Growth and N₂ fixation in mixed cropping of Medicago arborea and Atriplex halimus grown on a salt-affected soil using a ¹⁵N tracer technique

Abstract

The objective of this study was to evaluate dry matter (DM), nitrogen yield, N₂ fixation (Ndfa) and soil N uptake (Ndfs) in the shrubby medic (Medicago arborea) and saltbush (Atriplex halimus) grown in pots either solely or in a mixture on a salt-affected soil, using ¹⁵N dilution method. The combined DM of both species was considerably higher than that of solely grown shrubs. The inclusion of saltbush in the mixed cropping system decreased Ndfs by shrubby medic and enhanced % Ndfa without affecting amounts of N₂ fixed. It can be concluded that the use of mixed cropping system of shrubby medic and saltbush could be a promising bio-saline agricultural approach to utilize salt affected soils in terms of forage yield and N₂ fixation.

Keywords:

• Atriplex halimus
• Medicago arborea
• mixed cropping
• salinity stress
• ¹⁵N

Introduction

Salt-affected soils are widespread in many parts of the world, resulting in large areas of agricultural land being withdrawn from agricultural production annually. Attempts are being made to make these areas productive again by adopting the bio-saline agriculture approach (International Atomic Energy Agency [IAEA] 2003; Wallin and Pedroza 2003). The importance of bio-saline agriculture lies in the integration of genetic resources and improvement of agricultural practices to make better use of saline land and saline irrigation water on a sustainable basis (Qureshi and Barrett-Lennard 1998; Kurdali et al. 2003 ,2007). Among genetic resources, salt-tolerant shrubby plants are commonly used to revegetate salt affected soil. Several species belonging to the genus Atriplex are well adapted to extreme environmental conditions. Atriplex species are widely distributed in many semi-arid and arid parts of the world and used as protein supplements for low quality forage diets for sheep (Ben Salem et al. 2002; Norman et al. 2008) and Damascus does (Al-Masri et al. 2007). Moreover, Legumes are a large, diverse family ranging from herbaceous annuals to woody perennials that, because of their capacity to fix nitrogen, are essential components in natural and managed terrestrial ecosystems. The shrubby medic (Medicago arborea L.) is one of the most potentially valuable leguminous fodder shrubs in the Mediterranean environment because of its high preference by small ruminants and its nutritive value (Amato et al. 2004). This species is an evergreen, cold, drought and salt-tolerant plant species (Sibole et al. 2003). It is native to Greece and Italy but naturalized over all Mediterranean areas. Similarly, the halophyte saltbush (Atriplex halimus L.) is by far the most widespread native shrub in the Mediterranean regions (Le Houerou 2000; Martinez et al. 2003). This latter species is able to accumulate high levels of sodium under salt stress conditions, and growth is even stimulated in response to moderate NaCl concentrations (Bajji et al. 1998). As in most species within the genus Atriplex, A. halimus is a facultative halophyte that achieves salt tolerance by accumulating ions against a concentration gradient, and then parting those ions in specific cells. Salt glands, an anatomically distinct group of cells, concentrate the salt and then secrete salt crystals to the outside of the leaf surface (Thomson and Liu 1967; Qureshi and Barrett-Lennard 1998). This property is important because it can be proposed that one of the benefits of halophytic shrubs may be to decrease the concentration of salt in the saline soils, thereby enabling such soil to grow other plant species. Hence, it is reasonable to suggest that mixed cropping systems of A. halimus and other shrubby plants such as M. arborea may represent promising agricultural practices in reducing the adverse effect of soil salinity on forage production and nitrogen fixation by the legume species. Moreover, such cropping systems of legume and non-legume plants may produce much more forage than the solely grown plants along with better quality through improved nutritive value (Al-Masri 1998).

This study presents the first report on the use of M. arborea (possessing the C3 metabolic pathway) in mixed cropping systems with A. halimus (C4 metabolic pathway) under saline conditions. Therefore, the objectives of this pot experiment were to: (i) evaluate dry matter production and total N yield in A. halimus and M. arborea grown either solely or as mixed plants; (ii) estimate N₂ fixation by the leguminous plant using ¹⁵N enrichment method; and (iii) assess the intra-species competition for soil N uptake.

