减量施氮对玉米-大豆套作系统下作物氮素吸收和利用效率的影响

为探索玉米-大豆套作系统中作物对N素吸收的差异特性,揭示减量施N对玉米-大豆套作系统的N高效利用机理。利用15N同位素示踪技术,结合小区套微区多年定位试验,研究了玉米单作(MM)、大豆单作(SS)、玉米-大豆套作(IMS)及不施N(NN)、减量施N(RN:180 kg N/hm2)、常量施N(CN:240 kg N/hm2)下玉米、大豆的生物量、吸N量、N肥利用率及土壤N素含量变化。结果表明,与MM(SS)相比,IMS下玉米茎叶及籽粒的生物量、吸N量降低,15N%丰度及15N吸收量增加,大豆籽粒及植株的生物量、吸N量及15N吸收量显著提高;IMS下玉米、大豆植株的N肥利用率、土壤N贡献率、土壤15N%丰度降低,15N回收率显著增加。施N与不施N相比,显著提高了单、套作下玉米、大豆植株的生物量、吸N量、15N丰度及15N吸收量;RN与CN相比,IMS下,RN的玉米、大豆植株总吸N量提高13.4%和12.4%,N肥利用率提高213.0%和117.5%,土壤总N含量提高12.2%和11.6%,土壤N贡献率降低12.0%和11.2%,玉米植株15N吸收量与15N回收率提高14.4%和52.5%,大豆的则降低57.1%和42.8%,单作与套作的变化规律一致。玉米-大豆套作系统中作物对N素吸收存在数量及形态差异,减量施N有利于玉米-大豆套作系统对N肥的高效吸收与利用,实现作物持续增产与土壤培肥。     

Nitrogen fixation and N transfer from peanut to rice cultivated in aerobic soil in an intercropping system and its effect on soil N fertility

The novel cultivation of paddy rice in aerobic soil reveals the great potential not only for water-saving agriculture, but also for rice intercropping with legumes and both are important for the development of sustainable agriculture. A two-year field experiment was carried out to investigate the yield advantage of intercropping peanut (Arachis hypogaea L., Zhenyuanza 9102) and rice (Oryza sativa L., Wuyujing 99-15) in aerobic soil, and its effect on soil nitrogen (N) fertility. A pot experiment was also conducted to examine the N₂-fixation by peanut and N transfer from peanut to rice at three N fertilizer application rates, i.e., 15, 75 and 150 kg N ha^–1 using a ¹⁵N isotope dilution method. The results showed that the relative advantage of intercropping, expressed as land equivalent ratio (LER), was 1.41 in 2001 and 1.36 in 2002. Both area-adjusted yield and N content of rice were significantly increased in the intercropping system while those of peanut were not significantly different between intercropping and monocropping systems. The yields of rice grain and peanut, for example, were increased by 29–37% and 4–7% in the intercropping system when compared to the crop grown in the monocropping system. The intercropping advantage was mainly due to the sparing effect of soil inorganic N contributed by the peanut. This result was proved by the higher soil mineral N concentration under peanut monocropping and intercropping than under the rice monocropping system.%Ndfa (nitrogen derived from atmosphere) by peanut was 72.8, 56.5 and 35.4% under monocropping and 76.1, 53.3 and 50.7% under the intercropping system at N fertilizer application rates of 15, 75 and 150 kg ha^–1, respectively. The ¹⁵N-based estimates of N transfer from peanut (%NTFL) was 12.2, 9.2 and 6.2% at the three N fertilizer application rates. N transferred from peanut accounted for 11.9, 6.4 and 5.5% of the total N accumulated in the rice plants in intercropping at the same three N fertilizer application rates, suggesting that the transferred N from peanut in the intercropping system made a contribution to the N nutrition of rice, especially in low-N soil.     

