Continuous film mulching enhances maize root growth and grain yield by improving soil aggregates characteristic in semiarid areas of China: An eight-year field experiment

doi:10.21203/rs.3.rs-1647442/v1

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Research Article

Continuous film mulching enhances maize root growth and grain yield by improving soil aggregates characteristic in semiarid areas of China: An eight-year field experiment

https://doi.org/10.21203/rs.3.rs-1647442/v1

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Aims

Film mulching has been widely used in agricultural production in semiarid areas. While studies regarding the long-term effects of different film mulching methods on soil aggregates and stability are still lacking.

Methods

In the Loess Plateau of China, a 8-year continuous field experiment was conducted on soil aggregate and stability, and crop yield with five different film mulching treatments: (i) a control method, flat planting without mulching (CK); (ii) alternate mulching and bare rows without ridges (P); (iii) planting furrows separated by consecutive plastic film-mulched ridges (S); (iv) double ridge-furrow with full plastic film mulching (D); and (v) traditional ridge-furrow rainwater harvesting planting (R). The indexes were used to expressed soil aggregate stability by mean-weight diameter (MWD), geometric mean diameter (GMD), fractal dimension (FD), percentage of aggregate destruction (PAD) and unstable aggregates index (E_LT).

Results

After 8 years continuous film mulching, soil quality was significantly improved. Compared with CK, mulching could decrease soil bulk density (2.96%-4.54%), and increase soil porosity (2.45%-4.00%), soil macroaggregate content (7.39%-22.92% by dry-sieving, 8.49%-21.67% by wet-sieving), soil aggregate stability and crop yield (29.98%-63.96%), especially under S and D treatments. D and S significantly improved soil structure (soil aggregate content and stability) with the increase of crop root biomass (43.59%, and 47.25%, respectively), compared with other mulching treatments. The root system characteristic parameters under S and D treatment were significantly higher than R and P, especially during 60–120 days after planting.

Conclusions

On the whole, the S treatment has a greater effect on crop yield increase and soil quality improve, and thus it was suggested as an effective mulching pattern on stimulate and stable crop yields in the Loess Plateau of China.

Dryland farming area

Film mulching

Soil aggregate stability

Crop yield

Root system characteristic

The Loess Plateau is a typical arid and semi-arid region, where receives abundant soil and photothermal resources, which drive the growth of crop (Zhang et al. 2019). However, the intense evaporation and a scarcity and inefficiency of water resources restricts agricultural development (Zhang et al. 2014). Thus, inhibiting the invalid soil evaporation by adopting effective cultivation methods is the main way to relieve drought stress and improve crop yield in semi-arid areas of China (Wu et al. 2017). At present, film mulching had been used most widely in dryland farming regions (Wu et al. 2017; Zhang et al. 2019; Zhang et al. 2020a), because of the characteristics of improving water and nutrient use efficiency through soil water retention and temperature regulation (Gao et al. 2021). However, there are a serious of problems with the increasing area and years of applying film mulching, such as plastic film residues, soil structure and soil quality change (Wang et al. 2017; Zhang et al. 2020c; Gao et al. 2021). And there are still some problems that limit the application area expansion and sustainable utilization of film mulching, such as continuous film mulching for many years whether change filmland soil structure and how to effect soil quality.

Soil structure has an important effect on the crop root growth (Bergmann et al., 2016; Jin et al. 2017) as the basis of maintaining soil function and soil quality (Yang et al. 2014; Ma et al. 2020). While soil aggregate, as a basic unit of soil structure, which content and structural characteristics can affect crop productivity by affecting growth, nutrient and water absorption of crop (Freitas et al. 1999; An et al. 2013; Zhang et al. 2020b). The increase of crop root biomass in turn affected soil aggregate structure and stability by improving root microbial activity and increasing the input of organic matter into soil (Tisdall et al. 1982; Sithole et al. 2019; Bondaruk et al. 2020). The previous researches had observed that the effect of film mulching on soil macroaggregate content and soil aggregate stability in topsoil are various according to the film mulching materials (Mulumba et al. 2008; Gao et al. 2021) and tillage measures (Obalum et al. 2010; Wang et al. 2020). In contrast, there are many film mulching methods to improve crop yield in semi-arid region at present, include: (1) flat-planting mulching, (2) double ridges and furrows with complete mulching, and (3) ridge and furrow rainfall harvesting with half mulching (Zhang et al., 2020a). But studies regarding the effect of different film mulching methods on soil aggregate structure and stability in same agro-ecological conditions are still lacking, which could improve our comprehensive understanding of sustainability of the film mulching on crop productivity and farmland eco-environment.

Thus, an 8-year continuous field experiment was conducted at the Dryland Agricultural Research Station, Pengyang County, Ningxia, China. And three existing mulching methods were selected, including large and small ridges with full mulching (D), ridge and furrow mulching (R), and flat mulching (F), together with the new mulching method (S). The objectives for this study are to examine the effects of application different film mulching methods on (i) the soil physical properties (bulk density and porosity), (ii) the soil aggregate composition and stability, and (iii) the root system characteristic and crop yield. In addition, we also explored the correlation between film mulching methods, soil physical properties, aggregate distribution and crop yield (especially root biomass). And then, to select an optimal mulching method, which can sustainably increase and stabilize crop yield, for dryland farming regions of the Loess Plateau, China. We hypothesized that long-term plastic film mulching would significantly affect soil aggregate distribution and stability, which would relate to the changes of crop yield.

