Increased N2O Fluxes Research Articles

Root exudation is involved in regulation of nitrogen transformation under mowing in a temperate steppe

Nitrogen (N) transformation in soil is an important process involved in regulation of functioning of grassland ecosystems by affecting soil N availability and nitrous oxide (N2O) emission from soil. As a common grassland management practice worldwide, mowing has impacts on nutrient status in soil and nutrient cycling in grassland ecosystems. Root exudates including the organic carbon (C) and N substances are stimulated by mowing and involved in regulation of microbial community in the rhizosphere. Despite reports of mowing effects on soil nitrogen transformation and root exudation in the literature, few studies have specifically evaluated whether and how the mowing-induced root exudates are involved in mowing-induced soil N transformation in grassland ecosystems. Here, we investigated the effects of mowing on root exudation and the role of root exudates in soil N transformation by both field experiments in a temperate steppe and greenhouse experiments. We found that mowing enhanced soil net N mineralization rate (NNMR) by 3.2-fold and N2O flux by 75.0% in field experiment. We further demonstrated that mowing stimulated exudation of organic acids, amino acids, sugars, and phenolics in both field and greenhouse experiments. To explore the function of the root exudates, microcosm experiments were conducted to evaluate the roles of exuded amino acids, organic acids, sugars, and phenolics in regulating NNMR, N2O flux, enzyme activity, and functional genes related to soil N transformation by adding these chemicals to soil. We discovered that the addition of glycine enhanced NNMR mainly by enhancing the abundance of ureC, gdh, and amoA related to soil N mineralization. The addition of oxalic acid and glucose increased N2O flux mainly by increasing the abundance of nirS related to denitrification. Our findings reveal that root exuded organic acids, amino acids and sugars play an important role in the mowing-induced increases in soil N transformation. These novel findings would contribute to development of mowing-based grassland management.

Experimental Ecosystem Eutrophication Causes Offsetting Effects on Emissions of CO2, CH4, and N2O from Agricultural Reservoirs.

Despite decades of research and management efforts, eutrophication remains a persistent threat to inland waters. As nutrient pollution intensifies in the coming decades, the implications for aquatic greenhouse gas (GHG) emissions are poorly defined, particularly the responses of individual GHGs: carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O). The biogeochemical controls of each gas can differ, making it difficult to predict the overall effect of nutrient pollution on the net radiative forcing of aquatic ecosystems. Here, we induced eutrophication of small nitrogen (N)-limited agricultural reservoirs and measured changes in diffusive GHG emissions within a before-after-control-impact (BACI) study design during June to September 2021. Each gas exhibited a unique response to 300% increases in primary production, with a shift from an overall CO2 source to a sink, a modest increase in N2O flux, and, unexpectedly, no significant change in CH4 emissions. The lack of net directional change in CO2-equivalent GHG emissions in fertilized reservoirs during the summer contrasts findings from empirical studies of eutrophic lakes. Our findings illustrate the difficulty in extrapolating among different sized ecosystems and suggest that forecast 2-fold increases in agricultural N fertilization by 2050 may not result in consistently elevated GHG emissions during summer, at least from small reservoirs in continental grassland regions.

Greenhouse gases emissions from aquaculture ponds: Different emission patterns and key microbial processes affected by increased nitrogen loading

