Subtropical Karst Region Research Articles

Conversion of farmland to forest or grassland improves soil carbon, nitrogen, and ecosystem multi-functionality in a subtropical karst region of southwest China

The conversion of farmland to forest in China has been recognized for its positive impact on above-ground vegetation and carbon sequestration. However, the impact on soil quality during land conversion, particularly in vulnerable karst areas, has received less attention. In this study conducted in a karst area of southwest China, eight different farmland conversion strategies were investigated to assess improvements in surface soil carbon, nitrogen, and ecosystem multi-functionality (EMF). Our results showed that farmland converted to afforestation areas or farmland that was abandoned contained higher amounts of carbon (total, organic, active) and ammonium nitrogen (NH4+-N) in the soil compared to farmland converted to grassland or maize crop. Soluble organic carbon levels were higher in afforestation and grassland areas compared to maize crop controls. By contrast, soil from grassland and abandoned land exhibited higher levels of nitrate nitrogen (NO3--N) compared to afforestation land or maize crop controls. There were no differences in NH4+-N content between any condition, except for afforestation land that specifically contained the Zenia insignis plant species. Afforestation land consistently exhibited higher EMF values than grassland. Pearson correlation analysis revealed positive relationships between soil indices and EMF scores, except for NO3--N.Random forest analysis explained 95% of the variation in soil EMF and identified specific soil factors: total carbon, organic carbon, active labile organic carbon, total nitrogen, and ammonium nitrogen, as the main drivers of soil multi-functionality. Our studies show how various reforestation strategies can enhance soil nutrient sequestration and improve soil multi-functionality of farmland in the karst areas.These findings provide insight into sustainable soil management practices for converting farmland into natural areas.

Afforestation improves soil organic carbon and total nitrogen stocks mainly through increasing > 2 mm aggregate fractions and stimulating carbon and nitrogen transformations within aggregates in subtropical karst region

Vegetation restoration can significantly increase soil organic carbon (SOC) and total nitrogen (TN) stocks, however, the response of SOC and TN stocks, as well as their distribution, transformation, and stability within aggregates, to afforestation in fragile karst ecosystems remains unclear. In this study, soil samples were collected from orchards, 10-, 20-, and 40-year Dodonaea viscosa plantations after afforestation following orchard abandonment in a karst rocky desertification area. A natural succession shrubland soil served as the control. Aggregate size distribution, SOC and TN contents, and δ13C and δ15N values in bulk soils and aggregate fractions were determined. The results showed that 0.25–2 mm aggregates were the main contributors to SOC and TN stocks, amounting to 47.7 %-55.6 % and 43.0 %-51.7 %, respectively. Compared to orchards, D. viscosa afforestation significantly increased SOC and TN stocks by 160 %-393 % and 184 %-416 %, respectively, more obviously with prolonged afforestation. This effect was mainly attributed to the increases in > 2 mm aggregate fractions and their associated SOC and TN contents. In addition, δ13C and δ15N values decreased with decreasing aggregate size in afforested and shrubland soils. The C flow pathways followed the trend from < 0.053 mm to 4–8 mm following afforestation, but this was not the case for orchard soils exhibiting pathways from 2-4 mm to < 0.053 mm progressively. These results indicated that large aggregates showed a faster turnover rate and contributed to higher SOC and TN contents following afforestation. The significant and positive relationships between SOC and TN contents and magnesium, aluminium and iron oxides suggested the stimulatory effect on SOC and TN accumulation. Noticeably, although SOC and TN stocks in bulk soils following 40-year afforestation reached the level of shrubland, the proportion of > 2 mm aggregates and the geometric mean diameter remained significantly lower than those of shrubland soils. Soil aggregate stability following D. viscosa afforestation may take longer to reach natural restoration levels.

