Soil microbial dynamics and genetic diversity in soil under monoculture wheat grown in different long-term management systems

Organic matter incorporation into soil can increase nutrient availability to plants but it can affect soil microbial communities. These in turn influence soil fertility and plant growth. Soil biochemical and microbiological properties are indicators of soil quality, but there is still no consensus as to how these should be used. Recent developments in molecular biology have provided new tools to obtain a view of the whole microbial community. The long-term impact of crop residue management on the microbial biomass, and on the activity and community structure of soil bacteria was evaluated in a clay soil of Southern Italy, where a monoculture of durum wheat (Triticum durum Desf.) was grown in semiarid conditions, and burning or incorporation of post harvest plant residues were typical practices. The role of N-mineral fertilization, simultaneously with the ploughing in of crop residues and during the plant growth cycle was also investigated. Total bacterial counts of viable cells, biomass C, ATP content of soil microorganisms, genetic fingerprinting of the total eubacterial community and of ammonia oxidizers were evaluated. Burning and incorporation did not affect microbial biomass C, ATP content, and total bacterial counts of viable cells although statistically relevant changes were detected among rhizosphere and bulk soil samples regardless of the crop residue management used. Molecular fingerprinting confirmed that: no significant change in the composition and diversity of total bacteria, as well as of ammonia oxidizers was induced by the crop residue managements; that soil bacteria were more sensitive to N fertilizer application during the plant growth cycle; and that rhizosphere soil samples were significantly different from those of the bulk soil. As microbiological and genetic factors related to soil fertility were not affected significantly, the long-term incorporation of crop residues, under the field conditions investigated, is a sustainable practice to manage post-harvest residues.     

Global sensitivity and uncertainty analysis of the VIP ecosystem model with an expanded soil nitrogen module for winter wheat-summer maize rotation system in the North China Plain

Accurately simulating the soil nitrogen (N) cycle is crucial for assessing food security and resource utilization efficiency. The accuracy of model predictions relies heavily on model parameterization. The sensitivity and uncertainty of the simulations of soil N cycle of winter wheat-summer maize rotation system in the North China Plain (NCP) to the parameters were analyzed. First, the N module in the Vegetation Interface Processes (VIP) model was expanded to capture the dynamics of soil N cycle calibrated with field measurements in three ecological stations from 2000 to 2015. Second, the Morris and Sobol'' algorithms were adopted to identify the sensitive parameters that impact soil nitrate stock, denitrification rate, and ammonia volatilization rate. Finally, the shuffled complex evolution developed at the University of Arizona (SCE-UA) algorithm was used to optimize the selected sensitive parameters to improve prediction accuracy. The results showed that the sensitive parameters related to soil nitrate stock included the potential nitrification rate, Michaelis constant, microbial C/N ratio, and slow humus C/N ratio, the sensitive parameters related to denitrification rate were the potential denitrification rate, Michaelis constant, and N₂O production rate, and the sensitive parameters related to ammonia volatilization rate included the coefficient of ammonia volatilization exchange and potential nitrification rate. Based on the optimized parameters, prediction efficiency was notably increased with the highest coefficient of determination being approximately 0.8. Moreover, the average relative interval length at the 95% confidence level for soil nitrate stock, denitrification rate, and ammonia volatilization rate were 11.92, 0.008, and 4.26, respectively, and the percentages of coverage of the measured values in the 95% confidence interval were 68%, 86%, and 92%, respectively. By identifying sensitive parameters related to soil N, the expanded VIP model optimized by the SCE-UA algorithm can effectively simulate the dynamics of soil nitrate stock, denitrification rate, and ammonia volatilization rate in the NCP.     

