Effects of Soil Carbon Amendment on Nitrogen Availability and Plant Growth in an Experimental Tallgrass Prairie Restoration

Restoration of tallgrass prairie on former agricultural land is often impeded by failure to establish a diverse native species assemblage and by difficulties with nonprairie, exotic species. High levels of available soil nitrogen (N) on such sites may favor fast‐growing exotics at the expense of more slowly growing prairie species characteristic of low‐N soils. We tested whether reducing N availability through soil carbon (C) amendments could be a useful tool in facilitating successful tallgrass prairie restoration. We added 6 kg/m² hardwood sawdust to experimental plots on an abandoned agricultural field in the Sandusky Plains of central Ohio, United States, increasing soil C by 67% in the upper 15 cm. This C amendment caused a 94% reduction in annual net N mineralization and a 27% increase in soil moisture but had no effect on total N or pH. Overall, plant mass after one growing season was reduced by 64% on amended compared with unamended soil, but this effect was less for prairie forbs (?34%) than for prairie grasses (?67%) or exotics (?62%). After the second growing season, only exotics responded significantly to the soil C amendment, with a 40% reduction in mass. The N concentration of green‐leaf tissue and of senescent leaf litter was also reduced by the soil C treatment in most cases. We conclude that soil C amendment imparts several immediate benefits for tallgrass prairie restoration––notably reduced N availability, slower plant growth, and lower competition from exotic species.     

细胞自噬在植物碳氮营养中作用的研究进展

自噬（autophagy）是真核生物细胞通过形成自噬体,回收利用胞内物质,维持细胞健康的高通量亚细胞降解途径。随着酵母和动物自噬研究的深入,植物自噬也受到越来越多的关注。近期的研究揭示了植物自噬的基本机制及其生理意义,也发现了植物特有的自噬形式与自噬相关基因。该文主要综述了自噬在植物碳、氮营养中的作用。     

Interactive Effects of Fire, Elevated Carbon Dioxide, Nitrogen Deposition, and precipitation on a California Annual Grassland

Although it is widely accepted that elevated atmospheric carbon dioxide (CO₂), nitrogen (N) deposition, and climate change will alter ecosystem productivity and function in the coming decades, the combined effects of these environmental changes may be nonadditive, and their interactions may be altered by disturbances, such as fire. We examined the influence of a summer wildfire on the interactive effects of elevated CO₂, N deposition, and increased precipitation in a full-factorial experiment conducted in a California annual grassland. In unburned plots, primary production was suppressed under elevated CO₂. Burning alone did not significantly affect production, but it increased total production in combination with nitrate additions and removed the suppressive effect of elevated CO₂. Increased production in response to nitrate in burned plots occurred as a result of the enhanced aboveground production of annual grasses and forbs, whereas the removal of the suppressive effect of elevated CO₂ occurred as a result of increased aboveground forb production in burned, CO₂-treated plots and decreased root production in burned plots under ambient CO₂.The tissue nitrogen–phosphorus ratio, which was assessed for annual grass shoots, decreased with burning and increased with nitrate addition. Burning removed surface litter from plots, resulting in an increase in maximum daily soil temperatures and a decrease in soil moisture both early and late in the growing season. Measures of vegetation greenness, based on canopy spectral reflectance, showed that plants in burned plots grew rapidly early in the season but senesced early. Overall, these results indicate that fire can alter the effects of elevated CO₂ and N addition on productivity in the short term, possibly by promoting increased phosphorus availability.     

几种不同碳、氮源对隐甲藻生长及二十二碳六烯酸产量的影响

目的：找出有利于隐甲藻生长和二十二碳六烯酸（DHA）积累的碳、氮源。方法：利用不同碳、氮源培养隐甲藻,收集藻体后提取脂肪酸并甲酯化,然后利用毛细管气相色谱法进行分析。结果：最适的单一碳、氮源分别为葡萄糖、酵母粉,在此培养条件下隐甲藻培养72h后的生物量（干重）和DHA产量分别为3．90和0．642g／L。结论：葡萄糖、酵母粉分别作为碳、氮源时更有利于隐甲藻的生长和DHA的积累。     

