Contribution of Actinorhizal Plants to Tropical Soil Productivity And

CONTRIBUTION OF ACTINORHIZAL PLANTS TO TROPICAL SOIL PRODUCTIVITY AND REHABILITATION kkkwl .Y.JY" DOMMERGUES* la BFST (ORSTOM-CIRAD-Forêt), 45 bis Avenue de Belle Gabrielle, 94736 , Nogent-sur-Marne. France

1996 (Accepted 5 Jiily i

Summary-The contribution of actinorhizal plants to soil productivity and rehabilitation depends not only on properties encountered in a number of non-NI-fising trees but also on the input of fixed N2 that is subsequently transferred to soil and ultimately to associated crops. The nitrogen-fixing potential of a number of actinorhizal plants (e.g. Casnarina sp. and .-Ums sp.) is high but the amount of N2 actually fixed in the field is often low because the expression of (his potential is limited by unfavorable environmental conditions or improper management practices. Assessing the amount of fixed N2 transferred to soil is difficult mainly because of the recycling of fixed N2 except in open ecosystems. Many examples of successful introductions of actinorhizal plants into various systems of land management are given. To increase the input of fixed N2 into ecosystems two strategies can be adopted: the first one is to use proper management practices; the second one is to improve the performances of the N2-fixing system. Practically, in addition to optimizing nctinorhizal fixation, it is recommended to develop the introduction of actinorhizal plants as soil iniproyers in a number of countries where they are not yet used, to domesticate hitherto negfected or overlooked actinorhizal plants, and to exploit their ability to contribute to the rehabilitation of wasted lonas and possibly to the phytoremediation of polluted sites. 0 1997 Elsevier Science Ltd

INTRODUCTlON ities in the near future. The most important actinor- Plants symbiotically associated with the N2-fixing hizal plants belong to the Aliiztu genus and to the actinomycete Fraiikia, collectively called actinorhi- Casuarinaceae family. In the humid tropics they are zal plants, belong to 24 genera distributed among Casuarina ciuininghan?iana, C. equisetzyolia, C. jun- gJiuhiana, C. ofigodon, Gyinnosronia sumatram; in eight plant families. Therefore, until recently, the semi-arid and arid regions Casuarina cristata, C. impression was one of taxonomic unrelatedness decaisneana, C. glauca (irrigated), C. cunninghami- (Bond, but recent molecular analyses suggest 1983), ana (irrigated), C. obesa (irrigated); in tropical high- that actinorhizal plants are more closely related lands Alnus jorullensislaciiiiiinata, A. nepalensis, A. than usual morphologically based classification sys- glttrimsa. Casuarina cimiYngJiamiaiia, C. equisetifo- tems indicate (Swensen and Mullin, 1995). It must lia, C. jiinghuhniana and in mediterranean regions be noted that many closely related taxa do not bear Alms glutinosa, A. subcordata, Allocasuurina verti- actinorhizal nodules and there is no clear infor- cillata, Casuarina cuiininghamiana, C. glauca. In ad- mation on the molecular nature of barriers or com- dition, a few tropical and temperate species mon factors in actinorhizal symbiosis (Berry, 1994). belonging to the following genera will be considered Native actinorhizal plants are not evenly distribu- here: Coriaria, Elueagnus, Hippophuë and Myrica. ted worldwide (Table 1). Whereas many species of actinorhizal plants are found in Australia, Asia, Europe, South and North America, Africa is par- ticularly lacking in native actinorhizal plants, with &MOUNT OF Nz FEED the possible exception of several species of Myrica The contribution of actinorhizal plants to soil (Baker and Mullin, 1992). productivity depends not only on properties found This paper is an attempt to evaluate the current in a number of non-N2-fixing trees (e.g. redistribu- contribution of the best known actinorhizal plants tion of nutrients through the soil profile, protection to soil productivity and rehabilitation of wasted from erosion, improvement of soil physical con- lands especially in tropical and mediterranean cli- ditions, shading and modification of the microcli- mates and to suggest the strategies that should be mate, weed suppressing effects) (Prinsley and Swift, developed to increase significantly their potential- 1986), but also on the input of fixed N2 that is subsequently transferred to the soil and possibly to as- 4 88 05 47. *Fax: 33 93 sociated crops or non-fixing trees. Therefore it is

