Seasonal variability of adsorption and exchange equilibria in soil waters

Chemical analyses for major ions have been conducted on waters, collected o n an approximately weekly basis over the period April, 1993 to November, 19 96, that drain three small experimental ecosystems ("sandboxes") at Hubbard Brook, New Hampshire. One sandbox is planted with pine trees, another with grass, and the third is left "bare" (actually it is covered sporadically b y bryophytes and lichens). Results show linear correlations, independent of discharge, between the concentrations of dissolved Na+ and K+ on the one h and and Ca++ and Mg++ on the other for all three sandboxes. No correlations between singly charged and doubly charged cations were found. These correl ations are interpreted to represent cation exchange equilibria between soil waters and clay minerals plus soil organic matter. The correlation slope, representing the exchange constant, for Na vs K is different for the pine-c overed sandbox than for the other two whereas for Ca vs Mg the correlation is independent of the presence or absence of trees. We interpret this as re presenting a shift of cation exchange equilibria in the pine sandbox by the activities of growing trees. Concentrations of Na, K, Ca, Mg, and H4SiO4 from the barren and grass-lined sandboxes were found to vary seasonally with a marked sinusoidal pattern w hich was independent of the discharge from each sandbox. (The discernment o f a similar pattern in the tree lined sandbox was difficult due to a lack o f discharge over much of the year.) Concentration maxima occurred in August and minima in February, and there is a close parallelism with soil tempera ture. We interpret this as representing temperature induced variations in c ation exchange equilibria and silica adsorption. Independence from highly v arying water discharge, e.g.,. that accompanying severe rainstorms, indicat es rapidly re-attained equilibrium. Variations in the concentrations of cat ions are likely due to exchange with unmeasured cations, probably H+ or dis solved Al species, as a result of possible seasonal changes in internal aci d production and external input of acid rain to the sandboxes. Internal pro duction may represent a response to seasonal changes in respiration rate as it responds to seasonally varying temperature. Added to this is the effect of temperature on exchange equilibrium. Seasonal variations in dissolved s ilica are most likely due to the dependence of adsorption/desorption equili bria on temperature. The temperature dependence of a number of silica-consu ming reactions are consistent with the measured values.