Root Inputs Influence Soil Water Holding Capacity and Differentially Influence the Growth of Native versus Exotic Annual Species in an Arid Ecosystem

Invasion by exotic annual species is increasingly impacting Southern California arid lands, altering ecosystem processes and plant community composition. With climate change, the Southwestern United States is expected to experience increasingly variable rainfall. Larger rainfall events could facilitate invasion by exotic species that can capitalize on high resource conditions. Exotic annual species also have dense shallow root systems that could create positive feedbacks to further invasion by increasing soil organic matter and water holding capacity. Alternatively, fine root inputs could create negative feedbacks to exotic plant growth if they stimulate microbial nutrient immobilization. The dual influences of rainfall regime and fine root inputs on species performance were evaluated in an experiment where native and exotic species were grown individually and in combination under varying watering regimes (large infrequent or small frequent pulses, holding total rainfall constant) and root additions (with or without sterilized exotic roots). Mean soil moisture increased with larger infrequent watering events, and also with root addition. Plant growth (both native and exotic) increased with larger watering events, but declined with root addition. Exotic species growth declined more than native species growth with root additions. Mechanistically, root addition lowered inorganic nitrogen (N) availability, and microbial N immobilization increased with soil moisture content. Together these results show that increased fine root production promotes negative feedbacks to growth of exotic species via microbial N immobilization, especially under conditions of high soil moisture. Further, our results suggest that organic carbon additions are a potentially effective strategy for suppressing growth of problematic desert invaders.     

不同氮素形态及水分胁迫对水稻苗期水分吸收、光合作用及生长的影响

采用室内营养液培养,聚乙二醇（PEG6000）模拟水分胁迫处理、HgCl2抑制水通道蛋白活性的方法,在3种供氮形态下（NH4^＋-N／NO36-N为100／0、50／50和0／100）,研究了水稻苗期水分吸收、光合及生长的状况。结果表明,在非水分胁迫下,水稻单位干重吸水量以单一供NO3^--N处理最高,加HgCl2抑制水通道蛋白活性后,单一供NO3^--N、NH4^＋-N和NH4^＋-N／NO3^--N为50,50处理的水稻水分吸收分别下降了9．6％、20．7％和16．0％;但在水分胁迫下,单一供N03^--N的处理水分吸收量显著降低,低于其它2个处理,加HgCl2抑制水通道蛋白活性后,水分吸收量分别降低了1．0％、18．8％和23．5％。在2种水分条件（水分胁迫与非水分胁迫）下,净光合速率、气孔导度、蒸腾速率和细胞间隙CO2浓度等指标均以单一供NH4^＋-N处理最大,NH4^＋-N／NO3^--N为50,50处理次之,单一供NO3^--N处理最小。HgCl2处理结果表明,不同形态氮素营养能够影响水稻幼苗根系水通道蛋白活性。在2种水分条件下,NH4^＋-N／N03^--N为50,50处理的生物量（干重）均最大。本研究为水稻苗期合理施肥以壮苗提供了理论依据。     

The Ratios of Carbon,Nitrogen, and Phosphorus in a Wetland Coastal Ecosystem of Southern India

The fertility of the coastal and estuarine waters is of great concern because of its influence on the productivity of these waters. Seasonal variations in the distribution of organic carbon, total nitrogen and total phosphorus in the sediments of Kuttanad Waters, a part of the tropical Cochin Estuary on the south west coast of India, are examined to identify the contribution of sediments to the fertility of the aquatic systems. The adjoining region has considerable agricultural activity. The fresh water zones had higher quantities of silt and clay whereas the estuarine zone was more sandy. Organic carbon, total phosphorus and total nitrogen were higher in the fresh water zones and lower in the estuarine zones. Total phosphorus and organic carbon showed the lowest values during monsoon periods. No significant trends were observed in the seasonal distributions of total nitrogen. Ratios of C/N, C/P and N/P, and the phosphorus and nitrogen content indicate significant modification in the character of the organic matter. Substantial amounts of the organic matter can contribute to reducing conditions and modify diagenetic processes.     

