The effects of wildfire on multiple individual soil functions and soil multifunctionality
The results of this study highlight the response of soil ecosystems to wildfire. Within selected indicators, we found that soil DOC and MBC at the 1-year-postfire site were significantly lower than those of control while the soil TC was slightly low but not significant (Fig. 1A, D, F), suggesting a negative impact of wildfire on soil C storage (Certini 2005; Mack et al. 2021). This negative effect on soil C storage has been observed in some boreal forests (Bond-Lamberty et al. 2007; Ludwig et al. 2018; Turetsky et al. 2010). These researchers suggested two reasons for this reduction: 1) combustion of aboveground plants and forest floor, 2) mortality of microorganisms though peak temperature and toxic organic pollutants during burning. In this study, the decline in soil C storage may be contributed to the removal of forest floor and fire-caused sterilization of the surface soil. Furthermore, this combustion also releases a large number of nutrients such as N and P bound in the litter and vegetation biomass (Brais et al. 2000; Certini 2005). Though part of nitrogenous gas may be lost by volatilization during combustion, most N and P in ash can deposit on the ground surface and thus can increase soil fertility (Bodí et al. 2012; Johnson et al. 2012). These processes may be responsible for the increases in soil N and P fertility in this study (Fig. 1B-C, E, G-J). The fire fertilization effect suggests a positive impact of wildfire on soil fertility (Ludwig et al. 2018; Turner et al. 2007). In addition, direct killing of aboveground plants due to combustion of themselves and forest floor may result in a large decrease in fine root biomass (Fig. 1K), suggesting a negative effect on belowground productivity (Ma et al. 2017). Totally, wildfire simultaneously exerted different effects on multiple individual soil functions and the trade-off of changes in soil C storage, fertility and belowground productivity potentially caused a decline in soil multifunctionality (Fig. 2), suggesting that the overall effect of wildfire on boreal soil ecosystems was negative. This result concurred with our expectation that wildfire could decrease soil multifunctionality. To our knowledge, this is the first study to show the effect of wildfire on soil multifunctionality in boreal forest ecosystems. This novelty is crucial for comprehensive understanding of the fire-caused ecological consequences.
The effects of wildfire on soil functions changed with time since the fire. Eleven years postfire, the soil TC increased to the prefire level but the soil DOC and MBC were still significantly lower relative to the control (Fig. 1A, D, F), suggesting a long-term and negative effect of wildfire on labile soil C pools (Pingree and DeLuca 2018). This long-term decrease in microbial C sources generally regulates microbial growth with potential consequences for postfire recovery of soil microorganisms (Holden et al. 2015). The soil N parameters, such as DON, AN and NMP, decreased to the prefire levels after the immediate pulse following fire, which were consistent with the results reported by Durán et al. (2008) about a wildfire and by Monleon et al. (1997) about a prescribed burning. The reduction in soil N availability may be related to the N loss resulting from erosion and leaching. Wildfires can burn all or part of forest floor and hence can expose the mineral soil layers to the ground surface, which may possibly increase the risk of runoff erosion (Simard et al. 2001). Our study area suffered heavy rains in the first growing seasons following the fire; thus, large leaching and surface runoff erosion at severely burned areas could result in great loss of ash deposition, which would decrease soil N availability. Additionally, these postfire leaching and erosion may be responsible for the loss of soil P pool and P availability (Fig. 1C, I). Another possible explanation was increased nutrient uptake by regenerated vegetation. These reductions in soil N and P availability after the immediate pulse following fire indicate a lasting and negative effect of wildfire on soil fertility. In contrast, we found that fine root biomass at the 11-year-postfire site increased but was still significantly lower than that of control, which may be related to a slow recovery of regenerated plants. In our field investigation, we observed that the regenerated vegetation was sparse at the 11-year-postfire site, particularly at the severely burned areas. The above results indicate that wildfire could exert long-term and negative effects on individual soil functions, such as soil C storage, soil fertility and belowground productivity, which may potentially exacerbate the negative impacts on soil multifunctionality (Fig. 2). This finding supports the growing idea that forest ecosystem multifunctionality has a low resilience to disturbance (Eyvindson et al. 2021; Pohjanmies et al. 2021).
