Denitrification and total fertilizer-N losses from an irrigated cotton field

 In a 2-year field study, denitrification loss was measured from an irrigated sandy-clay loam under cotton receiving urea-N at 158–173 kg ha^–1. An acetylene inhibition-soil core method was employed for the direct measurement of denitrification, considering also the N₂O entrapped in the soil. Taking into account the N₂O evolved from soil cores and that entrapped in the soil, a total of 65.7 kg N ha^–1 and 64.4 kg N ha^–1 was lost due to denitrification during the 1995 and 1996 cotton-growing seasons, respectively. Most (>70%) of the denitrification loss occurred during June–August, a period characterized by high soil temperatures and heavy monsoon rains. On average, 35% of the denitrification-N₂O was found entrapped in the soil and the amount of entrapped N₂O was significantly correlated with head space N₂O concentration and with water-filled pore space. ¹⁵N-balance during the 1996 growing season revealed a loss of 71.8 kg N ha^–1. It was concluded that a substantial proportion of the fertilizer-N applied to irrigated cotton is lost under the semiarid subtropical climatic conditions prevailing in the Central Punjab region of Pakistan and that denitrification is the major N loss process under irrigated cotton in this region. Received: 8 March 1999     

Soil N<Subscript>2</Subscript>O emissions and N<Subscript>2</Subscript>O/(N<Subscript>2</Subscript>O+N<Subscript>2</Subscript>) ratio as affected by different fertilization practices and soil moisture

The objective of this work was to evaluate the effect of the chemical nature and application frequency of N fertilizers at different moisture contents on soil N₂O emissions and N₂O/(N₂O+N₂) ratio. The research was based on five fertilization treatments: unfertilized control, a single application of 80 kg ha⁻¹ N-urea, five split applications of 16 kg ha⁻¹ N-urea, a single application of 80 kg ha⁻¹ N–KNO₃, five split applications of 16 kg ha⁻¹ N–KNO₃. Cumulative N₂O emissions for 22 days were unaffected by fertilization treatments at 32% water-filled pore space (WFPS). At 100% and 120% WFPS, cumulative N₂O emissions were highest from soil fertilized with KNO₃. The split application of N fertilizers decreased N₂O emissions compared to a single initial application only when KNO₃ was applied to a saturated soil, at 100% WFPS. Emissions of N₂O were very low after the application of urea, similar to those found at unfertilized soil. Average N₂O/(N₂O+N₂) ratio values were significantly affected by moisture levels (p = 0.015), being the lowest at 120% WFPS. The N₂O/(N₂O+N₂) ratio averaged 0.2 in unfertilized soil and 0.5 in fertilized soil, although these differences were not statistically significant.     

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     

N<Subscript>2</Subscript>O emission from conventional and minimum-tilled soils

In this study, we investigated N₂O emissions from two fields under minimum tillage, cropped with maize (MT maize) and summer oats (MT oats), and a conventionally tilled field cropped with maize (CT maize). Nitrous oxide losses from the MT maize and MT oats fields (5.27 and 3.64 kg N₂O-N ha⁻¹, respectively) were significantly higher than those from the CT maize field (0.27 kg N₂O-N ha⁻¹) over a period of 1 year. The lower moisture content in CT maize (43% water-filled pore space [WFPS] compared to 60–65%) probably caused the difference in total N₂O emissions. Denitrification was found to be the major source of N₂O loss. Emission factors calculated from the MT field data were high (0.04) compared to the CT field (0.001). All data were simulated with the denitrification decomposition model (DNDC). For the CT field, N₂O and N₂O + N₂ emissions were largely overestimated. For the MT fields, there was a better agreement with the total N₂O and N₂O + N₂ emissions, although the N₂O emissions from the MT maize field were underestimated. The simulated N₂O emissions were particularly influenced by fertilization, but several other measured N₂O emission peaks associated with other management practices at higher WFPS were not captured by the model. Several mismatches between simulated and measured \textNH₄⁺ {\text{NH}}_4^ + , \textNO₃^- {\text{NO}}_3^ - and WFPS for all fields were observed. These mismatches together with the insensitivity of the DNDC model for increased N₂O emissions at the management practices different from fertilizer application explain the limited similarity between the simulated and measured N₂O emissions pattern from the MT fields.     

