Effects of Combined Application of Biochar-based Organic Fertilizer and Reduced Nitrogen Fertilizer on Soil Enzyme Activity and Yield of Purple Cabbage (Brassica oleracea var. capita rubra)

Ben YANG, Xiaoying LI, Yuechao WANG, Mengjie CHEN, Xiaoqin CHEN

in Yuanmou County

Abstract ［Objectives］ In response to the issue of soil improvement in Yuanmou County， the effects of combined application of biochar-based organic fertilizer and reduced nitrogen fertilizer on soil nutrients， soil enzyme activity， and yield of purple cabbage （Brassica oleracea var. capita rubra） were investigated in the field base of Institute of Thermal Zone Ecological Agriculture， Yunnan Academy of Agricultural Sciences in Yuanmou County.

［Methods］ A total of 13 treatments were set up by applying biochar-based organic fertilizer at three levels of 15， 30 and 45 t/hm2 （T1， T2， T3）， combined with top application of nitrogen fertilizer （urea） at four levels： 375 （N1）， 300 （N2）， 225 （N3） and 0 kg/hm2， with non-fertilizing treatment as control check （CK）， in order to explore the optimal ratio for the combined application of biochar-based organic fertilizer with nitrogen fertilizer.

［Results］ The application of biochar-based organic fertilizer could significantly improve soil nutrients， enzyme activity， and purple cabbage yield. The improvement effect of combined application with nitrogen fertilizer was higher than that of single application of biochar-based organic fertilizer， and the improvement effect was enhanced with the application amount of biochar-based organic fertilizer increasing. The contents of organic matter and total nitrogen were the highest in treatment T3N3， of which the values increased by 81.39% and 56.09% compared with the CK， respectively. The contents of soil hydrolyzable nitrogen， available phosphorus， and available potassium were all the highest under treatment T3N2， with increases of 92.76%， 171.01% and 235.50%， respectively. There was a significant positive correlation between the activity of soil catalase， urease， and sucrase and organic matter， total nitrogen， and available nutrients. The overall soil enzyme activity was relatively higher in treatment T3N2. The yield of purple cabbage treated with biochar-based organic fertilizer combined with nitrogen fertilizer could reach 85 750 kg/hm2， which was 94.78% higher than that treated with biochar-based organic fertilizer alone. Based on comprehensive analysis， the optimal combination ratio was 45 t/hm2 of biochar-based organic fertilizer and 300 kg/hm2 of urea （T3N2）.

［Conclusions］ This study provides data support for the promotion of biochar-based organic fertilizers and reduced fertilizer in agricultural soil in the Dam area of Yuanmou County.

Key words Soil enzyme activity; Yield; Biochar-based organic fertilizer; Nitrogenous fertilizer; Purple cabbage

Received： January 4， 2023  Accepted： March 5， 2023

Supported by Key R&D Program of the Ministry of Science and Technology of China （2017YFC0505102-4）.

Ben YANG （1996-）， male， P. R. China， master， devoted to research about environmental ecology and soil and water conservation.

*Corresponding author. E-mail： Lxying92@126.com.

Yuanmou County in Yunnan Province belongs to the typical dry and hot valley of Jinsha River， with a subtropical monsoon valley dry and hot climate［1］. Due to its abundant light and heat conditions， it has become one of the largest winter early vegetable production areas in Yunnan， and the amount of vegetables sold to other provinces account for one fourth of the total amount of winter and spring vegetables sold from Yunnan Province to other provinces. Although the area has sufficient sunlight， it has a dry climate， uneven distribution of rainfall， strong evaporation， prominent water-heat contradiction， and low agricultural productivity. It is one of the typical fragile ecosystems in China. The soil in Yuanmou Basin is mainly dry red soil， with a high content of clay particles， which reduces the soils ability to maintain fertilizer and water， which is not conducive to crop growth［3］. Moreover， in pursuit of yield， the local government has heavily used chemical fertilizers， especially nitrogen fertilizers， which have led to a decrease in soil quality and nutrient loss. How to improve soil and effectively utilize land resources has always been the pursuit of scholars devoted to soil research.

