Yu FU, Cheng YANG*, Lifang CUI, Fanfei MENG, Aiai YANG, Guangneng ZENG

1. College of Eco-environmental Engineering, Guizhou Minzu University, Guiyang 550025, China; 2. Solid Waste Pollution Control and Resource Engineering Research Center, Guizhou Minzu University, Guiyang 550025, China; 3. Guizhou Vocational Education Public Training Center, Guiyang 551400, China

Abstract [Objectives] The research aimed to study the effect of sludge extract on the growth of Zinnia elegans, and evaluate its possibility of application. [Methods] Z. elegans was taken as research object, and the effect of sludge extract on the growth indicators of Z. elegans was studied. [Results] The sludge extract can obviously promote the growth of Z. elegans, and the growth indicators were significantly improved when compared with the control group. The application effect was close to that of foliar fertilizer purchased in the market. In each treatment group, the low concentration of sludge extract did not significantly promote the growth of Z. elegans, while too high concentration could inhibit its growth, and the optimal dilution ratio was 600 times. [Conclusions] Further preparation of amino acid foliar fertilizer from sludge extract and its application in planting garden plants can realize the resource utilization of sludge and has a good development and application prospect.

Key words Sludge extract, Zinnia elegans, Plant growth

1 Introduction

With the rapid development of urbanization, the amount of sludge produced by urban sewage treatment in China has increased significantly[1]. It is reported that the amount of urban sludge in China can reach 7 500 tpd[2]. Incineration, composting and landfill are common solutions to surplus sludge, but there are many limitations[3]. From the perspective of environmental protection and sustainability, the best way of sludge disposal is resource utilization[4]. The sludge contains relatively abundant nutrients required for plant growth, and high protein content[5], which has the potential of resource utilization and is an excellent resource of amino acid foliar fertilizer. Therefore, the development of protein in sludge, the preparation of amino acid foliar fertilizer and the study of its application in agriculture and forestry[6]could not only broaden the raw materials of amino acid foliar fertilizer and reduce the production cost of amino acid foliar fertilizer, but also realize the resource utilization of sludge from sewage treatment plant[7].

Zhang Aifangetal.[8]carried out the research on the direct application of sludge as fertilizer in agriculture and forestry. Yang Yangetal.[9-10]carried out the research on sludge composting. Deng Jinfangetal.[11-12]carried out the research on the preparation of cultivation substrate for agriculture and forestry by sludge and other materials. At present, the research on sludge mainly focuses on direct application, composting and cultivation substrate[13]. Foliar fertilizer has the advantages of fast fertilizer efficiency, high nutrient utilization rate and low environmental pollution, and has become an important part of modern agricultural fertilizer[14]. In the research on the development of surplus sludge into foliar fertilizer, only Shi Zhoufangetal.[15]carried out the research on the preparation of amino acid foliar fertilizer by using surplus activated sludge, and there is no report on the preparation of amino acid foliar fertilizer by sludge and its application in gardens. Plant treatment and agricultural utilization is a new field of urban sludge treatment technology at present. It has the advantages of low cost and no secondary pollution, and has gradually become a hotspot of international research and development[16].

Zinniaelegansbelongs toZinniaL. of Asteraceae, is annual plant, and is widely used in landscaping[17]. Therefore, the research on the application effect of sludge extract in the planting ofZ.elegans, the effect of sludge extract on the growth ofZ.elegans, and the possibility of its application could provide a theoretical basis for the preparation of amino acid foliar fertilizer from the surplus sludge of sewage treatment plant and its application in the garden, and a reference for the resource utilization of surplus sludge in the sewage treatment plant.

2 Materials and methods

2.1 Test materialsTest plant wasZ.elegans. The test soil was taken from the abandoned land soil of a village in Dangwu Township, Huaxi District, Guiyang City, Guizhou Province and the natural forest land soil in the new campus of Guizhou Minzu University. The soil was mixed evenly according to the mass ratio of 2∶3 and sieved for future use. Basic physical and chemical properties of the tested soil were: pH 6.78, organic matter 12.25%, total nitrogen 0.78 g/kg, total phosphorus 1.35 g/kg, total potassium 9.29 g/kg, and available phosphorus 15.87 mg/kg. The test sludge was collected from the surplus sludge of a sewage treatment plant in Guiyang, Guizhou Province, and its physical and chemical properties were as below: moisture content 58%, pH 6.85, organic matter 28.6%, total nitrogen 19.31 g/kg, total phosphorus 10.95 g/kg, total potassium 4.58 g/kg, and available phosphorus 397.79 mg/kg. In the pot experiment, 4 kg of sieved soil was put into each pot, and the pot was made of polyethylene plastic, with the specification of 25 cm height and 28 cm caliber. The test sludge extract was prepared by the laboratory. By referring to the optimum extraction conditions of Cui Jingetal.[17-18], and considering that the potassium content of sludge is generally low[19], KOH solution was taken as extractant. According to the solid-to-liquid ratio of 1∶8 (sludge∶extractant), it was extracted for 10 h under 100 ℃. After filtration, the sludge extract used in the experiment was obtained[20].

