Structure of biological soil crust communities in Callitris glaucophylla woodlands of New South Wales,Australia

Question: What is the nature of the relationships between cover, diversity and abundance of biological soil crusts, cover and diversity of vascular plants, and annual rainfall, soil texture and forestry practices in Callitris glaucophylla woodlands? Location: Arid and semi‐arid Callitris glaucophylla‐domi‐nated woodlands of eastern Australia. Methods: We documented soil crust‐forming mosses, lichens and liverworts at 83 woodland sites along a gradient of declining rainfall. Linear and non‐linear regression were used to examine relationships between soil crust species and attributes of vascular plant communities, and a similarity matrix (species abundance X sites) was subjected to Non‐metric Multi‐Dimensional Scaling (MDS), and Analysis of Similarities (ANOSIM) to show the degree of association between groups of taxa, and soil texture, rainfall classes and forestry practices. Results : We collected 86 taxa. Mosses were dominated by the family Pottiaceae, and lichens were dominated by squamulose forms. Average annual rainfall was highly correlated with soil crust community composition, and loamy soils supported a greater cover and diversity of taxa compared with sandy soils. Increases in tree cover were associated with significant, though weak, increases in abundance, but not diversity, of crusts. Crusts tended to be more diverse in areas that (1) had a sparse cover of ground‐storey plants; (2) were relatively stable ‐ as indicated by the proportion of perennial and/or native plants; (3) had more stable soil surfaces; and (4) were unlogged. Litter cover, overstorey thinning, and livestock grazing had no appreciable effect on crust diversity or cover. Conclusions : Callitris glaucophylla woodlands provide substantial habitat for soil crust organisms, and the dense tree cover and closed canopies of Callitris do not appear to have a major influence on the structure of biological crust communities. Unlike other woodland systems, relatively few patches would be required to reserve a high diversity of crust species.     

Green algae in alpine biological soil crust communities: acclimation strategies against ultraviolet radiation and dehydration

Green algae are major components of biological soil crusts in alpine habitats. Together with cyanobacteria, fungi and lichens, green algae form a pioneer community important for the organisms that will succeed them. In their high altitudinal habitat these algae are exposed to harsh and strongly fluctuating environmental conditions, mainly intense irradiation, including ultraviolet radiation, and lack of water leading to desiccation. Therefore, green algae surviving in these environments must have evolved with either avoidance or protective strategies, as well as repair mechanisms for damage. In this review we have highlighted these mechanisms, which include photoprotection, photochemical quenching, and high osmotic values to avoid water loss, and in some groups flexibility of secondary cell walls to maintain turgor pressure even in water-limited situations. These highly specialized green algae will serve as good model organisms to study desiccation tolerance or photoprotective mechanisms, due to their natural capacity to withstand unfavorable conditions. We point out the urgent need for modern phylogenetic approaches in characterizing these organisms, and molecular methods for analyzing the metabolic changes involved in their adaptive strategies.     

Carbon exchange in biological soil crust communities under differential temperatures and soil water contents: implications for global change

Biological soil crusts (biocrusts) are an integral part of the soil system in arid regions worldwide, stabilizing soil surfaces, aiding vascular plant establishment, and are significant sources of ecosystem nitrogen and carbon. Hydration and temperature primarily control ecosystem CO₂ flux in these systems. Using constructed mesocosms for incubations under controlled laboratory conditions, we examined the effect of temperature (5–35 °C) and water content (WC, 20–100%) on CO₂ exchange in light (cyanobacterially dominated) and dark (cyanobacteria/lichen and moss dominated) biocrusts of the cool Colorado Plateau Desert in Utah and the hot Chihuahuan Desert in New Mexico. In light crusts from both Utah and New Mexico, net photosynthesis was highest at temperatures >30 °C. Net photosynthesis in light crusts from Utah was relatively insensitive to changes in soil moisture. In contrast, light crusts from New Mexico tended to exhibit higher rates of net photosynthesis at higher soil moisture. Dark crusts originating from both sites exhibited the greatest net photosynthesis at intermediate soil water content (40–60%). Declines in net photosynthesis were observed in dark crusts with crusts from Utah showing declines at temperatures >25 °C and those originating from New Mexico showing declines at temperatures >35 °C. Maximum net photosynthesis in all crust types from all locations were strongly influenced by offsets in the optimal temperature and water content for gross photosynthesis compared with dark respiration. Gross photosynthesis tended to be maximized at some intermediate value of temperature and water content and dark respiration tended to increase linearly. The results of this study suggest biocrusts are capable of CO₂ exchange under a wide range of conditions. However, significant changes in the magnitude of this exchange should be expected for the temperature and precipitation changes suggested by current climate models.     

