Synthesis of an eco-friendly nanocomposite fertilizer for common bean based on carbon nanoparticles from agricultural waste biochar

Dina M.SALAMAMahmoud E.ABD EL-AZIZMehrez E.EL-NAGGAREssam A.SHAABAN and Mohamed S.ABD EL-WAHED

1Vegetable Research Department,NationalResearch Centre,33 ElBohouth Street,Dokki,Giza 12622(Egypt)

2Polymers&Pigments Department,NationalResearch Centre,33 ElBohouth Street,Dokki,Giza 12622(Egypt)

3Textile Research Division,NationalResearch Centre,33 ElBohouth Street,Dokki,Giza 12622(Egypt)

4Pomology Department,NationalResearch Centre,33 ElBohouth Street,Dokki,Giza 12622(Egypt)

5Botany Department,NationalResearch Centre,33 ElBohouth Street,Dokki,Giza 12622(Egypt)

(Received March 7,2020;Revised April 11,2020)

ABSTRACT The burning of agricultural waste is a major cause of environmental pollution.In this study,we sought to prepare biochar from agricultural waste as a source material for the preparation of carbon nanoparticles(CNPs).Surface morphology,hydrodynamic particle size,and purity and crystallinity of CNPs were extensively investigated using transmission electron microscopy(TEM),zeta sizing,and X-ray diffraction(XRD)spectroscopy,respectively.The CNPs were subsequently immersed in a solution of potassium nitrate(KNO3)to prepare a CNPs/NK nanocomposite(CNPs loaded with nitrogen(N)and potassium(K))as a nanocomposite fertilizer for common bean(Phaseolus vulgaris L.).The CNPs/NK nanocomposite was sprayed as a foliar fertilizer at 0,10,20,30,and 40 mg L-1 on common bean plants 25 d after sowing on a farm in Shebin El-Kom,El-Monifia,Egypt.The growth,yield,and quality of common bean were investigated during two successive growing seasons(2017 and 2018).The highest seed yields of 2.04 and 2.01 t ha-1 and the highest values of growth parameters including plant height of 61.5 and 59.2 cm,number of leaves per plant of 35 and 35,number of flowers per plant of 83.3 and 82.7,and plant fresh weight of 148.7 and 152.8 g plant-1 were obtained when using the CNPs/NK nanocomposite at a concentration of 20 mg L-1 during the 2017 and 2018 growing seasons,respectively.

Key Words: foliar fertilizer,N and K-loaded nanosized carbon particles,Phaseolus vulgaris L.,plant growth,yield

INTRODUCTION

The increasing amounts of waste generated by agricultural processing pose a potentially significant environmental threat.Although this waste material tends to be lightweight,it is generated in large quantities over extensive areas.Moreover, if burned, this waste can cause considerable environmental damage(Youssefet al.,2019).In an attempt to address these issues, researchers have sought to identify practical uses of this waste; a prominent example is the preparation of biochar for use as a fertilizer(Vaccariet al.,2011;Saxenaet al.,2014;Wanget al.,2018;Abd Elwahedet al., 2019), an energy source (biofuel), and an additive for soil amendment,removal of pollutants,and waste management(Oliveiraet al.,2017; Bianet al.,2018).Given that the global market for biochar in 2018 was worth US$1.3 billion, with an estimated demand of 395×103t per annum(Zeidabadiet al.,2018; Liuet al.,2019; Zhanget al.,2019),the production of biochar from agricultural waste would clearly be a financially viable option.

Biochar is preparedviaa carbonization process, involving the pyrolysis of leftover biomass waste at a high temperature(450—700°C)in the absence of air(Antal and Grønli,2003;Lehmann and Rondon,2006).The benefit of pyrolysis is that the volume and weight of the biomass are reduced,making the product easier to handle.Biochar has been widely applied as a soil conditioner to improve plant growth by increasing nutrient contents as well as enhancing soil physical and biological properties and consequently improving soil water-holding capacity(Lehmann and Rondon,2006;Ozores-Hamptonet al.,2011).In addition,the extract obtained from biochar is rich in organic molecules and mineral nutrients,which can be beneficially applied to promote plant growth(Tripathi and Sarkar,2015;Louet al.,2016;Bianet al.,2019).

