Early and repeated nutrient additions support far greater stemwood production in Norway spruce than traditional late-rotation fertilisation

Silvicultural techniques aimed at promoting forest biomass production can help meet the growing demand for renewable materials and mitigate climate change. One-time nitrogen (N) addition late in the rotation is a wellestablished method to stimulate growth in coniferous forests in northern Europe, but the potential gains from earlier and repeated fertiliser application remain uncertain. Here, we tested the impact of repeated fertilisation in juvenile Norway spruce stands across 9 sites covering a wide range of growing conditions over a 700 km stretch from central to southern Sweden. We tested the fertilisation effects using two separate studies: i) an interval trial with a fertilisation frequency of one (F1), two (F2), or three years (F3) performed at plot-level across five sites (2002-2014), and ii) a practice-oriented trial with a two-year fertilisation interval (F2) applied at stand-level and replicated at four sites (2003-2013). The composition of the nutrient mix in each plot was optimised based on foliar nutrient analyses. In the interval trial, all three fertilisation schedules strongly increased periodic annual increment (PAI) (F1: 105 %, F2: 93 %, F3: 79 %) relative to the unfertilised control, resulting in more than a doubling of stem volume yield in the F1 and F2 treatments (110 % and 120 %, respectively) and a significantly smaller but still sizeable yield stimulation of 82 % in the F3 treatment. Nitrogen use efficiency (NUE, stemwood volume increase per unit mass of N added) was similar among fertilisation intervals (on average 130 m3 ha-1 1000 kg N-1), indicating that the extra N provided through yearly fertilisation (F1) is redundant given the similar stemwood yields in the F2 treatment. In the practice-oriented trial, the sole F2 treatment increased PAI by 95 % over the control, translating into a yield stimulation of 114 % and an almost identical NUE to that of the interval trial. NUE greatly exceeded the figures typically observed with traditional late-rotation fertilisation and correlated inversely with baseline site productivity (using site index as a proxy) in the F1 and F2 treatments (the latter pooled across the two trials). Our results clearly indicate that nutrient limitation restricts growth and carbon (C) capture in young Norway spruce plantations in northern Europe to less than half of their potential, highlighting repeated fertilisation at nutrient-poor sites as an effective management tool to support a growing bioeconomy and enhance C sequestration.