4.1 Forests soils in the urban areas
In this study, we found that forests soil in Wuhan city was more acid but less nutritious than the surrounding nonurban areas, supporting the urban soil acidization hypothesis but not the urban soil fertility hypothesis. The result differs from the study of Li et al. (2013b), which detected an alkaline soil in roadside, institutional or park areas in Wuhan city. This study focused solely on the soil of the remained forests which were seldom modified by human beings (e.g. transporting new soils, adding cement, concrete and other construction waste during urban park construction, coating white limes on tree barks as a management for street trees). As a result, the soil of urban forests in this area is rarely neutralized by artificial materials.
An increase in N deposition is usually accompanied by a corresponding increase in soil N availability from rural to urban areas (Fang et al. 2011). In this study, we tested the soil N contents but did not trace the N deposition and we found a lower N content in urban forest soils. In the studied area, precipitation is rich, at approximately 1269 mm per year, resulting in the easy loss of the soluble elements from land to water and then causing water eutrophication (Liu et al. 2021). Therefore, we infer that the lower N content in forests is related to the large sum of N loss under high precipitation in the studied areas. In the future, more studies about N cycle in the urban areas are need to be conducted. In addition, it is essential to distinguish between nitrate nitrogen and ammonia nitrogen, which might show a difference trend along urbanization because of their different origin (Huang et al. 2015; Du et al. 2022a).
There is no difference in C or P content between urban and nonurban forest. However, we found a positive relationship between pH and P content and a negative relationship between pH and C content in urban forests, but not in nonurban forests. This suggests that soil acidification will further promote changes in soil chemical properties in Wuhan city. It has been reported P is generally lacking in the acidic red soil in central and south of China (Ao et al. 2014; Huang et al. 2019a; Peng et al. 2020). Leaves returns part of P to the tree body before falling in P-poor areas (Du et al. 2020; Peng et al. 2023), therefore, the similar lower values of litter P and soil P confirmed that the P limitation in urban forests where has more acidic soils.
An increase in soil C/N ratio but a decrease in soil C/P ratio and N/P ratio are common results of increasing temperature in terrestrial ecosystems (Jiao et al. 2016; Sun et al. 2022; Wan et al. 2023). We found the same trend of soil stoichiometries (e.g. higher soil C/N, lower soil C/P ratio and N/P ratio) in urban forests, suggesting urban heat island effect could play the same role on urban forest soils as climate warming did. Due to the combined effect of heat island and global warming, urban heat waves expected to intensify in the coming decades (Wouters et al. 2017). Urban forests can be considered as natural laboratories with extrema treatments for studying climate change.
4.2 The effect of reforested trees on soil pH
The dominant species in forest could affect the degree of soil acidification. The increasing soil pH along with the increasing of C. officinarum in a forest indicates planting more C. officinarum could weaken soil acidification in the urban areas. However, the percentage of deciduous tree Q. variabilis or coniferous tree P. massoniana did not affect soil pH. The results indicate that the effect of plants on soil pH is species-specific rather than leave-type-specific. In the leave-type-specific theory, leaf litter of deciduous trees is typically considered to have an exchange capacity to buffer the acid treatments, due to their higher exchangeable base cation to weaken acidification (de Schrijver et al. 2012), as well ages organic matter to consume the H + and bind cations (Jiang et al. 2018). Camphora officinarum were proved to own higher content of Ca2+ and Mg2+ in leaf litter than pines and then lead to a greater soil pH in its dominant forests (Ma et al. 2015, Chen et al. 2021). However, Q. variabilis forest soil had a lower cation exchange (Lee et al. 2022), although Q. variabilis is a deciduous tree. In contrast, P. massoniana contained many organic acid, e.g. acetic acid, hexadecanoic acid and 2-methoxy-4-vinylphenol (Feng et al. 2010), which could promote soil acidification through leaching (Yang et al. 2012; Van Nevel et al. 2013).
Our results support that only C. officinarum forests have the capital to resist soil acidification in urban area. We also detected C. officinarum dominant stand had the highest pH in both litter and soils among all the sampled forests, conforming the specific role of C. officinarum litter on urban forests soils. In urban areas, Feng et al. (2023) found the same result in Chongqing city, another megacity in southwest China. However, the role of C. officinarum on soils in areas between urban and nonurban areas are hardly to compare due to its rare use in reforestation in areas far from human colonies. Nerveless, this ecological function of C. officinarum in mitigating soil acidification should be concerned in urban areas and other places where soil acidification is severe.
4.3 The effect of reforested trees on soil nutrients
The dominant species in urban reforested forest also affect litter and soil chemical properties. There were significant differences in litter properties among forests dominant by three different species. For example, the litter in P. massoniana stand had lower N and P content but a higher organic C content than others, as reported in previous studies (Xu et al. 2017; Farooq et al. 2022). However, only the difference of P content among three stands in litter consistently exist in soils, in that, P. massoniana stands still have the lowest P content in soils. The P limitation in pine forests is more sensitive to acid treatments than that in deciduous forests (Huang et al. 2015; Huang et al. 2019a). This may be the reason that the role of planting P. massoniana on soil P and its related C/P and N/P ratios are only evident in urban forests, where soil acidification is more severe, but not in nonurban forests. Furthermore, the annual return of macronutrients from litterfall to soil in P. massoniana forest only half of that in deciduous forests, e.g. C. officinarum forest (Ni et al. 2021; Farooq et al. 2022). That means the decline of soil P of urban forests will be more serious after reforested by P. massoniana, because this species is more sensitive to soil acidification and it returned less nutrients into soils.