Nitrogen fixation,transfer and turnover in upland and lowland grass-clover swards,using15N isotope dilution

The stable isotope¹⁵N is particularly valuable in the field for measuring N fixation by isotope dilution. At the same time other soil-plant processes can be studied, including¹⁵N recovery, and nitrogen transfer between clover and grass. Three contrasting sites and soils were used in the present work: a lowland soil, an upland soil, and an upland peat. Nitrogen fixation varied from 12 gm^–2 on lowland soil to 2.7 gm^–2 on upland peat. Most N transfer occurred on upland soil (4.2 gm^–2) which, added to nitrogen fixed, made a total of 8.7 gm² input during summer 1985.¹⁵N recovery for the whole experiment was small, around 25%.Measurement of dead and dying leaves, stubble and roots, suggests that plant organ death is the first stage in N transfer from white clover to ryegrass, through the decomposer cycle. Decomposition was fastest on lowland soils, slowest on peat. On lowland soil this decomposer nitrogen is apparently subverted before transfer, probably by soil microbes.Variations in natural abundance of¹⁵N in plants were found in the two species on the different soils. These might be used to measure nitrogen fixation without adding isotope, but the need for many replicates and repeat samples would limit throughput.     

Use of 15N stable isotope to quantify nitrogen transfer between mycorrhizal plants

Aims Mycorrhizas (fungal roots) play vital roles in plant nutrient acquisition, performance and productivity in terrestrial ecosystems. Arbuscular mycorrhizas (AM) and ectomycorrhizas (EM) are mostly important since soil nutrients, including NH₄⁺, NO₃^? and phosphorus, are translocated from mycorrhizal fungi to plants. Individual species, genera and even families of plants could be interconnected by mycorrhizal mycelia to form common mycorrhizal networks (CMNs). The function of CMNs is to provide pathways for movement or transfer of nutrients from one plant to another. In the past four decades, both ¹⁵N external labeling or enrichment (usually expressed as atom%) and ¹⁵N naturally occurring abundance (δ¹⁵N, ‰) techniques have been employed to trace the direction and magnitude of N transfer between plants, with their own advantages and limitations.     

Climate and forest management affect 15N-uptake,N balance and biomass of European beech seedlings

We investigated the effect of (a) different local climate and (b) thinning of the forest canopy on growth and N status of naturally regenerated European beech seedlings in a beech forest on shallow rendzina soil in southern Germany. For this purpose, a ¹⁵N-tracing experiment was conducted during the growing season of the year 2000 with beech seedlings growing on a warm, dry SW-exposed site and a cooler, moist NE-exposed site, and in a thinned and a control stand at each site. Biomass, ¹⁵N uptake and partitioning and total N concentrations of beech seedlings were determined. Site and thinning produced clear differences, particularly at the end of the growing season. Biomass and cumulative ¹⁵N uptake of beech seedlings then increased due to thinning on the NE site and decreased on the SW site. Total N concentrations in leaves, roots and stems of beech seedlings responded similarly. Therefore, growth and N status of beech seedlings are found to be favoured by thinning under cool-moist conditions. However, under higher temperature and reduced water availability—conditions that are prognosticated in the near future—thinning reduces N uptake and plant N concentration and, thus, impairs N balance and growth of beech regeneration.     

The contribution of associative and symbiotic nitrogen fixation to the nitrogen nutrition of non-legumes

During the past 10 years estimates of N₂ fixation associated with sugar cane, forage grasses, cereals and actinorhizal plants grown in soil with and without addition of inoculum have been obtained using the ¹⁵N isotope dilution technique. These experiments are reviewed in this paper with the aim of determining the proportional and absolute contribution of N₂ fixation to the N nutrition of non-legumes, and its role as a source of N in agriculture. The review also identifies deficiencies in both the totality of data which are currently available and the experimental approaches used to quantify N₂ fixation associated with non-legumes.Field data indicate that associative N₂ fixation can potentially contribute agronomically-significant amounts of N (>30–40 kg N ha^-1 y^-1) to the N nutrition of plants of importance in tropical agriculture, including sugar cane (Saccharum sp.) and forage grasses (Panicum maximum, Brachiaria sp. and Leptochloa fusca) when grown in uninoculated, N-deficient soils. Marked variations in proportions of plant N derived from the atmosphere have been measured between species or cultivars within species.Limited pot-culture data indicate that rice can benefit naturally from associative N₂ fixation, and that inoculation responses due to N₂ fixation can occur. Wheat can also respond to inoculation but responses do not appear to be due to associative N₂ fixation. ¹⁵N dilution studies confirm that substantial amounts of N₂ can be fixed by actinorhizal plants.     