Relationships between soil functions and fire severity and bacterial diversity
Our study indicates the effect of wildfire on soil functions is closely related to fire severity at the recently burned sites. Consistent with previous research, we found the decreases in soil TC, DOC and TN with increased fire severity (Certini 2005; Knelman et al. 2015; Ludwig et al. 2018). Severe burning can volatize more C and nitrogenous gas into the atmosphere due to their low volatilization temperatures (Neary et al. 2005). This process may be responsible for our no significant relationship between soil N availability and fire severity (Fig. 3F, H, I, K), which has been reported by past studies (Certini 2005; Knelman et al. 2015). In contrast, severe burning can release orthophosphate P by burning more organic matter and can deposit more P to the ground surface due to its high volatilization temperature, which may increase soil P (Dzwonko et al. 2015; Ludwig et al. 2018). These processes may possibly explain the positive relationships between fire severity and TP and AP at the 1-year-postfire site (Fig. 3D, J). Similar to soil C storage, fine root biomass decreased with increased fire severity (Fig. 3L) because severe burning could kill more plants and their fine roots (Smirnova et al. 2008).
Our result showed that the effect of fire severity on soil functions changed with time since the fire. Eleven years postfire, we found no significant relationships between fire severity and soil C storage as well as soil TN, consistent with previous studies (Certini 2005; Dzwonko et al. 2015). Comparatively, the soil P fertility decreased with increased fire severity at the 11-year-postfire site relative to their positive relationship at the 1-year-postfire site (Fig. 3D, J), suggesting greater loss of soil P following the short-term pulse. Such loss of soil P is well known in forestry and has been confirmed by some researchers, who have contributed it to higher postfire leaching and lower plant uptake (Blake et al. 2010; Bodí et al. 2012; Johnson et al. 2012). In our study area, the severely burned areas had thinner organic soil layers and even exposed mineral soil layers, thus likely increasing the risk of runoff erosion in the heavy rain seasons. Additionally, Cai et al. (2013) reported that there were sparse tree recruits and understory vegetation at the severely burned areas in the Great Xing’an Mountains because severe burning killed more seed trees and destroyed soil seed banks, suggesting that the nutrients by plant uptake was small with potential consequences for postfire loss of soil P. Meanwhile, these processes may be attributable to explaining the decrease of fine root biomass with increased fire severity.
Importantly, this study is unique in examining the relationship between soil multifunctionality and fire severity. As we expected, the soil multifunctionality decreased with increased fire severity immediately after the fire (Fig. 3A), suggesting a negative effect of fire severity on soil multifunctionality in the postfire Great Xing’an Mountains. This may be related to the fire-caused simultaneous declines in soil C storage, N pools and fine root biomass at the severely burned areas (Fig. 3B-C, E, L). Eleven years postfire, we found a stronger association between soil multifunctionality and fire severity (Fig. 3A), inconsistent with our expectation. This result is likely attributable to slow recovery of postfire plants and large loss of soil nutrients through runoff erosion at the severely burned areas. In our study area, the sparse regenerated plants may retard the recovery of soil C storage and fine root growth; the short-term pulses in soil fertility immediately following the fire may be eliminated by postfire runoff erosion and leaching. This result indicates that fire severity can exert a relatively long-term and negative effect on soil multifunctionality. Therefore, restoration measures, such as mulching and hydroseeding, at the severely burned areas should be initiated to improve recovery rates of soils and plants by reducing soil erosion and increasing seeds (Santana et al. 2014; Vourlitis et al. 2017).
