Contributions from different microbial processes to N2O emission from soil under different moisture regimes

Nitrous oxide emissions from a sandy-loam textured soil wetted to matric potentials of either-1.0 or-0.1 kPa were determined in laboratory experiments in which the soil was incubated in air (control), air plus 10 Pa C₂H₂ (to inhibit nitrification), 100 kPa O₂ (to suppress denitrification), 10 kPa C₂H₂ (to inhibit N₂O reduction to N₂ in denitrification) or following autoclaving. The total N₂O production, consumption and net N₂O emission from the soils together with the contributions to N₂O emission from different processes of N₂O production were estimated. The rate of N₂O production was significantly greater in the wetter soil (282 pmol N₂O g^-1 soil h^-1) than in the drier soil (192 pmol N₂O g^-1 soil h^-1), but because N₂O consumption by denitrifiers was also greater in the wetter soil, the net N₂O emissions from the wetter and the drier soils did not differ significantly. Non-biological sources made no significant contribution to N₂O emission under either moisture regime and biological processes other than denitrification and nitrification made only a small contribution (1% of the total N₂O production) in the wetter soil. Denitrifying nitrifiers were the predominant source of N₂O emitted from the drier soil and other (non-nitrifying) denitrifiers were the predominant source of N₂O emitted from the wetter soil.     

Phytotron studies to compare nitrogen losses from corn-planted soil by the 15-N balance or direct dinitrogen and nitrous oxide measurements

Summary Containers filled with soil mixed with potassium nitrate highly enriched in ¹⁵N were planted with corn (Zea mays L.) and kept in a phytotron under controlled conditions for 79 days. Soil water content was normally maintained at exactly 60% water-holding capacity (–33 kPa), but it was increased several times to 85% (–5 kPa) for short periods to favour denitrification. The soil headspace was sealed from the phytotron atmosphere and aerated by a continuous stream of air. Nitrous oxide emission was measured by estimating the N₂O concentration differences in the air entering and leaving the containers. Emission of N₂ was estimated by mass spectroscopy from changes in the N₂ composition in the temporarily enclosed soil headspace. Both methods were carefully checked for accuracy by different tests. At specific times during the experiment the distribution of ¹⁵N between plants and soil was determined and a ¹⁵N balance established. Emission of N gases peaked at times of increased water content and reached maxima of 149 and 142 g N pot^–1 day^–1 for N₂O and N₂, respectively. While N losses of 5% ± 2% were indicated by the ¹⁵N balance, only 1.1% ± 0.3% loss from 2.7 g applied N was estimated from the N₂O and N₂ measurements after 79 days. Possible reasons for these differences are discussed.     

N2O and NO fluxes between a Norway spruce forest soil and atmosphere as affected by prolonged summer drought

Global change scenarios predict an increasing frequency and duration of summer drought periods in Central Europe especially for higher elevation areas. Our current knowledge about the effects of soil drought on nitrogen trace gas fluxes from temperate forest soils is scarce. In this study, the effects of experimentally induced drought on soil N₂O and NO emissions were investigated in a mature Norway spruce forest in the Fichtelgebirge (northeastern Bavaria, Germany) in two consecutive years. Drought was induced by roof constructions over a period of 46 days. The experiment was run in three replicates and three non-manipulated plots served as controls. Additionally to the N₂O and NO flux measurements in weekly to monthly intervals, soil gas samples from six different soil depths were analysed in time series for N₂O concentration as well as isotope abundances to investigate N₂O dynamics within the soil. N₂O fluxes from soil to the atmosphere at the experimental plots decreased gradually during the drought period from 0.2 to −0.0 μmol m⁻² h⁻¹, respectively, and mean cumulative N₂O emissions from the manipulated plots were reduced by 43% during experimental drought compared to the controls in 2007. N₂O concentration as well as isotope abundance analysis along the soil profiles revealed that a major part of the soil acted as a net sink for N₂O, even during drought. This N₂O sink, together with diminished N₂O production in the organic layers, resulted in successively decreased N₂O fluxes during drought, and may even turn this forest soil into a net sink of atmospheric N₂O as observed in the first year of the experiment. Enhanced N₂O fluxes observed after rewetting up to 0.1 μmol m⁻² h⁻¹ were not able to compensate for the preceding drought effect. During the experiment in 2006, with soil matric potentials in 20 cm depth down to −630 hPa, cumulative NO emissions from the throughfall exclusion plots were reduced by 69% compared to the controls, whereas cumulative NO emissions from the experimental plots in 2007, with minimum soil matric potentials of −210 hPa, were 180% of those of the controls. Following wetting, the soil of the throughfall exclusion plots showed significantly larger NO fluxes compared to the controls (up to 9 μmol m⁻² h⁻¹ versus 2 μmol m⁻² h⁻¹). These fluxes were responsible for 44% of the total emission of NO throughout the whole course of the experiment. NO emissions from this forest soil usually exceeded N₂O emissions by one order of magnitude or more except during wintertime.     

