From Litter to Humus in a Norwegian Spruce Forest: Long-Term Studies on the Decomposition of Needles and Cones

Abstract

Abstract:The aim of this review and synthesis is to illustrate the gradual transformation of needles and cones from litter to humus. Individual needles may follow quite different decomposition pathways, which contributes to a diverse humus structure. In the litter layer (Oi), about 40% of the needles were excavated by special mites that produced slowly decomposable excrements. In the fermentation layer (Oe), needles which happened to be in close contact with fine roots decomposed more rapidly. Cones decomposed slower than needles during the first 3-5 years, so the role of cones in carbon (C) storage may be greater than indicated by their fraction of fresh litter. Over a 13 years period, potassium (K), magnesium (Mg) and phosphorus (P) in cones was released, while the total amount of calcium (Ca), manganese (Mn), iron (Fe) and aluminium (Al) increased strongly. Nitrogen concentration increased but the total nitrogen content remained rather constant. After 13 years, the cones had sunk about 6 cm into the soil and lost 60% of their dry weight but were morphologically intact. A cone monitored for 28 years was fully recognizable and had not yet reached the stable Oa layer. The most inert decomposition products in the Oa layer were fragments of needles and cone scales, microarthropod excrements and chitinous insect remains.

Succession of Mite Assemblages (Acari, Mesostigmata) during Decomposition of Tree Leaves in Forest Stands Growing on Reclaimed Post-Mining Spoil Heap and Adjacent Forest Habitats

“…This may be explained by the fact that colonization rate is limited and impeded on newly formed soils [13]. Soil biological activity on degraded lands is relatively low [33,59] due to disturbed structure of the soil profile, and lack of appropriate water and air conditions [6] or amount of organic matter [7], whereas on undisturbed habitats, such as forest habitats, the upper layer of soils may be well covered by a litter layer [60] or vegetation which can be important buffers in ecologically stressful situations, for example droughts [19].…”

Ecological Spotlights on Mites (Acari) in Norwegian Conifer Forests: A Review

“…A considerable number of spruce needles were decomposed from the inside by certain specialized oribatid mites [22,26]. Smaller, deeper-living species became abundant after 5-10 years, when the needles had been more or less fragmented.…”

“…Juveniles of certain specialized mites excavated cavities in about 40% of newly fallen spruce needles. Their activity reduced the decomposition rate of the actual needles, at least temporarily, probably because their excrements decomposed slowly [22,26]. The adult mites, which hatched after about 2 years, attacked other needles from the outside and fragmented these ( Figure 5).…”

“…The present review is a synthesis of mite studies in coniferous forest soils of Southern Norway, published over a 40-year period [9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26]. In the 1970s, extensive studies on soil microarthropods were initiated as part of a large project, "Effects of acid precipitation on forest and soil," and certain subjects were followed up long after the project was ended.…”

Determining Forest Duff Water Content Using a Low-Cost Standing Wave Ratio Sensor

“…The forest floor plays a significant role in forest hydrological processes by affecting water and energy transfer between the sub-canopy atmosphere and the mineral soil [ 1 , 2 , 3 ]. It can be divided into the surface litter layer (Oi), with freshly fallen plant residue, the sub-surface fermentation layer (Oe), with partially decomposed but still recognizable organic material, and the underlying humus layer (Oa), with well decomposed organic matter [ 4 , 5 ]. Fermentation and humus are collectively known as forest duff.…”