Emission of N2O from Biogas Crop Production Systems in Northern Germany

PDF HTML阅读 XML下载 导出引用 引用提醒 京郊典型设施蔬菜地土壤N2O排放特征 DOI: 10.5846/stxb201306091534 作者: 作者单位: 中国农业科学院农业资源与农业区划研究所/农业部面源污染控制重点实验室,中国农业科学院农业资源与农业区划研究所/农业部面源污染控制重点实验室,中国农业科学院农业资源与农业区划研究所/农业部面源污染控制重点实验室,中国农业科学院农业资源与农业区划研究所/农业部面源污染控制重点实验室 作者简介: 通讯作者: 中图分类号: 基金项目: 国家自然科学青年基金（41201287）；公益性行业（农业）科研专项（201103039）；中央级公益性科研院所基本科研业务费（302-30，402-15） Characteristics of nitrous oxide emissions from typical greenhouse vegetable fields in Beijing suburbs Author: Affiliation: Key Laboratory of Non-point Source Pollution Control, Ministry of Agriculture / Institute of Agricultural Resources and Regional Planning, Chinese Academy of Agricultural Sciences,,Key Laboratory of Non-point Source Pollution Control, Ministry of Agriculture / Institute of Agricultural Resources and Regional Planning, Chinese Academy of Agricultural Sciences, 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:利用静态暗箱-气相色谱法对北京郊区设施蔬菜地典型种植模式（番茄-白菜-生菜）下土壤N2O排放特征进行了周年（2012年2月22日-2013年2月23日）观测，探讨了不同处理下（即不施氮肥处理（CK）、农民习惯施肥处理（FP）、减氮优化施肥处理（OPT）和减氮优化施肥+硝化抑制剂处理（OPT+DCD））N2O排放特征及土壤温度、土壤湿度、土壤无机氮含量对土壤N2O排放的影响。结果表明：每次施肥+灌溉之后设施蔬菜地会出现明显的N2O排放高峰，持续时间一般为3-5 d。不同处理N2O排放通量变化范围在-0.21-14.26 mg N2O m-2 h-1，平均排放通量0.03-0.36 mg N2O m-2 h-1。整个蔬菜生长季各处理N2O排放与土壤孔隙含水率（WFPS）均表现出极显著的正相关关系（P < 0.01）；不施氮处理5 cm深度土壤温度与N2O排放通量呈现显著的正相关关系（P < 0.05）；各处理N2O排放与土壤表层硝态氮含量具有较一致变化趋势。不同处理下N2O年度排放总量差异显著，依次顺序为FP（（20.66±0.91）kg N/hm2） > OPT（（12.79±1.33）kg N/hm2） > OPT+DCD（（8.03±0.37）kg N/hm2）。与FP处理相比，OPT处理和OPT+DCD处理N2O年排放总量分别减少了38.09%和61.13%。各处理N2O排放系数介于0.36%-0.77%，低于IPCC 1.0%的推荐值。在目前的管理措施下，合理减少施氮量和添加硝化抑制剂是减少设施蔬菜地N2O排放量的有效途径。 Abstract:Nitrous oxide (N2O) has been recognized as one of the most important trace gases in the atmosphere that causes global warming and stratospheric ozone depletion. Nitrogen (N) fertilizer is considered as the primary source of N2O emissions from agricultural soils. As a large agricultural country, China consumes the greatest amount of synthetic N fertilizer which accounts for 30% of the world consumptions. Therefore, quantifying N2O emissions from agricultural soils and seeking suitable mitigation measures have become a relatively hot issue in international global climate change studies. However, the task has proved to be uneasy because N2O production and emission processes are very complex and are influenced by a number of soil and environmental variables, interacting soil water and N processes, crop uptake and management practices. Especially the N2O emissions from the greenhouse vegetable systems are more complex because the system obtain relatively higher inputs of fertilizer (e.g., N fertilizer application rate can be as high as 1,500 kg N/hm2), more water irrigation and cultivation disturbance. This paper reported a field experiment with intensive measurements of N2O fluxes from a greenhouse vegetable system with varied management treatments in Fangshan District located in the western suburbs of Beijing, China. N2O fluxes in conjunction with the main environmental drivers (i.e., soil temperature, soil moisture, soil NO3--N and soil NH4+-N) were observed from Feb. 2012 to Feb. 2013. Four treatments, i.e., the control treatment (CK), the farmers' practice treatment (FP), the optimized fertilization treatment (OPT), and the OPT treatment with nitrification inhibitor amendment treatment (OPT+DCD), were implemented during the experimental period to test the impacts of fertilization on N2O fluxes from the agroecosystem. The CK had no fertilizer applied; FP consisted of 2,470 kg N/hm2 with 1,270 and 1,200 kg N/hm2 from synthetic fertilizer and manure, respectively; the OPT reduced synthetic fertilizer rate to 573 kg N/hm2. The field was planted with tomato, cabbage and lettuce rotation during the two experimental years. The results indicated that large amount of N2O emissions were observed in the spring and summer periods when the soil had relatively high temperatures and moisture. N2O emission peaks were measured following each event of fertilization or irrigation. The high peaks usually lasted for 3-5 days. During the experimental period, N2O emission rates ranged from -0.21-14.26 mg N2O m-2 h-1 with daily means ranging from 0.03-0.36 mg N2O m-2 h-1. The annual cumulative N2O emissions ranged from 1.69-20.66 kg N hm-2, based on which the annual emission factor was calculated to be 0.36%-0.77% of the fertilizer N. Compared to the FP treatment, the OPT and OPT+DCD treatments both significantly reduced the annual N2O emissions by 38.09% and 61.3%(P < 0.05), respectively. The N2O emissions during the tomato, lettuce, cabbage and fallow periods accounted for 60.65%, 26.32%, 10% and 3.3% of annual cumulative emissions, respectively. The fertilizer-induced N2O emissions varied across the N application rates, the crop growing periods and the management treatments. N2O fluxes were positively related to the soil water filled pore space (WFPS) when the WFPS values varied between 48.88%-79.88%(P < 0.05). There was no consistent correlation between N2O fluxes and the soil temperature at soil depth of 5 cm. Higher soil available nitrogen, especially nitrate, contributed higher N2O emissions. In conclusion, alternative management practices such as reduced fertilizer application rate and amendment of DCD could effectively reduce N2O emissions from the greenhouse vegetable field which usually emitted more N2O than other croplands in China. 参考文献 相似文献 引证文献

