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.

[Response of Soil Multifunctionality to Reduced Microbial Diversity].

Soil microbial communities play an important role in driving a variety of ecosystem functions and ecological processes and are the primary driving force in maintaining the biogeochemical cycle. It has been observed that soil microbial diversity decreases with land use intensification and climate change in the global background. It is essential to investigate whether the reduction in soil microbial diversity can affect soil multifunctionality. Thus, in this study, the dilution-to-extinction method was used to construct the gradient of soil microbial diversity, combined with high-throughput sequencing to explore the impact of the reduction in bacterial, fungal, and protist diversity on soil multifunctionality. The results showed that the soil microbial alpha diversity (richness and Shannon index) was significantly lower than that of the original soil. Principal coordinate analysis (PCoA) showed that the microbial community structure of original soil was significantly different from that of diluted soil, and the response of bacterial and fungal communities to diluted soil was higher than that of protists. The regression model showed that there was a significant negative linear relationship between the average response value of soil multi-function and the index of microbial diversity, indicating that the change in soil microbial community was the key factor in regulating soil multifunctionality. The regression model showed that there was a significant negative linear relationship between soil multifunctionality and microbial diversity, indicating that the change in soil microbial community was the key factor to regulate soil multi-kinetic energy. Through the aggregated boosted tree analysis (ABT) and regression model, we found that some specific microbial groups, such as the Solacocozyma and Holtermaniella of fungi and Rudaea of bacteria, could significantly promote the change in soil multifunctionality, which showed that key microbial taxa play an indicative role in biological processes. Furthermore, the structural equation model revealed that bacteria could affect soil multifunctionality through the interaction between microbiomes, which was the key biological factor driving the change in soil multifunctionality. This study provided experimental evidence for the impact of soil microbial diversity on soil multifunctionality, and promoted the notion that maintaining a certain diversity of soil microbial community in a single agricultural ecosystem, especially the diversity of key microbial taxa, is of great significance to the sustainable development of ecosystem function in the future.

[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.

[Response Characteristics of Soil Microbial Community Under Long-term Film Mulching].

Film mulching is an important practice to increase the yield and income in agricultural production. Soil samples were collected from four farmland sites with different mulching years to reveal the effect of long-term plastic mulching on characteristics of soil microbial community structure. In order to explore the long-term effect of soil microbial community change and its effect on the microbial ecological environment, high-throughput sequencing technology was used to analyze the changes in soil bacterial and fungal community structure. The results showed that long-term film mulching had no significant effect on soil bacterial diversity but decreased fungal diversity. Long-term film mulching decreased the abundance of Acidobacteriota and Mortierellomycetes and increased the abundance of Actinobacteriota. Long-term film mulching enriched the beneficial microbial communities such as Bacillus, Nocardioidaceae, Aspergillus, and Hypocreales in soil. However, long-term film mulching indued a simple and fragile soil fungal co-occurrence network pattern. The unidentified Sordariales under Ascomycota was the only key species in the fungal co-occurrence network, which resulted in potential risks to the ecological environment of the farmland soil. This study provided a theoretical basis for further understanding the effects of long-term film mulching on the ecological and environmental effects of microorganisms in farmland.

[Effects of Plastic Mulch Film on Soil Nutrients and Ecological Enzyme Stoichiometry in Farmland].

