Response of microbial diversity to C:N:P stoichiometry in fine root and microbial biomass following afforestation

Soil samples were collected in June and October from areas with three land-use types, i.e., Robinia pseudoacacia L. (RP), Caragana korshinskii Kom. (CK), and abandoned land (AL), of which the former two were afforested areas, whereas the latter was not. These areas were converted from similar farmlands 40 years prior. Illumina sequencing of 16S rRNA gene and fungal ITS gene was used to analyze soil bacterial and fungal diversity. Additionally, plant communities, soil properties, fine root biomass, and C, N, and P levels in fine root and microbial biomass were estimated. Compared to AL, the C:N:P stoichiometry in fine root and microbial biomass in the afforested lands was synchronously changed, especially the N:P ratio. Soil microbial diversities were affected by afforestation and were more related to N:P ratio than C:P and C:N ratios. Moreover, Alpha-proteobacteria, Gamma-proteobacteria, and Bacteroidetes were significantly more abundant in afforested soils than in the AL soil, and the abundances of Actinobacteria, Chloroflexi, Cyanobacteria, and Nitrospirae ranked as AL > RP or CK. For fungal taxa, Ascomycota abundance responded positively to afforestation, whereas Basidiomycota abundance responded negatively. Changes of soil microbial taxa were significantly correlated with the N:P ratio in fine root and microbial biomass, which explained 54.1 and 55% of the total variation in bacterial and fungal taxa, respectively. Thus, our results provide evidence that compositions of soil microbial communities are linked to the N:P ratio in the plant-soil system.     

Stoichiometric responses of soil microflora to nutrient additions for two temperate forest soils

The ratios of soil carbon (C) to nitrogen (N) and C to phosphorus (P) are much higher in Chinese temperate forest soils than in other forest soils, implying that N and P might limit microbial growth and activities. The objective of this study was to assess stoichiometric responses of microbial biomass, enzyme activities, and respiration to N and P additions. We conducted a nutrient (N, P, and N + P) addition experiment in two temperate soils under Korean pine (Pinus koraiensis) plantation and natural broadleaf forest in Northeast China and measured the microbial biomass C, N, P; the activities of β-glucosidase (BG), N-acetyl-β-glucosaminidase (NAG), and acid and alkaline phosphomonoesterase (AP); and the microbial respiration in the two soils. Nitrogen addition increased microbial biomass N and decreased microbial biomass C-to-N ratio and microbial respiration in the two soils. Nitrogen addition decreased NAG activity to microbial biomass N ratio, P addition decreased AP activity to microbial biomass P ratio, and N, P, and N + P additions all increased BG activity to microbial biomass C ratio. These results suggest that microbial stoichiometry is not strictly homeostatic in response to nutrient additions, especially for N addition. The responses of enzyme activities to nutrient additions support the resource allocation theory. The N addition induced a decline in microbial respiration, implying that atmospheric N deposition may reduce microbial respiration, and consequently increase soil C sequestration in the temperate region.     

Altered precipitation seasonality impacts the dominant fungal but rare bacterial taxa in subtropical forest soils

How soil microbial communities respond to precipitation seasonality change remains poorly understood, particularly for warm-humid forest ecosystems experiencing clear dry-wet cycles. We conducted a field precipitation manipulation experiment in a subtropical forest to explore the impacts of reducing dry-season rainfall but increasing wet-season rainfall on soil microbial community composition and enzyme activities. A 67% reduction of throughfall during the dry season decreased soil water content (SWC) by 17–24% (P < 0.05), while the addition of water during the wet season had limited impacts on SWC. The seasonal precipitation redistribution had no significant effect on the microbial biomass and enzyme activities, as well as on the community composition measured with phospholipid fatty acids (PLFAs). However, the amplicon sequencing revealed differentiated impacts on bacterial and fungal communities. The dry-season throughfall reduction increased the relative abundance of rare bacterial phyla (Gemmatimonadetes, Armatimonadetes, and Baoacteriodetes) that together accounted for only 1.5% of the total bacterial abundance by 15.8, 40, and 24% (P < 0.05), respectively. This treatment also altered the relative abundance of the two dominant fungal phyla (Basidiomycota and Ascomycota) that together accounted for 72.4% of the total fungal abundance. It increased the relative abundance of Basidiomycota by 27.4% while reduced that of Ascomycota by 32.6% (P < 0.05). Our results indicate that changes in precipitation seasonality can affect soil microbial community composition at lower taxon levels. The lack of community-level responses may be ascribed to the compositional adjustment among taxonomic groups and the confounding effects of other soil physicochemical variables such as temperature and substrate availability.     

