Changes in ecosystem carbon stocks in a grassland ash (Fraxinus excelsior) afforestation chronosequence in Ireland

Aims Government policy in Ireland is to increase the national forest cover from the current 10% to 18% of the total land area by 2020. This represents a major land use change that is expected to impact on the national carbon (C) stocks. While the C stocks of ecosystem biomass and soils of Irish grasslands and coniferous forests have been quantified, little work has been done to assess the impact of broadleaf afforestation on C stocks.Methods In this study, we sampled a chronosequence of ash (Fraxinus excelsior) forests aged 12, 20, 27, 40 and 47 years on brown earth soils. A grassland site, representative of the pre-afforestation land use, was sampled as a control.Important findings Our results show that there was a significant decline (P < 0.05) in the carbon density of the soil (0–30cm) following afforestation from the grassland (90.2 Mg C ha^-1) to the 27-year-old forest (66.7 Mg C ha^-1). Subsequently, the forest soils switched from being a C source to a C sink and began to sequester C to 71.3 Mg C ha^-1 at the 47-year-old forest. We found the amount of C stored in the above- and belowground biomass increased with age of the forest stands and offset the amount of C lost from the soil. The amount of C stored in the above- and belowground biomass increased on average by 1.83 Mg C ha^-1 year^-1. The increased storage of C in the biomass led to an increase in the total ecosystem C, from 90.2 Mg C ha^-1 at the grassland site to 162.6 Mg C ha^-1 at the 47-year-old forest. On a national scale, projected rates of ash afforestation to the year 2020 may cause a loss of 290 752 Mg C from the soil compared to 2 525 936 Mg C sequestered into the tree biomass. The effects of harvesting and reforestation may further modify the development of ecosystem C stocks over an entire ash rotation.     

Net primary production in three bioenergy crop systems following land conversion

Aims Identifying the amount of production and the partitioning to above- and belowground biomass is generally the first step toward selecting bioenergy systems. There are very few existing studies on the dynamics of production following land conversion. The objectives of this study were to (i) determine the differences in aboveground net primary production (ANPP), belowground net primary production (BNPP), shoot-to-root ratio (S:R) and leaf area index in three bioenergy crop systems and (ii) evaluate the production of these three systems in two different land use conversions.Methods This investigation included biometric analysis of NPP on three agricultural sites converted from conservation reserve program (CRP) management to bioenergy crop production (corn, switchgrass and prairie mix) and three sites converted from traditional agriculture production to bioenergy crop production.Important findings The site converted from conventional agriculture produced smaller ANPP in corn (19.03±1.90 standard error [SE] Mg ha^-1 year^-1) than the site converted from CRP to corn (24.54±1.43 SE Mg ha^-1 year^-1). The two land conversions were similar in terms of ANPP for switchgrass (4.88±0.43 SE for CRP and 2.04±0.23 SE Mg ha^-1 year^-1 for agriculture) and ANPP for prairie mix (4.70±0.50 SE for CRP and 3.38±0.33 SE Mg ha^-1 year^-1 for agriculture). The BNPP at the end of the growing season in all the bioenergy crop systems was not significantly different (P = 0.75, N = 8).     

Carbon and nitrogen dynamics in a Pinus densiflora forest with low and high stand densities

