Seasonal variations in abundance and activity of nitrifiers in four arable cropping systems

Ammonium and nitrite oxidizers were counted with the most probable number (MPN) method and potential ammonium- and nitrite-oxidation rates were determined with a chlorate inhibition technique in an arable soil over a 3-year period. Samples were taken from the topsoil once a month for 2 years and a few times during a third year in four cropping systems: unfertilized lucerne ley and barley, and nitrate fertilized grass ley and barley. The distribution of nitrifiers was determined and their activities measured at various soil depths and between and within plant rows of fertilized barley.The numbers and activities of ammonium oxidizers were highest in the spring and autumn samples. Numbers of ammonium oxidizers ranged from 0.2 to 19×10⁴ and nitrite oxidizers from 3 to 870×10⁴ cells g^–1 dry soil. Potential ammonium-oxidizer activities ranged from 120 to 1,060 and nitrite-oxidizer activities ranged from 280 to 680 ng N g^–1 dry soil hour^–1. Lucerne and grass leys generally showed the highest, whereas unfertilized barley had the lowest, abundances and activities.Abundance estimates and activities were 10–20 times higher in the plow layer than in underlying sand and clay layers. A strong correlation was found between organic matter content vs numbers and activities of both ammonium and nitrite oxidizers. Only nitrite oxidizer counts were significantly higher within plant rows compared to between plant rows.     

Simulation of field scale denitrification losses from soils under grass ley and barley

Denitrification losses from soils under barley and grass ley crops were simulated. The model, which includes the major processes determining inputs, transformations and outputs of nitrogen in arable soils, represents a scale compatible with information generally available in agricultural field research. The denitrification part of the model includes a field potential denitrification rate and functions for the effect of soil aeration status, soil temperature and soil nitrate content. Easily metabolizable organic matter is assumed not to limit denitrification. Simulated values were compared with denitrification measurements made during two growing seasons in the barley and grass ley treatments of a field experiment in central Sweden.Calibration revealed that the optimal parameter values describing the effect of soil aeration on denitrification rates were similar for both treatments. The response function derived agreed well with two data sets found in the literature. The potential denitrification rate constant, derived in the simulations, was higher for grass ley than for barley, which was consistent with the differences in overall rates of carbon and nitrogen turnover found between treatments.The simulated mean denitrification rates for the two seasons were within 20% of the mean of the measured values. However, simulated denitrification showed less temporal variability and a less skewed frequency distribution than measured denitrification. Some of the measured denitrification events not explained by the model could have been due to the stimulating effects of soil drying/wetting and freezing/thawing on microbial activity.     

Distribution of15N in the soil-plant system during a four-year field lysimeter study with barley (Hordeum distichum L.) and perennial meadow fescue (Festuca pratensis Huds.)

An annual cereal, barley, and a perennial grass ley, meadow fescue, were grown in field lysimeters in Sweden and fertilized with 12 and 20g Ca(NO₃)₂-N m⁻² yr⁻¹, respectively. Isotope-labeled (¹⁵N) fertilizer was added during year 1 of the study, whereafter similar amounts of unlabeled N were added during years 2 and 3. The grass ley lysimeters were ploughed after the growing season of year 3 and sown with barley during year 4. The barley harvest in year 1 removed 59% of the added fertilizer N, while the fertilizer N export by two meadow fescue harvests in year 1 was 65%. The labeled N export decreased rapidly after year 1, especially in the barley, but increased slightly after ploughing of the grass ley. The microbial biomass, measured with the chloroform fumigation method, incorporated a maximum of 1.4–1.7% of the labeled N during the first seven weeks after application. Later on, the incorporation stabilized at less than 1% in both cropping systems. The susceptibility of the residual labeled N to mineralization was evaluated three years after application by means of long-term laboratory incubations. The curves of cumulative mineralized N were described by a two-component first-order regression model that differentiated between an available and a more recalcitrant fraction of potentially mineralizable N. There was no difference in the amounts of potentially mineralizable N between the cropping systems. The labeled N comprised 5 and 2% of the amounts of potentially mineralizable N in the available and more recalcitrant fraction, respectively. The mineralization rate constants for the labeled N were almost twice as high as for the total potentially mineralizable N. The available fraction of the total potentially mineralizable N was 12%, while twice that proportion of the labeled N was available. It was concluded that the short-term ley did not differ from the annual crop with respect to the early disposition of the fertilizer N and the behaviour of the residual organic N.     

