Analysis of water- and nitrogen-use efficiency of wheat in a Mediterranean climate

Water-use efficiency (WUE [g grain yield m^–2 mm^–1 ET]) and nitrogen-use efficiency (NUE [ g grain yield g^–1 N_applied]) are important measures that can affect the productivity of crops in different environmental systems. However, measurement and interpretation of WUE and NUE in the field are often hampered by the high degree of complexity of these systems due to season-to-season variability in rainfall, the variation in crop responses to soil types and to agronomic management. To be able to guide agronomic practice, experimentally-derived measurements of WUE and NUE need to be extrapolated across time and space through appropriate modelling. To illustrate this approach, the Agricultural Production Systems Simulator (APSIM), which has been rigorously tested for wheat (Triticum aestivum L.) in a Mediterranean environment, was used to estimate and analyse the WUE and NUE of wheat crops in the Mediterranean-climatic region of the central Western Australian agricultural zone. The APSIM model was run for three locations (average annual rainfall of 461 mm [high rainfall zone], 386 mm [medium] and 310 mm [low]) and two soil types that had contrasting plant-available water-holding capacities in the rooting zone (sand: 55 mm, clay soil: 109 mm). Simulations were carried out with historical weather records (82–87 years) assuming current crop management and cultivars. The modelling analyses highlighted the inherently high degree of seasonal variability in yield, WUE and NUE of wheat, depending on soil type, N fertiliser input, rainfall amount and, in particular, rainfall distribution. The clay soil tended to be more productive in terms of grain yield, WUE and NUE in the high and medium rainfall zones, but less productive in most years in the low rainfall zone. The sandy soil was less productive in the high rainfall zone due to the high nitrate leaching potential of this soil type, but more productive than the clay in the low rainfall zone due to poorer pre-anthesis growth and less water use, less water loss by soil evaporation and relatively more water use in the post-anthesis phase. When a wheat crop was sown early on clay soil in the low rainfall zone, it yielded as high as in the other rainfall zones in seasons when rainfall was above average or there was a good store of water in the soil prior to sowing. The simulations confirmed findings from a limited number of field experiments and extended these findings both qualitatively and quantitatively across soil types, rainfall regions and crop management options. Furthermore, by using long-term historical weather records, the simulations extended the findings across the wide range of climatic scenarios experienced in mediterranean-climatic regions.     

三峡消落带不同坡度狗牙根及实生土壤生态化学计量特征

三峡库区消落带反季节水位波动驱使下植物的养分适应策略和内稳态特征研究对消落带植被恢复及生态功能调控有重要意义。然而,三峡消落带范围广、地形复杂,不同生境下植物适应特征仍存在不确定性。本研究以三峡消落带广泛分布的优势植物狗牙根为研究对象,选取28个地形差异较大的样地进行采样分析,重点探讨不同坡度（0-5°、5-10°、10-15°、15-20°、>20°）狗牙根及其实生土壤生态化学计量特征的变异关系,揭示坡度对狗牙根养分适应及内稳态的影响机制。结果表明：（1）坡度对狗牙根实生土壤的养分及化学计量特征具有显著影响,土壤有机碳（SOC）、全氮（STN）、速效氮（SAN）、全磷（STP）、速效磷（SAP）含量均随坡度增加而降低,<10°的缓坡消落带土壤养分显著高于10°以上的样地;土壤碳氮比（C ∶ N）、碳磷比（C ∶ P）均随坡度增加而降低,而氮磷比（N ∶ P）比相对稳定,表明坡度增大,土壤营养物的流失具有同步性,且流失速度较有机碳慢;（2）随坡度增加,狗牙根各营养器官C含量呈增加趋势,而氮（N）、磷（P）则呈降低趋势,且坡度变化改变了狗牙根各营养器官间的养分分配;植物根、茎C ∶ N、C ∶ P随坡度增加而增加,N ∶ P随坡度增加而降低,而叶片均没有明显变化,表明在坡度改变了土壤养分供应水平的情况下,狗牙根优先稳定其叶片化学计量比以提高适应能力;（3）坡度与土壤中SOC、STN、SAN含量呈现显著负相关,而土壤STN与狗牙根植物养分及化学计量特征均有显著相关关系,由此可见,坡度变化导致土壤氮素流失增加是影响狗牙根生态化学计量特征分异的主要机制;（4）狗牙根与实生土壤的碳氮磷元素内稳性整体表现为C>P>N,化学计量比内稳性表现为C ∶ P>C ∶ N>N ∶ P,随着坡度增加,狗牙根N、P的内稳性呈增强趋势,而C ∶ N、N ∶ P呈显著降低,表明坡度变化导致狗牙根形成了不同的适生策略。研究表明,在三峡库区复杂的地形条件下,狗牙根能有效维持体内化学计量平衡以响应不同的坡度条件,并且内稳性较好,是三峡库区消落带植被恢复与保护的优势植物物种。     