2.2 Experimental methodsThe pot experiment was conducted from May to August 2019, with 6 treatment groups and 1 control group. They were sludge extract diluted 800 times (S1), 700 times (S2), 600 times (S3), 500 times (S4), 400 times (S5), commercial foliar fertilizer (SS), and clear water as control (CK), respectively. Three replicates were set for each process. 10 seeds ofZ.eleganswere planted in each pot. When the plants grew well, intercropping treatment was carried out, and 3 seedlings ofZ.eleganswere kept in each pot. According to the set treatment groups, the sludge extract was diluted and then sprayed, 250 mL each time, once a week. It was sprayed for 10 times in the whole growth period. Moreover, it should do a good job in weeding, watering and other daily management, and observe and record the growth status ofZ.elegans.

2.3 Indicator determinationPlant height, flower diameter, stem diameter, flower yield, dry weight and plant mass ofZ.elegansin each pot were measured respectively. Plant height indicates length from stem base to growth point; flower diameter indicates the maximum value of petal extension in horizontal direction; stem diameter indicates width of plant stem; flower yield indicates fresh weight of flowers after picking; dry weight indicates the weight after flowers are put into the oven and dried at 75 ℃; plant mass indicates the weight of the whole plant after cleaning and drying[7].

2.4 Statistical analysisExcel software was used for data statistics and processing, and Origin 9.0 software was used for drawing map, and SPSS 18.0 was used for difference analysis.

3 Results and analysis

3.1 Plant heightPlant height is one of the indicators to measure the growth status of plants, and can prove the growth speed[21]. Seen from Fig.1, plant height ofZ.elegansin SS processing group in whole growth period was the best, and it increased by 15.66% than CK when harvesting. Plant height in S1, S2 and S3 processing groups increased by 8.57%, 10.97%, and 10.97% than CK. Plant height in S4 and S5 processing groups was all lower than CK at later period, and it may also be because that too high application concentration inhibited the growth of plants. The results showed that the application of sludge extract with proper concentration could promote the growth ofZ.elegans. Among them, S3 treatment group applied the best dilution concentration, while S4 or S5 treatment group caused greater damage to plants and inhibited their growth due to excessive concentration.

3.2 Stem diameterSeen from Fig.2, stem diameter ofZ.eleganssprayed by sludge extract was all higher than CK. Under the same spraying amount, stem diameter in S3 processing group was the best. When harvesting, stem diameter in S1, S2, S3, S4, S5 and SS processing groups was 0.35, 0.37, 0.41, 0.37, 0.34, and 0.39 cm, respectively, while stem diameter in CK was 0.34 cm. It was clear that sludge extract could promote the growth ofZ.elegansstem diameter. With the increase of the spraying concentration of sludge extract, the stem diameter ofZ.elegansshowed a trend of first increasing and then decreasing, and it respectively increased by 2.94%, 8.82%, 20.58%, 8.82%, 0, and 14.70% than CK. It showed that the effect of S3 processing group in each period was the best. It was clear that the sludge extract could make the stems of flowers strong, which was conducive to plant growth. The application effect of the sludge extract diluted 600 times was the best, without difference from the effect of commercial foliar fertilizer.

3.3 Yield and flower diameterIn the process of flower production, flower diameter is one of the important indicators to measure the quality and ornamental value of flowers[22]. Seen from Fig.3, flower diameter sorting ofZ.eleganswas: SS>S3>S1>S2>CK>S4>S5, and flower diameter in S1, S2, S3 and SS processing groups respectively increased by 6.03%, 2.71%, 8.89%, and 13.61% when compared with CK. Among them, flower diameter in SS and S3 processing groups was obviously larger than that in other processing groups, and there was no significant difference between S1, S2 and control group. Flower diameter in S4 and S5 processing groups was smaller than control. The main reason was that too high extract concentration could damage the flower bud of the plant, and then affect the flowering quality ofZ.elegans. The results showed that the appropriate dilution concentration of the extract had a promoting effect on the flowers ofZ.elegans, and the best dilution concentration of the sludge extract for the flower diameter ofZ.eleganswas 600 times.

Flower yield is also another indicator to measure the quality of flowers. Flower yield includes fresh weight and dry weight. The study showed that the fresh weight results ofZ.elegansflowers after applying the extracts of different concentrations were as follows: S3>SS>S2>S4>S1>CK>S5. Among them, fresh weight ofZ.elegansflowers in S2, S3 and SS processing groups was obviously larger than CK, and they increased by 54.23%, 41.90%, and 54.01%, respectively. When compared with CK, fresh weight of flower in S1 and S4 processing groups increased by 14.25% and 14.45%, and fresh weight of flower in S5 processing group was smaller than CK. Dry weight sequence ofZ.elegansflowers was: SS>S3>S1>S2>S4>CK>S5. The dry weight of flower in S3 processing group (1.94 g) was lower than SS processing group (2.75 g), but was still higher than CK, and they differed by 2.3 times. Except S5 processing group, dry weight of flower in other processing groups was all higher than CK. Therefore, spraying sludge extract with proper dilution concentration can improve the yield ofZ.elegansflowers and promote the unfolding of flowers.

