Effects of maize plant type and row spacing on the field microclimate and yield of intercropping ginger

ABSTRACT

To explore field microclimate changes under maize/ginger intercropping and select a reasonable field configuration, this study used the major ginger variety ‘Zhugen’ from Southwest China as the experimental material. Two maize plant types; tall (Zhenghong 311) and short (Zhenghong 6), with 2, 3, and 4 m row spacings, shading treatment (ST), and a control (pure) without shading (CK) were used for the study. The effects of the maize plant type and row spacing on soil temperature, humidity, canopy temperature, light transmittance, plant growth, and intercropped ginger yield were investigated. Soil moisture, ginger canopy humidity, and shading rate in the maize/ginger intercropping mode were significantly higher than those in the non-shading treatment. Conversely, the soil and ginger canopy temperatures of the maize/ginger intercropping mode were significantly lower than those of the non-shading treatment. As a result, plant height, stem diameter, branch number, and ginger yield were significantly higher under the intercropping mode than the non-shading treatment. Although maize/ginger intercropping improves ginger growth and yield, these parameters are dependent on maize plant type and row spacing. The ginger yield was significantly higher when intercropped with the tall maize ZH 311 than with short maize ZH 6. The yield improved with decreasing row spacing and was the highest at a row spacing of 2 m. At the aforementioned field configurations, the soil and canopy humidity were the highest whereas the soil and canopy temperatures were the lowest, thereby giving best results on parameters like plant height, stem diameter, branch number, and yield.

GRAPHICAL ABSTRACT

KEYWORDS:

• Intercropping
• ginger
• field microclimate
• plant type
• row spacing

1. Introduction

Ginger (Zingiber Officinale Rosce) is an important spice, a traditional Chinese medicine, and an edible vegetable (Ayodele & Sambo, 2014). China’s ginger planting area is 0.26 Mha, with a total output of 9 Mt, accounting for approximately 40% (the highest worldwide) of the total production in the world. Ginger is one of the major export crops of China with an annual export of more than 500,000 tons, accounting for over 70% of the total global export quantity (United Nations Trade Database [DB/OL], 2018; Wu et al., 2019). It is an important medicine and food homologous vegetable in the billion-level industrial chain of Chongqing’s ‘13^th Five-Year’ Agricultural Plan, as well as, a major source of income for farmers in the hilly and mountainous areas.

Ginger prefers wet and shady conditions; therefore, its production in summer requires artificial shading. Under shade; the light quantum flux density of the ginger canopy decreases, which improves the chlorophyll content and photosynthetic efficiency of leaves; increases the activity of protective enzymes; and reduces stomatal conductance, intercellular CO₂ concentration, the level of reactive oxygen species, and damage to membrane lipid peroxidation in leaves (Zhang & Xu, 2008). With an increase in labor costs, ginger shading gradually transitioned from an artificial shed to an intercropping mode. Cheng et al. (2018) used grape and ginger interplanting to improve land use efficiency and increase production efficiency. Intercropping passion fruit and ginger promotes the growth of passion fruit, provides shade and heat dissipation to ginger, and achieves a double harvest of both (Su, 2015). Maize is widely used as a high-position crop in intercropping patterns with low-position crops, such as Chinese herbal medicines, vegetables, and legumes (Hu, 2010; Huang, 2014; Wang, 1995). Maize favors high temperatures and strong light with a strong edge row advantage, whereas ginger favors high moisture, shade, and lower temperatures (Ren, 2016; Tang et al., 2014). The intercropping of maize and ginger can improve the utilization rate of land as well as light energy, provide a low-light-intensity and humid environment for ginger growth, reduce the cost of artificial shading, and improve economic benefits.

