Foliar nutrient resorption patterns of four functional plants along a precipitation gradient on the Tibetan Changtang Plateau

Nutrient resorption from senesced leaves as a nutrient conservation strategy is important for plants to adapt to nutrient deficiency, particularly in alpine and arid environment. However, the leaf nutrient resorption patterns of different functional plants across environmental gradient remain unclear. In this study, we conducted a transect survey of 12 communities to address foliar nitrogen (N) and phosphorus (P) resorption strategies of four functional groups along an eastward increasing precipitation gradient in northern Tibetan Changtang Plateau. Soil nutrient availability, leaf nutrient concentration, and N:P ratio in green leaves ([N:P]_g) were linearly correlated with precipitation. Nitrogen resorption efficiency decreased, whereas phosphorus resorption efficiency except for sedge increased with increasing precipitation, indicating a greater nutrient conservation in nutrient‐poor environment. The surveyed alpine plants except for legume had obviously higher N and P resorption efficiencies than the world mean levels. Legumes had higher N concentrations in green and senesced leaves, but lowest resorption efficiency than nonlegumes. Sedge species had much lower P concentration in senesced leaves but highest P resorption efficiency, suggesting highly competitive P conservation. Leaf nutrient resorption efficiencies of N and P were largely controlled by soil and plant nutrient, and indirectly regulated by precipitation. Nutrient resorption efficiencies were more determined by soil nutrient availability, while resorption proficiencies were more controlled by leaf nutrient and N:P of green leaves. Overall, our results suggest strong internal nutrient cycling through foliar nutrient resorption in the alpine nutrient‐poor ecosystems on the Plateau. The patterns of soil nutrient availability and resorption also imply a transit from more N limitation in the west to a more P limitation in the east Changtang. Our findings offer insights into understanding nutrient conservation strategy in the precipitation and its derived soil nutrient availability gradient.     

Nodules from Fynbos legume Virgilia divaricata have high functional plasticity under variable P supply levels

Legumes have the unique ability to fix atmospheric nitrogen (N₂) via symbiotic bacteria in their nodules but depend heavily on phosphorus (P), which affects nodulation, and the carbon costs and energy costs of N₂ fixation. Consequently, legumes growing in nutrient-poor ecosystems (e.g., sandstone-derived soils) have to enhance P recycling and/or acquisition in order to maintain N₂ fixation. In this study, we investigated the flexibility of P recycling and distribution within the nodules and their effect on N nutrition in Virgilia divaricata Adamson, Fabaceae, an indigenous legume in the Cape Floristic Region of South Africa. Specifically, we assessed tissue elemental localization using micro-particle-induced X-ray emission (PIXE), measured N fixation using nutrient concentrations derived from inductively coupled mass-spectrometry (ICP-MS), calculated nutrient costs, and determined P recycling from enzyme activity assays. Morphological and physiological features characteristic of adaptation to P deprivation were observed for V. divaricata. Decreased plant growth and nodule production with parallel increased root:shoot ratios are some of the plastic features exhibited in response to P deficiency. Plants resupplied with P resembled those supplied with optimal P levels in terms of growth and nutrient acquisition. Under low P conditions, plants maintained an increase in N₂-fixing efficiency despite lower levels of orthophosphate (Pi) in the nodules. This can be attributed to two factors: (i) an increase in Fe concentration under low P, and (ii) greater APase activity in both the roots and nodules under low P. These findings suggest that V. divaricata is well adapted to acquire N under P deficiency, owing to the plasticity of its nodule physiology     

Source of inorganic N affects the cost of growth in a legume tree species (Virgilia divaricata) from the Mediterrean-type Fynbos ecosystem

