Introduction

Dragon fruit (Selenicereus spp.), also known as pitahaya, is a perennial climbing plant in the family Cactaceae (Chen et al. 2023a). Since its introduction into China in the 1980s, the dragon fruit planting area has rapidly expanded, making China the second-largest dragon fruit producer worldwide (Zhou et al. 2023). The cultivated area of 23,000 ha in the Guangxi Autonomous Region accounts for 34.2% of Chinese dragon fruit production (Wang 2022). Within Guangxi, Long’an County contains 19% of the total dragon fruit production area (Meng and Lu 2023). Although dragon fruit has high economic value, its shallow root distribution means that it requires a narrow growth environment, with optimal growth temperatures in the range of 25–35°C and an optimal pH range of 5.5–6.5 (Nobel 2002; Reis et al. 2020). However, dragon fruit production areas in China are mainly concentrated within the country’s southern regions, which have high temperatures, rainy weather, vigorous weed growth, acidic soil, and relatively low soil nutrient contents. Therefore, increasing the efficiency of dragon fruit production requires intense weed control and scientific soil nutrient management in dragon fruit orchards.

The use of mulch represents an attractive alternative to costly manual weeding, as mulch suppresses weeds and plays important roles in soil nutrient regulation. For these reasons, this approach has become a main focus in research on agricultural production. Plants that are suitable as crop field cover include grasses (Nobel 2002; Reis et al. 2020; Tang et al. 2022) and clover, an herb with low stature that is highly adaptable and easy to manage (Rady et al. 2022); as field cover, these plants improve soil nutrient contents, promote the development of beneficial microorganisms, and reduce soil bulk density when planted in orchards (Wang et al. 2020). Eucalyptus (family Myrtaceae) is a genus that includes fast-growing tree species planted for afforestation worldwide (Kojima et al. 2009). Guangxi has approximately 3.03 million ha of artificially planted eucalyptus forests, comprising 67% of the total eucalyptus forest area in China (Qiang et al. 2023). Eucalyptus wood processing yields large amounts of bark as a byproduct. Owing to its low bulk density, large volume, loose distribution, high water content, and non-combustibility, as well as the inconvenience of its collection and storage, eucalyptus bark is often discarded (Li 2023). In China, discarded eucalyptus bark is sometimes crushed and used as a substrate for plants or edible fungi, with the assumption that it will regulate plant growth in a manner similar to straw or the bark of other species (Cavallazzi et al. 2004). However, the properties of eucalyptus bark are rarely considered in such applications. The main components of eucalyptus bark are cellulose and lignin (Sartori et al. 2019), which inhibit its decomposition and the effective release of nutrients under natural conditions. In contrast, the artificial decomposition of eucalyptus bark can hasten these decomposition and nutrient release processes. In recent years, while fruit farmers in Long’an County have covered dragon fruit orchard fields with fermented eucalyptus bark to control weeds.

In this study, we explored the use of fermented eucalyptus bark mulch as a field cover in a one-year-old dragon fruit orchard in Long’an County, Guangxi. The effects of applying bark mulch on dragon fruit yields and quality outcomes, as well as soil nutrients, were compared to the effects of clear tillage and clover cover treatments. The objectives of this study were to determine the effects of an application of mulching fermented eucalyptus bark (MFEB) on dragon fruit production along with the mechanisms underlying these effects and to identify the optimal field covering method for improved dragon fruit production.

Materials and Methods

Site description and experimental design

A field experiment was conducted from 2020 to 2021 at the dragon fruit orchard of the Guangxi Jinsui Agricultural Group in Long’an County, Guangxi Autonomous Region, China (23°00’02’’ N, 107°52’05’’ E). The study site experiences an average annual temperature of 22.4°C, average annual rainfall of 1300 mm, and a frost-free period of approximately 343 days. The soil is laterite, which is common under wet tropical conditions with a low pH. We used the dragon fruit cultivar ‘Jindu No. 1’, which is the primary dragon fruit cultivar planted in Long’an County. The orchard was established in September of 2020, with a total area of 0.32 ha, plant density of 18,000 plants ha^-1, and spacing of 0.3 m × 2.1 m in single rows. Dragon fruit was planted by cutting. The branches used for cutting were selected from robust five-year-old plants with the same length, width and health status. Dragon fruit production at the orchard was managed by expert personnel who used a unified standard, except for the field covering treatments.

