The effect of inorganic nitrogen on the added nitrogen interaction of soils in incubation experiments

A laboratory incubation experiment was conducted to study the effect of indigenous inorganic N on the immobilization of applied N and on the occurrence of an added N interaction (ANI). Samples of six Mollisols from Illinois were incubated with ¹⁵N-labelled (NH₄)₂SO₄ (100 or 200 mg N kg^-1 soil), with or without the use of 0.01 M CaCl₂ to extract inorganic N (mainly NO _inf3 ^sup- ) before incubation. From 6 to 49% of the N applied was immobilized, higher percentages being obtained with unextracted soils than with the extracted soils and with the higher rate of N addition. Net mineralization of native N occurred in both the unextracted and extracted soils, but was more extensive in the unextracted soil and increased with the addition of N. The increases were accompanied by a positive ANI, which usually exceeded the amount of applied N immobilized and increased with the rate of addition. The ANI values observed with extracted soils were attributed to increased mineralization of native organic N.     

Synthesis of 15N-labelled microbial biomass in soil in situ and extraction of biomass N

Summary A Pakistani soil (Hafizabad silt loam) was incubated at 30°C with varying levels of ¹⁵N-labelled ammonium sulphate and glucose (C/N ratio of 30 at each addition rate) in order to generate different insitu levels of ¹⁵N-labelled microbial biomass. At a stage when all of the applied ¹⁵N was in organic forms, as biomass and products, the soil samples were analysed for biomass N by the chloroform (CHCl₃) fumigation-extraction method, which involves exposure of the soil to CHCl₃ vapour for 24 h followed by extraction with 500 mM K₂SO₄. A correction is made for inorganic and organic N in 500 mM K₂SO₄ extracts of the unfumigated soil. Results obtained using this approach were compared with the amounts of immobilized ¹⁵N extracted by 500 mM K₂SO₄ containing different amounts of CHCl₃. The extraction time varied from 0.5 to 4 h.The amount of N extracted ranged from 27 to 270 g g^–1, the minimum occurring at the lowest (67 g g^–1) and the maximum at the highest (333 g g^–1) N-addition rate. Extractability of biomass ¹⁵N ranged from 25% at the lowest N-addition rate to 65%a for the highest rate and increased consistently with an increase in the amount of ¹⁵N and glucose added. The amounts of both soil N and immobilized ¹⁵N extracted with 500 mM K₂SO₄ containing CHCl₃ increased with an increase in extraction time and in concentration of CHCl₃. The chloroform fumigation-extraction method gives low estimates for biomass N because some of the organic N in K₂SO₄ extracts of unfumigated soil is derived from biomass.     

The release and plant uptake of nitrogen from some plant and animal manures

Summary Field and laboratory experiments were used to examine the efficiency of N uptake from various manure forms, and at different rates of application. In a field experiment, wheat was grown on soils with different amounts of ¹⁵N-labelled legume residues. The amount of N taken up by the crop was directly proportional to the amount applied, with a recovery of between 15% and 23% of the legume N. In a second field experiment, inorganic N was applied at rates varying from 0 to 120 kg N ha^-1 in the presence and absence of poultry manure. The uptake of N by barley was 11 kg ha^-1 greater in the manured plots when no inorganic N was applied, and 23 kg ha^-1 greater when N was applied at the top rate. N uptake in a pot experiment was again shown to be directly proportional to the rate of manure application, but the amount of N taken up was strongly related to the N content of the manure. An incubation experiment demonstrated that net N mineralisation reached a maximum where residue concentrations were 1,5%. The significance of added nitrogen interactions in the context of manure-N additions is discussed.     

Effect of ammonium fixation and displacement on the added nitrogen interaction in incubation experiments

A laboratory incubation experiment was conducted to study the effect of NH ₄ ⁺ fixation/defixation on the added N interaction (ANI) in three Illinois Mollisols fertilized with 100 or 200 mg N kg^-1 soil. A positive ANI was observed in all three soils, which was greater at the higher rate of applied N. However, very little exchange was observed between applied ¹⁵NH ₄ ⁺ and the native clay-fixed NH ₄ ⁺ , and the ANI observed were attributed largely to microbial immobilization-mineralization. The results suggested that variations in the NH ₄ ⁺ fixation capacity of soils will not have a significant bearing on the interpretation of data obtained from studies of the ANI.     