Crop growth is closely related to the field microclimate. Temperature directly determines the development rate of maize apical meristems. The higher the temperature, the faster the emergence. However, if the temperature is too high during the maize-filling period, the filling duration is shortened, and the number of grains per ear and 1000-grain weight are decreased (Tian et al., 2015). In a previous study, Huang et al. (2017) showed that the diurnal variation in air temperature in a winter wheat canopy showed a ‘convex’ trend, whereas the canopy air humidity showed a ‘concave’ trend. The relative humidity of the canopy was the lowest at 16:00. The ground and canopy temperatures were significantly negatively correlated with wheat yield, whereas canopy humidity was significantly positively correlated with yield. The intercropping of maize with soybean and sweet potato exhibited a phenomenon where maize had a high priority in photosynthesis, promoting its own growth. However, maize shaded the soybean, reducing light capacity, which in turn reduced soybean yield (Chen et al., 2016; Zhang et al., 2020). Therefore, field microclimatic characteristics are closely related to crop growth and yield. Several studies have been conducted to understand the effect of field temperature, humidity, light environment, and other microclimatic changes on intercropping modes. Although there is adequate research on short-type and sun-loving crops such as soybean and sweet potato, research on the field microclimate and yield changes of shade-loving crops like ginger under the intercropping mode with maize remains scarce. In this study, the effects of plant height and row spacing on soil temperature and humidity, ginger canopy temperature and humidity, light environment, plant growth characteristics, and yield under the maize/ginger intercropping system were studied to understand the effect of field microclimate changes, to provide a theoretical basis and technical support for exploring the maize/ginger intercropping system, and for reasonable row width selection.

2. Materials and methods

2.1. Test materials

The tested maize varieties were two different plant types (Table 1), and the tested ginger varieties were the major ‘Zhugen’ ginger varieties in Southwest China.

Table 1. Maize cultivars for the experiment.

NO.	Cultivar	Plant type	Plant height (cm)	Ear length (cm)	Relative maturity (d)
A (ZH 311)	Zhenghong 311	Tall	290	19.5	115
B (ZH 6)	Zhenghong 6	Short	270	17.8	110

2.2. General situation of the test site

The experiment was conducted at the ginger base of Wujiang Town, Chongqing University of Arts and Sciences (29.17°N, 105.84°E), from 2017 to 2018. The area had a humid subtropical monsoon climate at an altitude of 300 m. The meteorological conditions during the ginger growth period are shown in Figure 1. The soil available nitrogen, available phosphorus, available potassium, total nitrogen, total phosphorus, total potassium, organic matter, and pH were 40.8 mg kg⁻¹, 114.3 mg kg⁻¹, 297.6 mg kg⁻¹, 1.2 g kg⁻¹, 18.2 g kg⁻¹, 1.7 g kg⁻¹, 18.3 g kg⁻¹, and 6.73, respectively. Soil total and available nitrogen were determined using the semi-micro Kjeldahl and alkaline hydrolysis diffusion methods, respectively. Soil total phosphorus was determined using the perchloric acid-sulfuric acid-molybdenum antimony anti-colorimetric method, whereas soil available phosphorus was determined using the molybdenum antimony colorimetric method with 0.5 mol L⁻¹ NaHCO₃ extraction. Soil total potassium was determined by NaOH fusion flame spectrophotometry, whereas soil available potassium was extracted with l mol L⁻¹ NH₄OAc and determined by flame photometry. Soil organic matter was determined by potassium dichromate volumetric method-external heating method, and soil pH was determined by potentiometric method.

Figure 1. Meteorological conditions in ginger growing period.

2.3. Experimental design

Two maize varieties (ZH 311 and ZH 6), three row spacings (2, 3, and 4 m), and the single crop cultivation system of ginger were used as controls (ST: net shading, shading time from mid-July to late August, 50% shading; CK: pure without shading), There were eight treatments in total, with three replicates per treatment. The plot area was 48 m², and the length and width of each plot were 4 m × 12 m. In the 2 m row spacing treatment, 7 rows of maize were planted with a ginger row spacing of 0.4 m × 0.3 m. In the 3 m row spacing treatment, 5 rows of maize were planted, and the ginger row spacing was 0.43 m × 0.3 m. In the 4 m row spacing treatment, 4 rows of maize were planted with a ginger row spacing of 0.4 m × 0.3 m. Maize seedling transplanting, single-row cultivation, north-south row direction, plant spacing of 25 cm, row spacing according to the test treatment, and a field diagram of each row spacing are shown in Figure 2. Maize was transplanted on 10 May 2017 and 8 May 2018, and harvested on 30 August 2017 and 27 August 2018, respectively. Ginger was sown on 20 April 2017 and 25 April 2018, and harvested on 20 November 2017 and 23 November 2018, respectively. Before sowing, the experimental field was plowed evenly and maize and ginger were uniformly fertilized. The amounts of inorganic fertilizer nitrogen (N), phosphorus (P₂O₅), and potassium (K₂O) were 400 kg ha⁻¹, 200 kg ha⁻¹, and 600 kg ha⁻¹, respectively, and the amount of organic fertilizer (cattle manure) was 1500 kg ha⁻¹. Phosphorus, potassium, and organic fertilizers were simultaneously applied as base fertilizers at one time, 50% of nitrogen fertilizer was applied as base fertilizer, and 50% topdressing was used at the tuber expansion stage. The other field management measures were consistent with those of the local high-yield fields.

