The effect of sediment redox potential and soil acidity on nitrogen uptake,anaerobic root respiration,and growth of rice (Oryza sativa)

Summary Oryza sativa Loisel cultivar Mars., a common lowland rice variety was grown under controlled soil redox conditions (Eh) and acidity (pH). The effect of two variables (Eh and pH) on growth, anaerobic root respiration, and uptake of added labelled nitrogen, was investigated. Plant growth, estimated by dry weight showed significantly higher growth under reducing sediment redox potentials (−200 mV and 0 mV) and at a soil pH of 6.5 Using the activity of the inducible enzyme alcohol dehydrogenase (ADH) as an indicator of anaerobic root respiration, a decrease in redox potential resulted in an increase in root ADH. However, growth paralled increases in anaerobic root respiration suggesting nitrogen transformation in the soil to be a primary parameter governing growth. Labelled nitrogen uptake which was greater under anaerobic conditions apparently led to greater growth of lowland rice in the highly reduced or anaerobic soil treatments.     

苏北海滨湿地互花米草种子特征及实生苗生长

摘　要　在江苏盐城新洋港滩涂由海向陆建立样地：零星互花米草(Spartina alterniflora)斑块(SAP)?稳定互花米草滩下边缘(SAFI)?2003年互花米草定居处(SAF03)?1989年互花米草定居处(SAF89),对互花米草的种子特征及幼苗生长进行了研究。结果表明：(1) 各样地种子产量有极显著差异(p< 0.01),大小顺序为SAP > SAF03 > SAFI > SAF89,种子产量与植株结实率、穗长、单穗种子数成正比。(2) 4月份SAP、SAFI、SAF03和SAF89互花米草短暂土壤种子库分别为673.7 /m2、2328.7 /m2、5948.8 /m2和3990.4 /m2,种子保存率分别为0.5%、3.9%、6.9%和15.8%,且在各样地差异极显著(p< 0.01)。(3) 7月份SAP、SAFI、SAF03和SAF89实生苗数分别为72 /m2、5 /m2、0和0。SAP与SAFI种子萌发率显著高于SAF03与SAF89(p< 0.01)。(4)表层土壤水分含量和种群内部光照衰减是影响实生苗生长的关键因素。在表层土壤水分含量高和低光照衰减的生境中种子繁殖对互花米草种群延续和扩张贡献较大。     

互花米草海向入侵对土壤有机碳组分、来源和分布的影响

在江苏盐城新洋港互花米草(Spartina alterniflora)盐沼选择光滩(MF),互花米草入侵la(SAF-1),3a(SAF-3),5a(SAF-5)和12a(SAF-12)样地,采集0-20 cm表层土壤样品,分别测定土壤有机碳(SOC)、顽固性有机碳(RC)和活性有机碳(LC)含量,碳氮比(C/N),土壤有机碳和顽固性有机碳的δ13C值,分析互花米草海向入侵过程中土壤有机碳组分、分布及来源变化.结果表明:(1)SOC、RC、LC含量分别介于0.82-7.60 mg/g,0.58-4.02 mg/g和0.23-3.58 mg/g,由海向陆呈递增趋势:SAF-5＞SAF-12＞SAF-3＞MF＞SAF-1.入侵12 a的SAF-12样地表土SOC储量最大,年均碳汇积累速率为1.8 t/hm2.(2)互花米草来源SOC、RC和LC含量分别为0.06-3.01 mg/g、0.04-1.06 mg/g和0.03-2.00 mg/g,各占5.75％-47.40％、6.77％-31.77％和3.20％-64.40％.互花米草来源SOC、RC、LC由海向陆均呈递增趋势:SAF-12＞SAF-5＞SAF-3＞ SAF-1＞ MF.(3)互花米草植物来源SOC、RC、LC含量、比例与入侵时间显著正相关(P＜0.01).互花米草入侵对LC的影响较大,对RC的影响较小.(4)随着入侵时间的增长,互花米草来源有机碳的输入显著改变了土壤SOC组分.以上结果表明,短期内互花米草海向入侵能够提高土壤碳汇能力.     

