基于功能性状的毛乌素沙地不同演替阶段适生植物筛选

沙地生态系统修复是恢复生态学研究的热点问题,适生植物筛选是修复的关键。植物功能性状反映了植物在不同环境中的生存策略,探究沙地植物功能性状及其与环境之间的关系,有助于筛选用于植被恢复的物种,为保护沙地生态系统提供理论依据。以毛乌素沙地为研究区,分析了1983-2015年间沙地典型飞播样地群落演替特征及其对环境因子的响应,建立基于10个植物功能性状的毛乌素沙地潜在种库,进一步筛选飞播恢复下沙地不同演替阶段的适生植物。研究表明:(1)飞播恢复下的毛乌素沙地植物群落分为三个演替阶段:固沙先锋物种群落、沙生植物为主的杂类草群落、中生植物为主的杂类草群落。(2)土壤因子是群落演替的主要驱动力,其中土壤全氮、土壤总有机碳、土壤硝态氮是影响群落演替的关键因素。(3)基于功能性状筛选出29种适生物种用于植被恢复,演替第一阶段可用雾冰藜、猪毛菜等,演替第二阶段可用拂子茅、无芒隐子草等,演替第三阶段可用草地风毛菊、猪毛蒿等。通过物种功能性状特征可以快速选择适合沙地退化生态系统修复的候选物种,为植被恢复提供了一定的理论支持。     

ALLELOPATHIC EFFECTS OF ANDROPOGON VIRGINICUS AND ITS PERSISTENCE IN OLD FIELDS

Aqueous extracts of fresh roots and shoots of Andropogon virginicus (broomsedge) were found to be inhibitory to the growth of seedlings of Amaranthus palmeri, Bromus japonicus, Aristida oligantha, and Andropogon scoparius. The first two species are often important in the pioneer stage of old-field succession in eastern Oklahoma, Aristida is prominent in the second stage, and Andropogon scoparius is important later in succession including the climax Quercus stellata–Quercus marilandica savanna. Sterile dilute extracts of roots and shoots of broomsedge were inhibitory to two test species of Azotobacter, a free living nitrogen fixer, and to two species of Rhizobium, a symbiotic nitrogen fixer. Small amounts of decaying shoots of broomsedge (1 g per 454 g of soil) were very inhibitory to the growth of the four test species listed above and to Amaranthus retroflexus, another species often important in the first stage of succession. Similar amounts of decaying material in soil also significantly inhibited growth and nodulation of the two most important species of legumes in old-field succession in eastern Oklahoma, Lespedeza stipulacea and Trifolium repens. Broomsedge is known to compete vigorously and grow well on soils of low fertility, so the inhibition of nodulation of legumes could help keep the nitrogen supply low and give broomsedge a selective advantage in competition over species that have higher nitrogen requirements. The combined interference of broomsedge against other species resulting from competition and allelopathy could help explain why it invades old fields in 3–5 yr after abandonment from cultivation and remains so long in almost pure stands.     

河滨湿地不同植物群落根系分布特征与土壤理化性状的关系——以黄河中游荥阳段为例

在黄河中游郑州荥阳段,选择了5种河滨湿地植物群落进行根系和土壤性状特征研究,以期阐明不同植物群落的根系分布规律与土壤性状的关系,为河滨湿地植物群落组成以及土壤质量恢复提供科学参考。结果表明(1)在0—40 cm土层,根生物量密度与根长密度的平均值均表现为：芦苇群落(Phragmites australis)和芦苇-狗牙根(Cynodon dactylon)群落均大于芦苇-拂子茅(Calamagrostis epigeios)-狗牙根群落、拂子茅-狗牙根群落、拂子茅-狗牙根-水莎草(Juncellus serotinus)群落。拂子茅-狗牙根、芦苇-拂子茅-狗牙根、拂子茅-水莎草-狗牙根三种植物群落类型下根生物量密度、根长密度在0—20 cm表层土壤较大,芦苇群落和芦苇-狗牙根群落的根生物量密度、根长密度在10—40 cm的土层较大。(2)黄河河滨湿地芦苇群落、芦苇-狗牙根群落的土壤以粉粒为主,拂子茅-狗牙根群落、芦苇-拂子茅-狗牙根群落、拂子茅-狗牙根-水莎草群落的土壤主要以砂粒为主。在0—40 cm土层,芦苇群落、芦苇-狗牙根群落的土壤含水率、土壤有机质、有效氮和有效磷含量均显著高于...     

