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Article

Effects of Shredded Paper Mulch on Komatsuna Spinach under Three Soil Moisture Levels

by
May Myat Mon
1,* and
Hiroki Oue
2
1
The United Graduate School of Agricultural Sciences, Ehime University, 3-5-7 Tarumi, Matsuyama 790-8566, Japan
2
Graduate School of Agriculture, Ehime University, 3-5-7 Tarumi, Matsuyama 790-8566, Japan
*
Author to whom correspondence should be addressed.
Agronomy 2023, 13(10), 2502; https://doi.org/10.3390/agronomy13102502
Submission received: 6 September 2023 / Revised: 24 September 2023 / Accepted: 25 September 2023 / Published: 28 September 2023
(This article belongs to the Special Issue Farming in Harmony with Nature)
Browse Figures
Review Reports Versions Notes

Abstract

:
Mulch has been considered effective for saving water and promoting plant growth. However, little has been investigated about the effect of mulch from recycled shredded paper under different soil moisture conditions on spinach growth. A pot-grown Komatsuna spinach experiment with a factorial design was conducted under two main conditions, with and without shredded paper mulch. Each of these conditions was treated with three different soil moisture levels (SWC) 20%, 25%, and 30%. The smallest evapotranspiration amount was in the mulch-only, no-plant treatment with SWC 20% (92.88 mm), and the largest was in the plant-only, no-mulch treatment (226.19 mm). All biomass yield attributions were negatively influenced by increasing the frequency of irrigation and SWC levels. SWC 20% with shredded mulch resulted in the largest dry matter, although it was statistically non-significant. But it resulted in taller plant heights and a larger leaf area index (LAI) compared to soil with no shredded mulch (p < 0.05). For SWC 25% and 30% treatments, Komatsuna spinach with no shredded mulch resulted in slightly superior plant dry matter and plant height, compared to mulched plants. These results suggest that SWC 20% with shredded paper mulch has the highest potential for saving water among all treatments for growing spinach under limited water availability.

1. Introduction

The Brassica rap plant species consists of Komatsuna spinach (var. perviridis), Chinese cabbage (var. pekinensis), pak choi (var. chinensis), and turnip (var. rapa), which are popularly grown in Asia [1]. Among the species, growing Komatsuna spinach with careful monitoring and control of soil moisture plays a key role in ensuring crop development. It is thought that the growth and yields of spinach vary due to water availability or water stress, which is highly correlated with efficient irrigation management. Irrigation schemes are designed to provide plants with water when natural rainfall is insufficient.
Plants grown under water stress may lead to bitter-tasting leaves, early blooming, decreased membrane stability index, and increased antioxidant enzymes activities, soluble sugars [2] (which also affect sugar content [3]), and oil content [4], partly due to an increasing concentration of bioactive compounds in Brassica plant species. Deficit irrigation contributes to a 50% reduction in fresh leaf weight due to a reduction in stomatal conductance (gs) [5]. On the other hand, extreme moisture results in waterlogging stress, which leads to excess water in the plant’s root zone, decreased oxygen availability, reduction in plant growth, and in some cases, leading to plant death [6,7]. Soils are susceptible to waterlogging when the amount of rainfall exceeds the ability of the soil to drain away soil moisture. This susceptibility is increased by the strong texture contrast between sandy topsoil and clay subsoils; infiltration is higher through the topsoil than in the subsoil [7].
At the same time, studies have been conducted to determine the timing and volume of irrigation for vegetable crops. To gather this information, two main approaches are commonly used: one using the water balance method based on the estimation of crop evapotranspiration (ETc), and the other using soil plant sensors [8] or soil moisture [9]. In this experiment, the soil moisture method was used to decide irrigation timing. Thus, the possibility of extending spinach production is dependent on available water in the given area, which may be aided by some additional modification to the soil surface.
For modification of the soil surface in agriculture, different types of mulch have been used. Mulching is the practice of covering the soil around plants with a layer of organic or inorganic material, such as bark, leaves, or stones, and in this study, with recycled shredded paper mulch. Mulch and meteorological conditions are related in several ways. Meteorological conditions such as temperature, humidity, wind speed, and precipitation can affect the effectiveness of mulching. The selection of a specific mulching type for a particular purpose is of significant importance. In this research, shredded paper waste was applied to approach agricultural production sustainably and to reduce paper waste. Nowadays in Japan, the accumulation of paper garbage has become greater because of the increment in paper usage along with economic expansion. Paper components have been graded as lower-value recyclable material or equal municipal solid waste [10]. In nature, unlike plastic polymer films, paper mulch is hygroscopic, expanding and shrinking with changes in its moisture content [11,12], and paper mulch tends to give a more uniform temperature than other mulch treatments [13]. Additionally, paper mulch was recommended by a previous researcher to use for cool-season crops because of their lack of soil heating capacity [14]. Moreover, paper mulch incorporated into the soil broke down naturally and was found to have a faster degradation rate than straw mulch [15]. Additionally, another benefit of using paper mulch is that it supports plants in water-limited conditions and plant water status [16].
For growing the Komatsuna spinach species, there have been concerns about appropriate irrigation schedules for the conditions of with and without soil cover to avoid nutrient leaching due to overwatering and the extra expenses of mulching. As irrigated agriculture represents 20% of the total cultivated land, the water demand that comes from agriculture must be allocated efficiently with the help of technology such as soil moisture sensors [16]. By using irrigation practices together with mulching, gardeners and farmers can create healthy, sustainable growing environments for their plants.
The present study aims to understand how irrigation scheduling and soil moisture affects the vegetative development of Komatsuna spinach for the optimization of efficient water use to achieve a better-quality product. In this study, shredded paper mulch was applied in combination with three different soil moisture deficit levels. Therefore, shredded paper could then be considered not just a waste product but a cost effective, environmentally friendly resource to promote vegetable production. Moreover, changes were monitored in plant growth, especially dry matter and water use amount, irrigation day intervals, evapotranspiration, and water use efficiency under four different treatments: (a) plant plus mulch (MP), (b) plant only (P), (c) mulch only (M), and (d) bare soil (BS).
The following questions were investigated:
  • Does shredded paper mulch affect the irrigation schedules of the Komatsuna spinach plant?
  • How does using shredded paper mulch affect the growth of Komatsuna spinach plants?