Aims In Mediterranean-type ecosystem, the Cape Fynbos, legumes may be able to switch between soil N and atmospheric N 2 sources during growth to adjust the carbon costs of N acquisition. This study investigated the utilization of different inorganic N sources by Virgilia divaricata, a native legume from the Mediterranean-type ecosystem of the Cape Floristic Region.Methods Plants were cultivated in sterile quartz sand, supplied with 25% strength Long Ashton nutrient solution, modified to contain 500 μM Phosphate. At the phosphate level (500 μM), plants were treated with 500 μM NH 4 NO 3 (treatment named N), or grown in N-free nutrient solution and inoculated with effective Burkholderia sp. (Bact.) or treated with combined N sources (500 μM NH 4 NO 3) and inoculated with effective Burkholderia sp. (N+Bact.).Important findings The application of NH 4 NO 3 to the legumes resulted in a greater increase in plant dry matter. Carbon construction costs were higher in plants that were supplied with mineral and symbiotic N sources. Maximum photosynthetic rates per leaf area was maintained, irrespective of the N sources. Although the plant roots were nodulated, the plant dependence on N 2 fixation decreased with addition of N. Roots and nodules of the plants solely reliant on N 2 fixation showed an increase in glutamine content. These results show that V. divaricata is highly adapted for growth at the forest margin. Fynbos and possibly anthropic soils by utilizing both atmospheric and soil N sources.     

水分再分配对土壤-植物系统养分循环的生态意义

水分再分配(hydraulic redistribution, HR)作为一个普遍存在的生物物理过程, 在缓解植物干旱胁迫、调节植物种间关系和群落组成、影响生态系统水碳平衡等方面具有重要的生态意义。近年来, 同位素标记示踪技术的应用促进了HR的深入研究, 该文综述了HR对土壤-植被系统养分循环的影响。HR能改善干燥土层的水分状况, 防止根系栓塞, 促进细根存活与生长, 提高微生物活性, 从而促进植物对表层土壤养分(尤其是氮)的吸收; HR还通过水分下传作用促进植物对深层土壤中磷和金属离子的吸收。HR促进土壤养分库的上下交换与流动, 调节植物与土壤的氮磷比, 因此其影响可能具有全球意义。在全球变化(如氮沉降)背景下, 有必要深入探索HR在生物地球化学循环过程中的影响和作用, 并将其纳入生态系统模型中。     

Are patterns in nutrient limitation belowground consistent with those aboveground: results from a 4 million year chronosequence

Accurately predicting the effects of global change on net carbon (C) exchange between terrestrial ecosystems and the atmosphere requires a more complete understanding of how nutrient availability regulates both plant growth and heterotrophic soil respiration. Models of soil development suggest that the nature of nutrient limitation changes over the course of ecosystem development, transitioning from nitrogen (N) limitation in ‘young’ sites to phosphorus (P) limitation in ‘old’ sites. However, previous research has focused primarily on plant responses to added nutrients, and the applicability of nutrient limitation-soil development models to belowground processes has not been thoroughly investigated. Here, we assessed the effects of nutrients on soil C cycling in three different forests that occupy a 4 million year substrate age chronosequence where tree growth is N limited at the youngest site, co-limited by N and P at the intermediate-aged site, and P limited at the oldest site. Our goal was to use short-term laboratory soil C manipulations (using ¹⁴C-labeled substrates) and longer-term intact soil core incubations to compare belowground responses to fertilization with aboveground patterns. When nutrients were applied with labile C (sucrose), patterns of microbial nutrient limitation were similar to plant patterns: microbial activity was limited more by N than by P in the young site, and P was more limiting than N in the old site. However, in the absence of C additions, increased respiration of native soil organic matter only occurred with simultaneous additions of N and P. Taken together, these data suggest that altered nutrient inputs into ecosystems could have dissimilar effects on C cycling above- and belowground, that nutrients may differentially affect of the fate of different soil C pools, and that future changes to the net C balance of terrestrial ecosystems will be partially regulated by soil nutrient status.     

