Sugarcane bagasse amendment mitigates nutrient leaching from a mineral soil under tropical conditions

Nan XUJehangir H.BHADHA∗Abul RABBANYStewart SWANSONJames M.MCCRAYYuncong LISarah L.STRAUSS and Rao MYLAVARAPU

1Horticultural Sciences Department,Universityof Florida,Gainesville FL 32611(USA)

2Everglades Research and Education Center,Universityof Florida,Belle Glade FL 33430(USA)

3Department of Soil,Water,and Ecosystem Sciences,Universityof Florida,Gainesville FL 32611(USA)

4Institute of Food and Agricultural Sciences(IFAS)Extension,Universityof Florida,LaBelle FL 33935(USA)

5AgronomyDepartment,Universityof Florida,Belle Glade FL 33430(USA)

6Department of Soil,Water,and Ecosystem Sciences,Universityof Florida,Homestead FL 33031(USA)

7Department of Soil,Water,and Ecosystem Sciences,Universityof Florida,Immokalee FL 34142(USA)

ABSTRACT Large quantities of organic by-products are generated by the sugarcane industry during sugar extraction process.These by-products may be used as soil amendments to improve soil quality,as nutrient leaching is common in mineral soils of Florida in USA due to their sandy texture and frequent rain events.A soil column study was designed to evaluate the effects of bagasse application at 85 t ha−1 of fresh bagasse,170 t ha−1 of fresh bagasse,and 170 t ha−1 of fresh bagasse+nitrogen(N)on the leaching potential of carbon(C),N,phosphorus(P),and potassium(K).Bagasse was incorporated within the topsoil(0–15 cm)in outdoor soil columns exposed to natural conditions,with periodical irrigation during the experiment.After approximately 57 weeks,the distributions of C,N,P,and K were evaluated at three soil depths(i.e.,0–15,15–30,and 30–53 cm)within the soil columns.Total organic C(TOC),N,and P in leachates decreased significantly from the soils amended with bagasse compared with the control with no bagasse and no N.Based on K content changes in the columns,bagasse could also be utilized as a potential source of K for plants.Overall,application of bagasse as a soil amendment might be an effective way to sustainably reutilize organic by-products while reducing concerns regarding major nutrients entering groundwater resources.The results of this study could assist in developing nutrient management plans of using sugarcane bagasse as a potential soil amendment in mineral soils.

KeyWords:nutrient leaching potential,nutrient loss,pore volume,sandy soil,soil amendment,soil column,sugarcane by-product

INTRODUCTION

The mineral soils of Florida,USA are characterized by low water-holding capacity(WHC)and nutrient-holding capacity due to their sandy texture and low organic matter(OM)content(Xuet al.,2019),which results in the leaching of essential nutrients below the plant root zone.South Florida typically receives an average annual precipitation of 1 417 mm(1981–2010 National Oceanic and Atmospheric Administration National Climatic Data),which makes nutrient lossesvialeaching a common phenomenon in the region.In addition,the conventional crop production systems on the mineral soils of South Florida rely on frequent application of commercial chemical fertilizers to maintain high yields.However,more than 50%of the applied nitrogen(N)fertilizers are subject to potential leaching or runofflosses(Yanget al.,2007a).

Many studies have been carried out to evaluate best management practices to improve soil quality and reduce potential nutrient leaching risks(Liet al.,1997;Burgoset al.,2006;Stark and Richards,2008;Dereet al.,2012).One of the common management practices is adding organic amendments to soils(Burgoset al.,2006;Yanget al.,2007a;Colombaniet al.,2015),which can improve soil quality by increasing OM content,WHC,and cation exchange capacity(CEC),reduce soil bulk density(BD),and improve soil structure(Mbagwu,1989;Alvarez-Camposet al.,2018).Moreover,plant essential nutrients are added through organic amendment application,thus potentially reducing the requirements for commercial inorganic fertilizers(Burgoset al.,2006).

