Nitrous oxide emission from different fertilizers and its mitigation by nitrification inhibitors in irrigated rice

 N₂O emissions from a transplanted irrigated rice grown on a Typic Ustochrept soil at New Delhi, India, were studied to evaluate the effect of N fertilizers, i.e. urea and (NH₄)₂SO₄, alone and in combination with the nitrification inhibitors dicyandiamide (DCD) and thiosulphate. The addition of urea and (NH₄)₂SO₄ increased N₂O emissions considerably when compared to no fertilizer N application (control). N₂O measurement in the field was done by a closed-chamber method for a period of 98 days. The application of urea with DCD and thiosulphate reduced N₂O fluxes considerably. The highest total N₂O-N emission (235 g N₂O-N ha^–1) was from the (NH₄)₂SO₄ treatment, which was significantly higher than the total N₂O-N emission from the urea treatment (160 g N₂O-N ha^–1). DCD reduced N₂O-N emissions by 11% and 26% when applied with urea and(NH₄)₂SO₄, respectively, whereas thiosulphate in combination with urea reduced N₂O-N emissions by 9%. Total N₂O-N emissions were found to range from 0.08% to 0.14% of applied N. N₂O emissions were low during submergence and increased substantially during drainage of standing water. Received: 20 October 1999     

CO2/heat fluxes in rice fields: Comparative assessment of flooded and non-flooded fields in the Philippines

The seasonal fluxes of heat, moisture and CO₂ were investigated under two different rice environments: flooded and aerobic soil conditions, using the eddy covariance technique during 2008 dry season. The fluxes were correlated with the microclimate prevalent in each location. This study was intended to monitor the environmental impact, in terms of C budget and heat exchange, of shifting from lowland rice production to aerobic rice cultivation as an alternative to maintain crop productivity under water scarcity.The aerobic rice fields had higher sensible heat flux (H) and lower latent heat flux (LE) compared to flooded fields. On seasonal average, aerobic rice fields had 48% more sensible heat flux while flooded rice fields had 20% more latent heat flux. Consequently, the aerobic rice fields had significantly higher Bowen ratio (0.25) than flooded fields (0.14), indicating that a larger proportion of the available net radiation was used for sensible heat transfer or for warming the surrounding air.The total C budget integrated over the cropping period showed that the net ecosystem exchange (NEE) in flooded rice fields was about three times higher than in aerobic fields while gross primary production (GPP) and ecosystem respiration (R_e) were 1.5 and 1.2 times higher, respectively. The high GPP of flooded rice ecosystem was evident because the photosynthetic capacity of lowland rice is naturally large. The R_e of flooded rice fields was also relatively high because it was enhanced by the high photosynthetic activities of lowland rice as manifested by larger above-ground plant biomass. The NEE, GPP, and R_e values for flooded rice fields were −258, 778, and 521 g C m⁻², respectively. For aerobic rice fields, values were −85, 515, and 430 g C m⁻² for NEE, GPP, and R_e, respectively. The ratio of R_e/GPP in flooded fields was 0.67 while it was 0.83 for aerobic rice fields.This short-term data showed significant differences in C budget and heat exchange between flooded and aerobic rice ecosystems. Further investigation is needed to clarify seasonal and inter-annual variations in microclimate, carbon and water budget of different rice production systems.     

Nitrogen fixation and beneficial effects of some grain legumes and green-manure crops on rice

Studies were conducted on paddy soils to ascertain N₂ fixation, growth, and N supplying ability of some green-manure crops and grain legumes. In a 60-day pot trial, sunhemp (Crotalaria juncia) produced a significantly higher dry matter content and N yield than Sesbania sesban, S. rostrata, cowpeas (Vigna unguiculata), and blackgram (V. mungo), deriving 91% of its N content from the atmosphere. Dry matter production and N yield by the legumes were significantly correlated with the quantity of N₂ fixed. In a lowland field study involving sunhemp, blackgram, cowpeas, and mungbean, the former produced the highest stover yield and the stover N content, accumulating 160–250 kg N ha^-1 in 60 days, and showed great promise as a biofertilizer for rice. The grain legumes showed good adaptability to rice-based cropping systems and produced a seed yield of 1125–2080 kg ha^-1, depending on the location, species, and cultivar. Significant inter- and intraspecific differences in the stover N content were evident among the grain legumes, with blackgram having the highest N (104–155 kg N ha^-1). In a trial on sequential cropping, the groundnut (Arachis hypogaea) showed a significantly higher N₂ fixation and residual N effect on the succeeding rice crop than cowpeas, blackgram, mungbeans (V. radiata), and pigeonpeas (Cajanus cajan). The growth and N yield of the rice crop were positively correlated with the quantity of N₂ fixed by the preceding legume crop.     

