Effect of organic amendment amount on soil nematode community structure and metabolic footprints in soybean phase of a soybean-maize rotation on Mollisols

It has been well documented that organic amendment affects soil nematode community structure. However, little is known about the effect of organic amendment amount on soil nematodes. To assess the effect of the amount of organic amendments on soil nematode community structure and metabolic activity, the community composition, abundance, and metabolic footprints of soil nematodes were determined in a long-term field experiment with various amounts of organic amendment in Northeast China. Fertilization treatments included an unfertilized control (CK), chemical fertilizer without manure amendment (OM₀), manure applied at 7.5 Mg ha^-1 plus chemical fertilizer (OM₁), and manure applied at 22.5 Mg ha^-1 plus chemical fertilizer (OM₂). A total of 46 nematode genera were found. Treatments with the largest amount of organic amendment had the smallest number of plant parasite genera (5), but a largest number of dominant genera (7). Soil nematodes, bacterivores, and fungivores were the most abundant in OM₂, followed by OM₁, and the lowest in OM₀ and CK. Organic amendment increased the enrichment index (EI), and the large amount of organic amendment increased the metabolic footprints of bacterivore (Baf) and fungivore (Fuf) and enrichment footprint (Ef). The relationships between Baf (or Fuf) and the increases in soil organic carbon (ΔSOC) and total nitrogen (ΔTN) were stronger than those of bacterivore (or fungivore) abundance with ΔSOC and ΔTN, except for the relationship between bacterivore abundance and ΔSOC. The EI and Ef were positively correlated with ΔSOC and ΔTN. These findings suggest that the amount of organic amendment affects soil nematode activity and function at entry levels in soil food web, and that metabolic footprints of soil nematodes may be better indicators than their abundances in assessing their relationships with soil nutrients.     

Bacteria and microbial-feeders modify the performance of a decomposer fungus

We studied whether the presence of a bacterium can affect the functioning or stability of simple fungal-based decomposer food chains. We constructed microcosms with 60 g washed mineral soil and four different food webs: (1) a fungus (Cladosporium herbarum) alone; (2) a fungus and a fungal-feeding nematode (Aphelenchoides sp.); (3) a fungus and a bacterium (Escherichia coli); and (4) a fungus, a bacterium, a fungal- and a bacterial-feeding nematode (Aphelenchoides sp. and Acrobeloides tricornus). Glucose was supplied as the sole carbon source. One replicate set of microcosms was kept at −2°C for the sixth and seventh week as an experimental disturbance. The microcosms were destructively sampled 10 times for food web effects (non-disturbed controls) and three times for freezing treatment. Fungal biomass growing alone was less efficient (i.e. it had higher respiration rate per unit biomass) than the other food webs. This did not, however, result in the greatest C loss in the long term, because the fungal biomass declined when growing alone. Presence of the bacterium increased the decomposition of dissolved organic carbon as much as the presence of the fungal-feeder, but their effects were not additive. Ammonium nitrogen concentration of the medium was higher in systems containing bacteria. Nematode populations and the bacterial biomass were not sensitive to the disturbance of freezing. Although the freezing disturbance decreased total fungal biomass in the absence of the bacterial energy channel, dissolved organic carbon decomposition was only incidentally retarded. In spite of their distinct role in the system functioning, the bacteria did not significantly alter the stability properties of the system. Thus, our results contradict some recent food-web based decomposition models, which predict that the effects of parallel energy channels on decomposition are additive. We discuss nutrient limitation and the dependence of fungal properties on the food-web configuration as explanations for the contradiction of theory and data. We did not find evidence supporting a hypothesis that an additional energy channel should stabilize flow through a food web, and hence decomposition. This is explained by high resilience of the reference system with a fungus alone, and by redundancy, i.e. the fact that the fungal-feeding nematode induced similar changes in the fungus than the bacterium.     

