Woody species diversity influences productivity and soil nutrient availability in tropical plantations

We investigated the relationship between plant diversity and ecological function (production and nutrient cycling) in tropical tree plantations. Old plantations (65–72 years) of four different species, namely Araucaria cunninghamii, Agathis robusta, Toona ciliata and Flindersia brayleyana, as well as natural secondary forest were examined at Wongabel State Forest, in the wet tropics region of Queensland, Australia. Two young plantations (23 years) of Araucaria cunninghamii and Pinus caribaea were also examined. The close proximity of the older plantations and natural forests meant they had similar edaphic and climatic conditions. All plantations had been established as monocultures, but had been colonised by a range of native woody plants from the nearby rainforest. The extent to which this had occurred varied with the identity of the plantation species (from 2 to 17 species in 0.1 ha blocks). In many cases these additional species had grown up and joined the forest canopy. This study is one of the few to find a negative relationship between overstorey plant diversity and productivity. The conversion of natural forest with highly productive, low-diversity gymnosperm-dominated plantations (young and old Araucaria cunninghamii and Pinus caribaea) was found to be associated with lower soil nutrient availability (approximately five times less phosphorus and 2.5 times less nitrogen) and lower soil pH (mean = 6.28) compared to the other, less productive plantations. The dominant effects of two species, Araucaria cunninghamii and Hodgkinsonia frutescens, indicate that ecosystem functions such as production and nutrient availability are not determined solely by the number of species, but are more likely to be determined by the characteristics of the species present. This suggests that monoculture plantations can be used to successfully restore some functions (e.g. nutrient cycling and production), but that the level to which such functions can be restored will depend upon the species chosen and site conditions. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Mineral element content in Ulva lactuca L. with reference to eutrophication in Hong Kong coastal waters

The amounts of tissue nitrogen, phosphorus, potassium, sodium, calcium and iron were estimated in the green alga Ulva lactuca L. collected from 9 rural and 14 urban littoral sites in the waters around Hong Kong Island during 1978 and 1979. The mean levels of tissue nitrogen and phosphorus were respectively 65% and 87% more in urban sites than in rural ones. Very significant correlation (r = 0.920) was found between the logarithmic concentration of seawater inorganic nitrogen and that of tissue nitrogen. The same applied to soluble reactive phosphorus in seawater and tissue phosphorus (r = 0.886). The levels of potassium, sodium and calcium in the alga were relatively uniform. However considerable variation in the level of iron was detected. The potential use of Ulva as an indicator species for eutrophication is discussed.     

Diverse responses of pqqC- and phoD-harbouring bacterial communities to variation in soil properties of Moso bamboo forests

Phosphate-mobilizing bacteria (PMB) play a critical role in the regulation of phosphorus availability in the soil. The microbial genes pqqC and phoD encode pyrroloquinoline quinone synthase and bacterial alkaline phosphatase, respectively, which regulate inorganic and organic phosphorus mobilization, and are therefore used as PMB markers. We examined the effects of soil properties in three Moso bamboo forest sites on the PMB communities that were profiled using high-throughput sequencing. We observed differentiated responses of pqqC- and phoD-harbouring PMB communities to various soil conditions. There was significant variation among the sites in the diversity and structure of the phoD-harbouring community, which correlated with variation in phosphorus levels and non-capillary porosity; soil organic carbon and soil water content also affected the structure of the phoD-harbouring community. However, no significant difference in the diversity of pqqC-harbouring community was observed among different sites, while the structure of the pqqC-harbouring bacteria community was affected by soil organic carbon and soil total nitrogen, but not soil phosphorus levels. Overall, changes in soil conditions affected the phoD-harbouring community more than the pqqC-harbouring community. These findings provide a new insight to explore the effects of soil conditions on microbial communities that solubilize inorganic phosphate and mineralize organic phosphate.     

