Afforestation enhances glomalin-related soil protein content but decreases its contribution to soil organic carbon in a subtropical karst area.

Afforestation on degraded croplands has been proposed as an effective measure to promote ecosystem functions including soil organic carbon (SOC) sequestration. Glomalin-related soil protein (GRSP) plays a crucial role in promoting the accumulation and stability of SOC. Nevertheless, mechanisms underlying the effects of afforestation on GRSP accumulation have not been well elucidated. In the present study, 14 pairs of maize fields and plantation forests were selected using a paired-site approach in a karst region of southwest China. By measuring soil GRSP and a variety of soil biotic and abiotic variables, the pattern of and controls on GRSP accumulation in response to afforestation were explored. The average content of total GRSP (T-GRSP) and its contribution to SOC in the maize field were 5.22 ± 0.29 mg g-1 and 42.33 ± 2.25%, and those in the plantation forest were 6.59 ± 0.32 mg g-1 and 25.77 ± 1.17%, respectively. T-GRSP content was increased by 26.4% on average, but its contribution to SOC was decreased by 39.1% following afforestation. T-GRSP content decreased as soil depth increased regardless of afforestation or not. Afforestation increased T-GRSP indirectly via its positive effects on arbuscular mycorrhizal fungi biomass, which was stimulated by afforestation through elevating fine root biomass or increasing the availability of labile C and N. The suppressed contribution of T-GRSP to SOC following afforestation was due to the relatively higher increase in other SOC components than T-GRSP and the significant increase of soil C:N ratio. Our study reveals the mechanisms underlying the effects of afforestation on T-GRSP accumulation, and is conducive to improving the mechanistic understanding of microbial control on SOC sequestration following afforestation.

Strengthen interactions among fungal and protistan taxa by increasing root biomass and soil nutrient in the topsoil than in the soil-rock mixing layer.

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.

Correction to: Effects of long-term straw incorporation on lignin accumulation and its association with bacterial laccase-like genes in arable soils.

The first funding No. should be 41671298 instead of 41301298.

Effects of long-term straw incorporation on lignin accumulation and its association with bacterial laccase-like genes in arable soils.

In this study, we aimed to investigate lignin accumulation and its relationship with the composition of bacterial laccase-like genes in three arable lands (i.e., upland limestone soil (UL), upland red soil (UR), and upland-paddy rotation red soil (UPR)), which are subjected to long-term straw incorporation. After 9-13 years of straw incorporation, the lignin content significantly increased from 337.1, 414.5, and 201.6 mg/kg soil to 2096.5, 2092.4, and 1972.2 mg/kg soil in UL, UR, and UPR, respectively. The dominant lignin monomer changed from vanillyl (V)-type to cinnamyl (C)-type in UR. Both V- and C-types were the dominant monomers in UPR, and V-type monomer remained the dominant monomer in UL. Compared with the treatment without straw, straw incorporation significantly promoted the activity of laccase enzyme and the abundance of bacterial laccase-like genes in all soils. The redundancy analysis showed that the main influencing factors on lignin accumulation patterns with straw incorporation were the laccase enzyme activity, nitrogen availability, and some specific bacterial communities possessing the laccase-like genes (e.g., Thermotogae and Acidobacteria). The variation partitioning analysis confirmed that the strongest influencing factor on lignin accumulation was the composition of bacterial laccase-like genes (explained 31.4% of variance). The present study provides novel insights into the importance of bacterial laccase-like genes in shaping lignin monomer accumulation with straw incorporation in arable soils.

Laccase activity is proportional to the abundance of bacterial laccase-like genes in soil from subtropical arable land.

Laccase enzymes produced by both soil bacteria and fungi play important roles in refractory organic matter turnover in terrestrial ecosystems. We investigated the abundance and diversity of fungal laccase genes and bacterial laccase-like genes in soil from subtropical arable lands, and identified which microbial group was associated with laccase activity. Compared with fungal laccase genes, the bacterial laccase-like genes had greater abundance, richness and Shannon-Wiener diversity. More importantly, laccase activity can be explained almost exclusively by the bacterial laccase-like genes, and their abundance had significant linear relationship with laccase activity. Thus, bacterial laccase-like gene has great potential to be used as a sensitive indicator of laccase enzyme for refractory organic matter turnover in subtropical arable lands.

Community structure analysis of soil ammonia oxidizers during vegetation restoration in southwest China.

