Soil respiration (R_s) is the second largest carbon flux between land and the atmosphere (Yang et al., 2022). In terrestrial ecosystems, forests are important regulators of the global carbon balance, with trees storing up to 56% of both above- and below-ground terrestrial carbon, and forest soil respiration accounting for at least 30% of total terrestrial respiration (Wan et al., 2015; Yang et al., 2022). Therefore, even minor alternations in forest soil respiration can significantly change atmospheric carbon dioxide concentration and the global carbon cycle (Valente et al., 2021). Forest soil respiration is affected by forest management practices such as thinning (Sherman and Coleman, 2020), fertilization (Wang et al., 2017), understory removal (Yao et al., 2019), and litter manipulation (Dai et al., 2021). However, since the two main components of R_s: autotrophic respiration (R_a: from live roots and rhizosphere), and heterotrophic respiration (R_h: from microbial decomposition of soil organic matter) are site-specifically related to forest type, soil characteristics, local climate, and other variables, the impact of forest management practices on actual soil respiration are often variable (Mo et al., 2008; Wan et al., 2015). For example, Zhang et al. (2022) explored that thinning increased R_s in subtropical Cunninghamia lanceolata plantations. Nonetheless, Pang et al. (2013) found that thinning inhibited R_s in a pine plantation in the eastern Tibetan Plateau. Furthermore, multiple silvicultural practices may be applied simultaneously, potentially producing interactive effects on soil respiration (Sherman and Coleman, 2020). Therefore, a comprehensive assessment of the impact of forest management measures on soil respiration will not only help to mitigate climate warming, but also provide a scientific basis for formulating more rational forest management strategies.

Lime application is a common forestry practice to improve acrisols in humid tropical and subtropical regions (Zhao et al., 2015). In addition to preventing or reversing soil acidification, the lime application can profoundly alter soil microbial communities (Sridhar et al., 2022), although the reported results concerning the effect of lime application on soil respiration have so far been mixed. Shah et al. (1990) and Goupil (2014) reported that lime application increased R_h, likely because of enhanced acid intolerant bacterial abundance and fungal abundance, and consequently increased soil respiration. On the contrary, Melvin et al. (2013) reported a reduction in soil respiration by inhibiting soil basal respiration in a broadleaved forest after the application of lime, thereby increasing carbon storage in the forest soil. Meanwhile, Lochon et al. (2019) found that lime application produced a negligible effect on soil respiration because the increase in R_h was counteracted by the decrease in R_a. Taken together, it appears that responses of soil respiration to liming are highly dependent on the environment and the balance between R_a and R_h. High soil pH tends to favor the growth of acid-intolerant bacteria at the expense of both acid-tolerant bacteria and fungi (Wan et al., 2015), thus alternations of microbial communities in limed soils may at least partially explain changes in soil respiration. Meanwhile, the lime application can increase the availability of some nutrients (i.e., Ca and Mn), while decreasing the availability of some micro nutrients, such as Cu and Zn (Florentino et al., 2021). Therefore, alternations in plant growth under lime application may also explain changes in soil respiration. To date, the effect of lime application has been tested in grassland and agricultural soils (Lochon et al., 2019; Xu et al., 2022). However, there is a lack of information on the response of soil respiration to lime application in forest ecosystems, particularly in Eucalyptus plantations grown in strong acidic soil in South China.

Eucalyptus urophylla S.T.Blake is a fast-growing, high-yielding tree species cultivated in subtropical and tropical terrestrial regions around the world. In southern China, cultivation has been expanding rapidly in response to increasing demand for fiber and wood (Wan et al., 2019; Gao et al., 2021). In southern Chinese E. urophylla monoculture plantations, the native fern Dicranopteris dichotoma, tends to create a thick understory blanket (Yang et al., 2021). In practice, understory plants are traditionally removed as they are believed to compete with cultivated trees for both nutrients and water. However, studies have shown that D. dichotoma can have profound impacts on soil organisms (Gao et al., 2021), litter decay (Chen et al., 2019), soil microclimate (Wan et al., 2019), and canopy productivity (Wan et al., 2014). Although understory plants may be a substantial driver of both above and below-ground ecosystem function, few studies have quantitatively explored the effect of understory clearance on soil respiration (Wan et al., 2015; Zhao et al., 2022). Even less is understood regarding the potential interactive effects of combined plantation management practices, such as understory clearance and lime application, on soil respiration.