氮、硫互作对克隆植物互花米草繁殖和生物量累积与分配的影响

氮、硫均是影响入侵植物互花米草（Spartina alterniflora Loisel.）生长与繁殖的重要营养元素,研究其交互作用可能有助于揭示互花米草成功入侵机制。本文在人工控制条件下,以河沙为栽培基质,研究了氮、硫互作对互花米草繁殖和生物量积累与分配的影响。实验结果表明：中氮和高氮处理下的互花米草分支强度、初级分株数、次级分株数、根状茎数及其总长要显著高于低氮,不同硫水平及氮硫互作对互花米草无性繁殖没有显著影响;互花米草的种子数和种子生物量受硫以及氮硫交互作用的影响显著,在高氮水平下,中硫处理的互花米草种子数和种子生物量要远远高于其他处理。中氮和高氮处理下的互花米草叶、茎、根、根状茎和总生物量显著高于低氮处理;只有在高氮水平下,高硫处理的生物量才显著低于其他处理。在中氮和高氮营养条件下,高硫处理会促使互花米草对地上部分投入更多的生物量。中硫-高氮作用对互花米草有性繁殖的促进,使互花米草能在硫浓度较高的滩地保持高有性繁殖率;虽然过高的硫会抑制互花米草的有性繁殖,但却能促进根状茎数目的增加,从而加速其向外短距离扩张;这些特性均有利于互花米草对滨海盐沼的入侵。     

互花米草与短叶茳芏枯落物分解过程中碳氮磷化学计量学特征

为了揭示植物枯落物分解过程中元素生态化学计量学特征,对闽江河口湿地互花米草和短叶茳芏枯落物分解过程进行了测定.结果表明:整个分解期间内(2007年1-10月),在近潮沟生境和远潮沟生境,互花米草枯落物分解速率、氮磷养分含量低于短叶茳芏枯落物,但热值高于短叶茳芏枯落物;近潮沟生境,互花米草和短叶茳芏枯落物分解过程中平均C/N、C/P和N/P分别为70.5和34.7,2285.8和1210.7,32.8和35.4;远潮沟生境互花米草和短叶茳芏枯落物分解过程中平均C/N、C/P和N/P分别为72.7和33.2,2519.2和1167.0,34.0和35.9,两种生境下均表现为互花米草具有较高的C/N、C/P和较低的N/P;互花米草枯落物分解过程中具有较高的C/N和C/P,其分解速率较低.     

Effects of salinity and clonal integration on growth and sexual reproduction of the invasive grass Spartina alterniflora

The purpose of this study was to explore clonal integration of Spartina alterniflora under gradually changing substrate salinity conditions. We hypothesized that there might be a trade-off between growth and sexual reproduction influenced by soil salinity and, that clonal integration would change this trade-off. The experiment consisted of three levels of substrate salinity (5‰, 20‰ and 35‰), two clonal integration treatments (rhizomes severed or not), and three growth stages of daughter ramets (21, 40 and 60 cm tall). Both growth and sexual reproduction of S. alterniflora greatly decreased with increasing salinity. Clonal integration enhanced the survival, growth and sexual reproduction of daughter ramets experiencing salt stress, especially for young ramets, whereas the performance of mother ramets was reduced by clonal integration. Therefore, clonal integration did not affect performance of the whole clones. Contrary to expectations, there was no evidence for a trade-off between growth and sexual reproduction associated with salinity. In addition, clonal integration did not change the effect of salinity on the growth and sexual reproduction of mother and daughter ramets nor of the whole clones.     

