An arbuscular mycorrhizal fungus significantly modifies the soil bacterial community and nitrogen cycling during litter decomposition

Arbuscular mycorrhizal fungi (AMF) perform an important ecosystem service by improving plant nutrient capture from soil, yet little is known about how AMF influence soil microbial communities during nutrient uptake. We tested whether an AMF modifies the soil microbial community and nitrogen cycling during litter decomposition. A two‐chamber microcosm system was employed to create a root‐free soil environment to control AMF access to ¹³C‐ and ¹⁵N‐labelled root litter. Using a 16S rRNA gene microarray, we documented that approximately 10% of the bacterial community responded to the AMF, Glomus hoi. Taxa from the Firmicutes responded positively to AMF, while taxa from the Actinobacteria and Comamonadaceae responded negatively to AMF. Phylogenetic analyses indicate that AMF may influence bacterial community assembly processes. Using nanometre‐scale secondary ion mass spectrometry (NanoSIMS) we visualized the location of AMF‐transported ¹³C and ¹⁵N in plant roots. Bulk isotope ratio mass spectrometry revealed that the AMF exported 4.9% of the litter ¹⁵N to the host plant (Plantago lanceolata L.), and litter‐derived ¹⁵N was preferentially exported relative to litter‐derived ¹³C. Our results suggest that the AMF primarily took up N in the inorganic form, and N export is one mechanism by which AMF could modify the soil microbial community and decomposition processes.     

宁夏荒漠草原不同植物群落微斑块内土壤微生物区系特征

植物群落斑块化是天然放牧草地最基本的特征之一.为探索植物群落斑块化对土壤微生物群落组成及多样性的影响,本研究以宁夏荒漠草原为研究对象,采用磷脂脂肪酸(PLFA)法对比分析了不同植物群落微斑块内土壤微生物生物量和群落结构特征的变化.结果表明: 1) 4种植物群落微斑块内土壤微生物种类丰富,且都表现为细菌含量最高,真菌和放线菌含量较少,革兰氏阳性菌含量高于革兰氏阴性菌;2)4种植物群落中,甘草微斑块的土壤微生物总量显著高于猪毛蒿、苦豆子和黄芪;3)冗余分析表明,磷脂脂肪酸总量、革兰氏阳性菌、革兰氏阴性菌、真菌、厌氧菌、真菌/细菌均与土壤有机碳呈显著正相关,与pH呈显著负相关,表明土壤有机碳、pH是荒漠草原土壤微生物生长和发育的重要影响因素.     

内生真菌感染对禾草宿主生境土壤特性和微生物群落的影响

以羊草(Leymus chinensis)-内生真菌共生体为研究对象, 分别在野外样地和室内盆栽两种实验条件下研究了内生真菌感染对土壤特性和微生物群落结构的影响。结果显示：在处理时间较长并伴随有枯落物分解的羊草样地中, 内生真菌感染促进了土壤氮(N)的积累, 提高了30天培养时间内土壤初始碳(C)矿化速率和前3天土壤矿化量和土壤矿化总量; 而在处理时间较短且没有地上枯落物分解的盆栽羊草中, 内生真菌感染对土壤的C、N含量及C矿化均无显著影响。无论是野外样地还是室内盆栽实验, 内生真菌感染均未引起土壤微生物磷脂脂肪酸种类的变化, 但内生真菌感染均有提高土壤微生物生物量的趋势, 内生真菌显著增加了盆栽羊草土壤中细菌、革兰氏阴性细菌、真菌磷脂脂肪酸含量和磷脂脂肪酸总量, 增加了羊草样地土壤中革兰氏阳性细菌和放线菌的磷脂脂肪酸含量。总体看来, 内生真菌感染能够改变土壤N积累和C矿化率, 并且改变土壤中微生物群落的结构, 这有助于进一步认识内生真菌与羊草之间的共生关系及其在生态系统C、N循环中所起的作用。     

Soil microbial community structure and activity in a 100-year-old fertilization and crop rotation experiment