Variable impacts of enchytraeid worms and ectomycorrhizal fungi on plant growth in raw humus soil treated with wood ash

An increasing amount of evidence shows the context dependent nature of various biotic interactions across terrestrial and aquatic ecosystems. We established a laboratory experiment to study whether the effects of Cognettia sphagnetorum (Enchytraeidae) and ectomycorrhizal fungi on Scots pine (Pinus sylvestris) seedling growth are influenced by wood ash application. Acidic coniferous forest soil was treated with wood ash at 5000 kg ha⁻¹ or left as ash-free control and inoculated with soil saprotrophic microbes and nematodes. The microcosms were destructively sampled 26 and 51 weeks after initiation of the experiment. We measured enchytraeid and pine seedling biomass, abundance of nematodes and leaching of NH₄⁺-N and NO₃⁻-N at both samplings, and root length and N concentration of pine needles at the end of the experiment. On average enchytraeids and mycorrhizal fungi enhanced pine biomass production in the ash-free control soils, however, their impact was most pronounced when these organisms were alone in the systems. In fact, mycorrhizas tended to have a negative impact on the seedlings in the presence of enchytraeids. Wood ash had a clear negative impact on enchytraeid populations. Wood ash decreased pine growth when enchytraeids and EM-fungi were alone in the systems, but when together they apparently compensated for the negative effects of wood ash on the seedlings. It is concluded that interactions between soil fauna, mycorrhizal fungi and plants are context dependent, thereby rendering predictions of the outcome of species interactions in soil food webs a demanding task.     

Strong impacts of belowground tree inputs on soil nematode trophic composition

Trees have a key role in determining the composition of soil biota via both above and belowground resource-based mechanisms, and by altering abiotic conditions. We conducted an outdoor mesocosm experiment to investigate the relative impact of above and belowground tree inputs on soil nematode trophic composition, and examine whether tree-driven impacts differed between contrasting species (birch and pine). For both species, we created a factorial design of litter addition and root presence treatments. The litter addition treatment was equivalent to natural levels of litterfall; tree saplings were planted in mesocosms for the root presence treatment and an unplanted control treatment was established that had no litter or root inputs. Litter addition had a limited impact on soil nematode community composition: it primarily decreased omnivore and predatory nematode abundance in birch but had few other effects on the nematode community. By contrast, root presence markedly altered nematode community composition through changes in a range of trophic groups. For both birch and pine, there were significant increases in total, fungivore and predatory nematode abundance in root presence treatments, and furthermore, total and fungivore abundances were positively related to root biomass. Root presence of these contrasting tree species also had a distinctive impact on some specific nematode trophic groups; pine roots promoted bacterivore abundance while birch roots promoted root-hair feeding nematode abundance. These findings suggest strong bottom-up effects of belowground tree inputs, and indicate that particular components of the nematode community may be affected differently by resource quantity and quality. Consequently, we suggest that, in the short-term, belowground rather than aboveground tree inputs have a strong impact on soil food web structure and complexity.     

Non-target effects of entomopathogenic nematodes on soil microbial community and nutrient cycling processes: A microcosm study

Under microcosm conditions, changes in the soil microbial biomass, respiration rates, and nitrogen pools as indicators of potential non-target effects of entomopathogenic nematodes on soil, were evaluated. Two tests were conducted using soil collected from the field with no history of entomopathogenic nematode application. Treatments consisted of applications of Steinernema carpocapsae All strain in the presence or absence of the wax moth Galleria mellonella larva as a target insect host, compared with the untreated control (soil only). In the second experiment an insecticide treatment (trichlorfon) was added. Microbial biomass (total nitrogen), and mineral nitrogen (NH₄-N, NO₃-N) were measured using standard methods up to 32 days and soil respiration up to 64 days in both experiments. No negative effect was detected in the soil microbial biomass, respiration and nitrogen pools after application of S. carpocapsae. However, a significant increase in ammonium was measured during almost the entire period of the test in the nematode plus larva treatment, not shown in the other treatments. This high level of ammonium in the nematode plus larva treatment showed that entomopathogenic nematodes can indirectly affect system-level processes in soil and adds evidence on the importance of indirect interactions affecting functions in soil food webs. The application of the insecticide trichlorfon significantly suppressed the microbial biomass and nitrification process.     

