Distinct Assembly Patterns of Soil Antibiotic Resistome Revealed by Land-Use Changes over 30 Years.

Compared with the ever-growing information about the anthropogenic discharge of nutrients, metals, and antibiotics on the disturbance of antibiotic resistance genes (ARGs), less is known about how the potential natural stressors drive the evolutionary processes of antibiotic resistance. This study examined how soil resistomes evolved and differentiated over 30 years in various land use settings with spatiotemporal homogeneity and minimal human impact. We found that the contents of soil organic carbon, nitrogen, soil microbial biomass, and bioavailable heavy metals, as well as related changes in the antibiotic resistome prevalence including diversity and abundance, declined in the order of grassland > cropland > bareland. Sixty-nine remaining ARGs and 14 mobile genetic elements (MGEs) were shared among three land uses. Multiple factors (i.e., soil properties, heavy metals, bacterial community, and MGEs) contributed to the evolutionary changes of the antibiotic resistome, wherein the resistome profile was dominantly driven by MGEs from both direct and indirect pathways, supported by a partial least-squares path model analysis. Our results suggest that pathways to mitigate ARGs in soils can coincide with land degradation processes, posing a challenge to the common goal of managing our environment sustainably.

Land use indirectly affects the cycling of multiple nutrients by altering the diazotrophic community in black soil.

BACKGROUND: Diazotrophic bacteria, as one of most important group of soil microorganisms, play critical roles in multiple ecosystem functions (i.e., multifunctionality). However, little information is available about the diazotrophic community in driving soil nutrient cycling and multifunctionality at different depths with distinct vegetation in the black soil region of northeastern China. To learn the interactions among land use, cycling of multiple nutrients and the diazotrophic community, we performed this study in grassland (GL), forested land and a cropland (CL) in soils at depths of 0-15 cm and 15-35 cm. RESULTS: The highest nifH gene abundances were found in the CL treatment, while the highest diazotrophic species richness and diversity were detected in the GL in both soil layers. The nifH gene abundance was directly/positively correlated with soil bulk density and negatively correlated with land use and soil depth. The index of multiple nutrient cycling was directly/negatively affected by soil depth and indirectly/positively affected by land use. Land use directly/negatively affected soil pH and thus indirectly affected the diazotrophic community composition and the nutrient cycling index. The diversity and community composition of the diazotrophs together accounted for 95% of the differences in the multiple nutrient cycling index. CONCLUSION: Soil diazotrophic communities undertake important roles in maintaining nutrient cycling and soil multifunctionality at depths of 0-15 cm and 15-35 cm layers with different land uses of the black soil region of China. © 2021 Society of Chemical Industry.

Long-term continuous cropping affects ecoenzymatic stoichiometry of microbial nutrient acquisition: a case study from a Chinese Mollisol.

BACKGROUND: Soil- and plant-produced extracellular enzymes drive nutrient cycling in soils and are assumed to regulate supply and demand for carbon (C) and nutrients within the soil. Thus, agriculture management decisions that alter the balance of plant and supplemental nutrients should directly alter extracellular enzyme activities (EEAs), and EEA stoichiometry in predictable ways. We used a 12-year experiment that varyied three major continuous grain crops (wheat, soybean, and maize), each crossed with mineral fertilizer (WCF, SCF and MCF, respectively) or not fertilized (WC, SC and MC, respectively, as controls). In response, we measured the phospholipid fatty acids (PLFAs), EEAs and their stoichiometry to examine the changes to soil microbial nutrient demand under the continuous cropping of crops, which differed with respect to the input of plant litter and fertilizer. RESULTS: Fertilizer generally decreased soil microbial biomass and enzyme activity compared to non-fertilized soil. According to enzyme stoichiometry, microbial nutrient demand was generally C- and phosphorus (P)-limited, but not nitrogen (N)-limited. However, the degree of microbial resource limitation differed among the three crops. The enzymatic C:N ratio was significantly lower by 13.3% and 26.8%, whereas the enzymatic N:P ratio was significantly higher by 9.9% and 42.4%, in MCF than in WCF and SCF, respectively. The abundances of arbuscular mycorrhizal fungi and aerobic PLFAs were significantly higher in MCF than in WCF and SCF. CONCLUSION: These findings are crucial for characterizing enzymatic activities and their stoichiometries that drive microbial metabolism with respect to understanding soil nutrient cycles and environmental conditions and optimizing practices of agricultural management. © 2021 Society of Chemical Industry.