Changes in 15N abundance and amounts of biologically active soil nitrogen

 Estimation of the capacity of soils to supply N for crop growth requires estimates of the complex interactions among organic and inorganic N components as a function of soil properties. Identification and measurement of active soil N forms could help to quantify estimates of N supply to crops. Isotopic dilution during incubation of soils with added ¹⁵NH₄ ⁺ compounds could identify active N components. Dilution of ¹⁵N in KCl extracts of mineral and total N, non-exchangeable NH4₄ ⁺, and N in K₂SO₄ extracts of fumigated and non-fumigated soil was measured during 7-week incubation. Samples from four soils varying in clay content from 60 to 710 g kg^–1 were used. A constant level of ¹⁵N enrichment within KCl and K₂SO₄ extracted components was found at the end of the incubation period. Total N, microbial biomass C and non-exchangeable NH₄ ⁺ contents of the soils were positively related to the clay contents. The mineralized N was positively related to the silt plus clay contents. The active soil N (ASN) contained 28–36% mineral N, 29–44% microbial biomass N, 0.3–5% non-exchangeable NH₄ ⁺ with approximately one third of the ASN unidentified. Assuming that absolute amounts of active N are related to N availability, increasing clay content was related to increased N reserve for crop production but a slower turnover. Received: 7 July 1998     

Significance of organic nitrogen uptake from plant residues by soil microorganisms as affected by carbon and nitrogen availability

Soil microorganisms can use a wide range of nitrogen (N) compounds. When organic N sources are degraded, microorganisms can either take up simple organic molecules directly (direct route), or organic N may be mineralized first and taken up in the form of mineral N (mineralization-immobilization-turnover [MIT] route). To determine the importance of the direct route, a microcosm experiment was carried out. Two types of wheat residue were added to soil samples, including younger residue with a carbon (C) to N ratio of 12 and older residue with a C to N ratio of 29. Between days 1 and 4, the gross N mineralization rate reached 8.4 and 4.0 mg N kg⁻¹ dry soil day⁻¹ in the treatment with younger and older residue, respectively. During the same period, there was no difference in protease activity between the two residue amended treatments. The fact that protease activity was not related to gross N mineralization, even though the products of protease activity are the substrates for N mineralization, suggests that not all organic molecules released from residue or soil N passed through the soil mineral N pool. In fact, when leucine and glycine were added, only 10 and 53% of the amino acid-N, respectively, was mineralized. The fraction of N taken up via the direct route was estimated to be 55 and 62% for the young and older residue, respectively. After 28 days of incubation, the proportion of amino acid-N mineralized had increased especially in the soil amended with older residue, suggesting that the MIT route became increasingly important. This result is supported by an increase in the activities of enzymes responsible for the intracellular assimilation of ammonium (NH₄⁺). Our results suggest that in contrast to what is proposed by many models of soil N cycling, both the direct and MIT routes were operative, with the direct route being the preferred route of residue N uptake. The direct route became less important over time and was more important in soil amended with older residue, suggesting that the direct route is favored by lower mineral N availabilities. An important implication of these findings is that when the direct route is dominant, gross N mineralization underestimates the amount of N made available from the residue.     

Effects of earthworm casts and compost on soil microbial activity and plant nutrient availability