Taxon-specific responses of soil microbial communities to different soil priming effects induced by addition of plant residues and their biochars

Purpose

This work investigated changes in priming effects and the taxonomy of soil microbial communities after being amended with plant feedstock and its corresponding biochar.

Materials and methods

A soil incubation was conducted for 180 days to monitor the mineralization and evolution of soil-primed C after addition of maize and its biochar pyrolysed at 450 °C. Responses of individual microbial taxa were identified and compared using the next-generation sequencing method.

Results and discussion

Cumulative CO₂ showed similar trends but different magnitudes in soil supplied with feedstock and its biochar. Feedstock addition resulted in a positive priming effect of 1999 mg C kg^?1 soil (+253.7 %) while biochar gave negative primed C of ?872.1 mg C kg^?1 soil (?254.3 %). Linear relationships between mineralized material and mineralized soil C were detected. Most priming occurred in the first 15 days, indicating co-metabolism. Differences in priming may be explained by differences in properties of plant material, especially the water-extractable organic C. Predominant phyla were affiliated to Acidobacteria, Actinobacteria, Chloroflexi, Gemmatimonadetes, Firmicutes, Planctomycetes, Proteobacteria, Verrucomicrobia, Ascomycota, Basidiomycota, Blastocladiomycota, Chytridiomycota, Zygomycota, Euryarchaeota, and Thaumarchaeota during decomposition. Cluster analysis resulted in separate phylogenetic grouping of feedstock and biochar. Bacteria (Acidobacteria, Firmicutes, Gemmatimonadetes, Planctomycetes), fungi (Ascomycota), and archaea (Euryarchaeota) were closely correlated to primed soil C (R ²?=??0.98, ?0.99, 0.84, 0.81, 0.91, and 0.91, respectively).

Conclusions

Quality of plant materials (especially labile C) shifted microbial community (specific microbial taxa) responses, resulting in a distinctive priming intensity, giving a better understanding of the functional role of soil microbial community as an important driver of priming effect.

     

Growth and nitrogen uptake of jack pine seedlings in response to exponential fertilization and weed control in reclaimed soil

We evaluated the impact of exponential fertilization in nursery and weed removal in the field on growth and nitrogen (N) retranslocation and uptake from the soil of jack pine (Pinus banksiana Lamb.) seedlings planted on an oil sands reclaimed soil. Exponential fertilization is a method of supplying nutrients at an exponential rate to achieve constant internal nutrient concentrations in seedlings without changing their size during their growth in the nursery. The N retranslocation in seedlings was traced using ¹⁵N isotope labeling. Exponential fertilization increased nutrient reserve in the seedling in nursery production, and increased height (P = 0.003), root collar diameter (P < 0.001), total biomass (P < 0.001), and N content (P < 0.001) of seedlings at the end of first growing season in the field growth. Conventionally fertilized seedlings allocated a greater percent of biomass to roots than to current-year needles. The ¹⁵N isotope analysis showed that 59 to 82% of total N demand of new growth was met by retranslocation from old tissues. Exponential fertilization increased N retranslocation by 147% (P < 0.001) and N uptake from the soil by 175% (P = 0.012). Weed removal marginally increased (P = 0.077) N uptake from the soil but decreased (P = 0.046) N retranslocation with no net effect on total N content in new tissues. We conclude that exponential fertilization improves the early growth of jack pine and can help improve revegetation in reclaiming disturbed oil sands sites.     

Effects of cover crop in an apple orchard on microbial community composition,networks, and potential genes involved with degradation of crop residues in soil

Changes in the soil microbial communities and networks were monitored after planting the cover crop for 9 years. The field experiment included plots with a cover crop and without a cover crop but with weed control, and two subplots with or without chemical fertilizer (192 kg N ha^?1, 108 kg P₂O₅ ha^?1, and 168 kg K₂O ha^?1 each year). After applying the cover crop and chemical fertilizer for 9 years, the composition and activity of bacterial and fungal communities changed significantly (p?<?0.05), with the cover crop had greater effects than the chemical fertilizer on the composition of the soil microbial community. The relative abundances of 22 selected genera (in Firmicutes and Bacteroidetes) and two selected classes (Ascomycota) related to cover crop residue degradation increased significantly in the presence of the cover crop (p?<?0.05). Network analysis showed that the cover crop decreased the number of positive links between bacterial and fungal taxa by 25.33%, and increased the negative links by 22.89%. The positive links among bacterial taxa increased by 16.63% with the cover crop, mainly among Proteobacteria (increase of 39), Firmicutes (16), Actinobacteria (five), and Bacteroidetes (10). The links among fungal taxa were less than among bacterial taxa and were not significantly affected by cover crop. Taxa such as Thaumarchaeota, unidentified_Nitrospiraceae, unidentified_Nitrosomonadaceae, Faecalibacterium, Coprococcus_3, and Ruminococcaceae_NK4A214_group dominated the network without the cover crop but they were not dominant with the cover crop. The relative abundances of potential genes involved with the degradation of cellulose, hemicellulose, and cello-oligosaccharides increased significantly with the cover crop. Therefore, the SOC and TN contents were enhanced by the cover crop with the increase of the soil enzyme activities. Thus, the apple yield was improved by the cover crop.     

