Effects of kangaroo rat exclusion on vegetation structure and plant species diversity in the Chihuahuan Desert

Long-term (1977–90) experimental exclusion of three species of kangaroo rats from study plots in the Chihuahuan Desert resulted in significant increases in abundance of a tall annual grass (Aristida adscensionis) and a perennial bunch grass (Eragrostis lehmanniana). This change in the vegetative cover affected use of these plots by several other rodent species and by foraging birds. The mechanism producing this change probably involves a combination of decreased soil disturbance and reduced predation on large-sized seeds when kangaroo rats are absent. Species diversity of summer annual dicots was greater on plots where kangaroo rats were present, as predicted by keystone predator models. However, it is not clear whether this was caused directly by activities of the kangaroo rats or indirectly as a consequence of the increase in grass cover. No experimental effect on species diversity of winter annual dicots was detected. Our study site was located in a natural transition between desert scrub and grassland, where abiotic conditions and the effects of organisms may be particularly influential in determining the structure and composition of vegetation. Under these conditions kangaroo rats have a dramatic effect on plant cover and species composition.     

Vertical leaf nitrogen distribution in relation to nitrogen status in grassland plants

Vertical gradients of leaf nitrogen (N) per unit leaf area (NLA) are viewed as plastic responses that optimize N utilization with respect to carbon assimilation. However, it has been shown that plant species, sowing density and N availability affect the steepness of the NLA gradient relative to the photon flux density (PFD) gradient. This paper tests the hypothesis that such variation is related to the N status of the plant. The N status was analysed using the concept of the critical N concentration (Ncrit) in which shoot N per unit dry mass (NSM) decreases with shoot mass, and a negative deviation of actual NSM from Ncrit indicates N shortage in the plant. The hypothesis was tested with contrasting grassland species Medicago sativa, Dactylis glomerata and Taraxacum officinale by varying PFD and N availability, plant density and hierarchical positions of individuals within stands. Combinations of all treatments showed a general negative correlation between the N allocation coefficient (i.e. the slope of the NLA-PFD relationship) and NSM for all three species. Thus, NLA, relative to PFD, gradients became steeper with increasing shoot mass and increasing N shortage in the plant. These data are consistent with the view that internal N availability is an important factor in modifying the NLA gradient.     

接种丛植菌根真菌对湿生植物氮磷吸收能力的影响

丛枝菌根(Arbuscular mycorrhizal,AM)真菌是一类能够与大多数陆生植物共生,并改善植物生长和氮磷吸收的微生物.湿生植物在湿地污染净化过程中起着决定性的作用,但利用AM真菌改良湿生植物氮磷吸收能力的研究鲜有报道.本研究选取了3种湿生植物千屈菜、旱伞草和黄花鸢尾,在盆栽培养的基础上,分别接种根内球囊霉(RE)、摩西球囊霉(GM)、幼套球囊霉(CE)三种AM真菌,并和空白对照比较接种AM真菌对不同植物地上、地下及整株的生物量和氮磷吸收的影响.结果表明,接种AM真菌对植物生长和氮磷吸收的影响呈现出植物间和植物内的差异,促进与抑制效应表现不一.综合AM真菌对植物生物量和氮磷吸收的促进效应,最佳AM真菌-植物的组合为:千屈菜-RE,旱伞草-RE,黄花鸢尾接种-GM.三种植物接种最佳的AM真菌后植物生物量、TN量和TP量分别提高了17.7%~29.8%、15.7%~39.0%和22.3%~62.6%.本研究为今后强化湿生植物的氮磷吸收能力提供了一种新的可选择的途径.     

A floristic-structural gradient related to land forms in the southern Chihuahuan Desert

Abstract. The purposes of this study were to elucidate the floristic and structural characteristics of the vegetationin the Mapimi subdivision of the Chihuahuan Desert, Mexico, and to relate them to environmental variation. The main question addressed was: How does floristic composition, total species richness and life-form species richness vary in relation to environmental change? 154 sites, randomly selected and stratified over seven landscape units, were analyzed. Results showed the existence of a land form gradient along which vegetation types were ordered. Species richness varied along the gradient, the richest land form was bajada, indicating that the maximum species richness did not occur at one extreme of the water availability gradient but in a moderately poor situation. The lowest species richness was found in the playa land form. Cover-based life form spectra varied significantly with land forms, while presence-absence based life form spectra did not. It is suggested that this may be a consequence of the relatively young age of this desert.     