The Florida sugar industry has a sugarcane growing area of approximately 1 600 km2,employs over 14 000 people,and has an annual income of over US$800 million,and a total(direct and indirect)economic value of over US$2 billion(Alvarez-Camposet al.,2018;Orndorffet al.,2018).Nearly 2.5 million metric tons of bagasse are generated by the sugar extraction mills during the harvest seasons(Bhadhaet al.,2020).Bagasse is the dry and fibrous organic material that remains after the extraction of sugar-containing juice from sugarcane stalks(Dotaniyaet al.,2016;Bhadhaet al.,2017).Although bagasse is currently used as a fuel in the boilers of sugar factories to generate electricity in Florida,there is still a large surplus of bagasse that requires disposal.Application of these locally generated agricultural by-products as organic amendments to mineral soils in South Florida could not only improve soil quality and subsequently reduce nutrient leaching potential,but also provide a cost-effective method of disposal with minimum transportation costs.

Thus far,studies related to sugarcane by-products in South Florida have concentrated on demonstrating their effectiveness in improving crop production and yields(Gilbertet al.,2008;Alvarez-Camposet al.,2018;Orndorffet al.,2018),but few have focused on evaluating the potential of these sugarcane by-products to minimize nutrient leaching in this region.Studying the dynamics of soil nutrients,especially the major nutrients such as N,phosphorus(P),and potassium(K),in the soils with by-product application is critical for the evaluation of the effects on nutrient loss and the quality of adjacent water bodies.In this study,a soil column experiment was performed to determine the nutrient losses from the soil amended with bagasse.We hypothesized that the application of bagasse would improve the WHC and nutrient-holding capacity of mineral soils,resulting in more nutrients being retained in the soil for plant uptake and consequently reducing nutrients leached below the plant root zone.The objectives of this study were to:i)compare the nutrient losses in leachates from the plant root zones of soils incorporated with bagasse at different rates and ii)determine the effects of bagasse application on the distribution of nutrients at different soil depths.

MATERIALS AND METHODS

Characteristics of the soil and sugarcane bagasse

Margate soil(siliceous hyperthermic Mollic Psammaquent)was collected from the 0–30 cm depth of a commercial sugarcane production farm in Clewiston,Florida,USA.The soil texture is sand.The region has a tropical monsoon climate with an average annual temperature of 23.6◦C and an average annual precipitation of 1 246 mm in 2019(Florida Automated Weather Network data;https://fawn.ifas.ufl.edu/).After removing all plant roots,organic debris,and rocks,part of the soil was sieved and stored in air-tight plastic buckets at room temperature until used for the soil column experiment.Part of the soil was air-dried and analyzed for soil characteristics,including pH,BD,OM,CEC,WHC,active carbon(C),Mehlich-3-extractable P and K,and total Kjeldahl N(TKN)(Table I).The sugarcane bagasse used in this study was provided by United States Sugar Corporation and was produced by a sugarcane mill in Clewiston,Florida.It was stored in air-tight plastic buckets prior to the start of the experiment.Samples of this material were analyzed for characteristics,including pH,BD,moisture content(MC),OM,CEC,WHC,active C,dissolved organic C(DOC),total C(TC)and N(TN),C/N ratio,total P(TP)and K(TK),and Mehlich-3-extractable P and K(Table I).

TABLE IBasic characteristics of the Margate mineral soil collected from the 0–30 cm depth of a commercial sugarcane production farm in Clewiston,Florida,USA and bagasse used in this study

Experimental design

The soil column experiment was established in a completely randomized design with a control(CK)with no bagasse and no N added,and three fresh bagasse application treatments:1)85 t ha−1of fresh bagasse(85FB),2)170 t ha−1of fresh bagasse(170FB),and 3)170 t ha−1of fresh bagasse plus 387 kg ha−1of NH4NO3(170FBN).Each treatment had four replicates.The rate 85 t ha−1of fresh bagasse is the minimum amount that can be uniformly applied at field scale.The N fertilizer was split equally into two applications:the first at the beginning of the experiment and the second six months later.The addition of N fertilizer was to prevent the potential immobilization caused by the incorporation of high C/N ratio plant residues(Singh and Aulakh,2001).