2. Materials and Methods

2.1. Study Area

The pot experiment was conducted at Ehime University, Matsuyama City, Ehime prefecture, Japan, 33.83° N, 132.79° E. A net house was used to protect the crops from harmful insects while allowing rain and sunshine to filter into the net. Climatic data (Figure 1) were collected from a weather station set up at a 2 m sensor height in the middle of the experimental net house.
The specifications for weather data were collected with a sensor that has a unit type (ATMOS-41, METER Group, Inc., Pullman, WA, USA) for precipitation, air temperature, relative humidity, wind speed, and soil moisture sensors from the meter group soil sensor series 5 TE (METER Group, Inc., Pullman, WA, USA) and 5 TM (METER Group, Inc., Pullman, WA, USA) and measurements were taken with a handheld EM 50 logger (METER Group, Inc., Pullman, WA, USA) at 5 cm soil depth. Data were recorded using ZL6 Pro. METER Group, Inc., Pullman, WA, USA) and EM 50 data loggers (METER Group, Inc., Pullman, WA, USA).
During the experimental period from the transplanted date to harvest date, sunshine ranged between 11.1 to 205.9 W/m2 (Figure 1a), the average daily air temperature ranged from 11.3 (min) to 21.5 °C (max) (Figure 1b), humidity ranged from 38.6 to 96.6% (Figure 1a), and wind speed ranged from 0.4 to 1.8 m/s (Figure 1b). There was 110.9 mm of rainfall accumulated during the crop growing season (29 January–9 April 2022) (Figure 2). In the literature [17], the best growth for mustard was in a mild winter climate with temperatures between 12.0 and 25.0 °C. Another crop in the same family such as broccoli (Brassica oleraceae L.) grows well at temperatures of 15.5–24.9 °C [18]. Therefore, the timing of this study was favourable for growing Komatsuna spinach plants.

2.2. Study Design

The pot experiment was laid out as a factorial design. The main two treatments were two soil surfaces, which are mulch and the control without mulch (a total of four different soil surfaces: plant plus mulch (MP), plant only, (P) mulch only (M), and bare soil (BS) × 3 subplot treatments (three irrigation levels: 20%, 25%, and 30% SWC)). They were arranged in a split plot design with three replications for each treatment and five additional pots for the lowest irrigation. The spinach variety was the Japanese spinach cultivar Komatsuna (Brassica rapa var. perviridis). Ubagai Shubyou Engei Co., Ltd., Ehime, Japan were germinated in a nursery tray in a glasshouse from the 2nd week of December 2021 (28 December) and transplanted after 4 weeks (29 January 2022).
As shown in Figure 3, one plant was maintained in a pot, and three plants per treatment were sampled to measure growth parameters and water consumption. As shown in Figure 4, a total of 36 pots were used—18 pots had mulch, and the other 18 had no mulch.
The field capacity in this experiment resulted in an average of 2.23 pF or 0.335 m3/m3 of soil water content (33.5% SWC) after two days of drainage from the bare soil of the 6 pots used for the trial measurement. As for the wilting point of the soil used, it was not measured directly with laboratory-based measuring tools but was estimated as 11.16% SWC, which is one third of the soil moisture content of the field capacity average. The soil has a 0.41 to 0.6 mS/cm range of EC (electrical conductivity,), which can be defined as USDA class A, comprising 0 to 0.13 g of salt per 100 g of soil [19].
From this result, the SWC lower limit in this experiment was set at the level of 20%, and this seems in line with what other researchers had concluded based on their experimental SWC conditions. Researchers reported a volumetric water content between about 0.1 and 0.16 cm3/cm3 (10% and 16% SWC), which produced the most favourable yield for growing spinach in Tottori, Japan [20]. Other researchers set soil moisture deficit conditions at 7.5%, 17.5%, 25%, 30%, and 35% for growing microgreens and found that 17.5% can provide an optimal yield [21]. Some researchers [5] have used irrigation treatments based on a gravimetric method that was set up at 100% irrigation (control) and deficit levels of 75% and 50% for growing baby spinach. Moreover, for growing kale in a pot [22] researchers selected volumetric water content (VWC) corresponding to SWC levels of well-watered plants with high moisture (35%), intermediate drought with medium moisture (25%), and drought-stressed with low moisture (20%) conditions.
To determine the upper limit level of 30% soil water content out of all the SWC treatments at 20, 25, and 30% (0.2, 0.25, and 0.3 cm3/cm3), a previous experiment with similar protocols was reviewed and followed [23]. The unit for soil water content, pot weight changes, and volume (grams) per cm2 was calculated as the weight of the pot changes in grams divided by pot area ԯr2. The original unit for SWC with the sensor output is m3/m3 VWC.
This experiment used shredded paper mulch as it has economic advantages due to its availability from local offices and its biodegradable rate in the field [24], and it does not cost farmers anything for disposal. Other similar research [11] emphasized that shredded newspaper degraded slower than sheeted newspaper but faster than straw mulch. However, there is some evidence surrounding the limitations of shredded paper mulch and whether it is only useful for small-scale farming in mild environments as it is still not as widely applicable as plastic mulch to cover a field using a machine, and strong winds may pose considerable difficulty for paper mulch to stick to the soil’s surface [10]. On 28 January 2022, soil surfaces for MP and M were completely covered with shredded paper to reduce water loss through evaporation. The white shredded paper had a size of about 3.5 cm × 0.2 cm, and 20 g was incorporated into each pot. As for basal fertilizer and soil, a chicken fertilizer ratio of 450 g per pot was applied together with a 2 kg soil mixture (black soil and charcoal from the previous year: rice field soil (2:1)) to pots equally, which had a 10 cm2 surface with a 17.5 cm depth.
Soil water content at other depths was not measured, and only the surface layer root zone depth (0–7 cm) was measured. In this experiment, the height of the pot from top to bottom is 17 cm, and soil depth is about 14 cm beneath the surface. In the literature, it is stated that effective root depth for soil moisture measurement will depend on the growth stage of the plant and irrigation method [25]. Applying water only when it is required by the plant and applying it to the active root zone depth minimizes water loss and use [5]. Additionally, spinach itself has a shallow root system, which does not have much room to branch out, and root length results are reported differently according to nutrient treatments and experimental season, etc. Previous researchers reported that Komatsuna spinach root length averages were observed mostly at a 9 cm depth upon harvest at 21 DAS (days after sowing) in a pot culture experiment conducted in Tsukuba, Japan [26], while in the Kyushu region of Japan, the root length of Komatsuna appeared to be longer, ranging from about 19 cm to 37 cm after harvest at 37 DAS [27].
In this experiment, water was added to each pot until drainage occurred similar to previous research methodology. Previous studies employed similar irrigation techniques described as “water added to the plant until drainage occurring” [28,29,30], and the weight of each pot was assumed as field capacity weight (WFC) of that pot [31]. The water content of the pots after the drainage stopped was assumed to be field capacity (WFC), and each pot was weighed before each irrigation event [32].
The control of soil water conditions in this study replicated previous studies. The idea is to maintain soil moisture at a set level using soil moisture sensors with real-time information in the root zone. This proposed strategy could help farmers with a cost-effective assessment of SWC (soil water content)-based irrigation for agricultural water use.

