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Understanding microbial community succession is key to uncover the mechanisms driving variation in soil fertility under rice cultivation. To evaluate microbial community divergence along paddy soils after 9, 15 and 25 years of cultivation with and without intensive fertilization, 16S rRNA gene high throughput sequencing and network co-occurrence analysis were used. Except for the initial barren soil, three different fertilization treatments were applied to rice planted soil: control (CK, without fertilization), NPK (N + P + K), and NPKM (NPK plus manure). Fertilization increased the difference of microbial species abundance between rice paddy soil and the initial barren soil reflecting higher C input by roots under fertilization. The bacterial communities after 9 and 15 years of rice cultivation clustered together and differed from those in barren soil and after 25 years of rice cultivation. Oligotrophic bacterial groups in the fertilized soil were gradually substituted with copiotrophic microorganisms during rice cultivation. The redundancy analysis indicated that the divergence in the microbial community structure during rice cultivation increased with soil organic C and total N. The network co-occurrence analysis showed that microbial network in NPKM soil contained the largest ratio of positive to negative links, whereas the CK network contained the smallest. Hence, fertilization and its duration control the distribution of keystone species and the nature of the links established between them.

Methanogenesis is crucial in the carbon cycle, and conductive iron oxide minerals play an important role in methanogenesis in anoxic environments. The influence of iron oxides is ubiquitous, but the strength and nature of iron oxides effects are believed to vary in response to environmental conditions, especially the soil iron concentration. We evaluated the influence of conductive nanoFe3O4 on straw degradation in high-iron (HR) and low-iron (LR) paddy soil enrichments, as well as the corresponding responses of related microorganisms. Under conductive nanoFe3O4 amendment, methane production and straw degradation increased by 184%–285% and 7.8%–12.7%, respectively, in the two soil enrichments. The increase apparently coincided with increased interspecies interactions within the microorganismal community, especially enhanced syntrophic cooperation among fermenting bacteria, syntrophic bacteria and methanogenic archaea, and facilitated interspecies electron transfer, which improved carbon conversion and alleviated end-product inhibition. The methanogenic archaeal community also exhibited changes with nanoFe3O4 amendment, and the relative abundance of Methanosarcina increased. Pairwise comparisons revealed that although methanogenic straw degradation was higher in HR soil than in LR soil, the enhancement induced by the nanoFe3O4 amendment showed to a larger degree and the microbial responses were more distinct in LR soil than in HR soil. The abundances of Geobacter and Methanosarcina greatly increased, and the microbial community shifted towards a composition similar to that in HR soil. This work indicates that iron oxides facilitate the methanogenic degradation of rice straw and that the indigenous iron content significantly influences the microbial community composition. This study provides a technique for agricultural residue degradation and has clear implications for our understanding of soil C-Fe biogeochemical coupling.

Sequestration of soil carbon is considered as a promising strategy for mitigating climate change. As a source of recalcitrant carbon, biochar has been widely used in agricultural soil as a mean of stabilizing soil organic carbon (SOC). However, limited reports focused on the changes of biochar itself in soil when compared with the bulk SOC after biochar addition. To explore how environmental conditions influence the stability of biochar, isolated straw-derived biochar particles (0.25-2 mm) were embedded in an Anthrosol for 12 months under varied environmental conditions of incubation temperature (15 °C, 25 °C and 35 °C) and moisture (60 % and 150 % of saturated water content). Within the early 1 month of incubation, pH and inorganic nitrogen contents of biochar changed significantly as a function of moisture and temperature (p0.01), whereas water extractable organic carbon (WEOC) content was only influenced by moisture content (p0.01). The highest temperature (35 °C) and saturated water content (150 %) induced the largest aging response reflected by increases in oxygen-containing surface functional groups of biochar, including C-O-C (51.35 % - 149 %) and N-C-O (65.55 % - 119 %). Pearson correlation and RDA analysis indicated that the chemical properties of biochar contribute more to the carbon-source utilization properties of biochar colonized microbial community within 1 month of incubation, while the bulk soil chemical properties (pH, DOC, MBC and NO