<Emphasis Type="Italic">Trichoderma</Emphasis> improves the growth of <Emphasis Type="Italic">Leymus chinensis</Emphasis>

Bio-fertilizer application has been proposed as a strategy for enhancing soil fertility, regulating soil microflora composition, and improving crop yields, and it has been widely applied in the agricultural yields. However, the application of bio-fertilizer in grassland has been poorly studied. We conducted in situ and pot experiments to investigate the practical effects of different fertilization regimes on Leymus chinensis growth, with a focus on the potential microecological mechanisms underlying the responses of soil microbial composition. L. chinensis biomass was significantly (P?<?0.05) increased by treatment with 6000 kg ha^?1 of Trichoderma bio-fertilizer compared with other treatments. We found a positive (R² =?0.6274, P <?0.001) correlation between bacterial alpha diversity and L. chinensis biomass. Hierarchical cluster analysis and nonmetric multidimensional scaling (NMDS) revealed that soil bacterial and fungal community compositions were all separated according to the fertilization regime used. The relative abundance of the most beneficial genera in bio-fertilizer (BOF) (6000 kg ha^?1Trichoderma bio-fertilizer) was significantly higher than in organic fertilizer (OF) (6000 kg ha^?1 organic fertilizer) or in CK (non-amend fertilizer), there the potential pathogenic genera were reduced. There were significant negative (P?<?0.05) correlations between L. chinensis biomass and the relative abundance of several potential pathogenic genera. However, the relative abundance of most beneficial genera were significantly (P?<?0.05) positively correlated with L. chinensis biomass. Soil properties had different effects on these beneficial and on these pathogenic genera, further influencing L. chinensis biomass.     

Long-term fertilisation regimes affect the composition of the alkaline phosphomonoesterase encoding microbial community of a vertisol and its derivative soil fractions

Alkaline phosphomonoesterase (ALP) mainly originates from soil microbial secretion and plays a crucial role in the turnover of soil phosphorus (P). To examine the response of ALP-encoding microbial communities (analysed for the biomarker of the ALP gene, phoD) of soils and derivative soil fractions to different fertilisation regimes, soil samples were collected from a long-term experimental field (over 35 years). The different organic P (Po) pools of soil fractions and the ALP activity of soil were also determined. Compared with chemical-only fertilised soils, the ALP activity was 232–815% higher in organic-amended soils, and the highest enzyme activity was observed in the organic-only fertilised treatment. The abundance of the phoD gene harbouring in soil fractions, determined by quantitative PCR (qPCR), was affected by different fertilisations. The highest abundance of the phoD gene was generally detected in the 2–63-μm-sized fraction (silt), but most phoD-encoding microbial species were associated to the 0.1–2-μm-sized fraction (clay) in the chemical-only fertilised soil. The contents of labile Po (LPo), moderately labile Po (MLPo) and fulvic acid-associated Po (FAPo) were significantly correlated with the phoD gene abundance, whereas only LPo content was significantly correlated with the ALP activity. The dominant phoD-encoding phylas were Actinobacteria and Proteobacteria, according to a high-throughput sequencing. Bradyrhizobium, a N₂-fixer identified as a phoD-encoding genus, showed the highest abundance in fertilised soils. The abundance of Bradyrhizobium, Streptomyces, Modestobacter, Lysobacter, Frankia and Burkholderia increased with the organic-only amendment and was significantly correlated with the ALP activity. According to structure equation models (SEM), pH and LPo content significantly and directly affected the ALP activity; the soil organic C (C_org) content was related to composition and abundances of phoD-harbouring microbial communities; since both microbial properties were correlated to the ALP activity, the C_org content was indirectly related to the ALP activity. In conclusion, soil management practices can be used to optimise the contents of soil available P and the organic P with regulation of soil ALP activity and the community composition of corresponding microbes.     

