[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.

Flooding and straw returning regulates the partitioning of soil phosphorus fractions and phoD-harboring bacterial community in paddy soils.

Flooding and straw returning are effective agricultural practices in promoting phosphorus (P) availability in paddy soils. However, little is known about the effects of these practices and their interaction on the soil P pools and functional microbes responsible for soil P mobilization. Our 4-year paddy field experiment aimed to analyze the responses of soil P fractions and phoD-harboring bacterial communities in a double-rice cropping system to intermittent flooding (IF) and continuous flooding (CF), in plots with (+ S) and without (-S) straw return. Compared to IF, CF significantly increased soil citrate-P and marginally decreased the HCl-P fractions, suggesting that the stable inorganic P pools are transferred to labile inorganic P at lower redox potentials. Compared to the -S treatments, + S treatments significantly increased the labile organic fractions (enzyme-P). Correspondingly, a decreased soil total organic P concentration was observed in + S treatment. Additionally, + S treatment significantly increased the activity of acid phosphomonoesterase and alkaline phosphomonoesterase and the abundance of phoD-harboring bacteria. These results indicated that straw promoted organic P minimization to release orthophosphate. The diversity of the phoD-harboring bacteria and complexity of the co-occurrence network decreased under the CF + S treatment; however, all keystone species of the phoD-harboring bacteria were retained in this oxygen-deficient environment. This study highlights that irrigation regimes mediate the processes of inorganic P mobilization, while straw returns regulate the processes of organic P mineralization. Additionally, flooding could be a more effective agricultural practice than straw returning to promote soil P availability in paddy soils. KEY POINTS: •Soil P pools and phoD-harboring bacteria communities were assessed. •Straw return mainly affects the mineralization of organic P. •Continuous flooding mainly affects the mobilization of inorganic P.

[Characteristics of soil nitrogen cycle and mechanisms underlying the increase in rice yield with partial substitution of mineral fertilizers with organic manure in a paddy ecosystem: A review]. / 稻作系统有机肥替代部分化肥的土壤氮循环特征及增产机制.

Partial substitution of mineral fertilizers with organic manure is a key strategy for stable and increase crop yield accompanying with zero growth of mineral fertilizers. Based on recent stu-dies, we reviewed the effects of partial substitution of mineral fertilizers with organic manure on rice yield, nitrogen utilization efficiency, soil nitrogen fractions, and microbial nitrogen fixation, ammonification, nitrification, and denitrification in rice paddy ecosystems. We further compared the cha-racteristics of soil nitrogen cycle of mineral fertilizers alone and partial substitution of mineral fertili-zers with organic manure. The partial substitution altered key processes of nitrogen cycling, including enhancement of ammonification, mediation of nitrification and denitrification, reduction of ammonia volatilization and nitrogen loss, improved the status of nitrogen supplements (enriching the supplement of low-molecular-weight organic nitrogen, adjusting the distribution of inorganic nitrogen components, increasing the amount of microbial biomass nitrogen, and decreasing the loss of total nitrogen), improved soil nitrogen supply (increasing supply of small molecule organic nitrogen, coordinating inorganic nitrogen components and proportions, and increasing soil microbial biomass nitrogen and total nitrogen fixation), which promoted nitrogen uptake and regulated nitrogen allocation in rice plant to realize stability and enhancement of rice yield.

Effects of long-term fertilization on phoD-harboring bacterial community in Karst soils.

Phosphorus (P) acquisition by plants from soil organic P mainly relies on microorganisms. Examining the community of functional microbes that encode phosphatases (e.g. PhoD) under different fertilization managements may provide valuable information for promoting soil organic P availability. Here, we investigated how the abundance and community diversity of phoD-harboring bacteria responded to long-term fertilization in Karst soils. Six fertilization treatments were designed as follows: non-fertilized control (CK), inorganic fertilization only (NPK), and inorganic fertilization combined with low- and high amounts of straw (LSNPK and HSNPK), or cattle manure (LMNPK and HMNPK). We found that soil available phosphorus (AP) content and the activity of alkaline phosphatase (ALP) were significantly higher in all combined inorganic/organic fertilization treatments, while the abundance of the phoD gene was only higher in the HMPNK treatment, compared to NPK. The combination of inorganic/organic fertilizations had no effect on the diversity of phoD genes compared to NPK alone, but the phoD gene richness was greater in these treatments as compared to the control. Only organic fertilization combinations with high amounts of organic matter (both HSNPK and HMNPK) significantly affected the phoD community structure. A structure equation model demonstrated that soil organic carbon (SOC), rather than P, greatly affected the phoD community structure, suggesting that organic P mineralization in soils is decoupled from C mineralization. Our results suggested that optimized combinations of inorganic/organic fertilizations could promote P availability via regulating soil phoD-harboring bacteria community diversity and ALP activity.

Irrigation management and phosphorus addition alter the abundance of carbon dioxide-fixing autotrophs in phosphorus-limited paddy soil.