Many previous research have investigated the spatial distribution of the content of soil organic matter (SOM) and soil humic substance (HS), while few work focused on the chemical composition variation of HS across geographic distance. Paddy soils, collected from different long-term fertilization treatments across a wide geographic area in subtropical China, were used to evaluate the spatial variation of HS chemical composition. The 3-dimensional fluorescence spectroscopy combined with parallel factor analysis was used to evaluate the chemical composition of the soil extractable humic substance (EHS, including humic acid and fulvic acid). Analysis of distance-decay relationship (DDR) was introduced here to study the spatial variation of the chemical compositions of these EHSs. It was found that geographic location impose greater impact on the compositional variation of soil EHS than fertilization practice. Fertilization treatments including NPK (mineral N, P and K fertilizers) and NPKS (NPK fertilizers plus straw return) exhibited significant flattened DDR slopes as compared to CK (no fertilizers), which indicated fertilization (NPK and NPKS) decreased the variation of EHS chemical composition in paddy soils across geographic distance. A positive correlation was found between variations of soil bacterial community and EHS chemical composition, and fertilization (NPK and NPKS) enhanced their relationships. This work demonstrated that although the geographic location mainly dominated the spatial variation of the soil EHS chemical composition across geographic distance, the same long-term fertilization practice could gradually domesticate the paddy soils to display more similar EHS composition.

Recent works have shown that long-term fertilization has a critical influence on soil microbial communities; however, the underlying ecological assemblage of microbial community as well as its linkage with soil fertility and crop yield are still poorly understood. In this study, using analysis of high-throughput sequencing of 16S rRNA gene amplicons, we investigate mean pairwise phylogenetic distance (MPD), nearest relative index (NRI), taxonomic compositions and network topological properties to evaluate the assembly of the soil microbial community developed in 30-year fertilized soils. The phylogenetic signal indicates that environmental filtering was a more important assembly process that structure the microbial community than the stochastic process. Increase of soil fertility indexes, such as cation exchange capacity (CEC), soil organic matter (SOM) and available P (AP), driven by balanced fertilizations and straw returning amendment, result in the decrease of environmental filtering on the bacterial community assembly. Network parameters show that the amendment of straw returning provides with more niches, which lead to more complex phylotype co-occurrence. Increase of crop yield under balanced fertilizations might due to the increase of soil microbial function traits, which is associated with decreasing influence of environmental filtering. The significantly increased bacterial genera, Candidatus Koribacter, Candidatus Solibacter, and Fimbriimonas, in straw returning treatments, might be the key species in the competition caused by long-term environmental filtering. These results are helpful for a unified understanding of the ecological processes for microbial communities in different fertilized agroecosystem and the development of sustainable agriculture.

Dissolved organic matter (DOM) and microbial biomass (MB) are small but reactive pools of soil organic matter (SOM). The incorporation of carbon (C) from rice residue into DOM and MB in paddy soils under aerobic condition when rice straw remains in the field is poorly understood. A one-year incubation experiment was conducted, in which 13C-labelled rice straw was added to a cultivation chronosequence of paddy soils ranging from 0 to 100 years. Rice straw was rapidly decomposed during the first 30 days, after which 73% of the added straw C, on average, was retained in the soil and 46% of the straw C remained in the soil after one year. Throughout the entire incubation period, 0.2–0.9% of the added straw C was incorporated into DOM, and 2–5% was recovered in MB. The paddy cultivation age strongly affected the straw contribution to the organic C pools. In barren land (0 year), 32–60% of the DOM was derived from straw C, while 13–30% of the DOM was derived from straw C in 5–100-year paddy soils. On average, straw C contributed to 88% of the MB in barren land, 50% in 5-year soil, and 13% in 100-year soil. Consequently, over the duration of paddy cultivation, the contribution of rice straw C to the MB decreased, while the contribution of SOM increased. Our study has indicated that DOM in paddy soils mainly originates from SOM rather than from added plant residues but that plant residues are an important C source for microorganisms.

