Soil enzyme activity serves as a vital interface for numerous biochemical reactions within soil ecosystems, playing a pivotal role in driving ecological processes. The regulation of enzyme activity is influenced by a complex interplay of physical, chemical and biological factors, which together elucidates the mechanisms governing nutrient cycling and biochemical transformations in soils (Cotrufo et al., 2015). Abiotic factors such as soil temperature and composition, alongside biotic factors like microbial community structure, collectively shape enzymes activity, making it a reliable indicator of changes within soil ecosystems (Utobo & Tewari, 2015).

Key enzymes responsible for carbon and nitrogen cycling, including peroxidase (PER), dehydrogenase (DEH), urease (URE), and invertase (INV), exhibit high sensitivity to environmental fluctuations (Feng et al., 2019). As an extracellular enzyme, PER is mainly involved in the degradation of complex organic compounds and it plays a key role in the carbon cycle, an activity that supports microbial metabolism and helps to stabilise organic carbon in the soil, while being influenced by the soil microenvironment (Nakayama & Tateno, 2022). Dehydrogenase, as an indicator of microbial oxidative activity, reflects the metabolic potential of soil microbial communities and is often used as a proxy for overall microbial activity, whereas extremes like drought or waterlogging inhibit its activity (Makoi & Ndakidemi, 2008; Filipović et al., 2020). Urease plays an important role in the nitrogen cycle, and soil organic matter content and the availability of urea or other nitrogenous substrates greatly affect urease activity (Sun et al., 2021a). Invertase provides a source of energy for soil microorganisms, is an important participant in the carbon cycle, and is closely linked to soil organic matter dynamics, with its activity influenced by root exudates, organic carbon availability and soil microbial biomass ( Ma et al., 2023).

Despite significant progress in understanding the individual roles of these enzymes, the intricate relationships between nutrient availability and enzyme activity within ecosystems remain inadequately explored. Gaps persist in our knowledge of how environmental factors, particularly forest management practices like gap creation, affect enzyme dynamics. Addressing these gaps is essential for advancing forest management strategies. Specifically, understanding the impact of forest gap size on soil enzyme activity will provide valuable insights into optimizing forest ecosystems for biodiversity conservation, nutrient cycling, and overall ecological stability.

Within the rhizosphere, the phenomenon known as the rhizosphere effect (RE) encompasses alterations in nutrient cycling, enzyme activities, and microbial communities (Finzi et al., 2015; Chen et al., 2018). In soil ecosystems, enzyme and microbial activities are heightened in the rhizosphere compared to bulk soils devoid of roots (Pausch & Kuzyakov, 2011). Numerous studies have consistently demonstrated that rhizosphere soils exhibit distinctive microbial behaviors and physicochemical properties compared to bulk soils without roots (Philippot et al., 2013). The discrepancy is attributed to the substantial influx of plant photosynthetic products into the rhizosphere, which subsequently alters soil properties and enhances nutrient assimilation (Nakayama & Tateno, 2022). Concurrently, labile organic compounds, known as root exudates, are continuously released into the rhizosphere by the fine roots of plants (Kuzyakov& Razavi, 2019). These root exudates serve as a primary reservoir of energy and substrate for microbial activities, thereby establishing the rhizosphere as a hotspot for heightened microbial abundance and activity (Wang et al., 2019a). Consequently, microbial and enzyme activities are significantly augmented within the rhizosphere soils (Kuzyakov & Razavi, 2019).

Forest gaps play significant roles in forest succession and nutrient cycling (Gray et al., 2012; Yang et al., 2017). Upon the formation of forest gaps, environmental conditions such as light, temperature, and humidity undergo changes due to the sudden reduction of the forest canopy and a decrease in the degree of shading (d’Oliveira & Ribas, 2011). This alteration triggers the resurgence of understory vegetation, modifies soil nutrient cycling, and subsequently induces shifts in microbial and enzyme activities (Gray et al., 2012; Fei et al., 2023). In recent decades, there has been a growing emphasis on establishing silvicultural forest gaps that mimic natural disturbances, as they are more conducive to ecologically restoring forests (Muscolo et al., 2014). Creating the silvicultural forest gaps aims to conserve biodiversity and ecosystem functioning while optimising harvesting practices (Vajari et al., 2012). The size of the forest gap predicts the degree of the environmental heterogeneity it introduces (Muscolo et al., 2014). Forest gaps of different sizes also vary in the intensity of their impacts on environmental conditions and soils within and surrounding them (Nakayama & Tateno, 2022).