Annual N2O emissions from conventionally grazed typical alpine grass meadows in the eastern Qinghai–Tibetan Plateau

Enhanced efficiency nitrogen fertilizers (EENFs) can reduce nitrous oxide emissions and maintain high grain yields in a rain-fed spring maize cropping system

The interest on N2O emission has increased since the late 1980s after realizing that N2O is an important greenhouse gas (Lashof and Ahuja 1990; Bouwman 1990a) which destroys ozone in the stratosphere by catalytic reactions (Crutzen 1970). The high global warming potential (GWP) of N2O has increased the scientific research effort on assessing N2O fluxes from soils of terrestrial ecosystems (Andreae and Schimel 1989; Bouwman 1990a; Granli and Bockman 1994) because soils are the largest natural source of N2O (IPCC 2001). Studies in the 1980s suggested that tropical forests are larger sources for N2O than temperate and boreal forests, whereas recent studies have indicated that beech (Fagus Sylvatica L.) forests can have N2O fluxes similar to those observed in tropical forests (Brumme and Beese 1992; Papen and Butterbach-Bahl 1999; Zechmeister-Boltenstern et al. 2002). Beech forests with high annual N2O fluxes have a seasonal emission pattern with high N2O fluxes in summer and low N2O fluxes in winter. However, most temperate forests (beech, spruce, oak) have low background N2O emissions during the year which lack any seasonal trend (Brumme et al. 1999). There are some questions which need to be answered to understand the importance of forests with a seasonal emission pattern for the global balance of N2O (Brumme et al. 2005). In this chapter, temporal and spatial variations of N2O emissions from a beech forest ecosystem with a seasonal emission pattern will be provided, leading to a discussion on the mechanisms and processes responsible for seasonal and background patterns of N2O emissions. Attempts will be made to assess the effect of temperature change, forest management practices (harvesting, liming, soil compaction), and nitrogen inputs on N2O emissions.

