Responses of seedlings of selected woody species to soil oxidation-reduction conditions

Under greenhouse conditions, seedlings of baldcypress (Taxodium distichum), slash pine (Pinus elliottii), Loblolly pine (Pinus taeda) and pecan (Carya illinoensis) were subjected to two soil mixtures of (1) 100% soil, (2) 99% soil plus 1% organic matter; and two watering regimes: (1) continuously flooded to 5 cm above the soil surface and (2) watered frequently but non-flooded. The combination of watering and soil mixtures resulted in various intensities of soil reduction as determined by measuring soil redox potential (Eh, mV). Soil Eh averaged > +400 mV in control indicating a well-aerated condition. The two soil mixtures when flooded, resulted in average Eh of - 70 mV and - 160 mV. Plant gas exchange and growth responses to the treatments were measured. Mean values of net photosynthesis over the experimental period showed that photosynthesis in T. distichum was not affected significantly under - 70 mV treatment but was significantly lower in plants under - 160 mV treatment. In contrast, photosynthetic values were decreased significantly in other species in response to the reduced Eh treatments. Survival was 100% for T. distichum whereas it was as low as 10% in C. illinoensis under the most reducing soil treatment (-160 mV). Whole-plant biomass, as well as biomass components (leaf, stem, root) in T. distichum, did not change significantly in response to the treatments. In contrast, total biomass and biomass components decreased in the other species in response to low soil Eh treatments. Relative growth rate reflected the relative flood-tolerance ranking of the study species, i.e. T. distichum as the most flood-tolerant species displayed the least sensitivity, whereas other species displayed high levels of sensitivities to low soil Eh conditions. Based on the results, it was concluded that soil redox intensity imposes considerable influence on plant functioning and growth. The critical soil Eh intensity at which plant physiological functions and growth were adversely affected was species-specific. (C) 1998 Elsevier Science B.V. All rights reserved.