Anthropogenic global shifts in biospheric N and P concentrations and ratios and their impacts on biodiversity,ecosystem productivity,food security,and human health

The availability of carbon (C) from high levels of atmospheric carbon dioxide (CO₂) and anthropogenic release of nitrogen (N) is increasing, but these increases are not paralleled by increases in levels of phosphorus (P). The current unstoppable changes in the stoichiometries of C and N relative to P have no historical precedent. We describe changes in P and N fluxes over the last five decades that have led to asymmetrical increases in P and N inputs to the biosphere. We identified widespread and rapid changes in N:P ratios in air, soil, water, and organisms and important consequences to the structure, function, and biodiversity of ecosystems. A mass‐balance approach found that the combined limited availability of P and N was likely to reduce C storage by natural ecosystems during the remainder of the 21st Century, and projected crop yields of the Millennium Ecosystem Assessment indicated an increase in nutrient deficiency in developing regions if access to P fertilizer is limited. Imbalances of the N:P ratio would likely negatively affect human health, food security, and global economic and geopolitical stability, with feedbacks and synergistic effects on drivers of global environmental change, such as increasing levels of CO₂, climatic warming, and increasing pollution. We summarize potential solutions for avoiding the negative impacts of global imbalances of N:P ratios on the environment, biodiversity, climate change, food security, and human health.     

Nitrogen fixation in legumes and actinorhizal plants in natural ecosystems: values obtained using 15N natural abundance

Background: Nitrogen fixation has been quantified for a range of crop legumes and actinorhizal plants under different agricultural/agroforestry conditions, but much less is known of legume and actinorhizal plant N₂ fixation in natural ecosystems.

Aims: To assess the proportion of total plant N derived from the atmosphere via the process of N₂ fixation (%Ndfa) by actinorhizal and legume plants in natural ecosystems and their N input into these ecosystems as indicated by their ¹⁵N natural abundance.

Methods: A comprehensive collation of published values of %Ndfa for legumes and actinorhizal plants in natural ecosystems and their N input into these ecosystems as estimated by their ¹⁵N natural abundance was carried out by searching the ISI Web of Science database using relevant key words.

Results: The %Ndfa was consistently large for actinorhizal plants but very variable for legumes in natural ecosystems, and the average value for %Ndfa was substantially greater for actinorhizal plants. High soil N, in particular, but also low soil P and water content were correlated with low legume N₂ fixation. N input into ecosystems from N₂ fixation was very variable for actinorhizal and legume plants and greatly dependent on their biomass within the system.

Conclusions: Measurement of ¹⁵N natural abundance has given greater understanding of where legume and actinorhizal plant N₂ fixation is important in natural ecosystems. Across studies, the average value for %Ndfa was substantially greater for actinorhizal plants than for legumes, and the relative abilities of the two groups of plants to utilise mineral N requires further study.     

Long‐term nitrogen addition modifies microbial composition and functions for slow carbon cycling and increased sequestration in tropical forest soil

Nitrogen (N) deposition is a component of global change that has considerable impact on belowground carbon (C) dynamics. Plant growth stimulation and alterations of fungal community composition and functions are the main mechanisms driving soil C gains following N deposition in N‐limited temperate forests. In N‐rich tropical forests, however, N deposition generally has minor effects on plant growth; consequently, C storage in soil may strongly depend on the microbial processes that drive litter and soil organic matter decomposition. Here, we investigated how microbial functions in old‐growth tropical forest soil responded to 13 years of N addition at four rates: 0 (Control), 50 (Low‐N), 100 (Medium‐N), and 150 (High‐N) kg N ha^?1 year^?1. Soil organic carbon (SOC) content increased under High‐N, corresponding to a 33% decrease in CO₂ efflux, and reductions in relative abundances of bacteria as well as genes responsible for cellulose and chitin degradation. A 113% increase in N₂O emission was positively correlated with soil acidification and an increase in the relative abundances of denitrification genes (narG and norB). Soil acidification induced by N addition decreased available P concentrations, and was associated with reductions in the relative abundance of phytase. The decreased relative abundance of bacteria and key functional gene groups for C degradation were related to slower SOC decomposition, indicating the key mechanisms driving SOC accumulation in the tropical forest soil subjected to High‐N addition. However, changes in microbial functional groups associated with N and P cycling led to coincidentally large increases in N₂O emissions, and exacerbated soil P deficiency. These two factors partially offset the perceived beneficial effects of N addition on SOC storage in tropical forest soils. These findings suggest a potential to incorporate microbial community and functions into Earth system models considering their effects on greenhouse gas emission, biogeochemical processes, and biodiversity of tropical ecosystems.     