Responses of soil nematode community to monoculture or mixed culture of a grass and a legume forage species in China

Mixed cultivation of fast-growing grasses and nitrogen (N)-fixing legumes for forage production is widely considered effective for obtaining sustained high forage yields without depleting soil N levels. However, the effects of monoculture and mixed culture of these species on soil food webs are poorly understood. In this study, soil nematode communities were examined as indicators of the soil food web structure of monoculture and mixed culture of grass and legume at three N levels, i.e., 338 (low), 450 (moderate), and 675 (high) kg N ha^-1 year^-1, across 2 years in wet and dry seasons, using the grass Paspalum wetsfeteini and the legume Medicago sativa (alfalfa), both commonly cultivated worldwide. Repeated-measures analysis of covariance showed that compared with grass monoculture, legume monoculture and grass-legume mixture increased abundances of herbivorous, bacterivorous, and fungivorous nematodes in the soil food web under the low and moderate N fertilization levels. Principal response curve results showed that the abundance of Helicotylenchus, a plant parasite, was significantly higher under legume monoculture than other planting systems at the low N fertilization level. Structural equation model analysis indicated that the legume increased bacterivore abundance, while increasing N fertilization decreased omnivore abundance. The legume might increase the quantity and quality of food resources for soil biota, resulting in the bottom-up control of soil nematode communities. Our results indicate that targeted control of a soilborne pathogen, Helicotylenchus, is required in alfalfa-based planting systems. In addition, high inorganic N application, which is detrimental to legume-rhizobia symbiosis, nullified the otherwise positive effects of legumes on soil nematodes.     

A comparison of soil food webs beneath C<Subscript>3</Subscript>- and C<Subscript>4</Subscript>-dominated grasslands

Soil food webs influence organic matter mineralization and plant nutrient availability, but the potential for plants to capitalize on these processes by altering soil food webs has received little attention. We compared soil food webs beneath C₃- and C₄-grass plantings by measuring bacterial and fungal biomass and protozoan and nematode abundance repeatedly over 2 years. We tested published expectations that C₃ detritus and root chemistry (low lignin/N) favor bacterial-based food webs and root-feeding nematodes, whereas C₄ detritus (high lignin/N) and greater production favor fungal decomposers and predatory nematodes. We also hypothesized that seasonal differences in plant growth between the two grassland types would generate season-specific differences in soil food webs. In contrast to our expectations, bacterial biomass and ciliate abundance were greater beneath C₄ grasses, and we found no differences in fungi, amoebae, flagellates, or nematodes. Soil food webs varied significantly among sample dates, but differences were unrelated to aboveground plant growth. Our findings, in combination with previous work, suggest that preexisting soil properties moderate the effect of plant inputs on soil food webs. We hypothesize that high levels of soil organic matter provide a stable environment and energy source for soil organisms and thus buffer soil food webs from short-term dynamics of plant communities.     

Effects of arsenic on nutrient dynamics of grassland microcosms and field plots

Grassland field plots were compared to two sizes of excised microcosms (15 and 30 cm diameter) and 30-cm diameter homogenized microcosms to assess the effects of As on soil-plant nutrient dynamics. The goal of this experiment was to determine the feasibility of using microcosms for screening chemicals which are potentially toxic to terrestrial ecosystems. All experimental units were treated with As, at rates of 0, 0.5, and 5.0 mg cm^?2, by superficial application of dissolved Na₂AsO₄. Calcium, PO₄-P, NO₃-N, dissolved organic carbon (DOC), and NH₄-N were determined in soil leachate and soil extracts. Greater loss of PO₄-P, NO₃-N, NH₄-N, and DOC occurred via soil leachate from microcosms treated with the greatest level of As relative to untreated microcosms. Field plots were unaffected by As treatment. Excised microcosms were found to be more representative of field plots in reflecting microbial dynamics than were homogenized microcosms. Thus excised microcosms were recommended for testing potentially hazardous chemicals.     

土壤微生物体氮测定方法的研究

用熏蒸-0.5mol/LK2SO4 直接浸取NH4+-N法 (简称薰蒸 铵态氮法 ) ,熏蒸 淹水培养法和熏蒸 通气培养法测定了有机质、全氮和C/N比差异较大的 15种土壤的铵态氮增量 (FN)。结果表明 ,它们之间有极显著的正相关 ,在反映土壤微生物体氮上有相同趋势。两种培养方法测定的FN 近乎一致 ,由此而计算的微生物体氮也几乎相同。对红油土铵态氮法测定值仅为两种培养法的 1/ 10。把铵态氮法中的FN 校正后 ,其结果与 2种培养法测定的微生物体氮同样近乎一致。用 3种方法测定的微生物体氮均与土壤有机碳存在显著正相关性。淹水培养和铵态氮法水分条件易控制 ,又无NH3的挥发损失 ,比通气培养法更加优越。对培养试验和长期肥料定位试验的土样测定结果表明 ,土壤中易矿化新鲜有机物料也会使熏蒸 淹水培养法测定的FN 显著下降 ,由此而计算的微生物体氮也显著减少 ,但熏蒸 铵态氮法测定的FN 不受新鲜有机物质的影响。与土壤微生物数目进行比较后发现 ,土壤中含易分解有机物质少或微生物体氮含量低时 ,选用熏蒸 淹水培养法测定误差小 ;当土壤中富含新鲜有机物质时 ,熏蒸 铵态氮法测定的结果更加可靠。用这两种方法在同类土壤上测定的FN 的比值相对稳定 ,微生物体氮 (BN)的平均比值为 0.98～1.01,不受施肥的影响     