Soil Microbial Community in Relation to Soil Organic Carbon and Labile Soil Organic Carbon Fractions under Detritus Treatments in a Subtropical Karst Region during the Rainy and Dry Seasons

Climate and detritus influence soil organic carbon (SOC) and labile SOC fractions by affecting soil microbial communities. However, it is not clear how, or to what extent, different detritus treatments affect soil microbial communities and SOC content in karst landscapes during different seasons. Plots in a karst landscape were treated with different detritus input regimes (control, no litter, no roots, no litter or roots, and double litter), and samples were collected during the dry and rainy seasons. We used Illumina sequencing of 16S rRNA to examine shifts in the diversity and composition of the associated soil microbial communities. Additionally, labile SOC fractions, including dissolved organic carbon (DOC) and microbial biomass carbon (MBC), along with soil physicochemical properties and C-degrading enzyme activities, were analyzed. The results revealed that the responses of soil properties and labile SOC fractions to detritus treatments were more pronounced during the rainy season than during the dry season, which mainly reflected that the levels of available potassium (AK), DOC, and MBC were significantly increased during the rainy season. Moreover, SOC and total nitrogen (TN) demonstrated significant changes with the double litter (DL) treatment during the rainy season. The responses of soil microbial communities to detritus treatments varied with the season, as reflected primarily in changes in the relative abundance of Ascomycota, unclassified_K_fungi, Proteobacteria, and Actinobacteriota. Climate, detritus treatments, and their interactions had significant effects on the species richness of soil bacterial communities, but did not influence fungal community diversity. Furthermore, structural equation modeling (SEM) revealed that the soil bacterial composition had the largest total effects on SOC, DOC, and MBC. In addition to directly influencing SOC, DOC, and MBC, soil properties (TN, AK, and pH) indirectly affected SOC, DOC, and MBC by altering C-degrading enzyme activity and the microbial community. We conclude that detritus treatments affect the soil microbial community and labile carbon fractions during both the rainy and dry seasons. Relationships among SOC, labile SOC fractions, enzyme activities, microbial communities, and function differed between seasons and among treatment types. This research advances our knowledge of how variation in detritus treatments affects biogeochemical cycling in karst soils during the rainy and dry seasons.

Comparison of Soil Microbial Community between Managed and Natural Vegetation Restoration along a Climatic Gradient in Karst Regions

Managed and natural vegetation restorations are two vital measures of land restoration; however, their effects on soil microbial communities at a large scale are not clearly understood. Hence, changes in the microbial community composition after 15 years of vegetation restoration along a climatic gradient in the subtropical karst region of Southwest China were assessed based on phospholipid fatty acids (PLFAs) profiles. Managed (plantation forest) and natural (naturally recovered to shrubbery) vegetation restoration types were compared, with cropland and mature forest serving as controls. Soil microbial community abundance was significantly higher under the two vegetation restoration types than in the cropland; however, it was lower than in the mature forest. The abundance, composition, and structure of soil microbial communities did not differ significantly between plantation forest and shrubbery. Soil organic carbon or total nitrogen was the primary factor positively affecting soil microbial abundance, whereas the mean annual temperature (MAT) was recognized as the primary factor contributing to the variation in the soil microbial community structure. Moreover, temperature had opposite effects on different indicators of microbial community structure. That is, it positively and negatively affected the ratios of gram-positive to gram-negative bacterial PLFAs (GP:GN) and fungal to bacterial PLFAs (F:B), respectively. Our results show that both vegetation restoration types have the ability to improve soil productivity in karst areas. Furthermore, shifts in soil microbial community structure (GP:GN and F:B ratios) induced by warming are likely to lead to a higher proportion of labile carbon, which is sensitive to soil tillage. Hence, more attention should be paid to ecological restoration in warmer karst areas to alleviate the severe loss of soil carbon in croplands.

Relationships between Soil Nematode Communities and Soil Quality as Affected by Land-Use Type

Researchers have used both soil nematode data and soil quality index (SQI) data as indicators of soil quality. However, the relationship between soil nematodes and soil quality index is poorly understood. This study explored the relationship between soil nematode properties and soil quality in different land-use types in a subtropical karst region of Southwest China. We selected the following five typical land-use types that differ in the degree of soil disturbance: cropland (maize and soybean), sugarcane, mulberry, forage grass, and forest. SQI was calculated on the basis of bulk density (BD), soil pH, the ratio of soil organic carbon to total nitrogen (C:N), the contents of soil water (SWC), soil total nitrogen (TN), soil organic carbon content (SOC), calcium (Ca), magnesium (Mg), microbial biomass carbon (MBC), microbial biomass nitrogen (MBN), ammonium nitrogen (AN), nitrate nitrogen (NN), bacteria, fungi, actinomycetes (ACT), and arbuscular mycorrhizal fungi (AMF). We found that the abundance, Simpson dominance index, and metabolic footprint of soil nematodes were highest in the forest, followed by sugarcane, cropland (maize and soybean), forage grass, and mulberry. The SQI was highest in the forest and lowest in the cropland. There was no significant difference observed among the other three regions. In addition, the SQI was positively correlated with the total nematode biomass and abundances of total nematodes, fungivores, and herbivores, the abundances of total nematodes biomass and total nematode abundance, fungivores, and herbivores. A random forest model revealed that the dominant nematode genera (i.e., Coomansus and Acrobeloides) and the rare genera (i.e., Heterocephalobus) were closely associated with soil quality. Our results suggest that the soil nematodes (especially keystone genera) may mediate the effects of ecosystem disturbance on soil quality. These findings increase our understanding of the relationships between soil organisms and soil quality.