Impacts of land-use changes on the variability of microbiomes in soil profiles.

BACKGROUND: The conversion of arable land to grassland and/or forested land is a common strategy of restoration because the development of plant communities can inhibit the erosion of soil, increase biodiversity and improve associated ecosystem services. The vertical profiles of microbial communities, however, have not been well characterized and their variability after land conversion is not well understood. We assessed the effects of the conversion of arable land (AL) to grassland (GL) and forested land (FL) on bacterial communities as old as 29 years in 0-200-cm profiles of a Chinese Mollisol. RESULTS: The soil in AL has been a stable ecosystem and changes in the assembly of soil microbiomes tended to be larger in the topsoil. The soil properties and microbial biodiversity of arable land were larger following revegetation and reforestation. The conversion caused a more complex coupling among microbes, and negative interactions and average connectivity were stronger in the 0-20-cm layers in GL and in the 20-60-cm layers in FL. The land use dramatically influenced the assembly of the microbial communities more in GL than AL and FL. The bacterial diversity was an important component of soil multinutrient cycling in the profiles and microbial functions were not as affected by changes in land use. CONCLUSION: The spatial variation of the microbiomes provided critical information on below-ground soil ecology and the ability of the soil to provide crucial ecosystem services. © 2021 Society of Chemical Industry.

Genetic diversity of indigenous soybean-nodulating rhizobia in response to locally-based long term fertilization in a Mollisol of Northeast China.

The influences of five different fertilizer treatments on diversity of rhizobia in soybean nodule were investigated in a long-term experiment with with four replicates: (1) control (without fertilization), (2) balanced NPK fertilizer (NPK), and (3-5) unbalanced chemical fertilizers without one of the major elements (NP, PK, and NK) in Mollisol in Northeast China. The highest soybean yield was observed in the NPK treatment. Total of 200 isolates were isolated and grouped into four Bradyrhizobium genospecies corresponding to B. japonicum, B. diazoefficiens, B. ottawaense and Bradyrhizobium sp. I, based upon the multilocus sequence analysis of 6 housekeeping genes. The Bradyrhizobium sp. I was extensively distributed throughout the study site and was recorded as the dominant soybean rhizobia (82.5-87.5%). Except the NK treatment, the other fertilizer treatments had no effect on rhizobial species composition. Compared with the CK treatment, all the fertilizer treatments decreased species richness, diversity and evenness. The soil organic carbon contents, available N content and pH were the key soil factors to rhizobial community structure. Results suggest that long-term fertilization can decrease rhizobial species diversity, while balanced fertilization with NPK is the most suitable fertilization regime if taking both soybean yields and rhizobial diversity into account.

Change in soil organic carbon between 1981 and 2011 in croplands of Heilongjiang Province, northeast China.

BACKGROUND: Soil organic carbon (SOC) is fundamental for mitigating climate change as well as improving soil fertility. Databases of SOC obtained from soil surveys in 1981 and 2011 were used to assess SOC change (0-20 cm) in croplands of Heilongjiang Province in northeast China. Three counties (Lindian, Hailun and Baoqing) were selected as typical croplands representing major soil types and land use types in the region. RESULTS: The changes in SOC density (SOCD) between 1981 and 2001 were -6.6, -14.7 and 5.7 Mg C ha(-1) in Lindian, Hailun and Baoqing Counties respectively. The total SOC storage (SOCS) changes were estimated to be -11.3, -19.1 and 16.5% of those in 1981 in the respective counties. The results showed 22-550% increases in SOCS in rice (Oryza sativa L.) paddies in the three counties, but 28-33% decreases in dry cropland in Lindian and Hailun Counties. In addition, an increase of 11.4 Mg C ha(-1) in SOCD was observed in state-owned farms (P < 0.05), whereas no significant change was observed in family-owned farms. CONCLUSION: Soil C:N ratio and initial SOCD related to soil groups were important determinants of SOCD changes. Land use and residue returning greatly affected SOC changes in the study region. To increase the topsoil SOCD, the results suggest the conversion of dry croplands to rice paddies and returning of crop residue to soils.

Effects of the construction of fertile and cultivated soil layer on soil fertility and maize yield in Albic soil.

