Microbial activity and soil respiration under nitrogen addition in Alaskan boreal forest

Climate warming could increase rates of soil organic matter turnover and nutrient mineralization, particularly in northern high‐latitude ecosystems. However, the effects of increasing nutrient availability on microbial processes in these ecosystems are poorly understood. To determine how soil microbes respond to nutrient enrichment, we measured microbial biomass, extracellular enzyme activities, soil respiration, and the community composition of active fungi in nitrogen (N) fertilized soils of a boreal forest in central Alaska. We predicted that N addition would suppress fungal activity relative to bacteria, but stimulate carbon (C)‐degrading enzyme activities and soil respiration. Instead, we found no evidence for a suppression of fungal activity, although fungal sporocarp production declined significantly, and the relative abundance of two fungal taxa changed dramatically with N fertilization. Microbial biomass as measured by chloroform fumigation did not respond to fertilization, nor did the ratio of fungi : bacteria as measured by quantitative polymerase chain reaction. However, microbial biomass C : N ratios narrowed significantly from 16.0 ± 1.4 to 5.2 ± 0.3 with fertilization. N fertilization significantly increased the activity of a cellulose‐degrading enzyme and suppressed the activities of protein‐ and chitin‐degrading enzymes but had no effect on soil respiration rates or ¹⁴C signatures. These results indicate that N fertilization alters microbial community composition and allocation to extracellular enzyme production without affecting soil respiration. Thus, our results do not provide evidence for strong microbial feedbacks to the boreal C cycle under climate warming or N addition. However, organic N cycling may decline due to a reduction in the activity of enzymes that target nitrogenous compounds.     

Physiological shifts in the microbial community drive changes in enzyme activity in a perennial agroecosystem

Perennial agroecosystems have the potential to promote plant–microbial linkages by increasing the quantity of root carbon entering the soil. However, an understanding of how perennial cropping systems affect microbial communities remains incomplete. The objective of this study was to determine the potential for a fertilized perennial bioenergy cropping system to impact microbial growth and enzyme activity. Three times throughout the growing season we examined the activity of four enzymes involved in decomposition (ß-glucosidase, ß-xylosidase, cellobiohydrolase, and N-acetyl glucosaminidase) in replicated plots of an annual (corn) and perennial-based (switchgrass) cropping system. We also took simultaneous measurements of microbial biomass and potential rates of microbial respiration and net N mineralization. Microbial biomass was unaffected by cropping system. Mid-summer, however, we observed increases in enzyme activity and potential microbial respiration in the perennial system that were independent of microbial biomass, likely in response to labile carbon inputs. Further, we observed lower net N mineralization, higher microbial biomass nitrogen and higher activity of nitrogen liberating enzymes, which are indicative of a community with high nitrogen demands. Overall, our research demonstrates that perennial agroecosystems can affect the physiological capacity of the microbial community, yielding communities with greater nitrogen retention and greater rates of decomposition as a result of allocation of resources towards enzyme production and nitrogen mining. These results can inform biogeochemical models with respect to the importance of temporally dynamic changes in carbon and nitrogen availability and microbial carbon use efficiency as drivers of enzyme production.     

土壤微生物生物量碳氮磷与土壤酶化学计量对气候变化的响应机制

微生物和土壤酶是陆地生态系统中生物地球化学循环的重要驱动力,深入理解微生物在生态系统中的调节作用以及气候变化过程中微生物量和土壤酶的响应机制是生态学领域关注的重要科学问题.本研究从气候因素角度出发,基于生态化学计量学理论,综述了微生物和土壤酶在陆地生态系统碳氮磷循环中的作用,以及土壤微生物生物量碳氮磷和土壤酶化学计量对气候变化的响应机制,即: 改变微生物代谢速率和酶活性;调整微生物群落结构;调整微生物生物量碳氮磷与土壤酶化学计量特征;改变碳氮磷养分元素利用效率.最后分析当前研究的不足,并提出了该领域亟待解决的科学问题: 综合阐明土壤微生物和土壤酶对气候变化的响应机制;探究土壤微生物和胞外酶养分耦合机理;深入探究土壤微生物量和土壤酶化学计量特征对气候变化的适应对策.     

