Soil mineral nitrogen dynamics in plots of organically and conventionally grown potato

In the DOK long-term field experiment two organic (D: biodynamic; O: bioorganic) and two conventional (K: mineral fertilizer plus farmyard manure; M: mineral fertilizer only) farming systems have been compared. In 1999, 2001 and 2002 dynamics of mineral nitrogen (Nmin) in plots planted with potato was analysed in the 0 to 100 cm soil layer. The preceding crop was always clover grass. In spring the plots were prepared by using a mouldboard plough followed by a rotary harrow. In the D, O, K and M treatments, a respective total of 25, 60, 110 and 110 kg plant available N/ha was applied. Mineralization and leaching of N was not consistent among years, which may be explained by differences in climatic conditions. Therefore, maximal Nmin contents in the 0 to 60 cm soil layer measured in May varied between 50 (1999) and 170 kg N/ha (2002). In June, when N uptake of potato usually is highest, Nmin contents in soils decreased in all cases to about 10 kg N/ha and afterwards continuously increased until senescence of the potato plants. Despite different N input, the dynamic of mineral N was similar in all treatments except for D and O in 1999, when an early infestation by potato late blight resulted in early senescence and thus in a reduced N uptake. After harvest Nmin contents tended to be higher for D, O, and K with organic fertilizers than for M with mineral fertilizer only. In conclusion, the risk for N leaching is highest after harvest of potato, which might be prevented by cultivation of a fast-growing cover crop immediately after harvest.