Soil ammonia oxidizers play a critical role in nitrogen cycling and ecological restoration. The composition and structure of soil ammonia oxidizers and their impacting factors were studied in four typical ecosystem soils, tussock (T), shrub (S), secondary forest (SF), and primary forest (PF), during vegetation restoration in the Karst region of Southwest China. The composition and structure of the ammonia-oxidizing bacteria (AOB) and ammonia-oxidizing archaea (AOA) communities were characterized by sequencing the amoA and arch-amoA genes, respectively. The diversity of soil ammonia oxidizers (except in S) and plant Shannon diversity index gradually increased with vegetation restoration, and the ammonia oxidizer communities differed significantly (p < 0.001). Amplicons of AOA from the Nitrososphaera cluster dominated all four ecosystem soils. AOB Nitrosospira cluster 3b only appeared in PF and SF soils, while Nitrosospira cluster 3a species were found in all soils. Changes in AOB paralleled the changes in soil ammonium content that occurred with vegetation restoration. Redundancy analysis showed that the distribution of dominant AOB species was linked to pH, soil urease activity, and soil C/N ratio, whereas the distribution of dominant AOA species was mainly influenced by litter nitrogen content and C/N ratio. These results suggested that the composition and structure of the AOB community were more sensitive to changes in vegetation and soil ammonium content, and may be an important indicator of nitrogen availability in Karst ecosystem soils.

Assessing the Effect of Slope Position on the Community Assemblage of Soil Diazotrophs and Root Arbuscular Mycorrhizal Fungi.

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.

Impacts of Lithology and Slope Position on Arbuscular Mycorrhizal Fungi Communities in a Karst Forest Soil.

The influence of lithology and slope position on arbuscular mycorrhizal fungi (AMF) communities has been explored in various ecosystems, but there is a limited understanding of these mechanisms in karst regions. This study focused on typical karst hills with contrasting lithologies, specifically dolomite and limestone. Additionally, three slope positions (upper, middle, and lower) were investigated within each hill in karst forest ecosystems. Total phosphorus (TP) content in the soil was higher in dolomite compared to limestone. Conversely, exchangeable calcium (Ca) was lower in dolomite than in limestone. Notably, the lithology, rather than the slope position, exerted a significant impact on AMF diversity and abundance and the presence of specific AMF taxa. Dolomite exhibited greater AMF richness and a higher Shannon index in comparison to limestone when not accounting for slope position. The AMF community composition differed between dolomite and limestone. For instance, without considering slope position, the relative abundance of Acaulospora, Diversispora, and Paraglomus was higher in dolomite than in limestone, while the relative abundance of Claroideoglomus displayed an opposing trend. Furthermore, a more complex interaction among AMF taxa was observed in dolomite as compared to limestone, as evidenced by an increase in the number of nodes and edges in the co-occurrence networks within the dolomite. The genera Glomus, Claroideoglomus, and Diversispora exhibited a higher number of links with each other and with other AMF taxa. The study identified TP and Ca as the primary factors determining variations in AMF diversity between dolomite and limestone. Consequently, it is imperative to consider the underlying lithology and soil conditions when addressing the restoration of degraded karst hilly areas.

Effects of magnesium-modified biochar on soil organic carbon mineralization in citrus orchard.

In order to investigate the carbon sequestration potential of biochar on soil, citrus orchard soils with a forest age of 5 years was taken as the research object, citrus peel biochar (OBC) and magnesium-modified citrus peel biochar (OBC-mg) were selected as additive materials, and organic carbon mineralization experiments were carried out in citrus orchard soil. OBC and OBC-Mg were applied to citrus orchard soils at four application rates (0, 1, 2, and 4%), and incubated at a constant temperature for 100 days. Compared with CK, the cumulative mineralization of soil organic carbon decreased by 5.11% with 1% OBC and 2.14% with 1% OBC-Mg. The application of OBC and OBC-Mg significantly increased the content of soil organic carbon fraction, while the content of soil organic carbon fraction was higher in OBC-Mg treated soil than in OBC treated soil. Meanwhile, the cumulative mineralization of soil organic carbon was significantly and positively correlated with the activities of soil catalase, urease and sucrase. The enzyme activities increased with the cumulative mineralization of organic carbon, and the enzyme activities of the OBC-Mg treated soil were significantly higher than those of the OBC treated soil. The results indicated that the OBC-Mg treatment inhibited the organic carbon mineralization in citrus orchard soils and was more favorable to the increase of soil organic carbon fraction. The Mg-modified approach improved the carbon sequestration potential of biochar for citrus orchard soils and provided favorable support for the theory of soil carbon sink in orchards.