We examined the effects of fertile soil layer construction technology on soil fertility and maize yield with a 3-year field experiment in Albic soil in Fujin, Heilongjiang Province. There were five treatments, including conventional tillage (T15, without organic matter return) and fertile soil layer construction methods [deep tillage (0-35 cm) with straw return, T35+S; deep tillage with organic manure, T35+M; deep tillage with straw and organic manure return, T35+S+M; deep tillage with straw, organic manure return and chemical fertilizer, T35+S+M+F]. The results showed that: 1) compared with the T15 treatment, maize yield was significantly increased by 15.4%-50.9% under fertile layer construction treatments. 2) There was no significant difference of soil pH among all treatments in the first two years, but fertile soil layer construction treatments significantly increased soil pH of topsoil (0-15 cm soil layer) in the third year. The pH of subsoil (15-35 cm soil layer) significantly increased under T35+S+M+F, T35+S+M, and T35+M treatments, while no significant difference was observed for T35+S treatment, compared with T15 treatment. 3) The fertile soil layer construction treatments could improve the nutrient contents of the topsoil and subsoil layer, especially in the subsoil layer, with the contents of organic matter, total nitrogen, available phosphorus, alkali-hydrolyzed nitrogen and available potassium being increased by 3.2%-46.6%, 9.1%-51.8%, 17.5%-130.1%, 4.4%-62.8%, 22.2%-68.7% under the subsoil layer, respectively. The fertility richness indices were increased in the subsoil layer, and nutrient contents of the subsoil layer were close to those of topsoil layer, indicating that 0-35 cm fertile soil layer had been constructed. 4) Soil organic matter contents in the 0-35 cm layer were increased by 8.8%-23.2% and 13.2%-30.1% in the second and third years of fertile soil layer construction, respectively. Soil organic carbon storage was also gradually increased under fertile soil layer construction treatments. 5) The carbon conversion rate of organic matter was 9.3%-20.9% under T35+S treatment, and 10.6%-24.6% under T35+M, T35+S+M, and T35+S+M+F treatments. The carbon sequestration rate was 815.7-3066.4 kg·hm-2·a-1 in fertile soil layer construction treatments. The carbon sequestration rate of T35+S treatment increased with experimental periods, and soil carbon content under T35+M, T35+S+M and T35+S+M+F treatments reached saturation point in the experimental second year. Construction of fertile soil layers could improve the fertility of topsoil and subsoil and maize yield. In term of economic benefits, combination application of maize straw, organic material and chemical fertilizer within 0-35 cm soil, cooperating with conservation tillage, is recommended for the Albic soil fertility improvement.

Land use alters diazotroph community structure by regulating bacterivores in Mollisols in Northeast China.

Changes in land use can generate environmental pressures that influence soil biodiversity, and numerous studies have examined the influences of land use on the soil microbial communities. However, little is known about the effects of land use on ecological interactions of soil microbes and their predators. Diazotrophs are key soil microbes that play important functional roles in fixing atmospheric nitrogen. In this study, we investigated the co-association of diazotroph community members and patterns of diazotroph and bacterivore networks under different long-term land uses including cropland, grassland, and bare land. Diazotroph community was characterized by high-throughput sequencing. The results indicated that land use type influenced the dominant genera of diazotrophs and shaped the occurrence of specific indicator diazotroph taxa. Co-existing pattern analysis of diazotrophs and bacterivores indicated that grassland converted from cropland increased the complexity of diazotroph and bacterivore network structure. The number of nodes for diazotrophs and bacterivores was higher in grassland than in cropland and bare land. Random forest analysis revealed that six bacterivore genera Cephalobus, Protorhabditis, Acrobeloides, Mesorhabditis, Anaplectus, and Monhystera had significant effects on diazotrophs. Bacterivores were found to have predominantly negative effects in bare land. Different bacterivores had differing effects with respect to driving changes in diazotroph community structure. Structural equation model showed that land use could control diazotroph community composition by altering soil properties and regulating abundance of bacterivores. These findings accordingly enhance our current understanding of mechanisms underlying the influence of land use patterns on diazotrophs from the perspective of soil food webs.

31-year contrasting agricultural managements affect the distribution of organic carbon in aggregate-sized fractions of a Mollisol.