Aims Understanding carbon (C) and nitrogen (N) dynamics and their dependence on the stand density of an even-aged, mature forest provides knowledge that is important for forest management. This study investigated the differences in ecosystem total C and N storage and flux between a low-density stand (LD) and a high-density stand (HD) and examined the effects of stand density on aboveground net primary productivity (ANPP), total belowground C allocation (TBCA) and net ecosystem production (NEP) in a naturally regenerated, 65- to 75-year-old Pinus densiflora S. et Z. forest.Methods LD (450 trees ha^-1) and HD (842 trees ha^-1) were established in an even-aged, mature P. densiflora forest in September 2006. The forest had been naturally regenerated following harvesting, and the stand density was naturally maintained without any artificial management such as thinning. The diameter at breast height (DBH ≥ 5.0cm) of all live stems within the stands was measured yearly from 2007 to 2011. To compare C and N storage and fluxes in LD and HD, C and N pools in aboveground and belowground biomass, the forest floor, coarse woody debris (CWD) and soil; soil CO₂ efflux (R S); autotrophic respiration (R A); litter production; and soil N availability were measured. Further, ANPP, TBCA and NEP were estimated from plot-based measurement data.Important findings Ecosystem C (Mg C ha^-1) and N (Mg N ha^-1) storage was, respectively, 173.0±7.3 (mean ± SE) and 4.69±0.30 for LD and 162±11.8 and 4.08±0.18 for HD. There were no significant differences in C and N storage in the ecosystem components, except for soils, between the two stands. In contrast, there were significant differences in aboveground ANPP and TBCA between the two stands (P < 0.05). Litterfall, biomass increment and R S were major C flux components with values of, respectively, 3.89, 3.74 and 9.07 Mg C ha^-1 year^-1 in LD and 3.15, 2.94 and 7.06 Mg C ha^-1 year^-1 in HD. Biometric-based NEP (Mg C ha^-1 year^-1) was 4.18 in LD and 5.50 in HD. Although the even-aged, mature P. densiflora forest had similar C and N allocation patterns, it showed different C and N dynamics depending on stand density. The results of the current study will be useful for elucidating the effects of stand density on C and N storage and fluxes, which are important issues in managing natural mature forest ecosystems.     

Response of plant biomass to nitrogen addition and precipitation increasing under different climate conditions and time scales in grassland

生产力是草地生态系统重要的服务功能, 而生物量作为生态系统生产力的主要组成部分, 往往同时受到氮和水分两个因素的限制。在全球变化背景下, 研究草地生态系统生物量对氮沉降增加和降水变化的响应具有重要意义, 但现有研究缺乏对其在大区域空间尺度以及长时间尺度上响应的综合评估和量化。本研究搜集了1990-2017年间发表论文的有关模拟氮沉降及降水变化研究的相关数据, 进行整合分析, 探讨草地生态系统生物量对氮沉降和降水量两个因素的变化在空间和时间尺度上的响应。结果表明: (1)氮添加、增雨处理以及同时增氮增雨处理都能够显著地提高草地生态系统的地上生物量(37%, 41%, 104%)、总生物量(32%, 23%, 60%)和地上地下生物量比(29%, 25%, 46%)。单独增雨显著提高地下生物量(10%), 单独施氮对地下生物量影响不显著, 但同时增雨则能显著提高地下生物量(43%); (2)氮添加和增雨处理对草地生态系统生物量的影响存在明显的空间变异。在温暖性气候区和海洋性气候区的草地生态系统中, 氮添加对地上、总生物量及地上地下生物量比的促进作用更强, 而在寒冷性气候区和温带大陆性气候区的草地生态系统中, 则增雨处理对地下、总生物量的促进作用更强; (3)草地生态系统生物量对氮添加和增雨处理的响应也存在时间格局上的变化, 地下生物量随着氮添加年限的增加有降低的趋势, 地上、总生物量及地上地下生物量比则有增加的趋势。增雨年限的增加对总生物量没有明显的影响, 但持续促进地上生物量和地下生物量, 增加地上地下生物量比, 可见长期增氮、长期增雨对地上生物量的促进作用更明显。     

