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。     

氮肥和种植密度对达乌里胡枝子的生长与生物固氮的影响

氮供给和种植密度是影响植物生长的两个重要因素。豆科植物因其生物固氮能力而在受到氮限制的生态系统中具有重要作用, 氮含量增加促进植物生长的同时也会抑制豆科植物的生物固氮能力, 种植密度会通过种内竞争影响豆科植物的生长和生物固氮能力, 然而少有研究关注氮肥添加和种植密度对豆科植物生长和生物固氮能力的影响。该研究以达乌里胡枝子(Lespedeza davurica)为研究对象, 通过温室盆栽实验, 探究氮肥和种植密度对其生长和生物固氮的影响。实验设置4个氮添加水平(0、5、10、20 g·m^-2·a^-1)和3种种植密度(1、3、6 Ind.·pot^-1, 约32、96、192 Ind.·m^-2)。结果发现: 1)施肥和密度增加均影响了达乌里胡枝子的生长。叶片碳(C)、氮(N)含量、净光合速率随施氮量增加而增加, 氮添加也促进了植物的生长, 当施氮量为10 g·m^-2·a^-1时植物产量达到最大。叶片C、N含量、净光合速率随种植密度增加而下降, 密度增加可以促进每盆的总生物量, 但对单个植株的生长有负效应。2)氮肥对根瘤形成有抑制作用, 但种植密度增加会缓解氮肥对生物固氮能力带来的“氮阻遏”。该实验条件下, 当施氮量为10 g·m^-2·a^-1, 种植密度为3 Ind.·pot^-1, 或施氮量为5 g·m^-2·a^-1, 种植密度为6 Ind.·pot^-1时, 能最大程度发挥“施氮增产”和种植密度缓解“氮阻遏”的作用。氮添加降低了达乌里胡枝子的根瘤生物量和对根瘤形成的投资(根瘤生物量占总生物量的比例), 从而抑制达乌里胡枝子的生物固氮。种植密度增加导致达乌里胡枝子因种内竞争增加而使资源获取受限, 从而增加对根瘤的投资和根瘤生物量来获得更多来自大气中的氮。3)结构方程结果显示, 氮肥和种植密度通过直接或间接作用, 解释了64%的达乌里胡枝子生物量变化和42%的根瘤生物量变化。上述结果表明合理优化豆科植物的施肥量和种植密度可能对人工草地种植以及退化草地恢复管理具有重要意义。     

丛枝菌根真菌对豆科作物生长和生物固氮及磷素吸收的影响

本研究以大豆为材料,采用盆栽和田间试验,探讨丛枝菌根真菌(AMF)对豆科作物生长的影响。盆栽试验设置了接种(+AMF)和不接种(-AMF)丛枝菌根真菌处理,田间试验设置了AMF菌丝非限制与限制处理。盆栽试验结果表明: 接种AMF显著提高了大豆地上部生物量(16.5%)和大豆根瘤数(131.4%),地上部磷含量、磷吸收量、氮含量和氮吸收量也显著增加。田间试验中,AMF菌丝非限制处理下大豆的地上部生物量、根系生物量、根瘤数量分别比限制处理下显著提高了123.6%、61.5%和212.5%,地上部和根系磷吸收量、氮含量、氮吸收量均显著高于限制处理,大豆根际土壤速效氮和有效磷含量也均显著高于限制处理。本研究可为进一步认识豆科作物与AMF的共生关系及田间磷肥高效利用提供理论参考。     

隔离降水对杉木幼苗细根生物量和功能特征的影响

在福建三明陈大国有采育场杉木幼苗小区,采用土钻法和内生长环法,以非隔离降水为对照,对隔离降水50%处理一年的杉木幼苗细根生物量和形态、化学计量学、比根呼吸、非结构性碳水化合物等功能特征进行研究.结果表明: 与对照相比,隔离降水处理0～1 mm细根生物量显著降低,1～2 mm细根生物量差异不显著;隔离降水导致细根在形态上发生了适应性变化,0～1 mm和1～2 mm细根比根长分别增加21.1%和30.5%,0～1 mm细根组织密度显著降低,而比表面积显著增加.隔离降水导致细根氮的富集,但限制了对磷的吸收,氮磷比升高,导致营养失衡;隔离降水没有显著改变细根比根呼吸和非结构性碳水化合物含量,但导致1～2 mm细根可溶性糖、糖淀比显著降低,淀粉含量增加33.3%,表明其通过增加非结构性碳水化合物贮存比例以应对降水减少.     

