Relationship between the structure of root systems and resource use for 11 North American grassland plants

Eleven Midwest North American grassland plant species differed in theirconstruction, production, and placement of fine and coarse belowground biomassin the soil profile after having been grown in containers in the field for twoand a half growing seasons. Based on the patterns of root system structure andresource utilization, the species we examined could be classified as 1)legumes,2) high-nitrogen rhizomatous C₃ species, and 3) a separategradient of differentiation from tall- to short-statured species(i.e. tallgrass to shortgrass species). Legumes depleted water evenlythroughoutthe soil profile, with little capacity for acquisition of inorganic nitrogenthroughout the 1m soil profile. The three rhizomatous species had shallow fineroot distributions, a large relative investment in shallow rhizomes, andmoisture and NO₃ ^– levels were low in shallow soils,but high at depth. Tallgrass species maintained a large standing root biomassofhigh-density, low-nitrogen fine roots, and acquire nitrogen andwater from a large, deep volume of soil, in which inorganic nitrogen is presentin low concentrations. Root systems ofshortgrass species lacked coarse belowground biomass, had fine roots that werefiner than those of the tallgrass species, and had a shallow root distribution.There was little support for functional dichotomies between the C₃and C₄ species or between the grasses and forbs. For example,Solidago rigida (C₃ forb) andAndropogon gerardii (C₄ grass) were moresimilarto each other than to other C₃ forbs or C₄ grasses,respectively.Across all species and depths examined, there were strong relationships betweenthe amount of fine root biomass present in a unit of volume of soil and thedepletion of soil water and nitrogen, but there were no relationships withcoarse belowground biomass. This reaffirms that differentiation of coarse andfine root biomass is as important as differentiating stems and leaves inevaluating plant allocation and ecosystem functioning.     

Positive tree diversity effect on fine root biomass: via density dependence rather than spatial root partitioning

The importance of species richness to ecosystem functioning and services is a central tenet of biological conservation. However, most of our theory and mechanistic understanding is based on diversity found aboveground. Our study sought to better understand the relationship between diversity and belowground function by studying root biomass across a plant diversity gradient. We collected soil cores from 91 plots with between 1 and 12 aboveground tree species in three natural secondary forests to measure fine root (≤ 2 mm in diameter) biomass. Molecular methods were used to identify the tree species of fine roots and to estimate fine root biomass for each species. This study tested whether the spatial root partitioning (species differ by belowground territory) and symmetric growth (the capacity to colonize nutrient-rich hotspots) underpin the relationship between aboveground species richness and fine root biomass. All species preferred to grow in nutrient-rich areas and symmetric growth could explain the positive relationship between aboveground species richness and fine root biomass. However, symmetric growth only appeared in the nutrient-rich upper soil layer (0–10 cm). Structural equation modelling indicated that aboveground species richness and stand density significantly affected fine root biomass. Specifically, fine root biomass depended on the interaction between aboveground species richness and stand density, with fine root biomass increasing with species richness at lower stand density, but not at higher stand density. Overall, evidence for spatial (i.e. vertical) root partitioning was inconsistent; assumingly any roots growing into deeper unexplored soil layers were not sufficient contributors to the positive diversity–function relationship. Alternatively, density-dependent biotic interactions affecting tree recruitment are an important driver affecting productivity in diverse subtropical forests but the usual root distribution patterns in line with the spatial root partitioning hypothesis are unrealistic in contexts where soil nutrients are heterogeneously distributed.     

大气CO2浓度升高对红桦幼苗根系的影响

应用封闭式生长室系统,研究了CO2浓度升高对红桦(Betula albosinensis)幼苗的根/冠、粗根和细根的干质量、非结构性碳水化合物类含量、碳含量和碳/氮、氮和磷的含量及氮磷吸收量的影响。结果表明:CO2浓度升高使红桦幼苗粗根和细根的干质量增加,同时根/冠值显著升高,表明CO2浓度升高使红桦幼苗生物量向根系的分配增加;与对照相比,粗根的还原糖、蔗糖和总可溶性糖含量显著增加,而在细根中没有显著变化;粗根、细根的淀粉和总的非结构性碳水化合物含量显著增加;CO2浓度升高下粗根和细根的碳含量有升高的趋势但未达到显著水平,同时氮含量降低,碳/氮值升高;氮的吸收量在粗根和细根中均无显著变化。上述结果表明,CO2浓度升高下红桦幼苗根系氮含量下降是由非结构性碳水化合物(主要是淀粉)含量升高和(或)根系生物量增加产生的稀释效应引起的。     

