Metagenomic and 14 C tracing evidence for autotrophic microbial CO2 fixation in paddy soils.

Autotrophic carbon dioxide (CO2 ) fixation by microbes is ubiquitous in the environment and potentially contributes to the soil organic carbon (SOC) pool. However, the multiple autotrophic pathways of microbial carbon assimilation and fixation in paddy soils remain poorly characterized. In this study, we combine metagenomic analysis with 14 C-labelling to investigate all known autotrophic pathways and CO2 assimilation mechanisms in five typical paddy soils from southern China. Marker genes of six autotrophic pathways are detected in all soil samples, which are dominated by the cbbL genes (67%-82%) coding the ribulose-bisphosphate carboxylase large chain in the Calvin cycle. These marker genes are associated with a broad range of phototrophic and chemotrophic genera. Significant amounts of 14 C-CO2 are assimilated into SOC (74.3-175.8 mg 14 C kg-1 ) and microbial biomass (5.2-24.1 mg 14 C kg-1 ) after 45 days incubation, where more than 70% of 14 C-SOC was concentrated in the relatively stable humin fractions. These results show that paddy soil microbes contain the genetic potential for autotrophic carbon fixation spreading over broad taxonomic ranges, and can incorporate atmospheric carbon into organic components, which ultimately contribute to the stable SOC pool.

Increased microbial functional diversity under long-term organic and integrated fertilization in a paddy soil.

Microbes play key roles in diverse biogeochemical processes including nutrient cycling. However, responses of soil microbial community and functional genes to long-term integrated fertilization (chemical combined with organic fertilization) remain unclear. Here, we used pyrosequencing and a microarray-based GeoChip to explore the shifts of microbial community and functional genes in a paddy soil which received over 21-year fertilization with various regimes, including control (no fertilizer), rice straw (R), rice straw plus chemical fertilizer nitrogen (NR), N and phosphorus (NPR), NP and potassium (NPKR), and reduced rice straw plus reduced NPK (L-NPKR). Significant shifts of the overall soil bacterial composition only occurred in the NPKR and L-NPKR treatments, with enrichment of certain groups including Bradyrhizobiaceae and Rhodospirillaceae families that benefit higher productivity. All fertilization treatments significantly altered the soil microbial functional structure with increased diversity and abundances of genes for carbon and nitrogen cycling, in which NPKR and L-NPKR exhibited the strongest effect, while R exhibited the least. Functional gene structure and abundance were significantly correlated with corresponding soil enzymatic activities and rice yield, respectively, suggesting that the structural shift of the microbial functional community under fertilization might promote soil nutrient turnover and thereby affect yield. Overall, this study indicates that the combined application of rice straw and balanced chemical fertilizers was more pronounced in shifting the bacterial composition and improving the functional diversity toward higher productivity, providing a microbial point of view on applying a cost-effective integrated fertilization regime with rice straw plus reduced chemical fertilizers for sustainable nutrient management.

Anaerobic ammonium oxidation in sediments of surface flow constructed wetlands treating swine wastewater.

Anaerobic ammonium oxidation (anammox) was suggested to be involved in the nitrogen (N) removal process in constructed wetlands (CWs). Nevertheless, its occurrence and role in CWs treating swine wastewater have not been well evaluated yet. In this study, we investigated the diversity, activity, and role of anammox bacteria in sediments of mesoscale surface flow CWs (SFCWs) subjected to different N loads of swine wastewater. We found that anammox bacteria were abundant in SFCW sediments, as indicated by 7.5 × 105 to 3.5 × 106 copies of the marker hzsB gene per gram of dry soil. Based on stable isotope tracing, potential anammox rates ranged from 1.03 to 12.5 nmol N g-1 dry soil h-1, accounting for 8.63-57.1% of total N2 production. We estimated that a total N removal rate of 0.83-2.68 kg N year-1 was linked to the anammox process, representing ca. 10% of the N load. Phylogenetic analyses of 16S ribosomal RNA (rRNA) revealed the presence of multiple co-occurring anammox genera, including "Candidatus Brocadia" as the most common one, "Ca. Kuenenia," "Ca. Scalindua," and four novel unidentified clusters. Correlation analyses suggested that the activity and abundance of anammox bacteria were strongly related to sediments pH, NH4+-N, and NO2--N. In conclusion, our results confirmed the presence of diverse anammox bacteria and indicated that the anammox process could serve as a promising N removal pathway in the treatment of swine wastewater by SFCWs.

Long-term balanced fertilization increases the soil microbial functional diversity in a phosphorus-limited paddy soil.

