Recycling of isabgol(Plantago ovata Forsk.)straw biomass and mineral powder with bio-inoculants as an effective soil amendment for isabgol cultivation

Biraj Bandhu BASAK and Ajoy SAHA

1Indian Councilof AgriculturalResearch-Directorate of Medicinaland Aromatic Plants Research,Anand 387310(India)

2Indian Councilof AgriculturalResearch-CentralInland Fisheries ResearchInstitute,Bangalore ResearchCentre,Bengaluru560089(India)

ABSTRACT Efficient utilization of isabgol(Plantgoovata Forsk.)straw biomass through preparation of bio-active organic fertilizer in a natural composting process has not been studied.In this study,phosphate rock and silicate mineral(mica)powder were used as natural sources of phosphorus(P)and potassium(K),respectively.Cow dung slurry as a natural decomposer was mixed with the straw biomass at a 10:1(weight/weight)ratio along with mineral powder.Then,nutrient-mobilizing bio-inoculants were used in the composting process after attaining thermal stabilization.The agronomic effectiveness of the resulting bio-active compost(BAC)as a bio-organic fertilizer was compared with that of conventional organics(farmyard manure and vermicompost)and chemical fertilizer(CF)by growing isabgol under field conditions.Composting with the natural sources of P and K along with the bio-inoculants increased the total nitrogen(13.6 g kg-1),P(38.7 g kg-1),and K(31.2 g kg-1)contents in the final product(i.e.,BAC)compared with composting without the mineral powder and bio-inoculants.Application of BAC remarkably improved the seed yield(2.5%)and husk quality of isabgol in comparison with conventional organics and CF.Compared with CF,BAC significantly boosted the economic yield of isabgol by improving the husk recovery(2.5%)and mucilage yield(4.12%).Furthermore,BAC significantly improved the soil quality by increasing organic carbon(C),available nutrients,and microbial biomass C contents,as well as enzyme activity.The positive correlation between soil and plant parameters also highlighted the benefits of BAC for isabgol production through soil quality improvement.Therefore,it can be considered as a zero-waste technology,whereby a large quantity of straw biomass generated from isabgol cultivation,which contains essential nutrients,can be recycled back to the soil.Furthermore,BAC can be effectively used as a bio-active organic fertilizer,particularly in systems where chemical inputs are restricted,such as organic agriculture.

KeyWords: agronomic effectiveness,bio-active compost,crop residue,enriched compost,low-grade minerals,organic fertilizer,soil properties

INTRODUCTION

Isabgol(PlantagoovataForsk.),an annual herb belonging to the Plantaginaceae family,is a commercially important medicinal plant cultivated in India for seed husk(the main economic part).India is the main producer of isabgol and holds a monopoly in exporting isabgol seed husk to the international market(Basak,2017a).The country earns approximately US$0.5 million annually from the export of isabgol seed husks and other products (Janakiramet al.,2019).The seed coat(seed epidermis)of isabgol is referred to as husk or psyllium husk, and is a popular traditional Ayurvedic medicine for treating constipation. The husk,obtained by mechanical milling, is a very good source of mucilage and dietary fiber(Mandalet al.,2008).Currently,psyllium husk is used as a food additive in several processed products(Thakuret al.,2012).Isabgol is cultivated in drier parts of Western India,particularly North Gujarat and adjoining parts of Western Rajasthan and Madhya Pradesh,with a crop area of approximately 3.9×105ha(Janakiramet al.,2019).However,the crop is spreading into non-traditional areas,such as Punjab,Haryana,and Uttar Pradesh,owing to the increasing demand in the international market(Mandalet al.,2008).Therefore,a larger amount of isabgol straw(IS)biomass(crop residues)is being generated;growers could further benefit if the straw biomass can be easily recycled into value-added compost.

Isabgol is suited for growth in well-drained, lighttextured(sandy to sandy loam)soils.The nutrient requirements of isabgol have been reported to be low(Kalyanasundaramet al.,1982).However,positive responses of isabgol to different nutrient application levels have been reported in a few studies(Ashrafet al.,2006;Mandalet al.,2008;Rahimiet al.,2013).According to the guidelines of the National Medicinal Plants Board of India on Good Agricultural Practices for medicinal plants,the complete substitution or minimum use of chemical inputs in the production system is advised.Alternative inputs,such as organic manure and compost, may be more suitable for growing such medicinal herbs(Sainiet al.,2007).Recently,organically grown medicinal herbs have received more attention and fetch a premium price in the international market.Considering the above scenario,enriched organic fertilizer may have promising potential for quality isabgol production by avoiding the use of costly chemical fertilizers(CFs).

