Growth, Nitrogen and Phosphorus Uptake of Sorghum Plants as Affected by Green Manuring with Pea or Faba Bean Shell Pod Wastes Using ¹⁵N

2.1. Soil Properties

The experiment was conducted in pots, each containing 10 kg of soil collected from Deir AL-Hajar research station, located southeast of Damascus, Syria, (36° 28' E, 33° 21' N) at 617 m above sea level. The area is located within a semiarid region in which the average annual rainfall reaches 120 mm year^-1, and most precipitations occur between November and early April. For the last ten years, the average minimum temperature in winter was 1.3°C in January, while it increases to the average maximum temperature of 38.3°C in July. Some climatic data for the studied site collected during the growing season were close to those averaged over the last 10 years (2008 to 2017) as shown in Table 1. The soil is classified as clay loam, with an average 57.9% clay, 39.5% silt, and 2.6% sand. The main physical and chemical soil properties were pH 7.74; Electrical Conductivity (EC) 0.32 dS m^-1; organic matter 0.91%; total nitrogen 0.07%; available phosphorus 6.2 µgg^-1; ionic content (meq L^-1): chloride (Cl^-) 0.74, bicarbonate (HCO₃^-) 0.97, sulfate (SO₄^--) 1.27, calcium (Ca⁺⁺) 1.15, 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^-) 9.5 µgg^-1 and ammonium (NH₄⁺) 6.8 µgg^-1.

2.2. Chemical Composition of Green Manure and Treatments

Shell pod wastes of pea (PGM) and faba bean (FGM) were used as a source of Green Manure (GM) for the growth of sorghum plants. After a young pod harvest of pea and faba bean, the green seeds were removed. Pod shells residues of both legumes’ species were cut into 3 - 5 cm pieces, weighed, dried, and ground to a fine powder for total N determination by Kjeldahl method, in four replicates. The mean value of total N was 2.3% and 2.86% for faba bean and pea shell pod wastes, respectively. Moreover, total polyphenols in PGM and FGM was determined by the Folin-Ciocalteau method [18]. Lignin was determined using the Acid Detergent Fiber (ADF) method as outlined by Rowland and Roberts [19]. Total carbon in both pod shells was determined by a mass spectrometer (Integra-CN, PDZ Europea Scientific Instrument, UK). Data of polyphenols, lignin, ratio of polyphenols to total N, ratio of lignin /N, ratio of (lignin + polyphenols) to total N and C/N ratio are shown in Table 2.

Pea and faba bean pod shells were incorporated into the soil, fifteen days before sorghum planting, at equivalent rates of 0, 50, 100, and 150 kg N ha^-1 (Table 3). They were mixed with the soil of each pot as GM by adopting the following treatments:

• (PGM₀) or (FGM₀) control, without green manure (N0).
• (PGM₅₀) addition of shell pod wastes of pea at a rate of 50 kg N ha^-1 (N50).
• (PGM₁₀₀) addition of shell pod wastes of pea at a rate of 100 kg N ha^-1 (N100).
• (PGM₁₅₀) addition of shell pod wastes of pea at a rate of 150 kg N ha^-1 (N150).
• (FGM₅₀) addition of shell pod wastes of faba bean at a rate of 50 kg N ha^-1 (N50).
• (FGM₁₀₀) addition of shell pod wastes of faba bean at a rate of 100 kg N ha^-1(N100).
• (FGM₁₅₀) addition of shell pod wastes of faba bean at a rate of 150 kg N ha^-1(N150).

2.3. Planting Procedures and ¹⁵N Application

Seeds of sorghum plants (Sorghum bicolor L.) were sown in pots containing soil previously amended with GMs (i.e. 15 days before sowing). After germination, seedlings were thinned to two plants per pot. Since soil contained a small amount of available phosphorus, an equivalent amount of 100 kg P₂O₅ ha^-1 in the form of triple phosphate was applied to the soil prior to planting to all treatments. Pots were weighed every three days and water was added to maintain the soil moisture content at around 75% of field capacity throughout the experimental period. Pots were arranged in a randomized complete block design in four replicates and set outdoors under open field conditions.

To estimate nitrogen derived from green manure, using the indirect ¹⁵N isotopic dilution method, an equivalent fertilizer rate of 10 kg N ha^-1 (43.48 mg N pot^-1) in the form of urea enriched with 9.63% ¹⁵N atom excess was applied to the soils. The N fertilizer was applied to all treatments in two equally split applications (5 kg N ha^-1 for each application) at ten days intervals after planting. This procedure was followed to stabilize the ¹⁵N enrichment of the N pool and to minimize N immobilization.