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.

Since the development of effective N2O mitigation options is a key challenge for future agricultural practice, we studied the interactive effect of tillage systems on fertilizer-derived N2O emissions and the abundance of microbial communities involved in N2O production and reduction. Soil samples from 0–10 cm and 10–20 cm depth of reduced tillage and ploughed plots were incubated with dairy slurry (SL) and manure compost (MC) in comparison with calcium ammonium nitrate (CAN) and an unfertilized control (ZERO) for 42 days. N2O and CO2 fluxes, ammonium, nitrate, dissolved organic C, and functional gene abundances (16S rRNA gene, nirK, nirS, nosZ, bacterial and archaeal amoA) were regularly monitored. Averaged across all soil samples, N2O emissions decreased in the order CAN and SL (CAN = 748.8 ± 206.3, SL = 489.4 ± 107.2 μg kg−1) followed by MC (284.2 ± 67.3 μg kg−1) and ZERO (29.1 ± 5.9 μg kg−1). Highest cumulative N2O emissions were found in 10–20 cm of the reduced tilled soil in CAN and SL. N2O fluxes were assigned to ammonium as source in CAN and SL and correlated positively to bacterial amoA abundances. Additionally, nosZ abundances correlated negatively to N2O fluxes in the organic fertilizer treatments. Soils showed a gradient in soil organic C, 16S rRNA, nirK, and nosZ with greater amounts in the 0–10 than 10–20 cm layer. Abundances of bacterial and archaeal amoA were higher in reduced tilled soil compared to ploughed soils. The study highlights that tillage system induced biophysicochemical stratification impacts net N2O emissions within the soil profile according to N and C species added during fertilization.

Abstract. Nitrous oxide (N2O) is an important greenhouse gas. N2O emissions from soils vary with fertilization and cropping practices. The response of N2O emission to fertilization of agricultural soils plays an important role in global N2O emission. The objective of this study was to assess the seasonal pattern of N2O fluxes and the annual N2O emissions from a rain-fed winter wheat (Triticum aestivum L.) field in the Loess Plateau of China. A static flux chamber method was used to measure soil N2O fluxes from 2006 to 2008. The study included 5 treatments with 3 replications in a randomized complete block design. Prior to initiating N2O measurements the treatments had received the same fertilization for 22 years. The fertilizer treatments were unfertilized control (CK), manure (M), nitrogen (N), nitrogen + phosphorus (NP), and nitrogen + phosphorus + manure (NPM). Soil N2O fluxes in the highland winter wheat field were highly variable temporally and thus were fertilization dependent. The highest fluxes occurred in the warmer and wetter seasons. Relative to CK, m slightly increased N2O flux while N, NP and NPM treatments significantly increased N2O fluxes. The fertilizer induced increase in N2O flux occurred mainly in the first 30 days after fertilization. The increases were smaller in the relatively warm and dry year than in the cold and wet year. Combining phosphorous and/or manure with mineral N fertilizer partly offset the nitrogen fertilizer induced increase in N2O flux. N2O fluxes at the seedling stage were mainly controlled by nitrogen fertilization, while fluxes at other plant growth stages were influenced by plant and environmental conditions. The cumulative N2O emissions were always higher in the fertilized treatments than in the non-fertilized treatment (CK). Mineral and manure nitrogen fertilizer enhanced N2O emissions in wetter years compared to dryer years. Phosphorous fertilizer offset 0.50 and 1.26 kg N2O-N ha−1 increases, while manure + phosphorous offset 0.43 and 1.04 kg N2O-N ha−1 increases by N fertilizer for the two observation years. Our results suggested that the contribution of single N fertilizer on N2O emission was larger than that of NP and NPM and that manure and phosphorous had important roles in offsetting mineral N fertilizer induced N2O emissions. Relative to agricultural production and N2O emission, manure fertilization (M) should be recommended while single N fertilization (N) should be avoided for the highland winter wheat due to the higher biomass and grain yield and lower N2O flux and annual emission in m than in N.

