Introduction

Sugar beet (Beta vulgarias L.) is one of the two major sugar crops in the world, with a wide range of cultivation and considerable economic value. Roots are used for sugar production, and stems and leaves are used for feed, ethanol and biofuel production1,2. Inner Mongolia is the largest sugar beet production area in China and it plays an important role in sugar production of China. The planting area of sugar beet in Inner Mongolia has grown significantly in recent years due to the industry’s rapid development; it went from 2.67 × 104 hm2 in 2010 to 14.29 × 104 hm2 in 2020. In order to achieve high yield with sufficient sugar content, rational crop rotation and tillage systems are promoted in sugar beet producing areas3. However, due to economic interests, limited arable land, and limited cultivation conditions, continuous cropping is usually required to ensure the yield of sugar beet4,5. According the reported data, sugar beet is currently planted continuously for 30% of the total planting area in Inner Mongolia, China. Long term continuous cropping can lead to abnormal root development, low stress resistance, frequent occurrence of diseases and pests, decreased yield and quality, and even plant death6,7. Studies have shown that the yield and sugar content of sugar beet have been decreased sharply after continuous cropping8. Moreover, the effects of continuous cropping obstacles and its’ regulatory measures in sugar beet still lack in-depth to investigate. At present, the shortage of the arable land resources, inadequate planting management systems, and desire production environment constraints have led to increasingly serious obstacles in sugar beet continuous cropping9,10, which urgently need to be addressed.

Photosynthesis plays a decisive role in plant growth and decvelopment. It is one of the most important physiological processes for crop growth and yield production11. Plant growth, production, and yield depend on photosynthesis. Maintaining photosynthesis stability under stress condition is one of the important factors to evaluate plant resistance. Optimization of photosynthetic performance and resistance to abiotic stresses are essential for maintaining optimum growth12. Plant photosynthetic performance is based on chlorophyll contents, which plays an important role in photosynthesis as it is involved in the absorption and transfer of light energy13,14. as well as a vector for reflecting plant tolerance to environmental stress15. The amount of chlorophyll within leaves can be quantified with SPAD values, which in turn be sued for the capacity of plants to perform photosynthesis. So SPAD value is, therefore, a crucial measure assessing the plant growth. A key indicator of photosynthetic capacity and the foundation of high crop production is material buildup. A major element limiting the creation of crop yield is the distribution ratio of dry matter in different parts, which contributes significantly in yield16. Research has shown that the formation of dry matter is mostly caused by the process of photosynthesis. Improving photosynthetic capacity can also promote the building-up of dry matter17, which will ultimately result in higher crop yields and better quality. The precise evaluation of photosynthetic capacity of plants led to dry matter accumulation and distribution mechanism with parts, which is advantage and disadvantage of the cultivation techniques.

According to some of the research findings, continuous cropping lowers the recipient plants’ capacity for photossynthesis through lowering in the SPAD value, decreasing the ability of leaves to assimilate CO2, and blocking the rate at which light energy and electrons are17. This resulted in with lower biomass accumulation and eventually stunted plant growth18. Bio-organic fertilizer has a dual characteristics of both biological and organic fertilizers application. Bio-organic fertilizer, which is mostly made up of living microorganisms and biodegradable organic waste like animal or chicken manure, which offer a complete balance nutrient balance together with immediate and long-lasting fertility. It is extensively utilised in the soil improvement, polluted soil cleanup, and crop fertilisation19. The application of bio-organic fertilizer can significantly increase the leaf area, photosynthetic rate, transpiration rate, and accumulation of K, Ca, Mg, and Fe in plant leaves, thereby improving photosynthetic productivity and above-ground growth biomass20. Meanwhile, it provides a new way to solve the problem of sugar beet continuous cropping. On the other hand, not much is known about how bio-organic fertilizer regulates the photosynthetic properties of beetroot leaves in continuous cropping environments. In order to provide a theoretical foundation and technical support for preventing and controlling the challenges associated with continuous sugar beet farming, Goal of this research was to examine the effects of applying bio-organic fertiliser on photosynthetic performance, dry matter accumulation, and distribution of beet under high intensity continuous cropping conditions.