Aims Nitrogen (N) fertilization and lime addition may affect soil microbial and nematode communities and ecosystem functions through changing environmental conditions, such as soil pH and soil organic carbon. The objectives of this experiment were to examine the impact of N input and liming on soil microbial and nematode communities and to identify the key environmental determinant of community composition in a century-old fertilization and crop rotation experiment.Methods The field experiment consisting of a 3-year crop rotation regime was established in 1911 in southeastern USA. Four treatments, (i) no-input control, (ii) NPK with winter legume, (iii) PK with legume and lime and (iv) NPK with legume and lime, were included in this study. Soil samples collected at the 0–5cm depth were used to determine the bacterial growth rate by the 3 H-thymidine incorporation technique. Incorporation of 13 C into neutral lipids, glycolipids and phospholipid fatty acids (PLFAs) was measured after incubation of soil with 13 C-labeled acetate for 24h. Free-living nematodes in fresh soil were extracted using a density sucrose centrifugal flotation method and identified to trophic group level.Important findings Liming resulted in a 10-fold increase in bacterial growth rates compared with the no-input control, whereas N fertilization had no significant effect. Multivariate analysis of PLFA profiles showed that soil microbial community composition was different among the four treatments; the difference was primarily driven by soil pH. PLFAs indicative of Gram-negative bacteria covaried with soil pH, but not those of fungi and actinobacteria. Liming enhanced 13 C incorporation into neutral lipids, glycolipids and phospholipids by 2–15 times. In addition, 13 C incorporation into 16:0, 16:1ω9, 18:1ω9, 18:1ω7 and 18:2ω6 were greater than other PLFAs, suggesting that Gram-negative bacteria and fungi were more active and sensitive to simple C input. Bacterivorous nematodes were the dominant trophic group in the soil, but no significant differences in nematode communities were found among the treatments. Our results suggest that soil pH had a greater impact than N fertilization on soil microbial community composition and activity in a crop rotation system including legumes.     

Carbon allocation mediated by arbuscular mycorrhizal fungi alters the soil microbial community under various phosphorus levels

Studies have shown that arbuscular mycorrhizal fungi (AMF) can shape the rhizosphere microbial community of the host plant, but the underlying mechanisms are unclear. Here, we tested the hypotheses that AMF may affect the rhizosphere microbial community by mediating carbon (C) allocation of the host plant, and that this mediation may be modulated by the soil phosphorus (P) level. Using a split-root system, we conducted a microcosm experiment with three main effects (soil P level: 5 or 25 mg kg⁻¹; AMF: with or without inoculation; and spatial niche (i.e., rhizosphere, hyphosphere, and mycorrhizosphere). Host plant growth benefited from AMF under low soil-P conditions. ¹³CO₂ isotope labeling showed that AMF increased C allocation to the colonized root and AMF mycelia under low-P conditions, which promoted AMF growth. ¹³C-DNA-SIP and 16S rRNA sequencing further indicated that the enhanced C allocation from the host altered the soil microbial community. Our results suggest that AMF enhances the C allocation of the host plant below ground, which can shape microbial community composition. These AMF effects were greater with a low than with a high level of soil P.     