Vermicomposting differs from conventional composting because the organic material is processed by the digestive systems of worms. The egested casts can be used to improve the fertility and physical characteristics of soil and potting media. In this study, the effects of earthworm casts (EW), conventional compost (CP) and NPK inorganic fertilizer (FT) amendments on N mineralization rates, microbial respiration, and microbial biomass were investigated in a laboratory incubation study. A bioassay with wheat (Triticum aestivium L.) was also conducted to assess the amendment effects on plant growth and nutrient uptake and to validate the nutrient release results from the incubation study. Both microbial respiration and biomass were significantly greater in the CP treatment compared to EW treatment for the initial 35 days of incubation followed by similar respiration rates and biomass to the end of the study at 70 days of incubation. Soil NO₃⁻ increased rapidly in the EW and CP treatments in the initial 30 days of incubation, attaining 290 and 400 mg N kg⁻¹ soil, respectively. Nitrate in the EW treatment then declined to 120 mg N kg⁻¹ soil by day 70, while nitrate in the CP treatment remained high. While ammonium levels decreased in the CP treatment as nitrate level increased with increasing incubation time, a low level of ammonium was maintained in the EW treatment throughout the incubation. The wheat bioassay study included two additional cast treatments (EW-N and EW2) to have treatments with higher levels of N input. Plants grown with CP or FT treatment had a lower shoot biomass and higher shoot N content than in EW-N and EW-2 treatments, and also showed symptoms of salinity stress. Ionic strength and other salinity indicators in the earthworm cast treatments were much lower than in the CP treatment, indicating a lower risk of salinity stress in casts than in compost. All cast and compost amendments significantly increased wheat P and K uptake compared to either the non-amended control or the mineral fertilizer treatment. The results show that casts are an efficient source of plant nutrients and that they are less likely to produce salinity stress in container as compared to compost and synthetic fertilizers.     

Quantification and potential availability of non-symbiotically fixed 15N in soil

Summary Non-symbiotic N₂ fixation was studied under laboratory conditions in two soils from Pakistan (Hafizabad silt loam and Khurrarianwala silt loam) and one from Illinois, USA (Drummer silty clay loam) incubated in a ¹⁵N-enriched atmosphere. N₂ fixation was greatest with the Drummer soil (18–122 g g^–1 soil, depending upon the soil treatment) and lowest with the Khurrarianwala soil (4–81 g g^–1 soil). Fixation was increased by the addition of glucose, a close correlation being observed between the amount of glucose added and the amount of N₂ fixed in the three soils (r = 0.96). Efficiency of N₂ fixation varied with soil type and treatment and was greatest in the presence of added inorganic P. Application of Mo apparently had a negative effect on the amount and efficiency of N₂ fixation in all the soils. The percentage of non-symbiotically fixed ¹⁵N in potentially mineralizable form (NH ₄ ⁺ -N released in soil after a 15-day incubation period under anaerobic conditions) was low (2%–18%, depending upon the soil treatment), although most of the fixed N (up to 90%) was recovered as forms hydrolysable with 6N HCl. Recovery in hydrolysable forms was much greater for the fixed N than for the native soil N, indicating that the former was more available for uptake by plants.     

Fate of nitrogen derived from 15N-labeled plant residues and composts in rice-planted paddy soil

Pot experiments that lasted for 3 y were conducted to investigate the dynamics of nitrogen derived from plant residues (rice root, hull, straw, corn root, and rapeseed pod-wall), and composts (rice straw compost, cattle manure compost, and cattle manure sawdust compost), which were labeled with ¹⁵N. The rates of nitrogen uptake by rice (=N efficiency), denitrification, and immobilization derived from the organic materials incorporated before the first year of cultivation were investigated throughout 3 y of cultivation. At the end of the first year of cultivation, relatively high rates of N efficiency were obtained for rapeseed pod-wall (24.6%), rice straw (19.1%), and rice hull (18.6%), while corn root and cattle manure sawdust compost displayed a noticeably high denitrification rate. Corn root, cattle manure sawdust compost, rice hull, and rapeseed pod-wall exhibited remarkably high N mineralization rates ranging from 60 to 75% of the organic materials N applied. Cumulative rates of N efficiencies from the organic materials applied before the first year of cultivation fitted well to a first-order kinetic model and their asymptotes were compared among the organic materials. The asymptotic rates of N efficiency tended to depend on the rates at the end of the first year of cultivation.     

