Carbon emission from the soil surface in a mature blueberry pine forest of the middle taiga (Republic of Komi)

Data on the input of plant falloff and organic matter decomposition on the surface of the peaty podzolic-gleyic humus-illuvial (Gleyic Podzol) soil under a mature blueberry pine forest in the middle taiga are presented. The fractional composition of the falloff was determined, and constants of decomposition for its components were calculated. The carbon flux to the atmosphere due to the mineralization of plant residues is estimated at 251 g/m². A close positive correlation (r = 0.71; P < 0.05) was found between the carbon dioxide emission measured using a gas analyzer and the soil temperature at the depth of 10 cm. The CO₂ emission for a growing period calculated from the data on its dependence on soil temperature in different years varied from 243 to 313 g C/m² and was related to weather conditions.     

Effect of Cadmium Stress on Growth and Electrical Impedance Spectroscopy Parameters of <Emphasis Type="Italic">Cotinus coggygria</Emphasis> Roots

The use of plants for ecological remediation is an important method of controlling heavy metals in polluted land. Cotinus coggygria is a landscape plant that is used extensively in landscaping and afforestation. In this study, the cadmium tolerance level of C. coggygria was evaluated using electrical impedance spectroscopy (EIS) to lay a theoretical foundation for broad applications of this species in Cd-polluted areas and provide theoretical support to broaden the application range of the EIS technique. Two-year-old potted seedlings of C. coggygria were placed in a greenhouse to analyse the changes in the growth, water content and EIS parameters of the roots following treatment with different Cd concentrations (50, 100, 200, 500, 1000 and 1500 mg kg^?1), and soil without added Cd was used as the control. The roots grew well following Cd treatments of 50 and 100 mg kg^?1. The Cd contents increased with the increase in Cd concentration in the soil. However, the lowest root Cd content was found at 4 months of treatment. The extracellular resistance r_e and the intracellular resistance r_i increased first overall and then decreased with the increasing Cd concentration, and both parameters increased with a longer treatment duration. The water content had a significant negative correlation with the Cd content (P?<?0.01) and the r_e (P?<?0.05). C. coggygria could tolerate a soil Cd concentration of 100 mg kg^?1. There was a turning point in the growth, water content and EIS parameters of the C. coggygria roots when the soil Cd concentration reached 200 mg kg^?1. The root water content and r_e could reflect the level of Cd tolerance in C. coggygria.     

Mineralization of nitrogen compounds in a soil under an oxalis birch forest

The total mineralization of nitrogen in the AO-A1 (0–6 cm), A1 (6–11 cm), and A2 (11–21 cm) horizons of a soddy pale-podzolic soil under an oxalis birch forest in Yaroslavl oblast was measured from May to November in 2009 and 2010 and comprised 6.7 ± 0.9, 3.0 ± 0.4, and 5.5 ± 0.6 g of N/m² in 2009 and 5.6 ± 0.5, 2.5 ± 0.2, and 2.1 ± 0.5 g of N/m² in 2010, respectively. The total nitrification reached 0.4 ± 0.1, 1.1 ± 0.2, and 1.4 ±0.1 g of N/m² in 2009 and 1.0, 0.6, and 0.7 g of N/m² in 2010. Overall, the amount of mineralized nitrogen in the 21-cm-deep soil layer in 2009 and 2010 constituted 15.2 ± 1.1 and 10.2 ± 0.7 g of N/m², respectively. The contribution of nitrification to the nitrogen mineralization amounted to 20%. The seasonal variations in the soil temperature and moistening affected the concentrations of ammonium in the upper horizons and the accumulation of ammonium in the AO-A1 and A1 horizons. The combined effect of the temperature and moisture controlled the ammonification in the AO-A1 horizon (R = 0.83 at p = 0.16 in 2010), the nitrification in all the studied horizons (R = 0.86 at p= 0.13 in 2009), and the ammonia emission from the soil surface (R = 0.92 at p = 0.06 in 2010). A correlation between the seasonal dynamics of the ammonification and the CO₂ emission was found for the AO-A1 horizon (r = 0.64 at p = 0.16 in 2010) and was absent in the deeper layers of the soil profile. The nitrogen losses from the soil surface due to the ammonia emission in the investigated periods reached 95 ± 31 g of N/ha (2009) and 33 ± 30 g of N/ha (2010).     