小麦与蚕豆间作系统氮肥调控对小麦白粉病发生及氮素累积分配的影响

通过田间小区试验,设N₀(0 kg·hm^-2)、N₁(112.5 kg·hm^-2)、N₂( 225 kg·hm^-2)、N₃( 337.5 kg·hm^-2) 4个施氮水平,研究不同施氮水平下小麦与蚕豆间作对小麦白粉病发生、植株氮含量和氮素累积分配的影响,探讨间作系统氮肥调控下小麦植株氮素含量、氮素累积分配与白粉病发生的关系.结果表明: 无论单作还是间作,施氮(N₁、N₂和N₃)均增加了小麦籽粒产量,以N₂水平下产量最高,单、间作分别为4146和4679 kg·hm^-2;施氮加重了小麦白粉病的发生与危害,N₁、N₂和N₃水平下病害进展曲线下的面积(AUDPC)分别平均增加39.6%～55.6%(基于发病率DI)和92.5%～217.0%(基于病情指数DSI),病情指数受氮素调控的影响较发病率大;施氮显著提高小麦植株氮含量(8.4%～51.6%)和氮素累积量(19.7%～133.7%),对氮素分配比例无显著影响.与单作相比,间作小麦产量平均增加12.0%;AUDPC(DI)和AUDPC(DSI)分别平均降低11.5%和30.7%,间作对病情指数的控制效果优于发病率.间作显著降低发病盛期小麦氮含量、阶段累积量和叶片中氮素分配比例(降幅6.6%～12.5%、1.4%～6.9%和9.0%～15.5%).在本研究条件下,兼顾控病效果和产量效应,小麦施氮量不应超过225 kg·hm^-2.     

Below-ground nitrogen transfer between different grassland species: Direct quantification by 15N leaf feeding compared with indirect dilution of soil 15N

Nitrogen (N) transfer from one species to another is important for the N cycling in low-input grassland. In the present work, estimates obtained by an indirect ¹⁵N dilution technique were compared with estimates obtained by a direct ¹⁵N leaf feeding technique over two complete growing seasons in red clover-ryegrass and white clover-ryegrass mixtures under field conditions. The direct technique confirmed that N transfer between clovers and ryegrass is a bi-directional process. The transfer of N from both clovers to ryegrass occurred within 25 days upon the first labelling event. A very high N transfer occurred from white clover to the associated ryegrass, 4.5 and 7.5 g m⁻² in the 1st and 2nd production year, respectively. The corresponding values for transfer from red clover to the associated ryegrass were 1.7 and 3.6 g m⁻². Quantified relatively to the total above-ground N content of white clover- ryegrass and red clover-ryegrass mixtures, the N transfer exceeded 50% and 10%, respectively, in three out of seven harvests. The N transfer from ¹⁵N labelled grass to associated clovers constituted a relatively constant proportion of approx. 8% of the above-ground N content of the mixtures. Estimates based on the soil ¹⁵N dilution technique generally underestimated the net N transfer by more than 50% compared to the direct ¹⁵N labelling technique. Furthermore, the indirect ¹⁵N dilution technique estimated only marginal differences between red and white clover in the quantities of N transferred, whereas the direct ¹⁵N labelling technique showed the N transfer from white clover to the associated ryegrass to be significantly higher than that involving red clover. It is concluded that N transfer is a much more dynamic and quantitatively important process in grassland than previously recognised. This revised version was published online in June 2006 with corrections to the Cover Date.     