Preliminary Research on Agricultural Cultivation Decreasing Amino Sugar Accumulation in Calcareous Soils in Subtropical Karst Region of China

Soil microbial residues play an important role in the formation and stabilization of soil organic matter and can be quantitatively characterized by amino sugars. However, the response of soil microbial residues to agricultural cultivation in karst areas remains unclear. In this study, we collected soil samples from natural reserved land as well as five plantation forests dominated by Citrus trees cultivated for 0, 1, 5, 15, 30 years to examine the effects of agricultural cultivation on the content of microbial residues (amino sugar analysis). Results showed that: (1) Soil Amino Sugars (ASs) contents were significantly reduced after agricultural cultivation along with the sharp decrease in soil organic carbon (SOC). After 30 years of cultivation, the contents of total ASs, glucosamine (GluN), galactosamine (GalN), and muramic acid (MurA) in cultivated soils decreased by 58.22%, 55.30%, 27.11%, respectively, compared with 0 yr.; (2) Microbial residual carbon contribution to SOC increased from 34.11% to 81.33% after 30 years of cultivation, including fungal residual carbon (FRC) (25.79% to 48.6%) and bacterial residual carbon (BRC) (8.32% to 32.72%); (3) Soil GluN/MurA values tended to decrease with increasing cultivation years. The results highlight the significant effect of cultivation years on amino sugar accumulation. It indicates that the years of reclamation in karst areas have different impacts on the organic fractions derived from various microbial communities in the soil organic matter pool, and the microbial residues indicated by amino sugar are of great significance for the interception of soil organic matter.

Tillage-Induced Fragmentation of Large Soil Macroaggregates Increases Nitrogen Leaching in a Subtropical Karst Region

Tillage leads to rapid loss of soil nitrogen (N) over a short period of time in karst areas. N leaching is the primary pathway of soil N loss and therefore is key to understanding the mechanisms of N loss induced by tillage. However, the factors affecting N leaching under tillage are not fully understood. Effects of tillage at various frequencies on leached N were examined in a one-year in situ simulation experiment using five tillage treatments: no tillage (T0), semiannual tillage (T1), and tillage every four months (T2), two months (T3), and monthly (T4). Concentration and amount of leached N had peaks in dry–rewetting months. Tillage significantly increased total amounts of leached N during the one-year experiment, and the largest amount of leached N was under tillage at the highest frequency. The primary form of N in leachate was NO3¯ (88.49–91.11%), followed by DON (7.80–9.87%), and then NH4+ with the lowest amount (1.09–2.10%). Tillage increased the amount of leached NO3¯ and DON, but had no significant effect on leached NH4+. Additionally, the amount of leached N had significantly negative correlations with 5–8 mm soil aggregate, NO3¯, DON, and sand content, and positive correlations with 2–5 and 0.25–2 mm. Soil 5–8 mm aggregate and DON were the main factors explaining the variation in leached N according to the RDA analysis. Tillage increased the breakdown of large aggregates, appearing to have increased the mineralization of organic matter, which resulted in increased N leaching. Our results emphasize the importance of reducing or eliminating physical disturbance indued by tillage and maintenance of large soil aggregates for decreasing N leachate in lime soil of karst regions.

