Effects of plastic mulch on the radiative and thermal conditions and potato growth under drip irrigation in arid Northwest China

Soil & Tillage Research 172 (2017) 1–11

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Effects of plastic mulch on the radiative and thermal conditions and potato growth under drip irrigation in arid Northwest China

MARK

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You-Liang Zhanga, Feng-Xin Wanga, , Clinton Cleon Shockb, Kai-Jing Yanga, Shao-Zhong Kanga, Jing-Tao Qina, Si-En Lia a b

Center for Agricultural Water Research in China, China Agricultural University, No. 17 Qinghua East Road, Haidian, Beijing 100083, China Oregon State University, Malheur Experiment Station, 595 Onion Ave., Ontario, OR, USA

A R T I C L E I N F O

A B S T R A C T

Keywords: Potato growth stages Black plastic-film mulch Transparent plastic-film mulch Soil heat flux Net radiation Temperature

Drip irrigation and plastic-film mulch are useful water-saving tools for potato (Solanum tuberosum L.) production in arid Northwest China. Effects of the radiative and thermal conditions produced by the plastic-film mulch on potato growth can be positive or negative. The objective of this study is to know how radiative and thermal conditions and potato growth are affected by the two most commonly used plastic-film mulches (transparent and black) to efficiently use the positive effects of the plastic-film mulch. Field experiments were conducted at the experimental station located in Wuwei, Gansu Province to explore the effects of transparent mulch (TM), a nonmulched check (NM), and black mulch (BM) on the net radiation (Rn), soil heat flux (G), soil temperature, and potato growth under surface drip irrigation during different plant development stages in 2014 and 2015. Results indicated that the daily integral Rn in the BM treatment was greater than in the TM treatment, while the amplitude of G in the BM treatment was lower than in the TM treatment. The BM treatment had 3.0 and 3.9 °C greater maximum mulch surface temperature than the TM treatment and had greater longwave radiation on the canopy emitted from the mulch surface. The differences in Rn, G, and soil temperature among the treatments diminished with plant canopy enlargement. The potato plant height rankings were BM > TM > NM. The BM treatment had 26% higher jumbo plus large tuber yield than the TM treatment in 2014. Compared with the BM treatment, the TM treatment had the similar potato yield but 9% and 8% less evapotranspiration in 2014 and 2015. The results suggested that the black plastic-film mulch was more suitable for large potato tuber production, while the transparent plastic-film mulch was favorable for water-saving.

1. Introduction Potatoes are sensitive to both water and heat stress. Tuber yield, grade, and quality are reduced by inadequate or excessive soil water (Shock et al., 2007). Heat stress affects both plant top and tuber growth and development (Marinus and Bodlaender, 1975; Van Dam et al., 1996; Hijmans, 2003). Potato haulm growth is fastest in the air temperature range of 20–25 °C; whereas, the optimal range for tuberization and tuber growth is 15–20 °C (Rykaczewska, 2015) or 16–18 °C (Kooman et al., 1996; Kar and Kumar, 2007; Paul et al., 2014). The optimum air temperature for leaf photosynthesis in potato has been reported to be about 24 °C as leaf photosynthetic rate decreases rapidly with increasing air temperature (Wolf et al., 1990). High air temperature stress reduces tuber yield by limiting tuber induction and development through reduced photosynthesis and carbon portioning to growing tubers (Reynolds and Ewing, 1989; Van Dam et al., 1996;

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Corresponding author. E-mail address: [email protected] (F.-X. Wang).

http://dx.doi.org/10.1016/j.still.2017.04.010 Received 1 August 2016; Received in revised form 27 March 2017; Accepted 30 April 2017 0167-1987/ © 2017 Elsevier B.V. All rights reserved.

Gangadhar et al., 2014). High soil temperature during the subsequent stages of plant development can lead to malformed tubers (such as cracks and secondary tuber growth) and tubers sprouting in the soil before harvest (Gangadhar et al., 2014; Rykaczewska, 2015). High soil temperature can reduce tuber internal quality by decreasing tuber specific gravity (Zommick et al., 2014) and decreasing Vitamin C and protein content (Wang et al., 2011). The abiotic stress of heat and drought often happen simultaneously in many areas of the world, accounting in part for the far lower average global potato yield (19 t/ ha) than its potential yield (120 t/ha) (Haverkort and Struik, 2015). Therefore, it is important to avoid heat and water stress to achieve high yields of high quality tubers. Plastic-film mulch is popular in potato cultivation in Northwest China, where the potential crop evapotranspiration is far greater than rainfall and water resources for irrigation are seriously limited. The advantages of using plastic-film mulch may include reduced water loss

