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Bacteria, fungi, and protists occupy a pivotal position in maintaining soil ecology. Despite limited knowledge on their responses to managed vegetation restoration strategies in karst regions, we aimed to study the essential microbial communities involved in the process of vegetation restoration. We compared microbial characteristics in four land use types: planted forests (PF), forage grass (FG), a mixture of plantation forest and forage grass (FF), and cropland (CR) as a reference. Our findings revealed that the richness of bacteria and protists was higher in FF compared to PF, while fungal richness was lower in both PF and FF than in CR. Additionally, the bacterial Shannon index in FF was higher than that in CR and PF, while the fungal and protist Shannon indices were similar across all four land use types. Significant differences were observed in the compositions of bacterial, fungal, and protist communities between FF and the other three land use types, whereas bacterial, fungal, and protist communities were relatively similar in PF and FG. In FF, the relative abundance of bacterial taxa Acidobacteria, Firmicutes, and Gemmatimonadetes was significantly higher than in PF and CR. Fungal communities were dominated by Ascomycota and Basidiomycota, with the relative abundance of Ascomycota significantly higher in FF compared to other land use types. Regarding protistan taxa, the relative abundance of Chlorophyta was higher in FF compared to CR, PF, and FG, while the relative abundance of Apicomplexa was higher in CR compared to FF. Importantly, ammonium nitrogen, total phosphorus, and microbial biomass nitrogen were identified as key soil properties predicting changes in the diversity of bacteria, fungi, and protists. Our results suggest that the microbial community under FF exhibits greater sensitivity to vegetation restoration compared to PF and FG. This sensitivity may stem from differences in soil properties, the formation of biological crusts and root systems, and management activities, resulting in variations in bacterial, fungal, and protist diversity and taxa in PF. As a result, employing a combination restoration strategy involving plantation forest and forage grass proves to be an effective approach to enhance the microbial community and thereby improve ecosystem functionality in ecologically fragile areas. [ABSTRACT FROM AUTHOR]

Considering the crucial role of soil diazotrophs and root arbuscular mycorrhizal fungi (AMF) in soil nutrient cycling during ecosystem restoration, diazotroph and AMF communities may be determined by slope position. However, the effect of slope position on diazotroph and AMF abundance, diversity, and community composition of karst ecosystems remains unknown. In this study, soil diazotrophs and root AMF characteristics on varying slope positions were assessed in a karst shrub ecosystem. The results displayed that the abundance of soil diazotrophs and root AMF diversity were significantly affected by slope position. Diazotroph abundance accompanied by soil nutrient and plant richness was higher on the lower slopes than the upper slopes, whereas root AMF diversity displayed the opposite trend. The soil diazotroph and root AMF community composition differed among the upper, middle, and lower slopes. The dominant taxa of soil diazotrophs and root AMF at the order level were Rhizobiales and Glomerales, respectively. Moreover, the diazotroph order of Nostocales and the AMF order of Paraglomerales were richer on the upper slopes than on the lower slopes. The slope position directly affected the plant diversity and soil nutrient distribution, indirectly affecting the diazotroph and AMF communities. Increased available nitrogen on the lower slope caused great diazotroph abundance by stimulating plant growth with sufficient carbohydrates. However, low soil nutrients and plant diversity but high plant root biomass induced more root AMF diversity on the upper slope than on the lower slope. Therefore, this study expands the knowledge of soil diazotroph and root AMF ecological functions along different slope positions during vegetation recovery for the successive stages of grass and shrub in the karst region. [ABSTRACT FROM AUTHOR]

Lithology has such an important effect on the sustainability of soil carbon (C) pools. Forests are an important part of terrestrial C sinks; yet, it is unclear whether their soil carbon sensitivity to temperature changes is regulated by lithology, especially in karst ecosystems, which are widely distributed globally. Along a climate gradient in the subtropical region of southwest China, we compared the contents of soil organic C (SOC) and total N (TN) in karst and non-karst forests. The data were analyzed and processed using ANOVA, regression analysis, and random forest. The results showed that the karst forests had significantly higher SOC and TN contents but lower ratio of SOC to TN (C:N) than non-karst forests, mainly because of the higher soil calcium (Ca) content and microbial biomass. With rising mean annual temperature (MAT), SOC and TN contents in non-karst forests significantly decreased, whereas in karst forests they were not correlated with MAT; while, the opposite was true for C:N. In karst forests, soil Ca constrain warming induced decomposition of SOC and TN by forming stable complexes with SOM through exchangeable Ca, and by promoting aggregate stability through the role of calcium carbonate. The correlation between δ13C and the logarithm of SOC concentration also supported that conclusion. In karst forests compared to non-karst forests, soil C pools are larger and less sensitive to regional temperature change. Nevertheless, climate warming may still accelerate soil C loss in karst forests by increasing microbial C limitation. Thus, soil C sequestration potential and loss risk coexist in karst areas. The ratio of SOC to TN (C:N) is regulated through appropriate management measures in the process of karst vegetation restoration, thus promoting long-term stable sequestration of soil carbon pools. [ABSTRACT FROM AUTHOR]

Knowledge of water movement from the hillslope critical zone is important for water management. However, the runoff process resulting from extreme rainfall events remains unclear due to rare field‐based natural monitoring data. We investigated rainwater infiltration and runoff generation processes in two plots with deep and relatively shallow soil thicknesses (66 vs. 35 cm depth on average, respectively) on karst hillslopes. Rainwater, soil water and lateral discharge runoff in the soil‐epikarst system were monitored using hydrometric and 18O isotope methods in September 2020, which was the wettest month on record at the study s, including three normal rainfalls (29.6 mm in 8.5 h, 25 mm in 8.5 h, and 27.4 in 3.5 h) and three extreme rainstorms (335 mm in 11.5 h, 77.8 mm in 9 h, and 129.2 mm in 5 h). Soil moisture and water table responded rapidly (<30 min) during the extreme rainfall. However, the soil‐epikarst interface exhibited the strongest stability (coefficient variation [CV] < 8.18%) and collected more pre‐event water, reflected by the isotopic signature of the soil water. Accordingly, low permeability at the soil‐epikarst interface played an important role in buffering rainwater infiltration. Controlled by infiltration‐excess and saturation‐excess runoff mechanisms, the contribution of surface runoff to total discharge was less than 3% during normal rainfall events, which was significantly less than that during extreme rainfall events (13%–60%). This was supported by the evolution of δ18O values of surface runoff that were similar to rainwater and more 18O‐depleted than that of subsurface runoff. Higher rainfall intensity which exceeded soil permeability during extreme rainstorms further increased the possibility of exceeding infiltration‐saturation processes. For the deep soil plot (DSP), surface runoff (>44%) dominated the extreme rainfall process; while for the shallow‐soil plot (SSP), surface runoff (60%) dominated only during the maximum rainfall events. Moreover, the surface runoff contribution to total runoff in DSP was greater than that in SSP. Surface runoff from slopes with thicker soil layers is an important part of karst watershed flood processes in extreme rainfall events, which should be considered carefully for water resource management. Highlights: Hydrometric and isotope methods are used to assess extreme hydrological processes.The surface runoff of extreme rainfalls is much higher than that of normal rainfall.The infiltration‐excess mechanism dominates the surface runoff of extreme rainfall.Slopes with thicker soil layers are more vulnerable to flooding in the karst region. [ABSTRACT FROM AUTHOR]