Nitrous oxide production by nitrification and denitrification in soil aggregates as affected by O2 concentration

Nitrous oxide emitted by soils can be produced either by denitrification in anoxic conditions or by nitrification in presence of O₂. The relative importance of the two processes, particularly under varied partial pressures of O₂, is not always known. This paper focuses on the influence of O₂ concentration on N₂O production by nitrification and denitrification in an arable Orthic Luvisol. Soil aggregates (2-3 mm size), water unsaturated, received 116 mg N kg⁻¹ as ammonium sulphate labelled with ¹⁵N and were incubated during 14 days at different O₂ partial pressures: 0, 0.35, 0.76, 1.5, 4.3 and 20.4 kPa. A ¹⁵N tracing technique was used to quantify nitrification and denitrification rates. ¹⁵N₂O and ¹⁵N₂ were measured. Oxygen pressure appeared to strongly influence both nitrification and denitrification rates and also N₂O emissions. Nitrification rates were reduced by a factor of 6-9 when O₂ decreased from 20.4 to 0.35 kPa. They were highly correlated with O₂ consumption rates. Denitrification mainly occurred in complete anoxic conditions. The proportion of N₂O emitted by denitrification was estimated by two independent methods: one based on ¹⁵N tracing using isotope composition of NH₄, NO₃ and N₂O, the other based on the measurement of the ¹⁵N₂O:¹⁵N₂ ratio. The two methods gave close results. The highest N₂O emissions were obtained under complete anoxic conditions and were due to denitrification. However, N₂O emissions almost as important were obtained at day 14 with 1.5 kPa O₂ pressure, and they were due to nitrification. Nitrification was the main source of N₂O at O₂ concentrations greater than 0.35 kPa. The amounts of N₂O-N emitted by nitrification were linearly related to the amounts of N nitrified, but the slope of the regression was highly dependent on O₂ concentration: it varied from 0.16 to 1.48% when O₂ concentration was reduced from 20.4 to 0.76 kPa. Emissions of N₂O by nitrification may then be quite significant if nitrification occurs at a reduced O₂ concentration.     

Evolution of dinitrogen and nitrous oxide from the soil to the atmosphere through rice plants