中亚热带森林植物多样性增加导致细根生物量“超产”

细根在森林生态系统C分配和养分循环过程中发挥着重要作用, 但对地下细根与植物多样性之间关系的研究相对较少。该研究选择中亚热带从单一树种的杉木(Cunninghamia lanceolata)人工林到多树种的常绿阔叶林(青冈(Cyclobalanopsis glauca)-石栎(Lithocarpus glaber)林)的不同植物多样性梯度, 用根钻法采集细根并测定其生物量, 用Win-RHIZO 2005C根系分析系统测定细根形态参数, 以验证以下3个假设: 1)植物种类丰富度高的林分其细根生产存在“地下超产”现象; 2)根系空间生态位的分离水平是否随着植物多样性增多而增大? 3)细根是否通过形态可塑性对林木竞争做出响应?结果显示: 从单一树种的杉木人工林到植物种类较复杂的青冈-石栎常绿阔叶林, 0-30 cm土层的林分细根总生物量和活细根生物量均呈增加的趋势, 即细根总生物量为杉木林(305.20 g·m^-2) <马尾松(Pinus massoniana)林(374.25 g·m^-2) <南酸枣(Choerospondias axillaris)林(537.42 g·m^-2) <青冈林(579.33 g·m^-2), 活细根生物量为杉木林(268.74 g·m^-2) <马尾松林(299.15 g·m^-2) <南酸枣林(457.32 g·m^-2) <青冈林(508.47 g·m^-2), 各森林类型之间的细根总生物量差异显著(p < 0.05), 但活细根生物量差异不显著。土壤垂直剖面上, 除杉木林细根生物量随土层变化不显著外, 其他森林类型的活细根生物量和总细根生物量均随土层变化显著, 表层细根生物量随树种多样性的升高呈减小趋势, 据此推测树种间的生态位分离水平逐渐增大。植物多样性的不同对林分的细根形态及空间分布格局影响不显著, 细根形态可塑性对生物量变化响应不明显。     

模拟氮沉降对中亚热带杉木幼树根系生物量的影响

植物根系是全球陆地生态系统碳储量的重要组成部分,在全球生态系统碳循环中起着重要作用,日益加剧的氮沉降会影响根系生物量在空间和不同径级的分配,进而影响森林生态系统的生产力和土壤养分循环。以杉木幼树为研究对象,通过野外氮沉降模拟实验,研究氮沉降四年后对不同土层、不同径级根系生物量的影响。结果发现：（1）低氮和高氮处理总细根生物量较对照均无显著差异（P > 0.05）,高氮处理粗根生物量及总根系生物量较对照分别增加45%和40%（P < 0.05）;（2）与对照相比,施氮处理显著增加20-40 cm与40-60 cm土层细根和粗根生物量,且在低氮处理下,20-40 cm土层细根、粗根在总土层细根与粗根生物量的占比显著提高。（3）与对照相比,高氮处理显著增加了2-5 mm、5-10 mm及10-20 mm径级的根系生物量,低氮处理显著增加2-5 mm、5-10 mm径级根系生物量,且显著降低20-50 mm径级根系生物量。综上所述表明：氮沉降后杉木幼树通过增加较粗径级根系来增加对养分及水分的输送,同时通过增加深层根系生物量及其比例的策略来维持杉木幼树的快速生长;而根系生物量的增加,在一定程度上会增加根系碳源的输入,影响土壤碳循环过程。     

Large altitudinal increase in tree root/shoot ratio in tropical mountain forests of Ecuador