Seasonal dynamics of denitrification activity in two water meadows

Seasonal variation in denitrification activity was measured in twoflooded water meadows, one on peaty and one on sandy soil, over a three-yearperiod. Measurements were taken during flooded and drained periods, usingthe acetylene-blockage technique, and the rates were compared to massbalance estimates of nitrate removal in the percolating water.Denitrification activity was higher in sandy soil than in peaty soil. Higherwater infiltration rate and thereby higher nitrate load was considered to bethe cause of the higher denitrification in the sandy soil. Floodingsignificantly increased denitrification, and the rates were higher in autumnand winter than in spring. This was considered to be a result of highernitrogen concentration in inflowing stream water during winter. Annualdenitrification was estimated to 430–460 kg N ha^-1yr^-1 in the sandy soil meadow, and 220 kg N ha^-1yr^-1 in the peaty soil meadow. In the sandy soil there was alarge discrepancy between nitrate removal rates and denitrification rates,which can be explained by nitrification of ammonium released from the soil.In the peaty soil nitrate disappearance and denitrification correspondedfairly well.     

Field measurement of symbiotic nitrogen fixation in an established lucerne ley using15N and an acetylene reduction method

Summary Lucerne is an important forage legume in the south and south-east of Sweden on well-drained soils. However, data is lacking on the apparent amount of nitrogen derived through N₂ fixation by field-grown lucerne. This report provides basic information on the subject. The experiment was performed in a lucerne ley grown 40 km north of Uppsala. The input of nitrogen through fixation to the above-ground plant material of an established lucerne (Medicago sativa L.) ley was estimate by¹⁵N methodology during two successive years. The amount of fixed N was 242 kg N ha^–1 in 1982 and 319 kg N ha^–1 in 1983. The proportion of N derived from the atmosphere (%Ndfa) was 70% and 80% for the two years respectively. The first harvest in both years contained a lower proportion fixed N. Both N₂ fixation and dry matter production were enhanced during the second year, particularly in the first harvest. The Ndfa was 61% in the first harvest in 1982, compared to 72% Ndfa during the same period in 1983. This demonstrates the strong influence of environment on both dry matter production and N₂ fixation capacity of the lucerne.In addition anin situ acetylene reduction assay was used in 1982 to measure the seasonal distribution of the N₂ fixation and in 1983 to study the effect of soil moisture on the N₂ fixation process. The seasonal pattern showed great dependence on physiological development and harvest pattern of the lucerne ley. The maximum rate of N₂ fixation occurred at the bud or early flower stage of growth and was followed by a rapid decline as flowering proceeded. After harvest the nitrogenase activity markedly decreased and remained low during at least two weeks until regrowth of new shoots began. Irrigation doubled the nitrogenase activity of the lucerne in late summer 1983, when soil moisture content in the top soil was near wilting point. No changes in nitrogenase activity did occur in response to watering earlier during the summer, when the soil matric potential was around –0.30 MPa.     

Abundance of microbial genes associated with nitrogen cycling as indices of biogeochemical process rates across a vegetation gradient in Alaska

Nitrification and denitrification processes are crucial to plant nutrient availability, eutrophication and greenhouse gas production both locally and globally. Unravelling the major environmental predictors for nitrification and denitrification is thus pivotal in order to understand and model environmental nitrogen (N) cycling. Here, we sampled five plant community types characteristic of interior Alaska, including black spruce, bog birch, tussock grass and two fens. We assessed abundance of functional genes affiliated with nitrification (bacterial and archaeal amoA) and denitrification (nirK/S and nosZ) using qPCR, soil characteristics, potential nitrification and denitrification rates (PNR and PDR) and gross mineralization rates. The main chemical and biological predictors for PNR and PDR were assigned through path analysis. The potential N cycling rates varied dramatically between sites, from some of the highest (in fens) to some of the lowest (in black spruce) measured globally. Based on path analysis, functional gene abundances were the most important variables to predict potential rates. PNR was best explained by bacterial amoA gene abundance followed by ammonium content, whereas PDR was best explained directly by nosZ gene abundance and indirectly by nirK/S gene abundance and nitrate. Hence, functional gene abundance is a valuable index that integrates recent environmental history and recent process activity, and therefore is a good predictor of potential rates. The results of this study contribute to our understanding of the relative importance of different biological and chemical factors in driving the potential for nitrification and denitrification across terrestrial ecosystems.     