2.3. Irrigation and Water Use

Irrigation scheduling was conducted based on soil water content (SWC) and measured with a 5 TE soil moisture sensor, which shows the targeted range such as when it reaches 20% depletion at a 5 cm depth from the soil surface. Pots were irrigated to saturation until they started dripping water from the bottom. Rewatering started again when the SWC reached designated levels, depending on rainfall input.
For evapotranspiration (ET) measurement, every morning before each irrigation schedule, the daily ET was recorded by measuring the pots’ weight losses every day [33]. Water use efficiency (WUE) for a given treatment = [the total dry matter yield (g/plant)]/[the respective total consumptive water use amount in mm for the whole crop period] [17,34]. The amount of water applied to each treatment was determined until the water started draining from the bottom of the pot. Soil water content (SWC) was regularly observed every day, and SWC was maintained by adding water every day depending on the sensor output average for each treatment. Irrigation interval day was calculated as the total irrigation number divided by the number of growing days.

2.4. Measurement of Physiological Changes for the Spinach

Plant growth parameters were measured using the following schedule, weekly: leaf area via direct measurement of the total leaf area (the leaf surface length and width of the whole plant) average divided by occupied pot area [17], plant height, and leaf number, at harvest; dry matter of leaf, stem. Dry weight was measured using an electronic weighing scale (A&D Company, Limited, Tokyo, Japan with readability 0.01 to 0.001 g) at harvest [35]. For measuring dry weight, harvested plant samples were divided into leaves, stems, and flowers and were oven-dried at 80 °C for 2 days.

2.5. Data Analysis

The data were analysed using post hoc multiple comparisons between treatment groups. Those with mulch and without mulch were compared with a paired t-test to calculate mean differences at p < 0.05. By applying the Bonferroni test alpha values instead of 5% probability, statistical differences and symbols were noted for the average data set between different soil moisture conditions under the same soil surface.

3. Results

3.1. Meteorological Conditions, Soil Water Content, and Irrigation Intervals

At the experimental site, the weather can be characterized as a cool climate with a relatively ideal winter for growing spinach as described in Figure 1. The mean daytime air temperature from January to April was 11.04 °C, although March and April were warmer with an average of 14.98 °C. Solar radiation for the whole season averaged 217.35 W/m2, with relative humidity of 62%, wind speed of 0.91 m/s, and total precipitation of 139.61 mm from 13 rainy days. In general, the targeted soil moisture trend became less stable in the middle of the growth stage starting from 26 February (Figure 5).
The irrigation day intervals, also called irrigation frequency, refers to the number of days between irrigation. As shown in Table 1, the study period was divided into two periods, vegetative and flowering stages, depending on the plant’s development to investigate the relationship of the irrigation soil moisture controls and mulch according to plant growth stages. Throughout the season, SWC-3 treatments needed the highest frequency of watering—once a day for all soil surface conditions, followed by SWC-2. The lowest irrigation frequency (irrigation once in 4 days) was maintained under P1, M1, BS1, as well as once in 3 days at MP1 and once in 2 days at BS2.
Table 1 shows that the average number of days between irrigation events were longer for SWC-1 under all soil cover treatments in the vegetative stage than in the second stage (flowering stage). The soil moisture stored shows that the spinach plants in the initial stage required smaller ET demands; also, the observed climate changes were mild during the initial period and could have led to fewer water demands. The longest intervals resulted in MP1 with 5 days, followed by M1, P1, and BS1 with 4-day intervals, respectively. For SWC-1 in the reproductive stage, irrigation intervals decreased with the higher water demand along with the increase in temperature and the development of the crop. During the flowering stage, MP1 dried faster and required more irrigation (once a day) than in the earlier vegetative stage (irrigation every 5 days).
For SWC-2 in the vegetative stage, irrigation intervals were almost the same as the second stage (flowering stage) under all soil cover treatments. The soil moisture stored shows that the spinach plants in the initial stage required equal demands to reflect the seasonal changes and targeted soil moisture range. The longest intervals resulted in BS2 with 2 days followed by M2, P2, and MP2 with 1-day intervals, respectively. For SWC-2 in the reproductive stage, irrigation intervals decreased with more frequent water demands with the weather approaching summer and the continuous growth of the crop. During the flowering stage, BS2 dried slightly faster and required more irrigation (once a day) than the earlier vegetative stage (irrigation every 2 days), while the other treatments under SWC-2 showed no particular changes.
For SWC-3 in the vegetative stage, irrigation intervals were not particularly different from the second stage (flowering stage) under all soil cover treatments.