Global aquaculture production is expected to rise to meet the growing demand for food worldwide, potentially leading to increased anthropogenic greenhouse gases (GHG) emissions. As the demand for fish protein increases, so will stocking density, feeding amounts, and nitrogen loading in aquaculture ponds. However, the impact of GHG emissions and the underlying microbial processes remain poorly understood. This study investigated the GHG emission characteristics, key microbial processes, and environmental drivers underlying GHG emissions in low and high nitrogen loading aquaculture ponds (LNP and HNP). The N2O flux in HNP (43.1 ± 11.3 μmol m−2 d−1) was significantly higher than in LNP (−11.3 ± 25.1 μmol m−2 d−1), while the dissolved N2O concentration in HNP (52.8 ± 7.1 nmol L−1) was 150 % higher than in LNP (p < 0.01). However, the methane (CH4) and carbon dioxide (CO2) fluxes and concentrations showed no significant differences (p > 0.05). N2O replaced CH4 as the main source of Global Warming Potential in HNP. Pond sediments acted as a sink for N2O but a source for CH4 and CO2. The △N2O/(△N2O + △N2) in HNP (0.015 ± 0.007 %) was 7.7-fold higher than in LNP (0.002 ± 0.001 %) (p < 0.05). The chemical oxygen demand to NO2-N ratio was the most important environmental factor explaining the variability of N2O fluxes. Ammonia-oxidizing bacteria driven nitrification in water was the predominant N2O source, while comammox-driven nitrification and nosZII-driven N2O reduction in water were key processes for reducing N2O emission in LNP but decreased in HNP. The strong CH4 oxidization by Methylocystis and CO2 assimilation by algae resulted in low CH4 emissions and CO2 sink in the aquaculture pond. The Mantel test indicated that HNP increased N2O fluxes mainly through altering functional genes composition in water and sediment. Our findings suggest that there is a significant underestimation of N2O emissions without considering the significantly increased △N2O/(△N2O + △N2) caused by increased nitrogen loading.

Drainage ditches are significant sources of indirect N2O emissions regulated by available carbon to nitrogen substrates in salt-affected farmlands

Agriculture is a main source of nitrous oxide (N2O) emissions. In agricultural systems, direct N2O emissions from nitrogen (N) addition to soils have been widely investigated, whereas indirect emissions from aquatic ecosystems such as ditches are poorly known, with insufficient data available to refine the IPCC emission factor. In this contribution, in situ N2O emissions from two ditch water‒air interfaces based on a diffusion model were investigated (almost once per month) from June 2021 to December 2022 in an intensive arable catchment with high N inputs and salt-affected conditions in the Qingtongxia Irrigation District, northwestern China. Our results implied that agricultural ditches (mean 148 μg N m−2 h−1) were significant sources for N2O emissions, and were approximately 2.1 times greater than those of the Yellow River directly connected to ditches. Agronomic management strategies increased N2O fluxes in summer, while precipitation events decreased N2O fluxes. Agronomic management strategies, including fertilization (294–-540 kg N hm−2) and irrigation on farmland, resulted in enhanced diffuse N loads in drain water, whereas precipitation diluted the dissolved N2O concentration in ditches and accelerated the ditch flow rate, leading to changes in the residence time of N-containing substances in water. The spatial analysis showed that N2O fluxes (202–233 μg N m−2 h−1) in the headstream and upstream regions of ditches due to livestock and aquaculture pollution sources were relatively high compared to those in the midstream and downstream regions (100–114 μg N m−2 h−1). Furthermore, high available carbon (C) relative to N reduced N2O fluxes at low DOC:DIN ratio levels by inhibiting nitrification. Spatiotemporal variations in the N2O emission factor (EF5) across ditches with higher N resulted in lower EF5 and a large coefficient of variation (CV) range. EF5 was 0.0011 for the ditches in this region, while the EF5 (0.0025) currently adopted by the IPCC is relatively high. The EF5 variation was strongly controlled by the DOC:DIN ratio, TN, and NO3−-N, while salinity was also a nonnegligible factor regulating the EF5 variation. The regression model incorporating NO3−-N and the DOC:DIN ratio could greatly enhance the predictions of EF5 for agricultural ditches. Our study filled a key knowledge gap regarding EF5 from agricultural ditches in salt-affected farmland and offered a field investigation for refining the EF5 currently used by the IPCC.

Plant communities can attenuate flooding induced N2O fluxes by altering nitrogen cycling microbial communities and plant nitrogen uptake

