生物结皮分布格局对坡面流水动力特征的影响

生物结皮的分布格局是影响坡面产流产沙的因素之一,但其格局对侵蚀动力的影响机理尚不明确,妨碍了进一步解析生物结皮影响坡面产流产沙的机制。本研究通过室内模拟降雨试验,采用景观生态学方法,对带状、棋盘和随机3种分布格局下生物结皮坡面景观指数和水力学参数的变化规律与裸土坡面进行对比研究。结果表明: 生物结皮显著影响坡面侵蚀动力,其分布格局是影响坡面侵蚀动力的因素之一。与裸土相比,生物结皮显著降低了坡面流速(54.6%)和弗劳德数(67.0%),显著增加了径流深(86.2%)和阻力系数(10.68倍),但对雷诺数和径流功率的影响较小;除径流深外,3种格局的生物结皮坡面的各水力学参数均存在显著差异,其中,随机格局对坡面侵蚀动力的影响最大。基于因子分析和聚类分析,筛选出斑块占景观面积比例、斑块密度、景观形状指数、斑块连结度和分离度5个指标作为描述生物结皮斑块分布特征的指标。其中,生物结皮斑块间的斑块连结度和分离度是影响坡面侵蚀动力的主要格局因子。随着生物结皮斑块间斑块连结度减小、分离度增大,坡面流速增加,阻力减小,坡面侵蚀加剧。     

Pathways regulating decreased soil respiration with warming in a biocrust‐dominated dryland

A positive soil carbon (C)‐climate feedback is embedded into the climatic models of the IPCC. However, recent global syntheses indicate that the temperature sensitivity of soil respiration (R_S) in drylands, the largest biome on Earth, is actually lower in warmed than in control plots. Consequently, soil C losses with future warming are expected to be low compared with other biomes. Nevertheless, the empirical basis for these global extrapolations is still poor in drylands, due to the low number of field experiments testing the pathways behind the long‐term responses of soil respiration (R_S) to warming. Importantly, global drylands are covered with biocrusts (communities formed by bryophytes, lichens, cyanobacteria, fungi, and bacteria), and thus, R_S responses to warming may be driven by both autotrophic and heterotrophic pathways. Here, we evaluated the effects of 8‐year experimental warming on R_S, and the different pathways involved, in a biocrust‐dominated dryland in southern Spain. We also assessed the overall impacts on soil organic C (SOC) accumulation over time. Across the years and biocrust cover levels, warming reduced R_S by 0.30 μmol CO₂ m^?2 s^?1 (95% CI = ?0.24 to 0.84), although the negative warming effects were only significant after 3 years of elevated temperatures in areas with low initial biocrust cover. We found support for different pathways regulating the warming‐induced reduction in R_S at areas with low (microbial thermal acclimation via reduced soil mass‐specific respiration and β‐glucosidase enzymatic activity) vs. high (microbial thermal acclimation jointly with a reduction in autotrophic respiration from decreased lichen cover) initial biocrust cover. Our 8‐year experimental study shows a reduction in soil respiration with warming and highlights that biocrusts should be explicitly included in modeling efforts aimed to quantify the soil C–climate feedback in drylands.     

Biological soil crusts in ecological restoration: emerging research and perspectives

Drylands encompass over 40% of terrestrial ecosystems and face significant anthropogenic degradation causing a loss of ecosystem integrity, services, and deterioration of social‐ecological systems. To combat this degradation, some dryland restoration efforts have focused on the use of biological soil crusts (biocrusts): complex communities of cyanobacteria, algae, lichens, bryophytes, and other organisms living in association with the top millimeters of soil. Biocrusts are common in many ecosystems and especially drylands. They perform a suite of ecosystem functions: stabilizing soil surfaces to prevent erosion, contributing carbon through photosynthesis, fixing nitrogen, and mediating the hydrological cycle in drylands. Biocrusts have emerged as a potential tool in restoration; developing methods to implement effective biocrust restoration has the potential to return many ecosystem functions and services. Although culture‐based approaches have allowed researchers to learn about the biology, physiology, and cultivation of biocrusts, transferring this knowledge to field implementation has been more challenging. A large amount of research has amassed to improve our understanding of biocrust restoration, leaving us at an opportune time to learn from one another and to join approaches for maximum efficacy. The articles in this special issue improve the state of our current knowledge in biocrust restoration, highlighting efforts to effectively restore biocrusts through a variety of different ecosystems, across scales and utilizing a variety of lab and field methods. This collective work provides a useful resource for the scientific community as well as land managers.     