Disturbance processes and spatial patterns of two emergent conifers in New Caledonia

Abstract Araucaria laubenfelsii Corbasson and Araucaria montana Brongn et Gris are emergent conifers in maquis and forest communities which are subjected to a combination of fire and cyclonic disturbances. Both species are able to survive fire once stems reach a critical size, but most seedlings and saplings are killed. Both species were found to be clumped at most spatial scales for both saplings and trees in maquis, probably resulting from a combination of patchy fire and a limited ability to disperse seeds. Comparisons of burned and unburned A. montana sites suggest that clumping increases only slightly after fire. The degree of clumping in the A. laubenfelsii sites, suggested that fire disturbances may occur relatively frequently in the region. Cyclonic disturbances are infrequent but may result in blow‐down of large individuals within both maquis and forest. All of the individuals blown‐down during the study had been previously fire‐scarred. Around tree blow‐downs, seedling and sapling densities can be high. This likely reflects both the low dispersability of Araucaria seeds and enhanced moisture from the shading of the adult, when it was alive. Disturbance by fire and wind play an important role in the regeneration dynamics and spatial pattern of these species in maquis in New Caledonia.     

Revegetation of harvested peat surfaces in relation to substrate quality

Abstract. The relationship between substrate quality and pattern of revegetation of harvested peat surfaces was studied by means of a survey and a field experiment examining influences of modest NPK-fertilization on plant colonization of an initially bare peat surface. The harvested peat surfaces varied a great deal in their chemical and physical characteristics and the sites differed in revegetation pattern. Early successional vegetation was dominated by perennial species native to nutrient-poor habitats on all sites. Soluble phosphorus and ash content, mean particle size of surface peat, and thickness of peat layer had the strongest influence in a CCA-ordination of species. The species composition depended on the amount and form of soluble nitrogen in the surface peat. Sites with a high content of phosphorus and ammonium nitrogen, and with a thick peat layer were usually densely revegetated by Eriophorum vaginatum alone, while sites characterized by thin peat layers associated with a high ash content, large particle size and a high content of nitrate nitrogen were mainly dominated by different grass and weed species. Deschampsia cespitosa clearly favoured sites with a high potassium content and small particle sizes of the peat. The importance of nutrient availability for the rate and pattern of colonization was also demonstrated by the field experiment. Application of 20 g/m² of NPK-fertilizer resulted in a significant increase in the number of established plant individuals and marked differences in species composition compared to unfertilized plots.     

Araucaria Forest conservation: mechanisms providing resistance to invasion by exotic timber trees

Since only 12.6% of the Brazilian Araucaria Forest remains and timber tree monocultures are expanding, biological invasion is a potential threat to the conservation of natural forest remnants. Here, we test (1) the susceptibility of Araucaria Forest to invasion by Pinus taeda and Eucalyptus saligna, (2) the efficiency of different mechanisms controlling the early establishment of these two exotic timber tree species, and (3) the potential of the native timber tree Araucaria angustifolia to establish successfully in ecologically-managed monocultures of Araucaria, Pinus and Eucalyptus. In Araucaria Forest, more than a thousand Pinus seeds landed annually in a hectare; however, experimentally exposed seeds were 100% removed in only 6 days. Furthermore, all experimentally transplanted seedlings of Pinus taeda and Eucalyptus saligna died in less than a year in Araucaria Forest, but not in the monocultures. Correlative evidence suggests that this mortality was associated to plant community richness, plant abundance, and soil fertility. Araucaria angustifolia, in contrast, showed an establishment success in ecologically-managed tree monocultures as high as that exhibited in its natural habitat. The current resistance of Araucaria Forest to invasion by exotic timber trees is good news for the conservation of Araucaria Forest remnants and for its keystone species. The understanding of the mechanisms providing such resistance against invasion points towards management tools for minimizing future threats.     

Accumulation and release of nitrogen and phosphorus in relation to lignin decomposition in leaf litter of 14 tree species

