Nitrous Oxide Emissions from a Coal Mine Land Reclaimed with Stabilized Manure

Mine‐soil treatment using stabilized manure rapidly sequesters large quantities of organic carbon and nutrients. However, the nutrient‐rich soil conditions may become highly conducive for production and emission of N₂O. We examined this possibility in a Pennsylvania coal mine restored using poultry manure stabilized in two forms: composted (Comp) or mixed with paper mill sludge (Man + PMS) at C/N ratios of 14, 21, and 28 and compared those with the emissions from conventionally treated soil. The mine soil was extremely well drained with 59% coarse fragments. Soil–atmosphere exchange of N₂O and CO₂ was determined using a sampling campaign of ten measurements between 16 June and 14 September 2009 (90 days) and 13 measurements between 28 June and 9 November 2010 (134 days) using static vented chambers at ambient and increased moisture (water added) content. Potential denitrification was determined in a laboratory incubation experiment. While non‐amended mine soil did not have a measurable potential for denitrifying activity, the manure‐based amendments introduced the potential. Soil water filled pore space was less than 60% on most sampling days in both ambient and water‐added plots. Daily N₂O‐N emissions ranged between 40 and 70 g N ha⁻¹ with cumulative emissions of 2–4 kg N ha⁻¹ from non‐amended, lime and fertilizer (L + F) and Comp, and 3–10 kg N ha⁻¹ from Man + PMS treatments. The maximum emission obtained from Man + PMS represented <1% loss of applied N. Although stabilized manure‐treated soil exhibits the potential for N₂O production, the emission is limited when soils are excessively well drained and reducing conditions rarely develop. Copyright © 2015 John Wiley & Sons, Ltd.     

Towards an agronomic assessment of N2O emissions: a case study for arable crops

Agricultural soils are the main anthropogenic source of nitrous oxide (N₂O), largely because of nitrogen (N) fertilizer use. Commonly, N₂O emissions are expressed as a function of N application rate. This suggests that smaller fertilizer applications always lead to smaller N₂O emissions. Here we argue that, because of global demand for agricultural products, agronomic conditions should be included when assessing N₂O emissions. Expressing N₂O emissions in relation to crop productivity (expressed as above‐ground N uptake: ‘yield‐scaled N₂O emissions') can express the N₂O efficiency of a cropping system. We show how conventional relationships between N application rate, N uptake and N₂O emissions can result in minimal yield‐scaled N₂O emissions at intermediate fertilizer‐N rates. Key findings of a meta‐analysis on yield‐scaled N₂O emissions by non‐leguminous annual crops (19 independent studies and 147 data points) revealed that yield‐scaled N₂O emissions were smallest (8.4 g N₂O‐N kg⁻¹N uptake) at application rates of approximately 180–190 kg N ha⁻¹ and increased sharply after that (26.8 g N₂O‐N kg⁻¹ N uptake at 301 kg N ha⁻¹). If the above‐ground N surplus was equal to or smaller than zero, yield‐scaled N₂O emissions remained stable and relatively small. At an N surplus of 90 kg N ha⁻¹ yield‐scaled emissions increased threefold. Furthermore, a negative relation between N use efficiency and yield‐scaled N₂O emissions was found. Therefore, we argue that agricultural management practices to reduce N₂O emissions should focus on optimizing fertilizer‐N use efficiency under median rates of N input, rather than on minimizing N application rates.     

Carbon dioxide and gaseous nitrogen emissions from biochar‐amended soils under wastewater irrigated urban vegetable production of Burkina Faso and Ghana

To quantify carbon (C) and nitrogen (N) losses in soils of West African urban and peri‐urban agriculture (UPA) we measured fluxes of CO₂‐C, N₂O‐N, and NH₃‐N from irrigated fields in Ouagadougou, Burkina Faso, and Tamale, Ghana, under different fertilization and (waste‐)water regimes. Compared with the unamended control, application of fertilizers increased average cumulative CO₂‐C emissions during eight cropping cycles in Ouagadougou by 103% and during seven cropping cycles in Tamale by 42%. Calculated total emissions measured across all cropping cycles reached 14 t C ha^?1 in Ouagadougou, accounting for 73% of the C applied as organic fertilizer over a period of two years at this site, and 9 t C ha^?1 in Tamale. Compared with unamended control plots, fertilizer application increased N₂O‐N emissions in Ouagadougou during different cropping cycles, ranging from 37 to 360%, while average NH₃‐N losses increased by 670%. Fertilizer application had no significant effects on N₂O‐N losses in Tamale. While wastewater irrigation did not significantly enhance CO₂‐C emissions in Ouagadougou, average CO₂‐C emissions in Tamale were 71% (1.6 t C ha^?1) higher on wastewater plots compared with those of the control (0.9 t C ha^?1). However, no significant effects of wastewater on N₂O‐N and NH₃‐N emissions were observed at either location. Although biochar did not affect N₂O‐N and NH₃‐N losses, the addition of biochar could contribute to reducing CO₂‐C emissions from urban garden soils. When related to crop production, CO₂‐C emissions were higher on control than on fertilized plots, but this was not the case for absolute CO₂‐C emissions.     

