Due to projected increases in winter air temperatures in the northeastern USA over the next 100 years, the snowpack is expected to decrease in depth and duration, thereby increasing soil exposure to freezing air temperatures. To evaluate the potential physiological responses of sugar maple (Acer saccharum Marsh.) to a reduced snowpack, we measured root injury, foliar cation and carbohydrate concentrations, woody shoot carbohydrate levels, and terminal woody shoot lengths of trees in a snow manipulation experiment in New Hampshire, USA. Snow was removed from treatment plots for the first 6 weeks of winter for two consecutive years, resulting in lower soil temperatures to a depth of 50 cm for both winters compared to reference plots with an undisturbed snowpack. Visibly uninjured roots from trees in the snow removal plots had significantly higher (but sub-lethal) levels of relative electrolyte leakage than trees in the reference plots. Foliar calcium: aluminum (Al) molar ratios were significantly lower, and Al concentrations were significantly higher, in trees from snow removal plots than trees from reference plots. Snow removal also reduced terminal shoot growth and increased foliar starch concentrations. Our results are consistent with previous research implicating soil freezing as a cause of soil acidification that leads to soil cation imbalances, but are the first to show that this translates into altered foliar cation pools, and changes in soluble and structural carbon pools in trees. Increased soil freezing due to a reduced snowpack could exacerbate soil cation imbalances already caused by acidic deposition, and have widespread implications for forest health in the northeastern USA.

The decomposition and transformation of above- and below-ground plant detritus (litter) is the main process by which soil organic matter (SOM) is formed. Yet, research on litter decay and SOM formation has been largely uncoupled, failing to provide an effective nexus between these two fundamental processes for carbon (C) and nitrogen (N) cycling and storage. We present the current understanding of the importance of microbial substrate use efficiency and C and N allocation in controlling the proportion of plant-derived C and N that is incorporated into SOM, and of soil matrix interactions in controlling SOM stabilization. We synthesize this understanding into the Microbial Efficiency-Matrix Stabilization (MEMS) framework. This framework leads to the hypothesis that labile plant constituents are the dominant source of microbial products, relative to input rates, because they are utilized more efficiently by microbes. These microbial products of decomposition would thus become the main precursors of stable SOM by promoting aggregation and through strong chemical bonding to the mineral soil matrix.© 2012 Blackwell Publishing Ltd.

Winter climate is expected to change under future climate scenarios, yet the majority of winter ecology research is focused in cold-climate ecosystems. In many temperate systems, it is unclear how winter climate relates to biotic responses during the growing season. The objective of this study was to examine how winter weather relates to plant and animal communities in a variety of terrestrial ecosystems ranging from warm deserts to alpine tundra. Specifically, we examined the association between winter weather and plant phenology, plant species richness, consumer abundance, and consumer richness in 11 terrestrial ecosystems associated with the U.S. Long-Term Ecological Research (LTER) Network. To varying degrees, winter precipitation and temperature were correlated with all biotic response variables. Bud break was tightly aligned with end of winter temperatures. For half the sites, winter weather was a better predictor of plant species richness than growing season weather. Warmer winters were correlated with lower consumer abundances in both temperate and alpine systems. Our findings suggest winter weather may have a strong influence on biotic activity during the growing season and should be considered in future studies investigating the effects of climate change on both alpine and temperate systems.

本试验选择黄河源区果洛州玛沁县大武滩不同退化高寒沼泽湿地为研究对象，分层采集冻融丘和丘间土壤样品,分析退化过程中土壤腐殖质变化以及相关的环境因子。结果表明:冻融丘和丘间土壤腐殖质随着退化程度的加剧而下降，冻融丘腐殖质碳、胡敏素和胡敏酸未退化与轻度退化、重度退化差异显著(PPP＜0.01)；冻融丘中纤维二糖水解酶(CBH)、β-1,4-木糖苷酶(BXYL)、α-1，4-葡萄糖苷酶(αG)、β-1，4-葡萄糖苷酶(BG)、亮氨酸肽酶(LAP)、β-1，4-N-乙酰基氨基葡萄糖苷酶(NAG)和脲酶(UR)对胡敏素形成具有显著的促进作用，丘间酶活性对土壤腐殖质的形成具有显著的促进作用。综上所述，高寒沼泽湿地退化导致土壤腐殖质减少，致使碳功能的下降，土壤水分、全氮和土壤酶有利于湿地土壤腐殖质的形成，建议在高寒沼泽湿地修复中加强土壤水分和有机肥的补充。