2.4. Plant Harvest, Analysis and Calculations

Sorghum plants were harvested 90 days after planting (i.e. physiological maturity stage) and separated into shoots, roots, and panicles. Samples were dried to a constant weight at 70 ºC and ground to a fine powder. Kjeldahl method was used to determine total nitrogen in the samples, and the ¹⁵N/¹⁴N isotopic ratio was determined using the emission spectrometry (Jasco-150, Japan). Phosphorus content was determined by dry ashing and measured calorimetrically by spectrophotometer (Termo Specronic, England).

Nitrogen fractions derived from the available sources were calculated using the indirect ¹⁵N isotopic dilution method [11, 12, 20] as follows:

The percent N derived from green manure (Ndf_gm) for both PGM and FGM was calculated using the following equation:

%Ndf_gm= [1-(atom %¹⁵N excess _treatment/ atom %¹⁵N excess _control)]100.

Where treatment: plants amended with PGM or FGM; control: plants grown without any green manure application.

Amount of Ndf_gm (mg N pot^-1) was calculated as follows:

Ndf_gm = (%Ndf_gm/100) total N yield.

Nitrogen Use Efficiency (%NUE_gm) of added green manure was calculated by the following equation:

%NUE_gm= (Ndf_gm/N added as GM)100.

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

%Ndff = (atom%¹⁵N in excess _plant / atom%¹⁵N in excess _fertilizer)100.

Ndff = (%Ndff/100) total N

Percent (%) and amount (mg N pot^-1) of N derived from soil (Ndfs) were calculated as follows:

%Ndfs = 100 - (%Ndf_gm + %Ndff).

Ndfs = (%Ndfs/100) total N.

2.5. Statistical Analysis

Data were subjected to Analysis of Variance (ANOVA) using the statistical program Statview, 4.57^® Abacus Concepts, Berkley, Canada. Means were compared using the least significant difference (Fisher’s LSD) test at a probability level of P <0.05.

3.1. Dry Matter Yield, Nitrogen and Phosphorus Uptake

The data of dry matter yield (DM), nitrogen and phosphorus uptake in different plant parts of sorghum as affected by green manuring with pea pod shell wastes (PGM) are given in Table 4. Data in the whole plant of sorghum (leaves plus roots and panicles) are shown in Fig. (1). The addition of PGM at a rate of 50 kg Nha^-1 (PGM₅₀), significantly increased total DM yield in sorghum plants as compared with the control. Raising the N rate (e.g. 100 and 150 kgNha^-1) resulted in more significant increments in the DM, which did not significantly differ from each other. The percent increments in total DM were 60, 99, and 98% in PGM₅₀, PGM₁₀₀, and PGM₁₅₀, respectively, as compared with the control PGM₀. It is noteworthy that DM of panicles increased by 49, 109, and 106% as compared with the control, for the same afore-mentioned treatments, respectively. Also, there were no significant differences in DM between PGM₁₀₀ and PGM₁₅₀ for different plant parts of sorghum. These results may indicate that PGM₁₀₀ is the proper treatment for plant growth improvement. In comparison with the un-manured control treatment, PGM₁₀₀ almost doubled DM in different plant parts of sorghum. The pattern of N yield in sorghum plants was relatively similar to that of dry matter yield. Amounts of nitrogen accumulated in the whole plant of sorghum significantly increased by 63, 123, and 132% in PGM₅₀, PGM₁₀₀, and PGM₁₅₀, respectively, as compared with the control. In panicles, the increases were 56, 124, and 126%, for the same treatments. Increasing the rate of nitrogen in the form of PGM from 100 to 150 kg N ha^-1 did not significantly increase nitrogen yield. Also, total phosphorus content (mg P pot^-1) in the different plant parts of sorghum increased as a result of adding PGM. For panicles, the percent increments in total phosphorus content were 77, 185, and 200% in PGM₅₀, PGM₁₀₀, and PGM₁₅₀, respectively, as compared with the control PGM₀ (Table 4). For the whole plant (Fig. 1), the amounts of P uptake were 86, 142, and 148% higher in PGM₅₀, PGM₁₀₀, and PGM₁₅₀ than that in the control, for the abovementioned treatments, respectively. No significant difference was obtained between PGM₁₀₀ and PGM₁₅₀ treatments.

