Brian K. Sorrell

Are Phragmites-dominated wetlands a net source or net sink of greenhouse gases?

Phragmites australis wetlands act as a sink for greenhouse gases by photosynthetic assimilation of carbon dioxide (CO2) from the atmosphere and sequestration of the organic matter produced in the wetland soil. The wetlands also act as a source for greenhouse gases by emission of sediment-produced methane (CH4) to the atmosphere. In P. australis wetlands, the dominant mechanism of CH4 release to the atmosphere is internal gas transport in the plants, primarily by pressurized convective gas flow. The time periods of carbon fixation and CH 4 release therefore vary seasonally and diurnally. The balance between net CO2-assimilation and CH4 emission determines if a wetland can be regarded as a net sink or a net source of greenhouse gases, and hence, the function of the wetland in relation to global climate change. On an annual basis up to 15% of the net carbon fixed by the wetlands may be released to the atmosphere as CH 4. Because of the different infrared absorption characteristics and atmospheric longevity of CH4 and CO2, the warming effect of CH4 in the atmosphere is about 21 times higher on a mass basis than CO2 over a 100-year timescale. Thus, the immediate carbon balance, coupled with the different physical characteristics of the two gases, would suggest that although some wetlands function as a net sink for CO2, the wetlands still increase the greenhouse effect because of their release of CH4. However, the short adjustment time for CH4 in the atmosphere means that, over a longer time scale, the radiative forcing of CH4 is less relative to CO2 and the wetlands effectively become a sink for greenhouse gases. Wetlands may therefore be regarded as a source for greenhouse gases and so increase radiative forcing if evaluated on a short time scale (decades), but as a sink for greenhouse gases and thus attenuating radiative forcing if evaluated over longer time scales (>100 years).

GAS FLUXES ACHIEVED BY IN SITU CONVECTIVE FLOW IN PHRAGMITES AUSTRALIS

The Common Reed (Phragmites australis Cav. Trin. ex Steud.) possesses an outstanding capacity to vent its underground tissues by pressurized through-flow. Phragmites-dominated wetlands therefore potentially provide a significant source of trace gas emissions to the atmosphere. In this paper we present results of in situ studies on gas exchange through Phragmites, and evaluate various methodologies used for measuring gas transport and the fluxes they record. Gas exchange rates were related to atmospheric humidity, temperature and light. Green shoots were influx culms and dead culms and broken or damaged green shoots were efflux culms. Gas exchange through the plants fluctuated diurnally, with highest rates in the early afternoon (up to 11 l m−2 h−1) and lowest rates during the night. The net flux of O2 to the below-ground tissues and sediment was up to 5.7 1 m−2 day−1, and the net emissions of CO2 and CH4 up to 5.1 and 0.27 l m−2 day−1 respectively. Net gas exchange rates varied with season and sediment characteristics, being highest during hot and dry summer days, and on organic sediments with a high oxygen demand and high rates of microbial decomposition. Hence, the convective throughflow mechanism in Phragmites not only accelerates gas exchange between the sediment and the atmosphere, but the oxygen delivered through the plant may also affect the microbial processes in the sediment. Therefore, the role of the plants for rhizosphere oxidation and conveyers of gases should be further assessed in future studies. A comparison of current methods for measuring flow suggested that they need refining if they are to quantify gas exchange through Phragmites wetlands on a large scale or over longer time periods.

Growth and root oxygen release by Typha latifolia and its effects on sediment methanogenesis

Growth of Typha latifolia L. and its effects on sediment methanogenesis were examined in a natural organic sediment and a sediment enriched with acetate to a concentration of 25 mM in the interstitial water. The lower redox potential and higher oxygen demand of the acetate-enriched sediment did not significantly impede growth of T. latifolia despite some differences in growth pattern and root morphology. Plants grown in acetate-enriched sediment were ca. 15% shorter than plants grown in natural sediment, but the former produced more secondary shoots at earlier stages, which resulted in similar total biomasses after 7 weeks of growth in the two sediment types. Plants grown in acetate-enriched sediment had thicker and much shorter roots than plants grown in natural sediment. This difference did not significantly affect the release of oxygen from the roots when measured under laboratory conditions, which was 0.12‐0.20 mmol O2 g ˇ1 DW h ˇ1 . Enrichment with acetate resulted in much higher sediment methanogenesis rates (643 vs. 90 nmol CH4 g ˇ1 sediment DW h ˇ1 ). Growth of T. latifolia significantly reduced methanogenesis in both types of sediment, but the effect was twice as marked in the natural sediment (34%) as in the acetateenriched sediment (18%), although in absolute terms the reduction was higher in the enriched sediment. The data suggest that this effect of plant growth was via root oxygen release and its effect on redox conditions. In the natural sediment, oxygen release resulted in a significantly higher redox potential and lower sediment oxygen demand, whereas there were no significant changes in the acetate-enriched sediment. The very high oxygen demand of this sediment probably masked the effect of root oxygen release so that a significant reduction in methanogenesis occurred without any significant increase in the redox potential. This demonstrates how root oxygen release from plants like T. latifolia can significantly alter rates of biogeochemical processes such as methanogenesis,