Evaluation of soil organic carbon (SOC) dynamics is often limited by the complexity of soil matrix. Quantitative information on the distribution of SOC within aggregate hierarchy will help elucidate the carbon flow in soil matrix. However, this knowledge still needs to be documented. Soils were sampled from a surface Mollisol with plots under 100 years of continuous cropping, 31 years of simulated overgrazing to severely degraded bareland, and grassland restoration from cropped soil. A combined density and chemical fractionation procedure within water-stable aggregate was utilized to quantify the distribution of OC after long-term different land use patterns. Results showed that grassland significantly increased total SOC and mean aggregate associated OC compared to initial soil in 1985 with total SOC (g kg-1 soil) from 46.1 to 31.7 and mean aggregate associated OC (g kg-1 aggregate) from 31.6 to 44.7. Converting cropland to grassland also enhanced the formation of macroaggregates (>0.25 mm) (from 34.7% to 52.2%) and increased the OC concentrations in density and humic fractions by 48.3%-75.9% within aggregates. But the proportions of OC in density and humic fractions to SOC only increased in macroaggregates in grassland. Alternatively, converting cropland to bareland caused substantial depletion of total SOC, macroaggregates and their associated OC concentrations. The SOC (g kg-1 soil) and mean aggregate associated OC (g kg-1 aggregate) significantly decreased from 31.7 to 25.7 and from 31.6 to 26.2, respectively. While the OC concentration of density and humic fractions within aggregates in bareland did not show significant decreases. Principal component analysis demonstrated that the soils were developed by contrasting land use changes, with the grassland soil being more associated with labile OC fractions within macroaggregats and bareland soil more associated with recalcitrant OC fractions within microaggregates and silt-clay units. These findings highlighted the favorable preservation of plant-derived carbon within soil aggregates, particularly in the labile OC fractions within macroaggregates under high plant inputs with 31 years of grassland conversion. For the cropland and bareland soils without organic inputs, more OC was stabilized within fine aggregates via organo-mineral interactions, tending to be more recalcitrant.

[Characteristics of soil fertility during soil fertilization from parent material of a Mollisol in Northeast China]. / 东北黑土成土母质培肥过程中土壤肥力变化特征.

Based on a long-term field experiment located in the central region of Mollisol in Northeast China, we examined the changes of soil fertility and nutrient supply capacity of the newly-formed soils with pot experiment after 14-year different agricultural management practices from parent material (PM) (down to 2.0-3.0 m depth) of a Mollisol, and finally tried to optimize pre-ferential agricultural practices improving soil fertility of seriously eroded PM. After 14-year different agricultural practices, soil organic carbon, total nitrogen, stability of soil aggregate and other rela-ted soil nutrient contents improved compared with PM. Soil fertility level of the newly-developed soils significantly affected nutrient supply capacity for wheat in the pot experiment. Results from principal component analysis showed that soil fertility developed toward to the surface Mollisol after 14 years of different agricultural practices from PM. Surface soils with higher organic carbon inputs in arable soil with chemical fertilizer plus all aboveground biomass incorporated and those in no-tilled alfalfa soil were much approached to surface Mollisol, followed by arable soil with fixed amount of organic inputs and natural fallow soil, while soils without and with only chemical fertilizer were much closed to PM. Our results indicated that 14-year organic carbon inputs improved soil aggregate structure and the decomposition of organic carbon entering into soil, changed soil physical and biochemical properties, and hence caused maturing of soil fertility of PM. Straw returning, organic manure application, and planting alfalfa were recommended for improving soil fertility of eroded soil. These findings would give a better scientific basis for high-efficient fertilization and management practices in eroded Mollisol region.

Keystone Microbiomes Revealed by 14 Years of Field Restoration of the Degraded Agricultural Soil Under Distinct Vegetation Scenarios.

