Soil nitrate distribution,N2O emission and crop performance after the application of N fertilizers to greenhouse vegetables

The impacts of nitrogen (N) application rates on soil nitrate, electrical conductivity (EC), pH, soil N₂O emission, tomato yield and fruit quality were investigated in a 20‐yr vegetable field experiment under greenhouse cultivation in Northeast China. The treatments included no N control (N0), the recommended N rate (N1; mean annual 300 kg/ha) and the commonly used N rate by farmers (N2; mean annual 600 kg/ha). Soil nitrate content and EC increased significantly to 120 cm depth with increasing rates of applied N. An opposite trend was found for soil pH within the upper 20 cm. Cumulative N₂O emission and maximum N₂O emission rates both increased significantly with increasing N rates. Tomato yield increased significantly from the N0 to N1 rate, but remained constant from the N1 to N2 one. Nitrate concentration in tomatoes also increased significantly with N rates. A contrasting trend was found for soluble sugar and organic acid concentrations. The results indicate that the commonly used N2 rate is excessive and causes adverse effects on tomato quality and on the environment. The recommended N1 rate is optimal for sustainable greenhouse vegetable production. In greenhouses, daily soil N₂O emission rates are strongly influenced by N application and fluctuate with irrigation frequency and the incidence of wetting/drying cycles.     

Effects of N placement,carbon distribution and temperature on N2O emissions in clay loam and loamy sand soils

Changes in the profile distribution of soil C stocks for conventional versus no‐tillage can affect N₂O losses. Uncertainty remains whether deep N placement into a wetter layer in humid areas would affect N₂O losses. This study evaluated the effects of soil carbon profile distribution (inverted, normal), depth of nitrogen placement (5 cm, 15 cm), temperature (10, 20 and 30 °C) and soil texture (clay loam, loamy sand) on N₂O emissions from soil cores in a 216‐h incubation after simulated rainfall. N₂O losses were larger from the clay loam than from the loamy sand, and cumulative N₂O emissions from the inverted profile, with greater C levels at depth, were more than those from the profile with more C near the upper surface. Cumulative N₂O losses from the inverted clay loam profile with deep N placement (1.16 mg N per kg dry soil; 0.71% of applied N) on average were almost double those in the loamy sand (0.62 mg N per kg dry soil; 0.42%). The smallest N₂O losses were measured from the profiles with more C close to the upper surface with a shallow placement of N for the clay loam (0.19 mg N per kg dry soil; 0.12%) and loamy sand (0.33 mg N per kg dry soil; 0.23%). An exponential relationship between N₂O fluxes and temperature was measured. We conclude that large N₂O losses may occur under the combination of greater soil C content at deeper layers (ploughed soils) and moist profiles after N application (humid regions). Deep N placement appears to aggravate rather than ameliorate these concerns.     

Ammonium accumulation in soil: the long‐term effects of tillage,rotation and N rate in a Mediterranean Vertisol

Plant nutrition requires organic nitrogen to be mineralized before roots can absorb it. A 13‐year field study was conducted on typical rain‐fed Mediterranean Vertisol to determine the effects of tillage system, crop rotation and N fertilizer rate on the long‐term NH₄⁺–N content in the soil profile (0–90 cm). The experiment was designed as a randomized complete block with a split–split plot arrangement and three replications. The main plots tested the effects from the tillage system (no‐tillage and conventional tillage); the subplots tested crop rotation with 2‐year rotations (wheat–wheat, wheat–fallow, wheat–chickpea, wheat–faba bean and wheat–sunflower) and the sub‐subplots examined the N fertilizer rate (0, 50, 100 and 150 kg N/ha). Soil NH₄⁺–N content was greatest in the rainiest years and greater under the no‐tillage (NT) system than the conventional tillage (CT) system (57 and 48 kg/ha, respectively). The deepest soil (30–60 and 60–90 cm) contained a greater NH₄⁺–N content (21.0 and 21.4 kg/ha, respectively) than the shallowest soil (19.5 kg/ha in 0–30 cm). This observation may be related to Vertisol characteristics, especially crack formation that allows greater mineralization in the deepest layers by displacing organic matter.     

