Nutrient availability and flooding stress interact to affect growth and mercury concentration in Taxodium distichum (L.) Rich. seedlings

• Our data contradicted the idea that plants produce more roots to forage for nutrients when nutrients are less abundant.
• Nutrient availability and flooding stress interacted to influence Taxodium distichum growth with the interaction dominated by flooding stress.
• The C:N and N:P ratio in leaves could not be used to indicate nutrient limitation in Taxodium distichum because flooding also altered those ratios.
• Taxodium distichum leaves accumulated higher concentrations of Hg than soil; foliar Hg concentrations increased in response to flooding during the first year and in response to nutrient availability during the second year.

Plant growth can be limited by nutrient availability and/or flooding stress but their relationship is understood too poorly to predict when they act independently or to predict which dominates the relationship. Nutrient availability also reduces uptake of toxic metals under some conditions but enhances uptake under others. We studied how flooding stress and nutrient availability affected growth and mercury uptake by Taxodium distichum (L.) Rich., which is a tree that dominates freshwater wetlands in the southeastern United States. We hypothesized that nutrients would negate flooding stress and enhance mercury uptake. We also hypothesized that leaf chemistry could serve as an indicator of factors limiting growth. We planted 432 one-year old seedlings in soil with one of four nutrient availabilities (no enhancement, or enhanced with 57, 283, or 1132 g N m−2 and other nutrients) and allowed the seedlings to grow for two seasons with one of six flooding regimes ranging from <1% to 96% flooding each month. The highest level of nutrient availability killed 82% of the seedlings almost immediately but after one year, nutrient availability and flooding interacted such that seedlings given 57 g N m−2 negated effects of flooding stress on stem length. After two years, nutrient availability and flooding stress interacted such that seedlings given 283 g N m−2 grew more than less fertilized seedlings except in the most flooded treatment. Mercury in leaves was related to flooding after the first year but to nutrient availability after the second year. The C:N and N:P ratio in leaves could not be used to indicate nutrient limitation in T. distichum because flooding also altered those ratios. The Ca:Mg ratio was related to nutrient availability but we found no chemical indicator of flooding stress. Research is needed to determine if flooding enhanced uptake of atmospheric mercury by leaves or soil mercury by roots.