Plant communities comprising species with different growth strategies and belonging to different functional groups can ensure stable productivity under variable climatic conditions. However, how plant communities can influence the response of nitrogen (N) cycling, in particular, soil microbial N cycling communities, N leaching and N2O fluxes under flooding, and their capacity to suppress flooding-induced N2O fluxes, remains unresolved. The aim of this study was to examine the effect of different plant communities composed of grasses and/or legumes on N cycling soil microorganisms and N2O fluxes, and how these effects are influenced by flooding. Our field experiment consisted of monocultures and two- and four-species mixtures of two grass and two legume species with different growth strategies (slow- and fast-growing species), grown in a fertilised sandy soil in the Netherlands. One year after plant establishment, we imposed paired control and flooding treatments for three weeks. We found that flooding significantly reduced plant N uptake and increased N2O fluxes. This increase was associated with higher abundances of N cycling microbial communities (except for ammonia-oxidising bacteria). Legume presence increased N2O fluxes, irrespective of the legume growth strategy or flooding, but this was not driven by changes in N cycling microbial communities; instead, it was related to an increase in soil nitrate availability. Mixing grasses with legumes promoted high plant N uptake and reduced N losses under control and flooded conditions, in particular when combining slow-growing species, and in the four-species mixture. Our results show that flooding exerted a strong influence on N cycling by increasing N leaching, N2O fluxes, microbial community abundances and decreasing plant N uptake. However, plant communities with slow-growing strategy had lowest relative abundance of nosZII bacteria and ameliorated flooding effects by both reducing N losses and enhancing plant N uptake.

Stimulation of soil gross nitrogen transformations and nitrous oxide emission under Free air CO2 enrichment in a mature temperate oak forest at BIFoR-FACE

Forest ecosystems are considered globally important sinks for offsetting increasing anthropogenic atmospheric carbon dioxide (CO2), however, this may be limited by the soil nutrient supply, predominantly nitrogen and phosphorus. Uncertainty remains regarding how soil N cycling in mature forests may respond to changes in carbon availability, arising from enhanced photosynthesis under elevated CO2 (eCO2) due to lack of experimental data. Further, potential positive feedbacks of nitrous oxide emissions may offset benefits of additional carbon sequestration under eCO2. The Birmingham Institute of Forest Research Free Air Carbon Enrichment experiment (BIFoR-FACE) started fumigating a mature temperate deciduous forest in 2017 at +150 ppm CO2 above ambient. Soil N cycling responses to eCO2 were investigated using the 15N pool dilution approaches to assess gross N mineralisation, immobilisation and nitrification rates, in combination with the 15N-gas flux method to quantify and source partition N2O production from 2018 to 2020 (2nd to 4th year of fumigation). Soil gross N mineralisation increased by 20% under eCO2 (6.6 μg N g−1 d−1) compared to the control treatment (5.3 μg N g−1 d−1) and despite the trends being consistent over the three years (2018–2020), the high variability between arrays reduced statistical significance except in 2019. Ammonium immobilisation by microbes increased by 20% under eCO2 (3.5 μg N g−1 d−1) as well. Overall, gross mineralisation was 4 times higher than nitrification, indicating a much higher ammonium turnover rate compared to nitrate (1.5 vs. 12 days mean residence time). N2O emission from denitrification (0.18 ng N g−1 h−1) was significantly higher under eCO2. After four years of CO2 fumigation, there are modest indications of enhanced soil N transformation rates and N availability to support the observed enhanced canopy CO2 uptake. Increased N2O fluxes under eCO2 indicated the potential for positive feedbacks on C sequestration under rising atmospheric CO2. The overall implications for C sequestration will depend on how long upregulation of soil N transformations and N bioavailability will last to meet plant demands before manifestation of N limitation, if any.

Opposite response of N2O emissions in different seasons to warming and precipitation addition on a temperate steppe

AbstractClimate change is an important issue that affects both global warming and precipitation, and the main cause is increased N2O emissions. Temperature and moisture are key factors in grassland ecosystem's response to climate change, and they can affect N2O fluxes. To clarify the impacts of warming and precipitation changes on N2O fluxes, an experiment was conducted in a semiarid steppe in Inner Mongolia, China over a nine‐year period (2011–2019). Plant productivity and soil nutrient dynamics were examined concurrently from 2017 to 2019, and N2O fluxes were monitored in response to different treatment conditions: control (C), warming (W), precipitation addition (P), and warming and precipitation addition (WP). The results showed that N2O emissions in the growing season were higher than those in the nongrowing season. Warming and precipitation addition had no significant effect on N2O fluxes compared with ambient conditions. Compared with P treatment, warming increased N2O flux in nongrowing season and decreased it in growing season. N2O flux was positively correlated with soil temperature and moisture (p &lt; 0.05). Warming had a significant positive effect on soil NH4+‐N, whereas additional precipitation had a large positive effect on soil total nitrogen and soil nitrate nitrogen. With the WP treatment, soil microbial biomass nitrogen (MBN) and soil microbial biomass carbon (MBC) increased by 53.8% and 41.9%, respectively. The decrease in N2O emissions during the growing season in the W treatment compared with the P treatment may be largely attributed to the greater dominance of Leymus chinensis. The results highlight that changes in species dominance play an important role in regulating N2O emissions, and that the N2O fluxes in the nongrowing season account for a large proportion of the changes in N2O fluxes. Therefore, The warming effects on N2O emissions during nongrowing seasons should be further investigated.