Soil parent material—A major driver of plant nutrient limitations in terrestrial ecosystems

Because the capability of terrestrial ecosystems to fix carbon is constrained by nutrient availability, understanding how nutrients limit plant growth is a key contemporary question. However, what drives nutrient limitations at global scale remains to be clarified. Using global data on plant growth, plant nutritive status, and soil fertility, we investigated to which extent soil parent materials explain nutrient limitations. We found that N limitation was not linked to soil parent materials, but was best explained by climate: ecosystems under harsh (i.e., cold and or dry) climates were more N‐limited than ecosystems under more favourable climates. Contrary to N limitation, P limitation was not driven by climate, but by soil parent materials. The influence of soil parent materials was the result of the tight link between actual P pools of soils and physical–chemical properties (acidity, P richness) of soil parent materials. Some other ground‐related factors (i.e., soil weathering stage, landform) had a noticeable influence on P limitation, but their role appeared to be relatively smaller than that of geology. The relative importance of N limitation versus P limitation was explained by a combination of climate and soil parent material: at global scale, N limitation became prominent with increasing climatic constraints, but this global trend was modulated at lower scales by the effect of parent materials on P limitation, particularly under climates favourable to biological activity. As compared with soil parent materials, atmospheric deposition had only a weak influence on the global distribution of actual nutrient limitation. Our work advances our understanding of the distribution of nutrient limitation at global scale. In particular, it stresses the need to take soil parent materials into account when investigating plant growth response to environment changes.     

Responses of growth of four desert species to different N addition levels

Aims The increase in atmospheric N deposition has accelerated N cycling of ecosystems, thus altering the structure and function of ecosystems, especially in those limited by N availability. Studies on the response of plant growth to artificial N addition could provide basic data for a better understanding of how the structure of grasslands in northern China responds to increasing N deposition. Methods We investigated the seasonal dynamics of plant growth of four species after 2-year multi-level N addition in a field experiment conducted in a desert steppe of Ningxia in 2011. Plant biomass and the relative growth rate (RGR) of the studied species were measured and their relationships with C:N:P ratios of plants (community and leaf levels) and soils were analyzed. Important findings Results in 2012 showed that 2-year N addition promoted the growth of the four species and the effects were different among growth forms and were species-specific. In general, the plant biomass of the studied species was significantly correlated with leaf N concentration, leaf N:P ratio, community N pool, soil total N content and soil N:P ratio, while only weak relationships were observed between plant biomass and C:N and C:P ratios of plants and soils. In contrast, there was a significant linear relationship between RGR and N:P ratios both of plants and soils.Our results suggest that short-term N addition promoted the accumulation of plant biomass, and the species-specific responses to stimulated N addition can directly affect the structure of the desert steppe ecosystem. Plant N:P ratio and soil N:P ratio could indicate nutrient limitation of plant growth to a certain extent: N addition increased soil N content and N:P ratio, and thus relieved N limitation gradually. Once more N is available to plants, the growth of plants and the accumulation of community N was stimulated in turn.     

四种荒漠草原植物的生长对不同氮添加水平的响应

大气氮(N)沉降增加加速了生态系统N循环, 从而会对生态系统的结构和功能产生巨大的影响, 尤其是一些受N限制的生态系统.研究N添加对荒漠草原植物生长的影响, 可为深入理解N沉降增加对我国北方草原群落结构的影响提供基础数据.该文基于2011年在宁夏荒漠草原设置的N沉降增加的野外模拟试验, 研究了两年N添加下4个常见物种(牛枝子(Lespedeza potaninii),老瓜头(Cynanchum komarovii),针茅(Stipa capillata)和冰草(Agropyron cristatum))不同时期种群生物量和6-8月份相对生长速率的变化特征.并通过分析物种生长与植物(群落和叶片水平)和土壤碳(C),N,磷(P)生态化学计量学特征的关系, 探讨C:N:P化学计量比对植物生长养分限制的指示作用.结果显示N添加促进了4个物种的生长, 但具有明显的种间差异性, 且这种差异也存在于相同生活型的不同物种间.总体而言, 4个物种种群生物量与叶片N浓度,叶片N:P,群落N库,土壤全N含量和土壤N:P存在明显的线性关系, 与植物和土壤C:N和C:P的相关关系相对较弱.几个物种相对生长速率与植物和土壤N:P也呈现一定程度的正相关关系, 但与其他指标相关性较弱.以上结果表明, 短期N沉降增加提高了植物的相对生长速率, 促进了植物生长, 且更有利于针茅和老瓜头的生物量积累, 从而可能会逐渐改变荒漠草原群落结构.植物N:P和土壤N:P对荒漠草原物种生长具有较强的指示作用: 随着土壤N受限性逐渐缓解, 土壤N含量和N:P相继升高, 可供植物摄取的N增多, 因而有利于植物生长和群落N库积累.     

