonia‐oxidizing archaea (AOA) and bacteria (AOB) abundances, likely leading to substantial changes in soil nitrification. However, the factors and mechanisms determining the responses of soil AOA:AOB and nitrification to N loading are still unclear, making it difficult to predict future changes in soil nitrification. Herein, we synthesize 68 field studies around the world to evaluate the impacts of N loading on soil ammonia oxidizers and nitrification. Across a wide range of biotic and abiotic factors, climate is the most important driver of the responses of AOA:AOB to N loading. Climate does not directly affect the N‐stimulation of nitrification, but does so via climate‐related shifts in AOA:AOB. Specifically, climate modulates the responses of AOA:AOB to N loading by affecting soil pH, N‐availability and moisture. AOB play a dominant role in affecting nitrification in dry climates, while the impacts from AOA can exceed AOB in humid climates. Together, these results suggest that climate‐related shifts in soil ammonia‐oxidizing community maintain the N‐stimulation of nitrification, highlighting the importance of microbial community composition in mediating the responses of the soil N cycle to N loading. [ABSTRACT FROM AUTHOR]

The 15N gas flux (15NGF) method allows for direct in situ quantification of dinitrogen (N2) emissions from soils, but a successful cross-comparison with another method is missing. The objectives of this study were to quantify N2 emissions of a wheat rotation using the 15NGF method, to compare these N2 emissions with those obtained from a lysimeter-based 15N fertilizer mass balance approach, and to contextualize N2 emissions with 15N enrichment of N2 in soil air. For four sampling periods, fertilizer-derived N2 losses (15NGF method) were similar to unaccounted fertilizer N fates as obtained from the 15N mass balance approach. Total N2 emissions (15NGF method) amounted to 21 ± 3 kg N ha− 1, with 13 ± 2 kg N ha− 1 (7.5% of applied fertilizer N) originating from fertilizer. In comparison, the 15N mass balance approach overall indicated fertilizer-derived N2 emissions of 11%, equivalent to 18 ± 13 kg N ha− 1. Nitrous oxide (N2O) emissions were small (0.15 ± 0.01 kg N ha− 1 or 0.1% of fertilizer N), resulting in a large mean N2:(N2O + N2) ratio of 0.94 ± 0.06. Due to the applied drip fertigation, ammonia emissions accounted for < 1% of fertilizer-N, while N leaching was negligible. The temporal variability of N2 emissions was well explained by the δ15N2 in soil air down to 50 cm depth. We conclude the 15NGF method provides realistic estimates of field N2 emissions and should be more widely used to better understand soil N2 losses. Moreover, combining soil air δ15N2 measurements with diffusion modeling might be an alternative approach for constraining soil N2 emissions. [ABSTRACT FROM AUTHOR]

This study presents a novel plant-soil mesocosm system designed for cultivating plants over periods ranging from days to weeks while continuously measuring fluxes of N2, N2O and CO2. For proof of concept, we conducted a 33-day incubation experiment using six soil mesocosms, with three containing germinated wheat plants and three left plant-free. To validate the magnitude of N2 and N2O fluxes, we used 15N-enriched fertilizer and a 15N mass balance approach. The system inherent leakage rate was about 55 µg N m− 2 h− 1 for N2, while N2O leakage rates were below the detection limit (< 1 µg N m− 2 h− 1). In our experiment, we found higher cumulative gaseous N2 + N2O losses in sown soil (0.34 ± 0.02 g N m− 2) as compared to bare soil (0.23 ± 0.01 g N m− 2). N2 fluxes accounted for approximately 94–96% of total gaseous N losses in both planted and unplanted mesocosms. N losses, as determined by the 15N mass balance approach, were found to be 1.7 ± 0.5 g N m− 2 for the sown soil and 1.7 ± 0.6 g N m− 2 for the bare soil, indicating an inconsistency between the two assessment methods. Soil respiration rates were also higher in sown mesocosms, with cumulative soil and aboveground biomass CO2 respiration reaching 4.8 ± 0.1 and 4.0 ± 0.1 g C m− 2 over the 33-day incubation period, in sown and bare soil, respectively. Overall, this study measured the effect of wheat growth on soil denitrification, highlighting the sensitivity and utility of this advanced incubation system for such studies. [ABSTRACT FROM AUTHOR]