The influence of long-term chemical fertilization on soil microbial communities has been one of the frontier topics of agricultural and environmental sciences and is critical for linking soil microbial flora with soil functions. In this study, 16S rRNA gene pyrosequencing and a functional gene array, geochip 4.0, were used to investigate the shifts in microbial composition and functional gene structure in paddy soils with different fertilization treatments over a 22-year period. These included a control without fertilizers; chemical nitrogen fertilizer (N); N and phosphate (NP); N and potassium (NK); and N, P and K (NPK). Based on 16S rRNA gene data, both species evenness and key genera were affected by P fertilization. Functional gene array-based analysis revealed that long-term fertilization significantly changed the overall microbial functional structures. Chemical fertilization significantly increased the diversity and abundance of most genes involved in C, N, P and S cycling, especially for the treatments NK and NPK. Significant correlations were found among functional gene structure and abundance, related soil enzymatic activities and rice yield, suggesting that a fertilizer-induced shift in the microbial community may accelerate the nutrient turnover in soil, which in turn influenced rice growth. The effect of N fertilization on soil microbial functional genes was mitigated by the addition of P fertilizer in this P-limited paddy soil, suggesting that balanced chemical fertilization is beneficial to the soil microbial community and its functions.

[Response of Organic Carbon Mineralization to Nitrogen Addition in Micro-aerobic and Anaerobic Layers of Paddy Soil].

In field conditions, a micro-aerobic layer with 1 cm thickness exists on the surface layer of paddy soil owing to the diffusion of dissolved oxygen via flooding water. However, the particularity of carbon and nitrogen transformation in this specific soil layer is not clear. A typical subtropical paddy soil was collected and incubated with13C-labelled rice straw for 100 days. The responses of exogenous fresh organic carbon(13C-rice straw) and original soil organic carbon mineralization to nitrogen fertilizer addition[(NH4)2SO4]in the micro-aerobic layer(0-1 cm) and anaerobic layer(1-5 cm) of paddy soil and their microbial processes were analyzed based on the analysis of 13C incorporation into phospholipid fatty acid(13C-PLFAs). Nitrogen addition promoted the total CO2 and 13C-CO2 emission from paddy soil by 11.4% and 12.3%, respectively. At the end of incubation, with the addition of nitrogen, the total soil organic carbon (SOC) and13C-recovery rate from rice straw in the anaerobic layer were 2.4% and 9.2% lower than those in the corresponding micro-aerobic layer, respectively. At the early stage(5 days), nitrogen addition increased the total microbial PLFAs in the anaerobic layer with a consistent response of bacterial and fungal PLFAs. However, there was no significant effect from nitrogen on microbial abundance in the micro-aerobic layer. Nitrogen addition had no significant impact on the abundance of total 13C-PLFAs in the micro-aerobic and anaerobic layers, but the abundance of 13C-PLFAs for bacteria and fungi in the micro-aerobic layer was decreased dramatically. At the late stage(100 days), the effect of nitrogen addition on microbial PLFAs was consistent with that at the early stage. The abundances of total, bacterial, and fungal 13C-PLFAs were remarkably increased in the anaerobic layer. However, the abundance of 13C-PLFAs in the micro-aerobic layer showed no significant response to nitrogen addition. During the incubation, the content of NH4+-N in the anaerobic soil layer was higher than that in the micro-aerobic soil layer. This indicates that nitrogen addition increased microbial activity in the anaerobic soil layer caused by the higher NH4+-N concentration, as majority of microorganisms preferred to use NH4+-N. Consequently, the microbial utilization and decomposition of organic carbon in the anaerobic soil layer were accelerated. By contrast, richer available N existed in the form of NO3--N in the micro-aerobic soil layer owing to the ammoxidation process. Thus, the shortage of NO3--N preference microorganisms in the paddy soil environment prohibited the microbial metabolism of organic carbon in the micro-aerobic layer. As a whole, nitrogen fertilization enhanced organic carbon loss via microbial mineralization in paddy soil with a weaker effect in the micro-aerobic layer than that in the anaerobic layer, indicating the limited microbial metabolic activity in the surface micro-aerobic layer could protect the organic carbon stabilization in paddy soil. This study emphasizes the heterogeneity of paddy soil and its significant particularity of carbon and nitrogen transformation in micro-aerobic layers. Consequently, this study has implications for optimizing the forms and method for the application of nitrogen fertilizer in paddy cropping systems.

[Phosphorus Adsorption Characteristics of Different Biochar Types and Its Influencing Factors].