Materials and methods

Site description

Experiments were conducted at the experimental farm of Agriculture and Forestry Sciences of Ulanqab in Inner Mongolia (40.9232° N, 113.1196° E) in the year 2020 and 2021. According to meteorological conditions, this region is a typical temperate continental monsoon climate. The elevation is 1962 masl, and the annual rainfall with mean annual temperature are 376 mm and 4.5 °C, respectively. The precipitation and daily average temperature of the experimental site during the crop growing season of 2020 and 2021 is shown in Fig. 1. The soil texture is chestnut soil, the organic matter is 12.09 g kg−1, total nitrogen is 1.34 g kg−1, the total phosphorus is 0.74 g kg−1, the total potassium is 13.56 g kg−1, the alkali hydrolyzed N-content is 108.78 mg kg−1, the available potassium content is 145.35 mg kg−1, the available phosphorus content is 15.02 mg kg−1, and the pH value is 8.18.

Fig. 1
figure 1

Precipitation (mm) and mean temperature (°C) of the experimental sites.

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Experimental design

This study was conducted at a sugar beet positioning long-term experimental site. In the long-term experimental site, sugar beet farming began in 2016 and last until 2021. Moreover, the planting area was 2,000 m2 (25 m × 80 m) each year. The land plot planted in 2016 was grown continuously from 2016 to 2020, the land plot planted in 2017 was grown continuously from 2017 to 2021, the land plot planted in 2018 was grown continuously from 2018 to 2021, the land plot planted in 2019 was grown continuously from 2019 to 2021, and the land plot planted in 2020 was only grown in 2021 under the sugar beet crop.

The experiment was carried out in a split-plot design in three replication during the two growing seasons for this study from 2020 to 2021. The main plot was chosen to be sugar beet continuous cropping system for the past 1, 2, 3, and 4 years (designated as C1, C2, C3, and C4, respectively), with the new cropping of each growing season serving as the control (CK). The sub-plot consisted of two fertilization treatments designated as 0 kg ha−1 bio-organic fertilization (N) and 6000 kg ha−1 bio-organic fertilization (Y). The amount of bio-organic fertilizer is based on the results of our previous studies21. Additionally, Fig. 2 displays the detailed field text layout.

Fig. 2
figure 2

The layout of the experimental field during 2020 to 2021 (R = replicate; Y = bio-organic fertilizer added; N = no bio organic fertilizer added).

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Sowing and fertilizing

We acknowledge the use of plant materials in this manuscript complies with all relevant institutional, national, and international guidelines and legislation. Sugar beet (IM1162), which is widely grown in Inner Mongolia, was planted in all plots from 2020 to 2021. Sugar beet was sown on 1st of May and harvested on 8th of October of 2020 and 2021. For the plots that do not use bio-organic fertilizer, compound granular fertilizer (12 N-18P-15 K) was applied each year at 900 kg ha−1 resulting in 108 kg ha−1 N, 162 kg ha−1 P, and 135 kg ha−1 K. While for the plots that use bio-organic fertilizer, compound granular fertilizer (12 N-18P-15 K) was applied each year at 600 kg ha−1 resulting in 72 kg ha−1 N, 108 kg ha−1 P and 90 kg ha−1 K.

Bio-organic fertilizer was made by mixing digested microbial agents with sheep manure at a 1:250 ratio. The majority of the microbial agents are Bacillus subtilis and Trichoderma harzianum, which were produced and processed using modern microbial fermentation technology. The formulation was a powder that has an effective viable bacterial population of at least 500 million cells per gramme. Bio-organic fertilizer was compounded by Inner Mongolia Geng-yu Fertilizer Co., Ltd. Furthermore, the content of N, P, and K is ≥ 4% and the organic matter is ≥ 40%.

Before planting, compound granular fertilizer and bio-organic fertilizer were applied to the field once and buried in each plot’s at a depth of 15 cm. At a depth of 1.5 cm, seeds were sown at a density of 100 thousand plants ha−1, with a plant spacing 18 cm and row spacing 50 cm. Field management was carried out in accordance with the normal recommended method.