Soil microbial communities play a crucial role in regulating multiple ecosystem functions. However, the specific influence of functional microbiomes, such as protists, bacteria, and fungi, and their interactions in response to ecosystem multifunctionality along slope positions remains largely unknown. We investigated the relationship between ecosystem multifunctionality and microbial index in the downhill, middle, and upper slopes of karst ecosystems. Compared to the upper slope, ecosystem functions related to microbial biomass, nutrient storage, plant-soil mutualism, plant productivity, and multifunctionality were higher in the downhill position. Bacterial and fungal abundance showed higher levels in the downhill position and middle slope than in upper slope. However, the composition and diversity of protists, bacteria, and fungi did not show significant differences across the downhill, middle, and upper slopes. Co-occurrence network analysis revealed that the downhill position had a higher microbial network complexity among bacterial taxa (e.g., Proteobacteria and Actinobacteria), fungal taxa (e.g., Ascomycota and certain unclassified taxa), and protistan taxa (e.g., Conosa and certain unclassified taxa) compared to the upper slope. Furthermore, bacterial diversity and network properties (e.g., the number of nodes, edges, and modules) were positively associated with ecosystem multifunctionality. These findings suggest that microbial network complexity is a key driver of ecosystem multifunctionality, indicating the crucial role of interactions among protistan, bacterial, and fungal taxa in soil functioning. Consequently, this study highlights the importance of microbial networks in maintaining the sustainability of ecosystems, emphasizing their stronger implications compared to biodiversity alone. [Display omitted] • Higher bacterial and fungal abundance in the downhill position than in the upper slope. • Microbial diversity and community composition were similar across the slope positions. • Increased ecosystem multifunctionality and network complexity in the downhill position. • Microbial network complexity is the most biotic predictor on multifunctionality. • Highlight the crucial role of microbial interaction in ecosystem multifunctionality. [ABSTRACT FROM AUTHOR]

Summary: Root access to bedrock water storage or groundwater is an important trait allowing plant survival in seasonally dry environments. However, the degree of coordination between water uptake depth, leaf‐level water‐use efficiency (WUEi) and water potential in drought‐prone plant communities is not well understood.We conducted a 135‐d rainfall exclusion experiment in a subtropical karst ecosystem with thin skeletal soils to evaluate the responses of 11 co‐occurring woody species of contrasting life forms and leaf habits to a severe drought during the wet growing season.Marked differences in xylem water isotopic composition during drought revealed distinct ecohydrological niche separation among species. The contrasting behaviour of leaf water potential in coexisting species during drought was largely explained by differences in root access to deeper, temporally stable water sources. Smaller‐diameter species with shallower water uptake, more negative water potentials and lower WUEi showed extensive drought‐induced canopy defoliation and/or mortality. By contrast, larger‐diameter species with deeper water uptake, higher leaf‐level WUEi and more isohydric behaviour survived drought with only moderate canopy defoliation.Severe water limitation imposes strong environmental filtering and/or selective pressures resulting in tight coordination between tree diameter, water uptake depth, iso/anisohydric behaviour, WUEi and drought vulnerability in karst plant communities. [ABSTRACT FROM AUTHOR]

Understanding hydrological processes is essential for management of water resources and for promoting catchment sustainability. In karst regions, landscapes are spatially heterogeneous and include discontinuous soil distribution, and complex networks of matrices and conduits in hillslopes and depressions, which result in variation in hydrological processes. However, most previous studies have mainly focused on the effects of the distribution of soil depth and the fast‐slow flow in the matrices and conduits on hydrological processes and ignored the different hydrological processes related to hillslopes and depressions. This study improved the VarKarst model by adding randomly distributed soil and epikarst depths (RSE), fast‐slow flow (FS), and hillslopes and depressions (HD). The improved model was calibrated and validated in six large catchments (1,213–5,454 km2) and one small catchment (1.25 km2). Results showed that the schemes by combining FS and HD (Scenario FS + HD) and combining RSE, FS, and HD (Scenario RSE + FS + HD) improved model performance (calibrated and validated KGE ranged from 0.54 to 0.89 and AIC ranged from −336.49 to 669.77) compared models that included other schemes (original VarKarst, Scenario RSE, Scenario FS, Scenario HD, and Scenario RSE + FS), especially when reproducing discharge of peaks and recessions. These results suggest that there is a need to separate the hillslopes and depressions when modeling karstic hydrological processes. Key Points: Hillslopes and depressions have different hydrological processes in karst regionsImproved VarKarst model better reproduced discharge of peaks and recessions compared to the original modelThere is a need to separate the hillslopes and depressions in modeling hydrological processes in karst regions [ABSTRACT FROM AUTHOR]