Biochar is a new material widely used in soil improvement in recent years. Because of its good pore structure and strong adsorption capacity， biochar can not only improve soil structure， but also effectively ameliorate soil physical and chemical properties， maintain soil nutrients and improve soil［4］. Bochar-based fertilizers are a new type of functional ecological fertilizer with slow-release performance， which is made by mixing biochar and ordinary fertilizers in a certain proportion by specific methods［5］. Organic fertilizers contain a certain amount of nitrogen， phosphorus， potassium， and various trace nutrients， but their fertilizer efficiency is slow. Biochar-based organic fertilizers combine the advantages of biochar and organic fertilizers， and have superior characteristics compared with their raw materials. They can slowly release nutrients， maintain fertilizer efficiency， and continuously improve the soil environment. Therefore， they have been promoted and used. Yang et al.［6］ showed that the application of biochar-based slow-release fertilizers could significantly improve soil nitrogen， phosphorus， and potassium nutrient contents and peanut yield. Zhang et al.［7］ found that the combination of biochar-based fertilizers and reducing the application of nitrogen fertilizer by 15% could maintain stable cotton yield and improve nitrogen use efficiency. Therefore， the combination of biochar-based organic fertilizers with nitrogen fertilizers can reduce the amount of nitrogen fertilizers applied， while ensuring yield and avoiding various land problems caused by application of excessive nitrogen fertilizer.

Soil enzymes are catalysts that are released by microorganisms， living animals and plants， and their debris into the soil to participate in biochemical reactions in the soil. The activity of soil enzymes reflects the level of soil microbial activity， and is a characterization of the strength of soil nutrient transformation and transport capacity［8-10］. It is an important indicator for evaluating soil quality. The application of biochar-based organic fertilizers to the soil has a significant impact on soil physical and chemical properties and microbial activity. Hou et al.［11］ found through indoor cultivation experiments that the addition of a small amount of rice straw biochar improved the biological environment of soil， increased the activity of soil urease， sucrase， and neutral phosphatase， and decreased the activity of soil catalase. Gu et al.［12］ showed that applying corn straw biochar before soybean planting increased the activity of sucrase， catalase， and urease in the soil.

The fertilizer requirements of different crops in different regions need to be continuously tested and obtained. Purple cabbage （Brassica oleracea var. capita rubra） is a variety of Brassica in the Cruciferae family， which has the advantages of fast growth， early yield， long shelf life， and high economic value， and is one of the key vegetables cultivated in the dam area of Yuanmou County. In this study， certain amounts of biochar-based organic fertilizer and nitrogen fertilizer were applied to determine the changes in important enzyme activity in the soil and their impact on purple cabbage yield and select the optimal ratio for combined application of biochar-based organic fertilizer and nitrogen fertilizer， providing data support for the promotion of biochar-based organic fertilizer and reduced chemical fertilizers in agricultural soil in the Dam area of Yuanmou County.

Materials and Methods

Generation situation of the study area

The study area is located in Yuanma Town， Yuanmou County， Chuxiong Yi Autonomous Prefecture， Yunnan Province. The terrain is a dry and hot valley plain area， which has a subtropical dry and hot monsoon climate. This area has distinct dry and wet seasons， sufficient heat， a large temperature difference between day and night， and four indistinct seasons. There is basically no frost throughout the year. The perennial average temperature is 22.9 ℃， and the extreme maximum temperature is 42 ℃. The average annual rainfall is 562 mm. The rainy season usually starts in May and ends in October. The study area mainly has dry red soil， and the basic properties of the tested soil at 0-20 cm are shown in Table 1.

Test materials

The tested purple cabbage was ‘Puluomiyou cultivated by Institute of Thermal Zone Ecological Agriculture， Yunnan Academy of Agricultural Sciences. The tested biochar-based organic fertilizer （C：N：P2O5：K2O=15.9：1.3：1.4：2.9） was provided by Yunnan Tianjiu Agriculture and Forestry Company， and the tested nitrogen fertilizer was urea （N≥ 46.0%）.

Test design

The sample plots adopted a combination design of 4×3. No application of biochar-based organic fertilizer and topdressing served as the control check （CK）， and the experiment was set with 13 treatments （Table 2）， with three replicates for each treatment. There were 39 plots in the study area， and each plot had an area of 2.5 m2， and planted with 30 purple cabbage plants. On October 27， 2021， the soil in the study area was uniformly turned over and sun-dried， and the soil was taken back to the laboratory for natural air drying and screening. The basic background values of the soil were determined， and the biochar-based organic fertilizer was applied as the base fertilizer to the sample plots， and mixed evenly with the soil. Starting from November 8， borders were made with a high border surface and low ditches. The borders were 70 wide， and the ditches were 30 cm wide. The border surface was covered with a silver gray dual-color film， and the plant and row spacing of film holes was 30 cm×25 cm. Purple cabbage seedlings with consistent growth （7-8 cm in height） were planted in the sample plots. During the experiment， water should be poured once every 3-4 d on average， and the irrigation method for the experiment should be surface drip irrigation. Top application was carried out 10 d after planting for the first time， and then once every 30 d， totaling four times， involving urea and P and K fertilizers later. The application rate was 150 kg/hm2 in all plots except for the CK. Attention should be paid to keeping no weeds in the plots throughout the entire cultivation process.