Immobilization and mobilization of nitrogen and phosphorus were investigated in relation to the nitrogen (L/N) ratio and lignin to the phosphorus (L/P) ratio as indicators of the nitrogen and phosphorus dynamics. The present study was carried out on upper and lower parts of a forest slope in a cool temperate forest in Japan. Net immobilization and net mobilization characterized the dynamics of nitrogen and phosphorus in 14 litter types and were related to the changes in the L/N and L/P ratio. The critical values of the L/N and L/P ratio at which the mobilization began were 23–25 and 500–620, respectively. In litter types with the L/N and L/P ratio higher than critical values, nitrogen and phosphorus were immobilized until the ratios reached at the critical values and then nitrogen and phosphorus began decreasing. In litter types with initial L/N and L/P ratios lower than or equal to the critical values, nitrogen and phosphorus were released from litter. The critical values of the L/N and L/P ratios showed convergent trends among litter types as compared to their initial values, and were approached to those of underlying humus layers. These results indicated the usefulness of L/N and L/P ratios as indicators of the nitrogen and phosphorus dynamics in the study site. The general validity of the L/N ratio as an indicator of nitrogen dynamics and the convergent trend of critical L/N ratio at 25–30 were demonstrated by a review of literature on lignin and nitrogen dynamics in 47 litter types in temperate and boreal forests.     

Nodulation and growth response ofAllocasuarina andCasuarina species to phosphorus fertilization

To examine how soil phosphorus status affects nitrogen fixation by the Casuarinaceae —Frankia symbiosis,Casuarina equisetifolia and two species ofAllocasuarina (A. torulosa andA. littoralis) inoculated or fertilized with KNO₃ were grown in pots in an acid soil at 4 soil phosphate levels. InoculatedC. equisetifolia nodulated well by 12 weeks after planting and the numbers and weight of nodules increased markedly with phosphorus addition. Growth ofC. equisetifolia dependent on symbiotically fixed nitrogen was more sensitive to low levels of phosphorus (30 mg kg^–1 soil) than was growth of seedings supplied with combined nitrogen; at higher levels of phosphorus, the growth response curves were similar for both nitrogen fertilized and inoculated plants. The interaction between phosphorus and nitrogen treatments (inoculated and nitrogen fertilized) demonstrated that there was a greater requirement of phosphorus for symbiotic nitrogen fixation than for plant growth when soil phosphorus was low.WithAllocasuarina species, large plant to plant variation in nodulation occurred both within pots and between replicates. This result suggests genetic variation in nodulation withinAllocasuarina species. Nodulation ofAllocasuarina species did not start until 16 weeks after planting and no growth response due toFrankia inoculation was obtained at the time of harvest. Addition of nitrogen starter is suggested to boost plant growth before the establishment of the symbiosis. Growth ofAllocasuarina species fertilized with nitrogen responded to increasing levels of phosphorus up to 90 mg P/kg soil after which it declined by 69% forA. littoralis. The decrease in shoot weight ofA. littoralis, A. torulosa, C. equisetifolia andC. cunninghamiana at high phosphorus was confirmed in a sand culture experiment, and may be atributable to phosphorus toxicity.     

The ecology of Araucaria species in New Guinea. I. Ordination studies of forest types and environments

Previous studies of the ecology and regeneration strategies of Araucaria cunninghamii and A. hunsteinii in New Guinea rainforests have treated these species as ecologically identical and disagree concerning their ability to maintain a stable population size in forest communities dominated by angiosperms. Vegetation data from a number of 0.5 ha forest sites in New Guinea are analysed in order to characterize forest types and their environmental regimes in the altitudinal zone from 700 to 1500 m a.s.l. Particular attention is paid to forests containing Araucaria species. Nodal component analysis identifies a number of distinct forest types or ‘series’, each dominated by a different species. Continuity analysis displays the location of sites and species in non-linear ordination space. Subsequent regression of environmental data on the position of sites in ordination space shows that forest types are separated along gradients related to altitude and rainfall on the one hand, and soil nutrient concentrations on the other. A. cunninghamii is generally found in wetter and less nutrient-rich sites than A. hunsteinii. The analysis of species presence data, as opposed to quantitative data, shows that whilst forest types are easily differentiated according to their dominant species, changes in floristic composition are subtle along the altitudinal and other gradients recognized.     