Summary It is commonly assumed that a large fraction of fertilizer N applied to a rice (Oryza sativa L.) field is lost from the soil-water-plant system as a result of denitrification. Direct evidence to support this view, however, is limited. The few direct field, denitrification gas measurements that have been made indicate less N loss than that determined by ¹⁵N balance after the growing season. One explanation for this discrepancy is that the N₂ produced during denitrification in a flooded soil remains trapped in the soil system and does not evolve to the atmosphere until the soil dries or is otherwise disturbed. It seems likely, however, that N₂ produced in the soil uses the rice plants as a conduit to the atmosphere, as does methane. Methane evolution from a rice field has been demonstrated to occur almost exclusively through the rice plants themselves. A field study in Cuttack, India, and a greenhouse study in Fort Collins, Colorado, were conducted to determine the influence of rice plants on the transport of N₂ and N₂O from the soil to the atmosphere. In these studies, plots were fertilized with 75 or 99 atom % ¹⁵N-urea and ¹⁵N techniques were used to monitor the daily evolution of N₂ and N₂O. At weekly intervals the amount of N₂+N₂O trapped in the flooded soil and the total-N and fertilized-N content of the soil and plants were measured in the greenhouse plots. Direct measurement of N₂+N₂O emission from field and greenhouse plots indicated that the young rice plant facilitates the efflux of N₂ and N₂O from the soil to the atmosphere. Little N gas was trapped in the rice-planted soils while large quantities were trapped in the unplanted soils. N losses due to denitrification accounted for only up to 10% of the loss of added N in planted soils in the field or greenhouse. The major losses of fertilizer N from both the field and greenhouse soils appear to have been the result of NH₃ volatilization.     

不同碳源添加对钙质土高累积硝态氮向土壤有机氮库转化的研究

Excessive amounts of nitrate have accumulated in many soils on the North China Plain due to the large amounts of chemical N fertilizers or manures used in combination with low carbon inputs. We investigated the potential of different carbon substrates added to transform soil nitrate into soil organic N (SON). A 56-d laboratory incubation experiment using the 15 N tracer (K15 NO3 ) technique was carried out to elucidate the proportion of SON derived from accumulated soil nitrate following amendment with glucose or maize straw at controlled soil temperature and moisture. The dynamics and isotopic abundance of mineral N (NO3 and NH+4 ) and SON and greenhouse gas (N2O and CO2 ) emissions during the incubation were investigated. Although carbon amendments markedly stimulated transformation of nitrate to newly formed SON, this was only a substitution effect of the newly formed SON with native SON because SON at the end of the incubation period was not significantly different (P > 0.05) from that in control soil without added C. At the end of the incubation period, amendment with glucose, a readily available C source, increased nitrate immobilization by 2.65 times and total N2O-N emission by 33.7 times, as compared with maize straw amendment. Moreover, the differences in SON and total N2O-N emission between the treatments with glucose and maize straw were significant (P < 0.05). However, the total N2O-N emission in the straw treatment was not significantly (P > 0.05) greater than that in the control. Straw amendment may be a potential option in agricultural practice for transformation of nitrate N to SON and minimization of N2O emitted as well as restriction of NO3-N leaching.     

Evidence of N<Subscript>2</Subscript>O emission and gaseous nitrogen losses through nitrification-denitrification induced by rice plants (<Emphasis Type="Italic">Oryza sativa</Emphasis> L.)

A greenhouse experiment was conducted with a specially designed apparatus consisting of an upper and lower chamber where the treatment with rice was carried out (treatment 1). The apparatus also had a single chamber where treatment 2, without rice plants, was carried out. The scope of this study was to elucidate the influence of rice plant growth on gaseous N losses as N₂ and N₂O produced by nitrification-denitrification in a flooded soil fertilized with (NH₄)₂SO₄ (with 56.50 atom% ¹⁵N). Gas samples were withdrawn weekly and analyzed for (N₂ + N₂O)-¹⁵N losses by mass spectrometer and for N₂O by gas chromatograph. The gaseous (N₂ + N₂O)-¹⁵N losses of the treatment with rice plants were significantly (P =0.01) higher than those of the treatment without rice plants, as were the amounts of N₂O emitted. Rice plants facilitate the efflux of N₂ and N₂O from soil to atmosphere, as about half of the total gaseous ¹⁵N loss as N₂ and N₂O was found in the upper chamber. The proportion of N₂O-¹⁵N to (N₂ + N₂O)-¹⁵N in the upper chamber was 10.56%, much higher than that of the lower chamber in treatment 1 and the headspace of treatment 2.     