Catalase and Phosphatase Activities During Hydrocarbon Removal from Oil-Contaminated Soil Amended with Agro-Industrial By-products and Macronutrients

Microbiological activities are essential in the bioremediation of polluted soils. The enzymatic activities of microorganisms are usually used as a biological indicator of soil health. The aim of this work was to observe the catalase, acid phosphatase (AcP), and alkaline phosphatase (AlP) activities in soil that was amended with agro-industrial by-products and macronutrients during the process of total petroleum hydrocarbon (TPH) removal. To this end, microcosm tests were performed with soil and agro-industrial by-products ratios of 100:2:2, for soil:sugarcane bagasse pith:filter cake mud (SSF); 100:2, for both soil:sugarcane bagasse pith (SS); and for soil filter cake mud (SF). The macronutrients—carbon, nitrogen, and phosphorus—in the experimental treatments were adjusted to 100:10:1 with a solution of NH₄NO₃ and K₂HPO₄. The best TPH removal (51.4%) was obtained with SSF at 15 days. In addition, a significant correlation was observed between TPH removal and AlP as well as AcP (r = 0.74, p < 0.0001; r = 0.70, p < 0.0107, respectively). Fungi growth was also correlated with both AlP (r = 0.97, p < 0.0001) and AcP (r = 0.95, p < 0.0001) activities. Besides, bacterial and fungi growth showed a correlation with TPH (r = 0.86, p < 0.001; r = 0.77, p < 0.0034, respectively). It could be said that the agro-industrial by-products and macronutrients contributed to pollutant removal from the oil-polluted soil at relatively short amount of time. In addition, the enzymatic activities were increased after the treatment; in this study, the high sensitivity enzyme was AlP, and it could be used as an indirect indicator of oil pollutant removal.     

Contrasting response of two grassland soils to N addition and moisture levels: N<Subscript>2</Subscript>O emission and functional gene abundance

Purpose

Nitrification and denitrification processes dominate nitrous oxide (N₂O) emission in grassland ecosystems, but their relative contribution as well as the abiotic factors are still not well understood.

Materials and methods

Two grassland soils from Duolun in Inner Mongolia, China, and Canterbury in New Zealand were used to quantitatively compare N₂O production and the abundance of bacterial and archaeal amoA, denitrifying nirK and nirS genes in response to N additions (0 and 100 μg NH₄ ⁺–N g^?1 dry soil) and two soil moisture levels (40 and 80 % water holding capacity) using microcosms.

Results and discussion

Soil moisture rather than N availability significantly increased the nitrification rate in the Duolun soil but not in the Canterbury soil. Moreover, N addition promoted denitrification enzyme activities in the Canterbury soil but not in the Duolun soil. The abundance of bacterial and archaeal amoA genes significantly increased as soil moisture increased in the Duolun soil, whereas in the Canterbury soil, only the abundance of bacterial amoA gene increased. The increase in N₂O flux induced by N addition was significantly greater in the Duolun soil than in the Canterbury soil, suggesting that nitrification may have a dominant role in N₂O emission for the Duolun soil, while denitrification for the Canterbury soil.

Conclusions

Microbial processes controlling N₂O emission differed in grassland soils, thus providing important baseline data in terms of global change.

     

Linear Regression between CIE-Lab Color Parameters and Organic Matter in Soils of Tea Plantations

To quantify the relationship between the soil organic matter and color parameters using the CIE-Lab system, 62 soil samples (0–10 cm, Ferralic Acrisols) from tea plantations were collected from southern China. After air-drying and sieving, numerical color information and reflectance spectra of soil samples were measured under laboratory conditions using an UltraScan VIS (HunterLab) spectrophotometer equipped with CIE-Lab color models. We found that soil total organic carbon (TOC) and nitrogen (TN) contents were negatively correlated with the L* value (lightness) (r = –0.84 and –0.80, respectively), a* value (correlation coefficient r = –0.51 and –0.46, respectively) and b* value (r = –0.76 and –0.70, respectively). There were also linear regressions between TOC and TN contents with the L* value and b* value. Results showed that color parameters from a spectrophotometer equipped with CIE-Lab color models can predict TOC contents well for soils in tea plantations. The linear regression model between color values and soil organic carbon contents showed it can be used as a rapid, cost-effective method to evaluate content of soil organic matter in Chinese tea plantations.     