Changes in biocrust cover drive carbon cycle responses to climate change in drylands

Dryland ecosystems account for ca. 27% of global soil organic carbon (C) reserves, yet it is largely unknown how climate change will impact C cycling and storage in these areas. In drylands, soil C concentrates at the surface, making it particularly sensitive to the activity of organisms inhabiting the soil uppermost levels, such as communities dominated by lichens, mosses, bacteria and fungi (biocrusts). We conducted a full factorial warming and rainfall exclusion experiment at two semiarid sites in Spain to show how an average increase of air temperature of 2–3 °C promoted a drastic reduction in biocrust cover (ca. 44% in 4 years). Warming significantly increased soil CO₂ efflux, and reduced soil net CO₂ uptake, in biocrust‐dominated microsites. Losses of biocrust cover with warming through time were paralleled by increases in recalcitrant C sources, such as aromatic compounds, and in the abundance of fungi relative to bacteria. The dramatic reduction in biocrust cover with warming will lessen the capacity of drylands to sequester atmospheric CO₂. This decrease may act synergistically with other warming‐induced effects, such as the increase in soil CO₂ efflux and the changes in microbial communities to alter C cycling in drylands, and to reduce soil C stocks in the mid to long term.     

黄土高原生物结皮对SCS-CN模型初损率的影响

水文模型是水文过程研究的有效工具,初损率(λ)是径流模型SCS-CN模型的参数,对模拟流域水文过程具有重要意义。为了确定生物结皮对λ的影响,提高该模型在黄土高原生物结皮广泛分布的退耕地的预测精度,本研究以陕西省定边县鹰窝山涧流域不同盖度的生物结皮坡面为对象,采用模拟降雨试验,分析土壤潜在最大入渗量(S)与实际入渗量(F)的关系,以及生物结皮盖度对λ的影响,并修订了λ;在此基础上,采用陕西省安塞县纸坊沟流域生物结皮径流小区的模拟降雨试验数据校验了参数修订后的模型。结果表明: 生物结皮坡面S与F的关系式为: S/F=2.5×60/T(其中T为降雨历时);模型参数λ与生物结皮盖度(C_BSC)呈极显著负相关关系,二者关系式为: λ=0.0791×e^{(-0.015×C_BSC}),R²=0.60;较λ取标准值,依生物结皮盖度修订λ后,SCS-CN模型Nash效率系数提高338.7%,合格率提升16.1%。研究结果为黄土高原生物结皮坡面λ的确定提供了科学依据,对准确评估黄土高原退耕还林(草)工程的水文效应具有重要意义。     

Vascular Plants and Biocrusts Modulate How Abiotic Factors Affect Wetting and Drying Events in Drylands

Understanding how organisms control soil water dynamics is a major research goal in dryland ecology. Although previous studies have mostly focused on the role of vascular plants on the hydrological cycle of drylands, recent studies highlight the importance of biological soil crusts formed by lichens, mosses, and cyanobacteria (biocrusts) as a major player in this cycle. We used data from a 6.5-year study to evaluate how multiple abiotic (rainfall characteristics, temperature, and initial soil moisture) and biotic (vascular plants and biocrusts) factors interact to determine wetting and drying processes in a semi-arid grassland from Central Spain. We found that the shrub Retama sphaerocarpa and biocrusts with medium cover (25–75%) enhanced water gain and slowed drying compared with bare ground areas (BSCl). Well-developed biocrusts (>75% cover) gained more water, but lost it faster than BSCl microsites. The grass Stipa tenacissima reduced water gain due to rainfall interception, but increased soil moisture retention compared to BSCl microsites. Biotic modulation of water dynamics was the result of different mechanisms acting in tandem and often in opposite directions. For instance, biocrusts promoted an exponential behavior during the first stage of the drying curve, but reduced the importance of soil characteristics that accentuate drying rates. Biocrust-dominated microsites gained a similar amount of water than vascular plants, although they lost it faster than vascular plants during dry periods. Our results emphasize the importance of biocrusts for water dynamics in drylands, and illustrate the potential mechanisms behind their effects. They will help to further advance theoretical and modeling efforts on the hydrology of drylands and their response to ongoing climate change.     

