Effect of grazing intensity on protozoan community,microbial biomass,and enzyme activity in an alpine meadow on the Tibetan Plateau

Purpose

The alpine meadow has received mounting attention due to its degradation resulting from overgrazing on the Tibetan Plateau. However, belowground biotic characteristics under varied grazing stresses in this ecosystem are poorly understood.

Materials and methods

Here, the responses of soil protozoan abundance, community composition, microbial biomass, and enzyme activity to five grazing patterns including (1) artificial grassland without grazing (AG), (2) winter grazing (WG), (3) grazing for 7 months within a fence (GF), (4) continuous grazing for a whole year (CG), and (5) natural heavy grazing (HG) were investigated for two continuous years. Soil protozoan community composition was investigated using the most possible number (MPN) method, and soil microbial biomass and enzyme activity were analyzed using chloroform fumigation extraction and substrate utilization methods, respectively. Multivariate statistical analysis, the analysis of variance (ANOVA), multiple comparisons, and correlation analysis were together performed.

Results and discussion

The WG treatment had the highest abundance of total protozoa (2342–2524 cell g^?1). Compared with AG treatment, HG treatment significantly reduced the abundance of soil total, flagellate and ciliate protozoa, and protease activities in 2012 and 2013. Significantly, lower soil microbial biomass nitrogen (MBN) was also observed in the HG (6.60 and 14.6 mg N kg^?1) than those in other four treatments (22.3–82.9 mg N kg^?1) both in 2012 and 2013, whereas significantly higher microbial biomass carbon (MBC) was observed in HG than that in AG treatment in 2012. Moreover, significantly positive correlations were detected between the abundance of soil protozoa and soil moisture, pH, organic C, total N, and MBN. Our results indicated that soil protozoa showed a negative response to increasing grazing intensities and therefore, suggesting that aboveground grazing practices also exerted strong impact on belowground protozoa, not only on soil microbial characteristics.

Conclusions

Soil protozoan community composition was apparently different between the HG treatment and other four grazing patterns and was potentially impacted by altered soil properties and MBC and/or MBN. Our results suggested that moderate grazing may sustain better belowground biotic diversity and ecosystem functioning in this alpine meadow on the Tibetan Plateau.

     

An evaluation of a microbial inoculum in promoting organic C decomposition in a paddy soil following straw incorporation

Purpose

Impacts of a commercially available decay-facilitating microbial inoculum on carbon (C) and nitrogen (N) mineralization were evaluated during decomposition of rice straw in a paddy soil.

Materials and methods

Two incubation experiments were conducted for 105 days with a typical low-yield high-clay soil in central China to monitor effects of straw and the inoculum on CO₂ evolution, as well as dissolved organic C (DOC), NH₄ ⁺, NO₃ ^?, and pH under conditions of 15 °C 70 %, 25 °C 40 %, 25 °C 70 %, 25 °C 100 %, and 35 °C 70 % of water-holding capacity (WHC) with adequate N, supplied as urea or manure, respectively.

Results and discussion

Treatments of 25 °C 70 % WHC, 25 °C 100 % WHC, and 35 °C 70 % WHC generally achieved significant higher CO₂ evolution while treatment of 25 °C 40 % WHC had least. This was more evident with added manure compared to urea (P?<?0.05). The inoculum generally increased the decomposition of C inputs and the largest increases were in the initial 28 day in treatments 25 °C 70 % WHC, 25 °C 100 % WHC, and 35 °C 70 % WHC; only the 25 °C 40 % WHC actually immobilized C. The CO₂ release rates were positively correlated with DOC, but with different slopes within treatments. Despite equivalent N application rates, manure treatments had significantly less N (including NO₃ ^?, NH₄ ⁺, and total dissolved N) than those with urea. Incubation of 25 °C 40 % WHC decreased soil pH the least, probably due to relative low moisture causing delayed nitrification.

Conclusions

The results implied that the inoculum, especially fungi, would adjust to edaphic and N fertilization in regulating organic C mineralization, during which water potential would exhibit a great role in regulating substrate and nutrient availability.

     

Climate change affects soil labile organic carbon fractions in a Tibetan alpine meadow

Purpose

Changes in bioactive soil C pools and their temperature sensitivities will dominate the fate of soil organic C in a warmer future, which is not well understood in highland ecosystems. This study was conducted in order to evaluate climate change, especially cooling effects, on soil labile organic C (LOC) pools in a Tibetan alpine meadow.