Therefore, different sizes of forest gaps introduce varying degrees of environmental heterogeneity to forest ecosystems, resulting in alterations in understory environmental conditions. These environmental changes affect rhizosphere and bulk soils, prompting inquiries into how microbial communities, enzyme activities, and nutrient cycling respond to such shifts. This aspect necessitates further exploration and a deeper understanding and clarification. To investigate the effects of forest gap sizes on the RE of four soil enzymes in Platycladus orientalis (L.) Franco, 1950 plantation, three different sizes forest gaps were established in Xuzhou, China. A range of soil biological and physicochemical variables characterizing the soil, including microbial biomass nitrogen and carbon (MBN and MBC, respectively), total nitrogen and carbon (TN and TC, respectively), soil soluble organic carbon (SOC), NO₃ ⁻-N, and NH₄ ⁺-N, were determined.

We hypothesized that (1) The heterogeneity of the understory environment increases with the expansion of forest gap size, leading to a corresponding rise in the RE of soil enzyme activities, and (2) the larger the forest gap size, the broader its effect on the RE of activity extends. This study provides valuable insights into the intricate interactions between forest gap size and the RE of soil enzyme activity. Consequently, this study may contribute to enhancing forest management strategies and practices.

The RE on PER demonstrated a consistent positive trend across most seasons and sampling locations (Figure 8), showing a highly significant positive correlation with ST as indicated in Table 4. This finding suggests that the establishment of forest gaps had a marked stimulating effect on PER activity in the rhizosphere soil, with PER activity being greater sensitivity to fluctuations in soil temperature (Fei et al., 2023), particularly in warmer months (August). Furthermore, MBC and MBN in both bulk and rhizosphere soils exhibited positive correlations with the RE on PER (Table 4), with notable increases at the M2 and S2 locations in August (Figure 8). This suggests that M and S gap sizes may create conditions that enhance microbial activity, and that this increase could be linked to the greater availability of organic substrates for microbial oxidation and decomposition. The strong positive correlation between RE on PER and NH₄ ⁺-N (Table 4) suggests that PER may play a critical role in nitrogen cycling in the rhizosphere, potentially facilitating the mineralization of organic matter into available forms. Furthermore, the significant correlation with MBC and MBN (Table 4) suggests that RE on PER may be involved in the breakdown of certain organic compounds, indirectly supporting microbial nutrient cycling in both rhizosphere and bulk soils. However, contrary to the hypothesis of our study, larger forest gaps and increased environmental heterogeneity did not result in enhanced RE of enzyme activity, nor did they extend range of influence on RE of enzyme activity. This result challenges our original expectations but aligns with findings from previous studies which suggested that smaller forest gaps are more conducive to higher soil enzyme activities (Muscolo et al., 2007; Yang et al. 2017). Additionally, Sinsabaugh (2010) emphasized a positive association between PER activity and litter quantity, proposing that smaller gaps tend to have higher litter accumulation, which can enhance enzymatic activity due to increased substrate availability.

Our results indicate that the RE on DEH were consistently lower than those of CK at most locations, with the exception of L2 in August (Figure 9). These findings suggest that the establishment of forest gaps significantly impacted DEH activity, particularly at M and S gap sizes, where environmental changes such as PAR and ST likely influenced microbial processes. Interestingly, the RE on DEH was more sensitive to fluctuations in PAR and ST than to soil factors in the rhizosphere (Table 4), pointing to bulk soil conditions as the primary drivers affecting DEH activity. Seasonal effects also played a key role in the RE on DEH (Table 3), with significant variations observed across different months. These seasonal variations could explain the observed increases in the RE on DEH during June and August (Figure 9), as higher temperatures and increased PAR likely stimulated microbial activity, leading to enhanced enzyme activity. This is in line with findings by Steinweg (2012), who highlighted the importance of temperature and moisture in regulating soil enzyme activities. Additionally, the negative correlations between RE on DEH and TC, TN, and MBN suggest that the availability of carbon and nitrogen might limit DEH activity under certain conditions (Table 4), possibly due to microbial competition for resources or imbalances in substrate availability.