Understanding the soil dissolved organic matter (DOM) chemodiversity and its interaction with microbes is crucial to comprehend the biogeochemical processes in soil. In this study, ultrahigh-resolution fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) was employed to in detail characterize the DOM chemodiversity at a molecular level. The variation of the DOM chemodiversity from four agro-ecological experimental sites (paddy fields) subjected to different long-term fertilizations across different distance-points was evaluated. Geographic distance had a greater impact on DOM chemodiversity than anthropogenic fertilization. Distance-decay analysis showed that the dissimilarity of the DOM chemodiversity significantly increased with the increase in geographic distance. Long-term organic fertilizations homogenized the DOM chemodiversity as it made lipid-like compounds more similar regardless of geography. A network analysis combining DOM chemodiversity and bacterial community composition showed the significant interactions between bacteria species and DOM molecules. Variable partitioning analysis (VPA) showed that the bacterial community diversity dictates the soil DOM chemodiversity and not vice versa. Geographic distance indirectly affects the soil DOM chemodiversity by shaping the bacterial community.

An understanding of microbial community assembly and succession are keys to uncovering mechanisms underlying soil fertility development. The dynamics of microbial communities during a paddy soil chronosequence were investigated by phospholipid fatty acid profiling and amplicon high throughput sequencing. The upper 20 cm were sampled from soils after 5, 15, 30 and 100 years of paddy use and from adjacent barren land. Enzyme activities and contents of soil organic carbon, nitrogen and phosphorus of paddy fields strongly increased compared to barren land, and continued to increase at least up to 100 years of paddy cultivation. The increasing soil trophic status favored bacteria over fungi, and fast-growing copiotrophic bacteria gradually replaced slow-growing oligotrophic bacteria. The genus Ignavibacterium with versatile metabolism was identified as an indicator of the bacterial community in year 30 and 100. The variations of bacterial α-diversity tended to stabilize, but species richness continued to increase after 30 years of paddy use. The β-diversity indicated that bacterial community structure in paddy fields differed from that in barren land. The soils of 5 and 15 years of paddy cultivation clustered into one group separated from the group formed from the year 30 and 100. Redundancy analysis indicated that two stoichiometric ratios: C/N and C/P were the major factors affecting microbial community succession. We conclude that long-term paddy cultivation resulted in changes in biochemical properties and variations in trophic pattern of microbial communities, corresponding to increasing soil fertility.

Fertilization is one of the most common agricultural practices to meet an increasing global demand for food products. Few investigations have been reported on spatial variation of microbial community composition in response to fertilizations in agroecosystems at a large scale. To improve the related understandings, we have evaluated the taxonomic and phylogenetic diversities of bacterial taxa in response to three fertilization strategies in six paddy experiment sites spanning across subtropical China. We found the large-scale compositional variation of paddy bacterial community is shaped by both geographic location and environmental selection, and the former is the dominant factor. The slopes of distance-decay relationships (DDR) are flattened by fertilizations, NPK (mineral NPK fertilizers) and OM (mineral NPK fertilizers plus organic amendments) when compared to Control. A flattened DDR implies that bacterial community composition is greatly homogenized by fertilizations in paddies. It is also inferred that fertilization decreases sensitivity of bacterial community to geographic and environmental factors, which is speculated to be beneficial for agroecosystem stability and yield sustainability. Results from this investigation correlate microscopic agroecosystem with macroscopic agricultural practices.

Risk assessments for pesticides typically focus on the compound itself ignoring the impact of its transformation byproducts. Challenges in isolating such byproducts (i.e. after application of pesticide in soil) often lead to underestimation of the real risk from such substances. The toxicological properties of these byproducts may differ from those of the parent pesticides; hence, special attention is required for these new emerging contaminants. In this study, two transformation byproducts of chlorantraniliprole were isolated from soil and identified, using nuclear magnetic resonance and high resolution mass spectrometry, as products of dechlorination (Z1) and bromination (Z2). Kinetic experiments revealed both byproducts degrade faster than chlorantraniliprole in soil (half-lives 38 & 43 d vs. 58 d). The ecological risk evaluation of chlorantraniliprole and its byproducts on soil bacterial community showed that they were all potentially harmful but they imposed different impacts on both alpha and beta diversities and co-occurrence networks of the bacterial community. Z2 had the biggest potential impact on soil bacteria and accounted as a high potential risk. By comparing their impacts on soil bacterial community, we confirm that ecological risk assessment necessitates the understanding of the environmental impacts of a substance as well as of its transformation byproducts.

