1 Introduction

Protected cultivation such as greenhouses and solar houses has expanded rapidly to meet year‑round vegetable demand and increase farmers’ income, especially in China (Hu et al., 2017; Yan et al., 2024). However, the combination of high cropping intensity, large agrochemical inputs, and closed environments often accelerates soil degradation, leading to salinization, acidification, nutrient imbalance, and pollutant accumulation in greenhouse vegetable soils. Continuous monocropping of the same vegetable, notably cucumber, further aggravates these problems and has emerged as a major constraint on the sustainable development of protected agriculture. Understanding how long‑term cucumber monocropping alters soil properties is therefore critical for designing more resilient protected production systems.

Continuous cropping obstacles (CCOs) refer to yield decline, quality reduction, and heightened disease incidence that arise when the same crop is repeatedly grown on the same land, and are now recognized as a widespread problem in intensive vegetable systems. Meta‑analysis and reviews across crops show that CCOs are closely linked to deterioration of soil physical and chemical properties, disruption of soil food webs, and shifts in microbial community structure that favor soil‑borne pathogens and reduce beneficial biodiversity. In greenhouse vegetables, long‑term continuous monoculture promotes secondary salinization and acidification, accumulation of nitrate and other ions, and microbial “dysbiosis,” ultimately impairing soil ecological function and nitrogen use efficiency (Huang et al., 2023). For cucumber, studies under protected conditions have reported rising electrical conductivity, enrichment of nitrate and phosphorus, declining pH, and changes in dominant bacterial and fungal groups as cropping years increase (Zhao et al., 2020; Bian et al., 2022). These changes are often accompanied by soil “sickness,” weaker plant growth, and reduced fruit yield and quality.

Cucumber is a key greenhouse cash crop, and continuous cucumber cultivation is common due to its high economic value and market demand. Recent work has begun to characterize specific aspects of cucumber continuous cropping, including shifts in rhizosphere microbial diversity, changes in fungal communities, and accumulation of soil phosphorus fractions under repeated cucumber cycles. However, existing studies are often fragmented, focusing on single indicators (such as phosphorus, fungi, or general microbial diversity) or limited time scales, and there is still a lack of integrated understanding of soil degradation characteristics-covering physicochemical properties, nutrient status, and microbial responses-under long‑term continuous cucumber cropping in protected facilities (Huang et al., 2023; Zhao et al., 2020). Moreover, while some mitigation strategies such as crop residue return or organic amendments have been shown to partially alleviate continuous cropping obstacles in cucumbers and other vegetables, they have generally been proposed without a detailed, site‑specific diagnosis of degradation patterns in the target system (Yan et al., 2024).

In this context, the present study focuses on soil degradation under continuous cucumber cropping in a protected cultivation system, aiming to clarify how long‑term monoculture reshapes key soil properties and biotic components. The main objectives are: (1) to characterize temporal changes in soil physicochemical indicators (such as pH, salinity, organic matter, and major nutrients) associated with increasing years or rounds of continuous cucumber cropping; (2) to analyze corresponding changes in soil biological attributes, including microbial community diversity, composition, and activity that are closely tied to soil health and continuous cropping obstacles; and (3) to identify the principal soil factors driving these biotic shifts and to summarize the degradation patterns most strongly associated with cucumber growth decline. By linking multiyear changes in abiotic and biotic soil indicators, the study seeks to provide a comprehensive picture of degradation trajectories under continuous cucumber monoculture and a scientific basis for developing targeted soil management and amendment strategies to mitigate CCOs and support the sustainable productivity of protected cucumber systems. Protected cucumber production intensifies land use but often causes continuous cropping obstacles driven by soil degradation. Existing studies show clear chemical and microbial deterioration in long‑term cucumber monocropping, yet integrative analyses of degradation characteristics remain limited. This study targets those gaps by jointly examining physicochemical and biological changes in soils under continuous cucumber cropping, to clarify degradation patterns and support more sustainable management of greenhouse cucumber systems.

2 Concept and Development Status of Continuous Cropping in Cucumber

2.1 Definition of continuous cropping and its agricultural applications

Continuous cropping generally refers to planting the same or closely related crop on the same land for many consecutive seasons without sufficient rotation or fallow. In modern intensive systems this practice often takes the form of continuous monoculture, where one high‑value crop is repeatedly grown to maximize short‑term economic returns and utilization of protected facilities. As land resources become constrained and demand for vegetables rises, continuous planting has become common in facility agriculture, especially for high‑profit crops such as cucumber, peanut, and various medicinal plants. While agronomically convenient, long‑term monoculture has been repeatedly associated with soil degeneration, reduced yield and quality, and greater incidence of soil‑borne diseases (Pervaiz et al., 2020; Belete and Yadete, 2023). 

From a functional perspective, continuous cropping is both an agronomic strategy and a stress test of the soil-plant-microbe system. In the short term, reusing infrastructure, irrigation systems, and fertilizer regimes can improve labor efficiency and enable year‑round production in greenhouses. However, long‑term continuous cropping alters soil physicochemical properties and biological processes, including nutrient cycling and microbial food‑web structure (Pervaiz et al., 2020). Accumulation of specific root exudates and autotoxic compounds, shifts in microbial communities toward pathogens, and changes in soil organic matter dynamics are now recognized as core mechanisms behind continuous cropping obstacles across crops. Consequently, continuous cropping is increasingly used as a model scenario to evaluate soil degradation processes and to test mitigation technologies such as crop rotation, soil amendments, and biological regulation. 