黄土高原藓结皮土壤呼吸速率对降雨量变化的响应

全球气候变化加剧背景下,干旱和半干旱地区的降雨模式将进一步改变,其造成的土壤水分波动是引起土壤呼吸动态变化的重要因素,但生物结皮土壤呼吸响应降雨模式变化继而影响陆地生态系统碳源/汇功能的机制尚不明确。针对黄土高原风沙土发育的藓结皮,以自然降雨量为对照,分别进行幅度为10%、30%、50%的模拟增雨和减雨处理,并利用便携式土壤碳通量分析仪(LI-8100A)测定了模拟增减雨后的藓结皮土壤呼吸速率,对比分析了其对降雨量变化的响应及机制。结果表明：(1)整个实验周期(2018和2019)增雨和减雨分别显著提高(增幅分别为17.9%—48.2%和27.1%—54.2%)和降低了(降幅分别为1.8%—26.8%和5.2%—20.8%)土壤含水量,但对土壤温度的影响不显著;(2)增雨抑制了藓结皮土壤呼吸速率(降幅分别为7.8%—31.7%和14.7%—39.4%),且随梯度增大抑制作用越明显;减雨则取决于减雨梯度,减雨10%和30%会促进土壤呼吸速率(增幅分别为27.5%、9.6%和23.6%、9.7%)而减雨50%具有抑制作用(降幅分别为15.6%和18.5%)。不同实验周期和不同降雨处理间藓结...     

Microsite enhancements for soil stabilization and rapid biocrust colonization in degraded drylands

In dryland ecosystems, natural recovery of biological soil crusts (biocrusts) following disturbance may be slow or inhibited, necessitating active restoration practices. While biocrusts can be readily propagated under environmentally controlled conditions, rehabilitation in the field is complicated by environmental stresses which may be particularly acute in degraded, destabilized soils with harsh climatic conditions at the soil surface. In this study, we first present the results of a field trial at a severely degraded rangeland site examining the stabilizing effects of various soil amendments (polysaccharide glues and polyacrylamides) in combination with biocrust inoculum. We found that a psyllium compound was the only amendment to maintain effectiveness after 19 months, and the only treatment that maintained biocrust inoculum throughout the trial. In a subsequent short‐term experiment where plots were shaded and watered, we examined how biocrust inoculation rate (0, 20, and 40% initial cover) and the psyllium‐based amendment affected biocrust growth. After 4 months, visible biocrust cover in inoculated plots was greater than in controls, but only chlorophyll a exhibited a dosage‐response to inoculum application rate, indicating preferential establishment of cyanobacteria. Psyllium did not affect biocrust development but did improve soil stability. Shade and watering buffered against temperature extremes (up to 15°C) and increased the duration of moist surface conditions necessary for biocrust growth by up to 30%, mimicking conditions more common in the fall and winter months. Our results suggest that inducing early successional biocrusts on a highly degraded site is possible with suitable microclimate conditions.     

黄土丘陵区生物结皮群落组成与水分入渗的关系

黄土丘陵区坡面尺度生物结皮多是由藻、藓和地衣等以不同比例、不同方式组合的一个复杂群落结构,显著影响水分入渗,但目前混合生物结皮水分入渗与其群落结构之间的关系仍不清楚,妨碍了对坡面尺度生物结皮土壤渗透性的评估。本研究测定了藻结皮、藓结皮及藓结皮盖度分别为<15%、15%～30%、30%～45%、45%～60%、＞60% 5个不同藻藓比例的混合生物结皮的稳定入渗速率,采用主成分分析和通径分析揭示混合生物结皮水分稳定入渗速率的影响因素,明确混合生物结皮水分稳定入渗速率与群落结构之间的关系。结果表明: 藻结皮和藓结皮土壤饱和导水率分别为0.66和2.40 mm·min^-1。藓结皮盖度从<15%到>60%的混合生物结皮的稳定入渗速率为0.80～2.30 mm·min^-1。混合生物结皮水分稳定入渗速率主要与藓结皮盖度和藓结皮改善的土壤孔隙结构密切相关,相关系数分别为0.636(P=0.011)和0.835(P=0.000)。通过藻结皮和藓结皮土壤饱和导水率与盖度加权预测的混合生物结皮水分入渗量(y)与混合生物结皮实测水分入渗量(x)具有极显著相关关系(r=0.945),二者拟合的线性函数为y=0.85x(R²=0.98,P<0.05)。本研究明确了混合生物结皮水分入渗与单一组成生物结皮水分入渗之间的关系,为准确评估该区生物结皮水文过程提供了科学依据。     