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from the soil, increased soil temperature of cold soils limit growth, and improved root nutrient uptake (Kasirajan and Ngouajio, 2012). As a promising water-saving measure, plastic-film mulch in combination with drip irrigation is already used in Northwest China potato production (Wang et al., 2009, 2011; Yang et al., 2017; Zhang et al., 2017). Drip irrigation can be used to modulate soil water status while retaining soil aeration, another important environmental factor affecting potato growth. Plastic-film mulch can affect the above-ground and the belowground radiative and thermal conditions, including solar radiation (Liakatas et al., 1986; Ham and Kluitenberg, 1994), latent heat flux and sensible heat flux (Ham et al., 1991; Ding et al., 2013), soil heat flux (Liakatas et al., 1986; Ham and Kluitenberg, 1994; Bonachela et al., 2012), and soil temperature (Decoteau et al., 1988; Cavero et al., 1996; Chellemi et al., 1997; Liu et al., 2009; Hou et al., 2010). The potential positive and negative effects of plastic-film mulch may explain the seemingly contradictory potato tuber yield results from the use of plastic-film mulch in different climatic regions or even from the same region during different growing seasons (Wang et al., 2009; Hou et al., 2010). To efficiently use the positive effects of plastic-film mulch while avoiding its disadvantages, it is imperative to know how radiative and thermal conditions in a potato field are affected by the two most commonly used plastic-film mulches (transparent and black plastic-film mulches). The effects of plastic-film mulch on radiative and thermal conditions are dominated by the thermal and optical properties of plastic-film mulch. The plastic-film mulch effects on the radiative and thermal conditions can vary dramatically during different plant growth stages because progressively more of the mulch is covered by the plant canopy as it grows and the coverage diminishes as the canopy matures and senesces (Li et al., 1999; Hou et al., 2010). Findings on mulching effects on thermal conditions and plant growth reported in the literature are contradictory. The transparent plastic-film mulch use results in higher soil temperature than the black mulch as shown for taro grown in southeastern Nigeria (Anikwe et al., 2007), watermelon grown in Croatia (Ban et al., 2009) and southwestern Mexico (Farias Larios and Orozco Santos, 1997), and cucumber grown in Syria (Yaghi et al., 2013). Meanwhile, the black plastic-film mulch produces higher soil temperature than the transparent plasticfilm mulch as shown for potato grown in Spain (Ruiz et al., 1999, 2002; Baghour et al., 2003) and watermelon grown in Croatia (Romic et al., 2003). Some crop yields with black mulch are higher than with transparent mulch such as taro (Anikwe et al., 2007) and potato (Ruiz et al., 1999); whereas, other crop yields with black plastic-film mulch are lower than with transparent plastic-film mulch such as watermelon (Farias Larios and Orozco Santos, 1997) and cucumber (Yaghi et al., 2013). Black plastic-film mulch is better than the transparent plastic-film mulch for high yield in some years, while the transparent plastic-film mulch is better in other years at the same place (Ban et al., 2009). The yield with transparent plastic-film mulch is even lower than without mulch (Ruiz et al., 1999). These results suggest that the plastic-film mulch effects on radiative and thermal conditions are complex and it is more difficult to know how the dramatically-changed radiative and thermal conditions affect crop yield due to the difficulty in separating these mulching effects from other environmental influences. Plastic-film mulch is known to increase soil temperature on potato (Ruiz et al., 1999, 2002; Baghour et al., 2003; Wang et al., 2009; Hou et al., 2010), but to our knowledge, there are few reports that present a thorough investigation of the plastic-film mulch effects on radiative and thermal conditions and its influence on drip-irrigated potato growth during different growth stages. The purpose of this experiment was to determine: 1) the effects of the two main standard plastic-film mulches (transparent and black) on radiative and thermal conditions in potato under surface drip irrigation during potato’s five typical developmental stages in Northwest China, 2) how potato growth is affected by the two

Fig. 1. The date and water amount of each irrigation for the different mulch treatments: transparent mulch (TM), non-mulched check (NM), and black mulch (BM) in 2014 and 2015.

different plastic-film mulches and 3) which plastic-film mulch should be chosen for potato production. 2. Materials and methods 2.1. Experimental site Field experiments were conducted at the Shiyanghe Experimental Station (N 37°52′, E 102°50′, altitude 1581 m), China Agricultural University, Wuwei, Gansu Province, on the border of Tenger Desert in 2014 and 2015. The station is located in a typical continental temperate climate zone with a mean annual sunshine duration over 3000 h, mean annual air temperature of 8 °C and a frost free growing season of 150 d. The region has mean annual precipitation of 164 mm, mean annual pan evaporation of 2000 mm measured by a Class A evaporation pan, and limited water resources. The groundwater table varies between 25 and 30 m depth. The soil is sandy loam with mean soil bulk density 1.53 g/ cm3 at 0–1.0 m depth. 2.2. Experimental design Experiments were carried out from April to August in 2014 and 2015. Three soil surface treatments were tested: 1) transparent plastic mulch (TM) with a film thickness of 0.008 mm, 2) a non-mulched check (NM), and 3) black plastic mulch (BM) with a film thickness of 0.008 mm. For the mulched treatments, the entire surface of each potato bed was covered with mulch. The treatments were replicated three times using a randomized complete block design. 2.3. Agronomic practices Each plot was 6 m long and 5.6 m wide, containing 7 beds in northsouth orientation (0.8 m wide and 0.2 m high). In the center of the beds, 30 g seed potatoes (cv. Kexin No.1, Inner Mongolia Minfeng Potato Industry Co., Ltd., Ulanqab, China) were planted every 30 cm. Holes 8 cm in diameter were punched through the film and the seed potatoes were placed 15 cm deep. In 2014, the potatoes were planted on 22 April and harvested on 21 August. Before planting, 101 kg/ha N and 259 kg/ha P2O5 were applied. After planting, an additional 84 kg/ha N and 117 kg/ha K2O were applied through the drip irrigation system on three dates: 40% on 10 2

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Table 1 Summaries of solar radiation (Rs), air temperature (Ta), maximum air temperature (Tmax), minimum air temperature (Tmin) and days with Tmax ≥ 30 °C, during each stage of potato development in 2014 and 2015. Growth stage