Figure 2. Field layout of different row spacing intercropping patterns.

2.4. Index determination and method

2.4.1. Determination of morphological index

Before ginger harvest, 10 ginger plants with similar growth vigor (plant height and tillers were the same) were selected from each plot, and plant height, stem diameter, and branch number were measured. Plant height was measured on a meter scale from the base of the main stem to the highest point at the top, and the diameter of the ginger stem (1 cm from the ground) was measured using a vernier caliper. The number of branches was counted manually.

2.4.2. Determination of soil temperature and humidity

After shading, the soil temperature and moisture were measured in the 0–20, 20–40, and 40–60 cm soil layers between the ginger rows in sunny weather. Measuring was conducted at the center of the plot in July 2017 and July 2018. Sampling was carried out on every Wednesday, four times a month, and the average value was taken. Temperature was measured using a soil temperature and humidity automatic monitoring device (RR-7210) from 8:00 to 20:00, and data were recorded every 2 h. Three points were measured in each plot, and the average temperature of each soil layer was recorded at each time point. The soil moisture was measured using the drying method (soil samples were dried to constant weight at 105°C). Three points were recorded in each plot, and the humidity was calculated as the average of each point.

2.4.3. Determination of temperature and humidity of ginger canopy

The temperature and humidity of the soil and the ginger canopy in each treatment were measured between 8:00 and 20:00 using an infrared temperature sensor (SI-411). The data were recorded every 2 h at three points in each plot. Measuring was conducted at the center of the plot in July 2017 and July 2018.

2.4.4. Determination of the light environment of ginger

Photosynthetically active radiation of the maize and ginger canopies was measured using the LI-1400 light quantum instrument (LI-COR, USA) from 9:00 to 11:00 in sunny weather after ginger shading. On July 5, 15, and 25, five points were measured in each plot to obtain an average value.

$\mathrm{SR}\mathit{\hspace{0.17em}}=\mathit{\hspace{0.17em}}100\left(1\mathit{\hspace{0.17em}}-\mathit{\hspace{0.17em}}\mathrm{Lg}/\mathit{\hspace{0.17em}}\mathrm{Lm}\right)$

where SR, Lg, and Lm mean shading rate (%), ginger canopy effective light radiation, and maize canopy effective light radiation, respectively.

2.4.5. Yield determination

When ginger was harvested, yield determination is the fresh weight of all but the border plants in each plot, and then the final yield is calculated according to the harvest area.

2.5. Statistical analysis

The data were organized using Excel 2019 and analyzed using the least significant difference method with the SPSS software package (version 20.0; IBM Corp., NY, USA) to test whether significant differences existed among treatments. The data were plotted using GraphPad Prism 9 (GraphPad Software, CA, USA).

3. Results

3.1. Effects of row spacing and maize plant type on soil temperature and humidity

Row spacing and maize plant type significantly affected the soil temperature in the maize/ginger intercropping mode (Figure 3, Table 2). Soil temperature decreased significantly with increasing soil depth. In 2017, the average temperature of the 0–20 cm soil layer was 10.5% and 21.0% higher than those of the 20–40 cm and 40–60 cm soil layers, respectively. In 2018, the corresponding values were 9.1% and 17.1% higher, respectively. In 2017, the average temperature of each soil layer was 13.6% and 20.4% higher, and in 2018, it was 14.2% and 21.3% higher. These findings indicate that intercropping and shading treatments effectively reduced soil temperatures for ginger during the high-temperature stage in summer. In the intercropping mode, the temperature of each soil layer increased with increasing row spacing. In 2017, the average temperature in the 2 m row spacing plots of each soil layer for the two varieties was 5.8% lower than that in the 3 m plots and 9.3% lower than that in the 4 m plots. In 2018, the corresponding differences were 5.4% and 10.6%, respectively. This suggests that the excessive row spacing in the intercropping mode is not conducive to maintain suitable soil temperature for ginger. In addition, maize plant type had an important influence on soil temperature. The temperature of each row spacing and soil layer was lower when intercropped with ZH 311 compared to ZH 6. In 2017, when intercropped with ZH 311, the average temperature for row spacing of 2, 3, and 4 m for each soil layer was 2.9%, 2.1%, and 1.3% lower, respectively. In 2018, the corresponding temperature differences were 2.2%, 3.7%, and 3.6%, respectively.