The nitrogen mineralization rate of legume residues in soil as influenced by their polyphenol,lignin, and nitrogen contents

A 12-week greenhouse experiment was conducted to determine the effect of the polyphenol, lignin and N contents of six legumes on their N mineralization rate in soil and to compare estimates of legume-N release by the difference and ¹⁵N-recovery methods. Mature tops of alfalfa (Medicago sativa L.), round leaf cassia (Cassia rotundifolia Pers., var. Wynn), leucaena (Leucaena leucocephala Lam., deWit), Fitzroy stylo (Stylosanthes scabra Vog., var Fitzroy), snail medic (Medicago scutellata L.), and vigna (Vigna trilobata L., var verde) were incorporated in soil at the rate of 100 mg legume N kg^-1 soil. The medic and vigna were labeled with ¹⁵N. Sorghum-sudan hybrid (Sorghum bicolor, L. Moench) was used as the test crop. A non-amended treatment was used as a control. Net N mineralization after 12 weeks ranged from 11% of added N with cassia to 47% of added N for alfalfa. With the two legumes that contained less than 20 g kg^-1 of N, stylo and cassia, there was net N immobilization for the first 6 weeks of the experiment. The legume (lignin + polyphenol):N ratio was significantly correlated with N mineralization at all sampling dates at the 0.05 level and at the 0.01 level at 6 weeks (r²=0.866). Legume N, lignin, or polyphenol concentrations or the lignin:N ratio were not significantly correlated with N mineralization at any time. The polyphenol:N ratio was only significantly correlated with N mineralization after 9 weeks (r²=0.692). The (lignin + polyphenol):N ratio appears to be a good predictor of N mineralization rates of incorporated legumes, but the method for analyzing plant polyphenol needs to be standardized. Estimates of legume-N mineralization by the difference and ¹⁵N recovery methods were significantly different at all sampling dates for both ¹⁵N-labeled legumes. After 12 weeks, estimates of legume-N mineralization averaged 20% more with the difference method than with the ¹⁵N recovery method. This finding suggests that estimates of legume N available to subsequent crops should not be based solely on results from ¹⁵N recovery experiments.     

The fate of15N enriched throughfall in two coniferous forest stands at different nitrogen deposition levels

The stable isotope¹⁵N was added as (¹⁵NH₄)₂SO₄ to throughfall water for one year, to study the fate of the deposited nitrogen at different levels of N deposition in two N saturated coniferous forests ecosystems in the Netherlands. The fate of the¹⁵N was followed at high-N (44–55 kg N ha^–1 yr^–1) 1) and low-N (4–6 kg N ha^–1 yr^–1) deposition in plots established under transparent roofs build under the canopy in a Douglas fir (Pseudotsuga menziesii (Mirb.) Franco.) and Scots pine (Pinus sylvestris L.) forest.The applied¹⁵N was detectable in needles and twigs, the soil and soil water leaching below the rooting zone (90 cm depth). Total¹⁵N recovery in major ecosystem compartments was 71–100% during two successive growing seasons after the start of a year-round¹⁵N application to throughfall-N. Nine months after the year-round¹⁵N application, the¹⁵N assimilated into tree biomass was 29–33% of the¹⁵N added in the Douglas fir stand and less than 17% in the Scots pine stand. At the same time total¹⁵N retention in the soil (down to 70 cm) of the high-N plots was about 37% of the deposited¹⁵NH₄-N, whereas 46% and 65% of the¹⁵N was found in the soil of the low-N deposition plots at the Douglas fir and Scots pine stand, respectively. The organic layers accounted for 60% of the¹⁵N retained in the soil. The total N deposition exceeded the demand of the vegetation and microbial immobilization. Total¹⁵N leaching losses within a year (below 90 cm) were 10–20% in the high-N deposition plots in comparison to 2–6% in the lowered nitrogen input plots. Relative retention in the soil and vegetation increased at lower N-input levels.Species differences in uptake and tree health seem to contribute to lower¹⁵N recoveries in the Scots pine trees compared to the Douglas fir trees. The excessive N deposition and resulting N saturation lead to conditions were the health and functioning of biota were negatively influenced. At decreased N deposition, lower leaching losses together with increased soil and plant retention indicated a change in the fate of the¹⁵N deposited. This may have resulted from changes in ecosystem processes, and thus a shift along the continuum of N saturation to N limitation.     

