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1、豆类作物施用氮肥在生物学与经济学上的意义Paul W. Singleton* Nantakorn Boonkerd* James R. Hollyer*夏威夷大学热带农业和人类资源学院 Nif TAL 项目* Suranaree 技术大学 农业生物技术系* 夏威夷大学热带农业和人类资源学院农业和资源经济系*1. 引言?Introduction豆类作物同化氮素有两种形式，它不仅可以同化来自土壤及肥料的无机氮，也可以通过生物固氮从大气中捕获氮气。生物固氮过程是利用植物根瘤中产生的细菌酶把N2还原成有效形态。根瘤是共生细菌（根瘤菌属）侵入根毛后形成的，豆类作物光合作用为共生固氮共生体提供能量。Leg
2、umes have two modes of nitrogen assimilation. Although legumes assimilate mineral N from soil and fertilizer sources, they also capture N from the atmosphere through biological dinitrogen fixation (BNF). The BNF process reduces N2 to plant available forms using bacterial enzymes produced within root
3、 nodules. The root nodules are formed after the symbiotic bacteria Rhizobium invades root hairs. Legume photosynthesis provides the energy that drives the BNF symbiosis.豆类作物依靠生物固氮作为氮源的程度取决于以下因素的相互作用：The extent to which legumes rely on BNF as an N source is determined by an interaction between:1) 达到作
4、物产量潜力所需的氮素总量；1) total amount of N required to meet crop yield potential;2) 土壤中有效态无机氮的数量；2) amount of mineral N available in the soil, and;3) 形成豆类作物根系根瘤的根瘤菌的效果。 3) effectiveness of rhizobia forming nodules on legume roots大多数豆类作物管理策略目的是最大限度地促进生物固氮。在作物生长期间，生物固氮过程直接受土壤矿质氮供应比例的抑制，要加大生物固氮量，豆类作物必须利用少量矿质氮以在
5、生物固氮全面发挥前维持幼苗生长。Most legume management strategies aim to maximize BNF. During most of crop duration, the legume BNF process is inhibited in direct proportion to soil supply of mineral N. To maximize BNF, however, legumes must assimilate a small amount of mineral N to sustain early crop growth prior
6、to full expression of BNF.豆类作物的氮素管理有两种常用方法。一种是接种优良的根瘤菌使其生物固氮量足以满足作物的大部分需要。接种技术和接种剂产品很多，为农民提供了最大限度促进作物生物固氮的廉价选择。另一种选择是施氮肥。许多农民在种植豆类作物时施用少量氮肥（1050公斤N /公顷），而另一些农民并不依靠生物固氮，仍然施用大量氮肥。There are two common practices for managing legume N nutrition. Inoculating legumes with superior rhizobia ensures that BNF
7、 can meet most of the crops N requirement. Inoculation techniques and inoculant products are well developed and provide farmers an inexpensive option to attain maximum BNF by their crops. Another option is t apply fertilizer N. Small amounts of N (10-50 kg N ha-1)are applied by many farmers when pla
8、nting legumes (starter N). Others apply larger quantities of N fertilizer and do not rely on BNF to any significant extent. 对豆类作物推荐施种肥是因为我们知道在出现生物固氮之前豆科植株对矿质氮有生理依赖。然而，在对农民进行施氮肥推荐时，通常不能考虑土壤有效氮对作物早期生长对矿质氮的实际需要。在大多数情况下，上季作物残留的氮加上由有机物分解的矿质氮就足够豆类作物早期生长的需要。Recommendations for starter N originate from our
9、knowledge of the legumes physiological dependence on mineral N prior to onset of BNF. Starter N recommendations to farmers, however, usually fail to account for soil available N relative to the crops actual requirement for mineral N during early growth. Most often, residual N from previous crops plu
10、s mineralized N from organic matter are sufficient to meet the early N requirements of legumes.在各种土壤类型和气候条件的大面积上遵循施氮推荐在生物学和经济学上的潜在费用是巨大的。例如，1994年估计有538,165吨氮肥施在了中国17个省的1050万公顷的大豆及花生上（钾磷研究所，Sam Portch私人通信），仅这一氮管理措施的直接投入费用就高达26.32亿元人民币。生物固氮潜力捕获的氮减少了215000吨以上?-假设氮肥利用率为40%，如果对豆类作物生物固氮管理得当的话，只以小部分氮肥成本就可获