Materials and methods

Soil properties

A pot experiment was conducted at the research station of the AECS at Deir-Alhajar, south east of Damascus, Syria. Each pot contained 12 kg of saline soil collected from salt-affected land of the Euphrates valley in Syria, located about 20 km south east of Deir-Ezzor city. The main physical and chemical soil properties were: EC_e 6.8 dS/m; pH 7.6; organic matter 0.74 %; CaCO₃ 19.8 %; Ionic content: Cl⁻ 16.83, 1.28, 17.1, Na⁺ 24.6, K⁺ 0.2, Ca^{2 +} 4.73, Mg^{2 +} 5.61 meq/l; cation exchange capacity (CEC) 16.7 meq/100 g soil; Available P(Olsen) 1.5 µg/g; 59.7 µg/g; 7.7 µg/g; total N 0.8 mg/g.

Treatments and experimental design

Seeds of shrubby medic (Medicago arborea L.) and saltbush (Atriplex halimus) were sown in plates containing sand and soil (1:1) for two months. Seedlings were pricked out into individual pots when they were large enough to handle for seven months. Three treatments were evaluated: (a) sole crop of shrubby medic (Med.), (b) sole crop of saltbush (Atr.), and (c) one medic and one saltbush were grown in the same pot (Med.+Atr.). For solely grown treatments, each pot was planted with one plant of each species. Since shrubby medic and saltbush are commonly grown in a single hole under field conditions, one saltbush, one shrubby medic, and one saltbush plus one shrubby medic plants were sown in each pot to evaluate the advantages of growing two plant species in the same hole over growing one plant in separate holes in terms of forage yield, N₂ fixation along with a better land use. Such an approach was relatively similar to that applied in another cropping system using one maize, two cowpea or one maize plus two cowpea plants per pot (Ofori et al. 1987). Different numbers of legume and non-legume plants per tank were also used in maize/groundnut cropping system (Senaratne and Ratnasinghe 1993).

The pots were arranged in a randomized complete block design with four replicates and set outdoor under natural climatic conditions.

¹⁵N application, plant sampling and analysis

A rate of ¹⁵N-labelled ammonium sulphate (9.6337 % ¹⁵N atom excess) equivalent to 20 kg N/ha was applied. The N fertilizer was used in two split applications at one month interval after planting. This procedure was followed to stabilize the ¹⁵N enrichment of the N pool and to minimize N immobilization.

The plants were sampled after seven months from transplanting, then separated into its main components (shoots and roots). Plants from the mixed stands were separated into the component species. Samples were weighed, dried at 70°C and ground for determination of total N by Kjeldahl. ¹⁵N/¹⁴N-isotope ratio was measured using an emission spectrometry (Jasco-150, Japan). The N fractions derived from the atmosphere (% Ndfa) by medic, grown as a single crop and in intercropped treatment, were calculated using the equation of Fried and Middelboe (1977). The sole crop of saltbush served as a reference crop for measuring N₂ fixation by shrubby medic.

The data were subjected to analysis of variance (ANOVA) test, and means were compared using the Least Significant Difference (Fisher's PLSD) test at the 0.05 level confidence.

The relative yield total (RYT), which is mathematically the same as land equivalent ratio (LER), was used for estimating the efficiency of intercropping relative to sole cropping (de Wit 1960), i.e.

where Y is the dry matter or N yield, Yii and Yjj are sole crop yield for the component crops i (saltbush) and j (medic), and Yij and Yji are mixed-crop yield. For convenience, the term relative yield total (RYT) will be served here for measures based on plant biomass, while, the term LER will be used for measures based on grain yield. The two measures are calculated in exactly the same way (Tofinga et al. 1993).

Results

Dry matter and nitrogen yield

Dry matter yield in different plant parts of M. arborea and A. halimus is shown in Table 1. Dry matter yield of shrubby medic grown alone was significantly higher than that of the sole saltbush. However, the combined dry matter yield of both plant species grown together was significantly higher than that of the solely grown shrubs. The observed values of total dry matter yield were 83, 60 and 105 g/pot in the sole medic, sole saltbush and mixed treatment, respectively. The contribution of medic to total dry matter yield (55 %) in the mixed system was slightly higher than that of saltbush (45 %). Dry matter yield of shoots and roots of both species followed a similar trend to the total aboveground dry matter weight. These results indicate a good performance of shrubby medic when grown either alone or together with saltbush plants, under the prevailing saline conditions.

Table 1. Dry matter yield (g/pot) of Medicago arborea (Med.) and Atriplex halimus (Atr.) grown either solely (Sole) or as mixed plants (Mixed) on a salt-affected soil.