2.2 Development of greenhouse cucumber cultivation and continuous cropping systems

Protected cucumber cultivation has expanded rapidly as facility agriculture has become an important indicator of agricultural modernization, especially in regions such as China where solar greenhouses permit off‑season production (Chu et al., 2025). Cucumber is highly suitable for greenhouse systems due to its sensitivity to temperature and water conditions and its high economic return per unit area (Liu et al., 2020). New greenhouse designs, including sunken and improved solar structures, have been developed to optimize microclimate and reduce winter temperature stress, thereby supporting multiple cucumber seasons per year in the same soil profile. At the same time, intensive fertigation practices with high nitrogen rates have been widely adopted to maintain high yields but have also increased risks of soil salinization, nutrient accumulation, and associated environmental burdens. 

This technological and economic context has driven the formation of typical continuous cucumber cropping systems, where the same beds are used over many successive cycles with limited rotation. High‑input management in such systems often leads to substantial build‑up of salts and nutrients in topsoil, as documented for long‑term cucumber greenhouses in northern China. Life‑cycle assessments comparing greenhouse and open‑field cucumber production also show that facility systems, while productive, can have higher greenhouse gas emissions and lower carbon efficiency, in part due to concentrated input use and plastic materials. As greenhouses become the dominant mode of cucumber production in many regions, understanding how repeated cucumber planting affects soil health, microbial ecology, and sustainability has become a key scientific and practical issue (Figure 1) (Pervaiz et al., 2020; Yang et al., 2025). 

2.3 Research progress on continuous cropping obstacles at home and abroad

Internationally, research on continuous cropping obstacles has evolved from descriptive yield decline studies to mechanistic analyses of soil microecology and plant responses. Meta‑analyses and broad reviews show that continuous cropping can on average reduce crop production by more than 20%, mainly through soil degradation, shifts in microbial communities, and increased disease pressure. Across diverse crops-including medicinal plants, legumes, tobacco and peanuts-common patterns include deterioration of soil physical and chemical properties, accumulation of toxic metabolites, and enrichment of pathogenic fungi and bacteria in the rhizosphere (Haq et al., 2023). At the same time, global syntheses emphasize that not all continuous systems inevitably develop obstacles; under certain managements, beneficial microorganisms and disease‑suppressive soils can be enriched, highlighting the importance of system‑level regulation of the soil microbial food web (Tan et al., 2021). 

Domestic and broader regional studies have paid particular attention to facility vegetable and legume systems, where continuous cropping is widespread due to high economic returns. Research has shown that soil microbial community structure is strongly driven by continuous cropping, with reduced beneficial taxa and increased pathogens contributing to root diseases and growth inhibition. Reviews focusing on legumes and other specialty crops further point out that continuous cropping obstacles typically start with soil environment degradation and then trigger oxidative stress and defense responses in plants. In parallel, considerable progress has been made on mitigation strategies such as crop rotation, intercropping, organic and microbial amendments, compost application, and reductive soil disinfection, all aiming to restore soil fertility, rebalance microbial communities, and alleviate continuous cropping obstacles in intensive systems (Ding et al., 2021; Belete and Yadete, 2023). 

3 Types and Manifestations of Soil Degradation under Continuous Cropping

3.1 Degradation of soil physicochemical properties (structure, salinity, pH changes)

Under continuous cucumber cropping in greenhouses, soil physicochemical conditions change markedly, even when yields initially remain acceptable. Long-term monoculture commonly causes soil salinization and acidification, with electrical conductivity and total salt content rising while pH declines as cropping years increase (Zhao et al., 2020; Huang et al., 2023). In solar‑greenhouse and commercial systems, repeated cucumber cultivation was associated with significant enrichment of organic matter and mineral nutrients, but this was accompanied by higher salinity and lower pH, indicating progressive chemical degradation despite apparent fertility gains. Studies on different cultivation years showed that greenhouse cucumber soils tended to accumulate salts and become more acidic as the duration of protected cultivation increased, particularly after more than a decade of continuous use. 

Physical aspects of soil structure are also affected, though sometimes more subtly than chemical properties. In some recropping experiments, soil particle composition and aggregate stability changed little, yet salinity build‑up and acidification still drove functional degradation and yield decline (Liang et al., 2012). Other work in protected cucumber systems reported increases in water‑stable aggregates and organic matter with continuous cropping, but these structural “improvements” co‑occurred with deteriorating chemical properties and salt accumulation that undermined overall soil health. More generally, reviews of continuous cropping highlight that repeated monoculture can reduce aggregation, increase compaction, and alter water infiltration and aeration, especially when combined with heavy agrochemical use in intensive vegetable systems (Pervaiz et al., 2020). Thus, under continuous cucumber cultivation, soil structure, salinity, and pH interact, with chemical stress often overriding any local gains in physical aggregation. 

