Potential nitrogen transformations in mineral soils of two coniferous forests exposed to different N inputs

Abstract

The objective of this study was to quantify the net potential N mineralisation and nitrification rates in mineral soils of two coniferous forest soils subjected to different N additions. One study site was located at Åmli ( Pinus sylvestris L. forest), southern Norway and another at Gårdsjön ( Picea abies forest), southwestern Sweden. Mineral soil was collected from 14 to 19 and 24 to 29 cm depth in May at Gårdsjön and in May, July, September and November at Åmli. The soil was incubated in the dark at 15 °C for 2 months in the laboratory, and the NH 4 + and NO 3 − content were compared relative to pre-incubated values. The initial contents of NO 3 − and NH 4 + in the soil at the two sites were of about the same magnitude, but the potential net N mineralisation, ammonification and nitrification rates differed significantly. At Gårdsjön, the net N mineralistion ranged from 6 to 29 mg N kg −1 per 2 months, whereas a net N immobilisation generally occurred at Åmli. Additions of 0–50 kg N ha −1 per year had no effect on the transformation rates at the two sites. Applications of large single doses of 90 kg N ha −1 per year during the last 8 years have significantly increased the net nitrification rate in the soil at Åmli. However, a net N mineralisation was only observed in the soil 1 month after the N addition. At low N input levels, site-specific factors, such as the content of organic matter, clay, and moisture, seemed to a large degree to determine the transformation rates. Large spatial variability both within catchments and between catchments at Gårdsjön may have obscured the effects of small N inputs.