Effects of Alternate Irrigation with Saline and Non-Saline Water on Sorghum Crop Manured with Elaeagnus angustifolia Leaves Using ¹⁵N

2.1. Soil Properties

The experiment was conducted in pots, each containing 10 kg of throughly mixed soil collected from Deir AL-Hajar research station, located south east of Damascus, Syria (36° 28' E, 33° 21' N) at 617 m above sea level. Soil was sandy clay loam in texture with pH 7.5; Electrical Conductivity (EC) 0.42 dS m^-1; organic matter 0.91%; total nitrogen 0.07%; available phosphorus 6.1 µgg^-1; ionic content (meq L^-1) chloride (Cl^-) 0.74, bicarbonate (HCO3^-) 0.97, sulfate (SO₄^--) 1.27, calcium (Ca⁺⁺) 1.1, potassium (K⁺) 0.14, sodium (Na⁺) 1.27, and magnesium (Mg⁺⁺) 0.47; Cation Exchange Capacity (CEC) 29.08 meq 100 g^-1; nitrate (NO₃^-) 32 µgg^-1 and ammonium (NH₄⁺) 16.1 µgg^-1.

2.2. Green Manure Preparation

Young leaves of Elaeagnus angustifolia used as green manure were collected from man- made plantation trees [18], oven dried at 70 ^oC for 72 hours, milled to pass through 1 mm sieve for total N and C determinations. Mean values of total N and C in leaves were 3.75% and 49%, respectively (e.g., C/N ratio was 13).

2.3. Experiment Details

Seeds of sorghum plants (Sorghum bicolor L.), were sown in pots, and set outdoors under natural climatic conditions. After germination, seedlings were thinned to two plants per pot. The design was a split plot, with manure treatments being the main treats and the irrigation systems are the sub-main. The green manure was incorporated into the soil at planting in two rates: 0 (-GM) and 64 mg N kg^-1 soil (+GM). Five types of irrigation systems abbreviated as (F, 2F:1S, 1F:1S, 1F:2S and S) were applied within these two manure treatments, where:

• F: control, irrigation only with fresh water (EC_iw ≈ 1 dS m^-1).
• 2F:1S alternate irrigation with fresh (F) and saline (S) water (EC_iw ≈ 7 dS m^-1), laboratory prepared (NaCl), at a sequence (2:1).
• 1F:1S alternate irrigation with fresh (F) and saline (S) water, at a sequence (1:1).
• 1F:2S alternate irrigation with fresh (F) and saline (S) water, at a sequence (1:2).
• S: irrigation only with a saline water (EC_iw ≈ 7 dS m^-1).

Total number of pots was 40 (i.e. 2 manure × 5 irrigation systems × 4 replicates).

At the time of each irrigation, all pots were weighed to keep soil water content around 75% of field capacity throughout the experimental period. Fresh water was used from planting up to the complete germination, thereafter; plants were subjected to the above-mentioned five different irrigation systems. The crop required a total number of irrigations equal to 18, starting from complete germination to the harvest time at a rate of 0.81 L pot^-1 per irrigation (Table 1).

An equivalent fertilizer rate of 20 kg Nha^-1 (8.5 mg N kg^-1 soil) in the form of urea enriched with 5% ¹⁵N atom excess was applied to the soils. The N fertilizer was applied to all treatments in four equally split applications at 10 days intervals after planting. This procedure was followed to stabilize the ¹⁵N enrichment of the N pool and to minimize N immobilization.

After harvest, composite soil samples representing each treatment were taken to monitor mineral nitrogen, organic matter and electrical conductivity under the different treatments.

2.4. Plant Harvest and Analysis

Sorghum plants were harvested at physiological maturity and separated into shoots, roots, and panicles. Samples were dried to a constant weight at 70 ^oC and grounded to a fine powder. Kjeldahle procedure was used to determine total nitrogen in the samples, and the ¹⁵N/¹⁴N isotopic ratio was determined by the emission spectrometry (Jasco-150, Japan). The nitrogen fractions derived from the available sources were calculated using the indirect ¹⁵N isotopic dilution method [23, 25]. This method is based on the fact that plant received unlabelled organic residues (e.g. green manure) with ¹⁵N labeled fertilizer to the soil resulted in a lower atom %¹⁵N excess in plant tissues compared to the control receiving no residues. This indicates that a dilution effect is taken place, and that a portion of sorghum N is derived from green manure. Accordingly, the percent N derived from green manure (Ndfgm) using the indirect method was calculated using the following equation:

Where, Treatment: plants grown with green manure. Control: plants grown without any green manure application.