Sampling and measurement methods

We acknowledge that we have permission to collect plants or plant parts in this experiment. Samples were collected during rapid leaf growth stage (RS), root and sugar growth stage (RSS), and sugar accumulation stage and harvest stage (SAS). Samples were randomly selected at 3 points in each plot, among which 3 plants were selected at each point.

Determination of photosynthetic performance

At the rapid growth stage of foliage (RS), root and sugar growth stage (RSS), and sugar accumulation stage (SAS), net photosynthetic rate (Pn), stomatal conductance (Gs), transpiration rate (Tr) and intercellular CO2 concentration (Ci), were measured using Li-6400 (Li-Cor Inc., Lincoln, NE, USA). At 09:30 a.m to 11:30 h of a sunny day, ten uppermost fully expanded leaves with almost similar lengths and widths were selected from different plants in each plot to determine the photosynthetic performance. While in operation, the system had a leaf chamber temperature of 25 °C, the CO2 concentration in the leaf chamber was set at 380 µmol mol−1, photosynthetic active radiation was set at 1100 µmol m−2 s−121. Simultaneously, SPAD value was determined using a portable chlorophyll meter (SPAD-502, Min-olta, Japan)22.

Dry matter accumulation

Samples were collected at rapid leaf growth stage (RS), root and sugar growth stage (RSS), and sugar accumulation stage and harvest stage (SAS). Samples (n = 3) were randomly selected at 3 points in each plot, among which 3 plants were selected at each point. After sampling, the plant samples were bifercated into leaves, leaf petioles, and roots and thereafter they were brought back to the laboratory for green removal at 105 °C for 30 min, and then dried at 80 °C until reaching a constant weight. The root-to-shoot ration was calculated as shown below.

$${\text{Root - to - shoot}}\;{\text{ratio}} = {\text{root}}\;{\text{dry}}\;{\text{weight }}({\text{g plant}}^{{ - {\text{1}}}} )/{\text{aboveground}}\;{\text{dryweight}}\;({\text{g plant}}^{{ - {\text{1}}}} )$$

Yield and sugar content

To calculate root yield, 10 m2 area were chosen in a plot, green tops of plants were cut, roots were removed and clean from soil around roots. All beets were weighed separately for each plot. In order to measure the tuber brix, 15 sugar beet-roots were chosen at random from a plot and brix reading was measured through a Japanese-made Atago Refractometer PAL−1 digital handheld refractometer, and sugar content was converted. The sugar content and sugar yield were calculated as shown below23.

$$\begin{gathered} {\text{Sugar}}\;{\text{Content }}\left( \% \right)={\text{The}}\;{\text{root}}\;{\text{tuber}}\;{\text{brix}}\left( \% \right) \times 0.{\text{83}} \hfill \\ {\text{Sugar}}\;{\text{Yield}}={\text{Yield}} \times {\text{Sugar}}\;{\text{content}} \hfill \\ \end{gathered}$$

Data preprocessing and statistical analysis

All data are shown as mean value of the three plots replications. Statistical analyses were carried out in R (version 4.1.1). The analysis of variance (ANOVA) was performed using the R packages “lme4 ′′ and “lmer Test”24. Differences between treatments (i.e., the ten treatments obtained by the interaction of five cropping year treatments combined with two fertilizer treatments) were compared using the Fisher’s least significant difference (LSD) approach25 (P ≤ 0.05). Root yield, sugar content and sugar yield were analyzed using a three-way ANOVA approach, with cropping year, fertilizer, and year as a fixed factors. Pearson’s correlation was performed to elucidate and visualize the relationships among yield index and plant characteristics. Pearson’s correlations were determined using R packages Hmisc (version 4.6) and corrplot (version 0.9). R packages corrplot (version 0.9), Lattice (version 0.2−45)26, ggplot2 (version 3.3.5)27, and Factoextra (version 1.07)28 were used to visualize the results. Figures were produced using the Origin 2021 software.

Results

Effects of continuous cropping and fertilizer on SPAD value of sugar beet

Both treatments, continuous cropping and the application of bio-organic fertilizer both had a significant effects on leaf SPAD value for all three growth stages of the sugar beet (Fig. 3). Based on the ANOVA results, the sugar beet SPAD value was significantly (p ≤ 0.001 or p ≤ 0.01) affected by the cropping year (C), fertilizer (F), growth stage (G), and the interaction of C×F and C×G.