Changes in soil microbial substrate utilization in response to altered litter diversity and precipitation in a Mediterranean shrubland

This study aimed at quantifying the consequences of reduced precipitation and plant diversity on soil microbial community functioning in a Mediterranean shrubland of southern France. Across a natural gradient of shrub species diversity, we established a total of 92 plots (4 × 4 m) with and without a moderate rain exclusion treatment of about 12 % of total precipitation. Shrub diversity included all possible combinations of the four dominant species (Cistus albidus, Quercus coccifera, Rosmarinus officinalis, and Ulex parviflorus). Respective leaf litter mixtures of these species combinations were exposed in all plots over 2 years. We quantified how litter species richness and the reduction in precipitation affected the soil microbial substrate utilization (measured by CO₂ evolution using the MicroResp method) on soil samples collected underneath each individual litter mixture after 1 and 2 years of decomposition. Moderate precipitation reduction had a minor impact, but litter species richness and the dissimilarity in phenolic concentrations (estimated using Rao’s quadratic entropy) showed a positive effect on the diversity of substrates metabolized by the microbial communities. Moreover, litter species richness increased soil microbial activity by increasing the catabolic diversity of the soil microbial community. These effects were mostly driven by the presence of Quercus and Ulex leaf litter, which at the same time reduced microbial metabolic dominance, while the presence of Rosmarinus had opposite effects. Our data suggest that plant species loss can have stronger effects on the functioning of soil microbial communities than moderate drought, with potentially important feedbacks on biogeochemical cycling in Mediterranean shrubland ecosystems.     

Effects of biogas slurry application on peanut yield,soil nutrients,carbon storage,and microbial activity in an Ultisol soil in southern China

Purpose

Biogas slurry (BS) was known to influence soil–plant ecosystems when applied as a fertilizer, especially in combination with a chemical fertilizer (CF). Limited information was available regarding how this combination of BS–CF actually affected the soil–plant ecosystems. The purpose of this study was to evaluate the effects of BS–CF combinations on peanut yield, soil properties, and carbon (C) storage in a red soil (Ultisol) in southern China.

Materials and methods

The soil was fertilized with five treatments, including a control (T1), CF-only (T2) treatment, and three treatments with different BS–CF combinations (T3–T5). The final quantities of N/P₂O₅/K₂O applied in T2–T5 were 120:90:135 kg ha^?1. In T3–T5, 15 % (18 kg ha^?1), 30 % (36 kg ha^?1), and 45 % (54 kg ha^?1) of total N (TN), respectively, were applied with BS and the remaining TN was applied with CF. Crop yield, soil nutrients, C storage, and microbial activity were determined through field and laboratory experiments.

Results and discussion

In the field experiment, peanut grain yields of T3–T5 were higher than those of T1 (44.5–55.7 %) and T2 (10.8–19.4 %), with the highest yield from T4 (3588 kg ha^?1). The relationship between BS–TN inputs and peanut grain yield conformed to the linear-quadratic equation: y?=??1.14x ²?+?59.1x?+?2988 (R ²?=?0.98). The biomasses of peanut plants, at the flowering, pod production, and harvesting stages, were higher in T4 compared with those in T1 and T2. Moreover, T4 produced higher soil N and P (total and available) concentrations at the pod production and harvesting stages relative to other treatments, with increased soil microbial biomass C and N, and enhanced dehydrogenase and urease activities, at the flowering, pod production, and harvesting stages. Data from the incubation experiment were fitted to a first-order kinetic model, which showed that although the application of BS increased potentially mineralizable C, the additional C seemed to slowly degrade, and so would be retained in the soil for a longer period.

Conclusions

A BS–CF combination increased peanut grain yield and biomass, due to increases in soil N and P availability, microbial biomass C and N concentrations, and urease and dehydrogenase activities. Moreover, the organic C retention time in the red soil was extended. Combined application of BS–CF at a suitable ratio (36 kg BS–TN ha^?1), together with proper management practices, could be effective to improve the quality and nutrient balance of amended soils.

     

Effect of N addition,moisture, and temperature on soil microbial respiration and microbial biomass in forest soil at different stages of litter decomposition

Purpose

The objective of the present study was to investigate the interactive effects of nitrogen (N) addition, temperature, and moisture on soil microbial respiration, microbial biomass, and metabolic quotient (qCO₂) at different decomposition stages of different tree leaf litters.

Materials and methods

A laboratory incubation experiment with and without litter addition was conducted for 80 days at two temperatures (15 and 25 °C), two wetting intensities (35 and 50 % water-filled porosity space (WFPS)) and two doses of N addition (0 and 4.5 g N m^?2, as NH₄NO₃). The tree leaf litters included three types of broadleaf litters, a needle litter, and a mixed litter of them. Soil microbial respiration, microbial biomass, and qCO₂ along with other soil properties were measured at two decomposition stages of tree leaf litters.