Effect of nitrification inhibitors on rice protein

Abstract

The effects of ammonium sulfate and urea nitrogen (150 kg N/ha) applied with three levels (5, 10 and 15% of N) of the nitrification inhibitors karanjin and nitrapyrin on grain protein of rice (Oryza sativa L. cv. Bala) were studied in pot experiment. Karanjin at the 10 and 15 per cent levels and nitrapyrin at the 10 per cent level significantly increased grain protein. Rice protein levels were highest at the 15 per cent karanjin level.     

Glassy volcanic-ash soils in Japan

Introduction

In Tottori district, there can be seen an extensive sand-dune region, which develops on the coastal terrace of the volcanogenous diluvium running from east to west along the shore of the Japan Sea. There is about 9750 hectares in total area: 5265 hectares comprising numerous inter-dune depressions are used for tillage field, 1207 hectares afforested and 3278 hectares left barren.     

Effect of nitrogen fertilization on methane and carbon dioxide production potential in relation to labile carbon pools in tropical flooded rice soils in eastern India

In an incubation experiment with flooded rice soil fertilized with different N amounts and sampled at different rice stages, the methane (CH₄) and carbon dioxide (CO₂) production in relation to soil labile carbon (C) pools under two temperature (35°C and 45°C) and moisture (aerobic and submerged) regimes were investigated. The field treatments imposed in the wet season included unfertilized control and 40, 80 and 120 kg ha^?1 N fertilization. The production of CH₄ was significantly higher (27%) under submerged compared to aerobic conditions, whereas CO₂ production was significantly increased under aerobic by 21% compared to submerged conditions. The average labile C pools were significantly increased by 21% at the highest dose of N (120 kg ha^?1) compared to control and was found highest at rice panicle initiation stage. But the grain yield had significantly responded only up to 80 kg ha^?1 N, although soil labile C as well as gaseous C emission was noticed to be highest at 120 kg ha^?1 N. Hence, 80 kg N ha^?1 is a better option in the wet season at low land tropical flooded rice in eastern India for sustaining grain yield and minimizing potential emission of CO₂ and CH₄.     

Nitrification of ammonium in different components of a flooded rice soil system

Nitrification associated with the various components [subsurface soil from unplanted and planted (rhizosphere) fields, standing water and surface soil from planted and unplanted fields and leaf sheath suspensions] of submerged rice paddies was examined in incubation experiments with solutions inoculated with soil or water samples. Substantial nitrification occurred in all samples, standing water and surface soil samples in particular, during their 40-day incubation with NH ₄ ⁺ –N. Almost all the NH ₄ ⁺ –N, disappeared during incubation with standing water, was recovered as NO _inf3 ^sup- –N. This, compared to 70–80% from all soil samples and only 29% from leaf sheath suspensions. Significant loss of nitrogen, especially from leaf sheath suspensions, is probably due to nitrification-denitrification as evidenced by its complete recovery in the presence of N-Serve. Nitrification potential of the soil and water samples varied with the crop growth stage and was more pronounced at tillering and panicle inititation stages than at other stages. Nitrification potential of samples from green-manure-amended plots was distinctly less than that of samples from control and urea-amended plots. Most probable number (MPN) estimates of ammonium-oxidizing bacteria were always higher in surface soil in both planted and unplanted plots at all stages of crop growth.Dedicated to Professor J. C. G. Ottow on the occasion of his 60th birthday     

Effects of inorganic fertilizers and micronutrients on methane production from wetland rice (Oryza sativa L.)

We compared the effects of adding different forms of nitrogenous fertilizers on the production of CH₄ in soil and on CH₄ emission from rice plants, Urea and diammonium phosphate gave the highest rates of CH₄ production from the soil and emission through rice plants, followed by (NH₄)₂SO₄. NaNO₃ was the least effective. The effects of micronutrients like Mo, Ni, or B were more prominent than those of Fe, Mn, Zn, V, or Co. It is concluded that CH₄ emission from rice paddies is influenced by both macro- and micronutrients, through effects on both microbial methanogenesis in soil and elimination through rice plants as a consequence of the effects on plant growth.     