Recently,carbon nanoparticles(CNPs)have been utilized as fertilizers(Khodakovskayaet al.,2009,2012;Gaafaret al., 2018; Schroederet al., 2019; Qiuet al., 2019). It has been found that the germination of date palm embryo culture(Tahaet al.,2016),the growth of tobacco cell culture(Khodakovskayaet al., 2012), and the germination and growth of tomato seeds and seedlings(Khodakovskayaet al.,2009)are improved by treatment with CNPs.Furthermore,Cañaset al.(2008)have demonstrated that CNPs influence the root elongation of four crop species,namely,cucumber,onion,lettuce,and tomato.

As the world population continues to grow, there is an ever-increasing need for an increase in agricultural productivity. This can be achieved by expanding the area of the cultivated land and/or by augmenting the production of plantsviaimproving fertilizer efficiency(Sonkaret al.,2012; Salamaet al., 2019). In this latter regard, nanomaterials have extra potential due to their small particle size and large surface area(Sheblet al.,2019).These properties make ease the penetration of these nanofertilizers toward the leaves of plants and thus provide progressions in plant nutrition strategies(Mahmoudet al.,2018;Abd El-Azizet al.,2019;Salamaet al.,2019;Xieet al.,2019).Elements used in nanofertilizer preparation,such as carbon,are present in high quantities in wood waste(Antal and Grønli,2003;Sonkaret al.,2012;Youssefet al.,2019).Both biochar and the CNPs from biochar can improve soil physical properties and water-holding capacity and reduce nutrient lossvialeaching(Tripathiet al.,2011;Ghorbanpour and Hadian,2015;Naeemet al., 2018), thereby promoting plant growth and reducing crop disease incidence. Furthermore, given that biochar has the ability to form complexes with heavy metals,it can limit the bioavailability of heavy metals and toxins in soil (Abbaset al., 2018). The CNPs from biochar can accordingly be used to reduce the amounts of commercial fertilizer required for plant growth, which can in turn reduce the detrimental effect of soil contamination on humans,animals,and the environment.

Common bean (Phaseolus vulgarisL.) is one of the most important vegetables cultivated globally. As a rich source of proteins, carbohydrates, vitamins, and mineral elements,it makes a valuable contribution to the human diet.Indeed, it is the most important vegetable crop cultivated in Egypt,in terms of both the domestic and export markets(Abdel-Hakimet al., 2012; Paliloet al., 2018). In this study,we sought to prepare CNPs from agricultural waste biochar to be used as a foliar fertilizer for common bean after being loaded with nitrogen (N) and potassium (K),the most important macronutrients for plants, to obtain a CNPs/NK nanocomposite,and ultimately,the effect of the nanocomposite as a foliar fertilizer for common bean was examined.

MATERIALS AND METHODS

Synthesis and characterization of CNPs

As agricultural waste, we used trimmings from citrus trees,which were subjected to pyrolysis in the absence of air at 650°C to produce biochar(Lehmann and Rondon,2006).Thereafter,the prepared biochar was collected,ground,and then sieved with a 200-mesh sieve to yield a fine powder.The ground biochar was subsequently washed with 2 mol L-1concentrated nitric acid (HNO3) that was purchased from the Al Gomhoria Chemical Company,Egypt,followed by washing with distilled water, to remove the undesired compounds, including carbonate compounds and soluble salts. At this point, the pH of the filtrate became neutral.The collected fine powder was dispersed in water using vigorous stirring until the heavier portion containing silica and inorganic materials settled out,and then the upper portion containing floating CNPs was collected.This latter step was repeated three times to ensure that the suspension comprised only CNPs(Saxenaet al.,2014).The CNP suspension was subsequently centrifuged at 10 000 r min-1for 15 min,and the resulting pellet comprising the wet CNP powder was dried at 60°C for 6h.