The experiment followed a randomized complete block design with three replicates. Each plot had an area of 67 m² (9.6 m × 7 m). The treatments were as follows: clear tillage (CK1), clover planting (CK2), and mulching fermented eucalyptus bark (MFEB). To prevent interference between plots, the horizontal and vertical intervals between the plots were set to 4.8 and 3.5 m, respectively (Fig. 1). Field planting and bark mulch application were initiated in December of 2020, lasting until the end of the reproductive growth period in 2021. The field coverage (including grass) treatments extended 50 cm beyond each dragon fruit plant; MFEB coverage was 5 cm deep, and the clover coverage protocol is described in the literature (Yan et al. 2017). The dragon fruit plants experienced three stages during the experimental period: seedling, vegetative growth, and reproductive growth. Regarding the eucalyptus bark fermentation process and application quantities, the collected eucalyptus bark was crushed into pieces 20–30 cm in size. A mixture containing 5% molasses concentrate, 2% urea, and 1% of a decomposing agent was added to the eucalyptus bark. Water was incorporated into the mixture to achieve a moisture content of 60%, after which the mixture was left to ferment for 15 days. The amount of fermented eucalyptus bark applied in the field was 150 m³ ha^-1. The basic indicators of the fermented eucalyptus bark are detailed in Table 1.

Fig. 1.

Field distribution map of different mulching treatments. Where area of each plot is 67 m², and the interval between the plots is 3.5 × 4.8 m.

Soil sampling and analysis

To determine soil moisture amounts in the dragon fruit rhizospheres in summer, we used a soil temperature and humidity sensor (JXBS-4001-BXSZD, JXCT Electronics Technology, Weihai, China) to record the rhizosphere humidity at 7:00, 11:00, and 15:00 over seven-day intervals from June 30 to September 1.

In 2021, soil samples were collected from the 0–20 cm soil layer, within a distance of 20 cm from the dragon fruit plants in each plot during July (vegetative growth period) and December (reproductive growth period). In total, 18 soil samples (nine per growth period) were collected. Composite samples comprising five replicate soil cores were collected using a 5-cm-diameter auger. Stones, plant roots, and agricultural waste were removed from the soil samples, which were then air-dried, fully ground, and passed through a 2-mm sieve for nutrient content analyses. Soil organic matter (SOM) was measured using the potassium dichromate external heating method, the soil electrical conductivity (EC) value was measured using a conductivity meter after preparing the saturated soil slurry leachate with a water-to-soil ratio of 5:1, and the pH was measured using an acidity meter. The nitrogen content was determined using the alkaline solution diffusion method; available phosphorus was leached with hydrochloric acid and sulfuric acid and then determined by molybdenum–antimony colorimetry. Available potassium was leached with ammonium acetate and determined by flame photometry; exchangeable calcium and magnesium were leached by ammonium acetate and determined by spectrophotometry. Available iron (Fe), available zinc (Zn), and available copper (Cu) were leached with hydrochloric acid and sulfuric acid and then determined by atomic absorption spectrophotometry. Finally, the soil moisture content was determined using the drying method. Detailed methods for these procedures can be found in the literature (Bao, 2011).

Fruit yield and quality determination

Dragon fruit produced in each plot were collected in September and November of 2021 and were graded according to size; grades 1, 2, and 3 corresponded to large, medium, and small fruits, respectively (Table 2). Fruit yield was calculated as follows:

Oq=∑p=13Wp×Npq ,

where p is the size grade, O_q is the output (kg) of the q^th fruit batch, W_p is the weight (kg) of a single fruit of grade p, and N_qp is the number of fruits in the q^th batch of fruit of grade p.