Soil sample collection and determination methods

At one month after top application each time， soil samples were taken until purple cabbage was fully ripe and picked， and soil enzyme activity was measured after air drying， sieving， and numbering. The measurement methods for various indexes were as follows： ① soil pH：pH meter with a soil-to-liquid ratio of 1：2.5， ② soil organic matter： oil bath heating-potassium dichromate oxidation volumetric method， ③ soil total nitrogen： Kjeldahl method， ④ soil alkali-hydrolyzable nitrogen： alkali-hydrolyzed diffusing method， ⑤ soil available phosphorus： sodium bicarbonate leaching-molybdenum antimony anticolorimetric method， ⑥ soil available potassium： ammonium acetate-flame photometer method， ⑦ soil urease activity： indophenol colorimetric method， ⑧ soil catalase activity： potassium permanganate titration method， ⑨ soil sucrase activity： 3，5-dinitrosalicylic acid colorimetric method， and ⑩ yield： harvesting and weighing. For the determination of yield， ripe purple cabbage was harvested after maturation （March 2022）， and weighed to record to the weight of purple cabbage in each plot.

Data processing

Excel 2020 was adopted to collate the data， and SPSS 22.0 was used for single-factor， double-factor， and correlation analysis. Origin 2021 was used to draw bar charts， and the Waller Duncan method was applied to compare the significance of differences between treatments （P<0.05）.

Results and Analysis

Effects of combined application of biochar-based organic fertilizer and nitrogen fertilizer on soil physical and chemical properties

Fig. 1 shows that compared with the control without applying biochar-based fertilizer and nitrogen fertilizer （CK）， the combination of biochar-based organic fertilizer and nitrogen fertilizer improved soil pH， organic matter， total nitrogen， and available nutrient contents in purple cabbage fields after 4 months. Among them， pH only increased by 0.42-0.95 units， and there were no significant changes in pH value between different fertilization treatments.

The application amount of biochar-based organic fertilizer increased from 15 to 45 t/hm2， and the contents of total nitrogen and available nutrients increased with the application rate of biochar-based organic fertilizer increasing. Under the treatment of 45 t/hm2 （T3） of biochar-based fertilizer， the contents of soil total nitrogen， hydrolyzable nitrogen， available phosphorus， and available potassium increased by 6.47%， 48.22%， 57.60%， and 31.35%. respectively， compared with the treatment of 15 t/hm2 （T1） of biochar-based fertilizer. The soil organic matter content also significantly increased under the application of 45 t/hm2 of biochar-based organic fertilizer.

Under application of the same amount of biochar-based organic fertilizer， the effect of combining with nitrogen fertilizer on improving soil organic matter， total nitrogen and available nutrient contents was always higher than that of applying biochar-based fertilizer alone， and with the increase of topdressing amount， all showed a trend of first increasing and then decreasing. Topdressing treatments with N1 had a lower improving effect on soil nutrients than topdressing treatments with N2 or N3. The soil organic matter and total nitrogen contents were the highest under T3N3 （biochar-based fertilizer 45 t/hm2+urea topdressing 225 kg/hm2） treatment， which increased the contents by 2.89% and 12.36% compared with T3N1 （biochar-based fertilizer 45 t/hm2+urea topdressing 375 kg/hm2）， respectively， so reducing the application of nitrogen fertilizer was effective. The improvement of soil available nutrients was more significant. Treatment T3N2 （biochar-based fertilizer 45 t/hm2+ urea topdressing 300 kg/hm2） had the highest relative contents of available nitrogen， available phosphorus， and available potassium， which were 36.66%， 10.23% and 25.60% higher than T3N1， respectively. The application of 45 t/hm2 of biochar-based fertilizer combined with nitrogen fertilizer was more conducive to the accumulation of soil nutrients， which could reduce the amount of nitrogen fertilizer applied by at least 20%.

Effects of combined application of biochar-based organic fertilizer and nitrogen fertilizer on soil enzyme activity

Effects of combined application on soil catalase activity

Fig. 2 shows that， from the perspective of the entire growth period of purple cabbage， soil catalase activity showed a trend of first decreasing and then slowly increasing. Catalase activity reached its highest level during the seedling stage， and it increased significantly under various fertilization treatments compared with the CK， but the increasing effect differed under various treatments. The application of biochar-based organic fertilizer alone improved the activity of catalase at all stages， and the activity of catalase increased with the application amount of biochar-based organic fertilizer increasing.