草地生态系统生物量在不同气候及多时间尺度上对氮添加和增雨处理的响应

生产力是草地生态系统重要的服务功能, 而生物量作为生态系统生产力的主要组成部分, 往往同时受到氮和水分两个因素的限制。在全球变化背景下, 研究草地生态系统生物量对氮沉降增加和降水变化的响应具有重要意义, 但现有研究缺乏对其在大区域空间尺度以及长时间尺度上响应的综合评估和量化。本研究搜集了1990-2017年间发表论文的有关模拟氮沉降及降水变化研究的相关数据, 进行整合分析, 探讨草地生态系统生物量对氮沉降和降水量两个因素的变化在空间和时间尺度上的响应。结果表明: (1)氮添加、增雨处理以及同时增氮增雨处理都能够显著地提高草地生态系统的地上生物量(37%, 41%, 104%)、总生物量(32%, 23%, 60%)和地上地下生物量比(29%, 25%, 46%)。单独增雨显著提高地下生物量(10%), 单独施氮对地下生物量影响不显著, 但同时增雨则能显著提高地下生物量(43%); (2)氮添加和增雨处理对草地生态系统生物量的影响存在明显的空间变异。在温暖性气候区和海洋性气候区的草地生态系统中, 氮添加对地上、总生物量及地上地下生物量比的促进作用更强, 而在寒冷性气候区和温带大陆性气候区的草地生态系统中, 则增雨处理对地下、总生物量的促进作用更强; (3)草地生态系统生物量对氮添加和增雨处理的响应也存在时间格局上的变化, 地下生物量随着氮添加年限的增加有降低的趋势, 地上、总生物量及地上地下生物量比则有增加的趋势。增雨年限的增加对总生物量没有明显的影响, 但持续促进地上生物量和地下生物量, 增加地上地下生物量比, 可见长期增氮、长期增雨对地上生物量的促进作用更明显。     

Fine Root Biomass,Production, Turnover Rates,and Nutrient Contents in Boreal Forest Ecosystems in Relation to Species,Climate, Fertility,and Stand Age: Literature Review and Meta-Analyses

Fine roots <2 mm in diameter play a key role in regulating the biogeochemical cycles of ecosystems and are important to our understanding of ecosystem responses to global climate changes. Given the sensitivity of fine roots, especially in boreal region, to climate changes, it is important to assess whether and to what extent fine roots in this region change with climates. Here, in this synthesis, a data set of 218 root studies were complied to examine fine root patterns in the boreal forest in relation to site and climatic factors. The mean fine root biomass in the boreal forest was 5.28 Mg ha^?1, and the production of fine roots was 2.82 Mg ha^?1 yr^?1, accounting for 32% of annual net primary production of the boreal forest. Fine roots in the boreal forest on average turned over 1.07 times per year. Fine roots contained 50.9 kg ha^?1 of nitrogen (N) and 3.63 kg ha^?1 of phosphorous (P). In total, fine roots in the boreal forest ecosystems contain 6.1 × 10⁷ Mg N and 4.4×10⁶Mg P pools, respectively, about 10% of the global nutrients of fine roots. Fine root biomass, production, and turnover rate generally increased with increasing mean annual temperature and precipitation. Fine root biomass in the boreal forest decreased significantly with soil N and P availability. With increasing stand age, fine root biomass increased until about 100 years old for forest stands and then leveled off or decreased thereafter. These results of meta analysis suggest that environmental factors strongly influence fine root biomass, production, and turnover in boreal forest, and future studies should place a particular emphasis on the root-environment relationships.     

气候变化和林火干扰对大兴安岭林区地上生物量影响的动态模拟

预测森林地上生物量对气候变化和林火干扰的响应是陆地生态系统碳循环研究的重要内容,气温、降水等因素的改变和气候变暖导致林火干扰强度的变化将会影响森林生态系统的碳库动态.东北森林作为我国森林的重要组成部分,对气候变化和林火干扰的响应逐渐显现.本文运用LANDIS PRO模型,模拟气候变化对大兴安岭森林地上生物量的影响,并比较分析了气候变暖对森林地上生物量的直接影响与通过林火干扰强度改变所产生的影响.结果表明: 未来气候变暖和火干扰增强情景下,森林地上生物量增加;当前气候条件和火干扰下,研究区森林地上生物量为(97.14±5.78) t·hm^-2;在B1F2预案下,森林地上生物量均值为(97.93±5.83) t·hm^-2;在A2F3预案下,景观水平第100～150和150～200年模拟时期内的森林地上生物量均值较高,分别为(100.02±3.76)和(110.56±4.08) t·hm^-2.与当前火干扰相比,CF2预案(当前火干扰增加30%)在一定时期使景观水平地上生物量增加(0.56±1.45) t·hm^-2,CF3预案(当前火干扰增加230%)在整个模拟阶段使地上生物量减少(7.39±1.79) t·hm^-2.针叶、阔叶树种对气候变暖的响应存在差异,兴安落叶松和白桦生物量随气候变暖表现为降低趋势,而樟子松、云杉和山杨的地上生物量则随气候变暖表现出不同程度的增加;气候变暖对针阔树种的直接影响具有时滞性,针叶树种响应时间比阔叶树种迟25～50年.研究区森林对高CO₂排放情景下气候变暖和高强度火干扰的共同作用较为敏感,未来将明显改变研究区森林生态系统的树种组成和结构.     

