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Journal of Stress Physiology & Biochemistry, Vol. 14, No. 4, 2018, pp. 16-30 ISSN 1997-0838 Original Text Copyright © 2018 by F. Kurdali
ORIGINAL ARTICLE
OPEN /71 ACCESS
Performance of Sesbania-Sunflower under Different Cropping Systems using 15N and 13C
F. Kurdali
Atomic Energy Commission of Syria, Agriculture Department P.O. Box 6091, Damascus, Syria
*E-Mail: ascientific(uaec.org.sy
Received October 14, 2018
A field experiment on sesbania (ses) and sunflower (sun) plants grown in monocropping and intercropping systems (1ses:1sun; 1ses:2sun, and 2ses:1sun, row ratio) was conducted to evaluate growth and N2-fixation using 13C and 15N natural abundance techniques. The results showed that 1ses:1sun surpassed the other treatments in terms of nitrogen (N) and dry matter yields (DM). However, 1ses:2sun gave the highest seed and oil production and showed the greatest N2-fixation. Sesbania plants in the 1ses:2sun fixed almost identical amounts of N2 in mono and intercropping systems. Moreover, soil N-uptake was the lowest among other treatments. These results give an advantage to the 1ses:2sun treatment over other treatments in terms of soil N consumption and N2 fixation to meet sesbania's N requirements. Nevertheless, 2ses:1sun seemed to be an inappropriate treatment. On the other hand, it could be suggested from %oA13C data that plants in the intercropping systems appeared not to be subjected to any abiotic stress during the growing period.
Key words: Sesbania, Sunflower, intercropping, N2- fixation, 13C, 15N
Multiple cropping system of legumes and non legumes is a traditional farming practice in many countries and represents a promising agricultural practice enabling better use of lands for sustainable agriculture (Muturi et al., 2016). Benefits of this system may arise from increased nitrogen (N), dry matter yield and greater N2 fixation efficiency (Fujita et al., 1990; Tobita et al. 1994; Kurdali et al., 1996; Kurdali 2009). However, N2 fixed by the legume component in the intercropping system depends on the species, morphology, density of legume, type of management, and the competitive abilities of the component crops (Ofori and Stern, 1987).
Sunflower was evaluated as a component of strip, row, and relay intercropping systems that would be practical for mechanized farming (Robinson, 1984). Several researchers have reported on sunflower intercropping with legumes. Robinson (1984) reported that yield was greater for some intercropping systems than from sole cropping. Fieldbean (Phaseolus vulgaris L.) as a row intercrop with sunflower increased in yield as sunflower population decreased, but its highest intercrop yield was 59% that of sole cropped fieldbean. Strip intercropping sunflower with corn or soybean compared with sole cropping showed that the percentage gain in sunflower yield was offset by about the same percentage reduction in corn or soybean yield. Shivaramu and Shivashankar (1992) reported that legumes reduced sunflower seed yield, achen per head and head diameter when they were sown at the same time. However, Kandhro et al., (2007) showed that mungbean/ sunflower intercropping can be practiced to maximize the crop production from the unit area.
Dhaincha (Sesbania aculeata L. PersJ is a fast growing leguminous plant, and is adapted to a variety of soil conditions, varying from waterlogged to saline, and from sandy to clay soils (Sandhu and Haq, 1981; Kurdali and Al-Ain, 2002; Kurdali et al., 2003). It is a native to Pakistan and India and was introduced to Syria to produce green manure (Kurdali et al., 2007) and fodder (Zarkawi, Al-Masri and Khalifa 2003) under saline and non saline conditions. Moreover, the inclusion of sesbania in multiple cropping systems with non legumes represents promising agricultural practices enabling
better use of lands for sustainable agriculture (Kurdali et al., 2003). However, Kurdali, (2009) showed that growth, N2-fixation and soil N uptake in this legume species was affected by the companion crop (e.g., sorghum as C4 or sunflower as C3). Intercropping of two rows of sesbania with one row of sorghum (2sesbania/ lsorghum) showed a greater efficiency over monocropping; whereas, the 2sesbania/1sunflower intercropping was similar to that of the monocropping (Kurdali 2009). Hence, the assessment of different planting patterns of sesbania and sunflower is required. The objectives of this field experiment were to: 1) evaluate dry matter production, total N yield and land-use efficiency in Sesbania aculeata (ses.) and Helianthus annuus L. (sun.) grown either solely or as intercrops using three different combinations of sesbania and sunflower row ratio (2ses: 1sun; 1ses: 1sun and 1ses: 2sun), 2) measure N2-fixation in sesbania using the 15N natural abundance technique, 3) assess the intraspecific competition for soil N uptake and balance under intercropping system, 4) examine the possibility of N transfer from sesbania to the associated non-legumes, and 5) determine if the carbon isotope discrimination (A13C) can be used to assess factors responsible for crop performance variability in different cropping systems.