3.2 Nutrient imbalance and decline in soil fertility

Nutrient dynamics under continuous cucumber cropping are characterized by strong accumulation in soil but growing imbalance relative to crop demand. Along continuous gradients (from 0-12 years or 1-25 years of cropping), contents of organic matter, alkali‑hydrolyzed N, available P, and available K generally rise significantly compared with non‑continuous controls, reflecting high fertilizer inputs and low nutrient use efficiency in greenhouses. Long‑term monitoring in solar‑greenhouse systems showed that total N and total P can more than double, and available forms of these elements also increase, while total K often remains relatively unchanged, indicating skewed element stoichiometry (Zheng et al., 2021). Similar patterns of nutrient enrichment-particularly of N and P-were observed across multiple cropping cycles, even where cucumber yields had not yet collapsed, implying that “hidden” nutrient stress may precede visible productivity loss (Bian et al., 2022). 

Despite apparent enrichment, soil fertility can effectively decline because nutrient ratios, availability, and plant uptake become decoupled. An 11‑year continuous cropping study revealed that increasing soil C, N, and especially P did not translate into proportional changes in leaf N and K, while leaf P increased, leading to decoupling of P from N and K in cucumber nutrient utilization. Other long‑term greenhouse experiments with consecutive cucumber cultivation showed strong accumulation of different soil P fractions, alongside shifts in microbial communities that favored P fixation into less labile pools and lowered the phosphorus activation coefficient (Bian et al., 2022). In broader continuous cropping systems, similar nutrient imbalances, coupled with salinity and pH shifts, are linked to reduced nutrient mineralization, impaired root uptake, and eventual yield decline despite high soil test values (Pervaiz et al., 2020). Together, these findings indicate that, under continuous cucumber cropping, nutrient imbalance and mis‑matched supply-demand relations constitute a key form of soil fertility degradation. 

3.3 Degradation of soil biological properties

Soil biological properties show some of the most pronounced degradation signals under continuous cucumber monoculture. With increasing years or rounds of cropping, the diversity and richness of rhizosphere bacterial and fungal communities often decline, and community structure diverges clearly from that of non‑continuous soils (Huang et al., 2023; Li et al., 2021). In several greenhouse systems, both bacterial and fungal Shannon diversity and evenness decreased significantly under long‑term monoculture, with fungal communities generally more sensitive than bacterial ones. Functional analyses further indicated reduced bacterial functional potential with longer cropping histories, suggesting that critical processes such as nutrient cycling and organic matter turnover may be weakened. 

At the same time, the composition and functional balance of the microbial community are reshaped in ways that can promote soil “sickness.” Continuous cucumber cultivation tends to increase the relative abundance of some bacterial phyla (e.g., Chloroflexi, Gemmatimonadetes) while reducing beneficial groups such as Actinobacteria and certain N‑cycling genera, and it alters fungal communities by decreasing overall diversity but sometimes increasing specific phyla like Ascomycota. Long‑term consecutive cucumber cropping has also been linked to initial increases and then marked declines in microbial biomass and key groups (bacteria, actinomycetes, Gram‑negative and Gram‑positive bacteria), with fungi:bacteria ratios rising, which is often associated with higher disease pressure. Broader reviews of greenhouse vegetable systems emphasize that continuous monoculture, together with heavy fertilizer use, drives microbial deterioration-reduced beneficial biodiversity, altered enzyme activities, and weakened microbial networks-thereby lowering N use efficiency and increasing soil‑borne disease incidence (Liu et al., 2020; Shen et al., 2021). Overall, shifts in microbial diversity, composition, and function form a central manifestation of soil biological degradation under continuous cucumber cropping.

4 Characteristics of Changes in Soil Physicochemical Properties

4.1 Changes in soil structure and aggregate stability

Under continuous cucumber cropping in greenhouses, repeated tillage, heavy fertilization, and frequent irrigation gradually damage soil structure. Long-term greenhouse systems commonly show increased bulk density and reduced total porosity, reflecting compaction and loss of a stable pore network (Chen et al., 2022). In continuous cucumber soils, water-stable aggregates and their mean weight diameter decline when only chemical fertilizers are used, indicating that aggregate stability is particularly vulnerable under high-intensity protected cultivation. This structural degradation weakens pore connectivity and reduces infiltration, making soils more prone to surface crusting and waterlogging during irrigation (Yan et al., 2024). 

At the same time, studies also show that soil structure in continuously cropped cucumber systems is highly responsive to organic and biological management. Adding bio-organic fertilizers to continuous cucumber soil significantly reduces bulk density, increases total porosity, and enhances the content and stability of water-stable aggregates. Microbial inocula and organic materials increase macro- and microaggregates, with aggregate indices strongly and positively correlated with soil organic matter and nutrients, and negatively with total salt content (Peng et al., 2023). Improved aggregation under such amendments not only enhances structural stability but also supports higher cucumber yields in salinized greenhouse soils (Peng et al., 2025). 

4.2 Processes of soil salinization and acidification

Soil salinization is a key chemical degradation process in greenhouse vegetable systems. In double-cucumber greenhouse systems, excessive mineral fertilization leads to secondary salinization, with increased electrical conductivity and accumulation of Cl⁻, NO₃⁻, and SO₄²⁻ in the soil profile. Nutrients and salts tend to build up in the upper layers because the absence of rainfall limits natural leaching, especially under frequent but shallow irrigation in protected facilities (Figure 2) (Yan et al., 2024). Studies of vegetable cultivation under alternating open-field and greenhouse conditions show that nitrate and salt concentrations rise strongly during the greenhouse season, closely tracking increases in soil EC and indicating progressive salinity build-up (Shi et al., 2008). 