Research progress on microbial control of sugarcane smut.

Sugarcane is the most important sugar crop. Sugarcane smut is one of the major diseases, which could reduce sugarcane yield and quality and seriously threaten the sustainable and healthy development of sugar industry. Microbial control of sugarcane smut is a rapidly emerging green biocontrol technology, with advantage to increase environmental compatibility and soil fertility. In this review, we briefly described the characteristics of Sporisorium scitamineum which causes sugarcane smut, synthesized the the mechanisms underlying the infection of sugarcane by S. scitamineum, and presented the research status of microbial controls of sugarcane smut via the application of bio-organic fertilizers and biopesticides. We then reviewed the mechanisms underlying the suppression of sugarcane smut by microorganisms through competition with pathogens for nutrients and ecological niches, secreting antagonistic substances, and improving plant resistance. It is notable that there are still some problems in the application of microbial control technologies, including poor colonization ability and unstable biocontrol efficiency. Finally, the major directions of future research on the biocontrol of sugarcane smut were proposed from the perspective of improving the biocontrol efficiency. This review would benefit the microbial control of sugarcane smut and the healthy development of sugar industry.

Ancillary information improves kriging on soil organic carbon data for a typical karst peak cluster depression landscape.

BACKGROUND: Soil carbon management at landscape scale requires reliable information on the spatial distribution of soil organic carbon (SOC). However, how to improve the accuracy of spatial prediction is not well addressed in the karst region of southwestern China. This study evaluates the performance of univariate kriging (ordinary kriging (OK)) and hybrid kriging (co-kriging (CK), regression kriging (RK) and residual maximum likelihood (REML)) in mapping the spatial distribution of SOC at a depth of 0-15 cm. Terrain attributes and the normalised difference vegetation index (NDVI) were used as ancillary variables. RESULTS: The distribution of SOC was significantly related to NDVI and terrain attributes. Furthermore, geostatistical analyses reflected a moderately structured spatial correlation of SOC. Regression analyses identified the NDVI and slope as the best predictors for describing the spatial pattern of SOC. Combined with NDVI and slope gradient, REML and RK performed better in increasing map prediction accuracy and decreasing the soothing effect of kriging. CONCLUSION: The spatial pattern of SOC was controlled by topography and cultivation activity. The predictive abilities of OK and CK were limited. Combined with the auxiliary variables, REML and RK can improve the prediction accuracy. This study is beneficial for the further research of precise SOC management in the typical karst landscape.

Response of soil organic carbon mineralization in typical Karst soils following the addition of 14C-labeled rice straw and CaCO3.

BACKGROUND: Organic substrates and calcium are important factors controlling organic matter turnover in Karst soils. To understand their effects on soil organic carbon (SOC) mineralization, an incubation experiment was conducted involving a control treatment (CK), the addition of a (14)C-labeled rice straw (T1), CaCO(3) (T2), and both (14)C-labeled rice straw and CaCO(3) (T3) to two types of Karst soils (terra fusca and rendzina) and a red soil from southwestern China. RESULTS: Cumulative mineralization of the rice straw over 100 days in rendzina (22.96 mg kg(-1)) and terra fusca (23.19 mg kg(-1)) was higher than in the red soil (15.48 mg kg(-1); P < 0.05). Cumulative mineralization of native SOC decreased following addition of (14)C-labeled rice straw in the rendzina and terra fusca but increased in the red soil (negative and positive priming effects on native SOC). The turnover times of (14)C-labeled microbial biomass C (MBC) in the red soil, terra fusca and rendzina were 71 ± 2, 243 ± 20 and 254 ± 45 days, respectively. By adding CaCO(3), the accumulation of SOC was greater in the Karst soils than in the red soil. CONCLUSION: Although the interactions between rice straw decomposition and priming effects on native SOC are not yet understood, there was considerable variation between Karst and red soils. Soil calcium was a positive factor in maintaining SOC stability. MBC from rice straws was stable in terra fusca and rendzina, whereas it was active in the red soil. The Karst soils (terra fusca and rendzina) used in this study benefited SOC accumulation.

Land reclamation and short-term cultivation change soil microbial communities and bacterial metabolic profiles.