C and N turnover in structurally intact soils of different texture

The turnover of native and applied C and N in undisturbed soil samples of different texture but similar mineralogical composition, origin and cropping history was evaluated at −10 kPa water potential. Cores of structurally intact soil with 108, 224 and 337 g clay kg⁻¹ were horizontially sliced and ¹⁵N-labelled sheep faeces was placed between the two halves of the intact core. The cores together with unamended treatments were incubated in the dark at 20 °C and the evolution of CO₂-C determined continuously for 177 d. Inorganic and microbial biomass N and ¹⁵N were determined periodically. Net nitrification was less in soil amended with faeces compared with unamended soil. When adjusted for the NO₃-N present in soil before faeces was applied, net nitrification became negative indicating that NO₃-N had been immobilized or denitrified. The soil most rich in clay nitrified least N and ¹⁵N. The amounts of N retained in the microbial biomass in unamended soils increased with clay content. A maximum of 13% of the faeces ¹⁵N was recovered in the microbial biomass in the amended soils. CO₂-C evolution increased with clay content in amended and unamended soils. CO₂-C evolution from the most sandy soil was reduced due to a low content of potentially mineralizable native soil C whereas the rate constant of C mineralization rate peaked in this soil. When the pool of potentially mineralizable native soil C was assumed proportional to volumetric water content, the three soils contained similar proportions of potentially mineralizable native soil C but the rate constant of C mineralization remained highest in the soil with least clay. Thus although a similar availability of water in the three soils was ensured by their identical matric potential, the actual volume of water seemed to determine the proportion of total C that was potentially mineralizable. The proportion of mineralizable C in the faeces was similar in the three soils (70% of total C), again with a higher rate constant of C mineralization in the soil with least clay. It is hypothesized that the pool of potentially mineralizable C and C rate constants fluctuate with the soil water content.     

高氮和NO2-对中亚热带森林土壤N2O和NO产生的影响

利用15N同位素标记方法,研究在两种水分条件即60％和90％ WHC下,添加硝酸盐(NH4NO3,N 300 mg kg-1)和亚硝酸盐(NaNO2,N 1 mg kg-1)对中亚热带天然森林土壤N2O和NO产生过程及途径的影响.结果表明,在含水量为60％ WHC的情况下,高氮输入显著抑制了N2O和NO的产生(p＜0.01);但当含水量增为90％ WHC后,实验9h内抑制N2O产生,之后转为促进.所有未灭菌处理在添加NO2-后高氮抑制均立即解除并大量产生N2O和NO,与对照成显著差异(p＜0.01),在60％ WHC条件下,这种情况维持时间较短(21 h),但如果含水量高(90％ WHC)这种情况会持续很长时间(2周以上),说明水分有效性的提高和外源NO2-在高氮抑制解除中起到重要作用.本实验中N2O主要来源于土壤反硝化过程,而且加入未标记NO2-后导致杂合的N2O(14N15NO)分子在实验21 h内迅速增加,表明这种森林土壤的反硝化过程可能主要是通过真菌的“共脱氮”来实现,其贡献率可多达80％以上.Spearman秩相关分析表明未灭菌土壤NO的产生速率与N2O产生速率成显著正相关性(p＜0.05),土壤含水量越低二者相关性越高.灭菌土壤添加NO2-能较未灭菌土壤产生更多的NO,但却几乎不产生N2O,表明酸性土壤的化学反硝化对NO的贡献要大于N2O.     