Nutrient and carbon balances in organic vegetable production on an irrigated,sandy soil in northern Oman

Our understanding of nutrient and carbon (C) fluxes in irrigated organic cropping systems of subtropical regions is limited. Therefore, leaching of mineral nitrogen (N) and phosphorus (P), gaseous emissions of NH₃, N₂O, CO₂, and CH₄, and total matter balances were measured over 24 months comprising a total cropping period of 260 d in an organic‐cropping‐systems experiment near Sohar (Oman). The experiment on an irrigated sandy soil with four replications comprised two manure types (ORG1 and ORG2) characterized by respective C : N ratios of 19 and 25 and neutral detergent fiber (NDF)‐to‐soluble carbohydrates (SC) ratios of 17 and 108. A mineral‐fertilizer (MIN) treatment with equivalent levels of mineral N, P, and potassium (K) served as a control. The three treatments were factorially combined with a cropping sequence comprising radish (Raphanus sativus L.) followed by cauliflower (Brassica oleracea L. var. botrytis) or carrot (Daucus carota subsp. sativus). Over the 24‐months experimental period gaseous N emissions averaged 45 kg ha^–1 (59% NH₃‐N, 41%N₂O‐N) for MIN, 55 kg N ha^–1 (69% NH₃‐N, 31%N₂O‐N) for ORG1, and 49 kg N ha^–1 (59% NH₃‐N, 41% N₂O‐N) for ORG2. Carbon losses were 6.2 t ha^–1 (98% CO₂‐C, 2% CH₄‐C) for MIN, 9.7 t C ha^–1 (99% CO₂‐C, 1% CH₄‐C) for ORG1, and 10.6 t ha^–1 (98% CO₂‐C, 2% CH₄‐C) for ORG2. Exchange resin–based cumulative leaching of mineral N amounted to 30 kg ha^–1 for MIN, 10 kg ha^–1 for ORG1, and 56 kg ha^–1 for ORG2. Apparent surpluses of 361 kg N ha^–1 and 196 kg P ha^–1 for radish‐carrot and 299 kg N ha^–1 and 184 kg P ha^–1 for radish‐cauliflower were accompanied by K deficits of –59 kg ha^–1 and –73 kg ha^–1, respectively, for both cropping systems. Net C balances for MIN, ORG1, and ORG2 plots were –7.3, –3.1, and 1.5 t C ha^–1 for radish‐carrot and –5.0, 1.3, and 4.6 t C ha^–1 for radish‐cauliflower. The results underline the difficulty to maintain soil C levels in intensively cultivated, irrigated subtropical soils.     

Nitrous oxide emissions from an irrigated sandy-clay loam cropped to maize and wheat

 Nitrous oxide (N₂O) emissions were measured from an irrigated sandy-clay loam cropped to maize and wheat, each receiving urea at 100 kg N ha^–1. During the maize season (24 August–26 October), N₂O emissions ranged between –0.94 and 1.53 g N ha^–1 h^–1 with peaks during different irrigation cycles (four) ranging between 0.08 and 1.53 g N ha^–1 h^–1. N₂O sink activity during the maize season was recorded on 10 of the 29 sampling occasions and ranged between 0.18 and 0.94 g N ha^–1 h^–1. N₂O emissions during the wheat season (22 November–20 April) varied between –0.85 and 3.27 g N ha^–1 h^–1, whereas peaks during different irrigation cycles (six) were in the range of 0.05–3.27 g N ha^–1 h^–1. N₂O sink activity was recorded on 14 of the 41 samplings during the wheat season and ranged between 0.01 and 0.87 g N ha^–1 h^–1. Total N₂O emissions were 0.16 and 0.49 kg N ha^–1, whereas the total N₂O sink activity was 0.04 and 0.06 kg N ha^–1 during the maize and wheat seasons, respectively. N₂O emissions under maize were significantly correlated with denitrification rate and soil NO₃ ^–-N but not with soil NH₄ ⁺-N or soil temperature. Under wheat, however, N₂O emissions showed a strong correlation with soil NH₄ ⁺-N, soil NO₃ ^–-N and soil temperature but not with the denitrification rate. Under either crop, N₂O emissions did not show a significant relationship with water-filled pore space or soil respiration. Received: 11 June 1997     