During the seedling and rosette stages of purple cabbage， under the conditions of applying 15 and 30 t/hm2 of biochar-based organic fertilizer， the enzyme activity was higher when combined with nitrogen fertilizer than single application of biochar-based organic fertilizer. When the application of biochar-based organic fertilizer reached 45 t/hm2， single application of biochar-based organic fertilizer had the highest catalase activity， and the value increased by 45.21% and 96.01% compared with the CK， respectively， while the combined application with nitrogen fertilizer actually inhibited the increase of catalase activity. During the heading stage of purple cabbage， when 15 （T1） and 45 t/hm2 （T3） of biochar-based organic fertilizers were used as the base fertilizer， the treatments of combining with top application of urea at 375 kg/hm2 （T1N1， T3N1） significantly increased the activity of catalase， with an increase of 39.44% and 22.85% compared with single application of the same amount of biochar-based fertilizer （T1 and T3）， respectively. There were no significant differences between other fertilization treatments. In the maturation stage of purple cabbage， under the conditions of the same amount of biochar-based organic fertilizer， the combination of 45 t/hm2 （T3） of biochar-based organic fertilizer and nitrogen fertilizer increased the activity compared with single application of biochar-based organic fertilizer. The activity of catalase in the combination of 15 t/hm2（T1） and 30 t/hm2 （T2） of biochar-based organic fertilizer with 300 kg/hm2 urea （N2） was higher than that in the N1 and N3 topdressing treatments， and the activity of catalase in the treatment of 45 t/hm2 of biochar-based organic fertilizer with top-applying 375 kg/hm2 of urea （T3N1） was the highest， with an increase of 12.12% compared with treatment T3. Treatment T3 had the highest average catalase activity among the four stages， and the application of 45 t/hm2 of biochar-based organic fertilizer alone was more conducive to improving the activity of catalase.

Effects of combined application on soil urease activity

Fig. 3 shows that soil urease activity showed a trend of first decreasing and then increasing throughout the entire growth period of purple cabbage. The application of biochar-based organic fertilizer significantly improved soil urease activity compared with the CK. The average urease activity of the treatments with 15， 30， and 45 t/hm2 of biochar-based organic fertilizer alone increased by 21.47%， 28.31%， and 31.70% compared with CK， respectively. With the application rate of biochar-based organic fertilizer increasing， soil urease activity increased， but the improvement was not significant. The top application of nitrogen fertilizer had a more significant impact on soil urease activity.

During the seedling stage of purple cabbage， under the application of 15 and 30 t/hm2 of biochar-based organic fertilizer， soil urease activity was consistently higher in the combed application treatments with 300 kg/hm2 of urea （N2） and 225 kg/hm2 of urea （N3） than in single application of biochar-based organic fertilizer， while the combination with 375 kg/hm2 of urea （N1） was lower than that of single application of biochar-based fertilizer， indicating that application of excessive nitrogen fertilizer actually inhibited the improvement effect of biochar-based organic fertilizer on soil urease activity. When the application amount of biochar-based organic fertilizer reached 45 t/hm2， single application of biochar-based organic fertilizer showed a higher urease activity. Treatment T2N3 showed the highest soil urease activity， which increased by 14.74% compared with treatment T2. During the rosette stage of purple cabbage， under the application of equal amounts of biochar-based organic fertilizer， the soil urease activity with single application of biochar-based organic fertilizer was consistently higher than that with the combined application of nitrogen fertilizer. Urease activity ranked as T3>T1>T2， and the single application of 45 t/hm2 biochar-based organic fertilizer increased it by 12.70% compared with treatment T2. During the heading period， soil urease activity was the highest under treatment T1N1， which increased soil urease activity by 40.31% compared with treatment T1. In the treatments of 30 and 45 t/hm2 of biochar-based organic fertilizer combined with nitrogen fertilizer， except for treatment T2N1， the urease activity of the combined application treatments with nitrogen fertilizer was higher than that of single biochar-based fertilizer treatments， and urease activity was stronger under combined application with 225 kg/hm2 of urea （N3）. When purple cabbage reached its maturation stage， 30 （T2） and 45 t/hm2 （T3） of biochar-based organic fertilizers achieved the highest urease activity when being combined with 300 kg/hm2 of urea （N2）， while 15 t/hm2 （T1） of biochar-based organic fertilizer showed the highest urease activity when being combined with 225 kg/hm2 of urea （N3）. The urease activity of the combination of nitrogen fertilizer and biochar-based organic fertilizer was higher than that of single application of biochar-based organic fertilizer， and the combination with reduced nitrogen was better than the high-nitrogen combination. Among them， treatment T3N2 had the highest urease activity， which was 28.60% higher than treatment T3. Treatment T3N2 also had the highest average urease activity throughout the entire growth period.

Effects of combined application of biochar-based organic fertilizer and nitrogen fertilizer on soil sucrase activity

Fig. 4 showed that soil sucrase activity of purple cabbage reached its highest during the maturation stage. The combination of biochar-based organic fertilizer and reduced nitrogen fertilizer kept soil sucrase activity consistently high， and the sucrase activity of each fertilization treatment was significantly higher than that of the CK. Except for treatment T2， which showed lower soil sucrase activity than the CK treatment during the maturation stage， the application of biochar-based organic fertilizer alone effectively increased soil sucrase activity in purple cabbage at all stages， but did not show a regular change with the application amount of biochar-based fertilizer increasing.

