Vertical transport of decomposing spruce needles during nine years in a raw humus soil profile in southern Norway

Abstract

In a podsol profile, the amount of time passed after spruce ( Picea abies ) needles fall from the trees is correlated with their depth position in the organic soil layer. This relationship was examined by dropping marked needles in small patches on the ground and collecting them at various intervals during the following nine years. Needles dropped in moss-covered patches mostly reached the dense needle mat at the bottom of the green moss (“zero depth”) after about one year. After two years, it was estimated that about 60% of the initial dry mass was lost. After three years, the needles had an average depth position of 1 cm below “zero”, corresponding to the transition between the O l and O f layer. Needles of the same age were situated at varying depths, mainly because of variation between individual patches, but also because not all needles were caught by the moss vegetation at the start. The downward transport rate slowed down with time. After nine years, the mean needle depth was 3.1 cm below “zero”, about halfway to the bleached mineral layer and in the upper part of the fine root layer. Needles situated close to roots were often almost completely disintegrated. The total vertical transport through the organic layer evidently takes many decades. Needles dropped on bare needle mats underneath large trees had a mean depth position of 1.6 cm after nine years.