applied sciences

Article A Biodegradable Ramie Fiber-Based Nonwoven Film Used for Increasing Oxygen Supply to Cultivated Soil

Wanlai Zhou *,† , Yanbin Niu †, Chaoyun Wang *, Yuanru Yang, Zhijian Tan , Yongjian Yi, Wang Yu and Hongying Wang

Institute of Bast Fiber Crops, Chinese Academy of Agricultural Sciences, Changsha 410205, China; [email protected] (Y.N.); [email protected] (Y.Y.); [email protected] (Z.T.); [email protected] (Y.Y.); [email protected] (W.Y.); [email protected] (H.W.) * Correspondence: aruofl[email protected] (W.Z.); [email protected] (C.W.); Tel.: +86-731-8899-8517 (W.Z.); Tel./Fax: +86-731-8899-8501 (C.W.) † These authors contributed equally to this work and should be considered co-first authors.



Received: 3 September 2018; Accepted: 18 September 2018; Published: 3 October 2018


Featured Application: Short-term plant cultivation within containers where anoxia often occurs.

Abstract: Plastic agricultural nonwoven films are traditionally used as covering materials, and are prone to cause various ecological problems due to their poor biodegradability. In this paper, a ramie fiber/starch nonwoven film was prepared, and was used as bedding material, that was covered by cultivated soil as opposed to covering it. The biodegradability and porosity characteristics of the film were analyzed, and its effect on oxygen supply to soil was investigated. Results showed that the prepared film had good biodegradability (65.6% after 72 days), and had a loose and porous structure, with the main pore size being in the range of 250–300 µm. After the soil moisture content was reduced to about 44%, the oxygen concentration in the soil that was in close contact with the film, which padded the bottom surface of the plate, rose sharply and then kept stable at 20.1%, whereas soil directly in contact with the plate remained extremely anoxic (0.2%). It was concluded that use of the prepared film increased the oxygen supply to the soil in contact with it, which sufficiently compensated for the oxygen consumption caused by soil microbial activities. Thus, the prepared film is very suitable in short-term plant cultivation within containers where anoxia often occurs.

Keywords: agricultural film; nonwoven; ramie; soil; oxygen

1. Introduction Since it was first introduced to agriculture in the 1950s, plastic film mulching has become a globally applied agricultural practice because of its instant economic benefits, such as higher yields, earlier harvests, improved fruit quality, and increased water-use efficiency [1–4]. Plastic films are generally compact and low-permeable or impermeable, whereas nonwoven films, which are produced through bonding randomly-oriented micron-sized fibers together physically or chemically, are usually loose and porous. These structural differences give nonwoven films some unique physical properties (e.g., air permeability) that can be advantageous, compared to plastic film when used as covering materials. For example, the temperature changes under nonwoven film are shown to be more stable, therefore avoiding the extremely high temperatures at noon that often occur under plastic film [5–7], and the occurrence of disease is described as being reduced due to the decreased air humidity under nonwoven film [8–10]. Because of these advantages, nonwoven film is increasingly used for crop cultivation as a substitute for common plastic film in modern agriculture [11].

