Measured and simulated nitrous oxide emissions from ryegrass- and ryegrass/white clover-based grasslands in a moist temperate climate

There is uncertainty about the potential reduction of soil nitrous oxide (N(2)O) emission when fertilizer nitrogen (FN) is partially or completely replaced by biological N fixation (BNF) in temperate grassland. The objectives of this study were to 1) investigate the changes in N(2)O emissions when BNF is used to replace FN in permanent grassland, and 2) evaluate the applicability of the process-based model DNDC to simulate N(2)O emissions from Irish grasslands. Three grazing treatments were: (i) ryegrass (Lolium perenne) grasslands receiving 226 kg FN ha(-1) yr(-1) (GG+FN), (ii) ryegrass/white clover (Trifolium repens) grasslands receiving 58 kg FN ha(-1) yr(-1) (GWC+FN) applied in spring, and (iii) ryegrass/white clover grasslands receiving no FN (GWC-FN). Two background treatments, un-grazed swards with ryegrass only (G-B) or ryegrass/white clover (WC-B), did not receive slurry or FN and the herbage was harvested by mowing. There was no significant difference in annual N(2)O emissions between G-B (2.38±0.12 kg N ha(-1) yr(-1) (mean±SE)) and WC-B (2.45±0.85 kg N ha(-1) yr(-1)), indicating that N(2)O emission due to BNF itself and clover residual decomposition from permanent ryegrass/clover grassland was negligible. N(2)O emissions were 7.82±1.67, 6.35±1.14 and 6.54±1.70 kg N ha(-1) yr(-1), respectively, from GG+FN, GWC+FN and GWC-FN. N(2)O fluxes simulated by DNDC agreed well with the measured values with significant correlation between simulated and measured daily fluxes for the three grazing treatments, but the simulation did not agree very well for the background treatments. DNDC overestimated annual emission by 61% for GG+FN, and underestimated by 45% for GWC-FN, but simulated very well for GWC+FN. Both the measured and simulated results supported that there was a clear reduction of N(2)O emissions when FN was replaced by BNF.     

The fate of photosynthetically-fixed carbon in Lolium perenne grassland as modified by elevated CO2 and sward management

Prediction of the impact of climate change requires the response of carbon (C) flow in plant-soil systems to increased CO(2) to be understood. A mechanism by which grassland C sequestration might be altered was investigated by pulse-labelling Lolium perenne swards, which had been subject to CO(2) enrichment and two levels of nitrogen (N) fertilization for 10 yr, with (14)CO(2). Over a 6-d period 40-80% of the (14)C pulse was exported from mature leaves, 1-2% remained in roots, 2-7% was lost as below-ground respiration, 0.1% was recovered in soil solution, and 0.2-1.5% in soil. Swards under elevated CO(2) with the lower N supply fixed more (14)C than swards grown in ambient CO(2), exported more fixed (14)C below ground and respired less than their high-N counterparts. Sward cutting reduced root (14)C, but plants in elevated CO(2) still retained 80% more (14)C below ground than those in ambient CO(2). The potential for below-ground C sequestration in grasslands is enhanced under elevated CO(2), but any increase is likely to be small and dependent upon grassland management.     

Growth, nitrogen uptake, and metabolism in two semiarid shrubs grown at ambient and elevated atmospheric CO2 concentrations: effects of nitrogen supply and source