Coastal acidification alters estuarine sediment nitrous oxide and methane fluxes

AbstractThe impact of coastal acidification on sediment nitrous oxide (N2O) and methane (CH4) fluxes is largely unknown. We exposed temperate estuarine sediments to moderate (pH 7.3) and extreme (pH 6.3) acidification. Sediments were collected from two sites—one exposed to high and the other to low nitrogen loading. We demonstrate that low pH has a strong effect on greenhouse gas fluxes. The response, in terms of both magnitude and direction, was site specific. Sediments from the high nitrogen loading site exhibited increased N2O fluxes and decreased CH4 fluxes under moderate and extreme acidification. In contrast, sediments from the low nitrogen loading site exhibited decreased N2O fluxes under moderate and extreme acidification while CH4 fluxes both decreased (moderate) and increased (extreme). This study highlights the dynamic response of sediment N2O and CH4 fluxes to low pH and emphasizes the need for deeper understanding of ofcoastal acidification impacts on sediment biogeochemistry.

Nongrowing season soil nitrous oxide emissions as influenced by cover crops and fall tillage termination

Cropland soil is a major driver of global nitrous oxide (N2O) emissions. In cold climates, nongrowing season (NGS) emissions can be significant due to high fluxes during freeze–thaw (FT) cycles. Cover crops can alter key soil conditions that govern N2O-producing microbial processes, with multiple potential pathways to either increase or decrease N2O production during FT cycles. Cultivating cover crops in the fall to terminate may further disrupt these processes and the overall impact of cover crops on N2O emissions. Yet, few studies have touched on how termination practices of cover crops impact FT emissions over the NGS. Using the flux gradient method to continuously measure N2O emissions from a conventional corn–soybean rotation, we investigated the effects of summer-established cover crops (perennial ryegrass and crimson clover) (with cover crops, +CC; without cover crops, −CC) when terminated by fall cultivation (with fall cultivation, +FC; without fall cultivation, −FC) over a six-month NGS that was characterized by several freezing and thawing periods. Crimson clover cover crop was completely winterkilled, while the ryegrass survived on the +CC−FC field. Total NGS (Nov–Apr) emissions varied nearly 2.5-fold among treatments from 395.1 (−CC−FC) to 978.1 (+CC+FC) g N2O-N ha−1. Compared with the control treatment (−CC−FC), fall cultivation alone (−CC+FC) and cover crops alone (+CC−FC) increased total NGS N2O emissions, and fall cultivation with cover crops (+CC+FC) increased N2O fluxes even more. Careful CC species selection and management are important to avoid elevated NGS emissions.

Reclamation intensifies the positive effects of warming on N2O emission in an alpine meadow.

Climatic warming can alter grassland nitrous oxide (N2O) emissions due to soil property alterations. However, how the reclamation affect grassland N2O flux under warming conditions remains unclear in alpine meadow ecosystems. We conducted a long-term manipulative warming experiment in a natural alpine meadow and a cultivated grassland on the Qinghai-Tibetan Plateau to explore the separate and interactive effects of warming and reclamation on the soil N2O emission flux. N2O fluxes were measured under four treatments including control (CK), warming (W), reclamation (R) and warming under reclamation (WR) from August 2018 to July 2019. We measured the content of soil C, N nutrients and 5 enzymatic activities in 2018 and 2019. Correlation analysis and structural equation modeling were used to clarify how soil N availability and soil enzyme activities affect N2O emission. Our results indicated that compared to the ambient conditions for the growing and non-growing seasons, soil N2O flux was significantly increased 59.1% and 152.0% by warming and 28.4% and 142.4% by reclamation, respectively. Compared with W, WR significantly increased N2O flux by 18.9% and 81.1% during the growing and non-growing seasons, respectively. Soil moisture was negatively correlated to enzymatic activity and N2O flux. Both warming and reclamation promoted soil nitrification by increasing related enzymatic activities that acted to increase the N2O flux. Reclamation resulted in a greater sensitivity of the activity of ammonia monooxygenase and hydroxylamine oxidoreductase to warming, thus enhancing the effects of warming on increasing the N2O flux. Our research indicated that reclamation can additionally increase the effects of warming on N2O emissions for alpine meadows. Therefore, excessive expansion of arable land should be avoided, and new reclamation sites should be planned scientifically, as warming is expected to intensify in the future.