Cropping system	Shoots	Roots	Whole plant
Comparison between cropping systems
Sole Med.	57.3±2.6b	25.7±2.1a	83.0±1.8b
Sole Atr.	45.4±1.2c	15.0±0.7b	60.4±1.5c
Med.+Atr.	77.7±1.4a	27.1±1.1a	104.8±2.0a
LSD 0.05	5.77	4.6	5.63
Comparison between cropping systems for each plant species
Sole Med.	57.3±2.6a	25.7±2.1a	83.0±1.8a
Mixed Med.	40.8±1.6b	16.5±1.5b	57.3±3.0b
LSD 0.05	7.4	6.3	8.6
Sole Atr.	45.4±1.2a	15.0±0.7a	60.4±1.4a
Mixed Atr.	37.0±1.8b	10.6±0.7b	47.5±2.1b
LSD 0.05	5.2	2.3	6.2
Means within a column followed by the same letter are not significantly different (p<0.05).

Dry matter yield of each component crop within the mixed cropping treatment was significantly lower than that of the sole cropping (Table 1). However, both plant species in the mixed treatment produced much more forage than the solely grown plants. The relative yield index (RY) of each component plant in the mixed treatment was above 0.5, indicating a higher productivity in mixture when compared to monocultures. The relative yield total (RYT) for dry matter yield of shoots (1.52), roots (1.35) and the whole plant (1.48) under mixed cropping (Med.+Atr.) were > 1 (Table 2).

Table 2. Relative yield (RY) for dry matter and nitrogen yield in different plant parts of saltbush and shrubby medic grown in mixed cropping system.

	Dry matter yield
Relative yield (RY)	Shoots	Roots	Whole plant
RY (Saltbush)	0.81	0.71	0.79
RY (Shrubby Medic)	0.71	0.64	0.69
Total (RYT)	1.52	1.35	1.48
	Nitrogen yield
RY (Saltbush)	0.82	0.95	0.85
RY (Shrubby Medic)	0.71	0.68	0.70
Total (RYT)	1.53	1.63	1.55

Nitrogen content (mg/g) in shoots and roots of solely grown shrubby medic was significantly higher than those of solely grown saltbush (Table 3). However, no significant differences were obtained between sole and mixed cropping systems for each plant species, except for the mixed saltbush where N concentration in its roots was significantly higher than that of the sole saltbush.

Table 3. Nitrogen concentration (mg/g) and yield (mg/pot) in Medicago arborea (Med.) and Atriplex halimus (Atr.), grown either solely (Sole) or as mixed plants (Mixed) on a salt-affected soil.

	Shoots		Roots
Cropping system	N (mg/g)	mg/pot	N (mg/g)	mg/pot	Whole plant mg/pot
Comparison between cropping systems
Sole Med.	17.3±1.1a	992±44b	18.8±1.1a	475±8a	1467±38a
Sole Atr.	11.2±0.1b	507±7c	10.1±0.5b	152±12b	659±10b
Med.+Atr.	−	1125±43a	−	468±27a	1592±67a
LSD 0.05	0.5	113	3.1	56	143
Comparison between cropping systems for each plant species
Sole Med.	17.3±0.1a	992±44a	18.8±0.11a	475±8a	1467±38a
Mixed Med.	17.4±0.3a	708±44b	19.6±0.07a	324±32b	1032±68b
LSD 0.05	NS	141	NS	81	190
Sole Atr.	11.2±0.1a	507±7a	10.1±0.5b	152±12a	659±10a
Mixed Atr.	11.3±0.5a	417±22b	13.7±0.3a	144±8a	560±18b
LSD 0.05	NS	57	0.13	NS	48
Means within a column followed by the same letter are not significantly different (p<0.05).

Total nitrogen acquisition by plants in the two cropping systems behaved in a manner relatively similar to dry matter yield (Table 3). Total N in the sole medic (1467 mg N/pot) was significantly higher than that of sole saltbush (659 mg N/pot), whereas, that in the mixed treatment (1592 mg N/pot) did not significantly differ from sole medic but was higher than sole saltbush. The contribution of medic to total nitrogen yield (65 %) in the mixed system was considerably higher than that of saltbush (35 %). N yield in shoots of the mixed treatment was significantly higher than that of the solely grown medic, which was in turn higher than that of the solely grown saltbush. However, no significant difference in root N uptake was obtained between the sole medic and mixed treatment; whereas, that of the sole saltbush was significantly lower. Mixed cropping medic and saltbush generally resulted in a substantially lower N yield of each individual shrub than that of the solely grown plants with the exception of roots of the saltbush plants where N yield was not significantly different between the two cropping systems. Moreover, values of the relative yield total (RYT) for N yield of shoots (1.53), roots (1.63) and the whole plant (1.55) under mixed cropping (Med.+Atr.) were >1, reflecting a greater benefit of the mixed cropping system over sole cropping for N uptake (Table 2).