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.

     

Tillage system affects fertilizer-induced nitrous oxide emissions

Since the development of effective N₂O mitigation options is a key challenge for future agricultural practice, we studied the interactive effect of tillage systems on fertilizer-derived N₂O emissions and the abundance of microbial communities involved in N₂O production and reduction. Soil samples from 0–10 cm and 10–20 cm depth of reduced tillage and ploughed plots were incubated with dairy slurry (SL) and manure compost (MC) in comparison with calcium ammonium nitrate (CAN) and an unfertilized control (ZERO) for 42 days. N₂O and CO₂ fluxes, ammonium, nitrate, dissolved organic C, and functional gene abundances (16S rRNA gene, nirK, nirS, nosZ, bacterial and archaeal amoA) were regularly monitored. Averaged across all soil samples, N₂O emissions decreased in the order CAN and SL (CAN?=?748.8?±?206.3, SL?=?489.4?±?107.2 μg kg^?1) followed by MC (284.2?±?67.3 μg kg^?1) and ZERO (29.1?±?5.9 μg kg^?1). Highest cumulative N₂O emissions were found in 10–20 cm of the reduced tilled soil in CAN and SL. N₂O fluxes were assigned to ammonium as source in CAN and SL and correlated positively to bacterial amoA abundances. Additionally, nosZ abundances correlated negatively to N₂O fluxes in the organic fertilizer treatments. Soils showed a gradient in soil organic C, 16S rRNA, nirK, and nosZ with greater amounts in the 0–10 than 10–20 cm layer. Abundances of bacterial and archaeal amoA were higher in reduced tilled soil compared to ploughed soils. The study highlights that tillage system induced biophysicochemical stratification impacts net N₂O emissions within the soil profile according to N and C species added during fertilization.     

Changes in methane emission and methanogenic and methanotrophic communities in restored wetland with introduction of <Emphasis Type="Italic">Alnus trabeculosa</Emphasis>

Purpose

The dynamics and uncertainties in wetland methane budgets affected by the introduction of Alnus trabeculosa H. necessitate research on production of methane by methanogenic archaea and consumption by methane-oxidizing microorganisms simultaneously.

Materials and methods

This study investigated methane emission in situ by the closed chamber method, and methanogenic and methanotrophic communities using denatured gradient gel electrophoresis (DGGE) and quantitative PCR based on mcrA (methyl coenzyme M reductase), pmoA (particulate methane monooxygenase) genes in the rhizosphere and non-rhizosphere soils in the indigenous pure Phragmites australis T., and A. trabeculosa–P. australis mixed communities in Chongxi wetland.

Results and discussion

Methane flux rate from the pure P. australis community was 2.4 times larger than that of A. trabeculosa–P. australis mixed community in the rhizosphere and 1.7 times larger in the non-rhizosphere, respectively. The abundance of methanogens was lower in the mixed community soils (3.56?×?10³–6.90?×?10³ copies g^?1 dry soil) compared with the P. australis community (1.47?×?10⁴–1.89?×?10⁴ copies g^?1 dry soil), whereas the methanotrophs showed an opposite trend (2.08?×?10⁶–1.39?×?10⁶ copies g^?1 dry soil for P. australis and 6.20?×?10⁶–1.99?×?10⁶ copies g^?1 dry soil for mixed community soil). A liner relationship between methane emission rates against pmoA/mcrA ratios (R ²?=?0.5818, p?<?0.05, n?=?15) was observed. The community structures of the methane-cycling microorganism based on mcrA and pmoA suggested that acetoclastic methanogens belonging to Methanosarcinaceae and a particular type II methanotroph, Methylocystis, were dominant in these two plant communities.