In this study, we assessed the interactive effects of phosphorus (P) application and irrigation methods on the abundances of marker genes (cbbL, cbbM, accA and aclB) of CO2-fixing autotrophs. We conducted rice-microcosm experiments using a P-limited paddy soil, with and without the addition of P fertiliser (P-treated-pot (P) versus control pot (CK)), and using two irrigation methods, namely alternate wetting and drying (AWD) and continuous flooding (CF). The abundances of bacterial 16S rRNA, archaeal 16S rRNA, cbbL, cbbM, accA and aclB genes in the rhizosphere soil (RS) and bulk soil (BS) were quantified. The application of P significantly altered the soil properties and stimulated the abundances of Bacteria, Archaea and CO2-fixation genes under CF treatment, but negatively influenced the abundances of Bacteria and marker genes of CO2-fixing autotrophs in BS soils under AWD treatment. The response of CO2-fixing autotrophs to P fertiliser depended on the irrigation management method. The redundancy analysis revealed that 54% of the variation in the functional marker gene abundances could be explained by the irrigation method, P fertiliser and the Olsen-P content; however, the rhizosphere effect did not have any significant influence. P fertiliser application under CF was more beneficial in improving the abundance of CO2-fixing autotrophs compared to the AWD treatment; thus, it is an ideal irrigation management method to increase soil carbon fixation.

[Various effects on the Abundance and Composition of Arbuscular Mycorrhizal Fungal Communities in Soils in Karst Shrub Ecosystems].

Slope position is a key factor used in the restoration of vegetation in degraded karst ecosystems, and arbuscular mycorrhizal fungi (AMF) play an important role in improving this plant growth. However, little information is available regarding the effects of slope position on arbuscular mycorrhizal fungi. To test whether these fungal communities are impacted by slope position, the abundance, and composition of soil, AMF communities along the slope position were analyzed through terminal restriction fragment length polymorphism (T-RFLP) and real-time fluorescence-based quantitative polymerase chain reaction (real-time PCR). The diversity, richness, and evenness of plant species were evaluated through field surveys and soil properties were also measured. The results show that content of carbon, nitrogen, and phosphorus in the soil are different along the slope, and the trends identified were that the upper slope position ≈ middle slope position > lower slope position. The trend for AMF abundance was identified as upper slope position ≈ middle slope position < lower slope position. The available phosphorus content in the soil correlated significantly with the AMF abundance. A redundancy analysis showed that the structure of soil, AMF, and plant community compositions differed along the slope. The plant evenness index was shown to significantly contribute to the distribution of the AMF community structure, while the total nitrogen and total organic carbon content of the soil had a significant effect on the plant community structure. These results indicate that the interaction effects of soil nutrients and plant community structures on the soil AMF community structures suggest micro-morphology should be taken into account when AMF is used to restore vegetation in karst regions.

[Effects of Lithology on the Abundance and Composition of Soil Nitrogen-fixing Bacteria and Arbuscular Mycorrhizal Fungal Communities in Karst Shrub Ecosystem].

Lithology is a key factor when used to restore vegetation in karst degraded ecosystems, and arbuscular mycorrhizal (AM) fungi and nitrogen-fixing bacteria play an important role in improving plant growth. However, little information is available regarding the effects of lithology on these two groups of microorganisms. To test whether these microbial communities are impacted by lithology, the abundance and composition of soil AM fungal and nitrogen-fixing bacteria communities were determined through terminal restriction fragment length polymorphism (T-RFLP) and real-time fluorescence-based quantitative PCR (real-time PCR). Three types of lithology (dolomite, limestone and dolomite-limestone) were selected in this study. The diversity, richness, and evenness of plant species were evaluated through field surveys and soil properties were measured. The results showed that the abundances of soil nitrogen-fixing bacteria and arbuscular mycorrhizal fungal communities were significantly influenced by lithology. The abundances of these two groups of microorganisms were the lowest in dolomite soil, inferior to dolomite-limestone soil, while highest in limestone soil. Similarly, the composition of soil nitrogen-fixing bacteria and AM fungi communities varied among lithology. A significant linear correlation was observed among soil organic carbon, available phosphorus, clay content and nitrogen-fixing bacterial abundance (P<0.05), and a significant linear correlation among total nitrogen, clay content and AM fungal abundance (P<0.05). Redundancy analysis showed that the composition of nitrogen-fixing bacterial community was closely linked to plant evenness, and the AM fungal community composition was closely linked to plant diversity (plant evenness, Shannon-wiener and richness). These results indicated that lithology influenced the abundances and compositions of soil nitrogen-fixing bacteria and arbuscular mycorrhizal (AM) fungal communities mainly through plant and soil properties.

[Effect of long-term fertilization on lignin accumulation in typical subtropical upland soil].