Appl. Sci. 2018, 8, 1813; doi:10.3390/app8101813 www.mdpi.com/journal/applsci Appl. Sci. 2018, 8, x FOR PEER REVIEW 2 of 10 filmAppl. is Sci. increasingly2018, 8, 1813 used for crop cultivation as a substitute for common plastic film in modern2 of 10 agriculture [11]. Traditionally, agricultural nonwoven films are mainly made of fossil fuel-based plastics (e.g., polyester,Traditionally, polypropylene, agricultural vinylon) nonwoven due to filmstheir aregood mainly mechanical made properties of fossil fuel-based and low plasticscost [8]. However,(e.g., polyester, it is relatively polypropylene, expensive, vinylon) and thus due una tottractive, their good to mechanical remove and properties recycle used and films low costfrom [ 8the]. field.However, Consequently, it is relatively these expensive, agrotextiles and are thus often unattractive, intentionally to remove or unintentionally and recycle usedleft on films the from land, the field. Consequently, these agrotextiles are often intentionally or unintentionally left on the land, where they accumulate gradually in the soil and persist for years due to poor biodegradability, and where they accumulate gradually in the soil and persist for years due to poor biodegradability, ultimately cause various ecological problems [12–15]. and ultimately cause various ecological problems [12–15]. A solution to overcome these issues is to employ biodegradable polymers such as polybutylene A solution to overcome these issues is to employ biodegradable polymers such as polybutylene adipate terephthalate (PBAT), polycaprolactone (PCL), polybutylene succinate (PBS), PBS-co-adipate adipate terephthalate (PBAT), polycaprolactone (PCL), polybutylene succinate (PBS), PBS-co-adipate (PBSA), polylactic acid (PLA), and polyhydroxyalkanoate (PHA) as raw materials. PLA is most often (PBSA), polylactic acid (PLA), and polyhydroxyalkanoate (PHA) as raw materials. PLA is most often selected as the major feedstock of biodegradable nonwoven films because of its relatively low cost, selected as the major feedstock of biodegradable nonwoven films because of its relatively low cost, abundance, high mechanical strength, and frequent use [16]. Many studies have demonstrated that abundance, high mechanical strength, and frequent use [16]. Many studies have demonstrated that nonwoven films made of PLA perform satisfactorily in the field [6,17–19]. However, considering the nonwoven films made of PLA perform satisfactorily in the field [6,17–19]. However, considering the costcost and and richness richness of ofthe the raw raw materials, materials, biodegradable biodegradable natural natural plant plant fibers fibers such suchas cotton, as cotton, flax, hemp, flax, andhemp, ramie, and which ramie, have which excellent have excellentmechanical mechanical properties, properties, could be more could suitable be more for suitable use in agricultural for use in nonwovenagricultural films. nonwoven Some films.plant Somefiber-based plant fiber-basednonwoven nonwoven films have films been have reported been reported on, such on, as such cotton- as basedcotton-based [20], flax-based [20], flax-based [21], and [21 jute-based], and jute-based [22,23]. [ 22,23]. AA ramie ramie fiber/starch fiber/starch nonwoven nonwoven film film was was develope developedd in inour our previous previous study. study. Waste Waste fiber fiber from from the ramiethe ramie spinning spinning industry industry was was used used as the as the main main feedstock feedstock due due to to its its huge huge supply inin ChinaChina and and its its distinctivedistinctive characteristics characteristics (e.g., (e.g., excellent excellent air air permea permeabilitybility and hygroscopicity). AA continuouscontinuous process, process, mainlymainly consisting consisting of of air air laying laying web web formation, formation, bonding, bonding, and drying, was usedused toto prepareprepare thethe film. film. UnlikeUnlike the the traditional traditional agricultural agricultural nonwoven nonwoven films films that are usuallyusually usedused asas mulchingmulching materials materials to to covercover soil soil [24,25], [24,25], the the ramie ramie fiber/starch fiber/starch nonwoven nonwoven film film was used asas beddingbedding materialmaterial that that was was to to be be coveredcovered by by cultivated cultivated soil soil as as opposed opposed to to covering it. In a typical applicationapplication ofof ourour previousprevious work, work, thethe film film was was applied applied so so that that it it padded padded the the bottom bottom surfacesurface of thethe platesplates thatthat werewere to to be be used used to to raise raise machine-transplantedmachine-transplanted rice seedlings,seedlings, and and was was then then covered covered by seedling by seedling soil (Figure soil 1(Figure). Previous 1). Previous studies studiesshowed showed that the that growth the growth of rice seedling of rice seedling roots was roots significantly was significantly promoted promoted by the use by of thisthe use film, of as this it film,helped as it to helped form ato strong form a and strong not and easily not broken easily seedlingbroken seedling block, which block, meant which the meant efficiency the efficiency of the ofmachine the machine transplanting transplanting was improved—a was improved—a process process that was that delayed was delayed by the presence by the ofpresence broken of seedling broken seedlingblocks whereblocksthis where film this was film not was employed not employed [26–28]. [26–28]. Rice seedlings Rice seedlings raised in raised this way in this showed way showed many manycharacteristics characteristics similar similar to rice to seedlingsrice seedlings raised raised under under aerobic aerobic cultivation cultivation [29,30 [29,30],], suggesting suggesting that thethat thefunction function of theof the ramie ramie fiber/starch fiber/starch nonwoven nonwoven film wasfilm related was related to oxygen to oxygen supply supply in seedling in seedling soil. In this soil. Inpaper, this paper, the biodegradability the biodegradability and porosity and porosity characteristics characteristics of the ramie of fiber/starchthe ramie fiber/starch nonwoven nonwoven film were filmanalyzed were analyzed in detail, in and detail, its effect and on its oxygen effect on supply oxygen to soilsupply was to investigated. soil was investigated.