In order to understand the resource utilization of plant biomass, five types of biomass materials were used to produce biochar to treat wastewater containing phosphorus. The phosphorus adsorption capacity of five materials was preliminarily compared through laboratory experiments, and two materials with strong phosphorus adsorption capacity were screened out. The physicochemical characteristics of the selected biochar were analyzed using scanning electron microscopy and a BET specific surface area analyzer, and the effects of different pH values on phosphorus adsorption of the biochar were investigated. Furthermore, the phosphorus adsorption characteristics of the selected biochar were analyzed via isothermal adsorption and adsorption kinetics models. The results showed that among the five biochar materials, only rice straw and corn straw biochar had the ability to adsorb phosphorus. The Langmuir isothermal adsorption curve showed that the adsorption capacity of rice straw biochar for phosphorus in wastewater was stronger than that of corn straw biochar, and the theoretical maximum adsorption capacity was as follows:rice straw biochar (9.78 mg·g-1)>corn straw biochar (0.39 mg·g-1). The specific surface area (148.30 m2·g-1) and total pore volume (0.11 cm3·g-1) of rice straw biochar were much higher than those of corn straw biochar (8.26 m2·g-1 and 0.03 cm3·g-1, respectively), and the contents of Mg, Ca, Fe, and Al were higher in rice straw biochar. The best pH for phosphorus adsorption of rice straw biochar and corn straw biochar was acidic. In different pH ranges (3.0-11.0), the phosphorus adsorption capacity of rice straw and corn straw biochar decreased with the increase in pH. These results indicated that rice straw biochar has strong phosphorus adsorption capacity and has a better application prospect in wastewater treatment.

[Effect of Long-term Straw Returning on the Mineralization and Priming Effect of Rice Root-carbon].

Long-term straw returning to the field changes the environmental conditions of rice paddy soil, which affects the mineralization and priming effect of residual rice roots in the soil, but the direction and intensity of its influence is not clear. Therefore, based on a long-term fertilization field experiment, 13C-CO2 isotopic labeling technology and laboratorial incubation were used to analyze the characteristics of mineralization of rice roots and native soil organic carbon, the intensity and direction of the priming effect, and the source partitioning of CO2 emissions in three treatments, consisting of no fertilization (CK), chemical fertilizer (CF), and straw returning with chemical fertilizer (CFS). The results showed that after 120 days of flooding incubation, the root residue (R) increased the cumulative CO2 emissions by 617.41-726.27 mg·kg-1. The cumulative CO2 emissions from roots and root mineralized proportions in the CFS+R and CF+R treatments were 470.82 and 444.04 mg·kg-1, respectively, and 18.8% and 17.8%, respectively. These were significantly higher than those in the CK+R treatment (384.19 mg·kg-1, 15.4%). There was no significant difference in the cumulative CO2 emissions from native soil organic carbon among the three treatments. However, the mineralized proportion of native soil organic carbon in the CFS+R treatment (4.2%) was significantly lower than that in the CF+R and CK+R treatments (5.4% and 5.8%). The priming effect in the CFS+R treatment was 29.6%, which was significantly lower than that in the CK+R treatment (42.5%) and higher than that in the CF+R treatment (14.4%). A total of 23.47% to 27.59% of the cumulative CO2 emission of the flooded paddy soil was from the roots, and the remainder was from the soil. In addition, the proportion of CO2 emission caused by the priming effect was smaller in the CFS+R treatment than that in the CK+R treatment and larger than that in the CF+R treatment. In summary, the long-term straw returning in the flooded paddy soil will increase the mineralization potential of rice roots, but it is more conducive to the stability of the native soil organic carbon.

[Effects of Chemical Fertilizer Reduction Combined with Straw Application on Diazotrophic Communities in a Double Rice Cropping System].

Based on a three-year field experiment, the effects of reduced chemical fertilizer combined with straw application on paddy yield, soil fertility properties, and community structure of diazotrophs in a double-rice cropping field three years after straw application were examined. Three treatments were applied:conventional fertilizer application (CF), chemical fertilizer reduction combined with a low straw application rate (CFLS, 3 t·hm-2), and a high straw application rate (CFHS, 6 t·hm-2). The results showed that CFLS and CFHS did not significantly reduce rice grain yield (P>0.05); significantly neutralized soil acidification; increased soil microbial biomass carbon and nitrogen, dissolved organic carbon, and organic carbon content (P<0.05); and significantly reduced soil redox potential, ammonium nitrogen, and nitrate nitrogen contents (P<0.05). This was more conducive to improve soil nitrogen use efficiency. Compared with those under the CF treatment, the natural nitrogen fixation functional communities of CFLS and CFHS increased the Shannon, PD, and Evenness indexes (P<0.05) due to the improvement of conditions such as the increase in soil carbon storage and the decrease in acidification degree. The relative abundance of microbial communities with nitrogen fixation, carbon fixation, and plant growth promotion functions such as Ferrigenium, Sulfurivermis, Methylomonas, Methylovulum, Ectothiorhodospira, and Nostoc increased significantly (P<0.05). In conclusion, the reduction in chemical fertilizer combined with 3 t·hm-2 and 6 t·hm-2 straw application was an effective measure to improve the community structure of soil diazotrophs and the potential of soil nitrogen fixation.

[Annual Nitrogen Removal Efficiency and Change in Abundance of Nitrogen Cycling Microorganisms in Swine Wastewater Treated by Crop Straw Materials].