Conclusions

The introduction of A. trabeculosa would promote the proliferation of methanotrophs, especially the dominant Methylocystis, but not methanogens, ultimately diminishing methane emission in the wetland.

     

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.     

Inverse Estimation of the Hydrodispersive Properties of Unsaturated Soil Using Complex-Variable-Differentiation Method under Field Experiments Conditions

Because soil hydraulic properties are indispensable for determining soil water retention and soil solute movement, their input for simulation models is essential. Many of these parameters cannot be estimated directly at the scale of interest, but can only be derived through inverse modeling. During this process, the parameters are generally adjusted using least-squares approach with Levenberg–Marquardt (LM) algorithms in which numerically simulated models are fitted to measured data. In this study we used a new inverse method to estimate the unsaturated soil hydro-dispersive properties from in-situ experiments. The method employs complex-variable-differentiation method (CVDM) to accurately predict of the hydraulic properties of the van Genuchten–Mualem models (θ_r, θ_s, α, k_s, n). To the knowledge of the authors, it is first study use CVDM in soil physics. The optimization procedure was performed by using a continuous data set of daily in situ soil water content and bromide concentration measurements. Estimated parameters during the inversion showed high correlation (R² = 0.88, RMSE = 0.013 and the model efficiency CE = 0.77) by using the CVDM-methods with the actual field measurements, compared with the traditional LM-algorithm (R² = 0.81, RMSE = 0.021 and CE = 0.626). The results show that the new inverse analysis in the present work has the high accuracy, validity, uniqueness, and higher inversion efficiency. Meanwhile, the convergence and stability of the modified LM-algorithm are improved. Overall, it was concluded that the CVDM is promising method to estimate hydro-dispersive parameters in soil physics.     

Interactive effects of biochar addition and elevated carbon dioxide concentration on soil carbon and nitrogen pools in mine spoil

Purpose

This study aimed to assess the effects of biochar on improving nitrogen (N) pools in mine spoil and examine the effects of elevated CO₂ on soil carbon (C) storage.

Materials and methods

The experiment consisted of three plant species (Austrostipa ramossissima, Dichelachne micrantha, and Lomandra longifolia) planted in the N-poor mine spoil with application of biochar produced at three temperatures (650, 750, and 850 °C) under both ambient (400 μL L^?1) and elevated (700 μL L^?1) CO₂. We assessed mine spoil total C and N concentrations and stable C and N isotope compositions (δ¹³C and δ¹⁵N), as well as hot water extractable organic C (HWEOC) and total N (HWETN) concentrations.

Results and discussion

Soil total N significantly increased following biochar application across all species. Elevated CO₂ induced soil C loss for A. ramossissima and D. micrantha without biochar application and D. micrantha with the application of biochar produced at 750 °C. In contrast, elevated CO₂ exhibited no significant effect on soil total C for A. littoralis, D. micrantha, or L. longifolia under any other biochar treatments.

Conclusions

Biochar application is a promising means to improve N retention and thus, reduce environmentally harmful N fluxes in mine spoil. However, elevated CO₂ exhibited no significant effects on increasing soil total C, which indicated that mine spoil has limited potential to store rising atmospheric CO₂.

     

An overlooked nitrogen loss linked to anaerobic ammonium oxidation in estuarine sediments in China

Purpose

Despite its importance, anammox (anaerobic ammonium oxidation) in estuarine sediment systems remains poorly understood, particularly at the continental scale. This study aimed to understand the abundance, diversity, and activity of anammox bacteria and to determine the main factors influencing the anammox process in estuarine sediments in China.

Materials and methods

Estuarine sediments were collected from 18 estuaries spanning over 4000 km. Experiments using an ¹⁵ N–tracer, quantitative PCR, and clone library construction were used to determine the activity, abundance, and diversity of anammox bacteria. The impact of environmental factors on anammox processes was also determined.