1 93 1 n ORSTOM I 932 Y. R. Doinmergues 2000 Table 1. Distribution of representative genera of actinorhizal y-' in a plantation of trees lia-' plants (after Baker and Mullin, 1992) (Dommergues, 1995). This is indeed a high Nz-fix- Family Genus Native from following regions" ing potential for a 2-year-old actinorhizal plant. A Betulaceae Alnus NAm, SAm, Eur, NAS, SAS rough prediction of the Ns-fixing potential can be Casuarinaceae Allocasuarina Aus based on the assumption that the highest active Casuarina Aus nodule biomass observed reflects the N2-fixing po- Gymnostonin Aus Coriariaceae Coriaria Alls, NAm, S Am, Eur tential to a certain extent. Thus the following Datiscaceae Datisca NAm, SAS species could be considered as having a high N2-fix- Elaeagnaceae Elaeagnus NAS, NAm, Eur, SAS Hippophaë Eur, NAS ing potential: Altius glutinosa whose nodule biomass Shepherdia NAm was estimated to be up to 454 kg ha-' (Akkermans Myricaceae Myrica SAf, NAm, SAm, Aus, SAS, and Van Dijk, 1976) and nepalensis with NAS Abius Compronia NAm 307 kg nodules ha-' (Sharma and Ambasht, 1986). Rhamnaceae Adorphiab NAm With the exception of Casuarina equisetifolia and Ceanothus NAm CoNeria SAm some Altius spp that have been shown to have a Rosaceae Cercocarpus NAm high N2-fixing potential, data are still missing that Dryas NAm would allow one to classify the different tropical Purshia NAm actinorhizal plants according to their N2-fixing 'NAm = North America, SAm = South America, Eur = Europe, potential. Aus = Australia and/or Oceania, SAf = southern Africa, NAS = northern Asia, SAS = southern Asia. A certain number of evaluations of the amount bCruz-Cisneros and Valdes (1990). of Nn actually fixed in the field by tropical actinorhizal plants have been published during the last 10 essential to estimate the amount of N2 fixed in each years. The most reliable are presented in Table 2. It situation. appears that the actual N2 fixation of Casuarina It must be noted here that a distinction has to be equiserifolia varies considerably irrespective of the made between the N2-fixing potential of a given N2- mode of expression used: actual Ns, fixation fixing system and its actual N2 fixation. expressed on an area basis ranges from 15 to 94 kg By N2-fixing potential we designate the amount N2 fixed ha-' y-'; actual Nz fixation expressed per of N2 fixed in a constraint-free environment. This individual tree ranges from 6 to 47 g N2 fixed tree-' ideal value could theoretically be determined by y-'. Actual N2 fixation of Myrica faya was found growing the system under the most favorable con- to be IS kg ha-' y-', a figure lower than expected ditions. Such an evaluation has been attempted in since this small tree is known to be an aggressive the case of Casuarina equisetifolia by growing this colonizer. Generally the actual N2 fixation appears actinorhizal plant inoculated with an effective to be lower-and even much lower-than usually Frankia strain in a soil with a low content of avail- assumed. It is low not only in species with a poor able N, proper addition of P, K and trace elements, Na-fixing potential but also in species with a high careful irrigation and appropriate climatic con- N?-fixing potential whenever its expression is lim- ditions. The N2-fixing potential of 2-year old C. ited by unfavorable environmental conditions, such equisetifolia was shown to be ca. 42.4g N2 fixed as climate and soil constraints, improper manage- tree-' y-', which would be 84.8 kg N2 fixed ha-' ment techniques, absence of irrigation or fertiliza-

Table 2. Ndfa% (percentage of N derived from N2 fixation) and actual nitrogen fixation (kg NI fixed ha-' y-' and g N2 fixed tree-' y-') in Casuarina eqtrisetifolin and Myrica faya as assessed ín the field Species and Nz fixed N2 fixed country Age (years)b No. of trees ha-' Ndb% (kg ha-' y-') (g tree-ly-') Method and reference' Casuarina eqttisetifolia

Senegal MU 13 2000 58 29 Bal. Dommergues (1963) id. 6-38 I600 15 41 Bal. Mailly and Margolis (1992) Senegal NU 3 2500 39 6 NA Mariotti er al. (1992) 3 15 6 id. 2500 33 15 Dif. Mariotti er al. (1992) Puerto Rico (M) I O000 48-55 66-77 1-8 Enr. Parrotta et al. (1994) 2 so00 8-12 id. 2 (Cl 64-67 39-62 Enr. Parrotta er al. (1994) id. 2 (MI I O000 94 9 Dif. Parrotta et al. (1994) 62 12 id. 2 (Cl 5000 Dif. Parrotta er al. (1994) Myrica faya

Hawaii 18 Inc. Vitousek and Walker (1989)

"Senegal M: Malika, site close to the sea; Senegal N Notto, site distant from the sea. b(ìvl) monoculture of C. equisetifolia; (C) mixed stand or C. equiseiifofia and Eucalypriis robusta. 'Methods of assessment: ARA. acetylene reducing activity; Dif., total N difference; Bal., balance studies; Enr., enrichment with "N labelled fertilizer (also called isotope dilution method); NA. I5N natural abundance; Inc., increase of total N soil content. The results obtained through isotope methods either in Senegal or at Porto Rico are in close agreement with estimates made using the N difference method. Aclinorhizai plants and soil productivily 933