Nitrous oxide emissions and methane oxidation by soil following cultivation of two different leguminous pastures

 Nitrous oxide (N₂O) emissions and methane (CH₄) consumption were quantified following cultivation of two contrasting 4-year-old pastures. A clover sward was ploughed (to 150–200 mm depth) while a mixed herb ley sward was either ploughed (to 150–200 mm depth) or rotovated (to 50 mm depth). Cumulative N₂O emissions were significantly greater following ploughing of the clover sward, with 4.01 kg N₂O-N ha^–1 being emitted in a 48-day period. Emissions following ploughing and rotovating of the ley sward were much less and were not statistically different from each other, with 0.26 and 0.17 kg N₂O-N ha^–1 being measured, respectively, over a 55-day period. The large difference in cumulative N₂O between the clover and ley sites is presumably due to the initially higher soil NO₃ ^– content, greater water filled pore space and lower soil pH at the clover site. Results from a denitrification enzyme assay conducted on soils from both sites showed a strong negative relationship (r=–0.82) between soil pH and the N₂O:(N₂O+N₂) ratio. It is suggested that further research is required to determine if control of soil pH may provide a relatively cheap mitigation option for N₂O emissions from these soils. There were no significant differences in CH₄ oxidation rates due to sward type or form of cultivation. Received: 1 November 1998     

Nitrous oxide emission from an irrigated cotton field under semiarid subtropical conditions

In a 1-year study, quantification of nitrous oxide (N₂O) emission was made from a flood-irrigated cotton field fertilized with urea at 100kg N ha⁻¹ a⁻¹. Measurements were made during the cotton-growing season (May–November) and the fallow period (December–April). Of the total 95 sampling dates, 77 showed positive N₂O fluxes (range, 0.1 to 33.3g N ha⁻¹ d⁻¹), whereas negative fluxes (i.e., N₂O sink activity) were recorded on 18 occasions (range, −0.1 to −2.2g N ha⁻¹ d⁻¹). Nitrous oxide sink activity was more frequently observed during the growing season (15 out of 57 sampling dates) as compared to the fallow period (3 out of 38 sampling dates). During the growing season, contribution of N₂O to the denitrification gaseous N products was much less (average, 4%) as compared to that during the fallow period (average, 21%). Nitrous oxide emission integrated over the 6-month growing period amounted 324g N ha⁻¹, whereas the corresponding figure for the 6-month fallow period was 648g N ha⁻¹. Subtracting the N₂O sink activity (30.3g N ha⁻¹ and 3.8g N ha⁻¹ during the growing season and fallow period, respectively), the net N₂O emission amounted 938g N ha⁻¹ a⁻¹. Results suggested that high soil moisture and temperature prevailing under flood-irrigated cotton in the Central Punjab region of Pakistan though favor high denitrification rates, but are also conducive to N₂O reduction thus leading to relatively low N₂O emission.     

Emissions of nitrous oxide from a constructed wetland using a groundfilter and macrophytes in waste-water purification of a dairy farm