Nitrous oxide (N2O) emissions can be significantly affected by the amounts and forms of nitrogen (N) available in soils, but the effect is highly dependent on local climate and soil conditions in specific ecosystem. To improve our understanding of the response of N2O emissions to different N sources of fertilizer in a typical semiarid temperate steppe in Inner Mongolia, a 2-year field experiment was conducted to investigate the effects of high, medium and low N fertilizer levels (HN: 200 kg N ha-1y-1, MN: 100 kg N ha-1y-1, and LN: 50 kg N ha-1y-1) respectively and N fertilizer forms (CAN: calcium ammonium nitrate, AS: ammonium sulphate and NS: sodium nitrate) on N2O emissions using static closed chamber method. Our data showed that peak N2O fluxes induced by N treatments were concentrated in short periods (2 to 3 weeks) after fertilization in summer and in soil thawing periods in early spring; there were similarly low N2O fluxes from all treatments in the remaining seasons of the year. The three N levels increased annual N2O emissions significantly (P HN > LN compared with the CK (control) treatment in year 1; in year 2, the elevation of annual N2O emissions was significant (P 0.05). The three N forms also had strong effects on N2O emissions. Significantly (P 0.05). Annual N2O emission factors (EF) ranged from 0.060 to 0.298% for different N fertilizer treatments in the two observed years, with an overall EF value of 0.125%. The EF values were by far less than the mean default EF proposed by the Intergovernmental Panel on Climate Change (IPCC).

Reduced fertilization mitigates N2O emission and drip irrigation has no impact on N2O and NO emissions in plastic-shed vegetable production in northern China

A field experiment was conducted to evaluate the influence of the continuous application of organic and mineral N fertilizer on N2O and NO emissions under maize and wheat rotation on the North China Plain. This study included eight treatments: no fertilizer (control); mineral N fertilizer (Nmin) at a rate of 200 kg N ha−1 per season; 50% mineral fertilizer N plus 50% cattle manure N (50% CM), 50% chicken manure N (50% FC) or 50% pig manure N (50% FP); 75% mineral fertilizer N plus 25% cattle manure N (25% CM), 25% chicken manure N (25% FC) or 25% pig manure N (25% FP). The annual N2O and NO emissions were 2.71 and 0.39 kg N ha−1, respectively, under the Nmin treatment, with an emission factor of 0.50% for N2O and 0.07% for NO. Compared with the Nmin treatment, N2O emissions did not differ when 50% of the mineral N was replaced with manure N (50% CM, 50% FC and 50% FP), while annual NO emissions were significantly reduced by 49.0% and 27.8% under 50% FC and 50% FP, respectively. In contrast, annual N2O emissions decreased by 21–38% compared to the Nmin treatment when 25% of the mineral N was replaced with manure N (25% CM, 25% FC and 25% FP). Most of the reduction occurred during the maize season. The 25% CM, 25% FC and 25% FP treatments had no effect on NO emissions compared to the Nmin treatment. There was no obvious difference in annual N2O and NO emissions among the organic manures at the same application rate, probably due to their similar C/N ratio. Replacing a portion of the mineral fertilizer N with organic fertilizer N did not significantly affect crop grain yield, except for the 50% FC treatment in the wheat season. Overall, the results suggest that the combined application of 25% organic manure N plus 75% mineral fertilizer N had the most potential to mitigate N2O emissions while not affecting crop yield in the maize and wheat rotation system in this area of China.