In this study, a field experiment was conducted in a southern Chinese subtropical E. urophylla plantation to investigate the individual and combined effect of lime application and understory clearance on soil respiration and its two main components (autotrophic respiration and heterotrophic respiration). We hypothesized that: (1) lime application would decrease soil respiration by inhibiting soil total microbial PLFAs and associated R_h (Melvin et al., 2013; Wan et al., 2019); (2) understory clearance would reduce root respiration of understory vegetation and R_h by reducing labile carbon and nutrient substrates for soil microorganisms, thus consequently decreasing soil respiration (Wan et al., 2021; Xiao et al., 2022); (3) lime application combined with understory clearance would reduce soil respiration by inhibiting both R_h and R_a.

Studies have shown that lime application can increase R_s by increasing plant carbon input, improving soil properties and structure, and activating the activities of soil microorganisms (Ingvar Nilsson et al., 2001; Dada and Ewulo, 2011; Jiang et al., 2018; Abalos et al., 2020). However, in the present study, we did not find such a positive relationship between lime application and R_s, and lime application significantly reduced soil respiration rate irrespective of understory removal, which is consistent with the first hypothesis. Generally, soil respiration mainly includes soil heterotrophic respiration and autotrophic respiration, therefore, the effect of lime application on soil respiration depends on the balance of these two components. Although we found that lime application tended to increase R_h, this treatment also led to the consistent inhibition of R_s, primarily due to the inhibition of R_a (Figures 1A–C), with the extent of inhibition varying according to the occurrence of understory vegetation. Fine root respiration is the primary contributor to total root respiration (Chen et al., 2011). Nevertheless, lime application had no effect on Eucalyptus fine root biomass (Figure 4), thus the reduction in understory plant root respiration is likely responsible for the decreased R_a under lime application. This conclusion is further supported by the finding that lime application caused a greater reduction in R_s when the understory vegetation was intact (Figure 2A). Indeed, in the Eucalyptus plantation we studied, the understory vegetation was almost dominated by Dicranopteris dichotoma, which is considered as an indicator plant for acidic soils in tropical and subtropical regions. Its roots are densely distributed close to the soil surface, therefore, the root growth and associated root respiration are thus negatively affected by higher soil pH under lime application (Yang et al., 2021; Zhang et al., 2021). Børja and Nilsen (2009) and Lin et al. (2015) also explored that lime application can inhibit plant root growth and decomposition. Notably, lime application increased R_h (Table 1 and Figure 1B), but decreased both fungal and bacterial PLFAs (Table 2), implying that some microbial populations were inhibited at higher soil pH but others with low carbon use efficiency were promoted in growth and activity (Xiao et al., 2022). Consequently, lime application stimulated soil microbial activities and associated R_h following the findings of other researchers (Ingvar Nilsson et al., 2001; Dada and Ewulo, 2011). Given that the inhibitory effect of lime application on soil respiration was significantly greater when the understory was intact compared with when the understory was removed (Figure 2A), our results suggest that understory vegetation of Eucalyptus plantation can play a crucial role in regulating the effects of lime application on soil respiration. It should be noted that the effect of lime application on R_s has been found to vary with forest, lime formulation, treatment time, and lime application rate (Klaus et al., 2001; Børja and Nilsen, 2009).

