N2O and CH4 Emissions,and NO3 − Leaching on a Crop-Yield Basis from a Subtropical Rain-fed Wheat–Maize Rotation in Response to Different Types of Nitrogen Fertilizer

Guaranteeing high crop yields while reducing environmental impacts of nitrogen fertilizer use due to associated losses of N₂O emissions and nitrate (NO₃ ^?) leaching is a key challenge in the context of sustainable intensification of crop production. However, few field data sets are available that explore the effect of different forms of N management on yields as well as on N losses in the form of N₂O or NO₃ ^?. Here we report on a large-scale field lysimeter (8 × 4 m²) experiment, which was designed to determine soil CH₄ and N₂O emissions, NO₃ ^? leaching losses and crop yields from a subtropical rain-fed wheat–maize rotation in the Sichuan Basin, one of the most intensively used agricultural regions in China. One control and three different fertilizer treatments with the same total rate of N application (280 kg N ha^?1 y^?1) were included: NF: control (no fertilizer); NPK: synthetic N fertilizer; OMNPK: synthetic N fertilizer plus pig manure; RSDNPK: synthetic N fertilizer plus crop residues. As compared to the standard NPK treatment, annual NO₃ ^? leaching losses for OMNPK and RSDNPK treatments were decreased by 36 and 22%, respectively (P < 0.05). Similarly, crop yield-scaled NO₃ ^? leaching for NPK treatment was higher than those for either OMNPK or RSDNPK treatments (P < 0.05). Direct N₂O emissions for RSDNPK treatment were decreased as compared with NPK and OMNPK treatments (P < 0.05). Furthermore, the yield-scaled GWP (global warming potential) was lower for the treatments where either pig manure or crop residues were incorporated as compared to the standard NPK treatment (P < 0.05). Our study indicates that it is possible to reduce the negative environmental impact of NO₃ ^? leaching and N₂O emissions without compromising crop productivity. Yield-scaled NO₃ ^? leaching, similar to the yield-scaled GWP, represents another valuable-integrated metric to address the dual goals of reducing nitrogen pollution and maintaining crop grain yield for a given agricultural system.     

Nitrous oxide emissions during the non-rice growing seasons of two subtropical rice-based rotation systems in southwest China

Background and aims

High nitrous oxide (N₂O) emissions may occur during the non-rice growing season of Chinese rice-upland crop rotation systems. However, our understanding of N₂O emission during this season is poor due to a scarcity of available field N₂O measurements.

Methods

Using the static manual chamber-GC technique, seasonal N₂O emissions during the non-rice growing season were simultaneously measured at two adjacent rice-wheat and rice-rapeseed fields in southwest China for three consecutive annual rotation cycles (May 2005 to May 2008).

Results

Compared to the control, N fertilizer applications significantly enhanced soil N₂O emissions from both wheat and rapeseed systems. Seasonal cumulative N₂O fluxes from wheat systems were on average 2.6 kg N ha^?1 for the recommended practice (RP [150 kg N ha^?1]) and 5.0 kg N ha^?1 for the conventional practice (CP [250 kg N ha^?1]). Lower N₂O emissions were observed from the adjacent rapeseed systems. Average cumulative seasonal N₂O fluxes from rapeseed were 1.5 and 2.2 kg N ha^?1 for the RP and CP treatments, respectively. The first 3 weeks after N fertilization were the “hot moment” of N₂O emissions for both the wheat and rapeseed systems. The lowest yield-scaled N₂O fluxes for wheat were obtained at the RP treatment (mean: 0.81 kg N Mg^?1) while for rapeseed the CP treatment produced the lowest yield-scaled fluxes (mean: 0.79 kg N Mg^?1). On average, the direct N₂O emission factors (EF_d) for the wheat system (1.76 %) were over two times higher than for the rapeseed system (0.73 %).

Conclusions

Intercropping of rapeseed tends to result in lower N₂O emissions than wheat for rice-upland crop rotation systems of southwest China, indicating that either the N fertilization or the cropping system need to be considered not only for improving the estimate of regional and/or national N₂O fluxes but also for proposing the climate-smart agricultural management practice to reduce N₂O emissions from agricultural soils.     