Changes in precipitation regime lead to acceleration of the N cycle and dramatic N2O emission

Alpine meadows on the Qinghai-Tibetan Plateau are sensitive to climate change. The precipitation regime in this region has undergone major changes, “repackaging” precipitation from more frequent, smaller events to less frequent, larger events. Nitrous oxide (N2O) is an important indicator of responses to global change in alpine meadow ecosystems. However, little information is available describing the mechanisms driving the response of N2O emissions to changes in the precipitation regime. In this study, a manipulative field experiment was conducted to investigate N2O flux, soil properties, enzyme activity, and gene abundance in response to severe and moderate changes in precipitation regime over two years. Severe changes in precipitation regime led to a 12.6-fold increase in N2O fluxes (0.0068 ± 0.0018 mg m−2 h−1) from Zoige alpine meadows relative to natural conditions (0.0005 ± 0.0029 mg m−2 h−1). In addition, severe changes in precipitation regime significantly suppressed the activities of leucine amino peptidase (LAP) and peroxidase (PEO), affected ecoenzymatic stoichiometry, and increased the abundances of gdhA, narI and nirK genes, which significantly promoted organic nitrogen (N) decomposition, denitrification, and anammox processes. The increase in abundance of these genes could be ascribed to changes in the abundance of several dominant bacterial taxa (i.e., Actinobacteria and Proteobacteria) as a result of the altered precipitation regime. Decreases in nitrate and soil moisture caused by severe changes in precipitation may exacerbate N limitation and water deficit, lead to a suppression of soil enzyme activity, and change the structure of microorganism community. The N cycle of the alpine meadow ecosystem may accelerate by increasing the abundance of key N functional genes. This would, in turn, lead to increased N2O emission. This study provided insights into how precipitation regimes changes affect N cycling, and may also improve prediction of N2O fluxes in response to changes in precipitation regime.

Soil nitrous oxide emissions after the introduction of integrated cropping systems in subtropical condition

The assessment of impacts of GHG to mitigate emissions in Brazil is a significant challenge for the expansion of integrated cropping systems. In Brazil, most studies on integrated cropping systems were conducted in tropical regions and evaluated N2O fluxes, important GHG due to its global warming potential. However, the dynamics of N2O fluxes of these systems in subtropical climate conditions in Brazil are still unclear. Thus, we investigated N2O emissions under integrated cropping systems and monoculture systems and evaluated N2O fluxes in five consolidated systems: cropland, integrated crop-forest (ICF), pasture, integrated livestock-forest (ILF), and eucalyptus. N2O emissions were monitored weekly using six manual static chambers for each agricultural system. Soil-weather variables were observed consecutively during N2O sampling. We assessed the relation between soil moisture, water-filled porous space (WFPS), rainfall, soil NO3-, and soil NH4+ with N2O. Our results showed that seasonal water availability influenced N2O fluxes in all five systems. Fertilization with N increased N2O daily fluxes in cropland and ICF (N2O maximum from 30 to 50 μg N m−2 h−1). However, cumulative N2O in the second season was lower than the first season to all evaluated systems. Cropland, ICF, and eucalyptus showed an increase of more than 50% of cumulative N2O emissions compared to the dry to the rainy season, while pasture and ILF presented an increase of more than 200% of cumulative N2O from one season to another. However, the absolute cumulative value was higher for cropland, ICF, and eucalyptus than pasture systems. Thus, the use of annual crops or just monoculture could increase N2O fluxes due to the influence of weather-soil variables. The results showed that N2O emissions were similar between ICF and ILF systems and between cropland, pasture, and eucalyptus. Therefore, integrated cropping systems offer potential for reduced N2O losses to the atmosphere and may support national and international climate change initiatives to reduce GHG emissions in agriculture.