FigureFigure 1. 1. (left(left) )The The ramie ramie fiber/starch fiber/starch nonwoven nonwoven film; film; and (right)) ItsIts application.application. A: A: Empty Empty plate plate for for raisingraising rice rice seedlings seedlings for for machine-transplanting, machine-transplanting, B: B: The filmfilm isis paddedpadded onon thethe bottom bottom surface surface of of the the plate,plate, C: C: The The film film is is covered covered with with cultivated soil andand ricerice seedsseeds areare sown sown on on it. it.

Appl. Sci. 2018, 8, 1813 3 of 10

2. Materials and Methods

2.1. Materials and Reagents The ramie fiber/starch nonwoven film was prepared in our cooperative enterprise (Haerbin Jingzhu Agricultural Science and Technology Co. Ltd., Haerbin, China) by a continuous process that included web formation by air-laid web-forming machine, bonding with 3–4% (w/v) modified corn starch aqueous solution, and desiccation in a drying chamber. It had an approximate thickness of 0.25 mm and weight of 40 g/m2, and contained approximately 14% modified corn starch and 86% waste fibers from the textile industry (ramie and cotton fibers with a mass ratio of 4:1). The properties of the ramie fiber were as follows: approximately 2–5 cm in length, 30 µm in diameter, 1.49 g/cm3 in density, and 6.5% in moisture content. The properties of the cotton fiber were as follows: approximately 13 mm in length, 20 µm in diameter, 1.58 g/cm3 in density, and 7.2% in moisture content. The modified corn starch was purchased from Suzhou Shuanghuan Chemical Technology Co., Ltd. (Suzhou, China), with an average molecular weight of 15,000 and purity of ≥98%.

2.2. Evaluation of Biodegradability The biodegradability of the film was evaluated according to the China standard GB/T 19277.1-2011 [31]. In this evaluation, cellulose powder (particle size <20 µm) was employed as a positive control reference material, the mixture of film fragments (or cellulose powder) and activated vermiculite (inoculated with microbial flora) was placed under a controlled environment (58 ± 2 ◦C, water content ≈ 50%, sufficient oxygen supply) to undergo a strong aerobic composting, and the degree of biodegradation (Db) was assessed by measuring the amount of carbon dioxide emission during the composting process and calculated according to the following formula: Db = AcCO2/ThCO2 × 100%, where AcCO2 stands for actual release amount of carbon dioxide produced by experimental materials during the composting process and ThCO2 stands for theoretical release amount of carbon dioxide of experimental materials.

2.3. Analysis Related to Porosity Characteristics The microstructure of the film was analyzed through scanning electron microscopy (SEM, QUANTA FEG450, FEI, Hillsboro, OR, USA), and SEM images were obtained with an accelerating voltage of 15 kV and a working distance of 13.5 mm (for 200× magnification) and 13.6 mm (for 800× magnification). A capillary flow porometer (3H-2000PB, Beishide Instrument Technology (Beijing) Co., Ltd., Beijing, China) was used to measure pore size distribution of the film with anhydrous ethanol as infiltrating fluid. The air permeability of the film was assessed by an air permeability tester (TQD-G1, Labthink Instruments Co., Ltd., Jinan, China) at the pressure of 10 MPa, and the water vapor permeability of the film was assessed by a water vapor permeability tester (W3/031, Labthink Instruments Co., Ltd., Jinan, China) at the temperature of 38 ◦C and relative humidity of 90%.

2.4. Actual Detection of the Effect on Oxygen Supply to Soil A seedling cultivation plate with small holes (ϕ = 3 mm) at the bottom was used in this detection. The treatment and control were arranged in the same plate to ensure consistency of moisture and other soil conditions during the determination. Half of the plate was covered by the film (treatment) while the other half was not (control), and two fixed needle-type oxygen mini-sensors (OXF500PT, PyroScience GmbH, Aachen, Germany) were placed in the plate, with their ends in close contact with the film and the bottom surface of plate, respectively (Figure2). The seedling plate was then filled with crushed seedling soil (loam from paddy field) and the soil was watered well. The oxygen concentration was measured and recorded continuously and automatically with an optical oxygen meter (FSO2-4, PyroScience GmbH, Aachen, Germany) every 4 min. In the process of determination, the soil would be watered well again once it dried out. The above detection was carried out in an artificial climate box (MGC-400H, Bluepard Instruments Co., Ltd., Shanghai, China). The temperature of the climate box Appl. Sci. 2018, 8, 1813 4 of 10

Appl. Sci. 2018, 8, x FOR PEER REVIEW 4 of 10 was set at 25 ◦C and the relative humidity at 60%. A 3-day continuous measurement was conducted, duringmeasurement which the was soil conducted, was thoroughly during watered which the twice, soil and was some thoroughly soil samples watere wered twice, taken and from some the soil plate tosamples determine were moisture taken from content the plate using to the determine drying method.moisture content using the drying method.