Nitrous oxide emissions from arable soils in Germany — An evaluation of six long‐term field experiments

In this study emissions of N₂O from arable soils are summarized using data from long‐term N₂O monitoring experiments. The field experiments were conducted at six sites in Germany between 1992 and 1997. The annual N‐application rate ranged from 0 to 350 kg N ha^—1. Mineral and organic N‐fertilizer applications were temporarily split adapted to the growth stage of each crop. N‐fertilizer input and N‐yield by the crops were used to calculate the In/Out‐balance. The closed chamber technique was applied to monitor the N₂O fluxes from soil into the atmosphere. If possible, plants were included in the covers. Annual N₂O emission values were based on flux rate measurements of an entire year. The annual N₂O losses ranged from 0.53 to 16.78 kg N₂O‐N ha^—1 with higher N₂O emissions from organically fertilized plots as compared to minerally fertilized plots. Approximately 50% of the total annual emissions occurred during winter. No significant relationship between annual N₂O emissions and the respective N‐fertilization rate was found. This was attributed to site‐ and crop‐specific effects on N₂O emission. The calculation of the N₂O emission per unit N‐yield from winter cereal plots indicates that the site effect on N₂O emission is more important than the effect of N‐fertilization. From unfertilized soils at the sites Braunschweig and Timmerlah a N‐yield of 60.0 kg N ha^—1 a^—1 and N₂O emissions of 2 kg N ha^—1 a^—1 were measured. This high background emission was assigned to the amount and turnover of soil organic matter. For a crop rotation at the sites Braunschweig and Timmerlah the N In/Out‐balance over a period of four years was identified as a suitable predictor of N₂O emissions. This parameter characterizes the efficiency of N‐fertilization for crop production and allows for N‐mineralization from the soil.     

Modelling soil compaction impacts on nitrous oxide emissions in arable fields

Nitrous oxide (N₂O) emissions comprise the major share of agriculture's contribution to greenhouse gases; however, our understanding of what is actually happening in the field remains incomplete, especially concerning the multiple interactions between agricultural practices and N₂O emissions. Soil compaction induces major changes in the soil structure and the key variables controlling N₂O emissions. Our objective was to analyse the ability of a process‐based model (Nitrous Oxide Emissions (NOE)) to simulate the impact of soil compaction on N₂O emission kinetics obtained from field experiments. We used automatic chambers to continuously monitor N₂O and CO₂ emissions on uncompacted and compacted areas in sugar beet fields during 2 years. Soil compaction led to smaller CO₂ emissions and larger N₂O emissions by inducing anoxic conditions favourable for denitrification. Cumulative N₂O emissions during the crop cycles were 944 and 977 g N ha⁻¹ in uncompacted plots and 1448 and 1382 g N ha⁻¹ in compacted plots in 2007 and 2008, respectively. The NOE model ( Hénault et al., 2005 ) simulated 106 and 138 g N₂O‐N ha⁻¹ in uncompacted plots and 1550 and 650 g N₂O‐N ha⁻¹ in compacted plots in 2007 and 2008, respectively, markedly under‐estimating the nitrification rates and associated N₂O emissions. We modified the model on the basis of published results in order to better simulate nitrification and account for varying N₂O fractions of total end‐products in response to varying soil water and nitrate contents. The modified model (NOE2) better predicted nitrification rates and N₂O emissions following fertilizer addition. Using a fine vertical separation of soil layers of configurable, but constant, thickness (1 cm) also improved the simulations. NOE2 predicted 428 and 416 g N‐N₂O ha⁻¹ in uncompacted plots and 1559 and 1032 g N‐ N₂O ha⁻¹ in compacted plots in 2007 and 2008, respectively. These results show that a simple process‐based model can be used to predict successfully the post‐fertilizer addition kinetics of N₂O emissions and the impact of soil compaction on these emissions. However, large emissions later on during the cropping cycle were not captured by the model, emphasizing the need for further research.     

No-till only increases N2O emissions in poorly-aerated soils

Denitrification rates are often greater in no-till than in tilled soils and net soil-surface greenhouse gas emissions could be increased by enhanced soil N₂O emissions following adoption of no-till. The objective of this study was to summarize published experimental results to assess whether the response of soil N₂O fluxes to the adoption of no-till is influenced by soil aeration. A total of 25 field studies presenting direct comparisons between conventional tillage and no-till (approximately 45 site-years of data) were reviewed and grouped according to soil aeration status estimated using drainage class and precipitation during the growing season. The summary showed that no-till generally increased N₂O emissions in poorly-aerated soils but was neutral in soils with good and medium aeration. On average, soil N₂O emissions under no-till were 0.06 kg N ha⁻¹ lower, 0.12 kg N ha⁻¹ higher and 2.00 kg N ha⁻¹ higher than under tilled soils with good, medium and poor aeration, respectively. Our results therefore suggest that the impact of no-till on N₂O emissions is small in well-aerated soils but most often positive in soils where aeration is reduced by conditions or properties restricting drainage. Considering typical soil C gains following adoption of no-till, we conclude that increased N₂O losses may result in a negative greenhouse gas balance for many poorly-drained fine-textured agricultural soils under no-till located in regions with a humid climate.     