The SPAD value decreased as the number of continuous cropping year increased. While the application of bio-organic fertilizer had a major impact on the SPAD value in the CK, C1, and C2 treatments. In two growing seasons, no matter the bio-organic fertilizers were used or not, the SPAD values of C1, C2, C3, and C4 were significantly lower than those of CK. And compared with CKN, the SPAD value of C1N, C2N, C3N, and C4N were reduced by an average of 15.79–22.93%, 20.65–26.38%, 30.40–37.87%, and 37.24–46.05%. Compared with CKY, the SPAD value of C1Y, C2Y, C3Y, and C4Y were reduced by an average of 10.09–21.15%, 15.25–25.36%, 30.20–40.50%, and 38.66–48.21%. Additionally, the SPAD values of C1and C2 were significantly higher than those of C3 and C4. Bio-organic fertilizers considerably improved the SPAD value for cropping year treatments CK, C1, and C2. However, there were no significant impacts be observed for treatments C3 and C4.

Fig. 3
figure 3

Changes of SPAD value of sugar beet leaves under different treatments. RS: Rapid growth stage of foliage, RSS: Root and sugar growth stage, and SAS: Sugar accumulation stage. C: Cropping year, F: Fertilizer, G: Growth stage. The statistical significance is denoted by *P ≤ 0.05; **P ≤ 0.01; ***P ≤ 0.001, and NS, not significant. Numbers followed by different letters in each column of the same growth stage indicate significantly differences at α = 0.05 based on ANOVA test.

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Effects of continuous cropping and fertilizer on gas exchange parameters of sugar beet

Continuous cropping year and fertilizer treatments had certain effects on Pn, Gs, Tr and Ci of sugar beet, and also the results were basically consistent in 2 years (Fig. 4). Based on the ANOVA results, the cropping year (C), fertilizer (F), and growth stage (G) and the interaction of C×F and C×G significantly (p ≤ 0.001 or p ≤ 0.01) affected the gas exchange parameters of sugar beet.

The Pn, Gs, Tr, and Ci of sugar beet all exhibited a declining tendency during three growth stages with the extension of continuous cropping years. The Pn, Gs, Tr, and Ci were strongly impacted by the application of bio-organic fertilizer under treatments of CK, C1, and C2. Additionally, the effects of cropping year and fertilizer on the gas exchange parameters were generally consistent. The Pn, Ci, Tr, and Gs of continuous year treatments were significantly lower than CK during three growth stages. And compared with CKN, the Pn, Ci, Tr, and Gs decreased by 15.49-38.27%, 15.05-39.95%, 7.77-39.31%, and 9.61-37.68%. Compared with CKY, those decreased by 14.60-43.19%, 12.00-42.86%, 10.59-43.83%, 11.68–42.65%, respectively. The increasing number of cropping years that also increased the rate of decline. In addition, regardless of the use of bio-organic fertilizer, the Pn, Ci, Tr, and Gs of C1 and C2 were much higher than those of C3 and C4. For the cropping year treatments of CK, C1, and C2, applying bio-organic fertilizer led to a considerable rise in Pn, Ci, Tr, and Gs. Furthermore, compared with CKN, the Pn, Ci, Tr, and Gs of CKY increased by 11.26-12.93%, 10.77–14.19%, 8.08-10.39%, and 8.25-11.06%; compared with C1N, the Pn, Ci, Tr, and Gs of C1Y increased by 12.79-22.50%, 14.76-16.47%, 4.76-13.08%, and 8.51-12.70%; compared with C2N, the Pn, Ci, Tr, and Gs of C2Y increased by 13.69-18.91%, 12.17-14.60%, 5.71-15.14%, and 7.58-15.41%. Nevertheless, the application of bio-organic fertilizer to treatments C3 and C4 did not have any significant effects.

Fig. 4
figure 4

Changes of gas exchange parameters of sugar beet leaves under different treatments. RS: Rapid growth stage of foliage, RSS: Root and sugar growth stage, and SAS: Sugar accumulation stage. The different letters on bars show significant differences at 0.05 level of probability. The statistical significance is denoted by *P ≤ 0.05; **P ≤ 0.01; ***P ≤ 0.001, and NS, not significant. Numbers followed by different letters in each column of the same growth stage indicate significantly differences at α = 0.05 based on ANOVA.