For Faba bean pod shells wastes (FGM), soil amended with FGM significantly increased dry matter yield of sorghum (Table 5 and Fig. 1). Soil amended with FGM at a rate of 50 kg N ha^-1 (FGM₅₀), significantly increased DM yield in sorghum plants as compared with FGM₀. Raising the N rate (i.e. PGM₁₀₀ and PGM₁₅₀) resulted in more significant increments in the DM. Percent increments in DM were 19, 103, and 105% for panicles, and 14, 64, and 101% for the whole plant in FGM₅₀, FGM₁₀₀, and FGM₁₅₀, respectively, as compared with the un manured control treatment (FGM₀). Total nitrogen yield by sorghum responded to FGM in a manner relatively similar to dry matter yield. The percent increments in N yield were 29, 111, and 109% for panicles and 21, 69, and 104% for the whole plant, in FGM₅₀, FGM₁₀₀, and FGM₁₅₀, respectively. Phosphorus uptake was increased in all plant parts of sorghum as a result of soil incorporated with faba bean shell pod wastes (FGM). FGM increased P yield by 30, 92, and 101%, for the whole plant; and by 37, 156, and 205% for panicles in FGM₅₀, FGM₁₀₀, and FGM₁₅₀, respectively, as compared with the control (FGM₀).

3.2. Nitrogen Uptake from Various Sources

In comparison with the un-manured control plants, soil incorporated with both forms of GM (i.e. PGM or FGM), resulted in lower atom %¹⁵N excess values in the different plant parts of sorghum (Table 6). The decrease in ¹⁵N in sorghum plants was related to the amount of N in the applied GM. Regardless of plant parts, the highest dilution was obtained in the N₁₅₀ followed by N₁₀₀ and then by N₅₀ and N₀ treatments, for both PGM and FGM (Table 6). Percentages and amounts of nitrogen derived from fertilizer (Ndff), soil (Ndfs), and green manure (Ndf_gm) in different plant parts of sorghum as affected by green manuring with PGM or with FGM are shown in Tables 7 and 8, respectively.

For pea pod shells (PGM) treatments, %Ndff and %Ndfs values in the different plant parts as well as in the whole sorghum plant decreased with increasing rates of the applied PGM (Table 7). However, the percentage of nitrogen derived from PGM (%Ndf_gm) increased with increasing rates of the applied PGM. Values of %Ndf_gm in the whole plant of sorghum were 15.7, 48.7, and 53.7% in PGM₅₀, PGM₁₀₀, and PGM₁₅₀, respectively. In regard to the amounts of N derived from the available sources, it can be shown from (Table 7) that soil amendment with PGM generally increased amounts of Ndff and Ndfs, as compared with the control, particularly in PGM₁₀₀. The percent increments in the amounts of Ndff and Ndfs in the whole plant was about 15% higher than the control in the latter treatment. Moreover, significant amounts of nitrogen were derived from green manure (Ndf_gm) which were increased by increasing rates of added PGM. The Ndf_gm values in the whole plant of sorghum were 99, 183, and 211 mg N pot^-1 in PGM₅₀, PGM₁₀₀, and PGM₁₅₀, respectively (Table 7).

For faba bean pod shells (FGM) treatments, the addition of GM addition in the form of faba bean shell pod wastes (FGM) also decreased %Ndff and %Ndfs values. Whereas, %Ndf_gm increased with increasing rates of the applied FGM (Table 8). %Ndf_gm values in the whole plant were 37, 48, and 55% for FGM₅₀, FGM₁₀₀, and FGM₁₅₀, respectively. Moreover, it can be shown from Table 8 that the addition of FGM generally decreased the amounts of Ndff and Ndfs in the leaves, panicles, and the whole plant as compared with the control. However, N uptake from both sources (i.e. Ndff and Ndfs) increased in roots, particularly in FGM₁₀₀ and FGM₁₅₀, where the percent increments either in Ndff or Ndfs values were 24 and 40% for both treatments, respectively,

3.3. Nitrogen Use Efficiency (%NUE_gm) of Added Green Manure

Nitrogen use efficiency (%NUE_gm) of added green manure in the form of pea (PGM) or faba bean (FGM) pod shells in the different plant parts of sorghum are shown in Table 9. Regardless of GM forms, % NUE_gm values were high in leaves, followed by panicles and roots. In the whole plants, nitrogen use efficiency of added green manure decreased with increasing rates of applied GM. For pea pod shells (PGM) treatments, %NUE_gm values were 45.4, 42.2, and 32.2% in PGM₅₀, PGM₁₀₀, and PGM₁₅₀, respectively. This result indicated that sorghum utilized approximately half of the N applied as pea shell pod wastes in PGM₅₀ and PGM₁₀₀ treatments, and third of the N applied in the PGM₁₅₀ treatment.