During the seedling stage， except for treatment T1N1， the soil sucrase activity of all other combination treatments of biochar-based organic fertilizer and nitrogen fertilizer was higher than that of single biochar-based organic fertilizer， and the sucrase activity in treatments of the same amount of biochar-based organic fertilizer combined with nitrogen fertilizer showed a trend of first increasing and then decreasing with the topdressing amount increasing. The highest sucrase activity was observed in treatment T3N3， which increased sucrase activity by 67.71% compared with treatment T3. During the rosette stage of purple cabbage， under the application of 15 t/hm2 of biochar-based organic fertilizer， the combined application with nitrogen fertilizer could improve soil sucrase activity. When the application amount of biochar-based organic fertilizer reached 30 and 45 t/hm2， soil sucrase activity was higher when biochar-based fertilizer was applied alone， and with the increase of nitrogen fertilizer topdressing， soil sucrase activity decreased. The soil enzyme activity was highest under treatment T3， which was 45.35% higher than treatment T1. During the heading period of purple cabbage， except for treatments T1N1 and T2N2， among all other treatments under the same amount of biochar-based organic fertilizer， the activity of sucrase in the combined application treatments with nitrogen fertilizer was all lower than that of single application of biochar-based organic fertilizer. During the maturation stage of purple cabbage， there were no significant differences among different fertilization treatments under the application of 15 t/hm2 of biochar-based organic fertilizer. However， under the application of 30 and 45 t/hm2 of biochar-based organic fertilizer， the combined application with nitrogen fertilizer significantly increased sucrose enzyme activity. Among them， treatment T2N2 had the highest sucrose enzyme activity， which increased by 65.80% compared with treatment T2. Meanwhile， treatment T2N2 had the highest average sucrase activity throughout the entire growth period of purple cabbage， followed by treatment T3N2， which had a 5.41% difference in average sucrase activity from treatment T2N2.

Effects of combined application of reduced nitrogen fertilizer and biochar-based organic fertilizer on yield of purple cabbage

After picking the purple cabbage， the outer leaves were removed， and only the edible part was recorded as the yield. Fig. 5 showed that compared with the control group CK， the yield with the application of biochar-based fertilizer was improved to varying degrees， with an increase of 59.48%-201.55%. The yield increased with the application rate of biochar-based organic fertilizer increasing， and fertilization was a very effective measure for improving yield.

Under the same application amount of biochar-based organic fertilizer， the combined application with nitrogen fertilizer could significantly increase the yield of purple cabbage. Under the application of 15 and 45 t/hm2 of biochar-based organic fertilizer， the yield increased with the increase of topdressing amount， and the yield of the combined application treatment with 375 kg/hm2 （N2） of nitrogen fertilizer was higher than those of the treatments with 300 （N1） and 225 kg/hm2 （N3） of nitrogen fertilizer; and the yield of treatment T1N1 increased by 3.89% and 36.21%， respectively， compared with treatments T1N2 and T1N3， and the yield of treatment T3N1 increased by 0.41% and 26.88%， respectively， compared with treatments T3N2 and T3N3. When the application amount of biochar-based organic fertilizer reached 30 t/hm2， the yield of purple cabbage first increased and then decreased with the increase of nitrogen fertilizer topdressing， and the yield of treatment T2N2 increased by 9.14% and 18.63% compared with treatments T2N1 and T2N3， respectively. Under the same application amount of biochar-based organic fertilizer， the yields of the combined treatments with 300 （N2） and 375 kg/hm2 （N1） remained close. The highest yield of purple cabbage under treatment T3N1 was 85 750.05 kg/hm2， followed by treatment T3N2 with a yield of 85 402.5 kg/hm2， and there was no significant difference in yield therebetween， but treatment T3N2 could reduce the application of nitrogen fertilizer by 20% compared with treatment T3N1. It could be seen that the combination of 45 t/hm2 of biochar-based organic fertilizer with 300 kg/hm2 of urea was more conducive to the yield-increasing and nitrogen-reducing effects of purple cabbage.

Correlation between soil enzyme activity and soil nutrient elements

From Table 3， it can be seen that soil catalase and sucrase activity were in an extreme significant positive correlation with pH and organic matter， total nitrogen， hydrolyzable nitrogen， available phosphorus， and available potassium contents. Urease activity was also in an extreme significant positive correlation with organic matter， total nitrogen， available phosphorus， and available potassium contents， and in a significant positive correlation with pH， but not correlated with hydrolyzable nitrogen content.