Tropical rain forests decrease in tree height and aboveground biomass (AGB) with increasing elevation. The causes of this phenomenon remain insufficiently understood despite a number of explanations proposed including direct or indirect effects of low temperature on carbon acquisition and carbon investment, adverse soil conditions and impaired nutrient supply. For analysing altitudinal patterns of aboveground/belowground carbon partitioning, we measured fine (<2 mm in diameter) and coarse root (2–5 mm) biomass and necromass and leaf area index (LAI), and estimated AGB from stand structural parameters in five tropical mountain rain forests at 1050, 1540, 1890, 2380 and 3060 m along an altitudinal transect in the South Ecuadorian Andes. Average tree height and AGB were reduced to less than 50% between 1050 and 3060 m, LAI decreased from 5.1 to 2.9. The leaf area reduction must have resulted in a lowered canopy carbon gain and thus may partly explain the reduced tree growth in the high-elevation stands. In contrast, both fine and coarse root biomass significantly increased with elevation across this transect. The ratio of root biomass (fine and coarse) to AGB increased more than ten-fold from 0.04 at 1050 m to 0.43 at 3060 m. Under the assumption that fine root biomass does reflect root productivity, our data indicate a marked belowground shift in C allocation with increasing elevation. Possible explanations for this allocation shift are discussed including reduced N supply due to low temperatures, water logging or adverse soil chemical conditions. We conclude that the fine root system and its activity may hold the key for understanding the impressive reduction in tree size along tropical mountain slopes in Ecuador. Analyses of fine root turnover and longevity in relation to environmental factors along altitudinal transects in tropical mountains are urgently needed.     

Soil freezing alters fine root dynamics in a northern hardwood forest

The retention of nutrients within an ecosystem depends on temporal andspatial synchrony between nutrient availability and nutrient uptake, anddisruption of fine root processes can have dramatic impacts on nutrientretention within forest ecosystems. There is increasing evidence thatoverwinter climate can influence biogeochemical cycling belowground,perhaps by disrupting this synchrony. In this study, we experimentallyreduced snow accumulation in northern hardwood forest plots to examinethe effects of soil freezing on the dynamics of fine roots (< 1 mm diameter)measured using minirhizotrons. Snow removal treatment during therelatively mild winters of 1997–1998 and 1998–1999 induced mild freezingtemperatures (to –4 °C) lasting approximately three months atshallow soil depths (to –30 cm) in sugar maple and yellow birch stands.This treatment resulted in elevated overwinter fine root mortality in treatedcompared to reference plots of both species, and led to an earlier peak infine root production during the subsequent growing season. These shiftsin fine root dynamics increased fine root turnover but were not largeenough to significantly alter fine root biomass. No differences inmorality response were found between species. Laboratory tests on pottedtree seedlings exposed to controlled freezing regimes confirmed that mildfreezing temperatures (to –5 °C) were insufficient to directlyinjure winter-hardened fine roots of these species, suggesting that themarked response recorded in our forest plots was caused indirectly bymechanical damage to roots in frozen soil. Elevated fine root necromass intreated plots decomposed quickly, and may have contributed an excess fluxof about 0.5 g N/m²·yr, which is substantial relative tomeasurements of N fluxes from these plots. Our results suggest elevatedoverwinter mortality temporarily reduced fine root length in treatmentplots and reduced plant uptake, thereby disrupting the temporalsynchrony between nutrient availability and uptake and enhancing ratesof nitrification. Increased frequency of soil freezing events, as may occurwith global change, could alter fine root dynamics within the northernhardwood forest disrupting the normally tight coupling between nutrientmineralization and uptake.     

Biomass and distribution of fine and coarse roots from blue oak (Quercus douglasii) trees in the northern Sierra Nevada foothills of California

This research adds to the limited data on coarse and fine root biomass for blue oak (Quercus douglasii Hook and Arn.), a California deciduous oak species found extensively throughout the interior foothills surrounding the Central Valley. Root systems of six blue oak trees were analyzed using three methods — backhoe excavation, quantitative pits, and soil cores. Coarse root biomass ranged from 7 to 177 kg per tree. Rooting depth for the main root system ranged from 0.5 to 1.5 m, with an average of 70% of excavated root biomass located above 0.5 m. Of the total biomass in excavated central root systems, primary roots (including burls) accounted for 56% and large lateral roots (> 20 mm diameter) accounted for 36%. Data from cores indicated that most biomass outside of the root crown was located in fine roots and that fine root biomass decreased with depth. At surface depths (0–20 cm), small-fine (< 0.5 mm diameter) roots accounted for 71%, large-fine (0.5–2.0 mm) for 25%, and coarse (> 2 mm) for 4% of total root biomass collected with cores. Mean fine root biomass density in the top 50 cm was 0.43 kg m⁻³. Fine root biomass did not change with increasing distance from the trees (up to approximately 5 m). Thus, fine roots were not concentrated under the tree canopies. Our results emphasize the importance of the smallest size class of roots (<0.5 mm), which had both higher N concentration and, in the area outside the central root system, greater biomass than large fine (0.5–2.0 mm) or coarse (> 2.0 mm) roots. This revised version was published online in June 2006 with corrections to the Cover Date.     