Divergent Response of the Supply Capacity and Turnover of Inorganic Nitrogen to Pitaya Cultivation in the Subtropical Karst Region of Southwest China

Determining the availability and supply capacity of soil inorganic nitrogen (N) can effectively guide the appropriate application of N fertilizers during crop cultivation. However, the mechanism underlying soil inorganic N production remains unknown for cash crops in karst regions. In this study, the rates of organic N mineralization to ammonium (NH4+) and NH4+ nitrification to nitrate (NO3−) were determined using a 15N tracing technique to evaluate the supply capacity of inorganic N in soils from woodland and pitaya plantations with different cultivation years (3, 9, and 15 years) in the subtropical karst region of China. The conversion of woodland to pitaya plantations significantly decreased the content of soil organic carbon (SOC), total N, calcium (Ca), and magnesium (Mg), along with the soil pH and cation-exchange capacity (CEC), but significantly increased the content of available potassium, available phosphorus, iron, and aluminum, in a more pronounced fashion with the increasing length of pitaya cultivation. The conversion of woodland to pitaya plantations has not significantly changed soil NH4+ and NO3− content, but this land use has resulted in divergent effects on mineralization and nitrification rates. Compared to woodland (5.49 mg N kg−1 d−1), pitaya cultivation significantly reduced the mineralization rate to 0.62–2.38 mg N kg−1 d−1. Conversely, the nitrification rate significantly increased from 4.71 mg N kg−1 d−1 in soil under woodland to 9.32 mg N kg−1 d−1 in soil under 3-year pitaya cultivation, but this rate decreased to 1.74 mg N kg−1 d−1 under 15-year cultivation. Furthermore, the mean residence time of inorganic N was significantly higher in long-term than in short-term pitaya plantations, indicating the decline in inorganic N turnover with the increasing length of pitaya cultivation. Taken together, long-term pitaya cultivation could significantly decrease the supply capacity and turnover of inorganic N in soil. The Ca, Mg, SOC, and total N content, as well as CEC, were significantly and positively related to the mineralization rate, but negatively related to the mean residence time of NH4+ and NO3−, suggesting that the incorporation of organic matter can accelerate the soil inorganic N supply and turnover for long-term pitaya plantation in subtropical regions.

Assessing the quality of the soil around a shale gas development site in a subtropical karst region in southwest China

The construction of shale gas facilities disturbs large areas of land and affects soil quality and function. In this study, we investigated the properties (including physical, chemical, and microbiological indicators) of soil at three different distances from a shale gas development site (<30 m, 30–50 m, and 50–100 m) in a karst area in 2017 and 2020. Our results showed that the soil water content; available carbon, nitrogen, and phosphorus concentrations; total nitrogen and total phosphorus concentrations; microbial biomass, and enzyme activities increased (P < 0.05) as the distance from the well pad increased, and the total carbon content, pH, electrical conductivity, and some ions (magnesium, sodium, and potassium) decreased with distance from the well pad (P < 0.05). The differences in the soil properties were most noticeable in 2017. The increases in the available nutrients were greater than in the total nutrients. The overall soil quality after the shale gas well pad construction was limited by the microbial biomass and sodium contents. The soil properties recovered most quickly at 30–50 m from the well pad, because of local farmland management practices that improved the soil properties and microbial biomass, and reduced the microbial stress. Therefore, we recommend planting sodium-tolerant crops on the land closest to the well pads, to facilitate restoration of the soil that was disturbed during the construction period.

