Microbial communities in terrestrial surface soils are not widely limited by carbon.

Microbial communities in soils are generally considered to be limited by carbon (C), which could be a crucial control for basic soil functions and responses of microbial heterotrophic metabolism to climate change. However, global soil microbial C limitation (MCL) has rarely been estimated and is poorly understood. Here, we predicted MCL, defined as limited availability of substrate C relative to nitrogen and/or phosphorus to meet microbial metabolic requirements, based on the thresholds of extracellular enzyme activity across 847 sites (2476 observations) representing global natural ecosystems. Results showed that only about 22% of global sites in terrestrial surface soils show relative C limitation in microbial community. This finding challenges the conventional hypothesis of ubiquitous C limitation for soil microbial metabolism. The limited geographic extent of C limitation in our study was mainly attributed to plant litter, rather than soil organic matter that has been processed by microbes, serving as the dominant C source for microbial acquisition. We also identified a significant latitudinal pattern of predicted MCL with larger C limitation at mid- to high latitudes, whereas this limitation was generally absent in the tropics. Moreover, MCL significantly constrained the rates of soil heterotrophic respiration, suggesting a potentially larger relative increase in respiration at mid- to high latitudes than low latitudes, if climate change increases primary productivity that alleviates MCL at higher latitudes. Our study provides the first global estimates of MCL, advancing our understanding of terrestrial C cycling and microbial metabolic feedback under global climate change.

Trade-offs in carbon-degrading enzyme activities limit long-term soil carbon sequestration with biochar addition.

Biochar amendment is one of the most promising agricultural approaches to tackle climate change by enhancing soil carbon (C) sequestration. Microbial-mediated decomposition processes are fundamental for the fate and persistence of sequestered C in soil, but the underlying mechanisms are uncertain. Here, we synthesise 923 observations regarding the effects of biochar addition (over periods ranging from several weeks to several years) on soil C-degrading enzyme activities from 130 articles across five continents worldwide. Our results showed that biochar addition increased soil ligninase activity targeting complex phenolic macromolecules by 7.1%, but suppressed cellulase activity degrading simpler polysaccharides by 8.3%. These shifts in enzyme activities explained the most variation of changes in soil C sequestration across a wide range of climatic, edaphic and experimental conditions, with biochar-induced shift in ligninase:cellulase ratio correlating negatively with soil C sequestration. Specifically, short-term (<1 year) biochar addition significantly reduced cellulase activity by 4.6% and enhanced soil organic C sequestration by 87.5%, whereas no significant responses were observed for ligninase activity and ligninase:cellulase ratio. However, long-term (≥1 year) biochar addition significantly enhanced ligninase activity by 5.2% and ligninase:cellulase ratio by 36.1%, leading to a smaller increase in soil organic C sequestration (25.1%). These results suggest that shifts in enzyme activities increased ligninase:cellulase ratio with time after biochar addition, limiting long-term soil C sequestration with biochar addition. Our work provides novel evidence to explain the diminished soil C sequestration with long-term biochar addition and suggests that earlier studies may have overestimated soil C sequestration with biochar addition by failing to consider the physiological acclimation of soil microorganisms over time.

Linking microbial functional gene abundance and soil extracellular enzyme activity: Implications for soil carbon dynamics.

Emerging evidence indicates that enzyme-catalyzed transformation and degradation of soil organic matter at the ecosystem scale is more likely driven by microbial functional gene abundance, rather than short term induction/repression responses. In this paper, we are trying to highlight the potential links between microbial functional gene abundance and soil extracellular enzyme activity. Those links will likely offer a new path for optimizing the model performance of microbial-mediated soil C dynamics from microbial functional gene perspectives.

Long-term nitrogen loading alleviates phosphorus limitation in terrestrial ecosystems.