Results and discussion

The increase in soil cumulative carbon dioxide (CO₂) flux and microbial biomass during the incubation depended on types of tree leaf litters, N addition, and hydrothermal conditions. Soil microbial biomass carbon (C) and N and qCO₂ were significantly greater in all litter-amended than in non-amended soils. However, the difference in the qCO₂ became smaller during the late period of incubation, especially at 25 °C. The interactive effect of temperature with soil moisture and N addition was significant for affecting the cumulative litter-derived CO₂-C flux at the early and late stages of litter decomposition. Furthermore, the interactive effect of soil moisture and N addition was significant for affecting the cumulative CO₂ flux at the late stage of litter decomposition but not early in the experiment.

Conclusions

This present study indicated that the effects of addition of N and hydrothermal conditions on soil microbial respiration, qCO₂, and concentrations of labile C and N depended on types of tree leaf litters and the development of litter decomposition. The results highlight the importance of N availability and hydrothermal conditions in interactively regulating soil microbial respiration and microbial C utilization during litter decomposition under forest ecosystems.

     

Arbuscular mycorrhizal fungi optimize the acquisition and translocation of Cd and P by cucumber (<Emphasis Type="Italic">Cucumis sativus</Emphasis> L.) plant cultivated on a Cd-contaminated soil

Purpose

Fruiting vegetables are generally considered to be safer than other vegetables for planting on cadmium (Cd)-contaminated farms. However, the risk of transferring Cd that has accumulated in the stems and leaves of fruiting vegetables is a major issue encountered with the usage of such non-edible parts. The objective of this study was to resolve the contribution of arbuscular mycorrhizal (AM) fungi to the production of low-Cd fruiting vegetables (focusing on the non-edible parts) on Cd-contaminated fields.

Materials and methods

An 8-week pot experiment was conducted to investigate the acquisition and translocation of Cd by cucumber (Cucumis sativus L.) plants on an unsterilized Cd-contaminated (1.6 mg kg^?1) soil in response to inoculation with the AM fungus, Funneliformis caledonium (Fc) or Glomus versiforme (Gv). Mycorrhizal colonization rates of cucumber roots were assessed. Dry biomass and Cd and phosphorus (P) concentrations in the cucumber shoots and roots were all measured. Soil pH, EC, total Cd, phytoavailable (DTPA-extractable) Cd, available P, and acid phosphatase activity were also tested.

Results and discussion

Both Fc and Gv significantly increased (P?<?0.05) root mycorrhizal colonization rates and P acquisition efficiencies, and thus the total P acquisition and biomass of cucumber plants, whereas only Fc significantly increased (P?<?0.05) soil acid phosphatase activity and the available P concentration. Both Fc and Gv significantly increased (P?<?0.05) root to shoot P translocation factors, inducing significantly higher (P?<?0.05) shoot P concentrations and shoot/root biomass ratios. In contrast, both Fc and Gv significantly decreased (P?<?0.05) root and shoot Cd concentrations, resulting in significantly increased (P?<?0.05) P/Cd concentration ratios, whereas only Gv significantly decreased (P?<?0.05) the root Cd acquisition efficiency and increased (P?<?0.05) the root to shoot Cd translocation factor. Additionally, AM fungi also tended to decrease soil total and phytoavailable Cd concentrations by elevating plant total Cd acquisition and soil pH, respectively.

Conclusions

Inoculation with AM fungi increased the P acquisition and biomass of cucumber plants, but decreased plant Cd concentrations by reducing the root Cd acquisition efficiency, and resulted in a tendency toward decreases in soil phytoavailable and total Cd concentrations via increases in soil pH and total Cd acquisition by cucumber plants, respectively. These results demonstrate the potential application of AM fungi for the production of fruiting vegetables with non-edible parts that contain low Cd levels on Cd-contaminated soils.

     

Combined biochar and nitrogen fertilizer reduces soil acidity and promotes nutrient use efficiency by soybean crop

Purpose

Soil acidification is universal in soybean-growing fields. The aim of our research was to evaluate the effects of soil additives (N fertilizers and biochar) on crop performance and soil quality with specific emphasis on ameliorating soil acidity.