Site description

From 2013 to 2020, field experiment was conducted at the Dryland Agricultural Research Station, Pengyang County, Ningxia, China (Fig. 1). It is located on Liupan Mountain (35°79'N, 106°45'E, and 1800 m altitude), and characterized by a semiarid, warm and continental monsoon climate. The average annual precipitation was 539.75 mm, more than 60% of which occurred from July to September (Fig. 2). The mean annual air temperature was 8.63 ℃ year^− 1 (with a maximum of 20.87 ℃ in July and a minimum of -5.09 ℃ in January), the annual mean evaporation was 1753.2 mm year^− 1, the average sunshine duration was 2518 h year^− 1, and the frost-free period was 140–160 days.

Figure 1.

Figure 2.

The soil at the research site was loessic and classified as a Calcic Cambisol according to the FAO/UNESCO Soil Classification (FAO/UNESCO 1993). The texture of the soil comprised: sand = 14% (0.05-2.0 mm), silt = 26% (0.002–0.05 mm), and clay = 60% (< 0.002 mm). The initial soil properties in the 0–40 cm layers were as follows: pH = 8.5, organic matter = 9.99 g kg^− 1, total nitrogen = 1.06 g kg^− 1, available nitrogen = 51.77 mg kg^− 1, available phosphorus = 8.83 mg kg^− 1, and available potassium = 127.03 mg kg^− 1.

A completely randomized block design with three replicates was employed. Each plot area measured 39.6 m² (3.6 m × 11.0 m). The five treatments tested were as follows: (i) CK: a control method, flat planting without mulching; (ii) P: alternate mulching and bare rows without ridges; (iii) S: planting furrows separated by consecutive plastic film-mulched ridges; (iv) D: double ridge-furrow with full plastic film mulching; and (v) R: traditional ridge-furrow rainwater harvesting planting. Diagrams of each plastic film mulching method are presented in Fig. 3.

Figure 3.

Maize (Dafeng 30) was sown at a rate of 75,000 plants ha^− 1 during middle-to-late April in 2013–2020 using a hole sowing/fertilization (diameter = 3 cm) machine. Base fertilizer comprising 150 kg N ha^− 1 and 150 kg P₂O₅ ha^− 1 was applied using a hole-sowing/fertilization machine after planting. In addition, 150 kg N ha^− 1 was applied as a topdressing in late June, with a fertilization depth of 4–5 cm. Maize was harvested from the plots in late September or early October during 2013–2020. After harvesting, the maize stalks were removed but the film mulch was retained in the same location in each of the plots. The mulching film was cleaned approximately 30 days before the subsequent sowing operation (late March in each experimental year) and sown maize in the next month (mid-to-late April). The methods used in practice were the same during each experimental year (2013–2020). Irrigation was not applied in the experimental years. Pests and weeds were controlled during each crop growth season.

Soil bulk density

After maize harvesting in 2020, the bulk density was determined from three replicate of undisturbed soil samples of each treatment by using the cutting rings (soil core) methodology (volume = 100 cm³, inner diameter = 5.05 cm). The soil was sampled at intervals of 10 cm to a depth of 40 cm. Soil samples were dried to a constant weight in an oven at 105°C. Soil total porosity was calculated from bulk density and measured particle density (i.e., 2.65 g cm^− 3) with the following equation (Yang et al. 2019):

$$\text{soil total porosity = (1}-\frac{\text{soil bulk density}}{\text{2.65}}\text{) × 100\%}$$

Soil aggregate

After harvest in 2020, soil samples were collected from randomly selected positions of each treatment at four depths: 0–10, 10–20, 20–30, and 30–40 cm. Visible plant residues were removed and then the soil samples were air-dried in their clod forms, gently crushed and air-dried further to constant weight. The soil mechanical aggregates were performed by dry sieving method through a series of sieves (5, 2, 1, 0.5, and 0.25 mm in aperture) by using soil fractionator (FSD-200, Hunan Focucy Experimental Instrument Co., Ltd, China), following the procedure by Kemper and Rosenau (1986), with some minor modifications. The air-dried soil samples were also used to analyze the distribution of water-stable aggregates via the wet sieving method (Elliott 1986) into six size classes (> 5, 5 − 2, 2 − 1, 1-0.5, 0.5 − 0.25, and < 0.25 mm) by using soil aggregate analyzer (TPF-100, Hunan Focucy Experimental Instrument Co., Ltd, China).

Mean weight diameter (MWD) was determined as a sum of weighed mean diameters of all fraction classes using the following formula described by Sithole et al. (2019):

$$\text{MWD = }{\sum }_{\text{i}\text{ }\text{=}\text{ }\text{1}}^{\text{n}}{\stackrel{\text{-}}{\text{x}}}_{\text{i}}{\text{w}}_{\text{i}}$$

Where ${\stackrel{\text{-}}{\text{x}}}_{\text{i}}$ is mean diameter of each size faction, ${\text{w}}_{\text{i}}$ is proportion of total sample weight, n is number of size fractions.