Forest Productivity and Efficiency of Resource Use Across a Chronosequence of Tropical Montane Soils

We tested the hypothesis that plants adjust to nutrient availability by altering carbon allocation patterns and nutrient-use efficiency (NUE = net primary production [NPP] per unit nutrient uptake), but are constrained by a trade-off between NUE and light-use efficiency () = NPP per unit intercepted light). NPP, NUE and ) were measured in montane Metrosideros polymorpha forest across a 4.1 x 10⁶ yr space for time substitution chronosequence in which available soil N and P pools change with site age. Although the range of N and P availability across sites was broad, there was little difference in NPP between sites, and in contrast to theories of carbon allocation relative to limiting resources, we found no consistent relationships in production allocation to leaves, fine roots or wood. However, canopy nutrient pools and fluxes were correlated with the mass of fine roots per unit soil volume and there was a weak but positive correlation of NPP with LAI. Patterns of ) and NUE across the soil developmental sequence were opposite to each other. ) increased as nutrient availability and nutrient turnover increased, while NUE decreased in response to the same influences but reached its highest values where either N or P availability and turnover of both N and P were low. A negative correlation between ) and NUE supports the hypothesis that a trade-off exists between ) and leaf characteristics affecting NUE.     

Elevated carbon dioxide and ozone alter productivity and ecosystem carbon content in northern temperate forests

Three young northern temperate forest communities in the north‐central United States were exposed to factorial combinations of elevated carbon dioxide (CO₂) and tropospheric ozone (O₃) for 11 years. Here, we report results from an extensive sampling of plant biomass and soil conducted at the conclusion of the experiment that enabled us to estimate ecosystem carbon (C) content and cumulative net primary productivity (NPP). Elevated CO₂ enhanced ecosystem C content by 11%, whereas elevated O₃ decreased ecosystem C content by 9%. There was little variation in treatment effects on C content across communities and no meaningful interactions between CO₂ and O₃. Treatment effects on ecosystem C content resulted primarily from changes in the near‐surface mineral soil and tree C, particularly differences in woody tissues. Excluding the mineral soil, cumulative NPP was a strong predictor of ecosystem C content (r² = 0.96). Elevated CO₂ enhanced cumulative NPP by 39%, a consequence of a 28% increase in canopy nitrogen (N) content (g N m^?2) and a 28% increase in N productivity (NPP/canopy N). In contrast, elevated O₃ lowered NPP by 10% because of a 21% decrease in canopy N, but did not impact N productivity. Consequently, as the marginal impact of canopy N on NPP (?NPP/?N) decreased through time with further canopy development, the O₃ effect on NPP dissipated. Within the mineral soil, there was less C in the top 0.1 m of soil under elevated O₃ and less soil C from 0.1 to 0.2 m in depth under elevated CO₂. Overall, these results suggest that elevated CO₂ may create a sustained increase in NPP, whereas the long‐term effect of elevated O₃ on NPP will be smaller than expected. However, changes in soil C are not well‐understood and limit our ability to predict changes in ecosystem C content.     