The plant nutrient content in traditional organic sources,such as farmyard manure(FYM)and compost,is not suffi-cient to support crop demand,particularly in terms of major nutrients.One promising way of enriching the nutrient content in compost is blending low-grade minerals,such as rock phosphates(RPs)and silicate minerals,with straw biomass during composting. Most Indian RPs are considered lowgrade for the commercial production of phosphate fertilizers owing to their low phosphorus(P)content(Narayanasamy and Biswas, 1998). Furthermore, significant amounts of silicate mineral powders are generated in India as mining byproducts,e.g.,waste mica(WM),a promising source of potassium(K)(Basak,2019).However,the bioavailability of P and K from these mineral powders can be enhanced by biological interventions,such as composting(Biswaset al.,2009)and microbial inoculation(Girgiset al.,2008).Therefore,it was hypothesized that composting straw biomass with mineral powder coupled with nutrient-mobilizing microbial intervention would provide a nutrient-enriched organic formulation. However, the nutrient availability from such enriched compost and its role in the yield and quality of isabgol have not yet been studied.Therefore,the main objectives of this study were to recycle the straw biomass into enriched compost and subsequently evaluate its performance compared with that of CF, as well as with other potential organic sources,such as FYM and vermicompost(VC),for isabgol production and soil quality improvement.

MATERIALS AND METHODS

Collection of raw materials and bio-inoculants

In this study,RP and WM were used as natural sources of P and K, respectively. The RP sample was procured from Rajasthan State Mines & Minerals Ltd., Udaipur,Rajasthan,India,and was considered low-grade owing to its low total P content(≤250 g P2O5kg-1)(Narayanasamy and Biswas,1998).The WM sample(muscovite),generated as a byproduct of mica mining,was collected from the Nellore District,Andhra Pradesh,India.Both mineral samples were crushed to powder in a Wiley mill(150-μm size)for further use.The RP sample had 0.02 g kg-1water-soluble P(WSP),17 g kg-1citrate-soluble P,and 94.1 g kg-1total P(Basak,2017b). The WM sample contained 0.11 g kg-1watersoluble K,0.21 g kg-1exchangeable K,1.56 g kg-1nonexchangeable K,and 80.7 g kg-1total K(Basak,2019).

The IS biomass was harvested from an experimental field of the Indian Council of Agricultural Research(ICAR)-Directorate of Medicinal and Aromatic Plants Research(DMAPR),Anand.After harvesting the seeds,the sun-dried straw biomass was stored for further use.The IS biomass had nutrient contents of total carbon(C)of 379.8 g kg-1,total nitrogen(N)of 9.7 g kg-1,total P of 2.8 g kg-1,and total K of 7.4 g kg-1,with a C:N ratio of 40.8.Fresh cow dung was used as a natural inoculum(Zhuet al.,2015)for the decomposition of IS during composting.The cow dung was collected from a local cattle shed at Anand, India, with properties of pH of 7.3, total C of 412.3 g kg-1, total N of 20.8 g kg-1,total P of 3.3 g kg-1, total K of 4.1 g kg-1, and a C:N ratio of 19.8.Phosphate-solubilizing(Bacillus coagulans)and K-mobilizing(Enterobacter asburiae)bacteria obtained from the Biofertilizer Production Unit,Anand Agricultural University,Anand,India were used as bio-inoculants.The bio-inoculants were multiplied in the laboratory in broth culture.The cell concentration in the liquid was adjusted to 2×106colony forming units mL-1by centrifugation.

Preparation of bio-active composts(BACs)

The dried IS was chopped into small pieces(5–6 cm).The different composts from various treatment combinations of IS,mineral powder,and bio-inoculants were as follows:IS + cow dung (CD) (C1), IS + CD + mineral powder(2% RP + 2% WM) (C2), IS + CD + mineral powder(4% RP + 4% WM) (C3), IS + CD + mineral powder(2%RP+2%WM)+bio-inoculants(C4),and IS+CD+ mineral powder (4% RP + 4% WM) + bio-inoculants(C5).The enriched compost was prepared following a wellestablished method proposed by Biswaset al.(2009).Fresh CD was made into a slurry and applied at 10%of the straw biomass as a natural inoculum,and proportional quantities of IS,CD,and mineral powder were applied at two ratios,25:2.5:1 and 25:2.5:2.Accordingly,114 and 118 kg of initial raw materials were used for the composting experiment.Regular monitoring and sprinkling of the required quantity of water maintained the moisture content of the composting mixtures at approximately 60%on a dry weight basis.The bio-inoculants (B. coagulansandE. asburiae) were added to the compost mixtures (5 mL culture kg-1) after reaching thermal stabilization to avoid the adverse effects of heat on the bio-inoculants (Suthar, 2008). The whole composting process was continued for 120 d,comprising the bio-oxidative phase up to 60 d,followed by the stabilization(30 d)and maturing phases(30 d).The composting mixtures were turned periodically after 7,15,30,60,90,and 120 d to facilitate sufficient aeration(Biswaset al.,2009).In the biooxidative phase,frequent turning(four turns)was performed to stabilize the temperature,and six turnings were performed in total to obtain mature compost at 120 d.