Materials and methods

A short-term reciprocal translocation experiment was implemented to stimulate climate warming (downward translocation) and cooling (upward translocation) using an elevation gradient on the Tibetan Plateau. Variations in soil microbial biomass C (MBC), dissolved organic C (DOC) and LOC were analyzed.

Results and discussion

Over the range of soil temperature from 0.02 to 5.5 °C, warming averagely increased soil MBC, DOC and LOC by 15.3, 17.0 and 3.7 % while cooling decreased them by 11.0, 11.9 and 3.2 %, respectively. Moreover, warming generally increased the proportion of DOC in LOC but cooling had an opposite effect, while the response of the MBC proportion to DOC and LOC varied depending on vegetation type. Soil MBC, DOC and LOC pools were positively related to soil temperature and showed a hump-shaped relationship with soil moisture with a threshold of about 30–35 %. Although soil DOC was more sensitive to warming (5.1 % °C^?1) than to cooling (3.0 % °C^?1), soil LOC showed a symmetrical response due to regulation by soil moisture.

Conclusions

Our results indicated that climate change would not only change the size of soil LOC pools but also their quality. Therefore, cooling effects and regulation of soil moisture should be considered to evaluate the fate of soil organic C in Tibetan alpine meadows in a warmer future.

     

Changes in the activity and abundance of the soil microbial community in response to the nitrification inhibitor 3,4-dimethylpyrazole phosphate (DMPP)

Purpose

The application of organic and inorganic fertilizers to soil can result in increased gaseous emissions, such as NH₃, N₂O, CO₂, and CH₄, as well as nitrate leaching, contributing to climate warming and ground and surface water pollution, particularly in regions with hot climates, where high temperatures and high soil nitrification rates often occur. The use of nitrification inhibitors (NIs) has been shown to effectively decrease nitrogen (N) losses from the soil-plant system.

Materials and methods

Non-disruptive laboratory incubation experiments were conducted to assess the extent to which temperature (20 and 30 °C) and nutrient source (mineral and organic fertilizers) influence the rate of carbon (C)- and N-related microbial processes in soil in response to the NI 3,4-dimethylpyrazole phosphate (DMPP). Furthermore, short-term changes in the ability of microbes to degrade C substrates were evaluated in disruptive soil microcosms using microbial community-level physiological profiling and the abundance of the bacterial 16S rRNA gene as a measure of total bacterial population size.

Results and discussion

DMPP reduced net nitrification after 2 and 4 weeks of incubation at 30 and 20 °C by an average of 78.3 and 84.5 %, respectively, and with similar dynamics for mineral or organic fertilization. The addition of labile organic matter with cattle effluent led to a rapid increase in C mineralization that was significantly reduced by DMPP at both temperatures, whereas no changes could be detected after the addition of mineral fertilizer. The culturable heterotrophic microorganisms showed metabolic diversification in the oxidation of C sources, with organic fertilizer playing a major role in the substrate utilization patterns during the first week of incubation and the DMPP effects prevailing from day 14 until day 28. Furthermore, the copy number of the bacterial 16S rRNA gene was reduced by the application of DMPP and organic fertilizer after 28 days.

Conclusions

Our results show the marked efficiency of DMPP as an NI at elevated temperatures of incubation and when associated with both mineral and organic fertilization, providing support for its use as a tool to mitigate N losses in Mediterranean ecosystems. However, we also observed impaired C respiration rates and bacterial abundances, as well as shifts in community-level physiological profiles in soil, possibly indicating a short-term effect of DMPP and organic fertilizers on non-target C-related processes and microorganisms.

     

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.

     

Clay amendment to sandy soil—effect of clay concentration and ped size on nutrient dynamics after residue addition

Purpose

Crop growth on sandy soils can be increased by claying. In modified sandy soils, the added clay is in the form of peds ranging in size from millimetres to centimetres creating a highly non-uniform matrix where ped size could influence nutrient availability and organic C binding. The aim of the study was to determine the effect of clay addition rate and ped size in residue amended sandy soil on soil respiration, nutrient availability and organic C retention.

Materials and methods

In this study, clay peds of 1, 2 or 3 mm size derived from a clay-rich Vertosol (73 % clay) were added to a sandy soil (3 % clay) at clay addition rates of 10 and 20 % w/w. After the addition of ground faba bean residue (C/N 37) at 10 g kg^?1, the soils were incubated for 45 days at 80 % of water-holding capacity.