 In less populated rural areas constructed wetlands with a groundfilter made out of the local soil mixed with peat and planted with common reed (Phragmites australis) are increasingly used to purify waste water. Particularly in the rhizosphere of the reed, nitrification and denitrification processes take place varying locally and temporally, and the question arises to what extent this type of waste-water treatment plant may contribute to the release of N₂O. In situ N₂O measurements were carried out in the two reed beds of the Friedelhausen dairy farm, Hesse, Germany, irrigated with the waste water from a cheese dairy and 70 local inhabitants (12 m³ waste water or 6 kg BOD₅ or 11 kg chemical O₂ demand (COD_Mn) day^–1). During November 1995 to March 1996, the release of N₂O was measured weekly at 1 m distances using eight open chambers and molecular-sieve traps to collect and absorb the emitted N₂O. Simultanously, the N₂O trapped in the soil, the soil temperature, as well as the concentrations of NH₄ ⁺-N, NO₃ ^–-N, NO₂ ^–-N, water-soluble C and the pH were determined at depths of 0–20, 20–40 and 40–60 cm. In the waste water from the in- and outflow the concentrations of COD_Mn, BOD₅, NH₄ ⁺-N, NO₃ ^–-N, NO₂ ^–-N, as well as the pH, were determined weekly. Highly varying amounts of N₂O were emitted at all measuring dates during the winter. Even at soil temperatures of –1.5  °C in 10 cm depth of soil or 2  °C at a depth of 50 cm, N₂O was released. The highest organic matter and N transformation rates were recorded in the upper 20 cm of soil and in the region closest to the outflow of the constructed wetland. Not until a freezing period of several weeks did the N₂O emissions drop drastically. During the period of decreasing temperatures less NO₃ ^–-N was formed in the soil, but the NH₄ ⁺-N concentrations increased. On average the constructed wetlands of Friedelhausen emitted about 15 mg N₂O-N inhabitant equivalent^–1 day^–1 during the winter period. Nitrification-denitrification processes rather than heterotrophic denitrification are assumed to be responsible for the N₂O production. Received: 28 October 1998     

Nitrous oxide emissions from a fallow and wheat field as affected by increased soil temperatures

 In order to determine the effects of increased soil temperature resulting from global warming on microbiological reactions, a 21-month field experiment was carried out in the Bavarian tertiary hills. The major objective was to focus on N₂O releases as either a positive or negative feedback in response to global warming. The soils of a fallow field and a wheat field were heated 3  °C above ambient temperature and N₂O fluxes were measured weekly from June 1994 to March 1996. During the experimental period, measured temperature differences between the control plots and the heated plots were 2.9±0.3  °C at a depth of 0.01 m and 1.0–1.8  °C at a depth of 1 m. Soil moisture decreased with the elevated soil temperatures of the heated plots. The mean differences in soil moisture between the treatments were 6.4% (fallow field) and 5.2%_DW (wheat field dry weight, DW), respectively. Overall N₂O releases during the experimental period from the fallow field were 4.8 kg N₂O–N ha^–1 in the control plot against 5.0 kg N₂O–N ha^–1 in the heated plot, and releases from the wheat field were 8.0 N₂O–N ha^–1 in the control plot and 7.6 N₂O–N kg ha^–1 in the heated plot. However, on a seasonal basis, cumulated N₂O emissions differed between the plots. During the summer months (May–October), releases from the heated fallow plot were 3 times the rates from the control plot. In the winter months, N₂O releases increased in both the fallow and wheat fields and were related to the number of freezing and thawing cycles. Received: 1 December 1997     

Nitrous oxide emissions under different soil and land management conditions

Nitrous oxide (N₂O) emissions of three different soils – a rendzina on cryoturbed soil, a hydromorphic leached brown soil and a superficial soil on a calcareous plateau – were measured using the chamber method. Each site included four types of land management: bare soil, seeded unfertilized soil, a suboptimally fertilized rapeseed crop and an overfertilized rapeseed crop. Fluxes varied from –1g to 100g N₂O-nitrogen ha^–1 day^–1. The highest rates of N₂O emissions were measured during spring on the hydromorphic leached brown soil which had been fertilized with nitrogen (N); the total emissions during a 5-month period exceeded 3500gNha^–1. Significant fluxes were also observed during the summer. Very marked effects of soil type and management were observed. Two factors – the soil hydraulic behaviour and the ability of the microbial population to reduce N₂O – appear to be essential in determining emissions of N₂O by soils. In fact, the hydromorphic leached brown soil showed the highest emissions, despite having the lowest denitrification potential because of its water-filled pore space and low N₂O reductase activity. Soil management also appears to affect both soil nitrate content and N₂O emissions. Received: 4 April 1997     

Denitrification from the horticultural peats: effects of pH,nitrogen, carbon,and moisture contents