The visible appearance(color and texture)was used to determine the maturity of the composts. The appearance of a brownish black color,uniformity of the material,and earthy smell were considered indicators of compost maturity(Bernalet al.,2009).In addition,the C/N ratio(Goyalet al.,2005)and germination index(GI)(Tiquia,2005),two widely used indicators of compost maturity,were also considered.

Characterization of the BACs

The composts were harvested from each treatment at maturity(120 d)based on the appearance of a dark brown color on the surface and placed on polypropylene sheets for air-drying in the shade.The air-dried samples were pulverized and homogenized by passing through a 1-mm sieve.The samples were analyzed for their total C,N,P,K,and available P and K contents.Total C was measured using a CHN analyzer(Elementar,Germany),total N was analyzed using the Kjeldahl method(Bremner and Mulvaney,1982),and the C:N ratio was calculated.For analysis of total P and K,HNO3/HClO4mixture(9:4)was used to digest the compost samples(Jackson,1973).The P content was analyzed using a spectrophotometer by developing a vanadomolybdophosphate yellow-colored complex.The acid-digested solution was diluted with distilled water,and the K content was analyzed using a flame photometer.The available P was analyzed by extraction with 0.5 mol L-1NaHCO3solution (Olsenet al.,1954),followed by estimation with a spectrophotometer by developing a blue-colored complex (Watanabe and Olsen,1965).Neutral normal ammonium acetate solution(pH 7.0)was used to extract available K from compost samples (Hanway and Heidel, 1952), and K concentration in solution was analyzed using a flame photometer.

Agronomic evaluation

The agronomic effectiveness of the enriched compost was evaluated under field conditions in an experimental field of ICAR-DMAPR, Anand, India (22°35′N, 73°27′E) at an altitude of 45.1 m.The area falls in a semi-arid region(average annual precipitation of 860 mm)with mild winters and hot summers. The details of the weather parameters that prevailed during the field experiment are shown in Fig. 1. Fodder sorghum (Sorghum vulgare) was grown to exhaust the experimental soil before the commencement of the study.Some basic properties and nutrient contents of the exhausted soil are shown in Table I.The experimental soil was classified to the Fluventic Ustochrept subgroup (Soil Survey Staff,2010).

Fig.1 Weather parameters of the experimental field of the Indian Council of Agricultural Research-Directorate of Medicinal and Aromatic Plants Research,Anand,India during the experiment period.

TABLE I Initial physiochemical properties of the soil(sandy loam)before the commencement of the field experiment on a field of the Indian Council of Agricultural Research-Directorate of Medicinal and Aromatic Plants Research,Anand,India

The field experiment was conducted by growing isabgol(PlantagoovataForsk.)as a test crop during the winter season.Isabgol is a dry land crop best cultivated in well-drained light-textured soil with pH 7.0–8.5. The organic sources were analyzed for their total nutrient contents(Tables II and III), and applications of individual organic sources were calculated based on the recommended dose of N (30 kg N ha-1): FYM, 5.77 t ha-1, VC, 2.68 t ha-1, and BAC,2.20 t ha-1.Five treatments were set up in the field experiment with applications of different organics or CF:control(CK, no fertilization), FYM, VC, BAC, and CF (applied at recommended dose).Urea and diammonium phosphate(DAP)were applied at 30 kg N ha-1and 25 kg P2O5ha-1,respectively. Additionally, N application was balanced by subtracting the amount supplied by DAP.The field experiment was conducted by laying out treatment plots (4.2 m× 3.6 m) in a randomized block design (RBD) with four replications.Calculated amounts of chemical and organic fertilizers were mixed thoroughly into the soil 15 d before sowing.The isabgol seeds(Gujarat Isabgol 2)were broadcast in the experimental plot during the second week of November.Necessary intercultural operations and irrigation scheduling were performed to ensure the optimum stand of isabgol crops in the field experiment.

TABLE II Effects of mineral powder(a mixture of rock phosphate(RP)and waste mica(WM))and bio-inoculants(P and K solubilizers Bacillus coagulans and Enterobacter asburiae,respectively)on compost quality

TABLE III Basic properties of the organic sources used in the field study conducted on an experimental field of the Indian Council of Agricultural Research-Directorate of Medicinal and Aromatic Plants Research,Anand,India

Growthand yield parameters

Five plants from each plot were randomly tagged to observe the plant biometric parameters(height and number of tillers). Composite leaf samples were collected at the flowering stage from each plot for chlorophyll content analysis.Chlorophyll was extracted from samples taken from the center of fresh leaves using 80%(volume:volume)acetone.The absorbance of the filtrate was measured at 645 and 663 nm using a spectrophotometer, and the total chlorophyll content was determined according to Arnon’s equation(Arnon, 1949). Isabgol was harvested 120 d after sowing from each plot, and the total biomass yield was recorded after air-drying. Seeds were separated from the harvested biomass by mechanical thrashing and cleaning.The total dry biomass and seed yield were recorded separately from each plot.The test weight was estimated by taking the average of three measurements of the 1 000-seed weight from each plot(AOAC,1990).