Impacts of Low Disturbance Liquid Dairy Manure Incorporation on Alfalfa Yield and Fluxes of Ammonia, Nitrous Oxide, and Methane

Surface applied liquid dairy manure application (i.e., broadcasting) after alfalfa (Medicago sativa L.) harvest is a common practice. Low disturbance manure incorporation (LDMI) may offer multiple benefits including lower ammonia (NH3), greenhouse gas (GHG) and hydrologic nutrient losses compared to broadcast. However, few studies have simultaneously quantified LDMI impacts on alfalfa yield, NH3 and greenhouse gas (GHG) fluxes. We measured NH3, nitrous oxide (N2O), and methane (CH4) fluxes for liquid dairy manure treatments applied to alfalfa plots for broadcast and LDMI over three seasons (2014 to 2016) in central Wisconsin, USA. There were minor differences in alfalfa yield and nitrogen (N) uptake across treatments and years. Shallow disk injection and aerator/band reduced NH3 loss by 95 and 52% of broadcast, respectively, however both substantially increased N2O fluxes (6 and 4.5 kg ha−1 year−1 versus 3.6 kg ha−1 year−1 for broadcast, respectively). The magnitude and timing of N2O fluxes were related to manure application and precipitation events. Average CH4 fluxes were similar among methods and increased with soil moisture after manure application. Results highlight the importance of quantitatively evaluating agri-environmental tradeoffs of LDMI versus broadcast manure application for dairy farms.

Clear-cut and forest regeneration increase soil N2O emission in Cunninghamia lanceolata plantations

Forest regeneration after clear-cut has been practiced extensively to establish a new stand. However, the effects of forest regeneration on soil N2O emission and the related changes in soil environment factors have been rarely examined in managed plantation forests. A 21-month field trial was conducted to investigate the effects of two different regeneration practices following clear-cut on soil N2O emission and related environmental factors in a Cunninghamia lanceolata plantation. The trial included three treatments, clear-cut and natural regeneration (CN), clear-cut and artificial overall regeneration (CAO), and uncut control (UC). Compared with the control, CN, and CAO increased cumulative soil N2O emissions (P < 0.05) by 76%, and 170%, respectively. Soil N2O fluxes were positively (P < 0.05) correlated with soil temperature, moisture, microbial biomass C and N, NO3−-N, water-soluble organic C and N, and NH4+-N concentrations. We conclude that clear-cut and associated natural regeneration (CN) increased N2O fluxes due to the increased soil temperature, and the increased concentrations of NO3−-N, microbial biomass C and N, and water-soluble organic C and N; and artificial overall regeneration increased soil N2O fluxes due to the increased concentrations of soil NO3−-N, microbial biomass C and N. The results highlighted that artificial overall regeneration with higher soil disturbance intensity in clear-cut stands would exacerbate soil N2O emission.

Assessing Seasonal Methane and Nitrous Oxide Emissions from Furrow-Irrigated Rice with Cover Crops

Improved irrigation management is identified as a potential mitigation option for methane (CH4) emissions from rice (Oryza sativa). Furrow-irrigated rice (FR), an alternative method to grow rice, is increasingly adopted in the Mid-South U.S. However, FR may provide a potential risk to yield performance and higher emissions of nitrous oxide (N2O). This study quantified the grain yields, CH4 and N2O emissions from three different water management practices in rice: multiple-inlet rice irrigation (MIRI), FR, and FR with cereal rye (Secale cereale) and barley (Hordeum vulgare) as preceding winter cover crops (FRCC). CH4 and N2O fluxes were measured from May to September 2019 using a static chamber technique. Grain yield from FR (11.8 Mg ha−1) and MIRI (12.0 Mg ha−1) was similar, and significantly higher than FRCC (8.5 Mg ha−1). FR and FRCC drastically reduced CH4 emissions compared to MIRI. Total seasonal CH4 emissions decreased in the order of 44 &gt; 11 &gt; 3 kg CH4-C ha−1 from MIRI, FR, and FRCC, respectively. Cumulative seasonal N2O emissions were low from MIRI (0.1 kg N2O-N ha−1) but significantly higher from FR (4.4 kg N2O-N ha−1) and FRCC (3.0 kg N2O-N ha−1). However, there was no net difference in global warming potential among FR, FRCC and MIRI. These results suggest that the increased N2O flux from furrow-irrigated rice may not greatly detract from the potential benefits that furrow-irrigation offers rice producers.