FigureFigure 2. 2.Diagram Diagram of ofthe the experimentalexperimental device (not (not yet yet covered covered by by seedling seedling soil). soil). 2.5. Simulation Demonstration of the Effect on Oxygen Supply 2.5. Simulation Demonstration of the Effect on Oxygen Supply TheThe presence presence of of oxygen oxygen was was detected detected byby anan oxygenoxygen indicator which which cont containedained agar agar (1%, (1%, w/v), w/v), methylene blue (C H ClN S, 13 mg/L), and sodium dithionite (Na S O , 130 mg/L). It is originally methylene blue (C16 16H1818ClN33S, 13 mg/L), and sodium dithionite (Na22S2O2 4, 4130 mg/L). It is originally colorless,colorless, but but once once it it comes comes into into contact contact withwith oxygenoxygen it turns turns blue blue [32]. [32]. The The film film was was first first deoxidized deoxidized byby soaking soaking it init in aqueous aqueous solution solution that that contained contained methylenemethylene blue (13 (13 mg/L) mg/L) and and sodium sodium dithionite dithionite (260(260 mg/L). mg/L). Two Two kinds kinds of of petri petri dishesdishes (Petri dish A A and and Petri Petri dish dish B) B) were were used used in inthe the experiment. experiment. TheThe difference difference between between them was was that that the the bottom bottom of the of thePetri Petri dish dishB had B a hadhole a(φhole = 3 mm, (ϕ =sealed 3 mm, sealedbeforehand beforehand with withtape). tape). In an Inanaerobic an anaerobic operating operating box, the box, heated the heatedoxygen oxygenindicator indicator (in melted (in state) melted state)was was first first poured poured into intoPetri Petri dish dish A. In A. the In meanti the meantime,me, the deoxidized the deoxidized film was film laid was on laid the on bottom the bottom of ofPetri dish dish B. B. After After the the oxygen oxygen indicator indicator cooled cooled to be tocome become an agar an block, agar block, it was ittransferred was transferred into Petri into Petridish dish B so B that so that it was it wasin close in close contact contact with the with film. the Petri film. dish Petri B was dish then B was covered then coveredwith a lid with and asealed lid and sealedwith witha sealing a sealing membrane membrane to insulate to insulate it from it the from ou thetside outside air. During air. During this process, this process, the agar the block agar and block andfilm film were were always always kept kept isolated isolated from from oxygen, oxygen, and andtherefore therefore colorless. colorless. The operation The operation of the control of the control was wasthe the same same as above as above except except that there that therewas no was deoxidized no deoxidized film laid film on the laid bottom on the of bottom Petri dish of PetriB. Finally, dish B. Finally,all the all prepared the prepared Petri dish Petri Bs dish were Bs placed were placed in air an ind air the and tape the on tape the holes on the were holes torn were off, torn and off,the andcolor the changes of the agar blocks were observed and recorded. color changes of the agar blocks were observed and recorded.