The geometric mean diameter (GMD) is calculated approximately by the following equation (Kemper and Rosenau 1986):

$$\text{GMD = exp[}{\sum }_{\text{i}\text{ }\text{=}\text{ }\text{1}}^{\text{n}}{\text{w}}_{\text{i}}\text{log}{\stackrel{\text{-}}{\text{x}}}_{\text{i}}/{\sum }_{\text{i}\text{ }\text{=}\text{ }\text{1}}^{\text{n}}{\text{w}}_{\text{i}}\text{]}$$

Where ${\stackrel{\text{-}}{\text{x}}}_{\text{i}}$ is the mean diameter of a given size fraction (mm), ${\text{w}}_{\text{i}}$ is the proportion by weight of aggregates in the size fraction (g g^− 1), and n is the total number of size fractions of aggregates.

The soil aggregate mass fractal dimension (FD) for each mulching treatment was imposed, was calculated as Tagar et al. (2019):

$$\text{FD = 3 -}\text{log}\frac{{\text{M}}_{\text{(}\text{ }\text{r}\text{ }\text{<}{\text{R}}_{\text{i}}\text{)}}}{{\text{M}}_{\text{tot}}}/\text{log}\frac{{\text{R}}_{\text{i}}}{{\text{R}}_{\text{max}}}$$

Where r is grain size, ${\text{M}}_{\text{(r}\text{ }\text{<}\text{ }{\text{R}}_{\text{i}}\text{)}}$is the cumulative mass of aggregates of size r less than ${\text{R}}_{\text{i}}$, ${\text{M}}_{\text{tot}}$is the total mass, ${\text{R}}_{\text{i}}$ is the aggregate size class, and ${\text{R}}_{\text{max}}$ is the mean diameter of the largest aggregate class.

The percentage of aggregate destruction (PAD) is calculated by the method of Zhang et al. (2001):

$$\text{PAD = }\frac{{\text{m}}_{\text{d}}\text{-}{\text{m}}_{\text{w}}}{{\text{m}}_{\text{d}}}\text{ × 100\%}$$

where ${\text{m}}_{\text{d}}$ is mass fraction of aggregates > 0.25 mm after dry sieving; and ${\text{m}}_{\text{w}}$ is mass fraction of aggregates > 0.25 mm after wet sieving.

The unstable aggregates index (E_LT) is calculated approximately by the equation (Zhang et al. 2017):

$${\text{E}}_{\text{LT}}\text{ = }\frac{\text{m-}{\text{m}}_{{\text{wr}}_{\text{0.25}}}}{\text{m}}\text{×100\%}$$

where $\text{m}$ is weight of soil sample, ${\text{m}}_{{\text{wr}}_{\text{0.25}}}$ is weight of > 0.25 mm water stable aggregates.

Crop yield and root biomass

Grain yield and biomass yield were measured at October. Because plastic-film mulch advanced maize development, different treatments were harvested in different time. Ten adjacent plants in each row in three randomly-selected adjacent rows of maize in each plot were cut off at ground level, the number of cobs counted before all the parts were oven-dried at 75°C to constant weight. Thirty ears in each plot except for border rows from each plot were randomly sampled for the determination of yield components: cob number, kernel number per cob and 100-kernel weight.

Root biomass was measured at the time of harvesting. Three soil columns in each plot were dug to 40 cm depth for root sampling; each column included a single maize plant. Each soil column had a surface dimension of 60 cm (The left and right sides are half away from the next plant) long × 22 cm (11 cm to one adjacent plant and 11 cm to the other adjacent plant in the maize row) wide. Using a sieve with aperture 0.25 mm to washing roots, then mix three sub samples of roots from the same plot. Finally, oven dried all root samples at 75°C to constant weight (Wang et al. 2018).

Statistical analysis

SPSS 22.0 was used to perform an ANOVA. Data obtained from each sampling event were analyzed separately. Mean comparisons were performed using the Fisher’s LSD test at P < 0.05. Structural Equation Modeling (SEM) was performed using the AMOS.

Soil bulk density and porosity

The soil bulk density (BD) with mulching treatments were lower than pre-treatment (PS) value after 8-year continuous film mulching, average decreased by 3.85%, while CK was only decreased by 0.15% (Fig. 4). After maize harvest in 2020, the BD value of each treatment was ordered as CK > P > R > D > S, which was lowest in 20–30 cm, and highest in 0–10 cm depth. Compared with CK, the average BD under film mulching (P, R, D, and S) were significant decreased by 3.68% (P < 0.05). Only S was significantly lower than P and R by 1.83% and 1.65%, respectively, and no significant difference among other film treatments.

Contrary to BD, the soil porosity (SP) with mulching treatments were all higher than PS value, average increased by 3.27% in 0–40 cm depth, and the difference increase gradually with the deepening of the soil layer (Fig. 4). The order of the value in each soil depth was S > D > R > P > CK, the film mulching treatments were all significantly (P < 0.05) higher than CK, average increased by 7.76%, 5.70%, 1.72%, and 2.01% under S, D, R, and P, respectively, but no significant difference among the four film treatments.

