Wood Degradation by Fungi and Environmentally Benign Wood Preservatives

Abstract

The causes, processes, features and conditional factors of wood decay and degradation due to fungi were reviewed. The degradation path of wood varies depending on diverse fungi and wood species, as fungi possess selectivity in the degradation of versatile wood components. The chemical treatments and preservatives are reviewed to understand their correlation with the decay mechanisms of wood. Environmentally benign wood preservatives are discussed, e.g. one based on chicken feather protein combined with copper and boron salts to replace the traditional wood preservatives together with several environmentally friendly preservatives based on wood extractives as a source of natural raw materials. Excellent functionalities of the protein-based wood preservative suggested that this eco-formulation could offer great potential to be used as an environmentally benign wood preservatives with a more competitive cost. This new system of wood preservatives provides a theoretical basis for further research and the reasonable utilization and scientific protection of wood products.

1. Introduction

Wood, being a natural resource, exhibits its great value and importance, and can be used widely as a structural material and industrial raw material in many fields of the global economy. It is important to note that, despite wood is a remarkable material, it is susceptible to be biodegraded by the action of microorganisms, such as fungi and bacteria. Thus, wood is ready to be decomposed under proper or certain conditions and returns natural components to ecosystem cycling.

Wood is composed of three main components, namely cellulose, hemicellulose and lignin. The highest content of the three main components is cellulose, a long linear homopolymer, which is the main chemical component in wood cell walls, accounting for approx. 40–50% of the dry weight of wood substrates, composed of β-D-glucose molecules connected by [1, 2, 3, 4] glycosidic linkages, and can be broken down into reducing sugars by cellulase. Hemicellulose is also a kind of polysaccharide molecules similar to cellulose, but a heterogeneous material which consists of various monosaccharides, and as such easier to be degraded by microorganisms than cellulose. Lignin, an aromatic heteropolymer, is the third major component of wood, accounting for approx. 25–30% of the wood dry weight [1, 2]. Unlike cellulose and hemicellulose, lignin is a polymer composed of condensed phenylpropane units (including benzene ring and aliphatic structure) with an extremely complex structure and chemical properties. The main function of lignin is to provide intensity and durability, as well as resist attacks from microorganisms and insects. Due to its complex structure, lignin is difficult to be degraded, which is often seen as a difficult obstacle in the production and utilization of wood, and only a few microorganisms, such as white rot fungi, possess the ability to completely degrade lignin [3].

In nature, the decay and degradation of wood products is a complex process that involves the combined action of various microorganisms, which can produce many types of enzymes and disintegrate woody materials as an organic substrate by the secreted enzymes [4]. By the loss of the constituents of wood cell walls, such as cellulose, hemicellulose and lignin, wood will lose its valuable strength and stability, and ultimately be bio-deteriorated by microorganisms. Among the microorganisms, white rot fungi, brown rot fungi, soft rot fungi, and bacteria are the most common causes that contribute to the decay of wood [2, 4]. White rot fungi are a few microorganisms that can completely degrade lignin. Brown rot fungi mainly decompose carbohydrates (cellulose and hemicellulose). Soft rot fungi are always ready to degrade polysaccharides. In addition, there are other microorganisms, such as bacteria, which can also degrade the components of wood [5].

Therefore, the relationship between the chemical components of wood and its degradation mechanism is strong. The degree of wood decay, or the degradation of wood chemical components, and the wood decay mechanisms vary depending on different environmental conditions [2]. For example, humid environments are more prone to cause wood degradation since microorganisms are more ready to colonize and reproduce in humid environments. In addition, different types of wood can also affect diverse types of degradation, leading to different levels of decay.

In order to extend wood service life, some treatments can be performed, such as heat treatment, chemical preservation treatment or other methods [6]. Meanwhile, the in-depth research on the wood degradation mechanism could also provide a theoretical basis and technical support for relieving and preventing wood decay and is also meaningful for both proper protection and reasonable utilization of wood.

2. Wood degradation by fungi

2.1 Wood decay fungi species

Wood decay can mainly be caused by the infection of wood decay fungi. Basidiomycetes [4, 7], which are the most common wood decay fungi in wood and play predominant roles in terrestrial carbon recycling, are well known as members of Basidiomycota, one of two large divisions of Eumycota, together with the Ascomycota [8, 9]. Therefore, most wood decay fungi can be assigned to Basidiomycetes or Ascomycetes. In a terrestrial ecosystem, a large group of decomposers, wood rot fungi, has been found, with about 1500 species in Finland and about 2000 species reported in China [10].