生物土壤结皮演替对土壤生态化学计量特征的影响

生物土壤结皮(生物结皮)是干旱半干旱地区重要的地表覆被物,能够固定碳氮,影响养分循环,从而可能引起土壤化学计量特征的变化。以黄土丘陵区不同演替阶段生物结皮为研究对象,研究该区生态恢复初期生物结皮演替对土壤碳(C)、氮(N)、磷(P)含量及其化学计量特征的影响。结果表明:(1)生物结皮显著增加了生物结皮层C、N、P含量,后期藓结皮较初期藻结皮C、N、P含量分别增加了161%、127%和9%,均显著高于其下0—10 cm土壤;(2)土壤C/N随着生物结皮演替变化较小,范围为10.0—11.8,C/P和N/P在演替后期分别是初期的2.4倍和2.1倍,均显著高于其下0—10 cm土壤;(3)生物结皮土壤C/N、C/P与N/P受坡向影响较大,并与藓生物量显著正相关,与土壤容重显著负相关;(4)生物结皮显著影响土壤C、N、P含量及其化学计量特征,使生物结皮层显著不同于其下层土壤及裸地;(5)生物结皮演替加速表层土壤养分恢复,影响程度可至其下2 cm土层。研究初次从土壤化学计量特征的角度揭示生物结皮对土壤养分循环的贡献,对干旱半干旱地区生态修复和管理具有指导意义。     

生物土壤结皮对荒漠土壤线虫群落的影响

在干旱的沙漠生态系统中,生物土壤结皮对于沙丘的固定和土壤生物的维持起着相当重要的作用.土壤线虫能敏感的指示土壤的恢复程度,是衡量沙区生态恢复与健康的重要生物学属性,而目前关于生物土壤结皮与土壤线虫的关系研究很少.为探明生物土壤结皮对土壤线虫的影响,以腾格里沙漠东南缘的人工植被固沙区藻结皮和藓类结皮覆盖的沙丘土壤为研究对象,根据固沙年限的不同将样地分为4个不同的区进行采样(1956、1964、1981和1991年固沙区),以流沙区作为对照;同时,在不同季度(4、7、9和12月)分别采集腾格里沙漠东南缘的人工植被固沙区藻结皮和藓类结皮覆盖下不同土层(0-10、10-20和20-30 cm)的沙丘土壤,以沙坡头地区的红卫天然植被区为对照,分析生物土壤结皮下土壤线虫的时空变化.采用改良的Baermann漏斗法进行分离线虫,用光学显微镜鉴定并统计.研究表明:1956、1964、1981和1991年人工植被固沙区的藻结皮和藓类结皮均可显著提高其下土壤线虫多度、属的丰富度、Shannon-Weaver多样性指数、富集指数和结构指数(P＜0.05),这可能是因为生物土壤结皮的存在为土壤线虫提供了重要的食物来源和适宜的生存环境;固沙年限与结皮下土壤线虫多度、属的丰富度、Shannon-Weaver多样性指数、富集指数和结构指数存在显著的正相关关系(P＜0.05),这说明固沙年限越久,越有利于土壤线虫的生存和繁衍;结皮类型显著影响土壤线虫群落,相对于藻结皮而言,藓类结皮下土壤线虫多度与属的丰富度更高(P＜0.05),这说明演替后期的藓类结皮比演替早期的藻结皮更有利于土壤线虫的生存和繁衍.此外,藻结皮和藓类结皮均可显著增加其下0-10、10-20和20-30 cm土层线虫多度和属的丰富度(P＜0.05),但随着土壤深度的增加,这种影响逐渐减弱,表明生物土壤结皮更有利于表层土壤线虫的生存;而且,随着季节的变化,藻结皮和藓类结皮下土壤线虫多度基本表现为秋季＞夏季＞春季＞冬季,这些反映了生物土壤结皮的生物量、盖度和种类组成随着季节变化而变化.因此,腾格里沙漠东南缘的人工植被固沙区生物土壤结皮的存在与演替有利于土壤线虫的生存和繁衍,增加了线虫数量、种类和多样性,这指示了生物土壤结皮有利于该区土壤及其相应生态系统的恢复.     