施磷对干旱胁迫下箭竹根际土壤养分及微生物群落的影响

以箭竹及其根际土壤作为研究对象,采用两因素随机区组实验,设置2种水分处理（正常浇水和干旱胁迫）和2种施磷量处理（施磷和不施磷）,探究施磷对干旱胁迫下箭竹根际土壤养分及微生物群落结构和多样性的影响。结果表明：（1）干旱胁迫显著降低了箭竹根际土壤中微生物量碳、可溶性有机氮和有效磷的含量,虽对箭竹根际土壤微生物群落的多样性无显著影响,但显著降低了箭竹根际土壤中总PLFA（phospholipid fatty acid contents）的含量和真菌、细菌、革兰氏阳性菌与革兰氏阴性菌的PLFA含量以及革兰氏阳性菌/革兰氏阴性菌的PLFA比值,显著改变了箭竹根际土壤微生物群落结构,结果显著降低了箭竹的生物量。（2）施磷显著增加了受旱箭竹根际土壤中微生物量碳和有效磷的含量,虽大体上对受旱箭竹根际土壤微生物群落的多样性无显著影响,但显著增加了受旱箭竹根际土壤中总PLFA和真菌PLFA的含量,并在一定程度上增加了细菌、革兰氏阳性菌、革兰氏阴性菌和放线菌的PLFA含量以及革兰氏阳性菌/革兰氏阴性菌和真菌/细菌的PLFA比值,也在一定程度上改善了受旱箭竹根际土壤微生物群落结构,从而改善受旱箭竹的生长。（3）主成分分析表明,干旱对箭竹根际土壤微生物群落结构的影响显著,而施磷的影响不明显。（4）相关分析发现,箭竹根际土壤微生物群落结构与箭竹根际土壤微生物量碳、可溶性有机氮及箭竹生物量呈显著正相关。综上,干旱降低了箭竹根际土壤养分含量和微生物生物量,改变了箭竹根际土壤微生物群落结构,抑制了箭竹的生长;施磷能增加受旱箭竹根际土壤养分含量和微生物生物量,改善受旱箭竹根际土壤微生物群落结构,进而改善受旱箭竹的生长。     

丛枝菌根真菌参与下植物-土壤系统的养分交流及调控

近几年随着有机农业的发展,丛枝菌根的作用受到特别关注。丛枝菌根是由植物根系与丛枝菌根真菌(AMF)形成的一种共生体。在植物-AMF-土壤系统中,AMF为植物提供N、P等营养的同时从根系得到所需的C。概述了植物-AMF-土壤系统中C、N、P等营养物质的交流以及AMF与土壤微生物的互作关系。丛枝菌根的形成可显著提高植物对P的吸收,且在高P条件下多余的P可储存于AMF中。AMF对土壤N循环的影响相当复杂,可能参与调控N循环的多个过程,如硝化作用、反硝化作用和氨氧化作用等。在有机质丰富的土壤中AMF菌丝可快速扩增并吸收其中的N,主要供菌丝自身所需,只有一小部分传递给植物。AMF对土壤C库的影响尚存争议,可能存在时间尺度的差异。短期内可活化土壤C,而在长期尺度上可能有利于土壤C的储存。AMF能够通过改变土壤微生物群落结构而影响植物-土壤体系的物质交流。AMF与解磷菌、根瘤菌和放线菌的协同增效作用可促进土壤有机质的降解或增强其固氮能力;AMF对氨氧化菌的抑制作用可降低氨的氧化减少N2O的释放。AMF与外生共生真菌EMF共存时,表现出协同增效作用,但EMF的优先定殖会限制AMF的侵染。AMF不同类群之间则主要表现为竞争和拮抗关系。AMF与土壤微生物之间的互作关系受土壤无机环境的影响,在养分亏缺条件下微生物之间往往表现为竞争关系。因植物、AMF与土壤微生物之间存在复杂的互作关系,为此AMF并不总是表现出其对植物营养的促进作用。目前关于AMF的作用机理仍以假说为主,需要进一步的实验验证。在植物-AMF-土壤系统中N与C的交流和P与C的交流并未表现出一致性,对N、P循环相互关系的进一步探讨有助于深入理解植物-土壤体系中的养分循环。植物、AMF和土壤微生物的养分来源及其对养分的相对需求强度和吸收效率尚未可知,因此无法深入理解AMF在植物-土壤体系中养分交流和转化的作用。在方法上,传统的土壤学方法在养分动态研究中存在局限性,现代分子生物学手段和化学计量学的结合值得尝试。     

Soil Moisture Alters the Response of Soil Organic Carbon Mineralization to Litter Addition