δ15N–N2O signatures in response to N fertilization in a wheat–maize rotation

The responses of δ15N–N2O isotopic signatures to nitrogen (N) fertilization in in situ soils are not yet clear in agricultural field settings. We therefore conducted a 1-year field experiment to assess how N fertilization (0 and 400 kg N ha−1 year−1, N0 and N400) affects the δ15N–N2O isotopic signatures both at soil surface and in soil profile (0–3 m) in a wheat–maize rotation in North China. N fertilization increased N2O concentrations by 18–79% in the soil profile. It increased N2O fluxes by 9 times and 1–7 times at the soil surface and 0–0.9 m soil depth, respectively. In comparison, N fertilization reduced δ15N–N2O values by 10–74% in the soil profile. It reduced δ15N–N2O isotopic signatures at the soil surface and in the soil profile by 18% and 33%, respectively. The δ15N–N2O isotopic signature was dominated by substrate δ15N signature and N isotope fractionation during cold and warm seasons, respectively. Our results indicate that denitrification steps of NO3− → N2O dominated N2O production (over 99%) in the N0 treatment and the N400 treatment during N-fertilizer-free period. Following N fertilization, the NO3− → N2O dominated N2O production (over 99%) in the soil profile, N2O flux from the soil surface was mostly derived from NO3− → N2O and less from NH4+ → N2O. Overall, denitrification was found to dominate soil N2O transformation metabolism in the wheat–maize rotation cropland in North China.

Nitrous oxide emission reduced by coated nitrate fertilizer in a cool-temperate region

Application of coated nitrate fertilizers is expected to be a possible strategy to reduce nitrous oxide (N2O) emissions from soils of croplands. To evaluate the effects of coated nitrate fertilizer application on N2O emissions from soils, N2O fluxes were measured in a carrot field from 2016 to 2017. Ammonium sulfate (AS), coated urea (CU; only in 2017), coated calcium nitrate (CC), and no-nitrogen (NN) fertilizers were investigated. No recognizable increase in N2O flux was observed for about 45 days after fertilization in all the plots in 2016. In 2017, significant simultaneous increases in N2O and nitric oxide fluxes (up to 142 and 436 µg N m−2 h−1, respectively) were found only in the AS plots in spring, indicating enhancement of N2O production via nitrification. After mid-July, the N2O fluxes increased temporarily with the increase in soil water content after rainfall in all the plots including the NN plots in 2016 (up to 183–342 µg N m−2 h−1) and 2017 (up to 56–86 µg N m−2 h−1), indicating N2O production mainly via denitrification. Cumulative N2O emissions did not differ significantly among all the treatments in 2016, whereas that in the CC plots was significantly lower than that in the AS plots by 36% in 2017. These results suggest that the application of coated calcium nitrate fertilizers can reduce emissions of N2O produced via nitrification and can thus reduce cumulative N2O emissions in fields where nitrification is the major N2O production process in the soil.

Nitrogen turnover and N2O production in incubated soils after receiving field applications of liquid manure and nitrification inhibitors