3. Results3. Results and and Discussion Discussion

3.1.3.1. Biodegradability Biodegradability AsAs presented presented in in Figure Figure3, 3, the the biodegradation biodegradation ofof the the ramieramie fiber/starchfiber/starch nonwoven nonwoven film film started started withwith a rapida rapid degradation degradation periodperiod of 4 days, days, in in which which time time the the diurnal diurnal degradation degradation rate rate(4.08%/day (4.08%/day on average) was very close to that of the cellulose (reference material) (4.73%/day on average). This was on average) was very close to that of the cellulose (reference material) (4.73%/day on average). followed by a much slower, but roughly constant, (0.86%/day on average) degradation period, which This was followed by a much slower, but roughly constant, (0.86%/day on average) degradation continued until day 60 (at day 60 the degradation degree was 64.5%). In a slightly different way, the period, which continued until day 60 (at day 60 the degradation degree was 64.5%). In a slightly cellulose degraded at a roughly constant rate (3.29%/day on average) until day 20 (at day 20 the different way, the cellulose degraded at a roughly constant rate (3.29%/day on average) until day 20 degradation degree was 65.9%). After that, the degradation degree tended to be constant. At the end (atof day the 20 determination the degradation period degree (day was 72), 65.9%). the degradat Afterion that, degree the degradation of the cellulose degree reference tended and to be the constant. film Atwere the end 77.1% of theand determination 65.6%, respectively period. Because (day 72), the the degradation degradation degree degree of ofthe the film cellulose in the referencefirst 4-days and the(16.3%) film were was 77.1%slightly and greater 65.6%, than respectively. the mass ratio Because of corn the starch degradation in the film degree (14%), ofwe the inferred film inthat the the first Appl. Sci. 2018, 8, x FOR PEER REVIEW 5 of 10 Appl. Sci. 2018, 8, 1813 5 of 10 rapidAppl. Sci. degradation 2018, 8, x FOR PEERof the REVIEW film during the first 4-days was mainly due to the degradation5 of 10of the modified4-days (16.3%)corn starch, was slightlyas it degraded greater thanmore the easily mass than ratio ramie of corn fiber. starch In inother the filmwords, (14%), when we the inferred film is embeddedrapidthat degradation the rapid in the degradation soil,of the the film ofadhesive theduring film materialthe during first the 4-willdays first biodegrade 4-days was mainly was mainlyrapidl due y,to due butthe to thedegradation degradationfiber skeleton of ofthe the will remainmodifiedmodified and corn cornwork starch, starch, for aas relatively asit degraded it degraded longer more more time. easily easily than than ramie ramie fiber. fiber. In Inother other words, words, when when the the film film is is embeddedembedded in inthe the soil, soil, the the adhesive materialmaterial willwill biodegrade biodegrade rapidly, rapidl buty, but the the fiber fiber skeleton skeleton will remainwill remainand work and work for a relativelyfor a90 relatively longer longer time. time. 80 ） 90 % 70

（ 80 ）

% 7060 （ 6050 5040 4030 3020 20 degree of biodegradation 10 cellulose ramie fiber/starch nonwoven film

degree of biodegradation 100 cellulose ramie fiber/starch nonwoven film 0 0 1020304050607080 time （day） 0 1020304050607080 time （day） Figure 3. Biodegradation of the ramie fiber/starch nonwoven film. FigureFigure 3. Biodegradation 3. Biodegradation of ofthe the ramie ramie fiber/starch fiber/starch nonwoven nonwoven film. film. 3.2.3.2. Porosity Porosity Characteristics Characteristics 3.2. Porosity Characteristics FigureFigure 4a,b4a,b show show the the microstructure microstructure ofof ramieramie fiber/starch fiber/starch nonwoven nonwoven film film that that was was investigated investigated Figure 4a,b show the microstructure of ramie fiber/starch nonwoven film that was investigated byby SEM SEM analysis analysis at 200× at 200 and× and 800× 800 magnification,× magnification, respectively. respectively. It can It canbe clearly be clearly seen seen that thatramie ramie fibers by SEM analysis at 200× and 800× magnification, respectively. It can be clearly seen that ramie fibers crisscrossedfibers crisscrossed in the film, in theand film, gaps and between gaps betweenthem were them incompletely were incompletely filled with filled corn with starch, corn forming starch, a crisscrossed in the film, and gaps between them were incompletely filled with corn starch, forming a looseforming and porous a loose structure. and porous As structure. shown in As Figure shown 5a, in the Figure film’s5a, pore the film’s diameter pore was diameter mainly was distributed mainly loose and porous structure. As shown in Figure 5a, the film’s pore diameter was mainly distributed in distributedthe range 250–300 in the range μm, 250–300 accountingµm, accounting for 46.9% for of 46.9%the total, of the followed total, followed by 0–50 by 0–50μm, µ50–100m, 50–100 μm,µ m,and in the range 250–300 μm, accounting for 46.9% of the total, followed by 0–50 μm, 50–100 μm, and 100–150and 100–150 μm, accountingµm, accounting for 20.0%, for 20.0%, 18.3%, 18.3%, and and 13.1%, 13.1%, respectively. respectively. The The air permeabilitypermeability and and water water 100–150 μm, accounting for 20.0%, 18.3%, and 13.1%, respectively. The air permeability and water vaporvapor permeability permeability of of th thee film film were 2.712.71 L/(cmL/(cm22··min)min) andand 107.1107.1 g/(m g/(m22··h),h), respectivelyrespectively (Figure (Figure5b), 5b), vapor permeability of the film were 2.71 L/(cm2·min) and 107.1 g/(m2·h), respectively (Figure 5b), indicatingindicating that that the the porous porous structure structure prov providedided the filmfilm withwith good good permeability. permeability. indicating that the porous structure provided the film with good permeability.