The effect of differences in nitrogen (N) availability and source on growth and nitrogen metabolism at different atmospheric CO(2) concentrations in Prosopis glandulosa and Prosopis flexuosa (native to semiarid regions of North and South America, respectively) was examined. Total biomass, allocation, N uptake, and metabolites (e.g., free NO(3)(-), soluble proteins, organic acids) were measured in seedlings grown in controlled environment chambers for 48 d at ambient (350 ppm) and elevated (650 ppm) CO(2) and fertilized with high (8.0 mmol/L) or low (0.8 mmol/L) N (N(level)), supplied at either 1 : 1 or 3 : 1 NO(3)(-) : NH(4)(+) ratios (N(source)). Responses to elevated CO(2) depended on both N(level) and N(source), with the largest effects evident at high N(level). A high NO(3)(-) : NH(4)(+) ratio stimulated growth responses to elevated CO(2) in both species when N was limiting and increased the responses of P. flexuosa at high N(level). Significant differences in N uptake and metabolites were found between species. Seedlings of both species are highly responsive to N availability and will benefit from increases in CO(2), provided that a high proportion of NO(3)- to NH(4)-N is present in the soil solution. This enhancement, in combination with responses that increase N acquisition and increases in water use efficiency typically found at elevated CO(2), may indicate that these semiarid species will be better able to cope with both nutrient and water deficits as CO(2) levels rise.     

Soil nutrient heterogeneity interacts with elevated CO2 and nutrient availability to determine species and assemblage responses in a model grassland community

Interactive effects of atmospheric CO(2) concentration ([CO(2)]), soil nutrient availability and soil nutrient spatial distribution on the structure and function of plant assemblages remain largely unexplored. Here we conducted a microcosm experiment to evaluate these interactions using a grassland assemblage formed by Lolium perenne, Plantago lanceolata, Trifolium repens, Anthoxanthum odoratum and Holcus lanatus. Assemblages exhibited precise root foraging patterns, had higher total and below-ground biomass, and captured more nitrogen when nutrients were supplied heterogeneously. Root foraging responses were modified by nutrient availability, and the patterns of N capture by interactions between nutrient distribution, availability and [CO(2)]. Greater above-ground biomass was observed under elevated CO(2) only under homogeneous conditions of nutrient supply and at the highest availability level. CO(2) interacted with nutrient distribution and availability to determine foliar percentage N and below : above-ground biomass ratios, respectively. Interactions between nutrient distribution and CO(2) determined the relative contribution to above-ground biomass of four of the species. The responses of dominant and subordinate species to [CO(2)] were dependent on the availability and distribution of nutrients. Our results suggest that soil nutrient distribution has the potential to influence the response of plant species and assemblages to changes in [CO(2)] and nutrient availability.     

Evidence that carbon dioxide enrichment alleviates ureide-induced decline of nodule nitrogenase activity

The hypothesis that elevated [CO(2)] alleviates ureide inhibition of N(2)-fixation was tested. Short-term responses of the acetylene reduction assay (ARA), ureide accumulation and total non-structural carbohydrate (TNC) levels were measured following addition of ureide to the nutrient solution of hydroponically grown soybean. The plants were exposed to ambient (360 micromol mol(-1)) or elevated (700 micromol mol(-1)) [CO(2)]. Addition of 5 and 10 mM ureide to the nutrient solution inhibited N(2)-fixation activity under both ambient and elevated [CO(2)] conditions. However, the percentage inhibition following ureide treatment was significantly greater under ambient [CO(2)] as compared with that under elevated [CO(2)]. Under ambient [CO(2)] conditions, ARA was less than that under elevated [CO(2)] 1 d after ureide treatment. Under ambient [CO(2)], the application of ureide resulted in a significant accumulation of ureide in all plant tissues, with the highest concentration increases in the leaves. However, application of exogenous ureide to plants subjected to elevated [CO(2)] did not result in increased ureide concentration in any tissues. TNC concentrations were consistently higher under elevated [CO(2)] compared with those under ambient [CO(2)]. For both [CO(2)] treatments, the application of ureide induced a significant decrease of TNC concentrations in the leaves and nodules. For both leaves and nodules, a negative correlation was observed between TNC and ureide levels. Results indicate that product(s) of ureide catabolism rather than tissue ureide concentration itself are critical in the regulation of N(2)-fixation.     

