Comment on bg-2022-56

<strong class="journal-contentHeaderColor">Abstract.</strong> Efforts to develop effective climate mitigation strategies for agriculture require methods to estimate nitrous oxide (N₂O) emissions from soil. Process-based biogeochemical models have been used for such estimations but were mainly tested with field-scale measurements. In this study, results from a short-term (43-day) factorial incubation experiment were used to investigate the ability of a process-oriented model (CoupModel) to estimate N₂O and carbon fluxes, and soil mineral nitrogen (N) dynamics. This study identified the sensitivities of model parameters when estimating three output variables using a global sensitivity analysis approach. Our results suggested that important parameters regarding N₂O flux estimates were linked to the decomposability of soil organic matter (e.g. organic C pool sizes) and the denitrification process (e.g. Michaelis constant and denitrifier respiratory rates). The model was able to simulate low-magnitude daily and cumulative N₂O fluxes with model errors (MEs) close to zero, but tended to underestimate N₂O fluxes as observed daily values increased over 0.1 g N m^-2 day^-1. Besides, the response of N₂O emissions to soil moisture was not well reflected in the model, probably related to the indirect involvement of soil moisture response function in the denitrification process. We also evaluated ancillary variables regarding N cycling, which indicates that more frequent measurements and additional types of observed data such as soil oxygen content and the microbial sources of emitted N₂O are required to further evaluate model performance and biases. The current description of the N cycling process in the model may not consistently represent the temporal scale of nitrification and denitrification processes behind N₂O emissions. The major challenges for calibration are associated with high sensitivities of denitrification parameters to initial soil moisture abiotic conditions and residue amendment. For the development of process-based models, we suggest there is a need to address soil heterogeneity, and to revisit current subroutines of moisture response functions.

NO fluxes from soils of a periodically flooded tropical savanna in southern Africa were investigated and modeled. In the laboratory, NO production rates, NO consumption rate constants, NO mixing ratios, relationships between NO emissions and soil temperature and moisture were determined for nutrient‐poor, nutrient‐rich savanna soils and a floodplain soil. The NO production rate and consumption rate constants of the floodplain soil (1.96 ng N s−1 per kilogram of soil and 2.04×10−5 m3 s−1 per kilogram of soil, respectively) were significantly higher than those of the savanna soils (average of 1.28 ng N s−1 per kilogram of soil and 1.47×10−5 m3 s−1 per kilogram of soil, respectively), but there were no significant difference between the nutrient‐rich and nutrient‐poor soils. NO flux rates increased exponentially with soil temperature. NO flux rates increased with soil moisture reaching a maximum near the field capacity (7.5–10% and 31.2% gravimetric water content for savanna and floodplain soils, respectively), after which the NO flux rate declined. These laboratory data were used in a model to estimate field NO flux rates, which were compared with actual field NO emission measurements. The NO model was modified to incorporate NO “pulsing” after the first rains of the season. Correlation between the modeled and field NO fluxes from the nutrient‐poor savanna, nutrient‐rich savanna, and the floodplain soils showed r2 values of 0.91, 0.82, and 0.74, respectively. The NO model was linked with a soil moisture and temperature model to predict annual NO emission estimates from savannas. Annual NO flux from the nutrient‐poor and nutrient‐rich savannas was estimated to be 0.16×10−3 and 0.14×10−3 kg N m−2 yr−1, respectively, which agree well with estimates from other savanna studies.