In regard to faba bean pod shells (PGM) treatments, the highest nitrogen recovery was in FGM_50. There were slight but significant decreases in %Ndf_gm as a result of increasing rates of applied GM. Values of %NUE_gm in the whole plant were 35, 32, and 29%, in FGM₅₀, FGM₁₀₀, and FGM₁₅₀, respectively. This indicates that sorghum utilized approximately a third of the N applied in the form of faba bean shell pod wastes.

4.1. Effects of Green Manuring with Pea and Faba Bean Pod Shell on Dry Matter Production and N Yield of Sorghum

Performance of sorghum plants grown on the soil amended with pea (PGM) or faba bean (FGM) pod shells wastes as green manure (GM) was examined using indirect ¹⁵N isotope dilution. Sorghum plants responded positively and differently to the soil amendments with either GMs used. The results showed that dry weight and N yields of sorghum plants significantly increased by soil amendment with PGM or FGM as compared with un-manured treatments. The beneficial effects of leguminous green manures on the growth, yield, and N-uptake of the following crops, particularly, the sorghum plants, have been previously observed in different cropping systems [3, 21-24]. The increase in dry weight and that in N uptake caused by the application of pea (PGM) or faba bean (FGM) pod shells as green manures in sorghum plants may be attributed to the increase in available N released from GM. Moreover, other nutrient elements such as phosphorus could also promote plant growth. The beneficial effects following the green manure amendment on sorghum plants were affected by the mass of incorporated GM material in the soil. In comparison with the un-manured control treatment (N0), soil amendment with 3.5 t ha^-1 of PGM (PGM₁₀₀) or with 6.5 t ha^-1 of FGM (FGM₁₅₀) almost doubled the above-mentioned parameters in different plant parts of sorghum. Similarly, Fall et al. [17] reported that soil amendment with 4 to 6 t ha⁻¹ of peanut shells significantly increased seedlings growth of threes leguminous trees. On the other hand, our study showed that growth and N uptake by sorghum plants were affected by the form of added GM. The enhancement in dry-matter production and N yield of sorghum was more pronounced in plants amended with pea than with faba bean pod shell residues. As indicated by Giller and Wilson [4], such differences may have resulted from variations in decomposition and immobilization rates between the two forms of plant material incorporated in the soil.

4.2. Effects of Green Manuring with Pea and Faba Bean Pod Shell on P uptake in Sorghum

In addition to the effect green manure of pea and faba bean pod shell on N uptake, the incorporation of GMs improved P content in sorghum plants. Bah, et al. [9] reported that improved availability of soil P due to the incorporation of plant materials has been attributed to direct P release from the decomposing materials and the action of other decomposition products on native soil P. Several researchers reported that the phosphorus in green manure can potentially be delivered to the soil in a form that is readily available to plants and soil microorganisms [25, 26]. Baddeley et al. [27] showed that the incorporation of a legume green manure can enhance biological phosphorus cycling in soil and improve its availability. In this study, the increase of phosphorus content in sorghum plants was relatively related to the mass of applied GM. In comparison with the control (N0), total phosphorus content in sorghum plants increased by 86, 140, and 148% for PGM and by 30, 92, and 101% for FGM, in N₅₀, N₁₀₀, and N₁₅₀ treatments, respectively. Gao et al. [28] showed that the positive effect of alfalfa green manure on increasing yield of rice can be attributed to its good functions on increasing soil available P, promoting P uptake, and enhancing interactive effect of N and P. Such an organic amendment could increase some soil enzyme activities, such as dehydrogenase, urease, acid phosphatase, and β -glucosidase. The changes in soil enzyme activities affect the progress of nitrogen and phosphorus mineralization and release [28]. Pypers et al. [29] suggested that green manure with leguminous in crop rotation system increases the yield and growth of the maize plants, possibly for microbiological reasons, and the enhancement of P acquisition by plants resulted from improved soil P availability. On the other hand, phosphorus recovery following green manure crops may be derived from the decomposition and mineralization of incorporated plant material in the soil, from native P in the soil and from mineral P fertilizer. In this study, we did not estimate these values, only the total P content of the biomass. It is not possible to separate these P sources without using the ³²P isotope labeling technique [9]. However, it is evident that the incorporation of GMs improved P content in sorghum plants and may have contributed to yield increases. Based on the P-uptake under the two GM treatments (i.e. pea and faba bean pod shell), the phosphorus accumulation in total biomass of sorghum plants was 43, 29, and 20% higher in pea than faba bean, for N₅₀, N₁₀₀, and N₁₅₀ treatments, respectively. Such increments may be partly attributed to differences in soil enzyme activities between the two types of GMs [28].