In Division Basidiomycota, Gloeophyllales, an order of Class Agaricomycetes, is capable of producing brown rot of wood and contains several important species, among which Gloeophyllum trabeum and Gloeophyllum sepiarium are two important wood brown rot fungi species in common service above ground [9]. In particular, G. trabeum is well known as an important fungus for the test during the decay resistance trial, which has the tolerance of some kinds of organic wood preservatives.

Polyporales is another order of Class Agaricomycetes in Division Basidiomycota and includes many of the fungi species. In Polyporales, Rhodonia placenta is another most common species of brown rot fungi and a common test fungus applied in evaluating new wood preservatives. It is interesting that Trametes versicolor is another species in Polyporales, but a white rot fungus, which is also a test fungus commonly used for the assessment of the function of wood preservatives especially in hardwoods.

2.2 Inhabiting conditions of decay fungi

Wood decay fungi have certain requirements in inhabiting conditions for growth and survival [11], several factors affecting fungal colonization and propagation are as follows:

1. Water: free water or adequate moisture in wood cell lumina;

2. Oxygen: as living organisms, oxygen is a necessary condition for most wood-inhabiting fungi;

3. Favorable temperature: suitable range of survival temperature for most wood decay fungi is from 15 to 40°C;

4. Available nutritional supplies: sustainable nutrient source is also a necessary factor for long-term living in order to provide sufficient energy and metabolites for synthesis via metabolism;

5. pH range: favorable chemical conditions are necessary for fungal growth on wood, the range, the optimal pH condition, is from 3 to 6.

In-depth comprehension of the fungal inhabiting or living conditions is extremely significant as this information can provide us with a better understanding of how these wood decay fungi survive and multiply, and furthermore how to efficiently protect wood and prevent serious degradation. For example, reducing the surrounding humidity or keeping wood dry below the fiber saturation point (25–30%) could efficiently eliminate the fungi growth. In addition, scientific treatments could be carried out to better protect wood resources, and a new wood preservative system could be developed. Besides, novel biotechnology can also be developed to pretreat the woody raw materials or waste wood even including some organic waste, which can promote the development and progress of environmental protection.

2.3 Wood decay process and feature

2.3.1 Wood decay process

As mentioned before, wood rot fungi are traditionally divided into white rot and brown rot fungi, and different fungi can decompose the different wood chemical compositions due to their own selectivity and action mechanism [12, 13]. During the process of wood degradation, white rot fungi can mainly secrete a key group of extracellular oxidases (oxidative enzymes) to degrade lignin, i.e. lignin peroxidase, manganese (II)-dependent peroxidase, and laccase, which is the most typical and common oxidative enzymes possessing the relative strong degrading ability. Hydrolytic enzymes, such as cellulase, hemicellulase, amylase and pectinase can as well as be secreted by white rot fungi, in consequence, white-rot fungi can completely deconstruct the lignocellulose cell wall materials [4]. In addition, according to the degradation and removal of wood chemical components by white rot fungi, it can be classified into “selective” and “simultaneous” decay path [14]: (1) Selective rot initially degrade the hemicellulose and lignin, but retaining the cellulose [15]; (2) Simultaneous rot degraded cellulose, hemicellulose and lignin in a rather uniform depletion [16, 17]. It is noteworthy that the same white rot fungi can cause selective or simultaneous rot when it decayed different substrates, and even both types rot in the same substrates [18, 19].

Unlike the white-rot fungi, brown rot fungi can secrete a large amount of carbohydrate enzymes, such as cellulase and hemicellulase, pectinase including amylase. Brown rot fungi extensively depolymerize the carbohydrates (cellulose and hemicellulose), leaving the fragments of the degraded cellulose and hemicellulose but retaining the modified lignin which is not depolymerized seriously [20, 21, 22]. It has long been thought that these basidiomycetes do not decompose the lignin seriously, and their activities on lignin, the abundant aromatic biopolymer, are limited to minor oxidative modifications [23, 24].

As mentioned before, different enzymes display rather various effects on wood chemical components, especially from various fungi as well as different degradation stages, which could be attributed to the fungi’s instinctive motivation and selectivity. Thus, different fungi biodegrade wood in their own selective path, and different biodegradation paths vary between different wood species (soft and hard wood).