Increasing rainfall and longer drought conditions lead to frequent changes in soil moisture that affect soil organic carbon (SOC) mineralization. However, how soil moisture affects response of SOC mineralization to litter addition in forest ecosystems remains unexplored. We added ¹³C-labeled litter to subtropical forest soils with three mass water contents (L, 21%; M, 33%; H, 45%). Carbon dioxide production was monitored, and the composition of soil microbial communities was determined by phospholipid fatty acid (PLFA). When no litter was added, SOC mineralization was greater in the M-treated soil. Litter addition promoted SOC mineralization, but this promotion was altered by soil moisture and litter type. Priming effects induced by P. massoniana leaf litter in the M-moistened soil were significantly (P < 0.05) higher than those in other treatments. Litter-derived C was approximately 55% incorporated into 18:1ω9c and 16:0 PLFAs, and this proportion was not significantly affected by soil moisture. Soil moisture affected the distribution of litter-¹³C in i15:0, i17:0, and cy19:0 individual PLFAs. The primed C evolution was significantly related to the ratio of Gram-positive to Gram-negative bacteria. These results suggest that changes in soil moisture could affect SOC mineralization in forest ecosystems.     

Seasonal carbon allocation to arbuscular mycorrhizal fungi assessed by microscopic examination, stable isotope probing and fatty acid analysis

Background and Aim

Climate change models are limited by lack of baseline data, in particular carbon (C) allocation to – and dynamics within – soil microbial communities. We quantified seasonal C-assimilation and allocation by plants, and assessed how well this corresponds with intraradical arbuscular mycorrhizal fungal (AMF) storage and structural lipids (16:1ω5 NLFA and PLFA, respectively), as well as microscopic assessments of AMF root colonization.

Methods

Coastal Hypochoeris radicata plants were labeled with ¹³CO₂ in February, July and October, and ¹³C-allocation to fine roots and NLFA 16:1ω5, as well as overall lipid contents and AM colonization were quantified.

Results

C-allocation to fine roots and AMF storage lipids differed seasonally and mirrored plant C-assimilation, whereas AMF structural lipids and AM colonization showed no seasonal variation, and root colonization exceeded 80 % throughout the year. Molecular analyzes of the large subunit rDNA gene indicated no seasonal AMF community shifts.

Conclusions

Plants allocated C to AMF even at temperatures close to freezing, and fungal structures persisted in roots during times of low C-allocation. The lack of seasonal differences in PLFA and AM colonization indicates that NLFA analyses should be used to estimate fungal C-status. The implication of our findings for AM function is discussed.     

Relationships Between Soil Microorganisms, Plant Communities, and Soil Characteristics in Chinese Subtropical Forests

We analyzed the influence of above- and belowground factors on the soil microbial community in a Chinese subtropical forest, one of the most diverse biomes in the northern hemisphere. Soil samples were taken at different depths from four replicate comparative study plots in each of three forest age classes (young 10–40?years, medium 40–80?years, old ≥80?years). Microbial biomass and community structure were then determined using phospholipid fatty acid (PLFA) analysis, and basal respiration and microbial biomass carbon (C_mic) were determined by substrate-induced respiration. These data were then related to plant community and soil variables using non-metric multidimensional scaling analysis and post-hoc permutational correlations. We found that microbial lipid composition and abundance were not related to forest age class. Instead, microbial lipid composition and abundance were related to factors reflecting primary production, i.e., percent litter cover, percent dead wood cover, and percent tree layer cover. Specifically, the relative abundance (mol fraction) of indicators for arbuscular mycorrhizal fungi, Gram-positive and Gram-negative bacteria were positively significantly correlated with percent litter cover. We also found that the biomass of all microbial groups and total PLFA were negatively significantly related to percent deadwood cover. In addition, $ {\text{pH}}_{{{\text{H}}_{ 2} {\text{O}}}} $ was the only soil parameter that was correlated significantly to microbial biomass. Our results indicate that overarching ecological factors such as plant productivity and soil pH are important factors influencing the soil microbial community, both in terms of biomass and of community composition in this subtropical ecosystem.     