NH3 Volatilization,N2O Emission and Microbial Biomass Turnover from 15N-Labeled Manure Under Laboratory Conditions

On irrigated agricultural soils from semi-arid and arid regions, ammonia (NH₃) volatilization and nitrous oxide (N₂O) emission can be a considerable source of N losses. This study was designed to test the capture of ¹⁵N loss as NH₃ and N₂O from previous and recent manure application using a sandy, calcareous soil from Oman amended one or two times with ¹⁵N labeled manure to elucidate microbial turnover processes under laboratory conditions. The system allowed to detect ¹⁵N enrichments in evolved N₂O-N and NH₃-N of up to 17% and 9%, respectively, and total N, K₂SO₄ extractable N and microbial N pools from previous and recent ¹⁵N labeled manure applications of up to 7%, 8%, and 15%. One time manured soil had higher cumulative N₂O-N emissions (141 µg kg^?1) than repeatedly manured soil with 43 µg kg^?1 of which only 22% derived from recent manure application indicating a priming effect.     

Field evaluation of N2 fixation and N partitioning in climbing bean (Phaseolus vulgaris L.) using 15N

Summary The common bean (Phaseolus vulgaris L.) is generally regarded as a poor N₂ fixer. This study assessed the sources of N (fertilizer, soil, and fixed N), N partitioning and mobilization, and soil N balance under field conditions in an indeterminate-type climbing bean (P. vulgaris L. cv. Cipro) at the vegetative, early pod-filling, and physiological maturity stages, using the A-value approach. This involved the application of 10 and 100 kg N ha^-1 of ¹⁵N-labelled ammonium sulphate to the climbing bean and a reference crop, maize (Zea mays L.). At the late pod-filling stage (75 days after planting) the climbing bean had accumulated 119 kg N ha^-1, 84% being derived from fixation, 16% from soil, and only 0.2% from the ¹⁵N fertilizer. N₂ fixation was generally high at all stages of plant growth, but the maximum fixation (74% of the total N₂ fixed) occurred during the interval between early (55 days after planting) and late podfilling. The N₂ fixed between 55 and 75 days after planting bas a major source (88%) of the N demand of the developing pod, and only about 11% was contributed from the soil. There was essentially no mobilization of N from the shoots or roots for pod development. The cultivation of common bean cultivars that maintain a high N₂-fixing capacity especially during pod filling, satisfying almost all the N needs of the developing pod and thus requiring little or no mobilization of N from the shoots for pod development, may lead to a net positive soil N balance.     

Denitrification losses from puddled rice soils in the tropics

Summary Although denitrification has long been considered a major loss mechanism for N fertilizer applied to lowland rice (Oryza sativa L.) soils, direct field measurements of denitrification losses from puddled rice soils in the tropics have only been made recently. This paper summarizes the results of direct measurement and indirect estimation of denitrification losses from puddled rice fields and reviews the status of research methodology for measurement of denitrification in rice fields. The direct recovery of (N₂+N₂O)-¹⁵N from ¹⁵N-enriched urea has recently been measured at sites in the Philippines, Thailand, and Indonesia. In all 12 studies, recoveries of (N₂+N₂O)-¹⁵N ranged from less than 0.1 to 2.2% of the applied N. Total gaseous N losses, estimated by the ¹⁵N-balance technique, were much greater, ranging from 10 to 56% of the applied urea-N. Denitrification was limited by the nitrate supply rather than by available C, as indicated by the values for water-soluble soil organic C, floodwater (nitrate+nitrite)-N, and evolved (N₂+N₂O)-¹⁵N from added nitrate. In the absence of runoff and leaching losses, the amount of (N₂+N₂O)-¹⁵N evolved from ¹⁵N-labeled nitrate was consistently less than the unrecovered ¹⁵N in ¹⁵N balances with labeled nitrate, which presumably represented total denitrification losses. This finding indicates that the measured recoveries of (N₂+N₂O)-¹⁵N had underestimated the denitrification losses from urea. Even with a probable two-or threefold underestimation, direct measurements of (N₂+N₂O)-¹⁵N failed to confirm the appreciable denitrification losses often estimated by the indirect difference method. This method, which determines denitrification losses by the difference between total ¹⁵N loss and determined ammonia loss, is prone to high variability. Measurements of nitrate disappearance and ¹⁵N-balance studies suggest that nitrification-denitrification occurs under alternate soil drying and wetting conditions both during the rice cropping period and between rice crops. Research is needed to determine the magnitude of denitrification losses when soils are flooded and puddled for production of rice.     