Denitrification plays an important role in N-cycling. However, information on the rates of denitrification from horticultural growing media is rare in literature. In this study, the effects of pH, N, C, and moisture contents on denitrification were investigated using four moderately decomposed peat types (oligotrophic, mesotrophic, eutrophic, and transitional). Basal and potential denitrification rates (20°C, 18 h) from the unlimed peat samples varied widely from 2.0 to 21.8 and from 118.9 to 306.6 μg (N₂O + N₂)–N L⁻¹ dry peat h⁻¹, respectively, with the highest rates from the eutrophic peat and the lowest from the transitional one. Both basal and potential denitrification rates were substantially increased by 3.6–14- and 1.4–2.3-fold, respectively, when the initial pH (4.3–4.8) was raised to 5.9–6.5 units. Emissions of (N₂O + N₂)–N from oligotrophic, mesotrophic, and transitional peats were markedly increased by the addition of 0.15 g NO₃–N L⁻¹ dry peat but further additions had no effect. Denitrification rates were increased by increasing glucose concentration suggesting that the activity of denitrifiers in all peat types was limited by the low availability of easily decomposable C source. Increasing moisture contents of all peats from 40 to 50% water-filled pore space (WFPS) did not significantly (p > 0.05) increase (N₂O + N₂)–N emissions. However, a positive effect was observed when the moisture contents were increased from 60% to 70% WFPS in the eutrophic peat, from 70% to 80% in the transitional, from 80% to 90% in the oligotrophic and from 70% to 90% in the mesotrophic peat. It can be concluded that liming, N-fertilization, availability of easily decomposable C, and moist condition above 60% WFPS could encourage denitrification from peats although the rates are greatly influenced by the peat-forming environments (eutrophic > mesotrophic > oligotrophic > transitional types).     

Influence of different agricultural practices (type of crop, form of N-fertilizer) on soil nitrous oxide emissions

 N₂O emissions were periodically measured using the static chamber method over a 1-year period in a cultivated field subjected to different agricultural practices including the type of N fertilizer (NH₄NO₃, (NH₄)₂SO₄, CO(NH₂)₂ or KNO₃ and the type of crop (rapeseed and winter wheat). N₂O emissions exhibited the same seasonal pattern whatever the treatment, with emissions between 1.5 and 15 g N ha^–1 day^–1 during the autumn, 16–56 g N ha^–1 day^–1 in winter after a lengthy period of freezing, 0.5–70 g N ha^–1 day^–1 during the spring and lower emissions during the summer. The type of crop had little impact on the level of N₂O emission. These emissions were a little higher under wheat during the autumn in relation to an higher soil NO₃ ^– content, but the level of emissions was similar over a 7-month period (2163 and 2093 g N ha^–1 for rape and wheat, respectively). The form of N fertilizer affected N₂O emissions during the month following fertilizer application, with higher emissions in the case of NH₄NO₃ and (NH₄)₂SO₄, and a different temporal pattern of emissions after CO(NH₂)₂ application. The proportion of applied N lost as N₂O varied from 0.42% to 0.55% with the form of N applied, suggesting that controlling this agricultural factor would not be an efficient way of limiting N₂O emissions under certain climatic and pedological situations. Received: 1 December 1997     

Effect of three contrasting onion (Allium cepa L.) production systems on nitrous oxide emissions from soil

