Mechanisms Affecting Soil Solution Nutrient Concentrations Following Oak Removal

Soil solution data indicate that oak tree removal results in a rapid shift in soil solution nutrient concentrations toward that of the adjacent grassland soils. In addition to reductions in nutrient concentrations, soil solution pH and DOC concentrations were significantly decreased. It appears that the cut plots are quickly reverting to nutrient conditions similar to plots with no canopy influence. This suggests that the long-term trend is for rapid loss of nutrients from soils beneath oak canopies following oak removal. An important question raised by these data is: Why is there such a rapid decrease in soluble nutrient concentrations following tree removal?

The three primary mechanisms by which oak trees affect soluble nutrient concentrations are plant uptake, canopy throughfall returns, and litterfall returns. Removal of the above-ground tree biomass eliminates nutrient uptake which should result in increased concentrations of soluble nutrients as has been shown is several studies examining the effects of clearcutting. Therefore, we can eliminate plant uptake as the primary mechanism for decreased soluble nutrient concentrations. Inputs of soluble nutrients via canopy throughfall are only a small fraction of the concentrations found in the A horizon. Additionally, nutrients leached from the canopy must be replaced by plant uptake which results in little if any net effect on nutrient pools. Thus, canopy throughfall processes do not explain the rapid loss of soluble nutrients from the cut plots.

Annual litterfall returns are the major mechanism for return of nutrients to the soil surface by oak trees. Assuming that the organic matter pool is in steady-state, nutrient inputs must equal nutrient outputs meaning that the flux of nutrients returned via litterfall each year is released by organic matter decomposition/mineralization. Examination of the organic matter accumulated beneath the oak canopy indicates that the majority of the litterfall is decomposed to unrecognizable detritus within two years after falling to the soil surface. Since the oak trees were removed prior to senescence and acorn release, a large pool of nutrients was removed with the trees. Thus, we conclude that elimination of litterfall returns was the primary mechanism responsible for the rapid attenuation of soluble nutrients following oak tree removal.