Magnesium-modified biochar improves tea quality and growth of tea plant by improving soil properties and promoting nutrient uptake

doi:10.21203/rs.3.rs-4373613/v1

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Magnesium-modified biochar improves tea quality and growth of tea plant by improving soil properties and promoting nutrient uptake

https://doi.org/10.21203/rs.3.rs-4373613/v1

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Soil acidification affects the growth of tea plants and induces magnesium loss, which further reduces tea quality. In this study, magnesium-modified biochar was developed from discarded tea plant branches, via potting test to evaluate the effect on the red loam soil, and to study the quality of tea in acidified tea gardens. Four treatments were set up as control (no biochar added, CK), conventional magnesium fertilizer treatment (FC), tea plant branch biochar treatment (BC) and magnesium-modified biochar with acetic acid (BCY) respectively. Our results showed that soil pH was significantly increased by 0.3 and 0.42 units in BC and BCY treatments respectively, conventional magnesium fertilizer, biochar, and magnesium-modified biochar treatments could significantly increase soil organic matter, alkaline-dissolved nitrogen, effective phosphorus, quick-acting potassium, and exchanged calcium and magnesium content in acidified tea plantations. Compared with CK treatment, both conventional magnesium fertilizer treatment and biochar treatment could increase the root dry weight and tea plant height to some extent; the SPAD values of tea fresh leaves of BC and BCY treatments were significantly higher than those of CK and FC treatments by 17.6, 37.6 and 6.4, 26.4, respectively; and the increase in accumulation of nitrogen, phosphorus, potassium, calcium, and magnesium in tea leaves of FC, BC, and BCY treatments compared with that of CK was 2.49~8.04 g·kg^-1, 0.19~0.49 g·kg^-1, 0.30~3.27 g·kg^-1, 0.26~0.50 g·kg^-1, and 0.15~1.45 g·kg^-1; and SPAD value, tea polyphenols, water leachate, caffeine, and amino acid contents of tea leaves in BC and BCY treatments were significantly higher than CK treatment (P < 0.05). Our study showed that magnesium-modified biochar improved the quality of tea significantly via enhancing the pH, organic matter and nutrient content of soil, increasing the uptake of nitrogen, phosphorus, potassium, calcium and magnesium in the tea plant, and ascending photosynthesis, The best results were obtained with magnesium chloride modified biochar (BCL) treatment. This study plays a guiding role for the improvement and nutrient supplementation of acidified soil in tea plantations and promotes the healthy development of tea plantation soil..

Earth and environmental sciences/Ecology/Agri ecology

Biological sciences/Ecology/Agri ecology

biochar

magnesium-modified

acidified tea garden

soil nutrients

tea nutrient absorption

tea quality

Tea plant is one of the important cash crops, and the area planted of china ranks first in the world¹. According to data from the National Bureau of Statistics², in 2020 China's tea plantation area of 3,216,700 hectares accounted for 62.1% of global tea plantation area and tea production was 2,931,800 tons and represented 46.77% of global tea production. This represented increases of 66.51 and 100.46% than that of 10 years ago³.

Soil Mg availability is thus related to tea plant growth and tea quality⁴. In addition, the optimum pH range for tea plants is 4.5 to 6.0, and the growth of tea plant was inhibited when pH<4.0⁵. Soil acidification can also be detrimental by increasing exchangeable aluminum (Al³⁺) from the soil⁶. Accumulation of Al in tea garden soils can also interchange with Mg soil colloids resulting in Mg leaching losses that adversely affects the tea plant growth⁵ and limits tea yields and quality⁷. In addition, soil acidification leads to changes in soil microbial communities and reduces the decomposition rates of soil organic matter and adversely alters soil ecosystem health⁸.

Soil acidification in tea plantations has been increasing due to a large number of high-intensity anthropogenic activities and an estimated 52% of tea plantation soils possess pH values < 4.5⁹. This has been the result of the acidification effects of chemical fertilizers (especially nitrogen fertilizers) put into the tea plantation soils and increases Mg leaching¹⁰. Soil Mg content was lower than 50 mg kg^-1 in 73% tea gardens in China¹¹. These red soil hilly areas in southern China cause strong leaching due to the warm and humid rainy climate and when coupled with the large amount of nitrogen fertilizers applied, leads to soil acidification, soluble Mg loss and a decline in the effective Mg content and reservoir¹². Therefore, there is an urgent need to explore suitable amendments to alleviate and repair acidified tea garden soils and to increase soil Mg content to meet the Mg nutritional requirements of tea plants.