Based on previous research, using straw material to treat swine wastewater can effectively reduce the concentration of nitrogen (N); however, the annual N-removal efficiency and change in the abundance of N-cycling functional genes remain unclear. In this study, four treatments (wheat straw, rice straw, corn stalk, and CK) were set up, with the aim of studying the annual N-removal efficiency and change in the abundance of functional genes. Our results showed that:① the total nitrogen (TN) removal and NH4+-N removal efficiency were the best in the first six months and were significantly reduced in the following six months. In addition, the TN removal and NH4+-N efficiency in straw and wheat straw were better than those in corn straw. The TN-removal efficiency in straw and wheat straw were 32.81%±11.34% and 32.99%±9.60%, respectively. The NH4+-N removal efficiency in straw and wheat straw were 35.3%±13.23% and 34.97%±12.00%, respectively. ② The abundance of N-cycling functional genes significantly increased by the addition of straw materials, compared with that of the CK (P<0.05). The average abundances of nirK, nirS, and hzsB were 6.45×109 copies·L-1, 6.18×109 copies·L-1, and 2.31×109 copies·L-1, respectively. The average abundances of ammonia-oxidizing archaea (AOA) and ammonia-oxidizing bacteria (AOB) were 6.12×1010 copies·L-1 and 4.93×109 copies·L-1, respectively. The average hzsB gene abundance was 2.31×109 copies·L-1. The average abundance of 16S rRNA in the treatment was 8.90×1010 copies·L-1. The abundances of hzsB and nirS genes in the straw and wheat straw were higher than those in the other treatment, indicating that the activities of anaerobic ammonia oxidation and denitrifying microorganisms were significantly increased by the addition of straw and wheat straw (P<0.05). In addition, the abundance of AOA and AOB genes were increased in wheat straw, suggesting that wheat straw could promote nitrification. The results provided data supporting the molecular mechanism of nitrogen removal in swine wastewater treatment with straw materials.

[Nitrogen and Phosphorus Removal in Surface Flow Constructed Wetland Planted with Myriophyllum elatinoides Treating Swine Wastewater in Subtropical Central China].

The loss of nitrogen (N) and phosphorus (P) from aquaculture has caused eutrophication of freshwater systems. Here, surface flow constructed wetland (SFCW) planted with Myriophyllum elatinoides were used to treat swine wastewater from a medium-sized hoggery in subtropical Central China. Inflow concentrations of NH4+-N, TN, TP, and COD ranged from 535.4 to 591.09, 682.09 to 766.96, 57.73 to 82.29, and 918.4 to 1940.43 mg·L-1, respectively. The mean removal efficiencies of NH4+-N, TN, TP, and COD were 97.4%, 97.1%, 91.0%, and 90.2%, respectively, and CW1 had the largest contributions of 37.3%, 38.4%, 43.3%, and 27.4%, respectively. Plant N and P uptake ranged 23.87-79.96 g·m-2 and 5.34-18.98 g·m-2, accounting for 19.1% and 20.2% of removal, respectively. Sediment N and P accumulation ranged 19.17-56.62 g·m-2 and 10.59-26.62 g·m-2, accounting for 19.8% and 61.7% of removal, respectively. Multiple linear regression showed that environmental factors explained 79.9% of the N removal and 70.1% of the P removal; DO was the main factor affecting N removal, and sediment adsorption was the key process in P removal. These results show that M. elatinoides constructed wetland can efficiently treat swine wastewater, thereby reduce the discharge of pollutants downstream.

[Characteristics of Paddy Soil Organic Carbon Mineralization and Influencing Factors Under Different Water Conditions and Microbial Biomass Levels].