Simulated nitrogen dynamics and nitrate leaching in a perennial grass ley

A soil nitrogen model was used for a 4-year simulation of nitrogen dynamics and nitrate leaching, both during grass ley growth and after ploughing a grass ley. Model results were compared with field measurements of soil mineral-N status and leaching. A soil water and heat model provided daily values for abiotic conditions, which were used as driving variables in the nitrogen simulation. Simulated values for mineral-N levels in the soil agreed well with field data for the first 3 years of the simulation. During the final year the model predicted considerably higher levels of soil mineral-N content compared with measurements. To reach the mineral-N level measured at the time of ploughing the ley, the simulated N-uptake by plants had to be increased by 8 g N m⁻². Simulations of nitrate leaching suggested that estimates of leaching based on measurements in tile-drained plots can be considerably underestimated. Accurate quantification of leaching in tile-drained plots often requires additional information on water-flow paths. A substantial increase in simulated and measured values for the mineral-N content of the soil occurred after ploughing the ley. In the simulation, most of the increase was due to a high crop residue input and the absence of a growing crop after ploughing. Litter accumulations in the soil during the 4-year period contributed little to the increase in soil mineral-N.     

不同覆盖材料对夏秋苹果根区土壤硝酸盐代谢的影响

以3年生新红星苹果树为试验材料,在春季将稻草苫、农用地毯、透明塑料膜和园艺地布覆盖地表,于夏秋季调查根区土壤硝化-反硝化作用、硝酸还原酶(NR)和亚硝酸还原酶(NiR)活性以及铵态氮、硝态氮、亚硝态氮含量和植株生长的变化.结果表明: 4种覆盖处理均降低了夏季土壤硝化强度和夏秋之交的土壤NiR活性,提高了秋季土壤铵态氮含量以及夏秋之交的土壤反硝化强度、NR活性和铵态氮含量,降低了夏秋季土壤硝化强度、反硝化强度和NR活性的变异系数;稻草苫提高了夏季和秋季土壤反硝化强度与硝态氮含量,降低了夏季土壤NR和NiR活性;在4种处理中,稻草苫覆盖的土壤硝化与反硝化强度及NR活性在整个夏秋季的变异系数最低;农用地毯降低了夏季土壤反硝化强度,提高了夏季土壤NR和NiR活性、夏秋之交土壤硝态氮含量和秋季土壤反硝化强度;透明塑料膜降低了夏季土壤硝态氮含量,提高了夏季土壤亚硝态氮含量、夏秋之交土壤硝态氮含量以及秋季土壤硝化强度和NiR活性;园艺地布提高了夏季土壤反硝化强度、夏秋之交和秋季土壤的硝化强度以及秋季土壤硝态氮含量.4种覆盖处理均促进了植株生长,其中稻草苫和园艺地布促进新梢和干径增粗的效果更显著;4种覆盖处理对夏秋季土壤硝酸盐代谢的影响不同,但对土壤硝酸盐代谢与转化都具有稳定作用,其中稻草苫的稳定效果最好.     

Strategies to decrease nitrate leaching in the Brimstone Farm Experiment,Oxfordshire, UK, 1988–1993: the effects of winter cover crops and unfertilised grass leys

Nitrate losses in drainflow were measured over five years on eight hydrologically isolated field plots, pairs of which had the following cropping regimes: (a) a 3-yr unfertilised, ungrazed grass ley followed by winter and spring cereals, (b) mixed cropping including winter cover crops, spring cereals, winter cereals, winter fallow and spring beans, (c) a similar sequence to (b) but with a winter fallow replacing the cover crop in the first year and a winter cover crop replacing the fallow in the third year, and (d) continuous winter cereals (control plots). Less nitrate was lost in winter drainflow from winter cover crops than from the winter fallows, but over all five years less nitrate was leached from the continuous cereal plots than from those with mixed cropping. Most of the extra nitrate lost from the mixed cropping regimes probably resulted from mineralisation of the cover crop residues, which occurred at times when subsequent crops could not take advantage of the mineral nitrogen released. Crops grown after the grass ley and cover crops did not benefit from their residues, in terms of either grain yield or of total nitrogen uptake. We conclude that on heavy clay soils in UK a cropping regime of continuous winter cereals offers the best compromise between profitable crop production and minimised nitrate loss to surface waters.     

