Response of Coniferous Forest Ecosystems on Mineral Soils to Nutrient Additions: A Review of Swedish Experiences

Nitrogen (N) is the only nutrient that promotes forest growth when given individually. An extra stem growth of 15 m 3 ha -1 is obtained during a 10 yr period following an application of 150 kg N ha -1 . Larger growth increases have often been the result of more intensive N fertilization. Lime or wood ash give a minor growth stimulation on sites with a carbon (C) to N ratio below 30 in the humus layer, while the opposite effect prevails on N-poor sites. Nutrients given as soluble fertilizers are readily taken up by trees. Boron deficiency may be induced in northern Sweden after N fertilization or liming. The ground vegetation may be altered by single-shot N fertilization, but long-term effects occur only for intensive regimes. Lime or wood ash may modify the flora if soil pH is significantly altered: the change will be in response to N availability. Fruit-body production of mycorrhizal fungi is disfavoured by chronic N input, but also by lime or ash. However, the mycorrhizal structures on root tips are less affected. Faunistic studies are not common and those present are mostly devoted to soil fauna. A practical N dose of 150 kg N ha -1 has no clear effect, but higher doses may reduce the abundance in some groups. Hardened wood ash does not significantly affect the soil fauna. Lime favours snails and earthworms, while other groups are often disfavoured. The response of aquatic fauna to terrestrial treatments has hardly been studied. N fertilization generally results in insignificant effects on fish and benthic fauna. Lime and wood ash reduce the acidity of the topsoil, but practical doses (2-3 t ha -1 ) are too low to raise the alkalinity of runoff unless outflow areas are treated. N fertilizer use in forestry and N-free fertilizers lack effects on acidification. N fertilization may, however, be strongly acidifying if nitrification is induced and followed by nitrate leaching. N fertilization often results in increased long-term C retention in trees and soil, but does not promote significant N 2 O losses. N may temporarily reduce CH 4 oxidation in soil, but there are indications of a long-term opposite effect. Lime and poorly hardened wood ash may cause losses of C from N-rich soils. Only a few per cent of added N are leached to surface water following practical N fertilization, while N-free fertilizers do not induce N leaching. Soil incubations and soil-water studies suggest an increased risk for nitrate formation and leaching where lime or wood have been added to N-rich soils, but increased leaching to surface water has not been observed. Wood ash causes a temporal increase in bioavailability of cadmium (Cd). Other fertilizers may indirectly increase the availability of heavy metals. Wood ash may contain radioactive caesium 137Cs, but addition of such ash does not increase radioactivity in plants and soil.     

Fertilization of Danish Forests: A Review of Experiments

Fertilization experiments in Norway spruce stands on nutrient-poor sites have been practised in Denmark since the eighteenth century. Until 1950, the main aim was to find a nitrogen (N) source that improved the survival of plants in the early growth phase. The N supply could be improved by intercropping with N 2 -fixing plants, and fertilization with phosphorus (P) and potassium (K) improved the growth of the N 2 fixers, thereby improving tree growth and survival. Positive responses of P and K in spruce cultures were seen in the 1960s, and in most cases the N response was dependent on the supply of P and K. Fertilization experiments in mature spruce stands began in the 1960s. Despite 20 yrs of experimentation, no trends can be deduced. The results discouraged fertilization aimed at increasing growth in mature stands, and fertilization stopped in 1989. After 1980, fertilizers were used in experiments aimed at clarifying the causes and effects of forest decline. Fertilization and liming counteracted soil acidification and increased needle nutrient concentrations when they were below deficiency levels. However, positive growth responses have only been recorded in some of the experiments where nutrient deficiencies were overcome. The experiments have not resulted in practical revitalization programmes. Future fertilization of Danish forests may be limited to systems where nutrient extraction is in excess of the carrying capacity of the ecosystem, and presumably where spruce plantations are converted to broadleaved forests. Finally, future events of deposition of acidifying compounds may necessitate counteractive measures.     

The influence of different forest organic matter on the growth of one-year old planted Norway spruce seedlings in a greenhouse experiment

A greenhouse trial was carried out to improve the knowledge of how forest organic matter could be utilized in site preparation and the choice of planting spot for Picea abies (L.) Karst. One-year old Norway spruce seedlings were grown for 26 weeks in pots containing pure mineral soil, forest organic matter in three different states of decomposition and combined treatments where the organic matter was mixed with or placed on the surface of the mineral soil (volume proportions of organic:mineral of 1:2). Watering was adapted to the water retention characteristics of each growing medium in order to keep the soil water potential between -4 and -5 kPa in all pots.In its pure form as well as combined with mineral soil, the moderately decomposed mor organic matter gave a higher seedling dry weight increment than the corresponding growing media containing the more decomposed humus. Adding mor or humus to mineral soil increased the seedling dry weight increment by 30–140% and 10–40%, respectively. The largest seedlings grew in the pure mor and humus. Mixing these forms of organic matter with mineral soil appeared to reduce seedling growth as compared to placing them on the mineral soil surface. The increment increase compared to plain mineral soil mostly consisted of proleptic growth and was probably due to the improved nutrient availability of the growing media. Fresh organic matter (chopped needles and twigs) seemed to impair seedling root function and reduced the dry weight increment by 30–50% as compared with pure mineral soil.The results suggest that as long as the water and temperature requirements are fulfilled planting Norway spruce seedlings without scarification and/or gathering extra mor and humus to the planting position should improve seedling growth as compared to the growth following scarification. Unmixed fresh needles and twigs or fresh needles and twigs combined with plain mineral soil should be avoided when planting Norway spruce seedlings.