黔中北亚热带喀斯特次生林动态监测样地：物种组成与群落结构

多年来强烈的人为干扰导致我国西南喀斯特原生森林丧失殆尽并呈现植被灌丛化, 甚至石漠化现象。贵州普定县残存的片段化喀斯特次生林是黔中地区北亚热带森林植被的典型代表, 可作为监测该区域植被结构和动态的样本。在普定县天龙山建立2 hm²(200 m×100 m)森林固定样地, 对样地内胸径≥ 1 cm的所有木本植物进行了每木调查, 分析了木本植物的物种组成、区系特征、径级结构和空间分布格局。结果表明, 样地内有木本植物独立个体14, 025株, 隶属于34科55属66种。重要值最大的前3个科依次是壳斗科、胡桃科和樟科。在区系成分上, 温带区系成分占总属数的52.7%, 而热带区系成分占40.0%, 温带成分多于热带成分。稀有种有21种, 占总种数的31.82%。重要值≥ 1的物种共有13种, 贡献了88.81%的重要值, 重要值排名前三的物种分别是窄叶石栎(Lithocarpus confinis)、化香树(Platycarya strobilacea)和云南鼠刺(Itea yunnanensis)。样地内所有个体的平均胸径仅为5.12 cm, 整体径级分布呈倒“J”, 表明群落更新良好。部分优势种的空间分布表现出较明显的聚集性, 这可能与喀斯特生境的高度异质性有关。对北亚热带喀斯特次生林开展长期监测研究, 将有助于掌握亚热带喀斯特植被恢复过程中生物多样性维持和群落构建的机制。;China has the largest and widest karst regions in the world, which is mainly distributed in the subtropical mountainous regions of southwest China where a unique type of karst vegetation grows. However, the karst forests in these regions have been degraded seriously due to strongly anthropogenic disturbance over many years. Therefore, vegetation thicketization and even rocky desertification have occurred in most karst regions in southwest China. The fragmented karst secondary forest in Puding County is a typical representative of the north subtropical forest vegetation in central Guizhou Province, and can be used as a sample for monitoring the vegetation structure and dynamics in this region. In this study, a 2 hm² (200m×100m) permanent forest plot was established in Tianlongshan in Puding County. Within this plot, all woody plants with a diameter at breast height (DBH)≥1 cm were mapped and identified at species level. Based on these data, the community composition and spatial structure of the vegetation community were analyzed. Results indicated that there were 14, 025 woody plant individuals belonging to 66 species, 55 genera and 34 families. The families with high importance values (Ⅳ) are Fagaceae, Juglandaceae, Lauraceae, Saxifragaceae, Rosaceae, Betulaceae, Rhamnaceae, etc. There were more temperate elements (52.7%) than tropical elements (40.0%) found both at genus level, which showed that the flora was mainly the type of transition from tropical to temperate in the study area. Twenty-one rare species (those with individual number fewer than two in the plot) accounted for 31.82% of the species total. The distribution of rare species in Tianlongshan plot might be determined by multi-dimensional factors such as climate, topography, biogeographic processes, as well as anthropogenic disturbance. In the plot, the 13 species with Ⅳ≥1 contributed 88.81% to the Ⅳ of the community. The species with high Ⅳ are Lithocarpus confinis, Platycarya strobilacea Itea yunnanensis, Machilus cavaleriei, Carpinus pubescens, Pittosporum brevicalyx, Lindera communis, etc. The average DBH of all individuals in the plot was only 5.12 cm, and DBH size-class distribution followed a reverse J shape, indicating abundant regeneration. The spatial distribution of some dominant woody species showed obvious aggregation pattern in plot scales possibly related to high habitat heterogeneity in karst landscape. Monitoring and research on secondary forests in the north subtropical karst region will contribute to further understanding the mechanisms of biodiversity maintenance and community assembly in the process of vegetation restoration. Further studies should concentrate on the maintenance mechanisms of diversity and stability of forest community, the coupling relationships between above-ground and underground ecological processes, and the response and adaptation of vegetation to short-term disturbance events and long-term climate change in the north subtropical karst regions in southwest China.

Dynamics of soil nitrogen availability following vegetation restoration along a climatic gradient of a subtropical karst region in China

Soil nitrogen (N) availability plays an important role in determining ecosystem structure and function. However, the variation patterns of soil N availability under different vegetation restoration strategies along climatic gradients in the ecologically fragile karst region of China have been poorly understood. Soil N variables, including total N (TN), inorganic N [ammonium (NH4+) and nitrate (NO3-)], and rates of net N mineralization and nitrification were measured following managed (plantation forest) and natural (spontaneous regeneration) vegetation restoration along a climatic gradient in the karst region of southwestern China. Compared to that in the cropland, soil TN and NH4+ concentrations significantly increased in both vegetation restoration strategies after 15 years of cropland abandonment. The net N mineralization and nitrification rates were lower in both vegetation restoration strategies compared with those in cropland and secondary forest. There were no significant differences in the soil N variables between the two vegetation restoration strategies. Furthermore, soil NO3− was positively correlated with mean annual temperature (MAT) following vegetation restoration, whereas the net nitrification rate was negatively correlated with MAT. Generally, soil pH was the main factor affecting soil inorganic N, whereas elevation, soil microbial abundance, SOC or C:N, and climatic factors mainly affected soil net N mineralization and nitrification rates. Vegetation restoration was beneficial to soil N accumulation and is likely to decrease the risk of N loss in warmer regions given the reduced potential nitrification rate with increased MAT in the karst region of Southwest China.