Figure 4.

Soil mechanical and water stable aggregates

After the 8-year continuous plastic film mulching, only > 5 mm size fraction significant increased with the soil layer depth. And the proportion of the soil mechanical stable aggregates in < 0.25 mm size fraction was highest in 0–40 cm depth, then 1-0.5, 2 − 1 and 5 − 2 mm, and the 0.5 − 0.25 mm was the lowest. Compared with pre-treatment (PS), each mulching treatments was increased the macroaggregate (> 0.25 mm) content by 8.64% in average, and also was 15.68% (P < 0.05) higher than CK (Fig. 5). In 0–10 cm layer, the S was significantly increased each aggregate fraction-size than that with other mulching treatments (D, R, and P), the macroaggregate (> 0.25 mm) content average increased by 10.38% (P < 0.05). The difference between mulching treatments increased gradually as the soil layer deepened. In 10–20 cm, S or D treatment was significant (P < 0.05) increased the proportions of aggregates in the size fractions 5 − 2, 1-0.5 and 0.5 − 0.25 mm by 18.80%, 14.35%, 16.82%, respectively, compared to P treatment. Meanwhile, the S or D treatment increased (P < 0.05) the percentages of aggregates in the > 5 and 5–2 mm fractions to R or P treatment in 20–30 cm. In 30–40 cm, the S or D significant increased (P < 0.05) in the > 5 mm fraction, but decreased in other fractions by 8.10% or 15.97% in average to R or P treatment.

Figure 5.

The proportion of the soil water stable aggregates in < 0.25 mm size fraction was highest in 0–40 cm depth, and the five treatments increased the macroaggregate content (> 0.25 mm) by 34.63% to pre-treatment in average (Fig. 6). In 0–40 cm layer, every mulching treatment increased the proportions of macroaggregate by 24.67%, 17.99%, 11.85%, and 8.49% with S, D, R, and P, respectively, compared to CK. The proportions of aggregates in the size fractions under mulching treatments was ordered as 1-0.5 mm > 0.5 − 0.25 mm > 2 − 1 mm > 5 − 2 mm, however, the 0.5 − 0.25 mm size fraction was highest and 5 − 2 mm lowest under CK. The small variation of soil water stable aggregate fraction-size distribution was found between mulching treatments in different soil layers, except the 5 − 2 mm fraction significant increased (P < 0.05) by 16.10% and 7.20% in average in 0–10 and 20–30 cm depth under S or D compared to R or P treatment, respectively.

Figure 6.

Mean-weight diameter (MWD) and geometric mean diameter (GMD) of soil aggregates

The variation of each treatment on soil MWD was different after 8-year continuous mulching (Table 1). The MWD of the mechanical-stable aggregates (MMWD) was increased gradually with the layer depth. Compared with pre-treatment (PS), only S and D were increased by 16.04% and 11.76% in average, while CK was 12.83% decreased in 0–40 cm layer. In 0–40 cm layer, very mulching treatments significant (P < 0.05) increased the MMWD by 33.13%, 28.22%, 19.63%, and 9.20% with S, D, R, and P, respectively, compared to CK. Contrary to the MMWD, the MWD of the water-stable aggregates (WMWD) was decreased gradually with the layer depth, and the five treatments, which was ordered as S > D > R > P > CK in each depth in 0–40 cm, increased by 10.53% to pre-treatment in average (Table 1). Compared with CK, mulching treatments average increased by 6.25%, and nearly no variation of was found between mulching treatments in different soil layers, except S and D significant (P < 0.05) increased by 11.11% and 6.67% in average in 0–10 cm depth compared to P treatment, respectively.

Table 1

Mean weight diameter (MWD) of soil mechanical-stable and water-stable aggregates in 0–40 cm depth under different treatments after 8-year continuous film mulching.
Treatment	Mechanical-stable aggregates (MMWD)					Water-stable aggregates (WMWD)
Treatment	0–10 cm	10–20 cm	20–30 cm	30–40 cm	0–40 cm AVG	0–10 cm	10–20 cm	20–30 cm	30–40 cm	0–40 cm AVG
PS	1.38	1.65	2.09	2.34	1.87	0.39	0.37	0.39	0.38	0.38
S	1.39a	1.82a	2.71a	2.77a	2.17a	0.50a	0.44a	0.42a	0.39a	0.44a
D	1.36b	1.77a	2.57a	2.67a	2.09a	0.48a	0.43a	0.42a	0.39a	0.43a
R	1.35b	1.72b	2.17b	2.55a	1.95a	0.47ab	0.42ab	0.41ab	0.39ab	0.42ab
P	1.31b	1.66b	1.98b	2.16bc	1.78bc	0.45b	0.41ab	0.41ab	0.39ab	0.41ab
CK	1.26c	1.56c	1.67c	2.04c	1.63c	0.42c	0.40b	0.39b	0.38b	0.40b
Note: PS, pre-sowing; S, planting furrows separated by consecutive plastic film-mulched ridges; R, traditional ridge-furrow rainwater harvesting planting; D, double ridge-furrow with full plastic film mulching; P, alternate mulching and bare rows without ridges; CK, flat planting without mulching. Different lower-case letters indicate significant differences at P < 0.05.