Period

Total Rs (MJ/m2)

Mean Ta (°C)

Mean Tmax (°C)

Mean Tmin (°C)

Days Tmax ≥ 30 °C

Percentage of days Tmax ≥ 30 °C (%)

2014 Sprout developmenta Vegetative growth Tuber initiation Tuber bulking Maturation Overall

22 April–17 May 18 May–5 June 6 June–19 June 20 June–7 August 8 August–21 August 22 April–21 August

530.8 399.1 272.9 989.9 265.5 2458.2

13.1 20.0 18.9 20.7 18.7 18.3

20.7 27.6 25.7 27.9 26.1 25.6

5.2 11.3 12.2 13.6 11.8 10.8

1 8 0 16 3 28

4 42 0 33 21 23

2015 Sprout developmenta Vegetative growth Tuber initiation Tuber bulking Maturation Overall

15 April–6 May 7 May–27 May 28 May–10 June 11 June–4 August 5 August–20 August 15 April–20 August

421.5 433.4 291.5 1126.0 302.5 2574.9

14.9 17.5 18.1 21.0 19.5 18.2

22.2 25.3 24.9 28.6 27.6 25.7

6.9 9.8 11.4 13.9 11.8 10.8

1 3 1 20 6 31

5 14 7 36 38 24

a The duration of the sprout development stage was based on the potato plants in the black mulch (BM) and transparent mulch (TM) treatments. The emergence date of non-mulched check (NM) potatoes was about 7 days later than BM and TM treatments.

After planting, an additional 95 kg/ha N and 117 kg/ha K2O were applied through the drip irrigation system on two dates: 50% on 31 May and 50% on 23 June. 2.4. Irrigation system, plastic-film mulch, and irrigation scheduling Potatoes were irrigated by drip irrigation. Each plot had a drip irrigation sub-system, consisting of a sluice valve (Beijing Tianzhu North Butterfly Valve Manufacturing Co., Ltd., Beijing, China), a pressure gauge (Beijing Brighty Instrument Co., Ltd., Beijing, China), and a water meter (Lianyungang Water Meter Co., Ltd., Jiangsu, China). Drip tape (Beijing Luckrain Plastics Co., Ltd., Beijing, China) was placed on the center of the beds. The emitter spacing was 0.2 m and the emitter flow rate was 1.38 l/h at the operating pressure of 0.1 MPa. After the drip irrigation system was installed, the plastic films (polyethylene blown film, 1.2 m wide, 0.008 mm thick, Shandong Jining Zhongqu Double Star Plastic Products Factory, Jining, China) were laid tightly on the soil surface of the beds. The shortwave transmittance, reflectance, and absorbance of plastic films were measured using a UV–vis-near-infrared spectrophotometer (UV-3150, Shimadzu Co., Kyoto, Japan) between 300 nm and 1100 nm in 1 nm increments in the laboratory. The shortwave transmittance, reflectance, and absorbance for transparent plastic film were 0.90, 0.08, and 0.02, respectively, and for black plastic film they were 0.13, 0.04, and 0.83, respectively. The film edges were fixed in the furrows by covering with 2–5 cm of soil. Each treatment (three plots) had two tensiometers equipped with vacuum manometers (Beijing Waterstar Technology Co., Ltd., Beijing, China) installed in two different plots. Each tensiometer was installed at 0.2 m depth, directly under a drip tape in the middle of a bed. When the average of soil matric potentials (SMP) read from vacuum manometers of the two tensiometers in the same treatment reached −25 kPa (Wang et al., 2007), the drip irrigation system of this treatment was turned on. For each irrigation the amount of water m (in mm) was determined using the equation:

m = h (θa − θb ) P / η

(1)

where h is the planned wetted depth (mm), θa is the volumetric soil water content after irrigation (cm3/cm3), θb is the volumetric soil water content immediately before irrigation (cm3/cm3), P is the soil wetted percentage which is the ratio of actual wetted soil volume to the planned wetted total soil volume (Keller and Karmeli, 1974; Zur, 1996; Yang et al., 2017), η is the water utilization coefficient of the drip irrigation system which is the ratio between the water delivered to the crop and the water pumped from the well (Shen et al., 2013). In this experiment, the planned wetted depth was 25 cm during

Fig. 2. Seasonal variations of the daily average solar radiation (Rs), daily air temperature (Ta), daily maximum air temperature (Tmax), daily minimum air temperature (Tmin), and daily variation of 15-min air temperature (T′) before the plant emergence in 2014 and 2015.

June, 30% on 23 June, and 30% on 12 July. In 2015, the potatoes were planted on 15 April and harvested on 20 August. Before planting, 90 kg/ha N and 231 kg/ha P2O5 were applied.

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Fig. 3. Daily variation of average 30-min net radiation (Rn) during (a) sprout development, (b) vegetative growth, (c) tuber initiation, (d) tuber bulking, and (e) maturity for the different mulch treatments: transparent mulch (TM), non-mulched check (NM), and black mulch (BM) in 2014 and 2015.