Consistent with the second hypothesis, understory clearance caused a significant reduction in R_s (Figure 1), which is in accordance with the majority of previous work in subtropical plantations (Wan et al., 2015; Zhao et al., 2022). Studies on natural tropical forests, subtropical plantations, and boreal forests show that understory vegetation can affect R_s by modulating R_h and R_a (Fukuzawa et al., 2015; Subedi et al., 2019; Dacal et al., 2020). In the present study, understory clearance allowed a greater amount of solar radiation to reach the soil surface, and increased soil temperature by 0.74°C, which may stimulate R_h, as soil temperature was significantly positively correlated with R_h (Figure 5). Overall, however, we found that understory clearance had a negative effect on R_h. This result may be due to the reduction of DOC and soil NO₃^–-N content caused by the clearance of understory vegetation (Supplementary Table 1), because the growth of soil microbiota is often limited by the accessibility of soil nutrients and carbon substrates (Subedi et al., 2019). The decreased soil total PLFAs after understory clearance can further support this finding (Table 2). Furthermore, despite the effects of understory clearance on Eucalyptus root respiration and Eucalyptus fine root biomass were negligible (Figure 4), it decreased R_a, suggesting that the inhibition of R_a was mainly due to the reduction in understory root respiration. Overall, understory clearance reduced R_s by decreasing both R_h and R_a. These observations highlight that understory vegetation can play a crucial role in modulating soil microorganisms and carbon cycling in subtropical plantations. In addition, our previous studies have found that understory vegetation had a positive effect on Eucalyptus fine root biomass and productivity in 4-year-old plantations (Wan et al., 2014, 2015), while this was not the case in the present study with plantations in 6-year-old, implying that the positive effect of understory vegetation on the growth of Eucalyptus is age-dependent.

Following with the third hypothesis, R_s was decreased when lime application was combined with understory clearance (Table 1 and Figure 1). In the present study, the annual average of R_h under the L+UR treatment were significantly higher than those under the CK treatment (Figure 2B), indicating that the increase in R_h due to lime application was greater than that the decrease in R_h after understory clearance. Therefore, when lime application and understory clearance was combined, the increased R_h by lime application can be partially offset by the reduction in R_h caused by understory clearance, and the other by the reduction in R_a caused by understory clearance. Given that both lime application and understory clearance had negligible effects on Eucalyptus root respiration, we believe that the combined lime application and understory clearance can inhibit soil respiration, mainly by reducing root respiration associated with understory vegetation. In the context of the prevalence of lime application and understory clearance in subtropical plantations (Moore and Ouimet, 2021; Wan et al., 2021), our results suggest that the combined effect of lime application and understory clearance should be considered to understand and predict carbon cycling in the plantation ecosystems.

Overall, lime application and understory removal, and their combined application reduced soil respiration, suggesting that both forest management practices have the potential to increase soil carbon stocks of subtropical Eucalyptus plantations. However, whether lime application or understory removal contributes to carbon sequestration in subtropical Eucalyptus plantation requires further study. Soil microbial abundance and activity generally have positive effects on plant growth, however in this study, lime application and understory significantly reduced soil microbial biomass. Therefore, we advise that plant productivity and soil carbon stocks should be considered when taking lime application and understory removal measures in subtropical Eucalyptus plantations.

In conclusion, lime application inhibited soil respiration by decreasing the root respiration of understory vegetation. Understory clearance reduced soil respiration by inhibiting both soil heterotrophic respiration and understory root respiration. The interaction of lime application and understory clearance resulted in a reduction in soil respiration, which was mainly due to the decrease in understory root respiration. These results demonstrate that the effects of lime application on soil respiration are highly modulated by understory vegetation, highlighting the important role of understory vegetation in regulating soil carbon balance in subtropical Eucalyptus plantations. Given the critical role of soil respiration in regulating soil carbon cycling, our findings will contribute to understanding and predicting the effect of forest management practices on soil carbon storage and global carbon cycling in a changing climate.

The original contributions presented in this study are included in the article/Supplementary material, further inquiries can be directed to the corresponding author.

JL performed the study data collection, analyzed the results, wrote the original draft, and reviewed the manuscript. RL performed the formal analysis and visualization. JX analyzed the results and reviewed the manuscript. SF designed the experiment, reviewed the manuscript, and wrote the final manuscript. SW designed the experiment, writing—review and editing, and acquired funding. All authors contributed to the article and approved the submitted version.