Effects of water-fertilizer coupling on immobilization remediation technology using sepiolite

Yiyun Liu Tianjin Huakan Environmental Protection Technology Co.,Ltd Yingming Xu (  ymxu1999@163.com ) Agro-Environmental Protection Institute Qingqing Huang Agro-Environmental Protection Institute Xu Qin Agro-Environmental Protection Institute Lijie Zhao Agro-Environmental Protection Institute Xuefeng Liang Agro-Environmental Protection Institute Lin Wang Agro-Environmental Protection Institute Yuebing Sun Agro-Environmental Protection Institute


Introduction
Soil is the main gathering place for metals and acts as a barrier to prevent them from transferring through food chain to protect food safety and human health. It's reported that around 2.35×10 8 hectares farmland worldwide are contaminated with trace metal elements (Bermudez et al., 2012). In China, due to improper disposal of industrial wastes and the excessive use of pesticides, fertilizers and plastic lms, heavy metal contamination is getting serious and the pollution area is gradually expanding (Liang et al., 2016). According to a general survey of soil pollution status released in 2014, Cadmium (Cd) is the most widely distributed soil pollutant in China. In addition to natural sources (which cause fewer changes), human activities are changing soil functions and properties more rapidly and more intensively. More than 90% of Cd release in the environment is caused by man-made sources (Bi et al., 2006). Therefore, the prevention and control of Cd pollution has become a hotspot in the eld of soil environment.
Engineered soil remediation technology (such as mixing of soil method) and bioremediation technology (such as phytoremediation and microbial remediation) are di cult to realize in practical applications due to high cost and low remediation e ciency. Therefore, as soon as the in-situ chemical immobilization remediation technology appeared, it has attracted widespread attention for its advantages of high cost-effectiveness, easy to operate, suitable for large-area soil treatment, and environmental friendliness.
Immobilization remediation is a process of converting metals with high mobility into solid or physicochemical stable form through chemical interactions among heavy metals, soil particles, and binders while agricultural production ).
Numerous soil amendments, i.e., alkaline compounds (Liu et  Therefore, in the past two decades, the research on heavy metal pollution using immobilization remediation method has mainly focused on the screening and synthesis of immobilizing agents. Our previous study found that the application of natural sepiolite could decrease available content of Cd in soil and reduce the pollution risk of Cd on plant. However, natural sepiolite might not achieve the desired effect during planting and other soil biogeochemical process, like acid-soluble and complexation effects. As a consequence, development of agronomic management technologies to prevent Cd remobilization was an important pathway to control the food safety of agricultural products in soils. As we all know, continuous ooding irrigation dramatically reduced Cd accumulation in rice grains (Wiggenhauser et al., 2020). Because, radial oxygen loss from rice roots temporarily oxygenates the rhizosphere and causes redox oscillations in the soil (Maisch et al., 2019). Rinklebe(2016) and Ye(2018) found that the reduction of sulfate to sul de and the formation of insoluble CdS consequently reduced available Cd in the ooded soil. Intermittent ooding promote rice growth through regulate of soil permeability. The periodic variation in rhizosphere soil may affect the dominant biogeochemical reactions of redox-sensitive elements such as Fe, nitrogen (N), sulfur (S) and carbon (C)(Zhu et al., 2014). Meanwhile, intermittent irrigation improves rainfall utilization rate and facilitate water conservation (Lv et al., 2014).
As the main source of plant nutrition, the application of chemical and organic fertilizer also affects the distribution of soil heavy metal fractions  Pot experiments were performed in a greenhouse in the Agro-Environmental Protection Institute, Tianjin, China. The soil samples were air-dried, homogenized and passed through a 4mm sieve. Soil (6 kg) was placed in each pot (23.3 cm in diameter, 34 cm in height). Basal fertilizer, containing 3.87 g urea and 6.24 g monopotassium phosphate, was added to soil and mixed thoroughly (Xiao et al., 2015). SP and GM were mixed in to each pot as shown in Table 3. Then, soils were equilibrated for one month, maintaining approximately 75% of eld water-holding capacity with tap water (without Cd). In order to test and verify the long-term stability and effectiveness of sepiolite, the pot experiment with the same design was conducted in the second year. The pot have applied goat manure in the rst year were added with the same dosage of GM to replenish soil fertility.