Materials and methods

Four nitrogen treatments were applied as follows: no nitrogen (N0), urea (N1), potassium nitrate (N2), and ammonium sulfate (N3), each providing 30 kg N ha^?1. Half plot area of the N1, N2, and N3 treatments was also treated with biochar (19.5 t ha^?1) to form N-biochar treatments (N1C, N2C, N3C). Both bulk and rhizosphere soils were sampled separately for the following analyses: pH, exchangeable base cations (EBC), exchangeable acidity (EA), total inorganic N (IN), total N (TN), and microbial phospholipid fatty acids (PLFAs). Soybean biomass and nutrient contents were also determined. Correlation analysis was applied to analyze the relationships between soil chemical properties and soybean plant parameters.

Results and discussion

With N-biochar additions (N1C, N2C, N3C), soil chemical properties changed as follows: pH increased by 0.6–1.2 units, EBC, IN, and TN increased by 175–419, 38.5–54.7, and 136–452 mg kg^?1, respectively, and PLFAs increased by 23.6–40.9 nmol g^?1 compared to the N0 in the rhizosphere. Microbial PLFAs had positive correlations with soil pH; EBC; exchangeable K, Ca, Na, and Mg; TN; IN; NH₄ ⁺; and NO₃ ^? (r?=?0.66–0.84, p?<?0.01). There were negative correlations between PLFAs and EA or exchangeable Al (r?=??0.64, ?0.66, p?<?0.01), which indicated that the additives increased microbial biomass by providing a suitable environment with less acid stress and more nutrients. The additives increased soil NH₄ ⁺ and NO₃ ^? by promoting soil organic N mineralization and reducing NH₄ ⁺ and NO₃ ^? leaching. Moreover, the soybean seed biomass and the nutrient contents in seeds increased with N-biochar additions, especially in the N3C treatment.

Conclusions

N-biochar additions were effective in ameliorating soil acidity, which improved the microenvironment for more microbial survival. N-biochars influenced N transformations at the plant–soil interface by increasing organic N mineralization, reducing N leaching, and promoting N uptake by soybeans. The soil additive ammonium and biochar (N3C) were best in promoting soybean growth.

     

Effects of biochar on copper immobilization and soil microbial communities in a metal-contaminated soil

Purpose

Copper (Cu) contamination has been increasing in land ecosystems. Biochars (BCs) and arbuscular mycorrhizal fungi (AMF) are known to bind metals, and metallophyte can remove metals from soils. Will BC in combination with AMF contain the Cu uptake by a metallophyte growing in a metal-contaminated soil? The objective of this study was to investigate the effects of BCs on the Cu immobilization and over soil microbial communities in a metal-contaminated soil in the presence of AMF and metallophyte.

Materials and methods

Two BCs were produced from chicken manure (CMB) and oat hull (OHB). A Cu-contaminated sandy soil (338 mg kg^?1) was incubated with CMB and OHB (0, 1, and 5 % w/w) for 2 weeks. Metallophyte Oenothera picensis was grown in pots (500 mL) containing the incubated soils in a controlled greenhouse for 6 months. A number of analyses were conducted after the harvest. These include plant biomass weight, microbial basal respiration, and dehydrogenase activity (DHA), AMF root colonization, spore number, and glomalin production; changes in fungal and bacterial communities, Cu fractions in soil phases, and Cu uptake in plant tissues.

Results and discussion

The BCs increased the soil pH, decreased easily exchangeable fraction of Cu, and increased organic matter and residual fraction of Cu. The BCs provided favorable habitat for microorganisms, thereby increasing basal respiration. The CMB increased DHA by ～62 and ～574 %, respectively, for the low and high doses. Similarly, the OHB increased soil microbial activity by ～68 and ～72 %, respectively, for the low and high doses. AMF root colonization, spore number, and total glomalin-related soil protein (GRSP) production increased by ～3, ～2, and ～3 times, respectively, in soils treated with 1 % OHB. Despite being a metalophyte, O. picensis could not uptake Cu efficiently. Root and shoot Cu concentrations decreased or changed insignificantly in most BC treatments.

Conclusions

The results show that the BCs decreased bioavailable Cu, decreased Cu uptake by O. picensis, improved habitat for microorganisms, and enhanced plant growth in Cu-contaminated soil. This suggests that biochars may be utilized to remediate Cu-contaminated soils.