Atmospheric nitrogen deposition in world biodiversity hotspots: the need for a greater global perspective in assessing N deposition impacts

Increased atmospheric nitrogen (N) deposition is known to reduce plant diversity in natural and semi‐natural ecosystems, yet our understanding of these impacts comes almost entirely from studies in northern Europe and North America. Currently, we lack an understanding of the threat of N deposition to biodiversity at the global scale. In particular, rates of N deposition within the newly defined 34 world biodiversity hotspots, to which 50% of the world's floristic diversity is restricted, has not been quantified previously. Using output from global chemistry transport models, here we provide the first estimates of recent (mid‐1990s) and future (2050) rates and distributions of N deposition within biodiversity hotspots. Our analysis shows that the average deposition rate across these areas was 50% greater than the global terrestrial average in the mid‐1990s and could more than double by 2050, with 33 of 34 hotspots receiving greater N deposition in 2050 compared with 1990. By this time, 17 hotspots could have between 10% and 100% of their area receiving greater than 15 kg N ha^?1 yr^?1, a rate exceeding critical loads set for many sensitive European ecosystems. Average deposition in four hotspots is predicted to be greater than 20 kg N ha^?1 yr^?1. This elevated N deposition within areas of high plant diversity and endemism may exacerbate significantly the global threat of N deposition to world floristic diversity. Overall, we highlight the need for a greater global approach to assessing the impacts of N deposition.     

Theory for growth of plants derived from the nitrogen productivity concept

A theory is developed on the assumption that growth of plants is determined by the current amount of nitrogen in the plants. The nitrogen-growth relation is formalized in the nitrogen productivity concept (amount of biomass produced per amount of nitrogen in the biomass and per unit of time), which is essentially a constant for a given species under fixed environmental conditions. A number of results follow for increases in whole plant biomass: (A) The relative growth rate is a linear function of the internal nitrogen concentration. (B) The maximal relative growth rate uniquely determines the scaling of the time axis. (C) Exponential growth is consistent only with stable internal nitrogen concentration. Dose-response curves expressed in reduced variables (the ratio between a variable and the same variable for a plant growing under optimal conditions) are universal, so that all species and all environmental conditions yield the same curve. This is confirmed by experimental data. The shape (linear, exponential, etc.) of the nitrogen uptake curve is the only parameter differentiating these universal curves. The Mitscherlich curve or variations of it can be fitted very closely to the derived dose-response curves, except under exponential growth. A conclusion drawn from the analysis is that the results of nutrition experiments cannot be properly interpreted unless the variation with time of the amount of nitrogen in the plant is known. The theory can be extended to more complex situations, for example, time-varying environmental conditions.     

Effects of nitrogen application on beetle communities in tea plantations

In contrast to grassland and forest ecosystems, little is known about insect response to nitrogen deposition in agricultural ecosystems. This study was carried out to investigate the effects of short-term (1–2 years) nitrogen application (0, 172.5, 345.0, 690.0, and 1035.0 kg · N ha⁻¹· y⁻¹) on beetle communities in tea plantations. A collection of 34 families, 89 species of beetles, was obtained from a tea plantation in Wuyishan, China. Among them, herbivores, predators and detritivores had 52, 29, and eight species, respectively. Species richness, effective diversity and abundance (measured as the number of individuals and insect biomass) of the beetle community were not significantly related to the rate of nitrogen application. However, nitrogen application changed the species distribution and weakly increased the evenness of species distribution, while this did not significantly change the species evenness. Species richness and abundance of herbivores and predators were not significantly related to the rate of nitrogen application. However, there were some variations in trophic responses to nitrogen. Species richness and abundance of detritivores increased with increasing nitrogen application.     