11、得这些氮。The potential biological and economic costs of following recommendations for starter N across large areas with variable soil types and climates are enormous. For example, in 1994 it was estimated 538,165 metric tonnes (mt) of N were applied to 10.5 million ha of soybean and groundnut crops acro
12、ss 17 provinces of China (Potash & amp; amp; Phosphate Institute, Sam Portch personal communication). Direct input cost alone of this N-management option could be as high as 2,632 million RMB ($317 million U.S.). Reduction in potential capture of N from the BNF process could exceed 215,000 mt - assu
13、ming an FUE for N of 40%. This N could be obtained at a small fraction of the cost of N fertilizer by properly managing legume BNF.氮的生物学反应与其成本之比和将这些财力资源用于其他管理选择的预期经济收益决定着施氮肥是否作为一种推荐管理措施。本文在生物学及生态学方面评述豆类作物的氮肥管理，并进一步评估了在中国对豆类作物的氮肥管理选择的经济费用和收益，包括施用氮肥和非氮肥投入的机会成本。Whether N fertilizer should be a recommen
14、ded management practice is determined by biological response to N relative to its costs and the foregone economic benefit from using these financial resources for other management options. This paper reviews the biological and ecological aspects of N-management of legumes. Further, it evaluates the
15、economic costs and benefits of N-management options for legumes in China including opportunity costs of N fertilizer use in relation to non-N fertilizer inputs.2. 豆类作物对氮的需要和吸收?Nitrogen Requirements and Uptake by Legumes2.1 豆类作物在农业上的重要性 Importance of legumes in agriculture籽用豆类是世界农业中第二重要的作物。通过直接食用和转化为
16、动物产品，它们是重要的食物蛋白的来源。在许多发展中国家豆类蛋白占蛋白质消耗的大部分；在一些发达国家及那些经济快速增长较不发达国家，对豆类蛋白需求的增长速度超过了对谷物需要的增长。收入的增加加速对动物蛋白与豆类作物这种动物原料的需求。在很多情况下，豆类作物生产的增加反映了畜牧生产的变化。例如，在19801992年间，中国畜牧业迅速增长，在同一时期，大豆及花生生产也提高了，豆类作物生产的增加几乎完全是来自管理措施的改进。改进作物管理与产量潜力的一个标志是磷肥与钾肥的施用量的急剧增加。本文将说明管理的改进与豆类作物高产是农民从生物固氮中获得最大利益的条件。Grain legumes are the
17、second most important crop in world agriculture. They are an important source of dietary protein through direct consumption and conversion to animal products. In many developing countries legume protein accounts for a major portion of protein consumption. In developed countries and in those lesser d
18、eveloped countries that are experiencing rapid economic growth, demand for legume protein is accelerating faster than demand for cereals. Rising incomes accelerate demand for animal protein and legume crops, the raw materials for animal industries. In many cases increased legume production mirrors c
19、hanges in animal production. For example, animal production in China increased rapidly between 1980 and 1992 In the same period soybean and groundnut production also rose. Increased legume production was almost entirely due to higher yields from improved management. One indicator of improved crop ma