trees ha-' tion, diseases and enemies. Other factors can be Though carried out under a temperate climate high N,-fix- involved such as age: N2 fixation increases each (North America) the following experiment reported cal plant. A year in young plantations, then, after 10-20 y, by Friedrich and Dawson (1984) is worth mention- itial can be decreases with time for different reasons including ing. Comparing soil N concentration in plots of a :hest active the accumulation of available N in the soil. 14-year-old plantation of Jiiglaris izigrn mixed with ;-fixing po- different Nz-hing species (Abztw glufiriosa, % 4 following Elaeagnus iimbellata, Robitia pseudoacacia and high N2-fix- TRANSFER OF FIXED N2 TO THE SOIL AND COMPANION PLANTS Lespedeza striata), the authors found that inter- ule biomass plantings of Juglans with Robiizin and Elaeagizus Akkermans All cultivated soils are prone to losing nutrients, had the highest total N concentration in the top densis with especially N, either by erosion, removal in crops, 30 cm soil, followed by Alder, Lespedeza and con- asht, 1986). gaseous losses (denitrification and NH3 volatiliz- trol plots: the overall mean N concentration in the etifolia and ation), or leaching. In tropical soils N losses range horizon 0-30cm (expressed in mg N kg-' soil, dry to have a from 20 to 40 kg ha-' y-' but they can be up to weight) was 1022. Cj) for Robinia, 1006 Cj) for nissing that 70 kg ha-' y-'. The nutrient balance has lo be Elaeagnus, 995 Cjk) for Lespedeza, 936 (lc) for Altius, :nt tropical restored by proper management practices (especially and 933 (k) for control plots (N concentrations fol- r &fixing fertilization and recycling of agricultural wastes) lowed by the same letter are not significantly differ- I and by exploiting the N2 fixation process. Inputs ent at P = 0.05). Jziglans size was greatest by far in che amount from N2 fixation are usually through introducing Elneagnus plots, where the actinorhizal plants not La1 actinor- more annual or perennial legumes into farming sys- only improved the soil N status but also afforded the last 10 tems. Actinorhizal plants are seldom used for this shading and weed control. One complementary ben- Table 2. It purpose in spite of the fact that their role in efit following Juglans-Elaeagnus interplanting could Casuarina improving the soil fertility is clearly established in [ be the reduction of the spread of spores of the tive of the certain silvopastoral and agroforestry systems as foliar disease walnut anthracnose (Dawson, 1990). I:! fixation shown by the three following examples. The benefits of interplanting valuable Juglaiis nigra to 94 kg Under a 2-year-old plantation (1200 trees ha-') 1.5 with Elaeagnus utnbellata are illustrated in Table 3. pressed per of Altius acuminata (syn. A. jorullensis) in the Whereas it is fully acknowledged that Alnus fixed tree-' Colombian highlands the increase of soil N content acuminata. Casuarina oligodon and Elaeagïius iimbellata was found has been estimated to be about 279 kg ha-' n expected (Carlson and Dawson, 198.5). It is not yet known significantly improve soil fertility and increase the productivity of associated plants, nobody has yet aggressive whether such an improvement of the soil N status assessed the exact contribution of the N2 fixation 3n appears results only from N2 fixation or also from other tan usually processes, common to a number of trees, such as process alone. The reason is that assessing the sith a poor nutrient retrieval, reduction of losses from wind amount of fixed N2 transferred to soil and sub- 6th a high and water erosion. One undisputable fact is that, by sequently to non-N2-fixing plants is difficult. ion is lim- introducing A. aczmzinata in cattle pastures, farmers However, this type of investigation has already in Costa Rica got a significant increase in the pro- been carried out successfully, using an isotope tech- tions, such al. 'r manage- duction of milk (Budowski, 1957, 1979). nique by Van Kessel et (1994) in the case of a Ir fertiliza- Another well documented example of the .ability perennial legume (Leucaeria leucocephafa). The of actinorhizal plants to improve soil is that of major difficulty encountered results from the inter- Casuarina olìgodoii in the highlands of Papua ference of two processes taking place in most tree ed tree-' y-') Guinea (Ataia, 1983; Thiagalingam, 1983; Bourke, ecosystems: redistribution of N in internal pools 1985) and in Irian Jaya (Askin et al., 1990). This and recycling of fixed N2. The first process is trivial fecencec species of Casuarina has been successfully intro- in the case of young trees because of their relatively duced in agroforestry systems (annual and perennial low biomass. As to the recycling of N, it can also food crop gardens) since 1960 and has expanded be neglected but only when the decomposition rate

5s (1963) rapidly since about 1970. of the litter is low. r_rolis ( 1992) I¿.(1992) ( il.11. 1992) Table 3. Average height (ht in m), stem diameter at breast height of 1.5 m (dbh in cm) and annual dbh growth rate for Jugfans nigra (I994) d. (1994) intemlanted with actinorhizal ulants in Illinois" (Dawson. 1992) ~~ ~~ rl. ( 1994) Interplanted species Average annual dbh growth rate 11. (1994) Age 14 y Age 18y Age (Y) ht dbh (cm) ht dbh (cm) 0-14 'alker (m) (m) 14-18' 0-18 (1989) cm y-

Elaeagntis unibellata .o 10.07.6 IO15 12.2IO. 19 1.1 1 1.1 :nt with I5N AlIIlis g~ll~b~OS~ I 14 0.7 1.1 0.8 c. The results Control 3.4 4.9 4.9 8 0.4 0.6 0.4 the N differ- nTwenty-four J. nigra spaced at 9.8 m x 3.7 m with three actinorhizal plants between, giving an overall spacing of 2.4 m x 3.7 m. Control a plots without interplanting in row. 934 Y. R. Dommergues

Table 4. Propagation of actinorhizal plants by seeds (Seed), cuttings (Cut.), suckering (Suc.), air-layering (Air). stump sprouts (Spr,) and micropropagation (Mic.) (after MacDicken, 1994; Subba Rao and Rodriguez-Barrueco. 1995) Species Seed Cut. suc. Air Spr. Mic.