2.5. Evapotranspiration estimation

early vegetative growth and 50 cm for the remaining plant development stages. The field capacity (θa) was equal to 0.20 cm3/cm3 and 0.27 cm3/cm3 in 2014 and 2015, respectively. The θb was determined gravimetrically by sampling the soil at three points perpendicular to the drip line at 0, 20, and 40 cm and increments down to 10, 20, 30, 50, 70, and 90 cm depths in each plot for the first irrigation. For subsequent irrigations, θb was equal to 70% of the field capacity when the soil matric potential (SMP) measured with tensiometers reached −25 kPa, according to the soil water characteristic curve. The soil wetted percentage was estimated to be 55% for all treatments. The water utilization coefficient of the drip irrigation system was 0.97. In 2014, the first irrigation was 15 mm and subsequent irrigations were 17 mm (the operation error ≤ 2 mm) for all treatments (Fig. 1). The total irrigation in the TM, NM, and BM treatments was 293, 277, and 317 mm, respectively. In 2015, the first irrigation was 19 mm and subsequent irrigations were 23 mm (the operation error ≤ 2 mm) for all treatments (Fig. 1). The total irrigation in the TM, NM, and BM treatments was 344, 392, and 366 mm, respectively. The pH of the irrigation water was 7.8 and the electrical conductivity (EC) was 63 ms/ m.

Potato evapotranspiration (ETc) was calculated using the soil water balance:

ETc = I + P − ΔS − R − D

(2)

where ETc is the actual potato evapotranspiration, I is the irrigation amount, P is precipitation, ΔS is the change of soil water storage, R is the surface runoff, and D is the drainage below crop root zone, and all terms are in mm. The P was assumed to be as effective for mulched treatments and non-mulch treatments; ΔS was the difference between the soil water storage in the soil profile at harvest and the soil water before planting and was determined gravimetrically. The soil water measurements were made by sampling the soil at three points perpendicular to the drip line at 0, 20 and 40 cm and increments down to 10, 20, 30, 50, 70, 90 cm depths in each plot. The gravimetric soil water content was changed to volumetric soil water content by multiplying by the soil dry bulk density. The R was negligible because barriers blocked runoff along the furrows. The D was assumed to be negligible because the experimental site was in an arid area and each individual irrigation was small (Wang et al., 2011). 4

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in plant height, tuber grade, and tuber yield among treatments were determined by F-test. The differences in plant height, tuber grade, and tuber yield were compared between treatments using Duncan’s multiple range test. Due to the high cost of equipment, no replications were made in measurements of net radiation, soil heat flux, and soil temperature. The differences in net radiation, soil heat flux, and soil temperature between treatments were determined by paired t-test. The Statistical Product and Service Solutions software (SPSS version 20 for windows, SPSS Inc., Chicago, Illinois, USA) was used for the statistical analyses. 3. Results and discussion 3.1. Weather conditions Daily air temperature varied along with the daily solar radiation (Rs) (Fig. 1). Total Rs was 2458.2 and 2574.9 MJ/m2 with the average air temperature 18.3 and 18.2 °C during the 2014 and 2015 growing seasons, respectively (Table 1). However, the weather became warm earlier in 2015 than in 2014 which resulted in a longer growing season in 2015 (128 days) than in 2014 (122 days) (Fig. 2 and Table 1). During the sprout development stage, the weather was warmer in 2015 than in 2014; the minimum air temperature ranged from −0.6 to 16.9 °C and averaged 5.2 °C in 2014 and ranged from 2.9 to 10 °C and averaged 6.9 °C in 2015. During tuber bulking and maturity stages, there were 19 and 26 days with maximum air temperature above 30 °C in 2014 and 2015, respectively. The high maximum air temperature (above 30 °C) might have been detrimental to optimal tuber growth (Kar and Kumar, 2007).

Fig. 4. Daily integral of net radiation (Rn) differences during (a) sprout development, (b) vegetative growth, (c) tuber initiation, (d) tuber bulking, and (e) maturity for the different mulch treatments: transparent mulch (TM), non-mulched check (NM), and black mulch (BM) in 2014 and 2015.

2.6. Plant height and yield Ten plants in each plot were tagged to measure their heights over time. The middle three rows of plants were harvested for yield from each plot. Ten plants were sampled for grade analysis. 2.7. Measurements

3.2. Variation of net radiation (Rn)

Weather variables were measured with a standard automatic weather station, 2 m above the ground surface. Mulch surface temperature, soil surface temperature, and soil temperature at 5–50 cm depths were measured with sensors in the middle and at the side (20 cm from the center) of the beds in one replication of each treatment. The sensors on the mulch surface, on the soil surface, and at 5 cm depth were 0.25mm copper-constantan thermocouples (ST10, Beijing Unism Technologies, Inc., Beijing, China). The sensors at 10, 20, 30 and 50 cm depths were soil temperature/water sensors (FDS120, Beijing Unism Technologies, Inc.). The sensors were connected to a data logger (SMC6108, Beijing Unism Technologies, Inc.), and sampled at 10-s intervals and calculated and stored an average every 10-min. Variable G was measured with soil heat flux plates (HFP01, Hukseflux, Netherlands) in the middle of beds at 5 cm depth on one replication of each treatment. The Rn above the canopy was measured with a net radiometer (NR Lite2, Kipp & Zonen, Delft, Netherlands). Net radiometers were installed in the middle of one replication of each treatment. The heights of the net radiometers, adjusted according to the plant growth, were 0.4 m above the bed from 10 to 22 May, 0.8 m from 23 May to 21 June, 1.3 m from 22 June to 14 August, and 0.8 m from 15 to 21 August 2014. As the heights of radiometers changed, the area measured by net radiometers also changed. To eliminate this effect, the heights of net radiometers were kept the same (1.0 m above beds) in 2015. All the sensors were connected to a data logger (CR1000, Campbell Scientific Inc., Logan, UT, USA) and sampled at 10-s intervals and calculated an average every 10-min. The soil temperature, soil heat flux, and net radiation sensors were installed on 7 May 2014 (after the potato planting) and on 11 April 2015 (before the potato planting).

