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Investigation 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.

Cropland conversion has been suggested as a major strategy to improve the fertility of degraded croplands, but whether cropland conversion changes the status of microbial resource limitation remains poorly understood. Here, we studied three common cropland conversion strategies in a karst area of southwest China (i.e., conversion with i) Toona sinensis (TS), ii) Guimu-1 hybrid elephant grass (GG), and iii) a combination of Zenia insignis and Guimu-1 hybrid elephant grass (ZG) using cropland (CR) with maize-soybean rotation as reference. Enzymatic stoichiometry was calculated and used as an indicator of microbial resource limitation. Soil enzyme activity was altered after cropland conversions, which was strongly related to changes in soil properties. Enzymatic stoichiometry further showed that microbial carbon (C) limitation was mitigated under all three conversion strategies, but nitrogen (N) and phosphorus (P) limitations were aggravated under TS and GG. The mitigated limitation of microbial C may be due to the elevated soil C content, and the aggravated microbial N and P limitation may have been due to increased soil C/N and C/P, respectively. This study suggests that cropland conversion can change microbial resource limitation, which should be considered in the assessment and management of rebuilding ecosystems after cropland conversions.

Soil bacteria participate in nutrient cycling above and below ground to promote ecosystem stability and health. However, the relationship of soil bacteria and environmental factors following the Grain for Green (GfG) program remains poorly understood in southwest China. Soil samples were collected from seven Grain for Green sites that had been revegetated for 15 years. Four of these sites were afforested with a different tree species: Zenia insignis (ZI), Toona sinensis (TS), Castanea mollissima (CM), and Citrus reticulate (CR). One site was revegetated with Zenia insignis and Guimu-1 elephant grass (ZG), and one with only Guimu-1 elephant grass (GM). The remaining site, abandoned cropland (AC), was left to regenerate naturally. Here, we used Illumina sequencing of 16S rRNA genes to explore how the Grain for Green project affected soil bacterial community. We found that Actinobacteria, Proteobacteria, Chloroflexi, and Acidobacteria were the dominant phyla in these soils. The dominant genera at each revegetation site were also different. The CM, ZI, TS, and AC sites were dominated by Micromonospora, ZG was dominated by Streptomyces, and CR and GM were dominated by Subgroup 6. The bacterial structure was most similar in AC and TS. Correlation analysis showed that the ratio of C:P had positive effects on KD4-96, Intrasporangiaceae, and Gaiella. The ratio of soil N:P was significantly positively correlated with Cupriavidus and Kribbella. The combination of planting Zenia insignis and Guimu-1 elephant grass had the best edaphic benefits, and the approach of planting Citrus reticulate and Toona sinensis needs to be improved. Redundancy analysis (RDA) revealed that plant Simpson index, and soil N:P contributed to 16 and 15.7% of the total variations in the soil bacterial community composition, respectively. Our results suggested that plant diversity (Simpson index) and soil stoichiometric ratio (N:P) were the important factors affecting the bacterial community, and phosphorus was the limiting factor of the bacterial community in the Grain for Green karst region. In the future, revegetation should be accompanied with phosphorus fertilizer and polycultures should be considered.

Soil quality assessment is important for karst ecosystems where soil erosion is significant. A large amount of vegetation restoration has been implemented since the early 21st century in degraded karst areas across southwestern China. However, the impacts on soil quality of different restoration types rarely have been compared systematically. In the current study, we investigated the soil quality after a number of vegetation restoration projects as well as their adjacent cropland by analyzing soil samples. Six vegetation restoration types were evaluated, including one natural restoration (natural shrubland, protected for 13 years), three economic forests (4 years Eucalyptus robusta, 4 years Prunus salicina and 6 years Zenia insignis) and two mixed forests (1 year Juglans regia–crop and 13 years Toona sinensis-Pennisetum purpureum ). We evaluated the benefits of different restoration types more accurately by setting each adjacent cropland as the control and setting the variation between the corresponding restored and control site as the evaluation object so that the background differences of six sites could be eliminated. The results indicated that natural shrubland, Toona sinensis-Pennisetum purpureum and Zenia insignis were effective in improving soil quality index (SQI) in degraded karst cropland largely due to their higher SOC and TN content. The variation of SQI (VSQI) of natural shrubland was significantly higher than that in Eucalyptus robusta, Prunus salicina and Juglans regia-crop in total soil layer (0–30 cm) (P < 0.05), indicating natural shrubland had better capacity to improve soil quality. The boosting regression tree model revealed that vegetation restoration type explained 73.49% and restoration time explained 10.30% of the variation in VSQI, which confirmed that vegetation restoration type and restoration time are critical for achieving soil reserves. Therefore, it is vital to select appropriate vegetation type in restoration projects and recovery for a long time in order to achieve better soil quality. The current study provides a theoretical basis on which to assess the effects of different vegetation restoration types on the heterogeneous degraded karst areas.

