Organic carbon accumulation and pH changes in an Andic Gleysol under a long-term fertilizer experiment in Iceland

The effects of long-term fertilizer use on soil pH and accumulation of soil organic matter are assessed under permanent grassland. Ammonium nitrate, 75 and 120 kg N ha−1, and 120 kg N ha−1 each of ammonium sulphate and calcium nitrate were applied annually for 43 years in an experiment on permanent grassland established on a drained andic gleysol. Ammonium nitrate had little effect on soil pH, whereas ammonium sulphate decreased the pH from about 6.4 to 3.8 in the top 5 cm of the soil, and the pH to 60 cm depth was less than in other plots. Calcium nitrate caused a slight increase in pH to 6.9 in the top 5 cm and to >7 at greater depths. The fertilizers increased organic matter in the top 5 cm of the soil from 6.9–8.8% C in 1973 to 12–21% C in 1996. The accumulation is confined mainly to the top 10 cm, and in this layer, the annual increase in organic C is 0.6–1.0 t ha−1. With a C/N ratio of 12–15, this means a yearly increase in N of 45–65 kg ha−1. Organic C increased in the order ammonium nitrate>calcium nitrate>ammonium sulphate, whereas the increase in N followed the order ammonium nitrate>ammonium sulphate>calcium nitrate. The difference in accumulation of C and N leads to diverging C/N ratios, suggesting that the most favourable humus with a narrow C/N ratio occurs where calcium nitrate is used, and the most unfavourable humus occurs where ammonium sulphate is applied over a prolonged period.