不同热解温度茶渣生物质炭对茶园土壤吸附解吸NH4-N的影响

生物质炭对铵根的吸附解吸影响着土壤的固氮效果,为探讨茶渣生物质炭对茶园土吸附—解吸NH_4~+—N性能的影响,减少土壤中氮素的淋失,提高氮素利用效率,通过模拟培养试验,采用平衡吸附法及HCL解吸法,研究了不同热解温度下制备的茶渣生物质炭在不同添加比例(0.35%,0.70%,1.40%,2.80%)下,茶园土对NH_4~+—N吸附解吸的特性。结果表明:施用生物质炭能有效增强茶园土对NH_4~+—N的吸附,并随生物质炭添加量的增加而增强。同一生物质炭添加量下,4种生物质炭处理下茶园土对NH_4~+—N的吸附量大小表现为BC400BC300BC500BC600。生物质炭的CEC含量是影响土壤吸附NH_4~+—N能力的主要因素。土壤对NH_4~+—N的吸附过程均以Langmuir方程拟合达到显著水平(0.953 7R~20.995 5),以单层吸附为主。施用生物质炭后,土壤产生了解吸滞后,有效降低了茶园土对NH_4~+—N的解吸率,BC400的解吸率最低。茶渣生物质炭能够增强土壤对NH_4~+—N的吸附,降低对NH_4~+—N的解吸,有利于提高土壤对氮素的吸持能力,其中BC400,2.80%处理下效果最佳。     

Effects of methyl bromide fumigation,chloropicrin fumigation and steam sterilization on soil nitrogen dynamics and microbial properties in a pot culture experiment

Abstract

The effects of steam sterilization (SS), methyl bromide (MeBr) fumigation and chloropicrin (CP) fumigation on soil N dynamics and microbial properties were evaluated in a pot experiment. All disinfection treatments increased the NH⁺ ₄-N level and inhibited nitrification. The additional NH⁺ ₄-N in the CP treatment probably originated from the decomposition of microbial debris by surviving microbes, while that in the SS treatment was attributable to deamination processes of soil organic N occurring in a less labile fraction in addition to the decomposition of microbial debris. The MeBr fumigation increased the level of NH⁺ ₄-N without changing the soil microbial biomass. Based on the determinations of soil microbial biomass, substrate utilization activity (Biolog method) and microbial community structure (phospholipid fatty acid method), the effects of the MeBr, CP and SS treatments on the microbial community were compared. The MeBr fumigation had relatively mild and short-term effects on microbial biomass and activity, but altered the community structure drastically by promoting the growth of gram-positive bacteria. The CP fumigation had large and long-term impacts on microbial biomass and activity; the community structure remained unaffected except for the gram-negative bacteria. Steam sterilization had severe and persistent effects on all parameters. The severity of the effects decreased in the order SS ≥ CP > MeBr.     

Effect of density and species richness of soil mesofauna on nutrient mineralisation and plant growth

The aim of this study was to test the relative importance of changes in density and species richness of soil mesofauna as determinants of nutrient mineralisation and plant growth. The experiment was carried out using microcosms containing a mixture of plant litter and soil in which seedlings of Lolium perenne were planted, and a range of combinations of levels of density and species richness of microarthropods added. Over the duration of the experiment, nutrient release, measured as concentrations of NO₃ ^--N and total N in leachates, increased significantly with increasing microarthropod density, but decreased with increasing species richness. Leachate concentrations of NH₄ ⁺-N, dissolved organic N and C (DON and DOC) were not affected by the faunal treatments. Soil respiration, a measure of microbial activity, decreased with increasing density of microarthropods, whereas microbial biomass was not affected by microarthropods. Increasing density of soil animals had a negative effect on the shoot biomass of L. perenne while the effect of species richness was positive. Neither the species richness nor density of soil microarthropods was found to significantly influence root biomass. We conclude that variations in animal density had a greater influence on soil nutrient mineralisation processes than did species richness. Possible reasons for these opposing effects of animal density and diversity on soil N mobilization are discussed.     