Biochar is a novel soil conditioner and possesses unique physicochemical properties that promote soil structure and overall health. Biochar was found to be superior in enhancing tea garden soil buffering capacity and long-term applications increased the soil pH of tea garden soils¹³. The application of biochar in acidic red loam tea garden soil also effectively improved soil buffering and microbial activity and these were conducive to soil nutrient transformations¹⁴.

Conventional biochar has certain limitations due to its own properties and the by-products produced during the pyrolysis process. These included a smaller specific surface area and a limited number of surface functional group species¹⁵. Physical, chemical and biological methods have been used to modify traditional biochar to improve biochar efficiency. These treatments increased the specific surface area resulting in an increased number of surface adsorption sites and further improved its adsorption capacity for pollutants such as heavy metals and reactive aluminum¹⁶. In addition, biochar modified by impregnation with metal salt solutions resulted in metal oxide attachment to its surface that could effectively increase the content of soil salt-based ions after application and further increase the soil's ability to retain and supply fertilizer¹⁷. For instance, montmorillonite could be modified using Fe and Al and sludge-based biochar with Mg salt impregnation. These treatments increased the efficiency of adsorptive properties and enhanced catalytic activity^18,19.

There are few studies on the application of magnesium-modified biochar for tea plantations. Therefore, we investigates the effect of magnesium-modified biochar on soil nutrient content and tea quality in tea plantations, and introduces magnesium ions needed for the growth and development of tea plants at the same time of soil improvement, which provides more choices of materials for acidified soil improvement and improves tea quality.

Effect on root dry weight and plant height of tea plants

The results in Fig. 1 showed that both conventional magnesium fertilizer and biochar treatments could increase the root dry weight and plant height of tea plant to some extent compared with CK treatment. The root dry weight of tea plant in BC and BCY treatments was significantly increased by 16.67% and 57.41%, respectively, compared with the CK treatment (P < 0.05), and the root dry weight of tea plant in FC treatment was increased by 5% compared with the CK treatment, but the difference was not significant (P > 0.05).The root dry weight of tea plant in FC, BC and BCY treatments was significantly increased by 4.11%, 8.22% and 10.49%, respectively, compared with the CK treatment and BCY treatments significantly increased tea plant plant height by 4.11%, 8.22% and 10.49%, respectively, compared with CK treatment (P < 0.05). It can be seen that acidified tea garden species adding conventional magnesium fertilizer and biochar can promote tea plant root system and tea plant growth to a certain extent, in which the application of biochar on the tea plant root growth is significantly better than the conventional application of magnesium fertilizer, and the application of magnesium-modified biochar on the root system of the tea plant and the growth of the tea plant are better than the application of common biochar to promote the effect of the tea plant root system and the growth of the tea plant..

Effect on soil pH and organic matter in tea plantations

The effects of different treatments on soil pH and organic matter in tea plantations are shown in Fig. 2, BC and BCY treatments to tea plantation soils both increased soil pH and organic matter content, and the soil pH of conventional magnesium fertilizer treatment was lower than that of the control; the highest soil pH was found in the BCY treatment, and the soil pH of BC and BCY treatments was significantly increased by 0.3 and 0.42 units, respectively(P < 0.05) ); while FC treatment soil pH decreased by 0.02 units (P > 0.05). The effect of different treatments on the enhancement of soil organic matter content in tea plantations was BC > BCY > FC, and the soil organic matter content of tea plantations in BC and BCY treatments was significantly higher than that in CK treatment by 28.04 and 22.93 g·kg^-1 (P < 0.05). Thus, the application of biochar and magnesium-modified biochar in tea garden soil could significantly increase soil pH and organic matter content, thus slowing down the acidification of tea garden soil and providing a long-term source of organic carbon for the soil, in which the effect of magnesium-modified biochar in increasing soil pH was better than that of ordinary biochar.

