Indices for nitrogen status and nitrate leaching from Norway spruce (Picea abies (L.) Karst.) stands in Sweden

The aims of the study were to evaluate the influence of former N deposition and land-use on N status expressed as net N mineralisation and soil ON ratio, and to evaluate the relationship between N leaching and N status, N deposition or a combination of both. Net nitrogen (N) mineralisation and C/N ratio in the soil were measured at seven Swedish sites, all dominated by Norway spruce (Picea abies (L.) Karst). The atmospheric N deposition at the sites ranged from 3 to 24 kg N ha(-1) per year. At three of the sites, fertilised plots (30-75 kg N ha(-1) per year) were also included in the study. The rate of NO3- leaching ranged from 0 to 25 kg N ha(-1) per year. Measured net N mineralisation rate ranged from 4 to 104 kg N ha(-1) per year and was correlated with the soil C/N ratio, both depending on previous N inputs and former land-use. Nitrate leaching was poorly correlated with N input (atmospheric deposition + fertilisation), but more so with the net N mineralisation: i.e. enhanced NO3- leaching was always but only detected at sites with high net N mineralisation. The relative mineralisation rate, i.e. the ratio between the actual rate and a site-specific maximum rate, as simulated by the PnET-CN model, was no better an index of NO3- leaching than was the absolute mineralisation rate, mainly because only one tree species was considered. The soil flux density of mineral N (N input + net N mineralisation) was the best index of NO,leaching in our data set. There was a threshold value at about 90 kg N ha(-1) per year below which no NO3- leaching occurred and above which leaching and soil flux density of mineral N were positively correlated. (C) 2002 Elsevier Science B.V. All rights reserved.