Our results revealed a significant negative correlation between the RE on URE and NH₄ ⁺-N in bulk soil, while a positive correlation was observed in rhizosphere soil (Table 3). This differential response suggests that, under favorable hydrothermal conditions, substrate availability, particularly nitrogen, may limit URE activity in bulk soil following the forest gap establishment (Zuccarini et al., 2020, 2023). In comparison to the CK, the RE on URE showed an increasing trend in both L and S forest gaps in April and June, but declined in August and October (Figure 10). This temporal pattern highlights the greater sensitivity of URE activity to environmental changes in rhizosphere soils compared to bulk soils across different phases of the growing season (Xu et al., 2024). Notably, both in the rhizosphere and bulk soils, the RE of URE exhibited significant positive correlations with MBN, aligning with the findings of Fisher et al. (2017) and Corstanje et al. (2007), who emphasized the influence of microbial community composition on URE activity. These results further corroborate the observations by Wang et al. (2019b), who demonstrated the association between URE activity and soil nitrogen transformations, reinforcing the importance of microbial processes in mediating soil nitrogen cycling.

The changes in the RE on INV closely mirrored those of URE in our study, showing similar seasonal and spatial patterns (Figure 10 and 11). In April, the RE on INV increased across all forest gap sizes (Figure 11), indicating that forest gap creation positively influenced INV activity during the early growing season. However, by August and October, the RE on INV declined, following a distinct temporal pattern (Figure 11). This suggests that forest gaps initially enhanced INV activity by promoting favorable conditions for microbial activity, including increased substrate availability and rising temperatures in the rhizosphere soil (Zuccarini et al., 2023). In the later stage of the growing season, the decomposition of litter likely contributed substantial nutrients inputs to the bulk soil, potentially overshadowing the effects of forest gaps on INV activity in the rhizosphere. This shift could explain why INV activity in bulk soils was markedly higher than in the rhizosphere during the later months, and why the RE on INV was lower than CK in these periods. This finding aligns with previous research, which observed that INV activity was more responsive to changes in soil conditions during the early growing season, with a subsequent decline as the soil nutrient dynamics shifted, as seen in studies on organic mulching (Sun et al., 2021b) and straw returning practices (Wu et al., 2020). Additionally, in bulk soils, the RE on INV was negatively correlated with WC, TC, TN, SOC, MBN, and NO₃ ⁻-N (Table 4). These results are consistent with the findings of Yang et al. (2017), who noted a decrease in INV activity in response to NH₄ ⁺-N additions during the early stages of plant growth, followed by an increase as the season progressed. Our study further emphasizes the complex interactions between soil nitrogen, carbon, and microbial activity, as these factors influence enzyme activity in response to varying forest gap sizes and the associated environmental changes.

In addition to the environmental factors discussed above, plant fine roots and microorganisms play a crucial role in shaping both bulk and rhizosphere soils (Sun et al., 2021b). Plants contribute organic substrates to the rhizosphere through litter deposition and root exudates, which regulate nutrient composition, enhance microbial habitat quality, and stimulate microbial abundance and metabolic activities (Sasse et al., 2018). The establishment of forest gaps significantly impacts these dynamics by altering soil properties and microbial communities. Given the pivotal role of plants in shaping the rhizosphere environment, the observed changes in enzyme activity following forest gap establishment can be attributed, in part, to the direct and indirect effects of plant root activity and microbial interactions on the soil environment.

During the growing season, forest gaps initially stimulated the RE of PER, URE, and INV, but these effects diminished as the season progressed, while the RE of DEH consistently decreased in both June and August. Additionally, forest gaps of L and S sizes influenced the RE of enzyme activity, with L gaps having the most pronounced impact. This study identified that the RE of soil enzyme activity did not increase with the enlargement of forest gap size. However, in L forest gaps, its impact range was more extensive, aligning with the assumptions made in Hypothesis 2 outlined earlier in this text.