Evidence shows that nitrogen (N) addition may suppress or have no significant effect on asymbiotic N2 fixation (ANF) in soil, litter, or moss. However, the mechanisms underlying the differential responses of ANF to N addition are not well understood. The main objectives of the current study were to assess how ANF in soil, litter, and moss responds to N addition and whether topography modulates the responses of ANF in soil, litter, or moss to N addition. We conducted an N addition experiment at two topographic positions, that is, valley and slope of a secondary karst forest in southwest China. Three N addition treatments, that is, control (0 kg N·ha−1·year−1), moderate N addition (N50, 50 kg N·ha−1·year−1), and high N addition (N100, 100 kg N·ha−1·year−1) were included. Nitrogen addition had no significant effect on moss ANF at both topographic positions. Soil ANF was lower by 17.1% in the N100 plots relative to the control in the valley but was not significantly altered by N addition on the slope. In contrast, litter ANF was suppressed by N addition at both topographic positions by 77.9% to 87.4%. Our findings suggest that topography may modulate the responses of ANF to N addition, but the modulation effects likely differ among different ecosystem compartments. Plain Language Summary: Asymbiotic N2 fixation (ANF), a process carried out by free‐living N2‐fixing microorganisms to convert atmospheric N2 to ammonia, is an important source of new nitrogen for terrestrial ecosystems. Atmospheric nitrogen deposition has been found to suppress ANF but much uncertainty still exists. We evaluated the effects of nitrogen addition on ANF in soil, litter, and moss at two topographic positions, that is, valley and slope of a subtropical secondary forest in southwest China. We find that ANF in soil was inhibited by nitrogen addition in the valley but not on the slope, whereas ANF in litter was suppressed by nitrogen addition irrespective of topographic positions. ANF in moss was not affected by nitrogen addition. Our results imply that the responses of ANF to nitrogen addition depend on both topographic position and substrate. Key Points: N2 fixation in soil was inhibited by N addition in the valley but not on the slopeN2 fixation in litter was suppressed by N addition in the valley and on the slopeN2 fixation in moss was not affected by N addition in the valley or on the slope [ABSTRACT FROM AUTHOR]

Purpose: Tillage disturbance can significantly affect soil microbial metabolic activity and CO2 fluxes. Nevertheless, the influence of different tillage frequencies on microbial metabolic activity and short-term temporal changes of CO2 fluxes remains unclear. Materials and methods: We established an in situ experiment with the following treatments: no tillage (T0), semiannual tillage (T1), tillage every 4 months (T2), bimonthly tillage (T3), and monthly tillage (T4). The microbial metabolic activity (Biolog EcoPlate), short-term (hours to days) temporal changes in CO2 fluxes within 1 week, and soil properties were measured after 1 year of treatment. Results and discussion: The highest CO2 emissions occurred in the first 72 h after tillage treatment and were significantly higher in T3 and T4 than in T0, T1, and T2 within 1 week. Average well color development (AWCD) values reflect microbial metabolic activity and were significantly higher in the tillage treatments (T1, T2, T3, and T4) than under no tillage. There was no significant difference in the Shannon diversity index under all treatments. A higher Simpson diversity index was observed under high tillage frequency in T2, T3, and T4 compared with T0 and T1, while the highest was observed in T2. The highest utilization of carboxylic acids, amino acids, and polymers occurred in T3 and T4 soils, whereas T2 had the highest utilization of carbohydrates, amines, and miscellaneous carbon sources. AWCD values and short-term CO2 fluxes were significantly correlated with annual changes in soil organic carbon (△SOC), annual changes in dissolved organic carbon (△DOC), microbial biomass carbon (MBC), and large macroaggregates (> 1 mm). Conclusions: These results suggest that frequent tillage disturbance increases microbial metabolic activity, which can stimulate short-term CO2 emissions through changes in soil aggregates, SOC, DOC, and MBC. [ABSTRACT FROM AUTHOR]

• AMF richness increased at high temperature levels in shrubland and mature forest. • AMF community was more sensitive to climatic levels than that of diazotrophs. • High temperature and precipitation strengthened AMF-diazotroph interactions. • Glomus and Bradyrhizobium contributed to the most correlation links in co-occurrence patterns. Arbuscular mycorrhizal fungi (AMF) and diazotrophs have the potential for nutrient transfer and biological nitrogen fixation in ecosystems, respectively. However, their response to vegetation restoration remains unclear, especially under varying temperature and precipitation levels in karst ecosystems. This study aimed to understand the effects of three climatic levels within four natural and managed vegetation restoration types on the diversity and community composition of AMF and diazotrophs. The interactive effects of temperature, precipitation, and vegetation types affected AMF diversity, while diazotroph diversity was not affected. Under conditions of natural vegetation restoration, there was an increase in AMF diversity in response to increasing temperature and precipitation. AMF richness was higher in shrubland and mature forest than in cropland when temperatures were over 20 ℃ and precipitation was high. Thus, in terms of diversity, AMF were more responsive to changes in climatic conditions and vegetation recovery than diazotrophs. Both AMF and diazotroph community compositions were affected by temperature and vegetation type. The relative abundances of AMF groups (e.g., Gigaspora , Glomus , and Septoglomus) and diazotroph taxa (e.g., Frankia) increased at temperatures above 18 ℃. The relative abundances of the AMF genus Glomus and the diazotroph genus Bradyrhizobium in shrubland and mature forest were higher than those in cropland, while abundances of the AMF genus Septoglomus and diazotroph genus Anabaena increased in cropland. Network complexity increased with increasing temperature and precipitation between AMF and diazotroph taxa. Glomus and Bradyrhizobium showed the most links with other groups, confirming that the dominant genera perform well in the co-occurrence network. These results suggested high hydrothermal regions resist rapid nutrient decomposition by strengthening the interactions between AMF and diazotrophs, especially between the abundant groups Glomus and Bradyrhizobium. Management to increase AMF and diazotroph abundance during vegetation recovery in high climate level may stimulate nutrient absorption and transport. [ABSTRACT FROM AUTHOR]

Although many land-use patterns have been established to restore vegetation and eliminate poverty in the karst area in southwest China, the ecosystem services (ESs) of these patterns are still not fully understood. To compare the differences in seven typical monoculture patterns and three agroforestry patterns, their ESs and tradeoffs were analyzed within the Millennium Ecosystem Assessment Framework. Compared with the local traditional corn pattern, the marigold pattern improved provisioning, regulating, and cultural services by >100%. The pomegranate pattern provided far more provisioning services than the other patterns. The apple + soybean intercropping pattern reduced regulating services, and eventually, its Total ESs (TES) and ecosystem multifunctionality index (EMF) also decreased. Cultural services will be enhanced by the introduction of fruit trees, as well as intercropping. Orange + peach had the greatest negative tradeoffs between provisioning and regulating services (P-R), provisioning and supporting services (P-S), and provisioning and cultural services (P-C), which indicates that the provisioning services urgently require improvement. Peach + pumpkin intercropping decreased the negative tradeoffs of P-R, P-S, and P-C (all > 10%), while pomegranate + grass intercropping increased the negative tradeoffs of R-S and R-C (all > 100%). Our results suggest that all six of these patterns are worthy of promotion but the pomegranate pattern should be given priority. Among the three intercropping patterns studied herein, the apple + soybean pattern should be redesigned to improve performance. [ABSTRACT FROM AUTHOR]