BACKGROUND: Soil microbes play an important role in many critical ecosystem processes, but little is known about the effects of land reclamation and short-term cultivation on microbial communities in red soil. In this study, soil microbial communities under five land use patterns-artificial pine forest (Fp), tussock and shrub (TS), shrubbery (Sh), sugarcane (Su) and maize and cassava rotation (Ma)-were characterised by DNA fingerprinting and metabolic profiling to reveal how land reclamation and cultivation affect the underlying diversity and function of soil microbial communities in southwestern China. RESULTS: Eight years of reclamation and cultivation significantly affected population size, composition and structure, bacterial metabolic profiles and diversity values (Shannon-Wiener index) of soil microbial communities. Soil organic carbon and pH were the most important factors shaping the underlying microbial communities; however, with significant correlations between soil carbon/nitrogen ratio and bacterial taxonomic and metabolic diversities, soil total nitrogen was a potentially important factor for soil microbial composition and function, as well as soil moisture, cation exchange capacity and physical structure to a lesser extent. In addition, the lowest pH, lower nutrient availability and the most compact soil in pine forest resulted in the lowest microbial taxonomic and metabolic diversities among the five land use patterns studied. CONCLUSION: Soil organic carbon, nitrogen and pH appeared to be the most important factors influencing microbial biomass, composition and function in red soil of southwestern China. The study suggests that measures to lessen the impact of changes in this edaphic environment should be taken to avoid an imbalance of microbial function and improve ecological sustainability in southwestern China.

Soil bacterial community composition and diversity respond to cultivation in Karst ecosystems.

Soil microorganisms play vital roles in recovering and maintaining the health of ecosystems, particularly in fragile Karst ecosystems that are easily degraded after cultivation. We investigated the composition and diversity of soil bacterial communities, based on RFLP and 16S rDNA sequencing, in a cropland, a naturally revegetated land with former cultivation disturbance and a primeval forest in the subtropical Karst of southwest China. Our results illustrated that Proteobacteria accounted for 44.8% of the 600 tested clones, making it the most dominant phylum observed. This phylum was followed by Acidobacteria and Planctomycetes for the three Karst soils analyzed. Compared with the primeval forest soil, the proportions of Proteobacteria were decreased by 30.2 and 37.9%, while Acidobacteria increased by 93.9 and 87.9%, and the Shannon-Wiener diversity indices and the physicochemical parameters declined in the cropland and the revegetated land, respectively. Among the three soils, the proportion of dominant bacterial phyla and the diversity indices in the revegetated land were similar to the cropland, implying the bacterial community in the cropland was relatively stable, and the after-effects of cultivation were difficult to eliminate. However, similar distributions of the four Proteobacteria subphyla were observed between the revegetated land and the primeval forest soil. Furthermore, the proportion of Rhizobiales belonging to α-Proteobacteria was sharply decreased with cultivation compared to the primeval forest soil, while a small cluster of Rhizobiales recurred with vegetation recovery. These results indicated that although the subphyla of the dominant bacterial phylum had some positive responses to 20 years of vegetation recovery, it is a slow process. Our results suggest that priority should be given to conserve the primeval forest and inoculation of functional microorganisms on the basis of vegetation recovery may be more effective for the restoration of Karst ecosystems after cultivation.

Deciphering the dominant components and functions of bacterial communities for lignocellulose degradation at the composting thermophilic phase.

The decomposition and transformation of organic matters during composting process are performed by various microorganisms. However, the bacterial communities and their functions usually vary with composting materials. Here the dominant bacterial genera and their functions were identified at the thermophilic phase during composting of mulberry branches with silkworm excrement (MSE), pig manure (MPM) and cow manure (MCD). The activities of ß-glucosidase and endoglucanase were highest for MCD (1.31 and 17.15 µg g-1 min-1) and lowest for MPM (0.92 and 14.22 µg g-1 min-1). Random Forest model and correlation analysis revealed that Stenotrophomonas, Bacillus, and Sinibacillus were the dominant bacterial genera involved in lignocellulose degradation regardless of composting materials. Carbohydrate metabolism, amino acid metabolism, and DNA replication and repair were primary functions of the bacterial communities for the three types of composting. The quantification of lignocellulose degradation genes further verified the dominant functions of the bacterial communities.

Comparison of bacterial and fungal diversity and network connectivity in karst and non-karst forests in southwest China.