MATERIALS AND METHODS
Site description
The field experiment was conducted at Der-Alhajar Research Station located south east of Damascus, Syria (33°21'N, 36°28' E) at 617 m. above sea level. Soil texture was sandy clay (50.4% sand, 13.1% silt and 36.5% clay) with an average of pH 8.6, Ece 0.16 dS/m, organic matter 0.82%, available P 10.8 ^g /g, NH4+ 8.3 ^g /g, NO3" 4.6 ^g /g and total N 0.7 mg/g of the top 25 cm. The average minimum temperature in winter is 1.3 °C in Jan., while it increases to an average of 36 °C during August.
Field procedures and treatments
Seeds of dhaincha (Sesbania aculata) and sunflower (Helianthus annuus) were hand sown at 3 cm depth in the first week of July, in rows with row- to- row distance of 40 cm and plant to plant distance of 40 cm for
sesbania, and 20 cm for sunflower. Two crops and three intercropping systems were studied: 1) sole crop of sesbania, 2) sole crop of sunflower, 3) sesbania and sunflower grown in alternating rows of each species (1ses:1sun), 4) species were grown in alternating two rows of sesbania with one row of sunflower (2ses:1sun), and 5) sesbania and sunflower were grown in alternating one row for the former with two rows for the latter (1ses:2sun). Plant densities were 60,000 and 120,000 plant/ha for solely grown sunflower and sesbania, respectively. In the intercropping systems, plant density in the 1ses:1sun treatment was 1/2 that of the solely grown species. Whereas, sesbania and sunflower in the 1ses:2sun were 1/3 and 2/3 that of the sole cropping plants, respectively. In contrast, they were 2/3 and 1/3 that of the sole cropping in the 2ses:1sun treatment. Barley was the previous crop. Each plot, representing one replicate, was 5x5m. Since abundant nodules have been previously observed on the roots of Sesbania aculeata grown at the same site, the seeds were not inoculated. Surface irrigation was initiated immediately after planting and preceded according to soil moisture content which was determined by neutron scattering (CPN 503 DR) in access tubes installed to 105 cm depth in a central row of each treatment (Kurdali, 2009). Intercropping system effectiveness assessment
Land equivalent ratio (LER) was used for estimating the efficiency of intercropping relative to sole cropping (Mead and Willy, 1980), and was calculated using the following equation (Offori, and Stern, 1987):
LER= (Yij/Yii)+(Yji/Yjj)
Where, Y is the yield per unit area (kg/ha), Yii and Yjj are sole crop yield for the component crops i (sunflower) and j (sesbania). Yij and Yji are mixed-crop yield.
Plant sampling and isotopic compositions analyses
Plant samples were harvested 85 days after planting (DAP) coinciding with the physiological maturity stage. Whole aboveground plant samples were collected from the corresponding subplots by cutting the main stem immediately below the cotyledonary node, then separated into its main components (leaves, stem and reproductive parts), dried at 70 °C, weighed and ground to a fine powder. Plants from the mixed stands were separated into the component species. Furthermore,
grain yield of the different plant species was determined at maturity (115 DAP). In addition, oil content was determined in sunflower seeds originated from pure and mixed stands as extractable component in Soxhelt apparatus using standard method (A.O.A.C, 1990). Concentration of total N and C, and a15N and ct13C were determined on sub-samples (7 and 2 mg dry weight for N and C determination, respectively) of leaves, stem and reproductive parts using the continuous-flow isotope ratio mass spectrometry (Integra-CN, PDZ Europea Scientific Instrument, UK). Isotopic compositions are expressed using delta notation (a) in parts per thousand
(%o):
a (%o) = [(R sample/R standard ) -1] 1000
where R is the ratio of 15N/14N or 13C/12C. The nitrogen isotope ratios are expressed relative to atmospheric air as the standard. The carbon isotope ratio in the plant sample is expressed relative to Pee Dee belemnite (PDB) standard.
Although a13C provides information on the 13C/12C in tissues, it is often preferable to express the values as leaf carbon isotope discrimination (A13C) (Farquhar, O'Leary and Beny 1982):
A13C = (a 13Cair - a 13Csample) / (1 - a 13Csample/1000)
where a 13Cair is the a 13C value in air (-8%o) and a 13Csample is the measured value in the plant.
