Half-a-century effects of a slow-release nitrogen fertilizer, ureaformaldehyde, on stand growth and soil processes in a Scots pine stand

Slow-release nitrogen fertilizers have received little attention as alternatives to conventional fast-release fertilizers in N-limited boreal forests. Slow-release fertilizers better match the rate at which trees can uptake nutrients from the soil solution and are less likely to be vulnerable to losses to the environment. Ureaformaldehyde is a polymer that is synthesized from formaldehyde and urea and mainly used as a nitrogen fertilizer in lawns and greenhouse cultivations. Our goal was to assess ureaformaldehyde (UF) as a forest fertilizer. We aimed to determine its long-term effects on tree ring widths and stand basal area growth and how the effects differ from those of a conventional fast-release N fertilizer, ammonium nitrate (AN). We also attempted to determine whether the fertilizer additions changed soil organic matter properties in the long term. The study site was an unfertile Scots pine (Pines sylvestris L.) stand with study plots with both fertilizer treatments that were performed 47 years ago and unfertilized control plots. As expected, AN addition resulted in a sharp increase in tree ring widths and the annual basal area increment that lasted for approximately seven years. UF addition caused a slower increase that lasted much longer, approximately 17 years. In addition, the UF addition showed indications that it slightly enhanced the growth even during the last decade. Our results also suggested that different fertilization treatments resulted in differences in tree size distribution. After almost a half century, some signs of UF-induced effects were still visible in soil organic matter; for instance, the humus layer in the UF treatment had a lower C-to-N ratio by 7 units than that in the humus layer of the AN treatment. In conclusion, slow-release ureaformaldehyde has a considerably longer effect on tree growth than conventional ammonium-nitrate fertilizer and, after almost a half century, there were signs that it still increased nitrogen availability for trees.