The shape of CNPs was determined with a transmission electron microscope(model JEM-1230,JEOL,Japan),operated at 120 kV and with a resolution down to 0.2 nm.Samples were prepared by adding a drop of an aqueous dispersion of the CNPs to a carbon-coated copper grid,which was allowed to dry in the air prior to characterization.For further confirmation, the hydrodynamic particle size and polydispersity index (PDI) of a colloidal solution of CNPs was estimated using a zeta sizer (Mervaln, USA).Additionally,X-ray diffraction(XRD)patterns were determined with a Philips X-ray diffractometer coupled with a PW 1930 generator and a PW 1820 goniometer (Philips Japan,Ltd.,Japan),using a CoKαradiation source energized at 45 kV and a CuKαradiation source (0.154 18 nm) to determine the purity and crystallinity of CNPs.The chemical composition of the prepared biochar powders was determined by X-ray fluorescence(XRF)spectroscopy performed with an Axios sequential WDXRFspectrometer (PANalytical,The Netherlands).

Preparation of the CNPs/NK nanocomposite

The procedure for preparing the CNPs/NK nanocomposite used for foliar application to common bean plants in the present study is shown in Fig.1.To a freshly prepared 1 000 mg L-1solution of potassium nitrate (KNO3) purchased from the Al Gomhoria Chemical Company,Egypt,we gradually added 1 g of CNPs followed by continuous stirring for 24 h.Thereafter,the solution was subjected to ultrasonication for 1 h at room temperature(25°C)to disperse the particles in solution and to verify the dispersion of CNPs.Subsequently,to collect the CNPs/NK nanocomposite,the suspension was centrifuged at 10 000 r min-1.To estimate the uptake of N and K by the CNPs,the contents of N and K in the solution were determined before and adding CNPs,using the Kjeldahl method and flame spectrophotometry,respectively(Nielsen,2010).

Fig.1 Steps for the preparation of carbon nanoparticles(CNPs)and the CNPs/NK nanocomposite(CNPs doped with N and K)as a foliar fertilizer sprayed on common bean plants.

Controlled-release behavior of the CNPs/NK nanocomposite in water

One gram of the CNPs/NK nanocomposite was suspended in 100 mL of distilled water at pH 7(release medium)and incubated at room temperature,and the amounts of N and K released from the CNPs/NK nanocomposite were monitored over a period of 30 d(Liuet al.,2019).Nitrogen and K were determined using the Kjeldahl method and flame spectrophotometry,respectively(Nielsen,2010).

Field experiment

The seeds of common bean,provided by the Agricultural Research Centre, Ministry of Agricultural and Land Reclamation,Egypt,were sown at a rate of 120 kg ha-1on March 6in 2017 and 2018 on a farm in Shebin El-Kom,El-Monifia Governorate (30°33′N, 31°0′E, 20 m above sea level),Egypt.The field has a clay-textured soil and had been cleaned, plowed, leveled, and divided into plots in mid-February of each year. Two seeds were sown in each hill on one side of a ridge with a distance of 30 cm between hills and 60 cm between ridges.Selected soil physical and chemical properties (average of 2 seasons) are presented in Table I.The soil was analyzed according to Cottenieet al.(1982),using samples(0—20 cm depth)collected from random points in the center of each plot.

During the soil preparation,organic manure(47.6m3ha-1),calcium superphosphate(15.5%P2O5,476kg ha-1),agricultural sulfur (238 kg ha-1), and potassium sulfate(48%K2O,120 kg ha-1)that were obtained from El-Naser Company,Egypt,as intermediate chemicals,and ammonium sulfate(20.6%N,119 kg ha-1)that was supplied by Abu Qir Fertilizers Company,Egypt,were added prior to the initial irrigation.

TABLE I Selected soil physical and chemical properties of the experimental field

Common bean plants were sprayed with the CNPs/NK nanocomposite at 0,10,20,30,and 40 mg L-1as a foliar fertilizer 25 d after sowing.The experiment was performed in a completely randomized block design with five replicates for each treatment.