3.2. Plant Growth

3.2.1. Plant Height and Flowering

As shown in Figure 6, plant growth indices were differentiated by soil moisture content level variations, highlighting the immediate impact of moisture on plant height development. Overall plant height was tallest in P3 in un-mulched plants and then closely followed by P2 and P1 treatments. Over the whole season, plant height increased slowly until the vegetative stage and then rapidly increased in the later growth stages (Figure 6a).
Under the same SWC level, mulched Komatsuna’s tallest plant height at harvest was recorded in MP3 and then followed by MP2, and MP1 was the shortest. The overall height at harvest (DAT 97) resulted in 84 cm for mulched plants at MP3, MP2, and the plant-only treatment P3, while the shortest was 67 cm for the plant-only treatment P1, which also had the lowest moisture treatment. Spinach is a leafy green vegetable that grows best in cool and moist conditions. It has a taproot that might have absorbed water from deep in the soil, but it also needs regular and shallow watering to keep the surface soil moist as in the targeted level SWC-3.
In this study, plant heights are slightly higher than other’s reported data [35]. At DAT 60, the plant heights ranged between 81.9 cm for no post-sowing irrigation, a plant height of 88.2 cm for two irrigation events after sowing, and 88.3 cm for a one-time irrigation event. Another researcher [36] reported that the Komatsuna plant height at DAT 70, ranged between 20 cm and 30 cm, which met the shipment standard for the Japanese vegetable market, while others decided on marketability based on a leaf area of less than 20 cm [37]. As in Figure 6b, late flower initiation resulted under drier SWC-1 treatments for both with and without mulch. At SWC-1, flower numbers were significantly fewer compared with SWC-3. However, under all SWC-3 conditions, no significant differences were observed under all treatments.

3.2.2. Leaf Area Index (LAI) and Leaf Number

It is undeniable that soil moisture depletion has a great impact on the growth of vegetative parts in many crops. Among the growth traits, leaf area plays an important role in most studies of terrestrial ecosystems concerning light interception, evapotranspiration, photosynthetic efficiency, fertilizers, irrigation response, and plant growth [38].
The results shown in Figure 7a, in which the increased area of the leaves is plotted against the increased days after transplanting (DAT), each SWC level is measured under various moisture treatments starting 1 week after transplanting from 3 February to 6 April. This period represents the vegetative stage to nearly the reproductive stage. The largest leaf area was observed in MP1 and the smallest in P2 within the last three weeks before harvest. Larger LAIs were found in mulch treatments over no-mulch treatments, although there are no statistical differences in Figure 7a. During the primary growth stage, the LAI was not influenced by mulch application and differences in SWC treatments.
In Figure 7b, the greatest number of leaves was recorded in MP3 among mulch treatments and P2 in no-mulch treatments. Leaf numbers increased very noticeably from DAT 46 (16 March). Starting from 6 April (DAT 67), the leaves at the bottom turned yellow and dropped off due to age-diminished leaf numbers at the end of the growth period. However, all observations for leaf numbers per plant had no significant results throughout the season between mulch and no mulch nor SWC differentiation, although there were more narrow-pointed leaves in the higher soil moisture levels SWC-1 (Figure 7b).

3.3. Evapotranspiration

The combination of two separate processes whereby water is lost on the one hand from the soil surface through evaporation and on the other hand through plant body via transpiration is referred to as evapotranspiration (ET) [25].
In this experiment, ET accumulation for a total of 54 days was highest under P3 treatment with 246 mm and the smallest ET in M1 with 103 mm. During the initial growth stage (0–10 leaf/plant), the plant-only (P2) treatment shows the highest ET at 2.2 mm/day, while mulch-only (M1) was the lowest with 1.00 mm/day. In the mid-season stage, the plant continued to grow with higher leaf numbers (10–30 leaves/plant). The ET average under P2 was highest at 3.46 mm/day among all treatments, while MP1 ranked lowest with 1.56 mm/day. In the late season, after the plant reached a leaf number of more than 30 leaves/plant before harvest, the largest ET value in P3 was 9.17 mm/day, while M1 was the lowest at 2.26 mm/day. There were similar results reported in other studies with spinach production in Hokkaido at 4 mm/day [39], 7.68 mm/day in Nigeria, and 0.5 to 3 mm/day in South Texas [40].
According to Figure 8, ET varied with the growing stages. Daily evapotranspiration of spinach over the growing season ranged from 0.13 mm (BS3) in the initial week to 15.30 mm/day (MP3), and all treatments reached their peak starting from 10 March (DAT 40), 4 weeks after transplanting.
As shown in Figure 8a, the effectiveness of mulch on saving evapotranspiration at SWC-1 was significant on Mar 16 (DAT 46). When the daily daytime air temperature reached over 18 °C for 5 continuous days, M1 recorded its highest ET in DAT 46 at 2.53 mm, while the lowest was in DAT 7 and DAT 41 at 1.04 mm. As for BS1, ET reached the maximum in DAT 68 with 4.30 mm, while the smallest was 1.10 mm/day at the initial stage.
As shown in Figure 8b, the important use of mulch on soil water loss under SWC-2 treatment was significant in DAT 54 and DAT 61. There, the variation of ET could be due to increasing solar radiation, especially when it reached over 300 W/m2 for 4 continuous days in both weeks. The extraction of water from soil under M2 was highest in DAT 68 with 2.66 mm, while lowest was at DAT 17, 1.42 mm. The response of bare soil at SWC-2 reached its highest at DAT 16 (9.84 mm), while the lowest was at DAT 20 (0.76 mm).
As shown in Figure 8c, the minimal variation of weekly ET with mulch application was observed under SWC-3. Mulched soil under sufficient irrigation resulted in the largest ET at DAT 61 with 5.44 mm and the lowest in DAT 71 of 2.20 mm. Similarly, BS3 water demands were greatest in DAT 61 at 4.28 mm, while the smallest was in DAT 7 at 2.10 mm. These results show that the usefulness of the applied mulch was limited to soil-deficient moisture.
As shown in Figure 8d, the average weekly ET rate at SWC-1 with mulch application was relatively consistent throughout the period. It became obvious on DAT 34 when the plant heights were found to be shorter than other SWC levels with MP1 at 1.32 mm, while P1 was 2.26 mm. The largest ET for MP1 was at DAT 61, 6.72 mm, while the smallest was at DAT 7 (1.09 mm). On the other hand, P1 shows the largest in DAT 54 and the smallest in DAT 17 with 1.98 mm.
In Figure 8e, SWC-2 treatments’ responses to mulch and no-mulch show results with a consistently smaller ET in mulched treatments. However, the significant benefits of mulch were revealed at DAT 41 with an ET rate of 1.82 mm in MP2, while P2 was observed as 2.98 mm. As shown in Figure 8f, mulching’s effectiveness is limited under sufficient soil water conditions (SWC-3). No statistically significant difference was found between with and without mulch, which might be due to the movement of nutrients breaking down through irrigation water drainage in this treatment. The largest ET for MP1 was at DAT 61, 12.24 mm, while the smallest was at DAT 7 (1.95 mm). On the other hand, MP1 shows the largest in DAT 61 at 12.07 mm and the smallest in DAT 26 at 2.11 mm.