The effect of slurry application techniques and slurry N stabilizing strategies on nitrous oxide emission from grasslands is poorly understood and, therefore, can result in large uncertainties in national/regional inventories. Field experiments were, thus, conducted to estimate the effect of different fertilization techniques on nitrous oxide (N2O) emissions. Fertilizer was applied (135–270 kg N ha−1 year−1) as calcium ammonium nitrate (CAN), untreated or treated cattle slurry. The slurry was either treated with sulfuric acid (target pH = 6.0), applied using trailing shoes or treated with 3,4‐dimethyl pyrazole phosphate and applied via slot injection. N2O fluxes were sampled using the closed chamber technique. Cumulative N2O emissions ranged 0.1–2.9 kg N ha−1 year−1 across the treatment, sites and years. The N application techniques showed inconsistent effects on soil mineral N content, cumulative N2O emission and N yield. The fertilizer replacement value of slurry was low due to low N use efficiencies at the sites. However, a close positive relationship (r = 0.5; p = .013) between slurry value and biomass yield was observed, highlighting the benefit of high slurry value on crop productivity. N2O‐N emission factors were low for all treatments, including CAN, but were 2–6 times higher in 2019 than in 2020 due to lower precipitation in 2020. Variations in N2O emission were largely explained by soil and climatic factors. Even with the low N2O emissions, this study highlights the benefit (significant mitigation of N2O emissions) of replacing the increasingly expensive chemical fertilizer N with input from slurry under favourable conditions for denitrification.

For 3 years we followed the complete annual cycles of NO and NO2 flux rates from soil of a spruce control site, a limed spruce site, and a beech site at the Höglwald Forest, Bavaria, Germany, with high temporal resolution in order to gain detailed information about (1) the impacts of forest type, liming, and atmospheric N input by wet deposition on the magnitude of NO and NO2 flux rates and (2) the microbial processes involved in NO production and emission. In addition to identification of seasonal variations of flux rates the huge database allowed calculation of annual mean NO and NO2 fluxes with high accuracy and identification of interannual variations of fluxes. The long‐term annual mean NOx emission was 61.7 μg NOx N m−2 h−1 for the spruce control site, 17.3 μg NOx N m−2 h−1 for the limed spruce site, and 4.0 μg NOx N m−2 h−1 for the beech site. These extremely high soil NOx emissions from a temperate forest most likely reflect the status of N saturation of the Höglwald Forest as a consequence of year‐long heavy atmospheric N input. Multiple regression analyses revealed the following sequence of importance of environmental factors on NO flux: soil temperature to water‐filled pore space to soil NO3− concentrations to soil NH4+ concentrations. Nitrification was the dominating biotic modulator of NO emission at all sites: &gt;60% of the variation of NO emission rates were associated with variations of net nitrification rates. There was a strong positive correlation between amount of in situ N input by wet deposition and magnitude of in situ NO flux rates. Approximately 15% and 7% of the actual N input was lost as NO from the soil stocked with spruce and beech, respectively. Liming resulted in 49% reduction of NO emissions as compared to an unlimed spruce control site. The results indicate that the reduction in NO emission was due to an increase in NO consumption within the limed soil. In contrast to NO flux, NO2 flux was modulated by physico‐chemical rather than biological factors. Using the data of this study, we estimate that the contribution of N‐affected temperate coniferous and deciduous forests to the global NOx release is 0.3 Tg NOx N yr−1.