Acidification often co-occurs with salinization but follows partly different mechanisms. Surveys of greenhouse vegetable soils with various initial pH values show that pH decreases steadily with cultivation years, while EC increases, demonstrating simultaneous acidification and salinity development. Over-application of N and K fertilizers reduces the proportion of Ca²⁺ and HCO₃⁻ in soil exchange complexes, driving pH decline and weakening buffering capacity (Han et al., 2014). In solar greenhouse systems with flooding irrigation, surplus N is leached downward, and both surface and deep soil layers experience pH declines linked to mineral N dynamics (Lv et al., 2020). Across greenhouse vegetables, accumulated salts and strong acidifying fertilization together form a coupled process of salinization-acidification, which is now recognized as a major constraint on long-term soil productivity. 

4.3 Changes in soil water retention capacity

Continuous cropping and associated structural degradation also alter soil hydraulic behavior. In a karst region under different continuous cropping durations, long-term continuous systems showed reduced soil organic matter and clay contents, accompanied by deterioration of soil water characteristic parameters and lower water-holding capacity compared with rotation systems (Yang et al., 2023). These changes were consistent across depths, and soil water properties were positively correlated with organic matter, clay content, and pH, indicating that chemical and textural degradation directly impair soil hydraulic performance. 

In continuous cucumber greenhouse soils, poor structure and secondary salinization reduce effective water availability. However, where bio-organic fertilizers are applied, bulk density declines, total porosity rises, and hydraulic properties improve, including higher water-stable aggregates that favor better water storage and transmission in the root zone (Chen et al., 2022). Similar findings from other cropping systems show that conservation-oriented practices, such as residue retention and diversified rotations, increase porosity, plant-available water capacity, and infiltration by enhancing aggregate stability and fine pore fractions (Eze et al., 2020; Parker et al., 2024). These results suggest that in continuous cucumber systems, degradation of water retention is not irreversible; targeted structural and organic management can partly restore soil hydraulic functions and buffer plants against water stress under intensive greenhouse conditions. 

Under continuous cucumber cropping, soil structure tends to compact and aggregates destabilize, while salinization and acidification progress due to high fertilizer inputs and limited leaching. These physical and chemical degradations jointly reduce soil water-holding capacity and hydraulic quality. Evidence from greenhouse cucumber and related systems indicates that organic amendments, bio-organic fertilizers, and improved management can counteract these trends by rebuilding aggregates, lowering salt levels, and enhancing water retention, thereby mitigating soil degradation in long-term protected cucumber cultivation.

5 Soil Nutrient Dynamics and Their Effects on Cucumber Growth

5.1 Variation patterns of major nutrients (N, P, K)

Under continuous cucumber cropping in greenhouses, major nutrients tend to accumulate strongly in the topsoil due to high fertilizer inputs and relatively low uptake efficiency. Surveys along continuous‑cropping gradients show that soil alkali‑hydrolyzed N, available P, and available K are all significantly higher in long‑term cucumber plots than in non‑continuous controls, reflecting sustained over‑fertilization and low nutrient utilization in protected systems (Huang et al., 2023). In a long‑term solar‑greenhouse study, continuous cucumber cultivation increased soil total N and total P by more than 70% and 140%, respectively, over 11 years, while total K remained essentially unchanged, indicating a marked shift in soil N:P:K stoichiometry. Such nutrient enrichment frequently coincides with higher electrical conductivity and nitrate in soil and leachate, demonstrating that N, P, and K accumulation is accompanied by elevated environmental risk rather than balanced fertility (Tian et al., 2016). 

Despite these high soil test values, nutrient dynamics increasingly diverge from plant requirements as cropping years progress. Monitoring of cucumber leaves under continuous cropping showed that leaf P concentration rose while leaf N and K remained stable, leading to a decoupling of P utilization from N and K and suggesting that soil P oversupply distorts internal nutrient ratios rather than enhancing overall nutrition (Zheng et al., 2021). Other studies comparing soils with extremely high and lower nutrient levels found that in excessively fertilized cucumber soils, large pools of mineral N, P, and K translated into greater nutrient losses through leaching rather than proportional yield gains (Tian et al., 2016). This mismatch between soil nutrient build‑up and crop uptake implies that, under continuous cucumber monoculture, N, P, and K transitions from limiting factors to drivers of salinization, imbalance, and soil degradation. 

5.2 Micronutrient imbalance and deficiency issues

Compared with macronutrients, micronutrient behavior under continuous cucumber cropping has been less directly quantified, but broader soil surveys highlight that imbalanced fertilization and intensive production often aggravate hidden deficiencies. Large‑scale assessments in intensive agricultural regions show widespread shortages of Zn, B, and S, as well as co‑occurring deficiencies of multiple micronutrients, even where macronutrients are abundant (Shukla et al., 2021). Such patterns arise because repeated application of N-P-K fertilizers without corresponding micronutrient inputs gradually depletes plant‑available Zn, B, Fe, Mn, and Cu, particularly in high‑yielding systems that remove large amounts of nutrients in harvested products. In protected vegetable systems, similar trends are expected as continuous monoculture and high biomass export increase micronutrient demand, while fertilizer regimes remain focused on N, P, and K. 