Paddy soil often undergoes frequent dry-wet alternation. The change in water status not only affects the physical and chemical properties of the soil, but also changes the structure and diversity of the soil microbial communities, which in turn determines the rate of soil organic carbon mineralization. However, the effects of different water conditions and soil microbial biomass levels on the process of soil organic carbon mineralization and its mechanisms are still unclear. Therefore, this study took typical subtropical paddy soil as the research object, applied a laboratory incubation experiment with two water treatments of dry-wet and continuous flooding, and reduced the soil microbial biomass through chloroform fumigation, thereby obtaining high and low soil microbial biomass carbon contents, to elucidate the influencing mechanisms of microbial biomass and water conditions on organic carbon mineralization in paddy soil. The results showed that during the first 30 d of incubation, the dry-wet treatment was in a non-flooded stage and its cumulative CO2 emissions were significantly lower than those of the continuous flooded treatment. After 30 d, the dry-wet treatment entered the flooded stage. The difference in the cumulative CO2 emissions of the soils with a high microbial biomass carbon content between the dry-wet and continuous flooding treatments gradually decreased, and there was no significant difference on day 78. In the soil with a low microbial biomass carbon content, the cumulative CO2 emissions of the dry-wet treatment on day 78 was still significantly lower than that of the continuous flooded treatment. The soils with a low microbial biomass carbon content showed a faster CO2 emission rate at the beginning of the incubation period (first 20 d), which was 1.1-6.1 times greater than that of the high microbial biomass carbon soils owing to their high soil dissolved organic carbon (DOC) content, and the CO2 emission rate then gradually decreased until it was below that of the soil with a high microbial biomass carbon content. The soil organic carbon mineralization rate became stable later in the incubation period (days 45-78). The stable mineralization rate of the high microbial biomass carbon soil was 20%-30% higher than that of the low microbial biomass carbon soil. The multiple regression analysis results showed that the decrease in the soil DOC content (ΔDOC) and the increase in the Fe2+ content (ΔFe2+) significantly affected the change in cumulative CO2 emissions (ΔCO2) under continuous flooding conditions, but had no effect on ΔCO2 during the flooding stage of the dry-wet treatment. The correlation analysis showed that the daily CO2 emission rate of soils with high microbial biomass carbon was significantly positively correlated with glucosidase activity under dry-wet treatment and significantly negatively correlated with acetylglucosaminidase (NAG) and peroxidase activities under continuous flooding treatment. In the low microbial biomass carbon soils, the daily CO2 emission rate of the continuous flooding treatment was negatively correlated with the NAG activity, but showed no correlation with enzyme activities under dry-wet management. In summary, the cumulative CO2 emissions of dry-wet treatment were lower than those of continuous flooding treatment, and the difference was significant in soils with low microbial biomass carbon. The size of the soil microbial biomass determined the level of the stable soil organic carbon mineralization rate. The amount of soluble organic carbon and iron reduction affected the soil CO2 emissions under continuous flooding conditions, and the soil water conditions affected the daily CO2 emission rate and its key influencing enzymes. This study provides data and theoretical support for the carbon cycle and carbon sequestration potential in paddy soil.

[Effect of Water Management on Rice Growth and Rhizosphere Priming Effect in Paddy Soils].

The rhizosphere priming effect (RPE) caused by carbon inputs from crop rhizodeposits plays a key role in regulating the carbon emission flux and carbon balance of farmland soils. Due to frequent alternations between dry and wet conditions, CO2 and CH4 emissions and the RPE in paddy field ecosystems are significantly different to those of other ecosystems. Therefore, it is of great significance to determine the direction and intensity of the rice RPE under alternations of dry and wet to limit greenhouse gas emissions. In this study, using a 13C-CO2 continuous labeling method combined with a pot-based experiment, the response of rice growth and the RPE under alternating dry and wet and continuous flooding conditions was examined. The results showed that, compared with the continuous flooding treatment, the alternating dry and wet treatments significantly increased aboveground and root biomass and the root-to-root ratio, and also increased soil microbial biomass. Under continuous flooding conditions, fluxes of 13CO2 and 13CH4 increased with rice growth from 10.2 µg·(kg·h)-1 and 2.8 µg·(kg·h)-1 (63 d) to 16.0 µg·(kg·h)-1 and 3.2 µg·(kg·h)-1 (75 d), respectively. During the 12-day drying process, the emissions of 13CO2 and 13CH4 derived from rhizosphere deposited C decreased by 57.5% and 88.1%. Under continuous flooding conditions, the RPE for CO2 and CH4 were positive and increased with the growth of rice. Under the alternating dry and wet treatment, after 12 days of drying, the RPE for CO2 and CH4 was reduced from 0.29 mg·(kg·h)-1 and 12.3 µg·(kg·h)-1 (63 d) to -0.39 mg·(kg·h)-1 and 0.07 µg·(kg·h)-1 (75 d). Thus, alternating wet and dry treatment can effectively promote rice growth and reduce the cumulative emissions of CH4. Therefore, adopting appropriate field water management is of great significance for increasing rice yields and mitigating greenhouse gas emissions.

Hydrocarbon Generation and Chemical Structure Evolution from Confined Pyrolysis of Bituminous Coal.

The molecular composition of organic matter formed during pyrolysis is complex. Fourier transform infrared spectroscopy (FTIR) is a good technique to investigate the coal chemical structural evolution. However, reports on the effects of chemical structure on the n-alkane yields and their relative functional groups are scarce in the literature. In our case, the chemical structural evolution process of bituminous coal obtained by pyrolysis at two different heating rates has been analyzed by pyrolysis-gas chromatography (Py-GC) and FTIR. Furthermore, some of the small molecular compounds (e.g., n-alkanes 24 can generate n-alkanes 20 or low-weight compounds) generated by gold-tube pyrolysis were identified using other GC techniques. Biomarkers were analyzed and compared to generated n-alkanes from the gold-tube pyrolysis experiments. We present the results of the relationship between the FTIR parameters and the molecular compositions that were analyzed. A good linear relationship can be seen between the FTIR parameters (C=O, C=C, and C-factor values), the carbon preference index (CPI), and the ratio of the pristane content and n-C17 alkane content (Pr/n-C17). Furthermore, the n-alkane fraction of the pyrolysates, in particular pristane, phytane, n-C17 alkane, and n-C18 alkane, changed upon maturation. Our conclusions indicate that FTIR is applicable as a structural and chemical change probe to explore the pyrolysis process.