Plant and nutrient controls on microbial functional characteristics in a tropical Oxisol

Aims

Extracellular enzymes mediate the decomposition of organic matter and the release of plant-available nutrients. Current theory predicts that enzyme production by soil microbes is regulated by the stoichiometric demands of microbial biomass and the complexity of environmental resources, but most experiments ignore the potential effect of alleviated carbon limitation in the rhizosphere. Our objective was to investigate linkages between enzyme activities, soil nutrient availability and plant roots in a tropical Oxisol.

Methods

We conducted a greenhouse experiment using soils from the Luquillo Experimental Forest and seedlings of Tabebuia heterophylla. Planted and unplanted pots were fertilized with different combinations of phosphorus and either mineral nitrogen (ammonia chloride) or a nitrogen-rich organic compound (casein). We measured changes in plant and soil nutrients and five extracellular enzyme activities.

Results

Phosphatase activity declined by 28% in the P and 40% in the complex nitrogen treatment, while N-acetyl glucosaminidase increased 162% in the complex nitrogen treatment. Beta-glucosidase, beta-xylosidase, cellobiohydrolase and N-acetyl glucosaminidase all increased significantly over time in the simple nitrogen treatment (P?<?0.05).

Conclusions

Enzymatic responses support microbial resource allocation theory, that is, the concept that soil microbes regulate enzyme production based on scarcity of resources. However, we did not observe any additional effect of roots on extracellular enzyme activities. Enzymatic C:N, C:P and N:P ratios further support the notion that shifts in microbial stoichiometric demand drive responses to nutrients.     

Spatial polymorphism of bacteriocins and other allelopathic traits

Summary Many bacterial species carry plasmids that encode both the production of a highly specific toxin (bacteriocin) that kills competitors of the same species and immunity to that toxin. A great diversity of bacteriocins is produced within a single species. I present a model for the dynamics of competition between allelopathic and susceptible types. The model applies to most kinds of allelopathic competition. My primary goal is, however, to explain the widespread genetic polymorphism for bacteriocins. The model includes competition for scarce resources, competition through toxin production, spatial diffusion of individuals and toxins at varying rates and spatial variation in habitat quality. I draw three main conclusions from this reaction—diffusion model. (1) Polymorphism of toxin producers and susceptibles cannot be maintained within a single spatial location when individuals and the toxin mix randomly. (2) Susceptibles are generally favoured in poor habitats, where the rate of resource competition per interaction increases relative to the resource-independent rate of toxic killing. By contrast, toxic producers are generally favoured in good habitats, where the rate of resource competition is lower. (3) Spatial variation in habitat quality can lead to spatial polymorphism; susceptibles tend to win in poor habitats and producers tend to win in good habitats.     

The unseen majority: soil microbes as drivers of plant diversity and productivity in terrestrial ecosystems

Microbes are the unseen majority in soil and comprise a large portion of life's genetic diversity. Despite their abundance, the impact of soil microbes on ecosystem processes is still poorly understood. Here we explore the various roles that soil microbes play in terrestrial ecosystems with special emphasis on their contribution to plant productivity and diversity. Soil microbes are important regulators of plant productivity, especially in nutrient poor ecosystems where plant symbionts are responsible for the acquisition of limiting nutrients. Mycorrhizal fungi and nitrogen-fixing bacteria are responsible for c. 5–20% (grassland and savannah) to 80% (temperate and boreal forests) of all nitrogen, and up to 75% of phosphorus, that is acquired by plants annually. Free-living microbes also strongly regulate plant productivity, through the mineralization of, and competition for, nutrients that sustain plant productivity. Soil microbes, including microbial pathogens, are also important regulators of plant community dynamics and plant diversity, determining plant abundance and, in some cases, facilitating invasion by exotic plants. Conservative estimates suggest that c. 20 000 plant species are completely dependent on microbial symbionts for growth and survival pointing to the importance of soil microbes as regulators of plant species richness on Earth. Overall, this review shows that soil microbes must be considered as important drivers of plant diversity and productivity in terrestrial ecosystems.     

