N application to winter wheat at tillering and shooting: N balance at different growth stages

At two sites, microplots under winter wheat were given 140 kg N ha^–1 as labelled ammonium nitrate split in 80 kg N ha^–1 at tillering and 60 kg N ha^–1 at shooting. Soil and plant samples were analyzed at shooting, after anthesis and at grain harvest and a¹⁵N balance was established. The average recovery rate of 95% indicates that there were no marked N losses due to leaching and denitrification, which is attributed to the low rainfall in the two months after fertilizer application. Between 19 and 23% of the fertilizer N remained in the 0–30 cm soil layer as organically bound soil N. Up to 64% was taken up by the above-ground crop. On the loamy sand, 4% of the fertilizer N at harvest remained in the roots in the 0–30 cm layer and only 3% was found as inorganic N in the 0–90 cm soil layer. The fertilizer N applied diminished plant uptake of soil N in the period between fertilizer application and harvest. As compared with the control, the fertilized plants extracted 25 and 28% less soil N from loamy sand and loess soil, respectively. The results show that application of mineral N fertilizer helps to maintain the mineralizable N content of the soil, which has been accumulated in the course of long-term intensive crop production, by adding N to the soil organic pool and simultaneously reducing the supply of soil N to the plants.     

A model to predict changes in soil moisture,NO3-N content and N uptake by wheat

A simulation model to predict fertilizer N behaviour in a soil-plant (wheat) system has been developed and tested. The model predicts components of field water balance (evaporation, transpiration, drainage and run-off) and changes in soil nitrogen amounts due to N transformations (urea hydrolysis, mineralization, nitrification and volatilization), N movement and plant N uptake using information on N transformation coefficients for the soil, atmospheric evaporative demand (E_pan), leaf area development and root growth characteristics of the crop. The model predicts N uptake by wheat through mass flow using a new simplified crop cover function. The coefficients of correlation between the measured and predicted N uptake by wheat grown under three different moisture regimes in the two years (1987–88 and 1988–89) approached unity. The computed amount of residual NO₃-N in the soil profile at wheat harvest matched well with the measured amount with a root mean square error of 13.7 percent. The close matching of the measured and model predicted components of nitrogen and water balances under three widely different set of irrigation treatments suggests of model's capabilities to help in on-farm N management both under irrigated and rainfed conditions.     

Review and simplification of calculations in15N tracer studies

Calculations in nitrogen (N) balance research using¹⁵N involve several steps that require care to avoid errors. The objective of this paper is to provide examples of these calculations using established procedures and to present shortened alternative calculations that give the same result. The calculations examined include determination of the amount of N to apply, determination of the atom %¹⁵N abundance needed in the labeled fertilizer, preparation of the labeled fertilizer, and calculation of the fertilizer N recovered. Calculations needed in the preparation of the labeled fertilizer using established procedures include the determination of the mean atomic weight of the enriched source from which the labeled fertilizer is prepared. This determination is not needed in the shortened alternative calculations, because the procedure places the calculations on a mole basis rather than a mass basis.     

4. Reappraisal of the significance of ammonia volatilization as an N loss mechanism in flooded rice fields

The role of ammonia volatilization as a nitrogen loss mechanism in lowland rice (Oryza sativa L.) has recently been extensively reevaluated using techniques that do not disturb the field environment. This paper summarizes methodologies used in this research and discusses findings from recently conducted micrometeorological studies on ammonia volatilization. Factors affecting ammonia loss and the contribution of this process to the overall nitrogen loss from lowland rice systems are also outlined. Suggestions for future research are included.     

Seasonal runoff estimation of N and P in a paddy field of central Korea

The present study was carried out during a period of one year (from May 1, 1997 to April 30, 1998) to quantify seasonal runoff of N and P in a rice field with an area of 5,000 m². The total amount of runoff water was 1,043 mm during the cropping season and 281 mm during the non-cropping season. Nutrient concentrations in runoff water increased significantly during the period of fertilizer application and then decreased. During the non-irrigation period after harvest, however, the concentrations of tota -N were 3 to 4 mg l^–1. The annual runoff loading of total-N and total-P was 157.9 and 4.5 kg ha yr^–1. The runoff loading was 109.9 kg ha^–1 for total-N and 3.5 kg ha^–1 for total-P during the fertilizer application period (from May 13 to August 3, 1997). During the rainy season (from June 20 to July 20, 1997), the runoff loading was 66.1 kg ha^–1 for total-N and 1.9 kg ha^–1 for total-P. The runoff loading was 5.6 kg ha^–1 for total-N and 0.2 kg ha^–1 for total-P during the fallow stage (from October 1, 1997 to March 20, 1998) while it was 6.7 kg ha^–1and 0.4 kg ha^–1 for each nutrient during the plowing stage (March 20 to May 10, 1998). The loss of total-N and total-P was 68.2% and 63.9% of annual runoff loading during the fertilizer application stage, respectively. During the non-cropping season after harvest, however, the loss was 30.4% of total-N and 22.3% of total P. In summary, intensive long-term studies on various sites of nutrient management planning during the fertilizer application and rainy seasons are needed.     

