The karst landscape, one of the most fragile terrestrial ecosystems occupying 15–20% of the word’s land area, has been undergone severe soil/land degradation (rocky desertification) and re-vegetation or ecological restoration is thus vital to halt rocky desertification in karst regions. Karst soil has a relatively high pH, a high Ca and Mg content, with remarkably abundant organic matter. Numerous algae, lichens, mosses, ferns, grasses, and woody plants growing on shallow karst soils enhance rock weathering and soil fertility, as well as maintaining the micro-biota. A successful re-vegetation can therefore be manipulated by interactions among plants, microbes and soil in the harsh karst habitats. However, small- or local-scale studies on biophysical and biogeochemical processes are rarely combined with structural ecosystem properties (Silva and Lambers 2021), resulting in a lack of understanding of plant-microbe-soil interactions at an ecosystem level. As a result, we focused on plant-microbe-soil interactions within the process of re-vegetation under rocky karst desertification and gathered studies with state-of-art technologies, including stable and radioactive isotopes, molecular biological techniques, physical probes (heat pulse or diffuse), and even remote-sensing technology and big data or meta-analysis. This special issue composes 13 studies resulting from 57 manuscript submissions; we briefly introduce these studies as follows.

Impact of land surface characteristics on plant-soil processes

Six studies in this special issue focused on how land surface characteristics (such as soil characteristics, topography, vegetation and hydrological process) can modulate plant-soil interactions. Meng et al. (2022) show that a continuous soil catena along a toposequence in a dolomite peak-cluster depression catchment has led to a gradual transition of vegetation communities (herbs─herbs, shrubs─shrubs, trees─herbs and trees─trees, etc.). Liu et al. (2021) show that soil depth, species diversity, and different species combinations are significant predictors of plant productivity in herbaceous karst communities. Species diversity positively affects productivity at all three soil depths (0–5 cm, 5–15 cm and 15–30 cm), with the effect size of species diversity being greatest in the medium-depth soil. A complementarity effect (effect of positive interactions among species) would be the major mechanism by which diversity increased community productivity, particularly on the shallow soil. More attention should hence be paid to promoting species diversity with decreasing soil depth for adequate preservation of karst key functions. Slope gradient, fine soil mass ratio and nutrient stocks are important factors affecting plant diversity in karst areas. In a rock hill or valley, the side with the same or opposite slope direction and rock inclination is called a dip or anti-dip slope, where in general the bedrock is bare and the soil is scattered in both slopes. Xiao et al. (2021) observed different responses of ecohydrological process in the re-vegetation area between the dip and anti-dip slopes in a southwestern China rocky karst desertification area. They suggest that, considering the specific water-soil conservation and land carrying capacity, diverse vegetation restoration schemes and treatment measures should be implemented between the dip and anti-dip slopes. That is, natural restoration or priority restoration of pioneer species for vegetation growth (mosses, algae) instead of prickly ash trees should be selected for dip slopes; reasonable planting densities and precise fertilization should be managed to avoid land degradation and dissolved inorganic carbon (DIC) loss caused by excessive development in anti-dip slopes. Zhu et al. (2021) show significant differences in soil carbon (C) sequestration between karst and non-karst regions, with faster C sequestration occurring in karst regions. The rate of soil C change (Rs) was approximately 31% greater in karst than in non-karst soils, suggesting a significant increase in soil C accumulation following vegetation restoration. Yang et al. (2021) identify significant effects of land uses on nitrogen (N) transformation rates. Compared with natural grasslands, inorganic N supply capacity was increased in the cash crop pitaya plantations, while it was decreased in the Eucalyptus and corn plantations, in southwestern China rocky karst desertification area. Their results indicate that the application of organic fertilizer to agricultural plantations might be an effective practice for increasing labile organic C and improving soil structure to accelerate N cycling and inorganic N supply. In addition, Jiang et al. (2022) provided isotope-based practical recommendations for characterizing root water uptake in karst regions. They emphasized that the application of δD and δ18O isotope-based root water uptake analyses involving cryogenic water extraction have to be adapted to the specific properties of karst regions since isotopic deviations are caused by the cryogenic water extraction and the impediments associated with the isotopic calibration process.