Role of amendments on N cycling in Mediterranean abandoned semiarid soils

Soils found in semiarid areas of the Mediterranean Basin are particularly prone to degradation due to adverse climatic conditions with annual rainfall <300 mm and high temperatures being responsible for the scant vegetal growth and the consequent lack of organic matter. A three-year field experiment was conducted to test the potential of two organic amendments (sludge and compost) to improve soil quality and plant growth in a semiarid degraded Mediterranean ecosystem. Since little is known about N dynamics in such assisted ecosystems, we investigated the effects of this practice on key processes of the global N cycle. Besides soil chemical and biological parameters and vegetation cover, we measured absolute and specific potential nitrification and denitrification rates and quantified the size of the ammonia oxidising and denitrifying bacterial populations via quantitative PCR (amoA and nirS genes). At the end of the experiment soil fertility, microbial activity and plant growth had improved in treated plots. Amendments increased the amount of ammonia oxidisers and denitrifiers in soil, but the relative proportion of these groups varied in relation to the total microbial community, being higher in the case of ammonia oxidisers but not in the case of denitrifiers. As a consequence, significantly higher potential nitrification and denitrification rates were measured on a global basis in amended soils. Yet specific activities (potential rate/gene copy numbers) were lower for ammonia oxidisers in amended soils and for denitrifiers in sludge treated soils than those observed in control plots. Organic amendments influenced resource availability, the size and the activity patterns of microbial populations involved in long-term N dynamics. Therefore N cycling processes may play a key role to assist sustainable restoration practices in semiarid degraded areas.     

Denitrification potential in a grassland subsoil: effect of carbon substrates

We examined the potential of a subsoil to denitrify nitrate under optimal anaerobic conditions in a laboratory-based incubation when supplied with a range of C substrates of increasing recalcitrance. Both topsoil and its associated subsoil were supplied with nitrate and either glucose, starch or cellulose. Microbial respiration and the evolution of N₂O and N₂ were measured. The subsoil supported low amounts of microbial activity and responded only to the glucose treatment; with less than one-fifth of the N₂O production measured in the top soil. Overall, our findings demonstrated that the denitrification potential of this particular subsoil is relatively low and that only simple carbohydrates could be utilised readily by the resident microorganisms.     

土壤氮气排放研究进展

自20世纪初人类发明并掌握工业合成氨的技术以来,氮肥施用量迅速增长。在一部分国家或地区,氮肥的施入量已经超过作物对氮素的需求,导致大量氮素损失到环境中,造成氨挥发、氧化亚氮排放、地下水硝酸盐污染等环境问题。土壤在微生物的作用下可以通过反硝化、厌氧氨氧化等过程将活性氮素转化为惰性氮气,达到清除过多活性氮的目的。由于大气中氮气背景浓度太高,因此很难直接准确测定土壤的氮气排放速率,导致土壤氮气排放通量、过程与调控机制研究远远落后于土壤氮循环的其他方面。本文综述了土壤氮气排放主要途径(反硝化、厌氧氨氧化与共反硝化)及其对土壤氮气排放的贡献;测定土壤氮气排放速率的方法(乙炔抑制法、氮同位素示踪法、N2/Ar比率-膜进样质谱法、氦环境法与N2O同位素自然丰度法)及其优缺点;调控土壤氮气排放通量的主要因素(氧气、可溶性有机碳、硝酸盐、微生物群落结构与功能基因表达等)及其相关作用机制。最后指出研发新的测定原位无扰动土壤氮气通量的方法是推进本领域相关研究的关键;定量典型生态系统(如旱地农田、稻田、森林、草地与湿地)土壤氮气排放通量,阐明其中的微生物学机制,模拟并预测土壤氮气排放对全球变化的响应规律是本领域的研究热点与发展方向。     