Soil inorganic nitrogen composition and plant functional type determine forage crops nitrogen uptake preference in the temperate cultivated grassland,Inner Mongolia

ABSTRACT

Plant nitrogen (N)-acquisition strategy affects soil N availability, community structure, and vegetation productivity. Cultivated grasslands are widely established to improve degraded pastures, but little information is available to evaluate the link between N uptake preference and forage crop biomass. Here an in-situ ¹⁵N labeling experiment was conducted in the four cultivated grasslands of Inner Mongolia, including two dicots (Medicago sativa and Brassica campestris) and two monocots (Bromus inermis and Leymus chinensis). Plant N uptake rate, shoot- and root biomass, and concentrations of soil inorganic-N and microbial biomass-N were measured. The results showed that the root/shoot ratios of the dicots were 2.6 to 16.4 fold those of the monocots. The shoot N concentrations of the dicots or legumes were 40.6% to 165% higher than those of the monocots or non-legumes. The four forage crops in the cultivated grassland preferred to uptake more NO₃^?-N than NH₄⁺-N regardless of growth stages, and the NH₄⁺/NO₃^? uptake ratios were significantly lower in the non-legumes than in the legumes (p < 0.05). Significant differences in the NH₄⁺-N rather than NO₃^?-N uptake rate were observed among the four forages, related to plant functional types and growth stages. The NH₄⁺ uptake rate in the perennial forages exponentially decreased with the increases in shoot-, root biomass, and root/shoot ratio. Also, the plant NH₄⁺/NO₃^? uptake ratio was positively correlated with soil NH₄⁺/NO₃^? ratio. Our results suggest that the major forage crops prefer to absorb soil NO₃^?-N, depending on soil inorganic N composition and belowground C allocation. The preferential uptake of NO₃^?-N by forages indicates that nitrate-N fertilizer could have a higher promotion on productivity than ammonium-N fertilizer in the semi-arid cultivated grassland.     

Abundance,diversity and connectance of soil food web channels along environmental gradients in an agricultural landscape

Soil food webs respond to anthropogenic and natural environmental variables and gradients. We studied abundance, connectance (a measure of the trophic interactions within each channel), and diversity in three different channels of the soil food web, each comprised of a resource-consumer pair: the microbivore channel (microbes and their nematode grazers), the plant–herbivore channel (plants and plant-feeding nematodes), and the predator–prey channel (predatory nematodes and their nematode prey), and their associations with different gradients in a heterogeneous agricultural landscape that consisted of intensive row crop agriculture and grazed non-irrigated grasslands in central California. Samples were taken at three positions in relation to water channels: water’s edge, bench above waterway, and the adjacent arable or grazed field. Nematode communities, phospholipid fatty acid (PLFA) biomarkers, and soil properties (NH₄⁺-N, NO₃⁻-N, total N, total C, pH, P, bulk density and soil texture) were measured, and riparian health ratings were scored. Environmental variables were obtained from publicly-available data sources (slope, elevation, available water capacity, erodability, hydraulic conductivity, exchangeable cation capacity, organic matter, clay and sand content and pH).The abundance and richness in most food web components were higher in grazed grasslands than in intensive agricultural fields. Consumers contributed less than their resources to the abundance and richness of the community in all channels. The association between richness and abundance for each component was strongest for the lowest trophic links (microbes, as inferred by PLFA) and weakest for the highest (predatory nematodes). The trophic interactions for the predator–prey and plant–herbivore channels were greater in the grassland than in the cropland. Fields for crops or grazing supported more interactions than the water’s edge in the plant–herbivore and microbivore channels. Connectance increased with the total richness of each community. Higher connectance within the microbivore and predator–prey soil food web channels were associated with soil NO₃⁻-N and elevation respectively, which served as surrogate indicators of high and low agricultural intensification.     