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Effects of continuous cropping and fertilizer on dry matter accumulation of sugar beet

The dry matter weight of sugar beet was substantially different during three growing stages (Fig. 5). And with the growth going on, the dry matter weight all showed an increased trend. The ANOVA results for dry matter weight are shown in Table 1. The cropping year (C) and fertilizer (F) all significantly (p ≤ 0.001) affected the dry matter weight. The interaction of C×F had different significant effects on dry matter weight among different plant organs and growth stages.

Under different continuous cropping year, the changes of dry matter weight of sugar beet are shown in Fig. 5. In general, the dry matter weight decreased significantly in all growth stages due to continuous cropping from 2020 to 2021. At different growth stages, the significant differences among different cropping year showed differently. And in general, the dry weight of CK was significantly higher than that of four continuous cropping year treatments. Those of C1 and C2 were significantly higher than those of C3 and C4, and also there were a significant differences between C1 and C2, C3 and C4 (Table 2). Compared with CKN, the average dry weight of leaf, petiole, root, and the whole plant of four continuous cropping year treatments decreased by 11.86–47.12%, 11.32–43.94%, 19.11–53.11%, and 16.67–48.35%, respectively. Compared with CKY, the average dry weight of leaf, petiole, root, and the whole plant of four continuous cropping year treatments decreased by 12.41–49.61%, 9.58–44.77%, 17.26–53.76%, and 15.79–49.79%. Also the number of consecutive cropping year increases the degree of decline.

The dry weight of sugar beet treated with bio-organic fertilizer was higher than that treated without fertilizer in some stages. In 2020 and 2021, the effects of bio-organic fertilizer under CK, C1 and C2 were significantly. Compared with CKN, C1N, C2N, the dry weight of leaf, petiole, root, and the whole plant of CKY increased by 8.98–11.51%, 7.23–12.49%, 12.11–17.44%, and 9.80–14.15%; those of C1Y increased by 8.32–16.18%, 10.80–13.33%, 14.93–20.13%, and 14.12–15.43%; those of C2Y increased by10.65–15.32%, 6.58–15.63%, 15.37–20.94%, and 12.68–16.85% on average, respectively. For the treatments C3 and C4, the application of bio-organic fertilizer had no significant effects on the dry weight basically during these three growth stages.

Fig. 5
figure 5

Changes of dry matter of sugar beet under different treatments. RS: Rapid growth stage of foliage, RSS: Root and sugar growth stage, and SAS: Sugar accumulation stage. The different letters on bars of the same color show significant differences at 0.05 level of probability. And the different letters on the top of each bars of each treatemnt show significant differences for the dry matter weight for the whole plant at 0.05 level of probability.

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Table 1 ANOVA results of cropping year, fertilizer on dry weight of sugar beet.
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Effects of continuous cropping and fertilizer on dry matter distribution of sugar beet

Continuous cropping year and fertilizer had certain effects on the dry matter distribution and transport of sugar beet, and the results of 2 years were basically the same (Table 2). The ANOVA results for dry matter distribution were shown in Table 2. In the rapid growth stage of foliage, the cropping year (C) significantly (p ≤ 0.01 or p ≤ 0.001) affected the dry matter ration of above ground, dry matter ration of root tuber and root shoot ration in two years, while fertilizer (F) only significantly (p ≤ 0.01) affected the dry matter ration of above ground, dry matter ration of root tuber and root shoot ration in 2021 and the interaction of C×F had no significant effects on the dry matter distribution. In the tuber growth and sugar accumulation stage and sugar accumulation stage, the cropping year (C), fertilizer (F) and their interaction all significantly (p ≤ 0.05, p ≤ 0.01, or p ≤ 0.001) on the dry matter ration of above ground, dry matter ration of root tuber and root shoot ration.