Agricultural intensification accelerates the degradation of cropland, and restoration has been managed by changing its vegetation. However, the keystone microbiome that drives the decomposition of plant-associated organic matter in the restoration is poorly understood. In this study, we established a 14-year field restoration experiment on a degraded cropland with four treatments: (1) bare land soil without biomass input (BL), (2) maize cropland (CL) without fertilization and biomass input, (3) natural grassland (GL), and (4) alfalfa cropland (AL) with biomass left in the fields. The activity of total soil microbiome was assessed by community-level physiological profiling (CLPP) with Biolog EcoPlates analysis, and keystone microbiome was identified as phylotypes showing statistically significant increase in the restored soils (GL and AL) relative to the degraded BL soil. The results showed that GL and AL treatments improved soil fertility as indicated by significant increase in soil organic carbon, total nitrogen, and available phosphorus when compared to BL treatment. The significant difference was not observed between CL and BL treatments except for pH and available phosphorus, indicating that the input and microbial decomposition of plant-associated organic matter were the key for restoration of soil fertility. Similar results were obtained for soil microbial activities of carbon utilization efficiency via CLPP analysis, and real-time quantitative polymerase chain reaction of 16S rRNA genes further revealed significantly higher abundance of total soil microbial community in GL and AL soils than in BL and CL. High-throughput sequencing of total 16S rRNA genes revealed the Bacteroidetes as the only keystone taxa at phylum level, and 106 and 120 genera were keystone phylotypes. Compared with BL and CL, the genera that increase significantly in GL and AL are called keystone genera of ecological restoration. The dominant keystone genera included Reyranella, Mesorhizobium, Devosia, Haliangium, Nocardioides, and Pseudonocardia. Significantly higher abundance of Bacillus genus in BL soil implied it might serve as an indicator of agricultural land degradation. Statistical analysis showed that soil organic carbon and pH were significantly correlated with the input of plant-associated organic matters and dynamic changes of keystone taxa. These results suggest that the vegetation of natural grass (GL) and alfalfa plant (AL) and subsequent decomposition of plant-associated materials could serve as effective strategies for restoration of the degraded cropland by stimulating the keystone microbiomes and improving their physiological metabolisms of carbon utilization efficiency.

Long-term application of fertilizer and manures affect P fractions in Mollisol.

Application of phosphorus (P), a major plant nutrient, as fertilizer is critical to maintain P level for crop production and yield in most cultivated soils. While, it may impact the dynamics, limited studies have examined the long-term effects of fertilization on P fractions in a soil profile in Mollisol. A long-term field experiment was conducted at the State Key Experimental Station of Agroecology of the Chinese Academy of Sciences in Hailun county, Heilongjiang Province, China. A sequential fractionation procedure was used to determine the effect of fertilizer (types) treatments including no fertilizer (CK), chemical fertilizer (NPK), chemical fertilizer plus straw (NPK + S) and pig manure (OM) on fractions of P and their distribution within 0-100 cm soil profiles. Unlike CK treatment, the long-term application of fertilizers increased the concentration and accumulation of total and available P in 0-20 and 0-40 cm soil depths than deeper soils, respectively. The phosphorus activity coefficient (PAC) ranged from 1.5 to 13.8% within 0-100 cm soil depth. The largest PAC value was observed under OM treatment at 0-40 cm soil depth and under NPK + S treatment at 40-100 cm soil depth. The Ca2-P and Ca8-P concentrations increased significantly by 0.5-7.5 times and 0.5-10.4 times, respectively in OM treatment with the largest value in 0-40 cm soil depth over CK treatment. The Al-P concentration under NPK + S and OM treatments increased throughout the soil profile. The OM treatment increased all Po concentrations in the 0-40 cm soil depth, while NPK and NPK + S treatments increased labile organic P, moderately labile organic P, and highly stable organic P in the 0-20 cm soil depth. Thus, the application of fertilizer and straw, or organic manure may enhance inorganic and organic P pool in a Mollisol in Northeast China. Thus, organic manure application in the subsoil as a potential P source and their impact should be considered in developing management practices and policies regarding nutrient management.

[Effects of the construction of fertile and cultivated upland soil layer on soil fertility and maize yield in black soil region in Northeast China.] / 肥沃耕层构建对东北黑土区旱地土壤肥力和玉米产量的影响.