Table 1

The GMD of the mechanical-stable aggregates (MGMD) was far greater than that in water-stable aggregates (WGMD) after continuous 8-year mulching (Table 2). Compared with pre-treatment (PS), the MGMD under S, D, and R treatments was average increased by 28.26%, 21.74%, and 9.78%, respectively, however, CK was 13.04% lower. The MGMD under four mulching treatments was ordered as S > D > R > P in each depth in 0–40 cm, and average increased by 47.50%, 40.00%, 26.25%, and 11.25%, respectively, compared to CK treatment. Small variation was found between mulching treatments, and only S or D significantly higher than P in 20–30 or 30–40 cm depth by 48.7% and 35.40% in average. Different from MGMD, the WGMD of the five treatments were ordered as S > D > R > P > CK in each depth in 0–40 cm, increased by 7.59% to pre-treatment in average (Table 2). While each mulching treatments were all higher than CK, and average increased by 5.01%, however, no significant difference among four mulching treatments.

Table 2

Geometric mean diameter (GMD) of soil mechanical-stable and water-stable aggregates in 0–40 cm depth under different treatments after 8-year continuous film mulching.
Treatment	Mechanical-stable aggregates (MGMD)					Water-stable aggregates (WGMD)
Treatment	0–10 cm	10–20 cm	20–30 cm	30–40 cm	0–40 cm AVG	0–10 cm	10–20 cm	20–30 cm	30–40 cm	0–40 cm AVG
PS	0.66	0.78	1.02	1.22	0.92	0.30	0.29	0.29	0.29	0.29
S	0.72a	0.92a	1.51a	1.57a	1.18a	0.34a	0.32a	0.31a	0.30a	0.32a
D	0.69ab	0.88a	1.40a	1.49a	1.12a	0.33a	0.32a	0.31a	0.30a	0.32a
R	0.67ab	0.85a	1.10b	1.41a	1.01ab	0.33ab	0.31ab	0.31ab	0.30a	0.31ab
P	0.65ab	0.80ab	0.98b	1.13b	0.89bc	0.32ab	0.31ab	0.31ab	0.30a	0.31ab
CK	0.61b	0.72b	0.82c	1.04c	0.80c	0.31b	0.31b	0.30b	0.29a	0.30b
Note: PS, pre-sowing; S, planting furrows separated by consecutive plastic film-mulched ridges; R, traditional ridge-furrow rainwater harvesting planting; D, double ridge-furrow with full plastic film mulching; P, alternate mulching and bare rows without ridges; CK, flat planting without mulching. Different lower-case letters indicate significant differences at P < 0.05.

Table 2

FD Fractal dimension (FD) of soil Aggregate

The fractal dimension (FD) with mechanical- or water-stable aggregates have a range of values from 2.64–2.80 or 2.91–2.96, and each mulching treatments decreased FD value in 0–40 cm layer, compared to PS (Fig. 7). The FD with mechanical-stable aggregates (MFD) of five treatments was ordered as CK > P > R > D > S, mulching treatments decreased by 2.10% in average than CK in 0–40 cm layers. The variation between four mulching treatments was gradually decreased with the depth in 0–40 cm layer, in 0–10 cm, the S was significantly lower than D, R, or P by 1.25%, 1.89%, and 2.46%, respectively. Contrary to the MFD, the FD with water-stable aggregates (WFD) was gradually increase with soil depth. Mulching treatments was ordered as P > R > D > S in 0–40 cm depth, which were lower than CK by 0.36% in average, and no significant differences were observed among the four mulching treatments.

Figure 7. Fractal dimension (FD) of soil Aggregate at 0–40 cm depth under per-treatment and different film mulching after 8-year continuous film mulching.

Percentage of aggregate destruction (PAD) and unstable aggregates index (E_LT)

Continuous plastic film mulching decreased the percentage of aggregate destruction (PAD) in 0–40 cm layer by 6.39%, compared to pre-treatment (PS) (Table 3). Different variation was found in different soil depth. In 0–20 cm, each mulching treatment was higher than that in CK, and the greatest increase showed in 10–20 cm by 2.81%. Together with the soil layer's deepening, mulching treatments (except R) gradually lower than CK, and average decreased by 0.64% in 30–40 cm. Overall, nearly no difference observed among different four mulching treatments in different soil depth.