Regrowth and Nutrient Composition of Different Plant Organs in Grass-clover Canopies as Affected by Phosphorus and Potassium Availability

Regrowth after cutting and the distribution of nitrogen (N),phosphorus (P) and potassium (K) in different plant organs ofwhite clover and perennial ryegrass growing in pure or mixedswards were investigated under field conditions in a soil witha low-to-moderate availability of P and K. In all treatments,white clover constituted more than 70% of the above-ground biomassin the mixed swards. The petioles were the dominant pool ofdry matter throughout regrowth and contained the greatest amountsof N, P and K. Increased supply of P and K increased the growthof ryegrass, but not that of white clover in the mixed swards.The increased competition from ryegrass led to a decline inthe yield of white clover laminae as well as in the N contentper unit of dry matter in laminae, petioles and stolons. TheP content of all white clover organs also declined followingP application to the mixed swards, whereas K application increasedtheir K contents. In the pure swards of ryegrass and white clover,yields and contents of N, P and K in the dry matter were eithernot affected or increased following P and K application. Itwas concluded that commonly-used defoliation heights may remove80% or more of the nutrient and dry matter pools located inthe petioles but the remaining quantities of dry matter andnutrients in the petioles will normally exceed the correspondingquantities in the stolons. Copyright 2001 Annals of Botany Company Coexistence, competition, phosphorus, potassium, regrowth, ryegrass, white clover     

大气CO2浓度升高和N沉降对南亚热带主要乡土树种叶片元素含量的影响

大气CO₂浓度升高和N沉降以及二者之间的耦合作用对陆地森林生态系统的影响是当前国际生态学界关注的热点之一。该实验运用大型开顶箱(open-top chamber, OTC)研究: 1)高CO₂浓度(700 μmol×mol^-1) +高N沉降(100 kg N×hm^-2×a^-1) (CN); 2)高CO₂浓度(700 μmol×mol^-1)和背景N沉降(CC); 3)高N沉降(100 kg N×hm^-2×a^-1)和背景CO₂浓度(NN); 4)背景CO₂和背景N沉降(CK) 4种处理对南亚热带主要乡土树种木荷(Schima superba)、红锥(Castanopsis hystrix)、肖蒲桃(Acmena acuminatissima)、红鳞蒲桃(Syzygium hancei)、海南红豆(Ormosia pinnata)叶片元素含量的影响。研究结果表明, 大气CO₂浓度升高对5种乡土树种叶片元素含量有较大的影响, 除海南红豆叶片的Ca含量外, 其他树种的叶片元素含量在高CO₂浓度处理下都显著升高(p < 0.05); 而在N沉降处理下, 5个树种的叶片K和Ca含量都降低。大气CO₂浓度升高与N沉降处理对5种乡土树种植物叶片元素含量影响的交互作用不是很明显, 仅仅木荷和红鳞蒲桃的叶片Ca和Mn以及海南红豆的叶片Mn含量在大气CO₂浓度上升和N沉降交互处理下显著下降, 而肖蒲桃的叶片P含量在大气CO₂浓度上升和N沉降交互处理下显著上升。     

Pseudoviviparous reproduction of Poa alpina var. vivipara L. (Poaceae) during long-term exposure to elevated atmospheric CO2

Pseudovivipary is an asexual reproductive strategy exhibited by some arctic/alpine grasses in which leafy plantlets are produced in place of seeds, with genetic conservation an advantage for stress tolerators in these nutrient-poor habitats. Photosynthetic metabolism and the development of this reproductive system were investigated under varying nutrient availability and predicted future CO(2) partial pressure (pCO(2)). Poa alpina var. vivipara L., grown at present ambient pCO(2) or ambient plus 340 micro mol mol(-1) CO(2) (elevated pCO(2)), was supplied with either 0.05 mol m(-3) phosphorus and 0.2 mol m(-3) nitrogen, or 0.2 mol m(-3) phosphorus and 1.0 mol m(-3) nitrogen. Gas exchange measurements and determination of total non-structural carbohydrate (TNC), nitrogen and phosphorus contents revealed that parent plant leaf blade tissues experienced acclimatory loss of photosynthetic capacity after long-term growth at elevated pCO(2) (particularly so when nutrient availability was low); there were associated reductions in photosynthetic nitrogen and phosphorus use efficiencies (PNUE and PPUE). In addition, decreased PNUE and PPUE were exhibited by plantlets grown at elevated pCO(2) with low nutrient availability. Decreased reproductive dry matter in this treatment also resulted from a lack of reproductive initiation in daughter tillers, and altered phenology. Pseudoviviparous P. alpina is likely to be at a disadvantage in both vegetative and reproductive phases at predicted future elevated atmospheric CO(2) concentrations, particularly where nutrients are scarce and when in competition with species experiencing less acclimatory loss of photosynthetic capacity.     