土壤酶计量揭示了武夷山黄山松林土壤微生物沿海拔梯度的碳磷限制变化

了解土壤胞外酶活性和酶计量的变化对评估山地生态系统土壤养分有效性和微生物的营养限制状况具有重要意义.然而,亚热带山地森林土壤微生物的营养限制状况对海拔梯度变化的响应及其驱动因素尚不清楚.本研究以武夷山不同海拔(1200～2000 m)黄山松林为对象,测定了土壤基本性质、微生物生物量以及与碳(C)、氮(N)、磷(P)循环...     

长期施肥对华北典型潮土N分配和N2O排放的影响

利用长期定位肥料试验研究化学肥料N、有机肥料N以及化学肥料N和有机肥料N混合施用对N分配和N2O排放影响。处理包括化学肥料N、P、K的不同组合NPK、NP、NK、PK、全部施用有机肥料N（OM）、一半化学肥料N＋一半有机肥料N（1／2OM）及不施肥（CK）7个处理。结果表明,等N条件下,处理间N2O排放的差异不显著,N2O排放主要发生在玉米生长期。均衡提供N、P和K显著提高土壤N储量,有机肥料N的效果显著高于化学N肥。施肥也影响N的利用效率和N在作物中的分配。均衡的养分供应有利于N在子粒中积累,而养分缺乏的处理,秸秆中N含量相对较高。进入环境的N量以NK最多,1／20M最少。总体而言,施P肥和有机肥可减少N2O的间接排放,提高土壤N素肥力并能获得较高的产量。     

C:N:P stoichiometry in soil: is there a “Redfield ratio” for the microbial biomass?

Well-constrained carbon:nitrogen:phosphorus (C:N:P) ratios in planktonic biomass, and their importance in advancing our understanding of biological processes and nutrient cycling in marine ecosystems, has motivated ecologists to search for similar patterns in terrestrial ecosystems. Recent analyses indicate the existence of “Redfield-like” ratios in plants, and such data may provide insight into the nature of nutrient limitation in terrestrial ecosystems. We searched for analogous patterns in the soil and the soil microbial biomass by conducting a review of the literature. Although soil is characterized by high biological diversity, structural complexity and spatial heterogeneity, we found remarkably consistent C:N:P ratios in both total soil pools and the soil microbial biomass. Our analysis indicates that, similar to marine phytoplankton, element concentrations of individual phylogenetic groups within the soil microbial community may vary, but on average, atomic C:N:P ratios in both the soil (186:13:1) and the soil microbial biomass (60:7:1) are well-constrained at the global scale. We did see significant variation in soil and microbial element ratios between vegetation types (i.e., forest versus grassland), but in most cases, the similarities in soil and microbial element ratios among sites and across large scales were more apparent than the differences. Consistent microbial biomass element ratios, combined with data linking specific patterns of microbial element stoichiometry with direct evidence of microbial nutrient limitation, suggest that measuring the proportions of C, N and P in the microbial biomass may represent another useful tool for assessing nutrient limitation of ecosystem processes in terrestrial ecosystems.     