Tree species diversity interacts with elevated CO2 to induce a greater root system response

As a consequence of land‐use change and the burning of fossil fuels, atmospheric concentrations of CO₂ are increasing and altering the dynamics of the carbon cycle in forest ecosystems. In a number of studies using single tree species, fine root biomass has been shown to be strongly increased by elevated CO₂. However, natural forests are often intimate mixtures of a number of co‐occurring species. To investigate the interaction between tree mixture and elevated CO₂, Alnus glutinosa, Betula pendula and Fagus sylvatica were planted in areas of single species and a three species polyculture in a free‐air CO₂ enrichment study (BangorFACE). The trees were exposed to ambient or elevated CO₂ (580 μmol mol^?1) for 4 years. Fine and coarse root biomass, together with fine root turnover and fine root morphological characteristics were measured. Fine root biomass and morphology responded differentially to the elevated CO₂ at different soil depths in the three species when grown in monocultures. In polyculture, a greater response to elevated CO₂ was observed in coarse roots to a depth of 20 cm, and fine root area index to a depth of 30 cm. Total fine root biomass was positively affected by elevated CO₂ at the end of the experiment, but not by species diversity. Our data suggest that existing biogeochemical cycling models parameterized with data from species grown in monoculture may be underestimating the belowground response to global change.     

Reconciling root plasticity and architectural ground rules in tree root growth models with voxel automata

Dynamic models of tree root growth and function have to reconcile the architectural rules for coarse root topology with the dynamics of fine root growth (and decay) in order to predict the strategic plus opportunistic behaviour of a tree root system in a heterogeneous soil. We present an algorithm for a 3D model based on both local (soil voxel level) and global (tree level) controls of root growth, with development of structural roots as a consequence of fine root function, rather than as driver. The suggested allocation rules of carbon to fine root growth in each rooted voxel depend on the success in water uptake in this voxel during the previous day, relative to overall supply and demand at plant level. The allocated C in each voxel is then split into proliferation (within voxel growth) and extension into neighbouring voxels (colonisation), with scale-dependent thresholds and transfer coefficients. The fine root colonisation process defines a dynamic and spatially explicit demand for transport functions. C allocation to development of a coarse root infrastructure linking all rooted voxels depends on the apparent need for adjustment of root diameter to meet the topologically defined sap flow through this voxel during the previous day. The allometric properties of the coarse root system are maintained to be in line with fractal branching theory. The model can predict the dynamics of the shape and structure (fine root density, coarse root topology and biomass) of the root system either independently of soil conditions (purely genetically-driven) or including both the genetic and environmental effects of roots interacting with soil water supply and its external replenishment, linking in with existing water balance models. Sensitivity of the initial model to voxel dimensions was addressed through explicit scaling rules resulting in scale-independent parameters. The model was parameterised for two tree species: hybrid walnut (Juglans nigra × regia) and wild cherry (Prunus avium L.) using results of a pot experiment. The model satisfactorily predicted the root growth behaviour of the two species. The model is sparse in parameters and yet applicable to heterogeneous soils, and could easily be upgraded to include additional local influences on root growth (and decay) such as local success in nutrient uptake or dynamic soil physical properties.     

陇中黄土高原主要造林树种细根生物量分布

以2 mm为粗、细根的划分界限,采用根钻法对黄土高原安家沟流域油松、白杨、山杏、刺槐、沙棘和柠条6个主要造林树种细根分布进行研究,并测定不同林地下土壤含水率和土壤理化性质.结果表明： 在水平方向上,油松细根生物量呈先增大后减小的二次多项式分布,其他5个树种细根生物量均呈对数分布,水平根系发达,细根主要分布在冠幅半径2~3倍的范围内,表明各植被通过水平扩展来获取更多的土壤水分.在垂直方向上,随着土层深度的增加,细根生物量均呈减小趋势.6种植被细根生物量与土壤水分、容重呈显著负相关,与有机质、全N含量呈显著正相关.     