Figure 3. Differences in soil temperature in different treatments.

ST, shading treatment; CK, pure without shading. Values with different lowercase letters are significantly different with p < 0.05 according to the least significant difference test.

Table 2. Variance analysis of soil and canopy indexes.

	Treatments	Soil temperature	Soil moisture	Canopy temperature	Canopy humidity	Shading rate
F value	Years (Y)	852.68**	4000.78**	2.91^ns	4.63*	0.03^ns
	Variety (V)	100.70**	99.555**	0.72^ns	8.29**	34.62**
	Row spacing (R)	521.38**	958.20**	53.79**	137.02**	302.93**
	Y×V	3.38^ns	0.38^ns	1.26^ns	0.90^ns	0.19^ns
	Y×R	2.63^ns	22.01**	0.49^ns	0.16^ns	0.19^ns
	V×R	0.31^ns	1.43^ns	3.29^ns	2.20^ns	14.84**
	Y×V×R	3.12^ns	0.93^ns	0.39^ns	0.25^ns	3.45*

**p < 0.01; *p < 0.05; ^nsNot significant.

The results in Figure 4 show that row spacing and maize plant type significantly affected soil moisture content in the maize/ginger intercropping treatment (Table 2). As the soil layer deepened, the soil moisture content initially increased and then decreased. The highest soil moisture content was found in the 20–40 cm soil layer. The average soil moisture content in the 20–40 cm soil layer was 21.4% and 8.2% higher compared with the 0–20 cm and 40–60 cm soil layers in 2017 and 20.1% and 8.7% higher in 2018, respectively. The soil moisture content of each soil layer in the control group was significantly lower compared with the intercropping and shading treatment groups. The average soil moisture content of each soil layer in the non-shading group was 20.3% and 36.0% lower in 2017 and 11.7% and 26.1% lower in 2018, respectively. During intercropping, soil moisture in each soil layer decreased with increasing row spacing. The average soil moisture content of the plots with 2 m row spacing for each soil layer for the two varieties was 9.3% and 17.1% higher than those in the 3 m and 4 m row spacing in 2017, and 9.2% and 14.8% higher in 2018, respectively. This indicates that excessive row spacing of the intercropping mode was not conducive to the maintenance of suitable soil water content of ginger. Soil water content was significantly different among the different maize intercropping plant types. The water content of each soil layer in each row of ZH 311 was higher than that of ZH 6. The average water contents of the intercropped ZH 311 soil with row spacing of 2, 3, and 4 m were 2.1%, 2.6%, and 2.7% higher in 2017, and 2.1%, 4.7%, and 3.3% higher in 2018, respectively, relative to the intercropped ZH 6 soil.

Figure 4. Differences in soil moisture content in different treatments.

ST, shading treatment; CK, pure without shading. Values with different lowercase letters are significantly different with p < 0.05 according to the least significant difference test.