脱硫废弃物改良宁夏盐渍化土壤对细菌和氨氧化微生物丰度的影响

为探明脱硫废弃物改良盐渍化土壤对微生物群落的影响效果,在2009~2010年,采用田间试验,施用不同量的脱硫废弃物(0、0.74、1.49、2.25、3.00 kg·m-2),研究了脱硫废弃物对盐渍化土壤细菌、氨氧化细菌和氨氧化古菌的影响。试验结果表明:0~20 cm土层,Ca2+和NO-3-N含量随着施用量增加而增加;土壤p H值、电导率值显著下降。实时荧光定量PCR(q PCR)分析结果表明,微生物丰度随着脱硫废弃物的施用发生变化,但这种变化并不与脱硫废弃物的施用量呈线性关系。在0~20 cm土壤层,施脱硫废弃物使得细菌16S rRNA基因拷贝数处理组显著高于对照组。氨氧化古菌与氨氧化细菌基因拷贝数在T2和T4处理高于其它处理。20~40 cm土层各处理间微生物群落没有显著变化,或没有出现规律的变化趋势。因此,脱硫废弃物增加了土壤细菌和氨氧化功能基因丰度,且对上层土壤影响更为显著。本研究中施用脱硫废弃物1.49 kg·m-2(T2)是引起细菌和氨氧化功能基因丰度增加的施用量。     

Long-term impact of fertilization on activity and composition of bacterial communities and metabolic guilds in agricultural soil

To explore long-term impact of organic and inorganic fertilizers on microbial communities, we targeted both the total bacterial community and the autotrophic ammonia oxidizing bacteria (AOB) in soil from six treatments at an experimental field site established in 1956: cattle manure, sewage sludge, Ca(NO₃)₂, (NH₄)₂SO₄, unfertilized and unfertilized without crops. All plots, except the bare fallows, were cropped with maize. Effects on activity were assessed by measuring the basal respiration and substrate induced respiration (SIR) rates, and the potential activity of the AOB. To determine the bacterial community composition, 16S rRNA genes were used to fingerprint total soil communities by terminal restriction fragment length polymorphism analysis and AOB communities by denaturing gradient gel electrophoresis. The fertilization regimes had clear effects on both activity and composition of the soil communities. Basal respiration and r, which was kinetically derived as the exponentially growing fraction of the SIR-response, correlated well with the soil organic C content (r=0.93 and 0.66, respectively). Soil pH ranged from 3.97 to 6.26 in the treatments and was found to be an important factor influencing all microbial activities. pH correlated negatively with the ratio between basal respiration and SIR (r=0.90), indicating a decreased efficiency of heterotrophic microorganisms to convert organic carbon into microbial biomass in the most acid soils with pH 3.97 and 4.68 ((NH₄)₂SO₄ and sewage sludge fertilized plots, respectively). The lowest SIR and ammonia oxidation rates were also found in these treatments. In addition, these treatments exhibited individually different community fingerprints, showing that pH affected the composition of AOB and total bacterial communities. The manure fertilized plots harbored the most diverse AOB community and the pattern was linked to a high potential ammonia oxidation activity. Thus, the AOB community composition appeared to be more strongly linked to the activity than the total bacterial communities were, likely explained by physiological differences in the populations present.     

Long-term nitrogen fertilization alters microbial community structure and denitrifier abundance in the deep vadose zone

Purpose

The excessive use of nitrogen (N) fertilizer in intensive agriculture has increased nitrate leaching into groundwater, but its impacts on N transformation processes and the associated microbial communities in the deep vadose zone remain unclear.

Materials and methods

Soil samples from 0–1050 cm depth were collected from a 20-year field experiment with two N fertilization treatments: 0 (N0) and 600 kg N ha^?1 year^?1 (N600). Amplicon sequencing and quantitative PCR analyses were performed to profile the vertical distribution of soil microbial communities and denitrification genes.

Results and discussion

The soil microbial community structure and diversity were strongly influenced by soil depth and N fertilization. The 250 cm depth was identified as a threshold depth, as dramatically different microbial communities were found below and above this depth. Quantitative PCR results showed that the absolute abundance of denitrification genes decreased with increasing soil depth.

Conclusion

This study elucidated the profound effects of long-term N input on the composition and diversity of the microbial communities and the abundance of denitrifiers in the deep vadose zone. Our results provide basic information for use in mitigating nitrate leaching by enhancing microbial denitrification in deep vadose zones in intensive agricultural areas.