Organic amendment return could enhance soil fertility, improve soil structure, and increase crop yield. However, how construction of soil layers can affect soil fertility and crop yield are not fully understood. We examined the effects of constructions of fertile and cultivated soil layer on soil fertility and maize yield in the upland black soil region in Northeast China, to provide theoretical guidance in increasing soil fertility and sustainable development of agriculture. Based on the combination of field plot experiments and demonstration regions, nine study sites with different ecological characteristics were selected from Heilongjiang, Jilin and Liaoning provinces from northeast China, covering dark brown, black, meadow, chernozem, albic, brown and cinnamon soils. There were three treatments in each study site, including maize straw return within 0-35 cm soil layer (CFⅠ), the combination of maize straw and organic manure return within 0-35 cm soil layer (CFⅡ) and conventional agricultural practice without organic amendmentas control (CK). The rate of straw return in CFⅠ and CFⅡ treatments were 10000 kg·hm-2, and full straw for demonstration regions. The rate of organic manure in CFⅡ treatment was 30000 kg·hm-2. Considerable difference in soil fertility were recorded among the nine study sites with the trend of tillage layer > sub-tillage layer, especially for dark brown soil and albic soil. Soil fertility of tillage layer and sub-tillage layer was relatively low both for brown soil and cinnamon soil. The heavy clay and plow pan were pivotal limiting factors of soil fertility for the black soil and the meadow soil. Compared with CK, the concentrations of soil organic matter (SOM), available nitrogen (AN), available phosphorous (AP), and available potassium (AK) in tillage layers was increased on average by 1.85 g·kg-1, 20.16 mg·kg-1, 1.56 mg·kg-1 and 17.2 mg·kg-1 in the CFⅠ and CFⅡ treatments in five study sites with more than two years of treatments. The contents of SOM, AN, AP and AK in sub-tillage layer increased by 2.09 g·kg-1, 12.06 mg·kg-1, 2.18 mg·kg-1 and 3.84 mg·kg-1, compared with tillage layer. CFⅠ treatment significantly enhanced the contents of SOM and AP in both tested soil layers, while CFⅡ treatment significantly enhanced all fertility indices in both tested soil layers. This indicated that the increase of organic amendment return is an effective way to improve soil fertility. Maize yield fluctuated under the combined effect of climatic conditions and soil types. The significant differences in maize yield under CK, CFⅠ and CFⅡ treatments were observed with a trend of CFⅡ > CFⅠ > CK. This result indicated that the construction of fertile and cultivated soil layer could significantly increase maize yield independent of soil types. The construction of fertile and cultivated soil layer based on maize straw return or maize straw and organic manure combined return within 0-35 cm soil layer, could simultaneously increase soil fertility in both tillage and sub-tillage layer, as well as maize yield. We suggested that the selection of approaches of the constructions of fertile and cultivated soil layer should consider soil types and the sources of organic amendments. It should also give priority to soil layers rich in organic manure source to construct fertile and cultivated soil layers.

Labile and recalcitrant components of organic matter of a Mollisol changed with land use and plant litter management: An advanced 13C NMR study.

Soil organic matter (SOM) changes with land use and soil management, yet the controlling factors over the chemical composition of SOM are not fully understood. We applied quantitative 13C nuclear magnetic resonance and spectral editing techniques to measure chemical structures of SOM from different land use types. The land use types included a native grassland (nGL), a crop land with straw burning in the field (bCL), a restored grassland (rGL) and a cropland with straw removed out of the field (rCL) for 28years. The abundances of OCH groups from carbohydrates were higher in the SOMs of the nGL and rGL than in those of the rCL and bCL, while the abundances of OCH3 and aromatic CO groups from lignin were higher in the SOMs of the three-ever cultivated lands (rGL, rCL and bCL) than in that of the nGL. Although aromatic CC groups were most dominant in the Mollisols, they did not consistently decrease after the burnings of straw were ceased in the fields of the rCL and rGL compared to the bCL with continuous burning. In addition, the COO groups were bound with the aromatic CC groups in all the land use types, and the sizes of the aromatic clusters were affected by the land use types. The labile and recalcitrant components were correlated with SOC contents the mineral-associated and particular SOM in a contrasting way. Our results suggested that the chemical composition of SOM in the Mollisol depended on land use types, and that labile and recalcitrant components might be protected through mineral associations and aggregation, respectively. The most abundant aromatics in the Mollisols might not just be pyrogenic and could be oxidized to different extents, depending on field drainage conditions.

Removal of ciprofloxacin from aqueous solutions by ionic surfactant-modified carbon nanotubes.