The amount of N derived from green manure (mg N pot^-1) was calculated as follows:

The percent of N recovery from the green manure was calculated by the following equation:

Percent (%) and amount (mg N pot^-1) of N derived from fertilizer (Ndff), and N recovery from the added fertilizer were calculated using the following equations:

The percent N derived from soil (%Ndfs) was calculated as follows:

%Ndfs = 100 - (%Ndfgm + %Ndff)

Data were subjected to analysis of variance (ANOVA) test by using the statistical program Statview, and means were compared using the Least Significant Difference (LSD) test at a probability level of P <0.05.

3.1. Soil After Harvest

Table 2 shows that the Electrical Conductivity (EC) of soils increased under the different irrigation systems, relative to the control (F). Such increases were slightly higher in the unmanured treatments than those in the manured ones within the same irrigation system. In spite of the increased salinity, soils under the different treatments were classified as non-saline soils according to [26], indicating the safe employment of the different irrigation systems used in this study. On the other hand, soils mineral nitrogen content increased with increasing the number of irrigations with saline water, (i.e., increased NaCl concentration in those soils). This might be explained by the decreased nutrients assimilation with increasing soil solution salinity due to the competition between ions, particularly nitrate (NO₃^-) and chloride (Cl^-) [27, 28].

3.2. Dry Matter Production (DM) and Nitrogen Yield (NY)

Total dry matter yields were 59.9, 37.7, 26.4, 22.4 and 13.6 g pot^-1, in the F, 2F:1S, 1F:1S, 1F:2S and S treatments, respectively (Table 3). It was evident from the data that increasing the number of irrigations with saline water had negative effect on plant growth. However, GM addition increased dry matter productions by 9, 19, 43, 43 and 65% in F, 2F:1S, 1F:1S, 1F:2S and S, respectively. Moreover, there were similarity in (DM) production between (2F:1S -GM) and (1F:1S +GM); (1F:2S -GM) and (S +GM). These results reflected the positive role of GM in decreasing the harmful effect of salinity on dry matter production. Our results were in agreement with those of Rao and Pathak [24], Tejada et al. [29] and Kurdali and Al-Shamma'a [16] who mentioned the importance of GM in alleviating the negative impact of salinity on (DM) due its positive role in improving physical, chemical and biological soil properties. However, the reduction in (DM) yield caused by alternate irrigation could be attributed to a lower water availability in the rhizosphere, leading to a reduction ability of roots to absorb the water and essential nutrients required to plant growth [30]. Salinity can also cause nutrients deficiency or toxicity due to imbalance in nutrients uptake, and disturb many biological processes that affect plant growth [31-33].

Total Nitrogen Uptake (NY) by (Sorghum bicolor L.) responded to saline conditions in a manner similar to dry matter yield. The observed values were 711, 540, 403, 343 and 199 mg N pot^-1, in F, 2F:1S, 1F:1S, 1F:2S and S, respectively. The reduction in (NY) as the result of increasing the number of irrigations with saline water could be most probably due to increasing sodium chloride content in soil solution, leading to an increase in its osmotic potential and a reduction in NO₃^- and other nutrients availability [28, 33-35].

The amounts of N uptake in sorghum manured with E. angustifolia leaves were considerably higher than those of the unmanured treatments under the different irrigation systems. The percent increments of sorghum (NY) due to the addition of (GM), were 33, 30, 42, 36 and 60%, in F, 2F:1S, 1F:1S, 1F:2S and S, respectively. It could also be noticed that (GM) addition induced similarity in the amounts of (NY) in either (2F:1S +GM) and (F -GM) or (1F:2S -GM) and (S +GM). These results indicated a positive impact for the use of (GM) to enhance plant growth through providing a portion of its nitrogen demands, alleviating the adverse effect of salinity on (NY) [25, 36, 37], and increasing the available soil mineral nitrogen content [22, 38].

