1. Introduction
Controlled drainage has a long history of use and is widely applied globally [
1,
2,
3,
4]. In arid and semi-arid regions, soil salinization can be prevented by drainage. In humid and semi-humid regions, excessive water can be drained away to reduce the risk of yield loss, allowing farmers to cultivate a greater variety of crops. In temperate regions, drainage facilitates the reclamation of flood-stricken areas and provides better growth conditions for crops [
5]. In most cases, surface drainage not only causes an increase in agricultural production, but in many countries, it also leads to a reduction in retention and water loss, and it is ineffective in the event of a flood. The advancement of science and technology and the increasing attention to agriculture have encouraged the wide application of subsurface drainage technology in agricultural production [
6]. However, underground drainage has an adverse effect on surface water quality, and the excessive nutrients cause environmental pollution [
7]. Underground drainage systems have been identified as the main sources of nutrients, as well as pollutants [
8]. Nutrients, herbicides, and pesticides are discharged from the soil body through drainage, which reduces nutrient utilization efficiency and results in environmental pollution [
9]. The issues associated with environmental protection and soil pollution have attracted increasing attention. In addition to the use of subsurface pipes to reduce soil salinization and discharge excessive water to provide a suitable environment for crops, minimizing the unnecessary adverse effects of a drainage system on hydrology and water quality has become a main goal for the development of new drainage systems [
10,
11]. Controlled drainage (CD) is considered to be a sustainable management method that can be used to save water, decrease nutrient leaching, reduce drought stress, and increase yield, and it has therefore become a prospective trend in agricultural production.
In arid areas, controlled drainage has become increasingly popular in the treatment of saline soil due to its advantages, which include lower land space requirements, low pollution, long durability, stability, no weeds, lower labor requirements, less earthwork, easy management, operation, and maintenance, and convenience of mechanized construction [
12]. Studies have shown that soil salinization is one of the major causes of the decline of agricultural productivity in many arid and semi-arid regions in the world [
13,
14]. In most cases, conventional irrigation and crop management measures cannot reduce soil salinity [
15], so using subsurface drainage to reduce soil salinization offers a solution. In the second half of the 20th century, subsurface drainage was widely introduced in many parts of the world [
16]. Ghumman et al. [
17] reported that the long-term use of subsurface drainage effectively reduced soil salinity, reduced the area of saline soil by 10–40%, and increased social and economic benefits by 1.8–2.5%. A study performed by Ali et al. [
18] showed that subsurface drainage removed large amounts of salts from the soil. In the first three years after subsurface drainage was introduced, the soil salinity decreased linearly. The same result was obtained in a newly salinized area, whereby saline soils were reduced from 75% to 30% within four years, and the soil salinity was expected to decline further. The study performed by Li et al. [
19] indicated that if the groundwater was maintained at a level suitable for crop growth by using an engineered drainage system, the salts in the soil could be well balanced under natural rainfall conditions. The appropriate management of groundwater levels using a drainage system enables crops to effectively use groundwater and promotes the leaching of soil salinity through natural rainfall, thereby increasing water-use efficiency.
CD refers to the use of a control device to raise or lower the drainage outlet in order to adjust the draining intensity to meet the requirements of the agricultural field. CD can be used to maintain a higher groundwater level during the growing season, thus making it easier for crops to absorb and utilize shallow groundwater and nutrients at critical growth stages; meanwhile, it reduces the discharge of soil chemicals and nutrients, thereby facilitating environmental protection. In arid areas, a key issue that needs to be resolved is how to maintain the balance between water and salt in the field during the CD process [
20]. It is also very important to understand the migration of soil water and salts during the process. CD with subsurface pipes is a new management measure for farmland drainage, and a number of studies have shown that this measure is able to lower the amount of water drained away from farmlands, thereby reducing the amount of nitrogen lost [
11,
21,
22]. A study conducted by Wang et al. [
21] indicated that the implementation of CD reduced the loss of
-N in dryland soil by about 20.53%, reduced
-N loss by an average of 18.9%, increased crop yield by 0.11%, and reduced the amount of drained water by 19.23%. Using the DRAINMOD-NII simulation model, DRAINMOD-NII incorporates process-based modeling of carbon and nitrogen dynamics in addition to the hydrologic processes that were already present in the model. The model simulates the reactive transport nitrogen by using a finite difference solution to a multiphase form of the advection–dispersion–reaction equation. The nitrogen processes and transformations considered in the model include atmospheric deposition, fertilizer application, plant uptake, mineralization, immobilization, nitrification, denitrification, ammonia volatilization, and NO
3- and NHx-nitrogen losses via subsurface drainage and surface runoff. Luo et al. [
10] predicted that both shallow drainage and CD reduced the annual drainage amount and
-N loss by 20–30% and affected crop yield by −3% (reduced yield) to 2%, depending on the distance between drainage ditches or pipes. Youssef, M.A. et al. [
22] reported that CD reduced the amount of annual underground drainage by 86 mm (30%) on average and reduced the annual N loss by 10.9 kg N hm
−1 (32%). They also used the DRAINMOD model to predict the dynamics of drainage, and the DRAINMOD-NII prediction showed that the reduction of N loss under CD conditions was mainly caused by increased denitrification. The trend of the reduction of the annual drainage amount and nitrogen loss under CD conditions predicted by RZWQM-DSSAT was similar to that predicted by the DRAINMOD/DRAINMODNII model [
22]. (Similarly to DRAINMOD, RZWQM-DSSAT simulates the infiltration of water into the soil profile using the Green–Ampt equation, deep seepage using Darcy’s equation, and the tile drainage using the Hooghoudt equation. On the other hand, RZWQM-DSSAT applies a numerical solution to Richards’ equation in order to compute the soil water distribution in the vadose zone, which is different from the empirical “drained-to-equilibrium” approach implemented in DRAINMOD. The former model uses the Shuttleworth–Wallace PET method.)
The use of subsurface controlled drainage plays an important role in the improvement of salinized soil and environmental protection in the Hetao Irrigation District, and the desalinization efficiency and soil moisture conservation effects of controlled drainage are conducive to the control of soil secondary salinization and the improvement of agricultural water-use efficiency in the irrigation area [
23]. At the same time, the nutrient loss in the soil is relatively small, which has an important impact on the uptake and utilization of nutrients and the growth and development of crops, and it has a positive impact on environmental protection. Most studies show that after the cessation of this agrotechnical treatment and the liquidation of sunflower cultivation, the local ecosystem has a serious phenomenon of secondary salinization, and the degree of soil salinization is increasingly aggravated, which affects the growth and development of crops and, thus, affects the crop yield [
24,
25,
26]. A comprehensive study for the understanding of the response of crops, soil, salt, and fertilizers to CD using subsurface pipes has not been conducted so far in the Hetao Irrigation District, where the soil is typically salinized. Due to the severe salinization, the soil in the district has poor permeability, and as a result, irrigation at the later stage of growth of oilseed sunflower plants may cause a high rate of plant death. However, if the plants were to be under water deficit stress at later growth stages, CD could then be used to satisfy the needs of the plants for water and nutrients at these stages. Our study was performed in an area with moderately salinized soil in the Hetao Irrigation District. We aimed to examine the effects of different drainage systems on soil water content, salinity, mineral nitrogen content, and the quality of drainage water, to understand the regularity of the response of the soil–crop–environment system to drainage methods, and to determine the best drainage system for benefiting crop yield, reducing environmental pollution, and improving water- and fertilizer-use efficiency.