Plant-microbe-soil interaction under re-vegetation processes

Wang et al. (2021) indicate that soil bacterial and fungal diversities and compositions changed significantly during secondary succession in karst areas. Microbial compositions are driven by both plant and soil properties. With an increase in site age, bacterial diversity slightly decreases, while fungal diversity first increases and then decreases. For specific plant species in karst areas, Tang et al. (2021) found that rhizosphere fungal communities contribute to acclimation of Themeda japonica (a common perennial grass in karst regions of southwest China) and promote plant growth and ecological performance, slowing down rocky karst desertification. The growth of a deciduous shrub Broussonetia papyrifera, which has been selected as a typical tree species for afforestation of the rocky karst desertification ecosystem of Southwest China, is mainly limited by phosphorus (P) availability. Rhizosphere soil P concentration is the most significant factor leading to changes in fine root and leaf stoichiometric characteristics of B. papyrifera (Hu et al. 2020). Tian et al. (2020) indicate that recalcitrant P can be transformed into more labile fractions by soybean rhizosphere acidification in karst soils, even when P fertilizer is applied. A consequence is that P fertilization may suppress the growth of bacteria (Bacillales and Pseudomonadales) that contribute to P mobilization, due to a reduced demand on these taxa to release P. Moreover, as a key role in the initial stage of re-vegetation, moss biocrusts have a positive effect on all soil nutrients and buffer the negative effects of rocky karst desertification progression compared with bare soil, which might be adopted as a supplementary method in promoting ecological restoration to rocky karst desertification (Cheng et al. 2020).

Large-scale approaches promoting our understanding of plant-soil interactions

A global perspective of the karst ecological restoration (KER) publication characteristics, topic trends, and knowledge domains, based on the number of annual publications among 319 field and/or laboratory publications from 1991 to 2021, divided KER into the germination (GP), slow growth (SGP) and rapid growth (RGP) phases (Liu et al. 2022). The study/publication topics varied and flourished over time. From hydrology, plant, and vegetation characters in the GP to the SGP and RGP using remote sensing and model as the main study methods, especially for assessing vegetation recovery, vegetation dynamics under climate change, and the dynamics of C and N. In addition, new study methods including high-throughput sequencing and stoichiometry in the SGP and stoichiometry and isotopic tracing in the RGP have also emerged (Liu et al. 2022). Huang et al. (2021) investigated the greening trends in global karst areas from 2001 to 2020 by Seasonally Integrated Normalized Difference (SINDVI) and suggested that anthropogenic activities are mainly responsible for the increase of vegetation greenness in tailoring management measures (e.g., Ecological Engineering, the Grain to Green Project) in China, Europe, and Middle East. Coupling warming temperature and increasing precipitation in southeastern Asia and Russia show increasing trends in SINDVI. Climate factors are generally dominant drivers for variation in vegetation greenness in karst regions globally during the past two decades. Zhu et al. (2021) address the influence and potential mechanisms of vegetation restoration on soil C sequestration in karst and non-karst regions based on multiple sources of literature and remote sensing dataset analyses. Unexpectedly, a faster and more persistent C sequestration occurred in karst regions, which is attributed to differences in climate gradients and soil N between karst and non-karst regions. The Marschner Review of Silva and Lambers (2021) have pointed out that the challenge for scientists and policy makers to scale up the effect of ecological interactions quantified at small spatiotemporal scales remains difficult. Therefore, studies based on state-of-art technologies (e.g., remote-sensing, models, meta-analysis) would allow a large-scale perspective on studies of plant-soil interactions. In summary, a conceptual diagram showing a multiple scale integration of plant-microbe-soil interactions with a range of diverse approaches has been proposed to further enhance re-vegetation and slow down rocky karst desertification in the global karst areas (Fig. 1).

Fig. 1
figure 1

A conceptual diagram showing a multiple scale integration of plant-microbe-soil interactions with a range of diverse approaches to study plant-microbe-soil interactions at a large-scale. The mechanism of soil carbon (C) sequestration processes in karst areas was revised from Zhu et al. (2021). Anti-slope, the side with the opposite slope direction and rock inclination of a dip slope in a rock hill or valley; BD, soil bulk density; Dip slope, the side with the same slope direction and rock inclination in a rock hill or valley; N, soil nitrogen; P, soil phosphorus. The upward, downward, and horizontal arrows represent increase, decrease, and no change in the corresponding variables, respectively. The plus and minus signs next to the arrows between the variables indicate the positive and negative effects, respectively. The World Karst Map V3.0 was obtained from Geography and Environmental Science, University of Auckland (https://www.fos.auckland.ac.nz/our_research/karst/index.html)

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