Nutrient contents in plants

The plant sample was dried at 65°C for 48 h in a hot air oven and pulverized in a Wiley mill(5-mm size). The powder sample was digested on an electric hot plate with HNO3/HClO4(9:4)(Piper,1967).The K concentration in the digested solution was measured using a flame photometer(Model 128, Systronics, India). The P concentration in the digest was analyzed using a spectrophotometer(Model 117, Systronics, India) after developing a yellow-colored complex(vanadomolybdophosphate)(Jackson,1973).For estimation of total N, plant samples were digested in a Kjeldahl digestion block using H2SO4along with a catalyst mixture(K2SO4/CuSO4/Se,200:10:1)at 400±5°C.Then,an automatic distillation of the digested samples was followed by titration in the Kjeldahl unit for measuring the N content.

Seed husk recoveryand qualityanalysis

The husk recovery was recorded after the husk was detached from the seed.A hot water and dilute HCl treatment was used to dissolve the polysaccharide from the seed.Husk recovery was estimated based on the loss of seed weight.To estimate the swelling factor,1 g of seed was placed into a 25-mL measuring cylinder with 20 mL of distilled water.Seed swelling was calculated based on the volume increase after 24 h(Sharma and Koul,1986).The seed mucilage content was estimated using the techniques described by Sharma and Koul (1986) with minor modifications. Isabgol seeds(1 g)were added to 50 mL boiling solution of 0.5 mol L-1HCl and stirred for 30 min until the seeds changed color.To separate the mucilage,the hot solution was passed through a muslin cloth.The seeds were washed twice with hot water to remove traces of mucilage adhering to the seeds. The combined filtered solution consisting of dissolved mucilage was mixed with 60 mL acetone,stirred,and allowed to stand to obtain a substantial amount of precipitate.The precipitate was separated and dried,and the final weight was taken to represent the total mucilage content.

Soilsampling and analysis

After harvesting the isabgol crop,soil samples(0–15 cm)were collected from each plot.Immediately after collection,part of soil samples were kept separately in a refrigerator(4°C).The remaining soil samples were air-dried,ground,and processed for analysis of the soil organic C (SOC),mineral N,and available P and K contents as per the standard methods(Table I).The refrigerated samples were allowed to reach room temperature(25°C)for analysis of microbial biomass C (MBC), respiration, and enzyme activity. Soil MBC was measured following the procedure described by Jenkinson and Ladd (1981). The alkali trap method (Anderson,1982)was used to estimate soil respiration(SR)by determining the CO2-C release during incubation.Dehydrogenase activity(DHA)was assayed by measuring the rate of triphenylformazan production during incubation(Kleinet al.,1971).

Statisticalanalysis

The data generated in both the laboratory and field studies were expressed as the means of four replicates.Analysis of variance(ANOVA)was applied as per the experimental design(i.e.,RBD).Data calculation,tabulation,and graphical representations were performed using Microsoft Excel(Microsoft Corporation,USA).A Pearson’s correlation matrix between various plant and soil parameters was developed using SPSS software version 24(SPSS Inc.,Chicago,USA).RESULTS AND DISCUSSION

Nutrient composition of BACs

An overall improvement in nutrient contents and a decrease in C content were observed in the mature composts compared with the initial biomass(i.e.,IS)used(Table II).The IS had high C and low N content,which was reflected in the high C/N ratio(40.8).However,owing to composting,the C content was significantly decreased in the final product,irrespective of the treatment.On average,a 36.9%decrease in C content and 37.7%increase in N content were observed in the mature compost compared with the initial biomass.The C content decreased significantly in the composts enriched with mineral powder and bio-inoculants compared with C1.Total N increased in the different types of composts after composting,by 30.9%–40.2%.Total N was markedly higher in the compost enriched with microbial inoculation and mineral powder than in C1. The integration of mineral powder and bio-inoculant increased the total N by 7%.However,the total N content of the enriched compost was not influenced by the rate of mineral powder application.The dilution of composting biomass due to higher decomposition might have decreased the total C in the enriched compost (Basak, 2017b). This result corroborates earlier reports(Biswaset al.,2009;Guptaet al.,2016)of a decrease in the C content in BAC due to microbial decomposition and the mineralization of organic C.The significant increase in N content in the mature compost compared with the initial substrate might be due to the net loss of biomass in the form of CO2due to C mineralization(Dekaet al.,2011;Boruahet al.,2019).A further increase in N content can be explained by the contribution of N mineralization during composting.The high C/N ratio(40.8)of the initial substrate was found to decrease significantly and stabilize at 17.0–20.0 in the mature composts.The C/N ratio in C1(20.0)was higher than those in the other four composts(17.0–18.3).Generally,the C/N ratio is considered a key indicator of compost maturity.A C/N ratio＜20 is regarded as an acceptable limit of compost maturity for agronomic use(Goyalet al.,2005).In this investigation,all composts treated with bio-inoculants registered a C/N ratio much lower than the acceptable limit(＜20).The GI values ranged between 67.1%and 54.7%for all mature composts obtained in this study(data not shown).A GI value of 70%–80% was proposed as the threshold value,based on many studies involving manures/composts(Tiquia, 2005; Raj and Antil, 2011). Hence, a GI value＜70%may be accepted as an indicator of the disappearance of phytotoxic substances and maturity of compost prepared from straw biomass and mineral powder.