The occurrence of sulfonamides in the environment is a severe global threat to public health due to the increasing prevalence of antibiotic selection pressure that may lead to the development of antibiotic resistance. We report an enhanced biodegradation of sulfadimethoxine (SDM) by Phanerochaete chrysosporium (Pc) with lignocellulosic biomass (Lb) using Fe3O4-ZSM-5 as a catalyst (Pc/Fe3O4-ZSM-5/Lb). SDM was completely degraded within 4 days at pH 7.0 in the Pc/Fe3O4-ZSM-5/Lb system. Transcriptomic, metabolites and free radical analyses were performed to explore the detailed molecular mechanisms of SDM degradation. A total of 246 genes of Pc in the Pc/Fe3O4-ZSM-5/Lb system exhibited significant upregulation compared to that in Pc alone. Upregulated genes encoding cellulases, cytochrome P450, cellobiose quinone oxidoreductase, and cellobiose dehydrogenase were involved in SDM degradation in the Pc/Fe3O4-ZSM-5/Lb system. In addition, genes encoding glutathione S-transferase and cytochrome P450 genes related to oxidative stress and detoxification were all significantly upregulated (P OH suggesting a free radical pathway could be catalyzed by Fe3O4-ZSM-5 and the enzymes. These findings of catalyst-assisted SDM biodegradation will be valuable for remediation of antibiotics from contaminated wastewater.

The swine effluent studied was collected from scale pig farms, located in Yujiang County of Jiangxi Province, China, and duckweed (Spriodela polyrrhiza) was selected to dispose the effluent. The purpose of this study was to elucidate the effects of duckweed growth on the dissolved organic matter composition in swine effluent. Throughout the experiment period, the concentrations of organic matter were determined regularly, and the excitation-emission matrix (3DEEM) spectroscopy was used to characterize the fluorescence component. Compared with no-duckweed treatments (controls), the specific ultra-violet absorbance at 254nm (SUVA254) was increased by a final average of 34.4% as the phytoremediation using duckweed, and the removal rate of DOC was increased by a final average of 28.0%. In swine effluent, four fluorescence components were identified, including two protein-like (tryptophan, tyrosine) and two humic-like (fulvic acids, humic acids) components. For all treatments, the concentrations of protein-like components decreased by a final average of 69.0%. As the growth of duckweed, the concentrations of humic-like components were increased by a final average of 123.5% than controls. Significant and positive correlations were observed between SUVA254 and humic-like components. Compared with the controls, the humification index (HIX) increased by a final average of 9.0% for duckweed treatments. Meanwhile, the duckweed growth leaded to a lower biological index (BIX) and a higher proportion of microbial-derived fulvic acids than controls. In conclusion, the duckweed remediation not only enhanced the removal rate of organic matter in swine effluent, but also increased the percent of humic substances.

Microcalorimetry was used to study the effects of long-term (20 years) fertilization regimes on microbial activities in a paddy soil in southern China derived from infertile land. Managements of phosphorus fertilizer coupled with nitrogen fertilizer significantly promoted the contents of total and available P, mineral N and microbial biomass C (MBC) ( P Q T /MBC) showed that fertilization of P coupled with N, P-deficient fertilization and non-fertilized control significantly separated from each other. Redundancy analysis plot showed that rate of heat output ( Q T /t), peak power ( P max ) and constant of growth rate ( k ) were significantly correlated with soil total and available P, total and mineral N, which were greatly increased by the P fertilizer coupled with N fertilizer. In contrast, Q T /MBC and peak time ( t max ) were greatly increased by the P-deficient treatments. In addition, Q T /t as a new introduced parameter was negatively correlated well with Q T /MBC (R 2 = 0.93, P Q T /MBC, Q T /t and t max are useful to assess soil microbial activity. The higher Q T /t, lower Q T /MBC and t max indicate higher microbial activity and soil quality. In conclusion, long-term fertilization of P coupled with N, especially combined organic fertilizer greatly improved soil fertility and microbial activity; in contrast, deficiency of soil P had lower microbial activity in the paddy soil derived from infertile land.