Grass species and soil type effects on microbial biomass and activity

We evaluated plant versus soil type controls on microbial biomass and activity by comparing microbial biomass C, soil respiration, denitrification potential, potential net N mineralization and nitrification in different soils supporting four grass species, and by growing a group of 10 different grass species on the same soil, in two experiments respectively. In the first experiment, none of the microbial variables showed significant variation with grass species while all variables showed significant variation with soil type, likely due to variation in soil texture. In the second experiment, there were few significant differences in microbial biomass C among the 10 grasses but there were significant relationships between variation in microbial biomass C and potential net N mineralization (negative), soil respiration (positive) and denitrification (positive). There was no relationship between microbial biomass C and either plant yield or plant N concentration. The results suggest that 1) soil type is a more important controller of microbial biomass and activity than grass species, 2) that different grass species can create significant, but small and infrequent, differences in microbial biomass and activity in soil, and 3) that plant-induced variation in microbial biomass and activity is caused by variation in labile C input to soil.     

Development of lucerne (Medicago sativa L.) treated with mineral fertilizer and manure at optimal and water deficit conditions

A study on the effect of different rates of mineral fertilizer and manure on yield parameters of lucerne under optimal and water deficit conditions was carried out. Leached chernozem soil and lucerne cultivar Victoria were used. The soil was treated with ammonium nitrate and fully matured cattle manure. The plants were grown under optimum moisture content of 80% and 40% of field capacity. The water deficit stress decreased top and root biomass by 11-75% and 3-29% at mineral and organic fertilization, respectively. The applied mineral and organic N strongly depressed nodules development. Both mineral fertilizer and organic manure at dose of 210 mg N kg(-1) soil completely inhibited the appearance of nodules. Next to nitrogen, water deficit stress further inhibited the development of nodules. Nitrogen fertilization increased seed productivity in the two experimental moisture conditions. The water deficit stress decreased seed productivity by 18 to 33% as compared to optimum conditions. The plant treatments with manure were much more resistant to water deficit and recovering ability of plants was faster as compared to treatments with mineral fertilizer. The application of manure stimulates development of drought-stress tolerance in lucerne. However, the results obtained can be considered for the soil type and experimental conditions used.     

Denitrification Potential, Root Biomass, and Organic Matter in Degraded and Restored Urban Riparian Zones

Hydrologic changes associated with urbanization often lead to lower water tables and drier, more aerobic soils in riparian zones. These changes reduce the potential for denitrification, an anaerobic microbial process that converts nitrate, a common water pollutant, into nitrogen gas. In addition to oxygen, denitrification is controlled by soil organic matter and nitrate. Geomorphic stream restorations are common in urban areas, but their effects on riparian soil conditions and denitrification have not been evaluated. We measured root biomass, soil organic matter, and denitrification potential (anaerobic slurry assay) at four depths in duplicate degraded, restored, and reference riparian zones in the Baltimore, Maryland, U.S.A., metropolitan area. There were three main findings in this study. First, although reference sites were wet and had high soil organic matter, they had low levels of nitrate relative to degraded and restored sites and therefore there were few differences in denitrification potential among sites. Evaluations of riparian restorations that have nitrate removal by denitrification as a goal should consider the complex controls of this process and how they vary between sites. Second, all variables declined markedly with depth in the soil. Restorations that increase riparian water tables will thus foster interaction of groundwater nitrate with near-surface soils with higher denitrification potential. Third, we observed strong positive relationships between root biomass and soil organic matter and between soil organic matter and denitrification potential, which suggest that establishment of deep-rooted vegetation may be particularly important for increasing the depth of the active denitrification zone in restored riparian zones.     