Nitrate Transformation and N2O Emission in a Typical Intensively Managed Calcareous Fluvaquent Soil: A 15-Nitrogen Tracer Incubation Study

A 56-day aerobic incubation experiment was performed with 15-nitrogen (N) tracer techniques after application of wheat straw to investigate nitrate-N (NO₃-N) immobilization in a typical intensively managed calcareous Fluvaquent soil. The dynamics of concentration and isotopic abundance of soil N pools and nitrous oxide (N₂O) emission were determined. As the amount of straw increased, the concentration and isotopic abundance of total soil organic N and newly formed labeled particulate organic matter (POM-N) increased while NO₃-N decreased. When ¹⁵NO₃-N was applied combined with a large amount of straw at 5000 mg carbon (C) kg^?1 only 1.1 ± 0.4 mg kg^?1 NO₃-N remained on day 56. The soil microbial biomass N (SMBN) concentration and newly formed labeled SMBN increased significantly (P < 0.05) with increasing amount of straw. Total N₂O-N emissions were at levels of only micrograms kg^?1 soil. The results indicate that application of straw can promote the immobilization of excessive nitrate with little emission of N₂O.     

Influence of oxygen on denitrification and aerobic respiration in soil

Summary The influence of the partial pressure of oxygen on denitrification and aerobic respiration was investigated at defined P02 values in a mull rendzina soil. The highest denitrification and respiration rates obtained in remoistened, glucose- and nitrate-amended soil were 43 1 N₂0 h^–1g^–1 soil and 130 1 O₂ h^–1g^–1 soil, respectively. At -55 kPa matric water potential, corresponding to 40% water saturation, N₂0 was produced only below P0₂ 40 hPa. The K _m, for O₂ was 3.0 x 10^–6 M. Formation of N₂O and consumption of O₂ occurred simultaneously with half maximum rates at P0₂ 6.7–13.3 hPa. Nitrite accumulated in soil below 40 hPa and increased with decreasing pO₂. The upper threshold for N₂0 formation in amended soil was P0₂ 33–40 hPa (39-47 M O₂).     

Use of 15N isotope dilution for quantification of N2 fixation associated with roots of kallar grass (Leptochloa fusca (L.))

Summary Leptochloa fusca (L.) Kunth (kallar grass) has previously been found to exhibit high rates of nitrogen fixation. A series of experiments to determine the level of biological nitrogen fixation using ¹⁵N isotopic dilution were carried out in nutrient solution and saline soil. In the nutrient solution, E. coli inoculated plants were taken as non-nitrogen-fixing control. It was observed that nearly 60%–80% of the plant N was derived from atmospheric fixation. Estimations based on the N difference method gave much lower values (18%–35%). In experiments with saline soil which was initially sterilized with chloroform fumigation, a mixed culture of N₂-fixing rhizospheric isolates from kallar grass roots was inoculated and planted to kallar grass. Uninoculated treatments were regarded as controls. The soil was previously labelled with ¹⁵N by adding cellulose and (¹⁵NH₄)₂SO₄. The results of these studies showed fixation values of 6%–32% when estimated by ¹⁵N dilution, whereas by the N difference method 54% of the plant N was estimated to be derived from fixation. This discrepancy is due to the increase in root proliferation due to inoculation, which results in greater uptake of soil N. The distribution of ¹⁵N in different fractions of the soil-N indicted isotopic dilution due to bacterial fixation of atmospheric N₂.     

