Variation of Subsoiling Effect at Wing Mounting Heights on Soil Properties and Crop Growth in Wheat–Maize Cropping System

Abstract

Subsoiling is an effective practice to promote better soil water infiltration and crop growth. The information regarding the variation and persistence of subsoiling effects with different soil disturbance on soil properties and crop growth was absent in previous research. In this study, the effects of wing mounting height (h) (75–155 mm) during subsoiling on soil disturbance at various layers, soil properties and crop performance were investigated using in situ field experiments from 2019 to 2021 in winter wheat–summer maize rotations. The experimental field soil was covered with crop stubble and crop straw was removed before subsoiling or sowing the next crop. The analysis of variance (ANOVA) was used to assess different subsoiling treatment effects on tested variables, including soil moisture content, bulk density, plant diameter, plant height, dry root weight, root number, yield, and yield components of crops. Means between subsoiling treatments were compared using Duncan’s multiple range tests. Based on ANOVA outputs, the h significantly (p < 0.05) affected soil moisture content in the first growing season of winter wheat (WW1), soil bulk density, number of roots and panicle number and grain yield of WW1, and thousand kernel weight (TKW) of both WW1 and summer maize (SM). Decreasing h overall gave better soil properties and crop performance. Decreased subsoiling effects were found on aspects of insignificant difference in subsequent grain yield of SM and winter wheat in the second growing season (WW2). These findings had implications for designing higher-performance winged subsoilers, as well as selection of subsoiling frequency.

1. Introduction

With the improvement of the level of mechanization in China, soil is gradually compacted due to the frequent operation of heavy agricultural machinery and tools in the farmland [1,2,3]. As one of the core technologies of conservation tillage, subsoiling practice has been employed to remove soil compaction and restore soil productivity [4,5,6,7]. Subsoiling technology mainly consists of natural, chemical, biological, and mechanical methods [2,8]. The natural method generally relies on the natural recovery of field soil, e.g., the freeze–thaw effect of soil and the movement of earthworms in soil. The biological method mainly includes the mixing of soil and crop residue and continuous seeding of a variety of plants with highly developed root systems. The chemical method is generally achieved by applying fertilizer and lime to the soil. By contrast, various subsoiling tools can be used to break hardpans and reduce the soil bulk density in accordance with the mechanical method. The natural and biological methods require a long-term process, during which time crop cultivation should be stopped; thus, the crop production will be reduced. The effect of chemical subsoiling on crop growth is not significant and additional fertilization is economically inefficient and may cause environmental pollution. Mechanical subsoiling is the most widely used and the quickest way to disturb hardpans and remove soil compaction [8]. This subsoiling method could improve soil structure and properties and promote crop roots’ growth and the ability of crops to absorb nutrients and water from soil [2,9].

Winged subsoilers are important subsoiling tools during subsoiling practice [2,9,10]. A study from Song et al. [2] showed that a mole’s claw-inspired winged subsoiler could increase the disturbance width of hardpan soil as compared with a traditional winged subsoiler. Wang et al. [9] found that wings of the subsoiler mainly affected the disturbance area and fragmentation degree of soil above them. Hang et al. [11] reported that a winged subsoiler had better soil loosening performance on aspects of the soil fragmentation degree and disturbance area, as compared with a chisel subsoiler; a similar phenomenon was also found in the study of Wei et al. [12], who demonstrated that wings improved soil loosening quality by re-shearing the soil cut by a chisel tine of a subsoiler. Zhen et al. [13] found that a winged subsoiler could construct a stratified structure inside the soil. Gao and Li [14] reported that the penetrating distance of winged subsoiler was significantly affected by wing mounting height. Godwin et al. [10] showed that attaching wings to the shank opens the shank failure cracks and improves soil loosening quality near the soil surface. Xia [15] found that the ridge and width of disturbed soil after subsoiling was significantly affected by the wing rake angle and mounting height. The aforementioned studies indicated that wing parameters of the subsoiler are critical for the disturbance behaviours of soil.