A markedly improvement in WSP content was recorded in compost prepared with mineral powder(low-grade RP)as compared with the initial substrate(Table II).A simple addition of RP during the composting process significantly improved WSP and total P compared with C1.Further,the addition of bio-inoculants resulted in significantly higher WSP(0.28 and 0.51 g kg-1in C4and C5,respectively)in BAC than in the composts without addition of bio-inoculants(0.12 and 0.23 g kg-1in C2and C3,respectively).The increased WSP and total P contents in compost with an increase in the RP application rate from 2%to 4%indicated that RP was the key substrate for P enrichment.The significant release of P from RP during composting was reflected in the higher WSP content in RP compost.Kumariet al.(2008)reported that significant amounts of CO2and organic acids were released during composting due to the decomposition of the organic substrate.These acids accelerated the dissolution of P from RP,leading to an increase in WSP.Furthermore,the application of phosphate-solubilizing bacteria also plays a key role in the solubilization of P from RP(Goldstein,1986).The increased WSP content in the bio-inoculated compost clearly indicated bacteria-mediated P solubilization.

Water-soluble K(WSK)increased by 16.5%–40.1%due to addition of WM powder compared with C1(Table II).An increase in WSK content with an increase in WM application rate from 2% to 4% indicated that WM was the key contributing factor for K enrichment in BAC.The K enrichment might have occurred as a result of the dissolution of WM into available forms,owing to the prevalence of an acidic environment during composting.However,the inoculation of K-solubilizing bacteria plays a crucial role in solubilizing K from WM by releasing organic acids (Linet al., 2002;Biswas and Basak,2014).Organic acids are involved in the direct dissolution of silicate minerals or chelation with the structural cations of minerals (Basaket al., 2017). Significant increases(11.5%–16.5%)of WSK in bio-inoculated compost over uninoculated compost gave specific indication of bacteria-mediated K solubilization from WM.Because C5had the highest nutrient contents and availability,it was selected as the BAC treatment in the field experiment.

Agronomic evaluation of BAC

Plant growthand physiology.The results demonstrated that the plant height of isabgol was significantly increased by fertilization compared with CK(Table IV).The highest(35.1 cm)and lowest(28.9 cm)plant heights were recorded in the BAC and CK treatments, respectively. Fertilization with organic fertilizers(FYM and VC)or CF significantly increased plant height compared with CK. However, the plant heights recorded in the FYM,VC,and CF treatments were significantly lower than that in the BAC treatment.This improvement in plant height might be due to the increase in chlorophyll content with the application of balanced nutrition in the form of BAC.The improvement in plant height due to the application of enriched compost has been reported in senna(Basak and Gajbhiye,2018).A significantly higher number of tillers per plant was recorded in the BAC treatment than in the FYM and CK treatments. However, the BAC treatment did not show any significant differences with reference to the VC and CF treatments.The application of BAC,CF,and VC increased the number of tillers by 53%,42%,and 37%,respectively,compared with CK.However,the FYM treatment did not result in a significant increase in tiller number compared with CK.Tiller number is one of the most important yield-contributing characteristics of isabgol.The balanced supply of nutrients from BAC might have stimulated plant growth and physiology,which allowed the accumulation of more photosynthates and thereby increased the number of tillers per plant.These findings are in close agreement with the reports of Khalil(2006)and Raissiet al.(2012)on isabgol.Similarly,Naziret al.(2006)reported the highest runners per plant in strawberry with the combined application of poultry manure,Azotobacter,wood ash,VC,and oil cake among different treatment combinations.

Leaf chlorophyll content indicates plant photosynthetic capacity.Lower chlorophyll content limits photosynthetic potential and thus decreases plant biomass(Rahimiet al.,2013).In this study,different treatments had significant impacts on leaf chlorophyll content(Table IV).The application of BAC resulted in the highest leaf chlorophyll content.Although FYM application resulted in a lower chlorophyll content, it was still superior to CK. The application of balanced nutrition in the form of BAC may have led to a higher chlorophyll content. The CF application also increased chlorophyll content,corroborating a previous study in which N addition increased the chlorophyll content in isabgol(Aishwath and Chandra,2008;Rahimiet al.,2013).The increased vegetative growth with BAC application might be due to the production of growth regulators and hormones that stimulate nutrient assimilation by the roots(Khanet al.,2015).