Simulation of N2O emissions and nitrate leaching from plastic mulch radish cultivation with LandscapeDNDC

Radish is one of the major dry field crops in Asia commonly grown with plastic mulch and high rates of N fertilization, and potentially harming the environment due to N₂O emissions and nitrate leaching. Despite the widespread use of plastic mulch, biogeochemical models so far do not yet consider impacts of mulch on soil environmental conditions and biogeochemistry. In this study, we adapted and successfully tested the LandscapeDNDC model against field data by simulating crop growth, C and N turnover and associated N₂O emissions as well as nitrate leaching for radish cultivation with plastic mulch and in conjunction with different rates of N fertilization (465–765 kg N ha^?1 year^?1). Due to the sandy soil texture and monsoon climate, nitrate leaching with rates up to 350 kg N ha^?1 year^?1 was the dominant reason for overall low nitrogen use efficiency (32–43 %). Direct or indirect N₂O emissions (calculated from simulated nitrate leaching rates and IPCC EF_ind = 0.0075) ranged between 2 and 3 kg N ha^?1 year^?1, thus contributing an equal amount to total field emissions of about 5 kg N ha^?1 year^?1. Based on our results, emission factors for direct N₂O emissions ranged between 0.004 and 0.005. These values are only half of the IPCC default value (0.01), demonstrating the need of biogeochemical models for developing site and/or region specific EFs. Simulation results also revealed that changes in agricultural management by applying the fertilizer only to the rows would be an efficient mitigation strategy, effectively decreasing field nitrate leaching and N₂O emissions by 50–60 %.     

Nitrous oxide and nitric oxide emissions from an irrigated cotton field in Northern China

Cotton is one of the major crops worldwide and delivers fibers to textile industries across the globe. Its cultivation requires high nitrogen (N) input and additionally irrigation, and the combination of both has the potential to trigger high emissions of nitrous oxide (N₂O) and nitric oxide (NO), thereby contributing to rising levels of greenhouse gases in the atmosphere. Using an automated static chamber measuring system, we monitored in high temporal resolution N₂O and NO fluxes in an irrigated cotton field in Northern China, between January 1st and December 31st 2008. Mean daily fluxes varied between 5.8 to 373.0 µg N₂O-N m^?2?h^?1 and ?3.7 to 135.7 µg NO-N m^?2?h^?1, corresponding to an annual emission of 2.6 and 0.8 kg N ha^?1?yr^?1 for N₂O and NO, respectively. The highest emissions of both gases were observed directly after the N fertilization and lasted approximately 1 month. During this time period, the emission was 0.85 and 0.22 kg N ha^?1 for N₂O and NO, respectively, and was responsible for 32.3% and 29.0% of the annual total N₂O and NO loss. Soil temperature, moisture and mineral N content significantly affected the emissions of both gases (p?<?0.01). Direct emission factors were estimated to be 0.95% (N₂O) and 0.24% (NO). We also analyzed the effects of sampling time and frequency on the estimations of annual cumulative N₂O and NO emissions and found that low frequency measurements produced annual estimates which differed widely from those that were based on continuous measurements.     

Nitrous oxide emissions and nitrate leaching from a rain-fed wheat-maize rotation in the Sichuan Basin, China

Aims

A 3-year field experiment (October 2004–October 2007) was conducted to quantify N₂O fluxes and determine the regulating factors from rain-fed, N fertilized wheat-maize rotation in the Sichuan Basin, China.

Methods

Static chamber-GC techniques were used to measure soil N₂O fluxes in three treatments (three replicates per treatment): CK (no fertilizer); N150 (300 kg N fertilizer ha^?1 yr^?1 or 150 kg N?ha^?1 per crop); N250 (500 kg N fertilizer ha^?1 yr^?1 kg or 250 kg N?ha^?1 per crop). Nitrate (NO ₃ ^? ) leaching losses were measured at nearby sites using free-drained lysimeters.

Results

The annual N₂O fluxes from the N fertilized treatments were in the range of 1.9 to 6.7 kg N?ha^?1 yr^?1 corresponding to an N₂O emission factor ranging from 0.12 % to 1.06 % (mean value: 0.61 %). The relationship between monthly soil N₂O fluxes and NO ₃ ^- leaching losses can be described by a significant exponential decaying function.

Conclusions

The N₂O emission factor obtained in our study was somewhat lower than the current IPCC default emission factor (1 %). Nitrate leaching, through removal of topsoil NO ₃ ^? , is an underrated regulating factor of soil N₂O fluxes from cropland, especially in the regions where high NO ₃ ^- leaching losses occur.     