Effect on soil nutrient content in tea plantations

Adding BC and BCY treatments could improve the nutrient content of tea garden soil to a certain extent, and the alkaline nitrogen content of tea garden soil in BC and BCY treatments was increased by 3.04 mg·kg^-1 and 1.27 mg·kg^-1 compared with CK treatment(Table 1); FC、BC and BCY treatments could all increase the effective phosphorus and quick-acting potassium content of tea garden soils, and the effective phosphorus content of FC, BC and BCY treated soils was significantly higher than that of the CK treatment by 6.19 mg·kg^-1, 12.60 mg·kg^-1 and 23.06 mg·kg^-1, respectively (P < 0.05);Soil quick potassium content was increased by 1.87 mg·kg^-1 in FC treatment and significantly increased by 14.39 mg·kg^-1 and 5.00 mg·kg^-1 in BC and BCY treatments, respectively, over CK treatment (P < 0.05)..

Table 1

Effect on soil nutrient content in tea plantations
Treatment	Soil nutrient content in tea garden
Treatment	N(mg/kg)	P(mg/kg)	K(mg/kg)
CK	45.14 ± 1.67a	27.31 ± 1.28d	35.67 ± 0.84c
FC	34.09 ± 4.71b	33.50 ± 1.05c	37.54 ± 1.84b
BC	48.18 ± 1.40a	39.91 ± 2.22b	50.06 ± 3.19a
BCY	46.41 ± 2.79a	50.37 ± 1.05a	40.67 ± 0.88b
Note : Different lowercase letters in the same column indicate significant differences between treatments ( P < 0.05 ), the same below.

Effect of different treatments on exchangeable calcium and magnesium in tea garden soil

As can be seen from Fig. 3: the soil exchangeable calcium content with added biochar and magnesium-modified biochar treatments was significantly higher than CK and FC treatments, and the soil exchangeable calcium content with BC and BCY treatments was significantly increased by 16.96% and 21.74% and 16.45% and 21.21%, respectively (P < 0.05). The addition of magnesium-modified biochar to the tea garden soil could significantly increase the soil exchangeable magnesium content, and the BCY treatment had the highest soil exchangeable magnesium content of 4.71 cmol·kg^-1; the soil exchangeable magnesium content of the BCY treatment was significantly increased by 3.09 cmol·kg^-1 and 3.87 cmol·kg^-1 compared with that of the CK treatment and the BC treatment (P < 0.05). It can be seen that both added biochar and magnesium-modified biochar can improve the soil exchangeable calcium and magnesium content of acidified tea plantations to a certain extent, while the effect of magnesium-modified biochar treatment in enhancing soil exchangeable magnesium content was more obvious..

Effect of different treatments on photosynthesis of tea plant

The strength of photosynthesis of tea plant was indicated by the relative chlorophyll content (SPAD value) of tea fresh leaves, and the changes of SPAD value of tea fresh leaves in different treatments were shown in Fig. 4. As can be seen from Fig. 4, the size of SPAD value was BCY>BC>FC>CK, and conventional magnesium fertilizer and added biochar and magnesium-modified biochar treatments could significantly increase the SPAD value of tea plant fresh leaves, in which the SPAD value of tea plant fresh leaves of BC and BCY treatments were significantly higher than that of CK and FC treatments by 17.6, 37.6, and 6.4, 26.4, respectively (P<0.05). It can be seen that conventional magnesium fertilizer and biochar addition can significantly increase the SPAD value of tea plant fresh leaves, promote the photosynthesis of tea plants in acidified tea plantations, and facilitate the accumulation of carbohydrates in tea leaves, among which the magnesium-modified biochar treatment has a better effect.