<p id="p00005"><i><strong>Aims</strong></i> Forest litter are the main sources of soil humic substances, and different litter types can have differential effects on the formation and composition of soil humic substances. The aim of this study was to determine how variations in litter input would affect the dynamics of soil humic substances in subalpine forests of western Sichuan. <br><i><strong>Methods</strong></i> A field <i>in-situ </i>litter manipulation experiment incorporating litter removal was established on sites of coniferous, broad-leaved, and mixed coniferous and broad-leaved forest stands. Measurements were made on the contents of soil extractable humic substances, humic acid, and fulvic acid; ratios of humic acid to fulvic acid and humic acid to humic substances were also computed. <br><i><strong>Important findings</strong></i> The contents of extractable humic substances, humic acid and fulvic acid of soils varied significantly among forest types. The extractable humic substances content followed a descending order of coniferous forest &gt; mixed coniferous and broad-leaved forest &gt; broad-leaved forest, and the humic acid content of mixed coniferous and broad-leaved forest &gt; coniferous forest &gt; broad-leaved forest, and the fulvic acid content of coniferous forest &gt; broad-leaved forest &gt; mixed coniferous and broad-leaved forest. Fulvic acid was the predominant fraction of soil humic substances in all the three forest types. Time of measurements also significantly affected the contents of extractable humic substances, humic acid and fulvic acid, with an increasing trend for up to 1.5-year followed by decreases thereafter. With few exceptions, litter removal generally reduced the contents of extractable humic substances, humic acid and fulvic acid. The ratios of humic acid/fulvic acid and humic acid/extractable humic substances indicate low degree of humification for all the three forest types, which followed a descending order of mixed coniferous and broad-leaved forest &gt; broad-leaved forest &gt; coniferous forest. Litter removal improved the humic substances quality in the broad-leaved and mixed coniferous and broad-leaved forests to some extent. Correlation analysis showed significantly positive correlations of soil extractable humic substances with soil organic carbon, total nitrogen and soil water content, and a significantly negative correlation with temperature. In summary, short-term litter removal reduced the soil humic substances content, with differential effects by different litter types. We draw conclusion that the dynamics of soil humic substances are comprehensively regulated by litter type and environmental factors. Therefore, the impact of litter changes on soil humic substances needs further long-term research.</p>

森林凋落物作为森林土壤腐殖质的主要来源, 在土壤腐殖质的形成中发挥着重要作用, 但不同森林类型凋落物因其含量、组成等的不同, 对土壤腐殖质的影响也不同。该研究以川西亚高山针叶林、阔叶林和针阔混交林3种不同森林类型为对象, 采用凋落物原位控制实验, 对比研究不同关键期凋落物去除对土壤可提取腐殖质、胡敏酸和富里酸含量及胡敏酸/富里酸、胡敏酸/可提取腐殖质的影响。主要结果: (1)土壤可提取腐殖质、胡敏酸和富里酸含量在不同森林类型中差异显著。土壤可提取腐殖质含量总体表现为针叶林&gt;针阔混交林&gt;阔叶林, 胡敏酸含量总体表现为针阔混交林&gt;针叶林&gt;阔叶林, 而富里酸含量则表现为针叶林&gt;阔叶林&gt;针阔混交林, 其中3种林型中土壤腐殖质的主要成分为富里酸, 总体均表现为富里酸型。不同采样时期也显著影响了土壤可提取腐殖质、胡敏酸和富里酸含量, 总体均表现为先升高后下降的趋势。除个别采样时期外, 凋落物去除总体降低了土壤可提取腐殖质、胡敏酸和富里酸的含量。(2)胡敏酸/富里酸和胡敏酸/可提取腐殖质的结果显示3种林型土壤总体腐殖化程度均较低, 整体表现为针阔混交林&gt;阔叶林&gt;针叶林, 凋落物去除在一定程度上有利于提高阔叶林与针阔混交林的腐殖质品质。(3)相关分析表明不同凋落物处理间土壤可提取腐殖质与土壤有机碳含量、全氮含量和土壤含水量呈显著正相关关系, 与温度呈显著负相关关系。综上所述, 短期的凋落物去除会降低土壤腐殖物质的含量, 但不同林型间由于凋落物类型差异会导致土壤腐殖质的不同变化, 说明土壤腐殖质的动态变化受凋落物类型以及环境因素的综合调控。因此, 关于凋落物变化对土壤腐殖质的影响还需进一步的长期研究。