Convective gas flow in Eleocharis sphacelata R. Br.: methane transport and release from wetlands

Abstract We investigated the importance of lacunar gas transport for the release of methane from beds of Eleocharis sphacelata R. Br. in a freshwater wetland in southeastern Australia. Gases were transported in the E. sphacelata lacunar system by pressurized convective flow, which was apparently driven by humidity induced pressurization. Internal culm temperatures were 1.9–4.2° C lower than at the external culm surface, suggesting that thermal transpiration was not responsible for lacunar pressurization. Some of the culms (influx culms) provide a net gas flux from the atmosphere into the plant, whereas others (efflux culms) act as conduits for flux from the rhizosphere to the atmosphere. The mean gas influx was 1.0±0.2 (SE) ml min−1 per culm, and the mean gas efflux was 0.2±0.0 (SE) ml min−1 per culm. The difference in influx and efflux flow rates is due to unequal numbers of the two culm types, and the total gas flux through three adjacent E. sphacelata beds was estimated from this flow ratio and the total culm density. It ranged from 1.1 to 2.5 1 m−2 h−1. The methane concentration in the efflux culms was 2–3%, resulting in a total methane efflux from E. sphacelata of 22–75 ml CH4 m−2 h−1. These rates represented 1–15 times the rate of methane release from the E. sphacelata beds by ebullition of methane in bubbles released from the sediment. Diffusive methane fluxes in the lacunar s system (

CONTROLS ON SOIL CELLULOSE DECOMPOSITION ALONG A SALINITY GRADIENT IN A PHRAGMITES AUSTRALIS WETLAND IN DENMARK

Although soil organic matter decomposition is an important process determining nutrient transformations and availability in wetland ecosystems, few studies have attempted to assess which environmental factors are most important in controlling spatial differences in decomposition rates found along environmental gradients. Relative soil decomposition was determined in a Phragmites australis Cav. Trin ex Steudel dominated wetland in northern Jutland, Denmark along a natural salinity gradient, where nutrients, soil moisture, temperature and salinity among other factors also varied. Our objective was to identify which edaphic factors most limited rates of relative soil decomposition, as evaluated by measuring cellulose decomposition with the cotton strip technique. Replicate cotton strips were placed at seven marsh sites along the salinity gradient, and soil and interstitial water samples were collected and analyzed for major macro- and micronutrients (NH4‐N, NO3‐N, P, PO4, K, Mg, Ca, Na, S, Fe, Mn, Zn, Cu, Mo, B, Si), pH, Eh, conductivity, temperature, and soluble sulfides. Cellulose decomposition, expressed as cotton tensile strength loss (CTSL) per day, decreased with increasing salinity, except at the highest salinity site where a significant increase occurred. Mean CTSL values, averaged for each marsh site, varied 3-fold from 1.8 to 5.5% loss per day. Principal component and multiple regression analyses were used to prioritize the importance of the various factors that might control this spatial difference in CTSL rates. Although soil conductivity (salinity) accounted for the large percentage (45%) of the variation in the environmental data, soil fertility- and soil reduction-associated variables explained the greatest percentage (56%) of the spatial variation in cellulose decomposition. Univariate correlation analyses supported the conclusion that soil fertility, primarily inorganic nitrogen and phosphorus, is the major

Invasion strategies in clonal aquatic plants: are phenotypic differences caused by phenotypic plasticity or local adaptation?

BACKGROUND AND AIMS The successful spread of invasive plants in new environments is often linked to multiple introductions and a diverse gene pool that facilitates local adaptation to variable environmental conditions. For clonal plants, however, phenotypic plasticity may be equally important. Here the primary adaptive strategy in three non-native, clonally reproducing macrophytes (Egeria densa, Elodea canadensis and Lagarosiphon major) in New Zealand freshwaters were examined and an attempt was made to link observed differences in plant morphology to local variation in habitat conditions. METHODS Field populations with a large phenotypic variety were sampled in a range of lakes and streams with different chemical and physical properties. The phenotypic plasticity of the species before and after cultivation was studied in a common garden growth experiment, and the genetic diversity of these same populations was also quantified. KEY RESULTS For all three species, greater variation in plant characteristics was found before they were grown in standardized conditions. Moreover, field populations displayed remarkably little genetic variation and there was little interaction between habitat conditions and plant morphological characteristics. CONCLUSIONS The results indicate that at the current stage of spread into New Zealand, the primary adaptive strategy of these three invasive macrophytes is phenotypic plasticity. However, while limited, the possibility that genetic diversity between populations may facilitate ecotypic differentiation in the future cannot be excluded. These results thus indicate that invasive clonal aquatic plants adapt to new introduced areas by phenotypic plasticity. Inorganic carbon, nitrogen and phosphorous were important in controlling plant size of E. canadensis and L. major, but no other relationships between plant characteristics and habitat conditions were apparent. This implies that within-species differences in plant size can be explained by local nutrient conditions. All together this strongly suggests that invasive clonal aquatic plants adapt to a wide range of habitats in introduced areas by phenotypic plasticity rather than local adaptation.

