1. Introduction
The muddy coast is composed mainly of mud or clay and occupies a major part of the eastern coast of China [
1,
2]. The coastal zone is the most frequent and active zone of interaction between the hydrosphere, lithosphere, atmosphere and biosphere. The unique location of the coastal zone and global climate change cause this zone to be a frequent zone location for natural disasters and a fragile zone for ecosystems [
3,
4,
5,
6,
7]. The soil in the coastal zone is mainly coastal saline soil, which is rich in soluble salts. The salt content within one meter of the soil layer is generally greater than 4 g·kg
−1, and can reach up to 20–30 g·kg
−1. Soil desalination is difficult when there is a shallow terrain and slow internal drainage [
8,
9,
10,
11,
12]. Under these conditions, the growth of plants is limited, and under severe stress, plant death occurs. Underground pipe drainage has been implemented in the coastal area, but due to spatial nutrient limitations, plants are threatened by secondary salinization over long periods of time, and plants are prone to degradation, aging and death [
13,
14,
15,
16]. Therefore, how to develop ecologically green land scientifically and effectively is an important issue that needs to be solved in coastal areas.
In China, with the rapid development of cities, garden areas have increased rapidly. Moreover, the amounts of garden waste, such as dead branches, fallen leaves, grass clippings and flowers, has also increased as a result of landscape maintenance. For example, from 2006 to 2016, the area of green land in Beijing increased from 39,392 ha to 82,113 ha, and more than 2.37 million tons of garden waste are produced annually [
17]. Traditional garden waste disposal methods mainly involve incineration and landfill deposition, which likely cause environmental problems such as air pollution, land occupation and water pollution. Therefore, garden waste disposal has become a major environmental problem in urban construction [
18,
19,
20]. According to domestic and international studies, composting represents a significant way to utilize resources [
21,
22,
23,
24]. Through the composting of garden waste, fresh organic carbon is decomposed and converted into humus [
25]. Garden waste compost can be reused as a soil amendment or as an organic fertilizer to maintain soil moisture, improve soil fertility, and improve the soil structure [
26,
27,
28]. However, the improvement effect of garden waste on salt leaching in saline soil has yet to be verified. Some studies have shown that compost containing a high proportion of humified organic matter can decrease the bioavailability of metals in soil by absorption and by the formation of stable complexes with humic substances. This is due to the great capacity of humic acids to retain or to bind metals. Moreover, their molecular size is usually larger than the soil pore size, resulting in low mobility and little leaching through the soil profile [
29]. Therefore, the application of garden waste compost in saline soil will require new methods to improve the desalination effect.
Bentonite exists as a 2:1-type crystal structure composed of two silicon-oxygen tetrahedrons sandwiched between a layer of aluminum-oxygen octahedrons [
30]. Owing to the presence of Cu, Mg, Ca and other cations in the layered structure formed by the crystal, the interaction with the crystal is very unstable. In addition, it is easily exchanged with other cations, so it has better ion exchange. Bentonite can absorb hydrated cations (e.g., K
+, Na
+, Mg
2+), and its ion exchange capacity can reach 60~150 meq·100 g
−1 [
31]. The world is rich in bentonite resources, most of which are distributed in China, the United States, Germany, Italy and Greece. The exchangeable Ca
2+ ion of calcium bentonite acts mainly on Na
+ in the soil, which improves the desalination effect [
32].
Even though there are many studies concerning garden waste or bentonite as soil conditioners, very little is known about their effects on coastal saline soil. Thus, the question of which amendments are the most effective for saline soil remediation remains unanswered. Our study is a new attempt to investigate the application of garden waste compost and bentonite to coastal saline soil. Two materials were used, and four treatments were applied: no materials, garden waste compost alone, bentonite alone and a mixture of garden waste compost and bentonite. Through analysis of soil salinity, physicochemical properties and plant growth among the different treatments, this study aimed to evaluate the effects of different amendments and provide basic data for rational application of these materials in coastal areas.
4. Discussion
According to this study, the coastal saline soil of Tianjin is a typical NaCl-type saline soil that is highly saline and has an extremely poor structure. The soil permeability was only 0.63 × 10−5 cm·s−1, which can lead to a very long natural desalination process. The EC of the 0–20 cm soil layer in the CK treatment (without a soil amendment) remained as high as 3.21 dS·m−1. Therefore, effective measures must be taken to improve soil structure and increase the nutrient content and water conductivity to accelerate soil leaching and salt removal.
The exchangeable Ca
2+ ions of calcium bentonite act mainly on Na
+ ions in the soil, which improves the desalination effect. Studies have shown that bentonite reduces the hydraulic conductivity in the soil and increases the accessibility of the exchange surface, thus enhancing the cation exchange rate, which leads to the rapid release of cations from the soil colloid [
43].
According to our study, the soil salinity after the garden waste compost and bentonite were mixed together and applied was significantly lower than that in response to garden waste compost applied alone, which indicates that adding bentonite may help in leaching Na +.
During the second year after planting
Robinia pseudoacacia cv. Idaho, the mortality rate of the trees without any amendment (CK) and with the addition of bentonite alone (T2) was very high. Analysis of the soil physical and chemical properties of these two treatments revealed that the soil salinity in the bentonite treatment and the CK treatment was not reduced. The average annual precipitation in Tianjin is 516 mm, and the average surface water evaporation across many years is 1625 mm, which is three times the precipitation [
44]. The movement of soil salt depends mainly on the evaporation/deposition of surface water. The amount of water that is evaporated is much greater than the amount that falls as precipitation, which drives the rapid accumulation of soil salt near the surface. Even if the underground drainage desalination system could control the groundwater level effectively, salt will still accumulate in the soil because of the low soil desalination rate. The results of Zhang’s study showed that the high temperature in the coastal area in the summer aggravated the back-salt effect of the surface soil, and that it is necessary to apply suitable soil amendments to drainage desalination systems to improve them further [
45]. In addition to being involved in ion exchange, bentonite also undergoes strong hygroscopic expansion. Under the repeated evaporation and precipitation cycles in nature, hygroscopic expansion is greater than the ion exchange effect. The salt rises faster because of capillary action, and the soil is more likely to return to being saline. Eilenbrod’s research found that the application of bentonite greater than 20% will reduce the hydraulic conductivity of the soil due to the swelling effect, which in turn reduces the leaching efficiency [
46].
The mixed application of garden waste compost and bentonite can significantly increase the total porosity and saturated hydraulic conductivity. At the same time, the ion exchange of bentonite was promoted to prevent its hygroscopic swelling and salt accumulation. Studies have shown that the combination of organic and inorganic modifiers can produce interactions, thereby enhancing the effects of soil amendments [
47]. In the present study, the soil nutrient contents in the T3 treatment was greater than that in the T1 and T2 treatments. The microorganisms produced during the fermentation process also play a role in activating soil nutrients. The presence of bentonite improves the physical structure of the soil as well as the water and fertilizer retention abilities. Therefore, the addition of garden waste compost and bentonite helped to establish a self-sustaining ecosystem in saline soils [
48], ensuring the sustainability of green land in the Tianjin coastal area.