Plant sampling and analysis

Five common bean plants were randomly selected from each treatment 45 d after sowing(vegetative stage)during the two studied seasons to investigate the following vegetative growth parameters:plant height,root length,the numbers of leaves and branches per plant, and the fresh and dry weights of leaves and branches.The five plants selected from each treatment were weighted immediately after clipping to estimate the fresh weights of leaves, branches, and roots.Thereafter,they were dried at 70°C to a constant weight for dry weight determinations.At harvest(90 d after sowing),the numbers of pods and seeds per plant,weights of pods and seeds, seed index (100-seed weight), shelling percentage,shoot residues per plant, and weights of seeds and shoot residues were determined during each of the two studied seasons(Salamaet al.,2019).

Photosynthetic pigments were determined in the fresh leaves that were collected 45 d after sowing.The contents of chlorophylla(Chla),chlorophyllb(Chlb),and carotenoids were determined spectrophotometrically against 85%acetone as a blank at 663,644,and 452 nm,respectively,with a UV/Vis spectrophotometer (TG 80, Bruker, Germany)(Metzneret al.,1965).The concentrations of the pigments were calculated using the equations proposed by Jianget al.(2018).

The dried powder of plants was digested in a 10:1:4 mixture of concentrated nitric acid,sulfuric acid(H2SO4),and perchloric acid(HClO4)that were purchased from the Al Gomhoria Chemical Company, Egypt. The acid-digested solution was kept at constant volume with distilled water according to the method of Chapman and Pratt(1962)and then analyzed for determination of different elements as follows.The contents of phosphorus(P)and K were determined spectrophotometrically(Murphy and Riley,1962),the contents of zinc(Zn),copper(Cu),iron(Fe),and manganese(Mn)were determined using atomic absorption spectroscopy,and the content of N was determined using the Kjeldahl method(Nielsen,2010).

For determination of phenols in common bean plants,a known weight of the plant sample was extracted with cold methanol(CH3OH,85%),and the resulting filtrate was made up to a predetermined volume.Total phenols were determined calorimetrically according to the method reported by Snell and Snell(1955)as follows.Firstly,1 mL of ethanol extract was mixed with 10 drops of concentrated hydrochloric acid,heated rapidly in a boiling water bath for 10 min,and then cooled.Thereafter,1 mL of Folin’s reagent and 1.5 mL of 14%sodium carbonate(NaHCO3)were added.The mixture was made up to 5 mL with distilled water,shaken well,and placed in a boiling water bath for 5 min.The developed color was measured spectrophotometrically at 650 nm against a reagent blank (Sadaket al., 2019), and the total soluble phenols were calculated from a standard curve of pyrogallol.

The total flavonoids in common bean plants were determined according to Changet al.(2002).Methanol extracts of the plant sample (0.5 mL, 0.1 g mL-1) were prepared separately by mixing plant with 1.5 mL of 85%methanol,0.1 mL of 10%aluminum chloride(AlCl3),0.1 mL of 1 mol L-1potassium acetate(C2H3KO2),and 2.8 mL of distilled water,and incubated at room temperature for 30 min.The absorbance of the reaction mixture was measured at 415 nm with a double-beam Perkin-Elmer UV/Vis spectrophotometer(USA).

Statisticalanalysis

All obtained data were subjected to statistical analysis of variance(ANOVA)and analyzed for statistical significance using the least significant difference(LSD)test atP ＜0.05(Kobataet al.,2018).

RESULTS AND DISCUSSION

In this study,a nanofertilizer was applied as a foliar spray instead of the salts or soil fertilizers that are typically used in agriculture fields.The foliar application of fertilizers has been widely used in plant fertilization owing to the ease of application and low cost.Moreover,such application is more target oriented and gradually feeds plants in a sustained and controlled manner.Foliar application also reduces the toxicity symptoms that potentially occur following the application of soil fertilizer.Furthermore,nanofertilizers can be applied in smaller amounts than commercial soil fertilizers(Davarpanahet al.,2016).The nanosized carbon particles(CNPs)were obtained from the prepared biochar and immersed in a solution of potassium nitrate to facilitate the uptake of large amounts of N and K, resulting in the formation of the CNPs/NK nanocomposite. The deficiencies of N and K,two most important macronutrient for plants,would lead to the yellowing of plant leaves and reduced growth and yield(Hafsiet al., 2014; Živčáket al., 2014). The synthesized nanocomposite accordingly had a substantial positive effect on the growth of the common bean plants.Compared with the control plants,the common bean plants treated with the CNPs/NK nanocomposite were more healthy and green and showed better growth characteristics(Fig.1).