Drought is one of the most critical limiting environmental factors for plant survival and restoration in areas of rocky desertification in southwestern China. Here, we aimed to understand how arbuscular mycorrhizal fungi (AMF) affect the growth and drought tolerance of tree species used for afforestation, with the ultimate aim of using them to restore degraded karst ecosystems. A pot experiment was conducted in which Zenia insignis seedlings were inoculated separately with three AMF (Funneliformis mosseae, Rhizoglomus intraradices, and Diversispora versiformis) and a mixture of all three species under two water regimes (drought stress and normal water) in a greenhouse. The results showed that AMF have a positive effect on plant biomass, osmolytes, and antioxidant enzyme activity under drought conditions. Shoot/root biomass increased by 12.5–33.8% and plant height increased by 13.6–32.1% after inoculation with AMF under drought conditions. The phosphorus content of the shoots significantly increased in mycorrhized plants, whereas nitrogen content was minimally affected. Superoxide dismutase and catalase activity was higher under drought stress in mycorrhized plants than in non-mycorrhized plants. The same pattern was found for soluble sugar and proline content in mycorrhized seedings under water deficit conditions. Mixed AMF inoculation had a more beneficial effect on plant growth than single AMF inoculation. In conclusion, our results indicate that AMF inoculation could help restore degraded ecosystems in desertified karst regions.

Zenia insignisChun is a large, fast-growing deciduous tree. In this study, we successfully developed a reliable and efficient protocol for the regeneration of fertile plants via callus induction from leaf segments of young Z. insignis seedlings. The best results were obtained with a medium containing 11.00 μM 6-benzyladenine (6-BA), 1.20 μM indole-3-butytric acid (IBA), and 0.45 μM 2,4-dichlorophenoxyacetic acid (2,4-D), which yielded morphogenic callus within 2 weeks at a frequency of 62.23%. We tested the effect of IBA alone and in combination with 6-BA on the bud differentiation response of Z. insignis callus. Shoots differentiated normally when cultured on differentiation medium containing 6.00 μM 6-BA and 1.20 μM IBA. Regenerated buds elongated successfully in medium containing 1.20 μM gibberellic acid (GA3). The elongated shoots were then transferred to Murashige and Skoog basal medium supplemented with various combinations of naphthalene acetic acid (NAA) for root induction; well-developed roots were achieved on MS basal medium supplemented with 0.01 μM NAA at a rooting rate of 89.23%. Rooted plantlets were successfully acclimatised to a greenhouse at a survival rate exceeding 90.00%.

Soil carbon (C) sequestration through cropland conversion has been regarded as a major strategy to absorb atmospheric CO2 and thus mitigate global warming, but much uncertainty still exists in terms of restoration strategies. In this study, soil C and nitrogen (N) were measured 13 years after cropland conversion in a karst area, southwest China. Four restoration strategies were included, i.e., (i) restoration with an economic tree species Toona sinensis (TS), (ii) restoration with Guimu-1 hybrid elephant grass (GG), (iii) restoration with a combination of Zenia insignis and Guimu-1 hybrid elephant grass (ZG), and iv) spontaneous regeneration (SR). Cropland under maize-soybean rotation (CR) was used as reference and the space-for-time substitution approach was adopted to evaluate soil C and N stock (0–15 cm) change. Soil C stocks in TS and SR were elevated by 40% and 50%, respectively, relative to CR, while those in GG and ZG were not significantly changed. Soil N stocks in GG were not significantly changed, but stocks in TS, ZG and SR were enhanced by 130%, 81% and 117%, respectively, relative to CR. Significant correlation was found between soil C and N stock changes with rate of relative N stock change greater than that of C stock change. Similar to soil N stock, nitrate concentration, net nitrification rate and δ15N values in TS, ZG and SR were greater than those in GG or CR. Stepwise multiple linear regression indicated that exchangeable calcium was the soil variable mainly responsible for the dynamics of both soil C and N. Our results indicate that plantation with economic tree species and spontaneous regeneration are likely equally effective in soil C sequestration.

Zenia insignis Chun is a rare and threatened species of the monotypic genus Zenia Chun in the subfamily Dialioideae of Leguminosae. The complete plastid genome of Zenia insignis was sequenc...