Differences in Yields,Residue Composition and N Mineralization Dynamics of Bt and Non-Bt Maize

Cultivation of genetically modified crops may have several direct and indirect effects on soil ecosystem processes, such as soil nitrogen (N) transformations. Field studies were initiated in Northeast Missouri in 2002 and 2003 to determine grain and biomass yields and the effects of application of crop residues from five Bt maize hybrids and their respective non-Bt isolines on soil inorganic N under tilled and no-till conditions in a maize-soybean rotation. A separate aerobic incubation study examined soil N mineralization from residue components (leaves, stems, roots) of one Bt maize hybrid and its non-Bt isoline in soils of varying soil textural class. Three Bt maize hybrids produced 13–23% greater grain yields than the non-Bt isolines. Generally no differences in leaf and stem tissues composition and biomass was observed between Bt and non-Bt maize varieties. Additionally, no differences were observed in cumulative N mineralization from Bt and non-Bt maize residues, except for non-Bt maize roots that mineralized 2.7 times more N than Bt maize roots in silt loam soil. Incorporation of Bt residues in the field did not significantly affect soil inorganic N under tilled or no-till conditions. Overall Bt and non-Bt maize residues did not differ in their effect on N dynamics in laboratory and field studies.     

Crop nitrogen utilization and soil nitrate loss in a lettuce field

Low N use efficiency and high nitrate (NO ₃ ^- ) pollution potentials are problems in intensive vegetable production systems. The purpose of this study was to quantify N utilization by lettuce (Lactuca sativa L. cv Salinas), and identify periods of NO ₃ ^- loss in an on-farm study in the Salinas Valley in coastal California. During autumn and winter, surface moisture remained low, and NO ₃ ^- concentrations increased, reflecting high net mineralizable N, as determined by anaerobic incubation, and nitrification potential, as determined by the chlorate inhibition method. At the onset of a large winter storm, tracer levels of¹⁵NO ₃ ^- were injected in the top 5 mm of soil in 30 cm-deep cylinders. After two weeks, most of the¹⁵N was present as¹⁵NO ₃ ^- at 10–30 cm depth. By difference, losses to denitrification accounted for ~ 25% of the surface-applied¹⁵N. Leaching below 30 cm did not occur, since no¹⁵N enrichment of NO ₃ ^- -N was measured in anion-exchange resin membranes placed at the base of each cylinder. During the crop period, NO ₃ ^- losses were most pronounced after irrigation events. Uptake of N by two crops of lettuce (above- and belowground material) was approximately equal to fertilizer inputs, yet simulation of N fates by the Erosion/Productivity Impact Calculator (EPIC) model indicated losses of 14.6 g-N m^–2 by leaching and 2.5 g-N m^–2 by denitrification during the 6-month crop period. The large NO ₃ ^- losses can be attributed to accumulation of soil NO ₃ ^- during winter that was leached or denitrified during the irrigated crop period.     

Recovery of mineral N fertilizer in herbage and soil organic matter in grasslands of the Massif Central,France

Nitrogen fluxes in intensive grassland have been studied in lysimeters (3 m², 0.80 m deep) with¹⁵N labelled fertilizers (360 kg N/ha). The harvested fertilizer uptake efficiency was around 52% of which 45% was exported and 7% stored. Leaching losses were very low, despite considerable drainage. The immobilization of mineral N during the first year was very important (40–50%) and the remineralization was slight. The N recovered in soil after two years was 40% of N-fertilizer applied of which 19% is contained in free organic matter and 21% in organo-mineral fraction (< 50µ). The immobilization of N is greater in the surface top of soil: 68% in the first 5cm. Beneath a growing crop the in-depth migration of the N takes place exclusively through root growth. The global N balance and cycle in soil were discussed.

