Effects of newly developed biodegradable liquid mulch on soil temperature，cotton seed germination and seedling growth

MAKONI Jane, MAODZEKA Antony, SUN Can, DASS Devwattie, ZHAO Tianlun, ZHU Shuijin, CHEN Jinhong(Institute of Crop Science,College of Agriculture and Biotechnology,Zhejiang University,Hangzhou 310058,China)

Abstract In order to determine the effect of newly developed biodegradable liquid mulch (BDLM) on soil temperature, seed germination, and seedling growth of upland cotton, an experiment was carried out at the Agricultural Experiment Station of Zhejiang University in 2018 and 2019. The results showed that BDLM exhibited significant improvement with respect to soil temperature, cotton seed germination rate, stem and leaf biomass(P＜0.05)compared with bare soils.In comparison with plastic mulch,BDLM had no significant effects on soil temperature and cotton seed germination rate.In order to evaluate the effect of BDLM residues on cotton seedling growth under different soil types, a pot trial was carried out in a growth chamber. The results revealed that BDLM residues had significant effect on cotton seed germination rate. The interaction of soil types and BDLM residues had significant effects on chlorophyll content, leaf area index (LAI) and soil moisture content(P＜0.05). The highest chlorophyll content was observed from loam soils with BDLM residues, and the highest leaf area index was observed from clay soils with plastic residues, and lowest moisture content was observed from traditional loam soils. In conclusion, BDLM has positive effect on cotton growth, however further researches should be proposed during cotton harvesting stage.

Key words biodegradable liquid mulch;soil temperature;upland cotton;germination;seedling growth

Cotton (Gossypium hirsutum) is native to the global tropical and subtropical regions, including India, Africa and America with the greatest diversity found in America then Australia and Africa[1]. The plant can be grown in temperate, tropical and subtropical climates. Cotton farming started around 207 BC to 220 AD in the southern part of China[2].Cotton fibre is usually spun into thread or yarn which can be further processed into breathable textile.Cotton also provides animal feed and oil. The estimated world cotton production is 110 million bales (25 million tonnes), and China is the largest cotton producer in the world although most of its cotton is domestically used[3]. To increase the yield of cotton and other crops, plastic mulching was introduced as a result of advancement in China agricultural sciences and plastic mulch became an important agricultural production material, which assumed an important role in ensuring the supply of agricultural products in China[4]. High yields in China were obtained as a result of intensified farming technologies. Improved agricultural technologies have seen cotton yield increasing by 3.12% annually.Such technologies include plastic mulching. About 2.7 million hectares(70%of total China cotton fields)are covered with plastic mulch every year[4-5]. This is mainly done in the northern of China,where there are arid and semi-arid regions. Plastic mulch is also practised in North China as well as the coastal saline and alkaline regions.

Plastic mulch has various positive effects including increasing soil temperature, intensifying sunlight,preserving soil moisture to reduce drought,improving fertilizer use efficiency, conserving soil, minimizing soil salinization and suppressing weed growth[6].Various improved agricultural technologies including mulching film have increased cotton yield in China,for example, in the 2014/2015 season the global average lint was 768 kg/hm2, whilst in China it was 1 484 kg/hm2[7].

Although plastic mulching has played an important role in making China a giant in cotton production,it also has some challenges.Plastic mulch used is mainly composed of polythene, so mulching film does not readily degrade in soil. The problem is worsened by the low rate of plastic mulch recovery,due to mechanized cultivations and thin film. Great amounts of residual plastic mulch can make farmland unusable thereby affecting agricultural land[8-9]. In general, because plastic mulch on soil is weak and easily broken up, the extent of recovery is inadequate in the presence of straw, especially with a highdensity crop. Plastic remains are difficult to retrieve from soil at the end of farming season hence causing plastic pollution or white pollution. Therefore, plastic remains accumulate in soil year after year. These plastic residues destroy soil structure and disturb plant growth, and they are major contributors to environmental pollution in China. Accumulated plastic residues destroy soil integrity, soil water movement, soil ecology as well as crop growth[10-11].In some parts of China, the amount of plastic mulch residues have reached 60-260 kg/hm2in top soil(0-20 cm depth). Despite the benefits of plastic mulching technologies, its widespread use has generated large amounts of mulch residues, evolving from a‘white revolution’to‘white pollution’in the world[12]. Because of challenges caused by plastic mulch, many crop scientists have a rising interest in developing alternatives, thus biodegradable mulch was developed.