Applying abundant manure to soils can accelerate nitrogen (N) transformations and nitrous oxide (N2O) emissions. We conducted a laboratory incubation to examine the turnover of labile N in manured soils. Soils were collected from agricultural fields that had recently received spring-injected liquid dairy manure with or without admixing nitrification inhibitors. Bands and interbands of the manure plots were incubated separately. Time courses of ammonium (NH4+) and nitrate (NO3−) were used to derive and contrast zero-, first-, and second-order kinetics models. We found that nitrification rates were consistently better represented by first-order kinetics (k1). Furthermore, across all evaluated soils, the dependency of nitrification rate (k1 of NH4+) on initial NH4+ concentration was modelled by Michaelis–Menten kinetics reasonably well, with an affinity (Km) of 63 mg N·kg−1 soil (R2 = 0.82). Compared with the manure interbands, the initially NH4-enriched bands had a much faster nitrification rate, with half-life for NH4+ of only 4 d and rapid k1 (0.186 versus 0.011 d−1). Soil N substrate and k1 exerted control on N2O production. Nitrous oxide production increased linearly with both measured NH4+ intensity (R2 = 0.47) and modelled k1–NH4+ (R2 = 0.48). Conversely, N2O production increased non-linearly with NO3− intensity (R2 = 0.68), where high NO3− caused a saturation plateau with a threshold of 96 mg N·kg−1·d−1 — beyond which no additional N2O was produced. During peak N transformations, measured N2O-N flux was 1.4% ± 0.3% of the inorganic N undergoing nitrification. Heavily manured soils exhibited augmented N turnover that increased N2O fluxes, but inhibitors reduced these emissions by half.

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

Nitrous oxide (N2O) emissions were measured in irrigated wheat (Triticum aestivum) in an annual wheat- mungbean (Vigna radiata)-rice (Oryza sativa L) rotation that had been running for seven consecutive years. Effect of two soil disturbance levels (strip vs. conventional tillage; ST vs. CT both with 30 % residue retention) and three nitrogen (N) fertilizer rates (60, 100 and 140 % of the recommended N fertilizer dose, RD; called hereafter 60RD, 100RD and 140RD, respectively) were assessed. Mean N2O fluxes were about 110 % higher in ST than in CT. The rate of N fertilizer application influenced the mean and cumulative N2O fluxes with significantly higher fluxes in ST than in CT. Based on the respective maximum grain yields (CT: 140RD, 3.52 t ha−1; ST: 60RD, 3.19 t ha−1) yield-scaled N2O emissions were higher in ST than those in CT. However, tillage vs. N rate interactions showed both the highest and lowest yield-scaled N2O fluxes in ST with 140RD and 60RD, respectively. Soil microbial biomass carbon (MBC), organic carbon (SOC), total N (TN), nitrate (NO3- -N), aggregate mean weight diameter (MWD) and larger aggregate size classes (2.0‒0.85 and >2.0 mm) were significantly higher in ST and positively correlated with N2O fluxes. Our results highlight that, despite increased N2O emissions, ST with residue can trade-off emissions to improve soil macro aggregation, C sequestration and retention of N and crop yield with the lower N fertilizer and other energy inputs. Reduction of the recommended N fertilization rate could be considered if ST is adopted.

Rubber-leguminous shrub systems stimulate soil N2O but reduce CO2 and CH4 emissions

Rubber plantations represent 13 million hectares in the world. Many researchers have focused on the environmental sustainability of rubber plantations, with rubber-legume systems better approach for ameliorating soil environments. This study investigated the effect of introducing Flemingia macrophylla (a leguminous shrub) to different-aged rubber plantations on the emissions of CO2, N2O, and CH4. As trees aged in the rubber plantations, the annual emissions of CO2, N2O, and CH4 significantly decreased. The introduction of F. macrophylla to different-aged rubber plantations significantly decreased CO2 and CH4 flux but increased N2O flux. The CO2 and N2O fluxes were mainly affected by soil temperature at 10 cm depth, and CH4 flux was mainly affected by both soil water content in the 0–10 cm soil layer and soil temperature at 10 cm depth. Compared to the same-aged rubber plantations, annual total CO2 flux of young and mature rubber–F. macrophylla plantations decreased 154,000 and 64000 kg ha−1, CH4 flux decreased 0.50 and 0.78 kg ha−1 (17.0 and 26.5 kg CO2 eq ha−1), and N2O flux increased 0.15 and 0.55 kg ha−1 (44.7 and 163.9 kg CO2 eq ha−1), respectively. The rubber-leguminous shrub systems significantly improved soil organic carbon sequestration rate, relative to the same aged rubber plantations. In conclusion, the emissions of CO2, N2O and CH4 decreased as the trees aged in the rubber plantations, and rubber-leguminous shrub systems could mitigate local climate warming by reducing reduce greenhouse gas emissions and improving soil organic sequestration rate.