3.2. Effects of row spacing and maize plant type on temperature and humidity of the ginger canopy

The diurnal variation in the ginger interrow temperature first increased and then decreased over time. The temperature of each treatment was the highest at 14:00 and then gradually decreased (Figure 5, Table 2). The temperature of the intercropping and net shading treatments at each time point was lower than that of the net non-shading treatment, and the difference between treatments was the largest between 14:00 and 16:00, with the highest daily temperature. The average temperatures for the intercropped group at each time point were 6.4% and 14.4% lower in 2017 and 6.6% and 14.6% lower in 2018, respectively. This indicates that intercropping and shading treatments could effectively reduce the temperature between ginger rows during the high-temperature stage in summer, particularly in the afternoon. In the intercropping mode, row spacing and maize plant type had significant effects on the diurnal variation in ginger interrow temperature. As row spacing increased, the interrow temperature increased at each time point. On an average, the interrow temperature in the 3 m and 4 m row spacings were 2.2% and 6.3% higher than that of the 2 m row spacing in 2017 and 3.4% and 7.5% higher in 2018, respectively. This shows that with an increase in row spacing, the shading effect of maize on intercropping ginger decreased, and the temperature between rows of ginger increased. The interrow temperature of ZH 311 was lower than that of ZH 6 at each time point under the 2 m and 3 m row spacings; however, there was no significant difference between the two varieties at each time point under the 4 m row spacing. The results of the two-year test were consistent, and the average interrow temperatures at each time point were 1.0% and 1.3% lower in 2017 and 1.7% and 1.7% lower in 2018, respectively.

Figure 5. Diurnal variation in canopy temperature under different treatments.

A1, ZH 311-2 m; A2, ZH 311-3 m; A3, ZH 311-4 m; B1, ZH 6-2 m; B2, ZH 6-3 m; B3, ZH 6-4 m; ST, shading treatment; CK, pure without shading.

The diurnal variation trend of humidity between the rows of ginger was contrary to that of temperature. Over time, it initially decreased and then increased. The humidity of each treatment (except 2017 for CK) was the lowest at 14:00 and then gradually increased (Figure 6, Table 2). The humidity of the intercropping and net shading treatments at each time point was higher than that of the net non-shading treatment, and the difference in the humidity between treatments was the largest at 14:00–16:00, when the humidity was the lowest. The average humidity at each time point was 17.6% and 21.1% higher in 2017 and 13.1% and 17.9% higher in 2018, respectively. With increasing row spacing, humidity at each time point decreased significantly. On an average, the average humidity for 2 m and 3 m row spacings were 11.2% and 5.0% higher than in the 4 m spacing in 2017 and 10.4% and 4.9% higher in 2018, respectively. This shows that with an increase in row spacing, the shading effect of maize on intercropping ginger gradually weakened, and the water loss between ginger rows increased significantly. Humidity between the ginger rows of ZH 311 was higher than that of ZH 6 under the different row spacing treatments. The average humidity between the ginger rows of ZH 311 was 2.0% higher in 2017 and 1.6% higher in 2018, indicating that the tall maize variety had a better shading effect on intercropped ginger than the short maize variety.

Figure 6. Diurnal variation in canopy humidity under different treatments.

A1, ZH 311-2 m; A2, ZH 311-3 m; A3, ZH 311-4 m; B1, ZH 6-2 m; B2, ZH 6-3 m; B3, ZH 6-4 m; ST, shading treatment; CK, pure without shading.

3.3. Effects of row spacing and maize plant type on the light environment of ginger

The results in Figure 7 show that the shading rates of intercropping and net shading treatments on ginger were significantly higher than those of the net non-shading treatment, which were 8.0 and 48.6% points higher in 2017 and 9.0 and 50.4% points higher in 2018, respectively, indicating that intercropping and shading treatment had a certain shading effect on ginger (Table 2). The row spacing and maize plant type significantly affected the shading rate of ginger in the maize/ginger intercropping mode. The shading rate of intercropped ginger decreased significantly with increasing row spacing. In 2017, the shading rate of row spacing of 3 m spacing was 7.7% points lower than that of the 2 m spacing, and the shading rate of the 4 m spacing was 7.2% points lower than that of the 3 m spacing. In 2018, the corresponding shading rates were 8.2 and 6.5% points lower, respectively, indicating that the shading effect of intercropping on ginger decreased significantly with increasing row spacing. The greater the row spacing, the lesser the shading effect. The difference in the shading rate of ginger between the two maize varieties was significant at 2 m row spacing in 2017 and 2018, though not significant at 3 m and 4 m row spacings. The shading rate of ZH 311 under the 2 m row spacing was 8.0% points higher than that of ZH 6 in 2017 and 5.3% points higher in 2018, indicating that the tall maize variety had a better shading effect on intercropping ginger than the short variety, particularly at 2 m row spacing.

Figure 7. Shading rate of ginger in different treatments.

