Biocrusts modulate warming and rainfall exclusion effects on soil respiration in a semi-arid grassland

Soil surface communities composed of cyanobacteria, algae, mosses, liverworts, fungi, bacteria and lichens (biocrusts) largely affect soil respiration in dryland ecosystems. Climate change is expected to have large effects on biocrusts and associated ecosystem processes. However, few studies so far have experimentally assessed how expected changes in temperature and rainfall will affect soil respiration in biocrust-dominated ecosystems. We evaluated the impacts of biocrust development, increased air temperature and decreased precipitation on soil respiration dynamics during dry (2009) and wet (2010) years, and investigated the relative importance of soil temperature and moisture as environmental drivers of soil respiration, in a semiarid grassland from central Spain. Soil respiration rates were significantly lower in the dry than in the wet year, regardless of biocrust cover. Warming increased soil respiration rates, but this response was only significant in biocrust-dominated areas (>50% biocrust cover). Warming also increased the temperature sensitivity (Q₁₀ values) of soil respiration in biocrust-dominated areas, particularly during the wet year. The combination of warming and rainfall exclusion had similar effects in low biocrust cover areas. Our results highlight the importance of biocrusts as a modulator of soil respiration responses to both warming and rainfall exclusion, and indicate that they must be explicitly considered when evaluating soil respiration responses to climate change in drylands.     

黄土丘陵区退耕地生物结皮影响下的土壤腐殖质分异特征

本文对黄土丘陵区退耕地生物结皮影响下的土壤腐殖质分异特征进行了研究。结果表明,①该区生物结皮影响下的土壤腐殖质组分HA、FA和HM含量均有明显的分层特征,土层间均表现出:结皮层0～2cm土层2～5cm土层;不同年限生物结皮土壤腐殖质含量总体表现出:16年8年32年12年,表明退耕地生物结皮影响下的土壤腐殖质含量随时间变化呈非线性变化趋势;HA/FA平均值变化范围为0.34～0.70,低于1,该区生物结皮影响下的土壤腐殖质胡敏酸含量不高,腐殖酸以富啡酸为主;②结皮层土壤腐殖酸E4/E6值随时间变化总体上呈"抛物线"型变化趋势;而0～2cm和2～5cm土层土壤腐殖酸E4/E6值呈缓慢增长的趋势;③腐殖质组分与土壤养分因子的全N、NO3--N、NH4+-N和有效Zn之间存在极显著的线性相关关系(p0.01);④土壤腐殖质组分含量的变化与全N、NO3--N、NH4+-N和有效Zn含量间存在满足二项式的函数回归关系(p0.01),表明生物结皮对土壤C素与N素和有效Zn的影响密切相关。     

Impact of inorganic nitrogen additions on microbes in biological soil crusts

Many studies have shown that changes in nitrogen (N) availability affect the diversity and composition of soil microbial community in a variety of terrestrial systems, but less is known about the responses of microbes specific to biological soil crusts (BSCs) to increasing N additions. After seven years of field experiment, the bacterial diversity in lichen-dominated crusts decreased linearly with increasing inorganic N additions (ambient N deposition; low N addition, 3.5 g N m⁻² y⁻¹; medium N addition, 7.0 g N m⁻² y⁻¹; high N addition, 14.0 g N m⁻² y⁻¹), whereas the fungal diversity exhibited a distinctive pattern, with the low N-added crust containing a higher diversity than the other crusts. Pyrosequencing data revealed that the bacterial community shifted to more Cyanobacteria with modest N additions (low N and medium N) and to more Actinobacteria and Proteobacteria and much less Cyanobacteria with excess N addition (high N). Our results suggest that soil pH, together with soil organic carbon (C), structures the bacterial communities with N additions. Among the fungal communities, the relative abundance of Ascomycota increased with modest N but decreased with excess N. However, increasing N additions favored Basidiomycota, which may be ascribed to increases in substrate availability with low lignin and high cellulose contents under elevated N conditions. Bacteria/fungi ratios were higher in the N-added samples than in the control, suggesting that the bacterial biomass tends to dominate over that of fungi in lichen-dominated crusts after N additions, which is especially evident in the excess N condition. Because bacteria and fungi are important components and important decomposers in BSCs, the alterations of the bacterial and fungal communities may have implications in the formation and persistence of BSCs and the cycling and storage of C in desert ecosystems.     