4.3. Effect of Green Manuring with Pea and Faba Bean Pod Shell on N Derived from Various Sources in Sorghum

Sorghum plants amended with GM (i.e. pea or faba bean pod shells), had lower atom %¹⁵N excess values as compared to the un manured control treatment. This result indicated that a dilution effect had taken place and that some portions of sorghum N were derived from the incorporated GM in the soil [3, 10-12]. Regardless of GM used, estimated values of %Ndf_gm in sorghum plants, using the indirect ¹⁵N isotope dilution method, ranged from 35% to 55%. This result indicated that the use of pod shells of leguminous as green manures provided substantial portions of N requirements for sorghum. Amounts of nitrogen derived from GM (Ndf_gm) in the whole plant of sorghum were 99, 183, and 211 mg N/pot for PGM and 75, 138, and 188 mg/N pot for FGM in N₅₀, N₁₀₀, and N₁₅₀ treatments, respectively. As indicated by Rees et al. [30], the amounts of N taken up by plants were proportional to those applied in the form of legume residues. Moreover, based on amounts of N uptake from the two GM residues, (i.e. pea or faba bean pod shells), nitrogen accumulation in total biomass of sorghum plants were 31, 33, and 12% higher in pea than faba bean, for N₅₀, N₁₀₀, and N₁₅₀ treatments, respectively. Such divergence may result from differences in the decomposition and mineralization rates of organic N between the two forms of plant material incorporated in the soil [4]. Consequently, our results showed a beneficial effect of using pea or faba bean pod shells as GMs to meet some of the N-requirement in sorghum plants.

With regard to soil (Ndfs) or fertilizer (Ndff) N uptake values (i.e, proportions and amounts), sorghum plants grown in a soil amended with pea pod shells (PGM) had significantly higher values in the different plant parts than those in the un-manured plants. (Table 7). With the exception of roots, both Ndff and Ndfs values in shoots, panicles, and whole plant sorghum, amended with FGM were lower than those in the control, indicating that the enhancement of total N uptake of sorghum (Table 8) mainly resulted from N released from FGM. However, amounts of soil or fertilizer N uptake in roots of the FGM treated plants were higher than those of the control, particularly in FGM₁₀₀ and FGM₁₅₀, treatments. Moreover, a positive effect on sorghum root dry matter yield was also observed following the addition of both GMs (Tables 4 and 5). The increase in the amount of soil N uptake by sorghum following the addition of plant residues as GMs, particularly PGM, was demonstrated in this study, a phenomenon highlighted previously by several authors [3, 30-32]. Jenkinson et al. [33] introduced the term “Added Nitrogen Interaction” or “priming effect” to describe any effect that the addition of N (i.e. organic N in the green manure) may have on the native soil N. Azam [34] reported that the extra nitrogen comes from soil organic matter as a result of the interaction of the added nitrogen. An increase in N availability from sources like soil organic matter could be attributed to a priming effect of the added nitrogen [34]. In other words, the added nitrogen interacts with the N already present in the soil, in a way to increase the availability of the later. Such an effect may result from an increase in root growth enabling the plants to explore a greater soil volume thus increasing the uptake of nutrients ‘like soil N’ by the green manured crops [33]. In this study, a positive effect on sorghum root dry matter yield was observed following the addition of both GMs. On the other hand, as suggested by Azam [31-34], there is a reason to believe that a priming effect may occur in soils because the endogenous soil microorganisms will react to the addition of energy-rich materials, and the increased microbial activity will involve mineralization process of the organic N in the GM. In view of our results, it can be concluded that the beneficial effects of incorporating pod shells in soil on sorghum nitrogen accumulation, may be attributed not only to the additional N released from GMs to the plants but also to their effects on the enhancement of soil N uptake which was more evident in PGM than FGM treatments [3, 12, 23].