Soil column setup

The soil column experiment was conducted using 61-cm-long PVC columns with an inner diameter of 7.6 cm.Allcolumns were packed with the soil at a BD of 1.29 g cm−3to 53 cm,leaving approximately 8 cm of head space on the top.Bagasse and N fertilizer were thoroughly mixed with the top soil(0–15 cm)using a hand shovel.The soils in CK columns were also mixed in a similar manner.

Soil columns were placed outdoor in the Everglades Research and Education Center of University of Florida at Belle Glade,Florida under natural conditions for the entire experimental period.Daily temperature,precipitation,and evaporation throughout the duration of experiment from August 18,2018 to September 19,2019 are shown in Fig.1.In addition to natural precipitation,columns were irrigated periodically with deionized water and at least 24 h prior to each sampling.Leachate samples were collected every day for weeks 1 and 2,every two days for weeks 3 and 4,every seven days for weeks 5–10,every 14 d for weeks 11–40,and every 30 d from week 41 until the end of the experiment(week 57).All leachate samples were filtered using Fisher P5 filter paper and analyzed for total organic C(TOC),TN,TP,and TK concentrations.At the end of the experiment,each soil column was sectioned into three layers(0–15,15–30,and 30–53 cm),air-dried,and analyzed for TOC,active C,TKN,and Mehlich-3-extractable P and K.

Fig.1 Daily temperature,precipitation,and evaporation during the approximately 57-week outdoor soil column experiment conducted in the Everglades Research and Education Center of University of Florida at Belle Glade,Florida,USA from August 18,2018 to September 19,2019.Data were sourced from Florida Automated Weather Network(https://fawn.ifas.ufl.edu/).

Soil and water analyses

Soil pore volume(PV)was measured at the beginning of the experiment using the modified saturation method(Yu,2013).Briefly,the soil columns were slowly saturated with deionized water,raising the water table to soil surface.The amount of water used was the PV.The baseline pH of soil and sugarcane bagasse was determined at a 1:2(volume/volume)soil/bagasse to water ratio using an Accumet AB250 pH meter(Fisher Scientific,USA).Bulk density was calculated by dividing soil/bagasse mass in a given volume.Organic matter content was measured using the loss-on-ignition method(Mylavarapu,2009).The MC of bagasse was determined using the loss-on-drying method.Cation exchange capacity was estimated by the ammonium acetate method(Sumneret al.,1996),and ammonium concentration was quantified by flow injection analysis with a Lachat analyzer(Hach Company,USA).Water-holding capacity was determined by measuring the amount of water retained in soil/bagasse after saturation(Jenkinson and Powlson,1976).Soil TOC and total C(TC)and TN of bagasse were determined by the combustion method using an elemental analyzer(Costech ECS 4010,NC Technologies,USA).Soil available P and K were determined using the Mehlich-3-extraction method,while the TP,TK,and total calcium(Ca)of bagasse were determined by ashing bagasse samples in a muffle furnace at 550◦C for at least 5 h,followed by extraction with 6 mol L−1HCl(Bhadhaet al.,2018).Then,P,K,and Ca concentrations were analyzed using an Agilent 5110 inductively coupled plasma-optical emission spectrometer(Agilent Technologies,USA).For TKN,soil samples were digested,followed by colorimetric determination(USEPA,1993).Soil active C was determined based on quantifying potassium permanganateoxidizable C with a spectrophotometer(Moebius-Cluneet al.,2016).The TN concentrations of leachate samples were determined using a Shimadzu TN analyzer(Shimadzu Corporation,Japan).

Statistical analysis

The differences in C,N,P,and K levels of the soil and leachate samples among different treatments were compared by performing one-way analysis of variance(ANOVA)using the RStudio statistical software(RStudio,Inc.,USA).WhenF-test/ANOVA showed statistical significance,the Tukey’s test was used to separate the means of different treatments.