Microalgal species variation at different successional stages in biological soil crusts of the Gurbantunggut Desert,Northwestern China

Biological soil crusts (BSC), most notably lichen crusts, develop and diversify in the Gurbantunggut Desert, the largest fixed and semi-fixed desert in China. Four different successional stages of BSC, including bare sand, microalgal crusts, lichen crusts, and moss crusts, were selected to determine successional changes in microalgal species composition and biomass and formation of BSC. A 10 × 10-m observation plot was established in an interdune region of the Gurbantunggut Desert and data were collected over an 8-year study period. The main results were: (1) different successional stages of BSC significantly affected the content of soil organic C and total and available N but not the total and available P and K content of soil; (2) composition of microalgal communities differed among the four successional stages; (3) significant differences in microalgal biomass were observed among the four successional stages; (4) bare sand was mainly uncompacted sand gains; (5) filamentous cyanobacteria, particularly Microcoleus vaginatus, were the dominant species in the early phase of crust succession. The presence of fungal mycelium and moss rhizoids prevented water and wind erosion.     

Role of dissolved organic nitrogen (DON) in soil N cycling in grassland soils

Dissolved organic nitrogen (DON) represents a significant pool of soluble N in many soils and freshwaters. Further, the low molecular weight (LMW) component of DON represents an important source of N for microorganisms and can also be utilized directly by some plants. Our purpose was to determine which of the pathways in the decomposition and subsequent ammonification and nitrification of organic N represented a significant block in soil N supply in three agricultural grassland soils. The results indicate that the conversion of insoluble organic N to LMW-DON and not LMW-DON to NH₄⁺ or NH₄⁺ to NO₃⁻ represents a major constraint to N supply. We hypothesize that there are two distinct DON pools in soil. The first pool comprises mainly free amino acids and proteins and is turned over very rapidly by the microbial community, so it does not accumulate in soil. The second pool is a high molecular weight pool rich in humic substances, which turns over slowly and represents the major DON loss to freshwaters. The results also suggest that in NO₃⁻ rich soils the uptake of LMW-DON by soil microorganisms may primarily provide them with C to fuel respiration, rather than to satisfy their internal N demand.     

黄土丘陵区不同降水量带生物结皮对土壤氮素的影响

黄土丘陵区生物结皮广泛发育,可通过固氮作用影响土壤氮素水平,但该区生物结皮对土壤氮素水平的影响鲜见报道.本文通过野外调查结合采样分析,研究了黄土丘陵区不同降水量带生物结皮组成、覆盖度差异及其对土壤氮素水平的影响.结果表明,1)黄土丘陵区不同降水量带生物结皮覆盖度无显著差异,但组成有差别;2)不同降水量带土壤氮素含量剖面分布具有明显的分层特征,生物结皮显著增加了结皮层土壤氮素含量,对下层土壤影响较小,结皮层下0-2 cm、2-5 cm、5-10 cm土层中氮素含量差异不显著;3)生物结皮层土壤全氮、碱解氮及微生物氮在不同降水量带差异不显著,而0-2 cm、2-5 cm、5-10 cm土壤全氮、碱解氮及土壤微生物氮含量在200～300 mm降水量带小于300～600 mm降水量带.研究结果揭示了黄土丘陵区生物结皮对土壤氮素的贡献,而不同降水量带生物结皮对土壤氮素的贡献差异不显著的原因有待于进一步研究.     

Carbohydrates in cyanobacterial soil crusts as a source of carbon in the southwest Kalahari, Botswana

Semiarid ecosystems, also known as drylands, contain small amounts of carbon (C). To date only few studies have evaluated soil C dynamics in these ecosystems. Cyanobacterial soil crusts are considered a major source of organic C in semiarid ecosystems through photosynthesis, increasing soil organic C (SOC) pool as carbohydrates. This study considers cyanobacterial soil crusts as a source of C in the southwest Kalahari. Cyanobacterial soil crusts are well adapted to drylands because extracellular polysaccharide (EPS) secretions provide stabilization of soil and resistance to desiccation. The carbohydrate and chlorophyll a content were evaluated in the upper soil profile on Kalahari Sands and pan soils. Topsoil carbohydrate concentration decreased exponentially with depth. The carbohydrate content produced in cyanobacterial soil crusts in the southwest Kalahari represents up to 75% of the total SOC and is thus an essential component for the fertility of Kalahari Sand soils.     