Modelling mineralization of plant residues on soil: effect of physical protection

A mechanistic dynamic model (Verberne et al. 1990) was used to simulate mineralization of white-clover materials in a loam (25% clay) and a sandy loam soil (5% clay). I tested the model‘s ability to simulate the observed temporal patterns and to take account of altered physical protection as affected by soil compaction or spatial residue distribution. With default parameter values, the model greatly overestimated net N mineralization. The model was very sensitive to changes in the C/N ratio of the microbial biomass. Reducing this value from 8.0 to 6.0 improved the model performance. Nevertheless, initial N mineralization was appreciably overestimated. Two hypotheses may explain the discrepancies: (1) the C/N ratio of the microbial biomass is initially low (3–4) and gradually increases because of a succession from bacterial- to fungal-dominated biomass (H ₁); (2) the C/N ratio of the substrates first attacked by microorganisms, i.e. water-soluble components such as sugars and free amino acids, is higher than the average value (6.0) assumed for the readily decomposable fraction (H ₂). Conceptually, this fraction originally included N-containing polymers (proteins and nucleic acids), which in large part are water insoluble and probably attacked somewhat later than the monomers. Modification of the model, either by implementing a dynamic C/N ratio of the biomass and the effect of faunal grazing or by increasing the C/N ratio of the easily decomposable fraction, improved the model performance substantially. The two hypotheses need to be tested experimentally. The model adequately simulated measured effects of spatial residue distribution and soil compaction on N mineralization after adjustment or parameter values regulating physical protection of microbial biomass and metabolites. Moreover, there was a good agreement between simulated and measured microbial biomass N in the two soils. Received: 9 December 1996     

Carbon and net nitrogen mineralisation in two forest soils amended with different concentrations of biuret

Biuret is a known contaminant of urea fertilisers that might be useful as a slow release N fertiliser for forestry. We studied carbon (C), net nitrogen (N) mineralisation and soil microbial biomass C and N dynamics in two forest soils (a sandy loam and a silt loam) during a 16-week long incubation following application of biuret (C 23.3%, N 40.8%, O 30.0% and H 4.9%) at concentrations of 0, 2, 10, 100 and 1000 mg kg⁻¹ (oven-dried) soil to assess the potential of biuret as a slow-release N fertiliser. Lower concentrations of biuret specifically increased C mineralisation and soil microbial biomass C in the sandy loam soil, but not in the silt loam soil. A significant decrease of microbial biomass C was found in both soils at week 16 after biuret was applied at higher concentrations. C mineralisation declined with duration of incubation in both soils due to decreased C availability. Biuret at concentrations from 10 to 100 mg kg⁻¹ soil had a significantly positive priming effect on soil organic N mineralisation in both soils. The causes for the priming effects were related to the stimulation of microbial growth and activity at an early stage of the incubation and/or the death of microbes at a later stage, which was biuret-concentration-dependent. The patterns in NH₄⁺-N accumulation differed markedly between the two soils. Net N mineralisation and nitrification were much greater in the sandy loam soil than in the silt loam soil. However, the onset of net nitrification was earlier in the silt loam soil. Biuret might be a potential slow-release N source in the silt loam soil.     

甜玉米/白三叶草秸秆还田的碳氮矿化研究

豆科/禾本科作物间套作后进行秸秆还田能补充土壤养分,缓解集约化农业生产对环境的压力.根据田间甜玉米/白三叶草套种各作物的秸秆产量,在恒温恒湿条件下进行室内培养,探讨秸秆不同方式还田后土壤微生物量碳、微生物量氮、呼吸产生的CO2和矿化产生的无机氮的变化规律.研究发现,各施肥处理的土壤微生物量碳、微生物量氮均在培养前期出现峰值,后期平稳降低;甜玉米秸秆和白三叶草绿肥同时还田的土壤微生物量碳、微生物量氮在各培养时期均最大,峰值分别达529.57 mg·kg-1和75.50 mg·kg-1,土壤呼吸产生的CO2最多;白三叶草绿肥单独还田有利于土壤无机氮的释放,培养第26 d 无机氮达到最大值,为29.81 mg·kg-1,之后一直在对照处理的1.60倍以上,第80 d达到2.48倍;甜玉米秸秆单独还田不利于土壤无机氮的释放,培养的第26 d至结束,甜玉米秸秆处理的无机氮为对照的13%～53%,最大为7.51 mg·kg-1;甜玉米秸秆配施尿素,短期内不利于土壤无机氮矿化.结果表明,施用有机物料能引起土壤有机质的短期快速转化,甜玉米秸秆和白三叶草绿肥配施有利于维持较大基数的土壤微生物量,单施白三叶草绿肥土壤微生物活性强,最有利于土壤速效氮的释放.     