 N₂O emissions were measured from three contrasting onion (Allium cepa L.) production systems over an 8.5-month period. One system was established on soil where a clover sward had 3 months earlier been ploughed in (ploughed clover site). This production system followed conventional production management practices. The other two systems were established on soil where a mixed herb ley had 3 months earlier been either ploughed or rotovated. These last two production systems followed the guidelines of the International Federation of Organic Agriculture Movements (IFOAM). Cumulative N₂O emissions were significantly greater from the ploughed clover site compared to the ploughed ley site (3.8 and 1.6 kg N₂O-N ha^–1, respectively), while cumulative N₂O emissions from the ploughed ley and rotovated ley sites were not significantly different from each other. Emissions from all sites were dominated by episodes of high N₂O flux activity following seedbed preparation and drilling, when soil water suction (SWS) was shown to be the rate-controlling variable. The decline in the N₂O fluxes after these peak emissions followed clear exponential relationships of the form F=Ae^{– kt} (r≥0.91), where F is the daily flux and A is the y-intercept. First-order decay constants (k) during these periods of declining N₂O fluxes (corresponding to half-lives of 2.6–3.0 days) were not significantly different in magnitude from the first-order rate constants that characterised the increasing SWS. Gross differences in cumulative emissions between the clover and ley sites were attributed to the influence of differing soil pHs at the two sites on the N₂O:(N₂O+N₂) ratio in the denitrification products. It also appeared that fertiliser applications to the clover site had both direct and indirect effects on N₂O emissions by: (1) enhancing N₂O emissions via potential nitrification, (2) increasing the NO₃ ^– supply for enhanced N₂O emissions via denitrification, and (3) influencing the N₂O:(N₂O+N₂) ratio by lowering soil pH and increasing NO₃ ^– concentrations. Onion crop yields were greater at the clover site, mainly due to the higher density of planting made possible under a conventional production philosophy. Expressing the yield on the basis of net N₂O emissions, 23 t onions kg^–1 N₂O-N was obtained from the ploughed clover, which was double that obtained for the two systems based on the ley site. However, when the N₂O emissions from the cultivation of the soils prior to the sowing of the onions was included, all three systems produced a similar yield per kilogram of N₂O-N emitted, averaging 10 t kg^–1. Received: 6 January 1999     

Mineralization and denitrification in upland, nearly saturated and flooded subtropical soil I. Effect of nitrate and ammoniacal nitrogen

 The influence of fertilizer N applied through nitrate and ammoniacal sources on the availability of nitrate, supply of C, and gaseous N losses via denitrification (using acetylene inhibition technique) in a semiarid subtropical soil (Typic Ustochrepts) was investigated in a growth chamber simulating upland [60% water-filled pore space (WFPS)], nearly saturated (90% WFPS), and flooded (120% WFPS) conditions. The rate of denitrification was very low in the upland soil conditions, irrespective of fertilizer N treatments. Increasing water content to nearly saturated and flooded conditions resulted in four- to sixfold higher rates of denitrification within 2 days, suggesting that the denitrifying activity commences quickly. Results of this study reveal that (1) under restricted aeration, these soils could support high rates of denitrification (∼6 mg N kg^–1 day^–1) for short periods when nitrate is present; (2) application of fertilizer N as nitrate enhances N losses via denitrification (∼10 mg N kg^–1 day^–1) – however, the supply of available C determines the intensity and duration of denitrification; (3) when fertilizer N is applied as an ammoniacal form, nitrification proceeds slowly and nitrate availability limits denitrification in flooded soil; (4) the nearly saturated soil, being partially aerobic, supported greater nitrification of applied ammoniacal fertilizer N than flooded soil resulting in higher relative rates of denitrification; and (5) under aerobic soil conditions, 26 mg mineral N kg^–1 accumulated in control soil over a 16-day period, demonstrating a modest capacity of such semiarid subtropical soils, low in organic matter, to supply N to growing plants. Received: 7 June 1999     

Effects of biochar addition on N<Subscript>2</Subscript>O and CO<Subscript>2</Subscript> emissions from two paddy soils

Impacts of biochar addition on nitrous oxide (N₂O) and carbon dioxide (CO₂) emissions from paddy soils are not well documented. Here, we have hypothesized that N₂O emissions from paddy soils could be depressed by biochar incorporation during the upland crop season without any effect on CO₂ emissions. Therefore, we have carried out the 60-day aerobic incubation experiment to investigate the influences of rice husk biochar incorporation (50 t ha⁻¹) into two typical paddy soils with or without nitrogen (N) fertilizer on N₂O and CO₂ evolution from soil. Biochar addition significantly decreased N₂O emissions during the 60-day period by 73.1% as an average value while the inhibition ranged from 51.4% to 93.5% (P < 0.05–0.01) in terms of cumulative emissions. Significant interactions were observed between biochar, N fertilizer, and soil type indicating that the effect of biochar addition on N₂O emissions was influenced by soil type. Moreover, biochar addition did not increase CO₂ emissions from both paddy soils (P > 0.05) in terms of cumulative emissions. Therefore, biochar can be added to paddy fields during the upland crop growing season to mitigate N₂O evolution and thus global warming.     