4.4. Effect of Green Manuring with Pea and Faba Bean Pod Shells on N Recoveries (%NUE_gm) by Sorghum

Regardless of green manure used, nitrogen recoveries (%NUE_gm) in sorghum plants (uptake by plants) following the addition of pea or faba bean pod shells ranged from 29 to 45% indicating that both GM forms were used effectively. Release of N from pea GM which added to the soil 15 days before planting, seemed to be rapid with nearly half of N being utilized by the sorghum in PGM₅₀ and PGM₁₀₀, and third of N in PGM₁₅₀ treatments. For faba bean GM, around a quarter to third of its N content being used by the sorghum plants. Nearly, the same range of %NUE_gm values (20-52%) has been reported for sorghum manured with sesbania [3], Russian olive [12], or leucaena [23] leaves. However, the efficient use of green manure by sorghum plants was higher in treatment with pea than those with faba bean residues. It is well documented that organic nitrogen in the GM is released into the soil after incorporation through the process of mineralization by soil microorganisms [27]. The rate of this process is affected by many factors such as temperature, moisture, quantity, and quality (i.e. chemical composition) of the GM residue [4, 30, 35]. The ratio of carbon to nitrogen (C/N) is a useful guide for the decomposition and mineralization of the organic nitrogen in the added GM [4]. Abdelhamid et al. [36] reported that the lower the C/N the greater the mineralization rate. In this study, nitrogen recovery rates (uptake by sorghum) from pea pod shells were higher than those from faba bean probably because C/N of pea (15.2) was lower than that of faba bean (18). Moreover, Daimon [37] reported that not only the C/N ratio of the incorporated green manure involved in nitrogen release, but also other parameters such as lignin and polyphenols, ratio of lignin /N, ratio of polyphenols to total N, and ratio of (lignin + polyphenols) to total N are determining for GM quality regarding to N release after incorporation. Accordingly, GMs with a high ratio of these parameters are usually considered to compose more slowly than those of low ratio [37, 38]. In this study, all the above-mentioned parameters in PGM were lower than those in FGM (Table 2). Such results may interpret the beneficial effects of the former over the latter GM regarding growth and N uptake by sorghum plants. Moreover, it is worth mentioning that the difference in polyphenols and polyphenols /N values between the two forms of GM were much higher than those of the other analyzed parameters (Table 2). In other words, polyphenols and polyphenols /N mean values were 5 and 6 times higher in FGM than PGM, respectively. Whereas, other values were less high (i.e. 1.2, 1.2, 1.5, 1.4 times higher in FGM than PGM, for L, C/N, L+ph/N, and L/N, respectively), (Table 2). This result may indicate the importance of polyphenols and polyphenols /N criteria in N released from GM. Our observation agrees well with the results of Fan et al. [39] who found that the impact of leaf litter polyphenols concentration and polyphenols /N ratio on N release rate was stronger than that of the C/N or lignin/N ratio.

While green manuring has been demonstrated to have beneficial effects on many aspects of the cropping systems, the most obvious direct economic benefit from the utilization of GM in agriculture is N fertilizer saving. It is well known that legume GM can replace a portion of the fertilizer N requirements for the subsequent crops. For example, Kurdali [11] reported that 18-54 kg N ha^-1 of inorganic N fertilizer may be saved as a result of using dhaincha (Sesbania aculeata Pers.) plant residues as GM for sorghum growth. In this study, amounts of added nitrogen from GMs to sorghum were equivalent to 50, 100, and 150 kg N ha^-1 (Table 3). The use of PGM or FGM as green manures could substitute significant amounts of N fertilizer. For example, when PGM was used as a GM, recoveries of the added N by sorghum (%NUE_gm) were 45, 42, and 32% (Table 9), for the above-mentioned treatments, respectively. Thus, equivalent amounts to 23, 42, and 48 kg N ha^-1 of inorganic fertilizer may be saved using PGM as green manure for sorghum growth; whereas, 17, 32, and 43 kg N ha^-1 of inorganic N fertilizer may be saved as a result of using FGM.

Overall, this study demonstrated that the organic wastes of pea and faba bean pod shells applied to soil as GMs increased its fertility, like inorganic N fertilizer, and improved the growth of sorghum by enhancing nutrients uptake from soil and from organic sources. The use of these organic wastes as GMs is very useful to reduce chemical fertilizer application in cropping systems, maintain a sustainable N and P supply for crops and can be effectively used for soil rehabilitation and plant growth enhancement. In light of this study, the importance of using pea and faba bean pod shells wastes as green manures for plant growth enhancement is summarized in Fig. (2).