[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.

[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.

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 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.

[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.

[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.

[Dynamics of Rice Photosynthesized Carbon Input and Its Response to Nitrogen Fertilization at the Jointing Stage:13 C-CO2 Pulse-labeling].

Photosynthesized carbon (C) is an important source of soil organic C in paddy fields, and its input and distribution are affected by rice growth and soil fertility. Fertilizer application plays an important role in rice growth. The 13C pulse-labeling method was used to quantify the dynamics and distribution of input photosynthesized C in the rice-(rhizosphere-and bulk-) soil system and its response to nitrogen fertilizer (N) application. The results suggested that N fertilization significantly increased the rice aboveground and the root biomass and decreased the rice biomass root/shoot ratio. The amount of assimilated 13C gradually decreased in the rice plants but gradually decreased over 0-6 days and increased over 6-26 days in the rhizosphere and bulk soil during rice growth. N fertilization significantly increased the amount of assimilated 13C in the rhizosphere soil by 9.5%-32.6% compared with the control. In comparison to the unfertilized treatment, the application of N fertilization resulted in higher photosynthetic13C in rice aboveground and in the root by 24.5%-134.7% and 9.1%-106%, respectively. With the N fertilized and unfertilized treatments, 85.5%-93.2% and 91.3%-95.7%, respectively, of input photosynthetic 13C was distributed in the rice plants. The results suggested that N fertilization significantly affected the distribution of photosynthesized C in the rice-soil system (P<0.01). After 26 days of pulse labeling, the distribution of photosynthetic 13C into rice aboveground was increased by 13.4%, while the distribution into the rhizosphere and bulk soil were decreased by 21.9% and 52.2%, respectively, in the N fertilized treatments compared with the unfertilized treatments. Therefore, the N application increased the distribution of photosynthesized carbon in the soil-rice system but decreased the accumulation in the rhizosphere and bulk soil. The findings of this study provided a theoretical basis for our understanding of the dynamic of photosynthetic C in the plant-soil system and the assimilation of the soil organic matter pool in the paddy soil ecosystem.

[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.

[Responses of organic carbon mineralization and priming effect to phosphorus addition in paddy soils]. / ç¨»ç”°åœŸå£¤æœ‰æœºç¢³çŸ¿åŒ–åŠå ¶æ¿€å‘æ•ˆåº”å¯¹ç£·æ·»åŠ çš„å“åº”.

To understand the coupled controlling of carbon (C) and phosphorus (P) on the minera-lization of soil organic carbon and amended substrates in paddy soil, we investigated the effects of P addition on the decomposition of organic carbon and its induced priming effect by using 13C isotope probing technique in microcosm. The results showed that P addition accelerated the release of CO2 but inhibited the release of CH4, leading to 53.1% reduction of total accumulated CH4 and 70.5% reduction of the 13CH4 derived from exotic glucose-13C. P addition altered the carbon distribution during the microbial turnover progress, with 3.6% of glucose-13C being transferred into the labile carbon pool, therein significantly increased potential of the mineralization rate of exogenous C. A transient negative priming effect was observed in the early stage of incubation. With time prolonging, the priming effect on CO2 emission (PECO2) generally increased and then decreased after a peak. The priming effect on CH4 emission (PECH4) kept increasing and finally fluctuated at a relative stable value until the end of the experiment (100 days). P addition increased PECO2 by 32.3% but reduced PECH4 by 93.4%. Results from the RDA and Pearson analysis showed that electric conductivity, oxidation-reduction potential and dissolved organic carbon significantly affected soil C mineralization. There were significantly negative correlations between available phosphorus (Olsen-P) and 13CH4, and between Olsen-P and PECH4. In conclusion, with the addition of exogenous organic matter, P application could reduce CH4 emissions and inhibit its priming effect, acce-lerate the mineralization of SOC, probably improve the nutrient supply, and thus enhance the avai-lability of organic C and promote C cycling in paddy soil.

[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.

[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 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.

Fe(II)/Cu(II) interaction on goethite stimulated by an iron-reducing bacteria Aeromonas Hydrophila HS01 under anaerobic conditions.

Copper is a trace element essential for living creatures, but copper content in soil should be controlled, as it is toxic. The physical-chemical-biological features of Cu in soil have a significant correlation with the Fe(II)/Cu(II) interaction in soil. Of significant interest to the current study is the effect of Fe(II)/Cu(II) interaction conducted on goethite under anaerobic conditions stimulated by HS01 (a dissimilatory iron reduction (DIR) microbial). The following four treatments were designed: HS01 with α-FeOOH and Cu(II) (T1), HS01 with α-FeOOH (T2), HS01 with Cu(II) (T3), and α-FeOOH with Cu(II) (T4). HS01 presents a negligible impact on copper species transformation (T3), whereas the presence of α-FeOOH significantly enhanced copper aging contributing to the DIR effect (T1). Moreover, the violent reaction between adsorbed Fe(II) and Cu(II) leads to the decreased concentration of the active Fe(II) species (T1), further inhibiting reactions between Fe(II) and iron (hydr)oxides and decelerating the phase transformation of iron (hydr)oxides (T1). From this study, the effects of the Fe(II)/Cu(II) interaction on goethite under anaerobic conditions by HS01 are presented in three aspects: (1) the accelerating effect of copper aging, (2) the reductive transformation of copper, and (3) the inhibition effect of the phase transformation of iron (hydr)oxides.