20、nagement and yield potential is the dramatic increase in P and K fertilizer consumption in China. This paper will show improved management and higher yields of legume crops are precisely the conditions where farmers can gain the largest benefit from legume BNF.2.2 豆科作物吸收的总氮量 Total nitrogen assimilat
21、ion by legumes大多数豆类作物的产量及总生物量低于谷类作物，但豆类种子及叶片中的蛋白质含量却高几倍。蛋白质本质上就是带有氨基的碳链。豆类作物组织高浓度的蛋白质就意味着对氮素同化的高度需求。While total biomass and yield of most legume crops is lower than cereals, legume seed and leaf protein concentrations can be several times higher. Since proteins are essentially carbon chains with att
22、ached amino groups, the high concentration of protein in legume tissues translates into a high demand for nitrogen assimilation.豆类作物的产量与大多数粮食作物一样是同生长过程中氮的同化量密切相关的（Gassman etal, 1993, Herridge et al, 1984），即使要达到中等产量，豆类作物同化的氮量比谷类要多。豆类作物对氮素的高需要与其可同化来源于大气及无机氮的事实，使研究的焦点趋于提高氮的同化量以增加豆科作物产量。Yield of legumes
23、, as with most food crops, is linked to the amount of N assimilated during crop growth (Cassman et al., 1993, Herridge et al., 1984). To meet even moderate yield potentials, legumes must assimilate more N than cereals. Both the high N requirements of legumes and the fact that legumes can assimilate
24、N from both atmospheric and mineral sources has tended to focus research toward increasing N assimilation as a way to raise legume yields.表1给出了几种豆类与谷类作物同化氮的全球估计值。全球籽用豆类的产量不足谷类的10%，但豆类同化氮量占谷类的37%。别看大豆平均产量只有1919公斤/公顷，而谷有3000公斤/公顷，但单位面积豆类积累氮的总量却是谷类的2倍。Estimates of the global amount of N assimilated by
25、several grain legume and cereal species are given (Table 1) Global grain legume production is less than 10% of cereal production, yet grain legume N assimilation is 37% of the total N accumulated by cereal crops. Despite world average soybean yields of only 1919 kg ha-1 compared to more than 3000 kg
26、 ha-1 for most cereal crops, soybean crops accumulate more than twice the N per unit area as cereals.(表：表1 全球主要谷类、豆类的总产量、单产和氮产量 ) 总产量 Production 总氮量* Total N* 单产(kg/hm2) Yield (kg ha-1) 氮产量(kg/hm2) N yield (kg N ha-1) (百万吨) tonnes (millions) 豆类 Legumes 大豆 Soybean 108 8.444 1913 149 花生 Groundnut 23 1
27、.15 1156 58 豆类* Pulses* 41 2.05 809 40 合计 Total 172 11.64 谷物 Cereals 玉米 Maize 475 8.55 3682 66 水稻 Rice 518 7.77 3557 53 小麦 Wheat 595 14.28 3510 60 谷子 Miller 30 0.60 794 16 合计 Total 1618 31.20 注释：* 氮含量计算(gN/kg种子)：大豆是62.5；花生与豆类40.0；玉米14.4；水稻12.0，小麦19.2，小米16.0，基于所有作物收获指标的N值为0.8，FAO资料数据计算(1992)。*人们直接食用的
28、一般豆类作物：小扁豆、鹰嘴豆、菜豆、豌豆。 *N calculations based on N contents(g N kg-1 seed): soybean 62,5; groundnut and pulses 40.0, maize 14.4; rice12.0; wheat 19.2; and millet 16.0; N values based on N harvest index of all crops of 0.08, Figures based on data from FAO(1992), * Legumes generally used directil for hu
29、man consumption: lentils; chickpea; common bean, broad bean, and peas. 农业中豆类作物的生物固氮在全球农业的氮素平衡中扮演重要角色。豆类作物每年向农业系统中通过生物固氮提供3500万吨以上的氮素(Burns and Hardy, 1975)，这对年8000万吨的工业固氮量是一个不小的数字(FAO Year Book, Fertilizers, 1992)。当考虑工业氮肥相对较低的利用效率时，可能作物实际对工业与生物固氮氮源的利用量差不多是相等的。BNF by agricultural legumes plays a majo