Fluxes of nitrous oxide and methane in different coastal Suaeda salsa marshes of the Yellow River estuary, China

Designing efficient nitrous oxide sampling strategies in agroecosystems using simulation models

N2O emissions from humid tropical agricultural soils: effects of soil moisture, texture and nitrogen availability

Conventional agriculture is the dominant contributor to negative environmental impacts such as the growth in global greenhouse gas (GHG) emissions, and the challenges are likely to increase with the increasing global food demand as well as the agricultural expansion. Agroforestry is a sustainable management practice with strong potential to provide ecosystem services and environmental benefits through increasing carbon sequestration, nutrient availability, water use efficiency and biodiversity, and reducing soil erosion and nitrogen losses. Therefore, the establishment of agroforestry practices offers an opportunity to reduce GHG emissions. Previous studies have showed the effects of agroforestry on soil nitrous oxide (N2O), carbon dioxide (CO2), and methane (CH4) fluxes in many parts of the world. In temperate Europe, the information on the GHG mitigation potential of agroforestry compared to cropland monoculture is still unclear. The present thesis consists of two studies, which was designed to explore whether the conversion of cropland monoculture to agroforestry systems reduces trace gases N2O, CO2, and CH4 emissions from the soil. The study was carried out at three sites varied with soil types in Germany. Each site had adjacent alley cropping agroforestry and cropland monoculture systems and the trees in agroforestry system were planted 1 to 11 years prior to this research. We measured soil N2O, CO2, and CH4 fluxes monthly using vented static chambers at the three sites from March 2018 to January 2020. On each day of gas sampling, soil temperature, water-filled pore space and extractable mineral nitrogen (N) were measured in the top 5 cm. The objective of our first study was to quantify the spatial-temporal dynamics of soil N2O fluxes from cropland agroforestry and monoculture systems, following different crop rotations and fertilization rates. The pattern of soil N2O fluxes were predominantly controlled by soil mineral N in both agroforestry and monoculture systems. The positive relationship between water-filled pore space with soil N2O fluxes during the cropping seasons, indicating soil moisture acts as a limiting factor under N-sufficient conditions. The entire agroforestry systems tended to reduce soil N2O emissions by 9% to 56% compared to monocultures, during the corn phase of the rotation that had typically high fertilization rates. The lowest soil N2O emissions in the unfertilized tree rows (occupied 20% of the agroforestry area) represent a potential for mitigating N2O emissions from croplands. The objective of our second study was to investigate the changes in soil CO2 and CH4 fluxes after conversion from cropland monoculture to alley cropping agroforestry systems. Our results showed that seasonal variations of soil CO2 and CH4 fluxes were strongly regulated by soil temperature and moisture, and the spatial variations were mainly controlled by texture. The establishment of agroforestry systems had no effect on reducing soil CO2 emissions, possibly because there was no significant difference in soil temperature between management systems. Annual soil CH4 uptake in the agroforestry systems was increased by up to 300% compared to monocultures, which may be related to the regulation of trees on soil moisture in agroforestry systems. The present research provides the first insight into the systematic comparison of soil N2O, CO2 and CH4 fluxes from cropland agroforestry and monoculture systems, and it provides a unique dataset for estimating the net balance of carbon emissions after conversion of cropland monoculture to alley cropping agroforestry system in temperate regions. Although soil CO2 emissions showed no differences between management systems, the total annual soil emissions of non-CO2 GHG from agroforestry systems were reduced by 0.22 Mg CO2 eq ha-1 compared to the monocultures. Considering the driving function of soil moisture and mineral N on soil GHG fluxes from cropland agroforestry and monoculture systems, our findings suggest that improved system management (e.g. optimal adjustments of the areal coverages between tree and crop rows) and optimized fertilizer input will enhance the potential of cropland agroforestry for mitigating N2O emissions and increasing CH4 uptake and C sequestration in the long run.