[Responses of Extracellular Enzymes to Nitrogen Application in Rice of Various Ages with Rhizosphere and Bulk Soil].

Ecological enzyme activities are closely relevant to the carbon and nitrogen decomposition and mineralization of paddy soils, which can reflect the growth and metabolism of microorganisms. In order to clarify the response to nitrogen application by different enzymes in the rhizosphere and bulk soil of rice, the rhizosphere and bulk soil were identified using a rhizosphere bag. In addition, the ß-1,4-glucosidase (BG) enzyme; ß-1,4-N-acetylglucosaminidase (NAG) enzyme; and the effects of the rhizosphere, nitrogen application, and growth period on soil enzyme activities were analyzed. The results showed that the activity of the BG enzyme decreased by 7.4-13.5 nmol·(g·h)-1 in the jointing stage and increased by 7.0-31.4 nmol·(g·h)-1 in the maturity stage, and the activity of the BG enzyme in the rhizosphere and bulk soil also changed with the growth period of rice.compared with no nitrogen fertilizer, nitrogen application increased NAG enzyme activity by 1.1 times and rhizosphere soil by 0.3 times in the bulk soil in the maturity stage. Nitrogen application and growth period had significant effects on soil BG enzyme activity, whereas the interaction within rice growth stage, nitrogen application, and rhizosphere effect had significant effects on NAG enzyme activity. The result of RDA showed that the soil microbial biomass carbon (MBC) and dissolved organic carbon (DOC) contents mainly affected the activity of extracellular enzymes in the rice rhizosphere soil, whereas the activities of enzymes in the rhizosphere soil were mainly affected by microbial biomass nitrogen (MBN) and NH4+-N. The relationships between soil enzyme activity and various factors are complex, and the effects of nitrogen addition on microbial community composition needs to be considered after accounting for plant physiological characteristics, soil enzyme activity, and soil characteristics.

[Effect of Straw Application on the Dynamics of Exogenous Nitrogen and Microbial Activity in Paddy Soil].

Returning straw to the field provides an important source of fertilizer that can increase soil fertility. However, the rate of straw carbon utilization is low and large amounts of greenhouse gases are emitted due to the high carbon to nitrogen ratio of the straw mass. In this regard, the application of inorganic nitrogen and phosphate fertilizers can control the ratio of elements in the soil, increase the activity of microorganisms and their utilization of elements, and promote the improvement of soil fertility. In this study, straw application conditions were simulated, and inorganic nitrogen fertilizer labeled with 15N was added to examine the effects of different nutrient fertilizer additions on the transformation and distribution of exogenous nitrogen in the soil, and also the characteristics of the microbial response. The results showed that application of straw increased the contents of ammonia nitrogen and total nitrogen in the soil and soil solution. When both straw and inorganic nitrogen fertilizer were applied, the 15N-TN in the soil remained at 28 to 33 µg during the 100-day culture phase. In contrast, 15N-NH4+ increased gradually during the initial 30 days of the culture phase, but subsequently decreased gradually. Application of phosphate increased the contents of 15N-TN and 15N-NH4+ in the soil, but decreased the content of 15N in the soil solution by 28%. The distribution of inorganic nitrogen in the soil showed that the proportion of 15N in the soil remained at 52%-61%. Addition of phosphate fertilizer increased the distribution ratio of 15N in the soil by up to 16.5%, whereas the proportion of 15N in the soil solution decreased from 36% on the fifth day to 30% on the 100th day, thereby the loss amount of 15N reduced by 1.2-fold. Addition of straw promoted microbial activity and significantly increased the microbial biomass nitrogen (MBN) content of the soil. Addition of inorganic fertilizer further promoted the microbial activity of the soil. After the 100-day culture experiment, the addition of straw, inorganic nitrogen, and phosphate fertilizer increased MBN to between 2.0-fold and 2.2-fold that of the control treatments. Addition of phosphate fertilizer increased the utilization of 15N by microorganisms, so that the amount of 15N-MBN was 1.5-fold higher than that of treatments where only straw and nitrogen fertilizer were added. Examination of soil enzyme activity showed that nitrogen fertilizer reduced soil enzyme activity and substrate affinity. When both nitrogen and phosphate fertilizers were added, the enzyme activity was 48.1% higher than that when only straw was added. The findings of this study thus provide a theoretical basis for furthering our understanding on the nitrogen cycle of the paddy soil ecosystem, the improvement of soil fertility, and the reduction of greenhouse gas emissions.