黄土高原典型植物根际对土壤微生物生物量碳、氮、磷和基础呼吸的影响

选择黄土高原7种典型植物的根际与非根际土壤为研究对象,对土壤的养分含量、微生物生物量碳、氮、磷和基础呼吸的影响进行了初步研究。结果表明,7种不同植物根际土壤与非根际土壤的养分含量、微生物生物量和基础呼吸均存在显著差异;除冷蒿的土壤微生物生物量磷以外,其他各种植物的根际土壤的养分含量、微生物生物量和基础呼吸均比非根际土壤的高;土壤有机碳、全氮与土壤微生物生物量碳、氮及基础呼吸之间均具有极显著或显著相关关系,表明了土壤微生物生物量碳、氮可以作为判断土壤肥力状况的生物学指标,同时也可为提高土壤肥力水平和土壤培肥效果提供依据。     

内蒙古典型草原的生物量与生产力

氮素是植物生长和作物高产的限制因素。氮素来源主要是生物固氮和化学固氮。生物固氮是地球表面氮素的主要来源。在生物固氮中,目前最有效的还是豆科根瘤菌固氮。但近年来,禾本科植物根系与固氮菌的联合固氮作用引起了人们的广泛重视。联合固氮是自生固氮和共生固氮体系的中间类型,固氮细菌与相应联合植物之间具有较密切的相互影响,但又不象形成根瘤那样具有共生结构。这种联合固氮作用在自然界中广泛存在,各种作物,热带和亚热带牧草的根际和根表均有联合固氮菌存在,它们能提供土壤氮素。经测定水稻根际每个生长季非藻类的生物固氮量达25—30     

半干旱典型草原养分添加对优势物种叶片氮磷及非结构性碳水化合物含量的影响

在7年的养分添加(对照、低磷、低氮、低氮高磷和高氮低磷)试验基础上,测定了内蒙古半干旱典型草原优势种大针茅(Stipa grandis)和羊草(Leymus chinensis)叶片氮、磷、可溶性碳水化合物和淀粉含量。结果表明:两物种叶片氮、可溶性碳水化合物、淀粉及非结构性碳水化合物含量对养分添加有显著的响应(P0.05),养分添加与物种有显著的交互作用(P0.05);加氮显著增加了两物种植物叶片氮含量(P0.05),单加N处理显著降低了两物种叶片中的淀粉含量(P0.05),单加磷处理显著增加了羊草叶片可溶性碳水化合物含量(P0.05),高氮低磷处理显著降低两物种叶片非结构性碳水化合物总量(P0.05)。大针茅叶片各变量对磷添加无明显的响应,其叶片相对较高的C/N、C/P和可溶性碳水化合物/淀粉比,表明其具有相对较高的可直接利用的碳水化合物以及较高的氮、磷养分利用效率;羊草对外源养分的添加具有相对较强的竞争吸收能力。     

长脐红豆幼苗生长对不同土壤氮磷添加的响应及其对土壤养分的反馈

以热带豆科树种长脐红豆(Ormosia balansae Drake)幼苗为研究对象, 开展苗期控制试验, 共设置了5个添加氮(N, N1—N5)和2个添加磷(P, LP—HP)梯度, 观察长脐红豆在不同土壤N、P添加条件下的生长表现及对土壤养分的反馈。测定的植物和土壤响应指标包括: 植株地上和地下生长量、叶面积、叶和根的N、P含量、根际和非根际土壤N、P含量等。结果表明: (1)从苗高、总叶面积、地上生物量、地下生物量、总根长和根表面积这6个指标总体来看, 在低磷(LP)条件下, 中、高浓度的N添加(N4、N5)对长脐红豆生长有抑制作用; 高磷(HP)条件下, 高浓度的N添加(仅有N5)对长脐红豆生长有抑制作用; 表明长脐红豆的最适生长N浓度会随P添加浓度增加而升高; (2)随着N添加浓度的增加, 根和叶的全N含量呈先增加后减小的趋势, N4处理条件下达到最大; 根和叶的全P含量却呈现相反趋势, 符合元素稀释效应。叶的N:P>16, 且P浓度<1.0 mg•g-1, 表明长脐红豆生长受P限制; (3)基于氨态氮、硝态氮和有效P这3个肥力指标, 长脐红豆苗期非根际土壤肥力>根际土壤肥力, 表明豆科树种长脐红豆在其幼苗期共生根瘤尚未明显形成时期, 需补充适量N、P养分供给。     