Our previous research showed large amounts of nitrous oxide (N2O) emission (>200 kg N ha−1 year−1) from agricultural peat soil. In this study, we investigated the factors influencing relatively large N2O fluxes and the source of nitrogen (N) substrate for N2O in a tropical peatland in central Kalimantan, Indonesia. Using a static chamber method, N2O and carbon dioxide (CO2) fluxes were measured in three conventionally cultivated croplands (conventional), an unplanted and unfertilized bare treatment (bare) in each cropland, and unfertilized grassland over a three-year period. Based on the difference in N2O emission from two treatments, contribution of the N source for N2O was calculated. Nitrous oxide concentrations at five depths (5–80 cm) were also measured for calculating net N2O production in soil. Annual N fertilizer application rates in the croplands ranged from 472 to 1607 kg N ha−1 year−1. There were no significant differences in between N2O fluxes in the two treatments at each site. Annual N2O emission in conventional and bare treatments varied from 10.9 to 698 and 6.55 to 858 kg N ha−1 year−1, respectively. However, there was also no significant difference between annual N2O emissions in the two treatments at each site. This suggests most of the emitted N2O was derived from the decomposition of peat. There were significant positive correlations between N2O and CO2 fluxes in bare treatment in two croplands where N2O flux was higher than at another cropland. Nitrous oxide concentration distribution in soil measured in the conventional treatment showed that N2O was mainly produced in the surface soil down to 15 cm in the soil. The logarithmic value of the ratio of N2O flux and nitrate concentration was positively correlated with water filled pore space (WEPS). These results suggest that large N2O emission in agricultural tropical peatland was caused by denitrification with high decomposition of peat. In addition, N2O was mainly produced by denitrification at high range of WFPS in surface soil.

Forests cover about 4 billion ha globally. They are important regulators of carbon dioxide (CO2) fluxes, whereas the comprehensive understanding of their overall greenhouse gas (GHG) budgets, especially for methane (CH4) and nitrous oxide (N2O), are still largely unknown.Wetland forest soils are commonly recognized as emitters of CH4, whereas upland forest soils tend to consume CH4. However, several studies demonstrate that trees can emit a large amount of CH4 especially from tree stems and substantial amounts also from canopies through poorly studied and partly unidentified aerobic processes. Moreover, tree stems can have substantial concentrations of CH4 inside, which can originate from soil or be produced by methanogens within the wood, while canopy CH4 emissions are mostly abiotic and driven by light and temperature. Thus, forest vegetation can be a significant CH4 source.Various soil microbiological, chemical and physical properties influence N2O fluxes in forests. In general, N2O emissions from tropical wetland forest soils are significantly higher than those from tropical upland forests, temperate and boreal forests. High nitrogen (N) availability, coupled with high moisture content, makes tropical peatland soils especially likely to emit N2O. Similarly, forests on drained N-rich peatland soils in temperate and boreal areas can be significant N2O sources. In temperate zone, a considerable part of such emissions appears in winter.Understanding spatial and temporal dynamics of GHG emissions is crucial for adequate modelling and mitigation of emissions in forests. In comparison with CO2 fluxes, which are clearly temperature dependent, temporal and spatial variation of soil, tree stem and canopy CH4 and N2O emissions is more complex and poorly studied. Soil N2O emissions in wetland and upland forests are mainly determined by soil moisture (soil oxygen concentration), and N2O shows bell-shaped (unimodal) dependence on soil water content. In the wet periods, stem flux of CH4 can be the main source for ecosystem exchange, whereas in the dry periods, emission from canopy adds to the total fluxes from soil and stems. N2O fluxes from the soil and stems are normally low during the dry periods and peak during the wet periods and the freeze-thaw cycles.Only a few examples are available on ecosystem-level CH4 and N2O budgets (fluxes from the soil, tree stems and shoots + eddy covariance (EC) measurements above the canopy). Nevertheless, estimation of the GHG balance in different forest ecosystems under various environmental conditions is essential for understanding their impact on the Earth&amp;#8217;s climate.In this presentation, we will bring results from ecosystem-level CH4 and N2O flux studies in forests growing on both organic and mineral soils in temperate and tropical zones.