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Mots clés bilan de l'azote, lixiviation, minéralisation, immobilisation, matière organique, prairieRésumé La dynamique de l'azote sous une prairie temporaire a été étudiée en cases lysimétriques (3 m², 0,80cm profondeur) en utilisant de l'engrais marqué à¹⁵N (360 kg N/ha). Le coefficient d'utilisation par les parties aériennes a été d'environ 52% dont 45% exporté et 7% mis en réserve. Les pertes par lixiviation ont été minimes malgré un drainage important. L'organisation de Nmineral pendant la première année a été importante (40-50%) et la reminéralisation ultétieure très faible. Après deux ans, 40% de N apporté comme engrais a été retrouvé dans le sol dont 19% dans la matière organique libre et 21% dans les fractions organo-minerales (< 50µ). L'immobilisation de l'azote est supérieure dans les couches superficielles du sol: 68% à la profondeur 0–5 cm. Sous une culture en croissance la migration en profondeur a lieu exclusivement par la croissance racinaire. Le bilan de N et la cycle de l'azote dans le sol ont été établis.

     

工业结晶过程的多相流与粒数衡算的CFD耦合求解

考虑不同尺寸的晶体为不同的分散相,通过有限体积法差分不同尺寸组的晶体求解粒数衡算方程,建立了质量衡算与粒数衡算之间的联系,在考虑晶体成核和生长的条件下,建立了稳态结晶过程的粒数衡算方程与多相流方程的耦合求解方法,得到了工业结晶过程的计算流体力学模拟模型。使用商业软件ANSYS CFD,采用该模型对DTB 型工业结晶器中氯化钾-水体系的结晶过程进行了模拟,最终获得不同尺寸晶体的流场和固体含量的分布信息,从而实现结晶过程的仿真模拟。对部分模拟结果与实验值进行了比较,结果表明模拟结果与实验结果吻合较好。     

Quantifying the effect of soil organic matter on indigenous soil N supply and wheat productivity in semiarid sub-tropical India

A field experiment was conducted on a loamy sand soil for six years to quantify the effect of soil organic matter on indigenous soil N supply and productivity of irrigated wheat in semiarid sub-tropical India. The experiment was conducted by applying different combinations of fertilizer N (0–180 kg N ha⁻¹), P (0–39 kg P ha⁻¹) and K (0–60 kg K ha⁻¹) to wheat each year. For the data pooled over years, fertilizer N together with soil organic carbon (SOC) and their interaction accounted for 75% variation in wheat yield. The amount of fertilizer N required to attain a yield goal decreased as the SOC concentration increased indicating enhanced indigenous soil N supply with an increase in SOC concentration. Besides SOC concentration, the soil N supply also depended on yield goal. For a yield goal of 4 tons ha⁻¹, each ton of SOC in the 15 cm plough layer contributed 4.75 kg N ha⁻¹ towards indigenous soil N supply. An increase in the soil N supply with increase in SOC resulted in enhanced wheat productivity. The contribution of 1 ton SOC ha⁻¹ to wheat productivity ranged from 15 to 33 kg ha⁻¹ across SOC concentration ranging from 3 to 9 g kg^-1 soil. The wheat productivity per ton of organic carbon declined curvilinearly as the native SOC concentration increased. The change in wheat productivity with SOC concentration shows that the effect of additional C sequestration on wheat productivity will depend on the existing SOC concentration, being higher in low SOC soils. Therefore, it will be more beneficial to sequester C in soils with low SOC than with relatively greater SOC concentration. In situations where the availability of organic resources for recycling is limited, their application may be preferred in soils with low SOC concentration. The results show that an increase in C sequestration will result in enhanced wheat productivity but the increase will depend on the amount of fertilizer applied and the existing fertility level of the soil.     

Fate of nitrogen (15N) from oxamide and urea applied to turf grass: A lysimeter study

Slow release N fertilizers are receiving increasing attention for use on turf grass, but their fate in the plant-soil system is still poorly understood. We aimed to quantify the uptake and recovery of N by a mixture of grasses when applied as either urea or oxamide in different diameter granules using a tracer technique (¹⁵N). The effects of the N source on soil biomass, root density and amount of readily available organic C in soil were also evaluated.In a first experiment oxamide in 4–5 mm diameter granules was compared with urea. The initial N absorption, 40 days after fertilization (d.a.f.), was higher for urea (23.5%) than for oxamide (12.1%), but after 64 days absorption efficiencies were about the same (11%) for both fertilizers. Fertilizer-derived N lost by leaching was much greater from the urea-fertilized soil (1.57 g), compared with losses from oxamide-fertilized soil (0.05 g). The total residual fertilizer N remaining in the system at the end of the experiment was 26.7% of applied urea N and 39.6% of applied oxamide N. Cumulated absorption efficiencies, calculated after dismantling the lysimeters, were 43.1% for urea and 54.8% for oxamide (roots included). A priming effect caused by a larger uptake of soil N because of the better root development was found in the oxamide-treated lysimeter. Fertilization with oxamide also caused an increase in the amount of soil microbial biomass.In a second experiment, the efficiencies and fertilizer N uptake rates from oxamide applied at two different granule sizes (1–2 mm and 5–10 mm) were evaluated. The amount of soil N taken up by the grass was linearly related to root density (r = 0.92).     