2.3.2 Decay features

According to the shape, wood decay can be classified into white rot, brown rot, and soft rot [4]. Most wood decay fungi species are subordinate to Basidiomycota (Basidiomycetes), typically classified into two types, either white- or brown-rot fungi [7]. Brown rot fungi cause significant degradation of cellulose and hemicellulose but with little degradation of lignin, which can only be modified. The typical features of wood brown rot are shrinks and fragmentations, which easily to be decomposed into soft cubic shapes with brown discoloration, due to the lack of cellulose and hemicellulose, and the oxidation of lignin. Conversely, white rot fungi mainly degrade lignin, causing a whitish, needlelike texture or fibrous shape of the decayed wood [25]. Soft-rot fungi, broadly as “non-Basidiomycete” destroyers, resemble the brown-rot fungi which utilize exoglucanases, and endoglucanases to degrade cellulose, which was reported that the attack is limited to the amorphous cellulose zones in the microfibrils [26]. Generally, the attack of soft rot fungi is limited primarily to the carbohydrates in cell walls, and limited modification of lignin, such as demethoxylation.

2.4 Variation of wood property

Once wood is infected by fungi, wood degradation will be presented outside or/and inside of wood, with the result of alterations in chemical compositions, physical and mechanical properties, and changes of microstructures. Some alterations of wood properties, such as mass losses, chemical components and microstructures are summarized as below:

1. Mass alteration

   Wood mass loss is commonly accompanied by most fungal decay due to that fungi need nutrients as they grow through the wood by utilizing the various components of the wood cell wall. Thereby, most wood decay fungi can utilize the various chemical components of the wood substrate in the cell wall as they colonize and grow through the wood, leading to the reduction of the overall wood mass [27]. Wood mass losses can achieve 70% through brown rot, even exceed 95% after white rot and about 5–60% by soft rot. Mass loss will be generally expressed at the different trend of change due to the various decay conditions, which depends on the diverse wood species including the types of decay fungi as well as the different periods during wood decay.

   Mass losses of some wood species in different periods with both brown rot fungi and white rot fungi are presented in Figure 1 (derived from the research of previous research [28] and authors’ results [29]). As portrayed in Figure 1, the mass loss of wood clearly increased as the decay time prolongs. Besides, regarding softwood, brown rot fungi were stronger than white rot fungi. In contrast, in hardwood samples, the deconstruction capacity of white rot fungi to hardwood was more aggressive and vigorous than that of brown rot fungi, and this could be that the lignin types of hardwood mainly included guaiacyl lignin and syringyl lignin, and hardwood lignin contained more methoxyl groups, which was more easily decomposed [14, 30]. Furthermore, it was worth mentioning that, in hardwood group, Hevea brasiliensis showed more resistance against white rot fungi than Populus yunnanensis and Liquidambar styraciflua, which may be due to the density of H. brasiliensis higher than that of P. yunnanensis and L. styraciflua, attributing to the significant effect of density on wood properties (Table 1).

2. Changes in chemical components

   The chemical compositions of diverse wood species at different periods by brown rot and white rot fungi are summarized in Table 2.

   For P. yunnanensis wood, Trametes versicolor (white rot fungi) caused simultaneous rot, resulting in a rather uniform depletion of glucan, xylan and lignin. Conversely, in brown rot groups, fungi preferentially decomposed carbohydrates, which led lignin retained selectively. However, they decomposed cellulose and hemicellulose in their own selective approach. The xylan decreased from 9.73 to 2.97%, which decayed by G. trabeum, while the glucan decreased from 33.67 to 19.50% after Rhodonia placenta biodegradation, revealing that G. trabeum could degrade hemicellulose selectively, while Rhodonia placenta could preferentially attack cellulose. It is interesting that different fungi exhibited obviously different degradation in softwood species, e.g. the Picea jezoensis decayed by Porodaedalea pini, both cellulose and lignin were depolymerized due to the respective decreased contents, whereas, the increased content of hemicellulose showed the slight degradation of hemicellulose probably due to the weak influence on hemicellulose from this fungus. This also reveals that wood rot fungi depolymerize the chemical compositions of wood substrate through their own degradation pathway.

