Difference In N2O Emissions Research Articles

Effects of N levels on land productivity and N2O emissions in maize-soybean relay intercropping.

Relay intercropping of maize and soybean can improve land productivity. However, the mechanism behind N2O emissions in this practice remains unclear. A two-factor randomized block field trial was conducted to reveal the mechanism of N2O emissions in a full additive maize-soybean relay intercropping. Factor A was three cropping systems - that is, monoculture maize (Zea mays L.), monoculture soybean (Glycine max L. Merr.) and maize-soybean relay intercropping. Factor B was different N supply, containing no N, reduced N and conventional N. Differences in N2O emissions, soil properties, rhizosphere bacterial communities and yield advantage were evaluated. The land equivalent ratio was 1.55-2.44, and the cumulative N2O emission ( ) was notably lower by 60.2% in intercropping than in monoculture, respectively. Reduced N declined without penalty on the yield advantages. The relay intercropping shifted soil properties - for example, soil organic matter, total N, and protease activity - and improved the soil microorganism community - for example, Proteobacteria and Acidobacteria. Intercropping reduced by directly suppressing nirS- and amoA-regulated N2O generation during soil N cycling, or nirS- and amoA-mediated soil properties shifted to reduce indirectly. Reduced N directly reduced by decreasing soil N content and reducing soil microorganism activities to alleviate N2O produced in soil N cycling. Conducting a full additive maize-soybean relay intercropping with reduced nitrogen supply provides a way to alleviate N2O emissions without the penalty on the yield advantage by changing rhizosphere bacterial communities and soil N cycling. © 2024 Society of Chemical Industry.

Interaction of rice root Fe plaque with radial oxygen loss enhances paddy-soil N2O emission by increasing •OH production and subsequently inhibiting N2O reduction

Amorphous iron (Fe) oxides are commonly deposited on rice roots, forming a distinctive Fe plaque, which has been observed as a notable hotspot for increased nitrous oxide (N2O) emission from paddy fields. However, the precise mechanisms underlying this phenomenon have remained uncertain. We hypothesized that the interaction between Fe plaque and rice root radial oxygen losses (ROL) leads to the generation of hydroxyl radical (•OH), thereby inhibiting N2O reduction and increasing N2O emission. To test this hypothesis, we transplanted rice seedlings with induced Fe plaque coating alongside those without Fe plaque coating into the same paddy soil. The results showed that paddy soil with Fe plaque-coated rice seedlings exhibited significantly higher concentrations of •OH and increased N2O emission rates, but a reduced abundance of the microbial N2O reduction gene (nosZ), compared to paddy soil with Fe plaque-free rice seedlings. These observed differences in N2O emission disappeared when we introduced an •OH scavenger into the paddy soil or shaded the rice leaves to reduce ROL. Furthermore, the supplementation of Fe (II) into the paddy soil with Fe plaque-free rice seedlings markedly increased •OH concentrations and N2O emissions. Our results demonstrate that Fe plaque, in conjunction with rice root ROL, facilitates •OH generation through the Fenton reaction. This •OH production effectively inhibits microbial N2O reduction, ultimately leading to increased N2O emissions from paddy soils. Our study uncovers a new mechanism whereby Fe plaque enhances N2O emissions from paddy soil. Minimizing Fenton reactions on Fe plaque may offer a promising strategy for reducing N2O emissions from paddy fields.

Climatic zone effects of non-native plant invasion on CH4 and N2O emissions from natural wetland ecosystems