三峡库区苔藓生物结皮对土壤水分入渗的影响

在三峡库区王家桥小流域选取以苔藓为优势种的生物结皮样地,以附近无结皮发育的裸地为对照,设计5个盖度水平(1%～20%、20%～40%、40%～60%、60%～80%和80%～100%),采用环刀法测定土壤入渗过程,研究生物结皮盖度对入渗过程的影响。结果表明: 与裸地相比,生物结皮发育可显著提高表层土壤粘结力、孔隙度、黏粒含量、水稳性团聚体和有机碳含量,显著降低土壤容重和砂粒含量。生物结皮促进了土壤水分入渗,初始入渗率、稳定入渗率、平均入渗率和累积入渗量可达裸地的2.0倍及以上,土壤入渗性能随结皮盖度的增大呈先增加后减小的变化规律,在40%～60%盖度下最大。通径分析显示,土壤初始入渗率主要受结皮盖度、土壤容重和有机碳含量的影响,稳定入渗率主要受结皮盖度和土壤容重的影响。Horton模型对三峡库区生物结皮覆盖土壤的水分入渗过程拟合效果最佳。     

Strategies for restoring the structure and function of lichen‐moss biocrust communities

Biological soil crusts (biocrusts) are crucial components of dryland ecosystems, but they are slow to recover following disturbance. Herein, we evaluated several methods for restoring lichen‐moss biocrusts that included factorial applications of moss fragments in a water‐slurry (1) with and without lichen fragments (to restore biocrust taxonomic structure), (2) with and without clay (to facilitate establishment), and (3) with and without jute ground cloth (to facilitate establishment). Three and four years after inoculation, moss and lichen cover was up to five and eight times higher on jute ground cloth than on bare ground, respectively. Lichen cover was six times higher in plots where lichen fragments were added. Clay amendments did not increase moss or lichen establishment. To understand the effects of biocrust recovery on soil properties, we measured soil inorganic nitrogen, microbial biomass carbon, and soil water availability in restoration and control plots. Restored biocrusts decreased inorganic NH₄‐N availability by 67% when compared to controls 3 years after inoculation, but did not influence the availability of inorganic NO₃‐N, soil water, or microbial biomass carbon. Our results demonstrate that adding a biocrust inoculant to jute ground cloth can expedite recovery of lichen‐moss biocrust and reestablish its influence on soil properties within a few years.     

Amendments fail to hasten biocrust recovery or soil stability at a disturbed dryland sandy site

In most drylands, biological soil crusts (biocrusts), an assemblage of lichens, bryophytes, fungi, green algae, and cyanobacteria, are critical to healthy ecosystem function. However, they are extremely sensitive to disturbance and attempts to facilitate their recovery have had variable success. In this study, we applied soil amendments designed to improve soil surface stability and accelerate biocrust recovery on an area disturbed by oil/gas exploration vehicles. Treatments included: (1) control (one time water only); (2) biocrust‐only: biocrust inoculum + nutrients in water; (3) polyacrylamide gels (which are known to stabilize soils) + biocrust inoculum + nutrients in water; (4) gypsum + biocrust inoculum + nutrients in water; and (5) saline (NaCl) solution + biocrust inoculum + nutrients in water. Only the NaCl treatment showed any effects on soil properties and these were only short term. These effects included an increase in soil strength and a reduction in soil aggregate stability, unsaturated hydraulic conductivity (K_h), and cyanobacterial biomass. The inoculated biocrust material failed to develop and even after 10 years, there was only a very low natural recolonization of the plots. These results show that inoculating soils or applying these levels of soil amendments does not guarantee recovery of soil stability or biocrust, and that some sites are unlikely to recover without assistance. Thus, there is a need for more research into ways to enhance soil stability and identify the factors limiting biocrust establishment.     