3. Changes in microstructure

   The microstructure of wood will also undergo significant changes after being infected by decay fungi, such as the enlarged porosity in the wood due to the partially or entirely destroyed fibers, which could improve the permeability, decrease the density, and reduce the strength and toughness of wood as well, making it prone to fracture [2]. Some morphology observations are illustrated in Figure 2 (derived from the authors’ results [29]). In Figure 2, it can be seen that white rot fungi T. versicolor almost colonized in cell lumens at 4 weeks due to the obviously present hyphae in the wood cell lumina, and hyphae presented in large clusters after 8 weeks [29]. It can be suggested that white rot fungi T. versicolor grew along lumens of wood cell walls during its colonization then the lignin can as well be decomposed accordingly [8, 17]. In brown rot fungi, a large number of cell walls were deconstructed, revealing fungi were able to grow and reproduce, leading to the destruction of wood cell walls. It was also reported that decay fungi colonized and attacked through parenchyma cells via pits and the wood rays were the primary paths for the spread of mycelium [19].

   It can be concluded that there also are microstructure changes within wood during the degradation by fungi with the destruction of the cell wall materials, resulting in the enhancement of the accessibility of wood substrates as well as the improvement of the wood permeability.

4. Changes in physical and mechanical properties

   Decaying fungi can lead to the changes in the physical and mechanical properties of wood, such as a decrease in intensity, elastic modulus, hardness, etc. Simultaneously, the toughness of wood will also deteriorate. These changes will affect the stability and service life, even the safety of wood products.

   Wood decay by fungi can cause wood substrate losses, and changes in the chemical compositions, microstructures and physical and mechanical properties of wood. Among those properties, strength is a prior concern when study wood decay due to its critical significance in most structural uses. In summary, the degradation caused by fungi can damage the wood cell walls, leading to the destruction of organizational constituents and structure and the embrittlement of wood, thereby affecting its performance and service life.

Figure 1.

Mass loss of different wood species decayed by white and brown rot fungi.

Wood species	P. yunnanensis	Cunninghamia lanceolata	Pinus taeda	H. brasiliensis	P. yunnanensis	L. styraciflua
Densities/g.cm^−3	0.472	0.401	0.580	0.650	0.364	0.545

Table 1.

Densities of different wood species.

Wood densities of different tree species [31, 32, 33, 34].

P. yunnanensis [19]
Fungi	Time(d)	Glucan (%)	Xylan (%)	Lignin (%)
—	0	33.67	9.73	32.24
T. versicolor	30	25.93	11.32	29.90
	60	26.10	4.49	27.64
	90	33.17	7.18	25.90
G. trabeum	30	30.09	7.72	28.33
	60	25.76	4.36	27.62
	90	23.50	2.97	29.62
R.placenta	30	21.80	7.89	27.05
	60	20.12	6.57	26.89
	90	19.50	3.16	27.95
P. jezoensis [35]
Fungi	Time(d)	Glucan (%)	Xylan (%)	Lignin (%)
—	0	46.9	23.2	26.8
P. pini	30	45.9	22.2	26.4
	60	40.4	24.2	24.0
	90	31.2	26.3	19.4

Table 2.

Relative chemical compositions of control and degraded wood.

Figure 2.

SEM images of wood samples biodegraded by different fungi.

3. Wood preservatives

As aforementioned, one of the major drawbacks of wood is its susceptibility to biological deterioration. Wood can be attacked by decay fungi inadvertently in natural conditions and as such its durability is accordingly reduced, accompanied by some decay hazards and others such as losses amounting to billions of dollars each year. It is the primary reason that wood needs to be treated aiming to prolong its service life. Based on the background, the chemical wood preservative and protection technology have been greatly developed worldwide [36].

3.1 Conventional wood preservatives

Generally, conventional wood preservatives can commonly be classified into three types, i.e. oil type wood preservatives, oil-borne type wood preservatives and water-borne type wood preservatives. Coal tar and creosote, which are usually known as traditional oil preservatives, are toxic and effective in resistance against wood decay fungi, insects and other microorganisms. Oil-borne preservatives, which dissolve some toxic water-insoluble organic fungicide compounds in organic solvents [37, 38], are also efficient but limited utilized in certain wood products due to their volatile organic compound problems. During the actual production and application process, some traditional preservatives (oil preservatives or oil-borne preservatives) are poisonous to the environment and human health, due to their containing some toxic chemicals [39]. Therefore, water-borne preservatives have been developed and applied instead of traditional preservatives in many areas.