Measuring N2O emissions from organic soils by closed chamber or soil/snow N2O gradient methods

Drained organic soils contribute substantial amounts of nitrous oxide to the global atmosphere, and we should be able to estimate this contribution. We have investigated when the fluxes of N₂O from drained forested or cultivated organic soils could be determined by calculating the fluxes from the concentration gradients of the gas in soil or snow according to Fick's law of diffusion. A static chamber method was applied as a control technique for the gas gradient method. Concentrations of N₂O in soil varied from 296 nl l^?1 to 8534 nl l^?1 during the snow‐free periods and were greatest in the early summer. Our results suggest that the gas gradient method can be used to estimate N₂O emissions from drained organic soils. There was some systematic difference in the N₂O fluxes measured with these two methods, which we attributed to the differences in weather between years 1996 and 1997. In the wet summer of 1996 the chamber method gave greater flux rates than the gas gradient method, and the reverse was true in the dry summer of 1997. In the forest the N₂O fluxes measured with the two methods agreed well. The gas gradient is convenient and fast for measuring N₂O emissions from fairly dry organic unfrozen soil. In winter the diffusion calculation based on the N₂O gradients in snow and the chamber method gave fairly similar flux rates and provided adequate estimates of the fluxes of N₂O in winter.     

Compaction influences N2O and N2 emissions from 15N-labeled synthetic urine in wet soils during successive saturation/drainage cycles

Nitrous oxide emitted from urine patches is a key source of agricultural greenhouse gas emissions. A better understanding of the complex soil environmental and biochemical regulation of urine-N transformations in wet soils is needed to predict N₂O emissions from grazing and also to develop targeted mitigation technologies. Soil aeration, gas diffusion and drainage are key factors regulating N transformations and are affected by compaction during grazing. To understand how soil compaction from animal treading influences N transformations of urine in wet soils, we applied pressures of 0, 220 and 400 kPa to repacked soil cores, followed by ¹⁵N-labeled synthetic urine, and then subjected the cores to three successive saturation–drainage cycles on tension tables from 0 to 10 kPa.Compaction had a relatively small effect on soil bulk density (increasing from 0.81 to 0.88 Mg m⁻³), but strongly affected the pore size distribution. Compaction reduced both total soil porosity and macroporosity. It also affected the pore size distribution, principally by decreasing the proportion of 30–60 μm and 60–100 μm pores and increasing the proportion of micropores (<30 μm).Rates of urine-N transformations, emissions of N₂ and N₂O, and the N₂O to N₂ ratio were affected by the saturation/drainage cycles and degree of compaction. During the first saturation–drainage cycle, production of both N₂O and N₂ was low (<0.4 mg N m⁻² h⁻¹), probably because of anaerobic conditions inhibiting nitrification. In the second saturation/drainage cycle, the predominant product was N₂ at all compaction rates. By the third cycle, with increasing availability of mineral-N substrates, N₂O was the dominant product in the uncompacted (max = 4.70 mg N m⁻² h⁻¹) and 220 kPa compacted soils (max = 7.65 mg N m⁻² h⁻¹) with lower amounts of N₂ produced, while N₂ was produced in similar quantities to N₂O (max = 3.11 mg N m⁻² h⁻¹) in the 400 kPa compacted soil. Reduced macroporosity in the most compacted soil contributed to more sustained N₂ and N₂O production as the soils drained. In addition, compaction affected the rate of change of soil pH and DOC, both of which affected the N₂O to N₂ ratio.Denitrification during drainage and re-saturation may make a large contribution to soil N₂O emissions. Improving soil drainage and adopting grazing management practices that avoid soil compaction while increasing macroporosity will reduce total N₂O and N₂ emissions.     

