Oxygen dynamics and plant-sediment interactions in isoetid ...

Paper 4Hyphal densities of isoetid sediments arecomparable to terrestrial soils having typicallengths of hyphae between 2 and 40 m per cm -3(Smith and Read 2008). Densities were low inthe upper 1 cm of the isoetid sediment probablydue to disturbance by wave exposure andbioturbation and pH-changes due tophotosynthesis of microalgae (Jasper et al. 1991;Van Aarle et al. 2002). In the root zone, hyphaldensity was high and correlated positively withroot density. With lengths of hyphae relative toroots of 700 and conservative estimates ofhyphal surface area exceeding that of rootsurface areas by 1.7- 3.2 times in low-organicsediments the AMF-isoetid symbiosis results ina large increase of sediment contact for nutrientretrieval. Lengths of hyphae relative to infectedroots were also comparable to terrestrial potexperiments with Trifolium subterraneuminoculated with Glomus fasciculatum (450-900,Abbott et al. 1984) suggesting quantitativesimilarity to well-oxygenated nutrient-poorisoetid sediments.In contrast to AMF root colonizationpercentages, no decrease was observed inhyphal density in sediments following organicenrichment. In fact, addition of organic mattertended to increase hyphal density (Fig 4). Thisresult could, however, be due to staining of bothliving and dead hyphae because degradation ofdead hyphae can be slow in anaerobicsediments.Beck-Nielsen & Madsen (2001) havereported hyphal densities from aquaticsediments inhabited by Littorella at 3-20 timeslower density than in the present investigation.They found hyphal densities to decrease withlower redox potential of sediments. However,different redox potentials were found bysampling in sediments differing in plant densityand, therefore, relations could alternatively arisefrom differences in root densities which werehighly correlated to hyphal density in thepresent investigation. Sanders et al. (1998)reported of increased hyphal growth interrestrial soils under elevated CO 2 levels andsimilarly Andersen & Andersen (2006) foundincreased AMF colonization of Littorella grownsubmerged at 10 times CO 2 air saturation in thewater. Addition of organic matter and thefollowing increase in CO 2 could be responsiblefor the observed tendencies of higher hyphaldensities upon organic enrichments but coupledinvestigations of hyphae growth and AMFcolonization are needed to address this questionfurther.Sediment biogeochemistryAMF colonization decreased when organicadditions deprived sediments of O 2 and Fe 2+accumulated in the pore-water. Redox potentialswere not measured, but in sediments at normalpH levels (≈7) thermodynamics favor theoccurrence of Fe 2+ as the primary form below 0mV, while the presence of O 2 leads to rapid Fe 2+oxidation to Fe 3+ (half-life of minutes; Canfieldet al. 2005). With anoxic sediments andcertainly low redox potentials following organicenrichment the results are consistent with thoseof Beck-Nielsen & Madsen (2001) showingreduced AMF colonization in roots at redoxpotentials below 250 mV. Recent experimentsby Møller & Sand-Jensen (2011) showed thatanoxia occurred in oligotrophic isoetidsediments during nighttime, and anoxia evendeveloped in root and leaf aerenchyma ofLobelia late at night. This result implies thatAMF can cope with anoxia for at least some81