Most studies of the effects of manure amendment on the occurrence of antibiotics and antibiotic resistance genes (ARGs) in soil employ the investigation of grab samples or short-term laboratory studies. However, the effects of long-term manure applications on antibiotics, ARGs and their vertical distribution in paddy soil in field experiments are lacking. We assessed the concentrations of antibiotics, ARGs and their vertical distribution in paddy soil receiving long-term manure applications in four field experiments. High concentrations of tetracyclines were detected in most manured soils, while sulfonamides were not detectable. Long-term manure amendments generally increased the antibiotic concentrations and ARGs abundances in the paddy soil over decades. However, in some sites such significant trends of ARGs could not be observed. The abundance of ARGs was statistically correlated with antibiotics and soil properties including pH and soil organic matter (SOM), indicating their importance in the selection of resistance genes. Tetracyclines could be detected in soil at different depths and the concentrations of tetracyclines and abundance of ARGs generally decreased with increasing soil depths.

Although increasing number of studies have shown that microorganisms play important roles in plant residue decomposition, an important process for crop productivity and soil fertility in agroecosystems, the underlying ecological processes of microbial community assembly as well as the associated governing factors remain elusive. As such, we conducted three replicate paddy straw decomposition experiments, located across subtropical China. We used ecological null modeling to quantify assembly processes governing bacterial community turnover during straw decomposition. Consistent observations emerged across the experiments that indicated significant associations between bacterial community assembly processes and straw chemistry. Specifically, according to our framework, shifts in straw chemistry were associated with variable selection, which was inferred to drive community turnover between soil and straw surfaces. This resulted in bacterial subgroups from soil being deterministically selected to degrade straw. In turn, patterns were consistent with homogeneous selection governing community composition within straw decomposition stages and ecological drift being important across decomposition stages. Subsequently, shifts in community composition and assembly processes were linked to variation in functional aspects of straw decomposition. This study indicates that straw chemistry strongly influenced assembly processes governing microbial community turnover during straw decomposition. These outcomes are important for mechanistically understanding and predicting microbial-driven plant residue decomposition in terrestrial ecosystem.

Knowledge gap exists to understand the soil CO2 emission and microbial group response to substrates of whole plant residues and derived biochar. We used 13C-labelled substrates (rice straw, roots and biochar) to track influences of their decomposition on soil priming effect (PE) and phospholipid fatty acid (PLFA) composition during one-year incubation. Organic substrates at 1% (w/w) level increased soil pH, available nitrogen (AN) and available phosphorus (AP), especially during the first 45 days of incubation. After incubation, 44% of the added straw was mineralized to 13CO2, followed by roots (~35%) and biochar (~5%). Straw and roots amendment caused positive PE during 4–360 day of the incubation, where a lowest value of 41.9 mg C kg−1 was observed. Biochar amendment caused negative PE during 56–150 day of the incubation, where a largest value of −99.0 mg C kg−1 was observed. Analysis of 13C-labelled PLFA enabled the differentiation of microbial groups during substrates utilization. Gram positive bacteria (G+) and general bacteria groups were dominated in co-metabolizing both the native soil organic carbon (SOC) and substrates after straw and roots amendment. Gram negative bacteria (G−), especially identified by PLFA biomarkers cy17:0 and cy19:0, preferentially utilizes the 13C-labelled biochar but not promoting soil priming effect. Soil pH, SOC, AN and AP all explained changes of total and 13C-labelled PLFA contents (>75%, p

Phytopathogenic fungi are a major cause of plant disease. However, the interrelationships between phytopathogenic and non-phytopathogenic fungi in the peanut (Arachis hypogaea) monoculture system remain unclear. In this study, rhizosphere soils from four fields that had been monocropped with peanut for different durations were investigated, including monoculture for 1 year (CK), 4 years (P4), 10 years (P10), and 20 years (P20). Illumina sequencing of fungal ITS regions and FUNGuild were used to investigate the interrelationships between potential phytopathogenic and non-phytopathogenic fungi. Our results showed that the relative abundance of potential phytopathogenic fungi was significantly negatively correlated with that of non-phytopathogenic fungi (r = −0.650, P = 0.022); and the relative abundance of the key potential phytopathogenic species, Fusarium, was also significantly negatively correlated with that of the non-phytopathogenic fungi, Penicillium (r = −0.815, P = 0.001). The greater negativity between potential phytopathogenic and non-phytopathogenic fungi in the co-occurrence network implied an antagonism between them; and the greater negativity between Penicillium species and Fusarium species in the network combining with the results of confrontation experiment suggested the suppression of Penicillium species on Fusarium species. In conclusion, there is a trade-off between potential phytopathogenic and non-phytopathogenic fungi in the peanut monoculture system. Our results provide better insight towards understanding the obstacles of continuous peanut cropping.