Water soluble preservatives can protect wood efficiently in most environments, which dissolve some active chemical ingredients in an aqueous solution, which is valid in the inhibition of the harmful microorganisms to wood due to its toxicity to fungi, bacteria, insects, and other biological erosion. Besides, water-borne preservatives can also penetrate into wood cell walls due to their good permeability, and effectively protect the entire wood from the erosion of microorganisms, then provide long-lasting protective effects. Wood water-borne preservatives are usually prepared in liquid form, which can be applied to wood and/or wood surfaces through simple methods such as soaking, spraying, or brushing. Compared with other types of preservatives (oil preservatives or oil-borne preservatives), the application of water-borne preservatives is very convenient. Thus, water-borne preservatives attract increasing attention due to their well-treatment feature, lower toxicity, less environmental impact and pollution after use. Collectively, up to date, water-borne preservatives are the most widely applied wood preservative.

However, there also are some disadvantages to water-borne preservatives, including limited fixation and durability due to their high solubility in water, and low protection compared to some oil preservatives or oil-borne preservatives, especially in extreme environmental conditions, such as high humidity.

3.2 Environmentally benign wood preservatives

Wood is easy to be deteriorated for ubiquitous organisms, such as fungi, bacteria and insects. For this reason, wood products require chemical treatment rather than soil direct contact to prolong their operating lives. Though water-borne preservatives are widely utilized attributing to their inexpensive cost and good permeability, their high solubility also brings negative effects such as low stability and fixation, even risk to the environment due to the toxic leachable chemicals. Chromated copper arsenate (CCA) has been extensively used to effectively protect wood for nearly 100 years. Arsenic and copper compounds are used as toxic elements to the microorganisms and insects in CCA components, while the chromium salt is applied to fixable agents and prevent them from leaching from the CCA-treated lumber into the environment. However, CCA-treated wood products need careful use and cautious disposal for it is a toxic waste and harmful to humans, animals and the environment due to chromium and arsenate in CCA elements are inaccessible to standard toxicity characteristic leaching profile (TCLP) tests [40]. Since 2004 the U.S. Environmental Protection Agency prohibited CCA for residential purposes due to its hazard during manufacture and treatment. As a consequence, it is urgent to develop and research feasible, effective, environment-friendly and cost-competitive wood preservatives to substitute for traditional preservatives, such as CCA.

Under the above background, environmentally benign wood preservatives are researched and developed by many wood science researchers and wood preservative companies during the past few years. Copper salts, which are poisonous to microorganisms and insects, have been used most frequently in wood preservatives and could react with and/or bind to lignin, tannin, or protein consequently fixed in the wood. Boron salts are the oldest preservatives and are still used as effective fungicides and insecticides nowadays on account of their low toxicity [41, 42]. Due to the preservative active ingredients and low toxicity, copper and boron salts attracted more and more attention [43, 44, 45]. However, the leachates (copper and/or boron elements) from the treated products during the long-time application are inevitable for their water solubility [14]. Thus, copper and/or boron-based wood preservatives need to be developed to stabilize and fix the active ingredients (copper and/or boron elements) onto wood structures.

Recently, proteins such as soy isolates, okara protein, and feather protein, have been used to interact with the preservative active ingredients by coagulation, auto-condensation, and/or other chemical reactions to increase the durability of the preservative in treated wood [36, 46]. When the copper-boron-protein preservatives impregnate the wood, copper and boron can interact with wood components and be fixed in the wood matrix by gelling of protein via heating or other methods. Mazela et al. (2003) and Thevenon et al. (1998) used proteins and tannin compounds to fix the boric acid during two impregnation stages [47, 48, 49]. Sye et al. (2008) prepared a wood preservative by formulating copper and/or borax with organic waste okara to substitute the high-price copper azoles (CuAz) and alkaline copper quaternary (ACQ) [50]. Yang (2006) studied the feasibility of using soy protein instead of toxic chromium and arsenic to formulate wood preservatives with copper and boron [51]. The aforementioned studies proved that the protein-based wood preservatives could penetrate the wood block and protect the wood products against fungal attack as effectively as traditional preservatives, such as CuAz. In addition, they are environmentally friendly and have been considered as an interest alternative to CCA.