Plant invasion can significantly alter the carbon and nitrogen cycles of wetlands, which potentially affects the emission of greenhouse gases (GHGs). The extent of these effects can vary depending on several factors, including the species of invasive plants, their growth patterns, and the climatic conditions prevailing in the wetland. Understanding the global effects of plant invasion on the emission of methane (CH4) and nitrous oxide (N2O) is crucial for the climate-smart management of wetlands. Here, we performed a global meta-analysis of 207 paired case studies that quantified the effect of non-native plant invasion on CH4 and N2O emissions in tropical/sub-tropical (TS) and temperate (TE) wetlands. The average emission rate of CH4 from the TS wetlands increased significantly from 337 to 577 kg CH4 ha−1 yr−1 in areas where native plants had been displaced by invasive plants. Similarly, in TE wetlands, the emission rates increased from 211 to 299 kg CH4 ha−1 yr−1 following the invasion of alien plant species. The increase in CH4 emissions at invaded sites was attributed to the increase in plant biomass, soil organic carbon (SOC), and soil moisture (SM). The effects of plant invasion on N2O emissions differed between TS and TE wetlands in that there was no significant effect in TS wetlands, whereas the N2O emissions reduced in TE wetlands. This difference in N2O emissions between climate zones was attributed to the depletion of NH4+ and NO3− in soils and the lower soil temperature in temperate regions. Overall, plant invasion increased the global net CH4 emissions from natural wetlands by 10.54 Tg CH4 yr−1. However, there were variations in CH4 emissions across different climatic zones, indicated by a net increase in CH4 emissions, of 9.97 and 0.57 Tg CH4 yr−1 in TS and TE wetlands, respectively. These findings highlight that plant invasion not only strongly stimulates the emission of CH4 from TS wetlands, but also suppresses N2O emissions from TE wetlands. These novel insights immensely improve our current understanding of the effects of climatic zones on biogeochemical controlling factors that influence the production of greenhouse gases (GHGs) from wetlands following plant invasion. By analyzing the specific mechanisms by which invasive plants affect GHG emissions in different climatic zones, effective strategies can be devised to reduce GHG emissions and preserve wetland ecosystems.

Biochar and Manure from Cattle Fed Biochar as Agricultural Amendments Alter CH4 Oxidation in a Gray Luvisol

Greenhouse gases (GHG) emissions from agricultural practices contribute 14% of anthropogenic emissions to the atmosphere, and novel practices to reduce these emissions, including feeding cattle a modified diet, are of interest. This study examines how additions of manure from cattle fed a regular diet or a diet supplemented with 2% biochar, and biochar at 5 or 10 Mg ha−1, impact GHG emissions in a Gray Luvisol agricultural field experiment. Emissions of CH4 and N2O were monitored, and soil samples were collected to analyze exchangeable NPKS, microbial biomass, total C and N, electrical conductivity, and pH. Wheat (Triticum aestivum) was planted, and grain yield measured. We calculated the yield-based emission factor (EFyield) and cumulative area-based GHG emissions emission factor (EFarea). The results showed an up to 98.5% change in the inhibition of CH4 oxidation from biochar-manure + biochar at 5 and 10 Mg ha−1 compared to the CT. The biochar in biochar-manure may have acted as a biocide to methanotrophs, causing a reduction in the release of CH4 over time. Yet, there were no significant differences in N2O emissions amongst treatments. Therefore, biochar-manure + biochar at 5 and 10 Mg ha−1 applications may impact total GHG emissions and improve grain productivity and protein content compared to BM alone.

Integrated rice-crayfish farming system does not mitigate the global warming potential during rice season

Integrated rice-crayfish farming system (RCS) has become increasingly popular in China. However, previous research has largely ignored the effect of trench around the paddy field on GHG emissions, which may cause inaccurate estimation of the global warming potential (GWP) from the system. This study compared the GWP between rice monoculture (RM) and RCS. The results indicated that the field of RCS had significantly lower CH4 emissions compared with RM due to lower mcrA abundance and higher pmoA abundance, while there was no difference in N2O emissions. In addition, the trench resulted in remarkably more CH4 emissions due to higher mcrA abundance and lower pmoA abundance and less N2O emissions than the field in RCS. In general, RCS seems not to mitigate GWP compared with RM due to more CH4 emissions from the trench in the current mode. Furthermore, our results indicate that RCS can reduce GWP relative to RM only when the area ratio of the trench to the system is controlled to be lower than 13.9 %.

Mitigation potential and trade-offs for nitrous oxide emissions and carbon balances of irrigated mixed-species and ryegrass-clover pastures

Grazed grasslands are an important source of N2O emissions. Grasslands also store significant amounts of carbon (C), and net changes to these amounts contribute to their net greenhouse gas budgets. We investigated whether net N2O emissions from irrigated pasture grazed by dairy cows can be reduced by planting more diverse species, compared with conventional ryegrass-clover pasture, and whether there are co-benefits for greenhouse gas reduction by net C gains in the ecosystem, or trade-offs through net C losses. Fluxes of CO2 and N2O were measured by eddy covariance, near the boundary of adjacent five-species mixed pasture (MIX) and ryegrass-white clover pasture (RyWC). A split-footprint approach was applied to separate the CO2 and N2O exchange for the two pastures. The CO2 fluxes were gap-filled with marginal distribution sampling (MDS), and the N2O fluxes with k-nearest-neighbour (kNN) regression. To enable calculation of the net ecosystem C balances (NECB) of the two pastures, non-CO2 carbon imports and exports were also determined. Annual N2O emissions from the MIX pasture were 0.14 g N m−2 yr−1 (mean of two years), compared with 0.23 g N m−2 yr−1 from the RyWC pasture. The N2O emissions accounted for 0.34% and 0.61% of the nitrogen inputs to the MIX and RyWC pasture, respectively. Combining all C gains and losses, both pastures recorded net ecosystem C gains in the first year, of 44 and 282 g C m−2 for MIX and RyWC, respectively. In the second year, both recorded losses: −207 g C m−2 for MIX and −86 g C m−2 for RyWC. Thus, in both years, the net C balance gave the RyWC pasture a considerable advantage over the MIX pasture in the net greenhouse gas balance, and this advantage was one magnitude greater than the difference in N2O emissions, when expressed in CO2-equivalent units.