     

Effects of Endosulfan on the Populations of Cultivable Microorganisms and the Diversity of Bacterial Community Structure in Brunisolic Soil

Endosulfan, an organochlorine pesticide, has been applied ubiquitously worldwide. However, endosulfan has been identified as a type of persistent organic pollutants (POPs), and its ecotoxicity has drawn attentions from scientists. The present study was implemented to examine the effects of endosulfan on the diversity and structure of soil microorganism communities. A control treatment and three concentrations (0.1, 1.0, and 10.0 mg/kg) were set up in laboratory experiments and sampled on days 7, 14, 21, and 28. The results revealed that the populations of bacteria and actinomycetes decreased significantly after 1.0 and 10.0 mg/kg treatments and that the soil microbial biomass carbon (MBC) was increased by endosulfan compared with the control. Terminal restriction fragment length polymorphism (T-RFLP) results revealed that the soil bacterial diversity was decreased by endosulfan and that the soil microbial community structure became unstable after endosulfan application. Moreover, the results of a 16S rRNA clone library revealed that the phyla Proteobacteria, Actinobacteria, Bacteroidetes, Spirochaetes, and Firmicutes showed an obvious advantage and closely relative. In conclusion, the results of the present study indicated that 0.1–10.0 mg/kg endosulfan showed obvious influences on the diversity and structure of the soil microbial community.     

Straw amendment to paddy soil stimulates denitrification but biochar amendment promotes anaerobic ammonia oxidation

Purpose

Organic matter amendment is usually used to improve soil physicochemical properties and to sequester carbon for counteracting climate change. There is no doubt that such amendment will change microbial activity and soil nitrogen transformation processes. However, the effects of straw and biochar amendment on anammox and denitrification activity and on community structure in paddy soil are unclear.

Materials and methods

We conducted a 30-day pot experiment using rice straw and rice straw biochar to deepen our understanding about the activity, microbial abundance, and community structure associated with soil nitrogen cycling during rice growth.

Results and discussion

Regarding activity, anammox contributed 3.1–8.1% of N₂ production and denitrification contributed 91.9–96.9% of N₂ production; straw amendment resulted in the highest denitrification rate (38.9 nmol N g^?1 h^?1), while biochar amendment resulted in the highest anammox rate (1.60 nmol N g^?1 h^?1). Both straw and biochar amendments significantly increased the hzsB and nosZ gene abundance (p < 0.05). Straw amendment showed the highest nosZ gene abundance, while biochar amendment showed the highest hzsB gene abundance. Phylogenetic analysis of the anammox bacteria 16S rRNA genes indicated that Candidatus Brocadia and Kuenenia were the dominant genera detected in all treatments.

Conclusions

Straw and biochar amendments have different influences on anaerobic ammonia oxidation and denitrification within paddy soil. Our results suggested that the changes in denitrification and anammox rates in the biochar and straw treatments were mainly linked to functional gene abundance rather than microbial community structure and that denitrification played the more major role in N₂ production in paddy soil.

     

Different plant covers change soil respiration and its sources in subtropics

Heterotrophic soil respiration (R _H) and autotrophic soil respiration (R _A) by a trenching method were monitored in four vegetation types in subtropical China from November 2011 to October 2012. The four vegetation types included a shrubland, a mixed-conifer, a mixed-legume, and a mixed-native species. The average R _H was significantly greater in soils under the mixed-legume and the mixed-native species than in the shrubland and the mixed-conifer soils, and it affected the pattern of soil total respiration (R _S) of the four soils. The change in R _H was closely related to the variations of soil organic C, total N and P content, and microbial biomass C. The R _A and the percentage of R _S respired as R _A were only significantly increased by the mixed-native species after reforestation. Probably, this depended on the highest fine root biomass of mixed-native species than the other vegetation types. Soil respiration sources were differently influenced by the reforestation due to different changes in soil chemical and biological properties and root biomass.     

Effects of nitrification inhibitor 3,4-dimethylpyrazole phosphate and fungicide iprodione on soil fungal biomass and community: based on internal transcribed spacer region

Purpose

Nitrification inhibitors that impact soil nitrifying microorganisms have been widely applied in agricultural soils to enhance the efficiency of nitrogen fertilizers. However, little is known about their combined impact with other chemical applications, such as fungicides, on soil fungi. This study specifically examined the effects of the nitrification inhibitor 3,4-dimethylpyrazole phosphate (DMPP), alone or together with the fungicide iprodione, on fungi biomass and community in a typical farmland soil.

Materials and methods

Four treatments were set: (1) control of zero agrochemical applications (CK), (2) a single DMPP application (DAA), (3) repeated iprodione applications (4×IPR), and (4) combined applications of DMPP and iprodione (DAA+4×IPR). The agrochemicals were applied at the recommended intervals, and the soil samples were incubated in the dark for 28 days. During the incubation, soil sample DNA was extracted, and the effects of DMPP and iprodione applications on soil fungal internal transcribed spacer (ITS) abundances were determined with quantitative PCR (qPCR). At the end of the incubation, Illumina MiSeq method was employed to assess soil fungal community diversity and structure.