During the two growing seasons, the curve of daily variation for average 30-min Rn of each growth stage was bell-shaped for different mulch treatments (Fig. 3). Rn became positive between 7:00 and 7:30 A.M., peaked between 12:30 and 13:30 P.M., and became negative between 19:30 and 20:00 P.M. The Rn reached the peak almost at the same time of day for different treatments. The differences in Rn between treatments varied with the plant development stages (Figs. 3 and 4). During the two growing seasons, differences in average maximum 30-min Rn between treatments were greater before tuber bulking (41–62 W/m2 and 45–102 W/m2) and lower after tuber bulking (6–21 W/m2 and 12–31 W/m2) in 2014 and 2015. The differences in average maximum 30-min Rn between the BM treatment and the TM treatment were significant during all growth stages by t-test (P < 0.05). The differences were greater during early growth stages, because the plants were small and the optical properties of the mulch had a large effect on Rn. As the potato canopy grew, the effect of the optical properties of the mulch on Rn became less and less, reaching the lowest point during tuber bulking. Generally, during the two growing seasons, the order of daily integral Rn was BM > NM > TM (Fig. 4). During sprout development and vegetative growth, the difference of daily integral Rn between the BM and TM treatments ranged from 1 to 3 MJ/m2 and averaged 2 MJ/ m2 in 2014 and ranged from 1 to 4 MJ/m2 and averaged 3 MJ/m2 in 2015. The BM treatment had higher Rn than the TM and NM treatments in 2014 and 2015. This occurred because the black mulch had a lower albedo than transparent mulch and bare soil (Liakatas et al., 1986; Bonachela et al., 2012). 3.3. Variation of soil heat flux (G)

2.8. Statistical analyses During the two growing seasons, the amplitudes of daily G (30-min averages) at 5 cm depth in the middle of the beds varied with treatment and growth stage (Fig. 5). Generally, the average 30-min G from the BM

Data on plant height over time and tuber weight, number, yield for each plot were subjected to analysis of variance. Significant differences 5

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Fig. 5. Daily variation of average 30-min soil heat flux (G) in the middle of the bed at 5-cm depth for potato during (a) sprout development, (b) vegetative growth, (c) tuber initiation, (d) tuber bulking, and (e) maturation for the different mulch treatments: transparent mulch (TM), non-mulched check (NM), and black mulch (BM) in 2014 and 2015.

treatment had a lower amplitude (−15–46 W/m2 and −34–82 W/m2) than from the TM treatment (−25–114 W/m2 and −45–117 W/m2) in 2014 and 2015. The difference in average 30-min maximum G between the BM and TM treatments was significant by t-test (P < 0.05). This result was consistent with Liakatas et al. (1986), however, Ham and Kluitenberg (1994) found G of the TM treatment was comparable with the BM treatment. In contrast, Bonachela et al. (2012) observed G of the TM treatment to be lower than the BM treatment. This might be due to the difference of air gap, between the mulch and soil, blocking heat conduction between the black mulch and underlying soil (Liakatas et al., 1986; Ham and Kluitenberg, 1994). As the potato plants grew, their canopies shaded the beds and the amplitudes of G were lower during the tuber bulking for all treatments, similar to the report by Munguía-López et al. (2012). Interestingly, the TM treatment had the largest G while having the lowest Rn among the three treatments during daylight hours. This result can be explained by the shortwave transmissivity of transparent mulch being higher than that of black mulch and the lower heat convection and latent heat dissipation prevented by mulch than without mulch.

3.4. Variation of temperature 3.4.1. Mulch surface temperature The mulch surface temperature variation depended on the potato growth stage because of the canopy growth which could change the shaded area on the plastic-film (Figs. 6 and 7). During vegetative growth, the maximum average 30-min mulch surface temperature in the middle of the beds in the BM treatment were 3.0 and 3.9 °C greater than in the TM treatment in 2014 and 2015, respectively. The warmer black mulch could emit and reflect greater amounts of longwave radiation to the surrounding canopy. Ham et al. (1993) measured the longwave radiation reflected and emitted from different plastic-film mulch surfaces with an infrared transducer in the field and found the black mulch surface had the largest quantity of longwave radiation which could be in similar magnitude to the shortwave radiation. The air temperature near the surface could be increased by convective heat flux from the mulch surface (Ham et al., 1993). Higher air temperatures above black plastic mulch were observed by Bonachela et al. (2012) in greenhouse conditions. High air temperature accelerates growth of the above-ground part of the potato plant (Marinus and Bodlaender, 1975). After the vegetative growth, the BM treatment had lower mulch 6

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Fig. 6. Daily fluctuations in the average 30-min soil temperature in the middle of the bed at different depths during potato (a) sprout development (b) vegetative growth, (c) tuber initiation, (d) tuber bulking, and (e) maturation for the different mulch treatments: transparent mulch (TM), non-mulched check (NM), and black mulch (BM) in 2014 and 2015.