In this review, the evolution of Soil Organic Matter (SOM) research was traced back to outline the main achievement of understanding SOM in relation to its ecological functioning, particularly of carbon sequestration against climate change. The short-coming of soil humus theory, knowledge of SOM protection and stabilization, framework of newly emerged Humeomics as well as the increasingly active study of molecular organics in soils were analyzed and discussed, highlighting the importance of re-visiting SOM in term of structure-property-functions for the main mission of modern soil science. There were limitations of soil forming conditions, fraction separation procedure and single molecule identification for understanding the huge complex humus of larger sized synthesized molecules. Thanks to the ever-active studies of soil (organic) carbon sequestration and stabilization focusing on the association status of SOM with soil components, SOM has been increasingly recognized as an assemblage of metabolites from life activities on or in soil, with different allocation or protected in mineral/organic complex phases, which could be traced by biomarker molecules. Using such biomarker molecules as a target (like primer in molecular microbiology), all the molecules of SOM could be digested and isolated for qualitative or quantitative identification with GC/MS high resolution technologies. Such development has emerged a new paradigm of molecular SOM study, finally as SOMics as a modern soil science frontier. The functioning of SOM for stabilizing soil structure, enhancing reactivity and promoting biological resistance could be correlated to the paradigm of abundance, composition, structure and functions rather than the content and recalcitrance of SOM. This may deserve urgent studies to quantify and parameterize the defined paradigm based on the molecular composition of SOM. Again, such theory and technology development could provide a tool to manage SOM in term of carbon sequestration but revalorizing bioactivity in ecosystems, especially in agroecosystems. We believe such studies could rather depict the nature of SOM and of soil in relation to its ecological services and functioning, which will be the focus of soil science in serving the sustainable development of human society.

梳理了与土壤生态系统功能相联系的，特别是对固碳减排的土壤有机质本质认识的研究进展及路径，探讨了经典腐殖质学说存在的问题，概述了新近的有机质保护稳定学说及腐殖质组学学说，并追溯了生物标志物有机质分子研究，最后从土壤学的基本理念和理论出发讨论和重新认识土壤有机质的本质及其价值。从形成条件、分离条件和分子鉴定等多方面分析，土壤腐殖质形成和稳定学说越来越显示出局限性；而面向气候变化的碳固定研究可以深入探析土壤有机质的复杂存在状态。越来越认识到土壤有机质是投入土壤的有机物质经不同程度生物利用或降解的产物残留，只是被土壤不同程度地区隔和封闭，本质上仍是分子量变化极大的生命源有机物的集合。因此，可通过生物标志物分子作为靶标在土壤中提取和识别，该技术的发展将孕育萌生土壤有机质分子组学。后者可以用于判读土壤有机质的结构支撑、反应活性和促生功能等方面的本质差别，这些差别可能是由有机分子组成结构及存在状态所决定而不是由有机分子稳定性决定的。从这个概念出发，类似于土壤微生物分子生态，土壤有机质的丰度、组成、结构与功能间的联系可能是土壤有机质本质的核心问题。对这种关系的量化和参数化表征可用以探索土壤有机质永续固定，且可以保持生命活性的土壤有机质的管理策略及技术，并配合土壤的团聚体理论诠释土壤的本质和生态系统功能服务，这将是未来土壤学服务人类可持续发展的理论立足点。

