Forest gaps have received considerable attention in recent decades as a type of disturbance and are widely recognized as a significant factor in forest dynamics (Martinez-Ramos et al., 1989; Clark, 1990). Gaps commonly arise as part of forest management activities and are thought to play a crucial role in nutrient cycling and forest regeneration (Gray et al., 2012; Yang et al., 2017; Wang et al., 2019). Due to increased irradiance, gaps are brighter and warmer, but their surface soils contain more water due to the reduction in plant transpiration (Denslow, 1987). Furthermore, gaps have a significant impact on soil microbial biomass and activity by altering microclimatic conditions and reducing litter inputs, which can lead to changes in soil biochemical properties (Denslow, 1987).

The effects of gap size on soil microbial properties remains uncertain. Although several studies conducted in conifer stands have found no significant differences in mean air temperature during the growing season between gaps of varying sizes (Gray et al., 2002; Muscolo et al., 2007a,b), many others have demonstrated that soil temperature increases with increasing gap size (Gray et al., 2002; Gugliotta et al., 2006; Muscolo et al., 2007a,b). Gap opening has a particularly significant impact on soil moisture content, which is higher inside gaps than under the surrounding closed canopy, as observed in a wide range of forest types (Ostertag, 1998; Zhu et al., 2003; Albanesi et al., 2008; Sariyildiz, 2008).

Soil enzyme activity, as the main catalyst of many biochemical reactions in soil, is influenced by a series of physico-chemical and biological factors. Muscolo et al. (2007a) demonstrated that small gaps in pine forests, which had greater microbial biomass and larger populations of bacteria and fungi, contributed to a more rapid and balanced turnover of organic matter and nutrients, suggesting that creating small gaps is a silvicultural practice with minimal environmental impact. This study also suggested that small gaps enhance soil enzyme activity and concluded that creating small gaps was an appropriate method of forest management. Yang et al. (2017) found that approximately one year after creating gaps in the forest, small gaps promoted microbial communities and enzyme activity compared with closed canopies. They also observed that soil β-glucosidase activity and ι-leucine aminopeptidase activity were lowest in sites with large gaps and highest in those with small gaps in a Chinese pine forest. It is essential to gain a better understanding of how gap size affects soil microbial biomass and enzyme activities to develop effective forest management and conservation strategies accordingly.

Platycladus orientalis is one of the primary afforestation species in Jiangsu Province, and the creation of artificial gaps in Platycladus orientalis forests is a commonly employed silvicultural practice to reduce forest density. However, the relationship between gap size and soil enzyme activity in this species requires further investigation.

In this study, we investigated the effects of small gaps (4 m radius, S), medium gaps (8 m radius, M), and large gaps (12 m radius, L) on enzyme activity 1a after gap creation, in Platycladus orientalis plantation located in Xuzhou, eastern China. Along with characterizing soil physico-chemical and biological factors, such as soil soluble organic carbon (SOC), total carbon (TC) and total nitrogen (TN), microbial biomass carbon and nitrogen (MBC and MBN), we tested two hypotheses: (1) temperature-sensitive enzyme activity in forest gaps changes with gap size due to the variation in soil temperature, and (2) the establishment of forest gaps alters the surrounding microclimate conditions, and the impact of forest gaps on soil enzyme activity also increases with gap size. We anticipate that this study will provide valuable insights into the relationship between soil enzyme activity and forest gap size, as well as forest management practices.

Luo (2012) investigated the soil enzyme activity of Chinese fir plantation and found a significant correlation between peroxidase activity and soil pH value. However, in the present study, no significant correlation was found between peroxidase activity and pH value. One possible reason for this discrepancy is that the time since gap creation was relatively short, and soil pH, being a fundamental soil property, may not have been significantly influenced by the gaps in the short term. Therefore, soil pH may not be a driving factor for changes in peroxidase activity. The results of this study showed a negative correlation between soil peroxidase activity and soil temperature, possibly due to the higher solar radiation and temperature received by large forest gaps. As a temperature-sensitive enzyme, peroxidase activity was significantly increased in I and O positions of M and S gaps, indicating a promoting effect of the forest understory environment influenced by M and S gaps on peroxidase activity. These findings are consistent with previous studies by Yang et al. (2017) and Muscolo et al. (2007a) and contradict the hypothesis 1 of this study, which suggested that larger forest gaps would have a positive effect on peroxidase activity due to their higher temperature. Instead, it appears that M and S gaps have a positive effect on peroxidase activity in the surrounding soil. Sinsabaugh (2010) found that peroxidase activity is positively correlated with litter quantity. Accordingly, the smaller forest gaps had higher peroxidase activity due to their greater litter quantity compared to the larger ones.