模拟氮沉降对天山云杉细根分解及其养分释放的影响

采用野外模拟试验,设计4种氮处理——对照(不施氮,CK)、低氮(施氮5kg·hm-2·a-1,LN)、中氮(施氮10kg·hm-2·a-1,MN)、高氮(施氮15kg·hm-2·a-1,HN),研究氮沉降对天山云杉细根分解及养分释放的影响。结果表明:(1)不同氮处理分解2年后天山云杉细根残留率依次为74.044%(HN)、71.967%(MN)、68.156%(CK)、61.933%(LN),且差异显著。(2)天山云杉的细根月分解速率在试验前期不同氮处理下规律不明显;而在试验后期呈现为对照中氮低氮高氮。(3)4种氮处理下天山云杉细根分解50%需要的时间依次为3.31年(LN)、3.67年(CK)、4.28年(MN)、4.64年(HN),分解95%需要的时间依次为14.39年(LN)、15.93年(CK)、18.58年(MN)和20.17年(HN)。(4)天山云杉细根C元素迁移模式总体表现为直接释放,N元氮为富集-释放模式,残留率呈现波动式下降趋势。(5)不同氮处理下天山云杉细根分解率与C元素浓度间均呈线性负相关关系;对照和低氮处理下,天山云杉细根分解率与N元素浓度间均为线性负相关关系,中氮和高氮处理下,细根分解率随N元素浓度的增加呈先增加后降低的趋势。     

DMPP reduced nitrification,but not annual N2O emissions from mineral fertilizer applied to oilseed rape on a sandy loam soil

Direct field emissions of nitrous oxide (N₂O) may determine whether biodiesel from oilseed rape (Brassica napus L.) fulfills the EU requirement of at least 50% reduction of greenhouse gas emissions as compared to fossil diesel. However, only few studies have documented fertilizer N emission factors (EF) and mitigation options for N₂O emissions from oilseed rape cropping systems. We conducted a field experiment with three N levels (0, 171, and 217 kg/ha), where the N fertilizer was applied as ammonium sulfate nitrate with or without the nitrification inhibitor 3,4‐dimethylpyrazole phosphate (DMPP). N₂O fluxes were measured using static chambers technique and soil samples were analyzed for water and mineral N content during a monitoring period of 368 days. The DMPP treatments showed a significantly increased level of ammonium () for up to 18 weeks after spring fertilization as compared to the treatments without DMPP. However, this difference did not result in a corresponding decrease in soil content, and no differences in cumulative N₂O emissions were found between any fertilized treatments with or without DMPP (mean, 1.26 kg N₂O‐N ha^?1 year^?1). More field experiments are needed to clarify whether DMPP‐coated mineral fertilizers could mitigate N₂O emissions under different weather conditions, for example, under conditions where fertilization events concurred with rainfall events increasing water‐filled pore space to the assumed 60% threshold for denitrification. Emission factors for mineral N fertilizer were 0.28%–0.36% with a mean of 0.32% across the fertilized treatments. These data concur with recent European studies suggesting that the EF for mineral N fertilizers in oilseed rape cropping systems may typically be lower than the default IPCC value of 1%. Further studies are needed to consolidate an EF for oilseed rape under temperate conditions, which will be determining for the sustainability of Northern European oilseed rape cultivation for biodiesel.     

Emissions of nitrous oxide from three tropical forests in Southern China in response to simulated nitrogen deposition

Emissions of nitrous oxide (N₂O) from the soil following simulated nitrogen (N) deposition in a disturbed (pine), a rehabilitated (pine and broadleaf mixed) and a mature (monsoon evergreen broadleaf) tropical forest in southern China were studied. The following hypotheses were tested: (1) addition of N will increase soil N₂O emission in tropical forests; and (2) any observed increase will be more pronounced in the mature forest than in the disturbed or rehabilitated forest due to the relatively high initial soil N concentration in the mature forest. The experiment was designed with four N treatment levels (three replicates; 0, 50, 100, 150 kg N ha⁻¹ year⁻¹ for C (Control), LN (Low-N), MN (Medium-N), and HN (High-N) treatment, respectively) in the mature forest, but only three levels in the disturbed and rehabilitated forests (C, LN and MN). Between October 2005 to September 2006, soil N₂O flux was measured using static chamber and gas chromatography methodology. Nitrogen had been applied previously to the plots since July 2003 and continued during soil N₂O flux measurement period. The annual mean rates of soil N₂O emission in the C plots were 24.1 ± 1.5, 26.2 ± 1.4, and 29.3 ± 1.6 μg N₂O–N m⁻² h⁻¹ in the disturbed, rehabilitated and mature forest, respectively. There was a significant increase in soil N₂O emission following N additions in the mature forest (38%, 41%, and 58% when compared to the C plots for the LN, MN, and HN plots, respectively). In the disturbed forest a significant increase (35%) was observed in the MN plots, but not in the LN plots. The rehabilitated forest showed no significant response to N additions. Increases in soil N₂O emission occurred primarily in the cool-dry season (November, December and January). Our results suggest that the response of soil N₂O emission to N deposition in tropical forests in southern China may vary depending on the soil N status and land-use history of the forest.     