Similar strong impact of N fertilizer form and soil erosion state on N2O emissions from croplands

Soil erosion affects 20% of croplands worldwide. However, understanding the effect of soil erosion on N2O emissions, which is one of the most potent greenhouse gases, is still limited. This limitation is likely because the small-scale differences in soil properties and fertility induced by erosion (i.e. ranges of erosion states) have barely been considered in studies quantifying N2O emissions from croplands. There are, however, indications that the erosion state itself strongly impacts N2O emission, similar to the N fertilizer form. Therefore, our investigations aimed to further explore these indications.We measured N2O fluxes for three years and at five sites within an erosion affected field experiment. N2O emissions were quantified using a manual chamber system. Three sites were established on a summit position (Albic Luvisol; non-eroded) but differed in N fertilizer forms (organic biogas fermented residues, calcium ammonium nitrate and a mixture of both fertilizers). Two additional sites were established on an extremely eroded soil (Calcaric Regosol) and wet depositional soil in a depression (Endogleyic Colluvic Regosol) to measure the effect of soil erosion states on N2O emissions. Both additional sites were fertilized with calcium ammonium nitrate only.In case of the non-eroded soil (summit), organic fertilization resulted in the highest cumulative N2O emission (6.2 ± 0.21 kg N2O-N/ha y−1) compared to mixed (5.5 ± 0.18 kg N2O-N/ha y−1) and mineral (1.9 ± 0.17 kg N2O-N/ha y−1) fertilization. These high emissions were probably caused by soluble C and N substrates from organic fertilizer, resulting in microbial activities favoring high N2O emissions. Regarding the erosion status, we observed the highest N2O emissions in the depositional soil (2.8 ± 0.21 kg N2O-N/ha y−1), followed by the non-eroded (1.9 ± 0.17 kg N2O-N/ha y−1) and the extremely eroded soil (0.6 ± 0.03 kg N2O-N/ha y−1). These differences in N2O emissions were mainly due to the site-specific, erosion induced differences in soil properties such as soil texture, soil organic C and total N contents and stocks, water-filled pore space and soil pH. These results indicate that soil erosion state may indeed be of similar importance, as N fertilizer form, for the magnitude of N2O emissions from croplands.

Agriculture-Induced N2O Emissions and Reduction Strategies in China.

Greenhouse gases are one of the most important factors in climate change, their emissions reduction is a global problem. Clarifying the spatial patterns of N2O, as an important component of greenhouse gases, it is of great significance. Based on the planting and breeding data of China from 2000 to 2019, this paper measures the N2O emissions of agricultural systems, and uses kernel density to explore the spatial distribution differences between the eight major economic zones. Finally, the proposed emissions reduction countermeasures are provided. The research results show that the N2O emissions of China’s agricultural system showed a trend of increasing first and then decreasing, and in 2019, the national N2O emissions were 710,300 tons, agricultural land emissions and animal husbandry emissions were the main sources of N2O emissions. The difference in N2O emissions by province was significant, the concentration trend was more prominent, and the differences of N2O emissions between provinces and regions were diverse. In order to achieve the reduction in N2O emissions, it is necessary to carry out low-carbon production of staple grains for different parts and economic zones, and focusing on low-carbon production in the Central Part and the West Part, as well as the Northeast and the Greater Southwest zones, is essential.

Productivity and global warming potential of direct seeding and transplanting in double-season rice of central China.

