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

Paper 1Table 3. Retention of added C, N and P to sediments at termination of the c. 200-days longexperiments with Lobelia (Lob) and Littorella (Lit).C (mol m -2 ) TN (mol m -2 ) TP (mol m -2 )Treatmen Added LeftLobLeftLitAdded LeftLobLeftLitAdded LeftLobLeftLit0 % 0 0 0 0 0 0 0 0 00.1 % 5.8 4.6 -2.9 0.24 0.16 0.15 0.013 0.017 0.0080.2 % 11.6 3.5 5.8 0.49 0.24 0.60 0.026 0.024 0.0480.4 % 23.1 13.3 6.9 0.97 1.00 -0.10 0.051 0.047 0.0320.8 % 46.2 26.6 13.3 1.94 1.31 1.39 0.103 0.095 0.0631.6 % 92.4 58.9 39.3 3.89 4.02 1.98 0.206 0.121 0.149the two species, while retention wassignificantly smaller for C (44±35%), and alsotended to be lower for N (77±26%) than P (ttest;Table 3). Retentions were not significantlydifferent between Lobelia and Littorella.DiscussionIn natural low-organic sediments of low O 2consumption rates, high O 2 release frompermeable roots of isoetids builds-up O 2concentrations above air saturation in the porewaterin the light (Møller and Sand-Jensen2011a,b) and O 2 penetrates to more than 40 mmdepth in the sediments. Lobelia has leaf surfacesof low gas permeability and releases mostphotosynthetic O 2 in the light from the roots tothe sediment and takes up O 2 by the roots in thedark until the pore-water and the plant tissueturn anoxic (Møller and Sand-Jensen 2011a).Littorella’s leaf surfaces have 12-14 timeshigher permeability than Lobelia’s (Møller andSand-Jensen 2011b) and releases about 30% ofphotosynthetic O 2 from the roots in the light(Sand-Jensen et al. 1982), while O 2 uptake fromlake water in the dark ensures O 2 concentrationsin the leaf lacunae above 5-10 Pa (Møller andSand-Jensen 2011b), downstream transport toroots and release to sediments at about 20-40%of the rate in the light (Christensen et al. 1994).The more rapid recovery of O 2 penetrationdepths and disappearance of Fe 2+ after 0.1-0.4%organic enrichment of Littorella but not Lobeliasediments and the small efflux andaccumulation of Fe 2+ relative to DIC ofLittorella sediments suggest that O 2 supply ishigher to Littorella sediments than to Lobeliasediments. Three times higher leaf biomass, 1.3-1.5 times higher mass specific photosynthesis(Møller and Sand-Jensen 2011b) andappreciable dark release of Littorella(Christensen et al. 1994) can account for anestimated higher O 2 release to sediments withLittorella (20-40 mmol m -2 d -1 ) than Lobelia(10-20 mmol m -2 d -1 ).Well oxygenated conditions ofsediments with no or low organic enrichment(0.1%) ensure that aerobic respiration is theprominent respiration mode. O 2 depth profiles inthe light show one peak a few mm below thesediment surface due to photosynthesis ofbenthic microalgae and a second peak at 20 mmdue to O 2 release from isoetid roots (Pedersen etal. 1995, Sand-Jensen et al. 2005). High O 2concentrations permit conversion of NH +4 toNO - 3 that can drive denitrification during anoxiain two zones below a surface and a deeper zoneof maximum O 2 concentrations at depthintegratedrates of 500-700 µmol m -2 d -1 in fieldmeasurements on Littorella and laboratoryexperiments with Lobelia (Christensen andSørensen 1986, Risgaard-Petersen and Jensen1997). The experiments with Lobelia showedthat denitrification was stimulated by a factorgreater than six when compared to baresediments and this enhanced activity was due to+root mediated oxic stimulation of both NH 4-release and NO 3 formation driving35