Results and discussion

Clay addition had no consistent effect on cumulative respiration, but reduced NH₄ ⁺ availability with a greater reduction at 20 % compared to 10 % clay and with 1 and 2 mm compared to 3 mm peds. Sandy soil with clay peds had a greater maximum NH₄ ⁺ and P sorption capacity than sandy soil alone, and sorption capacity was higher at 20 % compared to 10 % clay addition and greater with 1 mm compared to 3 mm peds. Retrieval of clay peds at the end of the experiment showed ped breakdown during the experiment but also the formation of larger peds. Compared to the <53 μm fraction added at the start of the experiment, the total organic carbon (TOC) content of the <53 μm fraction was increased up to nearly two fold, particularly in the smaller peds (1 and 2 mm).

Conclusions

When sandy soils are amended with clay, N availability and organic C binding depend on both clay addition rate and ped size.

     

Effect of rice straw mulching on migration and transportation of Cd,Cu, Zn,and Ni in surface runoff under simulated rainfall

Purpose

Heavy metals in runoff from contaminated land are becoming a major environmental problem. The presented paper considers the effects of mulching with rice straw on the migration and transportation of heavy metals from the soil into runoff under conditions of simulated rainfall.

Materials and methods

A simulated rainfall experiment was conducted to investigate the impact of rice straw mulching on emissions of sediment and heavy metals in runoff. The soil box was in 20-cm depth with a surface area of 1 m² and the slope was set to 10°. The rainfall intensity was 90 mm h^?1with a 60-min rainfall duration. The study involved samples with different treatments of rice straw mulching: bare soil (BS), low mulching (LM), and high mulching (HM), which had straw contents of 0, 200, and 500 g m^?2, respectively.

Results and discussion

The results showed that compared with BS, the cumulative runoff volume declined by 31 and 50 % and cumulative sediment declined significantly by 93 and 97 % for the LM and HM treatments, respectively. Additionally, with an increase of straw mulching, the concentrations of total heavy metals in the LM and HM treatments declined by 79.90–82.84 and 81.90–90.07 %, and the cumulative total heavy metals decreased significantly by 86.5–87.0 and 90.3–94.6 %, respectively. Particulate-bound heavy metals decreased by 88.1–88.9 % for the LM and 94.5–97.1 % for the HM. Furthermore, Cd, Cu, Zn, and Ni migrated and transported mainly in particulate-bound form and had high enrichment in sediments.

Conclusions

Therefore, straw mulching on soil could reduce the sediment yields, and the loss of both particulate-bound heavy metals, especially for Cd and Ni, and cumulative total heavy metals in runoff. Accordingly, it can be used as an effective measure to control heavy-metal-contaminated soil posing pollution risk to environment through surface runoff.

     

Changes in the pH,EC, available P,SOC and TN stocks in a single rice paddy after long-term application of inorganic fertilizers and organic matters in a cold temperate region of Japan

Purpose

The objective of this study was to determine the changes in the main soil chemical properties including pH, electrical conductivity (EC), available phosphorus (P), soil organic carbon (SOC) and total nitrogen (TN) stocks after long-term (31 years) additions of two types of organic matters—rice straw and rice straw compost, combined with NPK fertilizers in single rice paddy in a cold temperate region of Japan.

Materials and methods

A long-term experiment on combined inorganic fertilizers and organic matters in paddy rice cultivation began in May 1982 in Yamagata, northeastern Japan. After the 31st harvest, soil samples were collected from five treatments [(1) PK, (2) NPK, (3) NPK + 6 Mg ha^?1 rice straw (RS), (4) NPK + 10 Mg ha^?1 rice straw compost (CM1), and (5) NPK + 30 Mg ha^?1 rice straw compost (CM3)] at five soil depths (0–5, 5–10, 10–15, 15–20, and 20–25 cm). Soil chemical properties of pH, EC, available P, SOC, and TN were analyzed.

Results and discussion

The pH decreased significantly only at the higher compost rate of 30 Mg ha^?1, while EC increased in all the organic matter treatments. Available P significantly increased in the CM1 and CM3 treatments by 55.1 and 86.4 %. The amounts of SOC stock increased by 67.2, 21.4, and 8.6 %, and soil TN stock by 64.1, 20.2, and 8.5 % in CM3, RS, and CM1, respectively, compared to NPK treatment.