Fine root morphological and functional traits in Fagus sylvatica and Fraxinus excelsior saplings as dependent on species, root order and competition

Aims

The mechanisms of belowground competition are not well understood. Addressing literature reports on competition-induced changes in tree fine root morphology, we conducted a growth experiment with tree saplings to investigate competition effects on important root morphological and functional traits in a root order-focused analysis.

Methods

European beech and European ash saplings were grown for 34 months in containers under greenhouse conditions in monoculture (2 conspecific plants), in mixture (1 beech and 1 ash) or as single plants. The root system was fractionated according to root orders and eight morphological and functional properties were determined.

Results

Root order was the most influential factor affecting the fine root traits (except for root diameter and δ¹³C); a significant species identity effect was found for root diameter, tissue density, N concentration and δ¹³C. Ash fine roots were thicker, but had lower tissue densities, contained more N and had systematically higher δ¹³C values than beech roots. The competition treatments had no significant effect on morphological root traits but altered δ¹³C in the 2nd root order.

Conclusion

Neither intra- nor interspecific root competition affected fine root morphology significantly suggesting that competition-induced root modification may not be a universal phenomenon in temperate trees.     

川滇高山栎灌丛萌生过程中的营养元素供应动态

萌生更新是森林更新的重要方式, 是硬叶栎林受到干扰后植被恢复的主要机制。以位于青藏高原东南缘的川西折多山东坡川滇高山栎(Quercus aquifoliodes)灌丛为研究对象, 调查分析了砍伐后灌丛萌生过程中基株根系和萌株生物量动态、营养元素含量, 以及基株根系和土壤对萌株生长过程中的营养元素供应动态。结果表明, 川滇高山栎灌丛平均地上和地下生物量分别为(11.25 ± 0.92) t·hm^-2和(34.85 ± 2.02) t·hm^-2, 具有较大的根冠比(3.10:1); 萌生过程中, 萌株生物量呈线性增加趋势, 以灌丛活细根生物量变化为最大, 其次是活中根和活粗根, 树桩和根蔸生物量变化最小; 萌生过程中, 灌丛细根和中根N、P含量表现为先增加、后降低的变化趋势, 萌生初期树桩、粗根和根蔸中N和K的含量明显下降, 根蔸中Ca含量略有下降, 而P没有明显下降, 根系Mg含量变化幅度较大, 灌丛地下根系储存了较多的营养元素; 土壤、树桩、粗根和根蔸是川滇高山栎灌丛砍伐后0-120天萌生生长的主要营养来源, 砍伐后60天, 萌株生长所需的营养除K元素主要来源于根系外, 其余营养元素主要来源于土壤; 在砍伐后60-120天, 基株根系对萌株生长所需的N、K和Ca贡献较大, 而对P和Mg的贡献较小; 在砍伐后120-180天, 根系除K元素对萌生生长还保持较大的贡献外, 对其余营养元素的贡献均较小。高山栎林管理要注重加强地下根系的保护。     

水曲柳根系生物量、比根长和根长密度的分布格局

采用连续钻取土芯法在生长季内对东北林业大学帽儿山实验林场17年生水曲柳人工林根系取样,研究水曲柳不同直径根系现存生物量、比根长和根长密度及垂直分布状况.结果表明,水曲柳人工林根系总生物量为1 637.6 g·m^-2,其中活根生物量占85%,死根占15%.在活根生物量当中,粗根(直径5～30 mm)占的比例最高（69.95%）,其次为活细根（直径<1 mm,13.53%）,小根(1～2 mm)和中等直径的根(2～5 mm)比例较小（分别为7.21%和9.31%）.直径<1 mm活细根的比根长为32.20 m·g^-1,直径5～30 mm粗根的比根长为0.08 m·g^-1.单位面积上活根的总长度为6 602.54 m·m^-2,其中直径<1 mm的细根占92.43%,其它直径等级则不到活根总长度的8%.直径<1 mm的细根生物量与根长密度具显著线性关系（R²=0.923）,但与比根长无显著相关关系（R²=0.134）.     