Aerial root nodules in the tropical legume,Pentaclethra macroloba

Summary Symbiotic nitrogen fixation in angiosperms normally occurs in buried root nodules and is severely inhibited in flooded soils. A few plant species, however, respond to flooding by forming nodules on stems, or, in one case, submerged roots with aerenchyma. We report here the novel occurrence of aerial rhizobial nodules attached to adventitious roots of the legume,Pentaclethra macroloba, in a lowland tropical rainforest swamp in Costa Rica. Swamp sapdings (1–10 cm diameter) support an average 12 g nodules dry weight per plant on roots 2–300 cm above water, and nodules remain in aerial positions at least 6 months. Collections from four swamp plants maintained linear activity rates (3–14 moles C₂H₄/g nodule dry weight/hr) throughout incubations for 6 and 13 hrs; excised nodule activity in most legumes declines after 1–2 hrs. Preliminary study of the anatomy and physiology suggest aerial nodules possess unusual features associated with tolerance to swamp conditions. High host tree abundance and nodulation in the swamp compared to upland sites indicate the aerial root symbiosis may contribute more fixed nitrogen to the local ecosystem than the more typical buried root symbiosis.     

Soil Saprotrophic Microfungi Associated with Roots of Calamagrostis epigeios on an Abandoned Deposit of Toxic Waste from Smelter Factory Processing Pyrite Raw Materials

Saprotrophic microfungi associated with roots of Calamagrostis epigeios growing in industrial waste contaminated with high concentrations of heavy metals (manganese, iron, zinc) were isolated on three different media. The isolates were obtained from three plots colonized by plant cenoses, representing three different succession series of plant cover: initial, intermediate, and advanced. A total of 60 species of saprotrophic microfungi were found in the roots during three samplings. In addition, unidentified dark and white sterile mycelia were frequently isolated. Trichoderma koningii was frequent in the roots formed under the advanced phytocenose. The abundance of dark sterile mycelium gradually decreased with progressing succession of the plant cover. Chrysosporium merdarium was rarely observed in the roots under the intermediate and advanced plant cenoses. Multivariate redundancy analysis showed that the effect of succession stage of phytocenoses on incidence of saprotrophic microfungi in mycocenoses present in roots was statistically significant. When the similarity of root mycocenoses of the three plots was studied using the similarity coefficients, the results proved that the similarities between the initial and intermediate or intermediate and advanced mycocenoses were higher than that between the initial and advanced mycocenoses. This corresponds to a different speed of succession of the mycocenoses. The results suggest that the dark sterile mycelia, abundant at slowly developing plots, play a dominant role during the first stages of succession where the plant cover is poor.     

Increased solar UV-B radiation may reduce infection by arbuscular mycorrhizal fungi (AMF) in dune grassland plants: evidence from five years of field exposure

An area of coastal dune grassland, dominated by the gramineous species Calamagrostis epigeios and Carex arenaria, was exposed to enhanced levels of UV-B radiation during a five year period. These species showed reduced AM-fungal infection percentages in their roots. In C. epigeios AM infection was reduced by 18%, C. arenaria showed a reduction by 20%. The major effect of enhanced UV-B on AM associations was a reduction of the number of arbuscules. This indicates a reduction in the exchange of nutrients between the symbionts. Since the effect of UV-B on AM associations may result from altered flavonoid levels in the root exudates of the host plants, flavonoid levels in the roots were investigated. No detectable flavonoid concentrations were found in the roots of C. epigeios and C. arenaria. Less effective AM associations can have pronounced negative effects on biodiversity and nutrient dynamics of the dune grassland ecosystem. The possible mechanisms causing these indirect effects of elevated UV-B on below ground AM associations are discussed. We conclude that UV-B induced changes in plant hormone levels are more likely to be the mechanism reducing AMF infection than UV-B induced alterations in flavonoid concentrations in the root exudates of the host plant.     