3.4. Dry Matter Production

The variation in dry matter accumulation of the Komatsuna crop influenced by irrigation regimes and mulch at different growths are shown in Figure 9a,b. Mulch had significant individual effects on the specific part of the plant and total dry matter.
In SWC-1, the use of applied mulch for the lowest soil moisture condition spinach favoured vegetative growth significantly over no-mulch with dry matter of 33 g/plant MP1 vs. 14 g/plant P1. The result of the dry leaf matter in MP1 revealed a three times higher dry matter (DM) accumulation with SWC-1 than that of the no-mulched spinach leaf, P1.
At SWC-1 treatment, between mulch and no-mulch treatments, stem DM and flower DM were approximately two times larger for mulched spinach than that of no-mulched spinach.
In SWC-2, the effect of applied mulch was statistically insignificant with the no- mulch plant having more dry matter than the mulched plants (24 g/plant MP2 vs. 28 g/plant P2). For P2, relatively larger leaf dry matter was found in SWC-2 and SWC-3 due to statistically significant longer stem elongation and development. Among no-mulch treatments, P2 produced the highest dry matter and the largest in dry stem weight; however, the leaves were small and leaf production was slow.
In SWC-3, the effect of mulch application was statistically insignificant but larger compared to mulch plants’ dry matter (17.9 g/plant MP3 vs. 18.0 g/plant P3). P3 resulted in the largest flowers DM but with the smallest leaf and dry stem matter over SWC-1 and SWC-2. MP3 resulted in the smallest amount of dry matter of all plant parts. The reason could be due to a problem of soil compaction achieved due to overwatering, and shading under mulch seems to reduce root development with less aeration in pore spaces.

3.5. Water Use Efficiency

Water use efficiency (WUE) is closely related to irrigation management and mulching practices. The reasonable combination of mulch and irrigation can improve Komatsuna’s WUE, which is a significant contribution in sustainable agriculture.
Under mulch treatments, statistically significant and with largest dry matter, the WUE among all treatments was observed under deficit-irrigation, MP1, which might have been attributed to the larger leaf area but with less water loss with the help of mulch through soil evaporation, increasing the photosynthesis mechanism in the plant. The results for treatments MP2 and MP3 are insignificant, indicating that saving irrigation water with the use of mulch was not efficient (Figure 10). Under no mulch treatments, significantly improved WUE at P2 occurred over P1 and P3.

4. Discussion

4.1. Mulch Effects on Irrigation Management

Understanding the relationship between meteorological conditions, soil water content, irrigation intervals, and mulch is crucial for effective irrigation management. This study shows mulching’s impact and the soil moisture contribution in reducing irrigation water needs. During the vegetative stage, the soil moisture shows a similar irrigation interval of once a day, which indicates that the impact of shredded paper mulch was not effective in helping to reduce the irrigation frequency to fall within the targeted soil moisture range. For SWC-3 in the reproductive stage, irrigation intervals decreased with more frequent water demands with the weather approaching summer and the continuous growth of spinach. During the flowering stage, MP3 and P3 treatments dried slightly faster, and more irrigation was required. This could probably be due to intense crop water needs as the leaf number with stomatal density increased, which is a direct response to light conditions and warmer air temperatures. The effect is in agreement with previous researchers saying climate aspects such as temperature and evapotranspiration also have an impact on soil moisture variations [21]. Factors affecting the irrigation intervals could be due to the variation of weather conditions and the water use rate of the crop, which changes throughout the season.
In this study, there was no significant interaction between shredded paper mulch and soil moisture levels for high moisture treatments, indicating that shredded paper mulch only had consistent beneficial effects under water deficit conditions especially at SWC-1 (20%).
The proportion of ET (evapotranspiration) increased with an increase in irrigated water whether with mulch or without mulch, and consequently, out of the mulched plants treatment, MP1 (144.34 mm) was the lowest followed by MP2 and MP3. Therefore, it confirmed that mulch treatment reduced the application of water due to less water loss to evaporation. Other research also mentioned the effectiveness of mulch on water saving. For example, straw mulch reduced the influence of meteorological conditions on soil water evaporation [41]; also, there was a 70% evaporation reduction in greenhouse-grown pak choi with plastic mulch compared with no mulch [42].
The accumulated evapotranspiration amount at the end of the season in SWC-3 levels with the different soil surfaces in Figure 8c,f (M3 143.66 mm vs. BS 154.63 mm and MP3 144.34 mm vs. P3 226.19 mm) agrees with researchers [40], who reported that spinach’s evapotranspiration rate increased with irrigated water and reported ETa in spinach as the lowest at 107.91 mm and highest at 251.82 mm per season. However, in this study [43], ET values were higher than other research works, with an accumulated ET of 156 mm in 2004 and 160 mm in 2003 [40].
Some studies [44] reported the effects of mulch on water saving over no-mulch treatment such as an overall gap of 47.4 mm between straw mulch and no mulch. To compare accumulated ET results among shredded paper mulch and no-mulch in this study, 37.14 mm at SWC-3, 42.16 mm at SWC-2, and 11.16 mm at SWC-1 were consumed more by the plant-only treatment. Under mulch-only with no-plant treatments, 31.03 mm, 56.56 mm, and 10.97 mm water were saved at SWC-3, SWC-2, and SWC-1 compared with bare soil treatments.
According to our hypothesis, the water demand reduction resulted in longer irrigation intervals under all mulch treatments than under no-mulch treatments. It indicates that the mulched plant with SWC-20% has saved more water and reduced the total ET of 56.90 mm over no mulch. For SWC-25%, less evapotranspiration occurred with a total of 47.02 mm, and for SWC-30%, 72.84 mm less water was used. Therefore, our study using paper mulch shows positive results for water savings, and these findings are in line with other researchers’ results [44] in which they indicated that the main advantage of using paper mulch is retaining soil moisture.