Conclusions and Discussion

Effects of biochar-based organic fertilizer combined with nitrogen fertilizer on soil physical and chemical properties

Purple cabbage is suitable for growing in high fertility and loose soil， and as a fertilizer-loving and tolerant crop， nitrogen， phosphorus， and potassium are essential elements in its growth process［13］. Purple cabbage has a high demand for nitrogen fertilizer， but excessive application of nitrogen fertilizer can easily lead to nutrient loss. Kang et al.［14］ showed that biochar-based fertilizers could significantly increase soil organic matter content， which increased with the increase of biochar added in auxiliary materials. Biochar-based fertilizers also increased the content of hydrolyzable nitrogen and available phosphorus nutrient in the soil. The study of Shi et al.［15］ showed that applying biochar-based fertilizers could significantly increase soil organic matter （13.5%-103.9%） and available nutrient content. Dong et al.［16］ showed that the application of biochar and nitrogen fertilizer increased soil organic carbon （SOC） by 20%-58% and total nitrogen content by 9.3%-15%. In this study， it was also found that biochar-based organic fertilizer had an increasing effect on soil pH and organic matter， total nitrogen， and available nutrient contents， which increased with the application amount of biochar-based organic fertilizer increasing， but under the application of equal amount of biochar-based organic fertilizer， combined application with nitrogen fertilizer was more conducive to improving soil nutrient contents. With the increase of the application rate of nitrogen fertilizer， the contents of soil organic matter， total nitrogen， hydrolyzable nitrogen， available phosphorus and available potassium showed a trend of first increasing and then decreasing. Under the application of 45 t/hm2 of biochar-based organic fertilizer and 225 kg/hm2 （T3N3） of urea， soil organic matter and total nitrogen contents were the highest， which increased by 2.89% and 12.36%，compared with the topdressing treatment of 375 kg/hm2 （T3N1） of urea， respectively; and the combined application with 300 kg/hm2 （T3N2） urea achieved the highest relative contents of hydrolyzable nitrogen， available phosphorus， and available potassium， which increased by 36.66%， 10.23%， and 25.60% compared with the T3N1， respectively. The combination of biochar-based organic fertilizer and reduced nitrogen fertilizer was more conducive to the transformation and accumulation of soil nutrients. Ruan et al.［17］ found that adding biochar to conventional nitrogen application and reducing nitrogen by 20% significantly increased the contents of organic carbon， available phosphorus， and available potassium compared with conventional nitrogen application， similar to the results of this study.  Perhaps it was because the biochar-based organic fertilizer itself contained some organic nitrogen， it could effectively reduce the application of chemical nitrogen fertilizer， and reduced nitrogen fertilizer reasonably coordinated the proportion of nitrogen， phosphorus， and potassium in the soil， avoiding the problem of slow growth of purple cabbage caused by insufficient nitrogen fertilizer in the soil.

Effects of combined application of biochar-based organic fertilizer with nitrogen fertilizer on soil enzyme activity

Catalase is an important oxidoreductase in soil， which can decompose H2O2 in the soil and protect crops from toxicity［18］. The role of urease in soil is to catalyze the hydrolysis of urea to produce ammonia， carbon dioxide， and water， so it can promote soil nitrogen conversion. Sucrase can decompose sucrose in the soil into glucose and fructose， providing energy for microbial activities［19］. Jia et al.［20］ found that adding straw biochar and organic fertilizer had a promoting effect on soil urease， sucrase， catalase， and alkaline phosphatase. Li et al.［21］ found an increasing trend in soil catalase and urease activity as the proportion of biochar in the biochar-based compound fertilizer formula increased. In this study， the activity of three soil enzymes was in an extreme significant positive correlation with the contents of soil organic matter， total nitrogen， available phosphorus， and available potassium. The application of biochar-based organic fertilizer increased the contents of soil organic matter， total nitrogen， and available nutrients， and compared with the non-fertilizing treatment （CK）， the application of biochar-based organic fertilizer improved soil enzyme activity to varying degrees at each stage， and the activity of catalase and urease also increased to certain extents with the amount of biochar-based organic fertilizer increasing. Compared with single application of biochar-based organic fertilizer， the combined application with nitrogen fertilizer showed higher enzyme activity at multiple stages. The treatments with the highest average activity of catalase and urease throughout the entire growth period of purple cabbage were treatments T3 and T3N2， respectively， and the average activity of sucrase was highest under treatment T2N2， followed by treatment T3N2. The research results of Qin et al.［22］ indicated that within a certain range， the increase of nitrogen fertilizer could activate the activity of these enzymes， but as the amount of nitrogen fertilizer applied further increased， the enzyme activity was inhibited， which is consistent with the conclusion of this study that the combination of 45 t/hm2 of biochar-based organic fertilizer with 300 kg/hm2 of urea was more conducive to improving soil enzyme activity.