Increased human-derived nitrogen (N) deposition to terrestrial ecosystems has resulted in widespread phosphorus (P) limitation of net primary productivity. However, it remains unclear if and how N-induced P limitation varies over time. Soil extracellular phosphatases catalyze the hydrolysis of P from soil organic matter, an important adaptive mechanism for ecosystems to cope with N-induced P limitation. Here we show, using a meta-analysis of 140 studies and 668 observations worldwide, that N stimulation of soil phosphatase activity diminishes over time. Whereas short-term N loading (≤5 years) significantly increased soil phosphatase activity by 28%, long-term N loading had no significant effect. Nitrogen loading did not affect soil available P and total P content in either short- or long-term studies. Together, these results suggest that N-induced P limitation in ecosystems is alleviated in the long-term through the initial stimulation of soil phosphatase activity, thereby securing P supply to support plant growth. Our results suggest that increases in terrestrial carbon uptake due to ongoing anthropogenic N loading may be greater than previously thought.

Soil carbon loss with warming: New evidence from carbon-degrading enzymes.

Climate warming affects soil carbon (C) dynamics, with possible serious consequences for soil C stocks and atmospheric CO2 concentrations. However, the mechanisms underlying changes in soil C storage are not well understood, hampering long-term predictions of climate C-feedbacks. The activity of the extracellular enzymes ligninase and cellulase can be used to track changes in the predominant C sources of soil microbes and can thus provide mechanistic insights into soil C loss pathways. Here we show, using meta-analysis, that reductions in soil C stocks with warming are associated with increased ratios of ligninase to cellulase activity. Furthermore, whereas long-term (≥5 years) warming reduced the soil recalcitrant C pool by 14%, short-term warming had no significant effect. Together, these results suggest that warming stimulates microbial utilization of recalcitrant C pools, possibly exacerbating long-term climate-C feedbacks.

A potential central role of Thaumarchaeota in N-Cycling in a semi-arid environment, Fort Stanton Cave, Snowy River passage, New Mexico, USA.

Low biomass and productivity of arid-land caves with limited availability of nitrogen (N) raises the question of how microbes acquire and cycle this essential element. Caves are ideal environments for investigating microbial functional capabilities, as they lack phototrophic activity and have near constant temperatures and high relative humidity. From the walls of Fort Stanton Cave (FSC), multicolored secondary mineral deposits of soil-like material low in fixed N, known as ferromanganese deposits (FMD), were collected. We hypothesized that within FMD samples we would find the presence of microbial N cycling genes and taxonomy related to N cycling microorganisms. Community DNA were sequenced using Illumina shotgun metagenomics and 16S rRNA gene sequencing. Results suggest a diverse N cycle encompassing several energetic pathways including nitrification, dissimilatory nitrate reduction and denitrification. N cycling genes associated with assimilatory nitrate reduction were also identified. Functional gene sequences and taxonomic findings suggest several bacterial and archaeal phyla potentially play a role in nitrification pathways in FSC and FMD. Thaumarchaeota, a deep-branching archaeal division, likely play an essential and possibly dominant role in the oxidation of ammonia. Our results provide genomic evidence for understanding how microbes are potentially able to acquire and cycle N in a low-nutrient subterranean environment.

Biocrusts benefit from plant removal.

PREMISE OF THE STUDY: Productivity in drylands may depend on the sensitivity of interactions between plants and biocrusts. Given future climate variability, it is essential to understand how interactions may be context-dependent with precipitation regime. Furthermore, little is known about the additional interactions of these producers with the belowground biota (e.g., roots, fungi, microarthropods). We evaluated the effect of removal (such as could occur following disturbance) and net interaction of plants and biocrusts and additionally manipulated the abiotic and biotic context. METHODS: We established field mesocosms containing grass (Bouteloua gracilis) and surrounding biocrusts, then clipped the plant or heat-sterilized the biocrust to simulate the loss of dryland producers. To test for context-dependency on the precipitation pattern, we imposed a large, infrequent or small, frequent precipitation regime. A mesh barrier was used to impede belowground connections that may couple the dynamics of producers. Productivity was assessed by plant biomass and biocrust chlorophyll content. KEY RESULTS: Biocrusts increased chlorophyll content more when plants were removed than when they were present in the first year, but only in the small, frequent precipitation regime. In contrast, plant growth slightly declined with biocrust removal. Plant biomass and biocrust chlorophyll content were negatively correlated in the second year, suggesting net competition. Belowground connectivity weakly promoted overall biocrust relative productivity, but was generally weakly detrimental to plant relative productivity. CONCLUSIONS: Altered precipitation patterns can amplify positive effects of plant removal on biocrust producers. Furthermore, we discovered that belowground networks contributed to dryland productivity by promoting biocrust performance.