Estimate of atmospheric N2 fixation by sesbania
The fractional contribution of fixed N2 derived from air (%Ndfa) in sesbania grown as a single crop and in intercropped stands was calculated from the 15N abundance of the legume and non N2-fixing reference plants (%o) as indicated in the following equation (Shearer and Kohl, 1986; Amarger et al., 1979; Domenach and Chalamet 1979):
%Ndfa= (a15N ref - a15N fp)/ (a15N ref - B) 100
where a15Nref represents the level of a15N detected in the reference plants (e.g., solely grown sunflower) growing on the same soil at the same time as the test legume, a15Nfp is the a15N of the test fixing plant (e.g., sesbania) and B is the value of a15N (-1.37%o) in sesbania plants that was solely dependent on atmospheric N2 (e.g., nodulated plants grown on N-free medium), (Kurdali, 2009). The sole crops of sunflower
served as a reference crop for measuring N2-fixation by sesbania plants.
Whole plant a15N(%o) was calculated as an average of shoots (Sh), stems (St) and reproductive parts (Rp) ct15N weighted by the total N content (kg) of shoots, stems and reproductive parts:
ct15N= [(Sh a15Nx Sh N) + (Sta15N x St N) + (Rp a15N x Rp N)]/ (Sh N + St N + Rp N)
Whole plant ct13C (%o) was calculated as follow: ct13C = [(Sh ct13C x Sh C) + (St ct13C x St C) + (Rp ct13C x Rp C)]/ (Sh C + St C + Rp C) Statistical analysis
The experimental design was a randomized complete block with four replicates. Data was subjected to analysis of variance (ANOVA) test, and means were compared using the Least Significant Difference (Fisher's PLSD) test at the 0.05 confidence level.
RESULTS AND DISCUSSION
Dry matter yield
It has been reported that production efficiency in intercrop systems could be improved by minimizing inter specific competition between the component crops for growth limiting factors (Offori and Stern, 1987). Therefore, a balance between non legumes and legumes in the mixed stand is desirable. Competition between component crops for growth limiting factors may be regulated by agronomic factors, such as the proportion of crops in the mixture (Offori and Stern, 1987; Kurdali et al., 2003; Yilmaz et al., 2008; Kurdali (2009) reported that when component crops in mixed cropping systems belong to different photosynthetic carbon dioxide metabolism pathway (e.g., sesbania, C3 and sorghum, C4), productivity and efficiency appeared to be determined by the more aggressive crop, usually the C4. However, the undertaken study showed that when component crops belong to the same photosynthetic pathway (C3), productivity appeared to be determined by their corresponding densities in the intercropping system. The 1ses:1sun treatment exhibited a similar distribution of total DM in the sesbania (4.7Mg/ha) and sunflower (4.9 Mg/ha), (Table 1). This result indicated that a balance in the dry matter yield between the legume and non legumes species
(C3) was obtained when they were grown in alternating rows. However, the contribution of sunflower to dry matter production (5.7 Mg/ha) was more than that of sesbania (3.7 Mg/ha) in the 1ses: 2sun treatment. In contrast, the contribution of sesbania to dry matter production (5.3 Mg/ha) in the 2ses: 1sun treatment was higher than that of sunflower (3.3 Mg/ha), (Table. 1). Similarly, the latter values were relatively close to those obtained in a previous study where DM yield of sesbania (5.9 Mg/ha) was almost two-fold that of sunflower (3.1 Mg/ha) using 2ses:1sun row ratio (Kurdali, 2009). Also, Shivaramu and Shivashankar (1992) showed that the soybean yield increased along with its increased density in mixed cropping system with sunflower. Therefore, it can be concluded that dry matter production in a given species is determined by their corresponding densities in the intercropping system and/or by the more competitive crop.
The total above ground dry matter accumulation in the solely grown sunflower was higher than that of solely grown sesbania as well as of the intercropping treatments. Both 1ses:1sun and 1ses:2sun treatments significantly (P<0.05) surpassed 2ses:1sun and solely grown sesbania in terms of total dry matter yield. Dry matter yield of each component crop within the intercropping treatment was significantly lower (P<0.05) than that of the sole cropping (Table 1). This indicates that the component crops compete with each other for the limited resources under the intercropped conditions. The reduced dry matter of the intercropped plants compared to solely grown plants in our study is a common observation in multiple cropping systems (Danso, Zapata, and Hardarson 1987; Tobita et al. 1994; Kurdali et al., 2003; Kurdali, 2009). Grain and oil yield
Grain yield data followed a similar trend to that of the dry matter yield (Table 2). Grain yield of each plant species grown in the mixed stands was lower than solely grown crops (Table 2). Moreover, oil content and yield in sunflower grown in the mixed stands were also lower than solely grown crops. Similarly, Kandel et al., (1997) showed that intercropping of hairy vetch, sweet clover alfalfa and medic with sunflower, at the same time, reduced sunflower achene's yield and head diameter.