坡向和海拔对高寒草甸山体土壤水热和植物分布格局的定量分解

为明晰坡向和海拔对山体土壤水热和植物分布格局的影响,以青藏高原东北缘山体高寒草甸为研究对象,利用回归分析、典型相关分析(CCA)排序和方差分解等方法,对阶地与同一山体不同坡向和海拔的189个样方内土壤温湿度和植物分布进行分析和定量分解.结果表明: 阴坡物种丰富度最高,阶地最低.随海拔升高,阴坡和山脊物种丰富度先增加后降低,而阳坡物种丰富度呈线性增加.阳坡土壤温度最高,不同坡向0～20 cm土层土壤温度随海拔升高基本不变;阴坡土壤湿度最高,不同坡向0～30 cm土层土壤湿度随海拔升高而增加.方差分解表明,在0～30 cm土层中,坡向和海拔共解释土壤温度变化的100%,土壤湿度变化的51.8%.坡向单独解释土壤温度变化的72.2%,贡献率最高;海拔单独解释土壤湿度变化的51.8%,贡献率最高.大多数植物倾向分布于中等海拔的阴坡与山脊之间.阴坡以莎草科为主,阳坡以禾本科为主,山脊为过渡地带.莎草科、禾本科和豆科主要分布于低海拔区.坡向和海拔共解释了山地植物多度变化的28.6%,坡向单独解释19.9%,贡献率最高.在山地高寒草甸生态系统,综合考虑小尺度地形造成的土壤及植物分布格局差异的基础上,在进行生产与生态恢复的分区管理时,应优先考虑坡向造成的土壤和植物差异.     

Simultaneous occurrence of denitrification and nitrate ammonification in sediments of the French Mediterranean Coast

Dissimilatory nitrate reductions in coastal marine sediment of Carteau Cove (French Mediterranean Coast) were studied between April 1993 and July 1994. Simultaneous determination of denitrification and dissimilatory nitrate reduction to ammonium was achieved by using a combination of acetylene blockage and 15N techniques. After short incubations (maximum 5 h), a part of 15N labelled nitrate added to the sediment was recovered as ammonium without incorporation in organic matter. The result indicate that a fraction of nitrate was reduced to ammonium by a dissimilatory mechanism instead of denitrifying. Denitrifying and nitrate ammonifying activities ranged from 0 to 19.8 μmol l-1 d-1 and from 2.3 to 83.2 μmol l-1 d-1, respectively. Denitrification rates were highest in early spring whereas nitrate ammonification were highest in fall. The recovery of nitrate reduced as N2O-N plus ammonium was between 40 and 100%, the highest nitrogen losses were recorded in July. Depending on the station and time of year denitrification accounted for between 0 and 43% of the total nitrate reduction whereas dissimilatory nitrate reduction to ammonium (DNRA) accounted for between 18 and 100%. The reduction rate data suggest that the pathway of nitrate reduction to ammonium may be important in coastal sediments. This revised version was published online in July 2006 with corrections to the Cover Date.     

DenNit – Experimental analysis and modelling of soil N2O efflux in response on changes of soil water content, soil temperature, soil pH, nutrient availability and the time after rain event

To quantify the effects of soil temperature (T_soil), and relative soil water content (RSWC) on soil N₂O emission we measured N₂O soil efflux with a closed dynamic chamber in situ in the field and from soil cores in a controlled climate chamber experiment. Additionally we analysed the effect of soil acidity, ammonium, and nitrate concentration in the field. The analysis was performed on three meadows, two bare soils and in one forest. We identified soil water content, soil temperature, soil nitrogen content, and pH as the main parameters influencing soil N₂O emission. The response of N₂O emission to soil temperature and relative soil water content was analysed for the field and climate chamber measurements. A non-linear regression model (DenNit) was developed for the field data to describe soil N₂O efflux as a function of soil temperature, soil moisture, pH value, and ammonium and nitrate concentration. The model could explain 81% of the variability in soil N₂O emission of all individual field measurements, except for data with short-term soil water changes, namely during and up to 2 h after rain stopped. We validated the model with an independent dataset. For this additional meadow site 73% of the flux variation could be explained with the model.     