铜尾矿生物结皮的生物固氮及其影响因素研究

在铜尾矿生态系统自然恢复过程中,生物结皮广泛存在并成为尾矿生态系统演替早期的重要阶段。本文采用乙炔原位还原法对藻类结皮、藻藓混合结皮和藓类结皮的生物固氮特征进行了系统研究。结果表明:(1)生物结皮显著提高了铜尾矿总氮含量,同时降低了铜的含量。(2)不同类型生物结皮的固氮能力差别较大,其中藻藓混合结皮的生物固氮量最高,在N 4.36～30.39 kg hm2 a–1之间;藻类结皮和藓类结皮的固氮量分别为N 1.32～8.78、0～16.34 kg hm2 a–1。(3)生物固氮能力随季节变化明显,夏季的生物固氮量最高,春季次之,秋冬季节相对较低。(4)铜尾矿基质pH、NH4+-N和水溶性有机碳(WSOC)等与生物固氮量呈显著正相关(p<0.05),而土壤容重、NO3--N和总铜等与生物固氮量呈显著负相关(p<0.05)。     

Feasibility of cyanobacterial inoculation for biological soil crusts formation in desert area

Practical testing of the feasibility of cyanobacterial inoculation to speed up the recovery of biological soil crusts in the field was conducted in this experiment. Results showed that cyanobacterial and algal cover climbed up to 48.5% and a total of 14 cyanobacterial and algal species were identified at the termination of inoculation experiment; biological crusts' thickness, compressive and chlorophyll a content increased with inoculation time among 3 years; moss species appeared in the second year; cyanobacterial inoculation increased organic carbon and total nitrogen of the soil; total salt, calcium carbonate and electrical conductivity in the soil also increased after inoculation. Diverse vascular plant communities composed of 10 and 9 species are established by cyanobacterial inoculation on the windward and leeward surface of the dunes, respectively, after 3 years. The Simpson index for the above two communities are 0.842 and 0.852, while the Shannon-Weiner index are 2.097 and 2.053, respectively. In conclusion, we suggest that cyanobacterial inoculation would be a suitable and effective technique to recover biological soil crusts, and may further restore the ecological system.     

生物结皮对荒漠地区土壤及植物的影响研究述评

 生物结皮是目前防沙治沙的一种重要措施。通过对国内外生物结皮研究的回顾,简要总结了生物结皮对荒漠地区土壤理化性质及植物的影响。生物结皮对其下层土壤的影响主要表现在土壤的机械组成、土壤盐分、土壤养分等方面。生物结皮具有明显的养分、盐分和细粒物质的聚集作用,对表层土壤发育有积极的意义。生物结皮及下部土壤细粒的增加,提高了土壤的吸湿性和持水性,但对降雨入渗的影响尚无定论。在生物结皮的发育过程中,其与维管植物萌发和定居间的关系也存在几种不同的观点。对生物结皮的研究是目前防沙治沙研究的一个重要领域,其对荒漠地区的土壤及植被某些方面的影响目前并未形成一致的结论,其形成机制、生物组成、演替规律、对后续植物定居的影响等,均是需要进一步深入研究的科学问题。     

土壤生物结皮在干旱半干旱地区退化生态系统恢复中的作用(英文)

 土壤生物结皮是由细菌、真菌、藻类、地衣和苔藓等形成的一种混合体,作为干旱半干旱地区生态系统的组成部分,它对生态系统镶嵌格局和生态过程有不可忽视的影响;同时,土壤生物结皮通过影响局部水分条件,可以起到稳定土壤表层、减少土壤侵蚀、增加土壤氮养分的作用:因此,土壤生物结皮能够为土壤表层提供一种天然保护的作用,对干旱半干旱地区退化生态系统的恢复具有非常重要的作用。     

Effects of biological soil crusts on soil enzyme activities in revegetated areas of the Tengger Desert,China

Biological soil crusts (BSCs) cover up to 70% of the sparsely-vegetated areas in arid and semiarid regions throughout the world and play a vital role in dune stabilization in desert ecosystems. Soil enzyme activities could be used as significant bioindicators of soil recovery after sand burial. However, little is known about the relationship between BSCs and soil enzyme activities. The objective of this study was to determine whether BSCs could affect soil enzyme activities in revegetated areas of the Tengger Desert. The results showed that BSCs significantly promoted the activities of soil urease, invertase, catalase and dehydrogenase. The effects also varied with crust type and the elapsed time since sand dune stabilization. All the soil enzyme activities tested in this study were greater under moss crusts than under cyanobacteria–lichen crusts. The elapsed time since sand dune stabilization correlated positively with the four enzyme activities. The enzyme activities varied with soil depth and season, regardless of crust type. Cyanobacteria–lichen and moss crusts significantly enhanced all test enzyme activities in the 0–20 cm soil layer, but negatively correlated with soil depth. All four enzyme activities were greater in the summer and autumn than in spring and winter due to the vigorous growth of the crusts. Our study demonstrated that the colonization and development of BSCs could improve soil quality and promote soil recovery in degraded areas of the Tengger Desert.     