Microbial communities contribute to sustaining the function of terrestrial ecosystems and are influenced by soil type and climate gradients. The effects of karst and non-karst soils on bacterial and fungal profiles for seven climate gradients were assessed to better understand bacterial and fungal diversity and community composition in response to soil type with changes in soil physicochemical properties under different temperatures and precipitations. Bacterial and fungal abundance, diversity, and community composition differed between karst and non-karst forests. Bacterial and fungal richness, Shannon index, and bacterial abundance in karst forests were higher than non-karst forests, but the fungal abundance was lower. Mean annual temperature was negatively correlated with bacterial diversity in the karst forest and fungal abundance in karst and non-karst forests. The community composition of bacteria and fungi differed among these two soil types. The karst forest had greater connectivity among bacterial and fungal communities than non-karst forests. The bacterial members of Acidobacteria, Proteobacteria, Actinobacteria, and fungal groups of Ascomycota and Basidiomycota were mainly connected with other taxa in the network, implying that taxa were associated with highly functional potential. The relative abundance of Actinobacteria and Ascomycota was higher in karst than in non-karst forests. Proteobacteria and Basidiomycota showed the opposite results. A random forest and multiple regression tree analyses revealed that soil properties, specifically pH, calcium, and total nitrogen, were the main factors influencing the variation in bacterial and fungal profiles between karst and non-karst forests. This study provides novel evidence that the abundant microbial taxa were kinless hubs in co-occurrence patterns. Controlling complex networks of species interactions may contribute to improving soil nutrient processes rather than microbial diversity, enhancing our understanding of developing sustainable recovery strategies in fragile ecosystems.

[Effects of Organic Materials on Phosphorus Fractions and phoD-harboring Bacterial Community in Karst Soil].

Efficient utilization of organic materials based on the rich resources in the karst region can promote soil fertility. Microorganisms have a crucial influence on soil phosphorus availability. phoD is considered to be the encoding phosphatase gene that can reflect the hydrolysis of organophosphorus compounds for the soil bacterial community. Molecular analysis of the phoD-harboring bacterial gene provides insight into promoting soil phosphorus availability under different fertilization managements. However, the effects of organic materials on soil phosphorus fractions associated with phoD-harboring bacterial communities are poorly understood. This study comprehensively investigated the effects of organic materials on soil phosphorus availability and explored environmental drivers of phoD-harboring bacteria in the Karst region. Here, six treatments were designed in the field as follows:non-fertilized control (CK), inorganic fertilization (NPK), inorganic fertilization combined with straw (NPKS), inorganic fertilization combined with manure (NPKM), inorganic fertilization combined with sludge (NPKL), and inorganic fertilization combined with sugarcane ash (NPKA). The phoD-harboring bacterial community in Karst region soil was analyzed using high-throughput sequencing. The results showed that the content of total P (TP), Olsen-P, and Ca2-P increased with the years after organic material application, whereas the content of CaCl2-P first decreased and then increased. Compared to that under the CK treatment, organic material application, especially NPKL treatment, significantly increased soil total nitrogen (TN), TP, Olsen-P, CaCl2-P, and Ca2-P contents, followed by those in the NPKA and NPKM treatments. Correlation analysis showed that the contents of CaCl2-P, Ca2-P, and Olsen-P were significantly positively correlated with soil exchangeable calcium (Ca-ex) content. Redundancy analysis (RDA) showed that TN, Ca-ex, soil organic carbon (SOC), and total potassium (TK) contents were the key factors affecting soil P fractions. Using high-throughput sequencing, we found that only NPKS increased the richness of phoD-harboring bacteria compared to that under the control treatment. No significant difference was observed in the phoD-harboring bacterial community among all treatments. The RDA model selected the Ca-ex, TK, Olsen-P, pH, and SOC as the key environmental predictors for the phoD-harboring bacterial community. In summary, soil phosphorus availability can be improved through the input of organic materials and inorganic fertilizer combined with manure, sludge, and ash. These additions were suitable for nutrient management and sustainable development in farmland soil in the Karst region of Guangxi.

[Characteristics of Microbial Utilization for Crop Residue-Derived C in Paddy and Upland Soils].