滨海沙地不同树种人工林叶片和土壤表层稳定碳氮同位素及水分利用效率研究

以福州市滨海后沿沙地人工营造的湿地松、木麻黄、尾巨桉、肯氏相思和纹荚相思防护林为研究对象,测定不同年龄(新叶、老叶)叶片、表层土壤(0~10cm)天然稳定碳、氮同位素丰度值(δ~(13) C、δ~(15)N),研究稳定碳、氮同位素丰度值与水分利用效率和土壤氮饱和程度的相互关系,以揭示不同树种水分利用效率、氮饱和程度和碳氮循环速率差异的机理。结果表明:(1)滨海沙地不同树种叶片δ~(13) C变化范围为-31.682‰~-29.323‰,其δ~(13) C大小为:湿地松肯氏相思木麻黄纹荚相思尾巨桉,除尾巨桉外各树种δ~(13) C均表现为新叶老叶;各树种叶片δ~(15)N变化范围为-5.548‰~-2.167‰,其δ~(15)N大小为:肯氏相思纹荚相思木麻黄湿地松尾巨桉,且各树种均表现为新叶老叶。(2)不同树种表层土壤δ~(15)N变化范围为-4.675‰~-2.975‰,表层土壤δ~(15)N大小为:纹荚相思肯氏相思木麻黄尾巨桉湿地松,但不同树种表层土壤C含量无显著差异。(3)滨海沙地湿地松、木麻黄、肯氏相思和纹荚相思的水分利用效率随叶龄增加均呈显著递减趋势;不同树种新叶的水分利用效率变化范围为39.09~76.57μmol·mol~(-1),其大小依次为:湿地松肯氏相思木麻黄纹荚相思尾巨桉;老叶的水分利用效率变化范围为38.56~62.59μmol·mol~(-1),其大小依次为:湿地松木麻黄肯氏相思尾巨桉纹荚相思。(4)不同树种人工林水分利用效率与其新叶水分利用效率呈显著正相关关系,说明林分水分利用效率主要体现在新叶的水分利用效率上,同时林分水分利用效率受林分类型的影响。     

Assessment of Water Pollution using Diatom Community Structure and Species Distribution — A Case Study in a Tropical River Basin

A study on biological assessment of water pollution using diatom community structure and species distribution was carried out in the Linggi River Basin, Malaysia which was polluted by various urban, industrial and agricultural wastes. A total of 86 diatom taxa belonging to 21 genera were recorded from all eight sampling stations located in the basin, of which 70 species were found on artificial substrates; the remaining 16 species were recorded exclusively on natural substrates. The number of diatom species observed between the stations varied from 22-47 species. The dominant diatom species in decreasing order of abundance were Eunotia vanheurckii, Nitzschia palea, Achnanthes saxonica, Gomphonema parvulum and Achnanthes minutissima. The most common clean water species were Achnanthes minutissima, A. linearis and Synedra rumpens. The most tolerant species were Nitzschia palea followed by Gomphonema parvulum and Pinnularia braunii. Eunotia vanheurckii and Navicula cryptocephala occurred in high densities at both unpolluted and polluted stations and can be considered as the common facultative or indifferent species. Although a large number of species were recorded at the unpolluted stations, equivalent number of species were also found at the moderately polluted stations. However, the number of species was reduced at grossly polluted stations. Nevertheless, a marked variation in species association exists between the unpolluted and polluted stations, but not among the polluted stations to distinguish the type and degree of pollution.     

The Effects of Water and Fertilizer Coupling on Plant and Soil Nitrogen Characteristics and Fruit Growth of Rabbiteye Blueberry Plants in a Semi-Arid Region in China