Due to the intracellular nature of dehydrogenase, which does not accumulate extracellularly in soil (Filipović et al., 2020), and the ability of enzymatic activity to rapidly respond to even small changes in environmental conditions (Dussault et al., 2008), dehydrogenase is commonly used as an indicator of soil microbial activity (Makoi and Ndakidemi, 2008). The results of this study showed that soil dehydrogenase activity was significantly higher in all gap sizes compared to CK, and was positively correlated with MBC (p < 0.01), and positively correlated with SOC (p < 0.05), consistent with Lungmuana et al. (2017) who found a strong positive relationship between MBC and enzyme activities in the surface soil layers. Thus, it can be said that the establishment of forest gaps has led to better soil chemical conditions and higher biological activity compared to the forest under canopy, which also supported by the study of Avazpoor et al. (2019). Soil enzymes play a key role in nutrient cycling and SOM mineralization (Gianfreda, 2015).

The findings of this study indicated that soil urease activity was positively correlated with SOC, MBC and MBN (p < 0.01), because many bacteria, fungi, archaea and plants present in the soil produce urease (Fisher et al., 2017). Consistent with the results of this study, Wang et al. (2005) also demonstrated that the transformation of nitrogen in soil was related to urease activity. Furthermore, this study revealed significantly higher urease activity in all types of gaps in July, with larger forest gaps exhibiting higher urease activity due to the humus-urease complexes extracted from soil being highly resistant to extreme temperatures (Makoi and Ndakidemi, 2008). These findings are in line with hypothesis 1, which proposed that enzyme activity in forest gaps changes with gap size due to soil temperature variations.

The study by Yu et al. (2017) showed that invertase activity in soil can hydrolyze sucrose into glucose and fructose as available energy sources for plants and soil microorganisms. Our study found that soil invertase activity was positively correlated with soil temperature, soil moisture, and MBN (p < 0.01), which could be attributed to the increased metabolism of soil microorganisms and the stimulation of microbial activity. In April and July, all three sizes of forest gaps had increased soil invertase activity, indicating that the establishment of forest gaps increased solar radiation and soil temperature. In particular, the soil invertase activity was significantly higher in larger forest gaps and their surroundings, consistent with hypothesis 2 that the impact of forest gaps on soil enzyme activity increases with gap size. The increase in soil invertase activity can increase the microbial population and provide energy and a favorable environment for soil enzymes, which is consistent with the results of the study by Wu et al. (2020).

In this study, forest gap size had different effects on different enzymes. The M and S gaps increased the activity of peroxidase, which is contrary to hypothesis 1 mentioned earlier. The L gaps and its surrounding area showed the most significant increase in dehydrogenase activity, consistent with hypothesis 2. Urease activity increased with gap size, consistent with hypothesis 1. The impact of forest gaps on invertase activity expanded with increasing gap size, consistent with hypothesis 2.

This study explored the impact of forest gap size on soil enzyme activities. and investigates their correlation with physico-chemical properties. The creation of gaps as a silvicultural approach has significant potential to promote ecologically sustainable forest development. Nonetheless, the existing body of research on the interplay between above-and below-ground nutrient cycling in forest gaps remains insufficient, which hinders our comprehensive understanding of gap dynamics and their impact on forest ecosystems. Further research in this area is essential to fill this knowledge gap and improve our understanding of forest ecology.

The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

FF: conceptualization, methodology, software, data curation, and writing–original draft preparation. QG: supervision. XC: writing–reviewing and editing. All authors contributed to the article and approved the submitted version.

This work was financially supported by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD) and the National Natural Science Foundation of China (No. 31971453).

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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