Distinct Microbial Limitations in Litter and Underlying Soil Revealed by Carbon and Nutrient Fertilization in a Tropical Rainforest

Human-caused alterations of the carbon and nutrient cycles are expected to impact tropical ecosystems in the near future. Here we evaluated how a combined change in carbon (C), nitrogen (N) and phosphorus (P) availability affects soil and litter microbial respiration and litter decomposition in an undisturbed Amazonian rainforest in French Guiana. In a fully factorial C (as cellulose), N (as urea), and P (as phosphate) fertilization experiment we analyzed a total of 540 litterbag-soil pairs after a 158-day exposure in the field. Rates of substrate-induced respiration (SIR) measured in litter and litter mass loss were similarly affected by fertilization showing the strongest stimulation when N and P were added simultaneously. The stimulating NP effect on litter SIR increased considerably with increasing initial dissolved organic carbon (DOC) concentrations in litter, suggesting that the combined availability of N, P, and a labile C source has a particularly strong effect on microbial activity. Cellulose fertilization, however, did not further stimulate the NP effect. In contrast to litter SIR and litter mass loss, soil SIR was reduced with N fertilization and showed only a positive effect in response to P fertilization that was further enhanced with additional C fertilization. Our data suggest that increased nutrient enrichment in the studied Amazonian rainforest can considerably change microbial activity and litter decomposition, and that these effects differ between the litter layer and the underlying soil. Any resulting change in relative C and nutrient fluxes between the litter layer and the soil can have important consequences for biogeochemical cycles in tropical forest ecosystems.     

A tripartite microbial reporter gene system for real-time assays of soil nutrient status

Plant-derived carbon is the substrate which drives the rate of microbial assimilation and turnover of nutrients, in particular N and P, within the rhizosphere. To develop a better understanding of rhizosphere dynamics, a tripartite reporter gene system has been developed. We used three lux-marked Pseudomonas fluorescens strains to report on soil (1) assimilable carbon, (2) N-status, and (3) P-status. In vivo studies using soil water, spiked with C, N and P to simulate rhizosphere conditions, showed that the tripartite reporter system can provide real-time assessment of carbon and nutrient status. Good quantitative agreement for bioluminescence output between reference material and soil water samples was found for the C and P reporters. With regard to soil nitrate, the minimum bioavailable concentration was found to be greater than that analytically detectable in soil water. This is the first time that bioavailable soil C, N and P have been quantified using a tripartite reporter gene system.     

Dominance of legume trees alters nutrient relations in mixed species forest restoration plantings within seven years

Failures in reforestation are often attributed to nutrient limitation for tree growth. We compared tree performance and nitrogen and phosphorus relations in adjacent mixed-species plantings of contrasting composition, established for forest restoration on Ultisol soil, originally covered by tropical semi-deciduous Atlantic Forest in Southeast Brazil. Nutrient relations of four tree species occurring in both planting mixtures were compared between a legume-dominated, species-poor direct seeding mixture of early-successional species (“legume mixture”), and a species-diverse, legume-poor mixture of all successional groups (“diverse mixture”). After 7 years, the legume mixture had 6-fold higher abundance of N₂-fixing trees, 177% higher total tree basal area, 22% lower litter C/N, six-fold higher in situ soil resin-nitrate, and 40% lower in situ soil resin-P, compared to the diverse mixture. In the legume mixture, non-N₂-fixing legume Schizolobium parahyba (Fabaceae-Caesalpinioideae) had significantly lower proportional N resorption, and both naturally regenerating non-legume trees had significantly higher leaf N concentrations, and higher proportional P resorption, than in the diverse mixture. This demonstrate forms of plastic adjustment in all three non-N₂-fixing species to diverged nutrient relations between mixtures. By contrast, leaf nutrient relations in N₂-fixing Enterolobium contortisiliquum (Fabaceae-Mimosoideae) did not respond to planting mixtures. Rapid N accumulation in the legume mixture caused excess soil nitrification over nitrate immobilization and tighter P recycling compared with the diverse mixture. The legume mixture succeeded in accelerating tree growth and canopy closure, but may imply periods of N losses and possibly P limitation. Incorporation of species with efficient nitrate uptake and P mobilization from resistant soil pools offers potential to optimize these tradeoffs.     