Formation of hybrid N2O in a suspended soil due to co‐denitrification of NH2OH

A few studies have shown that amine compounds (e.g., hydroxylamine) can be co‐metabolically introduced into the reaction pathway of denitrification. During this microbial process, the N atom of the amine species is bound to a N atom of nitrite. In case of hydroxylamine, this concomitant reaction ultimately results in the formation of hybrid N₂O. Due to its co‐metabolic character the process has been termed co‐denitrification. Hybrid N₂O production during co‐denitrification has been proven to occur in prokaryotic (e.g., Pseudomonas sp.) as well as eukaryotic (e.g., Fusarium sp.) species. Many of them are already well‐known as common denitrifiers. However, until now no clear evidence has been provided to show that N₂O production by co‐denitrification really takes place in a soil. In the present study, a formation of hybrid N₂O was revealed by an adapted ¹⁵N‐tracer model, when both hydroxylamine and ¹⁵N‐nitrate were applied (mol ratio 10:1) to an anaerobically incubated soil suspension from a Haplic Chernozem. The presence of hybrid N₂O was also indicated by a novel characteristic factor (R_binom) developed for a hybrid‐N‐N‐gas detection. By contrast, no hybrid N₂O was found when either an autoclaved soil suspension, only nitrate or only hydroxylamine was used. Thus, it appears that hybrid‐N₂O formation occurred due to co‐denitrification of hydroxylamine. Hence, this is the first study which demonstrates hybrid‐N₂O production by co‐denitrification beyond a microbial species level. The ¹⁵N‐tracer model revealed that under the given experimental conditions N₂O production by co‐denitrification prevailed against N₂O from denitrification and abiotic hydroxylamine decomposition. In addition, a formation of hybrid N₂ was also calculated by the model. However, the experimental results lead to the conclusion that it was most likely caused by a reduction of hybrid N₂O due to conventional denitrification.     

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.     

Symbiotic N<Subscript>2</Subscript> fixation and nitrate utilisation in irrigated lucerne (<Emphasis Type="Italic">Medicago sativa</Emphasis>) systems

Symbiotic N₂ fixation by lucerne (Medicago sativa) has capacity to provide significant inputs of N to agro-ecosystems, and the species has also been shown to scavenge soil mineral N and thus act as a sink for excess reactive N. The balance between these two N cycle processes was investigated in an extensive irrigated lucerne growing region where nitrate contamination of groundwater has been reported. We sampled 18 permanent pure lucerne stands under irrigation for standing dry matter, total shoot N, and N₂ fixation using ¹⁵N natural abundance along with activity of the inducible enzyme nitrate reductase as indicators of use of soil NO₃⁻ by lucerne. On average 65% of lucerne N was obtained from symbiotic N₂ fixation. Converting standing dry matter estimates to annual N₂ fixation amounts we calculated average N₂ fixation of 311 kg N/ha, including N in roots and nodules. Uptake of N from soil by lucerne was calculated to be 181 kg N/ha/year. We were not able to identify the source of this soil mineral N, although nitrate reductase activity of lucerne was higher than that of non-N₂ fixing species examined.     

Soil chemical properties after long‐term n fertilization of bromegrass: Nitrogen rate

Abstract

Nitrogen (N) fertilizers increase yield and quality of grass forage, and may also alter soil chemical properties. A field experiment was conducted in south‐central Alberta to determine the effect of long‐term application of ammonium nitrate to bromegrass on concentration and downward mobility of soluble NO₃‐N, extractable NH₄‐N, P, Ca, Mg, and K, and total C and N in a Thin Black Chernozemic loam soil. The fertilizer was applied annually in early spring for 16 years at 0 to 336 kg N/ha. There was little accumulation of NO₃‐N in the soil at N rates of 112 kg/ha or less. However, at rates higher than 112 kg N/ha there was accumulation of NO₃‐N in the 15–30 and 30–60 cm layers, but very little in the 90–120 cm depth. The NH₄‐N accumulated in the 0–5 cm layer when the fertilizer was applied at rates between 168 to 280 kg N/ha and in the 5–10 cm layer at N rates exceeding 280 kg/ha. There was a decline in extractable P in soil with N application up to 84 kg N/ha rate, while it increased with high N rates. The increasing amounts of applied N resulted in a decline in extractable soil Ca, Mg and K, and this decrease was more pronounced in the 0–5,5–10,10–15, and 15–30 cm layers for K, 0–5 and 5–10 cm layers for Ca, and 0–5, 5–10, and 10–15 cm layers for Mg. There was a build‐up of total C and N in the surface soil with increasing rate of applied N.     