Fertilizer and soil nitrogen uptake

Proportions and amounts of nitrogen in different plant parts of M. arborea and A. halimus, grown either solely or as mixed plants, acquired from soil (Ndfs) are presented in Table 4. Proportions of the Ndff and Ndfs were, in all cases higher, in saltbush than those in shrubby medic. Amounts of Ndfs in shoots of the sole saltbush were significantly higher than that of the sole medic, whereas, the contribution of soil N to medic's roots was more than that absorbed by saltbush. In the whole plant, soil N uptake by the mixed treatment did not significantly differ from that of the sole saltbush, but was higher than that of the sole medic. The observed values of Ndfs were 549, 628 and 742 mg N/pot in the sole medic, sole saltbush and mixed treatment, respectively. Amounts of nitrogen derived from soil (Ndfs) were generally reduced by each of the component plants in the mixed treatment compared with the sole treatments (Table 4). Nitrogen absorbed from soil by saltbush (528 mg N/pot) was substantially higher than medic plants (214 mg N/pot). Moreover, data of nitrogen derived from fertilizer (data not shown) followed almost a similar trend to the amounts of N derived from soil.

Table 4. Nitrogen derived from soil (Ndfs) of Medicago arborea (Med.) and Atriplex halimus (Atr.) grown either solely (Sole) or as mixed plants (Mixed) on a salt-affected soil.

	Shoots		Roots		Whole plant
Cropping system	% Ndfs	Ndfs (mg/pot)	% Ndfs	Ndfs (mg/pot)	% Ndfs	Ndfs (mg/pot)
Comparison between cropping systems
Sole Med.	31.5±2.8b	315±40b	49.2±3.9b	234±18a	37.3±3.1b	549±60b
Sole Atr.	95.0±0.3a	482±5.5a	96.2±0.1a	146±11b	95.3±0.3a	628±9ab
Med.+Atr.	−	491±26a	−	251±23a	−	742±42a
LSD 0.05	6.9	88.3	9.5	57.9	7.52	131
Comparison between cropping systems for each plant species
Sole Med.	31.5±2.8a	315±40a	49.2±3.9a	234±18a	37.3±3.0a	549±60a
Mixed Med.	14.0±2.5b	102±22b	33.3±5.6a	112±26b	20.1±3.6b	214±48b
LSD 0.05	9.2	111	NS	77.9	11.5	182
Sole Atr.	95.0±0.3a	482±5.5a	96.2±0.1a	146±11a	95.3±0.3a	628±09a
Mixed Atr.	93.5±0.22b	389±21b	96.2±0.3a	139±7.6a	94.2±0.26b	528±16b
LSD 0.05	0.92	52	NS	NS	0.88	44
Means within a column followed by the same letter are not significantly different (p<0.05).

Table 5. %¹⁵N in excess in shoots and roots of Medicago arborea (Med.) and Atriplex halimus (Atr.) grown either solely (Sole) or as mixed plants (Mixed) on a salt-affected soil.

Cropping system	Shoots	Roots	Whole plant
Comparison between solely grown plants
Sole Med.	0.1599b	0.1885b	0.1691b
Sole Atr.	0.4817a	0.3682a	0.4562a
LSD 0.05	0.081	0.044	0.070
Comparison between cropping systems for each plant species
Sole Med.	0.1599a	0.1885a	0.1691a
Mixed Med.	0.0712b	0.1275b	0.0890b
LSD 0.05	0.0035	0.06	0.05
Sole Atr.	0.4817b	0.3682a	0.4562b
Mixed Atr.	0.6259a	0.3626a	0.5571a
LSD 0.05	0.089	NS	0.086
Means within a column followed by the same letter are not significantly different (p<0.05).

N₂ fixation

The atom %¹⁵N excess values detected in shrubby medic were lower than those observed in saltbush plants and the ¹⁵N uptake from soil was diluted significantly by atmospheric N₂ (Table 5). A significantly lower atom %¹⁵N excess value was observed in the intercropped medic than the sole plant. Consequently, intercropped medics derived significantly higher N from fixation (79 %) than the sole plant (61 %) (Figure 1). However, amount of N₂ fixed by the sole medic (892 mg N/pot) was slightly but not significantly higher than that of the mixed plant (808 mg N/pot).