Effect of encapsulated calcium carbide on dinitrogen,nitrous oxide,methane, and carbon dioxide emissions from flooded rice

Summary The efficiency of N use in flooded rice is usually low, chiefly due to gaseous losses. Emission of CH₄, a gas implicated in global warming, can also be substantial in flooded rice. In a greenhouse study, the nitrification inhibitor encapsulated calcium carbide (a slow-release source of acetylene) was added with 75, 150, and 225 mg of 75 atom % ¹⁵N urea-N to flooded pots containing 18-day-old rice (Oryza sativa L.) plants. Urea treatments without calcium carbide were included as controls. After the application of encapsulated calcium carbide, 3.6 g N₂, 12.4 g N₂O-N, and 3.6 mg CH₄ were emitted per pot in 30 days. Without calcium carbide, 3.0 mg N₂, 22.8 g N₂O-N, and 39.0 mg CH₄ per pot were emitted during the same period. The rate of N added had a positive effect on N₂ and N₂O emissions, but the effect on CH₄ emissions varied with time. Carbon dioxide emissions were lower with encapsulated calcium carbide than without. The use of encapsulated calcium carbide appears effective in eliminating N₂ losses, and in minimizing emissions of the greenhouse gases N₂O and CH₄ in flooded rice.     

稻田反硝化速率测定方法研究进展

反硝化作用是淹水稻田肥料氮损失的主要途径之一.采用合适的反硝化测定方法是开展稻田反硝化作用研究的前提.然而,由于反硝化过程主要产物N2的大气背景值较高,以及反硝化作用具有高度时空异质性,淹水稻田反硝化作用损失氮量难以准确量化一直是阻碍科学评价稻田气态氮损失的关键难题.本文综述了研究稻田反硝化作用的4种方法(乙炔抑制法、...     

Nitrogen-15 balance in transplanted and direct-seeded flooded rice as affected by different methods of urea application

Summary Alternative N-fertilizer management practices are needed to increase productivity and the N-use efficiency of flooded rice (Oryza sativa L.). In the 1987 dry season, a field study using ¹⁵N-labeled urea evaluated the effect of the time and method of fertilizer-N application on grain yield and N-use efficiency in transplanted and direct-seeded flooded rice. Conventional fertilizer application (broadcasting and incorporation) was compared with band placement of liquid urea and point placement of urea supergranules. With band or point placement, the grain yields were significantly greater, and the partial pressure of NH₃ (pNH₃) in the floodwater was significantly reduced. In the transplanted rice, conventional fertilizer-N application gave a 64% total ¹⁵N recovery and 38% crop (grain and straw) recovery. Band placement of liquid urea N resulted in 92% total and 73% crop recovery. In the direct-seeded flooded rice, a conventional N application gave 72% total and 42% crop recovery; band placement, 98% total and 73% crop recovery; and urea supergranule point placement, 97% total and 75% crop recovery.Dedicated to Prof. Dr. K. Mengel, Giessen, FRG, in honor of his 60th birthday.     

Effect of placement of urea and coated urea fertilizers on yield and quality of soybean (Glycine max (L.) Merr.) seeds

The objective of the present study was to record the seed yield and to examine visually the quality of soybean seeds cultivated under different types and placements of urea fertilizers. In addition to the conventional fertilizer application (including ammonium sulfate 16 kg N ha^-1 broadcasting (100 kg N ha^-1 of urea (0B) and X00-d type coated urea CU-100 (CUB), and deep placement (100 kg N ha^-1) of urea (UD) and 100-d type coated urea CU-100 (CUD) was conducted in separate plots in a paddy field converted to an upland field located at Shindori Experimental Station of Niigata University. Soybean plant growth was periodically analyzed and the quality of harvested seeds was also visually examined (hereafter referred to as “visual quality”). It was found that the deep placement treatments were more conducive 1o nitrogen (N₂)fixation, based on the relative mreide N concentration in the xylem sap, which is a good indicator of N~fixation by soybean. Also the total seed yield was the highest in CUD (82 g plant^-1) and 0D (81 g plant^-1), compared to the control (62 g plant^-1), UB (68 g plant^-1), and CUB (68 g plant^-1). The visual quality of harvested seeds showed that CUD enhanced the quality of seeds compared to the other treatments, in which the percentage of good quality seeds, hereafter referred to as "good seeds," based on the dry weight was 51 (control), 65 (K3B), 61 (CUB), 61 (0D), and 6696 (CUD). In terms of diseased seeds, the percentage of turtle wrinkle and broken seed coats was found to decrease by N application compared to the control. Thus, it is suggested that N fertilization management is important for maximum yield of soybean as well as for the enhancement of seed quality.     