Understanding the driving factors of forest biomass are critical for further understanding the forest carbon cycle and carbon storage management in karst forests. This study aimed to investigate the distribution of forest aboveground biomass (AGB) and the effects of stand structural and abiotic factors on AGB in karst forests in Southwest China. We established a 25 ha plot and sampled all trees (≥1 cm diameter) in a subtropical mixed evergreen–deciduous broadleaf forest. We mapped the forest biomass distribution and applied a variation of partitioning analysis to examine the topographic, stand structural, and spatial factors. Furthermore, we used structural equation models (SEM) to test how these variables directly and/or indirectly affect AGB. The average AGB of the 25 ha plot was 73.92 Mg/ha, but that varied from 3.22 to 198.11 Mg/ha in the 20 m × 20 m quadrats. Topographic, stand structural, and spatial factors together explained 67.7% of the variation in AGB distribution. The structural variables (including tree density and the diameter at breast height (DBH) diversity) and topographic factors (including elevation, VDCN (vertical distance to channel network), convexity, and slope) were the most crucial driving factors of AGB in the karst forests. Structural equation models indicated that elevation, tree density, and DBH diversity directly affected AGB, and elevation also indirectly affected AGB through tree density and DBH diversity. Meanwhile, AGB was indirectly influenced by VDCN, convexity, and slope. The evaluation of stand structural and abiotic drivers of AGB provides better insights into the mechanisms that play a role in carbon storage in karst forests, which may assist in improving forest carbon management. [ABSTRACT FROM AUTHOR]

Soil nitrogen (N) and phosphorus (P) shortages limit the growth of shrubs, and P shortage limit the growth of trees in karst ecosystems. Changes in fine root functional traits are the important strategies for plants to respond to such nutrient shortages. However, such responses in karst ecosystems are poorly known. To determine the responses of fine root functional traits to soil N and P changes and define their resource-use strategies in the ecosystem, we tested the specific root length (SRL), root tips over the root biomass (RT/RB), and N concentration (Nroot) in the fine roots of four plant species (two shrubs (Alchornea trewioides and Ligustrum sinense) and two trees (Celtis biondii and Pteroceltis tatarinowii)) during the dry (January) and the wet (July) season. The results showed that the SRL, RT/RB, and Nroot in the fine roots of shrub species were lower than those of tree species, and the three parameters were higher in the wet season than in the dry season. Linear regression models revealed that the SRL, RT/RB, and Nroot of overall species increased with increasing soil N and P concentrations and availabilities, and were positively correlated with increasing rhizosphere soil oxalic acid, microbial biomass carbon (C), and the activities of hydrolytic enzymes. In addition, the individual plant species had unique patterns of the three fine root traits that resulted affected by the change of soil nutrients and biochemistry. Thus, the specific root length, root tips over the root biomass, and N concentrations of fine roots were species-specific, affected by seasonal change, and correlated with soil nutrients and biochemistry. Our findings suggests that fine root functional traits increase the ability of plant species to tolerate nutrient shortage in karst ecosystems, and possibly indicated that a P-exploitative strategy in tree species and an N-conservative strategy in shrub species were exhibited. [ABSTRACT FROM AUTHOR]

The cosmic-ray neutron sensing (CRNS) is an emerging method for continuously monitoring soil water content (SWC) at an intermediate scale. However, when multiple hydrologic units are present within its footprint, the potential application of CRNS in water resources management is restricted. Here we propose a new strategy to predict point-scale SWC in root zone established on CRNS-based soil moisture and improved relative difference method. A total of 768 days of soil moisture data were collected by CRNS at the intermediate scale and EC-TM sensors at the point scale in a karst catchment. The original and improved mean relative difference methods predicted point-scale SWCs within and without the effective measuring depth, respectively. The mean effective measuring depth was 13.16 cm, ranging from 10.13 to 19.23 cm. Both land use type and soil structure played essential roles in regulating point scale SWC in the soil profile. Point-scale SWC in root zone can be predicted accurately (P < 0.001) based on SWC data derived from the CRNS system. The prediction accuracy of point scale SWC can be improved by increasing the averaging time of the soil moisture values. Our results demonstrated that the proposed strategy was reliable for CRNS to predict SWC beyond the effective measurement depth. This study provides a good perspective for effectively managing of water resources in areas with complex hydrological processes. • Both land use type and soil structure have important effects on soil moisture. • CRNS can accurately predict deep soil moisture at point scale. • The prediction accuracy of soil moisture can be improved by increasing averaging time. • CRNS is an effective tool for water resources management in small watershed. [ABSTRACT FROM AUTHOR]

• Runoff and sediment yield responded to karst distribution patterns. • KC, COHESION, LPI, and SPLIT dominantly influenced runoff generation. • A highly fragmented carbonate rock landscape effectively increased runoff. • Distribution patterns explained more variations in runoff than sediment yield. Aquatic karst landscapes are highly heterogeneous and vulnerable to environmental conditions due to carbonate rock dissolution by flowing water, freshwater movement and sediment transport. However, the landscape metrics commonly used in landscape ecology do not consider the spatial distribution of carbonate rocks in karst watersheds because these metrics are derived from land use/land cover maps. Therefore, this study investigated the effects of distribution patterns of karst landscapes on annual runoff and sediment yield in 40 heterogeneous karst watersheds in southwest China. The results show that the distribution patterns of karst landscapes significantly influenced annual runoff and sediment yield. A general linear model explained 74% of the runoff variations, indicating that the distribution pattern was the principal factor affecting runoff. Although these patterns significantly affected sediment yield, the general linear model accounted for only 52% of the sediment yield variations. Thus, the distribution patterns showed larger effects on runoff than sediment yield. This may be because landscape metrics of karst distribution patterns could not fully reflect the three-dimensional (3D) hydrogeological structure of karst watersheds. It is necessary to investigate the effects of 3D hydrogeological structures and hydrological and sediment connectivity on sediment yield. This will facilitate a more in-depth understanding of soil erosion mechanisms in karst watersheds. [ABSTRACT FROM AUTHOR]