To evaluate the effects of nitrogen (N) and irrigation coupling on the soil N distribution, plant N utilization, and fruit yield of rabbiteye blueberries (Vaccinium virgatum), a field experiment was designed using two factors (water and fertilizer application) with four levels of irrigation and three levels of fertilization, and a control. Under the different water and fertilizer combinations, N primarily accumulated in the leaves. Irrigation and N application within appropriate ranges (pure N ≤ 29 g/plant and irrigation volume ≤ 2.5 L/plant) significantly improved the blueberry fruit yield. Increases in water and N within these ranges promoted the effective accumulation of N in various organs and the absorption and utilization of N in the plants, which ultimately promoted blueberry yield. With increased N application rate, the nitrate N content of the 0–20 cm and 20–50 cm soil layers increased. With increased irrigation volume, the nitrate N content of the 0–20 cm soil layer decreased, while the nitrate content in the 20–50 cm soil layer increased. Low N and moderate water treatments resulted in high fruit yields and reduced nitrate N retention in the soil. Under these conditions, the economic input-output ratio was high and the soil N accumulation was low, and thus the economic and ecological benefits were maximized.

     

The Role of Dissolved Organic Carbon, Dissolved Organic Nitrogen, and Dissolved Inorganic Nitrogen in a Tropical Wet Forest Ecosystem

Although tropical wet forests play an important role in the global carbon (C) and nitrogen (N) cycles, little is known about the origin, composition, and fate of dissolved organic C (DOC) and N (DON) in these ecosystems. We quantified and characterized fluxes of DOC, DON, and dissolved inorganic N (DIN) in throughfall, litter leachate, and soil solution of an old-growth tropical wet forest to assess their contribution to C stabilization (DOC) and to N export (DON and DIN) from this ecosystem. We found that the forest canopy was a major source of DOC (232 kg C ha^–1 y^–1). Dissolved organic C fluxes decreased with soil depth from 277 kg C ha^–1 y^–1 below the litter layer to around 50 kg C kg C ha^–1 y^–1 between 0.75 and 3.5m depth. Laboratory experiments to quantify biodegradable DOC and DON and to estimate the DOC sorption capacity of the soil, combined with chemical analyses of DOC, revealed that sorption was the dominant process controlling the observed DOC profiles in the soil. This sorption of DOC by the soil matrix has probably led to large soil organic C stores, especially below the rooting zone. Dissolved N fluxes in all strata were dominated by mineral N (mainly NO₃⁻). The dominance of NO₃^– relative to the total amount nitrate of N leaching from the soil shows that NO₃^– is dominant not only in forest ecosystems receiving large anthropogenic nitrogen inputs but also in this old-growth forest ecosystem, which is not N-limited.     

Fate and Transport of Organic Nitrogen in Minimally Disturbed Montane Streams of Colorado,USA

In two montane watersheds that receive minimal deposition of atmospheric nitrogen, 15–71% of dissolved organic nitrogen (DON) was bioavailable in stream water over a 2-year period. Discharge-weighted concentrations of bulk DON were between 102 and 135 μg/l, and the C:N ratio differed substantially between humic and non-humic fractions of DON. Approximately 70% of DON export occurred during snowmelt, and 40% of that DON was biologically available to microbes in stream sediments. Concentrations of bioavailable DON in stream water were 2–16 times greater than dissolved inorganic nitrogen (DIN) during the growing season, and bioavailable DON was depleted within 2–14 days during experimental incubations. Uptake of DON was influenced by the concentration of inorganic N in stream water, the concentration of non-humic DON in stream water, and the C:N ratio of the non-humic fraction of dissolved organic matter (DOM). Uptake of DON declined logarithmically as the concentration of inorganic N in stream water increased. Experimental additions of inorganic N also caused a decline in uptake of DON and net production of DON when the C:N ratio of non-humic DOM was high. This study indicates that the relative and absolute amount of bioavailable DON can vary greatly within and across years due to interactions between the availability of inorganic nutrients and composition of DOM. DOM has the potential to be used biotically at a high rate in nitrogen-poor streams, and it may be generated by heterotrophic microbes when DIN and labile DOM with low relative nitrogen content become abundant.     