Plant nutrient contents and nutrient uptake.The treatments with VC,BAC,and CF application showed significantly higher nutrient(N,P,and K)contents than CK and the FYM treatment(Fig.2).The highest nutrient contents were recorded in the BAC treatment.The N content in the CF treatment (18.2 g kg-1) was close to that in the BAC treatment(18.6 g kg-1).However,the P(8.5 g kg-1)and K(22 g kg-1)contents in the BAC treatment were significantly higher than those in the VC and CF treatments. Nutrient uptake by isabgol followed a trend similar to that of nutrient content in plants.The P and K uptake levels were found to be significantly higher in the BAC treatment than in the other treatments. However, N uptake in the BAC treatment was found to be close to that in the CF treatment.

The results clearly indicated that BAC application significantly increased the N,P,and K contents in plant tissue,which translated into the total N,P,and K uptake by isabgol(Fig. 2). Moreover, variations in nutrient content in planttissue with regard to organic sources were identical to those reflected in yield parameters(Table IV).The greater nutrient uptake by isabgol in the BAC treatment indicated that nutrient availability was well synchronized with plant demand throughout the growth period.This might be the crucial factor behind the better performance of BCA over CF and other organic sources.The increased nutrient content and uptake,particularly those of P and K,with BAC application can be explained by the higher P and K bioavailability throughout the plant growth period.The mobilization of P and K from mineral powder(RP and WM)during composting(Basak,2017b, 2018) might have contributed to the improved P and K bioavailability in BAC.Furthermore,microbial inoculation might have augmented P and K solubilization from RP(Zayed and Abdel-Motaal,2005)and WM(Badr,2006;Biswas and Basak,2014),resulting in higher available P and K in the final product of BAC.The organic matter present in the compost also protects the soluble P from fixation(Lata Verma and Marschner,2013),which ensures a continuous and steady supply of P,contributing to higher P content and uptake.The outcome of this investigation is in accordance with those of previous studies(Nishant and Biswas,2008;Basak and Gajbhiye,2018).

TABLE IV Effects of organic and chemical fertilizer applications on the growth and yield parameters of isabgol in the field study conducted on an experimental farm of the Indian Council of Agricultural Research-Directorate of Medicinal and Aromatic Plants Research,Anand,India

Fig.2 Plant nutrient contents(a)and nutrient uptake(b)in different treatments with organic or chemical fertilizer applications in the field study conducted on an experimental farm of the Indian Council of Agricultural Research-Directorate of Medicinal and Aromatic Plants Research,Anand,India.Bars are standard errors of the means(n=4).CK=control with no fertilization;FYM=farmyard manure;VC=vermicompost;BAC=bio-active compost;CF=chemical fertilizer.

Biomass and seed yield.Biomass yield was significantly improved by both organic and chemical fertilization(Table IV).The highest(1 949.8 kg ha-1)and lowest(1 710.8 kg ha-1) dry biomass yield was obtained in the BAC and CK treatments, respectively. The application of BAC increased straw yield by 14%compared with CK.The CF treatment resulted in significantly higher dry biomass yield (1 937.2 kg ha-1) than the FYM (1 740.2 kg ha-1)and VC(1 815.3 kg ha-1)treatments.However,BAC did not result in a significantly higher dry biomass yield than CF.Application of BAC might have resulted in a high photosynthetic rate,which helped to increase the tiller number per plant and,subsequently,the leaf number and overall yield.Similar results were also reported in isabgol(Khalil,2006),where a substantial improvement in plant growth parameters was due to the combined application of organic manures and biofertilizer. The biological yield of isabgol was also reported to improve after application with VC(Raissiet al.,2012),which contains both macro-and micro-nutrients.

The seed yield was significantly improved by the application of both organic fertilizers (FYM and VC) and CF(Table IV).However,the highest seed yield(833.2 kg ha-1)was observed in the BAC treatment,followed by CF(812.8 kg ha-1).The study revealed that the organic and CF treatments also significantly affected the 1 000-seed weight.However,the BAC(1.85 g)and CF(1.80 g)treatments resulted in significantly higher 1 000-seed weights than the FYM(1.59 g)and VC(1.70 g)treatments.The 1 000-seed weight is an indicator of the boldness of seeds, which is closely related to the husk (seed epidermis) yield. These observations indicate that BAC application resulted in the highest 1 000-seed weight, which might have contributed to the economic yield(husk).This may be an evidence of increased carbohydrate storage in the seed epidermis.The highest seed yield obtained in the BAC-treated plot may be due to bolder seeds and more siliqua owing to the high nutrient availability of BAC(Table III)throughout the plant growth period(Singhet al.,2003;Raissiet al.,2012).Previous studies also supported the finding that the application of manure and compost improved the seed yield of isabgol(Intodia and Tomar, 1998; Mirshekari and Forouzandeh,2015).