Integrated crop-livestock (ICL) systems can have a complex of effects on soil properties that can influence greenhouse gas emissions (GHG). The ICL aim to capture atmospheric CO2 and sequester it in the soil, holding promise for reducing GHG emission intensity from livestock products. Moreover, modeling N2O emissions can help assess the potential impact of N management on the ICL system to optimize the sustainability of agriculture production. Field data were obtained from an ICL experiment of EMBRAPA-Rice and Beans, located on Capivara farm, Santo Ant&amp;#244;nio de Goi&amp;#225;s/GO, Brazil (16&amp;#176;28&amp;#180;S; 49&amp;#176;17&amp;#180;W; 823 m alt.). The ICL experiment was evaluated for four years (2013-2016) with the following crop rotation sequence: pasture-fallow-maize, fallow-soybean, maize-fallow-maize, and beans-fallow. The N2O data was obtained from the 2013-14 season, which was measured in a static chamber during maize cultivation. The experiment consisted of 9 treatments (N sources and rates) with 5 replicates. The N2O was measured in 30 sampling events over almost 100 days. The daily N2O fluxes from the treatments control (No N), urea (UR), calcium ammonium nitrate (CAN), and ammonium sulfate (AS) at an N rate of 150 kg/ha were used to parametrize the DNDC. Model crop and soil parameters were adjusted to better simulate maize production and N2O emission according to observed data. DNDC simulated CO2 emissions, quantified as Net Ecosystem Exchange (NEE), were validated against CO2 emissions derived from eddy-covariance data, using statistical parameters such as R2, RMSE, MAE, and Bias. While data refinement is ongoing, preliminary findings indicate that DNDC shows promise for estimating CO2 emissions IPS under tropical conditions The DNDC had a satisfactory performance in predicting N2O emission in the ICL system, resulting in a significant correlation with the observed data (r = 0.63, p &lt; 0.001), MAE of 0.024, and RMSE of 0.036. The average daily N2O-N emission observed was 0.026 kg ha-1 day-1 and simulated was 0.025 kg ha-1 day-1. The UR, CAN and AS applications showed a peak of N2O emission on 31th day after sowing (2 days after fertilization) corresponding to 0.175, 0.217, and 0.163 kg ha-1 day-1, respectively, where the model simulated N2O peaks of 0.151, 0.123, and 0.173 kg ha-1 day-1. The accumulated N2O emissions were 0.513, 1.148 1.738, and 0.890 kg ha-1 for control, UR, CAN, and AS respectively, in which the simulated by DNDC were 0. 778, 1.612, 1.391, and 1.755 kg ha-1. In general, the model had a good fit with daily N2O emissions, but it tended to overestimate the N2O emission from UR and AS, and underestimate from CAN. Further model parametrization and calibration may be necessary to better predict N2O and CO2 emissions. The DNDC satisfactory simulated the N2O emissions from different N sources applied to ICL system, which can be used to evaluate the potential emissions and mitigation according to N management in ICL.

Nitrous oxide (N2O) is a powerful greenhouse gas and also the remaining threat to the ozone layer. N2O emission is mainly from cropland accounting for 82% of the global N2O increase, which is of great concern for policymakers making strategies for mitigating N2O emissions. One of such strategies is agroforestry systems which integrate trees into cropland and are considered as environmentally-friendly ecosystems, in particular in greenhouse gas mitigation (e.g. N2O emissions). The net balance of N2O flux is constrained by gross N2O emissions and uptake. However, we are still struggling to fully understand the complexity of gross N2O emissions and uptake due to its spatial- and temporal variation. No systematic comparison of gross N2O fluxes was conducted between cropland agroforestry and monoculture. Besides, N2O produced and consumed are not only in topsoil but also in subsoil and there is lacking information about how gross N2O emissions and uptake vary at depths in different types of agroforestry systems. The first study aimed to assess the impact of land-use change on gross N2O emissions and uptake and their associated controls between cropland agroforestry and monoculture. The study was conducted at three sites in Germany, of which two sites had paired cropland agroforestry and monoculture on a loam Calcaric Phaeozem soil and a clay Vertic Cambisol soil, and one site was a cropland monoculture on a sandy Arenosol soil. Gross N2O emissions and uptake were monthly measured by using the 15N2O pool dilution technique over two growing seasons (2018 - 2019). Our results showed that soil gross N2O emissions from the area-weighted tree and crop rows in the agroforestry did not differ from monoculture. Nonetheless, the unfertilized tree rows showed the lowest gross N2O emissions. Although tree rows only occupied 20% in the agroforestry, annual gross N2O emissions in the top 5-cm soil decreased by 6% to 36% in the agroforestry compared to monoculture. Gross N2O emissions were influenced by soil mineral N, available C, and moisture content rather than by denitrification gene abundance. Soil gross N2O uptake was highest in the tree row and decreased with distance into crop rows. The agroforestry tree row increased annual gross N2O uptake in the top 5-cm soil by 27% to 42% compared to monoculture. In the tree row, soil gross N2O uptake correlated with nirK gene abundance which, in turn, was correlated with nosZ clade II that was related to low mineral N-to-available C ratios. The second study aimed to compare gross N2O emissions and uptake between riparian tree buffer and tree row of alley cropping system, and between depths (0 – 5 vs. 40 – 60 cm), and to elucidate their associated abiotic and biotic controls. This study was conducted at two contrasting agroforestry systems in Germany: riparian tree buffer and tree row of the alley cropping system. We quantified gross N2O emissions and uptake using the 15N2O pool dilution technique in early spring (April), spring (June), summer (August), and fall (October). Our results showed that riparian tree buffer had higher gross N2O emissions and uptake in topsoil (0 – 5 cm) than the tree row of alley cropping but such differences were not observed in subsoil (40 – 60 cm). Although gross N2O emissions and uptake did not differ between the two depths in each agroforestry system, we observed a hot moment, i.e. early spring, for gross N2O emissions in topsoil of riparian tree buffer, with a large source of N2O observed. Gross N2O emissions were mainly controlled by mineral N, biodegradable organic carbon, and water-filled pore space rather than microbial population size between the two agroforestry systems and depths. Gross N2O uptake in topsoil was driven by available carbon and nirK gene abundance across agroforestry systems. But subsoil showed a sink of N2O due to low mineral N. Gross N2O uptake in subsoil was affected by soil temperature in each agroforestry system, indicating positive feedback of global warming. Overall, this research provides new insights into mitigation of N2O emissions from soil to atmosphere after conversion of cropland monoculture to agroforestry and also provides field-based rates of gross N2O fluxes at depths in contrasting agroforestry systems. Our research provides the first year-round quantification of gross N2O emission and uptake using 15N2O pool dilution for cropland agroforestry and monoculture, with key implications for support on greenhouse gas regulation function for policy implementation of agroforestry. Our findings emphasize that adjusting the tree and crop areal coverages of agroforestry can further optimize the benefits of agroforestry in reducing emissions and increasing uptake of N2O in soils. As discussed in the synthesis chapter, future studies should increase the measurement frequency of gross N2O fluxes at depths to capture hot moments and spots, especially in the riparian tree buffer, and further better constrain the contribution of subsoil to the ecosystem N loss although this area is relatively small.

