The effects of nutrients and moisture on soil nutrient availability, nutrient uptake, tissue nutrient concentration, and growth of Douglas-fir seedlings

A 4 x 4 factorial experiment was established to study the effects of soil nutrients (N) and soil moisture (M) on soil nutrient availability, seedling growth, nutrient uptake, and tissue nutrient concentration. Other objectives of the study were to test for the existence and significance of a nutrient x moisture interaction, to attempt to correlate some measures of seedling growth with levels of specific nutrients in seedling tissue and soil, and to ascertain whether certain correlations are affected by soil moisture level. The sixteen treatment combinations of nutrients and moisture were applied to potted Douglas-fir seedlings and arranged in a randomized complete block design on a greenhouse bench. Soil and tissue parameters generally displayed highly significant differences due to both moisture and nutrients. The magnitude of difference and the direction of change varied depending on the particular nutrient and moisture treatment (i.e. N x M interactions were generally highly significant), and the parameter in question. With tissue nutrient levels, differences also varied with the type of tissue (i.e. foliage, stems, or roots), and whether the level of the particular nutrient was expressed as concentration or content. Soil moisture was shown to have a strong indirect influence on the amount of available soil nitrogen as well as the form in which it is taken up by seedlings. The form of nitrogen not only appeared to influence seedling growth but may have also affected the uptake of other plant nutrients (e.g. P and Ca). High concentrations of available soil nitrogen appeared to upset delicate nutrient balances within seedlings, resulting in induced nutrient deficiencies and reduced seedling growth. Direct antagonisms between specific nutrients (e.g. N and P, K and Ca.) were also demonstrated. Increased soil nutrient supply generally had a strong negative influence on mean seedling root weight and root/shoot ratio. Mso, soil moisture and nutrients generally had favorable and unfavorable effects, respectively, on seedling mycorrhizae. Under conditions of moisture stress, foliar concentrations were shown to partially mask nutrient deficiencies. Only when the growth-limiting effect of soil moisture was alleviated did foliar concentrations of the limiting nutrient(s) fall to severe deficiency levels, even though the uptake of the deficient nutrient(s) generally increased. These results suggested a closer examination of the moisture-supplying ability of a site may be warranted when evaluating the results of foliar analysis. Critical foliar concentrations of some nutrients may in fact vary depending on site moisture conditions. Highly significant positive correlations between foliar nutrient content and seedling growth were generally obtained only for those soil nutrients which were inadequately supplied. Correlations were generally strongest in those treatments where soil moisture was not a growth-limiting factor. Results suggest that the higher site productivity often evident on 'seepage sites' might often be largely due to the favorable influence of soil moisture on nutrient availability and uptake, rather than to the increased amounts of nutrients supplied in the seepage water.