Nitrogen addition overrides the effects of Stellera chamaejasme litter on the growth of Leymus chinensis and its associated mycorrhizal fungi

氮添加掩盖了狼毒凋落物对羊草生长及其菌根真菌的影响 退化草地上迅速蔓延、生长的有毒植物能产生大量的凋落物,而氮输入能促进有毒植物的生长, 并改变其根际微生物特别是丛枝菌根真菌(AMF)的特征。然而,有毒植物的凋落物如何影响可食牧草的生长及其根际AMF,N沉降是否以及如何介导这些影响,目前尚不清楚。本文在中国东北的草甸草原上进行了温室实验以检验添加狼毒(Stellera chamaejasme)凋落物和氮素对优势植物羊草(Leymus chinensis) 的生长、AMF和土壤特征的影响。结果表明,添加凋落物显著增加了羊草分株数、地上生物量和土壤速效磷(AP)浓度,并降低了AMF孢子密度。然而,凋落物与氮素的交互作用并未显著影响羊草和AMF。狼毒凋落物通过提高AP对羊草产生正效应,并结合土壤养分平衡与凋落物诱导的化感成分的变化对AMF产生负效应。另外,高氮添加能缓解土壤的氮限制并抑制凋落物分解,从而减缓凋落物对羊草和AMF的影响。上述结果表明,除了关注有毒植物对家畜的有害作用外,还应客观地、全面地评价有毒植物的生态功能;同时,氮沉降应作为预测退化草地有毒植物与可食植物的种间关系的一项重要指标。     

凋落物和根系去除对滨海沙地土壤微生物群落组成和功能的影响

通过在亚热带滨海防护林湿地松、尾巨桉、纹荚相思和木麻黄人工林中设置去除凋落物、去除根系和对照处理,分析改变地上、地下碳输入对沙地人工林土壤微生物生物量、群落结构和功能的影响.2015年9月,在处理设置一年后采集各处理样方0～10 cm土壤样品,分别采用磷脂脂肪酸分析方法和微孔板酶检测技术对土壤样品的微生物群落组成和6种酶活性进行测定.结果表明: 碳输入处理对土壤微生物生物量的影响因树种而异,并主要取决于凋落物和根系的质量.在尾巨桉林中,去除根系使得脂肪酸总量、革兰氏阳性细菌、革兰氏阴性细菌、真菌和放线菌生物量分别显著减少了31%、30%、32%、36%和26%,去除凋落物使得革兰氏阳性细菌、真菌和放线菌生物量显著减少了24%、27%和24%,而其他树种处理对微生物生物量无显著影响.碳输入处理对土壤微生物群落结构的影响主要表现为真菌丰度下降而放线菌丰度上升.不同树种的土壤酶活性对处理的响应表现不一致:湿地松、纹荚相思和木麻黄林分去除凋落物显著降低土壤中纤维素水解酶、β-葡萄糖苷酶、酸性磷酸酶和乙酰氨基葡萄糖苷酶活性,去除根系仅分别降低和提高了湿地松和纹荚相思林β-葡萄糖苷酶的活性;湿地松、木麻黄人工林去除凋落物显著降低了多酚氧化酶和过氧化物酶活性;去除根系对所有树种土壤氧化酶活性影响不显著.不同树种的凋落物、根系特性是影响土壤微生物群落组成和酶活性的重要因子,碳输入处理造成的土壤温度、水分等微环境的改变也是土壤微生物性质变化的重要驱动力.     