Microbial communities play a key role in maintaining agroecosystem functioning and sustainability, but their response to excessive animal manure application and relevant mechanisms have not been thoroughly elucidated to date. This study investigated the responses of soil bacterial and fungal communities to pig manure (PM) amendment in red paddy soils. High-throughput sequencing revealed that PM amendment significantly reduced the relative abundance of Acidobacteria yet increased that of Bacteroidetes, Ignavibacteriae, Firmicutes, and Rozellomycota. The Cu and available phosphorus were the primary impact factors influencing bacterial and fungal diversity, respectively. Bacterial alpha-diversity tended to sharply decrease when the content of soil Cu was >30.70 mg kg−1, while fungal alpha-diversity did not continuously increase when the content of soil available phosphorus was >82.84 mg kg−1. Bacterial communities with a wider niche breadth showed significantly lower structural variation, whereas fungal communities with a narrower niche breadth showed greater variation in community structure. Soil heavy metals, primarily Cu and Zn, were the primary factors that affected bacterial communities, whereas soil fungal communities were mainly influenced by soil phosphorus. Bacterial and fungal communities showed distinct co-occurrence patterns, with bacterial communities showing a higher degree, a clustering coefficient, and betweenness centrality, but a lower closeness centrality. The findings highlighted that bacteria and fungi responded differently to PM amendment because of their discrepant niche breadth, interspecific relationships, and different tolerance to heavy metal and soil nutrient.

The impacts of nanoFe3O4 on the composition of degradation products, microbial community, and microbial metabolic functions during rice straw anaerobic degradation were investigated. Under nanoFe3O4 addition, CH4 production and straw degradation increased by 81% and 10.4%, respectively, in paddy soil enrichment. Coupling product chemistry and microbial community during straw degradation found that nanoFe3O4 effectively promoted the hydrolysis-acidification-methanogenesis of straw, which made lignin-, lipid-, protein-, tannin-like and VFAs products rapidly increase and then quickly decrease. Moreover, the relative abundance of Clostridiaceae and Methanosarcina corresponded with increased hydrolysis and acetoclastic methanogenesis with nanoFe3O4 addition. Cellular processes, environmental information processing and metabolism, especially energy metabolism, were enhanced functions of the microbial community during straw degradation with nanoFe3O4. The nanoFe3O4 addition may improve the electron transfer efficiency, stimulate energy release, reduce Gibbs free energy of the half reaction of organic carbon oxidation (ΔGcox0) and promote energy metabolism to accelerate straw degradation and CH4 generation.

The increasingly intensified animal industry in recent decades has resulted in the discharge of a large amount of wastewater with high concentrations of organic matter and nutrients into the ambient environment, which influences soil properties. In this study, we applied a multi-parameter approach to investigate changes in soil microbial properties and community compositions from three acid soil sites that differed in land-use patterns and histories of receiving wastewater. Wastewater had been applied to the sites for 2–20 years. Compared to controls, soil pH, EC and total nutrients were significantly higher in soils receiving wastewater, as well as average increases of 149 mg kg −1 for microbial biomass carbon and 0.19 mg CO 2 –C kg −1 h −1 for basal respiration; whereas the metabolic quotient and the ratio of saturated to monounsaturated phospholipid fatty acids decreased by 13% to 31%, and 32% to 61%, respectively. Soil microbial communities of all sites changed with the impact of wastewater application and showed significant increases in bacteria, especially Gram-negative bacteria. The differences in microbial metabolic profiles from all sites were reduced by wastewater application. Soil pH and EC were the two most important factors controlling microbial community composition under wastewater application. These results suggested that wastewater application could reduce stress on acid soil microorganisms by providing more organic carbon and nutrients, and through neutralization of soil acidity.