Greenhouse studies on cucumber fertilization indicate that nutrient management practices can substantially alter micronutrient availability and uptake. Experiments with peat-wood fiber substrates showed that increasing N fertilization improved the uptake of Mn and Cu by cucumber plants, with media containing mixtures of wood fiber and peat achieving particularly favorable micronutrient uptake profiles (Čepulienė et al., 2024). Integrated nutrient strategies that combine organic materials or bio‑organic fertilizers with reduced mineral inputs can also modify soil pH and organic matter, indirectly influencing micronutrient solubility and root access (Chen et al., 2022; Guan et al., 2025). At the same time, global syntheses of micronutrient status warn that simultaneous deficiencies of Zn, B, Fe, and Mn are increasingly common in intensively managed soils, underscoring the risk that cucumber grown under continuous cropping may experience subclinical micronutrient stress even in “fertile” soils saturated with N, P, and K. 

5.3 Nutrient use efficiency and crop response

Continuous cucumber cropping in nutrient‑enriched greenhouse soils is typically associated with low nutrient use efficiency (NUE) and high loss potential, particularly for nitrogen. Studies manipulating total available N (soil mineral N + mineralized N + applied fertilizer) showed that cucumber yield and dry matter production follow a linear‑plateau response: yields increase with total N supply up to about 220-230 kg N ha⁻¹, beyond which additional N does not improve yield but sharply increases residual mineral N and potential leaching losses. Similar patterns were found under different seasons and cultivars, indicating a consistent response where N uptake efficiency declines exponentially as N supply exceeds crop demand (Gallardo et al., 2020). Optimization of drip fertigation regimes further demonstrates that moderate N rates combined with appropriate irrigation (e.g., 80% ET₀) can maintain high yields while significantly improving water and N use efficiency compared with higher N inputs (Wang et al., 2019). 

Management innovations in continuous cucumber systems confirm that rebalancing nutrient supply can enhance both soil health and crop performance. Aerated irrigation combined with reduced N application increased cucumber yield and NUE relative to conventional drip irrigation at higher N, suggesting that improving root‑zone conditions allows plants to utilize applied N more effectively (Cui et al., 2020). Organic or bio‑organic amendments, composted crop residues, and biochar‑based strategies have been shown to increase microbial biomass, improve nutrient and water holding capacity, and reduce nitrate, P, and K concentrations in leachate or surface soil, thereby lowering environmental risk while maintaining or even increasing yield. Comprehensive reviews of cucumber fertilization consistently conclude that integrated nutrient management-combining organic amendments with rational N-P-K fertigation-offers the best route to higher nutrient use efficiency, stable yields, and reduced nutrient accumulation in continuous cropping systems (Al-Rubaie, 2025).

6 Soil Microbial Community and Ecological Function Changes

6.1 Changes in microbial diversity and community structure

Continuous cucumber cropping markedly reshapes rhizosphere microbial diversity. Along a gradient of 0-12 years, both bacterial and fungal diversity and richness declined after several years of monoculture, and community composition became clearly separated from non‑continuous controls in ordination analyses (Huang et al., 2023). Similar trends were observed in solar‑greenhouse soils where 7‑year continuous cropping had the lowest bacterial diversity, and predicted bacterial functional potential decreased as succession age increased (Li et al., 2021). 

Community structure shifts involve both bacteria and fungi. Long‑term greenhouse cucumber production increased relative abundances of Chloroflexi and Gemmatimonadetes while reducing beneficial taxa such as Actinobacteria and several N‑cycling genera (Liu et al., 2020). In another mono‑cropped system (1-22 years), Actinobacteria decreased, whereas Acidobacteria and Firmicutes, and fungal Ascomycota, increased with cultivation years (Zhao et al., 2020). High‑throughput studies also show that pH, organic carbon, nitrate, and available P are key drivers of these structural changes, linking altered soil chemistry under continuous cropping with microbiome reassembly. 

6.2 Dynamic balance between beneficial and harmful microorganisms

Continuous cucumber cultivation often disturbs the balance between beneficial and harmful microbes. Rhizosphere studies from 1-18 years of cropping showed reductions in nutrient‑cycling bacteria such as Bacillus and Sphingomonas, together with gradual declines in beneficial fungal antagonists (Chaetomium, Mortierella, Aspergillus, Penicillium). In long‑term greenhouse soils, continuous cropping weakened bacterial co‑occurrence networks, suggesting less stable and less functionally complementary bacterial consortia that may be more vulnerable to pathogen dominance (Liu et al., 2020). 

Cropping‑system adjustments can partially restore a favorable balance. Relay or strip intercropping of cucumber with garlic increased bacterial and actinomycete populations and reduced fungal populations, shifting the soil from a “fungal type” toward a “bacterial type” and alleviating soil sickness (Xiao et al., 2019). Diversified cover‑crop systems (e.g., spinach-cucumber, Chinese cabbage-cucumber) similarly enriched Proteobacteria, Actinobacteria and other active taxa, enhanced bacterial diversity, and improved cucumber yield in degraded continuous‑cropping soils (Ali et al., 2019). Organic amendments such as spent mushroom substrate or residue‑derived biochar also increased beneficial bacterial groups and adjusted fungal composition, further demonstrating that the beneficial-harmful balance is management‑sensitive. 