Soybean Residue Effect on Carbon Fractions and Gaseous Nitrogen Losses at Contrasting Moisture Contents

Most published studies related to crop effects on denitrification are not continuous and are based on the growing period. The objective of this work was to evaluate the effect of different amounts of soybean stubble, under different soil moisture contents, on gaseous nitrogen (N) losses by denitrification from an agricultural soil. The following soil moisture treatments were reached by adding distilled water to soil cores of a typic Hapludoll: 50 and 100% of water‐filled porosity space (WFPS). Residue treatments included no application of residues, amendment with 2600 kg ha^?1 of soybean residues, and amendment with 5200 kg ha^?1 of soybean residues. Cumulative nitrous oxide + dinitrogen (N₂O + N₂) emissions displayed great variability, ranging between 0 and 581.91 µg N kg^?1, which represented 0 to 3.93% of the N residue applied. Under 50% WFPS moisture conditions, statistical differences in cumulative N₂O + N₂ emissions between residue treatments were not detected (p = 0.21), whereas at saturation conditions, cumulative N₂O + N₂ emissions decreased with the application of increasing amounts of soybean residues (p = 0.017). Daily and cumulative N₂O + N₂ emissions significantly increased as soil moisture increased, except at soils amended with 5200 kg ha^?1 of soybean residues; this lack of statistical difference was probably due to the immobilization of native mineral N. Under 50% WFPS soil moisture contents, aeration seemed to be the main factor controlling redox conditions, limiting the denitrification process, and preventing differences in N emissions between residue treatments. The application of soybean residues to saturated soils notably decreased N₂O + N₂ emissions by denitrification through a strong mineral N immobilization into organic and nondenitrifiable forms.     

Annual emissions of nitrous oxide and nitric oxide from a wheat–maize cropping system on a silt loam calcareous soil in the North China Plain

Nitrogen amendment followed by flooding irrigation is a general management practice for a wheat–maize rotation in the North China Plain, which may favor nitrification and denitrification. Consequently, high emissions of nitrous oxide (N₂O) and nitric oxide (NO) are hypothesized to occur. To test this hypothesis, we performed year-round field measurements of N₂O and NO fluxes from irrigated wheat–maize fields on a calcareous soil applied with all crop residues using a static, opaque chamber measuring system. To interpret the field data, laboratory experiments using intact soil cores with added carbon (glucose) and nitrogen (nitrate, ammonium) substrates were performed. Our field measurements showed that pulse emissions after fertilization and irrigation/rainfall contributed to 73% and 88% of the annual N₂O and NO emissions, respectively. Soil moisture and mineral nitrogen contents significantly affected the emissions of both gases. Annual emissions from fields fertilized at the conventional rate (600 kg N ha⁻¹ yr⁻¹) totaled 4.0 ± 0.2 and 3.0 ± 0.2 kg N ha⁻¹ yr⁻¹ for N₂O and NO, respectively, while those from unfertilized fields were much lower (0.5 ± 0.02 kg N ha⁻¹ yr⁻¹ and 0.4 ± 0.05 kg N ha⁻¹ yr⁻¹, respectively). Direct emission factors (EF_ds) of N₂O and NO for the fertilizer nitrogen were estimated to be 0.59 ± 0.04% and 0.44 ± 0.04%, respectively. By summarizing the results of our study and others, we recommended specific EF_ds (N₂O: 0.54 ± 0.09%; NO: 0.45 ± 0.04%) for estimating emissions from irrigated croplands on calcareous soils with organic carbon ranging from 5 to 16 g kg⁻¹. Nitrification dominated the processes driving the emissions of both gases following fertilization. It was evident that insufficient available carbon limited microbial denitrification and thus N₂O emission. This implicates that efforts to enhance carbon sink in calcareous soils likely increase their N₂O emissions.     

CARBON BUDGET AND SEQUESTRATION POTENTIAL IN A SANDY SOIL TREATED WITH COMPOST

The effects of compost application on soil carbon sequestration potential and carbon budget of a tropical sandy soil was studied. Greenhouse gas emissions from soil surface and agricultural inputs (fertiliser and fossil fuel uses) were evaluated. The origin of soil organic carbon was identified by using stable carbon isotope. The CO₂, CH₄ and N₂O emissions from soil were estimated in hill evergreen forest (NF) plot as reference, and in the corn cultivation plots with compost application rate at 30 Mg ha⁻¹ y⁻¹ (LC), and at 50 Mg ha⁻¹ y⁻¹ (HC). The total C emissions from soil surface were 8·54, 10·14 and 9·86 Mg C ha⁻¹ y⁻¹ for NF, HC and LC soils, respectively. Total N₂O emissions from HC and LC plots (2·56 and 3·47 kg N₂O ha⁻¹ y⁻¹) were significantly higher than from the NF plot (1·47 kg N₂O ha⁻¹ y⁻¹). Total CO₂ emissions from fuel uses of fertiliser, irrigation and machinery were about 10 per cent of total CO₂ emissions. For soil carbon storage, since 1983, it has been increased significantly (12 Mg ha⁻¹) under the application of 50 Mg ha⁻¹ y⁻¹ of compost but not with 30 Mg ha⁻¹ y⁻¹. The net C budget when balancing out carbon inputs and outputs from soil for NF, HC and LC soils were +3·24, −2·50 and +2·07 Mg C ha⁻¹ y⁻¹, respectively. Stable isotope of carbon (δ¹³C value) indicates that most of the increased soil carbon is derived from the compost inputs and/or corn biomass. Copyright © 2011 John Wiley & Sons, Ltd.     