Perspectives on Screening Winter-Flood-Tolerant Woody Species in the Riparian Protection Forests of the Three Gorges Reservoir

The establishment of riparian protection forests in the Three Gorges Reservoir (TGR) is an ideal measure to cope with the eco-environmental problems of the water-level fluctuation zone (WLFZ). Thus, the information for screening winter-flood-tolerant woody plant species is useful for the recovery and re-establishment of the riparian protection forests in the TGR WLFZ. Therefore, we discussed the possibilities of constructing and popularizing riparian protection forests in the TGR WLFZ from several aspects, including the woody plant species distribution in the WLFZ, the survival rate analyses of suitable candidate woody species under controlled flooding conditions, the survival rate investigation of some woody plant species planted in the TGR WLFZ, and the physiological responses of some woody plant species during the recovery stage after winter floods. The results of woody species investigation showed that most woody plant species that existed as annual seedlings in the TGR WLFZ are not suitable candidates for the riparian protection forests. However, arbor species (e.g., Salix matsudana, Populus×canadensis, Morus alba, Pterocarya stenoptera, Taxodium ascendens, and Metasequoia glyptostroboides) and shrub species (e.g., Salix variegata, Distylium chinensis, Lycium chinense, Myricaria laxiflora, and Rosa multiflora) might be considered suitable candidates for the riparian protection forests in the TGR WLFZ by survival rate analyses under controlled winter flooding conditions, and survival rate investigations of woody plant species planted in the TGR WLFZ, respectively. Physiological analyses showed that P.×canadensis, M. alba, L. chinense, and S. variegata could develop specific self-repairing mechanisms to stimulate biomass accumulation and carbohydrate synthesis via the increases in chlorophyll pigments and photosynthesis during recovery after winter floods. Our results suggested these woody plant species could endure the winter flooding stress and recover well, and be used as candidate for the construction of riparian protection forests in the TGR WLFZ.     

Soil and fertilizer nitrogen transformations under alternate flooding and drying moisture regimes

Summary A study of changes in NH₄ ⁺ and NO₃ ^––N in Maahas clay amended with (NH₄)₂SO₄ and subjected to 4 water regimes in the presence and absence of the nitrification inhibitor N-Serve (Nitrapyrin) showed that the mineral N was well conserved in the continoous regimes of 50% and 200% (soil weight basis) but suffered heavy losses due to nitrification-denitrification under alternate drying and flooding. N-Serve was effective in minimizing these losses.Another incubation study with 3 soils showed that after 10 cycles of flooding and drying (either at 60°C or 25°C), the ammonification of soil N was enhanced. Nitrification of soil as well as fertilizer NH₄ ⁺ was completely inhibited upto 4 weeks by the treatments involving drying at high temperature. Flooding and air drying at 25°C, on the other hand, enhanced ammonification of soil N but retarded nitrification. These treatments, however, enhanced both ammonification and nitrification of the applied NH₄ ⁺ fertilizer N. Under flooded conditions rate of NH₄ ⁺ production was faster in soils that were dried at 60°C or 25°C and then flooded as compared to air dried soils.It is concluded that N losses by nitrification-denitrification and related N transformations may be considerably altered by alternating moisture regimes. Flooding and drying treatments seem to retard nitrification of soil N but conserve that of fertilizer NH₄ ⁺ applied after these treatments.     

Spatial Variation in Denitrification and N2O Emission in Relation to Nitrate Removal Efficiency in a N-stressed Riparian Buffer Zone

Spatial variability in hydrological flowpaths and nitrate-removal processes complicates the overall assessment of riparian buffer zone functioning in terms of water quality improvement as well as enhancement of the greenhouse effect by N₂O emissions. In this study, we evaluated denitrification and nitrous oxide emission in winter and summer along two groundwater flowpaths in a nitrate-loaded forested riparian buffer zone and related the variability in these processes to controlling soil factors. Denitrification and emissions of N₂O were measured using flux chambers and incubation experiments. In winter, N₂O emissions were significantly higher (12.4 mg N m⁻² d⁻¹) along the flowpath with high nitrate removal compared with the flowpath with low nitrate removal (2.58 mg N m⁻² d⁻¹). In summer a reverse pattern was observed, with higher N₂O emissions (13.6 mg N m⁻² d⁻¹) from the flowpath with low nitrate-removal efficiencies. Distinct spatial patterns of denitrification and N₂O emission were observed along the high nitrate-removal transect compared to no clear pattern along the low nitrate-removal transect, where denitrification activity was very low. Results from this study indicate that spots with high nitrate-removal efficiency also contribute significantly to an increased N₂O emission from riparian zones. Furthermore, we conclude that high variability in N₂O:N₂ ratio and weak relationships with environmental conditions limit the value of this ratio as a proxy to evaluate the environmental consequences of riparian buffer zones.     