Almis acumìnata/jorullensìs Q Q A. glutinosa Q @ o o A. nepalensis 8 Allocasuarina verticillata e3 bID fa Casuarina cristata @ o C. cunningliamiana CD 63 C. equise! folia. a4 6 O O o C. glauca QB B o C.jiinghuhniana o 8 Elneagnus angustifolia gP O 63 Gymnostoma papriana Q @ 0 Generally successful. 0 Infrequently successful.

Such an absence of recycling, which is character- from those occurring in their native habitat. istic of open ecosystems (Dommergues, 1995), was Thus Casuarina glauca, which is native to a observed in Casuarina eguisetifolia plantations narrow coastal belt of southeast Australia established on the coastal sand dunes of Senegal, where annual rainfall averages 500 mm, whose soil is very poor in N (N -2 0.01%) and thrives in Hawaii up to an altitude of 900111 where the decomposition of the litter is impeded by and with annual rainfall as much as drought (mean annual rainfall is ca. 300mm), the 4000 mm. The same species grows vigorously absence of an active soil microfauna, and nutrient in Egypt with annual rainfall less than deficiencies (especially P deficiency). In these planta- 50 mm, provided that it is correctly irrigated tions the accumulation rate of N in the soil and lit- (El-Lakany, 1990, 199 1). Alnus glufinosa, ter was estimated to be 23 kg ha-' y-* at one site native to climates with severe cold, may per- by Dommergues (1963) and 75 kg ha-' y-' at other form well in tropical highlands where unsea- sites by Mailly and Margolis (1992). N balance stu- sonal cold can destroy other Alnus species dies confirmed that the observed accumulation of N (NRC, 1980). This wide adaptability is resulted primarily from N2 fixation. shared by other actinorhizal plants, es- More investigations are obviously required, especially Casuarina cunninghamiana and C. pecially in the tropics and subtropics, (i) to quantify junghuhniana. the amount of fixed N2 added to the soil N and N (iv) Actinorhizal plants are fairly resistant to pests uptake by non-N2-6xing companion plants and (ii) and major diseases. to evaluate the other benefits such as those resulting (v) In addition, actinorhizal plants are easily pro- from the addition of organic matter, improvement pagated by seed or through diverse methods of soil structure, reduction of erosion, shading, of vegetative propagation including micro- ' weed control on disease control. propagation (Table 4). For all the reasons mentioned above actinorhizal SYSTEMS OF LAND USE AND MANAGEMENT plants can be successfully introduced in a number In addition to their ability of fix N2, actinorhizal of systems of land use: production forestry (es- plants possess four essential traits which are less often found in legume trees: Table 5. Management systems involving representative tropical, subtropical and mediterranean actinorhizal plants: production for- (i) They are able to thrive in poor and wasted estry (Prd.), agroforestry (Agr.), protective forestry (Prt.), tecla- lands (which is not the case for a number of mation forestry (Rcl.) and urban forestry (Urb.) legume trees like Leucaeiza spp), Prd. Agr. Prot. Rcl. Urb. (ii) Actinorhizal plants are tolerant or semi-toler- AIniis acuminafa + +++ ant to a range of toxic pollutants. Thus A. nepalensis + ++ Alnus glutinosa tolerates relatively high levels A. glufinosa + i! ++ Allocasuarina verricillata 4- + of boron, cadmium, lead and zinc (Wheeler Casuarina and Miller, 1990). Like Alnus glutinosa, cunninghamiana + + + :, Casuarina equisetifolia forms good barriers C. equisetifoolia +U + .+ C. glauca + + +u+ + for catchTng industrial dusts. Therefore acti- C.jungliuhniana + ++++ norhizal plants would be prime candidates C. oligodon + + Elneagnus angusfifolin + i- for achieving phytoremediation (Salt et al., E. umbellam +u 1995) of polluted sites. Myrica faja + (iii) Some actinorhizal plants are adapted to Hippophaë.. . rltamnoides + + environmental conditions that differ widely "Including mixed plantations with non-N2-fixing trees. Actinorhizal plants and soil productivity 935 its (Spr,) and pecially mixed-tree plantations), agroforestry ble Frankia strains. These problems will be dealt (enriched fallows, alley cropping and intercropping), with in the following section. Mic. sylvopastoral systems, protective forestry (especially sand dune stabilization, windbreaks and control of 4 water erosion), reclamation forestry (reclamation of STRATEGIES FOR INCREASING N INPUTS FROM eroded, salinized and wasted mine soils and phytor- ACTINORHIZAL PLANTS 8 emediation), and urban forestry (amenity planta- In actinorhizal plants the situation is similar to tions in cities and recreational areas) (Table 5). 