Nitrogen oxides (NO x ) play an important role for atmospheric chemistry and radiative forcing. However, NO x emissions from the vast northern circumpolar permafrost regions have not been studied in situ due to limitations of measurement techniques. Our goals were to validate the offline analytical technique, and based on this, to widely quantify in situ NO x emissions from peatlands in the southern Eurasian permafrost region. To this end, we conducted a comparison of online and offline flux measurements in 2018 and 2019 using the synthetic air flushing, steady-state opaque chamber method. With differences in annual average and cumulative fluxes less than 0.1 μg N m -2  h -1 and 0.01 kg N ha -1  year -1 , the online and offline fluxes were in good agreement, demonstrating the feasibility of conducting offline measurements in remote regions without power supply. The flux measurements over 2 years showed obvious NO x emissions of 0.05-0.14 and 0.13-0.30 kg N ha -1  year -1 in the hollow and hummock microtopography of permafrost peatlands, respectively. The rapid expansion of alder (Alnus sibirica) in the peatlands induced by permafrost degradation significantly increased soil mineral N contents and NO x emissions depending on the age of alder (0.64-1.74 and 1.44-2.20 kg N ha -1  year -1 from the alder forests with tree ages of 1-10 years and 11-20 years, respectively). Alder expansion also intensively altered the thermal state of permafrost including the sharp increases of soil temperatures during the non-growing season from October to April and active layer thickness. This study provides the first in situ evidences of NO x emissions from the northern circumpolar permafrost regions and uncovers the well-documented expansion of alders can substantially stimulate NO x emissions and thus, significantly affect air quality, radiative forcing, and ecosystem productivity in the pristine regions., (© 2024 John Wiley & Sons Ltd.)

Maintaining or even increasing crop yields while reducing nitrous oxide (N 2 O) emissions is necessary to reconcile food security and climate change, while the metric of yield-scaled N 2 O emission (i.e., N 2 O emissions per unit of crop yield) is at present poorly understood. Here we conducted a global meta-analysis with more than 6000 observations to explore the variation patterns and controlling factors of yield-scaled N 2 O emissions for maize, wheat and rice and associated potential mitigation options. Our results showed that the average yield-scaled N 2 O emissions across all available data followed the order wheat (322 g N Mg -1 , with the 95% confidence interval [CI]: 301-346) > maize (211 g N Mg -1 , CI: 198-225) > rice (153 g N Mg -1 , CI: 144-163). Yield-scaled N 2 O emissions for individual crops were generally higher in tropical or subtropical zones than in temperate zones, and also showed a trend towards lower intensities from low to high latitudes. This global variation was better explained by climatic and edaphic factors than by N fertilizer management, while their combined effect predicted more than 70% of the variance. Furthermore, our analysis showed a significant decrease in yield-scaled N 2 O emissions with increasing N use efficiency or in N 2 O emissions for production systems with cereal yields >10 Mg ha -1 (maize), 6.6 Mg ha -1 (wheat) or 6.8 Mg ha -1 (rice), respectively. This highlights that N use efficiency indicators can be used as valuable proxies for reconciling trade-offs between crop production and N 2 O mitigation. For all three major staple crops, reducing N fertilization by up to 30%, optimizing the timing and placement of fertilizer application or using enhanced-efficiency N fertilizers significantly reduced yield-scaled N 2 O emissions at similar or even higher cereal yields. Our data-driven assessment provides some key guidance for developing effective and targeted mitigation and adaptation strategies for the sustainable intensification of cereal production., (© 2024 John Wiley & Sons Ltd.)