With the increasing of continuous cropping year, the dry matter ration of above ground of sugar beet showed an increasing trend, while the dry matter ratio of root tuber and the root shoot ratio showed a decreasing trend. In the rapid growth stage of foliage, only the CK had significant effects on dry matter distribution. Compared with other four continuous cropping treatments, it showed an significantly increasing effects on dry matter ratio of root tuber and the root shoot ratio. In this stage, continuous cropping of sugar beet inhibited the transport of dry matter, but there were no significant differences among the four continuous cropping treatments. And also the application of bio-organic fertilizer didn’t have significant effects on the dry matter distribution. In root and sugar growth stage and sugar accumulation stage, the distribution direction of dry matter of sugar beet plants has changed, with assimilation products flowing more towards the root tubers. In general, the dry matter ratio of root tuber and the root shoot ratio of CK was significantly higher than those of four continuous cropping year treatments. And those of C1 and C2 was significantly higher than that of C3 and C4. Compared with CKN, the root shoot ration of C1N, C2N, C3N, and C4N decreased by 5.80–8.00%, 6.65–8.63%, 8.81–9.66%, and 13.74–15.38% on average, respectively. Compared with CKY, the root shoot ration of C1Y, C2Y, C3Y, and C4Y in two years decreased by 3.62–8.57%, 5.56–9.22%, 14.47–14.67% and 17.61–19.76% on average, respectively. So in these two growth stages, continuous cropping of sugar beet inhibited the transport of dry matter from the above ground to the root, increased the dry matter ration of above ground, decreased the dry matter ratio of root tuber and the root shoot ration. At the same time, in these two growth stages, the application of bio-organic fertilizer had significant effects on decreasing dry matter ration of above ground and increasing dry matter ratio of root tuber under the cropping year treatments of CK, C1, and C2, but these effects under the cropping year treatments of C3 and C4 didn’t significantly. Compared with CKN, C1N, and C2N, the root shoot ration of CKY, C1Y, and C2Y increased by 5.21-6.04%, 4.58-8.56% and 4.52-7.35% on average, respectively.

Table 2 Changes of dry matter distribution of sugar beet under different treatments.
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Correlation among photosynthetic characteristics, dry matter, yield, sugar content and sugar yield

The correlations among growth indexes and root yield are shown in Fig. 6. Obviously, photosynthetic characteristics (Pn, Ci, Tr, Gs, and SPAD) all had very significant correlations (p ≤ 0.001) with the dry matter of whole plant, root shoot ratio, yield, sugar content and sugar yield. The correlation results showed that through the adjustment of photosynthetic characteristics, the root yield and sugar yield of sugar beet can be improved cooperatively in the present study.

Fig. 6
figure 6

Pn, Gs, Ci, Tr, SPAD, DM, RS, Y, SS and SY indicate net photosynthetic rate, intercellular CO2 concentration, transpiration rate, stomatal conductance, SPAD value, dry matter of whole plant, root shoot ratio, yield, sugar content and sugar yield.

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Discussion

Effects of continuous cropping and application of bio-organic fertilizer on photosynthetic performance

In continuous cropping system, plants are in a “stress” state, in which the nutrients required for plant growth and development are insufficient, the accumulation of nutrients in body is difficult, the growth is slow, and the growth momentum is weak. Photosynthesis is one of the important physiological and metabolic processes of crop production, and its efficiency is one of the fundamental factors determining crop productivity. It was concluded that continuous cropping could cause harmful changes in crop rhizosphere microecological environment, significantly inhibit or damage physiological processes such as water and nutrient transport, substance synthesis and metabolism, and weaken photosynthetic physiological activities and photosynthetic capacity of crops29. Wang et al.30 reported that continuous cropping could inhibit photosynthesis and reduce chlorophyll content in leaves. In this study, continuous cropping inhibited the photosynthesis of sugar beet, which led to a significant decrease in leaf Pn, Gs, Tr, Ci, and SPAD values. With the increasing of continuous cropping year, the degree of reduction intensified. While the differences between C1 and C2 were not significant. These results were similar with the present studies on potato31, melon32, tomato33. These consequences could be linked to the depletion of soil health that stresses crop growth and results in a decline in soil fertility and nutrient delivery due to continuous cropping. Patterson34 observed that phenolic acids can lower the photosynthetic capacity and chlorophyll content of soybean leaves. The accumulation of soil allelopathic compounds during continuous cropping, which is another component of soil health impacting sugar beet growth and development, may be directly linked to the decline in net photosynthetic rate of sugar beet in this article with increasing continuous cropping year. However, additional findings also indicated that pepper’s chlorophyll content, photosynthetic traits, and physiological and biochemical traits exhibited a trend of initially increasing and then declining with the increasing number of continuous cropping year. Furthermore, the study’s results differed from the others; this may because various crops react differently to ongoing cropping.