Figure 1.  Nitrogen derived from atmosphere (Ndfa) of Medicago arborea grown either solely (Sole) or as mixed plants with Atriplex halimus.

Discussion

This study provided valuable information on: (a) the growth of shrubby medic (Medicago arborea) and saltbush (Atriplex halimus) on a saline soil, (b) the impact of mixed cropping system on N₂- fixation by the legume, and (c) the interaction between both species for soil N uptake. The combined dry matter yield of mixed medic and saltbush was considerably higher than that of solely grown shrubs, indicating the superiority of mixed cropping over sole cropping for biomass production. Similarly, Kurdali et al. (1996) reported that barley-vetch mixed stand significantly out-yielded plants in monoculture under rain fed conditions. On the other hand, the reduced dry matter and N yield of the shrubby medic and saltbush grown as mixed plants compared to solely grown plants in our study is a common observation in multiple cropping systems (Danso et al. 1987; Tobita et al. 1994; Kurdali et al. 2003; Kurdali 2009). This indicates that the component crops compete with each other for the limited resources under the intercropped conditions (Tobita et al. 1994). The relative yield index of each component plant in the mixed treatment was above 0.5, indicating a higher productivity in mixture when compared to monocultures. Moreover, the relative yield total (RYT) for dry matter and N yield (>1) reflects a greater benefit of the mixed cropping system over the sole cropping. These findings indicate that the use of mixed cropping systems of shrubby medic and saltbush could be a promising agricultural approach to reutilize salt affected soil for forage production.

In the present study, although saltbush contained a significantly less total of N than did medic under both cropping systems, it absorbed almost the same amount of N from soil (Ndfs) as did the sole medic, suggesting an equivalent performance of solely grown plant species regarding soil N uptake. Based on this result, the use of N difference approach could be used herein for estimating total N uptake from N₂ fixation. The estimated value was 808 mg N/pot, which is close to that estimated by the ¹⁵N isotope dilution method (Figure 1). Amounts of nitrogen derived from soil (Ndfs) were generally reduced by each crop component in the mixed treatment compared with sole treatments (Table 4). The reduction of total Ndfs was more pronounced by medic than by saltbush. Nitrogen absorbed from soil by saltbush (528 mg N/pot) was twice that of medic (214 mg N/pot). Such amounts represented 71 and 29 % of the total amount of N derived from soil in the mixed treatment for the above-mentioned species, respectively. Therefore, the use of the N difference approach cannot be considered an appropriate technique for estimating N-uptake from N₂ fixation under mixed cropping system. Nitrogen derived from fertilizer (Ndff) by plants in the two cropping systems behaved in a manner almost similar to Ndfs. The greater uptake of N from fertilizer and soil by the saltbush than that by the medic in the mixed treatment indicates the superiority of non-legumes over legumes in exploiting soil N (Kurdali et al. 2003). This was mainly due to the reduced N supply to the component shrubby medic (possessing the C3 metabolic pathway) associated with the higher competitive ability of the component saltbush (possessing the C4 metabolic pathway) from soil and from fertilizer nitrogen (Kurdali et al. 2003).

The percentage contribution of biological nitrogen fixation (BNF) to the amount of N accumulated by the component shrubby medic in the mixed treatment (79 %) was greater than that of mono cropped medic (61 %). Increased N₂ fixation in the intercrop medic mainly resulted in a more efficient depletion of N from soil and fertilizer by the saltbush compared with that absorbed by the associated legume as previously reported by Hardarson et al. (1988) for alfalfa-ryegrass swards, Izaurralde et al. (1992) for the pea-barley intercrop, Kurdali et al. (1996) for the vetch-barley mixed cropping, and Kurdali et al. (1990) for the alder-poplar associations. High soil N content has been demonstrated to reduce N₂ fixation, with no adverse effect on yield (Danso et al. 1987). Thus, N₂ fixation is capable of substituting for N from soil and fertilizer or vice versa. When a non-fixing plant competes with an N₂ fixing legume for soil N, the lowered soil N status attributable to high uptake by the non-legume could induce increased N₂ fixation in the legume plant. In this study, it was observed that the inclusion of saltbush in the mixed cropping system severely decreased soil and fertilizer N uptake by shrubby medic and enhanced % N₂ fixation. It was previously observed that % Ndfa of the legume increased due to an N-sparing effect (N concentration in legume rhizosphere is reduced) by the non legume (Danso et al. 1987). A similar observation was also reported by a recent study (Fan et al. 2006). Greater Ndfa % by intercropped legume could be further explained by the probable reduction of nitrate concentration in the rhizosphere due to higher non-legume uptake of nitrate, which may reduce its toxic effect on Rhizobium (Kurdali et al. 2003). It is also reasonable to suggest that saltbush inclusion in the mixed cropping may reduce salt concentration in the rhizosphere enabling the adjacent medics to fix more N₂. On the other hand, it has been previously reported that arbuscular mycorrhizal fungi (AMF) enhanced growth, nodulation and N₂ fixation of legume plants grown in multiple cropping systems (Barea et al. 1988). AMF was found to have an ability to form an association with M. arborea (Valdenegro et al. 2001). Therefore, it is possible that mixed cropping of saltbush and shrubby medic may stimulate the colonization of AMF in the rhizosphere enabling the adjacent shrubby medic to fix more N₂. This suggestion, however, merits further investigation to demonstrate the relationship between N₂ fixation and mycorrhizal fungi in this type of mixed cropping system.