8 that encountered in legume trees: the amount of N2 d Examples of successes abound. Some have actually fixed is often lower than expected (values already been mentioned. The following should also of the order of 10-20 kg N2 fixed ha-' y-' are not Q be given: infrequent), whereas losses from ecosystems are -Increase of productivity of Qiiercus robur grown on often much higher, as already stressed. Therefore it lignite spoil banks at Santa Barbara (Italy) is mandatory to devise efficient strategies to increase through its association with Alnus cordutu, an the input of N, which involves not only the re- actinorhizal plant which increases available N for duction of the effects of environmental constraints 3 habitat. associated timber trees both by its high capacity but also the establishment of a symbiosis with high ttive to a symbiotic performance and a sufficient tolerance to Lo fix N2 (80% of plant N derived from N2 fix- Australia the remaining environmental constraints, the main ation) and its low N uptake from soil of 500 mm, (ZOO/, ones being: soil nutrient deficiencies and factors as- plant derived from soil) (Buresti et al., 1991). of 900 m N sociated with soil acidity, salinity, excess of plant- -Reduction of the volume of excess salinized drain nuch as available N in the soil, drought, enemies and dis- water in the Joachin Valley (California) by igorously San eases. irrigating Casuarina glauca and C. cunizinglranzi- :SS than To attain this goal two strategies have to be irrigated ana with this water (Merwin, 1990). adopted: the first one is to use proper management :lutinosa, -Afforestation of clay and sand-filled reclaimed practices; the second one is to improve the perform- nay per- lands at Singapore with Acacia auriculiformis, ance of the N2-fixing system. .e unsea- Paraserianthes falcataria and Casuarina equiseti- F species folia (Lee er al., 1993). Alleviating soil constraints through proper nzanage- bility is -Rehabiliting quarry sites and limestone spoil from a ment practices nts, es- cement .factory near Mombasa, Kenya (Baobab Soil nutrient de$cieircies and acidity. Nutrient de- and C. Farm) by planting Casuarina equisetifolia ficiencies are frequent when the plantations have (Baumer et al., 1990). been established on very poor soils, which is the to pests -Protecting crops against wind by planting case for a Casuarina equisetifolia plantation in Casuarina equisetifolìa windbreaks in China Benin, West Africa (Zech and Kaupenjohann, sily pro- (Turnbull, 1983) or Casuarina glauca in Egypt 1990). Deficiencies appear quite often in soils whose nethods (El-Lakany, 1983) and Tunisia. nutrient reserves have been depleted by successive micro- -Stabilizing sand dunes in many countries such as removal of forest products, including litter, often China (Turnbull, 1983) or Senegal (Andéké- removed from Casuarina plantations in southern Lingui and Dommergues, 1983). China (Diem and Dommergues, 1990). Replenishing norhizal soil reserves by proper addition of fertilizers, Many other success stories have been reported in number especially P, is an absolute requirement. Casuarina- try (es- the books and reviews of Midgley et al. (1983), ceae potentially are well suited to utilize rock phos- NRC (1984), Dawson (1992), Pinyopusarerk and phates, since they require fairly large amounts of tropical, House (1993) and Subba Rao and Rodnguez- Ca and probably, like legumes (Giller and Cadisch, ction for- Barrueco (1995). Yet failures have been observed t.), recla- 1995), they could possibly acidify the soil. but infrequently reported. Some result from in- Acidity is a major constraint to nodulation, - herent characteristics of the actinorhizal plant to be Urb. because it affects the process of infection. Liming is planted, such as sensitivity of Casuarina to some often suggested to raise the soil pH, which has the pests, (Hassan, 1990; Pinyopusarerk and House, advantage of supplying Ca and reducing AI toxicity. 1993), sensitivity of Hippophaë rhamnoides to nema- However, the cost of liming is often prohibitive and todes (F. Zoon, Ph.D. thesis University of selection of acid tolerant host plants is preferable. Wageningen, 1995) or IOW salt tolerance of Drought. Water deficits affect symbiotic N2 fix- Casuariira oligodon (NPC, 1984). Other failures are ation through influences on host plant metabolism, the consequence of inappropriate management prac- nodulation and nodule function, as shown by var- tices, a frequent situation when dealing with mixed ious experiments with Casuarinaceae (Reddell, e plantations (Schlesinger and Williams, 1984). 1993). Since semi-arid and arid regions receive too Finally many failures are caused by unexpected or little precipitation to ensure good plant growth and disregarded soil constraints (especially nutrient de- adequate N2 fixation, irrigation is often required. A ficiencies and drought) and the absence of compati- variety of irrigation systems have been developed, 936 Y.R. Dommergues