喀斯特地区三种人工林土壤微生物群落结构特征

为揭示不同人工植被修复模式对喀斯特土壤微生物群落的影响,采用氯仿熏蒸提取法和磷脂脂肪酸(phospholipid fatty acid, PLFA)法研究人工构建的降香黄檀(Dalbergia odorifera)纯林(PDOP)、顶果木(Acrocarpus fraxinifolius)纯林(PAFP)、顶果木×降香黄檀混交林(MADP)对土壤微生物生物量及土壤微生物群落结构的影响。结果表明:(1)PDOP的土壤微生物生物量碳(MBC)和微生物生物量氮(MBN)含量显著高于PAFP和MADP,PAFP显著高于MADP。(2)三种人工林土壤真菌、丛枝菌根真菌和总PLFA含量无显著差异,但PDOP土壤细菌、放线菌、丛枝菌根真菌和总PLFA含量均高于PAFP和MADP,PAFP高于MADP。PDOP的土壤细菌、革兰氏阳性菌、革兰氏阴性菌、放线菌的PLFA含量显著高于MADP。MADP的真菌细菌比显著高于PDOP,但与PAFP无显著差异。(3)冗余分析表明,土壤阳离子交换量、pH和C:N是影响土壤微生物群落组成的最主要影响因子。从三种人工林的土壤微生物生物量及微生物群落结构来看,在喀斯特地区MADP并未显示出酸性土地区混交林提高土壤微生物生物量、改善土壤微生物群落结构的优势,但混交林的真菌细菌比最高,更有利于提高土壤生态系统的稳定性。     

Linking toluene degradation with specific microbial populations in soil

Phospholipid fatty acid (PLFA) analysis of a soil microbial community was coupled with (13)C isotope tracer analysis to measure the community's response to addition of 35 microg of [(13)C]toluene ml of soil solution(-1). After 119 h of incubation with toluene, 96% of the incorporated (13)C was detected in only 16 of the total 59 PLFAs (27%) extracted from the soil. Of the total (13)C-enriched PLFAs, 85% were identical to the PLFAs contained in a toluene-metabolizing bacterium isolated from the same soil. In contrast, the majority of the soil PLFAs (91%) became labeled when the same soil was incubated with [(13)C]glucose. Our study showed that coupling (13)C tracer analysis with PLFA analysis is an effective technique for distinguishing a specific microbial population involved in metabolism of a labeled substrate in complex environments such as soil.     

岷江干旱河谷优势灌丛对土壤微生物群落组成的影响

通过调查岷江干旱河谷两河口、飞虹、撮箕和牟托4个样地优势灌丛及其灌丛间空地的表土土壤物理化学性质和微生物群落组成,探讨植物灌丛群落对土壤微生物群落组成的影响。研究发现不同灌丛种类对土壤微生物群落组成以及土壤物理化学性质并没有显著影响,而同一样地灌丛与空地间的差异却较为显著。灌丛下比空地土壤中具有更高的有机质、养分含量,更高的土壤含水量和更低的容重,而灌丛下相对富集的养分资源是造成灌丛与空地间微生物群落组成差异的主要原因。不同样地影响微生物群落的主要因子存在一定差异,但与氮相关的因子(总氮、有效氮、碳/氮比)对土壤微生物群落着非常重要的影响,特别是对土壤微生物群落总生物量和细菌类群(革兰氏阳性菌、革兰氏阴性菌、细菌等)。虽然不同灌丛和空地下土壤中细菌群落都没有显著地变化,但真菌和菌根真菌却明显的在灌丛下富集。在飞虹和牟托样地,总磷和碳/磷比与真菌类群,主要指真菌和菌根真菌,表现出显著正相关性,这或许反映了真菌类群对于该区域磷循环的重要作用。研究结果揭示了灌丛植被在干旱河谷地区地下生态系统中的重要作用,以及氮、磷这两种养分元素对土壤微生物群落的重要影响。同时,未来对于干旱河谷地区植物-土壤关系的研究应该关注真菌和菌根真菌类群的作用。     

Winter climate change implications for decomposition in northeastern forests: comparisons of sugar maple litter with herbivore fecal inputs

