Cultivation of Norway spruce and Scots pine on organic nitrogen improves seedling morphology and field performance

► Effects of nitrogen source on seedling traits and performance were studied. ► Seedlings cultivated on organic nitrogen allocated relatively more biomass to roots. ► Seedlings with higher root: shoot ratios performed better in field. Nitrogen availability exerts a significant control on biomass allocation of plants including Norway spruce (Picea abies (L.) Karst.) and Scots pine (Pinus sylvestris (L.)) in boreal forest ecosystems. Recent studies suggest, however, this control differs for inorganic and organic nitrogen sources. The importance of the chemical form of nitrogen (inorganic or organic) for the morphology and growth of conifer seedlings was studied during production of seedlings in a forest nursery and subsequently in a field trial in northern Sweden. Seedlings were supplied with two different nutrient solutions; an inorganic conventional fertilizer and an organic, amino acid-based fertilizer. Seedlings cultivated on the organic nitrogen source displayed larger root systems resulting in a higher root: shoot ratio than did seedlings cultivated on the inorganic nitrogen source. The proportion of fine roots to lateral roots and the root tip proportion colonized by mycorrhiza were positively affected by the organic nitrogen source. Norway spruce seedlings cultivated on organic nitrogen displayed significantly increased shoot growth compared to seedlings cultivated on inorganic nitrogen. Our results suggest that the chemical form of nitrogen influences the allocation of biomass in conifer seedlings. The shift in allocation of resources to root biomass further leads to a competitive advantage in field conditions, resulting in a significant increase in shoot growth one year following transplant.