模拟氮沉降条件下木荷幼苗光合特性、生物量与C、N、P分配格局

设置模拟氮沉降的控制试验,以NH4NO3作为外加氮源,设计CK(0kg N hm-2·a-1)、LN(50 kg N hm-2·a-1)、MN(100 kg N hm-2·a-1)、HN(150 kg N hm-2· a-1)4个处理,历时9个月,测定木荷(Schima superba)幼苗的光合特性、生物量和C、N、P含量及其分配格局对氮沉降的响应.结果表明:(1)木荷幼苗的最大净光合速率和光饱和点随着氮处理水平增加呈先增加后减小的特点,在中氮处理下极显著增加(P＜0.01).氮处理降低了幼苗的光补偿点和暗呼吸速率,光补偿点在低氮处理下显著降低(P＜0.05),暗呼吸速率在低中氮处理下极显著降低(P＜0.01),高氮处理下显著降低(P＜0.05).未见氮处理对表观量子效率产生显著影响.(2)氮处理促进了木荷的全株生物量以及各部分生物量的增长.随着氮处理水平的增加,叶重比呈升高的趋势,而根重比和根冠比呈降低的趋势,在高氮处理下叶重比的增加和根重比、根冠比的降低都达到了显著水平(P＜0.05).(3)氮沉降促进各器官N含量的增加,在高氮处理下根和茎中N含量极显著增加(P＜0.01),叶中N含量显著增加(P＜0.05).而各器官C含量随着氮沉降程度的增加呈先增加后降低的趋势,在中氮处理下根和茎中C含量极显著增加(P＜0.01),叶中C含量显著增加(P＜0.05).但各器官P含量变化趋势各不相同,随着氮的增加,根中P含量是呈先增加后降低的趋势,而茎和叶中P含量是呈降低的趋势.氮沉降一定程度上降低了木荷各器官的C/N比值而增加了N/P比值.     

Effects of organic matter incorporation on nitrous oxide emissions from rice-wheat rotation ecosystems in China

Organic matter addition is thought to be an important regulator of nitrous oxide (N₂O) emissions from croplands. Contradictory effects, however, were reported in previous studies. To investigate the effects of crop residue management on N₂O emissions from rice-wheat rotation ecosystems, we conducted field experiments at three sites (Suzhou, Wuxi and Jiangdu) in the Yangtze River Delta, using static chamber and gas chromatography methods. Our data show that N₂O emissions throughout the rice season from plots treated with wheat straw application at a high rate (WS) prior to rice transplanting (1.1–2.0 kg N ha^?1) were significantly lower (P?<?0.05) than those from the control plots without organic matter addition or added with wheat straw at a moderate rate (1.6–2.9 kg N ha^?1). Furthermore, the WS treatments had a residual inhibitory effect on N₂O emissions in the following non-rice season, which consistently resulted in significantly lower emissions (P?<?0.05) compared to the control treatments (2.2–3.1 vs. 3.9–5.6 kg N ha^?1). In comparison to the control treatments, the WS treatments reduced both the seasonal and annual direct emission factors of the applied nitrogen (EF_d) by 50–68% (mean: 57%). The addition of compost (aerobically composted rice or wheat straw harvested in the last rotation) reduced the seasonal and annual EF_ds by 29–32%. Over the entire rice-wheat rotation cycle, annual N₂O emissions from the fertilized fields at the three sites ranged from 3.3?±?0.3 to 16.8?±?0.6 kg N ha^?1, with a coefficient of variation (CV) of 61%. Similarly, the EF_ds during the rice-wheat rotation cycle ranged from 0.4% to 2.5%, with a CV of 67%. These high spatial variations might have been related to: variations in soil properties, such as texture and soil organic carbon; management practices, such as straw treatments (i.e., compost versus fresh straw) and weather conditions, such as precipitation and rainfall distribution. Our results indicate that the incorporation of fresh wheat straw at a high rate during the rice season is an effective management practice for the mitigation of N₂O emissions in rice-wheat rotation systems. Whether this practice is also effective in reducing the overall global warming potential of net N₂O, CH₄ and CO₂ emissions needs to be seen through further studies.     