Nevertheless, the 1ses:2sun treatment gave the greatest seed and oil production of sunflower plants comparing to the other two intercropping treatments. Intercropping system effectiveness assessment
According to Offori and Stern (1987), the land equivalent ratio (LER) directly reflects the land performance under mixed cropping as a function of plant density, competitive ability of the component crops, land management and surrounding environment and water availability (Kurdali et al., 2003; Kurdali 2009). In this study, LERs (total) in the three intercropping systems were close to one indicating that the efficiency of producing dry matter in the sesbania /sunflower intercropping was similar to that in the monocropping system (Table 3). In the 1Ses:2Sun treatment, the LERi value of sunflower (0.51) was almost equal to that of sesbania (LERj 0.47). This indicates that both plant species performed well in this mixed stand from the land equivalent ratio point of view. The 1ses:1sun was next in order where LER values were 0.44 and 0.59 in sunflower and sesbania, respectively. Nevertheless, 2ses:1sun treatment seemed not to be an appropriate treatment due to the divergence of LER values in both species, where sunflower plants had a much smaller value (0.30) comparing to that of sesbania (0.66). Similarly, these values were relatively close to those obtained in a previous study (Kurdali, 2009) confirming that 2ses:1sun row ratio is not an appropriate intercropping treatment from the land equivalent ratio point of view.
Table 3 shows that the greatest LER total value of seed yield was in 1Ses:2Sun treatment (1.07). Moreover, LER values of sunflower seeds (0.69) and oil (0.62) yields in this intercropping treatment were greater than 0.5. These results indicate that 1ses:2sun treatment was more appropriate than the others because of its higher LER value for seed and oil yield of sunflower plants. Total Nitrogen Yield
Nitrogen yield of different plant parts of sesbania, and sunflower, grown either alone or in intercropping systems is given in Table 4. Whole plant N yield of the solely grown plants did not differ from each other (P<0.05). Nitrogen yield of each component crop within
the intercropping treatment was significantly lower than that of the sole cropping. With regards to the intercropping treatments, 1ses:1sun gave the greatest N yield and together with 1ses:2sun treatment were satisfactory in terms of N uptake in both species having almost similar values. The observed values of N yield were 110 and 100 kg N/ha in the 1ses:1sun and 87.7 and 88 kg N/ha in the 1ses:2sun treatment for sesbania and sunflower plant species, respectively (Table 4). Nevertheless, 2ses:1sun treatment seemed not to be an appropriate treatment due to the divergence of N yield in sesbania (119 kg N/ha) and sunflower (56 kg N/ha) species.
Atmospheric N2 fixation
a15N (%o) values in shoots and reproductive plant parts of sesbania, and sunflower, grown either alone or in intercropping systems are given in Table 5. Whole plant a15N (%o) data is summarized in Fig.1. The a15N value of fixed nitrogen determined in sesbania plants grown on a N-free medium was -1.37±0.5. This value was significantly different from those of sunflower plants (+14%o on the average); whereas, a15N in sesbania plants ranged between -0.32 and +7.19%o. The lower a15N values in sesbania plants compared to sunflowers indicated a significant contribution of N2 fixing in this plant species. Moreover, the lower a15N values (-0.32%o) in sesbania plants grown in the intercropping system (e.g., 1ses:2sun) compared to other cropping reflected a higher %Ndfa. Similar observations were reported in previous studies, where the natural abundance of the 15N in the intercropped legumes compared with non-legumes was found to be considerably less than that of sole cropping (Berkasm et al., 1988, Kurdali, 2009).
Data of the proportion and amounts of N derived from N2-fixation (Ndfa) in the different plant parts of sesbania are given in Table 6. The percentage contribution of biological nitrogen fixation (BNF) to the amount of N accumulated by sole sesbania (45%) did not significantly differ from intercropped sesbania plants in 1Ses:1Sun (43%) and 2Ses:1Sun (44%) treatments. However, a significant increase in %Ndfa rates (93%) was obtained when growing one row of sesbania with two rows of sunflower (1ses:2sun). The enhanced % N2
fixation in the 1ses:2sun stand compared with the other stands might be attributed to the depletion of soil N resulting from the greater apparent competitiveness of sunflower for soil N, and consequently, a greater sesbania dependence on N2 fixation as previously reported for other mixed cropping systems (Izaurralde et al., 1992; Kurdali et al., 1990; Kurdali et al., 1996; Hardarson et al., 1988).