三峡库区消落带落羽杉与立柳林土壤微生物生物量碳氮磷动态变化

为探究库区消落带人工乔木植被恢复重建后土壤质量及肥力的变化特征,于2016年6月(T_1)、2016年9月(T_2)、2017年6月(T_3)及2017年9月(T_4)选择165—175 m高程落羽杉与立柳土壤为研究对象,并以裸地作为对照,测定土壤微生物生物量碳、氮、磷和相关理化性质。结果表明:(1)经历水淹(T_2—T_3)会使土壤微生物生物量处于较低水平,落干期(T_1—T_2、T_3—T_4)落羽杉与立柳人工植被恢复生长能显著提高土壤微生物生物量,对土壤微生物恢复具有重要意义。(2)落羽杉与立柳土壤微生物生物量碳、氮占土壤有机碳、全氮百分比在4个时期均显著高于裸地,表明落羽杉与立柳土壤微生物对土壤碳、氮库的贡献大于裸地;落羽杉土壤微生物生物量磷及其占全磷百分比在T_1和T_3处于极低水平,T_2和T_4处于较高水平,应注意磷元素的迁移。(3)土壤微生物生物量碳、氮、磷与土壤有机碳和全氮有极显著相关性,与土壤pH值呈不同程度的负相关。在三峡库区消落带进行落羽杉与立柳乔木植被恢复重建能显著提高土壤微生物生物量及土壤肥力,进一步证实开展科学的植被修复与重建值得提倡和肯定。     

Genetic variation in eggplant for Nitrogen Use Efficiency under contrasting NO3‐ supply

Eggplant (Solanum melongena L.) yield is highly sensitive to N fertilization, the excessive use of which is responsible for environmental and human health damage. Lowering N input together with the selection of improved Nitrogen‐Use‐Efficiency (NUE) genotypes, more able to uptake, utilize, and remobilize N available in soils, can be challenging to maintain high crop yields in a sustainable agriculture. The aim of this study was to explore the natural variation among eggplant accessions from different origins, in response to Low (LN) and High (HN) Nitrate (NO₃^‐) supply, to identify NUE‐contrasting genotypes and their NUE‐related traits, in hydroponic and greenhouse pot experiments. Two eggplants, AM222 and AM22, were identified as N‐use efficient and inefficient, respectively, in hydroponic, and these results were confirmed in a pot experiment, when crop yield was also evaluated. Overall, our results indicated the key role of N‐utilization component (NUtE) to confer high NUE. The remobilization of N from leaves to fruits may be a strategy to enhance NUtE, suggesting glutamate synthase as a key enzyme. Further, omics technologies will be used for focusing on C‐N metabolism interacting networks. The availability of RILs from two other selected NUE‐contrasting genotypes will allow us to detect major genes/quantitative trait loci related to NUE.     

Enhanced‐efficiency fertilizers are not a panacea for resolving the nitrogen problem

Improving nitrogen (N) management for greater agricultural output while minimizing unintended environmental consequences is critical in the endeavor of feeding the growing population sustainably amid climate change. Enhanced‐efficiency fertilizers (EEFs) have been developed to better synchronize fertilizer N release with crop uptake, offering the potential for enhanced N use efficiency (NUE) and reduced losses. Can EEFs play a significant role in helping address the N management challenge? Here we present a comprehensive analysis of worldwide studies published in 1980–2016 evaluating four major types of EEFs (polymer‐coated fertilizers PCF, nitrification inhibitors NI, urease inhibitors UI, and double inhibitors DI, i.e. urease and nitrification inhibitors combined) regarding their effectiveness in increasing yield and NUE and reducing N losses. Overall productivity and environmental efficacy depended on the combination of EEF type and cropping systems, further affected by biophysical conditions. Best scenarios include: (i) DI used in grassland (n = 133), averaging 11% yield increase, 33% NUE improvement, and 47% decrease in aggregated N loss (sum of NO₃^‐, NH₃, and N₂O, totaling 84 kg N/ha); (ii) UI in rice‐paddy systems (n = 100), with 9% yield increase, 29% NUE improvement, and 41% N‐loss reduction (16 kg N/ha). EEF efficacies in wheat and maize systems were more complicated and generally less effective. In‐depth analysis indicated that the potential benefits of EEFs might be best achieved when a need is created, for example, by downward adjusting N application from conventional rate. We conclude that EEFs can play a significant role in sustainable agricultural production but their prudent use requires firstly eliminating any fertilizer mismanagement plus the implementation of knowledge‐based N management practices.     