Soil depth plays a crucial role in shaping the interactions between soil microbes and nutrient availability. However, there is limited understanding of how bacterial, fungal, and protistan communities respond to different soil depths, particularly in the unique geological context and soil properties of karst regions. Organic matter, total nitrogen, and phosphorus, ammonium, nitrate, and plant root biomass, as well as bacterial and fungal abundances, bacterial and protistan diversity were higher in the 0–20 cm soil layer than those in the 20–40 cm and soil-rock mixing layers. In contrast, soil pH was higher in the 20–40 cm and soil-rock mixing layers than that in the 0–20 cm soil layer. The soil exchange of calcium, nitrate, and root biomass were identified as the primary factors regulating microbial assemblages across the depth transect. Moreover, co-occurrence network analysis revealed a greater degree of connectivity between protistan taxa and fungal taxa in the 0–20 cm soil layer than those in the 20–40 cm and soil-rock mixing layers. In contrast, the number of association links between protist-bacteria and bacteria-bacteria was higher in the soil-rock mixing layers compared to the 0–20 cm soil layer. Actinobacteria, Ascomycota, and unclassified protistan taxa were identified as keystones, displaying the highest number of connections with other microbial taxa. Collectively, these results suggested that the increased plant root biomass, coupled with sufficient available nutrient inputs in the upper 0–20 cm soil layer, facilitates strong interactions among fungal and protistan taxa, which play crucial roles in the topsoil. However, as nutrients become less available with increasing depth, competition among bacterial taxa and the predation between bacterial and protistan taxa intensify. Therefore, these findings indicate the interactions among keystone taxa at different soil depths has the potential to generate ecological implications during vegetation restoration in fragile ecosystems. [Display omitted] • Fungal abundance decreased faster than bacterial abundance with soil depth. • Strengthened interactions among fungal and protistan taxa in the 0–20 cm soil layer. • The link numbers between protist-bacteria were increased in the deep soil layer. • Interaction among microbial taxa was regulated by root biomass and soil nutrient. [ABSTRACT FROM AUTHOR]

• We identified sedimentary history of soil magnetic susceptibility (MS) in karst depression. • The vertical distribution of soil MS is significantly affected by tillage practice. • Total phosphorus is a good predictor of soil MS in undisturbed sediment cores. • Soil MS decreases with the increase of sediment deposition rates. Tillage practices can greatly influence the temporal variation in soil erosion, deposition, and redistribution, and thus soil magnetic susceptibility (MS). In-depth investigation of the distribution characteristics of soil magnetic susceptibility (MS) is helpful to understand the dynamic process of soil redistribution and environmental changes. With the aid of the 137Cs and 210Pb ex deposition chronologies, the objective of this study is to investigate the historical changing pattern of MS in a karst depression, and to further evaluate the effects of soil properties, tillage practice, and soil erosion on the temporal variation in soil MS. With collecting representative soil profile samples from cropland and undisturbed bamboo forest land in the karst depression, the MS, soil particle distribution, nutrients, radionuclides and heavy metals were measured. The results showed that mass-specific low-frequency MS (χ lf) was positively correlated with percentage frequency-dependent MS (χ fd %) (P < 0.001), and the corrected χ fd % was close to the saturation value (11 %) after removing the influence of the initial aeolian signal. The relationship between MS and soil properties is different under cropland and bamboo forest land. For undisturbed forestland, χ lf and χ fd % were both positively related to nutrients concentration (P < 0.001), but negatively related to heavy metal concentrations (P < 0.001). For the cropland with frequent tillage practices, χ lf was positively related to total phosphorus (TP) and heavy metals concentration. Based on the identified MS deposition history, from 1949 to 2015, χ lf and χ fd % showed a significant increase at first and then tended to be stable, and were negatively related to sediment deposition rate (P < 0.001). This study emphasizes the pronounced influence of tillage practices and soil erosion on the temporal variation of soil MS, is helpful for targeted land management strategies in these environmentally vulnerable karst regions. [ABSTRACT FROM AUTHOR]

[Display omitted] • Soil nutrients and diazotroph and arbuscular mycorrhizal fungi (AMF) abundance were high in the top soil; • Free-living nitrogen fixation (FLNF) activity increased from topsoil to soil-rock mixing layer; • Increased FLNF activity via strengthen cooperation between diazotroph and AMF taxa; • Diazotroph and AMF keystone taxa were the key predictor of biological nitrogen fixation. Soil nitrogen (N) is a renewable resource of N fixed by free-living N fixation (FLNF) diazotrophs. Thus, understanding the microbial driving mechanism of FLNF activity can aid in the optimization of N input. However, the role of co-symbiosis between diazotrophs and arbuscular mycorrhizal fungi (AMF) in the FLNF activity at different soil depths has been largely overlooked, particularly in karst ecosystems. Thus, we investigated soil properties and the characteristics of diazotroph and AMF across soil depths, from topsoil to soil-rock mixing layer, based on the soil profile. Soil properties such as soil organic matter and ammonium N decreased with increasing soil depth, whereas pH showed the opposite trend. Similarly, diazotroph abundance and diversity and AMF abundance were higher by 8–73% in 0–20 cm soil than in 20–40 cm and soil-rock mixing layer. Despite high diazotroph abundance in the topsoil, the FLNF activity was higher by 30% at soil-rock mixing layer than in 0–20 cm. The co-occurrence network analysis revealed a strengthening of the cooperative relationship between the diazotroph and AMF taxa at the soil-rock mixing layer via an increase in the number of unique AMF taxa. A structural equation model indicated that increasing soil depth improves the FLNF activity due to increasing soil pH and mutualistic cooperations between diazotrophs and AMF taxa, such as Bradyrhizobium to AMF unclassified taxa and Bradyrhizobium to Racocetra. This study provides novel insights into the interspecific interactions between diazotrophs and AMF, rather than their abundance and diversity, which were found to be the most important driving factors of FLNF activity at the soil-rock mixing layer. Consequently, the roles of biotic factors in influencing mutually beneficial microbial taxa regulating FLNF activity should be considered during vegetation recovery in the fragile karst region. [ABSTRACT FROM AUTHOR]