After the fruit yield had been estimated in November, one fruit was randomly selected from the middle of each plot, ensuring that the fruits exhibited similar shape and size, for quality testing in the laboratory (n = 9). The sugar content was determined using the Anthrone colorimetric method, and titratable acid was determined by means of acid-base titration (citric acid, coefficient = 0.070 g mmol^-1). Color was measured using a hand-held color difference meter (NR110 Precision Colorimeter, 3NH, Guangzhou, China).

Data analyses

Experimental data were analyzed using SPSS v25.0 software (IBM Corp., Armonk, NY, USA). Means were compared using a one-way analysis of variance, followed by Fisher’s least significant difference test. Excel 2020 (Microsoft, Redmond, WA, USA) was used for graphing. The Mantel test and a Pearson correlation analysis were used to analyze the correlations of the soil nutrients with the dragon fruit yield and quality, as well as correlations between the soil nutrients. The linkET package in R software (R Core Team, Vienna, Austria) was used to draw the figures in this study (Sunagawa et al. 2015). Significant differences were assessed using a threshold of p < 0.05.

Results

Effects of MFEB on SOM of in dragon fruit soil

Compared to the two controls, the MFEB treatment increased the SOM content significantly in dragon fruit at different growth stages (Fig. 2). Compared to CK1 and CK2, the SOM contents in the MFEB treatment were correspondingly 14% and 13% higher during vegetative growth and 6% and 10% higher during reproductive growth.

Fig. 2.

Effects of different field mulching methods on soil organic matter in dragon fruit. Uppercase letters and lowercase letters represent significant differences in the nutrient elements of dragon fruit soil in the vegetative growth stage (VGS) and the reproductive growth stage (RGS) under different mulching methods, as determined by a LSD test at p < 0.05.

Effects of MFEB on the medium and micronutrients in dragon fruit soil

MFEB significantly increased soil available Fe, Zn, and Cu contents compared to both controls during the reproductive growth stage, but it did not show similar effects during the vegetative growth stage (Fig. 3F–3H). MFEB did not affect soil available nitrogen or exchangeable magnesium in dragon fruit during vegetative growth (Fig. 3A and 3E); moreover, available phosphorus and available potassium levels showed no clear association with the MFEB treatment (Fig. 3B and 3C). In dragon fruit during the reproductive growth period, the soil exchangeable calcium content was significantly higher at 48% and 25% in the MFEB treatment than in CK1 and CK2, respectively. During vegetative growth, the soil exchangeable calcium content was significantly lower (by 29%) in the MFEB treatment than in CK2; it did not significantly differ between the MFEB and CK1 treatments (Fig. 3D).

Fig. 3.

Effects of different field mulching methods on available nitrogen in the dragon fruit soil: (A), available phosphorus (B), available potassium (C), exchangeable calcium (D), exchangeable magnesium (E), available copper (F), available iron (G) and available zinc (H). Uppercase letters and lowercase letters represent the significant differences in the nutrient elements of dragon fruit soil in the vegetative growth stage (VGS) and the reproductive growth stage (RGS) under different mulching methods, as determined by a LSD test at p < 0.05.

Effects of MFEB on the soil environment of dragon fruit

During the dragon fruit growth cycle, the soil pH was significantly higher in the MFEB treatment than in CK1, whereas it did not differ significantly between the MFEB and CK2 treatments (Fig. 4A). Soil EC was significantly higher in the MFEB treatment than in the control treatments during both growth periods; it increased by 33% and 64% during vegetative growth compared with CK1 and CK2, respectively, and by 31% and 48% during reproductive growth compared with CK1 and CK2, respectively (Fig. 4B). In summer, the water content of dragon fruit root layer soil was higher in the MFEB treatment at each of the three time points (7:00, 11:00, and 15:00); it increased by 21%, 21%, and 23%, respectively, compared with CK1 and correspondingly by 6%, 3%, and 1% compared with CK2 (Fig. 4C).