Ecological enzyme stoichiometry can be used to evaluate the limit of soil microbial energy and nutrient resources. To illustrate the effects of plastic mulch film on soil ecological enzyme stoichiometry in farmland, this study collected soil with different amounts of mulching film residual and used the fluorescence analysis to determine the activities of key enzymes for the carbon, nitrogen, and phosphorus cycle processes including ß-1,4-glycosidase (BG), ß-1,4-N-acetyl amino glycosidase (NAG), and phosphatase (ACP) activity. This study investigated the effects of plastic mulch film on soil nutrient cycling and supply in farmland. The results showed that in the soil with chemical fertilizer, plastic film mulching decreased soil Olsen-P and NO3--N contents to 48%-62% and 16%-24% of those in the soil without plastic film mulching, respectively. In the soil with the combined application of organic-chemical fertilizers, plastic film mulching increased Olsen-P and NO3--N contents by 144%-203% and 1.9-5.1 times, respectively. In the organic-chemical fertilization soils, plastic film mulching decreased SOC:TN in soils by 6.6%-25.8%, whereas it increased SOC:TP and TN:TP significantly. MBC, MBN, and MBP contents in the soil with plastic film mulching were significantly lower than that in non-plastic film mulching farmland, and there were no significant differences in MBC:MBN and MBC:MBP between soil with and without plastic film mulching. The MBN:MBP was reduced by 36.6% and 23.8% in S1 and S2, and 5.4 and 1.3 times in S3 and S4 by plastic film mulching, respectively. The change pattern of NAG:ACP in soil was similar to that of the corresponding elements ratio in microbial biomass. In the soil from plastic film mulching, the ratio of BG:NAG was 1.3-15 times higher in organic-chemical fertilization soils than that with only chemical fertilizer. In conclusion, plastic film mulching reduced the availability of soil nutrients, and organic-chemical fertilization alleviated the limitation of soil nutrients to a certain extent. This study deepened the understanding of the response of soil microorganisms to nutrient cycling after plastic film mulching. It provides a theoretical basis for optimizing the farmland management in the use of plastic film.

[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.

[Consumption Capacity of N2O in Paddy Soil and the Response Mechanism of nosZ-I-containing Communities].

The long-term flooding anaerobic environment in paddy soils is conducive to denitrification, which is one of the most important reasons for N2O emissions. N2O can be transformed to nitrogen gas (N2) by bacteria and archaea containing nitrous oxide reductase (N2OR) encoded by the nosZ gene, which is the only known biological pathway of N2O consumption in soil. nosZ-I is known to be typical in denitrifying bacteria, which is one of the clades of the nosZ gene and is mainly possessed a Tat signal peptide motif. Although many researchers have studied N2O emission characteristics of paddy soil, the capacity of N2O consumption and the response mechanism of related functional microorganisms in paddy fields is not yet clear. To verify the effect of exogenous N2O on N2O consumption and nosZ-I gene, a pot trial experiment was performed under anaerobic conditions. We collected intact soil cores from flooding paddy fields at a 0-5 cm depth, and exogenous N2O gas was input through the bottom of flooding paddy soil cores. Meanwhile, a control treatment (CK) with no additional N2O gas was also performed. The dynamic characteristics of the added exogenous N2O concentration through the intact soil cores, the content of inorganic nitrogen, and DOC were systematically monitored. In addition, the change in the nosZ-I population diversity and community composition were investigated by high-throughput sequencing approaches, with the purpose of revealing the N2O uptake ability of flooded paddy soil and the response mechanism of the nosZ-I population. The results showed that 97.39% of exogenous N2O diffused into the soil cores, and only 0.72%-7.75% of exogenous N2O escaped from the soil surface. The N2O released in the headspace of soil cores could continue being absorbed and consumed by the flooding soil column. In addition, 67.10% of the N2O escaped to the headspace was consumed in exogenous N2O treatment after 192 h of incubation, which was higher than that in CK treatment, and the N2O consumption rate increased by 144.2% than that in CK treatment. Meanwhile, the consumption of NH4+-N, NO3--N, and DOC consumed during exogenous N2O addition treatment was 19.65%, 16.29%, and 8.41% higher than that in CK treatment, respectively. However, the diversity of the nosZ-I gene community had no significant difference; the community composition of nosZ-I-containing bacteria changed significantly after 192 h when exogenous N2O was input. The abundances of OTU5004, OTU5065, OTU960, and OTU1282 (Proteobacteria) significantly increased, which were the dominant bacterial strain of nosZ-I gene on the OTU level. Compared with the initial sample and CK, the abundance of the OTU5004 strain increased by 7.3% and 4.63%, and the abundance of the OTU5265 strain (Azoarcus sp.) increased by 0.33% and 0.15%, respectively. The result indicated that the flooding paddy soil column at the soil layer of 0-5 cm has a strong N2O absorption and consumption ability. In summary, compared with CK, the addition of exogenous N2O significantly accelerated the N2O consumption rate, improved the consumption potential of flooding paddy soil column, promoted carbon and nitrogen conversion, and changed nosZ-I community composition. These results would provide a new reference for reducing atmospheric N2O emissions.