3.3. Nitrogen Uptake from Various Nitrogen Sources

Percentages of nitrogen derived from fertilizer (%Ndff) in sorghum plants ranged between 9.30 and 11.23%; whereas, percentages of nitrogen derived from soil (%Ndfs) ranged between 88.77 and 90.7% under the different irrigation systems. E. angustifolia leaves, applied as green manure, resulted in significant decreases in both (%Ndff) and (%Ndfs) within all irrigation systems (Table 4). Values of %N derived from green manure (%Ndfgm) were: 30.91, 29.58, 27.92, 34.11 and 49.67% in F, 2F:1S, 1F:1S, 1F:2S and S, respectively. In this regards, it can be noted that %Ndff values under manured treatments were (6-8%), approximately in the same range of %Ndff values (4-6%) reported for sorghum plants manured with Sesbania aculeata [25]. The amounts of (Ndfgm) were reduced as the result of increasing the number of irrigations with saline water. The percent reductions in the amount of Ndfgm were 29, 45, 46 and 46% as compared to the control (F +GM) in 2F:1S, 1F:1S, 1F:2S and S, respectively. These results showed no significant differences in the amounts of Ndfgm in 1F:1S, 1F:2S and S, respectively. All treatments, receiving green manure, resulted in lower amounts of Ndff and Ndfs in sorghum under all irrigation systems as compared to the unmanured treatments. This indicated that (GM) provided a substantial portion of N requirements in sorghum. Similar results were reported by Kurdali [25] and Malik et al. [37] using different kinds of green manure. The reduction in the amounts of Ndff and Ndfs as the result of added E. angustifolia leaves (GM) is useful for maintaining soil fertility and substituting considerable amounts of the costly and environmentally pollutant inorganic N-fertilizer, particularly at early growth stages of sorghum [22, 38]. However, several reports [13, 39, 40] showed an increase in the amounts of Ndfs in plants receiving (GM), such increments could be explained as the mineralized N in soil originated from (GM), which is taken by plants and referred to as Ndfgm, has already been a part of soil N pool. Therefore, the absorbed N from resources other than N-fertilizer, is the sum of Ndfgm and Ndfs, and were: 872, 647, 527, 432 and 300 mg N pot^-1 in F, 2F:1S, 1F:1S, 1F:2S and S, respectively. In comparing the aforementioned values with the amounts of Ndfs in unmanured treatments (632, 480, 358, 305 and 181 mg N pot^-1 in F, 2F:1S, 1F:1S, 1F:2S and S, respectively), the percent increments in the amounts of Ndfs were 38, 35, 47, 42 and 66%, respectively. Such results indicate a positive effect of (GM) in the enhancement of soil N pool (Ndfgm and Ndfs) to meet sorghum N demands.

Jenkinson [41] has introduced the term “Added Nitrogen Interaction” (ANI) or priming effect. ANI may result from an increase in the volume of soil exploited by the roots thereby increasing the uptake of soil N by the green manured–plants. Furthermore, Rees et al. [40] reported that such effects are likely to be a result of either increased root growth leading to a greater uptake of N, or increased N mineralization. The study of Azam [39] using an organic N source demonstrated positive added N interactions to be associated with increased root growth. In this study, a positive effect on sorghum root dry matter yield was observed following the addition of green manure (data not shown) where the percent increments ranged between (25 and 40%) as compared to the control. Moreover, there is a reason to believe that a priming effect may occur in soils because soil microorganisms react to the addition of energy-rich materials, and the increased microbial activity will involve mineralization of the soil organic matter [39]. Therefore, it can be suggested that the beneficial effect of GM on total N uptake by sorghum may be attributed to root growth stimulation (i.e. DM) and N-mineralization of added GM (i.e., Ndfgm) [39-41].

3.4. Nitrogen Fertilizer Use Efficiency (%NUE)

Several researchers reported improvement in the percentage of nitrogen fertilizer use efficiency (%NUE) following the application of non organic N-fertilizer in the presence of organic manure. Such improvement was mainly attributed to greater water availability and greater biological activity resulting from the presence of green manure [13, 14, 16]. In this study, values of NUE ranged between 22 and 96% under the different treatments (Fig. 1). The increasing number of irrigations with saline water resulted in decreases in NUE values. Moreover, the application of (GM) did not lead to an improvement in NUE, probably due to the small added amount of N (e.g., 20 kg ha^-1 as ¹⁵N) which was mainly used to estimate %Ndfgm. Regardless of GM amendments, the efficient use of N-fertilizer was considerably higher in plants irrigated with non-saline than those irrigated with saline water.