Development of cyanobacterial blooms in Valencian rice fields

Seasonal variation of biomass, photosynthetic pigments and C and N contents of the cyanobacterial blooms developed during the crop cycles of 1998, 1999 and 2000 at two locations in Valencian rice fields (Spain) were studied to find their potential contribution to soil fertility. Blooms of Gloeotrichia sp., Gloeocapsa sp., Microchaete sp., and Nostoc sp. were small and dispersed and appeared only for a few weeks in the experimental fields of the Tancat de Malta location during the crop season of 1998. The biomass of these blooms ranged from 4.4 to 12.8 mg dry wt cm^–2. A bloom of Anabaena sp. was found in the same location during most of the crop cycles of 1998 and 1999, covering up to 80% of the experimental fields. The biomass of this bloom ranged from 2.7 to 11.4 mg dry wt cm^–2. In the moment of its maximal extension (July 1998) we estimate a total biomass of 2,100 kg dry wt for this bloom, equivalent to 420 kg dry wt ha^–1. In the Sueca location a bloom of Microchaete sp. was observed in August 1999 and during most of the crop cycle of 2000, when it covered a wide area (60–70%) of the experimental field. The biomass of this bloom ranged from 0.9 to 5.6 mg dry wt cm^–2. Carbon contents of the blooms ranged from 150 to 310 g mg^–1 dry wt and N contents ranged from 11 to 30.2 g mg^–1 dry wt. Taking into account their N content and their extension we estimate that the incorporation of cyanobacterial blooms could add 4–12 kg N ha^–1 to soil. From an agronomical point of view this means that, in spite of their impressive aspect, cyanobacterial blooms can only fulfill a small proportion of the N requirements of rice plants.     

The effects on N2 fixation (C2H2 reduction), bacterial population and rice plant growth of two modes of straw application to a wetland rice field

Summary The effects of incorporation and surface application of straw to a wetland rice field on nitrogen fixation (C₂H₂ reduction), bacterial population and rice plant growth were studied. Rice straw (5 t ha^–1) was chopped (10- to 15-cm pieces) and applied to the field 2 weeks before transplanting IR42, a long-duration variety, and IR50, a short-duration variety. The acetylene-reducing activity (ARA) of IR42 and IR50 measured at heading stage for 3 consecutive days showed significantly higher ARA in IR42 as a result of the 2 straw application methods. Mostly up to 20 days after straw surface application and incorporation, the dark ARA in the soil, total and N₂-fixing heterotrophs, and photoorganotrophic purple nonsulphur bacteria (POPNS) in the soil and in association with degrading straw were stimulated. Higher bacterial populations were associated with straw on the surface than with straw incorporated. The POPNS counts, in particular, were increased hundreds fold in the surface-applied straw treatment. Straw applications also increased the root, shoot and total plant biomass at heading stage and the total dry matter yield at harvest in both varieties. The data show the potentials of straw as a source of substrate for the production of microbial biomass and for the non-symbiotic N₂ fixation to improve soil fertility and plant nutrition.     

Nitrogen fixation in paddy soils

Several important features of the N. fixation in paddy fields which were reported previously were confirmed and some new additional results regarding the evaluation of the N₂ fixation in the rhizosphere were obtained by reinvestigation in the fields. In addition, rice plants were cultivated in the submerged soil in pots and various parts of the soil were analyzed for the N₂-fixing activity as well as several other properties. The results of the pot experiments were found to be fairly similar to those observed in the field investigations, indicating the validity of the submerged soil in a pot as a rather simulated model for the actual paddy field. By using this model system, the following facts were ascertained: (1) Water-percolation had almost no effect on the N₂-fixing activities of both the rhizosphere and the non-rhizosphere soils. (2) Suppressing effect of washing the root of rice plant on the N₂-fixing activity was slight in the seedling stage and marked in the tillering and flowering stages. (3) The N₂-fixing activity of a single rice root varied from tip to base.     