Characterization of the prepared biochar and CNPs from biochar

The XRF spectroscopy analysis indicated that Si4+,C4+, P5+, Al3+, Fe2+, Ca2+, Na+, Mn2+, Mg2+, and S6+were the major elements in the biochar(Table II).The elements such as Ca2+,Na+,Mn2+,and Mg2decreasing after treatment of the biochar with nitric acid and washing water to remove salts such as nitrates.The loss on ignition(LOI) values of biochar were 46.70% and 56.16% before and after treatment with nitric acid,respectively.

To confirm the successful preparation of CNPs,the shape of the prepared CNPs was initially determinedviatransmission electron microscopy(TEM)analysis.As shown in Fig.2a,b,these particles were spherical with a narrow size distribution, with most of them being less than 50 nm in diameter. The hydrodynamic particle sizes in a colloidal solution of CNPs (Fig. 2c) determinedviaa particle size analyzer were approximately 40 nm in diameter,in accordance with the TEM analysis.Moreover,the PDI value was 0.139, less than 0.5, confirming the mono-disparity and homogeneity of the prepared CNPs.

The XRD patterns obtained for the prepared CNPs(Fig.2d)showed characteristic diffraction peaks at 2θ=25°,44°,and 77°related to the C(002),C(100),and C(110)atoms of CNPs,respectively(Luet al.,2008;Saleh,2011),indicating that the CNPs were free from other impuritiesafter having been subjected to several washes with distilled water.

TABLE II X-ray fluorescence(XRF)spectroscopy analysis of agricultural compost biochar before and after treatment with nitric acid

Fig.2 Transmission electron microscopy(TEM)images of carbon nanoparticles(CNPs)at two different magnifications(a and b),hydrodynamic particle size(diameter)distribution by number in colloidal solution and polydispersity index(PDI)value(c)of CNPs,and X-ray diffraction(XRD)patterns of CNPs(d).

Release rates of N and K from the CNPs/NK nanocomposite

The release rates of N and K from the CNPs/NK nanocomposite over 30 d at room temperature are shown in Fig. 3. The CNPs can absorb N and K due to adsorption properties and porous structure of these particles.The concentrations of N and K absorbed on the CNPs were calculated and compared with those in the native compounds at different time intervals(Fig.3).The CNPs/NK nanocomposite released approximately 88%and 80%of N and K,respectively,over 15 d.The high absorption capacity of carbon in its nanoform can be attributed to its high surface area and numerous functional groups,in addition to the created pores(Liuet al.,2019).These properties give the CNPs their large capacity for K and N sorptionviaelectrostatic interactions and sorption-precipitation.

Fig.3 Release rates of N and K from the CNPs/NK nanocomposite(carbon nanoparticles(CNPs)doped with N and K)in water.