Ecological restoration is widespread in the karst region, southwest China, but the impacts of different restoration strategies on soil fertility indices have rarely been compared. Here soil nutrients and microbial communities were measured 16 years after agricultural abandonment in a karst area, southwest China. Three restoration strategies were included, i.e., i) restoration with an economic tree species Toona sinensis (TS), ii) restoration with Guimu-1 hybrid elephant grass (GG), iii) restoration with a combination of Zenia insignis and Guimu-1 hybrid elephant grass (ZG). Cropland under maize-soybean rotation (CR) was used as reference. Soil organic carbon level was more than doubled in TS, and that in GG and ZG was elevated by about 50% relative to CR. Soil total nitrogen concentration in GG was not significantly different from CR, but that in TS and ZG was increased by 93% and 55% relative to CR. Similar to nitrogen, soil total phosphorus concentration in GG was not changed relative to CR, but that in TS and ZG were significantly increased. Microbial biomass carbon and nitrogen concentrations were significantly increased in TS and GG by 124% and 82%, respectively, compared to CR, but those in ZG and CR were similar. The abundance of total PLFAs (phospholipid fatty acids) was significantly increased by 55-69% following agricultural abandonment, and there was no significant difference among the three restoration strategies. The patterns of the other microbial groups and the ratio of fungal to bacterial (F:B) PLFAs were largely similar to that of total PLFAs. Soil organic carbon was identified as the primary factor affecting the abundance of soil microbial communities. Our findings suggest that the three restoration strategies, particularly TS are efficient in improving soil fertility.

Soil microbial life in vivo is a major component of soils throughout the whole process of soil development.The living environment for soil microorganisms is a very sensitive early-warning indicator of change in soil ecosystems.Using slope cropland as a contrast,we investigated the main soil microbial populations,microbial biomass carbon(MBC),nitrogen(MBN),and phosphorus(MBP) and their fractal characteristic model,as well as soil enzyme activities under seven grain-for-green patterns(cultivation of Zenia insignis,Toona sinensis,Castanea mollissima,Citrus reticulate,Zenia insignis + Guimu No.1 forage,Guimu No.1 forage,and abandoned cropland) in depressions between karst hills.The main soil microbial populations differed under different grain-for-green patterns.Actinomycetes were 71.28% to 96.44% of the microbial population in the slope cropland,Zenia insignis + Guimu No.1,Guimu No.1,and abandoned cropland.Differences in their populations covered up to three orders of magnitude.Bacteria dominated in the other four soils,being 55.24% to 71.79% of the microbial population,but fungi were less than 1%.Bacteria were highest under Guimu No.1 and abandoned cropland,and were lowest in the slope cropland.Fungi populations were in the order: abandoned cropland Zenia insignis Guimu No.1 Citrus reticulate Castanea mollissima slope cropland Toona sinensis Zenia insignis + Guimu No.1,with significant differences between them.All the grain-for-green patterns except Zenia insignis increased total soil microbial populations.The differences between Guimu No.1,abandoned cropland,and Zenia insignis + Guimu No.1 were significant(P0.05),and were significantly higher than the others.Microbial biomass carbon(MBC) in the all grain-for-green patterns except Zenia insignis was significantly higher than in the slope cropland,and was highest in the abandoned cropland.Microbial biomass nitrogen(MBN) varied within a small range,showing a downward trend.Differences in MBN between cultivation of Zenia insignis + Guimu,Citrus reticulata,Zenia insignis and slope cropland were significant(P0.05).Microbial biomass phosphorus(MBP) varied widely in Toona sinensis,Castanea mollissima,abandoned cropland and Guimu No.1 but was significantly higher than in slope cropland.MBC was well correlated with bacteria(D=-4.07,R=0.81,P0.01),followed by the correlation between MBC and actinomycetes(D=3.82,R=0.44,P0.05) and that between MBN and fungi(D=0.58,R=0.61,P0.01).However,there was no fractal relationship between the other microbial biomass and microbial populations.Compared with the slope cropland,all the grain-for-green patterns except Guimu No.1 forage significantly increased the activities of soil sucrase,urease and phosphatase,suggesting an increase in carbon and nitrogen cycling.Under different grain-for-green patterns,the activity of soil sucrase was highest under Toona sinensis,reaching(29.10±7.73) mg g-1 d-1,while the activity of soil urease was highest in the abandoned cropland,reaching(540.7±34.77) μg g-1 d-1,and the activity of soil phosphatase was highest under Castanea mollissima,reaching(991.1±76.28) μg g-1 d-1.All the grain-for-green patterns improved soil microbial properties,suggesting important benefits arising from fragile ecosystem restoration and reconstruction in depressions between karst hills.