Labor and water scarcity requires crop establishment of double-season rice to be shifted from traditional transplanting to direct seeding. Owing to the limited thermal time, only ultrashort-duration cultivars of about 95 d can be used for direct-seeded, double-season rice (DDR) in central China. However, whether the shift in crop establishment of double-season rice can reduce greenhouse gas emissions without yield penalty remains unclear. Field experiments were conducted in Hubei province, central China with three treatments of crop establishment in the early and late seasons of 2017 and 2018. Treatments included DDR with ultrashort-duration cultivars (DDRU), transplanted double-season rice with ultrashort-duration cultivars (TDRU), or with widely grown cultivars which have short duration of about 110 d (TDRS). It was found that crop growth duration of DDRU was 6-20 days shorter than that of TDRU and TDRS, respectively. Ultrashort-duration cultivars under DDRU achieved 15.1t ha-1 of annual yield that was 9.4% higher than TDRU, and only 3.2% lower than TDRS. DDRU reduced the annual cumulative CH4 emission by 32.0-46.1%, but had no difference in N2O emission in comparison with TDRU and TDRS. The highest CO2 emission was TDRS followed by DDRU, and then TDRU. As a result, shifting from TDRU and TDRS to DDRU decreased global warming potential and yield-scaled greenhouse gas intensity by 28.9-53.2% and 20.7-63.8%, respectively. These findings suggest that DDR can be a promising alternative to labor- and water-intensive TDR in central China that offers important advantages in mitigating agricultural greenhouse gas emissions without sacrificing grain yield.

Farmers’ Participatory Alternate Wetting and Drying Irrigation Method Reduces Greenhouse Gas Emission and Improves Water Productivity and Paddy Yield in Bangladesh

In dry season paddy farming, the alternate wetting and drying (AWD) irrigation has the potential to improve water productivity and paddy production and decrease greenhouse gas (GHG), such as methane (CH4) and nitrous oxide (N2O), emissions when compared to continuous flooding (CF). Participatory on-farm trials were conducted from November 2017 to April 2018 in the Feni and Chattogram districts of Bangladesh. Total 62 farmers at Feni and 43 at Chattogram district, each location has 10 hectares of land involved in this study. We compared irrigation water and cost reductions, paddy yield, and CH4 and N2O emissions from paddy fields irrigated under AWD and CF irrigation methods. The mean results of randomly selected 30 farmers from each location showed that relative to the CF irrigation method, the AWD method reduced seasonal CH4 emissions by 47% per hectare and CH4 emission factor by 88% per hectare per day. Moreover, the AWD decreased the overall global warming potential and the intensity of GHG by 41%. At the same time, no noticeable difference in N2O emission between the two methods was observed. On the other hand, AWD method increased paddy productivity by 3% while reducing irrigation water consumption by 27% and associated costs by 24%. Ultimately it improved water productivity by 32% over the CF method.

Quality of Sediment Organic Matter Determines the Intertidal N2O Response to Global Warming

AbstractEstuaries and coasts are areas of intense biogeochemical cycling and are sensitive to global climate change. However, the effect of temperature increase on the emissions of the powerful greenhouse gas nitrous oxide (N2O) in different estuarine and coastal areas is still uncertain. In this study, we used laboratory incubation experiments to investigate increasing temperatures (12, 25, and 35°C) and tidal effects on N2O fluxes in intertidal sediments from the East China coast (ECC). Overall, the ECC acts as a net source of atmospheric N2O and exhibited considerable spatial variability over three orders of magnitude (from −0.17 to 8.4 μmol m−2 h−1). The warming promoted N2O emissions in most intertidal areas, while reducing N2O emissions at some sampling sites. In addition, the overall effect of flooding on N2O emissions changed from a positive to a negative effect with increasing temperature. By combining the sediment properties of all the sampling sites, we found that large differences in N2O emissions at the same amended floodwater nitrogen concentration were due to the quality and quantity of sediment organic matter. Sediment derived mainly from marine sources emitted more N2O than sediment derived from terrestrial sources. This suggests that the mangroves, salt marshes, and intertidal zones of estuaries have a mitigating effect on N2O emissions due to their elevated terrestrial organic matter inputs. This research improves our understanding of the impact of future global climatic changes on intertidal N2O fluxes, which can inform future studies and models and can be used to constrain intertidal N2O emissions.