黄土丘陵区草本植物覆盖下生物结皮对坡面径流流速的削减作用

生物结皮是干旱半干旱地区的常见地被物,与植物共同影响坡面径流及流速。迄今鲜有研究关注植物和生物结皮共同覆盖对流速的影响,是干旱半干旱地区坡面侵蚀驱动因素研究的薄弱环节。本研究以黄土丘陵区退耕草地为对象,通过野外模拟降雨试验,研究草本植物覆盖下有无生物结皮及不同组成生物结皮(多藻少藓、多藓少藻和藓)对径流流速的影响。结果表明: 植物和植物+生物结皮覆盖显著降低了流速,植物覆盖较裸土降低70.7%,植物+生物结皮覆盖较裸土降低83.1%;植物和生物结皮共同覆盖下,植物和生物结皮对径流流速的削减效益分别为70.7%和12.4%。植物覆盖下生物结皮对流速的影响程度与其组成有关,多藻少藓结皮、多藓少藻结皮和藓结皮对流速的削减效益分别为11.5%、12.4%和19.4%。流速与藓盖度呈显著负相关,与藻盖度呈显著正相关,藓结皮盖度(x)与流速(y)的关系式为:y=-2.081x+0.03(R²=0.469)。当植物盖度一定时(40%±10%),生物结皮组成中藓盖度是影响共同覆盖坡面流速的关键因子。综上,草本植物覆盖下生物结皮有显著减缓流速的作用,且作用程度与其组成有关。表明在研究退耕草地坡面侵蚀动力机制时,生物结皮的作用应予以考虑。     

Biological soil crust salvage for dryland restoration: an opportunity for natural resource restoration

Biocrusts' functional importance and vulnerability to disturbance have motivated consistent interest in biocrust restoration, as well as a recent increase in research to cultivate biocrusts in laboratory and greenhouse settings for use in ecological restoration. As part of a sustainable approach to developing biocrust restoration, we argue that a complementary step is to improve and accelerate methods for salvaging biocrusts that would otherwise be destroyed in a forthcoming disturbance. The increasing rate and scale of disturbance pressures in drylands where biocrusts flourish means that the supply of salvageable biocrust and demand for that material in restoration greatly exceed the present cultivable supply. In this article we describe the state of knowledge for biocrust salvage, present a simple set of steps for conducting a salvage harvest, discuss risks and benefits when considering using salvage, and suggest future research directions to facilitate scaling up biocrust restoration using salvaged material. A focus on the use of salvaged biocrust as a restoration source may prove an important step to improve ecological restoration in notoriously difficult to restore dryland ecosystems.     

Climate Change and Physical Disturbance Manipulations Result in Distinct Biological Soil Crust Communities

Biological soil crusts (biocrusts) colonize plant interspaces in many drylands and are critical to soil nutrient cycling. Multiple climate change and land use factors have been shown to detrimentally impact biocrusts on a macroscopic (i.e., visual) scale. However, the impact of these perturbations on the bacterial components of the biocrusts remains poorly understood. We employed multiple long-term field experiments to assess the impacts of chronic physical (foot trampling) and climatic changes (2°C soil warming, altered summer precipitation [wetting], and combined warming and wetting) on biocrust bacterial biomass, composition, and metabolic profile. The biocrust bacterial communities adopted distinct states based on the mechanism of disturbance. Chronic trampling decreased biomass and caused small community compositional changes. Soil warming had little effect on biocrust biomass or composition, while wetting resulted in an increase in the cyanobacterial biomass and altered bacterial composition. Warming combined with wetting dramatically altered bacterial composition and decreased Cyanobacteria abundance. Shotgun metagenomic sequencing identified four functional gene categories that differed in relative abundance among the manipulations, suggesting that climate and land use changes affected soil bacterial functional potential. This study illustrates that different types of biocrust disturbance damage biocrusts in macroscopically similar ways, but they differentially impact the resident soil bacterial communities, and the communities'' functional profiles can differ depending on the disturbance type. Therefore, the nature of the perturbation and the microbial response are important considerations for management and restoration of drylands.     