[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.

[Impact of Dicyandiamide (DCD) and 3,4-Dimethylpyrazole Phosphate (DMPP) on Ammonia-oxidizing Bacteria and Archaea in a Vegetable Planting Soil].

Nitrification inhibitors (NIs) dicyandiamide (DCD) and 3,4-dimethylpyrazole phosphate (DMPP) showed significant effects in the inhibition of nitrification and the improvement of the utilization efficiency of nitrogen fertilizer in agricultural soils. However, the effects of different NIs on ammonia-oxidizing bacteria (AOB) and archaea (AOA) is still unclear. To verify the inhibitory effect of DCD and DMPP on AOB and AOA, a pot experiment was performed, including Urea, Urea+DCD, and Urea+DMPP treatments. The dynamics of NH4+-N and NO3--N and nitrification potential among different treatments were measured. In addition, real-time PCR and high-throughput sequencing approaches were applied to investigate the changes in the AOB and AOA population abundance and composition. The results revealed that the concentrations of NH4+-N in Urea+DCD and Urea+DMPP treatments were 213% and 675% higher than that in the CK treatment, respectively. However, the concentrations of NO3--N and the nitrification potentials were 13.3% and 37.2%, and 20.4% and 82.4% lower than that in CK treatment, respectively; Furthermore, the copy numbers of the bacterial and archaeal amoA gene were 51.2% and 56.5%, and 6.0% and 27.0% lower than that in the CK treatment, respectively. However, the diversity indexes of AOB and AOA communities, including evenness and richness, exhibited no significant differences after addition of DCD and DMPP. The nork-environmental-samples, unclassified-Nitrosomonadaceae, unclassified-Bacteria, and Nitrosospira, were the predominant genera of the AOB community. The no rank-Crenarchaeota, no rank-environmental-samples and Nitrososphaera were the predominant groups in the AOA community. Summarily, application of DCD and DMPP significantly delayed the transformation of NH4+-N, decreased the formation of NO3--N, inhibited the abundance and changed the composition of AOB and AOA communities. DMPP had a stronger inhibitory effect on nitrification, and on AOB and AOA than DCD. Therefore, compared with DCD, DMPP had a better application prospect regarding the improvement of the nitrogen utilization efficiency in vegetable soil.

[Microbial Carbon Source Metabolic Profile in Rice Rhizosphere and Non-rhizosphere Soils with Different Long-term Fertilization Management].

Rhizosphere and non-rhizosphere soil samples under different long-term fertilization treatments including control without fertilizer (CK), chemical fertilization alone (NPK), rice residues combined with NPK (NPKS), 30% manure plus 70% chemical fertilizers (LOM), and 60% manure plus 40% chemical fertilizers (HOM) were collected from a paddy field in a red soil hilly area in Ningxiang City, Hunan Province, China. The characteristics of microbial carbon utilization in the soils were studied. Results of 18O-H2O tracer analysis showed that both soil microbial biomass carbon content (MBC) and microbial growth rate (CGrowth) were highest in the HOM treatment, whereas they were lowest in CK. In the rhizosphere soil, the highest basal respiration was observed in HOM, and the lowest values were in CK and NPK. Microbial carbon utilization efficiency (CUE) was highest in NPK but lowest in the LOM and HOM treatments. In non-rhizosphere soil, no significant differences between basal respiration and CUE were observed among the fertilization treatments. Results from MicroRespTM showed that the ability of microorganisms to metabolize exogenous carbon sources was higher in non-rhizosphere soil than in rhizosphere soil. The application of organic materials (rice residues or manure) increased the microbial metabolic rate of carboxylic acids, amino acids, and carbohydrates in the order carboxylic acids > amino acids and carbohydrates > complex compounds. Redundancy analysis of the microbial metabolism patterns of various carbon substrates showed that:① CK was well separated from the fertilization treatments; ② NPK was grouped with NPKS, whereas LOM and HOM were grouped together and were separate from NPK and NPKS. This indicates that the fertilization treatments changed the microbial carbon metabolism patterns. The above-mentioned results indicated that the fertilization treatments did not affect microbial CUE and basal respiration. However, exogenous carbon source input (such as root exudates) and the application of organic materials can increase microbial basal respiration, and thus, reduce microbial CUE.