Results and discussion

The abundance of the anammox-specific hydrazine synthase (hzsB) gene ranged from 1.8 × 10⁵ ± 3.4 × 10⁴ to 3.6 × 10⁸ ± 7.5 × 10⁷ copies g^?1 dw. Candidatus Scalindua, Brocadia, Kuenenia, Jettenia, and two novel unidentified clusters were detected, with Scalindua dominating the anammox population. Additionally, the abundances of Scalindua, Kuenenia, and Brocadia were found to be significantly correlated with latitude. The anammox rates ranged from 0.29 ± 0.15 to 13.68 ± 3.98 nmol N g^?1 dw h^?1 and contributed to 2.39–82.61% of total N₂ production. Pearson correlation analysis revealed that the anammox rate was positively correlated with total nitrogen, total carbon, and temperature, and was negatively correlated with dissolved oxygen (DO). The key factors influencing the hzsB gene abundance were ammonium concentration, salinity, and DO. Ammonium concentration, pH, temperature, and latitude were main variables shaping the anammox-associated bacterial community.

Conclusions

Our results suggested that anammox bacteria are ubiquitous in coastal estuaries in China and underline the importance of anammox resulting in N loss at a continental scale.

     

Responses of soil microbial community to nitrogen fertilizer and precipitation regimes in a semi-arid steppe

Purpose

Changes of nitrogen (N) cycle caused by N fertilization and precipitation regimes have affected the key ecosystem structure and functions in temperate steppe, which may modify the structure of soil microbial communities involved in N transformation. This paper was designated to examine the response of soil ammonia oxidizers and denitrifiers to the N fertilization and precipitation regimes in a semi-arid steppe where N and water contents are major limiting factors of the grassland productivity.

Materials and methods

This study was based on a long-term N fertilization and precipitation regimes experiment in Inner Mongolia (116° 17′ 20″ E, 42° 2′ 29″ N). The treatments including CK (control), R (reduced precipitation), W (30% increase in precipitation), N (10 g N m^?2 y^?1), RN (reduced precipitation and 10 g N m^?2 y^?1), and WN (30% increase in precipitation and 10 g N m^?2 y^?1). Soil basic chemical properties and microbial activities were analyzed. Molecular methods were applied to determine the abundance, structure and diversity of ammonia oxidizers and denitrifiers. Statistical analysis detected the main and interactive effect of treatments on soil microbial communities and revealed the relationship between soil microbial community structures and environmental factors.

Results and discussion

N fertilization significantly increased ammonia-oxidizing bacteria (AOB) abundance. Ammonia-oxidizing archaea (AOA) community structure was markedly changed in N fertilizer treatment and strongly affected by soil pH, while soil nitrate and water content correlated with AOB community structure. Soil nitrate was the key factor influencing nirK gene community structure, while soil pH and water content explained much of the variations of nosZ gene community. AOB-amoA and nosZ gene community diversities were influenced by precipitation regimes and interaction of N fertilization and precipitation regimes, respectively.

Conclusions

N fertilization and precipitation regimes had significant influences on the changes of soil properties and microbial functional communities. Soil nitrification was mainly driven by AOB in the semi-arid grassland. Changes of substrate content and soil pH were the key factors in shifting functional microbial communities. The non-synergistic effects of N fertilization and precipitation regimes on the microbial functional groups indicated that the negative effect of lower pH induced by N fertilization would be alleviated by precipitation regimes, which should be well considered in grassland restoration.

     

Geographic distance and amorphous iron affect the abundance and distribution of <Emphasis Type="Italic">Geobacteraceae</Emphasis> in paddy soils in China

Purpose

Geobacteraceae are important dissimilatory Fe (III)-reducing microorganisms, influencing the cycling of metals, nutrients as well as the degradation of organic contaminants. However, little is known about their distribution, diversity, and abundance of Geobacteraceae and the effects of environment factors and geographic distance on the distribution and diversity of Geobacteraceae in paddy soils remain unclear. Therefore, the objectives of this study were to investigate the distribution, diversity, and abundance of Geobacteraceae in paddy soils and to determine key factors in shaping the Geobacteraceae distribution, environmental factors, geographic distance, or both and to quantify their contribution to Geobacteraceae variation.

Materials and methods

Illumina sequencing and quantitative real-time PCR using a primer set targeting 16S rRNA genes of bacteria affiliated with the family Geobacteraceae were employed to measure the community composition, diversity, and abundance patterns of 16S rRNA genes of Geobacteraceae in 16 samples collected from north to south of China. MRT, Mantel test, and VPA were used to analyze the relationship between communities of Geobacteraceae and environmental factors and geographic distance.