Effect of nitrogen application on the A N value of soil

Pot experiments were conducted with two soils, from Rottenhaus and Seibersdorf in Austria, to ascertain whether the rate of fertilizer N application and the test crop would influence the amount of N available in the soil as assessed by the A-value method. ¹⁵N-labelled fertilizer was applied at rates of 10, 25, 40, 60, and 100 mg N kg^-1 soil, corresponding approximately to 20, 50, 80, 120 and 200 kg N ha^-1 respectively, and two crop species, barley (Hordeum vulgareL.) and non-nodulating soybean (Glycine max L.) were used to determine the soil A _N value under the various fertilizer regimes. The results showed that the Rottenhaus soil had a higher A _N value than the Seibersdorf soil, suggesting that the former was more fertile than the latter. The A _N values of both soils were significantly affected by the level of N application. When grown in the same soil, the two test crops showed significantly different fertilizer use efficiency and per cent N derived from fertilizer when the rate of N application exceeded 20 kg ha^-1. Thus, the A _N value as determined by the two test crops differed significantly for the same soil when the rate of N application was greater than 20 kg/ha. The difference was greater when the soil fertility level was high. The dependence of the A _N value on the level of N application and the species of crop seriously compromises the suitability of this method for determining plant-associated N₂ fixation. Hence, considerable caution is required when using this method to estimate plant-associated N₂ fixation.     

Field measurements of nitrogen fixation in leguminous trees used in agroforestry systems: Influence of 15N-labeling approaches and reference trees

Appropriate ¹⁵N-labeling methods are crucial for estimating N₂-fixation in trees used in agroforestry systems. A 4-year field experiment was conducted on an Alfisol in Southwestern Nigeria to compare the estimates of N₂ fixed in Leucaena leucocephala, using two non-N₂-fixing leguminous trees, Senna siamea and S. spectabilis, as reference plants and three different methods of introducing ¹⁵N into soil. The atom % ¹⁵N uptake pattern (as reflected in the leaves) was identical in both N₂- and non-N₂-fixing tree species irrespective of the ¹⁵N-application method. There was a significant decline in atom % ¹⁵N excess in the leaves of L. leucocephala (from 0.266 to 0.039), S. siamea (0.625 to 0.121), and S. spectabilis (from 0.683 to 0.118) from the first sampling 12 months after planting and the second sampling 18 months after sampling. From the second harvest in 1991 until the end of the experiment (fifth) harvest in 1993, however, the atom ¹⁵N % excess decline in leaves of the three species was less pronounced and depended on the method of ¹⁵N application. In those plants to which the tracer was applied once at planting, the ¹⁵N decline was steady between the second and the last prunings. In the split-application treatment, the atom ¹⁵N % excess increased slightly at the third pruning and decreased during the subsequent two prunings. The reference tree and the method of ¹⁵N application influenced the estimated proportion of N derived from atmospheric N₂ by L. leucocephala, calculated as 73 and 64%, corresponding to 119 and 98 kg N ha^-1 of N₂ fixed per 6 months, when S. spectabilis and S. siamea were used as reference trees, respectively. The approach by which ¹⁵N-labeled fertilizer was applied to the soil in three splits gave slightly higher estimates of N derived from the atmosphere but this was of little agronomic significance because total N₂ fixed was similar for all methods.     

Nitrapyrin effectiveness in reducing nitrous oxide emissions decreases at low doses of urea in an Andosol

In the tropics,frequent nitrogen(N)fertilization of grazing areas can potentially increase nitrous oxide(N₂O)emissions.The application of nitrification inhibitors has been reported as an effective management practice for potentially reducing N loss from the soil-plant system and improving N use efficiency(NUE).The aim of this study was to determine the effect of the co-application of nitrapyrin(a nitrification inhibitor,NI)and urea in a tropical Andosol on the behavior of N and the emissions of N₂O from autotrophic and heterotrophic nitrification.A greenhouse experiment was performed using a soil(pH 5.9,organic matter content 78 g kg^-1,and N 5.6 g kg^-1)sown with Cynodon nlemfuensis at 60%water-filled pore space to quantify total N₂O emissions,N₂O derived from fertilizer,soil ammonium(NH₄⁺)and nitrate(NO₃^-),and NUE.The study included treatments that received deionized water only(control,NI).No significant differences were observed in soil NH₄⁺content between the UR and UR+NI treatments,probably because of soil mineralization and NO₃^-produced by heterotrophic nitrification,which is not effectively inhibited by nitrapyrin.After 56 d,N₂O emissions in UR(0.51±0.12 mg N₂O-N concluded that the soil organic N mineralization and heterotrophic nitrification are the main processes of NH₄⁺and NO₃^-production.Additionally,it was found that N₂O emissions were partially a consequence of the direct oxidation of the soil's organic N via heterotrophic nitrification coupled to denitrification.Finally,the results suggest that nitrapyrin would likely exert significant mitigation on N₂O emissions only if a substantial N surplus exists in soils with high organic matter content.     

