1. Introduction
Salinity is one of the most important factors limiting the productivity of agricultural crops, and yield is greatly increased when soil salinity is managed to fall below the tolerance threshold of a crop [
1,
2]. The uncultivated coastal saline plains of the Western Bohai Rim in Northern China are about 670,000 ha, accounting for nearly a quarter of the area with low agricultural production in this region [
3,
4]. The soils of these plains are highly saline with low crop productivity and cannot meet local agricultural demands for food [
4]. Several methods have been introduced to solve the issues of many uncultivated and poorly productive fields and food shortages in this region, including comprehensive improvements such as river sediment amelioration, chemical remediation, and phytoremediation, all of which have been successful in salt alleviation [
5,
6,
7]. However, these remediation measurements can take effect only when they are combined with large amounts of irrigation [
8], and available irrigation resources are scarce in these saline areas [
9]. Appropriate planting patterns could influence water and salt transport by controlling evaporation and distributing rainfall [
10]; moreover, they are inexpensive and do not pollute the soil. Thus, planting patterns are an environmentally friendly way to moderate soil salinity [
11].
Flat (F), mini-ditch (MD), and ridge-furrow (RF) are three conventional planting patterns used around the world [
11,
12,
13]. Flat planting is widely accepted for its ease of implementation and is usually regarded as a control when investigating the influence of seeding depth, planting date, planting density, irrigation, or other factors on yield [
11,
14]. MD is another agronomic technique that is commonly used in arid and semiarid areas [
15]. The MD method can increase stand establishment and yield when the soil temperature is suitable [
16], while it might delay plant emergence and reduce seedling vigor and yield when the soil temperature is low [
17]. However, in other cases, the MD method has no significant influence on stand establishment and yield [
18]. When combined with plastic film mulching, the MD method shows a positive effect on crop growth by increasing soil moisture and temperature [
19,
20]. Ridge-furrow planting, which is well known for efficiently capturing rainfall and improving water-use efficiency, is widely used in the arid and semiarid areas [
21]. Li et al. [
22] found that RF can increase yield and water-use efficiency by 1.9 and 1.8 times, respectively, when compared to F in Northwestern China. Devkota et al. [
11,
23] compared different planting patterns for salinity management and found that RF could efficiently reduce salinity in the soil profile in Central Asia. Other studies have also indicated that RF could increase crop yield by 40%–150% when compared to F in arid and semiarid areas [
12,
21], but it is not known whether RF is suitable for cotton planting in coastal areas.
Around the Bohai Rim, F is the only planting pattern for cotton cultivation [
20]. Although cotton is regarded as the most salt-tolerant of the annual crops, the stand establishment rate, growth, and lint yield in this region are relatively low because of high salinity in soils [
24,
25,
26]. Reducing salt content and improving the soil environment for crop growth is important in increasing yield [
24,
25]. Though studies of F and MD or F and RF have been conducted to investigate the effects on soil water and salt dynamics as well as on crop production [
13,
24], most of these experiments were carried out in the inland with dry climates, not in the coastal region with a semi-humid climate [
11,
12]. Planting patterns such as MD and RF, which have proved beneficial for increasing yield in other parts of the world [
21], need to be introduced in this region. Water and salt movement, as well as crop production, respond differently to the same planting pattern under the dry and semi-humid conditions [
27].
Plastic film mulching is essential when soil temperature is low and evaporation is extensive [
20,
28]. According to Wang et al. [
29], soil temperature 5 cm under the plastic film mulching was 3–5 °C higher than that without plastic film mulching. Du et al. [
30] found that plastic film mulching can increase winter wheat yield from late winter to early spring in the North China Plain. When there was a gap between the plastic and soil, Dong et al. [
31] found that the temperature in the gap was 1.9 °C higher than that outside the mulch, and the seedling biomass and lint yield were significantly higher than when there was no gap. The effects of plastic film mulching on increasing soil temperature and moisture have been acknowledged, but the effects of plastic film mulching under different planting patterns are not well documented.
Few studies have compared the influence of F, MD, and RF on the soil environment (soil moisture and salt) and crop growth (plant height, leaf area, aboveground biomass, and root-length density) in coastal saline areas, and it is not known which planting pattern is more suitable for these areas. Therefore, the objective of this study was to investigate the response of soil water and salt movement as well as cotton growth to F, MD, and RF planting and to select the best pattern for cultivating the heavy soil in a coastal field along the Bohai Rim.