Enzyme activity responses to nutrient loading in subtropical wetlands

Eutrophication caused by anthropogenic nutrient inputs is one of the greatest threats to the integrity of freshwater wetlands. The resultant changes in organic carbon cycling and nutrient mineralization may be expressed through increased decomposition rates, which are ultimately dependent on the metabolism of the resident microbial community. Specifically, microbial nutrient acquisition is controlled through the activity of enzymes, which are in turn influenced by local biogeochemical conditions. This study examines enzyme activities along distinct North-South P gradients within four distinct hydrologic units of the Florida Everglades. The results indicate that nutrient enriched sites exhibit lower N and P limitations on microbially constrained C mineralization, in addition to enhanced cellulose decomposition rates. Nutrient loading resulted in decreased microbial mobilization of resources for P mineralization, resulting in greater energetic allocation for C mineralization. Additionally, N appears to become less limiting to C mineralization in the enriched sites within Everglades National Park, the least P enriched area within the Everglades. A simple two component model, incorporating total P and the relationship between the enzymes involved in C and P mineralization accounted for between 46 and 92% of the variability in measured cellulose decomposition rates and thus demonstrates the significant influence that P loading plays in these systems. These results also suggest there is an environmental threshold TP concentration below which changes in enzyme-based resource allocation will not occur.     

Ecoenzymatic stoichiometry of microbial nutrient acquisition in tropical soils

The relative activities of soil enzymes involved in mineralizing organic carbon (C), nitrogen (N), and phosphorus (P) reveal stoichiometric and energetic constraints on microbial biomass growth. Although tropical forests and grasslands are a major component of the global C cycle, the effects of soil nutrient availability on microbial activity and C dynamics in these ecosystems are poorly understood. To explore potential microbial nutrient limitation in relation to enzyme allocation in low latitude ecosystems, we performed a meta-analysis of acid/alkaline phosphatase (AP), β-1,4-glucosidase (BG), and β-1,4-N-acetyl-glucosaminidase (NAG) activities in tropical soils. We found that BG:AP and NAG:AP ratios in tropical soils are significantly lower than those of temperate ecosystems overall. The lowest BG:AP and NAG:AP ratios were associated with old or acid soils, consistent with greater biological phosphorus demand relative to P availability. Additionally, correlations between enzyme activities and mean annual temperature and precipitation suggest some climatic regulation of microbial enzyme allocation in tropical soils. We used the results of our analysis in conjunction with previously published data on soil and biomass C:N:P stoichiometry to parameterize a biogeochemical equilibrium model that relates microbial growth efficiency to extracellular enzyme activity. The model predicts low microbial growth efficiencies in P-limited soils, indicating that P availability may influence C cycling in the highly weathered soils that underlie many tropical ecosystems. Therefore, we suggest that P availability be included in models that simulate microbial enzyme allocation, biomass growth, and C mineralization.     

Leaf phenological shifts and plant–microbe–soil interactions can determine forest productivity and nutrient cycling under climate change in an ecosystem model

Climate change is expected to affect tree leaf phenology by extending the length of the growing season (LGS), which will affect the productivity and nutrient cycling of forests. Interactions between plants and microbes will mediate the ecosystem processes further through microbe-mediated plant–soil feedback (PSF). To investigate the possible consequences of interactions between the extension of the growing season (GS) and PSF under various conditions, we developed a simple theoretical model (LGS-PSF model). The LGS-PSF model predicts that microbe-mediated PSF will intensify the negative effects of increasing temperature on the size of soil carbon stock when compared with simulations without the PSF effect. The combined effects of increasing temperature and PSF on the size of soil carbon stock occurs through enhanced activity of individual microbes and increased microbial population size. More importantly, the model also demonstrated that a longer GS mitigates this negative effect on carbon accumulation in soil, not through increased net primary production, but through intensified competition for nutrients between plants and microbes, thus suppressing microbial population growth. Our model suggested that the interactive effects of the LGS and PSF on carbon and nitrogen dynamics in forests should be incorporated into larger scale quantitative models for better forecasting of future forest functions under climate change.     