[Input and Distribution of Photosynthesized Carbon in Soil-Rice System Affected by Water Management and Nitrogen Fertilization].

Fertilizer and water management are two key factors for rice growth. A better understanding of the carbon (C) cycling in paddy soil requires investigation into the input characteristics and distribution dynamics of photosynthesized carbon in rice-soil system. We grew rice (Zhongzao 39) in PVC pots and used the 13 C-CO2 continuous labeling method to quantify the allocation of photosynthesized carbon in rice-soil system under two regimes(Drying-rewetting vs. continuous watering) and N fertilization (250 mg·kg-1vs. no addition). The results showed that nitrogen fertilizer application increased rice shoot biomass and the amount of C and N, but had no significant influence on rice root biomass. Thus, nitrogen fertilizer application decreased rice biomass root/shoot ratio significantly. Drying-rewetting with N fertilizer treatment resulted in higher total C and N amount by 22% and 33%, respectively, in the shoot, and by 36% and 44%, respectively in the root than continuous watering with nitrogen fertilizer treatment. These results indicated that nitrogen fertilizer application promoted the growth of rice shoot. Nitrogen fertilizer application significantly increased the 13 C content in rice shoot by 32%-83% over the control without N addition. Nitrogen fertilizer application also increased the 13 C recovery in rice shoot by 6%-32%, but decreased that in the root by 18%-59%. Pertaining to water effect, drying-rewetting with N application increased the amount of 13 C in rice shoot and root. However, without N addition, the amount and the recovery of 13 C in shoot dropped by 10.3 mg·pot-1 and 12%, respectively, compared with the continuous watering treatment. The root, on the other hand, recorded increases in both the amount and the recovery of 13 C by 1.9 mg·pot-1 and 57%, respectively. Furthermore, the deposition of assimilated C into rhizosphere-soil increased by both the individual and the interactive effects of N fertilizer application and drying-rewetting treatment. Thus, combining N fertilizer and drying-rewetting water management led to more increased allocation and deposition of photosynthesized carbon in soil-rice system compared with combined continuous flooding and N application. This study was able to quantify the partitioning and allocation of rice photosynthesized carbon into different plant and soil pools under different water and N fertilizer treatments, and can serve as a useful guide for better water and nutrient management practices in paddy-rice production that can achieve both sustainable high yield and sequestration of more C within the paddy soil system.

[Effects of Soil Texture on Autotrophic CO2 Fixation Bacterial Communities and Their CO2 Assimilation Contents].

Autotrophic bacteria can assimilate atmospheric carbon dioxide (CO2) and convert CO2 into organic carbon. The CO2 fixation by autotrophic bacteria is important for the improvement of carbon sequestration in agricultural soils. However, the effect of soil texture on autotrophic CO2 fixation bacteria and their CO2 fixation capacity is still unknown. Here, two paddy soils with different textures (loamy clay soil and sand clay loam soil) were incubated with continuous 14C-CO2 in a glass chamber. The two soils were developed from the same parent. At the end of 110 days incubation, the 14C-CO2 incorporated in soil organic carbon (14C-SOC), microbial biomass carbon (14C-MBC) and dissolved organic carbon (14C-DOC) were measured to explore the effects of soil texture on the autotrophic bacterial CO2 fixation rates. The effect of soil texture on the composition and diversity of autotrophic CO2 fixation bacterial community was investigated using cloning and sequencing of the cbbL gene, which encodes ribulose-1,5-biphosphate carboxylase/oxygenase (RubisCO) in the Calvin cycle. The results showed that the average contents of 14C-SOC, 14C-MBC and 14C-DOC were 133.81, 40.16 and 8.10 mg·kg-1 in loamy clay soil, respectively, which were significantly higher than their corresponding contents in sand clay loam soil (P<0.05). This suggested that soil texture not only affected the amounts of autotrophic bacteria CO2 fixation but also had an effect on the transformation of microbial assimilated 14C in soil. The cbbL gene libraries of two soils were significantly different as revealed by libshuff analyses (P<0.05). Phylogenetic analysis showed that cbbL sequences from the loamy clay soil were closely affiliated with known cultures such as Rhodoblastus acidophilus, Blastochloris viridis, Thauera humireducens, Mehylibium sp.and Variovorax sp., whereas these sequences belonging to the sand clay loam soil were related to branching lineages originating from Rhizobiales and Actinomycetales.Rarefaction curve, clone library coverage and diversity index analysis based on bacterial cbbL clone libraries indicated that the loamy clay soil had higher cbbL gene diversity compared to the sand clay loam soil. These results suggested that soil texture had a pronounced effect on the composition and diversity of autotrophic CO2 fixation bacterial communities. The higher clay content, nutrient availability and cation exchange capacity may stimulate the growth and activity of autotrophic bacteria, and result in the higher amounts of 14C in loamy clay soil. These data broaden the understanding and knowledge of mechanisms of microbial carbon fixation and their influencing factors in agricultural soils.