     

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

Purpose

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

Materials and methods

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

Results and discussion

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

Conclusions

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

     

The chemical and functional characterization of soil N and its biotic components

The aim of this review is to describe and discuss the concepts that have been employed to interpret N mineralization-immobilization in soil, and how N turnover is related to the characteristics of organic N and the biota conducting the transformations. A brief survey of the period before the arrival of electronic searches became available provides access to the classical literature that can help interpret today's challenges. Classical (acid hydrolysis) and modern spectrometry and spectroscopy techniques indicate that protein N is the prevalent component of organic N in soil. The presence of heterocyclic N can indicate its abiotic, partial synthesis as in fire-affected soils. Clays and pedogenic oxides can protect organic N against microbial degradation. The evidence for such protection is mostly based on in vitro studies involving pure clays, and proteins and their relevance to field conditions requires further work. The proteomic approach, with extraction and characterization of proteins stabilised by soil colloids (structural proteomics) might give further insights into this area. Functional proteomics can improve our understanding of the degradation of organic pollutants and organic debris as well as identifying the molecular colloquia between microorganisms and between soil biota and plant roots. Subdivision of organic N into sub-pools has helped to interpret mechanistic studies and modelling of N dynamics. Uncomplexed organic matter, obtained by physical fractionation procedures, is considered a labile pool. The interpretation of N mineralization measurements is affected by immobilization during microbial attack especially in high-C environments. Transfer of materials among particle size fractions and changes in microbiological properties of aggregates also can occur during fractionation procedures. Classical mineralization-immobilization turnover (MIT) does not always occur since microorganisms (and plants) can take up amino acid N with intracellular deamination. Protozoa, due to their grazing activities, can influence not only N mineralization but also the composition of rhizosphere-plant growth stimulating communities. Differences between N-poor and N-rich microsites, occurring in the same soil, can markedly affect the competition for N between plants and microorganisms especially the nitrifiers. The use of molecular techniques has allowed the identification of unculturable microorganisms and functional genes in the N cycle. Archeae are probably capable of oxidising NH₄⁺ to NO₃⁻ and anerobic ammonia oxidation (Ammonox) bacteria have been identified in biofilms and probably also occur in soils. The use of nitrate as an electron acceptor is encoded by specific gene clusters but nitrate reduction also occurs in dissimilatory nitrate reduction.     

Heterotrophic N2-fixation in arid soil crusts

The cryptogamic soil crusts of the Great Basin Artemisia, Ceratoides, and Atriplex plant communities contain a significant heterotrophic N₂-fixing microbial population in addition to the predominating filamentous cyanobacteria. The bacterial association with the cyanobacteria exhibits a phycosphere-like effect. Heterotrophically fixed N gains reached 17.5 μg N· g^?1 of soil (23.1% increase above the initial soil N content) and 45.9 μg N·g^?1 of soil (57.4% increase) after 3 and 5 weeks, respectively. (NH₄)₂SO₄ and native plant material amendments to soil resulted in a 41–100% reduction in N₂-fixation. The potential input of N to soil crusts may be reduced in the presence of shrub-produced allelochemic agents and by concurrent denitrification.     

Mixed plantations can promote microbial integration and soil nitrate increases with changes in the N cycling genes