((aa)) analysisanalysis at at 200× 200× magnification magnification (b) ( banalysis) analysis at 800× at 800× magnification magnification Figure 4. Scanning electron microscopy (SEM) pictures of the ramie fiber/starch nonwoven film. FigureFigure 4. 4.ScanningScanning electron electron microscopy microscopy (SEM) (SEM) pictures ofof thethe ramie ramie fiber/starch fiber/starch nonwoven nonwoven film. film. Appl. Sci. 2018, 8, x FOR PEER REVIEW 6 of 10

Appl.Appl. Sci. Sci.2018 2018, 8,, 8 1813, x FOR PEER REVIEW 6 of6 of 10 10

(a) (b)

Figure 5. Pore size(a distribution) (a) and permeability (b) of the ramie fiber/starch(b) nonwoven film. FigureFigure 5. 5.Pore Pore size size distribution distribution (a ()a and) and permeability permeability (b ()b of) of the the ramie ramie fiber/starch fiber/starch nonwoven nonwoven film. film. 3.3. Effect on Oxygen Supply to Soil 3.3. Effect on Oxygen Supply to Soil 3.3.As Effect can on be Oxygen seen in Supply Figure to 6, Soil soil moisture content decreased linearly with time. The two roughly parallelAsAs fitting can can be be lines seen seen in(dotted in Figure Figure straight6, 6, soil soil moisture lines) moisture indicate content content that decreased decreased it decreased linearly linearly at approximately with with time. time. The The the two two same roughly roughly rate forparallelparallel the two fitting fitting water lines lines cycles. (dotted (dotted In straightwater-saturated straight lines) lines) indicate indicatesoil (about that that it 48% decreased it decreased moisture at approximately atcontent), approximately whether the samethe the same bottom rate forrate ofthefor the two the plate watertwo was water cycles. covered cycles. In by water-saturated In the water-saturated film or not, soil soil (aboutsoil oxygen (about 48% concentration 48% moisture moisture content), decreased content), whether rapidly whether the from bottomthe 20.1%bottom of totheof 0.2–0.5% platethe plate was in was covered about covered 7 by h, the byand filmthe then film or not, remainedor not, soil oxygensoil steady. oxygen concentration However, concentration when decreased decreased the soil rapidly rapidlymoisture from from 20.1%content 20.1% to decreased0.2–0.5%to 0.2–0.5% in to about aboutin about 7 h,44% and 7 (estimated h, then and remained then results remained steady. based However, steady.on linear However, when fitting the equation) when soil moisture the 19 soil h contentlater, moisture the decreased oxygen content concentrationtodecreased about 44% to (estimatedin about the soil 44% that results (estimated was based in close results on linearcontact based fitting with on equation)thelinear film fitting (i.e., 19 hoxygen equation) later, theconcentration oxygen19 h later, concentration atthe the oxygen film surface),inconcentration the soil padded that in was on the the in soil closebottom that contact was surface in close with of the thecontact plate, film withincreased (i.e., the oxygen film sharply (i.e., concentration oxygenfrom 0.5% concentration to at 20.1% the film in lessat surface), the than film 1padded surface),h, and onthen padded the remained bottom on the surfacesteady. bottom ofThe surface the same plate, of changes the increased plate, in increased oxygen sharply concentration sharply from 0.5% from to 0.5%at 20.1% the to film 20.1% in less surface in than less (i.e., 1than h, rapidand1 h, then anddecline—steady remainedthen remained steady. anoxic steady. The state—sharp same The changessame rise—steadychanges in oxygen in oxygen oxygen concentration concentration saturation at the state) filmat the surfaceoccurred film surface (i.e., again rapid (i.e.,in thedecline—steadyrapid second decline—steady water anoxic cycle. state—sharp Theanoxic oxygen state—sharp rise—steadyconcentration rise—steady oxygen in soil saturation thatoxygen was saturation state)directly occurred in state)contact again occurred with in the the again second plate in bottomwaterthe second cycle. (i.e., oxygenwater The oxygen cycle. concentration The concentration oxygen at theconcentration inplate soil bottom that in was soilsurface) directly that wasthat in directlywas contact not incovered with contact the with platewith the the bottom film, plate however,(i.e.,bottom oxygen (i.e.,always concentration oxygen remained concentration atat the 0.2% plate for at bottomtheboth plate water surface) bottom cycles, that surface) regardless was not that covered ofwas the not decline with covered the in film,soil with moisture however, the film, content.alwayshowever, remained always at remained 0.2% for bothat 0.2% water for cycles, both water regardless cycles, of regardless the decline of in the soil decline moisture in content.soil moisture content.