[Transformation and Distribution of Soil Organic Carbon and the Microbial Characteristics in Response to Different Exogenous Carbon Input Levels in Paddy Soil].

The turnover of soil organic carbon (SOC) and the activity of soil microbes can be influenced by exogenous carbon. However, microbial response characteristics of the transformation and distribution of available organic carbon under different levels remain unclear in paddy soils. 13C-labeled glucose was used as a typical available exogenous carbon to simulate indoor culture experiments added at different levels of soil microbial biomass carbon (MBC) (0×MBC, 0.5×MBC, 1×MBC, 3×MBC, and 5×MBC) to reveal the process of C-transformation and distribution. The characteristics of microbial response in the process of exogenous carbon turnover was also monitored. The 96-well microplate fluorescence analysis was adopted to determine the activities of cellobiose hydrolase (CBH) and ß-glucosidase (ß-Glu). The results showed that, in 2 d of incubation, the ratio of labeled glucose carbon to dissolved organic carbon (13C-DOC/DOC) or to SOC (13C-SOC/SOC) was positively correlated with the amount of glucose added. The incorporation of glucose C (13C) into MBC reached the highest value (18.96 mg·kg-1) at 3×MBC treatment but decreased thereafter. The 13C allocation rate was mainly positively correlated with MBC, Olsen-P, and DOC. At 60 d, 13C-DOC, 13C-MBC, and 13C-SOC decreased significantly to less than 0.02 mg·kg-1, 2 mg·kg-1, and 10 mg·kg-1 in soil, and it was positively correlated with the amount of glucose added. Compared with CK, CBH enzyme activity increased significantly after the addition of glucose, and for the 3×MBC treatment it was increased by 22.6 times, which was significantly higher than those of other treatments (P<0.05). However, ß-Glu enzyme activity increased only in the 3×MBC and 5×MBC treatments, wherein it decreased with increasing amounts of added glucose. NH4+-N, pH, ß-Glu, and CBH were the primary factors affecting the distribution rate of 13C. In conclusion, the conversion of exogenous carbon to SOC increased with increased amounts of added organic carbon. This changed the activity of soil enzymes; however, microbial utilization of exogenous carbon may have a saturation threshold. Within the saturation threshold, the conversion rate of organic matter was directly proportional to the amount of added organic matter. When the saturation threshold was exceeded, the conversion rate of organic matter decreased. Therefore, the appropriate addition of exogenous carbon is beneficial, as it can increase SOC in rice fields and improve the quality of the crop growth environment.