Effect of different treatments on nutrient accumulation in tea leaves

As shown in Table 2, FC、BC and BCY treatments could promote the uptake of N, P, K, Ca and Mg nutrients in tea leaves; the increase in the accumulation of N in tea leaves of FC, BC and BCY treatments ranged from 2.49 to 8.74 g·kg^-1 compared with CK, in which the accumulation of N in tea leaves of BC and BCY treatments increased significantly by 11.33% and 18.43% ( P < 0.05); the accumulation of P in tea leaves of FC, BC and BCY treatments increased in the range of 0.19–1.44 g·kg^-1, of which the accumulation of P in tea leaves of BCY treatment increased significantly by 16.61% (P < 0.05); Compared with CK, the increases of K and Ca accumulation in tea leaves of FC, BC and BCY treatments were 0.30–3.27 g·kg^-1 and 0.26–0.84 g·kg^-1, and the K and Ca accumulation in tea leaves was highest in BCY treatment, and the differences of Ca accumulation in tea leaves among treatments did not reach the significant level; Magnesium-modified biochar treatment was able to significantly increase tea Mg accumulation compared with other treatments, and tea Mg accumulation increased significantly by 57.09% in BCY treatment compared with CK. It can be seen that conventional magnesium fertilizer, added biochar and magnesium-modified biochar can all promote the uptake of N, P, K, Ca and Mg nutrients in tea leaves, thus promoting the growth of tea plants, of which the comprehensive results of magnesium-modified biochar treatment had the best effect.

Table 2

Difference analysis of nutrient accumulation in tea
Treatment	Nutrient accumulation of tea(g/kg)
Treatment	N	P	K	Ca	Mg
CK	43.62 ± 2.08d	2.95 ± 0.47c	4.45 ± 0.21c	8.77 ± 0.45a	2.54 ± 0.11b
FC	46.11 ± 1.01cd	3.14 ± 0.05bc	4.75 ± 0.20bc	9.03 ± 0.83a	2.69 ± 0.17b
BC	48.59 ± 1.72bc	3.26 ± 0.05bc	4.95 ± 0.05b	9.22 ± 0.02a	2.76 ± 0.06b
BCY	51.66 ± 1.58ab	3.44 ± 0.13a	7.72 ± 0.22a	9.27 ± 0.17a	3.99 ± 0.32a

Effect of different treatments on the main quality components of tea leaves

As shown in Table 3, FC、BC and BCY treatments could affect the tea quality, and the tea polyphenol content of tea leaves showed BC>BCY>FC, of which the tea polyphenol content of tea leaves in BC treatment was the highest (16.53%), which was significantly higher than that of CK and FC treatments (P<0.05); Compared with the CK treatment, the caffeine and amino acid contents of tea leaves of BCY, BC and FC treatments were significantly increased by 39.04%, 30.93%, 14.71% and, 15.81%, 15.20% and 12.16%, respectively (P < 0.05). The variation of water leachate content of tea leaves in different treatments ranged from 37–48%. Conventional magnesium fertilizer, added biochar and magnesium-modified biochar could increase the water leachate content of tea leaves to different degrees, and added magnesium-modified biochar BCY treatment increased the water leachate significantly by 25.16% compared with the CK treatment (P < 0.05).The phenol-ammonia ratio of tea leaves in different treatments was CK>BC>BCY>FC, and the lowest phenol-ammonia ratio of tea leaves in BCY treatment with magnesium-modified biochar was significantly lower than that in CK treatment. In conclusion, adding BC and BCY treatments can improve the content of various quality components of tea, in which the effect of improving the quality of tea is more obvious with the addition of magnesium-modified biochar BCY treatment.

Table 3

Difference analysis of main quality components content of tea
Treatment	Main Quality Components of Tea
Treatment	tea polyphenol(%)	caffeine(%)	amino acid(%)	water extracts(%)	phenol ammonia
CK	15.54 ± 0.42b	3.33 ± 0.15c	3.29 ± 0.10c	37.59 ± 1.26b	4.47 ± 0.22a
FC	15.53 ± 0.25b	3.82 ± 0.13b	3.69 ± 0.06b	39.22 ± 0.31b	4.21 ± 0.03b
BC	16.53 ± 0.22a	4.36 ± 0.18a	3.79 ± 0.06ab	43.66 ± 1.45b	4.36 ± 0.02b
BCY	16.51 ± 0.61a	4.63 ± 0.16aa	3.81 ± 0.05a	47.05 ± 1.65a	4.32 ± 0.11b

Effect of magnesium-modified biochar on the improvement of acidified tea plantation soil