氮磷添加对呼伦贝尔草甸草原4种优势植物根系和叶片C∶N∶P化学计量特征的影响

近年来大量的氮添加实验表明,持续氮沉降往往会造成植物生长由氮限制转变为磷限制或氮磷共限制,但目前还很少有报道涉及磷添加或氮磷共同添加以研究氮磷元素之间的平衡/失衡的生态后果。本研究依托额尔古纳氮磷添加平台,研究了草甸草原4种优势植物羊草(Leymus chinensis)、贝加尔针茅(Stipa baicalensis)、狭叶柴胡(Bupleurum scorzonerifolium)和披针叶黄华(Thermopsis lanceolata)根系及叶片碳(C)、氮(N)、磷(P)含量与计量特征的变化。结果表明:氮添加或磷添加对4种植物根、叶部的C含量均无显著影响;磷添加对羊草、贝加尔针茅和狭叶柴胡的根、叶部N含量和C∶N无显著影响,对羊草根部、狭叶柴胡叶部的P含量和C∶P也无显著影响,但显著增加了羊草叶、狭叶柴胡根以及贝加尔针茅根和叶部的P含量,降低了其C∶P;氮添加显著提高了羊草、贝加尔针茅和狭叶柴胡根、叶部的N含量,降低了其C∶N,对羊草和狭叶柴胡根、叶部的P含量和C∶P无显著影响,但显著增加贝加尔针茅根、叶部的P含量和降低其C∶P,同时显著提高了羊草、贝加尔针茅和狭叶柴胡根、叶部的N∶P;氮添加和磷添加对豆科植物披针叶黄华根、叶部的养分含量与计量特征均无显著影响;氮磷添加对4种植物根、叶部养分含量与计量特征的影响均不存在显著交互作用。研究结果说明,物种属性在决定植物养分和化学计量特征对养分富集的响应方面发挥着重要作用。不同物种养分含量和计量特征发生的改变对于预测未来养分富集情况下植物群落组成的改变将具有重要参考意义。     

凉水天然阔叶红松林植物叶片与细根的N∶P化学计量特征

叶片与细根是植物体地上和地下部分重要的营养器官,二者的化学计量学特征的关联性研究是探讨植物适应策略的重要方面。本研究对凉水国家级自然保护区天然阔叶红松林中34种常见植物叶片与不同根序级间氮含量、磷含量和氮磷比的差异性及相关性进行了研究。结果表明,氮、磷元素在不同器官中的含量有显著差异,且不同器官中的氮、磷元素具有一定的相关性;叶片的氮、磷含量和氮磷比高于细根,1~2级细根的氮、磷含量和氮磷比高于3~5级细根;植物叶片和各级细根的氮含量与其磷含量呈显著正相关,氮磷含量相关性高低顺序为:1~2级根叶片3~5级根;叶片与1~2级细根的氮含量、磷含量以及氮磷比均呈显著相关,叶片与3~5级根的氮含量呈显著相关;在不同生长型植物中,乔木、灌木和草本植物的叶氮、叶磷含量、叶氮磷比与不同级细根的氮、磷含量、氮磷比呈显著正相关。本研究阐明了植物叶片与不同根序级间氮磷化学计量学特征的相关性,有助于理解植物各性状之间的相互作用以及植物生长过程中对资源的利用和分配。     

Functional groups' performances as influenced by nitrogen,phosphorus and nodule inhibition of legumes

Aims Nitrogen (N) and phosphorus (P) constitute essential elements for plant growth and their availability influence species diversity in herbaceous plant communities. Legumes exhibit relatively high abundance in N-limited soils. Moreover, the legumes' N:P ratios are much higher than those of the other plant species grown in the same site, probably because they are able to fix atmospheric N 2. The objective of this study was to determine how the relative proportion in N and P availability and the restriction of legumes to fix atmospheric N 2 affect: (i) the primary productivity of plant species, (ii) species composition and (iii) N and P concentrations of species.Methods In an outdoor experiment, mixtures containing grasses, legumes and non-legume forbs were established in 32 containers under four soil treatments (control, N addition, P addition and disinfected soil), in a completely randomized design with eight replicates. Plant growth was examined when N and P were limited in the control soil:sand mixture, in a pot experiment sown with Plantago lanceolata .Important findings The pot experiment indicated that both N and P were limiting for the growth of P. lanceolata. Soil treatments affected primary productivity and species composition. Legumes had a relatively high abundance in the control and their growth was favoured, especially that of Medicago sativa, by P addition. Grasses' growth was increased by the addition of N. Inhibition of rhizobia resulted in poor growth of legumes and concomitant higher growth of grasses, in comparison to the control. The N:P ratios of non-legume species differed between treatments and were always higher in the legume species, even in the disinfected soil. The latter provides evidence that the high N concentrations found in legumes are a physiological characteristic of this specific group of plants.     

