4.1 Urbanization indirectly decreases GRSP
This study is the first to reveal the urban spatial distribution characteristics of GRSP in red soil in southern China. The average contents of EEG and TG in the 0-20 cm soil layer were 0.57 and 2.38 mg·g−1 (n=184), respectively. These values were lower than those in temperate forests, grasslands, and tropical rain forests, but higher than those in poorer ecosystems, such as farmland and deserts (Singh et al. 2013; Treseder and Turner 2007). Wang et al. (2020a) found that the average contents of EEG and TG were 0.56 and 6.19 mg·g−1 in black soil in Changchun, Northeast China. Comparatively, the average contents of EEG were similar, but TG was much higher in black soil than was found in this study. A possible explanation is that red soil is less fertile and has lower SOC and nutrients than other soil types, such as black soil (Wang et al. 2015b), resulting in a lower GRSP content. Climate may also play a role in regional variations of GRSP soil content. Rillig et al. (2010) found that GRSP decomposes more rapidly at higher temperatures, and there is a large difference in climate between the southern and northern parts of China. The average annual temperature in the north is relatively low (4°C), whereas it is reaches 17°C in the south (Chen 2013). The relatively lower temperature environment may allow for a slower turnover rate of GRSP secreted by AMF in soil, which can be preserved longer and thus lead to a higher TG content. In addition, the magnitude of GRSP content is influenced by various factors, such as net primary productivity, vegetation type, and soil properties, etc. (Jiˇrí et al. 2020; Udayakumar et al. 2021). Additionally, GRSP is also affected by human activities, such as soil compaction, household waste, and other anthropogenic disturbances (Gujre et al. 2021). Therefore, the factors affecting the changes in GRSP characteristics in urban greenspaces are more complex than those in other ecosystems.
EEG and TG showed decreases of 19.68% and 16.22%, respectively, from low to heavy urbanization areas (Fig. 2). Interestingly, our analysis revealed that this negative impact was caused by indirect effects (Fig. 6a), consistent with the results of Wang et al. (2018c) in a study of a northern city. This may be due to the following reasons. First, GRSP is a sensitive indicator of the soil carbon pool, closely related to carbon, nitrogen, and other nutrients, and its content changes are closely related to soil quality (Jin et al. 2021). In this study, low urbanization areas had high SOC and TN contents (Table 1), resulting in a change in GRSP content. The decrease in soil nutrients caused by urbanization may be due to low vegetation cover in heavy urbanization areas and surface runoff during precipitation events, which can easily cause soil nutrient loss (Qin and Xu 2018). Second, serious anthropogenic disturbances can affect soil microbial activity, affecting GRSP secretion and storage (Udayakumar et al. 2021; Xu et al. 2017). Third, soil compaction and disturbance during urbanization reduced soil porosity and permeability, further reducing the AMF growth on plant roots (Entry et al. 2003), thus affecting the production and turnover of GRSP. Additionally, urbanization increased IM and reduced VE, thus triggering the heat island effect. Temperatures are higher in heavy urbanization areas than in low urbanization areas (Meehl and Tebaldi 2004), resulting in faster decomposition and less accumulation of GRSP in soils. Overall, urbanization has a multifaceted negative impact on GRSP. Generally, in terms of improving urban soil quality, the functional aspects of GRSP require more attention.
4.2 GRSP plays a 0 vital role in increasing SOC sequestration in heavily urbanized areas
GRSP, as the largest mycorrhizal carbon pool (Wang et al. 2018a), accounts for 27% of SOC, while soil humus accounts for only 8%. The carbon contribution of GRSP is 2-24 times greater than that of soil humus (Comis 2002). Pearson correlation analysis and RDA showed that both EEG and TG were positively correlated with SOC, indicating that GRSP can be used as an indicator of the soil carbon pool. Moreover, EEG/SOC and TG/SOC were 3.94 and 15.89% (n=184), respectively, suggesting that GRSP contributes more C to the urban red soil carbon pool. Similar values were also observed by Staunton et al. (2020), who reported an average of 9.7% for TG/SOC and 3.8% for EEG/SOC.