Table 3

Percentage of aggregate destruction (PAD) and Unstable aggregates index (E_LT) in 0–40 cm soil depth under different treatments after 8-years continuous film mulching.
Treatment	Percentage of aggregate destruction (PAD)					Unstable aggregates index (E_LT)
Treatment	0–10 cm	10–20 cm	20–30 cm	30–40 cm	0–40 cm AVG	0–10 cm	10–20 cm	20–30 cm	30–40 cm	0–40 cm AVG
PS	71.78	78.50	81.67	84.93	78.92	85.28	87.77	88.42	89.43	87.23
S	63.34a	72.01ab	77.71a	81.58a	74.33a	77.91b	81.41b	83.15b	85.71b	82.05b
D	63.31a	71.77ab	77.33a	81.57a	74.29a	79.22ab	82.02ab	83.22b	85.90b	82.59ab
R	63.56a	72.30a	76.57a	82.11a	74.43a	80.13ab	83.12ab	84.19b	86.55ab	83.50ab
P	64.10a	71.81ab	76.25a	80.56a	73.81a	81.12ab	83.68ab	84.64ab	86.53ab	83.99ab
CK	63.08a	70.01b	77.60a	81.97a	74.07a	82.35a	84.25a	86.31a	88.06a	85.24a
Note: PS, pre-sowing; S, planting furrows separated by consecutive plastic film-mulched ridges; R, traditional ridge-furrow rainwater harvesting planting; D, double ridge-furrow with full plastic film mulching; P, alternate mulching and bare rows without ridges; CK, flat planting without mulching. Different lower-case letters indicate significant differences at P < 0.05.

Similar with PAD, the unstable aggregates index (E_LT) of each treatment in 0–40 cm soil layer was average decreased by 5.09%, compared to pre-treatment (PS) (Table 3). Four mulching treatments was ordered as S < D < R < P in 0–40 cm depth, which decreased the E_LT by 2.74% (P < 0.05), 2.51% (P < 0.05), 1.74%, and 1.77% with S, D, R, and P, respectively, compared to CK. And no significant difference (P > 0.05) observed among different four mulching treatments in different soil depth.

Table 3

Root system characteristic parameter

Maize roots system gradually increased with the growth process, and reached the maximum in 120 days after planting (DAP) of each treatments. During 2019–2020, film mulching treatments significantly (P < 0.05) increased root length (RL), root diameter (RD), root surface area (RS), and root volume (RV) compared with CK, and both of these in S and D were better than that with R and P (Fig. 8). In 2019, the RL of D treatment was 12.02%, 25.82% and 21.06% higher than that under S, R and P in early growth stage (0–30 DAP), respectively, while there were no significant difference in RD and RV between mulching treatments. After sowing 60 days, the RL of each mulching treatments increased to 1996.07-2284.03 cm with the growth process moving forward, and no significant difference between mulching treatments; the RD of S treatment was 11.11% (P < 0.05) and 18.86% (P < 0.05) higher than R and P treatment, respectively. And the RS of S and D were significantly higher than that under R and P by 32.20% and 20.72%; while the RV of S treatment significantly (P < 0.05) increased by 16.40% than R treatment. After sowing 60–120 days, the root growth under S treatment gradually better than others, i.e. the RL was significantly (P < 0.05) increased by 6.30%, 13.30% and 15.34% compared with D, R and P treatment, respectively, and RD, RS and RV average increased by 15.84%, 32.25%, and 24.50%, respectively. The effects of mulching treatment on root characteristics in 2020 was similar to 2019.

Figure 8.

Root biomass (RB), grain and biomass yield (GY and BY)

The film mulching significantly affected maize grain yield (GY) and biomass yield (BY) during experimental years (Fig. 9). Compared with CK (GY = 7.48 t ha^− 1 and BY = 17.92 t ha^− 1), the GY of each film mulching treatments (S, D, R, and P) were significantly (P < 0.05) increased by 63.96%, 61.81%, 30.99%, and 29.98%, and BY were increased by 44.31%, 44.22%, 22.14%, and 19.26%, respectively. The GY order of mulching treatments was S > D > R > P. The variation coefficient of GY with S and D were lower than R and P, and the S were significant (P < 0.05) increased by 25.17% and 26.14%, and D were 23.53% and 24.49%, respectively. Similar with GY, the BY of S and D were significantly higher than R and P, average increased by 18.12% (P < 0.05) and 20.97% (P < 0.05).

Figure 9.

Film mulching enhanced root dry mass during 2013–2020, and the interaction between the mulching and experimental year was significant (Fig. 10). The mean values with film mulching treatments were all higher than CK, the difference increased gradually with the increase of mulching years, which averaged increased by 47.25% (P < 0.05), 43.59% (P < 0.05), 25.15%, and 22.87% under S, D, R and P, respectively. In each experimental year (except 2013), the order of mulching treatments was S > D > R > P. S and D were significantly higher than R and P in most experimental years (except 2013 and 2018), average increased by 18.13% and 23.39%, and the variation coefficient was also significantly lower (28.61% in average).

Figure 10.

Correlativity

The structural equation modeling (SEM) indicated that all predictor variables explained 94% of variations in crop yield. Film mulching method showed significantly effects on root biomass and crop yield (0.76 and − 0.26, respectively) (Fig. 11A). Root biomass showed significantly positive effects on macroaggregate content, which showed significantly positive effects on soil aggregate stability and crop yield, and negative effects on soil bulk density. And soil bulk density suppressed crop yield (Fig. 11A). In general, film mulching method had positive total effects on root biomass, soil aggregate stability, macroaggregate content and crop yield, and negative total effects on soil bulk density (Fig. 11B).

Figure 11.

