Mineral nitrogen dynamics in poorly drained blanket peat

Summary

Mineral-N dynamics have been measured over a period of 3 years in PK- and NPK-treated plots (4 m²) laid out on an area of poorly drained, reseeded, blanket peat in the north of Scotland. Mineral-N, present in the peat almost entirely as NH ^sup+_in4 , accumulated in winter, reaching 42 kg N ha⁻¹ in the surface 10 cm in April before the application of 112.5 kg N ha⁻¹ as NH₄NO₃ or urea. In situ incubation of peat cores isolated to prevent leaching, and with grass tops removed, confirmed that net mineralization occurred between November and April, with the greatest rate, 1.2 kg N ha⁻¹ day⁻¹, recorded between March and April. During the period May to early June, immobilization of N predominated and rates of net immobilization ranged between 0.2 and 0.8 kg N ha⁻¹ day⁻¹. This coincided with a poor uptake into herbage, less than 16% of soil mineral N and fertilizer NH₄NO₃ in June of the first 2 years. The largest counts (most probable number) of ammonifying bacteria in the surface 5 cm were recorded in July for aerobes (27.1×10⁹ litre⁻¹) and August for anaerorbes (7.1×10⁹ litre⁻¹). N fertilizer increased these counts significantly (P<0.05) to 56×10⁹ aerobes and 13×10⁹ anaerobes. During July and August, in 2 out of the 3 years, mineralization predominated over immobilization and mean net rates of up to 0.9 kg N ha⁻¹ were recorded.