[Response of Extracellular Enzyme Activities to Substrate Availability in Paddy Soil with Long-term Fertilizer Management].

The availability of carbon (C), nitrogen (N), and other substrates in soil determines the growth and metabolism of microorganisms and affects the activity of extracellular enzymes. To study the activities of ß-1,4-glucosidase (BG) and ß-1,4-N-acetylglucosaminidase (NAG) in response to C and N availability, samples that underwent four treatments-non-fertilization (CK), chemical fertilizer (NPK), combination of organic manure and chemical fertilizer (OM), and mixture of straw and chemical fertilizer (ST)-were collected from long-term fertilization paddy soil and incubated for 0, 4, 8, and 12 months to obtain soil with different C and N availability gradients. The results showed that the dissolved organic carbon(DOC) content of OM and ST treatment samples was 2-3 times higher than that of CK and NPK treatment samples. With the increase of DOC and ammonium (NH4+-N) contents, the activities of BG and NAG and the contents of microbial biomass C (MBC) and N (MBN) showed no increase during incubation within each treatment. Fertilization treatments, incubation time, and their interaction are crucial factors varying the contents of DOC, NH4+-N, MBC, and MBN among different fertilization treatments (P<0.01). There was a positive correlation between MBC/MBN and DOC/NH4+-N of OM treatment (P<0.05) and a negative relationship between ln(BG)/ln(NAG) and DOC/NH4+-N of ST treatment (P<0.01), indicating that the availability of substrates played a key role in the potential activity of extracellular enzymes in paddy soil, and the carbon-nitrogen ratio of microbial biomass was controlled by the C/N stoichiometry of substrates in soil. The results have a certain guiding significance for further study on the variation of extracellular enzyme activity in paddy soil, regulating the balance of carbon and nitrogen, and improving the fertility of paddy soil.

Effects of top excision on the potassium accumulation and expression of potassium channel genes in tobacco.

The effects of the removal of the shoot apex of tobacco on the relative transcript levels of potassium channel genes, determined by real-time PCR, and on the relationship between the expression of genes encoding potassium channels and potassium concentration, were studied. The results from the study indicated that comparatively more assimilates of photosynthesis were allocated to the apex in control plants than in both decapitated and IAA-treated decapitated plants. By contrast, dry matter in the upper leaves, roots, and stems in both decapitated and IAA-treated plants was significantly increased relative to control plants. The potassium level in whole plants decreased post-decapitation compared with control plants, and so did the potassium concentration in middle and upper leaves, stem, and roots. Expression of NKT1, NtKC1, NTORK1, and NKT2 was inhibited by decapitation in tobacco leaves with a gradual reduction after decapitation, but was induced in roots. The relative expression of NKT1, NTORK1, and NKT2 in tobacco leaves was higher than that in roots, whereas the expression of NtKC1 was higher in roots. The levels of inhibition and induction of NKT1, NtKC1, NTORK1, and NKT2 in leaves and roots, respectively, associated with decapitation were reduced by the application of IAA on the cut surface of the decapitated stem. Further results showed that the level of endogenous auxin IAA in decapitated plants, which dropped in leaves and increased in roots by 140.7% at 14 d compared with the control plant, might be attributed to the change in the expression of potassium channel genes. The results suggest that there is a reciprocal relationship among endogenous auxin IAA, expression of potassium channel genes and potassium accumulation. They further imply that the endogenous IAA probably plays a role in regulating the expression of potassium channel genes, and that variations in expression of these genes affected the accumulation and distribution of potassium in tobacco.

[Influence of Biochar Amendment on Soil Denitrifying Microorganisms in Double Rice Cropping System].

