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

Abstract

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 C/N 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 NO 3 − 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 NO 3 − 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 NO 3 − 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 3 − leaching in our data set. There was a threshold value at about 90 kg N ha −1 per year below which no NO 3 − leaching occurred and above which leaching and soil flux density of mineral N were positively correlated.