Responses of soil gross nitrogen transformations to three vegetation restoration strategies in a subtropical karst region

AbstractInvestigation of soil internal nitrogen cycles is useful for unravelling the mechanisms responsible for soil nitrogen availability change. Here, gross nitrogen transformations were investigated in a subtropical karst area after 16 years of vegetation restoration. The experiment included four treatments with one control and three restoration strategies, that is: (i) maize‐soybean rotation field (CR, control); (ii) restoration with Toona sinensis (TS); (iii) restoration with Guimu‐1 hybrid elephant grass (GG, Pennisetum americanum (L.) Leeke × Pennisetum purpureum Schumach); and (iv) restoration with Guimu‐1 hybrid elephant grass and Zenia insignis as a mixture (ZG). Soil NH4+ and NO3− responded differentially to three restoration strategies, resulting in no significant change of total inorganic N content following vegetation restoration. Gross nitrogen mineralization (GNM) rate increased by 124.0%, 96.8%, and 60.3% in TS, GG, and ZG, respectively, relative to CR (2.88 ± 0.05 mg N kg−1 d−1). The strongest explanatory variable was microbial biomass carbon for both GNM (R2 = 0.82) and gross nitrification (R2 = 0.66). Dissimilatory NO3− reduction to NH4+ (DNRA) rate in TS or ZG, which was significantly greater than that in CR, was significantly correlated with SOC: NO3− ratio (R2 = 0.61), implying that DNRA was limited by SOC availability. Gross NH4+ immobilization rate, which was highest in GG, and lowest in CR, was best predicted by carbon to nitrogen ratio (R2 = 0.65). However, gross NO3− immobilization rate could not be well predicted by the measured soil properties. Our results suggest that vegetation restoration especially TS significantly enhanced soil N availability as reflected by gross N mineralization rate.

Difference in hydraulic resistance between planted forest and naturally regenerated forest and its implications for ecosystem restoration in subtropical karst landscapes

Drought-induced plant hydraulic failure is expected to increase under climate change, and this negatively affects reforestation under the fragile karst environment in subtropical China. Compared with naturally regenerated forests, planted forests can rapidly increase vegetation coverage and have considerable economic benefits; however, their degree of resistance to drought-induced hydraulic failure is still unclear. Our study aimed to fill this knowledge gap by examining the differences in hydraulic characteristics between planted forests and naturally regenerated forests. Sample sites for both types of forest were selected in the humid subtropical karst region, and the hydraulic safety margin, stomatal conductance, and species-specific growing season length were measured for statistical analyses. The planted forest had a narrower hydraulic safety margin and presented a greater vulnerability to hydraulic failure than the naturally regenerated forest; however, it can bud later and abscise leaves early to avoid hydraulic failure. This study is the first to provide evidence of coordination between plant safety margin and species-specific growing season length in fragile karst ecosystems. To increase vegetation coverage and reduce soil erosion, the combination of both forest types could be an optimal ecosystem restoration strategy. Our study provides critical information for ecosystem restoration and management in subtropical karst areas.

Influence of shale gas development on core forests in the subtropical karst region in southwestern China

Core forests are an important component of forest landscapes and wildlife habitat. Although the core forests were damaged during the development of shale gas sites, it remain unclear how much damage the shale gas development has caused to this ecologically vulnerable region. We analyzed high-resolution remote sensing images of a shale gas development area in 2012, 2014, and 2017 in the karst region in southwestern China. The results showed that the core forest area decreased by approximately 4.0% from 2012 to 2017. Of this decrease, approximately 32.3% was related to the shale gas development activities, while 67.7% was related to other human activities, i.e., agricultural lands and residential developments. Approximately 5.6% of the decrease in the core forest was for new pipelines, with 0.5 ha occurred in 2012–2014 and 248.6 ha occurred in 2014–2017. Of the shale gas development activities, the pipeline constructions were most detrimental to the core forest. The patchiness of the core forest increased by 8.2% from 2012 to 2017 by the expansions of dry fields, towns, and settlements. The core forest Effective Mesh Size (MESH) decreased by 86.3%, primarily caused by the shale gas development pipelines. In conclusion, human activities that were not directly related to shale gas development were the main driver of the core forest decreases. The pipelines caused most losses of the core forest among the shale gas activities and the impacts deteriorated as the shale gas development proceeds. Therefore, we propose that new shale gas pads should be placed adjacent to existing shale gas pipelines and new shale gas pipelines should be constructed in parallel with existing roads to reduce the damages on core forest.