A reasonably higher amount of soil disturbance is considered to favor the reduction of bulk density difference of topsoil [16]; moreover, the extent of the breakage and disturbance of hardpan soil plays an important role in promoting soil water infiltration [15,17], growth of crop roots, and the ability of crops to absorb nutrients and water from soil [2], respectively. However, an ideal subsoiling tool would disturb hardpans and leave most soil undisturbed, in terms of lower draft force requirement and energy consumption [18]. Additionally, less topsoil disturbance helps to conserve soil and reduce soil water evaporation [17,18,19]. The disturbance extent of soil at different depths often are used as an important indicator for subsoiler performance evaluation, and some studies have demonstrated that soil properties had significant effects on crop growth [6,16]. However, there are few reports on the variation and persistence of the impacts of soil disturbance amount at different depths (after subsoiling practice) with time.

Wing mounting height is one of the key wing parameters of the subsoiler in terms of soil disturbance area, hardpan loosening, and tillage forces, in accordance with previous studies [14,15]. Moreover, the winter wheat–summer maize rotation system tested in this study is one of the main cropping systems in North China [20,21,22]. Therefore, this study aimed to: (1) assess soil disturbance characteristics with various wing mounting heights and evaluate, in key crop growing stages, the variation of its effects on soil properties and crop growth using three-season field experiments and (2) provide implications for the selection of subsoiling frequency and optimization of the design of winged subsoilers on aspects of the variation of soil properties and the performance of crops.

2. Materials and Methods

2.1. Experimental Site

A field experiment was conducted from 2019 to 2021 at Wugong Dryland Agricultural Research Station (latitude 34°16′ N; longitude 108°12′ E) in Shaanxi Province. Winter wheat and summer maize are two common kinds of crops in the annual double cropping system and are grown every year in this region. Prior to the test, continuous winter wheat and summer maize rotations were carried out, and the conventional tillage with residue removal was found every year after crop harvest. The soil used in the experimental field is a Lou soil, which is a typical soil type in Northwest China [9]. In the top 40 cm layer of experimental field, soil bulk density and moisture content were 1.63 g cm⁻³ and 22.44%, respectively. The hardness of the hardpan soil in the depth of 16–30 cm was > 1.8 MPa. The high soil hardness of the hardpan might be a limitation to the growth of roots of the tested winter wheat and summer maize [23,24], which can usually reach a depth of >40 cm [25]. The precipitation and temperatures during the tests are shown in Figure 1.

2.2. Field Subsoiling Test Design

Before subsoiling test, the field soil was covered with maize stubble (0.77 t ha⁻¹, dry basis) and maize straw was removed. The subsoiler tested consisted of a pair of wings and an arc-shaped shank selected based on the Chinese standards (JB/T 9788-1999) (Figure 2A). The rake angle (α) and width (W_s) of the subsoiler were 23° and 202 mm, respectively. The summer maize and winter wheat varieties used in the field subsoiling test were Liangyu 99 and Xiaoyan 22, respectively, which were cultivated by Denghai Liangyu Seed Industry Co. Ltd. And Northwest A&F University, respectively. The study was carried out for 1.5 cycles, two crops of winter wheat and one crop of summer maize (i.e., winter wheat—summer maize—winter wheat). Seeding dates were 27 October 2019 for the first winter wheat growing season (WW1) and 28 October 2020 for the second winter wheat growing season (WW2). Harvesting dates were 7 June 2020 for WW1 and 5 June 2021 for WW2. Summer maize (SM) was grown during the period from 15 June 2020 to 25 October 2020. Only stubble was left, and crop straw was removed before sowing the next crop during the experiment.

Before the WW1, six subsoiling treatments were carried out with wing mounting height (h) ranging from 75 to 155 mm (Figure 2A); subsoiling with a non-winged subsoiler was used as a control treatment. During field subsoiling operations, tillage depth, working speed, and the spacing between neighboring tools were 30 cm, 0.83 m s⁻¹, and 55 cm, respectively (Figure 2B). Each plot is 50 m long and 9 m wide and each subsoiling treatment was repeated three times. After subsoiling treatments, the winter wheat was seeded at 5 cm depth with a seeding density of 4.81 million plants per hectare using a 2BMFD-6/12 machine (Figure 2C). After harvesting of WW1, SM was then seeded at 5 cm depth with a seeding density of 6.67 × 10⁴ plants per hectare using a 2BMFD-7/14 machine (Figure 2D). Fertilizers were applied once to soil before sowing by the above two machines. To avoid the effects of other tillage practices on subsoiling treatments, no-till seeding was conducted for the SM and the WW2.