Fig. 4.

Effects of different mulching methods on the soil pH (A), EC value (B) and moisture content (C) of dragon fruit. Uppercase letters and lowercase letters represent the significant differences in the nutrient elements of dragon fruit soil in the vegetative growth stage (VGS) and the reproductive growth stage (RGS) under different mulching methods, as determined by a LSD test at p < 0.05. 7:00 am, 11:00 am, and 15:00 pm in Fig. 3C are the measurement time points of the dragon fruit rhizosphere soil moisture in summer (June 30 to September 1) for each respective month.

The mechanism of MFEB on regulating the yield and quality of dragon fruit

Dragon fruit yields were 4% and 15% higher in the MFEB treatment than in CK1 and CK2, although the difference was not statistically significant compared to CK1 (Table 3). Similar to the yield trend, although the total sugar content of MFEB did not differ statistically from those of CK1 and CK2, it was 13% and 25% higher than the two, respectively (Table 3). MFEB did not significantly affect the total titratable acidity of dragon fruit. Fruit in the MFEB treatment had a greener, darker pericarp compared to those in CK1; it had redder, darker skin compared to both controls. Mantel tests revealed correlations of the dragon fruit yield and quality with the soil nutrients (Fig. 5). SOM was significantly correlated with the dragon fruit total sugar content (p < 0.01, r = 0.61). SOM and available Cu were significantly correlated with the dragon fruit yield and titratable acid content (p < 0.05), with respective correlation coefficients of 0.39 and 0.43. The remaining soil nutrient factors were not significantly correlated with the dragon fruit quality or yield. In summary, MFEB improved dragon fruit yields and quality levels by regulating soil nutrient efficiency.

Fig. 5.

Soil environmental drivers of dragon fruit yield and quality components. Yield and quality are correlated with each environmental factor according to a Mantel test. The edge width corresponds to Mantel’s r statistic of the corresponding distance correlation. Pairwise comparisons of environmental factors are shown, and the color gradient indicates the Spearman’s correlation coefficient. pH, soil pH; EC, soil electrical conductivity; SOM, Soil organic matter; AN, soil available nitrogen; AP, soil available phosphorus; AK, soil available potassium; Ca, soil exchangeable calcium; Mg, soil exchangeable magnesium; Zn, soil available zinc; Fe, soil available iron; Cu, soil available copper; SMC, soil moisture content.

Discussion

Plants require sufficient nutrients to support growth, development, and reproduction (Lambers et al. 2008; Sathyanarayan et al. 2023). The results of the present study demonstrated that the application of MFEB as a type of field covering increased soil nutrient contents in the dragon fruit root layer, improved the soil nutrient environment, and boosted dragon fruit yields. The MFEB application also restricted weed growth, thereby reducing the field management workload and improving the economic benefits of the dragon fruit crop.

The MFEB treatment increased the SOM content, likely through decomposition of the fermented eucalyptus bark, which led to increased soil nutrient input. Eucalyptus bark contains abundant carbohydrates and has a high carbon-to-nitrogen ratio (Sartori et al. 2019). Therefore, the use of MFEB as a field cover material effectively contributes to the soil organic carbon level, similar to the principle by which straw mulching increases the SOM content (Zhu et al. 2016). The improved dragon fruit yield and quality observed under the MFEB treatment were caused by the increased SOM content in the 0–20 cm soil layer, consistent with the results of a study focusing on banana plants in acidic soil (Zhang et al. 2020).