INHIBITION OF NODULATION OF INOCULATED LEGUMES BY LEAF LEACHATES FROM PIONEER PLANT SPECIES FROM ABANDONED FIELDS

Leaf leachates of three forbs, Ambrosia psilostachya, Euphorbia supina, and Helianthus annuus, and three grasses, Arislida oligantha, Bromus japonicus, and Digitaria sanguinalis, all of which are important in the pioneer stages of old-field succession, were tested to see if they would affect nodulation of heavily inoculated red kidney bean (Phaseolus vulgaris), white clover (Trifolium repens), and Korean lespedeza (Lespedeza stipulacea) growing in soil. Leachates of all inhibitor species except Ambrosia psilostachya and Digitaria sanguinalis significantly reduced the nodule number of Korean lespedeza in at least one experiment. Nodule numbers of red kidney bean and white clover were not significantly reduced by any leachate. It is significant that Korean lespedeza is the most important legume in our revegetating old fields. In almost all experiments in which the nodule number was significantly reduced, the nodules were small and grey or at least not a bright pink as were the control nodules. Leachate of Helianthus annuus reduced the hemoglobin content of Korean lespedeza nodules per plant by 36 %, and Euphorbia supina reduced the hemoglobin content per clover plant by 24 % even though the nodule number was not changed significantly in the latter case. Based on previous research, this indicated a definite reduction in the nitrogen-fixing capacity of the test nodules. Root exudates of all six inhibitor species were previously found to reduce nodule numbers significantly in the three legume species as were very small amounts of decaying material (1 g per 454 g of soil) of each forb. It appears likely, therefore, that the inhibition of nodulation of legumes in old fields by the six species under test would slow the rate of addition of nitrogen and thus the rate of plant succession, because it was previously demonstrated that the order in which species enter our old fields is the same as the order based on increasing nitrogen requirements.     

Mycorrhizae are present in cycad roots

We describe the occurrence of arbuscular mycorrhizae in the roots ofZamia pumila andDioon edule. Seedlings were grown on native, unsterilized soil taken from local pinelands of south Florida, whereZ. pumila occurs naturally. Arbuscules, hyphae, hyphal coils, and vesicles occur in the parenchyma cells of the root cortex, especially the half of the cortex next to the stele. Hyphae of the arbuscular mycorrhizal fungi (AMF) occur mainly in longitudinal intercellular spaces and conform to theAcorus type. The finest, ultimate roots have AMF, but these roots are extremely brittle, detach with the slightest disturbance, and are usually lost when plants are uprooted from the ground. No AMF were found in the cortex of coralloid roots. Vovides (1991) previously reported that AMF occur onDioon edule and Ceratozamia mexicana, and we reconfirm this inD. edule. In this species, AMF appear to be mostly associated with the outer and to a lesser extent the inner cortex. However, roots of a potted plant of C.hildae growing in native soil lacked AMF. When grown on low phosphorus soils, legumes are known to require AMF in order for theirRhizobium nodules to fix nitrogen. Without AMF, the legumes are deficient in phosphorus, which inhibits nodule production and nitrogen fixation. It is probable that cycads, with their nitrogen-fixing coralloid roots containingNostoc, may also require AMF for successful nitrogen fixation when phosphorus is limiting.     

15N natural abundance of plant and soil components of a Banksia woodland ecosystem in relation to nitrate utilization, life form, mycorrhizal status and N2-fixing abilities of component species

Studies of the variation in δ¹⁵N values for plants from a fire-prone Banksia woodland in South West Australia showed that pioneer herbaceous, non-mycorrhizal species which were active in nitrate reduction and storage, had the highest values (1.81%c). A detailed study of one such species Ptilotus polystachus demonstrated a close correspondence between the δ¹⁵N values of soil nitrate, xylem nitrate and leaf total nitrogen, suggesting an exclusive reliance on nitrate ions as nitrogen source. These pioneer species also showed a preponderance of the chloroplastic isoform of glutamine synthetase while woody species generally had higher activity associated with the cytosolic isoform. The group comprising monocotyledonous hemicryptophytes and geophytes contained species with slightly positive δ¹⁵N values and moderately active in nitrate reduction and storage. Nitrogen-fixing species had the lowest δ¹⁵N values (–0.36‰), irrespective of their apparent utilisation of nitrate. However, woody resprouter species which had low levels of nitrate reduction and storage had δ¹⁵N values which fell within the range of values obtained for the miscellaneous assemblage of N₂-fixing species. Consequently, ¹⁵N abundance values failed to distinguish N₂ fixing from non-fixing woody species, and therefore, could not be used in the ecosystem to determine the dependence of putative nitrogen fixing species on N₂ fixation. The study demonstrated complex patterns of nitrogen utilization in the ecosystem in which exploitation of different nitrogen resources related to plant life form and the physiological attributes of nitrogen assimilation by component species.     