To investigate the effect of long-term fertilization on lignin accumulation and clarify its influencing factors in subtropical agricultural upland soils, alkaline CuO oxidation and gas chromatography was performed to quantify the amount of lignin and its monomers components (V, S and C). The soil samples were collected from the fertilization treatments of NPK and NPKS (NPK combined with straw) in Huanjiang County, Guangxi Province (limestone soil) and Taoyuan County, Hunan Province (red soil). The results showed that NPK had no significant effect on the lignin content (Sumvsc) of limestone soil, whereas the content in red soil significantly increased by (55 ± 1)%. For the NPKS treatment, the lignin content in limestone and red soil increased by (328 ± 4)% and (456 ± 9)%, respectively. After the same fertilization treatment, the proportion of cinnamyl (C)-type significantly increased in red soil, while a significant increase of vanillyl (V)-type monomers occurred in limestone soil, indicating that lignin degradation in agricultural soils was monomer specific. Furthermore, the acid-to-aldehyde ratios of syringyl-type [(Ac/Al)] or vanillyl-type [(Ac/Al)v] monomers tended to decrease after long-term fertilization with the higher value for limestone soil, suggesting the degree of lignin degradation in limestone was higher than that in red soil. Soil organic matter and total nitrogen were not correlated with lignin content, but were significantly correlated with the composition of VSC monomers. Meanwhile, the available nutrient content in the soil (available nitrogen, phosphorus, and potassium) was closely related to the contents and components of V, S, and C-type monomers (P<0.05). It indicated that the availability of soil nutrition should be considered as a key factor for the accumulation of lignin.

Abundance and Diversity of CO2-Assimilating Bacteria and Algae Within Red Agricultural Soils Are Modulated by Changing Management Practice.

Elucidating the biodiversity of CO(2)-assimilating bacterial and algal communities in soils is important for obtaining a mechanistic view of terrestrial carbon sinks operating at global scales. "Red" acidic soils (Orthic Acrisols) cover large geographic areas and are subject to a range of management practices, which may alter the balance between carbon dioxide production and assimilation through changes in microbial CO(2)-assimilating populations. Here, we determined the abundance and diversity of CO(2)-assimilating bacteria and algae in acidic soils using quantitative PCR and terminal restriction fragment length polymorphism (T-RFLP) of the cbbL gene, which encodes the key CO(2) assimilation enzyme (ribulose-1,5-bisphosphate carboxylase/oxygenase) in the Calvin cycle. Within the framework of a long-term experiment (Taoyuan Agro-ecosystem, subtropical China), paddy rice fields were converted in 1995 to four alternative land management regimes: natural forest (NF), paddy rice (PR), maize crops (CL), and tea plantations (TP). In 2012 (17 years after land use transformation), we collected and analyzed the soils from fields under the original and converted land management regimes. Our results indicated that fields under the PR soil management system harbored the greatest abundance of cbbL copies (4.33 × 10(8) copies g(-1) soil). More than a decade after converting PR soils to natural, rotation, and perennial management systems, a decline in both the diversity and abundance of cbbL-harboring bacteria and algae was recorded. The lowest abundance of bacteria (0.98 × 10(8) copies g(-1) soil) and algae (0.23 × 10(6) copies g(-1) soil) was observed for TP soils. When converting PR soil management to alternative management systems (i.e., NF, CL, and TP), soil edaphic factors (soil organic carbon and total nitrogen content) were the major determinants of bacterial autotrophic cbbL gene diversity. In contrast, soil phosphorus concentration was the major regulator of algal cbbL community composition. Our results provide new insights into the diversity, abundance, and modulation of organisms responsible for microbial autotrophic CO(2) fixation in red acidic soils subjected to changing management regimes.

[Effects of Slope Position and Soil Horizon on Soil Microbial Biomass and Abundance in Karst Primary Forest of Southwest China].

To explore the effects of slope position and soil horizon on soil microbial biomass and abundance, chloroform fumigation extraction methods and real-time fluorescence-based quantitative PCR (Real-time PCR) were adopted to quantify the changes of soil microbial biomass C, N and abundance of bacteria and fungi, respectively. Soil samples were harvested from three horizons along profile, i. e., leaching horizon (A, 0-10 cm), transitional horizon (AB, 30-50 cm) and alluvial horizon (B, 70-100 cm), which were collected from the upper, middle and lower slope positions of a karst primary forest ecosystem. The results showed that slope position, soil horizon and their interaction significantly influenced the soil microbial biomass and abundance (P < 0.05). Different from A horizon, where SMBC was greater in lower than in upper slope position (P < 0.05), SMBC in AB and B horizons were highest in middle slope position. Similarly, SMBN was greater in lower than in upper slope position for A, AB and B horizons. Besides soil bacterial abundance in B horizon and fungal abundance in AB layer, the middle slope position had the highest value for all the three soil horizons (P < 0.05). Stepwise regression analysis showed that soil organic carbon, available nitrogen and pH were the key factors responsible for SMBC and SMBN variation, respectively, while the important factors responsible for the variation of bacteria abundance were available nitrogen and available phosphorus, and that for fungi abundance variation were available potassium.