Rates of decomposition of plant residues and available nitrogen in soil, related to residue composition through simulation of carbon and nitrogen turnover

The dynamics of inorganic N in soil following the application of plant residues depends on their composition. We assumed that all plant materials are composed of similar components, each decomposing at a specific rate, but differ in the proportions of the various components. The NCSOIL model that simulates C and N turnover in soil was used to link the rates of residue decomposition to their composition, defined as soluble, cellulose-like and lignin-like C and N, and thereby integrate short and long-term effects of residues on available N dynamics in soil. Five plant residues in a wide range of C:N ratios were incubated in soil for 24 weeks at 30 °C, during which C and N mineralization were measured. The materials with large C:N ratios (corn, rice hulls and wheat straw) were also incubated with NH₄⁺-N to avoid N deficiency. The residues were analyzed for total and soluble C and N. The partitioning of insoluble C and N between cellulose- and lignin-like pools was optimized by best fit of simulated C and N mineralization to measured results. The decomposition rate constants of the soluble and lignin-like pools were assumed to be 1.0 and 10⁻⁵ d⁻¹, respectively, and that of the cellulose-like pool, obtained by model optimization against mineralization of cellulose with NH₄⁺-N in soil, was 0.051 d⁻¹. The optimized, kinetically defined lignin-like pool of all residues was considerably larger than lignin contents normally found in plant residues by the Van Soest procedure. Gross N mineralization of tobacco and rape residues was similar, but N recovery from tobacco was larger, because a larger fraction of its C was in the lignin-like pool. N in rice hulls, corn and wheat residues was mostly recalcitrant, yet rice hulls did not cause N deficiency, because most of its C was recalcitrant too. The soluble components of the residues had strong short-term effects on available N in soil, but the cellulose-like pool was equally important for short and medium-term effects. Soluble and cellulose-like C were 29 and 42% of total C, respectively, in corn and 7 and 50% in wheat. Maximal net inorganic N losses, measured in both residue treatments after 2 weeks, were 42 mg g⁻¹ C applied as corn and 31 mg g⁻¹ C applied as wheat, or 84 and 110 mg g⁻¹ decomposed C of corn and wheat, respectively. Rice hulls immobilized N slowly, but by the end of 24 weeks all three residues immobilized 26-27 mg N kg⁻¹ C applied. The different dynamics of N immobilization demonstrated the need to determine the decomposability of C and N rather than their total contents in plant residues.     

Comparison of methods for extraction of organic N monomers from soil microbial biomass

One way of investigating the function of soil is via the pool of low molecular weight organic compounds in the soil microbial biomass. This is because low molecular weight organic compounds have key roles in metabolism of soil microbes, can function in osmotic adjustment and other stress responses, and are intermediates in the breakdown of polymers to inorganic nutrients. Methods for measuring low molecular weight microbial metabolites in soil rely upon extracting total metabolites and then subtracting the contribution from metabolites in the soil extracellular matrix (i.e. microbial = total − extracellular). Recent studies have tested methods for extracting organic N monomers from the extracellular matrix of soil, but there has not been similar testing of methods for extracting total organic N monomers. The aims of this study were to examine methods for extracting total organic N monomers by a) contrasting chloroform gas fumigation with chloroform direct extraction, and b) examining whether it is possible to extract soil with two methods that combine quenching of metabolic activity with extraction, namely cold methanol/chloroform/water and hot aqueous ethanol. To evaluate methods, organic N compounds were extracted from soil and then capillary electrophoresis–mass spectrometry identified and quantified 42 organic N monomers including amino acids, quaternary ammonium compounds, nucleobases and nucleosides, amines and polyamines. Absolute concentrations of 32 out of the 42 quantified organic N monomers were significantly different between soil extracted by chloroform gas fumigation and chloroform direct extraction. These differences were probably a function of gains and losses of compounds due to oxidation, hydrolysis and deamidation during the two-day chloroform gas fumigation. Cold methanol/chloroform/water yielded large amounts of the extremely labile compound ergothioneine, probably because the extraction method rapidly quenched metabolic activity. The primary limitation of extraction with methanol/chloroform/water is that it was ineffective at extracting strongly cationic compounds (e.g. polyamines). Extraction with hot aqueous ethanol was unsuccessful with soil presumably because soil microbes are difficult to lyse. It is recommended that future studies examining organic N monomers in soil microbial biomass use chloroform direct extraction or cold methanol/chloroform/water rather than chloroform gas fumigation.     