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.     

Pig slurry treatment modifies slurry composition,N2O,and CO2 emissions after soil incorporation

The treatment of manures may improve their agricultural value and environmental quality, for instance with regards to greenhouse gases mitigation and enhancement of carbon (C) sequestration. The present study verified whether different pig slurry treatments (i.e. solid/liquid separation and anaerobic digestion) changed slurry composition. The effect of the slurry composition on N₂O and CO₂ emissions, denitrification and soil mineral nitrogen (N), after soil incorporation, was also examined during a 58-day mesocosm study. The treatments included a non-treated pig slurry (NT), the solid fraction (SF), and the liquid fraction (LF) of a pig slurry and the anaerobically digested liquid fraction (DG). Finally, a non-fertilized (N0) and a treatment with urea (UR) were also present.The N₂O emissions measured represented 4.8%, 2.6%, 1.8%, 1.0% and 0.9% of N supplied with slurry/fertilizer for NT, LF, DG, SF and UR, respectively. Cumulative CO₂ emissions ranged from 0.40 g CO₂-C kg^?1 soil (0.38 Mg CO₂-C ha^?1) to 0.80 g CO₂-C kg^?1 soil (0.75 Mg CO₂-C ha^?1). They were highest for SF (56% of C applied), followed by NT (189% of C applied), LF (337% of C applied) and DG (321% of C applied). Ammonium was detected in the soil for all treatments only at day one, while nitrate concentration increased linearly from day 15 to day 58, at a rate independent of the type of slurry/fertilizer applied. The nitrate recovery at day 58 was 39% of the N applied for NT, 19% for SF, 52% for LF, 67% for DG, and 41% for UR. The solid fraction generally produced higher potential denitrification fluxes (75.3 for SF, 56.7 for NT, 53.6 for LF, 47.7 for DG and 39.7 mg N₂O + N₂-N kg^?1 soil for UR). The high variability of actual denitrification results obfuscated any treatment effect.We conclude that treatment strongly affects slurry composition (mainly its C, fibre and NH₄⁺ content), and hence N₂O and CO₂ emission patterns as well as denitrification processes and nitrate availability. In particular, the solid fraction obtained after mechanical separation produced the most pronounced difference, while the liquid fraction and the anaerobically digested liquid fraction did not show significant difference with respect to the original slurry for any of the measured parameters. Combining data from the different fractions we showed that separation of slurry leads to reduced N₂O emissions, irrespective of whether the liquid fraction is digested or not. Furthermore, our results suggested that the default emission factor for N₂O emissions inventory is too low for both the non-treated pig slurry and its liquid fraction (digested or not), and too high for the separated solid fraction and urea.     

Nitrate removal from drained and reflooded fen soils affected by soil N transformation processes and plant uptake

Knowledge about nitrate transformation processes and how they are affected by different plants is essential in order to reduce the loss of valuable N fertiliser as well as to prevent environmental pollution due to nitrate leaching or N₂O emission after fertilisation or the reflooding of degraded fens with nitrate-containing municipal sewage. Therefore four microcosm ¹⁵N tracer experiments were performed to evaluate the effect of common wetland plants (Phalaris arundinacea, Phragmites australis) combined with different soil moisture conditions (from dry to reflooded) on nitrate turnover processes. At the end of experiment, the total formation of gaseous N compounds was calculated using the ¹⁵N balance method. In two experiments (wet and reflooded soil conditions) the N₂O and N₂ emissions were also directly determined.Our results show that in degraded fen soils, which process mainly takes place—denitrification or transformation into organic N compounds—is determined by the soil moisture conditions. Under dry soil moisture conditions (water filled pore space: 31%) up to 80% of the ¹⁵N nitrate added was transformed into organic N compounds. This transformation process is not affected by plant growth. Under reflooded conditions (water filled pore space: 100%), the total gaseous N losses were highest (77-95% of the ¹⁵N-nitrate added) and the transformation into organic N compounds was very low (1.8% of ¹⁵N nitrate added). Under almost all soil conditions plant growth reduced the N losses by 20-25% of the ¹⁵N nitrate added due to plant uptake. The N₂ emissions exceeded the N₂O emissions by a factor of 10-20 in planted soil, and as much as 30 in unplanted soil. In the treatments planted with Phragmites australis, N₂O emission was about two times higher than in the corresponding unplanted treatment. 15% of the N₂O and N₂ formed was transported via the Phragmites shoots from the soil into the atmosphere. By contrast, Phalaris arundinacea did not affect N₂O emissions and no emission via the shoots was observed.     