Forests in northeastern North America are influenced by varying climatic and biotic factors; however, there is concern that rapid changes in these factors may lead to important changes in ecosystem processes such as decomposition. Climate change (especially warming) is predicted to increase rates of decomposition in northern latitudes. Warming in winter may result in complex effects including decreased levels of snow cover and an increased incidence of soil freezing that will effect decomposition. Along with these changes in climate, moose densities have also been increasing in this region, likely affecting nutrient dynamics. We measured decomposition and N release from ¹⁵N‐labeled sugar maple leaf litter and moose feces over 20 months in reference and snow removal treatment (to induce soil freezing) plots in two separate experiments at the Hubbard Brook Experimental Forest in New Hampshire, USA. Snow removal/soil freezing decreased decomposition of maple litter, but stimulated N transfer to soil and microbial biomass. Feces decomposed more rapidly than maple litter, and feces N moved into the mineral soil more than N derived from litter, likely due to the lower C : N ratio of feces. Feces decomposition was not affected by the snow removal treatment. Total microbial biomass (measured as microbial N and C) was not significantly affected by the treatments in either the litter or feces plots. These results suggest that increases in soil freezing and/or large herbivore populations, increase the transfer rate of N from plant detritus or digested plants into the mineral soil. Such changes suggest that altering the spatial and temporal patterns of soil freezing and moose density have important implications for ecosystem N cycling.     

Annual burning of a tallgrass prairie inhibits C and N cycling in soil,increasing recalcitrant pyrogenic organic matter storage while reducing N availability

Grassland ecosystems store an estimated 30% of the world's total soil C and are frequently disturbed by wildfires or fire management. Aboveground litter decomposition is one of the main processes that form soil organic matter (SOM). However, during a fire biomass is removed or partially combusted and litter inputs to the soil are substituted with inputs of pyrogenic organic matter (py‐OM). Py‐OM accounts for a more recalcitrant plant input to SOM than fresh litter, and the historical frequency of burning may alter C and N retention of both fresh litter and py‐OM inputs to the soil. We compared the fate of these two forms of plant material by incubating ¹³C‐ and ¹⁵N‐labeled Andropogon gerardii litter and py‐OM at both an annually burned and an infrequently burned tallgrass prairie site for 11 months. We traced litter and py‐OM C and N into uncomplexed and organo‐mineral SOM fractions and CO₂ fluxes and determined how fire history affects the fate of these two forms of aboveground biomass. Evidence from CO₂ fluxes and SOM C:N ratios indicates that the litter was microbially transformed during decomposition while, besides an initial labile fraction, py‐OM added to SOM largely untransformed by soil microbes. Additionally, at the N‐limited annually burned site, litter N was tightly conserved. Together, these results demonstrate how, although py‐OM may contribute to C and N sequestration in the soil due to its resistance to microbial degradation, a long history of annual removal of fresh litter and input of py‐OM infers N limitation due to the inhibition of microbial decomposition of aboveground plant inputs to the soil. These results provide new insight into how fire may impact plant inputs to the soil, and the effects of py‐OM on SOM formation and ecosystem C and N cycling.     

13C and 15N in microarthropods reveal little response of Douglas-fir ecosystems to climate change

Understanding ecosystem carbon (C) and nitrogen (N) cycling under global change requires experiments maintaining natural interactions among soil structure, soil communities, nutrient availability, and plant growth. In model Douglas-fir ecosystems maintained for five growing seasons, elevated temperature and carbon dioxide (CO₂) increased photosynthesis and increased C storage belowground but not aboveground. We hypothesized that interactions between N cycling and C fluxes through two main groups of microbes, mycorrhizal fungi (symbiotic with plants) and saprotrophic fungi (free-living), mediated ecosystem C storage. To quantify proportions of mycorrhizal and saprotrophic fungi, we measured stable isotopes in fungivorous microarthropods that efficiently censused the fungal community. Fungivorous microarthropods consumed on average 35% mycorrhizal fungi and 65% saprotrophic fungi. Elevated temperature decreased C flux through mycorrhizal fungi by 7%, whereas elevated CO₂ increased it by 4%. The dietary proportion of mycorrhizal fungi correlated across treatments with total plant biomass (n= 4, r²= 0.96, P= 0.021), but not with root biomass. This suggests that belowground allocation increased with increasing plant biomass, but that mycorrhizal fungi were stronger sinks for recent photosynthate than roots. Low N content of needles (0.8–1.1%) and A horizon soil (0.11%) coupled with high C : N ratios of A horizon soil (25–26) and litter (36–48) indicated severe N limitation. Elevated temperature treatments increased the saprotrophic decomposition of litter and lowered litter C : N ratios. Because of low N availability of this litter, its decomposition presumably increased N immobilization belowground, thereby restricting soil N availability for both mycorrhizal fungi and plant growth. Although increased photosynthesis with elevated CO₂ increased allocation of C to ectomycorrhizal fungi, it did not benefit plant N status. Most N for plants and soil storage was derived from litter decomposition. N sequestration by mycorrhizal fungi and limited N release during litter decomposition by saprotrophic fungi restricted N supply to plants, thereby constraining plant growth response to the different treatments.     