3.4 BiomassFlower biomass is the sum of plant quality and flower yield. Seen from Table 1, biomass sequence ofZ.elegansprocessed by different dilution concentrations was: S3>SS>S1>S2>CK>S4>S5. Biomass in S3 processing group was the highest (18.16 g), which increased by 24.63% than CK (14.57 g), and biomass in S1, S2 and SS processing groups was also higher than CK. Mass rule of plant processed by different dilution concentrations was S3>S1>S2>SS>CK>S4>S5, and there was no significant difference between plants. Moreover, biomass in S4 and S5 processing groups decreased than CK. It illustrated that too high spraying concentration can inhibit the quality ofZ.elegans. Flower yield sequence was: S3>SS>S1>S2>S4>CK>S5, and its rule was roughly consistent with biomass. When the concentration of sludge extract was too high, the flower yield increased first and then decreased. Seen from the results of biomass, plant mass and flower yield, the sludge extract can promote the growth ofZ.elegans, but it could inhibit its growth when the dilution concentration was too high. Among them, dilution ratio in S3 processing group was the best, with the most obvious effect.

Table 1 Effects of sludge extract application on plant weight of Zinnia elegans g

4 Discussion

Foliar fertilizer is a kind of fertilizer that plants absorb the needed nutrients through foliar spraying during the growth process and then play a basic role[23]. It could directly participate in metabolism and organic synthesis, and its fertilizer effect is better than that of soil fertilization. Foliar fertilizer could promote the synthesis of plant photosynthetic pigments and photosynthetic intensity, and make up for the lack of soil fertility[24]. According to difference of specific components, foliar fertilizer could be divided into six classes by the Ministry of Agriculture: macroelement water-soluble fertilizer, microelement water-soluble fertilizer, amino acid water-soluble fertilizer, humic acid water-soluble fertilizer, agricultural and forestry water retaining agent, and others[7,25]. According to the function, it is divided into nutritive and regulatory type[26]. Amino acid foliar fertilizer can not only promote plant growth, but also prevent and control pests and diseases. It can also chelate with trace elements to make full use of nutrients, so it is widely used[27]. Sludge is the waste discharged from the sewage treatment plant, with a huge output, and it contains relatively rich nutrients such as protein[28]. It can be prepared into amino acid fertilizer and applied to agriculture and forestry to realize the resource utilization of sludge.

Among the amino acid water-soluble fertilizers developed in the market at present, they need to be diluted 500-1 000 times when applied[29]. When studying the effects of plant growth regulator on the growth and flowering ofZ.elegans, Sun Chenetal.[30]obtained that the dilution concentration of plant growth regulator inZ.elegansplanting was 800-1 000 times. Wang Haiboetal.[31]studied the effects of amino acid selenium foliar fertilizer on growth quality of Muscat grape, and obtained that dilution concentration of amino acid selenium foliar fertilizer was between 500 and 600 times. The research by Liao Haiyanetal.[32]showed that when amino acid fertilizer was used in garden plants, the dilution of 500 times had a better effect on the growth of grass flowers and hanging plants. The research by Yuan Zuhuaetal.[33-34]showed that spraying amino acid water-soluble fertilizer on pepper and lettuce can significantly increase the number of leaves, plant height, plant width and other growth indicators. Fan Yanxiaetal.[35]studied the effects of several foliar fertilizers on the ornamental effect of chrysanthemum, and it showed that spraying foliar fertilizer had significant effect. Similar to the previous research results, spraying sludge extract with appropriate dilution concentration can better promote the growth of flowers. However, when the concentration was too high, the phenomenon of drug damage would occur, which could cause greater damage to plants and thus inhibit their growth. The research by Gao Xiangbinetal.[36-37]also showed that too low or too high application concentration was unfavorable to plant growth.

In this paper,Z.eleganswas taken as research object. Through plant height, stem diameter, flower yield, plant mass and other growth indicators, the effect of sludge extract on the growth ofZ.eleganswas judged, which could provide theoretical basis for plantingZ.elegansin the future.

5 Conclusions

Spraying the sludge extract with a dilution concentration of 600 times onZ.eleganscan improve the quality and ornamental value of its flowers, and the effect was close to or higher than that of commercial foliar fertilizer. The further preparation of the sludge extract into amino acid foliar fertilizer and its application in the garden plants can realize the resource utilization of sludge, bring economic benefits and provide a new way for sewage treatment plants in China, with a good development and application prospect.