Effects of Biochar and Nitrogen-Enriched Biochar on Tree Growth in Scots Pine Stands

Abstract

Biochar, a carbon-rich product of pyrolysis, is increasingly considered for soil amendment and climate change mitigation due to its potential to enhance soil properties and sequester carbon. However, its effects on tree growth in forest ecosystems remain uncertain. This study investigated the impact of biochar and nitrogen-enriched biochar on the growth of two middle-aged (approximately 60-year-old) Scots pine stands in southeastern Norway. Both stands were characterized by medium site indices and podzolic soils. In replicated field experiments, we applied four treatments consisting of an unfertilized control, and addition of biochar (2.5 t ha⁻¹), nitrogen fertilizer (150 kg N ha⁻¹), or biochar loaded with nitrogen (2.5 t biochar +150 kg N ha⁻¹). The biochar was produced by pyrolysis of Norway spruce wood chips at 600°C. After 5 years, only treatments containing added nitrogen (either as mineral fertilizer or as nutrient-enriched biochar) significantly increased basal area and volume growth compared to control. No significant difference in timing or magnitude of effects was observed between the nitrogen and biochar + nitrogen treatments, except better annual growth in the combined treatment the third year after fertilization, indicating rapid nitrogen release and uptake regardless of carrier. Pure biochar did not stimulate tree growth. While biochar did not negatively affect growth, its direct role as a growth stimulant was not supported under these conditions, at least in the short-term with the dose of 2.5 t ha⁻¹. Further research is needed across different forest ages, site types, and application rates to fully understand biochar's potential in forestry.