Tannin impacts on microbial diversity and the functioning of alpine soils: a multidisciplinary approach

In alpine ecosystems, tannin-rich-litter decomposition occurs mainly under snow. With global change, variations in snowfall might affect soil temperature and microbial diversity with biogeochemical consequences on ecosystem processes. However, the relationships linking soil temperature and tannin degradation with soil microorganisms and nutrients fluxes remain poorly understood. Here, we combined biogeochemical and molecular profiling approaches to monitor tannin degradation, nutrient cycling and microbial communities (Bacteria, Crenarcheotes, Fungi) in undisturbed wintertime soil cores exposed to low temperature (0°C/−6°C), amended or not with tannins, extracted from Dryas octopetala . No toxic effect of tannins on microbial populations was found, indicating that they withstand phenolics from alpine vegetation litter. Additionally at −6°C, higher carbon mineralization, higher protocatechuic acid concentration (intermediary metabolite of tannin catabolism), and changes in fungal phylogenetic composition showed that freezing temperatures may select fungi able to degrade D. octopetala 's tannins. In contrast, negative net nitrogen mineralization rates were observed at −6°C possibly due to a more efficient N immobilization by tannins than N production by microbial activities, and suggesting a decoupling between C and N mineralization. Our results confirmed tannins and soil temperatures as relevant controls of microbial catabolism which are crucial for alpine ecosystems functioning and carbon storage.     

Seasonal and Spatial Controls over Nutrient Cycling in a Pacific Northwest Prairie

West Coast prairies in the US are an endangered ecosystem, and effective conservation will require an understanding of how changing climate will impact nutrient cycling and availability. We examined how seasonal patterns and micro-heterogeneity in edaphic conditions (% moisture, total organic carbon, % clay, pH, and inorganic nitrogen and phosphorus) control carbon, nitrogen, and phosphorus cycling in an upland prairie in western Oregon, USA. Across the prairie, we collected soils seasonally and measured microbial respiration, net nitrogen mineralization, net nitrification, and phosphorus availability under field conditions and under experimentally varied temperature and moisture treatments. The response variables differed in the degree of temperature and moisture limitation within seasons and how these factors varied across sampling sites. In general, we found that microbial respiration was limited by low soil moisture year-round and by low temperatures in the winter. Net nitrogen mineralization and net nitrification were never limited by temperature, but both were limited by excessive soil moisture in winter, and net nitrification was also inhibited by low soil moisture in the summer. Factors that enhanced microbial respiration tended to decrease soil phosphorus availability. Edaphic factors explained 76% of the seasonal and spatial variation in microbial respiration, 35% of the variation in phosphorus availability, and 29% of the variation in net nitrification. Much of the variation in net nitrogen mineralization remained unexplained (R ² = 0.19). This study, for the first time, demonstrates the complex seasonal controls over nutrient cycling in a Pacific Northwest prairie.     

Effect of substrate on the regulation of exoprotease production by Pseudomonas aeruginosa ATCC 10145

Exoprotease production by Pseudomonas aeruginosa ATCC 10145 was growth-associated when cultures were grown on complex substrates such as proteins but it occurred during the decelerating growth phase when the organism was grown on amino acids, mixtures of amino acids or simple carbon sources. NH4Cl and simple carbon sources caused repression. Exoprotease was produced in chemostat cultures in response to growth under any of the nutrient limitations studied (carbon, nitrogen or phosphate). Furthermore, by growing at rates less than approximately 0.1 h-1, the repression of enzyme production could be overcome to a large degree. At low growth rates there was an inverse relationship between growth rate and exoprotease production. Thus, exoprotease production was depressed by available energy sources and was increased in response to any nutrient limitation.     