Tree girdling provides insight on the role of labile carbon in nitrogen partitioning between soil microorganisms and adult European beech

Nitrogen (N) cycling in terrestrial ecosystems is complex since it involves the closely interwoven processes of both N uptake by plants and microbial turnover of a variety of N metabolites. Major interactions between plants and microorganisms involve competition for the same N species, provision of plant nutrients by microorganisms and labile carbon (C) supply to microorganisms by plants via root exudation. Despite these close links between microbial N metabolism and plant N uptake, only a few studies have tried to overcome isolated views of plant N acquisition or microbial N fluxes. In this study we studied competitive patterns of N fluxes in a mountainous beech forest ecosystem between both plants and microorganisms by reducing rhizodeposition by tree girdling. Besides labile C and N pools in soil, we investigated total microbial biomass in soil, microbial N turnover (N mineralization, nitrification, denitrification, microbial immobilization) as well as microbial community structure using denitrifiers and mycorrhizal fungi as model organisms for important functional groups. Furthermore, plant uptake of organic and inorganic N and N metabolite profiles in roots were determined.Surprisingly plants preferred organic N over inorganic N and nitrate (NO₃⁻) over ammonium (NH₄⁺) in all treatments. Microbial N turnover and microbial biomass were in general negatively correlated to plant N acquisition and plant N pools, thus indicating strong competition for N between plants and free living microorganisms. The abundance of the dominant mycorrhizal fungi Cenococcum geophilum was negatively correlated to total soil microbial biomass but positively correlated to glutamine uptake by beech and amino acid concentration in fine roots indicating a significant role of this mycorrhizal fungus in the acquisition of organic N by beech. Tree girdling in general resulted in a decrease of dissolved organic carbon and total microbial biomass in soil while the abundance of C. geophilum remained unaffected, and N uptake by plants was increased. Overall, the girdling-induced decline of rhizodeposition altered the competitive balance of N partitioning in favour of beech and its most abundant mycorrhizal symbiont and at the expense of heterotrophic N turnover by free living microorganisms in soil. Similar to tree girdling, drought periods followed by intensive drying/rewetting events seemed to have favoured N acquisition by plants at the expense of free living microorganisms.     

Effect of increasing rates of 15N‐labelled fertilizer on recovery of fertilizer N in plant and soil N pools in a pot experiment with winter wheat

The effect of increasing rates of ¹⁵N‐labelled Ca(NO₃)₂ (N₀ = no N application, N₃₀₀ = 300 mg N/pot; N₆₀₀ = 600 mg N/pot; N₉₀₀ = 900 mg N/pot) on recovery of fertilizer N in winter wheat plants and soil (total soil N, soil microbial biomass N [N_mic], extractable organic N [N_org]) and on N mineralization (NM_soil) was investigated at milk‐ripe growth stage in a pot experiment. The N rates were equally split at tillering, stem elongation and ear emergence. Fertilizer N recovered in crops increased with increasing N rates (N₃₀₀: 223.5 mg N/pot [74.5% of applied fertilizer N], N₆₀₀: 445.6 mg N/pot [74.3%], N₉₀₀: 722.1 mg N/pot [80.2%]). NM_soil slightly increased from N₀ (43.8 mg N/pot) to N₉₀₀ (75.6 mg N/pot) indicating that N application enhanced availability of soil‐derived N for the plants. However, in fertilized treatments NM_soil is balanced by immobilization and losses (non‐recovered fertilizer N). Therefore the effective soil N mineralization is indicated by apparent net N mineralization (ANNM = NM_soil — fertilizer N immobilization — lost fertilizer N). Fertilizer N immobilization in soil increased from N₃₀₀ (38.7 mg N/pot) to N₆₀₀ (60.7 mg N/pot) and N₉₀₀ (65.5 mg N/pot). Lost fertilizer N increased from N₃₀₀ (14.8 mg N/pot) to N₆₀₀ (56.7 mg N/pot) and N₉₀₀ (62.1 mg N/pot). As a consequence negative ANNM values were calculated at N₆₀₀ and N₉₀₀. Due to the small differences between N₆₀₀ and N₉₀₀ fertilizer N immobilization and lost fertilizer N did not increase linearly with increasing N rates, i.e. both processes were limited by factors other than N rate. Only 5.6—7.4% of the immobilized fertilizer N was recovered in N_org and 5.4—9.3% in N_mic soil pools. It is assumed that most of the immobilized fertilizer N was in non‐extractable organic N forms. N_mic and N_org were weak indicators for the extent of fertilizer N immobilization.     