城市化地区典型水体的N2O排放通量特征——以南京市为例

PDF HTML阅读 XML下载 导出引用 引用提醒 城市化地区典型水体的N2O排放通量特征——以南京市为例 DOI: 10.5846/stxb202205061253 作者: 作者单位: 作者简介: 通讯作者: 中图分类号: 基金项目: 国家自然基金青年项目（42207268）；江苏省基础计划自然科学基金青年基金项目（SBK2022044914）；南京信息工程大学科研启动经费项目（1521052201006）；大学生创新训练项目（1514072101087） N2O emissions in typical urban waterbodies: example of Nanjing Author: Affiliation: Fund Project: 摘要 | 图/表 | 访问统计 | 参考文献 | 相似文献 | 引证文献 | 资源附件 | 文章评论 摘要:内陆淡水水体是大气中N2O的重要排放源，然而目前对于内陆典型城市水体N2O排放通量的监测数据依然匮乏，典型城市水体的N2O排放特征及驱动因素尚不清楚。本研究选取了南京市江北新区的典型水体，包括湖库、河流、养殖池塘和景观池塘，在2020年5月-2021年4月利用漂浮箱法连续监测了不同水体类型的水-气界面N2O排放特征，并通过测定水环境特征，探究驱动水体N2O排放通量的关键因素。研究结果表明，典型城市水体整体均表现为N2O排放源，河流和养殖池塘的日平均排放通量最大，分别为（503±1236）μg m-2 d-1和（508±797）μg m-2 d-1，其次为景观池塘（（179±989）μg m-2 d-1），而湖库的N2O排放通量最小，仅表现为微弱的N2O排放源（（54±212）μg m-2 d-1）。水体的N2O排放呈现季节性差异，河流和养殖池塘夏季的N2O排放通量显著高于其他季节（P<0.01）。水体全年N2O排放数据与水体温度和溶解氧含量（DO）呈显著相关。而在温度较高的5月份-9月份（>20℃），氮输入成为影响N2O排放通量的关键因素（P<0.01），因此控制城市水体的氮输入尤其是在水温较高的夏季是减少N2O排放的有利措施。此外，由于水文化学条件差异等因素，小型封闭水体包括养殖池塘和景观池塘的N2O排放通量差异较大，未来应加强监测不同水体的水文化学特征和N2O的时空排放特征，探讨影响小型封闭水体水-气界面N2O排放通量的具体驱动因素。此研究为城市区域N2O排放的精准核算提供了数据支撑，为N2O排放模型的修正提供了科学依据。 Abstract:Even though inland freshwater waterbodies are important atmospheric N2O sources, but current N2O emission data from those waterbodies are still limited. Furthermore, it is unclear about N2O spatiotemporal emission characters in different urban waterbodies and corresponding drivers. This study applied floating chambers to in-situ measure N2O emission fluxes in typical urban waterbodies including reservoirs, rivers, aquaculture ponds and scenic ponds of Nanjing starting from May of 2020 to April of 2021. Furthermore, physical and chemical water parameters were also measured to elucidate main N2O emission drivers. The result revealed that all types of urban waterbodies generally showed as emission sources. Rivers and aquaculture ponds had the largest N2O flux, with average values of (503±1236) μg m-2 d-1 and (508±797) μg m-2 d-1, respectively, following by scenic ponds ((179±989) μg m-2 d-1), while reservoirs only showed as a weak N2O emission source ((54±212) μg m-2 d-1). N2O emissions from all waterbodies showed the seasonal pattern with higher N2O fluxes in summer, which is especially pronounced in rivers and aquaculture ponds (P<0.01). The annual N2O flux positively correlated with temperature and negatively correlated with dissolved oxygen (DO) (P<0.01), respectively. However, the nitrogen input was shown as the key driver for N2O emissions starting from May to September with temperature higher than 20℃. Therefore, controlling nitrogen input especially in the Summer season was an important measure for reducing N2O emissions. Furthermore, ponds showed large differences in N2O emissions resulting from different hydro-geochemical conditions and other factors. Monitoring of hydro-geochemical conditions and N2O emission fluxes for small waterbodies should be strengthened in further studies, and specific drivers for N2O emissions had to be explored. This study contributed to N2O emission estimations in the city, and provided the basis for updating N2O emission models. 参考文献 相似文献 引证文献

N2O hot moments were not driven by changes in nitrogen and carbon substrates or changes in N cycling functional genes