The SOC and GRSP levels were significantly lower in the heavy urbanization areas than in the low urbanization areas (Fig. 2, Table 1). Linear regression analysis showed a negative relationship between SOC and GRSP/SOC (EEG/SOC and TG/SOC) (Fig. 3). We found that the rate of SOC loss was much higher than the decomposition rate of GRSP. This also confirmed that GRSP is a component of inert carbon in the soil carbon pool (Wang et al. 2018b). Hence, increasing GRSP content can reduce soil carbon loss due to urbanization. In addition, GRSP can reduce the decomposition of soil organic matter by binding to soil aggregates and sequestering GRSP-C within aggregates (Liu et al. 2020). Therefore, GRSP has important roles in SOC sequestration, especially in areas with heavy urbanization.
4.3 Key factors affecting GRSP change in the urbanization process
Compared with natural ecosystems, urban ecosystems are dominated by humans. Climate change, animals and plants, and land use pattern are strongly disturbed by human in urban ecosystem (Meng et al. 2021). In contrast to natural ecosystems (Jin et al. 2021), we should consider how various urban environmental factors (forest characteristics, soil properties, and land use configuration) affect GRSP soil content.
Soil properties were the key factors affecting GRSP content during the process of urbanization. Pearson correlation analysis showed that more than 80% of the soil factors were significantly correlated with GRSP content. Variation partitioning analysis showed that soil factors could explain 64.4% of the GRSP differences. In particular, the PLS-PM model showed that soil factors were one of the key indirect factors influencing the decrease in GRSP. For instance, GRSP was significantly and positively correlated with TP, SOC, and TN, confirming that GRSP can be a crucial indicator of the dynamics of soil carbon sequestration and nutrient retention (Wang et al. 2017b). AMF mycelia promote plant growth and improve soil quality by increasing the uptake of soil nutrients in exchange for photosynthetic products from the host plant (Smith and Read 2008). This also creates an important pathway for the transfer of mycorrhizal carbon to and stored in the soil over time (Clemmensen et al. 2013), which can lead to improved soil quality. GRSP was positively correlated with NO3− and NH4+ concentrations (Fig. 4). GRSP also showed a strong correlation with NO3− and NH4+, which are the main forms of nitrogen uptake by plants (Zhang et al. 2015). Moreover, GRSP can enhance nitrogen mineralization, and hyphae bridges can also help plants absorb nitrogen from the soil (Terrer et al. 2016). Urbanization significantly increased soil pH from slightly acidic to neutral and then to weakly alkaline (Table 1). This may due to the fact that most urban greenspace soils contain construction backfill, which contains construction waste, cement, lime, and other alkaline substances (Gujre et al. 2021). Consistent with the results of Singh et al. (2016); Wang et al. (2015b), GRSP was negatively correlated with BD and pH. Slightly acidic or neutral soils are favorable for GRSP accumulation, mainly because the optimal habitat for AMF is in slightly acidic soils, whereas alkaline soils are not favorable for spore production and therefore GRSP secretion (Bonfim et al. 2016; Wang et al. 2015a). Previous studies have shown that AMF is an aerophile fungi, and soil with better permeability is more suitable for AMF spores production and increases colonization of plant roots (Sun et al. 2011). In the present study, BD was significantly lower in low urbanization areas than in heavily urbanized areas. Since BD can indicate soil compaction, this result suggests that the reduction in GRSP caused by urbanization may be due to soil hardening and human disturbance, which reduced soil microbial activity and GRSP secretion.