施氮和增温对降香黄檀幼苗生长发育和生理特征的影响

采用添加氮素模拟氮沉降和人工气候箱模拟增温的方法,设置了对照、施氮、增温以及增温×施氮交互作用4个处理,分析不同处理对降香黄檀(Dalbergia odorifera T. Chen)幼苗生长形态变化、生物量的积累、叶片和根系生理方面的影响。结果显示:与对照相比,增温、施氮以及增温×施氮交互作用均促进了降香黄檀幼苗的生长形态发育、增加了生物量的积累、光合参数、最大光化学量子产量、叶片硝酸还原酶及根系活力,但降低了叶片相对电导率和丙二醛含量;增温×施氮交互作用下降香黄檀幼苗的生物量积累、光合参数、最大光化学量子产量、叶片硝酸还原酶及根系活力的增幅最大,而叶片相对电导率和丙二醛含量降幅最显著;隶属函数分析结果表明,增温×施氮交互作用对降香黄檀幼苗生长发育的促进效果最好,其次为单独增温,最后为施氮处理。     

模拟氮沉降对华西雨屏区苦竹林细根特性和土壤呼吸的影响

细根在森林生态系统地下碳循环过程中具有核心地位.2007年11月-2009年11月,对华西雨屏区苦竹人工林进行了模拟氮沉降试验.氮沉降水平分别为对照(CK,0 g N·m^-2·a^-1)、低氮(5 g N·m^-2·a^-1)、中氮(15 g N·m^-2·a^-1)和高氮(30 g N·m^-2·a^-1)处理,研究氮沉降对苦竹人工林细根和土壤根际呼吸的影响.结果表明：不同处理氮沉降下,<1 mm和1～2 mm细根特性差异较大,与< 1 mm细根相比,1～2 mm细根的木质素、磷和镁含量更高,而纤维素、钙含量更低;氮沉降显著增加了<2 mm细根生物量,对照、低氮、中氮和高氮处理的细根生物量分别为(533±89)、(630±140)、(632±168)和(820±161) g·m^-2,氮、钾、镁元素含量也明显增加;苦竹林各处理年均土壤呼吸速率分别为(5.85±0.43)、(6.48±0.71)、(6.84±0.57)和(7.62±0.55) t C·hm^-2·a^-1,氮沉降对土壤呼吸有明显的促进作用;苦竹林的年均土壤呼吸速率与<2 mm细根生物量和细根N含量呈极显著线性相关.氮沉降使细根生物量和代谢强度增加,并通过增加微生物活性促进了根际土壤呼吸.     