Bio-organic fertilizer not only contains nitrogen, phosphorus, and potassium elements contained in ordinary fertilizers, but also contains various trace elements. As a base fertilizer, they can improve soil and enhance crop quality. According to this study, the Pn, Ci, Tr, Gs, and SPAD value of sugar beet under continuous cropping can be improved to varying degrees by applying bio-organic fertilizer. These results were essentially in line with earlier researches on the photosynthetic properties of crops like wolfberry35, watermelon36, and cabbage37. This can be the results of soil nutrient proportions being balanced by the use of bio-organic fertilizer. Crops are encouraged to absorb and use the nutrients found in the soil, and other metabolic materials, such as phenols, enzymes, vitamins, and auxin, which were produced during the decomposition process can further improve root development and nutrient absorption, raising the amount of nutrients in crop leaves38. Studies have shown that the increase of nitrogen content in leaves can effectively improve the photosynthetic efficiency and photosynthetic capacity of crops39. At the same time, studies have shown that microorganisms in soil can promote the improvement of chlorophyll fluorescence parameters of crops and their utilization of effective light40,41,42, which is also one of the reasons why bio-organic fertilizer can improve the photosynthetic performance of continuous cropping beet leaves. And in this study, the application of bio-organic fertilizer had significant effects on improving the photosynthetic performance under the continuous cropping year of C1 and C2, while these effects under continuous cropping year of C3 and C4 were not significant. This may be caused by the fact that when the continuous cropping year reaches 3 years or more, the bio-organic fertilizer have some effects on reducing the harm caused by continuous cropping, but it’s not enough to completely suppress continuous cropping disorders. Also these findings were different with those related to cucumbers43, which showed that the longer the soil was continuously cropped after the application of bio-organic fertilizer, the greater the impact of the fertilizer on soil remediation for the cucumbers in facility. This could be brought about by variations in crop traits and kinds of bioorganic fertilizer.

Effects of continuous cropping and application of bio-organic fertilizer on dry matter accumulation and distribution of sugar beet

Dry matter formation, distribution, and transportation are significant indicators of a crop’s capacity to adjust to external elements like soil and climate. In addition to reflecting genetic traits and physiological and ecological adaptability, the accumulation and distribution of dry matter in various crop organs also reflects variations in growth and yield due to the movement of nutrients between organs after stimulation and regulation by the external environment. Strong dry matter production capability is necessary for a high crop yield, but effective dry matter distribution and transit throughout different organs are also essential for yield development44,45. The growth of the aboveground sections and the root tubers of sugar beet mutually encourage and restrain each other. Regulating the proportion of dry matter and increasing the root crown ratio play important roles in promoting yield formation46.

Continuous cropping can lead to changes in endogenous hormone levels during the early stages of crop growth, affecting the establishment of dry matter47. The root system may operate as a mediator in the process by which environmental stimuli modify endogenous hormone metabolism and impact the establishment of dry matter. The ability of the roots to synthesise essential metabolic compounds such proteins, amino acids, alkaloids, and hormones is reduced as a result of the soil stress caused by ongoing cropping48. In addition, it alters the nutritional status of plants used for continuous cropping, decreases plant absorption of soil nutrients, particularly nitrogen, and creates negative feedback control on material buildup49. The results of this study showed that continuous cropping significantly inhibited the accumulation of dry matter in different organs of sugar beet. Following continuous cropping, the restricted plant growth causes a reduction in the canopy population’s photosynthetic source area during the growth period, as well as insufficient and decreased photosynthetic capacity. These factors collectively lead to a decrease in the dry matter accumulation that occurs during continuous sugar beetroot cropping. In this study, we found that compared with CK, the dry matter weight decreased significantly with the increasing of continuous cropping year, while the differences between C1 and C2 were not significant. These results were similar with the study which resulted that continuous cropping leaded to a significant decrease in the accumulation of dry matter in various organs of potatoes50, but the study about potato showed that compared with the control, there was no significant decrease in dry matter accumulation in potatoes after 1–2 years of continuous cropping. This may be due to different crops reacting differently to continuous cropping.