We measured the activities of six kinds of enzyme, including β-glucosidase (BG), β-N-acetyl-glucosaminidase (NAG), leucine aminopeptidase (LAP), acid phosphatase (AP), polyphenol oxidase (POX), peroxidase (POD), as well as enzyme stoichiometric ratios and soil physical and chemical properties at 0-10 and 10-20 cm layers across typical <i>Pinus massoniana</i> plantation, <i>Pinus elliottii</i> plantation and mixed plantation of<i> P. massoniana</i> and <i>Schima superba</i> (broadleaved-conifer mixed plantation) in mid-subtropical China. Key factors driving the variation in soil enzyme activity and stoichiometry among different stand types were investigated. The results showed that the activities of soil BG and LAP were significantly affected by stand type. Soil BG activity at 10-20 cm soil layer was significantly higher in <i>P. elliottii</i> plantation than in <i>P. massoniana</i> plantation, while the activity of LAP was highest in the <i>P. massoniana</i> plantation. Soil BG/(NAG+LAP) and BG/AP at 10-20 cm layer of <i>P. elliottii</i> plantation were significantly higher than those of <i>P. massoniana</i> plantation, while (NAG+LAP)/AP of <i>P. massoniana</i> plantation was significantly higher than those of <i>P. elliottii</i> plantation and mixed plantation. The vector length of enzyme stoichiometry at 10-20 cm soil layer was significantly different among stand type, with an order of <i>P. elliottii</i> plantation &gt; broadleaved-conifer mixed plantation &gt; <i>P. massoniana</i>. The vector angles of enzyme stoichiometry in the three plantations were greater than 45°, with the vector angle in the<i> P. elliottii</i> plantation at 10-20 cm soil layer being significantly greater than that of the <i>P. massoniana</i> plantation. Results from redundancy analysis showed that soil carbon quality index and the ratio of soil organic carbon to total phosphorus (C/P), soil water content and C/P were the key factors affecting soil enzyme activity and stoichiometry at 0-10 and 10-20 cm soil layers, respectively. The quantity and quality of soil carbon and phosphorus, and soil water content played a key role in regulating nutrient cycling in mid-subtropical plantation ecosystem.

以中亚热带典型的马尾松林、湿地松林和马尾松-木荷混交林(针阔混交林)为研究对象,分析不同林分类型下0～10和10～20 cm土层的β-D-葡萄糖苷酶(BG)、β-N-乙酰氨基葡萄糖苷酶(NAG)、亮氨酸氨基肽酶(LAP)、酸性磷酸酶(AP)、多酚氧化酶(POX)、过氧化物酶(POD)6种土壤酶活性,以及酶化学计量比及土壤理化性质特征,分析驱动中亚热带典型林分类型土壤酶活性及其计量比变异的主要因素。结果表明: 林分类型显著影响了土壤BG和LAP活性,表现为湿地松林10～20 cm土层土壤BG显著高于马尾松林,而LAP在马尾松林最高;湿地松林10～20 cm土层土壤BG/(NAG+LAP)、BG/AP显著高于马尾松林,而马尾松林(NAG+LAP)/AP显著高于湿地松林和针阔混交林;林分类型间酶化学计量的向量长度在10～20 cm土层差异显著,表现为湿地松林&gt;针阔混交林&gt;马尾松林。3种人工林酶化学计量的向量角度均大于45°,其中在湿地松林10～20 cm土层向量角度显著大于马尾松林。冗余分析表明,土壤碳质量指数和有机碳与全磷的比值(C/P)以及土壤含水量和C/P分别是0～10和10～20 cm土层土壤酶活性及其化学计量的关键影响因素,土壤碳和磷的数量和质量,以及土壤含水量在调节中亚热带人工林生态系统养分循环中发挥关键作用。

