Dynamics of nitrogen (¹⁵N) in decomposing Scots pine (Pinus sylvestris) needle litter. Long-term decomposition in a Scots pine forest. VI

Labelled ( 15 N) Scots pine (Pinus sylvestris L.) needle litter was incubated in one laboratory and two field experiments and the nitrogen dynamics was followed during the decomposition process. Release of the litter's original nitrogen began immediately after the start of the decomposition process. Simultaneously an immobilization of nitrogen to the litter took place. Very little of the actual turnover of nitrogen in the litter, which was considerable, was reflected by the dynamics of total nitrogen. The dynamics of nitrogen in the lignin fraction (acid-insoluble substance) was partly different from that of whole litter. The dynamics of total nitrogen coincided so that a net loss of total nitrogen started simultaneously both in whole litter and in the acid-insoluble fraction. As the amount of acid-insoluble substance increased in the early stage, the amount of 15 N associated to this fraction increased and when a net disappearance of acid-insoluble substance started, a net release of 15 N from this fraction started simultaneously. This was interpreted to mean that some of the litter's own nitrogen was transformed into compounds that could be incorporated into humic and fulvic acids (e.g., NH 3 or amino acids), which were analytically recorded as acid-insoluble substance.