Some biodegradable mulching films had been developed as an alternative measure for reduction of plastic residue pollution in the past; however, they could not meet the expected standards[13]. Researches are still going on to come up with a suitable biodegradable film,which can give all the advantages provided by plastic mulch but should have the ability to completely degrade into end products which are harmless to soil ecosystem and do not cause environmental pollution at the end of cropping season.Also,this will have the advantage of reducing labor cost and it can be considered as a sustainable agriculture production[14-16].

Biodegradable liquid mulch (BDLM) is a kind of composite black-brown viscous liquid that is degradable, thus making it environmentally friendly.After being mixed with water and sprayed on soil, this liquid mulch can become a layer of black immobilizing film on soil surfaces, which can contribute significantly to enhancing soil temperature and inhibiting water evaporation.Hence,this research was done to evaluate the effect of newly developed BDLM on soil temperature, upland cotton seed germination,and seedling growth.

1 Materials and methods

1.1 Treatments

In 2018 and 2019 trials to evaluate the effects of newly developed BDLM on soil temperature, cotton seed germination and seedling growth were carried out during the summer at Zijingang Campus of Zhejiang University in Hangzhou, China.This site is located at 30°18′51″N and 120°05′09″E, which receives an annual rainfall of 1 000 mm, and the soil type is clay.Newly developed BDLM made up of hydrophobic polymer with biodegradability and three controls (no mulch, black plastic mulch, and white plastic mulch)were used.The experiment was done on an open field then a further growth chamber experiment was done to investigate the effects of BDLM residues on cotton seed germination and seedling growth under different soil types.

1.2 Investigation items and methods

1.2.1 Soil temperature

Temperature records were taken at the soil depths of 5,10 and 15 cm.Thermometers were inserted into soil on planting day at the angle of 90°, and then temperature data was measured at 6:00, 14:00 and 18:00,respectively.

1.2.2 Germination rate

Germination rate was obtained by counting the number of germinated seeds until the last seed germinated.The data was then converted to percentage.

1.2.3 Seedling height

Samples of 10 seedlings/plot were uprooted for height measurement. String and tape measure were used.Seedling height was measured at six weeks.

1.2.4 Seedling dry matter content

At six weeks, samples of 10 seedlings/plot were collected from the field. Seedlings were separated into different plant parts, then leaves, stems and roots were oven dried;and their masses were determined.

1.2.5 Nitrogen(N)content

Samples of 10 seedlings/plot were used for N analysis. Seedlings were harvested at six weeks then cut into roots, stems and leaves. The samples of roots, stems and leaves were then oven dried at 60 ℃for 48 h. Dried samples were ground into powder and then analysed according to Kjeldahl method or Kjeldahl digestion.

1.2.6 Seedling height trend

Plant height was measured at two weeks interval using a tape measure.

1.2.7 Leaf number trend

Leaf number was measured at two weeks interval by counting the number of leaves.

1.2.8 Chlorophyll content

Chlorophyll content was measured at four weeks using a SPAD-502 chlorophyll meter[Konica Minolta (Japan) Investment Ltd.]. The third leaf from the bottom was used for chlorophyll content measurement.

1.2.9 Leaf area index(LAI)

Leaf area index (LAI) was measured at four weeks. This was done by measuring the leaf area and covered ground area. The below formula was used to calculate the leaf area index[17]:

LAI=Leaf area/m2÷Ground area/m2

1.2.10 Soil moisture content

All plots were subjected to equal amount of water on planting day. At the end of the experiment,soil samples were taken from each plot.Fresh mass of all soil samples was measured using a mass scale.Then all soil samples were oven dried and their masses were being measured until it became constant.When the soil samples was fully dried, the difference between initial mass and final mass was calculated and then converted to percentage.

1.3 Statistical analysis

Data analysis was carried out using Genstat discovery Edition 18. Significant differences between treatments were tested with analysis of variance(ANOVA). Waller-Duncan's multiple range tests were used for mean separation at P＜0.05. Figures were plotted in Excel 2016.