Drought consistently alters the composition of soil fungal and bacterial communities in grasslands from two continents.

The effects of short-term drought on soil microbial communities remain largely unexplored, particularly at large scales and under field conditions. We used seven experimental sites from two continents (North America and Australia) to evaluate the impacts of imposed extreme drought on the abundance, community composition, richness, and function of soil bacterial and fungal communities. The sites encompassed different grassland ecosystems spanning a wide range of climatic and soil properties. Drought significantly altered the community composition of soil bacteria and, to a lesser extent, fungi in grasslands from two continents. The magnitude of the fungal community change was directly proportional to the precipitation gradient. This greater fungal sensitivity to drought at more mesic sites contrasts with the generally observed pattern of greater drought sensitivity of plant communities in more arid grasslands, suggesting that plant and microbial communities may respond differently along precipitation gradients. Actinobateria, and Chloroflexi, bacterial phyla typically dominant in dry environments, increased their relative abundance in response to drought, whereas Glomeromycetes, a fungal class regarded as widely symbiotic, decreased in relative abundance. The response of Chlamydiae and Tenericutes, two phyla of mostly pathogenic species, decreased and increased along the precipitation gradient, respectively. Soil enzyme activity consistently increased under drought, a response that was attributed to drought-induced changes in microbial community structure rather than to changes in abundance and diversity. Our results provide evidence that drought has a widespread effect on the assembly of microbial communities, one of the major drivers of soil function in terrestrial ecosystems. Such responses may have important implications for the provision of key ecosystem services, including nutrient cycling, and may result in the weakening of plant-microbial interactions and a greater incidence of certain soil-borne diseases.

Plant, microbial and ecosystem carbon use efficiencies interact to stabilize microbial growth as a fraction of gross primary production.

The carbon use efficiency of plants (CUEa ) and microorganisms (CUEh ) determines rates of biomass turnover and soil carbon sequestration. We evaluated the hypothesis that CUEa and CUEh counterbalance at a large scale, stabilizing microbial growth (µ) as a fraction of gross primary production (GPP). Collating data from published studies, we correlated annual CUEa , estimated from satellite imagery, with locally determined soil CUEh for 100 globally distributed sites. Ecosystem CUEe , the ratio of net ecosystem production (NEP) to GPP, was estimated for each site using published models. At the ecosystem scale, CUEa and CUEh were inversely related. At the global scale, the apparent temperature sensitivity of CUEh with respect to mean annual temperature (MAT) was similar for organic and mineral soils (0.029°C-1 ). CUEa and CUEe were inversely related to MAT, with apparent sensitivities of -0.009 and -0.032°C-1 , respectively. These trends constrain the ratio µ : GPP (= (CUEa  × CUEh )/(1 - CUEe )) with respect to MAT by counterbalancing the apparent temperature sensitivities of the component processes. At the ecosystem scale, the counterbalance is effected by modulating soil organic matter stocks. The results suggest that a µ : GPP value of c. 0.13 is a homeostatic steady state for ecosystem carbon fluxes at a large scale.

Global synthesis of the temperature sensitivity of leaf litter breakdown in streams and rivers.