内蒙古高原湖滨湿地优势植物功能性状特征及其适应性

受人类活动和气候变化的影响,湖泊湖滨带退化速度显著加快。植物功能性状的方法可以量化植物特征,预测植物对外界环境干扰的反应,有助于理解退化湖滨带湿地植物应对环境变化所表现出的适应机制,对湖泊湖滨湿地生态系统植被的恢复与重建具有重要意义。在内蒙古高原典型湖泊湖滨湿地选取芦苇（Phragmites australis）、赖草（Leymus secalinus）、毛茛（Ranunculus japonicus）、鹅绒委陵菜（Potentilla anserina）、碱蓬（Suaeda glauca）、盐角草（Salicornia europaea）和拂子茅（Calamagrostis epigeios）7种优势植物的叶片和根系作为研究对象,对不同湿地植物的11种功能性状变化规律及其与环境因子的关系进行研究。旨在探究环境变化影响下湖滨带湿地植物的物种分布和功能性状的差异,以及湿地植物在不同湖滨带湿地生境下的适应策略。在评估植物功能性状差异基础上,采用环境矩阵连接性状矩阵（RLQ）结合第四角分析（Fourth-Corner）的方法分析环境因子对植物功能性状的影响。结果表明,内蒙古湖滨带湿地中7种优势植物为了适应不同的环境的影响,植物的功能性状均产生不同程度的种间与种内变异,在湖滨带湿地中植物的植株高度、叶片碳含量、叶片氮含量、叶片碳氮比、比根长、根组织密度、根氮含量对环境变化的响应比较敏感,土壤pH与叶片干物质含量呈显著负相关;土壤盐分与植株高度、叶片碳含量和叶碳氮比显著负相关,与叶片氮含量、根组织密度显著正相关;土壤的总氮含量与植株高度显著正相关,与比根长显著负相关;土壤碳氮比与植株高度和叶片碳含量显著负相关,与植物比根长显著正相关;土壤容重与根氮含量显著负相关。研究表明内蒙古高原湖滨带湿地植物的功能性状受环境的作用强烈,植物采取了不同的性状策略来适应环境。     

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.     

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。     

The effects of lime-pelleting and lime-superphosphate fertilizer on the growth of three annual legumes in an acid sandy soil

Summary Three pioneer pasture legume species,Trifolium subterraneum, Trifolium glomeratum andOrnithopus compressus, were grown in a sandy soil of pH 5.0. The growth and nodulation of each species was examined in the presence and absence of lime pelleting and with superphosphate and lime-superphosphate fertilizer. In each species tested, plant yield, nodule number and nitrogen recovery as maximal where lime pelleting and lime-super were applied together. Plant calcium indicated that the response to lime was due to changes in soil pH rather than to a calcium response. T. glomeratum was the species most responsive to lime application but its growth was at best only half that ofT. subterraneum andO. compressus, which produced equally under both favourable (pH 5.9) and unfavourable (pH 4.4) conditions created by fertilizer applications.     