短期氮磷配施对刨花楠细根形态及其土壤微生物的影响

以1年生刨花楠幼苗为研究对象,通过不同的氮磷配施实验,采用扫描根系法和磷脂脂肪酸法,研究不同氮磷配施处理对刨花楠幼苗1—4级细根根序形态特征及其土壤微生物的影响。结果表明:(1)4种氮磷配施处理均显著增加了刨花楠1—2级根的比根长和比根面积(P0.05),降低了3—4级根的比根面积(P0.05);(2)通过不同梯度的氮磷配施,1—2级细根的根组织密度呈下降态势,而3—4级根的组织密度则显著增加(P0.05),体现低级根与高级根之间的权衡;(3)4种氮磷配施处理都显著降低刨花楠1—4级细根的平均直径(P0.05);(4)随着氮磷比的增加,微生物总量及细菌、真菌与放线菌数量等均呈现先增加后降低的趋势,并均在N∶P为10∶1时达到最大;(5)氮磷配施条件下,细菌、真菌等与1—2级细根的比根长和比根面积呈显著正相关,而与4级根的比根长和比根面积则呈显著负相关,革兰氏阳性菌、真菌等与3—4级根的组织密度存在显著正相关,而与1—2级根的组织密度无显著相关性。各级根序的平均直径均与土壤微生物无显著相关性。研究结果表明,短期氮磷配施以N∶P为10∶1的效果最好,其最有利于提高刨花楠苗木细根的养分吸收能力与养分吸收效率,苗木通过调整细根形态来适应氮沉降,其地下生物群落如土壤微生物及其与细根的关系也发生变化,进而影响地下生态系统碳氮循环和养分流动。     

Spatial changes and influencing factors of fine root carbon,nitrogen and phosphorus stoichiometry of plants in China

为了研究中国陆地植物细根碳(C)、氮(N)、磷(P)的空间变化模式,揭示细根在"温度-植物生理假说"及"生长速率假说"等方面的规律,该文收集已发表的有关中国陆地植物细根研究的文献,从中提取细根C、N、P元素含量及其相关数据,分析了细根C、N、P含量及其比例与经纬度之间的关系。结果表明:细根N、P元素含量均随纬度增加而增加,P含量随经度增加而降低,N:P随经度增加而增加。细根N、P含量与年平均气温、年降水量均呈负相关关系,与土壤养分呈正相关关系。在土壤养分、温度、降水量3个非生物因素中,土壤养分对细根N、P含量的影响最大。该文中细根和粗根的C:P、N:P差异变化不完全支持"生长速率假说"。根系和叶片一样,N、P含量与纬度呈正相关关系,支持"温度-植物生理假说",反映了植物对自然环境的适应策略。     

实验增温对陆地生态系统根系生物量的影响

根系是植物吸收土壤水分和养分的重要器官, 驱动着多个生态系统过程, 该研究揭示了实验增温对根系生物量的影响及机制, 可为气候变暖背景下土壤碳动态和生态系统过程的变化提供理论依据。该研究从已发表的151篇国内外研究论文中收集到611组数据, 通过整合分析(meta-analysis)方法研究了实验增温对根系生物量(根系总生物量、粗根生物量、细根生物量、根冠比)的影响, 并探讨了增温幅度、增温年限、增温方式的影响, 以及根系生物量对增温的响应与本底环境条件(生态系统类型、年平均气温、年降水量、干旱指数)的关系。结果表明: (1)模拟增温使细根生物量显著增加8.87%, 而对根系总生物量、粗根生物量、根冠比没有显著影响; (2)中等强度增温(1-2 ℃)使得细根生物量和根冠比分别提高14.57%和23.63%; 中短期增温实验(<5年)对细根生物量具有促进影响, 而长期增温实验(≥5年)使细根生物量有降低的趋势; 开顶箱增温和红外辐射增温分别使细根生物量显著提高了17.50%和12.16%, 而电缆加热增温使细根生物量和粗根生物量显著降低了23.44%和43.23%; (3)不同生态系统类型对于增温响应不一致, 模拟增温使苔原生态系统细根生物量显著提高了21.03%, 细根生物量对增温的响应与本底年平均气温、年降水量、干旱指数均呈显著负相关关系。     