Streams and rivers are important conduits of terrestrially derived carbon (C) to atmospheric and marine reservoirs. Leaf litter breakdown rates are expected to increase as water temperatures rise in response to climate change. The magnitude of increase in breakdown rates is uncertain, given differences in litter quality and microbial and detritivore community responses to temperature, factors that can influence the apparent temperature sensitivity of breakdown and the relative proportion of C lost to the atmosphere vs. stored or transported downstream. Here, we synthesized 1025 records of litter breakdown in streams and rivers to quantify its temperature sensitivity, as measured by the activation energy (Ea , in eV). Temperature sensitivity of litter breakdown varied among twelve plant genera for which Ea could be calculated. Higher values of Ea were correlated with lower-quality litter, but these correlations were influenced by a single, N-fixing genus (Alnus). Ea values converged when genera were classified into three breakdown rate categories, potentially due to continual water availability in streams and rivers modulating the influence of leaf chemistry on breakdown. Across all data representing 85 plant genera, the Ea was 0.34 ± 0.04 eV, or approximately half the value (0.65 eV) predicted by metabolic theory. Our results indicate that average breakdown rates may increase by 5-21% with a 1-4 °C rise in water temperature, rather than a 10-45% increase expected, according to metabolic theory. Differential warming of tropical and temperate biomes could result in a similar proportional increase in breakdown rates, despite variation in Ea values for these regions (0.75 ± 0.13 eV and 0.27 ± 0.05 eV, respectively). The relative proportions of gaseous C loss and organic matter transport downstream should not change with rising temperature given that Ea values for breakdown mediated by microbes alone and microbes plus detritivores were similar at the global scale.

Soil microbial responses to nitrogen addition in arid ecosystems.

The N cycle of arid ecosystems is influenced by low soil organic matter, high soil pH, and extremes in water potential and temperature that lead to open canopies and development of biological soil crusts (biocrusts). We investigated the effects of N amendment on soil microbial dynamics in a Larrea tridentata-Ambrosia dumosa shrubland site in southern Nevada USA. Sites were fertilized with a NO3-NH4 mix at 0, 7, and 15 kg N ha(-1) y(-1) from March 2012 to March 2013. In March 2013, biocrust (0-0.5 cm) and bulk soils (0-10 cm) were collected beneath Ambrosia canopies and in the interspaces between plants. Biomass responses were assessed as bacterial and fungal SSU rRNA gene copy number and chlorophyll a concentration. Metabolic responses were measured by five ecoenzyme activities and rates of N transformation. By most measures, nutrient availability, microbial biomass, and process rates were greater in soils beneath the shrub canopy compared to the interspace between plants, and greater in the surface biocrust horizon compared to the deeper 10 cm soil profile. Most measures responded positively to experimental N addition. Effect sizes were generally greater for bulk soil than biocrust. Results were incorporated into a meta-analysis of arid ecosystem responses to N amendment that included data from 14 other studies. Effect sizes were calculated for biomass and metabolic responses. Regressions of effect sizes, calculated for biomass, and metabolic responses, showed similar trends in relation to N application rate and N load (rate × duration). The critical points separating positive from negative treatment effects were 88 kg ha(-1) y(-1) and 159 kg ha(-1), respectively, for biomass, and 70 kg ha(-1) y(-1) and 114 kg ha(-1), respectively, for metabolism. These critical values are comparable to those for microbial biomass, decomposition rates and respiration reported in broader meta-analyses of N amendment effects in mesic ecosystems. However, large effect sizes at low N addition rates indicate that arid ecosystems are sensitive to modest increments in anthropogenic N deposition.

Assembly of root-associated bacteria communities: interactions between abiotic and biotic factors.

