Nutrient composition of Douglas-fir rhizosphere and bulk soil solutions

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Abstract

Rhizosphere soil solution is the direct source of nutrients for plant uptake. The nutrient composition of rhizosphere soil solution can be very different from that of bulk soil solution due to root exudation, nutrient uptake and rhizosphere microorganism activity. This study examined the nutrient composition of Douglas-fir rhizosphere soil solution in two soils belonging to the Nisqually and Pitcher soil series and compared rhizosphere solution with that of bulk soil solution. Fertilized and unfertilized Nisqually soils were also compared. Soil solutions were collected using centrifugation. Results indicated that nutrient concentrations in the rhizosphere solutions were typically higher than that of bulk soil solutions when no fertilizer was applied. Differences in the concentrations of nutrients between the rhizosphere and bulk soil solutions were masked by the addition of fertilizers. Rhizosphere solution pH also appeared to be affected by the concentration of NH4 and NO3 in the solution. With a higher concentration of NH4 relative to NO3 in the rhizosphere soil solution, the solution pH of the rhizosphere was lower than that of the bulk soil, but with a lower concentration of NH4 relative to NO3, the solution pH of the rhizosphere was higher than that of the bulk soil solution.

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