Retrotransposon-based genetic variation and population structure of Impatiens macrovexilla Y. L. Chen in natural habitats and the implications for breeding

Impatiens macrovexilla Y. L. Chen is a potential ornamental herbaceous plant originated from a relatively small area of subtropical Karst region in southern China, and natural populations of this species presented strong adaptability to a range of environmental conditions, with strong horticultural characteristics including bright colour, large flowers, early flowering, long flowering, and floriferousness. Overall, these adventive traits are significant for future selective breeding. Using a ‘jumping gene’ based or retrotransposon-based Inter-Primer Binding Site (iPBS) marker system to evaluate the genetic variation of nine I. macrovexilla populations collected from contrasting environments. The electrophoretic patterns of the PCR products produced for all the accessions were applied to evaluate the genetic diversity and differentiation between populations. The genotyping results distinguished all the 91 individual accessions in nine I. macrovexilla populations from varied microclimates, and population G was separated as a particular variety (I. macrovexilla var. yaoshannensis) which agrees with former botanical classifications about this species. Results from the AMOVA analysis showed that the greatest genetic variation (77 %) was observed within populations whilst 23 % was recorded among populations, indicating a wide range of biotypes at each location where germplasm collected. Results from our PCoA analysis, the phylogenetic dendrogram, and the Bayesian clustering analysis showed obvious subdivision among populations analyzed. The genetic relationships for population G (Jinxiu) and population H (Gantang) were the most different from each other and showed the highest genetic distance of 0.254. Characteristics of specific traits for each population and the genetic relationships of these populations could be utilized in optimizing potential parental candidate for cross combinations. Variations of the genetic pattern and the relationships among and within the populations will impact profoundly on our selective breeding of the species in the future.

Response of soil microbial communities to natural and managed vegetation restoration in a subtropical karst region

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.

Temporal changes in vegetation around a shale gas development area in a subtropical karst region in southwestern China

Over the past decade, various aspects of China's fragile karst environments, including net primary productivity (NPP), have been changed or threatened by shale gas development. This industry is still developing, so it is important to understand what drives environmental changes, particularly in NPP, when shale gas pads are constructed in sensitive areas. Few previous studies have addressed this issue, so we quantified how the NPP changed, and what drove the changes, when a large shale gas area was developed at the end of 2012 in a mountainous karst area in Sichuan Province. We calculated the trend in the normalized difference vegetation index (NDVI) from 2012 to 2017 and used the Carnegie-Ames-Stanford Approach (CASA) model to calculate the changes in NPP at different distances from the pads using remote sensing images for July 2012 and July 2017 and field survey data from July 2017. We then identified the factors that drove the changes with Geodetector. The results showed that the NDVI increased across 64.2% of the shale gas development area from 2012 to 2017 because of climate change, and only showed a significant decrease across 0.3% of the area, mainly because of the shale gas development. The NPP decreased by 110.1 t because of the shale gas development in July 2017, or by about 0.35% of the total NPP. Of this, 93.8 t were associated with the pad construction areas, and 16.3 t were associated with the area around the pads. The changes in NPP around the shale gas pads were mainly confined to within 150 m during the construction phase and 90 m once the construction was completed. The NPP at different distances from the pads during the construction period was related to the distance from the pad, slope, and land use. Once completed, the NPP mainly varied with distance, land use, and the distance from the pad to rural settlements. The NPP was most strongly influenced by the distance from the pad and the area of the pad. We suggest that, when planning the construction of shale gas pads, the pads should be sited on gently sloping areas, the number of wells on each pad should be optimized, land use type changes outside the pad should be limited, and the land beyond the pads should be reclaimed in a timely manner to allow the NPP to recover.