In addition to fire severity, the soil multifunctionality was significantly correlated with soil bacterial diversity at the two burned sites but this association was not obvious at the control site (Fig. 4A), suggesting that wildfire could strengthen the relationship between bacteria and soil multifunctionality. A previous study reported that soil multifunctionality was not significantly correlated with bacterial richness in the unburned Great Xing’an Mountains and attributed it to low bacterial diversity resulting from low soil pH and high recalcitrant C (Li et al. 2019). However, another previous study reported that soil bacterial diversity was positively correlated with soil microbial biomass and N cycle at a unburned Canadian boreal forest (Giguère-Tremblay et al. 2020). Consistent with this study, we found a stronger positive relationship between bacterial diversity and soil DON but a weak positive relationship between bacterial diversity and labile soil C as well as other N indicators at the control site (Fig. 4E-I). Interestingly, at the burned sites, we found a stronger positive association between soil multifunctionality and bacterial diversity, which may be related to higher resistance of bacteria to heat than fungi (Certini 2005). And bacteria possess a high adaptive capability and thus can rapidly colonize the burned environments relative to fungi (Holden et al. 2016). Additionally, postfire regenerated vegetation, such as birch, aspen and vascular plants, can provide high-quality C and N resources for bacterial activity. These processes may be responsible for the postfire enhanced association between bacterial diversity and soil multifunctionality.
The relative importance of direct and indirect effects of wildfire on soil multifunctionality
This study is a novel attempt to assess the direct and indirect effects of wildfire on soil multifunctionality in the Great Xing’an Mountains. As we expected, fire severity was the major driver of soil multifunctionality at the 1-year-postfire site (Fig. 5A), and the soil multifunctionality significantly decreased with increased fire severity (Fig. 3A). However, the thinner OML, lower soil moisture and bacterial diversity resulting from severe burning played minor roles in mediating soil multifunctionality (Fig. 5A). These results indicate that the direct impact of wildfire is greater than its indirect effect at the recently burned site.
The SEM analysis showed that the mechanisms of wildfire driving soil multifunctionality differed between the two burned sites (Fig. 5). Eleven years postfire, despite fire severity still having significant negative effects on soil multifunctionality, its cascading effects by affecting soil environments and vegetation recovery played more important roles (Fig. 5B). As the most important predictor of soil multifunctionality, the positive effect of soil moisture suggests that postfire soil water availability may limit recovery of soil multifunctionality. This conclusion is supported by some previous studies that have reported marked decreases in soil moisture after boreal forest fires (Certini 2005; Holden et al. 2015; Osawa et al. 2010). At our severely burned areas, the fire combusted almost all the forest floor and surface moss cover, which may not store abundant water in the thinner soil organic horizons and make the burned stand dry out quickly following precipitation events. Our results also showed soil bacterial community was suffering from water stress at the 11-year-postfire site (Fig. 5B). These processes would likely regulate microbial decomposition and regenerated plant growth. Additionally, we observed that recruit tree density was significantly positively related to soil multifunctionality (Fig. 5B), suggesting that postfire recruits of tree saplings may be conducive to recovery of soil functions (Alexander and Mack 2016; Mack et al. 2021). However, severe burning caused a lower recruit tree density (Fig. 5B), suggesting that fire severity could indirectly exert long-term negative effects on soil multifunctionality via decreasing tree recruitments. Comparatively, grass cover was negatively correlated with soil multifunctionality, suggesting great herbaceous vegetation may have adverse effects on soil multifunctionality. This result may be related to higher decomposition rates of herbaceous vegetation (Hart et al. 2005), which may further decrease soil C storage. Finally, our SEM analysis showed that soil bacterial diversity had a weaker positive association with soil multifunctionality (Fig. 5B), suggesting that recovery of bacteria community could produce favorable effects on soil multifunctionality. However, soil bacterial diversity decreased with increased fire severity (Fig. 5B), indicating that fire severity can mediate the effects of belowground biodiversity on soil multifunctionality. A possible explanation is that fire severity can affect structure and composition of microbial community directly by killing microbes and indirectly by altering abiotic soil environments (Certini 2005; Holden et al. 2015; Holden et al. 2016), which in turn may regulate microbial activity. These processes may be conducive to unravelling the mechanisms of wildfire driving soil multifunctionality. However, our knowledge of the ecological role to fire regimes is still in its infancy, therefore, future research should focus on how various fire regimes affect ecosystem multifunctionality of boreal forests.