Effects of various nitrogen fertilizers on emission of nitrous oxide from soils

Summary Field studies of the effects of different N fertilizers on emission of nitrous oxide (N20) from three Iowa soils showed that the N₂O emissions induced by application of 180 kg ha^–1 fertilizer N as anhydrous ammonia greatly exceeded those induced by application of the same amount of fertilizer N as aqueous ammonia or urea. On average, the emission of N₂O-N induced by anhydrous ammonia was more than 13 times that induced by aqueous ammonia or urea and represented 1.2% of the anhydrous ammonia N applied. Experiments with one soil showed that the N₂O emission induced by anhydrous ammonia was more than 17 times that induced by the same amount of N as calcium nitrate. These findings confirm indications from previous work that anhydrous ammonia has a much greater effect on emission of N₂O from soils than do other commonly used N fertilizers and merits special attention in research relating to the potential adverse climatic effect of N fertilization of soils.Laboratory studies of the effect of different amounts of NH₄OH on emission of N₂O from Webster soil showed that the emission of N₂O-N induced by addition of 100 g NH₄OH-N g^–1 soil represented only 0.18% of the N applied, whereas the emissions induced by additions of 500 and 1 000 g NH4OH-N g^–1 soil represented 1.15% and 1.19%, respectively, of the N applied. This suggests that the exceptionally large emissions of N₂O induced by anhydrous ammonia fertilization are due, at least in part, to the fact that the customary method of applying this fertilizer by injection into soil produces highly alkaline soil zones of high ammonium-N concentration that do not occur when urea or aqueous ammonia fertilizers are broadcast and incorporated into soil.     

Relationships between soil moisture content and nitrous oxide production during nitrification and denitrification

Summary The effect of soil water content [60%–100% water-holding capacity (WHC)] on N₂O production during autotrophic nitrification and denitrification in a loam soil was studied in a laboratory experiment by selectively inhibiting nitrification with a low C₂H₂ concentration (2.1 Pa). Nitrifiers usually produced more N₂O than denitrifiers. During an initial experimental period of 0–6 days the nitrifiers produced more N₂O than the denitrifiers by a factor ranging from 1.4 to 16.5, depending on the water content and length of incubation. The highest N₂O production rate by nitrifiers was observed at 90% WHC, when the soil had become partly anaerobic, as indicated by the high denitrification rate. At 100% WHC there were large gaseous losses from denitrification, while nitrification losses were smaller except for the first period of measurement, when there was still some O₂ remaining in the soil. The use of 10 kPa C₂H₂ to inhibit reduction of N₂O to N₂ stimulated the denitrification process during prolonged incubation over several days; thus the method is unsuitable for long-term studies.     

Interactive effects from combining fertilizer and organic residue inputs on nitrogen transformations

Concerns about sustainability of agroecosystems management options in developed and developing countries warrant improved understanding of N cycling. The Integrated Soil Fertility Management paradigm recognizes the possible interactive benefits of combining organic residues with mineral fertilizer inputs on agroecosystem functioning. However, these beneficial effects may be controlled by residue quality. This study examines the controls of inputs on N cycling across a gradient of (1) input, (2) residue quality, and (3) texture. We hypothesized that combining organic residue and mineral fertilizers would enhance potential N availability relative to either input alone. Residue and fertilizer inputs labeled with ¹⁵N (40–60 atom% ¹⁵N) were incubated with 200 g soil for 545 d in a microcosm experiment. Input treatments consisted of a no-input control, organic residues (3.65 g C kg⁻¹ soil, equivalent to 4 Mg C ha⁻¹), mineral N fertilizer (100 mg N kg⁻¹ soil, equivalent to 120 kg N ha⁻¹), and a combination of both with either the residue or fertilizer ¹⁵N-labeled. Zea mays stover inputs were added to four differently textured soils (sand, sandy loam, clay loam, and clay). Additionally, inputs of three residue quality classes (class I: Tithonia diversifolia, class II: Calliandra calothyrsus, class III: Z. mays stover) were applied to the clay soil. Available N and N₂O emissions were measured as indicators for potential plant N uptake and N losses. Combining residue and fertilizer inputs resulted in a significant (P < 0.05) negative interactive effect on total extractable mineral N in all soils. This interactive effect decreased the mineral N pool, due to an immobilization of fertilizer-derived N and was observed up to 181 d, but generally became non-significant after 545 d. The initial reduction in mineral N might lead to less N₂O losses. However, a texture effect on N₂O fluxes was observed, with a significant interactive effect of combining residue and fertilizer inputs decreasing N₂O losses in the coarse textured soils, but increasing N₂O losses in the fine textured soils. The interactive effect on mineral N of combining fertilizer with residue changed from negative to positive with increasing residue quality. Our results indicate that combining fertilizer with medium quality residue has the potential to change N transformations through a negative interactive effect on mineral N. We conclude that capitalizing on interactions between fertilizer and organic residues allows for the development of sustainable nutrient management practices.     