Effect of CNPs/NK nanocomposite as a foliar fertilizer on common bean plants

Vegetative growth characters.The effects of different concentrations of the CNPs/NK nanocomposite applied as a foliar fertilizer to common bean plants on the vegetative growth characters during two successive seasons in 2017—2018 are presented in Table III.At a concentration of 20 mg L-1,the CNPs/NK nanocomposite significantly improved the growth of common bean plants, as evidenced by increases in plant height to 61.5 and 59.2 cm, root length to 18.3 and 17.7 cm, number of leaves per plant to 35 and 35, number of flowers per plant to 83.3 and 82.7, and the fresh weight of plant to 148.7 and 152.8 g plant-1during the 2017 and 2018 growing seasons,respectively.Increasing the concentration of the CNPs/NK nanocomposite to 30 mg L-1resulted in an increase in plant dry weight to 29.3 and 29.2 g plant-1in 2017 and 2018,respectively.Our results clearly indicated that the CNPs/NK nanocomposite applied at a concentration of 20 mg L-1to common bean plants was optimal and sufficient for improving plant growth,which is consistent with the findings of Khodakovskayaet al.(2013),who reported that multi-walled carbon nanotubes(MWCNTs)with a concentration equal to or greater than 200 μg L-1promoted tomato plant growth and yield.Furthermore,Ghorbanpour and Hadian(2015)reported that the fresh and dry weights and growth ratio of calli in medicinal plantSatureja khuzestanicaincreased with increasing concentration of MWCNTs up to 50 mg L-1,but deceased when the concentration of MWCNTs increased to 500 mg L-1.Our data are also consistent with the results reported by Saxenaet al.(2014),who have demonstrated that the efficacy of biochar traditionally used to promote plant growth is due to its slow release of nutrients,the presence of hydrophilic carboxylic and hydroxyl groups on the biochar surface promotes water and ionic nutrient absorption,whereas CNPs retain nutrient ions such as ammonium nitrate and thereby enhance the growth rate of wheat plants.

Yield and yield components.Agricultural crops are among the major sources of essential nutrients for animals and humans.The advantage of using CNPs can be attributed to their high water retention capacity and biomass,as well as to the slow release of nutrients,which in turn positively affects plant health,thereby enhancing crop yield and quality(Khodakovskayaet al.,2009).The data presented in Table IV indicated that compared with the control,the yield and yield components of common bean significantly increased in response to treatment with different concentrations of the CNPs/NK nanocomposite.Common bean plants treated with the CNPs/NK nanocomposite at a concentration of 30 mg L-1were found to have the highest number of pods per plant of 31.3 and 30.3,weight of pods per plant of 109.8 and 104.8 g,and weight of shoot residues per hectare of 2 138 and 2 101 kg during the 2017 and 2018 growing seasons,respectively,whereas treatment with the CNPs/NK at 20 mg L-1nanocomposite provided the highest values of number of seeds per plant of 87.3 and 87.0, weight of seeds perplant of 37.2 and 36.5 g and per hectare of 2.04 and 2.01 t,and seed index of 42.6and 41.9,respectively.These results confirmed that the CNPs/NK nanocomposite applied at a concentration of 20 mg L-1was sufficient to increase the seed yield of common bean plants,which is in line with the findings of previous studies(Khodakovskayaet al.,2012;Ghorbanpour and Hadian,2015;Vermaet al.,2019),where the lowest concentration of carbon nanomaterials(CNMs)improved the vegetative growth and yields of fruits and seeds to a greater extent than the highest concentration of CNMs. Generally, the CNMs at lower concentrations can improve water uptake and transport,seed germination,and nitrogenase, photosystem, and antioxidant activities and promote the absorption of nutrients as well.These effects are reflected in the observed improvements in the growth and yield of common bean plants,whereas N and K are essential for plant growth and the source-to-sink translocation of metabolites in plant(Hafsiet al.,2014;Živčáket al.,2014).

TABLE III Effects of different CNPs/NK nanocomposite(carbon nanoparticles(CNPs)doped with N and K)concentrations on the vegetative growth characters of common bean plants during two successive growing seasons(2017 and 2018)

TABLE IV Effects of different CNPs/NK nanocomposite(carbon nanoparticles(CNPs)doped with N and K)concentrations on the yield and yield components of common bean plants during two successive growing seasons(2017 and 2018)