The formation of soil organic matter via humification of plant litter is important for long-term carbon sequestration in forests; however, whether soil fauna affects litter humification is unclear. In this study, we quantified the effects of soil fauna on the optical properties (i.e., Delta logK and E4/E6) of the alkaline-extracted humic acid-like solutions of four foliar litters by removing soil fauna via litterbags with different mesh sizes in two subtropical evergreen broad-leaved forests. Litterbags were collected at the leaf falling, budding, expanding, maturation, and senescence stages from November 2013 to October 2015 to assess whether the effects of soil fauna on litter humification vary in different plant phenology periods. The results showed that soil fauna significantly reduced the Delta logK and E4/E6 values in the leaf expanding stage of oak litter and in the leaf falling stage of camphor and fir litters. The richness index of soil fauna explained 21%, 55%, 19%, and 45% of the variations in the E4/E6 values for oak, fir, camphor, and pine litters, respectively. The effects of litter water content on these optical properties were greater than that of temperature. These results indicated that soil fauna plays a key role in litter humification in the leaf expanding and falling stages and are potentially involved in soil carbon sequestration in these subtropical forests.

During soil formation, the accumulation of humic substances such as humic acid and fulvic acid is an important way to maintain soil fertility and nutrient cycling, which is regulated by soil substrate quality, litter, and environmental factors. In a laboratory incubation experiment, we exa-mined the effects of litter addition on soil humic substances accumulation in freeze-thaw environment by controlling the freeze-thaw cycles and litter additions in soils from the typical coniferous forest, mixed forest and broadleaved forest in a subalpine forest in western Sichuan. The freeze-thaw events significantly increased the content of humic substances in the coniferous forest soils but decreased those in the mixed forest soil and broadleaved forest soil. Litter addition had no significant effect on the content of soil humic substances. Freeze-thaw events increased the content of humic acid, with the net accumulation of humic acid following the order of mixed forest &gt; coniferous forest &gt; broad-leaved forest. Freeze-thaw events decreased the content of fulvic acid in the three forest soils during the early stage of incubation, and the degree of fulvic acid degradation was broadleaved forest &gt; mixed forest &gt; coniferous forest. Litter addition had no significant effect on the content of soil humic acid and fulvic acid. With prolonged incubation, the content of humic acid and fulvic acid in the three types of forest soils all declined. These results indicated that litter had effects on soil humic substances, which were related to soil substrate quality and affected by the duration of soil freeze-thaw events in winter.

亚高山森林土壤形成过程中,胡敏酸、富里酸等腐殖物质的累积是维持土壤肥力及物质循环的重要途径,它受到土壤基质质量、凋落叶和环境因素的调控.本研究以川西亚高山典型的针叶林、针阔混交林和阔叶林土壤为对象,采用室内培养控制冻融环境和凋落叶添加的方法,研究冻融环境下凋落叶添加对土壤腐殖物质累积的影响.结果表明: 冻融循环环境下针叶林土壤腐殖质含量升高,而针阔混交林和阔叶林土壤腐殖质含量降低,且凋落叶对土壤腐殖质含量无显著影响.培养前期冻融环境下3种林型土壤胡敏酸净累积,净累积量大小为针阔混交林＞针叶林＞阔叶林,富里酸含量下降,下降程度为阔叶林＞针阔混交林＞针叶林,且凋落叶对土壤胡敏酸和富里酸含量无显著影响.随培养时间的延长,3种林型土壤胡敏酸及富里酸含量均下降.这表明,凋落叶对土壤腐殖质含量的影响与土壤基质质量存在密切关系,且受到冬季土壤冻融时间长短的影响.