Qualityof seed husk.The application of VC,BAC,and CF had a significant influence on husk recovery from isabgol seeds(Table V).Application of BAC resulted in the highest husk recovery(35.2%),which was significantly higher than that of VC(34.0%)and CF(33.6%).The highest mucilage content(212.3 g kg-1)and swelling factor(12.03%)were recorded in the treatment with BAC.The next best results for mucilage content (205.6 g kg-1) and swelling factor(11.70%)were obtained in the VC treatment,which produced similar results to the CF treatment.Higher mucilage content in isabgol by organic fertilizer application was due to the improvement of soil physiochemical properties(Singhet al.,2003),which,in turn,led to higher nutrient availability and plant assimilation(Yadavet al.,2003).The higher mucilage content in the BAC treatment as compared with the other organic treatments might be due to the significantly higher P and K uptake levels(Mirshekari and Forouzandeh,2015)(Table III). Swelling factor is the most important quality parameter of husk, and a significant positive relationship with the mucilage yield was observed.The strong correlation between the swelling factor and mucilage yield in isabgol found in this study reaffirmed previous reports (Ebrahim Zadehet al.,1998;Pouryousefet al.,2007).The mucilage content ofPlantagoafrawas found to be increased by the combined application of organic manures and biofertilizers(Khalil,2006).

Changes in soilproperties

Soilfertility status.The experimental plots treated with organic sources (FYM, VC, and BAC) of nutrients recorded 10.2%–17.3%higher SOC content than that treated with CF(Table VI).The highest SOC content was observed in the FYM treatment,which was 17.3%and 14%higher than that in CF and CK, respectively. However, the BAC treatment resulted in a significantly higher SOC (10.2%)than the CF treatment.A comparatively higher amount of organic substrate was supplied through FYM(5.77 t ha-1)than through VC(2.68 t ha-1)and BAC(2.20 t ha-1),which might have contributed to the higher SOC content. This observation corroborates the findings of previous studies where the application of FYM(Srinivasanet al.,2016)and enriched compost (Biswas, 2011) resulted in 15.2% and 16.7% increases in SOC content, respectively, compared with the application of CF.The application of organic inputs(manure and compost)in agricultural soils plays a synergistic role in the buildup of the soil C pool.In contrast,the rapid mineralization of native soil C in the CF treatment leads to a decrease in SOC content (Srinivasanet al., 2016). The results of this study are also supported by previous reports(Biswas,2011;Sharmaet al.,2014).

The application of organic fertilizers and CF resulted in a significant change in the available N content in soil(Table VI). The highest available N (47.6 mg kg-1) was observed in the BAC treatment,followed by the VC treatment.The treatments receiving BAC and VC recorded significantly higher available N content in soil than the treatment receiving CF.Therefore,the application of these composts might have enriched the soil N pool, which leads to increased N use efficiency by releasing more mineral N (Rees and Castle,2002).This result followed a similar trend to that in a maizewheat cropping system(Mandalet al.,2007;Sharmaet al.,2014),where significantly higher mineral N was built up in the organic treatment than in the CF treatment.

TABLE V Effects of organic and chemical fertilizer applications on husk recovery and quality of isabgol seeds in the field study conducted on an experimental farm of the Indian Council of Agricultural Research-Directorate of Medicinal and Aromatic Plants Research,Anand,India

TABLE VI Effects of organic and chemical fertilizer applications on soil chemical and biological propertiesa) in the field study conducted on an experimental farm of the Indian Council of Agricultural Research-Directorate of Medicinal and Aromatic Plants Research,Anand,India

The application of both organic fertilizers and CF significantly improved soil available P status compared with CK (Table VI). However, the BAC treatment resulted in the highest available P(24.1 mg kg-1)in soil,which was significantly higher than that in the CF treatment.The total P added through BAC was much higher than that added through other organics and DAP,which might be reflected in the higher soil available P. The organic matter present in BAC protects soluble P from fixation(Wuet al.,2007)in neutral and alkaline soils by occupying the adsorption site in the soil matrix and through complexing with cations responsible for secondary reaction products(Basak,2017b).The rapid transformation of P into microbial biomass may also protect available P from fixation and subsequently contribute to the available pool (Lata Verma and Marschner,2013).This result is in close agreement with other studies(Nishant and Biswas, 2008; Basak and Gajbhiye, 2018),where RP compost was found to significantly contribute to the accumulation of available P in soil.

The available K content in soil followed a pattern identical to that of available P(Table VI).The BAC-treated plots recorded the highest available K(121.5 mg kg-1)in soil,followed by the CF-treated plots(109.4 mg kg-1).The BAC treatment resulted in significantly higher available K than the other treatments,including the CF treatment.This can be explained by the significantly higher addition of total K through BAC and the synergistic effects of mineral, microbe, and organic interactions(Basaket al.,2017).The mobilization of K from the compost and subsequent bio-activation of mineral K (in WM) by organic matter and microbes (Badr, 2006)might have contributed to the higher available K in soil.This outcome is in accordance with previous studies,where the application of compost with silicate minerals(Basak,2018),as well as K-solubilizing bacteria(Badr, 2006), improved the available K content in soil.