6.3 Soil enzyme activities and ecological function changes

Soil enzyme activities respond sensitively to continuous cucumber cropping and associated microbial shifts. Along 0-12 years of monoculture, urease, sucrase and alkaline phosphatase activities changed significantly, and pH was negatively correlated with sucrase while available P was positively correlated with alkaline phosphatase, indicating tight coupling between enzymes and nutrient pools (Huang et al., 2023). In solar‑greenhouse rhizospheres, urease and catalase activities varied with cropping years, and redundancy or clustering analyses identified urease as a major factor shaping bacterial communities, highlighting feedbacks between enzymes and microbes (Li et al., 2021). 

Cropping pattern and amendment studies clarify functional consequences. Garlic-cucumber relay intercropping increased urease, invertase, phosphatase and catalase compared with continuous monoculture, alongside higher microbial biomass and basal respiration, reflecting more active C, N and P cycling (Du et al., 2017). Intercropping with garlic or onion also maintained higher urease and polyphenol oxidase activities over three seasons, with sustained yield benefits (Zhou et al., 2011). In continuous‑cropping cucumbers, incorporation of vegetable residues or mixtures of biochar, vinegar and mushroom residues enhanced microbial biomass and key enzymes, and structural equation modeling or functional predictions linked improved enzyme activities and microbial diversity with higher yields and better ecological functions such as nutrient cycling and detoxification. Overall, continuous monoculture tends to depress or destabilize enzyme‑mediated processes, while diversified rotations and organic amendments can rebuild microbial enzymatic capacity and soil ecological function.

7 Mechanisms of Continuous Cropping Obstacles

7.1 Root exudates and autotoxicity mechanisms

Autotoxicity is a central mechanism of continuous cropping obstacles in cucumber, driven largely by allelochemicals released from roots and decomposing residues. Phenolic acids such as cinnamic acid (CA), p‑coumaric acid, and p‑hydroxybenzoic acid have been identified as key autotoxins in cucumber root exudates and residues, where they accumulate in soil under long‑term monoculture and suppress growth of subsequent cucumber plants. These compounds inhibit root system development by reducing total root length, root surface area, and branching, leading to weaker nutrient and water uptake and ultimately growth retardation (Lyu et al., 2022). Root exudates and extracts also induce oxidative stress in root cells, increasing membrane peroxidation and damaging root tip structure, thereby decreasing root viability and absorptive capacity (Figure 3) (Yu et al., 2003).

At the physiological and molecular levels, autotoxins interfere with redox balance, membrane function, and root tip signaling. Cinnamic acid induces rapid overproduction of reactive oxygen species (ROS) in cucumber roots, activates NADPH oxidase and antioxidant enzymes, and triggers lipid peroxidation and loss of plasma membrane H⁺‑ATPase activity, which impairs ion transport and root growth (Ding et al., 2007). p‑Hydroxybenzoic acid inhibits root elongation by reducing ROS accumulation in root tips, upregulating ROS‑scavenging genes (POD, CAT, MT) and thereby disrupting the ROS homeostasis required for normal meristem activity and cell expansion (Huang et al., 2020). Exogenous silicon can partially alleviate CA‑induced autotoxicity by improving root morphology, enhancing N metabolism‑related enzyme activities, and reducing membrane peroxidation in roots and leaves, indicating that modifying root responses to allelochemicals can mitigate continuous cropping obstacles. 

7.2 Accumulation mechanisms of soil-borne pathogens

Continuous cucumber monoculture favors the build‑up of soil‑borne pathogens, especially Fusarium oxysporum f. sp. cucumerinum, which is closely associated with soil sickness. Long‑term pot and field experiments show that with successive cucumber cropping cycles, the abundance of Fusarium communities in the rhizosphere increases, and peak pathogen populations coincide with the cropping cycle where cucumber growth becomes notably retarded. RNA‑based analyses indicate that the sizes of active fungal and Fusarium communities are more strongly linked to poor plant performance than diversity shifts, suggesting that pathogen proliferation rather than simple compositional change is critical for disease expression (Zhou and Wu, 2012). Rhizosphere soils of Fusarium‑wilted cucumbers in long‑term greenhouses also contain more F. oxysporum than healthy rhizospheres, despite similar total fungal abundance, confirming pathogen enrichment under disease pressure (Liu and Zhang, 2020). 

Autotoxins from root exudates interact with pathogens and broader microbial communities, reinforcing pathogen accumulation. p‑Coumaric acid not only suppresses cucumber seedling growth but also increases soil dehydrogenase activity, microbial biomass, and the abundance of fungal taxa including Sordariomycetes, while specifically raising F. oxysporum population densities in soil. In continuously mono‑cropped cucumber systems, such autotoxins alter bacterial and fungal community structures, reduce the bacteria/fungi ratio, and promote conditions favorable to soil‑borne diseases. Comparative studies on other crops show that Fusarium wilt pathogens survive and accumulate primarily in residues and rhizospheres of susceptible hosts, and their colonization substantially modifies soil physicochemical properties and element cycling, creating feedbacks that further support pathogen survival (Yan et al., 2023). Thus, in cucumber continuous cropping, combined effects of host specialization, residue return, and autotoxin‑driven microbial shifts underlie the progressive build‑up of soil‑borne pathogens. 