4.2. Mulch Effects on Komatsuna Spinach Growth’ Performances

The main functions of shredded paper mulch are to help retain moisture in the soil and improve crop production parameters. Leaf area index (LAI), leaf number, and plant height are important indicators of radiation and precipitation interception, energy conversion, and water balance.
It is possible that the mulch application determines LAI at the pre-flowering stage from DAT 46 onwards with a comparatively larger LAI in all irrigation schedules with mulched spinach treatments over no mulch. From the viewpoint of the photosynthetic rate, leaves with a larger LAI could receive more light. When LAI becomes larger with time, it could also increase canopy respiration [38]. Another researcher also supported the idea of dry matter decreases accounting for decreased LAI and decreased respiration per unit of dry matter (see Section 3.4) [45]. Opposite results were recorded in other research with superior leaf area index results under full irrigation with LAI 2.13. Likewise, the differences in leaf number per plant increased due to the application of mulch under all irrigation treatments. As it is recorded in the literature [18] the soil moisture condition influences plants’ water absorption and leaf transpiration, which further corresponds to plant growth and dry matter accumulation. The leaf numbers per plant would probably be one of the attributable factors responsible for increased dry matter production due to larger leaf area development, providing larger photosynthesis potential.
In terms of plant height, the increase in plant height at SWC-3 might have been due to the supply of water coinciding with the plants’ water requirement, which might have been proportional to cell turgidity and thus potentially higher meristematic movement leading to more foliage improvement, a higher photosynthetic rate, a higher nutrient uptake, and more dry matter. Another study with Brassica spp. [46] investigated that the most frequent water application with a 6 cm irrigation depth resulted in a significantly taller plant height of 101 cm at harvest compared with 4 cm and 5 cm depth due to abundant moisture at all rapeseed plants’ critical stages. This research shows that spinach plants could have found a way to adjust to conditions of water deficit or excess water to maintain growth. Such adjustments show the morphological adaptation of the plant.
Early flowering stem production on crops before harvesting is known as bolting. Bolting is an event induced by the coordinated effects of various environmental factors and endogenous genetic components, which bring a large impact in terms of the quality and productivity of vegetable crops like spinach [47]. Due to consumers’ selection of fresh leaves of spinach, bolting or having a flower bud at harvest could have an adverse effect on the market [48]. To investigate the influence of irrigation management concerning bud number per plant, these experimental results suggest that soil moisture SWC-1 positively affects bud formation by having fewer buds in both mulch and no-mulch spinach plants.
Dry matter production is an important indicator of plant growth and productivity. In the P3 treatment, the lower dry matter compared with the P2 result could be due to water stress where the drought stress can change chlorophyll content, photosynthetic systems, and decrease nutrient uptake [49]. The lowest dry matter P3 might have potentially experienced oxidative stress damage due to overirrigation, which other researchers have found [50]. The successive increase in the dry matter accumulation with the decrease in irrigation frequency under mulch from MP3, MP2, and MP1 might be due to the availability of more nutrients with less nutrient loss through irrigation drainage for the proper growth of plants [51]. In this study, the results are in agreement with others. For example, a study [52] investigated the effect of different soil moisture regimes on the plant growth and water use efficiency of sunflowers. The study used a completely randomized block design with three treatments including T1 (30% soil moisture depletion), T2 (50% soil moisture depletion), and T3 (70% soil moisture depletion) with three replicates. The gypsum blocks were used to compute the daily soil moisture depletion. The study found that the total yield of crop under T3 was higher than T1 and T2 and that WUE under T3 was more, as compared to T1 and T2. In this study, shredded paper mulch significantly increased the plant height, leaf number, and dry weight of Komatsuna spinach under all soil moisture levels compared to the control (no mulch).
Water use efficiency could be related to the optimal water availability, which balanced the root growth and nutrient uptake in the soil. On the other hand, the highest soil moisture treatment P3 may have undesirable saturated soil pore spaces, and soil loses a lot of nutrients through quick drainage. The results of the experiments in this study shows that a larger water supply greatly increased ET, which has been proven in previous studies [53]. The combination of mulch with frequent irrigation practices at SWC-3 and SWC-2 resulted in a rapid stem growth stage, while deficient irrigation and the mulch combination might have allowed the absorption of more nutrients for leaf growth and increased the total WUE. In this study, the result of WUE demonstrated that the dry matter accumulation and irrigation amount in Komatsuna were positively correlated under water-deficit treatments.