Maas and Grattan [42] found reverse relationship between soil solution salinity and plant response to fertilization. This response depends on the degree of salt stress in the root zone. Similar trends were also reported by Albassam [43] and Irshad et al. [44] who mentioned that salinity could lead to a decrease in NUE. Moreover, Esmaili et al. [33] noted a negative effect of soil salinity on mineralization process. Therefore, the decreases in the %NUE of urea fertilizer as the result of increasing the number of irrigations with saline water most probably attributed to the application of green manure of high N-content with low ¹⁴N/¹⁵N ratio and to the decreasing in the microbial activity involved in N-mineralization process as salinity increased.

3.5. Nitrogen Recovery of Added Green Manure (%NUEgm)

The efficient use of green manure decreased with increasing the number of irrigations with saline water (Fig. 2) with values being 47, 33, 26, 25 and 25% in F, 2F:1S, 1F:1S, 1F:2S and S, respectively. Approximately, the same range of %NUEgm values (20-52%) has been reported for sorghum manured with sesbania leaves grown in saline soil [13, 25]. The reduction in %NUEgm could be attributed to the increase in soil salinity (Table 2), as salinity has a negative effect on green manure use efficiency. Several workers: Rees et al. [40], Biederbeck et al. [45], Pathak and Rao [46] and Morrissey et al. [47] reported that the benefit from the green manure to be dependent on the type of green manure and soil salinity. Azam [39] showed that %NUEgm was 19.3% in rice manured with Sesbania aculeata. For other Sesbania species, Diekmann et al. [48] reported that rice utilized 46% of the N applied in the form of Sesbania rostrata.

The mineralization rate of plant material depends on a number of factors including quality and quantity of the organic matter, soil type, temperature, and moisture [15, 16]. Giller and Wilson [49] reported that the mineralization rate of plant material with a C/N ratio lower than 30 is high. The application of green manure with low C/N ratio, such as legume plants, would lead to a high mineralization rate, if the application was made even at planting [13]. On the other hand, Rietz and Haynes [50] reported that agriculture-induced salinity greatly affects soil microbial and biochemical properties. Therefore, it can be concluded that the high %NUEgm in treatments irrigated with non-saline water may be mainly attributed to a high mineralization rate of Elaeagnus angustifolia leaves which attributed to have a low C/N ratio (13) [18], as well as to greater soil microbial activity under fresh conditions.

The indirect isotopic dilution method used herein is feasible and simple for measuring N release from organic residues (e.g. green manure). Hood et al. [51] reported that estimates of the percent N derived from alfalfa residues in the ryegrass were similar for the direct and indirect methods. Similary, Kurdali [25] showed that both methods gave similar results in estimating Ndfgm as a result of adding Sesbania leaf residues (C18: N1) prior to sorghum planting.

3.6. Water Study

From Table 1 it is clear that percentages of saving fresh water were 33, 50, and 67% in 2F:1S, 1F:1S, and 1F:2S, respectively. It is noteworthy that the reduction in dry matter production of manured sorghum in 2F:1S, 1F:1S, and 1F:2S Table 3 did not exceed 50% compared with the control (F -GM) which is considered producible yield [2]. This suggests the possibility of employment the aforementioned alternate irrigation systems, according to the availability of saline and non-saline water resources, along with the economic returns aimed to farmers. However, our results revealed unsuitable use of 1F:1S or 1F:2S treatments in the unmanured sorghum plants, since the reduction in (DM) yield exceeded 50% compared with the control. This reflects the necessity of applying (GM) to enhance dry matter production when using the abovementioned alternate irrigation treatments.

The use of saline water in irrigation could substitute a significant amount of fresh water, which in turn, enables us to irrigate additional agricultural lands and increase farmer^'s income. However, under the experiment conditions, irrigation of sorghum only with saline water (S) cannot be followed as the loss in dry matter production exceeded 50% as compared with the control (F).

Overall, the use of salt tolerant N₂- fixing plants could be suggested as a promising approach to revegetate salt affected lands. Elaeagnus angustifolia can be used as part of a larger set of strategies to exploit salt-affected irrigated croplands. The desirable characteristics of this plant species (e.g. high foliage N content, ability to fix atmospheric and high salt and drought tolerent) can contribute to an effective use of its leaves and pruning residues as green manures for growth of economic crops.