Fertilization generates nutrient patches that may impact soil microbial activity. In this study, nitrogen patches were generated by adding ammonium sulfate or urea to soil columns (length 25 cm; internal diameter 7.2 cm). Changes in nitrogen transformation, soil microbial biomass, and microbial functional diversity with the nitrogen gradients were investigated to evaluate the response of microbial activity to chemical fertilizer nutrient patches. After applying of ammonium sulfate or urea, the added nitrogen migrated about 7 cm. Microbial biomass carbon (MBC) was lower in fertilized soil than in the control (CK) treatment at the same soil layers. MBC increased with soil depth while microbial biomass nitrogen (MBN) decreased. BIOLOG analysis indicated that the average well color development (AWCD) and functional diversity indices of the microbial communities were lower in the 1 cm and 2 cm soil layers after application of ammonium sulfate; the highest values were in the 3 cm soil layer. AWCD and Shannon indices from the 1 to 5 cm soil layers were higher than those from other soil layers under urea application. Both principal component analysis and carbon substrate utilization analysis showed significant separation of soil microbial communities among different soil layers under application of ammonium sulfate or urea. Microbial activity was substantially decreased when NH 4 + -N concentration was higher than 528.5 mg kg −1 (1–3 cm soil layer under ammonium sulfate application) or 536.8 mg kg −1 (1 cm soil layer under urea application). These findings indicated that changes in soil microbial biomass and microbial functional diversity can occur with a nitrogen gradient. The extent of changes depends on the nitrogen concentration and the form of inorganic fertilizer.

For better understanding the development of infertile paddy soils in subtropical China, a long-term field experiment of paddy soil was developed from barren land in 1990. Experimental treatments including NPK, NPKRS (NPK and rice straw), NPK2RS (NPK and double amount of rice straw), NPKPM (NPK and pig manure), and NPKGM (NPK and green manure (Astragalus sinicus L.)) were employed with rice–rice (Oryza sativa L.) cropping system. Rice yields, soil organic C (SOC) and total N were analyzed. In all of the treatments, early rice yields increased along cultivation years steadily, however late and annual rice yield was fluctuant between different years. From 1991 to 2006, average annual yield ranged from 7795 to 8572 kg ha−1 among different fertilizer treatments. Organic amendments usually enhanced rice yields significantly except for the treatment with NPKRS. SOC and total N contents of surface soil increased linearly with cultivated years from 3.9–5.7 g kg−1 and 0.46–0.57 g kg−1 in initial stage to 7.1–9.2 g kg−1 and 0.87–0.95 g kg−1 in 2005 respectively. Quantity and quality of input organic matter affected soil C dynamics and N balance. SOC sequestration rates were well correlated to modified organic C input, while SOC mineralization rates were related with either organic C input or SOC contents. Annual soil N accumulation was 13–18% of total input. However the net lost N, which was calculated based on fertilizer inputs, crop outputs, and annual N accumulation, was 44–49% in our study. In general, after 17 years’ cultivation and fertilization management, rice yield reached a high level equivalent to the average yield of local high productivity paddy soils, whereas SOC and total N content were still less than half of those in high productivity paddy soils in this region.

CO2 efflux plays a key role in carbon exchange between the biosphere and atmosphere, but our understanding of the mechanism controlling its temporal and spatial variations is limited. The purpose of this study is to determine annual soil CO2 flux and assess its variations in arable subtropical soils of China in relation to soil temperature, moisture, rainfall, microbial biomass carbon (MBC) and dissolved organic carbon (DOC) using the closed chamber method. Soils were derived from three parent materials including granite (G), tertiary red sandstone (T) and quaternary red clay (Q). The experiment was conducted at the Ecological Station of Red Soil, The Chinese Academy of Sciences, in a subtropical region of China. The results showed that soil CO2 flux had clear seasonal fluctuations with the maximum value in summer, the minimum in winter and intermediate in spring and autumn. Further, significant differences in soil CO2 flux were found among the three red soils, generally in the order of G>T>Q. The average annual fluxes were estimated as 2.84, 2.13 and 1.41 kg CO2 m−2 year−1 for red soils derived from G, T and Q, respectively. Soil temperature strongly affects the seasonal variability of soil CO2 flux (85.0–88.5% of the variability), followed by DOC (55.8–84.4%) and rainfall (43.0–55.8%). The differences in soil CO2 flux among the three red soils were partly explained by MBC (33.7–58.9% of the variability) and DOC (23.8–33.6%).