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Soil samples were collected from paddy ecosystem under five long-term fertilization treatments, including control without fertilizer (CK), chemical fertilization alone (NPK), rice residue combined with NPK (NPKS), 30% manure plus 70% chemical fertilizer (LOM), and 60% manure plus 40% chemical fertilizer (HOM) in Ningxiang City, Hunan Province. The cha-racteristics of amino sugars accumulation in the rhizosphere and non-rhizosphere soils at rice tillering stage were analyzed. Results showed that the contents of soil organic carbon, total amino sugars and three amino monosaccharides (muramic acid, glucosamine and galactosamine) with long-term application of organic materials (rice residue or manure) were significantly higher compared with CK and NPK. The inconsistent accumulation trends of the three amino monosaccharides under different fertilization treatments indicated that different responses of microbial groups to various fertilization treatments. The content of total amino sugars was not significantly different between the rhizosphere soil and the non-rhizosphere soil, probably because the agricultural operations such as plowing could homogenize paddy soils. The contribution of amino sugar derived carbon to soil organic carbon ranged from 24.0 to 28.3 mg·g-1, which was highest in NPKS, and lowest in HOM and CK. The ratio of fungal to bacterial residues (fungal glucosamine/muramic acid) ranged from 24.4 to 36.6, indicating that fungi dominated the degradation and transformation of organic matter in all the soils. Compared with that under NPK and CK, the participation of organic matter transformation from fungi under NPKS treatment was increased, whereas the bacteria involved in organic matter transformation under HOM treatment was enhanced.

[Allocation and Stabilization Responses of Rice Photosynthetic Carbon in the Plant-Soil System to Phosphorus Application].

This research studied the response of the input and allocation of photosynthetic carbon (C) to phosphorus (P) in paddy soils. Two treatments were conducted in this experiment:no P application (P0) and the application of 80 mg·kg-1 of P (P80). The rice cultivar was the indica Zhongzao 39. The 13C-CO2 continuous labeling technique was used to identify the photosynthetic C distribution of the rice. The results showed that the application of P80 significantly increased the photosynthates allocation in the rice aboveground, but reduced their allocation in the rhizosphere soil (P<0.05). At the jointing stage, P80 application increased the photosynthetic C content of the rice by 70%, but the root dry weight decreased 31%. Compared with P0, the total C content of the aboveground rice was increased 0.31 g·pot-1 by P80. The ratio of rice roots to shoots decreased with the P80 treatment. Moreover, P80 application led to an increase in the photosynthetic microbial biomass in the non-rhizosphere soil C (13C-MBC) of 0.03 mg·kg-1, but still decreased its allocation in the rhizosphere soil. The allocation of photosynthetic C to the particulate organic matter fraction (POC) and mineral fraction (MOC) in the non-rhizosphere soil showed no significant differences between P0 and P80. Additionally, the P80 fertilization treatment significantly lowered the content of POC in the rhizosphere soil. In summary, P application increased the allocation of photosynthetic C in the soil-rice system, but reduced the accumulation of photosynthetic C in the soil. This research provided a theoretical basis and data supporting the rational application of P fertilizer, and was also of great significance as a study of the transportation and allocation of photosynthetic C and its sequestration potential response to the application of P to the rice soil.

[Characterization of Soil Organic Carbon Mineralization Under Different Gradient Carbon Loading in Paddy Soil].

Available carbon is the most active part of the soil carbon pool. It is also the main carbon source of soil microbes and plays an important role in the processes of soil organic carbon mineralization and accumulation. However, the mechanisms are still not clear how soil organic carbon mineralization and its priming effect (PE) are affected by different input levels of readily available carbon, based on the growth requirements of microbes in paddy soil. In this study, an incubation experiment was conducted by adding different levels (0.5, 1, 3, and 5 times of MBC) of exogenous source organic carbon (13C-glucose) to the soil. The mineralization dynamics of labile organic carbon and its priming effect was investigated. The mineralization rate of glucose-C increased significantly with the increasing carbon loading level. The distribution of glucose-C into rapid and slow C pools was also exponentially correlated with the carbon loading (R2=0.99, P<0.05 and R2=0.99, P<0.05, respectively). Negative PE was observed at high carbon loading (3×MBC and 5×MBC); while positive PE was induced by low carbon loading (0.5×MBC and 1×MBC). The cumulative PE was 160.0 mg·kg-1 and 325.1 mg·kg-1, respectively, at the end of the incubation. Redundancy analysis showed that the main factors affecting the cumulative PE were MBC, MBN, and DOC at the initial glucose mineralization stage, while ß-glucosidase, chitinase, and ammonium nitrogen were the main factors at later stages. Therefore, the readily available carbon loading has an important effect on the organic carbon mineralization and PE in paddy soil. Higher carbon loading was good for the accumulation of organic carbon sequestration in paddy soil. This study is of great scientific significance for revealing the activity of organic carbon in paddy fields and for its contribution to the development of sustainable agriculture.