Mosses form a ground layer with a thickness of nearly 1 cm during the first decade of vegetation restoration, but their effects on the belowground microbial community and soil properties and the associated soil carbon (C) and nitrogen (N) accumulation in subtropical areas are unclear. Here, we measured soil C and N variables (soil organic C [SOC], total N [TN], dissolved organic C, ammonium [NH 4 +-N] and nitrate [NO 3 −-N]), soil microbial community and soil properties (soil water content [SWC] and pH) under four treatments (bare soil [BS], bare soil with transplanted moss, moss-covered soil [MS] and moss-covered soil with moss removed) in three vegetation types (forest plantations, forage grasslands and mixed plantations and forage grasslands) in a subtropical climate. We analyzed the effects of native mosses, moss transplantation and removal using BS and MS as references. One year post-treatment, moss transplantation increased NO 3 −-N and NH 4 +-N in the 0–5 cm and 2–5 cm soil layers, respectively. Conversely, moss removal decreased SOC and TN in the 0–2 cm soil layer and SWC in the 0–5 cm soil layer. Compared to BS and MS, native moss presence, moss transplantation and removal decreased the total microbial, bacterial and fungal biomass and altered the soil microbial community composition (ratios of fungi and bacteria and Gram-positive bacteria to Gram-negative bacteria) to varying degrees. Moss properties (biomass, C and N concentrations, C:N ratio and saturated water absorption content), as well as SWC, soil microbial biomass and community composition regulated by mosses affected soil C and N. These findings underscore the crucial role of mosses in facilitating soil C and N accumulation during vegetation restoration. • Both the presence and removal of mosses decreased soil microbial biomass. • Mosses altered soil microbial community composition. • Mosses promoted soil C and N accumulation by affecting soil microbial biomass and community composition. • Mosses are particularly important for soil C and N accumulation in forest plantations. [ABSTRACT FROM AUTHOR]

Abstract Soil carbon (C), nitrogen (N), and phosphorus (P) are some of the most important indicators of soil quality, especially in fragile ecological environments with shallow soils. However, the differences in their variations and influencing factors related to scale in karst areas remain unclear. Therefore, variations in surface soil organic carbon (SOC), total nitrogen (TN), and total phosphorus (TP) distributions and their determinant factors were quantitatively analyzed at different scales (i.e., region, sub-region, and catchment) in a typical karst area, southwest China. Overall, both soil nutrient variations and the determinant factors involved varied with scale. The coefficients of variation for SOC, TN, and TP increased significantly from catchment to sub-region scales but changed only slightly between sub-region and region scales. At region and sub-region scales, lithology only affected TP significantly, while land use affected all soil nutrients significantly. However, the explanations for TP variation by lithology and for SOC, TN, and TP variations by land use increased by 12.2, 7.0, 11.0, and 12.8% at sub-region scale over the values observed at region scale, respectively. When lithology and land use were controlled, soil depth was observed playing an important role in soil nutrients at the three studied scales. At region and sub-region scales, other factors related to soil texture (clay, silt, or sand content), topography (elevation or TWI), and climate (MAT or MAP) were selected as significant variables and accounted for a larger proportion of nutrient variation. However, at catchment scale, bare rock rate and micro-topography (e.g., slope) explained soil nutrient variation to a larger proportion. Therefore, elucidation of the role of factors influencing soil nutrients at different scales can provide a guide for soil quality assessment in karst areas. Highlights • Both soil nutrient variation and its determinant factors varied with scale. • Lithology explained 12.2% more of TP variation at sub-region than at region scale. • Land use explained soil nutrient variation best at the sub-region scale. • Bare rock rate affected cropland soil nutrients significantly at the three scales. [ABSTRACT FROM AUTHOR]

• Multiple runoff components in a soil-epikarst system were directly monitored. • Critical zone structure exerted more evident influence on carbon loss than land uses. • Thicker soils with less undulating soil-epikarst contact facilitated carbon loss. • Artificial forage grasslands reduced carbon loss. In karst hillslopes, the soil-epikarst system is not only the main reservoir of soluble carbon, but also the hot spots of hydrological and biogeochemical processes. However, the soluble carbon loss mechanism through various runoff components occurring in the soil-epikarst system is not clear. We hypothesize that the karst critical zone structure determines the runoff and soluble carbon loss, and that these response processes can be regulated by land use. 12 experimental plots on a typical karst hillslope (each with 5 × 20 m projected area) of four land uses (naturally recovered shrubland, forage grassland, economic forest land, and cropland) were intensively instrumented for separately monitoring surface, subsurface, and epikarst seepage runoffs and their dissolved organic carbon (DOC) and dissolved inorganic carbon (DIC) concentration. Results show that economic forest land had the highest total runoff and soluble carbon loss, followed by cropland, naturally recovered shrubland, and forage grassland. Generally, karst critical zone structure was the most important factors for controlling DOC and DIC loss through surface, subsurface, and epikarst seepage runoffs. Critical zones with thicker soil and less undulating soil-epikarst contact were more susceptible to runoff and soluble carbon loss. Although not as important as critical zone structures, land use can effectively regulate runoff and prevent soluble carbon loss by affecting rainfall partitioning in karst hillslopes. Planting forage grassland is an effective method of reducing carbon loss, due to its remarkable ability of reducing runoff in the soil-epikarst systems. Our results provide useful insights for determining appropriate land uses and optimizing soil and water conservation strategies in karst hillslopes. [ABSTRACT FROM AUTHOR]