Variation of microbial residue contribution to soil organic carbon sequestration following land use change in a subtropical karst region

Microbial residues are an important constituent of persistent soil organic carbon (SOC). Nevertheless, how microbial residue contribution to bulk SOC sequestration changes following land use conversion remains poorly understood. Here, we investigated the variation of soil microbial residue C and its contribution to SOC following conversion of maize-soybean field to mulberry, forage grass and sugarcane fields in southwest China, with forest used as reference. Microbial residue C contents were estimated based on microbial cell wall biomarker amino sugars. The contents of fungal, bacterial and total microbial residue C were lowest in the sugarcane, maize-soybean and mulberry fields, intermediate in the forage grass field and highest in the forest. Fungal to bacterial residue C ratios were comparable among the five land use types, implying that the relative contribution of fungi and bacteria residues to SOC sequestration remained unchanged. Conversion of maize-soybean field to mulberry and sugarcane fields had no significant effect on microbial residue contribution to SOC sequestration. Nevertheless, the contribution of total microbial residues to SOC sequestration was marginally (p < 0.1) and significantly (p < 0.05) higher, respectively, in the forage grass field and forest than in the maize-soybean field. Levels of SOC and living microbial biomass were the major factors determining the dynamics of microbial residue contents. Stoichiometric imbalance of carbon: nitrogen ratios between soil organic matter and microbial biomass (i.e., C:N imbalance) was found to negatively correlate with the contribution of microbial residues to SOC sequestration. Our results suggest that increase of C:N imbalance may explain lowered microbial residue contribution to SOC sequestration following land use change.

Moss-dominated biological soil crusts modulate soil nitrogen following vegetation restoration in a subtropical karst region

Vegetation restoration often promotes the development of biological soil crusts (biocrusts), which have a significant role in the terrestrial nitrogen (N) cycle. However, biocrusts are not always considered when assessing soil N responses to restoration efforts, especially in subtropical regions. We assessed the effect of various restoration strategies on moss-dominated biocrust properties and soil N variables (0–5 cm topsoil and 5–10 cm sub-topsoil) in the subtropical karst region of southwestern China. Four restoration strategies were included: restoration with plantation forest, forage grass, and a combination of forest and grass, and spontaneous regeneration to natural grassland. Cropland under maize-soybean rotation (CR) was used as reference. The biocrust under combination restoration strategy had significantly higher biomass, saturated water adsorption ratio, and carbon content than did the other strategies; and the soil generally had the highest total N, ammonium, and microbial biomass carbon and N content but the lowest nitrate content. Redundancy analysis, variation partitioning, and stepwise multiple linear regression all indicated that biocrust properties play an important role in affecting soil N variables, especially in the topsoil. Vegetation restoration strategies that use various plant functional group assemblages, such as forest and grass, are preferable in enhancing the development of biocrusts and thus improving soil N properties.

Effects of tillage on soil N availability, aggregate size, and microbial biomass in a subtropical karst region

Tillage results in the loss of soil nitrogen (N) over the short term in a typical karst area, but the factors affecting soil N content under intensive agriculture are poorly understood. In this study, our objective was to examine the effect of tillage at various frequencies on soil aggregates and microbial biomass, and consequently on soil N content. A one-year in situ simulation experiment was conducted using five tillage treatments: no tillage (T0), semiannual tillage (T1), and tillage every four months (T2), two months (T3), and one month (T4). Tillage at various frequencies reduced the total N (TN) content of soil from 0.43 to 0.72 g kg−1, compared with the no-till treatment. Nitrate (NO3-) was the major form of dissolved N in this karst region probably owing to high soil pH and it declined sharply with increasing tillage. Tillage disintegrated large soil aggregates (5–8 mm). Microbial biomass carbon (MBC) decreased gradually with increasing tillage frequency, while microbial biomass nitrogen (MBN) was sharply reduced by all tillage treatments including T1. In addition, redundancy analysis (RDA) indicated that both 5–8 mm-sized aggregates and MBC significantly impacted the N cycle (P < 0.05). Structural equation model (SEM) analysis showed that tillage significantly affected total N loss (ΔTN) via the fragmentation of 5–8 mm aggregates. These findings highlight the importance of preserving large aggregates for maintaining the soil N pool in fragile ecosystems, and preventing the loss of soil N due to tillage in the karst region, as its high soil NO3- content and likely leaching may be a threat to groundwater quality.