The Utility of Carbon and Nitrogen Isotope Analyses to Trace Contributions from Fish Farms to the Receiving Communities of Freshwater Lakes: a Pilot Study in Esthwaite Water,UK

A pilot study was conducted to assess the potential for stable isotope analyses to reveal the fate of waste pelleted food material from fish farms in freshwater food webs. Esthwaite Water (Cumbria, UK) was selected as the study site, as it hosts an established salmonid farm, and a wealth of complementary limnological data exists. Salmonid pellet feed consists of primarily marine-derived material and thus exhibits carbon and nitrogen stable isotopic compositions distinct to most freshwater organic material. Comparison of the isotopic ratios of organisms at the cage site with an unaffected control site, supports incorporation of pellet-derived material to the diet of planktonic and benthic communities. Moreover, after allowing for a number of trophic transfers, stable isotope analyses revealed the predatory cladoceran Leptodora kindti also utilised pellet material, while roach were probably short-circuiting the food chain by directly consuming particulate pellet material, as well as via ingestion of their zooplankton prey. Isotope data substituted into a simple two-source mixing model suggested that approximately 65% of Daphnia, and >80% of roach body carbon may be derived from pellet material in the plankton, and that chironomid larvae may incorporate >50% in the sediment environs. However, contributions calculated from both ¹³C and ¹⁵N values were inconsistent, which may simply be due to the constraints of the model and parameters used, but may also reflect different routing of isotopes from the original pellet source, via soluble or particulate routes.     

Patterns of Carbon, Nitrogen and Phosphorus Dynamics in Decomposing Foliar Litter in Canadian Forests

We examined the patterns of nitrogen (N) and phosphorus (P) gain, retention or loss in ten foliar tissues in a litterbag experiment over 6 years at 18 upland forest sites in Canada, ranging from subarctic to cool temperate. N was usually retained in the decomposing litter until about 50% of the original C remained. The peak N content in the litter was observed at between 72 and 99% of the original C remaining, with C:N mass quotients between 37 and 71 (mean 55). The rate of N release from the litters was not related to the original N concentration, which may be associated with the generally narrow range (0.59–1.28% N) in the litters. P was immediately lost from all litters, except beech leaves, with critical litter C:P mass quotients for P release being in the range 700–900. The rate of P loss was inversely correlated with the original litter P concentration, which ranged from 0.02 to 0.13%. The soil underlying the litterbags influenced the pattern of N and P dynamics in the litters; there were weak correlations between the N and P remaining at 60% C remaining in the litters and the C:N and C:P quotients of the surface layer of the soil. There was a trend for higher N and P retention in the litter at sites with lower soil C:N and N:P quotients, respectively. Although there was a large variation in C:N, C:P and N:P quotients in the original litters (29–83, 369–2122 and 5–26, respectively), and some variation in the retention or loss of N and P in the early stages of decomposition, litters converged on C:N, C:P and N:P quotients of 30, 450 and 16, when the C remaining fell below 30%. These quotients are similar to that found in the surface organic matter of these ecosystems.     

铜、镉胁迫下施硫肥和有机肥对冬小麦碳氮运转的影响

采用盆栽试验,研究了铜、镉胁迫条件下施硫和有机肥对冬小麦碳氮运转的影响。结果表明,与各自对照相比,铜、镉胁迫下低施硫和有机肥的处理增加了小麦叶片、茎鞘、颖壳穗轴等营养器官花前贮藏物质、氮素的再运转量和运转率以及营养器官花前贮藏物质、氮素的总再运转量和总运转率,高施硫和有机肥的铜、镉处理则规律性不明显。在铜、镉胁迫条件下,施用硫肥和有机肥处理增加了小麦成熟期籽粒重和花后光合同化物输入籽粒量以及籽粒氮素含量和花后氮素积累量。与各自对照相比,铜胁迫下施硫和有机肥的处理与镉胁迫下低施硫和有机肥的处理增加了成熟期小麦的穗数、穗粒数和千粒重,提高了籽粒产量,其中以T\-5处理增产幅度最大;镉胁迫下高施硫和有机肥的处理则变化不大。铜、镉胁迫下低施硫和有机肥的处理均增加了籽粒淀粉含量,而高施硫和有机肥的铜、镉处理则未表现出此规律。此外,铜、镉胁迫下施硫和有机肥的各处理增加了籽粒蛋白质的含量。     