Dynamics of cover, UV-protective pigments, and quantum yield in biological soil crust communities of an undisturbed Mojave Desert shrubland

Biological soil crusts are an integral part of dryland ecosystems. We monitored the cover of lichens and mosses, cyanobacterial biomass, concentrations of UV-protective pigments in both free-living and lichenized cyanobacteria, and quantum yield in the soil lichen species Collema in an undisturbed Mojave Desert shrubland. During our sampling time, the site received historically high and low levels of precipitation, whereas temperatures were close to normal. Lichen cover, dominated by Collema tenax and C. coccophorum, and moss cover, dominated by Syntrichia caninervis, responded to both increases and decreases in precipitation. This finding for Collema spp. at a hot Mojave Desert site is in contrast to a similar study conducted at a cool desert site on the Colorado Plateau in SE Utah, USA, where Collema spp. cover dropped in response to elevated temperatures, but did not respond to changes in rainfall. The concentrations of UV-protective pigments in free-living cyanobacteria at the Mojave Desert site were also strongly and positively related to rainfall received between sampling times (R² values ranged from 0.78 to 0.99). However, pigment levels in the lichenized cyanobacteria showed little correlation with rainfall. Quantum yield in Collema spp. was closely correlated with rainfall. Climate models in this region predict a 3.5–4.0 °C rise in temperature and a 15–20% decline in winter precipitation by 2099. Based on our data, this rise in temperature is unlikely to have a strong effect on the dominant species of the soil crusts. However, the predicted drop in precipitation will likely lead to a decrease in soil lichen and moss cover, and high stress or mortality in soil cyanobacteria as levels of UV-protective pigments decline. In addition, surface-disturbing activities (e.g., recreation, military activities, fire) are rapidly increasing in the Mojave Desert, and these disturbances quickly remove soil lichens and mosses. These stresses combined are likely to lead to shifts in species composition and the local extirpation of some lichen or moss species. As these organisms are critical components of nutrient cycling, soil fertility, and soil stability, such changes are likely to reverberate throughout these ecosystems.     

厌氧氨氧化生物脱氮技术的研究进展

厌氧氨氧化是指在厌氧条件下,厌氧氨氧化混合菌直接以NH4 为电子供体,以NO3-或NO2-为电子受体,将NH4^ 、NO3-或NO2-转变成N2的过程。厌氧氨氧化作为一种新型的污水处理工艺具有较高的理论意义和良好的应用前景。本文主要阐述了厌氧氨氧化生物脱氮技术原理、厌氧氨氧化的可能途径、方法及其应用现状,并且讨论了厌氧氨氧化反应的微生物学机理和厌氧氨氧化工艺的开发,提出了今后研究的主要方向。     

生物土壤结皮对照相法测量植被覆盖度结果的影响

植被覆盖度测度的准确性很大程度上影响着研究结论是否科学合理。在干旱半干旱退化草原区,尤其是受采矿剧烈扰动的矿区,发育的生物土壤结皮(Biological soil crust,BSC)由于其颜色和光谱同绿色植被具有相似性,导致对植被覆盖度的测量存在一定的影响。以伊敏露天矿区为研究区,在西排土场和内排土场采集了含苔藓结皮、地衣结皮和藻结皮的样方相片各四组(每组中包含样方喷水前和喷水后的相片各一张),并采集了一组不含结皮的样方相片作为对照组,运用数码照相法提取植被覆盖度,通过不同的数据处理方法(最大似然分类法及RGB阈值法)进行植被覆盖度提取,设立对比试验,分析BSC对于植被覆盖度测度是否有影响,其影响大小如何,影响程度是否受BSC含水量大小的影响,并对比各常规处理方法的优劣,研究能否通过结合纹理特征与色彩信息剔除BSC对植被覆盖度提取值的影响。研究结论:1)基于照相法的常规数据处理方法提取植被覆盖度时,BSC的存在导致测得的植被覆盖度值偏高,且苔藓结皮、地衣结皮吸水后比吸水前影响更显著,藻结皮相反;2)3个演替阶段的BSC中,尤以含苔藓结皮的样方植被覆盖度高估最为明显,其次为地衣,而含藻结皮样方规律不明显;3)样方内BSC覆盖度越高,植被覆盖度越低,其植被覆盖度测度越不准确,因此在研究草原矿区这类草本植物覆盖度较低、结皮发育的区域时,应当注意BSC的影响;4)试通过应用纹理信息提出改进的提取方法,发现单纯的纹理分类精度极低,而结合了纹理信息与RGB色彩信息的分类精度较高;5)对两种常规分类方法的精度进行比较,RGB阈值法较最大似然分类法更为不准确,对植被覆盖度的高估接近最大似然分类法的2倍。对两种改进的提取方法的精度进行比较,二者都可以有效提高测量精度,基于波段合成的纹理分类方法最佳。四种方法精度由高到低的顺序为:纹理结合RGB法考虑生物土壤结皮的最大似然分类法普通最大似然分类法RGB阈值法。     