Mixed-species plantations of Eucalyptus and legume trees can symbiotically fix nitrogen and potentially improve the soil quality and biomass productivity compared with a conventional Eucalyptus monoculture. In this study, we evaluated changes in the structure and abundance of different microbial groups and nitrogen cycle genes in mixed and pure plantations of Acacia mangium and Eucalyptus urograndis in an experimental area in southeastern Brazil. Soil samples (0–10 cm) collected in two- and three-year-old stands were submitted to chemical characterization and molecular analyses using DGGE and real time-PCR for bacteria (16S rRNA), fungi (ITS), and genes involved in nitrogen cycling (nirK, amoA, nifH). The mixed plantation did not significantly change general soil fertility or total soil C and N content compared with the Eucalyptus monoculture. However, there was a significant increase in available phosphorus and soil nitrate content in both the A. mangium and mixed-species treatments. The multivariate ordination of the DGGE profiles of bacteria, fungi and archaea groups showed distinct community structures in each treatment. Significant differences in the abundance of copies of the target genes were found for fungi, with higher values in the Eucalyptus followed by the mixed and A. mangium plantations. The analysis of nitrogen cycle genes showed no clear difference in the communities of nitrogen fixing bacteria or nitrifying archaea among treatments. The nitrification activity was dominated by archaea because it was not possible to detect the presence of bacterial nitrifiers; the denitrifier community had a distinct profile in the Eucalyptus monoculture. The abundance of archaeal amoA and nirK genes suggests that the A. mangium treatment had higher nitrification and lower denitrification than the other treatments, which would explain the higher soil nitrate levels found in pure A. mangium treatments. Our data suggest that mixed plantations of E. urograndis and A. mangium result in a distinct microbial community relative to the respective monocultures with positive effects on soil phosphorus and nitrate content, which potentially reduces the need for anthropogenic fertilization.     

水分胁迫下土壤下层水分和硼对苔期油菜的作用

采用分层隔水盆栽试验 ,研究不同土壤状况下 ,表层土壤干旱 ,下层土壤湿润条件下 ,下层土壤中的水分和养分对油菜营养生理的影响 ,结果表明 ,土壤上下层干旱使油菜生长严重受抑 ,而下层湿润且硼含量充分的土壤上栽培的油菜可获得生长所需的水分和硼 ,从而改善其营养状况 ,增加其生物量 .土壤湿润情况下 ,油菜对土壤硼的利用效率增强 ,而土壤中硼各组分对油菜的有效性也有显著变化     

Stellera chamaejasme L. increases soil N availability, turnover rates and microbial biomass in an alpine meadow ecosystem on the eastern Tibetan Plateau of China

Plant species have been shown to have significant effects on soil nutrient pools and dynamics. Stellera chamaejasme L., a toxic perennial weed, has established and is now abundant in the alpine meadow on the eastern Tibetan Plateau of China since the 1960s. We quantified the effects of Stellera on carbon and nitrogen cycling in two topographic habitats, a flat valley and a south-facing slope, where Stellera was favored to spread within the study area. Aboveground litter biomass and tissue chemistry of aboveground litter and root were measured to explain the likely effects of Stellera on soil carbon and nutrient cycling. The sizes of various soil pools, e.g. nitrate, ammonium, inorganic phosphorus, microbial biomass, soil respiration and turnover rates including net mineralization, gross nitrification and denitrification were determined. The results showed that Stellera produced more aboveground litter than each of the co-occurring species. Aboveground litter of Stellera had higher tissue N and lower lignin:N than the other species. Stellera significantly increased surface soil (0-15 cm) organic matter, whereas no significant differences were found for organic C and total P in subsoil (15-30 cm) within and between patches of Stellera. Soil extractable nitrate concentrations in Stellera surface soil were 113% and 90% higher on the flat valley and on the south-facing slope, respectively. Both microbial biomass C and N were significantly higher in Stellera surface soil. Gross nitrification and microbial respiration were significantly higher in Stellera surface soil both on the flat valley and on the south-facing slope, whereas significant differences of denitrification were found only on the flat valley. The differences in the quantity and quality of aboveground litter are a likely mechanism responsible for the changes of soil properties.     