Disentangling the roles of resource availability and disturbance in fine and coarse root biomass in savanna

Savannas are among the most unknown biomes concerning the plant below‐ground system. Root biomass might be influenced by the availability of limiting resources and by the type and intensity of disturbances, mediated by the plant functional attributes related to environmental conditions. Fine and coarse roots should be affected differently: the former should be more responsive to resource supplies, whilst the latter should be related to changes in disturbance frequency. We studied the roles of soil fertility, topography of the plots indicating water availability, fire frequency and plant resistance to fire in affecting fine and coarse root biomass. We sampled the root biomass, environmental variables and functional attributes of all individuals present in 100 plots in savanna physiognomies of cerrado, in central Brazil. We used structural equation modelling to test our hypothesis and found that shallow root biomass, from 0 to 20 cm deep, was not caused by resource availability, by disturbances, as fire or drought, or by functional attributes. Biotic interactions were not considered in our study, but they may have a central role in affecting the shallow root biomass. In the deep layer, from 20 to 100 cm deep, we identified soil fertility and recent fires as the main factors causing changes in fine and coarse root biomass in the cerrado, respectively. Low nutrient availability in the soil caused higher fine root biomass, increasing the uptake of resources, whereas recent fires led to less coarse root biomass below 20 cm deep, probably due to the higher dominance of the herbaceous layer in the plots, with less coarse root biomass. According to our expectation, fine roots were mostly affected by nutrient availability in the soil, whereas coarse roots were more related to disturbance, in our case, recent fires.     

水氮耦合对辣木幼苗根系形态特征的影响

辣木(Moringa oleifera)主要分布在热带地区，属落叶乔木，具有极高的经济价值和栽培用途。为探究12种水氮组合处理对辣木根系形态的影响，该研究设置3种土壤水分梯度，分别为40%(低水，W1)、60%(中水，W2)、80%(高水，W3)的田间饱和持水量，以及4种氮水平，分别为无氮(N0,0 g·plant^-1)、低氮(N1,0.6 g·plant^-1)、中氮(N2,1.8 g·plant^-1)、高氮(N3,3.6 g·plant^-1),比较不同水氮组合处理对辣木幼苗粗根和细根性状(根长度、根表面积、根体积、根平均直径、比根长、比表面积等)的影响。结果表明：(1)水处理对辣木幼苗粗根和细根的性状没均有显著影响。(2)氮处理对粗根的根长、根表面积、根体积、根平均直径和比表面积有显著影响，而仅对细根的平均直径和比表面积有显著影响。(3)双因素方差分析显示，水氮交互作用仅对细根根长有显著影响；在相同的水处理下，低氮促进而高氮抑制辣木粗根和细根根长、根表面积和根体积的增加，说明低氮中水处理和低氮高水...     

Cover crop root contributions to soil carbon in a no‐till corn bioenergy cropping system

Crop residues are potential biofuel feedstocks, but residue removal may reduce soil carbon (C). The inclusion of a cover crop in a corn bioenergy system could provide additional biomass, mitigating the negative effects of residue removal by adding to stable soil C pools. In a no‐till continuous corn bioenergy system in the northern US Corn Belt, we used ¹³CO₂ pulse labeling to trace plant C from a winter rye (Secale cereale) cover crop into different soil C pools for 2 years following rye cover crop termination. Corn stover left as residue (30% of total stover) contributed 66, corn roots 57, rye shoots 61, rye roots 50, and rye rhizodeposits 25 g C m^?2 to soil. Five months following cover crop termination, belowground cover crop inputs were three times more likely to remain in soil C pools than were aboveground inputs, and much of the root‐derived C was in mineral‐associated soil fractions. After 2 years, both above‐ and belowground inputs had declined substantially, indicating that the majority of both root and shoot inputs are eventually mineralized. Our results underscore the importance of cover crop roots vs. shoots and the importance of cover crop rhizodeposition (33% of total belowground cover crop C inputs) as a source of soil C. However, the eventual loss of most cover crop C from these soils indicates that cover crops will likely need to be included every year in rotations to accumulate soil C.