Nitrification and nitrogen accumulation in the early stages of primary succession on Mt. Fuji

The relationship between nitrification potential and nitrogen accumulation was studied in an early successional sere on Mt. Fuji. Soil organic nitrogen accumulated with the invasion ofPolygonum cuspidatum and successively withMiscanthus oligostachyus and other species. Laboratory incubation experiments showed a higher nitrification potential at theM. oligostachyus state. The numbers of nitrifying bacteria increased with the progress of succession. No significant difference in nitrate reductase activity was found between pioneer and succeeding species. The soil solution at theM. oligostachyus stage contained a lower level of nitrate than rainwater, while that of the bare ground and theP. cuspidatum stage contained a higher nitrate level than rainwater. It was concluded that the high nitrate levels in the soil solution of the bare ground and theP. cuspidatum stage were due to lower nitrate-absorbing activity, leading to loss of nitrogen with precipitation, while the lower nitrate levels at theM. oligostachyus stage when higher nitrification activity occurred were due to higher nitrate-absorbing activity, preventing net loss of nitrogen from the ecosystem.     

Ineffectiveness of Two Annual Legumes as Nurse Plants for Establishment of Artemisia californica in Coastal Sage Scrub

Annual legumes are often used as nurse plants for restoration projects, but two commonly used legume species were competitors at all densities with Artemisia californica (California sagebrush), a dominant shrub of southern California coastal sage scrub. Survival of Artemisia was not reduced by the lowest densities of the native Lupinus succulentus (arroyo lupine) at ratios of Artemisia to Lupinus of 1:1 or 1:3 or by the exotic Trifolium hirtum (rose clover) at the 1:1 density, but its survival was as low as 4% at the highest densities of Trifolium (1:16) and 1:32). Overall, Trifolium was more detrimental to survival of Artemisia, but the biomass of Artemisia was reduced by 90% or more in mixtures with both legumes even at the lowest densities of 1:1. The total soil nitrogen either did not change or decreased in two of the mixtures between planting and harvest dates, indicating that the legumes not only did not add nitrogen to the soil within one growing season but even depleted it in these two cases. Whereas Lupinus had greater aboveground bio-mass than Trifolium, it had a lower root density than Trifolium. The Artemisia root system was more shallow than either Trifolium or Lupinus, possibly explaining the poor growth of Artemisia in mixtures, The legumes were one to two orders of magnitude greater in aboveground biomass than Artemisia at the 1:1 ratio and therefore may be inappropriate choices as nurse plants. There is no evidence from this study that either of these legumes can act as nurse plants, even at the lowest ratio of one nurse plant to on shrub. Nurse plants are probably more important in harsher environments than in coastal sage scrub.     

Colonization and early succession on anthropogenic soils

Abstract. This paper reports on vegetation development on permanent experimental plots during five years of succession. Nine (1 m²) plots were filled with three typical substrates from man-made habitats of urban and industrial areas in the region of Berlin. The three substrates (a commercial ‘topsoil’, a ruderal ‘landfill’ soil and a sandy soil), differ in organic matter and nutrient contents. Relevés of species composition and percent cover of each species present were made monthly during the growing season from the start of vegetation development. This paper describes the different successional pathways on topsoil and ruderal soil and the colonization process on sandy soil. On topsoil, ruderal annuals are dominant in the first year and are replaced by short-lived perennials from the second year. Those species were replaced by long-lived perennial herbs (Ballota nigra or Urtica dioica) from the third year of succession onwards. On the ruderal land-fill soil the early successional stages are less sharp and the perennial Solidago canadensis is able to dominate within one year after the succession was initiated. On sandy soil there is still an ongoing colonization process, where pioneer tree species like Betula pendula and Populus nigra play a main role. The importance of ‘initial floristic composition’, the role of substrate for community structure and the peculiarities of successional sequences on anthropogenic soils in the context of primary and secondary successions are discussed.     