Interaction effects of nitrogen and phosphorus fertilizer on nitrogen mineralization of wheat residues in a calcareous soil

Abstract

Because of the high pH of the soil in semiarid regions, phosphorus adsorption is unfavorable. So, considerable amounts of phosphorus fertilizers are used annually, where this fertilizer may affect the plant residues' decomposition. To examine the interaction effects of nitrogen and phosphorus on nitrogen mineralization in calcareous soil, a factorial experiment was performed in a completely randomized design with three replications. The first factor consisted of various C:N ratios (20, 40, and 60 or three levels of nitrogen N₁:0, N₂:11, and N₃:43?kg N ha^?1, respectively) and the second factor consisted of various C:P ratios (87, 174, and 260 or three levels of phosphorus P₁:0, P₂:12, and P₃:45?kg P ha^?1, respectively), under incubation conditions. The results indicated that the cumulative mineral nitrogen content in all treatments, except for N1P2 and N1P3 treatments, started from a positive amount and remained positive until the end of the incubation period. The highest amount of cumulative mineral nitrogen among treatments was related to N3P1 treatment, while the lowest was associated with N2P3 treatment. Mineralization of nitrogen during 60?d of incubation was the dominant phenomenon, except for the N1P2 and N1P3 treatments which remained in the organic phase. The effect of phosphorus on the cumulative mineralization of nitrogen was significant. With increasing the amount of phosphorus, the total inorganic nitrogen diminished. Nitrogen release begins earlier with lower C:N ratios, and therefore the available nitrogen can be released more quickly to the plant. It is generally concluded that, in calcareous soil, the use of nitrogen fertilizer to adjust C:N ratio and to improve the mineralization of wheat residues will be a suitable option.     

Plant and microbial uptake of nitrogen and phosphorus affected by drought using 15N and 32P tracers

Competition for nutrients between plants and microbes is an important determinant for plant growth, biodiversity and carbon cycling. Perturbations such as drought affect the availability of nitrogen (N) and phosphorus (P), and may cause shifts in uptake of N and P between plants and microbes. Competitiveness for these nutrients may depend on how flexible plants and microbes are in taking up N and P. We used a novel dual isotope labelling technique (¹⁵N and ³²P) to assess short-term uptake of N and P by plants and microbes affected by drought in two different plant–soil systems. Mesocosms were extracted from two grassland sites differing in soil nutrient availability and plant species. Half of the mesocosms were subjected to drought one week prior to injection of ¹⁵N (as KNO₃) and ³²P (as H₃PO₄) tracers. Uptake rates of ${{\text{NO}}_3}^{-}$ and P in plants and microbes were estimated based on average source pool enrichment during the labelling period and on plant and microbial recovery of ¹⁵N and ³²P measured after 4 days of labelling. Overall competition for N and P was reduced with drought as less ${{\text{NO}}_3}^{-}$ and P was taken up in plants and microbes. However, plant uptake of ${{\text{NO}}_3}^{-}$ was more sensitive to drought than microbial ${{\text{NO}}_3}^{-}$ uptake, while microbial P uptake was more sensitive than plant P uptake. These different sensitivities to drought by plants and microbes may decouple the N and P cycle with increased drought conditions.     