Dissolved soil organic nitrogen and carbon in a Norway spruce stand and an adjacent clear-cut

 The aims of this study were to characterize dissolved soil organic N (DON) and C (DOC) in a coniferous stand and an adjacent clear-cut, and to evaluate the importance of DON in N leaching. The study was carried out in a Norway spruce stand and a clear-cutting treatment in the same forest stand. Concentrations of DON in soil solution were monitored for 5 years after clear-cutting with gravity lysimeters. In the Norway spruce stand DON comprised 62–83% of the total N in soil solution over the 5-year period. The concentrations of DON in the clear-cut were higher than in the forest stand, but the proportion of total N was lower. To characterize dissolved organic matter, soil samples were aerobically incubated for 6 weeks in the laboratory, and the quantity, molecular size distribution and chemical nature of both DON and DOC were determined from water extracts made before and after the incubation. In the soil samples from the Norway spruce stand, C-rich compounds with a high C/N ratio and large molecular size were formed. In contrast, after the incubation the major carriers of DON in soil samples from the clear-cut were N-rich organic compounds with a low C/N ratio and a small molecular size. The distribution of different chemical fractions of DOC in soil did not differ much whether recovered from the Norway spruce stand or the clear-cut. It was (from highest to lowest concentration): hydrophobic acids>hydrophilic acids>phenols>hydrophilic neutrals. A major part of DON was also carried by these fractions. During incubation the concentration of N-containing hydrophilic acids increased, especially in the soil from the clearcut. In soil samples from the Norway spruce stand, the rate of net N mineralization was low and no NO₃ was formed, whilst the rate of net N mineralization was high and net nitrification was intensive in soil from the clear-cut. Received: 12 June 2000     

Seasonal variation in soluble soil carbon and nitrogen across a grassland productivity gradient

Understanding the fate and turnover of the pools that comprise dissolved organic nitrogen (DON) in soil is key to determining its role in ecosystem functioning. We investigated seasonal changes of dissolved organic carbon (DOC) and nitrogen (DON) concentrations within four molecular weight (MW) size fractions across an altitudinal gradient (from lowland to montane systems), and quantified individual amino acids and amino acid constituents of oligopeptidic-N, as well as nitrate and ammonium. We tested two ideas: first, that DON is more abundant than DIN in low-productivity relative to high-productivity grassland ecosystems; and second, that the abundance of peptides and amino acids is likewise greater in low- than high-productivity grassland. The most productive site had a history of inorganic fertiliser application, and hence in this site alone DIN was more abundant than DON. Plant productivity varied 3-fold between the other sites, and DON was generally at higher concentrations in the sites of lower productivity both in absolute terms as well as relative to DIN, with a large increase observed in spring. The fraction containing the highest concentration of the DON had a MW of >100 kDa, and in summer and autumn this fraction was more abundant at the lowest productivity site. We conclude that relationships between the abundance of DON relative to DIN and ecosystem productivity is dependent on season, and hence more complex than previously suggested, and that peptides are a dynamic and potentially nutritionally significant component of DON.     

Revisiting classic water erosion models in drylands: The strong impact of biological soil crusts

Soil erosion and subsequent degradation has been a contributor to societal collapse in the past and is one of the major expressions of desertification in arid regions. The revised universal soil loss equation (RUSLE) models soil lost to water erosion as a function of climate erosivity (the degree to which rainfall can result in erosion), topography, soil erodibility, and land use/management. The soil erodibility factor (K) is primarily based upon inherent soil properties (those which change slowly or not at all) such as soil texture and organic matter content, while the cover/management factor (C) is based on several parameters including biological soil crust (BSC) cover. We examined the effect of two more precise indicators of BSC development, chlorophyll a and exopolysaccharides (EPS), upon soil stability, which is closely inversely related to soil loss in an erosion event. To examine the relative influence of these elements of the C factor to the K factor, we conducted our investigation across eight strongly differing soils in the 0.8 million ha Grand Staircase-Escalante National Monument. We found that within every soil group, chlorophyll a was a moderate to excellent predictor of soil stability (R² = 0.21–0.75), and consistently better than EPS. Using a simple structural equation model, we explained over half of the variance in soil stability and determined that the direct effect of chlorophyll a was 3× more important than soil group in determining soil stability. Our results suggest that, holding the intensity of erosive forces constant, the acceleration or reduction of soil erosion in arid landscapes will primarily be an outcome of management practices. This is because the factor which is most influential to soil erosion, BSC development, is also among the most manageable, implying that water erosion in drylands has a solution.     