Seasonal snow cover may change the characteristics of freezing, leaching and freeze-thaw cycles in the scenario of climate change, and then play important roles in the dynamics of water soluble and organic solvent soluble components during foliar litter decomposition in the alpine forest. Therefore, a field litterbag experiment was conducted in an alpine forest in western Sichuan, China. The foliar litterbags of typical tree species (birch, cypress, larch and fir) and shrub species (willow and azalea) were placed on the forest floor under different snow cover thickness (deep snow, medium snow, thin snow and no snow). The litterbags were sampled at snow formation stage, snow cover stage and snow melting stage in winter. The results showed that the content of water soluble components from six foliar litters decreased at snow formation stage and snow melting stage, but increased at snow cover stage as litter decomposition proceeded in the winter. Besides the content of organic solvent soluble components from azalea foliar litter increased at snow cover stage, the content of organic solvent soluble components from the other five foliar litters kept a continue decreasing tendency in the winter. Compared with the content of organic solvent soluble components, the content of water soluble components was affected more strongly by snow cover thickness, especially at snow formation stage and snow cover stage. Compared with the thicker snow covers, the thin snow cover promoted the decrease of water soluble component contents from willow and azalea foliar litter and restrain the decrease of water soluble component content from cypress foliar litter. Few changes in the content of water soluble components from birch, fir and larch foliar litter were observed under the different thicknesses of snow cover. The results suggested that the effects of snow cover on the contents of water soluble and organic solvent soluble components during litter decomposition would be controlled by litter quality.

季节性雪被可能通过冻结、淋溶以及冻融循环等对高山森林凋落物水溶性和有机溶性组分含量产生影响.本文采用凋落物分解袋法，以川西高山森林典型乔木（四川红杉、岷江冷杉、红桦、方枝柏）和灌木（高山杜鹃、康定柳）凋落物为研究对象，研究了雪被覆盖不同时期（雪被形成期、雪被覆盖期和雪被融化期）和雪被厚度（厚型雪被、中型雪被、薄型雪被和无雪被）下凋落物水溶性和有机溶性组分含量的动态变化特征.结果表明： 在一个冬季的分解过程中，6种凋落物水溶性组分含量在雪被形成期和融化期降低而雪被覆盖期增加，但除高山杜鹃凋落物有机溶性组分含量在雪被覆盖期增加外，其他5种凋落物有机溶性组分含量在整个冬季呈降低趋势.相对于凋落物有机溶性组分含量，不同厚度雪被斑块对凋落物水溶性组分含量变化的影响更大，且主要表现在雪被形成期和雪被覆盖期.相对于其他雪被斑块，薄型雪被斑块更加显著地促进了高山柳和高山杜鹃凋落物水溶性组分含量降低，但显著抑制了方枝柏凋落物水溶性组分含量降低，而其他凋落物水溶性组分含量变化在不同斑块间无显著差异.冬季高山森林雪被对凋落物水溶性和有机溶性组分含量的影响主要受控于凋落物质量.

Bacterial communities are the primary engineers during litter decomposition and related material cycling, and they can be strongly controlled by seasonal changes in temperature and other environmental factors. However, limited information is available on changes in the bacterial community from winter to the growing season as litter decomposition proceeds in cold climates. Here, we investigated the abundance and structure of bacterial communities using real-time quantitative PCR and denaturing gradient gel electrophoresis (DGGE) during a 2-year field study of the decomposition of litter of 4 species in the winter and growing seasons of an alpine forest of the eastern Tibetan Plateau. The abundance of the bacterial 16S rRNA gene was relatively high during decomposition of cypress and birch litter in the first winter, but for the other litters 16S rRNA abundance during both winters was significantly lower than during the following growing season. A large number of bands were observed on the DGGE gels, and their intensities and number from the winter samples were lower than those from the growing season during the 2-year decomposition experiment. Eighty-nine sequences from the bands of bacteria that had been cut from the DGGE gels were affiliated with 10 distinct classes of bacteria and an unknown group. A redundancy analysis indicated that the moisture, mass loss, and elemental content (e.g., C, N, and P) of the litter significantly affected the bacterial communities. Collectively, the results suggest that uneven seasonal changes in climate regulate bacterial communities and other decomposers, thus affecting their contribution to litter decomposition processes in the alpine forest.