Based on this theory, the authors developed a kind of environmental friendly wood preservative based on chicken feather protein, which was used as the source of protein for its environmental benign character and low cost. The preservative formulations were composed of hydrolyzed chicken feather protein, copper sulfate (CuSO₄·5H₂O) and sodium borate (Na₂B₄O₇·10H₂O). Chicken feather powder was hydrolyzed at 140°C for 4 h, then the protein hydrolysate was obtained. The condensed hydrolyzate was added into the suspension of copper sulfate and sodium borate. Then the wood preservative solution based on feather protein was achieved with the dissolving agent ammonium hydroxide (NH₄OH) [36].

The results showed that chicken feather proteins can be successfully used to prepare the protein-based wood preservative, which can penetrate wood structures and are stable against water leaching. The interactions between chicken feather protein-based wood preservatives and wood components were also confirmed. Therefore, chicken feather protein could be used as a source of protein and an efficient chelating agent to prepare low-cost, effective and environmentally benign wood preservatives, and the chicken feather protein-based preservative can effectively protect the wood against decay fungi and prolong the service life of the treated wood blocks, which provides a new source of protein using natural components as potential wood preservatives. For exploring the ground-contact protection of the chicken feather protein-based preservative, field trials with much longer processing time need to be conducted in the future in order to evaluate the long-term effectiveness of this kind of preservative.

Natural materials attract more and more interest as a source of preservatives due to their simple way to obtain, low cost and environmentally friendly characteristics. There are some new preservatives prepared from natural materials, due to their competitive cost, low toxicity and low environmental impact, such as plant-derived wood preservatives. For instance, Tiina Belt investigated the extractives of heartwood of Scots pine, containing extractives, such as pinosylvins, and suggested that pine heartwood extractives have the potential to inhibit the white rot fungi [52]. The ethanol extractives of teak heartwood residues also showed promising antifungal abilities as wood preservatives [6]. Tchinda reported that plant essential oil showed positive antifungal activities, using natural plant extracts to protect wood [38, 53]. Senmiao Fang mixed chitosan and cinnamaldehyde as a kind of natural wood preservative and proved that the new kind of preservative can effectively protect the test sample, which can be easily used and overcomes the volatilization problem. Salicylic acid, also a natural organic substance extracted from plants, possessing antibacterial functions, can also be utilized and formulated as a kind of wood preservative. Li Yan formulated a salicylic acid/silica microcapsule and studied its decay resistance as well as the stability of modified poplar wood. The decay resistance of treated poplar was greatly improved compared to untreated poplar [54].

4. Conclusions

Wood is susceptible to being infected, decayed and deteriorating in the natural environment due to the ubiquitous microorganisms. Many microorganisms, such as wood decay fungi and bacteria, could attack and damage wood products leading to enormous commercial waste and property losses. Wood decay, an inevitable natural phenomenon caused by the activity of some microorganisms, brought about various hazards, such as wood structural deformation, and the losses of original strength and stability, which could lead to deconstruction and collapse of wooden products, causing economic impact and property damage even harm and danger to citizens. The decay mechanisms of wood are different depending on the different decay organisms (fungi, bacteria or insects) and various wood species including diverse decay stages. It is very important to understand the decay mechanisms of wood. Through the elucidation of the decay mechanisms of wood, scientific protection technology or measures could be performed in order to prolong the service life of wood products, which is crucial both for sustainable forest resource management and the research and development of wood preservatives. To enhance the resistance of wood against the decay fungi or bacteria, chemical treatments and preservatives have frequently been applied in the wood industry. Environmental concerns have prompted the development of wood preservatives based on natural materials, which are with high efficacy, low cost, low health risks and low environmental impacts. Some protein-based wood preservatives or wood extractives have received great attention due to their low cost, toxicity and environmental impact. As scientific research and technologies advance, the decay mechanism of wood and its relationship with wood preservatives have been further developed, providing an improved understanding of the wood degradation process by various microorganisms (fungi, bacteria) and promoting scientific wood protection and maintenance.

Acknowledgments

The authors would like to express their sincere gratitude to Prof. Mizi Fan and Dr. Yonghui Zhou from the Department of Civil and Environmental Engineering at Brunel University London for their valuable suggestions in reviewing the manuscript.

The authors also are grateful for the financial support by the Regional Project of National Natural Science Foundation of China (32260362, 31860186, 31360157), the Joint project of Yunnan Agricultural Basic Research (202101BD070001-058), and the 111 Project (D21027).

Conflict of interest

The authors declare no conflict of interest.