Interplanting Some Soybean Cultivars with Mandarin Trees in Sandy Soil

Two-year field experiments were carried out at El-Kassasen Horticultural Research Station, Agricultural Research Center (ARC), Ismailia government, Egypt, during 2018 and 2019 summer seasons to evaluate some soybean cultivars for interplanting with mandarin trees to achieve high productivity of both crops, land usage and profitability under sandy soil conditions. The treatments were the combinations of two cropping systems (interplanting and solid cultures) and five soybean cultivars (Giza 21, Giza 22, Giza 35, Giza 82 and Giza 111). A strip plot design with three replications was used. Interplanting soybean cultivar Giza 22 with mandarin trees gave a higher total count of rhizobia in rhizosphere of mandarin roots after 75 days from soybean sowing than the other treatments in both seasons. The highest fruit weight and volume, total soluble solids (T.S.S.), fruit yields per tree and per ha were obtained by growing soybean cultivar Giza 22 or Giza 111 with mandarin compared with the other treatments in both seasons. With respect to soybean crop, interplanting soybean with mandarin trees decreased percentages of light intensity at the middle and bottom of the plant, chlorophylls a and b, as well as, plant dry weight after 75 days from soybean sowing compared with those of solid culture in both seasons. Soybean cultivars Giza 22 and Giza 82 had higher light intensity at the middle and bottom of the plant, as well as chlorophyll a, meanwhile soybean cultivars Giza 22 and Giza 111 had higher plant dry weight than the other soybean cultivars after 75 days from soybean sowing in both seasons. Soybean cultivar Giza 22 and Giza 111 had higher plant dry weight than the other soybean cultivars under interplanting and solid plantings in both seasons. Interplanting soybean with mandarin trees decreased soybean seed yield and its attributes compared with soybean solid culture in both seasons. Soybean cultivars Giza 111 and Giza 22 gave a higher number of pods per plant, seed yields per plant and per ha than the other cultivars in both seasons. Soybean cultivars Giza 111 and Giza 22 followed by Giza 82 recorded a higher number of pods per plant, seed yields per plant and per ha under interplanting and solid cultures than the other treatments in both seasons. Interplanting soybean cultivar Giza 22 and Giza 111 with mandarin trees achieved higher LER, LEC, total return and MAI than solid culture of mandarin. Growing four ridges of soybean cultivars Giza 22 or Giza 111 between mandarin trees cultivar Fremont had higher productivity, land usage and profitability than mandarin solid culture under sandy soil conditions.