Clonal integration enhances flood tolerance of Spartina alterniflora daughter ramets

Recently, considerable attention has been paid to the invasion of the clonal plant Spartina alterniflora into coastal wetlands at lower elevations. In this experiment, we tested whether clonal integration improved flood tolerance in S. alterniflora daughter ramets. Daughter ramets at two growth stages (young and old ramets) were flooded to water levels of 0, 9 and 18 cm above the soil surface, and the rhizomes between mother and daughter ramets were either severed or left intact. Biomasses of connected ramets grown in controls or in shallow and deep water treatments were 119%, 108% and 149% higher in the old ramet group than those of severed ramets, respectively, whereas they were 3.0, 3.3 and 11.2 times higher in the young ramet group, respectively. At the end of the experimental period, the shoot height, connected with young ramets, in shallow and deep water treatments increased by 19% and 26%, respectively, over that in the control treatments, whereas the old ramets increased by 11% and 39%, respectively. In contrast, the shoot height of the severed young ramets was 27% and 26% lower in shallow and deep water treatments than in the control treatment, respectively. However, the shoot height of the severed old ramets remained constant with increasing water depth. We conclude that clonal integration enhances the flood tolerance of S. alterniflora daughter ramets, and the trait of clonal integration plays more important roles in severe flooding stress conditions and at early growth stages.     

苏北海滨湿地互花米草种群繁殖方式

在江苏盐城新洋港滩涂由海向陆建立样地:光滩(Mudflat)、零星米草斑块(SAP)、稳定米草滩下边缘(SAFI)、2003年米草定居处(SAF03)、1989年米草定居处(SAF89)、碱蓬滩(SS)及禾草滩(Grass flat),在每个样地分别建立6个样方,构建原地栽培系统(长1 m×宽1 m×深0.7 m),其中3个用于无性繁殖体栽培,3个用于有性繁殖体栽培,对互花米草的种子萌发、无性分株及实生苗的定居、生长、繁殖进行了研究。结果表明:(1)受海浪物理冲刷、淹水胁迫、受潮浸频率及土壤水分的影响,实验系统各样地种子萌发率均较低,只有样地SAFI处互花米草种子萌发率为17.8%,显著高于其他样地(P0.01)。SAP、SAFI、SAF03、SAF89和Grass flat处实验最初栽培的互花米草无性分株植株存活率较高(大于60.0%)且样地间差异性不显著(P0.05);Mudflat处无性分株存活率极低(7.8±2.2)%,SS处无性分株全部死亡(0.0%)。无性分株和实生苗相比较,除Mudflat和SS样地二者存活率接近,其余样地的无性分株成活率均远高于有性分株。(2)互花米草无性分株在SAP、SAFI和SAF89样地中的株高、叶长、叶数、叶宽和叶厚均较高;实生苗在SAFI处长势较好,其他样地植株矮小、基径细或者死亡。无性分株生长指标显著高于实生苗。(3)各样地成功定居的无性分株均有结实植株,而实生苗只有Mudflat和SAFI处结实。样地Mudflat、SAP、SAFI、SAF03、SAF89和Grass flat无性分株分蘖数分别为21.7、23.0、25.5、6.8、9.0和4.1,实生苗的分蘖数除在Mudflat处达到14.0外,在其他样地均未分蘖。(4)各样地互花米草的无性分株单株生物量显著大于实生苗。两种繁殖方式地上生物量分配都呈现出向海陆两边递减的趋势,而地下生物量分配表现出相反的情况。综合对比各项指标,可以看出,在研究区内互花米草无性繁殖方式的定居成功率、生长、繁殖等指标均优于有性繁殖方式。研究区内互花米草种群的维持和扩张以无性繁殖方式为主。     

Advantages of NH4 + on growth, nitrogen uptake and root respiration of Phragmites australis

