Nitrogen transformations in the former subaqueous soils of polders recently reclaimed from Lake Ijssel

Summary

In incubation experiments, carried out at 29°C with a calcareous loam soil which had been enriched with ammonium nitrogen and kept in shallow jars, losses of mineral nitrogen, due to denitrification of nitrates previously formed at a slow rate, were found to occur when the moisture content was 90 to 100 per cent of the water capacity (W.C.).

When the soil was filled more compactly into the jars losses due to denitrification occurred even when the moisture content was higher than 75% W.C.

In experiments in which columns of the same soil, 1.20 m in length and enriched with added ammonium nitrogen, were incubated at temperatures of 18 to 23°C it was found that losses of nitrogen, due to denitrification, also occurred in certain layers at 80 to 100 per cent of saturation with water. Such soil layers appear to contain aerobic and anaerobic zones side by side whereby nitrification and denitrification can occur simultaneously. Compactness of the soil and the depth of the soil layer below the surface obviously play an important role, so that under some circumstances denitrification may take place even at 70% W.C.

Large losses of nitrogen due to denitrification were recorded when a heavy soil, initially having a high moisture and ammonium content, was incubated at room temperature and allowed to dry out slowly in tubes 46 cm in length.

In 1959 about 22 kg nitrogen per hectare were lost by leaching from the East Flevoland polder whilst in 1960 and 1961, two exceptionally rainy years, about 30 and 23.8 kg per hectare respectively were lost.