Generally, the TM treatment had greater maximum soil temperature than the BM treatment. Compared with the BM treatment, the maximum soil temperature in the TM temperature was 3.8 and 1.4 °C greater in the middle of the beds and 4.1 and 1.5 °C in the side of the beds at 5–30 cm depths during tuber initiation in 2014 and 2015. These temperature results were consistent with the results of G. They were also in agreement with the results of Liakatas et al. (1986) who found that black plastic-film mulch produced lower daytime soil temperature than transparent plastic-film mulch. Although the black mulch had the maximum solar radiation absorbance, the contact between mulch and underlying soil may not have been enough for efficient heat conduction due to the lower air heat conductivity than the soil heat conductivity with the findings of others (Liakatas et al., 1986; Ham and Kluitenberg, 1994). The maximum soil temperature for the BM treatment was 2.2 and 1.8 °C lower than for the NM treatment at 5–30 cm depths during tuber bulking in 2014. During maturation stage, the BM treatment had the greatest maximum soil temperature in 2015. It might be caused by the early canopy senescence in the BM treatment in 2015.

surface temperature than the TM treatment in 2014. However, the result in 2015 was reversed. It might be caused by the canopy difference between these two growing seasons.

3.4.2. Soil temperature The daily fluctuations in average 30-min soil temperature depended on the potato growth stages and soil depths both years (Figs. 6 and 7). During sprout development and vegetative growth, soil temperature fluctuated more than during the later growth stages, and the fluctuations were greater on the soil surface, and at the 5–10 cm depths than at 20–50 cm depths as should be expected. The differences of average maximum temperature between the BM and TM treatments were significant at soil surface and 5–10 cm depths by t-test (P < 0.05). At 20–50 cm depths, the fluctuations of soil temperature were very small. During sprout development stage, the plastic-film mulch treatments had greater minimum soil temperature than the NM treatment in 2014 and 2015. The minimum 30-min soil temperature at 5–30 cm depths (near the seed potatoes) in the middle of the beds was greater (3.0 and 0.9 °C for the BM treatment and 2.9 and 3.3 °C for the TM treatment) in the plastic-film mulch treatments than in the NM treatment in 2014 and 2015 (Table 2). That should be the reason why the plastic-film mulch treatments had earlier emergence (7 days) than the NM treatment. Anikwe et al. (2007) found that the plastic-film mulch treatment had 7 days earlier emergence in cocoyam. During tuber initiation, the minimum 30-min soil temperature at 5–30 cm depths in the middle of the beds was 3.2 and 2.9 °C greater in the TM treatment (1.1 and 2.5 °C in the BM treatment) than in the NM treatment in 2014 and 2015, respectively. The result meant that the plastic-film mulch could keep soil warm at night.

3.5. Plant height Both the BM and TM treatments had greater plant heights than the non-mulched treatment in 2014 and 2015 (Fig. 8). Before the maturation stage, the heights of potato plants grown with plastic-film mulch were significantly taller (the maximum difference was 23 cm) than those in the NM treatment in 2014 and 2015 (Duncan’s test, P < 0.05). Earlier emergence of the potatoes grown with plastic-film mulch than those without mulch may have caused the height differences similar to the results of Hou et al. (2010). During the latter part of vegetative growth, the heights of potato 7

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Fig. 7. Daily fluctuations in the average 30-min soil temperature at the side of the bed (20 cm from the center of the bed) at different depths at potato (a) sprout development (b) vegetative growth, (c) tuber initiation, (d) tuber bulking, and (e) maturation for the different mulch treatments: transparent mulch (TM), non-mulched check (NM), and black mulch (BM) in 2014 and 2015. Table 2 The average 30-min minimum and maximum temperature differences between different mulch treatments: transparent mulch (TM), non-mulched check (NM), and black mulch (BM) at 5–30 cm depths in the middle of the bed and the side of the bed (20 cm from the center of the bed) during different growth stages in 2014 and 2015. Growth stage

In the middle of the bed TM-NM

In the side of the bed BM-NM

TM-BM

TM-NM

BM-NM

TM-BM

Min.

Max.

Min.

Max.

Min.

Max.

Min.

Max.

Min.

Max.

Min.

Max.

2014 Sprout development Vegetative growth Tuber initiation Tuber bulking Maturation

2.9 2.9 3.2 1.0 1.5

0.9 1.8 1.4 −0.2 2.4

3.0 3.1 1.1 −0.1 1.2

−0.1 0.3 −2.4 −2.2 0.0

−0.1 −0.2 2.2 1.1 0.2

1.0 1.5 3.8 2.0 2.4

2.6 2.9 3.3 1.5 2.3

1.4 2.7 1.0 0.0 2.6

3.2 3.0 1.2 0.6 1.9

0.6 1.1 −3.0 −1.8 1.0

−0.6 0.0 2.1 0.9 0.5

0.8 1.6 4.1 1.8 1.7

2015 Sprout development Vegetative growth Tuber initiation Tuber bulking Maturation

3.3 2.9 2.9 0.2 1.2

4.5 3.7 2.9 −0.8 −1.3

0.9 1.5 2.5 0.3 2.2

−0.1 0.0 1.5 −0.5 1.0

2.4 1.4 0.5 −0.2 −1.0

4.6 3.8 1.4 −0.3 −2.3

2.9 2.5 2.4 −0.6 0.0

3.7 2.5 1.9 −1.2 −0.4

0.8 1.2 2.2 0.1 1.2

−0.9 −0.5 0.4 −0.5 2.1

2.2 1.2 0.1 −0.6 −1.2

4.5 3.0 1.5 −0.7 −2.5

suggests that the greater potato plant height might not be caused by higher soil temperature but by greater longwave radiation onto the canopy from mulch surface and higher air temperature near the mulch surface. In addition, the potato height differences between the BM and TM treatments were greater in 2014 than in 2015. The weather becoming warm earlier and being more favorable for plant elongation in 2015 than in 2014 may have caused this difference. The plants grown with the TM treatment grew faster in 2015 than in 2014. The different potato