GREENHOUSE GAS EMISSIONS FROM AN ALKALINE SALINE SOIL CULTIVATED WITH MAIZE (ZEA MAYS L.) AND AMENDED WITH ANAEROBICALLY DIGESTED COW MANURE: A GREENHOUSE EXPERIMENT

Sludge derived from cow manure anaerobically digested to produce biogas (methane; CH₄) was applied to maize (Zea mays L.) cultivated in a nutrient-low, alkaline, saline soil with electrolytic conductivity 9.4 dS m^?1 and pH 9.3. Carbon dioxide (CO₂) emission increased 3.1 times when sludge was applied to soil, 1.6 times when cultivated with maize and 3.5 times in sludge-amended maize cultivated soil compared to the unamended uncultivated soil (1.51 mg C kg^?1 soil day^?1). Nitrous oxide (N₂O) emission from unamended soil was -0.0004 μg nitrogen (N) kg^?1 soil day^?1 and similar from soil cultivated with maize (0.27 μg N kg^?1 soil day^?1). Application of sludge increased the N₂O emission to 4.59 μg N kg^?1 soil day^?1, but cultivating this soil reduced it to 2.42 μg N kg^?1 soil day^?1. It was found that application of anaerobic digested cow manure stimulated maize development in an alkaline saline soil and increased emissions of CO₂ and N₂O.     

Denitrification in the rooting zone of cropped soils with regard to methodology and climate: A review

Summary Denitrification N losses can be determined by three methods. The first is by estimating the non-recovery of ¹⁵ N-labelled compounds (¹⁵N-balance method). Using this method, denitrification losses are deduced from the balance of an N budged (¹⁵N-labeled fertilizer), having accounted for transformations in soil, plant uptake, and leaching losses. The evolution of gaseous N from native soil N is not taken into account by this procedure. Studies on arable land with annual crops in the temperate zone have shown that of the fertilizer N applied, about 20–500% (10–70 kg N^* ha^–1) is not recovered at the end of the growth period. The second method of determining denitrification N losses is by in situ field measurement of ¹⁵ N ₂ and ¹⁵ N ₂ O production. Under this procedure, ¹⁵N-enriched N is applied to a plot and the denitrification N losses are determined by covering the soil. The method allows a quantitative estimate of the relative contributions to the emitted gas by both the original enriched source and the native soil N. N-evolution rates measured on arable land under a temperate climate are approximately the same order of magnitude as the N losses estimated by the non-recovery of ¹⁵ N method. The third measuring procedure is based on the acetylene inhibition phenomenon. This principle uses the inhibition of bacterial N₂O reduction to N₂ in the presence of acetylene (C₂H₂). The methoddetermines the denitrification of all NO₃ ^–-N irrespective of its source. Measurements on classical crop production systems show maximum N losses in the temperate climate of about 20–30 kg N^* ha^–1 during the growth period of annual crops. A similar level of denitrification is estimated for grassland sites under the same climate. In the subtropics (mediterranean climate with hot summers and mild winters), from both intensively cultivated arable land and grassland sites, N losses may exceed 200 kg^* ha^–1 year^–1. Without the use of irrigation the denitrification flux is negligible in spite of the high temperatures in this climate.