We investigated growth, N nutrition, and root respiration in Phragmites australis (Cav.) Trin. ex Steud. grown under conditions with different N sources, and evaluated the advantages of NH₄ ⁺ nutrition in relation to adaptation to anaerobic soil conditions. Hydroponics culture was carried out for 2 months under two treatment conditions with different N sources, NH₄ ⁺ and NO₃ ^?. The relative growth rate (RGR) of the roots, shoot and whole plant, net N uptake rate (NNUR), and root respiration rate were examined. Shoot RGR, shoot to root (S/R) ratio, and NNUR were obviously higher with the NH₄ ⁺ treatment. High S/R ratio of plants grown in the NH₄ ⁺ treatment contributed to repression of whole-root oxygen consumption. In consequence, NNUR per root respiration rate was higher with the NH₄ ⁺ treatment, which clearly suggested efficient oxygen consumption in the roots. In conclusion, higher S/R ratio due to higher NNUR enable to efficiently use oxygen for N nutrition through the repression of whole-root oxygen consumption, which is consequently achieved by NH₄ ⁺ nutrition. Therefore, we suggest that NH₄ ⁺ nutrition is indispensable for hydrophytic species growing in anaerobic soil because it enables both sufficient N nutrition and efficient oxygen consumption.     

Intercropping effect on root growth and nitrogen uptake at different nitrogen levels

Aims Intercropping legumes and non-legumes may affect the root growth of both components in the mixture, and the non-legume is known to be strongly favored by increasing nitrogen (N) supply. The knowledge of how root systems affect the growth of the individual species is useful for understanding the interactions in intercrops as well as for planning cover cropping strategies. The aim of this work was (i) to determine if different levels of N in the topsoil influence root depth (RD) and intensity of barley and vetch as sole crops or as an intercropped mixture and (ii) to test if the choice of a mixture or the N availability in the topsoil will influence the N uptake by deep roots.Methods In this study, we combined rhizotron studies with root extraction and species identification by microscopy with studies of growth, N uptake and 15 N uptake from deeper soil layers, for studying the root interactions of root growth and N foraging for barley (Hordeum vulgare L.) and vetch (Vicia sativa L.), frequently grown in mixtures as cover crops. N was added at 0 (N0), 50 (N1) and 150 (N2) kg N ha^-1. The roots discrimination relying on the anatomical and morphological differences observed between dicots and monocots proved to be a reliable method providing valuable data for the analysis.Important findings The intercrop and the barley attained slightly higher root intensity (RI) and RD than the vetch, with values around 150 crosses m^-1 and 1.4 m, respectively, compared to 50 crosses m^-1 and 0.9 m for the vetch. At deep soil layers, intercropping showed slightly larger RI values compared to the sole-cropped barley. The barley and the intercropping had larger root length density (RLD) values (200–600 m m ?3) than the vetch (25–130) at 0.8–1.2 m depth. The topsoil N supply did not show a clear effect on the RI, RD or RLD; however, increasing topsoil N favored the proliferation of vetch roots in the intercropping at deep soil layers, with the barley:vetch root ratio ranging from 25 at N0 to 5 at N2. The N uptake of the barley was enhanced in the intercropping at the expense of the vetch (from ~100mg plant^-1 to 200). The intercropped barley roots took up more labeled nitrogen (0.6mg 15 N plant^-1) than the sole-cropped barley roots (0.3mg 15 N plant^-1) from deep layers.     

Effects of salinity and temperature stress on Ecophysiological characteristics of exotic cordgrass, Spartina alterniflora

Salintiy and temperature are two important ecological factors which affect the distribution and abundance of Spartina alerniflora Loisel. To find out how cordgrass adapts to the environmental conditions in the introduced range, we studied the dynamics of a series of important physiological components including superoxide dismutase (SOD), guaiacol peroxidase (POD), catalase (CAT), malondialdehyde (MDA), soluble sugar and free proline under different salinity and temperature stresses. The results showed that low NaCl concentration (lower than 100 mmol·L⁻¹) enhanced the growth of S. alterniflora. On the contrary, high NaCl concentration (higher than 100 mmol·L⁻¹) inhibited the growth of S. alterniflora. To a certain extent, S. alterniflora was able to be acclimated to the osmotic pressure created by external solution concentration by adjusting the activities of POD, SOD and CAT, and the contents of free proline and soluble sugar. S. alterniflora varied in its responses to environment in different parts of the plant under 5°C and 38°C temperature stress. Compared with roots, leaves accumulated more soluble sugar, and CAT activities in leaves were higher, whereas SOD and POD activities in leaves were much lower than those in roots.     