增温下土壤资源异质分布对杉木幼苗生长的影响

在福建三明森林生态系统与全球变化研究站陈大观测点开展大气温度控制、土壤温度控制和土壤资源分布3因子试验,探讨土壤资源异质分布和增温对杉木幼苗地下和地上生长的影响,以及增温是否能改变杉木幼苗细根对土壤资源异质分布的识别度,以明确杉木人工林在全球变暖背景下对土壤资源异质分布的响应.结果表明:杉木对土壤资源异质分布的识别度主要体现在吸收根(0~1 mm径级)上,而1~2 mm径级细根则不具有识别度.除了单独大气增温处理对杉木1~2 mm径级细根的避贫系数具有显著影响外,不同增温处理均未对杉木幼树细根的贫富比、趋富系数和避贫系数产生显著影响.与土壤资源均质分布相比,土壤资源异质分布增加了0~1 mm径级细根生物量,降低了树高.与无大气增温相比,大气增温降低了0~1和0~2 mm径级细根生物量,增加了树高.与无土壤增温相比,土壤增温降低了1~2 mm径级细根生物量,但增加了树高和侧枝长度.大气增温控制、土壤增温控制和土壤资源异质分布对杉木地下、地上生长都无显著交互作用.杉木幼苗吸收根本身对土壤资源异质分布具有识别度,但增温并不会改变杉木幼苗细根对土壤资源异质分布的识别度.     

增温施氮对高寒草甸生产力及生物量分配的影响

在青藏高原高寒草甸区设置模拟增温和氮添加处理,研究长期增温与外源氮输入对高寒草甸群落生产及其分配的影响.结果表明:开顶箱增温装置造成小环境暖干化,即显著提高地表空气温度1.6℃,提高表层土壤温度1.4℃,降低土壤含水量4.7%.2012、2013和2014年不施氮处理下增温分别降低地上生物量61.5%、108.8%和77.1%,在高氮(40和80kg N·hm^-2·a^-1)处理下增温对群落地上生物量无显著影响,这说明增温的影响依赖于氮添加水平,且施氮补偿了增温导致的土壤氮损失.增温导致根冠比增加,2012、2013和2014年不施氮处理下增温分别增加根冠比98.6%、60.7%和97.8%.在不增温处理下,植物群落地上、地下生物量的变化率均表现出低氮(10、20 kg N·hm^-2·a^-1)促进、高氮抑制的趋势,达到饱和阈值时的氮添加剂量分别为56.0和55.5 kg N·hm^-2·a^-1;而在增温处理下,地上、地下生物量随施氮量增加呈线性增加趋势.这说明增温改变了高寒草甸生物量分配对外源氮输入的响应模式,增温导致的土壤无机氮含量变化是生物量分配模式改变的主要原因.由氮添加试验估算的高寒草甸氮饱和阈值表明,高寒草甸对氮输入的敏感性高于其他类型草地.     

Litter decomposition in grasslands of Central North America (US Great Plains)

One of the major concerns about global warming is the potential for an increase in decomposition and soil respiration rates, increasing CO₂ emissions and creating a positive feedback between global warming and soil respiration. This is particularly important in ecosystems with large belowground biomass, such as grasslands where over 90% of the carbon is allocated belowground. A better understanding of the relative influence of climate and litter quality on litter decomposition is needed to predict these changes accurately in grasslands. The Long‐Term Intersite Decomposition Experiment Team (LIDET) dataset was used to evaluate the influence of climatic variables (temperature, precipitation, actual evapotranspiration, and climate decomposition index), and litter quality (lignin content, carbon : nitrogen, and lignin : nitrogen ratios) on leaf and root decomposition in the US Great Plains. Wooden dowels were used to provide a homogeneous litter quality to evaluate the relative importance of above and belowground environments on decomposition. Contrary to expectations, temperature did not explain variation in root and leaf decomposition, whereas precipitation partially explained variation in root decomposition. Percent lignin was the best predictor of leaf and root decomposition. It also explained most variation in root decomposition in models which combined litter quality and climatic variables. Despite the lack of relationship between temperature and root decomposition, temperature could indirectly affect root decomposition through decreased litter quality and increased water deficits. These results suggest that carbon flux from root decomposition in grasslands would increase, as result of increasing temperature, only if precipitation is not limiting. However, where precipitation is limiting, increased temperature would decrease root decomposition, thus likely increasing carbon storage in grasslands. Under homogeneous litter quality, belowground decomposition was faster than aboveground and was best predicted by mean annual precipitation, which also suggests that the high moisture in soil accelerates decomposition belowground.     