Global pattern and controls of biological nitrogen fixation under nutrient enrichment: A meta‐analysis

Biological nitrogen (N) fixation (BNF), an important source of N in terrestrial ecosystems, plays a critical role in terrestrial nutrient cycling and net primary productivity. Currently, large uncertainty exists regarding how nutrient availability regulates terrestrial BNF and the drivers responsible for this process. We conducted a global meta‐analysis of terrestrial BNF in response to N, phosphorus (P), and micronutrient (Micro) addition across different biomes (i.e, tropical/subtropical forest, savanna, temperate forest, grassland, boreal forest, and tundra) and explored whether the BNF responses were affected by fertilization regimes (nutrient‐addition rates, duration, and total load) and environmental factors (mean annual temperature [MAT], mean annual precipitation [MAP], and N deposition). The results showed that N addition inhibited terrestrial BNF (by 19.0% (95% confidence interval [CI]: 17.7%?20.3%); hereafter), Micro addition stimulated terrestrial BNF (30.4% [25.7%?35.3%]), and P addition had an inconsistent effect on terrestrial BNF, i.e., inhibiting free‐living N fixation (7.5% [4.4%?10.6%]) and stimulating symbiotic N fixation (85.5% [25.8%?158.7%]). Furthermore, the response ratios (i.e., effect sizes) of BNF to nutrient addition were smaller in low‐latitude (<30°) biomes (8.5%?36.9%) than in mid‐/high‐latitude (≥30°) biomes (32.9%?61.3%), and the sensitivity (defined as the absolute value of response ratios) of BNF to nutrients in mid‐/high‐latitude biomes decreased with decreasing latitude (p ≤ 0.009; linear/logarithmic regression models). Fertilization regimes did not affect this phenomenon (p > 0.05), but environmental factors did affect it (p < 0.001) because MAT, MAP, and N deposition accounted for 5%?14%, 10%?32%, and 7%?18% of the variance in the BNF response ratios in cold (MAT < 15°C), low‐rainfall (MAP < 2,500 mm), and low‐N‐deposition (<7 kg ha^?1 year^?1) biomes, respectively. Overall, our meta‐analysis depicts a global pattern of nutrient impacts on terrestrial BNF and indicates that certain types of global change (i.e., warming, elevated precipitation and N deposition) may reduce the sensitivity of BNF in response to nutrient enrichment in mid‐/high‐latitude biomes.     

Partitioning of P and the activity of root acid phosphatase in white clover (Trifolium repens L.) are modified by increased atmospheric CO2 and P fertilisation