Danso et al. (1987) reported that greater N₂ fixation may be achieved by the two forms, i.e. increase in the actual amount of N₂ fixed or the proportion fixed. The former is normally associated with increased total dry matter yield or total N uptake. Since mixed cropping normally results in reduced dry matter yield and N yield of the legume plant, as found in our study, it is not surprising that the total amount of N fixed in shrubby medic was slightly lower in the intercrop than that of the sole crop, with both being not significantly different from each other. However, the %Ndfa in medics was greatly increased when intercropped with saltbush, illustrating a greater ability of shrubby medic plants in the intercrop to fix N₂. The successful symbiosis between M. arborea and the local rhizobia in terms of N₂ fixation, exhibited in our experiments, implies that the indigenous rhizobial strains were able to form effective nodules on its roots and fixed atmospheric nitrogen under saline conditions. This underlines the need to isolate these local rhizobia and to select better salt-tolerant strains.

One of the methods used for assessing N transfer is based on the ¹⁵N-isotope dilution concept, which implies that such a nitrogen transfer in mixed stands has a significantly lower %¹⁵N excess than that of the same non-legume grown solely in pure stands. However, under pot experiment conditions, the possibility of N-transferred from shrubby medic to the adjacent saltbush was excluded because the mixed salt-bush had significantly higher ¹⁵N excess values than that of the solely grown plant (Table 5). This anomaly could be the result of differences in N uptake patterns of the pure and mixed non-legume plants, as suggested by Hardarson et al. (1988), who reported a higher atom ¹⁵N excess in the 2:1 alfalfa-ryegrass sward compared with that of pure grass. Therefore, the present pot experiment failed to demonstrate N-transfer from shrubby medic to saltbush using ¹⁵N isotope dilution technique. The experiment of Izaurralde et al. (1992) illustrated the problem of non-uniform distribution of applied ¹⁵N in the field when they reported significantly higher ¹⁵N enrichments in barley intercropped with pea compared with the barley mono-crop. Since our experiment was conducted in pots, the labeling should be uniform and the natural ¹⁵N-abundance method may offer advantages in this respect (Chalk 1993).

Overall, to make better use of salt-affected soil in agricultural practices, the use of mixed c ropping system of shrubby medic and saltbush is suggested to be a promising approach in managing bio-saline agriculture to reutilize salt affected lands in terms of forage yield and N₂ fixation. Moreover, such species may alleviate feed shortages, or even fill feed gaps in winter and summer, when grassland growth is limited or dormant due to unfavorable weather conditions in areas with dry to semi-dry Mediterranean climates (Papanastasis et al. 2008). In addition, a mixed cropping system is suggested to be nutritionally superior to mono cropping and may provide a significant contribution to ruminants feeding system for long periods of the year. Therefore, further detailed studies are needed under field conditions to determine green biomass and nutritive value of this type of cropping system throughout the growing season and to illustrate the impact of the mixed cropping system of Medicago arborea and A. halimus from an ecological and economical point of view.

Conclusion

The undertaken study represents a new approach of managing salt-affected soil using a mixed cropping system of shrubby medic (Medicago arborea) and saltbush (Atriplex halimus):

• The combined dry matter yield of both species was considerably higher than that of solely grown shrubs.

• Intercropping M. arborea with A. halimus resulted in lower soil N uptake by the legume, and higher % Ndfa than the solely grown plants without affecting the amount of N₂ fixed.

Acknowledgements

The author would like to thank Professor I. Othman, Director General of AECS, for his support. The technical assistance of the staff at the AECS, Department of Agriculture, is greatly acknowledged.