but most of them do not guarantee a sustained Whereas conventional plant breeding methods economic return. Therefore, low input practices are have been applied to improve growth of Casuarina recommended, especially rainwater harvesting sys- at different sites (El-Lakany, 1983), improvement tems that have been successfully used to direct programs have not yet taken into account the N2- infrequent runoff into the rooting zone of trees or fixing capability of the host (Diem and to assist groundwater recharge. Temporary watering Dommergues, 1990). may be necessary during the establishment phase of Do host-strain interactions that may exist in plantations to assist root systems in reaching per- legume Nz-fixing symbioses, also occur in actinorhi- manently moist soil horizons. This practice is cur- zal symbioses? To elucidate this problem the follow- rently applied to ensure the success of Casuarina ing experiment was carried out. The objective was eguisetifolia plantations on the Senegalese coast and to compare the amount of NI fixed by nine combi- C. glauca windbreaks in Egypt. nations involving three clones of Casuarina equiseti- Soil pathogens. Among soil pathogens nematodes folia and three strains of Frankia. Analysis of are probably the most dangerous, at least for some variance showed a very significant effect (P < 0.01) species of actinorhizal plants. Sterilization of nur- of the actinorhizal clones and Frankia strains and a sery soils using simple methods such as solarization less significant effect (P < 0.05) of the “C. eguiseti- is therefore recommended; unfortunately such a folia clone x Frankia strain” interaction measure is seldom applied. (Sougoufara et al., 1992). Since the ranking of clones and strains was not affected by the “clone x - Improving the performaiices of Frankia and kost strain” interaction in this experiment, the authors plant suggested that the identification of the best N2-fix- The two main criteria for improvement are higher ing combination “C. eguisetifolia clone x Frankia N2 fixation (generally assessed in growth chamber strain” could be achieved through a simple pro- or greenhouse) and, when environmental conditions cedure involving only two comparisons: comparison cannot readily be altered, increased tolerance to en- of the clones associated with one Frarikia strain and vironmental constraints. Examples illustrating these comparison of the Fraiikia strains associated with approaches are given below. one clone. Such a simplified procedure cannot be used when “actinorhizal plant genotype x Frankia Development of higher N2jxation. It is now clear that the N2-fixing potential depends on host plant strain” interactions are significant. This last situ- genotype, symbiotic bacterial strain and their inter- ation probably exists, but has not yet been reported actions. By selecting both partners of the symbiosis in the case of actinorhizal plants. it is thus possible to increase the performances of Development of insensivity to cornbined N (plant- N2-fixing systems. available N). Field as well as most laboratory ex- The usual screening procedures based on the periments indicate that nodulation and N2 fixation exploitation of the variability of Frankia is currently are inhibited by combined N (that is the plant- used to improve the symbiotic performances (com- available N in the soil) (Huss-Daneil, 1990). How- petitive ability and effectivity) of the actinomycete. ever, a continuous supply of combined N in small Theoretically the techniques of molecular biology doses, which were continuously increased to match have great potential to achieve this goal. However, the plant uptake, stimulated growth and N2-fixation in spite of recent advances, practical results cannot in Alnus incana seedlings (Ingestad, 1980). Field ob- be expected in the short term, because investigations servations show that the nodule biomass is high in on the transformation of Frankia still lag behind old Alntu, Casuarifia and Allocasuarina plantations those concerning the rhizobia1 symbiosis and the (several hundred kg ha-‘) in spite of the accumu- streptomyces (Dawson, 1992; Benson and Silvester, lation of plant-available N, whereas nodules tend to 1993). disappear under most ageing stands of legume trees. With the exception of some species like These observations would suggest that the actinor- Ceanothus sp. (Nelson and Lopez, 1989), actinorhi- hizal plants mentioned above would be less sensitive zal plants exhibit enormous variability, and exploi- to combined N than most legumes. However, this tation of this is promising. Provenance selection insensivity does not hold for all actinorhizal plants should be performed as a first stage, followed by species; thus in Hippophaë rhamnoides vegetation clonal selection. This procedure is facilitated by the the nodule numbers m-2 markedly decrease at fact that a number of actinorhizal plants are readily higher ages of the plants (Stewart and Pearson, 4.), propagated vegetatively (Table which allows for 1967: A.D.L. Akkermans, unpubl. Ph.D. thesis, the development of micropropagation techniques University of Leiden 1971). In fact, recent investi- (Duhoux et al., 1993). Thus a highly effective Nz- gations suggest that the reduction of the number of fixing clone of Casuariiia equisetifolia (clone p) actinorhizal nodules in H. rhamnoides could result obtained by screening a large number of seedlings from the proliferation of plant parasitic nematodes according to their nodulation, was easily propa- following increased availability of N in the soil and gated by cuttings (Sougoufara et al., 1987). increased food quality of plant cell contents and Actinorhizal plants and soil productivity 937 g methods also from possible interactions between nematodes tinosa (Mackay et al., 1988) have already been f Casuarina and root-rot fungi (Zoon, 10c.cit.) developed (whereas such systems are not yet avail- lprovement The development