FigureFigure 6. 6. ChangesChanges in in oxygen oxygen concentration concentration and and soil soil moisture content with time. Figure 6. Changes in oxygen concentration and soil moisture content with time. Appl. Sci. 2018, 8, 1813 7 of 10 Appl. Sci. 2018, 8, x FOR PEER REVIEW 7 of 10

TheThe rapid rapid decline decline in oxygenin oxygen concentration concentration in water-saturatedin water-saturated soil soil in thisin this study study fully fully accorded accorded with thewith reported the reported fact that fact water-saturated that water-saturated soil becomes soil becomes anoxic anoxic as soonas soon as as there there is is persistent persistent depletion depletion of oxygenof oxygen by soil by aerobic soil aerobic microorganisms. microorganisms. This is becauseThis is thebecause diffusion the diffusion of oxygen of into oxygen water-saturated into water- soil is extremelysaturated soil slow is (theextremely effective slow diffusion (the effectiv coefficiente diffusion is on coefficient the order ofis 10on− the9 to order 10−8 ofm 102/s),−9 to meaning 10−8 m2/s), that anmeaning adequate that compensation an adequate ofcompensation oxygen supply of oxygen cannot supply be obtained cannot be [33 obtained]. Considering [33]. Considering that the oxygen that depletionthe oxygen by soil depletion aerobic by microorganisms soil aerobic microorganisms continued when continued the soil when water the content soil water reduced, content the reduced, persistent highthe oxygen persistent concentration high oxygen at concentration the film surface at the indicates film surface that there indicates was a that continuous there was adequate a continuous oxygen supply.adequate When oxygen considering supply. theWhen contemporaneous considering the constantcontemporaneous anoxia at constant the bottom anoxia surface at the of thebottom plate, it cansurface be safely of the concludedplate, it can that be safely it was concluded the film that that increasedit was the film the supplythat increased of oxygen the supply to the bottom of oxygen soil. to the bottom soil. 3.4. Process of Oxygen Into the Soil 3.4. Process of Oxygen Into the Soil As shown in Figure7a,b, when the holes were sealed, whether the bottom of the petri dish was coveredAs by shown the film in orFigure not, 7a,b, the agar when blocks the holes within were them sealed, remained whether colorless, the bottom indicating of the petri that dish no oxygen was covered by the film or not, the agar blocks within them remained colorless, indicating that no oxygen entered the petri dish. However, after tearing off the tape, color change occurred in the agar block in entered the petri dish. However, after tearing off the tape, color change occurred in the agar block in the petri dish covered by the film. A blue round plaque formed rapidly at the hole in the bottom of the the petri dish covered by the film. A blue round plaque formed rapidly at the hole in the bottom of dish, and gradually expanded and deepened in color, indicating that radial oxygen diffusion occurred the dish, and gradually expanded and deepened in color, indicating that radial oxygen diffusion in the bottom of the agar block. At the same time, no obvious color change occurred in the other agar occurred in the bottom of the agar block. At the same time, no obvious color change occurred in the block,other where agar block, the petri where dish the was petri not dish covered was not by covere the film,d by except the film, for except a small for blue a small plaque blue at plaque the central at hole,the indicating central hole, oxygen indicating diffusion oxygen was di dependentffusion was ondependent the film. on the film.

(a) sealed with adhesive tape, 0 min (b) sealed with adhesive tape, 15 min later

(c) 5 s after tearing off the adhesive tape (d) 60 s after tearing off the adhesive tape

(e) 2 min after tearing off the adhesive tape (f) 20 min after tearing off the adhesive tape

FigureFigure 7. Color7. Color change change of of agar agar block. block. The The bottom bottom surface surface of of the the petri petri dish dish on on the the right right was was covered covered by theby ramie the ramie fiber/starch fiber/starch nonwoven nonwoven film whereasfilm whereas the leftthe wasleft was not, not, the diameterthe diameter of the of petrithe petri dish dish was was 9 cm. 9 cm. Appl. Sci. 2018, 8, 1813 8 of 10 Appl. Sci. 2018, 8, x FOR PEER REVIEW 8 of 10