At present, it is not explicit how biochar regulates the microbial process of denitrification in paddy fields. Therefore, a field experiment was carried out in a double rice cropping system with three wheat straw biochar treatments:no biochar treatment (CK), added 24 t·hm-2 biochar (LC), and added 48 t·hm-2 biochar (HC). Real time PCR (qPCR) and terminal-restricted fragment length polymorphism (T-RFLP) technology were adopted to analyze the abundances and microbial community structures of denitrification functional genes (narG, nirK and nosZ) in the fallow season and rice season. Due to its alkalinity, biochar amendment increased soil pH by 0.2-0.8. Biochar amendment also increased soil NH4+-N and NO3--N contents by 21.1%-32.5% and 63.0%-176.0% in the fallow season due to the presence of soluble N. Nevertheless, it reduced NH4+-N content by 48.8%-60.1% in the rice season due to the adsorption of biochar. The amendment increased soil MBN content in the fallow season, which may be a result of the large surface of biochar supplying nutrients and a suitable survival environment for the microorganisms. In the fallow season, compared to CK treatment, the increased soil NH4+-N and NO3--N with biochar amendment promoted the conversion of NH4+-N to NO3--N, and thus increased the abundances of narG and nosZ (P<0.05). The higher soil pH with biochar addition increased the abundances of nosZ and altered the community structures of narG and nosZ in the fallow season. Biochar amendment altered the denitrification process, but it did not change N2O emissions in the fallow season, which might reduce NO3--N leaching losses. In the rice season, biochar amendment increased nosZ abundance (P<0.05). HC increased the nirK gene abundance, which contributed to increased N2O emission in the rice season (P<0.05). Biochar converted the community structures of nirK and nosZ by decreasing the NH4+-N content in the rice season. The changes of the narG community structure with HC treatment contributed to the increased N2O emission. In conclusion, biochar amendment can influence the microbes involved in soil denitrification by changing the soil properties, and thus impact the N2O emissions and NO3--N leaching.

[Effects of Biochar Amendment on Soil Microbial Biomass Carbon, Nitrogen and Dissolved Organic Carbon, Nitrogen in Paddy Soils].

Biochar can influence soil microbial biomass. It is not clear how biochar amendment affects soil microbial biomass carbon and nitrogen (MBC and MBN) and dissolved organic carbon and nitrogen (DOC and DON) in double-cropping rice soils. To address this problem, two subtropical double-cropping rice soils (S1 and S2) were selected for an incubation experiment. S1 is developed from granite-weathered red soil and S2 is developed from Quaternary red clay. The following three wheat straw-derived biochar application rates were used, without N fertilizer, in each paddy soil:0%, 1%, and 2% of soil weight, represented by CK, LB, and HB, respectively. After a 70 d incubation, soil mean MBC was 877.03 mg·kg-1, 832.11 mg·kg-1, and 849.30 mg·kg-1 in S1 for the three application rates, and 902.94 mg·kg-1, 874.19 mg·kg-1, and 883.22 mg·kg-1, respectively, in S2. S1+LB, S1+HB, and S2+LB treatments reduced soil mean MBC compared to the CK treatment (P<0.05). This may be attributed to biochar inhibiting microbial growth by adsorbing soil organic carbon and other low-molecular-weight organic matter. Low biochar application rates decreased mean soil MBN by 9.45% compared to the CK treatment in S1 (P<0.05). No significant differences in mean MBC/MBN were observed among the S1 treatments, but LB reduced MBC/MBN in S2 (P<0.05). Due to the soluble organic carbon content and strong alkalinity of biochar, biochar amendment increased mean soil DOC by 4.42%-22.20% and 10.57%-35.47% in S1 and S2, respectively (P<0.05). However, biochar amendment (except for the S2+HB treatment) decreased mean soil DON in both paddy soils. This may have resulted from the adsorption of soil organic nitrogen by biochar and N consumption during the decomposition of the organic carbon within biochar. Biochar amendment increased mean soil DOC/DON in both paddy soils (P<0.05) and mean DOC/DON increased with an increase in the biochar application rate. Based on these results, biochar amendment increased soil dissolved organic carbon, decreased soil microbial biomass, and enhanced the nitrogen deficit in double-cropping paddy soils. Therefore, biochar should be combined with the application with fertilizer in double-cropping rice systems in subtropical central China.

[Atmospheric Nitrogen Dioxide, Nitric Acid, Nitrate Nitrogen Concentrations, and Wet and Dry Deposition Rates in a Double Rice Region in Subtropical China].

Nitrogen dioxide (NO2) and nitric acid (HNO3) are nitrogen-containing acidic gases in the atmosphere, and they are important precursors of nitrate in aerosol and rainwater. The emission intensity of atmospheric nitrogen oxides is high in the subtropical region of China, but the concentrations and deposition rates of atmospheric nitrogen dioxide, nitric acid, particulate nitrate-nitrogen (NO3--Np), and rainwater nitrate-nitrogen (NO3--Nr) in a double rice region in subtropical China are still unclear,. In this study, the atmosphere concentrations of NO2-N, HNO3-N, NO3--Np in PM10, and NO3--Nr and related meteorological parameters were simultaneously monitored in a typical double rice region within a subtropical hilly region of China, with the aim of determining the characteristics and influencing factors of NO2-N, HNO3-N, NO3--Np, and NO3--Nr concentrations and quantifying the wet and dry deposition rates. The results showed that the annual mean concentrations of NO2-N, HNO3-N, NO3--Np, and NO3--Nr were 4.2 µg·m-3, 0.7 µg·m-3, 4.0 µg·m-3, and 1.0 mg·L-1, respectively, and the deposition rates were 1.5, 3.2, 2.3, and 6.1 kg·hm-2, respectively. The NO2-N concentrations were negatively correlated with air temperatures, and the HNO3-N concentrations were negatively correlated with wind speeds. TheNO3--Np concentrations were negatively correlated with air temperatures, positively correlated with NO2-N concentrations, but not significantly correlated with HNO3-N concentrations, thus indicating that NO2-N concentrations were an important limiting factor forNO3--Np pollution in this study area. The NO3--Nr concentrations were negatively correlated with rainfall, as well as the concentrations of HNO3-N and NO3--Np. The annual total dry and wet depositions of the atmospheric NO2-N, HNO3-N, NO3--Np, and NO3--Nr were 13.0 kg·hm-2, which indicates that these compounds are important sources of nitrogen in paddy fields and may have significant impacts on paddy fields and surrounding ecosystems.