三峡水库秭归段消落带植物群落特征及其与环境因子的关系

消落带植被群落特征及物种多样性的研究一直是植物生态学和恢复生态学研究的热点。三峡水库的运行对消落带生态系统造成了极大的影响,如引起绝大部分河岸带土著植物消失、生物多样性降低以及土壤侵蚀严重等一系列生态学问题。研究消落带植物群落特征及其影响机制,旨在丰富消落带新生湿地生态学研究内容,为消落带植被的恢复、湿地保护和管理提供科学依据。通过对三峡水库秭归段香溪河与童庄河陡坡消落带植物群落及其环境因子的实地调查,并结合双向聚类分析及典范对应分析(CCA),研究了三峡水库消落带植物群落特征、物种多样性及其与环境因子之间的关系。结果表明:(1)在三峡水库秭归段消落带共发现维管植物34种,隶属于15科32属,其中菊科(Compositae)和禾本科(Gramineae)种类数较多,为优势物种,植物群落组成以草本植物为主,一年生和多年生草本植物为23种和11种,分别占67.65%和32.35%,一年生植物占绝对优势;(2)随海拔升高,物种数、Shannon多样性指数、Pielou均匀度从低海拔到高海拔随着海拔的增加而升高,而Simpson优势度指数指数从低海拔到高海拔呈下降趋势;(3)双向聚类显示消落带...     

Physiological responses of cotton to a single waterlogging at high and low N-levels

Surface-irrigated cotton (Gossypium hirsutum L.) grown on slowly draining clay soil is subjected to short-term periods of waterlogging at each irrigation which generally results in reduced productivity. The sequence of above- and below-ground plant responses to transient waterlogging and the role of N availability in modifying the immediate responses were studied. Lysimeters of Marah clay loam (a Natrustalf) were instrumented to monitor soil and plant responses to a 7-day waterlogging event beginning 67 days after sowing. Cotton (‘Deltapine 61’) plants (8 per lysimeter) were grown with two levels of added N (300 kg ha⁻¹ and 30 kg ha⁻¹) and two irrigation treatments (flooded and control). Measured soil-O₂ levels decreased rapidly upon surface flooding because water displaced air and root zone respiration consumed O₂. The rate of O₂ consumption was 2.7 times greater in the high-N treatment than the low-N treatment. This difference was associated with a 1.8 fold difference in numbers of observed roots. Root growth was only slightly affected by flooding. Leaf growth decreased by 28%, foliage temperature increased 2.3% and apparent photosynthesis decreased by 16%. It is suggested that flooding reduced photosynthetic activity within 2 days while other stress symptoms became apparent after about 6 days. Although this stress was reflected in a trend for decreased plant productivity, the effect of flooding on boll dry mass at harvest was not significant at the level of replication used. The single waterlogging did not cause yield reductions comparable to those observed elsewhere when several waterlogging events were imposed. Contribution from the CSIRO, Centre for Irrigation Research, Griffith, NSW, Australia and USDA-ARS, Morris, MI, USA, in cooperation with the univ. of Minnesota.     