The amount of N symbiotically fixed by solely grown sesbania (83 kg N/ha) was not significantly different from that of 1ses:2sun (81 kg N/ha) in spite of higher %Ndfa values in the intercrop (Table 6). This could be attributed to the decrease in total dry matter yield when intercropped (Table 1). This observation is consistent with studies on cereal/legume intercropping systems (Danso et al., 1987; Tubita et al., 1994). Although there were no significant differences in the amount of Ndfa by sesbania plants among monocropping and intercropping (1ses:2sun) treatments, the number of rows in the mixed stand was only 1/3 of that in the pure stand. This gives an advantage to this intercropping system over sole cropping with regards to N2-fixation. Nevertheless, 1ses:1sun and 2ses:1sun treatments seemed to be unappropriate in terms of N2-fixation due to the lower amounts of fixed N (48 and 53 kg N/ha, respectively). On the other hand, the possibility of N-transferred from sesbania to the adjacent sunflower is excluded, due to the insignificant differences between ct15N values in the whole plant of sunflower plants grown in the different cropping systems (Fig. 1).
Soil nitrogen uptake
The percentage contribution of soil N (Ndfs) to the amount of N accumulated by sole sesbania (55%) did not significantly differ from sesbania plants in 1ses:1sun (57%) and 2ses:1sun (56%) treatments. However, a significant decrease in %Ndfs rate (7%) was obtained in sesbania when growing one row of sesbania with two rows of sunflower (1ses:2sun). Amount of N derived from soil in solely grown sesbania was 100 kg N/ha, whereas, it decreased to 62 and 66 kg N/ha in 1ses:1sun and 2ses:1sun treatments, respectively. However, soil N uptake by sesbania in 1ses:2sun treatment was only 7 kg N/ha (Table 6).
Soil N uptake in the 1ses:2sun treatment (95 kg
N/ha) was less than that of the other treatments (162, 122, 176 and 100 kg N/ha in 1ses:1sun, 2ses:1sun, sole sunflower, and sole sesbania, respectively) (Tables 6 & 4). Correspondingly, soil N uptake by sunflower (88 kg N/ha) in the latter intercropping system was 12.5 times greater than that of sesbania (7 kg N/ha) indicating a high competitiveness for soil N among the component crops when grown together. Such a decrease in soil N uptake by sesbania plants was associated with a higher amount of N2-fixation (81 kg/N ha). These results give an advantage to the 1ses:2sun treatment over other treatments in terms of soil N consumption and N2 fixation to meet sesbania's N requirements. Consequently, total N uptake was almost the same between the two plant species (88 kg N/ha).
Competitiveness for soil N among the component crops in the 1Ses:1Sun treatment seemed to be less than that of 1ses:2sun because soil N uptake by sunflower (100 kg N/ha) was only 1.6 times greater than that of sesbania (62 kg N/ha). However, amounts of Ndfs were almost the same between the two species (56 and 66 kg N/ha for sunflower and sesbania, respectively) in the 2ses:1sun treatment (Tables 4&6). Similarly, Kurdali et al., (2003) reported a relatively similar amount of soil N uptake when growing two rows of sesbania with one row of sorghum plants. Overall, from ecological standpoint, the best intercropping treatment seemed to be 1ses:2sun which showed greatest N2- fixation and lowest soil N consumption. Soil nitrogen balance
A positive soil N balance would be normally expressed when the total amount of N2 fixed in plant residues exceeded the total amount of N removed. Therefore, it is important to adopt suitable intercropping system to obtain a high potential yield of residual N, which gives an advantage to the subsequent crops. The total amount of fixed N2 in shoots and stems of sesbania plants were: 24, 12, 17 and 14 kg N/ha in sole sesbania, 1ses:1sun, 1ses:2sun and 2ses:1sun, respectively (Tables 6 & 7). While, for the same treatments, amounts of nitrogen absorbed from soil and stored in sesbania's pods were: 77, 50, 3, 52, kg N/ha (Table 7). Consequently, estimated N balance values of sesbania plants following pod harvest were -53, -38, +14.3 and -
38 kg N/ha for the above mentioned treatments, respectively.
Regarding sunflower plants, amounts of nitrogen absorbed from soil and stored in the reproductive parts were 105, 66, 57 and 38 kg N/ha in sole sunflower, 1ses:1sun, 1ses:2sun and 2ses:1sun, respectively
(Tables 4 & 6). Consequently, a negative N balance resulted in all treatments. The net soil N balance in different cropping system were -43, -53, -76, -104 and -105 kg N/ha for 1ses:2sun, sole sesbania, 2ses:1sun, 1ses:1sun and sole sunflower, respectively. However, the best treatment was 1ses:2sun because of its highest N balance and N2-fixation.