The influence of two agricultural biostimulants on nitrogen transformations, microbial activity, and plant growth in soil microcosms

The influence of two experimental soil treatments, Z93 and W91, on nitrogen transformations, microbial activity and plant growth was investigated in soil microcosms. These compounds are commercially marketed fermentation products (Agspectrum) that are sold to be added to field soils in small amounts to promote nitrogen and other nutrient uptake by crops in USA. In laboratory microcosm experiments, soils were amended with finely ground alfalfa-leaves or wheat straw, or left unamended, in an attempt to alter patterns of soil nitrogen mineralization and immobilization. Soils were treated in the microcosms with Z93 and W91 at rates equivalent to the recommended field application rates, that range from 0.2 to 1.1 l ha⁻¹, (0.005-0.03 μl g⁻¹ soil). We measured their effects on soil microbial activity (substrate-induced respiration (SIR), dehydrogenase activity (DHA) and acid phosphatase activity (PHOS)), soil nitrogen pools (microbial biomass N, mineral N, dissolved organic N), and transformations (net N mineralization and nitrification, ¹⁵N dilution of the mineral N pool, and accumulation of mineral N on ion-exchange resins), and on wheat plant germination and growth (shoot and root biomass, shoot length, N uptake and ¹⁵N enrichment of shoot tissues), for up to 56 days after treatment. To follow the movement of nitrogen from inorganic fertilizer into plant biomass we used a ¹⁵N isotopic tracer. Most of the soil and plant responses to treatment with Z93 or W91 differed according to the type of organic amendment that was used. Soil treatment with either Z93 or W91 influenced phosphatase activity strongly but did not have much effect on SIR or DHA. Both chemicals altered the rates of decomposition and mineralization of organic materials in the soil, which was evidenced by significant increases in the rates of the decomposition of buried wheat straw, and by the acceleration of net, rates of N mineralization, relative to those of the controls. Soil nitrate availability increased at the end of the experiment in response to both chemical treatments. In alfalfa-amended soils, the final plant biomass was decreased significantly by treatment with W91. Increased plant growth and N-use efficiency in straw-amended soil, resulting from treatments with Z93 or W91, was linked to increased rates of N mineralization from indigenous soil organic materials. This supports the marketing of these compounds as promoters of N uptake at these low dosage inputs.     

Soil N transformations after application of 15N‐labeled biomass in incubation experiments with repeated soil drying and rewetting

The effects of repeated soil drying and rewetting on microbial biomass N (N_bio) and mineral N (N_min) were measured in incubation experiments simulating typical moisture and temperature conditions for soils from temperate climates in the post‐harvest period. After application of in vitro ¹⁵N‐labeled fungal biomass to a silty loam, one set of soils was exposed to two drying‐rewetting cycles (treatment DR; 14 days to decrease soil moisture to 20 % water‐holding capacity (WHC) and subsequently 7 days at 60 % WHC). A control set (treatment CM) was kept at constant moisture conditions (60 % WHC) throughout the incubation. N_bio and N_min as well as the ¹⁵N enrichment of these N pools were measured immediately after addition of ¹⁵N‐labeled biomass (day 0) and after each change in soil moisture (day 14, 21, 35, 42). Drying and rewetting (DR) resulted in higher N_min levels compared to CM towards the end of the incubation. Considerable amounts of N_bio were susceptible to mineralization as a result of soil drying (i.e., drying enhanced the turnover of N_bio), and significantly lower N_bio values were found for DR at the end of each drying period. Immediately after biomass incorporation into the soil (day 0), 22 % of the applied ¹⁵N was found in the N_min pool. Some of this ¹⁵N_min must have been derived from dead cells of the applied microbial biomass as only about 80 % of the microbes in the biomass suspension were viable, and only 52 % of the ¹⁵N_bio was extractable (using the fumigation‐extraction method). The increase in ¹⁵N_min was higher than for unlabeled N_min, indicating that added labeled biomass was mineralized with a higher rate than native biomass during the first drying period. Overall, the effect of drying and rewetting on soil N turnover was more pronounced for treatment DR compared to CM during the second drying‐rewetting cycle, resulting in a higher flush of mineralization and lower microbial biomass N levels.