7.3 Effects of soil environmental changes on crop physiology

Soil environmental changes induced by continuous cropping-nutrient imbalance, salinization-acidification, and microbial dysbiosis-translate into pronounced physiological stress in cucumber plants. In nutrient‑enriched solar greenhouse systems, long‑term continuous cropping leads to substantial accumulation of soil organic C, total N, and especially total P, with increasing availability of these nutrients over time (Zheng et al., 2021). However, leaf nutrient stoichiometry responds differently: P concentration increases while N and K remain stable, resulting in decoupling of P from N and K utilization and causing imbalanced internal nutrition that can reduce harvest index and yield. In diseased cucumber rhizospheres, soil acidity and available nutrient contents are higher than in healthy rhizospheres, but microbial activity and nitrification capacity are lower, implying that chemical enrichment is accompanied by functional decline of soil processes that support plant nutrition (Liu and Zhang, 2020). 

Autotoxicity and associated oxidative stress further disrupt photosynthesis and water relations. Cinnamic acid and related allelochemicals decrease leaf relative water content and water potential, increase cell sap concentration, and inhibit uptake of macro‑ and micronutrients, thereby impairing leaf osmotic regulation and mineral nutrition (Lyu et al., 2022). Root exudates, extracts, and phenolic acids markedly reduce leaf stomatal conductance and transpiration, elevate leaf temperature, and depress net photosynthetic rate and intercellular CO₂ concentration, indicating both stomatal and non‑stomatal limitations to photosynthesis under allelopathic stress (Yu et al., 2003). CA‑mimicked autotoxicity also increases membrane lipid peroxidation in roots and leaves, damages chloroplast ultrastructure, and reduces PSII activity, whereas silicon supplementation restores PSII efficiency, enhances chlorophyll content, and improves biomass accumulation, highlighting the central role of oxidative and photosynthetic damage in continuous cropping obstacles and their partial reversibility with appropriate amendments.

8 Case Study: Soil Degradation under Continuous Cucumber Cropping in Typical Regions

8.1 Overview of the study area and research methods

Typical case studies have been carried out in major greenhouse cucumber production zones such as the Loess Plateau region of northern China, the Yellow River irrigation area in Gansu, and long‑term plastic shed or solar‑greenhouse bases in Shandong and Jiangsu. In Yan’an on the Loess Plateau, a solar greenhouse trial filled plots with soils that had already undergone 0, 1, 4, or 8 years of winter-spring cucumber recropping, in order to track subsequent degradation and productivity over multiple seasons (Liang et al., 2013). Similar fixed‑site greenhouse experiments with 15-18 years of continuous cucumber monoculture have been used as source soils for pot or plot trials on amendments and cultivation modes in Shandong and Jiangsu (Chen et al., 2022). 

These regional studies commonly combine measurements of soil physicochemical properties, microbial communities and enzyme activities with yield and quality monitoring of cucumber. In the Yellow River irrigation area, rhizosphere soils from 0, 4, 8 and 12 years of continuous cropping were analyzed for salinity, nutrients, pH, enzyme activities, and bacterial and fungal diversity using high‑throughput sequencing (Huang et al., 2023). Other greenhouse trials compared different fertilizer or organic amendment regimes (e.g., vermicompost, biochar, composts, vegetable residues) over several consecutive cucumber crops, measuring yield, fruit quality, basic soil properties and microbial community structure to assess how management modifies degradation trajectories (Zhao et al., 2017). 

8.2 Analysis of measured soil degradation characteristics

Across these typical regions, continuous cucumber cropping leads to characteristic deterioration of soil chemical and biological conditions. Along the Yellow River irrigation gradient (0-12 years), total salt content increased steadily and pH declined significantly, while organic matter and available N, P and K rose during the early continuous‑cropping years, indicating nutrient enrichment but progressive salinization and acidification (Huang et al., 2023). On the Loess Plateau, continuous greenhouse cucumber similarly caused salt accumulation and worsening chemical properties even where water‑stable aggregates and some nutrient indices increased, revealing a mismatch between apparent fertility and functional soil quality. 

Biological indicators corroborate this degradation. In the Yellow River irrigation area, continuous cropping reduced bacterial and fungal diversity and richness after 4 and 12 years, and principal coordinates analysis showed clear separation of microbial community structure from non‑continuous controls (Huang et al., 2023). In the Yan’an case, recropping years were associated with marked shifts in the proportions of bacteria, fungi and actinomycetes, with fungi and bacteria increasing and actinomycetes decreasing after about 5 years, which was linked to a rise in soil‑borne and pathogenic microbes (Liang et al., 2012). Greenhouse pot experiments using soil from a 15‑year continuous cucumber site further showed low pH, high EC and degraded microbial structure that required substantial amendment to be reversed. 

8.3 Effects of continuous cropping obstacles on yield and quality

Yield responses in these case regions share a similar pattern: initial stabilization or increase, followed by a marked decline as degradation intensifies. In Yan’an, cucumber fruit productivity first increased with recropping up to the fourth year, but yields decreased significantly after about five years of continuous greenhouse cropping, signaling that soil quality had crossed a degradation threshold (Liang et al., 2013). Long‑term continuous cropping under farmers’ conventional fertilization also led to pronounced yield decline in facility cucumbers, whereas optimized fertilization regimes slowed the rate of decline but did not completely remove the underlying obstacle (Yang, 2014).