Conclusions

Significant changes in soil properties were observed after 31 years of organic matter applications with reference to PK- and NPK-fertilized rice paddy soils. A significant decrease in pH was observed with the application of a high rate (30 Mg ha^?1) of rice straw compost but not with the conventional rate of 10 Mg ha^?1. However, EC increased significantly relative to that of the PK- and NPK-fertilized plots in all the organic matter treatments. Available P significantly increased in the CM1 and CM3 treatments by 55.1 and 86.4 %. The amounts of SOC stock expressed as a percentage of total C applied to the soil were higher from 10 Mg ha^?1 compost (28.7 %) than that from 6 Mg ha^?1 rice straw (17.4 %), indicating a more effective soil organic C accumulation from rice straw compost than that from original rice straw.

     

Interactive effects of biochar and polyacrylamide on decomposition of maize rhizodeposits: implications from <Superscript>14</Superscript>C labeling and microbial metabolic quotient

Purpose

The applications of biochar (BC) and polyacrylamide (PAM) may have interactive effects on carbon (C) dynamics and sequestration for improving the soil quality and achieving sustainable agriculture. Relative to BC and PAM, rhizodeposits act as C and energy source for microorganisms and may change the mineralization dynamics of soil organic matter (SOM). No attempt has been made to assess the effects of BC, anionic PAM, or their combination on the decomposition of different aged ¹⁴C-labeled rhizodeposits. The objective of this study was to investigate the effects of the treatments mentioned above on the decomposition of different aged ¹⁴C-labeled maize rhizodeposits.

Materials and methods

biochar (BC) at 10 Mg ha^?1 or anionic PAM at 80 kg ha^?1 or their combination (BC + PAM) was applied to soils with/without 2-, 4-, 8-, and 16-day-aged ¹⁴C-labeled maize rhizodeposits. After that, the soil was incubated at 22 °C for 46 days.

Results and discussion

After 2 days of incubation, the total CO₂ efflux rates from the soil with rhizodeposits were 1.4–1.8 times higher than those from the soil without rhizodeposits. The cumulative ¹⁴CO₂ efflux (32 % of the ¹⁴C input) was maximal for the soil containing 2-day-aged ¹⁴C-labeled rhizodeposits. Consequently, 2-day-aged rhizodeposits were more easily and rapidly decomposed than the older rhizodeposits. However, no differences in the total respired ¹⁴CO₂ from rhizodeposits were observed at the end of the incubation. Incorporation of ¹⁴C into microbial biomass and 66–85 % of the ¹⁴C input remained in the soil after 46 days indicated that neither the age of ¹⁴C-labeled rhizodeposits nor BC, PAM, or BC + PAM changed microbial utilization of rhizodeposits.

Conclusions

Applying BC or BC + PAM to soil exerted only minor effects on the decomposition of rhizodeposits. The contribution of rhizodeposits to CO₂ efflux from soil and MBC depends on their age as young rhizodeposits contain more labile C, which is easily available for microbial uptake and utilization.

     

The effect of biochar loading rates on soil fertility,soil biomass,potential nitrification,and soil community metabolic profiles in three different soils

Purpose

Biochar is increasingly being used as a soil amendment to both increase soil carbon storage and improve soil chemical and biological properties. To better understand the shorter-term (10 months) impacts of biochar on selected soil parameters and biological process in three different textured soils, a wide range of loading rates was applied.

Materials and methods

Biochar derived from eucalypt green waste was mixed at 0, 2.5, 5, 10 % (wt/wt) with a reactive black clay loam (BCL), a non-reactive red loam (RL) and a brown sandy loam (BSL) and placed in pots exposed to the natural elements. After 10 months of incubation, analysis was performed to determine the impacts of the biochar rates on the different soil types. Also, microbial biomass was estimated by the total viable counts (TVC) and DNA extraction. Moreover, potential nitrification rate and community metabolic profiles were assayed to evaluate microbial function and biological process in biochar-amended soils.