Concepts of sulfur,carbon, and nitrogen transformations in soil: evaluation by simulation modeling

The dynamics of sulfur immobilization and mineralization in soil were simulated to test hypotheses about their regulation by the availability of carbon and nitrogen. The concept of chemical bond classes was incorporated into the model to account for variation in composition of carbon, nitrogen, and sulfur compounds. Microbial biomass was differentiated into bacteria and fungi, and the element ratios of both groups were assumed to vary. Organic residues were divided between dead microbes plus microbial products, and the more labile fraction of stabilized soil organic matter. Concepts and hypotheses in the model were tested by applying it to data on microbial biomass, sulfate, nitrate, and CO₂ evolution obtained in laboratory incubations of two soils amended with sulfate and cellulose. An important mechanism of regulation tested in the model was the stimulation of sulfohydrolase enzyme production depending on sulfur stress in microbial biomass. The hypothesis that excess sulfate is stored as ester sulfate was supported by model dynamics.     

Turnover of labile and recalcitrant soil carbon differ in response to nitrate and ammonium deposition in an ombrotrophic peatland

The effects of 4 years of simulated nitrogen deposition, as nitrate (NO₃^?) and ammonium (NH₄⁺), on microbial carbon turnover were studied in an ombrotrophic peatland. We investigated the mineralization of simple forms of carbon using MicroResp^? measurements (a multiple substrate induced respiration technique) and the activities of four soil enzymes involved in the decomposition of more complex forms of carbon or in nutrient acquisition: N‐acetyl‐glucosaminidase (NAG), cellobiohydrolase (CBH), acid phosphatase (AP), and phenol oxidase (PO). The potential mineralization of labile forms of carbon was significantly enhanced at the higher N additions, especially with NH₄⁺ amendments, while potential enzyme activities involved in breakdown of more complex forms of carbon or nutrient acquisition decreased slightly (NAG and CBH) or remained unchanged (AP and PO) with N amendments. This study also showed the importance of distinguishing between NO₃^? and NH₄⁺ amendments, as their impact often differed. It is possible that the limited response on potential extracellular enzyme activity is due to other factors, such as limited exposure to the added N in the deeper soil or continued suboptimal functioning of the enzymes due to the low pH, possibly via the inhibitory effect of low phenol oxidase activity.     

Implication of <Emphasis Type="Italic">Bacillus</Emphasis> sp. in the production of pectinolytic enzymes during cocoa fermentation

The role of bacilli in cocoa fermentation is not well known. Their potential of production of pectinolytic enzymes during this process was evaluated. Bacillus growth was monitored and pectinolytic strains were screened for their use of pectin as sole carbon source. Effects of cocoa fermentation parameters susceptible to influence on enzyme production were analysed. Among 98 strains isolated, 90 were positive for pectin degradation and 80% of them presented detectable pectinolytic activities in submerged fermentation. Forty-eight strains produced polygalacturonase (PG), 47 yielded pectin lyase (PL) and 23 strains produced both enzymes. Bacilli growth was not significantly affected during fermentation. PL production was favoured by galactose, lactose, glucose as sugars, and arginine, glutamine, cysteine and ammonium sulphate as nitrogen compounds. Pectin at low concentration (0.05%) and iron stimulated PL production. It was strongly repressed by galacturonic acid (1%), and negatively affected by nitrogen starvation, zinc and temperatures above 45°C. PL yield was very weak below pH 4.0 and in anaerobic conditions. PG production was weakened by sucrose and cation depletion. It was increased slightly by cysteine, ammonium nitrate and nitrogen starvation and significantly above 40°C. PG synthesis was not affected by acidic pH (3.0–6.0) or oxygen availability. As fermentation products, lactate and acetate lowered the production of both enzymes while ethanol had no effect. The high proportion of pectinolytic producers among the strains studied and analysis of factors influencing pectinolytic enzymes production, suggest that Bacillus sp. is liable to produce at least one enzyme during cocoa fermentation.     

