Growth responses following nitrogen and N–P–K–Mg additions to previously N-fertilized Scots pine and Norway spruce stands on mineral soils in Sweden

To study growth responses to refertilization, 12 Scots pine (Pinus sylvestris L.) and 6 Norway spruce (Picea abies (L.) Karst.) stands used in long-term nitrogen (N) fertilization experiments were refertilized with N, either alone or with various combinations and doses of phosphorus (P), potassium (K), and magnesium (Mg). Many of the experimental plots had previously been subjected to heavy N fertilization regimes over a period of 20–30 years. On average, for all the experiments, the latest N addition resulted in a significant growth increase, corresponding to 57% of the mean annual volume increment and comparable with the response to the initial fertilization. Differences in growth response between fertilization with N alone or in combination with P–K–Mg were in most cases insignificant for both tree species. Overall, the joint addition of P–K–Mg resulted in a nonsignificant additional growth increase of 0.2 m3·ha–1·year–1, corresponding to 6% of the N fertilization effect. Repeated additions of N alone had no effect on the P, K, and Mg concentrations in current-year needles. It was concluded that the repeated N fertilizations did not cause any serious nutrient deficiencies.