Effects of combined application of biochar-based organic fertilizer with reduced nitrogen fertilizer on yield of purple cabbage

Adequate nutrients can ensure high crop yield， and numerous studies have shown that organic fertilizers can replace and reduce the use of some chemical fertilizers， and the combination of biochar and fertilizers can significantly improve crop yield. Ye et al.［23］ found that under three consecutive experimental years， the yield of maize treated with 80% of nitrogen fertilizer combined with organic fertilizer significantly increased by 32.39%， 28.06%， and 51.42% compared with 100% of nitrogen fertilizer， respectively. Guo et al.［24］ found that compared with the treatment without biochar， reduced application of nitrogen fertilizer combined with biochar also increased rice yield by 17.61% to 39.58%. In this study， it was found that the application of biochar-based fertilizer could improve the yield of purple cabbage compared with the non-fertilizing treatment， and the yield increased with the application amount of biochar-based organic fertilizer increasing. However， purple cabbage did not yield highly when biochar-based fertilizer was applied alone， while the combination of biochar-based organic fertilizer and nitrogen fertilizer significantly increased the yield of purple cabbage， which is similar to the research conclusion of Zeng et al.［25］. Under the same application amount of biochar-based organic fertilizer， the yields of topdressing with 375 and 300 kg/hm2 of nitrogen fertilizer remained close， and when 45 t/hm2 of biochar-based organic fertilizer was combined with 375 kg/hm2 of nitrogen fertilizer， purple cabbage achieved the highest yield， only increasing by 0.41% compared with topdressing with 300 kg/hm2 of nitrogen fertilizer， which maintained a high yield while reducing the application of nitrogen fertilizer by 20% and was thus a choice of fertilization for promotion.

Conclusions

The results of this study showed that in the planting of purple cabbage in the dry and hot river valley area of Yuanmou County， as the application amount of biochar-based organic fertilizer increased， soil nutrients， enzyme activity， and yield were all improved， and under the same amount of biochar-based organic fertilizer， the combination with nitrogen fertilizer achieved more significant effects than single application of biochar-based organic fertilizer. Under the application of 45 t/hm2 of biochar-based organic fertilizer， the combined application treatment with 300 kg/hm2 of urea was more conducive to the accumulation of soil organic matter and the conversion of soil available nutrients than the combined application treatment with 375 kg/hm2 of urea， and it always maintained the high activity of soil catalase， sucrase， and urease， achieved a high yield of purple cabbage and more long-lasting fertilizer efficiency， and continuously improved the soil， thereby enabling crops to obtain stable and balanced nutrients at various growth stages. In addition， it effectively reduced the application rate of nitrogen fertilizer by 20%， avoiding soil problems caused by excessive nitrogen fertilizer application in agricultural production.

References

［1］ LI YM， WANG KQ， LIU ZQ， et al. Effect of reconstructed micro-slope on soil moisture dynamics in Yunnan dry-hot river valley［J］. Journal of Zhejiang Forestry College， 2005（3）： 259-265. （in Chinese）.

［2］ WEI YL， GUO FF， CHEN AQ， et al. Effects of vegetation restoration on soil enzyme activity in Yuanmou Dry-hot Valley， China［J］. Journal of Agro-Environment Science， 2011， 30（1）： 100-106. （in Chinese）.

［3］ WANG XM， YAN BG， ZHAO G， et al. Responses of Dodonaea viscosa growth and soil biological properties to nitrogen and phosphorus additions in Yuanmou dry-hot valley［J］. Journal of Mountain Science， 2018， 15（6）： 1283-1298.

［4］ XU T. Effects of bamboo charcoal on soil carbon-nitrogen-phosphorus and microbial diversity in pear orchards［D］. Beijing： Chinese Academy of Agricultural Sciences， 2012. （in Chinese）.

［5］ WANG CJ， TANG LN， HU ZL， et al. Application and prospect of biochar and biochar-based fertilizer in tobacco agriculture［J］. Acta Agriculturae Nucleatae Sinica， 2021， 35（4）： 997-1007. （in Chinese）.

［6］ YANG JF， JIANG T， HAN XR， et al. Effects of continuous application of biochar-based fertilizer on soil characters and yield under peanuts continuous cropping［J］. Soil and Fertilizer Sciences in China， 2015（3）： 68-73. （in Chinese）

［7］ ZHANG YS， TANG GM， PU SH， et al. Effect of biochar-based compound fertilizer on soil nutrients and cotton yield in irrigated sandy soil under nitrogen reduction condition［J］. Acta Agriculturae Boreali-occidentalis Sinica， 2020， 29（9）： 1372-1377. （in Chinese）.