A1, ZH 311-2 m; A2, ZH 311-3 m; A3, ZH 311-4 m; B1, ZH 6-2 m; B2, ZH 6-3 m; B3, ZH 6-4 m; ST, shading treatment; CK, pure without shading. Values with different lowercase letters are significantly different with p < 0.05 according to the least significant difference test.

3.4. Effects of row spacing and maize plant type on ginger growth and yield

Intercropping and shading significantly affected the plant height, stem diameter, branch number, and ginger yield (Table 3). Plant height, stem diameter, branch number, and ginger yield under the intercropping and shading treatments were significantly higher than those under the no shading treatment. In 2017, the plant height was 11.6% and 39.4% higher, stem diameter was 22.3% and 52.3% higher, branch number was 16.0% and 47.3% higher, and yield was 19.1% and 79.5% higher under the intercropping and shading treatments, respectively, compared to those under the no shading treatment. In 2018, plant height was 11.9% and 39.7%, 19.4% and 52.3%, 16.0% and 45.8%, and 29.4% and 90.6% higher, under the intercropping and shading treatments, respectively, compared to the no shading treatment. Row spacing and maize plant type significantly affected the growth characteristics and yield of intercropped ginger. Plant height, stem diameter, branch number, and ginger yield decreased significantly with increasing row spacing over two years. In the two years (2017 and 2018), for 2 m row spacing, the average plant height was 8.4% and 16.1%, the stem diameter was 11.4% and 20.1%, the branch number was 14.8% and 20.6%, and the yields were 20.7% and 30.5% higher than those of 3 m and 4 m row spacings, respectively. Plant height, stem diameter, branch number (except in 2017), and yield of ginger under the ZH 311 treatment were significantly higher than those under the ZH 6 treatment. The average row spacing was 3.07%, 7.7%, 3.1%, and 6.1% higher in 2017 and 4.2%, 8.1%, 5.7%, and 4.9% higher in 2018, respectively. This indicates better ginger growth and high shading rate under the intercropping system with tall maize variety.

Table 3. Morphological indices and ginger yield under different intercropping patterns.

Treatments	Plant height (cm)	Stem diameter (mm)	Branching number	Yield (t ha^−1)
2017
A1	95.94 a	11.36 a	16.28 a	49.65 a
A2	88.04 bc	9.87 bc	14.44 bc	39.34 c
A3	80.76 de	9.24 de	13.61 c	37.32 cd
Average	88.25 A	10.16 A	14.78 A	42.10 A
B1	91.57 ab	10.25 b	15.72 ab	45.03 b
B2	85.01 cd	9.32 cd	13.83 c	38.74 c
B3	80.28 e	8.71 e	13.46 c	35.33 d
Average	85.62 B	9.43 B	14.34 A	39.70 B
IC	86.93 b	9.79 b	14.56 b	40.90 b
ST	108.53 a	12.19 a	18.49 a	61.65 a
CK	77.87 c	8.01 c	12.55 c	34.35 c
2018
A1	92.62 a	10.86 a	16.51 a	47.34 a
A2	84.93 c	9.81 c	14.17 c	39.36 b
A3	79.13 de	9.13 d	13.27 de	37.00 c
Average	85.56 A	9.93 A	14.65 A	41.23 A
B1	87.97 b	10.06 b	15.50 b	45.64 a
B2	81.61 d	9.17 d	13.32 d	38.07 bc
B3	76.80	8.33 e	12.76 e	34.19 d
Average	82.12 B	9.19 B	13.86 B	39.30 B
IC	83.84 eb	9.56 b	14.26 b	40.27 b
ST	104.65 a	12.19 a	17.91 a	59.30 a
CK	74.91 c	8.01 c	12.29 c	31.11 c
F value
Years (Y)	21.27**	8.57**	2.45^ns	2.57^ns
Variety (V)	20.49**	85.97**	10.24**	29.80**
Row spacing (R)	122.09**	169.20**	73.12**	272.08**
Y×V	0.36^ns	0.02^ns	0.82^ns	0.35^ns
Y×R	0.17^ns	0.74^ns	0.69^ns	0.16^ns
V×R	1.80^ns	1.88^ns	0.53^ns	2.69^ns
Y×V×R	0.15^ns	1.18^ns	0.03^ns	2.38^ns

A1, ZH 311–2 m; A2, ZH 311–3 m; A3, ZH 311–4 m; B1, ZH 6–2 m; B2, ZH 6–3 m; B3, ZH 6–4 m; ST, shading treatment; CK, pure without shading. Values with different lowercase letters are significantly different with p < 0.05 according to the least significant difference test. **p < 0.01; *p < 0.05; ^ns Not significant.