The optimal pH range for tea plant growth is 4.5 to 6.0, and our study showed that the application of biochar could increase the pH of acidified tea plantation soil. Biochar is alkaline and contains alkaline groups, which can neutralize the H⁺ contained in the soil solution after being applied to the soil, which is consistent with the results of Xie and Wu^21,22. Biochar is a carbon-containing material with a porous structure and stable, and the pH of the tea garden soil decreased significantly after a long period of time in the potting experiment, which may be related to the fact that the nitrogen in the organic matter was converted by the microorganisms into ammonia nitrogen and ammonium root ions (NH₄⁺) and further nitrified to release H^+ 23. We also found that BCY treatment was more effective than BC treatment in increasing the pH of acidified tea plantation soil, which was attributed to the fact that BCY treatment was introduced with compounds (magnesium oxide, etc.) in the process of preparation, which were able to form more chemical bonds with the biochar indicating that the magnesium-modified biochar had a higher specific surface area, contained soluble alkaline matrix substances²⁴, which could adsorb the H⁺ in the soil solution and increase soil salt-based ion content²⁵.

Organic matter is an important component of soil, which contains various nutrients needed for plant growth and plays a very important role in soil fertility and sustainable agricultural development²⁶. Some studies have shown that the direct addition of crop residues²⁷, organic fertilizers, and other agricultural and forestry waste organic matter to the soil²⁸. Biochar can increase soil organic carbon content²⁹. In this experiment, the biochar applied to the soil of acidified tea plantation, which itself belongs to the biomass material with high carbon content, was directly applied to the soil of tea plantation which is equivalent to the direct input of a large amount of organic carbon into the soil, which is conducive to the effect of increasing the accumulation of soil organic matter. This is similar to the results of many studies, such as Li³⁰, which showed that both rice and corn stover charcoal could improve the acidity of red loamy rice soil, and increase soil organic carbon and soil nutrient content. Xu³¹found that soil application of biochar could increase the total soil carbon stock of moso bamboo forests, thus promoting the carbon sequestration capacity of moso bamboo forest ecosystems through long-term field experiments in moso bamboo forests. As a carbon-rich biomass material, biochar plays a significant role in sequestering carbon and increasing sinks³².

Soil nutrient content is a direct source of nutrient uptake for tea plant growth, and some studies have shown that the application of biochar can significantly increase soil nutrient content³³. This study showed that BC and BCY treatment increased the alkaline dissolved nitrogen content of tea garden soil compared with CK treatment, indicating that soil addition of biochar may promote the mineralization of soil organic nitrogen, thus releasing inorganic nitrogen³⁴. In addition, it has been found that soil addition of biochar reduces leaching and volatilization loss of soil nitrogen, thus increasing soil nitrogen content³⁵. In this study, it was found that both BC and BCY treatment could increase the effective phosphorus content of tea garden soils, which was attributed to the fact that, on the one hand, the solubility of PO₄^3- was low in the more acidic soil conditions, and the biochar increased the soil pH thereby promoting the dissolution of PO₄^3- in the soil³⁶; on the other hand, the biochar itself carried phosphorus, which was released by adding it to the soil thereby increasing the effective phosphorus content of the soil³⁷. Changes in soil quick potash in tea plantations may be related to changes in soil pH in tea plantations³⁸, and the results of this study showed that all biochar treatments increased soil quick potash content, the reason for this is because biochar itself contains a certain amount of potassium, so that the application of all biochar increased soil quick potash content in tea plantations³⁹.

Soil exchangeable calcium and magnesium is an important component of soil salt-based substances and is a very important chemical property of soil⁴⁰. In this study, both BC and BCY treatment could increase the soil exchangeable calcium and magnesium content of acidified tea plantations to a certain extent, which could be attributed to the fact that biochar contains a large number of salt-based ions, which were added to the soil to increase the soil exchangeable calcium and magnesium content on the one hand⁴¹, and on the other hand, it might be due to the fact that the soil acid solubilization promotes certain soil minerals to release salt-based ions⁴². This study also found that the addition of BCY treatment to the soil significantly increased the soil exchangeable magnesium content⁴³, suggesting that the magnesium element loaded on the biochar was converted into exchangeable Mg²⁺ in the soil, similar to the results of other studies on the amelioration of soil acidification⁴⁴. Dai⁴⁵found that modified biochar prepared by mixing raw materials could increase the pH, salinity ions and pH buffering properties of the soil, as well as reduce the exchangeable aluminum concentration of the soil. Qin⁴⁶used magnesium-modified biochar for indoor soil simulation tests and found that magnesium-modified biochar prepared at different pyrolysis temperatures not only had larger porosity but also had more -OH and -COOH functional groups compared with unmodified biochar, which could reduce the soil acidity and at the same time, effectively improve the soil fertility, soil exchangeable calcium and magnesium content.