Long-term Effects of Pre-harvest Burning and Nitrogen and Vinasse Applications on Yield of Sugar Cane and Soil Carbon and Nitrogen Stocks on a Plantation in Pernambuco, N.E. Brazil

Since the 1970s the area under sugarcane in Brazil has increased from 2 million to over 5 million ha (M ha), and it is expected to pass the 7 M ha mark in 2007. More than half of the cane is harvested to produce bioethanol as a fuel for light vehicles. The distilleries produce approximately 13 L of distillery waste (vinasse) for each litre of ethanol produced. In the 1980s there was considerable concern over the long-term effects of the disposal of this material (containing about 1% carbon and high in K) on cane yields if it was applied to the field. At the same time there was a growing movement to abandon the practice of pre-harvest burning and some research was showing that some Brazilian varieties of sugar cane were able to obtain significant contributions of N from plant-associated biological nitrogen fixation (BNF). For these reasons an experiment was installed on a cane plantation in the state of Pernambuco, NE Brazil to investigate the long-term effects of vinasse and N fertiliser additions and the practice of pre-harvest burning on crop and sugar yield, soil fertility parameters, N balance and soil C stocks. The results showed that over a 16-year period, trash conservation (abandonment of burning) increased cane yields by 25% from a mean of 46 to 58 Mg ha⁻¹. Vinasse applications (80 m³ ha⁻¹ crop⁻¹) increased mean cane and sugar yield by 12 to 13% and the application of 80 kg N ha⁻¹ as urea increased cane yields by 9%, but total sugar yield by less than 6% (from 7.0 to 7.4 Mg ha⁻¹ crop⁻¹). The total N balance for the soil/plant system when only the surface 20 cm of the soil was considered was positive in plots where no N fertiliser was added. However, the data indicated that during the 16 years of the study considerable quantities of soil organic matter were accumulated below 20 cm depth such that the N balance considering the soil to 60 cm depth was strongly positive, except where N fertiliser was added. The data indicated that there were considerable BNF inputs to the system, which was consistent with its low response to N fertiliser and low N fertiliser-use-efficiency. There were no significant effects of vinasse or urea addition, or trash conservation on soil C stocks, although the higher yields proportioned by trash conservation had potentially significant benefits for increased mitigation of CO₂ emissions where the main use of the cane was for bioethanol production.     

祁连山冷龙岭南坡移地植物叶片的碳氮特征

在祁连山冷龙岭南麓坡地进行不同海拔高度土壤(0～40(30)cm土层)和植被的整体双向移地实验,以探讨气候变化对主要物种和群落碳、氮特征的影响。结果表明,3 200 m的麻花艽移地至高海拔时,叶片碳、氮含量分别呈现出先增加后减少和一直增加的趋势,而碳/氮值下降明显。3 400 m的兰石草、珠芽蓼、垂穗披碱草、金露梅和鹅绒委陵菜5种植物大部分叶片碳、氮素含量及碳/氮呈现出随海拔增高而下降的趋势。3 600 m的矮嵩草、雪白委陵菜和重齿风毛菊3种植物叶片碳、氮含量及碳/氮在各海拔之间变化不明显。3 800 m的矮嵩草和矮火绒草的叶片碳、氮含量随海拔高度增加而增加,碳/氮下降。整个群落来讲碳含量随海拔升高而降低,氮含量和碳/氮比变化较小。研究发现,各群落和物种对移地的响应方式因物种组成、原生状态不同而存在差异;温度条件(以海拔升降模拟温度降升)对植被群落及主要植物叶片碳、氮含量和碳/氮比有一定影响,尤其是当高海拔稀疏植被移地到低海拔时,其叶片碳、氮和碳氮比与原生状况的植被群落及主要植物种相对变化较大。     