Spartina alterniflora invasions and effects on crab communities in a western Pacific estuary

We investigated Spartina alterniflora invasions, their relationship with shoreline dynamics and effects on crab communities in the Yellow River estuary, China, where shoreline dynamics have been accelerated due to human-mediated estuarine sediment deposition and sea-level rise. We determined the distribution of Spartina with extensive ground surveys, and quantified shoreline dynamics between 2001 and 2009 by interpreting satellite images. We used pitfall traps to sample crab populations in Spartina-invaded habitats and non-invaded mudflats in 2009 and 2010. Large areas (>0.5 km²) of Spartina plants were found at three locations in the estuary. The seaward limit of Spartina at each location generally coincided with the present shoreline, regardless of historical shoreline advance or retreat. Crab communities in Spartina-invaded habitats significantly differed from those in non-invaded habitats. The total number and biomass of crabs caught per trap were much higher in Spartina-invaded habitats than in non-invaded habitats, however, species richness and Shannon diversity were much lower. These results suggest that Spartina invasions are likely to keep pace with shoreline dynamics accelerated by global change and have significant ecological consequences for crab communities. These issues should be taken into account to improve the use and management of Spartina, especially in rapidly accreting estuaries with large-area mudflats that are important habitats but prone to Spartina invasions.     

Body temperature and desiccation constrain the activity of Littoraria irrorata within the Spartina alterniflora canopy

Behavioral patterns of motile ectotherms are often constrained by their microclimate conditions. For intertidal ectotherms, thermal and desiccation stresses are primary limiting factors. In this study, we developed and tested a steady-state heat budget model to calculate the duration of time that the salt marsh snail, Littoraria irrorata (Say), would maintain active behaviors (crawling or attached on stalks of marsh grass Spartina alterniflora) before switching to an inactive state (retracted and glued with a mucus holdfast on the stalks) due to desiccation. The snails' water loss tolerance limit was found to be 43.6±16.0 mg in a laboratory experiment using 5 temperature treatments (25-45 °C in 5 °C increments) with a vapor density (VD) deficit of ∼15 g/m³ (saturated VD-air VD). We found that snails attached to S. alterniflora at lower heights in the canopy had higher body temperatures during daytime hours but lower water loss rates. Furthermore, we found that calculated activity times generally matched daily and seasonal patterns of life history behaviors reported in the literature. If tidal emersion began at night (∼20:00-4:00 h), calculated activity times were much higher than if emersion began in the daytime. The total monthly activity times for 2005-2010 were the highest in May, the lowest in July, and increased from July to September. Therefore, L. irrorata's behaviors appear to be constrained by microclimate conditions within the S. alterniflora canopy as predicted by the heat budget model. The extent to which the snails' life history traits are controlled by environmental conditions will have important implications for their population dynamics as climate change progresses, and heat budget models can help to predict future changes in behavioral responses.     

Effects of nitrogen nutrition and root medium water potential on growth, nitrogen uptake and osmotic adjustment of rice