Modeling N2O flux from an Illinois agroecosystem using Monte Carlo sampling of field observations

We modeled the expected range of seasonal and annual N₂O flux from temperate, grain agroecosystems using Monte Carlo sampling of N₂O flux field observations. This analysis is complimentary to mechanistic biogeochemical model outcomes and provides an alternative method of estimating N₂O flux. Our analysis produced a range of annual N₂O gas flux estimates with mean values overlapping with results from an intermodel comparison of mechanistic models. Mean seasonal N₂O flux was 1–4% of available N, while median seasonal N₂O flux was less than 2% of available N across corn, soybean, wheat, ryegrass, legume, and bare fallow systems. The 25th–75th percentile values for simulated average annualized N₂O flux rates ranged from 1 to 12.2 kg N ha^?1 in the conventional system, from 1.3 to 8.8 kg N ha^?1 in the cover crop rotation, and from 0.8 to 9.3 kg N ha^?1 in the legume rotation. Although these modeling techniques lack the seasonal resolution of mechanistic models, model outcomes are based on measured field observations. Given the large variation in seasonal N gas flux predictions resulting from the application of mechanistic simulation models, this data-derived approach is a complimentary benchmark for assessing the impact of agricultural policy on greenhouse gas emissions.     

Assessing the impacts of biochar-blended urea on nitrogen use efficiency and soil retention in wheat production

Improving nitrogen (N) use efficiency (NUE) in crop plants is important to reduce the negative impact of excessive N on the environment. Although biochar-blended fertilizer had been increasingly tested in crop production, the fate of fertilized N in soil and plant had not been elucidated in field conditions. In this study, a novel biochar-blended urea (BU) was prepared by pelleting maize straw biochar, bentonite, sepiolite, carboxymethylcellulose sodium, and chitosan with urea (commercial urea without biochar [CU]). N fertilization in a winter wheat field was treated with BU and CU at both 265 kg N ha^?1 (HL) and 186 kg N ha^?1 (LN). Within a treatment plot, a microplot was fertilized with ¹⁵N-labeled urea at a relevant N level. We investigated the influence of fertilizer management on biomass, grain yield, bioaccumulation of nutrient, soil properties, ¹⁵N isotopic abundance, and greenhouse gas emissions. Microscopic and spectroscopic analysis showed that micro/nanonetwork of biochar could bind N to form a loss control agglomerated particle, and organo-mineral coatings on BU may protect N from quick release. Compared with CU, BU significantly increased grain yield by 13% and 38%, and grain N allocation by 19% and 55%, respectively, at HN and LN level. The total recovery of urea ¹⁵N in wheat plant (¹⁵N based NUE) was 32.8% under CU regardless of N rates but increased to 41.7% (HN rate) and 56.3% (LN rate) under BU. Whereas, the soil proportion (soil residual ¹⁵N) was 20.1% and 13.4% under CU but 32.5% and 18.8% under BU, in 0-20cm topsoil, respectively, at HN and LN rate. Compared with the CU, BU had no effect on CO₂ and CH₄ emissions but significantly reduced the total N₂O emission by 23%–28%. These important findings suggested that BU can be beneficial to uplift plant NUE to reduce reactive N loading but boost crop production.     

Trace gas flux and the influence of short-term soil water and temperature dynamics in Australian sheep grazed pastures of differing productivity

Temperate pastures are often managed with P fertilizers and N₂-fixing legumes to maintain and increase pasture productivity which may lead to greater nitrous oxide (N₂O) emissions and reduced methane (CH₄) uptake. However, the diel and inter-daily variation in N₂O and CH₄ flux in pastures is poorly understood, especially in relation to key environmental drivers. We investigated the effect of pasture productivity, rainfall, and changing soil moisture and temperature upon short-term soil N₂O and CH₄ flux dynamics during spring in sheep grazed pasture systems in southeastern Australia. N₂O and CH₄ flux was measured continuously in a High P (23 kg P ha^?1 yr^?1) and No P pasture treatment and in a sheep camp area in a Low P (4 kg P ha^?1 yr^?1) pasture for a four week period in spring 2005 using an automated trace gas system. Although pasture productivity was three-fold greater in the High P than No P treatment, mean CH₄ uptake was similar (?6.3?±?SE 0.3 to ?8.6?±?0.4 μg C m^?2 hr^?1) as were mean N₂O emissions (6.5 to 7.9?±?0.8 μg N m^?2 hr^?1), although N₂O flux in the No P pasture did not respond to changing soil water conditions. N₂O emissions were greatest in the Low P sheep camp (12.4 μg?±?1.1 N m^?2 hr^?1) where there were also net CH₄ emissions of 5.2?±?0.5 μg C m^?2 hr^?1. There were significant, but weak, relationships between soil water and N₂O emissions, but not between soil water and CH₄ flux. The diel temperature cycle strongly influenced CH₄ and N₂O emissions, but this was often masked by the confounding covariate effects of changing soil water content. There were no consistently significant differences in soil mineral N or gross N transformation rates, however, measurements of substrate induced respiration (SIR) indicated that soil microbial processes in the highly productive pasture are more N limited than P limited after >20 years of P fertilizer addition. Increased productivity, through P fertilizer and legume management, did not significantly increase N₂O emissions, or reduce CH₄ uptake, during this 4 week measurement period, but the lack of an N₂O response to rainfall in the No P pasture suggests this may be evident over a longer measurement period. This study also suggests that small compacted and nutrient enriched areas of grazed pastures may contribute greatly to the overall N₂O and CH₄ trace gas balance.     