2 Results

2.1 Effect of newly developed BDLM on soil temperature

Soil temperatures were measured for five consecutive days. Temperature records are shown on Fig.1.The highest average soil temperature trend was observed with white plastic mulch in the morning,afternoon and evening throughout the five days. Both white and black plastic controls gave higher temperatures compared with newly developed BDLM. However, soils under newly developed BDLM treatment gave higher temperatures in comparison with traditional bare soils. The same results were obtained for both average temperature trend and the highest temperature trend.

2.2 Effect of newly developed BDLM on germination rate of upland cotton seed

The highest germination rate was observed in seedlings under black plastic mulch treatment (Fig. 2).It was not significantly different from seedlings under white plastic mulch and BDLM treatments (P＞0.05).The lowest germination rate was observed under traditional bare soils. However, the germination rate under newly developed BDLM was greater than that on traditional bare soils.

2.3 Effect of newly developed BDLM on soil moisture content and seedling physiological parameters

BDLM had significant effects on biomass of root, stem, leaf and seedling height (P＜0.05), but no significant effects on soil moisture content and N contents of root and stem (P＞0.05) (Table 1), when compared with white and black plastic mulch treatments.

Relative to soil moisture content, the highest water loss was obtained from traditional bare soils,which was not significantly different from soils under white plastic mulch and BDLM treatments. The lowest water loss was obtained from soils under black plastic mulch which was not significantly different from soils under BDLM and white plastic mulch treatments.

BDLM had significant effect on seedling biomass accumulation. Relative to root biomass, the highest biomass was obtained from seedlings under white plastic mulch treatment, which had a mass of 0.62 g. However, it was not significantly different from black plastic mulch treatment. The lowest root biomass was observed from seedlings under bare soils, but it was not significantly different from seedlings under BDLM treatment. Besides, BDLM had a significant effect on seedlings' leaf biomass(P＜0.05). The highest leaf biomass was observed from seedlings under white plastic mulch treatment but it was not significantly different from seedlings under black plastic mulch treatment. The lowest leaf biomass was observed from seedlings grown on bare soils. As for stem biomass,BDLM had significant effect on stem biomass compared with other treatments.

BDLM had a significant impact on seedling N content, including root and stem compared with these in bare soils.The highest root N content was observed from seedlings under traditional bare soil treatment;however, it was not significantly different from seedlings under white plastic mulch. The lowest root N content was observed from seedlings under black plastic mulch treatment;however,it was not significantly different from seedlings under BDLM treatment.Relative to stem N content, the highest stem N content was observed from seedlings grown on traditional bare soil which was not significantly different from seedlings under white plastic mulch treatment. The lowest stem N content was obtained from seedlings under black plastic mulch treatment which was not significantly different from seedlings under BDLM treatment. The highest leaf N content was observed from seedlings under white plastic mulch treatment, which was not significantly different from seedlings under bare soil treatment. The lowest leaf N content was observed from seedlings under BDLM treatment,which was not significantly different from seedlings under bare soil and black plastic mulch treatments.

2.4 Effect of BDLM residues on cotton plant height trend under growth chamber conditions

Mulching residues have effects on soil plant relations with respect to seedling height. Results reviewed high seedling height(growth rate)from clay soil under BDLM residue treatments from four weeks up to 10 weeks (Fig. 3). At 10 weeks, there were no differences between clay soil under plastic mulch residues and clay soil under BDLM residue treatments.Also different soil types had effects on seedling heights. This research reviewed clay soils giving taller plants compared with loam soils.

Table 1 Effect of newly developed BDLM on soil moisture content,physiological parameters of cotton seedlings

Fig.3 Effect of different soil types and BDLM on plant height trends

2.5 Effect of BDLM residues on cotton leaf number under growth chamber conditions

Mulching residues had effect on cotton with respect to leaf number (Fig. 4). The highest leaf number of seedlings was observed from clay soil under BDLM, followed by seedlings from loam soil under BDLM. The lowest leaf number was observed from loam soil(CK).

2.6 Effect of BDLM residues on cotton seed germination，chlorophyll content，LAI and soil moisture content

Results reviewed an interaction between BDLM residues and soil types (Table 2). BDLM had a significant effect with respect to chlorophyll content,leaf area index (LAI), soil moisture content and seed germination rate.