RESULTS AND DISCUSSION

Total water loading,PV,and porosityassessments

Total water loading,which was defined as the total amount of natural precipitation plus accumulated irrigation at the end of the experiment,was not significantly different between the bagasse treatments and CK(Table II).However,soil porosity was significantly different between treatments(170FB,170FBN>85FB>CK),resulting in significant differences in PV.The increases in PV and porosity might be attributed to the OM derived from bagasse.This result was similar to those of previous studies which reported that OM addition decreases soil BD and increases PV,which improves soil structure and increases WHC(Huesoet al.,2011;Bongiornoet al.,2019;Chachaet al.,2019).

Effects of bagasse on nutrient leaching patterns

The leaching patterns of TOC,TN,TP,and TK are presented in Fig.2.Leachate TOC,TN,and TP concentrationswere significantly different between the bagasse treatments and CK,whereas there were no significant differences between the treatments with different rates of bagasse.The leaching patterns of TOC showed that the TOC leached from CK became significantly higher than those from the bagasse treatments approximately eight weeks after bagasse application.A similar pattern was seen with TP,as TP leached from the bagasse treatments became significantly lower than that from CK approximately eight weeks after bagasse application.The leaching pattern of N was very different from those of the other nutrients.During the first two weeks,TN concentration in leachate decreased by approximately 97%for all treatments and was 3–20 mg L−1during weeks 3–23.After 24 weeks of leaching,the TN from CK increased gradually and became significantly higher than those from the bagasse treatments.Potassium presented a different leaching pattern from the other nutrients as well.During the first eight weeks,the 170FB treatment had the highest leachate TK concentration compared with the other treatments.This may be due to the higher amount of K(1 g kg−1)derived from the higher application rate of bagasse,as would be discussed further below.Total K concentration in the leachate from CK began to be significantly higher than those from the bagasse treatments at week 19,indicating that bagasse began to enhance K retention in the soils approximately four months after application.

Fig.2 Concentration changes of total organic C(TOC),total N(TN),total P(TP),and total K(TK)in the leachates from different treatments during the approximately 57-week outdoor soil column experiment conducted in the Everglades Research and Education Center of University of Florida at Belle Glade,Florida,USA from August 18,2018 to September 19,2019 using a Margate mineral soil.CK=control with no bagasse and no N;85FB=85 t ha−1 of fresh bagasse;170FB=170 t ha−1 of fresh bagasse;170FBN=170 t ha−1 of fresh bagasse+387 kg ha−1 of NH4NO3.The error bars represent standard deviations(n=4).

TABLE IITotal water loading,pore volume,and porosity in different treatments at the end of the approximately 57-week outdoor soil column experiment conducted in the Everglades Research and Education Center of University of using a Florida at Belle Glade,Florida,USA from August 18,2018 to September 19,2019 using a Margate mineral soil

Effects of bagasse on nutrient leaching losses in leachate

Though all treatments received the same volume of water,the cumulative TOC,TN,TP,and TK losses from CK were significantly higher than those from the bagasse treatments,while no significant differences were observed between the bagasse treatments over the entire experimental period(Table III).The lowest leaching loss ratios of TOC,TN,and TP between the bagasse treatments and CK were found in the 170FB treatment.The reductions in leaching losses in the bagasse treatments might be attributed to the high sorption capacity of bagasse,as it has a large surfacearea and a high porosity.Rasulet al.(1999)reported a porosity of 66%for bagasse and Cruzet al.(2018)reported that sugarcane bagasse had a high density of pores especially mesopores,with a specific surface area of approximately 0.63 m2g−1.