Carbon, nitrogen and Greenhouse gases budgets over a four years crop rotation in northern France

Croplands mainly act as net sources of the greenhouse gases carbon dioxide (CO₂) and nitrous oxide (N₂O), as well as nitrogen oxide (NO), a precursor of troposheric ozone. We determined the carbon (C) and nitrogen (N) balance of a four-year crop rotation, including maize, wheat, barley and mustard, to provide a base for exploring mitigation options of net emissions. The crop rotation had a positive net ecosystem production (NEP) of 4.4?±?0.7 Mg C ha^-1 y^-1 but represented a net source of carbon with a net biome production (NBP) of -1.3?±?1.1 Mg C?ha^-1 y^-1. The nitrogen balance of the rotation was correlated with the carbon balance and resulted in net loss (?24?±?28 kg N ha^-1 y^-1). The main nitrogen losses were nitrate leaching (?11.7?±1.0 kg N ha^-1 y^-1) and ammonia volatilization (?9 kg N ha^-1 y^-1). Dry and wet depositions were 6.7?±?3.0 and 5.9?±0.1 kg N ha^-1 y^-1, respectively. Fluxes of nitrous (N₂O) and nitric (NO) oxides did not contribute significantly to the N budget (N₂O: -1.8?±?0.04; NO: -0.7?±?0.04 kg N ha^-1 y^-1) but N₂O fluxes equaled 16% of the total greenhouse gas balance. The link between the carbon and nitrogen balances are discussed. Longer term experiments would be necessary to capture the trends in the carbon and nitrogen budgets within the variability of agricultural ecosystems.     

黄河上游灌区稻田N2O排放特征

黄河上游灌区稻田高产区过量施肥现象十分突出,氮肥过量施用引起土壤氮素盈余,导致N₂O排放量增大,由此引起的温室效应引起广泛关注。采用静态箱-气相色谱法研究黄河上游灌区稻田不同施肥处理下N₂O排放特征。试验设置5个施肥处理,包括常规氮肥300 kg/hm²下单施尿素和有机肥配施2个处理,分别用N300和N300-OM代表;优化氮肥240 kg/hm²下单施尿素和有机肥配施2个处理,分别用N240和N240-OM代表;对照不施氮肥用N0代表。试验结果得出,灌区水稻生长季稻田土壤N₂O排放主要集中在水稻分蘖前及水稻生长的中后期,稻田氮肥施用、灌水及土壤温度的变化对N₂O排放通量影响较大,不同处理水稻各生育阶段N₂O累积排放量与稻田土壤耕层NO^-₃-N含量动态变化显著相关。稻田N₂O排放不是黄河上游灌区稻田氮素损失的主要途径,但灌区稻田N₂O排放的增温潜势较大;稻田氮肥过量施用会显著增加N₂O排放量,在相同氮素水平下,有机肥配施会显著增加稻田土壤N₂O的排放量(P＜0.01)。优化施氮能有效减少灌区稻田水稻生长季N₂O排放量。稻田不同处理的水稻整个生长季土壤N₂O排放总量为2.69-3.87 kg/hm²,肥料氮通过N₂O排放损失的百分率仅为0.43%-0.64%。在灌区习惯灌水和高氮肥300 kg/hm²时,N300-OM处理的稻田N₂O排放量达3.87 kg/hm²,在100 a时间尺度上的全球增温潜势(GWPs)为20.76×10⁷ kg CO₂/hm²;优化施氮240 kg/hm²水平下,N240和N240-OM处理的N₂O累计排放量较N300-OM处理,分别降低了1.18 kg/hm²和0.57 kg/hm²,在100 a尺度上每年由稻田N₂O排放引起的GWPs分别降低了6.33×10⁷ kg CO₂/hm²和3.06×10⁷ kg CO₂/hm²。     