AbstractIf a dry soil is wetted it can produce a large flush of nitrous oxide (N2O), which can be 10 times above the background rate, this phenomenon is called a hot moment. However, there is uncertainty in the literature regarding the mechanisms that cause hot moments. Therefore, this study aimed to induce two different hot moments on soil cores to investigate changes in nitrogen (N) cycling functional genes, and correlate these changes with subsequent N2O emissions, and transformations in soil chemistry. We induced a ‘pre‐dry’ treatment which wetted soils from 50% to 90% water‐filled pore space (WFPS) and ‘pre‐wet’ treatment which wetted soils from 70% to 90% WFPS to analyse changes in gas fluxes (N2O, nitric oxide [NO] and carbon dioxide [CO2]), soil chemistry (pH, dissolved organic carbon [DOC], total oxidised nitrogen [TON], ammonium [NH4+]) and microbiology (bacterial 16S, fungal ITS and nitrogen cycling functional genes associated with denitrification). The soil produced emissions of N2O that were significantly higher for the pre‐dry treatment compared with the pre‐wet treatment (p = &lt;0.001; 9 times that of the pre‐wet). The pre‐dry treatments did not prime the soil any more than the pre‐wet treatments for CO2 and NO emissions, and the results showed that the soils TON and DOC concentrations were significantly higher in the pre‐wet conditions despite its lower N2O emissions. This suggests that the cause of the hot moment was not as dependant on the soil's substrate concentrations as previously thought. Moreover, the amount of denitrification genes in the pre‐dry soil was statistically similar to the pre‐wet (nirK, norB, nifH, nosZI, nxrA), apart from nirS which was marginally higher (p = 0.03). Contrary to other experiments, this study provides evidence that changes in soil substrates and functional gene abundance are not the cause of N2O hot moments. We therefore suggest that future studies should investigate changes in mRNA abundance related to N cycling genes.Highlights Rewetting following drought caused a significant release of N2O. The pre‐dry soil produced higher N2O fluxes but contained less key substrates (TOC, NH4+ and DOC) than the pre‐wet soil. The pre‐dry and pre‐wet soil had a similar quantity of denitrification functional genes. Differences in soil chemistry and denitrification functional genes between the pre‐dry and pre‐wet soil cannot explain the differences in N2O emissions.

End water content determines the magnitude of N2O pulse from nitrifier denitrification after rewetting a fluvo-aquic soil

Large nitrous oxide (N2O) emissions pulses have been observed after rewetting dry soil. However, few studies have uncoupled the effects of drought severity from the degree to which the soil is saturated. In this study, we conducted three aerobic incubation experiments to investigate the effects of soil rewetting on N2O emissions from a dryland soil. The results showed that, at constant soil moisture, total N2O emissions in soil with 90% water-holding capacity (WHC) were significantly higher than those in 30%, 45%, 60% and 75% WHC treatments. In the dry–wet group, the soil moisture content was adjusted from 30%, 45% and 60% WHC to the end content of 75% and 90% WHC, respectively; the cumulative N2O emissions in the 30–90%, 45–90% and 60–90% WHC nitrogen (N) treatments were significantly higher than those in the 30–75%, 45–75% and 60–75% WHC N treatments. Regarding fertilizer N types, there was no significant difference in N2O emissions from soil at 90% WHC when (NH4)2SO4 or urea was applied. Nitrification inhibitor significantly reduced N2O emissions in soil applied with NH4+-N fertilizer, indicating that nitrification played a major role in N2O emissions from soils. The contribution of denitrification was negligible, according to the low emission rate of soils with only NO3− additions. High N2O emissions occurred in soil treated with NO2−, accounting for about 83.6% of those of the NH4+ treatment. Therefore, in this study we concluded that the end water content of soil was more important than the role of drought severity in the dry-wet process and that nitrifier denitrification was probably the main pathway of N2O production under the condition of 90% WHC moisture after rewetting soil.

Evidence for microbial rather than aggregate origin of substrates fueling freeze-thaw induced N2O emissions

Soil freeze-thaw induces a pulse of nitrous oxide (N2O) emissions fueled by a concomitant increase in available organic carbon (C) and nitrogen (N) substrates. These substrates are hypothesized to originate from the disruption of aggregates and microbial biomass, but experiments designed to falsify these hypotheses have been scarce. We therefore conducted a series of column experiments using intact soil cores of silt loam and loamy sand under different freezing rates and durations, factors previously identified as controlling the magnitude of subsequent N2O emissions. We used a slower freezing rate and shorter freeze duration (6 days) as a Control, to which we compared the same freeze duration but faster freezing rate (Fast, 6 days) and the same freezing rate but longer freeze duration (Long, 21 days). All soils were frozen to ~ -2.5 °C. Upon thaw, only the silt loam emitted N2O; we therefore focused on silt loam C, N, and aggregate dynamics. Control and Fast soils emitted similar amounts of N2O, even though Fast soils maintained aggregate mean weight diameter (MWD). Long soils emitted 1.8 times more N2O than Control, even though Long and Control soils exhibited similar decreases in aggregate MWD. We suggest that because in no instance were differences in N2O emissions between treatments proportional to differences in aggregate disruption, and some soils emitted N2O without any detectable aggregate disruption, that aggregate disruption does not play a significant, universal role in providing substrates for N2O emissions. In contrast, larger N2O emissions due to longer freezing durations were associated with a delayed decrease in microbial biomass. Although we did not assess microbial communities for features other than their biomass, our observations are consistent with longer freezing durations selecting for slower growing microorganisms and freeze-thaw induced N2O emissions being fueled by microbial osmolytes.