Nitrogen (N) deposition in many areas of the world is over an order of magnitude greater than it would be in absence of human activity. We ask how abiotic (N)and biotic (plant host and neighborhood) effects interact to influence root-associated bacterial (RAB)community assembly. Using 454 pyrosequencing, we examined RAB communities from two dominantal pine tundra plants, Geum rossii and Deschampsia cespitosa, under control, N addition and D. cespitosa removal treatments, implemented in a factorial design. We hypothesized that host would have the strongest effect on RAB assembly, followed by N,then neighbor effects.The most dominant phyla were Proteobacteria (mostly Gammaproteobacteria), Actinobacteria,Bacteroidetes and Acidobacteria. We found RAB communities were host specific, with only 17% overlap in operational taxonomic units. Host effects on composition were over twice as strong as Neffects. D. cespitosa RAB diversity declined with N, while G. rossii RAB did not. D. cespitosa removal did not influence G. rossii RAB community composition, but G. rossii RAB diversity declined with N only when D. cespitosa was absent. We conclude that RAB of both hosts are sensitive to N enrichment, and RAB response to N is influenced by host identity and plant neighborhood.

Nitrogen deposition alters plant-fungal relationships: linking belowground dynamics to aboveground vegetation change.

Nitrogen (N) deposition rates are increasing globally due to anthropogenic activities. Plant community responses to N are often attributed to altered competitive interactions between plants, but may also be a result of microbial responses to N, particularly root-associated fungi (RAF), which are known to affect plant fitness. In response to N, Deschampsia cespitosa, a codominant plant in the alpine tundra at Niwot Ridge (CO), increases in abundance, while Geum rossii, its principal competitor, declines. Importantly, G. rossii declines with N even in the absence of its competitor. We examined whether contrasting host responses to N are associated with altered plant-fungal symbioses, and whether the effects of N are distinct from effects of altered plant competition on RAF, using 454 pyrosequencing. Host RAF communities were distinct (only 9.4% of OTUs overlapped). N increased RAF diversity in G. rossii, but decreased it in D. cespitosa. D. cespitosa RAF communities were more responsive to N than G. rossii RAF communities, perhaps indicating a flexible microbial community aids host adaptation to nutrient enrichment. Effects of removing D. cespitosa were distinct from effects of N on G. rossii RAF, and D. cespitosa presence reversed RAF diversity response to N. The most dominant G. rossii RAF order, Helotiales, was the most affected by N, declining from 83% to 60% of sequences, perhaps indicating a loss of mutualists under N enrichment. These results highlight the potential importance of belowground microbial dynamics in plant responses to N deposition.

Diversity of Ammonia Oxidation (amoA) and Nitrogen Fixation (nifH) Genes in Lava Caves of Terceira, Azores, Portugal.

Lava caves are an understudied ecosystem in the subterranean world, particularly in regard to nitrogen cycling. The diversity of ammonia oxidation (amoA) and nitrogen fixation (nifH) genes in bacterial mats collected from lava cave walls on the island of Terceira (Azores, Portugal) was investigated using denaturing gradient gel electrophoresis (DGGE). A total of 55 samples were collected from 11 lava caves that were selected with regard to surface land use. Land use types above the lava caves were categorized into pasture, forested, and sea/urban, and used to determine if land use influenced the ammonia oxidizing and nitrogen fixing bacterial communities within the lava caves. The soil and water samples from each lava cave were analyzed for total organic carbon, inorganic carbon, total nitrogen, ammonium, nitrate, phosphate and sulfate, to determine if land use influences either the nutrient content entering the lava cave or the nitrogen cycling bacteria present within the cave. Nitrosospira-like sequences dominated the ammonia-oxidizing bacteria (AOB) community, and the majority of the diversity was found in lava caves under forested land. The nitrogen fixation community was dominated by Klebsiella pneumoniae-like sequences, and diversity was evenly distributed between pasture and forested land, but very little overlap in diversity was observed. The results suggest that land use is impacting both the AOB and the nitrogen fixing bacterial communities.

Field and lab conditions alter microbial enzyme and biomass dynamics driving decomposition of the same leaf litter.

