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Repeated waterlogging affects soil conditions and growth and physiology of Scots pine saplings

Timo Domisch, Tapani Repo, Sirpa Piirainen, Leena Finer, Qian Ji, Tarja Lehto and Izabela Sondej

Soil waterlogging (WL) during the growing season is known to have adverse effects on soil conditions and plant growth. This is the case especially on peatlands, where groundwater tables are high and can vary within the growing season, even on drained peatlands. For assessing the effects of repeated WL periods on soil conditions and root and shoot growth of 4-year-old Scots pine saplings, we conducted a laboratory experiment lasting for 3 simulated growing seasons. During the second growing season, the plants were subjected either to a 2.5 week anoxic period or three repeated WL periods of the same total time of anoxia. We measured concentrations of soil oxygen (O2), carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O). In addition we assessed shoot and root growth and biomass and foliage carbohydrate status. Waterlogging had immediate and distinctive effects on soil gas conditions: anoxic soil conditions were observed during the WL periods, but O2 concentrations returned immediately to normoxia when WL ceased. Respectively, CO2, CH4 and N2O concentrations increased instantaneously with WL and decreased thereafter. Waterlogging decreased root growth and survival and increased the proportion of dead roots. Waterlogging reduced also shoot growth, especially one growing season later as an after-effect. Starch concentrations in the needles increased during WL, indicating a missing sink as root growth was impaired. The WL saplings showed increased raffinose and pinitol concentrations in the needles. These sugars act as antioxidants under abiotic stress conditions and their role might be connected with the mitigation of the effects of reactive oxygen species during re-oxygenation after WL ceased. The rather short WL periods in our experiment did not lead to lethal effects on the Scots pine saplings but our results suggest that even short-termed WL periods can have significant consequences on soil conditions and plant growth.