Fruit quality deteriorates alongside yield. In continuous‑cropping facilities, increasing years of cultivation raised cucumber fruit nitrate content while reducing vitamin C, soluble sugar, soluble protein and other quality indices under traditional fertilization patterns. Another study of continuous cucumber systems reported that extended cropping increased fruit nitrate and total acidity and decreased vitamin C, soluble sugar and protein, linking quality loss to both soil degradation and excessive chemical fertilizer use (Sun et al., 2021). Conversely, regional trials with vermicompost, biochar, compost or residue incorporation showed that improving degraded soils-by raising pH, lowering EC, enhancing organic carbon and rebuilding microbial communities-could maintain or increase yields and significantly enhance fruit quality, even in soils with 5-20 years of continuous cucumber history (Zhao et al., 2017).

9 Soil Degradation Control and Sustainable Management Strategies

Adjusting cropping systems is a core strategy to alleviate continuous‑cropping obstacles in cucumber. Pea-cucumber rotation with straw return reshaped soil microbial communities, sharply reduced Fusarium abundance, and increased available N, P and K by more than 1.3‑fold compared with continuous cucumber, thereby improving cucumber growth and yield. Green garlic-cucumber rotation over three seasons enhanced soil catalase and invertase activities, increased fungal richness, and achieved the highest cucumber yield at intermediate garlic bulb inputs, showing that appropriate rotation intensity can both improve soil health and sustain production. Intercropping and multi‑cropping patterns further optimize the soil-plant system. Garlic or leafy‑vegetable integration into continuous cucumber fields reduced soil salt content, increased microbial numbers, and inhibited Fusarium reproduction, while boosting overall economic returns relative to monoculture. A phytoremediation-production system that intercropped Sedum alfredii with cucumber, then rotated with low‑accumulator water spinach and autumn cucumber under rational water management, suppressed Fusarium wilt by up to 57% and increased autumn cucumber yield by more than threefold, while simultaneously reducing soil Cd and enhancing photosynthetic and antioxidant traits. Long‑term comparisons of six cultivation patterns showed that paddy-upland and garlic rotations steered soil microbial succession toward pathogen‑antagonistic communities and mitigated soil acidification, supporting their use as sustainable alternatives for degraded cucumber soils. 

Organic amendments are effective tools to rebuild soil quality in continuously cropped cucumber systems. Application of Caragana microphylla‑straw compost in excessively fertilized soils increased soil water and nutrient‑holding capacity, total microbial biomass, root length and fruit yield, while simultaneously lowering nitrate, P and K leaching and reducing Fusarium oxysporum populations, thus decreasing environmental risk. Bio‑organic fertilizer combined with reduced chemical fertilizer application decreased soil bulk density, increased total porosity and water‑stable aggregates, and improved hydraulic properties; modeling indicated that 20 t ha⁻¹ bio‑organic fertilizer with slightly reduced chemical N maximized cucumber yield in greenhouse continuous cropping. Biochar‑based strategies have shown particular promise for reversing physicochemical and biological degradation. In soils with different continuous‑cropping histories, co‑application of biochar and vermicompost most effectively raised pH and dissolved organic carbon, decreased electrical conductivity, increased fungal and bacterial abundance, and boosted cucumber yield by 29-56%, with structural equation modeling linking higher DOC, nutrients and microbial biomass to improved fruit quality. Mixing crop‑residue biochar with vinegar and mushroom residues elevated pH, soil water content and organic C, enriched N, P and K, and lowered total phenols and salinity; microbial diversity increased, and cucumber yield correlated more strongly with microbial indices than with physicochemical variables, indicating that composite organic-biochar amendments can reconfigure soil microecology to overcome continuous‑cropping stress. Sweet‑pepper straw biochar at 5% further improved cucumber growth and enriched beneficial microbial groups, illustrating the potential of residue‑derived biochars as sustainable amendments in monoculture greenhouses. 

Digital and precision technologies provide new tools to manage soil degradation processes and optimize inputs in continuous cucumber systems. Reviews of precision agriculture highlight that sensor‑based monitoring of soil moisture, pH and other properties, combined with GIS and variable rate application, can tailor irrigation and fertilization to spatial variability, improving soil health while reducing excess agrochemical use. Integration of AI with IoT platforms enables high‑throughput phenotyping, remote sensing and automated robots to detect plant stress and soil condition in real time, supporting timely interventions that limit salinization, nutrient accumulation and disease pressure in intensive vegetable production. Smart sensor networks and proximal sensing can further refine management under greenhouse conditions. IoT‑enabled soil and plant sensors deliver continuous data on moisture, nutrient status and plant stress, which can drive decision‑support systems for optimized irrigation, fertigation and pest control, lowering inputs and mitigating degradation of soil structure and fertility. At the same time, daily fertigation schemes combined with biochar have been shown to enhance soil water content, available P, microbial biomass and overall soil quality index, while markedly increasing cucumber water‑fertilizer productivity in semi‑arid alkaline soils, demonstrating how precise water-nutrient management synergizes with amendment‑based rehabilitation. Collectively, the integration of digital monitoring, variable‑rate input control and soil‑improving amendments offers a pathway toward intelligent, sustainable management of continuously cropped cucumber soils.

Acknowledgments

I extend our sincere gratitude to the anonymous reviewers for their valuable and insightful comments, which have greatly strengthened this paper.

Conflict of Interest Disclosure

The author affirms that this research was conducted without any commercial or financial relationships that could be construed as a potential conflict of interest.

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