Results and discussion

Quantitative PCR showed that the abundance of 16S rRNA genes of Geobacteraceae ranged from (1.20?±?0.18)?×?10⁸ to 1.13?×?10⁹?±?2.25?×?10⁸ copies per gram of soil (dry weight) across different types of soils. Illumina sequencing results showed Geobacter was the dominant genus within the family of Geobacteraceae. Multivariate regression tree (MRT) analysis showed that soil amorphous iron contributed more (22.46 %) to the variation of dominant species of Geobacteraceae than other examined soil chemical factors such as pH (14.52 %), ammonium (5.12 %), and dissolved organic carbon (4.74 %). Additionally, more geographically distant sites harbored less similar communities. Variance partitioning analysis (VPA) showed that geographic distance contributed more to the variation of Geobacteraceae than any other factor, although the environmental factors explained more variation when combined. So, we detected the uneven distribution of Geobacteraceae in paddy soils of China and demonstrated that Geobacteraceae community composition was strongly associated with geographic distance and soil chemical factors including aFe, pH, Fe, DOC, C:N, and NO₃ ^?-N. These results greatly expand the knowledge of the distribution of Geobacteraceae in environments, particularly in terrestrial ecosystems.

Conclusions

Our results showed that geographic distance and amorphous iron played important roles in shaping Geobacteraceae community composition and revealed that both geographic distance and soil properties governed Geobacteraceae biogeography in paddy soils. Our findings will be critical in facilitating the prediction of element cycling by incorporating information on functional microbial communities into current biogeochemical models.

     

The effects of silicon fertilizer on denitrification potential and associated genes abundance in paddy soil

This study evaluated the effect of silicate fertilizer on denitrification and associated gene abundance in a paddy soil. A consecutive trial from 2013 to 2015 was conducted including the following treatments: control (CK), mineral fertilizer (NPK), NPK plus sodium metasilicate (NPK + MSF), and NPK plus slag-based silicate fertilizer (NPK + SSF). Real-time quantitative PCR (qPCR) was used to analyze the abundances of nirS, nirK, and nosZ genes. Potential N₂O emissions and ammonium and nitrate concentrations were related to the nirS and nirK gene abundance. Compared with the NPK treatments, the addition of a Si fertilizer decreased N₂O emission rates and denitrification potential by 32.4–66.6 and 22.0–59.2%, respectively, which were probably related to increased rice productivity, soil Fe availability, and soil N depletion. The abundances of nirS and nirK genes were decreased by 17.7–35.8% and 21.1–43.5% with addition of silicate fertilizers, respectively. Rates of total N₂O and N₂O from denitrification (DeN₂O) emission were positively correlated with the nirS and nirK gene abundance. Nitrate, exchangeable NH₄ ⁺, and Fe concentrations were the main factors regulating the nirS and nirK gene abundance. Silicate fertilization during rice growth may serve as an effective approach to decreasing N₂O emissions.     

Distinct effect of nitrogen fertilisation and soil depth on nitrous oxide emissions and nitrifiers and denitrifiers abundance

The nitrous oxide and molecular N emissions from 5-cm length subsamples taken from 20-cm length sample corers containing eutric Cambisol soil fertilised either with urea, ammonium or nitrate for 1 year have been examined using gas chromatography. At the beginning of the incubation, the same N rate (260 kg N/ha) was added to the soil and kept constant during the experiment. The total abundance of the soil Bacteria and Archaea and that of nitrifiers and denitrifiers was estimated by quantitative PCR of the corresponding biotic variables 16S rRNA, amoA and napA, narG, nirK, nirS, norB, nosZI and nosZII genes. The abiotic variables dissolved oxygen, pH, exchangeable NH₄⁺-N and NO₃^?-N contents and total C and total N were also analysed. None of the three fertilisers affected the total abundance of Bacteria and Archaea and nitrification was the main driver of nitrous oxide production in the 0- to 5-cm and 5- to 10-cm soil layers while denitrification was in the 10- to 15-cm and 15- to 20-cm soil horizons. Parallel to the reduction in the content of dissolved oxygen along the soil profile, there was a decrease in the total and relative abundance of the bacterial and archaeal amoA gene and an increase in the abundances of the denitrification genes, mainly in the 10- to 15-cm and 15- to 20-cm soil layers. A non-metric multidimensional scaling plot comparing the biotic and abiotic variables examined in each of the four 5-cm soil subsamples and the whole 20-cm sample showed a disparate effect of N fertilisation on N gas emissions and abundance of nitrifiers and denitrifiers bacterial and archaeal communities.     