Fluctuations in climate and edaphic factors influence field decomposition rates and preclude a complete understanding of how microbial communities respond to plant litter quality. In contrast, laboratory microcosms isolate the intrinsic effects of litter chemistry and microbial community from extrinsic effects of environmental variation. Used together, these paired approaches provide mechanistic insights to decomposition processes. In order to elucidate the microbial mechanisms underlying how environmental conditions alter the trajectory of decay, we characterized microbial biomass, respiration, enzyme activities, and nutrient dynamics during early (<10% mass loss), mid- (10-40% mass loss), and late (>40% mass loss) decay in parallel field and laboratory litter bag incubations for deciduous tree litters with varying recalcitrance (dogwood < maple < maple-oak mixture < oak). In the field, mass loss was minimal (<10%) over the first 50 days (January-February), even for labile litter types, despite above-freezing soil temperatures and adequate moisture during these winter months. In contrast, microcosms displayed high C mineralization rates in the first week. During mid-decay, the labile dogwood and maple litters in the field had higher mass loss per unit enzyme activity than the lab, possibly due to leaching of soluble compounds. Microbial biomass to litter mass (B:C) ratios peaked in the field during late decay, but B:C ratios declined between mid- and late decay in the lab. Thus, microbial biomass did not have a consistent relationship with litter quality between studies. Higher oxidative enzyme activities in oak litters in the field, and higher nitrogen (N) accumulation in the lab microcosms occurred in late decay. We speculate that elevated N suppressed fungal activity and/or biomass in microcosms. Our results suggest that differences in microbial biomass and enzyme dynamics alter the decay trajectory of the same leaf litter under field and lab conditions.

Carbon use efficiency of microbial communities: stoichiometry, methodology and modelling.

Carbon use efficiency (CUE) is a fundamental parameter for ecological models based on the physiology of microorganisms. CUE determines energy and material flows to higher trophic levels, conversion of plant-produced carbon into microbial products and rates of ecosystem carbon storage. Thermodynamic calculations support a maximum CUE value of ~ 0.60 (CUE max). Kinetic and stoichiometric constraints on microbial growth suggest that CUE in multi-resource limited natural systems should approach ~ 0.3 (CUE max /2). However, the mean CUE values reported for aquatic and terrestrial ecosystems differ by twofold (~ 0.26 vs. ~ 0.55) because the methods used to estimate CUE in aquatic and terrestrial systems generally differ and soil estimates are less likely to capture the full maintenance costs of community metabolism given the difficulty of measurements in water-limited environments. Moreover, many simulation models lack adequate representation of energy spilling pathways and stoichiometric constraints on metabolism, which can also lead to overestimates of CUE. We recommend that broad-scale models use a CUE value of 0.30, unless there is evidence for lower values as a result of pervasive nutrient limitations. Ecosystem models operating at finer scales should consider resource composition, stoichiometric constraints and biomass composition, as well as environmental drivers, to predict the CUE of microbial communities.

Diversity and distribution of soil fungal communities in a semiarid grassland.

The fungal loop model of semiarid ecosystems integrates microtopographic structures and pulse dynamics with key microbial processes. However limited data exist about the composition and structure of fungal communities in these ecosystems. The goal of this study was to characterize diversity and structure of soil fungal communities in a semiarid grassland. The effect of long-term nitrogen fertilization on fungi also was evaluated. Samples of rhizosphere (soil surrounding plant roots) and biological soil crust (BSC) were collected in central New Mexico, USA. DNA was amplified from the samples with fungal specific primers. Twelve clone libraries were generated with a total of 307 (78 operational taxonomic units, OTUs) and 324 sequences (67 OTUs) for BSC and rhizosphere respectively. Approximately 40% of soil OTUs were considered novel (less than 97% identity when compared to other sequences in NCBI using BLAST). The dominant organisms were dark-septate (melanized fungi) ascomycetes belonging to Pleosporales. Effects of N enrichment on fungi were not evident at the community level; however the abundance of unique sequences, sampling intensity and temporal variations may be uncovering the effect of N in composition and diversity of fungal communities. The fungal communities of rhizosphere soil and BSC overlapped substantially in composition, with a Jaccard abundance similarity index of 0.75. Further analyses are required to explore possible functions of the dominant species colonizing zones of semiarid grassland soils.