Transitioning from standard to minimum tillage: Trade-offs between soil organic matter stabilization, nitrous oxide emissions, and N availability in irrigated cropping systems

Few studies address nutrient cycling during the transition period (e.g., 1–4 years following conversion) from standard to some form of conservation tillage. This study compares the influence of minimum versus standard tillage on changes in soil nitrogen (N) stabilization, nitrous oxide (N₂O) emissions, short-term N cycling, and crop N use efficiency 1 year after tillage conversion in conventional (i.e., synthetic fertilizer-N only), low-input (i.e., alternating annual synthetic fertilizer- and cover crop-N), and organic (i.e., manure- and cover crop-N) irrigated, maize–tomato systems in California. To understand the mechanisms governing N cycling in these systems, we traced ¹⁵N-labeled fertilizer/cover crop into the maize grain, whole soil, and three soil fractions: macroaggregates (>250 μm), microaggregates (53–250 μm) and silt-and-clay (<53 μm). We found a cropping system effect on soil N_new (i.e., N derived from ¹⁵N-fertilizer or -¹⁵N-cover crop), with 173 kg N_new ha⁻¹ in the conventional system compared to 71.6 and 69.2 kg N_new ha⁻¹ in the low-input and organic systems, respectively. In the conventional system, more N_new was found in the microaggregate and silt-and-clay fractions, whereas, the N_new of the organic and low-input systems resided mainly in the macroaggregates. Even though no effect of tillage was found on soil aggregation, the minimum tillage systems showed greater soil fraction-N_new than the standard tillage systems, suggesting greater potential for N stabilization under minimum tillage. Grain-N_new was also higher in the minimum versus standard tillage systems. Nevertheless, minimum tillage led to the greatest N₂O emissions (39.5 g N₂O–N ha⁻¹ day⁻¹) from the conventional cropping system, where N turnover was already the fastest among the cropping systems. In contrast, minimum tillage combined with the low-input system (which received the least N ha⁻¹) produced intermediate N₂O emissions, soil N stabilization, and crop N use efficiency. Although total soil N did not change after 1 year of conversion from standard to minimum tillage, our use of stable isotopes permitted the early detection of interactive effects between tillage regimes and cropping systems that determine the trade-offs among N stabilization, N₂O emissions, and N availability.     

The effect of nitrogen fertilization and no‐till duration on soil nitrogen supply power and post–spring thaw greenhouse‐gas emissions

With a world population now > 7 billion, it is imperative to conserve the arable land base, which is increasingly being leveraged by global demands for producing food, feed, fiber, fuel, and facilities (i.e., infra‐structure needs). The objective of this study was to determine the effect of varying fertilizer‐N rates on soil N availability, mineralization, and CO₂ and N₂O emissions of soils collected at adjacent locations with contrasting management histories: native prairie, short‐term (10 y), and long‐term (32 y) no‐till continuous‐cropping systems receiving five fertilizer‐N rates (0, 30, 60, 90, and 120 kg N ha^–1) for the previous 9 y on the same plots. Intact soil cores were collected from each site after snowmelt, maintained at field capacity, and incubated at 20°C for 6 weeks. Weekly assessments of soil nutrient availability along with CO₂ and N₂O emissions were completed. There was no difference in cumulative soil N supply between the unfertilized long‐term no‐till and native prairie soils, while annual fertilizer‐N additions of 120 kg N ha^–1 were required to restore the N‐supplying power of the short‐term no‐till soil to that of the undisturbed native prairie soil. The estimated cumulative CO₂‐C and N₂O‐N emissions among soils ranged from 231.8–474.7 g m^–2 to 183.9–862.5 mg m^–2, respectively. Highest CO₂ fluxes from the native prairie soil are consistent with its high organic matter content, elevated microbial activity, and contributions from root respiration. Repeated applications of ≥ 60 kg N ha^–1 resulted in greater residual inorganic‐N levels in the long‐term no‐till soil, which supported larger N₂O fluxes compared to the unfertilized control. The native prairie soil N₂O emissions were equal to those from both short‐ and long‐term no‐till soils receiving repeated fertilizer‐N applications at typical agronomic rates (e.g., 90 kg N ha^–1). Eighty‐eight percent of the native soil N₂O flux was emitted during the first 2 weeks and is probably characteristic of rapid denitrification rates during the dormant vegetative period after snowmelt within temperate native grasslands. There was a strong correlation (R² 0.64; p < 0.03) between measured soil Fe‐supply rate and N₂O flux, presumably due to anoxic microsites within soil aggregates resulting from increased microbial activity. The use of modern no‐till continuous diversified cropping systems, along with application of fertilizer N, enhances the soil N‐supplying power over the long‐term through the build‐up of mineralizable N and appears to be an effective management strategy for improving degraded soils, thus enhancing the productive capacity of agricultural ecosystems. However, accounting for N₂O emissions concomitant with repeated fertilizer‐N applications is imperative for properly assessing the net global warming potential of any land‐management system.     