[Effects of Varying Long-term Fertilization on Organic Carbon Mineralization and Priming Effect of Paddy Soil].

A laboratory incubation experiment was conducted using the 14C isotope labeling technique to study the characteristics of organic carbon mineralization and their response to glucose addition when treated with a combination of straw and chemical fertilizer (ST), inorganic fertilizer (NPK), and non-fertilization (CK). The cumulative mineralization rate (ratio of accumulated mineralization amount to total organic carbon content) in CK reaches 1.64% at the end of incubation (56 days). The cumulative mineralization rate during NPK and ST treatments is significantly lower than that in CK (by 0.34% and 0.39%, respectively). This indicates that long-term fertilization affects the soil carbon sequestration. Varying long-term fertilization influences the response of paddy soil to glucose addition and leads to different levels of the priming effect. The priming effect on soil organic carbon mineralization of the three treatments gradually changes from negative to positive with increasing incubation time. The significantly negative cumulative priming effect in ST and NPK after 56 d is 22.07 and 9.05 times higher than that in CK, respectively. The results of the structural equation model indicate that the NH4+-N and DOC contents indirectly influence the cumulative priming effect on soil organic carbon by affecting the MBC and MBN contents. The NH4+-N concentration has a direct and significant negative effect on the cumulative priming effect. In conclusion, long-term fertilization treatments reduce the cumulative organic carbon mineralization rate of paddy soil. Fertilizer, especially the combination of straw and chemical fertilizer, enhances the soil carbon sequestration and accumulation.

[Effect of Phosphorus Addition on the Abundance of Autotrophic CO2-Fixation Microorganisms in Rhizospheric Soil from a Phosphorus-Limited Paddy Field].

A rice pot experiment was conducted to investigate the effect of phosphorus addition on the abundance of autotrophic CO2-fixation microorganisms using phosphorus-limited paddy soil from the Changsha Observation and Research Station for the Agricultural Environment. Rice seedlings were transplanted in the paddy soil with or without phosphorus addition, corresponding to P-treated-pot (P) or control pot (CK), respectively. Rhizosphere soils were collected from the P and CK treatments during the tillering and shooting stages. The physical and chemical soil properties were measured and the abundance of autotrophic CO2-fixation microorganisms was quantified with a real-time PCR technique based on four functional genes (cbbL, cbbM, accA, and aclB) involved in three CO2-fixation pathways (CBB cycle, rTCA cycle, and 3-hydroxypropionate/4-hydroxybutyrate cycle). The results show that phosphorus addition improves the concentrations of DOC and Olsen-P and the pH value, whereas negative effects on the MBC and NH4+-N concentrations are revealed during the tillering stage. The effect of phosphorus addition on the NO3--N concentration in the tillering and shooting stages differs. Phosphorus addition significantly increases the abundances of the cbbL, cbbM, accA, and aclB genes, which are 156%, 99%, 110%, and 193% higher than those of the CK treatment in the tillering stage. However, this positive effect is not notable for the cbbL, accA, and aclB genes during the shooting stage. Redundancy analysis (RDA) shows that Olsen-P is the environmental factor that most significantly affects the abundance of autotrophic CO2-fixation microorganisms.

[Effect of CO2 Doubling and Different Plant Growth Stages on Rice Carbon, Nitrogen, and Phosphorus and Their Stoichiometric Ratios].