Chitin decomposition in a model soil system

The effect of various organisms on the decompositon of chitin in a gnotobiotic soil system was investigated. Chitin decomposers were isolated from the short grass prairie in Colorado and selected by their ability to use chitin as a source of both C and N. Three bacteria, a fungus, and an actinomycete were grown for 45 days in sterile chitin amended (3 mg g^?1 chitin-C) and unamended soil microcosms. Net mineralization of ammonium was greatest in the chitin-ainended microcosms. The greatest increases in N mineralization occurred in chitin-amended microcosms containing the fungus and the actinomycete. A second series of sterile soil microcosms amended with chitin (3 mg g^?1 chitin-C) were inoculated with decomposers, a fungus and a bacterium, and a nematode and an amoeba (bacteriophagic grazers) in various combinations. Bacterial and grazer populations, NH₄⁺ CO₂ evolution, and residual chitin were measured periodically for 80 days. Bacterial grazing reduced bacterial populations, increased N mineralization, but had no effect on the decomposition of chitin.     

间伐对杉木人工林土壤碳氮及其组分特征的影响

为探究不同间伐强度对杉木人工林土壤碳氮及其组分特征的影响,以福建省三明市官庄国有林场11年生杉木(Cunninghamia lanceolata)人工林为研究对象,采用弱度间伐(LIT)、中度间伐(MIT)、强度间伐(HIT)等3种间伐强度,研究不同间伐强度林分0—10,10—20,20—40,40—60,60—80,80—100 cm土层总有机碳(SOC)、全氮(TN)及易氧化有机碳(ROC)、硝态氮(NO_3~--N)、铵态氮(NH_4~+-N)、微生物量碳(MBC)、微生物量氮(MBN)、微生物熵碳(qMBC)、微生物熵氮(qMBN)的变化特征,以探讨不同间伐强度对杉木人工林土壤碳氮及其组分特征的影响。结果表明:间伐降低了土壤SOC和TN的含量,降低幅度分别为1.4%～36.9%,3.1%～45.7%。间伐增加了土壤MBC、NO_3~--N的含量,而对ROC、NH_4~+-N和MBN的程度在不同土层有差异,qMBC和qMBN随着间伐强度的增加而增大。相关性分析表明,土壤SOC分别与TN、qMBC、ROC、NH_4~+-N、MBC、MBN呈极显著正相关(P0.01);TN与qMBN、ROC、NH_4~+-N、MBC、MBN呈极显著正相关(P0.01)。杉木人工林间伐处理降低了土壤表层SOC和TN含量,增加了土壤SMBC和qMBC、qMBN,同时也增加了土壤表层(0—10 cm)SMBN。抚育间伐导致土壤SOC和TN含量降低主要是由于活性碳、氮含量的增加,提高土壤中有机质分解速率,最终导致土壤SOC和TN含量降低。     

Foliage application of nitrogen has less influence on soil microbial biomass and community composition than soil application of nitrogen

Purpose

Many studies have shown the simulated effects of nitrogen (N) deposition on soil microbial community composition by adding N directly to the forest floor but have ignored the N retention process by the canopy. This study was conducted to compare the responses of soil microbial biomass and community composition between soil application of N (SAN) and foliage application of N (FAN).

Materials and methods

A pot experiment was designed with (1) two N application methods (SAN and FAN), (2) three N application levels (5.6, 15.6 and 20.6 g N m^?2 year^?1), and (3) two tree species (Schima superba Gardn. et Champ. and Pinus massoniana Lamb.) following a nested factorial design. Soil microbial biomass and community composition were determined using phospholipid fatty acids (PLFAs) techniques after 1 and 1.5 years of treatments.

Results and discussion

Nitrogen addition increased (P?<?0.05) soil NH₄⁺-N content and soil NO₃^?-N content and decreased (P?<?0.05) soil pH and soil microbial (bacterial, fungal, and actinomycete) biomass for both N application methods. Compared with the SAN treatment, the FAN treatment had higher (P?<?0.05) pH and lower (P?<?0.05) contents of soil NH₄⁺-N and soil NO₃^?-N. Soil microbial biomass and community composition were significantly different between the different N addition levels under the SAN treatment, but they showed no significant difference (P?<?0.05) between the different N addition levels under the FAN treatment. The soil microbial biomass in the S. superba soil was higher (P?<?0.05) than that in the P. massoniana soil for the FAN treatment, with the opposite trend observed under the SAN treatment. Moreover, redundancy analysis showed that soil microorganisms were significantly correlated with soil pH, soil water content, NH₄⁺-N, and NO₃^?-N.