Vegetation,fire and soil feedbacks of dynamic boundaries between rainforest,savanna and grassland

At fine spatial scales, savanna‐rainforest‐grassland boundary dynamics are thought to be mediated by the interplay between fire, vegetation and soil feedbacks. These processes were investigated by quantifying tree species composition, the light environment, quantities and flammability of fuels, bark thickness, and soil conditions across stable and dynamic rainforest boundaries that adjoin grassland and eucalypt savanna in the highlands of the Bunya Mountains, southeast Queensland, Australia. The size class distribution of savanna and rainforest stems was indicative of the encroachment of rainforest species into savanna and grassland. Increasing dominance of rainforest trees corresponds to an increase in woody canopy cover, the dominance of litter fuels (woody debris and leaf), and decline in grass occurrence. There is marked difference in litter and grass fuel flammability and this result is largely an influence of strongly dissimilar fuel bulk densities. Relative bark thickness, a measure of stem fire resistance, was found to be generally greater in savanna species when compared to that of rainforest species, with notable exceptions being the conifers Araucaria bidwillii and Araucaria cunninghamii. A transect study of soil nutrients across one dynamic rainforest – grassland boundary indicated the mass of carbon and nitrogen, but not phosphorus, increased across the successional gradient. Soil carbon turnover time is shortest in stable rainforest, intermediate in dynamic rainforest and longest in grassland highlighting nutrient cycling differentiation. We conclude that the general absence of fire in the Bunya Mountains, due to a divergence from traditional Aboriginal burning practices, has allowed for the encroachment of fire‐sensitive rainforest species into the flammable biomes of this landscape. Rainforest invasion is likely to have reduced fire risk via changes to fuel composition and microclimatic conditions, and this feedback will be reinforced by altered nutrient cycling. The mechanics of the feedbacks here identified are discussed in terms of landscape change theory.     

Role of nurse plants in Araucaria Forest expansion over grassland in south Brazil

Abstract Araucaria Forest expansion over grassland takes place under wet climate conditions and low disturbance and it is hypothesized that isolated trees established on grassland facilitate the establishment of forest woody species beneath their canopies. Forest with Araucaria angustifolia is a particular type of Brazilian Atlantic Forest and the main forest type on the highland plateau in south Brazil, often forming mosaics with natural Campos grassland. The objectives of this paper were to evaluate the role of isolated shrubs and trees as colonization sites for seedlings of Araucaria Forest woody species on grassland, to determine which species function as preferential nurse plants in the process and the importance of vertebrate diaspore dispersal on the structure of seedling communities beneath nurse plants. The study was conducted in São Francisco de Paula, Rio Grande do Sul State, where we sampled isolated shrubs and trees in natural grassland near Araucaria Forest edges. Seedlings were counted and identified, and seedling diaspore dispersal syndromes, size and colour were registered. We detected 11 woody species with a potential role in nucleating grassland colonization by forest species. Beneath the canopies of nurse plants more forest species seedlings were found compared with open field grassland and the seedlings had diaspores mostly dispersed by vertebrates. Also, more seedlings were found under the canopy of A. angustifolia than beneath other nurse plant species. We conclude that A. angustifolia trees established on grassland act as nurse plants, by attracting disperser birds that promote colonization of the site by other forest species seedlings, and that under low level of grassland disturbance, conservation of frugivorous vertebrate assemblages may increase forest expansion over natural grassland and also facilitate the regeneration of degraded forest areas.     

Asymbiotic Nitrogen Fixation and Litter Decomposition on a Long Soil-Age Gradient in Hawaiian Montane Rain Forest1

We determined rates of decomposition and asymbiotic nitrogen fixation in the leaf litter of Cheirodendron spp. on the Hawaiian Islands. Leaf litter was collected from four sites on a long soil-age gradient (300 yr to 4.1 M yr) and decomposed at two sites that differed widely in substrate age and nutrient availability. Rates of decomposition were higher in litter decomposed at the older site, where nutrient availability was greater. A substantial amount of nitrogen and phosphorus immobilization occurred in litter decomposed at the older site, with more immobilization occurring in litter with lower initial nitrogen and phosphorus concentrations, suggesting both supply and demand controls on nutrient immobilization. Potential rates of nitrogen fixation were very low in the first 25 d (0–5 nmol acetylene/gdw/h), rose to much higher rates by 70 d (20–45 nmol), and then declined by 140 d. We found no significant difference in rates of potential nitrogen fixation between sites of decomposition, but there was a strong substrate effect, with higher rates in litter with low lignin, low nitrogen, and high phosphorus. Where significant immobilization of nitrogen occurred for decomposing Cheirodendron, nitrogen fixation could have comprised no more than 10 percent of immobilized nitrogen. Overall, rates of nitrogen fixation were dependent on the source of the decomposing substrate but not on the site of decomposition, while short-term decomposition and nutrient immobilization were strongly dependent on the site of decomposition but not as much on the source of the decomposing substrate.     