• Subsurface runoff dominates the runoff components in karst hillslope. • Thinner soils decrease surface runoff and increase subsurface runoff. • Bedrock topography controlled subsurface runoff generation in the thinner soils. • Thinner soils had a higher contribution of new water than that in the thick soils. Hydrological processes in the critical zone are closely related to the soil–bedrock structures. However, the effect of soil thickness on the rainfall-runoff relationship on the hillslope with complex topography remains unclear. Surface runoff, lateral subsurface runoff from soil–epikarst interface, and epikarst runoff from the epikarst–bedrock interface were monitored on two adjacent plots with deep and shallow (66.0 vs. 35.4 cm) soil thicknesses from June 2019 to December 2020 in the karst region of southwest China. During the monitoring period, surface and subsurface runoff account for 20% and 37% of the total runoff in the deep-soil plot (DSP), and 3% and 43% in the shallow-soil plot (SSP). This demonstrates that runoff from the soil-epikarst system is predominant compared to the relatively small contribution of surface runoff. In the SSP, the surface topography wetness index (TWI) was highly coupled with bedrock TWI , and the bedrock TWI had a significant negative linear relationship (p < 0.01) with subsurface runoff. Moreover, isotope hydrogen-separation results showed that subsurface and epikarst runoff were dominated by pre-event water, but a higher contribution of event water was observed in the SSP than in the DSP. These findings supported the hypothesis that rainwater could infiltrate the epikarst more easily in shallow soil slopes. Rainfall and surface runoff exhibited a linear relationship in the dry season and a non-linear relationship in the rainy season, indicating the occurrence of threshold rainfall–runoff behavior. The rainfall amount threshold for surface runoff was higher in DSP (44.7 mm) than in SSP (39.5 mm), and the corresponding variation of rainfall intensity interpretation was greater (54% vs. 38%). For subsurface runoff, the rainfall amount threshold was higher in the DSP than in the SSP (91.0 vs 79.4 mm), and the corresponding variation of soil moisture interpretation was higher (56% vs. 20%). This demonstrated that runoff can be better predicted at deeper soil hillslopes by rainfall and antecedent soil moisture. Accordingly, this study emphasizes the importance of evaluating the spatial heterogeneity of soil thickness in hydrological process research. [ABSTRACT FROM AUTHOR]

Understanding the effects of straw return and N fertilization on soil organic matter (SOM) transformations will help maintain crop production and soil function, which can ultimately contribute to mitigating climate change. In this study, we conducted a 100-day soil incubation experiment with the addition of 13C-labeled maize straw and/or N fertilization. Soils that were used in the study included soil without fertilizers (Control), with mineral fertilizer alone (NPK), and with mineral fertilizer and straw (NPK+Straw). Compared with the control, the NPK- and NPK+Straw-treated soils showed higher straw decomposition by 59% and 55%, and SOM mineralization by 27% and 37%, respectively, although the priming effect was decreased by 59% and 39%, respectively. The priming effect (PE) was higher with increased N content and lower with decreased N additions because of an improved C/N ratio for microorganisms. Straw additions compared to without straw increased the bacterial and fungal abundance by 1.4- and 4.9-fold, respectively. N fertilization lowered C/N ratios resulting in decreased fungal diversity. Although the bacterial abundance decreased, the diversity increased with the duration of incubation as the bacteria preferred to utilize the labile organic compounds that were abundant in the initial stages of incubation. In addition to the depletion of labile organic compounds, the fungal abundance increased. Bacteria (Firmicutes, Actinobacteria, and Proteobacteria phyla) and fungi (Ascomycota, Basidiomycota, and Mucoromycota phyla) dominated straw and SOM decomposition. Firmicutes were mostly involved in straw and SOM mineralization during the first day after straw addition. The edge number and ratio for pairwise correlations between environmental factors and fungal taxa (22.1–24.6%) were greater than those with bacterial taxa (1.0–2.9%) in the microbial correlation network. Overall, straw combined with a low level of added N benefited soil C sequestration by decreasing the PE. Compared to bacteria, the functional role of fungi in SOM mineralization is more prominent and should be considered during agricultural management. • Straw with low nitrogen addition increased carbon sequestration by a low priming effect. • Firmicutes are mainly involved in the initial stage of organic matter mineralization. • Firmicutes, Actinobacteria, and Proteobacteria of bacteria aid organic matter mineralization. • Fungal phyla Ascomycota, Basidiomycota, and Mucoromycota dominate organic matter decomposition. • Fungal taxa correlate closer with organic matter mineralization than bacterial groups. [ABSTRACT FROM AUTHOR]

• The mean footprint radius and effective depth of CRNS were 311.51 m and 11.98 cm. • CRNS can accurately estimate intermediate scale soil moisture. • CRNS can be used for field scale water management in karst area. • Temporal stability analysis is an effective tool for trans -scale soil water prediction. Cosmic-ray neutron sensing (CRNS) is a newly-developed method for continuously measuring soil water content (SWC) at the hectometer horizontal scale. However, it is unknown whether CRNS-based SWC could be used for soil moisture prediction at field scale. In this study, we try to predict field scale SWC using the CRNS combined with the temporal stability analysis in a karst watershed. The CRNS was installed in a karst watershed as well as capacitance-based SWC sensors located in five land uses. The CRNS was calibrated by eighteen manual sampling procedures. A weather station was established about 200 m away from the CRNS for monitoring basic meteorological variables. The mean footprint radius and effective depth of the CRNS were 311.51 m and 11.98 cm, respectively. The CRNS can accurately estimate intermediate scale soil moisture, with a coefficient of determination (R 2) of 0.89. Significant correlations (P < 0.01) were observed between SWC measured by the CRNS and Em-50 instruments. Though the SWC derived from the CRNS was significantly lower than that from Em-50 instruments in different land uses, the SWCs at field scale can be predicted accurately. The influences of landforms and land uses on prediction accuracy were not obvious from our data. The CRNS has great potential to improve water resources management in areas with greater heterogeneity. [ABSTRACT FROM AUTHOR]

Vegetation restoration effectively prevents soil degradation. However, the changes in soil properties that occur in fragile ecosystems in response to vegetation restoration are unclear. The aim of this study was to investigate the effects of vegetation restoration on soil physicochemical properties and saturated hydraulic conductivity (Ks) at different elevations. Restoration strategies included natural shrubland (NR), managed plantation forest (PF), cropland (CR), and secondary forest (SF). Topsoil (0–15 cm) samples were collected from seven counties in the karst region of southwest China. The soil nutrients were significantly (p < 0.05) higher in both NR and PF than in CR but were lower in SF. Soil physical properties (bulk density, porosity, texture, and Ks) were significantly (p < 0.05) different between CR and SF but not significantly different between NR and PF. Hence, the short restoration period (~15 years) had a stronger influence on soil nutrients than on soil physical properties. The soil properties following different land-use types were affected by elevation. For SF and PF, soil Ca, total nitrogen, and sand content were significantly (p < 0.05) negatively correlated with elevation. For NR, only sand content was significantly (p < 0.05) negatively correlated with elevation. No soil parameters were significantly correlated with elevation in CR. Soil nutrients increased to a greater extent in the low-elevation area (150–500 m) than in the high-elevation (1000 to 1200 m) areas. This discrepancy could be explained by the relatively increased biomass after vegetation restoration. Clay content variation was the main factor controlling Ks variation. The change in clay content was greater in low-elevation areas than in higher-elevation areas, where higher weathering and erosion rates were caused by strong hydrothermal conditions. Accordingly, the present study showed that the variations in soil properties observed after vegetation restoration are influenced by both land-use types and elevation. • The influence of elevation & land use type on karst soil properties were assessed. • The short restoration period (~15 y) had a stronger influence on soil nutrients. • Soil nutrients increased greater in low-elevation than in high-elevation areas. • Clay content was the main factor controlling Ks variation under restoration. [ABSTRACT FROM AUTHOR]