黄土丘陵区微地形对草地植物群落结构组成和功能特征的影响

对黄土丘陵区微地形(阳坡坡上、中、下部,坡顶,阴坡坡上、中、下部)条件下草地植物群落进行群落组成调查和地上、地下生物量测定,分析微地形对草地植物群落结构组成和功能特征的影响.结果表明： 研究区内草地植物群落主要由菊科、禾本科和豆科物种组成.群落地上和地下生物量以及根系年生长量分别为164.12 g·m^-2、1044.87 g·m^-2、731.77 g·m^-2·a^-1.群落地上和地下生物量以及根系年生长量在不同坡向的大小均为：阴坡>阳坡>坡顶.在阴坡,群落生物量和根系年生长量的大小为：坡下部>坡中部>坡上部>坡顶部,阳坡群落生物量在不同坡位的大小顺序与阴坡不同.根系生长主要集中在0～20 cm土层,且从上到下逐渐减小.根系周转率的平均值为0.75 a^-1,在不同微地形条件下不同土层内大小不同.     

Temperature and substrate availability regulate soil respiration in the tropical mountain rainforests,Hainan Island,China

Aims Tropical forest plays a key role in global C cycle; however, there are few studies on the C budget in the tropical rainforests in Asia. This study aims to (i) reveal the seasonal patterns of total soil respiration (R T), litter respiration (R L) and soil respiration without surface organic litter (R NL) in the primary and secondary Asian tropical mountain rainforests and (ii) quantify the effects of soil temperature, soil moisture and substrate availability on soil respiration.Methods The seasonal dynamics of soil CO₂ efflux was measured by an automatic chamber system (Li-8100), within the primary and secondary tropical mountain rainforests located at the Jianfengling National Reserve in Hainan Island, China. The litter removal treatment was used to assess the contribution of litter to belowground CO₂ production.Important findings The annual R T was higher in the primary forest (16.73±0.87 Mg C ha^-1) than in the secondary forest (15.10±0.26 Mg C ha^-1). The rates of R T, R NL and R L were all significantly higher in the hot and wet season (May–October) than those in the cool and dry season (November–April). Soil temperature at 5cm depth could explain 55–61% of the seasonal variation in R T, and the temperature sensitivity index (Q 10) ranked by R L (Q 10 = 3.39)> R T (2.17)> R NL (1.76) in the primary forest and by R L (4.31)> R T (1.86)> R NL (1.58) in the secondary forest. The contribution of R L to R T was 22–23%, while litter input and R T had 1 month time lag. In addition, the seasonal variation of R T was mainly determined by soil temperature and substrate availability. Our findings suggested that global warming and increased substrate availability are likely to cause considerable losses of soil C in the tropical forests.     

Response and correlation of above- and below-ground functional traits of Leymus chinensis to nitrogen and phosphorus additions