黄土丘陵区生物结皮坡面产流产沙与其分布格局的关联

生物结皮在黄土丘陵区退耕地广泛发育,并在干扰等因素下形成不同分布格局。为明确生物结皮分布格局对坡面产流产沙的影响,采用模拟降雨试验研究了生物结皮条带状、棋盘状和随机3种分布格局对坡面产流产沙的影响,借助景观生态学方法,计算了生物结皮斑块的分布格局指数,解析了生物结皮分布格局与坡面产流产沙的关联。结果表明,（1）生物结皮分布格局显著影响坡面初始产流时间、径流率和土壤侵蚀速率。相同盖度条件下,生物结皮坡面的随机分布格局较带状和棋盘式分布格局显著降低了坡面径流量和产沙量,其径流率与土壤侵蚀速率分别是带状格局的71.7%、12.9%,是棋盘格局的89.6%、31.8%。（2）坡面径流率与生物结皮斑块最大斑块指数、斑块连结度呈极显著负相关,与分离度呈极显著正相关,与景观分裂指数呈显著正相关。（3）土壤侵蚀速率与生物结皮斑块最大斑块指数、斑块连结度呈显著负相关,与景观分裂指数、分离度呈显著正相关。（4）坡面径流率（Y₁）和土壤侵蚀速率（Y₂）与分离度（SPLIT,Splitting Index）的关系式为：Y₁=1.116+0.017SPLIT,R²：0.957;Y₂=22.767+17.936SPLIT,R²：0.801。（5）以上结果表明分布格局主要是通过影响生物结皮在坡面的分离度进而影响坡面产流产沙,随着生物结皮斑块分离度增大,生物结皮斑块破碎度增大,坡面产流产沙增多。从分布格局的角度解析了生物结皮对产流、产沙的影响,量化了生物结皮坡面径流率与侵蚀率与格局指数之间的关系,为量化认识生物结皮的水土保持功能提供了科学依据。     

古尔班通古特沙漠生物土壤结皮对氨氧化微生物生态位的影响

生物结皮作为荒漠地表的重要覆被类型, 在荒漠生态系统的氮素循环中扮演重要角色。融雪期为古尔班通古特沙漠生物结皮的复苏和生长提供了充足的水分, 也成为该沙漠氮素固定和转化的重要时期, 但该时期生物结皮如何影响驱动氨氧化转化的微生物群落动态尚未明确。因此, 我们利用荧光定量PCR (fluorescent quantitative PCR, qPCR)方法分析融雪期生物结皮与去除结皮不同土层(0-2, 2-5, 5-10和10-20 cm)氨氧化菌群丰度特征, 结合潜在硝化速率和土壤理化参数, 探究融雪期生物结皮对荒漠土壤氮素转化作用。结果表明: 氨氧化古菌(ammonia-oxidizing archaea, AOA)是古尔班通古特沙漠土壤优势氨氧化菌, 生物结皮对0-2 cm层土壤中AOA、氨氧化细菌(ammonia-oxidizing bacteria, AOB) amoA基因丰度具有显著抑制作用(P < 0.01), 对10-20 cm层土壤中AOA amoA基因丰度具有显著促进作用(P < 0.01)。冗余分析(redundancy analysis, RDA)表明, AOA、AOB amoA基因丰度主要受土壤含水量和铵态氮含量的影响, 占总条件效应的54.90%。氨氧化速率分析发现, 去除生物结皮显著降低古尔班通古特沙漠土壤硝化作用潜力(P < 0.001), 证实生物结皮对荒漠土壤氮素转化具有重要的调控作用。综上所述, 古尔班通古特沙漠氨氧化微生物的分布规律受环境因子调控, 特别是生物结皮可以通过调节土壤含水量和铵态氮含量影响AOA和AOB的空间生态位分化, 促进沙漠土壤的硝化作用。     

Simulated climate change reduced the capacity of lichen-dominated biocrusts to act as carbon sinks in two semi-arid Mediterranean ecosystems