For tea garden soils with high acidification and low organic matter content, using the qualities of magnesium-modified biochar for amendment can significantly reduce soil acidity⁴⁷and increase the organic matter content of acidified tea garden soils^48,49while also increasing soil nutrient content ⁵⁰, so that the tea garden soils develop in the direction of being more conducive to the growth of tea plants, and improve the yield of tea gardens and the quality of tea leaves^51,52.

Effect of magnesium-modified biochar on the growth of tea seedlings and tea quality

The addition of biochar improves soil acidity, increases soil organic matter content, and improves soil nutrient content so as to meet the nutrient demand of tea plant during its reproductive period⁵³, and significantly enhances the absorption and accumulation of nitrogen, phosphorus, potassium, and calcium and magnesium nutrients in tea. Plant root system is an important organ for absorbing soil nutrients, and its growth and development will directly affect the absorption of soil nutrients by plants. In this experimental study, it was found that biochar in acidified tea garden soil could promote the growth of tea plant roots, which was due to the fact that on the one hand, biochar was able to improve soil acidity, reduce soil bulk weight, and increase soil aeration pore space⁵⁴; on the other hand, it was able to improve soil pH, salinity ions, and alleviate the toxic effect of aluminum in acidified soils, which in turn promoted the growth of tea plant roots⁵⁵. This study also found that BCY treatment promoted the root growth of tea seedlings significantly better than BC treatment, which is because magnesium-modified biochar can increase the magnesium necessary for the growth of tea plants while alleviating aluminum toxicity, thus promoting the root growth and development of tea plants.

In this study, BC and BCY treatment could promote the uptake of nitrogen, phosphorus, potassium, and calcium and magnesium nutrients in tea leaves to different degrees, and tea plant is an ammonium-loving plant, and the addition of biochar significantly increased the supply of alkaline dissolved nitrogen (ammonium nitrogen) in the soil, which promoted the uptake of nitrogen by tea plant⁵⁶; Magnesium is an important component of tea plant chlorophyll and affects tea plant leaf photosynthesis⁵⁷, due to the addition of biochar increased the effective magnesium content of acidified tea garden soil, promoting the absorption of magnesium by tea plants and the synthesis of tea chlorophyll, thus enhancing tea photosynthesis. In this study, magnesium-modified biochar, because of its large magnesium loading, was added to the soil to significantly increase the absorption of magnesium by tea plants, significantly increase the SPAD value of the fresh leaves of tea plants in acidified tea gardens, and contribute to the promotion of photosynthesis and the synthesis of carbohydrates in tea leaves.

Tea garden soil nutrient content directly affects the growth and development of tea plants, which in turn affects the quality of tea, tea quality is generally determined by the tea polyphenols, caffeine, amino acids, water leachate and phenol-ammonia ratio and other indicators⁵⁸. Tea polyphenol consists of various phenolic compounds, mainly affects the aroma and taste of tea, tea unique tea flavor and sweetness from tea polyphenol, tea polyphenol content is appropriate at about 20%, too high will lead to tea bitterness and astringency directly affect the quality of tea⁵⁹. In this study, the tea polyphenol content of all treatments ranged from 15.50–17.00%, and the addition of biochar was able to increase the tea polyphenol content, of which the tea polyphenol content of BCY treatment was the highest and significantly higher than that of the CK and FC treatments (P < 0.05). This is due to the fact that tea polyphenol content is closely related to the nutrients (nitrogen, phosphorus and potassium) in tea leaves, and an increase in the accumulation of nitrogen, phosphorus and potassium nutrients increases the tea polyphenols synthesized by the tea plant⁶⁰. The study of Jiang⁶¹found that the application of biogas and biochar can increase the amino acid content of tea and help to improve the quality of tea, which is the same as the results of this study. Caffeine and amino acids are important substances for the freshness of tea broth, and water leachate is important for the strong flavor of tea broth⁶². The results of this study showed that BC and BCY treatment could increase the content of caffeine, amino acids and water leachate in tea, and the magnesium-modified biochar enhancement effect was significantly better than that of unmodified biochar, which could make the tea broth fresh and refreshing, with a high and long aroma, and better quality.