Two typical subtropical agricultural soils, a flooded paddy soil and its adjacent upland, were collected and then incubated with or without 13C-labeled crop residue (maize straw) for 40 days. During the incubation, the mineralization rate of the crop residue was monitored, and the 13C incorporated into fungal and bacterial phospholipid fatty acid (PLFA) was quantified. At the early stage (0.25-1 days), the mineralization rate of crop residue was faster in paddy soil than that in upland soil, whereas the opposite trend was observed from 2 to 20 days. At the late stage (21-40 days), the mineralization rate was similar in both soils. At the end of incubation, 11% of the total crop residue was mineralized in paddy soil, which was about half of that in upland soil (20%). Although paddy soil had a higher amount of microbial biomass (indicated by total PLFA), the total amounts of 13C-PLFA were comparable in both soils, and the enrichment ratio (proportion of 13C to total C in PLFA) was lower in paddy soil than that in upland soil. This indicated that the microbial community in paddy soil was less active in the uptake of crop residue C than that in upland soil. During the incubation, the residue-derived 13C was mainly distributed in bacterial PLFA (up to 86% of total 13C-PLFA, including 59% in gram-positive and 27% in gram-negative bacteria) in paddy soil, and up to 75% of total 13C-PLFA distributed in fungal PLFAs was in upland soil. Thus, bacteria dominated the utilization of crop residue in paddy soil versus fungi in upland soil. Compared with that in upland soil, the microbial activity was suppressed in the anaerobic condition caused by flooding in paddy soil, with a stronger inhibition of fungi than bacteria. Considering the discrepancies of life strategies and necromass turnover between bacteria and fungi, the different dominant microbial groups in the utilization of crop residue in water-logged and well-drained conditions could lead to the distinct accumulation and stabilization of microbial-derived organic matter in paddy and upland soils.

Soil carbon accumulation with increasing temperature under both managed and natural vegetation restoration in calcareous soils.

Vegetation restoration has been proposed as an effective strategy for increasing soil organic carbon (SOC) sequestration. However, the responses of SOC to managed and natural vegetation restoration strategies at a large scale are poorly understood due to the varying SOC components and changing climatic conditions. Here, we measured bulk SOC, particulate organic carbon (POC), and mineral-associated organic carbon (MOC) after 15 years of vegetation restoration along an elevation gradient with a corresponding temperature gradient in the calcareous soils of karst region, Southwest China. We compared managed plantation forest and naturally recovered shrubland vegetation restoration strategies, using cropland and mature forest as references. Overall, we found that the SOC and POC densities in both plantation forest and shrubland were significantly higher than in the cropland but lower than in the mature forest. There were no significant differences in the SOC pool between the plantation forest and shrubland. Furthermore, the relative changes in the SOC and POC densities increased with increasing mean annual temperature in the plantation forest and shrubland. Our results showed that both vegetation restoration strategies, characterized by higher soil microbial abundance and exchangeable Ca concentration, were beneficial to POC but not MOC accumulation, and sufficiently compensated SOC decomposition at lower elevation with higher MAT. Our results highlight the potential of both vegetation restoration strategies for promoting SOC accumulation in warmer karst regions and emphasize the necessity to understand soil carbon stabilization mechanisms in calcareous soils.

Nitrogen fertilizer and Amorpha fruticosa leguminous shrub diversely affect the diazotroph communities in an artificial forage grassland.

Soil diazotrophs have been known to be essential in biological nitrogen (N) fixation, which contributes to the sustainability of agricultural ecosystems. However, there remains an inadequacy of research on the effects of different N inputs from N fertilization and from symbiotic N fixation associated with legumes on the diazotroph communities in agricultural ecosystems. Hence, we investigated the variations in diazotroph abundance and community composition as well as the soil properties with different N inputs in the Guimu-1 hybrid elephant grass cultivation on karst soils in China. We conducted six different N treatments: control, Amorpha fruticosa planting at a spacing of 1.5 × 2 m (AFD1), A. fruticosa planting at a spacing of 1 × 2 m (AFD2), N fertilization (N), A. fruticosa planting at a spacing of 1.5 × 2 m with N fertilization (AFD1N), and A. fruticosa planting at a spacing of 1 × 2 m with N fertilization (AFD2N). Our results showed that the interaction between sampling time and N fertilization significantly affected the diazotroph abundance. In July, the diazotroph abundance significantly decreased in the N fertilization treatments: N, AFD1N, and AFD2N, compared to that in the control. The richness and Chao1 estimator of diazotrophs significantly increased in AFD2N and AFD1 correspondingly in December and July, relative to those in the control. Co-occurrence networks showed species-species interactions with high negative correlations that occurred more in the control than in the N input plots. The N input from N fertilization and legume planting directly increased the ammonium N and nitrate N and consequently affected the dissolved organic N and pH of the soil, thereby altering the diazotroph abundance and richness. Our findings demonstrated that both N fertilization and legumes could reduce the interspecific competition among diazotroph species by providing greater N availability in the forage grass.