Forest regeneration operations increase the concentration of nitrogen (N) in watercourses especially outside the growing season when traditional biological water protection methods are inefficient. Biochar adsorption-based water treatment could be a solution for nutrient retention. We studied the total nitrogen (TN) and nitrate–nitrogen (NO3−–N) adsorption–desorption properties of spruce and birch biochar. The adsorption test was performed under four different initial concentrations of TN (1, 2, 3, and 4 mg L−1) using forest runoff water collected from ditch drains of boreal harvested peatland. The results showed that the TN adsorption amount increased linearly from the lowest to the highest concentration. The maximum adsorption capacity was 2.4 and 3.2 times greater in the highest concentration (4 mg L−1) compared to the lowest concentration (1 mg L−1) in spruce and birch biochar, respectively. The NO3−–N adsorption amount of birch biochar increased linearly from 0 to 0.15 mg NO3−–N g biochar−1 when the initial concentration of NO3−–N increased from 0.2 to 1.4 mg L−1. However, in spruce biochar, the initial concentration did not affect NO3−–N adsorption amount. The results indicate that concentration significantly affects the biochar’s capacity to adsorb N from water. The desorption test was performed by adding biochar extracted from the adsorption test into the forest runoff water with low TN concentration (0.2 or 0.35 mg L−1). The desorption results showed that desorption was negligibly small, and it was dependent on the TN concentration for birch biochar. Therefore, biochar can be a complementary method supporting water purification in peatland areas.

<p>In boreal peatland forests, drainage and harvesting increase nitrogen (N) export to watercourses and the highest N concentration in runoff water occurs outside the growing season when traditional water protection methods based on biological activity are inefficient. In these conditions, water purification based on adsorption could offer a solution. Biochar can be an effective sorbent material for removal of nutrients from water due to its high specific surface area, porous structure and high cation and anion exchange capacity. We tested adsorption capacity for total N (TN) of spruce and birch biochar using water collected from ditch drains of boreal harvested peatland. The water was collected outside the growing season when TN concentration was 4.6 mg L<sup>-1</sup>. During the growing season, TN concentration varies from 0.5 to 2 mg L<sup>-1</sup>. To study the effect of TN concentration on adsorption capacity, we diluted water samples to concentrations 1, 2, 3 and 4 mg L<sup>-1</sup>. We added 5 g of biochar to 1 L of water and shook the samples for 180 h. TN adsorption capacity increased monotonously from the smallest to the highest concentration. Adsorption capacity was 2.6 and 3.7 times greater in the highest concentration compared to the lowest concentration in spruce and birch, respectively. This indicates that concentration affects significantly the capacity of biochar to adsorb N from forest runoff water. Therefore, biochar can be an effective water protection tool in areas, which have high TN concentration, and it can be a complementary method supporting water purification outside the growing season.</p>