杉木幼苗生物量分配格局对氮添加的响应

大气氮(N)沉降的急剧增加可能会对植物碳(C)固定和分配产生深远影响。然而, N添加如何影响碳水化合物在植物不同器官之间的分配动态并不十分清楚。该研究利用杉木(Cunninghamia lanceolata)幼苗盆栽试验, 设置N添加处理, 测定分析幼苗非结构性碳水化合物(NSC)与结构性碳水化合物(SC)含量和库的变化, 以探讨N添加后杉木幼苗不同器官中NSC与SC的分配模式及调控机制。结果发现: (1) N添加虽然显著增加叶片净光合速率(143.96%), 但却降低了叶片中的NSC含量和库; N添加导致一年生茎的淀粉含量显著下降, 而可溶性糖含量的变化不显著, 当年生茎的NSC组分含量和库没有显著变化; 幼苗根系的NSC及其组分含量和库也有降低的趋势。(2) N添加后地下与地上生物量的比值降低22.09%, 其中SC库比值降低31.07%, 而NSC库比值无显著变化。(3) N添加使地上部分的磷(P)库显著增加, 使地下与地上P库的比值降低了57.02%, 而N库的比值无显著变化。(4) N添加后土壤pH由(4.94 ± 0.09)显著降低到(4.02 ± 0.04), 铵态N和硝态N含量分别增加7.17倍和11.55倍, 土壤有效P含量也增加了42.86%, 而土壤中脲酶(62.75%)和酸性磷酸酶(56.52%)的活性显著降低。研究表明, 低养分条件下杉木幼苗主要通过构建根系结构增加养分吸收, 而非通过向根系分配更多的NSC, 而N添加驱动的养分缓解使更多的碳水化合物分配到地上器官, 导致地上部分SC积累。     

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。     

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.     

Using an ecophysiological analysis to dissect genetic variability and to propose an ideotype for nitrogen nutrition in pea

BACKGROUNDS AND AIMS: Nitrogen nutrition of legumes, which relies both on atmospheric N2 and soil mineral N, remains a major limiting factor of growth. A decade ago, breeders tried to increase N uptake through hypernodulation. Despite their high nodule biomass, hypernodulating mutants were never shown to accumulate more nitrogen than wild types; they even generally displayed depressed shoot growth. The aim of this study was to dissect genetic variability associated with N nutrition in relation to C nutrition, using an ecophysiological framework and to propose an ideotype for N nutrition in pea. METHODS: Five pea genotypes (Pisum sativum) characterized by contrasting root and nodule biomasses were grown in the field. Variability among genotypes in dry matter and N accumulation was analysed, considering both the structures involved in N acquisition in terms of root and nodule biomass and their efficiency, in terms of N accumulated through mineral N absorption or symbiotic N2 fixation per amount of root or nodule biomass, respectively. KEY RESULTS: Nodule efficiency of hypernodulating mutants was negatively correlated to nodule biomass, presumably due to the high carbon costs induced by their excessive nodule formation. Root efficiency was only negatively correlated to root biomass before the beginning of the seed-filling stage, suggesting competition for carbon between root formation and functioning during the early stages of growth. This was no longer the case after the beginning of the seed-filling stage and nitrate absorption was then positively correlated to root biomass. CONCLUSIONS: Due to the high C costs induced by nodule formation and its detrimental effect on shoot and root growth, selecting traits for the improvement of N acquisition by legumes must be engineered (a) considering inter-relationships between C and N metabolisms and (b) in terms of temporal complementarities between N2 fixation and nitrate absorption rather than through direct increase of nodule and/or root biomass.     