The nitrogen budget of a pine forest under free air CO2 enrichment

Elevated concentrations of atmospheric CO₂ increase plant biomass, net primary production (NPP) and plant demand for nitrogen (N). The demand for N set by rapid plant growth under elevated CO₂ could be met by increasing soil N availability or by greater efficiency of N uptake. Alternatively, plants could increase their nitrogen-use efficiency (NUE), thereby maintaining high rates of growth and NPP in the face of nutrient limitation. We quantified dry matter and N budgets for a young pine forest exposed to 4 years of elevated CO₂ using free-air CO₂ enrichment technology. We addressed three questions: Does elevated CO₂ increase forest NPP and the demand for N by vegetation? Is demand for N met by greater uptake from soils, a shift in the distribution of N between plants, microbes, and soils, or increases in NUE under elevated CO₂? Will soil N availability constrain the NPP response of this forest as CO₂ fumigation continues? A step-function increase in atmospheric CO₂ significantly increased NPP during the first 4 years of this study. Significant increases in NUE under elevated CO₂ modulated the average annual requirement for N by vegetation in the first and third growing seasons under elevated CO₂; the average stimulation of NPP in these years was 21% whereas the average annual stimulation of the N requirement was only 6%. In the second and fourth growing seasons, increases in NPP increased the annual requirement for N by 27-33%. Increases in the annual requirement for N were largely met by increases in N uptake from soils. Retranslocation of nutrients prior to senescence played only a minor role in supplying the additional N required by trees growing under elevated CO₂. NPP was highly correlated with between-plot variation in the annual rate of net N mineralization and CO₂ treatment. This demonstrates that NPP is co-limited by C availability, as CO₂ from the atmosphere, and N availability from soils. There is no evidence that soil N mineralization rates have increased under elevated CO₂. The correlation between NPP and N mineralization rates and the increase in the annual requirement for N in certain years imply that soil N availability may control the long-term productivity response of this ecosystem to elevated CO₂. Although we have no evidence suggesting that NPP is declining in response to >4 years of CO₂ fumigation, if the annual requirement of N continues to be stimulated by elevated CO₂, we predict that the productivity response of this forest ecosystem will decline over time.     

Patterns in groundwater nitrogen concentration in the floodplain of a subtropical stream (Wollombi Brook,New South Wales)

The sources of groundwater and the patterns in groundwater dissolved N and DOC concentration in the floodplain of a subtropical stream (Wollombi Brook, New South Wales) were studied over a 2-year period using three piezometer transects. While the stream was generally a discharge area for regional groundwater, this source represented only a small contribution to either the water or N budget of the alluvial aquifer. Groundwater–surface water interactions appeared mostly driven by cycles of bank recharge and discharge between the stream and the alluvial aquifer. DON and NH₄⁺ were the principal forms of dissolved N in groundwater, consistent with the primarily suboxic to anoxic conditions in the alluvial aquifer. A plume of groundwater NO₃⁻ was found at one transect where oxic conditions persisted within the riparian zone. The origin of the NO₃⁻ plume was hypothesized to be soil NO₃⁻ from the riparian zone flushed to the water table during recharge events. When present, NO₃⁻ did not reach surface water because conditions in the alluvial aquifer in the vicinity of the stream were always reduced. The concentration of groundwater DOC was variable across the floodplain and may be related to the extent of the vegetation cover. Overall, transformation and recycling of N during lateral exchange processes, as opposed to discharge of new N inputs from regional groundwater, appears to primarily control N cycling during groundwater–surface water interactions in this subtropical floodplain.     

Nitrogen fertilization enhances water‐use efficiency in a saline environment

Effects of salinity and nutrients on carbon gain in relation to water use were studied in the grey mangrove, Avicennia marina, growing along a natural salinity gradient in south‐eastern Australia. Tall trees characterized areas of seawater salinities (fringe zone) and stunted trees dominated landward hypersaline areas (scrub zone). Trees were fertilized with nitrogen (+N) or phosphorus (+P) or unfertilized. There was no significant effect of +P on shoot growth, whereas +N enhanced canopy development, particularly in scrub trees. Scrub trees maintained greater CO₂ assimilation per unit water transpired (water‐use efficiency, WUE) and had lower nitrogen‐use efficiency (NUE; CO₂ assimilation rate per unit leaf nitrogen) than fringe trees. The CO₂ assimilation rates of +N trees were similar to those in other treatments, but were achieved at lower transpiration rates, stomatal conductance and intercellular CO₂ concentrations. Maintaining comparable assimilation rates at lower stomatal conductance requires greater ribulose 1·5‐bisphosphate carboxylase/oxygenase activity, consistent with greater N content per unit leaf area in +N trees. Hence, +N enhanced WUE at the expense of NUE. Instantaneous WUE estimates were supported by less negative foliar δ¹³C values for +N trees and scrub control trees. Thus, nutrient enrichment may alter the structure and function of mangrove forests along salinity gradients.     