Large contribution of arbuscular mycorrhizal fungi to soil carbon pools in tropical forest soils

The origins and composition of soil organic matter (SOM) are still largely uncertain. Arbuscular mycorrhizal fungi (AMF) are recognized as indirect contributors through their influence on soil aggregation, plant physiology, and plant community composition. Here we present evidence that AMF can also make large, direct contributions to SOM. Glomalin, a recently discovered glycoprotein produced by AMF hyphae, was detected in tropical soils in concentrations of over 60 mg cm^–3. Along a chronosequence of soils spanning ages from 300 to 4.1 Mio years, a pattern of glomalin concentrations is consistent with the hypothesis that this protein accumulates in soil. Carbon dating of glomalin indicated turnover at time scales of several years to decades, much longer than the turnover of AMF hyphae (which is assumed to be on the order of days to weeks). This suggests that contributions of mycorrhizae to soil carbon storage based on hyphal biomass in soil and roots may be an underestimate. The amount of C and N in glomalin represented a sizeable amount (ca. 4–5%) of total soil C and N in the oldest soils. Our results thus indicate that microbial (fungal) carbon that is not derived from above- or below-ground litter can make a significant contribution to soil carbon and nitrogen pools and can far exceed the contributions of soil microbial biomass (ranging from 0.08 to 0.2% of total C for the oldest soils).     

改变碳输入对亚热带人工林土壤微生物生物量和群落组成的影响

通过在亚热带杉木(Cunninghamia lanceolata)和米老排(Mytilaria laosensis)人工林中设置互换凋落物、去除凋落物、去除凋落物+去除根系和对照处理来分析改变地上、地下碳输入对人工林土壤微生物生物量和群落组成的影响。结果显示,改变地上、地下碳输入对土壤微生物生物量碳、氮的影响因树种而异。在米老排林中,土壤微生物生物量不受碳源的限制。而在杉木林中,加入米老排凋落物、去除凋落物和去除凋落物+去除根系3种处理中土壤微生物生物量碳、氮具有明显增加的趋势。磷脂脂肪酸分析结果显示,杉木林中,添加高质量的米老排凋落物后,革兰氏阳性细菌、阴性细菌、丛枝菌根真菌、放线菌和真菌群落生物量分别显著增加了24%、24%、53%、25%、28%,革兰氏阴性细菌和丛枝菌根真菌的相对丰度均有显著增加。与对照相比,杉木林中去除凋落物后革兰氏阳性细菌、阴性细菌、丛枝菌根真菌、放线菌和真菌群落生物量分别显著增加了22%、29%、44%、25%、52%,真菌与细菌比值显著增加了21%。但是,去除凋落物+去除根系处理对两个树种人工林土壤微生物群落组成均无显著影响。米老排和杉木林土壤微生物生物量碳、氮的季节变化格局不同,土壤养分有效性可能是驱动土壤微生物生物量季节变化的主要因子。未来研究需要关注凋落物和根系在不同树种人工林中对土壤微生物群落的相对贡献。