Formation of unidentified nitrogen in plants: an implication for a novel nitrogen metabolism

Plants take up inorganic nitrogen and store it unchanged or convert it to organic forms. The nitrogen in such organic compounds is stoichiometrically recoverable by the Kjeldahl method. The sum of inorganic nitrogen and Kjeldahl nitrogen has long been known to equal the total nitrogen in plants. However, in our attempt to study the mechanism of nitrogen dioxide (NO₂) metabolism, we unexpectedly discovered that about one-third of the total nitrogen derived from ¹⁵N-labeled NO₂ taken up by Arabidopsis thaliana (L.) Heynh. plants was converted to neither inorganic nor Kjeldahl nitrogen, but instead to an as yet unknown nitrogen compound(s). We here refer to this nitrogen as unidentified nitrogen (UN). The generality of the formation of UN across species, nitrogen sources and cultivation environments for plants has been shown as follows. Firstly, all of the other 11 plant species studied were found to form the UN in response to fumigation with ¹⁵NO₂. Secondly, tobacco (Nicotiana tabacum L.) plants fed with ¹⁵N-nitrate appeared to form the UN. And lastly, the leaves of naturally fed vegetables, grass and roadside trees were found to possess the UN. In addition, the UN appeared to comprise a substantial proportion of total nitrogen in these plant species. Collectively, all of our present findings imply that there is a novel nitrogen mechanism for the formation of UN in plants. Based on the analyses of the exhaust gas and residue fractions of the Kjeldahl digestion of a plant sample containing the UN, probable candidates for compounds that bear the UN were deduced to be those containing the heat-labile nitrogen–oxygen functions and those recalcitrant to Kjeldahl digestion, including organic nitro and nitroso compounds. We propose UN-bearing compounds may provide a chemical basis for the mechanism of the reactive nitrogen species (RNS), and thus that cross-talk may occur between UN and RNS metabolisms in plants. A mechanism for the formation of UN-bearing compounds, in which RNS are involved as intermediates, is proposed. The important broad impact of this novel nitrogen metabolism, not only on the general physiology of plants, but also on plant substances as human and animal food, and on plants as an integral part of the global environment, is discussed.Abbreviations NO Nitric oxide - NO₂ Nitrogen dioxide - RNS Reactive nitrogen species - UN Unidentified nitrogen - TNNAT, RNNAT, INNAT and UNNAT Total, Kjeldahl, inorganic and unidentified nitrogen in naturally fed plants, respectively - TNNIT, RNNIT, INNIT and UNNIT Total, Kjeldahl, inorganic and unidentified nitrogen derived from nitrate, respectively - TNNO2, RNNO2, INNO2 and UNNO2 Total, Kjeldahl, inorganic and unidentified nitrogen derived from NO₂, respectively     

云南省高等植物多样性与分布状况

云南省为我国乃至世界生物多样性的热点区域之一.为深入认识和保护云南的植物多样性,作者以《云南植物志》为基础,对其中记载的各物种在云南的县级分布情况进行了统计.结果显示:云南省高等植物多样性最丰富的为滇西北地区,其中的玉龙县、贡山县、香格里拉县物种数分别高达4,358种、3,981种和3,874种,其次为以西双版纳州为主体的南部热带边缘,其中的勐腊、景洪的物种数也都在3,000种以上;而以云南高原为主体的滇东和滇中地区植物多样性则较为贫乏,作者认为这与人们的关注度和野外调查的强度直接相关.云南分布有4,008种特有物种,4,509种狭域物种,特有物种的分布规律与总物种相似;含狭域物种最多的为贡山县(556种),是唯一超过500种的县.云南省多数物种的分布范围极为狭窄,64.1％的物种仅分布在1-5个县,46.0％的特有物种只分布在1个县的范围内.通过以上分析,作者指出由于此前对云南植物多样性的调查仍不完善,结果并不能全面反映真实的地带性规律,建议加大调查深度和标本采集量.如果条件许可,一些极易沦为濒危种的狭域分布种也应得到优先关注.     