Introduction
Citrus is a genus from Rutaceae family, subfamily Aurantoideae1 and there are several species in this genus; but there are major species such as sweet orange (Citrus sinensis (L.) Osbeck), mandarins group, grapefruits (Citrus paradisi), lime (Citrus aurantifolia) and sour orange (Citrus aurantium L.) as reported by Salem and Sheta [1]. Citrus is an important cash crops and an essential source of vitamin C for human diet. Citrus trees are of outstanding economical importance among fruit crops in Egypt. Mandarin (Citrus reticulata Blanco) is popular among citrus fruit trees grown in Egypt. Mandarin occupies the second planted citrus species after orange [2] and Egypt represents about 15% of the total citrus production in the Mediterranean Basin and occupies the 3rd rank after Spain and Italy. Mandarin fruit is the cheap and rich source in vitamin C and A, as well as it contains small amounts of vitamin B6, carbohydrates and proteins. Usually, the fruits are eaten fresh; so internal fruit quality like fruit weight, peel colour, juice quantity and quality and easy peeling play an important role in fruit marketing. Adjusting nitrogen (N) nutrition of citrus is considered an important and limited factor for improving production and fruit quality [3]. In this concern, Helail et al. [4] concluded that fertilizing Balady mandarin trees with different sources of N fertilizers enhanced tree fruiting traits expressed as fruit set, fruit drop, yield and fruit quality. Particularly, the major mandarin production in Egypt is confined to the local cultivar Balady that belongs to common Mediterranean mandarin group. Thus, the global production of mandarins in 2016 reached 32.8 million ton [5].
On the other hand, Lachungpa [6] revealed that interplanting some crops with mandarins provided farmers with increased food security and opportunities for cash flow. Thus, intercropping legumes in citrus orchards is beneficial for the citrus production. According to Srivastava et al. [7] and Aziz et al. [8], legumes improved fruit yield compared with sole orchard and such crops increased the yield of the main crop by fixing N biologically in the soil. The legume has been rated as the highest contribution of biological N fixation among the seed producers, with reports of rates of up to 450 kg N/ha [9]. About 90% of the species within the family Leguminosae (Fabaceae) can fix atmospheric N through a symbiotic association with soil bacteria known as rhizobia [10]. Currently, the subject of biological N fixation is of great practical importance because the use of mineral N fertilizers has resulted in unacceptable levels of water pollution [11] [12]. It has been reported that various cultivars of legumes show significant differences in their ability to support BNF. Although N fixed by heterotrophic free-living bacteria is of minor importance as a mechanism for N input in arid soils [13], the potential of symbiotic N fixers is significantly higher than of those that are free living. Thus, the rate of potential N fixation is estimated by either nodule biomass [14], or aboveground biomass [15]. Consequently, a hallmark trait of legumes is their ability to develop root nodules and to fix N in symbiosis with compatible rhizobia [16]. However, biological N fixation itself is considered a sensitive indicator that precedes declining soybean [Glycine max (L.) Merrill] growth in response to environmental stressors [17]. Shading of citrus trees resilience in soybean has naturally been linked to bacterial driven N fixation. Most of N requirement for soybean is supplied through the N-fixation process, which is a result of a beneficial relationship between the plant and specific soil bacteria.
Early maturing soybean cultivars often have reduced N-fixation ability and shorter N-fixation periods than full-season cultivars [18]. Active fixation usually begins within 10 -14 days later, around second and three trifoliate growth stages, at which point they can supply most of the plant's N requirements [19]. Accordingly, BNF in crop legumes not only reduces fertilizer costs but also improves soil fertility through crop rotation and intercropping. Therefore, the objective of this investigation was to evaluate some soybean cultivars for interplanting with mandarin trees to achieve high productivity of both crops, land usage and profitability under sandy soil conditions.

Soybean Cultivars
Five soybean cultivars (Giza 21, Giza 22, Giza 35, Giza 82 and Giza 111) were used. A strip plot design with three replications was used. Cropping systems were randomly assigned to the vertical strips and soybean cultivars were allocated in the horizontal strips. Each strip plot was 48 m 2 (8 m in length and 6 m in width). Soybean cultivars were sown on May 28th and 23rd at 2018 and 2019 summer seasons, respectively. Mechanical and chemical analyses of the soil (0 -60 cm) were done by Water, Soil and Environment Research Institute, ARC (Table 1). Mechanical and chemical properties of the soil were determined using the methods described by Chapman and Pratt [20]. The average monthly temperature for the two years ranged from 16.8˚C to 35.9˚C in the first season and from 17.2˚C to 36.4˚C in the second season, the average relative humidity ranged from 50.0% to 61.0% in the first season and from 50.0% to 63.0% in the second season, meanwhile, the average sunshine hours ranged from 9.0 to 12.5 hours (Table 2). Table 3 shows pedigree, maturity group and growth habit of the tested soybean cultivars.
Drip irrigation system was used in all tested treatments by separated nets for each crop owing to control the amounts, time and methods of supply the fertilization (fertigation) request. Drip irrigation system was used in this study established on both sides of the tree trunk at a distance of one meter. Each tree provided with two droppers (discharge 4 L/h) and the time of operation was 4 hours/day (32 L/tree/day) throughout the period of study. At the beginning of each season, the experimental trees which subjected to solid or intreplanting culture received 0.5 kg calcium super phosphate (15.5% P 2 O 5 ) per tree mixed with 10 kg/tree organic manure added in rounded trenches close to the root system around the tree canopy. In addition, Nitrogen (N) fertilizer was added at a rate of 178.5 and 357.0 kg N per ha as urea (46

The Studied Mandarin Traits
Total count of rhizobia in rhizosphere of mandarin roots (colony forming unit "cfu"/g soil) after 75 days from soybean sowing was performed Regional Center for Food & Feed, Agricultural Research Center, Giza, Egypt. The culture medium was yeast extract mannitol agar, counting method was done by dilution plate count and incubation condition was 30˚C/2 -3 days. Methods of microbial analysis were described by Alexander and Clark [21]. At harvest, samples of ten fruits per tree were collected at harvesting date from plot to determine some fruit traits; fruit weight (g), fruit volume (cm 3 ), total soluble solids (T.S.S. %) of the fruit was determined by using Zeiss hand refractometer, fruit yield per tree (kg) and fruit yield per ha (ton).