Biochar suppresses N2O emissions while maintaining N availability in a sandy loam soil

Nitrous oxide (N₂O) from agricultural soil is a significant source of greenhouse gas emissions. Biochar amendment can contribute to climate change mitigation by suppressing emissions of N₂O from soil, although the mechanisms underlying this effect are poorly understood. We investigated the effect of biochar on soil N₂O emissions and N cycling processes by quantifying soil N immobilisation, denitrification, nitrification and mineralisation rates using ¹⁵N pool dilution techniques and the FLUAZ numerical calculation model. We then examined whether biochar amendment affected N₂O emissions and the availability and transformations of N in soils.Our results show that biochar suppressed cumulative soil N₂O production by 91% in near-saturated, fertilised soils. Cumulative denitrification was reduced by 37%, which accounted for 85–95 % of soil N₂O emissions. We also found that physical/chemical and biological ammonium (NH₄⁺) immobilisation increased with biochar amendment but that nitrate (NO₃⁻) immobilisation decreased. We concluded that this immobilisation was insignificant compared to total soil inorganic N content. In contrast, soil N mineralisation significantly increased by 269% and nitrification by 34% in biochar-amended soil.These findings demonstrate that biochar amendment did not limit inorganic N availability to nitrifiers and denitrifiers, therefore limitations in soil NH₄⁺ and NO₃⁻ supply cannot explain the suppression of N₂O emissions. These results support the concept that biochar application to soil could significantly mitigate agricultural N₂O emissions through altering N transformations, and underpin efforts to develop climate-friendly agricultural management techniques.     

N2O fluxes from a Haplic Luvisol under intensive production of lettuce and cauliflower as affected by different N‐fertilization strategies

Vegetable‐production systems often show high soil mineral‐N contents and, thus, are potential sources for the release of the climate‐relevant trace gas N₂O from soils. Despite numerous investigations on N₂O fluxes, information on the impact of vegetable‐production systems on N₂O emissions in regions with winter frost is still rare. This present study aimed at measuring the annual N₂O emissions and the total yield of a lettuce–cauliflower rotation at different fertilization rates on a Haplic Luvisol in a region exposed to winter frost (S Germany). We measured N₂O emissions from plots fertilized with 0, 319, 401, and 528 kg N ha^–1 (where the latter three amounts represented a strongly reduced N‐fertilization strategy, a target value system [TVS] in Germany, and the N amount fertilized under good agricultural practices). The N₂O release from the treatments was 2.3, 5.7, 8.8, and 10.6 kg N₂O‐N ha^–1 y^–1, respectively. The corresponding emission factors calculated on the basis of the total N input ranged between 1.3% and 1.6%. Winter emission accounted for 45% of the annual emissions, and a major part occurred after the incorporation of cauliflower residues. The annual N₂O emission was positively correlated with the nitrate content of the top soil (0–25 cm) and with the N surpluses of the N balance. Reducing the amount of N fertilizer applied significantly reduced N₂O fluxes. Since there was no significant effect on yields if fertilization was reduced from 528 kg N ha^–1 according to “good agricultural practice” to 401 kg N ha^–1 determined by the TVS, we recommend this optimized fertilization strategy.     