Evaluating the effects of gross nitrogen mineralization,immobilization, and nitrification on nitrogen fertilizer availability in soil experimentally contaminated with diesel

Sandy clay loam soil was contaminated with 5000 mg kg⁻¹ diesel, and amended with nitrogen (15.98 atom% ¹⁵N) at 0, 250, 500, and 1000 mg kg⁻¹ to determine gross rates of nitrogen transformations during diesel biodegradation at varying soil water potentials. The observed water potential values were −0.20, −0.47, −0.85, and −1.50 MPa in the 0, 250, 500, and 1000 mg kg⁻¹ nitrogen treatments respectively. Highest microbial respiration occurred in the lowest nitrogen treatment suggesting an inhibitory osmotic effect from higher rates of nitrogen application. Microbial respiration rates of 185, 169, 131, and 116 mg O₂ kg⁻¹ soil day⁻¹ were observed in the 250, 500, control and 1000 mg kg⁻¹ nitrogen treatments, respectively. Gross nitrification was inversely related to water potential with rates of 0.2, 0.04, and 0.004 mg N kg⁻¹ soil day⁻¹ in the 250, 500, and 1000 mg kg⁻¹ nitrogen treatments, respectively. Reduction in water potential did not inhibit gross nitrogen immobilization or mineralization, with respective immobilization rates of 2.2, 1.8, and 1.8 mg N kg⁻¹ soil day⁻¹, and mineralization rates of 0.5, 0.3, and 0.3 mg N kg⁻¹ soil day⁻¹ in the 1000, 500, and 250 mg kg⁻¹ nitrogen treatments, respectively. Based on nitrogen transformation rates, the duration of fertilizer contribution to the inorganic nitrogen pool was estimated at 0.9, 1.9, and 3.2 years in the 250, 500, and 1000 mg kg⁻¹ nitrogen treatments, respectively. The estimation was conservative as ammonium fixation, gross nitrogen immobilization, and nitrification were considered losses of fertilizer with only gross mineralization of organic nitrogen contributing to the most active portion of the nitrogen pool.     

Bi-directional transfer of nitrogen between alfalfa and bromegrass: Short and long term evidence

Transfer of N from legumes to associated non-legumes has been demonstrated under a wide range of conditions. Because legumes are able to derive their N requirements from N₂ fixation, legumes can serve, through the transfer of N, as a source of N for accompanying non-legumes. Studies, therefore, are often limited to the transfer of N from the legume to the non-legume. However, legumes preferentially rely on available soil N as their source of N. To determine whether N can be transferred from a non-legume to a legume, two greenhouse experiments were conducted. In the short-term N-transfer experiment, a portion of the foliage of meadow bromegrass (Bromus riparius Rhem.) or alfalfa (Medicago sativa L.) was immersed in a highly labelled ¹⁵N-solution and following a 64 h incubation, the roots and leaves of the associated alfalfa and bromegrass were analyzed for ¹⁵N. In the long-term N transfer experiment, alfalfa and bromegrass were grown in an ¹⁵N-labelled nutrient solution and transplanted in pots with unlabelled bromegrass and alfalfa plants. Plants were harvested at 50 and 79 d after transplanting and analyzed for ¹⁵N content. Whether alfalfa or bromegrass were the donor plants in the short-term experiment, roots and leaves of all neighbouring alfalfa and bromegrass plants were enriched with ¹⁵N. Similarly, when alfalfa or bromegrass was labelled in the long-term experiment, the roots and shoots of neighbouring alfalfa and bromegrass plants became enriched with ¹⁵N. These two studies conclusively show that within a short period of time, N is transferred from both the N₂-fixing legume to the associated non-legume and also from the non-legume to the N₂-fixing legume. The occurrence of a bi-directional N transfer between N₂-fixing and non-N₂-fixing plants should be taken into consideration when the intensity of N cycling and the directional flow of N in pastures and natural ecosystems are investigated.     