呼伦贝尔草原辉河湿地不同淹水状态的土壤碳氮磷特征比较

碳(C)、氮(N)、磷(P)生态化学计量比是生态系统过程及其功能的重要特征。为了比较不同淹水状态下草原区河岸湿地土壤生态化学计量特征差异,探讨用土壤生态化学计量特征来表征草原河岸湿地的退化状态,以呼伦贝尔草原辉河河岸湿地为研究对象,分别选择完全退化和未退化河岸湿地,从中选取常年淹水区域(W)、年际干湿交替区域(WD)以及河岸带边缘多年未曾淹没的干燥区域(D)为观测样地,对比研究退化与未退化草原河岸湿地土壤C、N、P化学计量比在不同淹水状态下的表征。结果表明:(1)随着淹水状态的改变,不论是退化河岸湿地还是未退化河岸湿地,不同淹水状态下的土壤C/N、C/P、N/P均差异显著(P0.05);(2)退化河岸湿地的C/N、C/P和N/P的平均值分别为9.05、273.08和28.25,与未退化河岸湿地相比(7.85、95.48、11.41)值显著升高;(3)相关性分析结果显示,土壤总磷与有机碳、全氮显著正相关,与C/N、C/P、N/P负相关,这可能是限制退化河岸湿地土壤养分元素循环的主要因素。研究结果表明:呼伦贝尔草原辉河河岸湿地不同淹水状态的土壤碳、氮、磷特征差异明显(P0.05),尤其是土壤C/P和N/P。与未退化河岸湿地相比,退化河岸湿地的C/N、C/P、N/P均明显高于未退化河岸湿地(P0.05),因此可以将土壤碳、氮、磷化学计量比特征作为草原区河岸湿地退化的评价指标。     

丹江口水库水滨带植物群落空间分布及环境解释

探讨了环境因素对丹江口水库(南水北调中线水源地)水滨带植物群落空间分布的影响。通过对水滨带植物群落和环境因素的实地调查,用双向指示种分析(TWINSPAN)对201个水滨带植物群落进行分类;结合地形、土壤和水文因素用除趋势典范对应分析法(DCCA)分析环境因素对水滨带植物群落的影响;并对环境因素的解释能力进行定量分离。结果表明:(1)水滨带植物群落包括7种类型,分别是萹蓄群落、苘麻群落、细叶水芹+狗牙根群落、狗牙根群落、响叶杨-狗牙根群落、杜梨-白刺花-狗牙根群落和侧柏-牡荆-三穗苔草群落;(2)海拔和水淹影响对水滨带植物群落空间分布具有主导作用。海拔升高,水淹影响减弱,植物群落呈现由草本植物群落向木本植物群落变化的格局;(3)土壤因素的解释能力大于地形因素,水文因素的解释能力最小。各类环境因素之间存在交互作用,地形、水文和土壤因素三者间的交互作用最大,地形和土壤因素之间的交互作用最小。环境因素共解释水滨带植物群落空间分布的21.99%,未解释部分为78.01%。结果证明环境对植被的解释能力是由植被的复杂程度决定的,植被越复杂,环境的解释能力越低。     

Diversity and Above-Ground Biomass Patterns of Vascular Flora Induced by Flooding in the Drawdown Area of China's Three Gorges Reservoir

Hydrological alternation can dramatically influence riparian environments and shape riparian vegetation zonation. However, it was difficult to predict the status in the drawdown area of the Three Gorges Reservoir (TGR), because the hydrological regime created by the dam involves both short periods of summer flooding and long-term winter impoundment for half a year. In order to examine the effects of hydrological alternation on plant diversity and biomass in the drawdown area of TGR, twelve sites distributed along the length of the drawdown area of TGR were chosen to explore the lateral pattern of plant diversity and above-ground biomass at the ends of growing seasons in 2009 and 2010. We recorded 175 vascular plant species in 2009 and 127 in 2010, indicating that a significant loss of vascular flora in the drawdown area of TGR resulted from the new hydrological regimes. Cynodon dactylon and Cyperus rotundus had high tolerance to short periods of summer flooding and long-term winter flooding. Almost half of the remnant species were annuals. Species richness, Shannon-Wiener Index and above-ground biomass of vegetation exhibited an increasing pattern along the elevation gradient, being greater at higher elevations subjected to lower submergence stress. Plant diversity, above-ground biomass and species distribution were significantly influenced by the duration of submergence relative to elevation in both summer and previous winter. Several million tonnes of vegetation would be accumulated on the drawdown area of TGR in every summer and some adverse environmental problems may be introduced when it was submerged in winter. We conclude that vascular flora biodiversity in the drawdown area of TGR has dramatically declined after the impoundment to full capacity. The new hydrological condition, characterized by long-term winter flooding and short periods of summer flooding, determined vegetation biodiversity and above-ground biomass patterns along the elevation gradient in the drawdown area.     