• Natural and managed vegetation restoration were compared in a karst region in China. • PLFAs in topsoil were higher after 15 years of restoration than in cropland. • AMF was significantly higher in natural than in managed restored vegetation. • SOC and the SOC:N ratio affected the abundance of soil microbial communities. • Natural vegetation restoration has more potential to improve soil fertility. The soil microbial community is a key indicator to evaluate the effectiveness of vegetation restoration in degraded ecosystems. However, few studies have simultaneously compared the effects of managed and natural vegetation restoration strategies on the soil microbial community in the ecologically fragile karst region using signature lipid biomarker analysis to provide information on a variety of microbial properties. Here, phospholipid fatty acids (PLFAs) in the soil microbial community were measured following different restoration strategies in a karst region of southwest China. Managed vegetation restoration (including restoration with plantation forest, forage grass, and a combination of plantation forest and forage grass) and natural vegetation restoration (spontaneous regeneration to natural grassland) were compared, using cropland as a reference. The abundance of total PLFAs and the PLFAs of four microbial functional groups (bacteria, fungi, actinomycetes, and arbuscular mycorrhizal fungi [AMF]) significantly increased in the topsoil (0–5 cm soil depth) following vegetation restoration. In the sub-topsoil layer (5–10 cm soil depth), the abundance of PLFAs was not significantly higher than that in the cropland, except in AMF in natural vegetation restoration. Principal component analysis indicated that the microbial PLFA composition differed between the restoration strategies, and the abundance of AMF PLFAs was significantly higher in natural than in managed vegetation restoration for both soil depths. The ratio of soil organic carbon to total nitrogen and dissolved organic carbon content were the primary factors affecting the abundance of soil microbial communities in the topsoil and sub-topsoil, respectively. In addition, soil pH served as another explanatory variable for the topsoil. Our findings suggest that vegetation restoration over a relatively short period (15 years) mainly improved topsoil fertility. Moreover, natural vegetation restoration may be superior to managed vegetation restoration for the maintenance of multiple ecosystem functions in the short term in the karst regions, owing to the potentially significant role of AMF. [ABSTRACT FROM AUTHOR]

• Annual precipitation and runoff showed similar changes with turning points in 2002. • Ecological restoration had a great impact on runoff in karst basins. • Increases in NDVI negatively related to the proportions of karst landscape. • Revegetation decreased flood season runoff and increased non-flood season runoffs. Ecological restoration changes land cover types and management practices. However, understanding of their hydrological response is limited, especially in karst ecosystems, which are vulnerable to environmental changes. This study aimed to identify the spatial variability of the hydrological response in a mesoscale karst basin to climate change and ecological restoration. We examined changes in hydrological variables and normalized difference vegetation index (NDVI) from 1985 to 2015 in Longjiang River Basin (LRB) and its three main sub-basins, namely Huanjiang (HJB), Zhongzhouxiaojiang (ZZB), and Dongxiaojiang (DXB) sub-basins. The proportions of karst landscapes (POK) were 81, 66, 61, and 26%, respectively. Three independent quantitative methods were employed to distinguish the relative contributions of climate change and human activity. Annual runoff showed a good linear correlation with precipitation in all the basins, with insignificantly lower average values during 2003–2015 than during 1985–2002. The relative contribution of precipitation to annual runoff change varied from 17.5 to 52.9%. At a seasonal scale, flood season precipitation and runoff decreased with similar patterns of annual variation, whereas non-flood season values increased from 1985–2007 to 2008–2015. The quantitative results showed that the relative contribution rates of human activity to non-flood season runoff change were strongly linked to POK values and related vegetation changes. Annual NDVI improved after the implementation of ecological restoration projects, with change rates negatively related to POK values. Human activity was the dominant cause of increased non-flood season runoff in the LRB and HJB (accounting for 57 and 78%, respectively), whereas increases in non-flood season runoff in the ZZB and DXB were mainly caused by precipitation changes (accounting for 84 and 172%, respectively). These findings highlighted the facts that ecological restoration had changed hydrological processes and seasonal-scale analyses might therefore facilitate a better understanding of runoff changes in karst basins. [ABSTRACT FROM AUTHOR]

• N and P addition affect the dry-cold and wet-hot seasonal dynamics of diazotroph communities. • Relative abundance of Rhizobiales and Rhodospirillales was high in July. • Relative abundance of Nostocales and Burkholderiales was high in December. • N addition decreased diazotroph abundance; P addition increased diazotroph diversity. • P availability affected diazotroph diversity and community composition the most. Diazotrophs have a vital role in nitrogen (N) supply to ecosystems. Yet, response of diazotroph community profiles and N 2 -fixation to N deposition and phosphorus (P) addition remains poorly understood. Two-year nutrient addition experiments (control, N, P, and a combination of N plus P) were conducted in a karst grassland. The relative abundances of diazotroph orders Rhizobiales and Rhodospirillales were higher in July, while those of Nostocales and Burkholderiales were higher in December. Diazotroph abundances and N 2 -fixation activity in July were significantly higher compared with December under control, while an opposite pattern was observed for diazotroph richness and Shannon diversity index. N, P, and N plus P plots showed no significant changes between July and December in terms of diazotroph abundance and diversity. Diazotroph abundances and N 2 -fixation activity in N and NP plots were significantly reduced compared with control plots in July. P addition plots in July showed a significant increase in diazotroph diversity and stronger interaction patterns among diazotroph taxa compared with control plots. Diazotroph diversity and community compositions were primarily affected by P availability. Overall, our findings suggested diazotroph diversity and community compositions were more sensitive to P addition than to N addition, and P availability played an important role in regulating N 2 fixation by increasing diazotroph diversity in karst ecosystems. N and P addition may alter the dry-cold and wet-hot seasonal dynamics of diazotroph communities, while sampling months also affect diazotroph community changes among treatments (no difference between treatments in December). [ABSTRACT FROM AUTHOR]