plants grown with the BM treatment were significantly greater than those with the TM treatment both years (Duncan’s test, P < 0.05). On 14 June 2014, the plant heights grown with the BM treatment (52 cm) were 44% greater than those with the TM treatment (36 cm) and on 11 June 2015, the plant heights grown with the BM treatment (57 cm) were 16% greater than with the TM treatment (49 cm). This result was consistent with Decoteau et al. (1988) who reported that tomato plants grown with black mulch were greater. However, the soil temperature of the BM treatment was not the highest during vegetative growth. This 8

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P < 0.05). The plants from the BM treatment produced significantly 26% greater yield of large plus jumbo tubers (W ≥ 200 g) than those from the TM treatment (Duncan’s test, P < 0.05). Lower soil temperature of the BM treatment than the TM and NM treatments during tuber bulking stage and a more favorable range for tuber bulking in 2014 may have caused this result. According to Van Dam et al. (1996) high soil temperature delays tuber initiation and bulking. The total potato tuber number in ten plants from the TM treatment was significantly more than from the BM and NM treatments (Duncan’s test, P < 0.05). Plants from both the BM and TM treatments produced greater tubers in weight than plants from the NM treatment. In 2015, the tuber grade differences between the BM and TM treatments were not significant (Duncan’s test, P > 0.05). The tuber grade results in 2015 were different from the result in 2014. It could be explained by the smaller soil temperature differences between the BM and TM treatments during tuber initiation in 2015. However, both the plants in the BM and TM treatments had significantly more large plus jumbo tubers (W ≥ 200 g) than those in the NM treatment in weight and number (Duncan’s test, P < 0.05). Potato tubers from the BM and TM treatments had significantly 35% and 34% more total weight than those from the NM treatment, respectively. These results were consistent with the results of Wang et al. (2009) who found that the plasticfilm mulch had a positive effect on tuber production in Gansu Province. 3.7. Potato evapotranspiration (ETc), yield, and water use efficiency (WUE) Fig. 8. Plant height of potatoes grown with different mulch treatments: transparent mulch (TM), non-mulched check (NM), and black mulch (BM) in 2014 and 2015.Note: The letters (“a”, “b”, “c”) in the two graphs meant differences among different treatments were significant by F-test (P < 0.05). Values in a date with the same letter were statistically similar according to Duncan’s multiple range test (P > 0.05). “NS” meant difference among different treatments was not significant by F-test (P > 0.05).

The ETc of potato grown with the BM treatment (532 and 517 mm) were 9% and 8% more than the ETc of potato grown with the TM treatment (487 and 478 mm) in 2014 and 2015, respectively, and the difference was significant in 2015 (Duncan’s test, P < 0.05) (Table 5). This result was probably found because the plant canopy in the BM treatment received a greater amount of longwave radiation from the mulch surface than the TM treatment. In addition, the air temperature near the black plastic-film mulch surface should have been increased by convective heat flux. The radiative and thermal conditions caused more vigorous plant growth and more water was consumed by the plants in the BM treatment than in the TM treatment. The differences in ETc between potato grown with the NM treatment and those grown with plastic-film mulch treatments varied in 2014 and 2015 consistent with the climate conditions which caused different soil surface evaporation and utilization of rain, consistent with the results of Zhang et al. (2017). The potato yields of plastic-film mulch treatments were significantly higher than those of the NM treatment in 2014 and 2015 (Duncan’s test, P < 0.05) (Table 6). The potato yields from the TM treatment (63,031 and 81,968 kg/ha) were 23% and 32% significantly higher than from the NM treatment (51,206 and 62,191 kg/ha) in 2014 and 2015,

heights between the two growing seasons could explain the differences in temperature variations between treatments over years. For example, the lowest maximum soil temperature in the BM treatment during tuber bulking could have been caused by the greater plant canopy coverage in the BM treatment in 2014. 3.6. Tuber grade On 21 August 2014 and 16 August 2015, when plant leaves and stems of all treatments were completely senescent, ten plants were harvested for tuber grade (Table 3 and 4). In 2014, the potatoes from the BM treatment had significantly 24% and 28% more jumbo tubers (W ≥ 300 g) than those from the TM treatment in number and weight, respectively (Duncan’s test, Table 3 Potato tuber number from different mulch treatments. Treatment

Tuber number of ten plants W ≥ 300 g

300 g > W ≥ 200 g

W ≥ 200 g

200 g > W ≥ 100 g

100 g > W ≥ 50 g

W ≥ 50 g

W < 50 g

Total

2014 Transparent mulch Non-mulched check Black mulch

14b* 13b 17a

14NS 13 17

28NS 26 34

30a* 23b 24b

16NS 13 10

74NS 62 69

19NS 15 9

93a* 77b 78b

2015 Transparent mulch Non-mulched check Black mulch

26a* 15b 25a

17NS 14 20

43a* 29b 46a

23NS 24 25

6b* 16a 10ab

72NS 69 81

7NS 21 9

79NS 90 90

Sampling date: 21 August 2014 and 16 August 2015. W: weight per tuber. NS: difference among different treatments was not significant by F-test (P > 0.05). * Difference among different treatments was significant by F-test (P < 0.05). Values in a column with the same letter were statistically similar according to Duncan’s multiple range test (P > 0.05).