The Studied Soybean Traits
The traits on vegetative growth at 75 days of soybean sowing recorded on five plants from each plot were as follows: light intensity (lux) inside each canopy (at the middle of the plant and at the bottom of the plant at 20 cm from the soil surface) by Lux-meter apparatus at 12 h and expressed as percentage from light intensity measured above the plant. Leaf chlorophyll a and b contents (mg/g leaves fresh weight) and plant dry weight (g) were recorded as analyzed by the General Organization for Agricultural Equalization Fund, ARC, Giza, Egypt, according to Holden [22]. At harvest, the following traits were measured on ten plants

Competitive Relationships 1) Land equivalent ratio (LER):
LER is the ratio of area needed under sole cropping to one of interplanting at the same management level to produce an equivalent yield [23]. LER is calculated as follows: 2) Land Equivalent coefficient (LEC): LEC is a measure of interaction concerned with the strength of relationship [24]. It is calculated as follows: LEC = La × Lb, where La = relative yield of crop a (mandarin) and Lb = relative yield of crop b (soybean).

Financial Return (US$ per ha)
It was calculated as a difference between total net returns from interplanting and solid cultures. Soybean seeds and mandarin fruits prices presented by market price (2018)

Statistical Analysis
Analysis of variance of the obtained results of each season was performed. The measured variables were analyzed by ANOVA using MSTATC statistical package [26]. Mean comparisons were performed using the least significant differences (L.S.D) test with a significance level of 5% [27].

Total Count of Rhizobia in Rhizosphere of Mandarin Roots after 75
Days from Soybean Sowing Total rhizobia count in rhizosphere of mandarin trees after 75 days from soybean sowing was affected significantly by interplanting soybean cultivars with mandarin trees in both seasons (Table 5)   With respect to soybean cultivar Giza 111, it had higher leaves dry weight per plant (late maturing cultivar) than the other soybean cultivars [29], probably resulted in higher biological N fixation ability during vegetative growth and development. Accordingly, it is expected that nodules ability in roots of soybean cultivar Giza 22 and Giza 111 had a longer duration than those of the other soybean cultivars (Table 3) led to differences in their harvested dates (Table 4). At the time of pod fill, nodules on annual legumes generally lose their ability to fix N, because the plant feeds the developing seed rather than the nodule [30].

Mandarin Fruit Yield and Its Attributes
All the studied mandarin traits were affected significantly by interplanting soybean cultivars with mandarin trees in both seasons (  interplanting soybean with mandarin that reduced soil pH and thereby increased availability of other elements in the soil. According to Lovatt et al. [31], higher contents of N in the buds and leaves increased the number of flowers per tree, which explained the increase in fruit yield per tree. It is known that, legumes played a major role in the farming system [32]. Total soil N was enhanced, especially legumes in sole or intercropping culture because of symbiotic atmospheric N fixation [33].
So, it is expected that total soil N was been enhanced in rhizosphere of mandarin roots by interplanting with soybean cultivar Giza 22 or Giza 111 compared with the others where the populations of rhizobia were considered as a biological pool influencing soil nutrient dynamics. Phosphorus improved growth, nutritional status of trees, yield and fruit quality of citrus, macronutrient in nutrition of citrus trees regulated large-scale production of high fruit quality [34]. Particularly, Postgate [35] showed that the fixed N is available to other plants and helps to fertilize the soil. On the other hand, rhizobia promoted plant growth by solubilization of mineral phosphates and other nutrients [36]. N fixing bacteria,

2) Soybean cultivars
Percentages of light intensity at the middle and bottom of the plant, chlorophylls a and b, as well as, plant dry weight after 75 days from soybean sowing were different significantly among soybean cultivars in both seasons (Table 7).  (Table 7). It is worthy to note that maturity group of all the studied soybean cultivars played a major role in dry matter accumulation, where the early maturing cultivars Giza 35 and Giza 82 gave lower dry matter accumulation as a result of completed their life cycle quickly than the late maturing cultivars (Table 3).