水氮耦合对绿洲灌区土壤硝态氮运移及甜瓜氮素吸收的影响

在地处沙漠绿洲的甜瓜种植区,研究不同水、 氮输入量对土壤氮素平衡和运移的影响,为当地甜瓜生产的水肥管理提供科学依据。通过2009、 2010连续两年田间裂区试验,研究了不同灌水量（1500、 2100、 2700、 3300 m3/hm2,以W1500、 W2100、 W2700和W3300表示）和施氮量（N 0、 120、 240、 360 kg/hm2,以N0、 N120、 N240和N360表示）对土壤硝态氮分布、 累积和甜瓜的水、 氮吸收以及产量的影响。结果表明,甜瓜收获后各处理土壤硝态氮含量在040 cm土层最高, 0200 cm土层呈现先减少后增加再减少的变化趋势,且施氮量越大,硝态氮在80120 cm土层大量累积的趋势越明显。土壤硝态氮累积量随施氮量的增加而增加,随灌水量的增加而减少,灌水量超过2700 m3/hm2 时,仅有不到53%的硝态氮留存在0100 cm土层。甜瓜产量和果实氮素吸收量随灌水量和施氮量的增加而提高,但在W3300N360处理略有下降。氮素回收率随施氮量的增加持续降低,氮收获指数以处理W2700N240最大,水分利用效率以W1500N240处理最大。W2700N240处理能够兼顾甜瓜产量,平衡氮素吸收运移与土壤中硝态氮的留存空间3个方面,是绿洲灌区甜瓜种植的高产高效的水氮输入模式。     

Effect of improved fallow on crop productivity, soil fertility and climate-forcing gas emissions in semi-arid conditions

The impacts of fallow on soil fertility, crop production and climate-forcing gas emissions were determined in two contrasting legumes, Gliricidia sepium and Acacia colei, in comparison with traditional unamended fallow and continuous cultivation systems. After 2 years, the amount of foliar material produced did not differ between the two improved fallow species; however, grain yield was significantly elevated by 55% in the first and second cropping season after G. sepium compared with traditional fallow. By contrast, relative to the unamended fallow, a drop in grain yield was observed in the first cropping season after A. colei, followed by no improvement in the second. G. sepium had higher foliar N, K and Mg, while A. colei had lower foliar N but higher lignin and polyphenols. In the third year after fallow improvement, a simulated rainfall experiment was performed on soils to compare efflux of N₂O and CO₂. Improved fallow effects on soil nutrient composition and microbial activity were demonstrated through elevated N₂O and CO₂ efflux from soils in G. sepium fallows compared with other treatments. N₂O emissions were around six times higher from this nitrogen-fixing soil treatment, evolving 69.9 ngN₂O–N g⁻¹soil h⁻¹ after a simulated rainfall event, compared with only 8.5 and 4.8 ngN₂O–N g⁻¹soil h⁻¹ from soil under traditional fallow and continuous cultivation, respectively. The findings indicate that selection of improved fallows for short-term fertility enhancement has implications for regional N₂O emissions for dry land regions.     

Amino acid 15N in long-term bare fallow soils: influence of annual N fertilizer and manure applications