Conclusions

The results showed that N addition affected soil properties, microbial biomass, and the composition of microbial communities; however, the FAN treatment had less influence on soil properties and soil microorganisms than did the SAN treatment over short time scales, and the extent of this effect was different between coniferous and broadleaf trees.

     

Analysis of manure and soil nitrogen mineralization during incubation

Understanding the N-cycling processes that ensue after manuring soil is essential in order to estimate the value of manure as an N fertilizer. A laboratory incubation of manured soil was carried out in order to study N mineralization, gas fluxes, denitrification, and microbial N immobilization after manure application. Four different manures were enclosed in mesh bags to allow for the separate analysis of manure and soil. The soils received 0.15 mg manure N g^–1 soil, and the microcosms were incubated aerobically and sampled throughout a 10-week period. Manure addition resulted in initial NH₄-N concentrations of 22.1 to 36.6 mg kg^–1 in the microcosms. All manured microcosms had net declines in soil mineral N. Denitrification resulted in the loss of 14.7 to 39.2% of the added manure N, and the largest N losses occurred in manures with high NH₄-N content. Increased soil microbial biomass N amounted to 6.0 to 8.6% of the added manure N. While the microcosms as a whole had negative N mineralization, all microcosms had positive net nitrification within the manure bags. Gas fluxes of N₂O and CO₂ increased in all manured soils relative to the controls. Our results show that measurement of microbial biomass N and denitrification is important to understand the fate of manure N upon soil application.     

Arbuscular mycorrhizal fungi reduce ammonia emissions under different land-use types in agro-pastoral areas

Ammonia (NH₃) emissions, the most important nitrogen (N) loss form, always induce a series of environmental problems such as increased frequency of regional haze pollution, accelerated N deposition, and N eutrophication. Arbuscular mycorrhizal (AM) fungi play key roles in N cycling. However, it is still unclear whether AM fungi can alleviate N losses by reducing NH₃ emissions. The potential mechanisms by which AM fungi reduce NH₃ emissions in five land-use types (grazed grassland, mowed grassland, fenced grassland, artificial alfalfa grassland, and cropland) were explored in this study. Results showed that AM fungal inoculation significantly reduced NH₃ emissions, and the mycorrhizal responses of NH₃ emissions were determined by land-use type. Structural equation modeling (SEM) showed that AM fungi and land-use type directly affected NH₃ emissions. In addition, the reduction in NH₃ emissions was largely driven by the decline in soil NH⁺₄-N and pH and the increases in abundances of ammonia-oxidizing archaea (AOA) amoA and bacteria (AOB) amoB genes, urease activity, and plant N uptake induced by AM fungal inoculation and land-use type. The present results highlight that reducing the negative influence of agricultural intensification caused by land-use type changes on AM fungi should be considered to reduce N losses in agriculture and grassland ecosystems.     

西南地区冬水田剖面的微生物空间分异规律

为研究冬水田土壤基本理化性质对微生物多样性和冬水田生态系统功能与结构的作用,以西南地区3个不同样地的冬水田为研究对象,采用烘干法、电位法、静态室内培养法、稀释涂布平板法和氯仿熏蒸法对土壤理化和生物学指标进行检测。结果表明:(1)各样地含水量均沿垂直深度逐渐降低,pH为6.3～7.1,呈微酸—中性生境,铵态氮(NH4+—N)含量总体呈现为合川区冬水田沙坪坝区冬水田北碚区紫色土基地,且表层(0—10cm)亚表层(10—20cm)底层(20—40cm),整体为36.97～52.02mg/kg,3种冬水田各土层硝态氮(NO3-—N)含量差异不显著(2.13～2.61mg/kg)。(2)微生物量碳(MBC)和微生物量氮(MBN)含量呈现为北碚区紫色土基地合川区冬水田沙坪坝区冬水田,不同层次表现为表层(0—10cm)亚表层(10—20cm)底层(20—40cm)。(3)各样地间微生物丰度表现为细菌放线菌真菌,微生物数量沿土层垂直深度降低,各土层细菌、放线菌和真菌数量均呈极显著负相关(p0.01),不同样地间表现为合川区冬水田沙坪坝区冬水田北碚区紫色土基地。