Nitrous oxide emissions from a cropped soil in a semi-arid climate

Understanding nitrous oxide (N₂O) emissions from agricultural soils in semi‐arid regions is required to better understand global terrestrial N₂O losses. Nitrous oxide emissions were measured from a rain‐fed, cropped soil in a semi‐arid region of south‐western Australia for one year on a sub‐daily basis. The site included N‐fertilized (100 kg N ha^?1 yr^?1) and nonfertilized plots. Emissions were measured using soil chambers connected to a fully automated system that measured N₂O using gas chromatography. Daily N₂O emissions were low (?1.8 to 7.3 g N₂O‐N ha^?1 day^?1) and culminated in an annual loss of 0.11 kg N₂O‐N ha^?1 from N‐fertilized soil and 0.09 kg N₂O‐N ha^?1 from nonfertilized soil. Over half (55%) the annual N₂O emission occurred from both N treatments when the soil was fallow, following a series of summer rainfall events. At this time of the year, conditions were conducive for soil microbial N₂O production: elevated soil water content, available N, soil temperatures generally >25 °C and no active plant growth. The proportion of N fertilizer emitted as N₂O in 1 year, after correction for the ‘background’ emission (no N fertilizer applied), was 0.02%. The emission factor reported in this study was 60 times lower than the IPCC default value for the application of synthetic fertilizers to land (1.25%), suggesting that the default may not be suitable for cropped soils in semi‐arid regions. Applying N fertilizer did not significantly increase the annual N₂O emission, demonstrating that a proportion of N₂O emitted from agricultural soils may not be directly derived from the application of N fertilizer. ‘Background’ emissions, resulting from other agricultural practices, need to be accounted for if we are to fully assess the impact of agriculture in semi‐arid regions on global terrestrial N₂O emissions.     

Plant and soil responses of an alpine steppe on the Tibetan Plateau to multi-level nitrogen addition

Background

Although plant growth in alpine steppes on the Tibetan Plateau has been suggested to be sensitive to nitrogen (N) addition, the N limitation conditions of alpine steppes remain uncertain.

Methods

After 2 years of fertilization with NH₄NO₃ at six rates (0, 10, 20, 40, 80 and 160 kg N ha^?1 yr^?1), the responses of plant and soil parameters as well as N₂O fluxes were measured.

Results

At the vegetation level, N addition resulted in an increase in the aboveground N pool from 0.5?±?0.1 g m^?2 in the control plots to 1.9?±?0.2 g m^?2 in the plots at the highest N input rate. The aboveground C pool, biomass N concentration, foliar δ¹⁵N, soil NO₃ ^?-N and N₂O flux were also increased by N addition. However, as the N fertilization rate increased from 10 kg N ha^?1 yr^?1 to 160 kg N ha^?1 yr^?1, the N-use efficiency decreased from 12.3?±?4.6 kg C kg N^?1 to 1.6?±?0.2 kg C kg N^?1, and the N-uptake efficiency decreased from 43.2?±?9.7 % to 9.1?±?1.1 %. Biomass N:P ratios increased from 14.4?±?2.6 in the control plots to 20.5?±?0.8 in the plots with the highest N input rate. Biomass N:P ratios, N-uptake efficiency and N-use efficiency flattened out at 40 kg N ha^?1 yr^?1. Above this level, soil NO₃ ^?-N began to accumulate. The seasonal average N₂O flux of growing season nonlinearly increased with increased N fertilization rate and linearly increased with the weighted average foliar δ¹⁵N. At the species level, N uptake responses to relative N availability were species-specific. Biomass N concentration of seven out of the eight non-legume species increased significantly with N fertilization rates, while Kobresia macrantha and the one legume species (Oxytropics glacialis) remained stable. Both the non-legume and the legume species showed significant ¹⁵N enrichment with increasing N fertilization rate. All non-legume species showed significant increased N:P ratios with increased N fertilization rate, but not the legume species.

Conclusions

Our findings suggest that the Tibetan alpine steppes might be N-saturated above a critical N load of 40 kg N ha^?1 yr^?1. For the entire Tibetan Plateau (ca. 2.57 million km²), a low N deposition rate (10 kg N ha^?1 yr^?1) could enhance plant growth, and stimulate aboveground N and C storage by at least 1.1?±?0.3 Tg N yr^?1 and 31.5?±?11.8 Tg C yr^?1, respectively. The non-legume species was N-limited, but the legume species was not limited by N.     

Contribution of 15N-labelled leaf litter to N turnover, nitrous oxide emissions and N sequestration in a beech forest during eleven years

Aims

Decomposition of leaf litterfall plays a major role for nitrogen (N) dynamics in soils. However, little is known as to which extent beech leaf litter contributes to N turnover and nitrous oxide (N₂O) emissions within one decade after litterfall.

Methods

In 1997, we exchanged recently fallen leaf litter by ¹⁵N-labelled litter in a beech stand (Fagus sylvatica) at the Solling, Germany. Measurements were conducted 2–3 and 10–11 years after litter exchange.