The growth response of white clover (Trifolium repens L.) to the expected increase in atmospheric partial pressure of CO₂ (p_CO2) may depend on P availability. A decrease in the rate of transpiration due to increased p_CO2 may reduce the amount of P transported to the shoot, thereby causing a change in the partitioning of P between the root and shoot. To test these hypotheses, four concentrations of P in the nutrient solution, combined with two p_CO2 treatments, were applied to nodulated white clover plants. Compared to ambient p_CO2 (35 Pa), twice ambient p_CO2 (70 Pa) reduced the rate of transpiration but did not impair the total P uptake per plant. However, at twice ambient p_CO2 and a moderate to high supply of P, concentrations of structural P and soluble P (Pi) were lower in the leaves and higher in the roots. The activity of root acid phosphatase was lower at twice ambient p_CO2 than at ambient p_CO2; it depended on the Pi concentration in the roots. At the highest P concentration, twice ambient p_CO2 stimulated photosynthesis and the growth rate of the plant without affecting the concentration of nonstructural carbohydrates in the leaves. However, at the lower P concentrations, plants at twice ambient p_CO2 lost their stimulation of photosynthesis in the afternoon, they accumulated nonstructural carbohydrates in the leaves and their growth rate was not stimulated; indicating C-sink limitation of growth. P nutrition will be crucial to the growth of white clover under the expected future conditions of increased p_CO2. This revised version was published online in June 2006 with corrections to the Cover Date.     

不同氮素水平下二氧化碳加富对草莓叶片光抑制的影响

用便携式调制叶绿素荧光仪和光合仪研究了强光下不同供氮水平（12、4和0.4 mmol·L^-1）和不同CO₂浓度下（700和390 μl·L^-1）丰香草莓叶片的荧光参数及净光合速率的变化.结果表明,CO₂和氮素对草莓叶片光抑制有明显的互作效应.在富CO₂下,12 mmol·L^-1供氮水平的草莓叶片净光合速率升高了62.7%,4和0.4 mmol·L^-1供氮水平则分别降低了7.4%和21.3%;12 mmol·L^-1供氮水平的F_m和F_v/F_m在强光胁迫时降辐减小,暗恢复时F_m和F_v/F_m恢复程度提高,而4和0.4 mmol·L^-1供氮水平却相反.表明氮素供应不足时草莓叶片在富CO₂环境下光合作用出现适应性下调,光抑制增强.     

Does nitrogen supply affect the response of wheat (Triticum aestivum cv. Hanno) to the combination of elevated CO(2) and O(3)?

Spring wheat (Triticum aestivum cv. Hanno) was grown at ambient (350 micromol mol(-1)) or elevated CO(2) (700 micromol mol(-1)) in charcoal/Purafil-filtered air (CFA <5 nmol mol(-1)) or ozone (CFA +75 nmol mol(-1) 7 h d(-1)) at three levels of N supply (1.5, 4 and 14 mM NO(-3)), to test the hypothesis that the combined impacts of elevated CO(2) and O(3) on plant growth and photosynthetic capacity are affected by nitrogen availability. Shifts in foliar N content reflected the level of N supplied, and the growth stimulation induced by elevated CO(2) was dependent on the level of N supply. At 60 d after transfer (DAT), elevated CO(2) was found to increase total biomass by 44%, 29%, 12% in plants supplied with 14, 4 and 1.5 mM NO(-3), respectively, and there was no evidence of photosynthetic acclimation to elevated CO(2) across N treatments; the maximum in vivo rate of Rubisco carboxylation (V(cmax)) was similar in plants raised at elevated and ambient CO(2). At 60 DAT, ozone exposure was found to suppress plant relative growth rate (RGR) and net photosynthesis (A) in plants supplied with 14 and 4 mM NO(-3). However, O(3) had no effect on the RGR of plants supplied with 1.5 mM NO(-3) and this effect was accompanied by a reduced impact of the pollutant on A. Elevated CO(2) counteracted the detrimental effects of O(3) (i.e. the same ozone concentration that depressed RGR and A at ambient CO(2) resulted in no significant effects when plants were raised at elevated CO(2)) at all levels of N supply and the effect was associated with a decline in O(3) uptake at the leaf level.     

