The effects of nitrogen (N) deposition on plant litter and soil organic matter decomposition differ depending on the stage of decomposition (early, late, and final stages). The effects can be divided further into direct and indirect ones. Direct effects: additions of ammonium and nitrate to fresh, newly shed litter stimulate the initial decomposition of celluloses and solubles. By contrast, addition of the same compounds to humus (final stages) clearly suppresses activity. This was seen in all studies reviewed and for several types of humus. Indirect effects: long-term deposition leads to increases in litter concentrations of N and other nutrients. This N in litter forms "natural" organic compounds and the resulting effects are similar to those resulting from natural variation among litter types. Thus, initial decomposition is generally higher for N (nutrient) rich plant litters than for litters with a lower N (nutrient) content. In later stages, at which lignin-degradation rates regulate litter decomposition, N has a retarding effect on decomposition. Significant negative correlations have also been found between N concentrations in humus and respiration rate. There probably is a sink for deposited N in the humus. We may conclude that N storage in humus is regulated by a positive feedback mechanism. Raised levels of N resulting from N deposition cause more humus to be left in the system, and the resulting lower levels of Mn further retards humus decomposition, thus leading to an increased storage of N in humus. Thus, when calculating critical loads it would be incorrect to assume that N pools in the humus remain at a steady state.