Availability of organic carbon for denitrification of nitrate in subsoils

Summary Previous work in our laboratory indicated that the slow rate of denitrification in Iowa subsoils is not due to a lack of denitrifying microorganisms, but to a lack of organic C that can be utilized by these microorganisms for reduction of NO ₃ ^– . This conclusion was supported by studies showing that drainage water from tile drains under agricultural research plots contained only trace amounts of organic C and had very little, if any, effect on denitrification in subsoils. Aqueous extracts of surface soils promoted denitrification when added to subsoils, and their ability to do so increased with increase in their organic C content. Amendment of surface soils with corn and soybean residues initially led to a marked increase in the amounts of organic C in aqueous extracts of these soils and in the ability of these extracts to promote denitrification in subsoils, but these effects were short-lived and could not be detected after incubation of residue-treated soils for a few days. We conclude from these observations that water-soluble organic C derived from plant residues is decomposed so rapidly in surface soils that very little of this C is leached into subsoils, and that this largely accounts for the slow rate of denitrification of nitrate in subsoils.     

Frontiers in the microbial processes of ammonia oxidation in soils and sediments

Purpose

Two recent discoveries in nitrogen (N) cycling processes, i.e., archaeal ammonia oxidizers and anaerobic ammonia (ammonium) oxidation (anammox), have triggered great interest in studying microbial ammonia oxidation processes. The purpose of this review is to highlight recent progress in ammonia oxidation processes in soils and sediments and to propose future research activities in this topic.

Results and discussion

Aerobic ammonia oxidation and anammox processes are linked through the production and consumption of nitrite, respectively, thereby removing the reactive N (NH₄ ⁺, NO₂ ^?, NO₃ ^?) from soil and sediment ecosystems. Ammonia-oxidizing microorganisms are widely distributed in soils and sediments, and increasing evidence suggests that ammonia-oxidizing archaea and bacteria are functionally dominant in the ammonia oxidation of acid soils and other soils, respectively. The widespread occurrence and great variation in the abundance of anammox bacteria indicate their heterogeneous distribution and niche differentiation. Therefore, the worldwide distribution of both microbial groups in nature has stimulated researchers to investigate the physiology and metabolism of related groups, as well as appraising their contribution to N cycling.

Conclusions

We summarized the current progress and provided future perspectives in the microbiology of aerobic and anaerobic ammonia oxidation in soils and sediments. With increasing concern and interest in soil and sediment ammonia oxidation processes, studies in the microbial mechanisms underlying nitrification and anammox, as well as their interactions, are essential for understanding their contribution to the loss of N either through nitrate leaching or N-related gas emissions.     

The role of ammonium oxidizing communities in mediating effects of an invasive plant on soil nitrification

Invasive plants often benefit from changes that they impose on soil microbes via positive plant–soil feedback, but the mechanisms that underlie these changes, and the legacy of their effects, remain poorly quantified. We investigated the impacts of an invasive annual grass, Microstegium vimineum, on the structure and functioning of soil microbial communities in a multi-year, field-based common garden experiment. Given previous reports that M. vimineum can both elevate nitrification rates in soil and benefit from enhanced nitrate availability, we sought to answer the following questions: 1) Does M. vimineum alter the abundance or composition of soil nitrifying microbial communities (ammonia oxidizing archaea and bacteria, AOA and AOB, respectively)? 2) Are such effects reversible or do soil legacy effects persist after M. vimineum is no longer present? After three years, invaded plots had greater AOA abundances than uninvaded native dominated plots, as well as different AOA community structure. However, after seven years, and following a period of M. vimineum replacement by native plants in the invaded plots, AOA abundances and nitrification rates declined toward levels found in uninvaded plots. Collectively, our results suggest that while the impacts of M. vimineum invasions on nitrogen cycling likely relate to their association with AOA, these effects may not persist if M. vimineum declines over time and native plants and their associated microbes are able to re-establish.     