Crop performance, nitrogen and water use in flooded and aerobic rice

Irrigated aerobic rice is a new system being developed for lowland areas with water shortage and for favorable upland areas with access to supplementary irrigation. It entails the cultivation of nutrient-responsive cultivars in nonsaturated soil with sufficient external inputs to reach yields of 70–80% of high-input flooded rice. To obtain insights into crop performance, water use, and N use of aerobic rice, a field experiment was conducted in the dry seasons of 2002 and 2003 in the Philippines. Cultivar Apo was grown under flooded and aerobic conditions at 0 and at 150 kg fertilizer N ha^–1. The aerobic fields were flush irrigated when the soil water potential at 15-cm depth reached –30 kPa. A ¹⁵N isotope study was carried out in microplots within the 150-N plots to determine the fate of applied N. The yield under aerobic conditions with 150 kg N ha^–1 was 6.3 t ha^–1 in 2002 and 4.2 t ha^–1 in 2003, and the irrigation water input was 778 mm in 2002 and 826 mm in 2003. Compared with flooded conditions, the yield was 15 and 39% lower, and the irrigation water use 36 and 41% lower in aerobic plots in 2002 and 2003, respectively. N content at 150 kg N ha^–1 in leaves and total plant was nearly the same for aerobic and flooded conditions, indicating that crop growth under aerobic conditions was limited by water deficit and not by N deficit. Under aerobic conditions, average fertilizer N recovery was 22% in both the main field and the microplot, whereas under flooded conditions, it was 49% in the main field and 36% in the microplot. Under both flooded and aerobic conditions, the fraction of ¹⁵N that was determined in the soil after the growing season was 23%. Since nitrate contents in leachate water were negligible, we hypothesized that the N unaccounted for were gaseous losses. The N unaccounted for was higher under aerobic conditions than under flooded conditions. For aerobic rice, trials are suggested for optimizing dose and timing of N fertilizer. Also further improvements in water regime should be made to reduce crop water stress.     

植被恢复过程中芒萁覆盖对侵蚀红壤氮组分的影响

氮素是限制陆地生态系统生产力的重要因子。采用时空代换法,以红壤侵蚀区未治理、恢复12年和30年的马尾松林为研究对象,对比分析了林下芒萁覆盖地与裸地表层土壤之间氮同位素、不同形态氮组分含量以及不同组分氮含量所占比例之间的差异。结果表明:在所有马尾松林中,芒萁覆盖增加了表层土壤的全氮含量,δ~(15)N值则比林下裸地显著降低了33. 8%—83.1%(P0.05)。随着恢复年限增加,林下芒萁覆盖地表层土壤δ~(15)N值显著下降,而林下裸露地δ~(15)N值没有显著变化(P0.05)。不同恢复年限马尾松林的芒萁覆盖地表层土壤微生物生物量氮、可溶性有机氮和铵态氮含量显著高于林下裸地(P 0.05),而硝态氮含量则显著低于林下裸地(P0.05)。随恢复年限增加,表层土壤微生物生物量氮、可溶性有机氮、铵态氮含量均呈增加趋势,而硝态氮含量则呈下降趋势,不同形态氮占全氮比例表现为:微生物生物量氮铵态氮可溶性有机氮硝态氮。相关分析表明土壤δ~(15)N值与硝态氮极显著正相关,与其他氮组分极显著负相关(P0.01)。由此可见,与林下裸地相比,芒萁覆盖在植被恢复过程中有助于提高表层土壤中全氮、微生物生物量氮、可溶性有机氮和铵态氮含量,降低硝态氮的淋溶损失风险,促进土壤氮保持和积累,从而有利于退化红壤生态系统的恢复。     

Variability in nitrogen fixation of common bean (Phaseolus vulgaris L.) intercropped with maize

Thirty one selected bean lines were evaluated in the field for ability to support N₂ fixation when intercropped with maize which received 0, 30 and 60 kg N ha^–1 as ammonium sulphate. The amount of fixed N₂ was estimated using the natural variation of ¹⁵N and wheat as the standard non-fixing crop. Nitrogen as low as 15 kg N ha^–1 at sowing suppressed nodule weight and activity (acetylene reduction activity) but not nodule number, suggesting that the main effect of mineral N was on nodule development and function. ¹⁵N data revealed a high potential of the bean genotypes to fix N₂, with the most promising ones averaging between 50–60% of seed N coming from fixation. Bean lines CNF-480, Puebla-152, Mexico-309, Negro Argel, CNF-178, Venezuela-350 and WBR22-3, WBR22-50 and WBR22-55 were ranked as good fixers.     