Fig.4 Effect of different soil types and BDLM residues on upland cotton leaf number

First, residues of clay and loam soils under BDLM had significantly positive effects on seed germination rate compared with soils without BDLM.Besides, residues of clay soil under BDLM had significantly positive effect on cotton chlorophyll content compared with clay soil. The highest chlorophyll content of seedlings was observed from loam soil by BDLM treatment. But, it was not significantly different from all other treatments except clay soil (CK). For LAI, residues of BDLM also had significant effects. The highest LAI of seedlings was observed from clay soil under plastic mulch treatment.The lowest LAI of seedlings was observed from traditional loam soil. Besides, BDLM residues had a significant effect on soil moisture content.The highest soil moisture content was observed from clay soil under plastic mulch residues treatment which was not significantly different from that in clay soil under BDLM residue treatment. The lowest soil moisture content was observed from traditional loam soil treatment.

Table 2 Effect of BDLM on cotton seedling growth parameters

3 Discussion

As expected BDLM had some improvement in cotton seedling growth when compared with bare soil. However, plastic mulch performed better than BDLM.In 2019,DENG et al.did a similar research in Xinjiang, China, using maize and cotton[18]. Our research results are consistent with this. BDLM has the capacity to improve soil temperature, soil moisture conservation and crop growth parameters.Previously, another experiment was carried out to research on the effects of polyadipic acid/butylene terephthalate (PBAT) biodegradable plastic mulch on soil properties and yields of cotton and maize[19].Their results reviewed no significant differences between biodegradable plastic and polythene, which are in accordance with the present result. It was indicated that BDLM was a potential alternative to synthetic plastic mulch and could be used in a wide range of crops such as maize. Also, there is still room for improving BDLM so that it will have all the advantages we get from plastic mulch and have the ability to degrade without harming the environment at the end of the cropping season.The useage of BDLM proves to be a positive step towards sustainable crop production[20].

BDLM has the potential to solve challenges of plastic mulch since the mulching solution can be easily applied to any agricultural domains. Compounds of BDLM may be applied through spraying, which is an added advantage since one can apply the film according to required coat thickness and can be easily adjusted. BDLM can be ploughed directly into soil for biodegradation since soil microorganisms can transform them to water,biomass and carbon dioxide.Degradation of BDLMs occurs from the natural action of soil microorganisms like bacteria,fungi,and algae. Unlike synthetic plastics which only breaking into small pieces that will accumulate in the soil,newly developed BDLM will completely degrade within 12-18 months.Although BDLM offers solutions to challenges caused by polythene, there is a raised concern about their rate of biodegradability after being applied to soil and their effects on different crops[21-22].

Interaction of temperature, sunlight, moisture and mechanical strength promotes deterioration of BDLM above ground and can also affect biodegradation which occurs below ground[23-26]. Fragments of BDLM are either physical or biogeochemical input. This is unique to BDLM and can have effects on soil ecosystem. Fragments of BDLM can physically modify soil before they are fully biodegraded[11].Hence, BDLM is capable of creating conditions which are not good for microbial activity e.g. water scarcity to affecting plant growth. Also, BDLM fragments can accumulate in soil causing the same effects like polythene.

BDLM is quite promising, but there are considerable gaps in our understanding of their longterm impacts on soil ecosystems that are critical to crop productivity. For future in-depth researches,there is a need to determine the long-term effects of BDLM, including their effects on soil ecosystems,effects of BDLM residues on soil physical properties[27]. Many important gaps remain since many researches to date have never focused on the effects of BDLMs for a long time. Besides,researches need to assess the impacts of BDLM on soil ecosystems especially with respect to soil carbon and sustainable crop production. Moreover,the research needs to be done up to cotton harvesting stage and observe if BDLM can maintain its functionality throughout the cropping season. Effects of BDLM can also be evaluated using different crops.

4 Conclusions

This experiment reviewed that BDLM had the potential to work as an alternative to synthetic plastics. In comparison with traditional bare soils,BDLM revealed soil temperature improvement as well improvement in soil moisture content and seedling growth parameters, thus chlorophyll content,leaf area index, seedling height and leaf number.These results are not entirely conclusive, so a followup study reviewed that BDLM residues had positive effect on different soil types. Since BDLM is promising, further studies can be carried out in cotton production test. Also researches to understand longterm effects of BDLM should be done before recommending it for use.