Effect of seasonal rainfall,N fertilizer and tillage on N utilization by dryland wheat in a semi-arid environment

Crop yield and N uptake in semi-arid environments are typically limited by available water and/or N. Since remobilization of shoot N is a major source of grain N, an understanding of how it is influenced by soil N and water supply, and tillage, is required. In 2003, 2005 and 2006, we determined the influence of N supply (0 or 60 kg fertilizer N ha⁻¹) and tillage [no tillage (NT) or conventional tillage (CT)] on N translocation and N use efficiency of wheat (Triticum aestivum L.) at Scott, Saskatchewan, Canada. Wheat production and N use, and their response to N fertilizer or tillage, were largely influenced by water availability. Wheat N uptake and remobilization were strongly correlated with normalized rainfall in May and June (r = 0.985 and 0.935, respectively, both significant at the P = 0.01 level). In a moisture-stressed year (2003), grain yield was higher under NT than CT, and fertilizer N was ineffective due to low N demand. Nitrogen application increased shoot dry matter (DM), and N uptake and remobilization only in 2006, a year with near-average precipitation. In a wet and cool year (2005), wheat showed no response to tillage or fertilizer N as available soil N was high. Root DM and N content varied slightly only with year or treatment. When N uptake at heading was substantially greater than 100 kg ha⁻¹, N loss occurred during plant senescence, and it was higher with N fertilization: in 2005 and 2006, N-fertilized wheat lost 33–35 kg N ha⁻¹. Nitrogen use efficiency was: (1) higher under NT than CT, due to higher N utilization efficiency, (2) higher with no added N due to higher uptake and utilization efficiencies, and (3) low when water availability was low or excessive. Tillage system had little effect on the uptake, remobilization or loss of N. Fertilizer N application in a year with average rainfall increased wheat production, N accumulation and remobilization, and N loss during senescence.     

Nitrogen input, <Superscript>15</Superscript>N balance and mineral N dynamics in a rice–wheat rotation in southwest China

A field experiment and farm survey were conducted to test nitrogen (N) inputs, ¹⁵N-labelled fertilizer balance and mineral N dynamics of a rice–wheat rotation in southwest China. Total N input in one rice–wheat cycle averaged about 448 kg N ha⁻¹, of which inorganic fertilizer accounted for 63% of the total. The effects of good N management strategies on N cycling were clear: an optimized N treatment with a 27% reduction in total N fertilizer input over the rotation decreased apparent N loss by 52% and increased production (sum of grain yield of rice and wheat) compared with farmers’ traditional practice. In the ¹⁵N-labelled fertilizer experiment, an optimized N treatment led to significantly lower ¹⁵N losses than farmers’ traditional practice; N loss mainly occurred in the rice growing season, which accounted for 82% and 67% of the total loss from the rotation in farmers’ fields and the optimized N treatment, respectively. After the wheat harvest, accumulated soil mineral N ranged from 42 to 115 kg ha⁻¹ in farmers’ fields, of which the extractable soil NO₃ ⁻–N accounted for 63%. However, flooding soil for rice production significantly reduced accumulated mineral N after the wheat harvest: in the ¹⁵N experiment, farmers’ practice led to considerable accumulation of mineral N after the wheat harvest (125 kg ha⁻¹), of which 69% was subsequently lost after 13 days of flooding. Results from this study indicate the importance of N management in the wheat-growing season, which affects N dynamics and N losses significantly in the following rice season. Integrated N management should be adopted for rice–wheat rotations in order to achieve a better N recovery efficiency and lower N loss.     