30、r role in the nitrogen economy of global agriculture. Agricultural legumes contribute more than 35 million metric tonnes (mmt) of nitrogen derived from BNF to agricultural systems (Burns and Hardy, 1975). This contribution is a significant portion of the 80 mmt of N captured through industrial proce
31、sses (FAO Yearbook, Fertilizers, 1992). When the relatively low use efficiency of manufactured N fertilizer is considered, actual assimilation of N by crops from industrial and legume BNF sources may be nearly equal.3. 豆类作物的氮素管理?Nitrogen Management of Legume Crops对任何作物而言，建立作物氮素管理策略因氮素吸收随以下因素变化而复杂化：
32、Developing nitrogen management strategies for any crop is complex since N assimilation varies with:1）土壤氮的有效性；1) availability of soil N;2）作物生长发育；2) crop development;3）产量潜力；3) yield potential, and; 4）作物和可同化氮的生理效率。4) physiological efficiency with which crops can assimilate N.所有因素都受到植物基因型、管理及环境的影响。在氮肥的管
33、理上，豆类比谷类要复杂，它可利用两种氮源以满足氮素的需要。在科学家与农民之间一直存在着豆类作物生产系统中有关氮素管理的最佳方式这种有意义的争论。就任何农作管理策略而言，豆类作物的氮素管理受到风险调节，农民从各种管理中选择可以获得经济回报的那种。All these factors are influenced by plant genotype, management and the environment. Nitrogen management of legumes can be more complicated than for cereals since two alternative
34、sources of N can be tapped to meet N uptake requirements. There is significant controversy among scientists and farmers about the optimum way to manage nitrogen in legume production systems. As with any agriculture management strategy, legume nitrogen management should be driven by the risk adjusted
35、 financial return farmers can obtain from the various management options.4. 生物固氮-引物?Biological Dinitrogen Fixation (BNF) - a Primer4.1 生物固氮的过程 The BNF process通过与土壤根瘤菌的共生，豆类作物把大气中的氮气还原为氨供植物形成蛋白质。首先，根瘤菌属或慢生根瘤菌属的细菌侵染豆科作物根毛并在根毛内增殖形成根瘤。根瘤菌只有在特定的寄主上才可能形成根瘤并进行生物固氮。波顿（1984）根据对根瘤菌的专一性把豆类作物可分为45组，例如，大豆被大豆慢生根瘤
36、菌细菌侵染，而豌豆被豌豆根瘤菌侵染结瘤，三叶草被三叶草根瘤菌侵染。能使相同寄主结瘤的根瘤菌属于同一互接种族。Through symbiosis with the soil bacteria rhizobia, legumes reduce atmospheric N2 to ammonia for production of plant proteins. First, bacteria of the genera Rhizobium or Bradyrhizobium infect root hairs of legumes and induce formation of root nodu
37、les where the bacteria proliferate. Rhizobia have specific host requirements for nodulation and BNF Burton, (1984) listed 45 different legume groups based upon their specificity for rhizobia. For example, soybean is infected by Bradyrhizobium japonicum while peas nodulate with Rhizobium leguminosaru
38、m and clover with Rhizobium trifolii. Rhizobia that nodulate the same hosts belong to a cross inoculation group.根瘤菌先接触被侵染的作物根的表面，互相交换生化信号后，根瘤菌通过侵入线侵入根毛。根瘤菌沿着侵入线增殖，最终侵入皮层细胞的细胞质。在此，寄主在类菌体膜中把根瘤菌包裹起来，并形成与根的维管束连接。 Infection of legume roots begins with attachment of rhizobia to the root surface Biochemica
39、l signals are exchanged between the partners and then rhizobia invade the root hairs through an infection thread. Rhizobia proliferate along the infection thread and eventually invade the cytoplasm of root cortical cells. There, the host envelopes the rhizobia in a peribacteriod membrane and connect