4. Discussion

Intercropping can effectively improve the utilization rates of light energy and soil moisture, thereby significantly improving the grain output efficiency. However, crop species and field configuration have regulatory effects on the field microclimate, such as the temperature, humidity, and the light environment of crop populations. Therefore, it is important to coordinate the field configuration in the maize/ginger intercropping mode and improve the field microclimate to enhance the production efficiency of maize/ginger intercropping systems (Xu, 2007).

4.1. Effect of field microclimate on intercropping ginger yield

Light is an energy source for plant life activities, organic synthesis, and the metabolic cycle, and is the material basis of plant growth and yield (Zhang, 2022). Appropriate shading can promote chlorophyll synthesis in shade-loving plants, thereby improving the photosynthetic ability of the leaves. The results of this study show that the shading rates of the shading and intercropping treatments were significantly higher than those of the no-shading treatment, consistent with the result of the intercropping experiment using grape and ginger (Peng et al., 2023). Further analysis showed that the higher the shading rate, the better the growth performance of ginger and the higher the yield. Therefore, ginger yield was significantly positively correlated with the shading rate (R² = 0.94). When ginger is not shaded, stem elongation, leaf and branch development, and plant growth rate, plant biomass, the number of ginger rhizomes per plant, and yield decreases (Tang et al., 2014). This is contrary to the response of the photosynthetic characteristics of sun-loving plants, such as soybean (Liang, 2000), maize (Chen, 2021), and rice (Yuan et al., 2005) under shading.

The growth and yield of crops are affected by the combined effects of various ecological factors (Bai, 2021). Under the shading treatment, the temperature also changed significantly, and the field temperature directly affected crop growth. Field temperature is an important factor affecting crop growth. The results of this experiment showed that the temperature of soil and ginger canopy under the shading and intercropping treatments were significantly lower than those under the no-shading treatment, consistent with the results of Chen et al. (2016) for maize/soybean intercropping. If the bandwidth is too large and the field temperature is too high, the stem or internode is excessively elongated, causing lodging, and the maize yield decreases significantly (Chen et al., 2016; Liu, 2013). This study also found that shading and intercropping significantly increased ginger yield, showing a linear relationship between the shading rate and ginger yield (R² = 0.85, Figure 8), which is inconsistent with previous research findings on maize (Zhang, 2019), since maize, soybean, and sweet potato are light-loving crops and are insensitive to high temperatures. The photosynthetic capacity and nitrogen and phosphorus absorption of soybean and sweet potato were reduced by the shading of maize leaves in the late growth stage when intercropped. Consequently, the yield of soybean and sweet potato decreased significantly (Chen et al., 2016; Zhang et al., 2020). Ginger is a shade-loving crop, and its yield is negatively impacted by higher temperatures. Shading and intercropping can effectively avoid heat damage caused by high-temperature stress, creating favorable growth conditions for ginger and leading to higher ginger yield (Tang et al., 2014; Wang et al., 2018).

Figure 8. Correlation analysis between field microclimate and ginger yield.

Adequate water is a basic condition for the normal growth of crops, and moderate water level is beneficial for plants to improve their water retention capacity and water-use efficiency, thereby increasing crop yield. Insufficient water during the filling period of maize shortens the filling duration and reduces the 1000-grain weight. Lower soil moisture condition restricts the crop growth speed, dry matter accumulation, and ultimately yield. Soil water content is significantly positively correlated with ginger yield (Liu, 2017). The results of the present experiment showed that soil moisture and ginger canopy moisture under the shading and intercropping treatments were significantly higher than those under the no-shading treatment. The smaller the row spacing, the higher the humidity. Hence, the tall-type varieties contributed to a higher soil humidity than the short-type varieties. Consistent with previous studies in other crops, sufficient water can promote rapid crop growth and increase dry matter accumulation. However, the present study also found that soil moisture (R² = 0.46) and canopy humidity (R² = 0.52) had the lowest correlations with ginger yield among the field microclimate parameters. The results showed that the key climatic factors limiting the yield of shade-loving ginger could be light (R² = 0.94) and temperature (R² = 0.85), as opposed to the humidity of light-loving crops (Li et al., 2020).