2.3. Variables Measurement and Analysis

2.3.1. Disturbance Area of Soil

Soil disturbance profiles after subsoiling tests were measured by a soil disturbance profiler with free-dropping pins (1 cm wide). The specific measuring method can be found in previous studies [9,11,15]. The soil disturbance areas of both the top layer and hardpan soil were finally calculated in accordance with the areas within the corresponding soil disturbance profiles measured above (Figure 3A). The measurement of soil disturbance areas at various layers were repeated three times. The soil disturbance area ratio was used to reflect the characteristics of soil disturbance at various layers, and it was defined as A_h/A_t, where A_h and A_t stand for disturbance areas of hardpan and top layer, respectively.

For winter wheat, soil moisture content in filling and wintering stages greatly affects thousand kernel weight (TKW) and panicle number, respectively [22,23]. By contrast, for SM, soil moisture content in tasseling and filling stages seriously affects grain number per panicle and TKW, respectively [22,24]. Therefore, all soil samples were collected (3 repetitions) in wintering and filling stages for winter wheat and in tasseling and filling stages for SM. Moreover, soil samples for moisture content measurement were collected from depths of 0–10, 10–20, 20–30, 30–40, 40–60, 60–80, and 80–100 cm. To investigate the effects of subsoiling treatments on the variation of soil bulk densities, samples for soil bulk density measurement were collected from depths of 0–10, 10–20, 20–30, and 30–40 cm in wintering, filling, and harvesting stages for winter wheat, and in tasseling, filling, and harvesting stages for SM. The soil samples from various depths were weighed wet, dried for 48 h at 105 °C, and weighed again to determine the soil bulk density and moisture content according to the soil weight before and after drying (Equation (1)).

$$M_s=\frac{m_w-m_d}{m_d}\times 100$$

(1)

where M_s was soil moisture content, %; m_w and m_d were wet and dried soil weight, respectively (g).

2.3.2. Crop Growth Conditions

The emergence rate (ER) was measured in accordance with the seeding density and crop density in seeding stage (about three weeks after seeding). Three random 1 m² areas of winter wheat seedlings and three random 3 m² of SM seedlings were collected respectively, for the ER measurement in each region. In the filling stage of winter wheat, the number of roots at various depths (ranging from 0–40 cm) was measured at 3 random locations using a transparent film (TF) (Figure 3B). First, a trench (100 cm in length and 60 cm in depth) was excavated at a 5 cm lateral location from the row center of winter wheat; then, the TF was placed on the trench profile; eventually, the locations of winter wheat roots at various depths were recorded on the TF. In filling stage of SM, twelve maize plants with similar growth conditions were selected for the measurement of roots, height, and diameter of maize plant in the middle of each plot. Root samples were dug out and collected from a 35 cm deep soil depth and then they were oven-dried at 65 °C to constant weight and weighed again to determine the root dry weight.

2.3.3. Crop Yield and Yield Components

Winter wheat initially was taken randomly from three 1 m² areas in each plot; then they were manually harvested, threshed, and dried to determine wheat grain yield at 14% moisture content, panicle number, and thousand kernel weight (TKW). SM grain yield was determined (14% moisture content) by manually harvesting three 3 m long rows taken randomly in each plot. The maize yield components, including length (L_p) and diameter (D_p) of panicles (Figure 3C), kernel number and kernel weight per panicle, and TKW, were measured from the collected maize plants.

2.4. Statistical Analysis

The SPSS analytical software package (SAS Inst., Cary, NC, USA) was used for all of the statical analyses. Average values were calculated for each set of experimental data, and ANOVA was used to assess different subsoiling treatment effects on tested variables. Means between subsoiling treatments were compared using Duncan’s multiple range tests.

3. Results and Discussion

3.1. Characteristics of Disturbance Area of Soil

The soil disturbance area ratio increased with decreasing wing mounting height (h) (Figure 4); additionally, the soil disturbance area ratios from the winged subsoiler were much larger compared to that of the non-winged subsoiler. Soil cutting tools with a smaller tillage depth/tool width ratio (<5) would loosen soil in a crescent manner [9]. The soil was loosened in a crescent manner by winged subsoiler, as the corresponding critical depth is larger than the tillage depth of 30 cm, as indicated by a tillage depth/tool width ratio of 1.49. Removing two wings from the shank of the subsoiler gave a much larger tillage depth/tool width ratio of 7.5 and only a small crescent near to the surface was produced, which resulted in much a smaller soil disturbance area of hardpan. This explained the much smaller soil disturbance area ratio from the non-winged subsoiler. With the decrease of h, the soil disturbance area of hardpan was increased, which resulted in larger soil disturbance area ratios with decreasing h.