Results and discussion

The results showed that biochar additions had a significant impact on NH₄ and NO₃, total C and N, pH, EC, and soil moisture content in both a soil type and loading-dependent manner. In the heavier and reactive BCL, no significant impact was observed on the available P and K levels, or the total exchangeable base cations (TEB) and CEC. However, in the other lighter soils, biochar addition had a significant effect on the exchangeable Al, Ca, Mg, and Na levels and CEC. There was a relatively limited effect on microbial biomass in amended soils; however, biochar additions and its interactions with different soils reduced the potential nitrification at the higher biochar rate in the two lighter soils. Community metabolic profile results showed that the effect of biochar on carbon substrate utilization was both soil type and loading dependent. The BCL and BSL showed reduced rates of substrate utilization as biochar loading levels increased while the opposite occurred for the RL.

Conclusions

This research shows that biochar can improve soil carbon levels and raise pH but varies with soil type. High biochar loading rates may also influence nitrification and the function and activity of microbial community in lighter soils.

     

Functional and structural responses of bacterial and fungal communities from paddy fields following long-term rice cultivation

Purpose

Rice paddy soils undergo pedogenesis driven by periodic flooding and drainage cycles that lead to accumulation of organic matter and the stratification of nutrients and oxygen in the soil profile. Here, we examined the effects of continuous rice cultivation on microbial community structures, enzyme activities, and chemical properties for paddy soils along a chronosequence representing 0–700 years of rice cropping in China.

Materials and methods

Changes in the abundance and composition of bacterial and fungal communities were characterized at three depths (0–5, 5–10, and 10–20 cm) in relation to organic carbon, total nitrogen, dissolved organic carbon, microbial biomass carbon/nitrogen, and activities of acid phosphatase, invertase, and urease.

Results and discussion

Both soil organic carbon and total nitrogen increased over time at all three depths, while pH generally decreased. Microbial abundance (bacteria and fungi) and invertase and urease activity significantly increased with the duration of rice cultivation, especially in the surface layer. Fungal abundance and acid phosphatase activity declined with depth, whereas bacterial abundance was highest at the 5–10-cm soil depth. Profiles of the microbial community based on PCR-DGGE of 16S rRNA indicated that the composition of fungal communities was strongly influenced by soil depth, whereas soil bacterial community structures were similar throughout the profile.

Conclusions

Soil bioactivity (microbial abundance and soil enzymes) gradually increased with organic carbon and total nitrogen accumulation under prolonged rice cultivation. Microbial activity decreased with depth, and soil microbial communities were stratified with soil depth. The fungal community was more sensitive than the bacterial community to cultivation age and soil depth. However, the mechanism of fungal community succession with rice cultivation needs further research.

     

Short-term effects of organo-mineral biochar and organic fertilisers on nitrogen cycling,plant photosynthesis,and nitrogen use efficiency

Purpose

Organo-mineral biochar fertiliser has the potential to replace conventional biochar and organic fertiliser to improve soil quality and increase plant photosynthesis. This study explored mechanisms involved in nitrogen (N) cycling in both soil and ginger plants (Zingiber officinale: Zingiberaceae) following different treatments including organic fertiliser, commercial bamboo biochar fertiliser, and organo-mineral biochar fertiliser.

Materials and methods

Soil received four treatments including (1) commercial organic fertiliser (5 t ha^?1) as the control, (2) commercial bamboo biochar fertiliser (5 t ha^?1), (3) organo-mineral biochar fertiliser at a low rate (3 t ha^?1), and (4) organo-mineral biochar fertiliser at a high rate (7.5 t ha^?1). C and N fractions of soil and plant, and gas exchange measurements were analysed.

Results and discussion

Initially, organo-mineral biochar fertiliser applied at the low rate increased leaf N. Organo-mineral biochar fertiliser applied at the high rate significantly increased N use efficiency (NUE) of the aboveground biomass compared with other treatments and improved photosynthesis compared with the control. There was N fractionation during plant N uptake and assimilation since the ¹⁵N enrichment between the root, leaf, and stem were significantly different from zero; however, treatments did not affect this N fractionation.

Conclusions

Organo-mineral biochar fertiliser has agronomic advantages over inorganic and raw organic (manure-based) N fertiliser because it allows farmer to put high concentrations of nutrients into soil without restricting N availability, N uptake, and plant photosynthesis. We recommend applying the low rate of organo-mineral biochar fertiliser as a substitute for commercial organic fertiliser.

     

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.