InIn orderorder toto detectdetect thethe actualactual effecteffect ofof thethe filmfilm onon thethe oxygenoxygen supplysupply toto soil,soil, loamloam waswas usedused toto covercover the film. film. It It is is well well known known that that this this kind kind of soil of soil has hasan enormous an enormous number number of capillary of capillary pores, pores, with withan equivalent an equivalent pore size pore range size of range 2–20 ofμm, 2–20 andµ thusm, and has thusgood has water good holding water capacity holding [34]. capacity However, [34]. However,the main pore the main size porerange size of rangethe ramie of the fiber/starch ramie fiber/starch nonwoven nonwoven film was film 250–300 was 250–300 μm. Thatµm. was That non- was non-capillary capillary porosity, porosity, which which means means that thatwater water in the in pore the poress of the of thefilm film were were inclined inclined to be to absorbed be absorbed by bythe thesoil soil when when in close in close contact. contact. After After the loss the lossof moisture, of moisture, the film the filmreturned returned to its to initial its initial porous porous state stateand air and diffusion air diffusion occurred. occurred. As the As theeffective effective diffus diffusionion coefficient coefficient in inthe the film film was was of of the the same inin −5 −4 2 magnitudemagnitude as that that in in air air (10 (10− to5 to10 10 m−4/s),m2 the/s), oxygen the oxygen transfer transfer rate in rate the inporous the porous film was film much was higher much higherthan that than in water-saturated that in water-saturated soil. As soil.shown As in shown Figure in 8, Figure the outside8, the outsideoxygen entered oxygen enteredthe film thefrom film the fromhole, thespread hole, over spread the film over rapidly, the film then rapidly, the diffusion then the diffusionof oxygen of into oxygen the soil into occurred the soil across occurred the acrossentire thefilm entire surface, film rather surface, than rather only than at the only hole at the(which hole occurred (which occurred in the plate in the that plate was that not was covered not covered by the byfilm). the Consequently, film). Consequently, more moreoxygen oxygen diffused diffused into into the thebottom bottom soil soil and and compensated compensated for for the the oxygen consumptionconsumption there.there.

FigureFigure 8.8. Process ofof oxygenoxygen intointo soilsoil throughthrough diffusiondiffusion inin thethe ramieramie fiber/starchfiber/starch nonwoven film.film. 4. Conclusions 4. Conclusions This study confirmed the good biodegradability of the ramie fiber/starch nonwoven film. This study confirmed the good biodegradability of the ramie fiber/starch nonwoven film. Thus, Thus, it can be safely used for some short-term crop production without worrying about its long-term it can be safely used for some short-term crop production without worrying about its long-term residual pollution in the soil. Moreover, due to its unique structure (i.e., a large number of non-capillary residual pollution in the soil. Moreover, due to its unique structure (i.e., a large number of non- pores), when covered by soil, the water in the film tends to be absorbed by the soil. This makes the capillary pores), when covered by soil, the water in the film tends to be absorbed by the soil. This film permeable so that oxygen can spread quickly within the film and diffuse into the soil from the makes the film permeable so that oxygen can spread quickly within the film and diffuse into the soil entire film surface, therefore increasing the supply of oxygen to the bottom soil and compensating for from the entire film surface, therefore increasing the supply of oxygen to the bottom soil and the oxygen consumption there. It would also be very beneficial to the growth and development of compensating for the oxygen consumption there. It would also be very beneficial to the growth and rice seedlings raised using plates. Rice seedling soil is often too wet to remain permeable in the early development of rice seedlings raised using plates. Rice seedling soil is often too wet to remain stage of cultivating rice seedlings, which means anoxia tends to occur in the soil. Thus, it would be permeable in the early stage of cultivating rice seedlings, which means anoxia tends to occur in the particularly useful to apply the ramie fiber/starch nonwoven film during this stage. Finally, we deduce soil. Thus, it would be particularly useful to apply the ramie fiber/starch nonwoven film during this that the film would also increase the oxygen supply to the soil next to the inner surface of any other stage. Finally, we deduce that the film would also increase the oxygen supply to the soil next to the cultivation container where anoxia often occurs, if padded with the film, and accordingly benefit the inner surface of any other cultivation container where anoxia often occurs, if padded with the film, growth and development of any plants in the container, as their roots could get more oxygen. and accordingly benefit the growth and development of any plants in the container, as their roots could get more oxygen. Appl. Sci. 2018, 8, 1813 9 of 10

Author Contributions: W.Z., Y.N. and C.W. conceived and designed the experiments; W.Z., Y.N., Y.Y. (Yuanru Yang), Y.Y. (Yongjian Yi), W.Y. and H.W. performed the experiments; W.Z. and Z.T. analyzed the data; W.Z. wrote and revised the paper. Funding: This research and the APC werefunded by the Young Scientists Fund of the National Natural Science Foundation of China (No. 31701372) and the Natural Science Foundation of Hunan Province (No. 2018JJ3583). Conflicts of Interest: The authors declare no conflict of interest.

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