Denitrification in lucerne nodules and bacteroids supplied with nitrate

Nodulated lucerne plants ( Medicago sativa L. cv. Aragón) were supplied with 20 m M nitrate. Anaerobically isolated bacteroids of Rhizobium meliloti from these plants were able to denitrify after 48 h treatment. R. meliloti bacteroids behave as total denitrifiers, reducing nitrate to dinitrogen: when acetylene was omitted from the assay medium very little nitrous oxide was recovered. The onset of denitrification activity was coincident with the induction of nitrite reductase activity (EC 1.7.99.3) whereas nitrate reductase activity (EC 1.7.99.4) was constitutive. Whole nodules from plants receiving several doses of nitrate were assayed, in a nitrate-free medium, to monitor denitrification activity dependent on nitrate within the nodules. Denitrification activity was detected after 2 days of 20 m M nitrate supply or after 3 days in the presence of 10 or 5 m M nitrate. These results are discussed in relation to current controversy about nitrate entry into the infection region of nodules. It is concluded that this process occurs more rapidly than suggested in recent research.     

Effects of climate change on nitrogen dynamics in upland soils. 2. A soil warming study

A new warming technique has been developed in a field experimental study of the potential effects of climatic change on N leaching from hill land plant/soil systems. Thermocouple compensating cable has been utilized to provide a small cross-section, flexible, low voltage heating cable, mounted on a framework of stainless steel mesh, to provide uniform heating at the vegetation/soil interface of zero-tension lysimeters and surrounding turf. We describe a specially designed heat controller capable of maintaining a temperature differential of 3 °C above ambient at a soil depth of 0.8 cm. The equipment raises temperatures down the soil profile and within the grass sward, whilst tracking normal diurnal temperature variation. Results presented here illustrate the efficacy of the warming technique, together with the consequences for the release of nitrate from lysimeters. The responses of soil solution concentrations of nitrate varied markedly between soil types, but showed a significant decrease in the brown earth during the first 5 months of additional heating. This suggests that increased nutrient release is masked by plant uptake in this soil, but the responses in the other two soils were less marked.     

Dinitrogen and nitrous oxide produced by denitrification and nitrification in soil with and without barley plants

Summary To examine the effect of barley roots on denitrification, a pot experiment was designed to compare N₂O production and denitrification in soils with and without barley plants. Denitrification, N₂O resulting from denitrification and nitrification, and respiration were estimated by incubating pots with soil with and without intact plants in plastic bags at high moisture levels. C₂H₂-inhibition of nitrous oxide reductase (partial pressure of 10 kPa C₂H₂) was used to determine total denitrification rates while incubations with ambient air and with C₂H₂ at partial pressures of 2.5–5 Pa were used to estimate the amounts of N₂O released from autotrophic nitrification and from denitrification processes. Other sources of N₂O were presumed to be negligible. Potential denitrification, nitrification and root biomass were measured in subsamples collected from four soil depths. A positive correlation was found between denitrification rates and root biomass. N₂ was the predominant denitrification product found close to roots; N₂O formed by non autotrophic nitrifiers, assumed to be denitrifiers originated in soil not affected by growing roots. Apparently, roots promote denitrification because they consumed oxygen, thereby increasing the anaerobic volume of the soil. The ratio of actual to potential denitrification rates increased over time, especially in the presence of roots.     

The soil Acari of barley plots with different cultural treatments

Sixty-four species of Acari were extracted by wet sieving and salt flotation from soil in four barley plots sampled on five occasions between 23 May and 15 August 1975. Over half the species were Prostigmata. The most abundant species wereScutacarus longiusculus (Karafiat) (Scutacaridae, Prostigmata),Bakerdania gracilis (Krczal) (Pygmephoridae, Prostigmata), andTyrophagus longior (Gervais) (Acaridae, Astigmata).Only three species (B. gracilis, T. longior andPseudopygmephorus sellnicki (Krczal) (Pygmephoridae)) contributed more than 2% of the individuals and occurred in more than 25% of the samples in all plots.Mean population densities ranged from 41 to 216 thousand/m², with highest density occurring in a conventionally ploughed and cultivated plot and lowest in a direct drilled plot. Prostigmata accounted for 46–85% of the total populations in the various plots.Twelve species were recorded from decaying crop residues on the soil surface; these included some of the most abundant species recorded from growing barley in a related study. Soil and crop residues were the main sources of most of the dominant species colonizing the growing barley crop.The barley plots had greater numbers of Prostigmata and Astigmata but fewer Cryptostigmata and Mesostigmata than were extracted by a high gradient canister technique from adjacent grass ley pasture sampled in its second and third years (1976 and 1977). Biomass in the older ley was far greater than in the barley. There were higher numbers of species of intermediate relative abundance (0.1–5%) in the ley, indicating more mature communities than in the barley plots.