Assessment of nutrient availability and limitation using macroalgae

Discs of the macroalga,Ulva lactuca L., were transplanted around an ocean outfall and at a reference site in Køge Bay, Denmark, to assess the influence of the outfall on the nutrient availability. At 2-wk intervals, samples were collected and analyzed for growth, nitrogen, and phosphorus content.The tissue concentrations of nitrogen and phosphorus decreased with distance to the outfall, showing that the tissue concentrations are suitable for monitoring nutrient availability in coastal areas and provide a time-integrated measure of the nutrient availability. The lowest tissue concentrations of nitrogen were recorded at the reference station, where the internal concentrations generally were below the critical concentration level, showing that nitrogen limited the growth. At the station located close to the outfall, the flux of nitrogen was sufficient to maintain the maximum growth rate. The tissue concentrations of phosphorus were only below the critical concentration level on one occasion, and the result showed a net uptake throughout the study period.It was concluded that in the Køge Bay, nitrogen was the main limiting factor for macroalgae growth during the summer. The applicability of tissue concentrations for assessment of nutrient availability is discussed and it is considered that the method, when evaluated against established critical concentrations, provides a valuable tool for assessing ecosystem health with regard to eutrophication.     

Nitrogen and phosphorus dynamics and retention in ecotones of Lake Titicaca,Bolivia/Peru

We are studying present conditions and consequences of material movement from land to water in the Lake Titicaca basin, and how fluxes are affected by human activities. The principal objective of this research is to describe and explain the variability in the Andean Altiplano of (a) water, nutrient and sediment fluxes from land and (b) composition, nutrient limitation and other important features of nearshore lake communities, and compare the effects of different agricultural practices (especially traditional and modern) on these factors. We are focusing on a comparison of the impacts of two forms of agriculture in this region: ancient raised fields currently under rehabilitation, and flat pastures and fields, which are more common. Results of the first year of study indicate there is substantial variability in nitrogen and phosphorus dynamics in relation to ecotone complexity (simple vs. intermediate vs. complex). Raised field sites have the beneficial effect of reducing high available nutrient concentrations (nitrate and soluble reactive phosphorus) and sediment load (measured as turbidity) as the water passes through them enroute to the lake. Aquatic vegetation (algae and macrophytes) reflect well ambient total nitrogen and phosphorus concentrations. Experimental nutrient limitation bioassays indicate that nitrogen is the most important limiting nutrient, though there is important spatial variability within the landscape, and phosphorus as well as nitrogen can be limiting.     

Environmental influences on akinete germination and development in Nodularia spumigena (Cyanobacteriaceae), isolated from the Gippsland Lakes, Victoria, Australia

This study was carried out to investigate the genesis of N. spumigena blooms by specifically studying the effects of environmental variables (salinity, nitrogen, phosphorus and light) on the germination of N. spumigena akinetes. Optimal conditions for maximum germination and germling growth were determined by exposing akinetes to a range of salinities and nutrient (nitrogen and phosphorus) concentrations under two different irradiances. At pre-determined time periods, treatments were sampled and the percent germination and length of germlings assessed. The results indicated that akinete germination and germling growth were optimal at salinities from 5 to 25 and significantly reduced outside this range. A positive correlation in germination was observed with increasing nutrient (phosphorus and nitrate) concentration. Similarly, germling growth increased with increasing concentrations of both nutrients. Irradiance significantly influenced both germination and growth during salinity experiments, whereas in nutrient addition experiments, irradiance had no effect on germination; however, growth was significantly influenced during phosphorus addition experiments. Consequently, salinity and light appeared to be most critical in the germination process for N. spumigena akinetes, with phosphorus most important for germling growth. The study showed that N. spumigena may be able to germinate under environmental conditions outside its optimal range, but the growth of the germling is significantly reduced, which in turn suggests that its ability to form a bloom outside its optimal environmental conditions would also be greatly reduced.     