Nitrogen monoxide production and consumption in an organic soil

 Factors controlling NO production, consumption, and emission rates were examined in an organic soil. Emission rates were measured in the enclosed headspaces of intact soil cores under three fertilisation treatments (unfertilised or 100 kg N ha^–1 as NH₄Cl or as NaNO₃), with and without the nitrification inhibitor C₂H₂ (20–70 μl l^–1). Nitrification was always the main source of NO emitted across the soil surface, even when the soil was nearly saturated. Fertilisation of soil with NH₄Cl increased NO emission both by stimulating NO production from nitrification, and by decreasing the NO consumption rate constant. Addition of NaNO₃ also stimulated the production of NO and N₂O during nitrification in aerobic soil slurry experiments. This effect was eliminated by adding C₂H₂ and was therefore not related to denitrification. In loose soil samples, the increase in NO-N production after NH₄Cl addition represented as much as 26% of the added N. However, in intact cores, 95% of the NO produced through nitrification was oxidised within the soil column rather than emitted to the atmosphere. We concluded that nitrification is the primary NO source from this organic soil, that surface NO emissions are much lower than gross NO production rates, and that gaseous N oxide (NO and N₂O) losses during nitrification can be affected by both soil NH₄ ⁺ and NO₃ ^–. Received: 15 December 1998     

Factors influencing nitrous oxide and methane emissions from minerotrophic fens in northeast Germany

 At two field sites representing northeastern German minerotrophic fens (Rhin-Havelluch, a shallow peat site; Gumnitz, a partially drained peat site) the influence of different factors (N fertilization, groundwater table, temperature) on N₂O and CH₄ emissions was investigated. The degraded fens were sources or sinks of the radiatively active trace gases investigated. The gas fluxes measured were much higher than those found in other terrestrical ecosystems such as forests. Lowering the groundwater table increased the release of N₂O and the oxidation of CH₄. High CH₄ emission rates occurred when the groundwater tables and soil temperatures were high (>12  °C). N fertilization stimulated the release of N₂O only when application rates were very high (480 kg N ha^–1). A moderate N supply (60 or 120 kg N ha^–1) hardly increased the release of N₂O in spite of high soluble soil NO₃ ^– contents. Received: 31 October 1997     

Crop residues and fertilizer nitrogen influence residue decomposition and nitrous oxide emission from a Vertisol

Crop residues with high C/N ratio immobilize N released during decomposition in soil, thus reducing N losses through leaching, denitrification, and nitrous oxide (N₂O) emission. A laboratory incubation experiment was conducted for 84 days under controlled conditions (24°C and moisture content 55% of water-holding capacity) to study the influence of sugarcane, maize, sorghum, cotton and lucerne residues, and mineral N addition, on N mineralization–immobilization and N₂O emission. Residues were added at the rate of 3 t C ha⁻¹ to soil with, and without, 150 kg urea N ha⁻¹. The addition of sugarcane, maize, and sorghum residues without N fertilizer resulted in a significant immobilization of soil N. Amended soil had significantly (P < 0.05) lower NO₃⁻–N, which reached minimum values of 2.8 mg N kg⁻¹ for sugarcane (at day 28), 10.3 mg N kg⁻¹ for maize (day 7), and 5.9 mg N kg⁻¹ for sorghum (day 7), compared to 22.7 mg N kg⁻¹ for the unamended soil (day 7). During 84 days of incubation, the total mineral N in the residues + N treatments were decreased by 45 mg N kg⁻¹ in sugarcane, 34 mg kg⁻¹ in maize, 29 mg kg⁻¹ in sorghum, and 16 mg kg⁻¹ in cotton amended soil compared to soil + N fertilizer, although soil NO₃⁻–N increased by 7 mg kg⁻¹ in lucerne amended soil. The addition of residues also significantly increased amended soil microbial biomass C and N. Maximum emissions of N₂O from crop residue amended soils occurred in the first 4–5 days of incubation. Overall, after 84 days of incubation, the cumulative N₂O emission was 25% lower with cotton + N fertilizer, compared to soil + N fertilizer. The cumulative N₂O emission was significantly and positively correlated with NO₃⁻–N (r = 0.92, P < 0.01) and total mineral N (r = 0.93, P < 0.01) after 84 days of incubation, and had a weak but significant positive correlation with cumulative CO₂ in the first 3 and 5 days of incubation (r = 0.59, P < 0.05).     