The effects of nitrogen (N) nutrition on growth, N uptake and leaf osmotic potential of rice plants (Oryza sativa L. ev. IR 36) during simulated water stress were determined. Twenty-one-day-old seedlings in high (28.6 × 10 ^?4M) and low (7.14 × 10 ⁴M) N levels were exposed to decreased nutrient solution water potentials by addition of polyethylene glycol 6000. The roots were separated from the solution by a semi-permeable membrane. Nutrient solution water potential was ?0.6 × 10⁵ Pa and was lowered stepwise to ?1 × 10⁵, ?2 × 10⁵, ?4 × 10⁵ and ?6 × 10⁵ Pa at 2-day intervals. Plant height, leaf area and shoot dry weight of high and low nitrogen plants were reduced by lower osmotic potentials of the root medium. Osmotic stress caused greater shoot growth reduction in high N than in low N plants. Stressed and unstressed plants in 7.14 × 10⁴M N had more root dry matter than the corresponding plants in 28.6 × 10⁴M N. Dawn leaf water potential of stressed plants was 1 × 10⁵ to 5.5 × 10⁵ Pa lower than nutrient solution water potential. Nitrogen-deficient water-stressed plants, however, maintained higher dawn leaf water potential than high nitrogen water-stressed plants. It is suggested that this was due to higher root-to-shoot ratios of N deficient plants. The osmotic potentials of leaves at full turgor for control plants were about 1.3 × 10⁵ Pa higher in 7.14 × 10^?4M than in 28.6 × 10^?4M N and osmotic adjustment of 2.6 × 10⁵ and 4.3 × 10⁵ Pa was obtained in low and high N plants, respectively. The nitrogen status of plants, therefore, affected the ability of the rice plant to adjust osmotically during water stress. Plant water stress decreased transpiration and total N content in shoots of both N treatments. Reduced shoot growth as a result of water stress caused the decrease in amount of water transpired. Transpiration and N uptake were significantly correlated. Our results show that nitrogen content is reduced in water-stressed plants by the integrated effects of plant water stress per se on accumulation of dry matter and transpiring leaf area as well as the often cited changes in soil physical properties of a drying root medium.     

The effect of rhizoctonia root disease and applied nitrogen on growth,nitrogen uptake and nutrient concentrations in spring wheat

Root disease caused by Rhizoctonia solani is a common problem of spring wheat in South Australia. There are reports that nitrogen applications can reduce the incidence and severity of the disease. A glasshouse trail in pots examined the effects of disease and of applied nitrogen on wheat growth, and evaluated the utility of the basal stem nitrate concentration in diagnosing deficiency in plants with and without root disease. Plants were harvested at the mid-tillering stage. Shoot growth was increased by applied nitrogen until a maximum yield was attained, after which additional N had no effect on shoot yield. Root growth, however, responded positively only to low levels of applied N, after which it declined, and in the highest N treatment root mass was less than in the plants without applied N. Root disease caused severe reductions in plant growth, and both root and shoot mass were affected similarly. Even though growth of diseased plants responded positively to applied nitrogen the response was less than that of disease-free plants. The critical concentration of basal stem nitrate-N did not appear to be affected by root disease, and was estimated at 1200 mg kg^-1, consistent with other glasshouse data. The basal stem nitrate-N concentration, either in fresh or dried tissue, appeared a better diagnostic tool of N stress than did total shoot N concentration or content, because of sharper definition of critical concentrations. Concentrations of other nutrients in shoot tissue were affected differentially by both applied nitrogen and root disease, but generally did not reach critical levels, although phosphorus and magnesium appeared deficient in very disease-stressed plants.     

Interactive effects of salt and alkali stresses on seed germination,germination recovery,and seedling growth of a halophyte Spartina alterniflora (Poaceae)

Soil salinization and alkalinization frequently co-occur in nature, but there is little information on the interactive effects of salt and alkali stresses on plants. Seed germination and early seedling growth are crucial stages for plant establishment. We investigated the interactive effects of salt and alkali stresses on seed germination, germination recovery and seedling growth of a halophyte Spartina alterniflora. Seed germination percentage was not significantly reduced at low salinity (≤ 200 mM) at pH 6.63–9.95, but decreased with increased salinity and pH. Ungerminated seeds germinated well after transfer to distilled water from treatment solutions, indicating that seeds can remain viable in high salt–alkaline habits. Shoot growth was stimulated at low salinity and pH, but decreased with increased salinity and pH. Radicle elongation decreased sharply with increased salinity and pH, and was significantly inhibited when pH ≥ 9.0, indicating that the radicles are very sensitive to salt–alkaline stress. The deleterious effects of salinity or high pH alone were less than when combined. A reciprocal enhancement of salt and alkali stresses is a characteristic feature for salt–alkaline stress. Stepwise regression analysis indicates that salinity is the dominant factor, while pH and buffer capacity are secondary for salt–alkaline mixed stress.     