4.2. Effects of maize/ginger intercropping on ginger growth and yield

Plants adapt to external environmental changes by altering their morphology, structure, and physiological functions (Fan et al., 2017). Several studies have shown that plant height, dry matter accumulation, and reproductive growth are hindered in soybean, cotton, and sweet potato under intercropping conditions (Li et al., 2022; Wang et al., 2015; Yu et al., 2016). The results of this study showed that the plant height, stem diameter, branch number, and yield of ginger treated with shading and intercropping were significantly higher than those under ginger without shading, indicating that shading and intercropping can improve the growth and yield of ginger. This is inconsistent with previous results for the intercropping of soybeans (Yu et al., 2016), peanuts (Lin, 2020), sweet potatoes (Wang et al., 2015), and other light-loving plants. Under intercropping, light for low-position crops is limited, photosynthesis is weakened, and biomass is reduced. Appropriate shading can promote the growth of shade-loving crops. After shading, the chlorophyll content of ginger increased, and photosynthesis was enhanced (Peng et al., 2023). Therefore, plant height, stem diameter, branch number, and yield of ginger under shade and intercropping treatments increased significantly, which is consistent with the results of Wang et al. (1999).

The spatial layout of crops primarily regulates the field microclimate through plant type, row spacing, and row orientation, thereby regulating crop yield (Yu et al., 2013). The results of the present experiment showed that the plant height, stem diameter, branch number, and yield of ginger in the ZH 311 intercropping were significantly higher than in the ZH 6 intercropping, indicating that tall maize had a greater effect on the growth and yield of intercropped ginger than short maize. With increasing intercropping row spacing, plant height, stem diameter, branch number, and ginger yield decreased. The growth characteristics and yield of ginger under the 2 m row spacing were higher compared to other row spacings, indicating that row spacing had an important effect on the growth and yield of intercropping ginger. Therefore, the tall maize variety had the best shading effect on intercropping ginger under a 2 m row spacing. In intercropping ginger with 2 m row spacing, soil and canopy humidity and ginger plant height, stem diameter, branch number, and yield were the highest, whereas the soil and canopy temperatures were the lowest, relative to the other intercropping treatments.

5. Conclusion

The field microclimate, including light, temperature, and humidity in the intercropping mode, changed drastically, particularly for low-position crops, which directly affected their growth and yield. The soil moisture, ginger canopy humidity, and shading rate of the maize/ginger intercropping mode were significantly higher than those of the non-shading treatment. Whereas, the soil and ginger canopy temperatures of the maize/ginger intercropping mode were significantly lower than those of the non-shading treatment. Plant height, stem diameter, branch number, and ginger yield under the intercropping mode were also significantly higher than those under the non-shading treatment. Maize/ginger intercropping has a good shading effect on ginger, which can improve growth and increase yield; however, these effects were dependent on the maize type and row spacing. The shading effect of tall maize ZH 311 on intercropped ginger was significantly greater than that of short maize ZH 6, and the shading effect of ginger improved with decreasing row spacing. Therefore, compared with 3 m and 4 m row spacings, the shading effect of the tall maize varieties on intercropping ginger had better shading effect under 2 m row spacing, with higher soil and canopy humidity, lower soil and canopy temperature, and higher ginger plant height, stem diameter, branch number, and yield. Further analysis showed that the most important climatic factor affecting ginger yield could be light (R² = 0.94), followed by temperature (R² = 0.85) and humidity (R² = 0.52). Light intensity and temperature in ginger canopy may affect ginger yield, but further detailed investigations are required in the future.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

This study was supported by the Scientific and Technological Research Program of Chongqing Municipal Education Commission [KJQN202201305], the Natural Science Foundation of Chongqing [cstc2019jcyj-msxmx0803], the Projects for Innovative Research Groups of Chongqing Universities [CXQT21028]. The funders have no role in studydesign, data collection, data analysis, data interpretation, or writing of the manuscript