向日葵种群中植株个体大小对其氮素利用策略的影响

我们利用Berendse和Aerts提出的氮素利用效率(NUE)概念及原理研究了高密度一年生草本植物向日葵(Helianthus annuus L.)种群中植株个体大小对其氮素吸收利用的影响,并对种内竞争进行了分析.结果表明,植株对氮素的吸收与其个体大小不成线性关系,说明种群内不同植株个体对土壤氮素的竞争属于非对称竞争.植株的氮素损失随着个体大小的增加而增加.个体较大的植株具有较高的氮素输入率和较低的氮素输出率,因而具有较高的氮素净增加值.植株的氮素生产力(NP)和氮素平均滞留时间(MRT)均与植株个体大小呈正相关.较大的植物个体具有较高的NP和较长的MRT,由于NUE为NP和MRT二者的乘积,因而较大个体植株的NUE高于个体较小的植株.同种植物的不同个体的NP和MRT之间不存在协衡关系.氮素回收效率(NRE)与植株个体大小密切相关.在个体水平上,较大的植株个体具有较高的NUE与其较高的NRE有关.种群内植株个体对土壤氮素的非对称竞争主要由于植株对氮素的吸收和利用效率不同所致.因此,Berendse和Aerts提出的氮素利用效率概念不仅适用于研究种间的养分利用策略,对于种内不同植株的养分策略研究也同样适用.     

向日葵种群中植株个体大小对其氮素利用策略的影响

我们利用Berendse和Aerts提出的氮素利用效率(NUE)概念及原理研究了高密度一年生草本植物向日葵(Helianthus annuus L.)种群中植株个体大小对其氮素吸收利用的影响,并对种内竞争进行了分析。结果表明,植株对氮素的吸收与其个体大小不成线性关系,说明种群内不同植株个体对土壤氮素的竞争属于非对称竞争。植株的氮素损失随着个体大小的增加而增加。个体较大的植株具有较高的氮素输入率和较低的氮素输出率,因而具有较高的氮素净增加值。植株的氮素生产力(NP)和氮素平均滞留时间(MRT)均与植株个体大小呈正相关。较大的植物个体具有较高的NP和较长的MRT,由于NUE为NP和MRT二者的乘积,因而较大个体植株的NUE高于个体较小的植株。同种植物的不同个体的NP和MRT之间不存在协衡关系。氮素回收效率(NRE)与植株个体大小密切相关。在个体水平上,较大的植株个体具有较高的NUE与其较高的NRE有关。种群内植株个体对土壤氮素的非对称竞争主要由于植株对氮素的吸收和利用效率不同所致。因此,Berendse和Aerts提出的氮素利用效率概念不仅适用于研究种间的养分利用策略,对于种内不同植株的养分策略研究也同样适用。     

Woody Plant Encroachment by <Emphasis Type="Italic">Juniperus virginiana</Emphasis> in a Mesic Native Grassland Promotes Rapid Carbon and Nitrogen Accrual

The cover and abundance of Juniperus virginiana L. in the U.S. Central Plains are rapidly increasing, largely as a result of changing land-use practices that alter fire regimes in native grassland communities. Little is known about how conversion of native grasslands to Juniperus-dominated forests alters soil nutrient availability and ecosystem storage of carbon (C) and nitrogen (N), although such land-cover changes have important implications for local ecosystem dynamics, as well as regional C and N budgets. Four replicate native grasslands and adjacent areas of recent J. virginiana encroachment were selected to assess potential changes in soil N availability, leaf-level photosynthesis, and major ecosystem C and N pools. Net N mineralization rates were assessed in situ over two years, and changes in labile soil organic pools (potential C and N mineralization rates and microbial biomass C and N) were determined. Photosynthetic nitrogen use efficiencies (PNUE) were used to examine differences in instantaneous leaf-level N use in C uptake. Comparisons of ecosystem C and N stocks revealed significant C and N accrual in both plant biomass and soils in these newly established forests, without changes in labile soil N pools. There were few differences in monthly in situ net N mineralization rates, although cumulative annual net N mineralization was greater in forest soils compared to grasslands. Conversely, potential C mineralization was significantly reduced in forest soils. Encroachment by J. virginiana into grasslands results in rapid accretion of ecosystem C and N in plant and soil pools with little apparent change in N availability. Widespread increases in the cover of woody plants, like J. virginiana, in areas formerly dominated by graminoid species suggest an increasing role of expanding woodlands and forests as regional C sinks in the central U.S.