Table 1. Dry matter yield (Mg/ha) in leaves, stems, reproductive parts and whole plant of sesbania (Ses) and sunflower (Sun) grown either solely or as intercropping systems
Cropping system leaves stem Rep. parts Whole plant
(A) Comparisons among cropping systems for each plant species
Sesbania
Sole 0.91a 2.81a 4.3a 8.00a
(1ses:1sun) 0.53c 1.66b 2.51b 4.69b
(1ses:2sun) 0.46c 1.33c 1.95c 3.73c
(2ses:1sun) 0.68b 1.82b 2.75b 5.25b
LSD 0.05 0.13 0.29 0.46 0.76
Sunflower
Sole 2.48a 4.29a 4.34a 11.12a
(1ses:1sun) 1.20b 1.82c 1.91b 4.93c
(1ses:2sun) 1.23b 2.45b 2.05b 5.71b
(2ses:1sun) 0.72c 1.29d 1.30c 3.30d
LSD 0.05 0.23 0.29 0.36 0.50
(B) Comparisons among cropping systems
Sole crop
Sesbania 0.91d 2.81c 4.28a 8.00c
Sunflower 2.48a 4.29a 4.34a 11.12a
intercropped
(1ses:1sun) 1.73b 3.47 b 4.42a 9.62b
(1ses:2sun) 1.67b 3.78b 4.00a 9.44b
(2ses:1sun) 1.39c 3.11c 4.05a 8.56c
LSD 0.05 0.23 0.35 N.S 0.77
Note. For each crop species (A), and the cropping systems (B), means within a column followed by the same letter are not significantly different (P>0.05).
■ Sesbania ■ Sunflowers
IS
-4
fix. sole lses:lsuri lses:2sun 2ses:lsun
Figure 1. ö15N (%o) values of the entire plants of sesbania (Ses) and sunflower (Sun) grown either solely or as intercropping systems. ö15N (%o) of N2-fixed (fix) was obtained from sesbania plants grown on N-free medium. Bars mean standard deviations.
Table 2. Seed yield (Mg/ha) of sesbania (Ses), and sunflower (Sun) grown either solely or as an intercropping system, in addition to oil content and yield in sunflower seeds under different cropping systems
Cropping system Sesbania Sunflower
Seed yield (Mg/ha) Seed yield (Mg/ha) Oil content (mg/g) Oil yield (Mg/ha
Sole 1.58a 3.03a 478a 1.45a
(1ses:1sun) 0.93b 1.38c 437b 0.60c
(1ses:2sun) 0.59c 2.09b 427b 0.89b
(2ses:1sun) 1.02b 1.06d 431b 0.46d
LSD 0.05 0.17 0.26 22.04 0.13
Note. Means within a column followed by the same letter are not significantly different (P>0.05).
Sesbania ■ Sunflowers
Si
20 19.5
19 18.5 18 17.5 17 16.5 16 15.5 15
H
sole
lses:lsuri lses:2sun 2ses:lsun
Figure 2. Carbon isotope discrimination (A13C%o) in the whole plants of sesbania (Ses) and sunflower (Sun) grown either solely or as intercropping systems. Bars mean standard deviations.
Table 3. Land Equivalent Ratio (LER) of the dry matter, seed and oil yields for sunflower (LERi) and Sesbania (LERj) either solely or as intercropping systems
LER Intercropping
1ses:1sun 1ses:2sun 2ses:1sun
Total dry matter yield
LERi (sunflower) 0.44 0.51 0.30
LERj (sesbania) 0.59 0.47 0.66
LER (total) 1.03 0.98 0.96
Seed yield
LERi (sunflower) 0.46 0.69 0.35
LERj (sesbania) 0.59 0.38 0.65
LER (total) 1.05 1.07 1.00
Oil yield
LERi (sunflower) 0.42 0.62 0.31
Table 4. Nitrogen yield (kg N /ha) in leaves, stem, reproductive parts and whole plant of sesbania (Ses) and sunflower (Sun) grown either solely or as intercropping systems
Cropping system leaves stem Rep. parts Whole plant
(A) Comparisons among cropping systems for each plant species
Sesbania
Sole 28.7a 18.3a 136.3a 183.3a
(1ses:1sun) 13.5c 10.5b 86.0bc 110.1b
(1ses:2sun) 12.4c 8.5b 66.7c 87.7c
(2ses:1sun) 18.4b 10.1b 90.6b 119.1b
LSD 0.05 4.2 4.5 19.3 21.9
Sunflower
Sole 50.4a 20.5a 105.0a 175.9a
(1ses:1sun) 25.3b 8.8b 65.7b 99.8b
(1ses:2sun) 21.8b 9.3b 57.2b 88.2b
(2ses:1sun) 12.3c 5.7b 37.9c 55.8c
LSD 0.05 8.7 3.8 14.7 20.6
(B) Comparisons among cropping systems
Sole crop
Sesbania 28.7c 18.3a 136.3ab 183.3b
Sunflower 50.4a 20.5a 105.0c 175.9b
intercropped
(1ses:1sun) 38.8b 19.4a 151.7a 209.9a
(1ses:2sun) 34.2bc 17.8a 123.9bc 175.9b
(2ses:1sun) 30.7bc 15.7a 128.6b 175.0b
LSD 0.05 8.4 4.8 20.9 25.8
Note. For each crop species (A), and the cropping systems (B), means within a column followed by the same letter are not significantly different (P>0.05).