Effect of urease and nitrification inhibitors on N transformation, gaseous emissions of ammonia and nitrous oxide, pasture yield and N uptake in grazed pasture system

Nitrogen (N) losses via nitrate (NO₃⁻) leaching, ammonia (NH₃) volatilization and nitrous oxide (N₂O) emissions from grazed pastures in New Zealand are one of the major contributors to environmental degradation. The use of N inhibitors (urease and nitrification inhibitors) may have a role in mitigating these N losses. A one-year field experiment was conducted on a permanent dairy-grazed pasture site at Massey University, Palmerston North, New Zealand to quantify these N losses and to assess the effect of N inhibitors in reducing such losses during May 2005-2006. Cow urine at 600 kg N ha⁻¹ rate with or without urease inhibitor N-(n-butyl) thiophosphoric triamide (nBTPT) or (trade name “Agrotain”) (3 L ha⁻¹), nitrification inhibitor dicyandiamide (DCD) (7 kg ha⁻¹) and the use of double inhibitor (DI) containing a combination of both Agrotain and DCD (3:7) were applied to field plots in autumn, spring and summer. Pasture production, NH₃ and N₂O fluxes, soil mineral N concentrations, microbial biomass C and N, and soil pH were measured following the application of treatments during each season. All measured parameters, except soil microbial biomass C and N, were influenced by the added inhibitors during the three seasons. Agrotain reduced NH₃ emissions over urine alone by 29%, 93% and 31% in autumn, spring and summer respectively but had little effect on N₂O emission. DCD reduced N₂O emission over urine alone by 52%, 39% and 16% in autumn, spring and summer respectively but increased NH₃ emission by 56%, 9% and 17% over urine alone during those three seasons. The double inhibitor reduced NH₃ by 14%, 78% and 9% and N₂O emissions by 37%, 67% and 28% over urine alone in autumn, spring and summer respectively. The double inhibitor also increased pasture dry matter by 10%, 11% and 8% and N uptake by the 17%, 28% and 10% over urine alone during autumn, spring and summer respectively. Changes in soil mineral N and pH suggested a delay in urine-N hydrolysis with Agrotain, and reduced nitrification with DCD. The combination of Agrotain and DCD was more effective in reducing both NH₃ and N₂O emissions, improving pasture production, controlling urea hydrolysis and retaining N in NH₄⁺ form. These results suggest that the combination of both urease and nitrification inhibitors may have the most potential to reduce N losses if losses are associated with urine and improve pasture production in intensively grazed systems.     

具有保水功能的缓释肥料的制备研究

以尿素、高吸水性树脂、低品位磷矿、风化煤、膨润土等为主要材料,制备了一种既有吸水保水功能又有缓释功能的双层包膜尿素。对其工艺条件进行了优化,结果表明,当以低品位磷矿为缓释材料,粘结剂浓度为50%,石蜡用量为10%,保水剂粒径为0.15 mm时,保水缓释肥在水中24 h和168 h氮素累计释放率分别为4.36%和26.5%,在土壤中64 d氮素累计释放率为49.34%,常压下吸附肥料自身质量20.3倍的水分。加有保水缓释肥的土壤保水率曲线表明,该保水缓释肥可明显降低水分蒸发率,提高土壤水分的有效性,改善土壤释水、保水的能力。保水缓释肥兼有吸水、保水和养分缓释性能,具有较好开发和应用前景。