Spatial heterogeneity of eukaryotic microbial communities in an unstudied geothermal diatomaceous biological soil crust: Yellowstone National Park, WY, USA

Knowledge of microbial communities and their inherent heterogeneity has dramatically increased with the widespread use of high-throughput sequencing technologies, and we are learning more about the ecological processes that structure microbial communities across a wide range of environments, as well as the relative scales of importance for describing bacterial communities in natural systems. Little work has been carried out to assess fine-scale eukaryotic microbial heterogeneity in soils. Here, we present findings from a bar-coded 18S rRNA survey of the eukaryotic microbial communities in a previously unstudied geothermal diatomaceous biological soil crust in Yellowstone National Park, WY, USA, in which we explicitly compare microbial community heterogeneity at the particle scale within soil cores. Multivariate analysis of community composition showed that while subsamples from within the same soil core clustered together, community dissimilarity between particles in the same core was high. This study describes a novel soil microbial environment and also adds to our growing understanding of microbial heterogeneity and the scales relevant to the study of soil microbial communities.     

Practices of biological soil crust rehabilitation in China: experiences and challenges

Biological soil crusts (biocrusts) are a central component of dryland ecosystems. However, they are highly vulnerable to disturbance and natural recovery may be slow. Therefore, finding ways to enhance the reestablishment of biocrusts after disturbance has been of great interest to researchers. This article provides a review of the laboratory cultivation and field inoculations of biocrust materials in China (mostly published in Chinese). Larger filamentous cyanobacteria (e.g. Microcoleus) are relatively easy, although slow, to grow in culture compared to other biocrust components. Thus, most researchers have focused their efforts on the cyanobacteria and a few species of mosses that are also easily grown but at smaller scale. For all the studies, a small amount of biocrust material was collected and its biomass enhanced under controlled conditions. However, the enhancement was done using various methods and techniques in different regions. These materials were then applied to disturbed field sites, again with various methods. Results show that keeping the inoculated soil surface wet for some time period after inoculation was crucial for restoration success. Cyanobacterial establishment was improved by installing automatic sprinkling using micro‐irrigation techniques and/or physical structures that reduced sediment moving onto the inoculated area. Experimental applications in China showed that cyanobacteria can be successfully inoculated at a large scale (hundreds of ha). Moss inoculation, on the other hand, was only accomplished at a small scale (several m²). To assess whether biocrust restoration can enhance the establishment of a self‐supporting ecosystem, further research is needed on how inoculation affects vegetation diversity and structure and ecological processes.     

Activation of photosynthesis and resistance to photoinhibition in cyanobacteria within biological desert crust

Filamentous cyanobacteria are the main primary producers in biological desert sand crusts. The cells are exposed to extreme environmental conditions including temperature, light, and diurnal desiccation/rehydration cycles. We have studied the kinetics of activation of photosynthesis during rehydration of the cyanobacteria, primarily Microcoleus sp., within crust samples collected in the Negev desert, Israel. We also investigated their susceptibility to photoinhibition. Activation of the photosynthetic apparatus, measured by fluorescence kinetics, thermoluminescence, and low temperature fluorescence emission spectra, did not require de novo protein synthesis. Over 50% of the photosystem II (PSII) activity, assembled phycobilisomes, and photosystem I (PSI) antennae were detected within less than 5 min of rehydration. Energy transfer to PSII and PSI by the respective antennae was fully established within 10 to 20 min of rehydration. The activation of a fraction of PSII population (about 20%-30%) was light and temperature-dependent but did not require electron flow to plastoquinone [was not inhibited by 3-(3,4-dichlorophenyl)-1,1-dimethylurea]. The cyanobacteria within the crusts are remarkably resistant to photoinhibition even in the absence of protein synthesis. The rate of PSII repair increased with light intensity and with time of exposure. Consequently, the extent of photoinhibition in high-light-exposed crusts reached a constant, relatively low, level. This is in contrast to model organisms such as Synechocystis sp. strain PCC 6803 where PSII activity declined continuously over the entire exposure to high illumination. Ability of the crust's organisms to rapidly activate photosynthesis upon rehydration and withstand photoinhibition under high light intensity may partly explain their ability to survive in this ecosystem.     