) The interaction between cropping systems and soybean cultivars
Percentages of light intensity at the middle and bottom of the plant, chlorophylls a and b, as well as plant dry weight after 75 days from soybean sowing were affected significantly by the interaction between cropping systems and soybean cultivars in both seasons (Table 7). Interplanting all the studied soy-  (Table 3), BNF ability was similar between them (Table 5).

Seed Yield and Its Attributes 1) Cropping systems
Plant height, numbers of branches and pods per plant, seed yields per plant and per ha were affected significantly by the cropping systems in both seasons (Table 8). Interplanting soybean with mandarin trees increased plant height but it decreased numbers of branches and pods per plant, seed yields per plant and per ha compared with those of solid one in both seasons. Interplanting soybean with mandarin trees increased plant height by 12.04% and 12.37% in the first and second seasons, respectively, compared with soybean solid planting.
It is obvious that interplanting formed unfavorable conditions for soybean plants which reflected on the severe decrease in intercepted light intensity by soybean canopy during soybean growth (Table 7) and consequently more amounts of plant hormones. So, the observed response in plant height of soybean may be primarily attributed to an increase of internode elongation of shaded soybean plant as a result of increasing plant hormones [29]. It is important to note that interplanting soybean with mandarin trees increased shading intensity around soybean canopy and consequently lower dry matter accumulation in the other parts of soybean plant. According to Board and Harville [40], pod number per plant as the yield component was most influenced by changing cultural and environmental conditions. It is known that shading during early flowering stage decreased assimilates availability to the developing reproductive structures, and thereby pod number at harvest [41]. These results are in accordance with those obtained by Abdel-Wahab and Abd El-Rahman [42] who showed that intercropping decreased the number of pods per plant. Also, shading of mandarin trees decreased seed yield per plant by 16.57% and 14.83% in the first and second season, respectively, seed yield per ha by 59.41% and 58.13% in the first and second seasons, respectively compared with soybean solid culture. This may be due to the adverse effects of interplanting increased inter-specific competition between mandarin and soybean plants for basic growth resources, which reflected negatively on soybean productivity per unit area compared with soybean solid culture. These results are in the same context  [43], Egbe [44] and Metwally et al. [45] who investigated that intercropped soybean produced lower seed yield than their sole crop counterparts.

2) Soybean cultivars
Soybean cultivars differed significantly for plant height, seed yields per plant and per ha, meanwhile numbers of branches and pods per plant were not different among soybean cultivars in both seasons (Table 8) 3

) The interaction between cropping systems and soybean cultivars
The interaction between cropping systems and soybean cultivars affected significantly all the studied traits of soybean in both seasons, except plant height in the second season (Table 8) (Table 7).
It is likely that canopies architectures of soybean cultivars Giza 111 and Giza 22 were more compatible with adverse effects of mandarin shading, which reflected positively on their productivity per unit area. With regard to soybean cultivars Giza 82, it had self regulation mechanism of redistributing the available assimilates to components, in an attempt to maintain or improve yield under high shading intensity [42]. These data show that each of these two factors acts dependently on all the studied traits of soybean. These results are in accordance with those observed by Gadallah and Selim [45] and Metwally et al. [46] who found that soybean cultivars responded deferentially to the cropping system.

Land Equivalent Ratio (LER)
The values of land equivalent ratio (LER) were estimated by using data of recommended solid cultures of both crops. LER of more than 1.00 indicates yield advantage, equal to 1.00 indicates no gain or no loss and less than 1.00 indicates yield loss [47]. It can be used both for replacement and additives series of interplanting. The results obtained strongly coincided with the definition of LER.
Land equivalent ratio values were greater than one for all soybean cultivars under interplanting with mandarin trees in both seasons (Table 9 and Figure 1). crop to produce the same yield as interplanting).
The advantage of the highest LER by interplanting soybean cultivars Giza 22 and Giza 111 with mandarin trees probably due to the late maturing cultivar Giza

Total Return
The financial return of interplanting soybean with mandarin trees compared with solid culture of mandarin is shown in Table  Giza 111 with mandarin trees is more profitable than solid culture of mandarin for Egyptian farmers. These results are in agreement with Lachungpa [6].

Monetary Advantage Index (MAI)
The economic performance of the interplanting was evaluated to determine if soybean and mandarin combined yields are high enough for the farmers to adopt

Conclusion
It can be concluded that improving production and fruit quality of mandarin