植物物种丰富度对水培微宇宙中硝态氮去除的影响

在沙基质的人工湿地中植物多样性能够提高污水去除效果,但无基质水生系统中植物多样性对除氮效应的影响还未知.本研究在45个水培微宇宙(53 cm×37.5 cm×18.5cm)中配置了4个物种丰富度梯度(1、2、3和4),并定期供给硝氮为唯一氮形态的模拟污水,氮载荷率为548.5 gN· m-2 ·a-1.结果表明:物种丰富度对出水中氮去除有显著效应,4个种系统出水总无机氮浓度(54.3 mg·L-1)明显低于单种系统(129.0 mg·L-1);物种丰富度显著提高群落生物量,4个种微宇宙系统群落总生物量为1621.6 g·m-2,高于单种群落的1032.7 g·m-2.水培微宇宙的氮平均移除速率为466.8 g N·m-2·a-1,不低于已有报道的全尺度人工湿地的去除能力,同时,4个种的系统比单种系统约高13％,因而可以通过物种多样性配置提高人工湿地效能.     

Associative effect of cellulolytic fungi andAzospirillum lipoferum on yield and nitrogen uptake by wheat

The associative effect of cellulolytic fungi, such asAspergillus awamori andA. niger, with the nitrogen fixer,Azospirillum lipoferum was studied in a soil amended with rice straw. All the inoculants gave significantly higher grain and straw yield and nitrogen uptake by wheat crop than did the uninoculated treatment. The doubling of chemical nitrogen dose significantly increased the yield and nitrogen uptake. It was observed thatA awamori performed significantly better followed byA. niger andA. lipoferum. The maximum benefit was obtained with combined inoculation ofA. awamori andA. lipoferum. Another experiment was conducted in the subsequent year in soil amended with and without rice straw using cellulolytic culture eitherA. awamori orSclerotium rolfsii, andA. lipoferum. Application of straw in soil significantly reduced the yield and N-uptake by wheat crop as compared to the controls. All the inoculants exceptS. rolfsii gave significantly higher grain yield. However, N-uptake by grain was significantly increased only by combined inoculation ofA. lipoferum and either one of the cellulolytic fungi. Similar trends on yield and N-uptake of straw due to inoculants were observed. The maximum benefit was obtained with combined inoculation ofA. awamori andA. lipoferum followed byA. awamori alone on grain yield and only combined inoculants on N-uptake by the crop.     

Carbon and nitrogen content of congeneric annual and perennial grass species: relationships with growth

Seven annual-perennial pairs of grass species (six congeneric and one pair taken at random) were grown under productive conditions in the laboratory in order to investigate which plant characters were responsible for the higher relative growth rate (RGR) of annuals as compared to perennials under these conditions. The nitrogen and carbon concentrations of shoot organs and of the whole plant were higher in annuals than in perennials. This was also the case for the specific absorption rate for nitrate and nitrogen productivity (on whole plant and leaf basis). The range of RGR displayed by the 14 species was large enough (0.15–0.33d⁻¹) to examine the general relationships between RGR and the various parameters measured in the present study. RGR was positively related to plant, leaf blade and sheath nitrogen concentrations, but there was no relationship between RGR and any of the carbon concentrations. RGR also strongly correlated with specific absorption rate for nitrate and with nitrogen productivity. A new factorization of this latter parameter led to the definition of the ‘leaf nitrogen productivity’ (N_LP), which is likely to depend on photosynthetic nitrogen use efficiency. RGR was shown to be strongly correlated with N_LP, but not with the second term of the factorization, namely the proportion of plant nitrogen allocated to the leaves.     