Photosynthetic pigments.To gain a better understanding of the mechanisms underlying the effect of the CNPs/NK nanocomposite as a foliar fertilizer for common bean plants,we determined the contents of photosynthetic pigments(Chla, Chlb, total chlorophylls, and carotenoids) in leaves at different concentrations of the CNPs/NK nanocomposite applied as a foliar fertilizer during the 2017 and 2018 growing seasons(Fig.4).We found that the average content of total chlorophylls(Chla+Chlb)in the leaves of common bean plants increased with increasing concentration of the CNPs/NK nanocomposite up to 20 mg L-1, whereas the content of carotenoids decreased owing to the positive effect of the CNPs/NK nanocomposite on plant physiology.These results are consistent with the findings of a previous study conducted by Siddiquiet al. (2015), who concluded that CNPs increased the absorption of carbon dioxide,which subsequently entered the chloroplasts,promoting the absorption of various radiations and thereby improving photosynthesis.In addition,N and K are essential elements for chlorophyll synthesis in the leaves(Živčáket al.,2014).

Fig.4 Effects of different CNPs/NK nanocomposite(carbon nanoparticles(CNPs)doped with N and K)concentrations on the average contents of chlorophylls a(Chl a)and b(Chl b),total chlorophylls(Chl a+Chl b),and carotenoids(Caro)in the leaves of common bean plants during two successive growing seasons (2017 and 2018). The asterisk (*) indicates significance at P ＜0.05 compared with the control.

Mineralcontents of the seeds and leaves.The minerals present in plant seeds or leaves play important roles in altering plant metabolism, which affects plant growth,yield,and quality.In the present study,we speculated that the CNPs/NK nanocomposite passed through the plant cell wallviathe stomata,owing to the small size of the nanocomposite particles,less than the diameter of the stomatal pores(Torneyet al.,2007;Mahmoudet al.,2018;Abd El-Azizet al.,2019).The contents of N,P,Fe,Cu,and Mn in the leaves significantly increased with increasing CNPs/NK nanocomposite concentrations, whereas the contents of Fe and Zn significantly decreased with increasing CNPs/NK nanocomposite concentrations(Table V).The contents of N,Fe,Zn,Cu,and Mn in seeds showed a significant increase,whereas the P and K contents decreased with increasing CNPs/NK nanocomposite concentrations. The common bean plantstreated with the CNPs/NK nanocomposite at 10 mg L-1had the highest Fe contents of 810.3 and 807.0 mg kg-1and Zn contents of 21.10 and 21.03 mg kg-1in the leaves and the highest N content of 41.5 and 41.8 g kg-1in the seeds during the 2017 and 2018 growing seasons,respectively.When the concentration of the CNPs/NK nanocomposite increased to 30 mg L-1,the contents of N,P,K,Cu,and Mn in the leaves increased to 45.5 g kg-1,3.4 g kg-1,33.1 g kg-1,11.80 mg kg-1, and 54.1 mg kg-1in the 2017 growing season and to 45.2 g kg-1,3.3 g kg-1,32.4 g kg-1,11.73 mg kg-1,and 53.7 mg kg-1in the 2018 growing season,respectively.We also observed that using the CNPs/NK nanocomposite at 30 mg L-1resulted in the production of common bean seeds with high Zn contents of 23.5 and 23.3 mg kg-1and high Cu contents of 12.7 and 12.5 mg kg-1in the 2017 and 2018 growing seasons,respectively.Furthermore,the seeds of common bean plants treated with the CNPs/NK nanocomposite at 40 mg L-1were found to have the highest Fe contents of 89.0 and 88.4 mg kg-1and the highest Mn contents of 7.8 and 7.7 mg kg-1during the 2017 and 2018 growing seasons, respectively. Compared with the control,the contents of all minerals in common bean seeds,with the exception of P(3.2 and 3.2 g kg-1)and K(25.7 and 25.5 g kg-1),were found to be higher in each of the 2017 and 2018 growing seasons.