黄土丘陵区生物结皮对土壤可蚀性的影响

在采样分析生物结皮对土壤理化属性影响的基础上,采用EPIC模型估算与模拟降雨试验相结合的方法,研究了黄土丘陵区不同生物量、不同土壤质地和不同季节的生物结皮对土壤可蚀性(K值)的影响.结果表明： 生物结皮显著降低了土壤可蚀性,生物结皮层土壤可蚀性较下层土壤降低17%;土壤可蚀性随生物结皮生物量的增加呈降低趋势,藓结皮土壤可蚀性K值较藻结皮土壤降低21%;生物结皮土壤可蚀性在不同季节因其生物活性不同而存在差异,雨季中显著高于雨季前和雨季末;不同质地土壤上生物结皮对可蚀性的影响不同,可蚀性K值为砂壤>粉壤>砂土;模拟降雨条件下测定表明,生物结皮的发育使土壤可蚀性较对照（下层5~10 cm土壤）降低约90%.
     

Inoculation and habitat amelioration efforts in biological soil crust recovery vary by desert and soil texture

As dryland degradation continues, it is increasingly important to understand how to effectively restore biocrust communities. Potential techniques include the addition of biocrust inoculum to accelerate biocrust recovery. Enhanced erosion typical of degraded environments creates a challenge for these approaches, due to loss by wind or water and burial by saltating particles. To retain and protect added inoculum, the inclusion of habitat‐amelioration techniques can improve recovery rates. This study tested three different types of inoculum (field‐collected, greenhouse‐cultivated, and laboratory‐cultivated biocrust) coupled with two treatments to augment soil stability and ameliorate habitat limitations: soil surface polyacrylamide additions and installation of straw barriers. This was done across two deserts (Great Basin and Chihuahuan) and separated into generally coarse‐ or finer‐textured soils in each desert, with results monitored for 3 years (2015, 2016, 2017). While the inoculum type, coupled with habitat ameliorations, occasionally enhanced biocrust growth across years and treatments, in other cases, it made no difference compared to natural recovery rates. Rather, the desert location and soil texture groupings were the most prominent factors in determining recovery trajectories. Recovery proportions were similar in the finer‐textured sites in both the Great Basin and the Chihuahuan deserts, while the coarser‐textured site in the Great Basin did show some recovery over time and the Chihuahuan coarser‐textured site did not. This study demonstrates the importance of understanding site potential and identifying key limitations to biocrust recovery for successful restoration projects.     

冻融条件下生物结皮覆盖对土壤饱和导水率的影响

生物结皮(BSC)是广泛分布的地被物,每逢冬春季节,受冻融交替作用影响,结皮土壤的理化性质和水文学特征明显改变且与裸土差异显著,从而影响该地区土壤可蚀性评估和土壤侵蚀防治。采用室内模拟实验,以蓝藻结皮土壤为对象,研究不同冻融交替次数和初始含水量下,土壤三相对温度变化的响应特征并定量分析结皮覆盖土壤在此条件下饱和导水率(Ks)的变化趋势和突变点。结果表明:初始含水量对Ks无显著影响(P>0.05),冻融交替次数对Ks有极显著影响(P<0.01),冻融条件下裸土的平均Ks为1.941 mm/min,结皮覆盖土壤平均Ks为0.325 mm/min,两者具有极显著差异(P<0.01),且随交替次数增加,Ks差异逐渐增大,并在10次时达到最大值为10.13倍。不同冻融含水量下的结皮土壤的Ks在冻融10—20次时趋近,平均值为0.219mm/min。冻融作用显著改变土壤结构,且在冻融7次时土壤结构变化较明显,冻融过程中<0.1 mm的土壤颗粒显著变化。试验条件下,Ks受因子影响程度大小为:冻融交替次数>土壤结构>结皮厚度>结皮容重>下层土壤容重>...     