Oxygen stress in wetland plants : comparison of de-oxygenated and reducing root environments

1. Growth, photosynthesis and root adenine nucleotides were compared in two wetland plants, Phalaris arundinacea and Glyceria maxima, grown in aerated, de-oxygenated or reduced (redox potential = -250 mV) nutrient solutions, to test the hypothesis that the stress of de-oxygenated conditions is mild compared with that of naturally reducing sediments. 2. Relative growth rate (RGR) was not significantly different between plants in the aerated and de-oxygenated treatments. However, plants in the reducing treatment stopped growing and some lost mass (RGR negative). Differences in root porosity were not significant between treatments. 3. Rates of net photosynthesis in both species were stable in the de-oxygenated and aerated treatments, at 3-8 μmol CO 2 m -2 s -1 . However, net photosynthesis in the reducing treatment declined over 5 days, becoming negative in P. arundinacea and falling below 1 μmol CO 2 m -2 s -1 in G. maxima. 4. Concentrations of adenine nucleotides in the roots of both species were significantly but only slightly lower in the de-oxygenated treatment than in the aerated treatment (0.65 times as much ATP and 0.70-0.87 times as much total adenine nucleotides). However, nucleotide concentrations were much lower in the reducing treatment (0.10-0.19 times as much ATP and total adenine nucleotides as the aerated treatment). Both species tolerated the de-oxygenated treatment but were sensitive to the reducing treatment.

Die-back of Phragmites australis : influence on the distribution and rate of sediment methanogenesis

Methanogenesis was measured during the summer of 1994, in sediment coresand bulk samples from a Phragmites australis wetland in northern Jutland,Denmark. We compared sediment from healthy reed and dying-back reed, andan open lagoon resulting from die-back. Cores revealed variability withdepth and between sites, with the highest rates coinciding with layers oforganic gyttja, and negligible methane production from the underlying sandbase. Methanogenesis rates in the lagoon and die back sites were higher(up to 100–150 nmol h-1 g-1dry wt. sediment) than in the healthy reed (50–80 nmolh-1 g-1), with the highest rates being recordedfrom May to July. At these times, methanogenesis was markedly temperature-limited; samples incubated at 30 °C anon-limiting temperature, gave rates as high as 200–400nmol h-1 g-1 for the lagoon and die-backareas and 150 nmol h-1 g-1 for the healthyarea. Addition of 8 mM acetate and H2/CO2headspace suggested that both acetate-fermenting andCO2-reducing bacteria were present. Acetate additions suggested some co-limitation by substrate availability, with acetate limitation occurring in the healthy site during July and in the die-back site during August. Lower rates during August, especially in the healthy area, were associated with low water levels which resulted in more oxidized sediments. The data reveal highly variable methanogenesis in the sediment which, when considered with sediment depths, indicates that sites of Phragmites die-back have significantly greater rates of anaerobic mineralization than surrounding healthy wetland, and may be intense sources of methane.

Regime shifts between clear and turbid water in New Zealand lakes: Environmental correlates and implications for management and restoration

Abstract We reviewed lakes in New Zealand reported to have undergone regime shifts between macrophyte‐dominated clear water states and de‐vegetated, turbid states. Regime‐shifting lakes (RSLs) occurred along a wide latitudinal gradient. We obtained catchment land‐use data as well as data on the occurrences of introduced (non‐indigenous) macrophytes and herbivorous and benthivorous fish for the 37 RSLs and for 58 lakes with similar maximum depths and climates, but which had not been reported to have undergone regime shifts. All RSLs had a maximum depth <20 m and mean annual surface air temperature between 9 and 16°G Regime shifts were positively related to the percentage of the catchment in pasture and negatively related to the percentage of the catchment in forest. The occurrences of the introduced macrophyte Egeria densa and the introduced fish, Ameiurus nebulosus (catfish), Carassius auratus (goldfish), Scardinius erythrophthalmus (rudd), Cyprinus carpio (koi carp), and Tinca tinca (tench), were significantly correlated to regime shifts in lakes. Although the presence of other introduced aquatic macrophytes was not significantly correlated with RSLs, the number of exotic fish taxa present in lakes was strongly positively correlated with increasing prevalence of regime shifts. The strength of the correlations between land use and introduced species versus regime shifts illustrates a number of factors which could be managed to reduce the susceptibility of lakes to regime shifts and to restore lakes that have become de‐vegetated. Our findings also suggests that regime shifts in lakes were unlikely to have been common in New Zealand lakes before anthropogenic deforestation and introductions of introduced aquatic taxa.

Exploring the borders of European Phragmites within a cosmopolitan genus

European Phragmites australis is one of four main cp-DNA haplotype clusters present worldwide. The European gene pool extends from North America to Far East Asia and South Africa. Extensive gene flow occurs only within the temperate region of Europe.