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Table 4 Potato tuber weight from different mulch treatments. Treatment

Tuber weight of ten plants W ≥ 300 g

300 g > W ≥ 200 g

W ≥ 200 g

200 g > W ≥ 100 g

100 g > W ≥ 50 g

W ≥ 50 g

W < 50 g

Total

2014 Transparent mulch Non-mulched check Black mulch

5646b* 5142b 7241a

3514NS 3166 4281

9160b* 8308b 11,521a

4250NS 3344 3780

1192NS 1024 808

14,603NS 12,676 16,109

466NS 423 253

15,069NS 13,099 16,362

2015 Transparent mulch Non-mulched check Black mulch

12,050NS 6759 10,789

4249NS 3359 5111

16,298a* 10,118b 15,900a

3526NS 3343 3696

437NS 1207 806

20,261NS 14,668 20,402

201a* 598b 275a

20,462NS 15,266 20,677

Sampling date: 21 August 2014 and 16 August 2015. W: weight per tuber. NS: difference among different treatments was not significant by F-test (P > 0.05). * Difference among different treatments was significant by F-test (P < 0.05). Values in a column with the same letter were statistically similar according to Duncan’s multiple range test (P > 0.05).

4. Summary and conclusion

Table 5 Potato evapotranspiration (ETc) calculated with precipitation (P), irrigation (I), change of soil water storage (△S), and drainage below crop root zone (D) for different mulch treatments. Treatment

P(mm)

I(mm)

△S(mm)

D(mm)

ETc(mm)

2014 Transparent mulch Non-mulched check Black mulch

176 176 176

293 277 317

−18 −19 −39

0 0 0

487NS 472 532

2015 Transparent mulch Non-mulched check Black mulch

124 124 124

344 392 366

−10 −32 −27

0 0 0

478b* 548a 517a

The BM treatment had higher Rn than the TM and NM treatments, especially during sprout development and vegetative growth. The amplitude of G in the BM treatment was lower than in the TM treatment at 5 cm depth in the middle of beds. The average maximum mulch surface temperature in the BM treatment was greater than in the TM treatment in the middle of the beds. As the potato canopy grew, the effect of the mulch became less and less, reaching the lowest point during the tuber bulking stage. The plant height in the BM treatment was greater than in the TM treatment. However the height difference between the BM treatment and the TM treatment was greater in 2014 than in 2015. Lower air temperature at the sprout development stage in 2014 than in 2015 may have caused this difference. Although the soil temperature in the TM treatment was higher than in the BM treatment, the plant heights in the TM treatment were not higher. It indicated that the above-ground radiative and thermal conditions might be the main factor that affected the plant height during vegetative growth. In 2014 the BM treatment produced significantly 26% more large plus jumbo tubers (W ≥ 200 g) than the TM treatment. However, in 2015 there was no significant difference between the BM and TM treatments in tuber grade. Although the potato yield of the TM treatment was almost the same with the BM treatment, the BM treatment had 9% and 8% (significantly) higher potato evapotranspiration than the TM treatment in 2014 and 2015, respectively. In conclusion, the transparent plastic-film mulch was favorable for water-saving in potato production. However, the black plastic-film mulch might be more suitable for large potato tuber production in a growing season with the lower air temperature at the sprout development stage, such as 2014.

NS: difference among different treatments was not significant by F-test (P > 0.05). * Difference among different treatments was significant by F-test (P < 0.05). Values in a column with the same letter were statistically similar according to Duncan’s multiple range test (P > 0.05). Table 6 Potato tuber yield, evapotranspiration (ETc), and water use efficiency (WUE) of different mulch treatments. (Reproduced from Zhang et al., 2017). Treatment

Tuber yield (kg/ha)

ETc (mm)

WUE (kg/ha/mm)

2014 Transparent mulch Non-mulched check Black mulch

63,031a* 51,206b 62,353a

487NS 472 532

129a* 109b 117ab

2015 Transparent mulch Non-mulched check Black mulch

81,968a* 62,191b 82,704a

478b* 548a 517a

171a* 113b 160a

NS: difference among different treatments was not significant by F-test (P > 0.05). * Difference among different treatments was significant by F-test (P < 0.05). Values in a column with the same letter were statistically similar according to Duncan’s multiple range test (P > 0.05).

Acknowledgements This study was supported by Program 201501017 of the Ministry of Water Resources of China and Programs 51579240 and 51621061 of National Natural Science Foundation of China.

respectively (Duncan’s test, P < 0.05). The potato yields from the BM treatment (62,353 and 82,704 kg/ha) were similar to those from the TM treatment in 2014 and 2015. The potato WUE of plastic-film mulch treatments (129 and 171 kg/ha/mm for the TM treatment; 117 and 160 kg/ha/mm for the BM treatment) were significantly higher than the potato WUE of the NM treatment (109 and 113 kg/ha/mm) in 2014 and 2015 (Duncan’s test, P < 0.05). The increases in potato yield and WUE with plastic-film mulch reported here were consistent with previous reports (Zhao et al., 2012; Qin et al., 2014; Zhang et al., 2017).

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