Open access publications in the SLU publication database

Abstract

Nitrate leaching from growing forests is not yet a widespread problem in Sweden, but nitrogen (N) deposition and high N retention in the soil during recent decades might have changed forest ecosystems towards N saturation. In this thesis net N mineralisation (measured with in situ incubations) was tested as an index of N status. It was shown that historic land use and N deposition, natural or simulated in fertilisation experiments, had caused great differences in this index (4-104 kg N ha -1 yr -1 ) and that N leaching occurred at sites with high N status (net N mineralisation > 60 kg N ha -1 yr -1 ). The net N mineralisation was well correlated to C/N ratio, which could also be used as a N status index. The amount of nitrate lost was not dependent on N status or current N deposition, but rather to the total N flux density (deposition + net mineralisation). The relatively fast change (30 years) from a highly N limited ecosystem to a system apparently approaching the first stage of N saturation upon moderate N fertilisation (35 kg N ha -1 yr -1 ) motivates continued concern about N saturation in Sweden. Predicting future response to N deposition and forest management necessitates a thorough understanding of all important N fluxes, but the importance of organic N uptake in Swedish spruce forest ecosystems is still unclear. Budget calculations indicated that uptake of organic N is of great importance (80%) in low input areas. In high N-input plots the relative importance appeared lower (35%). However, soil solution samples from the forest floor contained low concentrations of both amino acids and inorganic N, and the lack of a clear dominance of amino acids might be explained by a tight circulation of amino acids, never releasing the amino acids into the soil solution fraction sampled with tension lysimeters