Previous studies have shown that plastic film mulching can reduce soil bulk density and improve soil porosity (Tomasz et al. 2008; Gao et al. 2021). In our study, it was also found that plastic film mulching significantly reduced soil bulk density (3.68%) and increased soil porosity (3.27%) compared with PS after eight years. This may be because the soil surface is covered with plastic film could significantly reduce soil water evaporation (Zhang et al. 2020a) to increase the soil water content, while soil bulk density decreased with increasing water content (Eden et al. 2020). Therefore, the soil bulk density under S and D mulching treatments was lower than that under the other two mulching treatments, because the soil covered areas under S and D mulching treatments were larger than those under R and P treatments, and the improve effects on soil moisture were more significant. In addition, we found that S and D treatments were higher than R and P treatments in terms of soil porosity, because the underground partial with S and D treatments growth vigorous, root system developed, and making a loose and porous soil for crop growth (Oliveira et al. 2001). Our study also found that soil bulk density in 0–10 cm depth was higher than other soil layers after 8 years of continue film mulching, which might be due to the interference of human factors in the surface soil, and the compacted soil by human increased the soil bulk density.

Numerous studies have shown that the > 0.25 mm aggregates (macroaggregate) content was positively correlated with soil fertility, and it can be used as an evaluation index of soil aggregate stability and soil erosion resistance (Ai et al. 2021). Wang et al. (2017) found that continued film mulching significantly increased the content of macroaggregates in semiarid regions. In our study also found that the content of macroaggregates was significantly increased in 0–40 cm depth after 8 years of continuous film mulching. This because film mulching could significantly improve the crop root properties in topsoil by promoting the soil hydrothermal condition (Zhang et al. 2020a), and that could significantly influence soil aggregation characteristic from physical, chemical and biological (Wang et al. 2017). Our study also found that the soil macroaggregates content under S treatment is highest among all film mulching treatments. Because large amount of root biomass will lead to a significant increase in the organic matter that returned to topsoil under S treatment (Fig. 10). Many previous studies showed that the organic matter is an important intrinsic factor in the formation of soil aggregates (Tisdall et al. 1982; Duan et al. 2021), and its increase promoted the formation of soil macroaggregates, especially those with > 5 and 5 − 2 mm sizes in our experiment. In this study, we also found that the content of mechanical stable aggregate was significantly higher than that of water stable aggregate (Fig. 4, 5), this was similar with mostly studies in the same regions (Zhang et al. 2014; Fu et al. 2016). This is probably due to the soil type is loessial in our experimental site, which most of its components were silt and clay, and therefore the ability of soil to resist water erosion is lower than that to resist external mechanical damage.

Soil aggregate stability is an important index for evaluating soil structure, which was generally expressed by mean weight diameter (MWD) and geometric mean diameter (GMD) (Ai et al. 2021), and the fractal dimension (FD) was also introduced in recently studies (Tagar et al. 2019). Previous researches shown that plastic film mulching could increase the stability of soil aggregate (Wang et al. 2017; Huo et al. 2017). In our study, after 8-year continuous film mulching, the value of MWD and GMD were increased and FD decreased in all soil layers compared with CK, which was due to the significantly root biomass increase (an average increase of 34.72%, Fig. 10) by comfortable soil hydrothermal condition. That is partly because the role of root during aggregate formation can not only as a connection network (Li et al. 2014), but also can released root exudates as cementing substance to promote the macroaggregate formation (Bronick et al. 2005). Moreover, the good soil hydrothermal conditions promoted the increase of soil microbial community (Jastrow et al. 1998; Yang et al. 2012), especially the fungus (Jastrow et al. 1998), and the fungal mycelium has benefits for soil small- and micro-aggregates cementing into the large aggregates (Tian et al. 2022). Our study also found that different film mulching treatments have different effects on soil aggregate stability, which soil aggregate stability under S and D treatments were all higher than that under R and P. Because the mulching area of S and D were almost twice larger than that under R and P. The bigger mulching area could provide a better soil hydrothermal conditions during crop vegetative growth stage to promote crop root and shoot growth (Zhang et al. 2020a) in the semiarid cold area, which would result in more crop residues (mostly is plant root) remained in soil, further a binding agent influencing soil macroaggregate contents and stability. But dissimilar results were found by Zhang et al. (2015), the different mulching areas with vegetables significantly affected soil organic matter content in red soil but not effectively affected soil aggregate stability. This is mainly related to the different soil textures and use patterns in different studies. In addition, compared with CK, film mulching could protect soil from physical damage, i.e. the value of E_LT was significantly decreased in 0–40 cm soil layers. The mulching film can keep rain drops from hitting the soil directly and therefore improve the stability of topsoil structure (Wang et al. 2017). The protective effect was different between different mulching treatments. The S, D and R were more efficient than that under P, which was a result of using ridge–furrow system in field. Because the rainfall flows along the mulching ridges into furrows in ridge-furrow system, which have a remarkable weakening in the impaction of rainwater on soil. Thus, the soil aggregate stability under these three treatments (S, D, and R) were better than that under P treatment, especially under S and D, which could be because the bigger mulching area have the stronger impact resistance. In our study, the influence of mulching treatment on PAD was various in different soil layers, and the PAD was increased in topsoil (0–10 cm) but decreased in deep layer (20–40 cm). This may be related to the fact that film mulching stimulates topsoil wet-dry alternation (Zhang et al. 2019), which leads to the destruction of soil aggregates (Park et al. 2007). Moreover, all the tillage practices were applied above 20 cm layer, which also significant increase the PAD of topsoil.

