4.1. Responses of SOC mineralization to vegetation restoration, moisture and temperature
Similar to earlier studies(Parsapour et al. 2018; Yang et al. 2018; Pang et al. 2019; Wu et al. 2019), we found that land use changes from vegetation degraded land to forest substantially increased SOC, TN, TP, MBC, DOC, POC, and total PLFAs contents (Table 2). These changes in soil properties among restoration stages, such as SOC content, nutrients, and microbes significantly affect SOC mineralization (Cates et al. 2019; Wang et al. 2019; Zhang et al. 2019; Li et al. 2021; Xu et al. 2021). As we expected, significantly lower SOC mineralization in the DS than that in the PS and SFS (Fig. 1, Table 1) was found. Lower SOC mineralization in the DS would be attributed to lower total SOC or lower labile SOC, mainly POC, DOC and MBC contents (Table 2), which supported lower populations of soil microbes and through which thus led to lower SOC mineralization (Sheng et al. 2010; Chen et al. 2014; Yang et al. 2017; Huang et al. 2019). An alternative explanation is that extremely lower (P < 0.001) soil microbial biomass in the DS than that in the PS and SFS (Table 2). Previous studies which presented a positive microbial biomass effect on SOC decomposition (Fig. 6) also indicating that less soil microorganisms were involved in the decomposition in the DS (Wei et al. 2014; Fang et al. 2015).
Despite very different soil properties between the PS and SFS, the Cumulative SOC mineralization did not differ. The possible reason is that, although total SOC was significantly lower in the PS than that in the SFS, the amount of POC and MBC was slightly smaller (Table 2, P > 0.05) in the PS than in the SFS, which likely resulted in similar amount of SOC mineralization in the PS and SFS. Likewise, Huang et al (2019) showed that although total SOC was higher in the natural forest soil, the amount of labile SOC was 27–28% greater in the plantation soil leading to greater SOC mineralization in the forest plantation soil than that in the natural forest soil. There are also some incubation experiments revealed that the fluxes of CO2 were positively correlated with DOC during the several months of incubation suggested that soil CO2 emission was mainly from DOC (Conant et al. 2008; Gershenson et al. 2009). The DOC content did not differ between the PS and SFS which could partly explain the similar amount of SOC mineralization in the PS and SFS (Fig. 3, 6).
The property differences among the soils of three vegetation restoration stages did not result in logical responses of SOC mineralization to the rising temperature. In detail, increased temperature significantly increased (63.4%-102.6%) the SOC mineralization in the PS and SFS under both of the two soil moisture levels, but solely significantly increased SOC mineralization under 60%MWHC level in the DS. This is partial in line with most previous observations. For instance, many studies showed that increased temperature would stimulate SOC decomposition (Fissore et al. 2008; Huang et al. 2019; Wang et al. 2019; Fang et al. 2020), due to increased temperature would decrease SOC use efficiency by changing the physiological processes of microorganisms (Schindlbacher et al. 2011; Streit et al. 2013). Consistent with the observed responses of SOC mineralization to rising temperature, increased temperature significantly increased qCO2 in the PS and SFS under both of the two soil moisture levels, but solely significantly increased SOC mineralization under 60%MWHC level in the DS (Fig. 4). Such changes in qCO2 suggesting that increased temperature stimulated microbial respiration in the in the PS, SFS, and 60%MWHC level in the DS in our study. Although microbial biomass and the abundance of major microbial groups were not remarkably altered by temperature (Fig. 3, 5), Schindlbacher et al (2011) also reported that five years of soil warming did not affect microbial biomass or the abundance of major microbial groups, but significantly increased qCO2, thus leading to an increase in soil respiration.
However, increased temperature did not enhance SOC mineralization under 30%MWHC level in the DS which may be relate to the interactive effect of soil water deficit and substrate limitation (Suseela et al. 2012; Moyano et al. 2013). Most studies have demonstrated that the optimum for soil respiration is frequently found at intermediate soil water content, with decreases in rate both above and below the optimum soil water content (Craine et al. 2011; Suseela et al. 2012; Zhang et al. 2015). At intermediate soil moisture, which not only facilitates the diffusion of soluble C substrates, extracellular enzymes, and microbes in water film (Davidson et al. 2006), but also may help to improve microbial substrate availability through increasing labile C and nutrients pools (Balogh et al. 2011). Therefore, under the 30%MWHC treatment in the DS, the extremely lower labile SOC (i.e. POC and DOC) content (Table 4) combined with lower soil water content may hinder the diffusion of limited soluble C substrates, resulting in insensitive responses of SOC mineralization to rising temperature. Although we cannot provide conclusive evidence, the lowest MBC concentrations under the 30%MWHC treatment in the DS observed in this study (Fig. 3) does support this postulated mechanisms. Without substrate limitation, even at 20% MWHC, zhou et al (2014) also revealed that SOC mineralization could still increase with temperature. The results meant that effects of temperature on SOC mineralization at different soil moisture levels would be affected by soil properties.