Nitrous oxide emission from Swedish forest soils in relation to liming and simulated increased N-deposition

Fluxes of N₂O were studied in a Norway spruce forest in the southwest of Sweden. Three differently treated catchments were compared: Limed (6 t dolomite ha^–1), Nitrex (additional N-deposition corresponding to 35 kg ha^–1 year^–1, in small doses) and Control (used as control site). The N-retention was still high (95%) after 2years of N-addition at the Nitrex site when the flux measurements were performed. Each catchment contained both well-drained and poorly drained soils (covered with Sphagnum sp.). The emissions of N₂O were in general low with both a high spatial and temporal variation for all three sites. The measured emissions were 25, 71 and 96 (gN₂O-N ha^–1 year^–1) for the well-drained Limed, Control and Nitrex sites, respectively. The average emissions of N₂O from the wet areas were significantly higher than the well-drained areas within the catchments. For the wet areas the measured emissions were larger: 90, 118 and 254 (g N₂O-N ha^–1 year^–1) for the Limed, Control and Nitrex sites, respectively. Comparison between treatments showed the wet Nitrex site to have a significantly higher emission than all other sites. The increased N-deposition at the Nitrex site increased the N₂O emissions by 0.2% of the added N for the well-drained soils and about 1% for the wet areas, compared with the control site. Since the wet areas represented only a small part of the forest, their larger emissions did not contribute significantly to the overall emission of the forest. Neither temperature nor water content of the soil was well correlated with the N₂O emissions. Soil gas samples showed that most of the N₂O was produced below a 0.3-m depth in the soil. Received: 27 September 1996     

Nitrous oxide emission derived from soil organic matter decomposition from tropical agricultural peat soil in central Kalimantan,Indonesia

Our previous research showed large amounts of nitrous oxide (N₂O) emission (>200?kg?N?ha^?1?year^?1) from agricultural peat soil. In this study, we investigated the factors influencing relatively large N₂O fluxes and the source of nitrogen (N) substrate for N₂O in a tropical peatland in central Kalimantan, Indonesia. Using a static chamber method, N₂O and carbon dioxide (CO₂) fluxes were measured in three conventionally cultivated croplands (conventional), an unplanted and unfertilized bare treatment (bare) in each cropland, and unfertilized grassland over a three-year period. Based on the difference in N₂O emission from two treatments, contribution of the N source for N₂O was calculated. Nitrous oxide concentrations at five depths (5–80?cm) were also measured for calculating net N₂O production in soil. Annual N fertilizer application rates in the croplands ranged from 472 to 1607?kg?N?ha^?1?year^?1. There were no significant differences in between N₂O fluxes in the two treatments at each site. Annual N₂O emission in conventional and bare treatments varied from 10.9 to 698 and 6.55 to 858?kg?N?ha^?1?year^?1, respectively. However, there was also no significant difference between annual N₂O emissions in the two treatments at each site. This suggests most of the emitted N₂O was derived from the decomposition of peat. There were significant positive correlations between N₂O and CO₂ fluxes in bare treatment in two croplands where N₂O flux was higher than at another cropland. Nitrous oxide concentration distribution in soil measured in the conventional treatment showed that N₂O was mainly produced in the surface soil down to 15?cm in the soil. The logarithmic value of the ratio of N₂O flux and nitrate concentration was positively correlated with water filled pore space (WEPS). These results suggest that large N₂O emission in agricultural tropical peatland was caused by denitrification with high decomposition of peat. In addition, N₂O was mainly produced by denitrification at high range of WFPS in surface soil.     

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.