Shifting fungal endophyte communities colonize Bouteloua gracilis: effect of host tissue and geographical distribution.

Communities of root-associated fungi (RAF) commonly have been studied under the auspices of arbuscular mycorrhizal fungi (AMF) or ectomycorrhizal fungi. However many studies now indicate that other groups of endophytic RAF, including dark septate endophytes (DSE) are more abundant in some plants and environments. The common forage grass, Bouteloua gracilis, was used as a model to examine whether RAF also colonize different organs within the same plant and to compare RAF communities from sites across North America, spanning the latitudinal range of B. gracilis (from Canada to Mexico). We compared the RAF communities of organs within individual plants at one site and within plant roots among six sites. With the possible exception of one group related to genus Paraphaeosphaeria there was little evidence that RAF colonized vertically beyond the crowns. Furthermore, although there was some variation in the constitution of rare members of the RAF communities, several taxonomically related groups dominated the RAF community at all sites. These dominant taxa included members in the Pleosporales (related to the DSE, Paraphaeosphaeria spp.), Agaricales (related to Moniliophthora spp., or Campanella spp.) and Hypocreales (related to Fusarium spp.). AMF were notable by their near absence. Similar phylotypes from the dominant groups clustered around adjacent sites so that similarity of the RAF communities was negatively correlated to site inter-distance and the RAF communities appeared to group by country. These results increase the possibility that at least some of these common and widely distributed core members of the RAF community form important, intimate and long lasting relationships with grasses.

Ecoenzymatic stoichiometry in relation to productivity for freshwater biofilm and plankton communities.

The degradation of detrital organic matter and assimilation of carbon (C), nitrogen (N), and phosphorus (P) by heterotrophic microbial communities is mediated by enzymes released into the environment (ecoenzymes). For the attached microbial communities of soils and freshwater sediments, the activities of ß-glucosidase, ß-N-acetylglucosaminidase, leucine aminopeptidase, and phosphatase show consistent stoichiometric patterns. To determine whether similar constraints apply to planktonic communities, we assembled data from nine studies that include measurements of these enzyme activities along with microbial productivity. By normalizing enzyme activity to productivity, we directly compared the ecoenzymatic stoichiometry of aquatic biofilm and bacterioplankton communities. The relationships between ß-glucosidase and α-glucosidase and ß-glucosidase and ß-N-acetylglucosaminidase were statistically indistinguishable for the two community types, while the relationships between ß-glucosidase and phosphatase and ß-glucosidase and leucine aminopeptidase significantly differed. For ß-glucosidase vs. phosphatase, the differences in slope (biofilm 0.65, plankton 1.05) corresponded with differences in the mean elemental C:P ratio of microbial biomass (60 and 106, respectively). For ß-glucosidase vs. leucine aminopeptidase, differences in slope (0.80 and 1.02) did not correspond to differences in the mean elemental C:N of biomass (8.6 and 6.6). ß-N-Acetylglucosaminidase activity in biofilms was significantly greater than that of plankton, suggesting that aminosaccharides were a relatively more important N source for biofilms, perhaps because fungi are more abundant. The slopes of ß-glucosidase vs. (ß-N-acetylglucosaminidase + leucine aminopeptidase) regressions (biofilm 1.07, plankton 0.94) corresponded more closely to the estimated difference in mean biomass C:N. Despite major differences in physical structure and trophic organization, biofilm and plankton communities have similar ecoenzymatic stoichiometry in relation to productivity and biomass composition. These relationships can be integrated into the stoichiometric and metabolic theories of ecology and used to analyze community metabolism in relation to resource constraints.