Ionic surfactants may impact removal efficiency of organic contaminants from aqueous solution, but research regarding the adsorption mechanisms on surfactant-modified carbon nanotubes (CNTs) was limited. In this study, three multi-walled and one single-walled CNTs were used as adsorbents to investigate the adsorption behavior and mechanisms of ciprofloxacin (CIP) on CNTs modified by ionic surfactants (cationic CTAB (Cetyltrimethylamnonium bromide) or anionic SDS (Sodium dodecyl sulfate)). More than 80% (82-88%) of the total removed CIP on CTAB-modified CNTs occurred within the first 6 h, much higher than that on SDS-modified CNTs (57-78%). Modeling adsorption kinetics demonstrated that CIP adsorption on surfactant-modified CNTs was controlled by multiple and faster processes, and both external mass transfer and intraparticle diffusion are limiting factors. Relative to SDS, CTAB was significantly (P < 0.001) concentration-dependent in suppressing CIP removal. Besides, the increase in 1/n values of Freundlich model with increasing CTAB concentration suggested that CTAB could be a stronger competitor for CIP adsorption. Hydrophobic interactions predominated zwitterionic CIP adsorption on all CNTs tested, while electrostatic interactions could help control ionizable CIP adsorption on surfactant-modified CNTs depending upon pH. CIP adsorption on modified SWCNTs significantly declined with increasing ionic strength from 1 mM to 100 mM relative to those multi-walled CNTs because the more favorable aggregation of SWCNTs reduced the CIP adsorption, irrespective of which surfactant was added. Significant desorption hysteresis of adsorbed CIP released by SDS and water was observed, but not by CTAB, by which 32.6-54.4% of adsorbed CIP were removed. For SDS-modified CNTs, the mean release ratio (RR) followed an order of MWCNTs (0.075) > MHCNTs (0.058) > SWCNTs (0.057) > MCCNTs (0.049), significantly (P < 0.001) lower than CTAB-CNTs (0.37-0.56). It can be predicted that the tested surfactants co-existing with CNTs depress removal efficiency of diverse contaminants similar to CIP in aqueous systems.

Response of Soil Fungal Community Structure to Long-Term Continuous Soybean Cropping.

Long-term continuous soybean cropping can lead to the aggravation of soil fungal disease. However, the manner in which the fungal community and functional groups of fungi are affected by continuous soybean cropping remains unclear. We investigated the fungal abundance, composition and diversity during soybean rotation (RS), 2-year (SS) and long-term (CS) continuous soybean cropping systems using quantitative real-time PCR and high-throughput sequencing. The results showed that the fungal abundance was significantly higher in CS than in SS and RS. CS altered the fungal composition. Compared with RS, SS had an increase of 29 and a decrease of 12 genera in fungal relative abundance, and CS increased 38 and decreased 17 genera. The Shannon index was significantly higher in CS and SS than in RS. The result of principal coordinate analysis (PCoA) showed that CS and SS grouped together and were clearly separated from RS on the PCoA1. A total of 32 features accounted for the differences in fungal composition across RS, SS, and CS. The relative abundance of 10 potentially pathogenic and 10 potentially beneficial fungi changed, and most of their relative abundances dramatically increased in SS and CS compared with RS. Our study indicated that CS results in selective stress on pathogenic and beneficial fungi and causes the development of the fungal community structure that is antagonistic to plant health.

Priming effects on labile and stable soil organic carbon decomposition: Pulse dynamics over two years.

Soil organic carbon (SOC) is a major component in the global carbon cycle. Yet how input of plant litter may influence the loss of SOC through a phenomenon called priming effect remains highly uncertain. Most published results about the priming effect came from short-term investigations for a few weeks or at the most for a few months in duration. The priming effect has not been studied at the annual time scale. In this study for 815 days, we investigated the priming effect of added maize leaves on SOC decomposition of two soil types and two treatments (bare fallow for 23 years, and adjacent old-field, represent stable and relatively labile SOC, respectively) of SOC stabilities within each soil type, using a natural 13C-isotope method. Results showed that the variation of the priming effect through time had three distinctive phases for all soils: (1) a strong negative priming phase during the first period (≈0-90 days); (2) a pulse of positive priming phase in the middle (≈70-160 and 140-350 days for soils from Hailun and Shenyang stations, respectively); and (3) a relatively stabilized phase of priming during the last stage of the incubation (>160 days and >350 days for soils from Hailun and Shenyang stations, respectively). Because of major differences in soil properties, the two soil types produced different cumulative priming effects at the end of the experiment, a positive priming effect of 3-7% for the Mollisol and a negative priming effect of 4-8% for the Alfisol. Although soil types and measurement times modulated most of the variability of the priming effect, relative SOC stabilities also influenced the priming effect for a particular soil type and at a particular dynamic phase. The stable SOC from the bare fallow treatment tended to produce a narrower variability during the first phase of negative priming and also during the second phase of positive priming. Averaged over the entire experiment, the stable SOC (i.e., the bare fallow) was at least as responsive to priming as the relatively labile SOC (i.e., the old-field) if not more responsive. The annual time scale of our experiment allowed us to demonstrate the three distinctive phases of the priming effect. Our results highlight the importance of studying the priming effect by investigating the temporal dynamics over longer time scales.