浑善达克沙地生物土壤结皮及其下层土壤中固氮细菌群落结构和多样性

【背景】荒漠化是一个重大环境问题,生物土壤结皮(Biological soil crusts,BSCs)可遏制荒漠化,其中的固氮微生物对BSCs的形成和发育有重要作用,但目前BSCs中固氮细菌群落结构和多样性尚不十分清楚。【目的】阐明浑善达克沙地中不同类型生物土壤结皮及其下层土壤固氮细菌的群落结构、多样性及其影响因素。【方法】利用稀释热法和碱解扩散法检测土壤的有机质(Organic matter,OM)和速效氮(Available nitrogen,AN)含量;利用高通量测序对nifH基因进行测序,基于nifH序列比较分析固氮细菌群落结构和多样性;利用典范对应分析(Canonical correlation analysis,CCA)分析群落、样品和土壤理化参数的相关性。【结果】固氮细菌优势菌门除在苔藓结皮(HSM)中为Cyanobacteria和Proteobacteria外,在其他类型BSCs中均只为Cyanobacteria;苔藓结皮下层土壤(HSMs)(下层土壤中只有HSMs检测到了nifH)优势菌门为Proteobacteria,优势菌纲为Alphaproteobacteria和Betaproteobacteria;优势菌属差异较大,藻结皮(HSA)中Unclassified_f_Nostocaceae占90.99%;地衣结皮(HSL)中Scytonema和Unclassified_f_Nostocaceae分别占45.85%和44.14%;HSM中Unclassified_f_Nostocaceae、Scytonema、Nostoc、Skermanella、Unclassified_o_Nostocales分别占29.21%、22.57%、15.34%、14.74%和10.60%;HSMs中Skermanella、Azohydromonas、Unclassified_p_Proteobacteria、Unclassified_c_Alphaproteobacteria分别占33.80%、25.66%、18.20%和10.62%;固氮细菌多样性随结皮的发育逐渐提高;OM和AN对结皮的发育起促进作用。【结论】藻结皮、地衣结皮和苔藓结皮及其紧邻下层土壤中的固氮细菌群落结构和多样性差异明显,且固氮细菌类群和多样性指数随BSCs发育阶段的提高而增加。本研究为认识和利用生物土壤结皮相关固氮细菌提供了基础依据。     

黄土丘陵区生物结皮对土壤可蚀性的影响

在采样分析生物结皮对土壤理化属性影响的基础上,采用EPIC模型估算与模拟降雨试验相结合的方法,研究了黄土丘陵区不同生物量、不同土壤质地和不同季节的生物结皮对土壤可蚀性(K值)的影响.结果表明： 生物结皮显著降低了土壤可蚀性,生物结皮层土壤可蚀性较下层土壤降低17%;土壤可蚀性随生物结皮生物量的增加呈降低趋势,藓结皮土壤可蚀性K值较藻结皮土壤降低21%;生物结皮土壤可蚀性在不同季节因其生物活性不同而存在差异,雨季中显著高于雨季前和雨季末;不同质地土壤上生物结皮对可蚀性的影响不同,可蚀性K值为砂壤>粉壤>砂土;模拟降雨条件下测定表明,生物结皮的发育使土壤可蚀性较对照（下层5~10 cm土壤）降低约90%.
     

Non-cyanobacterial diazotrophs mediate dinitrogen fixation in biological soil crusts during early crust formation

Biological soil crusts (BSCs) are key components of ecosystem productivity in arid lands and they cover a substantial fraction of the terrestrial surface. In particular, BSC N₂-fixation contributes significantly to the nitrogen (N) budget of arid land ecosystems. In mature crusts, N₂-fixation is largely attributed to heterocystous cyanobacteria; however, early successional crusts possess few N₂-fixing cyanobacteria and this suggests that microorganisms other than cyanobacteria mediate N₂-fixation during the critical early stages of BSC development. DNA stable isotope probing with ¹⁵N₂ revealed that Clostridiaceae and Proteobacteria are the most common microorganisms that assimilate ¹⁵N₂ in early successional crusts. The Clostridiaceae identified are divergent from previously characterized isolates, though N₂-fixation has previously been observed in this family. The Proteobacteria identified share >98.5% small subunit rRNA gene sequence identity with isolates from genera known to possess diazotrophs (for example, Pseudomonas, Klebsiella, Shigella and Ideonella). The low abundance of these heterotrophic diazotrophs in BSCs may explain why they have not been characterized previously. Diazotrophs have a critical role in BSC formation and characterization of these organisms represents a crucial step towards understanding how anthropogenic change will affect the formation and ecological function of BSCs in arid ecosystems.