Long‐term dynamics of amino acids (AAs), from a bare fallow soil experiment (established in 1928 at INRA‐Versailles, France), were examined in unamended control (Con) plots and plots treated with ammonium sulphate (Amsul), ammonium nitrate (Amnit), sodium nitrate (Nanit) or with animal manure (Man). Topsoil (0–25 cm) from 1929, 1963 and 1997 was analysed for C, N and ¹⁵N content and distribution of 18 amino acids recovered after acid hydrolysis with 6 m HCl. With time, soil N, C and AA content were reduced in Con, Amsul, Amnit and Nanit, but increased in Man. However, the absolute N loss was 3–11 times larger in Man than Nanit, Amsul, Amnit and Con, due to the much higher N annual inputs applied to Man. From 1929 to 1997 in Con, Amsul, Amnit and Nanit the whole soil and non‐hydrolysable‐N pool δ¹⁵N increased associated with the loss of N (indicative of Rayleigh ¹⁵N^/14N fractionation). No δ¹⁵N change from 1929 to 1997 was found in the hydrolysable AA‐N (HAN) pool. Fertilizer N inputs aided stabilization of soil AA‐N, as AA half‐life in the mineral N fertilizer treatments increased from 34 years in 1963 to 50 years in 1997. The δ¹⁵N values of alanine and leucine reflected both source input and ¹⁵N^/14N fractionation effects in soils. The δ¹⁵N increase of ornithine (～6‰) was similar to the whole soil. The δ¹⁵N change of phenylalanine in Con (decrease of 7‰) was related to its proportional loss since 1929, whereas for Amsul, Amnit, Nanit and Man it was associated with isotope effects caused by the fertilizer inputs. However, the soil δ¹⁵N value of most individual amino acids (IAAs) did not significantly change over nearly 70 years, even with mineral or organic N inputs. We conclude for these bare fallow systems that: (i) δ¹⁵N changes in the whole soil and non‐hydrolysable AA pool were solely driven by microbial processes and not by the nature of fertilizer inputs, and (ii) without plant inputs, the δ¹⁵N of the HAN pool and (most) IAAs may reflect the influence of plant–soil interactions from the previous (arable cropping) rather than present (fallow) land use on these soil δ¹⁵N values.     

CO2 evolution and N mineralization after biogas slurry application in the field and its yield effects on spring barley

The objective of this study was to investigate the effects of biogas slurry derived from straw-rich farmyard manure on the soil microbial biomass, on the mineralization in the field and on the related crop yield. The experiment was carried out in the following four treatments: (1) fallow, (2) fallow + biogas slurry, (3) spring barley, and (4) spring barley + biogas slurry. The CO₂ evolution rate ranged between 15 and 120 mg C m⁻² h⁻¹ in both fallow treatments and showed a significant exponential relationship with the soil temperature at 5 cm depth. According to the extrapolation of the CO₂ evolution rates into amounts per hectare, approximately 200 kg C ha⁻¹ or 27% of the biogas slurry derived C were mineralized to CO₂ during a 50 days’ period to 18 June in the fallow treatment with biogas slurry. An additional amount of up to 29.5 kg inorganic N ha⁻¹ could be calculated as the sum of NH₄-N already present in biogas slurry at the time of amendment and from the amount of biogas slurry mineralized in the soil to NO₃-N. A good agreement between measured and modelled stocks of inorganic N at 0–60 cm depth was obtained after having five-fold increased soil organic C turnover compared to the default values of the model DNDC. The mineralization data are in line with an amount of up to 21 kg ha⁻¹ more N transferred by the barley plants to their aboveground biomass in biogas slurry treatment. The N not accounted for by the aboveground plant biomass could be explained by the belowground plant-derived N. CO₂ evolution from the soil surface, inorganic N content at 0–60 cm depth and N transfer into barley aboveground biomass lead apparently to similar results after the application of biogas slurry. The soil ATP content after harvest of the barley was significantly larger in the two treatments with biogas slurry, especially in the fallow treatment indicating a positive effect on the soil microbial community.     

Soil N Losses by Denitrification Evaluated Using the 15N Tracer Method

Molecular nitrogen (N₂) and nitrous oxide (N₂O) generated by denitrification increase N losses in the soil–plant system. This study aimed to quantify N₂ and N₂O from potassium nitrate (K¹⁵NO₃) applied to soils with different textures and moisture contents in the absence and presence of a source of carbon (C) using the ¹⁵N tracer method. In the three soils used (sandy texture (ST), sandy clay loam texture (SCLT), and clayey texture (CT)), three moisture contents were evaluated (40%, 60%, and 80% of the water holding capacity (WHC)) with (D⁺) and without (D^?) dextrose added. The treatments received 100 mg N kg^?1 (KNO₃ with 23.24 atom% ¹⁵N). N₂ emissions occurred in all of the treatments, but N₂O emissions only occurred in the D⁺ treatment, showing increases with increasing moisture content. SCLT with 80% WHC in the D⁺ treatment exhibited the highest accumulated N emission (48.26 mg kg^?1). The ¹⁵N balance suggested trapping of the gases in the soil.     