Continuous cropping also leads to changes in the distribution of dry matter in sugar beet. The standout characteristic is that while the amount of dry matter accumulated in root is less than in the control, it is more in the aboveground portions of the system. This change becomes more pronounced with the increasing of continuous cropping years. This is also consistent with previous research results51. The adjustment of root shoot ratio is considered as the main strategy for plants to cope with biotic or abiotic stress. It can show how crops are developing and growing as well as the various ways that the environment affects the growth of roots and aboveground components52,53. In this study, continuous cropping dramatically decreased the sugar beet root shoot ratio when compared to the CK. As sugar beet mature, dry matter transfers to the root, and the root shoot ratio of C1 and C2 treatments are much higher than that of C3 and C4 treatments, the differences between the four continuous cropping treatments were not significant in the early growth stage. Changes in the root to shoot ratio54 may be brought on by an increase in pests and root diseases brought on by prolonged, continuous cropping, as well as by nutrition and water stress and endogenous hormone imbalances.

Previous studies have shown that bio-organic fertilizer can provide crops with nutrients necessary for the entire growth period, which is beneficial for increasing their dry matter accumulation55,56. Research by Liu et al.57 has shown that, on the basis of 30% reduction in chemical fertilizers, the application of bio-organic fertilizer can increase the accumulation of dry matter in corn, while promoting the transfer of dry matter from nutrient organs to grains, and improving the accumulation of dry matter after corn flowering. According to research by Wang et al.44, applying a specific quantity of bio-organic fertilizer can increase the dry matter accumulation in chives. Additionally, within a certain range, the dry matter accumulation shows an increasing trend as bio-organic fertilizer application increases. Application of bio-organic fertilizer in continuous cropping shown that bio-organic fertilizer may encourage cucumber58 and cotton59 growth and dry matter accumulation. Wang et al.60 stated that the primary role of bio-organic fertilizer is to boost photosynthesis and postpone the aging of functioning leaves, which in turn promotes the accumulation and transfer of dry matter. Simultaneously, microorganisms present in bio-organic fertilizer have the ability to activate mineralized nutrients in soil and convert them into forms that crops can absorb and use. This process helps to increase crop development by promoting nutrient absorption by crops. The application of bio-organic fertilizer in this study resulted in a considerable rise in the dry matter accumulation in the various organs of continuous cropping beet. It is evident that the results of this study’s continuous cropping conditions and the use of bio-organic fertilizer on the accumulation of dry matter in beet are essentially the same as those of previous studies. This is so that bio-organic fertilizer may control the amount of nutrients in soil used for continuous cropping, maintain a balance between the C and N ratios of crops, control the soil environment in the root zone of crops, increase the plants’ ability to withstand stress, and encourage growth. And also, the application of bio-organic fertilizer had significant effects on improving dry matter accumulation and distribution of sugar beet under the continuous cropping year of C1 and C2, while these effects under continuous cropping year of C3 and C4 were not significant. And these were the same with the effcts on photosynthetic performance.

Conclusions

Continuous cropping inhibited the photosynthesis of sugar beet and hindered the ability of dry matter accumulation and distribution in various organs. With the extension of continuous cropping years, Pn, Tr, Gs, Ci and SPAD value of sugar beet significantly decreased, and the dry matter mass and distribution ratio of leaves, petioles and roots significantly decreased. And these effects had very significant correlation ship with root yield and sugar yield. Under continuous cropping conditions, the application of 6000 kg ha−1 bio-organic fertilizer could effectively promote the photosynthesis of sugar beet, and better regulate the distribution and transport of dry matter of sugar beet, and the effects of bio-organic fertilizer was significant under continuous cropping for 1–2 years. And while bio-organic fertilizer did have some impacts, they were not statistically significant for cropping that was continued for three years or more.