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

NewPaper 2Phytologist Research 323Värsjö as used for collection of sediment turf for laboratoryexperiments to make sure that patterns observed in laboratoryexperiments resembled natural sediment and plantresponses in the field.A long-term organic enrichment experiment was madeover 80 d in the field (10 July–1 October) in which sedimentchemistry and plant performance were comparedbetween triplicate controls and treatments (20 · 20 cm perplot) enriched with organic matter to 0.8% DW asdescribed for the laboratory experiment. At the end of theexperiment, pore-water concentrations and sediment compositionwere measured on sediment cores (5 cm indiameter, 10 cm in depth) taken from each plot andbrought back to the laboratory in Perspex tubes with rubberstoppers preventing disturbance of sediment and porewater. Cores were stored in the laboratory at 15°C ina12 : 12 h light : dark cycle for 1–4 d before pore water wasextracted and analysed as already described. Likewise, netphotosynthesis was measured in the laboratory incubator onleaf samples from each plot. Leaf samples derived fromharvested field plants were transported to the laboratory insealed moist plastic bags and stored at 8°C and used forexperiments within 2 d.O 2 dynamics was measured continuously over a full 24 hlight–dark cycle in the field in late August in duplicated experimentsby inserting microelectrodes into leaf lacunae of twoplants and inserting mini-electrodes at 1 cm depth in thesediment next to these two plants using the same type of equipmentas for laboratory experiments. HOBO data loggers wereused to measure water temperature and irradiance (OnsetComputer Corporation, Bourne, Massachusetts, USA).Data treatment and statistical analysisData were processed in Excel 2007 and statistical analysesand graphs were made in Graph Pad Prism 5. Data arepresented as means ± SD. P < 0.05 was considered significant.The in situ experiment was a block design in triplicatesuitable for t-test analysis, whereas the laboratory experimentwas a gradient study with six amounts of added organicmatter to single sediment turfs inhabited by 20 plants(pseudo-replicates) suited for regression ⁄ correlation analysis.Values from the laboratory experiment are presented asmeans ± SD of usually three replicates from each sediment turf.ResultsSediment processes and chemistryOrganic enrichment of sediments in long-term laboratoryexperiments stimulated O 2 use for degradation of organicmatter and reduced O 2 depth penetration (Fig. 1). O 2 penetrationdepth exceeded 40 mm in control sediments butdeclined to < 3 mm after 25 d in sediments fertilized withFig. 1 Depth of O 2 penetration in Lobelia sediments as a functionof time after addition of different amounts of labile organic matter(per sediment DW) (open circles, control; closed circles, 0.1%; opensquares, 0.2%; closed squares, 0.4%; open diamonds, 0.8%; closeddiamonds, 1.6%). Measurements were made 10–12 h into the 12 hlight period. Measurements could not extend deeper than 40 mminto the sediment, when O 2 penetration in control sediments wasdescribed as > 40 mm. Values are mean of two (0–170 d) and three(194 d) measurements ± SD.‡ 0.4% organic matter. In sediments receiving only 0.1 and0.2% organic matter, O 2 penetration depth had onlydeclined to 21 and 10 mm, respectively, 25 d after fertilization.As degradation of organic matter progressed overtime, O 2 gradually penetrated deeper into the sediments.However, only at the lowest organic dose did O 2 penetrationrecover to the same value (> 40 mm) as in the controlsediment within the 195 d of experiments. At the end ofthe experiment, O 2 penetration depth was significantlyand negatively correlated to the magnitude of addition(Spearman’s r, P < 0.001). Thus, organic enrichment hadprofound and long-lasting effects on O 2 availability anddecomposition processes.This persistent enrichment effect was evident in sedimentchemistry even at the lowest enrichment after 194 d inthe laboratory experiment (Table 1). Across the gradient,organic content, TN and TP increased two- to threefoldand they were significantly correlated to the amount oforganic matter added (linear regression, P < 0.0001), whilewater content and TFe did not change significantly.Approximately 14.5 mg organic matter, 368 lg TN and43 lg TP (all g –1 dry sediment) were added with the highest1.6% organic enrichment and the treated sediment stillcontained an extra 10.2 mg organic matter, 382 lg TNand 25.5 lg TP relative to the control sediment after194 d. Likewise, the 0.4% organic treatment received anadditional 3.6 mg organic matter, 92 lg TN and 9.8 lgTP per g dry sediment and still contained extra 2.3 mgorganic matter, 96 lg TN and 9.8 lg TP after 194 d,stressing that organic matter was lost by degradation butmost N and P remained in the sediment.Ó 2010 The AuthorsNew Phytologist Ó 2010 New Phytologist Trust44New Phytologist (2011) 190: 320–331www.newphytologist.com