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