5. Conclusions

This study demonstrated the effectiveness of shredded paper on spinach crop production, along with the water demands under three different soil moisture levels. From the point of growth impact, the positive effect of shredded paper mulch on spinach produced better leaf growth, dry matter, and water use efficiency compared to spinach with no mulch. Also, there was a downward trend in dry matter with the increase in the irrigation frequency under mulch, while the decrease was particularly obvious under P3 and MP3 treatments. From the point of mulch and SWC on water use, an increase in the ET gap between mulch and without mulch can be found under drier soil moisture conditions. Therefore, among many factors, our study indicates that the effect of soil moisture levels on water loss through evapotranspiration was strongly dependent on the provided water status and soil cover. To sum up, irrigation scheduling set at SWC-1 with the use of shredded paper mulch is the recommended way to improve WUE in Komatsuna spinach production in areas of limited water. Since this study only focused on shredded paper mulch and bare soil (control), future research needs to consider a wider range of mulch types to compare shredded paper mulch from the point of soil health, nutrients, and degradation pathways.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/agronomy13102502/s1, Table S1: Daily climatic conditions change in experimental net house (For Figure 1 for St,Ta, RH,u and Figure 2 for rainfall data in the manuscript); Table S2: Weekly averaged manual soil water content measurement (For Figure 5 in manuscript); Table S3: Irrigation interval days in each growth stages of spinach. (Table 1 in the manuscript); Table S4: The whole season total irrigation times for each treatment (For Table 1 in manuscript); Table S5: p values for comparison of irrigation interval days between mulch plant and no mulch plant (two-tailed t-test at 5% level) (For Table 1 in manuscript); Table S6: p values for comparison of irrigation interval days between mulch plant and no mulch plant (two-tailed t-test at 5% level) (For Table 1 in manuscript); Table S7: Weekly plant height measurement (cm) (For Figure 6 in the manuscript); Table S8: p values for comparison of plant height between mulch plant and no mulch plant (two-tailed t-test at 5% level) (For Figure 6a in the manuscript); Table S9: p values for comparison of weekly plant height changes between three soil moisture levels at alpha value 0.017 with Bon-feronni test (For Figure 6a in the manuscript); Table S10: Average flower number per plant in each treatment (For Figure 6b in the manuscript); Table S11: p values for comparison of daily flower numbers per plant changes between three soil moisture levels at alpha value 0.017 with Bon-feronni test (For Figure 6b in the manuscript); Table S12: p values for comparison of flower number per plant between mulch plant and no mulch plant (two-tailed t-test at 5% level) (For Figure 6b in the manuscript);Table S13: Weekly LAI (For Figure 7a in the manuscript); Table S14: p values for comparison of weekly LAI per plant changes between three soil moisture levels at alpha value 0.017 with Bon-feronni test, (For Figure 7a in the manuscript); Table S15: p values for comparison of weekly LAI per plant between mulch plant and no mulch plant (two-tailed t-test at 5% level) (For Figure 7a in the manuscript); Table S16: Weekly leaf number per plant (For Figure 7b in the manuscript); Table S17: p values for comparison of daily leaf number per plant changes between three soil moisture levels at alpha value 0.017 with Bon-feronni test (For Figure 7b in the manuscript); Table S18: p values for comparison of weekly leaf number per plant between mulch plant and no mulch plant (two-tailed t-test at 5% level), (For Figure 7b in the manuscript); Table S19: Average ET (Weekly average for mm/day) (For Figure 8 in the manuscript); Table S20: p values for comparison of weekly from daily ET per plant changes between three soil moisture levels at alpha value 0.017 with Bon-feronni test (For Figure 8 in the manuscript); Table S21: Dry matter per plant at harvest (g/plant) (For Figure 9 in manuscript); Table S22: p values for comparison of dry matter parts per plant between mulch plant and no mulch plant (two-tailed t-test at 5% level) (For Figure 9 in manuscript); Table S23: p values for comparison of dry matter parts per plant changes between three soil moisture levels at alpha value 0.017 with Bon-feronni test (For Figure 9 in manuscript); Table S24: WUE at harvest (For Figure 10 in manuscript); Table S25: p values for comparison of WUE from above ground dry matter parts total per plant between mulch plant and no mulch plant (two-tailed t-test at 5% level) (For Figure 9 in manuscript); Table S26: p values for comparison of WUE from above ground dry matter parts per plant changes between three soil moisture levels at alpha value 0.017 with Bon-feronni test; Table S27: Daily soil temperature under different soil moisture status.

Author Contributions

Writing—original draft, M.M.M.; Supervision, H.O. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

Data are available within Supplementary Materials.

Acknowledgments

The authors would like to thank the Environmental Hydro-met lab members, Ehime University who helped us during the experiment and the co-supervisor Hideto UENO for supporting the experimental net house.

Conflicts of Interest

The authors declare no conflict of interest.