Evolution of soil respiration depends on biological soil crusts across a 50-year chronosequence of desert revegetation

Despite intensive study in recent decades, soil respiration rate (Rs) and its evolution accompanying vegetation succession remain perplexing. Using a 50-year chronosequence of sand-fixing revegetation in the Tengger Desert of China, we took intact soil columns of 20 cm in depth, incubated them at 12 levels of soil water content (0–0.4 m³ m^?3) and at nine temperatures (5–45°C) in a growth chamber, and measured Rs. The results showed that Rs increased rapidly 15 to 20 years following revegetation but stabilized after 25 years. Rs for soils covered with moss crusts were markedly higher than those covered with algal crusts. Further, Rs correlated significantly with sand content (negatively) and fine particle contents (positively), and increased exponentially with increased soil organic matter (SOM) and total nitrogen (TN) contents. Soil texture had a stronger influence on Rs than did SOM and TN. Also, Rs increased linearly with increased coverage and depth of biological soil crusts, which had a more pronounced influence on Rs than did soil physicochemical properties. Our results suggest that the capacity of carbon sequestration likely increases during the 50-year period after revegetation because the linear increase in SOM outweighs the limited sigmoidal increase in Rs.     

Archaeal populations in biological soil crusts from arid lands in North America

Archaea are common and abundant members of biological soil crust communities across large-scale biogeographic provinces of arid North America. Regardless of microbial community development, archaeal populations averaged 2 × 10⁷ 16S rRNA gene copies per gram of soil, representing around 5% of the prokaryotic (total calculated bacterial and archaeal) numbers assessed by quantitative-PCR. In contrast, archaeal diversity, determined by denaturing gradient gel electrophoresis fingerprinting and clone libraries of 16S rRNA genes, was very restricted. Only six different phylotypes (all Crenarchaea) were detected, three of which were very dominant. Some phylotypes were widespread, while others were typical of Southern desert areas.     

Carbon-to-nitrogen ratio is a poor predictor of low molecular weight organic nitrogen mineralization in soil

Carbon-to-nitrogen ratio (C:N) has frequently been shown to be a good predictor of the speed of organic residue decomposition and N mineralization in soil. While this relationship appears to work well for complex organic materials (e.g. plant litter), its applicability to smaller organic substrates containing N remains unknown. Here we evaluated whether the intrinsic properties of amino acids and peptides could be used to predict their rate of microbial uptake and subsequent N mineralization. In an agricultural grassland soil we found that C:N, molecular weight, aromaticity and sulphur content provided poor indicators of amino acid bioavailabilityand subsequent NH₄⁺ release into soil. We therefore hypothesize that the position of amino acids along microbial biosynthetic pathways together with internal demand for individual amino acids rather than their C or N content is the primary determinant of N mineralization.     

降雨对荒漠草原生物土壤结皮化学计量的影响

生物土壤结皮是干旱地区地表景观的基本组成部分,对生物地球化学循环具有重要影响。在中国北方荒漠化地区,生物土壤结皮化学计量在很大程度上是未知的,特别是降雨如何影响荒漠草原生物土壤结皮化学计量仍然不确定。该研究以自然降雨为对照,通过使用遮雨棚和喷灌系统控制降水输入,开展增水和减水处理野外控制性试验,研究降雨量对荒漠草原生物土壤结皮化学计量的影响。试验结果表明：1）减水处理增加了结皮层C∶N、C∶P和N∶P的比率,增水处理增加了结皮层下垫面C∶N、C∶P和N∶P比率;2）减水处理增大了结皮层与下垫面之间C含量的差异,同时减小了N和P含量的差异,增水处理增大了结皮层与下垫面之间N和P含量的差异,减水处理有利于结皮层C的积累,而增水后结皮层中磷的有效性降低;3）适宜的土壤水分条件促进了结皮层及下垫面土壤微生物生物量碳（Soil Microbial Biomass Carbon,SMBC）和土壤微生物生物量氮（Soil Microbial Biomass Nitrogen,SMBN）的积累,而过高的降雨量导致土壤养分损失,不利于SMBC和SMBN的积累。相对干旱的土壤环境有利于结皮层土壤C、N的富集,为土壤微生物呼吸提供较多的营养物质,有利于SMBC和SMBN的积累。总之,在中国北方荒漠化地区,生物土壤结皮和下垫面的C∶N∶P化学计量对降雨量有不同的响应。