Carbon isotope discrimination
Data presented in Table 5 showed that 13C discrimination (A13C%o) in the above ground plant materials (shoots, stems and reproductive parts) was affected by the plant species and the cropping system. Moreover, A13C%o values of the whole plants are shown in Fig.2. It has been reported that carbon isotope discrimination (A13C%o) was considered to be a powerful tool for studying the effect of different abiotic parameters on C3 plant stomata functioning. Water deficit, temperature, salinity and sunlight (Farquhar et al., 1989; Ehleringer et al., 1986) decrease %oA13C through their influence on stomatal aperture. In this study, %oA13C values did not decrease in
the intercropping compared to monocropping treatments, indicating that, under prevailing conditions, plants in the intercropping systems appeared not to be subjected to any abiotic stress during the experimental period. On the other hand, in some cases, whole plant %oA13C values in mixed stands (e.g., 1ses:2sun) were higher than that of solely grown plants. Such increments may be related to the amount of light intercepted by the component crops in the mixed stands. Ehleringer et al., (1986) reported that carbon isotope discrimination decreased with increasing sunlight Correspondingly, Kurdali, (2009) suggested that stomata of the partly shaded leaves in mixed stands were more opened and carbon isotope discrimination values were higher.
Table 5. Natural abundance of nitrogen (515N%o) and carbon isotope discrimination (A13C%o) in leaves, stems and reproductive parts of sesbania (Ses) grown either alone or as intercropping systems with sunflower (Sun)
Cropping system leaves stem Rep. parts
Natural abundance of nitrogen (ö15N%o)
Sesbania
Sole +6.89a +4.50ab +7.17a
(1ses:1sun) +6.89a +6.07a +7.37a
(1ses:2sun) -0.41b +3.62b -0.82b
(2ses:1sun) +7.51a +4.03b +7.3a
LSD 0.05 1.7 1.7 1.74
Sunflower
Sole +12.35ab +13.31ab +14.40a
(1ses:1sun) +13.72ab +13.58ab +15.70a
(1ses:2sun) + 9.28b +16.79a +14.06a
(2ses:1sun) +14.20a +10.17b +15.63a
LSD 0.05 4.6 4.6 N.S
Carbone isotope discrimination (A13C%)
Sesbania
Sole 16.37b 16.50c 15.71a
(1ses:1sun) 17.13a 16.65bc 15.86a
(1ses:2sun) 16.44b 18.36a 16.38a
(2ses:1sun) 17.46a 17.68b 16.06a
LSD 0.05 0.71 0.47 N.S
Sunflower
Sole 18.86b 18.26b 18.16ab
(1ses:1sun) 18.71b 18.01b 17.63b
(1ses:2sun) 18.45b 19.96a 18.38a
(2ses:1sun) 19.52a 19.04a 17.97ab
LSD 0.05 0.53 0.71 0.72
Note. For each crop species, means within a column followed by the same letter are not significantly different (P>0.05). Mean value of ö15N of fixed N2 determined in sesbania plants grown on N-free medium was %o-1.37 (Kurdali 2009)
In sesbania plants, the inclusion of sunflower in the intercropping stands; particularly in 1ses:2sun treatment is accompanied with increments of A13C values (Fig. 2) and with a decrease of ct15N (Fig.1). Such an observation was previously reported by Kurdali and Al-Shammaa (2010) in lentil plants subjected to different soil moisture levels. It is worthy to mention that the decline of ct15N was an
indication of dilution of the 15N isotope through a higher N2 fixation. Moreover, the increase of A13C values (Fig. 2) was associated with an enhancement in the amount of N2 fixed (Table 7). This observation is consistent with that of Knight et al., (1993) who reported a positive correlation between A13C and the amount of N2 fixed in lentil inoculated with different strains of rhizobia.