Fate of fertiliser N applied to winter wheat growing on a Vertisol in a Mediterranean environment

A field study using 15N was conducted on a Vertisol in semi-arid Morocco to assess the fate and efficiency of fertiliser N split applied to winter wheat (Triticum aestivum L.). Splitting of fertiliser N is highly crucial in semi-arid regions, considering the increased moisture stress towards the end of the growing season. A N fertilisation rate of 100 kg N ha-1 was split according to two schemes: i) 25% at planting, 50% at tillering and 25% at stem elongation; or ii) 50% at tillering and 50% at stem elongation. The application of 100 kg N ha-1increased the vegetative dry matter production with more than 2000 kg dry matter ha-1 in comparison with the control treatment. Nitrogen fertilisation had no significant effect on the grain yield production. Moreover, the 1000 grain weight decreased from 32 to 26 g due to N fertilisation. Total N uptake was about 50 kg N ha-1 higher for the fertilised plants in comparison with the unfertilised plants, but it was not affected by the splitting pattern of the fertiliser N. Recoveries of 15N-labelled fertiliser by the plant (above-ground plant parts plus roots from the upper 20 cm layer) were low (31% and 24% for the 3-split and 2-split application, respectively). More N in the plant was derived from fertiliser when applied early in the growing season than when applied late in the season. About 13% of the N in the plants was derived from the 50 kg N ha-1 at tillering, while only 5% was derived from the N application (50 kg N ha-1) at stem elongation. At harvest, a high residual of fertiliser-derived N was found in the 0–90 cm profile (62% and 72%, for the 3-split and 2-split application, respectively). Less than 10% of the applied N could not be accounted for, the amount being highest for the application at tillering. This N not accounted for was mainly ascribed to denitrification after an important rainfall event. The application of fertiliser N led to an increase of about 20 kg N ha-1 in soil N uptake by the crop (positive ANI). The results suggested a dominant influence of moisture availability on the fertiliser N uptake by wheat.     

Effects of soil solution on the dynamics of N2O emissions: a review

In this review, which consists of two parts, major interactions between nitrous oxide (N₂>O) and soil solution are described. In the first part, as an introduction, concentrations of dissolved N₂>O in different aqueous systems are summarized. An inventory of data on maximal N₂>O concentrations in soil solution (up to 9984 g N₂>O-N l^–1>) and in soil air (up to 8300 ppm) from literature is presented. The peak N₂>O concentrations represent a N₂>O supersaturation in the soil solution up to 30000 times with respect to ambient air and a soil air N₂>O concentration about 25000 times higher than in the atmosphere. The main physico–chemical parameters (solubility, diffusion) controlling N₂>O distribution between soil solution and soil air are outlined. The influences of cultivation practice, nitrogen turnover, water content and temperature on N₂>O a ccumulation in soil solution and soil air are reviewed. In the second part some models of N₂>O dynamics in soils are discussed with emphasis on N₂>O transport processes. A simple qualitative scheme is developed to categorize the effects of the soil solution on N₂>O dynamics in soils. In this scheme the temporary, intensive N₂>O oversaturation of the soil solution is interpreted as a result of gas diffusion inhibition by water (barrier function of soil solution) resulting in an accumulation of N₂>O. In addition, N₂>O supersaturation is an indication that transitory much N₂>O can be stored in the soil solution (storage function of soil solution). Where the soil solution flows up-, down- or sidewards it can act as a relevant transport medium for dissolved N₂>O (transport function of soil solution). This scheme is applied to examples from the literature.     

Influence of long-term changes in nitrogen flows on the environment: A case study of a city in Hokkaido, Japan

There is an urgent need to establish sustainable nutrient cycling. Changes in amounts of N flow and separation of production and consumption sectors are becoming a serious environmental problem. In this study, the yearly N in- and outflow of a city in northern Japan from 1912 to 2002 was investigated based on the statistics and inventory data. Based on the characteristics of the N flow, the period was divided into manure-based period (MBP, 1912–1950), transition period from manure- to chemical fertilizer-based period (TP, 1950–1970), and chemical fertilizer-based period (CBP, 1970–2002). The highest amount of N inflow (up to 350 Mg N y^–1) was observed at the end of the MBP, and the second peak (about 300 Mg N y^–1) at the beginning of the CBP. The N application rate on farmland increased from 68 kg N ha^–1 in 1912 to above 250 kg N ha^–1 in the 1950s, then decreased to 168 kg N ha^–1 in 2002. The farmland productivity increased from 30 kg N ha^–1 at the end of the 1950s to 90 kg N ha^–1 in 2002, due to improvement in crop varieties and management methods. In MBP surplus N in farmland and NH₃ volatilization accounted for 90% of the N outflow from the city, then in CBP, disposal N and surplus N in farmland became the main N outflow. All these outputs are considered to increase the N concentration in rivers and/or underground water. In the case of surplus N in farmland, it exceeded the amount of optimum N management (<50 kg N ha^–1; , Agricult. Ecosyst. Environ. 72: 35–52) during 1935–1970 and 1981–1997. In order to prevent degradation of the environment through artificially altered nutrient flow, we need to be aware of the environmental impact of the N flow and establish proper N management practices.     