Long- and short-term effects of crop residues on aluminum toxicity,phosphorus availability and growth of pearl millet in an acid sandy soil

Pasture swards containing perennial ryegrass (Lolium perenne L.) alone or with one of five different white clover (Trifolium repens L.) cultivars were examined for production and transfer of fixed nitrogen (N) to grass under dairy cow grazing. Grass-only swards produced 21% less than mixed clover-grass swards during the second year after sowing. Production from grass-only plots under a mowing and clipping removal regime was 44% less than from grass-only plots under grazing. Much of this difference could be attributed to N transfer. In swards without clover, the ryegrass component also decreased in favour of other grasses.The average amount of fixed N in herbage from all clover cultivars was 269 kg N ha^–1 yr^–1. Above-ground transfer of fixed N to grasses (via cow excreta) was estimated at 60 kg N ha^–1 yr^–1. Below-ground transfer of fixed N to grasses was estimated at 70 kg N ha^–1 yr^–1 by ¹⁵N dilution and was similar for all clover cultivars. Thus, about 50% of grass N was met by transfer of fixed N from white clover during the measurement year. Short-term measurements using a ¹⁵N foliar-labelling method indicated that below-ground N transfer was largest during dry summer conditions.     

Interactions between white clover and ryegrass under contrasting nitrogen availability: N2 fixation,N fertilizer recovery,N transfer and water use efficiency

Seasonal variation in N₂ fixation, N transfer from clover to ryegrass, and soil N absorption in white clover–ryegrass swards were investigated under field conditions over three consecutive years. The plots were established with different seeding ratios of clover and ryegrass and contrasting fertilizer N ranging from 3 to 72 kg ha^-1 year^-1.An initially poor clover population needed at least one growing season to reach the same yield output as an initially well established clover population. The clover content of the sward decreased by the annual application of 72 kg N ha^-1 but not by smaller N dressings.The total amount of atmospherically derived N in clover growing in mixture with ryegrass was, on average over the three years equal to 83, 71, 68 and 60 kg N ha^-1 for the treatments of 3, 24, 48 and 72 kg N ha^-1, respectively. The proportion of atmospherically derived N declined with increasing N application, but never became smaller than 80% of total clover N. The proportion of atmospherically derived N in a pure stand white clover amounted to 60–80% of the total N content, equivalent to 109, 110, 103 and 90 kg N ha^-1 for the treatments of 3, 24, 48 and 72 kg N ha^-1, respectively.Only small amounts of atmospherically derived N was transferred to the associated ryegrass during the first production year, while in each of the following years up to 21 kg ha^-1 was transferred. The average amount of N transferred from clover to ryegrass was equivalent to 3, 16 and 31% of the N accumulated in ryegrass in the first, second and third production year, respectively. Expressed relative to the total amount of fixed N₂ in the clover–ryegrass mixture, the transfer amounted to 3, 17 and 22% in the first, second and third production year, respectively. Thus transfer of atmospherically derived N from clover contributed significantly to the N economy of the associated ryegrass.The clover–ryegrass mixture absorbed constantly higher amount of soil derived N than the pure stands of the two species. Only 11% of the total accumulated fertilizer N and soil derived N in the mixture was contained within the clover component. Lower water use efficiencies for the plants grown in mixture compared to pure stands were mainly related to the increased N uptake in the mixture, with the subsequent increase in growth compared to the pure stands.It is concluded that positive interactions between clover and ryegrass growing in mixture ensure a more efficient fixation of atmospheric N₂ and absorption of fertilizer N and soil derived N than pure stands of the same species.     

Effects of white clover (Trifolium repens L.) on plant and soil nitrogen and soil organic matter in mixtures with perennial ryegrass (Lolium perenne L.)