TABLE IIITotal amounts of organic C,N,P,and K leached from the different treatmentsa)and their leaching loss ratiosb)at the end of the approximately 57-week outdoor soil column experiment conducted in the Everglades Research and Education Center of University of Florida at Belle Glade,Florida,USA from August 18,2018 to September 19,2019 using a Margate mineral soil

The significant reduction of TOC leaching loss in the bagasse treatments demonstrated that bagasse did not increase TOC loss through leaching even though it had a high TC of 339.1 g kg−1.Bagasse application may help prevent DOC from leaching loss due to the sorption of DOC to the bagasse-mineral complexes formed in soil aggregates(Linet al.,2012).Nekiret al.(2019)have shown that the additional OM from bagasse facilitates the formation of soil aggregates because the additional OM acts as a cementing agent that helps bind soil particles together.Improved soil aggregation would help decrease macro-porosity while increasing pore volume(Abd El-Halim and Kumlung,2015).Thus,soil water retention increases,and the soil can hold more DOC and reduce its leaching.In the study of Mukherjee and Zimmerman(2014),sorption of C to mineral complexes is also believed to be responsible for the lower DOC leaching losses from the biochar-and water treatment residual-amended soils.In this study,it was more likely that the leached DOC originated from the soil instead of from the added bagasse(Hagedornet al.,2002;De Troyeret al.,2011).Studies have shown that the incorporation of organic amendments such as soybean and corn residues to agricultural soils initially increases the DOC in surface soils,but DOC would then be rapidly mineralized and only very little would be leached into subsurface soil layers(McCarty and Bremner,1992;Marschner and Kalbitz,2003).Similar results were found in this study with bagasse.Therefore,although organic amendments are an additional C source,the organic C derived from them would not contribute significantly to that leaching into groundwater.

The significant reduction in TN leaching loss from the bagasse treatments might be attributed to the high CEC(70.12 cmolckg−1)of bagasse.Researchers have pointed out that reduction in N leaching loss after the addition of organic amendments such as biochar and manure is due to the high CEC values of the organic materials,although the percentage of reduction varies with the type of organic amendment(Lairdet al.,2010;Dereet al.,2012;Liet al.,2019).In addition,other biochemical processes might be responsible for the reduction in N leaching loss,such as denitrification.Bagasse provides a C source for microbial communities such as denitrifiers(Tangsiret al.,2017),and the increases in denitrifying organisms could result in increased dinitrogen(N2)efflux,therefore reducing N leaching loss from soils.

The reduction in P leaching might be attributed to soil pH change,microbial activity,and P mineralization and complexation(Liet al.,2019).Although bagasse has a low pH(3.79 in this study),its incorporation would not decrease the soil pH to below 7,because the mineral soil from South Florida has a high buffering capacity due to years of mixing with the underlying limestone bedrock(Xuet al.,2019).The addition of organic materials may have enhanced P incorporation into microbial biomass.Calciumbridged phosphate sorption to the negatively charged soil surface could also enhance P retention in soils and eventually reduce P leaching(Heet al.,1992;Yanget al.,2007b).In addition,the rich Ca in sugar industry by-products such as mill-mud and bagasse(2.10 g kg−1in the bagasse of this study,Table I)would convert the dissolved P to less solubleforms by precipitating with it,thereby decreasing its loss in leachate(Gilbertet al.,2008;Sarwaret al.,2010;Gomez,2013;Chachaet al.,2019).

The significant reduction in TK leaching loss may also be due to the high CEC(70.12 cmolckg−1)of bagasse,which can increase the K sorption capacity of soil(Hue and Silva,2000).According to Chachaet al.(2019),the CEC of a sandy soil increased five-fold when the soil was mixed with sugarcane bagasse at 1:1(weight/weight)ratio.