In summary, the results of our research showed that film mulching treatments, especially S, could increase soil macroaggregate (> 0.25 mm) content and soil aggregate stability in 0–40 cm soil layer. It can strengthen soil structure, and improve the capability of soil anti-erosion, which is particularly important for the Loess Plateau of China, an area with serious soil erosion.

Root system is the most important part for crop growth which affecting crop productivity (Gong et al., 2020). In this study, film mulching significantly improved maize root system characteristic parameter, because water is the main restrictive factor for root growth in semi-arid areas, while film mulching significantly increased soil water content (Zhang et al., 2020), thus improved maize root growth (Zhang et al., 2022). Gao et al. (2014) found that film mulching could affect root growth and development by improving soil structure, and physical and chemical properties, increasing soil temperature and enhancing nutrient cycling in the soil. In our study, different film mulching methods had different effects on root growth after 8 consecutive years. Overall, the root system characteristic parameters under S and D treatment were significantly higher than R and P, and R was better than P treatment. Compared with flat mulching (P), ridge-furrow film mulching (S, D, R) probably improved root growth water conditions by reducing soil water evaporation, and accumulating and storing rainfall, while S and D treatment had lager mulching area than R which made a better effects and retained more water in maize root area, thus improved the water use efficiency and promoted maize root growth (Gao et al., 2014, Zhang et al., 2020). We also found that in the early growth stage (0–30 DAP), the RL of D treatment was higher than S; but in the later growth stage, the RL, RD, RV and RS of S treatment were all higher than these in D. Because full-film mulching (D) probably caused oxygen deprivation after maize grown for a period of time, which limited root respiration and reduced root growth rate (Zhang et al., 2021). In addition, the difference of root growth under different film mulching treatments during 0–60 DAP was less than that during 60–120 DAP. In 0–60 DAP, each film mulching treatments can provide sufficient soil water for root growth due to the lower filed water consumption. In 60–120 DAP, the decrease of rainfall and the increase of temperature lead to the increase of water evaporation and filed water consumption, so the difference of water retention performance among different film mulching treatments increased gradually (Zhang et al., 2020), which lead to the increase of difference in root growth. And this is probably another reason of the advantage of root growth under S and D will increased gradually with crop growth process.

Lots of previous studies have shown that film mulching could significantly increase crop yield by improving soil hydrothermal condition and soil physical and chemical properties (Zhou et al. 2015; Wu et al. 2017; Haque et al. 2018). Similar in our study, after 8 years continuous film mulching, the maize grain and biomass yield both significantly higher than that under CK (Fig. 9). Because film mulching could significantly improve topsoil hydrothermal condition to promote crop growth and development, thereby increasing maize photosynthetic efficiency (Jia et al. 2018). In addition, film mulching may also increase crop yields by improving the topsoil physical structure (Tomasz et al. 2008; Jia et al. 2018), as our study also found that maize yield has significantly related to soil water stable aggregates stability and PAD. We also found that, between different four film mulching treatments, the S and D treatments were all significantly produced the higher grain and biomass yield as compared with R and P treatments in each experimental year. Similar result was indicated for comparison of different film mulching areas (Zhang et al. 2019). Because with the mulching area increase, the water use efficiency and water productivity of maize growth can be significantly improved (Zhang et al. 2020a), thus promoted crop shoot properties (i.e. leaf area, biomass, plant height, etc.). On the other hand, the soil structure (bulk density, soil porosity, soil aggregates content and stability, etc.) also can be significantly improved, which has been proved to be beneficial to the crop root growth and development (Jia et al. 2018).

This eight-year field experiment demonstrates that, compared with non-mulched treatment (CK), the long-term film mulching decreased the soil bulk density, improved soil porosity, and reduced the damage of the soil. The aggregation of microaggregate and soil aggregate stability were also generally improved under film mulching. Moreover, the soil aggregate fractions and stability were significantly correlation with crop yield and root biomass, and the treatment effects were more apparent for S than for others (D, R and P). Our findings suggested that the S treatment enhanced soil aggregate stability and crop yield and improved the anti-erosion ability. Therefore, the S (planting furrows separated by consecutive plastic film-mulched ridges) treatment represents an effective mulching pattern to mitigate crop yield change and improve soil quality in dryland of Northwest China.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This research was supported by the National Key Research and Development Program of China (2021YFE0101302), the National Natural Science Foundation of China (Nos. 31801314, 31901475), and the Postdoctoral Science Foundation of China (2021M702697, 2019T120951). We are grateful to Junfeng Nie, Hui Li, Baoping Yang, and Ruixia Ding for help with the experiments. We also would like to thank every reviewer.

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Continuous film mulching enhances maize root growth and grain yield by improving soil aggregates characteristic in semiarid areas of China: An eight-year field experiment

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