Increases in soil CO<Subscript>2</Subscript> and N<Subscript>2</Subscript>O emissions with warming depend on plant species in restored alpine meadows of Wugong Mountain,China

Purpose

Ecosystem restorations can impact carbon dioxide (CO₂) and nitrous oxide (N₂O) emissions which are important greenhouse gasses. Alpine meadows are degraded worldwide, but restorations are increasing. Because their soils represent large carbon (C) and nitrogen (N) pools, they may produce significant amounts of CO₂ and N₂O depending on the plant species used in restorations. In addition, warming and N deposition may impact soil CO₂ and N₂O emissions from restored meadows.

Materials and methods

We collected soils from degraded meadows and plots restored using three different plant species at Wugong Mountain (Jiangxi, China). We measured CO₂ and N₂O emissions when soils were incubated at different temperatures (15, 25 or 35 °C) and levels of N addition (control vs. 4 g m^?2) to understand their responses to warming and N deposition.

Results and discussion

Dissolved organic C was higher in restored plots (especially with Fimbristylis dichotoma) compared to non-restored bare soils, and their soil inorganic N was lower. CO₂ emission rates were increased by vegetation restorations, decreased by N deposition, and increased by warming. CO₂ emission rates were similar for the three grass species at 15 and 25 °C, but they were lower with Miscanthus floridulus at 35 °C. Soils from F. dichotoma and Carex chinensis plots had higher N₂O emissions than degraded or M. floridulus plots, especially at 25 °C.

Conclusions

These results show that the effects of restorations on soil greenhouse gas emissions depended on plant species. In addition, these differences varied with temperature suggesting that future climate should be considered when choosing plant species in restorations to predict soil CO₂ and N₂O emissions and global warming potential.

     

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.

     

Bacterivore nematodes stimulate soil gross N transformation rates depending on their species

We conducted a microcosm experiment with soil being sterilized, reinoculated with native microbial community and subsequently manipulated the bacterivorous nematodes, including three treatments: without (CK) or with introducing one species of the two bacterivores characterized with different body size but similar c-p (colonizer-persister) value (Rhabditis intermedia and Protorhabditis oxyuroides, accounted for 6 and 59% of bacterivores in initially undisturbed soil, respectively). We monitored the N₂O and CO₂ emissions, soil properties, and especially quantified gross N transformation rates using ¹⁵N tracing technique after the 50 days incubation. No significant differences were observed on soil NH₄ ⁺ and NO₃ ^? concentrations between the CK and two bacterivores, but this was not the case for gross N transformation rates. In comparison to CK, R. intermedia did not affect soil N transformation rates, while P. oxyuroides significantly increased the rates of mineralization of organic N to NH₄ ⁺, oxidation of NH₄ ⁺ to NO₃ ^?, immobilization of NO₃ ^? to organic N and dissimilatory NO₃ ^? reduction to NH₄ ⁺. Furthermore, the mean residence time of NH₄ ⁺ and NO₃ ^? pool was greatly lowered by P. oxyuroides, suggesting it stimulated soil N turnover. Such stimulatory effect was unrelated to the changes in abundance of bacteria and ammonia-oxidizing bacteria (AOB). In contrast to CK, only P. oxyuroides significantly promoted soil N₂O and CO₂ emissions. Noticeably, bacterivores increased the mineralization of recalcitrant organic N but decreased soil δ¹³C_-TOC and δ¹⁵N_-TN values, in particular for P. oxyuroides. Combining trait-based approach and isotope-based analysis showed high potential in moving forward to a mechanistic understanding of bacterivore-mediated N cycling.