Primed N2O emission from native soil nitrogen: A 15N‐tracing laboratory experiment

Soils can naturally be a source of the potent greenhouse gas nitrous oxide (N₂O). By contrast, the largest anthropogenic source of N₂O is the application of nitrogen (N) fertilizer on agricultural soil, but it is unclear if fertilizer‐supported N₂O emission only originates from the fertilizer N directly or through additionally stimulated N₂O production from native soil N. Even though native soil N also includes mineral N already in soil before fertilizer application, organic N is the principal native N pool and thereby provides for mineral N cycling and N₂O emission. Here, we tested (1) the contribution of native soil N to N₂O emission after mineral N fertilizer application and (2) whether it is affected by different soil organic matter (SOM) contents by conducting a laboratory ¹⁵N‐tracing experiment with agricultural soil from a long‐term field trial with two treatments. Both field treatments are fertilized with mineral N, whereas only one of the two receives liquid manure causing higher SOM content. Soil sampling was conducted in March 2016 shortly before fertilizer application in the field. The application of ¹⁵N‐labeled fertilizer more than doubled the N₂O production from native N sources compared to the non‐fertilized control incubations. This primed N₂O production contributed by 5–8% to the fertilizer‐induced N₂O emission after one week of incubation and was similar for both field treatments regardless of liquid manure application. Therefore, further research is needed to link N₂O priming to its potential production pathways and sources. While the observed effect may be important in soils, the amount of applied N fertilizer remains the largest concern being responsible for the majority of N₂O emission.     

N2O emissions from humid tropical agricultural soils: effects of soil moisture,texture and nitrogen availability

We studied soil moisture dynamics and nitrous oxide (N₂O) fluxes from agricultural soils in the humid tropics of Costa Rica. Using a split-plot design on two soils (clay, loam) we compared two crop types (annual, perennial) each unfertilized and fertilized. Both soils are of andic origin. Their properties include relatively low bulk density and high organic matter content, water retention capacity, and hydraulic conductivity. The top 2–3 cm of the soils consists of distinct small aggregates (dia. <0.5 cm). We measured a strong gradient of bulk density and moisture within the top 7 cm of the clay soil. Using automated sampling and analysis systems we measured N₂O emissions at 4.6 h intervals, meteorological variables, soil moisture, and temperature at 0.5 h intervals. Mean daily soil moisture content at 5 cm depth ranged from 46% water filled pore space (WFPS) on clay in April 1995 to near saturation on loam during a wet period in February 1996. On both soils the aggregated surface layer always remained unsaturated. Soils emitted N₂O throughout the year. Mean N₂O fluxes were 1.04±0.72 ng N₂O-N cm⁻² h⁻¹ (mean±standard deviation) from unfertilized loam under annual crops compared to 3.54±4.31 ng N₂O-N cm⁻² h⁻¹ from the fertilized plot (351 days measurement). Fertilization dominated the temporal variation of N₂O emissions. Generally fluxes peaked shortly after fertilization and were increased for up to 6 weeks (‘post fertilization flux’). Emissions continued at a lower rate (‘background flux’) after fertilization effects faded. Mean post-fertilization fluxes were 6.3±6.5 ng N₂O-N cm⁻² h⁻¹ while the background flux rate was 2.2±1.8 ng N₂O-N cm⁻² h⁻¹. Soil moisture dynamics affected N₂O emissions. Post fertilization fluxes were highest from wet soils; fluxes from relatively dry soils increased only after rain events. N₂O emissions were weakly affected by soil moisture during phases of low N availability. Statistical modeling confirmed N availability and soil moisture as the major controls on N₂O flux. Our data suggest that small-scale differences in soil structure and moisture content cause very different biogeochemical environments within the top 7 cm of soils, which is important for net N₂O fluxes from soils.     