The effect of root temperature on the induction of nitrate reductase activities and nitrogen uptake rates in arctic plant species

We investigated whether six arctic plant species have the potential to induce nitrate reductase (NR) activity when exposed to NO₃ ^--nitrogen under controlled environment conditions, using an in vivo assay that uses the rate of NO₂ ^--accumulation to estimate potential NR activity. We also assessed the effect of low root temperatures on NR activity, growth and nitrogen uptake (using ¹⁵N applications) in two of the selected species. Five of the six species (Cerastium alpinum, Dryas intergrifolia, Oxyria digyna, Saxifraga cernua and Salix arctica) were capable of inducing NR activity when exposed to solutions containing 0.5 mM NO₃ ^- at 20°C for 10 days. Although in vivo NR activity was not induced in Saxifraga oppositifolia under controlled conditions, we conclude that it was capable of growing successfully on NO₃ ^-, due to the presence of moderate rates of NR activity observed in both NH₄ ⁺-grown and NO₃ ^--treated plants. Exposure of O. digyna and D. integrifolia to 3°C root temperatures for two weeks, with the shoots kept at 20°C, resulted in root and leaf NR activity rates of NO₃ ^--treated plants being reduced to rates exhibited by NH₄ ⁺-grown plants. Although these decreases in NR in both species appeared to be due to limitations in NO₃ ^--uptake and growth rate (rather than direct low-temperature inhibition of NR synthesis per se), direct low-temperature inhibition of root NR synthesis could not be ruled out. In contrast to the temperature insensitivity of NH₄ ⁺ uptake in D. integrifolia, NO₃ ^--uptake in D. integrifolia was inhibited by low root temperatures. We conclude that the selected arctic species have the genetic potential to utilize NO₃ ^--nitrogen, and that low root temperatures, in conjunction with other environmental limitations, may be responsible for the lack of induction of NR in D. integrifolia and Salix arctica under field conditions.     

Evaluation of15N methods to measure nitrogen transfer from alfalfa to companion timothy

Four different methods: direct¹⁵N₂ exposure, legume leaf labeled with¹⁵N,¹⁵N dilution and total N balance were applied to assess the nitrogen transfer (NT) from alfalfa to companion timothy. Evidence of NT was obtained in all cases, which represents about 3% of total N fixed by alfalfa or 10% of N content in timothy at the first cycle of growth. All the three¹⁵N methods gave identical results, while the conventional calculation of NT from the difference of N content in timothy from mixture and monoculture resulted in an over-estimation. The advantages and disadvantages of each method as applied to field conditions are discussed.Contribution No 1158 from the Plant Research Centre.     

Effects of nitrogen supply rate on photosynthesis,nitrogen uptake and growth of seedlings in a Eucalyptus/Dalbergia odorifera intercropping system

• The introduction of N₂-fixing species into a Eucalyptus plantation resulted in a successful planting system. It is essential to understand the contribution of nitrogen (N) competition and photosynthetic efficiency to plant dry matter yield to shed more light on the growth mechanism of the Eucalyptus/legume system.
• We compared N competition, photosynthesis and dry matter yield of Eucalyptus urophylla × E. grandis and the N₂-fixing tree species Dalbergia odorifera in intercropping and monoculture systems under different N levels.
• The photosynthesis of E. urophylla × E. grandis was improved, while that of D. odorifera was inhibited in the intercropping system. Intercropped E. urophylla × E. grandis increased the N utilization and the dry matter yield by 6.57–48.46% and 7.59–97.26%, and decreased those of D. odorifera by 10.21–30.33% and 0.48–13.19%, respectively. Furthermore, N application enhanced the competitive ability of E. urophylla × E. grandis relative to D. odorifera and changed the N contents and chlorophyll synthesis to optimize the photosynthetic structure of both species.
• Our results reveal Eucalyptus for photosynthesis, N absorption and increasing the growth benefit from the introduction of N₂-fixing species, which hence can be considered to be an effective sustainable management option of Eucalyptus plantations.