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Agronomy 13 02502 g001
Figure 1. Daily daytime climate changes of (a) solar radiation (W/m2) and relative humidity (%). (b) Wind speed (m/s) and air temperature (°C) during growing season (21 January to 9 April 2022).
Figure 1. Daily daytime climate changes of (a) solar radiation (W/m2) and relative humidity (%). (b) Wind speed (m/s) and air temperature (°C) during growing season (21 January to 9 April 2022).
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Figure 2. Daily accumulated rainfall over the season.
Figure 2. Daily accumulated rainfall over the season.
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Figure 3. Four soil surface treatments in experimental pots. (a) Plant with mulch (MP), (b) plant only (P), (c) mulch (M), and (d) bare soil (BS) under SWC1 at DAT 42 (photos taken on 11 March 2022, DAT = days after transplanting).
Figure 3. Four soil surface treatments in experimental pots. (a) Plant with mulch (MP), (b) plant only (P), (c) mulch (M), and (d) bare soil (BS) under SWC1 at DAT 42 (photos taken on 11 March 2022, DAT = days after transplanting).
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Figure 4. Illustrations for mulch treatments used in pot experiment inside the net house; 1 is SWC-1 set at soil moisture 20%, 2 is SWC-2 set at soil moisture 25%, and 3 is SWC-3 set at soil moisture 30%. (E stands for extra pots).
Figure 4. Illustrations for mulch treatments used in pot experiment inside the net house; 1 is SWC-1 set at soil moisture 20%, 2 is SWC-2 set at soil moisture 25%, and 3 is SWC-3 set at soil moisture 30%. (E stands for extra pots).
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Figure 5. Average weekly soil moisture content maintained at 3 levels. (a) MP, plant with mulch; (b) P, plant only; (c) M, Mulch only; and (d) BS, bare soil only (1, 2, and 3 in legend indicates SWC at 20%, 25%, and 30%).
Figure 5. Average weekly soil moisture content maintained at 3 levels. (a) MP, plant with mulch; (b) P, plant only; (c) M, Mulch only; and (d) BS, bare soil only (1, 2, and 3 in legend indicates SWC at 20%, 25%, and 30%).
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Figure 6. Weekly changes of (a) plant height and (b) total average flower number per plant.
Figure 6. Weekly changes of (a) plant height and (b) total average flower number per plant.
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Figure 7. Weekly trend of (a) LAI changes and (b) leaf number per plant throughout season.
Figure 7. Weekly trend of (a) LAI changes and (b) leaf number per plant throughout season.
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Figure 8. (a–c) Comparison of weekly average ET between mulch and no-mulch under 3 different SWC levels for evaporation and (d–f) for evapotranspiration. ns = non-significant level at 0.05 and ** = significant level at 0.01 for daily data set comparison within a week’s differences between mulch and no mulch under the same SWC level. (3 February = DAT 6 to 5 April = DAT 67).
Figure 8. (a–c) Comparison of weekly average ET between mulch and no-mulch under 3 different SWC levels for evaporation and (d–f) for evapotranspiration. ns = non-significant level at 0.05 and ** = significant level at 0.01 for daily data set comparison within a week’s differences between mulch and no mulch under the same SWC level. (3 February = DAT 6 to 5 April = DAT 67).
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Figure 9. Dry matter of (a) plant with mulch and (b) no-mulch spinach plant at harvest. (The letters a,b on top of each bar are for the whole plant total dry matter comparisons between three different SWCs. The letters a and b inside the bars show the results at significantly different 5% level of probability, which were adjusted from α values using Bonferroni’s method and two-tailed t-test. The comparison is conducted for the same plant part at different SWC levels. (SWC-1 indicates SWC at 5 cm depth controlled at 20%, SWC-2 at 25%, and SWC-3 at 30%, respectively).
Figure 9. Dry matter of (a) plant with mulch and (b) no-mulch spinach plant at harvest. (The letters a,b on top of each bar are for the whole plant total dry matter comparisons between three different SWCs. The letters a and b inside the bars show the results at significantly different 5% level of probability, which were adjusted from α values using Bonferroni’s method and two-tailed t-test. The comparison is conducted for the same plant part at different SWC levels. (SWC-1 indicates SWC at 5 cm depth controlled at 20%, SWC-2 at 25%, and SWC-3 at 30%, respectively).
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Figure 10. Water use efficiency of spinach under different SWCs. Note: The letters on top of each bar for comparison between 3 SWCs under the same soil surface (MP1 vs. MP2 vs. MP3 and P1 vs. P2 vs. P3) symbols were from the result of adjusted α values using Bonferroni’s method and two-tailed t-test. Different letters inside bars for difference comparisons between mulch and no mulch under the same SWC level, significantly different at 5% level of probability (MP1 vs. P1, MP2 vs. P2, and MP3 vs. P3. SWC-1, 2, and 3 means soil moisture conditions controlled at SWC 20%, 25%, and 30% of plant with mulch (MP) and no-mulch plant (P).
Figure 10. Water use efficiency of spinach under different SWCs. Note: The letters on top of each bar for comparison between 3 SWCs under the same soil surface (MP1 vs. MP2 vs. MP3 and P1 vs. P2 vs. P3) symbols were from the result of adjusted α values using Bonferroni’s method and two-tailed t-test. Different letters inside bars for difference comparisons between mulch and no mulch under the same SWC level, significantly different at 5% level of probability (MP1 vs. P1, MP2 vs. P2, and MP3 vs. P3. SWC-1, 2, and 3 means soil moisture conditions controlled at SWC 20%, 25%, and 30% of plant with mulch (MP) and no-mulch plant (P).
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Table 1. Irrigation interval days in each growth stage of spinach.
Table 1. Irrigation interval days in each growth stage of spinach.
TreatmentsMP1P1MP2P2MP3P3M1BS1M2BS2M3BS3
Vegetative stage5.304.081.000.981.341.294.083.790.982.211.281.29
Flowering stage1.303.250.940.931.071.083.253.250.941.631.151.63
Days interval (d)341111441211
p-values0.430.110.330.250.090.24
nsnsnsnsnsns
Note: MP stands for a plant with mulch, P for plant only, M for mulch-only covered soil, and BS for soil only. The two sub-periods are the vegetative stage from 29 January to 26 March and the flowering stage from 27 March to harvest on 9 April.
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Mon, M.M.; Oue, H. Effects of Shredded Paper Mulch on Komatsuna Spinach under Three Soil Moisture Levels. Agronomy 2023, 13, 2502. https://doi.org/10.3390/agronomy13102502

AMA Style

Mon MM, Oue H. Effects of Shredded Paper Mulch on Komatsuna Spinach under Three Soil Moisture Levels. Agronomy. 2023; 13(10):2502. https://doi.org/10.3390/agronomy13102502

Chicago/Turabian Style

Mon, May Myat, and Hiroki Oue. 2023. "Effects of Shredded Paper Mulch on Komatsuna Spinach under Three Soil Moisture Levels" Agronomy 13, no. 10: 2502. https://doi.org/10.3390/agronomy13102502

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