AimsLeymus chinensis is a constructive and dominant species in typical steppe of northern China. The structure and functions of L. chinensis grassland ecosystem has been degenerated seriously due to long-term overgrazing in recent decades. As an effective measure to restore the degraded grasslands, the effects of nutrient addition on plant growth and ecosystem structure and functioning have been paid more attention in manipulation experimental research. The effects of nutrient addition, especially P addition on the above- and below-ground functional traits of L. chinensis have rarely been studied; particularly the underpinning mechanisms remain unclear. Our objective is to examine the responses and adaptive mechanisms of L. chinensis to different levels of N and P additions. MethodsWe conducted a culture experiment in the greenhouse, with three levels of N (50, 100 and 250 mg N·kg^-1) and P (5, 10 and 25 mg P·kg^-1) addition treatments. The above- and below-ground biomass, leaf traits (e.g., specific leaf area, leaf N and P contents) and root traits (e.g., specific root length, root N and P contents) of L. chinensis were determined in this study.Important findings Our results showed that: 1) the aboveground biomass and total biomass of L. chinensis were mostly affected by N addition, while the belowground biomass was mainly affected by P addition. N addition greatly enhanced the aboveground biomass of L. chinensis, while P addition reduced the belowground biomass at the moderate and high N levels. The root-shoot ratio of L. chinensis was influenced by both N and P additions, and root-shoot ratio decreased with increasing N and P levels. N and P additions promoted more biomass and N and P allocations to aboveground and leaf biomass. 2) Leymus chinensis showed different responses and adaptive mechanisms to P addition at low and high N levels. At low N level, L. chinensis exhibited high photosynthetic rate and specific root length (SRL) to improve photosynthetic capacity and root N acquisition, which promoted aboveground biomass. High root P content was favorable for belowground biomass. At high N level, P addition did not significantly affect plant growth of L. chinensis, even reduced its belowground biomass. Leymus chinensis showed high specific leaf area (SLA) and SRL to improve light interception and N acquisition in order to maintain stable aboveground biomass. 3) P addition greatly impacted below-ground than above-ground functional traits. SLA exhibited a weakly positive correlation with SRL, indicating L. chinensis exhibited relatively independence of resource acquirement and utilization between leaf and root functional traits.     

羊草的地上-地下功能性状对氮磷施肥梯度的响应及关联

羊草(Leymus chinensis)是我国北方典型草原群落的主要建群种和优势种, 由于长期的过度放牧, 羊草草原生态系统的结构和功能严重退化。养分添加作为恢复草地生态系统的一种管理措施, 其应用目前还处于实验性研究阶段。关于羊草的地上-地下功能性状对养分添加, 尤其是P添加的响应研究较少, 相关机制尚不十分清楚。为此, 该文以羊草为研究对象, 通过温室栽培进行N (50, 100, 250 mg N·kg^-1)和P (5, 10, 25 mg P·kg^-1)各3个水平的养分添加实验, 研究羊草的地上-地下功能性状对N、P添加的响应及适应机制。主要研究结果表明: 1)羊草的地上生物量和总生物量主要受N添加的影响, N添加显著提高了羊草的地上生物量, 而地下生物量主要受P添加的影响, 尤其在中N和高N水平, P添加显著降低了羊草的地下生物量。羊草的根冠比受N、P添加的共同影响, 随着N、P添加梯度加大, 根冠比显著降低, N、P添加促进了羊草生物量向地上部分的分配和N、P向叶片的分配。2)在低N和高N水平, 羊草对P添加的响应与适应机制不同。低N水平, 羊草主要通过增加光合速率和比根长(SRL), 提高光合能力和根系对N的获取能力促进地上部分的生长, 而根系对P的吸收有利于地下部分的生长; 在高N水平, P添加对羊草的个体生长无明显促进作用, 甚至地下生物量明显受到P素抑制, 羊草主要通过保持较高的比叶面积(SLA)和SRL, 提高对光资源的截获能力和根系对N的获取和吸收能力, 维持地上部分的生长。3)相对于地上性状, P添加对羊草的地下性状影响更大, 羊草的SLA与SRL呈较弱的正相关关系, 表明叶片与根系在资源获取和利用方面具有相对独立性。     

Biomass allocation and carbon density of Sophora moorcroftiana shrublands in the middle reaches of Yarlung Zangbo River,Xizang, China