     

Residual nitrogen contribution from grain legumes to succeeding wheat and rape and related microbial process

The residual N contribution from faba bean (Vicia faba L.), pea (Pisum sativum L.) and white lupin (Lupinus albus L.) to microbial biomass and subsequent wheat (Triticum aestivum L.) and oilseed rape (Brassica napus L.) was studied in a greenhouse experiment. The grain legumes were ¹⁵N labelled in situ with a stem feeding method before incorporated into the soil, which enables the determination of N rhizodeposition. Wheat and rape were subsequently grown on the soil containing the grain legume residues (incl. ¹⁵N-labelled rhizodeposits) and were harvested either twice at flowering and at maturity or once at maturity, respectively. The average total N uptake of the subsequent crops was influenced by the legume used as precrop and was determined by the residue N input and the N₂-fixation capacity of the legume species. The succeeding crops recovered 8.6–12.1% of the residue N at maturity. Similar patterns were found for the microbial biomass, which recovered 8.2–10.6% of the residue N. Wheat and rape recovered about the same amount of residue N. The absolute contribution of soil derived N to the subsequent crops was similar in all treatments and averaged 149 mg N pot^–1 at maturity. At flowering 17–23% of the residue derived N was recovered in the subsequent wheat and in the microbial biomass; 70% of the residue N was recovered in the microbial biomass in the flowering stage and decreased to about 50% at maturity. In contrast, the recovery in wheat and rape constituted only 30% at flowering and increased to 50% at maturity in all treatments, indicating that the residual N uptake by the subsequent wheat was apparently supplied by mobilisation of residue N temporarily immobilised in the microbial biomass.     

干旱胁迫下胡杨实生幼苗氮素吸收分配与利用

胡杨(Populus euphratica)是塔里木河流域荒漠河岸林的建群种,水分和氮素是限制胡杨幼苗的存活及早期生长的主要因子。利用~(15)N同位素示踪技术分析水和氮素的交互作用对胡杨幼苗不同生长阶段氮素的吸收分配利用及幼苗生长的影响,进一步探究氮素对胡杨实生苗早期形态建成的作用及对干旱胁迫的缓解效应,以期提高幼苗的存活率。实验以一年生胡杨实生幼苗为研究对象,采用温室内盆栽实验,设置4个干旱处理(D_1、D_2、D_3、D_4,土壤相对含水量为:20%—25%、40%—45%、60%—65%、80%—85%)和3种氮素水平(N_0、N_1、N_2:0、3、6 g/盆)测定胡杨幼苗的生长指标和各部分的Ndff、分配率及利用率。结果表明:胡杨幼苗在土壤相对含水量60%—65%(D_3)、氮素添加量3 g/盆(N_1)时,其生长表现为最佳状态;干旱胁迫下,不同氮素添加量对胡杨幼苗各部分的Ndff值存在显著差异,N_2低于N_1;随干旱胁迫减弱(D_3、D_4),植株在生长早期(25 d)根部吸收的~(15)N优先向地上部分转运,生长后期(75 d)植株Ndff最高,其中以根系中Ndff最高;不同生长期幼苗各部分的~(15)N分配存在显著差异,根系~(15)N分配率较高,但不同氮量处理间差异不显著;随生长期的推移,植株对~(15)NH_4~(15)NO_3的利用率表现为粗根最大,各处理中D_3N_1处理均显著高于其他处理。结论:轻度干旱胁迫下添加适量的氮素能够增强植株对氮素的吸收征调能力,优化水资源获取以维持生存的重要机制。