3.2. Soil Moisture Content

An overall decreasing trend of soil moisture contents was found from Figure 5, with increasing soil depths from 0 to 100 cm in filling stage of both WW1 and WW2. This might be due to the increased demand of winter wheat for soil water at deeper locations during the period. In contrast, soil moisture contents initially decreased and then increased with the increase of soil depths in the filling stage of SM. The soil moisture contents were mostly larger for regions of subsoiling with smaller h values in various stages of WW1 and SM. However, for WW2, soil moisture contents at various depths fluctuated frequently with the increase of h values and no particular trend was found in various stages.

For WW1, the h significantly (p < 0.05) affected mean moisture contents of the top 40 cm (D40) and the top 100 cm (D100) soils (Figure 6); overall lower soil moisture contents of D40 and D100 were found with the increase of h values. Moreover, compared with larger h in the wintering stage of WW1, soil moisture contents of D40 and D100 with smaller h (≤115 mm) were 1.71–10.93% and 1.10–9.63% higher, respectively. Furthermore, soil moisture contents of D40 and D100 in the filling stage of WW1 with smaller h (≤95 mm) were 2.04–7.26% and 3.85–7.99% higher, respectively. By contrast, for SM and WW2, mean moisture contents of D40 and D100 soils were not significantly affected by h, except for the significantly lower mean soil moisture content of D40 in the tasseling stage of SM, with h of 135 mm. This may result from the decreased subsoiling effects due to three-season no-till sowing practices. The above results implied that the soil moisture content at various stages of crops was more or less affected by h, especially in the WW1.

As shown in Figure 5 and Figure 6, the soil wilting point moisture content of <7.2% [26] was much lower than the above soil moisture contents measured in various stages of both winter wheat (WW1 and WW2) and SM. Moreover, the collected soil moisture contents at various depths were mostly smaller than the soil field capacity moisture content (22%) [26]. This implied that for the given soil, the growth conditions of winter wheat and SM could be improved by optimizing subsoiling treatment and raising soil moisture content.

3.3. Soil Bulk Density

Subsoiling can be used to overcome soil compaction constraints and restore the soil bulk density for better water infiltration and gas exchange [23,27,28]. Variations of soil bulk densities at various depths were different. The bulk density of 0–10 cm soil was not significantly (p > 0.05) affected by h during the three crop growing seasons tested, and bulk density of 30–40 cm soil fluctuated with varying h in most stages of crops (Figure 7). The same level of soil bulk density in the 0–10 cm from the surface may be attributable to the similar disturbance of top layer soil under different subsoiling treatments and high standard deviations (SDs). In contrast, soil bulk densities in depths of 10–20 cm or 20–30 cm were significantly smaller for these with lower h (≤115 mm) in most cases. These differences could be due to the delay in soil re-compaction because of more hardpan disturbance. The above results and comparisons may show that decreasing h during subsoiling practices is favorable for reducing subsequent bulk density of 10–30 cm soil in key growing stages of crops.

3.4. Emergence Rate and Crop Growth

As shown in Figure 8A, the various subsoiling treatments showed no significant effects on emergence rate (ER) of both winter wheat and summer maize (p > 0.05). The ERs with the same levels under different subsoiling treatments might be attributed to highly variable data with high SDs. The ERs of SM were 5.59–31.23% and 8.02–27.15% higher than these of WW1 and WW2, respectively. A similar phenomenon was also found in the study of [29] which showed that the ERs of maize was 20.3–45.2% higher than those of wheat under various treatments. These phenomena could result from a combination of the following factors: moisture contents of soil in wintering stage, the performance of sowing machines for winter wheat and SM, and the surface flatness before and after subsoiling treatments.

The SM growth conditions, including root dry weight, plant diameter, and plant height, were not significantly affected by wing mounting height (h) (Figure 8B). In the filling stage of winter wheat, root number initially increased and then decreased with increasing soil depths (0–40 cm), except for h of 135 mm and 155 mm in the second growing season (2020–2021) (

Table 1. Number of roots of winter wheat from two growing seasons.