Effects of two wood-based biochars on the fate of added fertilizer nitrogen\u2014a 15N tracing study

A 15N tracing pot experiment was conducted using two types of wood-based biochars: a regular biochar and a Kon-Tiki-produced nutrient-enriched biochar, at two application rates (1% and 5% (w/w)), in addition to a fertilizer only and a control treatment. Ryegrass was sown in pots, all of which except controls received 15N-labelled fertilizer as either 15NH4NO3 or NH415NO3. We quantified the effect of biochar application on soil N2O emissions, as well as the fate of fertilizer-derived ammonium (NH4+) and nitrate (NO3−) in terms of their leaching from the soil, uptake into plant biomass, and recovery in the soil. We found that application of biochars reduced soil mineral N leaching and N2O emissions. Similarly, the higher biochar application rate of 5% significantly increased aboveground ryegrass biomass yield. However, no differences in N2O emissions and ryegrass biomass yields were observed between regular and nutrient-enriched biochar treatments, although mineral N leaching tended to be lower in the nutrient-enriched biochar treatment than in the regular biochar treatment. The 15N analysis revealed that biochar application increased the plant uptake of added nitrate, but reduced the plant uptake of added ammonium compared to the fertilizer only treatment. Thus, the uptake of total N derived from added NH4NO3 fertilizer was not affected by the biochar addition, and cannot explain the increase in plant biomass in biochar treatments. Instead, the increased plant biomass at the higher biochar application rate was attributed to the enhanced uptake of N derived from soil. This suggests that the interactions between biochar and native soil organic N may be important determinants of the availability of soil N to plant growth.

Enhanced N2O emission rate in field soil undergoing conventional intensive fertilization is attributed to the shifts of denitrifying guilds

The emission of greenhouse gas N2O in agricultural soil is modulated by N fertilization that could be converted to N2O by denitrifiers under anaerobic condition. Nevertheless, the effect of denitrifiers on N2O emission has not been thoroughly elucidated. In this study, we explored the denitrifying gas kinetics, nitrate content, transcribed denitrifying functional genes (narG, nirS, nirK, and nosZ), and the active bacteria during anaerobic incubation of soils with conventional intensive N fertilization (CNS) and reduced N fertilization (RNS), both sampled from a vegetable greenhouse experimental site. The CNS sample showed significantly higher N2O emission rates relative to the RNS sample. However, the difference in N2O emission between the soils was neither because of the cumulative nitrate content nor the quantity of denitrifying gene transcripts. The distinct fertilization regimes shaped the significantly different bacterial communities in these soils. The absolute abundance of bacteria that produce N2O but lack the nosZ gene for N2O reduction (for example, the dominant Kaistobacter) was higher in CNS than in RNS. Meanwhile, the abundance of two operational taxonomy units (OTUs), namely Rhodanobacter, belonging to the most abundant genus in denitrifying guilds, was strongly enriched in CNS and showed significant positive correlation with N2O/(N2O + N2). The predominance of these bacterial OTUs in the CNS denitrifying guild strongly suggested that high N2O emission from the soil with long-term conventional intensive fertilization might be primarily attributed to the reshaping of distinct denitrifying guilds in their bacterial communities.

Crop residues contribute minimally to spring-thaw nitrous oxide emissions under contrasting tillage and crop rotations