Long-term effects of mineral fertilizers on soil microorganisms – A review

Increasing nutrient inputs into terrestrial ecosystems affect not only plant communities but also associated soil microbial communities. Studies carried out in predominantly unmanaged ecosystems have found that increasing nitrogen (N) inputs generally decrease soil microbial biomass; less is known about long-term impacts in managed systems such as agroecosystems. The objective of this paper was to analyze the responses of soil microorganisms to mineral fertilizer using data from long-term fertilization trials in cropping systems. A meta-analysis based on 107 datasets from 64 long-term trials from around the world revealed that mineral fertilizer application led to a 15.1% increase in the microbial biomass (C_mic) above levels in unfertilized control treatments. Mineral fertilization also increased soil organic carbon (C_org) content and our results suggest that C_org is a major factor contributing to the overall increase in C_mic with mineral fertilization. The magnitude of the effect of fertilization on C_mic was pH dependent. While fertilization tended to reduce C_mic in soils with a pH below 5 in the fertilized treatment, it had a significantly positive effect at higher soil pH values. Duration of the trial also affected the response of C_mic to fertilization, with increases in C_mic most pronounced in studies with a duration of at least 20 years. The input of N per se does not seem to negatively affect C_mic in cropping systems. The application of urea and ammonia fertilizers, however, can temporarily increase pH, osmotic potential and ammonia concentrations to levels inhibitory to microbial communities. Even though impacts of fertilizers are spatially limited, they may strongly affect soil microbial biomass and community composition in the short term. Long-term repeated mineral N applications may alter microbial community composition even when pH changes are small. How specific microbial groups respond to repeated applications of mineral fertilizers, however, varies considerably and seems to depend on environmental and crop management related factors.     

Identification of bacterial sources of soil peptidases

 Four topsoils and three subsoils from an arable field, two grasslands and a beech forest with different ecophysiological properties were investigated for the most abundant proteolytic bacteria. The number of proteolytic bacteria was estimated with the most probable number (MPN) assay using a gelatin-based medium. Subsequent isolations of bacteria were performed on a gelatin-based agar medium. No coherence was observed between site specific properties, MPN counts of proteolytic bacteria and proteolytic activities at the different sites. In the subsoils proteolytic activity was considerably lower than in the corresponding topsoils. Differences in MPN counts of proteolytic bacteria were only significant for the arable field profile in March, with lower values in the subsoil. Pseudomonas fluorescens was the most abundant proteolytic species in all investigated horizons except for the acidic topsoil of the beech forest. Bacillus cereus and B. mycoides were also prominent especially in the topsoils and were less abundant in the subsoils. Flavobacterium-Cytophaga bacteria were enriched in autumn, but were lacking in the beech forest horizons and in the topsoil of the arable field. The results of inhibition assays suggest that the extracellular peptidases formed by these species were metalloenzymes. Received: 20 May 1999     

Temporal variability in plant and soil nitrogen pools in a high-Arctic ecosystem

This study determined temporal variability in N pools, both aboveground and belowground, across two contrasting plant communities in high-Arctic Spitsbergen, Svalbard (78°N). We measured N pools in plant material, soil microbial biomass and soil organic matter in moist (Alopecurus borealis dominated) and dry (Dryas octopetala dominated) meadow communities at four times during the growing season. We found that plant, microbial and dissolved inorganic and organic N pools were subject to significant, but surprisingly low, temporal variation that was controlled primarily by changes in temperature and moisture availability over the short growing season. This temporal variability is much less than that experienced in other seasonally cold ecosystems such as alpine tundra where strong seasonal partitioning of N occurs between plant and soil microbial pools. While only a small proportion of the total ecosystem N, the microbial biomass represented the single largest of the dynamic N pools in both moist and dry meadow communities (3.4% and 4.6% of the total ecosystem N pool, respectively). This points to the importance of soil microbial community dynamics for N cycling in high-Arctic ecosystems. Microbial N was strongly and positively related to soil temperature in the dry meadow, but this relationship did not hold true in the wet meadow where other factors such as wetter soil conditions might constrain biological activity. Vascular live belowground plant parts represented the single largest plant N pool in both dry and moist meadow, constituting an average of 1.6% of the total N pool in both systems; this value did not vary across the growing season or between plant communities. Overall, our data illustrate a surprisingly low growing season variability in labile N pools in high-Arctic ecosystems, which we propose is controlled primarily by temperature and moisture.