Improving soil nutrient availability increases carbon rhizodeposition under maize and soybean in Mollisols.

Rhizodeposited carbon (C) is an important source of soil organic C, and plays an important role in the C cycle in the soil-plant-atmosphere continuum. However, interactive effects of plant species and soil nutrient availability on C rhizodeposition remain unclear. This experiment examined the effect of soil nutrient availability on C rhizodeposition of C4 maize and C3 soybean with contrasting photosynthetic capacity. The soils (Mollisols) were collected from three treatments of no fertilizer (Control), inorganic fertilizer only (NPK), and NPK plus organic manure (NPKM) in a 24-year fertilization field trial. The plants were labelled with 13C at the vegetative and reproductive stages. The 13C abundance of shoots, roots and soil were quantified at 0, 7days after 13C labelling, and at maturity. Increasing soil nutrient availability enhanced the C rhizodeposition due to the greater C fixation in shoots and distribution to roots and soil. The higher amount of averaged below-ground C allocated to soil resulted in greater specific rhizodeposited C from soybean than maize. Additional organic amendment further enhanced them. As a result, higher soil nutrient availability increased total soil organic C under both maize and soybean systems though there was no significant difference between the two crop systems. All these suggested that higher soil nutrient availability favors C rhizodeposition. Mean 80, 260 and 300kgfixedCha-1 were estimated to transfer into soil in the Control, NPK and NPKM treatments, respectively, during one growing season.

[Profile distribution and storage of soil organic carbon in a black soil as affected by land use types].

Taking soils in a long-term experimental field over 29 years with different land uses types, including arable land, bare land, grassland and larch forest land as test materials, the distribution and storage of soil organic carbon (SOC) in the profile (0-200 cm) in typical black soil (Mollisol) region of China were investigated. The results showed that the most significant differences in SOC content occurred in the 0-10 cm surface soil layer among all soils with the order of grassland > arable land > larch forest land > bare land. SOC contents at 10-120 cm depth were lower in arable land as compared with the other land use types. Compared with arable land, grassland could improve SOC content obviously. SOC content down to a depth of 60 cm in grassland was significantly higher than that in arable land. The content of SOC at 0-10 cm in bare land was significantly lower than that in arable land. Although there were no significant differences in SOC content at 0-20 cm depth between larch forestland and arable land, the SOC contents at 20-140 cm depth were generally higher in larch forestland than that in arable land. In general, SOC content showed a significantly negative relationship with soil pH, bulk density, silt and clay content and an even stronger significantly positive relationship with soil total N content and sand content. The SOC storage in arable land at 0-200 cm depth was significantly lower than that in the other three land use types, which was 13.6%, 11.4% and 10.9% lower than in grassland, bare land and larch forest land, respectively. Therefore, the arable land of black soil has a great potential for sequestering C in soil and improving environmental quality.

Adsorption of antibiotic ciprofloxacin on carbon nanotubes: pH dependence and thermodynamics.

The environmental risks of antibiotics have attracted increasing research attention, but their environmental behaviors remain unclear. In this study, functionalized carbon nanotubes (CNTs), namely, hydroxylized (MH), carboxylized (MC), graphitized multi-walled CNTs (MG) and single-walled CNTs (SW) were used as adsorbents and ciprofloxacin (CIP) as an adsorbate to investigate the effect of pH and temperature on sorption and desorption processes. Sorption isotherms of CIP were fitted well by Freundlich and Dubinin-Ashtakhov models. Highly nonlinear isotherms of CIP were observed, indicating the highly heterogeneous site energy distribution on CNTs. At all pHs, SW had the highest sorption for CIP due to its largest surface area among all CNTs. Sorption distinction between MH and MC was explained by π-π electron donor-acceptor interactions. For SW, CIP sorption was thermodynamically favorable and endothermic associated with an entropy driven process, while the reverse process occurred for MC and MG. The rearrangement of CNTs bundles/aggregates and covalent bond formation may be responsible for CIP desorption hysteresis on CNTs. Desorption of antibiotics from CNTs may lead to potential exposure, particularly under changing environmental conditions such as temperature and pH.