     

Iron–manganese concretions contribute to benthic release of phosphorus and arsenic in anoxic conditions in the Baltic Sea

Purpose

Deposits of iron–manganese (Fe, Mn) concretions forming a large storage of phosphorus (P) and arsenic (As) are frequently under pressure of oscillating oxygen conditions in the eutrophic Gulf of Finland, the Baltic Sea. Yet, there is a poor understanding how anaerobic microbial processes regulate the cycling of elements in the concretions. The objective of this study was to highlight how the microbial processes control the release of elements from the concretions to brackish water during anoxia.

Materials and methods

Spherical concretions were collected from the oxic bottoms of the Gulf of Finland in the summer. Concretions and autoclaved controls were incubated in anoxic artificial brackish seawater with and without labile carbon, plus supplied with ammonium at 5, 10, and 20 °C for 15 weeks. Concentrations of Fe, Mn, P, and As were measured from the intact concretions and the ambient solutions during the experiment. Also, the consumption of the added ammonium and organic carbon and the formation of dissolved inorganic carbon were measured.

Results and discussion

At near-bottom temperature 5 °C, the concretions released at highest 0.12, 0.42, 0.02, and 0.0002 μmol g^?1 day^?1 of Fe, Mn, P, and As, respectively. The rates were significant only in the microcosms with added labile carbon, and only minor proportions (0.1–0.4 %) of their total contents were released during the incubations. The concretions removed completely the supplied ammonium only without carbon addition. We find that concretion deposit may form a local hot spot for the microbial reduction of Fe and Mn and release significant amounts of P and As, and participate in N cycling besides the bottom sediments of the Gulf of Finland during prolonged anoxia. However, the concretions may maintain their binding capacity for P and As longer than the fine-grained organic-rich sediment during anoxia.

Conclusions

During anoxia concretion deposits may form a temporal source of bioavailable P having ecological significance in the Gulf of Finland when concretions have access to labile organic carbon. Concretions from the Baltic Sea, the oceans, lakes, and soils contain high concentrations of Mn and Fe, but their proportions vary considerably. Anaerobic microbial processes may thus affect the stability of concretions from the different environments, but the outcome may depend on the ambient geochemical conditions.

     

Effects of temperature on soil organic carbon fractions contents,aggregate stability and structural characteristics of humic substances in a Mollisol

Purpose

Under a global warming scenario, understanding the response of soil organic carbon fractions and aggregate stability to temperature increases is important not only for better understanding and maintaining relevant ecosystem services like soil fertility and crop productivity, but also for understanding key environmental processes intimately related with the maintenance of other regulatory ecosystem services like global climate change mitigation through carbon sequestration. An increase in temperature would accelerate the mineralization of soil organic carbon. However, the properties of organic carbon remained in soil after mineralization is not well known.

Materials and methods

Mollisol was collected at 0–20-cm depth from maize (Zea mays L.) field in Northeast China. A 180-day incubation experiment was conducted at three different temperatures (10, 30, and 50 °C) under constant soil moisture (60 % water holding capacity). Soil samples were assayed for total organic carbon (TOC), water-soluble organic carbon (WSOC), easily oxidizable organic carbon (EOC), humic fractions carbon, aggregate-associated carbon, and water stability of aggregates. Elemental analysis and solid-state ¹³C nuclear magnetic resonance spectroscopy were used to characterize humic acid and humin fractions.

Results and discussion

The contents of soil TOC, EOC, humic fractions carbon, and aggregate-associated carbon decreased with the increase in temperature. The proportion of 2–0.25-mm macroaggregate and the mean weight diameter (MWD) of aggregates also decreased. The C, H, N, S, alkyl C, and O-alkyl C contents of humic acid and humin decreased, whereas the O, aromatic C, and carbonyl C contents increased. The H/C, aliphatic C/aromatic C, and O-alkyl C/aromatic C ratios in humic acid and humin fractions decreased.

Conclusions

The increase in temperature has a negative impact on soil organic carbon content, soil aggregation, and aggregate stability. Moreover, humic acid and humin molecules become less aliphatic and more decomposed with the increase in temperature.

     

Effects of biochar and polyacrylamide on decomposition of soil organic matter and <Superscript>14</Superscript>C-labeled alfalfa residues

Purpose

Various soil conditioners, such as biochar (BC) and anionic polyacrylamide (PAM), improve soil fertility and susceptibility to erosion, and may alter microbial accessibility and decomposition of soil organic matter (SOM) and plant residues. To date, no attempts have been made to study the effects of BC in combination with PAM on the decomposition of soil SOM and plant residues. The objective of this study was to evaluate the effects of BC, PAM, and their combination on the decomposition of SOM and alfalfa residues.