Response of irrigated dry-seeded rice to nitrogen and phosphorus in a semi-arid environment

Summary Under semi-arid conditions, three field experiments were conducted at Gezira Research Station to determine response of irrigated dry-seeded rice (Oryza sativa L. var IR 2053-206-1-3-6) to addition of nitrogen and phosphorus fertilizers. The experimental treatments included the factorial combinations of seven levels of nitrogen applied as urea and four levels of phosphorus applied as super phosphate. Plant growth and grain yield were significantly and progressively increased with the rise in the levels of added nitrogen and phosphorus. However, response to added phosphorus was restricted by the applied level of nitrogen. The responses of grain yield to nitrogen and phosphorus levels are given by quadratic regression equations. Without addition of nitrogen or phosphorus grain yield averaged 1.52 t/ha compared to 6.07 t/ha with addition of the optimal levels (160 kg N plus 35 kg P/ha). The high potential for rice production in semi-arid environment is evidently restricted by addition of relatively high rates of nitrogen and phosphorus.     

Invasive plant removal increases insect herbivory pressure on a native tree due to an increase in resource quality

Insect predator densities and plant nutritional status can affect insect herbivore densities, but these effects have not yet been assessed in the context of an invasive species management. This study investigated the influence of (i) altered predatory arthropod abundance and community composition (top-down-effects); and (ii) altered leaf nitrogen and phosphorus levels (bottom-up-effects) as possible causes of fluctuations in herbivore abundance and herbivory on the native tree Brabejum stellatifolium in riparian sites that differ in invasion status (near pristine, heavily invaded by Acacia mearnsii, and restored). Species richness, abundance and composition of herbivorous and predatory arthropods were compared between categories. Herbivore and predator abundance, species richness and assemblage composition were significantly influenced by invasion status. As expected, herbivore abundance was significantly and positively correlated to levels of leaf damage. Leaf nitrogen levels were the highest at restored sites where herbivorous arthropods were also most abundant and the abundance of predators was intermediate. These results suggest that altered leaf nitrogen content is likely a key influence for the increased herbivore pressure on B. stellatifolium at sites under pressure from invasive A. mearnsii and its status. Bottom-up influences therefore may be more important than top-down effects on controlling herbivore abundances in these altered riparian environments.

     

Can zinc influence nutrient resorption? A test with the drought-deciduous desert shrub Fouquieria splendens (ocotillo)

Paradoxically low nitrogen resorption efficiency in the drought-deciduous desert shrub Fouquieria splendens Engelm (ocotillo) triggered tests of the hypotheses that resorption is often low in this species and that resorption is influenced by zinc. Resorption efficiency and proficiency were measured in 1989 and 1994 at two sites in the Chihuahuan Desert in plants to which zinc, or zinc and nitrogen were added. Resorption of nitrogen, phosphorus, and zinc in unfertilized plants varied temporally and spatially, but was both efficient (66%, 49%, and 40%, respectively) and proficient (0.55%, 0.09%, and 9.4 μg g−1, respectively) as determined by comparison to worldwide resorption patterns in a wide variety of other species. Applications of zinc had no significant effect on the resorption of nitrogen and phosphorus, but did influence the resorption of zinc. Resorption of zinc was significantly less efficient in zinc-treated plants than controls at only one of the two sites in one of the 2 years, yet resorption of zinc was significantly less proficient in zinc-treated plants than controls in both years and at both sites. This pattern of zinc resorption adds insight into the continuing debate regarding the relationship between fertility and resorption because the data used to fuel the debate have almost exclusively described macronutrients, not trace metals. The high variability in resorption among individuals, sites, and years observed for F. splendens may well be an attribute of many desert-dwelling, drought-deciduous plants. When senescence is controlled primarily by water availability rather than photoperiod, especially in a landscape characterized by unpredictable amounts and timing of precipitation, high variability in associated processes such as resorption may be inevitable.     