Nitrous oxide (N2O) emissions at the air-water-sediment interfaces of cascade reservoirs in the Yunnan-Guizhou Plateau: Spatial patterns and environmental controls

Reduced tillage with residue retention and nitrogen application rate increase N2O fluxes from irrigated wheat in a subtropical floodplain soil

Nitrous oxide (N2O) flux measurements from an Irish spring barley field managed under conventional and reduced tillage and different N fertilizer rates at the Teagasc Oak Park Research Centre were made for two consecutive seasons. The aims were to investigate the efficacy of reduced tillage, reduced N fertilizer and climate change on N2O fluxes and emission factors and to study the relationship between crop yield and N-induced fluxes of N2O. The soil is a sandy loam with a pH of 7.4 and organic carbon and nitrogen content at 15 cm of 19 and 1.9 g kg-1 dry soil, respectively. Three climate scenarios, a baseline of measured climatic data from a nearby weather station and a high and low temperature sensitive scenarios predicted by the Hadley Global Climate Model were investigated. The Field-DeNitrification DeComposition (DNDC) was tested against measured nitrous oxide flux from the field, and then used to estimate future fluxes. Reduced tillage had no significant effect on N2O fluxes from soils or crop grain yield. Soil moisture and soil nitrate are the main significant factors affecting N2O flux. The derived emission factor was 0.6% of the applied N fertilizer. By reducing the applied nitrogen fertilizer by 50 % compared to the normal field rate, N2O emissions could be reduced by 57% with no significant decrease on grain yield or quality. DNDC was found suitable to estimate N2O fluxes from Irish arable soils however, underestimated the flux by 24%. Under climate change, using the high temperature increase scenario, DNDC predicted an increase in N2O emissions from both conventional and reduced tillage, ranging from 58 to 88% depending upon N application rate. In contrast annual fluxes of N2O either decreased or increased slightly in the low temperature increase scenario relative to N application (-26 to +16%). Outputs from the model indicate that elevated temperature and precipitation increase N mineralisation and total denitrification leading to greater fluxes of N2O. Annual uncertainties due to the use of two different future climate scenarios were significantly high, ranging from 74 to 95% and from 71 to 90% for the conventional and reduced tillage respectively.

Effect of the soil water dynamics on nitrous oxide emissions

Contrasting potential impact patterns of unique and shared microbial species on nitrous oxide emissions in grassland soil on the Tibetan Plateau

N2O flux short-term response to temperature and topsoil disturbance in a fertilized crop: An eddy covariance campaign

Emissions of reactive N compounds produced during terrestrial N cycling can be an important N loss pathway from ecosystems. Most measurements of this process focus on NO and N(2)O efflux; however, in alkaline soils such as those in the Mojave Desert, NH(3) production can be an important component of N gas loss. We investigated patterns of NO and NH(3) emissions in the Mojave Desert and identified seasonal changes in temperature, precipitation and spatial heterogeneity in soil nutrients as primary controllers of soil efflux. Across all seasons, NH(3) dominated reactive N gas emissions with fluxes ranging from 0.9 to 10 ng N m(-2) s(-1) as compared to NO fluxes of 0.08-1.9 ng N m(-2) s(-1). Fluxes were higher in April and July than in October; however, a fall precipitation event yielded large increases in both NO and NH(3) efflux. To explore the mechanisms driving field observations, we combined NO and NH(3) soil flux measurements with laboratory manipulations of temperature, water and nutrient conditions. These experiments showed a large transient NH(3) pulse (~70-100 ng N m(-2) s(-1)) following water addition, presumably driven by an increase in soil NH(4) (+) concentrations. This was followed by an increase in NO production, with maximum NO flux rates of 34 ng N m(-2) s(-1). Our study suggests that immediately following water addition NH(3) volatilization proceeds at high rates due to the absence of microbial competition for NH(4) (+); during this period N gas loss is insensitive to changes in temperature and soil nutrients. Subsequently, NO emission increases and rates of both NO and NH(3) emission are sensitive to temperature and nutrient constraints on microbial activity. Addition of labile C reduces gaseous N losses, presumably by increasing microbial immobilization, whereas addition of NO(3) (-) stimulates NO and NH(3) efflux.