Soil Information

The test soils were taken in June 2022 from a tea plantation base in Lin'an District, Hangzhou City, Zhejiang Province, which was planted with tea plants for 10 years. The area has a subtropical monsoon climate with an annual average temperature of 15.9℃, an average annual precipitation of 1420 mm and a red loam soil texture. Soil samples were taken according to the "S" type multi-point mixed collection at 0 ~ 20 cm depths, air-dried followed by stone and dead leave removal. The soil was then ground and passed through a 2 mm sieve and used for basic chemistry testing (Table 4).

Table 4

Basic physical and chemical properties of tea plantation soil used in these experiments
	pH	Organic matter (g kg^− 1)		Available N (mg kg^− 1))	Available P (mg kg^− 1)	Available K (mg kg^− 1)	Exchangeable Mg (cmol kg^− 1)
red loam soil	4.51		12.3	32.93	27.67	67.33	0.78

Biochar preparation

Discarded tea plant branches were collected, rinsed 2–3 times with distilled water then dried at ambient temperature. The branches were then crushed and pyrolyzed under anaerobic conditions at 500°C for 2h in a muffle furnace. This biochar was designated BC. Modified biochars were prepared using tea plant twig powder was homogeneously mixed with Mg(CH₃COO)₂ (designated ‘Y’ treatment)solutions at 1 g:10 mL (Table 5). The mixtures were oscillated at frequency of 220 rpm at 30°C for 1 h, sealed and impregnated for 23 h, filtered, dried at 105°C, and placed in a muffle furnace and pyrolyzed under anaerobic conditions at 500°C for 2 h. The furnace was then allowed to cool to room temperature and the obtained product was ground and passed through a 100-mesh sieve to produce Mg-modified biomass charcoal (MgBC).

Table 5

Biochar-modified salt solutions
Serial number	Modifier	Concentration (M)
BCY	Mg(CH₃COO)₂	1.5

Experimental design

Four treatments were set up: control (no biochar added, CK), conventional magnesium fertilizer treatment (FC), tea plant branch biochar treatment (BC), and, magnesium acetate modified biochar (BCY), with three replications for each treatment. High-quality tea seedlings with good growth and basic uniform plant size were selected, and the seedlings were taken to ensure that the root system was intact and able to survive. Pour 5 kg of soil into the potting bowl, mix the biomass charcoal and soil (2%) thoroughly, fertilizer as a basal fertilizer applied at once, each potting bowl transplanted 4–6 tea seedlings, the first watering watered thoroughly, field water holding capacity to maintain 70%. Cultivation of the early need to be based on the survival of tea seedlings, there is a need to replenish seedlings, the 30th d of the test each pot of 1.25 g urea, usually potted tea plants in accordance with the daily management of the tea garden field, the experimental management measures of the treatment is the same.

The entire treatment process for a total of 3 months, the collection of tea plant samples, tea seedlings with the root system complete take out and bring back to the laboratory, with deionized water to quickly wash the root system attached to the soil after the tea roots and stems and leaves are separated, the quality of tea sampling standards for the first bud and two leaves of the tea fresh leaves, packed into envelopes and marking, put into the oven at 105 ℃ to kill the green for 1h in 50 ℃ drying to constant weight, the plant samples placed in a pulverizer, and then dried to constant weight. The plant samples were ground in a pulverizer and sieved, and stored in plastic self-sealing bags for the determination of tea-related indexes. Soil samples were taken by broken ring sampling, soil samples were dried naturally, ground and sieved, and then stored for the determination of related indexes.

Indicators and methods

The analytical methods of soil samples were referred to Soil Agrochemical Analysis (Third Edition)²⁰: soil pH was determined by potentiometric method with soil-water ratio of 1:2.5; soil organic matter was determined by volumetric method with externally heated potassium dichromate; effective soil phosphorus was determined by hydrochloric acid-ammonium fluoride dissolution-molybdenum antimony antimony colorimetry; quick-acting potassium of the soil was determined by ammonium acetate leaching and flame photometric method; alkaline nitrogen of the soil was measured by alkaline dissolution diffusion method Soil cation exchange capacity (CEC) was determined by 1 mol/L ammonium acetate exchange method; soil exchangeable salty ions were extracted by 1 mol/L ammonium acetate solution, and Ca and Mg in the extract were determined by atomic absorption spectrophotometry.