Soilbiologicalstatus.The results presented in Table VI indicate that the soil biological parameters MBC,respiration,and DHA were significantly influenced by organics and CF application.The MBC was significantly higher in the BAC treatment than in the CF,FYM,and VC treatments. The MBC represents the living component of soil organic matter and responsible for mineralization and nutrient turnover (Mandalet al., 2007). Here, a significant increase in MBC in the BAC treatment indicates high microbial proliferation due to the synergistic role of organic substrate and nutrient availability (Kukrejaet al., 1991;Goyalet al.,1993).This outcome agrees with a soil incubation study(Basak,2017b)and a field experiment(Meena and Biswas,2014),where higher MBC contents due to the application of enriched compost were reported.The values of SR and DHA also followed the same pattern as MBC.The highest values of SR(3.43 mg CO2-C kg-1soil d-1)and DHA(42.3 μg triphenylformazan g-1h-1),recorded in the BAC treatment,were significantly higher than those in the CF treatment.This result indicates the negative impacts of CF on soil biological properties.The parameters SR and DHA are important indicators of soil microbial activity and specifically represent the physiologically active microbial population (Nannipieriet al., 1990). Here, the supply of both organic matter and nutrients from BAC stimulated the metabolic activity of soil microbes,which was reflected in the higher SR and DHA values.This result was supported by the findings of previous studies (Nishant and Biswas,2008;Meena and Biswas,2014),where a higher soil enzyme activity was found with enriched compost application.

Correlation matrix.Pearson’s correlation matrix revealed that plant growth,yield,and soil quality parameters were significantly (P ＜0.01) correlated with each other(Table VII). There were significant positive correlations between plant dry matter and the contents of chlorophyll(r=0.93,P ＜0.01), available N(r=0.99,P ＜0.01),available P(r=0.98,P ＜0.01),and available K(r=0.84,P ＜0.01).Similarly,seed yield was also highly correlated with plant chlorophyll content (r= 0.97,P ＜0.01) and nutrient(N,P,and K)contents.There were also significant positive correlations between chlorophyll content and plant N (r= 0.95,P ＜0.01), P (r= 0.98,P ＜0.01), and K(r=0.86,P ＜0.01)contents.These results corroborate the findings of Aishwath and Chandra(2008),who reported a strong correlation between chlorophyll content and leaf nutrient contents.A significant and positive correlation between husk recovery and plant nutrient contents indicated their important impacts on the economic yield of isabgol.Husk quality parameters, such as swelling factor and mucilage content, were also significantly and positively correlated with plant nutrient contents.A strong positive correlation(r=0.93,P ＜0.01)was found between mucilage content and swelling factor.This result indicated a strong influence of mucilage content on swelling factor,which was confirmed in earlier studies(Ebrahim Zadehet al.,1998;Pouryousefet al.,2007).Soil available nutrients(N,P,and K)were well correlated with soil biological properties.Significant positive correlations were observed between plant nutrient content and soil available nutrients.Therefore,available nutrients in soil might have contributed to plant growth and nutrition,which is reflected in the yield and quality of isabgol.The close relationship between available nutrients and biological properties suggests that BMC and DHA play crucial roles in soil nutrient mobilization (Mandalet al., 2007).Therefore,the application of BAC might have contributed to plant growth and yield by improving the available nutrient status in soil during the plant growth period.These resultsare in accordance with other studies in which value-added compost contributed to the yield and quality of senna(Basak and Gajbiye, 2018) and kalmegh (Basaket al., 2020) by improving soil properties.

TABLE VII Correlation coefficients among soil properties and the yield and quality attributes of isabgol in the field study conducted on an experimental farm of the Indian Council of Agricultural Research-Directorate of Medicinal and Aromatic Plants Research,Anand,India

CONCLUSIONS

This study demonstrates the successful utilization of waste biomass and low-grade mineral powder for the production of inexpensive BAC,which can be a precursor for environmental sustainability with the potential of phasing out CFs in isabgol cultivation. Therefore, this can be referred to as zero-waste approach,whereby a large quantity of straw biomass generated from isabgol cultivation,which contains essential nutrients,can be recycled back into the soil through composting.Furthermore,the blending of lowgrade minerals(RP and WM)with composting biomass and bio-inoculants resulted in significant enrichment of major essential nutrients in BAC compared with other organic fertilizers(FYM and VC).The BAC had a remarkable positive impact on isabgol plant growth and nutrition,even more so than CFs. The BAC also improved soil quality by enhancing available nutrient contents and biological activities.Moreover, BAC application reduces the use of CFs while utilizing natural resources.However,a detailed field evaluation is required to determine the economic feasibility of the composting technology for organic isabgol production.

ACKNOWLEDGEMENTS

This research work was supported by the Indian Council of Agricultural Research (No. DMAPR/2015/P-6/4). The authors gratefully acknowledge the ICAR-DMAPR for providing the facilities required to undertake this study.