of actinorhizal genotypes toler- able in Frankia). One of the first targets will prob- int the Nz- ant of combined N is probably essential in some ably be the introduction of insect resistance genes + >iem and situations, especially in the presence of large into species of Casuarina (Franche et al., 1994; amounts of combined N. TWOapproaches could be Diouf et al., 1995). .y exist in adopted to develop genotypes with this trait: screen- i actinorhi- ing genotypes for their ability to nodulate in the the follow- presence of combined N or for their ability to form INOCULATION WITH SELECTED STRAINS OF FRANKIA iective was aerial nodules, a possibility which has been already AND MYCORRHIZAL FUNGI ine combi- reported in Casuarina cunizingharniana and C. As in the case of legumes, the need for inocu- ia equiseti- glauca; it has been postulated that aerial nodulation lation depends mainly on the specificity of the host nalysis of would make these host plants more independant of plant and on the size of the resident population of [P < 0.01) soil constraints and allow them to express their NS- the symbiotic compatible bacteria. The distinction ains and a fixing potential even in soil with unfavorable between promiscuous (also called non-specific) and :. equiseti- characteristics, especially excess of combined N non-promiscuous (also called specific) species is well nteraction (Prin el al., 1991). known for legumes and also holds for actinorhizal inking of Developinent of tolerance to acidity or alkalinity. x trees. Promiscuous actinorhizal genera, i.e. plants “clone - Like a number of legumes, actinorhizal plants prob- which enter into symbiosis with a broad range of e authors ably exhibit inter- and intraspecific variations in Frankia, are Myrica (Myricaceae) and to a lesser :st Nz-fix- their adaptation to acidity or alkalinity, Actinorhi- x extent Alnus (Betulaceae) and Gymnostonia Fraiikia zal plant selection in this direction would certainly (Casuarinaceae). Specific actinorhizal genera are nple pro- be rewarding. Allocasuarina and Casuarina (Maggia and imparison Developinent of tolerance to salinity. Casuarina Bousquet, 1994). Response to inoculation is to be itrain and glauca, and to a lesser degree C. equisetifolia have expected more frequently in specific actinorhizal ated with been used to reclaim salt-affected areas. Their toler- plants than in non-specific ones. annot be ance can be enhanced by selection of the host (e.g. The size of the resident soil-borne Compatible % Frankia provenance screening); the role of associated Fran- Frankia population is highly variable. There is no last situ- kia appears to be negligible. Frankia strain Thr, general rule to predict it and therefore it is necess- reported poorly salt tolerant in vitro, appeared to improve ary to evaluate the number of infective units of Nz fixation better than other strains (which were V Frankia using methods such as those proposed by (plant- more salt tolerant iiz vitro) when associated with a Arveby and Huss-Dane11 (1988) or Dawson et al. Itory ex- provenance of C. glauca (E) known for its salt tol- (1989). When Fraiikia infective units are few or fixation erance in the field (Girgis et al., 1992). absent the response to inoculation is spectacular ie plant- Development of tolerance to drought. A number of provided there is no major limiting factor. A clear )). HOW- actinorhizal plants are drought-resistant such as positive response to inoculation was recorded in the in small Allocasuarina decaisneana, A. campesiris, A. dielsi- semi-arid and arid soils of Senegal, Egypt and o match ana, Casuarina obesa and C. cristata ssp pauper Zimbabwe, which are devoid of Casuarina-compati- -fixation (Reddell et al., 1991), and South American Rham- ble Frankia. By contrast, inoculating Alnus giutiizosa ?eld ob- naceae. Some species, such as Casuarina equisetifo- planted in soils already harboring Alnus-compatible high in lia, are able to obtain their moisture from foliar Frankia did not significantly affect host growth n tations water absorption from dews or ocean sprays. (Table 6). sccumu- Usually Alnus spp have high water requirements The development of Frankia inoculants has been tend to but some, like A. cordata, are fairly drought resist- delayed for a number of years because isolating ne trees. ant and others, like A. acunziizata, are able to meet Frankia is sometimes very difficult and the culture actinor- their moisture requirements from mists and clouds of this microorganism is not easy because Frankia sensitive through the dry season (Russo, 1994). Breeding grows slowly with doubling times of 15 h ’er, this techniques have been successfully used to improve (Schwencke, 1991) and low increase of biomass 1 plants drought tolerance of Casuarina (El-Lakany, 1983). with respect to the amount of inoculum. The pat- zetation Development of resistance to diseases and pests. tern of growth varies with the inoculum density :ase at Though actinorhizal plants are reputed to be rather (Frioni et al., 1994), medium composition, and ’earson, resistant to pathogenic fungi, bacteria, viruses, degree of agitation (Benson and Silvester, 1993). thesis, insects and nematodes, one cannot exclude possible Advances have been made recently in the mass cul- investi- damage by these agents (Hassan, 1990; Pinyopusar- ture of Frankia (Diem and Dommergues, 1990) and nber of erk and House, 1993). One of the most elegant the use of polymeric carriers (Diem et al., 1988) I result approaches to this problem is to introduce resist- which allows the production of Frankia inoculants iatodes ance genes into the host plants. This is no longer a of high quality. The most convenient formulation oil and remote possibility since gene transfer systems for of Frankia inoculum is obtained by entrapping a ts and Casuarinaceae (Franche et al., 1994) and Alnus glu- Frankia culture containing a large number of spor- 938 Y. R. Dommergues