To increase our insight into the above- and belowground N flows in grass and grass-clover swards relations between crop and soil parameters were studied in a cutting trial with perennial ryegrass (Lolium perenne) monocultures and ryegrass–white clover (Trifolium repens) mixtures. The effects of clover cultivar on herbage yield, the amount of clover-derived nitrogen, apparent N transfer to companion grass, dynamics of N and organic matter in the soil were estimated.The grass monocultures had very low DM yields (<2.1 t ha^-1) and a low N concentration in the harvested herbage. During 1992–1995 the annual herbage DM yield in the mixtures ranged from 7.0 to 14.3 t ha^-1, the white clover DM yield from 2.4 to 11.2 t ha^-1 and the mean annual clover content in the herbage DM harvested from 34 to 78%. Mixtures with the large-leaved clover cv. Alice yielded significantly more herbage and clover DM and had a higher clover content than mixtures with small/medium-leaved cvs. Gwenda and Retor. Grass cultivar did not consistently affect yield, botanical composition or soil characteristics.The apparent N₂ fixation was very high, ranging from 150 to 545 kg N ha^-1 in the different mixtures. For each tonne of clover DM in the harvested herbage 49 to 63 kg N was harvested, while the apparent N transfer from clover to grass varied between 55 and 113 kg N ha^-1 year^-1.The net N mineralization rate was lower under monocultures than under mixtures. The C mineralization and the amounts of C and N in active soil organic matter fractions were similar for monocultures and mixtures, but the C:N ratio of the active soil organic matter fractions were higher under grass than under mixtures. This explains the lower N mineralization under grass.     

一种高效研究大豆根瘤共生固氮的营养液栽培体系

为建立一种既可高效结瘤固氮, 又具有一定产量的大豆(Glycine max)营养液栽培系统, 设计并进行了2个试验。首先在不同供氮条件下, 研究了接种根瘤菌对大豆的结瘤状况、固氮能力、生物量及产量的影响。结果表明, 供氮过高或过低, 均影响大豆生长、产量形成及根瘤固氮; 并且植物生长所需的最适供氮水平远高于生物固氮所需的最适供氮水平。此外, 大豆生物固氮活性最高的时期在生殖期第一期(R1期)之前。由此推断, 大豆R1期前, 供应较低的氮, 有利于根瘤形成及固氮; 而从R1期起, 应提高供氮水平, 以促进植物生长及产量的形成。在此基础上开展第2个试验, 对供氮条件进行了优化处理(即R1期前低氮供应、R1期开始中氮供应)。结果表明, 与持续供应高氮相比, 优化供氮处理不仅可获得较多固氮酶活性较高的大根瘤, 还能保持较好的生长、获得更高的百粒重及维持80%左右的产量。研究结果不仅可为高效研究大豆根瘤共生固氮提供营养液配方, 还可为大豆高产高效栽培提供试验依据。     

红豆草与土壤氮含量对大气二氧化碳浓度升高的响应

在封闭的植物培养箱中,通过盆栽实验,研究了红豆草和土壤氮含量对CO₂浓度增加的响应.结果表明,与正常CO₂浓度(355～370 μmol·mol^-1)相比,CO₂浓度升高(700 μmol·mol^-1),植物生物量增加25.1%(P＜0.01),但植物体氮浓度降低25.3%(P＜0.001),植物全氮没有显著的变化.经3个月盆栽实验后,与原始土壤相比,两种CO₂浓度处理土壤全N、NO₃^--N和NH₄⁺-N都有所降低,而土壤微生物氮则显著增加,这可能与植物生长有关.不同CO₂浓度处理土壤NH₄⁺-N浓度基本一致,但在高CO₂浓度下,土壤NO₃^--N浓度显著降低,而微生物生物氮显著增加.对整个土壤-植物系统而言,盆栽实验后,整个系统全氮有少量增加,但变化不显著,特别是在高CO₂浓度条件下,土壤-植物系统全氮最大,这可能与培养材料红豆草为豆科植物,而且在高CO₂浓度下生物量增加,导致氮的固定量增加有关.     