In the PS and SFS, we found that SOC mineralization was significant higher at 60%MWHC level (almost at an optimum soil water content for microbial respiration) than that at 30%MWHC level. As mentioned above, suitable soil moisture can facilitate the diffusion of soluble C substrates (Davidson et al. 2006) and improve microbial substrate availability (Balogh et al. 2011), which can stimulate the microbial decomposition of SOC. Compared to 30%MWHC, significantly increased MBC and DOC content at 60%MWHC level in the PS and SFS (Fig. 3, 6) could support this point. But increased soil moisture did not stimulate SOC mineralization in the DS, which may due to the limited soluble C substrates in the DS (Moyano et al. 2012). In the DS, 60%MWHC treatment did not increase DOC content under both two incubation temperatures also indicated the substrate limitation (Fig. 3). In addition, the inherent lower labile SOC content (Table 1) in the DS also should be responsible for the substrate limitation for microbes. These findings suggested that, to some extent, rising moisture and temperature would stimulate SOC decomposition, but these responses are highly dependent on soil properties or the quantity and quality of the substrates.
4.2. Responses of Q10 to vegetation restoration and soil moisture
Generally, soils with longer vegetation restoration years always have greater biodiversity and stability of biogeochemical processes (Yang et al. 2018; Zhang et al. 2019), we expected which may lead to lower Q10 values with the extension of vegetation restoration years (Deng et al. 2012; Xu et al. 2021). However, we found that the response of Q10 to vegetation restoration was highly dependent on soil moisture. In detail, at the 60%MWHC treatment, Q10 was decreased with vegetation restoration age (Fig. 2), this partly supports our expectation. Inversely, at the 30%MWHC treatment, Q10 was significantly lower in the DS than in the PS and SFS (Fig. 2). Likewise, previous studies presented soil property may interact with numerous climatic variables, such as temperature and precipitation, to influence Q10 (Deng et al. 2012; Davidson and Janssens 2006).
The optimum SWC was usually somewhere at 60% water-filled pore space, where the macropore spaces were mostly air filled, thus facilitating O2 diffusion; the micropore spaces were mostly water filled, thus facilitating diffusion of soluble substrates (Suseela et al. 2012; Zhou et al. 2014; Zhang et al. 2015). Under the 60%MWHC condition, microbes should be no longer constrained by various substrates, thus the inherent properties of the soils converted to the predominant factor associated with the variations in soil respiration and Q10 (Craine et al. 2011; Suseela et al. 2012). Firstly, although water limitation was removed, quite limited labile substrate in the DS would be rapidly depleted (Table 2), which may lead to the longer the incubation time, the more time microbes had to consume the recalcitrant C, thus resulting in higher Q10 in the DS than that in the PS and SFS (Davidson and Janssens 2006; Wang et al. 2019). Secondly, soils with longer vegetation restoration years always have greater biodiversity (Yang et al. 2018), and Xu et al (2020) reported that high microbial diversity can stabilize the responses of SOC decomposition to warming. Thirdly, some studies showed that soils with more fungi or GP: GN ratio would have larger Q10 (Wang et al. 2018; Huang et al. 2019; Fang et al. 2020). This hypothesis is supported, to a certain extent, by our observations of the inherent higher abundance of fungi and GP: GN ratio in the DS (Table 2), and the incubation experiment done little to reverse this trend (Fig. 5).
We found Q10 was significantly lower in the DS than in the PS and SFS at the 30%MWHC treatment (Fig. 2). Under a water deficit condition, substrate availability of the soils may convert to the predominant factor associated with the variations in soil respiration and Q10 (Craine et al. 2011; Suseela et al. 2012). Although water deficit also existed in the PS and SFS, to some extent, the inherent higher labile SOC content could support microbial consumption in the PS and SFS (Table 2). On the contrary, at the 30%MWHC treatment in the DS, limited diffusion of soluble substrates combined with lower inherent higher labile SOC ultimately may inhabit the growth of microbes (Craine et al. 2011). The significantly lower MBC content at 30%MWHC level in the DS (Fig. 3) could support this point.
The absence of consensus on the responses of Q10 to soil moisture may be caused by the differences in soil properties across a variety of ecosystem types (Craine et al. 2011; Deng et al. 2012; Zhang et al. 2015). The result that increased soil moisture did not affect Q10 in the PS and SFS, but enhanced Q10 in the DS, which is consistent with our expectations that soil moisture had a less effect on Q10 in the SFS than in the DS. The potential mechanism could also explained by the substrate availability. In our study, water deficit limited microbial biomass at the 30%MWHC in the DS which could lead to a lower Q10 (Fig. 3, 6). Previous studies also have shown that drying can decrease Q10 of soil respiration, and which was attributed to substrate limitation caused by the limited diffusion of solutes in thin soil water films (Davidson and Janssens 2006; Suseela et al. 2012; Moonis er al. 2021). At the 60%MWHC in the DS, water condition facilitated but substrates limited the microbial growth (Table 2, Fig. 3), leading to a disproportionately increases in microbial respiration and qCO2 in the 28°C, which may also lead to a higher Q10 (Schindlbacher et al. 2011). In a modeling study, Moyano et al (2012) showed that the decomposition of SOC response to moisture depends on soil properties. Thus, these results also partly support the positive biodiversity-ecosystem stability hypothesis (Xu et al. 2021).