Effect of nitrogen fertilization, cropping and irrigation on soil air composition and nitrous oxide emission in a loamy clay

Most of the nitrous oxide (N₂O) in the atmosphere, thought to be involved in global warming, is emitted from soil. Although the main factors controlling the production of N₂O in soil are well known, we need more quantitative data on the interactions of soil and the environment in the soil that affect the emission. We therefore studied the effects of irrigation, cropping (fallow, barley with grass undersown) and N fertilization (unfertilized, 103 kg N ha^?1) on the composition of soil air and direct N₂O emission from soil (using the closed chamber method) in a factorial field experiment on a well‐structured loamy clay soil during 1 June?22 October 1993. The measurements were made weekly during the growing season and three times after harvesting. The composition of the soil air did not indicate severe anoxia in any treatment or combination of treatments, but the accumulation of N₂O in the soil air indicated that hypoxia was common. At the start of the irrigation the emissions were small, even though there was much ammonium and nitrate in the soil and therefore a potential for emission of N₂O produced by both nitrification and denitrification. Larger emissions occurred later. The largest emissions were found when 60–90% of the soil pore space was filled with water. Irrigation and fertilization with N both roughly doubled the cumulative N₂O emission. Growing a crop decreased it by a factor of 3–7. Most N₂O was lost from the irrigated fertilized soil under fallow (3.5 kg N ha^?1), and least from the unirrigated unfertilized soil under barley (0.1 kg N ha^?1).     

Evaluation of nitrate and ammonium as sources of NO and N2O emissions from black earth soils (Haplic Chernozem) based on 15N field experiments

Nitrous oxide (N₂O) and nitric oxide (NO) released from soil is a concern since it can act as a potential atmospheric pollutant and it represents a loss of N from the soil. These gases are present in the atmosphere in trace amounts and are important to atmospheric chemistry and earth's radiative balance. Nitric oxide (NO) does not directly contribute to the greenhouse effect, but it contributes to climate forcing through its role in photochemistry of hydroxyl radicals and ozone and plays a key role in air quality issues. Nitrification and denitrification have been identified as major controlling microbial processes in soils responsible for the formation of NO and N₂O. To elucidate the contribution of both processes to the release of NO and N₂O from loess-black earth soils under field conditions—i.e. to evaluate nitrate and ammonium as sources of NO and N₂O emission—two field experiments with either [¹⁵N] nitrate (NO₃^?) or [¹⁵N] ammonium (NH₄⁺) labelling have been conducted at two sites differing in soil organic matter content (high and normal SOM). [¹⁵N] nitrate treatments revealed that denitrification of NO₃^? represents the main pathway of soil N₂O release. On average 76% and 54% of N₂O was emitted during denitrification from soils with high and normal SOM content, respectively. Contrarily, denitrification contributed on average only 17% and 12% of released NO from soil with high and normal SOM content, respectively. The [¹⁵N]ammonium treatments revealed that nitrification of NH₄⁺ is the major process responsible for soil NO emission. SOM content of the loess-black earth soil significantly influenced NO and N₂O emission. The soil with the higher SOM content showed lower NO emission but drastically increased N₂O emission after nitrate fertilisation. In particular the soil with high SOM content exhibited a high sorption capacity for ammonium ions which led to unexpected results after fertilisation with [¹⁵N]ammonium. To explain this results a revised concept containing three different interacting soil ammonium pools have been hypothesised.     

Effects of moisture and temperature on greenhouse gas emissions and C and N leaching losses in soil treated with biogas slurry

The objective of this study was to examine the effects of soil moisture, irrigation pattern, and temperature on gaseous and leaching losses of carbon (C) and nitrogen (N) from soils amended with biogas slurry (BS). Undisturbed soil cores were amended with BS (33 kg N ha⁻¹) and incubated at 13.5°C and 23.5°C under continuous irrigation (2 mm day⁻¹) or cycles of strong irrigation and partial drying (every 6 weeks, 1 week with 12 mm day⁻¹). During the 6 weeks after BS application, on average, 30% and 3.8% of the C and N applied with BS were emitted as carbon dioxide (CO₂) and nitrous oxide (N₂O), respectively. Across all treatments, a temperature increase of 10°C increased N₂O and CO₂ emissions by a factor of 3.7 and 1.7, respectively. The irrigation pattern strongly affected the temporal production of CO₂ and N₂O but had no significant effect on the cumulative production. Nitrogen was predominantly lost in the form of nitrate (NO₃⁻). On average, 16% of the N applied was lost as NO₃⁻. Nitrate leaching was significantly increased at the higher temperature (P < 0.01), while the irrigation pattern had no effect (P = 0.63). Our results show that the C and N turnovers were strongly affected by BS application and soil temperature whereas irrigation pattern had only minor effects. A considerable proportion of the C and N in BS were readily available for soil microorganisms.