Results and discussion

DMPP application had a negligible effect on fungal ITS abundance. However, repeated iprodione applications significantly decreased fungal ITS abundances. After 28 days of incubation, the fungal ITS abundances in the 4×IPR and DAA+4×IPR treatments were 43.6 and 56.2% of that measured from the CK treatment, respectively. Shannon indices of fungal communities demonstrated the treatment-induced gradients, with the DAA+4×IPR treatment harboring the highest Shannon index. Fungal community structures following the DAA and 4×IPR treatments remained overlapping with that in the CK treatment, but repeated iprodione applications markedly enriched the family Teratosphaeriaceae. Relative to the CK treatment, fungal community structure in the DAA+4×IPR treatment was significantly changed, with the families Cephalothecaceae, Hypocreaceae, and Cordycipitaceae harboring a linear discriminant analysis value >3.

Conclusions

DMPP application had negligible effects on soil fungal biomass, community diversity, and structure, potentially indicating that the DMPP is “bio-safe.” Conversely, repeated iprodione applications significantly decreased fungal ITS abundances. Moreover, the family Teratosphaeriaceae could be further investigated as a potential biomarker of the impacts of iprodione on soil fungi. The combined applications of DMPP and iprodione stimulated the Shannon diversity index and markedly changed soil fungal community structure.

     

Soil pre-fumigation could effectively improve the disease suppressiveness of biofertilizer to banana <Emphasis Type="Italic">Fusarium</Emphasis> wilt disease by reshaping the soil microbiome

Two seasonal pot experiments were conducted to investigate the effect of biofertilizer application after mixture of lime and ammonium bicarbonate (LA) fumigation, on banana Fusarium wilt disease suppression and soil microbial community composition. Biofertilizer application after LA fumigation decreased 80% of disease incidence compared to control of biofertilizer application to non-fumigated soil. Biofertilizer application after fumigation clearly manipulated soil microbial community composition as revealed by non-metric multidimensional scaling and Venn diagram. LA fumigation significantly reduced the abundance of F. oxysporum while biofertilizer application after fumigation could further decrease it. Furthermore, indigenous microbes, e.g., Bacillus, Pseudomonas, and Mortierella, were associated with disease suppression. Biofertilizer application after fumigation significantly (p?<?0.05) increased the soil pH and content of soil total C and available P and K, and this probably reshaped soil microbial community as revealed by redundancy analysis and variance partitioning analysis. The observed disease suppression due to biofertilizer application after soil fumigation can be attributed to the reduced abundance of F. oxysporum by general suppression resulting from manipulated soil properties and recovered soil microbiome.     

The foliar spray of <Emphasis Type="Italic">Rhodopseudomonas palustris</Emphasis> grown under <Emphasis Type="Italic">Stevia</Emphasis> residue extract promotes plant growth via changing soil microbial community

Purpose

The extract of Stevia residue is an ideal substitute for cultivation of the purple nonsulfur bacterium, like Rhodopseudomonas palustris (R. palustris). But the influence of R. palustris grown under residue extract on its downstream application is still not well-characterized. The objective of this study was to assess the effect of foliar spray of R. palustris grown under Stevia residue extract on the plant growth and soil microbial properties.

Materials and methods

A pot experiment was carried out under the greenhouse condition, consisting of four treatments varying in the sprayed substances: sterilized water (control), R. palustris grown under the chemical medium supplemented with L-tryptophan (SyT), R. palustris grown under Stevia residue extract supplemented with L-tryptophan (ExT), and R. palustris grown under Stevia residue extract supplemented with NH₄Cl (ExT). The net photosynthesis rate of the uppermost leaves was measured with a portable photosynthesis system. Soil microbial activity was analyzed by microcalorimetry. Soil bacterial community components were determined by real-time quantitative PCR (qPCR) and high-throughput sequencing techniques.

Results and discussion

Compared with SyT, the R. palustris grown under Stevia residue extract not only improved the plant biomass and the net photosynthetic rate to a large extent, but also increased soil microbial metabolic activity and altered community compositions as well. The treatments receiving R. palustris, especially ExT and ExN, increased the relative abundances of some functional guilds involved in C turnover and nutrient cycling in soil, including Acidobacteria, Actinobacteria, Proteobacteria, Gemmatimonadaetes, Nitrospirae, and Planctomycetes.

Conclusions

R. palustris grown under the Stevia residue extract showed advantages over that under the chemical medium on both plant growth and soil microbial properties. One of the possible reasons could result from the increases in microbial activity and several bacterial keystone guilds involved into C and nutrient cycling, both of which potentially contribute to the improved plant growth. The results would be conducive to the downstream application of R. palustris in an economical way.