Measurement of tea-related indexes: SPAD value of tea leaves in each treatment was determined by chlorophyll meter; total nitrogen of tea leaves was determined by carbon and nitrogen analyzer method; total phosphorus was determined by H₂SO₄-H₂O₂ digestion-molybdenum antimony antimony colorimetry method; total potassium was determined by H₂SO₄-H₂O₂ digestion-flame photometer method; total calcium and magnesium were determined by H₂SO₄-H₂O₂ cooking-atomic absorption spectrophotometry was used for total potassium; total calcium and magnesium were used for total calcium and magnesium. Tea polyphenols were determined by the colorimetric method of forintol (GB /T 8313 − 2018); caffeine was determined by ultraviolet spectrophotometry (GB /T 8312 − 2013); free amino acids were determined by the colorimetric method of ninhydrin (GB /T 8314 − 2013); water extracts were determined by the boiling water extraction method (GB /T 8314 − 2013); water extracts were determined by the flame photometric method of HSO4H2O2 digestion and atomic absorption spectrophotometry. leachate was determined by boiling water extraction-weight method (GB /T 8305 − 2013).

Statistical analyses

One-way analysis of variance (ANOVA) was performed using IBM SPSS Statistics 23 software. Duncan’s method was used to make multiple comparisons of the experimental data. The significance level of the difference was P < 0.05 using Origin 2021. The experimental data were expressed as mean ± standard deviation (SD).

Research involving plants statement

Experimental research and feld studies on plants (either cultivated or wild), including the collection of plant material, comply with relevant institutional, national, and international guidelines and legislation.

Tea trees purchased by Zhejiang A&F University, Plant samples are stored in the School of Environment and Resources, Zhejiang A&F University

Our study revealed the effects of magnesium-modified biochar on soil nutrient content and tea quality in acidified tea gardens. Biochar (both ordinary Biochar, magnesium-modified biochar) could improve soil pH and soil organic matter and our study showed that magnesium-modified biochar improved the quality of tea significantly via enhancing the pH, organic matter and nutrient content of soil, increasing the uptake of nitrogen, phosphorus, potassium, calcium and magnesium in the tea plant, and ascending photosynthesis, The best results were obtained with BCY treatment.

This study plays a guiding role for the improvement and nutrient supplementation of acidified soil in tea plantations and promotes the healthy development of tea plantation soil.

Funding

This work was financed by the National Natural Science Foundation of China (32371543), Zhejiang High-level Talents Special Support Program (2020R52026), Research and Science Development Fund of Zhejiang A&F University(2023FR012).

Institutional Review Board Statement

No humans or animals were involved in this study.

Informed Consent Statement

Not applicable.

Data Availability Statement

The datasets generated and/or analysed during the current study are not publicly available due [For the sake of privacy of the individuals and team labs involved in the research] but are available from the corresponding author on reasonable request.

Conflicts of Interest

The authors declare no conflict of interest.

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No competing interests reported.

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Magnesium-modified biochar improves tea quality and growth of tea plant by improving soil properties and promoting nutrient uptake

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Abstract

Figures

Introduction

Results

Effect on root dry weight and plant height of tea plants

Effect on soil pH and organic matter in tea plantations

Effect on soil nutrient content in tea plantations

Effect of different treatments on exchangeable calcium and magnesium in tea garden soil

Effect of different treatments on photosynthesis of tea plant

Effect of different treatments on nutrient accumulation in tea leaves

Effect of different treatments on the main quality components of tea leaves

Discussion

Effect of magnesium-modified biochar on the improvement of acidified tea plantation soil

Effect of magnesium-modified biochar on the growth of tea seedlings and tea quality

Materials and Methods

Soil Information

Biochar preparation

Experimental design

Indicators and methods

Statistical analyses

Research involving plants statement

Conclusions

Declarations

References

Additional Declarations

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