The importance of biological soil crusts (biocrusts) for the biogeochemistry of drylands is widely recognized. However, there are significant gaps in our knowledge about how climate change will affect these organisms and the processes depending on them. We conducted a manipulative full factorial experiment in two representative dryland ecosystems from central (Aranjuez) and southeastern (Sorbas) Spain to evaluate how precipitation, temperature and biocrust cover affected the assimilation and net C balance of biocrusts. Chlorophyll fluorescence, net photosynthesis and dark respiration were measured in situ bimonthly during a year. We also conducted daily cycle measurements of net photosynthesis in winter and at the end of spring. In Sorbas, warming reduced the fixation of atmospheric C in biocrust-dominated microsites throughout the year. In Aranjuez, there was an interaction between the three factors evaluated; during winter, net photosynthesis was significantly greater in high biocrust cover plots under natural conditions and in the rainfall exclusion treatment. During the daily surveys, rainfall exclusion and warming reduced C fixation in Sorbas and in Aranjuez respectively. The effects of the treatments evaluated varied with the rainfall and non-rainfall water inputs (NRWIs) registered before the measurements. Our results suggest that changes in NRWI regimes as consequence of warming could have a greater impact on the C balance of biocrusts than changes in rainfall amounts. They also indicate that climate change may reduce the photosynthetic ability of lichens, with a consequent reduction of their dominance in biocrust communities at the mid to long term. This could reduce the ability of dryland ecosystems to fix atmospheric C.     

黄土高原封禁林地藓结皮呼吸速率对放牧踩踏干扰强度的响应

放牧踩踏造成的土壤属性变化是引起土壤呼吸速率和碳固排波动的重要原因,但目前有关不同强度放牧踩踏对生物结皮呼吸的影响尚不明确。本研究针对黄土高原风沙土发育的藓结皮,以未干扰为对照,分别进行强度为10%、30%、50%和70%的模拟放牧踩踏干扰,连续测定了藓结皮呼吸速率的变化,分析了藓结皮呼吸速率对不同干扰强度的响应规律。结果表明: 1)适度干扰会激发藓结皮呼吸速率,而重度干扰则会抑制其呼吸速率。与未干扰相比,干扰10%和30%的藓结皮呼吸速率分别增加了41.1%和22.2%,而干扰50%和70%的藓结皮呼吸速率则分别降低了8.9%和15.3%。2)踩踏干扰显著改变了土壤温度,但对土壤含水量的影响不显著。与未干扰相比,干扰10%和30%的土壤温度分别显著降低了0.4和1.2 ℃,干扰50%和70%则分别显著增加了1.1和1.0 ℃。3)不同干扰强度下藓结皮呼吸速率与土壤温度和含水量分别呈显著指数和线性正相关关系,但与藓结皮基本特性无显著相关关系,土壤温度和水分可分别解释藓结皮呼吸速率动态变化的70.6%～96.3%和49.1%～70.0%。综上,放牧踩踏显著影响了藓结皮呼吸速率,短期适度放牧踩踏会激发藓结皮呼吸速率,而过度放牧踩踏则会抑制其呼吸速率。因此,在未来黄土高原地区土壤碳收支平衡研究中应考虑不同强度放牧踩踏对生物结皮呼吸的影响。     

毛乌素沙地生物土壤结皮对油蒿水分吸收的影响模拟

植物可利用水分是决定沙生灌木生长的主要因子,生物土壤结皮(简称生物结皮)在降雨期影响降水入渗,而在干旱期改变土壤蒸发,从而影响土壤水分分布,最终可能影响灌木水分吸收。然而,关于不同降水条件下生物结皮对灌木水分吸收和水分胁迫的影响机制认识不清。以油蒿为研究对象,基于试验数据和1990—2019年气象数据,采用数学模拟,定量研究了毛乌素沙地不同降水条件下生物结皮对土壤水分分布和油蒿水分吸收的影响,评价干旱期生物结皮对油蒿水分胁迫的影响。结果表明：与无结皮处理相比,生物结皮处理的土壤蒸发降低了5.1%;生物结皮改善了干旱期的土壤水分条件;生物结皮降低了植物水分胁迫的比例,平均降低比例为8.1%;生物结皮提高了植物水分吸收,平均增加比例为12.8%;生物结皮和对照植物水分吸收的比值随季节降水量的增加而降低,均值为1.13。综上,生物结皮的出现并未消极地影响沙生灌木的水分吸收。研究结果有助于理解生物结皮与灌木的共生或竞争关系。