[Decomposition of Myriophyllum aquaticum and the Associated Release of Nitrogen and Phosphorus].

Decomposition of wetland plants could release pollutants, which may affect the removal efficiency and effluent quality of constructed wetlands. The experimental decomposition test of Myriophyllum aquaticum was carried out for 60 d using nylon bags, and release characteristics of nitrogen and phosphorus during the decomposition process were studied. The results showed that the decomposition rate of M. aquaticum was fastest during the first 0-4 d, with a weight loss of 30%, while the degradation rate slowed gradually during the period 4-60 d, with weight loss of 31%. The fitting first-order kinetic decomposition rate constant was 0.0142 d-1, and the calculated time to degrade 50% of dry matter was 48.8 d. The water pH decreased rapidly from 7.60 to 5.63 during 0-4 d, stabilized during 4-32 d, and finally increased to 7.03 (which was close to the control sample without M. aquaticum). The dissolved oxygen concentration decreased rapidly from 6.30 mg·L-1 to 0.61 mg·L-1 during 0-4 d, and remained in an anaerobic state. The total nitrogen concentration in the water increased rapidly to 12.7 mg·L-1 within 2 h, gradually decreased to 5.80 mg·L-1 during 2 h-32 d, and then finally increased slightly. The phosphorus concentration increased rapidly to 18.4 mg·L-1 at the beginning of the experiment, and then gradually stabilized. The main forms of nitrogen and phosphorus released by M. aquaticum were organic nitrogen (accounting for 65.7%-94.7% of total nitrogen) and inorganic phosphorus (accounting for 61%-89% of total phosphorus), respectively. The total nitrogen content of M. aquaticum increased from 24.3 mg·g-1 to 60.5 mg·g-1 with increasing degradation time; the total phosphorus decreased initially from 6.09 mg·g-1 to 2.94 mg·g-1 and then remained constant. These trends may have been related to the fixation of nitrogen by attached microorganisms. Therefore, suitable harvesting and management strategies should be adopted for wetland plants to reduce secondary pollution.

[Removal Performance and Mechanism for Treating Phosphorus in Agricultural Wastewater by Three Adsorbents].

To screen the optimal absorbents for P removal from agricultural wastewater, the P adsorption capacity of bentonite, red soil, and slag was studied using synthetic wastewater. Combing the properties of three adsorbents measured by SEM, XDS, and BET methods, the isothermal adsorption, adsorption kinetics, and Ca2+ release capacity were analyzed to elucidate the mechanisms of P adsorption. The results showed that the P adsorption capacity of slag was higher than that of bentonite and red soil, and the Langmuir isotherm model was able to better fit the adsorption data (R2>0.96). The P theoretical saturation sorption capacity of slag was higher (16.87 mg·g-1) than that of bentonite (1.21 mg·g-1) and red soil (0.92 mg·g-1) (P<0.05). The results for adsorption kinetics indicated that slag rapidly removed 95.6% of P from 10 mg·L-1 solution, and the Elovich equation fit the data well (R2=0.812). The adsorption kinetics of P on bentonite and red soil were better described by the pseudo-second-order kinetic equation (R2=0.982 and 0.959, respectively). The Ca2+ release capacity of slag (10.46 mg·g-1) was significantly higher compared to bentonite (0.31 mg·g-1) and red soil (0.03 mg·g-1) (P<0.05). The P adsorption capacity of red soil was 0.26 mg·g-1 when the pH value was 3, and it decreased as the pH values increased. At the initial pH of 7.0, the P adsorption capacity of bentonite was about 0.01 mg·g-1, lower than 0.04 mg·g-1 at pH 3, and 0.05 mg·g-1 at pH 11. The initial pH value had little effect on the P adsorption capacity of slag. The P-loaded bentonite, red soil, and slag were effectively regenerated by using CaCl2 solution, and bentonite was easier to reuse compared to red soil and slag. The key factors affecting the P adsorption capacity of the three adsorbents were physical and chemical properties, such as crystal structure and the content of metal ions, Ca2+ release capacity, and initial pH. These findings demonstrated that slag was a better choice for P removal compared to bentonite and red soil and could be used as an effective P adsorbent for agricultural wastewater treatment.