Effects of bagasse on nutrient distribution in different soil layers

There were no significant differences in soil TOC between CK and the bagasse treatments for all soil layers at the end of the approximately 57-week column experiment(Table IV),indicating that most of the C derived from bagasse had likely been lost through gaseous emissions.A similar result was found that 73%of maize straw C was released as CO2after maize straw was incorporated in soil(De Troyeret al.,2011).While we did not measure CO2directly,we measured soil active C(Table IV),a soil labile C pool(Bongiornoet al.,2019).As a food and energy source of the soil microbial community,active C is positively correlated with such soil biological properties as respiration and microbial biomass.Active C content was significantly higher in the 170FBN treatment than in the 85FB treatment and CK in the 0–15 cm soil layer,indicating that bagasse applied at a high rate is likely to increase active C in the topsoil,potentially enhancing microbial biomass and activity and resulting in C loss through microbial CO2release.Although bagasse application could have a priming effect on soil OM mineralization due to the stimulated microbial activity,C loss as DOC from the soilvialeaching is not expected to be substantial since the mineral soil in this study had a low OM content(<50 g kg−1).However,C loss through gaseous CO2emissions could be increased in the long run.

Higher TKN levels were expected in the bagasse treatments where significant decreases in N leaching loss were found.However,there were no significant differences in TKN between the bagasse treatments and CK for any of the soil layers(Table IV).Thus,the reduced TN leaching losses were more likely due to the enhanced denitrification process and gaseous loss of N caused by bagasse application.In addition,although extra N was added in the 170FBN treatment,the amount of N added(approximately 0.05 g)did not significantly increase the soil TKN level compared with the other three treatments.Part of the added N might have been leached out from the soil column since slightly higher TN was observed in the leachate from the 170FBN treatment compared with the 170FB treatment(Table III).

In the 0–15 cm soil layer,Mehlich-3-extractable P content was significantly lower in 170FB and 170FBN than in CK(Table IV).This suggests that the reduced TP leaching losses in 170FB and 170FBN were possibly due to P immobilization(Abhilash and Singh,2008;Silviaet al.,2014).However,as we did not quantify the various fractions of P,further studies are required to substantiate this supposition or identify other possible mechanisms.

Compared with CK,the incorporation of bagasse at 170 t ha−1significantly increased the Mehlich-3-extractable Kcontent in the subsurface soil layers(Table IV),indicating that considerable amount of K derived from bagasse had been leached into the deeper soil layers despite the high CEC of bagasse.Unlike N and P,K is readily released from crop residues because its release is less dependent on microbial mineralization(Lupwayiet al.,2007).Plants generally have a TK content of 5–50 g kg−1on a dry weight basis(Mahler 2004),and the Mehlich-3-extractable K of the Margate mineral soil used in this study was only 22 mg kg−1(Table I).Thus,the rich K(1 g kg−1)in bagasse could be a potential source of K for plant uptake.

TABLE IVNutrient contents in different soil layers of different treatmentsa)at the end of the approximately 57-week outdoor soil column experiment conducted in the Everglades Research and Education Center of University of Florida at Belle Glade,Florida,USA from August 18,2018 to September 19,2019 using a Margate mineral soil

CONCLUSIONS

Total N concentrations of the leachates decreased dramatically for all treatments during the first two weeks of leaching,while the higher application rate of bagasse(170 t ha−1)caused greater K leaching losses compared with CK at the early stage of leaching.The leaching loss patterns of C and P showed similar trends,in which bagasse began to enhance C and P retention approximately two months after application.Overall,sugarcane bagasse reduced TOC,TN,and TP leaching out of the plant root zone compared with CK.Bagasse application at 170 t ha−1increased active C in the surface soil layer and Mehlich-3-extractable K in the subsurface soil layers,but did not affect TOC and TKN compared with CK.The addition of bagasse to mineral soils could therefore be an effective way to increase nutrient availability with low risk of groundwater pollution by major nutrients.Bagasse could also be utilized as a potential source of K for plant uptake.To further help develop nutrient management plans to use sugarcane bagasse as an amendment on mineral soils under the hot and humid conditions in South Florida,field-scale monitoring studies are warranted in the future.

ACKNOWLEDGEMENTS

This research was supported by the United States Sugar Corporation(No.PRO00015244)and United States Department of Agriculture(USDA)Hatch Project(No.FLAERC-006097).The authors would like to thank Mr.Salvador Galindo and Dr.Naba Amgain from the Everglades Research and Education Center,University of Florida,USA with technical assistance in the field and in the laboratory.