The variation characteristics of ecological stoichiometric ratios can reflect the nature of plant adaptation to environmental changes. The C, N, and P contetns, and their stoichiometric ratios in different organs of rice were studied using a CO2 continuous labeling system, by simulating the increase of atmospheric CO2 concentration (800×10-6). The results showed that CO2 doubling promoted the growth of rice organs and increased the root/shoot ratio. CO2 doubling reduced the shoot TN content in different growth periods, increased the C/N ratio in the rice root, shoot, and grain, decreased the N use efficiency, and improved the P use efficiency. Multiple comparison and Venn diagram analyses showed that CO2 concentration only has a significant impact on the TN content in the rice shoot; it contributed little to the variation in rice nutrient content and their stoichiometric ratios, indicating that CO2 doubling had no effect on these. Under the condition of elevated atmospheric CO2 concentrations, the C, N, and P contents and their stoichiometirc ratios, in rice organs had good homeostasis, and the stoichiometric change during growth periods was consistent with "the Growth Rate Theory". In farmland management, appropriate nitrogen fertilizers can alleviate the nutrient balance pressure caused by the increase in CO2 concentration.

[Effects of Long-term Fertilization on Enzyme Activities in Profile of Paddy Soil Profiles].

The enzyme activity, which is closely related to soil material cycling (mineralization, transformation, etc.), can reflect soil quality and nutrient status. In order to explore the effect of long-term fertilization on the enzyme activity in paddy soil profile (0-40 cm), soils with organic fertilizer and inorganic fertilizer, and non-fertilized soils were selected, and the carbon and nitrogen contents, and the activities of ß-1,4-glucosidase (BG), and ß-1,4-N-acetylglucosaminidase (NAG) in 10cm depths of soil were analyzed. The results showed that the activities of BG and NAG in the soils treated with inorganic fertilizer and organic fertilizer increased by 0.73-47.87 nmol·(g·h)-1 and 1.33-128.81 nmol·(g·h)-1, and 0.19-9.72 nmol·(g·h)-1 and 0.92-57.66 nmol·(g·h)-1, respectively, compared to those for non-fertilized soil. Soil enzyme activity decreased with increasing soil depth. Soil enzyme activity in soil from 0-20 cm was significantly higher than that of soil from 20-40 cm. Soil enzyme activities were significantly affected by long term fertilization at different soil depths. RDA analysis showed that soil carbon and nitrogen contents had significant positive relationships with the activities of BG and NAG in the 0-20 cm soil profiles, however, negative relationships were observed in the 20-40 cm soil profiles. The long-term application of organic fertilizer significantly increased soil biomass and enzyme activity, both of which decreased with the increase in soil depth. Long-term fertilization could increase soil nutrient contents, microbial biomass, and extracellular enzyme activities, which has important theoretical significance for optimizing farmland fertilizer management and improving soil productivity.

[Allocation of rice photosynthates in plant-soil system in response to elevated CO2 and nitrogen fertilization.] / æ°´ç¨»å ‰åˆç¢³åœ¨æ¤ç‰©-åœŸå£¤ç³»ç»Ÿä¸­çš„åˆ†é åŠå ¶å¯¹CO2升高和施氮的响应.

To examine the allocation of rice photosynthates and its response to the elevated CO2 (800 µL·L-1) and N fertilization (100 mg·kg-1) at both tillering stage and booting stage in plant-soil system, rice was continually labelled with 13CO2. The results showed that the rice root biomass at the tillering stage and the shoot biomass at the booting stage were significantly increased under elevated CO2. Elevated CO2 increased the rice biomass and root-shoot ratio at tillering stage, but reduced it at booting stage. Under elevated CO2, N fertilization promoted shoot biomass during rice growth, but significantly decreased the root biomass at booting stage. Elevated CO2 significantly increased the allocation of assimilated 13C to the soil at the booting stage. N fertilization did not promote the elevated CO2-induced stimulation of assimilated 13C allocated to the soil, and it even decreased the proportion of assimilated 13C in the soil. In summary, elevated CO2 increased the photosynthetic C allocation into soil and promoted the turnover of soil organic carbon in paddy soil. N fertilization enhanced rice shoot biomass but decreased the belowground allocation of photosynthetic C.