Rivers have been largely considered as the source of nitrous oxide (N2O) to the atmosphere. N2O emissions from rivers could be seriously influenced by damming and exhibit unique spatiotemporal patterns in river-reservoir systems. Multiple research studies report N2O emissions from rivers with single reservoirs, but the spatiotemporal patterns and controls of N2O emissions from cascaded river-reservoir system remain unclear. In this study, we investigated the spatiotemporal variations of N2O concentrations and fluxes along a cascade damming river (Wubu River) in Southwest China. Our results showed that N2O concentrations in the Wubu River ranged from 2.5 to 283.2 nmol L−1 with a mean of 50.7 ± 52.3 nmol L−1 and were generally supersaturated with gas fluxes ranging from 11.8 to 805.6 μmol m−2 d−1. N2O concentrations and fluxes showed a significant longitudinal variation with increasing fluxes from upstream to downstream. Meanwhile, for each river-reservoir-released water continuum, local variation of N2O concentrations was also prominent. Reservoir sections and released water sections had 2.7 (1.2–7.9) and 3.4 (1.3–12.2) times higher N2O concentrations than the corresponding upstream river reaches and acted as hotpots for N2O emission. The N2O concentrations had significant correlations with organic carbon, phosphorus, and Chl-a in surface water. Furthermore, the N2O concentrations and fluxes in reservoirs had a significant correlation with hydraulic residence time and hydraulic load, suggesting that fragmentation of hydrologic conditions was an important driver for the spatial variations of N2O concentrations in the Wubu River cascade reservoirs. Our results suggested that hydraulic residence time could predict the variation pattern of N2O fluxes in this small river basin. Seasonal variations of N2O concentrations and fluxes were the highest in autumn and lowest in winter and were mainly attributed to temperature and rainfall. N2O fluxes were much higher in the Wubu River than the average levels of China’s reservoirs and global reservoirs, acting as enhanced N2O emitter. Our study highlighted that the cascade reservoirs not only act as exciters for N2O production and emissions but also form cumulative effects and local hotpots along the longitudinal dimension, which could significantly increase the complexity of the spatiotemporal variability in riverine N2O emissions. Given the increasing construction of new river dams due to growing energy demand, more research should be done to quantify the contribution of cascaded damming to riverine N2O budgets.