［8］ ZHOU ZF， WANG JC， RAO XX. Impact of adding biochar on enzyme activity in soil［J］. Acta Agriculturae Jiangxi， 2015， 27（6）：110-112. （in Chinese）.

［9］ NANNIPIERI PAOLO， TRASAR CEPEDA CARMEN， DICK RICHARD P. Soil enzyme activity： A brief history and biochemistry as a basis for appropriate interpretations and meta-analysis［J］. Biology and Fertility of Soils， 2018， 54（1）：11-19..

［10］ LIU SP， NIE XT， ZHANG HC， et al. Effects of tillage and straw returning on soil fertility and grain yield in a wheat-rice double cropping system［J］. Transactions of the Chinese Society of Agricultural Engineering， 2006（7）： 48-51. （in Chinese）.

［11］ HOU JW， XING CF， DENG XM， et al. Effect of biochar on soil microbe numbers and enzyme activities in pepper bush［J］. Journal of Northwest A & F University： Natural Science Edition， 2020， 48（4）： 89-96. （in Chinese）.

［12］ GU SY， LI XJ， WEI D， et al. Effects of biochar on soil nutrient contents and microorganism of soybean rhizosphere［J］. Soybean Science， 2014， 33（3）： 393-397. （in Chinese）.

［13］ LIU XP. Characteristics of pollution-free cabbage fertilizer requirements and formula fertilization techniques［J］. Northwest Horticulture， 2010（4）： 50-51.

［14］ KANG RF， ZHANG NM， SHI J， et al. Effects of biochar-based fertilizer on soil fertility， wheat growth and nutrient absorption［J］. Soil and Fertilizer Sciences in China， 2014（6）： 33-38. （in Chinese）.

［15］ SHI ZL， GUO YQ， ZHOU ZD， et al. Effects of biochar based fertilizer and stumping measure on karst soil nutrient content and heat energy of Robinia pseudoacacia seedlings［J］. Journal of Plant Resources and Environment， 2019， 28（2）： 71-78. （in Chinese）.

［16］ DONG ZJ， LI HB， XIAO JN， et al. Soil multifunctionality of paddy field is explained by soil pH rather than microbial diversity after 8-years of repeated applications of biochar and nitrogen fertilizer［J］. The Science of the total environment， 2022（853）.

［17］ RUAN ZB， WANG LG， LAN WKN， et al. Effects of nitrogen reduction and biochar on nitrogen uptake by rice and soil physiochemical properties［J］. Acta Agriculturae Zhejiangensis， 2023， 35（2）： 394-402. （in Chinese）.

［18］ CHU SZ， ZHANG NM， SHI J. Research on the variation trend of greenhouse soil hydrogen peroxidase activities in Yunnan Province［J］. Chinese Agricultural Science Bulletin，2015， 31（15）： 220-225. （in Chinese）.

［19］ GUAN SY. Soil enzymes and their research methods［M］. Beijing： Agriculture Press， 1986. （in Chinese）.

［20］ JIA JX， XIE YH. Effects of biochar on soil nutrient and enzyme activity from coal mining subsidence reclamation area［J］. Journal of Irrigation and Drainage， 2016， 35（11）： 88-91. （in Chinese）.

［21］ LI CB， FU DL， CAO TM， et al. Effects of carbon-based compound fertilizer formula on soil enzyme activity and characteristic of physiology， nutrient accumulation and yield of flue-cured tobacco［J］. Hunan Agricultural Sciences， 2019（7）： 48-50， 54. （in Chinese）.

［22］ QIN WX， SI GC， LEI TZ， et al. Effect of nitrogen fertilizer addition on soil microbial biomass and enzyme activity［J］. Jiangsu Agricultural Sciences， 2021， 49（1）： 170-175. （in Chinese）.

［23］ YE SJ， ZHENG CM， ZHANG Y， et al. Effects of reduced chemical nitrogen input combined with organic fertilizer application on the productivity of winter wheat and summer maize rotation and soil properties in central Henan Province［J］. Chinese Journal of Eco-Agriculture， 2022， 30（6）： 900-912. （in Chinese）.

［24］ GUO QB， WANG XL， DUAN JJ， et al. Effects of nitrogen reduction combined with biochar application on organic carbon mineralization and enzyme activity in paddy field［J］. Journal of Soil and Water Conservation， 2021， 35（5）： 369-374， 383. （in Chinese）.

［25］ ZENG M， PAN YR， PENG Y， et al. Effects of reduced fertilizer combined with carbon-based organic fertilizer on growth of cut rose and soil properties in greenhouse［J］. Guangxi Agricultural Sciences， 2021， 52（8）： 2202-2210. （in Chinese）.