Table 6. Effect of inoculation with pure Frankia cultures on field-grown Camrim cunnirrglzomiq~m in Zirnb:th\ve (Rcd&.li <, <,,., ,ogsk Cnsrlarirln equisetTo/icr in ~enegd (Sougoubra et d., 1989) and ALII~~ &inoso III France (Pmt. 1992) Species (provenance or - CIOXX) Treatment Age Height (III) Weight (g trw-‘) h~lnn!c” I,,,’ ha-Q Casuarilm cutmirrghandatm (Zimbabwe) -

41Gympie IIIOC. III id. Uninoc. f N id. s-k Mareeba IIIOC. id. id. Uninoc. + N id. J”‘: Mt Morgan IIIOC. id. b-l id. Uninoc. + N id. 30 Cusnarina eouisetifolia (Senegal)

Senegal Inocb 7” 2846 id. Uninoc. 2025 id. Inoc.b 4607 id. Uninoc. 329 I ‘4,nus glurlnosa (rrance,

Najybajom hoc. 4Y 2.4 id. Uninoc. id. I.8 clone 11 Inoc. id. 2.0 id. Uninoc. id. 1.3 clone 142-2 IllOC. id. 2.1 id. Uninoc. id. 2.0 “Wood volume expressed as m’ ha-’ was estimated assuming Y = l/3 d'h where d= Stem diameter at ground Irvcl 111lci ), ';, ,,‘r*' hcirht For example the mean diameter of inoculated Gympie Cusuorina cl~nn@~aflIia~a was 0.225m and their height u:\s s.2 ,,, ,,,&h :; stocking rate of 800 trees ha- ’ SO V = l/3 (0.2125)~ x 8.2 x 800 = 111 m’ ha-‘. “Inoculation with Frankia entrapped in alginate beads (l-year storage). . . . The effect of inoculation was significant for C. cwminglzamia~ta (tree volume). C. e~lse@~ (weight but not height) and n,)t siji,ritic:,nt for A. glutiuosa.

angia in alginate beads, with kaolinite added to the inoculants carrying, ectomycorrhizal fungi lm;rpIt. gel. This type of inoculant is not only easy to trans- to specific actmorhizal plants grown in certnirt tl’o- port, it also has a remarkably long shelf life (Diem logical conditions. and Dommergues, 1990). An example of field inoculation with Frankia entrapped in alginate beads is given in Table 6. CONCLUSION In P-deficient soils, dual inoculation with Frankia In spite of their remarkable adaptability :rnd their and VA endomycorrhizal fungi can improve P oustanding performance In harsh sites. ;rctirrorhiz,tl uptake by increasing the volume of soil explored plants have not yet been exploited sufticiently. -l-he and subsequently can enhance Na fixation and benefits that they can provide can be dr:tmatic\rllv growth of actinorhizal plants (Diem and Gauthier, increased in the near future provided that the foi- 1982; Gardner et al., 1984). Since mycorrhizae are lowing strategies are adopted: also assumed to improve drought resistance of host plants by increasing water supply and water use effi- (i) Optimizing actinorhizal N-, fixation in the field ciency, inoculating actinorhizal plants with selected using the approaches described itI detail mycorrhizal strains can also be beneficial under arid above. and semi-arid conditions. Indculants obtained by . (ii) Introducing actmorhizal plants ahe:{+ known entrapping chopped roots of VA mycorrhizal plants for their performance into countries ilnd c.,..- with their spores in alginate beads are as efficacious systems where they are now absent, Thuq as the Frankia inoculants prepared according to the tropical Alms could be planted in miu4 same procedure (Gamy et al., 1985). It must be plantations and in pastures in Africun and noted that a positive response to inoculation can be Madagascan highlands. Casrmrirta o~~goc~o,,~ expected only in soils with very low populations of which IS used as a soil improver in pnpua VA endomycorrhizal fungi, a situation which is New Guinea and has been successfully intro- found in partially $erilized nursery soils. duced in Uganda @eden ef al., 1993) could Records of ectotrophic mycorrhizal associations be expanded to other African countries and with tropical actinorhizal plants are still few (an probably to some parts of South America example is that of Pisolithus associated with Casuarina equisetifolia and C. &I;WQ planta: AlIoca&arina) and further fundamental investi- tions could be enlarged along most African gations are obviously required before developing coasts, providing much needed fuel to the in- 939 1 Actinorhizal plants and soil productivity

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Special Issue I NTE R N AT1O NA L SY M POS IU IVI-S USTAI NAB LE Vol. 29 No. 5/6 AGRICULTURE FOR THE TROPICS: THE ROLE OF Rnay/June 1997 BI O LOG ICAL NITR O GEN FIXATION lSSS-BISS-IBG Editor-in-ChiefJ S VVAlD