Nutrient supply changes within a growing season in two deciduous forest soils

Little information is available on how the indices of nutrient availability used in mechanistic nutrient uptake models might be influenced by the temperature at which the equilibrium soil solution (C,) is extracted or how this parameter, and the solid-phase nutrient concentration (C s ) might be altered by the progression of the growing season. Objectives of this study were to determine: (i) if the laboratory extraction temperature would influence C l estimates, and (ii) the degree of change in C l and C s during the growing season. Soil samples were collected from two forest sites in eastern Tennessee in April, July, and October. Samples were returned to the laboratory and equilibrium soil solution concentrations of NO 3 -N, NH 4 -N, P, K, Ca, and Mg determined at the average soil temperature (11-18°C) or at laboratory temperature (24°C). Solid-phase (C s ) concentrations were also determined and both values used to calculate the buffer power (b) and the effective diffusion coefficient (D e ). Extraction temperature did not alter C l values. All solution nutrient concentrations except P declined by 16 to 99% after the July sample. Increases in P in the A horizon at one site were as high as 100%. Solid-phase nutrient concentrations exhibited declines ranging from 6 to 91% while solid-phase P concentrations increased by up to 40%. Disproportionate changes in the C l and C s values lead to changes in the calculated b and D e estimates. Results indicate that efforts directed at establishing soil supply parameters needed for mechanistic nutrient uptake models should take into consideration changes that occur in solution and solid-phase nutrient concentrations during the growing season.