The influence of forest characteristics on GRSP content should not be neglected. Unlike natural systems, urban vegetation is subject to more human modifications and disturbances, such as transplanting and pruning. Urbanization intensity could indirectly influence GRSP changes through forest characteristics (Fig. 7a). In particular, the interaction between forest characteristics and soil factors could explain 24.1% of the GRSP differences (Fig. 6). Correlation analysis and RDA also indicated that WSWI, WH, HR, DBH, and CS were positively correlated with GRSP content. In addition, the PLS-PM model selected these five factors as sufficient indicators of forest characteristics (Fig. 7b), which indicates that these factors had a significant impact on GRSP soil content. WSWI is a measure of woody diversity, which indicates that high vegetation diversity can increase GRSP soil content. This was consistent with the findings of Sousa et al. (2013), who found that the composite planting method can increase GRSP content compared to the traditional monoculture pattern. WH, DBH, and CS are tree growth parameters. Generally, the large and dense root systems of large woody plants facilitate the formation of symbioses between AMF and plant roots, thereby increasing AMF biomass and GRSP content (Gujre et al. 2021; Nautiyal et al. 2019). Trees with large canopies have large shade areas, which not only reduces the negative effects of high temperature and precipitation on SOC sequestration (Zhang et al. 2021), but also increases surface litter, which increases the soil organic mulch, and thus facilitates AMF colonization and GRSP production (Udayakumar et al. 2021). In general, there is a relatively strong correlation between GRSP content and plant biomass (Liu et al. 2020). Interestingly, we found that TD was not correlated with GRSP, and TD increased linearly from low urbanization areas to heavy urbanization areas. This result can be attributed to that most of the heavy urbanization areas are newly built urban areas with small and single tree species, resulting in higher TD. Low urbanization areas are not overdeveloped and built up, and some old trees are preserved, so the WH and DBH of the areas with low urbanization are higher. This phenomenon is consistent with the results of Zhang et al. (2017b).
Land use factors indirectly affected GRSP changes by influencing forest characteristics and soil factors (Fig. 7). The Pearson correlation analysis and RDA together indicated that GRSP content was closely related to the percentage of VE. There are some negative effects on GRSP accumulation in with increasing urbanization because increased impervious surface (e.g. buildings and roads) and a decrease in vegetative cover. Therefore, in the context of rapid urbanization, it is necessary to manage and protect urban greenspace soils by improving the GRSP content.
4.4 Implications and suggestions
China has experienced a rapid and continuous urbanization process over the last few decades and the urban population continues to increase, with an average annual growth rate of 0.16 billion (Guan et al. 2018). Resource-driven population growth in cities threatens limited natural resources by improper disposal of waste, resulting in soil and water pollution (Deus et al. 2020). Arbuscular mycorrhizal fungi (AMF) are ubiquitous soil microbial communities (Smith and Read 2008; Stürmer et al. 2018). GRSP secreted by AMF is a key indicator for maintaining plant growth and improving soil quality (Singh et al. 2020). Therefore, to increase urban soil GRSP content, we put forward the following suggestions and their implications.
Soil factors had the greatest impact on GRSP, and some soil nutrients were positively correlated with GRSP. Due to the development and utilization of land in the process of urbanization, the consumption of soil organic matter also decreased. In this study, nutrients, such as SOC, N, and P, were significantly higher in low urbanization areas than in heavily urbanized areas. Therefore, in heavy urbanization areas, appropriate fertilization can be used in greenspace management to promote plant growth and improve basic soil nutrients. In addition, GRSP was negatively correlated with pH and BD, especially in areas with heavy urbanization. The dumping of construction and domestic waste should be minimized to keep the soil pH slightly acidic, which will be beneficial to AMF growth and sporulation. Urban greenspaces in China are public spaces managed by local governments, and their soil compaction will continue to increase due to human trampling, crushing, etc. To increase soil permeability in heavy urbanization areas, greenspace soil should be loosened regularly to reduce BD. These soil management measures can increase the accumulation of GRSP and contribute to the improvement of soil quality.
Greenspace vegetation plays an increasingly important role in urban areas, greatly contributing to ecosystems and biodiversity (Kendal et al. 2020), and it is essential that greenspaces are properly managed to protect floristic health. The percentage of vegetation should be increased in areas with heavy urbanization, which can lead to an increase in GRSP content. In addition, GRSP is positively correlated with WSWI, WH, DBH, etc. The existing larger trees should be protected and managed, try not to transplant the original tree. Tree species in new urban development areas should be diversified under the premise of a beautification to effectively increase soil GRSP. In addition, the effects of tree species composition impact the organisms living in the soil. How to effectively select different plant ratios to maximize their functions requires further study. In addition, we should adopt a near-natural forest management pattern to manage urban greenspaces and minimize human interference. For example, reducing soil compaction and retaining forest litter can effectively increase the sequestration of GRSP and soil carbon. Overall, we can consider improving the soil quality of greenspace from the perspective of enhancing GRSP content to maintain the sustainable development of urban ecosystems in the future.