Nitrogen response efficiency increased monotonically with decreasing soil resource availability: a case study from a semiarid grassland in northern China

The concept of nutrient use efficiency is central to understanding ecosystem functioning because it is the step in which plants can influence the return of nutrients to the soil pool and the quality of the litter. Theory suggests that nutrient efficiency increases unimodally with declining soil resources, but this has not been tested empirically for N and water in grassland ecosystems, where plant growth in these ecosystems is generally thought to be limited by soil N and moisture. In this paper, we tested the N uptake and the N use efficiency (NUE) of two Stipa species (S. grandis and S. krylovii) from 20 sites in the Inner Mongolia grassland by measuring the N content of net primary productivity (NPP). NUE is defined as the total net primary production per unit N absorbed. We further distinguished NUE from N response efficiency (NRE; production per unit N available). We found that NPP increased with soil N and water availability. Efficiency of whole-plant N use, uptake, and response increased monotonically with decreasing soil N and water, being higher on infertile (dry) habitats than on fertile (wet) habitats. We further considered NUE as the product of the N productivity (NP the rate of biomass increase per unit N in the plant) and the mean residence time (MRT; the ratio between the average N pool and the annual N uptake or loss). The NP and NUE of S. grandis growing usually in dry and N-poor habitats exceeded those of S. krylovii abundant in wet and N-rich habitats. NUE differed among sites, and was often affected by the evolutionary trade-off between NP and MRT, where plants and communities had adapted in a way to maximize either NP or MRT, but not both concurrently. Soil N availability and moisture influenced the community-level N uptake efficiency and ultimately the NRE, though the response to N was dependent on the plant community examined. These results show that soil N and water had exerted a great impact on the N efficiency in Stipa species. The intraspecific differences in N efficiency within both Stipa species along soil resource availability gradient may explain the differences in plant productivity on various soils, which will be conducive to our general understanding of the N cycling and vegetation dynamics in northern Chinese grasslands.     

三峡库区消落带4种典型草本植物的生态化学计量特征

为研究三峡水库消落带优势植物的养分利用特征及其对生境的适应策略,选择消落带分布最多的4种草本植物为研究对象,分析了植物根、茎、叶的碳(C)、氮(N)、磷(P)、钾(K)含量和化学计量比特征。结果表明:(1)相比陆地系统和自然湿地系统,消落带植物具有较低的C含量和较高的N、P、K含量,C/N、C/P、C/K均较低,表明植物具有低固碳和高养分积累、低养分利用效率和高生长速率的特征;(2)4种植物的养分含量和计量比存在一定差异,其中狗牙根具有较低的N、P、K含量和较高的C/N、C/P、C/K,且变异系数均低于其他3种植物,其低养分需求、高养分利用效率以及较强的内稳性可能是其在库区分布最广的重要机制;(3)4种植物在不同器官的养分分配策略相似,均表现为叶片C含量低于根和茎,而N、P、K含量则显著高于根、茎;同时,与根、茎相比,叶片C/N、C/P、C/K较低,N/P、N/K较高,且在不同生境条件下变异系数较小,表现出相对较高的稳定性;(4)落消带植物的养分含量及计量比从全库区上游至下游的空间变异性较强,其中N、C/N、N/P变异性较大,而C、P、K变异性较小,表明植物N含量受生境变化的影响较大,加之消落带不同植物生长均受到严格的N限制,因此N供给可能是影响消落带生态系统结构的重要因子。三峡库区消落带植物生态化学计量特征具有明显的变异性和特殊性,是植物群落演替及生态系统功能变化的重要驱动因素。