Previous studies have demonstrated that both trace and common elements are important factors in efforts to improve crop nutritional quality outcomes (White and Broadley 2003; Geng et al. 2021; Ali et al. 2022). We found that MFEB increased the contents of trace elements (e.g., Zn, Fe, and Cu) and common elements (e.g., Ca) in dragon fruit root layer soil during the reproductive growth period, consistent with previous reports (Yu et al. 2015; Feng and Ling 2015; Martins et al. 2019). These findings imply that MFEB can partially improve dragon fruit nutritional quality levels. This greater soil microelement abundance may have been caused by eucalyptus bark decomposition, which increases the soil nutrient input and therefore the soil microelement content (Chen et al. 2014; Sartori et al. 2019), or by an increase in the soil pH due to the application of MFEB. Under acidic conditions, available Cu, available manganese, and available Zn contents significantly increase as the soil pH increases (Zu et al. 2014). Intriguingly, the Cu content was associated with the quality of the dragon fruit in the present study. The maximum available Cu content in soil was approximately 3 mg kg^-1, which is far below the ecological safety threshold for Cu in agricultural and forestry soil in China (Chen et al. 2023b). As the active center of various enzymes, Cu is widely involved in numerous biological processes, including protein transport, cell wall metabolism, respiration/photosynthesis electron transfer, and hormone signal transduction (Gong et al. 2021), which may explain the lower fruit acid content observed in the present study. Changes in the soil exchangeable calcium content may have been related to the MFEB-induced increase in the pH, whereas soil calcium saturation increases with the pH in acidic soils (Reis et al. 2020).

We found that MFEB increased the soil pH, although not to optimal levels, thereby improving the root nutrient environment and increasing soil nutrient availability, perhaps due to the production of organic acids through eucalyptus bark decomposition. Most organic acids are weak acids, and organic acid anions contribute to alkalinity at a lower pH, causing soil H⁺ to increase the soil pH (Rukshana et al. 2014). Soil EC indicates the concentration of soluble salts in soil, which affects nutrient absorption by crop roots (Vyavahare et al. 2023). At EC < 350 µS cm^-1, dragon fruit growth is limited (Li et al. 2004). Our experimental results showed that after several months of sun exposure, rain, and absorption by plant roots, the concentration of soluble salts in the soil declined. Furthermore, soil EC values in CK1 and CK2 were insufficient to maintain normal plant growth, whereas the soil EC was significantly higher in the MFEB treatment, possibly due to the release of eucalyptus bark nutrients, as observed in a previous study where eucalyptus bark served as a nutrient substrate for edible fungus (Cavallazzi et al. 2004). Our experiments showed that in the summer heat, the soil water content was always higher in the MFEB treatment than in the controls, indicating that MFEB contributed to the maintenance of soil moisture in this case.

In the present study, the application of the MFEB treatment was found to enhance the yield of dragon fruit, and it improved the quality of the fruit as well to a certain extent due to the increase in soluble sugar, although the difference was not statistically significant. The lack of significance is attributable to the brief cultivation period and treatment duration of the dragon fruit in this particular study, which lasted for one year. Typically, dragon fruit production batches are limited in the first year to allow the plants to accumulate sufficient nutrients and prolong the fruiting period; because only two fruit batches were included in our study period, differences between the treatments may have been limited. Similarly, the use of a single year of grass or clover growth and eucalyptus bark covering may have led to non-significant differences between some treatments. Previous studies showed that both grass and straw mulching significantly improved apple and maize yields and quality levels (Akhtar et al. 2018; Tang et al. 2022). Yield improvement is one available method for increasing the economic value of dragon fruit production; furthermore, field coverage with eucalyptus bark inhibits weed growth by blocking sunlight, substantially reducing the workload associated with weeding in the field. These factors can decrease production costs and increase economic benefits.

Conclusion

Compared with clear tillage and clover planting, the application of MFEB had greater effects on dragon fruit production in the acidic soils of Long’an County, China. MFEB application significantly increased the root layer SOM and available Cu content in the dragon fruit orchard, which in turn increased the fruit yield. These findings provide a scientific basis for covering dragon fruit crop fields with fermented eucalyptus bark. They also add a new option for field mulching and improve the utilization efficiency of eucalyptus resources, ultimately reducing waste.