Sample preparation and analysis
After 135 d of growth, the plants were harvested and separated into root, straw, rachis, husk, and rice grains. The rice grains were oven-dried at 75 °C and weighed immediately after removing the husk and un lled grains. Each part of the plant were ground and passed a 1.85-mm sieve for further analysis. Cd contents in plant samples were digested using a block digester and soil solutions was determined by inductively coupled plasma mass spectrometry (iCAP Q; Thermo Scienti c, Waltham, MA, USA). .
Rhizosphere soil was collected from a small amount of soil shaken directly from the plant roots. Soil samples were air-dried at room temperature and sieved to <1 mm, and stored in a plastic container for further analysis.

Quality control
To monitor the accuracy and quality of chemical analyses, quality control measures were adopted using soil

Effects of GM on grain yield under different water management treatments
As shown in Fig.1, grain yield was higher in intermittent irrigation condition, comparing to aerobic and ooded condition.
The application of GM increased grain yield by 9.4% and 30.1% in the aerobic and intermittent condition, but decreased grain yield by 20.3% in the ooded paddy soil. In sepiolite amended soil, the application of GM increased grain yield by 34.3% and 40.2% in the aerobic and ooded paddy soil, while decreased grain yield by 11.9% in intermittent irrigation. Song et al (2021) observed higher grain yield in intermittent irrigation than ooded condition. Further, grain yield improved with the increase in P rates.
3.2 Effects of GM on soil DOM and pH under different water management treatments As shown in Fig.2, soil pH slightly increased in intermittent and ooded paddy soil. The application of sepiolite and goat manure also increased soil pH, The application of GM increased DOM by 244.3%, in contrast, the application of sepiolite decreased DOM by 31.5%. So, the application of GM in SP amended soil increased DOM by 135.2%. Dissolved organic matters refers to the organic matter that can be extracted by water passing 0.45μm membrane. Bian et al (2020) found that the application of silkworm excrement organic fertilizer increased DOM by 124.6%.

Effects of GM on Cd accumulation factor in different parts
The translocation factors of different parts were directly related to the Cd accumulation character. The translocation factors (TF) from roots to straws were between 0.08-0. 24. The transfer factors from straws to rachis were between 0.31-0.55.
The translocation factors from rachis to husk showed great disparity under different water management. The translocation factors from rachis to husk were between 0.31-0.67 under aerobic and intermittent irrigation condition, whilst the translocation factors from rachis to husk were 0.86-2.00 under ooded condition. As for the translocation factor from husk to unpolished rice, the only TF 1 was obtained for sepiolite-amended soil with the application of goat manure under intermittent irrigation condition.

