Rhizospheric P and K in forest soil manipulated with ammonium sulfate and water

This study shows how the stress manipulations, ammonium sulfate, drought and irrigation changed the P and K status of three soil fractions (bulk soil, rhizosphere and soil-root interface) from E, Bh and Bs horizons of a Norway spruce stand in southwestern Sweden. The results indicate that dynamic linkages exist between the three soil fractions and tree growth. Accumulation rather than depletion of P and K were observed in the vicinity of the roots which we attribute primarily to accumulated organic matter and high rates of mineralization. Relative depletion of P and K around roots resulted from treatments that stimulated tree growth and P and K uptake. The ratio of organic P, the largest extractable P fraction, to inorganic P indicated that organic P is an important P resource when inorganic P is heavily utilized in treatments where growth has increased. There are indications that K supply was reduced by stimulating growth with ammonium sulfate and irrigation, suggesting that K rather than P can become the second limiting factor to growth at this site after N.