黄土区六种植物凋落物与不同形态氮素对土壤微生物量碳氮含量的影响

黄土高原丘陵沟壑区进行的以退耕还林还草为主的生态环境建设,使得进入土壤生态系统有机物的种类及数量发生变化,其对土壤微生物量碳、氮的影响是值得关注的问题。采用室内培养法研究了采自该区6种不同植物凋落物（碳氮比在15.1-50.7之间）及其与不同形态氮素（NH+4-N及NO-3-N）配合对土壤微生物量碳、氮及矿质态氮含量的影响。结果表明,加入不同凋落物均显著提高了土壤微生物量碳、氮含量,其中加入柠条、沙打旺等碳氮比低的凋落物在培养的一段时期内土壤微生物量碳、氮均高于碳氮比高的凋落物（刺槐、沙柳和长芒草）。在加入凋落物再施用NH+4或NO-3,也提高了土壤微生物量碳、氮含量,其中铵态氮处理土壤微生物量碳、氮含量的增加达显著水平,说明微生物更易利用铵态氮。加入C/N高的凋落物后土壤中的矿质氮发生固持,矿质态氮固持量与凋落物的C/N比呈显著的正相关关系。建议在黄土高原丘陵沟壑区植被恢复过程中,有必要考虑不同植物凋落物的碳、氮养分含量及转化特性,以协调土壤碳、氮转化过程。     

Nitrogen, phosphorus and potassium nutritional status of semiarid steppe grassland in Inner Mongolia

In grazed semiarid steppe ecosystems, much attention has been paid to aspects of growth limitation by water. So far, potential limitation of primary production by plant nutrients was rarely considered. This knowledge is essential for identification of sustainable land-use practices in these large and important ecosystems on the background of over-exploitation and climate change. In the present study plant nutrient concentrations and ratios were investigated with factorial additions of water and N fertilizer at two sites with contrasting soil nutrient availability. Combined analysis of nutrient concentrations, contents, biomass production, and plant N:P ratios consistently confirmed primary growth limitation by water and a strong N limitation when sufficient amounts of water were supplied. P limitation only occurred at the site with low P availability when in addition to the natural supply, water and N fertilizer were given. According to reported thresholds of N:K and K:P ratios, K was not limiting in any plot. The observed nutritional patterns in the plant community were related to the dynamics of species composition and their specific nutrient status. Stipa grandis had the highest N:P ratio whereas Artemisia frigida showed lowest N:P. These nutrient characteristics were related to growth strategies of dominant species. Accordingly, the relative biomass contribution of S. grandis and A. frigida strongly affected the nutrient status of the plant community. Plant N:P ratios indicate the relative limitation by N or P in the semiarid grasslands under sufficient water supply, but other methods of nutritional diagnosis should be used when plant N:P ratios remain below critical values.     

Phosphorus conservation during post‐fire regeneration in a Chilean temperate rainforest

Nitrogen and phosphorus are the main elements limiting net primary production in terrestrial ecosystems. When growing in nutrient‐poor soils, plants develop physiological mechanisms to conserve nutrients, such as reabsorbing elements from senescing foliage (i.e. nutrient retranslocation). We investigated the changes in soil N and P in post‐fire succession in temperate rainforests of southern Chile. In this area, forest recovery often leads to spatially scattered, discrete regeneration with patches varying in age, area, species richness and tree cover, representing different degrees of recovery from disturbance. We hypothesized that soil nutrient concentrations should differ among tree regenerating patches depending on the progress of forest regeneration and that nutrient resorption should increase over time as colonizing trees respond to limited soil nutrients. To evaluate these hypotheses, we sampled 40 regeneration patches in an area of 5 ha, spanning a broad range of vegetation complexity, and collected soil, tree foliage and litter samples to determine N and P concentrations. Nutrient concentrations in leaf litter were interpreted as nutrient resorption proficiency. We found that soil P was negatively correlated with all the indicators of successional progress, whereas total soil N was independent of the successional progress. Foliar N and P were unrelated to soil nutrient concentrations; however, litter N was negatively related to soil N, and litter P was positively related with soil P. Finally, foliar N:P ratios ranged from 16 to 25, which suggests that P limitation can hamper post‐fire regeneration. We provide evidence that after human‐induced fires, succession in temperate forests of Chile can become nutrient limited and that high nutrient retranslocation is a key nutrient conservation strategy for regenerating tree communities.