TABLE V Effects of different CNPs/NK nanocomposite(carbon nanoparticles(CNPs)doped with N and K)concentrations on mineral contents in leaves and seeds of common bean plants during two successive growing seasons(2017 and 2018)

Previously,it has been demonstrated that the application of biochar to wheat(Abbaset al.,2018)and maize(Naeemet al., 2018) plants improves their growth differently on different soils,which may be related to differences in soil pH and water-soluble K.The application of biochar is considered as an appropriate approach for conditioning soil from the perspective of enhancing soil K availability, although the uptake of K by crops varies depending on soil type.Soils abundant in K-bearing minerals tend to spin the effect of biochar on crop K uptake, which might promote mineral K weathering in these soils(Wanget al.,2018).Similarly,Pavlíkováet al.(2017)found that the addition of biochar to soil significantly improved spinach plant growth by 102%in spring and by 353%in autumn,while enhancing the growth of mustard plants by only 69%,biochar treatment tended to limit the contents of other minerals such as calcium(Ca),magnesium (Mg), and sodium (Na) although the plant K content increased in all plants,and the content of total free amino acids was found to be higher in mustard plants than in autumn spinach plants under all treatments.On the basis of the findings of previous studies,it can thus be deduced that plants tend to differ with respect to the sensitivity of their responses to biochar(Vaccariet al.,2011;Pavlíková.,2017;Naeemet al.,2018;Wanget al.,2018).

Contents of totalphenols and flavonoids in the leaves and seeds.Natural phenols and flavonoids are important for leaf and seed growth,owing to their specific biochemical and biological activities.Phenols can modify gene expression in plant and also function as anticarcinogenic,antioxidant,and antimutagenic agents(Sulaiman and Balachandran,2012).Flavonoids have numerous valuable biological properties,including antiulcer,antimicrobial,mitochondrial adhesion inhibition, antiangiogenic, anticancer, and protein kinase inhibition activities,and can also protect against cardiovascular diseases.However,the regulation of phenols in plant is notably complex,given that these compounds are produced in different cell types and at different stages of plant growth(Sulaiman and Balachandran,2012;Linet al.,2016).Indeed,Abd Elwahedet al.(2019)observed that the total phenol content at different stages of wheat plant growth increased with increasing biochar rates,whereas Ghorbanpour and Hadian(2015)found that the total phenol and flavonoid contents of callus extracts fromS.khuzestanicaincreased significantly upon exposure to 100 μg mL-1MWCNTs. Furthermore,Tahaet al.(2016)reported a reduction in total phenol content in Sewy date palm plants with increasing concentrations of MWCNTs,whereas there were no significant changes in total flavonoids.In the present study,we found that utilization of CNPs loaded with N and K by immersion in a solution of potassium nitrate had more advantages,because of increased surface area, which led to greater activity compared with biochar.Our assessment of the effect of foliar application of the CNPs/NK nanocomposite on total phenols and flavonoids in the leaves and seeds of common bean plants during two successive growing seasons indicated that treatment with this nanocomposite at a concentration of 20 mg L-1was sufficient to maximize the contents of phenols and flavonoids(Fig.5).

Fig.5 Effects of different CNPs/NK nanocomposite(carbon nanoparticles(CNPs)doped with N and K)concentrations on total phenol and flavonoid contents in the leaves and seeds of common bean plants during two successive growing seasons(2017 and 2018).

CONCLUSIONS

Agricultural waste, which can have a detrimental effect on the environment when burned in the open air,was used to prepare biochar, a rich source of carbon, for the preparation of CNPs.The prepared CNPs were subsequently doped with N and K, important macronutrients for plant growth, to obtain the CNPs/NK nanocomposite for foliar application to common bean plants. The growth parameters,yield,mineral contents,and total phenol and flavonoid contents of the seeds and leaves of common bean plants were significantly improved by treatment with the CNPs/NK nanocomposite compared with the control.The foliar application of the CNPs/NK nanocomposite at a concentration of 20 mg L-1was sufficient to improve the growth and yield of common bean plants.Thus,the application of the CNPs/NK nanocomposite as a foliar fertilizer can provide significant benefits in terms of improving food security and minimizing the dependency on traditional fertilizers,which is of considerable environmental and economic advantages.

ACKNOWLEDGEMENT

We would like to thank the National Research Centre,Egypt for supporting the project“Polymer nanocomposite and their applications in controlled release fertilizer and the health impacts of nanoparticles on rats” (project No.11090108).