帽儿山地区不同土地利用方式下土壤-微生物-矿化碳氮化学计量特征

土地利用方式的变化导致土壤碳氮含量及其化学计量关系的变化,然而土壤微生物化学计量及其驱动的碳氮矿化过程如何响应这种变化仍不明确。以帽儿山地区天然落叶阔叶林、人工红松林、草地和农田4种不同土地利用类型为对象,测定其土壤有机碳(C_(soil))、全氮(N_(soil))、微生物生物量碳和氮(C_(mic)和N_(mic))、土壤碳和氮矿化速率(C_(min)和N_(min)),旨在比较不同土地利用方式对土壤、微生物碳氮化学计量特征及矿化速率的影响,探索土壤-微生物-矿化之间碳氮化学计量特征的相关性,揭示微生物对土壤碳氮化学计量变化的响应和调控机制。结果显示:C_(soil)、N_(soil)、C_(mic)、N_(mic)和C_(min)均呈现天然落叶阔叶林人工红松林草地农田,而天然落叶阔叶林和草地的N_(min)显著高于人工红松林和农田。土地利用方式显著影响土壤和微生物碳氮比(C∶N_(soil)和C∶N_(mic)),均呈现农田最高。不同土地利用方式的数据综合分析发现:碳氮矿化速率比与C∶N_(mic)呈负相关,而和微生物与土壤碳氮化学计量不平衡性(C∶N_(imb))显著正相关。单位微生物生物量的碳矿化速率(qCO_2)随着C∶N_(mic)的增加而降低,而单位微生物生物量的氮矿化速率(qAN)随着C∶N_(mic)的增加而增加。C∶N_(imb)与qCO_2正相关,与qAN负相关。以上结果表明,微生物会通过改变自身碳氮化学计量、调整碳氮之间相对矿化速率,以适应土地利用变化导致的土壤碳氮及其化学计量的变异性,以满足自身生长和代谢的碳氮需求平衡。     

The influence of dissolved nutrients and particulate organic matter quality on microbial respiration and biomass in a forest stream

1. Although dissolved nutrients and the quality of particulate organic matter (POM) influence microbial processes in aquatic systems, these factors have rarely been considered simultaneously. We manipulated dissolved nutrient concentrations and POM type in three contiguous reaches (reference, nitrogen, nitrogen + phosphorus) of a low nutrient, third‐order stream at Hubbard Brook Experimental Forest (U.S.A). In each reach we placed species of leaves (mean C : N of 68 and C : P of 2284) and wood (mean C : N of 721 and C : P of 60 654) that differed in elemental composition. We measured the respiration and biomass of microbes associated with this POM before and after nutrient addition. 2. Before nutrient addition, microbial respiration rates and biomass were higher for leaves than for wood. Respiration rates of microbes associated with wood showed a larger response to increased dissolved nutrient concentrations than respiration rates of microbes associated with leaves, suggesting that the response of microbes to increased dissolved nutrients was influenced by the quality of their substrate. 3. Overall, dissolved nutrients had strong positive effects on microbial respiration and fungal, but not bacterial, biomass, indicating that microbial respiration and fungi were nutrient limited. The concentration of nitrate in the enriched reaches was within the range of natural variation in forest streams, suggesting that natural variation in nitrate among forest streams influences carbon mineralisation and fungal biomass.     