Fine root dynamics and trace gas fluxes in two lowland tropical forest soils

Fine root dynamics have the potential to contribute significantly to ecosystem‐scale biogeochemical cycling, including the production and emission of greenhouse gases. This is particularly true in tropical forests which are often characterized as having large fine root biomass and rapid rates of root production and decomposition. We examined patterns in fine root dynamics on two soil types in a lowland moist Amazonian forest, and determined the effect of root decay on rates of C and N trace gas fluxes. Root production averaged 229 (±35) and 153 (±27) g m^?2 yr^?1 for years 1 and 2 of the study, respectively, and did not vary significantly with soil texture. Root decay was sensitive to soil texture with faster rates in the clay soil (k=?0.96 year^?1) than in the sandy loam soil (k=?0.61 year^?1), leading to greater standing stocks of dead roots in the sandy loam. Rates of nitrous oxide (N₂O) emissions were significantly greater in the clay soil (13±1 ng N cm^?2 h^?1) than in the sandy loam (1.4±0.2 ng N cm^?2 h^?1). Root mortality and decay following trenching doubled rates of N₂O emissions in the clay and tripled them in sandy loam over a 1‐year period. Trenching also increased nitric oxide fluxes, which were greater in the sandy loam than in the clay. We used trenching (clay only) and a mass balance approach to estimate the root contribution to soil respiration. In clay soil root respiration was 264–380 g C m^?2 yr^?1, accounting for 24% to 35% of the total soil CO₂ efflux. Estimates were similar using both approaches. In sandy loam, root respiration rates were slightly higher and more variable (521±206 g C m² yr^?1) and contributed 35% of the total soil respiration. Our results show that soil heterotrophs strongly dominate soil respiration in this forest, regardless of soil texture. Our results also suggest that fine root mortality and decomposition associated with disturbance and land‐use change can contribute significantly to increased rates of nitrogen trace gas emissions.     

Differences in rhizosphere carbon-partitioning among plant species of different families

Interspecific variations in carbon (C) allocation and partitioning in the rhizosphere were investigated on 12 Mediterranean species belonging to different family groups (grasses, legumes, non-legume forbs) and having different life cycles. Plants grown individually in artificial soil, in a greenhouse and inoculated with rhizosphere microflora were labelled with ¹⁴CO₂ for 3 h at the vegetative stage. Rhizosphere respiration was measured during 6 days after which labelled C partitioning between shoots, roots, soil, root washing solution and respiration was estimated. The percentage of assimilated ¹⁴C allocated below ground differed significantly between species (41 – 76%) but no significant difference was found between grasses, legumes and non-legume forbs. When expressed as percentage of below-ground ¹⁴C, rhizosphere respiration was significantly smaller for non-legume forbs (42%) than for grasses (46%) and legumes (51%). Consequently more ¹⁴C was incorporated into root biomass in the former. Half-life of ¹⁴CO₂ evolution through respiration ranged from 23 h in legumes to 27 h for non-legume forbs and 37 h for grasses. This suggested differences in microbial activities due to quantities and quality of root exuded C. Rhizosphere respiration was positively correlated with the amount of ¹⁴C in the solution used to wash the roots on one hand, and root N concentration on the other hand. This led to a functional hierarchy between plant family groups of the overall rhizosphere activity. It went from non-legume forbs being the less active (except Crepis sancta)in terms of respiration and exudation, to grasses and then legumes, the most active but also the richest in nitrogen.     

Nodulation of native woody legumes in Hong Kong, China

Woody legumes can play an important role in forest restoration on degraded land but the knowledge of woody legumes has lagged behind their uses. This study is a pioneer investigation to explore the ability of native woody legumes to form root nodules and fix nitrogen in Hong Kong. Nine sites of different habitat types were surveyed during both wet and dry seasons for two years. Young plants of woody legumes along studied transects were excavated. The patterns of nodulation and nodule morphology were recorded and the nitrogen fixing ability was tested by acetylene-reduction-assay. Twenty-eight species in 16 genera were examined, of which 20 species were nodulating and eight non-nodulating, including all six species in the Caesalpinioideae. Five species were new records to the world’s nodulation inventory. Bowringia callicarpa was a new species and genus examined, which was non-nodulating. The overall nodulation pattern was consistent with previous studies. Nodulation was more profuse in some shrub species while inconsistent in most tree species. Species with higher proportion of nodulated individual plants also tended to have more nodules in each plant. Spherical nodules were common in shrub and woody climber species whilst tree species usually had woody indeterminate nodules. Seasonal difference in the amount of senescent nodules was noted in most species. All the nodules tested by acetylene-reduction-assay were effectively nitrogen-fixing, with nitrogenase activity ranging from 4 μmol C₂H₄ g^?1 h^?1 to 20 μmol C₂H₄ g^?1 h^?1, which was comparable to other tropical tree species. The findings in nodulation pattern and nitrogen fixing ability of these species are essential in their application in forest restoration on degraded lands.