Table 6. Proportions (%) and amounts (kg N/ha) of nitrogen derived from atmosphere (Ndfa) and from soil (Ndfs) in leaves, stems, reproductive parts and the whole plant of sesbania aculeata (Ses) grown either solely or as intercropping systems with sunflower (Sun)
Cropping system leaves stem Rep. parts Whole plant
%Ndfa
Sole 45.3b 61.1ab 43.5b 45.3b
(1ses:1sun) 45.3b 50.7b 42.2b 43.4b
(1ses:2sun) 93.6a 67.0a 96.4a 93.1a
(2ses:1sun) 41.3b 64.3a 42.6b 44.2b
LSD 0.05 11.2 11.5 11.5 8.9
Ndfa (kg/ha)
Sole 12.7a 11.2a 59.1a 83.0a
(1ses:1sun) 6.1b 5.6b 36.0b 47.6b
(1ses:2sun) 11.6a 5.7b 63.7a 81.1a
(2ses:1sun) 7.5b 6.5b 38.8b 52.8b
LSD 0.05 1.9 3.5 14.0 15.6
%Ndfs
Sole 54.7a 38.9ab 56.5a 54.7a
(1ses:1sun) 54.7a 49.3a 57.9a 56.6a
(1ses:2sun) 6.4b 33.0b 3.7b 6.9b
(2ses:1sun) 58.8a 35.7b 57.4a 55.8a
LSD 0.05 11.2 11.5 11.5 8.9
Ndfs (kg/ha)
Sole 16.0a 7.1a 77.2a 100.3a
(1ses:1sun) 7.4b 5.0b 50.1b 62.4b
(1ses:2sun) 0.8c 2.8c 3.0c 6.6c
(2ses:1sun) 10.9b 3.6bc 51.9b 66.3b
LSD 0.05 4.3 1.5 15.7 15.7
Note. Means within a column followed by the same letter are not significantly different (P>0.05).
Table 7. Nitrogen balance (kg/ha) in sesbania (ses) and sunflower (Sun) grown either alone or as intercropping systems
N-Status Sole cropping intercropping systems
1ses:1sun 1ses:2sun 2ses:1sun
Ses Sun Ses Sun Ses Sun Ses Sun
N input From N2 fixation (leaves and stem) +24 - +12 - +17 - +14 -
N loses from soil in reproductive parts -77 -105 -50 -66 -3 -57 -52 -38
N balance in each species -53 -105 -38 -116 +14 -57 -38 -38
Net N balance in the cropping systems -53 -105 -104 -43 -76
CONCLUSUSION
This study provides valuable information on the impact of intercropping systems of dhaincha (Sesbania aculeata) and sunflower (Helianthus annuus) on their growth and N2-fixation by the legume, and the interaction between both species for soil N uptake. The following conclusions were obtained from this research: From a productivity standpoint (dry matter, N, seed and oil yield):
The 1ses:1sun surpassed the other treatments in terms of N and DM yields and exhibited a similar distribution of total DM and N uptake in the sesbania and sunflower plant species. The 1ses:2sun was second in terms of DM and N uptake showing also a similar distribution of total N in both plant species.
The 1ses:2sun gave the highest seed and oil production and together with 1ses:1sun treatment, were satisfactory in terms of LER for DM in both species having almost similar values. However, the former treatment was more appropriate than the latter because of its higher LER value for seed and oil yield of sunflower plants.
The 2ses:1sun treatment seemed to be inappropriate due to the divergence of LER values in both species, where sunflower plants had a low value as compared to sesbania.
From an ecological standpoint (N2-fixation, soil N-uptake and balance):
The o15N method seemed to be suitable for estimating symbiotic nitrogen fixation in sesbania plants grown in monocropping or intercropping systems with non-legumes. The best treatment was 1ses:2sun which showed the maximum N2-fixation. Sesbania plants fixed almost identical amounts of atmospheric N2 in both the monocropping and intercropping systems although the density of these plants in the latter was only 1/3 that of the former system.
Soil N-uptake in the 1ses:2sun was the lowest among other treatments. These results give an advantage to the 1ses:2sun over other treatments in terms of soil N consumption and N2 fixation to meet sesbania's N requirements.
Based on %oA13C data, plants in the intercropping systems appeared not to be subjected to any abiotic
stress.
ACKNOWLEDGEMENT
I would like to thank Professor I. Othman, General Director of the Atomic Energy Commission of Syria (AECS) for his support. The technical assistance of the staff at the AECS Department of Agriculture is greatly acknowledged.
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