漓江水陆交错带典型灌木群落根系分布与土壤养分的关系

在漓江流域水陆交错带选取一叶萩、细叶水团花、枫杨3种典型灌木群落,对不同土层深度的根长密度,以及根长密度与土壤有机质、全氮、有效磷的相关关系进行了研究,旨在为漓江流域生态修复过程中灌木植被恢复、主要灌木植被配置、快速绿化材料选取等提供科学依据。结果表明:(1)漓江水陆交错带典型灌木群落各土层根长密度差异性显著,分布规律均为:枫杨细叶水团花一叶萩。0—10 cm到40—60 cm土层,各灌木群落根长密度均减小,但不同灌木群落根长密度的差异程度逐渐缩小,表明地形、地表植物类型及生长状况对根长密度分布的影响也随土层深度的增加而逐渐减小。(2)枫杨(25.10g/kg)灌木群落底层腐殖质层厚,有机质含量最高,分别是细叶水团花、一叶萩的1.24、1.87倍。各灌木群落全氮含量从大到小依次是枫杨、细叶水团花、一叶萩,其值分别为:3.23、2.41、1.46 g/kg。土壤有效磷分布规律为一叶萩(11.56 mg/kg)细叶水团花(5.37 mg/kg)枫杨(3.99 mg/kg);一叶萩灌木群落有效磷含量远远大于枫杨和细叶水团花,原因是漓江水长期受人为洗漱影响,导致受江水干扰大的一叶萩灌木群落有效磷含量高。(3)根长密度与有机质、全氮含量呈正相关,与有效磷含量呈负相关,说明适量增加土壤有机质、全氮含量,减少土壤有效磷,有利于土壤根系的生长。根长密度与0L≤1 mm径级的根系所占比例呈极显著正相关,与1L≤2 mm径级的根系所占比例呈显著正相关,表明根系细根越多,根长密度越大。     

温带荒漠灌丛和藓类结皮对土壤养分的贡献

灌丛与生物土壤结皮镶嵌分布是温带荒漠常见的地表景观之一,二者的发育均显著影响了地表土壤养分的空间分布特征及循环过程。然而,灌丛和生物土壤结皮对荒漠土壤表层养分的影响存在怎样的差异,二者对养分变化的贡献度如何尚不清楚。因此,选择中国北方典型温带荒漠古尔班通古特沙漠为研究区,以荒漠中的优势灌丛膜果麻黄(Ephedra przewalskii)灌丛和生物土壤结皮发育高级阶段的藓类结皮的结皮层及结皮下层土壤为研究对象,采集不同微生境(裸露地、灌丛下)的裸沙与藓类结皮的土壤样品。为探究不同微生境下不同土层碳、氮、磷和钾养分变化特征,测定了不同土层土壤有机碳(SOC)、全氮(TN)、全磷(TP)、全钾(TK)、速效氮(AN)、速效磷(AP)和速效钾(AK)含量。结果表明:(1)相较于裸露地,灌丛显著提高了藓类结皮不同土层有机碳、全氮和全钾的含量和裸沙全磷的含量,降低了灌丛下藓类结皮土壤全磷的含量。(2)对于速效养分而言,与裸沙相比,裸露地藓类结皮覆盖降低了土壤速效氮含量,增加了速效磷和速效钾含量;而灌丛下藓类结皮覆盖提高了土壤速效氮和速效钾的含量,但降低了速效磷的含量。 (3) 相关性分析显示,在0-2 cm土壤中速效磷与速效氮呈现显著负相关,而在2-6 cm速效磷与速效氮呈现显著正相关(P<0.01)。(4)贡献度分析表明,土壤中灌丛效应对养分的贡献(42.54%)要远大于藓类结皮的贡献(2.43%),但二者交互作用却降低了除速效氮以外的其他土壤养分含量。综上,灌丛、藓类结皮覆盖和土层深度变化对土壤碳、氮、磷和钾养分均具有显著影响(P<0.05),但三者间的交互效应对养分的影响不显著(P>0.05)。相对于裸露地,灌丛与藓类结皮的覆盖均对表层土壤碳、氮、磷和钾养分具有促进作用,且随着土层深度的下降,土壤养分含量呈现显著的下降趋势(P<0.05)。因此,在荒漠生态系统中耐旱灌丛与生物土壤结皮这两个最重要的斑块单元联合调控了微尺度土壤养分的空间异质性变化。