Physiological variation among native and exotic winter annual plants associated with microbiotic crusts in the Mojave Desert

An experiment was established in 1992 in eastern Ontario, Canada to determine the effects of crop rotation (continuous maize, soybean-maize and alfalfa-maize) and nitrogen (N) amendment [0, 100 and 200 kg N ha^–1 of fertilizer (NH₄NO₃), and 50 and 100 Mg ha^–1 (wet wt.) each of stockpiled and rotted dairy manure] on maize production and soil properties. From 1997 to 1999, an additional study was added to the experiment to investigate treatment effects on the susceptibility of maize hybrids to gibberella ear rot. A moderately resistant and a susceptible hybrid were planted in each plot and inoculated with a macroconidial suspension of Fusarium graminearum by both the silk channel injection and the kernel-wound techniques. At harvest, ears were rated for the severity of disease symptoms and harvested kernels were analyzed for the mycotoxin deoxynivalenol (DON). The greatest number of significant N effects were found in the continuous maize treatments and with the susceptible hybrid. Most N amendments decreased both disease severity and DON accumulation in the susceptible hybrid. The most consistent effect was a decrease in disease severity with 100 kg N ha^–1 fertilizer and an increase in disease severity with the higher rate of 200 kg N ha^–1. This study is the first to report on the effects of soil N amendments on gibberella ear rot susceptibility.     

种植密度和施氮水平对小麦吸收利用土壤氮素的影响

2011-2013小麦季,在大田条件下设置2个氮肥水平(180和240kgN· hm-2)和3个种植密度(135、270和405万·hm-2),并将15N-尿素分别标记在20、60和100 cm土层处,研究种植密度-施氮互作对小麦吸收、利用土壤氮素及硝态氮残留量的影响.结果表明:种植密度从135万·hm-2增加至405万·hm-2,小麦在20、60和100 cm土层的15N吸收量分别增加1.86、2.28和2.51 kg·hm-2,地上部氮素积累量和吸收效率分别提高12.6％和12.6％,氮素利用效率降低5.4％;施氮量由240 kg N·hm-2降至180 kg N·hm-2,小麦在20、60 cm土层的15N吸收量分别降低4.11和1.21 kg·hm-2,在100 cm土层的15N吸收量增加1.02 kg·hm-2,地上部氮素积累量平均降低13.5％,氮素吸收效率和利用效率分别提高9.4％和12.2％.施氮180kg N·hm-2+种植密度为405万·hm-2处理与施氮240 kg N·hm-2+种植密度为270或405万·hm-2处理相比,其籽粒产量无显著差异,深层土壤氮素的吸收量显著提高,氮素吸收效率和利用效率分别提高13.4％和11.9％,O～ 200 cm土层的硝态氮积累量及100～ 200 cm土层硝态氮分布比例降低.在适当降低氮肥用量条件下,通过增加种植密度可以促进小麦吸收深层土壤氮素,减少土壤氮素残留,并保持较高的产量水平.     

Efficacy of organic wastes amendment on the establishment and biomass yield of grass species on a degraded Alfisol

Abstract

Deficiencies of organic matter and essential nutrients are important features of derelict/degraded lands; these characteristics have profound effects on the establishment of functional soil-plant systems on such lands. This work reports on the impact of organic wastes amendments on the establishment, growth and biomass yield of grass mixtures grown on a degraded soil. The experimental site was a heavily compacted and eroded land due to excavation (scrapping) of topsoil layer for urban construction activities. Zero, 10 and 20cm layers each of substrates collected from an old (abandoned) municipal refuse and from site affected by long-term surface disposal of livestock litter and topsoil collected under fallow vegetation were applied on soil surface, before the establishment of a mixture of grass species. Substrates collected from old municipal refuse contained high amount of essential nutrients and metals and, on application, produced upward shift in soil pH and electrical conductivity. Nevertheless, these properties neither significantly retarded plant growth nor impaired biomass yield. Plant establishment and biomass yield were comparable under topsoil application and organic waste amended soil and produced significantly higher biomass yield when spread at 20cm layers were made of them. This experimental group also produced superior biomass yield over the unamended control. The agronomic benefits of organic waste incorporation stemmed from improved soil properties in surface horizon and contents of essential nutrients supportive of the establishment and development of plant cover. Nutrient supply is related to soil organic matter status, organic wastes are therefore important to the reconstruction of effective nutrient cycling and the eventual functional soil-plant system on this degraded ecosystem. The goal of these revegetation efforts is to improve soil and plant productivity, plant diversity, conservation of native grasslands and aesthetic.