Interactive influence of phosphorus and iron on nitrogen fixation by soybean

Adequate supplies of phosphorus (P) and iron (Fe) to legumes have been shown to be crucial in obtaining high nitrogen fixation rates and growth. These responses are anticipated as a result of the high requirement for P in energy transfer processes in the nodule and for Fe as a constituent of nitrogenase and leghemoglobin. However, little attention has been given to documenting the response of nitrogen fixation rates resulting from concentrations of P and Fe that actually exist in nodules. In particular, an open question is whether there is an interaction between nodule P and Fe concentrations that maximize nitrogen fixation activity. This study was designed to induce various concentrations of P and Fe in the nodule and to measure the resultant nitrogen accumulation and nitrogen fixation rates. Plant nitrogen accumulation was linearly correlated with both nodule P and Fe concentration, and with total plant nitrogen fixation rate as measured by acetylene reduction rate. Therefore, total nitrogen fixation rate was also correlated with nodule P and Fe concentrations, but a higher linear correlation was obtained for Fe as compared to P concentration. Surprisingly, nodule ureide concentration, which is generally assumed to be a positive index of nitrogen fixation rate, was negatively correlated with nodule P and Fe concentrations. These results indicated that higher concentrations of P and Fe in the nodules not only stimulated higher nitrogen fixation rates, but were associated with an enhanced ability to export ureides from the nodules. Since there was a linear response to both P and Fe over the range of nodule concentrations induced in these experiments, no evidence for optimum interactive concentrations of these two elements in the nodules was obtained.     

The role of legumes as a component of biodiversity in a cross-European study of grassland biomass nitrogen

To investigate how plant diversity loss affects nitrogen accumulation in above‐ground plant biomass and how consistent patterns are across sites of different climatic and soil conditions, we varied the number of plant species and functional groups (grasses, herbs and legumes) in experimental grassland communities across seven European experimental sites (Switzerland, Germany, Ireland, United Kingdom (Silwood Park), Portugal, Sweden and Greece). Nitrogen pools were significantly affected by both plant diversity and community composition. Two years after sowing, nitrogen pools in Germany and Switzerland strongly increased in the presence of legumes. Legume effects on nitrogen pools were less pronounced at the Swedish, Irish and Portuguese site. In Greece and UK there were no legume effects. Nitrogen concentration in total above‐ground biomass was quite invariable at 1.66±0.03% across all sites and diversity treatments. Thus, the presence of legumes had a positive effect on nitrogen pools by significantly increasing above‐ground biomass, i.e. by increases in vegetation quantity rather than quality. At the German site with the strongest legume effect on nitrogen pools and biomass, nitrogen that was fixed symbiotically by legumes was transferred to the other plant functional groups (grasses and herbs) but varied depending on the particular legume species fixing N and the non‐legume species taking it up. Nitrogen‐fixation by legumes therefore appeared to be one of the major functional traits of species that influenced nitrogen accumulation and biomass production, although effects varied among sites and legume species. This study demonstrates that the consequences of species loss on the nitrogen budget of plant communities may be more severe if legume species are lost. However, our data indicate that legume species differ in their N₂ fixation. Therefore, loss of an efficient N₂‐fixer (Trifolium in our study) may have a greater influence on the ecosystem function than loss of a less efficient species (Lotus in our study). Furthermore, there is indication that P availability in the soil facilitates the legume effect on biomass production and biomass nitrogen accumulation.     

Inorganic soil nitrogen under grassland plant communities of different species composition and diversity

We measured aboveground plant biomass and soil inorganic nitrogen pools in a biodiversity experiment in northern Sweden, with plant species richness ranging from 1 to 12 species. In general, biomass increased and nitrate pools decreased with increasing species richness. Transgressive overyielding of mixed plant communities compared to the most productive of the corresponding monocultures occurred in communities with and without legumes. N₂-fixing legumes had a fertilizing function, while non-legumes had a N retaining function. Plant communities with only legumes had a positive correlation between biomass and soil nitrate content, whereas in plant communities without legumes they were negatively correlated. Both nitrate and ammonium soil pools in mixed non-legume communities were approximately equal to the lowest observed in the corresponding monocultures. In mixed legume/non-legume communities, no correlation was found for soil nitrate with either biomass or legume biomass as percentage of total biomass. The idea of complementarity among species in nitrogen acquisition was supported in both pure non-legume and mixed non-legume/legume communities. In the latter, however, facilitation through increased nitrogen availability and retention, was probably dominating. Our results suggest that diversity effects on biomass and soil N pools through resource use complementarity depend on the functional traits of species, especially N₂ fixation or high productivity.