Chronic catchment nitrogen enrichment and stoichiometric constraints on the bioavailability of dissolved organic matter from leaf leachate

1. Chronic nitrogen (N) deposition may alter the bioavailability of dissolved organic matter (DOM) in streams by multiple pathways. Elevated N deposition may alter the nutrient stoichiometry of DOM as well as nutrient availability in stream water. 2. We evaluated the influence of a decadal‐scale experimental N enrichment on the relative importance of DOM nutrient content and inorganic nutrient availability on the bioavailability of DOM. We measured the consumption of dissolved organic carbon (DOC) and changes in nutrient concentration, DOM components and enzyme activity in a bottle incubation assay with different DOM and nutrient treatments. To evaluate the effect of DOM stoichiometry, we used leaf leachates of different carbon/N/phosphorus (C : N :P) ratio, made from leaf litter sourced in the reference and N‐enriched catchments at the Bear Brook Watershed in Maine (BBWM). We also manipulated the concentration of inorganic N and P to compare the effect of nutrient enrichment with DOM stoichiometry. 3. DOC from the N‐enriched catchment was consumed 14% faster than that from the reference catchment. However, mean DOC consumption for both leachates was more than doubled by the simultaneous addition of N and P, compared to controls, while the addition of N or P alone increased consumption by 42 and 23%, respectively. The effect of N and/or P enrichment consistently had a greater effect than DOM source for all response variables considered. 4. We subsequently conducted DOC uptake measurements using leaf leachate addition under ambient and elevated N and P in the streams draining the reference and N‐enriched catchments at BBWM. In both streams, DOC uptake lengths were shorter when N and P were elevated. 5. Although both DOM stoichiometry and inorganic nutrient availability affect DOM bioavailability, N and P co‐limitation appears to be the dominant driver of reach‐scale processing of DOM.     

矿质养分输入对森林生物固氮的影响

生物固氮是森林生态系统重要的氮素来源,并且在全球氮循环中占有重要的地位。近代以来,因人类活动加剧而导致氮沉降的增加以及其它矿质养分元素(如磷、钼、铁等)输入的改变已成为影响森林生态系统生物固氮的重要因素之一,并引起了学术界的普遍关注。综述了国内外关于森林生物固氮对矿质养分输入的响应及机理。主要内容包括:(1)森林生物固氮的概念及主要的测定方法;(2)矿质养分输入对森林生物固氮的影响。整体上讲,氮素输入抑制了森林生物固氮,磷和其他营养元素输入则表现为促进作用。氮和磷、磷和微量元素同时添加均提高了森林的固氮量;(3)矿质养分改变森林生物固氮的机理。包括生物作用机制(如改变地表层固氮菌的数量或群落丰度、改变结瘤植物的根瘤生物量和附生植物的丰度或盖度)和环境作用机制(如引起土壤酸化、改变碳源物质的含量);(4)探讨了矿质养分输入对森林生物固氮影响研究中所存在的问题,并对未来该领域的研究提出建议。     

Gas exchange and carbon metabolism in two Prosopis species (Fabaceae) from semiarid habitats: effects of elevated CO2, N supply, and N source

Predicting future plant and ecosystem responses to elevated CO(2) also requires an understanding of the role of other factors, especially soil nitrogen. This is particularly challenging for global aridlands where total N and the relative amounts of nitrate and ammonia vary both spatially and seasonally. We measured gas exchange and primary and secondary C metabolites in seedlings of two dominant aridland shrub species (Prosopis flexuosa [S America] and P. glandulosa [N America]) grown at ambient (350 ppm) or elevated (650 ppm) CO(2) and nitrogen at two levels (low [0.8 mM] and high [8.0 mM]) and at either 1 : 1 or 3 : 1 nitrate to ammonia. Whereas elevated CO(2) increased assimilation rate, water use efficiency, and primary carbon metabolites in both species, these increases were strongly contingent upon nitrogen availability. Elevated CO(2) did not increase secondary metabolites (i.e., phenolics). For these important aridland species, the effects of elevated CO(2) are strongly influenced by nitrogen availability and to a lesser extent by the relative amounts of nitrate and ammonia supplied, which underscores the importance of both the amount and chemical composition of soil nitrogen in mediating the potential responses of seedling growth and establishment of aridland plants under future CO(2)-enriched atmospheres.