Genetic diversity and population genetic structure of saltmarsh Spartina alterniflora from four coastal Louisiana basins

Seventy-two Spartina alterniflora accessions originating from four coastal Louisiana basins (18 accessions per basin) were used to evaluate the genetic structure of this native perennial low-intertidal plant species. The objective of this study was to determine the population genetic structure and diversity of S. alterniflora accessions originating from these four basins using amplified fragment length polymorphism (AFLP) markers. A total of 250 unambiguous and highly repeatable AFLP markers, 186 of which (74.4%) were polymorphic, were obtained using four primer combinations. Overall, pairwise similarity estimates between accessions ranged from 0.70 to 0.93 (average = 0.80) with only a small portion of alleles (0.54–1.08%) unique to each basin. The average H_s (genetic diversity within coastal basins) was 0.20 with an H_s values of 0.19, 0.20, 0.20, and 0.21 for Mermentau, Terrebonne, Calcasieu, and Barataria-Breton basin, respectively. AMOVA analysis showed no genetic structure among basins, with the majority of genetic variation, 96.6%, residing within the basins. There was no indication of isolation by distance. Our results suggest that maintaining high levels of genetic diversity can be accomplished through the use of an adequate number of S. alterniflora samples collected within any large basin. Choosing parental lines from several Louisiana coastal basins for breeding purposes may not significantly increase genetic variability among the progeny lines.     

A growth chamber for the production of 14C-labeled salt marsh plants and its application to smooth cordgrass, Spartina alterniflora Loisel

An inexpensive growth chamber of simple construction is described and is shown suitable for the culture of Spartina alterniflora Loisel in a labeled (¹⁴CO₂) atmosphere. The smallest plants were least tolerant to transplant and trimming. The biomass which survived transplant was ≈ 684 g dry weight/m². Carbon dioxide uptake and O₂ evolution by growing plants and associated benthic microalgae were followed for 2 months. Increase of Spartina biomass indicated a whole plant net production of 713 g dry weight/m²/year. Spartina net production was 25 % of its gross production and 86 % of total net production (Spartina plus benthic microalgae). These Spartina production values and ratios are similar to those observed in natural Spartina marshes. Average specific activities of plant tissues were 42.4 gmCi/g C for new leaves and 16.0 and 14.0 μCi/g C for stem bases and roots, respectively. The tissues are suitable for experimental studies of breakdown and utilization of Spartina by microbial heterotrophs and detritus consumers. The chamber could be used in conjunction with existing techniques for seed germination and seedling growth to produce uniformly labeled Spartina plants.     

Positive interactions of the smooth cordgrass Spartina alterniflora on the mud snail Heleobia australis, in South Western Atlantic salt marshes

The role of positive interactions is often crucial in communities with intense abiotic stress such as intertidal environments. Grasses acting as ecosystem engineers, for example, may ameliorate intertidal harsh physical conditions and modify the community structure. The mud snails Heleobia australis d'Orbigny frequently inhabit the SW Atlantic marshes, mainly associated to intertidal marsh plants (mainly the smooth cordgrass Spartina alterniflora Loisel) probably due to the plant indirect effects. The purpose of this work was to investigate the magnitude of these association and the processes that generate the pattern. Samples of the snail abundance in six SW Atlantic coastal marshes show that H. australis is associated to coastal areas of low energy and low or none freshwater input. This result is important because this species is being used as bioindicator of coastal estuarine systems during the Holocene. Thus the paleontological interpretation based on this species should be revised. Within the studied areas, snails are associated to intertidal marsh plants. However, stable isotope analysis shows that neither plant nor their epiphytes are their main food sources. Field experiments show that snails actively select areas with plants, although tethering experiments show that plants do not provide shelter from predators. However, plants do buffer physical stress factors such as temperature, which generate important mortality outside plants covered areas. These positive interactions have large effects on H. australis distributions in marsh communities; increasing the habitats available for colonization and affecting their local distribution.