Feeding by omnivores increases food available to consumers

Consumers are usually thought of as negatively affecting producers, but they can affect them positively by releasing nutrients (nutrient regeneration). The net effects of consumers on producers should depend on the balance between the effects of consumption and nutrient regeneration. In aquatic habitats, nutrient regeneration by consumers may increase microbial activity on leaf detritus as well as algal production, which in turn may stimulate further nutrient release and benefit herbivores or detritivores by increasing food quantity or quality. Omnivores can regenerate nutrients from animals, algae and detritus, creating diverse nutrient pathways. Many tadpoles are omnivores, and their nutrient regeneration may be important in aquatic food webs. To reveal the nutrient pathways created by tadpoles and examine whether omnivorous tadpoles can have positive effects on producers and consumers, we experimentally examined the effects of nutrient regeneration by three densities of tadpoles on primary producers, leaf litter, and other consumers in tank mesocosms. Tadpole exclosures were placed inside each mesocosm, allowing us to separate direct consumption effects from indirect nutrient regeneration effects. Nutrient regeneration caused by the herbivorous and carnivorous feeding activities of tadpoles positively affected rates of production of benthic algae, phytoplankton, and herbivorous benthic chironomid larvae, and rates of mineralization of leaf litter. The increased production of benthic algae and chironomid larvae was consumed by the tadpoles themselves, leaving no net change in the standing biomass of these resources. Our experiment thus demonstrated that omnivores created complicated nutrient pathways and accelerated rates of primary production and growth rates of other consumers, leading to increased rates of food availability to the omnivores themselves. Interactions of this nature may be common in many systems and could strongly moderate the effects of consumers on their resources and each other.     

酶化学计量揭示5年氮添加加剧毛竹林土壤微生物碳磷限制

土壤胞外酶活性和酶化学计量比能很好地反映土壤养分有效性和微生物对养分的需求变化。然而,氮(N)沉降对亚热带森林土壤微生物养分相对限制情况的影响尚不清楚。通过在亚热带毛竹林进行N添加试验来模拟N沉降,并在试验满5年时进行取样,测定不同处理下土壤养分和与碳(C)、N、磷(P)循环相关的酶活性,利用酶化学计量比及矢量分析探究微生物的养分分配情况。结果表明: N添加显著降低土壤可溶性有机碳、有效磷含量,显著提高有效氮含量。此外,N添加显著降低β-N-乙酰氨基葡糖苷酶(NAG)活性和NAG/微生物生物量碳(MBC),显著提高酸性磷酸酶(ACP)和ACP/MBC。低N和中N处理显著提高酶C/N、矢量长度和矢量角度,但显著降低酶N/P。冗余分析表明,N添加下,土壤有效磷含量的变化是影响土壤酶活性及酶化学计量比变化的主要因子。综上可知,N添加改变了微生物的养分获取策略,即通过减少分配给合成N获取酶的养分来增加合成P获取酶的养分。此外,N添加还加剧了微生物的C、P限制,未来可以施加适量P肥来提高亚热带毛竹林的土壤肥力。     

Bacterial quorum sensing and nitrogen cycling in rhizosphere soil

Plant photosynthate fuels carbon-limited microbial growth and activity, resulting in increased rhizosphere nitrogen (N) mineralization. Most soil organic nitrogen is macromolecular (chitin, protein, nucleotides); enzymatic depolymerization is likely rate limiting for plant nitrogen accumulation. Analyzing Avena (wild oat) planted in microcosms containing sieved field soil, we observed increased rhizosphere chitinase and protease-specific activities, bacterial cell densities, and dissolved organic nitrogen (DON) compared with bulk soil. Low-molecular-weight (MW) DON (<3000 Da) was undetectable in bulk soil but comprised 15% of rhizosphere DON. Extracellular enzyme production in many bacteria requires quorum sensing (QS), cell-density-dependent group behavior. Because proteobacteria are considered major rhizosphere colonizers, we assayed the proteobacterial QS signals N-acyl-homoserine lactones (AHLs), which were significantly increased in the rhizosphere. To investigate the linkage between soil signaling and nitrogen cycling, we characterized 533 bacterial isolates from Avena rhizosphere: 24% had chitinase or protease activity and AHL production; disruption of QS in seven of eight isolates disrupted enzyme activity. Many Alphaproteobacteria were newly found with QS-controlled extracellular enzyme activity. Enhanced specific activities of nitrogen-cycling enzymes accompanied by bacterial density-dependent behaviors in rhizosphere soil gives rise to the hypothesis that QS could be a control point in the complex process of rhizosphere nitrogen mineralization.