Nitrification and ammonification in acid forest litter and humus as affected by peptone and ammonium-n amendment

Abstract

Samples of strongly acid forest litter and humus from beneath Sitka spruce, heather, Scots pine and larch from two sites in north-east Scotland were incubated aerobically at 20°C in the laboratory. At the Glen Tanar site, spruce litter and larch humus showed significant nitrification and ammonification whereas spruce humus and Scots pine humus produced only NH₄⁺-N. Heather humus showed no net mineralization. At the Fetteresso site, application of fertilizer N, P and K to Sitka spruce up to 3 yr previously, significantly stimulated the production of NO₃⁻-N in both litter and humus. Amendment of the samples with organic N as peptone caused significant increases in NO₃⁻-N production in those samples that already showed nitrification. The increases in NO₃⁻-N generally represented a low proportion of the added peptone-N. Amendment with NH₄⁺-N as (NH₄)₂SO₄ either had no effect or significantly reduced NO₃⁻-N production (in larch humus). The results suggest the occurrence of heterotrophic nitrification in some of these forest samples. Net immobilization of NH₄⁺-N was typically greater in NH₄⁺-N amended than in peptone amended samples, except for heather humus which showed complete immobilization of both N sources. Total mineral N produced at the end of the aerobic incubation was correlated (P < 0.01) with NH₄⁺-N produced during a 30-day anaerobic incubation at 30°C. Net NO₃⁻-N production was greater in litter than in the corresponding humus samples and was correlated (P < 0.001) with initial organic N soluble in 1 m KCl.