Aims The expansion of shrublands is considered as one of the key reasons leading to the increase of carbon density in terrestrial ecosystems in China. In the present study, our aims were to explore the biomass allocation and carbon density of Sophora moorcroftiana shrublands in Xizang.
Methods We sampled the biomass of S. moorcroftiana shrubs from 18 sites in the middle reaches of Yarlung Zangbo River, Xizang. Using concentrations of different organs, we estimated the carbon density of different layers in S. moorcroftiana shrublands.
Important findings The plant cover rather than biomass volume (the product of cover and height) provided the best fit for aboveground biomass. The average of the total biomass was 5.71 Mg·hm^-2, ranging from 2.32 to 8.96 Mg·hm^-2. The average biomass of shrub layer, the main component of shrub ecosystem, was 4.08 Mg·hm^-2, accounting for 71% of the total biomass. The belowground biomass of shrub and herb layers was 2.08 and 0.86 Mg·hm^-2, respectively, which was higher than the corresponding aboveground biomass. The average biomass carbon density was 2.48 Mg·hm^-2. Shrub vegetation in the eastern part of the middle reaches has lower carbon density than that in the western part. The relatively high biomass allocation to roots to increase water and nutrient undertake as well as physical support for plants is an important strategy of S. moorcroftiana to cope with the arid environment on the Qinghai-Xizang Plateau. Moreover, the lower carbon density in the eastern part of the middle reaches might be due to the dry environment resulted from high temperature and evapotranspiration and enhanced human activities at low altitudes. The continuous decrease of evapotranspiration under scenarios of future climate change may lead to increase in carbon density in S. moorcroftiana shrublands.     

Effects of short-term nitrogen addition on fine root biomass,lifespan and morphology of Castanopsis platyacantha in a subtropical secondary evergreen broad-leaved forest

Aims Fine roots are the principal parts for plant nutrients acquisition and play an important role in the underground ecosystem. Increased nitrogen (N) deposition has changed the soil environment and thus has a potential influence on fine roots. The purpose of this study is to reveal the effect of N deposition on biomass, lifespan and morphology of fine root.Methods A field N addition experiment was conducted in a secondary broad-leaved forest in subtropical China from May 2013 to September 2015. Three levels of N treatments: CK (no N added), LN (5 g·m^-2·a^-1), and HN (15 g·m^-2·a^-1) were applied monthly. Responses of fine root biomass, lifespan, and morphology of Castanopsis platyacantha to N addition were analyzed by using a minirhizotron image system from April 2014 to September 2015. Surface soil sample (0-10 cm) was collected in November 2014 and soil pH value, and concentrations of NH₄⁺-N and NO₃^--N were measured.Important findings The biomass and average lifespan of the fine roots of C. platyacantha were 128.30 g·m^-3 and 113-186 days, respectively, in 0-45 cm soil layer. Nitrogen addition had no significant effect on either fine root biomass or lifespan in 0-45 cm soil layer. However, LN treatment significantly decreased C. platyacantha root superficial area in 0-15 cm soil layer. HN treatment significantly decreased soil pH value. Our study indicated that short-term N addition influences soil inorganic N concentration and thus decreased pH value in surface soil, and thereafter affect fine root morphology. Short-term N addition, however, did not affect the fine root biomass, lifespan and morphology in subsoil.     

Root Biomass Dynamics Under Experimental Warming and Doubled Precipitation in a Tallgrass Prairie

Human-induced climate change is expected to increase both the frequency and severity of extreme climate events, but their ecological impacts on root dynamics are poorly understood. We conducted a 1-year pulse warming and precipitation experiment in a tallgrass prairie in Oklahoma, USA to examine responses of root dynamics. We collected data in the pre-treatment year of 2002, imposed four treatments (control, 4°C warming, doubled precipitation, and warming plus doubled precipitation) in 2003, and observed post-treatment effects in 2004. Root biomass dynamics (for example, root growth and death) were measured using sequential coring and ingrowth coring methods. Treatment effects were not significant on standing root biomass in 2003, although root growth rate was significantly higher in the warmed than control plots. However, in the post-treatment year, the warmed plots had significantly lower standing root biomass than the controls, likely resulting from increased root death rate. Root death rate was significantly lower in the doubled precipitation and warmed plus doubled precipitation plots than that in the warmed plots in 2004. The root:shoot ratio showed similar responses to the post-treatments as standing root biomass, whereas aboveground biomass changed relatively little, indicating that roots were more sensitive to lagged effects than aboveground biomass. Our results demonstrate that root growth and death rates are highly sensitive to extreme climate events and lagged effects of extreme climate on root dynamics are important in assessing terrestrial carbon-cycle feedbacks to climate change.     

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.