Biologically Fixed N2 as a Source for N2O Production in a Grass–clover Mixture, Measured by 15N2

The contribution of biologically fixed dinitrogen (N₂) to the nitrous oxide (N₂O) production in grasslands is unknown. To assess the contribution of recently fixed N₂ as a source of N₂O and the transfer of fixed N from clover to companion grass, mixtures of white clover and perennial ryegrass were incubated for 14 days in a growth cabinet with a ¹⁵N₂-enriched atmosphere (0.4 atom% excess). Immediately after labelling, half of the grass–clover pots were sampled for N₂ fixation determination, whereas the remaining half were examined for emission of ¹⁵N labelled N₂O for another 8 days using a static chamber method. Biological N₂ fixation measured in grass–clover shoots and roots as well as in soil constituted 342, 38 and 67 mg N m⁻² d⁻¹ at 16, 26 and 36 weeks after emergence, respectively. The drop in N₂ fixation was most likely due to a severe aphid attack on the clover component. Transfer of recently fixed N from clover to companion grass was detected at 26 and 36 weeks after emergence and amounted to 0.7 ± 0.1 mg N m⁻² d⁻¹, which represented 1.7 ± 0.3% of the N accumulated in grass shoots during the labelling period. Total N₂O emission was 91, 416 and 259 μg N₂O–N m⁻² d⁻¹ at 16, 26 and 36 weeks after emergence, respectively. Only 3.2 ± 0.5 ppm of the recently fixed N₂ was emitted as N₂O on a daily basis, which accounted for 2.1 ± 0.5% of the total N₂O–N emission. Thus, recently fixed N released via easily degradable clover residues appears to be a minor source of N₂O. An erratum to this article is available at .     

N relationship in a legume nonlegume association grown in an intercropping system

Sorghum grown in a mixture with legumes viz. groundnut, mungbean and cowpeas took up more N than sorghum grown as sole crop. In a mixture with mungbean the total N uptake by sorghum was 8.65 g m^–2, while with sole sorghum it was 6.79 g m^–2. The per cent N derived from fertilizer (% Ndff) was highest with sole sorghum and the lowest when grown in mixture with legumes. It is possible that sorghum derived part of the N from the soil pool enriched by concurrently grown legumes in the mixture.     

Modelling nitrogen dynamics in a plant-soil system with a simple model for advisory purposes

A simple functional computer model for advisory purposes is described. Results of simulation indicate some limitations of the model especially in handling the water regime in soils with fluctuating water tables. A major problem seems to be the disappearance of fertilizer N. Measurements by the fumigation-extraction method of microbial N during the growing season show that disappearance of fertilizer N can partly be explained by immobilization by the microbial biomass.     

Effect of crop residue management on nitrogen dynamics and balance in a lowland rice cropping system

Two field experiments were conducted in a rice–fallow–rice cropping sequence during consecutive dry and wet seasons of 1997 on a Fluvic Tropaquept to determine the fate and efficiency of broadcast urea in combination with three residue management practices (no residue, burned residue and untreated rice crop residue). Ammonia volatilization losses from urea (70 kg N ha^–1) broadcast into floodwater shortly after transplanting for 11 d were 7, 12 and 8% of the applied N from no residue, burned residue and residue treated plots, respectively. During that time, the cumulative percent of N₂ + N₂O emission due to urea addition corresponded to 10, 4.3 and nil, respectively. The ¹⁵N balance study showed that at maturity of the dry season crop, fertilizer N recovery by the grain was low, only 9 to 11% of the N applied. Fifty to 53% of the applied ¹⁵N remained in the soil after rice harvest, mainly in the upper 0–5 cm layer. The unaccounted for ¹⁵N ranged from 27 to 33% of the applied N and was unaffected by residue treatments. Only 4 to 5% of the initial ¹⁵N-labeled urea applied to the dry season rice crop was taken up by the succeeding rice crop, to which no additional N fertilizer was applied. Grain yield and N uptake were significantly increased (P=0.05) by N application in the dry season, but not significantly affected by residue treatments in either season.