Materials and methods

An 80-day incubation experiment was carried out to investigate the effects of oak wood biochar (BC; 10 Mg ha^?1), PAM (80 kg ha^?1), and their combination (BC?+?PAM) on decomposition of SOM and ¹⁴C-labeled alfalfa (Medicago sativa L.) residues by measuring CO₂ efflux, microbial biomass, and specific respiration activity.

Results and discussion

No conditioner exerted a significant effect on SOM decomposition over the 80 days of incubation. PAM increased cumulative CO₂ efflux at 55–80 days of incubation on average of 6.7 % compared to the soil with plant residue. This was confirmed by the increased MBN and MB¹⁴C at 80 days of incubation in PAM-treated soil with plant residue compared to the control. In contrast, BC and BC?+?PAM decreased plant residue decomposition compared to that in PAM-treated soil and the respective control soil during the 80 days. BC and BC?+?PAM decreased MBC in soil at 2 days of incubation indicated that BC suppressed soil microorganisms and, therefore, decreased the decomposition of plant residue.

Conclusions

The addition of oak wood BC alone or in combination with PAM to soil decreased the decomposition of plant residue.

     

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.

     

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.

     

Competitive sorption of linear alkylbenzene sulfonate (LAS) surfactants and the antibiotics sulfamethoxazole and ciprofloxacin in wastewater-irrigated soils of the Mezquital Valley,Mexico

Purpose

The increasing reuse of wastewater for irrigation introduces surfactants and antibiotics into the environment. How these two kinds of compounds interact with regard to their sorption processes in soil is not clear.

Materials and methods

We performed batch experiments to investigate the sorption of linear alkylbenzene sulfonates (LAS) and its effect on sorption of sulfamethoxazole and ciprofloxacin in irrigated and non-irrigated soils with different organic matter (OM) contents.

Results and discussion

LAS sorption was non-linear in the presence of the antibiotics, and as general trend, it increased with rising OM content of soils. Free LAS was also removed from solution by complexation with Ca²⁺. Dissolved organic compounds released from soils with OM contents ≥18.4 g kg^?1 further reduced LAS sorption. Sorption of sulfamethoxazole was reduced by LAS sorption only in one soil with a small OM content of 9.5 g kg^?1.

Conclusions

The strong sorption of ciprofloxacin is not affected by LAS. Sulfamethoxazole sorption only competes with LAS sorption in organic matter-poor soils. Accumulation of organic matter in soils, for example due to long-term wastewater irrigation, provides extra sorption capacity for LAS and sulfamethoxazole so that competition for sorption sites is reduced.

     

Impacts of continuous excessive fertilization on soil potential nitrification activity and nitrifying microbial community dynamics in greenhouse system

Purpose

Sampling and analysis of greenhouse soils were conducted in Shouguang, China, to study continuous excessive fertilization effect on nitrifying microbial community dynamics in greenhouse environment.

Materials and methods

Potential nitrification activity (PNA), abundance, and structure of nitrifying microbial communities as well as the correlations with soil properties were investigated.

Results and discussion

Short-term excessive fertilization increased soil nutrient contents and the diversity of nitrifying microbial communities under greenhouse cultivation. However, the abundance and diversity of nitrifying communities decreased greatly due to the increase of soil acidity and salinity after 14 years of high fertilization in greenhouse. There was a significant positive correlation between soil PNA and the abundance of ammonia-oxidizing bacteria (AOB) but not that of ammonia-oxidizing archaea (AOA) in topsoil (0–20 cm) when pH ≥7. Soil PNA and AOB were strongly influenced by soil pH. The groups of Nitrososphaeraceae, Nitrosomonadaceae, and Nitrospiraceae were predominant in the AOA, AOB, and nitrite-oxidizing bacteria (NOB) communities, respectively. Nitrifying community structure was significantly correlated with soil electrical salinity (EC), organic carbon (OC), and nitrate nitrogen (NO₃ ^?–N) content by redundancy analysis (RDA).

Conclusions

Nitrification was predominated by AOB in greenhouse topsoil with high fertilizer loads. Soil salinity, OC, NO₃ ^?–N content, and pH affected by continuous excessive fertilization were the major edaphic factors in shaping nitrifying community structure in greenhouse soils.