Straw amendments did not induce high N2O emissions in non-frozen wintertime conditions: A study in northern Germany

An increasing area of oilseed rape cultivation in Europe is used to produce biodiesel. However, a large amount of straw residue is often left in the field in autumn. Straw mineralization provides both carbon (C) and nitrogen (N) sources for emission of soil nitrous oxide (N₂O), which is an important greenhouse gas with a high warming potential. Some studies have focused on soil N₂O emissions immediately post-harvest; however, straw mineralization could possibly last over winter. Most field studies in winter have focused on freeze-thaw cycles. It is still not clear how straw mineralization affects soil N₂O emissions in unfrozen wintertime conditions. We carried out a field experiment in northern Germany in winter 2014, adding straw and glucose as a source of C with three rates of N fertilizer (0, 30, and 60 kg N ha⁻¹). During the 26 days of observation, cumulative N₂O emission in treatments without C addition was negative at all N fertilizer levels. Straw addition produced –3.2, 11.2, and 5.0 mg N₂O-N m⁻² at 0, 30, and 60 kg N ha⁻¹, respectively. Addition of glucose surprisingly caused –1.5, 74.6, and 165 mg N₂O–N m⁻² at 0, 30, and 60 kg N ha⁻¹, respectively. This study demonstrates that oilseed rape straw does not cause high N₂O emissions in wintertime when no extreme precipitation or freeze-thaw cycles are involved, and soil organic C content is low. However, N₂O emission could be intensively stimulated, when both easily available organic C and nitrate are not limited and the soil temperature between 0 and 10°C. These results provide useful information on potential changes to N₂O emissions that may occur due to the increased use of oilseed rape for biodiesel combined with less severe winters in the northern hemisphere driven by global warming.     

Nitrous oxide emissions from agricultural land use in Germany— a synthesis of available annual field data

The nations that have ratified the Kyoto Protocol must set up an appropriate national inventory on N₂O emissions from agricultural land use, in order to report properly on the achievements made in reducing greenhouse‐gas emissions. The search for the appropriate method is a controversial topic as it is subject to high uncertainty in particular associated to the upscaling from site measurements. In this study, all available data from Germany on annual N₂O‐emission rates derived from field experiments of at least an entire year are summarized. From each study, only differences in soil properties on N input qualified as an individual data set. Under these premises, 101 treatments from 27 sites were found equally spread across Germany. The annual N application ranged from 0 to 400 kg N ha^–1 and the annual emission rates from 0.04 to 17.1 kg ha^–1. Annual emission factors (EFs), uncorrected for background emission, varied considerably from 0.18% to 15.54% of N applied. There was no nationwide correlation found for the relationship between N₂O losses and N application, soil C, soil N, soil texture, or soil pH. However, site‐specific trends in the relationship between emission factor and mean soil aeration status, as expressed by the soil type and/or mean climatic conditions, were revealed. Regularly water‐logged soils were characterized by low emission factors as were soils from the drier regions (<600 mm y^–1), whereas well‐aerated soils from the frost‐intensive regions showed exceptionally high emission factors. Since purely physical and chemical parameterization failed to describe N₂O emissions from agricultural land use on the national scale, there must be a biological adaptation to mean site conditions, i.e., different microbial communities react differently to similar actual conditions in terms of N₂O dynamics. Regardless of the point of view, the chapter on N₂O soil dynamics cannot be closed yet, and new additional model concepts, process studies, and field measurements are needed.     

Nitrogen fertilization and liming increased CO2 and N2O emissions from tropical ferralsols,but not from a vertisol

The application of nitrogen (N) fertilizers and liming (CaCO₃) to improve soil quality and crop productivity are regarded as effective and important agricultural practices. However, they may increase greenhouse gas (GHG) emissions. There is limited information on the GHG emissions of tropical soils, specifically when liming is combined with N fertilization. We therefore conducted a full factorial laboratory incubation experiment to investigate how N fertilizer (0 kg N ha⁻¹, 12.5 kg N ha⁻¹ and 50 kg N ha⁻¹) and liming (target pH = 6.5) affect GHG emissions and soil N availability. We focussed on three common acidic soils (two ferralsols and one vertisol) from Lake Victoria (Kenya). After 8 weeks, the most significant increase in cumulative carbon dioxide (CO₂) and nitrous oxide (N₂O) fluxes compared with the unfertilized control was found for the two ferralsols in the N + lime treatment, with five to six times higher CO₂ fluxes than the control. The δ¹³C signature of soil-emitted CO₂ revealed that for the ferralsols, liming (i.e. the addition of CaCO₃) was the dominant source of CO₂, followed by urea (N fertilization), whereas no significant effect of liming or of N fertilization on CO₂ flux was found for the vertisol. In addition, the N₂O fluxes were most significantly increased by the high N + lime treatment in the two ferralsols, with four times and 13 times greater N₂O flux than that of the control. No treatment effects on N₂O fluxes were observed for the vertisol. Liming in combination with N fertilization significantly increased the final nitrate content by 14.5%–39% compared with N fertilization alone in all treatment combinations and soils. We conclude that consideration should be given to the GHG budgets of agricultural ferralsols since liming is associated with high liming-induced CO₂ and N₂O emissions. Therefore, nature-based and sustainable sources should be explored as an alternative to liming in order to manage the pH and the associated fertility of acidic tropical soils.