Michael Starr (firstname.lastname@example.org) & Liisa Ukonmaanaho (email@example.com)
Finnish Forest Res. Inst., P.O . Box 18, FIN-01301 Vantaa, Finland.
As part of the UN/ECE Integrated Monitoring programme, the concentrations of nitrate and ammonium in bulk precipitation (BD), throughfall (TF) and soil water (SW) has been monitored at 4 forested catchments in Finland since 1989 (see Ukonmaanaho et al. this volume). The sites are located in remote, undisturbed, old-growth forests and can therefore be expected to have relatively low N requirements compared to aggrading, managed forests. Although the deposition of NO3- and NH4+at the sites was low (1.2 and 1.0 kg N ha-1 yr-1, respectively, averaged across all sites and study years), there were clear differences between catchments. These differences were mainly related to differences in precipitation, i.e. deposition decreased northwards. We present monthly BD, TF, and SW concentration data (volume weighted) for the period 1989-95, together with the total N concentrations in needles collected annually (NC), litterfall collected monthly during the snow-free periods in 1990-95 (LF), and forest floor (Of+Oh) sampled at the beginning of the monitoring period (SC). TF for the first 5 years and SW were for the snow-free periods only. Calculation of organic N concentrations as well as N pools and fluxes are under preparation.
The mean concentrations of NO3--N and NH4+-N in BD and TF decreased northwards (Fig. 1). BD-TF values were negative, indicating canopy uptake, but the difference decreased northwards and disappeared at Vuoskojärvi. Seasonal Kendall statistics indicated a significant (P<0.05) linear annual trend (negative) in BD NH4+-N concentrations at all catchments except Vuoskojärvi. The only significant trend (negative) for NO3--N was at Hietajärvi. There were no significant trends in TF NO3--N and NH4+-N concentrations. Relatively high concentrations at Vuoskojärvi in May 1995 were thought to be due to pollen. SW concentrations of NO3--N and NH4+-N at all sites were less than in BD and TF, indicating no net leaching. There was little difference in SW concentrations between catchments and sampling depths (15 and 35 cm), and no trends observed. In contrast to BD, mean SW NH4+-N concentrations were greater than NO3--N concentrations. Scots pine needle N concentrations varied little between catchments and years; 1 year old needles had lower mean concentrations compared to current needles (Table 1). Mean N concentrations in litterfall were lower than in needles and the humus layer.
With the exception of Vuoskojärvi, there appeared to be effective retention of NO3- and NH4+ within the ecosystems. The similarity in BD, TF and SW inorganic N concentrations at Vuoskojarvi, the northernmost catchment with a growing season averaging 57 d, may indicate that this catchment is approaching nitrogen saturation in spite of having the lowest levels of deposition.
Fig. 1. Monthly mean ammonium and nitrate nitrogen concentrations in BD, TF (two plots), and SW (15 and 35 cm) at each catchment (error bars are standard deviations).
Table 1. Current (c) and 1-year-old (c+1) Scots pine needle (NC), litterfall (LF) and humus layer N concentrations (%). Humus later N pools are also given.
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