Results

Two years after litter exchange, 92 % of added ¹⁵N was recovered in the surface 10 cm of the soil. The labelled N was primarily found in the upper part of the F layer of the moder type humus. Eleven years after litter exchange, 73 % of the added ¹⁵N was lost and the remaining 27 % was mainly recovered in the lower part of the F layer indicating N sequestration. The remaining leaf litter N was subject to measurable N mineralisation (2–3 % of litter N) and N₂O production (0.02 %). Between 0.3 % (eleventh year) and 0.6 % (second year) of total annual N₂O emissions were attributed to beech leaf litter of a single year.

Conclusions

Most of the annual N₂O emissions (1.33–1.54 kg N ha^?1 yr^?1) were probably derived from older soil N pools.     

Role of maize stover incorporation on nitrogen oxide emissions in a non-irrigated Mediterranean barley field

Aims

Agricultural soils in semiarid Mediterranean areas are characterized by low organic matter contents and low fertility levels. Application of crop residues and/or manures as amendments is a cost-effective and sustainable alternative to overcome this problem. However, these management practices may induce important changes in the nitrogen oxide emissions from these agroecosystems, with additional impacts on carbon dioxide emissions. In this context, a field experiment was carried out with a barley (Hordeum vulgare L.) crop under Mediterranean conditions to evaluate the effect of combining maize (Zea mays L.) residues and N fertilizer inputs (organic and/or mineral) on these emissions.

Methods

Crop yield and N uptake, soil mineral N concentrations, dissolved organic carbon (DOC), denitrification capacity, N₂O, NO and CO₂ fluxes were measured during the growing season.

Results

The incorporation of maize stover increased N₂O emissions during the experimental period by c. 105 %. Conversely, NO emissions were significantly reduced in the plots amended with crop residues. The partial substitution of urea by pig slurry reduced net N₂O emissions by 46 and 39 %, with and without the incorporation of crop residues respectively. Net emissions of NO were reduced 38 and 17 % for the same treatments. Molar DOC:NO ₃ ^? ratio was found to be a robust predictor of N₂O and NO fluxes.

Conclusions

The main effect of the interaction between crop residue and N fertilizer application occurred in the medium term (4–6 month after application), enhancing N₂O emissions and decreasing NO emissions as consequence of residue incorporation. The substitution of urea by pig slurry can be considered a good management strategy since N₂O and NO emissions were reduced by the use of the organic residue.     

Greenhouse gas fluxes from an Australian subtropical cropland under long‐term contrasting management regimes

The long‐term effects of conservation management practices on greenhouse gas fluxes from tropical/subtropical croplands remain to be uncertain. Using both manual and automatic sampling chambers, we measured N₂O and CH₄ fluxes at a long‐term experimental site (1968–present) in Queensland, Australia from 2006 to 2009. Annual net greenhouse gas fluxes (NGGF) were calculated from the 3‐year mean N₂O and CH₄ fluxes and the long‐term soil organic carbon changes. N₂O emissions exhibited clear daily, seasonal and interannual variations, highlighting the importance of whole‐year measurement over multiple years for obtaining temporally representative annual emissions. Averaged over 3 years, annual N₂O emissions from the unfertilized and fertilized soils (90 kg N ha^?1 yr^?1 as urea) amounted to 138 and 902 g N ha^?1, respectively. The average annual N₂O emissions from the fertilized soil were 388 g N ha^?1 lower under no‐till (NT) than under conventional tillage (CT) and 259 g N ha^?1 higher under stubble retention (SR) than under stubble burning (SB). Annual N₂O emissions from the unfertilized soil were similar between the contrasting tillage and stubble management practices. The average emission factors of fertilizer N were 0.91%, 1.20%, 0.52% and 0.77% for the CT‐SB, CT‐SR, NT‐SB and NT‐SR treatments, respectively. Annual CH₄ fluxes from the soil were very small (?200–300 g CH₄ ha^?1 yr^?1) with no significant difference between treatments. The NGGF were 277–350 kg CO₂‐e ha^?1 yr^?1 for the unfertilized treatments and 401–710 kg CO₂‐e ha^?1 yr^?1 for the fertilized treatments. Among the fertilized treatments, N₂O emissions accounted for 52–97% of NGGF and NT‐SR resulted in the lowest NGGF (401 kg CO₂‐e ha^?1 yr^?1 or 140 kg CO₂‐e t^?1 grain). Therefore, NT‐SR with improved N fertilizer management practices was considered the most promising management regime for simultaneously achieving maximal yield and minimal NGGF.