Phosphorus and nitrogen fertilization of a Norway spruce forest-effects on needle concentrations and acid phosphatase activity in the humus layer

Increased atmospheric deposition of N might eventually lead to P deficiency. The relation between needle P concentration and acid phosphatase activity in the humus layer was studied during 1990–93 in a Norway spruce stand where the water and N and P supplies had been experimentally manipulated since 1988. Treatments included control (C), yearly application of ammonium sulphate (NS), N-free fertilizer (V), granulated wood ash (A), irrigation (I), drought (D) and water plus nutrients in an optimum combination (IF). We found indications of a feed-back mechanism for P, where low concentrations in the needles were associated with increased acid phosphatase activity in the humus layer. Acid phosphatase estimations made during moist soil conditions were much more informative than those made during dry conditions. We further argue that a site-specific base-line exists for acid phosphatase activity in the soil, mainly originating from enzymes immobilized in the field, but active in the assay. Increased phosphatase activity, above the base line, was generally found in the A, I and NS treatments, but in some cases also in C. Although P and N concentrations were significantly higher in the IF treatment as compared to the C and the D treatments, the P as fraction of N was 0.10 and thus balanced in all cases. In the A and I treatment P:N was around 0.09, while it was only 0.07 in the NS treatment, mainly due to high N concentrations. The latter treatment thus created an imbalanced situation where P additions most likely would have increased tree growth.