40、ions to the root vascular system are formed.在根瘤中，根瘤菌在形态改变的细胞中大量增殖紧密排列，这些，细胞组织被称作类菌体。类菌体合成固氮酶，一种钼铁蛋白，这种酶能催化氮气还原为氨。植株通过维管束连接系统把光合产物输入根系并将其氧化，为根瘤菌还原氮气提供能量(ATP)。植株在根瘤中合成豆血红蛋白来固持氧气，氧气要维持在低水平时，固氮酶才能正常发挥作用。固氮酶通过加入6个质子和6个电子使1个氮气分子还原为2个NH3分子，然后氨被合成为谷酰胺；谷酰胺可被转化为多种化合物供在植株内运移并合成蛋白质。Within the nodule, rhizobia p
41、roliferate in very dense clusters of cells with altered morphology. These cell forms are called bacteroids. Bacteroids synthesize nitrogenase, a Mo-Fe protein, which is the enzyme that catalyzes reduction of N2 to ammonia. The plant supplies the energy (ATP) for N2 reduction by rhizobia from oxidati
42、on of photosynthate imported through the vascular connections to the root. The plant synthesizes leghaemoglobin in the no.dule to bind oxygen which must be maintained at low tensions for proper function of nitrogenase. Nitrogenase reduces one molecule of N2 to two molecules of NH3 by addition of six
43、 protons and six electrons. Ammonia is then used to synthesize glutamine which can be converted to various compounds for transport within the plant for production of proteins.生物固氮的生化耗能估计为1741.7千焦耳/mol N2，比哈伯-勃赤（Haber-Bosch）法的1455.4千焦耳/mol N2稍高一些(Postgte and Hill, 1979)。生物固氮的这些消耗不包括根瘤的形成与维持的消耗，固定每公斤氮
44、可能需要植株提供近10公斤碳水化合物。The biochemical costs of BNF are estimated at 416 kcal/mole N2, somewhat higher than the Haber-Bosch process at 350 kcal/mole N2 (Postgate and Hill, 1979). These estimated costs for BNF do not include the costs of nodule formation and maintenance. The entire process may require as
45、 much as 10 kg carbohydrates on a whole plant basis, per kg N2 fixed.4.2 根瘤菌 Rhizobia土壤中根瘤菌群体由许多菌株组成，在一些土壤中有许多种。相同互接种族内的土壤根瘤菌株通常是异源的，与寄主引发共生反应的范围从完全无效到高度有效 (Singleton and Tavares 1986)。这些差异是选择优良共生体用于接种豆类作物的生物学基础。Rhizobia populations in soils are composed of many strains and, in some soils, many spec
46、ies. Strains of rhizobia in soil within the same cross inoculation group are usually heterogeneous, eliciting symbiotic responses with the host ranging from completely ineffective to a highly effective (Singleton and Tavares 1986). This variation in rhizobial effectiveness is the biological basis fo
47、r selecting superior symbionts commonly used to inoculate legumes.4.3 豆科作物固定多少氮气 How much N2 do legumes fix 这个常提的问题有着令人惊奇简单的回答：提供了高效的根瘤菌后，豆科作物固定的氮等于作物达到产量潜力的总需氮量与从土壤中吸收的矿质氮之差。生物学和环境因素当然剧烈影响豆类作物的实际固氮量，文献中有一年生豆类作物固氮的总结(Peoples and Craswell 1992；George et al, 1988)（表2）。在该表中，固氮量（N）从3到237kg/hm2，占豆科作物总氮量的
48、16%到几乎100%。非常低的生物固氮值往往与低的总氮同化相联系（见印度尼西亚和巴西的数据）。其它地区（肯尼亚）报道了很低的固氮量但高的氮同化量。肯尼亚的土壤一定含有植物可吸收的矿质氮。This commonly asked question has a surprisingly simple answer: Provided there are highly effective rhizobia available, legumes will fix N2 equal to the difference between the total N required to meet plant yield potential and the mineral N provided from soil and fertilizers. Biologica