Soil Depth (cm)		h0	h75	h95	h115	h135	h155
0–8	WW1	40.7 a	66.3 a	47.0 a	53.7 a	54.3 a	50.3 a
	WW2	35.3 a	36.3 a	42.0 a	41.0 a	43.3 a	45.3 a
8–16	WW1	56.7 a	93.7 a	95.0 a	84.3 a	89.3 a	55.7 a
	WW2	36.3 a	45.7 a	53.7 a	49.0 a	37.7 a	32.7 a
16–24	WW1	24.7 b	50.0 a	55.7 a	49.7 a	59.0 a	32.7 b
	WW2	19.0 a	30.3 a	22.3 a	23.3 a	29.3 a	19.0 a
24–32	WW1	15.7 ab	24.3 a	27.0 a	16.7 ab	18.3 a	6.00 b
	WW2	13.7 a	16.7 a	19.3 a	13.7 a	12.7 a	12.3 a
32–40	WW1	2.3 b	20.7 a	20.0 a	10.7 ab	9.3 ab	6.7 b
	WW2	6.3 a	4.7 a	6.3 a	6.7 a	4.3 a	5.3 a

The different superscripted letters in the table stand for (a–b) mean significant difference at p < 0.1.

). A study from [30] investigated the effects of tillage methods on crop root distribution and found similar phenomena. Increasing h values overall reduced winter wheat roots, possibly due to larger bulk densities of soil resulting from more hardpan disturbance (Figure 4 and Figure 7). Furthermore, the h values only affected the number of winter wheat roots at the deeper locations (ranging from 16 to 40 cm) significantly (p < 0.1) in the first growing season of winter wheat (WW1). The above analyses indicated that better growing conditions of winter wheat were associated with smaller h values in terms of larger wheat root number at various depts.

3.5. Crop Yield and Some Yield Components

Overall, increasing h values gave lower grain yields of both winter wheat (WW1 and WW2) and SM (Figure 9). A same level of grain yields from both SM and WW2 was found among the subsoiling treatments examined, due to the reduced subsoiling effects with time. In contrast, for WW1, the grain yields under subsoilers with larger h values (≥135 mm) or non-winged subsoiler were significantly smaller than these under subsoilers with lower h (≤95 mm). The higher yield generally results from a better combination of the yield components. Decreasing h values resulted in an overall larger panicle number, but an overall smaller TKW, possibly indicating that TKW is less influential for the grain yield of winter wheat as compared with panicle number. This is in line with Li et al., who found that panicle number and TKW were positively and negatively correlated with wheat grain yield, respectively [25]. The results from this study indicated that for the given soil, reasonably reducing wing mounting height during subsoiling practice can raise winter wheat grain yield significantly in the first growing season. However, variation of subsoiling treatments did not make a significant difference in the subsequent grain yield of summer maize and winter wheat in the second growing season.

4. Conclusions

In this study, variation and persistence of the effects of mounting height (h) (75–155 mm) of the subsoiler’s wing during subsoiling practice on soil properties and crop growth were investigated by three growing seasons (i.e., winter wheat–summer maize–winter wheat). The conclusions were drawn:

(1) Decreasing h gave a larger soil disturbance area ratio. In key stages of the first two crop growing seasons (i.e., winter wheat in the first growing season (WW1) and summer maize (SM)), moisture contents of soil subsoiling with smaller h values were higher in most cases.

(2) The h had significant (p < 0.05) effects on mean moisture contents of both the top 40 cm and top 100 cm soils in key stages of WW1, soil bulk density, number of roots and panicle number and grain yield of WW1, and thousand kernel weight of both WW1 and summer maize (SM). Decreasing h values during subsoiling practice increased the number of roots and panicles of winter wheat, improved moisture content of soil and crop grain yield, and decreased bulk density of soil and thousand kernel weight.

(3) Given the facts that subsoiling with a lower wing mounting height increased the soil disturbance area ratio, which gave better soil properties (e.g., moisture content and bulk density) and crop grain yield, raising soil disturbance area ratio appropriately is recommended during subsoiling practice by designing or optimizing winged subsoilers (e.g., smaller h values).