Crop residues are sources of carbon and nitrogen (N) after harvest, releasing inorganic N through mineralization or protecting soil N through immobilization. Inorganic N controls nitrous oxide (N2O) emissions, a potent greenhouse gas (GHG) from agriculture. Hence, crop residues are accounted for as N2O sources in national GHG inventories. For locations where post-harvest N2O emissions occurs due to freeze-thaw, it is not known if crop residues contribute to emissions, and if tillage or residue type impact this contribution. This is of concern since crop residue and freeze-thaw emission factors (EF) may be ‘double-counting’ N2O sources. We conducted an experiment over two non-growing seasons (NGS) in a long-term corn, soybean and winter wheat trial to (i) compare N2O emissions for different crop residues within simple or diverse crop rotations under no-tillage (NT) or conventional tillage (CT); (ii) determine the importance of above- and below-ground residue addition to spring-thaw N2O emissions as affected by rotation and tillage. A15N residue enrichment study was used to directly trace above- and below-ground residue 15N into 15N2O fluxes and derive EF. Higher N2O emissions were observed for CT than NT, regardless of rotation. Soybeans induced higher N2O emissions than corn residue and the same crop residue (e.g. corn or soybean) showed trends of higher N2O in the long-term diverse rotation. In all cases, crop residues contributed minimally to spring-thaw N2O emissions (<2%), meaning differences in N2O emissions were due to tillage and rotation effects on soil N availability, rather than by affecting crop residue N release for N2O production. The NGS 6-month EF for crop residues never surpassed 0.05%, a minimal fraction of the annual 0.6% recommended EF. Refinement in emission inventories for cold climates should focus on freeze-thaw substrate release for N2O production from other sources than crop residue.

Effects of cover crops on soil CO2 and N2O emissions across topographically diverse agricultural landscapes in corn-soybean-wheat organic transition

Agriculture represents about 10 % of total greenhouse gases (GHG) emission in the US, in which carbon dioxide (CO2) and nitrous oxide (N2O) can greatly contribute to global warming and climate change. Organic farming has been increasingly popular as it provides long-term environmental benefits including GHG mitigation. Judicious use of cover crops in crop rotation systems has been considered an effective approach to sustain crop productivity during organic transition period. Here, we examined effects of cover crops on soil CO2 and N2O fluxes after the first year of organic transition in corn-soybean-wheat rotation across three contrasting topographies, particularly depression, slope, and summit at Kellogg Biological Station located in Southwest Michigan, USA. Three cover cropping systems used in this study were (1) cereal rye; (2) a mixture of cold susceptible species, namely, oat, winter pea, and radish, frost-killed in winter (WK); and (3) a winter hardy mixture of cold tolerant species, namely annual ryegrass, Dwarf Essex rapeseed, and crimson clover (WH). We also examined the effect of the size of incorporated plant residue fragments on soil CO2 and N2O emissions. Soil GHG emissions were the lowest on slopes as compared to depressions and summits. Cover crop mixtures including WH and WK increased soil CO2 fluxes emitted from the soil while cereal rye decreased it. Cutting plant residues prior to incorporation tended to decrease CO2 emission, suggesting that reducing fragment size of incorporated residue has a potential to mitigate GHG emissions from agricultural soils. In contrast, differences in N2O emissions for any of the studied effects were not statistically significant, likely due to very high variability of N2O fluxes. Soil temperature, moisture and total aboveground biomass were strongly associated with soil CO2 and N2O fluxes from soil.

Effects of Different Rotation Patterns of Oil-Rice on Methane and Nitrous Oxide Emissions in Rice Fields

A field experiment was carried out to investigate the effects of different rice-rape rotation systems on methane and nitrous oxide emissions, which were measured using the static chamber/gas chromatography method, prediction of their global warming potentials (GWP), and greenhouse gas intensity (GHGI) in paddy fields. The results showed that the average cumulative emissions of CH4from a double cropping paddy field, single season rice field (including middle or late), rape field, and leisure land were 135.25, 55.64, 5.05, and 1.89 kg ·hm-2, respectively. The CH4 emission during the rice season accounted for 91.8%-98.5% of the annual CH4 emission in different rotation years, and the contents of dissolved organic carbon in paddy soil exhibited a significantly positive correlation with CH4 emission. The CH4 emission in conventional late rice paddy was 18.7% higher than that of hybrid late rice paddy (P<0.05). The average cumulative emissions of N2O from double cropping paddy field, single season rice field (including middle or late), rape field, and leisure land were 0.94, 0.64, 1.38, and 0.24 kg ·hm-2, respectively. Out of the total annual N2O emission, 57.2% to 70.2% was from the rape field; 17.8% and 30.6% was due to the winter fallow treatments with previous crop type of double corpping rice and single season rice, respectively. There was no significant difference in N2O emission between hybrid rice and conventional rice paddy fields. The GWP of double cropping rice-winter fallow and double cropping rice-rape was higher than that of rice-rape and rice-winter fallow, and the GWP of CH4 in rice season accounted for 71.2% to 90.9% of the annual GWP of rotation. The highest treatment of GHGI was rice-rice-rape, and the treatment of rice-oil and rice-winter slack was lower. According to the comprehensive environmental and economic benefits, the late hybrid rice-rape patterns should be selected to reduce greenhouse gases in multiple cropping rice fields of South China.