Discussion
Yield is the chief element to indicate the growth and development of crops that are affected by excess concentration of noxious substance in soil. Water management affects the bioavailability of cadmium (Cd) in the soil and hence their accumulation in rice and grain yields. Lv (2014) found that the intermittent irrigation increased grain yield by 7.55%-29.58%.
Compared with ooding, both aerobic and intermittent irrigation enhanced Cd distribution in the root and reduced it in the straw and grain. It is reported that Cd might promoted the growth of plants when the concentration below 5 mg·kg -1 (Tang et al., 2020). Song (2020) found that comparing to continuous ooding, grain yield was higher under alternate wetting and drying (intermittent) irrigation condition. Besides, the application of phosphorus fertilizer slightly increased grain yield.
Coincidentally, we found the same tendency in our research. Grain yield were higher under intermittent water condition comparing with ooded and aerobic water condition. The application of goat manure showed different effect on grain yield in unamended and sepiolite amended soil. It might ascribe to the substantial reduction of dissolved Cd in soil.
Soil acidity is a major yield-limiting factor that adversely affects crop productivity (Nanda and Adriano, 2014). In our study, the application of sepiolite alone decreased grain yield under aerobic and ooded irrigation condition. As a result, grain yield decreased by 9.7%-18.3%. Lahori (2017) found that tobacco biochar and zeolite signi cantly decreased the dry biomass yield of Brassica campestris L. due to the increase in the soil pH compared to the control. However, Ran (2019) reported that the combine application of sepiolite and organic fertilizer increased soil pH by 1.0-1.2, yet, some treatments appeared to increase the dry biomass yield relative to the CK. In our study, the application of sepiolite and goat manure increased soil pH by 1. organic matter fraction on distribution of Cd in long-term polluted paddy soils, the results showed that SOM fractions had higher Cd concentration than other soil constituents. In our study, the application of goat manure slightly increased available Cd extracted by DTPA under aerobic intermittent irrigation condition, but the effect did not reach a signi cant level.
The root is widely recognized as the most major connection between rice plant and soil environment. Compared to aerobic and intermittent irrigation, ooded irrigation decreased Cd content of root by 73.4%-93.0%. Meanwhile, the accumulation pattern of Cd in other parts of rice plant was similar to that in root. Cd content in different part of rice ranked in the following order root straw rachis husk.
Translocation factors (TFs) were also the most perceptual intuition of the concentration between different parts. A decrease of Cd concentrations in unpolished rice was relevant to TFs in all parts of the rice plant, but different organs had different TFs. With the addition of different amendment, the TFs of Cd from roots to straw, straw to rachis, rachis to husks, husks to unpolished rice, and roots to unpolished rice changed to different degrees (Table 4). However, TFs are inversely proportional to biomass of different plant parts. We found that TFs from roots to straw all lower than 0.20. Because, the biomass of straws were much higher than roots.  found that the application of hydroxyapatite decreased TF of maize, besides the effect increased with the increase of application rate.  found that the amendments the ability of Cd to be transferred to the grains was effectively inhibited as the values of TF G were lower than those of TF S (TF G , Translocation factor from straw to grains; TF S , Translocation factor from root to straw). In our study, the TFs from husks to unpolished rice exceeded 1.0 in most of the treatments. This results indicated that ability of husks to transport Cd was greater than that of other rice organs. Table 4 showed that under intermittent water condition, the combined application of sepiolite and goat manure inhibited the transfer of Cd from roots to unpolished rice to the greatest extent.
In the rst year, the combined application of sepiolite and goat manure decreased Cd by 83.1%, but did not reach the It is well known that bioavailable fraction of metal is more important for plant uptake than the total concentrations of heavy metals in the soil which might be due to the presence of different chemical forms of heavy metals (Rizwan et al., 2016). Bioavailable Cd is the fraction of total Cd in the interstitial water and soil particles that is readily available to the receptor organisms. The DTPA-extractable metal concentration in the 10-20 cm pro les is adapted to assess the phytoavailable proportion of Cd. Therefore, the uptake and transportation of Cd by rice plants was also greatly in uenced by DTPA-Cd (Fellet et al., 2014).  reported that the application of biochar decreased DTPA-Cd by 14%-51%, as a result, Cd in edible part decreased by 46%-86%. In this study, the application of goat manure increased DTPA-Cd in most treatments.
Because organic manure increased -COOH of soil particle, which can promoted the mobility of Cd by forming complexes. Zhou (2018b) found that rice straw(RS) increased phytoextraction e ciency of Cd, and RS has positive effects on plant Cd uptakes.
In our study, the application of sepiolite decreased exchangeable Cd and increased the fraction bound to organic matter. The reason is that CaCO 3 occupied a large proportion in sepiolite. Besides, the application of goat manure increased exchangeable Cd and decreased residual form of Cd in unamended soil. But in the sepiolite-amended soil, the application of goat manure decreased exchangeable Cd and increased residual form of Cd under intermittent irrigation condition. The results showed that intermittent irrigation regime was a safe and effective pattern for Cd remediation using sepiolite. Besides, the application of goat manure in sepiolite amendment was also a recommended measure to promote crop growth and achieve safe production in Cd polluted soil.

Conclusion
1. The application of sepiolite alone increased soil pH, as a consequence, sepiolite decreased available Cd under ooded condition.
2. The application of goat manure alone increased DOM content in unamended and sepiolite amended soil. As a result, goat manure increased available Cd under aerobic and ooded water condition in sepiolite amended soil.
3. Under intermittent water condition, the combined application of sepiolite and goat manure inhibited the transfer of Cd from roots to unpolished rice to the greatest extent. Note: TF 1 , Translocation factor from root to straw; TF 2 , Translocation factor from straw to rachis; TF 3 , Translocation factor from rachis to husk; TF 4 , Translocation factor from husk to unpolished rice, TF p , product of TF 1 , TF 2 , TF 3 and TF 4 . Table 5 Effects of GM on Cd of unpolished rice Effects of goat manure on pH and DOM under different water management treatments Available Cd extracted by DTPA